Schalm's Veterinary Hematology

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S C H A L M ’ S

VETERINARY HEMATOLOGY

S C H A L M ’ S

VETERINARY HEMATOLOGY SIXTH

EDITION

EDITORS

...

DOUGLAS J. WEISS DVM, PhD, DACVP Emeritus Professor College of Veterinary Medicine University of Minnesota St. Paul, Minnesota

K. JANE WARDROP DVM, MS, DACVP Professor and Director, Clinical Pathology Laboratory Department of Veterinary Clinical Sciences College of Veterinary Medicine Washington State University Pullman, Washington

A John Wiley & Sons, Ltd., Publication

Sixth Edition first published 2010 © 2010 Blackwell Publishing Ltd Fifth Edition © 2000 Lippincott Williams & Wilkins, First printing Fifth edition © 2006 Blackwell Publishing, Second printing. Blackwell Publishing was acquired by John Wiley & Sons in February 2007. Blackwell’s publishing programme has been merged with Wiley’s global Scientific, Technical, and Medical business to form Wiley-Blackwell. Editorial Office 2121 State Avenue, Ames, Iowa 50014-8300, USA For details of our global editorial offices, for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at www.wiley.com/wiley-blackwell. Authorization to photocopy items for internal or personal use, or the internal or personal use of specific clients, is granted by Blackwell Publishing, provided that the base fee is paid directly to the Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923. For those organizations that have been granted a photocopy license by CCC, a separate system of payments has been arranged. The fee codes for users of the Transactional Reporting Service are ISBN-13: 978-0-8138-XXXX-X/2007. Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought. Library of Congress Cataloguing-in-Publication Data Schalm’s veterinary hematology. – 6th ed. / editors, Douglas J. Weiss, K. Jane Wardrop. p. cm. Includes bibliographical references and index. ISBN 978-0-8138-1798-9 (hardback : alk. paper) 1. Veterinary hematology. I. Weiss, Douglas J. Jane. III. Schalm, O. W. (Oscar William), 1909– IV. Title: Veterinary hematology. [DNLM: 1. Hematologic Diseases–veterinary. SF 769.5 S2981 2010] SF769.5.S3 2010 636.089′615–dc22 2009020168 A catalogue record for this book is available from the British Library. Set in 10 on 11 pt Palatino by Toppan Best-set Premedia Limited Printed in Singapore 1

2010

II. Wardrop, K.

A S S O C I AT . E. .E D I T O R S Mary K. Boudreaux DVM, PhD Marjory B. Brooks DVM, DACVIM Mary Beth Callan VMD, DACVIM Joanne B. Messick VMD, PhD, DACVP Jaime F. Modiano VMD, PhD Andreas Moritz Dr. med. vet. PD, DECVIM-CA Assoc. ECVCP Rose E. Raskin DVM, PhD, DACVP Erik Teske DVM, PhD, DECVIM-CA K. Jane Wardrop DVM, MS, DACVP Douglas J. Weiss DVM, PhD, DACVP Maxey L. Wellman DVM, PhD, DACVP

D E D I C AT I O N

To those veterinary professionals who made the critical observations and performed the key experiments that provided the intellectual foundation for the discipline of Veterinary Hematology. Whether or not their names appear as authors in this text they have contributed to the concepts presented herein and have inspired those of us whose names appear. Douglas J. Weiss To my friends and colleagues for their continual support, and to those mentors who have nurtured my interest in clinical and investigative hematology. I am indeed fortunate to be able to work in an area that both challenges and rewards me. Also dedicated to my family: Marv, Jessica, Michael, and Evan, for bringing such joy and balance into my life. K. Jane Wardrop

CONTENTS

Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . xv Preface to the Sixth Edition . . . . . . . . . . . xxiii SECTION I Hematopoiesis . . . . . . . . . . . . . . . . . . . . . . . . . . 1

C H A P T E R

1 0

Lymphopoiesis

61

MARY JO BURKHARD C H A P T E R

1 1

Vasculogenesis/Endothelial Progenitor Cells

65

DEBRA A. KAMSTOCK

Maxey L. Wellman C H A P T E R

SECTION II Hematotoxicity . . . . . . . . . . . . . . . . . . . . . . . . 69

1

Embryonic and Fetal Hematopoiesis 3 KELLI L. BOYD and BRAD BOLON

Douglas J. Weiss C H A P T E R

2

Structure of Bone Marrow

8

C H A P T E R

C H A P T E R

3

WILLIAM J. REAGAN, FLORENCE M. POITOUT-BELISSENT, and ARMANDO R. IRIZARRY ROVIRA

14

Stem Cell Biology

1 2

Design and Methods Used for Preclinical Hematotoxicity Studies 71

LESLIE C. SHARKEY and SARA A. HILL

JED A. OVERMANN, JAIME F. MODIANO, and TIMOTHY O. O’BRIEN C H A P T E R

C H A P T E R

4

Cluster of Differentiation (CD) Antigens MELINDA J. WILKERSON and CINZIA MASTRORILLI

20

1 3

Interpretation of Hematology Data in Preclinical Toxicological Studies 78 FLORENCE M. POITOUT-BELISSENT and JEFFREY E. MCCARTNEY

C H A P T E R

5

The Hematopoietic System

27

BRUCE D. CAR C H A P T E R

1 4

Preclinical Evaluation of Compound-Related Cytopenias 85

6

Erythropoiesis

C H A P T E R

36

LAURIE G. O’ROURKE

CHRISTINE S. OLVER C H A P T E R C H A P T E R

7

Granulopoiesis

43

M. JUDITH RADIN and MAXEY L. WELLMAN C H A P T E R

Preclinical Evaluation of Compound-Related Alterations in Hemostasis 92 KAY A. CRISWELL

8

Monocytes and Dendritic Cell Production and Distribution 50 TRACY L. PAPENFUSS C H A P T E R

1 5

9

Thrombopoiesis MARY K. BOUDREAUX

C H A P T E R

Drug-Induced Blood Cell Disorders

98

DOUGLAS J. WEISS C H A P T E R

56

1 6

1 7

Myelonecrosis and Acute Inflammation

106

DOUGLAS J. WEISS

vii

viii

CONTENTS

C H A P T E R

C H A P T E R

1 8

Chronic Inflammation and Secondary Myelofibrosis 112

Congenital Dyserythropoiesis C H A P T E R

3 1

Anemias Caused by Rickettsia, Mycoplasma, and Protozoa 199

1 9

118

Infectious Injury to Bone Marrow

196

DOUGLAS J. WEISS

DOUGLAS J. WEISS C H A P T E R

3 0

K. JANE WARDROP

ROBIN W. ALLISON and JAMES H. MEINKOTH C H A P T E R

SECTION III Erythrocytes . . . . . . . . . . . . . . . . . . . . . . . . . 121

3 2

Anemia Associated with Bacteria and Viral Infections 211 CASEY M. RIEGEL and STEVEN L. STOCKHAM

Joanne B. Messick

C H A P T E R

C H A P T E R

Immune-Mediated Anemias in the Dog

2 0

Erythrocyte Structure and Function

123

131

2 2

136

JOHN A. CHRISTIAN

144

C H A P T E R

2 4

HAROLD TVEDTEN

152

3 7

Anemia of Inflammatory, Neoplastic, Renal, and Endocrine Diseases 246 MICHAEL M. FRY

2 5

C H A P T E R

162

Erythrocytosis and Polycythemia

JOHN F. RANDOLPH, MARK E. PETERSON, and TRACY STOKOL C H A P T E R

239

MICHEL DESNOYERS

Laboratory and Clinical Diagnosis of Anemia C H A P T E R

3 6

Anemias Associated with Oxidative Injury

ANNE M. BARGER C H A P T E R

3 5

KATHY K. SEINO C H A P T E R

2 3

Erythrocyte Morphology

226

Immune-Mediated Anemias in Ruminants and Horses 233

Erythrokinetics and Erythrocyte Destruction C H A P T E R

3 4

Immune-Mediated Anemias in the Cat C H A P T E R

JOHN W. HARVEY C H A P T E R

C H A P T E R TRACY STOKOL

2 1

Erythrocyte Biochemistry

216

MICHAEL J. DAY

CHRISTINE S. OLVER, GORDON A. ANDREWS, JOSEPH E. SMITH, and J. JERRY KANEKO C H A P T E R

3 3

2 6

Pure Red Cell Aplasia

251

DOUGLAS J. WEISS C H A P T E R

Iron and Copper Deficiencies and Disorders of Iron Metabolism 167

3 8

3 9

Aplastic Anemia

256

DOUGLAS J. WEISS

DOUGLAS J. WEISS C H A P T E R

2 7

The Porphyrias and Porphyrinurias

172

J. JERRY KANEKO C H A P T E R

SECTION IV Leukocytes . . . . . . . . . . . . . . . . . . . . . . . . . . 261 Erik Teske

2 8

C H A P T E R

Hereditary Erythrocyte Enzyme Abnormalities 179

4 0

263

Neutrophil Structure and Biochemistry MARY B. NABITY and SHASHI KUMAR RAMAIAH

URS GIGER C H A P T E R C H A P T E R

Erythrocyte Membrane Defects MUTSUMI INABA and JOANNE B. MESSICK

4 1

Neutrophil Distribution and Function

2 9

187

DOUGLAS J. WEISS, SHASHI KUMAR RAMAIAH, and BRUCE WALCHECK

268

ix

CONTENTS C H A P T E R

C H A P T E R

4 2

Neutrophil Function Disorders

275

Systemic Lupus Erythematosus

DOUGLAS J. WEISS C H A P T E R

383

LUC S. CHABANNE

4 3

C H A P T E R

Eosinophils and Their Disorders

281

393

HANS LUTZ and MARGARET HOSIE

4 4

C H A P T E R

Basophils, Mast Cells, and Their Disorders

5 5

Feline Immunodeficiency Virus

KAREN M. YOUNG and RICHARD L. MEADOWS C H A P T E R

5 4

290

LISA M. POHLMAN

5 6

T cell, Immunoglobulin, and Complement Immunodeficiency Disorders 400 PETER J. FELSBURG

C H A P T E R

4 5

Monocytes and Macrophages and Their Disorders 298

C H A P T E R

DOUGLAS J. WEISS and CLEVERSON D. SOUZA C H A P T E R

5 7

406

Severe Combined Immunodeficiencies STEVEN E. SUTER

4 6

C H A P T E R

Interpretation of Ruminant Leukocyte Responses 307

5 8

Benign Lymphadenopathies

412

C. GUILLERMO COUTO, RANCE M. GAMBLIN, and ERIK TESKE

SUSAN J. TORNQUIST and JOHANNA RIGAS C H A P T E R

SECTION V Hematologic Neoplasia . . . . . . . . . . . . . . . . 419

4 7

Interpretation of Equine Leukocyte Responses 314

Jaime F. Modiano

ELIZABETH G. WELLES C H A P T E R C H A P T E R

4 8

5 9

Cell Cycle Control in Hematopoietic Cells

Interpretation of Canine Leukocyte Responses 321

421

JAIME F. MODIANO and CATHERINE A. ST. HILL C H A P T E R

A. ERIC SCHULTZE

6 0

Epidemiology of Hematopoietic Neoplasia C H A P T E R

4 9

427

MICHELLE G. RITT

Interpretation of Feline Leukocyte Responses 335

C H A P T E R

6 1

Genetics of Hematopoietic Neoplasia

AMY C. VALENCIANO, LILLI S. DECKER, and RICK L. COWELL

433

JAIME F. MODIANO and MATTHEW BREEN C H A P T E R

5 0

Determination and Interpretation of the Avian Leukogram 345

440

358

6 3

Bone Marrow-Derived Sarcomas

447

LESLIE C. SHARKEY and JAIME F. MODIANO

MICHAEL J. DAY C H A P T E R

Transforming Retroviruses C H A P T E R

5 1

Biology of Lymphocytes and Plasma Cells

6 2

MARY JO BURKHARD

KENNETH S. LATIMER and DOROTHEE BIENZLE C H A P T E R

C H A P T E R

C H A P T E R

5 2

Structure, Function, and Disorders of Lymphoid Tissue 367

6 4

Classification of Leukemia and Lymphoma V.E. TED VALLI and WILLIAM VERNAU

V.E. TED VALLI and ROBERT M. JACOBS C H A P T E R C H A P T E R

5 3

Disorders of the Spleen

376

JOHN L. ROBERTSON and ERIK TESKE

6 5

General Features of Leukemia and Lymphoma 455 STUART C. HELFAND and WILLIAM C. KISSEBERTH

451

x

CONTENTS

C H A P T E R

C H A P T E R

6 6

Myelodysplastic Syndromes

467

DOUGLAS J. WEISS C H A P T E R

586

Immune-Mediated Thrombocytopenia MICHAEL A. SCOTT and L. ARI JUTKOWITZ C H A P T E R

6 7

Acute Myeloid Leukemia

475

7 9

Non-Immune-Mediated Thrombocytopenia

596

JENNIFER S. THOMAS

LAURA A. SNYDER C H A P T E R

7 8

C H A P T E R

6 8

Mast Cell Cancer

8 0

Essential Thrombocythemia and Reactive Thrombocytosis 605

483

CHERYL LONDON

TRACY STOKOL C H A P T E R

6 9

B Cell Tumors

C H A P T E R

491

MARJORY B. BROOKS and JAMES L. CATALFAMO 7 0

C H A P T E R

511

Plasma Cell Tumors ANTONELLA BORGATTI C H A P T E R

8 2

Inherited Intrinsic Platelet Disorders

619

MARY K. BOUDREAUX

7 1

C H A P T E R

Hodgkin’s Lymphoma

520

V.E. TED VALLI C H A P T E R

612

Von Willebrand Disease

V.E. TED VALLI C H A P T E R

8 1

8 3

Acquired Platelet Dysfunction

626

MICHAEL M. FRY 7 2

T Cell Lymphoproliferative Diseases 525 DAVIS M. SEELIG, PAUL R. AVERY, WILLIAM C. KISSEBERTH, and JAIME F. MODIANO

SECTION VII Hemostasis . . . . . . . . . . . . . . . . . . . . . . . . . . . 633 Marjory B. Brooks

C H A P T E R

7 3

Histiocytic Proliferative Diseases 540 PETER F. MOORE C H A P T E R

C H A P T E R

8 4

Overview of Hemostasis

635

STEPHANIE A. SMITH 7 4

Gene Therapy

550

BRUCE F. SMITH and R. CURTIS BIRD

SECTION VI Platelets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 559

C H A P T E R

8 5

Acquired Coagulopathies

654

MARJORY B. BROOKS and ARMELLE DE LAFORCADE C H A P T E R

8 6

Hereditary Coagulopathies

661

MARJORY B. BROOKS

Mary K. Boudreaux C H A P T E R C H A P T E R

Platelet Structure

8 7

Thrombotic Disorders

7 5

561

668

SUSAN G. HACKNER and BARBARA DALLAP SCHAER

MARY K. BOUDREAUX C H A P T E R C H A P T E R

7 6

Platelet Biochemistry, Signal Transduction, and Function 569 MARY K. BOUDREAUX and JAMES L. CATALFAMO C H A P T E R

7 7

Platelet Kinetics and Laboratory Evaluation of Thrombocytopenia 576 KAREN E. RUSSELL

8 8

Disseminated Intravascular Coagulation TRACY STOKOL C H A P T E R

8 9

Vascular Diseases

689

SEAN P. MCDONOUGH C H A P T E R

9 0

Treatment of Hemostatic Defects DANIEL F. HOGAN and MARJORY B. BROOKS

695

679

xi

CONTENTS C H A P T E R

C H A P T E R

9 1

Overview of Avian Hemostasis

703

1 0 3

Clinical Use of Hematopoietic Growth Factors 790

KENDAL E. HARR

STEVEN E. SUTER

SECTION VIII Transfusion Medicine . . . . . . . . . . . . . . . . . 709

SECTION IX Species Specific Hematology . . . . . . . . . . 797

Mary Beth Callan

Rose E. Raskin and K. Jane Wardrop

C H A P T E R

C H A P T E R

9 2

711

Erythrocyte Antigens and Blood Groups GORDON A. ANDREWS and MARIA CECILIA T. PENEDO C H A P T E R

9 3

Granulocyte and Platelet Antigens

THERESA E. RIZZI, KENNETH D. CLINKENBEARD, and JAMES H. MEINKOTH

9 4

Principles of Canine and Feline Blood Collection, Processing, and Storage 731 ANTHONY C.G. ABRAMS-OGG and ANN SCHNEIDER C H A P T E R

1 0 5

Normal Hematology of the Cat 811

JENNIFER S. THOMAS C H A P T E R

799

THERESA E. RIZZI, JAMES H. MEINKOTH, and KENNETH D. CLINKENBEARD C H A P T E R

725

1 0 4

Normal Hematology of the Dog

C H A P T E R

1 0 6

Normal Hematology of the Horse and Donkey 821 TANYA M. GRONDIN and SHANE F. DEWITT

9 5

C H A P T E R

Red Blood Cell Transfusion in the Dog and Cat 738

1 0 7

Normal Hematology of Cattle

829

DARREN WOOD and GERARDO F. QUIROZ-ROCHA

MARY BETH CALLAN C H A P T E R C H A P T E R

1 0 8

Normal Hematology of Sheep and Goats

9 6

Transfusion of Plasma Products 744

836

STACEY R. BYERS and JOHN W. KRAMER

MARJORY B. BROOKS C H A P T E R C H A P T E R

Hematology of the Pig

9 7

Platelet and Granulocyte Transfusion

751

C H A P T E R

9 9

C H A P T E R

Blood Transfusion in Exotic Species 763 1 0 0

C H A P T E R

769

C H A P T E R

776

1 1 3

Hematology of the Mongolian Gerbil

899

KURT L. ZIMMERMAN, DAVID M. MOORE, and STEPHEN A. SMITH

1 0 2

RAQUEL M. WALTON

893

KURT L. ZIMMERMAN, DAVID M. MOORE, and STEPHEN A. SMITH

ANNE S. HALE and JOHN A. GERLACH

Hematopoietic Stem Cell Transplantation

1 1 2

Hematology of the Guinea Pig

1 0 1

Major Histocompatibility Complex Antigens C H A P T E R

888

KURT L. ZIMMERMAN, DAVID M. MOORE, and STEPHEN A. SMITH

NICOLE M. WEINSTEIN C H A P T E R

1 1 1

Hematology of the Ferret

JAMES K. MORRISEY

Transfusion Reactions

852

ANNE PROVENCHER BOLLIGER, NANCY E. EVERDS, KURT L. ZIMMERMAN, DAVID M. MOORE, STEPHEN A. SMITH, and KIRSTIN F. BARNHART

MARGARET C. MUDGE

C H A P T E R

1 1 0

Hematology of Laboratory Animals

9 8

Blood Transfusion in Large Animals 757 C H A P T E R

843

CATHERINE E. THORN

ANTHONY C.G. ABRAMS-OGG C H A P T E R

1 0 9

C H A P T E R

783

1 1 4

Hematology of the Syrian (Golden) Hamster

904

STEPHEN A. SMITH, KURT L. ZIMMERMAN, and DAVID M. MOORE

xii

CONTENTS

C H A P T E R

1 1 5

SECTION X Quality Control and Laboratory Techniques . . . . . . . . . . . . . . . . . . . . . . . . . 1019

910

Hematology of Camelids SUSAN J. TORNQUIST C H A P T E R

1 1 6

Andreas Moritz

918

Hematology of Cervids

C H A P T E R

KATIE M. BOES C H A P T E R

1 1 7

TAMARA B. WILLS 1 1 8

1027

936

1034

MADS KJELGAARD-HANSEN and ASGER LUNDORFF JENSEN 1 3 2

1039

ANDREAS MORITZ, NATALI B. BAUER, DOUGLAS J. WEISS, ANNE LANEVSCHI, and ALAA SAAD

1 2 0

C H A P T E R

942

Hematology of Elephants

1 3 1

Reference Intervals

Evaluation of Bone Marrow

BRIDGET C. GARNER

KENDAL E. HARR, RAMIRO ISAZA, and JULIA T. BLUE 1 2 1

THOMAS H. REIDARSON

1 3 3

Hematopoietic Cell Culture

1047

CHRISTINE S. OLVER C H A P T E R

Hematology of Marine Mammals 950 C H A P T E R

C H A P T E R

C H A P T E R

1 1 9

Hematology of Reindeer

C H A P T E R

1 3 0

ASGER LUNDORFF JENSEN and MADS KJELGAARD-HANSEN

KURT L. ZIMMERMAN, DAVID M. MOORE, and STEPHEN A. SMITH

C H A P T E R

C H A P T E R

Diagnostic Test Validation

Hematology of the American Bison (Bison bison) 931 C H A P T E R

1021

MADS KJELGAARD-HANSEN and ASGER LUNDORFF JENSEN

Hematology of Water Buffalo (Bubalia bubalis) 927 C H A P T E R

1 2 9

Quality Control

1 3 4

Radiolabeling and Scintigraphic Imaging of Platelets and Leukocytes 1051 DOUGLAS J. WEISS

1 2 2

Hematology of Chickens and Turkeys 958 PATRICIA S. WAKENELL

C H A P T E R

1 3 5

Automated Hematology Systems

1054

ANDREAS MORITZ and MARTINA BECKER C H A P T E R

1 2 3

Hematology of Psittacines

C H A P T E R

968

Reticulocyte and Heinz Body Staining and Enumeration 1067

TERRY W. CAMPBELL C H A P T E R

HAROLD TVEDTEN and ANDREAS MORITZ

1 2 4

Hematology of Waterfowl and Raptors 977 TERRY W. CAMPBELL, STEPHEN A. SMITH, and KURT L. ZIMMERMAN C H A P T E R

987

1074

1 3 8

Laboratory Testing of Coagulation Disorders 1082 GEORGE LUBAS, MARCO CALDIN, BO WIINBERG, and ANNEMARIE T. KRISTENSEN

1 2 6

Hematology of Fishes

1 3 7

Flow Cytometry C H A P T E R

ALICE BLUE-McLENDON and ROBERT A. GREEN C H A P T E R

C H A P T E R

DOUGLAS J. WEISS and MELINDA J. WILKERSON

1 2 5

Hematology of Ratites

1 3 6

994

C H A P T E R

TERRY C. HRUBEC and STEPHEN A. SMITH

1 3 9

Clinical Blood Typing and Crossmatching C H A P T E R

1 2 7

Hematology of Reptiles

K. JANE WARDROP

1004

C H A P T E R

ARMANDO R. IRIZARRY ROVIRA C H A P T E R

1 2 8

Hematology of Elasmobranchs MICHAEL K. STOSKOPF

1 4 0

Testing for Immune-Mediated Hematologic Disease 1106 1013

K. JANE WARDROP, MELINDA J. WILKERSON, and CINZIA MASTRORILLI

1101

xiii

CONTENTS C H A P T E R

C H A P T E R

1 4 1

Evaluation of Neutrophil Function

1114

STEFANO COMAZZI C H A P T E R

1123

INGE TARNOW and ANNEMARIE T. KRISTENSEN 1 4 3

ANNE C. AVERY

1141

ROSE E. RASKIN C H A P T E R

1 4 7

Molecular Techniques and Real-Time PCR

1165

C H A P T E R

1 4 8

Genetic Evaluation of Inherited and Acquired Hematologic Diseases 1170 CHRISTIAN M. LEUTENEGGER and URS GIGER

1 4 4

Cytochemical Staining

C H A P T E R

CHRISTIAN M. LEUTENEGGER

Immunophenotyping and Determination of Clonality 1133 C H A P T E R

Measurement of Serum Iron Concentration, TIBC, and Serum Ferritin Concentration 1162 GORDON A. ANDREWS

1 4 2

Evaluation of Platelet Function C H A P T E R

1 4 6

1 4 5

Electrophoresis and Acute Phase Protein Measurement 1157 JOSE J. CERÓN, MARCO CALDIN, and SILVIA MARTINEZ-SUBIELA

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1176

CONTRIBUTORS

Anthony C.G. Abrams-Ogg, DVM, DVSc, DACVIM

Martina Becker, Dr. Med. Vet., DECVCP

Department of Clinical Studies Ontario Veterinary College University of Guelph Guelph, Ontario Canada

Clinical Pathology Novartis Institute for BioMedical Research Basel, Switzerland

Robin W. Allison, DVM, PhD Department of Veterinary Pathobiology College of Veterinary Medicine Oklahoma State University Stillwater, Oklahoma, USA

Dorothee Bienzle, DVM, MSc, PhD, DACVP Department of Pathobiology Ontario Veterinary College University of Guelph Guelph, Ontario Canada

R. Curtis Bird, BSc, PhD Gordon A. Andrews, DVM, PhD, DACVP Department of Diagnostic Medicine/Pathobiology College of Veterinary Medicine Kansas State University Manhattan, Kansas, USA

Anne C. Avery, VMD, PhD Department of Microbiology, Immunology, and Pathology Colorado State University Fort Collins, Colorado, USA

Department of Pathobiology Auburn University Auburn, Alabama, USA

Alice Blue-McLendon, DVM Department of Veterinary Physiology and Pharmacology College of Veterinary Medicine Texas Texas A&M University College Station, Texas, USA

Julia T. Blue, DVM, PhD, DACVP Paul R. Avery, VMD, PhD, DACVP Department of Microbiology, Immunology, and Pathology Colorado State University Fort Collins, Colorado, USA

Anne M. Barger, DVM, MS, DACVP Veterinary Diagnostic Laboratory University of Illinois Urbana, Illinois, USA

DEXX Reference Laboratories Lutz, Florida, USA

Katie M. Boes, DVM Department of Comparative Pathobiology Purdue University School of Veterinary Medicine West Lafayette, Indiana, USA

Brad Bolon, DVM, PhD, DACVP, DABT Kirstin F. Barnhart, DVM, PhD, DACVP MD Anderson Cancer Center Keeling Center for Comparative Medicine and Research Bastrop, Texas, USA

Natali B. Bauer, Dr. Med. Vet., DECVCP Department of Veterinary Clinical Sciences Faculty of Veterinary Medicine Justus-Liebig-University Giessen Giessen, Germany

GEMpath Inc. Cedar City, Utah, USA

Antonella Borgatti, DVM, MS, DACVIM (Oncology) DECVIM Department of Veterinary Clinical Sciences College of Veterinary Medicine University of Minnesota St Paul, Minnesota, USA

xv

xvi

CONTRIBUTORS

Mary K. Boudreaux, DVM, PhD

James L. Catalfamo, MS, PhD

Department of Pathobiology College of Veterinary Medicine Auburn University Auburn, Alabama, USA

Department of Population Medicine & Diagnostic Sciences College of Veterinary Medicine Cornell University Ithaca, New York, USA

Kelli L. Boyd DVM, PhD, DACVP

Jose J. Cerón, DVM, PhD, DECVCP

St. Jude Children’s Research Hospital Animal Resources Center Memphis, Texas, USA

Department of Animal Medicine and Surgery Faculty of Veterinary Medicine University of Murcia Murcia, Spain

Matthew Breen, PhD, CBIOL, FIBIOL Department of Molecular Biomedical Sciences College of Veterinary Medicine North Carolina State University Raleigh, North Carolina, USA

Marjory B. Brooks, DVM, DACVIM Director, Comparative Coagulation Section Department of Population Medicine & Diagnostic Sciences College of Veterinary Medicine Cornell University Ithaca, New York, USA

Luc Chabanne, DVM, PhD Laboratoire d’Hematologie Clinique et Unité de Médécine Interne Département des Animaux de Compagnie Ecole Nationale Vétérinaire de Lyon Lyon, France

John A. Christian, DVM, PhD School of Veterinary Medicine Purdue University West Lafayette, Indiana, USA

Mary Jo Burkhard, DVM, PhD, DACVP

Kenneth D. Clinkenbeard, PhD, DVM

Department of Veterinary Biosciences College of Veterinary Medicine The Ohio State University Columbus, Ohio, USA

Department of Veterinary Pathobiology College of Veterinary Medicine Oklahoma State University Stillwater, Oklahoma, USA

Stacey R. Byers, DVM, MS, DACVIM

Stefano Comazzi, DVM, PhD, DECVCP

Department of Clinical Sciences College of Veterinary Medicine Colorado State University Fort Collins, Colorado, USA

Dipart. Patol. Animale Igiene e Sanita publica Veterinaria Sezione die Patologia Generale Veterinaria e Parassitologia Milan, Italy

Rick L. Cowell, DVM, MS, MRCVS, DACVP Marco Caldin, DVM, DECVCP Clinica Veterinaria Privata “San Marco” Laboratorio d’Analisi Veterinarie “San Marco” Padova, Italy

Department of Anatomy, Pathology, and Pharmacology Index Reference Laboratories Stillwater, Oklahoma, USA

Kay A. Criswell, BS Mary Beth Callan, VMD, DACVIM Department of Clinical Studies School of Veterinary Medicine University of Pennsylvania Philadelphia, Pennsylvania, USA

Terry W. Campbell, MS, DVM, PhD Zoological Medicine Service Chief Veterinary Teaching Hospital College of Veterinary Medicine Colorado State University Fort Collins, Colorado, USA

Bruce D. Car, BVSc, MVS, PhD, DACVP, DABT Bristol-Myers Squibb Company Rt 206 & Provinceline Rd Princeton, New Jersey, USA

Pfizer Global Research and Development Ann Arbor, Michigan, USA

C. Guillermo Couto, DVM, DACVIM Department of Veterinary Clinical Sciences College of Veterinary Medicine Ohio State University Columbus, Ohio, USA

Barbara Dallap Schaer, VMD, DACVS, DACVECC Department of Clinical Studies University of Pennsylvania, School of Veterinary Medicine New Bolton Center Kennett Square, Pennsylvania, USA

CONTRIBUTORS

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Michael J. Day, BSc, BVMS(Hons), PhD, DSc, DECVP, FASM, FRCPath, FRCVS

Urs Giger, PD, Dr. Med. Vet. MS, FVH, DACVIM, DECVIM, DECVCP (Clinical Pathology)

School of Clinical Veterinary Science University of Bristol Langford, United Kingdom

Department of Clinical Studies School of Veterinary Medicine University of Pennsylvania Philadelphia, USA

Lilli S. Decker, DVM, MS, DACVP ALX Laboratories 510 East 62nd Street New York, USA

Armelle de Laforcade, DVM, DACVP Department of Clinical Sciences/Emergency/Critical Care Cummings School of Veterinary Medicine Tufts University North Grafton, Massachusetts, USA

Michel Desnoyers, DVM, IPSAV, MVSC, DACVP Department of Pathology and Microbiology University of Montreal St. Hyacinthe, Quebec Canada

Shane F. DeWitt, DVM, MS, DACVIM Department of Clinical Sciences College of Veterinary Medicine Kansas State University Manhattan, Kansas, USA

Nancy E. Everds, DVM, DACVP

Robert A. Green, DVM, PhD, DACVP Department of Veterinary Pathobiology College of Veterinary Medicine Texas A&M University College Station, Texas, USA

Tanya M. Grondin, DVM, DACVP Department of Diagnostic Medicine/Pathobiology Kansas State University Manhattan, Kansas, USA

Susan G. Hackner, BVSc, MRCVS, DACVIM, DACVECC Veterinary Specialty Consulting New York, USA

Anne S. Hale, DVM Director Animal Blood Resources International Stockbridge, Michigan, USA

Kendal E. Harr, DVM, MS, DACVP

Department of Pathology Amgen Inc. Seattle, Washington, USA

Phoenix Central Laboratory for Veterinarians Everett, Washington, USA

Peter J. Felsburg, VMD, PhD

Department of Physiologic Sciences College of Veterinary Medicine University of Florida Gainesville, Florida, USA

Department of Clinical Studies School of Veterinary Medicine University of Pennsylvania Philadelphia, Pennsylvania, USA

John W. Harvey, DVM, PhD, DACVP

Stuart C. Helfand, DVM, DACVIM Michael M. Fry, DVM, MS, DACVP Department of Pathobiology College of Veterinary Medicine University of Tennessee Knoxville, Tennessee, USA

Rance M. Gamblin, DVM, DACVIM Metropolitan Veterinary Hospital Akron, Ohio, USA

Department of Clinical Sciences Oregon State University Corvalis, Orgeon, USA

Sara A. Hill, DVM Department of Veterinary Medicine College of Veterinary Medicine St. Paul, Minnesota, USA

Daniel F. Hogan, DVM, DACVIM Bridget C. Garner, DVM, PhD, DACVP Veterinary Medical Diagnostic Laboratory University of Missouri Columbia, Missouri, USA

Department of Veterinary Clinical Sciences School of Veterinary Medicine Purdue University West Lafayette, Indiana, USA

John A. Gerlach, PhD, DABHI

Margaret Hosie, BSM&S, BSc, PhD, MRCS

Biomedical Laboratory Diagnostics Program and Department of Medicine Michigan State University East Lansing, Michigan, USA

Faculty of Veterinary Medicine University of Glasgow Glasgow, United Kingdom

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CONTRIBUTORS

Terry C. Hrubec, DVM, PhD

William C. Kisseberth, DVM, MS, DACVIM

Department of Biomedical Science Virginia College of Osteopathic Medicine Blacksburg, Virginia Department of Biomedical Sciences and Pathobiology VA-MD Regional College of Veterinary Medicine Virginia Polytechnic Institute and State University Blacksburg, Virginia, USA

Department of Veterinary Clinical Sciences College of Veterinary Medicine The Ohio State University Columbus, Ohio, USA

Mutsumi Inaba, MS, DVM, PhD Laboratory of Veterinary Clinical Pathobiology Department of Veterinary Medical Sciences Graduate School of Agriculture and Life Sciences University of Toyko Bunko-ku Tokyo, Japan

Mads Kjelgaard-Hansen, DVM, PhD Department of Small Animal Clinical Sciences Faculty of Life Sciences University of Copenhagen, Frederiksberg C, Copenhagen, Denmark

John W. Kramer, DVM, PhD, DACVP Department of Veterinary Clinical Sciences College of Veterinary Medicine Washington State University Pullman, Washington, USA

Armando R. Irizarry Rovira, DVM, PhD, DACVP Toxicology and Drug Disposition A Division of Eli Lilly and Company Lilly Corporate Center Indianapolis, Indiana, USA

Ramiro Isaza, DVM, MS Department of Small Animal Clinical Sciences College of Veterinary Medicine University of Florida Gainesville, Florida, USA

Robert M. Jacobs, DVM, PhD, DACVP Department of Pathobiology Ontario Veterinary College University of Guelph Guelph, Ontario Canada

Asger Lundorff Jensen, DVM, PhD, DrVetSci, DECVCP Department of Basic Animal and Veterinary Sciences Faculty of Life Sciences University of Copenhagen Frederiksberg C, Denmark

L. Ari Jutkowitz, VMD, DACVECC College of Veterinary Medicine Michigan State University East Lansing, Michigan, USA

Debra A. Kamstock, DVM, PhD, DACVP Department of Microbiology, Immunology, and Pathology College of Veterinary Medicine Colorado State University Fort Collins, Colorado, USA

J. Jerry Kaneko, DVM, PhD, DVSc (hc), DACVP Department of Pathology, Microbiology, and Immunology School of Veterinary Medicine University of California-Davis Davis, California, USA

Annemarie T. Kristensen, DVM, PhD, DACVIM, DECVIM-CA Department of Small Animal Clinical Sciences Faculty of Life Sciences University of Copenhagen Frederiksberg C, Copenhagen, Denmark

Anne Lanevschi DVM, MS, DACVP, DECVCP Clinical Pathology, Safety Assessment Astrazeneca Alderley Park Macclesfield, United Kingdom

Kenneth S. Latimer, DVM, PhD, DACVP College of Veterinary Medicine University of Georgia Athens, Georgia, USA

Christian M. Leutenegger, Dr. Med. Vet., PhD, FVH Regional Head of Molecular Diagnostics IDEXX Reference Laboratories West Sacramento, California, USA

Cheryl London, DVM, PhD, DACVIM Department of Veterinary Bioscience College of Veterinary Medicine Ohio State University Columbus, Ohio, USA

George Lubas, DVM, DECVIM-CA, Assoc. member ECVCP Department of Veterinary Clinics University of Pisa Pisa, Italy

Hans Lutz, Dr. Med. Vet. PhD, FVH, FAMH Clinical Laboratory Department of Internal Veterinary Medicine University of Zurich Zurich, Switzerland

CONTRIBUTORS

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Silvia Martinez-Subiela, DVM, PhD

David M. Moore MS, DVM, DACLAM

Department of Animal Medicine and Surgery Faculty of Veterinary Medicine University of Murcia Murcia, Spain

Department of Biomedical Sciences and Pathobiology VA-MD Regional College of Veterinary Medicine Virginia Polytechnic Institute and State University Blacksburg, Virginia, USA

Cinzia Mastrorilli, DVM, PhD

Andreas Moritz, Dr. Med. Vet., DECVIM-CA, Assoc. member ECVCP

Department of Pathobiology College of Veterinary Medicine Auburn University Auburn, Alabama, USA

Department of Veterinary Clinical Sciences Faculty of Veterinary Medicine Justus-Liebig-University Giessen Giessen, Germany

Jeffrey E. McCartney, DVM, MVSC, DACVP, DABT Preclinical Services Charles River Laboratories Preclinical Services Montreal, Quebec Canada

James K. Morrisey, DVM, DAVP

Sean P. McDonough, DVM, PhD, DACVP

Margaret C. Mudge, VMD, DACVS, DACVECC

Chief of Anatomic Pathology Department of Biomedical Sciences College of Veterinary Medicine Cornell University Ithaca, New York, USA

Department of Veterinary Clinical Sciences The Ohio State University Veterinary Teaching Hospital Columbus, Ohio, USA

Department of Clinical Sciences College of Veterinary Medicine Cornell University Ithaca, New York, USA

Mary B. Nabity, DVM, DACVP Richard L. Meadows, DVM, DABVP Department of Veterinary Medicine and Surgery College of Veterinary Medicine University of Missouri-Columbia Columbia, Missouri, USA

Department of Veterinary Pathobiology College of Veterinary Medicine Texas A&M University College Station, Texas, USA

Timothy D. O’Brien, DVM, PhD, DACVP James H. Meinkoth, DVM, PhD, DACVP Department of Veterinary Pathobiology College of Veterinary Medicine Oklahoma State University Stillwater, Oklahoma, USA

Joanne B. Messick, VMD, PhD, DACVP Department of Comparative Pathobiology Purdue University School of Veterinary Medicine West Lafayette, Indiana, USA

Jaime F. Modiano, VMD, PhD Department of Veterinary Clinical Sciences College of Veterinary Medicine Masonic Cancer Center University of Minnesota Minneapolis/St. Paul, Minnesota, USA

Veterinary Diagnostic Laboratory St. Paul, Minnesota, USA

Christine S. Olver, DVM, PhD, DACVP Department of Microbiology, Immunology and Pathology Colorado State University Ft. Collins, Colorado, USA

Laurie G. O’Rourke, DVM, PhD, DACVP, DECVCP Department of Biomedical Sciences and Pathobiology Virginia-Maryland Regional College of Veterinary Medicine Virginia Technical Institute Blacksburg, Virginia, USA

Jed Overmann, DVM, DACVP Department of Veterinary Medicine College of Veterinary Medicine St. Paul, Minnesota, USA

Peter F. Moore, BVSc, PhD, DACVP Department Pathology Microbiology and Immunology University of California-Davis School of Veterinary Medicine Davis, California, USA

Tracey L. Papenfuss, DVM, PhD, DACVP Department of Veterinary Biosciences College of Veterinary Medicine The Ohio State University Columbus, Ohio, USA

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CONTRIBUTORS

Maria Cecilia T. Penedo, PhD

William J. Reagan, DVM, PhD, DACVP

Veterinary Genetics Laboratory University of California-Davis Davis, California, USA

Biomarker Development and Translation Laboratory Lead Pfizer Global Research and Development Groton, Connecticut, USA

Mark E. Peterson, DVM, DACVIM Animal Medical Center New York, New York, USA

Lisa Pohlman, DVM, MS, DACVP Department of Diagnostic Medicine/Pathobiology College of Veterinary Medicine Kansas State University Manhattan, Kansas, USA

Florence M. Poitout-Belissent, DVM, DACVP, DECVCP Preclinical Services Charles River Laboratories Montreal, Quebec Canada

Anne Provencher Bolliger, DVM, MSc, DACVP Charles River Laboratories Preclinical Services Sherbrooke Quebec, Canada

Gerardo F. Quiroz-Rocha, DVM, MSc, CLASVCP Facultad de Medicina Veterinaria Universidad Nacional Autónoma de México Mexico

M. Judith Radin, DVM, PhD, DACVP College of Veterinary Medicine Department of Veterinary Biosciences Ohio State University Columbus, Ohio, USA

Thomas H. Reidarson, DVM, DACZM Director of Veterinary Services SeaWorld of California San Diego, California, USA

Casey M. Riegel, DVM, DACVP Antech Diagnostics Dallas, Texas, USA

Johanna Rigas, MS, DVM College of Veterinary Medicine Oregon State University Corvallis, Oregon, USA

Michelle G. Ritt DVM, DACVIM (Small Animal) Department of Veterinary Clinical Sciences College of Veterinary Medicine St. Paul, Minnesota, USA

Theresa E. Rizzi, DVM, DACVP Department of Veterinary Pathobiology Center for Veterinary Health Sciences Oklahoma State University Stillwater, Oklahoma, USA

John L. Robertson, VMD, MS, PhD Director of the Center for Comparative Oncology Department of Biomedical Sciences and Pathobiology Virginia-Maryland Regional College of Veterinary Medicine Virginia Technical Institute Blacksburg, Virginia, USA

Karen E. Russell, DVM, PhD, DACVP Biomarker and Clinical Pathology Lead Pfizer-Drug Safety Research and Development St. Louis, Missouri, USA

Department of Veterinary Pathobiology College of Veterinary Medicine & Biomedical Sciences Texas A & M University College Station, Texas, USA

John F. Randolph, DVM, DACVIM

Alaa Saad, DVM, PhD

Department of Clinical Sciences College of Veterinary Medicine Cornell University Ithaca, New York, USA

AstraZeneca R&D Safety Assessment Södertälje, Sweden

Shashi Kumar Ramaiah, DVM, PhD, DAVCP

Ann Schneider, DVM Rose E. Raskin, DVM, PhD, DACVP Department of Comparative Pathobiology School of Veterinary Medicine Purdue University West Lafayette, Indiana, USA

Director Eastern Veterinary Blood Bank Inc. Severna Park, Maryland, USA

CONTRIBUTORS

A. Eric Schultze, MT (ASCP), DVM, PhD, DACVP, Fellow IA Lilly Research Laboratories A Division of El-Lilly and Company Lilly Corporation Indianapolis, Indiana, USA

Michael A. Scott, DVM, PhD, DACVP Michigan State University East Lansing, Michigan, USA

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Catherine A. St. Hill, DVM, PhD Department of Veterinary Clinical Sciences College of Veterinary Medicine St. Paul, Minnesota, USA

Steven L. Stockham, DVM, MS, DACVP Department of Diagnostic Medicine/Pathology College of Veterinary Medicine Manhattan, Kansas, USA

Tracy Stokol, BVSc, PhD, DACVP Department of Microbiology, Immunology, and Pathology Colorado State University Fort Collins, Colorado, USA

Department of Population Medicine and Diagnostic Sciences College of Veterinary Medicine Cornell University Ithaca, New York, USA

Kathy K. Seino, DVM, PhD

Michael S. Stoskopf, DVM, PhD, DACZM

Washington State University Pullman, Washington, USA

Environmental Medicine Consortium College of Veterinary Medicine North Carolina State University Raleigh, North Carolina, USA

Davis M. Seelig, DVM

Leslie C. Sharkey, DVM, PhD, DACVP Department of Veterinary Medicine College of Veterinary Medicine St. Paul, Minnesota

Bruce F. Smith, VMD, PhD Department of Pathobiology and Scott-Ritchey Research Center College of Veterinary Medicine Auburn University, Auburn, Alabama, USA

Joseph E. Smith, DVM, PhD, DACVP (Deceased) Department of Diagnostic Medicine/Pathobiology College of Veterinary Medicine Kansas State University Manhattan, Kansas, USA

Stephanie A. Smith, DVM, MS, DACVIM Department of Biochemistry College of Medicine University of Illinois at Urbana-Champaign Urbana, Illinois, USA

Stephen A. Smith, DVM, PhD Department of Biomedical Sciences and Pathobiology VA-MD Regional College of Veterinary Medicine Virginia Polytechnic Institute and State University Blacksburg, Virginia, USA

Laura A. Snyder, DVM, DACVP Department of Veterinary Medicine College of Veterinary Medicine St. Paul, Minnesota, USA

Cleverson D. Souza, DVM, PhD Department of Veterinary Clinical Medicine College of Veterinary Medicine University of Minnesota St. Paul, Minnesota, USA

Steven E. Suter, VMD, MS, PhD, DACVIM Department of Clinical Sciences College of Veterinary Medicine North Carolina State University Raleigh, North Carolina, USA

Inge Tarnow, DVM, PhD Department of Small Animal Clinical Sciences Faculty of Life Sciences University of Copenhagen Frederiksberg C, Copenhagen, Denmark

Erik Teske, DVM, PhD, DECVIM-CA Department of Clinical Sciences of Companion Animals Faculty of Veterinary Medicine Utrecht University Utrecht, The Netherlands

Jennifer S. Thomas, DVM, PhD, DACVP Pathobiology and Diagnostic Investigation Diagnostic Center for Population and Animal Health College of Veterinary Medicine Michigan State University East Lansing, Michigan, USA

Catherine E. Thorn, DVM, DVSc, MSc, DACVP Antech Diagnostics Smyrna, Georgia, USA

Susan J. Tornquist DVM, PhD, DACVP College of Veterinary Medicine Oregon State University Corvallis, Oregon, USA

xxii

CONTRIBUTORS

Harold Tvedten DVM, PhD, DACVP, DECVCP

Douglas J. Weiss, DVM, PhD, DACVP

Faculty of Veterinary Medicine Department of Clinical Sciences Swedish University of Agricultural Sciences Uppsala, Sweden

College of Veterinary Medicine University of Minnesota St Pauls, Minnesota, USA

Elizabeth G. Welles, DVM, PhD, DACVP Amy C. Valenciano, DVM, MS, DACVP IDEXX Reference Laboratories Dallas, Texas, USA

College of Veterinary Medicine Auburn University Auburn, Alabama, USA

V.E. Ted Valli, DVM, MSc, PhD, DACVP

Maxey L. Wellman, DVM, PhD, DACVP

VDx Pathology Davis, California, USA

Department of Veterinary Biosciences College of Veterinary Medicine The Ohio State University Columbus, Ohio, USA

William Vernau, BSc, BVMS, DVSc, PhD, DACVP Department of Pathology, Microbiology, and Immunology School of Veterinary Medicine University of California-Davis Davis, California, USA

Patricia S. Wakenell DVM, PhD, DACVP Department of Comparative Pathobiology School of Veterinary Medicine Purdue University West Lafayette, Indiana, USA

Bo Wiinberg, DVM, PhD Department of Small Animal Clinical Sciences Faculty of Life Sciences University of Copenhagen Frederiksberg C, Copenhagen, Denmark

Melinda J. Wilkerson, DVM, MS, DACVP Department of Diagnostic Medicine and Pathobiology Kansas State University Manhattan, Kansas, USA

Bruce Walcheck, PhD Department of Veterinary and Biomedical Sciences College of Veterinary Medicine University of Minnesota St. Paul, Minnesota, USA

Tamara B. Wills, MS, DVM, DACVP Department of Veterinary Clinical Sciences College of Veterinary Medicine Washington State University Pullman, Washington, USA

Raquel M. Walton, VMD, PhD, DACVP Department of Pathobiology School of Veterinary Medicine University of Pennsylvania Philadelphia, Pennsylvania, USA

K. Jane Wardrop, DVM, MS, DACVP Department of Veterinary Clinical Sciences College of Veterinary Medicine Washington State University Pullman, Washington, USA

Nicole M. Weinstein, DVM, DACVP Department of Biomedical Sciences and Pathobiology Virginia-Maryland Regional College of Veterinary Medicine Virginia Technical Institute Blacksburg, Virginia, USA

Darren Wood, DVM, DVSc, DACVP Ontario Veterinary College Department of Pathobiology University of Guelph Guelph, Ontario Canada

Karen M. Young, VMD, PhD Department of Pathobiological Sciences School of Veterinary Medicine University of Wisconsin-Madison Madison, Wisconsin, USA

Kurt L. Zimmerman, DVM, PhD, DACVP Department of Biomedical Sciences and Pathobiology Virginia-Maryland Regional College of Veterinary Medicine Virginia Polytechnic Institute and State University Blacksburg, Virginia, USA

P R E FA C E T O T H E SIXTH EDITION

T

he sixth edition of Schalm’s Veterinary Hematology has undergone significant updating, reorganization, and refocusing. Several global changes have been made with the goal of making the information more accessible and improving cohesiveness and readability of the text. First, we grouped topics within established disciplines in hematology. Secondly, we introduced informative chapter outlines at the beginning of each chapter and extensively refer the reader to other chapters within the text. Finally, we edited the text in an attempt to make the presentation as clear and uniform as possible. In addition to organizational changes, we added two new sections; Hematotoxicity and Quality Control and Laboratory Techniques. The Hematotoxicity section was thought necessary because of the large number of clinical pathologists now involved in preclinical safety assessment and toxicology and because of present concerns about environmental toxicants. In that regard, we also expanded the laboratory animal hematology chapter in the book. The Quality Control and Laboratory Techniques section was added for several reasons. First, this section acknowledges and supports the growing group of veterinary laboratory technicians. Secondly,

this section may help to standardize testing and quality control in clinical and industrial laboratories. Thirdly, this section may support and encourage research in veterinary and comparative hematology. We have also expanded other sections in the book. The Hematologic Neoplasia section was extensively reorganized using the World Health Organization classification system as the basis for classification of leukemias and lymphomas. This expansion reflects the dramatic progress made in this discipline in recent years. The Species Specific Hematology section has also been expanded by increasing the number of photographs of blood cells and by adding discussions of bone marrow where appropriate. This should be helpful when veterinary clinical pathologists encounter blood or bone marrow preparations from species with which they are not familiar. We hope that you will find the many changes made in this edition to be beneficial. We thank the previous editors, section editors, and authors for their wisdom, knowledge, time, and commitment. Douglas J. Weiss K. Jane Wardrop

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SECTION I

Hematopoiesis Maxey L. Wellman

CHAPTER

1

Embryonic and Fetal Hematopoiesis KELLI L. BOYD and BRAD BOLON Basic Principles of Hematopoietic Development Cell Structure and Function Primitive Hematopoiesis Erythroid Cells Other Cells

Definitive Hematopoiesis Hemoglobin Switching Molecular Mechanisms Regulating Hematopoietic Development

Acronyms and Abbreviations AGM, aorta-gonad-mesonephros; Bmp, bone morphogenetic protein; 2,3-DPG, 2,3-diphosphoglycerate; E#, day of embryonic development, where the number indicates age of the embryo in days after conception; EPO, erythropoietin; fL, femtoliter; Gata1, 2, and 4, GATA-binding proteins 1, 2, and 4; HSC, hematopoietic stem cell; Ihh, Indian hedgehog; IL, interleukin; P#, day of postnatal development, where the number indicates age of the neonate in days after delivery; pg, picogram; PU.1, purine box-binding transcription factor; Scl/Tal-1, stem cell leukemia/T-cell acute leukemia factor.

T

he complexities of hematopoietic system development have been highly conserved throughout vertebrate evolution. Understanding the embryonic and fetal origins of hematopoiesis provides important insights regarding the function of the adult hematopoietic system. Hematopoiesis in embryonic and fetal animals has been studied intensively for several decades as a model for hematopoietic progression in humans. Recent technical advances have allowed researchers to characterize the spatial and temporal relationships as well as the cellular and molecular mechanisms of hematopoietic development. This chapter reviews the basic biology of hematopoietic development in the mouse (Mus musculus). This appraisal will emphasize hematopoietic events during the embryonic and fetal stages of development, but also will cover selected features of neonatal hematopoiesis. BASIC PRINCIPLES OF HEMATOPOIETIC DEVELOPMENT Cell Structure and Function Blood cells produced at different stages of development differ in morphology and function. Thus, primitive (“fetal”) cells fabricated early in gestation have mark-

edly different properties from their definitive (“adult”) counterparts produced during late gestation and in postnatal life. This principle has been characterized most completely in erythroid lineage cells. Primitive erythrocytes (RBCs) are formed in the yolk sac, whereas definitive RBCs are produced by the liver and later spleen and bone marrow. Primitive RBCs are nucleated in circulation until approximately day 12.5 (E12.5) of gestation, after which nuclei gradually become condensed before being shed between E14.5 to E16.5.35 Enucleated primitive RBCs retain their large size and can remain in circulation until as late as postnatal day 5 (P5). Both primitive and definitive RBCs are released during most of the latter half of gestation (E10 to E18), although the ratio shifts as time progresses from mainly primitive to mainly definitive RBCs. Primitive and definitive RBCs can be distinguished by their size. The volume of primitive RBCs varies from 465 to 530 femtoliters (fL) which is approximately six times larger than that of definitive RBCs.35 The hemoglobin content of primitive RBCs, 80 to 100 picograms (pg)/cell, also is nearly six times the amount found in definitive RBCs.35 Both primitive and definitive RBCs have basophilic cytoplasm when first produced due to abundant rough endoplasmic reticulum, but basophilia recedes as maximal hemoglobin content is achieved. 3

4

SECTION I: HEMATOPOIESIS

Other hematopoietic lineages also differ in cell structure and function during the course of development. Primitive megakaryocytes from the yolk sac contain fewer nuclei of lower ploidy, are about half the size, and respond differently to cytokine stimulation relative to definitive megakaryocytes.47 Primitive macrophages that originate in the yolk sac42 lack certain enzyme activities, are capable of division, and survive for extended periods compared to definitive monocyte-derived macrophages. These functional differences are related to the roles that the two cell populations appear to play. Primitive macrophages are the source for many tissue macrophages in embryonic through juvenile stages of development, whereas definitive macrophages are the source for circulating monocytes and resident macrophages characteristic of the adult immune system. PRIMITIVE HEMATOPOIESIS The processes that drive primitive and definitive stages of hematopoiesis as well as the events that regulate transition between the two stages are mediated by a constellation of factors.1,30,45,46 Cell adhesion factors, growth factors, and transcription factors that participate in this process often support differentiation of multiple hematopoietic cell types,10,29,36 and the dependence of a given cell lineage on any particular molecule may differ between primitive and definitive hematopoiesis. Erythroid Cells Hematopoiesis occurs at multiple sites within the embryo and in extraembryonic tissues. The first phase of blood cell production, referred to as primitive hematopoiesis, is responsible for producing blood elements during the earliest stage of embryogenesis. Primitive hematopoiesis takes place in the visceral yolk sac beginning at approximately E7.0.15,34 Thus, primitive hematopoietic cells are among the earliest distinct tissues to differentiate in the embryo. Formation of primitive cells declines rapidly after E11. The visceral yolk sac or extraembryonic splanchnopleure (the term for a structure in which mesoderm and endoderm are directly apposed) arises from the migration of extraembryonic mesoderm streaming from the caudal primitive streak along the inner surface of the visceral endoderm. The mesodermal cells committed to initiate and support hematopoiesis have been termed hemangioblasts because the contiguity of primitive hematopoiesis and vasculogenesis in both space and time suggests that primitive hematopoietic and endothelial cells in the yolk sac share a common ancestor.1,9 Hemangioblasts arise as undifferentiated cells at the primitive-streak stage and commit to producing a particular cell lineage before blood island formation.34,44 These pluripotent cells also can differentiate into other mesenchyme-derived tissues. Between E7.5 and E9.0, hemangioblasts form multiple aggregates termed blood islands.35 Each blood island contains a central core of unattached inner hemangiob-

lasts (hematopoietic progenitors) surrounded by a rim of spindle-shaped outer hemangioblasts (endothelial progenitors).15 Nucleated erythroid cells are first recognized in the cores of the blood islands at E8.0 and are evident circulating in the cardiovascular system starting at E8.25.18 At this stage embryonic erythroblasts enter the circulation, where they continue to divide until approximately E13.0. The majority of cells produced during primitive hematopoiesis are of the erythroid lineage. Committed erythroid colony-forming cells arrive in the yolk sac at approximately E7.25. These cells expand until E8.0 and then differentiate into primitive erythroblasts; all colony-forming cells have regressed completely by E9.0,34 which corresponds approximately to the earliest phase of definitive erythropoiesis. Primitive erythroblasts serve as the sole source of RBCs in the early embryo from E8.0 to approximately E10.534 and remain an important source of RBCs until E13. Thus, embryos with a developmental age between E8.0 and E11 that are anemic suffer from a defect in primitive erythropoiesis.26,38 Interestingly, seemingly profound defects in primitive hematopoiesis leading to persistent functional defects in adulthood may not elicit an aberrant hematologic profile in the embryo. Other Cells Recent studies suggest that other hematopoietic cell lineages also are generated in the yolk sac during this primitive stage of hematopoietic development. Primitive lymphoid precursors and even some adult stem cells evolve at E7.5 and subsequently seed other sites of hematopoiesis, including the aorta-gonadmesonephros region (AGM), umbilical vessels, and liver.40 Primitive macrophages have been identified in the yolk sac by E8.04 to E9.0.34 In vitro experiments have demonstrated that E7.5 yolk sac cells can give rise to functional megakaryocytic precursors by E10.5.47 Many hemangioblasts actually serve as bi- or oligo-potent progenitors, including those capable of commitment to erythrocytic/myeloid,4 erythrocytic/megakaryocytic,27 granulocyte/macrophage,34 and lymphoid (B cell and T cell)/myeloid lineages. Stem cells for mast cells have also been reported to arise in the yolk sac during primitive hematopoiesis.34 DEFINITIVE HEMATOPOIESIS The second stage of blood cell production, termed definitive hematopoiesis, is thought to arise primarily from the AGM.3,27 The AGM is an amorphous band of intraembryonic splanchnopleure that encompasses the dorsomedial wall of the abdominal cavity. The AGM domain is the main source of mesenchyme-derived, definitive hematopoietic stem cells (HSCs) that will serve the developing animal during late gestation and postnatal life. Initiation of definitive hematopoiesis ranges between E8.5 and E9.25, with definitive HSCs evident in the AGM by no later than E10. Peak production of

CHAPTER 1: EMBRYONIC AND FETAL HEMATOPOIESIS

HSCs in the AGM occurs between E10.5 and E11.5, at which time they comprise almost 10% of all AGM cells. Although controversial, some AGM-independent HSCs may also arise from the allantois, chorion, definitive placenta, umbilical arteries, and yolk sac. The actual contribution of these secondary sites to the overall HSC population has yet to be defined. However, the placenta appears to serve a particularly important role. The yolk sac also appears to be an essential secondary site because it is a source of multiple progenitor cell lineages and remains for at least a day after the AGM has halted HSC production.28 Regardless of their original site of de novo synthesis, HSCs migrate to seed other locations that support definitive hematopoiesis: embryonic liver, followed by embryonic thymus, fetal spleen, and bone marrow (in that order). These latter destinations do not produce HSCs de novo but rather contain niches suitable for expansion of newly arrived HSCs.33 The suitability of such niches is controlled by specific characteristics of their stromal support cells.33 The embryonic liver is colonized first, apparently because it shares many molecular and functional similarities with the yolk sac.31 It provides the major locus for definitive hematopoiesis from E12 to E16.39 The HSCs enter the embryonic liver in several succeeding waves between E9.0 or E10.0 and E12.12 The first HSCs to enter the liver are pluripotent and can form any type of hematopoietic cell. Their first step in intra-hepatic maturation is to commit to a more limited range of lineage options, typically as either an erythromyeloid progenitor or a common myelolymphoid progenitor.22 Definitive erythroid precursors mature and become enucleated within erythroid islands in the liver before entering the circulation.27 Liver-derived myelolymphoid progenitors subsequently develop into bi-potent cells (B cell and myeloid, or T cell and myeloid) before committing to produce a single cell lineage.22 Some T cell progenitors have a bi-potent commitment to NK cell lineage. T cell precursors destined for transfer to the embryonic thymus are produced even in athymic mice, indicating that the fetal liver may play a role in promoting early T cell differentiation.20,21 Embryonic thymus and fetal spleen are seeded either from the liver or AGM, or both, beginning about E13 for thymus and E15 for spleen. The thymus typically accepts only those HSCs that are committed to make T cells, whereas other multi-potent myelolymphoid elements are directed to other sites.20 The number of T cell precursors in liver is abundant at E12 but decreases thereafter, whereas the population of intra-hepatic B cell progenitors exhibits a reverse trend.19 Most types of definitive hematopoietic cells in the spleen arise from precursor cells that commit to a specific lineage before leaving the liver. Multi-potent HSCs entering the spleen cease proliferating and differentiate into mature macrophages. These cells may regulate intra-splenic erythropoiesis. The bone marrow first receives HSCs from hepatic depots at about E16.39,45 Thereafter, the allocation of colony-forming hematopoietic precursors shifts from a

5

primarily hepato-centric localization at E18 through a more dispersed distribution (bone marrow, liver, and spleen in approximately equal numbers) at P2 to a profile favoring bone marrow and to a lesser extent spleen at P4 and after.49 Thus the bone marrow, liver, and spleen function cooperatively to regulate definitive hematopoiesis. While cooperating, each organ supports a molecularly distinct subset of hematopoietic progenitors. Committed hematopoietic progenitors necessary to foster all lineages observed in adult animals arise during definitive hematopoiesis. The AGM-derived HSCs contribute to all major hematopoietic cell lineages. The HSC population from the placenta reportedly supports the genesis of erythroid, lymphoid (both B cell and T cell lineages), and myeloid elements. By comparison, the lineages sustained by yolk sac-derived HSCs are limited to lymphoid and myeloid cells.40 Whether or not progenitors for a given definitive cell lineage arising from distinct HSC populations exhibit different functional and molecular properties during late fetal and/ or postnatal life has yet to be determined. Late-stage embryos (E13 to E15), fetuses (E16 to birth), and neonates which present with anemia are afflicted with a defect in definitive erythropoiesis. Abnormalities associated with this presentation include the total absence of definitive hematopoiesis,25,41 and an inability of progenitor cells to properly colonize intraembryonic sites of hematopoiesis. Multiple cell lineages may be affected; such a combined effect suggests that the hematopoietic defect occurs in a bi- or multi-potent stem cell rather than in one committed to forming a specific cell lineage.43 Presentation with late-stage anemia also might result from a general delay in growth and development rather than a focused anomaly in the erythrocytic lineage.7 Young animals have circulating blood cell numbers is that are different from adults.39 RBC numbers more than double between birth and young adulthood. Circulating leukocyte counts at birth are approximately 20% of adult levels before increasing to adult numbers by 6 to 7 weeks of age. Platelet counts in neonates are approximately one-third numbers. HEMOGLOBIN SWITCHING Primitive and definitive RBCs bear a battery of seven α- and β-globins, the mix of which varies with the developmental stage. The α-globins are encoded by three genes (ζ, α1, and α2), whereas β-globins are encoded by four main genes (εγ, βH1, βmin, and βmaj). The globins of a given type (e.g. α- or β-globins) typically exist as a series of closely linked homologous genes and related pseudo-genes located on the same chromosome;16,24 mouse globin genes are carried on chromosomes 7 (β-globins) and 11 (α-globins). All seven mouse globin genes are transcribed during erythroid development, but the production of three—ζ, εγ, and βH1—is limited to primitive RBCs.23 A consequence of this limitation is that mouse β-globin genes, although

6

SECTION I: HEMATOPOIESIS

closely related to human globins in most respects, do not follow the human pattern of up-regulation in the sequence of their chromosomal arrangement.23,24 The extent of individual globin gene expression and the blend of globin genes that are expressed vary over time. For example, enucleated primitive RBCs contain relatively more βmin than do definitive RBCs. At E11.5, βmin constitutes approximately 80% of the β-globin in circulation. This level is reduced by approximately 60% at birth. Primitive RBCs express increasing levels of adult globins as gestation progresses, whereas definitive RBCs harbor only the adult protein variants. This evolution indicates that the pattern of globin expression switches as the primitive RBCs are replaced by definitive RBCs. Molecular mechanisms which regulate the switching process are complex.17 The timing of this switch, between E10.5 and E12.5, coincides with the initial escalation in definitive erythropoiesis. Perturbed timing of this switch is a feature of some murine models of hematopoietic disease.6 Successful maintenance of the developing conceptus depends on preferential capture of oxygen in embryonic and fetal tissues. Therefore, primitive RBCs generally have a higher affinity for oxygen than do maternal RBCs, although domestic cats are an exception. This sequestration of oxygen is mediated by two primary mechanisms. The mechanism pertinent to the early embryonic period is the greater affinity of embryonic hemoglobin in primitive RBCs for oxygen relative to that of adult hemoglobin.2 Alternatively, definitive RBCs in the late embryo and fetus possess a lower concentration of 2,3-diphosphoglycerate (2,3-DPG) than do maternal RBCs. Higher levels of 2,3-DPG facilitate oxygen release into tissues. After birth, levels 2,3-DPG content of RBCs rise to adult levels within 10 to 15 days. MOLECULAR MECHANISMS REGULATING HEMATOPOIETIC DEVELOPMENT A wide spectrum of growth factors, hormones, and transcription factors are required to specify the various stages of hematopoietic development in mammals. The entire meshwork responsible for directing any given event has not been completely characterized. Shifting levels of several transcription factors have been shown to modify blood cell production. Insufficiencies in many of these molecules act by forestalling primitive hematopoiesis in the yolk sac. For example, genesis of erythroid precursors is impacted by deficits in GATA-binding protein 1 (Gata-1),13 shown in vivo to prevent erythroid maturation; Gata-2,48 demonstrated in vivo to abort precursor expansion; and Gata-4,5 for which an in vitro shortage thwarts hemangioblast-mediated specification of blood islands and their associated vessels. These effects occur because the GATA consensus elements are critical regulatory regions in many erythroid-specific genes. All cell lineages are affected by stem cell leukemia/T-cell acute leukemia factor 1 (Scl/Tal-1).38 Abnormal levels of transcription factors can also act later in gestation to disrupt definitive hematopoiesis.

For example, purine box-binding transcription factor 1 (PU.1) is required for production of definitive (monocyte-derived) macrophages but not their primitive (yolk sac-derived) counterparts. This disparity in response is intriguing in that PU.1 is highly expressed during early hematopoiesis but fluctuates in various cell lineages as time progresses.10 Normal genesis of many progenitor cells, including bi-potent erythroid/megakaryocytic progenitors as well as B cell and T cell progenitors, requires that PU.1 levels be reduced, whereas production of myeloid progenitors necessitates an increase in PU.1.10 Secreted molecules also are important regulators of hematopoietic development during gestation. For example, erythropoietin (EPO) sustains both primitive and definitive erythropoiesis by stimulating proliferation and differentiation of immature primitive and definitive RBCs.25 Reduction in EPO activity within the yolk sac greatly reduces the number of colony-forming cells and erythroblasts via excessive apoptosis. Thrombopoietin fulfills a similar function for megakaryocytes, although other cytokines (interleukin-3 [IL-3], IL-6) and growth factors (granulocyte-colony stimulating factor, stem cell factor) also are required.47 Other ligand/receptor signaling pathways shown to affect hematopoietic development include the endoderm-derived molecule Indian hedgehog (Ihh)8 and bone morphogenetic protein 4 (Bmp4),11 both of which participate in blood island production and vasculogenesis in the yolk sac. In general, secreted molecules act via their interaction with a specific transcription factor. Cell adhesion molecules of the integrin family are essential for the proper migration of hematopoietic precursors. For instance, β1-integrins are essential if HSC are to reach the embryonic liver, and later the fetal spleen and bone marrow, at the appropriate developmental stages.37 A loss of β1-integrins prevents adhesive interactions between HSCs and endothelial cells, thereby stranding the HSCs within vessels.32 Some integrins have functions in addition to their targeting role. For example, β4-integrins are required not only for correct homing but also for expansion and differentiation of erythroid and B cell precursors in liver, spleen, and bone marrow. As with secreted factors, the activities of some integrins relate more to late gestation and neonatal stages rather than earlier stages of hematopoietic development. This chronology has been documented for β4-integrin with respect to lymphoid and myeloid differentiation.14

REFERENCES 1. Baron MH. Embryonic origins of mammalian hematopoiesis. Exp Hematol 2003;31:1160–1169. 2. Bauer C, Tamm R, Petschow D, et al. Oxygen affinity and allosteric effects of embryonic mouse haemolglobins. Nature 1975;257:333–334. 3. Bertrand JY, Giroux S, Golub R, et al. Characterization of purified intraembryonic hematopoietic stem cells as a tool to define their site of origin. Proc Natl Acad Sci USA 2005;102:134–139. 4. Bertrand JY, Jalil A, Klaine M, et al. Three pathways to mature macrophages in the early mouse yolk sac. Blood 2005;106:3004–3011. 5. Bielinska M, Narita N, Heikinheimo M, et al. Erythropoiesis and vasculogenesis in embryoid bodies lacking visceral yolk sac endoderm. Blood 1996;88:3720–3730.

CHAPTER 1: EMBRYONIC AND FETAL HEMATOPOIESIS 6. Bolch SL, Shinpock SG, Wawrzyniak CJ, et al. A comparison of stem cell populations and hemoglobin switching in normal versus β-thalassemic mice. Exp Hematol 1989;17:340–343. 7. Brotherton TW, Chui DH, McFarland EC, et al. Fetal erythropoiesis and hemoglobin ontogeny in tail-short (Ts/+) mutant mice. Blood 1979;54:673–683. 8. Byrd N, Becker S, Maye P, et al. Hedgehog is required for murine yolk sac angiogenesis. Development 2002;129:361–372. 9. Choi K. The hemangioblast: a common progenitor of hematopoietic and endothelial cells. J Hematother Stem Cell Res 2002;11:91–101. 10. DeKoter RP, Kamath MB, Houston IB. Analysis of concentrationdependent functions of PU.1 in hematopoiesis using mouse models. Blood Cell Mol Dis 2007;39:316–320. 11. Dyer MA, Farrington SM, Mohn D, et al. Indian hedgehog activates hematopoiesis and vasculogenesis and can respecify prospective neurectodermal cell fate in the mouse embryo. Development 2001;128:1717– 1730. 12. Everds NE. Hematology of the laboratory mouse. In: Fox JG, Barthold SW, Davisson MT, et al. eds. The Mouse in Biomedical Research, 2nd edn. Boston: Elsevier, 2007; 133–170. 13. Fujiwara Y, Chang AN, Williams AM, et al. Functional overlap of GATA-1 and GATA-2 in primitive hematopoietic development. Blood 2004;103:583–585. 14. Gribi R, Hook L, Ure J, et al. The differentiation program of embryonic definitive hematopoietic stem cells is largely α4 integrin independent. Blood 2006;108:501–509. 15. Haar JL, Ackerman GA. A phase and electron microscopic study of vasculogenesis and erythropoiesis in the yolk sac of the mouse. Anat Rec 1971;170:199–223. 16. Jahn CL, Hutchison CA 3rd, Phillips S, et al. DNA sequence organization of the β-globin complex in the BALB/c mouse. Cell 1980;21:159–168. 17. Jane SM, Cunningham JM. Molecular mechanisms of hemoglobin switching. Intl J Biochem Cell Biol 1996;28:1197–1209. 18. Ji RP, Phoon CK, Aristizábal O, et al. Onset of cardiac function during early mouse embryogenesis coincides with entry of primitive erythroblasts into the embryo proper. Circ Res 2003;92:133–135. 19. Kawamoto H, Ikawa T, Ohmura K, et al. T cell progenitors emerge earlier than B cell progenitors in the murine fetal liver. Immunity 2000;12:441–450. 20. Kawamoto H, Ohmura K, Fujimoto S, et al. Emergence of T cell progenitors without B cell or myeloid differentiation potential at the earliest stage of hematopoiesis in the murine fetal liver. J Immunol 1999;162:2725–2731. 21. Kawamoto H, Ohmura K, Hattori N, et al. Hemopoietic progenitors in the murine fetal liver capable of rapidly generating T cells. J Immunol 1997;158:3118–3124. 22. Kawamoto H, Ohmura K, Katsura Y. Direct evidence for the commitment of hematopoietic stem cells to T, B and myeloid lineages in murine fetal liver. Intl Immunol 1997;9:1011–1019. 23. Kingsley PD, Malik J, Emerson RL, et al. “Maturational” globin switching in primary primitive erythroid cells. Blood 2006;107:1665–1672. 24. Leder P, Hansen JN, Konkel D, et al. Mouse globin system: a functional and evolutionary analysis. Science 1980;209:1336–1342. 25. Lin C-S, Lim S-K, Dagati V, et al. Differential effects of an erythropoietin receptor gene disruption on primitive and definitive erythropoiesis. Genes Dev 1996;10:154–164. 26. Lux CT, Yoshimoto M, McGrath K, et al. All primitive and definitive hematopoietic progenitor cells emerging before E10 in the mouse embryo are products of the yolk sac. Blood 2008;111:3435–3438. 27. McGrath K, Palis J. Ontogeny of erythropoiesis in the mammalian embryo. Curr Top Dev Biol 2008;82:1–22.

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28. McGrath KE, Palis J. Hematopoiesis in the yolk sac: more than meets the eye. Exp Hematol 2005;33:1021–1028. 29. McKercher SR, Torbett BE, Anderson KL, et al. Targeted disruption of the PU.1 gene results in multiple hematopoietic abnormalities. EMBO J 1996;15:5647–5658. 30. Medvinsky AL, Dzierzak EA. Development of the definitive hematopoietic hierarchy in the mouse. Dev Comp Immunol 1998;22:289–301. 31. Meehan RR, Barlow DP, Hill RE, et al. Pattern of serum protein gene expression in mouse visceral yolk sac and foetal liver. EMBO J 1984; 3:1881–1885. 32. Mizgerd JP, Kubo H, Kutkoski GJ, et al. Neutrophil emigration in the skin, lungs, and peritoneum:different requirements for CD11/CD18 revealed by CD18-deficient mice. J Exp Med 1997;186:1357–1364. 33. Oostendorp RA, Harvey KN, Kusadasi N, et al. Stromal cell lines from mouse aorta-gonad-mesonephros subregions are potent supporters of hematopoietic stem cell activity. Blood 2002;99:1183–1189. 34. Palis J, Robertson S, Kennedy M, et al. Development of erythroid and myeloid progenitors in the yolk sac and embryo proper of the mouse. Development 1999;126:5073–5084. 35. Palis J, Yoder MC. Yolk-sac hematopoiesis: the first blood cells of mouse and man. Exp Hematol 2001;29:927–936. 36. Pevny L, Simon MC, Robertson E, et al. Erythroid differentiation in chimaeric mice blocked by a targeted mutation in the gene for transcription factor GATA-1. Nature 1991;349:257–260. 37. Potocnik AJ, Brakebusch C, Fässler R. Fetal and adult hematopoietic stem cells require β1 integrin function for colonizing fetal liver, spleen, and bone marrow. Immunity 2000;12:653–663. 38. Robb L, Lyons I, Li R, et al. Absence of yolk sac hematopoiesis from mice with a targeted disruption of the scl gene. Proc Natl Acad Sci USA 1995; 92:7075–7079. 39. Rugh R. The Mouse:Its Reproduction and Development. Oxford: Oxford University Press, 1990. 40. Samokhvalov IM, Samokhvalova NI, Nishikawa S. Cell tracing shows the contribution of the yolk sac to adult haematopoiesis. Nature 2007;446: 1056–1061. 41. Samokhvalov IM, Thomson AM, Lalancette C, et al. Multifunctional reversible knockout/reporter system enabling fully functional reconstitution of the AML1/Runx1 locus and rescue of hematopoiesis. Genesis 2006;44:115–121. 42. Shepard JL, Zon LI. Developmental derivation of embryonic and adult macrophages. Curr Opin Hematol 2000;7:3–8. 43. Shivdasani RA, Orkin SH. Erythropoiesis and globin gene expression in mice lacking the transcription factor NF-E2. Proc Natl Acad Sci USA 1995; 92:8690–8694. 44. Silver L, Palis J. Initiation of murine embryonic erythropoiesis: a spatial analysis. Blood 1997;89:1154–1164. 45. Speck NA, Peeters M, Dzierzak E. Development of the vertebrate hematopoietic system. In: Rossant J, Tam PPL, eds. Mouse Development: Patterning, Morphogenesis, and Organogenesis. San Diego: Academic Press, 2002:191–210. 46. Teitell MA, Mikkola HK. Transcriptional activators, repressors, and epigenetic modifiers controlling hematopoietic stem cell development. Pediatr Res 2006;59:33R–39R. 47. Tober J, Koniski A, McGrath KE, et al. The megakaryocyte lineage originates from hemangioblast precursors and is an integral component both of primitive and of definitive hematopoiesis. Blood 2007;109:1433–1441. 48. Tsai FY, Keller G, Kuo FC, et al. An early haematopoietic defect in mice lacking the transcription factor GATA-2. Nature 1994;371:221–226. 49. Wolber FM, Leonard E, Michael S, et al. Roles of spleen and liver in development of the murine hematopoietic system. Exp Hematol 2002; 30:1010–1019.

CHAPTER 2

Structure of Bone Marrow LESLIE C. SHARKEY and SARA A. HILL Supporting Structures Vasculature and Sinus Architecture Innervation

Cellular Organization Megakaryocytes Erythroblastic (Rubriblastic) Islands Granulocytes Lymphoid Cells and Macrophages Stem Cell Niches

Acronyms and Abbreviations HSC, hematopoietic stem cell; RBC, red blood cell; WBC, white blood cell.

B

one marrow is the major hematopoietic organ in adults, and a primary lymphoid organ. Bone marrow is a diffuse organ which constitutes approximately 3% of the body mass in rats, 2% in dogs, and 5% in humans.39 Marrow tissue is present in the central cavities of axial and long bones, and consists of a sinusoidal system, hematopoietic cells, adipose tissue, supporting reticular cells, and extracellular matrix. The complex vasculature and rich innervation of the marrow reflect the multiplicity of signals contributing to the control of hematopoiesis. Bone marrow is a dynamic organ capable of structural and functional remodeling in response to nutritional factors, endocrine signals, and variations in demand for the production of red blood cells (RBCs), white blood cells (WBCs), and platelets. This chapter will review the structure of bone marrow with a brief conceptual framework for structural and functional relationships among the different components of bone marrow. For a more thorough discussion of the biochemical and molecular control of hematopoiesis and the hematopoietic microenvironment, the reader is referred to Chapters 1 and 5. SUPPORTING STRUCTURES Hematopoietic tissue resides within a rigid boney cortex, and is further supported by a meshwork of trabecular bone that serves as a partial scaffold for additional structural components including adipose, reticular cells, and extracellular matrix. In addition to providing physical support, each of these structures contributes to the biochemical microenvironment of 8

hematopoietic tissue, either directly or via vascular connections. A one to two cell thick layer of flat endosteal cells with a thin layer of connective tissue lines all of the boney surfaces within the medullary cavity. This layer is punctuated with occasional osteoblasts and osteoclasts, and may be traversed by endosteal blood vessels connecting the hematopoietic space with bone. Osteoblasts contribute to bone production and are derived from multipotent mesenchymal stromal progenitor cells that also give rise to bone marrow stromal cells and adipocytes.30 Osteoclasts are multinucleated cells derived from fused monocyte-macrophage precursors under the influence of numerous signals, including those from osteoblasts.30 Osteoblasts and osteoclasts remodel bone within the marrow space, influencing the endosteal environment and probably contributing to regulation of hematopoietic stem cell proliferation and trafficking.25 Osteoblasts and osteoclasts also produce hematopoietic cytokines, and interplay between bone and hematopoietic cells can influence bone turnover and remodeling.30,36 Fine, spindloid to stellate stromal cells extend from endosteal regions into the parenchyma of hematopoietic tissue. These cells probably derive from fibroblasts in the bone marrow, and form a supporting meshwork for hematopoietic cells, adipose tissue, and blood vessels. Stromal cells produce soluble factors that contribute to regulation of hematopoiesis, and communicate with hematopoietic precursors via direct cell to cell contact. Bone marrow stromal cells produce structural fibrils such as collagen, reticulin, laminin, and fibronectin, and ground substance composed of water, salts,

CHAPTER 2: STRUCTURE OF BONE MARROW

glycosaminoglycans, and glycoproteins, which collectively are called the extracellular matrix. Like the supporting cellular structures of the marrow, the extracellular matrix participates in both the structural and biochemical support of hematopoiesis.26 Bone marrow contains predominantly types I and III collagen, which take their final form after secretion into the extracellular space, where they undergo enzymatic modification. Reticulin is distinguished from collagen in the marrow by the presence of fine argyrophilic fibers, which are composed of a core of type I collagen surrounded by type III collagen fibrils embedded in a matrix of glycoproteins and glycosaminoglycans.26 Although the fine connective tissue structure of bone marrow is not prominent on routinely processed histology sections, special stains can enhance its visualization. Collagen can be visualized with Mallory’s or Masson’s trichrome stains, whereas Gomori’s silver stain highlights the presence of reticulin. This can be important in pathologic conditions in which production of excess matrix material contributes to disease. Differentiation of collagen and reticulin fibrosis may have diagnostic implications and influence the likelihood of reversibility of the lesion. Expanded extracellular matrix characterizes other bone marrow disorders beyond classical syndromes of myelofibrosis.26,41 Adipose tissue interspersed with hematopoietic tissue is enmeshed in the same supporting structures. The relationship between formation of bone and adipose tissue is not clearly understood.15 Adipocytes probably are derived from the same mesenchymal progenitors that produce stromal cells and osteoblasts, and there is some evidence for interconversion of cells originating from committed osteoblastic and adipogenic lineages

9

derived from the mesenchymal progenitor population.3 Adipocytes are the most numerous stromal cells in bone marrow. In health, adipose tissue takes up approximately 25% to 75% of the bone marrow space, depending on the age of the animal. Adipose tissue also contains other cell types that are less visible than adipocytes on routine histologic sections. These cells also contribute to the structural and functional roles of adipose tissue in hematopoiesis, and include endothelial cells, macrophages, and adipocyte progenitor cells.24 Both brown and white adipose tissue are present in bone marrow; differences in biological function of these types of fat are not fully understood. Bone marrow adipose tissue tends to be relatively resistant to lipolysis during starvation compared with adipose tissue elsewhere in the body. In addition to providing structural support, adipose tissue also may participate in the hematopoietic microenvironment. Cells derived from bone marrow adipose tissue are capable of supporting differentiation of hematopoietic progenitors in vitro.8,10 The endocrine and paracrine functions of adipose tissue also are important.12,37 Adipokines, which are biologically active substances produced by adipose tissue, include regulators of hematopoiesis and the immune response.10,24,27,28,35 VASCULATURE AND SINUS ARCHITECTURE The nutrient artery provides the major blood supply to bone marrow (Fig. 2.1). Nutrient arteries enter the medullary cavity via one or more nutrient canals, which also may contain one or two nutrient veins. There often are two nutrient arteries for long bones, and flat bones may contain several.39 Once the vessels have penetrated the FIGURE 2.1 Organization of the venous vasculature of the marrow of a long bone. Thin-walled vascular sinuses originate at the periphery from termination of transverse branches of the nutrient artery (not shown). The vascular sinuses run transversely toward the center to join the CV. Hemopoiesis takes place in the space between the vascular sinuses. Adventitial processes project into the hematopoietic space, producing partial compartmentalization. (Reproduced from Lichtman MA. The ultrastructure of the hemopoietic environment of the marrow: a review. Exp Hematol 1981;9:391–410, with permission.)

10

SECTION I: HEMATOPOIESIS

cortex, ascending and descending branches bifurcate from the main vessels, coiling around the main venous bone marrow channel and central longitudinal vein. These branches form numerous arterioles and capillaries that penetrate the endosteal surface of the bone to communicate with cortical capillaries derived from arteries that supply surrounding muscle tissue (Fig. 2.1). These interactions facilitate communication and reciprocal regulation between hematopoietic cells and bone.30 Capillaries derived from the nutrient artery extend as far as the Haversian canals before coursing back to the bone marrow and opening into the venous sinuses. Periosteal arterioles penetrate cortical bone to form a second arterial system for the bone marrow. These vessels form branching networks of medullary venous sinuses. Medullary venous sinuses collect into the large central venous sinus from which blood enters the systemic circulation via the emissary vein, which exits through the nutrient foramen.1 Hematopoiesis occurs in the extravascular spaces between the venous sinuses, and has a close morphologic and functional relationship with the cells that line the venous sinuses. Venous sinuses are lined by a complete luminal layer of broad, flat endothelial cells and an incomplete outer layer of abluminal reticular cells (Fig. 2.2). Reticular cells maintain close physical relationships with the hematopoietic cells close to the sinus walls, frequently wrapping around or otherwise contacting hematopoietic precursors. The basement lamina between the sinusoids and the hematopoietic space is thin and interrupted to facilitate release of mature hematopoietic cells into the circulation.29 Sinus endothelial cells may regulate translocation of cells and other substances into the systemic circulation.

FIGURE 2.2 Scanning electron micrograph of a rat bone marrow showing developing cells in hematopoietic spaces (HSs), anastomosing venous sinusoids (VS), and central vein (CV). 290×. (Prepared with assistance of Dr. Prem Handagama.)

Ultrastructurally, sinus endothelial cells have distinct cell junctions that are not tight, and egress of hematopoietic cells through migration pores in endothelial cells has been observed. Other particulate matter may traverse sinus endothelial cells by the process of endocytosis.29 INNERVATION Primary innervation of the bone marrow is via myelinated and more numerous non-myelinated fibers. These fibers originate in the spinal nerve corresponding to the location of the nutrient foramen, although some innervation may originate from the epiphyseal and metaphyseal foramina.6,39 Once inside the medullary cavity, the mixed myelinated and non-myelinated nerve bundles, surrounded by a thin perineurium, divide to parallel the arterial vasculature of the bone marrow.6 The main branches of the arterial vessels are surrounded by several nerve bundles, whereas arterioles and capillaries may be accompanied by only a single fiber, with nerve endings contacting vascular smooth muscle cells or periarterial adventitial and reticular cells.43 The sinusoidal system is less richly innervated than the arterial vasculature, with nerve endings frequently contacting the walls of sinusoids. Other nerve fibers appear to terminate within the hematopoietic parenchyma or along the endosteum.6,11 It is not clear if there is shared or separate innervation of bone and hematopoietic tissue, although at least some nerve fibers in the bone marrow appear to originate from mineralized bone.31,34 Bone marrow contains efferent noradrenergic and peptidergic sympathetic and presumptive sensory nerve fibers.9,34 There is some evidence that

CHAPTER 2: STRUCTURE OF BONE MARROW

signals from the sympathetic nervous system may contribute to regulation of hematopoiesis, immune function, and hematopoietic stem cell trafficking.2,5,9,20,22,32,38 CELLULAR ORGANIZATION An intricate three-dimensional complex of hematopoietic cells forms cords or wedges between vascular sinuses within the medullary cavity (Fig. 2.3). Different cell lineages occupy specific locations: granulocytes, lymphocytes, and macrophages are concentrated near the endosteum and arterioles, and megakaryocytes and erythroid cells are located near venous sinuses.1,14,33 Hematopoietic cells are derived from a common pluripotent stem cell which gives rise to lymphoid and myeloid progenitor cells. Lymphoid progenitor cells generate lymphocyte progeny, whereas the myeloid progenitor cells generate erythroid cells, megakaryocytes, basophils, eosinophils, and a common granulocyte-macrophage cell that produces neutrophils and macrophages. Each cell line exhibits a pyramidal progression of cell numbers with the least mature cells

11

present in the smallest numbers and cells in subsequent stages of development present in increasing proportions. Megakaryocytes Megakaryocyte development begins with the megakaryoblast, progresses to promegakaryocytes and basophilic megakaryocytes, and culminates with formation of mature megakaryocytes. Megakaryocyte precursors progressively enlarge as they differentiate to become the largest cell in the bone marrow.4,16 Their nuclei mature from a single nucleus to a large multilobulated nucleus through a process termed endomitosis, which is replication of DNA without cellular division.4,16 The cytoplasm of early megakaryocytic precursors is scant and deeply basophilic, becoming more abundant, lightly basophilic, and filled with numerous eosinophilic granules as the cell matures.16 As megakaryocytes mature, they migrate toward the venous sinuses and may compose part of the endothelial cell layer.1,4,13,17,19,33,39,40 This location enables

FIGURE 2.3 Schematic depiction of the hematopoietic compartment of the medullary cavity. Hematopoiesis occurs in cords composed of differentiating hematopoietic cells, stromal cells, adventitial reticular cells, adipocytes, and endothelial cells. Megakaryocytes (Mega) reside near the sinuses, shedding platelets directly into the sinus. Erythropoiesis occurs around macrophages, termed erythroid islands. Some evidence suggests structural variation in the location of primitive versus differentiated cells, with more primitive cells located nearer the bone surface. (Reproduced from Sieff C, Williams D. Hemopoiesis. In: Handin R, Lux S, Stossel T, eds. Blood: Principles and Practice of Hematology. Philadelphia: JB Lippincott, 1995, with permission.)

12

SECTION I: HEMATOPOIESIS

cytoplasmic processes to extend through endothelial gaps and discharge of proplatelets directly into the lumen of the sinus. Platelets are released from proplatelet processes into the peripheral circulation (see Chapter 9).13,17,19,39 Erythroblastic (Rubriblastic) Islands Stages of erythropoiesis include rubriblasts, prorubricytes, rubricytes, metarubricytes, reticulocytes, and mature RBCs (see Chapter 6).13,16,17 As erythroid precursors mature, the cells become smaller, the nuclear to cytoplasmic ratio decreases, the cytoplasm becomes less basophilic and more polychromatophilic, and the nuclear chromatin becomes condensed. In mammals, the nucleus is extruded before maturation to a mature RBC.16,17 Erythropoiesis occurs in distinct erythroblastic islands (Fig. 2.3), which are clusters of cells that occasionally can be observed in cytologic samples of bone marrow.1,7,39,44 Erythroblastic islands form around a central macrophage that projects membranous processes to assist erythropoiesis by providing iron and probably other nutrients and hematopoietic cytokines. These macrophages also phagocytize extruded nuclei and defective cells.1,7,13 Erythroid progeny are found in concentric circles surrounding the central macrophage with younger forms closer to the macrophage.1 Central macrophages are recruited from a subset of resident macrophages derived from monocyte precursors. Erythroblastic islands are located near venous sinuses.1,39,40 A recent study of erythropoiesis in rat bone marrow suggests that erythroblastic islands are motile and migrate towards sinusoids as they mature, and that erythroblastic islands are composed of cells in similar stages of erythroid development.7,44 Granulocytes Neutrophils, eosinophils, and basophils develop in a parallel fashion from myeloblasts, promyelocytes, myelocytes, metamyelocytes, and band forms, to mature cells. Granulocytes, like RBCs, decrease their cellular size and nuclear to cytoplasmic ratio as they mature. Secondary specific granules appear in the myelocyte stage and allow differentiation of the various granulocytic lineages.16,17 At the metamyelocyte stage, the round nucleus elongates and indents to form a bean-shaped nucleus before ultimately forming segmentations at full maturation. The location of granulocytic cells is dependent on their stage of maturity. Immature forms are located near arterioles and boney trabeculae. As maturation proceeds, precursors migrate toward venous sinusoids, where they access the peripheral circulation by diapedesis.1,13,33,40 Lymphoid Cells and Macrophages Lymphoid progenitors produce B cells, which develop further in the bone marrow, and T/NK progenitors, which leave the bone marrow for further development

in the thymus and other tissues. Immature lymphoid cells and macrophages are located near the endosteum and arterioles,1,18,33 whereas mature lymphocytes are relatively uniformly distributed with the bone marrow parenchyma.1,18,33 Morphologic changes in lymphoid cells during maturation are relatively minimal compared to other lineages and include decreasing cell size, decreasing cytoplasmic basophilia, and increasing condensation of nuclear chromatin. Stem Cell Niches Hematopoietic stem cells (HSCs) are morphologically indistinct, and evaluation of surface proteins is required for definitive identification (see Chapter 3). HSCs reside in the bone marrow within stem cell niches, the specialized microenvironments created by supporting cells that provide the necessary signals for stem cell maintenance and function.21,23 Current evidence indicates that there are two niches: an endosteal or osteoblastic niche and a vascular niche.21,23,42 The endosteal niche is located adjacent to the endosteal surface of the bone marrow, in a location influenced by osteoblastic activity, and it promotes HSC quiescence and trans-marrow migration.21,23 The vascular niche is located near the sinusoidal endothelium and is involved in HSC expansion and egress into the circulation.21 The relationship between the two stem cell niches is still under investigation.23,42

REFERENCES 1. Abboud CN, Lichtman MA. Structure of the marrow and the hematopoietic microenvironment. In: Lichtman MA, Beutler E, Kipps TJ, Seligsohn U, Kaushandky K, Prchal JT, eds. Williams’ Hematology, 7th edn. New York: McGraw-Hill, 2006;29–59. 2. Afan AM, Broome CS, Nicholls SE, et al. Bone marrow innervation regulates cellular retention in the murine hematopoietic system. Br J Haematol 1997;98:569–577. 3. Anjos-Afonso F, Bonnet D. Flexible and dynamic organization of bone marrow stromal compartment. Br J Haematol 2007;139:373–384. 4. Battinelli EM, Hartwig JH, Italiano Jr JE. Delivering new insight into the biology of megakaryopoiesis and thrombopoiesis. Curr Opin Hematol 2007;14:419–426. 5. Broome CS, Whetton AD, Miyan JA. Neuropeptide control of bone marrow neutrophil production is mediated by both direct and indirect effects on CFU-GM. Br J Haematol 2000;108:140–150. 6. Calvo W. The innervation of the bone marrow in laboratory animals. Am J Anat 1968;123:315–328. 7. Chasis JA, Mohandas N. Erythroblastic islands: niches for erythropoiesis. Blood 2008;112:470–478. 8. Corre J, Barreau C, Cousin B, et al. Human subcutaneous adipose cells support complete differentiation but not self-renewal of hematopoietic progenitors. J Cell Physiol 2006;208:282–288. 9. Dénes Á, Boldogkoi Z, Uhereczky G, et al. Central autonomic control of the bone marrow: multisynaptic tract tracing by recombinant pseudorabies virus. Neuroscience 2005;134:947–963. 10. DiMascio L, Voermans C, Uqoezwa M, et al. Identification of adiponectin as a novel hemopoietic stem cell growth factor. J Immunol 2007;178:3511–3520. 11. Felten DL, Felten SY, Carlson SL, et al. Noradrenergic and peptidergic innervation of lymphoid tissue. J Immunol 1985;135:755–765. 12. Fischer-Posovszky P, Wabitsch M, Hochberg Z. Endocrinology of adipose tissue-an update. Hormone Metab Res 2007;39:314–321. 13. Fry MM, McGavin MD. Bone marrow, blood cells and lymphatic system. In: McGavin MD, Zachary JF, eds. Pathologic Basis of Veterinary Disease, 4th edn. St. Louis: Mosby, 2007;743–750. 14. Gasper PW. The hemapoietic system. In: Feldman BF, Zinkl JG, Jain NC, eds. Schalm’s Veterinary Hematology, 4th edn. Philadelphia: Lippincott, Williams & Wilkins, 2000;63–69. 15. Gimble JM, Zvonic S, Floyd ZE, et al. Playing with bone and fat. J Cell Biochem 2006;98:251–266.

CHAPTER 2: STRUCTURE OF BONE MARROW 16. Grindhem CB, Tyler RD, Cowell RL. The bone marrow. In: Cowell RL, Tyler RD, Meinkoth JH, DeNicola DB, eds. Diagnostic Cytology and Hematology of the Dog and Cat, 3rd edn. St Louis: Mosby, 2008;422–450. 17. Harvey JW. Atlas of Veterinary Hematology: Blood and Bone Marrow of Domestic Animals. Philadelphia: Saunders, 2001;87–91. 18. Hermans MHA, Hartsuiker H, Opstelten D. An in situ study of βlymphocytopoiesis in rat bone marrow. J Immunol 1989;142:67–73. 19. Italiano Jr JE, Patel-Hett S, Hartwig JH. Mechanics of proplatelet elaboration. J Thrombosis Haemostasis 2007;5(Suppl 1):18–23. 20. Iverson PO, Hjeltnes N, Holm B, et al. Depressed immunity and impaired proliferation of hematopoietic progenitor cells in patients with complete spinal cord injury. Blood 2000;96:2081–2083. 21. Kaplan RN, Psaila B, Lyden D. Niche-to-niche migration of bone marrowderived cells. Trends Mol Med 2007;13:72–81. 22. Katayama Y, Battista M, Kao WM, et al. Signals from the sympathetic nervous system regulate hematopoietic stem cell egress from bone marrow. Cell 2006;124:407–421. 23. Kiel MJ, Morrison SJ. Uncertainty in the niches that maintain hematopoietic stem cells. Nat Rev Immunol 2008;8:290–301. 24. Kilroy GE, Foster SJ, Wu X, et al. Cytokine profile of human adiposederived stem cells: expression of angiogenic, hematopoietic, and proinflammatory factors. J Cell Physiol 2007;212:702–709. 25. Kollet O, Dar A, Lapidot T. The multiple roles of osteoclasts in host defense: bone remodeling and hematopoietic stem cell mobilization. Annu Rev Immunol 2007;25:51–69. 26. Kuter DJ, Bain B, Mufti G, et al. Bone marrow fibrosis: pathophysiology and clinical significance of increased bone marrow stromal fibers. Br J Haematol 2007;139:351–362. 27. Lago F, Dieguez C, Gómez-Reino J, et al. Adipokines as emerging mediators of immune response and inflammation. Nat Clin Pract Rheumatol 2007;3:176–724. 28. Lam QL, Lu L. Role of leptin in immunity. Cell Mol Immunol 2007;4:1–13. 29. Lichtman MA. The ultrastructure of the hemopoietic environment of the marrow: a review. Exp Hematol 1981;9:391–410. 30. Lorenzo J, Horowitz M, Choi Y. Osteoimmunology: interactions of the bone and immune system. Endocr Rev 2008;29:403–440.

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31. Mach DB, Rogers SD, Sabino MC, et al. Origins of skeletal pain: sensory and sympathetic innervation of the mouse femur. Neuroscience 2002;113:155–166. 32. Méndez-Ferrer S, Lucas D, Battista M, et al. Haematopoietic stem cell release is regulated by circadian oscillations. Nature 2008;452:442–447. 33. Naito K, Tamahashi N, Chiba C, et al. The microvasculature of the human bone marrow correlated with the distribution of hematopoietic cells: a computer-assisted three-dimensional reconstruction study. Tohoku J Exp Med 1992;166:439–450. 34. Nance DM, Sanders VM. Autonomic innervation and regulation of the immune system (1987–2007). Brain Behav Immun 2007;21:736–745. 35. Payne MWC, Uhthoff HK, Trudel G. Anemia of immobility: caused by adipocyte accumulation in bone marrow. Med Hypoth 2007;69:778– 786. 36. Porter RL, Calvi LM. Communications between bone cells and hematopoietic stem cells. Arch Biochem Biophys 2008;473:193–200. 37. Sethi JK, Vidal-Puig AJ. Adipose tissue function and plasticity orchestrate nutritional adaptation. J Lipid Res 2007;48:1253–1261. 38. Shepherd AJ, Downing JEG, Miyan JA. Without nerves, immunology remains incomplete-in vivo veritas. Immunology 2005;116:145–163. 39. Travlos GS. Normal structure, function, and histology of the bone marrow. Toxicol Pathol 2006;34:548–565. 40. Valli VEO. Normal and benign reactive hematopoietic tissues. Veterinary Comparative Hematopathology, Ames: Wiley-Blackwell, 2007;98–102. 41. Weiss DJ. 2008;Bone marrow pathology in dogs and cats with non-regenerative immune-mediated haemolytic anaemia and pure red cell aplasia. J Comp Pathol 2007;138:46–53. 42. Wilson A, Trumpp A. Bone-marrow haematopoietic-stem-cell niches. Nat Rev Immunol 2006;6:93–106. 43. Yamazaki K, Allen TD. Ultrastructural morphometric study of efferent nerve terminals on murine bone marrow stromal cells, and the recognition of a novel anatomical unit: the “neuro-reticular complex.” Am J Anat 1990;187:261–276. 44. Yokoyama T, Etoh T, Kitagawa H, et al. Migration of erythroblastic islands toward the sinusoid as erythroid maturation proceeds in rat bone marrow. J Vet Med Sci 2003;65:449–452.

CHAPTER 3

Stem Cell Biology JED A. OVERMANN, JAIME F. MODIANO, and TIMOTHY O. O’BRIEN Defining Stem Cells Characteristics Tests and Markers CD34 Stem cell antigen Dye efflux c-Kit Lin− Transcription factors Bone Marrow-Derived Stem Cells Stem Cell Biology Regulation of Survival and Pluripotency Niche

Molecular mechanisms Leukemia inhibitory factor Bone morphogenic protein and fibroblast growth factor Wnt Tyrosine kinase with immunoglobulin-like and EGF-like domains 2 and angiopoietin-1 Other cytokines Transcription factors Microribonucleic acids Regulation of Differentiation Stem Cell-Associated Diseases Stem Cell Failure Stem Cells and Proliferative Disorders

Acronyms and Abbreviations ABC transporter, ATP-binding cassette transporter; BMP, bone morphogenic protein; CD, cluster of differentiation; EPC, endothelial precursor cell; EPO, erythropoietin; ERK, extracellular signal related kinases; ESC, embryonic stem cell; FGF, fibroblast growth factor; GM-CSF, granulocyte/macrophage colony stimulating factor; HSC, hematopoietic stem cell; IGF-2, insulin-like growth factor 2; IL, interleukin; JAK/STAT, Janus kinase/signal transducers and activators of transcription; LIF, leukemia inhibitory factor; Lin−, lineage negative; miRNA, microribonucleic acid; MSC, mesenchymal stem cell; PE, phycoerythrin; PI3k, phosphoinositide-3 kinase; Sca-1, stem cell antigen 1; SCF, stem cell factor; SCID, severe combined immunodeficiency; SP, side population; Tie, tyrosine kinase with immunoglobulin-like and EGF-like domains 1; TNK, T cell and natural killer cell progenitor; TPO, thrombopoietin.

DEFINING STEM CELLS Characteristics Stem cells are a population of unspecialized precursor cells that have capacity for self-renewal and the ability to differentiate, leading to formation of mature cells and tissues. This latter function is clearly evident in embryonic stem cells (ESCs), as they lead to the establishment of the numerous different cells and tissues of the mature organism. Small numbers of stem cells are retained throughout life as adult stem cells and are a reservoir for replacement of short-lived cells or regeneration of damaged tissues. Hematopoietic stem cells (HSCs) are the reservoir for replacement of blood cells and are present in a frequency of 1 in every 10,000 to 100,000 blood cells.3 (Fig. 3.1; see Chapters 6–10). 14

Two general functional characteristics are used in defining stem cells. The first of these is the ability of long term self renewal. Stem cells have the capability, through mitotic cell division, to maintain a population of undifferentiated cells within the stem cell pool for months to years, and over many cycles of cell division. As stem cells divide, on average, one daughter cell is a replica and remains in an undifferentiated state, while the second daughter cell is programmed to differentiate. This production of two daughter cells with different properties is termed asymmetric cell division. The second characteristic of stem cells is the capacity to form differentiated or specialized cell types. Potency is a term that is used to describe the degree or extent to which multiple functional cell lines can be formed. Totipotent stem cells are those cells that have the ability to form entire organisms, including extra-

CHAPTER 3: STEM CELL BIOLOGY

1000

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FIGURE 3.1 Hematopoietic progenitor cells in the peripheral circulation. Hematopoietic progenitor cells can be defined by expression of specific cell surface markers such as CD34, c-Kit, and CD133. In this case, such cells are detectable in blood from a normal dog using flow cytometry. Each panel shows a two-dimensional dot plot of FL2 (fluorescence channel-2 set to detect wavelength emission maxima at 575 ± 13 nm) vs. right angle side scatter. Cells were stained using routine protocols; dead cells were excluded using a vital dye. The left panel shows cells stained using an isotype control antibody. The right panel shows cells stained using a mix of antibodies against CD34, c-Kit, and CD133, each labeled with phycoerythrin (PE). In this healthy adult dog, approximately 0.25%, or ∼2/1,000 viable leukocytes expressed one or more of the progenitor markers. While the frequency is almost 20-fold greater than that seen in most healthy dogs, this case serves to illustrate the presence of hematopoietic progenitor cells in circulation with no associated pathology. (Analysis and figure courtesy of Megan Duckett, Masonic Cancer Center, University of Minnesota.)

embryonic tissues (e.g. placenta). This type of stem cell can be derived from the zygote or early blastomere. Pluripotent stem cells are able to form all cell types of the body (e.g. ESCs). Multipotent stem cells generate all differentiated cells of a particular lineage (e.g. HSCs), and will be of particular interest to the topics in this text. Finally, unipotent stem cells give rise to only a single cell line (e.g. spermatogonial stem cells).13 Tests and Markers Functional assays to demonstrate the pluripotent or multipotent nature of stem cells have been achieved through in vitro formation of embryoid bodies, in vivo generation of teratomas in mice with severe combined immunodeficiency (SCID) after grafting of ESCs, and, in the case of HSCs, repopulation of the hematopoietic system of lethally irradiated mice following transplantation of unpurified bone marrow derived cells.12,21,24 The ability to identify and isolate stem cells, however, relies largely on the use of a variety of markers such as surface molecules, transcription factors, and dye efflux. Numerous markers are available, and frequently are used in combination, to identify pluripotent cells and stem cells within certain types of tissue. While some markers and tests are used more universally to recognize stem cells, special attention will be paid to those used in identifying bone marrow-derived stem cells. Cluster of Differentiation (CD)34 CD34 is a cell surface glycoprotein that has traditionally been used in identification and purification of HSCs

and progenitor cells.1 This marker appears to be highly conserved among mammalian species. Experimental evidence suggests that CD34 may be involved in cell adhesion of hematopoietic cells to stromal cells in the bone marrow microenvironment.10 More recently, however, CD34-negative HSCs called side population (SP) cells were identified. SP cells are thought to be some of the most primitive HSCs because of their high proliferative potential and extreme efficiency at homing to sites of hematopoiesis when injected into recipient mice.19 CD34 expression on HSCs may thus be related to the degree of activation of these cells, with CD34negative cells being the most primitive and quiescent.1 Stem Cell Antigen Stem cell antigen-1 (Sca-1) is a cell surface protein often used in identification of murine HSCs. This molecule may play a role in lineage determination.5 Dye Efflux The ability of some primitive HSCs to efflux florescent dye allows for identification of this population, termed SP cells, by flow cytometry.8,19 This ability appears to be due to increased number or activity of membrane pumps (e.g. ATP-binding cassette transporter [ABCtransporter]), a hypothesis supported by the finding of blockage of dye efflux by the drug verapamil, a known inhibitor of these efflux pumps.8 SP cells lack CD34 expression and have been described in multiple species.9 As stated earlier, these CD34-negative cells have been proposed to be some of the most primitive HSCs.

16

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c-Kit c-Kit is a transmembrane tyrosine kinase receptor found on HSCs of multiple species. It binds the ligand stem cell factor (SCF, also called Steel factor), and is important in the maintenance, proliferation, and differentiation of HSCs.28 Lin− As an adjunct to the presence of certain markers (e.g. CD34, c-Kit, Sca-1), the absence of markers present on differentiated cells has been used to isolate and purify HSCs. A lineage negative (Lin−) classification generally indicates that cells are negative for a combination of anywhere from 6 to 14 different lineage markers of mature blood cells. Transcription Factors Transcription factors that appear to be important in regulation of stem cell pluripotency and their undifferentiated state have been identified. Most notable are the transcription factors Oct-4, Nanog, and Sox-2, which have been used as markers of embryonic and adult stem cells.6 BONE MARROW-DERIVED STEM CELLS Within the bone marrow, there appear to be at least three different types of stem cell. HSCs are multipotential stem cells that give rise to the mature cellular elements of the blood (e.g. RBCs, neutrophils, monocytes, platelets, etc.). The stromal components of bone marrow such as bone, cartilage, fat, and fibrous connective tissue are derived from mesenchymal stem cells (MSCs), also termed marrow stromal cells. Finally, endothelial precursor cells (EPCs) are a population of bone marrowderived cells that function in angiogenesis. EPCs are mobilized from the bone marrow into the peripheral blood, where they home to sites of neovascularization such as those present in areas of inflammation, tumor vascularization, or wound repair (see Chapter 11).1

STEM CELL BIOLOGY Regulation of Survival and Pluripotency Niche The niche concept is important to the discussion of stem cell survival and differentiation. Niches are local tissue microenvironments that function to support, maintain, and regulate stem cells. These microenvironments are found in various tissues throughout the body, for example the bulge region of the hair follicle, near the base of crypts in the gastrointestinal tract, and, in the case of HSCs, adjacent to endosteum and bone marrow

sinusoids.20 Regulation of stem cells by their niche occurs through physical contact and cell-cell interactions with adjacent cells, as well as elaboration of soluble factors.20 Evidence also indicates that stem cells have the ability to influence the cellular elements of their niche. For example, HSCs from mice subjected to an acute hematopoietic stress have an increased ability to direct bone marrow mesenchymal cells toward osteoblastic differentiation as a result of HSC-derived bone morphogenic proteins (BMPs).15 Molecular Mechanisms What are the factors that cause stem cells to go down a pathway of self-renewal and remain undifferentiated versus progression toward lineage differentiation and mature cell phenotypes? The answer to this question is constantly evolving as specific molecular mechanisms are elucidated. Several factors important in the maintenance of stem cell survival, self-renewal, and pluripotency have been revealed in murine ESCs. These factors have been divided into extrinsic factors (e.g. cytokines) and intrinsic factors (e.g. transcription factors). Leukemia Inhibitory Factor (LIF) LIF is an interleukin (IL) 6 class cytokine that prevents differentiation of mouse ESCs in culture. Binding of LIF to its membrane receptor results in activation of multiple molecular signaling pathways such as Janus kinase/signal transducers and activators of transcription (JAK/STAT), phosphoinositide-3 kinase (PI3K), and extracellular signal related kinases (ERK). Although these pathways are common downstream signals of many cytokines, in this context, their activation tends to promote maintenance of self-renewal and pluripotency. Activation of ERK in this example, however, appears to favor differentiation of mouse ESCs. Thus, LIF can activate signals that either promote or inhibit maintenance of an undifferentiated state, and it is the balance between these downstream effects (generally favoring self-renewal and pluripotency) that determines the outcome.6 Bone Morphogenic Protein 4 (BMP4) and Basic Fibroblast Growth Factor (FGF) BMP4 and basic FGF are additional examples of extrinsic factors that promote self-renewal and pluripotency in mouse ESCs. In the case of BMP4, it appears to work in a synergistic state with LIF.6 Discovery of additional signaling pathways and factors is likely, as factors important for mouse ESCs are not universal when applied to human ESCs. Wnt Specific extrinsic factors involved in selfrenewal of HSCs have been identified. The Wnt signaling pathway stimulates self-renewal of HSCs while concurrently inhibiting HSC differentiation. Inducing β-catenin activation, a downstream component of the Wnt signaling pathway, results in increased selfrenewal of murine HSCs and limits differentiation of these cells. When inhibitors of the Wnt pathway are added to murine HSCs and growth factors, HSC proliferation is repressed.22

CHAPTER 3: STEM CELL BIOLOGY

Tyrosine Kinase with Immunoglobulin-Like and Endothelial Growth Factor-Like Domains 2 (Tie2) and Angiopoietin-1 Tie2/Angiopoietin-1 signaling has also been implicated in survival of HSCs. Tie2 is a receptor tyrosine kinase expressed on some HSCs. Angiopoietin-1 is the ligand for the Tie2 receptor and promotes quiescence and increased adhesion of murine HSCs to bone marrow stromal cells. Regulation of the quiescent state and maintenance within the HSC niche is thought to be important in HSC survival through a protective effect against myelosuppresive stresses.2 Other Cytokines Other cytokines important in regulation of HSC survival include SCF, thrombopoietin (TPO), BMP, FGF, insulin-like growth factor 2 (IGF-2), and interleukin (IL)-10. SCF and TPO are common components of most cytokine combinations used in the culture and propagation of HSCs. Although TPO is the primary cytokine involved in megakaryocyte and platelet production, it also has been shown to have significant effects on HSCs. In vitro, TPO promotes survival and expansion of HSCs, and mice that have been genetically altered to lack TPO or its receptor have significantly fewer stem cells.16,29 Transcription Factors The intrinsic factors governing the undifferentiated state of ESCs consist primarily of transcription factors. Most notable are Oct-4, Nanog, and Sox-2. These factors are found in pluripotent cell lines and, in general, down-regulation results in differentiation of stem cells. Presence of Oct-4 and Sox-2 appears to be essential for pluripotency; however, the target genes for these transcription factors have not been completely characterized.6 Several transcription factors and cell cycle regulators governing self-renewal of HSCs also have been described. Microribonucleic Acids (miRNAs) miRNAs are an additional intrinsic molecular mechanism proposed to be involved in maintenance of pluripotent stem cells. miRNAs are short, single-stranded RNA molecules that regulate gene function by suppression of translation through annealing and sometimes degradation of mRNA. Novel miRNAs have been found that appear to be expressed preferentially in undifferentiated ESCs. In addition, evaluation of miRNA expression profiles from ESCs of varying degrees of differentiation as well as cells from mature tissues show repression or loss of specific miRNAs as cells progress to a more differentiated state.11 Regulation of Differentiation Differentiation of stem cells into specific lineages is controlled or directed by factors including cytokines, niche interaction, and regulators of self-renewal and pluripotency. Cytokines influence and guide lineage determination of stem cells and many have been described in the context of HSCs and hematopoietic progenitor cell differentiation (Fig. 3.2; see Chapters 6–11). Stromal cells that constitute the stem cell niche influence differentiation and lineage determination through

17

physical cell-cell interaction and elaboration of soluble or cell-bound factors (e.g. cytokines). Finally, as previously described, there are regulators that promote selfrenewal and the pluripotent state of stem cells. For differentiation to occur, these regulators must be inhibited or suppressed. Mechanistically, there may be two general categories by which cells restrict lineage commitment.23 The first of these involves the spectrum of surface receptors, adhesion proteins, and signaling pathways expressed by a given cell. For example, cytokines play an important role in lineage development. However, if a stem or progenitor cell lacks a cytokine receptor then that cytokine would have little or no effect on its target. Gene silencing is a second mechanism by which lineage restriction may occur. For cells to differentiate, specific genes are activated or silenced, guiding cells toward a particular lineage. This may be accomplished through mechanisms such as DNA methylation and histone modification, which alter the transcriptional state of the chromatin. STEM CELL-ASSOCIATED DISEASES Stem Cell Failure The hematopoietic system offers a clear illustration of the effects of stem cell failure. HSCs are responsible for the constant replacement of all cellular components of blood, with HSC failure, cytopenias (e.g. anemia, leukopenia, thrombocytopenia) and their associated clinical manifestations (e.g. lethargy, infection, hemorrhage) ensue. HSC failure can be the result of a number of underlying pathologic processes, including toxic or drug-mediated damage, immune-mediated damage, infectious agents (e.g. parvovirus, feline leukemia virus, and Ehrlichia spp.), insufficient stimulation by cytokines and growth factors, and disruption of or damage to the stem cell niche (e.g. myelophthisis, ischemia, inflammation).4,7,25,26 (See Section II.) Stem Cells and Proliferative Disorders Just as adult stem cells are responsible for replacement of mature cells and tissues, there is strong evidence that cells with stem cell properties underlie the pathology of at least some types of cancer. The hypothesis of cancer stem cells is based on a few basic observations. The first of these is the observation of tumor heterogeneity. Many tumors comprise cells with different morphologies and phenotypes that in some cases loosely resemble the tissue of origin. This suggests a certain degree of differentiation within a population of tumor cells, leading to variability in structure and function. A more primitive precursor cell (i.e. cancer stem cell) could presumably give rise to the different phenotypes within a tumor. The second observation is that transplantation of a tumor required relatively large numbers of cancerous cells, an indication that only small numbers of cells in a given tumor have the ability to form a tumor. Cancer stem cells are present in small numbers within

18

SECTION I: HEMATOPOIESIS

Multipotent stem cell Self renewal HSC SCF TFO

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Monocytes

Platelets Erythrocytes

Neutrophils, eosinophils, basophils

NK Cells

FIGURE 3.2 A general model of hematopoiesis. Blood cell development progresses from a hematopoietic stem cell (HSC), which can undergo either self-renewal or differentiation into a multilineage committed progenitor cell: a common lymphoid progenitor (CLP) or a common myeloid progenitor (CMP). These cells then give rise to more differentiated progenitors, comprising those committed to two lineages that include T cells and natural killer cells (TNKs), granulocytes and macrophages (GMs), and megakaryocytes and erythroid cells (MEPs). Ultimately, these cells give rise to unilineage committed progenitors for B cells (BCPs), NK cells (NKPs), T cells (TCPs), granulocytes (GPs), monocytes (MPs), erythrocytes (EPs), and megakaryocytes (MkPs). Cytokines and growth factors that support the survival, proliferation, or differentiation of each type of cell are shown in red. For simplicity, the three types of granulocyte progenitor cells are not shown; in reality, distinct progenitors of neutrophils, eosinophils, and basophils or mast cells exist and are supported by distinct transcription factors and cytokines (e.g. interleukin-5 in the case of eosinophils, stem-cell factor [SCF] in the case of basophils or mast cells, and G-CSF in the case of neutrophils). IL denotes interleukin, TPO thrombopoietin, M-CSF macrophage colony-stimulating factor, GM-CSF granulocyte-macrophage CSF, and EPO erythropoietin. (Reprinted from Kaushansky K. Lineage-specific hematopoietic growth factors. New Engl J Med 2006;354:2034–2045, with permission. ©Massachusetts Medical Society 2006.)

CHAPTER 3: STEM CELL BIOLOGY

a tumor, and thus relatively large amounts of tissue would be needed to ensure the presence of these cells. Just like normal stem cells, cancer stem cells share the basic functional properties of self-renewal and the ability to differentiate. In humans, evidence for cancer stem cells has been shown in hematopoietic, brain, breast, colon, prostate, bone, and ovarian cancers, and there is some evidence for the existence of cancer stem cells in animals.14,17,18,27 Support for the existence of cancer stem cells in humans consists of identification of a subset of tumor cells that express stem cell markers and have exclusive or enhanced ability to form tumors in vitro or in vivo. Evidence exists that cancer stem cells may arise from normal stem cells and/or progenitor cells that have reacquired the ability of self-renewal. The origins of cancer stem cells continue to be explored. The existence of cancer stem cells has clear implications for understanding cancer biology and treatment in at least certain types of cancers. For example, many chemotherapeutics target rapidly dividing cells. However, cancer stem cells are relatively slowly cycling, thus allowing them to persist with these conventional treatments. Newer therapeutic modalities directed at elimination of cancer stem cells will be important for effective treatment of these types of neoplasia.

REFERENCES 1. Alison MR, Brittan M, Lovell MJ, et al. Markers of adult tissue-based stem cells. Handbook Exp Pharmacol 2006;174:185–227. 2. Arai F, Hirao A, Ohmura M, et al. Tie2/Angiopoietin-1 signaling regulates hematopoietic stem cell quiescence in the bone marrow niche. Cell 2004;188:149–161. 3. Bonnet D. Haematopoietic stem cells. J Pathol 2002;197:430–440. 4. Boosinger TR, Rebar AH, DeNicola DB, et al. Bone marrow alterations associated with canine parvoviral enteritis. Vet Pathol 1982;19:558–561. 5. Bradfute SB, Graubert TA, and Goodell MA. Roles of Sca-1 in hematopoietic stem/progenitor cell function. Exp Hematol 2005;33:836–843. 6. Darr H, Benvenisty N. Factors involved in self–renewal and pluripotency of embryonic stem cells. Handbook Exp Pharmacol 2006;174:1–19. 7. Dornsife RE, Gasper PW, Mullins JI, et al. Induction of aplastic anemia by intra-bone marrow inoculation of molecularly cloned feline retrovirus. Leuk Res 1989;13:745–755.

19

8. Goodell MA, Brose K, Paradis G, et al. Isolation and functional properties of murine hematopoietic stem cells that are replicating in vivo. J Exp Med 1996;183:1797–1806. 9. Goodell MA, Rosenzweig M, Kim H, et al. Dye efflux studies suggest that hematopoietic stem cells expressing low or undetectable levels of CD34 antigen exist in multiple species. Nat Med 1997;3:1337–1345. 10. Healy L, May G, Gale K, et al. The stem cell antigen CD34 functions as a regulator of hemopoietic cell adhesion. Proc Natl Acad Sci USA 1995;92:12240–12244. 11. Houbaviy HB, Murray MF, and Sharp PA. Embryonic stem cell-specific microRNAs. Dev Cell 2003;5:351–358. 12. Itskovitz–Eldor J, Schuldiner M, Karsenti D, et al. Differentiation of human embryonic stem cells into embryoid bodies comprising the three embryonic germ layers. Mol Med 2000;6:88–95. 13. Jaenisch R and Young R. Stem cells, the molecular circuitry of pluripotency and nuclear reprogramming. Cell 2008;132:567–582. 14. Jordan CT, Guzman ML, and Noble M. Cancer stem cells. New Engl J Med 2006;355:1253–1261. 15. Jung Y, Song J, Shiozawa Y, et al. Hematopoietic stem cells regulate mesenchymal stromal cell induction into osteoblasts thereby participating in the formation of the stem cell niche. Stem Cells 2008;26:2042–2051. 16. Kaushansky K. Lineage-specific hematopoietic growth factors. New Engl J Med 2006; 354:2034–2045. 17. Lamerato-Kozicki AR, Helm KM, Jubala CM, et al. Canine hemagiosarcoma originates from hematopoietic precursors with potential for endothelial differentiation. Exp Hematol 2006;34:870–878. 18. Lobo NA, Shimono Y, Qian D, et al. The biology of cancer stem cells. Annu Rev Cell Devel Biol 2007;23:675–699. 19. Matsuzaki Y, Kinjo K, Mulligan RC, et al. Unexpectedly efficient homing capacity of purified murine hematopoietic stem cells. Immunity 2004;20:87–93. 20. Morrison S and Spradling AC. Stem cells and niches:mechanisms that promote stem cell maintenance throughout life. Cell 2008;132:598–611. 21. Reubinoff BE, Pera MF, Fong CY, et al. Embryonic stem cell lines from human blastocysts:somatic differentiation in vitro. Nat Biotechnol 2000;18:399–404. 22. Reya T, Duncan AW, Ailles L, et al. A role for Wnt signaling in selfrenewal of haematopoietic stem cells. Nature 2003;423:409–414. 23. Theise ND and Harris R. Postmodern biology:(adult)(stem) cells are plastic, stochastic, complex, and uncertain. Handbook Exp Pharmacol 2006;174:389–408. 24. Till JE and McCulloch EA. A direct measurement of the radiation sensitivity of normal mouse bone marrow cells. Rad Res 1961;14:1419–1430. 25. Weiss DJ. A retrospective study of the incidence and the classification of bone marrow disorders in the dog at a veterinary teaching hospital (1996– 2004). J Vet Int Med 2006;20:955–961. 26. Weiss DJ and Klausner JS. Drug–associated aplastic anemia in dogs:eight cases (1984–1988). J Am Vet Med Assoc 1990;196:472–479. 27. Wilson H, Huelsmeyer M, Chun R, et al. Isolation and characterization of cancer stem cells from canine osteosarcoma. Vet J 2008;175:69–75. 28. Zayas J, Spassov DS, Nachtman RG, et al. Murine hematopoietic stem cells and multipotent progenitors express truncated intracellular form of c-Kit receptor. Stem Cells Devel 2008; 17:351–361. 29. Zhang CC and Lodish HF. Cytokines regulating hematopoietic stem cell function. Curr Opin Hematol 2008;15:307–311.

CHAPTER 4

Cluster of Differentiation (CD) Antigens MELINDA J. WILKERSON and CINZIA MASTRORILLI Definition and History Structure and Integration of Membrane Antigens Transmembrane Proteins Single Pass Type I Transmembrane Protein (I) Single Pass Type II Transmembrane Protein (II)

Multipass Transmembrane Protein (III) Glycosyl Phosphoatidylinositol Transmembrane Protein (V) Tissue Distribution

Acronyms and Abbreviations CD, cluster differentiation; CLAW, Canine Leukocyte Antigen Workshop; ER, endoplasmic reticulum; FIV, feline immunodeficiency virus; GP, glycoprotein; GPI, glycolyl phosphatidylinositol transmembrane protein; Ig, immunoglobulin; IL, interleukin; Mab, monoclonal antibody; MDR-1, multidrug resistance transporter protein; MHC, major histocompatibility antigen; NK, natural killer; PROW, The International Protein Reviews on the Web; VLA, very late antigen.

DEFINITION AND HISTORY Cluster differentiation (CD) nomenclature was first introduced at an international conference in Paris (1982) during the boom in monoclonal antibody (MAb) technology.2 This nomenclature was established to standardize the classification of cell surface antigens and to define the biological functions of molecules expressed by various hematopoietic lineages. Several international workshops have been held to exchange human and other species-specific MAbs and to compare their reactivity with cells and cell proteins from veterinary species.7,27,28,40,46 Based on these workshops, MAbs that have similar reactivity with tissues or cell types are assigned to a cluster group. Therefore, an antigen that is recognized by a cluster of MAbs is assigned a “cluster of differentiation” (CD) number. If MAbs defining a cluster of antigens are derived from the same laboratory, the suffix “w” is appended to the CD designation. Only eight CD antigens are internationally accepted as defined by the 1st Canine Leukocyte Antigen Workshop (CLAW) and include the following homologues to the human system: CD4, CD5, CD8, CD11a/18, CDw41, CD44, CD45, and CD45R.7 The final meeting of the Human Leukocyte Differentiation Antigen 8-workshop was held in December, 2004, in Adelaide, Australia, and 376 MAbs from various companies, mainly directed against human leukocytes, were tested for their reactivity with cells from 17 different animal species. A special 20

issue of the journal Veterinary Immunology and Immunopathology described these efforts.39 Today there are over 339 CD molecules defined. This explosion in CD molecules is the result of the use of molecular biology techniques to identify new molecules. The purpose of this chapter is to provide the most current listing of CD antigens recognized by various antibodies in veterinary species. Table 4.1 summarizes the current knowledge of important veterinary CD antigens, including MAb clones that react with dog, cat, ruminant (cattle, sheep, and goats), pig, and horse CD antigens; relative molecular size(s); topology in the membrane; tissue distribution; known physiology; and species reactivity of MAbs. Selected key references for each CD antigen are included. More in depth information is available for the currently described CD molecules on several websites, including the taxonomic key program maintained by Washington State University that is designed to provide information of the specificity of MAbs specific for leukocyte differentiation molecules intra- and crossspecies (http://www.vetmed.wsu.edu/tkp/). There are links from this website to commercial companies that are the major suppliers of veterinary-specific MAbs, including Serotec (http://www.ab-direct.com/ antibodies/_-500.html) and Veterinary Medical Research & Development (VMRD) (http://www.vmrd. com/). The International Protein Reviews on the Web (PROW) (http://mpr.nci.nih.gov/prow/) provides an

CHAPTER 4: CLUSTER OF DIFFERENTIATION (CD) ANTIGENS

21

TABLE 4.1 Cluster Differentiation Antigens

CD Antigen (MAb Clone) CD1a (CA9.AG5) CD1a8.2 (CA13.9H11) CD1a6 (Fe1.5F4) CD1a (Fel5.5C1) CD1b (CC20) CD2 (MCA833F) (HB88A)b

MAb reactivity (primary species in italic)

Size (Mr)

Topology

Canine

49

I

Canine

43

Feline

49

Bovine

Thymocytes dendritic cells, Langerhans cells

Nonpolymorphic CD1 family of glycoproteins includes CD1a, b, c, d, and e isoforms involved in presentation of foreign and self lipid antigens and glycolipids; 43–49 subunits interact with a 12 kDa subunit (α2-microglobulin)

45 Enhances adhesion between T cells and antigen presenting cells

13, 23

I

T cells

A family of proteins that forms a signal transduction complex for T cell receptor when it binds antigen)

11, 50

60 55

Canine

67

I Neutrophil, T cells T cells I

T cells

Receptor for MHC class II, facilitates recognition of peptide antigens Receptor for CD72, facilitates signals transduced by T cell receptor for antigen

Equine

CD8 beta (CA15.4G2) (vpg9) (HT14A)b

Canine Feline Equine

26, 53

T cells, NK cells

Canine Feline

Canine Feline Equine

26

I

CD4 (CA13.1E4) (vpg34)

CD8 alpha (CA9.JD3) (Fe1.10E9) (73/6.9.1)b

35, 26

45–58

Bovine

Canine

Selected References

19

Bovine Equine 12, 16, 22, 32, 44

CD9 (vpg15) (MM2/57)

Physiology

Feline

CD3 (CA17.2A12) (MM1A)b

CD5 (YKIX322.3), (DH13A)b (HB19A), (HT23A)b

Distribution

7, 34 51 3, 7

21, 23 32, 36

39

Feline Human, canine, feline, bovine, horse Bovine

30–45

CD11a (Ca11.4D3, Ca11.7H11)

Canine, feline

CD11b (CCA16.3E10)

I

Thymocytes, T cells

T cells

Forms a heterodimer with CD8β; receptor for MHC class I; facilitates recognition of peptide antigens Forms a heterodimer with CD8α (see above)

34 53 49 34 44 49

III

Activated lymphocytes, monocytes, lymphocytes, granulocytes, platelets

Important for signal transduction, cell activation, adhesion, aggregation; co-receptor for FIV; associates with tetraspan superfamily (CD63, CD81, CD82)

180, 95

I

Monocytes, granulocytes, lymphocytes

Associates with CD18 to form a heterodimner receptor to facilitate adhesion

9

Canine, ruminants, porcine, feline, mink, human

180

I

Granulocytes, monocytes, lymphocyte subset

Associates with CD18 to form a heterodimer receptor to facilitate adhesion

3, 9, 10

CD11c (Ca11.6A1, Ca11.7D1)

Canine

150, 95

I

Dendritic cells, granulocytes, macrophages, lymphocytes

Associates with CD18 to form a heterodimer receptor to facilitate adhesion

9

CD11d (Ca11.8H2)

Canine

155

I

CD8+ T cells, γδ− T cells, macrophages in splenic red pulp

Associates with CD18 to form a heterodimner receptor to facilitate adhesion

10

(RHIA)b

24

I

18, 52 39, 41

47

22

SECTION I: HEMATOPOIESIS

TABLE 4.1 Continued

CD Antigen (MAb Clone)

MAb reactivity (primary species in italic)

Size (Mr)

Topology

150

II

Distribution

Physiology

CD13 (CVS19)

Equine

(CC81)

Bovine

CD14 (CAM36)b

Ruminant, canine, feline, porcine, llama

53

V

Monocytes, myeloid cells

Receptor for lipopolysaccharide that transduces signals, leading to oxidative burst and proinflammatory cytokine synthesis

3, 31

CD18 (CA1.E49)

Canine Bovine Equine

95

I

All leukocytes

Beta subunit of the integrin heterodimer and combines with the alpha subunit of either CD11a, CD11b, CD11c, CD11d; plays a role in adhesion to endothelium

33 21 41

145–160

I

B-cells, monocytes follicular dendritic cells

Receptor for C3d fragment and CD23; enhances B cell antigen receptor signal transduction

3, 4 3, 4

(LB21)

Canine Equine, feline, human Feline

CD22 (RFB-4) (Mc64-12)

Human Canine Bovine

130, 140

I

Early B-cell stages, mature B-cells

Binds sialoglyco-conjugates on some CD45 isoforms to modulate B cell signal transduction

16, 42, 47

CD23 (M763)a

Human Ruminants, canine, feline, porcine

44 52

II

B-cells and monocytes

Involved in the regulation of IgE synthesis following binding to IgE Fc and IgE-containing immune complexes

3

50–55

I

Mitogen stimulated T or B lymphocytes, LPS stimulated monocytes

IL2-receptor-alpha subunit

36–38

(H20A)b

CD21 (CA2.1D6)

CD25 (CACT116A, CACT108A)b (9F23)

Granulocytes and monocytes Dendritic cells in afferent lymph

Aminopeptidase N, a metalopeptidase in humans which removes NH2 terminal amino acids from peptides

Selected References 22, 23 17, 20

31

Ruminants Feline

CD29 (12G10, 3S3)

Human, canine

110

I

Platelets

Beta chain of VLA, platelet GPIIa

42

CD34 (1H6)

Canine

110

I

Lymphohematopoietic stem cells and progenitors, endothelial cells

Leukocyte-endothelial interactions through binding with CD62L and CD62E

29, 30

CD35 (To5)a

Feline

190, 220

I

B-cells,erythrocytes, granulocytes, monocytes

Receptor for C3b and C4b (CR1) bound to immune complexes

31

CD41 (Canine 20-4), (CL2A)b, (M7057)a

Canine

125

I

Platelets, megakaryocytes

Integrin αIIB CD41/CD51 complex, receptor for fibrinogen

3, 7

CD44 (YKIX337.8) (BAG40A)b

Canine

90

I

Most cell types including epithelial cells, activated T cells

Receptor for hyaluronate that facilitates lymphocytic binding to high endothelial venules

7

Membrane-bound tyrosine phosphatase critical for antigenreceptor-mediated activation of leukocytes Largest of the CD45 isoforms

Feline, bovine

CD45 (CA12.10C12)

Canine

180, 200, 220

I

Pan leukocyte

CD45RA (CA4.1D3)

Canine

205, 220, 180– 240

I

Naïve T/B lymphocytes

7, 47 4, 6, 7, 50

4, 6, 7, 50

CHAPTER 4: CLUSTER OF DIFFERENTIATION (CD) ANTIGENS

23

TABLE 4.1 Continued

CD Antigen (MAb Clone) CD47 (HUH69A), (HUH71A)b

MAb reactivity (primary species in italic) Human

Size (Mr)

Topology

47–55

III

Canine Bovine

Distribution

Physiology

Selected References

Lymphocytes, macrophages, granulocytes

Associates with CD61 integrins to form receptor for thrombospondin; role in chemotaxis and adhesive interactions with leukocytes and endothelial cells

39 42 47

CD49d (Fe 2.9F2), (P4G9)a

Feline, canine, bovine

180

I

Lymphocytes, macrophages, granulocytes

VLA binds with 39CD29; binds fibronectin and mucosal addressin

3, 31

CD49e (JBS5)

Human, canine

155

I

Macrophages, granulocytes

VLA antigen binds to CD29 and together is the fibronectin receptor and binds to RGD sequence of fibronectin

42

CD56 (MOC-1), (T199)a

Human, canine

175–220

I or V

Subset of lymphocytes

Homotypic adhesion and natural killer cell cytotoxicity

42

CD61 (Y2/51)a

Human, canine

90

I

Platelets, megakaryocytes, monocytes

Associates with CD41 to form the GPIIb-IIIa heterodimer that facilitates platelet aggregation

42

CD79a (HM57)a

Human, canine

44–49

I

B lymphocytes

Cytoplasmic molecule that mediates sIg expression and B cell receptor cell transduction

16, 32

CD88 (S5/1)

Human, bovine

30–45

III

Monocytes, B cells, T-cell subset

C5a receptor homolog; G-coupled receptor that triggers chemotaxis, respiratory burst and degranulation of granulocytes in humans

45

CD90 (CA1.4G8), (CA9.GA11)

Canine, equine Porcine, mink Guinea pig

18–44

V

Pro-thymocytes, T-cells, monocytes; weak on granulocytes, renal tubular cells

May contribute to formation of neuron memory and to growth regulation of hematopoietic stem cells

3, 4, 7

CD91 (A2MR-2)b

Human, canine

600

I

Macrophages of liver, lung and lymphoid tissues

Member of low density lipoprotein receptor family that binds to α2 macroglobulin

39, 42

CD94 (HP-3D9)b

Human, canine

30

II

Lymphocyte subset (NK)

Binds to NKG2 and plays a role in recognition of MHC class I molecules by NK cells and some cytotoxic T cells; ligation of CD94 can inhibit or stimulate killing by NK cells

42

CDw119 (BB1E2)

Bovine

90–100

I

Monocytes, B cells, epithelial cells

Interferon gamma receptor

47

CD134 (7D6)

Feline

43

I

CD4+ activated T cells

Tumor necrosis factor receptor superfamily; regulator of T celldependent immune responses; receptor for FIV in conjunction with CXCR4

14, 43

CD163 (Ber MAC3)

Bovine

130

I

Monocytes

Scavenger receptor cysteine rich family

47

CD172a (DH59b)b

Bovine, canine, equine, feline

90

I

Monocytes, granulocytes

Member of the signal regulatory protein family involved in negative regulation of receptor tyrosine kinase-coupled signaling processes

12, 15, 31, 39, 42

CD235a (JC159)a

Human Canine

35 20

I

Erythrocytes

Glycophorin A, major sialoglycoprotein on erythrocytes

42

24 or 26 27

Column 1: CD designation of the antigen with the monoclonal antibody (MAb) clone published to have primary reactivity shown below in parentheses. Column 2: primary species of reactivity (italics) and those species with cross reactivity (all antibodies are commercially available; those with superscripts are obtainable from Serotec, Inc.a and VMRDb). Column 3: relative molecular mass of the reduced CD antigen. Column 4: integration of antigen to plasma membrane. Column 5: cell types and tissues known to express a CD. Column 6: proposed or known physiology of a CD antigen based on studies in animals or humans. Column 7: key references concerning identification and characterization of the CD antigen and/or characterization of the antibodies.

24

SECTION I: HEMATOPOIESIS

exhaustive database of the complete listing of CD antigens and links to primary nucleic acid and protein sequences of human CD antigens.

STRUCTURE AND INTEGRATION OF MEMBRANE ANTIGENS CD antigens are principally membrane proteins defined by their location within or at the surface of the phospholipid bilayer. Membrane proteins are classified into three categories: integral, lipid-anchored, and peripheral, depending on the nature of membrane-protein interactions.24 The CD antigens are grouped as integral membrane proteins or transmembrane proteins and consist of cytosolic, membrane spanning, and exoplasmic (luminal) domains. The cytosolic and exoplasmic domains have hydrophilic exterior surfaces with either C-terminus or N-terminus group endings. The membrane-spanning domains usually contain hydrophobic amino acids and consist of one or more alpha helix or multiple beta strands.24 Most integral membrane proteins fall into one of five classes, depending on how they anchor themselves in the membrane.1 Type I and II proteins have a single transmembrane region, whereas Types III and IV have multiple transmembrane regions also referred to as tetraspanins. Type IV proteins (not shown in the table) are distinguished from type III proteins by the presence of a water-filled transmembrane channel. Type V proteins use lipid to attach to membranes. These lipids are either a glycosyl-phophatidylinositol (GPI) anchor or lipid moieties such as myristoyl groups, which include cytoplasmic signaling proteins that will not be described in the this chapter.1,25 Each class pertinent to CD antigens used in veterinary medicine will be described briefly, and examples of common CD antigens used in diagnostic veterinary medicine will be highlighted in the text and are listed in Table 4.1.

TRANSMEMBRANE PROTEINS Single Pass Type I Transmembrane Protein (I) Type I and II transmembrane proteins have only one membrane-spanning α-helix containing 20–25 hydrophobic amino acids. Type I proteins have an N-terminal endoplasmic reticulum (ER) signal sequence that is cleaved after the molecule passes into the ER. This is the most common mode of membrane integration among the CD antigens listed in Table 4.1. The protein is glycosylated in the Golgi apparatus if the protein has a glycosylation site and then is expressed on the cell surface. Type I proteins are anchored in the membrane with their hyprophilic N-terminal region on the exoplasmic face and their hydrophilic C-terminal region on the cytoplasmic face. These proteins commonly represent cell surface receptors and/or ligands (like CD4), of which many belong to the immunoglobulin (Ig) superfamily.1

Single Pass Type II Transmembrane Protein (II) Type II transmembrane proteins lack a cleavable ER signal sequence and are oriented with their hydrophilic C-terminus on the exoplasmic face and the hydrophilic N-terminus on the cytoplasmic face. These proteins have an internal hydrophobic ER signal and membrane anchor sequence. Because these proteins can be released from the cell surface, they also can act as plasma proteins with physiologic effect(s) on cells bearing the counter ligands.1 CD13, a zinc-binding metalloprotease that acts to facilitate antigen presentation by trimming the N-terminal amino acids from MHC Class II-bound peptides, is an example of a type II protein.1 Multipass Transmembrane Protein (III) Type III transmembrane proteins cross the membrane multiple (2, 3, 4, 5, 7, or 12) times and also are called tetraspanins. The most frequently found tetraspanins include those that span the membrane four and seven times. Structural studies indicate that the transmembrane regions are alpha helices. If the type III protein has an even number of transmembrane alpha helices, its N- and C-temini are oriented toward the same side of the membrane. Many of these cell surface molecules function as receptors for soluble molecules such as prostaglandins and chemokines. Examples of seven transmembrane type III proteins are interleukin-8 (IL8) receptor (CD128) and C5a receptor (CD88). CD9 is an example of a four multi-pass transmembrane protein.1 CD47 is an example of type III protein with five transmembrane sequences. The multidrug resistance transporter protein MDR-1 (CD243) has 12 transmembrane regions.1 Gpi Transmembrane Protein (V) Type V transmembrane proteins utilize GPI anchors attached to the C-terminal residue of the protein. GPIanchored molecules have a secretion signal sequence at their N-terminus and C-teminus that is cleaved and replaced by the GPI-anchor after synthesis of the molecule and entry into the ER.25 CD14, a surface protein involved in the clearance of Gram-negative pathogens bound to lipid polysaccharide binding protein (Fig. 4.1), is an example of a GPI-linked glycoprotein. Other examples of GPI-linked glycoproteins include CD56 and CD90.1 TISSUE DISTRIBUTION Cell surface antigens commonly used in immunophenotyping of hematologic neoplasia initially were determined by studies of hematopoietic cell differentiation and maturation.8,50 Lineage-associated markers can be broadly classified into groups that recognize B cell, T cell, natural killer (NK) cell, myeloid/monocytic, and erythroid lineages. Noncommitted hematopoietic stem cells express CD34, a glycoslyated surface glycoprotein.

CHAPTER 4: CLUSTER OF DIFFERENTIATION (CD) ANTIGENS

25

FIGURE 4.1 Major integral membrane proteins and their topology or interaction in the lipid bilayer. The types of membrane proteins are indicated at the top. CD4 is a type I transmembrane protein that passes through the membrane once, has four extracellular immunoglobulin domains, a carboxyl terminus on the cytoplasmic face of the bilayer, and N-terminus (NH2) on the exoplasmic face. CD13 is a type II transmembrane protein with the N-terminus on the cytoplasmic face and the carboxyl terminus on the exoplasmic face. The extracellular domain of CD13 is heavily N-glycosylated (thin pegs extending from the polypeptide backbone). CD9 is a type III multi-pass membrane protein with four transmembrane regions, and N- and C-termini on the cytoplasmic face. CD14 is a type V glycosylphosphatidylinositol (GPI) anchored protein. (Courtesy of Mal Rooks Hoover, Graphic Design Specialist.)

This marker is frequently used to differentiate acute immature leukemias of lymphoid or myeloid origin from chronic lymphocytic leukemia or leukemic stages of lymphoma.1 Lineage-specific cell surface antigens that are useful for delineating leukocyte cell lineages include CD3, which is exclusively expressed on mature T cells, and CD79α and β, which form part of the B cell surface antigen receptor. Non-lineage restriction surface antigens include antigens such as CD45, which is found on all leukocyte lineages (myeloid and lymphoid). Although the last CLAW was in 1993, it was an important collaborative effort because anomalous expression of the CD4 antigen was identified in dogs, which express a high density of CD4 on neutrophils in addition to helper T cells.34 Many other differences in CD antigen expression are unique among domestic animals compared to humans (e.g. high expression of the γδ T cell receptor in pigs and ruminants5,36 and co-expression of CD4 and CD8 on mature T cells in swine).48

REFERENCES 1. Barclay AN, Brown MH, Law SKA, et al. The architecture and interactions of leukocyte surface molecules. In: The Leucocyte Antigen Facts Book, 2nd edn. San Diego: Academic Press. 1997;101–129. 2. Bernard A, Boumsell L. Human leukocyte differentiation antigens. Presse Med 1984;13:2311–2316. 3. Brodersen R, Bijlsma F, Gori K, et al. Analysis of the immunological cross reactivities of 213 well characterized monoclonal antibodies with specificities against various leucocyte surface antigens of human and 11 animal species. Vet Immunol Immunopathol 1998;64:1–13. 4. Caniatti M, Roccabianca P, Scanziani E, et al. Canine lymphoma: immunocytochemical analysis of fine-needle aspiration biopsy. Vet Pathol 1996;33:204–212. 5. Carr MM, Howard CJ, Sopp P, et al. Expression on porcine gamma delta lymphocytes of a phylogenetically conserved surface antigen previously restricted in expression to ruminant gamma delta T lymphocytes. Immunology 1994; 81:36–40.

6. Cobbold S, Holmes M, Willett B. The immunology of companion animals: reagents and therapeutic strategies with potential veterinary and human clinical applications. Immunol Today 1994;15:347–353. 7. Cobbold S, Metcalfe S. Monoclonal antibodies that define canine homologues of human CD antigens: summary of the First International Canine Leukocyte Antigen Workshop (CLAW). Tissue Antigens 1994;43:137–154. 8. Comazzi S, Gelain ME, Spagnolo V, et al. Flow cytometric patterns in blood from dogs with non–neoplastic and neoplastic hematologic diseases using double labeling for CD18 and CD45. Vet Clin Pathol 2006;35:47–54. 9. Danilenko DM, Moore PF, Rossitto PV. Canine leukocyte cell adhesion molecules (LeuCAMs): Characterization of the CD11/CD18 family. Tissue Antigens 1992;40:13–21. 10. Danilenko DM, Rossitto PV, Van der Vieren M, et al. A novel canine leukointegrin, alpha d beta 2, is expressed by specific macrophage subpopulations in tissue and a minor CD8+ lymphocyte subpopulation in peripheral blood. J Immunol 1995;155:35–44. 11. Davis WC, MacHugh ND, Park YH, et al. Identification of a monoclonal antibody reactive with the bovine orthologue of CD3 (BoCD3). Vet Immunol Immunopathol 1993;39:85–91. 12. Davis WC,Marusic S, Lewin HA, et al. The development and analysis of species specific and cross reactive monoclonal antibodies to leukocyte differentiation antigens and antigens of the major histocompatibility complex for use in the study of the immune system in cattle and other species. Vet Immunol Immunopathol 1987;15:337–376. 13. Davis WC, Splitter GS. Individual antigens of cattle: bovine CD2 (BoCD2). Vet Immunol Immunopathol 1991;27:43–50. 14. de Parseval A, Chatterji U, Sun P, et al. Feline immunodeficiency virus targets activated CD4+ T cells by using CD134 as a binding receptor. Proc Natl Acad Sci USA 2004;101:13044–13049. 15. Falyna M, Leva L, Knotigova P, et al. Lymphocyte subsets in peripheral blood of dogs – a flow cytometric study. Vet Immunol Immunopathol 2001;82:23–37. 16. Faldyna M, Samankova P, Leva L, et al. Cross-reactive anti-human monoclonal antibodies as a tool for B-cell identification in dogs and pigs. Vet Immunol Immunopathol 2007;119:56–62. 17. Hope JC, Sopp P, Collins RA, et al. Differences in the induction of CD8+ T cell responses by subpopulations of dendritic cells from afferent lymph are related to IL–1 alpha secretion. J Leukocyte Biol 2001;69:271–279. 18. Hosie MJ, Willett BJ, Dunsford TH, et al. A monoclonal antibody which blocks infection with feline immunodeficiency virus identifies a possible non-CD4 receptor. J Virol 1993;67:1667–1671. 19. Howard CJ, Sopp P, Bembridge G, et al. Comparison of CD1 monoclonal antibodies on bovine cells and tissues. Vet Immunol Immunopathol 1993;39:77–83. 20. Howard CJ, Sopp P, Brownlie J, et al. Identification of two distinct populations of dendritic cells in afferent lymph that vary in their ability to stimulate T cells. J Immunol 1997;159:5372–5382.

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21. Ibrahim S, Saunders K, Kydd JH, et al. Screening of anti–human leukocyte monoclonal antibodies for reactivity with equine leukocytes. Vet Immunol and Immunopathol 119:63–80. 22. Ibrahim S, Steinbach F. Non–HLDA8 animal homologue section anti– leukocyte mAbs tested for reactivity with equine leukocytes. Vet Immunol Immunopathol 2007;119:81–91. 23. Kydd, JH. Report of the First International Workshop on Equine Leucocyte Antigens, Cambridge, UK. Equine Immunology, 1991;4. 24. Berk Lodish H, Kaiser A, Krieger CA, et al. Biomembranes: protein components and basic functions. In: Ahr K, ed., Molecular Cell Biology, 6th edn. New York: W.H. Freeman and Company, 2008;421–428. 25. Berk Lodish H, Kaiser A, Krieger CA, et al. Several topological classes of Integral membrane proteins are synthesized on the ER. In: Ahr K, ed., Molecular Cell Biology, 6th edn. New York: W.H. Freeman and Company, 2008;543–549. 26. Looringh van Beeck FA, Zajonc DM, et al. Two canine CD1a proteins are differentially expressed in skin. Immunogenetics 2008;60:315–324. 27. Lunn DP, Holmes MA, Antczak DF. Summary report of the Second Equine Leucocyte Antigen Workshop. Vet Immunol Immunopathol 1996;54:159– 161. 28. Lunn DP, Holmes MA, Antczak DF, et al. Report of the Second Equine Leucocyte Antigen Workshop, Squaw Valley, California, July 1995. Vet Immunol Immunopathol 1998;62:101–143. 29. McSweeney PA, Rouleau KA, Storb R, et al. Canine CD34: cloning of the cDNA and evaluation of an antiserum to recombinant protein. Blood 1996;88:1992–2003. 30. McSweeney PA, Rouleau KA, Wallace PM, et al. Characterization of monoclonal antibodies that recognize canine CD34. Blood 1998;91: 1977–1986. 31. Meister RK, Taglinger K, Haverson K, et al. Progress in the discovery and definition of monoclonal antibodies for use in feline research. Vet Immunol Immunopathol 2007;119:38–46. 32. Milner RJ, Pearson J, Nesbit JW, et al. Immunophenotypic classification of canine malignant lymphoma on formalin-mixed paraffin wax-embedded tissue by means of CD3 and CD79a cell markers. Onderstepoort J Vet Res 1996;63:309–313. 33. Moore PF, Rossitto PV, Danilenko DM. Canine leukocyte integrins: characterization of a CD18 homologue. Tissue Antigens 1990;36:211– 220. 34. Moore PF, Rossitto PV, Danilenko DM. Monoclonal antibodies specific for canine CD4 and CD8 define functional T-lymphocyte subsets and high-density expression of CD4 by canine neutrophils. Tissue Antigens 1992;40:75–85. 35. Moore PF, Schrenzel MD, Affolter VK, et al. Canine cutaneous histiocytoma is an epidermotropic Langerhans cell histiocytosis that expresses CD1 and specific beta 2-integrin molecules. Am J Pathol 1996;148:1699–1708. 36. Naessens J., Howard CJ, Hopkins J, et al. Nomenclature and characterization of leukocyte differentiation antigens in ruminants. Immunol Today 1997;18:365–368.

37. Naessens J, Sileghem M, MacHugh N, et al. Selection of BoCD25 monoclonal antibodies by screening mouse L cells transfected with the bovine p55-interleukin-2 (IL-2) receptor gene. Immunology 1992;76:305– 309. 38. Ohno K, Goitsuka R, Kitamura K, et al. Production of a monoclonal antibody that defines the alpha-subunit of the feline IL-2 receptor. Hybridoma 1992;11:595–605. 39. Saalmuller, Aasted B. Summary of the animal homologue section of HLDA8. Vet Immunol Immunopathol 2007;119:2–13. 40. Saalmuller A, Denham S, Haverson K, et al. The Second International Swine CD Workshop. Vet Immunol Immunopathol 1995;54:155–158. 41. Saalmuller A, Lunney JK, Daubenberger C, et al. Summary of the animal homologue section of HLDA8. Cell Immunol 2005;236:51–58. 42. Schuberth HJ, Kucinskiene G, Chu RM, et al. Reactivity of cross–reacting monoclonal antibodies with canine leukocytes, platelets and erythrocytes. Vet Immunol Immunopathol 2007;119:47–55. 43. Shimojima M, Miyazawa T, Ikeda Y, et al. Use of CD134 as a primary receptor by the feline immunodeficiency virus. Science 2004;303: 1192–1195. 44. Shimojima M, Pecoraro MR, Maeda K, et al. Characterization of anti-feline CD8 monoclonal antibodies. Vet Immunol Immunopathol 1998;61:17–23. 45. Sopp P, Howard CJ. Cross-reactivity of monoclonal antibodies to defined human leucocyte differentiation antigens with bovine cells. Vet Immunol Immunopathol 1997;56:11–25. 46. Sopp P, Kwong LS, Howard CJ. Cross-reactivity with bovine cells of monoclonal antibodies submitted to the 6th International Workshop on Human Leukocyte Differentiation Antigens. Vet Immunol Immunopathol 2001;78:197–206. 47. Sopp P, Werling D, Baldwin C. Cross-reactivity of mAbs to human CD antigens with cells from cattle. Vet Immunol Immunopathol 2007; 119:106–114. 48. Summerfield A, Rziha HJ, Saalmuller A. Functional characterization of porcine CD4+CD8+ extrathymic T lymphocytes. Cell Immunol 1996;168:291–296. 49. Tschetter JR, Davis W C, Perryman LE, et al. CD8 dimer usage on alpha beta and gamma delta T lymphocytes from equine lymphoid tissues. Immunobiology 1998;198:424–438. 50. Vernau W, Moore PF. An immunophenotypic study of canine leukemias and preliminary assessment of clonality by polymerase chain reaction. Vet Immunol Immunopathol 1999;69:145–164. 51. Willett BJ, de Parseval A, Peri E, et al. The generation of monoclonal antibodies recognising novel epitopes by immunisation with solid matrix antigen-antibody complexes reveals a polymorphic determinant on feline CD4. J Immunol Methods 1994;176:213–220. 52. Willett BJ, Hosie MJ, Jarrett O, et al. Identification of a putative cellular receptor for feline immunodeficiency virus as the feline homologue of CD9. Immunology 1994;81:228–233. 53. Woo JC, Moore PF. A feline homologue of CD1 is defined using a felinespecific monoclonal antibody. Tissue Antigens 1997;49:244–251.

CHAPTER

5

The Hematopoietic System BRUCE D. CAR Hematopoietic Stem Cells Hematopoietic Progenitors and Precursors Species Specificity of Hematopoiesis The Bone Marrow Microenvironment The Hematopoietic Stem Cell Niche The Erythropoietic Niche Cytokines and Cytokine Signaling in Hematopoiesis Negative Regulation of JAK-STAT Signal Transduction

Evaluation of Hematopoietic Function Bone Marrow Evaluation in Mice Evaluation of Hematopoiesis with Bone Marrow Culture Lethal Irradiation and Bone Marrow Transplantation Genetically Altered Mice Models of Accelerated Hematopoiesis

Acronyms and Abbreviations Ang1, angiopoietin 1; BFU-E, burst-forming unit erythroid; CD, cluster of differentiation; CFC-S, splenic colonyforming cell; CFU-E, colony forming unit erythroid; CFU-GEMM, colony forming unit granulocyte erythroid monocyte megakaryocyte; CFU-GM, granulocyte-macrophage colony forming cell; CLP, common lymphoid progenitor; CXCR4, CXC chemokine receptor 4; ECM, extracellular matrix; EDTA, ethylenediaminetetraacetic acid; EPO, erythropoietin; G-CSF, granulocyte colony stimulating factor; GM-CSF, granulocyte macrophage colony stimulating factor; GTPase, guanosine triphosphatase; HSC, hematopoietic stem cell; ICAM-4, intercellular adhesion molecule-4; IL, interleukin; JAK, Janus tyrosine kinase; JH, Janus homology; NK cell, natural killer cell; PCR, polymerase chain reaction; PI3K, phosphoinositol 3 kinase; PTH, parathyroid hormone; qPCR, quantitative polymerase chain reaction; Rac1 and Rac2, Ras-related C3 botulinum toxin substrate 1 and 2; RBC, red blood cell; SCF, stem cell factor; SCID, severe combined immunodeficiency; SDF-1, stromal derived factor 1 (CXCL12); SH, src homology; SOCS, suppressor of cytokine signaling; STAT, signal transducers and activators of transcription; TGF-β, transforming growth factor beta; Tie2, tyrosine kinase with immunoglobulin-like and EGF-like domains 2; TNFα, tumor necrosis factor alpha; TPO, thrombopoietin; TRAIL, TNF-related apoptosis inducing ligand; VCAM-1, vascular cellular adhesion molecule 1; VLA-4, very late antigen 4.

C

urrent understanding of the hematopoietic system draws heavily from clinical observations and research in humans and mice. Numerous gene-deleted mice targeting transcription factors, hematopoietic cytokines and their receptors, and extracellular matrix (ECM) components and their receptors are central to our knowledge of hematopoiesis at the molecular level.7 More recent studies have contributed information about the critical role of the bone marrow microenvironment. In domestic animals, research on spontaneous hematopoietic neoplasia, the roles of retroviruses and viral oncogenes that mimic hematopoietic tyrosine kinases, cyclic neutropenia of gray collies,

and other species-specific hematopoietic diseases, has added to our understanding of hematopoiesis, which is remarkably conserved across mammalian species.32 HEMATOPOIETIC STEM CELLS Hematopoietic stem cells (HSCs), which are considered to have lost their potential for mesenchymal differentiation, arise first in the embryonic yolk sac, then in fetal para-aortic splanchnopleura, from which liver and finally bone marrow are seeded (see Chapter 1). In the conventional view of hematopoiesis, pluripotential 27

28

SECTION I: HEMATOPOIESIS

Common lymphoid progenitors Common granulocyte monocyte lymphoid progenitor

Hematopoietic stem cell

Colony forming unit spleen

Common myeloid progenitor

Megakaryocyteerythroid progenitors

Pre-B cell

B-lymphocyte

Pre-T cell

T-lymphocyte

Burst forming unit-erythroid

Colony forming uniterythroid

Megakaryocyte colony forming cells

Blast colonyforming cell

Megakaryocyte

Eosinophil colony forming cells Granulocytemacrophage progenitors

Erythrocyte

Eosinophil Basophil

Basophil colony forming cells Granulocytecolony forming cells Granulocytemacrophage colony forming cells

Platelet

Monocyte colony forming cells

Neutrophil

Monocyte

Macrophage

Mast cell colony forming cells Mast cells

FIGURE 5.1 Hierarchical scheme of hematopoiesis. This figure depicts the conventional view of human and murine hematopoiesis. Larger block arrowheads indicate differentiation from pluripotent to oligopotent hematopoietic stem cells. Smaller block arrowheads indicate lineage commitment of oligopotent stem cells driven by cytokines and the JAK-STAT pathway. Dotted line arrows illustrate the multiple stages of differentiation that can be visually discriminated in Wright-Giemsa-stained bone marrow specimens. The figure has been modified from human and murine illustrations to indicate the mouse-specific GM-lymphoid precursors. (Modified from Iwasaki H, Akashi K. Hematopoietic development pathways: on cellular basis. Oncogene 2007;26:6687–6696 and Metcalf D. Hematopoietic stem cells and tissue stem cells: current concepts and unanswered questions. Stem Cells 2007;25:2390–2395.)

HSCs with unlimited self-generative capacity progressively differentiate to multipotent or oligopotent stem cells with reduced self-replicative capacity, to lineage committed progenitors with minimal ability to self renew, to lineage-specific precursors with no self-regenerative ability, and finally to the mature cells of blood (Fig. 5.1).26 In a recent modification in this scheme in mice, hematopoiesis matures through a common granulocyte-monocyte-lymphoid oligopotent progenitor that has not been identified in higher species.18 The strict division of common granulocyte-monocyteerythroid-megakaryocytic progenitors and common natural killer (NK)/B and T cell progenitors occurs for human hematopoiesis, and likely applies to hematopoiesis in domestic species. Lineage commitment typically follows expression of lineage-restricted transcription factors (Table 5.1). Induction of these transcription factors occurs through a combination of specific cytokine receptor and ligand interactions, less specific signal transduction pathways,

and an overlay of highly specific, permissive microenvironmental influences from stromal cells, endothelial cells, adipocytes, osteoblasts, ECM proteins, adherent cytokines, and trabecular bone. Because of the short life-span of differentiated hematopoietic cells, mature blood cell production is an ongoing process; estimates suggest production of 1.5 × 106 cells/second in humans. It is unlikely that HSCs supply a continuous flux of cells from left to right as shown in Figure 5.1 because HSCs and multipotent cells are non-dividing or very slowly dividing during normal hematopoiesis. Newer evidence suggests that information determined from experiments in lethallyirradiated mice may not necessarily recapitulate normal or physiologically accelerated hematopoiesis and that there may be other homeostatic mechanisms that control hematopoiesis.26 A somewhat confusing nomenclature has evolved around early committed hematopoietic progenitors based on in vivo experiments in mice. Splenic colonies

CHAPTER 5: THE HEMATOPOIETIC SYSTEM

29

TABLE 5.1 Hematopoietic Transcription Factorsa Early hematopoietic development SCL (TAL 1) (stem cell leukemia, T-cell acute lymphocytic leukemia-1) GATA-2 LMO2 (Rbtn-2) (Lim finger protein) AML-1 (acute myeloid leukemia 1 protein) Tel Notch 1 Erythropoiesis HIF-1 (Hypoxia-inducible factor-1) GATA-1 EKLF-1 (erythroid kruppel-like factor 1) p45/NF-E2 (nuclear factor erythroid-2) STAT5a, STAT5b CBP/p300 (CREB binding protein/p300) SP1 Erythropoietic regulators FOG-1 (friend of GATA-1) TRAP220 (thyroid hormone receptor-associated protein 220) BRG1 (Brahma-related gene 1) CBP/p300 (CREB binding protein/p300) Myelopoiesis Notch GATA-1 C/EBPα (CCAAT/enhancer binding protein α) STAT5a, STAT5b Granulocytopoiesis RAR (Retinoic acid receptor) C/EBPα and C/EBPε CBF (core binding factor) c-Myb STAT3 NFκB (nuclear factor of kappa B) – late precursor AML1 – precursor

Osteoclast differentation from monocyte precursor NFκB Megakaryocytopoiesis Notch GATA-1 PU.1 Fli-1/NF-E2 (platelet production) FOG SCF (stem cell factor) STAT5a, STAT5b Lymphoid PU.1 Ikaros STAT3, STAT5a, STAT5b, STAT4, STAT6 B-cell E2A EBF (early B-cell factor) Pax 5 NFκB (progenitors and late maturation) T-cell HEB (TCF12 – T-cell factor 12) Notch/HES-1 (Hairy Enhancer of Split-1) NFAT (Nuclear Factor of Activated T cells) late maturation GATA-3 TCF-1 (T cell factor 1) NFκB (progenitors and late maturation) Mast cell GATA-2+ Elf-1 MITF (microphthalmia-associated transcription factor) Eosinophilopoiesis STAT1, STAT3, STAT5a, STAT5b GATA-1

Monocytopoiesis EGR-1, EGR-2 (early growth response genes 1 and 2) Vitamin D receptor c-Fos, c-Fos PU.1 RAR (retinoic acid receptor) C/EBPβ and C/EBPε MafB/c PU.1 a

All transcription factors are involved in early lineage commitment and exert their effects on committed progenitors unless otherwise stated.

that appear in lethally irradiated mice reconstituted with HSCs are called splenic colony forming cells (CFC-Ss), which have limited capacity for self-renewal. Whether similar cells occur in other species is unknown, because CFC-Ss are defined by specific experimental conditions in mice. HEMATOPOIETIC PROGENITORS AND PRECURSORS The term progenitors typically refers to cells whose presence is inferred from cytokine-driven differentia-

tion of colonies in culture, such as granulocytemacrophage colony forming cells (CFU-GMs) and erythroid colony-forming units (CFU-Es), whereas the term precursors refers to stages of hematopoietic differentiation recognized by cytologic evaluation. HSCs, oligopotent stem cells, and committed progenitors appear similar to small lymphocytes, although they can be separated by flow cytometry based on expression of surface proteins. Towards the right of Figure 5.1, clonigenic cell types exist putatively without any capacity for selfgeneration. However, mature cells types such as macrophages and mast cells may have substantial

30

SECTION I: HEMATOPOIESIS

self-generative properties. The initial model of hematopoiesis did not include the concept of plasticity.26 In the current model, the concept of plasticity suggests that lineage fidelity and progressive restriction of proliferative capacity in the traditional model are not absolute. Plasticity is recognized as a property of both proliferation and lineage commitment. How plasticity is regulated under physiologic and pathologic conditions is not well understood. It is clear though that neither HSC nor any of the oligopotent progenitors have the unlimited capacity for self-generation possessed by embryonic stem cells.

