Cellular Molecular Immunology and Cellular Molecular Immunology and SEVENTH EDITION Abul K Abbas, MBBS Distinguished Professor in Pathology Chair, Department of Pathology University of California San Francisco San Francisco, California Andrew H Lichtman, MD, PhD Professor of Pathology Harvard Medical School Brigham and Women’s Hospital Boston, Massachusetts Shiv Pillai, MBBS, PhD Professor of Medicine and Health Sciences and Technology Harvard Medical School Massachusetts General Hospital Boston, Massachusetts Illustrations by David L Baker, MA Alexandra Baker, MS, CMI DNA Illustrations, Inc 1600 John F Kennedy Blvd Ste 1800 Philadelphia, PA 19103-2899 CELLULAR AND MOLECULAR IMMUNOLOGY 978-1-4377-1528-6 International Edition 978-0-8089-2425-8 Copyright © 2012, 2007, 2005, 2003, 2000, 1997, 1994, 1991 by Saunders, an imprint of Elsevier Inc Cover image © Suzuki et al., 2009 Originally published in Journal of Experimental Medicine doi: 10.1084/jem.20090209 All rights reserved No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein) Notices Knowledge and best practice in this field are constantly changing As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility With respect to any drug or pharmaceutical products identified, readers are advised to check the most current information provided (i) on procedures featured or (ii) by the manufacturer of each product to be administered, to verify the recommended dose or formula, the method and duration of administration, and contraindications It is the responsibility of practitioners, relying on their own experience and knowledge of their patients, to make diagnoses, to determine dosages and the best treatment for each individual patient, and to take all appropriate safety precautions To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein Library of Congress Cataloging-in-Publication Data Abbas, Abul K Cellular and molecular immunology/Abul K Abbas, Andrew H Lichtman, Shiv Pillai; illustrations by David L Baker, Alexandra Baker — 7th ed p ; cm Includes bibliographical references and index ISBN 978-1-4377-1528-6 (pbk : alk paper) 1. Cellular immunity. 2. Molecular immunology. I. Lichtman, Andrew H. II. Pillai, Shiv. III. Title [DNLM: 1. Immunity, Cellular. 2. Antibody Formation—immunology. 3. Antigens— immunology. 4. Immune System Diseases—immunology. 5. Lymphocytes—immunology QW 568] QR185.5.A23 2012 616.07'97—dc22 2011003193 Publishing Director: William Schmitt Managing Editor: Rebecca Gruliow Editorial Assistant: Laura Stingelin Publishing Services Manager: Patricia Tannian Senior Project Manager: Sarah Wunderly Design Manager: Lou Forgione Printed in the United States of America Last digit is the print number: 9 8 7 6 5 4 3 2 Working together to grow libraries in developing countries www.elsevier.com | www.bookaid.org | www.sabre.org D EDICATION To Ann, Jonathan, Rehana Sheila, Eben, Ariella, Amos, Ezra Honorine, Sohini PREFACE T his seventh edition of Cellular and Molecular Immunology has been significantly rewritten and revised as part of our continuing effort to make the textbook current and, at the same time, preserve the easily understandable style that readers have enjoyed in past editions We have added new information while striving to emphasize important concepts without increasing the length of the book We have also changed many sections, when necessary, for increased clarity, accuracy, and completeness Among the major changes is a reorganization of the chapters in order to consolidate topics and present information in a more accessible manner The chapter reorganization includes: a new chapter that discusses immune responses in mucosal tissues and other specialized sites; a new chapter on leukocyte migration, which brings together concepts that were previously discussed in multiple chapters; another new chapter that consolidates the discussions of immune receptors and signaling, which were also previously in several chapters; incorporation of discussions of cytokines into the relevant chapters rather than one chapter cataloguing all cytokines; and moving the discussion of autoimmunity into the chapter on tolerance, so the establishment and failure of immunologic tolerance is discussed as one cohesive theme In addition, the entire book has been updated to include many recent advances in immunology Some of the topics that have been significantly revised are the inflammasome, the biology of TH17 cells, and the development and functions of follicular helper T cells It is remarkable and fascinating to us that new principles continue to emerge from analysis of the complex systems that underlie immune responses Perhaps one of the most satisfying developments for students of human disease is that basic principles of immunology are now laying the foundation for rational development of new immunologic therapies Throughout the book, we have tried to emphasize these new therapeutics and the fundamental principles on which they are based Another major change in the seventh edition is a new illustration program, in which every figure in the book has been revised The style of the new figures is based on the strengths of our popular illustrations in past editions, but incorporates many new features such as three dimensionality and new labeling conventions intended to enhance clarity and aesthetics A large number of new illustrations have been added We have also continued to improve the clarity of tables, and kept design features such as the use of bold italic text to highlight “take-home messages,” to make the book easy and pleasant to read The lists of selected readings continue to emphasize recent review articles that provide in-depth coverage of particular topics for the interested reader We have divided the lists into sections based on themes to help readers find the most useful articles for their needs A new table listing cytokines, their receptors, and their major cellular sources and functions has been added (Appendix II) Many individuals have made valuable contributions to this edition Drs Richard Blumberg, Lisa Coussens, Jason Cyster, Francis Luscinskas, and Scott Plevy reviewed various sections, and all were generous with advice and comments We thank Drs Thorsten Mempel, Uli von Andrian, and Jason Cyster for help with cover illustrations for this and previous editions Our illustrators, David and Alexandra Baker of DNA Illustrations, vii viii Preface remain full partners in the book and provide invaluable suggestions for clarity and accuracy Several members of the Elsevier staff played critical roles Our editor, Bill Schmitt, has been a source of support and encouragement Our managing editor, Rebecca Gruliow, shepherded the book through its preparation and into production Lou Forgione is responsible for the design, and Sarah Wunderly took charge of the production stage Finally, our students were the original inspiration for the first edition of this book, and we remain continually grateful to them, because from them we learn how to think about the science of immunology, and how to communicate knowledge in the clearest and most meaningful way ABUL K ABBAS ANDREW H LICHTMAN SHIV PILLAI CHAPTER Properties and Overview of Immune Responses INNATE AND ADAPTIVE IMMUNITY, TYPES OF ADAPTIVE IMMUNE RESPONSES, CARDINAL FEATURES OF ADAPTIVE IMMUNE RESPONSES, CELLULAR COMPONENTS OF THE ADAPTIVE IMMUNE SYSTEM, CYTOKINES, SOLUBLE MEDIATORS OF THE IMMUNE SYSTEM, OVERVIEW OF IMMUNE RESPONSES TO MICROBES, 10 The Early Innate Immune Response to Microbes, 10 The Adaptive Immune Response, 10 SUMMARY, 13 The term immunity is derived from the Latin word immunitas, which referred to the protection from legal prosecution offered to Roman senators during their tenures in office Historically, immunity meant protection from disease and, more specifically, infectious disease The cells and molecules responsible for immunity constitute the immune system, and their collective and coordinated response to the introduction of foreign substances is called the immune response The physiologic function of the immune system is defense against infectious microbes However, even noninfectious foreign substances can elicit immune responses Furthermore, mechanisms that normally protect individuals from infection and eliminate foreign substances are also capable of causing tissue injury and disease in some situations Therefore, a more inclusive definition of the immune response