SPECIES SPECIFICITY OF HEMATOPOIESIS In contrast to detailed information about murine hematopoiesis,45 there are few detailed descriptions about the structure and function of the bone marrow in domestic species. Stem cell function, the biochemical nature of the bone marrow microenvironment, hematopoietic cytokines and their receptors, and lineage specific and non-specific transcription factors are assumed to apply to species other than mice or humans.5,21 Studies have confirmed some common cytokines between domestic species and humans.14,24,25,27,28,30,31,38,39 Research involving animal models of human diseases, toxicology, or other unique diseases such as cyclic hematopoiesis in dogs has provided additional information about hematopoiesis in domestic animals.4,11,20,26 The gene sequences for many hematopoietic cytokines, receptors, and transcription factors for multiple species now are available in publically accessible databases so they can be assessed specifically by high density microarray or more broadly by quantitative reverse transcription polymerase chain reaction (qPCR).10,12,33 Certain critical differences exist between human and murine hematopoiesis. Human HSCs express Flt3, the tyrosine kinase receptor for FLT3 ligand, whereas murine HSCs do not.18 However, both murine and human common lymphocyte progenitors (CLPs) express Flt3. Such differences are important because certain human acute myelogenous leukemias are associated with constitutive activation of Flt3.8 Whether other species express similar mutations governs the potential utility of therapy with Flt3-kinase inhibiting drugs. Although the hierarchical relationships observed in murine hematopoiesis are generally preserved in other species (Fig. 5.1), expression of a number of surface antigens at each developmental stage is different between mice and humans, and these and other differences may occur in other species. However, important similarities also have been recognized. Based on the recent molecular and genetic understanding of cyclic neutropenia in gray collie dogs and cyclic hematopoiesis in humans, some features of hematopoiesis appear similar. For example, allometric scaling correctly predicts granulopoietic cyclicity in mice (3 days), dogs (14 days), humans (19–21 days), and elephants (60 days).4,26

THE BONE MARROW MICROENVIRONMENT The heterogeneous cellular elements in Figure 5.1 are intimately associated with adipocytes, macrophages, endothelial cells, nerves, osteoclasts, osteoblasts, ECM, sinusoids, and cytokines, collectively termed the bone marrow microenvironment. Cell-cell and cell-ECM interactions are important in the regulation of cell proliferation and differentiation of hematopoietic cells.9 These interactions involve receptors and integrins, which are regulated by growth factors, cytokines, and transcription factors. The ultrastructure of the interactions between hematopoietic cells, stromal cells and noncellular components of the bone marrow microenvironment were described in detail in 1978,29 but the molecular mechanisms have more recently been characterized. The best characterized integrin on HSC is very late antigen 4 (VLA-4 or α4 β1) which binds to fibronectin in the ECM as well as to vascular cellular adhesion molecule 1 (VCAM-1) on adjacent stromal cells. The binding of VLA-4 to fibronectin mediates adhesion of HSC and progenitors to the microenvironment, homing of circulating HSC and progenitors to specific areas within the bone marrow, and signal transduction. The Rho guanosine triphosphatases (GTPases), Ras-related C3 botulinum toxin substrate 1 (Rac1), and Rac2 also are key regulators of adhesion and migration of cells in the hematopoietic microenvironment. The bone marrow microenvironment has highly specialized and integrated microanatomic functional units called niches. The two most clearly elucidated hematopoietic niches are the HSC niche (see Chapter 3) and the erythroblastic islands (Figs. 5.2 and 5.3; see Chapter 6).1 The Hematopoietic Stem Cell Niche Survival, proliferation, and differentiation of HSCs depend on their spatial and functional relationships with the cells and ECM of the microenvironment.29 HSCs are enriched at sites adjacent to the endosteal surface of bone. A specialized subset of activated osteoblasts display ligands and receptors that facilitate homing and transient docking of HSC, and regulate slow cycling or rapid mobilization of HSCs as needed.1 The interaction between stromal-derived factor 1 (SDF-1 or CXCL12) elaborated by osteoblasts and CXC chemokine receptor 4 (CXCR4), its cognate chemokine receptor on HSC, is central to this HSC niche. SDF-1 recruits quiescent progenitors, participates in their cycling and survival, and sensitizes them to further synergistic action of cytokines, thus contributing to hematopoietic homeostasis under both physiologic and stress conditions. HSC function also is regulated by osteoblasts through parathyroid hormone (PTH), the Notch signaling pathway, and interactions between angiopoietin 1 (Ang1) and its tyrosine kinase with immunoglobulin-like and EGF-like domains 2 (Tie2) receptor.6 Disruption of this HSC niche may result in abnormal cell mobilization and contribute to extramedullary infiltration in leukemia.

CHAPTER 5: THE HEMATOPOIETIC SYSTEM

osteoblast N-cadherin Jagged-1 Ang-1 CXCL-12 Kit ligand Dickkopf-1 TGF/BMP Hedgehog

endosteal trabecula

↑ [Ca++] osteopontin

hematopoietic stem cell N-cadherin Notch-1 Tie-2 CD44 CXCR4 VLA-4 Kit VLA-5 Frizzled Annexin II TFG-R/BMP-R Patched osteopontin ↑ [Ca++]

+ Ferritin Insulin-like growth factor-1 Burst-promoting activity

bone marrow stromal cell

- TNFa, TRAIL, IL-6, TGFb fibronectin S M

E

31

FIGURE 5.2 Hematopoietic stem cell niche (reciprocal molecular interactions). The relationship between a specialized subset of osteoblasts and the hematopoietic stem cell is shown, with reciprocal molecular interactions depicted between the two cell types, and between these cell types and endosteal bone.

FIGURE 5.3 The erythropoietic niche. The erythroblastic island is illustrated with rubriblasts of graded levels of maturity surrounding macrophage (M) cytoplasm. Specific molecular interactions between macrophages and rubriblasts, between the erythroblastic island and marrow stromal cell(s), and of all these cellular elements with marrow stroma are shown. Autocrine cytokine loops involved in stimulation of erythropoiesis (green bordered box) and in the negative regulation of erythropoiesis (red bordered box) are shown. Dotted lines indicate extracellular matrix components. Arrow indicates factors elaborated by macrophages that influence growth and maturation of rubricytes.

VCAM-1/VLA-4, ICAM-4 collagen types I, IV R thrombospondin

fibronectin laminin

E-cadherin/Ca++ VLA-5 CD36 CD44 Erythrocyte-macrophage protein

The Erythropoietic Niche Erythropoiesis occurs in distinct niches called erythroblastic islands that consist of a central macrophage surrounded by a ring of developing erythroblasts in bone marrow, fetal liver, and spleen, and even in long term marrow cultures.17 The macrophage contributes important signals to developing rubriblasts, phagocytoses expelled metarubricyte nuclei, and transfers iron to developing rubriblasts. Unlike megakaryocytes that localize exclusively to marrow sinusoids, erythroblastic islands are located throughout the marrow.9 Adhesion molecules mediating important structural and functional interactions between developing erythroid cells and central macrophages include VCAM-1/ VLA-4, α4β1/VLA-4, intercellular adhesion molecule-4 (ICAM-4)/αv, and E-cadherin. For example, ICAM-4 is postulated to enable reticulocytes to detach from central macrophages, allowing them to enter the circulation.9 Laminin and fibronectin and their receptors expressed on late-stage rubriblasts also are key components in differentation of reticulocytes. VLA-4 and VLA-5 are

involved in binding of erythroid burst forming units (BFU-Es) to hematopoietic stromal cells. Expression of these adhesion molecules is highest on BFU-Es and erythroid colony forming units (CFU-Es) and is progressively lost during erythroid maturation. Hemonectin and collagen type I support binding of BFU-Es in vitro, and a role for tenascin-C has been inferred from knockout mice. Hemonectin is absent from anemic mice with Kit ligand or c-kit deficiency. The surface antigen cluster differentiation 44 (CD44) is highly expressed on almost all hematopoietic cells in bone marrow and is responsible for interaction of these cells with collagen types I and IV, fibronectin, and hyaluronate.44 Reticulocytes express only low levels of a few surface adhesion receptors, such a CD36 (thrombospondin receptor) and VLA-4. Thrombospondin serves as an adhesive ligand for committed progenitors including colony forming units-granulocyte erythroid monocyte megakaryocyte (CFU-GEMM) and BFU-E. Adhesion molecule interactions of reticulocytes with bone marrow stromal cells and ECM may facilitate their egress into bone marrow sinuses. Mature red blood

32

SECTION I: HEMATOPOIESIS

cells (RBCs) do not express adhesion molecules under normal conditions. CYTOKINES AND CYTOKINE SIGNALING IN HEMATOPOIESIS Hematopoiesis is the cumulative result of intricately regulated signaling pathways mediated by soluble cytokines and their receptors (Table 5.2).2 Evaluation of cytokine-receptor interactions in hematopoiesis has largely been achieved through creation of gene-deleted mice and conditional knock-out mice, the phenotypes of which range from severe lethal embryonic, fetal, and neonatal defects to redundant null phenotypes.7 These mice have provided much of our current insight into the regulation of physiologic and pathologic alterations in hematopoiesis. Most hematopoietic cytokine receptors are multiple subunit complexes, with the exception of those that signal through a single chain, such as erythropoeitin (EPO), granulocyte-macrophage colony-stimulating factor (GM-CSF), and thrombopoietin (TPO). Hematopoietic cytokines are approximately 200 amino acids in length and carry a conserved sequence of tryptophan-serine-X-tryptophan-serine (W-S-X-W-S) in their extracellular domain, which functions as part of the ligand binding domain. Cytokine receptors contain docking regions for Janus tyrosine kinases (JAK1, JAK2, JAK3, TYK2) in their cytoplasmic termini, that when attached to the ligand-

TABLE 5.2 Hematopoietic Cytokinesa Lineage/Function Early hematopoiesis

Key Hematopoietic Cytokine Stem Cell Factor (SCF) Interleukin 3 (IL-3) Wnt ligands (Jagged) and receptors Kit ligand SDF-1/FGF-4

Common myeloid progenitor/ myelopoiesis

Stem cell factor (SCF) Thrombopoietin (TPO)

Erythropoiesis

Erythropoietin (EPO)

Megakaryocytopoiesis

Thrombopoietin Interleukin-6 (IL-6)

Lymphocytopoiesis T cells NK cells B cells

IL-7 IL-7, IL-2 IL-7, IL-15 IL-4

Regulatory

Transforming growth factor-β (TGF-β) Bone morphogenetic protein (BMP) Activin

a Note that this table oversimplifies the actions of these cytokines. With further refined analyses of murine models it has become apparent that even lineage restricted cytokines such as EPO exert pleiotrophic effects within and external to the hematopoietic system.

bound form of the cytokine receptor, recruit JAKs which then autophosphorylate. The JAK kinase then phosphorylates tyrosine residues on a specific signal transducer and activator of transcription (STAT) protein, which is STAT1, 3, or 5 for most hematopoietic cytokine receptors. JAKs contain a catalytic Janus homology (JH) 1 domain and a JH2 catalytic-like but inactive domain critical to the ability of JAKs to regulate themselves and to mediate cytokine-induced responses. Src kinase activation of STATs also is important for myeloid cell proliferation. In unstimulated cells, STATs are present as cytoplasmic monomers in the unphosphorylated state. Phosphorylation by JAK kinases leads to dimerization through reciprocal interactions of SH2 domains with phosphotyrosine residues, and thereby activation of STAT, which then translocates to the nucleus.15 STATs are transcription factors that prevent apoptosis or positively regulate prosurvival genes of late progenitor and early precursor cells.15 Although there are numerous STAT responsive genes in many different cell types, STAT regulation of hematopoietic precursors occurs in a cell-type restricted manner. Lineage-committed colony-forming cells respond to cytokines in an absolute lineage-restricted fashion. The specificity of hematopoietic cytokines is determined by progenitor and precursor cell expression of their cognate receptors. For example, rubriblasts express EPO receptors but myeloblasts do not, which is different from the relatively promiscuous expression of JAK/STAT pathways. Given the importance of JAK-phosphorylation events in driving proliferation of hematopoietic precursors, it is not surprising that mutations leading to constitutively active JAK2 result in myeloproliferative disorders and leukemia. In addition to the prominent role of JAK-STAT interactions in hematopoiesis, the functional involvement of Ras and phosphoinositol 3 kinase (PI3K) pathways also has been shown following interleukin (IL)-3/IL-5 and GM-CSF stimulation of bone marrow cultures in vitro.2 Negative Regulation of JAK-STAT Signal Transduction Suppressors of cytokine signaling (SOCSs) are a family of proteins that regulate the strength and duration of the hematopoietic cytokine-driven signaling cascade.42 They are transcriptionally induced by JAK-STAT signaling. SOCS proteins contain src homology (SH) 2 domains and a SOCS-box which mediate binding to cytokine receptors and associated JAKs, and attenuate signal transduction directly. In addition to their transcriptional induction by hematopoietic cytokines and subsequent self-limiting stimulation of hematopoiesis, other cytokines including tumor necrosis factor-α (TNF-α), IL-1, and Toll-like receptor ligands (e.g. lipopolysaccharide) also induce SOCS expression, providing negative regulation for granulocyte colony stimulating factor (G-CSF) signaling. Another level of regulation is provided by phosphatases. The signaling and subsequent hematopoiesis

CHAPTER 5: THE HEMATOPOIETIC SYSTEM

induced by phosphorylated dimers of STAT proteins is terminated by removal of phosphates from STAT tyrosines by three specific protein tyrosine phosphatases. Transforming growth factor-β (TGF-β) is perhaps the most potent endogenous negative regulator of hematopoiesis.19 TGF-β suppresses expression of the SCF receptor, the response of progenitors to SCF, and cell cycle progression of progenitors. The expression of receptors for TGF-β on primitive hematopoietic progenitors and subsequent stages of maturation suggests a broad role for this cytokine. Negative regulation of erythropoiesis by TNF-α, TNF-related apoptosis inducing ligand (TRAIL), IL-6, and TGF-β occurs when chronic inflammatory disease increases systemic and local bone marrow concentrations of these cytokines (see Chapter 37).9 EVALUATION OF HEMATOPOIETIC FUNCTION Hematopoiesis is studied to gain insight into mechanisms of cytopenias, leukemias, and other pathophysiologic responses. Evaluation of hematopoiesis begins with careful examination of peripheral blood and is complemented by cytologic or histologic assessment of bone marrow. Short and long term in vitro culture of bone marrow-derived cells, genetically modified mice, animal models of retarded and accelerated hematopoiesis, syngeneic and xenogeneic hematopoietic stem cell transplantation, retroviral-mediated gene transfer, and gene therapy also have been used to study hematopoiesis.43 Gene-deleted and transgenic mice provide a special challenge to the hematologist because these mice frequently die during the embryonic, fetal, or early neonatal period. Peripheral blood examination may still provide valuable information. A 3 μL volume of heart blood obtained with a fine gauge needle is sufficient to prepare a blood smear; as little as 20 μl of heart blood may be diluted with 2 mg/mL ethylenediaminetetraacetic acid (EDTA) in saline and analyzed with an electronic counter. Potential artifacts of dilution of small quantities can be overcome by comparing results from treated or genetically altered mice with similarly diluted volumes from control or wild type mice. Bone Marrow Evaluation in Mice Direct examination of bone marrow complements information gained from assessment of blood. The technique of bone marrow collection and analysis in mice is described in Chapter 132. In fetal mice, which lack developed medullary hematopoiesis, cytologic examination of liver imprints for hematopoietic precursors is useful in assessing early hematopoietic function. Evaluation of Hematopoiesis with Bone Marrow Culture Hematopoietic interactions are best evaluated in vitro where culture systems permit evaluation of effects of individual cytokines, growth factors or their regulators,

33

and combinations of factors (see Chapter 133). Target gene expression may be transiently induced by gene transfection or inhibited by transfection with antisense RNA. The significance of in vitro studies must be confirmed in vivo. For example, deficiency of hematopoietic cytokines or growth factors which would predictably have severe phenotypes based on in vitro work, such as IL-2 and GM-CSF, have much milder hematologic phenotypes than expected (i.e. failure to develop lymphopenia and neutropenia), underscoring the redundancy and pleiotrophy which characterize many of these factors. Bone marrow cells can be cultured from aspirates or core samples obtained from diseased or healthy animals. These cells are cultured in semisolid methylcellulosebased media with cocktails of cytokines, EPO, transferrin, bovine fetal serum, and albumin. Whereas EPO is active across species and has been cloned from a variety of species, other cytokines have more limited cross-species effects. When a species-specific cytokine is not available, conditioned spleen cell media obtained from phytohemaglutinin, endotoxin, or phorbol ester-stimulated cells may serve as a useful source of cytokines. Conditions for culture of bone marrow progenitors from dogs, cats, sheep, cattle, horses, chickens, and other species have been described.13,16,40,37 In general, methodologies applicable to murine and human culture systems apply to those of other species, when care is taken to ensure the quality of collected bone marrow specimens and species-specific reagents or appropriate substitutes are used. Short-term bone marrow cultures from mice, rats, and dogs are routinely performed to assess the potential toxicity of xenobiotics and define hematopoietic phenotypes of genetically altered animals.37 These culture systems may be used to study hematopoiesis by the addition of individual cytokines, growth factors or their regulators, or combinations thereof. Alternatively, neutralizing antibodies to these components or small molecules ( monkey > rat > dog. Additionally, aPTT appeared to be the more sensitive biomarker in all species. Multiple pharmacological models of thrombosis in rats, dogs, and pigs were also conducted with Otamixaban. In rats, thrombus mass was markedly reduced by nearly 95% with a corresponding increase in aPTT of 2.5-fold and PT of 1.6fold.4 In contrast, intravenous administration of 1, 5, or 15 μg/mL of Otamixaban in the pig model effectively eliminated coronary flow reserves related to this stenosis model at the middle and high dose. PT was also prolonged at the middle and high dose, but aPTT was only prolonged at the high dose. Ex Vivo Experiments (New Anticoagulant Development) Development of new anticoagulants provides a unique opportunity of assessing drug efficacy ex vivo before conducting preclinical studies. This is important in planning preclinical studies because factor X concentrations vary between species and the level of druginduced FXa inhibition produced is also variable. In typical studies, human or animal plasma is spiked with various concentrations of test compound. Chromagenic anti-FXa assays and factor X activity, using the factor X clotting (FX : C) assay can be used to determine efficacy of the drug. The FX : C assay provides several unique features that may make it a valuable biomarker for monitoring FXa inhibitor therapy:

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1. The assay provides a rapid, reliable assessment of drug concentration and the percent inhibition of FXa achieved during drug inhibitor administration. 2. The assay can be performed on a high throughput automated system that is available in most hospital or veterinary coagulation laboratories. 3. Individual factor X concentrations range from 60% to 150% among human or preclinical subjects. This fairly high level of inter-subject variability suggests that a standard dose of drug may have a substantially different effect on total Factor X inhibition. The FX : C assay defines baseline factor X activity and thereby allows continued dosing to achieve a targeted factor X concentration. 4. Literature is available concerning factor X concentrations and bleeding history in patients with factor X deficiency, so minimally there is some understanding that correlates the impact of reductions in FX : C evaluations and bleeding potential in humans and animals.3

Concentrations selected provided assessment of drug concentrations that induced FXa inhibition of approximately 20% to >90%, showing that the targeted range could be predicted and achieved in all species but the

By determining the actual concentration of functional factor X remaining, individuals conducting preclinical or clinical trials have increased confidence in the administration of new FXa inhibitors. As with other coagulation biomarkers used for monitoring FXa inhibition, it was not immediately clear whether the FX : C assay was applicable in multiple species. Ex vivo experiments allowed this evaluation. To provide effective anticoagulant activity, a 30% reduction in FX : C activity was predicted to be a minimal requirement. Table 15.3 shows the intended concentrations of a FXa inhibitor in each species, the resulting FX : C activity, and percent inhibition achieved.

TABLE 15.3 Factor X Activity and Percent Inhibition in Plasma Samples Containing Increasing Concentrations of the FXa Inhibitora

Species

Intended Drug Concentration (μg/mL)

FX:C Activityb (%)

0 0.2 0.6 1.2 1.8 6.0

106.1 64.3 32.2 16.5 10.0 2.3

± ± ± ± ± ±

1.9 1.6 1.0 0.7 0.5 0.2

NA 39.4 69.7 84.4 90.6 97.8

Dog

0 0.4 2.0 8.0 15.0

143.0 112.9 42.4 11.1 5.4

± ± ± ± ±

4.5 8.6 4.3 1.4 0.8

NA 21.0 70.3 92.2 96.2

Rat

0 1.0 4.0 12.0 24.0

84.8 52.6 26.2 11.8 6.7

± ± ± ± ±

2.8 1.8 0.9 0.6 0.3

NA 38.0 69.1 86.0 92.1

Human

BLQ, below limits of quantification; NA, not applicable. a Samples spiked with a FXa inhibitor in vitro. b Mean ± SD of 10 samples/concentration. c Calculated from species-specific control value.

TABLE 15.4 In Vitro Effect of an Experimental FXa Inhibitor on Absolute Prothrombin Time in Human, Dog, and Rat Plasma using Rabbit Brain Thromboplastin of Variable Potency (Defined By ISI Rating) Concentration of FXa inhibitor (μg/mL)

0.98 ISI

1.24 ISI

1.55 ISI

PT (s) – Human Plasma 12.9 ± 0.13a 13.4 ± 0.17a 23.8 ± 0.50a 23.4 ± 0.44a 37.3 ± 0.86a 39.9 ± 0.70a 51.4 ± 1.51a 58.9 ± 1.08a 61.8 ± 1.58 74.8 ± 1.54a 111.4 ± 3.17a 152.3 ± 3.08a

0 0.2 0.6 1.2 1.8 6.0

11.4 18.3 30.8 47.0 61.2 133.0

± ± ± ± ± ±

0.11 0.37 0.76 1.98 2.04 3.82

0 0.4 2.0 8.0 15.0

7.8 11.0 17.6 32.0 45.4

± ± ± ± ±

0.14 0.26 0.42 0.98 1.52

PT (s) – Dog 8.4 ± 0.07 11.0 ± 0.13 16.6 ± 0.23a 27.6 ± 0.49a 36.5 ± 0.73a

0 1.0 4.0 12.0 24.0

9.1 13.3 20.1 31.1 42.7

± ± ± ± ±

0.05 0.06 0.29 0.72 1.09

15.1 20.7 30.8 46.2 60.6

a

Inhibitionc (%)

Plasma 7.1 ± 10.5 ± 17.9 ± 33.6 ± 46.7 ±

PT (s) – Rat Plasma ± 0.07a 17.1 ± ± 0.13a 31.3 ± ± 0.23a 51.8 ± ± 0.49a 83.6 ± ± 0.73a 109.4 ±

2.21 ISI

10.9 16.8 27.0 38.2 48.3 94.8

± ± ± ± ± ±

0.14 0.43a 0.93a 1.39a 2.08a 4.12a

0.07 0.19 0.41 0.92 1.35

6.7 10.2 15.0 27.1 36.8

± ± ± ± ±

0.06 0.15 0.31a 0.63a 0.88a

0.15a 0.27a 0.52a 0.84a 0.79a

13.1 23.2 36.9 56.2 75.4

± ± ± ± ±

0.07a 0.19a 0.52a 0.84a 1.60a

Significantly different from 0.98 ISI thromboplastin values at the 5 % level as determined by a t-test.

CHAPTER 15: PRECLINICAL EVALUATION OF COMPOUND-RELATED ALTERATIONS IN HEMOSTASIS

drug concentrations required to produce similar levels of FXa inhibition across species were markedly different. The FX : C assay helped determine drug concentration required for complete inhibition of FXa in these species and the relative bleeding risk associated with a range of factor X concentrations.

Effect of Thromboplastin on Absolute Prothrombin Time Typically, the higher the international sensitivity index (ISI) value for thromboplastin, the less sensitive the reagent and the longer the PT produced. The most commonly used thromboplastin reagents for PT evaluation are either rabbit brain thromboplastin (variable ISI values depending on manufacturer) or human recombinant thromboplastin (typical ISI of 1.0). To more fully evaluate the effect of FXa inhibitors, PT was evaluated using rabbit brain thromboplastin with ISI values of 1.24, 1.55, and 2.21 and a human recombinant thromboplastin (0.98 ISI). The source and sensitivity of thromboplastin used in the assay affected the absolute PT value in all species, demonstrating the need to standardize this reagent in preclinical assessment and to be cognizant of the impact in clinical trials or in post-marketing where reagents are less likely to be standardized (Table 15.4).

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CONCLUSIONS Understanding compound-related effects of hemostasis continues to be an important aspect of marketing safe compounds and developing new anticoagulant therapies. Further, understanding of species-specific differences in hemostatic processes and interactions in assays typically optimized for human samples is critical in translating preclinical study data for the accurate prediction of safety and efficacy in humans.

REFERENCES 1. Car BD, Eng VM. Special considerations in the evaluation of the hematology and hemostasis of mutant mice. Vet Pathol 2001;38:20–30. 2. CMR International. 2006/7 Pharmaceutical Research & Development Factbook. CMR International. 2006. 3. Gentry PA. Comparative aspects of blood coagulation. Vet J 2004;168:238–251. 4. Guertin KR, Choi YM. The discovery of the factor Xa inhibitor Otamixaban: from lead identification to clinical development. Curr Med Chem 2007;14:2471–2481. 5. Manning KL, Novinger S, Sullivan PS, et al. Successful determination of platelet lifespan in C3h mice by in vivo biotinylation. Lab Anim Sci 1996;46:545–548. 6. Nelson OL. Use of the D-dimer assay for diagnosing thromboembolic disease in the dog. J Am Hosp Assoc 2005;41:145–149. 7. Prater MP. Acquired coagulopathy I: Avitaminosis K. In: Feldman BV, Zinkl JG, Jain NC, eds. Schalm’s Veterinary Hematology, 5th ed. Baltimore: Lippincott Williams & Wilkins. 2000;556–559. 8. Weigand K, Brown G, Hall R. Harmonization of animal clinical pathology testing in toxicity and safety studies. The Joint Scientific Committee for International Harmonization of Clinical Pathology Testing. Fund Appl Toxicol 1996;29:198–201.

C H A P T E R 16

Drug-Induced Blood Cell Disorders DOUGLAS J. WEISS Mechanisms of Drug Toxicity Evaluation of Suspected Adverse Drug Reactions Adverse Drug Reactions in Dogs Antineoplastics Estrogen Anti-inflammatory Phenylbutazone/meclofenamic acid Carprofen Azathioprine Naproxen Antibacterials Sulfonamides Chloramphenicol Cephalosporins Anticonvulsants Phenobarbital/Primidone Phenytoin

Antiparasitics Cardiovascular Other Drugs Adverse Drug Reactions in Cats Antineoplastics Acetaminophen/Aspirin (see Chapter 36) Benzocaine/Cetacaine/Propofol (see Chapter 36) Phenazopyridine/DL-dmethionine (see Chapter 36) Azathioprine Propylthiouracil/Methimazole Griseofulvin Albendazole Azidothymidine Adverse Drug Reactions in Horses Drug-induced Immune-mediated Hemolytic Anemia Drug-induced Hemolysis Heparin

Acronyms and Abbreviations ADR, adverse drug reaction; FDA, Food and Drug Administration; IMHA, immune-mediated hemolytic anemia; RBC, red blood cell.

A

growing number of therapeutic drugs have been incriminated in adverse drug reactions (ADRs) associated with the hematologic system, with the greatest number being reported in dogs and cats.39,64,69 No attempt has been made to standardize the definition of drug-induced cytopenias in animals or to quantify the severity of these reactions. Standardized definitions of human drug-induced hematologic dyscrasias and evaluation of their severity have been described.8 This document defines cutoff values for neutropenia, leukopenia, thrombocytopenia, and anemia. It also defines the likelihood that a drug caused a hematologic dyscrasia as suggestive, compatible, incompatible, or inconclusive based on assessment of the time course between the beginning of exposure to the drug and discovery of the hemato98

logic dyscrasia. Neutropenia is defined as 1,500/μL within 1 month after stopping drug treatment is also suggestive of an ADR. Thrombocytopenia is defined as 15% of all nucleated cells) was observed in 15.6% of the specimens.36 In some cats, lymphocyte numbers exceeded 50% of all nucleated cells in bone marrow.35 Greater than 80% of cats with lymphocytosis had a diagnosis of non-regenerative immune-mediated hemolytic anemia (IMHA) or pure red cell aplasia (PRCA).35,36 Differentiating benign lymphocytosis from chronic lymphocytic leukemia can be problematic. Lymphocyte distribution and phenotype may be useful in differentiating these conditions. Benign lymphocytes are frequently arranged in small multifocal aggregates in bone marrow, whereas, in chronic lymphocytic leukemia, they are diffusely distributed throughout the marrow.35,43 Immunophenotyping of a few cases has revealed that most lymphocytes in reactive lymphocytosis are B cells, whereas malignant lymphocytes in chronic lymphocytic leukemia are usually T cells.35,43 Unlike the cat, only 1% of canine clinical bone marrow specimens had lymphocytosis.37 These cases were associated with immune-mediated diseases including IMHA and PRCA. Therefore, at least in the Northern USA, bone marrow lymphocytosis in both cats and dogs correlates with immune-mediated hematologic diseases.

CHAPTER 18: CHRONIC INFLAMMATION AND SECONDARY MYELOFIBROSIS

FIGURE 18.1 Lymphoid aggregate (arrows) in a bone marrow core biopsy section from a cat with nonregenerative immunemediated hemolytic anemia. Hematoxylin & Eosin stain; bar, 200 μm.

Plasmacytosis Plasmacytosis is usually evaluated subjectively in bone marrow aspirates or core biopsy specimens because of their low numbers and uneven distribution.12 Plasmacytosis has been observed in approximately 10% of canine clinical bone marrow specimens and in 6% of cat bone marrow specimens.36,37 Plasmacytosis in cats has been associated with IMHA, PRCA, immunemediated thrombocytopenia, and feline infectious peritonitis. Plasmacytosis in dog bone marrow has been associated with IMHA, PRCA, and immune-mediated thrombocytopenia. Other causes of plasmacytosis that should be considered include Ehrlichia and Leishmania infections. Lymphoid Aggregates In a review of 17 species of mammals, lymphoid aggregates were identified only in humans and cats.13,14 Lymphoid aggregates are small clusters of mature small lymphocytes with no evidence of organized structure (Fig. 18.1). However, results of large retrospective studies indicate that lymphoid aggregates occur infrequently in cat and dog bone marrow core biopsy specimens.36,37 When present, lymphoid aggregates correlate with the presence of immune-mediated hematologic diseases or other chronic systemic immunologic stimulation.35,41 Macrophage Proliferation The presence of increased numbers of macrophages in bone marrow is associated with a variety of disorders.

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Macrophages can range from relatively immature cells with minimal phagocytosis to mature cells that have a foamy cytoplasm or contain cells, cytoplasmic debris, or hemosiderin. Four general types of mononuclear phagocyte proliferations can be observed in bone marrow. Differentiating these types can be problematic and may require the use of flow cytometry or immunophenotyping.33 The four types of proliferation include reactive macrophage proliferation, reactive hemophagocytic syndrome16,39 (see Chapter 45), lipid storage diseases, and malignant proliferations of monocytes (acute monocytic or myelomonocytic leukemia, chronic myelomonocytic leukemia; see Chapters 65 and 67), and dendritic cells (malignant histiocytosis, see Chapter 73). Reactive macrophage proliferations are macrophages responding to a need to remove necrotic debris or to infectious agents. Increased necrotic debris is present in the subacute stages of myelonecrosis and in conditions of ineffective hematopoiesis.35 Ineffective hematopoiesis is present in instances of myelodysplastic syndromes, immune-mediated hematologic diseases, and in some leukemias. In each of these conditions the presence of increased numbers of macrophages is commonplace. A variety of infectious agents cause reactive macrophage proliferations in bone marrow. These vary with the species and geographic distribution of infectious organisms. In dogs, leishmaniasis, parvovirus infection, and deep fungal infections have been associated with macrophage proliferations (see Chapter 19).4,11 In leishmaniasis and deep fungal infections macrophages appear to be part of an inflammatory response in that organisms are found in the marrow in these conditions. Additionally, the presence of increased numbers of neutrophils is consistent with a diagnosis of a pyogranulomatous inflammation. In parvovirus infection, macrophages may be reacting to the need to remove necrotic cells. Lysosomal storage disorders are a group of genetic deficiencies in lysosomal enzymes classified by the substance that accumulates in lysosomes. This results in the presence of large numbers of foamy macrophages in many tissues including the bone marrow. These cells tend to have a characteristic appearance. Granulomatous Inflammation Granulomatous inflammation can consist of a mixed infiltrate of macrophages, giant cells, small lymphocytes, or plasma cells, or can consist of distinct granulomas. Granulomatous inflammation has been associated with disseminated histoplasmosis in dogs and cats and Mycobacterium avium subsp. avium infection in dogs (see Chapter 19).7,8,18 Granulomas are distinct compact aggregates of macrophages (Fig 18.2). These may contain epithelioid macrophages that have a large amount of pale cytoplasm and elongated nuclei with a dispersed chromatin pattern. Epithelioid cells may fuse to form giant cells. Other cells associated with the granuloma may include lymphocytes, plasma cells, and neutrophils. Granulomas

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FIGURE 18.2 Granuloma (arrows) in a bone marrow core biopsy section from a dog with a disseminated fungal infection. Hematoxylin & Eosin stain; bar, 200 μm.

FIGURE 18.3 Secondary myelofibrosis in a bone marrow core biopsy section from a dog. Hematoxylin & Eosin stain.

are infrequently seen in dog and cat bone marrow.36,37 Granulomas have been associated with systemic fungal infections in dogs and a horse with an idiopathic systemic granulomatous disease.42 Lipid granulomas have been observed in a cow with serous atrophy of fat.42 Lipid granulomas consist of aggregates of lipid-laden macrophages. Other cells that may be present in lipid granulomas include lymphocytes, plasma cells, epithelioid cells, and giant cells.10 In humans, bone marrow granulomas have been associated with a variety of drug hypersensitivities. These drugs include phenytoin, procainamide, phenylbutazone, chlorpropamide, sulfasalazine, ibuprofen, indometacin, allopurinol, and carbamazepine.2 SECONDARY MYELOFIBROSIS Secondary myelofibrosis is a bone marrow disorder characterized by proliferation of fibroblasts or collagen or reticulin fibers in the hematopoietic space (Fig. 18.3).1,2 Secondary myelofibrosis must be differentiated from idiopathic myelofibrosis that is a rare chronic myeloproliferative disease (see Chapter 65). Secondary myelofibrosis occurs relatively frequently and is associated with moderate to severe non-regenerative anemia and less frequently with thrombocytopenia or leukopenia.32 Collagen and fibroblastic myelofibrosis has been observed in 4.2% of canine clinical bone marrow specimens and in 9% of feline clinical bone marrow specimens evaluated at a veterinary teaching hospital.36,37 The distribution of fibrosis in marrow sections varies. Reticulin fibers are most frequently diffusely distributed in the marrow but in some early lesions can be focally distributed around blood vessels. Fibrosis can be random and multifocal, diffuse, or paratrabecular. When paratrabecular myelofibrosis is detected, renal osteodystrophy and primary hyperparathyroidism should be considered (Fig. 18.4).2 The fibrosis is thought

FIGURE 18.4 Paratrabecular fibrosis (arrows) in a bone marrow core biopsy section from a dog. Hematoxylin & Eosin stain; bar, 200 μm.

to be a reactive change frequently resulting from acute bone marrow injury.32,34,38 As such, the fibrosis is regarded as a stage of the repair process much as occurs in other tissues. If the noxious agent is removed or effectively treated, the fibrosis is frequently reversible with the tissue reverting to normal hematopoietic tissue.28,32 Secondary myelofibrosis has been identified in several species including dog, cat, goat, and horse.1,34,38 Multiple causes of secondary myelofibrosis have been identified. Fibrosis is frequently present in the subacute or chronic stages of myelonecrosis.34,38 The necrosis may or may not be evidenced when the fibrosis is identified but if present is evidence of ongoing bone marrow injury. In dogs, secondary myelofibrosis has been associated with IMHA, acute leukemias/lymphomas, idiopathic

CHAPTER 18: CHRONIC INFLAMMATION AND SECONDARY MYELOFIBROSIS

myelonecrosis, congenital pyruvate kinase deficiency, congenital non-spherocytic anemia, non-hemic malignant tumors, whole body irradiation, and certain drug treatments.20,27,32,38,41 Drugs associated with myelofibrosis include phenobarbital, phenylbutazone, and cholchicine (see Chapter 16).32 Approximately half the cases of secondary myelofibrosis are associated with IMHA and intramedullary and extramedullary neoplasia.32 Most affected dogs have a moderate to severe nonregenerative anemia, some are thrombocytopenic, but leukopenia is a rare finding. Ovalocytes are infrequently seen in the peripheral blood. In one study of 19 dogs with secondary myelofibrosis, half of the dogs recovered from their anemia with supportive therapy. When the tumor-associated group was eliminated, 66% of dogs survived more than 8 months. Treatments with immunosuppressive doses of prednisone or with erythropoietin may be useful. A periparturient myelofibrosis, associated severe non-regenerative anemia, has been reported in young female beagle dogs.21 Myelonecrosis was identified in 1 of 3 dogs reported; therefore the fibrosis may have occurred secondary to acute bone marrow injury. Secondary myelofibrosis is a frequent finding in cat bone marrow.3,38 Both collagen and reticulin fibrosis are frequent causes of failure to obtain a sample when aspirating cat bone marrow. Major causes of secondary myelofibrosis in cats include myelodysplastic syndromes, acute myeloid leukemia, IMHA, feline infectious peritonitis, and chronic renal failure.3,32 Myelonecrosis is present in many cats with secondary myelofibrosis suggesting that the fibrosis occurs in response to necrosis. Myelofibrosis has been reported as an inherited disorder in pigmy goats.5 At birth, these goats have a non-regenerative anemia, neutropenia, and thrombocytopenia. All animals die at 6–12 weeks of age. Severe myelofibrosis was identified at necropsy. Other necropsy findings included extramedullary hematopoiesis and megakaryocytic hyperplasia and dysplasia. Myelofibrosis has been described in aged mice.25 Myelofibrosis was observed in 43% of female mice but 90% of females and ovariectomized females had evidence of myelofibrosis as well as some castrated males.24 This study indicates that estrogen may not be a major factor in development of myelofibrosis. The etiopathogenesis of secondary myelofibrosis is uncertain. The coexistence of myelofibrosis and myelonecrosis in many dog and cat bone marrow specimens indicates that fibrosis may be equivalent to scar tissue in other damaged tissues. Several fibrogenic cytokines have been identified. These include transforming growth factor-β, platelet-derived growth factor, and epidermal growth factor. In bone marrow, fibrogenic cytokines are produced by megakaryocytes and macrophages. Excessive production of fibrogenic cytokines by megakaryocytes is thought to induce myelofibrosis in human idiopathic myelofibrosis, acute megakaryoblastic leukemia, and subtypes of myelodysplastic

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syndrome characterized by dysmegakaryopoiesis.23 Constant high levels of thrombopoietin in mice also induce megakaryocyte hyperplasia and myelofibrosis.6,44 Myelofibrosis has been induced in rodents by injection of a variety of cytokines, including thrombopoietin, erythropoietin, and megakaryocyte growth factor.44,45 Increased megakaryocytes or dysmegakaryopoiesis has been seen infrequently in dogs with secondary myelofibrosis.32 Hemosiderosis has also been incriminated as a cause of secondary myelofibrosis. Increased hemosiderin has been identified in 28% of dogs with secondary myelofibrosis. OSTEOSCLEROSIS Osteosclerosis (also termed myelosclerosis) is a condition of excessive and unorganized production of trabecular bone. Alternatively osteopetrosis is a congenital condition of dogs and humans characterized by severe non-regenerative anemia or pancytopenia and generalized increased bone density.17 Osteosclerosis appears as irregular fingers or spurs of bone extending from the trabeculae into the hematopoietic space (Fig. 18.5). These sites usually contain increased numbers of osteoblasts and osteoclasts.20 When severe, the associated marked thickening of bone can be visualized radiographically.9,15,20 Osteosclerosis is almost always associated with concurrent severe myelofibrosis. Severe osteosclerosis can obliterate or markedly reduced the size of the hematopoietic space. Osteosclerosis along with severe myelofibrosis occurs terminally in dogs with pyruvate kinase deficiency and has been associated with feline leukemia virus (FeLV) infection.9,15,20 These changes are seen in core biopsy sections as markedly thickened trabeculae with irregular edges that contain increased numbers of osteoblasts.

FIGURE 18.5 Osteosclerosis in a bone marrow core biopsy section from a feline leukemia virus-infected cat. Hematoxylin & Eosin stain; bar, 200 μm.

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FIGURE 18.6 Gelatinous transformation in a bone marrow core biopsy section from a cat with chronic renal failure. Hematoxylin & Eosin stain; bar, 100 μm.

SEROUS ATROPHY OF FAT/GELATINOUS TRANSFORMATION The paratrabecular space within normal marrow is occupied almost totally by hematopoietic cells or adipocytes. In processing paraffin-embedded tissues the lipid is dissolved out. Therefore, the space within adipocytes normally appears clear and homogeneous. Serous atrophy of fat/gelatinous transformation is a condition in which there is loss of fat cells and hematopoietic cells from bone marrow and replacement by increased amounts of ground substance (Fig. 18.6).26 This is usually associated with starvation or cachexia. Foci of gray or pink gelatinous material may be grossly visible in bone marrow at necropsy. Histopathologic sections of bone marrow are characterized by marrow hypoplasia/aplasia, fat atrophy, and amorphous granular extracellular material. This material is thought to be composed of acid mucopolysaccharides.26 Serous atrophy of fat/gelatinous transformation has been documented in sheep, goats, primates, and cats.29,40 In sheep and goats, it has been associated with neglect or loss of teeth. In cats, it has been associated with cachexia associated with the combination of chronic disease and prolonged anorexia. Affected cats had pancytopenia in the blood and marked hypoplasia of bone marrow. Associated conditions included chronic renal failure and oral and gastric ulcers.

REFERENCES 1. Angel KL, Spano JS, Schiumacher J, et al. Myelophthisic pancytopenia in a pony mare. J Am Vet Med Assoc 1991;198:1039–1042. 2. Bain BJ, Clark DM, Lampert IA, Wilkins BS. Infection and reactive changes. In: Bone Marrow Pathology, 3rd ed. Ames: Blackwell Publishing, 2001;90–131. 3. Blue JT. Myelofibrosis in cats with myelodysplastic syndromes and acute myelogenous leukemia. Vet Pathol 1988;25:154–160. 4. Boosinger TR, Rebar AH, DeNicola DB, et al. Bone marrow alterations associated with canine parvoviral enteritis. Vet Pathol 1982;19:558–561. 5. Cain GR, East N, Moore PF. Myelofibrosis in young pygmy goats, Comp Haematol Intl 1994;4:167–172.

6. Chagraoui H, Komura E, Tulliez M, et al. Prominent role of TGF-beta 1 in thrombopoietin-induced myelofibrosis in mice. Blood 2002;100:3495–3503. 7. Clinkenbeard KD, Cowell RL, Tyler RD. Disseminated histoplasmosis in cats: 12 cases (1981–1986). J Am Vet Med Assoc 1987;190:1445–1448. 8. Clinkenbeard KD, Cowell RL, Tyler RD. Disseminated histoplasmosis in dogs: 12 cases (1981–1986). J Am Vet Med Assoc 1988;193:1443–1447. 9. Flecknell PA, Gibbs C, Kelly DF. Myelosclerosis in a cat. J Comp Pathol 1978;88:627–631. 10. Frisch B, Lewis SM, Burkhardt R, et al. The cytopenias: nonhaematopoietic components. Biopsy Pathology of Bone and Bone Marrow. New York, NY: Raven Press, 1985;58–71. 11. Gavazza A, Lubas G, Gugliucci B, et al. Hemogram and bone marrow patterns in canine leishmaniasis. Vet Clin Pathol 2002;31:198. 12. Harvey JW. Canine bone marrow: normal hematopoiesis, biopsy techniques, and cell identification and evaluation. Compend Contin Educ Pract Vet 1984;6:909–926. 13. Hamaguchi H. Lymph nodules in cat bone marrow. J Kyushu Hematol Soc 1955;5:241–296. 14. Hashimoto M. Pathology of bone marrow. Acta Haematol 1962;27: 193–216. 15. Hoover EA, Kociba GJ. Bone lesions in cats with anemia induced by feline leukemia virus. J Natl Cancer Inst 1974;53:1277–1284. 16. Larroche C, Mouthon L. Pathogenesis of hemophagocytic syndrome (HSP). Autoimmun Rev 2004;3:69–75. 17. O’Brien SE, Riedesel EA, Miller LD. Osteopetrosis in an adult dog. J Am Anim Hosp Assoc 1987;23:213–216. 18. O’Toole D, Tharp S, Thomsen BV, et al. Fatal mycobacteriosis with hepatosplenomegaly in a young dog due to Mycobacterium avium. J Vet Diagn Invest 2005;17:200–204. 19. Penny RHC, Carlisle CH, Davidson HA. The blood and marrow picture of the cat. Br Vet J 1970;126:459–463. 20. Prasse KW, Crouser D, Beutler E, et al. Pyruvate kinase deficiency anemia with terminal myelofibrosis and osteosclerosis in a beagle. J Am Vet Med Assoc 1975;166:1170–1175. 21. Reagan WJ. A review of myelofibrosis in dogs. Toxicol Pathol 1993;21:164–169. 22. Rebar AH. General responses of the bone marrow to injury. Toxicol Pathol 1993;21:118–129. 23. Reilly JT. Pathogenesis and management of idiopathic myelofibrosis. Bailliere’s Clin Haematol 1998;11:751–767. 24. Rittinghausen S, Kohler M, Kamino K, et al. Exp Toxicol Pathol 1997;49:351–353. 25. Sass B, Montali RJ. Spontaneous fibro-osseous lesions in aging female mice. Lab Anim Sci 1980;30:907–909. 26. Seaman JP, Kjeldsberg CR, Linker A. Gelatinous transformation of the bone marrow. Hum Pathol 1978;9:685–692. 27. Seed TM, Chubb GT, Tolle DV, et al. The ultrastructure of radiationinduced endosteal myelofibrosis in the dog. Scanning Electron Microsc 1982;1:377–391. 28. Stokol T, Blue J, French TW. Idiopathic pure red cell aplasia and nonregenerative immune-mediated anemia in dogs: 43 cases (1988–1999). J Am Vet Med Assoc 2000;216:1429–1436. 29. Valli VE. Bone marrow. In: Veterinary Comparative Hematopathology. Ames: Blackwell Publishing. 2007;114–115. 30. Valli VE, Villeneuve DC, Reed B, et al. Evaluation of blood and bone marrow, rat. In: Haschek WM, Rousseau, GG, eds. Hematopoietic System. Berlin: Springer Verlag, 1990;9–26. 31. Weiss DJ. A review of the techniques for preparation of histopathologic sections of bone marrow. Vet Clin Pathol 1987;16:90–94. 32. Weiss DJ. A retrospective study of 19 cases of canine myelofibrosis. J Vet Int Med 2002;16:174–178. 33. Weiss DJ. Evaluation of canine hemophagocytic disorders by use of flow cytometric scatter plots and monoclonal antibodies. Vet Clin Pathol 2002;31:36–41. 34. Weiss DJ. Bone marrow necrosis in dogs: 34 cases (1996–2004): J Am Vet Med Assoc 2005;227:263–267. 35. Weiss DJ. Differentiating benign and malignant causes of lymphocytosis in feline bone marrow. J Vet Int Med 2005;19:855–859. 36. Weiss DJ. A retrospective study of the incidence and classification of bone marrow disorders of cats (1996–2004). Comp Clin Pathol 2006;14:179– 185. 37. Weiss DJ. A retrospective study of the incidence and the classification of bone marrow disorders in the dog at a veterinary teaching hospital (1996– 2004). J Vet Int Med 2006;20:955–961. 38. Weiss DJ. Feline myelonecrosis and myelofibrosis: 22 cases 1996–2006. Comp Clin Pathol 2007;16:181–185. 39. Weiss DJ. Hemophagocytic syndrome in dogs: 24 cases (1996–2005). J Am Vet Med Assoc 2007;230:697–701. 40. Weiss DJ. Aplastic anemia in the cat – clinicopathologic features and associated disease conditions 1996–2004. J Feline Med Surg 2007;8:203–206. 41. Weiss DJ. Bone marrow pathology in dogs and cats with nonregenerative immune-mediated hemolytic anemias. J Comp Pathol 2008;138:46–53. 42. Weiss DJ, Greig B, Aird B, et al. Inflammatory disorders of bone marrow. Vet Clin Pathol 1992;21:79–84.

CHAPTER 18: CHRONIC INFLAMMATION AND SECONDARY MYELOFIBROSIS 43. Workman HC, Vernau W. Chronic lymphocytic leukemia in dogs and cats: the veterinary perspective. Vet Clin N Am Small Anim Pract 2003;33:1379–1399. 44. Yan XQ, Lacey D, Hill D, et al. A model of myelofibrosis and osteosclerosis in mice induced by overexpressing thrombopoietin (mpl ligand): reversal

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of disease by bone marrow transplantation. Blood 1996;88:402– 409. 45. Yanagida M, Ide Y, Imai A, et al. The role of transforming growth factorbeta in PEG-rHuMGDF-induced reversible myelofibrosis in rats. Br J Haematol 1999;99:739–745.

C H A P T E R 19

Infectious Injury to Bone Marrow K. JANE WARDROP See Chapters 17 and 18 Rickettsial Infection Protozoal Infection – Leishmania Viral Infection Parvovirus

Retrovirus Equine Infectious Anemia virus Bovine Viral Diarrhea Virus and Classical Swine Fever Virus Fungal Infection

Acronyms and Abbreviations BLV, bovine leukemia virus; BMSC, bone marrow stromal cell; BVDV, bovine viral diarrhea virus; CFU-GM, colony-forming units-granulocyte-monocyte; CPV, canine parvovirus; CSFV, classical swine fever virus; EIAV, equine infectious anemia virus; FeLV, feline leukemia virus; FIV, feline immunodeficiency virus; FPV, feline panleukopenia virus; PCR, polymerase chain reaction.

T

he response of the bone marrow to infectious agents is variable, depending on the organism involved, the nature and chronicity of infection, and the presence of other diseases. Responses are generally nonspecific, and can occur with other, noninfectious disease. Visualization of the organism on marrow cytology or histology can occur in some infections, facilitating the diagnosis. The marrow response to common companion or domestic animal infectious diseases known for their marrow involvement is described below.

but are difficult to find, with 1–4 morulae per 1,000 oil immersion fields seen in one study of acute ehrlichiosis.11 Chronic ehrlichiosis produces a pancytopenia, secondary to severe bone marrow hypocellularity (see Chapter 31). Marrow aspirates are poorly cellular, with cells consisting of low numbers of stromal cells, fibroblasts, and macrophages. Hematopoietic precursors are few or absent. Core biopsies show a severely hypocellular marrow filled with adipose tissue.3 Mastocytosis also has been reported in the chronic phase of ehrlichiosis.13 Bone marrow plasmacytosis has been reported with ehrlichiosis, and is independent of the phase.12,25

RICKETTSIAL INFECTION PROTOZOAL INFECTION – LEISHMANIA Bacteria within the families Rickettsiaceae and Anaplasmataceae of the order Rickettsiales cause a variety of diseases in dogs, cats, horses, and cattle; however, marrow changes are best documented for infections caused by Ehrlichia canis. Ehrlichia canis is the etiologic agent of canine monocytic ehrlichiosis, and is transmitted most commonly by the vector Rhipicephalus sanguineus. The disease has acute, chronic, and subclinical syndromes. Bone marrow in the acute phase of the disease is typically hypercellular. An increase in the bone marrow myeloid to erythroid ratio was described 3–4 weeks post-infection in experimental canine ehrlichiosis.17 Megakaryocytes were increased 1 week after infection, reflecting the hematopoietic response to the persistent thrombocytopenia seen with this disease (see Chapter 78). Morulae can be present in bone marrow 118

Leishmaniasis is a disease caused by protozoal organisms of the genus Leishmania, and is transmitted in Mediterranean regions by the bite of an infected female sand fly. The vector in North America is not known. Several forms of the disease, including cutaneous, mucocutaneous, and visceral forms, have been identified in dogs. A mild anemia and thrombocytopenia are the most frequent hematologic findings. Microscopic examination of fine needle aspirates of lymph nodes or bone marrow, along with serology and polymerase chain reaction (PCR), are frequently used diagnostic methods (Fig. 19.1). In one study, Leishmania spp. amastigotes were observed in the marrow of 92.6% of clinically affected dogs.20 Severe histiocytosis of the marrow has been reported.14

CHAPTER 19: INFECTIOUS INJURY TO BONE MARROW

FIGURE 19.1 Leishmania organisms in bone marrow from a dog; Wright-Giemsa stain. (Courtesy of Dr. Rick Cowell, Idexx Laboratories.)

VIRAL INFECTION Parvovirus Feline panleukopenia virus (FPV) and canine parvovirus (CPV) are both parvoviruses that cause bone marrow injury and leukopenia, neutropenia, and lymphopenia. These viruses have a broad tropism for mitotically active cells. It is unknown whether the reduction in marrow cells is due primarily to virus replicating in and killing cells, or due to more indirect effects. FPV can kill both erythroid and myeloid colony progenitor cells in bone marrow cultures, although at low viral doses there is greater suppression of the formation of myeloid colony-forming units-granulocyte-monocyte (CFU-GM).7 Viral replication of FPV in myeloid cells may lead to the neutropenia seen after FPV infection. In a study of 76 cats with clinical FPV, 84% showed complete bone marrow depopulation and dilation of the vascular sinuses, 5% had no significant marrow change and 6% had marrow hypercellularity with granulocytic hyperplasia.4 In 75 dogs with clinical CPV, 92% had marrow depopulation, one dog showed no changes, and five dogs had marrow hypoplasia.4 In another study using experimentally infected puppies, leukopenia and neutropenia only occurred in dogs with severe enteric disease. Viral replication in the bone marrow was sparse.9 In both FPV and CPV, the severe neutropenia observed is probably the result of both neutrophil loss or consumption in the infected gut and bone marrow suppression.15 Retrovirus Feline leukemia virus (FeLV, subfamily Oncornavirinae) and feline immunodeficiency virus (FIV, subfamily Lentivirinae) are retroviruses that are major infectious

119

pathogens in domestic cats (see Chapters 55 and 62). FeLV infects hematopoietic cells, lymphoid cells, and accessory cells in the hematopoietic microenvironment. Marrow nucleated cells become infected with the virus 2–6 weeks after exposure, and virus can be detected in circulating leukocytes at that time.8 After regression, one third to one half of cats transiently harbor latent virus in marrow myelomonocytic cells and stromal fibroblasts.8 The types of marrow change seen in FeLV are highly variable. Early viremia can be accompanied by pancytopenia and marrow hypocellularity.16 Marrow necrosis, with nuclear swelling and karyolysis has rarely been described.22 FeLV-C-induced pure red cell aplasia shows almost a total lack of marrow erythroid precursor cells, with normal myeloid and megakaryocytic precursors (see Chapters 38 and 62).21 FeLVinduced myelodysplastic syndromes can show marrow hypercellularity, megaloblastic maturation abnormalities, increased reticulin fibrosis, and increased numbers of immature cells and blast cells (see Chapter 66).2,10 Marrow myelophthisic disease, where the marrow is filled with blast cells, can be observed in FeLV-induced leukemias or lymphoma. FIV produces progressive CD4 T cell lymphopenia. Marrow morphologic abnormalities, marrow suppression, and peripheral blood cytopenias can be seen. Cytopenias (neutropenia, anemia) have been attributable to direct infection of the bone marrow stromal cell (BMSC) population.19 BMSC types include fibroblastic/ advential/reticular cells, macrophages, endothelial cells, adipocytes, and myofibroblasts, of which macrophages are productively infected with FIV.1 Bovine leukemia virus (BLV) is an oncogenic retrovirus which produces lymphoma in adult cattle. Bone marrow involvement is not a prominent feature. Equine Infectious Anemia Virus The equine infectious anemia virus (EIAV) is a lentivirus of the family Retroviridae (see Chapters 32 and 35). Infection of horses with EIAV produces a persistent viremia and recurrent episodes of anemia, thrombocytopenia, and fever. In one study using experimentally infected foals with severe combined immunodeficiency, bone marrow changes were described after the onset of thrombocytopenia.24 The post-thrombocytopenia samples had a moderate to marked decrease in bone marrow cellularity, with an increase in intercellular homogeneous eosinophilic material. Individual cell necrosis or apoptosis was not observed. A moderate increase in adipocytes and reticulin fibers was noted. Denuded megakaryocyte nuclei were also observed. In another study, the total megakaryocyte area and megakaryocyte nuclear area were increased. Numbers of megakaryocytes were slightly increased.18 Bovine Viral Diarrhea Virus and Classical Swine Fever Virus Bovine viral diarrhea virus (BVDV) and classical swine fever virus (CSFV) are related viruses (family

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In the bone marrow, large numbers of organisms can be found within macrophages (Fig. 19.2). Anemia is a frequent finding, but pancytopenia accompanied by granulomatous inflammation of the bone marrow has also been reported.5

REFERENCES

FIGURE 19.2 Histoplasma organisms in bone marrow from a cat; Wright-Giemsa stain. (Courtesy of Dr. James Meinkoth, Oklahoma State University.)

Flaviviridae, genus Pestivirus) both known to infect and damage megakaryocytes of the marrow and produce a subsequent thrombocytopenia, but other marrow lesions are not present. In a study using calves infected with type II BVDV, histologic evaluation of the bone marrow showed an increased number of megakaryocytes. Approximately one-half of the megakaryocytes were immature or had undergone nuclear loss or pyknosis.23 Infection of calves with type II BVDV also reduced the number of GFU-GM cultured from bone marrow. This suggests that suppression of granulopoiesis may contribute to the prolonged neutropenia associated with BVD infection. Experimental infection of pigs with a virulent strain of CSFV also produced changes in megakaryocytes, with abnormal nuclei and micromegakaryocytes seen.6 FUNGAL INFECTION Blastomycosis, histoplasmosis, coccidiomycosis, and cryptococcosis are common systemic fungal diseases of the dog and cat. Although all of these diseases can involve the bone marrow, marrow involvement is most frequently described in histoplasmosis. Histoplasma capsulatum is a soil-borne, dimorphic fungus that infects dogs and cats through inhalation of conidia from the mycelial phase, which subsequently convert to yeast in the body. The disseminated form of the disease predominantly affects the liver, spleen, gastrointestinal tract, bone, bone marrow, integument, and eyes. Primary gastrointestinal and pulmonary histoplasmosis can also occur.

1. Beebe AM, Gluckstern TG, George J, et al. Detection of feline immunodeficiency virus infection in bone marrow of cats. Vet Immunol Immunopathol 1992;35:37–49. 2. Blue JT. Myelofibrosis in cats with myelodysplastic syndrome and acute myelogenous leukemia. Vet Pathol 1988;25:154–160. 3. Brazzell JL, Weiss DJ. A retrospective study of aplastic pancytopenia in the dog: 9 cases (1996–2003). Vet Clin Pathol 2006;35:413–417. 4. Breuer W, Stahr K, Majzoub M, et al. Bone marrow changes in infectious diseases and lymphohaemopoietic neoplasias in dogs and cats—a retrospective study. J Comp Pathol 1998;119:57–66. 5. Gabbert NH, Campbell TW, Beiermann RL. Pancytopenia associated with disseminated histoplasmosis in a cat. J Am Anim Assoc 1984;20:119–122. 6. Gomez-Villamandos JC, Ruiz-Villamor E, Salguero FJ, et al. Immunohistochemical and ultrastructural evidence of hog cholera virus infection on megakaryocytes in bone marrow and spleen. J Comp Pathol 1998;119:111–119. 7. Kurtzman GJ, Platanias K, Lustig L, et al. Feline parvovirus propagates in cat bone marrow cultures and inhibits hematopoietic colony formation in vitro. Blood 1989;74:71–81. 8. Linenberger ML, Abkowitz JL. Haematological disorders associated with feline retrovirus infections. Bailliere’s Clin Haematol 1995;8:73–112. 9. Macartney L, McCandlish IAP, Thompson H, et al. Canine parvovirus enteritis 1: clinical, haematological and pathological features of experimental infection. Vet Rec 1984;115:201–210. 10. Maggio L, Hoffman R, Cotter SM, et al. Feline preleukemia: an animal model of human disease. Yale J Biol Med 1978;51:469–476. 11. Mylonakis ME, Koutinas AF, Billinis C, et al. Evaluation of cytology in the diagnosis of acute canine monocytic ehrlichiosis (Ehrlichia canis): a comparison between five methods. Vet Microbiol 2003;91:197–204. 12. Mylonakis ME, Koutinas AF, Breitschwerdt EB, et al. Chronic canine ehrlichiosis (Erhlichia canis): A retrospective study of 19 natural cases. J Am Anim Hosp Assoc 2004;40:174–184. 13. Mylonakis ME, Koutinas AF, Leontides LS. Bone marrow mastocytosis in dogs with myelosuppressive monocytic ehrlichiosis (Ehrlichia canis): a retrospective study. Vet Clin Pathol 2006;35:311–314. 14. Owens SD, Oakley DA, Marryott K, et al. Transmission of visceral leishmaniasis through blood transfusions from infected English Foxhounds to anemic dogs. J Vet Med Assoc 2001;219:1076–1083. 15. Parrish, CR. Pathogenesis of feline panleukopenia virus and canine parvovirus. Bailliere’s Clin Haematol 1995;8:57–71. 16. Pedersen NC, Theilen G, Keane MA, et al. Studies of naturally transmitted feline leukemia virus infection. Am J Vet Res 1977;38:1523–1531. 17. Reardon MJ, Pierce KR. Acute experimental canine ehrlichiosis: I. Sequential reaction of the hemic and lymphoreticular systems. Vet Pathol 1981;18:48–61. 18. Russell KE, Perkins PC, Hoffman MR, et al. Platelets from thrombocytopenic ponies acutely infected with equine infectious anemia virus are activated in vivo and hypofunctional. Virology 1999;259:7–19. 19. Sandy JR, Robinson WF, Bredhauer B, et al. Productive infection of the bone marrow cells in feline immunodeficiency virus infected cats. Arch Virol 2002;147:1053–1059. 20. Saridomichelakis MN, Mylonakis ME, Leontides LS, et al. Evaluation of lymph node and bone marrow cytology in the diagnosis of canine leishmaniasis (leishmania infantum) in symptomatic and asymptomatic dogs. Am J Trop Med Hyg 2005;73:82–86. 21. Shelton GH, Linenberger ML. Hematologic abnormalities associated with retroviral infections in the cat. Semin Vet Med Surg (Small Anim) 1995;10:220–233. 22. Shimoda T, Shiranaga N, Mashita T, et al. Bone marrow necrosis in a cat infected with feline leukemia virus. J Vet Med Sci 2000;62:113–115. 23. Walz PH, Bell TG, Steficek BA, et al. Experimental model of type II bovine viral diarrhea virus-induced thrombocytopenia in neonatal calves. J Diagn Invest 1999;11:505–514. 24. Wardrop KJ, Baszler TV, Reilich E, et al. A morphometric study of bone marrow megakaryocytes in foals infected with equine infectious anemia virus. Vet Pathol 1996;33:222–227. 25. Woody BJ, Hoskins JD. Ehrlichial diseases of dogs. Vet Clin N Am Small Anim Pract 1991;21:75–98.

SECTION III

Erythrocytes Joanne B. Messick

CHAPTER

20

Erythrocyte Structure and Function CHRISTINE S. OLVER, GORDON A. ANDREWS, JOSEPH E. SMITH, and J. JERRY KANEKO Erythrocyte Structure Overall Structure Membrane Structure Membrane lipids Lipid rafts Maintenance of membrane fluidity Cytoskeletal proteins Integral membrane proteins Hemoglobin Structure, Synthesis, and Metabolism Hemoglobin Structure Hemoglobin Synthesis Control of Hemoglobin Synthesis Hemoglobin Turnover Heme Catabolism and Bilirubin Formation Fetal Hemoglobin Hemoglobin Types in Animals

Iron Metabolism Absorption in Intestine Intestinal absorption of non-heme iron Cellular Iron Uptake Molecules Involved in Iron Metabolism Divalent metal iron transporter–1 Ferrireductase duodenal cytochrome b Hephaestin Ferroportin Hepcidin Transferrin receptor Transport and Storage of Iron Transport of iron Storage of iron Genetic Control of Iron Regulatory Proteins

Acronyms and Abbreviations dCytb, duodenal cytochrome B; DMT-1, divalent metal iron transporter-1; Hgb, hemoglobin; ID, iron deficiency; IRP, iron regulatory protein; kD, kilodalton; MPS, mononuclear phagocyte system; NADH, nicotinamide adenine dinucleotide, reduced form; NADPH, nicotinamide adenine dinucleotide phosphate, reduced form; PC, phosphatidylcholine; PE, phosphatidylethanolamine; PS, phosphatidylserine; RBC, red blood cell; SM, sphingomyelin; TfR, transferrin receptor.

T

he main function of red blood cells (RBCs) is to carry oxygen to tissues. All energy in RBCs that is devoted to maintaining cell shape, membrane structure, enzymatic functions, reduced iron in hemoglobin and other functions does so to optimize oxygen delivery to tissues.

of 61% water, 32% protein (mostly hemoglobin), 7% carbohydrates, and 0.4% lipids. Isolated RBC membranes in most animals are composed of approximately 20% water, 40% protein, 35% lipid and 6% carbohydrate.43 Membrane Structure

ERYTHROCYTE STRUCTURE

Membrane Lipids

Overall Structure

The RBC membrane phospholipid bilayer is composed of phosphatidylcholine (PC), phosphatidylethanolamine (PE), sphingomyelin (SM), and phosphatidylserine (PS), as well as other less abundant phospholipids.59 This composition varies with species. For instance, sheep have no PC and camellids have very low PC,

RBCs have no nuclei and no organelles, and thereby no ability to synthesize proteins. The full complement of functional proteins must be present by the time the reticulocyte matures. RBCs are composed (by mass)

123

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whereas rats have more PC.39,51,59 The phospholipids have polar head groups and hydrophobic tail groups. The tail groups associate together in the bilayer to provide a hydrophobic barrier to water while the polar groups face the hydrophilic exterior. The bilayer is vertically asymmetrical, with PS and PE located almost exclusively on an inner PL layer. This asymmetry is maintained by the activities of enzymes termed flippase14 and floppase.33,50 There is also horizontal asymmetry, so that there is lipid composition variability within different domains of the bilayer. Other lipids in the bilayer include glycolipids and cholesterol, and these are embedded in the phospholipids. Lipid Rafts

FIGURE 20.1 Lattice structure provided by spectrin tetrameters in the cytoskeleton of erythrocytes.

Lipids rafts are specialized membrane microdomains that are rich in cholesterol and sphingolipids and also contain specific membrane proteins. They are isolated as detergent resistant membranes because of their insolubility in non-ionic detergents.29 Examples of proteins localized in these microdomains include glycophosphatidylinositol-anchored proteins, flotillins, stomatin, and aquaporin-1.45

20.1). These complexes are made up of actin, 4.1 protein (named for its migration during one dimensional electrophoresis), and the integral membrane protein glycophorin. Spectrin is also affixed to the inner leaflet via an interaction with ankyrin (a peripheral protein) that is bound to band 3 (anion transporter, an integral membrane protein). Additional peripheral membrane proteins comprising the cytoskeleton include adducin, tropomyosin, tropomodulin, and palladin.

Maintenance of Membrane Fluidity

Integral Membrane Proteins

Anything that increases the “packing” of the membrane components together will “harden” the membrane, and anything that decreases the packing will “soften” the membrane. Membrane fluidity is, therefore based on several factors. One factor is the degree to which fatty acid chains are esterified with cholesterol. Esterified cholesterol is bulkier, and therefore “packs” more loosely, and creates a more fluid membrane. A second factor is the class of phospholipid and the type of fatty acid contained within the membrane. A third factor is the molar ratio of cholesterol to phospholipids. A fourth factor is the degree of saturation of phospholipid fatty acids (saturation makes PLs “pack” better and, therefore decreases fluidity).

Band 3 or the anion exchange protein is an integral membrane protein composed of dimers, tetramers and other large oligomers. It is responsible for anchoring the cytoskeleton to the membrane, anion exchange and binding of glycolytic enzymes, hemoglobin and hemichromes. The anion exchange function is primarily for delivering CO2 to the lungs in exchange for Cl−. Glycophorins are heavily glycosylated integral membrane proteins. They carry most of the sialic acid residues and, therefore negative charge on the surface of the RBC. RBCs carry other cell surface molecules, and new ones are being discovered all the time. A select list of these is shown in Table 20.1.

Cytoskeletal Proteins

HEMOGLOBIN STRUCTURE, SYNTHESIS, AND METABOLISM

Cytoskeletal proteins are located on the cytoplasmic side of the plasma membrane and function to help maintain cell shape, membrane deformability, membrane stability, and the lateral mobility of some integral membrane proteins.62 They are called “peripheral proteins” because they are not embedded within the membrane but rather are anchored to the cytoplasmic side of the bilayer. Alpha and beta spectrin subunits are the largest and most abundant proteins in the cytoskeleton. They are elongated and self-associate to form α2β2 tetramers which then associate end to end into a hexagonal network of molecules that lies just underneath the plasma membrane. The spectrin lattice is anchored to other proteins and to the bilayer at select areas of the hexagon. The spectrin molecules are secured to the lipid bilayer at “nodes” called junctional complexes (Fig.

Hemoglobin is an iron-porphyrin-protein complex. Other iron-porphyrin-protein complexes include myoglobin and heme-containing enzymes such as catalase, peroxidase, and cytochromes. The porphyrin-ironprotein hemoglobin molecule occupies a central role in physiology by binding, transporting, and delivering oxygen to tissues. Hemoglobin is synthesized within developing RBCs and its synthesis is coordinated with the developmental stages of the erythroid precursors. Hemoglobin Structure Hemoglobin is composed of two α- and two βpolypeptide chains and is approximately 64 kDa in molecular weight. Each chain contains a heme prosthetic group held firmly within a hydrophobic cleft

CHAPTER 20: ERYTHROCYTE STRUCTURE AND FUNCTION

125

TABLE 20.1 Selected Erythrocyte Surface Molecules in the Human and/or Mouse Molecule CD35 CD36 CD38 CD44 CD47 CD49d/CD29a CD55 CD59 CD71a CD147 CD239

Synonyms

Function

Complement receptor 1 Platelet glycoprotein IV N/A H-CAM, gp85 Integrin associated protein

Binds complement fragments Unknown NAD glycohydrolase Hyaluronic acid binding May be a senescence marker

VLA-4, α4β1 integrin Decay accelerating factor Complement regulatory protein

Adhesion to fibronectin, V-CAM Neutralizes complement activation Inhibits complement membrane attack

Transferrin receptor N/A Basal cell adhesion molecule/Lutheran

Iron acquisition Recirculation of RBC from spleen Laminin binding

a

Reticulocytes only.

(otherwise known as the “heme pocket”). The entire molecule is, therefore a globular tetramer. This globular structure permits a cooperative interaction of oxygen binding that gives the sigmoid oxygen-Hgb saturation curve.

Succinate

B6

Glycine

1

Hemoglobin Synthesis Heme is a planar molecule composed of two elements, a tetrapyrrole protoporphyrin IX and a central iron molecule. The iron is supplied by ferritin, the cytosolic storage form of iron. Heme biosynthesis is an enzymatic process requiring both mitochondrial and cytosolic enzymes. Figure 20.2 shows the sequence of reactions leading to the synthesis of PROTO IX, heme, and Hgb. The initial step of heme synthesis occurs in mitochondria. It requires vitamin B6 (pyridoxine) and is catalyzed by δ-aminolevulinic acid (ALA) synthase. ALA is then translocated to the cytosol. In the cytosol, two moles of ALA are condensed to form porphobilinogen (PBG), a reaction catalyzed by ALA-dehydrase (ALA-D). ALA-D is strongly inhibited by lead, leading to the anemia associated with lead poisoning. The remainder of the process is depicted in Figure 20.2. Synthesis of α and β globin chains occurs in the ribosomes and polyribosomes in the cytoplasm. Each globin chain contains a cleft or heme pocket that is lined by largely nonpolar amino acid side chains that impart a hydrophobic nature to the pocket. The nonpolar (vinyl and methyl groups) side of heme is positioned in the hydrophobic pocket of each globin chain. The addition of heme to the pockets of globin is followed by a dimerization of the α and β globin chains, which is then followed by spontaneous formation of hemoglobin tetramers. Control of Hemoglobin Synthesis Synthesis of heme and globin chains is finely coordinated so that there is little or no free heme or globin in the cytoplasm of developing erythroid cells. Heme

M

d ALA

nd ho itoc

d ALA

×2 2

PBG ×4 3 4

ria

UROgen III Cytosol

5 COPROgen III

Ribosomal RNA aa t RNA Globin Hemoglobin HEME dria ho n toc i M

COPROgen III 6 PROTOgen III 7

HEME

Fe+2 8

PROTO IX

FIGURE 20.2 Pathway for protoporphyrin, heme, and hemoglobin synthesis emphasizing the partitioning of the pathway between the mitochondrial and the cytolic compartments of the cell. The circled numbers represent the following enzymes of heme synthesis: 1, ALA-Syn; 2, ALA-D; 3, UROgenI-Syn; 4, UROgenIIICosyn; 5, UROgen-D; 6, COPROgenIII-Ox; 7, PROTOgenIII-Ox; 8, FER-Ch. (Courtesy of Christine S. Olver)

governs ribosomal translation of globin chain synthesis.57 Moreover, α and β chain synthesis is coordinated. Excess α chains inhibit their own synthesis while stimulating β-chain synthesis, while excess β chains inhibit their own synthesis.27,54

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Hemoglobin Turnover Hemoglobin is turned over after the extra- or intravascular destruction of RBCs. In health, extravascular hemolysis occurs as RBCs age and are subsequently trapped in the spleen and phagocytosed by splenic macrophages. Hemoglobin is catabolyzed within a cell. When RBCs undergo intravascular hemolysis, the hemoglobin is released into the circulation in its free form. Free plasma Hgb is quickly cleared by several mechanisms. The most important is binding of free Hgb to haptoglobin followed by clearance of the complex by macrophages.26 The half-time for free Hgb clearance is 20–30 minutes. Minor pathways for Hgb clearance are (1) oxidation to methemoglobin (MetHgb) that can be excreted, (2) hydrolysis of the MetHgb to release its ferriheme (ferric heme, hematin, hemin) that is complexed to hemopexin for transport to the mononuclear phagocyte system (MPS), and (3) binding of ferriheme to albumin (methemalbumin) for transport to the MPS. When there is excess intravascular hemolysis, such that the binding capacity of plasma haptoglobin is exceeded, Hgb may be cleared via glomerular filtration. In this event, hemoglobinuria may occur, or the proximal renal tubular cells may catabolyze Hgb.16 When Hgb is internalized into macrophages, it is degraded by hydrolysis into its globin and heme moieties. The globin chains are systematically degraded by proteolytic enzymes to release the constituent amino acids.

of bilirubin glucuronide into the bile canaliculi and thence the biliary system. This canalicular transport is the rate-limiting step in hepatic bilirubin metabolism.28 A small amount of the glucuronides may also be transported back into the circulation so that normally, both unconjugated and conjugated bilirubins are present in the circulation. Fetal Hemoglobin The synthesis of α globin chains remains constant during stages of gestational development and throughout adult life. Different types of non-α globin chains are synthesized and account for the different types of Hgb appearing during development and in adult life. At the embryonic stage, most animals synthesize only embryonic chains designated ε chains (i.e. embryonal Hgbs). Fetal Hgb (HgbF) is composed of two γ chains and two α chains, although not all animals produce HgbF. Ruminants possess a mixture of adult and fetal Hgb at birth, and HgbF is replaced by adult Hgb in the first few months of life.3 Although HgbF rapidly declines after birth, the capacity to synthesize it remains, and HgbF may appear in responsive anemias. Dogs, cats, horses, and pigs lack HgbF and are born with adult Hgb that has replaced the embryonal Hgb during gestation.9,31 Hemoglobin Types in Animals

Heme Catabolism and Bilirubin Formation The initial step in the heme catabolic pathway is enzymatic cleavage of the heme ring at the α methene bridge to release linear tetrapyrrole biliverdin, iron, and carbon monoxide. This reaction is catalyzed by microsomal heme oxygenase in the presence of cytochrome P-450, oxygen, and reduced NADPH. Heme oxygenase activity is highest in the spleen, and there is some activity in the liver, bone marrow, and renal tubular cells. Iron is oxidized to the ferric form, released and transported as transferrin for storage as hemosiderin or ferritin in the liver and as ferritin in the bone marrow for subsequent reuse.55 Biliverdin, a green pigment, is reduced in macrophages to bilirubin, a yellow pigment, by the action of the enzyme biliverdin reductase in the presence of NADPH. The virtual absence of biliverdin reductase in birds accounts for the green color of avian bile. Bilirubin is a nonpolar compound and must be bound to albumin to remain soluble in aqueous plasma. The bilirubin-albumin complex is transported to the liver, and at the hepatocyte surface, albumin is released. The first step involving the hepatocyte surface is the uptake or transport of bilirubin across the hepatocyte membrane by a transporter system. The second step in the hepatocyte is conjugation of bilirubin to glucuronic acid, primarily as the diglucuronide and with some monoglucuronide. This reaction is catalyzed by the enzyme glucuronyl transferase. The bilirubin glucuronides are highly polar and are therefore soluble compounds. The third step in the hepatocyte is the transport

With the possible exception of pigs, two or more types of Hgb normally occur in several domestic animal species.8,30 Most polymorphisms are determined genetically and are usually associated with multiple amino acid substitutions.31 The Hgb of cats has a unique structure containing 8–10 reactive sulfydryl groups per molecule compared to other animals that have only 2–4 per Hgb molecule.36 The presence of the readily oxidizable groups is regarded as the basis for the ease of Heinz body formation in the cat (see Chapter 36). Sheep and goats synthesize HgbC in response to severe anemia.25 Sheep normally have HgbA and goats HgbA and HgbB, and the switch to HgbC during anemia is mediated by erythropoietin.4 Carbon dioxide decreases oxygen affinity for HgbC more than it does for normal adult Hgbs.25,63 IRON METABOLISM Iron is an essential component not only of hemoglobin and myoglobin, but also of many enzymes. Iron containing enzymes include NADH dehydrogenase, lipoxygenases, superoxide dismutase, ribonucleotide reductase, fatty acid desaturases and phosphatases.37 These enzymes function in energy generation, prostaglandin synthesis, free radical detoxification, synthesis of DNA, synthesis of fatty acids, and signal transduction. Iron is provided to cells of the body by three sources: (1) absorption from the gut, (2) recycling from senescent RBCs, and (3) liver storage iron (Fig. 20.3).

CHAPTER 20: ERYTHROCYTE STRUCTURE AND FUNCTION

Fe3+ Fe2+ Fe2+ dCytb

DMT-1

Fe2+

Erythrocyte

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FIGURE 20.3 A schematic of iron metabolism including absorption from enterocytes, transport in circulation, utilization by the erythroid precursors, and recycling by macrophages.

Enterocyte

Haephestin Fe3+ Ferroportin Ferroportin Fe3+

Fe3+ Transferrin

Fe3+

Fe2+

Fe2+ Heme

Fe3+ DMT-1 Erythroid precursor Ferritin TfR

Macrophage Fe3+ Fe2+

Endosome Fe2+ DMT-1

Absorption in Intestine Absorption of iron is regulated; excretion is not (iron is lost through epithelial sloughing of gut, skin, urothelium). Iron absorption takes place in the proximal duodenum, and it is absorbed either as heme or non-heme iron. Although most iron utilized in physiologic mechanisms is that recycled from hemoglobin iron, total body iron stores are regulated by the duodenum to prevent whole-body iron accumulation or deficit. Intestinal Absorption of Non Heme Iron Iron is stabilized in the ferric form by the acidity of the gastric fluid. Apical duodenal cytochrome B (dCytb) reduces ferric to ferrous iron. Ferrous iron is transported to the basolateral membrane by intracellular mechanisms that are unclear. At the basolateral surface, hephaestin, a copper-containing enzyme, re-oxidizes iron to its ferric state. Ferroportin then exports iron out of the cells into the portal circulation, where it is carried bound to transferrin. Once in the circulation, iron is carried to the liver to be stored, or to the erythroid precursors in the bone marrow to be incorporated into hemoglobin. Heme iron is probably absorbed by an apical heme transporter (apical heme carrier protein 1).46 Cellular Iron Uptake Iron is taken up by most cells (i.e. not enterocytes) via the cell surface transferrin receptor (TfR). The expression level of this receptor is directly related to the iron

requirements of the cell.6,15 The cell surface transferrin receptor functions as a dimer, with each 90 kDa monomer embedded in the membrane with one single transmembrane-spanning region. Iron binding transferrin (holotransferrin) binds to the TfR at physiologic pH and the transferrin-iron-TfR complex is internalized into an endosome. The low pH of the endosome causes the release of Fe3+ from the transferrin and the iron is then reduced to Fe2+. Ferrous iron is then exported to the cytoplasm.5,34,41,47 A newly discovered protein, the product of the HFE gene, forms a complex with the TfR and appears to regulate its ability to import iron.7 Molecules Involved in Iron Metabolism The uptake of iron by the duodenum and by individual cells in the body is regulated by total body iron levels and intracellular iron levels, respectively.1 Many molecules involved in iron regulation have been discovered in recent years.11,20 Although there are still gaps in our knowledge of the physiologic processes, several important molecules have been identified in research involving humans and rodents, and are described below. Divalent Metal Iron Transporter-1 Divalent metal iron transporter-1 (DMT-1) is expressed in the enterocytes of the villi and not in the crypts,10 and is responsible for transporting iron from the enterocyte to the circulation, and from endosomes of macrophages to the cytoplasm. It is associated with phagosomal membranes in macrophages22 and plasma membranes

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and endosomes of cells that use the TfR complex for iron acquisition.22 One isoform of DMT-1 is strongly regulated by iron; it is markedly increased with an irondeficient diet and reduced by an iron replete diet.23

Inflammation, primarily through interleukin-1 and -6, and possibly transforming growth factor-beta, increases hepcidin expression, thus decreasing intestinal iron absorption and decreasing iron export from macrophages.

Ferrireductase Duodenal Cytochrome b DMT-1 transports Fe2+ into the enterocyte from the duodenal lumen. Since Fe3+ is the dietary form that exists in the lumen, it must be reduced. The duodenal cytochrome b, located on the apical membrane of the enterocyte, acts as such a reductase.18 Hephaestin Hephaestin, a copper dependent transmembrane ferroxidase, is an approximately 150–160 kDa protein with 50% homology to ceruloplasmin. It is highly expressed in the small intestine, primarily in the basolateral membrane of enterocytes, and is co-localized with ferroportin.44 Hephaestin is responsible for oxidizing Fe2+ to Fe3+ so that it can be transported out of the enterocyte into the plasma. Ferroportin Ferroportin is expressed on the membrane of duodenal enterocytes (basolateral), placental trophoblasts, and macrophages distributed in various tissues.1,13,17,18 It is the only mammalian iron exporter out of cells. It exports iron from enterocytes into the circulation (from dietary iron) and from macrophages to the circulation (from recycled erythrocyte iron). Its expression levels are regulated by hepcidin binding and internalization of the protein, and also by iron levels. Hepcidin Hepcidin should be considered a molecule that regulates iron systemically. Hepcidin regulates iron export from enterocytes and macrophages by binding to and causing degradation of ferroportin.21 It is synthesized as a pro-peptide in the liver and circulates as a 25 amino acid disulfide-rich peptide. Synthesis is increased by iron loading to decrease iron absorption, and decreased by anemia and hypoxia to increase iron absorption.42,60 It is also increased in inflammation, and causes the iron trapping in macrophages during anemia of inflammatory disease.21 Iron may control hepcidin synthesis through a signaling pathway dependent on proteins called hemojuvelin and bone morphogenic proteins (2 and 4).35 Kupffer cells also appear to regulate hepcidin expression; when Kupffer cells are eliminated, hepcidin expression is increased and serum iron is decreased.56 Anemia and hypoxia decrease hepcidin synthesis by increasing erythropoietic activity. Erythropoietin administration down-regulates hepcidin expression and this is probably due to the erythropoietic activity and not the direct effects of the hormone.32,60

Transferrin Receptor The transferrin receptor is described under the heading Cellular iron uptake. Transport and Storage of Iron Transport of Iron Transport of iron from one compartment or tissue to another through the circulation (e.g. enterocytes to bone marrow cells) is accomplished by the carrier protein transferrin. Serum transferrin is a soluble 78 kDa protein that binds one or two iron atoms in their ferric (3+) form. Storage of Iron Iron is stored in developing RBCs, and other cells, as ferritin, an iron-protein complex.38,48,49 The protein moiety, apoferritin, consists of at least 24 monomers of either H or L subunits. Each subunit is shaped like a short rod and these rods are assembled as a hollow sphere. Apoferritin has a molecular weight of 441 kDa and can store as much as 4500 molecules (31%) of iron.34 The maximal weight of ferritin can be approximately 800 kDa, but ferritin more commonly is less than that (620 kDa) with roughly 18% iron. The method of iron movement in and out of the molecule is not precisely known, but it apparently enters as the ferrous form and is oxidized to and stored as the ferric form.12 Iron is released either by a reversal of the process, or by digestion of ferritin by lysosomes and reduction to the ferrous form.24 Hemosiderin is found in cells of the monocytemacrophage system, predominantly in macrophages of the liver (Kupffer cells), spleen, and bone marrow. Hemosiderin can be seen as golden granules in hematoxylin and eosin stained histologic sections, and may appear green to black in Wright-Giemsa stained cytologic preparations. It is a “stripped down” version of ferritin, whereby the apoferritin protein shell has been removed, leaving approximately 25–30% of iron by weight.19 Genetic Control of Iron Regulatory Proteins Proteins involved in iron metabolism are predominantly regulated at the post-translational level by the interplay of iron-responsive elements (IREs) and cytosolic iron-binding proteins referred to as iron regulatory proteins (IRPs). The IREs are stem-loop or hairpin structures located in the 3′ or 5′ untranslated regions of the messenger RNAs (mRNAs) that act as nucleic acid binding sites for the IRPs. The binding affinity of IRPs is reversibly regulated by the intracellular

CHAPTER 20: ERYTHROCYTE STRUCTURE AND FUNCTION

concentration of iron. When the intracellular concentration of iron is low, IRPs have a high affinity for IREs, and when cells are iron replete, the affinity is low. Ferritin and aminolevulinate synthase (rate limiting enzyme in heme biosynthesis) mRNAs have a single IRE in the 5′ end at the beginning of the coding region. Binding of the IRP inhibits translation; thus when iron is low, the IRP binds to the IRE and translation is inhibited, whereas when iron is high, IRPs lose affinity and translation of those proteins occurs.2,52,53,58,61 Thus, when ferritin is required for storage of increased intracellular iron, translation is up-regulated. On the other hand, the transferrin receptor mRNA has an IRE on the 3′ end. When iron is low, the binding of the IRP inhibits the breakdown of the mRNA and thereby stimulates the translation of the protein. Thus, low iron up-regulates TfR so that cells become more iron avid. In summary, IRPs regulate mRNA translation by binding to IREs, and binding is stimulated by low iron. However, sometimes binding stimulates translation (TfR) and sometimes binding inhibits translation (ferritin).40

REFERENCES 1. Abboud S, Haile DJ. A novel mammalian iron-regulated protein involved in intracellular iron metabolism. J Biol Chem 2000;275:19906–19912. 2. Andrew GA, Chavey PS, Smith JE. Enzyme-linked immunosorbent assay to measure serum ferritin and the relationship between serum ferritin and nonheme iron stores in cats. Vet Pathol 1994;31:674–678. 3. Aufderheide WM, Parker HR, Kaneko JJ. The metabolism of 2,3-diphosphoglycerate in the developing sheep (Ovis aries). Comp Biochem Physiol 1980;65A:393–398. 4. Barker JE, Pierce JE, Nienhuis AW. Hemoglobin switching in sheep: a comparison of the erythropoietin-induced switch to HbC and the fetal to adult hemoglobin switch. Blood 1980;56:488–494. 5. Baynes RD, Shih YJ, Hudson BG, et al. Production of the serum form of the transferrin receptor by a cell membrane-associated serine protease. Proc Soc Exp Biol Med 1993;204:65–69. 6. Beguin Y, Clemons GK, Pootrakul P, et al. Quantitative assessment of erythropoiesis and functional classification of anemia based on measurements of serum transferrin receptor and erythropoietin. Blood 1993;814:1067–1076. 7. Bhatt L, Horgan CP, McCaffrey MW. Knockdown of 2-microglobulin perturbs the subcellular distribution of HFE and hepcidin. Biochem Biophys Res Commun 2009;378:727–731. 8. Braend M. Hemoglobin polymorphism in the domestic dog. J Hered 1988;79:211–212. 9. Bunn HF, Kitchen H. Hemoglobin function in the horse: the role of 2,3-diphosphoglycerate in modifying the oxygen affinity of maternal and fetal blood. Blood 1973;42:471–479. 10. Canonne-Hergaux F, Gruenheid S, Ponka P, et al. Cellular and subcellular localization of the Nramp2 iron transporter in the intestinal brush border and regulation by dietary iron. Blood 1999;93:4406–4417. 11. Canonne-Hergaux F, Zhang AS, Ponka P, et al. Characterization of the iron transporter DMT1 (NRAMP2/DCT1) in red blood cells of normal and anemic mk/mk mice. Blood 2001;98:3823–3830. 12. Crichton RR, Charloteaux-Wauters M. Iron transport and storage. Eur J Biochem 1987;164:485–506. 13. D’Ann MC, Veuthey TV, Roque ME. Immunolocalization of ferroportin in healthy and anemic mice. J Histochem Cytochem 2009;57:9–16. 14. Daleke DL, Lyles JV. Identification and purification of aminophospholipid flippases. Biochim Biophys Acta 2000;1486:108–127. 15. Das Gupta A, Abbi A. High serum transferrin receptor level in anemia of chronic disorders indicates coexistent iron deficiency. Am J Hematol 2003;72:158–161. 16. de Schepper J. Degradation of haemoglobin to bilirubin in the kidney of the dog. Tijdschr Diergeneeskd 1974;99:699–707. 17. Donovan A, Brownlie A, Zhou Y, et al. Positional cloning of zebrafish ferroportin1 identifies a conserved vertebrate iron exporter. Nature 2000;403:776–781. 18. Donovan A, Lima CA, Pinkus JL, et al. The iron exporter ferroportin/ Slc40a1 is essential for iron homeostasis. Cell Metab 2005;1:191–200. 19. Fairbanks VF, Beutler E. Iron metabolism. In: William’s Hematology. Beutler E, Lichtman MA, Coller BS, Kipps TJ, eds. New York: McGraw-Hill, 1995;369–380.

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20. Ferguson BJ, Skikne BS, Simpson KM, et al. Serum transferrin receptor distinguishes the anemia of chronic disease from iron deficiency anemia. J Lab Clin Med 1992;119:385–390. 21. Ganz T. Hepcidin and its role in regulating systemic iron metabolism. In: Hematology. Washington DC: American Society of Hematology. 2006;29–35. 22. Gruenheid S, Canonne-Hergaux F, Gauthier S, et al. The iron transport protein NRAMP2 is an integral membrane glycoprotein that colocalizes with transferrin in recycling endosomes. J Exp Med 1999;189:831–841. 23. Gunshin H, Allerson CR, Polycarpou-Schwarz M, et al. Iron-dependent regulation of the divalent metal ion transporter. FEBS Lett 2001;509:309–316. 24. Harrison PM, Treffry A, Lilley TH. Ferritin as an iron-storage protein: mechanisms of iron uptake. J Inorg Biochem 1986;27:287–293. 25. Huisman TH, Kitchens J. Oxygen equilibria studies of the hemoglobins from normal and anemic sheep and goats. Am J Physiol 1968;215:140–146. 26. Jain NC. Schalm’s Veterinary Hematology, 4th ed. Philadelphia: Lea & Febiger, 1986. 27. Jandl JH. Blood: Textbook of Hematology. Boston: Little Brown, 1987. 28. Jansen PL, Chowdhury JR, Fischberg EB, et al. Enzymatic conversion of bilirubin monoglucuronide to diglucuronide by rat liver plasma membranes. J Biol Chem 1977;252:2710–2716. 29. Kamata K, Manno S, Ozaki M, et al. Functional evidence for presence of lipid rafts in erythrocyte membranes: Gsα in rafts is essential for signal transduction. Am J Hematol 2008;83:371–375. 30. Kitchen H. Heterogeneity of animal hemoglobins. Adv Vet Sci Comp Med 1969;13:247–330. 31. Kitchen H, Brett I. Embryonic and fetal hemoglobin in animals. Ann NY Acad Sci 1974;241:653–671. 32. Kong WN, Chang Y, Wang M, et al. Effect of erythropoietin on hepcidin, DMT1 with IRE, and hephaestin gene expression in duodenum of rats. J Gastroenterol 2008;43:136–143. 33. Kuypers FA. Phospholipid asymmetry in health and disease. Curr Opin Hematol 1998;52:122–131. 34. Leibold EA, Guo B. Iron-dependent regulation of ferritin and transferrin receptor expression by the iron-responsive element binding protein. Ann Rev Nutr 1992;12:345–368. 35. Lin L, Valore EV, Nemeth E, et al. Iron transferrin regulates hepcidin synthesis in primary hepatocyte culture through hemojuvelin and BMP2/4. [see comment]. Blood 2007;110:2182–2189. 36. Mauk AG, Taketa F. Effects of organic phosphates on oxygen equilibria and kinetics of -SH reaction in feline hemoglobins. Arch Biochem Biophys 1972;150:376–381. 37. Mims MP, Prchal JT. Divalent metal transporter 1. Hematology 2005;10:339–345. 38. Miyata Y, Furugouri K, Shijimaya K. Developmental changes in serum ferritin concentration of dairy calves. J Dairy Sci 1984;67:1256–1263. 39. Montes LR, Lopez DJ, Sot J, et al. Ceramide-enriched membrane domains in red blood cells and the mechanism of sphingomyelinase-induced hotcold hemolysis. Biochemistry 2008;47(43):11222–11230. 40. Muckenthaler MU, Galy B, Hentze MW. Systemic iron homeostasis and the iron-responsive element/iron-regulatory protein (IRE/IRP) regulatory network. Ann Rev Nutr 2008;28:197–213. 41. Nathanson MH, Muir A, McLaren GD. Iron absorption in normal and iron-deficient beagle dogs: mucosal iron kinetics. Am J Physiol Gastrointest Liver Physiol 1985;249: G439–448. 42. Oates PS, Ahmed U. Molecular regulation of hepatic expression of iron regulatory hormone hepcidin. J Gastroenterol Hepatol 2007;22:1378– 1387. 43. Pennel RB. Composition of normal human red cells. In: Surgenor DM, ed. The Red Blood Cell. New York: Academic Press, 1:93–146. 44. Petrak J, Vyoral D. 2005; Hephaestin – a ferroxidase of cellular iron export. Intl J Biochem Cell Biol 1974;37:1173–1178. 45. Salzer U, Prohaska P. Stomatin, flotillin-1, and flotillin-2 are major integral proteins of erythrocyte lipid rafts. Blood 2001;97:1141–1143. 46. Shayeghi M, Latunde-Dada GO, Oakhill JS, et al. Identification of an intestinal heme transporter. Cell 2005;122:789–801. 47. Shih Y, Baynes RD, Hudson BG, et al. Serum transferrin receptor is a truncated form of tissue receptor. J Biol Chem 1990;26531:19077– 190781. 48. Smith JE., Moore K, Boyington D. Enzyme immunoassay for serum ferritin of pigs. Biochem Med 1983;29:293–297. 49. Smith JE, Moore K, Cipriano JE, et al. Serum ferritin as a measure of stored iron in horses. J Nutr 1984;114(4): 677–681. 50. Soupene E, Kuypers FA. Identification of an erythroid ATP-dependent aminophospholipid transporter. Br J Haematol 2006;133:436–438. 51. Spengler MI, Bertoluzzo SM, Catalani G, et al. Study on membrane fluidity and erythrocyte aggregation in equine, bovine and human species. Clin Hemorheol Microcirculat 2008;38:171–176. 52. Steinberg JD, Olver CS. Hematologic and biochemical abnormalities indicating iron deficiency are associated with decreased reticulocyte hemoglobin content (CHr) and reticulocyte volume (rMCV) in dogs. Vet Clin Pathol 2005;34:23–27.

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53. Stoffman N, Brugnara C, Woods ER. An algorithm using reticulocyte hemoglobin content (CHr) measurement in screening adolescents for iron deficiency. J Adolescent Health 2005;36:529.e1–529.e6. 54. Stohlman F Jr., Howard D, Beland A. Humoral regulation of erythropoiesis. XII. Effect of erythropoietin and iron on cell size in iron deficiency anemia. Proc Soc Exp Biol Med 1963;113:986–988. 55. Tennant BC. Hepatic function. In: Kaneko JJ, Harvey JW, Bruss ML, eds. Clinical Biochemistry of Domestic Animals. San Diego: Academic Press: pp. 329–333. 56. Theurl M, Theurl I, Hochegger K, et al. 2008; Kupffer cells modulate iron homeostasis in mice via regulation of hepcidin expression. J Mol Med 1997;86:825–835. 57. Traugh JA. Heme regulation of hemoglobin synthesis. Semin Hematol 1989;26:54–62.

58. Ullrich C, Wu A, Armsby C, et al. “Screening healthy infants for iron deficiency using reticulocyte hemoglobin content. J Am Med Assoc 2005;294:924–930. 59. Van Deenen L, de Gier J. Lipids of the red cell membrane. In: Surgenor DM, ed. The Red Blood Cell. New York: Academic Press. 1:147–211. 60. Vokurka M, Krijt J, Sulc K, et al. 2006; Hepcidin mRNA levels in mouse liver respond to inhibition of erythropoiesis. Physiol Res 1974;55:667–674. 61. Weeks BR, Smith JE, Phillips RM. Enzyme-linked immunosorbent assay for canine serum ferritin, using monoclonal anti-canine ferritin immunoglobulin G. Am J Vet Res 1988;49:1193–1195. 62. Winkelmann JC, Forget BG. Erythroid and nonerythroid spectrins. Blood 1993;81:3173–3185. 63. Winslow RM, Swenberg ML, Benson J, et al. Gas exchange properties of goat hemoglobins A and C. J Biol Chem 1989;264:4812–4817.

CHAPTER

21

Erythrocyte Biochemistry JOHN W. HARVEY Membrane Transport Carbohydrate Metabolism Embden-Meyerhof Pathway and ATP Production Diphosphoglycerate Pathway and Oxygen Affinity of Hemoglobin

Oxidants and Oxidative Injury Pentose Phosphate Pathway and Protection Against Oxidant Methemoglobin Formation and Reduction

Acronyms and Abbreviations Cb5R, cytochrome-b5-reductase; DPGM, diphosphoglycerate mutase; DPGP, diphosphoglycerate phosphatase activity; 1,3DPG, 1,3-diphosphoglycerate; 2,3DPG, 2,3-diphosphoglycerate; EMP, Embden-Meyerhof pathway; FAD, flavin adenine dinucleotide; GAPD, glyceraldehyde phosphate dehydrogenase; G6P, glucose 6-phosphate; G6PD, glucose-6-phosphate dehydrogenase; GPx, glutathione peroxidase; GR, glutathione reduction; GSH, reduced glutathione; GSSG, oxidized glutathione; HK, hexokinase; HK+, high potassium; LK+, low potassium; LMB, leukomethylene blue; MB, methylene blue; O −2 , superoxide; PFK, phosphofructokinase; 3PG, 3-phosphoglycerate; Pi, inorganic phosphate; PGK, phosphoglycerate kinase; PK, pyruvate kinase; PPP, pentose phosphate pathway; RBC, red blood cell; RNS, reactive nitrogen species; ROS, reactive oxygen species; SOD, superoxide dismutase.

E

rythrocytes or red blood cells (RBCs) provide vital functions of oxygen transport, carbon dioxide transport, and buffering of hydrogen ions. These functions do not require energy per se, but energy in the form of ATP, NADH, and NADPH is needed to keep the cells circulating for months in a functional state despite repeated exposures to mechanical and metabolic insults. Mature mammalian RBCs do not have nuclei; consequently, they cannot synthesize nucleic acids or proteins. The loss of mitochondria during the maturation of reticulocytes results in a loss of Krebs’ cycle and oxidative phosphorylation capabilities, and prevents the synthesis of heme or lipids de novo in RBCs. Energy needs of mature RBCs are met solely by anaerobic glycolysis in the Embden-Meyerhof pathway (EMP). Although metabolic demands are lower than in other blood cell types, RBCs still require energy in the form of ATP for maintenance of shape, deformability, phosphorylation of membrane phospholipids and proteins, active membrane transport of various molecules, partial synthesis of purine and pyrimidine nucleotides, and synthesis of glutathione.9 A minor shunt for carbohydrate metabolism, the pentose phosphate pathway, pro-

vides the RBC with additional critical protection against oxidative damage. MEMBRANE TRANSPORT The RBC lipid bilayer is impermeable to most molecules. Consequently, various membrane protein transport systems are utilized for movement of molecules into and out of RBCs. Water and carbon dioxide are transported across RBC membranes using water channels called aquaporins.4 Band 3 appears to function as a channel for the movement of anions, especially bicarbonate and chloride, certain non-electrolytes, and probably cations to some extent. Defective anion transport and marked spherocytosis with membrane instability occurs in anemic cattle with an inherited deficiency of band 3 protein (see Chapter 29).11 Major interspecies, and in some cases intraspecies, differences occur in cation transport and subsequently in intracellular Na+ and K+ concentrations.3 Animal species with high intracellular K+ concentrations (horse, pig, and some ruminants) have an active Na+,K+-pump that exchanges intracellular Na+ for extracellular K+ 131

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with the hydrolysis of ATP. In addition to individuals with high potassium (HK+) RBCs, some sheep, goats, buffalo, and most cattle RBCs have relatively low potassium (LK+) and, consequently, high sodium RBCs. These LK+ RBCs have low Na+,K+-pump activity and high passive K+ permeability. Red blood cells from cats and most dogs do not have Na+,K+-pump activity and have Na+ and K+ concentrations near those predicted for the Donnan equilibrium with plasma. However, many clinically normal Japanese and Korean dogs have HK+ RBCs.5 Red blood cells from these dogs have substantial Na+,K+-ATPase activity, and some of these dogs also have increased glutamate transport, which results in high reduced glutathione (GSH) concentrations. These HK+, high GSH RBCs promote Babesia gibsoni replication compared to LK+, normal GSH RBCs.24 Other pathways of sodium and potassium transport occur to variable degrees in certain species.9 Excessive intracellular Ca2+ promotes the suicidal death of RBCs (eryptosis);13 consequently, RBCs actively extrude Ca2+ using a calcium pump having Ca2+activated, Mg2+-dependent ATPase activity. The calcium pump is activated by a calcium-binding protein called calmodulin.9 Amino acid transport in RBCs provides amino acids for synthesis of glutathione. In addition, amino acid transporters may be responsible for efflux of amino acids during reticulocyte maturation.21 Species vary in their permeability to glucose, with human RBCs being very permeable and pig RBCs being poorly permeable. Red blood cells of other domestic animals appear to be intermediate between these extremes.9 Glucose movement into RBCs is not regulated by insulin; rather facilitative glucose transporters mediate the passive diffusion of glucose into RBCs.18 Red blood cells from adult pigs lack a functional glucose transporter and, therefore, have limited ability to utilize glucose for energy.2 Red blood cell membranes from most animal species have a nucleoside transporter. Rabbit, pig, and human RBCs exhibit substantially more adenosine uptake than those of other species studied. Red blood cells from dogs exhibit more adenosine uptake than those of cats, goats, or cattle, and RBCs from horses and most sheep appear to be nearly impermeable to adenosine. While dog RBCs are permeable to adenosine, they are impermeable to inosine. Dog and cat RBCs exhibit adenine uptake and incorporation into nucleotides, but values are much lower than those of human, rabbit, or rodent RBCs.9 CARBOHYDRATE METABOLISM Although substrates such as ribose, fructose, mannose, galactose, dihydroxyacetone, glyceraldehyde, adenosine, and inosine may be metabolized to some extent, depending on the species, glucose is the primary substrate for energy needs of RBCs from all species except the pig. Inosine appears to be the major substrate for pig RBCs. Its production by the liver is sufficient to meet RBC energy requirements.25

Once glucose enters the cell, it is phosphorylated to glucose 6-phosphate (G6P) utilizing the hexokinase (HK) enzyme. The G6P is then metabolized through either the Embden-Meyerhof pathway (EMP) or the pentose phosphate pathway (PPP) as shown in Figure 21.1. EMBDEN-MEYERHOF PATHWAY AND ATP PRODUCTION A net of two molecules of ATP is produced for each molecule of glucose metabolized to two molecules of lactate in the EMP. Because mature RBCs lack mitochondria, the EMP is the only source of ATP production in these cells. Reactions catalyzed by HK, phosphofructokinase (PFK), and pyruvate kinase (PK) appear to be rate-limiting steps in glycolysis, with the PFK reaction being most important under physiologic steady-state conditions.9 At physiologic pH values, high concentrations of inorganic phosphate (Pi) stimulate glycolysis through the EMP by reducing the ATP inhibition of PFK. Conversely, glycolysis is inhibited by short-term phosphate deficiency, primarily by decreasing intracellular Pi for glyceraldehyde phosphate dehydrogenase (GAPD). Decreased glycolytic rates result in decreased RBC ATP concentrations and hemolytic anemia in experimental dogs made severely hypophosphatemic by hyperalimentation. Hemolytic anemia associated with hypophosphatemia has also been reported in diabetic cats and a diabetic dog following insulin therapy, in a cat with hepatic lipidosis, and in postparturient cattle in which decreased RBC ATP concentrations have been measured.9 Because mature RBCs depend solely on anaerobic glycolysis for ATP generation, deficiencies of enzymes involved in glycolysis result in shortened RBC survival. Insufficient ATPgeneration in deficient RBCs can result in echinocyte formation. PK-deficient dogs and cats have mild to severe regenerative hemolytic anemia (see Chapter 28). PK-deficient dogs exhibit marked iron accumulation in the liver and die from liver failure or myelofibrosis.8 PFK-deficient dogs have compensated hemolytic anemia with sporadic episodes of intravascular hemolysis and hemoglobinuria (see Chapter 28).8 PFK-deficient dog RBCs are alkaline fragile, because 2,3-diphosphoglycerate (2,3DPG) concentration is decreased in these cells. A decrease in 2,3DPG, the major impermeant anion in dog RBCs, results in higher intracellular pH. Episodes of intravascular hemolysis occur when PFK-deficient dogs develop alkalemia secondary to hyperventilation.8 DIPHOSPHOGLYCERATE PATHWAY AND OXYGEN AFFINITY OF HEMOGLOBIN Molecules of 1,3-diphosphoglycerate (1,3DPG), produced by the GAPD reaction, may be utilized by the phosphoglycerate kinase (PGK) reaction in the EMP or

CHAPTER 21: ERYTHROCYTE BIOCHEMISTRY

FIGURE 21.1 Metabolic pathways of the mature

O2– + O2– + 2H+

RBCs. HK, hexokinase; GPI, glucose phosphate isomerase; PFK, phosphofructokinase; TPI, triosephosphate isomerase; GAPD, glyceraldehyde-3-phosphate dehydrogenase; PGK, phosphoglycerate kinase; MPGM, monophosphoglycerate mutase; DPGM, diphosphoglycerate mutase; PK, pyruvate kinase; G6PD, glucose-6-phosphate dehydrogenase; 6PGD, 6-phosphogluconate dehydrogenase; LDH, lactate dehydrogenase; LMB, leukomethylene blue; MB, methylene blue; GR, glutathione reductase; GPx, glutathione peroxidase; TK, transketolase; TA, transaldolase; GSSG, oxidized glutathione; G6P, glucose 6-phosphate; F6P, fructose 6-phosphate; FDP, fructose 1,6-diphosphate; DHAP, dihydroxyacetone phosphate; GAP, glyceraldehyde 3-phosphate; 1,3-DPG, 1,3-diphosphoglycerate; 2,3-DPG, 2,3-diphosphoglycerate; 3PG, 3-phosphoglycerate; 2PG, 2-phosphoglycerate; PEP, phosphoenolpyruvate; ADP, adenosine diphosphate; ATP, adenosine triphosphate; NAD, nicotinamide adenine dinucleotide; NADH, reduced nicotinamide adenine dinucleotide; NADP, nicotinamide adenine dinucleotide phosphate; NADPH, reduced nicotinamide adenine dinucleotide phosphate; NADPH-D, reduced nicotinamide adenine dinucleotide phosphate diaphorase; GSH, reduced glutathione; Pi, inorganic phosphate; SOD, superoxide dismutase. (Courtesy of John Harvey. This figure was published in Clinical Biochemistry of Domestic Animals, 6th ed., p. 196. ©Elsevier 2008.)

SOD O2 H2O2

GPx

GSH

ATP

2H2O GSSG

NADP

HK

Glucose

GR

NADPH 6PG G6PD NADP

G6P GPI

ADP

NADPH

TK + TA FDP TPI

MB

CO2

PFK

DHAP

LMB NADPH-D

6PGD

F6P ATP

ADP

Pentose PO4

G3P NAD + Pi GAPD

NADH 1,3DPG ADP DPGM 2,3DPG

PGK ATP

Pi

3PG ATP

ADP 2PG

PEP

133

PK

NADH Pyruvate

may be converted to 2,3DPG by the diphosphoglycerate mutase (DPGM) reaction (Fig. 21.1). 2,3DPG degradation to 3-phosphoglycerate (3PG) is catalyzed by diphosphoglycerate phosphatase activity (DPGP). The DPG pathway or shunt bypasses the ATP-generating PGK step in glycolysis; consequently, no net ATP is generated when glucose is metabolized through this pathway. Red blood cells of dogs, horses, pigs, and man normally contain high concentrations of 2,3DPG, whereas those of cats and domestic ruminants have low concentrations. In RBCs from most mammalian species, 2,3DPG decreases the oxygen affinity of hemoglobin. When the oxygen affinity of hemoglobin is studied in hemolysates, removal of 2,3DPG and ATP (termed “stripping”) from species with low 2,3DPG RBCs results in considerably lower oxygen affinities than removal from hemoglobin of species with high 2,3DPG RBCs. Because stripped hemoglobins from species with high 2,3DPG RBCs have high oxygen affinities, it appears that 2,3DPG is needed within RBCs of these species to maintain hemoglobin oxygen affinity within a physiologically useful range.9 The flow through the DPG pathway is regulated by the overall glycolytic rate. The formation of 2,3DPG is stimulated by increased phosphate concentration and increased pH. Hypoxic conditions stimulate 2,3DPG

NAD

LDH

Lactate

synthesis primarily by inducing hyperventilation with resulting alkalosis. Conversely, acidosis and hypophosphatemia result in decreased 2,3DPG concentrations. PFK deficiency inhibits glycolysis above the DPG shunt and results in decreased 2,3DPG concentration. In contrast, the concentration of 2,3DPG is increased in dogs with PK deficiency because the metabolic block occurs below the DPG shunt.8 Erythrocyte 2,3DPG increases in anemic dogs and cats. The resultant decrease in hemoglobin oxygen affinity would seem to be beneficial in response to anemia in the dog. 2,3DPG concentration is much lower in cat RBCs than in dog erythrocytes, and cat hemoglobins are generally less responsive to 2,3DPG than dog hemoglobin; consequently, the physiologic significance of this increase in cats in unclear.9 Increased 2,3DPG has also been reported in RBCs from horses with hypoxic conditions.6 In the case of severe hypoxic hypoxemia the response might be detrimental, because hemoglobin cannot be fully saturated.12 OXIDANTS AND OXIDATIVE INJURY Animals are exposed to low levels of oxidants in their environment and from normal metabolic processes in

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the body. Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are formed as products of normal cellular metabolism. At low to moderate concentrations, nitric oxide and superoxide (O2−) free radicals are involved in signal tranductions between cells. When generated at higher concentrations in disease states, these free radicals (and even more potent oxidative metabolites that they produce) can overwhelm protective systems within the body, producing cellular injury and/or destruction.22 Cumulative injury from these exposures may account for the normal aging and removal of circulating RBCs (see Chapter 22). Some metabolic disorders, including diabetes, inflammation, hyperthyroidism, neoplasia, RBC parasites, intense exercise, and ischemia/reperfusion can generate oxidants in sufficient amounts to result in increased oxidant injury and shortened erythrocyte lifespans, but the anemia (when present) is generally mild (see Chapter 36). A wide variety of drugs and environmental agents either exist as free radicals or can be converted to free radicals by cellular metabolic processes. These free radicals can be more damaging than ROS and RNS.9 Oxidants may damage RBC hemoglobin, enzymes (especially sulfhydryl groups), and membranes (especially polyunsaturated lipids). Methemoglobin forms when hemoglobin iron is oxidized from the ferrous to the ferric state. Methemoglobin is unable to bind oxygen, but its presence alone does not result in shortened erythrocyte lifespan.9 Heinz bodies are inclusions that form within RBCs following the oxidative denaturation of the globin portion of hemoglobin (see Chapter 36). They bind to the inner surface of RBC membranes, and their presence can result in premature RBC phagocytosis. Heinz bodies are frequently recognized in RBCs from cats because of the susceptibility of cat hemoglobin to form Heinz bodies, combined with a poor ability of the cat spleen to remove Heinz bodies from RBCs. Even normal cats may have low numbers of Heinz bodies (30 μg/dL (1.45 μmol/L) that is currently regarded as diagnostic for lead poisoning in children. Marked elevations in RBC PROTO IX are also found in experimental lead poisoning in calves.6 Modern fluorometers specifically designed to measure porphyrins have greatly simplified the assay. For these reasons, the current test of choice to monitor lead exposure is the blood ZnPROTO IX concentration. The final diagnosis of lead poisoning ultimately rests upon the measurement of blood lead concentration and this is best done using atomic absorption spectrophotometry. It is clear that the heme synthetic pathway is affected by blood lead concentrations well below those that are considered normal. The reported reference interval for blood lead in the dog is 10–50 μg/dL (0.48– 2.41 μmol/L) and a blood lead concentration of >60 μg/ dL (2.90 μmol/L) is diagnostic of lead poisoning.25 In the light of current knowledge, a blood level of 30 μg/ dL (1.45 μmol/L) should be considered diagnostic of

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lead poisoning in the dog as well as in all veterinary species.

REFERENCES 1. Badcock NR, O’Reilly DA, Zoanetti GD, et al. Childhood porphyrias: implication and treatments. Clin Chem 1993;39:1334–1340. 2. Badcock NR, Szep DA, Zoanetti GD, et al. Fecal coproporphyrin isomers in sporadic and familial porphyria cutanea tarda. Clin Chem 1995;41:1315–1317. 3. Brandt A, Doss M. Hereditary porphobilinogensynthase deficiency in humans associated with acute hepatic porphyria. Human Genet 1981;58:194–197. 4. Farant JP, Wigfield DC. Biomonitoring lead exposure with deltaaminolevulinate activity ratios. Intl Arch Occup Environ Health 1982;51:15–24. 5. Flyger V, Levin EY. Anim model: Normal porphyria of fox squirrels (Sciurus niger). Am J Pathol 1977;87:269–272. 6. George JW, Duncan JR. Erythrocyte protoporphyrin in experimental lead poisoning in calves. Am J Vet Res 1981;42:1630–1637. 7. George JW, Duncan JR. Pyrimidine 5’ nucleotidase activity in bovine erythrocytes: Effect of phlebotomy and lead poisoning. Am J Vet Res 1982;43:17–20. 8. Giddens WE, Jr, Labbe RF, Swango LJ, et al. Feline congenital erythropoietic porphyria associated with severe anemia and renal disease. Am J Pathol 1975;80:367–386. 9. Hift RJ, Davidson BP, Van der Hooft C, et al. Plasma fluorescence scanning and fecal porphyrin analysis for the diagnosis of variegate porphyria: precise determination of sensitivity and specificity with detection of protoporphyrinogen oxidase mutations as a reference standard. Clin Chem 2004;50:915–923. 10. Kaneko JJ. Porphyrins and the porphyrias. In: Kaneko JJ, Harvey JW, Bruss ML, eds. Clinical Biochemistry of Domestic Animals, 6th ed. San Diego: Elsevier, 2008. 11. Kaneko JJ, Mills R. Erythrocyte enzyme activity, ion concentration, osmotic fragilitiy, and glutathione stability in bovine bovine erythropoietic porphyria and its carrier state. Am J Vet Res 1969;30:1805–1810. 12. Kaneko JJ, Matthews DRG. Iron metabolism in normal and porphyric calves. Am J Vet Res 1966;27:923–929.

13. Kaneko JJ, Zinkl JG, Keeton KS. Erythrocyte porphyrin and erythrocyte survival in bovine erythropoietic porphyria. Am J Vet Res 1971;32:1981–1985. 14. Levin EY. Uroporphyrinogen 3 cosynthetase activity in bovine erythropoietic porphyria. Science 1968;161:907–908. 15. McManus J, Blake D, Ratnaike S. Assay of uroporphyrinogen decarboxylase in erythrocytes. Clin Chem 1988;34:2355–2357. 16. Poh-Fitzpatrick MB. Pathogenesis and treatment of photocutaneous manifestations of the porphyrias. Semin Liver Dis 1982;2:164–176. 17. Pimstone NR. Porphyria cutanea tarda. Semin Liver Disease 1982;11:132–142. 18. Romeo G. Enzymatic defects of hereditary porphyrias: An explanation of dominance at the molecular level. Hum Genet 1977;39:261–276. 19. Romeo G, Glenn BC, Levin EY. Uroporphyrinogen 3 cosynthetase in asymptomatic carriers of congenital erythropoietic porphyria. Biochem Genet 1970;4:719–726. 20. Rudolph WG, Kaneko JJ. Kinetics of erythroid bone marrow cells of normal and porphyric calves in vitro. Acta Haematol 1971;45:330–335. 21. Ruth GR, Schwartz S, Stephenson B. Bovine protoporphyria: first nonhuman model of this hereditary photosensitizing disease. Science 1977;198:199–201. 22. Sassa S. Modern diagnosis and management of the porphyrias. Br J Haematol 2006;135:281–292. 23. Sassa S, Kappas A. Mol aspects of the inherited porphyrias. J Intern Med 2000;247:169–178. 24. Smith JJ, Kaneko JJ. Rate of heme and porphyrin synthesis by bovine porphyric reticulocytes in vitro. Am J Vet Res 1966;27:931–940. 25. Zook BC, McConnell G, Gilmore CE. Basophilic stippling of erythrocytes in dogs with special reference to lead poisoning. J Am Vet Med Assoc 1970;157:2092–2099. 26. Valentine WN, Paglia DE, Fink K, et al. Lead poisoning: Association with hemolytic anemia, basophilic stippling, erythrocyte pyrimidine 5’ nucleotidase deficiency, and intranuclear accumulations of pyrimidines. J Clin Invest 1976;58:926–932. 27. Zuijderhoudt FM, Kamphuis JS, Kluitenberg WE, et al. Precision and accuracy of a HPLC method for measurement of fecal porphyrin concentrations. Clin Chem Lab Med 2002;40:1036–1039. 28. Zuijderhoudt FM, Koehorst S, Kluitenberg WE, et al. On accuracy and precision of a HPLC method for measurement of urine porphyrin concentrations. Clin Chem Lab Med 2000;38:227–230.

CHAPTER

28

Hereditary Erythrocyte Enzyme Abnormalities URS GIGER Phosphofructokinase Deficiency Canine Phosphofructokinase Deficiency

Pyruvate Kinase Deficiency Canine Pyruvate Kinase Deficiency Feline Pyruvate Kinase Deficiency

Acronyms and Abbreviations ADP, adenosine diphosphate; ATP, adenosine triphosphate; BFU-E, burst forming units-erythroid; CFU-E, colony forming unit-erythroid; DNA, deoxyribonucleic acid; DPG, diphosphoglycerate; EDTA, ethylenediaminetetraacetic acid; G6PD, glucose-6-phosphate dehydrogenase; L-M-P-PFK, liver, muscle, platelet phosphofructokinase; L-M1/ M2-R-PK, liver, muscle, red blood cell pyruvate kinase; PFK, phosphofructokinase; PK, pyruvate kinase; RBC, red blood cell.

A

s the committed erythroid precursor cells proliferate and develop into mature red blood cells (RBCs) in the bone marrow, their cellular metabolism undergoes many changes. While the burst forming units-erythroid (BFU-E) and colony forming unit-erythroid (CFU-E) cells are geared to massively expand the erythroid cell line, which is under the control of erythropoietin, major metabolic and structural shifts occur, particularly during the later erythroid stages, to achieve the highly specialized functions of mature RBCs (see Chapters 6, 20, and 21).40 To that end, during rubricyte and metarubricyte (also known as normoblasts) stages until the reticulocyte stage, heme and alpha- and betaglobin syntheses are in full force to produce hemoglobin, reaching one-third of a mature RBC’s mass. Heme synthesis occurs in both mitochondria and cytoplasm involving several enzymatic steps. Hereditary enzymatic defects in heme synthesis, also known as porphyrias, cause tissue (bone and teeth) accumulation and urinary as well as fecal excretion of varied porphyrins, and can lead to hematological, neurological, gastrointestinal, and cutaneous signs. Porphyrias and their classification and characterization to the molecular level in cats, pigs, sheep, and cattle are described in Chapter 27. While sickle cell disease and thalassemia are the most common RBC disorders in humans, interestingly, no hemoglobinopathies causing anemia have thus far been documented in domestic animals. However, hereditary methemoglobinemias, either due to a cytochrome b5 (methemoglobin) reductase deficiency or a yet unknown mechanism, have been observed in dogs, cats, and even a Rhesus monkey, and are typically asso-

ciated with cyanosis and erythrocytosis rather than anemia (see Primary erythrocytosis in Chapter 25).22,32,34,39 Glucose-6-phosphate dehydrogenase (G6PD) of the hexose-monophosphate shunt is the most common hereditary (and only x-chromosomal) RBC enzyme deficiency in humans (Fig. 28.1); these patients are generally not anemic, but can develop hemolytic crises with oxidative stress (drugs, infections, flava beans [flavism]).2 While in a large canine survey only a single Weimeraner dog was found to have a reduced G6PD activity without any hematological abnormalities,71 an American saddlebred colt with G6PD had anemia with eccentrocytosis.39,59 Furthermore, horses with RBC flavin adenine dinucleotide deficiency have both eccentrocytosis (attributable to severe deficiency in glutathione reductase activity) and methemoglobinemia (attributable to cytochrome b5 reductase deficiency); the dual enzyme deficiency occurs because flavin adenine dinucleotide is a required cofactor for both enzymes.29 Although various erythrocytic membrane defects have also been described affecting cytoskeletal proteins, such as spectrin and band 4.1, others remain undefined and may involve enzymatic pumps to maintain electrolyte and calcium homeostasis as suspected for stomatocytosis in the Alsaka Malamute and Miniature and Middle Schnauzer (see Chapter 29).76 Devoid of a nucleus and mitochondria the metabolism of the mature mammalian RBC is fairly restricted, and its energy is solely generated by anaerobic glycolysis, also known as the Embden-Meyerhof pathway (Fig. 28.1; see Chapter 21).40 Metabolism of one molecule of glucose to two molecules of lactate leads to the net 179

180

SECTION III: ERYTHROCYTES

Embden-Meyerhof Pathway glucose ATP Hexokinase

Pentose-Phosphate Pathway

ADP Glucose-6-phosphate dehydrogenase glucose-6-phosphate

6-phosphogluconate NADP 6-Phosphogluconate

Glucosephosphate isomerase

NADPH

dehydrogenase

Pentose-phosphate

fructose-6-phosphate ATP

Phosphofructokinase ADP fructose-1,6-diphosphate

dihydroxyacetone phosphate Triosephosphate isomerase

Cytochrome b5 reductase

glyceraldehyde-3phosphate

NAD

Glyceraldehyde-3-phosphate dehydrogenase

NADH

1,3-diphosphoglycerate 2,3-biphosphoglycerate (DPG)

ADP ATP 3-phosphoglycerate

Rapoport-Luebering Shunt

Monophosphoglycerate mutase 2-phosphoglycerate Enolase phosphoenolpyruvate ADP

Pyruvate kinase

ATP pyruvate NADH NAD

FIGURE 28.1 Embden-Meyerhof (anaerobic glycolytic) pathway with hexose-monophosphate and Rapoport-Luebering shunt in RBCs (simplified). ATP, adenosine triphosphate; ADP, adenosine diphosphate.

production of two molecules of ATP. The rate of glycolysis depends on the need for ATP of RBCs to maintain the shape, deformability, membrane transport, and metabolic functions such as phosphorylation and synthesis of purines, pyrimidines, and glutathione. Over a dozen enzymes are involved in glycolysis, which are

associated with the two ancillary pathways, unique to RBCs, the pentose-phosphate (hexose-monophosphate) pathway and Rapoport-Luebering shunts. Some of these enzymes exist in different isoforms allowing for cell and tissue specific expression and regulation. The key regulatory enzyme of anaerobic glycolysis is

CHAPTER 28: HEREDITARY ERYTHROCYTE ENZYME ABNORMALITIES

phosphofructokinase (PFK). Whereas ATP is the most important inhibitor of PFK activity, inorganic phosphate, AMP and ADP action is stimulatory. In addition, glucose-1,6-biphosphate and fructose-2,6-biphosphate are activators. Under maximally activating conditions for the enzyme PFK, an enzyme distal in glycolysis, pyruvate kinase (PK), becomes rate-limiting (Fig. 28.1). It is, therefore not surprising that a deficiency of either PFK or PK activity will lead to RBC malfunction and premature RBC destruction, thereby causing hemolytic anemia. Although both erythroenzymopathies impair the same metabolic pathway, their metabolic and hematological abnormalities and clinical presentations are distinctly different and can vary between species. Subsequently, both erythroenzymopathies will be reviewed separately and contrasting features between the two disorders as well as affected species will be highlighted. PHOSPHOFRUCTOKINASE DEFICIENCY Phosphofructokinase (PFK; EC 2.7.1.11) catalyzes the regulatory phosphorylation step of fructose-6-phosphate to fructose-1,6-biphosphate. There are three isoforms of PFK, referred to as muscle (M-PFK), liver (L-PFK), and platelet (P-PFK) that are encoded by three different genes.78 The active PFK enzyme is a homo- or heterotetramer composed of one or more isoforms; each of these combinations provides unique enzyme kinetic properties. Their expression is cell and tissue specific and developmentally regulated. Skeletal muscle contains exclusively M-PFK homotetramers.54 Human RBCs express equal amounts of M- and L-PFK, whereas in canine RBCs, the M-PFK isoform predominates in a ratio of 86:14 over P-PFK.78 Furthermore, during myogenesis7 and erythropoiesis as well as postnatal development,30 the isoform composition in these cells changes from L-PFK to M- and P-PFK. Deficiency of M-PFK has been associated with hemolysis and myopathy in humans and dogs, whereas in horses and Holstein calves PFK deficiency apparently did not cause any hematologic abnormalities75 and will, therefore, not be discussed here. Muscle-type PFK deficiency, also known as TaruiLayzer syndrome, is a rare genetic disorder in humans characterized by metabolic myopathy and a well-compensated hemolytic disorder.25,62 PFK-deficient humans exhibit exercise intolerance, muscle weakness, and muscle cramping on exertion. Because of the residual half-normal PFK activity in affected RBCs, contributed by L-PFK isozymes and lack of alkaline fragility of human RBCs, overt clinical signs of hemolysis occur rarely. In fact, human patients can over-compensate their hemolytic component, and develop a mild erythrocytosis, because PFK-deficient erythrocytes do not readily release oxygen from hemoglobin. The ensuing tissue hypoxia accelerates erythropoiesis. Several mutations in the M-PFK gene have been described in human patients.55

181

Canine Phosphofructokinase Deficiency Canine M-PFK deficiency was first described as an autosomal recessive trait in English Springer Spaniels in 1985 and was the first defect characterized at the molecular level for a common inborn error in dogs.9,10,12,13,23,68 PFK-deficient dogs have a nonsense mutation in the last exon of the M-PFK gene.69,70 The resulting change from a tryptophan codon to a stop codon causes truncation of the M-PFK protein by 40 amino acids, leading to rapid degradation and complete deficiency of the M-PFK enzyme.53,70 Affected dogs completely lack PFK activity in muscle and have 8–22% of control PFK activity in RBCs due to residual P- and L-PFK expression (other tissues appear less affected as they express higher levels of other PFK isoforms).15,30,33,36,78 The metabolic block at the PFK step results in deficiency of ATP and DPG in RBCs, reduced blood lactate production, and accumulation of sugar phosphates and glycogen in muscle; hence also the term glycogenosis or glycogen storage disease type VII.10,11,15 This metabolic pattern is also known as a metabolic crossover, indicating the specific metabolic block in a pathway. In erythroenzymopathies, the mechanism of accelerated lysis is generally not known, but it is assumed to be caused by energy depletion. In dogs, the compensated hemolytic disorder of PFK deficiency is accentuated by life-threatening hemolytic crises. A unique mechanism has been documented to be responsible for intravascular lysis, namely an increased alkalineinduced hemolysis.9 Although incompletely understood and possibly related to an aberrant RBC calcium homeostasis, canine RBCs are more alkaline fragile than cells from other species.41,79 RBC calcium homeostasis is disturbed in PFK deficiency, which results in an energy imbalance that accelerates adenine nucleotide pool depletion. Ca2+-calmodulin activation of RBC AMP deaminase contributes to this metabolic dysregulation and limits ATP, thereby likely contributing to increased RBC lysis.63 In vivo, the intracellular pH of RBCs is determined by organic phosphate and chloride anions. Because PFK-deficient RBCs have markedly decreased DPG concentrations, chloride ions move in and increase RBC pH.9,11,29 Thus, PFK-deficient RBCs start lysing at about pH 7.4 (Fig. 28.2A). Consequently, even minor systemic alkalemia or hyperthermia can induce intravascular hemolysis. Alkalemia is associated with any form of hyperventilation.9,13 The low DPG content of PFK-deficient RBCs markedly increases oxygen affinity and this is reflected in a left shift of the hemoglobin-oxygen dissociation curve.9,10 The relative tissue hypoxia impairs, for instance, muscle and other tissue metabolism and stimulates renal erythropoietin synthesis and erythropoiesis. Therefore, aside from the occasional hemolytic crises associated with anemia, the chronic hemolytic disorder can be fully compensated with a robust regenerative response even at normal hematocrit. However, erythrocytosis as seen in PFK-deficient humans does not occur in affected dogs.

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SECTION III: ERYTHROCYTES

100

PFK-deficient dog PK-deficient dog

100

% Oxygen Saturation

% Hemolysis

Control

50

50 PFK-deficient dog PK-deficient dog Control

0

0 7.2

7.4

7.6

7.8

8.0

8.2

0

pH

A

20

40

60

80

100

mm Hg

B

FIGURE 28.2 (A) Alkaline fragility of RBCs from a phosphofructokinase (PFK)-deficient English Springer Spaniel and pyruvate kinase (PK)-deficient Beagle compared to a healthy control dog. RBCs were incubated at various pH values for 1 hour at 37 °C 24 hours after collecting the blood samples. Note that RBCs from PFK-deficient dog are very alkaline fragile, while those from PK-deficient dogs behave like controls. (B) Hemoglobin-oxygen dissociation curve of RBCs from phosphofructokinase (PFK)-deficient English Springer Spaniel and pyruvate kinase (PK)-deficient Beagle. Note the severe left shift of the curve of the PFK-deficient cells (higher hemoglobin-oxygen affinity, P50 = 17 mmHg), while pyruvate kinase (PK)-deficient canine RBCs are slightly right-shifted (lower hemoglobin-oxygen affinity, P50 = 32 mmHg) compared to canine control blood (28 mmHg).

Sporadic intravascular hemolytic crises associated with pigmenturia, anemia, or jaundice are the hematological hallmark findings of PFK deficiency in dogs.9,13,35 These episodes are induced by hyperventilation-associated events such as excessive panting and barking, strenuous exercise and high environmental temperatures. Thus, guests or a new pet at the patient’s home, a visit to the veterinary clinic, boarding, and training for field trialing can precipitate hemolysis. These crises can be first observed at a few months of age, although some animals may not experience any problems until several years of age, or they may be completely missed until the owner looks for them. During these episodes, which generally last one to several days, affected animals can have pale to severely icteric mucous membranes, are lethargic, inappetent, and often febrile, and splenomegaly may be noted. Thus, PFK deficiency can readily be confused with immune-mediated and other acquired hemolytic anemias. Pigmenturia is characterized by hemoglobinuria during crises, whereas hyperbilirubinuria is persistently strong. The anemia may become life-threatening with hematocrits as low as 5%, but is always macrocytic, hypochromic and strongly regenerative with corrected and absolute reticulocyte counts of 4–25% and 200,000–1,200,000/μL, respectively. The apparent half-life of chromium-labeled PFKdeficient RBCs is 16 days compared to 20–28 days for

normal canine cells. Beside polychromasia, marked anisocytosis and normoblastosis (but no poikilocytosis) are observed on microscopic examination of a blood smear. Yet unexplained, leukocytosis and hyperglobulinemia also may be present. The plasma may be discolored because of hemoglobinemia and bilirubinemia. Transient hyperkalemia is due to lysis of high potassium containing reticulocytes and young RBCs.9,13 The debilitating exertional metabolic myopathy typically seen in human PFK-deficient patients is less commonly observed and frequently milder in affected dogs. PFK-deficient dogs may develop muscle signs on exertion, even only for a short distance.3,14,15,51,52 In fact, muscle cramping in one of the limbs even after mild exercise is the most dramatic presentation.14 Affected dogs may suddenly refuse to run and have high serum creatine kinase activity. Despite a complete lack of PFK activity in muscle, clinical signs of myopathy in deficient dogs are generally mild and rare, presumably due to the high oxidative rate of canine skeletal muscle.72 Nevertheless, major impairments in oxidative and anaerobic muscle metabolism have been demonstrated experimentally.3,51,52 Aside from the acute signs of cramping, affected dogs may only show clinically mild exercise intolerance and intermittent muscle wasting. The serum creatine kinase activity is normal to slightly increased. PFK-deficient skeletal muscle accumulates

CHAPTER 28: HEREDITARY ERYTHROCYTE ENZYME ABNORMALITIES

slightly more glycogen and occasionally amylopectin, an abnormal glycogen.14,31 Interestingly a couple of Whippets with PFK deficiency also showed clinical signs of a cardiomyopathy probably resulting from the M-PFK isozyme deficiency in heart muscle.8 PFK deficiency is an autosomal recessive trait10 and occurs commonly in the English Springer Spaniel breed.20,44 Since the first description in 1985,9 over 200 cases have been documented, but the gene frequency remains unknown. A randomized survey of American Kennel Club (AKC)-registered championed and bred English Springer Spaniels in 1998 revealed that 4% of field trial and and 2.7% of show (bench) English Springer Spaniels were still carriers. Based on extensive pedigree analyses, a common ancestor goes at least back to the 1960s, if not earlier. PFK deficiency also has been reported in American Cocker Spaniels,19 and several mixed breed dogs and Whippets.8 As they all had the same disease-causing DNA mutation, these animals must have a common ancestor. PFK deficiency may be suspected based upon signalment, family history, typical clinical signs, and suggestive laboratory test abnormalities. While the pigmenturia is generally marked (i.e. orange to black), it may not be noted by the owners, particularly with a female dog, and other signs may be mild unless the dog is rigorously exercised. A definitive diagnosis requires proof of the PFK mutation or reduced RBC PFK enzyme activity. A simple polymerase-chain reaction (PCR)-based DNA test accurately identifies PFK-deficient dogs (homozygotes, affected; two mutant alleles) and carriers (heterozygotes; one mutant and one normal allele) in the English Springer Spaniel, American Cocker Spaniel, and Whippet as well as the mixed breeds.8,18,19,70 EDTAanticoagulated blood, but also a drop of blood dried on special filter paper (Guthry paper), a buccal swab with a cytobrush, and even semen from a semen bank or formalized tissues can be suitable sources for DNA extraction and PCR testing. Whereas the PCR test and differentiation either by sequencing or restrictionenzyme digest are simple and permit a diagnosis of deficient, carrier, and normal animals from the first day of life, the PFK enzyme activity test is cumbersome and not accurate until two months of age. Normal and affected neonatal dogs express large quantities of L-PFK in RBCs, which increases the overall PFK activity.30 As the PCR-based test is mutation-specific, the measurement of PFK enzyme activity may, however, be indicated in other breeds and species, where the same disease is suspected. The PCR-based PFK test is recommended for use as a screening test for English Springer Spaniels (1) with suspicious clinical signs, (2) related to affected or carrier dogs, (3) used for breeding, and (4) prior to training for field trialing. In Whippets and American Cocker Spaniels, PFK deficiency seems to be more isolated and thus general screening is not recommended. PFK-deficient dogs can have a normal life expectancy, if crises-inducing situations are avoided.13 Dogs experiencing a hemolytic crisis are provided supportive care including, if needed, type-matched blood transfu-

183

sions and in case of repeat transfusions cross-matched blood. Fever and excessive panting or barking should be avoided whenever possible. Diamox, a carbonic anhydrase inhibitor, may acidify the blood, and aspirin and dipyrone may counter the fever accompanying hemolysis, thereby preventing further intravascular lysis during an acute crisis; however, they have not been proven effective in clinical practice. Similarly, because of disturbed purine metabolism, adenine could be considered as treatment, but efficacy and safety data are lacking.63 Despite massive intravascular hemolysis, hemoprotein-induced acute nephropathy has not been observed. Nevertheless, hemolytic dogs should be well hydrated. Affected dogs recover from these crises within days. The management of the cardiomyopathy seen in PFK deficient Whippets may represent one of the breedspecific complications limiting their life expectancy.8 Finally, experimental bone marrow transplantation has successfully corrected the hematological abnormalities and abolished the hemolytic crises. Moreover, the corrected RBCs also normalized the hemoglobinoxygen affinity and, thereby, positively affected oxygenation of muscle and aerobic muscle function.51,52 Because of the morbidity and mortality associated bone marrow transplantations even when a dog leukocyte antigen (DLA)-matched donor (PFK carrier or normal) sibling is available and the relatively mild signs, when preventative measures are instituted, bone marrow transplantation cannot be recommended for this inborn error of metabolism. PYRUVATE KINASE DEFICIENCY Pyruvate kinase (PK; EC 2.7.1.40) catalyzes the ATPgenerating conversion of phosphenolpyruvate to pyruvate and is, therefore, an important regulator in the terminal glycolytic pathway. There are two different genes coding by alternative splicing a total of four developmentally dependent and tissue-specific PK isoforms.42,50,57,58 The PKLR gene encodes RBC (R-PK) and liver (L-PK) isoenzymes, whereas the PKM gene generates the muscle (M1-PK and M2-PK) isoforms. R-PK is expressed almost exclusively in mature RBCs and has a different first exon making the amino-terminal sequence longer when compared to that of L-PK. In contrast, erythroid precursors express M2-PK and switch to R-PK isoforms during differentiation to mature RBCs.50 Erythrocytic PK deficiency causing hemolytic anemia has been described in humans, dogs, cats, and mice.43,55,56,73 Pyruvate kinase deficiency is the most prevalent hereditary non-spherocytic hemolytic anemia caused by a glycolytic enzymopathy.55,74 Many mutations in the PKLR gene have been found to cause R-PK deficiency in human patient; some of them are common in certain ethnic (Amish) and geographical regions.5,55 Most of the patients, however, are compound heterozygotes with two different mutant alleles. Clinical signs in PKdeficient humans are very variable, ranging from a mild compensated hemolytic disorder to severely anemic

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patients who become transfusion-dependent, may develop iron overload (particularly when receiving regular transfusions), and die during early childhood. Splenectomy has been shown to be helpful in some human patients with severe anemia; iron chelation and bone marrow transplantation are other options.25,74 Canine Pyruvate Kinase Deficiency In 1971, erythrocytic PK deficiency was first recognized in Basenji dogs.64 It served as the classic example of an inborn error of metabolism in veterinary medicine, despite the fact that the biochemical derangements turned out to be more complex, making a clinical diagnosis more difficult. R-PK deficiency occurred most frequently in Basenjis,17,64,65 but has also been reported in Beagles,16,28,60 West Highland White4,67 and Cairn Terriers,64 Miniature Poodles, Dachshunds,45 Chihuahuas, and American Eskimo Toy dogs. In Basenjis, a single base-pair deletion in exon 5 of the R-PK gene has been identified causing a frameshift and severe truncation of the protein.81,82 In contrast, a six base insertion ambiguously positioned at the 3′-end of exon 10 in the R-PK sequence was found in West Highland White Terriers.67 This insertion results in the in-frame addition of two amino acids, threonine and lysine, and is likely also the causative mutation in Cairn Terriers,63 a related breed. In the Beagle, an exon 1 mutation has been found while in the other breeds a diseasecausing mutation has yet to be confirmed. Despite the varied mutations in R-PK, the biochemical changes and clinicopathologic manifestations of PK deficiency in any of the reported canine breeds appear identical. The R-PK deficiency in RBCs of affected dogs can be demonstrated by agarose gel electrophoresis, immunoblotting, and, immunoprecipitation studies.17,37,38 However, all affected dogs express M2-PK in RBCs, which increases total RBC PK activity of affected dogs, and, therefore, is misleading. For instance, the Miniature Poodles with non-spherocytic anemia and osteosclerosis may well have had R-PK deficiency,61 despite high RBC PK activity in vitro. In contrast M2-PK expression is rarely observed in human patients. This M2-PK isozyme, which is normally present in erythroid precursor cells, appears to be heat-labile and dysfunctional in vivo, because affected RBCs show a severe metabolic block at the PK step.17 Beside the accumulation of proximal glycolytic metabolites in RBCs, the DPG content is also increased. The resulting right shift of the hemoglobin-oxygen dissociation curve allows for improved oxygen delivery, thereby ameliorating the clinical signs of severe anemia in PK-deficient dogs. PK-deficient dogs frequently present for evaluation at a few months to a couple of years of age because of intermittent weakness and exercise intolerance. They appear to adapt very well to the anemia, presumably due to the favorable tissue oxygenation. The hematocrit ranges between 12% and 28%; the anemia is highly regenerative with reticulocyte counts of 10–90%. Ecchinocytes have been documented.64 In contrast, canine PK-deficient RBCs have normal alkaline fragility

and readily release oxygen compared to canine PFKdeficient RBCs (Fig. 28.2). The erythrocyte half-life is only 5.8 days compared to 19–28 days in normal dogs.17 Hepatosplenomegaly, due to extramedulary erythropoiesis and hemosiderin deposition, may also be noted. Affected dogs die because of anemia or hepatic failure between 1 and 9 years of age, with Basenjis having a more severe form than Beagles and West Highland White Terriers. Interestingly, all PK-deficient dogs develop a progressive myelofibrosis and osteosclerosis that remain unexplained and do not occur in other species with PK deficiency.4,16,17,28,45,63,64,66 The osteosclerosis may be identified by radiography of long bones at 1 year of age and completely obstructs the marrow cavities by 3 years in Basenjis, but occurs later in other breeds. A diagnosis of PK deficiency is strongly suggested by the concurrent occurrence of a highly regenerative severe anemia and osteosclerosis in a dog. Generally, routine screening tests rule out or are not supportive of acquired anemias and the anemia is persistent and appears well-tolerated. PCR-based PK tests are available for the Basenji,80,81 Beagles, and West Highland White Terrier breeds.67 These DNA tests are accurate to detect affected and carrier dogs in these breeds. However, because they are mutation-specific, they cannot be used for other breeds. In other breeds, cumbersome PK enzyme activity assays using agarose gel electrophoresis, immunologic, and heat-lability methods specific for R-PK are required due to M-PK expression. PK carriers do not express M-PK and, therefore, have intermediate (∼50% of normal) RBC PK activity. Unfortunately, the PK activity assays are not very accurate in detecting carriers, as the carrier and normal ranges overlap.17 PK deficiency has most commonly been reported in the Basenji, West Highland White Terrier, and Beagle breeds, but the PK mutation frequency remains unknown in all canine breeds. Affected dogs are managed symptomatically in clinical practice. Because of the excellent DPG-facilitated oxygen release from hemoglobin, PK-deficient dogs adapt well despite severe anemia. They rarely need transfusions except for their terminal stage. Bone marrow transplantation, performed experimentally,80 corrects the hemolytic anemia and halts the osteosclerosis and hemosiderin deposition, whereas splenectomy has not been shown to be effective in slowing the hemolysis and anemia. Feline Pyruvate Kinase Deficiency In the early 1990s, erythrocytic pyruvate kinase deficiency was identified and characterized in the Abyssinian, Somali and Domestic Shorthair cats.6,21 A 13 base-pair deletion at the 3′-end of exon 5 in the R-PK cDNA, but not the genomic sequence is caused by a splicing defect and results in a severe reduction in RBC PK enzyme activity. In cats there is no anomalous M2-PK expression in RBCs and no osteosclerosis, which are observed in all PK-deficient dogs. Since the original observations of the PK-deficient Abyssinian

CHAPTER 28: HEREDITARY ERYTHROCYTE ENZYME ABNORMALITIES

and Somali cats, over 3,000 have been screened; approximately 12% are affected and 24% are carriers, making this one of the most frequent causes of anemia in these breeds. Since then, PK-deficient Abyssinian and Somali cats have been reported from Europe26,27,47,48,77 and Australia.1,49 In addition, a few Domestic Shorthair cats have also been found with the same mutation in the United States. Interestingly, a specific pathogen-free research colony of domestic cats also carries this same PK mutation and is responsible for an unexplained mild to moderate intermittent anemia. Affected cats have chronic intermittent hemolytic anemia that might not first be recognized until advanced age. The anemia is intermittent, mild to severe with PCV ranging from 5 to 35%. It is slightly macrocytichypochromic and slightly to strongly regenerative (reticulocyte count 45,000–290,000/μL).24,47,48 Moreover, PK-deficient cats frequently have a marked lymphocytosis and hyperglobulinemia. Many of these features are similar to cats with increased RBC osmotic fragility with splenomegaly in the same breeds.46 However, PK-deficient cats typically have a greater regenerative response and only mild splenomegaly and mild osmotic fragility (Fig. 28.3) Extrahepatic biliary obstruction due to bilirubin calculi is seen in affected cats.8,27 It should be noted that many deficient cats remain completely asymptomatic for years and the associated anemia may not be the primary reason for a PK-deficient cat’s illness. PK-deficient cats frequently do not require treatment, just like PK- and PFK-deficient dogs. However,

% Hemolysis

100

50

Cat with OF PK-deficient cat Normal

0 0

0.20

0.40

0.60

0.80

NaCl Concentration

FIGURE 28.3 Erythrocyte osmotic fragility (OF) curve from a PK-deficient Abyssinian cat and an Abyssinian with increased OF and splenomegaly. RBCs were exposed in vitro to different saline concentrations from 0% (water) to 0.85% (physiological saline) at room temperature for 30 min.

185

during a crisis, deficient cats may need to be transfused with AB blood-type compatible blood. In case of severe splenomegaly or persistently severe anemia, splenectomy appears to reduce the severity of the hemolytic crises.24

REFERENCES 1. Barrs VR, Giger U, Wilson B, et al. Erythrocytic pyruvate kinase deficiency and AB blood types in Aust Abyssinian and Somali cats. Aust Vet J 2009;87:39–44. 2. Beutler E. Red Cell Metabolism: A Manual of Biochemical Methods. Orlando: Grune & Stratton, 1984;42–82. 3. Brechue WF, et al. Metabolic and work capacity of skeletal muscle of PFKdeficient dogs studied in situ. J Appl Physiol 1994;77:2456–2567. 4. Chapman BL, Giger U. Inherited erythrocyte pyruvate kinase deficiency in the West Highland White terrier. J Small Anim Pract 1990;31:610–616. 5. Demina A, Varughese KI, Barbot J, et al. Six previously undescribed pyruvate kinase mutations causing enzyme deficiency. Blood 1998;92:647–652. 6. Ford S, Giger U, Duesberg C, et al. Inherited erythrocyte pyruvate kinase (PK) deficiency causing hemolytic anemia in an Abyssinian cat. J Vet Intern Med 1992;6:123. 7. Gekakis N, Gehnrich SC, Sul HS. Phosphofructokinase isozyme expression during myoblast differentiation. J Biol Chem 1989;264:3658–3661. 8. Gerber K, Harvey JW, D’Agorne, et al. Hemolysis, myopathy and cardiac disease associated with hereditary phosphofructokinase deficiency in two whippets. Vet Clin Pathol 2009;38:46–51. 9. Giger U, Harvey JW, Yamaguchi RA, et al. Inherited phosphofructokinase deficiency in dogs with hyperventilation-induced hemolysis: Increased in vitro and in vivo alkaline fragility of erythrocytes. Blood 1985;65:345–351. 10. Giger U, Reilly M, Asakura T, et al. Autosomal recessive inherited phosphofructokinase deficiency in English springer spaniels. Anim Genet 1986;17:15–23. 11. Giger U, Teno P, Reilly MP, et al. Phosphofructokinase-deficient canine erythrocytes studied in vitro. Blood 1986;68:35a. 12. Giger U. Survival of phosphofructokinase-deficient erythrocytes in a canine model [abstract]. Blood 1987;70(Suppl):52a. 13. Giger U, Harvey JW. Hemolysis caused by phosphofructokinase deficiency in English Springer Spaniels: seven cases (1983–86). J Am Vet Med Assoc 1987;191:453–459. 14. Giger U, Argov Z, Schnall M, et al. Metabolic myopathy in canine muscletype phosphofructokinase deficiency studies by P-NMR. Muscle Nerve 1988;11:1260–1265. 15. Giger U, Kelly AM, Teno PS. Biochemical studies of canine muscle phosphofructokinase deficiency. Enzyme 1988;40:25–29. 16. Giger U, Mason GD, Wang P. Inherited erythrocyte pyruvate kinase deficiency in a Beagle. Vet Clin Pathol 1991;20:83–86. 17. Giger U, Noble NA. Inherited erythrocyte pyruvate kinase deficiency in Basenji dogs. J Am Vet Med Assoc 1991;198:1755–1761. 18. Giger U, Smith BF, Griot-Wenk M, et al. The molecular basis of canine muscle phosphofructokinase deficiency is a point mutation. Blood 1991;78:365a. 19. Giger U, Smith BF, Woods CB. Inherited phosphofructokinase deficiency in the Am Cocker spaniel. J Am Vet Med Assoc 1992;201:1569–1571. 20. Giger U, Smith BF, Rajpurohit Y. PCR-based screening test for phosphofructokinase (PFK) deficiency: a common inherited disease in English springer spaniels. J Vet Intern Med 1995;9:187. 21. Giger U, Rajpurohit Y, Wang P, et al. Molecular basis of erythrocyte pyruvate kinase (R-PK) deficiency in cats. Blood 1997;90S:5b. 22. Giger U, Wang P, Boyden M. Familial methemoglobin reductase deficiency in domestic shorthair cats [abstract]. Feline Pract 1999;31(Suppl):14. 23. Giger U. Hereditary erythrocyte disorders. In: Kirk’s Current Veterinary Therapy XIII. Philadelphia: WB Saunders. 2000;414–420. 24. Giger U. Hereditary erythrocyte disorders. In: Consultations in Feline Internal Medicine 4. Philadelphia: WB Saunders, 2001;484–489. 25. Glader BE, Lukens JN. Hereditary hemolytic anemias associated with abnormalities of erythrocyte glycolysis. In: Wintrobe’s Clinical Hematology, 10th ed. Baltimore: Wilkins, 1999;1160–1175. 26. Harvey AM, Helps CR, Seng AS, et al. Survey of erythrocyte pyruvate kinase deficiency in Somali cats from the United Kingdom. Congress of the European College of Veterinary Internal Medicine – Companion Animals, 2007. 27. Harvey AM, Holt PE, Barr FJ, et al. Treatment and long-term follow-up of extrahepatic biliary obstruction with bilirubin cholelithiasis in a Somali cat with pyruvate kinase deficiency. J Feline Med Surg 2007;9:424–431 28. Harvey JW, Kaneko JJ, Hudson EB. Erythrocyte pyruvate kinase deficiency in a Beagle dog. Vet Clin Pathol 1977;6:13–17. 29. Harvey JW, Sussman WA, Pate MG. Effect of 2,3-diphosphoglycerate concentration on the alkaline fragility of phosphofructokinase-deficient canine erythrocytes. Comp Biochem Physiol B 1988;89:105–109.

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30. Harvey JW, Reddy GR. Postnatal hematologic development in phosphofructokinase-deficient dogs. Blood 1989;74:2556–2561. 31. Harvey JW, Calderwood-Mays MB, Gropp KE, et al. Polysaccharide storage myopathy in canine phosphofructokinase deficiency (type VII glycogen storage disease). Vet Pathol 1990;27:1–8. 32. Harvey JW, King RR, Berry CR, et al. Methaemoglobin reductase deficiency in dogs. Comp Haematol Intl 1991;1:55–59. 33. Harvey JW, Pate MG, Mhaskar Y, et al. Characterization of phosphofructokinase-deficient canine erythrocytes. J Inherit Metab Dis 1992;15:747–759. 34. Harvey JW, Dahl M, High ME. Methemoglobin reductase deficiency in a cat. J Am Vet Med Assoc 1994;205:1290–1291. 35. Harvey JW, Smith JE. Haematology and clinical chemistry of English Springer Spaniel dogs with phosphofructokinase deficiency. Comp Hematol Intl 1994;4:70–75. 36. Harvey JW. The erythrocyte – physiology, metabolism, and biochemical disorders. In: Clinical Biochemistry of Domestic Animals, 5th ed. San Diego: Academic Press, 1997;157–203. 37. Harvey JW. Hereditary methemoglobinemia. In: Schalm’s Veterinary Hematology. 5th ed. Philadelphia: Lippincott Williams & Wilkins, 2000;1008–1011. 38. Harvey JW, Stockham SL, Scott MA, et al. 2003; Methemoglobinemia and eccentrocytosis in equine erythrocyte flavin adenine dinucleotide deficiency. Vet Pathol 1997;40:632–642. 39. Harvey JW. Pathogenesis, laboratory diagnosis, and clinical implications of erythrocyte enzyme deficiencies in dogs, cats, and horses. Vet Clin Pathol 2006;35:144–156. 40. Harvey JW. The erythrocyte: physiology, metabolism and biochemical disorders. In: Clinical Biochemistry of Domestic Animals, 5th ed. San Diego: Academic Press, 2008;157–203. 41. Iampietro PF, Burr MJ, Fiorica V, et al. pH-dependent lysis of canine erythrocytes. J Appl Physiol 1967;23:505–510. 42. Imamura K, Tanaka T. Multimolecular forms of pyruvate kinase from rat and other mammalian tissues. J Biochem 1972;71:1043–1051. 43. Kanno H, Morimoto M, Fujii H, et al. Primary structure of murine red blood cell-type pyruvate kinase (PK) and molecular characterization of PK deficiency identified in the CBA strain. Blood 1995;86:3205. 44. Kimmel A. Survey of phosphofructokinase deficiency in English Springer spaniels. Thesis, University of Zurich, Switzerland. 45. Kohn B, Freistedt R, Pekrun A, et al. Chronische hämolytische Anämie und Osteosklerose aufgrund einer Erythrozyten-Pyruvatkinase-Defizienz bei einem Langhaardackel. Kleintierpraxis 1999;44:437–445. 46. Kohn B, Goldschmidt MH, Hohenhaus AE, et al. Anemia, splenomegaly, and increased osmotic fragility of erythrocytes in Abyssinian and Somali cats. J Am Vet Med Assoc 2000;217:1483–1491. 47. Kohn B, Fumi C, Seng A, et al. Anemia due to erythrocytic pyruvate kinase deficiency and its incidence in Somali and Abyssinian cats in Germany. Kleintierpraxis 2005;50:305–312. 48. Kohn B, Fumi C. Clin course of pyruvate kinase deficiency in Abyssinian and Somali cats. J Feline Med Surg 2008;10:145–153. 49. Mansfield CS, Clark P. Pyruvate kinase deficiency in a Somali cat in Australia. Aust Vet J 2005;83:483–485. 50. Max-Audit I, Kechemir D, Mitjavila MJ, et al. Pyruvate kinase synthesis and degradation by normal and pathologic cells during erythroid maturation. Blood 1988;72:1039. 51. McCully K. In: vivo determination of altered hemoglobin saturation in dogs with M-type phosphofructokinase deficiency. Muscle Nerve 1999;22:621–627. 52. McCully K, Giger U. Using near infrared spectroscopy coupled to magnetic resonance spectroscopy to evaluate canine muscle oxygen saturation: evaluation and treatment of M-type phosphofructokinase deficiency. In: International Book of In Vivo Imaging in Vertebrates. London: John Wiley & Sons, 2007;265–269. 53. Mhaskar Y, Giger U, Dunaway GA. Presence of truncated M-type subunit and altered kinetic properties of 6-phosphofructo-1-kinase isozymes in canine brain affected by glycogen storage disease type VII. Enzyme 1991;45:137–144. 54. Mhaskar Y, Harvey JW, Dunaway GA. Developmental changes of 6phosphofructo-1-kinase subunit levels in erythrocytes from normal dogs and dogs affected by glycogen storage disease type VII. Comp Biochem Physiol 1992;101:303–307. 55. Miwa S, Fujii H. Molecular basis of erythroenzymopathies associated with hereditary hemolytic anemia: Tabulation of mutant enzymes. Am J Hematol 1996;51:122.

56. Morimoto M, Kanno H, Asai H, et al. Pyruvate kinase deficiency of mice associated with non-spherocytic hemolytic anemia and cure of the anemia by marrow transplantation without host irradiation. Blood 1995;86:4323. 57. Noguchi T, Inoue H, Tanaka T. The M1 and M2-type isozymes of rat pyruvate kinase are produced from the same gene by alternative RNA splicing. J Biol Chem 1986;261:13807–13812. 58. Noguchi T, Yamada K, Inoue H, et al. The L and R-type isozymes of rat pyruvate kinase are produced from a single gene by the use of different promoters. J Biol Chem 1987;262:14366–14371. 59. Nonneman D, Stockham SL, Shibuya H, et al. A missense mutation in the glucose-6-phosphate dehydrogenase gene associated with hemolytic anemia in an American saddlebred horse [abstract]. Blood 1993;82(Suppl 1):466a. 60. Prasse KW, Crouser D, Beutler E, et al. Pyruvate kinase deficiency with terminal myelofibrosis and osteosclerosis in a Beagle. J Am Vet Med Assoc 1975;166:1170–1175. 61. Randolph JF, Center SA, Kalfelz FA, et al. Familial non-spherocytic hemolytic anemia in poodles. Am J Vet Res 1986;47:687–695. 62. Rowland LP, DiMauro S, Layzer RB. Phosphofructokinase deficiency. In: Myology. New York: McGraw-Hill. 1986;1603–1617. 63. Sabina RL, Woodliff JE, Giger U. Disturbed erythrocyte calcium homeostasis and adenine nucleotide dysregulation in canine phosphofructokinase deficiency. Comp Clin Pathol 2008;17:117–123. 64. Schaer M, Harvey JW, Calderwood-Mays M, et al. Pyruvate kinase deficiency causing hemolytic anemia with secondary hemochromatosis in a Cairn terrier. J Am Anim Hosp Assoc 1992;28:233–239. 65. Searcy GP, Miller DR, Tasker JB. Congenital hemolytic anemia in the Basenji dog due to erythrocyte pyruvate kinase deficiency. Can J Comp Med Vet Sci 1971;35:67–70. 66. Searcy GP, Tasker JB, Miller DR. Animal model: pyruvate kinase deficiency in dogs. Am J Pathol 1979;94:689–692. 67. Skelly B, Wallace M, Rajpurohit Y, et al. Identification of a 6 base pair insertion in West Highland White Terriers with erythrocyte pyruvate kinase deficiency. Am J Vet Res 1999;60:1169–1172. 68. Skibild E, Dahlgaard K, Rajpurohit Y, et al. Haemolytic anaemia and exercise intolerance due to phosphofructokinase deficiency in related springer spaniels. J Small Anim Pract 1999;42:298–300. 69. Smith BF, Henthorn PS, Rajpurohit Y, et al. A cDNA coding canine muscle type phosphofructokinase. Gene 1996;168:275–276. 70. Smith BF, Stedman H, Rajpurohit Y, et al. The molecular basis of canine muscle-type phosphofructokinase deficiency. J Biol Chem 1996;271:20070–20074. 71. Smith JE, Ryer K, Wallace L. Glucose-6-phosphate dehydrogenase deficiency in a dog. Enzyme 1976;21:379–382. 72. Snow DH, et al. No classical type IIB fibres in dog skeletal muscle. Histochemistry 1982;75:53–65. 73. Stockham SL, Harvey JW, Kinden DA. Equine glucose-6-phosphate dehydrogenase deficiency. Vet Pathol 1994;31:518–527. 74. Tanaka KR, Paglia DE. Pyruvate kinase and other enzymopathies of the erythrocyte. In: The Metabolic and Molecular Basis of Inherited Disease, 7th ed. New York: McGraw-Hill, 1995;3488–3493. 75. Tvedten H. Macrocytic hypochromic RBC are not always reticulocytes. European Society of Veterinary Clinical Pathology – Congress, 2008. 76. Valberg SJ, Mickelson JR, DiMauro S. Nonlysosomal glycogenoses in horses and cattle. Muscle Nerve 1998;7(Suppl):S89. 77. van Geffen C, Savary-Bataille K, Chiers K, et al. Bilirubin cholelithiasis and haemosiderosis in an anaemic pyruvate kinase-deficient Somali cat. J Small Anim Pract 2008;49:479–482. 78. Vora S, Giger U, Turchen S, et al. Characterization of the enzymatic lesion in inherited phosphofructokinase deficiency in the dog: an animal analogue of human glycogen storage disease type VII. Proc Natl Acad Sci USA 1985;82:8109–8113. 79. Waddell WJ. Lysis of dog erythrocytes in mildly alkaline isotonic media. Am J Physiol 1956;186:339. 80. Weiden PL. Long-term survival and reversal of iron overload after marrow transplantation in dogs with congenital hemolytic anemia. Blood 1981;57:66–70. 81. Whitney KM, et al. The molecular basis of canine pyruvate kinase deficiency. Exp Hematol 1994;22:866–874. 82. Whitney KM, et al. Genetic test for pyruvate kinase deficiency in Basenjis. J Am Vet Med Assoc 1995;207:918–921.