is a reaction to components of microbes as well as to macromolecules, such as proteins and polysaccharides, and small chemicals that are recognized as foreign, regardless of the physiologic or pathologic consequence of such a reaction Under some situations, even self molecules can elicit immune responses (so-called autoimmune responses) Immunology is the study of immune responses in this broader sense and of the cellular and molecular events that occur after an organism encounters microbes and other foreign macromolecules Historians often credit Thucydides, in the fifth century BC in Athens, as having first mentioned immunity to an infection that he called plague (but that was probably not the bubonic plague we recognize today) The concept of protective immunity may have existed long before, as suggested by the ancient Chinese custom of making children resistant to smallpox by having them inhale powders made from the skin lesions of patients recovering from the disease Immunology, in its modern form, is an experimental science, in which explanations of immunologic phenomena are based on experimental observations and the conclusions drawn from them The evolution of immunology as an experimental discipline has depended on our ability to manipulate the function of the immune system under controlled conditions Historically, the first clear example of this manipulation, and one that remains among the most dramatic ever recorded, was Edward Jenner’s successful vaccination against smallpox Jenner, an English physician, noticed that milkmaids who had recovered from cowpox never contracted the more serious smallpox On the basis of this observation, he injected the material from a cowpox pustule into the arm of an 8-year-old boy When this boy was later intentionally inoculated with smallpox, the disease did not develop Jenner’s landmark treatise on vaccination (Latin vaccinus, of or from cows) was published in 1798 It led to the widespread acceptance of this method for inducing immunity to infectious diseases, and vaccination remains the most effective method for preventing infections (Table 1-1) An eloquent testament to the importance of immunology was the announcement by the World Health Organization in 1980 that smallpox was the first disease that had been eradicated worldwide by a program of vaccination Anatomy and Functions of Lymphoid Tissues Stem cells Multipotent progenitors Committed precursors Common lymphoid progenitor Late precursors and mature forms Erythropoietin Proerythroblast Lymphopoiesis Erythrocyte Thrombopoietin SCF, IL-6, Flt3L Platelet Megakaryoblast Myelopoiesis Thrombopoietin, IL-11 IL-3, GM-CSF, IL-6 Common myeloid progenitor Basophil Immature basophil IL-5 IL-5 Immature eosinophil Eosinophil Monoblast Monocyte GM-CSF M-CSF Flt3L Flt3L G-CSF Pre-dendritic cell Myeloblast Selfrenewal c-KIT+ CD34+ LIN– Dendritic cell Neutrophil Cell division Lineagespecific markers FIGURE 2–9 Hematopoiesis The development of the different lineages of blood cells is depicted in this “hematopoietic tree.” Also shown are the principal cytokines that drive the maturation of different lineages The development of lymphocytes forming the common lymphoid precursor is described later in this chapter and in Figure 8-2, Chapter SCF, stem cell factor; Flt3L, Flt3 ligand; G-CSF, granulocyte colony-stimulating factor; GM-CSF, granulocyte-macrophage colony-stimulating factor; LIN−, negative for lineage-specific markers; M-CSF, macrophage colony-stimulating factor division of the HSCs HSCs give rise to two kinds of multipotent cells, the common lymphoid and common myeloid progenitors The common lymphoid progenitor is the source of committed single-lineage precursors of T cells, B cells, or NK cells Most of the steps in B cell maturation take place in the bone marrow, but the final events may occur after the cells leave the marrow and enter secondary lymphoid organs, particularly the spleen T cell maturation occurs entirely in the thymus and therefore requires that common lymphoid progenitors or some poorly characterized progeny of these cells migrate out of the marrow into the blood and then into the thymus NK cell maturation is thought to take place entirely in the bone marrow The common myeloid progenitors give rise to committed single-lineage progenitors of the erythroid, megakaryocytic, granulocytic, and monocytic lineages, which give rise, respectively, to mature red cells, platelets, granulocytes (neutrophils, eosinophils, basophils), and monocytes Most dendritic cells arise from the monocytic lineage The proliferation and maturation of precursor cells in the bone marrow are stimulated by cytokines (see 27 28 Chapter – Cells and Tissues of the Immune System TABLE 2–4 Hematopoietic Cytokines Cytokine Size Principal Cellular Sources Principal Cellular Targets Principal Cell Populations Induced Stem cell factor (c-Kit ligand) 24 kD Bone marrow stromal cells Hematopoietic stem cells All Interleukin-7 (IL-7) 25 kD Fibroblasts, bone marrow stromal cells Immature lymphoid progenitors B and T lymphocytes Interleukin-3 (IL-3) 20-26 kD T cells Immature progenitors All Granulocyte-monocyte colony-stimulating factor (GM-CSF) 18-22 kD T cells, macrophages, endothelial cells, fibroblasts Immature and committed myeloid progenitors, mature macrophages Granulocytes and monocytes, macrophage activation Monocyte colony-stimulating factor (M-CSF) Dimer of 70-90 kD; 40-kD subunits Macrophages, endothelial cells, bone marrow cells, fibroblasts Committed progenitors Monocytes Granulocyte colonystimulating factor (G-CSF) 19 kD Macrophages, fibroblasts, endothelial cells Committed granulocyte progenitors Granulocytes Fig 2-9) Many of these cytokines are called colonystimulating factors because they were originally assayed by their ability to stimulate the growth and development of various leukocytic or erythroid colonies from marrow cells Hematopoietic cytokines are produced by stromal cells and macrophages in the bone marrow, thus providing the local environment for hematopoiesis They are also produced by antigen-stimulated T lymphocytes and cytokine-activated or microbe-activated macrophages, providing a mechanism for replenishing leukocytes that may be consumed during immune and inflammatory reactions The names and properties of the major hematopoietic cytokines are listed in Table 2-4 In addition to self-renewing stem cells and their differentiating progeny, the marrow contains numerous antibody-secreting plasma cells These plasma cells are generated in peripheral lymphoid tissues as a consequence of antigenic stimulation of B cells and then migrate to the marrow, where they may live and continue to produce antibodies for many years Some longlived memory T lymphocytes also migrate to and may reside in the bone marrow Thymus The thymus is the site of T cell maturation The thymus is a bilobed organ situated in the anterior mediastinum Each lobe is divided into multiple lobules by fibrous septa, and each lobule consists of an outer cortex and an inner medulla (Fig 2-10) The cortex contains a dense collection of T lymphocytes, and the lighter-staining medulla is more sparsely populated with lymphocytes Bone marrow–derived macrophages and dendritic cells are found almost exclusively in the medulla Scattered throughout the thymus are nonlymphoid epithelial cells, which have abundant cytoplasm Thymic cortical epithelial cells provide IL-7 required early in T cell development A subset of these epithelial cells found only in the medulla, called thymic medullary epithelial cells (often abbreviated as TMEC), play a special role in presenting self antigens to developing T cells and causing their deletion This is one mechanism of ensuring that the immune system remains tolerant to self and is discussed in detail in Chapter 14 In the medulla are structures called Hassall’s corpuscles, which are composed of tightly packed whorls of epithelial cells that may be remnants of degenerating cells The thymus has a rich vascular supply and efferent lymphatic vessels that drain into mediastinal lymph nodes The epithelial component of the thymus is derived from invaginations of the ectoderm in the developing neck and chest of the embryo, forming structures called branchial pouches Dendritic cells, macrophages, and lymphocyte precursors are derived from the bone marrow Humans with DiGeorge syndrome suffer from T cell deficiency because of mutations