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29

Erythrocyte Membrane Defects MUTSUMI INABA and JOANNE B. MESSICK Hemolytic Anemias Caused by Hereditary Red Cell Membrane Defects Hereditary Spherocytosis Hereditary Band 3 Deficiency in Cattle (Band 3Bov.Yamagata) Spectrin and Ankyrin Deficiencies in Animals Hereditary Elliptocytosis Hereditary Protein 4.1 Deficiency in Dogs Spectrin Anomaly with Elliptocytosis in a Dog Hereditary Stomatocytosis Hereditary Stomatocytosis in Dogs and Cats

HSt in Miniature and Standard Schnauzers HSt in chondrodysplastic Alaskan Malamute dwarf dogs Familial stomatocytosis-hypertrophic gastritis in dogs Hemolytic anemia with increased erythrocyte osmotic fragility in cats Diagnostic Approaches to Erythrocyte Membrane Defects Membrane Transport Defects Amino Acid Transport Deficiency in Animals High Membrane Na,K-ATPase Activity in Dogs

Acronyms and Abbreviations DHS, dehydrated hereditary stomatocytosis; HE, hereditary elliptocytosis; HPP, hereditary pyropoikilocytosis; HS, hereditary spherocytosis; HSt, hereditary stomatocytosis; OHS, overhydrated hereditary stomatocytosis; PCR-RFLP, polymerase chain reaction-restriction fragment length polymorphism; PCR-SSCP, PCR-single strand conformation polymorphism; RBC, red blood cell; SDS-PAGE, sodium dodecyl sulfate-polyacrylamide gel electrophoresis.

R

ed blood cells (RBCs) must be very durable and flexible to undergo marked deformation under high shear stress condition so that they can survive during repeated passages through the microcirculation. These important properties are determined by three major elements of their membranes: a lipid bilayer, integral or transmembrane proteins, and a membrane skeletal network (Fig. 29.1). A lipid bilayer provides a permeability barrier between the cytosol and the external environment. Major constituents of the membrane skeleton mechanically support the plasma membrane and are organized into a lattice-like meshwork that is linked with both integral membrane components and cytoskeletal elements. Some of the transmembrane proteins embedded within the lipid bilayer ensure selective permeability to maintain RBC homeostasis. Accelerated destruction of erythrocytes (hemolytic anemia) may occur when these properties of the RBC membrane are affected or deficient by some genetic defects. This chapter reviews the current progress in the investigation of inherited red cell membrane disorders in animals.

HEMOLYTIC ANEMIAS CAUSED BY HEREDITARY RED CELL MEMBRANE DEFECTS The membrane protein-protein and protein-lipid interactions are the critical determinants of red cell morphology and mechanical stability, as evidenced by the numerous hereditary RBC disorders in humans attributed to mutations of the membrane.2,51–53,73,74 These interactions are divided into two categories (Fig. 29.1): (1) vertical interactions involving the band 3-ankyrinspectrin and glycophorin C-protein 4.1-spectrin binding, which attach the spectrin-actin network to the plasma membrane and stabilize the lipid bilayer, and (2) horizontal interactions involving spectrin dimer-dimer association (tetramer formation), and contact of the distal ends of spectrin with F-actin by the aid of protein 4.1 and adducin within the junctional complex. In general, the loss of vertical linkage between membrane skeleton and lipid bilayer causes decreased membrane cohesion leading to membrane loss (e.g. hereditary spherocytosis). Weakening of horizontal linkages 187

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Glycophorin C-protein-specrin linkage

Band 3-ankyrin-spectrin linkage Vertical interaction

Band 3 (AE1) tetramer

Protein 4.2

Glycophorin A

Band 3 dimer

p55 Dematin Adducin

Ankyrin

β-Spectrin

A

Glycophorin C

Toropomyosin Actin short filament Protein 4.1 Junctional complex

α-Spectrin

Horizontal interaction Junctional complex a2b2-spectrin tetramer

B FIGURE 29.1 A schematic diagram illustrating structural and functional organization of RBC membrane proteins. Membrane proteinprotein associations are divided into two categories: (1) vertical interactions involving band 3-ankyrin-spectrin linkage and glycophorin C-protein 4.1 (P4.1)-spectrin linkage, and (2) horizontal interactions involving the spectrin heterodimer contact (tetramer formation) and association of spectrin tetramer at the distal ends with F-actin at the junctional complex consisting of actin, P4.1, and adducin. In general, a defect of vertical interactions leads to hereditary spherocytosis, whereas a defect of horizontal interactions causes hereditary elliptocytosis and pyropoikilocytosis.

between skeletal proteins results in decreased membrane mechanical stability leading to membrane loss (e.g. hereditary elliptocytosis).1,51 The severity of the anemia is directly related to the extent of the membrane surface area loss in both of these defects; loss of membrane not only compromises the ability of the RBC to deliver oxygen to the tissues, but also results in premature removal from the circulation by the spleen. In human patients, splenectomy results in correction of the anemia.1 The reader is directed to Chapter 20 for details of the structural organization of the normal RBC membrane. HEREDITARY SPHEROCYTOSIS The cardinal features of hereditary spherocytosis (HS) are hemolytic anemia of varying severity, spherocytosis, increased RBC osmotic fragility, and splenomegaly.59 Pathophysiology of HS involves two major factors: intrinsic membrane defect, and selective sequestration of HS cells in the normal spleen. Hence, anemia can be corrected by splenectomy in human HS patients. Typical forms of HS need to be distinguished from other hemolytic anemias manifesting moderate to small numbers of spherocytes, such as autoimmune hemolytic anemia by Coombs’ test as well as unstable hemoglobin and oxidative damage by Heinz body screening. HS is now

considered to be a disorder of vertical interactions of the membrane proteins, although the primary molecular defects are heterogeneous including deficiencies or dysfunctions of spectrin, ankyrin, band 3, and protein 4.2. Consequently, the lipid bilayer is destabilized, leading to membrane and surface area loss, and spherocyte formation.25,53,74 Hereditary Band 3 Deficiency in Cattle (Band 3Bov.Yamagata) Band 3 (anion exchanger 1, AE1) is the most abundant transmembrane protein in mammalian RBCs.56 Hereditary band 3 deficiency in Japanese black cattle (band 3Bov.Yamagata) is associated with HS and is inherited by an autosomal dominant trait. Homozygous affected animals totally lack band 3 due to a nonsense mutation Arg664 → Stop (R664X) of the AE1 gene (SLC4A1)29 and show mild to moderate, chronic hemolytic anemia (hematocrit, 25–35%); slight acidosis; and growth retardation. Carrier cattle heterozygous for the R664X mutation also have abnormal RBC morphology and impaired anion transport activity due to partial deficiency of RBC band 3 (about 30%). However, spherocytosis in heterozygotes is mild, and the hemolysis is well compensated. In heterozygous animals, mutant band 3 is rapidly degraded in the endoplasmic reticulum without

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translocation to the plasma membrane and there is selective reduction of mutant band 3 mRNA.30,31 Genotypes for R664X mutation are easily determined using genomic DNA as the template by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP)29,30 or PCR-single strand conformation polymorphism (PCR-SSCP) techniques. A protocol to determine the genotype for band 3Bov.Yamagata and the sex of embryos before embryo tranfer has been developed.37 The ancestral origin of this genetic defect has not yet been identified. Band 3 has two major functions:34,56,71,72 (1) it mediates a rapid Cl−/HCO3− exchange across the plasma membrane to increase five-fold the capacity of the blood to carry CO2 from tissues to lungs and maintains blood acid-base homeostasis together with the renal band 3 function; and (2) it may also participate in maintaining mechanical properties of RBC membranes by forming the band 3-ankyrin-spectrin complex. Various mutations leading to disorders and partial deficiency of RBC 3 associated with abnormal RBC morphology (spherocytosis and ovalocytosis) have been reported in humans,32,35,53 but none of them exhibited complete lack of the protein or its function. The most surprising finding, therefore, is that cattle with total lack of band 3 survived to and thrived in adulthood. They suffer from extremely severe hemolytic anemia shortly after birth and exhibit jaundice and splenomegaly. The mortality rate is high during this period, particularly in the first week after birth. Once they overcome this neonatal crisis, jaundice subsides and hemolysis becomes modest. Until 1998, a half dozen homozygous affected cattle older than 3 years of age had been found. One of three affected females had two normal parturitions. The studies on bovine hereditary band 3 deficiency have demonstrated the importance of band 3 in RBC morphology and homeostasis. Red blood cells from homozygotes of band 3Bov.Yamagata are also deficient in spectrin, ankyrin, actin (by 20–50%), and protein 4.2, resulting in a distorted and disrupted membrane skeletal network (Fig. 29.2). Their RBC membranes are extremely unstable and demonstrate the spontaneous loss of surface area by invagination, vesiculation, extrusion of microvesicles, and fragmentation, thereby leading to the formation of spherocytes with irregular contours, gouging, and pitting (Fig. 29.3). RBCs from homozygous and heterozygous cattle constantly show considerably increased osmotic fragility with 50% hemolysis at 0.75% and 0.65–0.70% NaCl, respectively (normal, 0.45–0.55% NaCl), demonstrating that the surface area/volume ratios of RBCs from both homozygotes and heterozygotes are remarkably reduced due to the mechanical instability of their RBC membranes (Fig. 29.4). Despite these severe changes, the affected animals show no reticulocytosis and no noticeable intravascular hemolysis. However, the protein 4.1a/4.1b ratio, which is a good marker of RBC aging,24,27,28 is remarkably reduced and paralleled by increased erythropoiesis. These findings demonstrate the functional importance of band 3-ankyrin-spectrin

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FIGURE 29.2 Abnormal RBC membrane structures in bovine band 3 deficiency (band 3Bov.Yamagata). (A) SDS-PAGE profiles of RBC membrane proteins from the normal (N) and the homozygote for R664X mutation (Ho). Note that spectrin (+ankyrin) and actin levels are reduced compared to protein 4.1. Protein 4.2 is almost missing. A 66-kDa protein only found in the homozygote was albumin. gp155 indicates a transmembrane protein characteristic to ruminant RBCs.65 (B) Markedly reduced number of intramembrane particles in homozygous RBCs (Ho) compared to normal RBC on electron micrographs by the freeze fracture method. (C) Disrupted membrane skeletal network in band-3–deficient RBCs visualized by the quick-freeze deep etching method. The membrane skeletons in band-3-deficient RBCs (Ho) are disrupted and distorted with filaments of uneven length and width compared with the wellorganized normal RBCs (N). Scale bars = 0.1 μm.

association in maintaining mechanical stability of the membrane41 but not in assembly of the membrane skeletal architecture; they also indicate that accelerated and continual destruction of RBCs occurs in homozygous cattle and also in heterozygotes though it is less severe. Total deficiency of band 3 also results in defective Cl−/HCO3− exchange. The Cl− influx into the RBCs from homozygotes requires approximately 2 hours to reach transmembranous equilibrium even at 37°C. Band 3 deficiency causes mild acidosis with decreases in the HCO3− concentration and total CO2 in the blood. However, these values remain in the normal range. As CO2 in blood rarely reaches saturation, it is suggested that the additional CO2 carrying capacity facilitated by band 3 is probably not as critical as has been believed previously,33,70 except under high stress conditions such as vigorous exercise or high altitude. Transgenic mice with complete band 3 deficiency have similar RBC features.57,68 These studies demonstrate that band 3 indeed contributes to RBC membrane organization, CO2 transport, and acid-base homeostasis. In band 3 knockout mice, glycolytic enzymes that normally exist as multienzyme complexes on the inner surface of RBC membranes, are not membrane-associated but distributed throughout the cytoplasm. The assembly of glycolytic enzymes on the membrane is likely a general phenomenon of mammalian RBCs and stability of these interactions depends primarily on band 3.6

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FIGURE 29.3 Morphologic anomaly in bovine band 3 deficiency. Scanning (A, B) and transmission (C–F) electron micrographs of RBCs from a homozygous animal of band 3Bov.Yamagata. (A) The proband RBCs look like potatoes and greatly vary in size, principally being spherocytic and stomatocytic with irregular contours. (B) When blood is allowed to stand for several hours at ambient temperature, numerous small globules are observed on the surface. (C–F) Marked endocytosis-like invagination, exocytosis-like projections, fusion of vesicles inside the cell, and extrusion of microvesicles. The vesicles in the cytoplasm contain plasma proteins as determined by immunoelectron micrography using anti-bovine albumin antibodies (D). Scale bars = 5 μm (A and B), 1 μm (C), and 0.2 μm (D–F).

Spectrin and Ankyrin Deficiencies in Animals

FIGURE 29.4 Increased RBC osmotic fragility in bovine band 3 deficiency. Data for normal cattle (n = 15) as well as homozygotes (n = 4) and heterozygotes (n = 8) of band 3 deficiency.

Several mutations that affect components of the RBC membrane skeleton have also been reported in mice.3,79 These include spectrin deficiencies in house21,42 and deer mice63 as well as ankyrin deficiency in nb mice79 with moderate to life-threatening hemolytic anemia and fragile short-lived spherocytes. A quantitative spectrin deficiency with an autosomal dominant inheritance was reported in Dutch Golden Retrievers.66 The affected dogs had markedly reduced RBC spectrin concentration that was 50–65% of unaffected dogs and exhibited increased RBC osmotic fragility. However, the disorder was not associated with either spherocytosis or elliptocytosis. The exact mechanism for this occult spectrin deficiency without morphological anomaly remains to be clarified. Bovine α– and β-spectrin genes possess polymorphisms leading to generation of spectrin proteins with different amino acid sequences. One of these genotypes appears to cause mild reduction in RBC spectrin contents without morphological lesions and

CHAPTER 29: ERYTHROCYTE MEMBRANE DEFECTS

probably has a modulatory role in severity of RBC phenotypes in band 3Bov.Yamagata (M. Inaba, unpublished observation). HEREDITARY ELLIPTOCYTOSIS The principal lesion of human hereditary elliptocytosis (HE) involves heterogeneous defects in horizontal membrane protein interactions (Fig. 29.1) resulting in a mechanically unstable membrane (e.g. abnormal spectrin structure affecting the spectrin heterodimer contacts, deficiency or dysfunction of protein 4.1).51,53,74 Common HE is morphologically characterized by elliptocytes and rod-shaped RBCs in some patients. In severe cases, aberrant disruption of the horizontal interactions results in fragmentation of RBCs, leading to hereditary pyropoikilocytosis (HPP). Ovalocytosis, spherocytic HE, is a rare condition in which both round oval RBCs lacking central concavity and spherocytes are present on the blood film. It is likely that elliptocytes and poikilocytes are permanently stabilized in their abnormal shape. The weakened horizontal connections facilitate reorganization of skeleton, which follows axial deformation of cells by a prolonged shear stress.51,53,74 Although camelid RBCs are elliptocytic by nature, the molecular basis and physiologic importance of this unique feature remains unknown.

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proband RBC membranes were deficient in protein 4.1 (4.1a and 4.1b). The parents of the proband had decreased amounts (approximately 50% of normal) of protein 4.1 and some elliptocytes. Protein 4.1 forms a tertiary complex with spectrin and actin within the junctional complex. This spectrinactin binding region was mapped to the 10-kDa domain9 containing amino acid sequences encoded by a 21amino acid alternative exon and a 59-amino acid constitutive exon.7 Immunoblotting of RBC membrane proteins from the proband showed the presence of a small amount of 76-kDa polypeptide lacking the 21 amino acid segment and a very faint band of normal protein 4.1. RT-PCR analysis and sequencing of cloned reticulocyte protein 4.1 cDNA consistently showed that mRNA with a deletion of the alternative exon encoding 21 amino acid peptide was the predominant form with only a small quantity of normal mRNA. Therefore, the functional defect in the HE dog is likely due to the combined influences of two factors: a quantitative deficiency of protein 4.1 and a failure to activate efficiently expression of an alternatively spliced exon encoding 21 amino acids in the spectrin-actin binding region during erythropoiesis.8 The primary cause leading to inefficient RBC-specific alternative splicing has not been defined. Spectrin Anomaly with Elliptocytosis in a Dog

Hereditary Protein 4.1 Deficiency in Dogs (HE in Dogs) Canine HE was discovered by Smith et al.67 The proband exhibited elliptocytosis, membrane fragmentation, microcytosis, and poikilocytosis without anemia (Fig. 29.5). RBC mechanical stability was markedly decreased, and osmotic fragility was remarkably increased. The

An elliptocytosis was recently reported in a mixedbreed dog.11 Although the elliptocytosis was asymptomatic and was not associated with hemolytic anemia, the dog’s RBCs had decreased deformability and stability when subjected to shear stress. This is consistent with non-hemolytic HE in humans due to structural abnormality in either the α– or β-spectrin. Molecular analysis revealed that the elliptocytosis was probably due to a β-spectrin mutation in codon 2110 (T2110M), a region of spectrin that is critical for the self-association of spectrin dimmers. This would be expected to result in reduced formation or reduced stability of tetramers. HEREDITARY STOMATOCYTOSIS

FIGURE 29.5 Scanning electron micrograph of RBCs from the proband with hereditary elliptocytosis. The proband RBCs reveal biconcave elliptocytes. (Reproduced from Smith JE, Moore K, Arens M, Rinderknecht GA, Ledet A. Hereditary elliptocytosis with protein band 4.1 deficiency in the dog. Blood 1983;61:373–377, with permission.) Scale bar = 5 μm.

Stomatocytes are uniconcave or bowl-shaped RBCs in suspension but have a slit-like appearance, an artifact on dried blood films (Fig. 29.6). Two distinct phenotypes having abnormalities in membrane cation permeability leading to changes in changes in RBC volume have been identified in humans: dehydrated hereditary stomatocytosis (DHS) and overhydrated hereditary stomatocytosis (OHS).1,51 DHS is associated with a wellcompensated hemolytic anemia with less than 10% stomatocytes usually seen on blood smears. In contrast, stomatocytes are a major feature of RBC morphology in OHS. An important characteristic of these stomatocytosis patients is a marked predisposition to thrombocytosis and development of hypercoagulability after splenectomy.1

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A

B

FIGURE 29.6 Erythrocyte morphology of canine hereditary stomatocytosis (hydrocytosis). (A) Numerous stomatocytes (arrow) with a slit-like appearance are present on a Wright-Giemsa-stained blood smear from a Miniature Schnauzer. (Reproduced from Brown DE, Weiser MG, Thrall MA, Giger U, Just CA. Erythrocyte indices and volume distribution in a dog with stomatocytosis. Vet Pathol 1994;31:247–250, with permission.) (B) Scanning electron micrograph of a typical stomatocyte from an affected Alaskan Malamute. (Reproduced from Pinkerton PH, Fletch SM, Brueckner PJ, Miller DR. Hereditary stomatocytosis with hemolytic anemia in the dog. Blood 1974;44:557–567, with permission.) Scale bar = 10 μm. (Courtesy of Mitsumi Inaba and Joanne B. Messick.)

OHS (hydrocytosis) is a heterogeneous group of disorders in humans characterized by moderate to severe hemolytic anemia with stomatocytes, an elevated mean corpuscular volume, and a reduced mean corpuscular hemoglobin concentration. The principal lesion involves a remarkable increase of Na+ influx into RBCs, resulting in a marked increase of intracellular Na+ and water content and a corresponding decrease of K+.50,86 The molecular basis of this permeability defect is unknown. Human OHS is frequently associated with reduced contents of a 31-kDa integral membrane protein 7.2b (stomatin), which may function in regulation of cation transport and a stretch- or pressure-sensitive system, in the RBC membrane from OHS patients.16,40,69 It is reported that stomatin-actin association is necessary for maintaining the structure and modulating the function of stomatin in RBCs.80 Hereditary Somatocytosis in Dogs and Cats Hereditary stomatocytosis (HSt) is recognized in dogs and cats with undefined etiologies. All disorders appear to be transmitted as autosomal recessive traits. HSt in Miniature and Standard Schnauzers Stomatocytosis inherited by autosomal recessive trait has been reported in Miniature and Standard Schnauzers (Fig. 29.6A) without clinical signs of disease.4,5,20 Hereditary stomatocytosis in Schnauzers is characterized by macrocytosis, relatively high packed cell volume, remarkably decreased mean corpuscular

hemoglobin concentration, and increased osmotic fragility. Erythrocyte survival is only slightly shortened. Affected Miniature Schnauzers are of normal stature. Stomatin is not deficient from the erythrocyte membrane in affected Standard Schnauzers.54 HSt in Chondrodysplastic Alaskan Malamute Dwarf Dogs In Alaskan Malamutes, chondrodysplasia (short-limbed dwarfism) occurs along with stomatocytosis14,15 (Fig. 29.6B). The affected Malamutes have macrocytosis, decreased mean corpuscular hemoglobin concentration, increased osmotic fragility, shortened RBC survival, reticulocytosis, erythroid hyperplasia, and increased iron turnover.58 Although heterozygous carriers have minor changes in their RBCs, they have a normal RBC lifespan and no dwarfism. RBC Na+ concentration and water content are increased in affected dogs. The pathogenesis of stomatocyte formation in Malamutes and Schnauzers is attributed to an increase in monovalent cation and, consequently, increased water content of RBCs as is reported in human HSt. However, the exact nature of membrane defects leading to the changes in RBC indices in these breeds is obscure. Familial Stomatocytosis-Hypertrophic Gastritis in Dogs Familial stomatocytosis-hypertrophic gastritis is a multiorgan disease with hemolytic anemia, HSt, and hyper-

CHAPTER 29: ERYTHROCYTE MEMBRANE DEFECTS

trophic gastritis described in the Drentse Partrijshond breed.64,65 The main clinical signs are diarrhea, jaundice, and ataxia and paresis of the pelvic limbs. Pathologic findings involve hypertrophic gastritis, progressive liver disease, renal cysts in aged subjects, and polyneuropathy.64 Erythrocytes from affected dogs show increased osmotic fragility. In contrast to Malamutes and Schnauzers with stomatocytosis, RBCs from affected Partrijshonds have normal mean corpuscular volume, slightly increased cell water with basically normal content, and fluxes of Na and K, suggesting a different mechanism for stomatocyte formation.65 Affected dogs have normal RBC membrane protein profile, but a decrease of phosphatidylcholine with altered fatty acid composition and simultaneous increase of sphingomyelin in both RBCs and plasma. It is suggested that this polysystemic disease is a disorder of lipid metabolism in which defective membrane function and RBC shape change are induced by abnormal phospholipid composition of the plasma. This is supported by a shortened half-life of RBCs from normal dogs after transfusion into dogs with this syndrome. Stomatocytic shape change may be attributed to membrane surface loss, presumably due to abnormal phospholipid composition of the bilayer. The exact relation between anomaly in phospholipid and hypertrophic gastritis and the primary cause of this syndrome are unknown. Hemolytic Anemia with Increased Erythrocyte Osmotic Fragility in Cats A hereditary RBC defect is suspected in Abyssinian and Somali cats with Coombs’ negative hemolytic anemia.38,39 The affected cats exhibited recurrent anemia (hematocrit, 15–25%); severe splenomegaly with extra medullary hemopoiesis, hemosiderosis, congestion, and lymphoid hyperplasia; loss of body mass; macrocytosis; and a few stomatocytes. The anemia was variably regenerative. The osmotic fragility of their RBCs was markedly increased. Splenectomy partially corrected the anemia and prevented hemolytic crises, but longterm survival remains unknown. DIAGNOSTIC APPROACHES TO ERYTHROCYTE MEMBRANE DEFECTS Genetic aberrations of RBC membranes described above are feasibly surveyed by evaluating RBC morphology, osmotic fragility, and RBC parameters. The osmotic fragility test is a simplified means to estimate the surface area/volume ratio of RBCs. It is most valuable in the diagnosis of HS but is also useful in evaluation of most forms of HE and overhydrated HSt (hydrocytosis). Several laboratory tests, such as Coombs’ test and Heinz body screening, may be required to eliminate a possibility that hemolytic anemia and abnormal RBC shapes result from extrinsic factors. As exemplified for bovine HS in Figure 29.2A, SDS-PAGE analysis followed by some immunochemical, biochemical, and biophysical

193

techniques often provides insights into primary defects of membrane skeletal and integral proteins resulting in HS, HE, and HPP. MEMBRANE TRANSPORT DEFECTS Deficiency and dysfunction of membrane transport systems may affect RBC homeostasis. Particularly, defects in transport of amino acids involved in glutathione metabolism have been reported to generate hemolytic anemia when RBCs are exposed to extrinsic factors including oxidants. Amino Acid Transport Deficiency in Animals Red blood cell glutathione deficiency, inherited as an autosomal recessive trait, occurs in Finnish Landrace sheep.75 Affected animals are not anemic but have shortened RBC lifespan.76 This is possibly caused by increased oxidant sensitivity as exemplified by Heinz bodies. Affected sheep are more likely to become anemic after the administration of oxidants in vivo.78 RBCs from the affected animals are defective in the transport system for various amino acids including cysteine.84,85 Consequently, cysteine uptake and glutathione synthesis are limited, and glutathione concentrations in RBCs are decreased to approximately 30% of normal. The transport deficiency appears to develop during reticulocyte maturation.77 A similar defect of amino acid transport is found in about 30% of thoroughbred horses13 but seems to cause no clinical signs. The lesion appears to result in increased amino acid levels and glutathione deficiency in some cases. High Membrane Na,K-ATPase Activity in Dogs Although canine reticulocytes have a considerable amount of membrane Na,K-ATPase (Na/K-pump) activity, its activity is rapidly lost during maturation into mature RBCs.46 Proteolytic degradation23 and extrusion of vesicles (exosomes)36 are likely involved in this process. As a consequence, dogs usually have RBCs with low K+ and high Na+ concentrations (LK RBCs).55 However, some Japanese Shiba and mongrel dogs have HK RBCs with high K+ and low Na+ concentrations, because the Na,K-ATPase protein and its activity are retained in mature RBCs.23,41,45 This HK phenotype representing immaturity of erythroid precursor cells26,47 is inherited in an autosomal recessive manner and has also been found in Japanese Akita10 and several breeds of Korean dogs.18,49 Although dogs with HK RBCs are not anemic, their RBCs have shortened lifespans,47 increased osmotic fragility, increased mean corpuscular volume, and normal mean corpuscular hemoglobin values, suggesting an increase in RBC water.45 The molecular basis for RBC HK and LK phenotypes is unknown. Due to the leak of K+ from RBCs, blood from HK dogs may cause pseudohyperkalemia in vitro after storage or on delaying plasma or serum separation.10

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Thus, care should be taken, particularly when stored blood from HK dogs is used for transfusion. Canine RBCs possess a high-affinity Na+-dependent transport system for glutamate and aspartate12,22 which resembles the kinetic and pharmacologic properties of the transporter in the brain.60–62 The increased concentration gradients of Na+ and K+ across the membrane produced by the presence of Na,K-ATPase with high activity accelerates transport of glutamate and aspartate into HK RBCs.23 The concentration of reduced glutathione is increased 5–7 times that of normal because the feedback inhibition of γ-glutamylcysteine synthetase by glutathione is released by glutamate accumulated in these cells at about 90 times that in normal RBCs.44 Some variant dogs of HK phenotype that lack the increase of RBC glutathione have been reported,17,19 suggesting that several independent mutations have emerged in these breeds.61 The accumulation of glutathione in the canine HK RBCs only provides improved protection against oxidative damage induced by acetylphenylhydrazine, but these RBCs are more susceptible to oxidative damage induced by 4-aminophenyl disulfide,48 onions,43,81 and sodium n-propylthiosulfate,83 one of the hemolytic thiosulfate compounds isolated from onions.82 The increased glutathione concentration accelerates the generation of superoxide through its redox reaction with the aromatic disulfide,48 but the exact mechanism by which HK RBCs are more sensitive to the thiosulfates remains to be clarified.

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Kohn B, Goldschmidt MH, Hohenhas AI, et al. Anemia, splenomegaly, and increased osmotic fragility of erythrocytes in Abyssinian and Somali cats. J Am Vet Med Assoc 2005;217:1483–1491. 40. Lande WM, Thiemann PVW, Mentzer WC. Missing band 7 protein in two patients with high Na, low K erythrocytes. J Clin Invest 1982;70:1273–1280. 41. Low PS, Willardson BM, Mohandas N, et al. Contribution of the band 3-ankyrin interaction to erythrocyte membrane mechanical stability. Blood 1991;77:1581–1586. 42. Lux SE. Spectrin-actin membrane skeleton of normal and abnormal red blood cells. Semin Hematol 1979;6:121–51. 43. Maede Y. High concentration of blood glutathione in dogs with acute hemolytic anemia. Jpn J Vet Sci 1977;39:187–189. 44. Maede Y, Kasai N, Taniguchi N. Hereditary high concentration of glutathione in canine erythrocytes associated with high accumulation of glutamate, glutamine, and aspartate. Blood 1982;59:883–889. 45. Maede Y, Inaba M, Taniguchi N. Increase of Na-K-ATPase activity, glutamate, and aspartate uptake in dog erythrocytes associated with hereditary high accumulation of GSH, glutamate, glutamine, and aspartate. Blood 1983;61:493–499.

CHAPTER 29: ERYTHROCYTE MEMBRANE DEFECTS 46. Maede Y, Inaba M. (Na,K)-ATPase and ouabain binding in reticulocytes from dogs with high K and low K erythrocytes and their changes during maturation. J Biol Chem 1985;260:3337–3343. 47. Maede Y, Inaba M. Energy metabolism in canine erythrocytes associated with inherited high Na+-and K+-stimulated adenosine phosphatase activity. Am J Vet Res 1987;48:114–118. 48. Maede Y, Kuwabara M, Sasaki A, et al. Elevated glutathione accelerates oxidative damage to erythrocytes produced by aromatic disulfide. Blood 1989;73:312–317. 49. Maede Y, Amano Y, Nishida A, et al. Hereditary high-potassium erythrocytes with high Na,K-ATPase activity in Japanese Shiba dogs. Res Vet Sci 1990;50:123–125. 50. Mentzer WC, Smith WB, Goldstone J, et al. Hereditary stomatocytosis: membrane and metabolism studies. Blood 1975;46:659–669. 51. Mohandas N, Gallagher PG. Red cell membrane: past, present, and future. Blood 2008;112:3939–3948. 52. Palek J, Sahr KE. Mutations of the red blood cell membrane proteins: from clinical evaluation to detection of the underlying genetic defect. Blood 1992;80:308–330. 53. Palek J, Jarolim P. Hereditary spherocytosis, elliptocytosis, and related disorders. In: Beutler E, Lichtman MA, Coller BS, Kipps TJ, eds. William’s Hematology, 5th ed. New York: McGraw-Hill, 1995;536–557. 54. Paltrinieri S, Comazzi S, Ceciliani F, et al. Stomatocytosis of Standard Schnauzers is not associated with stomatin deficiency. Vet J 2007;173:200–203. 55. Parker JC. Solute and water transport in dog and cat red blood cells. In: Ellory JC, Lew VL, eds. Membrane Transport in Red Cells. London: Academic Press, 1977;427–465. 56. Perrotta S, Borriello A, Scaloni A, et al. The N-terminal amino acids of human erythrocyte band 3 are critical for aldolase binding and protein phosphorylation: implications for band 3 function. Blood 2005;106: 4359–4366. 57. Peters LL, Shivdasani RA, Liu S-C, et al. Anion exchanger 1 (band 3) is required to prevent erythrocyte membrane surface loss but not to form the membrane skeleton. Cell 1996;86:917–927. 58. Pinkerton PH, Fletch SM, Brueckner PJ, et al. Hereditary stomatocytosis with hemolytic anemia in the dog. Blood 1974;44:557–567. 59. Ribeiro ML, Alloisio N, Almeida H, et al. Severe hereditary spherocytosis and distal renal tubular acidosis associated with the total absence of band 3. Blood 2000;96:1602–1604. 60. Sato K, Inaba M, Maede Y. Characterization of Na+-dependent l-glutamate transport in canine erythrocytes. Biochim Biophys Acta 1994;1195:211–217. 61. Sato K, Inaba M, Suwa Y, et al. Inherited defects of sodium-dependent glutamate transport mediated by glutamate/aspartate transporter in canine red cells due to a decreased level of transporter protein expression. J Biol Chem 2000;275:6620–6627. 62. Sato K, Inaba M, Baba K, et al. Cloning and characterization of excitatory amino acid transporters GLT-1 and EAAC1 in canine brain. J Vet Med Sci 2001;63:997–1002. 63. Shohet SB. Reconstitution of spectrin-deficient spherocytic mouse erythrocyte membranes. J Clin Invest 1979;64:483–494. 64. Slappendel RJ, van der Gaag I, van Nes JJ, et al. Familial stomatocytosis– hypertrophic gastritis (FSHG), a newly recognized disease in the dog (Drentse partrijshond). Vet Q 1991;13:30–40. 65. Slappendel RJ, Renooij W, de Bruijne JJ. Normal cations and abnormal membrane lipids in the red blood cells of dogs with familial stomatocytosis–hypertrophic gastritis. Blood 1994;84:904–909.

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66. Slappendel RJ, van Zwieten R, van Leeuwen M, et al. Hereditary spectrin deficiency in GoldenRetriever dogs. J Vet Intern Med 2005;19:187–192. 67. Smith JE, Moore K, Arens M, et al. Hereditary elliptocytosis with protein band 4.1 deficiency in the dog. Blood 1983;61:373–377. 68. Southgate CD, Chishti AH, Mitchell B, et al. Targeted disruption of the murine erythroid band 3 gene results in spherocytosis and severe haemolytic anaemia despite a normal membrane skeleton. Nat Gen 1996;14:227–230. 69. Stewart GW, Hepworth-Jones BE, Keen JN, et al. Isolation of cDNA coding for a ubiquitous membrane protein deficient in high Na, low K stomatocytic erythrocytes. Blood 1992;79:1593–1601. 70. Tanner MJA. Molecular and cellular biology of the erythrocyte anion exchanger (AE1). Semin Hematol 1993;30:34–57. 71. Tanner MJA. The structure and function of band 3 (AE1): recent developments (review). Mol Membr Biol 1997;14:155–165. 72. Tanner MJA. Band 3 anion exchanger and its involvement in erythroid and kidney disorders. Curr Opin Hematol 2002;9:133–139. 73. Toye AM, William’son RC, Khanfar M, et al. Band 3 Courcouronnes (Ser667Phe): a trafficking mutant differentially rescued by wild-type band 3 and glycophorin A. Blood 2008;111:5380–5389. 74. Tse WT, Lux SE. Hereditary spherocytosis and hereditary elliptocytosis. In: Scriver CR, Beaudet al., Sly WS, Valle D. eds. The Metabolic and Molecular Bases of Inherited Disease. 8th ed. New York: McGraw-Hill, 2001;4665–4727. 75. Tucker EM, Kilgour L. An inherited glutathione deficiency and a concomitant reduction in potassium concentration in sheep red cells. Experientia 1970;26:203–204. 76. Tucker EM. A shortened life span of sheep red cells with a glutathione deficiency. Res Vet Sci 1974;16:19–22. 77. Tucker EM, Young JD. Biochemical changes during reticulocyte maturation in culture. A comparison of genetically different sheep erythrocytes. Biochem J 1980;192:33–39. 78. Tucker EM, Young JD, Crowley C. Red cell glutathione deficiency: clinical and biochemical investigations using sheep as an experimental model system. Br J Haematol 1981;48:403–415. 79. White RA, Birkenmeier CS, Lux SE, et al. Ankyrin and the hemolytic anemia mutation, nb, map to mouse chromosome 8: Presence of nb allele is associated with a truncated erythrocyte ankyrin. Proc Natl Acad Sci USA 1990;87:3117–3121. 80. Wilkinson DK, Turner EJ, Parkin ET, et al. Membrane raft actinb deficiency and altered Ca2+-induced vesiculation in stomatin-deficient overhydrated hereditary stomatocytosis. Biochim Biophys Acta 2008;1778:125–132. 81. Yamato O, Maede Y. Susceptibility to onion-induced hemolysis in dogs with hereditary high erythrocyte reduced glutathione and potassium concentrations. Am J Vet Res 1992;53:134–137. 82. Yamato O, Hayashi M, Yamasaki M, et al. Induction of onion-induced haemolytic anaemia in dogs with sodium n-propylthiosulfate. Vet Rec 1998;142:216–219. 83. Yamato O, Yoshihara T, Ichihara A, et al. Novel Heinz body hemolysis factors in onion (Allium cepa). Biosci Biotechnol Biochem 1994;58:221–222. 84. Young JD, Ellory JC, Tucker EM. Amino acid transport defect in glutathione-deficient sheep erythrocytes. Nature 1975;254:156–157. 85. Young JD, Ellory JC, Tucker EM. Amino acid transport in normal and glutathione-deficient sheep erythrocytes. Biochem J 1976;154:43–48. 86. Zarkowsky HS, Oski FA, Shaafi R, et al. Congenital hemolytic anemia with high sodium, low potassium red cells. I. Studies of membrane permeability. New Engl J Med 1968;278:573–581.

C H A P T E R 30

Congenital Dyserythropoiesis DOUGLAS J. WEISS Poodle Macrocytosis Inherited Selective Malabsorption of Cobalamin Clinical Condition Congenital Dyserythropoiesis, Polymyopathy, and Cardiac Disease in English Springer Spaniels

Hereditary Stomatocytosis in Alaskin Malmutes (see Chapter 29) Congenital Anemia, Dyskeratosis, and Progressive Alopecia in Polled Hereford Calves

Acronyms and Abbreviations RBC, red blood cell; RDW, RBC distribution width.

ongenital conditions associated with dysplastic features in red blood cells (RBCs) and bone marrow include Poodle macrocytosis,2,8 inherited selective malabsorption of cobalamin,4,5,7 congenital dyserythropoiesis, polymyopathy, and cardiac disease in English Springer Spaniels,6 hereditary stomatocytosis in Alaskan Malamutes (see Chapter 29), and congenital anemia, dyskeratosis, and progressive alopecia in polled Hereford calves.9,10 In general, these conditions are characterized by dysplastic changes in RBCs or erythroid precursor cells associated with ineffective erythropoiesis.11 The mechanisms associated with accelerated intra-marrow cell death are discussed in Chapter 64.

C

of dyserythropoiesis include megaloblasts, binucleation, multinucleation, irregular nuclear shapes, nuclear fragmentation, nuclear-cytoplasmic asynchrony, and nuclear bridging (Fig. 30.2).2 Nuclear bridging is a unique feature seen in Poodle macrocytosis that is rarely seen in other congenital or acquired types of dyserythropoiesis or in myelodysplastic syndromes (Fig 30.3). Mitotic figures are increased in number. The nature of the hematopoietic defect has not been investigated. Bone marrow cytologic abnormalities in Poodle macrocytosis have some similarity to those seen in congenital dyserythropoietic anemia type I of children as well as those in Hereford cattle with congenital anemia, dyskeratosis, and progressive alopecia.5,9,10

POODLE MACROCYTOSIS

INHERITED SELECTIVE MALABSORPTION OF COBALAMIN

Poodle macrocytosis is a familial condition that occurs mostly in Toy and Miniature Poodles.2,8 The abnormality affects both sexes. Affected dogs do not have associated clinical signs, are not anemic, and do not have reticulocytosis. The most distinctive feature in the blood is marked macrocytosis with mean cell volumes varying between 85 and 105 fL.2,8 Metarubricytosis is a frequent finding and some of these cells show nuclear-cytoplasmic asynchrony. Additionally, Howell-Jolly bodies are increased in number and tend to be large and multiple (Fig. 30.1). Bone marrow is characterized by marked dyserythropoiesis and the morphologic changes resemble those associated with vitamin B12 deficiency. Features 196

An inherited selective malabsorption of cobalamin (vitamin B12) has been identified in Giant Schnauzers and has been reported in Border Collies, and in a Beagle and a cat.3,4,7 Cobalamin is obtained in all species by absorption from the intestinal tract. Cobalamin can be produced by microorganisms in the gastrointestinal tract of ruminants but must be liberated from digestion of foodstuffs by other species. Cobalamin is essential as a cofactor for the activity of the enzymes methylmalonicCoA mutase and methionine synthase.1 Deficiency of methylmalonic-CoA mutase results in methylmalonic acidemia and aciduria.1 Deficiency of methionine syn-

CHAPTER 30: CONGENITAL DYSERYTHROPOIESIS

197

FIGURE 30.3 Bone marrow aspirate from a dog with Poodle macrocytosis. Notice the presence of a rubricyte with nuclear bridging (center of photo).

FIGURE 30.1 Blood from a dog with poodle macrocytosis. Notice the presence of an RBC with multiple large Howell-Jolly bodies.

macrocytic anemia with cobalamin deficiency, the anemia in dogs, rodents, horses, pigs, and ruminants is normocytic and normochromic.3,4 In dogs, both large and small RBCs are present in the blood with cobalamin deficiency.3,4 This results in an increase in red cell distribution width (RDW) but the mean cell volume remains within the reference interval. Clinical Condition

FIGURE 30.2 Erythroid precursor cells in bone marrow from a dog with Poodle macrocytosis. Notice the presence of misshapen and fragmented nuclei in rubricytes.

thase causes homocysteinemia and inhibits conversion of 5-methyl-tetrahydrahydrofolate to tetrahydrahydrofolate.1 Tetrahydrahydrofolate is an essential cofactor for enzymes of purine and pyrimidine synthesis.1 Resultant inhibition of nucleic acid synthesis is most noticeable as inhibition of hematopoiesis in adult animals.3,4 Inhibition of nuclear maturation typically results in a non-regenerative anemia, with megaloblastosis, neutropenia, and hypersegmented neutrophils.3,4 Although humans and non-human primates develop a

Inherited selective malabsorption of cobalamin in Giant Schnauzers is an autosomal recessive condition caused by lack of a receptor for intrinsic factor-cobalamin complex (i.e. cubilin) on the brush border of the small intestine and renal tubular epithelial cells.3,4 Affected dogs develop anorexia, lethargy, and cachexia by 2–3 months of age. Blood cell morphology is characterized initially by a mild neutropenia followed by development of a chronic non-regenerative normocytic normochromic anemia, acanthocytosis, and increased RDW.3,4 RBC morphology is characterized by many small RBCs, ovalocytes, and occasional megaloblasts (Fig. 30.4). Occasional hypersegmented neutrophils and giant platelets are also present. Bone marrow granulocytes are characterized by hypersegmented neutrophils and giant bands and metamyelocytes. Erythroid cells in marrow are characterized by nuclear-cytoplasmic asynchrony and abnormal nuclear chromatin clumping. Diagnosis of cobalamin deficiency is best made by determining serum cobalamin concentrations.3,4 Low serum cobalamin concentration appears to be the earliest indicator of cobalamin deficiency. Increased serum methylmalonic acid and serum total homocysteine concentrations can also be measured. Affected dogs respond well to daily injections of vitamin B12.3,4 Alternatively, one megadose of 1 mg of vitamin B12 is sufficient to maintain an affected dog in remission for 1 month. Complete resolution of hematologic signs and growth retardation occurs with parenteral administration of cyanocobalamin.

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CONGENITAL ANEMIA, DYSKERATOSIS, AND PROGRESSIVE ALOPECIA IN POLLED HEREFORD CALVES

FIGURE 30.4 Blood from a dog with selective malabsorption of cobalamin. Notice the presence of a megaloblast exhibiting an immature nucleus and a fully hemaglobinized cytoplasm.

Hereford calves with congenital anemia, dyskeratosis, and progressive alopecia have a congenital syndrome characterized by dyserythropoiesis and progressive alopecia associated with dyskeratotic hyperkeratosis.9,10 A familial pattern has been demonstrated which is consistent with a simple autosomal recessive mode of inheritance. Clinically, the condition is slowly progressive but is frequently fatal.9 Iron, cobalamin, and folate deficiencies were ruled out as causes and viral agents were not identified in affected calves. Affected calves have a macrocytic normochromic anemia with a mild reticulocytosis. Dyserythropoiesis in bone marrow is characterized by multinuclearity, nuclear bridging, irregular nuclear shapes, and irregular chromatin patterns. These dysplastic changes resemble those associated with Poodle macrocytosis and congenital dyserythropoiesis type I in humans.2,5

REFERENCES CONGENITAL DYSERYTHROPOIESIS, POLYMYOPATHY, AND CARDIAC DISEASE IN ENGLISH SPRINGER SPANIELS Congenital dyserythropoiesis, polymyopathy, and cardiac disease has been described in three related English Springer Spaniels.6 Clinical signs include regurgitation, stunted growth, stiff gait, skeletal muscle wasting, cardiomegaly, megaesophagus, and generalized muscle atrophy that is most pronounced in temporal muscles. All dogs have a mild to moderate microcytic normochromic non-regenerative anemia with marked metarubricytosis. Alterations in RBC morphology included spherocytes, schistocytes, dacryocytes, codocytes, and vacuolated RBCs. Bone marrow is characterized by erythroid hyperplasia and dyserythropoiesis. Features of dyserythropoiesis include lobulated nuclei, marked binucleation, arrested mitotic figures, and cytoplasmic vacuolization. The hematologic condition remain stable but the polymyopathy is slowly progressive.

1. Allen RH, Stabler SP, Savage DG, et al. Metabolic abnormalities in cobalamin (vitamin B12) and folate deficiency. FASEB J 1993;7:1344–1353. 2. Canfield PJ, Watson ADJ. Investigations of bone marrow dyscrasia in a poodle with macrocytosis. J Comp Pathol 1989;101:269–278. 3. Fyfe JC, Giger U, Hall CA, et al. Inherited selective intestinal cobalamin malabsorption and cobalamin deficiency in dogs. Pediatr Res 1991;29:24–31. 4. Fyfe JC, Jezyk PF, Giger U, et al. Inherited selective malabsorption of vitamin B12 in giant schnauzers. J Am Anim Hosp Assoc 1989;25:533–539. 5. Heimpel H, Forteza-Vila J, Queisser W, et al. Electron and light microscopic study of the erythroblasts of patients with congenital dyserythropoietic anemia. Blood 1971;37:299–308. 6. Holland CT, Canfield PJ, Watson ADJ, et al. Dyserythropoiesis, polymyopathy, and cardiac disease in three related English springer spaniels. J Vet Intern Med 1991;5:151–159. 7. Outerbridge CA, Myers SL, Giger U. Hereditary cobalamin deficiency in border collie dogs. J Vet Intern Med 1996;10:169–175. 8. Schalm OW. Erythrocyte macrocytosis in miniature and toy poodles. Canine Pract 1976;3:55–57. 9. Steffens DJ, Elliot GS, Leipold, HW, et al. Congenital dyserythropoiesis and progressive alopecia in Polled Hereford calves – hematologic, biochemical, bone marrow cytologic, electrophoretic, and flow cytometric findings. J Vet Diagn Invest 1992;4:31–37. 10. Steffen DJ, Leipoid HW, Elliott GS, et al. Ultrastructural findings in congenital anemia, dyskeratosis, and progressive alopecia in Polled Hereford calves. Vet Pathol 1992;29:203–209. 11. Weiss DJ. Recognition and classification of dysmyelopoiesis in the dog: A review. J Vet Intern Med 2005;19:147–154.

CHAPTER

31

Anemia Caused by Rickettsia, Mycoplasma, and Protozoa ROBIN W. ALLISON and JAMES H. MEINKOTH Rickettisa Ehrlichiosis Canine monocytic ehrlichiosis Canine granulocytic ehrlichiosis Anaplasmosis Granulocytic anaplasmosis Granulocytic anaplasmosis in dogs Granulocytic anaplasmosis in horses Granulocytic anaplasmosis in ruminants Erythrocytic anaplasmosis in ruminants Hemotrophic Mycoplasmas (Hemoplasmas) Canine hemotrophic mycoplasmas Feline hemotrophic mycoplasmas Swine hemotrophic mycoplasmas Ruminant hemotrophic mycoplasmas Camelid hemotrophic mycoplasmas

Protozoa Theileriosis Ruminants Equines Canines Babesiosis Canines Large canine Babesia Small canine Babesia Ruminants Equines Felines Feline Cytauxzoonosis Trypanosomiasis Sarcocystosis Haemoproteus Plasmodium

Acronyms and Abbreviations APBT, American Pit Bull Terrier; CME, canine monocytic ehrlichiosis; CNS, central nervous system; EDTA, ethylenediaminetetraacetic acid; FeLV, feline leukemia virus; FIV, feline immunodeficiency virus; HGE, human granulocytic ehrlichiosis; PCR, polymerase chain reaction; PCV, packed cell volume; RBC, red blood cell.

A

seemingly ever-increasing number of rickettsial, mycoplasmal and protozoal agents are being recognized as the cause of anemia in veterinary species.2 The anemia is frequently hemolytic in nature; however, with some organisms the anemia is nonregenerative resulting either from cytokine suppression of hematopoiesis or bone marrow pathology. Of importance to the veterinary hematologist is that many of these organisms can be visualized on blood films, providing a definitive diagnosis.

species. Many of the genera in this order have undergone reclassification. The order now contains two families, Anaplasmataceae and Rickettsiaceae. The Anaplasmataceae are intracellular parasites that grow within a cytoplasmic vacuole in the host cell as opposed to the Rickettsiaceae that infect the host cell cytoplasm or nucleus and are not bounded by a vacuole. Ehrlichiosis Canine Monocytic Ehrlichiosis (CME)

RICKETTSIA The order Rickettsiales contains many organisms that cause significant hematologic disease in veterinary

Ehrlichia canis is the most significant cause of canine monocytic ehrlichiosis worldwide. Transmission is by tick vectors (primarily Rhipicephalus sanguineous) and direct blood inoculation. Organisms replicate in cells of 199

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Morulae of E. ewingii are found predominantly in neutrophils and rarely in eosinophils. Morulae are similar in appearance to those of E. canis, although they sometimes have a “packeted” appearance rather than fine punctate granulation typical of E. canis. Organisms are seen in the blood transiently during acute infection, but parasitemia is frequently high enough to allow diagnosis by microscopy.21 Organisms may be identified in neutrophils of synovial fluid samples in dogs with polyarthritis. No specific serologic test exists for E. ewingii, but there is cross-reaction with E. canis serology. Infection can also be confirmed by PCR.21 Anaplasmosis

FIGURE 31.1 Blood film from a dog with E. canis infection. A morula is present in a disrupted large granular lymphocyte. The morula has the distinctive punctuate, granular appearance. Romanowsky stain.

Historically, members of the genus Anaplasma infected red blood cells (RBCs) of ruminants causing hemolytic anemia. However, with the reclassification of several Ehrlichia spp. as Anaplasma spp., members of this genus may infect RBCs (A. marginale), granulocytes (A. phagocytophilum) or platelets (A. platys). Anemia is not a main feature of A. platys infection. Granulocytic Anaplasmosis (A. phagocytophilum)

the mononuclear phagocyte system and can be seen in lymphocytes, monocytes or macrophages.42 Within mononuclear cells, organisms multiply by binary fission in a cytoplasmic vacuole. They appear by light microscopy as round, variably-sized, basophilic cytoplasmic inclusions termed morulae that have a punctate, granular appearance (Fig. 31.1). Parasitemia in peripheral blood is low and transient following initial infection. Diagnosis is generally made by serology or PCR, rather than visualization of organisms. Hematologic findings depend on the stage of infection. Acute infection is associated with thrombocytopenia, that may be marked, and non-regenerative anemia. Leukopenia and neutropenia are less consistent. After acute infection, there is a subclinical phase during which anemia typically resolves but thrombocytopenia may be persistent. Dogs that develop the severe chronic phase typically have profound pancytopenia. Bone marrow aspirates obtained in these animals are hypocellular with marked reductions of all hematopoiectic cell lines (see Chapter 19).8 Canine Granulocytic Ehrlichiosis The causative agent of granulocytic ehrlichiosis is E. ewingii.1 Transmitted by Amblyomma americanum, E. ewingii has been reported from the southern and southeastern United States.21 Polyarthritis is a common clinical manifestation of infection.21 Clinical disease is considerably milder than that caused by E. canis. Hematologic findings include thrombocytopenia and a mild to moderate non-regenerative anemia.21 E. ewingii infection is not associated with a chronic-phase pancytopenia.

Anaplasma phagocytophilum now includes the organisms previously described as Ehrlichia phagocytophila, Ehrlichia equi, and the human granulocytic ehrlichiosis (HGE) agent. Granulocytic Anaplasmosis in Dogs A. phagocytophilum infection in dogs is widespread. In the United States, it occurs most commonly in the upper Midwest and along the Pacific coast.22 Clinical signs are non-specific and include fever, anorexia, depression, lymphadenopathy, and splenomegaly.22 Some dogs may manifest either a neutrophilic polyarthritis or acute CNS signs, including seizures. Thrombocytopenia, mild to severe, is the most frequent hematologic abnormality and occurs in the majority of cases.22 Mild to moderate non-regenerative anemia may also occur.22 Morulae are similar to those of E. ewingii and can be seen in neutrophils at a fairly high parasitemia during the acute phase of infection. Granulocytic Anaplasmosis in Horses A. phagocytophilum infection in horses has been reported from many states in the USA as well as Canada, Brazil, and Europe.32 It causes a febrile illness characterized by anorexia, limb edema, mild petechiation, and reluctance to move. The predominant hematologic finding is a profound thrombocytopenia, variable neutropenia, and mild anemia may be present.18 The parasitemia may be high with morulae readily seen in blood neutrophils during acute infection. Granulocytic Anaplasmosis in Ruminants Anaplasma phagocytophilum is associated with abortion in cattle and increased susceptibility to other infections in lambs throughout Europe. Thrombocytopenia and leukopenia

CHAPTER 31: ANEMIA CAUSED BY RICKETTSIA, MYCOPLASMA, AND PROTOZOA

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are common hematologic findings, but anemia is not a prominent feature.

HEMOTROPHIC MYCOPLASMAS (HEMOPLASMAS)

Erythrocytic Anaplasmosis in Ruminants

Epierythrocytic parasites previously known as Haemobartonella and Eperythrozoon have been reclassified to the genus Mycoplasma and renamed, some with the designation Candidatus indicating they are incompletely described. As a group, these hemoplasmas have worldwide distribution, infect a wide variety of vertebrate animals, and share similar characteristics and morphologic features.38 On routine blood films they are small basophilic round, rod, or ring-shaped organisms that appear on RBCs individually or in chains. Electron microscopy has shown them to have a single limiting membrane and lack nuclei or distinct organelles (Fig. 31.3). Organisms are typically found in depressions and invaginations in RBC membranes and appear connected to the membrane by delicate fibrils (Fig. 31.4).57 Hemoplasmas vary in their ability to cause clinically apparent disease, but infected animals may remain carriers despite antibiotic therapy; parasitemia may reemerge in times of stress. Hypoglycemia secondary to glucose consumption by the bacteria has been reported in heavily parasitized pigs, sheep, llamas, and calves; however, rapid bacterial glycolysis in vitro may also cause artifactually decreased blood glucose concentrations.9 Historically, diagnosis has relied on detection of hemoplasmas on blood films. Organisms are generally found associated with the RBC membrane, but sometimes are free in the plasma (Fig. 31.5). Hemoplasmas

Anaplasmosis is a significant cause of disease in cattle.29 Two species are described: Anaplasma marginale is widespread and Anaplasma centrale occurs in South America and South Africa. Wild ruminants are reservoirs, but do not usually manifest disease. A. marginale infection causes fever and mild to marked hemolytic anemia. Numerous tick species serve as vectors and mechanical transmission may also occur. The hemolysis is primarily extravascular; hemoglobinuria is not seen. Disease is most pronounced in older animals; calves 6–9 months old are relatively resistant and disease is mild. Anaplasma centrale is generally less pathogenic than A. marginale. Organisms are visible in erythrocytes during acute infection. They are round, 0.5–1 μm, basophilic bodies frequently present on the periphery of RBCs (Fig. 31.2). They resemble Howell-Jolly bodies, from which they must be differentiated. Anaplasma centrale organisms are similar, but not peripherally located in RBCs. Although long thought to occur only in RBCs, A. marginale has been shown to invade microvascular endothelial cells both in vivo and in vitro.11 After infection, parasitemia increases until the hemolytic crisis, frequently with more than 50% of RBCs infected.29 After development of the anemia, parasite numbers decline due to removal of infected RBCs. Cattle that survive acute infection become chronic carriers and act as reservoirs. Anaplasma ovis causes anaplasmosis in sheep and goats. The disease occurs in tropical and subtropical regions throughout the world. Morphologically, A. ovis is similar to A. marginale.

FIGURE 31.2 Blood film from a steer experimentally infected with A. marginale. Marked parasitemia with more than 50% of RBCs containing inclusions. Note the marginal position of most organisms. Romanowsky stain.

FIGURE 31.3 Transmission electron micrograph of a hemoplasma parasite (M. haemofelis) illustrating a single limiting membrane separating the cytoplasm of the organism from the host RBC. Delicate fibrils attach the organism to the host cell. (Reprinted from Messick JB. Hemotrophic mycoplasmas (hemoplasmas): a review and new insights into pathogenic potential. Vet Clin Pathol 2004;33:2–13, with permission.)

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FIGURE 31.4 Scanning electron micrograph of several hemoplasma parasites (M. suis) within shallow depressions on the surface of an RBC. (Reprinted from Messick JB. Hemotrophic mycoplasmas (hemoplasmas): a review and new insights into pathogenic potential. Vet Clin Pathol 2004;33:2–13, with permission.)

FIGURE 31.6 Blood film from a cat with M. haemofelis infection prepared from aged blood. Organisms have detached from RBCs and are degenerating, appearing as clumps of pink granular material in the background. Romanowsky stain.

dissociate from RBCs and die after a variable amounts of time in EDTA.43 In aged samples, dead organisms can appear as pale pink clumped granular material in the background of the blood film (Fig. 31.6). Development of sensitive PCR assays capable of discriminating between various hemoplasmas has greatly enhanced diagnosis of these parasites, and has led to the identification of several new Mycoplasma species.

haematoparvum”, infect domestic canines. M. haemocanis causes acute hemolytic anemia in dogs that are splenectomized, immunocompromised, or have concurrent infections.39 These dogs usually have sufficient circulating parasites for them to be easily demonstrated on blood films. Anemia may be severe; spherocytes may be observed and Coombs’ test results may be positive. By contrast, healthy dogs typically develop chronic, asymptomatic infections with sporadic, low-grade parasitemia.39 Transmission is presumed to occur via arthropod vectors and has been experimentally demonstrated with the brown dog tick, Rhipicephalus sanguineus. Transmission of organisms may also occur via blood transfusion from asymptomatic carriers, but the recipient must be splenectomized or otherwise severely compromised for hemolytic disease to occur.39 On routine blood films M. haemocanis organisms are quite pleomorphic, but have a tendency to form long chains across the surface of RBCs, sometimes with “violin bow” morphology. Individual organisms are 1–3 μm and can appear round, rod-shaped, or in ring form (Fig. 31.7).57 “Candidatus Mycoplasma haematoparvum” was recently identified in a California dog with a T cell lymphoproliferative disease that had undergone splenectomy and blood transfusions.51 Molecular characterization found the organism to be most similar to “Candidatus Mycoplasma haemominutum” in cats. These organisms are smaller than M. haemocanis or M. haemofelis (15.0 μmol/L) left shift neutrophilia (bands ≥3.0 × 109/L) thrombocytopenia (platelets 5 days), the absence of regeneration has been attributed to ineffective erythropoiesis or PRCA (see below).46,51 These two bone marrow conditions appear to occur with similar frequency in cats with nonregenerative IMHA.2,51 Since bone marrow aspirates were not done in the 19 cats of one report32 and some cats had been sick for 2,900 lymphocytes/μL of blood, is an uncommon occurrence in the dog (Table 48.5). Physiologic leukocytosis may cause a transient lymphocytosis. Physiologic lymphocytosis does not occur frequently in the adult dog but occurs more frequently in puppies. This form of lymphocytosis can be avoided if animals are not overly excited at time of venipuncture. Examination of a second sample of blood, collected from the calmed or tranquilized dog, can be used to distinguish transient lymphocytosis of

TABLE 48.5 Causes of Lymphocytosis and Lymphopenia in Dogs Lymphocytosis Chronic antigenic stimulation

Hypoadrenocorticism Lymphoid neoplasia

Aspergillosis, actinomyces, Babesia canis infection, blastomycosis, brucellosis, ehrlichiosis, encephalitozoonosis, leishmaniasis, pneumocystis pneumonia, Rocky Mountain spotted fever, Trypanosoma cruzi gambiense infection Lymphocytic leukemia (acute or chronic), lymphosarcoma, thymoma

Physiologic leukocytosis (not common in the dog) Lymphopenia Acute systemic bacterial infections Corticosteroids

Disruption of lymph node architecture Immunodeficiency syndromes

Immunosuppressive drugs Loss of lymphocyte rich fluids

Malignant neoplasia Radiation Viral infections (acute stages usually)

Septicemia, endotoxemia Stress-induced leukocytosis (pain, extremes in body temperature), hyperadrenocorticism (Cushing’s syndrome), exogenous corticosteroid therapy or ACTH administration Generalized granulomatous disease, multicentric lymphosarcoma Combined T- and B-cell deficiency of Basset hounds, combined immunodeficiency of Jack Russell terriers Protein-losing enteropathy (lymphangiectasia), ulcerative enteritis, granulomatous enteritis, chylothorax, chyloperitoneum Lymphosarcoma, lymphocytic leukemia Canine distemper, infectious canine hepatitis, coronavirus enteritis, canine parvovirus

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physiologic leukocytosis from lymphocytosis of pathologic origin.36 Persistent antigenic stimulation in chronic infections or inflammatory reactions may cause lymphocytosis in dogs. Chronic canine ehrlichiosis11 and Rocky Mountain spotted fever are diseases that result in proliferation of lymphocytes and expansion of the blood lymphocyte pool. Trypanosomiasis,3 leishmaniasis,24 and brucellosis61 also may cause lymphocytosis by a similar mechanism. Lymphoid neoplasia can cause lymphocytosis.51 Malignant lymphoma, acute or chronic lymphocytic leukemia, and thymoma have been associated with lymphocytosis (see Chapters 69 and 77). The highest lymphocyte counts usually are associated with chronic lymphocytic leukemia.41 Malignant lymphoma, a neoplastic proliferation of lymphocytes within tissue, also can produce lymphocytosis in approximately 10% of dogs that have this disease. Hypoadrenocorticism (Addison’s disease) has been reported to cause lymphocytosis in 11–20% of affected dogs.66 The lack of lymphopenia in a severely stressed dog provides supportive evidence for glucocorticoid deficiency.

rent infections such as mycobacteriosis (see Chapter 57).8 Autosomal recessive severe combined immunodeficiency of Jack Russell terriers results in puppies with marked lymphopenia and decreases in the concentrations of serum immunoglobulins secondary to hypoplasia of all lymphoid tissues.4

Causes and Mechanisms of Lymphopenia

Morphologic Alterations in Lymphocytes

Lymphopenia, defined as 50,000/μL of blood, may be detected in monocytic or myelomonocytic leukemia. Abnormal monocyte morphology and numerous blast cells can be detected in the blood or bone marrow during these neoplastic diseases. Causes and Mechanisms of Monocytopenia Reference intervals for monocyte counts in the blood from healthy dogs are fairly broad. Therefore, it is difficult to document monocytopenia. Thus, this alteration is of little clinical significance. However, monocytopenia may be detected in some cases of acute pancytopenia due to several causes.36 LEUKEMIA The definition of leukemia is the neoplastic proliferation of hemopoietic cells originating in the bone marrow (see Section V). Leukemias (i.e. myeloproliferative disorders) are classified by cell type and differentiation, time course of disease process (acute or chronic), and presence or absence of neoplastic cells in circulation. Myeloproliferative disorders reported in the dog include granulocytic (myeloid) leukemia, myelomonocytic leukemia, monocytic leukemia, basophilic leukemia, mast cell leukemia, erythremic myelosis, polycythemia vera, megakaryocytic leukemia, and essential thrombocythemia. Lymphoproliferative diseases reported in the dog include lymphocytic leukemia and plasma cell leukemia. Diagnosis of these disorders is facilitated by examination of Romanowsky-stained blood films and bone marrow aspirates. Histologic examination of bone marrow core biopsies may be required in some cases. Use of immunohistochemistry greatly aids diagnosis in poorly differentiated cell types.31,36,39,40

REFERENCES 1. Aroch I, Perl S, Markovics A. Disseminated eosinophilic disease resembling idiopathic hypereosinophilic syndrome in a dog. Vet Rec 2001;149:386–389. 2. Ballmer Rusca E, Hauser B. Case report: persistent eosinophilia in a dog. Hypereosinophilic syndrome? Kleintierpraxis 1993;38:137–138. 3. Barr SC, Gossett KA, Klei TR. Clinical, clinicopathologic, and parasitologic observations of trypanosomiasis in dogs infected with North American Trypanosoma cruzi isolates. Am J Vet Res 1991;52:954–960. 4. Bell TG, Butler KL, Sill HB, et al. Autosomal recessive severe combined immunodeficiency of Jack Russell terriers. J Vet Diagn Invest 2002;14:194–204. 5. Bertram TA. Neutrophilic leukocyte structure and function in domestic animals. Adv Vet Sci Comp Med 1985;30:91–129. 6. Cain GR, Kawakami T, Taylor N, et al. Effects of administration of recombinant human interleukin-2 in dogs. Comp Hematol Intl 1992;2:201–207. 7. Campbell KL. Canine cyclic hematopoiesis. Comp Cont Educ Pract Vet 1985;7:57–60. 8. Carpenter JL, Myers AM, Conner MW, et al. Tuberculosis in five Basset Hounds. J Am Vet Med Assoc 1988;192:1563–1568. 9. Chinn DR, Myers RK, Matthews JA. Neutrophilic leukocytosis associated with metastatic fibrosarcoma in a dog. J Am Vet Med Assoc 1985;186:806–809. 10. Clinkenbeard KD, Cowell RL, Tyler RD. Disseminated histoplasmosis in dogs: 12 cases (1981–1986). J Am Vet Med Assoc 1988;193:1443–1447. 11. Codner EC, Farris-Smith LL. Characterization of the subclinical phase of ehrlichiosis in dogs. J Am Vet Med Assoc 1986;189:47–50.

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12. Couto CG. Tumor-associated eosinophilia in a dog. J Am Vet Med Assoc 1984;184:837. 13. Couto CG, Kallet AJ. Preleukemic syndrome in a dog. J Am Vet Med Assoc 1984;184:1389–1392. 14. Domina F, Giudice E, Britti D. Tetracycline-induced eosinophilia in the dog. J Vet Pharmacol Ther 1997;20(Suppl):259–260. 15. Felsburg PJ, Jezyk PF, Haskins ME. A canine model for variable combined immunodeficiency. Clin Res 1982;30:347A. 16. Fossum TW, Birchard SJ, Jacobs RM. Chylothorax in 34 dogs. J Am Vet Med Assoc 1986;188:1315–1318. 17. Fyfe JC, Giger U, Hall CA, et al. Inherited selective intestinal cobalamin malabsorption and cobalamin deficiency in dogs. Pediat Res 1991; 29:24–31. 18. Gaunt SD, Baker DC. Hemosiderin in leukocytes of dogs with immunemediated hemolytic anemia. Vet Clin Pathol 1986;15:8–10. 19. Gaunt SD, Pierce KR. Effects of estradiol on hematopoietic and marrow adherent cells of dogs. Am J Vet Res 1986;47:906–909. 20. Giger U, Boxer LA, Simpson PJ, et al. Deficiency of leukocyte surface glycoproteins Mol, LFA-1, and Leu M5 in a dog with recurrent bacterial infections: an animal model. Blood 1987;69:1622–1630. 21. Gossett KA, Carakostas MC. Effect of EDTA on morphology of neutrophils of healthy dogs and dogs with inflammation. Vet Clin Pathol 1984;13:22–25. 22. Gossett KA, MacWilliams PS. Ultrastructure of canine toxic neutrophils. Am J Vet Res 1982;43:1624–1637. 23. Gossett KA, MacWilliams PS, Cleghorn B. Sequential morphological and quantitative changes in blood and bone marrow neutrophils in dogs with acute inflammation. Can J Comp Med 1985;49:291–297. 24. Groulade P. Canine leishmaniasis. Clinical haematology and biology of leishmaniasis. Anim Compagn 1977;12:121–128. 25. Halliwell REW, Schemmer KR. The role of basophils in the immunopathogenesis of hypersensitivity to fleas (Ctenocephalides felis) in dogs. Vet Immunol Immunopathol 1987;15:203–213. 26. Haskins ME, Desnick RJ, DiFerrante N, et al. Beta-glucuronidase deficiency in a dog: a model of human mucopolysaccharidosis VII. Pediatric Research 1984;18:980–984. 27. Hoenig M. Six dogs with features compatible with myelonecrosis and myelofibrosis. J Am Anim Hosp Assoc 1989;25:335–339. 28. Hopkins J, McConnell I. Immunological aspects of lymphocyte recirculation. Vet Immunol Immunopathol 1984;6:3–33. 29. Horwitz M, Benson KF, Duan Z, et al. Hereditary neutropenia: dogs explain human neutrophil elastase mutations. Trends Mol Med 2004;10:163–170. 30. Huntley JF. Mast cells and basophils: a review of their heterogeneity and function. J Comp Pathol 1992;107:349–372. 31. Jain NC. Schalm’s Veterinary Hematology, 4th ed. Philadelphia: Lea & Febiger, 1986. 32. Jain NC. Essentials of Veterinary Hematology. Philadelphia: Lea & Febiger, 1993. 33. Jensen AL, Nielson OL. Eosinophilic leukemoid reaction in a dog. J Small Anim Pract 1992;33:337–340. 34. Kull PA, Hess RS, Craig LE, et al. Clinical, clinicopathologic, radiographic and ultrasonographic characteristics of intestinal lymphangiectasia in dogs: 17 cases (1996–1998). J Am Vet Med Assoc 2001;219:197–202. 35. Lappin MR, Latimer KS. Hematuria and extreme neutrophilic leukocytosis in a dog with renal tubular carcinoma. J Am Vet Med Assoc 1988;192:1289–1292. 36. Latimer KS. Leukocytes in health and disease. In: Ettinger SJ, Feldman EC, eds. Textbook of Veterinary Internal Medicine. Diseases of the Dog and Cat, 4th ed. Philadelphia: WB Saunders, 1995;1892–1946. 37. Latimer KS, Duncan JR, Kircher IM. Nuclear segmentation, ultrastructure and cytochemistry of blood cells from dogs with Pelger-Huët anomaly. J Comp Pathol 1987;97:61–72. 38. Latimer KS, Kircher IM, Lindl PA, et al. Leukocyte function in Pelger-Huët anomaly of dogs. J Leuk Biol 1989;45:301–310. 39. Latimer KS, Prasse KW. Leukocytes. In: Latimer KS, Mahaffey EA, Prasse KW, eds. Duncan and Prasse’s Veterinary Laboratory Medicine: Clinical Pathology, 4th ed. Ames: Iowa State Press, 2003;46–79. 40. Latimer KS, Rakich PM. Clinical interpretation of leukocyte responses. Vet Clin N Am Small Anim Pract 1989;19:637–668. 41. Leifer CE, Matus RE. Chronic lymphocytic leukemia in the dog: 22 cases (1974–1984). J Am Vet Med Assoc 1986;189:214–217. 42. Lilliehöök I. Diurnal variation of canine blood leukocyte counts. Vet Clin Pathol 1997;26:113–117. 43. Losco PE. Local and peripheral eosinophilia in a dog with anaplastic mammary carcinoma. Vet Pathol 1986;23:536–538. 44. Makimura S, Kinjo H. Cytochemical identification of canine circulating leukocytes parasitized with the gametocyte of Hepatozoon canis. J Vet Med Sci 1991;53:963–965. 45. Mears EA, Raskin RE, Legendre AM. Basophilic leukemia in a dog. J Vet Int Med 1997;11:92–94. 46. Meyer DJ, Coles EH, Rich LJ. Veterinary Laboratory Medicine: Interpretation and Diagnosis. Philadelphia: WB Saunders, 1992.

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47. Neer TM, Dial SM, Pechman R, et al. Clinical vignette. Mucopolysaccharidosis VI in a miniature pinscher. J Vet Int Med 1995;9:429–433. 48. O’Keefe DA, Couto CG, Burke-Schwartz C, et al. Systemic mastocytosis in 16 dogs. J Vet Intern Med 1987;1:75–80. 49. Pollock RVH. The parvoviruses. Part II. Canine parvovirus. Comp Cont Educ Pract Vet 1984;6:653–664. 50. Pryor WH Jr, Bradbury RP. Haemobartonella canis infection in research dogs. Lab Anim Sci 1975;25:566–569. 51. Raskin RE, Krehbiel JD. Prevalence of leukemic blood and bone marrow in dogs with multicentric lymphoma. J Am Vet Med Assoc 1989;194:1427–1429. 52. Rudmann DG, McNerney ME, VanderEide SL, et al. Epididymal and systemic phospholipidosis in rats and dogs treated with the dopamine D3 selective antagonist PNU-177864. Toxicol Pathol 2004;32:326–332. 53. Schalm OW. Uncommon hematologic disorders: spirochetosis, trypanosomiasis, leishmaniasis, Pelger-Huët anomaly. Canine Pract 1979;6:46–49. 54. Shull RM, DeNovo RC, McCraken MD. Megakaryoblastic leukemia in a dog. Vet Pathol 1986;23:533–536. 55. Stockham SL, Schmidt DA, Curtis KS, et al. Evaluation of granulocytic ehrlichiosis in dogs of Missouri including serologic status to Ehrlichia canis, Ehrlichia equi and Borrelia burgdorferi. Am J Vet Res 1992;53:63–68. 56. Stockham SL, Scott MA. Leukocytes. In: Fundamentals of Veterinary Clinical Pathology. Ames: Iowa State Press, 2002;49–83. 57. Teske E. Estrogen-induced bone marrow toxicity. In: Kirk RW, ed. Current Veterinary Therapy IX. Philadelphia: W.B.Saunders, 1986;495–498.

58. Thompson JP, Christopher MM, Ellison GW, et al. Paraneoplastic leukocytosis associated with a rectal adenomatous polyp in a dog. J Am Vet Med Assoc 1992; 201:737–738. 59. Tomlinson MJ, Jennings PB, Wendt JB, et al. Adenocarcinoma of the lung with secondary pericardial effusion and leukemoid response in a dog. J Am Vet Med Assoc 1973;63:257–258. 60. Tvedten HW, Walker RD, DiPinto NM. Mycobacterium bacteremia in a dog: diagnosis of septicemia by microscopical evaluation of blood. J Am Anim Hosp Assoc 1990;26:359–363. 61. Villalba EJ, Garrido A, Molina JM, et al. Haemogram in canine brucellosis. Med Vet 1990;9:99–100, 102–104. 62. Weiser MG, Thrall MA, Fulton R, et al. Granular lymphocytosis and hyperproteinemia in dogs with chronic ehrlichiosis. J Am Anim Hosp Assoc 1991;27:84–88. 63. Weiss DJ. An indirect flow cytometric test for detection of anti-neutrophil antibodies in dogs. Am J Vet Res 2007;68:464–467. 64. Weiss DJ, Armstrong PJ. Secondary myelofibrosis in three dogs. J Am Vet Med Assoc 1985;187:423–425. 65. Weiss DJ, Henson M. Pure white cell aplasia in a dog. Vet Clin Pathol 2007;36:373–375. 66. Willard MD, Schall WD, McCaw DE, et al. Canine hypoadrenocorticism: report of 37 cases and review of 39 previously reported cases. J Am Vet Med Assoc 1982;180:59–62. 67. Yanay O, Brzezinski M, Christensen J, et al. An adult dog with cyclic neutropenia treated by lentivirus-mediated delivery of granulocyte colonystimulating factor. Hum Gene Ther 2006;17:464–469.

CHAPTER

49

Interpretation of Feline Leukocyte Responses AMY C. VALENCIANO, LILLI S. DECKER, and RICK L. COWELL Leukocyte Morphologic Artifacts Nonpathologic Leukocyte Responses Physiologic Leukocytosis Stress Leukogram Pathologic Leukocyte Responses Inflammatory Leukogram Inflammatory Leukogram with a Left Shift Neutrophil Responses Pathologic Changes in Neutrophil Morphology Toxic change Pelger-Huët anomaly Hypersegmented neutrophils Mucopolysaccharidosis Chédiak-Higashi syndrome Cytoplasmic vacuolization Neutrophil granulation anomaly Intracellular infectious agents Intracellular pigment Neutrophilia

Neutropenia Increased use or destruction Deficient production Redistribution and sequestration Monocyte Responses Monocytosis Monocytopenia Lymphocyte Responses Lymphocytosis Lymphopenia Eosinophil Responses Eosinophilia Eosinopenia Basophil Responses Basophilia Basopenia Leukemia (Myeloproliferative and Lymphoproliferative Disorders) Myelodysplastic Syndrome Aplastic Pancytopenia (Aplastic Anemia)

Acronyms and Abbreviations CBC, complete blood count; CNP, circulating neutrophil pool; EDTA, ethylenediaminetetraacetic acid; FeLV, feline leukemia virus; FIV, feline immunodeficiency virus; MDS, myelodysplastic syndrome; MPD, myeloproliferative disease; nRBC, nucleated red blood cell; WBC, white blood cell.

T

he complete blood cell count (CBC) is a vital component of the minimum laboratory database used in the evaluation of feline illnesses. The CBC provides quantitative and qualitative information regarding the status of red blood cells (RBCs), platelets , and white blood cells (WBCs); it is also termed the leukogram. The leukogram includes a total WBC count with absolute and relative quantification of WBC types and comments on WBC morphology. Although some general rules regarding leukogram interpretation may be applied to all species, some unique characteristics must be considered and recognized during evaluation of the feline leukogram. In normal cats, the total WBC population is composed primarily of mature neutrophils and lesser numbers of lymphocytes. Monocytes, eosinophils, and

basophils usually do not contribute significantly to the total WBC count. Chapter 105 offers a detailed review of normal feline WBC morphology. LEUKOCYTE MORPHOLOGIC ARTIFACTS Peripheral blood samples that are collected in EDTA anticoagulant and allowed to sit for several hours before a blood smear is made tend to develop artifactual leukocyte nuclear hypersegmentation, pyknosis, or cytoplasmic vacuolization (Fig. 49.1). Once these changes occur, the cells are difficult or impossible to identify. Artifactual cytoplasmic vacuolation also may be confused for toxic changes of neutrophils. When nucleated cells rupture or are stripped of their cytoplasm during 335

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Physiologic Leukocytosis

FIGURE 49.1 Improper sample handling has resulted in altered WBC morphology. A WBC shows pyknosis, karyorrhexis and karyolysis. Wright-stained blood smear. (Courtesy of Oklahoma State University, College of Veterinary Medicine teaching files.)

blood smear preparation, the nuclear chromatin spreads out and stains eosinophilic. These eosinophilic masses are referred to as basket cells. Attempts should not be made to identify basket cells. When more than 10% of WBCs are artifactually affected, the WBC differential count is invalid. NONPATHOLOGIC LEUKOCYTE RESPONSES Various physiologic and pharmacologic processes can cause changes in the total leukocyte count. Because neutrophils are the predominant blood leukocyte in the cat, alterations in the WBC count frequently parallel alterations in the absolute neutrophil count. Nonpathologic leukogram changes primarily result from shifts in neutrophil populations related to changes in neutrophil kinetics. Granulopoiesis is directed by cytokines primarily secreted from macrophages within the bone marrow. Maturation of neutrophils occurs in approximately 3.5–6 days.18 Three distinct pools of neutrophils exist. The first pool is the bone marrow pool of developing and mature neutrophils. Neutrophils are released from the bone marrow in an age-ordered fashion. Once in the systemic circulation, they are distributed between the marginated neutrophil pool and the circulating neutrophil pool (CNP). Marginated neutrophils adhere to endothelium of venules and capillaries primarily in the spleen, lungs, and splanchnic vessels, whereas neutrophils of the CNP remain free in the vasculature. Only circulating neutrophils are collected during venipuncture sampling. In the cat, the ratio of the marginated neutrophil pool to circulating neutrophil pool is approximately 3 : 1.9 Blood neutrophils remain in a dynamic equilibrium, and many processes can cause a redistribution of neutrophils between these pools. Shifts are dynamic and may occur rapidly, altering circulating neutrophil numbers as reflected by the total WBC and neutrophil counts. Two distinct patterns of nonpathologic leukograms have been identified.

In physiologic leukocytosis, a mild to moderate leukocytosis characterizes the leukogram. Mild, mature neutrophilia and lymphocytosis exist. The lymphocytosis is frequently of greater magnitude than the neutrophilia.9,18 Lymphocytosis is probably caused by redistribution of lymphocytes between the blood, lymphatics, and lymphoid organs. The neutrophilia reflects the redistribution of neutrophils from the marginated to the circulating pool due to the effects of epinephrine. This leukogram pattern is generally seen in young, healthy cats that become excited or frightened by environmental stresses (e.g. fear, excitement, restraint, venipuncture). Leukogram changes are immediate but transient, lasting approximately 20–30 minutes. The short duration of the physiologic lymphocytosis helps to distinguish this physiologic state from other pathologic processes that cause a persistent lymphocytosis (e.g. lymphoma and lymphocytic leukemia). Young cats typically have a higher number of circulating lymphocytes than older cats, and a physiologic lymphocytosis rarely exceeds 20,000 lymphocytes/μL of blood.9 Stress (Glucocorticoid-Induced) Leukogram Exogenous glucocorticoids or elevations in endogenous glucocorticoids tend to cause a mild to moderate leukocytosis characterized by a mature neutrophilia, lymphopenia, and eosinopenia. Monocytosis generally is not observed in cats, and an eosinopenia is difficult to document in any species. Glucocorticoids induce neutrophilia primarily by enhancing the release of mature neutrophils from the bone marrow. Secondarily, glucocorticoids promote the shift of marginated neutrophils into the circulating neutrophil pool, decreasing the egress of circulating neutrophils into tissue. The normal blood transit time of circulating neutrophils is approximately 10–12 hours, but this blood transit time is prolonged by glucocorticoid administration. Finally, chronic glucocorticoid excess stimulates granulopoiesis. The lymphopenia is the result of redistribution of circulating lymphocytes; lysis of immature or uncommitted lymphocytes may occur with high dosages of corticosteroids given over prolonged periods. Glucocorticoids also exert a neutralizing effect on histamine, a major chemoattractant for eosinophils.13 The corticosteroid response is initially seen at approximately 4 hours and peaks 6–8 hours post-stress or after administration of corticosteroids. Neutrophil values return to reference intervals within 24 hours after a single 5 mg dose of prednisolone and within 48–96 hours after cessation of prolonged corticosteroid therapy.18 PATHOLOGIC LEUKOCYTE RESPONSES Because neutrophils are the predominant leukocyte in the cat, alterations in the total WBC count frequently parallel alterations in neutrophil numbers. Many factors, including physiologic, pharmacologic, and

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pathologic processes can cause changes in the absolute neutrophil count in blood. Inflammatory Leukogram In a normal animal, homeostasis is maintained between production and egress of neutrophils into tissues. Bone marrow production, margination, and tissue demand for neutrophils all can be affected by various disease processes. With an increased tissue demand for neutrophils, the bone marrow responds by increasing the release of mature neutrophils into circulation. In the cat, a significant, mature neutrophilic leukocytosis indicates inflammation. With established inflammation, a sustained neutrophilia indicates that bone marrow release of mature neutrophils exceeds neutrophil emigration into tissues. Bone marrow evaluation usually reveals granulocytic hyperplasia. These findings reflect the bone marrow’s capacity to produce enough mature neutrophils to combat the inciting agent and also suggests that the inflammatory process has been present for at least several days. The demand for neutrophils depends on the chronicity and severity of the disease process. Inflammatory Leukogram With a Left Shift When the demand for neutrophils exceeds bone marrow reserve capacity, the storage pool of mature neutrophils becomes exhausted. Immature neutrophils are then released into the circulation. An increase in the number of band cells and occasionally earlier precursor cells into the blood is referred to as a left shift. A clinically significant left shift is the hallmark of an inflammatory leukogram. In the cat, a left shift is considered clinically significant when immature neutrophils exceed 500–1000 bands/μL with a normal or elevated total WBC count.22 A left shift may be categorized as either regenerative or degenerative, which may be useful clinically as a prognostic indicator of disease severity. Additionally, the severity of the left shift can be assessed by evaluating the numbers of immature granulocytes and degree of immaturity of the cells, as an increase in both parameters generally parallels the severity of the condition. A regenerative left shift is defined as a neutrophilic leukocytosis in which the number of immature neutrophils does not exceed the number of segmented neutrophils. A regenerative left shift indicates that the bone marrow is meeting the body’s need for neutrophils. In contrast, a degenerative left shift is present when the number of immature neutrophils exceeds the number of segmented neutrophils.13 The number of segmented neutrophils is typically within the reference interval or decreased. A degenerative left shift indicates that the bone marrow cannot meet the tissue demand for neutrophils, indicating a severe disease process and suggests a guarded to poor prognosis. Degenerative left shifts can occur in conditions such as septicemia, endotoxemia, and severe inflammation of large-surface areas (e.g. peritonitis, pleuritis, pneumonia, gastroenteritis,

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placentitis). Frequently, a left shift is limited to band neutrophils, but with severe disease, metamyelocytes, myelocytes, and promyelocytes (progranulocytes) can appear in the blood. These immature stages are more likely to be associated with a degenerative left shift than with a regenerative left shift. Giant neutrophils or metamyelocytes are more frequent in the cat than in other species and can be seen as cats recover from severe neutropenia. Toxic neutrophils are frequently seen with inflammatory responses and are discussed later. Occasionally, an inflammatory process may induce an extreme neutrophilia (>50,000 cells/μL) with an associated left shift that may include band, metamyelocyte, myelocyte, and promyelocyte stages. This is termed a leukemoid response and can be difficult to distinguish from granulocytic or myelomonocytic leukemia (see Section V).

NEUTROPHIL RESPONSES Pathologic Changes in Neutrophil Morphology Toxic Change Toxic change occurs in neutrophils during maturation in the bone marrow and primarily affects the myelocytic and metamyelocytic stages of development. Toxic change is a maturational defect caused by toxic substances generated by strong inflammatory conditions including severe bacterial infections (pyothorax, pyoabdomen, sepsis, pneumonia), septicemia, acute inflammatory conditions, and extensive burns. Toxic changes can be seen in both mature and immature neutrophils and generally are associated with a left shift. Toxicity can be observed before quantitative changes in the leukogram, thus serving as a harbinger of disease, and may be the only hematological change evident to indicate an infectious/inflammatory condition.23 Toxic change can be present with a neutrophilia or a neutropenia and with or without a left shift. Toxicity is most frequently secondary to systemic rather than localized infectious/ inflammatory conditions.26,29 Although both nuclear and cytoplasmic toxic changes occur, the cytoplasmic changes are more reliable and occur more frequently. Although all cell types are exposed to the same insult, toxic change is evaluated only in neutrophils. The type of toxic change is indicated and semiquantitated as either 1+ to 4+, or as slight, mild, moderate, or severe. Cytoplasmic toxic changes include diffuse cytoplasmic basophilia, cytoplasmic vacuolation, Döhle bodies, and toxic granulation, with the latter change infrequently observed. Diffuse cytoplasmic basophilia occurs when polyribosomes are retained in the cytoplasm. Cytoplasmic vacuolation generally occurs in association with diffuse basophilia (Fig. 49.2). The mechanism of cytoplasmic vacuolization is thought to stem from damage to the cell membrane and subsequent loss of integrity.26,29 Döhle bodies are intracytoplasmic clumps of endoplasmic reticulum that stain a light blue with most Romanowsky-

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FIGURE 49.2 Feline neutrophil showing toxic change denoted by increased cytoplasmic basophilia, Döhle bodies, and foamy cytoplasm. Wright-stained blood smear. (Courtesy of IDEXX Reference Laboratories teaching files.)

FIGURE 49.4 Pelger-Hüet anomaly. Two neutrophils show mature chromatin patterns and lack of segmentation. Wrightstained blood smear. (Courtesy of IDEXX Reference Laboratories teaching files.)

anomaly, hyposegmented nuclei characteristically have a mature chromatin pattern, in contrast to nuclear hyposegmentation of infection where band cells have finely granular chromatin (Fig. 49.4). The lack of toxic change in the face of a significant degenerative left shift is also a hint to consider Pelger-Huët anomaly, and to carefully assess the nuclear chromatin pattern of the granulocytes. Neutrophil function is normal in affected cats, and cats with this anomaly are not predisposed to infection. Hypersegmented Neutrophils

FIGURE 49.3 A feline neutrophil contains Döhle bodies. Wright-stained blood smear. (Courtesy of Oklahoma State University, College of Veterinary Medicine teaching files.)

type stains (Fig 49.3). Döhle bodies occur frequently in cat neutrophils. Regardless of the number of neutrophils containing Döhle bodies, their presence should never be interpreted as indicating anything more than a mild toxic change. Toxic granulation occurs when primary granules within neutrophils retain sufficient amounts of mucopolysaccharide so that these granules stain reddish-purple. Nuclear toxic changes can include: vacuolization, hyposegmentation, ring formation, karyorrhexis, karyolysis and giant neutrophils.26,29

Hypersegmented neutrophils contain ≥5 distinct nuclear lobes. Hypersegmentation can occur as an in vivo aging change caused by prolonged blood transit time. More frequently, hypersegmentation, as well as pyknosis, represents an in vitro artifactual aging change that is seen in smears of EDTA-anticoagulated blood that has been allowed to sit for several hours. Mucopolysaccharidosis Mucopolysaccharidosis is a metabolic storage disease caused by inborn errors of mucopolysaccharide (glycosaminoglycan) metabolism.6,7,10,11 Circulating neutrophils contain few to many coarse, reddish-purple, intracytoplasmic granules on Romanowsky-stained blood smears (Fig. 49.5). Basophilic granules frequently are enlarged and metachromatic.

Pelger-Huët Anomaly Pelger-Huët anomaly is an inherited or acquired (pseudo Pelger-Huët) defect in granulocyte nuclear segmentation (see Chapter 42).19,20,31 In Pelger-Huët

Chédiak-Higashi Syndrome This syndrome occurs in Persian cats (see Chapter 42).15,16 Circulating neutrophils and lymphocytes contain

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A

A

B

B

FIGURE 49.5 Blood smear from a cat with mucopolysaccharidosis type VI. (A) Three neutrophils contain granular, purple, intracytoplasmic inclusions. (B) A neutrophil with intracytoplasmic inclusions and a basophil with dark blue-black granules. Wright stain. (Courtesy of Oklahoma State University, College of Veterinary Medicine teaching files.)

single to multiple, pink to reddish, intracytoplasmic granules (Fig. 49.6). Eosinophilic granules also may be enlarged.

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FIGURE 49.6 Blood smear from a cat with Chédiak-Higashi syndrome. (A) The neutrophils have large, pink cytoplasmic lysosomes. (B) The basophils have large, round, lavender granules. Wright stain. (Courtesy of Oklahoma State University, College of Veterinary Medicine teaching files.)

anomaly is inherited as an autosomal recessive trait and is innocuous. Intracellular Infectious Agents

Cytoplasmic Vacuolization Cytoplasmic vacuolization of neutrophils can be observed with drug toxicities including high doses of chloramphenicol or phenylbutazone. Also, cholesteryl ester storage disease is associated with subtle cytoplasmic vacuolation of neutrophils, lymphocytes, and monocytes on fresh blood smears.28 Neutrophil cytoplasmic granulation is also seen after storage of blood in EDTA. Neutrophil Granulation Anomaly A neutrophil granulation anomaly has been identified in Birman cats (see Chapter 42). The anomaly is characterized by the presence of azurophilic cytoplasmic granules that resemble toxic granulation.11 The

Histoplasma capsulatum yeast can rarely be seen in blood and buffy coat leukocytes in cats with disseminated histoplasmosis (see Chapter 19). These organisms are identified by their small size and round shape. They are 2–4 μm in diameter with a darkly staining, eccentrically located nucleus and a thin clear cell wall that resembles a halo (Fig. 49.7). The presence of intracellular bacteria is uncommon, and could be consistent with contamination of the specimen or with sepsis. Correlation with clinical signs, history, laboratory data, and results of blood culture are necessary to distinguish between these differentials. One or more morulae of Ehrlichia spp. are rarely identified in the cytoplasm of circulating neutrophils or monocytes in cats (see Chapter 31). Morulae are found in cytoplasmic vacuoles, are round with an approximate diameter of

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FIGURE 49.7 Feline neutrophil with two intracellular yeast forms of Histoplasma capsulatum. Wright-stained blood smear. (Courtesy of IDEXX Reference Laboratories teaching files.)

1.5–4 μm, and are composed of many small intracellular organisms that stain basophilic with Wright stains. Intracellular Pigment Rarely in hemolytic anemia, circulating sideroleukocytes can be identified. Sideroleukocytes are neutrophils and occasionally monocytes with intracellular hemosiderin, staining either brown-yellow, or blue-green on Romanowsky-stained blood films. The presence of intracellular hemosiderin can be confirmed with iron stains. Neutrophilia As previously stated, blood neutrophil counts may be increased by physiologic leukocytosis, corticosteroidinduced leukocytosis or pathologic mechanisms. Inflammation is the most common cause of a pathologic neutrophilia. Inflammation may stem from infectious or noninfectious origins. Infectious disease processes include bacterial, viral, fungal, and parasitic infections that can be localized or generalized (systemic).18 The magnitude of the neutrophilia and the presence or absence of a left shift depends on the severity and duration of disease, as well as the competence of the bone marrow to produce and release neutrophils. With prolonged duration, localized inflammatory processes may result in very high neutrophil counts. These cases are frequently associated with a left shift and toxic change of neutrophils. Occasionally, the inflammatory nidus is obscure. In such cases, exudative loss of neutrophils into large-surface areas should be considered. Such sites of loss include the skin, gastrointestinal tract, genitourinary tract, respiratory tract, and joints. Surgical removal of the inciting cause (e.g. abscess) often results in a transient increase in neutrophil numbers due to

granulocytic hyperplasia in the bone marrow. Medical treatment involving the administration of glucocorticoids may induce a transient glucocorticoid (stress) leukogram. Generalized or systemic infectious processes also can cause a neutrophilia, but the magnitude of the neutrophilia is generally less severe than localized infectious processes. Some specific examples include feline viral rhinotracheitis and feline infectious peritonitis. These diseases infrequently present with neutropenia. Various noninfectious causes of tissue necrosis can induce mild to moderate neutrophilia. Any process that leads to tissue necrosis can result in an inflammatory response. As expected, the magnitude of the neutrophilia and the presence of a left shift depend on disease severity and chronicity. A few examples include soft-tissue trauma, immune-mediated diseases (e.g. hemolytic anemia), hemorrhage, and nonleukemic neoplasia. Neutropenia Neutropenia (850 cells/μL is considered a monocytosis.9,18 Monocytosis is a nonspecific finding that has been reported to occur in approximately 11% of the leukograms of hospitalized cats.18 Monocytosis is not a characteristic feature of the stress leukogram in cats as it is in dogs. Monocytosis occurs in many conditions, including both acute and chronic inflammation, tissue destruction, and neutrophilia. Some of the reported causes of a monocytosis include trauma-related injuries, suppuration, necrosis, pyogranulomatous inflammation, hemolysis, hemorrhage, malignancy, and immune-mediated disorders. Monocytopenia Persistent monocytopenia is clinically unimportant in cats and is seldom documented, because the low end of the reference interval is zero cells/μL of blood. With severe leukopenia, the clinical focus is on neutropenia and prevention of sepsis; little attention is given to coexisting monocytopenia. LYMPHOCYTE RESPONSES Lymphocytes are the second most frequent leukocyte in the blood of healthy cats. Most blood lymphocytes are small-sized, long-lived, T cells that are capable of recirculation. Antigenically-stimulated lymphocytes (e.g. reactive lymphocytes) are larger and have more abundant, dark-blue cytoplasm. Rarely, immune-stimulated lymphocytes may exhibit plasmacytoid differentiation. Such cells have a round, eccentric nucleus with coarse nuclear chromatin; abundant dark-blue cytoplasm; and a pale-staining Golgi zone located between the nucleus and the largest volume of cytoplasm. Granular lymphocytes, that are natural killer cells and in the null cell group, may be observed. These cells are identified by distinctive azurophilic granules, usually in the vicinity of the nuclear indentation. In contrast, lymphoblasts (immature lymphocytes) are large lymphocytes (equal to or larger than neutrophils) with finely stippled nuclear chromatin and multiple, prominent nucleoli. Lymphocyte vacuolation has been associated with cholesteryl ester storage disease of Siamese cats and mannosidosis of Persian cats.21,28 Lymphocytes are evaluated by their morphology and absolute number. Blood lymphocyte numbers are influenced by physiologic states, disease, and drug administration, causing changes in lymphocyte

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production, distribution, margination, recirculation, sequestration, destruction, or loss. Lymphocytosis Lymphocytosis (>7000 lymphocytes/μL) can be secondary to epinephrine-mediated physiologic leukocytosis, hypoadrenocorticism, chronic antigenic stimulation (e.g. FeLV), hyperthyroidism, or lymphoid neoplasia. Young animals also have higher absolute lymphocyte counts compared to middle-aged or old cats.9 Young, healthy cats (especially those 50,000 eosinophils/μL).5 Eosinopenia Eosinopenia is difficult to recognize clinically because the lower end of the reference interval for feline eosinophil counts is 0 cells/μL. Eosinopenia usually is associated with exogenous administration or stress-related endogenous release of glucocorticoids. Acute infections are associated with eosinopenia, but this is likely glucocorticoid-induced. BASOPHIL RESPONSES Basophils are the least numerous of the blood leukocytes in healthy cats, accounting for less than 2% of the leukocyte differential count.18 Basophilia

Lymphopenia Lymphopenia is recognized by an absolute lymphocyte count of less than 1,500 lymphocytes/μL of blood in adult cats or less than 2,500 lymphocytes/μL of blood in young cats.9,18 The most frequent cause of lymphopenia is corticosteroid-induced redistribution of lymphocytes. Some other causes include viral infections (e.g. panleukopenia, FeLV, and FIV), septicemia or endotoxemia, lymphocyte-rich thoracic effusions (chylothorax from thoracic duct rupture, cardiovascular disease, nonexfoliating neoplasia, malignant lymphoma, or thymoma), and gastrointestinal disease (e.g. ulcerative enteritis, granulomatous enteritis, malignant lymphoma, and other neoplasias).

A basophilia is recognized by an increased absolute basophil count (>200 basophils/μL) and generally occurs with a concomitant eosinophilia. Some causes of basophilia in cats include allergic respiratory conditions, heartworm disease, eosinophilic granuloma complex, basophilic leukemia, myeloid leukemia, mast cell neoplasia and polycythemia vera.3,9,18 Basopenia Basophils are rarely observed in blood smears of cats; basopenia is not a recognized clinical problem. LEUKEMIAS (MYELOPROLIFERATIVE AND LYMPHOPROLIFERATIVE DISORDERS)

EOSINOPHIL RESPONSES Like neutrophils, the bone marrow has a storage pool of eosinophils. Eosinophils have many functions, including the destruction of parasites, the modulation of hypersensitivity reactions, and a proinflammatory function.

In depth discussion, including pathogenesis, classification, general and detailed features of acute and chronic myeloid and lymphoid leukemias can be found in Section V. Leukemias and myeloproliferative disorders (MPDs) originate in the bone marrow. Most of the MPDs and leukemias in cats are FeLV-associated and

CHAPTER 49: INTERPRETATION OF FELINE LEUKOCYTE RESPONSES

some also have been associated with FIV infection. Myeloproliferative disorders include neoplastic proliferation of granulocytes, monocytes, erythrocytes, and megakaryocytes. The acute MPDs include acute undifferentiated leukemia and the acute myeloid leukemias (French-American-British classification M1–M7).8 Chronic MPD categories include chronic myeloid leukemias, polycythemia vera, idiopathic myelofibrosis, myeloid metaplasia, and essential thrombocythemia.8 Leukemias are encountered less frequently than lymphoma in the cat.8 Cats that have leukemia generally have vague and variable clinical signs, including anorexia, loss of body mass, and lethargy. Clinical findings often include organomegaly, anemia, fever, emaciation, and petechiae.18 Leukemia usually is associated with a high total WBC count, but it occasionally may present with a normal WBC count or leukopenia. In chronic leukemias composed of well-differentiated cells, the type of leukemia often can be identified by examination of Wright-stained blood and bone marrow smears. However, in acute leukemias, blast cells can appear similar morphologically.14 Therefore, immunophenotypic assessment may be needed to identify the neoplastic cell line.14 Granulocytic, myelomonocytic, and monocytic leukemias all occur in cats. Granulocytic leukemia must be differentiated from a leukemoid response or extreme neutrophilic leukocytosis secondary to an inflammatory focus (e.g. infection and tumor with a necrotic center). Eosinophilic leukemia is rare and difficult to differentiate from idiopathic hypereosinophilic syndrome. Basophilic leukemia also is rare and should not be confused with mast cell neoplasia. In cats, mast cell neoplasia usually is associated with splenic enlargement or gastrointestinal involvement and with circulating mast cells. Lymphoproliferative disease includes acute lymphoblastic leukemia and chronic lymphocytic leukemia. The latter is characterized by many small, well-differentiated lymphocytes, and must be differentiated from a physiologic lymphocytosis and from a reactive lymphocytosis secondary to antigenic stimulation. MYELODYSPLASTIC SYNDROMES Hallmarks of myelodysplastic syndromes (MDSs) are: ineffective hematopoiesis leading to peripheral cytopenias and evidence of dysplasia in the peripheral blood or bone bone marrow (Fig. 49.8; see Chapter 66).32 Some cases of MDS in the cat occur secondary to FeLV infection, resulting in retroviral induced mutations in bone marrow stem cells. MDS in the cat is most frequently associated with a moderate to severe anemia, but bicytopenia and pancytopenia are common.12 Dysplastic changes observed in peripheral blood neutrophils can include: giant neutrophils, ring-shaped nucleated neutrophils, and hypersegmented neutrophils.12,25 Transition into acute myeloid leukemia is frequent in some types of MDS.

343

FIGURE 49.8 Blood smear of a dysplastic nucleated RBC. Wright stain. (Courtesy of IDEXX Reference Laboratories teaching files.)

APLASTIC PANCYTOPENIA (APLASTIC ANEMIA) Similar to MDS, aplastic anemia results in peripheral cytopenias; however, the marrow is severely hypocellular or acellular and the hematopoietic space is replaced by adipose tissue (see Chapter 39). Etiologies of aplastic pancytopenia in the cat include: infectious agents (Ehrlichia sp., parvovirus, FeLV, FIV), drugs (chemotherapeutics, sulfadiazine, griseofulvin, albendazole) and idiopathic causes.32

REFERENCES 1. Bortnowski HB, Rosenthal RC. Gastrointestinal mast cell tumors and eosinophilia in two cats. J Am Anim Hosp Assoc 1992;28:271–275. 2. Breitschwerdt EB, Abrams-Ogg ACG, Lappin MR, et al. Molecular evidence supporting Ehrlichia canis-like infection in cats. J Vet Int Med 2002;16:642–649. 3. Center SA, Randolph JF. Eosinophilia. In: Consultations in Feline Internal Medicine. Philadelphia: WB Saunders, 1991;349–358. 4. Center SA, Randolph JF, Erb HN, et al. Eosinophilia in the cat: a retrospective study of 312 cases (1975 to 1986). J Am Anim Hosp Assoc 1990; 26:349–358. 5. Gelain ME, An toniazzi E, Bertazzolo W, et al. Chronic eosinophilic leukemia in a cat: cytochemical and immunophenotypical features. Vet Clin Pathol 2006;35:454–459. 6. Gitzelmann R, Bosshard NU, Superti-Furga A, et al. Feline mucopolysaccharidosis VII due to β-glucuronidase deficiency. Vet Pathol 1994;31:435–443. 7. Glew RH, Basu A, Prence EM, et al. Biology of disease: lysosomal storage diseases. Lab Invest 1985;53:250–269. 8. Grindem CB. Classification of myeloproliferative diseases. In: Consultation in Feline Internal Medicine, 3rd ed. Philadelphia: WB Saunders, 1997;499–508. 9. Hall RL. Interpreting the leukogram. In: Consultations in Feline Internal Medicine, 2nd ed. Philadelphia: WB Saunders, 1994;489–494. 10. Haskins ME, Aguirre GD, Jezyk PF, et al. The pathology of the feline model of mucopolysaccharidosis VI. Am J Pathol 1980;101:657–674. 11. Hirsch VM, Cunningham TA. Hereditary anomaly of neutrophil granulation in Birman cats. Am J Vet Res 1984;45:2170–2174. 12. Hisasue M, Okayama H, Okayama T, et al. Hematologic abnormalities and outcome of 16 cats with myelodysplastic syndromes. J Vet Intern Med 2001;15:471–477. 13. Jain NC. Interpretation of leukocyte parameters. In: Essentials of Veterinary Hematology. Philadelphia: Lea & Febiger, 1993;295–306.

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14. Jain NC, Blue JT, Grindem CB, et al. Proposed criteria for classification of acute myeloid leukemia in dogs and cats. Vet Clin Pathol 1991;20:63–82. 15. Kramer JW, Davis WC, Prieur DJ. An inherited disorder of Persian cats with intracytoplasmic inclusions in neutrophils. J Am Vet Med Assoc 1975;166:1103–1104. 16. Kramer JW, Davis WC, Prieur DJ. The Chédiak-Higashi syndrome of cats. Lab Invest 1977;36:554–562. 17. Lapointe JM, Higgins RJ, Kortz GD, et al. Intravascular malignant T-cell lymphosarcoma (malignant angioendotheliomatosis) in a cat. Vet Pathol 1997;34:247–250. 18. Latimer KS. Leukocytes in health and disease. In: Ettinger SJ, Feldman EC, eds. Textbook of Veterinary Internal Medicine: Diseases of the Dog and Cat, 4th ed. Philadelphia: WB Saunders, 1995;1892–1929. 19. Latimer KS, Rakich PM. Clinical interpretation of leukocyte responses. Vet Clin N Am Small Anim Pract 1989;19:637–668. 20. Latimer KS, Rakich PM, Thompson DF. Pelger-Huët anomaly in cats. Vet Pathol 1985;22:370–374. 21. Maenhout T, Kint JA, Dacremont G, et al. Mannosidosis in a litter of Persian cats. Vet Rec 1988;122:351–354. 22. Prasse KW. Clinical, hematological and postmortem findings in feline leukovirus infected cats: a retrospective study of 95 naturally occurring cases. In: Proceedings of the 31st Annual Meeting of the American College of Veterinary Pathologists, New Orleans, 1980. 23. Segev G, Klement E, Aroch, I. Toxic neutrophils in cats: clinical and clinicopathologic features, and disease prevalence and outcome – a retrospective case control study. J Vet Int Med 2006;20:20–31.

24. Sellon RK, Rottman JB, Jordan HL, et al. Hypereosinophilia associated with transitional cell carcinoma in a cat. J Am Vet Med Assoc 1992;201:591–593. 25. Shimoda T, Shiranaga N, Mashita T, et al. A hematological study on thirteen cats with myelodysplastic syndrome. J Vet Med Sci 2000;62:59–64. 26. Smith GS. Neutrophils. In: Feldman BF, Zinkl JG, Jain NC, eds. Schalm’s Veterinary Hematology, 5th ed. Philadelphia: Lippincott Williams & Wilkins, 2000;281–296. 27. Swenson CL, Kociba GJ, O’Keefe DA, et al. Cyclic hematopoiesis associated with feline leukemia virus infections in two cats. J Am Vet Med Assoc 1987;191:93–96. 28. Thrall MA, Mitchell T, Lappin M, et al. Cholesteryl ester storage disease in two cats. In: Proceedings of the 42nd Annual Meeting of the American College of Veterinary Pathologists, Orlando, FL, 1991. 29. Tyler RD, Cowell, RL, Clinckenbreard KD, et al. Hematologic values in horses and interpretation of hematologic data. Vet Clin N Am Equine Pract 1987;3:461–484. 30. Watson ADJ, Middleton DJ. Chloramphenicol toxicosis in cats. Am J Vet Res 1978;39:1199–1203. 31. Weber SE, Feldman BF, Evans DA. Pelger-Huët anomaly of the granulocytic leukocytes in two feline littermates. Feline Pract 1981;11:44–47. 32. Weiss DJ. New insights into the physiology and treatment of acquired myelodysplastic syndromes and aplastic pancytopenia. In Vet Clin N Am Small Anim Pract 2003;33:1317–1334.

CHAPTER

50

Determination and Interpretation of the Avian Leukogram KENNETH S. LATIMER and DOROTHEE BIENZLE Leukocyte Count Stained Blood Smear Leukocyte Differential Count Changes in Leukocyte Morphology Toxic Changes Infectious Disease Reference Intervals Hematopoiesis with Emphasis on Leukocyte Production Leukocyte Function and Response in Health and Disease Heterophil or Lymphocyte Predominance in the Blood in States of Relative Health Leukocytosis and Leukopenia Leukocytosis Leukopenia Heterophil Responses Physiologic Heterophilia Corticosteroid-induced Heterophilia Inflammation- or Infection-induced Heterophilia Heteropenia

Deficient Heterophil Production Heterophil Shifts from the Circulating to Marginal Pool Severe Tissue Demands for Heterophils Lymphocyte Responses Lymphocytosis Lymphopenia Monocyte Responses Monocytosis Monocytopenia Basophil Responses Basophilia Basopenia Eosinophil Responses Eosinophilia Eosinopenia Remarks on Prognosis and Hematologic Trends Heterophil:Lymphocyte Ratios Unfavorable Prognostic Features Favorable Hematologic Trends New Techniques for Assessment of Avian Leukocytes

Acronyms and Abbreviations ACTH, adrenocorticotropic hormone; CBC, complete blood count; EDTA, ethylenediamine tetraacetic acid; H:L, heterophil:lymphocyte ratio; WBC, white blood cell.

M

ammalian hematology has advanced rapidly over the past three decades owing to the introduction of semi-automated or automated technology (that provides a total leukocyte count and complete or 2- or 3-part differential leukocyte count). Published literature exists detailing clinical hematologic responses to disease in mammalian species. In contrast, avian hematology is still in its infancy. Descriptive accounts of avian blood-cell morphology9,39,42,44 and techniques of performing avian white blood cell (WBC) counts10,21,57,77 have been published. However, the progress of avian hematology has been inhibited by lack of automated technology, lack of

appropriate reference intervals for most major avian species, and lack of carefully controlled experimental studies to interpret avian WBC responses. Presented in this chapter is a brief overview of avian WBCs with respect to quantitation of WBC data, establishment of reference intervals, and interpretation of WBC responses in health and disease. Much of the research to date has been done in domestic poultry, which provides a model for avian WBC development and function.43 Although generalizations can be drawn concerning avian leukocyte production, morphology, function, and response in health and disease, differences are sometimes apparent among and between species of birds. 345

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LEUKOCYTE COUNT In avian hematology, only the red blood cell (RBC) count can be determined routinely by automated methods (electronic particle counter).21 Therefore, three manual approaches are used to determine or estimate the WBC count. These include (1) the direct WBC count with a hemacytometer and Natt and Herrick’s solution, (2) the indirect WBC count using a hemacytometer and eosinophil Unopette® in conjunction with the stained blood smear, and (3) the estimated WBC count using the stained blood smear. Specific technical procedures for these counting methods can be found in the literature.9,22,33,36,77 The avian WBC count can be determined directly with a Neubauer-ruled hemacytometer, pipettes, and methyl violet 2B diluent.57 The advantage of this technique is that all WBCs are visualized and enumerated. The disadvantages are that this WBC counting technique is labor intensive, care must be taken to differentiate thrombocytes from small lymphocytes, and WBCs may distort if analysis of the diluted specimen is delayed.43 This method works well for an in-house laboratory but may prove difficult with samples that are mailed to a diagnostic laboratory. In the latter instance, prolonged exposure to ethylenediaminetetraacetic acid (EDTA) anticoagulant can cause erythrolysis and viscosity changes of hematologic specimens from crowned cranes, crows, jays, brush turkeys, hornbills, magpies, and some ratites.21,64 In addition, WBC morphology can be altered upon prolonged exposure to EDTA. Specimens collected in heparin eventually clot, and heparin also interferes with Romanowsky staining of blood smears. The WBC count can be quantitated indirectly with the eosinophil Unopette #5877 system. In this technique, heterophils and eosinophils are selectively stained by phloxine dye and counted in a hemacytometer.10,65 Based on the WBC differential cell count of a stained blood smear, the combined heterophil and eosinophil hemacytometer count is mathematically corrected to account for basophils and mononuclear cells (i.e. lymphocytes and monocytes) that were not counted. Thus, the total WBC count per microliter of blood has been determined indirectly. Advantages of this system are that heterophils and eosinophils are directly identified and counted in a hemacytometer. There also is no confusion between lymphocytes and thrombocytes because they are not counted with the hemacytometer. However, hemacytometer counts are labor intensive, a greater potential for mathematical errors exists, and WBC distribution on stained blood smears may not be random. One can estimate WBC counts from the stained blood smear by multiplying the average number of WBCs per field of view by a given factor (the factor is usually 1,500 for the 40× to 45× high-dry microscope objective or 2,000 for the 50× oil-immersion microscope objectives, respectively). Advantages of this technique include the stability of the blood film and the labor-efficient process of WBC count estimation. The major disadvantage is that the WBC count is estimated instead of quantitated.

Thus, estimated WBC counts may not be reproducible, poor smear technique may influence the estimate because of cell lysis or maldistribution. Coefficient of variation for hemacytometer-derived WBC counts (eosinophil Unopette method) are 12.7% as opposed to 28% for estimated WBC counts from stained blood smears.68 However, the latter technique may be useful to indicate hematologic trends for which the lability or volume of the blood specimen would otherwise preclude hematologic study. Ultimately, the future of avian hematology depends on the development of semi-automated or automated techniques to determine avian WBC counts in a rapid, accurate, economical, and laborefficient manner. STAINED BLOOD SMEAR The stained blood smear is used to perform the WBC differential cell count and morphologic examination of WBCs. Although considerable discussion has revolved around the best method to produce acceptable avian blood smears, both the wedge technique (with two glass slides) and coverslip technique (with two square glass coverslips) can be used. The major factors in producing high-quality smears are manual dexterity and practice. It is generally assumed that WBCs are randomly distributed in blood smears, which may not be the case. Hematologic studies have shown that WBCs are more randomly distributed in coverslip smears than in wedge smears where large cells, such as monocytes, tend to be carried to the lateral margins and feathered edge of the smear. Furthermore, this problem is magnified with poor smear technique. Depending on the type of smear and the method of examination, WBC differential counts may vary. The accuracy of the WBC differential count can be increased by counting more cells. To increase the accuracy of the differential count significantly, one must perform a 400-cell count as opposed to 100 cell WBC differential count. In summary, the smear quality directly influences the accuracy of the differential cell count and estimated WBC counts. Also, smears that are made at the time of blood specimen collection reflect the morphologic changes in WBCs at that point. If smears are made from the blood specimen at a later time, potential artifacts include increased numbers of smudge cells, distortion of WBCs and thrombocytes, vacuolation of monocytes, swelling of lymphocytes, and changes in WBC tinctorial properties.43 LEUKOCYTE DIFFERENTIAL COUNT White blood cell morphology may vary between species of birds (see Chapters 122–125). Examples include the appearance of granule morphology and cytoplasmic coloration of raptor eosinophils4 and the shape of heterophil granules in certain species of birds.7 With currently accepted criteria of WBC identification,

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347

FIGURE 50.1 Blood smear from an African grey parrot. A left shift is indicated by the presence of immature heterophils (band, metamyelocyte, and myelocyte). All of these cells display mild toxic change (cytoplasmic basophilia).

FIGURE 50.3 Blood smear from a great horned owl. A monocyte (lower left) and lymphocyte (upper right) are present.

FIGURE 50.2 Blood smear from a great horned owl. A monocyte (lower left), thrombocyte (center), and lymphocyte (top) are present.

performance of the WBC differential count in healthy and sick birds may be challenging and requires practice. Problems may be encountered in identifying left shifts in the heterophil population43 (Fig. 50.1) and in distinguishing some small lymphocytes from thrombocytes73 (Fig. 50.2), monocytes from large lymphocytes (Fig. 50.3), reactive lymphocytes from rubricytes,5 heterophils from eosinophils21,42 (Fig. 50.4), and extremely toxic promyelocytes (mesomyelocytes) from basophils68 (Fig. 50.5). Detection of left shifts in avian blood is more difficult because avian heterophils have less nuclear segmentation than mammalian neutrophils.43 Furthermore, heterophil granules tend to obscure nuclear morphology in Romanowsky-stained blood smears. With these preparations, only marked left shifts can be easily identified (Fig. 50.1). The presence of promyelocytes, that contain both basophilic and eosinophilic granules,28

FIGURE 50.4 Blood smear from a red-tailed hawk. The myelocytic heterophil (top right) has needle-shaped, dull red granules, whereas the eosinophil (left) has round, bright, red-orange granules.

indicates an intense left shift43 (Fig. 50.5). Subtle left shifts can be accurately quantitated by examination of hematoxylin-stained blood smears in which the nucleus is stained but the granules remain unstained.43 This technique permits determination of a nuclear lobulation score, a sensitive indicator of a left shift. CHANGES IN LEUKOCYTE MORPHOLOGY Changes in leukocyte morphology are important observations that permit assessment of the severity of the disease process. Occasionally, morphologic changes in leukocytes may provide a definitive medical diagnosis.

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FIGURE 50.5 Blood smear from a chicken. Segmented heterophil and toxic heterophil promyelocyte. Note purple and red granules in the promyelocyte.

FIGURE 50.7 Blood smear form a Bali mynah that has

FIGURE 50.6 Blood smear from an Amazon parrot that has septicemia. Note the phagocytosed bacterial cocci within a heterophil.

FIGURE 50.8 Blood smear from a great horned owl. Leukocytozoon

Toxic Changes

cytoplasm of monocytes (Fig. 50.7), Leukocytozoon sp. organisms in the cytoplasm of WBCs or erythrocytes (Fig. 50.8), and elementary bodies of Chlamydophila psittaci in the cytoplasm of various WBCs (Fig. 50.9). After exposure to antigens and stimulation of the immune system, scattered lymphocytes may become larger (a prelude to blast transformation or plasmacytoid differentiation) and have dark blue granular cytoplasm. Such cells are classified as reactive lymphocytes morphologically. In lymphoid neoplasia, the majority of lymphocytes may appear immature or reactive. The clinician also should be familiar with staininduced changes in cellular morphology. Stain-induced artifacts, such as degranulation and intensified cytoplasmic coloration of heterophils in Diff-Quik®-stained preparations, should not be mistaken for toxic changes.43 Also, partial heterophil degranulation may leave a round granule core, causing affected cells to resemble eosinophils.43,65 This condition is frequently seen with aqueous-based stains when short fixation times are

Variable degrees of degranulation, cytoplasmic basophilia, cytoplasmic vacuolation, and cellular swelling constitute toxic changes in avian heterophils40 (Figs. 50.1 and 50.5). Ultrastructurally, cellular swelling and cytoplasmic vacuolation are the result of intracellular edema. Degranulation is a sequel to dissolution of the granule matrix. Cytoplasmic basophilia is explained by the persistence of ribosomes. In Romanowsky-stained avian blood smears, cytoplasmic basophilia is the last manifestation of toxic change to disappear with convalescence.40 Infectious Disease Inclusions in avian WBCs are seen infrequently but may provide a definitive diagnosis when observed. Examples include phagocytosed bacteria in heterophils of septicemic birds (Fig. 50.6), Atoxoplasma sp. organisms in the

disseminated toxoplasmosis. Note round to oval intracytoplasmic organisms in the mononuclear cells.

sp. gametocyte is present in a leukocyte. The affected cell is enlarged, elongated, and has pointed cell margins.

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HEMATOPOIESIS WITH EMPHASIS ON LEUKOCYTE PRODUCTION

FIGURE 50.9 Blood smear from parrot that has chlamydiosis. Intracytoplasmic elementary bodies of Chlamydophila psittaci are present in a monocyte. (Courtesy of Dr. Michele Menard, Veterinary Cytopathology, Gresham, OR.)

used. Cytochemical staining has the potential to reliably distinguish heterophils from eosinophils for which cellular identification is uncertain.3 As avian hematology progresses, laboratories should undertake a coordinated effort to modernize avian leukocyte nomenclature, uniformly evaluate morphologic changes in blood cells, and consistently report semiquantitative hematologic data. Standardized guidelines will facilitate comparison of hematologic data between laboratories. REFERENCE INTERVALS Reference intervals are used to identify laboratory test results or values that discriminate between health and disease (see Chapter 131). Establishment of meaningful reference intervals requires data collection from an adequate number of individuals constituting a precisely defined population. The population must be defined precisely because test values can be influenced by species of bird, age, sex, health and reproductive status, diet, and climate or environment. In addition, the method of specimen collection and analytical technique can influence test results. With these intervals, laboratory test values suggestive of health or disease may be distinguished more reliably. Reference intervals for various species of companion and exotic birds have been published in journals and textbooks. Several of these publications are especially noteworthy based on the large numbers of individuals sampled, a defined reference population, and attention to age-related changes in hematologic data.19–23 Sometimes, avian clinicians are presented with ill patients that are rare exotic birds or juvenile birds of common species for which reliable reference intervals do not exist. In such cases, analysis of control blood samples from clinically healthy birds of the same species, age, sex, and environment permit identification of abnormal hematologic trends.29–31,74

Primitive cells in the yolk sac initiate blood-cell development during the first few days of embryonic incubation. Early erythroblasts are recognizable by day 5 in the developing lung, and their appearance precedes vasculogenesis.45 By 10–15 days of incubation, hemopoietic activity peaks in the yolk sac and is widespread in other tissues, including bone marrow, liver, kidney, spleen, thymus, bursa of Fabricius, aorta, heart, pharynx, cranial nerves, spinal ganglia, subcutaneous tissues, and muscles.47 Although the bone marrow is the major site of hematopoietic activity post-hatching, foci of hematopoiesis also may be observed in the liver, spleen, and kidney of companion birds.69 In birds, extramedullary hematopoiesis is widespread and involves granulopoiesis, wherein the majority of the cells contain eosinophilic granules and nuclei in various stages of lobulation. Granulopoiesis is especially prominent in the liver, kidney, and spleen, but it may involve the heart and subdural spaces. Granulopoiesis, especially in the spleen, bursa, and thymus, involves production of heterophils.47 In bird bone marrow, granulopoiesis occurs in the extravascular spaces, whereas erythropoiesis and thrombopoiesis occur in the bone marrow vascular sinuses. Sites of hematopoiesis in birds involve nonpneumatized long bones and the axial skeleton. A practical example is acquisition of bone marrow aspirates from the tibia because the humerus and femur frequently are pneumatized. During early life, hematopoiesis is distributed throughout the skeleton, except for the skull.63,70 Lymphocyte production occurs in primary (thymus, bursa of Fabricius) and secondary (spleen; gutassociated lymphoid tissue, including cecal tonsils; bronchial-associated lymphoid tissues; paraocular tissues, including conjunctival-associated lymphoid tissue; paranasal sinuses, and miscellaneous lymphoid follicles along lymphatic vessels) extramedullary tissues. Lymphocytes are present in the thymus and bursa of the embryo by 10–14 days of incubation. As lymphopoiesis progresses, the secondary lymphoid tissues produce the majority of the lymphocytes, whereas the primary lymphoid tissues (thymus and bursa) involute with sexual maturity.

LEUKOCYTE FUNCTION AND RESPONSE IN HEALTH AND DISEASE Interpretation of avian leukograms should be based on absolute leukocyte values per microliter of blood. This results in fewer erroneous interpretations compared to reliance on relative percentages alone.39 Although diurnal rhythms of leukocyte counts have been demonstrated occasionally in birds (e.g. chickens),35 these fluctuations have little effect on leukogram interpretations in a clinical setting.

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Few scientifically-controlled studies have been performed to evaluate avian WBC responses.43 Therefore, interpretation of avian leukograms has relied on extrapolations from the veterinary literature and individual clinical experience. Although hematologic studies of patients with spontaneous disease have increased our knowledge of avian hematology, precise interpretation of avian WBC responses is obscured by numerous variables and incomplete patient data concerning sample collection, method of analysis, restraint, drug administration, etc. The following discussion is intended to provide basic guidelines for leukogram interpretation. HETEROPHIL OR LYMPHOCYTE PREDOMINANCE IN THE BLOOD IN STATES OF RELATIVE HEALTH Birds are similar to rodents in that the primary circulating WBC in some species is the heterophil, whereas in other species the lymphocyte predominates. Examples of birds that have a predominance of heterophils include greater sulfur-crested cockatoos, herring gulls, hyacinth macaws, rainbow lorikeets, yellow-crowned Amazon parrots, African grey parrots, and pigeons. Examples of birds that have a predominance of lymphocytes include budgerigars, canaries, cockatiels, finches, rose-ringed parakeets, and many species of Amazon parrots. Hematologic studies of some species of adult birds, such as flamingos and white-naped cranes, are contradictory in that either heterophils or lymphocytes have been identified as the major circulating leukocyte in health. Possible explanations for these divergent observations include stress of capture, handling, caging, social interactions, and environmental conditions. These variables, singly or in combination, could induce heterophilia or lymphopenia, resulting in aberrant reference intervals. Hematologic study of neonatal and juvenile birds indicates that heterophilic leukocytosis can be observed with some frequency.15–17 The presumed mechanism for this observation is stress. Furthermore, some birds that have a predominance of lymphocytes in the adult leukogram frequently have a predominance of heterophils during the post-hatch and juvenile periods. Transition to the normal adult leukogram eventually occurs with age (generally 8–12 weeks of age), a similar process typically occurs in young ruminants by weaning age. Total WBC counts frequently are elevated post-hatch (a stressful period of neonatal life) and decrease with age. From the discussion and examples given above, one can readily appreciate the need for reference intervals so that leukograms from birds of various species and ages are interpreted correctly. In the case of expensive or rare birds, an annual physical examination and CBC provide individual reference data to detect subtle changes in individual health status. This procedure is expensive, but has proven to be effective in human health care.

LEUKOCYTOSIS AND LEUKOPENIA Leukocytosis Leukocytosis, is frequently the result of physiologic processes, infection, or inflammation. Physiologic leukocytosis is precipitated by excitement, fear, forced flight, and excessive muscular activity. Increases in heterophils or lymphocytes may account for the leukocytosis. If the mechanism in birds mimics that of mammals, leukocytosis could be related to epinephrine release or muscular exertion, both of which cause increased cardiac output, increased blood pressure, and a washout of leukocytes from the microvasculature into the mainstream of circulation. In inflammatory and infectious conditions of birds, heterophilia frequently is observed and the degree of heterophilia may be more pronounced than comparable neutrophilia in mammals.13 Examples include acute chlamydiosis, mycobacteriosis, and disseminated mycosis where the total WBC count may exceed 100,000 cells/μL of blood. In the case of birds that have a predominance of lymphocytes such as chickens, a heterophil-lymphocyte reversal may be observed.11,43 In some infectious conditions of birds, a lymphocytosis may be noted. An example is chronic chlamydiosis where the immune system has been stimulated as observed by plasmacytosis, which is most notable in the liver and spleen. Leukopenia Leukopenia in birds that have a predominance of heterophils usually is the result of heteropenia. In birds that have a predominance of lymphocytes, leukopenia often is synonymous with lymphopenia. Causes of leukopenia vary, depending on whether heterophils, lymphocytes, or both cell lines are affected. Specific causes of heteropenia and lymphopenia, including those mechanisms resulting in leukopenia, are discussed in detail below. HETEROPHIL RESPONSES Heterophils provide a first line of defense against infection (Table 50.1). Avian heterophils and mammalian neutrophils differ both morphologically and biochemically. Morphologically, avian heterophils are identified by the presence of large, dull red, frequently needleshaped granules (Fig. 50.4). The presence of these eosinstaining granules has given rise to the term heterophil (heteros from the Greek meaning different), as opposed to the mammalian neutrophil whose granules are generally neutral in Romanowsky-stained blood smears. A left shift exists when excess band heterophils are present in the blood. Because avian heterophils are less segmented in health compared with mammalian neutrophils, identification of left shifts is more challenging in avian species.43

CHAPTER 50: DETERMINATION AND INTERPRETATION OF THE AVIAN LEUKOGRAM

TABLE 50.1 Causes of Heterophilia and Heteropenia in Birds Heterophilia Physiologic response Infection Tissue destruction or necrosis Drug administration Miscellaneous

Heteropenia Infection

Drug administration Predictable Idiosyncratic Neoplasia

Bacteria, viruses, fungi, parasites Thrombosis and infarction, inflammation Corticosteroids, estrogen Acute, severe stress, foreign bodies, hemorrhage or hemolytic disease Overwhelming bacterial infection, viral-induced hemopoietic cell destruction Cyclophosphamide, progesterone Piperacillin or doxycycline Leukemia, metastatic disease (disseminated lymphosarcoma), radiation exposure

Toxic changes of heterophils include cytoplasmic basophilia, cytoplasmic vacuolation, variable degranulation, and cellular swelling (Figs. 50.1 and 50.5). Toxic degranulation must be distinguished from staininduced degranulation. Toxic degranulation is the result of severe inflammation or infection and is associated with cytoplasmic basophilia. Stain-induced degranulation results from partial or complete granule dissolution when heterophils on the blood smear are exposed to aqueous-based stains (such as Diff-Quik® stain) with a short fixation time.43 Biochemically, avian heterophils lack myeloperoxidase, an enzyme that is largely responsible for the efficient oxidative bactericidal activity of mammalian neutrophils.3,62 A measurable oxidative burst has been observed for avian heterophils, but it is insignificant in bacterial killing. Avian heterophils accomplish effective nonoxidative bacterial killing by myeloperoxidaseindependent methods with granule-derived proteins.62 These substances include lysozyme, acid phosphatase, cathepsin, and β-glucuronidase (hydrolase) activities, and cationic proteins.18,19 The cationic proteins include β-defensins (e.g. gallinacins, chicken heterophil antimicrobial peptides, and turkey heterophil antimicrobial peptides), which are natural antimicrobials.71 In addition to bacteria, avian heterophils have been shown to kill yeast (e.g. Candida albicans). Avian heterophil function (adherence, chemotaxis, phagocytosis, and bacterial killing) is fairly efficient in host defense against bacteria in conjunction with the humoral immune system.4,5,6,38 Avian heterophil functions are less well understood than mammalian neutrophil function and constitute an area for future research.

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or flight response). The heterophilia occurs rapidly but is transient. The presumed mechanism, based on studies in other species, is an epinephrine- or exercise-mediated increase in microvascular blood flow that shifts marginated heterophils into the mainstream of circulation. Concurrent lymphocytosis also may be observed and may overshadow the heterophilia, especially in those species that have higher lymphocyte counts in health. Because of the frequent occurrence of physiologic leukocytosis, it is wise to obtain hematologic specimens before the patient is unduly perturbed. Physiologic responses are frequently confused with stress leukograms where lymphopenia is expected.20 Corticosteroid-induced Heterophilia Corticosteroid-induced heterophilia is observed sporadically in diseased or severely stressed birds and is the result of corticosterone release from the adrenal cortex. An example of developing heterophilia in response to forced confinement has been reported in captive herring gulls.2,7 Corticosteroid-induced heterophilia is observed more frequently with exogenous corticosteroid administration or injection of adrenocorticotropic hormone (ACTH), which stimulates endogenous corticosterone release. The phenomenon is dosage and route dependent.1 Developing or concurrent lymphopenia separates this response from physiologic leukocytosis. Inflammation- or Infection-induced Heterophilia Heterophilia is frequently observed in conjunction with tissue damage induced by inflammation or bacterial infection (including chlamydiosis).2,7,43 Occasionally, the source of tissue damage provoking heterophilia may be obscure. Examples include hemorrhage, oxidant-induced RBC destruction, and lead poisoning.34,41,46 Experimental studies have shown that significant heterophilia can occur within 6 hours of induced inflammation and that cell counts peak 4-fold greater than baseline values by 12 hours post-inflammation.43 As bone marrow reserves of segmented heterophils are depleted, a left shift accompanied by toxic changes of heterophils may appear within 24 hours. The presence of promyelocytes (cells with round nuclei and a mixture of purple and red cytoplasmic granules) indicates an intense shift.43 As the bone marrow responds to tissue demands for heterophils, the leukocytosis and heterophilia intensify. A return toward baseline values with disappearance of the left shift and toxic change indicates convalescence. In companion birds, total leukocyte counts may exceed 100,000 cells/μL in chronic chlamydiosis and mycobacteriosis, especially in gray cheek parakeets and macaws. Heteropenia

Physiologic Heterophilia One can observe physiologic heterophilia after excitement, fear, or short-term strenuous exercise (the fright

The three most common mechanisms for production of neutropenia in mammals include deficient neutrophil production in the bone marrow, a shift in neutrophils

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from the circulating to marginal cell pool, and emigration of neutrophils from the blood into the tissues at a rate that exceeds neutrophil replacement from the bone marrow.39 Similar mechanisms undoubtedly exist in birds but have not received detailed study. However, anecdotal reports in the literature and personal observations tend to support these mechanisms. Any instance of heteropenia suggests a guarded prognosis until favorable resolution of the condition can be ascertained by additional hematologic study.

species (Figs. 50.2 and 50.3). Characterization of the blood lymphocyte population in mammals indicates that the majority of these cells are small, mature, longlived, memory T cells. This probably is the case in avian species. For example, chicken blood lymphocytes have shown to be approximately 14% B cells and 72% T cells by direct and indirect immunofluoresence, respectively.60 The remaining blood lymphocytes (approximately 14%) are probably natural killer or null cells. Lymphocytosis

Deficient Heterophil Production Deficient heterophil production can be the result of hemopoietic stem cell destruction by infectious agents, drugs, or ionizing radiation and myelophthisis-associated loss of hemopoietic space. Leukopenia and heteropenia have been reported in pet birds due to viral infections (herpesvirus, polyomavirus, and psittacine reovirus); however, such reports largely represent undocumented clinical or laboratory impressions.24,25,66 Drug-induced myelosuppression may be predictable, as observed in cyclophosphamidetreated chickens and turkeys,24 or may be an idiosyncratic adverse drug reaction, as observed in a budgerigar after antibiotic (piperacillin or doxycycline) treatment.67 Toxic change of heterophils and vacuolation of hematopoietic precursor cells may be observed with drug toxicity. Myelophthisis-associated loss of hematopoietic space is observed infrequently in clinical practice. In our experience, this cause of heteropenia has been associated with disseminated lymphoid neoplasia wherein proliferating neoplastic lymphocytes replace normal hemopoietic cells. Heterophil Shifts from the Circulating to Marginal Pool

Physiologic lymphocytosis represents a transient phenomenon in birds after excitement, fright, or struggling during venipuncture (Table 50.2). Lymphocytosis may overshadow any heterophilia. This response may be especially noticeable in healthy birds that have high circulating lymphocyte counts, and may be more prominent in young birds (Fig. 50.10). TABLE 50.2 Causes of Lymphocytosis and Lymphopenia in Birds Lymphocytosis Physiologic response Antigenic stimulation

Lymphoid neoplasia Lymphopenia Acute systemic infection Corticosteroid-induced

Immunosuppression

Chronic viral infection, chronic bacterial infection, chronic fungal infection, parasitic diseases Lymphosarcoma, lymphocytic leukemia

Exogenous corticosteroid administration Endogenous corticosterone release (acute, severe stress) Immunosuppressive drugs, radiation

In mammals, early endotoxemia or Gram-negative sepsis results in a shift of neutrophils from the circulating to marginal cell pools where they are not quantitated by the WBC count. The neutropenia is transient (1–3 hours duration) and is followed by a rebound leukocytosis if the patient survives. This mechanism may occur in birds but is not well documented. Severe Tissue Demands for Heterophils Overwhelming tissue demand for heterophils at a rate that exceeds cell replacement by the bone marrow also produces heteropenia. Examples include acute severe peritonitis and necrotizing enteritis. Heteropenia may be accompanied by a degenerative left shift and toxic change of heterophils. LYMPHOCYTE RESPONSES As mentioned previously, lymphocytes may be the major circulating leukocyte in the blood of some avian

FIGURE 50.10 Relationship of the lymphocyte count with age in 169 blood samples from clinically healthy psittacine birds. Relatively high lymphocyte counts were common in birds less than 4 years of age.

CHAPTER 50: DETERMINATION AND INTERPRETATION OF THE AVIAN LEUKOGRAM

Lymphocytosis secondary to antigenic stimulation is frequently observed in birds that have chronic infectious (bacterial, viral, fungal, or parasitic) or inflammatory diseases wherein the blood lymphocyte pool is expanded because of persistent antigen exposure, most notably in chronic bacterial or viral diseases.43 Occasionally, the lymphocytosis is extreme as in a crane that had granulomatous disease and an absolute lymphocyte count of 45,900 cells/μL. Lymphoid neoplasia such as lymphosarcoma with a leukemic blood picture27,58 (Fig. 50.11), or lymphoid leukemia (Fig. 50.12), may be associated with lymphocytosis. In our experience, lymphoid leukemias produce extreme elevations in the absolute lymphocyte count (as much as 200,000 lymphocytes/μL). Lymphopenia Lymphopenia is frequently the result of severe stress-induced endogenous corticosterone release or

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exogenous corticosteroid administration (Table 50.2). Corticosteroid administration produces a rapid, predictable, transient redistribution of mammalian T cells to the bone marrow or other tissue compartments, resulting in lymphopenia.1 Studies in chickens have documented a rapidly developing but transient lymphopenia after corticosteroid administration. Stressassociated lymphopenia is more variable and often difficult to document hematologically in apparently clinically stressed birds. Heterophilia and concurrent lymphopenia are the hallmarks of a stress leukogram and may be observed in diseased or severely stressed birds, such as those subjected to forced molting through severe feed restriction.32 Lymphopenia of acute infection may have a complex origin, involving one or more mechanisms. These mechanisms include endogenous corticosterone release with temporary lymphocyte redistribution, temporary trapping of recirculating lymphocytes within lymphoid tissues to promote antigen contact, and direct destruction of lymphoid tissue, especially during viral infection.39 Last, drug- or toxin-induced lymphopenia may be observed infrequently. Examples include cyclophosphamide administration in chickens and turkeys, and crude oil ingestion by gulls. MONOCYTE RESPONSES

FIGURE 50.11 Blood smear from an emu that has disseminated lymphoma. Immature, neoplastic lymphocytes within the blood smear have dark blue cytoplasm with broad pseudopodia.

FIGURE 50.12 Blood smear from a cockatoo that has lymphocytic leukemia. The leukocytes are predominately mature, welldifferentiated lymphocytes.

The monocyte-macrophage system is composed of stem cells, monoblasts, and promonocytes in the bone marrow; monocytes in the bone marrow and blood; and macrophages within various body tissues. Studies in turkeys indicate that phagocytic uptake of blood particulate material occurs predominantly by macrophages in the liver, spleen, and bone marrow.55 Monocytes are produced in the bone marrow and released into the blood at an early age compared with heterophils. A bone marrow storage pool of monocytes apparently does not exist. Once released into the blood, monocytes circulate for a short time and emigrate from the blood vessels into the tissues (Fig. 50.3 and Fig. 50.13). Those monocytes that mature into tissue macrophages may have an extended lifespan ranging from days to months. Tissue demands for macrophages are met primarily by recruiting monocytes from the blood and increasing monocyte production in the bone marrow.60 Macrophages can undergo limited mitosis in situ, but such activity generally accounts for less than 5% of the total macrophage population. Monocytes are generally the largest circulating leukocyte in avian blood in health and typically account for 1–3% of the circulating WBC population. These cells have round to oval nuclei, slightly condensed chromatin, gray cytoplasm, and occasional pseudopodia. Cytoplasmic vacuolation also may be observed, but is more prominent if the blood stands for a while before smears are made. Difficulty can be encountered in distinguishing monocytes from large lymphocytes in some

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birds. In such cases, cytochemistry can be used for definitive cell identification.61 Monocytosis Monocytosis frequently is associated with chronic diseases such as granulomatous lesions, nonspecific tissue necrosis, bacterial infections (e.g. mycobacteriosis and chlamydiosis), deep mycosis (e.g. aspergillosis, parasitism), and zinc-deficient diets (Table 50.3).26,76 A frequent misconception among avian clinicians, however, is that monocytosis is only seen in chronic diseases and, when present, is indicative of granulomatous inflammation. In fact, monocytosis can be observed in both acute and chronic diseases. Experimental studies in chickens have shown that significant monocytosis can be observed in the blood within 12 hours after induced inflammation or bacterial airsacculitis, with peak monocyte counts occurring within 24 to 48 hours.14,43 Monocytopenia Monocytopenia is clinically unimportant and difficult to document because of the wide range in avian monocyte counts. In instances of pancytopenia, such as viral-induced hematopoietic cell damage, heteropenia deserves more clinical attention because of the possibility of secondary infections.

BASOPHIL RESPONSES Avian basophils are recognized by their round to oval nucleus and prominent, round, purple granules in Romanowsky-stained blood smears (Fig. 50.13). In healthy birds, basophils usually are more numerous in the blood than are eosinophils. This observation has been made in many avian species. Functions of avian basophils appear to be similar to those of mammalian basophils (see Chapter 44) but investigations of cell function are sparse or anecdotal. The involvement of avian basophils in acute inflammation is well documented. In chickens, basophilic infiltration of tissues has been observed with early (1.5–3 hours) inflammation of skin, wattle, skeletal muscle, and mesentery.12,54,72 Involvement of basophils has also been demonstrated in avian cutaneous hypersensitivity and systemic anaphylaxis.52 Clinical and experimental evidence also suggests that avian basophils are involved in the host response to internal and external parasites, including schistosomes, soft-bodied ticks, and air sac mites. Evidence also exists for basophil-associated tumor cytotoxicity wherein intense basophilic infiltrates have been observed in experimentally induced Rous sarcomas in chicken wing webs. In contrast to mammalian basophils, avian basophils also are actively phagocytic.37 Basophilia Generally, only sustained overt basophilia can be detected by routine hematologic methods wherein basophils constitute at least 3–6% of the leukocyte differential cell count. Although absolute basophil counts, determined using a hemacytometer and toluidine blue diluent, have been performed in birds,52 reports of blood basophil responses in disease are sporadic (Table 50.4). Basophilia is reported to occur with respiratory diseases and severe tissue damage in pet birds.59 This finding is partially corroborated by a trend toward development of basophilia in chickens that have experimentally-induced salmonellosis wherein basophil counts tended to increase within the first week of infection. Although the mechanism of basophilia is unknown, it may be related to the proinflammatory response observed in humans.65 Blood basophilia also has been

FIGURE 50.13 Blood smear from a red-tailed hawk. A heterophil, monocyte, and basophil are present.

TABLE 50.4 Causes of Basophilia in Birds

TABLE 50.3 Causes of Monocytosis in Birds Granulomatous inflammation Acute inflammation Bacterial infection Fungal infection Parasitism Zinc-deficient diets

Mycobacteriosis Chlamydiosis (chronic) Aspergillosis

Tissue damage or perturbation

Stress Parasitism Miscellaneous

Acute inflammation (skin, wattle, muscle, mesentery) Severe nonspecific tissue damage Bacterial infection Respiratory disease Systemic anaphylaxis Feed restriction, starvation Forced molting Schistosomes, ticks, air sac mites Ingestion of mycotoxin-contaminated feed

CHAPTER 50: DETERMINATION AND INTERPRETATION OF THE AVIAN LEUKOGRAM

observed with internal and external parasitism. Examples include experimental infection of chickens that have Austrobilharzia variglandis, and natural infestation of canaries and finches with air sac mites. Basophilia can occur alone or in conjunction with eosinophilia. Basophilia also has been observed in chickens49,53 and ducks after significant feed restriction, and is presumed to be a harbinger of severe stress. Apparently birds differ from mammals in that basophilia occurs in stress and that blood basophil counts apparently are unaffected by corticosteroid administration.49 Basophilia also has been reported in chickens, presumably in response to mycotoxin-contaminated feed.50 Basopenia Basopenia cannot be detected reliably without performing absolute basophil counts. The presence of basopenia in other species is of limited clinical importance in routine health care, as may be the case in avian patients. EOSINOPHIL RESPONSES Avian eosinophils are recognized by their round, often red-orange granules, and light blue cytoplasm in Romanowsky-stained blood smears. However, raptor eosinophil granules may occasionally be rod shaped, and granule tinctorial properties may vary slightly between species. Eosinophils generally are the least frequently encountered leukocyte in avian blood smears, with the exception of raptors in which these cells may account for 15% of the leukocyte differential cell count. In some species of birds, such as Amazon parrots, a leukocyte with round, colorless to light blue granules may be observed in the stained blood smear. The common practice is to identify such cells as eosinophils because typical eosinophils with red-orange granules have not been observed. Ultimate identification of this unusual leukocyte will require both ultrastructural and cytochemical study. Eosinophilia Avian species are similar to horses in that eosinophilia is observed infrequently in species other than raptors, although parasitism is observed frequently (Table 50.5). Eosinophilia has been observed in chickens that have TABLE 50.5 Causes of Eosinophilia and Eosinopenia in Birds Eosinophilia Facial edema Parasitism Exposure to foreign antigens Eosinopenia Severe stress Corticosteroid administration

Horse serum, bovine serum albumin

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facial edema; chickens experimentally infected with schistosomes or naturally infected with Trichostrongylus tenuis; and in quails, chickens, and ducks injected intraperitoneally with horse serum or bovine serum albumin. The magnitude of the eosinophilia ranges from 6% to 56% of the leukocyte differential count in both experimental and natural disease. Furthermore, lymphocyte– eosinophil interaction has been demonstrated indirectly. Marked secondary eosinophilia has been observed in chickens and ducks after repeated antigen administration.48 Irradiation has been shown to depress development of eosinophilia in chickens, whereas thymectomy had little effect.51 The reason for this apparent paradoxical observation is that total-body irradiation affects all lymphocytes and it is difficult to remove all thymic tissue. Eosinopenia Eosinopenia is best defined by clinical experience. Because eosinophils are infrequently encountered in avian blood except for raptors, absolute eosinophil counts are necessary for a quantitative study of avian eosinopenia. A problem is readily apparent in that it may be difficult to distinguish eosinophils from heterophils in absolute counts unless cytochemistry is used. In mammals, eosinopenia is observed after corticosteroid or ACTH administration. Single-dose administration results in sequestration of eosinophils and delayed eosinophil release from the bone marrow. Prolonged high-dose corticosteroid treatment causes decreased eosinophil production. Eosinophil responses in sick birds are seldom mentioned, although indication of eosinopenia sometimes is apparent. Injection of long-acting ACTH into immature chickens has shown a suggestion of slight eosinopenia, but routine hematologic methods lacked the sensitivity needed for adequate quantitation of the cellular response. In summary, clarification of avian eosinophil responses in blood relies on evaluation of absolute cell counts. In addition, the importance of avian eosinophil responses within the tissues will only be clarified by immunohistochemical staining to reliably distinguish eosinophils from heterophils. REMARKS ON PROGNOSIS AND HEMATOLOGIC TRENDS Heterophil:Lymphocyte Ratios Introduced as an indicator of stress in chickens, the heterophil : lymphocyte (H : L) ratio has been calculated on the basis of both absolute heterophil and lymphocyte counts or on relative percentages of these WBCs from the stained blood smear. When interpreting H:L ratios, one must remember that the ratio can increase with absolute heterophilia when the lymphocyte count is in the reference interval or with absolute lymphopenia when heterophil counts are in the reference interval. The true stress response, that is observed after

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corticosteroid administration, is absolute heterophilia in conjunction with developing lymphopenia. In an experiment on feed restriction, ducks showed no change in H : L ratios; however, H : L ratios were increased in chickens and turkeys. In this study, it was felt that developing basophilia in chickens, turkeys, and ducks was a more uniform hematologic change denoting severe stress. In our opinion, H : L ratios currently represent another mathematical manipulation of limited clinical value in assessing stress in birds. Unfavorable Prognostic Features A guarded prognosis is suggested with heteropenia of any cause, especially if a left shift and toxic changes of heterophils are observed. If a left shift intensifies or becomes degenerative (where band heterophils and younger forms outnumber segmented heterophils), a guarded prognosis is suggested. Extreme leukocytosis also suggests a guarded prognosis until chlamydiosis, mycobacteriosis, deep mycosis, and leukemia are excluded. Favorable Hematologic Trends Hematologic trends in birds that are suggestive of recovery include resolution of extreme leukocytosis, leukopenia, heteropenia, left shift, toxic changes of heterophils, and lymphopenia. NEW TECHNIQUES FOR ASSESSMENT OF AVIAN LEUKOCYTES A flow cytometric technique for enumeration of quail, chicken and goose WBCs has been described.76,77 The technique relies on the differential affinity of avian RBCs and WBC types for fluorescent lipophilic dyes such as 3,3-dihexyloxacarbocyanine [(DiOC6(3)], permitting cell separation based on fluorescence.56,75 This technique holds promise for application in other avian species because antibodies specific for cell surface markers are not required; however, a flow cytometer capable of fluorescence detection is necessary. Classification of avian lymphocytes into subtypes of T cells and B cells,1 and distinction of thrombocytes from lymphocytes with antibodies specific for chicken thrombocytes,8 are established research techniques, but their incorporation into routine hematology is limited by the need for species-specific antibodies to WBC antigens and need of a flow cytometer.

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4. Andreasen JR, Andreasen CB, Anwar M, et al. Heterophil chemotaxis in chickens with natural staphylococcal infections. Avian Dis 1993;37:284–289. 5. Andreasen CB, Latimer KS, Harmon BG, et al. Heterophil function in healthy chickens and in chickens with experimentally induced staphylococcal tenosynovitis. Vet Pathol 1991;28:419–427. 6. Andreasen CB, Latimer KS, Steffens WL. Evaluation of chicken heterophil adherence. Avian Dis 1990;34:639–642. 7. Bounous DI, Schaeffer DO, Roy A. Coagulase-negative Staphylococcus sp. septicemia in a lovebird. J Am Vet Med Assoc 1989;195:1120–1122. 8. Bohls RL, Smith R, Ferro PJ, et al. The use of flow cytometry to discriminate avian lymphocytes from contaminating thrombocytes. Devel Comp Immunol 2006;30:843–850. 9. Campbell TW. Avian Hematology, 2nd ed. Ames: Iowa State University Press, 1995. 10. Campbell TW, Dein FJ. Avian hematology: the basics. Vet Clin N Am Small Anim Pract 1984;14:223–248. 11. Campo JL, Prieto MT, Davila SG. Effects of housing system and cold stress on heterophil-to-lymphocyte ratio, fluctuating asymmetry, and tonic immobility duration of chickens. Poult Sci 2008;87:621–626. 12. Carlson HC. The acute inflammatory reaction in chicken breast muscle. Avian Dis 1972;16:553–558. 13. Carlson HC, Allen JR. The acute inflammatory reaction in chicken skin: blood cellular response. Avian Dis 1969;14:817–833. 14. Chand N, Carlson HC, Eyre P. Passive cutaneous anaphylaxis in domestic fowl. Intl Arch Allergy Immunol 1975;51:508–517. 15. Clubb SL, Schubot RM, Joyner K, et al. Hematologic and serum biochemical reference intervals in juvenile eclectus parrots (Eclectus roratus). J Assoc Avian Vet 1990;4:218–225. 16. Clubb SL, Schubot RM, Joyner K, et al. Hematologic and serum biochemical reference intervals in juvenile cockatoos. J Assoc Avian Vet 1991;5:16–26. 17. Clubb SL, Schubot RM, Joyner K, et al. Hematologic and serum biochemical reference intervals in juvenile macaws (Ara sp.). J Assoc Avian Vet 1991;5:154–162. 18. Evans EW, Beach FG, Moore KM, et al. Antimicrobial activity of chicken and turkey heterophil peptides CHP1, CHP2, THP1, and THP3. Vet Microbiol 1995;47:295–303. 19. Evans EW, Beach GG, Wunderlich J, et al. Isolation of antimicrobial peptides from avian heterophils. J Leuk Biol 1994;56:661–665. 20. Davison TF, Rpwell JG, Rea J. Effects of dietary corticosterone on peripheral blood lymphocyte and granulocyte populations in immature domestic fowl. Res Vet Sci 1983;34:236–239. 21. Dein FJ. Laboratory Manual of Avian Hematology. East Northport: Association of Avian Veterinarians, 1984. 22. Ferris M, Bacha WJ. A new method for the identification and enumeration of chicken heterophils and eosinophils. Avian Dis 1984;28:179–182. 23. Ficken MD, Barnes HJ. Effect of cyclophosphamide on selected hematologic parameters of the turkey. Avian Dis 1988;32:812–817. 24. Fulton RM, Reed WM, Thacker HL, et al. Cyclophosphamide (Cytoxan)induced hematologic alterations in specific-pathogen-free chickens. Avian Dis 1996;40:1–12. 25. Godwin JS, Jacobson ER, Gaskin JM. Effects of Pacheco’s parrot disease virus on hematologic and blood chemistry values of Quaker parrots (Myopsitta monachus). J Zoo Anim Med 1982;13:127–132. 26. Graham DL. Histopathologic lesions associated with chlamydiosis in psittacine birds. J Am Vet Med Assoc 1989;195:1571–1573. 27. Gregory CR, Latimer KS, Mahaffey EA, et al. Lymphoma and leukemic blood picture in an emu (Dromaius novaehollandiae). Vet Clin Pathol 1996;25:136–139. 28. Hamre CJ. Origin and differentiation of heterophils, eosinophil and basophil leukocytes of chickens. Anat Rec 1952;112:339–340. 29. Hawkey C, Hart MG, Samour HJ, et al. Haematological findings in healthy and sick captive rosy flamingos (Phoenicopterus ruber ruber). Avian Pathol 1984;13:163–172. 30. Hawkey CM, Hart MG, Samour HJ. Normal and clinical haematology of greater and lesser flamingos (Phoenicopterus roseus and Phoeniconaias minor). Avian Pathol 1985;14:537–541. 31. Hawkey C, Samour HJ, Henderson GM, et al. Haematological findings in captive gentoo penguins (Pygoscelis papua) with bumblefoot. Avian Pathol 1985;14:251–256. 32. Holt PS. Effects of induced moulting on immune responses of hens. Br Poult Sci 1992;33:165–175. 33. Kariyawasam HH, Robinson DS. The eosinophil: the cell and its weapons, the cytokines, its locations. Semin Resp Crit Care Med 2006;27:117–127 34. Katavolos P, Staempfli S, Sears W, et al. The effect of lead poisoning on hematologic and biochemical values in trumpeter swans and Canada geese. Vet Clin Pathol 2007;36:341–347. 35. Kondo Y, Cahyaningsih U, Abe A, et al. Presence of the diurnal rhythms of monocyte count and macrophage activities in chicks. Poult Sci 1992;71:296–301. 36. Lane R. Basic techniques in pet avian clinical pathology. Vet Clin N Am Small Anim Pract 1991;21:1157–1179.

CHAPTER 50: DETERMINATION AND INTERPRETATION OF THE AVIAN LEUKOGRAM 37. Latimer KS. Diseases affecting leukocytes. In: Colahan PT, et al., eds. Equine Medicine and Surgery, 4th ed., Vol 2. Goleta: American Veterinary Publications, 1991;1809–1815. 38. Latimer KS, Harmon BG, Glisson JR, et al. Turkey heterophil chemotaxis to Pasteurella multocida (serotype 3,4)-generated chemotactic factors. Avian Dis 1990;34:137–140. 39. Latimer KS, Rakich PM. Clinical interpretation of leukocyte responses. Vet Clin N Am Small Anim Pract 1989;19:637–668. 40. Latimer KS, Steffens WL, Goodwin M. Ultrastructural changes in avian toxic heterophils. In: Proceedings of the 46th Annual Meeting of Electron Microscopic Society of America, San Francisco, CA, 1988;360–361. 41. Latimer KS, Tang KN, Goodwin MA, et al. Leukocyte changes associated with acute inflammation in chickens. Avian Dis 1988;32:760–772. 42. Leighton FA. Clinical, gross, and histological findings in Herring gulls and Atlantic puffins that ingested Prudhoe Bay crude oil. Vet Pathol 1986;23:254–263. 43. Lind PJ, Wolff PL, Petrini KR, et al. Morphology of the eosinophil in raptors. J Assoc Avian Vet 1990;4:33–38. 44. Lucas AM, Jamroz C. Atlas of Aviam Hematology. USDA Agricultural Monograph 25. Washington DC: US Government Printing Office, 1961. 45. Maina JN. Systematic analysis of hematopoietic, vasculogenetic, and angiogenetic phases in the developing embryonic avian lung, Gallus gallus variant domesticus. Tissue Cell 2004;36:307–322. 46. Maxwell MH. Production of a Heinz body anaemia in the domestic fowl after ingestion of dimethyl disulfide: a haematological and ultrastructural study. Res Vet Sci 1981;30:233–238. 47. Maxwell MH. Granulocyte differentiation in the lymphoid organs of chick embryos after antigenic and mitogenic stimulation. Devel Comp Immunol 1985;9:93–106. 48. Maxwell MH. Fine structural and cytochemical studies of eosinophils from fowls and ducks with eosinophilia. Res Vet Sci 1985;41:135–148 49. Maxwell MH, Burns RB. Blood eosinophilia in adult bantams naturally infected with Trichostrongylus tenuis. Res Vet Sci 1985;39:122–123. 50. Maxwell MH, Burns RB. Experimental eosinophilia in domestic fowls ducks following horse serum stimulation. Vet Res Commun 1982;5:369–376. 51. Maxwell MH, Burns RB. Experimental stimulation of eosinophil production in the domestic fowl (Gallus gallus domesticus). Res Vet Sci 1986;41:114–123. 52. Maxwell MH, Robertson GW, Spence S, et al. Comparison of haematological values in restricted-and ad libitum-fed domestic fowls: white blood cells and thrombocytes. Br Poult Sci 1990;31:399–405. 53. Maxwell MH, Siller WG, MacKenzie GM. Eosinophilia associated with facial oedema in fowls. Vet Rec 1979;105:232–233. 54. McCorkle F, Olah I, Glick B. The morphology of the phytohemagglutininduced cell response in the chicken’s wattle. Poult Sci 1980;59:616–623. 55. McEntee MF, Ficken MD. Blood clearance of radiolabeled gold colloid by the turkey mononuclear phagocytic system. Avian Dis 1990;34:393–397. 56. Morimoto T, Minami A, Inoue Y, et al. A new method for counting of quail leukocytes by flow cytometry. J Vet Med Sci 2002;64:1149–1151.

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57. Natt MP, Herrick CA. A new blood diluent for counting the erythrocytes and leukocytes of the chicken. Poult Sci 1952;31:735–738. 58. Newell SM, McMillan MC, Moore FM. Diagnosis and treatment of lymphocytic leukemia and malignant lymphoma in a Pekin duck (Anas platyrhynchos domesticus). J Assoc Avian Vet 1991;5:83–86. 59. Nordio C, Rosi F. Possible significance of the increase of basophils in blood differential count of farm animals. Atti Soc Ital Sci Vet 1978;32:254. 60. Nowak JS, Kai O, Peck R, et al. The effects of cyclosporin A on the chicken immune system. Eur J Immunol 1982;12:867–876. 61. Odend’hal S, Player EC. Histochemical localization of T-cells in tissue sections. Avian Dis 1980;24:886–895. 62. Rausch P, Moore TG. Granule enzymes of polymorphonuclear neutrophils: a phylogenetic comparison. Blood 1975;46:913–919. 63. Riddell C. Avian Histopathology. Kennett Square, PA: American Association of Avian Pathologists, 1987. 64. Robertson GW, Maxwell MH. Importance of optimal mixtures of EDTA anticoagulant blood for the preparation of well-stained avian blood smears. Br Poult Sci 1993;34:615–617. 65. Robertson GW, Maxwell MH. Modified staining techniques for avian blood cells. Br Poult Sci 1990;31:881–886. 66. Rosskopf WJ, Woerpel RW, Howard EB, et al. Chronic endocrine disorder associated with inclusion body hepatitis in a sulfur-crested cockatoo. J Am Vet Med Assoc 1981;179:1273–1276. 67. Rosskopf WJ, Woerpel RW. Avian diagnosis: laboratory interpretations and case reports, part 3. Companion Anim Pract 1989;19:41–48. 68. Russo EA, McEntee L, Applegate L, et al. Comparison of two methods for determination of white blood cell counts in macaws. J Am Vet Med Assoc 1986;189:1013–1016. 69. Santos MD, Yasuike M, Hirono I, et al. The colony-stimulating factors of fish and chicken. Immunogenetics 2006;58:422–432. 70. Schepelmann K. Erythropoietic bone marrow in the pigeon: development of its distribution and volume during growth and pneumatization of bones. J Morphol 1990;203:21–34. 71. Sugiarto H, Yu PL. Avian antimicrobial peptides. Biochem Biophys Res Commun 2004;22:721–727. 72. Stadecker MJ, Lukic M, Dvorak A, et al. The cutaneous basophil response to phytohemagglutinin in chickens. J Immunol 1977;118:1564–1568. 73. Swayne DE, Johnson GS. Cytochemical properties of chicken blood cells resembling both thrombocytes and lymphocytes. Vet Clin Pathol 1986;15:17–24. 74. Tell LA, Citino SB. Hematologic and serum chemistry reference intervals for Cuban amazon parrots (Amazona leucocephala leucocephala). J Zoo Wildlife Med 1992;23:62–74. 75. Uchiyama R, Moritomo T, Kai O, et al. Counting absolute number of lymphocytes in quail whole blood by flow cytometry. J Vet Med Sci 2005;57:441–444. 76. Wight PAL, Dewar WA, MacKenzie GM. Monocytosis in experimental zinc deficiency of domestic birds. Avian Pathol 1980;9:61–66. 77. Zinkl JG. Avian hematology. In: Jain NC, ed. Schalm’s Veterinary Hematology, 4th ed. Philadelphia: Lea & Febiger, 1986;256–273.

C H A P T E R 51

Biology of Lymphocytes and Plasma Cells MICHAEL J. DAY The Innate and Adaptive Immune Response T Cells Definition and Development T Cell Activation T cell functions Function of CD4+ T cells

Function of CD8+ T cells Function of γδ T cells NKT cells Natural Killer Cells B Cells and Plasma Cells

Acronyms and Abbreviations ADCC, antibody-dependent cell-mediated cytotoxicity; APC, antigen presenting cell; BCR, B cell receptor; CD, cluster of differentiation; CTLA-4, cytotoxic T lymphocyte antigen 4; DAMP, damage-associated molecular pattern; Foxp3, forkhead box P3; GITR, glucocorticoid-induced TNF-receptor regulated gene; HEV, high endothelial venule; IFN, interferon; Ig, immunoglobulin; IL, interleukin; LGL, large granular lymphocyte; MAMP, microbe-associated molecular pattern; MHC, major histocompatibility complex; NFκB, nuclear factor kappa B; NK, natural killer; NOD, nucleotide-binding oligomerisation domain; PAMP, pathogen-associated molecular pattern; PRR, pattern recognition receptor; RIG, retinoic-acid-inducible gene; SCID, severe combined immunodeficiency; SmIg, surface membrane immunoglobulin; STAT, signal transducer and activator of transcription; TCR, T cell receptor; Th, T helper cell; TGF, transforming growth factor; TNF, tumor necrosis factor; Tr or T-reg, T regulatory cell.

T

he lymphoid cells are key components of the immune system. An increasing number of lymphoid subsets are now defined (Table 51.1). Some have roles in the innate immune response (natural killer [NK] cells and T cells expressing the γδ T cell receptor [TCR]), but most are responsible for mediating adaptive immunity. These latter cells regulate the production of antibody (humoral immunity) or are responsible for cell-mediated immune effects such as cytotoxicity or delayed-type hypersensitivity. The fundamental importance of lymphoid cells is clearly demonstrated in animals with genetic inability to produce such cells (e.g. severe combined immunodeficiency; SCID) that are profoundly immunodeficient and readily succumb to infectious disease.34,36 Lymphocyte subsets may be defined: ■ phenotypically,

by the expression of surface molecules ■ anatomically, by their developmental pathways and distribution within lymphoid tissue ■ functionally, by their end-effects in an immune response which relates to expression of particular membrane molecules and release of specific cytokines. 358

Lymphocyte biology is best defined in humans and rodents, but most features are highly conserved and have parallels in veterinary species. THE INNATE AND ADAPTIVE IMMUNE RESPONSE The innate immune response encompasses an evolutionarily older, more simplistic form of immunity that is continually present to provide immediate first-line defense from potential pathogens.25 As the encounter with pathogens is most likely to occur at mucocutaneous surfaces, these are the anatomical regions that are particularly enriched for components of the innate immune system. A key part of the innate response is the presence of an epithelial barrier that has specific regional adaptations to enhance its effectiveness (e.g. the mucociliary escalator of the upper respiratory tract, keratinization of the cutaneous squamous epidermis). Mucocutaneous surfaces also frequently are bathed in antimicrobial secretions that contain a cocktail of enzymes (e.g. lysozyme), small antibacterial molecules

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359

TABLE 51.1 Lymphocyte Subsets Cell Type B cell Plasma cell T cell

Phenotypic Subtype

αβ TCR+ CD4+

Functional Subtype

Th0 Th1 Th2 Th3 Th17 Treg Tr1

γδ TCR+ CD4+

NKT cell NK cell

αβ TCR+ CD8+ Types I and II

(e.g. defensins, alternative pathway complement molecules) and low-specificity polyreactive antibodies of the immunoglobulin (Ig) A and IgM classes. Within, and immediately beneath, the epithelial barrier are populations of innate immune cells including phagocytic cells (e.g. neutrophils, macrophages), NK cells and γδ TCR+ T cells. Another important leukocyte involved in innate immunity is the dendritic cell. In recent years, this cell has become the focus of intense interest with the recognition that it provides the key link between the innate and adaptive immune systems.16,20 The dendritic cell (so named because of its characteristic elongate cytoplasmic dendrites) is located within or beneath the epithelial barrier where it is well placed to encounter and sample foreign antigen as it is first exposed to the body. The means by which this reaction occurs, and the way in which dendritic cells subsequently direct the nature of the ensuing adaptive immune response, will be discussed in detail below. Whereas the innate immune system is continually present and able to react immediately to foreign antigen, one major feature of the adaptive response is a delay in onset. This delay is accounted for by the requirement for a sequence of complex intercellular interactions leading to production of antigen-specific T and B effector lymphocytes that must migrate from organized lymphoid tissue (where they are generated) to the site of antigenic exposure via the lymphatic and blood vascular systems. The adaptive immune response also encompasses the generation of populations of antigen-specific regulatory (i.e. suppressor) lymphocytes that control the immune response by down-regulating (suppressing) effector cells when they are no longer required to

Function Plasma cell precursor Immunoglobulin secretion Precursor to functional CD4+ T cell subsets described below Cell-mediated immunity; limited help for antibody production. Mediates “type 1” immune response Help for antibody production, particularly IgE, IgG subclass and IgA. Mediates “type 2” immune response Regulatory T cell mediates oral tolerance Mediates proinflammatory immunopathology, response to infections and autoimmunity CD25+ CD4+ “natural suppressor” cell. Normally present to control autoimmune and allergic responses IL-10 producing “induced suppressor” cell stimulated during immune response to exogenous antigen Predominantly surface intraepithelial population important in first-line defence to bacterial pathogens. May be type 1 and 2 subsets Cytotoxic T cells. May be type 1 and 2 subsets Recognize lipid antigen expressed by CD1d Cytotoxic cell; performs antibody-dependent cellmediated cytotoxicity (ADCC)

be active. Immunological memory is the lasting sequel to this regulatory process.

T CELLS Definition and Development T cells are defined by the expression of a T cell receptor (TCR) that confers unique antigen specificity to the cell. The TCR complex comprises a central two chain transmembrane portion involved in antigen recognition that is associated with a series of minor components (collectively called CD3 and comprising γ, δ, ε and ζ chains) that act as signal transducers following recognition of specific antigen (Fig. 51.1). The majority of T cells utilise α and β chains in forming the central portion of the TCR complex, and a second group of T cells have γδ TCRs. The diverse specificity of TCRs is achieved by the variable genetic combination of an array of gene segments that encode portions of the TCR chains (see Chapter 10). The total repertoire of TCRs in the body is sufficient to permit an animal to potentially recognise any possible antigen that it may encounter. Most mature T cells also express one of two mutually exclusive surface molecules, CD4 or CD8 that define non-overlapping T cell subsets; however, double negative (CD4− CD8−) or double-positive (CD4+ CD8+ ) T cells are found at certain stages of development. All lymphoid cells, including T cells, arise from the common bone marrow stem cell (see Chapter 10). Immature T cell precursors are then exported from the marrow to undergo development and maturation in the thymus.4,37 Intrathymic development of T cells is

360

SECTION IV: LEUKOCYTES

SmIg α

Igβ

Igα

Igα

ε

Igβ

β

δ

γ

ζ A

B

BCR

ε

ζ TCR

FIGURE 51.1 Diagrammatic representation of the B cell receptor (BCR) and T cell receptor (TCR). Antigen recognition is mediated by the surface membrane immunoglobulin of the BCR, or the αβ chains (alternative γδ chains not shown) of the TCR. The signal transduction molecules of the BCR are collectively known as CD79, and those of the TCR as CD3.

Immature thymocyte

T

Express functional TCR? (positive selection)

T

No

Yes

APC

Express autoreactive TCR? (negative selection)

T

Yes

Apoptosis and phagocytosis

No

T

Exit thymus

FIGURE 51.2 Intrathymic development of T cells. Developing thymocytes must express a T cell receptor able to productively interact with MHC and peptide, but must not recognize selfantigens that are capable of inducing an autoimmune response. Only T cells that satisfy these requirements are permitted to leave the thymus and enter the peripheral recirculation pathway.

Mature CD4+ or CD8+ T cells leave the thymus and seed the peripheral lymphoid tissue, particularly the lymph node paracortex, splenic periarteriolar lymphoid sheath, or perifollicular regions of mucosa-associated lymphoid tissue (Fig. 51.3). However, lymphocytes do not remain static within these locations. There is continual and massive recirculation of lymphoid cells throughout the body, via the pathway depicted in Figure 51.4. Such recirculation is necessary in order to optimize the chance of contact with specific antigen, and to orchestrate and regulate an immune response in appropriate areas of the body. Lymphocyte recirculation and vascular egress of these cells is carefully regulated by a complex network of adhesion molecules expressed by the recirculating lymphocytes and modified vascular endothelium (high endothelial venules; HEV) found normally in some lymphoid tissues or induced at sites of inflammation.1,35 Lymphocytes are considered functionally naïve until they encounter the antigen that they are programmed to recognize via their specific TCR; following participation in an immune response, a population of memory lymphocytes persists for generation of the more effective anamnestic (memory) immune response. T Cell Activation

complex, and involves a series of interactions between the developing thymocytes and populations of antigenpresenting cells (APCs) such as thymic epithelial cells or dendritic cells. These tests of developing thymocytes determine whether the cells express a functional TCR capable of antigen recognition (positive selection) or a TCR that recognizes self antigens and induces autoimmunity (negative selection); T cells that fail either test undergo apoptosis (programmed cell death or cell suicide) within the thymus and the remnants of the cell are phagocytosed by macrophages (Fig. 51.2).

T cells have specialized requirements for activation. Intact antigen is generally unable to stimulate T cells and must first be processed and presented by populations of APCs.6 Naïve T cells are most effectively activated when antigen is presented to them by dendritic cells, but macrophages and B cells can present antigen, and in some circumstances a wide variety of other cells (e.g. endothelia, epithelia, fibroblasts) can be induced to present antigen (non-professional APC). Major breakthroughs have been made in expanding understanding of the key role of dendritic cells in T cell activation. Although considered part of innate immu-

CHAPTER 51: BIOLOGY OF LYMPHOCYTES AND PLASMA CELLS

A

B FIGURE 51.3 Sections of canine lymph node immunohistochemically labelled for expression of (A) CD79 and (B) CD3. The two antibodies clearly delineate cortical aggregates of B cells (A) and surrounding paracortical T cells (B).

Tissue

Afferent lymph Lymph node

Spleen

Efferent lymph

Thoracic duct

FIGURE 51.4 Pathways of lymphocyte recirculation. Lymphoid cells from the tissues drain to regional lymph nodes via the afferent lymph and exit the lymph node through efferent lymph, subsequently entering the blood via the thoracic duct. Lymphocytes circulate in the bloodstream and may leave the vasculature to enter lymphoid tissue or other body tissues when there is expression of appropriate adhesion molecules by the local vascular endothelium.

361

nity, it is now known that the dendritic cells direct the nature of the specific adaptive immune response and that this in turn is dependent on the nature of the stimulating antigen. The dendritic cell is endowed with a series of surface and cytoplasmic molecules collectively termed pattern recognition receptors (PRRs). The surface PRRs are also known as Toll-like receptors (TLRs) after the molecule Toll that was identified in the fruit fly Drosophila as a key immune-defense molecule in that insect.33 TLRs are few in number and are highly conserved throughout evolution. Cytoplasmic PRRs include the NOD- and RIG-like receptor molecules.39 The ligands for PRRs are antigenic epitopes expressed by microbes or damaged tissue cells. The former are known as pathogen-associated molecular patterns (PAMPs) or microbe-associated molecular patterns (MAMPs) and the latter as damage-associated molecular patterns (DAMPs). These are structurally conserved molecules that may be protein, carbohydrate, or nucleic acid in origin. Particular PAMPs interact with combinations of specific PRRs; for example, the peptidoglycan of Gram-positive bacteria binds to TLR-2, whereas the lipopolysaccharide of Gram-negative bacteria interacts with TLR-4. The surface PRRs are associated with signal transduction molecules such as MyD88 and ligation of the PRR triggers an intracytoplasmic and intranuclear signaling pathway that leads to differential gene expression via nuclear factor kappa B (NF κB) and other transcription factors. This gene expression determines how the APC will signal the responding T cell (as discussed below) and therefore determines the nature of the ensuing T cell response. In parallel with these signalling events, antigen is also internalized by the APC, processed and presented on the cell surface in association with molecules of the major histocompatibility complex (MHC). There are two broad pathways of antigen presentation. Most exogenous antigens (the majority of antigens such as infectious agents or allergens) are taken up by phagocytosis or macropinocytosis and placed in a phagosome where they are enzymatically degraded to small peptide fragments that associate with Class II molecules of the MHC. The combination of antigenic peptide-MHC is then expressed on the surface of the APC in a manner that can be recognised by the TCR. Endogenous antigens, that are derived from the cell cytoplasm, (e.g. viral, tumor or self molecules), undergo an alternative process involving degradation within a cytoplasmic proteasome and transport into the endoplasmic reticulum (mediated by transporter proteins), where they associate with Class I MHC molecules. After transfer to the Golgi apparatus, the combination of antigenic peptide-MHC is similarly expressed on the surface of the APC (Fig. 51.5). Endogenous peptides also can be presented by MHC Class II molecules and exogenous peptides by MHC Class I molecules in a phenomenon known as crosspresentation. Certain lipid antigens (e.g. derived from Mycobacterium) are presented via a third pathway involving endosomal process and presentation in association with molecules of the CD1 family.7 T cells that

362

SECTION IV: LEUKOCYTES

Antigen Presentation Peptides Cytoplasmic compartment A

MHC Class II

MHC Class II blocked

cytosolic signal transducers and activators of transcription (STATs), subsequently migrate to the nucleus, and initiate gene activation via the effects of specific transcription factors. Following activation, the T cell undergoes blast transformation and cytokine secretion (e.g. interleukin 2; IL-2), and the process of clonal proliferation and differentiation, whereby large numbers of T cells with identical antigen-MHC specificity are generated to participate as effector cells in the immune response. Such T cell activation generally occurs within the secondary lymphoid tissue (e.g. lymph node paracortex) and activated T lymphocytes enter the recirculation pathway to arrive at the site of antigen exposure, or other lymphoid tissues.

Antigen Presentation

Transporter Proteasome protein B

MHC Class I ER

Golgi

FIGURE 51.5 Processing and presentation of (A) exogenous antigen and (B) endogenous antigen. Exogenous antigen is degraded in a cytoplasmic compartment where there is association between peptide fragments of antigen and class II molecules of the major histocompatibility complex. Endogenously derived antigen is degraded within cytoplasmic proteasomes and transported to the endoplasmic reticulum where peptide fragments become associated with MHC class I molecules. In each case, the processed antigen is presented on the surface of the antigen-presenting cell by MHC for T cell recognition.

recognize lipid antigen expressed by CD1a, CD1b, or CD1c are likely similar in nature to peptide-specific T cells, whereas only NKT cells (see below) recognize antigen expressed by CD1d.13 T cell activation is, therefore, a complex event that is driven primarily by the APC. A series of molecular interactions occurs between these two cells that lead to activation of T cells of a specific type that is most relevant to the inciting antigen. These molecular events include: (1) Recognition of the combination of antigenic peptide and MHC residues by the TCR leading to signalling via CD3; (2) cognate interaction of the APC and T cell via an array of other surface molecules (e.g. B7 and CD28). In one such interaction, the CD4 molecule binds to MHC Class II and CD8 to MHC Class I. This renders the CD4+ T cells susceptible to stimulation only when antigen is presented by Class II, and CD8+ T cells to activation only following recognition of peptide in the context of Class I; (3) release of co-stimulatory cytokine/s by the APC that bind cytokine receptors on the T cell. The nature of cytokines released by the APC is determined by the PAMP-PRR interaction. APCderived co-stimulatory cytokines bind specific cytokine receptors on the surface of the T cell, and when aggregated such receptors signal via activating cytoplasmic tyrosine kinases (Janus kinases) that phosphorylate the

T Cell Functions The end effect of T cell activation is to orchestrate some component of an antigen-specific immune response. T cells may be regulatory, providing either positive (helper) or negative (suppressor) signals to other leukocytes, or be cytotoxic in function. Function of CD4+ T Cells T cells bearing the CD4 molecule regulate the function of a variety of leukocytes or cross-regulate the function of each other. The recognition of CD4+ T cell subsets has fundamentally reshaped study of the immune response in recent years.32 First recognized were two subsets of αβ TCR+ CD4+ T cells, known as Th1 (T helper) and Th2. Although phenotypically indistinguishable, these subpopulations have distinct function conferred upon them by the secretion of two non-overlapping cytokine profiles (Fig. 51.6). Th1 cells selectively secrete the cytokines IL-2 and interferon gamma (IFN-γ) and enhance the cytotoxic effects of CD8+ and natural killer (NK) cells, the killing of intracytoplasmic pathogens (e.g. Leishmania, Mycobacterium) by macrophages, and the selective production of antibody of a specific IgG subclass (IgG2a in mice). Th2 cells selectively secrete IL-4, -5, -6, -9, -10, and -13, provide help for B cell and plasma cell development, and promote secretion of IgE, IgA, and another subclass of IgG (IgG1 in mice). The two subsets are mutually exclusive in function as the cytokines produced by each are antagonistic of the other population. For example, IFN-γ will down-regulate the function of Th2 cells, whereas IL-4, -10, and -13 are suppressive of Th1 function either directly or indirectly through influencing the function of the APC. In practice, this exclusivity (known as immune-deviation) is not absolute, as elements of Th1 and Th2 immunology are involved in many immune responses, although some examples of distinctly polarized responses exist (e.g. Th1 requirement for resolution of intracellular infections; Th2 requirement for expression of type I hypersensitivity disease). Th1 and Th2 cells may follow selective recirculation pathways to different tissue sites as they may express unique adhesion molecules,5 and be chemotac-

CHAPTER 51: BIOLOGY OF LYMPHOCYTES AND PLASMA CELLS

PAMP-PRR interaction STAT signalling Dendritic cell

Th0

Molecular interactions & Co-stimulatory cytokines

Th1

IL-2, IFNγ, CMI, IgG2a (mice), IgG1 & IgG3 (man)

Th17

IL-17, IL-21, IL-22, immunopathology

IL-12 IL-18 IFNγ IL-6 TGFβ IL-23 IL-4

Th2

TGFβ IL-10

Th3

IL-4, IL-5, IL-9, IL-13 IgG1 and IgE (mice) IgE (man) TGFβ, regulatory, oral tolerance

363

FIGURE 51.6 The functional subsets of CD4+ T cells produce distinct cytokine profiles and mediate different immune functions. The subsets have a common precursor (Th0) and a number of factors determine which of the subsets will predominate in any particular immune response, in particular the nature of the stimulating antigen and its interaction with dendritic cell pattern recognition receptors, which in turn determines the type of co-stimulatory cytokine and STAT signalling that stimulates the naïve precursor lymphocyte. Th1 cells regulate cell-mediated immunity whereas Th2 cells control humoral immune responses. Th17 cells have a proinflammatory effector role and are frequently activated in parallel to Th1 cells. The three key regulatory populations are T-reg “natural” suppressors, Tr1 “induced” suppressors, and Th3 regulatory cells.

Tr1 IL-10, induced regulatory Treg

IL-10, natural regulatory

tically attracted by specific molecules (chemokines) for which they bear receptors.11 Th1 and Th2 cells are now characterized in several domestic animal species and their role in particular diseases has been defined.22 Th1 and Th2 subsets are proposed to be related through a common Th0 precursor that secretes elements of both cytokine profiles. In any particular immune response, the response can be driven towards either type 1 (Th1) or type 2 (Th2) function by a range of factors including: ■ antigen type (e.g. the nature of an infectious agent),

dose, and route of delivery ■ the nature of the APC presenting the antigen (e.g.

dendritic cells or non-professional APC) ■ local cytokine and hormonal environment in the lym-

phoid tissue generating the response; for example, the presence of high concentrations of endogenous corticosteroid or IL-4 may preferentially activate a Th2 response, whereas IL-12 will drive forward a Th1 response ■ APC signalling as determined by the PAMP-PRR interaction. For example, viral RNA acting as a PAMP will induce dendritic cell production of IL-12 and IL-18 as co-stimulatory cytokines, resulting in signalling through STAT-4 and regulation through the transcription factor T-bet with induction of a Th1 immune response most appropriate for the viral pathogen. By contrast, antigen derived from a helminth induces IL-4 and IL-6 signalling, activation via STAT-6 and the transcription factor GATA3 with generation of a Th2 response best equipped to counteract the effects of such parasites. A further CD4+ effector T-cell subpopulation has recently been identified. The Th-17 cell is engendered when Th0 precursors are signalled via IL-6, IL-23, and transforming growth factor (TGF)-β. Th-17 cells produce IL-17, IL-21, and IL-22 and have proinflammatory effects that are important in the clearance of certain

classes of pathogen and the pathogenesis of some cellmediated autoimmune diseases.2,9 The final CD4+ T cell subsets are regulatory (suppressive) rather than effector in nature. The most important of these is the CD4+ CD25+ natural T regulatory cell (T-reg).2,23,30 These cells are spontaneously present and have a key role in controlling potentially deleterious autoimmune and allergic immune responses. They produce IL-10 but require direct physical contact (cognate interaction) with the cell that they are suppressing. T-reg also express Foxp3 (forkhead box P3 gene), GITR (glucocorticoid-induced TNF-receptorregulated gene), CTLA-4 (cytotoxic T lymphocyte antigen 4), and some PRRs. T-reg have now been characterized in cats, dogs, and cattle.10,40 By contrast, the induced regulatory T cells (Tr1 cells) appear as part of an immune response to foreign antigen (as opposed to being present naturally) and function mainly through production of the immunoregulatory cytokine IL-10 rather than requiring direct cognate interaction with their targets.19 These cells have recently been characterized in the horse.41 Intriguing recent studies have shown that in the late stages of some infectious diseases (classically leishmaniosis) protective Th1 cells may switch their functional phenotype from an effector IFN-γ production to an IL-10 producing regulatory population. This has the effect of permitting persistence of infection but limiting immune-mediated damage to tissue that may be secondary to an over-exuberant protective immune response.3,38 Finally, the Th3 cell preferentially produces TGF-β and has been suggested to mediate the phenomenon of oral tolerance (failure to respond to systemic administration of antigen after prior feeding of the antigen).42 Function of CD8+ T Cells The major function of CD8+ T cells is mediating cytotoxicity, and these cells are positively influenced by the

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Function of γδ T Cells

TCR CD8

Th1

NK IFNγ

NKR ?

Target cell

FcR NK ADCC

A

TNF

TNFR

Cytotoxic cell

Apoptosis

Target cell

FasL Fas

In most species, T cells expressing the γδ TCR with a CD3 complex are primarily located in the skin and mucosal sites of the body; however, in ruminants a significant proportion of blood T cells may be of this phenotype21 and the spleen of cattle and dogs are also enriched for these cells.27 γδ T cells are poorly characterized but are known to develop in the thymus early in ontogeny. The γδ TCR has limited heterogeneity and may largely recognize conserved microbial (especially bacterial) molecules and thus have a role in the early immune response to such agents. The receptor may recognize antigen directly, or in association with particular forms of MHC class I molecules. Functional subsets of γδ T cells able to produce either IFN-γ or IL-4 have been proposed, and may be important in creating a cytokine milieu for subsequent development of the CD4+ αβ+ T-cell response.15 NKT Cells

Granzymes Perforin B

FIGURE 51.7 (A) Cytotoxic destruction of a target cell can be mediated by CD8+ T cells or natural killer cells. These populations are positively influenced by Th1 CD4+ T cells. (B) The cytotoxic process involves induction of target cell apoptosis by the interactions of Fas-Fas ligand and TNF-TNF receptor, membrane pore formation and osmotic imbalance, and cytoplasmic damage caused by granzymes released from the cytotoxic cell.

effects of the Th1 regulatory population. However, it has been recognized that CD8+ T cells are capable of producing type 2 cytokines (e.g. IL-4) and subsets of type 1 and type 2 CD8+ T cells are proposed.14 The cytotoxic effect of lymphoid cells follows a similar sequence of events (Fig. 51.7) that are initiated following recognition of the target cell (e.g. virally infected, neoplastic, or incompatible graft cell) by cytotoxic cells.12 In the case of CD8+ T cells, the recognition involves TCR interaction with endogenous peptide presented by MHC Class I. A range of other surface molecular interactions between the two cells is also required. After recognition, the cytotoxic effects are mediated by: ■ induction of target cell apoptosis triggered by intra-

cellular signals delivered following molecular interactions between Fas (on the target cell) and Fas-ligand (on the cytotoxic cell), or cytotoxic cell-derived TNF binding to TNF receptor on the target cell ■ osmotic and enzymatic effects produced by the release of substances from the cytotoxic cell that form membrane channels in the target cell (perforins) permitting osmotic imbalance, or the delivery of toxic substances (granzymes) to the target cell cytoplasm. Cytotoxic cells may then disengage from the killed target cell and subsequently attack other targets.

NKT cells are a T cell subset distinct from CD4+ and CD8+ T cells that are not yet recognized in domestic animals. Two NKT subsets are reported. Type I NKT cells express a T cell receptor that comprises an invariant α chain combined with a limited number of β chains. Type II NKT cells have greater variability in α chain usage. Both cell types are also defined by recognition of antigen expressed by the CD1d molecule, and therefore predominantly respond to glycolipid molecules.17 NATURAL KILLER CELLS The NK cell is observed microscopically as a large granular lymphocyte (LGL) and is defined by the presence of a series of NK stimulatory receptors that mediate the cytotoxic interaction with the target cell (Fig. 51.7). Such receptors are not encoded by genes that undergo recombination in the same way as those that encode T cell and B cell receptors (via the recombination-activating gene; RAG). This feature indicates that NK cells are considered part of the innate immune system. The cytotoxic function of an NK cell can be inhibited by the interaction of a second class of receptor (killer inhibitory receptors; KIRs) which recognize MHC Class I molecules on the target cell.28 The balance between stimulatory and inhibitory receptors determines whether the NK cell will be activated to mediate cytotoxic destruction of the target.31 The NK cell also may recognize the target cell via interaction of the membrane Fc immunoglobulin receptor and antibody bound to the target cell in the process of antibody-dependent cell-mediated cytotoxicity (ADCC; Fig. 51.7). B CELLS AND PLASMA CELLS B cells are defined by the expression of surface membrane immunoglobulin (SmIg) with a transmembrane

CHAPTER 51: BIOLOGY OF LYMPHOCYTES AND PLASMA CELLS

FIGURE 51.8 Activation of B cells requires recognition of

Cytokine

APC

365

antigen by surface membrane immunoglobulin and a range of interactions with helper Th2 cells. These latter include direct interaction of membrane molecules and binding of T cell-derived cytokines by specific receptors. B cell differentiation involves transformation to a plasma cell that secretes immunoglobulin.

Th2

Cognate interaction

Cytokine

B

Antigen recognition

Plasma cell Antibody

domain that is associated with the signal transducing molecules Igα and Igβ (CD79). The complex is collectively referred to as the B-cell receptor (BCR). Each B cell expresses a unique BCR of single specificity and this diversity in the B cell repertoire is achieved by genetic combination of BCR gene segments in a similar fashion to formation of the TCR. A range of other surface molecules (e.g. Fc and complement receptors) are expressed by B cells. B cells are derived from the bone marrow stem cell and are thought to undergo their development and maturation within the fetal liver and bone marrow of mammals. In some species, evidence suggests that the intestinal tract may be an alternative site of B cell development. The stages of B cell development are less well understood than for the T cell, but the principle of cellular interactions with marrow stroma, with deletion of nonfunctional or autoreactive B cells by apoptosis, is likely similar. B cell development is reliant upon direct contact with stromal cells that also provide appropriate growth factors (e.g. IL-7).29 Developing B cells first express cytoplasmic immunoglobulin μ heavy chain, and following development to the naïve stage, express antigen-specific SmIg of both the IgM and IgD classes. B cells are exported from the marrow to particular anatomical locations (lymphoid follicles of the lymph node cortex, splenic white pulp, or mucosal lamina propria; Fig. 51.3) and recirculate throughout the body in the manner described for T cells. B cell activation occurs in lymphoid tissue and requires a similar array of signals to those described for the T cells. The SmIg of the BCR recognizes antigenic epitopes directly, without the need for antigen processing or presentation by APC. The epitope may be larger and have conformational or planar shape, although peptides may also be recognized. B cells require costimulatory signals delivered by CD4+ helper T cells (Th2 or Th1). These take the form of cognate interactions between surface membrane molecules on the two cells (including TCR recognition of antigenic peptide presented on MHC Class II by the B cell) and cytokines derived from the T cell that bind receptors on the B cell

(Fig. 51.8). Some antigens (thymus-independent antigens) are able to directly activate B cells in the absence of T cell help. After activation, the B cell undergoes transformation to a lymphoblast (that may secrete IgM) and clonal proliferation and differentiation as described for T cells. As part of the differentiation process, the B cell undergoes the immunoglobulin class-switch, involving DNA rearrangement with substitution of the μ and δ genes by one of the other constant region genes to produce SmIg of the IgG, IgA or IgE class. This process appears to be cytokine-directed. The initial contact of antigen-specific B and Th2 cells with APC (dendritic cells) and antigen occurs in the T cell zones of lymphoid tissue (e.g. lymph node paracortex) following egress of the T and B cells from the HEV. At this point, there is B cell proliferation and activated B and T cells migrate from this primary focus to the B cell area (primary follicle) to form a germinal centre within the follicle.26 These activated B cells (centroblasts) accumulate within the dark zone of the germinal center of this secondary follicle, and then migrate to the edge of the germinal center (light zone) where as centrocytes they re-encounter antigen on follicular dendritic cells. The mantle zone of this secondary follicle is composed of inactive B cells that are not specific for the antigen driving the immune response. Most of the centrocytes have BCRs that recognise antigen with low affinity and this interaction causes them to undergo apoptosis; however, those centrocytes with appropriate receptors undergo further interactions with Th cells, and leave the germinal centers to differentiate into plasma cells (that secrete immunoglobulin but do not have SmIg), or the population of memory B cells that mediate the secondary immune response on re-encounter with antigen.24 Plasma cells migrate as precursor plasmablasts and are largely located in lymph node medullary cords, splenic red pulp, bone marrow and the mucosal lamina propria. They have a limited lifespan (weeks) at these sites. Memory T and B cells are long-lived cells that may be periodically restimulated by depots of antigen that persist associated with

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dendritic cells in lymphoid tissue, or are reintroduced to the body by vaccination or exposure to microorganisms bearing cross-reactive epitopes.8,18

REFERENCES 1. Agace WW. Tissue-tropic effector T cells: generation and targeting opportunities. Nat Rev Immunol 2006;6:682–692. 2. Afzali B, Lombardi G, Lechler R, Lord GM. The role of T helper 17 (Th17) and regulatory T cells (Treg) in human organ transplantation and autoimmune disease. Clin Exp Immunol 2007;148:32–46. 3. Anderson CF, Oukka M, Kuchroo VK, Sachs D. CD4+ CD25−FoxP3− Th1 cells are the source of IL-10-mediated immune suppression in chronic cutaneous leishmaniasis. J Exp Med 2007;204:285–297. 4. Ardavin C. Thymic dendritic cells. Immunol Today 1997;18:350–361. 5. Austrup F, Vestweber D, Borges E, et al. P- and E-selectin mediate recruitment of T-helper-1 but not T-helper-2 cells into inflamed tissues. Nature 1997;385:81–83. 6. Banchereau J, Steinman RM. Dendritic cells and the control of immunity. Nature 1998;392:245–251. 7. Barral DC, Brenner MB. CD1 antigen presentation: how it works. Nat Rev Immunol 2008;7:929–941. 8. Bell EB, Sparshott SM, Bunce C. CD4+ T-cell memory, CD45R subsets and the persistence of antigen-a unifying concept. Immunol Today 1998;19:60–64. 9. Bettelli E, Korn T, Kuchroo VK. Th17: the third member of the effector T cell trilogy. Curr Opin Immunol 2007;19:652–657. 10. Biller BJ, Elmslie RE, Burnett RC, et al. Use of FoxP3 expression to identify regulatory T cells in healthy dogs and dogs with cancer. Vet Immunol Immunopathol 2007;116:69–78. 11. Bonecchi R, Bianchi G, Bordignon PP, et al. Differential expression of chemokine receptors and chemotactic responsiveness of type 1 T helper cells (Th1s) and Th2s. J Exp Med 1998;187:129–134. 12. Chouaib S, Asselin-PatureMami-Chouaib F, et al. The host-tumor immune conflict: from immunosuppression to resistance and destruction. Immunol Today 1997;18:493–497. 13. De Libero G, Mori L. Recognition of lipid antigens by T cells. Nat Rev Immunol 2005;5:485–496. 14. Erard F, Le Gros G. Th2-like CD8 T cells: their role in protection against infectious diseases. Parasitol Today 1994;10:313–315. 15. Ferrick DA, Schrenzel MD, Mulvania T, et al. Differential production of interferon-γ and interleukin-4 in response to Th1- and Th2-stimulating pathogens by γδ T cells in vivo. Nature 1995;373:255–257. 16. Fritz JH, Le Bourhis L, Gamelas Magalhaes J, et al. Innate immune recognition at the epithelial barrier drives adaptive immunity: APCs take the back seat. Trends Immunol 2008;29:41–49. 17. Godfrey DI, Berzins SP. Control points in NKT-cell development. Nat Rev Immunol 2007;7:505–518. 18. Gray D, Matzinger P. T cell memory is short-lived in the absence of antigen. J Exp Med 1991;174:969–974. 19. Groux H, O’Garra A, Bigler M, et al. A CD4+ T-cell subset inhibits antigenspecific T-cell responses and prevents colitis. Nature 1997;389:737–741. 20. Hammad H, Lambrecht BN. Dendritic cells and epithelial cells: linking innate and adaptive immunity in asthma. Nat Rev Immunol 2008;8:193–204.

21. Hein WR, Dudler L. TCRγδ+ cells are prominent in normal bovine skin and express a diverse repertoire of antigen receptors. Immunology 1997;92:9158–9164. 22. Horiuchi Y, Nakajima Y, Nariai Y, et al. Th1/Th2 balance in canine peripheral blood lymphocytes – a flow cytometric study. Vet Immunol Immunopathol 2007;118:179–185. 23. Izcue A, Powrie F. Special regulatory T-cell review: regulatory T cells and the intestinal tract – patrolling the frontier. Immunology 2008;123:6–10. 24. Klein U, Dalla-Favera R. Germinal centres: role in B-cell physiology and malignancy. Nat Rev Immunol 2008;8:22–33. 25. Kupper TS, Fuhlbrigge RC. Immune surveillance in the skin: mechanisms and clinical consequences. Nat Rev Immunol 2004;4:211–222. 26. Lindhout E, Koopman G, Pals ST, de Groot C. Triple check for antigen specificity of B cells during germinal centre reactions. Immunol Today 1997;18:573–577. 27. McDonough SP, Moore PF. Clinical, hematologic, and immunophenotypic characterization of canine large granular lymphocytosis. Vet Pathol 2000;37:637–646. 28. Mingari MC, Moretta A, Moretta L. Regulation of KIR expression in human T cells: a safety mechanism that may impair protective T-cell responses. Immunol Today 1998;19:153–157. 29. Nagasawa T. Microenvironment niches in the bone marrow required for B-cell development. Nat Rev Immunol 2006;6:107–116. 30. O’Garra A, Vieira P. Regulatory T cells and the mechanisms of immune system control. Nat Med 2004;10:801–805. 31. Raulet DH, Vance RE. Self-tolerance of natural killer cells. Nat Rev Immunol 2006;6:520–531. 32. Romagnani S. Regulation of the T cell response. Clin Exp Allergy 2006;36:1357–1366. 33. Sabroe I, Read RC, Whyte MKB, et al. Toll-like receptors in health and disease: complex questions remain. J Immunol 2003;171: 1630–1625. 34. Shin EK, Perryman LE, Meek K. A kinase-negative mutation of DNAPKCS in equine SCID results in defective coding and signal joint formation. J Immunol 1997;158:3565–3569. 35. Springer TA. Adhesion receptors of the immune system. Nature 1990;346:425–434. 36. Suter SE, Gouthro TA, O’Malley T, et al. Marking of peripheral T-lymphocytes by retroviral transduction and transplantation of CD34+ cells in a canine x-linked severe combined immunodeficiency model. Vet Immunol Immunopathol 2007;117:183–196. 37. Takahama Y. Journey through the thymus: stromal guides for T-cell development and selection. Nat Rev Immunol 2006;6:127–135. 38. Trinchieri G. Interleukin-10 production by effector T cells: Th1 cells show self control. J Exp Med 2007;204:239–243. 39. Trinchieri G, Sher A. Cooperation of Toll-like receptor signals in innate immune defence. Nat Rev Immunol 2007;7:179–190. 40. Vahlenkamp TW, Tompkins MB, Tompkins WAF. Feline immunodeficiency virus infection phenotypically and functionally activates immunosuppressive CD4+CD25+ T regulatory cells. J Immunol 2004;172:4752–4761. 41. Wagner B, Hillegas JM, Brinker DR, et al. Characterization of monoclonal antibodies to equine interleukin-10 and detection of T regulatory 1 cells in horses. Vet Immunol Immunopathol 2008;122:57–64. 42. Weiner HL. Oral tolerance: immune mechanisms and treatment of autoimmune diseases. Immunol Today 1997;18:335–343.

CHAPTER

52

Structure, Function, and Disorders of Lymphoid Tissue V.E. TED VALLI and ROBERT M. JACOBS Hemopoietic System Cells and Organs Functional Anatomy of the Hemopoietic System Thymus

Spleen Lymph Nodes Lymphoid Systems of Body Surfaces

Acronyms and Abbreviations BLV, bovine leukemia virus; FeLV, feline leukemia virus; MALT, mucosa-associated lymphoid tissue; MHC, major histocompatibility complex; TCR, T cell receptor genes; RBC, red blood cell.

HEMOPOIETIC SYSTEM CELLS AND ORGANS The hematopoietic system consists of the cascade of cells produced by the bone marrow as well as their specialized conducting and supporting systems consisting of vascular endothelium and the connective tissue cells of the marrow, lymph nodes and spleen.5,15,20,21,25 Other highly specialized supporting structures of the hematopoietic system include the epithelial cells of the thymus which sheath the blood vessels and form the saccular structures of the thymic cortex by which developing T cells gain their recognition of normal self antigens. The hematopoietic cells consist of the full range of differentiated products of pluripotential stem cells, including monocyte-macrophage and granulocytic cells of the neutrophil, eosinophil, and basophil type as well as the precursors of red blood cells (RBCs), platelets, and lymphocytes. The latter, including the thymic and bone marrow dependent arms of the lymphoid system are respectively responsible for cellular and humoral immunity.15 The vascular endothelium of the hematopoietic system includes the apparently regionally undifferentiated cells lining the lymphatics as well as the regionally differentiated endothelial cells of the blood vascular system which include the high endothelial venules of the lymph node paracortex with specific cell surface markers permitting the adhesion of lymphocytes in transit and their transmural migration to enter the node paracortex.14 The specialized circulating blood cells and their conducting and supporting structures are uniquely packaged in a series of organs of either separate design like the thymus, lymph nodes,

and spleen or are incorporated into other organs like the bone marrow. These also include the free and fixed tissue macrophages and dendritic cells. These cells and organs of the blood vascular system constitute remarkably interrelated and integrated systems

FUNCTIONAL ANATOMY OF THE HEMOPOIETIC SYSTEM Thymus The thymus is cytologically simple but unique in containing both lymphocytes and epithelial cells, but is architecturally complex in having a lobular structure differentiated into cortical and medullary areas (Figs. 52.1 and 52.2). The epithelial cells of the thymus are derived from the third and fourth pharyngeal pouches that, in the embryo, migrate in two streams to form paired lobes of the thymus within the anterior mediastinum. This migration occurs very early in embryological development and is immediately followed by seeding of the thymus with progenitor cells from the blood islands of the yolk sac.15 The first stream of epithelial migration forms the isolated reticular epithelial cells of the cortex and medulla of the adult thymic lobule. The second epithelial migration forms the thymic duct epithelium and later the Hassall’s corpuscles of the thymic medulla (Fig. 52.3).21 Early reticular epithelial cells form loose cuffs around small vessels that persist in adult life and become obvious in conditions of lymphoid atrophy. 367

368

SECTION IV: LEUKOCYTES

FIGURE 52.1 Thymus from a young adult male rat. The organ is composed of closely faceted lobules with a sharp distinction between the darker cortex and the lighter medullary areas of each lobule. Hematoxylin & eosin stain; bar = 1.0 mm.

FIGURE 52.2 Mammalian thymus. In mammals, the medulla and cortex is sharply delineated in young healthy animals with the width of the cortices and medulla approximately equal to one-third the width of the lobe. Hematoxylin & eosin stain; bar = 100 μm.

Also, in early development, the ductular component of epithelium forms the branching system that communicates between the lobules of a single thymic lobe. These embryological relationships are important due to the mimicry of embryological events in thymic lesions of adult life. Thus, a thymic lesion with loss of corticomedullary distinction that might be medullary hyperplasia or thymoma can be differentiated by the presence of the reticular cuffs around the vessels seen in a thymoma. These epithelial cuffs of thymic vessels form a barrier to blood-borne antigens and naïve bone marrow lymphocytes receive their immunologic training solely from the reticular epithelial cells. Lymphoid germinal centers occurring in the thymus are not necessarily pathologic or an indication of autoimmune disease if

FIGURE 52.3 Hassall’s corpuscle in the thymic medullary area consisting of concentric laminations of squamous epithelial cells. The surrounding lymphocytes are characteristically a mixture of small and medium cells and epithelial cells with large pale nuclei (arrow). These cells with moderately abundant cytoplasm separating the nuclei give the medulla a less dense appearance than the cortex on histological examination. Hematoxylin & eosin stain; bar = 10 μm.

they occur near a blood vessel and lie within the vascular epithelial sheath and are thus immunologically outside of the thymus. Detection of this relationship requires application of a cytokeratin stain. The medullary epithelium produces trophic hormones that assist lymphocytic colonization and are the source of cysts lined by ciliated epithelium that frequently develop in adult life but are rarely of clinical significance. An additional cellular component are myoid cells. They surround the Hassall’s corpuscles and are important in the pathogenesis of myasthenia gravis.21 In most species, the thymus reaches its maximal development about the time of puberty and then slowly decreases in size throughout adult life. An unusual antigenic stimulation during adolescent life may result in benign thymic hyperplasia, which, in the calf, may result in a chain of thymic lobules that extend from the rami of the mandibles to the base of the heart. At the physiological level, the thymic cortex receives a continuous stream of uncommitted lymphocytes of bone marrow origin which undergo immunological selection for tolerance to self-antigens in contact with the reticular epithelial cells which form thin-walled pouches (caveolae) in cortical tissue (see Chapter 51). Paradoxically, the thymic cortical lymphocytes have small densely stained nuclei without apparent nucleoli, yet this is a region of intense cellular proliferation in which the great majority of the progeny die and are removed by tingible body macrophages (Fig. 52.4). These latter cells become more prominent in conditions resulting in cortical lympholysis such as certain viral infections, irradiation, or corticosteroid therapy. Marrow-derived lymphocytes have the cell surface molecules to selectively home to thymic cortical

CHAPTER 52: STRUCTURE, FUNCTION, AND DISORDERS OF LYMPHOID TISSUE

FIGURE 52.4 Thymic cortex composed of densely packed small lymphocytes. A macrophage in the center has cytoplasm that contains several pyknotic nuclear fragments (tingible bodies) of apoptotic cortical thymocytes. The larger vesicular nuclei above and to the left of the macrophage are probably epithelial cells involved in the self-recognition selection process of developing T cells. Hematoxylin & eosin stain; bar = 10 μm.

vascular endothelium. The naïve T cells rearrange their T cell receptor genes (TCR) most to αβ and some to γδ type (see Chapter 51).5 Medullary lymphocytes are larger than the cortical cells, having larger and more vesicular nuclei, more cytoplasm, and reduced cell density. These cells give the medullary area less density on histological examination when routine stains are applied. Physiologic development continues when lymphocytes enter the medullary areas with exposure resulting in preferential homing of cells to the intestinal mucosa and Peyer ’s patches. In young animals, particularly rabbits, heterophils are frequently found in the thymus, particularly in lobular connective tissue. Eosinophils may occasionally be found in the thymic connective tissues of other species and mast cells are present in the thymic capsules in most species and are particularly frequent in rats. Thymic hyperplasia occurs largely through an increase in number of lobules rather than increased lobule size, while thymic atrophy tends to result in a blurring of the corticomedullary distinction. Thymic function does not necessarily vary in proportion to thymic size and in the adult a small thymic remnant may be responsible for a persistent autoimmune disease. Recent work suggests that terminal maturation of T cells may occur in the intestinal tract as well as in the thymus.13 It would not be surprising if similar activity was found in both the lung and skin which would provide a more comprehensive explanation for aberrant lymphoid reactions in both benign and malignant states. Spleen The spleen filters blood through a sinusoidal system. It has additional functions in lymphopoiesis and antibody

369

FIGURE 52.5 Cross-section of spleen from a young adult male rat. At the architectural level, the spleen is occupied by multiple, dispersed round dark areas representing lymphoid nodules with the intervening lighter areas consisting of sinuses. The lymphoid or white pulp areas may have a germinal center in the middle of the mantle cell cuff (arrows). These areas are surrounded by the lighter mantle and marginal zone areas. Hematoxylin & eosin stain; bar = 1.0 mm.

production and hemopoiesis under conditions of increased demand for blood cells by colonization of sinusoids with pluripotential stem cells. Species with the genetic potential for sustained activity such as humans, dogs, cats, and horses have spleens with a contractile muscular capsule supported by equally contractile internal trabeculae. In contrast, species such as ruminants, which tend to group together for protection against their more agile predators tend to have spleens whose capsules are largely connective tissue with less capability for contraction. The major hazard to life after splenectomy appears to be septicemia. The spleen forms early in embryonic life and is a site of active erythropoiesis during the fetal period. The spleen is both architecturally and cytologically complex. It contains a wide variety of cells that vary in proportion in reactive and disease states. The spleen also has a diverse regional anatomy based on a complex vascular system able to alter internal anatomy with changes in overall size and volume. The spleen is unique in having efferent but no afferent lymphatics. Thus, all antigen enters the spleen through the blood vascular system. Both the major arterial supply and venous outflow enter through the hilus of the spleen and arborize together throughout its length. This arborization is relatively random in mammals but in reptiles there is a system somewhat analogous to the bone marrow with a major central venous sinus. At the level of small and medium-sized arterioles, the vessels are sheathed in a cuff of small thymic-derived lymphocytes known as the periarteriolar lymphoid sheaths (Fig. 52.5). Very small branches from these “central” arteries give rise to germinal centers that are foci of B cell proliferation. Surrounding the dense cuff of lymphocytes of the periarteriolar lymphoid sheath is a more loosely aggregated area of lymphocytes of the mantle cell layer that is a mixture of T and B cells. This mantle cell layer surrounds the

370

SECTION IV: LEUKOCYTES

FIGURE 52.6 Details of Figure 52.5. Unlike larger mammals, rats tend not to have well-formed splenic germinal centers. In the middle of the image, a lighter zone constitutes the dendritic cell bed of an ill-defined germinal center eccentric to and abutting a small muscular arteriole at lower right (arrows). The germinal center is surrounded by a cuff of darker mantle lymphocytes that has an outer envelope of lighter-staining marginal zone cells that interface with the surrounding splenic red pulp. Hematoxylin & eosin stain; bar = 100 μm.

germinal centers that arise adjacent to the central arterioles. A marginal zone of B cells lies outside the mantle cells (Fig. 52.6). Whereas the cells of the periarteriolar sheath and germinal center are composed of resident cells with movement largely through negative selection and replenishment, the lymphocytes of the mantle cell layer interchange with cells of the germinal center. The width of marginal zone cells varies widely with immune activity and is itself surrounded by a trough or sinus area that receives blood coming through the germinal center. This area may appear more red in histological section and contains cells currently in the general circulation; it may be viewed to assess the levels of cells in circulation. In humans and the cat this peripheral sinus or perifollicular zone lies between the mantle cell cuff and the marginal zone layer. The rat and mouse have a definite stromal appearing boundary around the mantle cells and germinal center that is not present in humans or domestic animals. The small branches of the central arteriole may terminate in germinal centers or pass through mantle and marginal zones to terminate in a penicillary array of small branches that feed directly into the sinusoids. The penicillary vessels are ensheathed by a few plump reticular cells forming a contractile ellipsoid which under neural and hormonal control adjusts the level of blood entering venous sinuses. The venous sinuses of the spleen constitute the expansile regions of the organ and thus may vary greatly in their content of blood in species with a high level of smooth muscle in the splenic capsule. The walls of the splenic sinusoids are unique in consisting of elongated endothelial cells arranged like the staves of a barrel which are without junctions to their neighbors

FIGURE 52.7 Splenic sinus area demonstrating the interposition of a large multinucleated macrophage and a thin walled sinus facilitating the filtering system of the spleen. Hematoxylin & eosin stain; bar = 10 μm.

but are maintained in alignment by a discontinuous encirclement of fine reticular fibers (Fig. 52.7). The lumen of these sinuses communicates directly to the exiting vein of the spleen while the exterior of these veins is in contact with the filtering area of spleen that consists of macrophages suspended in a loose reticular supporting framework. Under normal circumstances 97% of the arteriolar blood entering the spleen exists directly via the penicillary vessels into a major sinus and quickly re-enters the central circulation. The remaining 3% passes into the filtering extrasinusoidal area to regain the central circulation by passing between the walls of the sinus endothelial cells. Reticulocytes and normal RBCs and leukocytes readily achieve this transit back into the circulation while senescent RBCs, those containing Howell-Jolly bodies, senescent leukocytes and foreign material, are phagocytosed by the macrophages of the red pulp areas. By this process, the entire blood volume passes through the filtering system of the spleen once each day. In a muscular spleen in the contracted state, all of the blood would pass directly from the arteriole to the venous system in a “closed” circulation, permitting maximal utilization of blood cells in the systemic circulation. In contrast, any influence that causes the splenic capsule to dilate (e.g. anesthesia, portal hypertension due to hepatic fibrosis or marked increase in extrasinusoidal macrophages) results in a greater proportion of blood passing through the filtering system. Due to the slow flow of blood through the red pulp, cells are exposed to lower glucose, cholesterol, and pH than in the central circulation. Collectively, these biochemical changes can contribute to premature aging of RBC and platelets resulting in accelerated destruction. Thus, an enlarged spleen of any cause is a distinct hazard to RBC and platelet lifespan.1 A variety of changes occur in the spleen in response to systemic states. Acute toxic diseases will result in

CHAPTER 52: STRUCTURE, FUNCTION, AND DISORDERS OF LYMPHOID TISSUE

lysis of lymphocytes in the germinal centers and replacement by residual proteinaceous debris referred to as follicular hyalinosis. Diagnosticians are aided by the fact that nuclear debris after acute lympholysis is cleared within 24 hours but the hyalin debris may remain for many months. In systemic amyloidosis, the germinal centers may become sites of amyloid deposition, while atrophic changes due to starvation, aging, cancer, or chemotherapy may result in atrophy which impacts one or both of the thymic-dependent arteriolar sheaths or bone marrow-dependent germinal center systems. In chronic hemolytic anemias, a marked increase in hemosiderin-bearing macrophages in sinus areas and iron deposition can occur in the connective tissue of the spleen, particularly in old dogs. Splenic infarction occurs in focal areas of the spleen in dogs with hemangiosarcoma and is always of hemorrhagic type. Ischemic infarction can occur in dogs with myeloma due in part to the greater viscosity of blood. Splenic torsion occurs primarily in dogs and humans and rarely in adult pigs and is surprisingly compatible with life. These events in the dog are apparently painful and dogs with focal infarction due to hemangiosarcoma resist activity and become anorexic even without abdominal hemorrhage. Lymph Nodes Histogenetically, lymph nodes form at the confluence of an afferent lymphatic sprig with a dilated sheath of vascular serosa. The blood vessels covered by the serosal sheath then produce a fine arborization of vessels within this area of dilation, thus forming an architectural framework for lymphocytic colonization. The vascular distribution to lymph nodes is highly organized with the arteriolar and venous branches arborizing through the medullary cords to form microcirculatory units that become the functional basis for germinal center formation. Specialized high endothelial venules form in the paracortical areas between, and never within, the germinal centers and present specific adhesion molecules by which transmural lymphocyte traffic from blood to node paracortex is regulated.3 The normal development of lymph nodes is dependent upon cells and antigens entering through the afferent lymphatics that drain into the peripheral capsule of the node. Virgin (non-activated) lymphocytes apparently circulate randomly in the blood; however, once these cells are involved in antigen recognition in the node cortex, their further migration becomes altered. Activated B cells migrate preferentially to mucosa-associated lymphoid tissues (MALT), while T cells migrate to peripheral lymph nodes.5,7 This system of organ-specific lymphocyte homing receptors functions in neoplastic as well as inflammatory conditions, with the spread of malignant lymphocytes limited by their capacity to migrate into various tissues. Malignant lymphocytes tend to mimic their benign counterparts with B cell neoplasms like reactive B cells binding to endothelial venules in certain anatomic sites, including tonsil and lymph node. Thus, the spread of lymphoma to

371

lymphoid structures is not a function of the tumor cells being able to enter the peripheral blood, but rather an indication that the cells have successfully bound to endothelial venules in order to migrate into additional lymphoid structures.15 By this understanding, lymphoid neoplasms circulate in the blood, but their degree of dissemination is limited to their binding capability; this has led to the new assessment of leukemia versus lymphoma based simply on the tissue with the greatest volume of tumor. The afferent lymphatics and the peripheral capsule and the subcapsular sinus are the delivery systems whereby lymph, blood cells, and antigens are delivered to the interior of the lymph node (Fig. 52.8). The peripheral capsule will be thin and taut in conditions in which there has been rapid enlargement of a lymph node of benign or malignant cause. In contrast, lymph node atrophy results in the histologic appearance of a capsule that is thickened and wavy, with the peripheral sinus widened. Chronic stimulation of a node from either hyperimmune or septic cause results in thickening and sclerosis of the capsule with thickening of the fibrous raphe which spans the cortex and terminate in a dense collagenous medullary fibrovascular network.19 In cattle and likely in other species, the lymph node capsule is contractile and under neural control, thus assisting lymph flow.12 The lymph node cortex and germinal centers form the first order of filtering system through channels extending from the inner lining of the peripheral sinus to the microcirculatory units in the node cortex. A fully developed germinal center is a highly organized structure consisting of a progression of cell types in a proliferative gradient that provides the germinal center with an easily recognized polarity (Figs. 52.9 and 52.10).16

FIGURE 52.8 Lymph node outer cortex from a healthy dog. An afferent lymphatic (arrow) leads into the peripheral sinus beneath the node capsule. The sinus is open and contains a few cells in transit, whereas to the left and right, the sinus is compressed by developing germinal centers. Hematoxylin & eosin stain; bar = 100 μm.

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SECTION IV: LEUKOCYTES

FIGURE 52.9 Architecture of the lymph node in Figure 52.8 showing follicular hyperplasia and densely cellular medullary cords and sinuses (arrows). Hematoxylin & eosin stain; bar = 1.0 mm.

FIGURE 52.10 Lymph node cortex of Figure 52.9 with subcapsular germinal center having a polarity directed toward the antigens being delivered from the peripheral sinus into the superficial pole of small lymphocytes. In the middle of the germinal center, there are small and large lymphocytes with macrophages removing apoptotic lymphocyte nuclei. Near the bottom of the picture, the deep pole area consists of lymphocytes with large nuclei and a narrow rim of highly basophilic cytoplasm. Hematoxylin & eosin stain; bar = 100 μm.

The polarity gradient of the germinal center consists of a mantle cuff of small lymphocytes at the superficial pole, just beneath the inner lining of the peripheral sinus. Antigen sorting and processing occurs through a focusing process, with the antigen presenting dendritic reticular cells in the middle of the germinal center accompanied by tingible body macrophages. Morphologically, the lymphocytes of the superficial pole have small dense nuclei with abundant lightly stained cytoplasm, while those in the deep pole have large nuclei with multiple prominent usually peripheral nucleoli and only a narrow rim of very basophilic cytoplasm. The mantle cell cuff is eccentrically thickened at the superficial or capsular pole and thinned at

the base of the deep pole. In follicular hyperplasias, the germinal centers beneath the node capsule are usually sectioned in a plane that allows their antigen orientation to be recognized, but those in medullary areas are oriented in a variety of planes such that all levels of the proliferative progression of cells will be viewed.8 This point is worth emphasizing since it is essential to recognize polarity of germinal centers to distinguish benign and atypical follicular hyperplasias from follicular lymphomas. Follicular lymphomas whether of small cleaved cells of centrocytes, or large cells of centroblasts, or more often a mixture of these, completely lack a mantle cell cuff and are surrounded by a few residual small benign lymphocytes and the aggregations of postcapillary venules. A further distinction of follicular lymphomas is that whatever combination of cell types are present, each follicle will have only that same clonal assembly and lack the variety of benign follicles. Also, unlike benign follicles, those of follicular lymphoma have few or no tingible body macrophages as the tumor cells, unlike the benign cell selection process, are not undergoing apoptosis. A further distinction between true follicular lesions, whether benign or malignant, and pseudofollicular lesions is that the post-capillary venules are always between the follicular or nodular proliferations and never within them. The dendritic reticulum cells are derived from a variety of sources and can derive from either myeloid or lymphoid precursors.4,9,10,17 Dendritic cells at various anatomic sites have different phenotypic characteristics.10 Ultrastructurally, the dendritic cells are characterized by cytoplasmic tendrils that are joined by tight junctions. In conditions of lymphoid depletion, the dendritic cells become exposed in germinal centers as the eosinophilic background of large pale cells. One of the more difficult decisions in examining lymph nodes is the distinction between florid hyperplasia and lymphoma.24 The syndrome of transformation of germinal centers to malignant disease is described in humans6,14 and it would appear that in animals chronic lymphoid proliferation of any cause can be a serious risk factor for lymphoma. Examples of this type of interaction in animals include infection with FeLV or BLV and immunoproliferative small intestinal disease of idiopathic cause. A type of nodular or follicular lymphoma in which the malignant cells are derived from the corona of cells surrounding the germinal center occurs in both humans and animals and is known as mantle zone lymphoma. Morphologically, the disease is characterized by coalescing nodular proliferations of moderate-sized lymphocytes surrounding fading germinal centers. In the paracortex of lymph nodes, there are nodular structures distinct from germinal centers,24 that are particularly common in the mesenteric nodes of mature rats.3 These deep cortical units form as a result of antigenic stimulation, with cellular input from a distinct afferent lymphatic radical (Fig. 52.11). They form semicircular or oval structures beneath the germinal centers of the outer cortex and are most easily seen in lymph nodes with some degree of paracortical atrophy, making the nodules more apparent. Central areas of these

CHAPTER 52: STRUCTURE, FUNCTION, AND DISORDERS OF LYMPHOID TISSUE

373

FIGURE 52.11 Cross-section of rat mesenteric lymph node. The upper area of the node is occupied by typically poorly defined germinal centers. The medullary cords and sinuses lie along the bottom of the image and abut a large uniformly dense oval deep cortical unit that lies across the cut surface of the node. The unit is lightly outlined by a rim of less dense cellularity. Hematoxylin & eosin stain; bar = 200 μm.

FIGURE 52.12 Paracortex of rat mesenteric lymph node with a high endothelial post-capillary venule crossing the center. Several lymphocytes within the wall of the venule can be seen to be in transmural transit exiting the blood to enter the node cortex (near arrow). Hematoxylin & eosin stain; bar = 20 μm.

nodules are somewhat mottled in appearance due to varying proportions of large and small lymphocytes with some macrophages and post-capillary venules (Fig. 52.12). Edges of these nodules gradually meld with the surrounding medullary cords and sinuses and are apparently the areas where cells exiting these nodules enter the efferent lymph channels.3 The main significance of the deep cortical nodules is to recognize that they are normal structures and not to confuse them with a malignant lymphoid proliferation.3 The medullary cords and medullary sinuses are best considered together because they form an interwoven matrix that waxes and wanes in unison or at the expense of each other (Fig. 52.13). A medullary unit consists of a radical of vascular structures from the deep pole of

FIGURE 52.13 Medulla of a lymph node from a dog with normal cords and sinuses. The cords are densely cellular and surrounded by a thin endothelial membrane with the sinuses containing a few fixed macrophages and cells in transit in a background of lymph. Hematoxylin & eosin stain; bar = 100 μm.

the node hilus consisting of an artery, vein, and lymphatic surrounded by a variable amount of connective tissue and invested with a thin endothelial covering. These vascular structures form the center of many medullary sinuses as they are seen in a cross-sectional profile. The sinus area constitutes the space between the endothelial wall of the vascular structures and the similar endothelial wall of the medullary cords. The medullary sinuses may extend up to the inner surface of the subcapsular sinus in nodes in which there is severe paracortical and follicular atrophy. Such a circumstance occurs in the mesenteric nodes of cattle dying of acute bovine viral diarrhea virus infection with massive B cell lysis in which the sinuses tend to be filled with macrophages, thus forming sinus histiocytosis. The cellularity of the medullary sinuses will vary widely. In diseases characterized by chronic immune stimulation, there usually are large numbers of sinus macrophages which are apparently suspended in a syncytial network. In nodes draining areas of chronic dermatitis, the macrophages of the superficial cortex and those of the medullary sinuses frequently contain abundant normal-appearing melanin granules.18 Similarly, nodes draining areas of hemorrhage have numerous hemosiderin-bearing macrophages in medullary areas. The transit of RBCs to the lymph node and the process of RBC intracellular degradation is remarkably rapid; thus any amount of erythrophagocytosis by medullary macrophages must be looked on as a very recent event. The medullary cords contain recirculating lymphocytes that have received antigenic orientation by passage through the outer cortex. The medullary cords may contain small or medium lymphocytes rather than plasma cells, even in reactions that have persisted for many weeks. The fine reticular network of the medullary cords forms a fertile microenvironment for extramedullary hematopoiesis. In myeloid leukemias, the malignant cells also colonize the medullary cords,

374

SECTION IV: LEUKOCYTES

and under these circumstances, there may be both tumor and benign extramedullary hematopoiesis coexisting. LYMPHOID SYSTEMS OF BODY SURFACES In the context referred to herein, body surfaces include the skin as well as the lining of the upper and lower respiratory and enteric systems. Intraepithelial lymphocytes in all these locations are not primarily cells on the way to being lost to the exterior but rather cells in training, which, following their specific sensitization, return to deeper lymphoid structures to perform a specific function in differentiation and proliferation. In the skin, keratinocytes constitute 85% of the epidermis with the remaining cells constituting melanocytes, Langerhans’ cells, Merkel cells and dendritic cells of uncertain type. In humans and cattle, the Langerhans’ cells contain a typical cytoplasmic organelle visible on ultrastructure that is rod-shaped with a unipolar bulb, the Birbeck granule. The Langerhans’ cells are products of a variety of precursors and constitute the major antigen presenting cell of the epidermis.1 The presence of Birbeck granules permitted identification of Langerhans’ cells in the paracortex of lymph nodes draining skin. These cells ingest foreign molecules without degrading them and migrate to the local lymph nodes where these antigens are exposed to naïve lymphocytes in specific MHC context. Both T and B cells are present in the skin where their function is largely unknown.2 A fairly consistent proportion of cutaneous lymphocytes are suppressor T cells which may be resident in situ to dampen cutaneous immune sensitization. The mucosa-associated lymphoid tissue of the respiratory and enteric systems is in common in the tonsils and pharyngeal areas, which in humans, are collectively known as Waldeyer ’s ring. In the respiratory system, the epithelium of the nasal mucosa as well as the larynx, trachea, and bronchi are infiltrated to a variable extent with lymphocytes and to a lesser extent with neutrophils and macrophages. Mucosal infiltration is also more prominent above areas of submucosal lymphoid proliferation such as occurs in the tonsil and in bronchiolar-associated lymphoid tissues.2 In general, the level of reactivity in these areas is related to the level of antigenic stimulation. Lymphoid proliferation in the tonsil is usually characterized by prominent germinal center formation, while in the lower lung, in conditions of health, the lymphoid tissue may be minimal and restricted to diffuse accumulations of cells between the bronchial epithelium and the supporting smooth muscle and cartilaginous plates (Fig. 52.14).20 The impact of ambient air quality in terms of freedom from dust particles and airborne infectious agents is immediately apparent in the histology of laboratory animals. Animals maintained in facilities where the air is well filtered have minimal bronchiolar-associated lymph node tissue and few intra-epithelial lymphocytes. In contrast, animals maintained in less pristine circumstances char-

FIGURE 52.14 Lung from a normal rat with longitudinal section of a major bronchus. Note that the focal areas of lymphoid proliferation are both peribronchiolar and subepithelial. Hematoxylin & eosin stain; bar = 100 μm.

acteristically have diffuse lymphoid proliferation in the proximal trachea and laryngeal areas with heavy and mixed cellular infiltration of the surface epithelium. In the lower lung, the lymphoid proliferation, which may contain germinal centers but is usually diffuse, may be of sufficient volume to partially occlude airways. As in other areas, the lymph nodes draining the lung at the base of the heart and in the caudal mediastinum bear the evidence of past environmental experiences including contact with smoke and other inhaled particulate matter. The gut-associated lymphoid tissue forms a relatively continuous chain from the oral cavity to the anus in characteristic areas of proliferation. There are generally few lymphocytes in the epithelium and mucosa of the esophagus. Lymphoid proliferation with germinal center formation is not normal but relatively common beneath the gastric epithelium where lymphomas are relatively frequent in both humans and animals. In recent years, with discovery of the association between Helicobacter and human gastric ulceration, it has become apparent that most of the lymphoid tissue present in these cases is antigen dependent and rapidly recedes with long-term antibiotic therapy. Even more remarkably, cases of human gastric lymphoma in at least early stages of development appear to be antigen-dependent and may regress with antibiotic treatment alone. The organized areas of lymphoid tissue in the lower small intestine including the Peyer ’s patches are estimated to equal the thymus gland in young animals (Fig. 52.15). The epithelium overlying the intestinal germinal centers is generally devoid of goblet cells and villi, and glandular crypts are absent (Fig. 52.16). In these areas, the epithelium contains specialized cells with complex folding of the surface epithelium known as M cells.23 M cells enfold lymphocytes within their membranes and function as antigen trephocytic cells able to metabolize particulate antigen to a level of digestion or nucleotide message that instructs the associated lymphocytes to specific immune reaction. Much of the immunoglobulin

CHAPTER 52: STRUCTURE, FUNCTION, AND DISORDERS OF LYMPHOID TISSUE

375

produced in these areas is IgA-type with luminal and mucosal concentrations higher than that in blood.

REFERENCES

FIGURE 52.15 Lower small intestine of normal rat. A focal submucosal proliferation of lymphocytes in a dome area characterized by absent villi and modified epithelium to assist antigen recognition and sorting. Hematoxylin & eosin stain; bar = 100 μm.

FIGURE 52.16 Details of Figure 52.15. The epithelium above the lymphoid follicle is modified to contain M cells that have deeply folded cellular membranes that enclose intraepithelial lymphocytes. The M cells are capable of uptake and of processing antigen from the intestinal lumen and providing immune instruction to maturing lymphocytes. In this area there are lymphocytes in the epithelium undergoing apoptosis presumably similar to the selection process in node germinal centers. Hematoxylin & eosin stain; bar = 20 μm.

1. Anjuere F, Martin P, Ferrero I, et al. Definition of dendritic cell subpopulations present in the spleen, Peyer ’s patches, lymph nodes, and skin of the mouse. Blood 1998;93:590–598. 2. Beagley K, Husband AJ. Intraepithelial lymphocytes: origins, distribution and function. Crit Rev Immunol 1998;18:237–254. 3. Belisle C, Sainte-Marie G, Peng F-S. Tridimensional study of the deep cortex of the rat lymph node. Am J Pathol 1982;107:70–78. 4. Cumberbatch M, Gould SJ, Peters SW, et al. MHC class II expression by Langerhans’ cells and lymph node dendritic cells: possible evidence for maturation of Langerhans’ cells following contact sensitization. Immunology 1991;74:414–419. 5. Duijvestijn A, Hamann A. Mechanisms and regulation of lymphocyte migration. Immunol Today 1989;10:23–28. 6. Egeler RM, Neglia JP, Aricò M, et al. The relation of langerhans cell histiocytosis to acute leukemia, lymphomas, and other solid tumors. Hematol Oncol Clin N Am 1998;12:369–378. 7. Farr AG, De Bruyn PPH. The mode of lymphocyte migration through postcapillary venule endothelium in lymph node. Am J Anat 1975;143:59–92. 8. Ferry JA, Zukerberg LR, Harris NL. Florid progressive transformation of germinal centers. A syndrome affecting young men, without early progression to nodular lymphocyte predominance Hodgkin’s disease. Am J Surg Pathol 1992;16:252–258. 9. Fonseca R, Tefferi A, Strickler JG. Follicular dendritic cell sarcoma mimicking diffuse large cell lymphoma: a case report. Am J Hematol 1997;55:148–155. 10. Fossum S. Lymph-borne dendritic leucocytes do not recirculate, but enter the lymph node paracortex to become interdigitating cells. Scand J Immunol 1988;27:97–105. 11. Hoshi J, Kamiya K, Aijima H, et al. Histological observations on rat popliteal lymph nodes after blockage of their afferent lymphatics. Arch Histol Jap 1985;48:135–148. 12. Hughes GA, Allen JM. Neural modulation of bovine mesenteric lymph node contraction. Exp Physiol 1993;78:663–674. 13. Lefrancois L, Puddington L. Extrathymic intestinal T-cell development: virtual reality? Immunol Today 1995;16:16–21. 14. Mebius RE, Bauer J, Twisk AJT, et al. The functional activity of high endothelial venules: a role for the subcapsular sinus macrophages in the lymph node. Immunobiology 1991;182:277–291. 15. Metcalf D, Moore MAS. Hematopoietic cells. In: Embryonic Aspects of Haemopoiesis. New York: Elsevier, 1971;172–266. 16. Millikin PD. Anatomy of germinal centers in human lymphoid tissue. Arch Pathol 1966;82:499–505. 17. Reid CDL. The dendritic cell lineage in haemopoiesis. Br J Haematol 1997;96:217–223. 18. Sasaki K. Three-dimensional analysis of erythrophagosomes in rat mesenteric lymph node macrophages. Am J Anat 1990;188:373–380. 19. Schnitzer B. The reactive lymphadenopathies. In: Knowles, DM, ed. Neoplastic Hematopathology, 2nd ed. Philadelphia: Lippincott Williams & Wilkins, 2001;537–568. 20. Sell S. Immunology, Immunopathology and Immunity, 4th ed. New York: Elsevier, 1987; 9–13. 21. Shier KJ. 1981; The thymus according to Schambacher: medullary ducts and reticular epithelium of the thymus and thymomas. Cancer 2001;48:1183–1199. 22. Stauder R, Hamader S, Fasching B, et al. Adhesion to high endothelial venules: a model for dissemination mechanisms in non-Hodgkins’ lymphoma. Blood 1993;82:262–267. 23. Tohno M, Shimosato T, Kitazawa H, et al. Toll-like receptor 2 is expressed on the intestinal M cells in swine. Biochem Biophys Res Commun 1995;330:547–554. 24. Valli V, Vernau W, de Lorimier P, et al. Canine indolent nodular lymphoma. Vet Pathol 2006;43:241–256. 25. Valli VE, Villeneuve DC, Reed B, et al. Evaluation of blood and bone marrow, rat. In: Jones TC, et al., eds. Hematopoietic system. Monographs on Pathology of Laboratory Animals. Berlin: Springer-Verlag, 1990;9–26.

C H A P T E R 53

Disorders of the Spleen JOHN L. ROBERTSON and ERIK TESKE Spectrum of Spontaneous Splenic Disease in Animals Diagnostics Disorders of the Spleen Developmental Defects Circulatory Disturbances – Infarction Traumatic Lesions Infectious, Inflammatory, and Immune Disorders Infectious agents that affect the spleen Erythrotrophic pathogens Lymphotrophic pathogens Other pathogens affecting the spleen

Inflammatory and immune disorders affecting the spleen Toxic Injury Degenerative Conditions Splenomegaly and Hypersplenism Splenic histiocytosis Fibrohistiocytic nodules Splenic neoplasms Asplenia and Hyposplenism Summary

Acronyms and Abbreviations BSE, bovine spongiform encephalopathy; BVD-MD, bovine viral diarrhea-mucosal disease; GDV, gastric dilation and volvulus; IMHA, immune-mediated hemolytic anemia; RBC, red blood cell.

T

he spleen serves many important roles in mammals. These include the production of cells in fetal and adult life, phagocytosis and opsonization of particulate materials and pathogens, filtering of blood, storage and release of blood cells to meet physiologic demands, participation in the recycling and metabolism of iron, and immune reactivity. The spleen is critically important in defense against a variety of pathogens. A detailed review of anatomy and physiology of the spleen is found in Chapter 52.

SPECTRUM OF SPONTANEOUS SPLENIC DISEASE IN ANIMALS There is a large amount of published information on disorders of the spleen in humans, but considerably less for spontaneous diseases of the spleen in domesticated animals. Good data on the prevalence of disease is available for dogs and cats. In two papers, Spangler and Culbertson 53,54 surveyed neoplastic and non-neoplastic splenic lesions in dogs and cats. Table 53.1 describes major neoplastic and non-neoplastic lesions in samples of dog spleens from three different sources. These data show that non-neoplastic disease accounts for approxi376

mately 60–80% of splenic lesions in dogs, depending on the source of the sample. A number of lesions of the canine spleen can be considered uncommon, occurring in 3% or less of samples submitted for diagnostic evaluation (Table 53.2). Spangler and Culbertson also published data on the incidence of neoplastic and non-neoplastic lesions in the spleen of cats. Their data were formulated from evaluation of 455 specimens sent to the diagnostic laboratory of a teaching hospital (Table 53.3).54 It is clear that in cats, neoplastic disease of the spleen is significant and that hyperplasia of various elements is noteworthy. The relative importance of a diagnosis of splenic congestion is problematic, given that many specimens collected from either species may have been done with benefit of anesthetic agents or from animals euthanized with barbiturates. We have conducted a comprehensive review of the literature on splenic and hemolymphatic disease in horses.44 Single and small cohort case reports on splenic rupture, abscessation, and infarction have appeared over the past 100 years, but given the paucity of this information, it appears that splenic disease is rarely a problem in horses. Horses, as dogs, show splenic enlargement when euthanized with barbiturate overdose, although to a lesser degree.

CHAPTER 53: DISORDERS OF THE SPLEEN

TABLE 53.1 Prevalence of Major Lesions Seen in the

TABLE 53.3 Prevalence of Major Splenic Diseases in Cats

Spleens of Dogs Submitted for Evaluation Diagnosis

Group 1a (n = 1,372)

Group 2b (n = 92)

10% 23% 10% 10% 11%

19% 6% 19% 24% 21%

6% 7% 4%

2%

obtained as Diagnoses from 455 Specimens Submitted to a Teaching Hospital Diagnostic Laboratory54

Group 3c (n = 105) Diagnosis

Splenic hematoma Hyperplastic nodules Hematoma/nodules Hemangiosarcoma Neoplasm, not hemangiosarcoma Extramedullary hematopoiesis Congestion Lymphoid hypoplasia

377

59% 9% 22%

a Group 1. Specimens submitted to a diagnostic laboratory for evaluation by referring veterinary practitioners. b Group 2. Specimens obtained by splenectomy at a teaching hospital. c Group 3. Specimens obtained from a colony of Beagle dogs studied throughout their lifetime.53

Neoplastic disease Congestion Lymphoid hyperplasia Lymphoid hypoplasia/atrophy Capsulitis/peritonitis Extramedullary hematopoiesis Accessory spleen Hyperplastic nodule Splenitis Infarct Necrosis Lymphoid necrosis Reticuloendothelial hyperplasia Fibrosis Amyloidosis

% of Total Submissions 54% 9% 7% 7% 5% 4% 4% 3% 2% 1% 1% 1% 1% 20% of NECs are myeloblasts Most common type of erythroleukemia in humans Acute erythroleukemia with erythroid predominance (AML-M6b / Pure erythroleukemia ) >50% of ANCs are erythroid origin May have left shifting and dysplasia of the erythroid lineage >20% of ANCs are myeloblasts or rubriblasts Most common type of erythroleukemia in veterinary species Acute megakaryoblastic leukemia (AML-M7) >20% of ANCs are blasts >50% of blast cells are megakaryoblasts Megakaryoblasts are CD41/61 positive and stain with acetylcholinesterase, ANAE and PAS Marrows are often severely sclerotic and dry tap may result

479

480

SECTION V: HEMATOLOGIC NEOPLASIA

exhibit primary cytoplasmic granulation (type I and some type II myeloblasts). Though this granulation allows for distinction from ALL, cytochemical and immunologic markers may still be necessary for accurate identification. A normal or mildly elevated leukocyte count is typically present in these patients.13,17,37,38 AML-M2 is distinguished from AML-M1 by the presence of ≥10% of bone marrow cells maturing to the promyelocyte stage and beyond. Primary cytoplasmic granulation is common and prominent due to the presence of many type II and III myeloblasts (though the latter were not identified or characterized by the ALSG). To distinguish AML-M2 from AMLs with a monocytic component, 3% of blasts stain positively for MPO and/or SBB. Thrombocytopenia and nonregenerative anemia are typically present. A leukocytosis of up to 50,000 WBC/μL, including type I, II, and III myeloblasts, promyelocytes, and rare myelocytes is also commonly noted.13,17,37,38 Acute Promyelocytic Leukemia (AML-M3) Acute promeylocytic leukemia (AML-M3) is characterized by the presence of atypical promyelocytes in bone marrow or blood. As in other types of AML, the blast percentage must comprise ≥20% of ANCs; however, in AML-M3, both myeloblasts and atypical promyelocytes are included in the blast percentage. In humans, these atypical promyelocytes often outnumber the myeloblasts and have two distinct microscopic appearances: the more common hypergranular variant, and the less common micro- or hypogranular variant. The hypergranular variant typically presents with anemia, thrombocytopenia, and leukopenia due to myelophthisis. The hypogranular variant is reported to present with a marked leukocytosis of approximately 50,000/μL, consisting predominantly of abnormal promyelocytes. Coagulopathies may be noted at the time of presentation or may develop after initiation of therapy. Diagnosis may be confirmed with positive staining for SBB, MPO and CAE.13,17,37,38 In humans, AML-M3 has been reclassified as AML with a recurrent genetic abnormality as it is associated with the chromosomal translocation t(15;17)(q22;q1112). This results in the fusion of the retinoic receptoralpha (RARA) gene with the zinc finger binding transcription factor, also known as the promyelocytic leukemia (PML) gene. This fusion product is known as PML-RARA. It results in impaired maturation and uncontrolled proliferation including down-regulation of repressor genes through limited hypermethylation, heterochromatin formation, and gene silencing. This unique pathogenesis allows for highly effective targeted therapy using alltrans-retinoic acid (ATRA) resulting in durable remission by stimulating differentiation of the neoplastic clones.2,40,18,19 AML-M3 is a rare condition in animals that to date has only been reported in the literature in a single boar and as a transgenic mouse model.7,19 Peer reviewed case reports do not yet exist in dogs, cats, horses, cattle or

sheep. Anecdotal reports suggest the hypogranular variant is more common in domestic animals, possibly presenting with signs referable to a coagulopathy. The severity of clinical disease due to pancytopenia and coagulopathy may hinder a full diagnostic evaluation and contribute to the apparent rarity of this condition.37,38 Acute Myelomonocytic Leukemia (AML-M4) In acute myelomonocytic leukemia (AML-M4), leukemic cells appear to be of mixed lineage (Fig. 67.1). Three criteria must be met in the bone marrow to achieve a diagnosis of AML-4: ≥20% of ANCs must be blast cells, including myeloblasts, monoblasts and promonocytes; ≥20% of ANCs must be of monocytic lineage (CD14+, nonspecific esterase [NSE]-positive that is sensitive to fluoride inhibition); and ≥20% of ANCs must be of granulocytic lineage (SBB/MPO positive).13,15,17,37,38 Acute Monocytic Leukemia (AML-M5) Acute leukemias of the monocytic lineage may be divided to the more immature acute monoblastic leukemia (AML-M5a) and the more mature acute monocytic leukemia (AML-M5b). Both subtypes are characterized by ≥80% of ANCs being of monocytic origin, and ≥20% of ANCs being myeloblasts, monoblasts, or promonocytes (Fig. 67.2). However, in AML-M5a ≥50% of all monocytic cells are monoblasts, whereas in AML-M5b ≥50% of all monocytic cells are promonocytes.2,17,37,38,40 The ALSG did not recommend separating these AML-M5 subtypes because there was poor diagnostic agreement among the group pathologists due to the complexity of cellular identification.17 Confirmation of monocytic lineage is achieved by demonstrating expression of CD14, and/or fluoride-sensitive NSE. Reportedly,

FIGURE 67.1 Acute myelomonocytic leukemia (AML-M4) in bone

marrow from a dog. Notice infiltration of >20% blast cells with apparent granulocytic and monocytic differentiation but with maturation arrest evident by the paucity of mature forms. Note primary granules in myeloblast near the center of the image. Wright-Giemsa stain; original magnification ×1,000.

CHAPTER 67: ACUTE MYELOID LEUKEMIA

FIGURE 67.2 Acute monoblastic leukemia (AML-M5) in blood smear from a dog. Notice a population of monomorphic blast cells with large convoluted nuclei, lacy to finely stippled chromatin, and moderate to abundant vacuolated, basophilic cytoplasm. The cells resemble monocytic progenitors with no visible cytoplasmic granules. Wright-Giemsa stain; original magnification ×1,000.

AML-M5a is more common in young animals, whereas AML-M5b is more common in mature and older animals. Leukocyte counts vary from 50,000/μL terminally.13,17,37,38 Acute Erythroleukemia (AML-M6) The acute erythroleukemias are characterized by the presence of greater than 50% erythroid precursors in the bone marrow and can be further divided into two groups based on the blast composition (Fig. 67.3). In acute erythroleukemia (AML-M6a) there is myeloblastic predominance and ≥20% of all NECs in the bone marrow are myeloblasts. AML-M6a is the type of erythroleukemia most commonly observed in human patients. In contrast, acute erythroleukemia with erythroid predominance (AML-M6b) is characterized by a prominent rubriblastic component. In AML-M6b, ≥20% of ANCs in the marrow are myeloblasts and rubriblasts. AML-M6b is the type of erythroleukemia most commonly observed in veterinary patients, with many cases occurring in FeLV-positive cats during the FeLV epidemic (see Chapter 62). In both subtypes of AML-M6, left shifting and dysplasia of the erythroid lineage may be evident. The total nucleated cell count in the blood is commonly elevated, sometimes markedly, and may contain blast cells of the erythroid and myeloid lineage as well as other dysplastic erythroid precursor cells.2,12,13,16,17,28,37,38,40 Acute Megakaryoblastic Leukemia (AML-M7) Like other types of AML, acute megakaryoblastic leukemia (AML-M7) is diagnosed on the basis of ≥20% blast

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FIGURE 67.3 Erythroleukemia (AML-M6b) in blood smear from a cat. Notice a slightly pleomorphic population of cells that resemble erythroid progenitors, including erythroblasts, prorubricytes, rubricytes, and metarubricytes. There is apparent asynchronous maturation of these cells, and there are fewer than expected mature RBCs, WBC, and platelets. Wright-Giemsa stain; original magnification ×1,000.

cells in bone marrow. At least 50% of the blast cells must be megakaryoblasts. While features such as cytoplasmic blebbing, increased amounts of cytoplasm, and binucleation or multinucleation may be observed in megakaryoblasts, immunologic markers and cytochemical stains are essential for lineage determination. Though nonspecific, megakaryoblasts will often stain positive for acetylcholinesterase (AChE), periodic acid Schiff (PAS), and alpha-naphthylacetate esterase (ANAE) on cytochemistry.2,13,20,37,38,40 Expression of CD41, which represent the glycoprotein (Gp) IIb and GpIIIa subunits (i.e. platelet integrin receptor for fibrinogen and factor VIII-related antigen) are considered the most specific lineage identifiers.36 Additional features that may be observed in AML-M7 include thrombocytopenia and sclerotic bone marrow, which may make obtaining a bone marrow aspirate difficult.37,38 Species specific Information All AML subtypes have been reported in veterinary species. As mentioned previously, published reports of AML-M3 only exist in a boar and a transgenic mouse model, but its occurrence cannot be excluded in other veterinary species.7,19 The most commonly reported myeloid leukemias in the dog and cat appear to be the acute myeloid, myelomonocytic, and monocytic leukemias.13,17,37,38 Erythroleukemia has been reported in both the dog and cat; however, reports and incidence in cats were more frequent before the FeLV epidemic was controlled. Acute myelomonocytic leukemia is believed to be the most common myeloid leukemia in horses.5,9,25,31 Sporadic reports of AML in other mammalian species exist, including a recent report of AML with multilineage dysplasia in an alpaca.35 Finally, retroviral induced

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AML has been reported in poultry, primarily in conjunction with the avian leukosis/sarcoma virus (see Chapter 62). Myeloid leukosis in poultry has recently been attributed to the newly identified J envelope subgroup of the slowly transforming ALSVs.29 FUTURE DIRECTIONS IN VETERINARY MEDICINE Complete immunophenotypic, cytochemical, and (rarely) ultrastructural characterization of all leukemic processes are necessary to determine the true incidence of AML in veterinary species. Other modalities that may allow for more precise and detailed characterization include cytogenetics, gene expression and microRNA profiling, proteomics, and stem cell biology (see Chapters 3 and 148). Additionally, recent studies have identified evolutionarily conserved chromosomal aberrations that link several canine hematologic malignancies in dogs to their human counterparts. These findings imply that the neoplasms are pathologically comparable and suggest that cellular and molecular analysis of canine leukemias may allow for improved characterization, classification, prognostication, and treatment.6,23

REFERENCES 1. Bain BJ. Diagnosis from the blood smear. New Engl J Med 2005;353:498–507. 2. Bennett JM. World Health Organization classification of the acute leukemias and myelodysplastic syndrome. Intl J Hematol 2000;72:131–133. 3. Bennett JM. Proposals for the classification of the acute leukaemias. French-American-British (FAB) co-operative group. Br J Haematol 1976;33:451–458. 4. Blue J, Perdrizet J, Brown E. Pulmonary aspergillosis in a horse with myelomonocytic leukemia. J Am Vet Med Assoc 1987;190:1562–1564. 5. Boudreaux MK, Blue JT, Durham SK, et al. Intravascular leukostasis in a horse with myelomonocytic leukemia. Vet Pathol 1984;21:544–546. 6. Breen M, Modiano JF. Evolutionarily conserved cytogenetic changes in hematological malignancies of dogs and humans – man and his best friend share more than companionship. Chromosome Res 2008;16:145–154. 7. Breuer W, Heinritzi K, Hermans W. Akute myeloische Leukose (Promyelozyten-Leukose) mit Nachweis von Viruspartikeln bei einem Eber. Histologische, histochemische und ultrastrukturelle Befunde [Acute myeloid leukemia (promyelocytic leukemia) with detection of virus particles in a boar: histologic, histochemical and ultrastructural findings]. Berl Munch Tierartzl Wochenschr 1995;108:380–384. 8. Breuer W, Hermanns W, Thiele J. Myelodysplastic syndrome (MDS), acute myeloid leukaemia (AML) and chronic myeloproliferative disorder (CMPD) in cats. J Comp Pathol 1999;121:203–216. 9. Buechner-Maxwell V. Intravascular leukostasis and systemic aspergillosis in a horse with subleukemic acute myelomonocytic leukemia. J Vet Int Med 1994;8:258–263. 10. Campbell PJ, Green AR. The myeloproliferative disorders. New Engl J Med 2006;355:2452–2466.

11. Clark P. Myeloblastic leukaemia in a Morgan horse mare. Equine Vet J 1999;31:446–448. 12. Comazzi S, Paltrinieri S, Caniatti M, et al. Erythremic myelosis (AML6er) in a cat. J Feline Med Surg 2000;2:213–215. 13. Grindem CB. Acute myeloid leukemia. In: BF Feldman, JG Zinkl and NC Jain, eds. Schalm’s Veterinary Hematology, Baltimore: Lippincott, Williams and Wilkins, 2000; 717–726. 14. Haferlach T. Genetic classification of acute myeloid leukemia (AML). Ann Hematol 2004;83(Suppl 1):S97–S100. 15. Hisasue M. A dog with acute myelomonocytic leukemia. J Vet Med Sci 2008;70:619–621. 16. Hisasue M. Hematologic abnormalities and outcome of 16 cats with myelodysplastic syndromes. J Vet Int Med 2001;15:471–477. 17. Jain NC, et al. Proposed criteria for classification of acute myeloid leukemia in dogs and cats. Vet Clin Pathol 1991;20:63–82. 18. Jurcic JG, Soignet SL, Maslak AP. Diagnosis and treatment of acute promyelocytic leukemia. Curr Oncol Rep 2007;9:337–344. 19. Kogan SC. Acute promyelocytic leukemia: a view from a mouse. Blood Cell Mol Dis 2000;26:620–625. 20. Ledieu D, Palazzi X, Marchal T, et al. Acute megakaryoblastic leukemia with erythrophagocytosis and thrombosis in a dog. Vet Clin Pathol 2005;34:52–56. 21. Lee EJ, Schiffer CA. Leukemias of indeterminant lineage. Clin Lab Med 1990;104:737–754. 22. Lowenberg B, Downing JR, Burnett A. Acute myeloid leukemia. New Engl J Med 1999;341:1051–1062. 23. McManus PM. Classification of myeloid neoplasms: a comparative review. Vet Clin Pathol 2005;34:189–212. 24. McManus PM, Hess RS. Myelodysplastic changes in a dog with subsequent acute myeloid leukemia. Vet Clin Pathol 1998;27:112–115. 25. Mori T. Acute myelomonocytic leukemia in a horse. Vet Pathol 1991;284:344–346. 26. Mrozek K, Heerema NA, Bloomfield CD. Cytogenetics in acute leukemia. Blood Rev 2004;18:115–136. 27. Olsen RJ. Acute leukemia immunohistochemistry: a systematic diagnostic approach. Arch Pathol Lab Med 2008;132:462–475. 28. Park S, Picard F, Dreyfus F. Erythroleukemia: a need for a new definition. Leukemia 2002;16:1399–1401. 29. Payne LN. Retrovirus-induced disease in poultry. Poult Sci 1998;77:1204–1212. 30. Ringger NC. Acute myelogenous leukaemia in a mare. Aust Vet J 1997;75:329–331. 31. Savage CJ. Lymphoproliferative and myeloproliferative disorders. Vet Clin N Am Equine Pract 1998;14:563–578. 32. Schoch C, Haferlach T. Cytogenetics in acute myeloid leukemia. Curr Oncol Rep 2002;4:390–397. 33. Spier SJ, Madewell BR, Zinkl JG. Acute myelomonocytic leukemia in a horse. J Am Vet Med Assoc 1986;188:861–863. 34. Staudt LM. Molecular diagnosis of the hematologic cancers. New Engl J Med 2003;348:1777–1785. 35. Steinberg JD. Acute myeloid leukemia with multilineage dysplasia in an alpaca. Vet Clin Pathol 2008;37:289–297. 36. Suter, SE, Vernau W, Fry MM, et al. CD34+, CD41+ acute megakaryoblastic leukemia in a dog. Vet Clin Pathol 2007;36:288–292. 37. Valli VE. Acute Myeloid Leukemias. In: Veterinary Comparative Hematopathology. Ames: Blackwell, 2007. 38. Valli VE. Hematopoietic system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer ’s Pathology of Domestic Animals. Philadelphia: Elsevier, 2002. 39. van’t Veer MB. The diagnosis of acute leukemia with undifferentiated or minimally differentiated blasts. Ann Hematol 1992;64:161–165. 40. Vardiman JW, Harris NL, Brunning RD. The World Health Organization (WHO) classification of the myeloid neoplasms. Blood 2002;122: 2292–2302. 41. Villiers ES, Baines A, Law AM, Mallows V. Identification of acute myeloid leukemia in dogs using flow cytometry with myeloperoxidase, MAC387, and a canine neutrophil-specific antibody. Vet Clin Pathol 2006;35:55–71.

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Mast Cell Cancer CHERYL LONDON Mast Cell Tumor in the Dog Incidence, Signalment, Etiology History and Clinical Signs Diagnosis Staging CBC, biochemistry profile, urinalysis Buffy coat smear Bone marrow aspiration Lymph node aspiration Evaluation of the abdominal and thoracic cavities Clinical staging system for canine MCTs Prognostic Factors Histologic grade Clinical stage Anatomic location Growth rate Breed Markers of proliferation Kit mutations

Treatment Surgery Radiation therapy Chemotherapy Corticosteroids CCNU (lomustine) Vinca alkaloids Kit inhibitors Supportive Care H2 antagonists H1 antagonsits Feline Mast Cell Tumors Cutaneous Feline MCT Splenic (Visceral) MCT Feline Intestinal MCT Equine Mast Cell Tumors

Acronyms and Abbreviations AgNORs, agryophilic nucleolar staining organizing regions; CBC, complete blood count; GI, gastrointestinal; ITD, internal tandem duplication; MCT, mast cell tumor; MI, mitotic index; PCNA, proliferating cell nuclear antigen.

MAST CELL TUMOR IN THE DOG Incidence, Signalment, Etiology The mast cell tumor (MCT) is the most frequent skin tumor of the dog, and the second most frequent malignant tumor noted in the canine population. While MCTs are usually found in older dogs (mean age 8–9 years), they have also been reported in younger dogs and there is no apparent sex predilection. Several breeds appear to be at risk for the development of MCT including dogs of bulldog decent (boxer, Boston terrier, English bulldog, pug), Labrador and golden retrievers, cocker spaniels, Schnauzers, Staffordshire terriers, Beagles, Rhodesian ridgebacks, Weimeraners, and Sharpeis.34 The etiopathogenesis of MCTs in the dog is unknown. The

increased incidence of MCTs in certain breeds suggests the possibility of an underlying genetic cause and studies are ongoing to identify putative genetic risk factors. Interestingly, while dogs of bulldog ancestry are at higher risk for MCT development, it is generally accepted that MCTs in these dogs are more likely to be benign. Pugs develop multiple mast cell tumors that behave in a benign fashion.29 In contrast, anecdotal evidence suggests that Sharpeis develop MCTs that are biologically aggressive. Several authors have recently identified the presence of Kit mutations in dog MCTs and these result in uncontrolled signaling.10,22,25,50 In the majority of affected dogs, the Kit mutations consist of internal tandem duplications (ITDs) in the juxtamembrane domain of Kit (encoded by exons 11–12). This region of Kit is 483

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responsible for negatively regulating receptor activation and evidence suggests that the ITDs disrupt the structure of this domain, resulting in a loss of this function. Up to 30% of all dog MCTs may carry Kit mutations and these have been shown to be significantly associated with tumor grade: mutations are rarely identified in well-differentiated tumors while approximately 35% of poorly differentiated tumors carry an ITD.10,51 The Kit mutations are not germ-line in nature (i.e. are not inherited) and, therefore are not responsible for the observed breed predispositions to the development of MCTs. However, they do represent a target for therapy.

sional biopsy is required for actual histologic grading of the tumor. If cytologic diagnosis proves difficult, a needle or punch biopsy of the tumor can be obtained before surgery. This is preferable to a larger incisional biopsy, because local release of mast cell mediators may inhibit healing, resulting in excessive bleeding. Staging CBC, Biochemistry Profile, Urinalysis Dogs with MCTs may be anemic due to gastrointestinal bleeding.

History and Clinical Signs Most MCTs in the dog occur in the dermis and subcutaneous tissue.2 However, primary MCTs may present in other sites such as the oral cavity, nasopharynx, larynx, and gastrointestinal (GI) tract.35 Visceral MCT, involving the spleen, liver, and/or bone marrow (often referred to as disseminated mastocytosis), is usually the result of systemic spread of an aggressive primary cutaneous MCT, although it can occur as an independent syndrome.37 Cutaneous MCTs usually occur as solitary nodules, although roughly 10% to 15% of dogs will present with multiple tumors.16 Approximately 50% of cutaneous MCTs occur on the trunk and perineal region, 40% on limbs, and 10% on head and neck.16 Perhaps most importantly, the clinical appearance of MCTs can vary widely and can resemble many other neoplastic and non-neoplastic lesions. Mast cell tumors that arise in subcutaneous tissue are frequently poorly circumscribed and may resemble lipomas, sometimes delaying diagnosis. In general, MCTs that are slow growing and present for at least 6 months are more likely to behave in a benign manner, while those that are rapidly growing large tumors are more likely to behave in a malignant manner.2 Clinical signs of MCTs are largely due to release of histamine, heparin and other vasoactive amines. Mechanical manipulation of the tumor during physical examination can induce degranulation leading to erythema and wheal formation (termed Darrier ’s sign) and occasionally, an owner will report that the tumor appears to change in size over short periods of time. Gastrointestinal ulceration is also a potential complication of MCTs; between 35% and 83% of dogs with MCTs that underwent necropsy had evidence of gastric ulcers, and plasma histamine concentrations were found to be elevated in dogs with MCT.17 Elevated histamine levels presumably lead to stimulation of H2 receptors on parietal cells, excessive gastric acid production, and development of ulcers.

Buffy Coat Smear It was originally believed that while the buffy coat smear was not a sensitive test, it was specific for mast cell neoplasia. However, it is now clear that this is not the case, because dogs with many different kinds of disease, including pneumonia, parvovirus, pancreatitis, skin disease and GI diseases may have mast cells circulating in the blood. Therefore, this test is no longer routinely performed in the staging of MCT patients. Bone Marrow Aspiration In the normal bone marrow, mast cells are found infrequently. In a recent report evaluating 157 dogs with MCTs, the incidence of bone marrow infiltration at initial staging was only 2.8%.11 While the presence of bone marrow involvement is indicative of systemic mast cell disease, it is usually easier to find evidence of systemic involvement in other organs (liver, spleen). Therefore, routine bone marrow aspiration is not recommended for most MCT patients. Lymph Node Aspiration All regional lymph nodes should be examined for signs of enlargement and suspicious nodes should be aspirated for cytologic examination. Because metastatic nodes may palpate within normal size, it is recommended that all accessible regional lymph nodes be aspirated. If necessary, ultrasound can be used to localize lymph nodes for aspiration. Malignant mast cells in metastatic lymph nodes are frequently found in clusters/aggregates rather than singly, aiding in a diagnosis of metastasis. If possible, lymph node aspiration should be performed before surgery, because post-operative inflammation can result in mast cell migration to local nodes and thus confuse the interpretation.

Diagnosis

Evaluation of the Abdominal and Thoracic Cavities

Cytologic evaluation of fine needle aspirates is probably the easiest method to diagnose MCT. Poorly differentiated malignant mast cells may contain few, if any, granules in which case special stains (toluidine blue, Giemsa) may be necessary to observe granules. However, exci-

Thoracic radiographs may be included as part of staging, although pulmonary involvement is uncommon. Abnormalities reported include lymphadenopathy (sternal, hilar), pleural effusion, and anterior mediastinal masses, although these are rare. Evaluation of the

CHAPTER 68: MAST CELL CANCER

TABLE 68.1 Proposed Clinical Staging for Canine Dermal Mast Cell Tumors Stage IA IB

II

III IV

Tumor(s) 1 tumor, confined to skin, 1 tumor, confined to skin, 10 cm 1 or more skin tumors, either >3 cm or ill circumscribed or ulcerated or with satellites Any Any

Regional LN

Metastasis

Negative

Negative

Negative

Negative

Negative

Negative

Positive Any

Negative Positive

A, without clinical signs; B, with clinical signs.

abdominal cavity is important in dogs with MCTs, as spread to the liver and spleen and abdominal lymph nodes may be noted. It is recommended that fine needle aspiration of the liver and spleen be performed if abnormalities are detected during ultrasound examination, or if the dog possesses negative prognostic indicators (i.e. rapidly growing tumor or evidence of lymph node metastasis.15 For typical uncomplicated solitary MCTs, cytologic evaluation of an ultrasonographically normal liver or spleen was not found to be a clinically useful staging tool. Clinical Staging System for Canine MCTs Based on recent clinical studies, a revised staging system for dermal MCTs has been proposed (Table 68.1). Prognostic Factors Histologic Grade The histologic grade of a MCT is determined after excisional biopsy of the tumor. It is the most consistent and reliable prognostic factor and correlates significantly with survival, but it will not predict the behavior of every MCT. Furthermore, there is disagreement about tumor grading among pathologists; in one study there was significant variation among pathologists in grading the MCTs (P < 0.001), although this was found to be less so if all pathologists strictly employed the system described by Patnaik.34,35 Grade 1: These MCTs are considered to behave in benign manner and complete surgical excision is usually curative.2,34 Grade 2: These represent at least 45% of all MCTs reported and their biologic behavior is more difficult to predict.2,16,34 Many dogs are cured with complete excision of a Grade 2 MCT, and radiation therapy following incomplete excision of solitary Grade 2 MCTs can cure more than 80% of affected patients.36 However, it is important to note that Grade 2 MCTs have the ability to spread to local lymph nodes, as well as distant sites, and a proportion of dogs that

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undergo definitive therapy (surgery and radiation) may go on to develop metastatic disease. Furthermore, some dogs that present with Grade 2 MCTs will already have evidence of metastatic disease making appropriate staging important. Given the wide variation in biologic behavior among Grade 2 tumors, there is now an effort to identify subcategories of Grade 2 tumors using additional prognostic indicators described below. Grade 3: These represent between 20% and 40% of all MCT reported.2,16,34 They often behave in a biologically aggressive manner, exhibiting metastasis early on in the course of disease. The mean survival time of dogs with Grade 3 MCT has been reported as 18 weeks when treated with surgery alone.2 In one study, the percentage of dogs with Grade 3 MCTs surviving at 1500 days was reported as 6%, and in another study, the percentage of dogs surviving at 24 months was 7%, indicating that these tumors are particularly malignant.34 With the recent addition of post-operative chemotherapy, survival times of Grade 3 MCT patients may be improved. Lastly, evidence suggests that radiation therapy of incompletely excised Grade 3 MCTs can result in prolonged tumor control (median disease free interval of 27.7 months).14 Clinical Stage Recent evidence suggests that the historical staging system for MCTs is not reflective of tumor biology, and a new system has been proposed (Table 68.1). In two studies, the presence of mast cells in the regional lymph node was a negative prognostic factor for survival and disease-free interval.20 However, an additional study revealed that dogs with Grade 2 tumors and lymph node metastasis treated with radiation post-surgery achieved long-term survival.8 Other studies have shown that dogs with Grade 2 MCTs with lymph node metastasis may have a good prognosis if the affected lymph node is removed and adjuvant chemotherapy is administered.6,46 Lastly, while it would seem intuitive that dogs with multiple cutaneous MCTs would experience a worse outcome compared to dogs presenting with a solitary MCT, two separate studies have demonstrated that the presence of multiple tumors does not affect prognosis.32 Anatomic Location MCTs that develop in the oral cavity, nail bed, inguinal, preputial, and perineal regions were originally reported to behave in a more malignant fashion regardless of histologic grade.13 Two reports now demonstrate that at least for MCTs in the inguinal, preputial, and perineal regions this is probably not true.5 However, MCTs located in the muzzle are biologically aggressive tumors with higher regional metastatic rates than previously reported for MCT in other sites.13 MCTs that originate in the viscera (GI tract, liver, spleen) or bone marrow have a grave prognosis.45

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Growth Rate Tumors present for long periods of time are more likely to be benign. In one study, 83% of dogs with tumors present for longer than 28 weeks before surgery survived for at least 30 weeks, compared to only 25% of dogs with tumors present for less than 28 weeks.2 Breed Boxer and pug dogs have a high incidence of MCTs, but these tend to be more well differentiated and carry a better prognosis.2,29 However, every MCT should be treated as potentially malignant, regardless of breed.

TABLE 68.2 Schematic for Treatment of Canine Mast Cell Tumors Grade I Complete Incomplete

Grade II Complete Incomplete

Markers of Proliferation Several proliferative indices have been evaluated in an attempt to predict the outcome of canine MCTs. Perhaps the most useful is Ki-67, a protein found in the nucleus, the levels of which appear to correlate with cell proliferation. In one study, the mean number of Ki-67 positive nuclei was significantly higher for dogs that died of MCTs than for those that survived. For dogs with Grade 2 tumors, the number of Ki-67 positive nuclei was significantly associated with outcome. This was recently confirmed by an additional study that demonstrated the Ki-67 score can be used to divide Grade 2 MCTs into two groups with markedly different expected survival times.42 A recent study showed that mitotic index (MI, number of mitoses per 10 highpower fields) may be useful for predicting the biologic behavior of canine MCTs.41 When dogs presenting with metastatic disease were excluded from analysis, those with tumors possessing a MI ≤ 5 had a median survival time of 80 months, compared to 3 months for those possessing a MI > 5, suggesting that MI is a strong predictor of overall survival for dogs with MCTs. Other proliferation markers such as assessment of agryophilic nucleolar staining organizing regions (AgNORs) and PCNA have been used to try to determine biologic behavior of MCTs, although these may not be as reliable.3 One study sought to establish a new grading scheme for MCTs using Kit immunohistochemical staining patterns as an indicator of tumor aggressiveness but it did not correlate well with survival time.49 Lastly, investigators attempted to correlate histologic grading of mast cell tumors with a combined Ki67/ PCNA/AgNOR/Kit immunohistochemical scoring.1 No significant correlation was found for Kit staining and MCT grade, but high Ki67/PCNA/AgNOR scores all positively correlated with tumor grade (i.e. higher scores for higher grade). This study suggests that proliferation indices increase with increasing grade and are ultimately reflected in the eventual biologic behavior of the tumor. Kit Mutations As previously mentioned, mutations in Kit have been found in canine MCTs and research suggests they are

Grade III Complete Incomplete

No further therapy Wider excision or radiation therapy if surgery not possible; may consider no further therapy Chemotherapy only if negative prognostic factors present Wider excision or radiation therapy if surgery not possible; may consider no further therapy if there are no negative prognostic indicators; chemotherapy if negative prognostic factors present Chemotherapy Chemotherapy +/− radiation therapy

associated with an increased risk of local recurrence, metastasis, and death of affected dogs.10,51 Treatment See Table 68.2. Surgery Historically, it has been recommended that the margins need to be at least 3 cm in each direction; deep margins are as important as the lateral margins. Recent studies demonstrated that all Grade 1 MCTs were completely excised with 1 cm of normal tissue around the MCT (lateral margins) and one fascial plane included in the excision (deep margin).5,26,44 With respect to Grade 2 MCTs in both studies, 75% and 68% were completely excised with a 1 cm lateral margin and one fascial plane deep, while 100% and 89% of Grade 2 MCT were completely excised with a 2 cm lateral margin and one fascial plane deep. Neither of the studies evaluated Grade 3 MCTs. Because tumor grade is usually not known prior to surgery, it appears prudent to still recommend a 3 cm lateral margin and one fascial plane for the deep margin when feasible. All of the excised tissue should be submitted and margins should be labeled so the pathologist is able to specifically identify any areas of incomplete excision. However, even histologically clean margins do not guarantee that a tumor will not recur. In one study, 83% of dogs with Grade 1 MCT, 44% of dogs with Grade 2 MCT, and 6% of dogs with Grade 3 MCT were alive 1500 days after surgical excision.34 A proportion of Grade 2 tumors that are incompletely excised will not recur post surgery. In a recent report, the estimated proportions of Grade 2 tumors that recurred locally at 1, 2, and 5 years were 17.3%, 22.1%, and 33.3% respectively.43 Eleven (39.3%) dogs developed MCT at other cutaneous locations and median overall survival was 1426 days.

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Radiation Therapy Substantial data suggest that radiation therapy is effective in eliminating remaining microscopic disease after incomplete excision of Grades 1 and 2 MCT (greater than 90% 3 year control rate).20,36 Radiation therapy also was found to be useful for dogs with MCTs metastatic to local lymph node. In this report, administration of radiation to the incompletely excised tumor and to affected lymph node resulted in a median disease-free survival of 1240 days.8 With respect to incompletely excised Grade 3 tumors, evidence suggests that radiation therapy can result in prolonged tumor control (median disease free interval of 27.7 months) although dogs with tumors greater than 3 cm before surgery did not fare as well.14 It is likely that multi-modality therapy (i.e. combining surgery, radiation, and chemotherapy) would enhance this observed survival time. Radiation therapy has also been used to treat solid MCTs when surgery is not an option. Varying degrees of success have been found; in one study, a 60% 1-year control rate was obtained in dogs with non-resectable Grades 1–3 MCTs on the head or limb treated with prednisone and radiation. Coarse fractionated radiation has been used to treat dogs with non-resectable bulky, high grade MCTs, although it is important to note that systemic effects of degranulation following radiation may lead to vomiting, hypotension, and death. Chemotherapy The use of adjuvant chemotherapy is indicated after excision of Grade 3 MCTs, metastatic MCTs, non-resectable high-grade tumors, or for any other MCT with negative prognostic indices. While radiation therapy is the treatment of choice for incompletely excised Grades 1 and 2 MCTs, data indicate that post-operative chemotherapy may prevent local recurrence and, therefore should be considered for patients who are not candidates for radiation, if such therapy is not available or if the owners cannot afford the cost of therapy. Corticosteroids The reported response rate of canine MCT to prednisone is 20–70%, although the study demonstrating the highest response rate was of short duration (median 10 days) precluding assessment of response durability.27 Partial remissions are more common than complete remissions, and at least some of the observed response may be due to a decrease in tumor-associated edema secondary to stabilization of mast cell granules and a reduction in mast cell mediator production. CCNU (Lomustine) In one study, 8 out of 19 dogs (42%) with measurable MCTs had an objective response to single agent CCNU for a median duration of 77 days.38 Preliminary unpublished data suggest that CCNU given post-surgery, either alone or with prednisone and vinblastine, can significantly prolong survival times of dogs with high grade tumors or tumors with negative prognostic indicators. CCNU can induce hematopoietic and hepatic toxicity including neutrope-

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nia, thrombocytopenia, and liver failure. Patients receiving this drug should be monitored closely. In general, CCNU is dosed at 50–70 mg/m2 orally every 3–4 weeks. A complete blood count (CBC) and liver panel should be performed before each dose. Vinca Alkaloids Single agent response rates of vincristine, vinblastine and vinorelbine are 7%, 12% and 13%, respectively, suggesting that vinca alkaloids are not effective as sole agents for the treatment of MCTs.27 Vinblastine has been combined with prednisone in other studies, inducing objective responses ranging from 27% to 47%.46 A combination of vinblastine, cyclophosphamide and prednisone resulted in a 64% (7 out of 11) response rate in one study.6 The dose of vinblastine is 2–3 mg/m2 given intravenously every 1–3 weeks. The major toxicity of this drug is neutropenia (in approximately 5–6% of patients) and occasional gastrointestinal upsets. It is frequently used in an alternating manner with CCNU. Kit Inhibitors Orally bioavailable small molecule inhibitors of Kit (SU11654 [Palladia] and AB1010 [Kinavet]) have recently been demonstrated to have activity against canine MCT.22,23,24 In a placebo-controlled randomized study, response rate in Palladia-treated dogs was 37.2% versus 7.9% in placebo-treated dogs.24 Of 58 dogs that received Palladia after placebo escape, 41.4% experienced an objective response. The overall response rate for dogs in this study receiving Palladia was 42.8%. Additionally, the commercially available Kit inhibitor imatinib mesylate (Gleevec) has been used to treat canine MCTs. A recent study demonstrated some response to therapy in 10 of 21 dogs treated with imatinib; the objective response rate was 100% in dogs whose MCTs possessed a Kit ITD.17 Another study reported partial responses to therapy in three dogs with systemic mast cell disease treated with imatinib.26 However, it is important to note that imatinib can cause severe idiosyncratic hepatotoxicity and is extremely expensive, thereby limiting its use. Supportive Care H2 Antagonists Mast cell tumors may cause GI ulceration because histamine stimulates gastric acid production by parietal cells. This is especially relevant for dogs with evidence of systemic involvement. To prevent this, any of the standard H2 antagonists (cimetidine, ranitidine, or famotidine) should be administered to affected dogs. Alternatively, proton pump inhibitors such as omeprazole may be utilized; these inhibitors are probably more useful in the setting of gross mast cell disease where standard H2 antagonists may be less effective. H1 Antagonists Massive mast cell degranulation can lead to hypotensive shock and death. Therefore, all patients with gross

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mast cell disease, or evidence of systemic involvement should be placed on the H1 antagonist diphenhydramine. FELINE MAST CELL TUMORS Mast cell neoplasia in the cat occurs in three basic forms: cutaneous MCT, splenic mast cell disease (sometimes referred to as lymphoreticular) and intestinal MCT. The biologic behavior of these diseases is strikingly different, so they will each be considered separately. Cutaneous Feline MCT Cutaneous MCTs represent the second most commonly encountered cutaneous tumor in the cat.30 Two forms have been reported: mastocytic MCTs that appear histologically similar to those found in dogs, and histiocytic MCTs, a rare variant known to spontaneously regress, that possess features of histiocytic cells.9 In general, feline cutaneous MCTs are solitary, raised, firm, hairless, well-circumscribed dermal nodules between 0.5 and 3 cm in diameter.7,30 Occasionally, the surface of the tumor may be ulcerated; some tumors will present as plaque-like lesions, and tumors can be multiple.7 These tumors are most likely to occur on the head and neck (frequently involving the pinna near the base of the ear), trunk, and limbs.7,30 Affected cats usually do not exhibit clinical signs of disease other than pruritus. As with canine tumors, feline MCTs are usually easily diagnosed by cytologic examination of fine needle aspirates. Cats with cutaneous MCT should be evaluated for evidence of additional tumors, as well as potential splenic involvement as a recent study found that some cats with multiple cutaneous MCTs also have splenic disease.21 In addition, a minimum database is recommended, along with careful examination of local lymph nodes for evidence of lymphadenopathy. With respect to the typical mastocytic variant of feline MCT, they are usually categorized as either compact (representing 50–90% of all cases) or diffuse (histologically anaplastic).7 Several studies have demonstrated that well-differentiated compact tumors tend to behave in a benign manner and metastasis is uncommon.21,31 In contrast, anaplastic tumors may have a high MI, and marked cellular and nuclear pleomorphism, with infiltration into subcutaneous tissues. These behave in a more malignant manner, metastasizing to local lymph nodes, spleen, blood, and bone marrow. However, a more recent study evaluated pleomorphic cutaneous MCTs from 15 cats and found that the majority were behaviorally benign; only one cat was euthanized due to disease progression.19 Interestingly, in both studies, tumors with a high MI were most likely to recur or exhibit metastasis, suggesting that this feature is useful for predicting biologic behavior. The definitive treatment for cutaneous feline MCT is surgical excision. In a series of 32 cats with cutaneous MCT, five recurred after surgical excision, but no cats

died of the disease. In this study, completeness of excision and histopathologic factors, such as nuclear pleomorphism and MI, were not associated with tumor recurrence.31 Other reports have demonstrated local recurrence rates after excision of between 0% and 24%.7,19,21 Systemic spread of cutaneous tumors has been reported in 0% to 22% of cases, although those that metastasized may have been anaplastic tumors.19,31 As previously discussed, those tumors with a high MI are more likely to recur post-surgery or metastasize.19 Radiation therapy may be considered for tumors that are incompletely excised. A recent study demonstrated that feline cutaneous MCTs treated with strontium 90 irradiation exhibited a 98% control rate, with median survival times greater than 3 years.48 Limited information exists concerning the utility of chemotherapy in cats with MCT. It is generally believed that feline MCTs are less responsive to prednisone than their canine counterparts. Responses to CCNU have been reported in cats: of 20 cats with cutaneous MCTs, 10 exhibited complete (n = 2) or partial (n = 8) response to CCNU.39 Some investigators have utilized a combination of prednisone and chlorambucil to treat metastatic or multiple tumors. This is generally well-tolerated, although its effectiveness is unclear. Splenic (Visceral) MCT This form of neoplastic mast cell disease is also referred to as visceral or lymphoreticular MCT. It is the most frequent disease of the spleen of cats, affecting older animals (mean age 10 years) with no sex or breed predilection.7,21 While the spleen is the primary site affected by this disease, other organs may also be involved including the liver, mesenteric lymph nodes, bone marrow, lung, and intestine.7,12 Circulating mast cells may be found in many cases and pleural and/or peritoneal effusions may be noted.7,12 The majority of cats with splenic MCTs do not have a history of cutaneous MCT, although recent evidence suggests that cats with multiple cutaneous MCTs may also have splenic involvement.21 Cats with splenic MCT frequently present with signs of systemic illness including vomiting, anorexia, and weight loss.7,12 Dyspnea may be evident if pleural effusion is present. Abdominal palpation usually reveals a markedly enlarged spleen and/ or liver. Clinical signs associated with release of mast cell mediators include GI ulceration, hemorrhage, hypotensive shock, and labored breathing. Cats with suspected splenic MCT should undergo a standard clinical evaluation including a CBC, chemistry profile, urinalysis, abdominal ultrasound, and thoracic radiographs. Unlike the cutaneous form of MCT in cats and dogs, splenic MCT in the cat is frequently associated with circulating mast cells and up to 50% of cats will have evidence of bone marrow infiltration by mast cells.7,12 It is important to note that the finding of multiorgan involvement does not necessarily indicate a poorer prognosis. Anemia is also a frequent hematologic finding, with eosinophilia less likely to be observed.7,12 Lastly, in one report of 43 cats with splenic

CHAPTER 68: MAST CELL CANCER

mast cell disease, 90% had an abnormal coagulation profile, although there was no evidence of clinical bleeding.12 Splenectomy is the treatment of choice for cats with splenic MCT, even if other organ involvement is noted. Pre-treatment with H1 and H2 blockers before therapy is indicated; intraoperative death may occur due to the release of mast cell mediators. Median survival times of 12–19 months after splenectomy alone have been reported in cats with splenic MCT with bone marrow/ blood involvement.12 While mastocytosis in the circulation usually does not completely resolve, it does decline significantly and cats experience good quality of life for long periods of time. Cats should be followed postoperatively with buffy coat smears, as a rise in the number of mast cells in the blood may indicate disease progression. Anorexia, significant loss of body mass and the male sex were found to be negative prognostic indicators in one study.12 Adjunctive chemotherapy with prednisone, lomustine, and/or chlorambucil has been attempted in a limited number of cases, but clinical value is uncertain. Recently, a cat with splenic mast cell disease and systemic mastocytosis was treated with the Kit inhibitor imatinib and exhibited a significant response to therapy including complete resolution of tumor masses and marked reduction in circulating mast cells.18 Feline Intestinal MCT Mast cell tumors represent the third most common intestinal tumor in the cat, with lymphoma and adenocarcinoma first and second, respectively.7 Most cats have a history of vomiting, diarrhea and anorexia, and a solitary palpable abdominal mass is usually evident on physical examination.7 As metastasis is common with this disease, enlarged mesenteric lymph nodes and/or hepatomegaly may also be noted. A peritoneal effusion may be present, and this frequently contains mast cells and eosinophils. Diagnosis may be made by fine needle aspiration of the mass or involved organs. As with splenic MCT, these cats should be staged by performing a CBC, chemistry profile, urinalysis, thoracic radiographs, and abdominal ultrasound. Buffy coat smear and bone marrow aspiration may also be performed, although unlike splenic MCT, intestinal MCT rarely involves these organs. However, it may be useful to distinguish splenic MCT with intestinal involvement from the distinct syndrome of intestinal MCT. Surgery is the treatment of choice for intestinal MCT. Wide surgical margins are necessary (5–10 cm) as the tumor frequently extends well beyond the observable gross disease.7 The historical survival times of cats with intestinal MCT are poor, as metastasis is common at the time of diagnosis. Limited information regarding the use of chemotherapy in these cases is available, although anecdotal responses to lomustine and chlorambucil have been reported. Recently, two cats had objective responses (one complete, one partial) after treatment with lomustine.39

489

EQUINE MAST CELL TUMORS Mast cell tumors are known to occur in the horse and the majority are cutaneous in nature.28,40 The largest series of tumors reported evaluated 32 cutaneous MCTs and found that 22 of 25 did not recur for up to 6 years after removal, two recurred at the surgical site, and one spontaneously regressed within 3 months after biopsy.28 The majority of published data indicate that equine cutaneous MCTs are benign in behavior, seldom recurring after surgical excision.

REFERENCES 1. Bergman PJ, Craft DM, Newman SJ, et al. Correlation of histologic grading of canine mast cell tumors with Ki67/PCNA/AgNOR/c-Kit scores: 38 cases (2002–2003). Vet Comp Oncol 2004;2:98–98. 2. Bostock DE. The prognosis following surgical removal of mastocytomas in dogs. J Small Anim Pract 1973;14:27–41. 3. Bostock DE, Crocker J, Harris K, et al. Nucleolar organiser regions as indicators of post-surgical prognosis in canine spontaneous mast cell tumors. Br J Cancer 1989;59:915–918. 4. Buttner C, Henz BM, Welker P, et al. Identification of activating c-kit mutations in adult-, but not in childhood-onset indolent mastocytosis: a possible explanation for divergent clinical behavior. J Invest Dermatol 1998;111:1227–1231. 5. Cahalane AK, Payne S, Barber LG, et al. Prognostic factors for survival of dogs with inguinal and perineal mast cell tumors treated surgically with or without adjunctive treatment: 68 cases (1994–2002). J Am Vet Med Assoc 2004;225:401–408. 6. Camps-Palau MA, Leibman NF, Elmslie R, et al. Treatment of canine mast cell tumours with vinblastine, cyclophosphamide and prednisone: 35 cases (1997–2004). Vet Comp Oncol 2007;5:156–167. 7. Carpenter JL, Andrews LK, Holzworth J. Tumors and tumor like lesions. In: Holzworth J, ed. Diseases of the Cat: Medicine and Surgery, Philadelphia: WB Saunders, 1987;406–596. 8. Chaffin K, Thrall DE. Results of radiation therapy in 19 dogs with cutaneous mast cell tumor and regional lymph node metastasis. Vet Radiol Ultrasound 2002;43:392–395. 9. Chastain CB, Turk MA, O’Brien D. Benign cutaneous mastocytomas in two litters of Siamese kittens. J Am Vet Med Assoc 1988;193:959–960. 10. Downing S, Chien MB, Kass PH, et al. Prevalence and importance of internal tandem duplications in exons 11 and 12 of c-kit in mast cell tumors of dogs. Am J Vet Res 2002;63:1718–1723. 11. Endicott MM, Charney SC, McKnight JA, et al. Clinicopathological findings and results of bone marrow aspiration in dogs with cutaneous mast cell tumors: 157 cases (1999–2002). Vet Comp Oncol 2007;5:31–37. 12. Feinmehl R, Matus R, Maulden GN, Patnaik A. Splenic mast cell tumors in 43 cats (1975–1992). Proc Annu Conf Vet Cancer Soc 1992;12:50. 13. Gieger TL, Theon AP, Werner JA, et al. Biologic behavior and prognostic factors for mast cell tumors of the canine muzzle: 24 cases (1990–2001). J Vet Int Med 2003;17:687–692. 14. Hahn KA, King GK, Carreras JK. Efficacy of radiation therapy for incompletely resected grade-III mast cell tumors in dogs: 31 cases (1987–1998). J Am Vet Med Assoc 2004;224:79–82. 15. Hahn KA, King GK, Harris FD, et al. The usefulness of hepatic and splenic fine needle aspiration cytology in the clinical staging of canine cutaneous mast cell tumors. An evaluation of 88 dogs (1987–1998). Proc Mid-Year Conf Vet Cancer Soc 2000;2:23. 16. Hottendorf GH, Nielsen SW. Pathologic survey of 300 extirpated canine mastocytomas. Zentralbl Veterinarmed A 1967;14:272–281. 17. Isotani M, Ishida N, Tominaga M, et al. Effect of tyrosine kinase inhibition by imatinib mesylate on mast cell tumors in dogs. J Vet Int Med 2008;22:455–459. 18. Isotani M, Tamura K, Yagihara H, et al. Identification of a c-kit exon 8 internal tandem duplication in a feline mast cell tumor case and its favorable response to the tyrosine kinase inhibitor imatinib mesylate. Vet Immunol Immunopathol 2006;114:168–172. 19. Johnson TO, Schulman FY, Lipscomb TP, et al. Histopathology and biologic behavior of pleomorphic cutaneous mast cell tumors in fifteen cats. Vet Pathol 2002;39:452–457. 20. LaDue T, Price GS, Dodge R, et al. Radiation therapy for incompletely resected canine mast cell tumors. Vet Radiol Ultrasound 1998;39:57–62. 21. Litster AL, Sorenmo KU. Characterisation of the signalment, clinical and survival characteristics of 2006;41 cats with mast cell neoplasia. J Feline Med Surg 8:177–183. 22. London CA, Galli SJ, Yuuki T, et al. Spontaneous canine mast cell tumors express tandem duplications in the proto-oncogene c-kit. Exp Hematol 1999;27:689–697.

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23. London CA, Hannan AL, Zadovoskaya R, et al. Phase 1 dose-escalating study of SU 11654 a small molecule receptor tyrosine kinase inhibitor in dogs with spontaneous malignancies. Clin Cancer Res; 9(7):2755–68. 24. London CA, Henry CJ, Rusk AW, et al. Multi-center, placebo-controlled, double-blind, randomized study of oreal toceranib phosphate (Palladia [TM], SU11654), a receptor tyrosine kinase inhibitor, for the treatment of dogs with recurrent mast cell tumors. Proc 28th Annu Conf Vet Cancer Soc, 2008. 25. Ma Y, Longley BJ, Wang X, et al. Clustering of activating mutations in c-KIT’s juxtamembrane coding region of canine mast cell neoplasms. J Invest Dermatol 1999;112:165–170. 26. Marconato L, Bettini G, Giacoboni C, et al. Clinicopathological features and outcome for dogs with mast cell tumors and bone marrow involvement. J Vet Int Med 2008;22:568–574. 27. McCaw DL, Miller MA, Ogilvie GK, et al. Response of canine mast cell tumors to treatment with oral prednisone. J Vet Intern Med 1994;8:406–408. 28. McEntee MF. Equine cutaneous mastocytoma: morphology, biological behaviour and evolution of the lesion. J Comp Pathol 1991;104:171–178. 29. McNiel EA, Prink AL, O’Brien TD. Evaluation of risk and clinical outcome of mast cell tumours in pug dogs. Vet Comp Oncol 2004;4:2–8. 30. Miller MA, Nelson SL, Turk JR, et al. Cutaneous neoplasia in 340 cats. Vet Pathol 1991;28:389–395. 31. Molander-McCrary H, Henry CJ, Potter K, et al. Cutaneous mast cell tumors in cats: 32 cases (1991–1994). J Am Anim Hosp Assoc 1998;34:281–284. 32. Murphy S, Sparkes AH, Blunden AS, et al. Effects of stage and number of tumours on prognosis of dogs with cutaneous mast cell tumours. Vet Rec 2006;158:287–291. 33. Northrup NC, Harmon BG, Gieger TL, et al. Variation among pathologists in histologic grading of canine cutaneous mast cell tumors. J Vet Diagn Invest 2005;17:245–248. 34. Patnaik AK, Ehler WJ, MacEwen EG. Canine cutaneous mast cell tumors: morphologic grading and survival time in 83 dogs. Vet Pathol 1984;21:469–474. 35. Patnaik AK, MacEwen EG, Black AP, et al. Extracutaneous mast-cell tumor in the dog. Vet Pathol 1982;19:608–615. 36. Poirier VJ, Adams WM, Forrest LJ, et al. Radiation therapy for incompletely excised grade II canine mast cell tumors. J Am Anim Hosp Assoc 2006;42:430–434. 37. Pollack MJ, Flanders JA, Johnson RC. Disseminated malignant mastocytoma in a dog. J Am Anim Hosp Assoc 1991;27:435–440.

38. Rassnick KM, Moore AS, Williams LE, et al. Treatment of canine mast cell tumors with CCNU (lomustine). J Vet Int Med 1999;13:601–605. 39. Rassnick KM, Williams LE, Kristal O, et al. Lomustine for treatment of mast cell tumors in cats: 38 cases (1999–2005). J Am Vet Med Assoc 2008;232:1200–1205. 40. Riley CB, Yovich JV, Howell JM. Malignant mast cell tumours in horses. Aust Vet J 1991;68:346–347. 41. Romansik EM, Reilly CM, Kass PH, et al. Mitotic index is predictive for survival for canine cutaneous mast cell tumors. Vet Pathol 2007;44:335–341. 42. Scase TJ, Edwards D, Miller J, et al. Canine mast cell tumors: correlation of apoptosis and proliferation markers with prognosis. J Vet Int Med 2006;20:151–158. 43. Seguin B, Besancon MF, McCallan JL, et al. Recurrence rate, clinical outcome, and cellular proliferation indices as prognostic indicators after incomplete surgical excision of cutaneous grade II mast cell tumors: 28 dogs (1994–2002). J Vet Int Med 2006;20:933–940. 44. Simpson AM, Ludwig LL, Newman SJ, et al. Evaluation of surgical margins required for complete excision of cutaneous mast cell tumors in dogs. J Am Vet Med Assoc 2004;224:236–240. 45. Takahashi T, Kadosawa T, Nagase M, et al. Visceral mast cell tumors in dogs: 10 cases (1982–1997). J Am Vet Med Assoc 2000;216:222–226. 46. Thamm DH, Turek MM, Vail DM. Outcome and prognostic factors following adjuvant prednisone/vinblastine chemotherapy for high-risk canine mast cell tumour: 61 cases. J Vet Med Sci 2006;68:581–587. 47. Tsai M, Takeishi T, Thompson H, et al. Induction of mast cell proliferation, maturation, and heparin synthesis by the rat c-kit ligand, stem cell factor. Proc Natl Acad Sci USA 1991;88:6382–6386. 48. Turrel JM, Farrelly J, Page RL, et al. Evaluation of strontium 90 irradiation in treatment of cutaneous mast cell tumors in cats: 35 cases (1992–2002). J Am Vet Med Assoc 2006; 228:898–901. 49. Webster JD, Kiupel M, Kaneene JB, et al. The use of KIT and tryptase expression patterns as prognostic tools for canine cutaneous mast cell tumors. Vet Pathol 2004;41:371–377. 50. Zemke D, Yamini B, Yuzbasiyan-Gurkan V. Characterization of an undifferentiated malignancy as a mast cell tumor using mutation analysis in the proto-oncogene c-KIT. J Vet Diagn Invest 2001;13:341–345. 51. Zemke D, Yamini B, Yuzbasiyan-Gurkan V. Mutations in the juxtamembrane domain of c-KIT are associated with higher grade mast cell tumors in dogs. Vet Pathol 2002;39:529–535.

CHAPTER

69

B Cell Tumors V.E. TED VALLI Precursor B Cell Neoplasms B Cell Acute Lymphoblastic Leukemias (ALL) L1 Clinical features Acute Lymphoblastic Leukemia L2 Acute Lymphoblastic Leukemia L3 Mature (Peripheral) B Cell Neoplasms B Cell Chronic Lymphocytic Leukemias/Small Lymphocytic Lymphomas Pathologic features Clinical features Plasma Cell Myeloma (see Chapter 70) B Cell Prolymphocytic Leukemias Clinical features Pathologic features Lymphoplasmacytic Lymphoma Pathologic features

Marginal Zone Lymphoma Clinical presentation Pathologic features Clinical features Mantle Cell Lymphoma Pathologic features Clinical features Follicular Lymphoma Clinical presentation Pathologic features Clinical features Diffuse Large B cell Lymphomas Pathologic features Clinical features Burkitt’s Lymphoma Pathologic features Clinical features

Acronyms and Abbreviations ALL, acute lymphoblastic leukemia; AML, acute myeloid leukemia; CB, centroblastic; CHOP, cyclophosphamide, vincristine, doxorubicin, prednisone; CLL, chronic lymphocytic leukemia; DLBCL, diffuse large B cell lymphoma; EBV, Epstein-Barr virus; FAB, French-American-British; FL, follicular lymphoma; IB, immunoblastic; ICAM, intercellular adhesion molecule; LPL, lymphoplasmacytic lymphoma; MCL, marginal cell lymphoma; MZL, marginal zone lymphoma; PCR, polymerase chain reaction; PLL, prolymphocytic leukemia; RBC, red blood cell; SLL, small lymphoblastic leukemias; SNP, single nucleotide polymorphism; WBC, white blood cell; WHO, World Health Organization.

L

ymphoid tumors have been enigmatic neoplasms for much of the time that they have been diagnosed by pathologists. A problem for clinicians and diagnosticians has always been the basis for distinguishing leukemia from lymphoma. Both diseases were found to have cells of identical immunophenotypes. This difficulty was overcome in the revised WHO classification system that pragmatically linked the diagnosis to the topographical area with the greatest volume of tumor. By this system, if most of the tumor was in bone marrow the disease was termed leukemia and if most was in peripheral tissues it was termed lymphoma. A further advance in our understanding of lymphomas is that there are always neoplastic cells in circula-

tion with the risk of dissemination related to surface adhesion molecules (ICAMs) that function like the address on an envelope and determine where the cells bearing those markers can adhere to the vessel wall and exit the blood vascular system. In this context, the cells of acute leukemias lack most adhesion sites and tend to remain in circulation. The other extreme is MALT-type lymphomas that may remain so tightly adhered to a single location that they can be successfully managed by surgical removal. A generality of lymphoid proliferation is that malignancy of poorly differentiated lymphocytes is likely to occur in bone marrow of young individuals and present as leukemia, while malignancy of mature lymphocytes 491

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will likely occur in peripheral tissues of mature humans or animals and present as lymphoma (R. Brunning, personal communication). An interesting exception to that thumb rule is seen in the gamma delta T cell lymphomas of humans, dogs, and cats that arise in intestine and involve the spleen. When bone marrow is invaded by a hematopoietic neoplasm, some degree of marrow failure will be apparent when 50% or greater volume of marrow is involved with neoplastic cells. Significant clinical cytopenias may be apparent with much less than 50% of marrow involved in some peripheral T cell lymphomas where the neoplastic cells induce suppression of normal marrow cell production. Cats may present with weight loss, vomiting, diarrhea, and inflammatory bowel-type disease with tumor cells usually spreading to spleen

and marrow at the time of diagnosis. Similar conditions seem to occur in dogs; however, neutropenia is not a feature of large granular lymphomas in either dogs or cats. B and T cell leukemias are classified according to the French-American-British (FAB) system (Table 69.1). With this system the cells of acute lymphocytic leukemia are designated as L1, L2, or L3 types based on cell morphology. This system is less commonly applied today in human medicine because of their reliance on genetics and immunocytochemistry. The FAB system still has relevance in veterinary pathology where early diagnostic interpretation is based on the appearance of leukemic cells in Wright-Giemsa-stained preparations. Differentiating acute myeloid from acute lymphoid leukemias in animals remains a major difficulty. The

TABLE 69.1 Revised European-American Classification of Lymphoid Neoplasia (REAL) 1994a B-cell neoplasms Precursor B-lymphoblastic leukemia/lymphoma Mature (peripheral) B-cell neoplasms B-cell chronic lymphocytic leukemia/prolymphocytic leukemia/small lymphocytic lymphoma Lymphoplasmacytoid lymphoma/immunocytoma Mantle cell lymphoma Follicle center lymphoma, follicular Provisional cytologic grades: I, small cell; II, mixed small and large cell; III, large cell Provisional subtype: diffuse, predominantly small cell type Marginal zone B-cell lymphoma, extranodal mucosa-associated lymphoid tissue type (± monocytoid B-cells) Provisional subtype: Nodal marginal zone lymphoma (± monocytoid B-cells) Provisional entity: Splenic marginal zone lymphoma (± villous lymphocytes) Hairy cell lymphoma Plasmacytoma/plasma cell myeloma Diffuse large B-cell lymphoma Subtype: Primary mediastinal (thymic) B-cell lymphoma Burkitt’s lymphoma Provisional entity: High-grade B-cell lymphoma, Burkitt’s-like T-cell and putative natural killer-cell neoplasms Precursor T-cell neoplasm Precursor T-lymphoblastic lymphoma/leukemia Mature (peripheral) T-cell and natural killer-cell neoplasms T-cell chronic lymphocytic leukemia/prolymphocytic leukemia Large granular lymphocyte leukemia (LGL) T-cell type Natural killer-cell type Mycosis fungoides/Sezary syndrome Peripheral T-cell lymphomas, unspecified Provisional cytologic categories: medium-sized cell, mixed medium- sized and large cell, large cell, lymphoepithelioid cell Provisional subtypes: hepatosplenic γδT-cell lymphoma; subcutaneous panniculitic T-cell lymphoma Angioimmunoblastic T-cell lymphoma Angiocentric lymphoma Intestinal T-cell lymphoma (± enteropathy-associated) Adult T-cell lymphoma/leukemia Anaplastic large cell lymphoma, CD30+, T- and null cell types Provisional entity: anaplastic large cell lymphoma, Hodgkin’s-like Hodgkin’s lymphoma Lymphocyte predominance Lymphocyte-rich (classical Hodgkin’s lymphoma) Nodular sclerosis Mixed cellularity Lymphocyte depletion a Reproduced with permission from Knowles DM, ed. Neoplastic Hematopathology, 2nd ed. Philadelphia: Lippincott Williams and Wilkins, 2001;699, table 19.13.

CHAPTER 69: B CELL TUMORS

493

availability of a myeloperoxidase stain that works in both immunocytochemistry and immunohistochemistry on canine and feline cells has greatly assisted our ability to recognize acute myeloid leukemia (AML). PRECURSOR B CELL NEOPLASMS B Cell Acute Lymphoblastic Leukemias (ALL) L1 B cell lymphoblastic leukemias are clonal diseases primarily involving bone marrow that present with mild to moderate levels of leukemic lymphocytosis but can be characterized by leukopenia.7,37 The disease is associated with splenomegaly but not with a mediastinal mass seen in T cell-type ALL. Clinical Features L1 B cell ALL are relatively infrequent lymphomas usually of young animals.31 It is most often recognized in dogs and cats but probably occurs in all mammals. L1-type B cell ALL is a disease of acute onset. Affected animals are in good condition with acute loss of appetite. Organomegaly is usually mild but spleen is symmetrically enlarged and nodal changes are irregular or absent. Leukemia blast cells are always present in the blood varying between 10,000 and 50,000/μL and the total white blood cell (WBC) count may be normal or decreased. The key to the diagnosis is the recognition of a homogeneous population of cells with round densely stained nuclei only slightly larger than RBCs. The nucleoli are obscured by the dispersed chromatin and are only recognized if carefully looked for. The cytoplasm is scant and visible over about one-third of the nuclear circumference. Mitoses are usually present in the blood. Marrow is always involved and there may be some degree of phthisis with reduced erythropoietic cells and megakaryocytes (Fig. 69.1). Hemorrhage is present in marrow and is usually identified on gross examination of marrow. In early stages, lymph nodes may have few changes but, if involved, there will be atrophy of germinal centers with colonization of the inner paracortical areas and medullary cords with cells as described for marrow (Fig. 69.2). With progression and marrow failure, there will be extramedullary hematopoiesis in both medullary cords and sinuses. The spleen will have uniform involvement with resultant rounded borders and may have focal areas of hemorrhagic infarction. There is atrophy of the lymphoid follicles and of periarteriolar lymphoid sheaths with variable sinus colonization (Fig. 69.3). There may be colonization of subendothelial areas of large muscular veins. The liver also may be involved. The involvement is tightly periportal and tends to spread in solid proliferation. Other tissues that may be involved include skeletal muscle, intestine, brain, and testis. In terms of phenotype, L1 may be of B cell- or T celltype; not all cells of L1 type will stain with CD3 (T cell

FIGURE 69.1 Bone marrow aspirate from a dog with L1 acute lymphoblastic leukemia. There is phthisis of normal cells and replacement by small neoplastic lymphocytes. Note the size of the nuclei of the smaller cells in comparison to RBCs. The chromatin is densely stained and uniformly distributed with nucleoli not apparent. Cytoplasm is limited to a very narrow ellipse not visible on all cells. The larger cells with more red stained nuclei are the dividing cells. Wright-Giemsa stain.

marker) or CD79a (B cell marker), but almost none of them stain with both. Therefore, even slight staining is taken as a specific indication of lineage. These cells stain equally well in cytologic and histologic preparations with CD20 (B cell marker) that is more reliable in cats than CD79a. CD20 will occasionally stain T cells and if the population is questionably marked by both CD20 and CD3 the cells should be considered to be T cells. In flow cytometric analysis, L1 B cell ALL is identified by CD45-positive blasts of very small cell size. In addition, in human medicine, the B cell blasts are positive for CD10, CD19, and HLA-DR and negative for the T cell marker CD7. In contrast, the L1-type T cells are identified in flow cytometry as small blast cells that are positive for CD7 and are doubly positive with CD4 and CD8. Few molecular changes are defined in animals with L1-type ALL. Hyperdiploidy is present in nearly half of human cases occurring in children. The most common chromosomal alteration in children occurs in about 10% of cases and involves translocations and deletions in chromosome 12. Presumably the benign counterpart cell of origin is a primitive precursor in marrow in the early B cell lineage. Human subsets are identified as pre-B cell ALL and B acute ALL based on the level of immunoglobulin gene expression. The L1-type ALL is defined as tumors in which the neoplastic nuclei are larger than RBCs but less than

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FIGURE 69.2 Lymph node from a dog with L1 acute lymphoblastic leukemia. The nuclei are round and small to intermediate in size with dispersed chromatin. Nucleoli are obscured and only irregularly apparent. There are three mitotic figures in the upper left and two in left and right center. Note that the metaphase chromatin of lymphoblastic lymphoma in histological preparations do not have the sharp outline of chromosomes as is seen in other subtypes of lymphoma. Hematoxylin & Eosin stain.

FIGURE 69.3 Splenic aspirate from a dog with L1 acute lymphoblastic leukemia. Note the size of the smaller cells as compared to RBCs with cell type identical to that noted in bone marrow. Wright-Giemsa stain.

1.5 RBC diameters. The chromatin pattern is coarsely stained and may have a coarse cribriform chromatin structure with the nucleoli obscured. This is important because in lymphoblastic lymphoma the chromatin is dispersed and the nucleoli are present but obscured. Additionally, L1-type ALL cells have very little cytoplasm. In the FAB system the L1 form is essentially the leukemic form of lymphoblastic lymphoma. In histopathologic preparations, both L1 and L2 are virtually indistinguishable from lymphoblastic lymphoma. B cell ALL must be distinguished from T cell ALL that is morphologically identical. B cell ALL is not associated with hypercalcemia or a mediastinal mass both of which occur in a proportion of L1-type T cell ALL. CD3 is the best marker for T cell ALL. The most important topographic finding in T cell ALL is an anterior mediastinal mass. Staging of B cell ALL in animals is primarly based on the degree of bone marrow involvement and on the presence of cytopenias in the blood. All cases of B cell ALL can be expected to advance rapidly. Treatment usually consists of multi-agent protocols with both B cell and T cell ALL considered to have a very poor prognosis in dogs and cats; aggressive therapy is generally not given to other species. Evaluation of treated cases is essentially monitoring the levels of tumor cells in the blood and monitoring developing cytopenias. Most cases progress to marrow failure. Acute Lymphoblastic Leukemia L2 B cell lymphoblastic leukemia of L2 type is a clonal disease involving bone marrow that presents with mild to moderate levels of leukemic lymphocytosis but can be characterized by leukopenia. The disease is associated with organomegaly that includes at least moderate involvement of spleen, liver, and lymph nodes, and may involve the kidneys, intestinal tract, and reproductive tract. Most acute lymphoid leukemias of animals are of the L2 ALL type. The L2 type of B cell ALL is seen in the dog, cat, calves, cattle, horses, and laboratory species. In human cases, the L1 and L2 ALL do not appear to be biologically different diseases. Consequently, L2 B cell ALL can be considered to be like L1 B cell ALL. The L2-type lymphomas are characterized by nuclei that are fully 2 RBCs in diameter and have sharp shallow nuclear indentation. The chromatin pattern is coarsely cribriform with dense staining and occasional chromatin aggregations. These aggregates frequently surround the nucleolus giving it added prominence (Fig. 69.4). Mitoses are usually seen in cytologic preparations. There is a high N:C ratio with cytoplasm seen around no more than half of the nucleus. In L2 ALL cases without nuclear indentations, the cells resemble AML of the FAB M0 or M1 blast type. Some cases of L2 ALL may be of the large granular lymphocyte type and these tend to have a worse prognosis in humans and it appears that this also applies in cats and dogs. In general the same immunologic char-

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FIGURE 69.4 Blood smear from a dog with L2 acute lymphoblastic leukemia. The nuclei are generally 2–2.5 RBCs in diameter with frequent deep nuclear indentations and irregular rounded projections. The chromatin is deeply stained with irregular formation of dark chromocenters that outline some nucleoli. The cytoplasm is generally minimal in volume without an apparent complete covering of nuclei. There is moderate cytoplasmic basophilia without vacuolation or granulation. Wright-Giemsa stain.

acteristics can be applied to L2 ALL as for L1 ALL with the exception of differences due to cell size and cytoplasmic volume. Acute Lymphoblastic Leukemia L3 Acute lymphoblastic leukemias of the L3 type are acute clonal neoplasms involving marrow and generally are associated with organomegaly. L3-type ALL occurs in most mammals, being relatively frequent in dog, cat, and cattle, and less common in horses. L3-type ALL cells have fine peripheral cytoplasmic vacuolization that may stain positively for fat (Fig 69.5). The cytoplasm is similar in volume to the L2-type ALL and usually deeply basophilic. Nuclei vary from 2 to 3 RBCs in diameter. The chromatin is deeply stained and may be dense and dispersed or cribriform with some dense chromocenters that may encircle nucleoli that are multiple and moderately well defined. In some individual cases, nuclei mold into oval shapes and there may be deep nuclear indentations particularly in cattle. Mitoses are generally present. In both cytologic and histologic preparations L3 ALL is not reliably distinguishable from Burkitt-type lymphoma or Burkitt-like lymphoma. The anisokaryosis that is seen in the cytologic preparations of L3-type ALL is also evident in the Burkitt-like lymphomas. The L3 ALL cells also may be confused with large T cell lymphoma, megakaryocytic leukemia, and monocytic leukemias, all of which may have cytoplasmic vacuolizations. Like L3 ALL, these tumors are negative with myeloperoxidase staining. L3 B cells stain positively for CD79a and CD20 and negatively for CD3. In

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FIGURE 69.5 Lymph node aspirate from a dog with L3 acute lymphoblastic leukemia. The nuclei have mild anisokaryosis with deeply stained chromatin with apparent large densely stained chromocenters. Nucleoli are small and multiple and most apparent in the nucleus in lower right. The cytoplasmic vacuolation usually appears beneath the cytoplasmic membrane. Wright-Giemsa stain.

humans, almost all L1 and L2 ALL have the terminal deoxynucleotidyl transferase (TdT) marker typical of cells of primitive differentiation and as an indication of maturation. This marker is absent in L3 ALL. In human cases the L3 type ALLs are felt to respond poorly to therapy when compared to L1 and L2 ALL. Acute lymphoblastic leukemia type distinction has not had sufficient attention in animals for comment to be made on biological differences in response to treatment. MATURE (PERIPHERAL) B CELL NEOPLASMS B Cell Chronic Lymphocytic Leukemias/Small Lymphocytic Lymphomas Chronic lymphocytic leukemia (CLL) of B cell type is a clonal disease with primary involvement of the bone marrow. The disease is primarily one of accumulation of long-lived cells of low proliferative rate and not of excess proliferation.12,17,41,46 The neoplastic cells are small and uniform in appearance and are immunologically competent. Small B cell lymphocytic lymphoma (SLL) is a solid clonal neoplasm of low grade primarily involving lymph nodes, spleen, and all parenchymal organs that is morphologically indistinguishable from B cell CLL. Pathologic Features In CLL, lymphocytosis is present in blood (frequently >100,000/μL) and bone marrow. In cats, where the

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FIGURE 69.6 Blood smear from a cat with chronic lymphocytic leukemia. The leukocyte count was >600,000/μL with the cat likely entering an accelerated phase of the disease. The smaller nuclei are more typical of the chronic disease with the larger nuclei part of a dividing population. The chromatin is quite densely stained with a narrow rim of quite basophilic cytoplasm. Nucleoli are not apparent in most nuclei. Wright-Giemsa stain.

disease appears most often, the cell type is of small cells with round nuclei typically 1 to 1.5 RBCs in diameter with a very narrow rim of cytoplasm. In dogs, the disease may be of similar cell type but, like humans, there is a second type with larger nuclei and much more abundant cytoplasm that is relatively lightly stained. Nucleoli are usually absent or very small and indistinct, and the chromatin is dense with little internal nuclear detail. Larger cells increase with progression of the disease (Fig. 69.6). These cells may have eosinophilic granules present indicating that they are large granular lymphocytes. Bone marrow is always heavily involved in CLL and there may be relatively solid involvement with almost complete phthisis of normal marrow cells. In early stages of CLL the bone marrow will be involved in a random fashion, unlike acute or chronic myeloid leukemias that tend to involve subendosteal areas first. Marrow is usually diagnostic with CLL and there is usually moderate involvement with SLL. In human CLL, with solid marrow involvement there are lighter stained areas that consist of slightly larger cells that have been recognized and described as proliferation centers. These consist of clusters of dividing cells and constitute part of the diagnostic criteria for CLL. In cats with CLL, lymph nodes are rarely significantly enlarged and may be small or atrophic. In SLL, the nodes are enlarged with the capsule thinned and taut and the node has a diffuse architecture (Fig. 69.7). A remarkable finding in these nodes is numerous dilated lymphatics in what is likely a residual hilar area of the node that is distended with small lymphocytes of uniform type. In CLL, the spleen is uniformly enlarged, primarily due to filling of the sinus areas with fading germinal centers arising from the arteriolar system, with an archi-

FIGURE 69.7 Lymph node from a cat with chronic lymphocytic leukemia. Note the small size of the neoplastic cells in comparison to surrounding RBCs. The small more dense cells would be more typical during chronic stages of the disease with the larger nuclei being the dividing population. The cytoplasm is minimal with moderate staining density. Mitoses are infrequently encountered. Hematoxylin & Eosin stain.

tectural presentation of a fine coalescing multifocal nodular proliferation.47 There is frequently subendothelial involvement of the large muscular veins. In contrast small lymphocytic lymphoma tends to be multifocal and regional without uniform enlargement. Chronic lymphocytic leukemia tends to involve most other tissues but most prominently the liver that presents with a periportal involvement. Other tissues involved include the choroid plexus of the brain, adrenal gland, and pancreas. B cell CLL and SLL are uniformly positive with CD79a and CD1c and weakly positive for CD1a. They are negative for T cell markers. The molecular specificities of CLL and SLL are largely undefined in animals, but probing of the immunoglobulin genes demonstrates clonality.49 In humans, there is usually rearrangement of both the light and heavy immunoglobulin genes with some human cases of SLL also having rearrangement of the beta chain of the T cell receptor gene. About 10% of dogs with indolent B cell lymphomas have rearrangement of the T cell receptor gene. The cell of origin for B cell CLL and SLL in animals is unknown. In humans, there is a lack of hypermutation of the variable portion of the immunoglobulin gene in both SLL and CLL that suggests that the origin of the tumor is a population of naive pre-germinal center B cells.39 Chronic lymphocytic leukemia and SLL need to be differentiated from benign reactive hyperplasia because the cells lack atypia and individually cannot be recognized as neoplastic. The degree of increase in cell numbers in tissue or blood and the homogeneity of proliferation with cells of small cell type are the most important criteria in defining a neoplastic condition.

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The diagnosis of B cell CLL is made on finding a leukemic blood picture with a very high number of circulating lymphocytes that consist of small mature-appearing cells with a very low mitotic rate. If the diagnosis is in doubt, the safest route is to retest to prove that there is a sustained marked lymphocytosis that can then be reliably assumed to be clonal. If the blood lymphocyte count is sustained above a level of about 10,000/μL, a bone marrow biopsy is indicated. Heavy infiltration of bone marrow with small cells of similar type confirms the diagnosis. In terms of specific diagnosis it is important to determine phenotype because T cell-type small cell lymphomas are more common than B cell-type at least in the cat. The diagnosis of SLL is based on irregular enlargement of lymph nodes with focal lesions in both the liver and spleen and all affected areas being involved with an infiltration of very small mature-appearing lymphocytes. The blood picture may appear relatively normal in SLL in early stages. Clinical Features Chronic lymphocytic leukemia and SLL are slowly progressive clonal neoplasms of mature small lymphocytes: CLL begins in bone marrow and SLL begins in peripheral tissues. SLL is usually T cell type and is recognized most frequently in cats, less often in dogs, and infrequently in cattle and horses. It is seen in mature animals 5–10 years of age and in dogs 10–15 years. The disease may occur in younger animals but seldom in animals under 5 years old. The accumulation of small cells of CLL may cause few clinical signs and is most often diagnosed incidentally in animals undergoing routine blood examination. Chronic lymphocytic leukemias and SLL affect all breeds of dogs and cats. In a review of 600 lymphoid neoplasms in cats, 23 cases (3.8%) were felt to be CLLor SLL-type with these related entities present in about even proportions. In dairy cattle, clinical signs may include reduced feed intake and milk production, lethargy, loss of appetite, and melena if there is an ulcerated gastric tumor. Affected animals are usually thin and have hepatosplenomegaly. Staging has not been applied in animals with CLL, but the Binet system of human medicine is straightforward. This system defines the stage of disease progression once the diagnosis is made. The human stages of progression can be roughly transposed in this manner: 1. absence of anemia or thrombocytopenia, tumor present in fewer than three lymphoid areas 2. absence of anemia or thrombocytopenia and involvement of more than three lymphoid areas 3. presence of anemia with less than 10 g/dL of hemoglobin and the presence of thrombocytopenia as defined by levels of less than 100,000/μL. In animals this process involves comparing the current blood picture and topographic distribution of tumor with previous examinations based on the duration between examinations and rate of progression. It should

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be remembered that if chemotherapeutic treatment is given, this will cause changes in cell type usually due to smaller cells that may mask the presence of conversion to an accelerated phase where there will be at least focal areas consisting of larger cells. The survival of animals with SLL is largely undefined because the animals may have had the neoplasm for several years before diagnosis. When animals are presented in an accelerated phase, the tumor tends to be more responsive to treatment than when in a more indolent stage of disease. In general it is considered best to treat animals conservatively if they are not showing clinical signs. Treatment of non-clinical human cases of CLL may induce immune hemolytic anemia. B Cell Prolymphocytic Leukemias Prolymphocytic leukemia (PLL) is a clonal neoplasm involving the bone marrow and is characterized by a very high level of leukemic cells at presentation with marked splenomegaly, characteristic cellular morphology, and chronic course.23,32 The disease in animals is largely based on cell type without definition of the clinical entity. PLL can be of B cell- or T cell-type and in humans the ratio is about 80% B cell and 20% T cell. Clinical Features Prolymphocytic leukemia occurs in cattle and rarely in sheep and goats, and is seen most frequently in dogs and cats where it is usually considered to be chronic lymphocytic leukemia that has progressed to an accelerated phase.47,48 In humans, the disease has a well-defined presentation in elderly males that present with high levels of leukemic cells and marked splenomegaly. Most cases described occur in older animals that are presented in poor condition with weight loss and marked splenomegaly. Lymph node involvement is seen. Typically the liver is not grossly enlarged but has histological involvement on a lobular basis. Pathologic Features Like CLL there is marked leukocytosis in excess of 50,000/μL typically present in the blood. The cells have a “hand mirror” shape and represent a stage of differentiation that is seen in both B and T cell types of PLL. The nuclei are round and the size of 2.0 RBCs with a moderate envelope of lightly stained cytoplasm. The chromatin has multiple (15–20) large densely stained chromocenters that are about 2 μm in diameter and are outlined by clear areas of parachromatin joined by short, thin chromatin bands. Most cells lack nucleoli. There are occasional cells present that have slightly larger nuclei with less well-defined chromocenters that have nucleoli, and are considered to be the dividing population (Fig. 69.8). The pattern of involvement in lymph nodes is typical of the B cell infiltration in CLL. In human nodes, the

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FIGURE 69.8 Lymph node aspirate from a dog with prolymphocytic leukemia. The nuclei are of intermediate size (1.5 RBCs in diameter). The chromatin pattern is characteristic with large chromocenters that are joined by broad chromatin bands with irregular parachromatin clearing that tends to give the nuclei a “spotted” nuclear pattern. Nucleoli are only visible in the larger nuclei and the cytoplasm is moderate in volume and staining density. Wright-Giemsa stain.

early disease begins around fading germinal centers like mantle cell lymphoma with gradual involvement of the paracortex. Involvement of the spleen is present with a fine multinodular pattern related to the arteriolar sheath vessels. B cell type PLL is positive for CD79a and probably for CD20 and negative for CD3. There is clonal rearrangement of the immunogloblulin gene. The tumor is derived from a relatively mature progenitor of either B cell or T cell lineage. Because this appears to be a stage of morphologic differentiation, the normal benign counterpart probably passes through this stage without ever forming a recognizable tissue mass. Prolymphocytic leukemia must be differentiated from CLL that may have some larger sized cells in blood or tissue, but the bulk of the tumor population will have small compact nuclei with uniformly dense chromatin. Leukemic phases of mantle cell lymphoma may appear similar but nucleoli differ in most cells. Lymphoplasmacytic Lymphoma Lymphoplasmacytic lymphoma (LPL) is a clonal neoplasm of intermediate size cells that have nuclei similar to those of CLL and SLL.19 The cell type varies in having a greater cytoplasmic volume that is deeply amphophilic and gives the tissues a deep reddish tincture on architectural examination. The disease is otherwise similar to SLL with most nuclei lacking nucleoli and having a low mitotic rate (Fig. 69.9). Lymphoplasmacytic lymphoma occurs in all domestic animals but is seen most frequently in older cats and horses.38,40,48,49 It occurs in the intestines of horses. About

FIGURE 69.9 Kidney from cat with lymphoplasmacytic lymphoma. Histologically the lesion resembles chronic lymphocytic leukemia but with greater cytoplasmic volume that is more deeply amphophilic. Note the size of the nuclei in comparison to the RBC in the lower right. The cytoplasmic volume accounts for the wider spacing of the nuclei. Mitoses are infrequently encountered and cell boundaries are irregularly distinct. Wright-Giemsa stain.

half of the cases in the cat are also intestinal and some are mediastinal. The disease tends to be indolent.28 Pathologic Features Lymphoplasmacytic lymphoma does not usually have a leukemic phase; however, hyperglobulinemia may be present. Most cases of LPL have both surface and cytoplasmic IgM, are positive with CD79a and CD20, and are negative for T cell markers. However, it is possible for cells to have plasmacytoid morphology and stain for T cell markers. The diagnosis of lymphoplasmacytic lymphoma is made on the recognition of the specific cell types in lymph node, spleen, or bone marrow. The cytoplasmic volume and deep staining density are the unique features of cells from animals with this disease. Marginal Zone Lymphoma Marginal zone lymphoma (MZL) is a B cell lymphoma of distinctive architecture and cytologic features that develops in a concentric layer of proliferation outside the mantle cell cuff of germinal centers (see Chapter 52). In human pathology, cells of marginal zone-type are referred to as “monocytoid B cells.”3,6 Marginal zone lymphomas probably occur in all animals but are recognized most frequently in mature dogs and less frequently in cats.10,13,43 Marginal zone lymphoma appears to constitute approximately 15% of lymphomas in dogs and about 11% of non-Hodgkin’s lymphomas in humans. In human pathology, where MZL is divided into three disease presentations, the extranodal MALT-type lymphoma constitutes about

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FIGURE 69.10 Lymph node from a dog with marginal zone

FIGURE 69.11 Lymph node from a dog with marginal zone

lymphoma. The node has well advanced marginal zone lymphoma in which there is atrophy of surrounding paracortex with sinus ectasia. The small densely stained cells in the center are benign mantle cells that have collapsed into the dendritic bed of a fading germinal center. The surrounding cells are a malignant population of marginal zone cells. Note the lack of tingible body macrophages. Wright-Giemsa stain.

lymphoma; CD79a stain. Cells at bottom of the lesion are mantle cuff cells at the edge of a fading germinal center. The marginal zone nuclei are 1.5 RBCs in diameter with a prominent single central nucleolus and with irregularly peripheralized chromatin with marked parachromatin clearing. The cytoplasm is relatively abundant which results in wider spacing of nuclei. Wright-Giemsa stain.

8% of all cases of non-Hodgkin’s lymphoma, while the nodal-type is about 1.8%, and the splenic-type about 1% of total cases. A major species difference is that the disease always has diffuse involvement in humans and virtually always has focal and multinodular involvement in dogs.

The splenic involvement invariably is multifocal and locally extensive. The distribution of tumor foci of MZL is related to splenic end arterioles that dictate a regular periodicity of nodular distribution. These nodules then irregularly coalesce with disease progression (Fig. 69. 11). Splenic MZL is usually accompanied by internodular areas of plasmacytosis often with atypical features and accompanied by focal areas of sclerosis that are usually identified as “fibrohistiocytic nodules.” Because progression of MZL is slow, it is likely that many cases have been identified as areas of nodular hyperplasia. In general, when nodules are small and discrete, clonality studies tend to indicate that the condition is benign. Alternatively, when nodules are large and multifocal clonality studies usually indicate malignancy. Marginal zone lymphomas in animals are strongly positive for B cell markers including CD79a and CD20, and are negative for T cell markers (Fig. 69. 12). Consistent cytogenetic and molecular changes in MZL have not been identified in animals. In humans, the most consistent features of this type are seen in the splenic form in which trisomy 3 and trisomy X and the deletion of (q22q24) have been reported in a small group of patients. The cell of origin in MZL is not known in dogs. In humans, the close association between chronic immune stimulation caused by Helicobacter sp. gastritis and MALT lymphoma strongly suggests an origin from a post-germinal center B cell. In all tissue areas the key to diagnosis of MZL is based on the recognition of cellular proliferation arising outside the mantle cell layer. In advanced cases, most areas will consist entirely of marginal zone cells but a few areas may be identified with fading marginal zone

Clinical Presentation Marginal zone lymphoma occurs in mature large breed dogs, most frequently in submandibular lymph node, with only one site involved. The animals almost invariably feel well with normal appetite and activity. Nodes are always fully mobile. The second most frequent site in the dogs is the spleen with spread to hilar lymph nodes. Dogs eventually develop generalized lymphadenopathy but continue to feel well. Pathologic Features Leukemic manifestations of MZL have not been identified in animals. Bone marrow is seldom involved in MZL. The capsule of affected lymph nodes is thin and taut with the peripheral sinus preserved. Architecturally there is follicular hyperplasia with germinal centers extending into medullary areas. Usually there is some degree of medullary sclerosis indicating a long period of hyperplasia. In early stages, some germinal centers may still retain antigen-related polarity but with progression there is collapse of germinal centers with only a central area of mantle cells remaining. Marginal zones then appear as a lighter staining area of cells surrounding these dense clusters of smaller mantle cells (Fig. 69.10).