in genes required for thymus development In the “nude” mouse strain, which has been widely used in immunology research, a mutation in the gene encoding a transcription factor causes a failure of differentiation of certain types of epithelial cells that are required for normal development of the thymus and hair follicles Consequently, these mice lack T cells and hair The lymphocytes in the thymus, also called thymocytes, are T lymphocytes at various stages of maturation Cells that are committed to the T cell lineage are believed to develop in the bone marrow from common lymphoid progenitor cells, enter the circulation, and home to the thymic cortex through the blood vessels Further maturation in the thymus begins in the cortex, and as thymocytes mature, they migrate toward the medulla, so that the medulla contains mostly mature T cells Only mature T cells exit the thymus and enter the blood and peripheral lymphoid tissues The details of thymocyte maturation are described in Chapter Anatomy and Functions of Lymphoid Tissues A B Medulla Cortex C Blood vessels Thymocytes Hassall's corpuscle Medulla Cortex FIGURE 2–10 Morphology of the thymus A, Low-power light micrograph of a lobe of the thymus showing the cortex and medulla The darker blue-stained outer cortex and paler blue inner medulla are apparent B, High-power light micrograph of the thymic medulla The numerous small blue-staining cells are developing T cells called thymocytes, and the larger pink structure is Hassall’s corpuscle, uniquely characteristic of the thymic medulla but whose function is poorly understood C, Schematic diagram of the thymus illustrating a portion of a lobe divided into multiple lobules by fibrous trabeculae The Lymphatic System The lymphatic system, which consists of specialized vessels that drain fluid from tissues into and out of lymph nodes and then into the blood, is essential for tissue fluid homeostasis and immune responses (Fig 2-11) Interstitial fluid is constitutively formed in all vascularized tissues by movement of a filtrate of plasma out of capillaries, and the rate of local formation can increase dramatically when tissue is injured or infected The skin, epithelia, and parenchymal organs contain numerous lymphatic capillaries that absorb this fluid from spaces between tissue cells The lymphatic capillaries are blind-ended vascular channels lined by overlapping endothelial cells without the tight intercellular junctions or basement membrane that are typical of blood vessels These distal lymphatic capillaries permit free uptake of interstitial fluid, and the overlapping arrangement of the endothelial cells and one-way valves within their lumens prevents backflow of the fluid The absorbed fluid, called lymph once it is within the lymphatic vasculature, is pumped into convergent, ever larger lymphatic vessels by the contraction of perilymphatic smooth muscle cells and the pressure exerted by movement of the musculoskeletal tissues 29 30 Chapter – Cells and Tissues of the Immune System Cervical nodes Thoracic duct Intercostal vessels Axillary nodes Draining lymph node Cisterna chyli Paraaortic nodes Vessels from intestines Infection site Inguinal nodes FIGURE 2–11 The lymphatic system The major lymphatic vessels, which drain into the inferior vena cava (and superior vena cava, not shown), and collections of lymph nodes are illustrated Antigens are captured from a site of infection and the draining lymph node to which these antigens are transported and where the immune response is initiated These vessels merge into afferent lymphatics that drain into lymph nodes, and the lymph drains out of the nodes through efferent lymphatics Because lymph nodes are connected in series by lymphatics, an efferent lymphatic exiting one node may serve as the afferent vessel for another The efferent lymph vessel at the end of a lymph node chain joins other lymph vessels, eventually culminating in a large lymphatic vessel called the thoracic duct Lymph from the thoracic duct is emptied into the superior vena cava, thus returning the fluid to the blood stream Lymphatics from the right upper trunk, right arm, and right side of the head drain into the right lymphatic duct, which also drains into the superior vena cava About liters of lymph are normally returned to the circulation each day, and disruption of the lymphatic system may lead to rapid tissue swelling The lymphatic system collects microbial antigens from their portals of entry and delivers them to lymph nodes, where they can stimulate adaptive immune responses Microbes enter the body most often through the skin and the gastrointestinal and respiratory tracts All these tissues are lined by epithelia that contain dendritic cells, and all are drained by lymphatic vessels The dendritic cells capture some microbial antigens and enter lymphatic vessels Other microbes and soluble antigens enter the lymphatics independently of dendritic cells In addition, soluble inflammatory mediators, such as chemokines, produced at sites of infection enter the lymphatics The lymph nodes are interposed along lymphatic vessels and act as filters that sample the soluble and dendritic cell–associated antigens in the lymph before it reaches the blood and permit the antigens to be seen by the adaptive immune system Lymph Nodes Lymph nodes are encapsulated, vascularized secondary lymphoid organs with anatomic features that favor the initiation of adaptive immune responses to antigens carried from tissues by lymphatics (Fig 2-12) Lymph nodes are situated along lymphatic channels throughout the body and therefore have access to antigens encountered at epithelia and originating in interstitial fluid in most tissues A lymph node is surrounded by a fibrous capsule, beneath which is a sinus system lined by reticular cells, cross-bridged by fibrils of collagen and other extracellular matrix proteins and filled with lymph, macrophages, dendritic cells, and other cell types Afferent lymphatics empty into the subcapsular (marginal) sinus, and lymph may drain from there directly into the connected medullary sinus and then out of the lymph node through the efferent lymphatics Beneath the inner floor of the subcapsular sinus is the lymphocyte-rich cortex The outer cortex contains aggregates of cells called follicles Some follicles contain central areas called germinal centers, which stain lightly with commonly used histologic stains Follicles without germinal centers are called primary follicles, and those with germinal centers are secondary follicles The cortex around the follicles is called the parafollicular cortex or paracortex and is organized into cords, which are regions with a complex microanatomy of matrix proteins, fibers, lymphocytes, dendritic cells, and mononuclear phagocytes Anatomic Organization of B and T Lymphocytes B and T lymphocytes are sequestered in distinct regions of the cortex of lymph nodes, each region with its own unique architecture of reticular fibers and stromal cells (Figs 2-13 and 2-14) Follicles are the B cell zones They are located in the lymph node cortex and are organized around FDCs, which have processes that interdigitate to form a dense reticular network Primary follicles contain mostly mature, naive B lymphocytes Germinal centers develop in response to antigenic stimulation They are sites of remarkable B cell proliferation, selection of B cells producing high-affinity antibodies, and generation of memory B cells and long-lived plasma cells The T lymphocytes are located mainly beneath and more central to the follicles, in the paracortical cords These T cell–rich zones contain a network of fibroblastic reticular cells (FRCs), which are arranged to form the outer layer of tube-like structures called FRC conduits The conduits range in diameter from 0.2 to 3 µm and contain organized arrays of extracellular matrix molecules, including innermost parallel bundles of collagen fibers embedded in a meshwork of fibrillin microfibers, all tightly Anatomy and Functions of Lymphoid Tissues A A Antigen B cell zone (follicle) High endothelial venule (HEV) Afferent lymphatic vessel Dendritic cell High endothelial venule Naive B cell B cell specific chemokine Subcapsular sinus Afferent lymphatic vessel B cell zone T cell zone T cell zone Germinal center B Artery Medulla Medullary Efferent lymphatic Vein sinus vessel Capsule Trabecula Lymphocytes Naive T cell T cell and dendritic cell specific chemokine Artery B cell T cell B Primary lymphoid follicle (B cell zone) T cell zone (parafollicular cortex) B cell zone (lymphoid follicle) FIGURE 2–13 Segregation of B cells and T cells in a lymph node A, The schematic diagram illustrates the path by which Parafollicular cortex (T cell zone) Secondary follicle with germinal center FIGURE 2–12 Morphology of a lymph node A, Schematic diagram of a lymph node illustrating the T cell–rich and B cell–rich zones and the routes of entry of lymphocytes and antigen (shown captured by a dendritic cell) B, Light micrograph of a lymph node illustrating the T cell and B cell zones (Courtesy of Dr James Gulizia, Department of Pathology, Brigham and Women’s Hospital, Boston, Massachusetts.) surrounded by a basement membrane produced by a continuous sleeve of FRCs These conduits begin at the subcapsular sinus and extend to both medullary sinus lymphatic vessels and cortical blood vessels, called high endothelial venules (HEVs) Naive T cells enter the T cell zones through the HEVs, as described in detail in Chapter T cells are densely packed around the conduits in the lymph node cortex Most (∼70%) of the cortical T cells are CD4+ helper T cells, intermingled with relatively sparse CD8+ cells These proportions can change dramatically during the course of an infection For example, during a viral infection, there may be a marked increase naive T and B lymphocytes migrate to different areas of a lymph node The lymphocytes enter through an artery and reach a high endothelial venule, shown in cross section, from where naive lymphocytes are drawn to different areas of the node by chemokines that are produced in these areas and bind selectively to either cell type Also shown is the migration of dendritic cells, which pick up antigens from the sites of antigen entry, enter through afferent lymphatic vessels, and migrate to the T cell–rich areas of the node B, In this section of a lymph node, the B lymphocytes, located in the follicles, are stained green; the T cells, in the parafollicular cortex, are red The method used to stain these cells is called immunofluorescence (see Appendix IV for details) (Courtesy of Drs Kathryn Pape and Jennifer Walter, University of Minnesota School of Medicine, Minneapolis.) The anatomic segregation of T and B cells is also seen in the spleen (see Fig 2-15) in CD8+ T cells Dendritic cells are also concentrated in the paracortex of the lymph nodes, many of which are closely associated with the FRC conduits The anatomic segregation of B and T lymphocytes in distinct areas of the node is dependent on cytokines that are secreted by lymph node stromal cells in each area and that direct the migration of the lymphocytes (see Fig 2-13) Naive T and B lymphocytes are delivered to a node through an artery and leave the circulation and enter the 31 32 Chapter – Cells and Tissues of the Immune System A Sinus lining cell Subcapsular sinus HEV Fibroblastic reticular cell (FRC) conduit Reticular fiber Fibroblastic reticular cell Subcapsular macrophage or dendritic cell Dendritic cell Trabecular sinus Capsule Perivenular channel B C FIGURE 2–14 Microanatomy of the lymph node cortex A, Schematic of the microanatomy of a lymph node depicting the route of lymph drainage from the subcapsular sinus, through fibroreticular cell conduits, to the perivenular channel around the high endothelial venule (HEV) B, Transmission electron micrograph of an FRC conduit surrounded by fibroblast reticular cells (arrowheads) and adjacent lymphocytes (L) (From Gretz JE, CC Norbury, AO Anderson, AEI Proudfoot, and S Shaw Lymph-borne chemokines and other low molecular weight molecules reach high endothelial venules via specialized conduits while a functional barrier limits access to the lymphocyte microenvironments in lymph node cortex The Journal of Experimental Medicine 192:1425-1439, 2000.) C, Immunofluorescent stain of an FRC conduit formed of the basement membrane protein laminin (red) and collagen fibrils (green) (From Sixt M, K Nobuo, M Selg, T Samson, G Roos, DP Reinhardt, R Pabst, M Lutz, and L Sorokin The conduit system transports soluble antigens from the afferent lymph to resident dendritic cells in the T cell area of the lymph node Immunity © 22:19-29, 2006 Copyright 2005 by Elsevier Inc.) stroma of the node through the HEVs, which are located in the center of the cortical cords The type of cytokines that determine where B and T cells reside in the node are called chemokines (chemoattractant cytokines), which bind to chemokine receptors on the lymphocytes Chemokines include a large family of 8- to 10-kD cytokines that are involved in a wide variety of cell motility functions in development, maintenance of tissue architecture, and immune and inflammatory responses We will discuss the general properties of chemokines and their receptors in Chapter Naive T cells express a receptor called CCR7 that binds the chemokines CCL19 and CCL21 produced by stromal cells in the T cell zones of the lymph node These chemokines attract naive T cells to move from the blood, through the HEVs, into the T cell zone Dendritic cells that drain into the node through lymphatics also express CCR7, and this is why they migrate from the subcapsular sinus to the same area of the node as naive T cells (see Chapter 6) Naive B cells express another chemokine receptor, CXCR5, that recognizes a chemokine, CXCL13, produced only in follicles by FDCs Thus, B cells are attracted into the follicles, which are the B cell zones of lymph nodes Another cytokine (which is not a chemokine) called lymphotoxin plays a role in stimulating CXCL13 production, especially in the follicles The functions of chemokines and other cytokines in regulating where lymphocytes are located in lymphoid organs and in the formation of these organs have been established by numerous studies in mice For example, CXCR5 knockout mice lack B cell–containing follicles in lymph nodes and spleen Similarly, CCR7 knockout mice lack T cell zones The development of lymph nodes as well as of other peripheral lymphoid organs requires the coordinated actions of several cytokines, chemokines, transcription factors, and lymphoid tissue inducer cells During fetal life, lymphoid tissue inducer cells, which are cells of hematopoietic origin with phenotypic features of both lymphocytes and NK cells, stimulate the development of lymph nodes and other secondary lymphoid organs This function is mediated by various proteins expressed by the inducer cells, the most thoroughly studied being the cytokines lymphotoxin-α (LTα) and lymphotoxin-β (LTβ) Knockout mice lacking either of these cytokines not develop lymph nodes or secondary lymphoid organs in the gut Splenic white pulp development is also disorganized in these mice The LTβ produced by the inducer cells acts on stromal cells in different locations of a developing secondary lymphoid organ, and these stromal cells are activated to produce the chemokines CXCL13 or CCL19 and CCL21 In areas where CXCL13 is induced, circulating B cells are recruited into nascent B cell follicles; and in the areas where CCL19 and CCL21 are induced, T cells and dendritic cells are recruited to form T cell zones There are several other proteins expressed by lymphoid tissue inducer cells that are required for their function, including transcription factors, but their roles in lymphoid organogenesis are not well defined The anatomic segregation of T and B cells ensures that each lymphocyte population is in close contact with the appropriate APCs, that is, T cells with dendritic cells and B cells with FDCs Furthermore, because of this precise segregation, B and T lymphocyte populations are kept apart until it is time for them to interact in a functional way As we will see in Chapter 11, after stimulation by antigens, T and B cells lose their anatomic constraints and begin to migrate toward one another Activated T cells may either migrate toward follicles to help B cells or exit the node and enter the circulation, whereas activated B cells migrate into germinal centers and, after differentiation into plasma cells, may home to the bone marrow Anatomy and Functions of Lymphoid Tissues Antigen Transport Through Lymph Nodes Lymph-borne substances that enter the subcapsular sinus of the lymph node are sorted by molecular size and delivered to different cell types to initiate different types of immune responses The floor