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FIGURE 69.12 Spleen from a dog with marginal zone lymphoma. There are irregularly coalescing areas in the center and a large area of diffuse proliferation in the upper right. Note the atrophy of periarteriolar lymphoid sheaths. Wright-Giemsa stain.

cells. Marginal zone cells have nuclei 1.5–2 RBCs in diameter with moderately hyperchromatic chromatin that tends to be partially peripheralized onto the nuclear membrane with mild irregular parachromatin clearing. The other consistent finding is a prominent single central nucleolus. In addition, MZL tends to have relatively abundant and lightly stained cytoplasm. This combination of vesicular nuclei and abundant cytoplasm gives marginal zone areas a lighter staining aspect on architectural examination. A consistent cytologic characteristic of early MZL is that mitoses will be absent in most 400× fields. With progression in the dog (i.e. after 18 months to 2 years), there will be 3–4 mitoses per 400× field with an equivalent number of tingible body macrophages. In lymph nodes, MZL needs to be differentiated from T cell lymphoma that may look similar at the architectural level because of the appearance of fading germinal centers. These centers are peripheralized in T cell lymphoma and in MZL the proliferating cells are surrounding the fading centers. Both types of lymphoma share a low mitotic rate. Differentiation is made on the basis of the T cell lymphomas tending to have smaller nuclei with more compact chromatin and less cytoplasm. Immunophenotyping is useful in differentiating these two types of lymphoma. Clinical Features Marginal zone lymphoma is generally not associated with clinical signs, but with current usage of ultrasound, splenic masses are readily identified. Because the most common form of focal splenic mass in the dog is hemangiosarcoma, splenectony is the most frequent approach to treatment. Almost invariably MZL has not spread to other sites at the time of splenectomy. In contrast, MZL in lymph nodes in the dog tends to be diagnosed in more advanced cases, generally at stage

2a or 3a. In nodes that are completely effaced by MZL, the nodular nature of the disease may not be apparent until immunophenotyping is done. Most cases of nodal MZL in dogs involve peripheral nodes but occasionally internal nodes may be involved. Marginal zone lymphoma can progress from focal to multifocal to generalized over time. At the architectural level, the lesion progresses from multinodular to diffuse and the cytologic changes include appearance of low numbers of mitotic figures and an accompanying presence of tingible body macrophages. MZL tends to respond well to chemotherapy at all stages of disease but may respond better as the number of mitotic figures increases. Animals with a prior diagnosis of MZL tend to remain in phase 3a or 4a and tend not to become leukemic. Animals in advanced stages tend to retain normal appetite and activity and all nodes appear to be equally diagnostic for sampling purposes. A major diagnostic concern with MZL is that the large nucleoli in intermediate-sized cells are cytologically interpreted as high-grade lymphoma. MZL can be accurately diagnosed if a 2 mm diameter Tru-cut biopsy needle is used in most tissues except liver and kidney. Some cases of canine MZL will continue to respond to the same treatment protocol with prolonged remissions over a period of 1–2 years. An indication of the very slow spread of MZL is indicated by three cases treated only by splenectomy that did not have subsequent recurrence in other tissues. In humans, 5-year survival with the splenic form of MZL is 65%. Mantle Cell Lymphoma Mantle cell lymphoma (MCL) is a distinct subtype of lymphoma in humans and animals that is characterized by architectural origin around fading germinal centers with nuclei characteristically uniformly round and 1.5 RBCs in diameter with dense chromatin with little internal detail.24,30,51 Most nuclei lack visible nucleoli and the tumor has a low mitotic rate. Mantle cell lymphoma has been recently recognized in human pathology and is identified as a distinct entity arising from a translocation T(11;14) and by over expression of cyclin D1 protein. Mantle cell lymphoma is recognized in cats and dogs and probably occurs in all mammals. Mantle cell lymphoma constitutes 5% of human lymphoma in North America and twice that level in Europe with a 3:1 male predominance. In humans, the disease occurs in elderly patients. Eleven cases of canine MCL were identified in a collection of 461 cases with an incidence in the dog of about 2%. The age ranged from 1.5 to 15 years with a mean of just under 8 years. In a comparable period, since the diagnosis began to be recognized in 1992, one case has been recognized in 150 cases of lymphoma in cats. Like MZL, MCL is much more likely to occur in the spleen in the dog. Mantle cell lymphoma tends to present as a multinodular and multifollicular prolifera-

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tion that then undergoes coalescence. Associated changes in spleen include atypical plasmacytosis with fibrohistiocytic nodules and occasionally focal hemorrhagic infarction. In both humans and dogs, a “blastoid variant” of MCL occurs that is identified by slightly larger nuclei with consistent small nucleoli. This form tends to become leukemic and develop bone marrow involvement. In the dog, this type of presentation has a characteristic diffuse multinodular architecture. The nodules enlarge in size and in so doing appear to compress their own blood supply undergoing ischemic degeneration followed by hemorrhage. Pathologic Features In dogs, the blastoid-type of MCL will present with febrile disease accompanied by anemia and gammopathy that is reversed on splenectomy.5 In humans, a variable pattern is seen that tends to be paratrabecular with progression to a diffuse pattern. Lymph node MCL in the dog is generally identified in animals with generalized lymphadenopathy. Typically the nodes are markedly enlarged without involvement of perinodal tissues. Architecturally the nodes have a diffuse architecture with loss of germinal centers and proliferation compressing medullary structures generally without medullary sclerosis (Fig. 69.13). The follicular nature of MCL may only be evident on phenotypic staining. Mantle cell lymphoma is more difficult to recognize histologically. The architectural identity becomes dependent on identifying a few slightly smaller more densely stained benign mantle cells at the center of rounded areas of cellular proliferation of slightly larger

FIGURE 69.13 Lymph node from a dog with mantle cell lymphoma. Architecturally the pattern resembles marginal zone lymphoma with the proliferating cells surrounding a fading germinal center. The proliferation of cells around the mantle cell layer rather than eccentric to it identifies the neoplastic cells as B cells. Identification of the cells as small to intermediate nuclear type with small nucleoli is essential to recognize mantle cell lymphoma. Wright-Giemsa stain.

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cells. Cytologically, MCL has round nuclei that are densely stained and generally lack nucleoli. The cells are of the same size or slightly larger than those of T-zone lymphoma and lack nuclear indentations. Nucleoli are small or inapparent and cells have a relatively wide rim of cytoplasm giving the nuclei a neatly separated appearance similar to T-zone lymphoma. Like other indolent lymphomas, the mitotic rate is low and none may be found in a 400× field with the small type of mantle cell lymphoma, whereas 3–4 per 400× field can be found with the blastoid-type of MCL. Typically tingible body macrophages are not present. In dog and cat spleen, MCL is characterized by multifocal proliferation and is locally extensive (Fig. 69.14). With the small cell-type of MCL there will be progressive coalescence of splenic nodules and internodular areas of atypical plasmacytosis and fibrosis as occurs with marginal zone lymphoma. As noted earlier, the blastoid variant of MZL tends to involve the spleen uniformly in the dog and has not been identified in the cat. Cytologically the blastoid-type of MCL has nuclei that are 1.5 RBCs in diameter with much more variation in shape than with MZL. The chromatin pattern is hyperchromatic and coarse granular with irregular parachromatin clearing and each nucleus having 1–2 small nucleolus that are much smaller than the large single nucleolus of MZL. In splenic blastoid-type MCL in the dog there is typically subendothelial colonization of larger venous sinuses. Other tissues that are typically involved include lymph node, bone marrow, and liver. The liver involvement is by portal cuffing and occasionally implantation eccentric to central veins. MCL is uniformly positive with the B cell markers CD79a and CD20, and negative with CD3 and other

FIGURE 69.14 Spleen from a dog with mantle cell lymphoma. A cohesive nodule of coalescing foci is a typical presentation that is frequently accompanied by focal areas of hemorrhagic infarction as is apparent in lower right. The proliferating areas are lightly and irregularly outlined by more deeply stained cells consisting mostly of small benign plasma cells. Note the atrophy of periarteriolar lymphoid sheaths in surrounding more normal spleen (above and to the right).

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Clinical Features

FIGURE 69.15 Lymph node from a dog with mantle cell lymphoma; CD79a stain. The closely aggregated nuclei at the bottom are the edge of the fading mantle cell cuff evident at lower power in Figure 69.13. Note that the major difference between the benign and malignant mantle cells is only a slight enlargement of nuclei with the presence of prominent small nucleoli, but with more abundant cytoplasm that is deeply stained with CD79a. While the nucleoli are apparent and occasionally even two are present, they are much smaller than those typical of marginal zone lymphoma and the nuclei themselves are smaller.

T-cell markers (Fig. 69.15). In dogs, MCL tends to stain lightly and uniformly for lambda light chain. Nothing is known about genetic changes characteristic of MCL in animals. In human MCL, translocation (t11;t14)(q13;q32) is usually present and is considered presumptive for diagnosis of MCL. In human cases, the oncogene dysregulated by the translocation has been termed CCND1 and encodes for the cyclin D1 protein that is over expressed in tissues. The cell of origin in animals is not known. In humans, it is hypothesized that MCL is derived from naive marrow B cells that are pre-germinal center and migrate from the marrow to the mantle cell area. Mantle cell lymphoma is a type of lymphoid neoplasm with a nodular gross pattern in all tissues that is initially focal and becomes coalescent as the disease progresses. In early cases, apparent origin from germinal centers is evident by fading follicles within some foci of proliferation. An additional useful finding is the presence of areas of protein insudation that are present as follicular hyalinosis in a germinal center that has receded with the protein remaining. Mantle cell lymphoma must be differentiated from other indolent lymphomas that have a nodular architecture including follicular lymphoma and MZL. MCL might be confused with follicular lymphoma grade I where the concentric sites tend to have nuclei that are more irregular in shape than those of MCLs. MCL is differentiated from MZL because of smaller nuclei without the prominent single central nucleolus characteristic of MZL. In advanced cases MCL needs to be differentiated from chronic lymphocytic leukemia.

Mantle cell lymphoma in animals presents at advanced stages of 3–4 and staging is based on recognizing the number of areas of nodal involvement of both internal and peripheral nodes as well as hepatosplenomegaly. On fine needle aspirate, the cells are slightly larger than those of T-zone lymphoma and lack the shallow nuclear indentations. In advanced cases, tumor is present in the liver. Little is known about the progression of MCL in animals. In humans, the most significant prognostic factor is shortened remission time after treatment preceding recurrence. Poor prognostic signs in humans are old age, poor performance at the time of diagnosis, advanced stage of the disease, increased lactate dehydrogenase levels, splenomegaly, and anemia. Other symptoms, consisting of fever, night sweats, and loss of body mass, are important negative factors in humans as well as for the appearance of leukemia. Nothing is known about treatment or survival in animals with MCL. In human cases of MCL, survival is 3 years with an absence of long-term survivors and survival with the blastoid variant of MCL is generally less than 2 years. This seems to be at variance with very limited experience in dogs where splenectomy of dogs with blastoid-type MCL has been followed by longterm survival. Follicular Lymphoma Follicular lymphoma (FL) is a type of B cell neoplasm composed of centrocytes and centroblasts with at least partial follicular architecture.14,20,42 A major distinguishing feature of follicular lymphoma is that the cell composition of each follicle must be exactly similar to that in other follicles. This serves to distinguish true FL from various stages of follicular hyperplasia. True FL in animals is an uncommon diagnosis. In a collection of 502 cases of canine lymphoma, there were 20 cases of follicular lymphoma and 15 cases of benign and atypical follicular hyperplasia. The true incidence of the disease in dogs is likely somewhere between 2% and 4% of total lymphoma cases. In one review of 176 cases from the New York City area, there were eight cases of FL-type 3, two cases of FL-type 2, and none of FL-type 1 for a total incidence of 5.7%. Increased incidence in that population may have been because these cases had routine immunophenotyping carried out assisting recognition of advanced cases. In a major review of the WHO classification system of human lymphomas, it was found that architecture alone was adequate for diagnosis of FL. In a review of 602 cases of feline lymphoma by histopathology alone, there were six cases (1%) of FL. A review of 1198 cases of bovine lymphoma, identified by inspection at Canadian slaughter plants, identified four cases (0.3%) of FL.50 Follicular lymphoma appears to be rare in the horse, where the incidence appears to be less than 1%.27 In humans in North America and Western Europe, 25–40% of lymphoma cases are FL. Follicular lymphoma is infrequent

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in developing countries: it is likely that there are local influences involved. In support of that concept, a review of 238 cases of canine lymphoma from Ohio, in which immunophenotyping was carried out, found one case of FL. Clinical Presentation Affected dogs vary between 2 years and greater than 10 years of age.45 Animals with FL tend to have both peripheral and internal lymph node involvement. These nodes tend not to be enlarged, suggesting early spread and slow progression. In the dog, the nodes most likely involved are those around the head and neck, including prescapular and popliteal areas as well as mediastinal and sublumbar nodes. The nodes are always nonpainful and they are not fixed to the skin or deeper structures. Follicular lymphoma, like chronic lymphocytic leukemia, is usually found on incidental examination for dental care or other routine examination. With extensive lymph node involvement there is usually at least focal involvement of liver and multifocal involvement of spleen. Bone marrow may be involved but it is not apparent on examination of blood. Pathologic Features Virtually nothing is known about the presence of cells in blood of animals with follicular lymphoma. In humans, neoplastic cells are seen in blood in about 30% of cases. These neoplastic cells resemble those seen in nodes of dogs with T-zone lymphoma and have very sharp shallow nuclear indentations. In the few cases of FL examined in which bone marrow was involved, nonspecific changes of cancer were present including an increased coarse hemosiderin and plasmacytosis. The follicular pattern is maintained when FL involves the bone marrow and the tumor tends to have a paratrabecular distribution. In FL, the disease usually occupies an entire lymph node, but rarely nodes have partial involvement.48 The pattern consists of follicles that are tightly faceted (Fig. 69.16). A characteristic of FL is that there is complete absence of mantle cell cuffs and internodal areas tend to consist of the post-capillary venules and small arterioles with increased numbers of RBCs and a variable complement of small, medium, and large lymphocytes (Fig. 69.17). The peripheral capsule is characteristically thin and taut, with the peripheral sinus compressed and compromised in areas where there is colonization of extranodal tissues. Cytologically most cases in the dog are recognized at a grade II level when there is a mixture of centrocytes being the smaller cells with small or inapparent nucleoli and centroblasts that are the large more rapidly dividing population with multiple usually peripheralized nucleoli. The same proportion of centrocytes to centroblasts will be present in each nodule: that is the primary distinction between FL and follicular hyperplasia (Fig. 69.16 and Fig. 69.17). In addition, whereas tingible body macrophages are a characteristic of benign follicular

FIGURE 69.16 Lymph node from a dog with grade III follicular lymphoma; CD79a stain. With hematoxylin and eosin staining the definition of the follicles is lost and the node largely appears to have a diffuse proliferation except for follicles delineated near the node capsule. Note that in the upper right there is diffuse lymphoma proliferating outside of the node capsule with focal areas of diffuse progression acceptable in grade 3 lesions. A characteristic of follicular lymphoma, that distinguishes it from hyperplasia at the architectural level, is the complete absence of mantle cell cuffs. Note that follicular lymphoma cells of similar type extend right to the edge of each follicle. Detailed examination of the node reveals that small arterioles and post-capillary venules are all located between the nodules and never within them.

FIGURE 69.17 Lymph node from a dog with grade II follicular lymphoma. The cells in the upper half of the image are primarily centroblastic B cells of neoplastic follicles. A light stroma separates the follicle from the intrafollicular area below. There are two post-capillary venules in the internodal tissue that have heavy transmural cellular traffic. Note the variation in cell type in the interfollicular areas that includes plasma cells as well as small, medium, and large lymphocytes. Wright-Giemsa stain.

hyperplasia, they tend to be rare or absent in FL. Occasionally in areas where tingible body macrophages are still present these may be identified as areas of fading follicles.

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The WHO system of classifying FLs is a system of grading the proportion of centroblasts in each field. Basically, a level of 0–5 centroblasts per high power field (400×) is considered grade I, with 6–15 per high power field grade II, and more than 15 centroblasts per high power field is grade III. There is quite wide variation in grading between individuals. Because these diseases are so rare in animals, little effort has been made to count specific cell types other than to note that there is a general mixture present without centroblasts predominating. Follicular lymphoma in the spleen is largely undescribed in animals. In humans, FL tends to symmetrically involve the entire spleen. On the basis of the few cases described in animals, involvement of the spleen in FL is characteristically diffuse with involvement on the entire body of the spleen. In cat and mouse, the sinus areas are spared with the focal proliferations quite sharply defined. Follicular lymphoma in the spleen is characterized by complete loss of mantle cell cuffs and, as in lymph nodes, there is the same distribution of cell types in all regions of each follicle. The other organ most consistently involved is the liver. Follicular lymphoma in the liver tends to exclude areas of hematopoiesis from portal areas and similar changes are present in marrow with the follicles tending to be identified on low power examination by complete phthysis of fat cells and megakaryocytes. Follicular lymphomas are strongly positive with CD79a and CD20 and are negative with CD3. In most cases of animal FL, cytoplasmic immunoglobulin can be identified by immune staining and most can be shown to stain positively with lambda light chain and negative with kappa light chain, which is also a useful means of identifying clonality. It is well established in human pathology that FL is a result of neoplastic transformation of a post-germinal center cell. No specific characteristics have been identified for FLs in animals. In humans with FL, the heavy and light immunoglobulin chains are rearranged with the variable regions and there are extensive somatic mutations. In addition, there is extensive intraclonal diversity, indicating that mutations are ongoing similar to those that occur in benign germinal centers. In human FL, characteristic genetic alterations are chromosonal translocations of the long arms of chromosomes 14 and 18 with t(14;18)(q32;q21). A co-migrated segment that is transcribed is called BCL2 and this gene is over expressed in FL so providing a survival advantage by preventing the normal process of turning off the antiapoptosis gene. The major differential diagnosis for FL is benign or atypical follicular hyperplasia. In benign follicular hyperplasia there is usually some level of mantle cell cuff remaining, even in chronic follicular hyperplasia. A further distinction in follicular hyperplasia is that many of the follicles within the tissue have antigenrelated polarity that signals that these structures are responding normally to antigen stimulation. In follicular hyperplasia, the involvement of perinodal tissue is

frequently present but is almost never nodular or follicular as usually occurs in FL. Clinical Features Animals with FL, as is typical of all indolent lymphomas, retain normal appetite and activity and frequently present with advanced disease as a result of the owners noticing enlarged peripheral lymph nodes. Little is known about management of FL in animals, but the general management in humans appears logical. Animals having grades I–II lymphomas could be treated conservatively while those with grade III FL should be considered to have aggressive large B cell lymphoma and given multi-agent therapy. Nothing is known of risk factors for survival of animals diagnosed with FL. Changes considered important in predicting survival in human patients with FL are age, time since diagnosis, presence of nodes larger than 3 cm in diameter, and the number of extranodal sites involved. Negative factors accompanying these are the presence of B cell symptoms including night sweats and elevations of serum lactic dehydrogenase. Little is known about specific treatment for FL in animals. In humans, the major management therapy for patients in stage I or II of the disease is radiation which may provide relapse-free survival for 10 years or more. Alternatively, alkylating agents, including chlorambucil and cyclophosphamide, provide remission rates of 30–60% in previously untreated human patients. Newer strategies of therapy include anti-B cell antibodies. The current median survival of human patients with FL is about 9 years and after relapse about 4.5 years. In dogs with FL it is likely they have had the disease for up to 2 years before diagnosis and have a good chance of living 2–3 years after recognition and treatment. Diffuse Large B Cell Lymphomas Diffuse large B cell lymphomas (DLBCL) are tumors of large transformed lymphocytes with nuclei at least twice the size of two small lymphocytes.2,18,29 The nuclei can vary markedly in shape and the cells generally have vesicular chromatin 21,22 The cytoplasmic volume and staining density vary with the subtypes. DLBCL are heterogeneous and have variable numbers of proliferating cells with wide variation in antigenicity, molecular and genetic features, and clinical behavior.4,9 Diffuse large B cell lymphomas occur in all domestic animals, including birds, and are the largest and most frequent lymphoid neoplasms in most species including humans.33,44 In cats and cattle, DLBCL is associated with retroviral infection but lymphomas also occur in the absence of viral involvement. It is likely that in all species chronic benign lymphoid hyperplasia that occurs in association with a variety of chronic diseases may be a risk factor for malignant transformation of benign B cells.

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A histological study of 1,198 lymphomas in cattle, without using immunohistochemistry, found that 366 were large cell-type and 424 were of large cleaved celltype, together comprising 66% of the total cases. Characteristics of the population were not known, but because these were adult cattle sent for slaughter, most were assumed to be female and of dairy breed. In 136 cases of lymphomas in domestic pigs, 81 (60%) were of large cell-type and in 502 cases of canine lymphoma and leukemia there were 69 cases (14%) of large cell lymphoma including 19 cases of large cleaved cell-type. The median age of 47 of these dogs was 6.1 years and approximately equal numbers were male and female. Of 751 hematopoietic neoplasms of domestic cats, there were 314 (42%) cases of large cell lymphoma with 76 of these of large cleaved cell type, 184 cases of immunoblastic large cell lymphoma, and 115 cases of immunoblastic polymorphous type. In the total cat population, 58% of cats were male or neutered males with the remainder female or neutered females. The ages of cats with lymphoma ranged from 1 to 22 years. In 90 horses with lymphoid neoplasms there were 19 cases of diffuse large cell type and two cases of large cleaved cell type. In equines there was a slight majority of female cases, most of which were older than 10 years. Pathologic Features It is currently felt that neoplastic cells are always present in the blood with the conditions of spread determined by the intercellular adhesion molecules on the surface of malignant cells. In general, about 15% of cats with lymphoma are frankly leukemic with associated lymphocytosis. There is little prognostic value in identifying leukocytes in blood of an animal that has an established diagnosis of lymphoma and is not suffering any type of cytopenia. Examination of blood and marrow is an important and essential step in staging animals and predicting response to therapy. It is easiest to recognize neoplastic cells in blood when such cells are large, and have nuclei with prominent nucleoli and uniformly finely distributed chromatin. That said, it is important to note that the neoplastic cells in blood frequently differ morphologically from the neoplastic population in marrow or in other tissues. Bone marrow is always involved in DLBCL if the disease has progressed for some time. The manner in which neoplastic cells involve the marrow may vary, including diffuse, paratrabecular, and multifocal interstitial. With focal heavy involvement of marrow there is usually exclusion of fat cells and phthisis of normal marrow lineages that is indicative of tumor formation. The advent of immunocytochemistry now permits the identification of minor populations of neoplastic cells in bone marrow that are not identified on routine examination. A number of types of large B cell lymphomas are recognized in the recent WHO classification. Three of these are distinguished on the basis of their anatomic location including intravenous or intravascular, medi-

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astinal, and primary effusion types of lymphoma. A new entity based cytology alone, is the plasmablastictype that has general characteristics of plasma cells but with larger nuclei and nucleoli and a higher dividing fraction. Two subtypes of large B cell lymphoma are of mixed-type including the T cell-rich large B cell lymphoma and a closely related type known as lymphomatoid granulomatosis that tends to occur in the lung. In general, lymph nodes enlarged in large B cell lymphomas have diffuse architecture, frequent bridging of the node capsule, and colonization of perinodal tissues. Large cell lymphomas tend to compress and destroy medullary structures and are not associated with medullary sclerosis as is usually found in the indolent lymphomas that appear to develop after a long period of benign hyperplasia. A common division of subtypes of large cell lymphomas is on the basis of nucleolar number and position. With this classification the immunoblastic lymphomas (IB) have a large single central nucleolus while the centroblastic (CB) types have multiple peripheral nucleoli (Figs. 69.18 and 69.19). These were recognized as distinct types in the working formulation with the assumption that the immunoblastic-type was of higher grade; however, little difference was found between these two in subsequent trials. Currently it seems reasonable to divide lymphomas cytologically on the basis of nucleolar type until it is possible to determine if this distinction infers a survival advantage in treated animals. In a study of 1,017 canine lymphomas, there were 150 cases of large cell lymphoma of CB-type

FIGURE 69.18 Lymph node aspirate from a dog with diffuse large B cell lymphoma; Wright-Giemsa stain. The nuclei are 1.5–2 RBCs in diameter with densely stained chromocenters that encircle multiple largely central nucleoli. The nuclei of intermediate size have a similar chromatin pattern with multiple nucleoli that suggest these are part of the same neoplastic population. There are a number of small (benign) lymphocytes present that have densely stained nuclei similar in diameter or slightly larger than RBCs. The cytoplasm is relatively abundant and highly basophilic. There is a cell in mitosis at the center right margin.

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FIGURE 69.19 Lymph node from a dog with diffuse large B cell lymphoma. The large oval nucleus in the center with peripheralized nucleoli is a typical centroblastic nuclear type with mild parachromatin clearing. There is a mitotic cell in the lower left and an apoptotic cell in the upper right. There is more anisokaryosis in this field than is typically present with this subtype of lymphoma. Wright-Giemsa stain.

in which the mitotic rate was determined to be 12.13 per 400× field while in the same study 55 cases of IB-type had a mean mitotic rate of 8.7 mitoses per 400× field. These differences were statistically significant, suggesting that this distinction might relate to biological behavior. Cytologically, both the immunoblastic and centroblastic cell types tends to have round to oval nuclei 2–2.5 RBCs in diameter and coarse granular or branched chromatin patterns with mild parachromatin clearing. The cytoplasm is almost always moderate in volume and staining density with cell boundaries irregularly distinct. There are usually about the same number of apoptotic nuclei and mitoses as is seen in Burkitttype lymphoma but not as many tingible body macrophages. The plasmablastic variant of large B cell lymphoma occurs in dogs, cats, and horses. The distribution of the tumor can vary widely and involve almost all tissues including lungs and skin. Major distinguishing features of this type of lymphoma are large nuclei (>2.0 RBC diameters), marked variation in chromatin distribution, and very abundant eccentrically placed and highly amphophilic cytoplasm. A major difference between plasmablastic lymphoma and plasmacytoma is the presence of nucleoli and a higher mitotic rate in the plasmablastic subtype. There are some differences in homing patterns with the plasmablastic-type of lymphomas tending to mimic plasma cells in homing to medullary cords where they may be mistaken for benign plasma cell expansion if not examined in detail. The intravascular variant of large B cell lymphoma is rare. It may be found in any tissue but is most frequent in the central nervous system. Animals with this form of lymphoma usually present in good condition

with a rapid onset of nervous signs that begins with caudal paresis and rapidly ascends to convulsions and collapse with death occurring in 1–2 days after onset. Histologically, there is dilation of veins in the cerebral meninges and subdural vessels of the spinal cord. Oddly, the cells appear in clusters that distend vessels but do not appear to be attached to vessel walls; however, the tumor is not apparently leukemic. There is marked anisokaryosis with multinucleated and binucleated cells present. The nuclei are vesicular with peripheralized and branched chromatin patterns and multiple prominent nucleoli. The cytoplasm is moderate in volume and staining, and cell boundaries are generally distinct. Similar intravascular lymphomas may be found in the heart and lungs. Mediastinal large B cell lymphomas are rarely encountered or described but occur in dogs very similarly to what is described in humans. These lymphomas also occur in the cat and perhaps other species. In humans, the disease occurs in young adults with a female to male ratio of 3 : 1. Specific identification of the neoplasm is important because mediastinal large B cell lymphoma rapidly invades surrounding tissues. This disease is usually identified by ultrasound-guided fine needle aspiration and Tru-cut biopsy. Cytologically, the cells vary markedly in size and shape and are supported by a dense fine fibrovascular network that surrounds cells down to groups of two or three. Binucleated forms are occasionally present. The primary effusion-type large B cell lymphoma is rare in humans and has recently had a single report in animals. The disease is defined as an effusion of fluid containing neoplastic large B cells in serous cavities including the pleura, pericardium, or peritoneum in the absence of solid tumors in the underlying tissues. In humans, the disease is seen more frequently in immunosuppressed and post-transplant patients. Involvement of the spleen in large B cell lymphoma is initially multifocal and rapidly becomes coalescing. The multifocal involvement may occur as 1–2 mm foci grossly termed “sago spleen”, or the tumor may be a single large isolated mass. Focal large cell splenic lymphoma is an entity in the horse and the tumor may obtain a mass of 11.3–15.9 kg (25–35 lb). Diffuse involvement of the spleen is prominent in cows with lymphoma and rupture of the capsule may be the first indication that the tumor is present with the cow becoming recumbent due to anemia. In general, the larger the tumor the more likely it is to be associated with hemorrhage and infarction. Large B cell lymphoma frequently also involves the liver and kidney, but almost any other tissue in the body may be affected including uterus, skeletal muscle, tonsil and rarely bone. Large B cell lymphoma of domestic animals labels positively with CD79a and CD20 and is negative for CD3. Most large B cell lymphomas can be shown to have surface or cytoplasmic immunoglobulin and most are clonal for lambda light chain. For some subsets of lymphoma, CD20 is a more useful reagent than CD79a, particularly for plasmablastic lymphoma and T cell-rich large B cell lymphoma.

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Studies are now underway to specifically define the changes in B cell lymphoma of dogs that are unique to different breeds and subtypes of lymphoma.11 Remarkably, after years of comparing animal lymphomas to those in humans there is now interest in studying large cell lymphoma in the dog in order to gain information of comparative value in treating human lymphoma. This interest arises because it is easier to define a single nucleotide polymorphism (SNP) in relatively inbred purebred dogs than in outbred human species. In large cell lymphoma there are frequent chromosomal alterations involving the 3q27 band that apparently is a translocation involving regions of other chromosomes within the sites of immunoglobulin genes. The cell of origin is not known in animals but it can be assumed that this is roughly similar to humans where in large B cell lymphoma there is usually hypermutation of the IgH region compatible with origin from a post-germinal center cell. Clonality is now determined by polymerase chain reaction (PCR) application of the VDJ region in dogs and cats, and this is now routinely carried out in a number of institutions. Clinical Features Of 50 cases of large cell lymphoma described in dogs, the topography included generalized lymphadenopathy in 18, enteric or abdominal in nine, a single enlarged node in seven, multicentric in four, skin in four, tonsillar in three, spleen in two, mediastinum in two, and central nervous system in one. In 314 cats with large cell lymphoma, there were 85 cases with gastrointestinal involvement, 68 multicentric, 56 mediastinal, 35 in small intestine only, 25 renal, 20 with one or two nodes only, seven in nasal cavity, six with subcutaneous mass, three in skin, three in heart, two in urinary bladder, and one in liver. In 118 cases of equine lymphoma, there were 25 cases of large cell lymphoma with 20 of large centroblastic-type, four of immunoblastic-type, and one diffuse large cleaved-type. Presentation was not known in most of the horses, but one had generalized lymphadenopathy, one had splenic only, and four had enteric involvement. There was little information relating clinical signs to topography in these cases. About one-third of human patients with large B cell lymphoma have symptoms that include recurrent fever and weight loss. Large B cell lymphoma must be distinguished from lymphomas of similar type and T cell phenotype, with the variation in T cell types bridging most of the cytologic descriptions previously given. Large B cell lymphoma must also be distinguished from indolent lymphomas of intermediate subtypes like MZL. One of the most difficult diagnostic decisions is the distinction between large B cell lymphoma and Burkitt-type lymphoma that is of intermediate size and characterized by a very high mitotic rate and presence of numerous tingible body macrophages. One of the reasons this distinction is difficult is that there is a large variation in nuclear size in both large B cell lymphoma and in inter-

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mediate type Burkitt’s lymphoma, so that in morphometric comparisons there is overlap in nuclear volume but not in mean volume or cell size. Large B cell lymphomas generally progress with no change in cell morphology. There appears to be progression of changes at the molecular level that may relate to the progression and aggressiveness of tumor with time. Unlike benign B cells, where somatic hypermutation permits antibody diversity, in large B cell lymphoma, dysfunction of this process drives disease progression. Changes in the immunoglobulin genes appear to affect both non-translated as well as coding regions leading to changes in amino acid sequence and ultimately malignant transformation. Treated animals need to be restaged for subsequent management planning. In human cases, doxorubricinbased therapies are frequently used followed by radiation therapy. Even late-stage disseminated large B cell lymphoma in human patients has been followed by apparently long-term remission using combination chemotherapy, with CHOP being the most common initial regimen. Advancing age is considered an important negative factor in treatment of human patients with large B cell lymphoma; gene activation profiling of human lymphomas suggests that patients who have gene expression that mimics reactive germinal centers have a better survival profile than those whose gene profile reflects the pattern of activated blood lymphocytes. Treatment and survival is currently under study in animals without firm data. One-third of human patients with large B cell lymphoma have night sweats and irregular fever with weight loss at the time of diagnosis. An international prognostic index developed for large B cell lymphoma, divides human cases into stage 0–1 (35%), stage 2–3 (45%)and stage 4–5 (20%). The 5-year survival for stage 0–1 was 73% and for stage 4–5 was 26%. An almost equivalent estimate of survival was determined by measuring only serum lactate dehydrogenase and β-2 microglobulin levels. The rapidity of achieving complete response on first cycles of treatment is prognostically favorable in humans and apparently has equal value in animals. In humans, a high proliferative rate of the tumor cells tends to correlate with poor survival, while retention of a normally functioning immune system is correlated with longer survival. In humans, over expression of the bcl-2 protein is associated with more frequent relapse but not the presence of BCL-2 gene rearrangement. Burkitt’s Lymphoma Burkitt’s lymphoma is a high grade and multicentric B cell lymphoma characterized by diffuse architecture, heavy tingible body infiltration (starry sky appearance) at low magnification.8,16,22,25 Cytologically, cells are of intermediate size with vesicular nuclei, multiple nucleoli, moderate parachromatin clearing, and high mitotic rates. Burkitt’s lymphoma was the first human tumor shown to be curable by chemotherapy and the first shown to be associated with the Epstein-Barr virus

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(EBV). Burkitt’s lymphoma was also the first solid tumor to be associated with a non-random chromosomal translocation and also the first in which genetic deregulation was shown to be associated with an oncogene (c-myc). It was also recognized that the geographic distribution of Burkitt’s lymphoma in Africa was associated with both the presence of endemic malaria and the mosquito vector. Later the association of chronic immune stimulation related to malaria infection was shown to increase susceptibility to chromosomal injury by EBV. There are two forms of the Burkitt’s lymphoma. The classic type has very uniform cells, described as being of monotonous type (i.e. like “peas in a pod”): this form is very rare in animals. In contrast, the variant type (i.e. Burkitt-like lymphoma), also known as small noncleaved cell lymphoma, is very common in animals, particularly in dogs.

FIGURE 69.20 Lymph node with Burkitt’s-like lymphoma. Note the “starry sky” appearance. Wright-Giemsa stain.

Pathologic Features Because the cells of Burkitt’s lymphoma are of intermediate type, the only abnormality that is likely to be noticed in the blood is lymphocytes of intermediate size that have nucleoli, and these may be interpreted as reactive. The typical architecture of lymph nodes in Burkittlike lymphoma in dogs is of a multifocal proliferation that compresses normal cells between them.36 Benign areas are identified by more darkly stained cells, usually of smaller type, and with a higher number of RBCs present. The foci of proliferation are irregularly coalescing and are characterized by many tingible body macrophages in areas of apparent homogeneous cell proliferation. Cytologically, the cells are of intermediate type with round to oval nuclei 1.5 RBCs in diameter. There are irregular areas of chromatin distribution with parachromatin clearing and 1–4 small but prominent nucleoli. Cytoplasm is moderate in volume and staining density, with cell boundaries generally indistinct. There are usually more than 10 mitoses per 400× field. The numbers of apoptotic cells will almost always equal or exceed the number of mitoses with the number of tingible body macrophages usually 2–3 times the number of mitoses (Fig. 69.20). Nuclei can be oval but are not indented. This relates to this same population of cells being identified in the older working formulation as “small non-cleaved cells.” If excisional or incisional biopsy tissue is examined, there are fading germinal centers, usually only with a mantle cell cuff remaining in the subcapsular areas. The cortex appears diffuse with compression of medulla, and medullary sclerosis is not present. In the classical form of Burkitt’s lymphoma, the cells are of monotonously uniform-type lacking the mild anisokaryosis of the Burkitt-like lymphoma. The interface of the cells in fixed and processed tissues usually results in separation of cytoplasmic membranes along hexagonal lines, which is known as the “squaring-off” phenomenon typically seen in human Burkitt-type lymphomas. Architectural characteristics are similar,

although the Burkitt-type usually presents with solid tumor involvement with high tingible body macrophage infiltration, whereas the Burkitt-like lymphoma is more typical of the multifocal areas of proliferation with foci of coalescing tumor cells. The mitotic rate is similar in both tumor types. Other organs likely to be involved in the dog include the liver, mesenteric lymph nodes, and intestinal wall. Although the spleen may be involved, the histopathology has not been extensively described. Burkitt-type and Burkitt-like lymphomas stain strongly with CD79a and CD20, and are negative with CD3.1,15,26 Currently, specific information on molecular or chromosonal changes in Burkitt-type lymphoma in animals is lacking. Burkitt-type lymphoma in humans has the c-myc translocation, whereas the Burkitt-like variant in humans has variable staining for CD10 and Bcl-6 that is strongly positive in Burkitt-type lymphoma and stains positive for Bcl-2 which is not expressed in the classical form. Burkitt-type lymphoma and Burkitt-like lymphoma must be distinguished from acute lymphoblastic leukemia and lymphoblastic lymphoma.34,35 This is largely based on the presence of aggregated chromatin with parachromatin clearing and prominent small nucleoli in Burkitt’s tumors rather than the uniformly dispersed chromatin and obscured nucleoli of acute lymphoblastic leukemia and lymphoblastic lymphoma. The Burkitttype lymphoma is distinguished from Burkitt-like lymphoma on the basis of very uniform cell type in Burkitt-type lymphoma with mild anisokaryosis in the Burkitt-like subtype. Clinical Features About 20% of human Burkitt’s lymphoma cases have bone marrow involvement and oncologists are now finding that dogs also have early involvement of bone marrow. The general presentation of dogs with Burkitt’s

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lymphoma is of an animal in good condition that suddenly has reduced appetite and is noticed to have one or more enlarged peripheral lymph nodes. Unlike diffuse large B cell lymphoma, Burkitt-like lymphoma tends to present in dogs less than 10 years of age and generally 5–7 years old with no gender preference. Burkitt-like lymphoma, as well as Burkitt-type lymphoma, tends to be widespread and extranodal at the time of diagnosis. Careful staging is important to determine all sites of involvement. Restaging is essential to determine new or recurrent sites of tumor involvement. That is usually done by x-ray of the chest and ultrasound examination of the abdomen as well as digital palpation. In human cases, serum lactate dehydrogenase and IL-2 receptor levels correlate well with tumor burden and progressively rise in concert with increasing tumor cell volume. Bone marrow aspirate and core biopsy are indicated if the animals have a nonregenerative anemia and the anemia is not likely to be due to prior treatment. Treatment of Burkitt-like lymphoma appears to be one of the areas where veterinary oncology is making progress, with long-term survival exceeding 2 years in some cases. Human cases that present in early stages of the disease can be confidently cured with current treatments, and even cases that present as advanced have only slightly less probability of long-term survival.

15. 16. 17. 18. 19. 20. 21.

22. 23. 24. 25. 26. 27.

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Hodgkin’s Lymphomas, 2nd ed. Philadelphia: Lippincott Williams and Wilkins, 2004;367–388. Gabor LJ, Canfield PJ, Malik R. Immunophenotypic and histological characterization of 109 cases of feline lymphosarcoma. Aust Vet J 1999;77:436–441. Gaulard P, Delso, G, Callat MP, et al. Cytogenetic and clinicopathologic features of B-cell lymphomas associated with the Burkitt translocation T(8;14)(q24;q32) or its variants. Ann Oncol 2002;13(Suppl 2):33. Hamblin T. Autoimmune complications of chronic lymphocytic leukemia. Semin Oncol 2006;33:230–239. Hans CP, Weisenburger DD, Greiner TC, et al. Expression of PKC-beta or cyclin D2 predicts for inferior survival in diffuse large B-cell lymphoma. Mod Pathol 2005;18:1377–1384. Harris NL, Bhan, AK. B-cell neoplasms of the lymphocytic, lymphoplasmacytoid, and plasma cell types: Immunohisto logic analysis and clinical correlation. Hum Pathol 1985;16:829–837. Harris NL, Ferry JA. Follicular lymphomas. In: Knowles DM, ed. Neoplastic Hematopathology, 2nd ed. Philadelphia, Lippincott Williams and Wilkins, 2001;823–853. Harris NL, Jaffe ES, Diebold J, et al. World Health Organization classification of neoplastic disease of the hematopoietic and lymphoid tissues: Report of the Clinical Advisory Committee meeting Arlie House, Virginia, November 1997. J Clin Oncol 1999; 17:3835–3849. Harris NL, Jaffe ES, Stein H, et al. A revised European-American classification of lymphoid neoplasm: a proposal from the International Lymphoma Study Group. Blood 1994;84:1361–1392. Hercher C, Robain M, Davi F, et al. A multicentric study of 41 cases of B-prolymphocytic leukemia: two evolutive forms. Leuk Lymphoma 2001;42:981–987. Hui D, Reiman T, Hanson J, et al. Immunohistochemical detection of cdc2 is useful in predicting survival in patients with mantle cell lymphoma. Mod Pathol 2005;18:1223–1231. Hummel M, Bentink S, Berger H, et al. A biologic definition of Burkitt’s lymphoma from transcriptional and genomic profiling. New Engl J Med 2006;354:2419–2500. Hutchison RE, Finch C, Kepner J, et al. Burkitt lymphoma is immunophenotypically different from Burkitt-like lymphoma in young persons. Ann Oncol 2000;11(Suppl 1):35–38. Kelley LC, Mahaffey EA. Equine malignant lymphomas: morphologic and immunohistologic classification. Vet Pathol 1998;35:241–252. Kiupel M, Teske E, Bostock D. Prognostic factors for treated canine malignant lymphoma. Vet Pathol 1999;36:292–300. Kojima M, Nakamura S, Ichimura K, et al. Centroblastic and centroblastic/centrocytic lymphoma associated with a prominent epithelioid granulomatous response: a clinicopathologic study of 50 cases. Mod Pathol 2002;15:750–758. Lai R, Lefresne SV, Franko B, Hui D, et al. Immunoglobulin VH somatic hypermutation in mantle cell lymphoma: mutated genotype correlates with better clinical outcome. Mod Pathol 2006;19:1498–1505. Leifer CE, Matus RE. Lymphoid leukemia in the dog. Vet Clin N Am Small Anim Pract 1985;15:723–739. Merchant S, Schlette E, Sanger W, et al. Mature B-cell leukemias with more than 55% prolymphocytes. Arch Pathol Lab Med 2003;127:305–309. McDonough SP, Van Winkle TJ, Valentine BA, et al. Clinicopathological and immunophenotypical features of canine intravascular lymphoma (malignant angioendotheliomatosis). J Comp Pathol 2002;126:277–288. Mukhopadhyay S, Readling J, Cotte PD, et al. Transformation of follicular lymphoma to Burkitt-like lymphoma within a single lymph node. Hum Pathol 2005;36:571–575. Nakamura N, Nakamine H, Tamara J, et al. The distinction between Burkitt lymphoma and diffuse large B-cell lymphoma with c-myc rearrangment. Mod Pathol 2002;15:771–776. Ponce F, Magnol JP, Ledieu D, et al. Prognostic significance of morphological subtypes in canine malignant lymphomas during chemotherapy. Vet J 2004;167:158–166. Pui CH, Relling MV, Downing JR. Mechanisms of disease: acute lymphoblastic leukemia. New Engl J Med 2004;350:1535–1548. Ramos-Vara JA, Miller MA, Pace LW, et al. Intestinal multinodular A-amyloid deposition associated with extramedullary plasmacytoma in three dogs: clinicopathological and immunohistochemical studies. J Comp Pathol 1998;119:239–249. Reiniger L, Bodor C, Bognar A, et al. Richter ’s and prolymphocytic transformation of chronic lymphocytic leukemia are associated with high mRNA expression of activation-induced cytidine deaminase and aberrant somatic hypermutation. Leukemia 2006;20:1089–1095. Richter KP. Feline gastrointestinal lymphoma. Vet Clin N Am Small Anim Pract 2003;33:1083–1098. Sieler T, Dohner H, Stilgenbauer S. Risk stratification in chronic lymphocytic leukemia. Semin Oncol 2006;33:186–194. Soubeyran P, Debleb M, Tchen N, et al. Follicular lymphomas – A review of treatment modalities. Crit Rev Oncol Hematol 2000;35:13–32. Spangler WL, Kass PH. Pathologic and prognostic characteristics of splenomegaly in dogs due to fibrohistiocytic nodules: 98 cases. Vet Pathol 1998;35:488–498.

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44. Steinberg H. Multisystem angiotropic lymphoma (malignant angioendotheliomatosis) involving the humerus in a dog. J Vet Diagn Invest 1996;9:804–808. 45. Teske E, van Heerde P, Rutteman GR, et al. Prognostic factors for treatment of malignant lymphoma in dogs. J Am Vet Med Assoc 1994;205:1722–1728. 46. Tobin G, Rosen A, Rosenquist R. What is the current evidence for antigen involvement in the development of chronic lymphocytic leukemia? Hematol Oncol 2006;24:7–13. 47. Valentine BA, McDonough SP. B-cell leukemia in a sheep. Vet Pathol 2003;40:117–119. 48. Valli VE, Jacobs RM, Norri A, et al. The histologic classification of 602 cases of feline lymphoproliferative disease using the National Cancer Institute Working Formulation. J Vet Diagn Invest 2000;12:295–306.

49. Vernau W, Moore PF. An immunophenotypic study of canine leukemias and preliminary assessment of clonality by polymerase chain reaction. Vet Immunol Immunopathol 1999;69:145–164. 50. Vernau W, Valli VEO, Dukes TW, et al. Classification of 1,198 cases of bovine lymphoma using the National Cancer Institute Working Formulation for human non-Hodgkin’s lymphomas. Vet Pathol 1992;29:183–195. 51. Weisenburger DD, Vose JM, Greiner TC, et al. Mantle cell lymphoma. A clinicopathologic study of 68 cases from the Nebraska lymphoma study group. Am J Hematol 2000;64:190–196.

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Plasma Cell Tumors ANTONELLA BORGATTI Multiple Myeloma Epidemiology Etiology Pathogenesis and Disease Evolution Clinical Manifestations of Disease Diagnosis Treatment Prognosis

Solitary and Extramedullary Plasmacytic Tumors Clinical Manifestations of Disease Diagnosis Treatment Prognosis Waldenström’s Macroglobulinemia

Acronyms and Abbreviations BM, bone marrow; BMSC, bone marrow stromal cell; CCNU, 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (Lomustine); CD, cluster of differentiation; CEMP, cutaneous extramedullary plasmacytoma; CT, computed tomography; EMP, extramedullary plasmacytoma; FELV, feline leukemia virus; FIV, feline immunodeficiency virus; HVS, hyperviscosity syndrome; IFE, immunofixation electrophoresis; IFN, interferon; Ig, immunoglobulin; IL, interleukin; IRF, interferon receptor factor; IRF4, interferon receptor factor 4; MGUS, monoclonal gammopathy of unknown significance; MM, multiple myeloma; MRD, myeloma related disorders; MRI, magnetic resonance imaging; NCEMP, non-cutaneous extramedullar plasmacytoma; NFκB, nuclear factor kappa B; OAF, osteoclast activating factor; PBSC, peripheral blood derived stem cell; PET, positron emission tomography; PT, prothrombin; PTT, partial thromboplastin; RANK, receptor activator nuclear factor Kappa B; RANKL, receptor activator nuclear factor kappa B ligand; RBC, red blood cell; SOP, solitary osseous plasmacytoma; SPB, solitary plasmacytoma of bone; SSA, sulfosalicylic acid; TNF-α, tumor necrosis factor-α; VAD, vincristine adriamycin (doxorubicin) prednisone; VBAP, vincristine BCNU adriamycin (doxorubicin) prednisone; VBMCP, vincristine BCNU melphalan cyclophosphamide prednisone; VEGF, vascular endothelial growth factor; VEGF-R1, vascular endothelial growth factor receptor 1; WM, Waldenström’s macroglobulinemia.

P

lasma cell tumors represent a spectrum of diseases characterized by clonal (predominantly monoclonal) proliferation of immunoglobulin (Ig)producing plasma cells or B cells.21 They encompass clinically indolent conditions such as monoclonal gammopathy of unknown significance (MGUS) and Waldenström’s macroglobulinemia (WM) as well as malignant processes, including multiple myeloma (MM) and solitary plasmacytoma (solitary osseous plasmacytoma and extramedullary plasmacytoma). All these disorders share common cytomorphologic features, Ig production patterns, and immune dysfunction. Five major classes of Ig: IgG, IgA, IgM, IgD, and IgE, are synthesized by normal B cells and plasma cells, whereas atypical plasma cells secrete one of these molecules or, occasionally, only light chain molecules.

When the production of heavy and light chain molecules is unbalanced, the excess of light chain proteins is excreted in the urine (i.e. Bence Jones proteinuria; see Chapter 145). In non-secretory-type myeloma, plasma cells do not secrete these proteins. MULTIPLE MYELOMA Epidemiology Multiple myeloma (MM) is estimated to account for 100,000 cells/μL at presentation is associated with inferior survival,88 and early precursor phenotypes are indicators of poor prognosis.31,35 Lymphoblastic T Cell Lymphoma Lymphoblastic T cell lymphomas are highly aggressive tumors of immature T cells that are seen more commonly in children and young adults. Almost all lymphoblastic lymphomas in humans are LBTL.75 Anterior (cranial) mediastinal lymphoma appears to be a relatively common, unique entity within the T cell ALL/ LBTL complex. Lymphoblastic T cell lymphoma originates from a precursor cell and can, therefore, have CD4/CD8 double negative or double positive phenotypes.63,84 Similar to T cell ALL, surface CD3 expression can be absent, making the detection of cytoplasmic CD3 necessary. The tumors show diffuse effacement of nodal architecture, invasion into the subcapsular sinus and often extension into perinodal tissues by a cytologically bland population (Fig. 72.1). Despite their description as blasts, cells are of intermediate size, have round to indented euchromatic nuclei with indistinct nucleoli, and scant to modest amounts of lightly basophilic cytoplasm (Fig. 72.2). Unlike low-grade T cell lymphomas, these tumors can exhibit high mitotic activity and prominent staining with proliferation-specific markers such as Ki-67. A recent study showed that the CDKN2 locus that encodes the cyclin kinase inhibitors p16 and p15, and the p53 regulator p14/ARF, was deleted in 10/10 highgrade canine T cell lymphomas, including 7/7 LBTL.60 In contrast, this abnormality was not present in any of 11 low-grade T cell lymphomas (T zone lymphoma [TZL] or small lymphocytic lymphomas) or 26 highgrade and low-grade B cell lymphomas. These tumors also are genetically unstable, harboring numerous additional cytogenetic abnormalities.110,193 The mechanisms that account for genetic instability in these tumors are unknown, but recent gene expression profiling experiments suggest canine LBTL resembles other high-grade tumors, showing enrichment of genes that control metabolism, proliferation, and survival at the expense of genes that comprise functional ontogenetic pathways (A. Frantz, T.L. Phang, J.F. Modiano, unpublished data).

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FIGURE 72.1 Lymphoblastic T cell lymphoma in a lymph node from a dog. Notice diffuse effacement of nodal architecture and invasion into the subcapsular sinus by a monomorphic population of lymphocytes. H&E stain; original magnification 40×.

FIGURE 72.2 Lymphoblastic T cell lymphoma in a lymph node from a dog. Notice pleomorphic cells, generally of intermediate size with indented euchromatic nuclei, and modest amounts of lightly basophilic cytoplasm. In this case, nucleoli are apparent in many cells, a rare feature in these tumors. H&E stain; original magnification 500×.

Therapy for LBTL in dogs is generally unrewarding unless it is confined to the mediastinal space. The median survival with cyclophosphamide, vincristine, doxorubicin, prednisone (CHOP)-based chemotherapy for the dogs described above with LBTL and CDKN2 deletion was less than 4 months, and none of the dogs survived 1 year.60 Cranial Mediastinal Lymphoma In humans, LBTL of the mediastinum is a defined subtype of precursor T cell lymphoma that usually

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presents as an anterior mediastinal mass with or without blood and bone marrow involvement.53,117 In domestic animals, primary mediastinal lymphoma (also known as thymic lymphoma) has been reported in the dog, cat, horse, cow, ferret, and pig, most commonly as a T cell disease,29,63,66,99,112,113,140,141,217 although this diagnosis may include a more heterogeneous group of diseases. Incidence data are not published in all species, but cranial mediastinal lymphoma is considered, respectively, as the most common cause of thymic pathology in the cat and the second most common cause of thymic pathology in the dog.41 This is also the most common thoracic neoplasm in the horse.122,187 There may be heritable predisposition in the Boxer breed,121 in Siamese cats (specifically as a form of juvenile thymic lymphoma not associated with FeLV infection),67,120 and perhaps in golden retrievers. Most cases of mediastinal LBTL in humans occur in adolescents and young males, and they are the most common mediastinal tumors in children.8,185,199 Mediastinal lymphoma does not have an age predilection in dogs,41 but it is most commonly seen in younger cats.41,120 In cows, mediastinal LBTL is one of three forms of sporadic bovine lymphoma (not associated with BLV infection), and is most common in animals between 6 months and 2 years of age.217 As is noted for other peripheral lymphomas outside the gut, primary mediastinal lymphoma has become less common in cats as the prevalence of FeLV infection has decreased. Prior to 1980, mediastinal lymphoma was reported to account for up to 40% of feline lymphoma in the United States, 50% in the UK, and 70% in Japan.33,81,177,189 More recently, mediastinal lymphoma was reported to represent less than 12% of cases in the United States and approximately 25% of cases in Australia.30,67,120,198 In people, there can be immunophenotypic overlap between cells from LBTL and cells from other non-lymphomatous lesions in the mediastinum, most notably those from cases of thymoma and thymic hyperplasia. As such, a diagnosis of LBTL requires the identification of a thymocyte-like (i.e. CD4/CD8-double positive) population demonstrating loss of one or more of the normal thymocyte cell surface antigens (CD2, CD3, CD5, CD7, and/or CD45), disproportionately high numbers of CD4/CD8-double negative, CD4-single positive, or CD8-single positive cells, or expression of CD34 by an atypical population of mature thymocytes.78,117 Flow cytometric light scatter also can help distinguish neoplastic LBTL cells from normal thymic components.78,117 The diagnostic challenge of mediastinal LBTL is magnified in animals. Imaging alone is insufficient to distinguish lymphoma from other mediastinal lesions such as thymoma, carcinoma, chemodectoma, and sarcoma.157,218 Fine needle aspirate (FNA) cytology is useful as an adjunct to diagnosis, but distinguishing thymic lymphoma from thymoma in FNA samples from dogs is difficult because of the large number of non-neoplastic lymphocytes present in the latter.9,112

Lymphoblastic T cell lymphoma cells in mediastinal lymphomas of domestic animals can also show phenotypic heterogeneity, although they frequently have a single positive CD3, CD4 phenotype in dogs.63,112,121,134 In cats, the majority of cases of feline anterior mediastinal lymphoma consist of CD3, CD5, CD8-positive T cells.66,116,148,177,198 In cows, anterior mediastinal lymphoma is a CD2, CD3, and/or CD5-positive T cell disease, most commonly of CD4/CD8-double negative cells.4,169,217 Similar characterization is lacking in other species. MATURE T CELL NEOPLASMS T cell Large Granular Lymphocytic Leukemia T cell large granular lymphocytic (LGL) leukemias account for 2–3% of small lymphocytic leukemias in people.144 It is typically a disease of older adults (median age 61 years) with equal gender distribution.50 Autoimmune diseases including rheumatoid arthritis and Sjogren’s syndrome, and detection of anti-nuclear antibodies are reported as common co-morbidities with T-cell LGL leukemia.50,144 Anti-platelet antibodies, antineutrophil antibodies, and positive Coombs’ tests have been reported in subsets of people with LGL leukemia.50 T cell LGL leukemia is typically an indolent disease in people, and a significant proportion of affected patients do not require therapy at diagnosis. With treatment, median survivals of 161 months have been reported.50 T cell LGL leukemia has been reported in the dog, cat, horse, and cow, with much of the published literature focusing on the first two species.107,109,163,167 Large granular lymphocytic leukemia of the dog and cat is most commonly a T cell disease, as more than 90% of cells in lesions from each are of CD3-positive T cell origin.163 In the dog, T cell LGL leukemia is reported to be a common form of CLL, representing 54% (39/73) of cases in one study. It is a clinically heterogeneous disease in dogs with a broad spectrum of presentations and outcomes.126 In canine LGL leukemia, neoplastic cells are commonly seen in the blood.126 Infection with Ehrlichia canis needs to be excluded as it can result in a selective increase in circulating LGLs.210 In contrast, feline LGL malignancies are typically aggressive. Gastrointestinal tract-associated conditions are frequent with fewer than 10% of cats presenting with a leukemic component.109 Median survival time in treated cats has been reported to be 57 days.109 Clinical features in the dog are variable; splenomegaly (12 of 20 dogs) is reported as the most common physical examination finding.127 In contrast, feline LGL is typically characterized by anorexia, lethargy, weight loss, and vomiting, with an abdominal mass, described in 52% of cases, reported as the most common physical examination finding.109,163 A diagnosis of LGL leukemia in the dog and cat is generally made through the cytologic detection of the characteristic neoplastic cell in blood or tissue. These cells are typically large (10–35 μm) and pleomorphic

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with nuclear variability and a variable number of irregularly shaped and sized magenta, intracytoplasmic granules. The majority of feline cases of LGL disease are composed of CD18 (21/21), CD3 (19/21), homodimeric CD8 (13/21), and CD103 (11/19)-positive cells, consistent with intestinal origin.163 Rare, CD4 single positive, and CD4, CD8 double-positive cases have been reported.163 In the dog, T cell LGL is described as a phenotypically heterogeneous disease in which CD3, leukointegrin αdβ2-positive cells predominate (>90% of cases), with variable TCR expression. The leukointegrin expression has been proposed to indicate that canine LGL leukemia originates in the splenic red pulp with bone marrow involvement only seen in the advanced stages. An etiology has not been demonstrated for domestic animal T cell LGL leukemia, as the vast majority of affected cats are negative for FeLV and FIV infection.109,163 In dogs, a single report describes the presence of retroviral particles, although the significance of this remains uncertain.72

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FIGURE 72.3 Chronic lymphocytic leukemia (CLL) in a blood smear from a dog. Notice an expanded population of small lymphocytes characterized by round to convoluted nuclei without nucleoli and scant light cytoplasm. In this case, cells are agranular, but azurophilic granules are commonly seen in CLL cells. Diff-Quik stain; original magnification ×560.

T Cell Chronic Lymphocytic Leukemia/ Prolymphocytic Leukemia and Small Lymphocytic Lymphoma In the late 1980s, the French-American-British (FAB) classification system subdivided mature T cell leukemias into four categories; T cell chronic lymphocytic leukemia (CLL); T cell prolymphocytic leukemia; human T lymphotropic virus type 1-positive (HTLV-1) adult T cell leukemia/lymphoma; and Sézary syndrome.13 The original category of T cell CLL was updated in 2001 to T cell prolymphocytic leukemia (TPLL) where leukemias with small, mature-appearing lymphocytes are now considered as a small cell variant of prolymphocytic leukemia. There is no consensus for the use of prolymphocytic leukemia versus CLL in domestic animals, and CLL is used mostly by convention to describe an expanded population of small lymphocytes in the circulation. When the disease is confined to lymphoid organs, it is called small lymphocytic lymphoma (SLL). The characteristic cell in human TPLL is described as a small to medium sized prolymphocyte with a prominent single nucleolus and basophilic cytoplasm. In domestic animals, T cell CLL is characterized by small to medium sized cells with round to oval nuclei, smooth chromatin, and scant to moderate clear to lightly basophilic cytoplasm that may contain azurophilic granules (Fig. 72.3). In humans, the median age at diagnosis of TPLL is 63 years.46 In contrast to the more common B cell CLL, TPLL commonly presents with peripheral lymphadenopathy, splenomegaly, and peripheral lymphocyte counts greater than 10,000/μL; a subset of patients will present with cutaneous involvement, particularly of the head and neck.34,125 In animals, T cell CLL manifests as an indolent disease that is often diagnosed incidentally. Unlike acute leukemias, bone marrow infiltration is

inconsistent and peripheral cytopenias are rare, although splenomegaly and lymphadenopathy are reported in up to 30% of affected dogs.115,126,213 The phenotypic characteristics of TPLL in humans and T cell CLL in dogs are consistent with post-thymic origin. In humans, TPLL is most often associated with a helper/inducer phenotype (CD4),46 and while it is possible some of these tumors arise from CD4/CD25/ FoxP3 regulatory T cells, this manifestation may be more common in the Sézary syndrome (see below). In dogs and cats, CLL is most often a disease of T cells, including large granular lymphocytes.197,205,216 Canine T cell CLL usually involves expansion of a CD8 clone, whereas in cats and horses, CLL often comprises CD4 T cells.37,130,158,216 A peculiar feature of TPLL in humans and CLL in dogs and cats, is aberrant expression of cell surface markers.94 For example, in a study of 87 T cell tumors, Gorczyca et al. found that complete loss of any T cell antigen (CD2, CD5, CD7) or the pan-leukocyte antigen CD45 was diagnostic for malignancy.79 Loss of CD45 appears to be the most common form of aberrant antigen expression in canine T cell CLL, and loss of CD4 and CD8 also may occur in some cases of canine and feline T cell CLL (Anne Avery, personal communication).205,213 Management strategies for CLL are discussed in Chapter 63. Recent data suggest that the phenotype and number of circulating cells are prognostic in dogs. In one study, dogs with less than 30,000 lymphocytes/μL in the peripheral circulation had significantly longer survival (mean = 1,098 days) than dogs with more than 30,000 lymphocytes/μL (mean = 131 days).213

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Enteropathy-type T Cell Lymphoma/Intestinal T Cell Lymphoma Enteropathy-type T cell lymphoma (ETTCL) is a relatively rare condition that accounts for approximately 5–10% of NHL and 1–4% of all gastrointestinal tumors in people.51 In 2001, the WHO classification of intestinal T cell lymphoma was modified to include ETTCL because it was deemed more specific, although ETTCL can occasionally arise from extra-intestinal sites.26 O’Farrelly originally coined the term enteropathyassociated T-cell lymphoma to denote the association of this lymphoma with jejunal mucosal atrophy.144 Enteropathy-type T cell lymphoma is now recognized as a tumor that affects the gastrointestinal tract causing villous atrophy and crypt hyperplasia, with involvement of the regional lymph nodes, and potentially the spleen and liver. The small intestine is the primary affected site, but this form of lymphoma also can involve stomach and colon. Males tend to be overrepresented, and the disease has a worse prognosis than intestinal lymphomas arising from B cells.104 Most ETTLs are believed to originate from intraepithelial CD8 (CD103-negative) T cells with large anaplastic morphology; however, about 20% arise from small, atypical, cytotoxic lymphocytes or NK T cells that express CD3, CD8, CD56, and CD103, and that do not cause enteropathy-related lesions.219 While these tumors may have distinct clinicopathologic features,139 they are both characterized by aggressive biological behavior.3 Enteropathy-type T cell lymphoma has not been described as a specific entity in animals, although intestinal T cell lymphoma is relatively common in both epitheliotropic (lymphocytes infiltrate the epithelial layer) and non-epitheliotropic (lymphocytes do not cross the basement membrane) forms that also can involve one or more areas in the gut, regional lymph nodes, spleen, and liver. In the dog, intestinal T cell lymphoma accounts for more than 70% of intestinal lymphoid tumors and is frequently epitheliotropic, but the immunophenotype of these tumors (T cell versus B cell) was not prognostically significant in the small number of cases examined to date.64 In cats, the reduced incidence of retroviral-induced lymphomas has led to an apparent increase in the frequency of alimentary lymphoma. Almost 40% of feline lymphomas from a retrospective study spanning 20 years affected the gastrointestinal tract, and the authors noted an increase in the incidence of intestinal lymphoma in the last 10 years of the study.120 However, these data should be interpreted cautiously given the limitations of case-control and retrospective studies. The majority of alimentary lymphomas in cats have been described as having a B cell phenotype; yet, recent reports indicate tumors likely to have originated in the small intestine are more commonly T cell lymphomas.66,91,147,149,208 Epitheliotropism is seen in most lowgrade feline tumors.24,108 When cats with increased large granular lymphocytes in the circulation or in aspirates of abdominal organs were examined, they uniformly

had T cell intestinal lymphoma arising from CD8-single positive cells, consistent with their putative intraepithelial lymphocyte origin.162 The pathogenesis of ETTCL in humans and intestinal T cell lymphoma in animals is incompletely understood. There seems to be a spectrum of infiltrative, lymphoplasmacytic infiltrative lesions that range from mild inflammation with a diagnosis of inflammatory bowel disease (IBD) to effacement of intestinal architecture by monomorphic populations that are undoubtedly lymphoma. There may be a progression from antigen driven lymphocytic inflammation to lymphoma. For example, 5–10% of patients with an inflammatory enteropathy called celiac disease (“gluten allergy”) are refractory to gluten withdrawal therapy, and patients with refractory celiac disease are at high risk to develop intestinal T cell lymphoma, suggesting these tumors may originate from intraepithelial lymphocytes expanded as part of the inflammatory response.204 A similar progression of IBD to intestinal lymphoma has been hypothesized to occur in cats.24,155 For example, one cat diagnosed with alimentary lymphoma survived 28 months with conservative management using prednisone and a novel protein diet.24 Clinical signs associated with ETTCL are referable to gastrointestinal dysfunction and depend on the anatomical site and extent of infiltration. Although morphologically the cells are “bland” in appearance, the disease is highly aggressive in people, with one study reporting less than 20% 5 year survival and another study reporting less than 30% 2 year survival.38,68 Clinical signs of intestinal lymphoma in animals also are referable to the anatomic site and extent of infiltration. Paraneoplastic eosinophilia and eosinophil infiltration of the tumor have been reported in canine, feline, and equine T cell intestinal lymphoma. In humans, eosinophil production and recruitment are associated with tumor production of interleukin-5 (IL-5),11,18 but such association remains to be documented in veterinary species. Diagnosis of ETTCL or intestinal lymphoma requires complementary use of clinical signs, imaging, and microscopic pathology. Aspiration cytology of thickened bowel or enlarged mesenteric lymph nodes often can lead to a diagnosis if a uniform, atypical population of lymphocytes is seen. The cytomorphology of human ETTCL is highly variable. The cells are generally medium-sized to large but in a subset of cases, they can be anaplastic and express surface CD30, a characteristic of anaplastic large cell lymphoma (ALCL).56 Morphologic distinction between lymphocytic inflammation and lymphoma of the gut can be challenging, underscoring the utility of immunophenotyping and clonality assays.39,208 High-grade alimentary lymphomas are poorly responsive to therapy, especially because gastrointestinal signs can be intractable. Indolent intestinal T cell lymphomas in dogs and cats can be successfully managed with low-intensity chemotherapy, but lack of prognostic indicators makes this disease a therapeutic challenge.

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Cutaneous T Cell Lymphoma Cutaneous lymphoma (LPD) are relatively uncommon peripheral neoplastic diseases that are, in both humans and veterinary patients, characterized by a widely heterogeneous clinical presentation. Cutaneous lymphomas represent 1% and 2.8% of all canine and feline skin tumors, respectively.76 Cutaneous lymphoma is predominantly a disease of older animals, with a mean age of 9–12 years in the dog, 8–17 years in the cat, and 3–19 years in the horse.40,43,54,70,87,136,137,153,173,195,215 In cattle, cutaneous lymphoma most commonly develops in animals between 2 and 3 years of age. In no species has a sex predilection been identified. In veterinary medicine, cutaneous LPD is most commonly subdivided into epitheliotropic and non-epitheliotropic forms, based on the presence or absence of invasion of the epithelium of the epidermis and/or adnexae.129,172 In contrast, cutaneous lymphoma in humans comprises a broader spectrum of conditions including T cell, NK cell, and B cell neoplasms as well as immature hematopoietic malignancies and Hodgkin’s lymphomas.21 The WHO classification scheme considers these as independent entities, and the presence or absence of epitheliotropism is only one part of a multifaceted diagnostic algorithm that includes clinical presentation, histology, immunophenotyping, and molecular analysis.21 Despite these classification differences, there are many similarities between the cutaneous LPDs of humans and domestic animals. Most notably is the observation that most arise from T cells, and are therefore denoted as cutaneous T-cell lymphomas (CTCLs).40,137 Here, we have used the WHO classification scheme as a framework to organize the descriptive properties of CTCL in animals, recognizing that not all of the current veterinary literature fits neatly into this scheme. Those entities within veterinary medicine with WHO counterparts are: mycosis fungoides (MF), pagetoid reticulosis, and Sézary syndrome (SS). Mycosis Fungoides Mycosis fungoides (MF), in both humans and most domestic species, is the most common form of CTCL.19,21,40,137 It is characterized by invasion and effacement of the epidermal and/or adnexal epithelium by neoplastic cells, a property which has led to its more descriptive designation, epitheliotropic cutaneous lymphoma (ECL). Mycosis fungoides is reported in dogs, cats, horses, ferrets, cows, hamsters, squirrels, and a coati.87,89,129,137,164,171,175,180,195 It is the most common cutaneous LPD in the dog and cow.19,40 Briards, English cocker spaniels, bulldogs, Scottish terriers, and golden retrievers may be predisposed to develop cutaneous lymphoma.76 In both humans and domestic animals, an underlying etiology for MF remains to be identified. In humans, retroviruses, environmental exposures, genetic mutations and infection with Staphylococcus aureus and

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Chlamydia spp. have been implicated, but studies have yielded inconsistent results.1,73,90 In the cat, attempts to link retroviral infection to MF have been unsuccessful.23,57,136,142,173,195 An infectious cause of MF also has not been identified in dogs, although retroviral particles have been described in a cell line.71 In the cow, the majority of cases of MF are manifestations of sporadic bovine lymphoma, which is not associated with BLV infection. In addition to infectious, environmental, and genetic causes, a link between atopic dermatitis and MF has been proposed in humans based on the common phenotype of the infiltrating lymphocytes in both diseases (CD3/CD4/CD45RO- TCRαβ-positive and CD8/CD30negative memory T cells), an overlap in their cytokine profiles, and increased circulating levels of IgE in patients with MF.127,156,174 Although the results of such human studies are mixed, work in dogs supports a similar hypothesis. In addition to the phenotypic similarities between the lymphoid population in canine atopic dermatitis and MF, canine MF predominantly develops in anatomic areas affected with allergic disease.52 Moreover, a retrospective study of canine MF and atopic dermatitis suggests that there is an incidence association between the two diseases.127,168 The clinical presentation of MF is heterogeneous, and has been described in detail elsewhere.80,176 Generally, it involves an exfoliative dermatitis that is refractory to therapy. In humans, MF is most commonly a disease of mature, memory helper T cells (a CD2, CD3, CD4, TCRαβ, CD5, CD45RO-positive phenotype), while only rare cases of CD8 and TCRαβ MF have been reported.21 Although particularly aggressive cases might originate from regulatory T cells.83 In contrast, MF in dogs is predominantly a disease of cytotoxic CD8, TCRαβ-positive T cells, with few CD4-positive or CD4/CD8-double negative cases reported.58,137 A high-grade variant of CTCL in humans originates from anaplastic large cells that express CD30. The authors have yet to encounter a homologous CD30positive T cell-derived ALCL but the morphological features of cutaneous ALCL have been described.200 Intriguingly, the majority of MF cases in the dog do not express CD5.135 Information about feline MF is limited, but this also appears to be primarily a disease of CD3-positive T cells.22,23,96,142,173,195 Expression of perforin, a cytotoxic protein stored in the cytoplasm of cytotoxic T cells and NK cells, was documented in one case,118,142 but others state that CD4-, CD8-double negative cells predominate in this disease.80 Mycosis fungoides in horses and cows is also poorly defined.19,36,87,100,153,170,181 Skin biopsy with histopathology is required to diagnose MF in people and domestic animals, although neoplastic lymphoid cells that merit including MF in a differential list can be identified in fine needle aspiration samples. The pathognomonic features of MF include the presence of a population of neoplastic round cells demonstrating selective tropism for the epidermal, mucosal, and/or adnexal epithelium. Additionally, small aggregates of intraepithelial lymphocytes (i.e.

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Pautrier ’s microabscesses) can occasionally be identified throughout the epidermis.80 In early lesions, light microscopic diagnosis may be difficult due to the presence of an infiltrate that contains relatively few overt neoplastic cells. As the disease progresses, the cellular infiltrate becomes more homogeneous, with small cells that resemble those seen in T zone lymphomas. Adjunctive diagnostic modalities such as immunohistochemistry and molecular analysis may be necessary to diagnose MF.59 Pagetoid Reticulosis Pagetoid reticulosis is a low-grade variant of MF with characteristic histomorphologic appearance.21 Microscopically, pagetoid reticulosis is characterized by prominent epitheliotropism of vacuolated neoplastic cells into a thickened epithelium.21 In contrast to MF, pagetoid reticulosis has been described as a form of epitheliotropic lymphoma in which the neoplastic population is confined almost completely to the epidermal layer and in which the underlying dermis is infiltrated by a pleocellular, inflammatory infiltrate.80 A single case of pagetoid reticulosis is reported in a dog.95 Non-epitheliotropic Lymphoma In the WHO classification, non-epitheliotropic cutaneous lymphoma (NECL) is not recognized as a specific entity, but rather, the lack of significant epidermal invasion is a histologic feature of various diseases such as subcutaneous panniculitis-like T cell lymphoma and other forms of primary, cutaneous T cell lymphoma.21 NECL has been reported in dogs, cats, horses, and cows, and it appears to be the least common form of cutaneous lymphoma in the dog and the most common form in the cat and cow.43,48,77,106,159,178,196 The etiology of NECL is unknown,23,57 and the clinical features resemble other types of CTCL. The cellular phenotypes in NECL also are similar to those of MF and other CTCL, consisting primarily of CD3-positive T cells, which in the case of the dog can be CD8-single positive or CD4/ CD8-double negative cells.40,136 In the cat, most published cases described CD3-positive neoplastic cells.40,106 In the cow and horse, limited data also indicate that NECL is a T cell neoplasm.19,43,175 Microscopically, this disease is characterized by infiltration of the dermis and subcutis by a population of neoplastic round cells with relative sparing of the epithelium of the epidermal and adnexal structures.80 As is true for ECL, histopathology is necessary to demonstrate the presence or absence of epitheliotropism.9 Sézary Syndrome In the WHO-European Organization for Treatment and Research of Cancer (EORTC) classification scheme, Sézary syndrome (SS) is described as the leukemic form of CTCL.21 The disease is characterized by the presence of neoplastic T cells (Sézary cells) in the blood and

FIGURE 72.4 Sézary cells in a blood smear from a dog. Notice a population of pleomorphic lymphocytes characterized by round, lobulated, or complex nuclei with coarsely stippled to reticular chromatin and moderate amounts of vacuolated, lightly basophilic cytoplasm. Sézary cells in blood tend to show significantly greater pleomorphism than MF cells in the skin. Diff-Quik stain; original magnification ×500.

lymph nodes (Fig. 72.4). It is a very rare manifestation of CTCL and, in domestic animals, has been reported in dogs, cats, and horses.16,150,160,173,194,215 The clinical presentation of human SS includes generalized reddening and scaling of the skin (erythroderma) with lymphadenopathy. In animals, SS causes exfoliative, erythematous dermatitis that can cause severe pruritus and lymphadenopathy.61,80,173,215 In humans, the cellular infiltrate arises from mature helper T cells with the same phenotype reported for MF.21 There is limited information on the immunophenotype in animals, but two reports in dogs suggest either a prototypical MF phenotype (i.e. CD3, CD8-positive) or NK-like phenotype (CD3, CD4, CD8, TCR-negative).71,215 In cats, there is a single case report where cells in the circulation were CD3+, CD8+, CD4− T cells.215 In humans, the International Society for Cutaneous Lymphoma criteria for a diagnosis of SS are: an absolute Sézary cell count of at least 1,000 cells/μL; an expanded CD4+ T cell count resulting in a CD4/CD8 ratio of more than 10 where the cells may lack expression of CD2, CD3, CD4, or CD5; or the demonstration of a T cell clone in the blood by molecular or cytogenetic studies.209 Explicit criteria have yet to be established in veterinary hematopathology; thus, a diagnosis of SS in domestic animals relies on the identification of: (1) cutaneous lesions consistent with MF, and (2) circulating neoplastic T cells. T Zone Lymphoma T zone lymphoma as a distinct disease entity was introduced in the Kiel classification of 1974, but abandoned in the most recent WHO classifications because of

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FIGURE 72.5 T-zone lymphoma in a lymph node from dog. Notice an expansion of lymphocytes in the parafollicular zone, residual follicular structures, and compression of the peripheral sinus. Also notice the prominent small vessels interlaced within the tumor. H&E stain; original magnification ×40.

significant morphologic and biologic overlap with angioimmunoblastic T cell lymphomas (AITL) and other indolent T cell lymphomas. Retaining this category in the classification of domestic animal lymphomas appears to have merit in the case of the dog, as TZL may be the most common indolent lymphoma at least in some breeds.60,200 The disease is characterized by expansion of small to intermediate lymphocytes in the parafollicular zone, leading to follicular collapse (i.e. fading follicles), compression of the peripheral sinus and some degree of medullary sclerosis (Fig. 72.5). In contrast to indolent B cell lymphomas arising from the marginal zone, the cellular population responsible for TZL is eccentric to the follicles, rather than surrounding them. The main distinction between the TZL and AITL is the prominent vascular component in AITL, although these two diagnoses may represent a spectrum of the same disease. Morphologically, TZL cells have few distinguishing characteristics: they tend to have nuclear clefts or indentations, smooth to stippled chromatin that may form prominent chromocenters, indistinct nucleoli, scant, clear to amphophilic cytoplasm, and low mitotic activity (Fig. 72.6). T zone lymphomas generally have a helper (CD4) phenotype and seem to display slow clinical progression, but controlled studies are lacking to determine the most appropriate therapy for this disease. It is clear that TZL does not have the same poor duration of response and short patient survival commonly associated with high-grade T cell lymphoma. In fact, a recent study showed that, among 40 canine lymphomas, these tumors had by far the best response to standard care.60 Furthermore, these tumors may respond even better to conservative management with low intensity chemotherapy, although data to support this are only anecdotal.

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FIGURE 72.6 T-zone lymphoma in a lymph node from a dog. Notice the typical monomorphic appearance of the cells, which are small in size, have round nuclei with occasional indentations, smooth to stippled chromatin, prominent chromocenters, no apparent nucleoli, and modest amphophilic cytoplasm. Also notice the low mitotic activity and prominent association with a vascular network, suggestive of angioimmunoblastic lymphoma. H&E stain; original magnification ×500.

Extranodal NK/T Cell Lymphoma, Nasal Type Extranodal NK/T cell lymphoma (ENKL) is a rare form of human lymphoma, although the incidence is higher in East Asia and Latin America where the majority of cases are associated with EBV infection.186 Most cases of ENKL in humans arise from NK cells,118 although both the NK cell and T cell variants are considered as a single disease entity that share clinical and prognostic features. Nasal lymphoma is rare in dogs, but it is the most common tumor of the nasal cavity in cats, with increased incidence since the inception of routine FeLV vaccination.5,86,120,131,138 Common clinical signs in dogs and cats include sneezing, upper respiratory stridor, and nasal discharge that can be purulent, serous, and bloody; facial pain and/or deformity are rare.109,196 The association of nasal lymphoma with systemic lymphadenopathy or spread to other distant sites is unclear. A recent publication reported 10 of 22 cats with multi-organ involvement of brain, gut, liver, lung, kidney, eye, and bone marrow.109 Two independent case reports in dogs supported T cell ontogeny.102,196 Nasal lymphoma in the cat is predominantly a B cell tumor (70–90% of the cases).42,109,138 Histology is preferred for diagnosis of nasal lymphoma, but brush cytology of the nasal passages can provide diagnostic samples if there is minimal inflammation and hemorrhage.109 Squash preparations from endoscopic biopsies are also a reliable source of tissue for cytologic assessment, although it can be difficult to distinguish between lymphoma and lymphoid hyperplasia.45

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Hepatosplenic Gamma-Delta T Cell Lymphoma In humans, hepatosplenic T cell lymphoma (HSTCL) is a clinically aggressive neoplasm that was first described as an entity in 1981.98 It is characterized by hepatosplenic involvement, sinusoidal tropism, and an infiltrating population of αβTCR-positive T cells that also express CD2, CD3, and CD56.93,122,165,183,203 Although selected cases of TCRαβ have been identified, the gamma-delta variant of the disease is recognized as a distinct clinicopathologic entity in the WHO classification system.93,122,183 As TCRαβ HSTCL has not been reported in the veterinary literature, it will not be discussed. In both the human and veterinary literature, HSTCL is considered to be rare. In humans, approximately 200 cases of HSTCL have been reported, which represents less than 1% of all human lymphomas. The incidence of HSTCL in veterinary medicine is uncertain, but the small number of published reports since its initial description in 2003 suggests that it is similarly rare. HSTCL has been described in three reports, encompassing four dogs and one horse.27,65,161 Although a definitive diagnosis of HSTCL in the cat has yet to be published, a report of erythrophagocytic T cell lymphoma, which shares clinical and clinicopathologic features of HSTCL has been published.25 HSTCL in humans may be associated with immunodeficiency, particularly in patients undergoing longterm immunosuppression associated with organ transplantation or inflammatory bowel disease.12 Approximately one-third of human cases have been associated with organ transplantation and other conditions that require immune-modifying agents. The mechanism is unknown, but may be related to disruption of T cell negative regulation. Human patients with HSTCL typically present with marked hepatomegaly, splenomegaly, or both, in the absence of lymphadenopathy.12,27,92,209 They also have cytopenias of more than one lineage.12,209,212 In animals with HSTCL, the clinical features include lethargy, fever, splenomegaly, anorexia, diarrhea, and pancytopenia. The diagnosis of human HSTCL generally requires histological and immunohistologic evaluation of bone marrow and spleen, tissues in which the vascular tropism characteristic of the neoplastic cells can be identified.12,165,209 Infiltration by atypical cells within marrow sinusoids is common, but cellular identification is often difficult, because the infiltrating population is often subtle and requires immunohistochemistry. In humans, the neoplastic cells in HSTCL are most commonly CD2, CD3, CD38, CD56-positive cells with variable expression of CD7 and CD16.165,203 According to the limited veterinary patient phenotype data, the cells in four cases of canine HSTCL are described as CD3, CD11d, and TCR γδ-positive.27,65 Additionally, activated hemophagocytic histiocytes may be admixed with the neoplastic T cells.12 Despite its common involvement, evaluation of liver is not recommended to diagnose HSTCL in people because the histologic pattern is more

commonly consistent with an inflammatory rather than a neoplastic process. Given the limited number of cases in animals, an algorithm to diagnose veterinary patients with this disease remains to be constructed. Despite this, features similar to human HSTCL (prominent sinusoidal tropism in liver, spleen and bone marrow, and neoplastic cell CD3-positivity) have been reported in cases of canine HSTCL. However, in contrast to the humans, the antemortem evaluation of bone marrow through aspiration cytology, was not helpful in the two dogs in which it was performed.27,65 The prognosis of HSTCL is poor; most human patients die within 16 months of diagnosis irrespective of therapy, and three of the five animals with this disease (two dogs and the horse) survived less than 5 days after diagnosis.12,27,65,161,209 Peripheral T Cell Lymphoma, Not Otherwise Specified PTCL-NOS is a WHO category created to encompass those peripheral T cell malignancies that cannot be readily classified into another defined group based on morphology, phenotype, or molecular characteristics. As such, the diagnosis of PTCL-NOS encompasses a heterogeneous group that includes 40–50% of all human T cell lymphomas, representing some of the most aggressive cases of NHL that often present as advanced disease in middle-aged to older people with involvement of lymph nodes, skin, liver, spleen, and bone marrow or blood.2,74,101 This disease is commonly associated with paraneoplastic syndromes including hypercalcemia, skin rashes, vasculitis, eosinophilia, and hemophagocytosis.17 The morphology of PTCL-NOS cells can range from uniform small nondescript lymphoid cells with indistinct nucleoli, to large anaplastic cells, to combinations of the two, but there is no correlation between cytological atypia and biological behavior.105 Often, there are prominent reactive infiltrates admixed with the tumor cells, and not surprisingly, the latter have variable phenotypes, with expression of CD3 being the only consistent finding.211 PTCL-NOS in people is most commonly a nodal disease of CD4 cells; however, extranodal tumors, generally of CD8 origin, also are seen.105 The heterogeneous nature of this NHL subgroup is reinforced by their gene expression profiles, although these clearly distinguish PTCL-NOS from the two next most common human T cell lymphomas, AITL and ALCL.2 Treatment for this disease is generally unrewarding, and intensification of chemotherapy does not seem to improve outcomes over treatment with standard CHOP-based protocols.55 Given the paucity of immunolabeling reagents and the absence of large-scale studies describing precise subcategories of lymphoma in domestic animals, it would be tempting to include many T cell tumors in this category. However, dogs are the only species where some rigor has been applied to reach this diagnosis.60,63 PTCL-NOS appears to affect middle-aged to older dogs,

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FIGURE 72.7 Peripheral T cell lymphoma, not otherwise specified in a lymph node from a dog. Notice the large, pleomorphic cells with prominent irregular nuclear contours and variable amounts of pale, foamy cytoplasm. Nucleoli are generally indistinct, although in this case they are apparent in some cells. Features that distinguish them from diffuse large B cell lymphomas (DLBCL) and LBTL, are that PTCL has only occasional macrophages (these are a prominent feature of DLBCL and very rare in LBTL) and it can have a prominent vascular network. H&E stain; original magnification ×500.

causing regional or generalized lymphadenopathy. Morphologically, the tumor cells are pleomorphic, have prominent nuclear contour irregularity, indistinct nucleoli, and variable amounts of pale cytoplasm (Fig. 72.7). They mostly arise from CD4 T cells,63 and in the authors’ experience, they are frequently associated with paraneoplastic syndromes such as hypercalcemia. Like LBTL, these tumors are cytogenetically unstable, seem to harbor deletions of the CDKN2 locus, and respond poorly to multi-agent chemotherapy.60 Natural Killer Cell Neoplasms Natural killer cells are incompletely characterized in domestic animals other than the cow because there is a paucity of useful reagents that define their phenotype in humans and mice. Until now, a diagnosis of NK leukemia or lymphoma has been based on absence of T or B cell markers, and undetectable clonal rearrangements of lymphocyte antigen receptors. Yet, cells with morphologic characteristics and functions of NK cells can be isolated from canine blood,82 and the recent characterization of canine and feline CD56 may help to confirm the incidence of NK malignancies in these species.179 Unclassifiable, High-grade Plasmacytoid Type Lymphoma This subcategory of canine lymphoma was only recently described and is of particular interest given the disparity between its cytologic features and immunopheno-

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type. The tumor cells resemble plasma cells; yet, they consistently express T cell markers. There is no category of human T cell lymphoma with comparable features, so this tumor may warrant classification in a separate category in dogs.63 High-grade plasmacytoid T cell lymphoma appears to affect younger dogs and displays aggressive clinical behavior.152 Dogs have regional or generalized lymphadenopathy along with hepatosplenomegaly, mediastinal involvement, and bone marrow infiltration with infrequent atypical cells in the circulation.63,151 In one study, hypercalcemia was present in four out of nine cases and response to multi-agent chemotherapy was unrewarding, with a median disease-free survival of 3 months.151 High-grade plasmacytoid T cell lymphoma has a diffuse histological appearance with high mitotic index and frequent tingible body macrophages that create the typical starry sky appearance associated with diffuse B cell lymphomas.63,151 The malignant lymphocytes are of intermediate size, occasionally binucleate with variably clumped chromatin and abundant basophilic cytoplasm with perinuclear clearing, all morphologic features consistent with plasma cells. They express CD3 and CD8, but some cases may co-express CD4.63,151 The presence of nuclei with irregular, flower-shaped contours consistent with T cell origin in a tumor with plasmacytoid morphology is one feature that should encourage the pathologist to perform additional tests to reach a definitive diagnosis.

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C H A P T E R 73

Histiocytic Proliferative Diseases PETER F. MOORE Overview Histiocytic Differentiation and Canine Histiocytosis Canine Cutaneous Histiocytoma Complex Solitary Cutaneous Histiocytoma Immunophenotypic Studies Regression of Histiocytomas Metastatic Histiocytoma Langerhans Cell Histiocytosis Treatment of Histiocytoma Complex Canine Histiocytic Sarcoma Complex Localized and Disseminated HS Morphological Features of HS Immunophenotypic Studies Treatment of HS Complex

Canine Reactive Histiocytoses Systemic Histiocytosis Cutaneous Histiocytosis Morphological Features of SH and CH Immunophenotypic Studies Pathogenesis Treatment Options in SH and CH Feline Histiocytic Proliferations Feline Progressive Histiocytosis Immunophenotypic studies Feline Pulmonary Langerhans Cell Histioctyosis Immunophenotypic studies Feline Histiocytic Sarcoma Complex Immunophenotypic studies

Acronyms and Abbreviations APC, antigen-presenting cell; CCNU, 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (Lomustine); CH, cutaneous histiocytosis; CNS, central nervous system; DC, dendritic cell; EMH, extramedullary hemopoiesis; FPH, feline progressive histiocytosis; HHS, hemophagocytic histiocytic sarcoma; HS, histiocytic sarcoma; IHC, immunohistochemistry; LC, Langerhans cell; LCH, Langerhans cell histiocytosis; MF, mycosis fungoides; MH, malignant histiocytosis; NECL, non-epidermotropic cutaneous lymphoma; PCR, polymerase chain reaction; PLCH, feline pulmonary Langerhans cell histiocytosis; SH, systemic histiocytosis.

OVERVIEW There are at least four well-defined histiocytic proliferative diseases that have been recognized in dogs. They are a frustrating group of diseases because it may be difficult to differentiate them from granulomatous, reactive, inflammatory, or lymphoproliferative diseases by examination of regular paraffin sections. The clinical presentation and behavior and responsiveness to therapy vary between the syndromes observed (Table 73.1). Clinical and pathological images of canine histiocytic diseases, and details of histiocytic lineages are available on a web site maintained by the authors (http://www.histiocytosis.ucdavis.edu). Canine cutaneous histiocytoma usually occurs as a single lesion in young dogs and spontaneously regresses. Metastatic histiocytoma is a rare example of aggressive 540

behavior of this solitary tumor. Langerhans cell histiocytosis covers a spectrum of disease from multiple cutaneous lesions, which individually resemble solitary histiocytomas, to multiple cutaneous lesions with progressive systemic involvement. Cutaneous histiocytosis (CH) presents with single or multiple lesions, which tend to wax and wane, and may even spontaneously regress. Few cases respond to corticosteroids, the remainder persist and may require more aggressive immunosuppressive therapy. Systemic histiocytosis (SH) is a familial disease of Bernese mountain dogs and also occurs sporadically in other breeds. Systemic histiocytosis presents with prominent skin manifestations identical to those seen in CH, but mucous membranes (ocular and nasal) and a variety of other organ systems, including lymphoid organs, lung, and bone marrow may also be involved. Although the lesions may wax

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TABLE 73.1 Canine Histiocytic Disease Classification Disease Group

Disease

Histiocytoma complex

Histiocytoma Langerhans cell histiocytosis

Epidermal LC Epidermal LC

Spontaneous regression Widespread cutaneous/systemic disease. Spread beyond skin poor prognostic indicator

Reactive histiocytosis

Cutaneous histiocytosis

Interstitial DC (dermal DC)

Systemic histiocytosis

Interstitial DC

Immunoregulatory disorder – limited to skin and lymph node. Responds to immunosuppression Immunoregulatory disorder – skin, lymph node, mucous membranes, internal organs. Responds to immunosuppression

Localized histiocytic sarcoma Disseminated histiocytic sarcoma Hemopohagocytic histiocytic sarcoma

Interstitial DC

Initial solitary site – rapid dissemination

Interstitial DC

Progression of localized disease; multicentric origin (equivalent to malignant histiocytosis) Originates in spleen (± bone marrow); rapid spread to liver and lung

Histiocytic sarcoma complex

Cell of Origin

Macrophage – splenic red pulp and bone marrow

and wane, SH is a progressive disease that often requires continuous immunosuppressive therapy. Histiocytic sarcoma (HS) and malignant histiocytosis (MH) occur with high incidence in Bernese mountain dogs, Rottweilers, Flat Coat Retrievers, Golden Retrievers and sporadically in many other breeds. Histiocytic sarcomas occur as localized lesions in spleen, lymph nodes, lung, bone marrow, skin and subcutis, brain, and periarticular tissue of large appendicular joints. Histiocytic sarcomas can also occur as multiple lesions in single organs (especially spleen). Localized HS lesions rapidly disseminate to involve multiple organs. Hence, disseminated HS is difficult to distinguish from MH, which is a multisystem, rapidly progressive disease in which there is simultaneous involvement of multiple organs such as spleen, lymph nodes, lung, bone marrow, skin and subcutis. Response of HS and MH to chemotherapy is at best brief. HISTIOCYTIC DIFFERENTIATION AND CANINE HISTIOCYTOSIS The development of canine specific monoclonal antibodies for many of the functionally important molecules of macrophages and dendritic antigen-presenting cells (DCs) has enabled the identification of cell lineages involved in canine histiocytic disorders.1,3,14,19 The majority of histiocytic disorders involve proliferation of various DC lineages. Histiocytes differentiate from CD34+ committed stem cell precursors into macrophages and several DC lineages, which include epitheliotropic DCs or Langerhans cells (LCs), interstitial DCs in many organs (e.g. dermal DCs in skin), and interdigitating DCs of T cell domains in peripheral lymphoid organs (see Chapter 8). Dendritic cells are the most potent antigenpresenting cells (APCs) for induction of immune responses in naïve T cells. Canine DCs have been best defined in canine skin. They occur in two major locations: within the epidermis LCs, and within the dermis,

Disease Progression

especially adjacent to post-capillary venules (i.e. interstitial DCs or dermal DCs). Canine DCs abundantly express CD1 molecules,12,14,19 which together with MHC class I and MHC class II molecules, are responsible for presentation of peptides, lipids and glycolipids to T cells. Hence, DCs are best defined by their abundant expression of molecules essential to their function as APCs. Of these, the family of CD1 proteins is largely restricted in expression to dendritic APCs in skin,12 while MHC classes I and II are more broadly expressed. The beta-2 integrins (CD11/CD18) comprise the major family of adhesion molecules on leukocytes, and as such are useful markers of leukocytic differentiation. CD11/CD18 expression is highly regulated in normal canine macrophages and DCs. CD11c is frequently expressed by DCs, while macrophages predominately express CD11b (or CD11d in the splenic red pulp and bone marrow).5,6 In diseased tissues, these beta-2 integrin expression patterns may be diversified. Langerhans cells (intraepithelial DCs) and interstitial DCs are distinguishable by their differential expression of E-cadherin (LC+) and Thy-1 (CD90) (interstitial DC+). Lineage distinctions among histiocytes are best made via immunohistochemistry (IHC) performed on frozen sections (CD1, CD11b, CD11c, CD11d, CD18, CD90, MHC II, and E-cadherin expression). Less definitive, but useful distinctions can also be attained via IHC on formalin-fixed paraffin-embedded sections with panels of leukocytic markers developed for use in this format (CD3, CD11d, CD18, CD45, CD45RA, CD79a, and E-cadherin). Successful interaction of dendritic APCs and T cells in response to antigenic challenge also involves the orderly appearance of costimulatory molecules (B7 family – CD80 and CD86) on dendritic APC, and their ligands (CD28 and CTLA-4) on T cells. Defective interaction of dendritic APC and T cells appears to contribute to the development of reactive cutaneous histiocytic proliferative diseases (cutaneous and systemic histiocytosis), which are related DC disorders arising out of disordered immune regulation (see below).

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SECTION V: HEMATOLOGIC NEOPLASIA

FIGURE 73.1 Canine cutaneous histiocytoma. Nest of histiocytes in the epidermis. Hematoxylin & Eosin stain.

FIGURE 73.2 Canine cutaneous histiocytoma. E-cadherin

CANINE CUTANEOUS HISTIOCYTOMA COMPLEX

phoma with the aid of IHC. Histiocytomas have the phenotype of epidermal LC.19 They express CD1a, MHC class II, CD11c/CD18, and often E-cadherin. In formalin-fixed tissue, histiocytomas consistently express CD18 and commonly E-cadherin (Fig. 73.2). Among skin leukocytes, E-cadherin expression is unique to LCs. Langerhans cells utilize E-cadherin to localize in the epidermis via homotypic interaction with E-cadherin expressed by keratinocytes. E-cadherin expression has only rarely been observed in HS in canine skin and subcutis. Histiocytomas lack expression of CD4 and Thy-1, which are consistently expressed by histiocytes in CH and SH.

Solitary Cutaneous Histiocytoma Histocytoma is a common, benign, cutaneous neoplasm of the dog. Histiocytomas usually occur as solitary lesions, which undergo spontaneous regression. The age-specific incidence rate for histiocytomas drops precipitously after 3 years, although histiocytomas do occur in dogs of all ages.24 Recurrence of histiocytomas at the same or other sites is uncommon. The occurrence of multiple tumors is also uncommon. Epidermal invasion by cells of histiocytoma frequently occurs (incidence about 60%) and intra-epidermal nests of histiocytes resemble Pautrier ’s aggregates, characteristically found in epidermotropic cutaneous T cell lymphoma (mycosis fungoides or MF; Fig. 73.1). Epidermal invasion in histiocytoma, or presence of simultaneous multiple histiocytomas, especially in aged dogs, can present a diagnostic dilemma and distinction from MF and non-epidermotropic cutaneous lymphoma (NECL) is difficult without immunostaining for CD3 (T cell lymphoma) and CD18 (histiocytoma).17–19 Multiple histiocytomas are also readily confused with cutaneous histiocytosis on clinical appearance, although morphologically, histiocytomas are consistently epidermotropic and commonly epidermally invasive; these are not features of cutaneous histiocytosis, which is not focused on the epidermis. Instead the bulk of the lesion is formed by coalescence of perivascular infiltrates in the deep dermis and subcutis.

expression by epitheliotropic histiocytes and by epidermal keratinocytes.

Regression of Histiocytomas The factors that determine the onset of regression in canine histiocytomas are unknown. Evidence of regression is usually observed in lesions that have been present for only a few weeks, although regression can be delayed for many months. Regardless, regression is mediated by CD8+ αβ T cells; only scant numbers of CD4+ T cells are observed in histiocytoma lesions. Migration of tumor histiocytes and/or tumor infiltrating reactive DC to draining lymph nodes could activate CD4+ T cells, which would assist in CD8+ cytotoxic T cell recruitment. Because massive CD8+ T cell infiltration is observed in all instances of histiocytoma regression, therapeutic intervention with the aim of immunosuppression should be avoided once a definitive diagnosis of histiocytoma has been reached, to allow unfettered cytotoxic T cell function.

Immunophenotypic Studies Immunohistochemistry is best performed on frozen sections of tumor or cytological preparations (not formalin-fixed material). Histiocytoma is readily distinguished from other histiocytic disorders and cutaneous lym-

Metastatic Histiocytoma We have observed several dogs with solitary histiocytomas in which neoplastic histiocytes had migrated to draining lymph nodes resulting in complete oblitera-

CHAPTER 73: HISTIOCYTIC PROLIFERATIVE DISEASES

tion. In three of four instances regression of these lesions occurred spontaneously within 3–4 weeks.19 Langerhans Cell Histiocytosis If histiocytomas occur in multiple cutaneous sites, there is a spectrum of clinical disease ranging from skin involvement only to skin, lymph node, and internal organ involvement. This spectrum of disease best fits under the umbrella of LCH. Multiple histiocytomas, limited to skin or skin and draining lymph nodes, are best classified as cutaneous LCH. Multiple histiocytomas limited to skin appear to be more common in Shar Pei dogs, but can occur in any breed. Delayed regression of multiple histiocytomas can occur and lesions can persist for up to 10 months before onset of regression. In about 50% of instances, dogs with multiple histiocytomas are euthanized due to lack of regression of lesions and complications in management of the extensive ulcerated lesions that are frequently present. Multiple histiocytomas with lymph node metastasis have a poor prognosis, because spontaneous regression has not been encountered, and all affected dogs have been euthanized. Multiple cutaneous histiocytomas with progression to lymph node and internal organ involvement are best classified as systemic LCH. Clinically, the lesions may be almost confluent in affected skin regions. Affected animals have all been euthanized and the time course is shorter than cutaneous LCH. There is one published account of a similar case without necropsy to verify internal lesions,20 and the author has data on nine dogs with a similar presentation. Langerhans cell histicytosis is also recognized as a rare disease of humans, in which marked variation in clinical behavior is recognized.7,22 Treatment of Histiocytoma Complex Solitary histiocytomas are either surgically removed or undergo spontaneous regression. Cutaneous LCH and systemic LCH are refractory to therapeutic intervention. Treatment with CCNU (Lomustine), which is commonly used to treat HS, has not been effective in LCH. Corticosteroids are contraindicated for reasons alluded to above. Spontaneous remission is possible in cutaneous LCH and this occurs in about half of the cases, unless lymph node involvement occurs. In one published report, response of cutaneous LCH to immunomodulatory therapy with griseofulvin was reported.20 Others have tried immunomodulation with levamisole without success in cutaneous LCH. CANINE HISTIOCYTIC SARCOMA COMPLEX Localized and Disseminated HS The HS complex encompasses a number of distinctive clinical entities. Histiocytic neoplasia, which originates at a single site, is called localized HS. This form of histiocytic sarcoma, which is often encountered on the

543

extremities, has the best prognosis if treated early by surgical excision or by amputation of a limb. When spread to sites beyond the local lymph node occurs, the disease is then termed disseminated HS; this is more likely to occur unnoticed when primary lesions occur in cryptic sites (e.g. spleen, lung, and bone marrow). This latter form of HS is most like MH. Malignant histiocytosis is an aggressive, histiocytic neoplasm, which arises in multiple sites simultaneously. Most lesions previously defined as MH are probably more correctly termed disseminated HS. The occurrence of true MH is difficult to establish because the lesions often occur in cryptic sites, and the existence of histiocytic neoplasia is only recognized after clinical signs have appeared and disease progression is advanced. HS and MH are capable of widespread metastasis; hence in time the two syndromes merge clinically and it is not always possible to differentiate true multicentric origin (MH) from widespread metastasis of disseminated HS. Also, it is never possible to know exactly how long the disease process has been operative. Hence, the perception is that both disseminated HS and MH follow a rapid clinical progression despite therapeutic intervention. This is certainly true once clinical signs are apparent, but the sub-clinical period is of unknown duration. The HS complex of diseases is best recognized in the Bernese Mountain Dog, in which a familial association is apparent. Other breeds are predisposed to HS complex diseases and include Rottweilers, Golden Retrievers, and Flat-coated Retrievers.3 HS complex is not limited to these breeds and can occur sporadically in any breed. Primary lesions of HS occur in spleen, lymph node, lung, bone marrow, skin, and subcutis especially of extremities, and in periarticular tissues of the limbs. Secondary sites are widespread, but consistently include liver and lung when the spleen is the primary site, and hilar lymph node when lung is the primary site. Clinical signs include anorexia, weight loss, and lethargy. Other signs depend on the organs involved and are a consequence of destructive mass formation. Accordingly, pulmonary symptoms such as cough and dyspnea have been seen. Central nervous system (CNS) involvement (primary or secondary) can lead to seizures, incoordination, and paralysis. Lameness is often observed in periarticular HS. Regenerative anemia, thrombocytopenia, hypoalbuminemia and hypocholesterolemia have been consistently documented in hemophagocytic HS.16 Many of the cases have been mistaken for Evans’ syndrome despite the lack of demonstrable IgG on the surface of red blood cells (RBCs). Morphological Features of HS Lesions of HS are typically destructive mass lesions with a uniform, smooth cut surface and are white/ cream to tan in color. Lesions have a soft consistency and may contain discolored areas (typically yellow), which indicate area of necrosis. Lesions can be solitary or multiple within an organ (especially spleen). Periarticular HS has a distinctive appearance: it occurs

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SECTION V: HEMATOLOGIC NEOPLASIA

FIGURE 73.3 Histiocytic sarcoma. Cytologically atypical mononuclear and multinucleated giant cells in HS. Many cells are phagocytic. Hematoxylin & Eosin stain.

FIGURE 73.4 Hemophagocytic HS. Prominent

as multiple tan nodules located in the sub-synovium. These lesions may encircle the affected joint. Hemophagocytic HS does not initially form mass lesions in the primary sites (spleen and bone marrow). Typically, diffuse splenomegaly and ill-defined mass lesions are observed; the cut surface is dark red and the consistency is firm. The liver is usually bile stained (jaundice) and disruption of the lobular pattern due to metastasis is observed: marked liver involvement can occur in the absence of destructive liver masses. The histological appearance of HS lesions is consistent regardless of location. Lesions are most frequently composed of sheets of large, pleomorphic, mononuclear cells and multi-nucleated giant cells, which show marked cytological atypia and numerous bizarre mitotic figures (Fig. 73.3). Some lesions may include spindle cell forms either alone, or mixed with the mononuclear cells and multinucleated giant cells. Pure spindle cell lesions resemble spindle cell sarcomas of diverse cell lineage. Confirmation of histiocytic lineage can only be achieved with IHC in these instances. Phagocytosis of RBCs, leukocytes and tumor cells occurs, but is not prevalent in most forms of HS. However, in hemophagocytic HS this behavior is amplified. Neoplastic histiocytes manifest marked erythrophagocytosis and the infiltrates obliterate the splenic red pulp and invade red pulp sinuses (Fig. 73.4). Foci of extramedullary hemopoiesis (EMH) occur within and adjacent to the tumor infiltrates in the splenic red pulp. Simultaneous involvement of bone marrow is frequent, and erythrophagia is observed here as well; these cases are probably equivalent to hemophagocytic MH in recognition of the simultaneous involvement of multiple sites. In some instances, the neoplastic infiltrates can be deceptively cytologically bland. The cytological appearance can be asynchronous between sites (e.g. spleen and bone marrow), which can contribute to diagnostic ambivalence if only one site is evaluated. Invasion of splenic red pulp sinuses portends invasion of the

hepatic sinusoids. In the early stages, liver metastases can easily be overlooked grossly and histologically, because histiocytic infiltrates creep along sinusoids and do not form discrete masses. Neoplastic histiocytes in hemophagocytic HS express a distinctive surface antigen profile much like that expressed by macrophages in splenic red pulp and bone marrow (see below). In some instances, a secondary hemophagocytic syndrome can be confused with hemophagocytic HS. This has occurred most frequently with hepatosplenic T cell lymphoma, which is frequently accompanied by responsive anemia and thrombocytopenia. Demonstration of proliferative foci of CD3+ neoplastic T cells in spleen and liver in association with cytologically normal, hemophagocytic macrophages is necessary to resolve these cases.9

erythrophagocytosis by cytologically atypical histiocytes in the splenic red pulp. Hematoxylin & Eosin stain.

Immunophenotypic Studies Histiocytic sarcoma lesions express leukocyte surface molecules characteristic of DC (CD1, CD11c/CD18 and MHC II) (Fig. 73.5). Diffuse expression of E-cadherin, Thy-1, and CD4 has not been observed in HS or MH in any site; this together with cytomorphology assists in the distinction of MH and HS from histiocytoma and reactive histiocytosis (SH and CH).1,3 In histiocytoma, the phenotype is quite similar to that of HS except for the expression of E-cadherin, which occurs in histiocytoma especially in the cellular infiltrate immediately adjacent to the epidermis. However, E-cadherin expression is often visibly weaker in tumor cells in the deep dermis. In reactive histiocytosis, the activated interstitial (dermal) DCs (CD1+, CD11c+, MHC II+, E-cadherin-), consistently express CD4, and Thy-1 expression occurs. In hemophagocytic HS, histiocytes express CD11d instead of CD11c, and MHC II (Fig. 73.6). Expression of CD1 molecules is uniformly low, or occasionally moderate but with a patchy distribution. This phenotype is

CHAPTER 73: HISTIOCYTIC PROLIFERATIVE DISEASES

FIGURE 73.5 Histiocytic sarcoma. CD18 expression by atypical histiocytes. This most likely represents expression of CD11c/CD18.

545

been apparently successfully supplemented by local radiation therapy. In the case of periarticular HS, which occurs in the sub-synovial tissues of the extremities, amputation of the affected limb is necessitated by the inoperable nature of the primary lesions. It is important to establish that evidence of metastasis is absent via thoracic radiographs, abdominal ultrasound, and draining lymph node aspiration cytology before embarking on limb amputation. Disseminated HS (including MH) is not readily treated surgically, because even in the splenic form, early metastasis to the liver has often occurred. Chemotherapy with CCNU has been reported; the success depended on disease load. A small number of dogs with minimal residual disease had prolonged survival (>431 days); however, the median survival for all dogs was only 106 days. The authors identified hypoalbuminemia and thrombocytopenia as negative prognostic variables; dogs with these features survived less than 1 month.23 The latter group of dogs may have included dogs with hemophagocytic HS.16 CANINE REACTIVE HISTIOCYTOSES Systemic Histiocytosis

FIGURE 73.6 Hemophagocytic HS. CD11d expression by atypical histiocytes in the splenic red pulp – reflects expression of CD11d/ CD18.

consistent with macrophage differentiation rather than DC differentiation, in which abundant expression of CD1 and CD11c is expected.16 The exact sublineages of DC involved in HS have not been determined in most instances. The most likely candidates include interdigitating DC in lymphoid tissues and perivascular interstitial DC in other involved tissues. Immunophenotyping and careful morphological assessment should also avoid confusion of HS and MH with the large cell form of T cell lymphoma (CD3+), and poorly differentiated mast cell tumors (CD18+ variable, CD45+, CD45RA+, Tryptase+, c-kit+). Treatment of HS Complex Localized HS affecting skin and subcutis has been cured by early surgical excision, which in some instances, has

Systemic histiocytosis was originally recognized in related Bernese mountain dogs.15 Systemic histiocytosis is a generalized histiocytic proliferative disease with a marked tendency to involve skin, ocular and nasal mucosae, and peripheral lymph nodes. The disease predominately affects young to middle aged dogs (2–8 years old). Systemic histiocytosis has been observed less frequently in other breeds (Irish wolfhounds, Basset hounds and others). Clinical signs vary with the severity and extent of disease and include anorexia, marked weight loss, stertorous respiration, and conjunctivitis with marked chemosis. Multiple cutaneous nodules may be distributed over the entire body, but are especially prevalent in the scrotum, nasal apex, nasal planum, and eyelids. Ulceration of the skin overlying the nodules is common. Peripheral lymph nodes are often palpably enlarged. The disease course may be punctuated by remissions and relapses, which may occur spontaneously especially early in the disease course. In severe disease, lesions become persistent and do not respond to immunosuppressive doses of corticosteroids. Cutaneous Histiocytosis Cutaneous histiocytosis is a histiocytic proliferative disorder that primarily involves skin and subcutis and does not extend beyond the local draining lymph nodes.13 Cutaneous histiocytosis occurs in a number of breeds. Evidence of spread beyond the skin would invoke the diagnosis of SH, a closely related disorder. Lymphadenopathy has not been emphasized in published reports, and has only been documented in a small number of our cases. The lesions occur as multiple cutaneous and subcutaneous nodules up to 4 cm in

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FIGURE 73.7 Cutaneous histiocytosis. Cohesive infiltrates of well differentiated histiocytes and fewer lymphocytes in the subcutis. Lymphocytes and histiocytes infiltrate a vessel wall (arrow). Hematoxylin & Eosin stain. FIGURE 73.8 Cutaneous histiocytosis. Histiocytic infiltrates spare

diameter. Overlying skin ulceration is common. Lesions may disappear spontaneously, or regress and appear at new sites simultaneously. Topographically, lesions may be found on the face, ears, nose, neck, trunk, extremities (including foot pads), perineum. and scrotum. Morphological Features of SH and CH The lesions of SH in most tissues consist of perivascular infiltrates of large histiocytes and variable populations of lymphocytes, neutrophils, and eosinophils (Fig. 73.7). The histiocytes frequently invade vessel walls and this may lead to vascular compromise and infarction of surrounding tissues, which contributes to ulceration of the cutaneous lesions. The widespread distribution of lesions of SH is only fully appreciated at necropsy. Histiocytic lesions have been observed in skin, lung, liver, bone marrow, spleen, peripheral and visceral lymph nodes, kidneys, testes, orbital tissues, nasal mucosa and other sites. In skin, the lesions of SH and CH are virtually identical. The lesions usually involve the deep dermis and subcutis. Involvement of the superficial dermis is inconsistent and epidermotropism of the histiocytes is not observed (Fig. 73.8). In CH the lesions are limited to the skin, but may involve the draining lymph nodes. It is important to note that the lesion topography of SH and CH are the inverse of histiocytoma, and this is an often overlooked feature in diagnostic decision-making. Immunophenotypic Studies Histiocytes in SH and CH express markers expected of DCs, including CD1, C11c, and MHC II. However, lack of E-cadherin expression, and expression of Thy-1 (expressed by dermal DCs) and CD4 (a marker of DC activation) are consistent with an activated interstitial type DC phenotype.1 In skin, dermal DCs are mostly of

the superficial dermis and coalesce in the deep dermis and subcutis. Hematoxylin & Eosin stain.

interstitial DC type. In contrast, histiocytomas express an epidermal LC phenotype. Langerhans cells are intraepithelial DCs; they express CD1, CD11c, MHC II and E-cadherin. Langerhans cells lack expression of Thy-1, and do not express CD4 in the non-activated state. Pathogenesis The clinical behavior and consistent clinical response to immunosuppressive therapy with agents capable of profoundly inhibiting T cell activation has reinforced the concept that SH and CH occur in the context of disordered immune regulation. This may arise from defective interaction of DC and T cells in the resolution phase of an immune response. The end result of this dysregulated immune interaction is chronic proliferation of DC and T cells. The initiation of the process is probably antigen-driven, although studies to identify the nature of antigens involved have not been exhaustively conducted. Hence, it is important to perform tests to rule out infectious agents in the workup of a reactive histiocytosis case (culture and/or special stains for microorganisms in tissue). The lesions can wax and wane over time and spontaneous regression without therapy has been observed. The lymphoid component of the lesion consists of predominately CD8+ αβT cells, whose numbers can vary markedly between lesions. These T cells can comprise up to 50% of the cells in some instances. The role played by T cells is unknown. T cells may be involved in a key way in the exaggerated proliferation and activation of DC via T cell derived cytokines such as GM-CSF and TNFα, which are known to influence the proliferation and differentiation of DC. The author believes that the continued distinction of SH and CH as separate entities is no longer justifiable.

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It would be preferable to consider them within the spectrum of reactive histiocytoses of interstitial DC origin, in which clinical outcome is predictable more by the distant migratory potential of proliferating histiocytes beyond the skin. In this view, CH and SH would be regarded as skin limited and systemic interstitial DC proliferations, respectively. A wide range of clinical behavior is to be expected within each grouping, with SH usually exhibiting more aggressive disease. Cutaneous histiocytosis and SH should not be confused with malignant DC disorders (HS and MH), which can occur in the same topographical locations, and in littermates in genetically susceptible breeds. Cytological and immunophenotypic differences can distinguish these diseases in most instances. There has been very little direct evidence of progression of reactive histiocytosis to HS. On a cautionary note, inflamed T cell lymphoma of skin can look very much like reactive histiocytosis. In such cases, polymerase chain reaction (PCR)-based analysis of the T cell receptor gamma locus will identify clonal T cell expansion in the lesion with good sensitivity (about 80%).25 Treatment Options in SH and CH Systemic histiocytosis has proven to be a difficult and frustrating condition to treat. Consequently, many of the early cases were euthanized. Originally we treated dogs with thymosin (derived from bovine thymus) because of reports of its effectiveness in human LCH cases.21 Some dogs appeared to respond to thymosin, but not consistently. The original rationale for using thymosin was that SH was likely an immunoregulatory disorder and not cancer.15 Once modern immunosuppressive drugs became available, we abandoned the use of thymosin. In the majority of instances corticosteroid treatment is ineffective, although steroids are effective in 10% or more of cases in controlling lesions. Hence, steroids are worth trying given the expense of alternative treatments. Intractable cases are best treated with immunosuppressive doses of cyclosporine A (Neoral, Novartis, East Hanover, NJ) or leflunomide (Arava, Aventis Pharmaceuticals, Bridgewater, NJ). These drugs are potent inhibitors of T cell activation, and their ability to abrogate clinical disease supports the hypothesis that SH and CH are disorders of immune regulation. Treatment with these drugs is expensive and may be needed for life in dogs with continuously active disease. It is preferable not to invoke such powerful immunosuppressive therapy until disease progression is evident or troublesome sites are involved, since in some cases of CH (and even SH) spontaneous regression of lesions or episodic disease activation can occur. Cost of treatment can be substantially reduced by co-administration of cyclosporine A and ketaconazole. It is imperative to measure 12 hour plasma trough levels of cyclosporine A (with twice daily dosing); this is especially important if ketaconazole is co-administered. The 12 hour plasma trough target for cyclosporine A is 500–600 ng/mL. Neoral is the preferred cyclosporine A drug, because it

547

is a microemulsion preconcentrate with superior gastrointestinal absorption compared to Sandimmune oral concentrate (Novartis), which consists of cyclosporine A in an olive oil base. Hence, Neoral can be used at a lower dose.10 Neoral is marketed as Atopica (Novartis) for treatment of dogs with atopic dermatitis; its main advantage over Neoral is the convenient capsule sizes optimized for veterinary use. FELINE HISTIOCYTIC PROLIFERATIONS True histiocytic proliferative diseases in cats have not been extensively documented in the veterinary literature. At least three different histiocytic proliferative diseases have been recognized in cats; these include progressive histiocytosis, pulmonary LCH, and HS/ MH. Feline Progressive Histiocytosis Feline progressive histiocytosis (FPH) behaves as a lowgrade histiocytic sarcoma, which originates in skin from resident DCs.2 The initial clinical course is indolent and morphologically the histiocytes are cytologically bland. However, in time, the lesions become more troublesome and morphologically there is a higher frequency of cytological atypia more consistent with HS. The lesions have been considered an analogue of canine histiocytoma, although spontaneous regression is not observed, unlike canine histiocytoma. Canine histiocytoma is a LC proliferation, but the cell of origin in FPH has features in common with dermal DCs, which are interstitial DCs. The initial presentation of FPH many be a solitary skin nodule, although usually multiple papules, nodules or plaques develop, measuring up to 1.5 cm in diameter. The nodules are firm, non-pruritic and non-painful. The surface is often alopecic and may be ulcerated. The lesions are mostly located on the head, lower extremities or trunk. Occasionally, the lesions are limited to one extremity. Feline progressive histiocytosis is a disease of middle-aged to older cats, the age ranging from 7 to 17 years. Sex or breed predilection has not been seen. The lesions may wax and wane but spontaneous regression does not occur. In general, the nodules progress in size and may coalesce to large plaques. In addition, new lesions may develop. Some cats develop lesions in lymph nodes and internal organs including the lungs, kidneys, spleen, and liver. Additional clinical signs vary depending on the internal organ systems involved. Feline progressive histiocytosis has a poor long-term prognosis as no successful treatment has been recognized to date. Lesions consist of diffuse dermal histiocytic infiltrates, which may extend into the subcutis. The overlying epidermis is either intact or ulcerated. Histiocytes have irregular, vesicular nuclei and finely dispersed chromatin. Cytological atypia is present in a minority of lesions early in the course. Biopsies of later lesions reveal a higher incidence of multinucleated tumor cells, and occasional intra-lymphatic tumor cell aggregates

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FIGURE 73.9 Feline progressive histiocytosis. Cohesive sheets of immature histiocytes infiltrate the superficial dermis and extend into the epidermis as a solitary focus. Hematoxylin & Eosin stain.

FIGURE 73.10 Feline pulmonary LCH. Cohesive sheets of histiocytes fill the alveolar duct and extend to the alveoli and pleura. Hematoxylin & Eosin stain.

may be present. The mitotic activity varies and atypical mitoses are seen. Some cases are epitheliotropic, characterized by intra-epidermal single cells or cell aggregates; other cases lack epithelial involvement (Fig. 73.9). As lesions progress, cells may exhibit numerous cytoplasmic vacuoles and hence have a foamy appearance. The extent of reactive infiltrates, composed of dispersed lymphocytes and fewer neutrophils, varies between cases. Feline progressive histiocytosis has to be differentiated from xanthoma as well as multicentric round cell tumors, such as lymphoma and mast cell proliferation. Immunophenotypic Studies Histiocytes of FPH consistently express CD18, CD1, and MHC II. This immunophenotype is consistent with a DC origin.14 However, both the epitheliotropic and nonepitheliotropic lesions mostly lack expression of E-cadherin, and Birbeck’s granules could not be found in the only case evaluated by electron microscopy. These features indicate that FPH is not composed of LCs, despite the existence of epitheliotropic infiltrates. The DCs are most likely of interstitial (dermal) type. The reactive lymphocytes are CD8+ cytotoxic T cells. Feline Pulmonary Langerhans Cell Histiocytosis Pulmonary Langerhans cell histiocytosis (PLCH) is a disease of aged cats (10–15 years old), which causes progressive respiratory failure leading to euthanasia.4 The cats have severe respiratory distress characterized by tachypnea and open mouth breathing. The symptoms can be acute or present for several months. Thoracic radiographs reveal a diffuse broncho-interstitial pattern of miliary to nodular opacities throughout all lung lobes. The original report described PLCH in three cats. Since then, the author has encountered two more cases. All have been diagnosed at necropsy. An

FIGURE 73.11 Feline pulmonary LCH. Moderately pleomorphic histiocytes with complex nuclear membranes invade the bronchial lumen, which is filled with foamy macrophages. Hematoxylin & Eosin stain.

infiltrative process involved all lung lobes, which were diffusely firm and entirely effaced by ill-defined, coalescing nodular masses (2–5 mm maximum dimension). Extra-pulmonary spread to pancreas and kidney was variably observed. Draining lymph nodes were also effaced. Pulmonary lesions are characterized by histiocytic infiltrates within terminal and respiratory bronchioles. The infiltrates partially obliterate the airway walls and fill the lumens. Extension of the infiltrates into adjacent alveolar ducts and alveoli occurs (Fig. 73.10). Histiocytes form cohesive infiltrates with indistinct cell borders. They are moderately pleomorphic in cell size and nuclear morphology, which is often complex (Fig. 73.11). Transmission electron microscopy was used to

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demonstrate characteristic Birbeck’s granules in the cytoplasm of lesional histiocytes; these structures are only present in LCs and are formed in an endocytic process involving the C-type lectin, langerin.

sublineage of DC has yet to be determined. The tumor cells lack expression of E-cadherin and hence, are not of LC origin. Feline HHS is reported to lack CD1 expression, which supports a macrophage origin.8

Immunophenotypic Studies

REFERENCES

The lesional histiocytes expressed CD18 and E-cadherin. E-cadherin is expressed by LCs, which use it to localize within epithelia. Feline Histiocytic Sarcoma Complex Localized HSs have been observed in cats. The poorly demarcated tumor masses were located in the subcutis of the ventral abdomen or extremities. Metastasis to draining lymph nodes occurs. These lesions may be the end stage of FPH, which would be indistinguishable from HS. Alternatively, primary HS may occur in the spleen.11 Two of the cats in this study had lesions quite similar to those in canine HHS.16 Cats presented with anemia, thrombocytopenia, and hypoalbuminemia and splenomegaly like the canine cases, but studies of the cell of origin were limited due to lack of discriminating cell markers. We have seen similar cases in which erythrophagocytosis by bizarre histiocytes was the dominant feature, and the diagnosis was HHS. Investigation of cell lineage was conducted in a recent feline HHS case and a macrophage lineage was indicated based on lack of CD1 expression.8 Expression of CD11d by hemophagocytic histiocytes in feline HHS has not been evaluated; hence it is not known if feline HHS originates in splenic red pulp macrophages. Feline HS complex resembles the canine counterpart in terms of location of lesions and disease progression, but the incidence of feline HS is much lower. Feline HSs have a poor prognosis and most cats have been euthanized. Feline HS shares morphological features with canine HS, including variable cytology encompassing mononuclear and multinucleated round cells, and discrete to aggregated spindle cells. Anisocytosis and anisokaryosis exist in tumor cells of all types. A moderate to high mitotic rate with bizarre mitotic figures is common. Prominent erythrophagocytosis is observed in feline HHS. Immunohistochemistry is required to differentiate histiocytic sarcomas from vaccine-induced sarcomas or anaplastic sarcomas with giant cells (incorrectly referred to as “malignant fibrous histiocytoma”). Immunophenotypic Studies The round and spindle-shaped tumor cells in feline HS express CD18, CD1, and MHC II. This immunophenotype is consistent with a DC origin, although the precise

1. Affolter VK, Moore PF. Canine cutaneous and systemic histiocytosis: reactive histiocytosis of dermal dendritic cells. Am J Dermatopathol 2000;22:40–48. 2. Affolter VK, Moore PF. Feline progressive histiocytosis. Vet Pathol 2006;43:646–655. 3. Affolter VK, Moore PF. Localized and disseminated histiocytic sarcoma of dendritic cell origin in dogs. Vet Pathol 2002;39:74–83. 4. Busch MD, Reilly CM, Luff JA, et al. Feline pulmonary Langerhans cell histiocytosis with multiorgan involvement. Vet Pathol 2008;45:816–824. 5. Danilenko DM, Moore PF, Rossitto PV. Canine leukocyte cell adhesion molecules (LeuCAMS): characterization of the CD11/CD18 family. Tissue Antigen 1992;40:13–21. 6. Danilenko DM, Rossitto PV, Van der Vieren M, et al. A novel canine leukointegrin, alpha d beta 2, is expressed by specific macrophage subpopulations in tissue and a minor CD8+ lymphocyte subpopulation in peripheral blood. J Immunol 1995;155:35–44. 7. Favara BE, Feller AC, Pauli M, et al. Contemporary classification of histiocytic disorders. The WHO Committee On Histiocytic/Reticulum Cell Proliferations. Reclassification Working Group of the Histiocyte Society. Med Pediatr Oncol 1997;29:157–166. 8. Friedrichs KR, Young KM. Histiocytic sarcoma of macrophage origin in a cat: case report with a literature review of feline histiocytic malignancies and comparison with canine hemophagocytic histiocytic sarcoma. Vet Clin Pathol 2008;37:121–128. 9. Fry MM, Vernau W, Pesavento PA, et al. Hepatosplenic lymphoma in a dog. Vet Pathol 2003;40:556–562. 10. Gregory C. Immunosuppressive agents. In: Bonagura J. ed. Kirk’s Current Veterinary Therapy XIII – Small Animal Practice. Philadelphia: W.B. Saunders, 2000;509–513. 11. Kraje AC, Patton CS, Edwards DF. Malignant histiocytosis in 3 cats. J Vet Intern Med 2001;15:252–256. 12. Looringh van Beeck FA, Zajonc DM, Moore PF, et al. Two canine CD1a proteins are differentially expressed in skin. Immunogenetics 2008;60:315–324. 13. Mays MB, Bergeron JA. Cutaneous histiocytosis in dogs. J Am Vet Med Assoc 1986;188:377–381. 14. Moore P, Affolter V, Olivry T, et al. The use of immunological reagents in defining the pathogenesis of canine skin diseases involving proliferation of leukocytes. In: Kwotchka K, Willemse T, von Tscharner C. eds. Advances in Veterinary Dermatology, Vol. 3. Oxford: Butterworth Heinemann, 1998;77–94. 15. Moore PF. Systemic histiocytosis of Bernese mountain dogs. Vet Pathol 1984;21:554–563. 16. Moore PF, Affolter VK, Vernau W. Canine hemophagocytic histiocytic sarcoma: a proliferative disorder of CD11d+ macrophages. Vet Pathol 2006;43:632–645. 17. Moore PF, Olivry T. Cutaneous lymphomas in companion animals. Clin Dermatol 1994;12:499–505. 18. Moore PF, Olivry T, Naydan D. Canine cutaneous epitheliotropic lymphoma (mycosis fungoides) is a proliferative disorder of CD8+ T cells. Am J Pathol 1994;144:421–429. 19. Moore PF, Schrenzel MD, Affolter VK, et al. Canine cutaneous histiocytoma is an epidermotropic Langerhans cell histiocytosis that expresses CD1 and specific beta 2-integrin molecules. Am J Pathol 1996;148:1699–1708. 20. Nagata M, Hirata M, Ishida T, et al. Progressive Langerhans cell histiocytosis in a puppy. Vet Dermatol 2000;11:241–246. 21. Osband ME, Lipton JM, Lavin P, et al. Histiocytosis-X. New Engl J Med 1981;304:146–153. 22. Schmitz L, Favara BE. Nosology and pathology of Langerhans cell histiocytosis. Hematol Oncol Clin N Am 1998;12:221–246. 23. Skorupski KA, Clifford CA, Paoloni MC, et al. CCNU for the treatment of dogs with histiocytic sarcoma. J Vet Intern Med 2007;21:121–126. 24. Taylor DN, Dorn CR, Luis O. Morphologic and biologic characteristics of the canine cutaneous histiocytoma. Cancer Res 1969;29:83–92. 25. Valli VE, Vernau W, de Lorimier LP, et al. Canine indolent nodular lymphoma. Vet Pathol 2006;43:241–256.

C H A P T E R 74

Gene Therapy BRUCE F. SMITH and R. CURTIS BIRD General Approaches to Gene Therapy in Hematologic Neoplasia Vector Structures and Strategies Tumor Targeting Vector Detargeting Cell Specific Targeting Expression Targeting Tumor Cell Killing Suicide Gene Therapy Oncolytic Viruses Tumor Vaccination Immune Modulation Enhanced Antigen Presentation Enhanced T-Helper Activity

Modulation of Peripheral Tolerance and Regulatory T Cells Cytokine/Chemokine and Antibody Modulation Transplantation Therapy Non-specific Immune Stimulation Neoplastic Outcomes Resulting From Hematologic Gene Therapy Gene Therapy for Specific Veterinary Hematologic Neoplasms Leukemias and Lymphomas Osteosarcomas Future Directions in Veterinary Hematologic Gene Therapy Clinical Trials in Veterinary Species

Acronyms and Abbreviations Ad5, Adenovirus serotype 5; APC, antigen presenting cell; CAV2, canine adenovirus 2; CRAd, conditionally replicative adenoviruses; CTL, cytotoxic T-lymphocyte; IACUC, Institutional Animal Care and Use Committee; IBC, Institutional Biosafety Committee; IFN, interferon; IL, interleukin; MHC, major histocompatibility complex; NK, natural killer cell; PEG-3, progression elevated gene-3; SCFV, single-chain Fv fragment; TAAs, tumor-associated antigens; TGF, transforming growth factor; TNF, tumor necrosis factor; Treg, regulatory T-lymphocyte.

GENERAL APPROACHES TO GENE THERAPY IN HEMATOLOGIC NEOPLASIA Therapy for disease based on the introduction of novel genetic material has been discussed since the early 1980s. Initially, the concept of gene therapy was limited to replacement therapy in inherited metabolic diseases. However, that concept has been greatly expanded to include therapies for acquired diseases, including cancer. Now a maturing field, gene therapy has progressed into the clinics, with both human and animal patients enrolled in trials. Many innovative approaches have been identified including genetic vaccination, oncolytic replication competent viruses, and vector delivery of toxins or toxin converting genes. Much of the research that has been undertaken in the field of cancer gene therapy has focused on human cancers and a large amount of the preliminary evaluation of these strategies 550

has been performed in the laboratory mouse. These approaches have also been applied to a number of species of veterinary interest. The therapeutic application of genes in a variety of cancers in domestic animal species serves both to provide intermediate animal models in the evolution of these therapies for humans and to develop applicable gene therapies in these species themselves. VECTOR STRUCTURES AND STRATEGIES Gene therapy strategies are built around the delivery of genes to target tissues and cells. In most cases, this requires some further components in addition to the nucleic acids. These components are collectively known as vectors. Some common gene therapy vectors are shown in Table 74.1 and will not be discussed in detail here. Ideally, a vector should be able to survive the

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TABLE 74.1 Gene Therapy Vectors Vector

Insert Size (kb)

Target

Retrovirus

∼7

Lentivirus

∼7

Adenovirus

∼7

Depends on envelope used CD4 (can be altered by using other env genes) CAR

“Gutted” Adenovirus

∼30

Adeno-associated virus Alpha herpes virus Plasmid (naked DNA)

Fiber Knob

Fiber

Tropism

Comments

Dividing cells

Integrates into host chromosome

Quiescent and dividing cells

Integrates into host chromosome

CAR

Many cell types including liver, not tumor Same as Adenovirus

∼4–5

Variable

Depends on serotype

∼75

Unknown

Neurons

∼12

Variable

Depends on site of injection and any carrier molecules

Remains episomal, can be retargeted Requires helper virus, can be retargeted Integrates in mouse, episomal in other species Highly pathogenic – requires significant attenuation for use Extremely stable long-term

Penton Base Penton

Hexon

FIGURE 74.1 Structure of a typical adenovirus. The virus coat is an icosahedron consisting primarily of Hexon protein. At each vertex are five Penton and one Penton base proteins. The Penton base serves as the insertion site for the trimeric fiber protein, which forms a rod and a knob. Fiber knob is the binding site of CAR (cocksackie adenovirus receptor).

physiologic milieu, effect entry of the target cell across the plasma membrane and deliver its payload of nucleic acids to the nucleus, where gene expression occurs. A variety of different vectors have been identified, with most based on known viruses, which are essentially highly evolved gene delivery machines. These vector systems typically use engineered viruses, where critical genes for viral replication have been removed, rendering the virus replication incompetent (Fig. 74.1). In order to produce infectious virus particles, these genes must be provided in trans, by engineered packaging cell lines. As will be discussed in more detail below, subtle variations in these lines can result in innovative changes in vector targeting.

TUMOR TARGETING The need to specifically target tumor cells within the body is obvious. Because of the nature of gene therapy, it presents unique opportunities to target tumor cells on a systemic basis, promoting tumor killing without affecting intervening cells. This can be particularly useful in treating widely disseminated metastases which might not be practical to approach either surgically, or with radiation. In gene therapy, targeting can be accomplished at several levels by combining targeting modalities, including vector detargeting, ligand based targeting and the use of expression targeting. Ideally, targeting of gene therapy for cancer patients consists of a combination of approaches, with each approach adding a layer of specificity and therefore safety and efficacy to the eventual therapy (Fig. 74.2). Vector Detargeting Many vectors have specific endogenous affinities that may or may not be appropriate for therapy. For example, Adenovirus serotype 5 (Ad5) may infect a wide variety of cell types through the CAR receptor; however, this receptor is often poorly expressed in many tumors. To assure cell-specific uptake of the vector, this receptor binding can be ablated to “detarget” non-tumor cells. There are several approaches to accomplish this. Antibodies to the cell-specific portion of the vector can be used to mask the binding site. For viral vectors, the viral site that binds to the target cell may either be modified to change binding or ablated to eliminate binding. Cell Specific Targeting There are multiple ways to achieve cell-specific targeting of gene therapy vectors including physical restriction of the vector, pseudotyping, genetic modification

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LTR

ψ

LTR gag pol env

A LTR

ψ

LTR payload gene

B

or

promoter

payload gene

FIGURE 74.2 Genome structure and map of a typical mammalian retrovirus and a derived gene therapy vector. (A) A typical retrovirus encodes three major genes including gag, encoding the membrane glycoprotein, pol, encoding the reverse transcriptase (RNA-dependent DNA polymerase) and env, encoding the viral capsid protein. The retroviral genome also includes two long terminal repeats (LTRs) which encode the enhancer/promoter complex regulating viral gene expression and which are responsible for viral genome integration into the host genome. The bidirectional enhancer component of the LTR is also responsible for the insertional oncogenic activations associated with retroviral genome insertion events observed in gene therapy trials. (B) In gene therapy vectors derived from such viruses, the three major viral genes can be replaced with a gene therapy payload gene as long as the LTRs and the packaging sequence (ψ) are included and the viral particles are assembled in a packaging cell line that contains a helper retrovirus to provide the gag, pol, and env gene products. Expression of the payload gene may be driven off either the intrinsic LTR promoter (top) or an exogenous promoter can be inserted (bottom).

and redirection with antibodies to cell surface markers. Depending on their mode of action, vectors can be injected into specific and restricted body compartments that will limit their spread. These include direct injection into the tumor. In many tumors, direct injection requires multiple injections at numerous locations to treat the entire mass. A regional administration approach may also be used. Commonly, this method isolates the vasculature to a single limb and allows intravascular administration to the limb. Briefly increasing intravascular pressure in the isolated limb may allow vectors to freely cross the vascular endothelium and enter tissue beds. When perfusion is re-established, there may be some washout of vector into the general circulation. Vectors may also be administered systemically and targeted to specific cells using cell surface markers. This approach requires that markers be identified that are restricted to a particular cell type, or a limited subset of other cells. In the case of tumor cells, these may be tumor-associated antigens (TAAs), which are typically cell surface proteins that are normally expressed during development. Given the variety in natural tissue tropism among virus families, it may be possible to exploit the tissue specificity of a particular virus, either through using that virus, or through pseudotyping, a process whereby one virus is given the “coat” proteins of another virus. For example, the tropism of human Ad5, which has an extensive range of target tissues that

express CAR, can be restricted to a different subset of cells if the Ad5 fiber is replaced by the Ad3 fiber protein.30 Receptors that are not specifically viral receptors may be used to target cells through ligand-receptor interactions. These interactions are often highly specific, and will only target those cells expressing the receptor. This approach may also result in activation of the cellular signaling pathway that is normally the target of the receptor. Depending upon the receptor, cell type and pathway, this may be beneficial, or it may prove detrimental. If the target is not a receptor, or activation of a receptor is undesirable, antibodies may be used to target those cells. Conjugation strategies to attach the antibody to the vector can also add complications. It is also possible to identify peptides with targeting efficacy without any prior knowledge of the target cell’s protein inventory. Phage display libraries can be used to screen millions of potential peptides for their capacity to bind to specific cell types. These libraries can be screened in vitro with cultured cells or in vivo by injecting the library and then harvesting the appropriate tissues or cells. By screening for several rounds in the target tissue, it is possible to enrich for peptides or families of peptides with similar sequences and enhanced affinity for target tissue. Targeting molecules, including peptides, ligands, and antibodies, can by used to target in several ways. Viral vectors can be engineered so that these moieties are expressed in an appropriate location that both detargets the vector and retargets it at the same time. Care must be taken though that the added targeting molecule does not alter or destroy the ability of the virus to assemble, bind, infect or replicate. Non-genetic approaches to targeting vectors usually include “biphasic” adapters of some type. These adapters are often engineered fusion proteins that consist of two binding moieties, one to bind the vector and the other to bind the target cell. This approach has been demonstrated using Ad5 vectors. In this case, the adapter was created from an SCFV (single-chain Fv fragment) that binds to the Ad5 fiber knob region, blocking CAR binding and therefore detargeting the vector, and either a second SCFV, or a receptor ligand, such as CD40L. These adapters have the advantage of allowing the vector to be produced as a generic reagent using the common packaging lines or techniques, and then mixed at the bedside with the targeting adapter to allow customized targeting of the vector for each patient. Expression Targeting Targeting may also be achieved at the level of gene expression through the use of specific promoters. These may consist of tumor or tissue specific promoters such as the tyrosinase promoter in pigmented epithelium and melanoma. The ideal promoter for use in tumor cells is one that is normally active early in development and is therefore not active in normal adult cells. One example of this is the progression elevated gene-3

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(PEG-3) which is active in many different tumor cells.27 One disadvantage to these promoters is that they cannot be controlled. Conditional promoters are promoters that can be either induced or suppressed by the addition of another substance. Among the most frequently used are the Tet-on and Tet-off promoters, that are engineered bacterial promoters that have been adapted for use in eukaryotic cells. These promoters are regulated by the antibiotic tetracycline and can either deactivate or induce gene expression in the presence of the antibiotic. Such constructs permit the expression of the therapeutic gene in a temporally controlled fashion, either providing the needed protein at the correct time, or allowing protein synthesis to be disabled as a safety measure should toxicity occur. These promoters do not offer any tissue specificity. TUMOR CELL KILLING Ultimately, the goal of any genetic therapy is to kill cancer cells while sparing normal tissues. There are many different approaches to achieve this including the use of toxins, the induction of apoptosis, lysis of tumor cells with replication competent viruses and activation of an antitumor immune response. Suicide Gene Therapy Perhaps the most classical approach is known as “suicide gene therapy.” This method consists of transferring a prodrug converting enzyme gene to the tumor cells, followed by the administration of the prodrug. Expression of the gene results in the presence of the enzyme, which converts the prodrug to a toxic drug, in turn killing the target cell. An example of an enzyme/ prodrug combination is herpes simplex virus thymidine kinase (HSV-TK) and the anti-herpetic “X”-cyclovir prodrugs (e.g. gancyclovir). HSV-TK metabolizes the prodrug into a nucleotide analog, that is then incorporated into the cell’s DNA during replication of the genome. The drug may also pass to neighboring cells through gap junctions resulting in a “bystander effect.” This results in killing greater numbers of cells than actually express the transferred enzyme. Killing is usually confined to tumor cells.24 Oncolytic Viruses Oncolytic virotherapy consists of the administration of replication competent viruses to tumor cells. These viruses are selected or engineered to specifically replicate in tumor cells, resulting in the eventual lysis of tumor cells. When the cell lyses, new viral particles are released to circulate and potentially infect additional tumor cells. The most commonly used oncolytic viruses are conditionally replicative adenoviruses (CRAd). In general, because of the species-specificity of viral replication, the CRAd must be derived from a virus that infects the target species. In humans, this is typically

553

Ad5. In dogs, canine adenovirus 2 (CAV2) has been adapted for use as a CRAd.13 Normally, this virus causes a mild respiratory disease in dogs, when inhaled, and is used as a vaccine for CAV1, which causes canine hepatitis. In this case, the E1 gene, an early gene product of the virus that is critical for replication, was placed under the control of the osteocalcin promoter to restrict replication to osteosarcoma cells. CRAds can be injected either intra-tumorally or intravenously. Intravenously injected CRAd has been shown to remain in the circulation longer than 48 hours, even in the face of pre-existing immunity.25 Because the virus circulates, CRAds are considered to be an excellent method to target multiple distant metastases. Tumor Vaccination Tumor vaccines have been used with varying success for many years. These vaccines may consist of tumor cells or pieces, typically killed or fixed, that are injected back into the patient. In a refinement of this approach, specific tumor antigens, such as tyrosinase (or P100) in melanoma have been identified in a few tumors and have been used as antigens for tumor vaccines. The first DNA vaccine conditionally licensed for any species in the United States employs P100 as an antigen in an attempt to create an immune response to melanoma cells in the dog.1 The type of immune response may also be critical to the success of this approach. A predominantly T-helper 2 (Th2) response with a high level of circulating IgG1 antibody, may not have any effect on tumor cells, while a predominantly T-helper 1 (Th1) response, with IgG2 and activation of cytotoxic T-lymphocytes (CTLs) may produce significant cell killing. For this reason, antigen presentation and adjuvants have critical effects on the ability of a tumor vaccine to raise a therapeutic response. IMMUNE MODULATION Several different approaches to immune modulation have been developed including enhanced antigen presentation strategies where tumor-specific antigens are introduced for presentation on antigen presenting cells or in which existing antigen recognition is enhanced to promote the breaking of immune tolerance. Systemic or local administration of antigen complexes, or nucleotides encoding them, has been attempted as have strategies whereby immune system components which suppress immunity are themselves suppressed. Gene therapy promoting the local expression of cytokines has also been adopted. Gene therapy by introducing a foreign gene, such as green-fluorescent protein (GFP), has been used in a variety of species to promote immune targeting of neoplasias. This approach has been shown to promote immune responses to both the foreign gene target as well as tumor-associated antigens. Both cytotoxic T lymphocytes (CTLs) and antibody-based reactions have been promoted by this approach.

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Enhanced Antigen Presentation There is an abundance of data suggesting that tumors express specific antigens recognized by the immune system and in some instances these antigens induced immune responses to cause natural remission. A therapeutic strategy that could stimulate or enhance the effectiveness of immune recognition and induction of an effective immune response would be of immense value in the treatment of neoplastic disease, particularly in those cases of dissemination which are not surgically manageable. The basic strategy for forcing an immune response has involved different means of promoting or forcing tumor antigen presentation by antigen presenting cells (APCs). Such strategies have involved technologies such as protein antigen feeding to APC populations or transfection of DNA expression constructs or even tumor cell-derived mRNA population loading into APCs. Various APC populations, including allogeneic and autologous dendritic cells or even CD40 activated B cell populations have been antigen loaded.15 Another approach, designed to improve immune recognition, is based on creation of hybrid-cell vaccines through fusion of APCs with tumor cells.4 Hybrid-cell fusion constructs express both tumor-specific antigens and the necessary machinery needed for antigen presentation. Additionally, if they are MHC matched, T cell activation is also possible. Because dendritic cells are able to transport presented antigens to activate effector T cells in lymph nodes they have been suggested as promising candidates for the APC component of such hybrid-cell vaccines.4,5,10 Such experiments have been performed in normal healthy laboratory dogs providing proof-of-principle for successful development of tumor-specific immunity in this species.4,5 Enhanced T-Helper Activity Approaches to modify the T cell response to cancer have been attempted in dogs for a variety of cancers or to demonstrate proof-of-principle for strategies designed to enhance immune recognition of pan-tumor associated antigens associated with canine melanoma and mammary cancer.5 Application of such strategies to hematopoietic cancers is in a more formative stage; however, successful antigen loading of canine CD40activated B cells has been demonstrated.21 Specific T cell responses were detected to model antigens and the intent is to apply this technology to spontaneous canine lymphoma. Modulation of Peripheral Tolerance and Regulatory T Cells The importance of regulatory T (Treg) cells in the recognition and development of a killing immune response to cancer has only recently been recognized, especially their contributions to suppression of the immune response to what would be otherwise considered selfantigens.2 However, lack of appropriate antibodies to Treg antigens such as CD25 have inhibited investiga-

tions in species other than mouse and human until recently. A murine anti-FoxP3 antibody, which identifies this Treg-enriched transcription factor, has recently been shown to cross-react with canine FoxP3 putatively identifying canine Tregs.3 Such studies should provide the means to investigate the role of Tregs in canine intermediate models developed to investigate immunomodulatory strategies for treatment of cancer. Cytokine/Chemokine and Antibody Modulation The use of gene therapy to alter immune recognition through modification of cytokine, chemokine, or antibody expression has been an early approach attempted in a variety of cancer treatment strategies.19 Relevance has been demonstrated as canine lymphomas have been shown to express interleukin (IL)-2 receptors.12 However, the toxicity and the prohibitive costs associated with systemic treatment with cytokines and other immunomodulatory molecules in intermediate and larger species has driven efforts to find technical solutions designed to force localized expression from inserted genes only at desired locations or tissues. Interferon (IFN)-gamma has been shown to modify major histocompatibility complex (MHC) and tumorassociated antigen expression and to induce antigen specific T cell responses. Interferon-gamma also appears to synergize with effects of interleukins, such as IL-6, and growth factors, such as transforming growth factor (TGF)-beta 1. There is also evidence that regression of canine Langerhans (dendritic) cell tumors are associated with expression of several cytokines including IL-2, tumor necrosis factor (TNF)-alpha and IFNgamma.17 These investigations have suggested that such molecules have the potential to improve immune recognition, particularly if they are used in conjunction with other strategies designed to force presentation of tumor associated antigens. There are also indications that IL-6 can antagonize TGF-beta 1-dependent inhibition of T cell activation. Canine tumor-infiltrative lymphocytes also synthesize IL-6 and may be recruitable to promote an antitumor activity. Transplantation Therapy There is a well developed history of immune modulation in dogs associated with a variety of transplant models as well as those approaches specifically designed to improve management of canine cancer. This has resulted in the development of a new transplantation approach to treatment of canine T cell lymphoma based on aggressive chemotherapy to induce remission followed by transplantation of hematopoietic cell lineages supported by subcutaneous injections of canine granulocyte colony-stimulating factor.11,20 This approach has also been demonstrated following radiation ablation.16 Although based on transplantation of related and at least partially DLA-matched hematopoietic populations, such approaches have been shown to promote disease-free survival of such patients for more than a

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year. Additionally, the chimeric nature of the transplant was maintained in a stable form. Non-Specific Immune Stimulation Some effort has been applied to investigating the nature of nonspecific immune stimulation after the application of Toll-like receptor 9 ligands such as CpG containing oligonucleotides in primates, rodents, and dogs. Such nonspecific immunostimulants have a measurable effect on the innate immune system including natural killer (NK) cell activity. They also tend to promote Th1 type responses from the immune system and have been promoted for use as adjuvants. NEOPLASTIC OUTCOMES RESULTING FROM HEMATOLOGIC GENE THERAPY Gene therapy is a potent tool to alter the genomes of the target cell and organism, and it can be an extremely useful tool in the treatment of neoplasia. However, gene therapy has also been linked to the induction of tumors. In a clinical trial using retroviral vectors to treat X-linked severe combined immunodeficiency (X-SCID), at least four of ten boys developed leukemia. The therapeutic aspects were extremely successful, resulting in lifesaving immune reconstitution in nine of the ten patients. However, integration of the retrovirus in or near several proto-oncogenes, including LM02 in two patients, perhaps combined with expression of the transferred gene product, a cytokine, caused proliferation of those cells and subsequently, leukemia.31 A similar serious adverse event has been reported in a patient in an X-SCID gene therapy trial in England.14 GENE THERAPY FOR SPECIFIC VETERINARY HEMATOLOGIC NEOPLASIAS The instances of successful treatment of veterinary hematologic neoplasias are few, but attempts have been made with varying degrees of success and notable problems with unanticipated side effects. Although many strategies have been investigated in cell culture in vitro, few have been translated to preclinical laboratory animal models or clinical cases. Among non-human and non-murine animal species, these neoplasias include leukemias of both the spontaneous and induced forms, and the more distantly associated osteosarcomas. Leukemias and Lymphomas Although few attempts to directly treat canine lymphoma using a gene therapy approach have been reported, there have been several investigations identifying gene targets that appear to play a causative role in development of canine non-Hodgkin’s lymphoma and may be amenable to a gene therapy strategy based on genetic complementation in trans. The cyclin-

555

dependent kinase inhibitor p16/INK4A has been identified as a defective tumor suppressor gene in many of these canine lymphomas and could be amenable to repair by genetic complementation.9,23 Such an approach would seek to restore cyclin/CDK modulating activity to G1 phase of the cell cycle with the aim of providing the cell with the means to arrest or exit cell cycle via the p16/INK4A and retinoblastoma pathway. These cells appear also to harbor p53 defects making it likely that p53-dependent apoptosis is also defective.26 Additionally, the IL-2 receptor has been identified as dysfunctional in canine lymphoma and may provide a surface marker for targeting or an additional target for gene therapy.8 Canine distemper virus is known to infect such lymphoma cells and has the potential to be used as a gene therapy vector if safety considerations can be addressed.28 Although at a more formative stage than dogs, investigation of these tumors in cats has revealed some oncogene targets associated with feline lymphomas.18 Experimental treatment of leukemias has been attempted in dogs with mixed results owing to the nature of the side effects encountered. This strategy was based on a retroviral vector system and the introduction of a gene involved in imposing pattern formation on phenotype in developing animals. Homeobox (HOX) genes are a clustered family of genes involved in higher order developmental pattern regulation in multicellular animals. As such, these HOX genes play a critical role in the regulation of normal differentiation and appear to guide cells through an ever narrowing range of cellular fates or outcomes. The corollary for such an effect is thought to be a lack of control over phenotype should such genes become dysfunctional. Recent evidence has shown the potential of gamma-retroviral vectors in delivering HOXB4 genes to hematopoietic stem cells and the expansion of repopulating cells.33 The strategy was designed to replace neoplastic populations with genetically modified cells in which HOXB4 cDNA constructs were inserted in an effort to impose such differentiated phenotypes on expanded populations of stem cells. This effect can be demonstrated in a variety of species and includes expanded proliferative potential and perhaps immortalization in some species, a trait only recently attributed to the HOX genes.33 Control of aging appears to be highly homologous among mammals as the canine SIRT2 gene (a gene silencing regulator), an important regulator of lifespan in most multicellular species, has been identified and appears to be aberrant in function in spontaneous canine mammary tumor-derived cell lines.7 Transfer of HOXB4 to hematopoietic stem cells using a gamma-retroviral vector in both dogs and macaques has revealed clear potential for leukemogenesis due to the presence of the genetic construct.32 All of the dogs transplanted (two survived transplantation) and half the non-human primates (1 of 2 macaques) developed leukemias related to the presence of the construct within 2 years of transplantation. There was clearly a difference among species as this effect had not been detected in prior mouse investigations and the potential for leukemia development was not predicted from these

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SECTION V: HEMATOLOGIC NEOPLASIA

FIGURE 74.3 Gene therapy clinical trial patient. A 10-year-old female spayed Rottweiler with osteosarcoma is shown receiving an oncolytic canine adenovirus intravenously following amputation of her affected forelimb. The high titer CRAd vector is administered slowly to reduce the potential of anaphylaxis.

experiments, although other experimental gene therapy trials in humans have uncovered such potential when using retroviral vectors. Innovative vector constructs have recently been reported for the investigation of ways to mitigate the predisposition for insertional mutagenesis of viral vectors in a mouse model.6 These vector constructs incorporated suicide genes to enhance drug sensitivity for transduced cells providing a means to eliminate leukemic cells. Osteosarcomas Osteosarcoma is among the most frequently occurring bone malignancies in dogs and has been promoted as a model application for the development of new conditionally replicative adenoviral gene therapy vectors.13,25 These adenoviral vectors have been targeted to deliver gene payloads to canine osteosarcoma cells in osteosarcoma patients. Although early in its development, preclinical trials in normal dogs have been conducted to determine dosage, safety, and levels of viral vector tolerance. To facilitate delivery but suppress uncontrolled spread, conditionally replication competent (CRAd) constructs have been developed. Measurable replication in several serial but diminishing peaks of the viral vector have been detected following treatment of tumor bearing canine patient animals (Dr. B.F. Smith, personal communication). Figure 74.3 shows the initial clinical patient to receive this vector.5,29 Trials using replication defective adenoviral vectors to deliver pro-apoptotic and immunomodulatory therapies also are in early phase clinical trials in dogs with osteosarcoma.22 FUTURE DIRECTIONS IN VETERINARY HEMATOLOGIC GENE THERAPY Gene therapy as a field is rapidly approaching its 25th anniversary. As such, the approaches being used are

maturing and gaining sophistication. Is gene therapy a magic bullet? Unquestionably, it is not. As with other therapeutic approaches to cancer, it will, in the end, be a combination of approaches, tailored to the specific tumor in the patient and designed to maximize tumor cell killing that will provide the most successful cancer management strategies. Novel vectors are continually being introduced. In the next 5–10 years, vector development will focus on issues of enhancement of function. In particular, camouflaging vectors against immediate immune recognition and creation of vectors with novel tumor cell ligands are likely developments. In veterinary medicine, cost will drive the use of targeting molecules with the broadest possible application, especially if that is to treat multiple tumor types. To that end, cocktails of vectors with multiple and different targeting molecules may be developed to allow a single preparation to be used in all patients with a given tumor type. Targeting at the level of transcription may also become more broadly applicable with the use of promoters that are expressed in multiple tumors. Vectors are also likely to undergo significant development in the coming years. Novel fusion proteins with enhanced specificities for prodrug conversion are currently being evaluated. In addition, payloads focused on inciting immune responses or activating cellular apoptotic pathways are gaining favor. Clinical Trials in Veterinary Species At the point that it becomes possible to conduct a “clinical trial,” that is an experimental administration of a novel therapeutic in client-owned animals, there is a critical need to interact with multiple constituencies. Depending on the organization, experiments in clientowned animals may be covered by the Institutional Animal Care and Use Committee (IACUC) which by law is required to review experiments with institutionally-owned animals, but which may expand its jurisdiction to client-owned animals. Alternatively or in addition to the IACUC, it may be necessary to satisfy a specific clinical trials committee, or other hospital-based quality assurance and patient safety regulatory bodies. Additionally, in the case of many biologics and certainly in the case of any vector using recombinant DNA, it is necessary to secure the approval of an Institutional Biosafety Committee (IBC). Once institutional approvals are in place, most trials proceed in a manner similar to human trials. Initially, a Phase 1 trial is conducted, which is designed specifically to address safety. Some data may be generated in healthy animals in the preclinical phase; however, since these animals do not have tumors, it is necessary to evaluate safety in tumor bearing animals as well, as there can be important differences in physiologic response and immune status. In many cases, Phase 1 trials involve dose escalation to determine the maximum tolerable dose. If the Phase 1 trial is successful, then a Phase 2 trial may commence. Typically, this is a larger study designed to continue to gather safety

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and dosing data, and to examine therapeutic efficacy. Some studies may combine Phase 1 and 2 trials to obtain efficacy data at an earlier time point. Following successful completion of the phase 2 trial, a larger, typically multi-center trial, known as a phase 3 trial, is undertaken to gather more definitive efficacy data, and to compare the experimental therapy to the current standard of care in a larger patient population. In pet animals, the ability to perform clinical trials has been bolstered by the creation of the National Cancer Institute’s (NCI) Comparative Oncology Trial Consortium. As of 2008, 16 veterinary schools were members of the consortium. When a party is interested in performing a large-scale clinical trial, they negotiate directly with NCI, which then coordinates the trial with the consortium members that would like to participate. This approach can either identify large numbers of cases for a major trial, or it can help accumulate rare cases where an investigator may not have sufficient caseload at their own institution.

REFERENCES 1. Bergman PJ, McKnight J, Novosad A, et al. Long-term survival of dogs with advanced malignant melanoma after DNA vaccination with xenogeneic human tyrosinase: a phase I trial. Clin Cancer Res 2003;9:1284–90. 2. Beyer M, Schultze JL. Regulatory T cells in cancer. Blood 2006;108:804–811. 3. Biller BJ, Elmslie RE, Burnett RC, et al. Use of FoxP3 expression to identify regulatory T cells in healthy dogs and dogs with cancer. Vet Immunol Immunopathol 2007;116:69–78. 4. Bird RC, DeInnocentes P, Lenz S, et al. Cross-presentation of a hybrid-cell fusion vaccine against canine mammary cancer. Vet Immunol Immunopathol 2008;123:289–304. 5. Bird RC. Defects in genes regulating the cell cycle in spontaneous canine models of cancer. In: K. Yoshida, ed. Trends in Cell Cycle Research. Kerala, India: Research Signpost, 2009; pp 209–234. 6. Blumenthal M, Skelton D, Pepper KA, et al. Effective suicide gene therapy for leukemia in a model of insertional oncogenesis in mice. Mol Ther 2007;15:183–92. 7. DeInnocentes , Li LX, Sanchez RL, et al. Expression and sequence of canine SIRT2 and p53 alleles in canine mammary tumor cells – effects on down stream targets Wip1 and p21/Cip1. Vet Comp Oncol 2006;4:161–177. 8. Dickerson EB, Fosmire S, Padilla ML, et al. Potential to target dysregulated interleukin-2 receptor expression in canine lymphoid and hematopoietic malignancies as a model for human cancer. J Immunother 2002;25:36–45, Erratum: J Immunother 2002;25:188. 9. Fosmire SP, Thomas R, Jubala CM, et al. Inactivation of the p16 cyclindependent kinase inhibitor in high-grade canine non-Hodgkin’s T-cell lymphoma. Vet Pathol 2007;44:467–78. 10. Grabbe S, Beissert S, Schwarz T, et al. Dendritic cells as initiators of tumor immune responses: a possible strategy for tumor immunotherapy? Immunol Today 1995;16:117–121. 11. Graves SS, Hogan W, Kuhr CS, et al. Stable trichimerism after marrow grafting from 2 DLA-identical canine donors and nonmyeloablative conditioning. Blood 2007;110:418–423.

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12. Helfand SC, Modiano JF, Moore PF, et al. Functional interleukin-2 receptors are expressed on natural killer-like leukemic cells from a dog with cutaneous lymphoma. Blood 1995;86:636–645. 13. Hemminki A, Kanerva A, Kremel EJ, et al. A canine conditionally replicating adenovirus for evaluating oncolytic virotherapy in a syngeneic animal model. Mol Ther 2003;7:163–73. 14. Howe SJ, Mansour MR, Schwarzwaelder K, et al. Insertional mutagenesis combined with acquired somatic mutations causes leukemogenesis following gene therapy of SCID-X1 patients. J Clin Invest 2008;118:3143–3150. 15. Inaba K, Metlay JP, Crowley MT, et al. Dendritic cells pulsed with protein antigens in vitro can prime antigen-specific, MHC-restricted T cells in situ. J Exp Med 1990;172:631–640. 16. Jochum C, Beste M, Zellmer E, et al. CD154 blockade and donor-specific transfusions in DLA-identical marrow transplantation in dogs conditioned with 1-Gy total body irradiation. Biol Blood Marrow Transplant 2007;13:164–171. 17. Kaim U, Moritz A, Failing K, et al. The regression of a canine Langerhans cell tumour is associated with increased expression of IL-2, TNF-alpha, IFN-gamma and iNOS mRNA. Immunology 2006;118:472–482. 18. Kano R, Sato E, Okamura T, et al. Expression of Bcl-2 in feline lymphoma cell lines. Vet Clin Pathol 2008;37:57–60. 19. Kruth SA. Biological response modifiers: interferons, interleukins, recombinant products, liposomal products. Vet Clin N Am Small Anim Pract 1998;28:269–295. 20. Lupu M, Sullivan EW, Westfall TE, et al. Use of multigeneration-family molecular dog leukocyte antigen typing to select a hematopoietic cell transplant donor for a dog with T-cell lymphoma. J Am Vet Med Assoc 2006;228:728–732. 21. Mason NJ, Coughlin CM, Overley B, et al. RNA-loaded CD40-activated B cells stimulate antigen-specific T-cell responses in dogs with spontaneous lymphoma. Gene Ther 2008;15:955–965. 22. Modiano JF, Breen M, Lana SE, et al. Naturally occurring translational models for development of cancer gene therapy. Gene Ther Mol Biol 2006;10:31–40. 23. Modiano JF, Breen M, Valli VE, et al. Predictive value of p16 or Rb inactivation in a model of naturally occurring canine non-Hodgkin’s lymphoma. Leukemia 2007;21:184–187. 24. Portsmouth D, Hlavaty J, Renner M. Suicide genes for cancer therapy. Mol Aspects Med 2007;28:4–41. 25. Smith BF, Curiel DT, Ternovoi VV, et al. Administration of a conditionally replicative oncolytic canine adenovirus in normal dogs. Cancer Biother Radiopharmacol 2006;21:601–606. 26. Sokouowska J, Cywiuska A, Malicka E. p53 expression in canine lymphoma. J Vet Med A 2005;52:172–175. 27. Su ZZ, Sarkar D, Emdad L, et al. Targeting gene expression selectively in cancer cells by using the progression-elevated gene-3 promoter. Proc Natl Acad Sci USA 2005;102:1059–1064. 28. Suter SE, Chein MB, von Messling V, et al. In vitro canine distemper virus infection of canine lymphoid cells: a prelude to oncolytic therapy for lymphoma. Clin Cancer Res 2005;11:1579–1587. 29. Thomas R, Wang HJ, Tsai P-C, et al. Influence of genetic background on tumor karyotypes: evidence for breed-associated cytogenetic aberrations in canine appendicular osteosarcoma. Chromosome Res 2009;(in press). 30. Tsuruta Y, Pereboeva L, Breidenbach M, et al. A fiber-modified mesothelin promoter-based conditionally replicating adenovirus for treatment of ovarian cancer. Clin Cancer Res 2008;14:3582–3588. 31. Woods NB, Bottero V, Schmidt M, et al. Gene therapy: therapeutic gene causing lymphoma. Nature 2006;440:1123. 32. Zhang XB, Schwartz JL, Humphries RK, et al. Effects of HOXB4 overexpression on ex vivo expansion and immortalization of hematopoietic cells from different species. Stem Cells 2007;25:2074–81. 33. Zhang XB, Beard BC, Beebe K, et al. Differential effects of HOXB4 on nonhuman primate short- and long-term repopulating cells. PLoS Med 2006;3:687–698.

SECTION VI

Platelets Mary K. Boudreaux

CHAPTER

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Platelet Structure MARY K. BOUDREAUX Membrane Structure Microdomain Lipid Rafts Receptors Cytoplasmic Structural and Membranous Elements Microtubules Open Canalicular System (OCS) Dense Tubular System (DTS) Mitochondria

Cytoskeleton Granules Dense Granules Alpha Granules Lysosomal Granules

Acronyms and Abbreviations Arp, actin-related protein; DTS, dense tubular system; GMP140, granule membrane protein 140; GP, glycoprotein; LAMP, lysosomal-associated membrane protein; LIMP, lysosomal integral membrane protein; OCS, open canalicular system; PADGEM, platelet-activated derived granule external membrane protein; PMCA, plasma membrane calcium ATPase; PtdIns, phosphatidylinositol; SERCA, sarco/endoplasmic reticulum calcium ATPase; VWF, von Willebrand factor; WASp, Wiskott-Aldrich syndrome protein.