of the subcapsular sinus is constructed in a way that permits cells in the sinus to contact or migrate into the underlying cortex but does not allow movement of soluble molecules in the lymph to freely pass into the cortex Viruses and other highmolecular-weight antigens are taken up by sinus macrophages and presented to cortical B lymphocytes just beneath the cortical sinus This is the first step in antibody responses to these antigens Low-molecular-weight soluble antigens are transported out of the sinus through the FRC conduits and passed to resident cortical dendritic cells located adjacent to the conduits The resident dendritic cells extend processes between the cells lining the conduits and into the lumen and capture and pinocytose the soluble antigens inside the conduits The contribution of this pathway of antigen delivery may be important for initial T cell immune responses to some microbial antigens, but larger and sustained responses require delivery of antigens to the node by tissue dendritic cells, as discussed in Chapter In addition to antigens, there is evidence that soluble inflammatory mediators, such as chemokines and other cytokines, are transported in the lymph that flows through the conduits; some of these may act on the penetrating dendritic cells, and others may be delivered to HEVs into which the conduits drain This is a possible way in which tissue inflammation can be sensed in the lymph node and thereby influence recruitment and activation of lymphocytes in the node A Marginal sinus Red pulp Follicular arteriole T cell zone (periarteriolar lymphoid sheath PALS) Trabecular Central artery arteriole Marginal zone B Periarteriolar lymphoid sheath (PALS) Germinal center of lymphoid follicle C B cell zone (lymphoid follicle) Spleen The spleen is a highly vascularized organ whose major functions are to remove aging and damaged blood cells and particles (such as immune complexes and opsonized microbes) from the circulation and to initiate adaptive immune responses to blood-borne antigens The spleen weighs about 150 g in adults and is located in the left upper quadrant of the abdomen The splenic parenchyma is anatomically and functionally divided into the red pulp, composed mainly of blood-filled vascular sinusoids, and the lymphocyte-rich white pulp Blood enters the spleen through a single splenic artery, which pierces the capsule at the hilum and divides into progressively smaller branches that remain surrounded by protective and supporting fibrous trabeculae (Fig 2-15) Some of the arteriolar branches of the splenic artery end in extensive vascular sinusoids, which form the red pulp, lined by macrophages and filled with large numbers of erythrocytes The sinusoids end in venules that drain into the splenic vein, which carries blood out of the spleen and into the portal circulation The red pulp macrophages serve as an important filter for the blood, removing microbes, damaged cells, and antibody-coated (opsonized) cells and microbes Individuals lacking a spleen are highly susceptible to infections with encapsulated bacteria such as pneumococci and meningococci This may be because such organisms are normally cleared by B cell zone (follicle) T cell zone (periarteriolar lymphoid sheath) FIGURE 2–15 Morphology of the spleen A, Schematic diagram of the spleen illustrating T cell and B cell zones, which make up the white pulp B, Photomicrograph of a section of human spleen showing a trabecular artery with adjacent periarteriolar lymphoid sheath and a lymphoid follicle with a germinal center Surrounding these areas is the red pulp, rich in vascular sinusoids C, Immunohistochemical demonstration of T cell and B cell zones in the spleen, shown in a cross section of the region around an arteriole T cells in the periarteriolar lymphoid sheath are stained red, and B cells in the follicle are stained green (Courtesy of Drs Kathryn Pape and Jennifer Walter, University of Minnesota School of Medicine, Minneapolis.) opsonization and phagocytosis, and this function is defective in the absence of the spleen The function of the white pulp is to promote adaptive immune responses to blood-borne antigens The white pulp consists of many collections of densely packed lymphocytes, which appear as white nodules against the 33 34 Chapter – Cells and Tissues of the Immune System background of the red pulp The white pulp is organized around central arteries, which are branches of the splenic artery distinct from the branches that form the vascular sinusoids Several smaller branches of each central artery pass through the lymphocyte-rich area and drain into a marginal sinus A region of specialized cells surrounding the marginal sinus, called the marginal zone, forms the boundary between the red and white pulp The architecture of the white pulp is analogous to the organization of lymph nodes, with segregated T cell and B cell zones In the mouse spleen, the central arteries are surrounded by cuffs of lymphocytes, most of which are T cells Because of their anatomic location, morphologists call these T cell zones periarteriolar lymphoid sheaths B cell–rich follicles occupy the space between the marginal sinus and the periarteriolar sheath As in lymph nodes, the T cell areas in the spleen contain a network of complex conduits composed of matrix proteins lined by FRC-like cells, although there are ultrastructural differences between the conduits in nodes and spleen The marginal zone just outside the marginal sinus is a distinct region populated by B cells and specialized macrophages The B cells in the marginal zone, known as marginal zone B cells, are functionally distinct from follicular B cells and have a limited repertoire of antigen specificities The architecture of the white pulp is more complex in humans than in mice, with both inner and outer marginal zones and a perifollicular zone Antigens in the blood are delivered into the marginal sinus by circulating dendritic cells or are sampled by the macrophages in the marginal zone The anatomic arrangements of the APCs, B cells, and T cells in the splenic white pulp promote the interactions required for the efficient development of humoral immune responses, as will be discussed in Chapter 11 The segregation of T lymphocytes in the periarteriolar lymphoid sheaths and B cells in follicles and marginal zones is a highly regulated process, dependent on the production of different cytokines and chemokines by the stromal cells in these different areas, analogous to the case for lymph nodes The chemokine CXCL13 and its receptor CXCR5 are required for B cell migration into the follicles, and CCL19 and CCL21 and their receptor CCR7 are required for naive T cell migration into the periarteriolar sheath The production of these chemokines by nonlymphoid stromal cells is stimulated by the cytokine lymphotoxin contain a major proportion of the cells of the innate and adaptive immune systems We will discuss the special features of these regional immune systems in Chapter 13 SUMMARY Y The anatomic organization of the cells and tissues Y Y Y Y Y Regional Immune Systems Each major epithelial barrier of the body, including the skin, gastrointestinal mucosa, and bronchial mucosa, has its own system of lymph nodes, nonencapsulated lymphoid structures, and diffusely distributed immune cells, which work in coordinated ways to provide specialized immune responses against the pathogens that enter at those barriers The skin-associated immune system has evolved to respond to a wide variety of environmental microbes The components of the immune systems associated with the gastrointestinal and bronchial mucosa are called the mucosa-associated lymphoid tissue (MALT) and are involved in immune responses to ingested and inhaled antigens and microbes The skin and MALT Y Y of the immune system is of critical importance for the generation of effective innate and adaptive immune responses This organization permits the rapid delivery of innate effector cells, including neutrophils and monocytes, to sites of infection and permits a small number of lymphocytes specific for any one antigen to locate and respond effectively to that antigen regardless of where in the body the antigen is introduced The cells that perform the majority of effector functions of innate and adaptive immunity are phagocytes (including neutrophils and macrophages), APCs (including macrophages and dendritic cells), and lymphocytes Neutrophils, the most abundant blood leukocyte with a distinctive multilobed segmented nucleus and abundant cytoplasmic lysosomal granules, are rapidly recruited to sites of infection and tissue injury, where they perform phagocytic functions Monocytes are the circulating precursors of tissue macrophages All tissues contain resident macrophages, which are phagocytic cells that ingest and kill microbes and dead host cells and secrete cytokines and chemokines that promote the recruitment of leukocytes from the blood APCs function to display antigens for recognition by lymphocytes and to promote the activation of lymphocytes APCs include dendritic cells, mononuclear phagocytes, and FDCs B and T lymphocytes express highly diverse and specific antigen receptors and are the cells responsible for the specificity and memory of adaptive immune responses NK cells are a distinct class of lymphocytes that not express highly diverse antigen receptors and whose functions are largely in innate immunity Many surface molecules are differentially expressed on different subsets of lymphocytes as well as on other leukocytes, and these are named according to the CD nomenclature Both B and T lymphocytes arise from a common precursor in the bone marrow B cell development proceeds in the bone marrow, whereas T cell precursors migrate to and mature in the thymus After maturing, B and T cells leave the bone marrow and thymus, enter the circulation, and populate peripheral lymphoid organs Naive B and T cells are mature lymphocytes that have not been stimulated by antigen When they encounter antigen, they differentiate into effector lymphocytes that have functions in protective immune responses Effector B lymphocytes are SUMMARY Y Y Y Y Y antibody-secreting plasma cells Effector T cells include cytokine-secreting CD4+ helper T cells and CD8+ CTLs Some of the progeny of antigen-activated B and T lymphocytes differentiate into memory cells that survive for long periods in a quiescent state These memory cells are responsible for the rapid and enhanced responses to subsequent exposures to antigen The organs of the immune system may be divided into the generative organs (bone marrow and thymus), where lymphocytes mature, and the peripheral organs (lymph nodes and spleen), where naive lymphocytes are activated by antigens Bone marrow contains the stem cells for all blood cells, including lymphocytes, and is the site of maturation of all of these cell types except T cells, which mature in the thymus Extracellular fluid (lymph) is constantly drained from tissues through lymphatics into lymph nodes and eventually into the blood Microbial antigens are carried in soluble form and within dendritic cells in the lymph to lymph nodes, where they are recognized by lymphocytes Lymph nodes are encapsulated secondary lymphoid organs located throughout the body along lymphatics, where naive B and T cells respond to antigens that are collected by the lymph from peripheral tissues The spleen is an encapsulated organ in the abdominal cavity where senescent or opsonized blood cells are removed from the circulation, and in which lymphocytes respond to blood-borne antigens Both lymph nodes and the white pulp of the spleen are organized into B cell zones (the follicles) and T cell zones The T cell areas are also the sites of residence of mature dendritic cells, which are APCs specialized for the activation of naive T cells FDCs reside in the B cell areas and serve to activate B cells during humoral immune responses to protein antigens The development of secondary lymphoid tissues depends on cytokines and lymph node inducer cells SUGGESTED READINGS Cells of the Immune System Geissmann F, MG Manz, S Jung, MH Sieweke, M Merad, and K Ley Development of monocytes, macrophages, and dendritic cells Science 327:656-661, 2010 Schluns KS, and L Lefrancois Cytokine control of memory T-cell development and survival Nature Reviews Immunology 3:269-279, 2003 Surh CD, and J Sprent Homeostasis of naive and memory T cells Immunity 29:848-862, 2008 Tissues of the Immune System Lane P, M-Y Kim, D Withers, F Gaspal, V Bekiaris, G Desanti, M Khan, F McConnell, and G Anderson Lymphoid tissue inducer cells in adaptive CD4 T cell dependent responses Seminars in Immunology 20:159-163, 2008 Mebius RE, and G Kraal Structure and function of the spleen Nature Reviews Immunology 5:606-616, 2005 Mueller SN, and RN Germain Stromal cell contributions to the homeostasis and functionality of the immune system Nature Reviews Immunology 9:618-629, 2009 Ruddle NH, and EM Akirav Secondary lymphoid organs: responding to genetic and environmental cues in ontogeny and the immune response Journal of Immunology 183:22052212, 2009 Von Andrian UH, and TR Mempel Homing and cellular traffic in lymph nodes Nature Reviews Immunology 3:867-878, 2003 35 CHAPTER 3 Leukocyte Migration into Tissues l Delivery of lymphocytes from their sites of maturation ADHESION MOLECULES ON LEUKOCYTES AND ENDOTHELIAL CELLS INVOLVED IN LEUKOCYTE RECRUITMENT, 39 Selectins and Selectin Ligands, 39 Integrins and Integrin Ligands, 40 CHEMOKINES AND CHEMOKINE RECEPTORS, 41 Chemokine Structure, Production, and Receptors, 41 (bone marrow or thymus) to secondary lymphoid organs, where they encounter antigens and differentiate into effector lymphocytes l Delivery of effector lymphocytes from the secondary lymphoid organs in which they were produced to sites of infection in any tissue, where they perform their protective functions MIGRATION OF NEUTROPHILS AND MONOCYTES TO SITES OF INFECTION OR TISSUE INJURY, 45 The migration of a particular type of leukocyte into a restricted type of tissue, or a tissue with an ongoing infection or injury, is often called leukocyte homing, and the general process of leukocyte movement from blood into tissues is called recruitment The migration of leukocytes to tissues follows several general principles MIGRATION AND RECIRCULATION OF T LYMPHOCYTES, 45 l Leukocytes that have not been activated by external Biologic Actions of Chemokines, 43 LEUKOCYTE-ENDOTHELIAL INTERACTIONS AND LEUKOCYTE EXTRAVASATION, 43 Recirculation of Naive T Lymphocytes between Blood and Secondary Lymphoid Organs, 46 Recirculation of T Cells through Other Lymphoid Tissues, 49 Migration of Effector T Lymphocytes to Sites of Infection, 50 Memory T Cell Migration, 51 MIGRATION OF B LYMPHOCYTES, 51 SUMMARY, 52 A unique property of the immune system that distinguishes it from all other tissue systems in the body is the constant and highly regulated movement of its major cellular components through the blood, into tissues, and often back into the blood again This movement accomplishes three main functions (Fig 3-1): l Delivery of leukocytes of myeloid lineage (mainly neu- trophils and monocytes) from their bone marrow site of maturation into tissue sites of infection or injury, where the cells perform their protective functions of eliminating infectious pathogens, clearing dead tissues, and repairing the damage stimuli (i.e., are considered to be in a resting state) are normally located in the circulation and lymphoid organs Only after activation are these cells rapidly recruited to where they are needed The activating stimuli typically are products of microbes and dead cells (during innate immune responses) and antigens (during adaptive immune responses) l Endothelial cells at sites of infection and tissue injury are also activated, mostly in response to cytokines secreted by macrophages and other tissue cells at these sites Endothelial activation results in increased adhesiveness of endothelial cells for circulating leukocytes; the molecular basis of this adhesiveness is described later l The recruitment of leukocytes and plasma proteins from the blood to sites of infection and tissue injury is called inflammation Inflammation is triggered by recognition of microbes and dead tissues in innate immune responses and is refined and prolonged during adaptive immune responses This process delivers the cells and molecules of host defense to the sites where offending agents need to be combated The same process is responsible for causing tissue damage and underlies many important diseases We will return to inflammation in the context of innate immunity in Chapter and in the discussion of inflammatory diseases in Chapter 18 37 38 Chapter – Leukocyte Migration into Tissues A Postcapillary venule Neutrophils and monocytes