MEMBRANE STRUCTURE Microdomain Lipid Rafts Mammalian platelets are derived from the cytoplasm of megakaryocytes. Platelet membranes are similar to those of other cells and are characterized by phospholipids arranged in a bilayer forming a hydrophobic core. Interspersed within the fluid lipid matrix are densely compacted microdomain lipid rafts primarily composed of sphingolipid and cholesterol molecules, and fewer numbers of protein molecules. These microdomain lipid rafts are capable of lateral mobility and enhance signaling in initial phases of platelet activation as well as enhance cytoskeletal reorganization events critical for normal clot retraction.9 Receptors Proteins and glycoproteins, serving as specialized receptors important in platelet responses are present both within and outside the lipid rafts. Glycoprotein (GP) Ib-IX-V and GPVI, receptors involved in von Willebrand factor (VWF) and collagen binding, respectively, are examples of proteins primarily located within lipid rafts. Glycoprotein complex IIb-IIIa (GPIIb-IIIa), also known as integrin αIbβ3,3 is the most prevalent glycoprotein complex on the surface of platelets and is

located outside of lipid rafts.25 Although GPIIb-IIIa is not located within lipid raft microdomains, the complex does up-regulate phosphatidylinositols, in particular phosphatidylinositol(4,5)bisphosphate (PtdIns(4,5)P2), as part of the outside-in signaling cascade. This process may be facilitated by interaction with tetraspanins located within microdomains distinct from lipid rafts.33 PtdIns(4,5)P2 induces the recruitment of several actinmodulating proteins resulting in the interaction of the cytoskeleton with the microdomains. The interaction of the cytoskeleton with lipid raft microdomains is necessary for sustaining the forces necessary for mediating clot retraction.9 GPIIb-IIIa receptors are important for mediating platelet aggregation by binding fibrinogen, although these complexes can also bind VWF and assist with early stages of platelet adhesion to the subendothelium. The major receptors present on the surface of platelets are summarized in Table 75.1. CYTOPLASMIC STRUCTURAL AND MEMBRANOUS ELEMENTS Microtubules Just beneath the platelet membrane is a circumferential band of microtubules.35 Microtubules are hollow, cylindrical structures composed of protofilaments formed by 561

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TABLE 75.1 Classification and Function of Major Platelet Membrane Receptorsa Classification Integrins αIIbβ3 (GPIIb-IIIa;CD41/ CD61)

αvβ3 (CD51/CD61)

α2β1 (GPIa-IIa;CD49b/ CD29) α5β1 (CD49e/CD29) α6β1 (CD49f/CD29) Leucine-Rich Repeat GPIb-IX-V

Seven Transmembrane Protease activated receptors (PAR) ADP receptors

Thromboxane receptor (TXA2/ PGH2)

Prostacyclin receptor (PGI2) Platelet activating factor (PAF) receptor Adenosine receptor Adrenergic receptor

Serotonin receptor Immunoglobulin/ITAM/ITIM GPVI-FcRγ FcγRIIA FcεRI ICAM-2

Function

References

Known as the fibrinogen receptor. 50,000–80,000 receptors per platelet. Primarily mediates platelet aggregation; platelet activation is required to induce a conformational change resulting in ligand binding. Ligands include several RGD containing proteins including fibrinogen, VWF, thrombospondin, fibronectin, and vitronectin. Glanzmann thrombasthenia (GT) is due to mutations involving genes encoding αIIb or β3 Known as the vitronectin receptor. 200–300 receptors per platelet. Platelet activation is required to induce a conformational change resulting in ligand binding. Ligands include RGD containing proteins such as vitronectin, fibrinogen, and osteopontin Known as VLA2 on lymphocytes. 2000–3000 receptors per platelet. Major collagen adhesion receptor; platelet activation required for collagen binding via this receptor Known as the fibronectin receptor. Functions to supplement platelet adhesion; platelet activation is not required for fibronectin binding Known as the laminin receptor. Functions to supplement platelet adhesion; platelet activation is not required for laminin binding

60

Known as the VWF receptor; also binds Mac-1 and P-selectin. Functions in platelet adhesion under high shear. 60,000 receptors per platelet. BernardSoulier syndrome is due to mutations in genes encoding GPIb or GPIX

2

Function as thrombin receptors. Platelets have 3 PAR receptor types; types vary with species. Human and dog platelets primarily have functional PAR1 and PAR4 types. Mouse platelets have functional PAR3 and PAR4 types P2Y1 and P2Y12 have been identified on platelets. P2Y1 is coupled to Gq and initiates calcium mobilization resulting in shape change and reversible platelet aggregation. P2Y12 is coupled to Gi resulting in inhibition of adenylate cyclase and lowering of cAMP levels. P2Y12 activation is associated with irreversible platelet aggregation Platelets possess two types of receptors, TPα and TPβ, which are coupled to G proteins; TPα is coupled to Gs while TPβ is coupled to Gi. TXA2 and PGH2 interact with both receptor types. While genomic studies suggest both types of receptors are present on platelets from cattle, horses, and cats, there is no genomic evidence for TPβ receptors on dog platelets PGI2 receptors are coupled to Gs. Prostacyclin binding results in activation of adenylate cyclase, enhanced cAMP production, and inhibition of platelet activation PAF mediates platelet shape change and aggregation. PAF receptors are coupled to Gi and Gq. PAF can also initiate signal-dependent translation pathways in human platelets 3 receptor subtypes have been detected on platelets. Adenosine receptor activation results in cAMP elevation and inhibition of platelet aggregation alpha2A-adrenergic receptors are present on platelets which are coupled to Gi. Epinephrine binding results in reduction of cAMP and potentiation of platelet activation. May play a role in enhancing thrombus stability Coupled to G-proteins leading to calcium mobilization and enhancement of platelet activation. Two subtypes have been detected on human platelets

10, 16

Major collagen receptor. Composed of two immunoglobulin C2 loops which bind collagen. Is complexed with FcRγ which contains an ITAM which is tyrosine phosphorylated during platelet activation Immune receptor expressed on human but not mouse platelets. Binds IgG. Plays a role in heparin-induced thrombocytopenia IgE immune receptor. Activation induces release of serotonin and RANTES and thus may play a role in allergic inflammatory responses Present on the surface of activated and non-activated platelets. The only β2 integrin ligand on platelets; may play a role in leukocyte-platelet interactions

4, 62

47

56 29, 74

61

79, MK Boudreaux, unpublished observations

75

85

1, 31 66

1

34

13 36 24

CHAPTER 75: PLATELET STRUCTURE

563

TABLE 75.1 Continued Classification PECAM-1 (CD31)

CD47

Tetraspanins CD9

CD82 CD151 (PETA-3) TSSC6

Others P2X1 CD36 (GPIV, GPIIIb)

C1q Serotonin reuptake

Glycosyl phosphatidylinositol (GPI) anchored proteins

Thrombopoietin receptor

Function

References

Composed of six C2 immunoglobulin domains, a transmembrane domain, and a short cytoplasmic tail containing two ITIM domains. Plays dual roles as an inhibitor and enhancer of platelet activation. Has an inhibitory effect on ITAM-based collagen responses and also promotes αIIbβ3 mediated outside-in signaling Thrombospondin receptor; also known as integrin-associated protein (IAP). TSP binding results in enhanced platelet activation via inhibition of nitric oxide/cGMP signaling

46, 80

Present on resting platelet plasma membrane in association with αIIbβ3. May be involved in recruiting phosphatidylinositol kinase activity, specifically PI4K, to integrin-based signaling complexes Studies in lymphocytes suggest a role in linking signaling through raft microdomains with the actin cytoskeleton. Presence on platelets not clear Present on resting platelet plasma membrane in association with αIIbβ3, α5β1, and α6β1. May play a role in integrin-mediated signaling Associated with αIIbβ3 in resting platelets. As with other tetraspanins are associated with distinct microdomains that associate with lipid rafts during platelet activation, thus facilitating signal transduction events, including outside-in integrin signaling

39

ATP receptor that enhances calcium entry. Plays a role in reinforcing and amplifying platelet activation Ligands include thrombospondin, modified phospholipids, and long-chain fatty acids. Contributes to thrombus formation by binding to phosphatidylserine exposed on endothelial cell-derived micro-particles resulting in enhanced platelet activation May play a role in classical complement pathway activation on platelet surfaces, particularly under high-shear induced platelet activation situations Receptor is distinct from serotonin activation receptor. Responsible for serotonin transport across the platelet plasma membrane. Selective serotonin reuptake inhibitors inhibit platelet reactivity Platelets contain at least 5 GPI anchored glycoproteins on their surface. These include CD55, CD59, CD109, and prion protein. Platelets contain >96% of prion protein in human blood. Prion proteins are primarily present within alpha granule membranes and become expressed with platelet activation. Prion proteins are associated with platelet-derived microparticles and exosomes following thrombin-induced platelet activation A member of the tyrosine kinase receptor family. Important for regulating platelet production. Receptors are internalized when bound by TPO. TPO binding does not activate platelets but does enhance platelet activation and may contribute to the adverse prothrombotic events observed in unstable angina in people

40

21 29 33

61 32

63 72

68

53

a

ITAM, immunoreceptor tyrosine-based activation motif; ITIM, immunoreceptor tyrosine-based inhibitory domains; PETA-3, platelet and endothelial cell tetraspan antigen; RANTES, regulated on activation, normal T expressed, and presumably secreted; PI4K, phosphatidylinositol 4 kinase; TPO,thrombopoietin.

alpha-beta tubulin dimers arranged in a helical headto-tail fashion. Microtubules are responsible for the maintenance of the disc-shaped form of the circulating platelet.43 Under normal physiologic conditions, β1tubulin is solely expressed as a component of microtubules within megakaryocytes and plays a role in the orderly fragmentation of platelets from the megakaryocyte cytoplasm.70 Open Canalicular System (OCS) Connected to the surface of the platelet and extending deep within its cytoplasm is an extremely tortuous

maze of open interconnecting channels called the surface connected canalicular system or open canalicular system (OCS).81 This system was thought to be a remnant of the demarcation membrane system of megakaryocytes but the absence of the OCS in some mammals86 and the presence of the OCS in thrombocytes of birds77 and fish18,52 has cast some doubt as to its origin. The OCS can act as a conduit for the uptake of particles and for the release of granule contents by activated platelets. Particles such as viruses or bacteria can be taken up by the OCS; this activity usually results in platelet activation. Alpha and lysosomal granules have been documented to fuse and evacuate their contents

564

SECTION VI: PLATELETS

into the OCS around engulfed bacteria or viruses, but communication with the outside is always maintained, unlike phagocytic vacuoles of leukocytes. Thus, platelets are not considered to be true phagocytes.82 The OCS is everted to some extent during platelet activation and can thus serve to increase the number of surface receptors available for ligand binding since many of the same receptors found on the platelet surface also line the OCS.27 Ruminant and equine platelets do not possess an OCS30 although their megakaryocytes do possess a demarcation membrane system.78 Platelets from these species do not enhance their surface area during activation, do not have a mechanism for uptake of particles, and release their granule contents by fusion of granules directly to the outer membrane. Figures 75.1 and 75.2 are electron micrographs of platelets

obtained from several species for comparison of overall appearance. Dense Tubular System (DTS) Another specialized cytoplasmic organelle is the dense tubular system (DTS). The DTS is a remnant of the smooth endoplasmic reticulum of the megakaryocyte and is located near the microtubules. Unlike the OCS, it does not communicate with plasma or granule membranes. DTS membranes contain cyclooxygenase and thromboxane synthase and thus participate in prostaglandin synthesis. The DTS is also a site for calcium sequestration by the platelet; calcium movement is facilitated by ATPase pumps termed SERCAs (sarco/ endoplasmic reticulum calcium ATPases) which are

A

B

C

D

FIGURE 75.1 Electron micrographs of platelets obtained from alpaca (A), cow (B), goat (C), and sheep (D) for overall comparison of platelet morphology. Alpha granules appear light to dark gray and are prevalent and variably sized and shaped in these species. The electron dense content of most of the dense granules has been lost leaving vacuolar-like spaces. Microtubules can be observed in the periphery of some platelets. An open canalicular system is not present in these species. Bar = 2 μm. (Courtesy of Dr. Maria Toivio-Kinnucan, Auburn University.)

CHAPTER 75: PLATELET STRUCTURE

565

tightly coupled to plasma membrane calcium ATPases (PMCAs). Plasma membrane calcium ATPases function to move calcium ions to the extracellular medium while SERCAs pump calcium ions into the DTS. Calcium stores in addition to the DTS also exist in platelets but their specific location is poorly defined.50 Mitochondria Platelets have mitochondria important for maintaining energy and providing metabolic energy requirements during aggregation and secretion events. Mitochondrial membrane potential collapse is associated with platelet aging in circulation.64 CYTOSKELETON A

B

Platelets possess a cytoskeleton on the cytoplasmic side of the plasma membrane composed of actin, spectrin (fodrin), adducin, filamin, and glycoprotein Ib-IX-V (the VWF receptor). The membrane cytoskeleton interacts with an extensive cytoplasmic actin network. Adducin links the ends of cytoplasmic actin filaments to membrane associated spectrin strands. Filamin subunits bind to actin and to the cytoplasmic tails of the GPIbα subunits resulting in strengthening of the interaction of the spectrin/actin network with the plasma membrane.35 During platelet activation adducin molecules dissociate from the ends of actin filaments allowing disassembly and reorganization of actin. Actin disassembly is promoted by gelsolin and cofilin. Gelsolin undergoes a conformational change in response to increased cytoplasmic calcium allowing the protein to interdigitate and disrupt actin filaments. Cofilin becomes activated as a result of dephosphorylation during platelet activation and functions to sever and promote actin disassembly along with gelsolin. Actin reassembly during the formation of filopodia and lamellipodia by activated platelets is mediated by the actinrelated protein (Arp) 2/3 complex. Proteins involved in activation of the Arp 2/3 complex include cortactin and Wiskott-Aldrich syndrome (WASp) family proteins. Actin reassembly and myosin II binding and activation are critical for platelet spreading, granule movement, granule release, and clot retraction. GRANULES

C FIGURE 75.2 Electron micrographs of platelets obtained from cat (A), dog (B), and horse (C). Alpha granules are numerous and variable in size and shape as seen with ruminant platelets. An open canalicular system is present in cat and dog platelets but absent in horse platelets. Bar = 2 μm. (Courtesy of Dr. Maria Toivio-Kinnucan, Auburn University.)

Platelets have three main types of storage granules, dense, alpha, and lysosomal, and they are heterogeneous in content and morphology.49 Alpha and dense granules are thought to be derived from a common multivesicular body intermediate, and thus their membranes share several common components.84 (Table 75.2). Dense Granules The dense granules or dense bodies, as their name implies, are electron dense when viewed using an elec-

566

SECTION VI: PLATELETS

TABLE 75.2 Granule membrane proteins Classification Alpha granules GTP-binding proteins Glycoproteins Ib-IX-V Glycoproteins IIb-IIIa (CD41/CD61; αIIbβ3) P-Selectin (CD62P) Glucose transporter-3 (Glut-3) PECAM-1 Vitronectin receptor GPIV (CD36) CD9 Osteonectin Dense granules Hydrogen ion-pumping ATPase GTP-binding proteins Glycoprotein Ib Glycoproteins IIb–IIIa P-Selectin Lysosome-associated membrane proteins (LAMP-2 and LAMP-3/CD63) Vesicular monoamine transporter (VMAT) Nucleotide transporter (MRP4) Orphan sugar nucleotide transporter (Slc34d3) Lysosomal granules LAMP-1 CD107a LAMP-2 CD107b LAMP-3 CD63, LIMP LIMP II LGP85

References

7, 48 8 8 41 37 17 65 6 17 11 20 54 83 83 41 41, 42 12 44 15 59 59 59 59

tron microscope, and serve as storage sites for adenine nucleotides, serotonin, calcium, and inorganic phosphates.57,67 The adenine nucleotide (ADP/ATP) ratio is roughly 1.5 within dense granules. The pool is metabolically inert and not available as an energy source. Serotonin is actively transported from plasma into dense granule cores and the serotonin concentration within dense granules is 1000 times higher than plasma concentrations. Dense granules contain 70% of the total calcium within platelets. This calcium pool is not mobilized during platelet activation, unlike the calcium pool within the DTS. Inorganic phosphates, calcium, serotonin, and adenine nucleotides are held together tightly within dense granules as a result of intermolecular forces, thus contributing to their stability and density.73 Human and pig platelet dense granules contain high levels of lysolecithin;23 however, lysolecithin was not detectable in the dense granules of rabbit platelets.19 Dense granules also contain ganglioside GM3 and phospholipids including phosphatidylethanolamine, phosphatidylinositol, phosphatidylcholine, and sphingosine.14 Organelle proteomics has led to the discovery of 40 proteins within dense granules, many of which had not been known to be present within this organelle. Dense granule protein categories include cell signaling proteins, molecular chaperones, cytoskeletal proteins, proteins involved in glycolysis, and proteins involved in platelet function.38 Dense granule membranes contain

several receptors that become expressed on the plasma membrane as a result of dense granule fusion with the surface during the platelet release reaction. Some of these receptors are shared with other granule membranes and with the plasma membrane (Table 75.2). Alpha Granules The alpha granules, the largest and most numerous of the platelet granules, correspond to the azurophilic granules viewed by light microscopy. They are unique in terms of structure and packaging function. Structurally, alpha granules are composed of two major compartments, a dark centrally located nucleoid region and a peripherally located electron-lucent gray matrix region, that can be viewed at the electron microscopic level. The nucleoid region contains proteoglycans which confer stability to the granules. The nucleoid region is where beta-thromboglobulin and platelet factor 4 as well as other proteins are localized. Von Willebrand factor, multimerin, and factor V colocalize within tubular structures located in the outer part of the gray matrix region. Fibrinogen, thrombospondin, and fibronectin are primarily located in the gray matrix region in an area between the nucleoid and outer gray matrix region. Compartmentalization of VWF and fibrinogen within alpha granules has been documented in mouse, human, and porcine platelets.69 Compartmentalization of proteins may serve to allow platelets to differentially release proteins in response to activation.71 Mouse alpha granules are more heterogeneous in terms of shape, size, and distribution compared to human alpha granules.69 Alpha granules contain proteins that are synthesized by megakaryocytes as well as proteins that are endocytosed as platelets circulate in blood. Alpha granule proteins that are synthesized by megakaryocytes fall into two categories, those that are megakaryocyte-specific (such as betathromboglobulin and platelet factor 4) and those that are synthesized by other cells but are concentrated within platelet alpha granules (such as factor V and PDGF).76 The latter proteins are sometimes referred to as platelet-selective proteins. The most well-recognized protein endocytosed by platelets is fibrinogen with uptake being mediated by the glycoprotein IIb-IIIa (integrin αIIbβ3) receptor. Proteins present in low levels within alpha granules such as albumin and immunoglobulin are passively taken up by platelets and do not require specific receptors. A proteomic analysis of human platelet alpha granule proteins has been recently published and provides a list of 284 proteins, 44 of which have not been previously identified within platelet alpha granules.55 Alpha granule membrane proteins, much like dense granule membrane proteins, become expressed on the platelet surface after platelet secretion as a result of fusion of granules with the OCS or outer platelet membrane. The most well known alpha granule membrane protein expressed on the surface of activated platelets is P-selectin or CD62P (formerly known as granule membrane protein 140 (GMP140)45 and as platelet-activated derived granule external membrane

CHAPTER 75: PLATELET STRUCTURE

protein (PADGEM)).51 Since its discovery, P-selectin has also been detected on the membranes of dense granules.41 Monoclonal antibodies have been developed to detect the presence of P-selectin on the surface of activated platelets in dogs.26 Other proteins present within alpha granule membranes include GPIb-IX-V, GPIIbIIIa, PECAM-1, vitronectin receptor, and GPIV (Table 75.2). Some of these membrane proteins have also been detected within the membranes of dense granules.83 Lysosomal Granules Lysosomal granules contain acid-dependent hydrolases including glycosidases, proteases, and lipases.22 Lysosomal granules can be identified in platelets and megakaryocytes using specific cytochemical stains.5 Secondary lysosomes have been detected in bovine megakaryocytes and platelets.58 Lysosomal membranes also contain proteins that become exteriorized during the platelet release reaction (Table 75.2). Membrane proteins, which include lysosomal integral membrane protein (LIMP or CD63) and lysosomal-associated membrane proteins 1 and 2 (LAMP-1 and LAMP-2),28 are heavily glycosylated to protect them from hydrolytic enzymes stored within these granules.59 While initially LAMPs were thought to be solely present within lysosomal granule membranes, LAMP-2 and LAMP-3 have been detected within dense granule membranes as well.42 Lysosomal membrane proteins which mediate transport of ions and amino acids across the membrane necessary for maintenance of an acidic luminal pH are also present.

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42. Israels SJ, McMillan EM, Robertson C, et al. The lysosomal granule membrane protein, LAMP-2, is also present in platelet dense granule membranes. Thromb Haemost 1996;75:623–629. 43. Italiano JE, Bergmeier W, Tiwari S, et al. Mechanisms and implications of platelet discoid shape. Blood 2003;101:4789–4796. 44. Jedlitschky G, Tirschmann K, Lubenow LE, et al. The nucleotide transporter MRP4 (ABCC4) is highly expressed in human platelets and present in dense granules, indicating a role in mediator storage. Blood 2004;104: 3603–3610. 45. Johnston GI, Cook RG, McEver RP. Cloning of GMP140, a granule membrane protein of platelets and endothelium; sequence similarity to proteins involved in cell adhesion and inflammation. Cell 1989;56:1033–1044. 46. Jones KL, Hughan SC, Dopheide SM, et al. Platelet endothelial cell adhesion molecule-1 is a negative regulator of platelet-collagen interactions. Blood 2001;98:1456–1463. 47. Jung SM, Moroi M. Signal-transducing mechanisms involved in activation of the platelet collagen receptor integrin alpha(2)-beta(1). J Biol Chem 2000;275:8016–8026. 48. Karniguian A, Zahraoui A, Tavitian A. Identification of small GTP-binding rab proteins in human platelets: thrombin-induced phosphorylation of rab3B, rab6, and rab8 proteins. Proc Natl Acad Sci USA 1993;90: 7647–7651. 49. King SM, Reed GL. Development of platelet secretory granules. Semin Cell Devel Biol 2002;13:293–302. 50. Kovacs T, Berger G, Corvazier E, et al. Immunolocalization of the multisarco/endoplasmic reticulum Ca2+ ATPase system in human platelets. Br J Haematol 1997;97:192–203. 51. Larsen E, Celi A, Gilbert GE, et al. PADGEM protein: a receptor that mediates the interaction of activated platelets with neutrophils and monocytes. Cell 1989;59:305–312. 52. Lewis JH. Comparative Hemostasis in Vertebrates. New York: Plenum Press, 1996. 53. Lupia E, Bosco O, Bergerone S, et al. Thrombopoietin contributes to enhanced platelet activation in patients with unstable angina. J Am Coll Cardiol 2006;48:2195–2203. 54. Mark BL, Jilkina O, Bhullar RP. Association of Ral GTP-binding protein with human platelet dense granules. Biochem Biophys Res Commun 1996;225:40–46. 55. Maynard DM, Heijnen HFG, Horne MK, et al. Proteomic analysis of platelet alpha-granules using mass spectrometry. J Thromb Haemost 2007;5:1945–1955. 56. McCarty OJ, Zhao Y, Andrew N, et al. Evaluation of the role of platelet integrins in fibronectin-dependent spreading and adhesion. J Thromb Haemost 2004;2:1823–1833. 57. McNicol A, Israels SJ. Platelet dense granules: structure, function and implications for haemostasis. Thromb Res 1999;95:1–18. 58. Menard M, Meyers KM, Prieur DJ. Demonstration of secondary lysosomes in bovine megakaryocytes and platelets using acid phosphatase cytochemistry with cerium as a trapping agent. Thromb Haemost 1990;63:127–132. 59. Metzelaar MJ, Clevers HC. Lysosomal membrane glycoproteins in platelets. Thromb Haemost 1992;68:378–382. 60. Nurden AT. Glanzmann thrombasthenia. Orphanet J Rare Dis 2006;1:10. 61. Oury C, Toth-Zsamboki E, Vermylen J, et al. The platelet ATP and ADP receptors. Curr Pharm Des 2006;12:859–875. 62. Paul BZ, Vilaire G, Kunapuli SP, et al. Concurrent signaling from Galphaq- and Galphai-coupled pathways is essential for agonist-induced alpha v beta3 activation on human platelets. J Thromb Haemost 2003;1: 814–820. 63. Peerschke EI, ulYin W, Grigg SE, et al. Blood platelets activate the classical pathway of human complement. J Thromb Haemost 2006;4:2035–2042. 64. Pereira J, Soto M, Palomo I, et al. Platelet aging in vivo is associated with activation of apoptotic pathways: studies in a model of suppressed thrombopoiesis in dogs. Thromb Haemost 2002;87:905–909.

65. Poujol C, Nurden AT, Nurden P. Ultrastructural analysis of the distribution of the vitronectin receptor (alpha v beta 3) in human platelets and megakaryocytes reveals an intracellular pool and labeling of the alphagranule membrane. Br J Haematol 1997;96:823–835. 66. Pozgajova M, Sachs UJH, Hein L, et al. Reduced thrombus stability in mice lacking the α2A-adrenergic receptor. Blood 2006;108:510–514. 67. Rendu F, Brohard-Bohn B. The platelet release reaction: granules’ constituents, secretion and functions. Platelets 2001;12:261–273. 68. Robertson C, Booth SA, Beniac DR, et al. Cellular prion protein is released on exosomes from activated platelets. Blood 2006;107:3907–3911. 69. Schmitt A, Guichard J, Masse J-M, et al. Of mice and men: comparison of the ultrastructure of megakaryocytes and platelets. Exp Hematol 2001;29: 1295–1302. 70. Schwer HD, Lecine P, Tiwari S, et al. A lineage-restricted and divergent beta-tubulin isoform is essential for the biogenesis, structure and function of blood platelets. Curr Biol 2001;11:579–586. 71. Sehgal S, Storrie B. Evidence that differential packaging of the major platelet granule proteins von Willebrand factor and fibrinogen can support their differential release. J Thromb Haemost 2007;5:2009–2016. 72. Serebruany VL, Glassman AH, Malinin AI, et al. Selective serotonin reuptake inhibitors yield additional antiplatelet protection in patients with congestive heart failure treated with antecedent aspirin. Eur J Heart Failure 2003;5:517–521. 73. Skaer RJ, Peters PD, Emmines JP. The localization of calcium and phosphorus in human platelets. J Cell Sci 1974;15:679–692. 74. Sonnenberg A, Gehlsen KR, Aumailley M, et al. Isolation of alpha 6 beta 1 integrins from platelets and adherent cells by affinity chromatography on mouse laminin fragment E8 and human laminin pepsin fragment. Exp Cell Res 1991;197:234–244. 75. Stitham J, Arehart E, Gleim SR, et al. New insights into human prostacyclin receptor structure and function through natural and synthetic mutations of transmembrane charged residues. Br J Pharmacol 2007;152: 513–522. 76. Suehiro Y, Veljkovic DK, Fuller N, et al. Endocytosis and storage of plasma factor V by human megakaryocytes. Thromb Haemostasis 2005;94:585–592. 77. Taffarel M, Oliveira MP. Cytochemical analysis of the content of chicken thrombocytes vacuoles. Cell Biol Intl 1993;17:993–999. 78. Topp KS, Tablin F, Levin J. Culture of isolated bovine megakaryocytes on reconstituted basement membrane matrix leads to proplatelet process formation. Blood 1990;76:912–924 79. Walsh M-T, Foley Jf, Kinsella T. Investigation of the role of the carboxyterminal tails of the α and β isoforms of the human thromboxane A2 receptor (TP) in mediating receptor:effector coupling. Biochim Biophys Acta 2000;1496:164–182. 80. Wee JL, Jackson DE. The Ig-ITIM superfamily member PECAM-1 regulates the “outside-in” signaling properties of integrin alpha(IIb)beta3 in platelets. Blood 2005;106:3816–3823. 81. White JG, Escolar G. The blood platelet open canalicular system: a twoway street. Eur J Cell Biol 1991;56:233–42. 82. White JG. Platelets are covercytes, not phagocytes: uptake of bacteria involves channels of the open canalicular system. Platelets 2005;16: 121–131. 83. Youssefian T, Masse JM, Rendu F, et al. Platelet and megakaryocyte dense granules contain glycoproteins Ib and IIb-IIIa. Blood 1997;89: 4047–4057. 84. Youssefian T, Cramer EM. Megakaryocyte dense granule components are sorted in multivesicular bodies. Blood 2000;95:4004–4007. 85. Zimmerman GA, McIntyre TM, Prescott SM, et al. The platelet-activating factor signaling system and its regulators in syndromes of inflammation and thrombosis. Crit Care Med 2002;30:S294–S301. 86. Zucker-Franklin D, Benson K, Myers K. Absence of surface-connected canalicular system in bovine platelets. Blood 1985;65:241–244.

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Platelet Biochemistry, Signal Transduction, and Function MARY K. BOUDREAUX and JAMES L. CATALFAMO Overview of Platelet Function Platelet Receptors and Activation Pathways ADP/ATP Receptors Collagen Receptors Thromboxane Receptors Thrombin Receptors

Platelet-Leukocyte Interactions Procoagulant Activity

Acronyms and Abbreviations AA, arachidonic acid; CalDAG-GEFI, calcium diacylglycerol guanine nucleotide exchange factor I; CCL2, chemokine ligand-2; cPLA2, cytoplasmic phospholipase A2; CXCL8, chemokine IL-8; DAG, 1,2 diacylglycerol; DTS, dense tubular system; GIRK, G protein-gated inwardly rectifying potassium channel; GP, glycoprotein; ICAM-1, intercellular adhesion molecule–1; IP3, inositol triphosphate; ITAM, immunoreceptor tyrosine-based activation motif; LAT, linker for activation of T cells; LFA-1, lymphocyte function associate antigen-1; MLC, myosin light chain; NHE1, Na/H exchanger; PAK, p21-activated kinase; PAR, protease activated receptor; PE, phosphatidylethanolamine; PI3K, phosphoinositide-3-kinase; PIP, phosphatidylinositol; PIP3, phosphatidyl-inositol(3,4,5) trisphosphate; PIP2, phosphatidylinositol 4,5-bisphosphate; PKC, protein kinase C; PLC, phospholipase C; PS, phosphatidylserine; PSGL-1, P-selectin glycoprotein ligand; ROCK, Rho associated coiled-coil-containing kinase; SLP-76, Src homology domain-containing leukocyte phosphoprotein of 76 kDa; TF, tissue factor; TP, thromboxane receptor; TRAP, thrombin receptor activation peptide; TXA, Thromboxane A2; VCAM-1, vascular cell adhesion molecule–1. OVERVIEW OF PLATELET FUNCTION Platelets are the first line of defense against bleeding at sites of vascular injury and are major contributors to thrombosis, inflammation and neoplasia. Platelets have cell surface receptors that recognize signals from their environment and communicate those signals to a complex network of biomolecules that include ions, proteins, nucleotides and phospholipids. While the details of many key networking pathways that elicit platelet responses are known, others remain to be elucidated. Platelet reactions orchestrated via these complex signaling networks include adhesion, aggregation, granule release, and procoagulant expression. When molecules specifically recognized by platelets (agonists) bind to platelet receptors, they induce insideout signaling resulting in structural changes in glycoproteins on the platelet membrane surface that then allow binding of proteins that mediate platelet adhesion and aggregation (Fig. 76.1; see also Chapter 81, Fig. 81.1). In turn, binding of adhesive proteins to receptors results in outside-in signaling events that promote and enhance platelet granule release, platelet aggregate and fibrin formation, and clot retraction.

This chapter reviews platelet signaling events communicated through the major activation receptors present on platelets as well as pathways and receptors involved in platelet-leukocyte interactions and expression of procoagulant activity. Cross-talk between platelet receptors and signaling molecules is a key component of platelet activation, and much is still to be learned about the players involved in these events. Added to the complexity of understanding platelet signaling events is the recognition that platelets are capable of synthesizing proteins.68 This capability, referred to as signal-dependent translation, will likely change current viewpoints of platelet involvement in many events including hemostasis, thrombosis, inflammation, and neoplasia.

PLATELET RECEPTORS AND ACTIVATION PATHWAYS ADP/ATP Receptors Platelets have three distinct P2 purinergic receptors, P2Y1, P2Y12, and P2X1.16 P2Y1 and P2Y12 are G 569

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αIIb β3

α2 β1 Resting state

P2Y1

Fibrinogen Resting state

Collagen Inside-out activation Outside-in activation Collagen PLATELET CYTOSOL

Inside-out activation

Hi Ca++ inactivation

Fibrinogen

Outside-in activation FFi i iibrinogen

ADP

Adhesion, Shape change, Aggregation, Secretion, Procoagulant Activity

P2Y12 Gαi Gαq Gα12/13

cAMP PAR1 Thrombin

Fibrinogen

Src, Syk

Collagen

PAR4

PS exposure

FIGURE 76.1 Platelet activation transforms the resting state of platelet membrane adhesion receptors for collagen (α2β1) and fibrinogen (αIIbβ3) by altering their conformation states. Effector molecules generated by various signaling pathways in response to outside-in or inside-out signaling control the strength, reversibility or irreversibility of collagen or fibrinogen binding. Formation of tight and irreversible complexes consolidates platelet adhesion and aggregation. When platelet ionized Ca2+ is elevated for prolonged periods αIIbβ3 becomes secondarily inactivated and can no longer bind fibrinogen.

protein-coupled ADP receptors, mediating platelet shape change and aggregation responses, while P2X1 is a ligand-gated cation channel ATP receptor responsible for fast calcium entry and may play a role in collagenand shear-induced platelet activation (Fig. 76.2).33,43,44 P2Y1 is coupled to a Gαq protein subunit; signaling via Gαq results in activation of phospholipase C β2 (PLCβ2), Rho A, Rac, and Src family kinases.25 PLCβ2 mediates intracellular calcium mobilization and protein kinase C (PKC) activation via hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) within platelet membranes, to inositol triphosphate (IP3) and 1,2 diacylglycerol (DAG).30,44 IP3 mediates calcium mobilization from intraplatelet stores sequestered within the dense tubular system (DTS). DAG and ionized calcium activate diacylglycerol guanine nucleotide exchange factor I (CalDAG-GEFI) which in turn facilitates the displacement of GDP and loading of GTP within Rap1b. GTP-bound Rap 1b plays an important role in mediating the change in conformation of αIIbβ3 necessary for fibrinogen binding and platelet aggregation. The importance of CalDAG-GEFI downstream of PLC β2 in mediating inside-out signaling events leading to activation of αIIbβ3 is illustrated in mice, dogs, and cattle with mutations in CalDAG-GEFI.5,6,14 All of these animals have a bleeding diathesis and impaired platelet responses to ADP and collagen. P2Y1 activation results in platelet shape change as well as a reversible wave of platelet aggregation. Platelet shape change occurs as a result of both calcium-sensitive and calcium-insensitive pathways contributing to phosphorylation of myosin light chain (MLC).45 The

GPIa α2 GPIIa β1 Collagen FcRγ GPVI

FIGURE 76.2 G protein-coupled receptors (GPCRs) link plateletsignaling pathways. The extracellular domains of paired receptors for ADP (P2Y1 and P2Y12), thrombin (PAR1 and PAR4) and collagen (α2β1 and GPVI) interact with their specific ligands to mediate outside-in signaling pathways. The thrombin receptor is activated when thrombin binds to and cleaves the extracellular N-terminal domain unmasking a new tethered N terminus that binds to the second extracellular loop of the receptor. In contrast to α2β1 which is a GPCR, GPVI signals collagen-initiated platelet activation through sequential activation of Src and Syk family tyrosine kinases.

calcium-sensitive pathway is mediated by calciumcalmodulin. The calcium-insensitive pathway is mediated by Rho A/p160ROCK downstream of Gαq. Rho associated coiled-coil-containing kinase (p160ROCK) is a serine/threonine protein kinase that is directly targeted by Rho A. p160ROCK activation alters the balance between MLC phosphatases and MLC kinases in favor of MLC kinases, resulting in MLC kinase mediated phosphorylation of MLC necessary for platelet shape change.15 The RhoA/p160ROCK pathway is important in mediating the disruption of the microtubule ring necessary for transformation of platelets from discs to spheres and is also critical for maintaining the change in shape once shape change has occurred.46 Rac, a Rho family GTPase that is activated by P2Y1, is thought to be important in mediating platelet spreading and anchoring via activation of a p21-activated kinase (PAK).4 Activated PAK is thought to activate the serine/ threonine kinase LIM which targets and phosphorylates cofilin, inhibiting cofilin’s ability to depolymerize actin.3 Rac activation by P2Y1 is potentiated by P2Y12 signaling.58 In contrast, P2Y1 signals Src kinase

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activation and the inhibition of the phosphoinositide-3kinase(PI3K)-dependent phase of P2Y12 signaling. This inhibition is thought to be an important negative feedback mechanism regulating platelet activation mediated by ADP.25 P2Y12 activation does not contribute to platelet shape change but is associated with platelet aggregation and granule release, thromboxane generation, expression of procoagulant activity and inhibition of adenylyl cyclase.59 Engagement of the P2Y12 is also required for amplification of platelet activation induced by ADP and low concentrations of other platelet agonists including TXA2, collagen and thrombin. P2Y12 is coupled to the heterotrimeric G protein complex Gαi2βγ.66 ADP binding to P2Y12 results in uncoupling of Gαi2 and Gβγ which act as separate signaling molecules. Gαi2 inhibits adenylate cyclase resulting in lowering of cAMP levels. Gβγ dimers activate PI3K, Akt/protein kinase B, Rap1b, and G protein-gated inwardly rectifying potassium channels (GIRKs).33 The PI3K isoforms that function downstream of Gβγ in platelets are PI3Kγ and PI3Kβ. The two isoforms seem to function in distinct but complementary roles leading to platelet aggregation and release. PI3Kγ plays a role in enhancing granule secretion while PI3Kβ is necessary for sustaining platelet aggregation and formation of stable thrombi.29 Akt/protein kinase B activation is thought to be mediated via PI3Kγ. Deficiency of Akt/protein kinase B in mice results in impaired platelet aggregation and release, and like PI3Kβ, it is required for formation of stable thrombi.65 PI3Kγ and PI3Kβ, via generation of phosphatidyl-inositol (3,4,5) trisphosphate (PIP3), activate Rap1b, a critically important step in mediating the αIIbβ3 conformational change required for fibrinogen binding and platelet aggregation.37 GIRK channels containing GIRK1 and GIRK4 subunits are involved in the activation of Src kinases. Channel activation is coupled to Gβγ and ADP occupancy of the P2Y12 receptor. Src kinases in turn phosphorylate and activate cytoplasmic phospholipase A2 (cPLA2).56 P2Y1 activation and signaling via Gq as well as signaling through αIIbβ3 are also necessary for cPLA2 activation.31 When phosphorylated, cPLA2 is translocated from the cytoplasm to membranes where it functions to release membrane-bound arachidonic acid leading to the generation of thromboxane. Human patients with congenital P2Y12 deficiency and P2Y12 knock-out mice have prolonged bleeding times and exhibit bleeding, illustrating the importance of this receptor in hemostasis. The absent aggregation response of P2Y12 deficient platelets can be restored if platelets are activated concomitantly with ADP and epinephrine, presumably due to epinephrine being negatively coupled to adenylate cyclase in a manner similar to P2Y12.10 The P2X1 receptor is an ATP receptor that is antagonized by ADP; early studies demonstrating P2X1 receptor responses to ADP were later shown to be a result of ATP contamination of ADP preparations.38 P2X1 activation is associated with the change in conformation of platelets from discs to spheres with the rapidly reversible formation of multiple short pseudopodia.50,51 ATP

571

binding to P2X1 is associated with a rapid, reversible calcium influx followed by rapid desensitization of the receptor.42 Calcium influx is thought to activate MLCkinase which is responsible for mediating the shape change response. P2X1 activation also results in centralization of granules without granule release and formation of platelet microaggregates.21,61 Microaggregate formation is likely a result of calcium influx coupled with protein kinase C activation leading to Erk2 activation.33 Like the P2Y12 receptor, P2X1 also participates in amplification of platelet responses to low concentrations of other agonists, including ADP, collagen, thrombin, and thromboxane.21,22,42 Dense granule release of ATP induced by low concentrations of agonists triggers P2X1-mediated calcium influx resulting in reinforcement and amplification of platelet activation. P2X1 also seems to be essential in thrombus formation and growth at sites of injury in vessels where high shear forces are generated.26 Collagen Receptors Platelets have two receptors that are primarily responsible for direct response to collagen; glycoprotein VI (GPVI) and integrin α2β1.49 (Fig. 76.2) The platelet glycoprotein Ib-IX-V complex, which interacts with collagen indirectly, is critically important in initiating platelet contact with collagen under high shear rate conditions. The glycoprotein Ib-IX-V complex mediates transient arrest of platelets from flowing blood and weak tethering of platelets on exposed subendothelial surfaces through the binding of von Willebrand factor (VWF) A1 domain to the GPIbα region of the receptor complex. As tethered platelets roll along exposed subendothelium they encounter collagen fibrils, which bind to glycoprotein VI. This leads to inside-out signaling and activation of integrins α2β1 and αIIbβ3 and the release of ADP and thromboxane formation which in turn reinforce GPVI interactions.41 Activated α2β1, transformed to a high affinity state, binds tightly to specific sequences in collagen, allowing firm platelet adhesion and spreading.34 Activated αIIbβ3, also transformed to a high affinity state, reinforces firm platelet adhesion and supports platelet aggregation.53 Integrin α2β1 and GPVI are thought to signal through a shared network of signal transduction proteins. The shared signaling pathway possibly explains why the absence of either GPVI or integrin α2β1 does not result in a significant bleeding diathesis. Glycoprotein VI is a member of the immunoglobulin superfamily Type I transmembrane proteins; it is noncovalently associated with the Fc receptor gamma-chain (FcR γ-chain). The FcR γ-chain has an immunoreceptor tyrosine-based activation motif (ITAM). When stimulated with collagen, the FcRγ-chain is tyrosine phosphorylated by the Src kinases, Lyn and Fyn.23 This leads to the recruitment and activation of Syk tyrosine kinases which lead to the phosphorylation and activation of other signaling molecules. Adaptor molecules such as linker for activation of T cells (LAT) and Src homology domain-containing

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leukocyte phosphoprotein of 76 kDa (SLP-76) coordinate assembly of a multiprotein signaling complex necessary for activation of phospholipase Cγ2 (PLCγ2) and phosphoinositide-3-kinase (PI3K). LAT is a membrane associated adapter molecule that is anchored to lipid rafts in platelet membranes.47 Activation of PLCγ2 leads to cleavage of phosphatidylinositol 4,5 bisphosphate into inositol 1,4,5-triphosphate (IP3) and 1,2 diacylglycerol (DAG). IP3 mediates the release of calcium from stores within the dense tubular system. DAG and calcium bind to calcium diacylglycerol guanine nucleotide exchange factor I (CalDAG-GEFI) resulting in activation of PKC and activation of Rap1b. Rap1b activation plays a role in inducing the change in conformation of αIIbβ3 to a high affinity state necessary for binding of fibrinogen and platelet aggregation. Although GPIbIX-V also stimulates PLCγ2 activation, the tyrosine phosphorylation pattern is distinct from that occurring via GPVI, resulting in weaker PLCγ2 activation and diminished phospholipase activity.60 Collagen induced platelet aggregation is largely dependent on ADP signaling through P2Y12. Thromboxane A2 (TXA) generation and resulting platelet aggregation and release result in signaling through the P2Y12 receptor by secreted ADP and a positive feedback on platelet secretion through the PI3K pathway.44 Thromboxane Receptors Thromboxane A2 (TXA) and prostaglandin H2 interact with the same receptors. Human platelets have two distinct thromboxane A2-prostaglandin H2 receptors, TPα and TPβ, that are products of alternative splicing. These receptors differ only in their C-terminal or tail domains.28 TPα is a high-affinity binding site for thromboxane and mediates platelet shape change and intracellular calcium movement. TPα couples positively to adenylate cyclase via Gq and activates PLCβ which hydrolyzes PIP2, resulting in generation of IP3 and DAG. TPβ is a lowaffinity binding site and mediates platelet aggregation and secretion. TPβ couples negatively to adenylate cyclase via Gi and inhibits cAMP formation. Platelets obtained from most dogs have a weak arachidonic acid (AA) response characterized by shape change and reversible platelet aggregation without granule release.32 Addition of epinephrine prior to AA corrects the impaired aggregation and release response.20 Canine platelets likely only possess TPα receptors based on the absence within the dog genome of the coding region necessary for synthesizing the alternative C-terminal domain for TPβ. This may partially explain the minimal response of the majority of canine platelets to AA stimulation in vitro. Epinephrine, by linking to Gi, may mimic the effects of TPβ thus allowing for full platelet aggregation and release. A bovine TPα receptor has been cloned and characterized.39 Platelets generate TXA from AA. AA is liberated from dense tubular membrane phospholipids by the action of phospholipase A2 in response to agonist stimulation. Cytosolic phospholipase A2 (cPLA2) is phos-

phorylated during thrombin, ADP, and collagen induced platelet activation. Phosphorylation of cPLA2, in combination with a rise in intracellular calcium, results in cPLA2 translocation to the membrane where it releases arachidonic acid from the sn-2 position of membrane-bound phospholipids. Arachidonic acid is converted sequentially to prostaglandin G2 (PGG2), prostaglandin H2 (PGH2), and TXA via cyclooxygenase-1, prostaglandin hydroperoxidase, and thromboxane synthetase. The resulting thromboxane generation results in platelet aggregation and dense granule release. TXA synthase inhibitors do not inhibit platelet reactivity due to the enhanced production of prostaglandin H2 under these conditions and the ability of prostaglandin H2 to activate the receptor.63 Thrombin Receptors Platelet thrombin receptors include the glycoprotein complex Ib-IX-V and protease activated receptors (PARs). The Ibα subunit of the Ib-IX-V complex has a thrombin binding site that overlaps with the binding site for VWF.48 The GP V subunit is cleaved and removed from the complex by thrombin during platelet activation. GP V is thought to negatively modulate platelet activation; cleavage of GP V is a necessary first step prior to binding of thrombin to Ibα. Thrombin binding to Ibα mediates platelet activation via induction of ADP release and signaling through P2Y12. Ibα is also thought to act as a cofactor in the activation of PAR1 by thrombin.17 Thrombin bound to Ibα is protected from inactivation by antithrombin.18 Protease activated receptors (PARs) are G-protein coupled receptors that are characterized by the presence of a tethered ligand (Fig. 76.2). Protease activated receptors are activated when enzymes cleave at a specific site near the N-terminal exodomain resulting in formation of a tethered peptide sequence that can in turn bind to a G-protein coupled receptor on the same molecule. Peptide sequences that match the tethered ligands generated by thrombin cleavage, thrombin receptor activation peptides or TRAPs, have been used with varying success in vitro to activate platelets.9,11 In most species, TRAPs are not as effective as the comparable tethered ligand in inducing platelet aggregation or release. Mammalian platelets contain three PAR types that vary with species. Human platelets have PAR1 and PAR4 while mouse platelets signal through PAR3 and PAR4. PAR1 couplings to Gq, G13, and Giz are thought to be of most importance in mediating thrombin effects on platelets.12 The PI3 kinase pathway is considered to be of importance in stabilizing thrombin-induced platelet aggregates, although thrombin can activate platelets independently of Gi activation. On human platelets PAR1 binding results in a rapid spike in calcium influx while PAR4 binding is associated with a slower and sustained calcium influx that is considered vital to eliciting secondary signals necessary for complete platelet activation.13 Canine platelet PARs have not been fully

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evaluated; however, it is likely that canine platelets are similar to human platelets and possess PAR1 and PAR4 receptor subtypes.5 Canine platelets do not respond to TRAPs designed from human or canine sequence19 (M.K. Boudreaux, personal observation). There is genomic evidence for PAR3 receptor subtypes on both human and canine platelets; however, PAR1 and PAR4 receptors are considered to be of primary importance in mediating thrombin-induced platelet activation. Mouse platelets do not possess PAR1. PAR3 on mouse platelets acts as a cofactor in supporting cleavage and activation of PAR4. PAR3 alone is unable to mediate signaling leading to platelet activation but rather acts as a cofactor for promoting PAR4 cleavage in the presence of low thrombin concentrations.40

PLATELET-LEUKOCYTE INTERACTIONS The GPIbα subunit of platelet glycoprotein complex Ib-IX-V is a ligand for endothelial P-selectin, as well as VWF, both of which translocate from Weibel-Palade bodies to endothelial surfaces in response to stimuli such as thrombin and histamine.2,52,67 The binding site on GPIbα for P-selectin is very similar to the binding site of P-selectin glycoprotein ligand (PSGL-1) which allows leukocytes to roll along the vascular endothelium. Both GPIbα and PSGL-1 are membrane mucins and possess a heavily O-glycosylated region that separates the ligand-binding region from the plasma membrane.36 Both receptors also contain an acidic region immediately amino-terminal to the mucin-like region that is important for ligand binding.35 Unlike PSGL-1, which requires extensive post-translational modification, including fucosylation of side chains in order to interact with P-selectin, GPIbα is capable of binding to endothelial P-selectin without modifications. GPIbα can also bind to integrin αMβ2 (Mac-1)57 likely via the I domain within the αM subunit which closely resembles the A1 domain of VWF. This interaction, which requires conversion of Mac-1 to the ligandbinding competent form, may contribute to neutrophil attachment to the endothelium or subendothelium at sites of endothelial activation or injury. Platelet P-selectin can bind to PSGL-1 on neutrophils, monocytes, and T-cells providing a mechanism for initial adhesion of leukocytes to platelets. Firm plateletleukocyte adhesion is mediated by binding of Mac-1 to fibrinogen bound to the surface of activated platelets. Firm leukocyte-platelet interactions can also be mediated by intracellular adhesion molecule 2 (ICAM-2) on platelets which can bind to αLβ2 (LFA-1; CD11a/CD18) on leukocytes. Platelets can also release CD40L which binds to CD40 on vascular endothelial cells resulting in chemokine secretion and up-regulation of adhesion molecules.27 Platelets also secrete IL-1β which can induce endothelial cell secretion of IL-6, CXCL8, and CCL2 and up-regulation of adhesion molecules such as E-selectin, VCAM-1, ICAM-1, and αvβ3.24 All of these events link platelets to the inflammatory process.

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PROCOAGULANT ACTIVITY Platelet activation triggers significant remodeling of the platelet cell surface. This is associated with the activation and clustering of receptors for adhesion molecules, receptor mediated outside-in signaling, relocation and binding of proteins and other biomolecules from the cytosol to membrane surfaces, secretory organelle movement and fusion, and a dramatic redistribution of membrane phospholipids and formation and release of platelet membrane derived microparticles. Mammalian cells including platelets exhibit an asymmetric distribution of aminophospholipids. In non-activated platelets, negatively charged aminophospholipids phosphatidylserine (PS) and phosphatidylethanolamine (PE) are located primarily on the inner leaflet of the plasma membrane. An aminophospholipid translocase is responsible for transporting PS and PE to the inner membrane leaflet and maintaining asymmetry. When platelets are activated by collagen or thrombin alone or in combination, PS is rapidly externalized and maintained on the outer leaflet of the membrane (Fig. 76.3). This redistribution is also accompanied by the release of PS-enriched procoagulant microparticles. Phosphatidylserine is also externalized in cells exposed to apoptotic triggers. Activation of the lipid transporter protein referred to as “scramblase” functions to move phospholipids regardless of charge in a bidirectional fashion resulting in loss of membrane asymmetry and rapid externalization of PS. Phosphatidylserine remains externalized due to the inhibition of aminophospholipid translocase when “scramblase” is activated. The presence of PS on the surface of platelets is essential for expression of platelet procoagulant activity. Rare bleeding disorders associated with a deficiency of platelet procoagulant activity have been reported in humans and dogs.7,62 Phosphatidylserine facilitates the binding of coagulation proteins, which when anchored to PS and the activated platelet surface form procoagulant reaction complexes. These include the extrinsic tenase (factor VIIa + tissue factor (TF) + factor X), intrinsic tenase (factor X + factor IXa + factor VIIIa) complexes and the prothrombinase complex (factor Va + factor Xa and factor II). Once assembled the complexes allow for protein associations, which improve catalytic efficiency of the reactions. For example the prothrombinase complex is about 300,000 times more efficient than Xa alone in generation of thrombin from prothrombin, and the intrinsic tenase complex is more than 100,000 times more active than factor IXa alone as an activator of factor X. The assembly of reaction complexes on the PS enriched surface of activated platelets serves to localize and concentrate procoagulant reaction products at sites of injury. Since AT preferentially targets uncomplexed reaction products for neutralization, the anchored reaction products, including FXa and thrombin, are protected from neutralization and inhibition by AT. Lipid scrambling is associated with formation of PSenriched procoagulant membrane microparticles which

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FIGURE 76.3 Signaling pathways activated by collagen and thrombin trigger externalization of phosphatidylserine (PS). When PS is externalized, specific coagulation proteins can bind to the platelet surface. The platelet then acts as a scaffold for assembly of the tenase and prothrombinase complexes, which accelerate thrombin (IIa) generation and clot formation at the site of vessel injury.

Unactivated Platelet Membrane PS In

>

Procoagulant Platelet Membrane

IX

platelet cytosol

platelet cytosol

++

IIa IIa

4. Azim AC, Barkalow K, Chou J, et al. Activation of the small GTPases rac and cdc4, after ligation of the platelet PAR-1 receptor. Blood 2000;95:959–964. 5. Boudreaux MK, Catalfamo JL, Klok M. Calcium diacylglycerol guanine nucleotide exchange factor I gene mutations associated with loss of function in canine platelets. Transl Res 2007;150:81–92. 6. Boudreaux MK, Schmutz SM, French PS. Calcium diacylglycerol guanine nucleotide exchange factor I (CalDAG-GEFI) gene mutations in a thrombopathic Simmental calf. Vet Pathol 2007;44:932–935. 7. Brooks MB, Catalfamo JL, Brown HA, et al. A hereditary bleeding disorder of dogs caused by a lack of platelet procoagulant activity. Blood 2002;99:2434–2441. 8. Bucki R, Pastore JJ, Giraud F, et al. Involvement of the Na+/H+ exchanger in membrane phosphatidylserine exposure during human platelet activation. Biochim Biophys Acta 2006;1761:195–204. 9. Catalfamo JL, Andersen TT, Fenton JW II. Thrombin receptor-activating peptides unlike thrombin are insufficient for platelet activation in most species. Thromb Haemost 1993;69:1195 (abstract). 10. Cattaneo M, Zighetti ML, Lombardi R, et al. Molecular bases of defective signal transduction in the platelet P2Y12 receptor of a patient with congenital bleeding. Proc Natl Acad Sci USA 2003;100:1978–1983. 11. Connolly TM, Condra C, Feng DM, et al. Species variability in platelet and other cellular responsiveness to thrombin receptor-derived peptides. Thromb Haemost 1994;72:627–633. 12. Coughlin SR. Protease-activated receptors in hemostasis, thrombosis and vascular biology. J Thromb Haemost 2005;3:1800–1814. 13. Covic L, Gresser AL, Kuliopulos A. Biphasic kinetics of activation and signaling for PAR1 and PAR4 thrombin receptors in platelets. Biochemistry 2000;39:5458–5467. 14. Crittenden JR, Bergmeier W, Zhang Y, et al. CalDAG-GEFI integrates signaling for platelet aggregation and thrombus formation. Nat Med 2004;10:982–986. 15. Daniel JL, Molish IR, Rigmaiden M, et al. Evidence for a role of myosin phosphorylation in the initiation of the platelet shape change response. J Biol Chem 1984;259:9826–9831. 16. Daniel JL, Dangelmaier C, Jin J, et al. Molecular basis for ADP-induced platelet activation. I. Evidence for three distinct ADP receptors on human platelets. J Biol Chem 1998;273:2024–2029. 17. De Candia E, Hall SW, Rutella S, et al. Binding of thrombin to glycoprotein Ib accelerates the hydrolysis of PAR-1 on intact platelets. J Biol Chem 2001;276:4692–4698. 18. De Cristofaro R, De Candia E, Rutella S, et al. The Asp(272)Glu(282) region of platelet glycoprotein Ibalpha interacts with the heparinbinding site of alpha-thrombin and protects the enzyme from the heparin-catalyzed inhibition by antithrombin III. J Biol Chem 2000; 275:3887–3895. 19. Derian CK, Santuli RJ, Tomko KA, et al. Species differences in platelet response to thrombin and SFLLRN. Receptor-mediated calcium mobilization and aggregation, and regulation by protein kinases. Thromb Res 1995;78:505–519.

CHAPTER 76: PLATELET BIOCHEMISTRY, SIGNAL TRANSDUCTION, AND FUNCTION 20. Dunlop P, Leis LA, Johnson GJ. Epinephrine correction of impaired platelet thromboxane receptor signaling. Am J Physiol Cell Physiol 2000;279:C1760–C1771. 21. Erhardt JA, Pillarisetti K, Toomey JR. Potentiation of platelet activation through the stimulation of P2X1 receptors. J Thromb Haemost 2003;1:2626–2635. 22. Erhardt JA, Toomey JR, Douglas SA, et al. P2X1 stimulation promotes thrombin receptor-mediated platelet aggregation. J Thromb Haemost 2006;4:882–890. 23. Ezumi Y, Shindoh K, Tsuji M, et al. Physical and functional association of the Src family kinases Fyn and Lyn with the collagen receptor glycoprotein VI-Fc receptor gamma chain complex on human platelets. J Exp Med 1998;188:267–276. 24. Gawaz M, Brand K, Dickfeld T, et al. Platelets induce alterations of chemotactic and adhesive properties of endothelial cells mediated through an interleukin-1-dependent mechanism. Implications for atherogenesis. Atherosclerosis 2000;148:75–85. 25. Hardy AR, Jones ML, Mundell SJ, et al. Reciprocal cross-talk between P2Y1 and P2Y12 receptors at the level of calcium signaling in human platelets. Blood 2004;104:1745–1752. 26. Hechler B, Lenain N, Marches P, et al. A role of the fast ATP-gated P2X1 cation channel in thrombosis of small arteries in vivo. J Exp Med 2003;198:661–667. 27. Henn V, Slupsky JR, Gräfe M, et al. CD40 ligand on activated platelets triggers an inflammatory reaction of endothelial cells. Nature 1998;391:591–594. 28. Hirata T, Ushikubi F, Kakizuka A, et al. Two thromboxane A2 receptor isoforms in human platelets. Opposite coupling to adenylyl cyclase with different sensitivity to Arg60 to Leu mutation. J Clin Invest 1996;97:949–956. 29. Jackson SP, Schoenwaelder SM, Goncalves I, et al. PI3-kinase p100beta: a new target for antithrombotic therapy. Nat Med 2005;11:507–514. 30. Jin J, Daniel JL, Kunapuli SP. Molecular basis for ADP-induced platelet activation. II. The P2Y1 receptor mediates ADP-induced intracellular calcium mobilization and shape change in platelets. J Biol Chem 1998;273:2030–2034. 31. Jin J, Quinton TM, Zhang J, et al. Adenosine diphosphate (ADP)-induced thromboxane A(2) generation in human platelets requires coordinated signaling through integrin alpha(IIb)beta(3) and ADP receptors. Blood 2002;99:193–198. 32. Johnson GJ, Leis LA, Rao GHR, et al. Arachidonate-induced platelet aggregation in the dog. Thromb Res 1979;14:147–154. 33. Kahner BN, Shankar H, Murugappan S, et al. Nucleotide receptor signaling in platelets. J Thromb Haemost 2006;4:2317–2326. 34. Knight CG, Morton LF, Onley DJ, et al. Identification in collagen type I of an integrin alpha2 beta1-binding site containing an essential GER sequence. J Biol Chem 1998;273:33287–33294. 35. Li F, Erickson HP, James JA, et al. Visualization of P-selectin glycoprotein ligand-1 as a highly extended molecule and mapping of protein epitopes for monoclonal antibodies. J Biol Chem 1996;271:6342–6348. 36. Lopez JA, Chung DW, Fujikawa K, et al. Cloning of the α chain of human platelet glycoprotein Ib: a transmembrane protein with homology to leucine-rich α2-glycoprotein. Proc Natl Acad Sci USA 1987;84:5615–5619. 37. Lova P, Paganini S, Hirsch E, et al. A selective role for phosphatidylinositol 3,4,5 trisphosphate in the Gi-dependent activation of platelet Rap1B. J Biol Chem 2003;278:131–138. 38. Mahaut-Smith MP, Ennion SJ, Rolf MG, et al. ADP is not an agonist at P2X(1) receptors: evidence for separate receptors stimulated by ATP and ADP on human platelets. Br J Pharmacol 2000;131:108–114. 39. Muck S, Weber AA, Meyer-Kirchrath J, et al. The bovine thromboxane A2 receptor: molecular cloning, expression, and functional characterization. Naunyn-Schmiedebergs Arch Pharmakol 1998;357:10–16. 40. Nakanishi-Matsui M, Zheng YW, Sulciner DJ, et al. PAR3 is a cofactor for PAR4 activation by thrombin. Nature 2000;404:609–613. 41. Nieswandt B, Watson SP. Platelet-collagen interaction: is GPVI the central receptor? Blood 2003;102:449–461. 42. Oury C, Toth-Zsamboki E, Thys C, et al. The ATP-gated P2X1 ion channel acts as a positive regulator of platelet responses to collagen. Thromb Haemost 2001;86:1264–1271. 43. Oury C, Sticker E, Cornelissen H, et al. ATP augments von Willebrand factor dependent shear-induced platelet aggregation through Ca2+calmodulin and myosin light chain kinase activation. J Biol Chem 2004;279:26266–26273.

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44. Oury C, Toth-Zsamboki E, Vermylen J, et al. The platelet ATP and ADP receptors. Curr Pharm Des 2006;12:859–875. 45. Paul BZ, Daniel JL, Kunapuli SP. Platelet shape change is mediated by both calcium-dependent and -independent signaling pathways. Role of p160 Rho-associated coiled-coil-containing protein kinase in platelet shape change. J Biol Chem 1999;274:28293–28300. 46. Paul BZ, Kim S, Dangelmaier C, et al. Dynamic regulation of microtubule coils in ADP-induced platelet shape change by p160ROCK (Rho-kinase). Platelets 2003;14:159–169. 47. Ragab A, Severin S, Gratacap M-P, et al. Roles of the C-terminal tyrosine residues of LAT in GPVI-induced platelet activation: insights into the mechanism of PLCγ2 activation. Blood 2007;110:2466–2474. 48. Ramakrishnan V, DeGuzman F, Bao M, et al. A thrombin receptor function for platelet glycoprotein Ib-IX unmasked by cleavage of glycoprotein V. Proc Natl Acad Sci USA 2001;98:1823–1828. 49. Roberts DE, McNicol A, Bose R. Mechanism of collagen activation in human platelets. J Biol Chem 2004;279:19421–19430. 50. Rolf MG, Brearley CA, Mahaut-Smith MP. Platelet shape change evoked by selective activation of P2X1 purinoceptors with alpha, beta-methylene ATP. Thromb Haemost 2001;85:303–308. 51. Rolf MG, Mahaut-Smith JP. Effects of enhanced P2X1 receptor Ca2+ influx on functional responses in human platelets. Thromb Haemost 2002;88:495–502. 52. Romo GM, Dong JF, Schade AJ, et al. The glycoprotein Ib-IX-V complex is a platelet counterreceptor for P-Selectin. J Exp Med 1999;190:803– 813. 53. Ruggeri ZM. The role of von Willebrand factor in thrombus formation. Thromb Res 2007;120:S5–S9. 54. Samson J, Stelmach H, Tomasiak M. The importance of Na+/H+ exchanger for the generation of procoagulant activity by porcine blood platelets. Platelets 2001;12:436–442. 55. Schwertz H, Tolley ND, Foulks JM, et al. Signal-dependent splicing of tissue factor pre-mRNA modulates thrombogenicity of human platelets. J Exp Med 2006;203:2433–2440. 56. Shankar H, Kahner BN, Prabhakar J, et al. G-protein-gated inwardly rectifying potassium channels regulate ADP-induced cPLA2 activity in platelets through Src family kinases. Blood 2006;108:3027–3034. 57. Simon DI, Chen Z, Xu H, et al. Platelet glycoprotein Ibα is a counterreceptor for the leukocyte integrin Mac-1 (CD11b/CD18). J Exp Med 2000;192:193–204. 58. Soulet C, Hechler B, Gratacap MP, et al. A differential role of the platelet ADP receptors P2Y1 and P2Y12 in Rac activation. J Thromb Haemost 2005;3:2296–2306. 59. Story RF, Sanderson HM, White AE, et al. The central role of the P(2T) receptor in amplification of human platelet activation, aggregation, secretion and procoagulant activity. Br J Haematol 2000;110:925–934. 60. Suzuki-Inoue K, Wilde JI, Andrews RK, et al. Glycoproteins VI and IbIX-V stimulate tyrosine phosphorylation of tyrosine kinase Syk and phospholipase Cγ2 at distinct sites. Biochem J 2004;378:1023–1029. 61. Toth-Zsamboki E, Oury C, Cornelissen H, et al. P2X1-mediated ERK2 activation amplifies the collagen-induced platelet secretion by enhancing myosin light chain kinase activation. J Biol Chem 2003;278:46661–46667. 62. TotSatta N, Fressinaud E, et al. Scott syndrome, characterized by impaired transmembrane migration of procoagulant phosphatidylserine and hemorrhagic complications, is an inherited disorder. Blood 1996;87:1409–1415. 63. Vezza R, Mezzasoma AM, Venditti G, et al. Prostaglandin endoperoxides and thromboxane A2 activate the same receptor isoforms in human platelets. Thromb Haemost 2002;87:114–121. 64. Wolfs JLN, Comfurius P, Rasmussen JT, et al. Activated scramblase and inhibited aminophospholipid translocase cause phosphatidylserine exposure in a distinct platelet fraction. Cell Mol Life Sci 2005;62:1514–1525. 65. Woulfe D, Jiang H, Morgans A, et al. Defects in secretion, aggregation, and thrombus formation in platelets from mice lacking Akt2. J Clin Invest 2004;113:441–450. 66. Yang J, Wu J, Jiang H, et al. Signaling through Gi family members in platelets. Redundancy and specificity in the regulation of adenylyl cyclase and other effectors. J Biol Chem 2002;277:46035–46042. 67. Zarbock A, Polanowska-Grabowska RK, Ley K. Platelet-neutrophilinteractions: Linking hemostasis and inflammation. Blood Rev 2007;21:99–111. 68. Zimmerman GA, Weyrich AS. Signal-dependent protein synthesis by activated platelets. New pathways to altered phenotype and function. Arterioscler Thromb Vasc Biol 2008;28:S17–S24.

C H A P T E R 77

Platelet Kinetics and Laboratory Evaluation of Thrombocytopenia KAREN E. RUSSELL Platelet Kinetics Megakaryocytopoiesis and Platelet Production Circulating Lifespan and Senescence Non-mammalian Species Basic Mechanisms of Thrombocytopenia Decreased or Defective Platelet Production Increased Platelet Loss or Consumption Disseminated intravascular coagulation Thrombocytopenic thrombotic purpura and hemolytic uremic syndrome Platelet destruction: immune mediated, nonimmune mediated, and complex mechanisms Abnormal Platelet Distribution Pseudothrombocytopenia

Clinical Signs of Thrombocytopenia and Risk of Hemorrhage Laboratory Evaluation of Thrombocytopenia Complete Blood Count Platelet Parameters on Automated Hematology Analyzers Microscopic Evaluation of the Blood Smear Additional Diagnostic Tests Coagulation panel Bone marrow Evaluation Laboratory Confirmation of Consumptive Thrombocytopenia

Acronyms and Abbreviations APTT, activated partial thromboplastin time; BFU-Meg, burst forming unit megakaryocyte; CBC, complete blood count; CFU-Meg, colony forming unit megakaryocyte; CMP, common myeloid progenitor cell; DIC, disseminated intravascular coagulation; DMS, demarcation membrane system; EDTA, ethylenediaminetetraacetic acid; ELISA, enzyme-linked immunosorbent assay; FDP, fibrin-fibrinogen degradation product; FXII, factor XII; GM-CSF, granulocyte-monocyte colony stimulating factor; HUS, hemolytic uremic syndrome; IL, interleukin; MPC, mean platelet component concentration; MPV, mean platelet volume; OSPT, one-stage prothrombin time; PCDW, platelet component concentration distribution width; PCT, plateletcrit; PDW, platelet distribution width; PT, prothrombin time; SDF-1, stromal cell-derived factor-1; TTP, thrombocytopenic thrombotic purpura; VWF, von Willebrand factor.

PLATELET KINETICS Megakaryocytopoiesis and Platelet Production Platelets are the second most numerous circulating cell in blood and are essential for coagulation, maintenance of vascular integrity, and control of hemostasis. In mammals, these small, anucleated cells originate in the bone marrow from megakaryocytes. Megakaryocytopoiesis progresses through many stages starting with a pluripotential stem cell and proceeds through subsequent development of committed precursors. The hematopoietic pluripotential stem cells and early megakaryocyte progenitor cells are positive for CD34 and have low expressions of CD41.42,49 Cells 576

that become committed to the megakaryocyte lineage acquire the expression of CD61 and CD42, and expression of CD41 increases.42,49 From the hematopoietic stem cell, megakaryocytopoiesis progresses through the common myeloid progenitor cell (CMP), followed by a bipotential progenitor cell for erythroid and megakaryocytic cells.20,41,58 The next stage, a primitive megakaryocytic burst-forming unit (BFU-Meg), has a high proliferative capacity and can produce anywhere from 100 to 500 megakaryocytes per colony in a 1 week time period.11,42 Following the BFU-Meg, the more mature megakaryocytic colony-forming unit (CFU-Meg) gives rise to approximately 3–50 megakaryocytes per colony. The BFU-Meg and CFU-Meg stages express CD34, CD33, and CD41. Later stages gain CD61 expression

CHAPTER 77: PLATELET KINETICS AND LABORATORY EVALUATION OF THROMBOCYTOPENIA

followed by CD42 and CD36. The first morphologically recognizable cell of the megakaryocyte lineage in marrow is the promegakaryoblast. Before this stage, the morphology of the earlier progenitors resembles a small lymphocyte. The promegakaryoblast has a high N : C ratio, dark blue cytoplasm, and is approximately 10 μm in diameter.42,103 The megakaryoblast has a kidney-bean shaped nucleus, dark blue cytoplasm and is approximately 15–50 μm in diameter. As the later stages of megakaryocyte precursors undergo endomitosis, the cells lose CD34 expression, and increase ploidy, cell size and volume. Megakaryocytes undergo DNA duplication as low as 2 N and can attain ploidy as high as 128 N, with the modal ploidy in most mammals being 16 N.13 Mature megakaryocytes are the largest hematopoietic cells in marrow, reaching diameters as high as 150 μm. The higher ploidy corresponds to an increased cell volume, and as a result, a higher number of platelets that are produced.13 Each megakaryocyte is capable of generating several thousand platelets. In humans, daily platelet production is estimated to be approximately 35,000 ± 4,300 platelets/μL or 1 × 1011 total platelets.13,45,49 It takes 4–5 days to complete the sequence of megakaryocyte development to the release of new platelets.45 Mature megakaryocytes produce platelets by cytoplasmic fragmentation through regulated and dynamic processes that involve the invagination of the megakaryocyte plasma membrane which forms the demarcation membrane system (DMS) and the association of the DMS with microtubules and an actin/myosin complex.69,74 The association with microtubules causes evagination of the DMS and elongation of pseudopodal extensions that result in proplatelet formation.13,69 The actin/myosin complex is thought to be important in proplatelet formation and the separation of platelets from proplatelets.82 In marrow, mature megakaryocytes migrate so that they are closely positioned near endothelial cells. This allows the long proplatelet pseudopods from the megakaryocyte to extend into marrow sinusoids and also allows the megakaryocyte to enter the peripheral circulation. The final process of platelet shedding has to occur in the circulation to prevent platelet trapping within the marrow.13,95 This latter observation may explain, in part, why megakaryocytes are often found in pulmonary circulation where significant platelet shedding occurs.98 Thrombopoietin is the major cytokine that regulates all stages of megakaryocyte and platelet production. In addition to thrombopoietin, megakaryocytes and platelet production are regulated by several growth factors and cytokines. Many of these affect hematopoiesis in general, or work synergistically with thrombopoietin. Megakaryocyte progenitors are stimulated by interleukin (IL)-3, IL-6, IL-11, IL-12, granulocytemonocyte colony stimulating factor (GM-CSF), and erythropoietin.32 Additional factors important in thrombopoiesis include IL-1, stem cell factor, and leukemia inhibitory factor.13,32 Stromal cell-derived factor 1 (SDF-1) is thought to play a role in megakaryocyte migration.34

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Platelet production is regulated by total platelet mass rather than platelet numbers. In situations of increased demand, platelet production can increase as much as 20-fold or higher.49 In health, platelet numbers are relatively stable and constant, and the thrombopoietinthrombopoietin receptor system is responsible for maintaining platelet mass. Thrombopoietin is secreted constitutively from the liver and to a lesser extent from the kidney. High affinity thrombopoietin receptors, present on platelet surfaces, bind thrombopoietin, which is then internalized and degraded.48,52 In thrombocytopenic states, a higher level of free thrombopoietin is available to stimulate platelet production. Circulating Lifespan and Senescence Platelet numbers remain fairly constant within a species, but platelet numbers vary widely between different species. In addition, approximately 30% of the platelet circulating mass is transiently compartmentalized in the spleen for a short period of time in a resting individual. In health, there is a steady state between the number of platelets produced and platelets destroyed. Platelets circulate for approximately 5–9 days in most mammalian species. As platelets age, they are removed from circulation by macrophages in the spleen and liver. Signals involving platelet senescence are not completely characterized.59 Exposure of phosphatidylserine on the outer platelet membrane, damaged or denatured platelet glycoproteins and proteoglycans, and gradual loss of platelet fragments or microparticles are thought to play important roles in platelet attrition.59 Phosphatidylserine may induce macrophage phagocytosis directly or through bridging proteins that bind macrophage and phosphatidylserine receptors.39,59 Similar to erythrocyte senescence, glycoproteins on the platelet surface may become denatured or damaged through changes in glycosylation or through the exposure of neoepitopes that are recognized by naturally occurring antibodies.59 A smaller percentage of platelets are continually removed from circulation because of their role in the maintenance of endothelial integrity.30,59 Non-mammalian Species The cellular equivalent of platelets in birds, reptiles, amphibians, and fish is the thrombocyte, which is a small, round, oval, or spindled nucleated cell. Similar to mammals, the primary function of thrombocytes is hemostasis and coagulation, and thrombocytes are the second most prevalent circulating cell in blood after erythrocytes. Avian thrombocytes may also be important in innate immunity since these cells have phagocytic capability and can remove foreign material from blood.33 In addition to hemostasis, reptilian thrombocytes play a role in wound healing.93 Thrombopoiesis is similar in avian and reptilian species in that thrombocytes originate in the bone marrow from a distinct mononuclear cell. The stages in thrombopoiesis include thromboblasts, immature

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thrombocytes (early, mid, and late stages), and mature thrombocytes. In general, as thrombocytes develop, they decrease in size, the cytoplasm changes from basophilic to pale blue or colorless, and the cell and nuclear shape change from round to oval. Thrombocyte numbers in most avian species range between 20,000 and 30,000/μL or approximately 10–15 thrombocytes per 1000 erythrocytes; 1–2 thrombocytes per monolayer area under oil immersion (100×) is considered normal or adequate in most birds.10,25 In normal reptiles, thrombocyte numbers in blood range between 25 and 350 thrombocytes per 100 leukocytes.93 Numbers of thrombocytes in fish blood range from 60,000 to 70,000/μL.78 Manual counts for thrombocytes in fish can be performed using a hemacytometer, but given the tendency to clump, accurate counts are difficult to obtain. Thrombocytes of certain fish species do not appear to have phagocytic ability.78 BASIC MECHANISMS OF THROMBOCYTOPENIA Thrombocytopenia is defined as a decrease in circulating platelets and is the most common acquired hemo-

static disorder in veterinary and human medicine. Thrombocytopenia results from one or a combination of the following basic mechanisms: decreased or defective platelet production, increased peripheral platelet loss or consumption, increased platelet destruction, or abnormal distribution. To understand a diagnostic approach (Fig. 77.1) to thrombocytopenia, a basic understanding of the mechanisms leading to thrombocytopenia is useful, and an overview is discussed below. The pathogenesis of thrombocytopenia is covered in more detail in Chapters 78 and 79. Decreased or Defective Platelet Production Platelets originate from megakaryocytes found in the bone marrow and to a lesser extent in the lung. Causes of decreased platelet production that result in thrombocytopenia can specifically target the megakaryocyte, but more frequently, other hematopoietic cells are also affected. Although rare, acquired megakaryocyte hypoplasia or aplasia has been reported in dogs and cats, and probably has an immune-mediated component.27,54,65 General etiologies for bone marrow disease leading to hypopla-

Decreased platelet numbers

Blood smear confirmation Platelet clumps present

No platelet clumps present

Consider: Improper collection of sample Improper mixing of sample Inadequate anticoagulant EDTA-dependent pseudothrombocytopenia

Consider: Breed-specific thrombocytopenia (Cavalier King Charles Spaniels, sight hounds)

No indication for a breed-specific thrombocytopenia Collect a fresh blood sample Consider use of citrate instead of EDTA

Perform coagulation panel

Abnormal coagulation results Consider: DIC Vitamin K antagonists

Bone marrow evaluation (see below)

Normal coagulation results Consider: Antiplatelet antibody test Serology, molecular diagnostics for infectious agents Rule out neoplasia (lymphoma, leukemia, myelodysplastic syndrome, hemangiosarcoma

Bone marrow evaluation

Megakaryocytic hypoplasia Consider: Production defect

Megakaryocyte normo- or hyperplasia Consider: Increased loss or consumption--DIC, External hemorrhage Increased destruction—Immune mediated (primary, secondary), Nonimmune-mediated Abnormal distribution—image internal organs (spleen)

FIGURE 77.1 Diagnostic approach to thrombocytopenia.

CHAPTER 77: PLATELET KINETICS AND LABORATORY EVALUATION OF THROMBOCYTOPENIA

sia of one or more hematopoietic cell lines include drugs, chemicals, or toxins that have cytotoxic effects or produce idiosyncratic reactions; irradiation resulting in cell death and marrow suppression; infection with certain viruses (canine parvovirus, feline leukemia virus, feline immunodeficiency virus, equine infectious anemia virus, bovine viral diarrhea virus, African swine fever virus) or rickettsial agents; myelophthisis from primary or metastatic neoplasia or myelofibrosis; or myelonecrosis. Increased Platelet Loss or Consumption Thrombocytopenia secondary to rapid, increased loss of platelets may occur with massive trauma, extensive external hemorrhage, or exchange blood transfusion. In situations of trauma or external hemorrhage, thrombocytopenia is generally mild to moderate, transient, and usually reversible without specific treatment.85 Accelerated consumption of platelets can be brought on by widespread activation of the coagulation system or endothelial damage. Disseminated intravascular coagulation (DIC), thrombocytopenic thrombotic purpura (TTP) and hemolytic uremic syndrome (HUS) are complex syndromes characterized by widespread consumption of platelets, resulting in moderate to significant thrombocytopenia. Disseminated intravascular coagulation is a common complication seen in both veterinary and human medicine. Thrombocytopenic thrombotic purpura and HUS occur in humans but are rare in veterinary species. A syndrome resembling HUS has been reported in dogs, cats, horses and a heifer.2,14,21,22,40,56,63,79 Animal models of TTP and HUS have been described.77,81,102 Disseminated Intravascular Coagulation Disseminated intravascular coagulation occurs secondarily to a wide variety of insults and diseases; DIC can be acute, subacute, or chronic and is characterized clinically by hemorrhage and microthrombosis. The pathophysiology of DIC is discussed in more detail in Chapter 88. Common disorders associated with DIC include vascular damage, septicemia with release of bacterial endotoxins, release of tissue thromboplastin from necrotic or malignant tissue, or release of other procoagulant proteins. Approximately 65% of DIC cases in humans occur secondarily to infection; bacterial infection is the most common.107 In Gram-negative infections, bacterial release of endotoxin (lipopolysaccharide) causes simultaneous activation of the coagulation system and inhibition of coagulation control mechanisms. Endotoxin stimulates intrinsic coagulation directly by activation of factor XII (FXII) or indirectly by activation of FXII via endothelial damage. Endotoxin stimulates the extrinsic coagulation pathway by causing generation and increased surface expression of thromboplastins or tissue factor by inflammatory cells, primarily monocytes. Activated monocytes release IL-1 and tumor

579

necrosis factor alpha (TNF-α), which decreases endothelial expression of thrombomodulin and prevents activation of protein C, thus impairing an important coagulation inhibitory pathway. Thrombocytopenic Thrombotic Purpura and Hemolytic Uremic Syndrome In humans, the thrombotic microangiopathies are exemplified by two syndromes, TTP and HUS. The pathogenesis of both is heterogeneous and often the origin is unknown; many consider them to be different expressions of the same disease mechanism. Thrombocytopenic thrombotic purpura and HUS may occur either as a primary condition or as a secondary complication. Most cases of HUS occur in childhood after bloody diarrhea caused by Shigella dysenteriae serotype I or various Escherichia coli serotypes. The classical triad of TTP is characterized by severe thrombocytopenia, intravascular hemolysis with schistocytes, and neurologic symptoms. Severe renal dysfunction is also a prominent feature of HUS. The thrombotic microangiopathies are characterized by endothelial damage and platelet aggregation with resultant thrombocytopenia, hemolytic anemia, and thrombosis. High plasma concentrations of thrombomodulin, tissue plasminogen activator, and von Willebrand factor (VWF) are observed in many TTP patients.94 Plasminogen activator inhibitor type I and urine endothelin concentrations are increased in HUS patients.5,87 Concentrations return to normal when patients enter remissions. Platelet aggregation may be caused by the presence in plasma of abnormally large VWF molecules that bind platelets and cause aggregation and activation much more actively than smaller VWF molecules. In TTP, endothelial damage may cause leakage of stored large VWF multimers into subendothelial tissue and plasma. Normal cleavage and clearance of these large VWF multimers is limited, causing increased concentrations and extensive platelet aggregation.61 This extensive, systemic platelet aggregation causes platelet consumption and moderate to significant thrombocytopenia. Hemolytic uremic syndrome and TTP are discussed in more detail in Chapter 79. Platelet Destruction: Immune Mediated, Nonimmune Mediated, and Complex Mechanisms Immune-mediated thrombocytopenia is covered in Chapter 78. Briefly, immune-mediated destruction of platelets may be primary (idiopathic) or may occur in association with infectious agents, neoplasia, drugs, autoimmune, isoimmune, or neonatal-immune diseases. A diagnosis of idiopathic or primary immunemediated thrombocytopenia is made after all other potential causes have been eliminated. Immune destruction of platelet occurs when circulating platelets coated with antibody, antigen-antibody complexes or complement are phagocytized by macrophages in the spleen, liver, and bone marrow. The result

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is decreased platelet survival and lifespan. The spleen is the largest lymphoid organ, the major site of antibody production and platelet removal. Platelet aggregation, phagocytosis, or lysis resulting in platelet destruction and thrombocytopenia can occur independently of immune-mediated events. Nonimmune-mediated platelet destruction occurs in some acute bacterial and viral infections and in cardiovascular disease. Thrombocytopenia occurring from venomous snakebites may be secondary to DIC or may result from direct aggregation of platelets. Platelet activation with thrombocytopenia is associated with severe and extensive burns. Platelet destruction in bacterial infections can occur as a result of platelet adherence or aggregation to activated monocytes or neutrophils. In Gram-negative infections, monocytes stimulated by endotoxin express tissue factor on their surface and generate thrombin.83 Thrombin, a potent platelet agonist, causes platelet activation and aggregation. Platelets adhere to monocytes and are phagocytized. Neutrophil stimulation is not believed to be a major cause of platelet removal and is probably more important with regard to platelet function and thrombus formation.36 Exotoxins released from Gram-positive bacteria may directly damage platelets and contribute to thrombocytopenia.31,47,76,97 The pathogenesis of thrombocytopenia associated with acute viral infections is often multifactorial, even though one mechanism may predominate. Direct platelet damage or lysis is one proposed mechanism of viralassociated thrombocytopenia. Myxovirus infections, including those due to Newcastle disease virus and influenza, decrease platelet survival by removal of platelet membrane sialic acid residues by viral neuraminidase.96,99 Other viruses in which platelet damage is suspected include bovine viral diarrhea virus and hog cholera virus.16,104 In humans, platelets can be damaged and destroyed by mechanical means in arterial disease with roughened endothelial surfaces and narrowed microcirculation, stenotic or prosthetic heart valves, or cardiac by-pass surgery.6,17,29 Turbulent circulation causes membrane damage and microparticle formation.28 Interactions of platelets with altered or damaged endothelial surfaces cause extensive platelet activation, clumping, and removal of platelets by the mononuclear phagocyte system.6 This may also occur in viral infections that infect or alter endothelial cells. In vitro, virally transformed endothelial cells show a significant increase in platelet adherence.18 Many of the rickettsial diseases are associated with thrombocytopenia. Several mechanisms contribute to thrombocytopenia, including immune-mediated platelet destruction, direct damage, vasculitis, and platelet production deficits. Disseminated intravascular coagulation may occur as a secondary complication. Abnormal Platelet Distribution In health approximately 30–40% of the total circulating platelet pool may be stored in the spleen, which is

referred to as physiologic platelet sequestration. The liver and bone marrow are additional tissue sites of platelet sequestration.3 Hypersplenism is a pathologic condition in which as much as 90% of circulating platelets become sequestered predominately in the spleen. Hypersplenism is characterized by the presence of one or more cytopenias with corresponding bone marrow hyperplasia and significant splenomegaly. Splenic size is probably the most important factor that determines the degree of thrombocytopenia; the spleen must be significantly enlarged to cause a severe decrease in circulating platelets.71 Although hypersplenism is characterized by splenomegaly, the presence of splenomegaly does not always signify hypersplenism. Thrombocytopenia that results from hypersplenism can be thought of as a displacement of the majority of platelets from peripheral circulation into a reversible, but slowly exchanging, splenic pool.29 Thrombocytopenia secondary to splenic pooling is different from that seen with idiopathic thrombocytopenic purpura in which there is active removal of platelets by the splenic macrophages.29 In hypersplenism, thrombocytopenia is evident on a complete blood count (CBC), but the total numbers of platelets and platelet mass are actually normal. Platelet survival is usually normal, as is platelet production.3,37 Hypersplenism is rare in animals but a few suspected cases have been described.44,51 All other causes of splenomegaly should be ruled out before a diagnosis of primary hypersplenism is made. Recommended treatment in these cases is splenectomy. Severe hypothermia adversely affects platelet morphology and can cause a mild transient thrombocytopenia owing to pooling of platelets in the spleen.44 Transient thrombocytopenia can also occur with endotoxemia and hypotension. Pseudothrombocytopenia Pseudothrombocytopenia (artifactual or spurious thrombocytopenia) is an in vitro phenomenon resulting from platelet clumping secondary to poor or difficult venipuncture and subsequent platelet activation or exposure of the blood to certain anticoagulants. Feline platelets are especially prone to activation during blood collection. Pseudothrombocytopenia can occur with overfilling of a blood collection vacutainer tube and subsequent improper or inadequate sample mixing.72 Uncommonly, in vitro clumping occurs in certain individuals when blood is collected into anticoagulants. This has been seen most commonly with ethylenediaminetetraacetic acid (EDTA) and rarely with other anticoagulants. EDTA-dependent pseudothrombocytopenia has been reported in humans, a horse, a miniature pig, and a dog.30,38,75,105 Although there may be several factors, the leading mechanism for EDTA-dependent pseudothrombocytopenia involves autoantibodymediated platelet agglutination. Hidden epitopes become exposed within the platelet IIb-IIIa complexes as a result of complex destabilization occurring with calcium removal from calcium binding domains.8,12,23

CHAPTER 77: PLATELET KINETICS AND LABORATORY EVALUATION OF THROMBOCYTOPENIA

Rarely, platelet clumping can occur with exposure to anticoagulants containing citrate, oxalate, or heparin.68,70,86 Accurate automated platelet counts in cats are often difficult to obtain due to clumping. Feline platelets are more reactive than those of other species. Several factors unique to feline platelets may be involved, including a larger platelet size, a higher concentration of serotonin, irreversible aggregation and granule release when exposed to serotonin, and irreversible aggregation in response to low concentrations of ADP.9,57,73 Because of their nature and small size and the fact that many cats resist handling and restraint in unfamiliar settings, venipuncture is often challenging, which may increase the likelihood of platelet activation in vitro.66 CLINICAL SIGNS OF THROMBOCYTOPENIA AND RISK OF HEMORRHAGE Clinical signs leading one to suspect thrombocytopenia include petechiation or ecchymosis in tissues or mucosal membranes, epistaxis, melena, hematochezia, hematuria, prolonged bleeding after venipuncture, or retinal hemorrhage or hyphema. However, clinical signs of bleeding are infrequently seen. More commonly, thrombocytopenia is usually identified on a routine CBC. Animals having a low platelet count may exhibit no clinical signs or present for lethargy, weakness, fever or other signs related to an underlying disease process. Despite the observation that hemorrhage is not a consistent feature, thrombocytopenic animals are at an increased risk of bleeding. Hemorrhage solely caused by thrombocytopenia does not usually occur until peripheral platelet numbers are severely decreased (50,000/μL) within 5–7 days, but recovery times of up to 35 days have been reported.4,30,52,56 Dogs that do not

CHAPTER 78: IMMUNE-MEDIATED THROMBOCYTOPENIA

respond to initial therapy may still have a favorable outcome given more time or alternative strategies, such as splenectomy. Reported mortality rates have ranged from 10% to 43%, with the lower rate52 more in line with the authors’ experience. Both acute and chronic forms of the disease are seen in dogs, perhaps analogous to the human conditions. Over 50% of dogs with IMT experience a single, acute bout of thrombocytopenia followed by recovery.30,52,69 However, a substantial percentage of dogs respond initially but relapse and require re-treatment or chronic therapy with low dose steroids or other immunosuppressive agents.30,52,69 There is currently no way to predict which outcome a particular dog will have. PRIMARY IMT IN CATS Signalment Primary IMT appears to be a rare condition in cats, with only a few probable cases reported.5,22,32,34,36,63 The ages of affected cats ranged from 18 months to 12 years with a median age of 6 years. A gender bias is not apparent. Affected cats have been domestic shorthairs and single cats of the following breeds: Abyssinian, Somali, and British shorthair. History and Physical Exam Presenting complaints and physical exam findings include epistaxis, petechiae, ecchymoses, hematochezia, hematuria, and hemoptysis.5,22,32,34,36,63 Decreased appetite and weight loss have also been present.5

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78.1).32 Acquisition of historical information, physical examination, and diagnostic testing should generally mirror those for the thrombocytopenic dog (see page 588). In the cat, additional consideration should be given to certain drug exposures (e.g. methimazole)50 and to infection with FIV or FeLV.36 In one retrospective study, thrombocytopenia was associated most commonly with viral infections, especially by FeLV, and with neoplasia, particularly hemic in origin.32 Therefore, bone marrow examination is indicated. Regenerative and nonregenerative anemias have been reported in cats with primary IMT, and they are not unexpected with blood loss.5 However, anemia may occur with concurrent IMHA (i.e. Evans’ syndrome)36 or other illness resulting in anemia and a non-immune or secondary immune-mediated thrombocytopenia. Relatively little work has been done to develop and validate feline assays for PSAIg,34,36,63 and, as in dogs and people, these tests will likely play a limited role in the clinical diagnosis of primary IMT. Antiplatelet antibodies may support an immunologic component to the thrombocytopenia if the assays are reliable, but current assays cannot differentiate primary IMT from secondary IMT. Increased PSAIg has been reported in cats with a wide array of disorders including fat necrosis, feline infectious peritonitis (FIP), FeLV infection, FIV infection, lymphoma, leukemia, hepatitis, pyelonephritis, and hyperthyroidism.36 When no underlying condition is found in a severely thrombocytopenic cat with megakaryocytic hyperplasia and a thorough diagnostic work-up, idiopathic IMT is a reasonable working diagnosis, particularly if a reliable assay indicates the presence of increased PSAIg. Positive response to immunosuppressive therapy may lend further support to the conclusion.

Pathogenesis An immunologic pathogenesis for reported cases of feline idiopathic thrombocytopenia has been supported by exclusion of other identifiable disorders, response to immunosuppressive therapy, or increased PSAIg.36 The presence of megakaryocytic hyperplasia in most reported cases supports a decrease in platelet survival with responsive megakaryopoiesis. Diagnosis The diagnosis of IMT in cats is one of exclusion. Most affected cats have severe thrombocytopenia (50,000/μL) has been 7 days, ranging from three days to 144.5,22,36,63 All reported cats with IMT required long-term therapy, and some never achieved normal platelet concentrations.5,22,36,63 Relapses were reported in cats weaned from therapy.

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PRIMARY IMT IN HORSES Immune-mediated thrombocytopenia is uncommon in horses, but it may occur as idiopathic thrombocytopenia (likely primary),11,28,43 neonatal alloimmune thrombocytopenia, or in association with infections, drugs, neoplasia, or IMHA. As in other species, the diagnosis of primary IMT is based on exclusion of other causes of thrombocytopenia. Thrombocytopenia may be suspected because of the presence of mucocutaneous hemorrhage (e.g. petechiae, epistaxis) and prolongation of bleeding from sites of trauma, including venipuncture. Complete blood count, chemistry, urinalysis, and hemostasis profiles should be performed to document thrombocytopenia and screen for underlying disorders. Thrombocytopenia is typically severe and associated with hemorrhage, but it should be confirmed with microscopic examination of a blood film to exclude pseudothrombocytopenia. Equine platelets tend to be pale staining and together with the relatively low numbers of platelets in horses, when compared to other species, blood film evaluation may yield a false impression of thrombocytopenia to the untrained eye. History and physical examination should be thoroughly assessed for known causes of thrombocytopenia (see Table 78.1). Bone marrow assessment may be useful to detect underlying diseases. Molecular or serologic diagnostics for infections with equine infectious anemia virus and Anaplasma phagocytophilum may also be indicated. A platelet antibody assay may be used to provide evidence of PSAIg.43 Most horses with idiopathic IMT respond favorably to corticosteroids, but azathioprine has been added in refractory cases with apparent benefit.28,43 Dexamethasone may be instituted intravenously at 0.1– 0.2 mg/kg every 24 hours. It may be continued with tapering doses over several weeks, or therapy may be switched to tapering doses of oral prednisolone. Prednisone should not be used because it has limited absorption in horses.49 Some horses experience only a single episode of thrombocytopenia while others may have recurrent bouts. Platelet-rich plasma may be used for life-threatening bleeding. SECONDARY IMT Secondary IMT refers to IMT that is part of a more widespread autoimmune disease or that is associated with known predisposing neoplastic, infectious, or drug-induced conditions. Classification as secondary IMT is justified when there is a clear association between thrombocytopenia and the underlying condition, and when there is strong evidence that immune mechanisms are responsible for the thrombocytopenia. Increased PSAIg in a patient with a particular disorder supports an immunologic component to the thrombocytopenia, but it would be insufficient evidence to conclude that the underlying disorder should be considered a definite cause of secondary IMT.

In patients with secondary IMT, PSAIg may be bound specifically to platelet autoantigens as antiplatelet autoantibodies (e.g. systemic lupus erythematosus) or it may be bound to adsorbed non-autoantigens associated with infectious agents, drugs, or neoplasia (Fig. 78.4). PSAIg may also be in the form of immune complexes bound to platelets by: (1) complement-mediated immune adherence; (2) Fc receptors for IgG in species with platelet Fcγ receptors; or (3) nonspecific interactions. Circulating immune complexes may arise from infectious diseases, vaccinations, drugs, neoplasia, or systemic autoimmune diseases. Therefore, secondary IMT may or may not be autoimmune. In veterinary medicine, the type of interaction between antibody and platelet surface is typically unknown for any particular patient with a positive platelet antibody result. Systemic Autoimmune Diseases Primary IMT is but one clinical presentation in a spectrum of somewhat indistinct autoimmune disorders with different antibody specificities. Systemic lupus erythematosus (SLE) associated with presumed IMT has been reported in dogs, cats, and horses (see Chapter 54). An immune-mediated pathogenesis of the thrombocytopenia in dogs with SLE has been supported by PSAIg positivity and positive results with D-MIFAs or indirect assays. Thrombocytopenia may accompany IMHA because of consumption or immune-mediated destruction (Evans’ syndrome) (see Chapters 33 and 34). Immune-mediated thrombocytopenia has been suspected in association with several other targets of the immune system. Positive indirect results for plateletbindable immunoglobulin have been reported in a dog with steroid responsive idiopathic neutropenia and thrombocytopenia, and in a dog with thrombocytopenia and red cell aplasia.37 Similarly, a syndrome of concurrent neutropenia, splenomegaly, and severe thrombocytopenia has been described in giant schnauzers.65 IMT has been clinically diagnosed or suspected in dogs with rheumatoid arthritis and pemphigus, and thrombocytopenia has accompanied juvenile-onset polyarthritis syndrome in Akitas.21,25 Although possible IMT was reported in association with canine inflammatory bowel disease, thrombocytopenia was severe in only one dog and moderate in one dog.55 Neoplasia Thrombocytopenia frequently occurs in association with neoplasia, and although its pathogenesis is multifactorial, immune-mediated platelet destruction may contribute. This would explain the documented shortened platelet survivals without concurrent decreases in fibrinogen survivals in dogs with multicentric or metastatic neoplasms.47 Lymphoma and other hemic and non-hemic tumors have been associated with IMT in dogs, and this is supported by results of D-MIFAs and indirect assays for platelet-bindable immunoglobulin.31,35,37 IMT also has been diagnosed in horses with lymphoma.

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FIGURE 78.4 Schematic diagram of mechanisms by which immunoglobulins and complement may bind to and opsonize platelets or other hemic cells. Note that platelet surface-associated immunoglobulin (PSAIg) is not necessarily caused by autoantibodies. 1. Autoantibody binding to specific autoantigens is Fab mediated; this occurs in primary immune-mediated thrombocytopenia (IMT). 2. Alloantibody binding is also Fab mediated, but antibodies bind to specific cell alloantigens; this occurs in transfusion reactions and neonatal alloimmune thrombocytopenia. 3–5. In secondary IMT, Fab-mediated binding may occur to adsorbed antigens from infectious agents, drugs, or neoplasms (3), to surface sites perturbed by other molecules (4), or to a combination of adsorbed antigen and perturbed membrane (5). 6. In secondary IMT, PSAIg may also be bound as immune complexes via Fc receptors in some species (e.g. primates). 7. Immunoglobulin may bind via complement-mediated immune adherence in species with platelet complement receptors. 8. With in vitro indirect assays of serum or plasma, increased PSAIg may be a storage artifact resulting from nonspecific binding of immunoglobulins that have aggregated during freezing. 9. Complement deposition may occur as an innocent bystander phenomenon, with cold-reacting antibodies that dissociate from the cells before testing, or along with any complement-activating PSAIg.

Infectious Diseases Thrombocytopenia is commonly associated with infections caused by viruses, bacteria (especially the rickettsials), protozoa, fungi, and nematodes.6,25,32,58 As with neoplasia, the pathogeneses of these infectious thrombocytopenias may be complicated, involving various combinations of suppressed platelet production, altered platelet distribution, increased consumption, or immune-mediated and non-immune platelet destruction. Rickettsial diseases are common infectious causes of thrombocytopenia in endemic areas, but the pathogeneses of the thrombocytopenias are poorly understood. Immune-mediated platelet destruction is thought to contribute to the thrombocytopenia of acute ehrlichiosis caused by Ehrlichia canis,67 and positive Coombs’ tests

have suggested that immune-mediated disease may not be restricted to platelets.66 PSAIg or serum platelet-bindable immunoglobulins have been increased in some naturally and experimentally infected dogs.41,67 Immune mechanisms may contribute to thrombocytopenias in other rickettsial infections, including infection with Anaplasma phagocytophilum3 and Rickettsia rickettsii.26 Cats with FIV and FeLV infections may develop thrombocytopenias with immune components (see Chapters 55 and 62). Immune-mediated platelet destruction appears to contribute to the thrombocytopenia associated with equine infectious anemia (EIA), but thrombocytopenia in EIA is likely multifactorial and caused partly due to impaired platelet production.15 Puppies experimentally infected with canine distemper virus develop marked thrombocytopenia and increased PSAIgG.1 A complement-independent, immune

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complex-induced destruction of platelets appeared to contribute to thrombocytopenia. Dogs60 and other species20 vaccinated with modified live virus (MLV) distemper vaccines may develop mild to occasionally marked transient decreases in platelet concentrations. While clinical purpura has reportedly followed 1–21 days after routine clinical MLV vaccinations,20 further studies of these clinical observations have not been published. Immune-mediated platelet destruction likely contributes to the thrombocytopenia occurring commonly in dogs with histoplasmosis, babesiosis, and leishmaniasis, and it may play a role in thrombocytopenic dogs with other fungal or nematode infections.8,14,42,62 Serum from dogs with leishmaniasis contains plateletbindable IgM and sometimes IgG, suggesting a role for immune-mediated platelet clearance, but the nature of the platelet-immunoglobulin interaction is not known and may involve immune complexes and immune adherence.64 Drugs Drugs produce thrombocytopenia by many mechanisms, including immune-mediated platelet destruction by drug-induced antibodies that may be detectable by indirect assays of serum or plasma (see Chapters 14–16). Drug-induced antibodies may be drug-dependent, requiring the presence of the drug or its metabolite for platelet binding, or they may occasionally be drugindependent, behaving like autoantibodies that bind in the absence of drug. Some drug-dependent antibodies bind to cells in the form of drug-antibody complexes. Others bind directly to either membrane cryptantigen exposed by the presence of the drug or to drug-protein neoantigens on the cell surface. Drug-induced IMT has been suspected with several drugs, most notably sulfonamides in dogs,40,42 methimazole in cats,50 and penicillin or trimethoprim-sulfadoxine in horses.43

ALLOIMMUNE THROMBOCYTOPENIAS Although alloimmune thrombocytopenias can be classified as types of secondary IMT, they are considered separately here because of their distinct pathogeneses. They result from the production of alloantibodies that target platelet alloantigens and cause platelet destruction, primarily by phagocytosis of the opsonized platelets. Exposure to foreign platelet alloantigens may occur with pregnancy or blood transfusion, causing neonatal alloimmune thrombocytopenia or post-transfusion purpura, respectively. Neonatal alloimmune thrombocytopenia occurs when maternal antibodies to paternal epitopes on neonatal platelets are passively transferred through the placenta or colostrum. These antibodies circulate in the blood where they bind to neonatal platelets and lead to their premature destruction. The disorder is selflimiting because of the inherent lifespan and amounts

of transferred immunoglobulins. Neonatal alloimmune thrombocytopenia is uncommon, but has been described in pigs, horses, and possibly mules.10,19,54 The diagnosis of neonatal alloimmune thrombocytopenia is mostly one of exclusion, bacterial or viral infections being the most common differential. Where available, antibody assays may be applied to support a diagnosis of alloimmune thrombocytopenia by documenting the presence of PSAIg in the neonate, the absence of maternal PSAIg, and the presence of maternal antibody reactivity with paternal and neonatal, but not maternal, platelets. Specific therapy may not be required for this self-limiting condition, but glucocorticoids may be useful in severely affected newborns. Platelet transfusions are indicated if hemorrhage is severe or if central nervous system hemorrhage is suspected. Compatible platelets can be obtained from the dam, but they should be washed to remove plasma antibodies. Future offspring from the same mating pair are at risk of developing the same disease. Post-transfusion purpura is characterized by severe thrombocytopenia developing several days after transfusion of a patient previously sensitized to a platelet alloantigen by pregnancy or transfusion. Re-exposure to the platelet alloantigen induces a high titer of antibodies targeting the transfused platelets. Severe thrombocytopenia occurs because the patient’s own platelets are also destroyed, despite being negative for the target alloantigen when tested after recovery. Proposed mechanisms include epitope spreading, the development of an oligoclonal immune response that targets non-polymorphic autoepitopes in addition to the inciting alloepitope. Development of thrombocytopenia within 1–2 weeks of a blood transfusion should prompt consideration of this clinical entity in veterinary species (see Chapter 100).42

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CHAPTER 78: IMMUNE-MEDIATED THROMBOCYTOPENIA 12. Campbell KL, George JW, Greene CE. Application of the enzyme-linked immunosorbent assay for the detection of platelet antibodies in dogs. Am J Vet Res 1984;45:2561–2564. 13. Chong BH, Ho SJ. Autoimmune thrombocytopenia. J Thromb Haemost 2005;3:1763–1772. 14. Clinkenbeard KD, Tyler RD, Cowell RL. Thrombocytopenia associated with disseminated histoplasmosis in dogs. Comp Cont Educ Small Anim Pract 1989;11:301–306. 15. Crawford TB, Wardrop K, Tornquist SJ, et al. A primary production deficit in the thrombocytopenia of equine infectious anemia. J Virol 1996;70:7842–7850. 16. Dale DC, Nichol JL, Rich DA, et al. Chronic thrombocytopenia is induced in dogs by development of cross-reacting antibodies to the MpL ligand. Blood 1997;90:3456–3461. 17. Darbes J, Colbatsky F, Minkus G. Demonstration of feline and canine platelet glycoproteins by immuno- and lectin histochemistry. Histochemistry 1993;100:83–91. 18. Davis B, Toivio-Kinnucan M, Schuller S, et al. Mutation in β1-tubulin correlates with macrothrombocytopenia in Cavalier King Charles spaniels. J Vet Intern Med 2008;22:540–545. 19. Dimmock CK, Webster WR, Shiels IA, et al. Isoimmune thrombocytopenic purpura in piglets. Aust Vet J 59:157–158. 20. Dodds WJ. 1983; Immune-mediated diseases of the blood. Adv Vet Sci Comp Med 1982;27:163–196. 21. Dougherty SA, Center SA, Shaw EE, et al. Juvenile-onset polyarthritis syndrome in Akitas. J Am Vet Med Assoc 1991;198:849–856. 22. Garon CL, Scott MA, Selting KA, et al. Idiopathic thrombocytopenic purpura in a cat. J Am Anim Hosp Assoc 1999;35:464–470. 23. Gowing GM. Idiopathic thrombocytopenic purpura in the dog. J Am Vet Med Assoc 1964;145:987–990. 24. Graham-Mize CA, Rosser EJ. Bioavailability and activity of prednisone and prednisolone in the feline patient. Vet Dermatol 2004;15(Suppl 1):7–10. 25. Grindem CB, Breitschwerdt EB, Corbett WT, et al. Epidemiologic survey of thrombocytopenia in dogs: a report on 987 cases. Vet Clin Pathol 1991;20:38–43. 26. Grindem CB, Breitschwerdt EB, Perkins PC, et al. Platelet-associated immunoglobulin (antiplatelet antibody) in canine Rocky Mountain spotted fever and ehrlichiosis. J Am Vet Med Assoc 1999;35:56–61. 27. Hoffman R, Briddell RA, van Besien K, et al. Acquired cyclic amegakaryocytic thrombocytopenia associated with an immunoglobulin blocking the action of granulocyte-macrophage colony-stimulating factor. New Engl J Med 1989;321:97–102. 28. Humber KA, Beech J, Cudd TA. Azathioprine for treatment of immunemediated thrombocytopenia in two horses. J Am Vet Med Assoc 1991;199:591–594. 29. Impellizeri JA, Howell K, McKeever KP, et al. The role of rituximab in the treatment of canine lymphoma: an ex vivo evaluation. Vet J 2006;171:556–558. 30. Jackson ML, Kruth SA. Immune-mediated hemolytic anemia and thrombocytopenia in the dog: a retrospective study of 55 cases diagnosed from 1969 through 1983 at the Western College of Veterinary Medicine. Can Vet J 1985;26:245–250. 31. Jain NC, Switzer JW. Autoimmune thrombocytopenia in dogs and cats. Vet Clin N Am Small Anim Pract 1981;11:421–433. 32. Jordan HL, Grindem CB, Breitschwerdt EB. Thrombocytopenia in cats: a retrospective study of 41 cases. J Vet Intern Med 1993;7:261–265. 33. Joshi BC, Jain NC. Detection of antiplatelet antibody in serum and on megakaryocytes of dogs with autoimmune thrombocytopenia. Am J Vet Res 1976;37:681–685. 34. Joshi BC, Raplee RG, Powell AL, et al. Autoimmune thrombocytopenia in a cat. J Am Anim Hosp Assoc 1979;15:585–588. 35. Keller ET. Immune-mediated disease as a risk factor for canine lymphoma. Cancer 1992;70:2334–2337. 36. Kohn B, Linden T. Platelet-bound antibodies detected by a flow cytometric assay in cats with thrombocytopenia. J Feline Med Surg 2006;8:254–260. 37. Kristensen AT, Weiss DJ, Klausner JS, et al. Detection of antiplatelet antibody with a platelet immunofluorescence assay. J Vet Intern Med 1994;8:36–39. 38. Kuter DJ, Bussel JB, Lyons RM, et al. Efficacy of romiplostim in patients with chronic immune thrombocytopenic purpura: a double-blind, randomised controlled trial. Lancet 2008;371:395–403. 39. Lachowicz JL, Post S, Moroff SD, et al. Acquired amegakaryocytic thrombocytopenia – four cases and a literature review. J Small Anim Pract 2004;45:507–514. 40. Lavergne SN, Trepanier LA. Anti-platelet antibodies in a natural animal model of sulphonamide-associated thrombocytopaenia. Platelets 2007;18:595–604. 41. Lewis DC, Meyers KM. Studies of platelet-bound and serum plateletbindable immunoglobulins in dogs with idiopathic thrombocytopenic purpura. Exp Hematol 1996;24:696–701.

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42. Lewis DC, Meyers KM, Callan MB, et al. Detection of platelet-bound and serum platelet-bindable antibodies for diagnosis of idiopathic thrombocytopenic purpura in dogs. J Am Vet Med Assoc 1995;206:47–52. 43. McGurrin MKJ, Arroyo LG, Bienzle D. Flow cytometric detection of platelet-bound antibody in three horses with immune-mediated thrombocytopenia. J Am Vet Med Assoc 2004;224:83–87. 44. McVey DS, Shuman WS. Detection of antiplatelet immunoglobulin in thrombocytopenic dogs. Vet Immunol Immunopathol 1989;22:101– 111. 45. Miller MD, Lunn KF. Diagnostic use of cytologic examination of bone marrow from dogs with thrombocytopenia: 58 cases (1994–2004). J Am Vet Med Assoc 2007;231:1540–1544. 46. Northern Jr J, Tvedten HW. Diagnosis of microthrombocytosis and immune mediated thrombocytopenia in dogs with thrombocytopenia: 68 cases (1987–1989). J Am Vet Med Assoc 1992;200:368–372. 47. O’Donnell MR, Slichter SJ, Weiden PL, et al. Platelet and fibrinogen kinetics in canine tumors. Cancer Res 1981;41:1379–1383. 48. Olsson B, Andersson PO, Jacobsson S, et al. Disturbed apoptosis of T-cells in patients with active idiopathic thrombocytopenic purpura. Thromb Haemost 2005;93:139–144. 49. Peroni DL, Stanley S, Kollias-Baker C, et al. Prednisone per os is likely to have limited efficacy in horses. Equine Vet J 2002;34:283–287. 50. Peterson ME, Hurvitz AI, Leib MS, et al. Propylthiouracil-associated hemolytic anemia, thrombocytopenia, and antinuclear antibodies in cats with hyperthyroidism. J Am Vet Med Assoc 1984;184:806–808. 51. Provan D, Newland A, Norfolk D, et al. Guidelines for the investigation and management of idiopathic thrombocytopenic purpura in adults, children and in pregnancy. Br J Haematol 2003;120:574–596. 52. Putsche JC, Kohn B. Primary immune-mediated thrombocytopenia in 30 dogs (1997–2003). J Am Anim Hosp Assoc 2008;44:250–257. 53. Ramanarayanan J, Brodzik F, Czuczman MS, et al. Efficacy and safety of rituximab in the treatment of refractory/relapsed idiopathic thrombocytopenic purpura (ITP): results of a meta-analysis of 299 patients. Blood 2006;108:1076 (abstract). 54. Ramirez S, Gaunt SD, McClure JJ, et al. Detection and effects on platelet function of anti-platelet antibody in mule foals with experimentally induced neonatal alloimmune thrombocytopenia. J Vet Intern Med 1999;13:534–539. 55. Ridgway J, Jergens, AE, Niyo Y. Possible causal association of idiopathic inflammatory bowel disease with thrombocytopenia in the dog. J Am Anim Hosp Assoc 2001;37:65–74. 56. Rozanski EA, Callan MB, Hughes D, et al. Comparison of platelet count recovery with use of vincristine and prednisone or prednisone alone for treatment for severe immune-mediated thrombocytopenia in dogs. J Am Vet Med Assoc 2002;220:477–481. 57. Scott MA, Kaiser L, Davis JM, et al. Development of a sensitive immunoradiometric assay for detection of platelet surface-associated immunoglobulins in thrombocytopenic dogs. Am J Vet Res 2002;63:124–129. 58. Sellon DC, Levine J, Millikin E, et al. Thrombocytopenia in horses: 35 cases (1989–1994). J Vet Int Med 1996;10:127–132. 59. Shelah-Goraly M, Aroch I, Kass PH, et al. A prospective study of the association of anemia and thrombocytopenia with ocular lesions in dogs. Vet J 2009;182:187–192. 60. Stokol T, Parry BW. The effect of modified-live virus vaccination on vonWillebrand factor antigen concentrations and platelet counts in dogs. Vet Clin Pathol 1997;26:135–137. 61. Sullivan PS, Evans HL, McDonald TP. Platelet concentration and hemoglobin function in greyhounds. J Am Vet Med Assoc 1994;205: 838–841. 62. Taboada J. Babesiosis. In: Greene CE, ed. Infectious Diseases of the Dog and Cat, 2nd ed. Philadelphia: WB Saunders, 1998;473–481. 63. Tasker S, Mackin AJ, Day MJ. Primary immune-mediated thrombocytopenia in a cat. J Small Anim Pract 1999;40:127–131. 64. Terrazzano G, Cortese L, Piantedosi D, et al. Presence of anti-platelet IgM and IgG antibodies in dogs naturally infected by Leishmania infantum. Vet Immunol Immunopathol 2006;110:331–337. 65. Vargo CL, Yaylor SM, Haines DM. Immune mediated neutropenia and thrombocytopenia in 3 giant schnauzers. Can Vet J 2007;48:1159–1163. 66. Waddle JR, Littman MP. A retrospective study of 27 cases of naturally occurring canine ehrlichiosis. J Am Anim Hosp Assoc 1988;24:615–620. 67. Waner T, Harrus S, Weiss DJ, et al. Demonstration of serum antiplatelet antibodies in experimental acute canine ehrlichiosis. Vet Immunol Immunopathol 1995;48:177–182. 68. Wang B, Nichol JL, Sullivan JT. Pharmacodynamics and pharmacokinetics of AMG 531, a novel thrombopoietin receptor ligand. Clin Pharmacol Therapeut 2004;76:628–638. 69. Williams DA, Maggio-Price L. Canine idiopathic thrombocytopenia: clinical observations and long-term follow-up in 54 cases. J Am Vet Med Assoc 1984;185:660–663. 70. Zini E, Hauser B, Meli ML, et al. Immune-mediated erythroid and megakaryocytic aplasia in a cat. J Am Vet Med Assoc 2007;230:1024–1027.

C H A P T E R 79

Non-Immune-Mediated Thrombocytopenia JENNIFER S. THOMAS Platelets and Normal Hemostasis Platelet Function Platelet Kinetics Diagnostic Assays for Thrombocytopenia Basic Mechanisms for Thrombocytopenia Platelet Loss Platelet Consumption Thrombotic microangiopathies Disseminated intravascular coagulation Platelet Destruction Immune-mediated platelet destruction Non-immune-mediated platelet destruction

Platelet Distribution Disorders Platelet Production Disorders Disorders with Complex Mechanisms Infectious thrombocytopenia Drug-induced thrombocytopenia Neoplasia associated thrombocytopenia Miscellaneous Causes of Thrombocytopenia Breed associated thrombocytopenia Pseudothromobcytopenia

Acronyms and Abbreviations APTT, activated partial thromboplastin time; AT, antithrombin; CBC, complete blood count; CDV, canine distemper virus; DIC, disseminated intravascular coagulation; EDTA, ethylenediaminetetraacetic acid; FDP, fibrin/fibrinogen degradation product; GPIIb-IIIa, glycoprotein IIb-IIIa; HUS, hemolytic uremic syndrome; IMT, immune mediated thrombocytopenia; MPC, mean platelet component; MPS, mononuclear phagocytic system; MPV, mean platelet volume; PCR, polymerase chain reaction; PSAIg, platelet surface associated immunoglobulin; PT, prothrombin time; TPO, thrombopoietin; TTP, thrombocytopenic thrombotic purpura; VWF, von Willebrand factor.

T

hrombocytopenia is generally an acquired pathologic process that accompanies an underlying disease or disorder and is a common finding in many species.22,27,64 Uncomplicated, mild to moderate thrombocytopenia is usually subclinical and affected animals are either asymptomatic or have clinical signs (e.g. fever, lethargy, anorexia) that are related to the underlying condition. Thrombocytopenia is often first recognized when a decreased platelet concentration is identified following review of a blood smear or a complete blood count (CBC). Spontaneous bleeding disorders due to uncomplicated thrombocytopenia are rare when platelet concentrations are greater than 20,000–30,000/μL.2,27 Animals with severe thrombocytopenia or thrombocytopenia complicated by a coagulopathy, vascular disorder, or functional platelet defect often present with clinical findings typical of a defect in primary hemostasis. Findings may include petechiae or ecchymoses (often on the ventral abdomen, inner thighs, or mucous 596

membranes), epistaxis, gastrointestinal bleeding, gingival bleeding, hematuria, vaginal bleeding, or ocular hemorrhage. Excess hemorrhage following trauma, surgery, and venipuncture is sometimes noted.56,66 PLATELETS AND NORMAL HEMOSTASIS Platelet Function Platelets play a central role in thrombus formation and maintenance of vascular integrity. Initially, platelets are recruited and help to form the primary hemostatic plug which can repair small lesions in the vascular wall. Subsequently, platelets play a role in fibrin production and formation of the more stable secondary hemostatic plug.66 When a blood vessel is injured, platelets initially adhere to the defect in a von Willebrand factor (VWF)dependent manner, particularly under high shear con-

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ditions. Following adhesion, platelets become activated and expose fibrinogen binding sites on membrane integrin αIIbβ3, also known as glycoprotein IIb-IIIa (GPIIb-IIIa). Adhesive proteins, such as fibrinogen and VWF, bind to αIIbβ3 on adjacent platelets and form a bridge to support platelet aggregation. Activated platelets release a variety of agonists that activate other platelets (e.g. thromboxane, adenosine diphosphate), promote vessel healing (e.g. platelet derived growth factor), or play a role in coagulation (e.g. fibrinogen, factor V). Activated platelets shift negatively charged phospholipids (e.g. phosphatidylserine) from the inner membrane surface to the outer membrane where they provide binding sites for coagulation enzymes and cofactors.28,38 Finally, platelets facilitate wound closure by undergoing contractile processes that lead to clot retraction.66 Platelet Kinetics Platelet concentration in the blood reflects a balance between platelet production and platelet consumption, destruction or redistribution to the vasculature of organs. In most healthy animals, platelet lifespans are approximately 5–10 days.66 Platelets are continually consumed during repair of small vascular defects. Aged platelets are removed by macrophages. The spleen appears to play a major role in determining platelet survival. Platelet lifespans are significantly longer in splenectomized dogs when compared to healthy dogs with spleens.11 Studies in people have shown that 1011 platelets are produced every day and platelet population turnover occurs every 8–9 days. Platelets are produced by cytoplasmic fragmentation of megakaryocytes, with each megakaryocyte producing thousands of platelets.14 In response to increased demand, platelet production can increase 20 fold or more.30 Platelet production is affected by a number of growth factors (see Chapter 9); however, thrombopoietin (TPO) plays the primary role in regulating megakaryopoiesis and thrombopoiesis.30 Thrombopoietin is primarily produced by hepatocytes, stromal cells in the bone marrow, and renal epithelial cells. In healthy animals, TPO is constituitively produced and the blood concentration is generally inversely related to platelet mass.14 Thrombopoietin is bound to receptors on platelets and megakaryocytes. When circulating platelet mass is decreased, the concentration of unbound TPO available to stimulate megakaryopoiesis and thrombopoiesis is increased. Production of TPO is increased in some disease states. In inflammation, interleukin 6 has been shown to induce TPO production by the liver.14 DIAGNOSTIC ASSAYS FOR THROMBOCYTOPENIA The laboratory evaluation of thrombocytopenia is covered in greater depth in Chapter 77. A list of common diagnostic assays is found in Table 79.1. When throm-

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TABLE 79.1 Diagnostic Tests for Thrombocytopenia Assay Complete blood count Platelet concentration Mean platelet volume Mean platelet component Blood smear examination

Commentsa

Automated or manual Confirm and track thrombocytopenia Increase suggests enhanced thrombopoiesis Decrease suggests platelet activation Detect platelet clumps Estimate platelet concentration Detect organisms or neoplastic cells Detect large platelets – suggests increased production Evaluate for inflammatory leukogram

Hemostasis profile

Evaluate for DIC

Bone marrow examination

Assess megakaryocyte density and morphology Detect organisms, neoplastic cells, necrosis, inflammation, myelofibrosis

Serology

Detect exposure to infectious agents

PCR

Detect infectious agents

Flow cytometry

Detect PSAIg Measure reticulated platelets to assess platelet production Detect activation markers

a DIC, disseminated intravascular coagulation; PSAIg, platelet surfaceassociated immunoglobulin.

bocytopenia is detected by an automated hematology analyzer, it is important to first examine a blood smear to check for platelet clumps, estimate platelet concentration to confirm the accuracy of an automated platelet count, and evaluate platelet morphology. If clumps are present then the automated platelet concentration is of minimal value. Collection of a fresh blood sample without clumping would be required to determine an accurate platelet concentration. Once thrombocytopenia is confirmed, additional qualitative and quantitative data are available from a routine CBC and provide useful diagnostic information. Identification of increased numbers of large platelets, also known as megaplatelets, on a blood smear suggests increased thrombopoiesis and may be associated with an increased mean platelet volume (MPV).66 Increased MPV in an animal with thrombocytopenia suggests adequate to increased platelet production in the bone marrow.29,69 Decreased mean platelet component (MPC) suggests in vivo platelet activation.48,62 Additional diagnostic tests are often required to determine the underlying cause of the thrombocytopenia. The decision of which tests to perform depends on clinical presentation and results of other diagnostic findings. A complete hemostasis profile (prothrombin time [PT], activated partial thromboplastin time [APTT], fibrinogen concentration, fibrin degradation products [FDPs], D-dimer, and antithrombin [AT] activity) is

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used to detect disseminated intravascular coagulation (DIC). Examination of blood smears, cytology smears, or bone marrow samples may reveal infectious organisms or neoplastic cells. Serology or polymerase chain reaction (PCR) testing is indicated to identify an infectious process. Flow cytometry is used to detect reticulated platelets, platelet surface-associated immunoglobulin (PSAIg), or activated platelets.75 Reticulated platelets are immature platelets with increased amounts of RNA. Measurement of reticulated platelets may help differentiate thrombocytopenia due to shortened platelet survival from thrombocytopenia due to platelet production disorders. Reticulated platelets were increased in dogs and horses with destructive or consumptive thrombocytopenia.57,78 Identification of PSAIg may support a diagnosis of immune-mediated disease but these assays do not differentiate primary from secondary immune-mediated thrombocytopenia (IMT).31,78 Detection of activated platelets may identify animals that are in a prothrombotic state or in DIC.48,62 Bone marrow examination may be useful if a platelet production problem is suspected. If thrombocytopenia is due to a production disorder, megakaryocytes should be decreased or totally absent. Neoplastic cells or etiologic agents may be identified. If thrombocytopenia is due to platelet destruction or consumption, megakaryocytes should be normal or increased in number. The value of bone marrow examination as an initial diagnostic assay in animals with thrombocytopenia is debatable. A recent study in thrombocytopenic dogs reported that bone marrow cytology provided useful diagnostic or prognostic information in a minority of cases, suggesting that bone marrow aspiration was most likely to be useful in animals with thrombocytopenia that was unresponsive to therapy or was accompanied by bicytopenia or pancytopenia.46 Immunofluorescence assays may be used to detect anti-megakaryocyte antibodies on bone marrow cytology preparations; however, the sensitivity and specificity of these assays are low.66 BASIC MECHANISMS FOR THROMBOCYTOPENIA Thrombocytopenia occurs when platelets are removed from circulation faster than they are replaced from the bone marrow. This occurs because of decreased platelet survival or decreased platelet production. Basic mechanisms for decreased platelet survival include loss, distribution disorders, consumption with excessive thrombi formation, or destruction. Decreased platelet production results from impaired megakaryopoiesis or thrombopoiesis. Often multiple mechanisms are involved in the pathogenesis of thrombocytopenia in an animal. Platelet Loss Thrombocytopenia, usually mild to moderate in severity, may follow external hemorrhage. Platelet concen-

tration decreased by up to 50% in dogs with experimental acute, severe blood loss.66 Thrombocytopenia due to blood loss is usually self-limiting and resolves when the hemorrhage resolves. When severe thrombocytopenia occurs in conjunction with hemorrhage, it is likely that the hemorrhage is due to thrombocytopenia and not that the thrombocytopenia is due to hemorrhage. Dogs that have anticoagulant rodenticide toxicity frequently have thrombocytopenia and the thrombocytopenia may be severe.39 Likely mechanisms for thrombocytopenia include loss associated with hemorrhage and platelet consumption during formation of hemostatic plugs at sites of hemorrhage.66 Platelet Consumption Accelerated platelet consumption occurs with disorders that cause widespread damage to endothelial cells or trigger massive activation of coagulation via other mechanisms. In people, well characterized platelet consumptive disorders include DIC and thrombotic microangiopathies such as thrombocytopenic thrombotic purpura (TTP) and hemolytic uremic syndrome (HUS). Thrombotic Microangiopathies Thrombotic microangiopathies are uncommon, lifethreatening conditions characterized by disseminated thrombosis in the microvasculature, secondary ischemic organ damage, microangiopathic hemolytic anemia with schistocytes, and thrombocytopenia. Laboratory evidence of abnormalities in coagulation and fibrinolysis pathways is generally lacking.42 Thrombotic microangiopathies are associated with inherited or acquired disorders and the underlying pathogenesis is often heterogeneous.80 Some people consider HUS and TTP to be variants of the same condition and differentiation of the two disorders is challenging.42,80 Based upon clinical presentation, HUS is associated with more severe renal insufficiency. The most common form of HUS occurs primarily in children and follows a period of infectious diarrhea, particularly involving enterohemorrhagic Escherichia coli. The atypical form of HUS in humans is often associated with inherited or acquired defects in complement regulation.26,49,80 Although rare, HUS syndromes have been described in dogs, cats, cattle, and horses.3,13,15,55 When compared to HUS, the clinical presentation of TTP is associated with frequent neurologic dysfunction, less severe renal insufficiency, and evidence of microthrombi that are rich in platelets and VWF. Recent findings indicate that many people with TTP have inherited or acquired deficiency of ADAMTS13, a metalloprotease that cleaves VWF and prevents accumulation of the most thrombogenic, high relative molecular mass forms of VWF in the blood.49,80 TTP has not been welldocumented in veterinary species; however, spontaneous TTP has been reported in pigs and there are animal models of experimentally induced TTP.43,59

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Disseminated Intravascular Coagulation Unlike HUS and TTP, DIC is commonly diagnosed in veterinary species. Disseminated intravascular coagulation differs from HUS and TTP in that patients with DIC typically have laboratory evidence of abnormalities in coagulation and fibrinolytic pathways.42 DIC is never a primary condition; it occurs secondary to a wide variety of disorders that are characterized by excessive and unregulated coagulation. DIC varies from overt or uncompensated to non-overt or compensated. In non-overt DIC, excess thrombin is generated but is balanced by inhibitory pathways. In overt DIC, there is widespread microthrombi formation and bleeding may result from consumption of coagulation factors and platelets.37,67 Clinical signs vary from subclinical in non-overt DIC to signs of severe, life threatening hemorrhage, shock or multiorgan failure in overt DIC. Clinical signs referable to the underlying condition are often present.66 The consequences of hemorrhage are easily recognized; however, clinical signs and death are often attributable to organ dysfunction resulting from microthrombi formation and resulting tissue hypoxia.37,67 The pathogenesis of DIC is further discussed in Chapter 88. Briefly, DIC is characterized by overwhelming activation of coagulation and subsequent disruption of the normal regulatory mechanisms. Widespread fibrin deposition results due to excessive thrombin generation, loss or inhibition of physiologic anticoagulants, impaired fibrinolysis, and release of proinflammatory cytokines.19 Triggers for thrombin generation generally include extensive vascular disruption or massive release of tissue factor from damaged tissue.66 Snake venoms may directly activate coagulation factors. Specific disorders associated with DIC include infections (e.g. septicemia, endotoxemia), malignant neoplasia, severe tissue injury (e.g. trauma, surgery, burns), immunemediated disorders (e.g. immune-mediated hemolytic anemia), conditions that cause blood stasis (e.g. shock, severe dehydration), heatstroke, systemic hypersensitivity reactions, pancreatitis, snakebites or hepatic failure.37,56,66 Thrombocytopenia due to DIC varies from mild to severe. Unfortunately, no single test is diagnostic for DIC. Tests that detect markers of thrombin activation (e.g. prothrombin fragments 1+2, thrombinantithrombin complexes) or fibrin generation (e.g. fibrinopeptide A) are sensitive; however, they are difficult to run and lack specificity.67 The International Society on Thrombosis and Haemostasis proposed a diagnostic algorithm to diagnose overt DIC in people using platelet concentration, PT, fibrinogen concentration, and FDP or D-dimer. A more complex diagnostic algorithm adding evaluation of abnormal trends in coagulation screening data, AT activity and protein C activity has been proposed to detect non-overt DIC.70 The diagnosis of DIC in veterinary species usually relies on identifying abnormalities involving multiple hemostatic pathways. Laboratory findings include evidence of thrombocytopenia, coagulation defects (pro-

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longed APTT or PT, hypofibrinogenemia), enhanced fibrinolysis (increased FDPs or D-dimer), and loss of natural anticoagulants (e.g. decreased AT activity). Identification of schistocytes or keratocytes on a blood smear is also supportive.66 Platelet Destruction Immune-mediated Platelet Destruction Immune mediated thrombocytopenia (IMT) is commonly diagnosed in dogs and sporadically recognized in other species. IMT occurs when antibodies bind to platelets and cause shortened platelet lifespan due to complement-mediated lysis or platelet removal by the mononuclear phagocytic system (MPS). Direct T cell mediated cytotoxicity as well as impaired megakaryopoiesis and thrombopoiesis are implicated in the pathogenesis of IMT.65,81 With primary IMT, the immune response is directed against autoantigens and no underlying disease is identified. In people with primary IMT, the most common target antigens are located on integrin αIIbβ3.81 Target antigens are rarely identified in animals with primary IMT. Antibodies directed against αIIbβ3 and GPIb were detected in dogs with primary IMT.40 With secondary IMT, the immune response is associated with an underlying disorder such as systemic immune-mediated diseases (e.g. immune-mediated hemolytic anemia), neoplasia (e.g. lymphoma, hemic neoplasia, various solid tumors), drug administration, and infections.40,66 The diagnosis of IMT generally relies on exclusion of other causes of thrombocytopenia and response to immunosuppressive therapy. Detection of PSAIg is supportive but testing is not frequently performed in veterinary species.66 The pathogenesis and diagnosis of IMT is covered in more depth in Chapter 78. Non-immune-mediated Platelet Destruction Non-immune platelet destruction occurs in disorders that cause platelet aggregation, phagocytosis, or lysis independent of antibody or complement. Non-immune platelet destruction is often a feature of thrombocytopenia due to infections, drugs, or neoplasia. These are disorders with complex mechanisms and are discussed separately below. Potential mechanisms for nonimmune destruction include direct damage to platelets causing lysis or premature removal, release of substances that directly activate platelets, cytokinemediated activation of the MPS leading to enhanced phagocytosis of platelets, hemophagocytic syndrome, or phagocytosis by neoplastic cells.1,66,76 Hemophagocytic syndrome is characterized by a non-neoplastic proliferation of activated macrophages and is associated with multiple cytopenias in the blood. Hemophagocytic syndrome occurs secondary to immune-mediated disorders, infections, and neoplasia. In some cases, an underlying condition cannot be identified.76

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Platelet Distribution Disorders The total platelet mass in any animal consists of platelets freely circulating in the blood and platelets reversibly distributed in the vascular systems of tissues, particularly the spleen, liver and bone marrow. In some healthy animals, 30% or more of total platelet mass is stored in the spleen.8 Redistribution thrombocytopenia occurs when platelets become reversibly sequestered in tissues. Total platelet mass is not affected so increased thrombopoiesis is not expected. Disorders associated with increased platelet destruction are not considered redistribution disorders. Platelet pooling has been reported in the liver and spleen of dogs with hypothermia and in the lungs of dogs with endotoxemia.66 Mild to moderate thrombocytopenia may result from sequestration in animals with any condition associated with splenomegaly or hepatomegaly and increased blood pooling.8 Clinical hemorrhage is not expected with platelet sequestration unless the underlying disorder is complicated by platelet dysfunction or a coagulation disorder. Clinical signs are generally limited to those associated with the underlying disorder. Diagnosis of redistribution thrombocytopenia relies on discovering the cause for organomegaly and ruling out other potential causes of thrombocytopenia. Platelet Production Disorders Bone marrow production disorders are covered in Chapters 16–19. Decreased platelet production may result from disorders that specifically target megakaryocytes or from diseases that have more generalized effects to impact production of multiple hematopoietic cell lines directly or by altering the microenvironment in the bone marrow. Bicytopenia or pancytopenia in the blood suggests generalized bone marrow disease.66 Selective megakaryocytic hypoplasia is uncommon. Acquired immune-mediated amegakaryocytic thrombocytopenia has been reported in dogs and cats.35,83 Megakaryocytic hypoplasia can be associated with drugs, infections or neoplasia. Neoplasia may be metastatic to the bone marrow or hemic in origin, causing replacement of the normal hematopoietic cells. Other causes of hypoplasia include irradiation and myelonecrosis.66 Hereditary causes of megakaryocytic hypoplasia are documented in people but have been rarely reported in veterinary species.5,32 High quality bone marrow samples are required to adequately assess bone marrow production. Decreased megakaryocytes should be interpreted with caution in an aspirate, and core biopsy samples are recommended for definitive diagnosis of megakaryocytic hypoplasia.63 Disorders with Complex Mechanisms Thrombocytopenia has been frequently associated with infection, neoplasia and drug therapy in veterinary species.22,27,64 In many cases, the pathogenesis of the

thrombocytopenia is unclear and likely multifactorial in origin. Infectious Thrombocytopenia Thrombocytopenia frequently accompanies infections. 22,25,27,64 Some of the organisms associated with thrombocytopenia are listed in Table 79.2. Many organisms impair platelet production in the bone marrow by direct infection of megakaryocytes, stimulation of an immune response targeting megakaryocytes, or stimulation of an inflammatory response in the bone marrow and production of myelosuppressive cytokines. Platelet survival is often decreased. Proposed pathophysiologic mechanisms include increased platelet consumption due to platelet activation and associated DIC, direct infection of platelets causing destruction or removal by the MPS, secondary IMT, or platelet sequestration in the spleen or other tissues.56,66 With endotoxemia or some bacterial infections, platelets are activated due to a direct effect of endotoxins or inflammatory cytokines.61 Inflammation leads to production of platelet stimuli (e.g. thrombin, platelet activating factor) by activated endothelial cells, neutrophils, monocytes and platelets.61,72 Activated platelets enhance tissue factor expression by monocytes, leading to activation of the coagulation cascade.54,61 Endothelial cells become injured and consequently decrease production of inhibitory substances such as prostaglandin I2. Activated platelets express P selectin which plays a role in platelet adhesion to neutrophils and monocytes.77 Inflammatory cytokines mediate activation of the MPS which may further enhance phagocytosis of platelets.1 Rickettsial bacteria are commonly associated with thrombocytopenia. Acute infection with Ehrlichia canis is associated with immune-mediated and non-immune platelet destruction in dogs.74 Chronic infection is associated with bone marrow hypoplasia.66 Other rickettsial bacteria associated with thrombocytopenia include Anaplasma platys in dogs, Rickettsia rickettsii in dogs, Neorickettsia risticii in horses, and Anaplasma phagocytophilum in several species.18,41 A. phagocytophilum, the cause of granulocytic anaplasmosis (granulocytic ehrlichiosis), was reported to directly infect megakaryocytes but to not alter platelet production. Immune responses were implicated in the pathogenesis of the thrombocytopenia.21 The pathogenesis of thrombocytopenia in A. platys infection is unknown but may involve sequestration or removal of infected platelets by macrophages. In experimentally infected dogs, PCR testing was positive for the organism in the spleen and bone marrow.16 Vasculitis and platelet consumption occur in R. rickettsii infection and may be the primary causes for thrombocytopenia. The cause of thrombocytopenia in N. risticii infection is unclear but likely involves consumption due to DIC.56 Other bacterial infections (e.g. leptospirosis, salmonellosis, borreliosis, bartonellosis) are sporadically associated with thrombocytopenia.6,18,56,66 Viral infections are well-recognized causes of thrombocytopenia in animals. In dogs, thrombocytopenia

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601

TABLE 79.2 Reported Causes of Thrombocytopenia in Dogs, Cats, Horses and Cattle Species

General Category

Dog

Infectious

Neoplasia Drugs/toxins

Miscellaneous Cat

Infectious

Neoplasia Drugs/toxins Miscellaneous Horse

Infectious Neoplasia Drugs/toxins Miscellaneous

Cattle

Infectious

Drugs/toxins Miscellaneous

Specific examplesa Viruses: distemper, herpes virus, infectious hepatitis, parvovirus Bacteria: septicemia, endotoxemia, Anaplasma phagocytophilum, A. platys, Ehrlichia canis, Rickettsia rickettsii, leptospirosis, salmonellosis, bartonellosis, borreliosis Fungi: histoplasmosis, candidiasis Protozoa: babesiosis, leishmaniasis Parasites: heartworm disease Hemangiosarcoma, lymphoproliferative and myeloproliferative neoplasia, various solid tissue neoplasms Antibiotics (e.g. sulfonamides, cephalosporin); chemotherapeutic drugs (e.g. cyclophosphamide, doxorubicin), anti-inflammatory drugs (e.g. phenylbutazone, carprofen), phenobarbital, thiacetarsamide, estrogen, snakebites IMT, DIC, distribution disorders Viruses: FeLV, FIV, FIP, parvovirus Bacteria: septicemia, endotoxemia, A. phagocytophilum, Mycoplasma sp., salmonellosis, bartonellosis Fungi: histoplasmosis Protozoa: cytauxzoonosis, toxoplasmosis Parasites: heartworm disease Lymphoproliferative and myeloproliferative neoplasia, various solid tissue neoplasms Chemotherapeutic drugs, griseofulvin, methimazole, propylthiouracil IMT, DIC, distribution disorders Viruses: EIA, EVA, VEE Bacteria: septicemia, endotoxemia, A. phagocytophilum, Neorickettsia risticii, salmonellosis Lymphoproliferative and myeloproliferative neoplasia, various solid tissue neoplasms Phenylbutazone, trimethoprim, penicillin, trichothecene mycotoxin, snakebites IMT, neonatal alloimmune thrombocytopenia, DIC, gastrointestinal disorders, distribution disorders Viruses: BVD Bacteria: septic metritis/mastitis, septicemia, endotoxemia, salmonellosis Protozoa: babesiosis, theileriosis Bracken fern toxicity, trichothecene mycotoxin IMT, DIC, distribution disorders, displaced abomasums

a BVD, bovine viral diarrhea; DIC, disseminated intravascular coagulation; EIA, equine infectious anemia; EVA, equine viral arteritis; FeLV, feline leukemia virus; FIV, feline immunodeficiency virus; FIP, feline infectious peritonitis; IMT, immune-mediated thrombocytopenia; VEE, Venezuelan equine encephalitis.

occurs with canine distemper virus (CDV), herpes virus, infectious hepatitis, or parvovirus infection. The thrombocytopenia in CDV infection results from viral damage to platelets, secondary IMT, and decreased production due to direct infection of megakaryocytes.4 Thrombocytopenia associated with herpes virus or infectious hepatitis virus infection results from endothelial damage and DIC.6 In cats, thrombocytopenia is associated with feline infectious peritonitis (FIP), feline leukemia virus (FeLV), feline immunodeficiency virus (FIV), and parvovirus infection. Thrombocytopenia in FIP is multifactorial and results from vasculitis, secondary IMT, direct viral damage to platelets, and DIC.7 In horses, thrombocytopenia occurs with equine infectious anemia (EIA), equine viral arteritis, and Venezuelan encephalitis virus infection. Thrombocytopenia in EIA results from cytokine mediated impairment of platelet production, secondary IMT, and non-immune platelet destruction.10,58 In cattle, thrombocytopenia is associated with bovine viral diarrhea virus and primarily results from megakaryo-

cyte damage and subsequent decreased platelet production.73 In pigs, thrombocytopenia follows infection with African swine fever or classical swine fever (hog cholera). In classical swine fever, thrombocytopenia results from direct damage to platelets, platelet activation and removal by the MPS, DIC, and impaired megakaryopoiesis.20,52 Other agents associated with thrombocytopenia include protozoa (e.g. babesiosis, cytauxzoonosis, leishmaniasis, theileriosis, and toxoplasmosis), fungi (e.g. disseminated candidiasis, histoplasmosis), and metazoan parasites (e.g. heartworm infection in dogs and cats).6,22,27,56,66 Infection-induced thrombocytopenia is often suspected based upon history and clinical findings. Thrombocytopenia varies from mild to severe. Microscopic examination of blood, bone marrow or cytology smears may identify infectious agents. If organisms are not visible microscopically, serology, culture, or molecular (e.g. PCR) techniques may be diagnostic.

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Drug-induced Thrombocytopenia Thrombocytopenia is often suspected following exposure to drugs or toxins. Some drugs associated with thrombocytopenia in veterinary species are included in Table 79.2. Drug-induced blood cell disorders are covered in Chapter 16 and only a brief summary is provided here. With some drugs (e.g. chemotherapeutic drugs), thrombocytopenia is dose-dependent and predictable.23,66 Chemotherapeutic-induced thrombocytopenia is usually transient and resolves when the drug is withdrawn or dosage is altered. With other drugs (e.g. potentiated sulfas), thrombocytopenia is an idiosyncratic and unpredictable result.71 Many drugs are known to have myelosuppressive effects. Secondary IMT, targeting either platelets or megakaryocytes, is frequently implicated as a mechanism for drug-induced thrombocytopenia. In most cases, the immune response occurs only when the drug or drug metabolite is present.82 Antibodies may target the drug or metabolite adsorbed to the platelet membrane, a complex formed between the drug and a platelet antigen, or a hidden antigen exposed on the platelet membrane.66,82 Subsequent binding of antibody targets the platelet for removal by the MPS. Rarely, drugs cause IMT that is drug-independent and persists after the drug is removed. These types of drugs appear to stimulate an immune response that targets unaltered platelet antigens.66,82 Drug-induced IMT is often difficult to prove in these situations. PSAIg has been detected in dogs following administration of sulfonamides and in horses following administration of trimethoprim and penicillin.36,45 Some drugs directly activate platelets causing increased consumption, or injure platelets causing removal by the MPS. Snake venoms may either directly activate platelets or may cause the production of substances such as thrombin that cause platelets to aggregate.24 Diagnosis of drug associated thrombocytopenia generally relies on a history of thrombocytopenia occurring following drug administration and normalization of platelet concentration following cessation of drug therapy. Neoplasia Associated Thrombocytopenia Thrombocytopenia is commonly attributed to neoplasia.22,23,27,63 In dogs with neoplasia, thrombocytopenia was detected in 10–36% of affected animals.8 Lymphoma, carcinoma, hemangiosarcoma, and hematopoietic neoplasia comprised the majority of cases. Thrombocytopenia was frequently attributed to chemotherapy, decreased platelet production, or DIC.23 Decreased platelet production may result from myelophthisis, secretion of estrogen by neoplastic cells or myelosuppressive chemotherapeutic drugs.8 Disseminated intravascular coagulation is a frequent complication of neoplasia. In one study, over 10% of dogs with malignant neoplasia had DIC. Hemangiosarcoma, mammary carcinoma and adenocarcinoma of

the lung were at greatest risk.44 Neoplasia is the most commonly recognized cause of DIC in cats.17 Secondary IMT has been implicated as a mechanism for thrombocytopenia in a variety of hemic and nonhemic neoplasias.8,34,63 Other proposed mechanisms for neoplasia-induced thrombocytopenia include distribution disorders with platelet pooling (particularly with large vascular tumors), platelet loss with tumor associated hemorrhage, or platelet destruction by neoplastic cells.8,66 Thrombocytopenia was identified in 88% of dogs with hemophagocytic histiocytic sarcoma and phagocytosis of platelets by neoplastic cells was believed to play a role.47 Clinical signs are often related to the underlying neoplasm; thrombocytopenia may or may not be severe enough to cause a defect in primary hemostasis. Diagnosis is based upon cytologic or histopathologic identification of neoplastic cells. If the neoplasia is hemic in origin, then abnormal cells may be present in the blood. Miscellaneous Causes of Thrombocytopenia Breed Associated Thrombocytopenia Thrombocytopenia is an incidental finding in certain dog breeds. Affected dogs are healthy and do not have evidence of a bleeding disorder. Greyhounds often have thrombocytopenia when compared to non-breed specific reference intervals. Platelet concentrations are typically greater than 100,000/μL. In one study, none of the samples tested had detectable PSAIg, suggesting that the thrombocytopenia is not immune-mediated in origin.60 An inherited thrombocytopenia occurs in approximately 50% of Cavalier King Charles Spaniels (see Chapter 82). Macrothrombocytes are present in many dogs, making automated platelet counts unreliable.9 Recent studies suggest that there is a mutation in the gene encoding beta1-tubulin, leading to unstable microtubule formation in megakaryocytes and altered proplatelet formation.12 Pseudothrombocytopenia Pseudothrombocytopenia occurs when platelets in a sample are excluded from counting for any reason. Decreased automated platelet concentration should always be confirmed by microscopic examination of a blood smear. Small clumps may not be detected by automated analyzers and still falsely lower measured platelet concentrations.68 A recent study in cats reported that platelet concentrations determined with an impedance hematology analyzer were decreased in a majority of animals tested; however, examination of a blood smear confirmed thrombocytopenia in only 3.1% of cats. The disparity was attributed to the presence of platelet clumps and inability to separate platelets and erythrocytes based upon size.51 When clumps are detected in a sample, the measured platelet concentration is of minimal value and collection of a new blood sample without clumping is required to obtain an accu-

CHAPTER 79: NON-IMMUNE-MEDIATED THROMBOCYTOPENIA

rate platelet concentration. Platelet concentration determined by analyzers that use a buffy coat technique appear to be less affected by platelet clumping than impedance hematology analyzers.33 Proper blood collection and handling are critical to minimize the clumping that occurs when platelets become activated and form aggregates. Clumping can be minimized by atraumatic venipuncture or addition of platelet inhibitors to the sample.50 Platelet clumps may form in vitro in an EDTA-dependent manner. In people, EDTA causes a conformational change in αIIbβ3 that exposes normally hidden epitopes that bind to circulating antibodies. EDTA-dependent pseudothrombocytopenia has rarely been reported in dogs, horses and pigs; however, the underlying mechanism has not been determined.79 Samples with increased megaplatelets may have falsely low platelet concentrations because large platelets are excluded by some automated hematology analyzers. In these animals, manual platelet counts using a hemocytometer and microscope may provide a more accurate platelet concentration.53

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20. Gomez-Villamandos JC, Salguero FJ, Ruiz-Villamor E, et al. Classical swine fever: pathology of bone marrow. Vet Pathol 2003;40:157– 163. 21. Granick JL, Reneer DV, Carlyon JA, et al. Anaplasma phagocytophilum infects cells of the megakaryocytic lineage through sialylated ligands but fails to alter platelet production. J Med Microbiol 2008;57:416–423. 22. Grindem CB, Breitschwerdt EB, Corbett WT, et al. Epidemiologic survey of thrombocytopenia in dogs: a report on 987 cases. Vet Clin Pathol 1991;20:38–43. 23. Grindem CB, Breitschwerdt EB, Corbett WT, et al. Thrombocytopenia associated with neoplasia in dogs. J Vet Intern Med 1994;8:400–405. 24. Hackett TB, Wingfield WE, Mazzaferro EM, et al. Clinical findings associated with prairie rattlesnake bites in dogs: 100 cases (1989–1998). J Am Vet Med Assoc 2002;220:1675–1680. 25. Irmak K, Sen I, Cöl R, et al. The evaluation of coagulation profiles in calves with suspected septic shock. Vet Res Commun 2006;30:497–503. 26. Johnson S, Taylor CM. What’s new in haemolytic uraemic syndrome? Eur J Pediatr 2008;167:965–971. 27. Jordon HL, Grindem CB, Breitschwerdt EB. Thrombocytopenia in cats: a retrospective study of 41 cases. J Vet Int Med 1993;7:261–265. 28. Jurk K, Kehrel BE. Platelets: physiology and biochemistry. Semin Thromb Hemostasis 2005;31:381–392. 29. Kaito K, Otsubo H, Usui N, et al. Platelet size deviation width, platelet large cell ratio, and mean platelet volume have sufficient sensitivity and specificity in the diagnosis of immune thrombocytopenia. Br J Haematol 2005;128:698–702. 30. Kaushansky K. Historical review: megakaryopoiesis and thrombopoiesis. Blood 2008;111:981–986. 31. Kohn B, Linden T, Leibold W. Platelet-bound antibodies detected by a flow cytometric assay in cats with thrombocytopenia. J Feline Med Surg 2006;8:254–260. 32. Kohn CW, Swardson C, Provost P, et al. Myeloid and megakaryocytic hypoplasia in related standardbreds. J Vet Intern Med 1995;9:315–323. 33. Koplitz SL, Scott MA, Cohn LA. Effects of platelet clumping on platelet concentrations measured by use of impedance or buffy coat analysis in dogs. J Am Vet Med Assoc 2001;219:1552–1556. 34. Kristensen AT, Weiss DJ, Klausner JS, et al. Detection of antiplatelet antibody with a platelet immunofluorescence assay. J Vet Intern Med 1994;8:36–39. 35. Lachowicz JL, Post GS, Moroff SD, et al. Acquired amegakaryocytic thrombocytopenia – four cases and a literature review. J Small Anim Pract 2004;45:507–514. 36. Lavergne SN, Trepanier LA. Anti-platelet antibodies in a natural animal model of sulphonamide-associated thrombocytopaenia. Platelets 2007;18:595–604. 37. Levi M. Current understanding of disseminated intravascular coagulation. Br J Haematol 2004;124:567–576. 38. Levi M. Platelets. Crit Care Med 2005;33:S523–S525. 39. Lewis DC, Bruyette DS, Kellerman DL, et al. Thrombocytopenia in dogs with anticoagulant rodenticide-induced hemorrhage: eight cases (1990– 1995). J Am Anim Hosp Assoc 1997;33:417–422. 40. Lewis DC, Meyers KM. Canine idiopathic thrombocytopenic purpura. J Vet Intern Med 1996;10:207–218. 41. Madigan JE, Pusterla N. Ehrlichial diseases. Vet Clin N Am Equine Pract 2000;16:487–499. 42. Mannucci PM. Thrombotic thrombocytopenic purpura and the hemolytic uremic syndrome: much progress and many remaining issues. Haematologica 2007;92:878–880. 43. Maratea KA, Snyder PW, Stevenson GW. Vascular lesions in nine Göttingen minipigs with thrombocytopenic purpura syndrome. Vet Pathol 2006;43:447–454. 44. Maruyama H, Miura T, Sakai M, et al. The incidence of disseminated intravascular coagulation in dogs with malignant tumor. J Vet Med Sci 2004;66:573–575. 45. McGurrin MK, Arroyo LG, Bienzle D. Flow cytometric detection of platelet-bound antibody in three horses with immune mediated thrombocytopenia. J Am Vet Med Assoc 2004;224:83–87. 46. Miller MD, Lunn KF. Diagnostic use of cytologic examination of bone marrow from dogs with thrombocytopenia: 58 cases (1994–2004). J Am Vet Med Assoc 2007;231:1540–1544. 47. Moore PF, Affolter VK, Vernau W. Canine hemophagocytic histiocytic sarcoma: a proliferative disorder of CD11d+ macrophages. Vet Pathol 2006;43:632–645. 48. Moritz A, Walcheck BK, Weiss DJ. Evaluation of flow cytometric and automated methods for detection of activated platelets in dogs with inflammatory disease. Am J Vet Res 2005;66:325–329. 49. Nangaku M, Nishi H, Fujita T. Pathogenesis and prognosis of thrombotic microangiopathy. Clin Exp Nephrol 2007;11:107–114. 50. Norman EJ, Barron RC, Nash AS, et al. Evaluation of a citrate-based anticoagulant with platelet inhibitory activity for feline blood cell counts. Vet Clin Pathol 2001;30:124–132. 51. Norman EJ, Barron RCJ, Nash AS, et al. Prevalance of low automated platelet counts in cats: comparison with prevalence of thrombocytopenia based upon blood smear estimation. Vet Clin Pathol 2001;30:137–140.

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52. Núñez A, Gómez-Villamandos JC, Sánchez-Cordón PJ, et al. Expression of proinflammatory cytokines by hepatic macrophages in acute classical swine fever. J Comp Pathol 2005;133:23–32. 53. Olsen LH, Kristensen AT, Qvortrup K, et al. Comparison of manual and automated methods for determining platelet counts in dogs with macrothrombocytopenia. J Vet Diagn Invest 2004;16:167–170. 54. Ouellette AL, Evans RJ, Heath MF. Platelets enhance endotoxin-induced monocyte tissue factor (TF) in the horse. Res Vet Sci 2004;76:31–35. 55. Roby KW, Bloom JC, Becht JL. Postpartum hemolytic-uremic syndrome in a cow. J Am Vet Med Assoc 1987;190:187–190. 56. Russell KE, Grindem CB. Secondary thrombocytopenias. In: Feldman BF, Zinkl JG, Jain NC, eds. Schalm’s Veterinary Hematology, 5th ed. Baltimore: Lippincott Williams & Wilkins, 2000;487–495. 57. Russell KE, Perkins PC, Grindem CB, et al. Flow cytometric method for detecting thiazole orange-positive (reticulated) platelets in thrombocytopenic horses. Am J Vet Res 1997;58:1092–1096. 58. Russell KE, Perkins PC, Hoffman MR, et al. Platelets from thrombocytopenic ponies acutely infected with equine infectious anemia virus are activated in vivo and hypofunctional. Virology 1999;259:7–19. 59. Sanders WE, Reddick RL, Nichols TC, et al. Thrombotic thrombocytopenia induced in dogs and pigs. The role of plasma and platelet VWF in animal models of thrombotic thrombocytopenic purpura. Arterioscler Thromb Vasc Biol 1995;15:793–800. 60. Santoro SK, Garrett LD, Wilkerson M. Platelet concentrations and plateletassociated IgG in greyhounds. J Vet Intern Med 2007;21:107–112. 61. Schouten M, Wiersinga WJ, Levi M, et al. Inflammation, endothelium, and coagulation in sepsis. J Leuk Biol 2008;83:536–545. 62. Segura D, Monreal L, Argmengou L, et al. Mean platelet component as an indicator of platelet activation in foals and adult horses. J Vet Intern Med 2007;21:1076–1082. 63. Sellon DC, Grindem CB. Quantitative platelet abnormalities in horses. Comp Cont Educ Pract Vet 1994;16:1335–1345. 64. Sellon DC, Levine J, Millikin E, et al. Thrombocytopenia in horses: 35 cases (1989–1994). J Vet Intern Med 1996;10:127–132. 65. Stasi R, Evangelista ML, Stipa E, et al. Idiopathic thrombocytopenic purpura: current concepts in pathophysiology and management. Thromb Haemostasis 2008;99:4–13. 66. Stockham SL, Scott MA. Platelets. In: Fundamentals of Veterinary Clinical Pathology, 2nd ed. Ames: Blackwell, 2008;223–257. 67. Stokol T, Brooks M. Diagnosis of DIC in cats: is it time to go back to the basics? J Vet Intern Med 2007;20:1289–1290.

68. Stokol T, Erb HN. A comparison of platelet parameters in EDTAand citrate-anticoagulated blood in dogs. Vet Clin Pathol 2007;36:148– 154. 69. Sullivan PS, Manning KL, McDonald TP. Association of mean platelet volume and bone marrow megakaryocytopoiesis in thrombocytopenic dogs: 60 cases (1984–1993). J Am Vet Med Assoc 1995;206:332–334. 70. Toh CH, Hoots WK. The scoring system of the Scientific and Standardisation Committee on Disseminated Intravascular Coagulation of the International Society on Thrombosis and Haemostasis: a 5-year overview. J Thromb Haemost 2007;5:604–606. 71. Trepanier LA. Idiosyncratic toxicity associated with potentiated sulfonamides in the dog. J Vet Pharm Ther 2004;27:129–138. 72. Tsuchiya R, Kyotani K, Scott MA, et al. Role of platelet activating factor in development of thrombocytopenia and neutropenia in dogs with endotoxemia. Am J Vet Res 1999;60:216–221. 73. Walz PH, Bell TG, Steficek BA, et al. Experimental model of type II bovine diarrhea virus-induced thrombocytopenia in neonatal calves. J Vet Diagn Invest 1999;11:505–514. 74. Waner T, Leykin I, Shinitsky M, et al. Detection of platelet-bound antibodies in beagle dogs after artificial infection with Ehrlichia canis. Vet Immunol Immunopathol 2000;23:145–150. 75. Weiss DJ. Application of flow cytometric techniques to veterinary clinical hematology. Vet Clin Pathol 2002;31:72–82. 76. Weiss DJ. Hemophagocytic syndrome in dogs: 24 cases (1996–2005). J Am Vet Med Assoc 2007;230:697–701. 77. Weiss DJ, Rashid J. The sepsis-coagulant axis: a review. J Vet Intern Med 1998;12:317–324. 78. Wilkerson MJ, Shuman W, Swist S, et al. Platelet size, platelet surfaceassociated IgG, and reticulated platelets in dogs with immune-mediated thrombocytopenia. Vet Clin Pathol 2001;30:141–149. 79. Wills TB, Wardrop KJ. Pseudothrombocytopenia secondary to effects of EDTA in a dog. J Am Anim Hosp Assoc 2008;44:95–97. 80. Zheng XL, Sadler JE. Pathogenesis of thrombotic microangiopathies. Annu Rev Pathol Mech Dis 2008;3:249–277. 81. Zhou B, Zhao H, Yang RC, et al. Multi-dysfunctional pathophysiology in ITP. Crit Rev Oncol Hematol 2005;54:107–116. 82. Zimmerman KL. Drug-induced thrombocytopenias. In: Feldman BF, Zinkl JG, Jain NC, eds. Schalm’s Veterinary Hematology, 5th ed. Baltimore: Lippincott Williams & Wilkins, 2000;472–477. 83. Zini E, Hauser B, Meli ML, et al. Immune-mediated erythroid and megakaryocytic aplasia in a cat. J Am Vet Med Assoc 2007;230:1024–1027.

CHAPTER

80

Essential Thrombocythemia and Reactive Thrombocytosis TRACY STOKOL Thrombopoiesis: A Brief Review Causes of Thrombocytosis Pseudothrombocytosis Physiologic Thrombocytosis Drug-induced Thrombocytosis Reactive Thrombocytosis

Essential Thrombocythemia in Humans Differentiating Essential Thrombocythemia from Reactive Thrombocytosis Differentiating Essential Thrombocythemia from other Chronic Myeloproliferative Disorders Essential Thrombocythemia in Animals Familial/Inherited Thrombocytosis

Acronyms and Abbreviations CIMF, Chronic idiopathic myelofibrosis; CML, chronic myeloid leukemia; CMPD, chronic myeloproliferative disease; ECMP, European clinical, molecular and pathological criteria; ET, essential thrombocythemia; GM-CSF, Granulocyte monocyte colony stimulating factor; IL, interleukin; JAK2, Janus kinase 2; MCV, mean cell volume; PV, Polycythemia vera; PVSG, Polycythemia Vera Study Group; SD, standard deviation; SDF-1, stromal cellderived factor-1; STAT, signal transducer and activator of transcription; TPO, thrombopoietin; WHO, World Health Organization.

T

hrombocytosis, or thrombocythemia, is defined as a high platelet count or, more specifically, a count that is above a reference interval established for the species. Various methods are available for measuring platelet counts, including estimates from stained peripheral blood smears, manual hemocytometer counts, quantitative buffy coat analysis, and automated methods based on impedance or laser-induced light scatter. These vary in precision and accuracy and results from the same animal may differ between methods.31,59 Therefore, most veterinary laboratories establish reference intervals specific for their own equipment for routinely analyzed species. For those animals in which laboratory-specific intervals are unavailable, interpretation of the platelet count is more difficult. Knowledge of the normal physiology of the animal, combined with published reference intervals, can be used as a guide to data interpretation. Platelet counts in healthy animals vary markedly between species. For instance, small rodents normally have very high counts, whereas horses have relatively low counts (Table 80.1). Thus, a normal platelet count in a mouse would be regarded as an extreme thrombocytosis in a dog or horse.

A potentially misleading consequence of thrombocytosis is a pseudohyperkalemia that occurs in serum samples. This is due to release of intracellular potassium during clotting and has been most thoroughly evaluated in the dog.13,49 Potassium concentrations can be 0.63 ± 0.17 mEq/L (mean ± SD) higher in serum than plasma in dogs with platelet counts between 150,000– 600,000/μL.49 This difference increases in dogs with higher counts (1.55 ± 0.73 mEq/L) and can be quite marked (up to 5 mEq/L) in dogs with extreme thrombocytosis (>1,000,000 platelets/μL).3,13,25,39,49 To avoid this artifact, heparinized plasma is the preferred sample for potassium measurement. Thrombocytosis can be due to an artifact (pseudothrombocytosis), altered trafficking of platelets (i.e. release from storage pools; also called physiologic) and increased platelet production (enhanced thrombopoiesis). Enhanced thrombopoiesis can be secondary to drugs, cytokines (also called reactive), clonal hematopoietic disorders, or familial/inherited megakaryocyte disorders (Table 80.2). To understand the pathophysiology of thrombocytosis, the factors governing thrombopoiesis are briefly reviewed below (see Chapter 9 for more details). 605

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THROMBOPOIESIS: A BRIEF REVIEW Platelets are produced from megakaryocytes in the bone marrow, by a unique process that involves fragmentation of long cytoplasmic extensions (proplatelets) that the megakaryocyte inserts between bone marrow sinusoidal endothelial cells. The precise mechanisms controlling platelet release from proplatelets are largely unknown, however this process does require adhesion of megakaryocytes to subendothelial matrix compo-

TABLE 80.1 Reference Intervals for Platelet Counts from Various Animal Species Reference Interval (range or mean ± SD) (×103/μL)

Species Canine Feline Equine Bovine Alpaca Caprine Ovine Porcine Mice Long Evans Rats (13–15 months)

186–545a 195–624a 94–246a 252–724a 220–947a 245–975b 250–750c 520 ± 195c 1163 ± 382c,d 993 ± 183c,d

a Intervals established for the 2120 ADVIA hematology analyzer at Cornell University (2008). b Intervals established for the 120 ADVIA hematology analyzer at Cornell University (2000). c Ref. 28. d Sex– and strain–dependent differences apply.

nents (particularly fibronectin and thrombospondin), cytoskeletal elements (actin, microtubules), pro-apoptotic enzymes,12 matrix metalloproteinases33 and shear forces (which facilitate proplatelet fragmentation). To date, the only known soluble mediator of platelet release is the chemokine, stromal cell-derived factor-1 (SDF1);33 none of the known thrombopoietic cytokines, including thrombopoietin (TPO), appear to stimulate this process.35 It has been estimated that approximately 2,000–3,000 platelets are produced from each megakaryocyte, with the residual megakaryocyte nucleus being degraded by phagocytes within the bone marrow.35 Thrombopoietin is the main thrombopoietic cytokine and is produced primarily in the liver and kidney. TPO binds to its receptor, MPL (a product of the proto-oncogene, c-mpl) to mediate downstream responses through the non-receptor tyrosine kinase, Janus kinase 2 (JAK2), which in turn activates the transcription factors signal transducer and activator of transcription (STAT 3 and 5).29,35 MPL is highly expressed on megakaryocytes and platelets, but is also expressed on other cell types, including hematopoietic stem cells. TPO levels in serum are thought to be regulated by binding to MPL, with subsequent internalization and degradation. TPO mainly acts as a differentiation factor, stimulating stem cells to differentiate along the megakaryocytic lineage and preventing apoptosis through the protein Bcl-XL.32 It is also required for complete megakaryocyte maturation. TPO acts in concert (additively or synergistically) with other hematopoietic cytokines, including stem cell factor, interleukin-3 (IL-3) and erythropoietin, to promote megakaryocyte proliferation and polyploidy.