migrate to sites of infection and tissue injury: inflammation Infected or injured tissue B Lymph node High endothelial venule (HEV) Naive T and B cells migrate into secondary lymphoid tissues Artery C Postcapillary venule Infected or injured tissue Effector and memory T cells migrate into sites of infection and tissue injury: cell-mediated immunity FIGURE 3–1 The main functions served by leukocyte migration from blood into tissues A, Neutrophils and monocytes that arise in the bone marrow are recruited into tissue sites of infection or injury, where they eliminate infectious pathogens, clear dead tissues, and repair the damage B, Naive lymphocytes that arise in bone marrow or thymus home to secondary lymphoid organs, such as lymph nodes (or spleen, not shown), where they become activated by antigens and differentiate into effector lymphocytes C, Effector lymphocytes arising in secondary lymphoid organs migrate into tissue sites of infection, where they participate in microbial defense Adhesion Molecules on Leukocytes and Endothelial Cells Involved in Leukocyte Recruitment Leukocyte recruitment from the blood into tissues depends first on adhesion of the leukocytes to the endothelial lining of postcapillary venules and then movement through the endothelium and underlying basement membrane into the extravascular tissue This is a multistep process in which each step is orchestrated by different types of molecules, including chemokines and adhesion molecules The same basic process occurs for different types of leukocytes (neutrophils, monocytes, and naive and effector lymphocytes) homing to different types of tissues (secondary lymphoid organs, infected tissues), although the specific chemokines and adhesion molecules vary in ways that result in different migration properties for each cell type Before describing the process, we discuss the properties and functions of the adhesion molecules and chemokines that are involved in leukocyte recruitment ADHESION MOLECULES ON LEUKOCYTES AND ENDOTHELIAL CELLS INVOLVED IN LEUKOCYTE RECRUITMENT The migration of leukocytes from the blood into tissues involves adhesion between the circulating leukocytes and vascular endothelial cells as a prelude to the movement of the leukocytes out of the vessels into the tissues This adhesion is mediated by two classes of molecules, called selectins and integrins, and their ligands The expression of these molecules varies among different types of leukocytes and in blood vessels at different locations We next describe the major selectins and integrins and their ligands and their roles in leukocyte recruitment into tissues Selectins and Selectin Ligands Selectins are plasma membrane carbohydrate-binding adhesion molecules that mediate an initial step of lowaffinity adhesion of circulating leukocytes to endothelial cells lining postcapillary venules (Table 3-1) The extracellular domains of selectins are similar to C-type lectins, so called because they bind carbohydrate structures (the definition of lectins) in a calcium-dependent manner Selectins and their ligands are expressed on leukocytes and endothelial cells Two types of selectins are expressed by endothelial cells, called P-selectin (CD62P) and E-selectin (CD62E) P-selectin, so called because it was first found in platelets, is stored in cytoplasmic granules of endothelial cells and is rapidly redistributed to the surface in response to microbial products, cytokines, histamine from mast cells, and thrombin generated during blood coagulation E-selectin is synthesized and expressed on the endothelial cell surface within to hours in response to the cytokines interleukin-1 (IL-1) and tumor necrosis factor (TNF) and microbial products such as lipopolysaccharide (LPS) We will discuss IL-1, TNF, and LPS in our discussion of inflammation in Chapter The ligands on leukocytes that bind to E-selectin and P-selectin on endothelial cells are complex sialylated carbohydrate groups related to the Lewis X or Lewis A family, present on various surface glycoproteins of granulocytes, monocytes, and some previously activated effector and memory T cells The best defined of these is the tetrasaccharide sialyl Lewis X (sLeX) A leukocyte membrane glycoprotein called P-selectin glycoprotein ligand (PSGL-1) is post-translationally modified to display the carbohydrate ligands for P-selectin Several different TABLE 3–1 Major Leukocyte-Endothelial Adhesion Molecules Family Molecule Distribution Ligand (molecule; cell type) Selectin P-selectin (CD62P) Endothelium activated by cytokines (TNF, IL-1), histamine, or thrombin Sialyl Lewis X on PSGL-1 and other glycoproteins; neutrophils, monocytes, T cells (effector, memory) E-selectin (CD62E) Endothelium activated by cytokines (TNF, IL-1) Sialyl Lewis X (e.g., CLA-1) on glycoproteins; neutrophils, monocytes, T cells (effector, memory) L-selectin (CD62L) Neutrophils, monocytes, T cells (naive and central memory), B cells (naive) Sialyl Lewis X/PNAd on GlyCAM-1, CD34, MadCAM-1, others; endothelium (HEV) LFA-1 (CD11aCD18) Neutrophils, monocytes, T cells (naive, effector, memory) ICAM-1 (CD54), ICAM-2 (CD102); endothelium (upregulated when cytokine activated) Mac-1 (CD11bCD18) Monocytes, dendritic cells ICAM-1 (CD54), ICAM-2 (CD102); endothelium (upregulated when cytokine activated) VLA-4 (CD49aCD29) Monocytes, T cells (naive, effector, memory) VCAM-1 (CD106); endothelium (upregulated when cytokine activated) α4β7 (CD49dCD29) Monocytes, T cells (gut homing, naive, effector, memory) VCAM-1 (CD106), MadCAM-1; endothelium in gut and gut-associated lymphoid tissues Integrin CLA-1, cutaneous lymphocyte antigen 1; GlyCAM-1, glycan-bearing cell adhesion molecule 1; HEV, high endothelial venule; ICAM-1, intracellular adhesion molecule 1; IL-1, interleukin-1; LFA-1, leukocyte function-associated antigen 1; MadCAM-1, mucosal addressin cell adhesion molecule 1; PNAd, peripheral node addressin; PSGL-1, P-selectin glycoprotein ligand 1; TNF, tumor necrosis factor; VCAM-1, vascular cell adhesion molecule 1; VLA-4, very late antigen 39 40 Chapter – Leukocyte Migration into Tissues molecules may display the carbohydrate ligands for E-selectin, including the glycoproteins PSGL-1 and E-selectin ligand and some glycolipids A third selectin, called L-selectin (CD62L), is expressed on leukocytes but not on endothelial cells The ligands for L-selectin are sialomucins displayed on high endothelial venules, collectively called peripheral node addressin (PNAd) A major recognition determinant that L-selectin binds to on these sialomucins is sialyl 6-sulfo Lewis X The expression of these ligands is increased by cytokine activation of endothelial cells L-selectin on neutrophils serves to bind these cells to endothelial cells that are activated by IL-1, TNF, and other cytokines produced at sites of inflammation In adaptive immunity, L-selectin is important for naive T lymphocytes to home into lymph nodes through high endothelial venules Leukocytes express L-selectin and the carbohydrate ligands for P-selectin and E-selectin at the tips of their microvilli, facilitating interactions with molecules on the endothelial cell surface Integrins and Integrin Ligands Integrins are heterodimeric cell surface proteins composed of two noncovalently linked polypeptide chains that mediate adhesion of cells to other cells or to extracellular matrix, through specific binding interactions with various ligands There are more than 30 different integrins, all with the same basic structure, containing one of more than 15 types of α chains and one of seven types of β chains The extracellular globular heads of both chains contribute to interchain linking and to divalent cationdependent ligand binding The cytoplasmic domains of the integrins interact with cytoskeletal components (including vinculin, talin, actin, α-actinin, and tropomyosin) The name of this family of proteins derives from the idea that they coordinate (i.e., integrate) signals generated when they bind extracellular ligands with cytoskeleton-dependent motility, shape change, and phagocytic responses In the immune system, the most important integrins are two that are expressed on leukocytes, called LFA-1 (leukocyte function-associated antigen 1, more precisely named β2αL or CD11aCD18) and VLA-4 (very late antigen 4, or β1α4, or CD49dCD29) (see Table 3-1) One important ligand for LFA-1 is intercellular adhesion molecule (ICAM-1, CD54), a membrane glycoprotein expressed on cytokine-activated endothelial cells and on a variety of other cell types, including lymphocytes, dendritic cells, macrophages, fibroblasts, and keratinocytes The extracellular portion of ICAM-1 is composed of globular domains that share some sequence homology and tertiary structural features of domains found in immunoglobulin (Ig) molecules and are called Ig domains (Many proteins in the immune system contain Ig domains and belong to the Ig superfamily, which is discussed in more detail in Chapter 5.) LFA-1 binding to ICAM-1 is important for leukocyte-endothelial interactions (discussed later) and T cell interactions with antigen-presenting cells (see Chapter 6) Two other Ig superfamily ligands for LFA-1 are ICAM-2, which is expressed on endothelial cells, and ICAM-3, which is expressed on lymphocytes VLA-4 binds to vascular cell adhesion molecule (VCAM1, CD106), an Ig superfamily protein expressed on cytokine-activated endothelial cells in some tissues, and this interaction is important for leukocyte recruitment into inflammatory sites Other integrins also play roles in innate and adaptive immune responses For example, Mac-1 (β2αm, CD11bCD18) on circulating monocytes binds to ICAM-1 and mediates adhesion to endothelium Mac-1 also functions as a complement receptor, binding particles opsonized with a product of complement activation called the inactivated C3b (iC3b) fragment, and thereby enhances phagocytosis of microbes An important feature of integrins is their ability to respond to intracellular signals by rapidly increasing their affinity for their ligands (Fig 3-2) This is referred to as activation and occurs in response to signals generated from chemokine binding to chemokine receptors and in lymphocytes by intracellular signals generated when antigen binds to antigen receptors The process of changes in the binding functions of the extracellular domain of integrins induced by intracellular signals is called inside-out signaling Chemokine- and antigen A Low affinity integrin (LFA-1) High affinity integrin Chemokine Chemokine receptor B ICAM-1 Extended (high affinity) Bent (low affinity) FIGURE 3–2 Integrin activation A, The integrins on blood leukocytes are normally in a low-affinity state If a leukocyte comes close to endothelial cells, such as when selectin-dependent rolling of leukocytes occurs, then chemokines displayed on the endothelial surface can bind chemokine receptors on the leukocyte Chemokine receptor signaling then occurs, which activates the leukocyte integrins, increasing their affinity for their ligands on the endothelial cells B, Ribbon diagrams are shown of bent and extended conformations of a leukocyte integrin, corresponding to low- and high-affinity states, respectively (B From Takagi J, and TA Springer Integrin activation and structural rearrangement Immunological Reviews 186:141-163, 2002.) Chemokines and Chemokine Receptors receptor–induced inside-out signaling involves GTPbinding proteins (described in more detail later), eventually leading to the association of RAP family molecules and cytoskeleton-interacting proteins with the cytoplasmic tails of the integrin proteins The resulting affinity changes are a consequence of conformational changes in the extracellular domains In the low-affinity state, the stalks of the extracellular domains of each integrin subunit appear to be bent over, and the ligand-binding globular heads are close to the membrane In response to alterations in the cytoplasmic tail, the stalks extend in switchblade fashion, bringing the globular heads away from the membrane to a position where they more effectively interact with their ligands (see Fig 3-2) Chemokines also induce membrane clustering of integrins This results in increased avidity of integrin interactions with ligands on the endothelial cells, and therefore tighter binding of the leukocytes to the endothelium CHEMOKINES AND CHEMOKINE RECEPTORS Chemokines are a large family of structurally homologous cytokines that stimulate leukocyte movement and regulate the migration of leukocytes from the blood to tissues The name chemokine is a contraction of “chemotactic cytokine.” We have already referred to the role of chemokines in the organization of lymphoid tissue and now we will describe the general properties of this family of cytokines and summarize their multiple roles in innate and adaptive immunity Table 3-2 summarizes the major features of individual chemokines and their receptors Chemokine Structure, Production, and Receptors There are about 50 human chemokines, all of which are 8- to 12-kD polypeptides that contain two internal disulfide loops The chemokines are classified into four families on the basis of the number and location of N-terminal cysteine residues The two major families are the CC (also called β) chemokines, in which the cysteine residues are adjacent, and the CXC (or α) family, in which these residues are separated by one amino acid These differences correlate with organization of the subfamilies into separate gene clusters A small number of chemokines have a single cysteine (C family) or two cysteines separated by three amino acids (CX3C) Chemokines were originally named on the basis of how they were identified and what responses they triggered More recently, a standard nomenclature, based in part on which receptors the chemokines bind to (see Table 3-2), is being used Although there are exceptions, most of the CC chemokines and their receptors mediate recruitment of neutrophils and lymphocytes, and most of the CXC chemokines and their receptors recruit monocytes and lymphocytes The chemokines of the CC and CXC subfamilies are produced by leukocytes and by several types of tissue cells, such as endothelial cells, epithelial cells, and fibroblasts In many of these cells, secretion of chemokines is induced by recognition of microbes through various cell receptors of the innate immune system discussed in Chapter In addition, inflammatory cytokines, mainly TNF and IL-1, induce chemokine production Several CC chemokines are also produced by antigen-stimulated T cells, providing a link between adaptive immunity and recruitment of inflammatory leukocytes The receptors for chemokines belong to the seventransmembrane, guanosine triphosphate (GTP)–binding (G) protein–coupled receptor (GPCR) superfamily These receptors initiate intracellular responses through associated trimeric G proteins In a resting cell, the receptorassociated G proteins form a stable inactive complex containing guanosine diphosphate (GDP) bound to Gα subunits Occupancy of the receptor by ligand results in an exchange of GTP for GDP The GTP-bound form of the G protein activates numerous cellular enzymes, including an isoform of phosphatidylinositol-specific phospholipase C that functions to increase intracellular calcium and activate protein kinase C The G proteins stimulate cytoskeletal changes and polymerization of actin and myosin filaments, resulting in increased cell motility These signals also change the conformation of cell surface integrins and increase the affinity of the integrins for their ligands Chemokine receptors may be rapidly downregulated by exposure to the chemokine, and this is a likely mechanism for termination of responses TABLE 3–2 Chemokines and Chemokine Receptors Chemokine Original Name Chemokine Receptor Major Function CCL1 I-309 CCR8 Monocyte recruitment and endothelial cell migration CCL2 MCP-1 CCR2 Mixed leukocyte recruitment CCL3 MIP-1α CCR1, CCR5 Mixed leukocyte recruitment CCL4 MIP-1β CCR5 T cell, dendritic cell, monocyte, and NK recruitment; HIV coreceptor CCL5 RANTES CCR1, CCR3, CCR5 Mixed leukocyte recruitment CCL7 MCP-3 CCR1, CCR2, CCR3 Mixed leukocyte recruitment CCL8 MCP-2 CCR3, CCR5 Mixed leukocyte recruitment CC chemokines Continued 41 ... Professor of Medicine and Health Sciences and Technology Harvard Medical School Massachusetts General Hospital Boston, Massachusetts Illustrations by David L Baker, MA Alexandra Baker, MS, CMI DNA... California San Francisco San Francisco, California Andrew H Lichtman, MD, PhD Professor of Pathology Harvard Medical School Brigham and Women s Hospital Boston, Massachusetts Shiv Pillai, MBBS, PhD... enhanced phagocytosis, and directly lyse microbes (As we shall discuss later, complement can also be activated by antibodies—called the classical pathway, for historical reasons—with the same