TABLE 80.2 Causes and Associated Mechanisms of Thrombocytosis23,51,54,56 Cause

Mechanism

Pseudothrombocytosis Red cell ghosts or fragments, fragile leukocytes

Non-platelet cell fragments are counted as platelets

Physiologic Epinephrine (trauma, exercise, excitement) Post-splenectomy

Splenic contraction Lack of sequestration

Drug-induced Epinephrine Vincristine

Splenic contraction Enhanced megakaryopoiesis

Reactive Inflammation, infection, neoplasia (hematopoietic and non-hematopoietic), trauma, rebound from thrombocytopenia Iron deficiency-related Neoplasia involving megakaryocytes Chronic myeloproliferative disease: essential thrombocythemia, chronic myeloid leukemia, chronic basophilic leukemia, polycythemia vera, chronic idiopathic myelofibrosis Acute megakaryocytic leukemia Hereditary/familial a

a

Not reported in animals.

Cytokine-mediated (directly or indirectly through TPO production) enhanced megakaryopoiesis Unknown Acquired or inherited genetic mutations causing TPO-independent proliferation, e.g. JAK2 signaling mutations

Acquired or inherited genetic mutations Inherited genetic mutations

CHAPTER 80: ESSENTIAL THROMBOCYTHEMIA AND REACTIVE THROMBOCYTOSIS

607

Other thrombopoietic cytokines are SDF-1,38 IL-6 superfamily members (IL-6, IL-11, leukemia inhibitory factor), GM-CSF, IL-1α and IL-9.29,35 Thus, potential mechanisms that would result in thrombocytosis due to megakaryopoiesis are increased concentrations of thrombopoietic cytokines, ligandindependent MPL activation, and abnormal activation of the JAK2/STAT signaling pathway.

it can potentially occur with any counting method. Thus, it is important to always verify platelet counts by estimating numbers from stained blood smears.21,58 Although estimates are of questionable accuracy, smear examination can quickly confirm an elevated platelet count, identify morphologic defects in platelets and potentially reveal abnormalities that may cause a pseudothrombocytosis.

CAUSES OF THROMBOCYTOSIS

Physiologic Thrombocytosis

Pseudothrombocytosis

Up to one-third of the platelet mass is normally sequestered in the splenic red pulp. These stores can be released upon splenic contraction, which is mediated by epinephrine.34,65 A transient or persistent thrombocytosis may occur post-splenectomy in humans,53 however this is not a consistent finding in dogs.50 Although some authors have suggested the presence of an adrenaline-responsive, platelet pool in the lungs,20 there is no definitive evidence for this.34

Pseudothrombocytosis can occur when small, fragmented or hemolyzed erythrocytes, leukocyte fragments (from normal or leukemic cells), or particulate cellular debris are erroneously counted as platelets (Fig. 80.1). Although this artifact can be analyzer-dependent,

A

Drug-induced Thrombocytosis Two retrospective studies have made associations between various drugs and thrombocytosis in dogs and cats, however a direct cause and effect has not been established.23,56 It is possible that the thrombocytosis documented in animals on drug therapy is secondary to their underlying disease or a rebound from thrombocytopenia, rather than a drug response per se. Drugs documented to cause a thrombocytosis in animals are vincristine and epinephrine.34,37 Vincristine likely functions through enhanced megakaryopoiesis, whereas epinephrine induces a rapid (within a few minutes), transient (less than 60 minutes) thrombocytosis in dogs through splenic contraction, although there is large inter-animal variability in the response.20,34 Data for glucocorticoids are conflicting. Although hyperadrenocorticism is associated with thrombocytosis in dogs,19 two studies have not shown a consistent increase in platelet counts in healthy dogs given daily doses of up to 2 mg/ kg prednisolone.37,46 Reactive Thrombocytosis B

FIGURE 80.1 Pseudothrombocytosis in a hemolyzed sample from a dog with immune-mediated hemolytic anemia. (A) Hemolyzed red cells (arrow) can be seen entering the upper left portion of the platelet cytogram (forward scatter versus side scatter dotplot) and are counted as platelets on the ADVIA 120 hematology analyzer (laser light scatter-based system). (B) The hemolyzed erythrocytes can also be visualized as a second population of macrocytic cells (arrow) in the platelet histogram (cell number versus cell volume) and will artifactually increase the mean platelet volume and platelet distribution width. The “true” platelet clusters are identified by the arrowhead in both cytograms.

Reactive thrombocytosis (or secondary thrombocytosis) is due to cytokine stimulation of thrombopoiesis. It is the most common cause of thrombocytosis in humans and animals and occurs secondary to various disorders (Table 80.2).6,22,23,51,54,56 Several retrospective studies in animals have shown that inflammation and neoplasia are the most common diseases associated with a thrombocytosis, although different platelet counts were used to define a thrombocytosis in these studies.23,51,54,56 A recent unpublished study at Cornell University further showed that 61% of 42 cats with a thrombocytosis (>750,000 platelets/μL) had a primary inflammatory or infectious disorder. Reactive thrombocytosis can be transient or persistent, depending on the underlying cause, so documenting a thrombocytosis more than

608

SECTION VI: PLATELETS

FIGURE 80.2 Peripheral blood smear from a 12 week old kitten with iron deficiency anemia secondary to flea infestation. Many platelets, which vary markedly in size, are seen in the smear. The kitten had an extreme thrombocytosis (platelet count = 1.9 × 106/μL), which was presumably reactive. Wright’s stain; 1000× magnification.

once (particularly if done less than 1–2 months apart) does not exclude a reactive condition.52 Also, the increase in platelet count can be quite marked in reactive conditions (>1,000,000/μL) (Fig. 80.2). There are a plethora of cytokines potentially responsible for a reactive thrombocytosis, including TPO, IL-6, GM-CSF, IL-3, IL-11 and erythropoietin. These can be constitutively secreted by tumors26,43 or secretion can be induced from host-derived cells as a result of inflammation, infection or neoplasia.5 The inflammatory cytokine IL-6 is considered one of the main mediators of a reactive thrombocytosis. IL-6 levels are increased in patients with thrombocytosis due to inflammation and neoplasia5,8,61 and persistently high levels are supportive of a reactive thrombocytosis.1,61 Rather than acting directly on megakaryocytes, IL-6 is thought to stimulate platelet production indirectly through induction of TPO production from hepatocytes.30 Unfortunately, in some disorders typically associated with thrombocytosis (e.g. iron deficiency), the precise mechanism for the thrombocytosis is unknown.11 The main differential diagnosis for reactive thrombocytosis in animals is essential thrombocythemia (ET). However, there are only a handful of reported cases of ET in dogs and cats, indicating that thrombocytosis in most animals (like humans) is likely reactive. Essential Thrombocythemia in Humans Essential thrombocythemia (also known as primary thrombophilia, hemorrhagic thrombocythemia/throm-

bocytosis, primary thrombocythemia and idiopathic thrombocythemia), is a clonal disorder of hematopoiesis, that primarily affects megakaryocytes. It is classified as a chronic myeloproliferative disease (CMPD), along with chronic myeloid leukemia (CML), polycythemia vera (PV), and chronic idiopathic myelofibrosis (CIMF), by the World Health Organization (WHO).27 ET is the most frequently diagnosed CMPD22,27,52 and usually affects the elderly (>50 years), with no sex predilection. There is a second peak of incidence in females around 30 years old.27,52 Most patients (up to two-thirds) are asymptomatic at diagnosis, but affected individuals can paradoxically suffer from microvascular thrombosis (which is more common) or hemorrhage. Thrombi predominantly occur in arteries supplying the brain, heart, and extremities.27,52 The duration, but not degree, of thrombocytosis has been linked to a higher risk of thrombosis. The mechanism for thrombosis is largely unknown, but may involve interactions between activated platelets and leukocytes.2 In contrast to thrombosis, hemorrhage is associated with an extreme thrombocytosis (>1,000,000 platelets/μL), although hemorrhage can occur in patients with lower counts.52,63 Hemorrhage is typically from the skin or mucosa and has been attributed to acquired type II von Willebrand disease (VWD), with a selective loss of the high molecular weight multimers of von Willebrand factor (due to their preferential adsorption to platelets or enhanced proteolysis).41,52 Essential thrombocythemia is characterized by a marked thrombocytosis in peripheral blood and a megakaryocytic hyperplasia in the bone marrow.27 Leukocyte and erythrocyte counts are generally within normal limits. Platelets can vary markedly in size, but are usually not dysplastic. A histologic hallmark of ET is the presence of giant hyperlobulated megakaryocytes, found diffusely or in small loose clusters, within a normo- to slightly hypercellular marrow. Other evidence of dysplasia is lacking and there is no to minimal reticulin fibrosis.27,63 The main differential diagnoses for ET are reactive thrombocytosis (which is far more common) and other CMPDs (see below). The discovery of a mutation in the JAK2 gene has advanced our understanding of the pathogenesis of ET and related CMPDs (PV and CIMF). JAK2 is a nonreceptor tyrosine kinase that has a critical role in megakaryocyte and other hematopoietic cell signaling. JAK2 upregulates anti-apoptotic proteins, is required for efficient trafficking of MPL, and induces hematopoietic cytokine (e.g. IL-3, TPO) production.15,52 The mutation causes a phenylalanine for valine substitution at residue 617 (V617F) of the autoinhibitory domain and results in constitutive activation of JAK2.52,63,64 The V617F mutation is not specific for ET and also occurs in PV (up to 90% incidence) and CIMF, perhaps explaining the phenotypic and biologic similarities between these CMPDs.42,63,64 Constitutive JAK2 activation may explain the demonstrated hypersensitivity of megakaryocyte colonies derived from ET patients to thrombopoietic cytokines and to their autonomous growth in vitro.15,63,64 Testing for the JAK2 V617F mutation has

CHAPTER 80: ESSENTIAL THROMBOCYTHEMIA AND REACTIVE THROMBOCYTOSIS

609

TABLE 80.3 Diagnostic Criteria for Essential Thrombocythemia in Humans PVSGa Positive criteria

Platelet count ≥600,000/μL

WHOb Sustained platelet count ≥600,000/μLd Megakaryocytic hyperplasia (enlarged, mature)

Exclusions Reactive thrombocytosis Iron deficiency

Polycythemia vera Chronic myeloid leukemia Chronic idiopathic myelofibrosis

No underlying disease Stainable iron in marrow, failure of 1 month iron trial (to raise red cell mass) Normal red cell mass or hemoglobin No Philadelphia chromosome Collagen fibrosis absent or 600,000 platelets/μL for >1 month), megakaryocytic hyperplasia, normal TPO levels, and no evidence of dysplasia, iron deficiency, or underlying inflammation. It is possible that ET is a different disorder in animals than in humans and diagnostic criteria for humans (including genetic mutations) may not be valid in animals. The main differential diagnosis for ET in animals is a reactive or cytokine-driven thrombocytosis, which is

Familial/Inherited Thrombocytosis This has been attributed to germline mutations in genes involved in thrombopoiesis (MPL, TPO, JAK2), resulting in non-neoplastic polyclonal thrombopoiesis. High platelet counts are observed at birth or an early age. Affected individuals may be asymptomatic or suffer from thrombohemorrhagic complications.4,52,53

REFERENCES 1. Alexandrakis MG, Passam FH, Moschandrea IA, et al. Levels of serum cytokines and acute phase proteins in patients with essential and cancerrelated thrombocytosis. Am J Clin Oncol 2003;26:135–140. 2. Arellano-Rodrigo E, Alvarez-Larran A, Reverter JC, et al. Increased platelet and leukocyte activation as contributing mechanisms for thrombosis in essential thrombocythemia and correlation with the JAK2 mutational status. Haematologica 2006;91:169–175. 3. Bass MC, Schultze AE. Essential thrombocythemia in a dog: case report and literature review. J Am Anim Hosp Assoc 1998;34:197–203.

CHAPTER 80: ESSENTIAL THROMBOCYTHEMIA AND REACTIVE THROMBOCYTOSIS 4. Bellanne-Chantelot C, Chaumarel I, Labopin M, et al. Genetic and clinical implications of the Val617Phe JAK2 mutation in 72 families with myeloproliferative disorders. Blood 2006;108:346–352. 5. Blay JY, Favrot M, Rossi JF, et al. Role of interleukin-6 in paraneoplastic thrombocytosis. Blood 1993;82:2261–2262. 6. Buss DH, Cashell AW, O’Connor ML, et al. Occurrence, etiology, and clinical significance of extreme thrombocytosis: a study of 280 cases. Am J Med 1994;96:247–253. 7. Canfield PJ, Church DB, Russ IG. Myeloproliferative disorder involving the megakaryocytic line. J Small Anim Pract 1993;34:296–301. 8. Ceresa IF, Noris P, Ambaglio C, et al. Thrombopoietin is not uniquely responsible for thrombocytosis in inflammatory disorders. Platelets 2007;18:579–582. 9. Ceron JJ, Eckersall PD, Martynez-Subiela S. Acute phase proteins in dogs and cats: current knowledge and future perspectives. Vet Clin Pathol 2005;34:85–99. 10. Colbatzky F, Hermanns W. Acute megakaryoblastic leukemia in one cat and two dogs. Vet Pathol 1993;30:186–194. 11. Dan K. Thrombocytosis in iron deficiency anemia. Intern Med 2005;44:1025–1026. 12. De Botton S, Sabri S, Daugas E, et al. Platelet formation is the consequence of caspase activation within megakaryocytes. Blood 2002;100:1310–1317. 13. Degen MA. Correlation of spurious potassium elevation and platelet count in dogs. Vet Clin Pathol 1986;15:20–22. 14. Degen MA, Feldman BF, Turrel JM, et al. Thrombocytosis associated with a myeloproliferative disorder in a dog. J Am Vet Med Assoc 1989;194:1457–1459. 15. Delhommeau F, Pisani DF, James C, et al. Oncogenic mechanisms in myeloproliferative disorders. Cell Mol Life Sci 2006;63:2939–2953. 16. Dunn JK, Heath MF, Jefferies AR, et al. Diagnostic and hematologic features of probable essential thrombocythemia in two dogs. Vet Clin Pathol 1999;28:131–138. 17. Evans RJ, Jones DRE, Gruffydd-Jones TJ. Essential thrombocythaemia in the dog and cat: a report of four cases. J Small Anim Pract 1982;23:457–467. 18. Favier RP, van Leeuwen M, Teske E. Essential thrombocythaemia in two dogs. Tijdschr Diergeneeskd 2004;129:360–364. 19. Feldman EC, Nelson RW. Canine hyperadrenocorticism (Cushing’s disease). In: Canine and Feline Endocrinology and Reproduction, 3rd ed. St Louis: Saunders, 2004;252–357. 20. Freedman M, Altszuler N, Karpatkin S. Presence of a nonsplenic platelet pool. Blood 1977;50:419–425. 21. George JW. Ocular field width and platelet estimates [letter]. Vet Clin Pathol 1999;28:126. 22. Griesshammer M, Bangerter M, Sauer T, et al. Aetiology and clinical significance of thrombocytosis: analysis of 732 patients with an elevated platelet count. J Intern Med 1999;245:295–300. 23. Hammer AS. Thrombocytosis in dogs and cats: A retrospective study. Comp Haematol Intl 1991;1:181–186. 24. Hammer AS, Couto CG, Getzy D, Bailey MQ. Essential thrombocythemia in a cat. J Vet Intern Med 1990;4:87–91. 25. Hopper PE, Mandell CP, Turrel JM, et al. Probable essential thrombocythemia in a dog. J Vet Intern Med 1989;3:79–85. 26. Hwang SJ, Luo JC, Li CP, et al. Thrombocytosis: a paraneoplastic syndrome in patients with hepatocellular carcinoma. World J Gastroenterol 2004;10:2472–2477. 27. Imbert M, Vardiman JW, Pierre R, et al. Essential thrombocythemia. In: Jaffe ES, Harris NL, Stein H, Vardiman JW, eds. World Health Organization Classification of Tumours: Pathology and Genetics, Tumours of Haematopoietic and Lymphoid Tissues. Lyon: IARC, 2001;39–41. 28. Jain NC (ed.). Schalm’s Veterinary Hematology, 3rd ed. Philadelphia: Lea & Febiger, 1975. 29. Jenkins BJ, Roberts AW, Greenhill CJ, et al. Pathologic consequences of STAT3 hyperactivation by IL-6 and IL-11 during hematopoiesis and lymphopoiesis. Blood 2007;109:2380–2388. 30. Kaser A, Brandacher G, Steurer W, et al. Interleukin-6 stimulates thrombopoiesis through thrombopoietin: role in inflammatory thrombocytosis. Blood 2001;98:2720–2725. 31. Koplitz SLS, Scott MA, Cohn LA. Effects of platelet clumping on platelet concentrations measured by use of impedance or buffy coat analysis in dogs. J Am Vet Med Assoc 2001;219:1552–1556. 32. Kozuma Y, Kojima H, Yuki S, et al. Continuous expression of Bcl-xL protein during megakaryopoiesis is post-translationally regulated by thrombopoietin-mediated Akt activation, which prevents the cleavage of Bcl-xL. J Thromb Haemost 2007;5:1274–1282. 33. Lane WJ, Dias S, Hattori K, et al. Stromal-derived factor 1-induced megakaryocyte migration and platelet production is dependent on matrix metalloproteinases. Blood 2000;96:4152–4159. 34. Ljungqvist U. Platelet response to adrenalin infusion in splenectomised and non-splenectomised dogs. Acta Chir Scand 1971;137:291–297. 35. Long MW, Hoffman R. Thrombocytopoiesis. In: Hoffman R, Benz EJ, Shattil S, et al., eds. Hematology: Basic Principles and Practice, 4th ed. Philadelphia: Elsevier, 2005;303–320.

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36. MacEwen EG, Drazner FH, McClelland AJ, et al. Treatment of basophilic leukemia in a dog. J Am Vet Med Assoc 1975;166:376–380. 37. Mackin AJ, Allen DG, Johnston IB. Effects of vincristine and prednisone on platelet numbers and function in clinically normal dogs. Am J Vet Res 1995;56:100–108. 38. Majka M, Janowska-Wieczorek A, Ratajczak J, et al. Stromal-derived factor 1 and thrombopoietin regulate distinct aspects of human megakaryopoiesis. Blood 2000;96:4142–4151. 39. Mandell CP, Goding B, Degen MA, et al. Spurious elevation of serum potassium in two cases of thrombocythemia. Vet Clin Pathol 1988;17:32–33. 40. Mesa RA. New insights into the pathogenesis and treatment of chronic myeloproliferative disorders. Curr Opin Hematol 2008;15:121–126. 41. Michiels JJ, Berneman Z, Schroyens W, et al. The paradox of platelet activation and impaired function: platelet-von Willebrand factor interactions, and the etiology of thrombotic and hemorrhagic manifestations in essential thrombocythemia and polycythemia vera. Semin Thromb Hemostasis 2006;32:589–604. 42. Michiels JJ, De Raeve H, Hebeda K, et al. WHO bone marrow features and European clinical, molecular, and pathological (ECMP) criteria for the diagnosis of myeloproliferative disorders. Leuk Res 2007;31:1031–1038. 43. Minetto M, Dovio A, Ventura M, et al. Interleukin-6 producing pheochromocytoma presenting with acute inflammatory syndrome. J Endocrinol Invest 2003;26:453–457. 44. Miyamoto T, Hachimura H, Amimoto A. A case of megakaryoblastic leukemia in a dog. J Vet Med Sci 1996;58:177–179. 45. Mizukoshi T, Fujino Y, Yasukawa K, et al. Essential thrombocythemia in a dog. J Vet Med Sci 2006;68:1203–1206. 46. Moore GE, Mahaffey EA, Hoenig M. Hematologic and serum biochemical effects of long-term administration of anti-inflammatory doses of prednisone in dogs. Am J Vet Res 1992;53:1033–1037. 47. Murphy S, Iland H, Rosenthal D, Laszlo J. Essential thrombocythemia: an interim report from the Polycythemia Vera Study Group. Semin Hematol 1986;23:177–182. 48. Pardanani AD, Levine RL, Lasho T, et al. MPL515 mutations in myeloproliferative and other myeloid disorders: a study of 1182 patients. Blood 2006;108:3472–3476. 49. Reimann KA, Knowlen GG, Tvedten HW. Factitious hyperkalemia in dogs with thrombocytosis. The effect of platelets on serum potassium concentration. J Vet Intern Med 1989;3:47–52. 50. Richardson EF, Brown NO. Hematological and biochemical changes and results of aerobic bacteriological culturing in dogs undergoing splenectomy. J Am Anim Hosp Assoc 1996;32:199–210. 51. Rizzo F, Tappin SW, Tasker S. Thrombocytosis in cats: a retrospective study of 51 cases (2000–2005). J Feline Med Surg 2007;9:319–325. 52. Sanchez S, Ewton A. Essential thrombocythemia: a review of diagnostic and pathologic features. Arch Pathol Lab Med 2006;130:1144–1150. 53. Schafer AI. Essential thrombocythemia and thrombocytosis. In: Lichtman MA, Beutler E, Kipps TJ, et al., eds. Williams Hematology, 7th ed. New York: McGraw-Hill, 1785–1794. 54. Sellon DC, Levine JF, Palmer K, et al. 1997; Thrombocytosis in 24 horses (1989–1994). J Vet Intern Med 2006;11:24–29. 55. Simpson JW, Else RW, Honeyman P. Successful treatment of suspected essential thrombocythaemia in a dog. J Small Anim Pract 1990;31:345–348. 56. Snyder LA, Neel JA, Grindem CB. Thrombocytosis in dogs: A retrospective study [abstract]. Vet Pathol 2007;44:736. 57. Spivak JL, Silver RT. The revised World Health Organization diagnostic criteria for polycythemia vera, essential thrombocytosis, and primary myelofibrosis: an alternative proposal. Blood 2008;112:231–239. 58. Tasker S, Cripps PJ, Mackin AJ. Estimation of platelet counts on feline blood smears. Vet Clin Pathol 1999;28:42–45. 59. Tasker S, Cripps PJ, Mackin AJ. Evaluation of methods of platelet counting in the cat. J Small Anim Pract 2001;42:326–332. 60. Tefferi A. Essential thrombocythemia: scientific advances and current practice. Curr Opin Hematol 2006;13:93–98. 61. Tefferi A, Ho TC, Ahmann GJ, et al. Plasma interleukin-6 and C-reactive protein levels in reactive versus clonal thrombocytosis. Am J Med 1994;97:374–378. 62. Tefferi A, Thiele J, Orazi A, et al. Proposals and rationale for revision of the World Health Organization diagnostic criteria for polycythemia vera, essential thrombocythemia, and primary myelofibrosis: recommendations from an ad hoc international expert panel. Blood 2007;110:1092–1097. 63. Thiele J, Kvasnicka HM. Clinicopathological criteria for differential diagnosis of thrombocythemias in various myeloproliferative disorders. Semin Thromb Hemostasis 2006;32:219–230. 64. Villeval JL, James C, Pisani DF, et al. New insights into the pathogenesis of JAK2 V617F-positive myeloproliferative disorders and consequences for the management of patients. Semin Thromb Hemostasis 2006;32:341– 351. 65. Wardyn GG, Rennard SI, Brusnahan SK, et al. Effects of exercise on hematological parameters, circulating side population cells, and cytokines. Exp Hematol 2008;36:216–223.

C H A P T E R 81

Von Willebrand Disease MARJORY B. BROOKS and JAMES L. CATALFAMO Disease Mechanism Disease Classification VWD Subtypes Affected species and breeds Inheritance and Expression Patterns Hereditary VWD Acquired VWD von Willebrand Factor Assays Quantitative VWF Assays

Functional and structural VWF Assays Point of Care Analyses Clinical Management of VWD Clinical Diagnosis Clinical Signs Diagnostic Evaluation Treatment of VWD Transfusion Therapy Non-transfusion Therapy

Acronyms and Abbreviations AVWS, acquired von Willebrand syndrome; ADAMTS13, a disinintegrin and metalloprotease with thrombospondin repeats; DDAVP, deamino 8-D-arginine vasopressin; ELISA, enzyme-linked immunosorbent assay; FVIII, coagulation factor VIII; LIA, latex immunoassay; OMIM, on-line inheritance in man; PFA, platelet function analyzer; MW, molecular weight; VWD, von Willebrand disease; VWF, von Willebrand factor; VWF : Ag, von Willebrand factor antigen; VWF : CB, von Willebrand factor collagen binding; VWF : RCo, von Willebrand factor ristocetin cofactor activity.

V

on Willebrand disease (VWD) is the most common hereditary bleeding disorder in dogs10 and people,21 and the trait has been reported in many other species. The disease is heterogeneous, caused by a variety of defects in von Willebrand factor protein (VWF) that influence clinical severity and complicate disease diagnosis and management.6,9,10,19–21

DISEASE MECHANISM The bleeding tendency of VWD is caused by quantitative and functional deficiencies of VWF, a large plasma glycoprotein required for platelet adhesion at sites of vessel injury (Fig. 81.1).21 Endothelial cells are the major site of VWF synthesis and storage. Platelets provide a secondary pool of VWF in some species; however, canine platelets contain only trace amounts of VWF. Mature VWF circulates as linear strings of subunits (multimers) assembled within the endoplasmic reticulum and Golgi apparatus. Initially, two VWF subunits join to form dimers, followed by the association of dimers into variably-sized multimers. 612

Multimers may be composed of more than 100 subunits, ranging in molecular weight (MW) from 500 to 20,000 kDa. Endothelial cells secrete VWF via steadystate constitutive pathways, and release the protein from storage organelles in response to stimuli such as thrombin and epinephrine. VWF acts as a carrier for coagulation factor VIII (FVIII), circulating with FVIII in a noncovalent complex. Upon vascular injury, VWF binds to subendothelial collagen and undergoes a conformation change that facilitates its interaction with platelet glycoprotein lb. VWF also mediates intraplatelet bridging via platelet glycoprotein IIbIIIa. High MW VWF multimers are most effective in supporting platelet adhesion. Deficiency of VWF, or loss of high MW forms, results in failure of platelet plug assembly, especially under high shear in the microvasculature. After secretion, ultra-high MW multimers are catabolized to smaller forms by a disintegrin and metalloprotease with thrombospondin repeats (ADAMTS13) (Fig. 81.1).21 This conversion takes place slowly in plasma under static conditions (12 hour half-life); however, ADAMTS13 more rapidly cleaves platelet-bound VWF under conditions of high shear.5,21

CHAPTER 81: VON WILLEBRAND DISEASE

613

Blood Flow Tethering

Activation

Tight anchoring and thrombus growth

exposed subendothelium VWF ADAMTS13

Fibrinogen Collagen

GPIbα GPVI

α IIb β3 Platelet

FIGURE 81.1 Schematic diagram illustrating the role of VWF in platelet adhesion, aggregation, and thrombus growth. At the site of blood

vessel injury, platelets transiently adhere to subendothelial VWF via platelet GP1bα receptors. This slows platelet movement and triggers the engagement of platelet receptors, including the collagen receptor. Receptor engagement results in platelet activation and stable attachment. Activated platelets secrete additional prostimulatory molecules and bind to plasma fibrinogen and VWF to form a tight anchoring matrix for platelet recruitment and ultimately thrombus formation. Upon release from endothelial cells, ultra large VWF multimers are rapidly cleaved by ADAMTS13.

TABLE 81.1 Classification of von Willebrand Disease Type

VWF Defect

Affected Species (Breeds)

1

Partial quantitative deficiency, residual VWF has normal structure and function

Dog (Airedale, Akita, Bernese mountain dog, Dachshund, Doberman pinscher, German shepherd, Golden retriever, Greyhound, Irish wolfhound, Kerry blue terrier, Manchester terrier, Miniature pinscher, Papillon, Pembroke Welsh Corgi, Poodles, Schnauzer, other purebreeds and mixed breed dogs) Horse (Arabian) Mouse (RIIIS/J)

2A

Selective loss of large VWF multimers, decreased VWF-platelet & collagen interactions

Dog (German shorthaired pointer, German wirehaired pointer) Cow (Simmental) Horse (Quarter Horse, Thoroughbred)

2B

Increased VWF affinity for platelet glycoprotein Ib

No animal cases

2M

Impaired VWF binding to platelet glycoprotein Ib, normal multimer structure

No animal cases

2N

Decreased VWF binding to factor VIII

No animal cases

3

Complete VWF deficiency

Dog (Dutch Kooiker, Scottish terrier, Shetland sheepdog; sporadic cases Border collie, Chesapeake Bay retriever, Cocker spaniel, Eskimo dog, Labrador retriever, Maltese, Pitbull, and mixed breed) Cat (Himalayan) Pig (Poland, China) Primate (Rhesus monkey)

DISEASE CLASSIFICATION VWD Subtypes Von Willebrand disease in people is classified into one of three categories based on plasma VWF concentration, function, and multimer structure (see Table 81.1).6,21 This classification scheme is broadly applicable for VWD in animals. Type 1 VWD is a partial quantitative deficiency. Plasma VWF concentration is low (A, D>N

αIIb αIIb αIIb αIIb

αIIb

Exon 13, 14-base duplication Exon 12, G>C, D>H Exon 2, G>C, R>P Exon/Intron 11, 10-base deletion; Exon 2, G>C, R>P Exon/Intron 11, 10-base deletion Exon 2, G>C, R>P

AP3β1

Exon 20, 1-base duplication

P2Y12

Exon 2, 3-base deletion

LYST/CHS1

6065A>G, H>R

αIIb

Not known CalDAG-GEFI CalDAG-GEFI CalDAG-GEFI CalDAG-GEFI CalDAG-GEFI Kindlin-3

Exon Exon Exon Exon Exon Exon

5, 3-base deletion 5, 1-base duplication 8, C>T, R>STOP 7, T>C, L>P 6, C>G, H>Q 12, 12-base insertion

Not known

a

CalDAG-GEFI, calcium diacylglycerol guanine nucleotide exchange factor I. D, Aspartic acid; H, histidine; R, arginine; L, leucine; P, proline; N, asparagine; Q, glutamine. Compound heterozygote.

b c

evaluated. The prolonged PFA-100 closure times reported in some CKCS are likely secondary to acquired VWD in dogs with concomitant mitral valve regurgitation and are not related to the inherited macrothrombocytopenia.51 The high shear conditions generated with mitral valve regurgitation and other types of heart valve disorders can result in enhanced cleavage of von Willebrand Factor (VWF) by ADAMTS13 (a disintegrin and metalloprotease with thrombospondin repeats), an enzyme that cleaves VWF under high shear conditions.52 Evaluation of plasma VWF antigen concentration and VWF collagen binding activity should be included in the diagnostic profile. The large platelets in affected CKCS can be equal in size or larger than erythrocytes and tend to be spherical to oval in shape with prominent alpha granule staining. A mis-sense mutation at coding nucleotide 745 (c.745G>A) in the gene encoding β1-tubulin correlates with the inherited macrothrombocytopenia in CKCS.22 The mutation is predicted to result in the substitution of an asparagine for aspartic acid at amino acid position 249 (D249N) (Tables 82.1 and 82.2). Aspartic acid 249 is part of a microtubule intraprotofilament binding site and the change in charge at this position is thought to impair microtubule assembly necessary for normal proplatelet formation and platelet production by megakaryocytes. The prevalence of the mutation in United States CKCS is high; in the study by Davis et al., 90% of the CKCS were found to be either homozygous or heterozygous for the muta-

tion. In the same study 65% of CKCS in Ireland were either heterozygous or homozygous for the mutation. Treatment is not necessary for this disorder; affected dogs do not have clinical signs. Recently two non-CKCS dogs were documented to be heterozygous for the β1-tubulin gene mutation described in CKCS (M.K. Boudreaux, personal observation). Mutations in β1-tubulin should be considered as part of the differential diagnosis in dogs with persistent thrombocytopenia in the absence of clinical signs. MEMBRANE RECEPTOR DISORDERS Glanzmann Thrombasthenia (GT) Glanzmann thrombasthenia is characterized by absence or marked reduction in platelet glycoprotein complex IIb-IIIa, also known as integrin αIIb-β3 and as the fibrinogen receptor.39 In people, GT is categorized into three main types: Type I, Type II, and Variant. In Type I less than 5% of the receptor is detectable on platelet surfaces and clot retraction is absent. In Type II 10–20% of the receptor is present on platelet surfaces and clot retraction is detectable but reduced. In Variant GT the receptor is present but dysfunctional; clot retraction may or may not be detectable. In veterinary medicine, Type I GT has been documented in horses and dogs.5,24,33,36 The αIIb-β3 complex primarily binds fibrin-

CHAPTER 82: INHERITED INTRINSIC PLATELET DISORDERS

621

TABLE 82.2 Comprehensive Description of Reported Mutations Causing Animal Platelet Disorders Macrothrombocytopenia Cavalier King Charles Spaniel

Glanzmann thrombasthenia Great Pyrenees

Otterhound

Thoroughbred-cross (England) and Oldenbourg (Canada) Quarter Horse (Alabama)

Peruvian Paso (Idaho) Cyclic hematopoiesis Gray collie

Chediak-Higashi syndrome Japanese Black cattle

CalDAG-GEFI disorder Basset hounds

Eskimo Spitz

Landseers-ECT

Simmental cattle Arabian horse

There is a substitution of an A for a G in the gene encoding β1-tubulin at coding nucleotide 745 (c.745G>A) resulting in the substitution of an asparagine for aspartic acid at amino acid 249 (D249N). Microtubule stability is impaired resulting in aberrant proplatelet formation and platelet release. Affected dogs do not have clinical signs A 14-base duplication near the end of exon 13 of the gene segment encoding for the fourth calcium binding domain of αIIb results in defective splicing of intron 13 and appearance of a premature stop codon A single nucleotide change at coding nucleotide 1100 (c.1100G>C) in exon 12 of the gene encoding for αIIb results in the substitution of a histidine for an aspartic acid in the third calcium binding domain of αIIb (D367H). This results in a charge change in a highly conserved region of the protein. Similar mutations in humans resulted in destabilization of the αIIb-β3 complex resulting in lack of expression of the complex on the platelet surface A single nucleotide change at coding nucleotide 122 (c.122G>C) in exon 2 of the gene encoding αIIb results in the substitution of a proline for an arginine (R41P) in a highly conserved region of the protein This horse is a compound heterozygote with the same mutation described in the Thoroughbred in England as well as a 10-base deletion encompassing the splice site at the end of exon 11 in the gene encoding for αIIb. This second mutation likely results in lack of synthesis of the protein due to the appearance of a premature stop codon leading to nonsense-mediated decay This horse is homozygous for the 10-base deletion in the gene encoding αIIb described in the Quarter horse in Alabama There is a 1-base duplication (A) in a tract of 9 adenine nucleotides in exon 20 of the gene encoding the beta subunit of adaptor protein complex 3. The mutation and resulting frameshift cause the appearance of a premature stop codon. The protein is part of a complex that directs trans-Golgi export of transmembrane cargo proteins to granules There is a single nucleotide change (c.6065A>G) resulting in the substitution of an arginine for a histidine (H2015R) in the gene encoding a lysosomal trafficking regulator protein referred to as LYST or CHS1 There is a deletion in coding nucleotides 509, 510, and 511 (c.509–511delTCT) of the gene encoding CalDAG-GEFI resulting in deletion of a phenylalanine at position 170 (p.F170del) in structurally conserved region 1 (SCR1) of the catalytic domain of the protein. This mutation results in a qualitative disorder; the protein can be detected in western blots There is a duplication of coding nucleotides 452 (c.452dupA) of the gene encoding CalDAG-GEFI resulting in a frame shift starting at the codon encoding aspartic acid 151 (D151) near the beginning of SCR1. This frame shift results in the appearance of a premature stop codon at nucleotide 796. There is a premature stop codon at the codon encoding arginine at position 328 (R328Stop) of the gene encoding CalDAG-GEFI due to a substitution at coding nucleotide 982 (c.982C>T) at the beginning of the sequence encoding SCR4. The protein is not synthesized likely due to nonsense mediated decay of mRNA. There is a substitution at coding nucleotide position 701 (c.701T>C) in the gene encoding CalDAGGEFI resulting in the change of a leucine to a proline in SCR2 at amino acid 234 (L234P). There is a single nucleotide change at coding nucleotide 687 (c.687C>G) predicted to result in the change of a histidine to a glutamine at position 229 (H229Q) in SCR2. Although this mutation is within a SCR, the platelet function phenotype is mild compared to CalDAG-GEFI disorders described in dogs and cattle.

ogen; however, it can also bind other RGD-containing adhesive proteins, including von Willebrand factor. Glanzmann thrombasthenia is inherited as an autosomal recessive trait and is characterized by the inability of platelets to aggregate in response to all agonists, including thrombin. The alpha and beta subunits of the receptor are encoded by separate genes; both subunits are necessary for the formation of a stable complex

on the platelet surface. All cases of GT described in domestic animals have been due to mutations in the gene encoding the αIIb subunit; mutations in the gene encoding β3 have not yet been identified.7,8,17,18,32 Most mutations are in areas of the gene encoding for calcium binding domains within αIIb (Tables 82.1 and 82.2). αIIb subunits are not detected in flow cytometric assays using antibodies specific to the αIIb subunit. Flow

622

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cytometric studies using antibodies to the β3 subunit or to the αIIb-β3 complex may detect β3 subunits associated with vitronectin receptors (αv-β3) which share the β3 subunit and may be increased in these patients.38 Platelets isolated from dogs with GT do not bind CAP1, a monoclonal antibody specific for a receptor-induced binding site (RIBS) epitope on canine fibrinogen, when activated by strong agonists.6 Glanzmann thrombasthenia (formerly known as thrombasthenic thrombopathia) has been reported in Otterhounds and Great Pyrenees dogs, and in horses in England, Australia, Canada, Japan, and the United States. The prevalence of carriers for the mutation in Otterhounds is between 25% and 30% based on genetic testing done at Auburn University. The prevalence of the mutation in Great Pyrenees dogs is not known; only sporadic testing is being conducted by owners and breeders. Great Pyrenees dogs that are carriers for the mutation have been detected in several states, including Mississippi, Alabama, Florida, Missouri, Minnesota, Indiana, Illinois, Oklahoma, and Washington. The horses described in Australia and Japan have not been documented at the molecular level.37,50 The Quarter Horse described at Auburn University was a compound heterozygote and had two distinct mutations in the gene encoding αIIb. One mutation was identical to that described in the horse in England. The other mutation was a 10-base deletion encompassing the exon 11–intron 11 splice site (Tables 82.1 and 82.2). Recently, a 10 yearold horse in Idaho with GT was documented to be homozygous for the same 10-base deletion (M.K. Boudreaux, personal observation). The prevalence of mutations causing GT in horses is unknown.

adaptor protein complex 3 beta-subunit (AP3β1) has been linked to this disorder.2 AP3 directs trans-Golgi export of transmembrane cargo proteins to granules. Chediak-Higashi Syndrome (CHS) Chediak-Higashi syndrome is an autosomal recessive genetic disorder characterized by abnormal leukocyte, melanocyte and platelet granulation. Affected animals have partial oculocutaneous albinism, increased susceptibility to infection, and a bleeding diathesis as a result of impaired platelet function. Platelets of affected animals lack discernable dense granules and are deficient in granule storage pools of adenine nucleotides, serotonin, and divalent cations. Platelet aggregation in response to collagen, which is largely dependent on thromboxane generation and granule release of ADP, is markedly reduced in affected animals.28 The disease was identified in a line of Persian cats: all of the affected animals exhibited a “blue smoke” hair color and pale irises with the development of bilateral nuclear cataracts in several animals as early as 3 months of age.30 Affected cats experienced prolonged bleeding at incision sites and the development of hematomas following venipuncture. Chediak-Higashi syndrome has also been diagnosed in Aleutian mink,40 three breeds of cattle,1,40,53 blue foxes,47 killer whales,44 and mice.35 CHS in Japanese Black cattle, mice, and people has been linked to mutations in the lysosomal trafficking regulator (LYST) gene which encodes a 425 kDa cytoplasmic protein that may be involved with incorporation of proteins into lysosomal membranes.31 Molecular studies have not been reported in Brangus or Hereford cattle or other species affected with CHS.

P2Y12 Receptor Disorder Recently a Greater Swiss Mountain dog in Calgary, Alberta, Canada was identified with a mutation in the gene encoding P2Y12. The dog experienced excessive bleeding after a routine spay. No other clinical signs were noted. (MK Boudreaux, personal observation). STORAGE POOL DISORDERS Cyclic Hematopoiesis Cyclic hematopoiesis is an autosomal recessive disorder described in gray collies characterized by cyclic fluctuations in the number of circulating neutrophils, reticulocytes, and platelets.16,23,34 Melanocytes are also affected in this disorder resulting in the gray color of affected animals. A bone marrow stem cell defect results in neutropenic episodes occurring approximately every 12 days. Mortality is high; most puppies die prior to 6 months of age due to fulminating infection. Thrombocytopenia does not occur and platelet numbers fluctuate between 300,000 and 700,000/μL. Platelet dense granules are absent. Platelet reactivity to several agonists is defective; clot retraction and platelet adhesiveness are impaired. A mutation in the gene encoding

Dense Granule Defect A dense granule defect has been described in a family of American Cocker Spaniels.13 Affected dogs had a moderate to severe bleeding diathesis, particularly after trauma or surgery. Three of five affected dogs had impaired platelet aggregation responses to ADP and collagen. Platelet 14C-serotonin uptake and retention were normal; ATP content was also normal. ADP content was low resulting in an increased ATP/ADP ratio. Platelet morphology at the electron microscopic level was normal suggesting a functional dense granule defect. The disorder has not been recognized in Cocker Spaniels since the initial report. Molecular studies have not been conducted. SIGNAL TRANSDUCTION DISORDERS Calcium Diacylglycerol Guanine Nucleotide Exchange Factor I (CalDAG-GEFI) Thrombopathias Basset hounds, Eskimo Spitz, Landseers of European Continental Type (ECT) and Simmental cattle have been identified with inherited signal-transduction platelet disorders that are virtually identical in presentation at

CHAPTER 82: INHERITED INTRINSIC PLATELET DISORDERS

the clinical and functional level.4,15,26,27,29,45,49 Terms describing these disorders in the literature include canine thrombopathia, Basset hound thrombopathia, Spitz thrombopathia, Landseer thrombopathia, Bovine thrombopathia, and Simmental thrombopathia. Platelet aggregation responses to ADP and collagen are minimal to absent in affected animals. Platelet aggregation in response to thrombin is rate-impaired and is characterized by a lag phase, lengthening the time to complete aggregation to 4–6 minutes (normal dog platelets aggregate fully within 3 minutes). Because thrombopathic platelets do respond to thrombin, clot retraction assays are normal. Flow cytometric assays using antibodies to αIIb or β3 show normal receptor density. CAP-1 binding is absent on ADP or PAF-activated thrombopathic canine platelets indicating that affected platelets either fail to bind fibrinogen or the αIIb-β3 receptor fails to induce changes in the conformation of fibrinogen necessary for detection by this RIBS antibody.9 Platelet function disorders, likely due to problems with signal transduction, have been described in other breeds, including mixed-breed dogs.14 These disorders have not been evaluated at the molecular level. As observed in people,43 signal-transduction disorders are probably the most common cause of inherited intrinsic platelet disorders in dogs and possibly other species. The molecular basis for the signal transduction-related platelet disorders in Basset hounds, Spitz, Landseers-ECT and Simmental cattle is due to distinct mutations in the gene encoding calcium diacylglycerol guanine nucleotide exchange factor I (CalDAG-GEFI).9,10 All of the mutations are within areas encoding structurally conserved regions (SCRs) of the catalytic domain (Tables 82.1, 82.2, and 82.3). CalDAG-GEFI is a signal transduction protein that functions as a guanine nucleotide exchange factor (GEF) and facilitates GDP/GTP switching by Rap1b. Rap-1b is a Ras-related low molecular mass guanine nucleotide binding protein (GTPase) that is activated by the binding of GTP. Activated Rap-1b is critically involved in activation and change of conformation of integrin αIIb-β3 necessary for fibrinogen binding and ensuing platelet aggregation.3 The ability of thrombin to activate mutant platelets is thought to be attributable to the ability of PAR4 signaling to bypass CalDAG-GEFI and activate Rap1b directly or act via a pathway independent of Rap1b.9,20 The mutation in Basset hounds is believed to be widespread; results of DNA testing at Auburn University indicate that between 25% and 30% of Basset hounds are carriers for the mutation. The mutation is

more prevalent in Basset hound lines used for show rather than for hunting. Landseer-ECT is a distinct breed recognized in Europe since 1960. Breeders within several European countries have tested their breeding stock for the CalDAG-GEFI mutation. The prevalence of the carrier state in Landseers-ECT, based on testing at Auburn University, is around 25%. Other than the original two Eskimo Spitz reported in 1994, no other affected or heterozygous Eskimo Spitz have been identified. Whether this small number results from a very low prevalence or lack of recognition of the disorder is not known. The prevalence of the CalDAG-GEFI gene mutation in Simmental cattle is also unknown; testing for the disorder has not been mandated by breed organizations. A mutation in CalDAG-GEFI was identified in an Arabian horse that presented with hemoabdomen and enlarged spleen at Auburn University (M.K. Boudreaux, personal observation). The mutation was a single nucleotide change in Exon 6 at coding nucleotide 687 (c.687C>G) and is predicted to result in the change of a histidine to a glutamine at position 229 (H229Q) in SCR2 (Table 82.3). Although this mutation was in an area encoding a highly conserved region and would be predicted to affect the structure of SCR2, platelet aggregation in response to ADP and collagen was only mildly diminished. The presence of this mutation may result in enhanced susceptibility of affected horses for abnormal hemorrhage, particularly if given platelet inhibitory medications or subjected to trauma. To eliminate confusion in the literature and to more precisely describe the nature of the platelet disorders described in Basset hounds, Spitz, Landseers-ECT, and Simmental cattle they should be referred to as CalDAGGEFI platelet disorders. Although CalDAG-GEFI is also expressed in brain, neurological signs in people or animals with CalDAG-GEFI disorders have not been reported. CalDAG-GEFI may play a role in signal transduction events in other hematopoietic cells, including neutrophils. Redundant pathways, however, are likely responsible for preventing significant impairment of neutrophil function in animals with CalDAG-GEFI mutations. Kindlin-3 Recently a mutation in the gene encoding Kindlin-3 was identified in a German Shepherd dog referred to Washington State University (MK Boudreaux, personal observation). The dog had a history of prolonged bleeding, persistently high leukocyte counts, and susceptibility to infections. Kindlin-3 is a signal transduction

TABLE 82.3 Amino Acids within Structurally Conserved Regions (SCR) of the Catalytic Domain of CalDAG-GEFI Species Basset and Spitz Cow Horse Landseer-ECT a

623

SCR

Mutationa

SCR1 SCR2 SCR2 SCR4

FDHLEPLELAEHLTYLEYRSFCKI RAGVITHFVHVAEELLHLQNFNTLMAVVGGLSHSSISRLKETH RAAVITHFVHVAEKLLELQNFNTLMAVVGGLSHSSISRLKETH RLNGAK

Underlined/bold amino acids are sites of mutations described in dogs, cattle, and horses.

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SECTION VI: PLATELETS

protein shared by all hematopoietic cells that plays an essential role in integrin activation.

affected animals but also providing a means of identifying heterozygous individuals.

PROCOAGULANT EXPRESSION DISORDERS

CASE MANAGEMENT

Scott Syndrome

Clinical signs in animals with inherited intrinsic platelet disorders can vary considerably, even in animals with the same type of disorder. On a day to day basis hemorrhages are typically mild and sporadic, and tend to be self-limiting. Transfusions are usually only required in animals scheduled for surgery or subjected to major trauma. Minor procedures such as ear and teeth cleaning should be performed with extreme care. Hemorrhage into the ear canal can result in a vicious cycle of trauma and hemorrhage as a consequence of head shaking and scratching that can be difficult to curtail. Bleeding in the oral cavity may initially be missed if the animal swallows the blood; melena may be the first indication of this type of bleeding. Bloating can occur with excessive swallowing of blood and air. Gingival bleeding can be controlled using topical thrombin in animals with CalDAG-GEFI thrombopathias; however, repeated use of thrombin may result in immune reactions to thrombin and should be avoided. Surgical adhesive can also be used for any of the inherited platelet disorders to control bleeding from small skin lesions. Irritants present in cedar shavings, harsh detergents, cigarettes, or solvents, may elicit epistaxis and exposure should be avoided. Oral or topical medications that inhibit platelet reactivity or result in thrombocytopenia should also be avoided. Blood loss through the gastrointestinal or urinary tracts can be insidious and can result in iron deficiency anemia. Iron levels should be monitored in affected dogs less than a year old with chronic insidious bleeding. The animals should also be evaluated for intestinal parasites. It may be necessary to supplement iron using oral supplements and if oral iron is not sufficient to restore iron levels then injections should be considered using the smallest gauge needle to minimize risk of bleeding at injection sites. Affected dogs should be evaluated by a veterinarian at least every 6 months, even in the absence of apparent clinical signs. Owners can be taught to periodically assess mucous membranes for lightening in color as an indication of anemia. Animals scheduled for surgery or subjected to trauma may require transfusion of multiple units of plateletrich plasma or platelet concentrates. These products, which can be difficult and costly to obtain, cannot be stored for longer than 5 days and should be maintained at room temperature with gentle rocking to avoid platelet activation. Whole blood transfusions can be administered, but when given in volumes designed to prevent circulatory overload they may not provide enough functional platelets to produce effective hemostasis. Repeated transfusions in animals with GT may result in an immune response against αIIb-β3 on transfused platelets. In people with inherited platelet disorders alternative treatments have been evaluated including administration of 1-desamino-8-D-arginine vasopressin (DDAVP) and recombinant human factor VIIa.19,42 The

A family of German shepherd dogs has been described with a bleeding disorder resulting from lack of procoagulant expression on the surface of platelets, similar to Scott syndrome described in people.11,12 Affected dogs experienced intramuscular hemorrhage, epistaxis, and hyphema. Platelet morphology at the light microscopic level and platelet numbers were normal. Platelet aggregation and release, clot retraction, and buccal mucosa bleeding times were also normal. Affected platelets were markedly impaired in their ability to externalize membrane phosphatidylserine or generate platelet prothrombinase activity in response to collagen, thrombin, or combinations of collagen and thrombin. Platelet microvesicle generation was also impaired in response to calcium ionophore, thrombin, and collagen. In flow cytometric experiments, Annexin V binding by affected platelets activated by calcium ionophore was markedly reduced, supporting the inability of platelets to express phosphatidylserine. Although this is a platelet disorder, the clinical signs are more typical of a coagulopathy due to the inability of the platelets to generate and support a surface for coagulation protein activation. The molecular basis for this disorder is not known in dogs or in people. DIAGNOSIS The existence of inherited thrombocytopenia should be considered in dogs, particularly in CKCS, with persistent macrothrombocytopenia in the absence of clinical signs. Documentation of a mutation in the gene encoding β1-tubulin is recommended to avoid subjecting these animals to unnecessary and potentially harmful drug treatments. The existence of a qualitative intrinsic platelet disorder should be considered in animals with platelet-type bleeding (mucosal bleeding, epistaxis, petechial/ecchymotic hemorrhages) in the absence of thrombocytopenia (platelet numbers greater than 50,000/μL) and in the presence of normal coagulation screening assays and VWF factor antigen levels of greater than 15%. Diagnosis of novel intrinsic platelet disorders requires specialized testing which may include platelet aggregation and release, flow cytometry, and electron microscopy. Some of these techniques are not readily available and several require that the affected animal be on the premises of the testing facility for sample collection and processing. As a consequence, novel intrinsic platelet disorders are often diagnosed based on exclusion. In recent years the molecular basis for several of the documented inherited intrinsic platelet disorders in animals has been determined, not only facilitating diagnosis of

CHAPTER 82: INHERITED INTRINSIC PLATELET DISORDERS

mechanism of action of DDAVP in shortening of bleeding times in people with inherited platelet disorders is thought to entail more than enhanced release of VWF from Weibel-Palade bodies; however, those mechanisms remain unclear. DDAVP was found to be more useful in people with platelet signal transduction and granule disorders than in people with GT; recombinant human factor VIIa was found to be more useful in the latter group. The use of non-plasma-derived reagents and species compatible purified plasma products in veterinary medicine is largely unexplored but may be attractive as an alternative method of treatment, particularly in dogs.

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24. Dodds WJ. Familial canine thrombocytopathy. Thromb Diath Haemorrh Suppl 1967;26:241–248. 25. Eksell P, Haggstrom J, Kvart D, et al. Thrombocytopenia in the Cavalier King Charles spaniel. J Small Anim Pract 1994;35:153–155. 26. Frojmovic MM, Wong T, Searcy GP. Platelets from bleeding Simmental cattle have a long delay in both ADP-activated expression of GPIIb-IIIa receptors and fibrinogen-dependent platelet aggregation. Thromb Haemost 1996;76:1047–1052. 27. Gentry PA, Cheryk LA, Shanks RD, et al. An inherited platelet function defect in a Simmental crossbred herd. Can J Vet Res 1997;61:128– 133. 28. Honda N, Ohnishi K, Fujishiro I, et al. Alteration of release and role of adenosine diphosphate and thromboxane A2 during collagen-induced aggregation of platelets from cattle with Chediak-Higashi syndrome. Am J Vet Res 2007;88:1399–1406. 29. Johnstone IB, Lotz F. An inherited platelet function defect in basset hounds. Can Vet J 1979;20:211–215. 30. Kramer JW, Davis WC, Prieur DJ. The Chediak-Higashi syndrome of cats. Lab Invest 1977;36:554–562. 31. Kunieda T, Nakagiri M, Takami M, et al. Cloning of bovine LYST gene and identification of a missense mutation associated with Chediak-Higashi syndrome of cattle. Mamm Genome 1999;10:1146–1149. 32. Lipscomb DL, Bourne C, Boudreaux MK. Two genetic defects in αIIb are associated with Type I Glanzmann’s thrombasthenia in a Great Pyrenees dog: a 14-base insertion in Exon 13 and a splicing defect of Intron 13. Vet Pathol 2000;37:581–588. 33. Livesey L, Christopherson P, Hammond A, et al. Platelet dysfunction (Glanzmann’s thrombasthenia) in horses. J Vet Intern Med 2005;19:917–919. 34. Lothrop CD, Candler RV, Pratt HL, et al. Characterization of platelet function in cyclic hematopoietic dogs. Exp Hematol 1991;19:916–922. 35. Lutzner MA, Lowrie CT, Jordan HW. Giant granules in leukocytes of the beige mouse. J Hered 1967;58:299–300. 36. Macieira S, Rivard GE, Champagne J, et al. Glanzmann thrombasthenia in an Oldenbourg filly. Vet Clin Pathol 2007;36:204–208. 37. Miura N, Senba H, Ogawa H, et al. A case of equine thrombasthenia. Nippon Juigaku Zasshi 1987;49:155–158. 38. Newman PJ, Seligsohn U, Lyman S, et al. The molecular genetic basis of Glanzmann thrombasthenia in the Iraqi-Jewish and Arab populations in Israel. Proc Natl Acad Sci USA 1991;88:3160–3164. 39. Nurden AT. Glanzmann thrombasthenia. Orphanet J Rare Dis 2006;1:10. 40. Padgett GA, Leader RW, Gorham JR, et al. The familial occurrence of the Chediak-Higashi syndrome in mink and cattle. Genetics 1964;49:505– 512. 41. Pedersen HD, Haggstrom J, Olsen LH, et al. Idiopathic asymptomatic thrombocytopenia in Cavalier King Charles Spaniels is an autosomal recessive trait. J Vet Intern Med 2002;16:169–173. 42. Poon MC, D’Oiron R, Von Depka M, et al. Prophylactic and therapeutic recombinant factor VIIa administration to patients with Glanzmann’s thrombasthenia: results of an international survey. J Thromb Haemost 2004;2:1096–1103. 43. Rao AK, Gabbeta J. Congenital disorders of platelet signal transduction. Arterioscler Thromb Vasc Biol 2000;20:285–289. 44. Ridgway SH. Reported causes of death of captive killer whales. J Wildlife Dis 1979;15:99–104. 45. Searcy GP, Frojmovic MM, McNicol A, et al. Platelets from bleeding Simmental cattle mobilize calcium, phosphorylate myosin light chain and bind normal numbers of fibrinogen molecules but have abnormal cytoskeletal assembly and aggregation in response to ADP. Thromb Haemostasis 1994;71:240–246. 46. Singh MK, Lamb WA. Idiopathic thrombocytopenia in Cavalier King Charles Spaniels. Aust Vet J 2005;83:700–703. 47. Sjaastad OV, Blom AK, Stormorken H, et al. Adenine nucleotides, serotonin, and aggregation properties of platelets of blue foxes (Alopex lagopus) with the Chediak-Higashi syndrome. Am J Med Genet 1990;35:373– 378. 48. Smedile LE, Houston DM, Taylor SM, et al. Idiopathic, asymptomatic thrombocytopenia in Cavalier King Charles Spaniels: 11 cases (1983–1993). J Am Anim Hosp Assoc 1997;33:411–415. 49. Steficek BA, Thomas JS, McConnell MF, et al. A primary platelet disorder of consanguineous Simmental cattle. Thromb Res 1993;72:145–153 50. Sutherland RJ, Cambridge H, Bolton JR. Functional and morphological studies on blood platelets in a thrombasthenic horse. Aust Vet J 1989;66:366–370. 51. Tarnow I, Kristensen AT, Texel H, et al. Decreased platelet function in Cavalier King Charles Spaniels with mitral valve regurgitation. J Vet Intern Med 2003;17:680–686. 52. Tarnow I, Kristensen AT, Olsen LH, et al. Dogs with heart diseases causing turbulent high-velocity blood flow have changes in platelet function and von Willebrand factor multimer distribution. J Vet Intern Med 2005;19:515–522. 53. Umemura T, Katsuta O, Goryo M, et al. Pathological findings in a young Japanese Black cattle affected with Chediak-Higashi syndrome. Jpn J Vet Sci 1983;45:241–246.

C H A P T E R 83

Acquired Platelet Dysfunction MICHAEL M. FRY Introduction Acquired Platelet Hypofunction Underlying Disease Uremia Anti-platelet Antibodies Infection, Inflammation, and Neoplasia Increased Fibrinolytic Products and Liver Disease Snake Envenomation Drugs and Other Exogenous Agents Platelet Inhibitors Anti-inflammatory Agents Other Drugs and Exogenous Agents

Acquired Platelet Hyperfunction Underlying Disease Infection, Inflammation, and Neoplasia Taurine Deficiency in Cats Heart Disease Nephrotic Syndrome Asthma Diabetes Mellitus Drugs and Other Exogenous Agents

Acronyms and Abbreviations ADP, adenosine diphosphate; BMBT, buccal mucosal bleeding time; BVDV, bovine viral diarrhea virus; CCL5, chemokine L5; CKCS, Cavalier King Charles Spaniel; COX, cyclooxygenase; DIC, disseminated intravascular coagulopathy; EPO, erythropoietin; FDP, fibrin degradation products; FeLV, feline leukemia virus; IMT, immunemediated thrombocytopenia; MR, mitral regurgitation; MVP, mitral valve prolapse; NSAIDs, non-steroidal anti-inflammatory agents; OHE, ovariohysterectomy; PAF, platelet activating factor; PFA-100, platelet function analyzer-100; PRP, platelet-rich plasma; TEG, thromboelastography; TXA2, thromboxane A2; VWD, von Willebrand disease; VWF, von Willebrand factor.

INTRODUCTION Impaired platelet function should be considered in any patient with a bleeding tendency in the absence of thrombocytopenia, low von Willebrand factor (VWF) levels, or laboratory evidence of coagulopathy (e.g. prolonged prothrombin time, activated partial thromboplastin time, or activated clotting time). Abnormal platelet function, also known as thrombopathia, thrombopathy, or thrombocytopathy, may be primary (congenital, usually inherited) or secondary (acquired). Inherited intrinsic platelet disorders and von Willebrand Disease (VWD) are discussed in other chapters in this text (see Chapters 81 and 82). Acquired disorders occur most commonly due to underlying disease or treatment with drugs or other exogenous agents. These conditions may be subcategorized into hypofunctional and hyperfunctional disorders, which may cause bleeding tendencies and thrombotic tendencies, respectively. Clinical manifestations of acquired 626

thrombopathies are highly variable. Ideally, therapy should be directed toward removing or managing the underlying disease process or exogenous stimulus causing secondary platelet dysfunction. Specific etiologies and mechanisms of acquired platelet dysfunction are discussed in more detail below. ACQUIRED PLATELET HYPOFUNCTION Acquired platelet hypofunction may occur secondary to underlying disease or treatment with drugs or other exogenous agents. Underlying Disease Many pathologic conditions are recognized as potential causes of impaired platelet function, including renal failure, anti-platelet antibodies, infections, malignancies, increased fibrinolytic products, and others.

CHAPTER 83: ACQUIRED PLATELET DYSFUNCTION

Uremia Uremia is recognized as a cause of platelet dysfunction in people and animals.29,34,61 Many molecules and pathways affecting platelet adhesion, aggregation, and secretion have been implicated.1 An experimental model of renal failure in dogs resulted in prolonged buccal mucosal bleeding time and markedly impaired platelet glass bead retention, but did not significantly affect platelet concentration, volume, or aggregation in response to several agonists, or coagulation assays (prothrombin time and activated partial thromboplastin time), findings consistent with defective platelet adhesion.15 Neither quantitative nor qualitative abnormalities in VWF have been shown consistently in people with uremia, suggesting that the impaired adhesion in the uremic dogs was due to abnormal platelet function.1 Anti-platelet Antibodies Anti-platelet antibodies not only may cause immunemediated thrombocytopenia (IMT), but are also recognized as a cause of platelet dysfunction. Specifically, antibodies against GPIIb-IIIa (αIIbβ3) and other key platelet surface receptors such as Ib-IX-V have been implicated in people.1 Experimental work in dogs and equids suggests a similar phenomenon. A study in which serum from normal dogs or dogs with IMT was added to platelet-rich plasma (PRP) from a normal dog found the serum from dogs with IMT to cause impaired aggregation to several platelet agonists.52 In another study, serum from dogs experimentally infected with Ehrlichia canis inhibited aggregation of platelets from a normal dog.43 Serum from mule foals with experimentally-induced neonatal alloimmune thrombocytopenia had evidence of higher concentration of platelet-associated IgG than serum from control mule foals, and addition of serum from affected foals to donkey PRP caused impaired aggregation in response to collagen.77 Exogenous anti-platelet antibodies also comprise a group of drugs designed to inhibit platelet function (see “Platelet Inhibitors”, below). Infection, Inflammation, and Neoplasia There is evidence that type II bovine viral diarrhea virus (BVDV), in addition to causing thrombocytopenia, may also impair platelet function. Experimental work in neonatal calves showed impaired platelet aggregation in affected animals.93,95 In contrast, work by the same research group found no evidence of altered platelet function in cattle persistently infected with BVDV.94 Feline leukemia virus (FeLV) can infect megakaryocytes and, in addition to causing many other hematologic abnormalities, may cause platelet abnormalities potentially associated with impaired function.81 Clear evidence of a causal relationship between FeLV and platelet hypofunction is lacking. However, abnormal platelet or megakaryocyte morphology is a feature of

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many FeLV-related conditions, both neoplastic and non-neoplastic, and it is plausible that this has a functional correlate.6,13 There is conflicting information about the effects of endotoxin or sepsis on platelet function, with data to suggest both decreased and heightened reactivity.92 Lipoteichoic acid from Staphylococcus aureus has been shown to impair intracellular calcium mobilization and aggregation in human platelets.82 Yersinia pestis expresses a surface protein, YopM, that is structurally similar to the VWF- and thrombin-binding domain of the platelet GPIb receptor, and has been shown to inhibit aggregation of human platelets, possibly via competitive binding with thrombin.54 The clinical significance of these findings is not clear. Studies of experimentally-induced pancreatitis in dogs have found evidence of both platelet hypofunction and hyperfunction. In one study, pancreatitis caused impaired platelet aggregation in response to several agonists and reduced ATP release in response to collagen.46 Another study found platelets to have increased sensitivity to ADP and PAF within the first hour after the insult to the pancreas, but diminished or normal sensitivity thereafter.56 Hyperglobulinemia, either due to antigenic stimulation (e.g. ehrlichial) or neoplasia (e.g. multiple myeloma), is commonly recognized in veterinary medicine as a potential cause of impaired platelet function, possibly by “coating” the platelet surface and impairing adhesion and aggregation.29,87,91 Platelet dysfunction is a recognized complication of IgA and IgG myeloma and Waldenström macroglobulinemia in people, and some in vitro experiments have shown paraprotein to be capable of interfering with platelet function.1 A dog with IgA myeloma had a mildly prolonged bleeding time but more specific platelet function assays were not performed.79 Increased Fibrinolytic Products and Liver Disease It is not clear how often or to what degree increased plasma concentration of fibrin degradation products (FDPs) causes platelet inhibition in the clinical setting. Purified low molecular mass fibrinogen degradation products have been shown to inhibit platelet aggregation in vitro, but may not occur in high enough concentrations in vivo to produce this effect, and patients with disseminated intravascular coagulopathy (DIC) or other conditions may have decreased in vitro platelet aggregation due to in vivo platelet activation.74,86 Nevertheless, increased plasma concentration of FDPs is commonly cited as a potential cause of impaired platelet function in animals.29,87,91 Increased FDPs may occur due to increased fibrinolysis (e.g. in DIC) or decreased clearance (with liver insufficiency). Certainly, factors other than FDPs contribute to a bleeding tendency in patients with DIC. A study of dogs with liver disease found impaired platelet aggregation in response to some agonists, and bleeding tendencies consistent with thrombopathy, but FDPs were not shown to be the causative agent, and experimentally-induced hyperammonemia

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SECTION VI: PLATELETS

Many drugs and other exogenous agents have been shown to have inhibitory effects on platelet function. Some of these agents have platelet inhibition as their main clinical application, while others cause platelet inhibition as a side-effect.

selectively inhibit COX-2. Phenylbutazone has been shown to inhibit platelet aggregation in horses for a short time after administration, without causing a prolongation in BMBT.62 A recent study evaluating effects of several non-aspirin NSAIDs in dogs found that carprofen inhibited platelet aggregation, and caused thromboelastography (TEG) changes suggesting hypocoagulability, but deracoxib and meloxicam did not; in fact, deracoxib caused TEG changes consistent with increased clot strength.14 However, information on effects of carprofen on platelet function in dogs is conflicting: other studies found no evidence of carprofen causing impaired aggregation or prolonged PFA-100 closure time.35,42 Ketoprofen has been found to impair platelet aggregation and prolong PFA-100 closure time in dogs.35,42 The clinical implications of this effect are likely to be mild. In another study, dogs given ketoprofen before ovariohysterectomy (OHE) had inhibited platelet aggregation but did not have prolonged BMBT; the authors concluded that ketoprofen was safe to give to healthy dogs pre-OHE, but any such dogs should be screened preoperatively for bleeding problems and closely monitored after surgery.53

Platelet Inhibitors

Other Drugs and Exogenous Agents

Some drugs are designed to inhibit platelet function. Most of these are platelet receptor antagonists. Examples that have been used in animals include peptides or antibodies that inhibit GPIIb/IIIa (αIIbβ3 integrin) and agents that inhibit the ADP receptor.4,5,9,16,17,24,25,27,33,45,98,100,103,109 New platelet inhibitors continue to be developed for use in people.2,3

There is a long list of other drugs or exogenous agents found to inhibit platelet function in animals or people. A partial list includes antibiotics (penicillins and cephalosporins); antihistamines; barbiturates; calcium channel blockers; chondroprotective agents; dextran 70; diethylcarbamazine; dipyridamole; fibrinolytic and anti-fibrinolytic agents; halothane; heparin; hydroxyethyl starch; local anesthetics; marine fatty acids; mycotoxins; propanolol; psychotropic drugs; radiographic contrast agents; and wine or grape juice.1,8,19,20,26,28,30,36,39,48,49,51,58,60,65,70,72,105

has been shown to inhibit platelet aggregation in rats.83,106 Snake Envenomation Snake venoms contain complex mixtures of bioactive molecules (e.g. proteases, phospholipases, disintegrins, lectins) that can affect hemostasis in many ways, including interference with platelet function via interaction with receptors, modulation of ADP release or TXA2 formation, and production of reactive oxygen species.23,69 Experimental work in animals has shown speciesdependent platelet sensitivity to some snake venom proteins.55 Drugs and Other Exogenous Agents

Anti-inflammatory Agents Many anti-inflammatory agents inhibit platelet function. The main inhibitory mechanism is inactivation of the cyclooxygenase (COX) enzyme, which results in decreased production of the potent platelet agonist, thromboxane A2 (TXA2). Aspirin, which irreversibly inactivates both COX-1 (expressed by both newly formed and mature platelets) and COX-2 (expressed only by immature platelets) isoenzymes, has been shown to inhibit platelet function in many animal models.8,14,48,63,64,75 Platelet responses to aspirin evidently are species-dependent. In some species, aspirin impairs platelet aggregation in response to collagen and ADP. In cattle, however, treatment with oral aspirin resulted in decreased TXA2 production but did not impair platelet aggregation in response to collagen, ADP, or PAF.37 Aspirin also does not inhibit ADP-induced platelet aggregation in most dogs. This is likely because platelet release is not a necessary co-component of ADP-induced irreversible platelet aggregation in most dogs.7,8,10 Aspirin will inhibit ADP-induced aggregation responses in the small subset of dogs in which ADP-induced platelet aggregation is accompanied by dense granule release. Other non-steroidal anti-inflammatory agents (NSAIDs) inactivate COX reversibly, and some of them

ACQUIRED PLATELET HYPERFUNCTION Underlying disease or drug treatment may result in secondary platelet hyperfunction as well as hypofunction. Some studies in animals have found an association between underlying pathology or strenuous exercise and platelet activation, without demonstrating that the platelet response to the stimulus is abnormal.67,68,80,97,99,102 This section will be limited to conditions shown to result in abnormally enhanced platelet reactivity. Underlying Disease Many pathologic conditions are recognized as potential causes of enhanced platelet reactivity, including infectious and inflammatory diseases, neoplasia, and others. Infection, Inflammation, and Neoplasia Several infectious diseases are associated with increased platelet reactivity in animals, including heartworm disease and Rocky Mountain spotted fever in dogs,

CHAPTER 83: ACQUIRED PLATELET DYSFUNCTION

feline infectious peritonitis in cats, and pasteurellosis in cattle.7–12,22,41,71 Platelets from experimentally infected animals have been found to be hyperaggregable in response to some agonists, although the mechanisms responsible for this enhanced reactivity are not well understood. There is conflicting data in the scientific literature about the effects of sepsis on platelet function in people, with evidence to suggest both inhibition and activation.92 Interleukin-6, in addition to stimulating thrombopoiesis, increases the sensitivity of canine platelets to activation by some agonists.18,76 Ponies with experimentally-induced laminitis from excess dietary carbohydrate had increased platelet aggregation and formation of platelet-neutrophil aggregates.100,101 However, there is evidence indicating that inflammation may impair as well as enhance platelet reactivity. As noted above, studies of dogs with experimentally-induced pancreatitis have found evidence of both platelet hypofunction and hyperfunction. Moreover, a study using bovine samples found that addition of C-reactive protein to platelets suspended in homologous plasma impaired aggregation in response to some agonists.21 Cancer may be a cause of acquired platelet hyperfunction. Platelet hyperaggregability was detected in two dogs with probable essential thrombocythemia.31 A study in dogs found that platelets from dogs with cancer (not all of which were hematologic malignancies) were more sensitive to some agonists, compared to platelets from control animals.59 Taurine Deficiency in Cats The effect of taurine deficiency on platelet function in cats has been the subject of investigation, at least in part because of the tendency for cats with cardiomyopathy associated with taurine deficiency to develop thromboembolic disease. One aggregometry study using collagen as an agonist found platelets from taurine-depleted cats to be hyperaggregable compared to platelets from cats receiving taurine, and platelets from people supplemented with taurine to be hypoaggregable compared to platelets from people with normal taurine status.44 However, another similar study in cats did not confirm these findings, showing mildly increased collagen-induced serotonin release but no difference in collagen-induced platelet aggregation between taurinedeficient and taurine-replete cats; moreover, in this study there was evidence of decreased ADP-induced platelet aggregation and serotonin release.104 A recent study using the PFA-100 instrument showed no evidence of shorter closure times in cats with hypertrophic cardiomyopathy, compared to normal cats.47 Heart Disease Mitral valve disease is associated with increased reactivity and decreased survival of platelets in humans, and a possible association between mitral valve disease and platelet dysfunction has been the subject of inves-

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tigation in dogs, particularly in Cavalier King Charles Spaniels (CKCS).73 Congenital platelet abnormalities in CKCS, as discussed in more detail in Chapter 82, complicate interpretation of platelet function testing results in this breed. One study in CKCS found no correlation between degree of mitral valve prolapse (MVP) and platelet aggregation responses.73 Another study found CKCS with MVP had enhanced aggregation responses regardless of mitral regurgitation (MR) status, but platelets did not appear to circulate in an activated state. In that study, CKCS with MR and dogs with subaortic stenosis had prolonged PFA-100 closure times but also had lower proportions of large VWF multimers, likely secondary to consumption under high shear conditions, suggesting a mechanism of impaired primary hemostasis other than platelet hypofunction.90 Nephrotic Syndrome Enhanced platelet responsiveness to some agonists has been described in people and dogs with nephrotic syndrome, and may increase the risk of thromboembolic complications in these patients.40,57,78,84 The basis of the enhanced sensitivity is probably multifactorial; one study found that hyperaggregability can be inhibited in vitro by increasing plasma albumin concentration.40 Asthma A study of people with asthma found increased sensitivity of platelets to some agonists, as evidenced by increased intracellular free calcium concentration, increased CD62P expression, and increased plasma concentration of the RANTES or CCL5 cytokine.66 This enhanced response was inhibited in people with asthma who were taking oral theophylline. It is not clear whether allergic respiratory disease in animals causes similar platelet hyperfunction. Diabetes Mellitus Diabetes is a risk factor for atherosclerosis in people, likely due in part to altered platelet function.32,89,96 Platelet hyperreactivity in people with diabetes probably occurs due to multiple mechanisms, including glycation of proteins, oxidative damage to lipids, increased GP Ib-IX receptor expression, and altered signal transduction pathways.32,85 Effects of diabetes on platelet function are evident in the laboratory by enhanced sensitivity to agonists. In one study, platelets from people with diabetes had increased adhesion to an extracellular matrix substrate composed of collagen, adhesive glycoproteins, and proteoglycans, but not VWF.50 Rats with streptozotocin-induced diabetes had markedly increased release of arachidonic acid in response to thrombin in one study, and enhanced aggregation to ADP and thrombin and increased release of serotonin in response to thrombin in another study.38,107 The prevalence and clinical significance of altered platelet function in domestic animals with diabetes is not known.

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Drugs and Other Exogenous Agents In dogs, administration of erythropoietin (EPO) increased platelet responsiveness to thrombin and increased the proportion of reticulated platelets.108 A study in humans found that EPO increased platelet reactivity but did not change the numbers of reticulated platelets.88,108 Platelet reactivity was increased after thiacetarsamide treatment of Beagles experimentally implanted with adult heartworms, but not after treatment of dogs with naturally occurring heartworm infections.7

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82. Sheu J, Lee C, Lin C, et al. Mechanisms involved in the antiplatelet activity of Staphylococcus aureus lipoteichoic acid in human platelets. Thromb Haemost 2000;83:777–784. 83. Shinya H, Matsuo N, Takeyama N, et al. Hyperammonemia inhibits platelet aggregation in rats. Thromb Res 1996;81:195–201. 84. Sirolli V, Ballone E, Garofalo D, et al. Platelet activation markers in patients with nephrotic syndrome. A comparative study of different platelet function tests. Nephron 2002;91:424–430. 85. Sobel B, Schneider D. Platelet function, coagulopathy, and impaired fibrinolysis in diabetes. Cardiol Clin 2004;22:511–526. 86. Solum N, Rigollot C, Budzy ski A, et al. A quantitative evaluation of the inhibition of platelet aggregation by low molecular weight degradation products of fibrinogen. Br J Haematol 1973;24:419–434. 87. Stockham SL, Scott MA. Hemostasis. In: Stockham SL, Scott MA, eds. Fundamentals of Veterinary Clinical Pathology, 2nd ed. Ames: Blackwell, 2008;259–321. 88. Stohlawetz P, Dzirlo L, Hergovich N, et al. Effects of erythropoietin on platelet reactivity and thrombopoiesis in humans. Blood 2000;95: 2983–2989. 89. Stratmann B, Tschoepe D. Pathobiology and cell interactions of platelets in diabetes. Diabetes Vasc Dis Res 2005;2:16–23. 90. Tarnow I, Kristensen A, Olsen L, et al. Dogs with heart diseases causing turbulent high-velocity blood flow have changes in platelet function and von Willebrand factor multimer distribution. J Vet Intern Med 2005;19:515–522. 91. Topper MJ, Welles EG. Hemostasis. In: Latimer KS, Mahaffey EA, Prasse KW, eds. Duncan & Prasse’s Veterinary Laboratory Medicine: Clinical Pathology, 4th ed. Ames: Iowa State Press, 2003;99–135. 92. Vincent J, Yagushi A, Pradier O. Platelet function in sepsis. Crit Care Med 2002;30:S313–317. 93. Walz P, Bell T, Grooms D, et al. Platelet aggregation responses and virus isolation from platelets in calves experimentally infected with type I or type II bovine viral diarrhea virus. Can J Vet Res 2001;65:241–247. 94. Walz P, Grooms D, Bell T, et al. Platelet function and association of bovine viral diarrhea virus with platelets of persistently infected cattle. Am J Vet Res 2005;66:1738–1742. 95. Walz P, Steficek B, Baker J, et al. Effect of experimentally induced type II bovine viral diarrhea virus infection on platelet function in calves. Am J Vet Res 1999;60:1396–1401. 96. Watala C. Blood platelet reactivity and its pharmacological modulation in (people with) diabetes mellitus. Curr Pharm Des 2005;11:2331–2365. 97. Weiss D, Brazzell J. Detection of activated platelets in dogs with primary immune-mediated hemolytic anemia. J Vet Intern Med 2006;20:682–686. 98. Weiss D, Evanson O. Comparison of three arginine-glycine-aspartatecontaining peptides as inhibitors of equine platelet aggregation. J Vet Pharm Ther 2004;27:377–379. 99. Weiss D, Evanson O, Fagliari J, et al. Evaluation of platelet activation and platelet-neutrophil aggregates in Thoroughbreds undergoing near-maximal treadmill exercise. Am J Vet Res 1998;59:393–396. 100. Weiss D, Evanson O, McClenahan D, et al. Effect of a competitive inhibitor of platelet aggregation on experimentally induced laminitis in ponies. Am J Vet Res 1998;59:814–817. 101. Weiss D, Evanson O, McClenahan D, et al. Evaluation of platelet activation and platelet-neutrophil aggregates in ponies with alimentary laminitis. Am J Vet Res 1997;58:1376–1380. 102. Weiss D, Evanson O, McClenahan D, et al. Shear-induced platelet activation and platelet-neutrophil aggregate formation by equine platelets. Am J Vet Res 1998;59:1243–1246. 103. Weiss D, Evanson O, Wells R. Evaluation of arginine-glycine-aspartatecontaining peptides as inhibitors of equine platelet function. Am J Vet Res 1997;58:457–460. 104. Welles E, Boudreaux M, Tyler J. Platelet, antithrombin, and fibrinolytic activities in taurine-deficient and taurine-replete cats. Am J Vet Res 1993;54:1235–1243. 105. Wierenga J, Jandrey K, Haskins S, et al. In vitro comparison of the effects of two forms of hydroxyethyl starch solutions on platelet function in dogs. Am J Vet Res 2007;68:605–609. 106. Willis S, Jackson M, Meric S, et al. Whole blood platelet aggregation in dogs with liver disease. Am J Vet Res 1989;50:1893–1897. 107. Winocour P, Lopes-Virella M, Laimins M, et al. Time course of changes in in vitro platelet function and plasma von willebrand factor activity (VIIIR : WF) and factor VIII-related antigen (VIIIR : Ag) in the diabetic rat. J Lab Clin Med 1983;102:795–804. 108. Wolf R, Peng J, Friese P, et al. Erythropoietin administration increases production and reactivity of platelets in dogs. Thromb Haemost 1997;78:1505–1509. 109. Wu G, Ruan C, Drouet L, et al. Inhibition effects of KRDS, a peptide derived from lactotransferrin, on platelet function and arterial thrombus formation in dogs. Haemostasis 1992;22:1–6.

SECTION VII

Hemostasis Marjory B. Brooks

CHAPTER

84

Overview of Hemostasis STEPHANIE A. SMITH Enzyme Biochemistry The Nature of Enzymatic Cascades Enzymatic Complexes (Cofactors and Enzymes) Inhibition of Enzymatic Activity Membrane Surface Interactions Coagulation Proteins that Generate Thrombin Contact Pathway (Kallikrein-Kinin Pathway) Factor XII Prekallikrein Kininogens Extrinsic (Tissue factor) Pathway Tissue factor Factor VII Intrinsic Pathway Factor XI Factor IX Factor VIII Common Pathway Factor X Factor V Prothrombin Coagulation Inhibitors Tissue factor pathway inhibitor C1-Inhibitor Antithrombin Protein C pathway Miscellaneous inhibitors Cells that Participate in Hemostasis Endothelial Cells Antithrombotic properties of endothelium

Prothrombotic properties of endothelium Other properties of endothelial cells Platelets Tissue Factor Bearing Cells Microparticles The Role of Cell Surface Membranes Cell Based Model of Thrombin Generation Initiation Amplification Propagation Thrombin Feedback Regulation of Coagulation Platelet Activation Fibrin Formation Activation of Factor XIII Fibrinogen and Fibrin Clot Structure Fibrinolysis Plasminogen Plasminogen Activators Tissue-type plasminogen activator Urokinase plasminogen activator Fibrinolysis Inhibitors Plasminogen activator inhibitor 1 α2-Antiplasmin Thrombin activatable fibrinolysis inhibitor Miscellaneous inhibitors Hemostasis and Inflammation Effects of Hemostasis on Inflammation Effects of Inflammation on Hemostasis

Acronyms and Abbreviations α1-PI, α1-Protease inhibitor; α2-MG, α2-macroglobulin; α2-AP, α2-antiplasmin; APC, activated protein C; AT, antithrombin; C1-INH, C1-inhibitor; C4BP, C4 binding protein; DDAVP, desamino-8-D-arginine vasopressin; EPCR, endothelial protein C receptor; ER, endothelial reticulum; FV/FVa, factor V/activated factor V; FVII/FVIIa, factor VII/activated factor VII; FVIII/FVIIIa, factor VIII/activated factor VIII; FIX/FIXa, factor IX/activated factor IX; FX/FXa, factor X/activated factor X; FXI/FXIa, factor XI/activated factor XI; FXII/FXIIa, factor XII/activated factor XII; FXIII/FXIIIa, factor XIII/activated factor XIII; FDP, fibrin degradation products; FpA, fibrinopeptide A; FpB, fibrinopeptide B; Gla, gammacarboxyglutamic acid; GPI, glycosylphosphatidylinositol; HC, heparin cofactor II; HK, high molecular weight kininogen; HSPG, heparan sulfate proteoglycan; LRP, low density lipoprotein receptorrelated protein; MP, microparticle; NO, nitric oxide; PAF, platelet activating factor; PAI-1, plasminogen activator inhibitor 1; PARs, protease activated receptors; PC, protein C; PChol, phosphatidylcholine; PCI, protein C inhibitor; PEth, phosphatidylethanolamine; PGI2, prostacyclin; PK, prekallikrein; PLG, plasminogen; PS, protein S; PSer, phosphatidylserine; PZ, protein Z; sctPA, single chain tissue plasminogen activator; scuPA, single chain urokinase 635

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plasminogen activator; serpin, serine protease inhibitor; TAFI/TAFIa, thrombin activatable fibrinolysis inhibitor/ activated thrombin activatable fibrinolysis inhibitor; TF, tissue factor; TFPI, tissue factor pathway inhibitor; TM, thrombomodulin; tPA, tissue plasminogen activator; uPA, urokinase plasminogen activator; uPAR, urokinase plasminogen activator receptor; VWF, von Willebrand factor; ZPI, Protein Z-dependent protease inhibitor.

H

emostasis is a vital protective mechanism that prevents blood loss by sealing sites of injury in the vascular system. Hemostasis must be controlled, however, so that blood does not coagulate within the vasculature. Such uncontrolled coagulation restricts normal blood flow, resulting in tissue hypoxia. Hemostasis consists of a tightly controlled and wellbalanced interplay among a large number of cellular and protein participants. Endothelial cells lining the vasculature, cells outside the vasculature, and platelets all play important roles. Hemostatic proteins include inactive precursors (zymogens) that are converted to active enzymes, regulatory proteins (cofactors) that enhance the functionality of their corresponding coenzymes, and inhibitors that interfere with protein function through a variety of mechanisms (see Table 84.1). Normally, coagulation does not occur within the vasculature because properties of the resting cellular participants render them inactive, and because the majority of protein participants circulate in their inert, zymogen form. The initiation of coagulation in response to injury depends on the exposure of extravascular components that are not present within the bloodstream under physiologic conditions. Exposure of these extravascular participants initiates an explosive cascade of cellular activation, changes in cell-surface properties, and generation of active enzymes that produce a stable clot. Some of the cells and enzymes also trigger vascular wall responses, inflammatory pathways, and immunedefense mechanisms. Coagulation also initiates fibrinolysis, the process by which the clot is removed to restore blood flow. Ultimately, coagulation triggers the cell migration and proliferation that promote healing.

ENZYME BIOCHEMISTRY The Nature of Enzymatic Cascades The clotting cascade consists of a series of enzymatic reactions. Protein participants circulate in the blood mostly as inert precursors, or zymogens. These zymogens are activated by limited proteolysis to form active enzymes. With the exception of thrombin, all of these enzymes have limited activity unless they bind to their protein cofactor and a procoagulant membrane surface. Once the complete enzymatic complex has assembled on a suitable membrane surface, the activity of the enzyme is markedly exaggerated, by as much as several hundred-thousand fold. Each cascade reaction is initiated when the active enzyme complex binds to its substrate, resulting in cleavage of the substrate to generate the next enzyme in the series. In many cases, positive feedback occurs

because a downstream enzyme is able to back-activate an upstream zymogen, thereby markedly amplifying the generation of downstream enzymes from minimal amounts of upstream enzymes. Negative feedback is also a common feature in the coagulation cascade, with downstream enzymes generating inhibitors that downregulate activity of upstream enzymes. The generation of a blood clot is carefully controlled through the complex interplay of the initial trigger, amplification, and inhibition. Enzymatic Complexes (Cofactors and Enzymes) The majority of enzymes participating in coagulation are serine proteases with homology to trypsin, the prototype enzyme of this class. Serine proteases cleave after arginine residues. Other coagulation proteins have no enzymatic capabilities, but rather play regulatory roles in enhancing enzymatic activity. These proteins circulate in inactive forms, called procofactors. A procofactor must be cleaved by the appropriate enzyme to result in a fully active cofactor. Proteolytic activation of zymogens to enzymes and procofactors to cofactors can take several forms (Fig. 84.1). Many of these proteins contain disulfide bonds between two cysteine residues within the protein. Enzymatic cleavage of an activation site that lies between the cysteine residues will convert the protein from a single chain protein to a two chain protein of identical molecular weight, without removing any of the amino acid sequence. When the activation site does not lie between disulfide bound cysteine residues, cleavage of a single site will result in two different fragments of the initial protein. The smaller non-functional fragment is the activation peptide, while the remaining fragment is the fully activated enzyme or cofactor. Some of the proteins contain multiple activation sites. If both lie between disulfide-bridged cysteine residues, then an activation peptide is released and a twochain enzyme is produced. The order in which the target sites are cleaved may result in inactive intermediates, or partially or fully active intermediate enzymes. Complete cleavage of all of the sites will generally result in multiple fragments. Fully functional enzymatic complexes often consist of both an enzymatic subunit (the serine protease) and a regulatory subunit (the cofactor). Inhibition of Enzymatic Activity Coagulation and fibrinolysis enzymes may be inhibited through several general mechanisms: cleavage of the

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TABLE 84.1 Hemostatic Proteins, Factors, Cofactors, and Inhibitors Functional Classification

Abbreviation

Extrinsic pathway

TF FVII FVIIa

Tissue Factor Factor VII Factor VIIa

Thromboplastin Proconvertin

Cofactor Zymogen Enzyme

Contact pathway

FXII FXIIa PK K HK

Factor XII Factor XIIa Prekallikrein Kallikrein High molecular weight kininogen

Hageman factor

Zymogen Enzyme Zymogen Enzyme Cofactor

FXI FXIa FIX FIXa FVIII FVIIIa

Factor Factor Factor Factor Factor Factor

Plasma thromboplastin antecedent

FX FXa FV FVa Pro Fg Fb FXIII FXIIIa

Factor X Factor Xa Factor V Factor Va Prothrombin Fibrinogen Fibrin Factor XIII Factor XIIIa

α1PI AT HC TFPI PC APC PS TM PZ ZPI

α1-Protease inhibitor Antithrombin Heparin Cofactor II Tissue factor pathway inhibitor Protein C Activated protein C Protein S Thrombomodulin Protein Z Protein Z dependent protease inhibitor

PLG PLM scUPA

Plasminogen Plasmin Single chain urokinase plasminogen activator Two-chain urokinase plasminogen activator Single chain tissue-type plasminogen activator Two-chain tissue-type plasminogen activator

Intrinsic pathway

Common

Coagulation inhibitors

Fibrinolysis

tcUPA (uPA) sctPA tctPA (tPA) Fibrinolysis inhibitors

Nonspecific inhibitors

Common Use Name

Previous Names

XI XIa IX IXa VIII VIIa

Williams-Fitzgerald-Flaujeac factor

Christmas factor Antihemophilic factor Stuart-Prower factor Labile factor Factor II Factor I

α1-Antitrypsin Antithrombin III Extrinsic pathway inhibitor

Prourokinase Urokinase

Zymogen Enzyme Zymogen Enzyme Procofactor Cofactor Zymogen Enzyme Procofactor Cofactor Zymogen Substrate Final product Zymogen Enzyme Serpin Serpin Kunitz inhibitor Zymogen Enzyme, inhibitor Cofactor Cofactor Cofactor Enzyme Zymogen Enzyme Zymogen (slight enzyme) Enzyme Enzyme Enzyme

α2-AP PAI-1 TAFIa

α2-Antiplasmin Plasminogen activator inhibitor-1 Thrombin activatable fibrinolysis inhibitor

C1-INH α2-MG PN1 PCI

C1 inhibitor α2-Macroglobulin Protease nexin 1 Protein C inhibitor

enzyme, blockage of the active site, formation of stable complexes, and modification of the substrate (Fig. 84.2).39 Some inhibitors are enzymes that act by cleaving the target protein, cutting it into pieces that are no longer

Fletcher factor

Characterization

Plasmin inhibitor Carboxypeptidase U, B, or R

Serpin Serpin Enzyme

Plasminogen activator inhibitor 3

Inhibitor Inhibitor Inhibitor Serpin

functional. An example of this kind of inhibition is the cleavage of activated factor V (FVa) by activated protein C (APC). Blockage of the active site is an inhibitory mechanism that is commonly employed by the pharmaceutical

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FIGURE 84.1 Enzymatic activation of coagulation proteins. Most coagulation proteins circulate as inactive, single chain polypeptides containing disulfide bridges between nonadjacent cysteine residues. Depending on the location of the disulfide bridges and the cleavage site (or sites), enzymatic cleavage may result in a two-chain activated protein of identical size to the precursor, a two-chain activated protein (smaller than the precursor) and a released activation peptide, or a single chain activated protein (smaller than the precursor) and a released activation peptide.

industry and by some naturally occurring inhibitors. Small molecule inhibitors may fit directly into the enzymatic site, limiting the access of the substrate. Other inhibitors act by binding to the enzyme and altering it so it can’t be released. As a result, the target enzyme and inhibitor form a stable complex that permanently inactivates the enzyme. An example of this mechanism is inhibition of thrombin by antithrombin-heparin. Antithrombin (AT) is a member of the serpin (serine protease inhibitor) class of inhibitors. These inhibitors employ a “suicide-substrate” mechanism. They contain a small peptide loop that acts as “bait” that mimics substrates of the target enzyme. However, binding of the target enzyme to the bait results in serpin structural modifications that shift the position and conformation of the enzyme. A few inhibitors work by changing the nature of the substrate, so that the enzyme cannot bind effectively to its target site. The thrombin activatable fibrinolysis inhibitor (TAFI) acts via this mechanism by cleaving fibrin to limit plasmin-mediated fibrinolysis.

FIGURE 84.2 Coagulation enzyme and cofactor inhibition. Inhibitors work through a variety of different mechanisms: (1) Activated coagulation proteins can be inhibited by enzymes that cleave the target protein, damaging the structure and resulting in an inactivated protein. (2) Active site inhibitors bind the enzymatic site of the target enzyme, preventing the protease from cleaving its target protein. (3) Other inhibitors, such as serpins, display a “bait” peptide that looks like a substrate for the target enzyme. When the enzyme binds to and attempts to cleave the bait, the inhibitor changes in structure, resulting in a shift in position and structure of the target enzyme, which inactivates the enzyme into a stable complex with the inhibitor. (4) Some inhibitors modify the substrate rather than the target enzyme. These changes usually decrease binding interactions between the enzyme and the substrate, slowing or eliminating the ability of the enzyme to cleave its substrate.

Membrane Surface Interactions The rate of coagulation is profoundly affected by the presence of appropriate membrane surfaces for enzymatic reactions. Membrane-binding enhances enzyme

CHAPTER 84: OVERVIEW OF HEMOSTASIS

activity because localization to a membrane surface helps properly align the participating proteins. Tissue factor (TF) is the only coagulation protein that is permanently attached to the membrane surface.36 Some coagulation proteins (factors VII, IX, X [FVII, FIX, FX]), prothrombin, proteins C, S, Z [PC, PS, PZ]) contain gammacarboxyglutamic acid (Gla) residues that enable protein binding to a membrane surface via interaction with calcium and negatively charged phospholipids. Calcium-binding of the Gla domains of these proteins requires that the glutamic acid residues are carboxylated via intra-hepatic post-translational modifications involving the vitamin K cycle (Fig. 84.3). Incompletely carboxylated Gla proteins do not properly bind calcium and are unable to assemble on an activated membrane surface. The importance of carboxylation in membrane binding is dramatically illustrated by the profound adverse impact of vitamin K antagonists such as warfarin (See Chapter 85). The

639

requirement of calcium for membrane-binding explains the ability of calcium chelators (e.g. citrate and EDTA) to prevent activation of many coagulation enzymes. Some cofactors (factors V and VIII [FV and FVIII]) also have regions that interact with phospholipids, allowing fully functional enzymatic complexes to assemble on a membrane surface. The mechanism of membrane binding is less well described than that for Gla proteins, but involves the stereochemical configuration of the phosphatidylserine (PSer) head group.21,23 COAGULATION PROTEINS THAT GENERATE THROMBIN Previous coagulation models divided coagulation into the extrinsic, intrinsic and common pathways (Table 84.1, Fig. 84.4). The extrinsic pathway consisted of generation of active factor X (FXa) by the complex of TF

FIGURE 84.3 The vitamin K cycle. In hepatocytes this cycle is necessary for production of fully functional vitamin K dependent coagulation proteins. These proteins (prothrombin, FVII, FIX, FX, protein C, protein S, protein Z) are produced as precursors containing glutamic acid residues. The enzyme vitamin K γ-glutamyl carboxylase modifies these residues to γ-carboxyglutamic acid (Gla), producing zymogens containing Gla residues which are capable of binding Ca2+ and mediating membrane interactions. As a result of this enzymatic action, the vitamin K is converted to an epoxide, and must be regenerated via the activity of vitamin K epoxide reductase. Warfarin (and its derivatives) interfere with vitamin K epoxide reductase, preventing recycling of vitamin K epoxide to vitamin K quinol.

FIGURE 84.4 The cascade model of fibrin formation. This outdated model divides the coagulation system into separate, seemingly redundant pathways (extrinsic and intrinsic) either of which can result in the generation of FXa. The common pathway consists of formation of the prothrombinase complex (FXa-FVa) and results in generation of thrombin, followed by cleavage of fibrinogen to form fibrin. Many of the enzymes and enzymatic complexes require calcium (Ca2+) and binding to active membrane surfaces (PL) for full activity. For simplicity, inhibitors of the various enzymes and feedback activation of procofactors to cofactors have been omitted. See Table 84.1 for abbreviations.

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and activated factor VII (TF-FVIIa). The terminology “extrinsic” pathway arose from the requirement for TF (an extravascular, cell-bound protein) as a clotting trigger. The intrinsic pathway consisted of contact activation of factor XII (FXII), followed by a cascade consisting of activation of the downstream proteases factor XI (FXI) then FIXa-FVIIIa, ultimately resulting in FXa generation. This pathway was termed the “intrinsic” pathway due to the fact that it appeared to be intrinsic to blood, not requiring the addition of any extravascular trigger. The downstream portion of this cascade is still referred to as the intrinsic pathway, even though it is not truly intrinsic to blood. The common pathway consisted of the formation of the FXa-FVa complex, which generates thrombin. Thrombin then cleaves fibrinogen to fibrin, which spontaneously polymerizes into an insoluble fibrin gel.9 These classifications have been useful for in vitro laboratory evaluation, but such divisions do not occur in vivo. Contact Pathway (Kallikrein-Kinin Pathway) The contact pathway consists of the zymogens FXII (Hageman factor) and prekallikrein (PK), and the cofactor high molecular weight kininogen (HK; see Table 84.1). Activation of the two zymogens occurs spontaneously (auto-activation) upon assembly of the proteins on a suitable negatively charged surface.7 Because FXIIa can cleave downstream coagulation proteases, contact activation ultimately results in generation of thrombin and therefore formation of a fibrin clot. Consequently, contact activation (i.e. creation of the enzymes FXIIa and kallikrein) occurs whenever blood comes in contact with an artificial surface. Contact activation always occurs, to some extent, when blood is removed from the vascular system. The contact pathway is the source of blood’s apparent “intrinsic” clotting potential, and the reason blood coagulates when collected into a syringe or tube in the absence of anticoagulants. The amount of FXIIa and kallikrein generated depends on the negative charge of the artificial surface. Therefore, blood clots relatively slowly in plastic, more quickly in glass, and most rapidly

FIGURE 84.5 The contact pathway. This involves the assembly of FXII and K on a negatively charged surface, resulting in autoactivation of zymogen FXII to the enzyme FXIIa. FXIIa can then activate PK to K, and K can back activate more FXII to FXIIa. K also cleaves HK to generate BK while FXIIa can activate FXI to FXIa. The major products of contact activation have important roles unrelated to coagulation, in inflammation, vasoactive regulation, and fibrinolysis. See Table 84.1 for abbreviations.

when in contact with a strong negative charge. Note that contact activation is not calcium dependent, and consequently is not abolished by collection of blood into calcium chelators such as citrate or EDTA. Traditional coagulation cascade models often depict the contact pathway and the extrinsic pathway as independent and redundant generators of FXa, and subsequent thrombin and fibrin production (see Fig. 84.4). Recent advances clearly indicate that the contact pathway, while an important feature of in vitro coagulation, is irrelevant for in vivo hemostasis. The evidence to support this concept includes comparative genomics that reveal a complete absence of the FXII gene in cetaceans and non-mammalian vertebrates (see Chapter 91). In addition, many mammalian species (e.g. humans, mice, cats, and dogs) with inactivating FXII or PK mutations have no bleeding tendency.37 Recent work suggests that the contact pathway plays a role in pathologic thrombus formation.16,37 The physiologically relevant negatively charged surface that allows contact activation to influence coagulation in vivo has not been identified, although RNA,22 polyphosphate,46 and misfolded proteins26 have been proposed to serve this function. Contact activation has a defined role, however, in several physiologic processes. Contact activation on negatively charged phospholipid cell membranes is primarily responsible for bradykinin generation. The contact pathway has additional anti-adhesive, profibrinolytic, proinflammatory, and antithrombotic properties, and it is involved in angiogenesis (Fig. 84.5).7 Factor XII Factor XII is produced in the liver and circulates as a single chain molecule with molecular weight of about 80 kDa. Cleavage by either kallikrein or plasmin generates a two-chain active enzyme (FXIIa) composed of a light and heavy chain. The heavy chain contains two sites that bind to negatively charged surfaces, and the light chain is a serine protease. Autoactivation to FXIIa may occur when FXII binds to a negatively charged

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surface, especially in the presence of Zn2+. Prekallikrein, HK, and FXI are FXIIa’s primary coagulation substrates; however, it also activates the complement pathway and cleaves single chain urokinase plasminogen activator (scuPA) into its two chain form. C1-inhibitor (C1-INH) is the major inhibitor of FXIIa.7,44 Prekallikrein Prekallikrein is a single chain molecule of about 86 kDa expressed in many tissues, with highest levels in the liver. Approximately 75% of PK circulates bound to HK. Conversion of PK to kallikrein by FXIIa occurs on a negatively charged surface when HK is also bound. This generates a light chain (containing the serine protease) and a heavy chain. Autoactivation of PK at a different cleavage site results in a less active enzyme, β-kallikrein. Kallikrein’s primary substrates are FXII and HK. Like FXIIa, kallikrein also cleaves scuPA. Kallikrein is primarily inhibited by C1-INH and α2macroglobulin (α2-MG). Kallikrein is also inhibited by protein C inhibitor (PCI) and, when bound to HK, by heparin-AT.7 Kininogens High molecular weight kininogen and low molecular weight kininogen are multi-domain, distinct protein products of the same gene produced by alternative splicing. Both proteins contain an identical heavy chain (domains 1, 2, and 3) and domain 4. The light chain of HK contains domains 5 and 6, while the light chain of its low molecular weight form contains a different domain 5 and no domain 6. Kininogens have a multitude of functions unrelated to coagulation. Domains 1, 2, and 3, respectively, inhibit atrial natriuretic peptide, prevent calpain-mediated platelet aggregation, and prevent thrombin binding to platelets. Upon cleavage by kallikrein, domain 4 releases the critical vasoactive peptide bradykinin. Domain 5 of HK contains the binding site for charged-surfaces, has anti-adhesive properties affecting protein and cellbinding to neutrophils and endothelial cells, and also influences apoptosis. Domain 6 is the binding site for kallikrein and affects neutrophil activation. Endothelial cells, granulocytes, and platelets contain HK and express binding sites for HK on their resting membrane surface. Platelet activation results in dramatic increase in surface HK, due to release and increased expression of HK binding sites. Zn2+ is absolutely required for binding of HK to membrane surfaces.7 Extrinsic (Tissue Factor) Pathway Tissue Factor Tissue factor, also known as thromboplastin, CD142, and factor III, is a glycosylated single polypeptide chain of approximately 29.5 kDa. It is synthesized in a variety of cells, and expressed on the cell surface as an integral membrane protein. The extracellular portion of the

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molecule is responsible for interaction with coagulation proteins. The transmembrane domain anchors TF to the cell surface and is required for procoagulant activity. A small cytosolic domain is dispensable for coagulation functions, but is involved with intracellular signaling in TF’s role as a cell-surface receptor.34,36 In physiologic circumstances, TF is expressed by adventitial cells surrounding blood vessels larger than capillaries, differentiating skin keratinocytes, and by other epithelial cells, particularly in mucous membranes and organ capsules. This distribution is consistent with TF’s function as a protective “hemostatic envelope” whose role is instant initiation of coagulation. Under pathologic conditions, TF expression can be upregulated in other cell types by cytokines and other inflammatory mediators. Endothelial cells, vascular smooth muscle cells, monocytes, and granulocytes may express TF in disease states. Tissue factor is found in atherosclerotic plaques and is expressed by some malignant cells. Based on the lack of reports of TF deficiency in humans or animals, and the lack of viability of mice when the TF gene is deleted, TF expression appears to be essential for life.36 Tissue factor serves as the regulatory subunit of the TF-FVIIa enzymatic complex. Cell-surface TF, unlike most cofactors, fully functions as a cofactor without proteolysis. Although FVIIa is a weak enzyme alone, the TF-FVIIa enzyme complex is the most potent activator of coagulation. Consequently, TF-FVIIa is the first enzyme in the clotting cascade. Tissue factor is primarily sequestered outside the vasculature to prevent intravascular coagulation; however, TF antigen has been detected at low levels in blood. The source and role of this “blood-borne” TF are currently a matter of intense investigation. Blood-borne TF is thought to be associated primarily with microparticles and monocytes. It appears to accumulate in thrombi and may play a role in thrombus propagation. Circulating TF appears to be of limited procoagulant activity under normal circumstances. Some investigators believe that circulating TF is “encrypted” and must be cleaved by an enzyme for activity. Others believe that circulating TF is not fully active because the membrane surface on which it resides is not an appropriate procoagulant membrane.34,36 Factor VII Factor VII is a glycosylated single chain polypeptide with a molecular weight of approximately 50 kDa. It is a vitamin K-dependent protein produced in the liver and contains 10 Gla residues. It has the shortest half-life of the Gla proteins, so that FVII levels decrease rapidly if the vitamin K cycle or liver function is impaired. Factor VII binding to TF and to membrane surfaces via its Gla domain are calcium-dependent interactions. The FVII zymogen is converted to active FVIIa by proteolysis of a single peptide bond resulting in a two-chain molecule of identical size. Factor VIIa can be generated by a variety of downstream proteases, including FIXa, FXa, FXIIa, FVII activating protease, thrombin, and

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plasmin. Factor VII bound to TF can also be autoactivated by TF-FVIIa. The most important physiologic FVIIa activator has yet to be definitively determined.36 Factor VIIa is unique among coagulation enzymes in that some of it circulates with a long plasma half-life of approximately 2 hours (as compared to other serine proteases whose enzyme half-lives are measured in seconds). Approximately 1% of total circulating FVII antigen is active FVIIa, although inter-individual FVIIa levels vary considerably.35 The intriguing question of how and where circulating FVIIa is generated remains unanswered at this time. Factor IXa may be an important contributor of FVIIa generation, as patients deficient in FIX (hemophilia B) have very low levels of FVIIa.50 Factor VIIa levels are higher in lymph than plasma, suggesting that some FVIIa may be generated outside the vasculature and returned to the bloodstream via lymphatic circulation.32 Because some FVII circulates as FVIIa, exposure of blood to TF as a result of injury leads to formation of both TF-FVII complexes and TF-FVIIa complexes. The initial TF-FVIIa formation and autoactivation of TFFVII to additional TF-FVIIa is believed to generate sufficient enzymatic activity to trigger the clotting cascade via cleavage of the primary TF-FVIIa substrates, FIX and FX.36 The TF-FVIIa complex is often referred to as extrinsic tenase when its target substrate is FX. Deficiency of FVII is associated with a highly variable, but potentially severe bleeding tendency.29 Free plasma FVIIa is not inactivated by any plasma protease inhibitor, hence its long half-life. The TF-FVIIa complex is inhibited by the tissue factor pathway inhibitor (TFPI)-FXa complex, and by heparin-AT (albeit very slowly).36 The inhibition of TF-FVIIa by heparinAT is unique, in that heparin-AT binding to FVIIa releases a stable FVIIa-AT complex and regenerates fully functional TF. Consequently, FVIIa-AT levels may be an indicator of systemic TF exposure.29 Intrinsic Pathway The intrinsic cascade primarily occurs on the activated platelet surface. Platelet activation is a critical component of the function of the intrinsic tenase complex (FIXa-FVIIIa). Upon activation, platelets expose many (approximately 6000 per platelet) binding sites for FIXa as well as 1000–2000 each for the cofactor, FVIIIa, and the substrate FX. It is unclear whether these binding sites are simply the negatively-charged membrane surface, or whether a specific protein receptor is also involved. Regardless, co-localization of FIXa, FVIIIa, and FX on the platelet surface increases the rate of FXa generation by many million fold. Factor XI Factor XI is an approximately 160 kDa protein that is produced in the liver. Its structure is unique among coagulation proteins as a disulfide-linked dimer of two identical polypeptide chains. Factor XI circulates in plasma complexed with HK, which is required for FXI binding to negatively charged surfaces. Prothrombin

can substitute for HK as a cofactor for binding to platelet surfaces. Factor XI can be activated by FXIIa, FXIa, and thrombin in calcium-dependent reactions. Cleavage results in a 160 kDa enzyme with two disulfide linked heavy chains and two active site-containing light chains. Because FXII deficiency does not cause a bleeding tendency, it is likely that thrombin (rather than FXIIa) is the physiologically relevant enzyme generating FXIa. The GP Ibα subunit of the platelet GP Ib/IX/V complex binds FXI, localizing it and thrombin on the platelet for efficient cleavage. After activation, FXIa remains surface-bound where it cleaves its substrates, FXI and FIX. Factor IX is FXIa’s preferred substrate. Inhibition of FXIa depends on its localization. Plasma inhibitors include its major inhibitor, α1 protease inhibitor (α1-PI), and C1-INH, α 2-antiplasmin (α 2-AP), AT, protease nexin 1, plasminogen activator inhibitor 1 (PAI-1), and PCI. Platelet-bound FXIa is protected from α1-PI inactivation. When bound to the endothelial cell surface, protease nexin 2, in concert with surface heparan sulfate proteoglycans (HSPGs), strongly inhibits FXIa. This differential inhibition may serve to restrict FXIa mediated cleavage of FIX to the platelet surface. Factor IX Factor IX’s structure and size (about 55 kDa) are similar to those of FVII. Factor IX is a glycosylated single-chain polypeptide, produced in the liver. Post-translational carboxylation of 12 residues by the vitamin K cycle is required for normal Gla domain function and consequently for full-membrane binding capability.2 Tissue factor-FVIIa or FXIa activate FIX bound (in the presence of calcium) to a procoagulant membrane surface. Cleavage of two FIX sites releases a 35-residue activation peptide, resulting in a two-chain enzyme of about 45 kDa. The catalytic domain is in the heavy chain portion of the molecule. The presence of detectable levels of the FIX activation peptide in blood suggests that coagulation is ongoing, but controlled by inhibitory pathways.2 The primary substrate for FIXa is FX. The FIXa-FVIIIa complex on a suitable membrane surface (usually the platelet surface) in the presence of calcium is a powerful generator of FXa. Either FIX or FIXa may bind to endothelial cells, via specific receptors (collagen IV). Binding to the endothelial surface supports activation of FX by FIXa-FVIIIa in the absence of platelets. Factor IXa alone (without its cofactor FVIIIa) is able to activate FVII in the absence of TF. FIXa is primarily inhibited by AT-HSPG, although at a relatively slow rate compared to AT’s inhibition of FXa or thrombin. Factor VIII Factor VIII synthesis and secretion are extremely complex. Two distinct interactions involving metal ions (copper and calcium/manganese) are vital for normal FVIII structure and function. The primary site of FVIII

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production is unclear, although the liver and the reticuloendothelial system are strongly implicated. Factor VIII is synthesized as a single chain protein containing three A domains, a B domain, and two C domains, with multiple free cysteine residues and disulfide bonds. After translation, FVIII is translocated to the endothelial reticulum (ER), for modification. Factor VIII is retained within the ER by two protein chaperone systems, immunoglobulin-binding protein and calnexin/calreticulin. Improperly folded FVIII is extracted from the ER and degraded. Factor VIII then transits to the Golgi apparatus for further processing, which includes cleavage at two sites. The resulting protein consists of a heavy chain (160 or 200 kDa) and a light chain (80 kDa) which are not connected to one another unless von Willebrand factor (VWF) is also bound.23 Binding to VWF enables proper association of the two FVIII chains and markedly improves its intracellular and plasma stability. The ratio of the two proteins is maintained at one molecule of FVIII to 50–100 subunits of VWF. Although somewhat species-dependent, VWF deficiency is associated with lower levels of circulating FVIII, and infusion of desamino-8-D-arginine vasopressin (DDAVP) to increase circulating VWF levels elicits a concomitant increase in FVIII. Some FVIII is stored with VWF in storage granules, such as WeibelPalade bodies of endothelial cells and α granules of platelets. Factor VIII can be dissociated from VWF by exposure to negatively-charged phospholipids. Thrombin cleavage of FVIII at multiple sites activates FVIII to FVIIIa and releases it from VWF. Factor VIII can be activated by FXa, although the maximal activation by thrombin is greater than FXa. Factor VIIIa interacts with PSer on membrane surfaces by stereo-selective recognition of the PSer head group via a phospholipid-binding site in the light chain. Factor VIIIa also interacts with two platelet receptors, GP Ib and αIIbβ3. Platelet binding of FVIII is inhibited by VWF. The FIXa-FVIIIa enzyme/ cofactor complex assembles in a calcium-dependent manner on procoagulant phospholipid surfaces, and is referred to as intrinsic tenase, because its target substrate is FX. Factor VIIIa is inherently labile due to spontaneous dissociation of its subunits; however, FIXa binding stabilizes FVIIIa and delays decay. Factor VIIIa is cleared from the circulation by low density lipoprotein receptor-related protein (LRP), a common cell-surface receptor found on a variety of cell types. Proteolytic cleavage by activated protein C (APC) and FXa also inactivates FVIIIa.23 Common Pathway Factor X Factor X is a Gla protein of approximately 59 kDa. It is a glycosylated single-chain polypeptide requiring posttranslational carboxylation of 11 glutamic acid residues. Factor X is activated to FXa by either extrinsic tenase (TF-FVIIa) or intrinsic tenase (FIXa-FVIIIa) on a suitable membrane surface in the presence of calcium. The acti-

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vation is due to a single cleavage site and results in a 46 kDa enzyme. Factor Xa forms the prothrombinase complex with its cofactor FVa. Factor Xa is strongly inhibited by ATHSPG and by TFPI. It is also inhibited by the enzyme/ cofactor protein Z-dependent protease inhibitor (ZPI)/ PZ. Inhibition by ZPI/PZ is calcium and phospholipid dependent. Factor V Factor V circulates in plasma as a glycoprotein of approximately 300 kDa. Portions of the molecule are very similar in structure to FVIII. Like FVIII, FV contains a copper ion, and requires post-translational modifications and molecular chaperones for successful cellular trafficking. Approximately 20% of FV in blood is contained within platelet α granules. The plasma half-life of FV in man is fairly long (approximately 13 hours).21 Factor V is activated to FVa by cleavage at two sites that releases a large activation peptide, and a resultant two-chain cofactor of about 160 kDa. Activation is primarily due to thrombin cleavage, but FXa can also activate FV when both are bound to a procoagulant membrane surface. Activation by FXa is less efficient than activation by thrombin; however, FV released from platelets is somewhat more susceptible to FXa activation than plasma FV.21 Factor Xa and FVa form an enzyme/cofactor complex, referred to as the prothrombinase complex, in the presence of calcium on PSer-containing membrane surfaces. While FVa binding to the membrane is not calcium dependent, the binding of FX and prothrombin to the membrane is calcium dependent, but of low affinity. Binding of FVa to a negatively charged membrane surface markedly increases the affinity of FXa for FVa, and enhances formation of the complex. Platelets and microparticles primarily provide this membrane surface in vivo.21 Proteolysis of FVa by APC results in its inability to bind prothrombin, and therefore loss of function of the prothrombinase complex. Inactivation of FVa by APC occurs more rapidly on endothelial cells than on activated platelet surfaces.12,21 Prothrombin Prothrombin is a 72 kDa glycoprotein synthesized in the liver that (like other Gla proteins), undergoes vitamin K-dependent post-translational carboxylation of 10 glutamic acid residues. Prothrombin is the second most abundant coagulation protein in plasma, after fibrinogen.20 In addition to the Gla domain, prothrombin contains two kringle domains and a serine protease precursor domain. Prothrombin cleavage at two distinct sites by prothrombinase generates its most abundant active form, α thrombin. The first in vivo cleavage site is generally between the A and B chains of its serine protease domain to form the important intermediate, meizothrombin. Meizothrombin contains all the domains of

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the prothrombin molecule, with a fully active enzyme site, but it is less active toward fibrinogen and platelets than α thrombin. After the second cleavage, prothrombin fragments 1 (containing the Gla domain and kringle 1) and 2 (containing kringle 2 domain) are removed. The A and B chains of the much smaller (36.7 kDa) α-thrombin molecule remain. Under some conditions the order of cleavage is reversed, releasing the membrane binding Gla domain prior to activation of the protease.20 Coagulation Inhibitors Tissue Factor Pathway Inhibitor Tissue factor pathway inhibitor is the key regulator of the extrinsic pathway. It is a Kunitz type inhibitor synthesized by endothelial cells and expressed on their surface. Alternative splicing creates two forms: TFPIα and TFPIβ. Both forms are bound to the cell surface: TFPIα via a not yet identified glycosylphosphatidylinositol (GPI)-linked protein, and TFPIβ via interactions between the C terminal region and an identified GPI anchor.41 Cell surface associated heparan sulfates also play a role in the binding of TFPI to the endothelial cell.1,30 The majority (80–85%) of TFPI is attached to the endothelial cell surface. A small amount (10%) of TFPI circulates in plasma associated with lipoproteins, and platelet α granules also contain TFPI.1 Intravenous heparin administration causes release of TFPI from the endothelial cell-surface and a marked increase in plasma TFPI.30 Free TFPI is cleared from the circulation primarily by the liver and kidneys. Both TF-FVIIa and FXa are susceptible to TFPI inhibition. In the absence of FXa, TFPI is only a weak inhibitor of TF-FVIIa. The second Kunitz domain of TFPI binds to FXa; however, the FXa-FVa complex is protected from inhibition by TFPI, particularly if the substrate prothrombin also binds.31 Recent work indicates that PS serves as a TFPI cofactor, enhancing its inhibitory function,43 and that polyphosphate released from platelet dense granules profoundly abrogates TFPI inhibition.46 The TFPI-FXa complex is a strong inhibitor of TFFVIIa, binding via the first Kunitz domain. Endocytosis and degradation of the quarternary complex (TFPIFXa-TF-FVIIa) is mediated by cell-surface associated LRP.1 Inherited deficiency of TFPI has not been described, suggesting that complete lack of this inhibitor is incompatible with life.

but C1-INH inhibition is potentiated by glycosaminoglycans such as heparin and heparan sulfates.49 The C1-INH is the major inhibitor of the contact pathway, acting on FXIIa and kallikrein, and the downstream protease FXIa. Hereditary deficiency of C1-INH has no impact on hemostasis. Rather, it causes excess episodic production of bradykinin, producing the disease phenotype hereditary angioedema.49 Antithrombin Antithrombin is the prototype serpin; however, it is a relatively inefficient inhibitor of its target enzymes. The “bait” portion of the AT molecule is available only if AT binds to its cofactor. This cofactor is a specific pentasaccharide sequence that is found on approximately 30% of pharmaceutical heparin molecules and on endothelial-bound HSPG molecules. Upon AT-pentasaccharide binding, the bait loop becomes available as a target for the enzyme to be inhibited. Antithrombin’s inhibitory activity is enhanced up to several thousand-fold by interaction with heparins, but the impact varies with heparin structure and the target protease. Pentasaccharide alone is sufficient for AT inhibition of some enzymes, while longer heparin molecules are required for inhibition of others. In particular, AT inhibition of thrombin requires, in addition to the specific pentasaccharide sequence, heparin molecules of at least 18 sugar units to interact with thrombin. Antithrombin molecules in circulation have limited inhibitory capabilities in the absence of the necessary pentasaccharide. Endogenous activation of AT occurs when it binds to heparans on the surface of the endothelial cell. Additionally, endothelial cell surface thrombomodulin (TM) can also bind AT, which enhances AT inhibition of thrombin approximately 8-fold.4,48 In addition to inhibiting thrombin, AT-heparin has broad inhibitory activity against a variety of serine proteases. These include FXIIa, FXIa, kallikrein, FIXa, and FVIIa. The primary targets are FXa and thrombin. Platelet-bound thrombin (via GP Ibα) and thrombin bound to the fibrin clot or fibrin degradation products are refractory to AT-heparin inhibition. In contrast, inhibition of FVIIa requires that the enzyme be bound to its cofactor (TF) on a cell surface, and kallikrein is more readily inhibited when bound to HK on a cell surface. Antithrombin bound to a target protease forms a stable complex with long half-life. The complexes are primarily cleared in the liver by LRP. Inherited or acquired AT deficiency is associated with development of pathologic thrombosis.4 Protein C Pathway

C1-Inhibitor C1-Inhibitor is the largest member of the serpin family. It has an unusual structure for this class of proteins, containing a typical serpin domain and an additional Nterminal domain of ill-defined structure and function. It is a relatively slow inhibitor of its target proteases,

The PC pathway involves a complex interplay among endothelial cell-surface molecules, thrombin, PC, and PS. Activated PC is an essential regulator of thrombin generation due to its inactivation of the critical cofactors, FVa and FVIIIa. The thrombotic disorders in people attributed to inherited and acquired PC deficiency demonstrate its important anticoagulant action.

CHAPTER 84: OVERVIEW OF HEMOSTASIS

Protein C is an approximately 62 kDa vitamin Kdependent protein, synthesized in the liver, containing a Gla domain with nine glutamic acid residues. It circulates as a two-chain zymogen that is activated by thrombin (or meizothrombin) to APC. Cleavage at two sites results in removal of a 12-residue activation peptide and formation of an approximately 56 kDa two-chain enzyme, APC. Plasmin activation also produces limited amounts of APC. As a Gla containing protein, PC activation is calcium dependent and markedly enhanced by negatively charged phospholipid surfaces, in particular phosphatidylethanolamine (PEth). Thrombin alone is a poor activator of PC unless it is bound to TM.12 Thrombomodulin is an endothelial transmembrane molecule that interacts with thrombin to influence its substrate specificity. Thrombomodulin inhibits thrombin’s interaction with fibrinogen and protease activated receptors, and markedly enhances the ability of thrombin to cleave PC. Cellular expression of TM is modulated by a large number of factors. It is upregulated in response to cyclic AMP, thrombin, vascular endothelial growth factor, and platelet factor 4. It is down-regulated by inflammatory mediators (e.g. tumor necrosis factor, interleukin 1, endotoxin), hypoxia, transforming growth factor β, and neutrophil activation.12 Protein S is an approximately 69 kDa vitamin K dependent protein with a Gla domain that interacts with membranes. Protein S is the cofactor for APC mediated cleavage of FVa and FVIIIa, and for TFPI’s inhibition of FXa.43 Approximately 60% of circulating PS is bound to a large (540 kDa) member of the complement system, C4 binding protein (C4BP). C4 binding protein is an acute phase reactant and therefore increases in inflammatory conditions. Protein S complexed with C4BP is unable to provide APC cofactor activity. Activated platelets and neutrophils contain proteases that cleave and inactivate PS.11,12 Endothelial protein C receptor (EPCR) is a transmembrane protein found primarily on arterial endothelium. The EPCR can substitute for negatively-charged phospholipids in promoting activation of PC. It binds directly to PC via the Gla domain, anchoring it to the cell surface. The APC-EPCR complex is unable to bind PS and does not have anticoagulant activity, but may interact with other receptors, potentially contributing to the anti-inflammatory effects of APC.12,13 Once APC is produced and dissociates from thrombin-TM or EPCR, it assembles with PS on a procoagulant membrane surface and rapidly inactivates FVa and FVIIIa. A mutation at one of the APC cleavage sites on FV (“FV Leiden”) confers resistance to APC inactivation, and is a common hereditary prothrombotic disorder in people. Activated PC cleaves FVa more efficiently on endothelial surfaces than on platelets,38 and FVIIIa is resistant to APC inactivation when bound to VWF. The anticoagulant activity of APC is highly species-specific.12 The PC pathway appears to have a major role in linking inflammation and coagulation, in part due to the ability of APC to down-regulate thrombin generation. Additionally, APC alters gene expression

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in endothelial cells via activation of membrane surface protease activated receptors (PARs), resulting in antiapoptotic and anti-inflammatory effects. Activated PC-PS also inactivates an important inhibitor of fibrinolysis (PAI-1).12 Activated PC has a relatively long half-life of about 15 minutes. It is inhibited by α1-PI, PCI, and α2-MG. Miscellaneous Inhibitors Heparin cofactor II (HC) is a 66 kDa serpin with some similarities to AT. It requires binding to proteoglycans for full function, but the mechanism differs from that of AT. Heparin cofactor II is strongly activated by binding to dermatan sulfate and has a narrow spectrum of activity restricted to thrombin inhibition. Although HCthrombin complexes are detectable in vivo, HC deficiency is not associated with increased risk of thrombosis. Roles for HC in inflammation and wound healing, rather than in regulation of coagulation, have been suggested.4 Protein Z-dependent protease inhibitor is a 72 kDa serpin that strongly inhibits FXa when bound with its cofactor, PZ, on a phospholipid surface in the presence of calcium. Protein Z is a 62 kDa vitamin K-dependent protein that binds to the membrane via its Gla domain. Thrombomodulin also acts as a cofactor for ZPI. Factor XIa is also susceptible to ZPI inactivation, in a heparindependent reaction that does not require calcium or phospholipids. α1-Protease inhibitor, PCI, and α2-MG are inhibitors that act on a variety of serine proteases in coagulation and other systems. α1-Protease inhibitor is a 52 kDa molecule, synthesized primarily in the liver, whose major physiologic role is the protection of alveolar tissue from proteolytic damage by enzymes like neutrophil elastase. It is an acute phase reactant, and is up-regulated by inflammatory mediators.14 Protein C inhibitor is a 53 kDa serpin that inhibits APC, FXa, FXIa, kallikrein, thrombin, thrombin-TM, urokinase plasminogen activator (uPA), and tissue plasminogen activator (tPA). Its inhibitory activity is enhanced by heparin and it is involved in modulating vascular permeability, tissue regeneration, proteolysis in the kidney, and tumor cell invasion.47 α2-Macroglobulin is a large, 725 kDa inhibitor of matrix metalloproteinases that works through a unique mechanism. The target enzyme cleaves an α2-MG “bait” region to produce a conformational change that provides sites for covalent enzyme attachment. The enzyme binds and is entrapped by the inhibitor, with resultant α2-MG-enzyme complexes internalized and degraded via LRP binding. CELLS THAT PARTICPATE IN HEMOSTASIS Endothelial Cells The endothelium, once considered an inactive barrier between the blood and the subendothelial tissues, is now recognized as a highly active organ. The endothe-

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lium has a variety of functions including regulation of coagulation and fibrinolysis, local control of vascular tone, systemic regulation of blood pressure, interaction with lipoprotein metabolism, production of angiotensin converting enzyme, presentation of histocompatibility antigens, and interaction with leukocytes. The majority of endothelial cells are in capillary beds due to the surface area distribution of the vascular system, and most interactions between the endothelium and flowing blood occur in capillaries where the ratio of endothelial cell surface to blood volume is high. Antithrombotic Properties of Endothelium The prevention of coagulation in flowing blood is normal endothelium’s primary role in hemostasis (Fig. 84.6). Non-activated, resting endothelial cells have anticoagulant properties, in part due to proteins expressed on their surface and via a neutral membrane charge that is incapable of supporting coagulation reactions. Heparan sulfate proteoglycans are glycosaminoglycans expressed by endothelial cells on the luminal surface, in contact with flowing blood. Much larger abluminal stores of HSPG act as a reservoir. Antithrombin bound to HSPGs (at pentasaccharide sequences) is then able to inactivate its serine protease targets (e.g. thrombin, FXa). Resting endothelial cells also express TM. Thrombin, once bound to TM, demonstrates anticoagulant rather than procoagulant properties. Thrombomodulin-bound thrombin is unable to cleave fibrinogen or activate PARs, and the TM-thrombin complex rapidly activates PC. Expression of TM is 100fold higher in capillary endothelium compared to endothelium in the major vessels. Thrombin circulating in large vessels is therefore quickly extracted when the blood passes through capillaries. Lastly, TFPI on the endothelial cell surface prevents thrombin generation by acting as an upstream inhibitor of FXa and TF-FVIIa.

Prothrombotic Properties of Endothelium Endothelial cells store VWF in granules called WeibelPalade Bodies. This protein has important roles in FVIII biology, and in mediating platelet adhesion and aggregation (Chapter 81). Endothelial cells express a specific G protein coupled receptor for thrombin. Thrombin binding to this receptor initiates a number of procoagulant and proinflammatory events. Intracellular signaling initiates endothelial secretion of VWF and P-selectin from Weibel-Palade bodies, nitric oxide (NO) release, and phospholipase A2 activation that results in platelet activating factor (PAF) production. Activated endothelial cells express intracellular adhesion molecule 1, generate interleukin 1, and secrete both plasminogen activators and PAI-1. Other Properties of Endothelial Cells Endothelial cells synthesize NO in response to a variety of stimuli including thrombin, histamine, ATP, bradykinin, and acetylcholine. Nitric oxide inhibits platelet activation and causes vasodilation. Thrombin and other stimuli also induce endothelial cell synthesis of prostacyclin (PGI2), a platelet inhibitor that also maintains vascular relaxation.8 Platelets Platelets are a fundamental requirement for normal hemostasis. Activated platelets adhere to the site of injury, secrete molecules necessary for coagulation and for wound healing, and form platelet aggregates that obstruct the site of injury, preventing further blood loss. Platelets provide the primary membrane surface for thrombin generation, allowing for formation of a platelet-fibrin clot (Chapter 76).

FIGURE 84.6 Anticoagulant properties of endothelial cells. Endothelial cells are thromboresistant under normal physiologic conditions. They express adenosine diphosphatase (ADPase) and synthesize and release prostacyclin (PGI2) and nitric oxide (NO), leading to inhibition of platelet aggregation. They also produce urokinase (uPA) and tissue-type (tPA) plasminogen activators which activate plasminogen to plasmin, the enzyme that lyses fibrin clots. Endothelial cells express thrombomodulin (TM), heparan sulfate proteoglycans (HSPG) and tissue factor pathway inhibitor (TFPI) on their membrane surface, which collectively inhibit generation of thrombin (Th). TM acts as a cofactor for Th-mediated activation of the protein C (PC) pathway; HSPG acts as a cofactor for antithrombin (AT) inhibition of FXa; TFPI inhibits FXa and TF-FVIIa.

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Tissue Factor Bearing Cells As previously discussed, exposure of TF on a TF-bearing cell is the physiologically relevant trigger for in vivo coagulation. Tissue factor-bearing cells are primarily outside the vasculature, particularly in the adventitia, skin, mucous membranes, and organ capsules. Under pathologic conditions leukocytes, endothelial cells, and neoplastic cells may also express TF. Microparticles Microparticles (MPs) are intact vesicles surrounded by membranes. They range in size from 2–20% of the size of a red blood cell (i.e. 0.1–1 μm diameter) and arise when activated or apoptotic cells shed bits of membrane. Cytokines (such as tumor necrosis factor and interleukin-6), thrombin, shear stress, and hypoxia can stimulate MP formation. Under normal conditions, MPs are primarily derived from endothelial cells, platelets, and monocytes, but in certain disease states, MPs may arise from granulocytes and erythrocytes. The quantity of circulating MPs is increased in certain illnesses such as diabetes mellitus, sepsis, and cardiovascular disease and may contribute to pathologic coagulation in a variety of disorders.33 Microparticles contain cell surface proteins similar to those found on their parent cell (e.g. ultra-large VWF monomers on endothelial cell-derived MPs, TF on monocyte-derived MPs) that can participate in coagulation reactions, especially when the MP expresses a procoagulant surface. The contribution of MPs to normal hemostasis is currently under active investigation.24,33,40 The Role of Cell Surface Membranes All cells are surrounded by a lipid membrane bilayer which contains a large number of membrane surface proteins. In most mammalian cells, membrane bilayers consist primarily of cholesterol and phospholipids with neutral head-groups such as phosphatidylcholine (PChol), sphingomyelin, sugar-linked sphingolipids and PEth. The membranes also contain phospholipids with negatively charged-head groups, primarily PSer. In the resting state, PEth and PSer are sequestered on the membrane’s inner surface. This membrane asymmetry is essential and tightly controlled by lipid transporters. Flippase actively transports PSer from the external to the internal leaflet while floppase transports PChol in the opposite direction. These ATP-dependent enzymes maintain cell membrane asymmetry in the resting state. Cell-injury or activation stimuli that produce a sustained increase in intracellular calcium induce a poorly characterized “scramblase” that shuffles phospholipids between the membrane leaflets. This results in the appearance of PSer and PEth on the external membrane surface.40,51 Although the mechanisms of coagulation on activated membrane surfaces are not fully understood, it is known that membrane-surface PSer markedly (often by a factor of thousands to hundreds of thousands) increases the rate of coagulation reactions. The

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presence of PEth enhances the properties of PSerexpressing membranes, so that less PSer is required for maximum speed when PEth is present. It has been proposed that PEth aids in grouping PSer into clusters that support preferential binding of coagulation proteins.36,53 Under normal physiologic conditions, cells do not express these procoagulant phospholipids on their outer membrane. As a consequence, thrombin production is limited to cell surfaces in an injured area that have been triggered to externalize PSer (and PEth). The ability of cells to control the composition of their membrane surface constitutes a powerful method of localizing coagulation reactions.53 CELL BASED MODEL OF THROMBIN GENERATION Our new understanding of hemostasis incorporates the role of cells. This model suggests that in vivo coagulation occurs in distinct overlapping phases that require the participation of two different cell types: TF-bearing cells and platelets. Initiation All evidence to date indicates that TF is the sole relevant initiator of coagulation in vivo. Cells expressing TF are generally localized outside the vasculature, which prevents initiation of coagulation under normal flow circumstances with an intact endothelium. Some circulating cells (e.g. monocytes) and MPs may express TF on their membrane surface, but this TF under normal conditions is thought to be inactive or “encrypted”.27,36 Upon injury, flowing blood is exposed to a TFbearing cell and FVIIa rapidly binds to TF.35,36 The TFFVIIa complex then activates additional TF-FVII to TF-FVIIa, which generates small amounts of FIXa and FXa. Although it occurs slowly, FV can be activated directly by FXa. The resultant enzyme and cofactor (FXa, FVa) form the prothrombinase complex, which subsequently cleaves prothrombin to generate a small amount of thrombin on the surface of TF-bearing cells. Any FXa that dissociates from the membrane surface is rapidly inactivated by either TFPI or AT-HSPG. Factor Xa, therefore, is effectively restricted to the surface of the TF-bearing cell on which it was generated. However, FIXa is not inhibited by TFPI, and much more slowly inhibited by AT than FXa. Consequently, trace amounts of generated FIXa can dissociate and move to the surface of nearby platelets or other cells.19,42 Since TF is always expressed in the perivascular space, any FVIIa that leaves the vasculature will bind to TF and potentially initiate coagulation. The gaps in the physiologic endothelial envelope under normal conditions are very small. Most of the upstream coagulation proteins are relatively small, whereas some of the downstream proteins are much larger. These downstream substrates, as well as platelets, are sequestered from the extravascular space. Coagulation progresses beyond initiation (and its generation of trace amounts

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of thrombin) only when the injury allows platelets and larger proteins to leave the vascular space and adhere to extravascular TF-bearing cells (Fig. 84.7A).19,42 Amplification Once a small amount of thrombin has been generated in the initiation phase, it is available to activate platelets that have leaked from the vasculature at the site of injury. Thrombin binding to platelet surface receptors causes extreme changes in the surface of the platelet, resulting in shape change, shuffling of membrane phospholipids to create a procoagulant membrane surface, and release of granule contents that provide additional “fuel for the fire.” Platelet granules contain a large number of proteins and other substances which include raw materials for clotting reactions, agonists to induce further platelet activation, and calcium. Calcium may induce membrane externalization and clustering of PSer, and promotes binding of coagulation proteins to the activated membrane surface. In addition to activating platelets, thrombin generated in the initiation phase cleaves FXI to FXIa and activates FV to FVa on the platelet surface. Thrombin also cleaves VWF from FVIII and activates FVIII to FVIIIa (Fig. 84.7B).19,42 Propagation The release of platelet granule contents recruits additional platelets to the site of injury. The propagation phase occurs on the surface of these platelets. Expression of ligands on their surface results in cell-cell interactions that lead to platelet aggregation. Factor IXa generated by TF-FVIIa in the initiation phase can bind to FVIIIa (generated in the amplification phase) on the platelet surface. Additional FIXa is generated due to cleavage of FIX by FXIa (generated during amplification on the platelet surface). Once the intrinsic tenase complex forms (FIXa-FVIIIa) on the activated platelet surface, it rapidly begins to generate FXa on the platelet. Since FXa generated on TF-bearing cells is rapidly inhibited if it moves away from the cell surface, it cannot easily reach the platelet surface. The majority of FXa in the propagation phase, therefore, must be generated directly on the platelet surface by the intrinsic tenase complex. This generated FXa then rapidly binds to FVa (generated by thrombin in the amplification phase) and cleaves prothrombin to thrombin. This prothrombinase activity results in a burst of thrombin. When enough thrombin is generated with enough speed to cleave fibrinogen, a clot forms (Fig. 84.7C).19,42 THROMBIN Thrombin is generated from prothrombin by cleavage at two activation sites. Thrombin is the terminal coagulation protease responsible for fibrin formation. Thrombin generation, however, is not the end of coagulation. Many steps vital to normal hemostasis occur temporally after generation of a fibrin gel. Approximately 5% of the

A

B

C FIGURE 84.7 Cell-based model of fibrin formation. In this model thrombin generation occurs in overlapping phases. (A) Initiation phase: This phase occurs on the TF-bearing cell and is initiated when injury exposes the TF-bearing cell to the flowing blood. It results in the generation of a small amount of FIXa and thrombin that diffuse away from the surface of the TF-bearing cell to the platelet. (B) Amplification phase: In the second phase, the small amount of thrombin generated on the TF-bearing cell activates platelets, releases VWF and leads to generation of activated forms of FV, FVIII, and FXI. (C) Propagation phase. In the third phase the various enzymes generated in earlier phases assemble on the procoagulant membrane surface of the activated platelet to form intrinsic tenase, resulting in FXa generation directly on the platelet surface. Prothrombinase complex forms and results in a burst of thrombin generation directly on the platelet. See Table 84.1 for abbreviations. (Reproduced from Smith S, The cell-based coagulation model. J Vet Emerg Crit Care 19:3–10, with permission.)

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types. Cleavage by thrombin untethers a portion of the PAR molecule that self inserts into the receptor, thereby initiating intracellular signalling. This PAR-mediated signaling is the major mechanism by which thrombin activates platelets. Thrombin also binds to the platelet GP Ib/IX/V complex via a binding site in GP Ibα, although how this binding mediates platelet activation is not clearly understood.10 Fibrin Formation FIGURE 84.8 Central role of thrombin. Thrombin plays a central role in hemostasis, acting as both a procoagulant and an anticoagulant. Procoagulant thrombin is generated in small quantities during the initiation phase (which occurs on the TF-bearing cell), then in larger quantities on the platelet surface in the propagation phase. Thrombin is responsible for platelet activation which provides the procoagulant surface needed for the burst of thrombin generation during propagation. Thrombin cleaves fibrinogen to fibrin, resulting in clot formation. Thrombin also impacts clot structure through activation of cross-linker FXIII, and fibrinolysis inhibitor TAFI. Thrombin becomes anticoagulant via thrombomodulin-mediated activation of the protein C system. See Table 84.1 for abbreviations. (Reproduced from Smith S, The cell-based coagulation model. J Vet Emerg Crit Care 19:3–10, with permission.)

prothrombin that will be cleaved has been converted to thrombin at the time of fibrin gel formation, as detected in routine plasma-based clotting assays.28 Free thrombin has an extremely short plasma halflife of less than 15 seconds due to rapid inhibition by AT-HSPG.10 Thrombin bound to the fibrin clot, however, is both enzymatically active and protected from inhibition. In addition to its function in cleavage of fibrinogen to fibrin, thrombin has many additional roles in coagulation and fibrinolysis, as well as functions in cell regulation (Fig. 84.8).28 Feedback Regulation of Coagulation Thrombin provides positive feedback to the process of coagulation by back-activating a number of upstream proteases and cofactors. These include FVII, FXI, FVIII, and FV. Thrombin bound to endothelial surface TM acts primarily to inhibit coagulation via activation of the PC anticoagulant pathways. Moreover, TM-binding inhibits the interactions between thrombin and fibrinogen, limits thrombin activation of platelet PARs, and promotes activation of the fibrinolytic mediators, TAFI and scuPA. Through these feedback mechanisms, thrombin influences the quantity and speed of its own generation, and the amount of fibrin deposition and degradation. Platelet Activation Thrombin, along with many other serine proteases, can bind and cleave PARs. These cell-surface associated G-coupled proteins are found on platelets, endothelial cells, smooth muscle cells, leukocytes, and other cell

The primary function of thrombin is conversion of fibrinogen to fibrin. Thrombin cleaves two short peptides from the fibrinogen molecule, fibrinopeptide A (FpA) and fibrinopeptide B (FpB), thus exposing binding sites that interact with pre-existing sites on other fibrin molecules. The interaction between multiple fibrin molecules results in spontaneous polymerization into an insoluble fibrin gel. Activation of Factor XIII Factor XIII (FXIII) is a 320 kDa protein that consists of five chains arising from transcription of two different genes. Half of the circulating pool of functional FXIII is free in plasma, and the remainder is released from platelet α granules. Factor XIII cleavage by thrombin releases an activation peptide, resulting in a 312 kDa active enzyme. Thrombin cleavage of free FXIII is slow; however, polymerized fibrin acts as a cofactor, markedly increasing the rate of FXIIIa generation. Consequently, FXIIIa is generated primarily after a critical weight of fibrin has polymerized. Factor XIIIa is a transglutaminase whose primary function is crosslinkage of fibrin fibrils. This activity is calcium dependent, and critical for normal clot strength and stability. Factor XIIIa also crosslinks fibronectin to fibrin and to collagen, which influences anchoring of the clot to the vessel wall. Additionally, FXIIIa has roles in cell adhesion, angiogenesis, and tissue repair. FIBRINOGEN AND FIBRIN CLOT STRUCTURE Fibrinogen is a soluble 340 kDa dimer consisting of three pairs of disulfide-bridge linked polypeptides. It is the most abundant coagulation protein in plasma and is abundant in platelet α granules. In addition to the role of fibrinogen as a precursor to fibrin, fibrinogen mediates intracellular interactions between platelets in conjunction with thrombospondin (Chapter 76). Thrombin cleaves fibrinogen at two sites, releasing FpA and FpB. Upon FpA release, soluble fibrin monomer spontaneously assembles into an insoluble dimer. This polymerization occurs because removal of FpA exposes a binding site “A knob” that interacts with a complementary binding “hole” on separate fibrin molecules. Initial polymerization produces a dimer with a halfstaggered structure. Dimers continue to associate to form longer, two-fibrin-wide strands, called protofibrils.3,18 Subsequent removal of FpB allows for lateral

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requirement for fibrin as a cofactor for many of its processes. Although the primary function of fibrinolysis is removal of fibrin, many components of the system are involved in tissue repair processes, such as cell adhesion, migration, and proliferation. Plasminogen

FIGURE 84.9 Fibrin formation. Fibrinogen consists of a central E domain connected by coiled chains to two lateral D domains. Thrombin cleaves fibrinogen to fibrin monomer by removing two short peptides from the E domain (fibrinopeptides A and B) exposing binding sites in the E domain. These exposed sites then interact with corresponding binding sites on the D domains of other fibrin molecules, resulting in spontaneous polymerization into a fibrin protofibril. Multiple protofibrils laterally polymerize, forming a fibrin fiber. FXIII is activated to FXIIIa by thrombin. FXIIIa creates cross-links between D domains of contiguous fibrin monomers.

aggregation of protofibrils of increasing width via the “B knob” interacting with its complementary binding site. Protofibrils are stabilized by the same A-a and B-b knob-hole interactions that allowed for initial dimer association. Protofibril growth continues, with branch points developing where multiple fibrin monomers interact laterally. The resulting fibrin polymer is a mesh-like network (Fig. 84.9).3,18 Thrombin mediated release of fibrinopeptides from fibrinogen is neither calcium nor membrane dependent. However, fibrinogen contains several calcium binding sites. Calcium influences fibrin assembly, enhancing the rate and extent of lateral protofibril association. Consequently, calcium influences the structure of the mature fibrin clot.3,18 As the fibrin network forms, some thrombin is bound into the developing mesh via specific binding sites on the two molecules. Fibrin binding is important in activation of FXIII, and clot-bound thrombin also acts as a cofactor that accelerates fibrin assembly.3,17,18 FIBRINOLYSIS Fibrin clot formation prevents hemorrhage, but the clot must be removed eventually for restoration of blood flow. Clot dissolution is referred to as fibrinolysis. The fibrinolytic system is normally quiescent due to the relative abundance of fibrinolysis inhibitors, and the

Plasminogen (PLG) is a single chain 92 kDa glycoprotein synthesized primarily in the liver. It circulates in plasma with a long half-life of approximately 2 days. Its complex structure consists of five kringle domains and a protease domain. Several of the kringle domains bind aminocarboxylic acids (such as α2-aminocaproic acid and tranexamic acid) that are used as pharmacologic fibrinolysis inhibitors (see Chapter 90). Plasminogen activators transform plasminogen to plasmin by cleavage at a single site, resulting in a twochain molecule with an N-terminal glutamic acid residue (Glu-plasmin). In vitro, plasmin cleaves other plasmin molecules to remove part of the N-terminus, leaving an N-terminal lysine residue (Lys-plasmin). This additional cleavage probably does not occur in vivo due to rapid neutralization of free plasmin by inhibitors. The half-life of free plasmin is extremely short (0.1 seconds). Fibrin is a strong modulator of PLG activation. It enhances PLG activation due to binding and colocalization of PLG and its activators to the terminal lysine residues of fibrin monomers. Partially degraded fibrin exposes additional lysine sites, enhancing PLG activation through a positive feedback loop. Aminocarboxylic acids interfere with PLG binding to the lysine residues of fibrin. The fibrinolysis inhibitor, TAFI acts by removing lysine residues, thereby limiting available PLG binding sites. The PLG molecule contains within its kringle domains the anti-proliferative protein angiostatin. Tumor mediated proteolysis or proteolysis by macrophages releases this fragment from plasmin. Angiostatin also inhibits endothelial cell and smooth muscle proliferation and migration, and induces apoptosis.5 Plasmin degrades polymerized fibrin to form heterogeneous fragments referred to collectively as fibrin degradation products (FDP). The D-dimer fragment is produced by degradation of the linked region between D domains of adjacent fibrin molecules. This linkage is created exclusively by FXIIIa; therefore the presence of D-dimers indicates that polymerized fibrin was subject to crosslinking. Plasminogen Activators Mammals produce two physiologic PLG activators, tPA and uPA, named for the source from which they were originally isolated (tissue and urine). Some bacteria and vampire bats also produce plasminogen activators.5 Tissue-type Plasminogen Activator Tissue-type plasminogen activator is a 68 kDa serine protease glycoprotein produced and secreted by endothelial cells in response to a variety of triggers,

CHAPTER 84: OVERVIEW OF HEMOSTASIS

including bradykinin, histamine, acetylcholine, αadrenergic agents, and PAF. Very little tPA circulates in the bloodstream and most is bound to its major inhibitor, PAI-1. Free tPA and tPA-PAI-1 complexes are rapidly removed from circulation by binding to endothelial cell and hepatocyte receptors. The α2-MGLRP interaction is a major clearance mechanism. Endothelial cells secrete tPA as a single chain molecule (sctPA) that is not a zymogen; however, its enzymatic activity is weak in the absence of fibrin. Plasmin cleaves sctPA to form the two chain mature enzyme form (tPA). Fibrin binds to either single chain or two chain tPA. The protease activity of fibrin-tPA is strong and highly specific for cleavage of PLG to form plasmin.5 Urokinase Plasminogen Activator Urokinase plasminogen activator is a 54 kDa serine protease glycoprotein synthesized by fibroblast-like cells, epithelial cells, monocytes, and endothelial cells. It is secreted as a single chain molecule (scuPA) with less than 1% protease activity of the two-chain mature enzyme, uPA. Plasmin, FXIIa, and kallikrein cleave scuPA to form uPA.5 Monocytes express an important cellular receptor for scuPA (uPAR). Colocalization of scuPA and uPAR on cell surfaces enhances PLG activation. Unlike tPA, uPA can activate PLG in the absence of fibrin. It appears that tPA is the major PLG activator within the vasculature, whereas uPA is the major extravascular PLG activator. Consequently uPA’s primary roles include degradation of extracellular matrix in the processes of cell migration, and healing.5

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It also binds to PLG, which interferes with PLG activation due to impaired PLG-fibrin interaction. Plasminbound fibrin is relatively resistant to α2-AP inhibition. Factor XIIIa cross-links some forms of α2-AP to fibrin, thereby making the fibrin more resistant to lysis.5 Thrombin Activatable Fibrinolysis Inhibitor Thrombin activatable fibrinolysis inhibitor is a 60 kDa protein synthesized in the liver and released as a zymogen.6 Thrombin and plasmin activate TAFI by cleavage at a single site; however, activation is extremely slow. Thrombomodulin-bound thrombin, however, increases the TAFI activation rate more than 1000-fold. Activated TAFI inhibits fibrinolysis by removing the C-terminal lysines of polymerized fibrin. These residues are the primary binding site on fibrin for both PLG and its activators; therefore modification of fibrin by TAFIa makes the fibrin clot resistant to fibrinolysis. Complement cascade proteins and adhesive proteins are secondary targets of TAFIa. No TAFIa inhibitors have been identified, rather TAFIa appears to be regulated via spontaneous loss of activity. This inherent lability is temperature dependent. At body temperature TAFIa has a half-life of about 10 minutes. Miscellaneous Inhibitors As a non-specific protease inhibitor, α2-MG acts to inhibit plasmin (especially in the absence of free α2-AP) and also binds to tPA and uPA. Protein C inhibitor and protease nexin 1 also inhibit tPA.

Fibrinolysis Inhibitors

HEMOSTASIS AND INFLAMMATION

Plasminogen Activator Inhibitor 1

Recent discoveries demonstrate apparent “cross-talk” between the inflammatory and hemostatic systems. This communication plays important roles in the healing processes necessary for tissue repair, and in the inflammatory and innate immune response required when infection develops as a consequence of tissue injury. Cross-talk may be responsible for inappropriate activation of hemostasis associated with systemic inflammation (Chapter 88).

Plasminogen activator inhibitor 1, a 52 kDa serpin, is the major inhibitor of PLG activators. The primary source of plasma PAI-1 is unknown; however, many cell types, including megakaryocytes, endothelial cells, and hepatocytes, synthesize PAI-1. It is produced as an unstable, active form found primarily in blood and tissues, and a more stable latent form present in platelets. Most of the circulating PAI-1 is complexed with vitronectin, which stabilizes PAI-1’s active conformation.52 The target enzymes of PAI-1 are sctPA, and the two chain forms of tPA and uPA. When bound to a PLG activator, PAI-1 loses its affinity for vitronectin, but gains affinity for cell-surface LRP. This shift results in rapid clearance of PAI-1 by internalization, mediated by LRP. Vitronectin-bound PAI-1 also inhibits thrombin, an activity enhanced by HSPG.52 In addition to its protease inhibitory activities, PAI-1 mediates vitronectin’s adhesive properties and its role in tissue remodeling.5,52 α2-Antiplasmin α2-Antiplasmin is a single chain, 70 kDa serpin that is the primary plasmin inhibitor. It is synthesized in the liver, circulating with a long half-life of approximately 3 days.

Effects of Hemostasis on Inflammation Contact pathway activation produces two enzymes (kallikrein and FXIIa) that activate complement and induce neutrophil activation, and stimulate monocytes to down-regulate Fc receptors and release inflammatory cytokines. Additionally, cleavage products of HK have effects on cellular adhesion, cellular proliferation, and apoptosis.13,25,45 The TF-FVIIa complex expressed on monocytes and endothelial cells activates cell surface PARs. This stimulation induces PAR-mediated signaling and increased expression of inflammatory cytokines (e.g. interleukins 1 and 6, tumor necrosis factor) and expression of chemotaxis proteins.13,25,45

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Thrombin is chemotactic for macrophages and neutrophils, stimulates selectin expression that promotes neutrophil adhesion to endothelial cells, and activates endothelial cells to produce PAF, a potent neutrophil agonist. Thrombin activation of platelets releases CD40 ligand which can induce TF formation and increase cytokine production. Thrombin also increases endothelial permeability.13,25,45 The inflammatory response is attenuated by APC’s enzymatic action on PAR-1, which down-regulates TF expression. Soluble EPCR, released by metalloproteinases from the endothelial cell surface, inhibits neutrophil adhesion. Down-regulation of TM expression increases endothelial intracellular signaling pathways responsible for production of proinflammatory cytokines, and limits activation of TAFI, adversely affecting clearance of some complement proteins.13,25,45 Fibrin can directly stimulate production of proinflammatory cytokines by monocytes, and release of chemokines by endothelial cells and fibroblasts.13,25,45 Effects of Inflammation on Hemostasis Inflammatory mediators, especially components of the complement system, induce transmembrane phospholipid movement, resulting in expression of a procoagulant surface. The acute phase response results in a shift in the balance of protein synthesis. Proteins whose synthesis increases include fibrinogen, PLG, tPA, uPA, C1INH, C4BP PS, vitronectin, PAI-1, and α1-PI. Proteins whose synthesis is decreased include FXII, AT, and PC.15,25 Inflammatory cytokines down-regulate the expression of EPCR and TM on the endothelial cell surface. This results in less activation of APC, and consequently less inactivation of FVa and FVIIIa. Proinflammatory cytokines also induce expression of TF by cell-types that do not express this antigen under resting conditions. Proinflammtory cytokines also down-regulate expression of HSPG on the endothelial cell surface, which decreases the anticoagulant activity of AT.13 Neutrophil elastase cleaves a variety of coagulation and fibrinolysis proteins, including AT, PC, and TM.25

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CHAPTER 84: OVERVIEW OF HEMOSTASIS 40. Piccin A, Murphy WG, Smith OP. Circulating microparticles: pathophysiology and clinical implications. Blood Rev 2007;21:157–171. 41. Piro O, Broze GJ. Comparison of cell-surface TFPIalpha and beta. J Thromb Haemostasis 2005;3:2677–2683. 42. Roberts HR, Hoffman M, Monroe DM. A cell-based model of thrombin generation. Seminars in Thromb Hemostasis 2006;32:32–38. 43. Rosing J, Maurissen LF, Tchaikovski SN, et al. Protein S is a cofactor for tissue factor pathway inhibitor. Thromb Res 2008;122:S60–63. 44. Schmaier AH. The elusive physiologic role of Factor XII. J Clin Invest 2008; 118:3006–3009. 45. Schouten M, Wiersinga WJ, Levi M, et al. Inflammation, endothelium, and coagulation in sepsis. J Leuk Biol 2008;83:536–545. 46. Smith SA, Mutch NJ, Baskar D, et al. Polyphosphate modulates blood coagulation and fibrinolysis. Proc Natl Acad Sci USA 2006;103:903–908. 47. Suzuki K. The multi-functional serpin, protein C inhibitor: beyond thrombosis and hemostasis. J Thromb Haemostasis 2008;6:2017–2026.

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48. Tollefsen DM, Zhang L. Heparin and vascular proteoglycans. In: Hemostasis and Thrombosis: Basic Principles and Clinical Practice. Philadelphia: Lippincott Williams and Wilkins, 2006;271–283. 49. Wagenaar-Bos IG, Hack CE. Structure and function of C1-inhibitor. Immunol Allergy Clin N Am 2006;26:615–632. 50. Wildgoose P, Nemerson Y, Hansen LL, et al. Measurement of basal levels of factor VIIa in hemophilia A and B patients. Blood 1992;80:25– 28. 51. Yamaji-Hasegawa A, Tsujimoto Mi. Asymmetric distribution of phospholipids in biomembranes. Biol Pharm Bull 2006;29:1547–1553. 52. Yepes M, Loskutoff DJ, Lawrence DaA. Plasminogen activator inhibitor-1. In: Hemostasis and Thrombosis: Basic Principles and Clinical Practice. Philadelphia: Lippincott Williams and Wilkins, 2006;365– 380. 53. Zwaal RF, Comfurius P, Bevers EM. Surface exposure of phosphatidylserine in pathological cells. Cell Mol Life Sci 2005;62:971–988.

C H A P T E R 85

Acquired Coagulopathies MARJORY B. BROOKS and ARMELLE DE LAFORCADE Pathogenetic Classification Coagulation Factor Deficiencies of Acquired Coagulopathies Synthetic failure Activation defects Factor consumption and dilutional coagulopathies Coagulation Inhibitors Fibrinolysis Defects Clinical Evaluation History Physical Examination

Quick Assessment and Point-of-Care Tests Coagulation Panel Specific Disorders Liver Disease Vitamin K Deficiency Disseminated Intravascular Coagulation Dilutional Coagulopathy Coagulation Inhibitors Factor VIII inhibitory antibodies Antiphospholipid syndrome inhibitors Heparin overdose

Acronyms and Abbreviations ACT, activated clotting time; APLA, antiphospholipid-protein antibody; APS, antiphospholipid syndrome; aPTT, activated partial thromboplastin time; AT, antithrombin; DIC, disseminated intravascular coagulation; FDP, fibrin degradation product; KH2, vitamin K hydroquinone; KO, vitamin K epoxide; PIVKA, proteins induced by vitamin K absence/antagonism; PT, prothrombin time; TCT, thrombin clotting time; TEG, thromboelastography; t-PA, tissue plasminogen activator; UFH, unfractionated heparin.

C

linical signs of hemorrhage are usually the result of a breach in vascular integrity that overwhelms an individual’s normal hemostatic response. For these cases, diagnosis and management is directed at defining the site and cause of blood vessel injury. A subset of patients, however, have abnormal bleeding due to impaired hemostatic plug formation. Thrombocytopenia (see Chapters 78 and 79) and acquired coagulopathies are the most common hemostatic defects encountered in clinical practice. An index of suspicion and consistent use of screening tests early in the diagnostic work-up are key to recognition and effective management of acquired coagulopathies. PATHOGENETIC CLASSIFICATION OF ACQUIRED COAGULOPATHIES

Acquired coagulopathies discussed in this chapter include disease processes that delay or prevent fibrin clot formation or increase the rate of fibrinolysis (Table 85.1). 654

Coagulation Factor Deficiencies Coagulation factor activation generates a local burst of thrombin that transforms soluble plasma fibrinogen into an insoluble fibrin clot. Quantitative and/or functional deficiencies of coagulation factors influence the rate and amount of thrombin produced. (see Chapter 84). The common acquired factor deficiencies, unlike hereditary coagulopathies, are characterized by combined deficiencies of multiple clotting factors. Synthetic Failure The liver is the sole or primary source of all the serine protease coagulation factors and anticoagulants (i.e. factors II, VII, IX, X, XI, XII, proteins C and S) and other critical hemostatic proteins including factors V, XIII, antithrombin, and plasminogen. Liver disease is associated with complex abnormalities, with combined and variable deficiencies of procoagulant and anticoagulant proteins.1,25 Liver failure, however, typically manifests as a hemorrhagic diathesis. A lack of hepatic synthetic

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TABLE 85.1 Pathogenesis of Acquired Coagulopathies Mechanism

Disease Syndromes

Associated Factor Defects

Liver disease: severe failure and markedly reduced functional hepatic mass

Portosystemic shunting

Most coagulation factors low but relative retention of factor VIII activity Hypofibrinogenemia and dysfibrinogenemia Antithrombin and protein C low Protein C low

Vitamin K dependent activation defect

Cholestasis, intestinal malabsorption, Vitamin K antagonists, neonates

Factors II, VII, IX, and X low Protein C low

Excessive rate of consumption

Overt (hemorrhagic) disseminated intravascular coagulation

Complex pattern of increased and decreased factor activities, hypofibrinogenemia Antithrombin low Hypofibrinogenemia, variable deficiency of other factors

Synthetic failure

Envenomation Coagulation inhibitors

Hyperfibrinolysis

Alloimmune and autoimmune factor inhibitors Phospholipid-protein antibodies (associated with thrombosis) Heparin overdose

Single factor deficiency (factor VIII deficiency most common) Coagulation factor activities normal

Disseminated intravascular coagulation, neoplasia, infectious agents, prostate surgery

Complex factor deficiencies High plasminogen activators Hypofibrinogenemia

capacity results in quantitative coagulation factor deficiencies, first detectable by decreased activities of factors with short plasma half-lives (e.g. factor VII, half-life of 6 hours). Plasma fibrinogen, as an acute phase reactant, is often normal or elevated in patients with inflammatory and cholestatic liver disease. Diseased hepatocytes, however, may produce a dysfunctional, “nonclottable” fibrinogen due to abnormal alpha chain conformation and high sialic acid content that impairs fibrin formation and polymerization. The development of hypofibrinogenemia in patients with liver disease is an indicator of severely compromised synthetic functional reserve, associated with poor prognosis.1

Serine protease factors neutralized (especially factor X, factor II)

Inactive Factors (II, VII, IX, X)

KH2 Carboxylase

Functional Factors

Vitamin K antagonists Reductase

KO

FIGURE 85.1 Vitamin K cycle. A vitamin K-dependent carboxylase simultaneously converts vitamin K quinol (KH2) to vitamin K 2,3 epoxide (KO) as it transforms glutamate to functional gamma carboxyglutamate residues on factors II, VII, IX, and X. Vitamin K 2,3 epoxide is then reduced back to a quinol by vitamin K epoxide reductase. This enzyme is inhibited by coumarin-type anticoagulants.

Activation Defects Vitamin K is essential for the post-translational processing of the prothrombin group of coagulation factors (factors II, VII, IX, X) and the anticoagulants, proteins C and S. All of these factors require the addition of gamma-carboxyglutamic acid residues in order to interact with calcium, bind to membrane surfaces, and form active enzyme complexes (see Chapter 84). Vitamin K participates in the carboxylation reaction through oxidation of vitamin K hydroquinone (KH2) to vitamin K epoxide (KO). The epoxide form of vitamin K is continually recycled to replenish KH2 by the enzyme vitamin K epoxide reductase (Fig. 85.1). Coagulation factors synthesized in the absence of vitamin K circulate in plasma as inactive, descarboxy

precursors referred to as proteins induced by vitamin K absence or antagonism (PIVKA). Factor Consumption and Dilutional Coagulopathies Acute or chronic hemorrhage does not result in coagulation factor deficiency in animals with normal liver function. Factor consumption outpaces hepatic synthesis, however, in animals with overt disseminated intravascular coagulation (DIC). The resultant coagulation factor depletion causes clinical signs of hemorrhage (see Chapter 88). Systemic amyloidosis in people is associ-

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ated with variable factor X deficiency, although the phenomenon has not been described in animals. Amyloid fibrils are believed to adsorb factor X and facilitate its rapid plasma clearance in some patients.7 The bleeding tendency of amyloidosis is multifactorial; due to amyloid tissue infiltration and deposition in the microvasculature, and potentially exacerbated by severe factor X deficiency (e.g. values below 25%). Many snake venoms contain proteases that activate clotting factors and/or directly cleave fibrinogen, resulting in systemic factor depletion and defibrination in a consumptive process.13 Dilutional hemostatic defects are common after aggressive fluid resuscitation or massive transfusion, commonly defined as replacement of more than one blood volume within a 24 hour time interval.21 Factor dilution develops secondary to fluid shifts from the interstitial and intracellular space into the intravascular space. These shifts are triggered by reduced intravascular hydrostatic pressure and are compounded by administration of large volumes of intravenous fluids. Activation of coagulation secondary to trauma-induced tissue factor expression also leads to factor consumption. Thrombocytopenia, acidosis, hypothermia and hypocalcemia (due to citrate-based anticoagulants in blood products) further contribute to hemostatic compromise.8,10 Coagulation Inhibitors The classification of coagulation inhibitors includes antibodies (usually IgG) directed against specific coagulation factors.19 The antibodies bind to and directly inhibit factor activity and/or speed the rate of factor clearance. Inhibitory alloantibodies develop in patients with hereditary factor deficiencies after transfusion exposure to the “foreign” antigen. Factor VIII inhibitory antibodies develop in approximately 20% of people with severe hemophilia A (factor VIII deficiency).19,20 Alloimmunization and development of factor VIII and IX inhibitors has also been described in dogs with hemophilia A and B, respectively.18 Autoantibodies directed against coagulation factors represent a loss of tolerance and can also develop in patients without hereditary factor defects, regardless of prior transfusion.9 Factor VIII is the most commonly implicated factor and the resultant coagulopathy is referred to as “acquired hemophilia.” The epitope specificity of factor VIII alloantibodies and autoantibodies generally overlap, with most binding to the protein’s light chain C domain. Recently, acquired factor V inhibitors have been recognized in people exposed perioperatively to topical bovine thrombin and fibrin glues.12 These products contain trace amounts of bovine factor V, sufficient to induce cross-reactive antibody production, especially in multiply exposed patients. The term “lupus anticoagulant” was formerly used to describe antibodies that prolong in vitro clotting times by blocking the interaction of coagulation factors with phospholipid reagents.15 The immunogenic epitopes are actually phospholipid binding proteins and the current terminology for these inhibitors is antiphospholipid-protein antibodies (APLAs).5 Para-

doxically, people with APLA develop signs of thrombosis, rather than hemorrhage. Although their prothrombotic action is not fully defined, APLAs are believed to displace natural anticoagulants on cell surfaces and induce exposure of procoagulant phospholipids in the vascular space. Additional classes of coagulation inhibitors include pharmacologic anticoagulants such as hirudin that act by directly impairing factor activity and heparin compounds that act indirectly to enhance antithrombin’s ability to neutralize active factors (see Chapter 90). Finally, high plasma concentration of fibrin degradation products (FDPs) may act as coagulation inhibitors by interfering with normal fibrin polymerization. Fibrinolysis Defects Accelerated fibrinolysis causes hemorrhage by degrading fibrin clots before vascular repair is complete. Disease syndromes that induce increased plasminogen activators, decreased plasminogen inhibitors, or these factors combined, produce a hyperfibrinolytic state.29 Hyperfibrinolysis, mediated by high tissue plasminogen activator (t-PA), develops in people infected with the agents causing Rocky Mountain spotted fever, African swine fever, and dengue fever, and secondary to extensive tissue trauma.8 Neoplasia is associated with hyperfibrinolysis due to specific elaboration of plasminogen activators by tumor cells (e.g. acute promyelocytic leukemia) or in the context of a DIC process (see Chapter 88). The prostate and uterus are rich in t-PA and urokinase plasminogen activators. Surgery on these organs risks hemorrhagic complications due to local hyperfibrinolysis and impaired intra- and postoperative hemostasis. CLINICAL EVALUATION The initial evaluation of animals with signs of hemorrhage should aim to differentiate blood loss caused by vessel injury or vascular disease from a systemic failure of normal hemostasis (i.e. bleeding diathesis). The location and nature of hemorrhage and prior history of abnormal hemorrhage or underlying disease conditions provide clues to guide subsequent laboratory evaluation. Some animals with acquired coagulopathies do not manifest overt hemorrhage at initial presentation. Detection of factor deficiency in these cases requires an index of suspicion and consistent inclusion of coagulation screening tests early in the diagnostic work-up. History A history of recurrent, episodic hemorrhage is suggestive of a hereditary hemostatic defect, rather than an acquired coagulopathy (see Chapters 81, 82, 86). Hemostasis questionnaires have been developed for medical studies with the goal of simplifying history gathering and generating bleeding “scores” for standard comparison among trials.22 A brief questionnaire with specific questions on hemostasis is also useful

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early in the clinical work-up of animals suspected of having a bleeding disorder. Pertinent questions include: Has this animal ever had spontaneous bleeding from the nose, mouth, urinary tract, or noticeable bleeding when teething? Has this animal ever had a surgical procedure, dentistry, or traumatic injury, and were there any bleeding complications? Are you aware of any bleeding problems in relatives or in this breed? Is this animal receiving any medication or dietary supplements? Does this animal have any ongoing medical problems? Additional questions regarding housing conditions and potential access to drugs and toxins are useful; however, a negative exposure history does not preclude toxicity.

proteins.6 The thromboelastography (TEG) analyzer is the viscoelastic instrument used most widely to study coagulopathies in dogs.18,28 This instrument provides a visual tracing and numeric parameters reflecting the rate and strength of fibrin formation. Clinically significant coagulopathies are expected to cause prolonged initiation times and diminished tracing amplitude. Evidence of hypocoagulability, in either simple or sophisticated point-of-care tests, requires follow-up with specific coagulation panel assays (and possibly more detailed factor and anticoagulant assays) for definitive diagnosis of acquired coagulopathies.6

Physical Examination

The basic screening tests to detect coagulopathies include activated partial thromboplastin time (aPTT), prothrombin time (PT), thrombin clotting time (TCT) and fibrinogen concentration (see Chapter 138). The assays are functional tests, configured with specific reagents that sequentially activate distinct series of coagulation factors (i.e. aPTT and PT) or with a thrombin-containing reagent that directly transforms plasma fibrinogen to fibrin (i.e. TCT and fibrinogen). Factor deficiencies are detected by prolongation of clotting times. The pattern of abnormalities in screening test results depends on which factor(s) the patient lacks (see Fig. 85.2). Coagulation panel results, combined with clinical presentation, are often sufficient for management of acquired coagulopathies. Complex cases may require specific analyses of individual coagulation factors, anticoagulant proteins, drugs, or measures of fibrinolysis, to better define pathogenesis or facilitate case management. Collection of pre-treatment samples is critical for timely and accurate diagnosis of coagulopathies, and clinicians must bear in mind that the validity of all hemostasis testing depends on appropriate sampling techniques (see Chapter 138).

The physical examination should define focal or multifocal sites of hemorrhage and detect evidence of gross and subtle hemorrhage. Funduscopic examination and careful inspection of mucuous membranes and nonhaired skin may reveal petechiae and ecchymoses, suggestive of platelet or von Willebrand factor defects or vasculopathies. Large vessel (arterial/venous) injury typically causes obvious blood loss from localized sites of vessel injury. Animals with severe coagulopathies may develop spontaneous hemorrhage into any body cavity or potential space, producing effusion, mass effect, or hematoma formation. Simultaneous hemorrhage from several sites, including prolonged hemorrhage from venipuncture or catheter sites is evidence of severe coagulopathy. The possibility of an acquired coagulopathy should be included in the differential for any patient with unexplained hemorrhage, effusion or mass lesion, or disease conditions associated with factor deficiencies (see Table 85.1). Routine coagulation screening of these cases will minimize delays in instituting appropriate management.

Coagulation Panel

Quick Assessment and Point-of-Care Tests The initial assessment of any patient suspected of having a hemostatic defect should include platelet count or platelet estimate (see Chapter 77) and a pointof-care coagulation test. The activated clotting time (ACT) is the traditional, and simplest coagulation screening test (see Chapter 138). The ACT detects moderate to severe deficiencies of fibrinogen and all coagulation factors, with the exception of factor VII. The ACT is prolonged, therefore, in patients with active hemorrhage due to most acquired coagulopathies, including liver failure, anticoagulant rodenticide intoxication, DIC, and heparin overdose (see Table 85.1). More advanced instruments are now available to perform additional point-of-care clotting time tests using whole blood samples. A clinical study of coagulopathies in dogs found similar diagnostic utility comparing results of laboratory and point-of-care coagulation panels.26 Recently, viscoelastic monitors have been developed with the goal of providing a global assessment of hemostasis that incorporates the contribution of cellular elements and procoagulant and anticoagulant plasma

SPECIFIC DISORDERS Liver Disease Animals with hepatobiliary disease demonstrate variable signs of coagulopathy, ranging from spontaneous bleeding to subclinical prolongation of in vitro clotting times.2,14,24 Liver disease causes major alterations in many hemostatic mechanisms, including platelet and endothelial cell reactivity, procoagulant and anticoagulant protein production and function, dysregulation of fibrinolysis, and complications arising from portal hypertension and splenomegaly. These changes often act as opposing procoagulant and anticoagulant forces, resulting in variable net bleeding risk. Clinically severe hemorrhage is generally seen in animals with fulminant or end-stage hepatic failure, or those with concomitant DIC. Other forms of liver disease may cause relatively milder bleeding tendencies, albeit sufficient to impair surgical hemostasis for animals undergoing biopsy or hepatobiliary surgery.

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Diagnostic Algorithm for Acquired Coagulopathies

COAGULATION SCREENING TESTS: APTT, PT, TCT

APTT: LONG PT, TCT: NORMAL

Factor VIII Deficiency (Factor IX, XI, XII Deficiency) Phospholipid-binding antibodies

PT: LONG APTT, TCT: NORMAL

APTT, TCT: LONG PT: NORMAL

APTT& PT: LONG TCT NORMAL

Factor VII Deficiency

Heparin Effect

Combined Deficiency: Factors II, VII, IX, X

Coumadin therapy Early rodenticide intoxication Cholestasis, malabsorption

Unfractionated heparin therapy

Vitamin K Deficiencies Coumadin overdose Rodenticide toxicity Cholestasis, malabsorption Neonatal vitamin K deficiency

Factor VIII alloantibodies Acquired hemophilia A Factor inhibitory antibodies Antiphospholipid antibody syndrome

ALL TESTS LONG

Hypofibrinogenemia Dysfibrinogenemia Combined deficiencies of factors and fibrinogen DIC Liver failure Envenomation Dilutional coagulopathy

FIGURE 85.2 Coagulation screening test algorithm for acquired coagulopathies.

Marked prolongation of coagulation screening tests (aPTT, PT, TCT) and hypofibrinogenemia indicate high risk of spontaneous and post-surgical hemorrhage, and are overall poor prognostic signs. Mild prolongation or the absence of abnormal clotting times, however, does not rule out the risk of procedure-related bleeding. Newer hemostasis tests, such as endogenous thrombin potential and thromboelastography are designed to assess net procoagulant and anticoagulant forces. Their utility for predicting bleeding risk in patients with liver disease is currently under clinical investigation.6 A preliminary study has shown that measurement of protein C activity may be useful as a biomarker of liver function and hepatoportal blood flow in dogs.24 The finding of low protein C activity helps differentiate porto-systemic vascular anomalies from microvascular dysplasia, and is a useful predictor of clinical outcome when combined with traditional biochemical assays. Management strategies for coagulopathy of liver disease include prophylactic and “rescue” therapies. Prophylactic administration of vitamin K1 is often included with specific and supportive medical therapy for liver disease. Pre-operative vitamin K1 administration is indicated for animals with cholestatic disorders and/or prolonged aPTT and PT (see Chapter 90). Current trends in human medicine favor more restrictive use of prophylactic transfusion therapy.1 Fresh

frozen plasma is transfused to patients with liver failure and signs of hemorrhage to supply active procoagulant and anticoagulant factors (see Chapters 90 and 96). The need for pre-operative transfusion is generally assessed on an individual patient basis in human and veterinary medicine. People undergoing liver transplantation invariably require transfusion and algorithms based on thromboelastographic parameters have been developed to guide the use of blood components in this patient population.6 Vitamin K Deficiency Vitamin K deficiency is a common cause of coagulopathy, resulting from inadequate intestinal absorption or impaired intrahepatic recycling.2,4,14,16 Biliary obstruction, intrahepatic cholestasis, chronic oral antibiotic administration, and infiltrative bowel disease may all reduce vitamin K absorption. Hemorrhagic disease of the newborn refers to vitamin K deficiency in infants within the first few days to weeks of life. Hemorrhage is attributed to inadequate placental transfer of vitamin K and inadequate hepatic recycling.11 Coumadin (warfarin) and second-generation anticoagulant rodenticides (e.g. brodifacoum) act by irreversibly blocking the activity of epoxide reductase, the critical hepatic vitamin K recycling enzyme (see Fig. 85.1). Severe coagulopathies due to anticoagulant rodenticide toxicity are com-

CHAPTER 85: ACQUIRED COAGULOPATHIES

monly encountered in small animal practice, have been reported in equine practice, and are increasingly recognized in wild birds (see Chapter 91). Factor deficiency and resultant signs of a bleeding diathesis develop over a period of several days post-ingestion of anticoagulant poisons as vitamin K stores become depleted. Signs of hemorrhage may be obvious as external blood loss, or may be nonspecifc as pleural or abdominal effusion or mass lesions anywhere in the body due to hematoma formation. Treatment delays often lead to death from hemorrhagic shock, or respiratory or central nervous system compromise. Prolongation of both the aPTT and PT screening tests, with normal TCT, are the hallmarks of hemorrhage caused by severe vitamin K deficiency (see Fig. 85.2). Specific factor analyses reveal that severe deficiencies of factors II, VII, IX, and X but normal fibrinogen cause this characteristic coagulation profile. Therapeutic dosages of the anticoagulant Coumadin cause only mild deficiencies of the vitamin K-dependent factors that should not cause prolongation of the aPTT. Coumadin monitoring to ensure adequate, but not excessive dosage, is based on PT analyses (see Chapter 90). Management of vitamin K deficiency requires replacement therapy with vitamin K1 (phytonadione), rather than vitamin K3 (menadione). Vitamin K therapy is often given empirically pending results of pre-treatment coagulation assays, because synthesis of sufficient active factors to correct the coagulopathy requires at least 12 hours from the start time of initiating vitamin K supplementation. Transfusion is also indicated for patients with severe signs at presentation (see Chapter 96). Disseminated Intravascular Coagulation Microvascular thrombosis is the characteristic feature of DIC; however, signs of a severe bleeding diathesis dominate some case presentations. Dogs with DIC secondary to neoplasia (e.g. hemangiosarcoma) often develop consumptive coagulopathies, with coagulation factor and fibrinogen depletion, and concomitant increase in FDP and D-dimer (see Chapter 88). Hemorrhagic, overt DIC is often transfusion resistant, requiring high volume plasma replacement to sustain hemostasis (see Chapter 96). Dilutional Coagulopathy The terms dilutional coagulopathy, or “traumatic coagulopathy” describe a hemostatic defect that develops after high volume fluid resuscitation. Traumatic coagulopathy occurs in up to 1 of 4 human trauma patients and is considered a negative prognostic indicator. Concomitant acidemia, hyperfibrinolysis, and hypothermia further exacerbate coagulopathy in these patients. The effect of crystalloids versus colloids in promoting a dilutional coagulopathy has been extensively studied, with colloids demonstrating a greater dilutional effect than crystalloids and a negative effect on clot stability.8 Clinical studies in people also reveal

659

that fibrinogen deficiency generally develops early in the course of hemodilution and signs of inadequate hemostasis manifest at fibrinogen concentration below 50–100 mg/dL (0.5–1.0 g/L).21 Although traumatic coagulopathies are not as well-characterized clinically in animals, a retrospective study of massive transfusion in dogs noted prolonged aPTT and PT in half the cases.10 Correction of hemostatic abnormalities is associated with improved survival in people, and factor replacement in the form of fresh frozen plasma is typically administered after recognition of coagulopathy.8 Coagulation Inhibitors Factor VIII Inhibitory Antibodies Antibodies that inhibit factor VIII are a well-recognized complication of factor replacement therapy for people with hemophilia A. This alloantibody formation is commonly, but not exclusively, found in patients with severe factor VIII deficiency (i.e. ≤1% factor VIII).19 Consequently, mutation types that abolish protein synthesis appear to increase the risk of inhibitor formation. The development of coagulopathy due to factor VIII inhibitory antibodies also occurs as an autoimmune phenomenon.9 Underlying conditions associated with acquired factor VIII inhibitor development include systemic lupus erythematosus, rheumatoid arthritis, drug hypersensitivity, inflammatory bowel disease, malignancy, and the post-partum state. Factor VIII inhibitors (alloantibodies and autoantibodies) often cause severe coagulopathies characterized by spontaneous hemorrhage refractory to standard replacement therapy. Factor VIII inhibitors are detected by specific prolongation of the aPTT. This prolongation is not corrected by mixing patient plasma with normal plasma, in contrast to congenital factor VIII deficiency (in the absence of factor VIII inhibitors). The Bethesda unit assay (BU) is used to quantify the titer of inhibitory antibodies. Values below 5 BU/mL are considered low titers.19,20 Active bleeding in patients with low BU titers often responds to transfusion of high dose factor VIII replacement therapy. In contrast, patients with high titer inhibitors often require “by-pass” therapy using recombinant activated human factor VII (see Chapter 96). Long-term management of high titer factor VIII inhibitors consists of specific therapy of underlying disorders (for autoantibody patients) and immune-modulating or immunosuppressive therapy.20 Antiphospholipid Syndrome Inhibitors The antiphospholipid syndrome (APS) is recognized as a thrombotic risk factor in people. The syndrome typically develops in patients with autoimmune disorders and is characterized by the production of APLAs that prolong in vitro clotting time.15 Long aPTT that fails to correct in a mixing study is compatible with the presence of APLA, and more specialized clotting time tests (e.g. dilute Russell’s viper venom time) and quantitative immunoassays to detect antibodies directed against

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lipid-binding proteins are used as confirmatory assays.5 Inhibitory antibodies were proposed as the cause of long clotting times in a case report of canine immunemediated hemolytic anemia (IMHA) and thrombosis; however, the prevalence and clinical relevance of APLA and APS in animals is not well-defined.23 Heparin Overdose Unfractionated heparin (UFH) is widely used in veterinary medicine; however, few clinical studies have been performed to define effective dosage regimens. The pharmacokinetic profile of UFH is complex due to its extensive protein and cell binding and dose-dependent half-life (see Chapter 90).3,17,27 Since hemorrhage is the major risk of high dose UFH therapy, individual patient monitoring is required to prevent excessive anticoagulant effect. The aPTT and TCT coagulation screening tests are both sensitive to UFH, whereas the PT is generally unaffected by therapeutic UFH levels. Adjustment of UFH dose to prolong aPTT to 1.5–2 times pretreatment values is the traditional means for guiding UFH therapy in people, and this strategy is applicable to prevent overdosage in animals. Clinical signs of hemorrhage or marked prolongation of aPTT (i.e. >2.5 times baseline) are indications for discontinuing UFH therapy. Due to its short (2–3 hour) plasma half-life, hemorrhage is usually transient. People with major hemorrhage due to UFH overdosage are infused with protamine sulfate, a basic polypeptide that binds tightly to UFH and immediately neutralizes its anticoagulant activity.27 Protamine administration causes systemic hypotension in dogs; therefore transfusion therapy is used to control life-threatening hemorrhage caused by excessive UFH anticoagulant effect (see Chapter 96).

REFERENCES 1. Caldwell SH, Hoffman M, Lisman T, et al. Coagulation disorders and hemostasis in liver disease: pathophysiology and critical assessment of current management. Hepatology 2006;44:1039–1046. 2. Center SA, Warner K, Corbett J, et al. Proteins invoked by vitamin K absence and clotting times in clinically ill cats. J Vet Intern Med 2000;14: 292–297. 3. Diquelou D, Barbaste C, Gabaig Am, et al. Pharmacokinetics and pharmacodynamics of a therapeutic dose of unfractionated heparin (200 U/kg) administered subcutaneously or intravenously to healthy dogs. Vet Clin Pathol 2005;34:237–242. 4. Edwards DF, Russell RG. Probable vitamin K-deficient bleeding in two cats with malabsorption syndrome secondary to lymphocytic-plasmacytic enteritis. J Vet Intern Med 1987;1:97–101. 5. Galli M. Clinical utilituly of laboratory tests used to identify antiphospholipid antibodies and to diagnose the antiphospholipid syndrome. Semin Thromb Hemostasis 2008;34:329–334. 6. Ganter MT, Hofer CK. Coagulation monitoring: current techniques and clinical use of viscoelastic point-of-care coagulation devices. Anesth Analg 2008;106:1366–1375.

7. Greipp PR, Kyle RA, Bowie EJW. Factor X deficiency in amyloidosis: a critical review. Am J Hematol 1981;11:443–450. 8. Hess J, Brohi K, Dutton R, et al. The coagulopathy of trauma: a review of mechanisms. J Trauma 2008;65:748–754. 9. Holme PA, Brosstad F, Tjønnfjord GE. Acquired haemophilia: management of bleeds and immune therapy to eradicate autoantibodies. Haemophilia 2005;11:510–515. 10. Jutkowitz LA, Rozanski EA, Moreau JA, Rush JE. Massive transfusion in dogs: 15 cases (1997–2001). J Am Vet Med Assoc 2002;220:1664– 1669. 11. Kumar D, Greer FR, Super DM, et al. Vitamin K status of premature infants: implications for current recommendations. Pediatrics 2001;108: 1117–1122. 12. Lawson JH. The clinical use and immunological impact of thrombin in surgery. Semin Thromb Haemostasis 2006;32:(S1)98–110. 13. Leisewitz AL, Blaylock RS, Kettner F, et al. The diagnosis and management of snakebite in dogs – a southern African perspective. J S Afr Vet Assoc 2004;75:7–13. 14. Lisciandro SC, Hohenhaus A, Brooks M. Coagulation abnormalities in 22 cats with naturally occurring liver disease. J Vet Intern Med 1998;12:71– 75. 15. Marlar RA, Husain S. The enigmas of the lupus anticoagulant: mechanisms, diagnosis, and management. Curr Rheumatol Rep 2008;10:74– 80. 16. McConnico RS, Copedge K, Bischoff KL. Brodifacoum toxicosis in two horses. J Am Vet Med Assoc 1997;211:882–886. 17. Mischke RH, Schuttert C, Grebe SI. Anticoagulant effects of repeated subcutaneous injections of high doses of unfractionated heparin in healthy dogs. Am J Vet Res 2001;62:1887–1891. 18. Prasad S, Lillicrap D, Labelle A, et al. Efficacy and safety of a new-class hemostatic drug candidate, AV513 in dogs with hemophilia A. Blood 2008; 15:672–679. 19. Reding MT. Immunological aspects of inhibitor development. Haemophilia 2006;12(Suppl 6):30–36. 20. Reipert BM, van den Heilden PM, Schwarz HP, et al. Mechanisms of action of immune tolerance induction against factor VIII in patients with congenital haemophilia A and factor VIII inhibitors. Br J Haematol 2006; 136:12–25. 21. Reiss RF. Hemostatic defects in massive transfusion: rapid diagnosis and management. Am J Crit Care 2000;9:158–167. 22. Rodeghiero F, Castaman G, Tosetto A, et al. The discriminant power of bleeding history for the diagnosis of type 1 von Willebrand disease. J Thromb Haemostasis 2005;3:2619–2626. 23. Stone MS, Johnstone IB, Brooks M, et al. Lupus-type anticoagulant in a dog with hemolysis and thrombosis. J Vet Intern Med 1994;8:57–61. 24. Toulza O, Center SA, Brooks MB et al. Evaluation of protein C activity for detection of hepatobiliary disease and portosystemic shunting in dogs. J Am Vet Med Assoc 2005;229:1761–1771. 25. Tripodi A, Manucci PM. Abnormalities of hemostasis in chronic liver disease: reappraisal of their significance and need for clinical and laboratory research. J Hepatol 2007;46:727–733. 26. Tseng LW, Hughes D, Giger U. Evaluation of a point-of-care coagulation analyzer for measurement of prothrombin time, activated partial thromboplastin time, and activated clotting time in dogs. Am J Vet Res 2001;62:1455– 1460. 27. Warkentin TE, Crowther MA. Reversing anticoagulants both old and new. Can J Anaesthesiol 2002;49:S11–25. 28. Wiinberg B, Jensen AL, Rozanski E, et al. Tissue factor activated thromboelastography correlates to clinical signs of bleeding in dogs. Vet J 2009; 179:121–129. 29. Zorio E, Gilabert-Estelles J, Espana F, et al. Fibrinolysis: the key to new pathogenetic mechanisms. Curr Med Chem 2008;15:923–929.

ON-LINE RESOURCES 1. Comparative Coagulation Laboratory. Animal coagulation, anticoagulant, and fibrinolysis assays. http://www.diaglab.vet.cornell.edu/coag/ 2. Massachusetts General Hospital. Overview of assays to aid in diagnosis of coagulopathies. http://www2.massgeneral.org/pathology/coagbook/ handbook.htm

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Hereditary Coagulopathies MARJORY B. BROOKS Hereditary Coagulation Factor Deficiencies X-Linked Factor Deficiencies: Hemophilia A and B Clinical Diagnosis of Hemophilia (see Chapter 138) Treatment of Hemaphilis (see Chapters 90, 96) Genetics Autosomal Factor Deficiencies Fibrinogen Deficiency Prothrombin Deficiency

Factor VII Deficiency Factor X Deficiency Factor XI Deficiency Factor XII and Contact Factor Deficiencies Combined Deficiencies of Vitamin K-dependent Factors Unrecognized Factor Deficiencies

Acronyms and Abbreviations ACT, activated clotting time; aPTT, activated partial thromboplastin time; coagulation factors: FII, factor II; FVII, factor VII; FVIII, factor VIII; FIX, factor IX; FX, factor X; FXI, factor XI; FXII, factor XII; FXIII, factor XIII; DIC, disseminated intravascular coagulation; HMWK, high molecular weight kininogen; PIVKA, proteins induced by vitamin K absence/antagonism; OMIA, on-line Mendelian inheritance in animals; OMIM, on-line Mendelian inheritance in man; PK, prekallikrein; PT, prothrombin time; RVVT, Russell’s viper venom time; TCT, thrombin clotting time; VKOR, vitamin K epoxide reductase.

HEREDITARY COAGULATION FACTOR DEFICIENCIES Hereditary coagulopathies arise from mutations within genes required for synthesis or processing of active coagulation factors. The bleeding tendency associated with each trait depends on the physiologic role of the deficient factors in fibrin clot formation and the degree of factor deficiency. Hemophilia A is the most common hereditary coagulopathy in human beings and animals, due in part to a high de novo mutation rate in the coagulation FVIII gene (F8).11 In addition, new factor deficiencies continually arise within breed populations with variable frequencies that change over time. The general breeding strategies to limit propagation of hereditary factor defects include exclusion of individuals expressing a bleeding tendency, avoidance of repeat matings that produce affected individuals, and familial screening (especially dams, sires, siblings) of affected patients to identify others with quantitative or functional factor deficiencies. In addition to the review of defects presented in this chapter (Table 86.1), public access databases, such as Online Mendelian Inheritance in Man and Animals (OMIM, OMIA) hosted by the

NIH, provide information on hereditary factor deficiencies in animals. X-LINKED FACTOR DEFICIENCIES: HEMOPHILIA A AND B Hemophilia A (OMIM 306700) and hemophilia B (OMIM 306900) are distinct X-linked bleeding disorders caused by functional or quantitative deficiencies of FVIII and FIX, respectively.11 Although both genes map to the long arm of the X chromosome, they are inherited independently. Factor IX (a serine protease enzyme) and FVIII (its coenzyme) play critical roles in the amplification phase of coagulation by participating in formation of the “tenase” enzyme complex (see Chapter 84). Clinical Diagnosis of Hemophilia Hemophilia A and B are diagnosed primarily in males because of the sex-linked, recessive inheritance pattern of these traits. Carrier (heterozygous) females do not express a bleeding tendency. Signs of hemophilia include lameness due to hemarthrosis, intramuscular 661

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TABLE 86.1 Hereditary Factor Deficiencies Screening Test Results Factor Deficiency

Abnormal

Normal

Factor II (prothrombin) Factor VII (proconvertin)

TCT,a fibrinogena All testsb aPTT, PT PT

TCT, fibrinogen aPTT, TCT, fibrinogen

Factor VIII (hemophilia A)

aPTT

PT, TCT, fibrinogen

Factor IX (hemophilia B)

aPTT

PT, TCT, fibrinogen

Factor X (Stuart Prower factor)

aPTT, PT

TCT, fibrinogen

Factor XI (PTA deficiency, hemophilia C) Factor XII (Hageman factor)

aPTT

PT, TCT, fibrinogen

aPTT

PT, TCT, fibrinogen

Fibrinogen

*aPTT, *PT

Species (Breeds) Dogs (Bichon Frise, Borzoi, collie), cats (DSH), goat (Saanen), sheep (Leicester) Boxer Dogs (Alaskan Klee Kai, Beagle, Deerhound, Malamute, Schnauzer), cats (DSH) Dogs (any breed, mixed breeds, German shepherd, Golden retriever), cats (any breed, DSH), horse, sheep Dogs (any breed, mixed breeds), cats (any breed, DSH), horse Dogs (Cocker spaniel, Jack Russell terrier), cats (DSH) Dogs (English springer spaniel, Kerry blue terrier), cats (DSH) Dogs (Miniature poodle, Shar Pei), cats (DSH, DLH, Siamese, Himalayan)

a

Mild fibrinogen deficiency. Severe fibrinogen deficiency.

b

FIGURE 86.1 Clinical signs of hemophilia. Intramuscular hematoma formation in the lateral thigh muscles of a hemophiliac German shepherd. Note distal limb swelling due to edema, venous obstruction, and extravasated blood.

and subcutaneous hematoma formation, and prolonged hemorrhage from minor wounds and gingiva at tooth eruption sites (Fig. 86.1). Severe hemophilia typically manifests within the first few months of life. Severely affected patients are at risk for spontaneous and fatal hemorrhage, whereas milder hemophilia may become apparent only after surgery or trauma. Hemophilia A is the more common form in all species, with an incidence of 1–2 cases per 10,000 male births (in human populations). Unlike many hereditary diseases in animals, hemophilia is not restricted to a single breed or inbred line. Cases of hemophilia A and B have been identified in mixed breed dogs and more than 50 different purebred lines.2 Both traits have been reported in domestic and purebred cats9,14 and horses,6 and hemophilia A has been characterized in sheep.1 The laboratory diagnosis of hemophilia is based on results of coagulation assays. Both forms of hemophilia cause specific prolongation of intrinsic pathway screening tests (ACT and aPTT), but do not affect tests of the extrinsic/common pathways or fibrinogen (see Table 86.1). Definitive diagnosis and differentiation of hemophilia A from hemophilia B requires specific measurements of FVIII and FIX coagulant activities (FVIII:C, FIX:C). Clinical severity relates to residual factor activity. Severe hemophilia is characterized by FVIII:C or FIX:C < 2% of normal, whereas factor activities of 2–20% are associated with moderate to mild hemophilia. Factor assay values are derived from comparisons of patient plasma with the activity of plasma standards. Reaction kinetics vary among species; therefore animal factor activity assays are best performed using same-species plasma standards (see Chapter 138).

CHAPTER 86: HEREDITARY COAGULOPATHIES

663

FIGURE 86.2 Inheritance of hemophilia A and X

Y

X

X

Carrier Female

X

X

X

X

Y

X

Carrier Female (50%)

A

X

X

B. (A) Propagation of hemophilia by a carrier female. If a carrier female is bred, on average one-half of her daughters and one-half of her sons will inherit her mutant gene (depicted as a purple bar on the X chromosome). (B) Propagation of hemophilia by an affected male. Males with mild forms of hemophilia may survive to reproduce. If bred, they transmit a mutation to all their daughters and none of their sons.

Y

(50%)

Y

X

X

A ected Male

X

B

X

Carrier Female (100%)

X

Y

Clear Male (100%)

Treatment of Hemophilia Transfusion is the primary means for controlling or preventing hemorrhage in animals affected with hemophilia A or B (see Chapters 88 and 96). Fresh frozen plasma supplies both FVIII and FIX, whereas cryoprecipitate is a more specific product for replacement of FVIII, and cryosupernatant supplies FIX. The transfusion interval for hemophiliacs varies depending on residual factor activity, and clinical features such as patient size, activity, and conditions that further impair hemostasis. Maintenance of patients with severe hemophilia (120 seconds), with normal PT, TCT, and fibrinogen. A prolonged aPTT that shortens by increasing activation time is a common characteristic of PK-deficient plasmas. Specific deficiencies of the contact group factors can be diagnosed based on functional assays of FXII, PK, and HMWK (see Chapter 138). Due to its high prevalence, FXII deficiency should be considered a likely cause of specific prolongation of aPTT in cats. FXII deficient cats do not require specific therapy. Long aPTT is a persistent in vitro finding but is not an indication for transfusion of any contact factor deficient patients. Contact factor deficiencies are recessive traits; however, causative mutations have not yet been identified in animals. Cats homozygous for FXII deficiency have residual FXII:C values of approximately 2–10%, whereas heterozygous carriers generally have moderate reduction in activity with residual FXII:C from 20% to 40%.17 Combined Deficiencies of the Vitamin K-Dependent Factors Factors II, VII, IX, and X are serine protease enzymes that require post-translational γ-carboxylation in order to form active complexes with their coenzymes and substrates. Defects in the hepatic vitamin K reductase or carboxylase enzymes impair vitamin K recycling and cause coagulopathy due to a persistent vitamin K deficiency state (see Chapter 84). Familial combined deficiency of FII, VII, IX, and X is a rare recessive trait in people and has been identified in cats, sheep, and recently in Labrador retrievers (Table 86.1; OMIM 277450 and 607473).15,20,22 Clinical signs range from moderate to severe coagulopathy, with reports of spontaneous hematoma formation, prolonged hemorrhage after surgery, and some fatal hemorrhage. Combined deficiency of the vitamin K-dependent factors causes prolongation of the aPTT and PT (and PIVKA) screening tests, but does not affect TCT and fibrinogen. Specific coagulation factor assays confirm deficiencies of FII, VII, IX, X, with normal activities of

CHAPTER 86: HEREDITARY COAGULOPATHIES

the non-vitamin K-dependent factors. Acquired vitamin K-deficiency is common (see Chapter 85); therefore initial diagnostic work-up should rule out exposure to vitamin K antagonists, and liver disease or malabsorptive disorders. After exclusion of acquired disorders, specific analyses of hepatic microsomal vitamin K epoxide reductase (VKOR) and carboxylase enzyme activities further define a hereditary trait. Patients with severe hemorrhage at presentation may require transfusion for rapid replacement of active factors (see Chapter 96). After an initial lag-time of 1–2 days, case reports describe normalization of aPTT and PT and control of clinical signs of hemorrhage in response to oral vitamin K1 at approximately 2.5 mg/kg per day. Long-term maintenance of hemostasis has been reported at reduced daily dosages (e.g. 1 mg/kg/ day) given at 3 day intervals. Familial studies to date in all species reveal a recessive inheritance and expression pattern. The defective hepatic enzyme in cats and sheep is γ-glutamyl carboxylase; however, causative mutations in the corresponding genes have not yet been identified. Unrecognized Factor Deficiencies Hereditary deficiencies of FV, FXIII, and antiplasmin have been reported as rare hemorrhagic defects in people, with an estimated case incidence of 1,000 ng/mL).47 Echocardiography is a valuable diagnostic tool for PTE in people, and is a rapid and non-invasive means to rule out primary cardiac disease in dyspneic patients.21,64 Abnormalities typical of human PTE include right ventricular dilation and hypokinesis, pulmonary arterial hypertension, tricuspid regurgitation, and paradoxic septal wall motion. Regional hypokinesis sparing the ventricular apex (McConnell sign) is a specific finding.38 Reported PTE-associated echocardiographic abnormalities in animals include dilation of the right atrium, ventricle, and pulmonary artery, pulmonary hypertension, paradoxic septal wall motion, and visualization of a thrombus.29,33,49 Pulmonary angiography, scintigraphy, and helical computerized tomography (CT) are definitive tests for PTE.31,61 Selective pulmonary angiography, performed by direct injection of iodinated dye into the pulmonary artery was the early gold standard.61,64 Intraluminal filling defects, abrupt termination of pulmonary arteries, and complete absence of arterial branches are diagnostic for PTE. Additional supportive findings include regional loss of vascularity, asymmetric blood flow, tortuous pulmonary arteries, and abrupt tapering of peripheral vessels. A negative selective arteriogram excludes clinically significant PTE. The invasive procedure requires general anesthesia, thus constituting a safety risk for compromised patients. Nonselective pulmonary angiography, performed by injecting contrast media into the jugular vein or right side of the heart, is a simpler and safer technique that does not require general anesthesia. Nonselective angiography, however, is relatively insensitive and difficult to interpret due to dilution of contrast medium by venous blood and the superimposition of vascular structures. Pulmonary scintigraphy is a sensitive and specific test of PTE; however, its requirement for radiolabeling and specialized equipment limit its routine veterinary use.31 The procedure involves sequential scans that assess pulmonary blood flow (perfusion scan) and air flow (ventilation scan) to identify regions of ventilated lung lacking perfusion (i.e. V/Q mismatch). Perfusion scans are performed by injecting technetium-labeled macroaggregated albumin (99 mTc-MAA) into a central vein. Distribution is proportional to blood flow; inho-

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mogeneous distribution is consistent with abnormal perfusion. Perfusion scanning is contraindicated in patients with severe pulmonary hypertension because 99 m Tc-MAA particles can occlude capillaries and cause right-sided heart failure. A normal perfusion scan excludes PTE; however, abnormal scans can result from nonthrombotic conditions (e.g. pneumonia, edema, contusions, obstructive pulmonary disease, atalectasis). In contrast to PTE, nonthrombotic conditions are associated with decreased regional ventilation. For this reason, interpretation of abnormal perfusion scans requires assessment of pulmonary ventilation. Ventilation scans are performed by the administration of a nontoxic radioactive gas, such as technetiumlabeled diethylene-tiamene-penta-acetic acid (99 mTcDTPA), via closed ventilation circuit.31 Hypoventilated lung regions appear photopenic. While PTE disrupts regional blood flow, ventilation scans are typically normal. Severely dyspneic animals may be unable to undergo ventilation scans. In these cases, an absence of radiographic abnormalities in regions of abnormal perfusion can be considered evidence of V/Q mismatch, compatible with PTE. Helical, or spiral, computed tomographic angiography (CTPA) has largely replaced angiography and scintigraphy for the definitive diagnosis of PTE in people.61,64,66 This rapid, noninvasive technique enables high resolution visualization of the pulmonary vasculature. Moreover, CTPA can define nonthrombotic pulmonary disease and is ideal for unstable patients. Advanced, “multislice” CTPA techniques enable quality images in awake or minimally sedated patients. The technique is extremely accurate in detecting thrombi in the main, lobar or segmental arteries. Recent studies describe CTPA to visualize the pulmonary vasculature and detect PTE in dogs.66 Aortic Thromboembolism Feline Aortic Thromboembolism Aortic TE is a common and devastating condition in cats, with reported survival-to-discharge rates of approximately 30%.35,55,60 The inciting cause is usually HCM, with additional cases attributed to other cardiac disease, or rarely neoplasia.57,60 Thrombi typically form in the left ventricle with embolization to the aortic trifurcation (“saddle thrombus”) causing blood flow obstruction of both hind limbs. Less frequently, emboli lodge proximal to the renal arteries, or obstruct a single iliac or brachial vessel. Acute obstruction of the left ventricle or proximal aorta may cause sudden death. The consequences of ATE include physical occlusion of the vessel and embolus-induced vasoconstriction that disrupts collateral flow. Serotonin and TxA2 have been implicated as mediators of this intense vasoconstriction.56 Aortic occlusion increases left ventricular afterload, which may precipitate heart failure. The diagnosis of ATE is usually based on characteristic history and clinical signs.60 Affected cats develop acute pain, vocalization, and loss of motor function.

Physical exam may reveal swollen, tense hind limb musculature, cool extremities, pale or dark purple pads and nailbeds, and weak or absent femoral pulses. Varying sites of occlusion produce variable signs such as paresis (or paralysis), loss of sensation, or renal dysfunction or failure. At presentation many cats are dyspneic due to heart failure, with heart murmurs and/or gallop rhythms. More specific assessment of limb perfusion can be performed with color flow Doppler imaging or by cutting a nail back to the “quick” and observing the presence or absence of normal blood color and flow. Angiographic and nuclear imaging are definitive PTE tests; however, the risks of these invasive procedures are rarely justified. Cats with suspect or confirmed ATE should undergo cardiac evaluation (i.e. echocardiography, electrocardiography, thoracic radiography). Up to 90% of cats have cardiomegaly, and 50–70% have evidence of heart failure.35 Electrocardiographic abnormalities include left ventricular enlargement and conduction abnormalities, left anterior fascicular block, and ventricular and supraventricular premature beats. Biochemical profiles often reveal hyperglycemia and other nonspecific abnormalities reflecting muscle ischemia and stress. Uremia may develop in patients with renal vascular TE. In the absence of cardiac disease, diagnostic efforts should be directed toward identification of other potential hypercoagulable states, especially neoplasia. Canine Aortic Thromboembolism Although canine ATE is relatively uncommon, it has been reported in association with cardiac disease (infective endocarditis, heartworm, cardiomyopathy, and valvular disorders), PLN, PLE, neoplasia, hyperadrenocorticism, atherosclerosis, and localized vascular injury (e.g. Spirocerca lupi).29,63,69 Clinical signs depend on TE site, the extent and rapidity of occlusion, and the degree of collateral circulation.22 Dogs with underlying cardiac disease tend to have acute onset and severe signs resembling those of ATE in cats. Hypercoagulable syndromes generally cause local thrombosis with more insidious onset. In these cases, exercise intolerance, ill-defined pain and mild locomotor deficits or ataxia may develop. Femoral pulses may be absent, weak, or asymmetric. Prognosis generally depends on the underlying condition. Definitive diagnosis of ATE is achieved via imaging.29,33 Abdominal ultrasonography is most commonly employed and is usually reliable, especially when combined with color flow Doppler assessment. Additional diagnostic techniques include conventional and radionuclide angiography, magnetic resonance imaging, thermography, and Doppler device to assess blood flow to the affected limbs. Since canine ATE is rarely a primary disease, the diagnosis should always prompt a thorough evaluation for underlying disorders. Biochemical profiles of affected dogs often reveal high creatinine kinase and aspartate aminotransferase; however, other biochemical abnormalities may provide more specific indication of a primary disease process.

CHAPTER 87: THROMBOTIC DISORDERS

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Disseminated Intravascular Coagulation TRACY STOKOL Causes Pathogenesis Initiation Progression/Dissemination Amplification of thrombin production Role of phosphatidylserine Dissemination of coagulation Fibrinolysis Perpetuation of DIC Inflammatory respose Endothelial cell dysfunction Other factors

Diagnosis of DIC Overt DIC Clinical diagnosis Laboratory criteria of overt DIC in humans Scoring System for Overt DIC in Humans Non-Overt DIC Treatment of DIC Replacement of Coagulation Factors and Platelets Anticoagulant Therapy Replacement of Inhibitors and Anti-inflammatory Therapy

Acronyms and Abbreviations APC, activated protein C; aPTT, activated partial thromboplastin time; AT, antithrombin; CT, computerized tomography; DIC, disseminated intravascular coagulation; EC, endothelial cells; FDP, fibrin(ogen) degradation product; FV/FVa, factor V/activated factor V; FVII/FVIIa, factor VII/activated factor VII; FVIII/FVIIIa, factor VIII/ activated factor VIII; FIX/FIXa, factor IX/activated factor IX; FX/FXa, factor X/activated factor X; FXI/FXIa, factor XI/activated factor XI; FXIII, factor XIII; IMHA, immune-mediated hemolytic anemia; ISTH, International Society of Thrombosis and Hemostasis; MP, microparticle; PAR, protease-activated receptor; PCI, protein C inhibitor; PS, phosphatidylserine; PT, prothrombin time; RBC, red blood cell; TAFI, thrombin-activatable fibrinolysis inhibitor; TAT, thrombin-antithrombin complexes; TCT, thrombin clotting time; TF, tissue factor; TFPI, tissue factor pathway inhibitor.

D

isseminated intravascular coagulation (DIC) describes a complex, dynamic state of hemostatic imbalance resulting in thrombus formation throughout the microvasculature. Many different primary disorders trigger DIC, and its clinical and laboratory manifestations vary among patients and change over time, complicating its recognition and treatment.6 Hematologists have recently developed guidelines delineating distinct phases of overt and non-overt DIC, with the goals of standardizing terminology and improving clinical diagnosis (see Table 88.1).54,57 The International Society of Thrombosis and Hemostasis (ISTH) has proposed a consensus definition of DIC as follows: “An acquired syndrome characterized by the intravascular activation of coagulation with loss of localization resulting from different causes. It can originate from and cause damage to the microvas-

culature, which if sufficiently severe, can produce organ dysfunction.”54 PRIMARY DISEASE CONDITIONS An underlying disease process always initiates DIC; it is not a primary disorder (see Table 88.2). DIC has been documented in most domestic animals, with the exception of camelids. In dogs and cats, neoplasia and systemic inflammation (e.g. sepsis, pancreatitis, immune-mediated hemolytic anemia [IMHA]) are the most common initiating diseases.13,17,46,48,49,56 Endotoxemia (secondary to gastrointestinal disorders) and sepsis are the main causes of DIC in adult horses and neonatal foals, respectively.3,15,50 Similarly, DIC is primarily due to endotoxemia or sepsis in ruminants.26,39 679

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TABLE 88.1 Characterization of Overt and Non-Overt DIC Overt DIC

Non-Overt DIC

Alternate names and subtypes

Uncompensated DIC Acute DIC Fulminant DIC Consumptive coagulopathy Hypocoagulable DIC

Compensated DIC Chronic DIC Subclinical DIC Pre-DIC

Pathophysiology

Decompensated hemostasis with fibrin deposition throughout the microvasculature. Platelet and factor depletion may develop Primary inciting disease Some or all of the following: Systemic thrombosis, PTE, MODSa Hemorrhage from multiple sites

Procoagulant excess opposed by coagulation inhibitors, partial compensation modulates fibrin deposition Primary inciting disease Subclinical or tissue thrombosis

Screening test abnormalities

Some or all of the following: Thrombocytopenia, prolonged clotting times (aPTT, PT, TCT), hypofibrinogenemia, low AT, high FDP and D-dimer

No screening test abnormalities and/or progressive fall in platelet count and increase in clotting times, high FDP and D-dimer

Tests of pre-thrombotic state

Not applicable

Byproducts of thrombin generation or thrombin activity: Prothrombin fragment 1+2, TAT complex, APC-PCI complex, Fibrinopeptide A & B Kinetics of thrombin or fibrin generation: Endogenous thrombin potential Thromboelastography

Clinical features

a

MODS, multiple organ dysfunction syndrome; PTE, pulmonary thromboembolism.

PATHOGENESIS Over the past decade, our understanding of DIC has evolved from recognition of a severe hemorrhagic disorder to appreciation of DIC as a disease continuum. The process is initiated by inappropriate activation of hemostasis, at first kept in check by natural inhibitors. If regulatory mechanisms become overwhelmed, DIC progresses to a full-blown decompensated disorder characterized by systemic thrombosis and ultimately, a consumptive coagulopathy.6,57 Much of the current knowledge of DIC pathogenesis is derived from experimental models of sepsis-induced DIC.31,54 Although disease-dependent differences influence the initiation and clinical course of DIC, the consistent pathogenic mechanism remains the generation of excessive thrombin through dysregulated activation of coagulation.54 For ease of understanding, the DIC continuum can be separated into three stages: initiation, progression/dissemination, and perpetuation.19,54,57 Initiation Since DIC represents normal hemostasis gone awry, the same triggering event underlies DIC and physiologic hemostasis, i.e. exposure of tissue factor (see Chapter 84).24 Tissue factor (TF, tissue thromboplastin, factor III) is a transmembrane protein whose expression is normally restricted to interstitial perivascular fibroblasts. Under physiologic conditions, exposure of TF to

its plasma ligand, factor VII (FVII), occurs only at focal sites of endothelial cell injury.23 In DIC triggered by massive tissue trauma, widespread endothelial injury exposes supraphysiologic amounts of extravascular TF. In conditions such as viral infections, intravascular hemolysis, and vasculitis, generalized endothelial injury exposes extravascular TF and/or induces TF expression on damaged endothelium. Inflammatory cytokines also contribute to TF expression in these latter disorders (see Table 88.2).19 In the absence of endothelial injury, DIC may be initiated by aberrant expression of TF on the surface of intravascular cells, particularly monocytes and tumor cells (see Table 88.2). The pathogenesis of DIC in sepsis involves up-regulation of TF expression on monocytes (and possibly endothelial cells) by proinflammatory cytokines (Fig. 88.1).19,25 Although cancer cells have many ways of activating hemostasis, some types of neoplasms constitutively express high levels of TF thereby promoting cancerassociated DIC.21 TF-independent mechanisms of initiating DIC also exist (see Table 88.2). For example, proteases identified in erythrocyte membranes, tumor cells, and snake venoms can directly activate coagulation factors.28,47,63 The development of DIC depends not only on the location and severity of the inciting stimulus, but on the ability of naturally occurring inhibitors to regulate the procoagulant hemostatic response. In physiologic hemostasis, the TF-FVIIa-FXa complex is rapidly neu-

CHAPTER 88: DISSEMINATED INTRAVASCULAR COAGULATION

TABLE 88.2 Inciting Diseases and Pathophysiologic Mechanisms of DIC in Animals Disease Categories and Specific Disorders

Potential Mechanisms

Infections Cytokine-induced TF expression Sepsis (Gram positive on monocytes (and EC) and negative bacteria), Endothelial injury Exposure of extravascular TF viruses, protozoa (Babesia), or induction of TF on EC parasites (Angiostrongylus), rickettsia (Rocky Mountain spotted fever) Neoplasia Solid tumors (hemangiosarcoma, mammary cancer) Hematopoietic malignancy (lymphoma, acute leukemia)

Expression of TF on cancer cells Constitutive or induced by hypoxia, cytokines, or apoptosis Induction of TF expression Tumor-secreted cytokines act on monocytes, fibroblasts, and EC Shedding of TF-bearing microparticles from tumor cells, hematopoietic cells, other cell types Expression/secretion of procoagulants Cancer procoagulant (a vitamin K-dependent cysteine protease) mucin, up-regulation of factor V receptor, factor XIII-like activity, platelet activators Chemotherapy-induced changes Tumor lysis, myelotoxicity

Inflammation/necrosis Trauma, IMHA, pancreatitis, heat stroke, hepatitis, vasculitis, gastric dilatation-volvulus, strangulating obstructions and inflammatory gastrointestinal disorders

Inflammatory cytokines EC injury Tissue injury/necrosis/apoptosis Exposure of TF; shedding of PS-enriched MPs by apoptotic cells Release of procoagulant proteases (e.g. trypsin)

Intravascular hemolysis IMHA, acute transfusion reactions, insect/snake bites

EC injury Erythrocyte procoagulant activity RBC membrane-associated elastase, shedding of PS-enriched MPs

Envenomation

Snake venom proteases Direct coagulation factor activation and fibrinogen cleavage, phospholipaseinduced tissue injury/cell lysis

tralized by tissue factor pathway inhibitor (TFPI). TFPI is primarily expressed on endothelial cells, with its activity enhanced by cell-surface heparin and heparinlike glycosaminoglycans.32 In the process of DIC, TFPI is rendered ineffective through a number of mechanisms, including cleavage of TFPI by granulocytic elastases,

681

cytokine-mediated suppression of TFPI expression, and generation of excess TF-FVIIa that overwhelms TFPI’s inhibitory capacity.22,43 Progression/Dissemination Amplification of Thrombin Production Thrombin is pivotal to DIC progression (Figs. 88.1 and 88.2). In a positive feedback cycle, trace amounts of generated thrombin are amplified by intrinsic pathway reactions to produce a massive burst of thrombin.24,57 The thrombin burst cleaves soluble fibrinogen to form fibrin polymers, and ultimately the cross-linked fibrin clot. Thrombin simultaneously inhibits fibrin degradation via thrombin-activatable fibrinolysis inhibitor (TAFI). In early DIC, as in physiologic hemostasis, the generation of thrombin and its actions are opposed by the plasma anticoagulants, antithrombin (AT) and protein C, and the endothelial cell surface receptor, thrombomodulin (see Fig. 88.2 and Chapter 84). This phase of systemically activated coagulation restrained by natural anticoagulants is referred to as “non-overt” DIC, a state of stressed, but compensated hemostasis (Table 88.1 and Fig. 88.1).54,55 Patients with non-overt DIC are hypercoagulable, i.e. at risk for widespread microvascular fibrin deposition. Role of Phosphatidylserine Coagulation is a membrane-anchored process involving assembly of coagulation factor complexes on the surface of cells expressing the negatively charged phospholipid, phosphatidylserine (PS; see Chapter 84).24 Resting cells restrict PS to the inner leaflet of their cell membrane; however, this asymmetry is lost when cells are activated, undergo apoptosis, or lyse.30 During physiologic hemostasis, activated platelets provide the membrane-surface PS to support formation of coagulation complexes; hence PS is referred to as “platelet factor 3” or platelet phospholipid. In the course of DIC, cell-surface PS may be expressed by a variety of activated or injured cell types, by membrane-derived microparticles (MPs), and circulating lipoproteins (native very low density lipoproteins and oxidized low density lipoproteins).14,20,45 Microparticles are tiny (100 x 109/L = 0; 1.0 g/L = 0; < 1.0 g/L = 1)

4.

Calculate score

5.

If ≥ 5: compatible with overt DIC; repeat scoring daily If < 5; suggestive (not affirmative) for non-overt DIC; repeat next 1-2 days

* Values corresponding to a moderate or strong increase are test- and laboratory-dependent.

FIGURE 88.3 ISTH scoring system for overt DIC in man. (Adapted from Taylor FB, Jr., Toh CH, Hoots WK, et al. Towards definition, clinical and laboratory criteria, and a scoring system for disseminated intravascular coagulation. Thromb Haemostasis 2001;86:1327–1330, with permission.)

also applies to the clinical designation of hypercoagulability or “chronic DIC” reflecting continuous low grade, but compensated, activation of coagulation (Table 88.1 and Fig. 88.1). Routine coagulation screening tests are insensitive to this phase because they detect coagulation factor deficiencies, rather than accelerated coagulation. Technically sophisticated assays that measure the rate and byproducts of thrombin generation, coagulation inhibitor complexes, and kinetics of fibrin formation provide mechanistic and prognostic information on non-overt DIC (Table 88.1).38,51,57 Unfortunately, these tests are not widely available for routine clinical diagnosis. Thus, the ISTH has developed a second algorithm for the diagnosis of non-overt DIC primarily based on routine laboratory tests.54 The scoring system requires serial testing to evaluate the dynamic nature of nonovert DIC and awards points for an underlying disease (Fig. 88.4). The ISTH scheme for non-overt DIC has been modified and applied to critically ill dogs.62 In a preliminary study, the sensitivity of the modified scheme for diagnosing non-overt DIC and predicting 28-day mortality rates was compared to that of traditional criteria for diagnosing DIC (i.e. three or more abnormalities in hemostatic tests). More dogs were diagnosed with DIC using traditional criteria; however, the classification of non-overt DIC was more closely related to mortality.62 Further evaluation is needed to identify the optimum tests and test cut-offs to include in a DIC scoring scheme. Furthermore, due to species-differences in underlying diseases and diagnostic test sensitivity for DIC, it is unlikely that a single scoring system will apply across species. The strategy of serial monitoring of routine laboratory tests (e.g. platelet count, APTT, AT, D-dimer, fibrinogen) should be evaluated for clinical diagnosis of

non-overt DIC. Trends toward abnormal values would indicate hemostatic system decompensation and progression of DIC, even if absolute values remain within assay reference intervals. Point-of-care “global tests of hemostasis” such as thromboelastography may also prove useful for identifying animals in hypercoagulable phases of DIC.16,61 Ultimately, there is a need to establish consensus guidelines for diagnosing non-overt and overt DIC in animals. This would serve as a basis for achieving consistency across studies to assess the impact of early diagnosis and various treatment protocols on defined clinical outcomes. TREATMENT OF DIC Affected animals should always be treated for their primary disease with the goal of breaking the DIC cycle. Supportive care aimed at alleviating metabolic/hemodynamic sequelae of DIC (shock, hypoperfusion and acidosis) helps minimize organ damage, inflammation, and continued activation of hemostasis. Effective treatment modalities beyond primary disease-specific and supportive care remain unproven. General treatment options include transfusion therapy and anticoagulant and anti-inflammatory drug therapy (see Chapters 90 and 96). Treatment recommendations are generally derived from the human literature and should be applied with the knowledge that they may not be suitable for animals, due to species-specific differences in hemostasis and drug pharmacokinetics and efficacy. Randomized, controlled clinical trials, with collaboration between multiple institutions or practices and consensus on testing criteria and a defined outcome (e.g. 28-day mortality), are urgently needed in veteri-

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1.

Risk assessment: Does the patient have an underlying disorder known to be associated with DIC?

score

Yes = 2, no = 0

2.

Major criteria

Platelet count

>100 x 109/L = 0; 10 >1000

EAC

18

A1 A2 H D Z′ B1 B2 G1 G2 G3 K I1 I2 O1 O2 O3 Ox P1 P2 Q T1 T2 Y1 Y2 A′ B′ D′E′1 E′2 E′3 F′ G′ I′1 I′2 J′1 J′2 K′ O′ P′ Q′ Y′ A″ B″ G″ I″ C1 C2 E R1 R2 W X1 X2 C′ L′X′ C″

EAF EAJ −EAL EAM EAS EAZ EAR ′ EAT ′

17 11 3 23 21 10 16 19

F1 F2 V1 V2 N′ J L M1 M2 M′ S H′ U1 U2 U′1 U′2 S″ H″ U″ Z R′ S′ T′

–, A1, A2, A1D, H, A1H, A1Z ′, etc. –, B1GKOxY2A′O′A″, B1O1, B1O1Y2D′, B1I1Q, B2GOxA′A″, O1Q′, I1Y2E′1Y′, I1Q′, Y1D′I′, GY2E′1Q′, O1T1E′3F′K′, O1T1E′3F′G′K′G″, OxQA′E′1O′, etc. –, C1, C1EW, C1R1W, C1WL′, C2, C2WX2, R1WL′, R2WX2L′, WX2, etc. F1, F2, FN′, V1, V2, V1N′ –, J –, L –, M1, M′ –, SH, H′, U1, U1H′H″, U′, U″, U″U1H′H″, U′2U″, etc. –, Z R′, S ′ –, T ′

>60 7 2 2 3 >15 2 2 2

a

From refs. 61 and 67. The absence of detectable factors (null allele, recessive) in any locus is represented by a dash (–).

b

TABLE 92.5 Horse Blood Group Loci, Chromosome Assignment (Chr), Factors and Alleles (Phenogroups) Defined Locus

Chra

Factors

Recognized Allelesb

EAA EAC EAD

20 Unknown 14

abcdefg a abcdefghiklmnopqr

EAK EAP EAQ EAU

2 Unknown 8 24

a abcd abc a

Aa Aadf Aadg Aabdf Aabdg Ab Abc Abce Ac Ace Ae A– Ca C− Dadl Dadlnr Dadlr Dbcmq Dcefgmq Dcegimnq Dcfgkm Dcfmqr Dcgm Dcgmp Dcgmq Dcgmqr Dcgmr Ddeklr Ddeloq Ddelq Ddfklr Ddghmp Ddghmq Ddghmqr Ddkl Ddlnq Ddlnqr Ddlqr Dq (D –) K a K− P a P ac P acd P ad P b P bd P d P − Qabc Qac Qa Qb Qc Q − Ua U −

a

From ref. 76. The absence of detectable factors is designated by the system letter followed by a dash (–).

b

if produced by a blood group incompatible pregnancy. Hemolytic screening for this antibody (and others) prior to foaling can provide an effective test to identify mares whose foals are at risk for NI. In EAQ sensitization, the mare is negative for factor Qa (for most breeds this means negative for Qabc), while the stallion has this blood group. The detectable antibody response for Qa has only been reported as a lysin and would be missed by antibody screening based solely on agglutination testing. Occasionally the antibody presence can be detected only very late in pregnancy. Antibodies for blood group factors in EAC, EAD, EAK, EAP and EAU pose little risk as they do not occur naturally and have only rarely been implicated in NI cases, possibly as a result of blood incompatible pregnancies. Five blood group loci have been mapped to chromosomes by linkage analysis.76 A summary of the seven horse blood group loci, factors and allelic variants is given in Table 92.5. Sheep and Goat Eight blood group systems have been described in sheep: EAA, EAB, EAC, EAD, EAM, EAR, EAF30 and

EAF41.74 The nomenclature of systems and factors follows the same rules as those used for the horse and pig. Although many antigen specificities have been identified, only 22 blood group factors are internationally recognized in sheep. Hemolytic tests are the method of choice for sheep blood group factors with exception of the D system whose antigens are detected by agglutination tests. The sheep EAB, EAC and EAR systems are homologous to cattle EAB, EAC and EAJ. Products of the EAR locus are not an intrinsic component of the RBC membrane. The antigens are soluble substances found in the serum and saliva which attach to the RBC membrane. The EARR allele is dominant to EARO and expression of both antigens is under the control of the suppressor gene I. Sheep with genotype i/i do not express R or O antigens in RBCs, serum or saliva. The EAC locus is closely linked to the Amino Acid Transport gene. This association is of interest because RBCs with defective transport are never Cb-negative. The EAM locus is associated with RBC potassium transport such that low-potassium cells are always Mb-positive and high-potassium cells are always Mb-negative. It has been postulated that Mb inhibits active potassium transport into cells.100 Several sheep blood group

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TABLE 92.6 Sheep Blood Group Loci, Chromosome Assignment (Chr), Factors and Alleles (Phenogroups) Defined Chra

Loci

Blood factors

Number of Alleles 3 >502

EAA EAB

6 10

ab abcdefghi

EAC EAD EAM EAR EAX

20 Unknown 18 Unknown Unknown

ab ab abc RO XZ

42 2 4 2 2

Allelesb –, a, b –, a, ab, abc, etc –, a, ab, b –, a –, a, b, ac, c R, O X, Z

a

From ref. 29. Number of alleles based on internationally recognized factors. Additional factors with provisional designations have been identified that increase number to 100 EAB and 20 EAC alleles; the absence of detectable factors (null allele, recessive) in any locus is represented by a dash (–).

acetylgalactosamine transferase that adds a terminal fucose residue to the H substance.69 The recessive EAAO allele contains a deletion within the gene and results in a non-functional enzyme product.106 The EAH and EAS systems correspond to fucosyl-transferase 1 (FUT1) and fucosyl-transferase 2 (FUT2) that are involved in the production of the H substance and are related to human H (FUT1) and Secretor (FUT2) genes.25,69 Antigens of the EAN system also occur as soluble substances in the serum and milk but are not secreted into the saliva. Blood group loci have been mapped to pig chromosomes by linkage analysis.78 A summary of pig blood group systems, factors and alleles is given in Table 92.7. Llama and Alpaca

b

systems have been mapped by linkage analysis. A summary of sheep blood group loci, factors and allelic variants is given in Table 92.6. Blood groups in goats are less developed than those of other farm animals. Six genetic systems have been identified and named EAA, EAB, EAC, EAE, EAF and EAR.73 Several goat blood group factors cross-react with sheep blood typing reagents. The EAA system has a single specificity A1 that is homologous to sheep EAAa. In the EAB system, 14 specificities were identified, four of which are homologous to sheep factors Bb, Be, Bd and Bi. The EAC system has a single specificity related to sheep EACa. The EAE system contains two factors E6 and E18 and the EAF system is defined by a single specificity F19. The EAR system is defined by a single specificity R related to sheep R. Pig Development and expansion of pig blood groups is largely due to work carried out in Denmark, Germany, Poland and Russia. The source of blood typing reagents is primarily from isoimmune sera with most antibodies behaving as agglutinins and a few as hemolysins. Sixteen genetic systems are internationally recognized: EAA, EAB, EAC, EAD, EAE, EAF, EAG, EAH, EAI, EAJ, EAK, EAL, EAM, EAN, EAO and EAP.66 Some of these, e.g. EAE and EAM, approach the EAB and EAC systems of cattle in complexity and diversity. The nomenclature of pig blood group factors follows that of the horse, except for the EAA system. The EAA system is related to cattle EAJ, human A and sheep EAR systems. Pig A and O factors occur as soluble substances in the serum and saliva of A-positive and O-positive animals, respectively, and attach to the RBC membrane a few weeks after birth. The genetic inheritance of A and O factors is similar to sheep EAR system. EAAA is dominant to EAAO and their expression on RBCs is controlled by the suppressor gene, EAS. The EAAA allele codes for uridine diphosphate N-

Little is known about blood group variation in the two domestic South American camelids, llama and alpaca. From iso- and heteroimmune sera developed for these animals, six blood group factors were identified and given alphabetical designations in order of discovery: A, B, C, D, E and F.77 Factors A and B are inherited as co-dominant alleles and assigned to blood group system EAA. Factors C, D, E and F were assigned to four separate systems as they appeared to be transmitted independently from each other and from the EAA system. Blood groups in llamas and alpacas are detected by hemolytic tests. Applications of Blood Groups Animal Breeding Accurate pedigree records are essential to many aspects of animal breeding, including selection of breeding stock, estimation of heritabilities, and breeding values based on progeny testing. Most breed registries throughout the world have mandatory parentage testing programs to validate pedigree records of registered animals. From the early 1950s through the mid 1990s, serological tests for blood groups, supplemented by electrophoretic assays that detected additional genetic variation in blood proteins, were the only methods available to verify parentage and to help breeders solve problems of questionable paternity or maternity. The high degree of variation of blood group antigens and protein variants resulted in probabilities of exclusion of incorrect parentage as high as 98% for most breeds.17 The use of blood typing tests in animal breeding came to an end in the mid 1990s when more powerful and cost effective DNA typing technologies emerged and became established as the preferred genetic testing method for applied and research purposes. Blood Transfusion Clinically, knowledge of blood groups in cattle, sheep, goat, pig and llama/alpaca has not affected conventional practice in transfusion. Large animals are not commonly transfused, and matching of blood types for antigenic compatibility is not practical because of the high variability of blood groups between individuals

CHAPTER 92: ERYTHROCYTE ANTIGENS AND BLOOD GROUPS

721

TABLE 92.7 Sixteen Loci of Pig Blood Groups, Chromosome Assignment (Chr), Test Method, Factors and Alleles (Phenogroups) Defined Locus

Chra

Test Methodb

Blood factors

Number of Alleles

EAA EAB EAC EAD EAE

1 Unknown 7d 12 9

a, h a h a a

AO ab a ab abcefghijklmnoprst

2 2 2 2 17

EAF EAG EAH EAI EAJ EAK EAL EAM EAN EAO EAP

8 15 6 18 7 9 4 11 9 6 Unknown

a a h c c h c, d c, h c d c

a a a a a a a a a a a

4 2 7 2 3 6 6 20 3 2 2

b b b b b b b b b b

cd cde

cdefg cdfghijklm cdefghijkm c

Allelesc A, O a, b –, a a, b aeglns, bdgkmps, defhkmnps, deghkmnps, aeflns, etc ac, ad, bc, bd a, b –, a, b, ab, bd, cd, be a, b –, a, b –, acf, acef, ade, adeg, bf adhi, bcgi, bdfi, agim, adhjk, adhjl –, ab, ade, aem, b, bd, bcdi, cd, etc a, b, bc a, b –, a

a

From ref.78. a, Saline agglutination; c, Coombs’ test; d, Dextran test; h, hemolytic test. c The absence of detectable factors (null allele, recessive) in any locus is represented by a dash (–). d Assignment based on linkage of C and J systems1. b

and limited availability of donors. Blood transfusions in large animals are indicated for certain acute, life threatening conditions and for plasma transfusion when failure of passive transfer occurs. Single, unmatched whole blood transfusions are generally safe and well tolerated. A gross crossmatch is recommended and sufficient when repeated transfusions are required. Matching is done with hemolytic and/or agglutination tests depending of the species and involves testing of the recipient’s serum for antibodies against the RBCs of potential donors (major) and of the donors’ sera for antibodies against the recipient’s RBCs (minor) (see Chapter 139). Pigs are often used as an animal model for organ or hematopoietic transplantation. Blood group compatibility is an important component of experimental protocols because of problems associated with immunemediated tissue rejection or the need to provide transfusion support to patients.85 In transplant experiments, mismatched transfusions for the EAA system result in adverse reactions that include disseminated intravascular coagulation, bleeding, and progressive hypotension60,82 rather than typical hemolytic reactions seen in humans. Horses merit special consideration because whenever a mare is given a whole blood transfusion, she is potentially being sensitized to blood group factors that may subsequently lead to NI problems for her foals. Selection of suitable donors that are negative for highly antigenic factors such as EAAa (and without circulating antibodies in the serum) is recommended although currently not practical because very few laboratories can perform the serological test for blood group specificities. Mares that received a blood transfusion should

always be screened for potential risk of NI prior to foaling. Plasma Transfusion A blood transfusion may be used to restore fluid loss and replace necessary proteins, and the RBC component may not be essential. In this case, a plasma transfusion may fulfill the clinical requirements. Potential donors can be selected in a hematology laboratory by screening their sera against a RBC panel from 10 to 20 horses to identify those without antibodies with antiblood group activity. Plasma can be collected and stored frozen to administer as needed. Neonatal Isoerythrolysis in Horses Neonatal isoerythrolysis is an acute hemolytic disease of newborn foals caused by immunologically-mediated RBC destruction resulting from maternal-fetal blood group incompatibility. Affected foals are healthy at birth but within 2–5 days develop signs of lethargy, elevated pulse and respiration rates, and clinical evidence of anemia. NI foals are usually from the second or later pregnancies of a mare but, on rare occasions, first foals can be affected. The antibodies that sensitize the RBCs of the foal are passively acquired from the dam’s colostrum. Recovery may be spontaneous or the disease may progress to severe anemia and death. The most common antibodies involved in NI are anti-Aa and anti-Qa, although others are infrequently found. Not all mares negative for EAAa or EAQa factors become sensitized, although they may have produced Aa-positive or Qa-positive foals. One source of sensiti-

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zation is whole blood transfusion, but this accounts for only a small number of cases. Mares negative for both Aa and Ca factors may be protected from sensitization to Aa by naturally produced anti-Ca antibodies.11 At present, no simple hypothesis explains why blood group sensitization occurs in only a few percent of mares at risk. A serum sample taken about 3 weeks before a pregnant mare is due to foal can be screened for evidence of blood group incompatibility. If the test results are positive for hemolytic anti-blood group activity, it is strongly advised to withhold the foal from its dam’s colostrum for 36–48 hours before putting it back to its dam’s milk. An alternative colostrum and milk source must be provided to the foal during that period. NI Disease in Mule Foals Mule breeders are keenly aware that mares bred to jacks (male donkeys) can become sensitized to blood group factors of asses.99 Monitoring of a pregnant mare’s serum for antibodies against the RBCs of the jack to which she is bred is a prudent precaution to identify mule neonates at risk for NI. Red Blood Cell Donors for Foals with NI Appropriate management for a severely anemic NI foal may be a RBC transfusion to alleviate the anemia and prevent death. The best blood donor is one whose RBCs lack the factor to which the mare is making antibodies. The sire of the foal is the worst possible donor since he has the factor to which the mare’s immune system has responded. The best immediate RBC donor is the mare. The mare’s RBCs should be administered to the foal in a suitable transfusion solution, but first they must be separated from the plasma and washed in saline to remove the antibodies reacting with the foal’s RBCs. The mare is unlikely to have a matching blood group type but it provides the foal with vital RBCs that will not be destroyed by the antibodies acquired from the colostrum. Hopefully, in a short time the blood group antibodies will be eliminated from circulation and the foal can make sufficient RBCs of its own to prevent a recurrence of the anemia crisis.

REFERENCES 1. Andresen E, Baker LN. The C blood group system in pigs and detection and estimation of linkage between C and J systems. Genetics 1964;49:379–386. 2. Andrews GA, Chavey PS, Smith JE. Production, characterization, and applications of a murine monoclonal antibody to dog erythrocyte antigen 1.1. J Am Vet Med Assoc 1992;201:1549–1552. 3. Andrews GA, Chavey PS, Smith JE. Reactivity of seed lectins with blood typed canine erythrocytes. Comp Haematol Intl 1992;2:68–74. 4. Andrews GA, Chavey PS, Smith JE, et al. N-Glycolylneuraminic acid and N-acetylneuraminic acid define feline blood group A and B antigens. Blood 1992;79:2485–2491. 5. Arikan S, Duru SY, Gurkan M, et al. Blood type A and B frequencies in Turkish Van and Angora cats in Turkey. J Vet Med Ser A 2003;50:303–306. 6. Arikan S, Gurkan M, Ozaytekin E, et al. Frequencies of blood type A, B and AB in non-pedigree domestic cats in Turkey. J Small Anim Pract 2006;47:10–13.

7. Auer L, Bell K. The AB blood group system of cats. Anim Blood Groups Biochem Genet 1981;12:287–297. 8. Auer L, Bell K. Transfusion reactions in cats due to AB blood group incompatibility. Res Vet Sci 1983;35:145–152. 9. Auer L, Bell K, Coates S. Blood transfusion reactions in the cat. J Am Vet Med Assoc 1982;180:729–730. 10. Bagdi N, Magdus M, Leidinger E, et al. Frequencies of feline blood types in Hungary. Acta Vet Hung 2001;49:369–375. 11. Bailey E, Albright DG, Henney PJ. Equine neonatal isoerythrolysis – Evidence for prevention by maternal antibodies to the Ca blood group antigen. Am J Vet Res 1988;49:1218–1222. 12. Bell K. The blood groups of domestic animals. In: NS Agar, PG Board, eds. Red Blood Cells of Domestic Mammals, Elsevier, 1983;133–164. 13. Bighignoli B, Niini T, Grahn RA, et al. Cytidine monophospho-N-acetylneuraminic acid hydroxylase (CMAH) mutations associated with the domestic cat AB blood group. BMC Genetics 2007;8:27. 14. Bird MS, Cotter SM, Gibbons G, et al. Blood groups in cats. Companion Anim Pract 1988;2:31–33. 15. Blais MC,Berman L, Oakley DA, et al. Canine Dal blood type: A red cell antigen lacking in some dalmatians. J Vet Intern Med 2007;21:281–286. 16. Bowdler AJ, Bull RW, Slating R, et al. Tr: A canine red cell antigen related to the A-antigen of human red cells. Vox Sang 1971;20:542–554. 17. Bowling AT, Eggleston-Stott ML, Byrns G, et al. Validation of microsatellite markers for routine horse parentage testing. Anim Genet 1997;28:247–252. 18. Bucheler J, Giger U. Alloantibodies against A and B blood types in cats. Vet Immunol Immunopathol 1993;38:283–295. 19. Bull R, Bowdler AJ, Swisher SN. Two additional antigens in the canine blood group system. Bulletin of the American Society of Vet Clin Pathol 1973;II(3):10–11. 20. Bull RW, Vriesendorp HM, Zweibaum A, et al. The inapplicability of CEA-7 as a canine bone marrow transplantation marker. Transplant Proc 1975;7:575–577. 21. Butler M, Andrews GA, Smith JE. Reactivity of lectins with feline erythrocytes. Comp Haematol Intl 1991;1:217–219. 22. Christian RM, Ervin DM, Swisher SN, et al. Hemolytic anemia in newborn dogs due to absorption of isoantibody from breast milk during the first day of life. Science 1949;110:443. 23. Christian RM, Ervin DM, Young LE. Observations on the in-vitro behavior of dog isoantibodies. J Immunol 1951;66:37–50. 24. Cohen C, Fuller JL. The inheritance of blood types in the dog. J Hered 1953;44:225–228. 25. Cohney S, Mouhtouris E, McKenzie IF, et al. Molecular cloning and characterization of the pig secretor type alpha 1,2 fucosyltransferase (FUT2). Intl J Mol Med 1999;3:199–207. 26. Colling DT, Saison R. Canine blood groups. 1. Description of new erythrocyte specificities. Anim Blood Groups Biochem Genet 1980;11:1–12. 27. Colling DT, Saison R. Canine blood groups. 2. Description of a new allele in the Tr blood group system. Anim Blood Groups Biochem Genet 1980;11:13–20. 28. Corato A, Mazza G, Hale AS, et al. Biochemical characterization of canine blood group antigens: immunoprecipitation of DEA 1.2, 4 and 7 and identification of a dog erythrocyte membrane antigen homologous to human Rhesus. Vet Immunol Immunopathol 1997;59:213–223. 29. Crawford AM, Dodds KG, Ede AJ, et al. An autosomal genetic linkage map of the sheep genome. Genetics 1995;140:703–724. 30. Di Stasio L. Biochemical genetics. In: Piper L, Ruvinsky A, eds. The Genetics of Sheep. Wallingford, Oxon: CAB International. 1997;133–148. 31. Ejima H, Kurokawa K, Ikemoto S. Comparison test of antibodies for dog blood grouping. Jpn J Vet Sci 1980;42:435–441. 32. Ejima H, Kurokawa K, Ikemoto S. Feline red blood cell groups detected by naturally occurring isoantibody. Jpn J Vet Sci 1986;48:971–976. 33. Ejima H, Kurokawa K, Ikemoto, S. Phenotype and gene frequencies of red blood cell groups in dogs of various breeds reared in Japan. Jpn J Vet Sci 1986;48:363–368. 34. Ejima H, Nomura K, Bull RW. Breed differences in the phenotype and gene frequencies in canine D blood group system. J Vet Med Sci 1994;56:623–626. 35. Eyquem A, Millot P, Podliachouk L. Blood groups in chimpanzees, horses, sheep, pigs, and other mammals. Ann NY Acad Sci 1962;97:320–328. 36. Ferguson LC. Heritable antigens in the erythrocytes of cattle. J Immunol 1941;40:213–242. 37. Ferguson LC, Stormont C, Irwin MR. On additional antigens in the erythrocytes of cattle. J Immunol 1942;44:147–164. 38. Forcada Y, Guitian J, Gibson G. Frequencies of feline blood types at a referral hospital in the south east of England. J Small Anim Pract 2007;48(10):570–573. 39. Giger U, Akol KG. Acute hemolytic transfusion reaction in an Abyssinian cat with blood type B. J Vet Intern Med 1990;4:315–316. 40. Giger U, Bucheler J. Transfusion of type-A and type-B blood to cats. J Am Vet Med Assoc 1991;198:411–418.

CHAPTER 92: ERYTHROCYTE ANTIGENS AND BLOOD GROUPS 41. Giger U, Bucheler J, Patterson DF. Frequency and inheritance of A and B blood types in feline breeds of the United States. J Hered 1991;82:15–20. 42. Giger U, Gelens CJ, Callan MB, et al. A. An acute hemolytic transfusion reaction caused by dog erythrocytic antigen 1.1 incompatibility in a previously sensitized dog. J Am Vet Med Assoc 1995;206:1358–1362. 43. Giger U, Gorman NT, Hubler M, et al. Frequencies of feline A and B blood types in Europe. Anim Genet 1992;23(Suppl 1):17–18. 44. Giger U, Griot-Wenk M, Bucheler J, et al. Geographical variation of the feline blood-type frequencies in the United-States. Feline Pract 1991;19:21–27. 45. Green JL, Andrews GA, Wyatt CR. Phenotypic differences within the AB blood type of the feline AB blood group system. Comp Clin Pathol 2005;14:138–145. 46. Green JL, Chavey PS, Andrews GA, et al. Production and characterisation of murine monoclonal antibodies to feline erythrocyte A and B antigens. Comp Haematol Intl 2000;10:30–37. 47. Griot-Wenk ME, Callan MB, Casal ML, et al. Blood type AB in the feline AB blood group system. Am J Vet Res 1996;57(10):1438–1442. 48. Griot-Wenk ME, Giger U. Feline transfusion medicine – blood types and their clinical importance. Vet Clin N Am Small Anim Pract 1995;25:1305–1322. 49. Griot-Wenk ME, Giger U. The AB blood group system in wild felids. Anim Genet 1999;30:144–147. 50. Griot-Wenk M, Pahlsson P, Chisholm-Chait A, et al. Biochemical characterization of the feline AB blood group system. Anim Genet 1993;24:401–407. 51. Haarer M, Grunbaum EG. Blood group serological examinations in cats in Germany. Kleintierpraxis 1993;38:195–204. 52. Hale AS. Canine blood groups and their importance in veterinary transfusion medicine. Vet Clin N Am Small Anim Pract 1995;25:1323–1332. 53. Han BK, Lee CG, Ikemoto S. Studies on the blood groups of Jindo dogs by dog erytrocyte antigen system. Korean J Anim Sci 1988;30(11):643–651. 54. Hara Y, Ejima H, Aoki S, et al. Preparation of monoclonal antibodies against dog erythrocyte antigen D1 (DEA-3). J Vet Med Sci 53:1105–1107. 55. Hashimoto Y, Yamakawa T, Tanabe Y. Further studies on the red cell glycolipids of various breeds of dogs. A possible assumption about the origin of Japanese dogs. J Biochem 1984;96:1777–1782. 56. Hines HC. Blood groups and biochemical polymorphisms. In: Fries R, Ruvinsky A, eds. The Genetics of Cattle. Wallingford: CAB International, 1999;77–121. 57. Hirota J, Usui R, Oyamada T, et al. The phenotypes and gene frequencies of genetic markers in the blood of Japanese crossbred cats. J Vet Med Sci 1995;57:381–383. 58. Holmes R. Blood groupuls in cats. J Physiol 1950;111(3–4):61P. 59. Hubler M, Arnold S, Casal M, et al. The distribution of blood groups in Swiss housecats. Schweiz Arch Tierheilkd 1993;135:231–235. 60. Hunfeld MA, Hoitsma HF, Meijer S, et al. The role of A-O-incompatible blood transfusions in porcine orthotopic liver transplantations. Eur Surg Res 1984;16:354–359. 61. Ihara N, Takasuga A, Mizoshita K, et al. A comprehensive genetic map of the cattle genome based on 3802 microsatellites. Genome Res 2004;14(10A):1987–1998. 62. Ikemoto S, Haruhiro Y, Watanabe Y, et al. Individual differences within animal blood groups detected by lectins. Proceedings of the XVIth International Conference on Animal Blood Groups and Biochemical Polymorphisms, 1979;8–11. 63. Ikemoto S, Sakurai Y, Fukui M. Individual difference within the cat blood group detected by isohemagglutinin. Jpn J Vet Sci 1981;43:433–435. 64. Ikemoto S, Yoshida H. Genetic studies of new blood group C system on red cells of beagles. Jpn J Vet Sci 1981;43:429–431. 65. Jensen AL, Olesen AB, Arnbjerg J. Distribution of feline blood types detected in the Copenhagen area of Denmark. Acta Vet Scand 1994;35:121–124. 66. Juneja RK, Vogeli P. Biochemical genetics. In: Rothschild MF, Ruvinsky A, eds. The Genetics of the Pig. Wallingford, Oxon: CAB International. 1998;105–134. 67. Kappes SM, Bishop MD, Keele JW, et al. Linkage of bovine erythrocyte antigen loci B, C, L, S, Z, R’ and T’ and the serum-protein loci PostTransferrin-2 (Ptf-2), Vitamin-D-Binding Protein (Gc) and Albumin (Alb) to DNA microsatellite markers. Anim Genet 1994;25:133–140. 68. Malik R, Griffin DL, White JD, et al. The prevalence of feline A/B blood types in the Sydney region. Aust Vet J 2005;83:38–44. 69. Meijerink E, Neuenschwander S, Dinter A, et al. Isolation of a porcine UDP-GalNAc transferase cDNA mapping to the region of the blood group EAA locus on pig chromosome 1. Anim Genet 2001;32:132–138. 70. Melzer KJ, Wardrop KJ, Hale AS, et al. A hemolytic transfusion reaction due to DEA 4 alloantibodies in a dog. J Vet Intern Med 2003;17:931–933. 71. Muchmore EA, Milewski M, Varki A, et al. Biosynthesis of N-Glycolyneuraminic acid – the primary site of hydroxylation of N-Acetylneuraminic acid is the cytosolic sugar nucleotide pool. J Biol Chem 1989;264:20216–20223.

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72. Mylonakis ME, Koutinas AF, Saridomichelakis M, et al. Determination of the prevalence of blood types in the non-pedigree feline population in Greece. Vet Rec 2001;149:213–214. 73. Nguyen TC. Genetic systems of red-cell blood groups in goats. Anim Genet 1990;21:233–245. 74. Nguyen TC, Bunch TD. Blood-groups and evolutionary relationships among domestic sheep (Ovis aries), domestic goat (Capra hircus), aoudad (Ammotragus lervia) and european mouflon (Ovis musimon). Ann Genet Select Anim 1980;12:169–180. 75. Ottenberg R, Thalhimer W. Studies in experimental transfusion. J Med Res 1915;33:213–229. 76. Penedo MC, Millon LV, Bernoco D, et al. International Equine Gene Mapping Workshop Report: a comprehensive linkage map constructed with data from new markers and by merging four mapping resources. Cytogenet Genome Res 2005;111:5–15. 77. Penedo MCT, Fowler ME, Bowling AT, et al. Genetic variation in the blood of llamas, Lama glama, and alpacas, Lama pacos. Anim Genet 1988;19:267–276. 78. Rohrer GA, Vogeli P, Stranzinger G, et al. Mapping 28 erythrocyte antigen, plasma protein and enzyme polymorphisms using an efficient genomic scan of the porcine genome. Anim Genet 1997;28:323–330. 79. Ruiz De Gopequi R, Velasquez M, Espada Y. Survey of feline blood types in the Barcelona area of Spain. Vet Rec 2004;154:794–795. 80. Saison R, Colling DT. A proposed nomenclature for canine red blood cell groups. Proceedings of the XVIth International Conference on Animal Blood Groups and Biochemical Polymorphisms, 1979;225–228. 81. Sandberg K, Cothran EG. Blood groups and biochemical polymorphisms. In: Bowling AT, Ruvinsky A, eds. The Genetics of the Horse. Wallingford, Oxon: CAB International, 2000;85–108. 82. Sheil AG, Halliday JP, Drummond JM, et al. A modified technique for orthotopic liver transplantation. Arch Surg 1972;104:720–724. 83. Silvestre-Ferreira AC, Pastor J, Almeida O, et al. Frequencies of feline blood types in northern Portugal. Vet Clin Pathol 2004;33:240–243. 84. Silvestre-Ferreira AC, Pastor J, Sousa AP, et al. Blood types in the non-pedigree cat population of Gran Canaria. Vet Rec 2004;155(24):778–779. 85. Smith DM, Newhouse M, Naziruddin B, et al. Blood groups and transfusions in pigs. Xenotransplantation 2006;13:186–194. 86. Stormont C. Linked genes, pseudoalleles and blood groups. Am Nat 1955;89:105–116. 87. Stormont C. Current status of blood groups in cattle. Ann NY Acad Sci 1962;97:251–268. 88. Stormont C. A note on linear subtyping relationships. Papers dedicated to Johannes Moustgaard on the occasion of his 70th birthday, 26 September 1981. Copenhagen: Royal Danish Agricultural Society, 1981;190–193. 89. Stormont C, Owen RD, Irwin MR. The B system and C system of bovine blood groups. Genetics 1951;36:134–161. 90. Stormont C, Suzuki Y. Distribution of Forssman blood factors in individuals of various artiodactyl species. J Immunol 1958;81:276–284. 91. Stormont C, Suzuki Y, Rhode EA. Serology of horse blood groups. Cornell Vet 1964;54:439–452. 92. Suzuki Y, Stormont C, Morris BG, et al. New antibodies in dog blood groups. Transplant Proc 1975;7:365–367. 93. Swisher SN, Bull R, Bowdler J. Canine erythrocyte antigens. Tissue Antigen 1973;3:164–165. 94. Swisher SN, Young LE, Trabold N. In vitro and in vivo studies of the behavior of canine erythrocyte-isoantibody systems. Ann NY Acad Sci 1962;97:15–25. 95. Swisher SN, Young LE. The blood grouping systems of dogs. Physiol Rev 1961;41:495–520. 96. Symons M, Bell K. Expansion of the canine A blood group system. Anim Genet 1991;22:227–235. 97. Symons M, Bell K. Canine blood groups: description of 20 specificities. Anim Genet 1992;23:509–515. 98. Thiele OW, Krotlinger F, Ohl C. Transfer of blood group determinants from plasma onto erythrocytes. Naturwissenschaften 1975;62:586–586. 99. Traub-Dargatz JL, McClure JJ, Koch C, et al. Neonatal isoerythrolysis in mule foals. J Am Vet Med Assoc 1995;206:67–70. 100. Tucker EM. Some physiological aspects of genetic variation in the blood of sheep. Anim Blood Groups Biochem Genet 1976;7:207–215. 101. Vriesendorp HM, Albert ED, Templeton JW, et al. Joint report of the second international workshop on canine immunogenetics. Transplant Proc 1976;8:289–314. 102. Vriesendorp HM, Westbroek DL, D’Amaro J, et al. Joint report of 1st international workshop on canine immunogenetics. Tissue Antigen 1973;3:145–172. 103. Weinstein NM, Blais MC, Harris K, et al. A newly recognized blood group in domestic shorthair cats: The Mik red cell antigen. J Vet Intern Med 2007;21:287–292. 104. Wilkerson MJ, Meyers KM, Wardrop KJ. Anti-A isoagglutinins in two blood type B cats are IgG and IgM. Vet Clin Pathol 1991;20:10–14. 105. Wilkerson MJ, Wardrop KJ, Meyers KM, et al. Two cat colonies with A and B blood types and a clinical transfusion reaction. Feline Pract 1991;19:22–26.

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106. Yamamoto F, Yamamoto M. Molecular genetic basis of porcine histoblood group AO system. Blood 2001;97:3308–3310. 107. Yasue S, Handa S, Miyagawa S, et al. Difference in form of sialic acid in red blood cell glycolipids of different breeds of dogs. J Biochem 1978;83:1101–1107. 108. Yoshida H. Individual difference of dog blood groups detected by Clerodendron trichotomum extract. J Vet Med 1979;691:85–87. 109. Young LE, Christian RM, Ervin DM, et al. Hemolytic disease in newborn dogs. Blood 1951;6:291–313. 110. Young LE, Ervin DM, Christian RM, et al. Hemolytic disease in newborn dogs following isoimmunization of the dam by transfusions. Science 1949;109:630–630.

111. Young LE, Ervin DM, Yuile CL. Hemolytic reactions produced in dogs by transfusion of incompatible dog blood and plasma.1. Serologic and hematologic aspects. Blood 1949;4:1218–1231. 112. Young LE, Obrien WA, Miller G, et al. Erythrocyte-isoantibody reactions in dogs. Trans NY Acad Sci 1951;13:209–213. 113. Young LE, Obrien WA, Swisher SN, et al. Blood groups in dogs – their significance to the veterinarian. Am J Vet Res 1952;13:207–213.

CHAPTER

93

Granulocyte and Platelet Antigens JENNIFER S. THOMAS Granulocyte Antigens Laboratory Techniques to Identify Neutrophil Antigens and Antigen-Antibody Interactions Clinical Disorders Associated with Neutrophil Antigens

Platelet Antigens Laboratory Techniques to Identify Platelet Antigens and Antigen-Antibody Interactions Clinical Disorders Associated with Platelet Antigens

Acronyms and Abbreviations EDTA, ethylenediaminetetraacetic acid; GP, glycoprotein; HLA, human leukocyte antigen; HNA, human neutrophil antigen; HPA, human platelet antigen; IMN, immune-mediated neutropenia; IMT, immune-mediated thrombocytopenia; LFA-1, leukocyte function antigen 1; PCR, polymerase chain reaction; PTP, post-transfusion purpura; TRALI, transfusion-related acute lung injury; VWF, von Willebrand factor.

C

ell surface antigens on granulocytes and platelets play a role in recognition of self. Antigens vary in their ability to stimulate an immune response and, therefore, in their association with disease. Much of the published research has focused on the role of platelet and granulocyte antigens in immunologic disorders; however, antigen identification is also used as a marker of disease. In veterinary medicine expression of platelet or granulocyte antigens is determined to characterize myeloproliferative disorders. Expression of some antigens, such as P-selectin (CD62P) on platelets, is used to determine cellular activation.29 Immune responses targeting granulocyte or platelet antigens have been implicated in the pathogenesis of primary (autoimmune or idiopathic) and secondary immune-mediated neutropenia (IMN) or thrombocytopenia (IMT), neonatal alloimmune neutropenia or thrombocytopenia, and transfusion reactions.9,10,40,41,50 Binding of antibodies to cell antigens may also cause dysfunction of the target cells.8,25 Immunologic disorders resembling those listed above have been identified in veterinary species; however, the pathogenesis of these conditions is generally better characterized in humans because transfusion therapy and neonatal cytopenias are more common.50 Most of the information provided in this chapter is based upon current understanding of the role of platelet and granulocytes antigens in immunologic disorders in humans. Whenever possible, comparisons between humans and veterinary species will be presented.

Platelet or granulocyte antigens can be specific to one cell type, shared by other blood cells, or widely distributed on systemic cells. Antigens are generally classified based upon their biochemical nature into glycolipids, glycoproteins (GPs), or proteins.9,41 The roles of some, but not all, of these antigens in normal cellular function are known. The molecular structures of some of these antigens have been characterized, allowing the development of molecular techniques to detect alterations in the nucleic acid structure of the genes encoding the antigens. Variations in the structure of an antigen that is present in some, but not all, individuals in a species are termed alloantigens. Alloantigens are the products of polymorphic genes. Most granulocyte and platelet alloantigens have been linked to single-point mutations.27 The prevalence of the different alloantigens in humans varies significantly between ethnic groups.8,41 It is likely that similar differences exist in the expression of alloantigens among different breeds and species of animals.50 Antibodies directed against alloantigens are termed alloantibodies. Alloantibodies may form when an individual receives a blood product that contains an alloantigen not present on self cells or when there is an incompatibility between fetal and maternal blood cells. Isoantibodies are antibodies that target a nonpolymorphic antigen found on the cells of the vast majority of normal individuals in a species. Generally, isoantibodies form when an individual who has an inherited defi725

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ciency of an antigen is transfused with blood products from a normal individual.50 Antibodies that are produced against self-antigens are termed autoantibodies. The production of autoantibodies may be idiopathic or may be secondary to an underlying disorder.10,55,65 Autoantibodies are sometimes produced following drug administration. In some cases the drug or its metabolite must be present for antibody binding to occur. In other cases the drug induces an antibody that persists after drug administration ceases.3 GRANULOCYTE ANTIGENS The granulocytes consist of neutrophils, eosinophils and basophils. Neutrophils are present in highest concentrations in the blood and are most commonly implicated in immunologic disorders.50 In humans immunologic disorders are associated with antigens in the human leukocyte antigen (HLA) or the human neutrophil antigen (HNA) systems.56,57 Human leukocyte antigens are widespread in distribution and found on many different cell types; HNA antigens are predominantly expressed on neutrophils (Table 93.1). It is currently unclear how many of the HNA antigens are expressed on other leukocytes.8 The HNA systems are numbered according to the GP site where they are located. Antigens within each HNA system are listed alphabetically in order of their identification.56 The HNA-1 antigens are found on FcγRIIIb (CD16b), a low affinity receptor for the Fc portion of immunoglobulins. They vary significantly in degree of glycosylation. The HNA-2 antigen is found on a 56– 64 kDa GP (CD177) that is involved in adhesion to endothelial cells and subsequent transmigration into the extravascular space. The HNA-3 antigen is located on a 70–95 kDa protein whose function is not known. The HNA-4 antigen is located on the CD11b/CD18 complex (Mac-1, CR3), a β2-integrin that plays a role in binding to endothelial cells and complement proteins. The HNA-5 antigen is found on the CD11a/CD18 complex (leukocyte function antigen 1 [LFA-1]), a β2integrin involved in adhesion to endothelial cells.8,57 Monoclonal antibodies are used in a variety of veterinary species to identify the GPs present on neutrophils. The relationship between the antigens rec-

ognized by these monoclonal antibodies and the antigens associated with immunologic disorders is not currently known.50 Monoclonal antibodies to CD11a (LFA-1) and CD11b (Mac-1; CR3) bind to bovine neutrophils.19,54 The monoclonal antibody to CD11a also binds to all other bovine leukocytes but not to erythroid cells or platelets. Monoclonal antibodies to CD11a also recognize antigens on neutrophils, lymphocytes and monocytes from dogs, horses, sheep, goats and rabbits.6,28,31,49,59 The antibody to CD11b also binds to bovine monocytes and a subpopulation of B lymphocytes.19,54 Antibodies to CD11b bind to neutrophils, monocytes and lymphocytes from sheep, goats, and pigs, as well as neutrophils and monocytes from cats and dogs.6,49 Anti-CD16 antibodies bind to canine neutrophils.31,59 Additional information on CD antigens is available in Chapter 4. Laboratory Techniques to Identify Neutrophil Antigens and Antigen-Antibody Interactions Identification of neutrophil antigens or anti-neutrophil antibodies has traditionally relied on agglutination or immunofluorescence assays.56,57 In agglutination assays neutrophils clump if a patient’s serum containing autoantibodies is added to neutrophils from a normal individual or if a patient’s neutrophils have an alloantigen that is targeted by a specific alloantibody in test serum. In microscopic immunofluorescence assays a fluorescence-conjugated secondary antibody is used to detect binding of a specific alloantibody to its target alloantigen or autoantibodies to target antigens on neutrophils. Genotyping assays using polymerase chain reaction (PCR) techniques are replacing agglutination or immunofluorescence assays in humans to identify many of the major neutrophil antigens.57 Similar assays are not available for veterinary species. Monoclonal-antibody capture assays have been developed to detect antibodies to specific HNAs.56 Flow cytometry is used to phenotype neutrophils in humans and animals.11,49,54,56,57 It is limited by the availability of mononclonal antibodies that specifically target the antigens of interest. Flow cytometry is also used to identify anti-neutrophil antibodies in animals, but specific target antigens have not been identified.62 Further discussion of methods to detect anti-neutrophil antibodies is found in Chapter 140.

TABLE 93.1 Human Neutrophil Antigens and Associated Disorders CD Designation

Antigen System

Clinical Conditions in People Associated with Antibody-Antigen Interactionsa

FcγRIIIb

CD16b

HNA-1

NB1 glycoprotein 70-,96-kDa protein MAC-1; CR3 LFA-1

CD177

HNA-2 HNA-3 HNA-4 HNA-5

Neonatal alloimmune neutropenia, primary IMN, TRALI, drug-induced neutropenia Primary IMN, drug-dependent neutropenia, TRALI Neonatal alloimmune neutropenia, TRALI, febrile transfusion reaction Neonatal alloimmune neutropenia, primary IMN Unknown

Antigen Location

a

CD11b CD11a

IMN, immune-mediated neutropenia; TRALI, transfusion-related acute lung injury.

CHAPTER 93: GRANULOCYTE AND PLATELET ANTIGENS

CLINICAL DISORDERS ASSOCIATED WITH NEUTROPHIL ANTIGENS In humans, clinical conditions associated with alloantibody interactions include neonatal immune neutropenia, transfusion-related acute lung injury (TRALI), alloimmune neutropenia after bone marrow transplantation, refractoriness to granulocyte transfusions, and febrile transfusion reactions.8,10,57 Febrile transfusion reactions occur when the recipient has alloantibodies directed against antigens present on the donor leukocytes. Febrile transfusion reactions can be minimized by removing leukocytes from RBC and platelet components.8 TRALI occurs when alloantibodies to HLA antigens or neutrophil specific antigens cause neutrophils to become entrapped in the pulmonary vasculature.57 Binding of alloantibodies to neutrophil antigens also cause functional defects, such as impaired adhesion and altered respiratory burst.8,48 Disorders associated with alloantibody production are not commonly recognized in veterinary species. Neonatal neutropenia is rare but occurs in pigs and foals.13,33 It occurs when the dam has been previously exposed to paternal antigens found on the fetal neutrophils and produces alloantibodies that are passed to the fetus in utero or to the neonate following ingestion of colostrum. Febrile reactions in dogs following transfusion of platelet concentrates have been associated with leukocyte contamination.1 Additional discussion of transfusion reactions is found in Chapters 97 and 100. In humans autoantibody production is implicated in the pathogenesis of primary IMN, secondary IMN, drug-dependent neutropenia, and autoimmune neutropenia after bone marrow transplantation.8 The cause of the autoantibody production is unknown in primary IMN. Studies in humans have implicated antibodies targeting FcγRIIIb (anti-HNA-1a and anti-HNA-1g) in many patients.10 Secondary IMN is associated with systemic immune-mediated disorders, infectious diseases, neoplasia, bone marrow or stem cell transplants, kidney transplants, and drug therapy.10 The antigenic target in secondary IMN is often unknown. Antibodies bind to antigens on neutrophils and lead to premature neutrophil removal by macrophages. In some cases

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autoantibodies target antigens on hematopoietic precursor cells in the bone marrow leading to myeloid hypoplasia. Patients may also have defective neutrophil phagocytosis, as well as concurrent thrombocytopenia or hemolytic anemia. Diagnosis often depends on identification of anti-neutrophil antibodies using agglutination, monoclonal antibody capture assays, immunofluorescence or flow cytometric assays.10,21,62 Immune-mediated neutropenia is infrequently reported in veterinary species.7,45 Antibodies generally target antigens on circulating neutrophils, but bone marrow directed immune response causing white cell aplasia has been reported.63 IMN is suspected when an animal has unexplained neutropenia that is responsive to immunosuppressive drugs. Identification of antineutrophil antibodies is supportive, but the diagnostic assays required to detect these antibodies are often not performed due to limited availability.45 Anti-neutrophil antibodies have been identified in dogs with IMN, but the specific target antigen has not been identified.60,62 Drug associated neutropenia is rarely reported. Antineutrophil antibodies have been identified in neutropenic dogs receiving an antipsychotic drug or cephalosporin.4,35 Antibodies to neutrophil isoantigens have been identified in human neonates born to mothers who had neutrophils deficient in FcγRIIIb.18 Although not reported, a similar disorder is possible in dogs or cattle with leukocyte adhesion deficiency syndrome, a deficiency of membrane CD11/CD18 complex.50

PLATELET ANTIGENS In humans platelet antigens commonly implicated in immunologic disorders are those in the HLA system and the human platelet antigen (HPA) systems (Table 93.2). HLA antigens are widespread in distribution and found on many different cell types. HPA antigens are found predominantly on platelets, though some are expressed on other blood cells.41 The HPA antigens are located on membrane GPs and include GPαIIbβ3 (IIbIIIa), GPIb/IX/V, GPα2β1 (GPIa/IIa) and CD109.14,16,40,41

TABLE 93.2 Human Platelet Antigens and Associated Disorders Antigen Location Site

CD Designation

Human Antigen Systems

GPβ3

CD61

GPIbα GPαIIb GPIa GPIbβ CD109

CD42b CD41 CD49b CD42c CD109

HPA-1, HPA-4, HPA-6, HPA-7, HPA-8, HPA-10, HPA-11, HPA-14, HPA-16w HPA-2 HPA-3, HPA-9 HPA-5, HPA-13 HPA-12 HPA-15

a

IMT, immune-mediated thrombocytopenia; PTP, post-transfusion purpura.

Clinical Conditions Associated in People with Antibody-Antigen Interactionsa Neonatal alloimmune thrombocytopenia; PTP; primary IMT

Primary IMT Neonatal alloimmune thrombocytopenia; PTP; primary IMT Neonatal alloimmune thrombocytopenia; PTP; primary IMT Primary IMT Neonatal alloimmune thrombocytopenia; PTP; refractoriness to transfusion

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GPαIIbβ3 (CD41/CD61) is an integrin that is critical for platelet aggregation. Following activation, GPαIIbβ3 undergoes a conformational change that allows binding to adhesive proteins such as fibrinogen and von Willebrand factor (VWF). Glanzmann’s thrombasthenia, a functional or physical deficiency of GPαIIbβ3, is found in humans, dogs, and horses. Affected individuals have a bleeding disorder characterized by impaired platelet aggregation.5,12 The GPα2β1 complex is a collagen receptor. Alterations in this receptor cause impaired adhesion to collagen and impaired collagen induced aggregation.40,42 The GPIb/IX/V complex binds to VWF and plays an important role in platelet adhesion to the vessel wall under high shear-stress conditions.14 An inherited functional or physical deficiency of GPIb/IX, termed Bernard-Soulier syndrome, is found in humans and is associated with thrombocytopenia, giant platelets, and functional defects.42 The role of CD109 is not clear. It is detected on activated platelets and may be involved in cell communication.16 The HPA alloantigens have been characterized at the molecular level. Most result from a single amino-acid substitution resulting from a single nucleotide change. The frequency of the different alloantigens varies in different ethnic populations.40,41 These major platelet GPs appear to be conserved between the species, though the number of different alloantigens and their role in immunologic disorders in veterinary species is unknown.12,34,38 Monoclonal antibodies are used to identify platelet antigens in a number of species. The association between antigens recognized by the monoclonal antibodies and the antigens associated with disease is not known. Monoclonal antibodies against human or porcine CD41 (GPαIIb), CD42a (GPIX) and CD61 (GPβ3) bind to canine and equine platelets.2,36,49 Monoclonal antibodies to porcine or ruminant CD41/CD61 (GPαIIbβ3) bind to bovine, porcine and equine platelets.36,43,51 Monoclonal antibodies against human CD61a and CD62P (P-selectin) bind to bovine platelets.54 Monoclonal antibodies against human GPIb react with feline platelets.58 Canine GPIb α was cloned and is 82% identical with the human sequence in the carboxyl-terminus.22 The nucleotide sequence of canine GPβ3 is 92% homologous with the human sequence.34 Laboratory Techniques to Identify Platelet Antigens and Antigen-Antibody Interactions Platelets are commonly typed in people for purposes of platelet transfusion. Serologic phenotyping is generally limited to HPA-1a and HPA-5b because of limited availability of reliable alloantibodies to other alloantigens.40 Agglutination tests similar to those used to phenotype neutrophils are not reliable for platelets, and platelet phenotyping more commonly utilizes immunoblotting or immunoprecipitation techniques.16 Flow cytometry is used in human and veterinary medicine but is limited by the availability of reliable monoclonal antibodies.40 Genotyping using PCR techniques to identify platelet antigens has become standard practice in human medicine.20

No single test has proven sufficient to detect antiplatelet antibodies in humans.53 Available techniques include platelet immunofluorescence assays, flow cytometry, or monoclonal antibody-specific immobilization of platelet antigen assays. Immunofluorescence, flow cytometry and immunopreciptation have been used to measure anti-platelet antibodies in veterinary species.23,25,30,50 Detection of anti-platelet antibodies is further discussed in Chapters 78 and 140. Clinical Disorders Associated With Platelet Antigens Clinical disorders associated with platelet alloantigens are most commonly associated with the ability of alloantigens to induce an immune response; however, polymorphisms in the platelet receptors that occur with the different alloantigens may also cause altered function.40 Though conflicting results have been reported, studies suggest that some alloantigens in the HPA-1 system are associated with increased platelet reactivity and increased risk for coronary heart disease and stroke.14 The binding of autoantibodies and alloantibodies to platelet GPs often causes platelet destruction and thrombocytopenia. These antibodies may also cause platelet inhibition and increased risk for bleeding or platelet activation and increased risk for thrombotic disorders. Functional defects may be present independent of any associated thrombocytopenia.15 One study suggested that platelets from dogs with IMT have impaired platelet function due to circulating antibodies.26 Platelet aggregation was inhibited in mule foals with neonatal alloimmune thrombocytopenia, possibly due to alloantibody competition with collagen binding sites.46 In humans, disorders associated with alloantibody production include neonatal alloimmune thrombocytopenia, post-transfusion purpura (PTP), and refractoriness to platelet transfusions.41,47 Neonatal alloimmune thrombocytopenia occurs when the mother produces alloantibodies against a paternal alloantigen expressed on fetal platelets. The majority of human cases are associated with an alloantibody against fetal GPβ3 (HPA1a). Alloantibodies against fetal GPIa (HPA-5a), CD109 (HPA-15b), or GPαIIb (HPA-3a) are less commonly implicated.47 Neonatal alloimmune thrombocytopenia has been described in pigs, horses, and mules.17,44,46 The specific target alloantigen was not identified in any of these reports. Post-transfusion purpura is a rare condition that occurs 5–12 days after an individual receives a blood transfusion. It is most common in HPA-1a negative women who have been previously sensitized by prior transfusion or pregnancy. Other platelet specific antigens or HLA antigens have been less frequently implicated. The immune response causes the destruction of the individual’s own platelets, as well as the transfused platelets.41 PTP has been described in dogs and pigs; however, an association with a specific alloantigen has not been identified.32,61 Platelet refractoriness occurs in humans who receive multiple platelet transfusions. These individuals

CHAPTER 93: GRANULOCYTE AND PLATELET ANTIGENS

produce alloantibodies that result in shortened survival of the transfused platelets. Anti-HLA antibodies are most commonly implicated; however, anti-HPA antibodies are found in a minority of individuals.41 Platelet refractoriness has been experimentally reproduced in dogs.52 The pathogenesis of IMT is covered further in Chapter 78. Briefly, IMT occurs when antibodies bind to platelet antigens and cause premature destruction of platelets by the mononuclear phagocytic system or lysis in a complement-dependent manner.24,65 Recent studies also implicate direct T cell mediated cytotoxicity and impaired megakaryopoiesis and thrombopoiesis in the pathogenesis of the thrombocytopenia.55 In humans, the target antigens are most commonly located on GPαIIbβ3.65 Other targets include specific regions on GPIb/IX, GPIV, and GPα2β1.24 Antibodies to multiple antigens are found in many patients.55 Cryptic epitopes, particularly involving GPαIIbβ3, have been implicated in both initiation and perpetuation of thrombocytopenia in humans with IMT.65 These epitopes are normally hidden but may become exposed and initiate an immune reaction following an insult, such as an infection. Exposure of cryptic antigens is also involved in EDTA-dependent pseudothrombocytopenia in humans. EDTA binds calcium which causes a conformational change in GPαIIbβ3, exposing normally hidden epitopes which bind to circulating antibodies. EDTA-dependent psuedothrombocytopenia has been rarely reported in dogs, horses and pigs. The underlying mechanism in these veterinary species has yet to be determined.64 Immune-mediated thrombocytopenia is commonly diagnosed in dogs and sporadically identified in other species. In primary IMT the immune response is directed against autoantigens and an underlying disease is not identified. In secondary IMT the immune response is associated with an underlying disorder (e.g. systemic immune-mediated disease, infection, neoplasia). The antibodies can be directed against either autoantigens, foreign antigens absorbed to platelet membranes, or altered platelet antigens. Platelet surface associated antibodies have been demonstrated in dogs, cats, and horses with IMT.23,25,30,39 In most veterinary cases the target antigens on the platelet membrane are not identified. GPαIIbβ3 and GPIb were identified as target antigens in some dogs with a clinical diagnosis of primary IMT.30 Isoantibodies have been detected in humans with an inherited deficiency of platelet membrane antigens. Humans with Glanzmann’s thrombasthenia produce isoantibodies following transfusion of platelets from normal individuals.37 The isoantibodies appear to target epitopes on β3 or intact αIIbβ3, leading to inhibited function and rapid removal of the transfused platelets. Although not reported, similar responses could occur in animals with inherited membrane GP deficiencies.50

REFERENCES 1. Abrams-Ogg ACG, Kruth SA, Carter RF, et al. Preparation and transfusion of canine platelet concentrates. Am J Vet Res 1993;54:635–642.

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2. Aree C, Moreno A, Pérez JM, et al. Expression of CD61 (β3 integrin subunit) on canine cells. Platelets 2001;12:69–73. 3. Aster RH. Drug-induced immune cytopenias. Toxicology 2005;209: 149–153. 4. Bloom JC, Thiem PA, Sellers TS, et al. Cephalosporin-induced immune cytopenia in the dog: demonstration of erythrocyte-, neutrophil- and platelet-associated IgG following treatment with cefazedone. Am J Hematol 1988;28:71–78. 5. Boudreaux MK, Lipscomb DL. Clinical, biochemical, and molecular aspects of Glanzmann’s thrombasthenia in humans and dogs. Vet Pathol 2001;38:249–260. 6. Brodersen R, Bijlsma F, Gori K, et al. Analysis of the immunological cross reactivities of 213 well characterized monoclonal antibodies with specificities against various leucocyte surface antigens of human and 11 animal species. Vet Immunol Immunopathol 1998;64:1–13. 7. Brown MR, Rogers KS. Neutropenia in dogs and cats: a retrospective study of 261 cases. J Am Anim Hosp Assoc 2001;37:131–139. 8. Bux J. Human neutrophil alloantigens. Vox Sang 2008;94:277–285. 9. Bux J. Molecular nature of granulocyte antigens. Transfusion Clin Biol 2001;8:242–247. 10. Capsoni F, Sarzi-Puttini P, Zanella A. Primary and secondary autoimmune neutropenia. Arthritis Res Ther 2005;7:208–215. 11. Chabanne L, Bonnefont C, Bernaud J, et al. Clinical applications of flow cytometry and cell immunophenotyping to companion animals (dog and cat). Methods Cell Sci 2000;22:199–207. 12. Christopherson PW, Insalaro TA, Van Santen K, et al. Characterization of the cDNA encoding alphaIIb and beta3 in normal horses and two horses with Glanzmann’s thrombasthenia. Vet Pathol 2006;43:78–82. 13. Davis EG, Rush B, Bain F, et al. Neonatal neutropenia in an Arabian foal. Equine Vet J 2003;35:517–520. 14. Deckmyn H, Ulrichts H, Van de Walle G, et al. Platelet antigens and their function. Vox Sang 2004;87:S105–S111. 15. Deckmyn H, Vanhoorelbeke K, Peerlinck K. Inhibitory and activating human platelet antibodies. Bailliere Clin Haematol 1998;11:343– 359. 16. Denomme GA. The structure and function of molecules that carry human red blood cell and platelet antigens. Transfusion Med Rev 2004; 18:203–231. 17. Forster LM. Neonatal alloimmune thrombocytopenia, purpura, and anemia in 6 neonatal piglets. Can Vet J 2007;48:855–857. 18. Fromont P, Bettaieb A, Skouri H, et al. Frequency of the polymorphonuclear neutrophil Fc gamma receptor III deficiency in the French population and its involvement in the development of neonatal alloimmune neutropenia. Blood 1992;79:2131–2134. 19. Howard CJ, Naessens J. Summary of workshop findings for cattle. Vet Immunol Immunopathol 1993;39:25–47. 20. Hurd CM, Cavanagh G, Schuh A, et al. Genotyping for platelet-specific antigens: techniques for the detection of single nucleotide polymorphisms. Vox Sang 2002;83:1–12. 21. Jain NC, Vegad JL, Kono CS. Methods for detection of immune-mediated neutropenia in horses, using antineutrophil serum of rabbit origin. Am J Vet Res 1990;51:1026–1031. 22. Kenny D, Morateck PA, Fahs SA, et al. Cloning and expression of canine glycoprotein Ib alpha. Thrombosis and Haemostasis 1999;82: 1327–1333. 23. Kohn B, Linden T, Leibold W. Platelet-bound antibodies detected by flow cytometric assay in cats with thrombocytopenia. J Feline Med Surg 2006;8:254–260. 24. Kravitz MS, Shoenfeld Y. Thrombocytopenic conditions – autoimmunity and hypercoagulability: commonalities and differences in ITP, TTP, HIT, and APS. Am J Hematol 2005;80:232–242. 25. Kristensen AT, Weiss DJ, Klausner JS, et al. Detection of antiplatelet antibody with a platelet immunofluorescence assay. J Vet Intern Med 1994;8:36–39. 26. Kristensen AT, Weiss DJ, Klausner JS. Platelet dysfunction associated with immune-mediated thrombocytopenia in dogs. J Vet Intern Med 1994;8:323–327. 27. Kroll H, Carl B, Santoso S, et al. Workshop report on the genotyping of blood cell alloantigens. Transfus Med 2001;11:211–219. 28. Kydd J, Antczak DF, Allen WR, et al. Report of the First International Workshop on Equine Leukocyte Antigens, Cambridge, UK, July 1991. Vet Immunol Immunopathol 1994;42:3–60. 29. Lalko CC, Deppe E, Ulatowski D, et al. Equine platelet CD62P (P-selectin) expression: a phenotypic and morphologic study. Vet Immunol Immunopathol 2003;91:119–134. 30. Lewis DC, Meyers KM. Studies of platelet-bound and serum plateletbindable immunoglobulins in dogs with idiopathic thrombocytopenia purpura. Exp Hematol 1996;24:696–701. 31. Lilliehöök I, Johannisson A, Håkansson L. Expression of adhesion and Fcγ-receptors on canine blood eosinophils and neutrophils studied by anti-human monoclonal antibodies. Vet Immunol Immunopathol 1998;61:181–193. 32. Linklater KA. Post-transfusion purpura in a pig. Res Vet Sci 1977;22: 257–258.

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33. Linklater KA, McTaggart HS, Imlah P. Haemolytic disease of the newborn, thrombocytopenia purpura and neutropenia occurring concurrently in a litter of piglets. Br Vet J 1973;129:36–46. 34. Lipscomb DL, Bourne C, Boudreaux MK. DNA sequence of the canine platelet beta3 gene from cDNA: comparison of canine and mouse beta3 to segments that encode alloantigenic sites and functional domains of beta3 in human beings. J Lab Clin Med 1999;134:313–321. 35. Lorenz M, Evering WE, Provencher A, et al. Atypical antipsychoticinduced neutropenia in dogs. Toxicol Appl Pharmacol 1999;155:227– 236. 36. Macieira S, Rivard GE, Champagne J, et al. Glanzmann thrombasthenia in an Oldenbourg filly. Vet Clin Pathol 2007;36:204–208. 37. Male C, Koren D, Eichelberger B, et al. Monitoring survival and function of transfused platelets in Glanzmann thrombasthenia by flow cytometry and thromboelastography. Vox Sang 2006;91:174–177. 38. Mateo A, Perez de la Lastra J, Moreno A, et al. Biochemical characterization of antigens detected with anti-platelet monoclonal antibodies. Vet Immunol Immunopathol 1996;52:363–370. 39. McGurrin MK, Arroyo LG, Bienzle D. Flow cytometric detection of platelet-bound antibody in three horses with immune-mediated thrombocytopenia. J Am Vet Med Assoc 2004;224:83–87. 40. Metcalfe P. Platelet antigens and antibody detection. Vox Sang 2004;87:S82-S86. 41. Norton A, Allen DL, Murphy MF. Review: platelet alloantigens and antibodies and their clinical significance. Immunohematology 2004;20:89– 102. 42. Nurden P, Nurden AT. Congenital disorders associated with platelet dysfunctions. Thromb Haemostasis 2008;99:253–263. 43. Pérez de la Lastra JM, Moreno A, Pérez J, et al. Characterization of the porcine homologue to human platelet glycoprotein IIb-IIIa (CD41/CD61) by a monoclonal antibody. Tissue Antigen 1997;49:588–594. 44. Perkins GA, Miller WH, Divers TJ, et al. Ulcerative dermatitis, thrombocytopenia, and neutropenia in neonatal foals. J Vet Intern Med 2005;19:211–216. 45. Perkins MC, Canfield P, Churcher RK, et al. Immune-mediated neutropenia suspected in five dogs. Aust Vet J 2004;82:52–57. 46. Ramirez S, Gaunt SD, McClure JJ, et al. Detection and effects on platelet function of anti-platelet antibody in mule foals with experimentally induced neonatal alloimmune thrombocytopenia. J Vet Intern Med 1999;13:534–539. 47. Roberts I, Stanworth S, Murray NA. Thrombocytopenia in the neonate. Blood Rev 2008;22:173–186. 48. Sachs UJ, Chavakis T, Fung L, et al. Human alloantibody anti-Mart interferes with Mac-1-dependent leukocyte adhesion. Blood 2004;104:727–734.

49. Schuberth HJ, Kucinskiene G, Chu RM, et al. Reactivity of cross-reacting monoclonal antibodies with canine leukocytes, platelets and erythrocytes. Vet Immunol Immunopathol 2007;119:47–55. 50. Scott MA. Granulocyte and platelet antigens. In: Feldman BF, Zinkl JG, Jain NC, eds. Schalm’s Veterinary Hematology, 5th ed. Baltimore: Lippincott Williams & Wilkins, 2000;783–788. 51. Simon M, Dusinsky R, Horovska L, et al. Immunohistochemical reactivity of anti-platelet monoclonal antibodies. Vet Immunol Immunopathol 1996;52:377–382. 52. Slichter SJ, Fish D, Abrams VK, et al. Evaluation of different methods of leukoreduction of donor platelets to prevent alloimmune platelet refractoriness and induce tolerance in a canine transfusion model. Blood 2005;105:847–854. 53. Smith GA, Ranasinghe E, Ouwehand WH. The importance of using multiple techniques for detection of platelet antibodies. Vox Sang 2007;93:306–308. 54. Sopp P, Howard CJ. Cross-reactivity of monoclonal antibodies to defined human leukocyte differentiation antigens with bovine cells. Vet Immunol Immunopathol 1997;56:11–25. 55. Stasi R, Evangelista ML, Stipa E, et al. Idiopathic thrombocytopenic purpura: current concepts in pathophysiology and management. Thromb Haemostasis 2008;99:4–13. 56. Stroncek D. Granulocyte antigens and antibody detection. Vox Sang 2004;87:S91–S94. 57. Stroncek DF, Fadeyi E, Adams S. Leukocyte antigen and antibody detection assays: tools for assessing and preventing pulmonary transfusion reactions. Transfusion Med Rev 2007;21:273–286. 58. Tablin F, Johnsrude JD, Walker NJ. Evaluation of glycoprotein Ib expression on feline platelets. Am J Vet Res 2001;62:195–201. 59. Trowald-Wigh G, Johannisson A, Håkansson L. Canine neutrophil adhesion proteins and Fc-receptors in healthy dogs and dogs with adhesion protein deficiency, as studied by flow cytometry. Vet Immunol Immunopathol 1993;38:297–310. 60. Vargo CL, Taylor SM, Haines DM. Immune mediated neutropenia and thrombocytopenia in 3 giant schnauzers. Can Vet J 2007;48:1159–1163. 61. Wardrop KJ, Lewis D, Marks S, et al. Posttransfusion purpura in a dog with hemophilia A. J Vet Intern Med 1997;11:251–263. 62. Weiss DJ. Evaluation of antineutrophil IgG antibodies in persistently neutropenic dogs. J Vet Intern Med 2007;21:440–444. 63. Weiss DJ, Henson M. Pure white cell aplasia in a dog. Vet Clin Pathol 2007;36:373–375. 64. Wills TB, Wardrop KJ. Pseudothrombocytopenia secondary to the effects of EDTA in a dog. J Am Anim Hosp Assoc 2008;44:95–97. 65. Zhou B, Zhao H, Yang RC, et al. Multi-dysfunctional pathophysiology in ITP. Crit Rev Oncol Hematol 2005;54:107–116.

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Principles of Canine and Feline Blood Collection, Processing, and Storage ANTHONY C.G. ABRAMS-OGG and ANN SCHNEIDER Blood Banking – Introduction Donor Considerations Canine Feline Blood Collection Principles Blood Collection Supplies Canine Feline Apheresis Preparation and Collection of Whole Blood Canine Feline

Separation of Whole Blood into Components Canine Packed Red Blood Cells and Plasma Canine Cryoprecipitate and Cryosupernatant Canine Platelet-rich Plasma and Platelet Concentrate Feline Blood Components Storage Red Blood Cell Products Plasma Products Platelet Products Quality Control

Acronyms and Abbreviations ACD, acid citrate dextrose; APS, anticoagulant-preservative solution; AS, additive solution; CPD, citrate-phosphatedextrose; CP2D, citrate-phosphate(2)-dextrose; CPDA-1, citrate-phosphate-dextrose-adenine-1; EBV, estimated blood volume; FFP, fresh-frozen plasma; FP, frozen plasma; FWB, fresh whole blood; Hgb, hemoglobin; MAP, mean arterial pressure; PCV, packed cell volume; pRBCs, packed red blood cells; PC, platelet concentrate; PRP, plateletrich plasma; RBC, red blood cell; RT, room temperature; WB, whole blood.

BLOOD BANKING – INTRODUCTION The advent of large-scale veterinary blood banks has made the practice of high-quality transfusion medicine possible by increasing availability of blood components. Strict adherence to blood collection, processing, and storage guidelines is essential to providing safe and effective transfusions. This chapter details the currently accepted blend of human protocol and veterinary modifications where necessary for the collection, processing, and storage of canine and feline blood components. DONOR CONSIDERATIONS Canine Veterinary blood banks currently use resident dogs, client-owned dogs, or a combination of the two to meet supply demands. Because human blood packs are used

to collect blood, a standard donation is one unit of blood (450 ± 45 mL). Based on an estimated blood volume (EBV) of 85 mL/kg body weight15 and a recommended collection volume of 15–22% EBV,11 the maximum donation volume is approximately 19 mL/ kg. In a study evaluating changes in systolic blood pressure in 19 Greyhounds (27–41 kg) after donating 450 mL blood, the mean systolic pressure dropped from 145 to 134 mmHg, not a clinically relevant decrease.11 Recommended minimal donor body weight varies among veterinary blood banks from 23 to 27 kg, with dogs of this size able to donate a unit as often as every 3–4 weeks without need for nutritional supplementation.21 Donor blood-type issues are discussed in Chapters 92 and 100. For the safety of both the blood donor and recipient, the health of a dog is evaluated prior to enrollment as a donor and then annually through a complete medical history, physical examination, and laboratory evaluation including a hemogram, serum biochemistry profile, 731

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and infectious disease screening. In addition, a brief history, physical examination, and measurement of packed cell volume (PCV) or hemoglobin (Hgb) concentration is performed prior to each blood donation; donor PCV or Hgb concentration should be at least 40% or 13 g/dL, respectively. Donors should be current on vaccinations and maintained on heartworm prevention and flea and tick control. Most other drugs are unacceptable for use during the donation period because of unknown effects on blood quality and potential for adverse reactions in the recipient. Minimal screening for infectious diseases based on the American College of Veterinary Internal Medicine Consensus Statement is recommended if there is risk for exposure.30 These diseases include Babesia canis and B. gibsoni, Leishmania spp., Ehrlichia canis and other Ehrlichia spp., Anaplasma or Neorickettsia spp., and Brucella canis. Donors should be excluded based on a positive serologic test for a vector-borne disease. Serologic negative donors may be further screened by polymerase chain reaction assays. Screening for Trypanosoma cruzi, Bartonella vinsonii and hemotropic Mycoplasma spp. may also be considered. Exclusion from a blood donor program does not imply that the animal should be treated for the disease. Prior transfusion is considered an exclusion criterion for dogs to become blood donors. Following a blood transfusion dogs may develop antibodies against foreign red blood cell (RBC) antigens, resulting in an increased risk of incompatibility reactions to their plasma. Previous pregnancy has long been considered an exclusion criterion in the event that a dog negative for a specific RBC antigen becomes sensitized by her pups positive for that antigen. This risk should be minimal considering the zonal placentation of dogs, and there is little evidence to support this exclusion criterion.5 Feline Because cats typically require sedation for blood collection, most blood banks rely on resident cats. The estimated blood volume of a cat varies according to the reference, ranging from 40 to 60 mL/kg in the anesthetized cat12 to as high as 67 mL/kg.24 Donation volume should be calculated based on lean body weight; recommendations for maximum donation range from 11 to 15 mL/kg, and standard volumes collected (including anticoagulant) range from 50 to 70 mL.1,13,14 In one study of 26 cats (5–8 kg), blood pressure was measured before and after donation of a 50 mL unit using sevoflurane anesthesia; mean arterial pressure (MAP) decreased from 87 mmHg (range 59–127 mmHg) to 71 mmHg (range 44–116 mmHg).14 In another study with eight cats (4.0–6.5 kg) sedated with butorphanol, acepromazine, ketamine and diazepam, MAP dropped from 108 mmHg (range 80–140 mmHg) to 53 mmHg (range 20–100 mmHg) after collection of a 50 mL unit.4 No cats in either study showed effects of hypotension upon recovery. However, in the second study, when the same volume was drawn in awake or mildly sedated cats

through vascular access ports, two cats exhibited transient distress, including vocalization, vomiting, and defecation.4 This is in contrast to another study of four awake cats using vascular access ports where no signs of hypotension were noted with donations of 10 mL/ kg.20 In the authors’ experience cats donating less than 11 mL/kg are less likely to have hypotension-related problems. Most blood banks give donors supplemental iron. Assuming a similar mean concentration of 0.5 mg iron/mL blood in cats as in humans, a 60 mL donation represents a loss of 30 mg of iron. This may be replaced with 150 mg oral ferrous sulfate, typically divided into several doses in between donations and adjusted based on pre-donation PCV. Initial and pre-donation health assessment and health maintenance are similar to canine donors. Donor PCV or Hgb concentration should be at least 30% or 10 g/dL, respectively. Feline donors should be kept indoors to minimize the risk of infectious disease transmission. Cats testing positive for feline leukemia virus and feline immunodeficiency virus by enzymelinked immunosorbent assay and for hemotropic Mycoplasma spp. by cytology or polymerase chain reaction test should not be used as donors.30 Screening for Cytauxzoon felis and rickettsial diseases may be considered based on exposure risk. As with dogs, exclusion from a blood donor program does not imply that the cat should be treated for the disease. In addition, one must consider the potential ramifications of falsepositive results for feline viral infections, namely unwarranted euthanasia. BLOOD COLLECTION PRINCIPLES Blood is collected as whole blood (WB) and then separated into components by centrifugation. Alternatively, single components may be collected by apheresis. For optimal quality, WB is collected into commercially available, sterile, airtight systems that allow for subsequent processing and storage of components without exposure to the environment. These systems, known as closed systems, prevent exposure of blood components to air except when the component unit is entered with an administration set for transfusion. Conversely, an open system has one or more additional sites of entry and possible bacterial contamination, such as occurs when preparing for blood collection into syringes, acidcitrate-dextrose (ACD) bottles, or bags to which anticoagulant is added after manufacture. Components prepared from blood collected into an open system are not intended for storage. If an open collection system is used, the American Association of Blood Banks and Council of Europe advise that RBC products (stored at 1–6 °C) should be used within 24 hours and platelet products (stored at 20–24 °C) should be used within 4 hours.6,10 The shelf-life of plasma from an open system is unaffected if it is frozen within 8 hours.6,10 Throughout the following sections, processing and storage will refer to closed systems unless otherwise specified. Detailed illustrated methods of blood collection and processing

CHAPTER 94: PRINCIPLES OF CANINE AND FELINE BLOOD COLLECTION, PROCESSING, AND STORAGE

have been previously described and will be summarized here.1,19,23 BLOOD COLLECTION SUPPLIES Canine Commercial 450 mL collection bags manufactured for human blood banking are used for canine donations. These contain an anticoagulant-preservative solution (APS) and a sterile collection line with a 16-gauge thinwalled needle attached to the bag. The bags are available in a variety of configurations consisting of a primary collection bag with attached satellite bags (0 to 4). These satellite bags, some of which may contain RBC nutrient additive solutions (AS), are used for the separation of components as discussed later. After centrifugation and removal of plasma from the main collection bag, the nutrient AS is added to the packed red blood cells (pRBCs). Various AS are composed of saline, adenine, and dextrose ± mannitol ± citrate-citric acid, and are intended to extend the shelf-life of stored pRBCs. There are several APS and AS available that allow storage of canine pRBCs for approximately 1 month (Table 94.1). Collection bags are available in a variety of sizes (150 mL, 250 mL, 350 mL, and 500 mL) and contain the appropriate amount of APS and AS. Another option for collection of 8 hours from collection

≤ -18° C, storage ≤ 1 year; ≤ -65° C, storage ≤ 7 years

2,000 × g, 10 min, 20° C

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Liquid Liquid Plasma Plasma

≤ -18° C, storage ≤ 5 years

Fresh Fresh Frozen Frozen storage ≥ 1 year ≤ 5 years Plasma Plasma

separation of canine fresh whole blood into components. (Adapted from Abrams-Ogg ACG. Practical blood transfusion. In: Day M, Mackin A, Littlewood J, eds. BSAVA Manual of Canine and Feline Haematology and Transfusion Medicine. Gloucester: British Small Animal Veterinary Association. 2000;263–303, Fig. 15.19, with permission.)

Frozen Frozen Plasma Plasma

5,000 × g, 7 min, 4° C

Platelet Platelet Concentrate Concentrate

Cryoprecipitate Cryoprecipitate

Cryosupernatant Cryosupernatant

an AS, the pRBCs must have a PCV ≤ 80%. This requires leaving ≥50 mL of plasma with the pRBCs, which may be accomplished by stopping the expression of plasma when the buffy coat/plasma interface is about 2 cm from the top of the bag. If the pRBCs are to be stored in citrate-phosphate-dextrose (CPD) or CP2D (which contains twice as much dextrose as CPD) with an AS, all of the plasma may be removed from the pRBCs, as the AS will provide the pRBCs with the necessary fluid and nutrients. Once the plasma has been removed to one or more satellite bags, the AS is added to the pRBCs. The seal in the tubing to the AS bag is broken, and the solution is transferred into the primary bag by gravity and gently mixed. If desired, half of the pRBCs can then be transferred to an additional empty satellite bag if available to prepare half-units. The tubing attached to each component bag is then sealed. Each bag of component is weighed and the weight of the component calculated by subtracting the weight of the bag. Volumes of plasma and pRBCs at room temperature (RT) are then estimated by dividing by 1.026 and 1.056, respectively.28 (These values are for humans, and specific gravity varies with temperature, PCV, and protein concentration, but effects on volume estimates are clinically negligible.) Each component bag is labeled as to product, blood type, donor, volume, and dates of collection and expiration. Fresh plasma contains albumin, globulins, and maximum possible quantities of all coagulation factors. If this plasma is stored below −18 °C within 8 hours of collection, it is labeled fresh-frozen plasma (FFP). If the plasma is not placed in a freezer within 8 hours of collection, it may still be frozen and labeled frozen plasma (FP). There are negligible differences between FFP and FP with respect to albumin, globulins and α-

macroglobulins, vitamin-K dependent factors, and antithrombin.6 Plasma products are discussed further in Chapter 96. Canine Cryoprecipitate and Cryosupernatant Once frozen, FFP may be further processed into cryoprecipitate and cryosupernatant, also referred to as cryopoor plasma and cryoprecipitate-reduced plasma. When separating plasma from FWB with the intention of further processing into cryoprecipitate, the plasma is transferred into a satellite bag and another empty satellite bag is left attached via tubing. The tubing connecting the two is temporarily obstructed by folding and securing with a rubber band. The two bags are frozen together in a cardboard plasma box. After freezing, the unit is slowly thawed in a refrigerator at 1–6 °C. When the unit reaches a slushy consistency, it is centrifuged at 5000 × g for 7 minutes and all but 10–15 mL of the supernatant plasma (cryosupernatant) is transferred to the empty bag, leaving the precipitate (cryoprecipitate) and small amount of plasma in the first bag. The bags are separated, sealed, labeled and immediately refrozen. Cryoprecipitate contains von Willebrand factor, factor VIII, fibrinogen, fibronectin, and factor XIII and is primarily indicated in the management of bleeding in dogs with von Willebrand disease and hemophilia A (see Chapter 96). Canine Platelet-rich Plasma and Platelet Concentrate Fresh whole blood can also be separated into plateletrich plasma (PRP) and pRBCs. The FWB may be

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allowed to rest for 1 hour after collection to minimize platelet activation, and the bag should be gently massaged before centrifugation to re-suspend platelets.17 Centrifuge temperature should be set at 20 °C. Centrifugation to make PRP uses lighter gravitational forces than for standard separation of pRBCs and plasma, and isolates the platelets in the plasma above the buffy coat. When the PRP is expressed into the satellite bag, expression should be stopped when the RBCplasma interface is 1 cm from the top of the bag to minimize leukocyte and RBC content of PRP while not sacrificing platelet yield.7 One unit of FWB (450 ± 45 mL) yields 1 unit of PRP. Percent platelet yield (platelets in PRP/platelets in FWB × 100) and PRP platelet count vary with the donor, donor platelet count, blood volume being centrifuged, technician, centrifuge, and centrifugation protocol. In a study comparing protocols to make PRP from small volumes of canine blood, protocols with shorter centrifugation times and higher gravitational forces had better yields than protocols with longer times and lower forces.8 Most current blood bank protocols use about 1000 × g for 4–6 minutes or 2000 × g to 2500 × g for 2.5–3 minutes.2 The brake on the centrifuge should be turned to a low setting or turned off. In the protocol used by the authors (1000 × g for 4 minutes), average yield is about 80% (range 35–97%), resulting in a mean of 6 × 1010 platelets/unit (range 3 × 1010 to 10 × 1010 platelets/unit).2,3 A second centrifugation of the FWB can be used to increase yield, and is particularly useful with greyhounds which have low normal platelet counts (W.J. Dodds, personal communication). Platelet concentrate (PC) may be prepared by centrifugation (2000 × g for 10 minutes) of PRP to pellet the platelets. The platelet-poor plasma (PPP) is expressed, leaving 35–70 mL of plasma and the platelet pellet behind in the satellite bag. The resulting PC is left undisturbed for 60–90 minutes to promote disaggregation. Gentle manual kneading and agitation are then used to disperse macroscopic leukocyte-platelet aggregates and re-suspend the platelets. The PPP may be used as fresh plasma or FFP. One unit of PRP yields 1 unit of PC. Units of PC can be pooled via transfer tubing. The pooled units are centrifuged at 570 × g for 15 minutes and PPP is expressed, leaving behind 8–10 mL per unit pooled. Once pooled, the concentrate needs to rest for at least 15 minutes at room temperature, prior to resuspension (K.J. Wardrop, personal communication). Feline Blood Components Feline FWB is placed in one bag, and the tubing between this bag and an empty satellite bag is obstructed by folding and securing with a rubber band or a hand sealer clamp. Due to the relatively small volume of blood being separated, the collection/satellite bags may be bound (via rubber band or tape) to a 500 mL bag of saline to prevent the bags from collapsing during centrifugation. The blood bags are centrifuged at 3000 × g for 10 minutes at 4 °C, and separation is continued as

for dogs. The pRBCs may be stored without addition of AS (in which case 10 mL 0.9% sodium chloride is added just prior to administration to the patient), or 10 mL AS may be added to pRBCs prior to storage. Feline PRP is rarely prepared due to a limited clinical need, as well as technical challenges; FWB is more likely to be administered to thrombocytopenic or thrombopathic cats with severe bleeding. However, preparation of feline PRP has been described, with the transfused platelets exhibiting in vivo efficacy.8,9 STORAGE Red Blood Cell Products Units of WB and pRBCs are stored at 1–6 °C. A refrigerator with an alarm to indicate unsafe temperatures is ideal. Alternatives include a regularly-checked thermometer placed in a container of fluid the volume of a small unit and HemoTemp II blood bag thermometer labels (Biosynergy, Elk Village, IL). Red blood cell viability and function decrease during storage as a result of physical and metabolic changes collectively known as the storage lesion (see Chapter 95). Shelf-life is defined as the number of days after collection, assuming proper closed system collection and storage, at which 75% RBC viability is maintained.6,10 Viability is measured as 24-hour post transfusion survival of radiolabeled or biotinylated RBCs.29,31,32 Shelflives for canine pRBCs prepared in closed systems are summarized in Table 94.1. Plasma Products All plasma products should be stored at −18 °C or lower in a cardboard plasma box which protects the bag from breakage. A freezer with an alarm to indicate unsafe temperatures is ideal. The shelf-life of FFP is 1 year from collection if frozen at −18 °C (some countries specify −30 °C) and 7 years if frozen at or below −65 °C.6 If a regular household freezer is used, exposure of the FFP to repeated freeze/thaw cycles will result in loss of the more labile plasma coagulation factors. Expired −18 °C stored FFP from dogs and cats may be relabeled as FP and stored for an additional 4 years.19 Expired FFP is used primarily for treatment of hypoproteinemia and vitamin-K antagonist poisoning. The shelf-life of cryoprecipitate and cryosupernatant is 1 year from the date of collection, regardless of when during that year it was made from FFP.6,19 Platelet Products Canine PRP and PC are most often produced on an asneeded basis. Current guidelines for human PC allow RT storage for 5–7 days, although there is increased risk for bacterial proliferation at RT storage.26 The PCs are stored in plastic bags under continuous gentle agitation. The composition of the plastic bag, high surface area : volume ratio, and agitation facilitate gas exchange

CHAPTER 94: PRINCIPLES OF CANINE AND FELINE BLOOD COLLECTION, PROCESSING, AND STORAGE

and sustain aerobic metabolism, and thereby platelet viability. Canine PRP-derived PCs stored for 7 days at RT with to-and-fro agitation in polyolefin bags maintained platelet numbers, metabolic activity, and pH above 6.0, although there was progressive reduction in in vitro aggregation.3 QUALITY CONTROL Units of pRBCs should be visually inspected during storage and prior to administration. The presence of murky, purple, brown, or red supernatant, RBC mass that appears purple in color, visible clots, or differences in color between the unit and the tubing segments gives reason to suspect bacterial contamination.13,18 Similarly, units of FFP should be inspected when issued for transfusion and discarded if cracks or leaks are noted. Inadvertent thawing during storage may significantly reduce levels of factor VIII and von Willebrand factor, and allow for bacterial proliferation. In the absence of a freezer alarm or chart recorder, the following techniques may be used to detect inadvertent thawing and refreezing of a unit after its original freezing: (1) Store the plasma box initially laying flat in the freezer. After complete freezing, shift the box to sit on one end. Air bubbles present in the bag will move from the front edge to the upper edge of the bag if thawing occurs, and the unit will also be thicker on the lower edge. (2) Place a rubber band around the unit before freezing, so that it makes an indentation in the bag. After complete freezing, cut the rubber band. If thawing occurs, the indentation left by the rubber band will be less obvious or will vanish.

REFERENCES 1. Abrams-Ogg ACG. Practical blood transfusion. In: Day M, Mackin A, Littlewood J eds. BSAVA Manual of Canine and Feline Haematology and Transfusion Medicine, Gloucester: British Small Animal Veterinary Association, 2000;263–303. 2. Abrams-Ogg AC, Kruth SA, Carter RF, et al. Preparation and transfusion of canine platelet concentrates. Am J Vet Res 1993;54:635–642. 3. Allyson K, Abrams-Ogg AC, Johnstone IB. Room temperature storage and cryopreservation of canine platelet concentrates. Am J Vet Res 1997;58:1338–1347. 4. Aubert I, Abrams-Ogg A, Johnstone I, et al. The use of totally implantable access ports for blood collection in feline donors. J Vet Intern Med 2002;16:348 (abstract). 5. Blais MC, Rozanski EA, Hale AS, et al. Lack of evidence of pregnancyinduced alloantibodies in dogs. J Vet Intern Med 2009;23:462–465. 6. Brecher ME ed. Technical Manual, 15th ed. Bethesda: American Association of Blood Banks, 2005.

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7. Champion AB, Carmen RA. Factors affecting white cell content in platelet concentrates. Transfusion 1985;25:334–338. 8. Clemmons RM, Bliss EL, Dorsey-Lee MR, et al. Platelet function, size and yield in whole blood and in platelet-rich plasma prepared using differing centrifugation force and time in domestic and food-producing animals. Thromb Haemostasis 1983;50:838–843. 9. Cowles BE, Meyers KM, Wardrop KJ, et al. Prolonged bleeding time of chediak-higashi cats corrected by platelet transfusion. Thromb Haemostasis 1992;67:708–712. 10. Council of Europe. Guide to the Preparation, Use and Quality Assurance of Blood Components, 14th ed. Strasbourg: Council of Europe Publishing, 2008. 11. Couto CG, Iazbik MC. Effects of blood donation on arterial blood pressure in retired racing greyhounds. J Vet Intern Med 2005;19:845–848. 12. Groom AC, Rowlands S. The cardiac output and blood volume of the anaesthetized cat. Phys Med Biol 1958;3:138–156. 13. Hohenhaus AE, Drusin LM, Garvey MS. Serratia marcescens contamination of feline whole blood in a hospital blood bank. J Am Vet Med Assoc 1997;210:794–798. 14. Iazbik MC, Gomez Ochoa P, Westendorf N, et al. Effects of blood collection for transfusion on arterial blood pressure, heart rate, and PCV in cats. J Vet Intern Med 2007;21:1181–1184. 15. Jahr JS, Lurie F, Bezdikian V, et al. Measuring circulating blood volume using infused hemoglobin-based oxygen carrier (Oxyglobin®) as an indicator: verification in a canine hypovolemia model. Am J Ther 2008;15:98–101. 16. Kaufman PM. Supplies for blood transfusions in dogs and cats. Probl Vet Med 1992;4:582–593. 17. Kelley DL, Fegan RL, Ng AT, et al. High-yield platelet concentrates attainable by continuous quality improvement reduce platelet transfusion cost and donor exposure. Transfusion 1997;37:482–486. 18. Kim DM, Brecher ME, Bland LA, et al. Visual identification of bacterially contaminated red cells. Transfusion 1992;32:221–225. 19. Lucas RL, Lentz KD, Hale AS. Collection and preparation of blood products. Clin Techn Small Anim Pract 2004;19:55–62. 20. Morrison JA, Lauer SK, Baldwin CJ, et al. Evaluation of the use of subcutaneous implantable vascular access ports in feline blood donors. J Am Vet Med Assoc 2007;230:855–861. 21. Potkay S, Zinn RD. Effects of collection interval, body weight, and season on the hemograms of canine blood donors. Lab Anim Care 1969;19:192–198. 22. Price GS, Armstrong PJ, McLeod DA, et al. Evaluation of citrate-phosphate-dextrose-adenine as a storage medium for packed canine erythrocytes. J Vet Intern Med 1988;2:126–132. 23. Schneider A. Blood components. collection, processing, and storage. Vet Clin N Am Small Anim Pract 1995;25:1245–1261. 24. Spink RR, Malvin RL, Cohen BJ. Determination of erythrocyte half-life and blood volume in cats. Am J Vet Res 1966;27:1041–1043. 25. Springer T, Hatchett WL, Oakley DA, et al. Feline blood storage and component therapy using a closed collection system. J Vet Intern Med 1998;12:248 (abstract). 26. Stroncek DF, Rebulla P. Platelet transfusions. Lancet 2007;370:427–438. 27. Troyer HL, Feeman WE, GraTL, et al. Comparing chemical restraint and anesthetic protocols used for blood donation in cats: One teaching hospital’s experience. Vet Med 2005;100:652–658. 28. Trudnowski RJ, Rico RC. Specific gravity of blood and plasma at 4 and 37 degrees C. Clin Chem 1974;20:615–616. 29. Wardrop KJ, Owen TJ, Meyers KM. Evaluation of an additive solution for preservation of canine red blood cells. J Vet Intern Med 1994;8:253–257. 30. Wardrop KJ, Reine N, Birkenheuer A, et al. Canine and feline blood donor screening for infectious disease. J Vet Intern Med 2005;19:135– 142. 31. Wardrop KJ, Tucker RL, Anderson EP. Use of an in vitro biotinylation technique for determination of posttransfusion viability of stored canine packed red blood cells. Am J Vet Res 1998;59:397–400. 32. Wardrop KJ, Tucker RL, Mugnai K. Evaluation of canine red blood cells stored in a saline, adenine, and glucose solution for 35 days. J Vet Intern Med 1997;11:5–8.

C H A P T E R 95

Red Blood Cell Transfusion in the Dog and Cat MARY BETH CALLAN Indications Transfusion Threshold Whole Blood versus Packed RBCs Fresh versus Stored RBCs Additional Benefits of RBC Transfusions Administration Warming of Blood

Blood Filters and Infusion Devices Route, Volume, and Rate of Transfusion Massive Transfusion Patient Monitoring

Acronyms and Abbreviations BT, bleeding time; 2,3-DPG, 2,3-diphosphoglycerate; FFP, fresh frozen plasma; FWB, fresh whole blood; Hct, hematocrit; Hgb, hemoglobin; [Hgb], hemoglobin concentration; PCV, packed cell volume; pRBCs, packed red blood cells; RBC, red blood cell; SWB, stored whole blood; WB, whole blood; WBC, white blood cell.

T

he term red blood cell (RBC) transfusion includes administration of packed red blood cells (pRBCs), fresh whole blood (FWB), or stored whole blood (SWB). With a growing knowledge and expertise in veterinary transfusion medicine, as well as establishment of more commercial animal blood banks, the administration of pRBCs rather than whole blood (WB) has become more commonplace for treatment of anemia in dogs4,11,24 and, more recently, cats.5,12,17 Blood component therapy addresses a patient’s specific transfusion needs while maximizing the use of a blood unit and decreasing potential adverse events associated with WB transfusion. INDICATIONS Red blood cell transfusions are indicated in the treatment of anemia caused by hemorrhage, hemolysis, or ineffective erythropoiesis. Because oxygen is poorly soluble in plasma, nearly all oxygen contained in blood is carried by hemoglobin (Hgb). Therefore, RBC transfusions increase the oxygen-carrying capacity of the anemic patient and thereby treat or prevent inadequate delivery of oxygen to tissues, with consequent tissue hypoxia. Hemorrhage (acute or chronic) was the major cause of anemia in 70% of dogs receiving RBC transfusions.4,11 Blood loss anemia was also the most common indication (44–52%) for RBC transfusions in cats, followed by ineffective erythropoiesis (38%).12,26 738

Transfusion Threshold The decision to administer a RBC transfusion is usually based on a measurement of the patient’s packed cell volume (PCV), hematocrit (Hct), or Hgb concentration ([Hgb]) and, more importantly, on clinical evaluation of the patient. A “transfusion trigger” or threshold PCV below which a RBC transfusion is administered, has not been clearly defined in human or veterinary medicine. While for many years the standard transfusion threshold for a normovolemic anemic human patient was an Hct of 30% and [Hgb] of 10 g/dL, a more recent trend has been to lower this threshold.13 In an analysis of 45 observational studies that assessed the independent effect of RBC transfusion on human patient outcomes, the risks of transfusion outweighed the benefits in 42 studies.15 Acknowledging the inherent limitation in their analysis of cohort studies, the authors concluded that in adult, intensive care unit, trauma, and surgical patients, RBC transfusions are associated with increased morbidity and mortality and recommended an [Hgb] < 7 g/dL as a transfusion threshold for hemodynamically stable patients.15 In a landmark multicenter, randomized, controlled clinical trial, the Transfusion Requirement in Critical Care (TRICC), “restrictive” and “liberal” RBC transfusion strategies in the management of 838 critically ill patients with [Hgb] < 9 g/dL were compared.7 The “restrictive” group received blood when the [Hgb] fell below 7 g/dL to maintain the [Hgb] at 7–9 g/dL, whereas the “liberal” group received blood

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to maintain the [Hgb] at 10–12 g/dL. Overall, the 30-day mortality was similar in both groups, but the mortality rate during hospitalization was significantly lower in the “restrictive” group, suggesting that a restrictive strategy of RBC transfusion is at least as effective as and possibly superior to a liberal transfusion strategy in critically ill patients.7 In a recent review of RBC transfusion in human clinical practice, the following generalizations were made: (1) the physiology of oxygen delivery and clinical data indicate little need to transfuse patients with [Hgb] ≥ 10 g/dL; (2) at [Hgb] = 8– 10 g/dL, the risk of hypoxic organ damage is low for most patients; and (3) patients with [Hgb] < 6 g/dL are usually at substantial risk, particularly if ongoing bleeding is a possibility.13 Currently, there are no clinical studies evaluating RBC transfusion threshold (or increased morbidity and mortality associated with RBC transfusion) in dogs and cats. However, it is well recognized that measurement of Hct, [Hgb], or PCV alone is an inadequate determinant of the threshold for RBC transfusion because many additional factors (e.g. cardiac output and oxygen consumption) are involved in adequacy of tissue oxygenation. Also, in patients with hypovolemic anemia, the PCV is falsely elevated: when the total blood volume normalizes because of an increase in the plasma volume, the PCV decreases. Animals with chronic anemia (e.g. anemia of chronic renal failure) typically better cope with a lower PCV than do animals with an acute onset of anemia (e.g. an acute blood loss or hemolytic crisis) because of cardiovascular and other compensatory mechanisms. Because many factors can influence the need for a RBC transfusion, it is imperative that clinical judgment, not PCV, be the ultimate factor in the decision to administer RBCs to a patient. Tachycardia, poor pulse quality, pallor, lethargy, weakness, and decreased appetite are important clinical signs and symptoms that may indicate that a patient may be in need of additional oxygen-carrying support. Whole Blood Versus Packed RBCs The majority of canine and feline blood collected at commercial blood banks and veterinary teaching hospitals is processed into pRBCs and fresh frozen plasma (FFP) (see Chapter 94). Administration of pRBCs is appropriate in the medical management of anemia resulting from any cause, but whole blood (WB) transfusion may be considered in certain situations. By definition, FWB is blood that is less than 8 hours old from the time of collection and has not been refrigerated; therefore, FWB contains functional platelets, coagulation factors, and plasma proteins in addition to RBCs. Potential indications for FWB include anemia and a combined hemostatic disorder; anemia and thrombocytopenia or thrombopathia resulting in uncontrolled or life-threatening bleeding; and possibly massive transfusion. Stored whole blood is more than 8 hours old: the length of storage depends on the anticoagulant/preservative solution used and varies from 48 hours for 3.8% sodium citrate (no pre-

739

servative) to 4 weeks for CPD-A1 (citrate, phosphate, dextrose, and adenine) (see Chapter 94). Stored whole blood contains plasma proteins and RBCs but not functional platelets or coagulation factors. A possible indication for SWB is anemia and hypoproteinemia (e.g. chronic gastrointestinal bleeding), although administration of pRBCs and, if there is a clinical need to increase the patient’s oncotic pressure, a synthetic colloid or plasma would be appropriate. Given the loss of important blood components with storage, it is much more efficient to separate WB into pRBC and FFP than to store as WB. Also, in anemic patients with underlying cardiac disease, pRBCs would be clearly preferable to WB in an effort to avoid circulatory overload. Fresh Versus Stored RBCs Although storage of blood components allows for a readily available supply of RBCs for transfusion, it has been well-documented that during storage RBCs undergo a number of physical and chemical changes, collectively referred to as a “storage lesion,” which reduce RBC function and viability after transfusion.14 The effects of prolonged storage on RBCs include decreased deformability, which can impede microvascular flow; depletion of 2,3-diphosphoglycerate (2,3DPG), which shifts the oxyhemoglobin dissociation curve to the left and reduces oxygen delivery; reduction in RBC ATP concentration, which may result in decreased phosphorylation of other proteins or lipid kinases important in maintaining RBC integrity; reduction in RBC-derived nitric oxide bioactivity, which may impair the vasodilatory response to hypoxia; and accumulation of proinflammatory bioactive substances.1,14,16,21 Addition of various preservative solutions containing dextrose, adenine, and phosphate (substrates for RBC energy metabolism) improve RBC post-transfusion viability. A study of the effect of storage on canine pRBCs collected in citrate-phosphate-dextrose solution and resuspended in the additive solution Adsol (containing adenine, dextrose, saline, and mannitol) has documented a steady decrease in mean 2,3-DPG concentration from 15.2 μmol/g Hgb on day 1 to 3.7 μmol/g Hgb on day 44.25 However, the decrease in 2,3-DPG concentration in stored human and presumably other animal RBCs is a reversible change, with complete restoration of 2,3-DPG levels post-transfusion taking up to 72 hours.21 Decrease in RBC 2,3-DPG concentration with storage is not an issue when transfusing cats because feline RBCs contain very low levels of 2,3-DPG and feline Hgb does not require 2,3-DPG for release of oxygen.3 Experimental data in canine models have yielded conflicting results regarding oxygen delivery by fresh compared to stored RBCs. Although a diminished release of oxygen to tissue was demonstrated following administration of stored (21 days) RBCs compared with fresh RBCs in an isolated hind-limb model in the dog,28 a study comparing the effects of administration of autologous stored (21 days) and fresh RBCs in restoring muscular tissue oxygenation after profound isovolemic

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hemodilution in dogs did not reveal a difference in tissue oxygenation between the two groups at comparable [Hgb].19 Currently, there are no clinical data in the dog to document increased morbidity or mortality associated with transfusion of stored rather than fresh RBCs. There is considerable controversy in human medicine over recommendations for administration of “newer blood” (units stored for 14 days or less) in preference to “older blood” (units stored for more than 14 days). In a single-center, retrospective analysis of outcomes in 6,000 patients who had undergone coronary artery bypass grafting, valve surgery, or both, transfusion of “older blood” (median duration of storage 20 days) was associated with a significantly increased risk of postoperative complications, as well as reduced short-term and long-term survival.14 On the other hand, a retrospective study evaluating age of transfused RBCs and early outcomes after similar cardiac surgeries in 901 patients found no correlation between duration of storage of RBCs and early adverse outcomes after cardiac surgery.27 A review of nine clinical studies that evaluated the consequences of prolonged RBC storage suggested a possible detrimental clinical effect associated with transfusion of stored RBCs to critically ill patients.21 However, one must consider that cardiac surgery and critically ill patients are not representative of all patients in need of RBC transfusions. In light of the consequences of shortening RBC storage time on the blood supply, both sides (for and against use of “older blood”) agree that randomized clinical trials to evaluate the clinical consequences of transfusing older stored RBCs are required before changing blood banking standards for RBC storage time. ADDITIONAL BENEFITS OF RBC TRANSFUSIONS Enhanced hemostasis is an often overlooked benefit of RBC transfusions. A correlation between anemia and prolonged bleeding time (BT) has been reported in humans, and correction of the anemia results in shortening of the BT.23 The effect of Hct on the volume of blood shed during a BT test can be dramatic: a reduction in Hct from 45% to 35% in normal, healthy humans resulted in a greater than two-fold increase in the volume of shed blood.6 In a study evaluating the effect of Hct and platelet count on the BT measurement, healthy human volunteers underwent either a 2-unit RBC apheresis procedure followed by return of plateletrich plasma from both units (resulting in a 15% reduction in the peripheral venous Hct and a 9% reduction in the platelet count) or a plateletpheresis procedure (resulting in a 32% decrease in platelet count and no change in peripheral venous Hct).22 Interestingly, there was no change in BT following the plateletpheresis procedure, yet the RBC apheresis procedure resulted in a 60% increase in the BT, which was subsequently corrected by transfusion of RBCs.22 Furthermore, the acute reduction in Hct following the RBC apheresis procedure was accompanied by a decrease in the shed blood

thromboxane B2 level at the template BT site, in agreement with results of in vitro studies showing that RBCs stimulate thromboxane production by platelets.22 Other potential mechanisms for RBCs leading to an improvement in primary hemostasis include: dispersion of platelets from the center of the blood vessel toward the endothelial cells of the vessel wall, facilitating the platelet-vessel adhesion; shear stress-induced release of ADP, a platelet agonist, from RBCs; and scavenging of endothelial cell nitric oxide by oxidation and by nitric oxide binding to Hgb.23 In light of the beneficial effects of RBCs on hemostasis, it has been recommended in human medicine that nonsurgical bleeding diatheses in anemic thrombocytopenic patients should be managed with RBC transfusions to increase the Hct to 35% prior to administering platelet transfusions.23 In addition to providing oxygen-carrying support and improving hemostasis, RBC transfusions are an excellent source of readily bioavailable iron, with 1 mL of pRBCs containing approximately 1 mg of iron. RBC transfusions, therefore, are of benefit in the initial treatment of patients with severe iron-deficiency anemia. However, repeated RBC transfusions to patients with anemia due to causes other than continuous blood loss may place them at risk for developing hemochromatosis, which manifests clinically as organ dysfunction secondary to iron-induced injury. There is a single case report of transfusional hemochromatosis in a dog with pure RBC aplasia that received RBC transfusions every 6–8 weeks for 3 years.18 ADMINISTRATION Careful attention to administration is essential to prevent damage of the blood product and harm to the patient. Prior to a RBC transfusion, blood typing and/ or crossmatching should be performed to assure RBC compatibility (see Chapter 139). Segments of tubing from the donor blood bag may be used for the blood crossmatch and for quality control and investigation of transfusion reactions (see Chapter 100). Canine pRBCs stored in a RBC preservative solution such as Adsol can be administered directly, whereas other pRBC products should be diluted by adding 100 mL of saline to the blood bag (or 10 mL of saline to feline pRBCs), thus decreasing the viscosity of the donor blood. Concurrent administration of any drugs or fluids other than physiologic saline through the same catheter should be avoided to prevent blood coagulation and lysis of RBCs induced by contact with calcium-containing solutions and hypotonic solutions, respectively. Warming of Blood In the routine administration of RBC products to normovolemic anemic patients, refrigerated blood components do not require warming prior to transfusion; in fact, warming may accelerate the deterioration of stored RBCs and may permit rapid growth of contaminating microorganisms.9 However, in patients that are hypo-

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thermic or receiving large volumes of blood, refrigerated RBC products should be prewarmed to temperatures between 22 °C and 37 °C immediately before transfusion to prevent exacerbation or development of hypothermia, the consequences of which may include cardiac arrhythmias and coagulopathies.9 Several types of blood warmers are available: thermostatically controlled waterbaths, dry heat devices with electric warming plates, and high-volume countercurrent heat exchange with water jackets.2 Such devices are not typically available in veterinary practices. Alternatively, blood units can be warmed by keeping the unit at room temperature for 30 minutes or by running the tubing of the blood administration set through warm water (37 °C) during the transfusion. While it is unnecessary (and not recommended) to warm blood for RBC transfusions at conventional rates, in situations requiring blood warming it is necessary to carefully monitor the temperature of the warming system to prevent RBC vesiculation and fragmentation, as well as hemolysis. Blood Filters and Infusion Devices Red blood cell transfusions must be administered through a filter designed to remove clots and particles potentially harmful to the patient. Standard blood infusion sets have in-line filters with a pore size ranging from 170 μm to 260 μm that trap large cells, cellular debris, and coagulated proteins. According to human blood banking standards, a filter may be used to administer 2–4 units of blood to a patient or for a maximum time limit of 4 hours; the combination of a high protein concentration at the filter surface and room temperature conditions promote proliferation of any contaminating microorganisms, and accumulated material slows the rate of flow.2 Other blood filters with a pore size of 20–40 μm remove microaggregates composed of degenerating platelets, white blood cells (WBCs), and fibrin strands that form in blood after 5 days or more of refrigerated storage. Microaggregate filters are designed primarily for transfusions of RBCs. A pediatric microaggregate blood filter (18 μm pore size, priming space