'I'HE IMMUNE BYBTEM IN HEALTH AND DISEASE
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`CHARLES A JANEWAY PAUL TnAvsn's
`MAnK WALPOHT
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`
`
`
`Lassen — Exhibit 1040, p. 1
`
`Lassen - Exhibit 1040, p. 1
`
`

`

`immuno
`iologya
`
`THE IMMUNE SYSTEM IN HEALTH AND DISEASE
`
`Charles A. Janeway, Jr.
`
`Yale University School of Medicine
`
`Paul Travers
`
`Anthony Nolan Research Institute. London
`
`I
`
`Mark Walport
`
`Imperial College School of Medicine, London
`
`.
`
`Mark J. Shlomchlk
`
`Yale University School of Medicine
`
`Lassen — Exhibit 1040, p. 2
`
`Lassen - Exhibit 1040, p. 2
`
`

`

`Vice President:
`Text Editors:
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`Bilnk Studio. London
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`
`© 2001 by Garland Publishing.
`All rights reserved. No part oi this publication may be reproduced. stored in a reirievsl
`system or transmitted in any form or by any means—electronic. mechanical. photocopying.
`recording. or otherwise—without the prior written permission of the copyright holder.
`
`Distributors:
`Ins/d8 North Amer/ca: Garland Publishing, 29 West 35th Street,
`New York. NY 10001 -2299.
`Ins/d9 Japan: Nankodo Co. Ltd.. 42-6. Hongo 3-Chrome. Bunkyo-ku.
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`
`iSBN 0 8153 3642 X (paperback) Garland
`iSBN 0 4430 7098 9 (paperback) Churchill Livingstone
`ISBN 0 4430 7099 7 (paperback) international Student Edition
`
`Library of Congress Cataloging-in-Pubilcation Data
`Immunobiology : the immune system In health and disease / Charles A. Janeway, Jr.
`[et al.].-- 5th ed.
`p. cm.
`includes bibliographical references and index.
`ISBN 0-8153-3642-X (pbk.)
`1. immunology. 2. Immunity. l. Janeway. Charles. II. Title.
`
`QFi181 .i454 2001
`616.07'9vd021
`
`2001016039
`
`Thls book was produced using QuarkXpress 4.11 and Adobe illustrator 9.0
`
`Published by Garland Publishing, a member cl the Taylor ti Francis Group.
`29 West 35th Street. New York. NY 10001-2299.
`
`Printed In the United States of America.
`15141312111098765432
`
`Lassen — Exhibit 1040, p. 3
`
`Lassen - Exhibit 1040, p. 3
`
`

`

`
`
`E
`
`CONTENTS
`
`PART 1
`
`AN INTRODUCTION TO IMMUNOBIOLOGY AND INNATE IMMUNITY
`
`Chapter 1
`Chapter 2
`
`Basic Concepts in Immunology
`Innate Immunity
`
`PART II
`
`THE RECOGNITION OF ANTIGEN
`
`Chapter 3
`
`Antigen Recognition by B-ceII and T-cell Receptors
`
`Chapter 4
`
`The Generation of Lymphocyte Antigen Receptors
`
`Chapter 5
`
`Antigen Presentation to T Lymphocytes
`
`PART III
`
`THE DEVELOPMENT OF MATURE LYMPHOCYTE RECEPTOR
`HEPEFITOIFIES
`
`Chapter 6
`
`Signaling Through Immune System Receptors
`
`Chapter 7
`
`The Development and Survival of Lymphocytes
`
`
`_
`.
`Chapter 8
`
`THE ADAPTIVE IMMUNE RESPONSE
`T Cell-Mediated Immunity
`
`Chapter 9
`
`The Humoral Immune Response
`
`Chapter 10 Adaptive Immunity to Infection
`
`PART V
`
`THE IMMUNE SYSTEM IN HEALTH AND DISEASE
`
`Chapter 11
`
`Failures of Host Defense Mechanisms
`
`Chapter 12 Allergy and Hypersensitivity
`
`Chapter 13 Autoimmunity and Transplantation
`
`Chapter 14 Manipulation of the Immune Response
`
`Aftenrvord Evolution of the Immune System: Pastl Present, and Future,
`by Charles A. Janeway, Jr.
`
`Appendix I
`
`Immunologists' Toolbox
`
`Appendixll
`
`CD Antigens
`
`Appendix III Cytokines and their Receptors
`
`Appendix IV Chemokines and their Receptors
`
`Appendix V
`
`Immunological Constants
`
`Biographies
`
`Glossary
`Index
`
`35
`
`93
`
`123
`
`155
`
`187
`221
`
`295
`
`341
`
`381
`
`425
`
`471
`
`501
`
`553
`
`597
`
`613
`
`661
`
`677
`
`680
`
`681
`
`682
`
`683
`
`708
`
`
`
`Lassen — Exhibit 1040, p. 4
`
`Lassen - Exhibit 1040, p. 4
`
`

`

`that
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`92,
`
`The Humoral Immune
`
`Response
`
`
`
`Many of the bacteria that cause infectious disease in humans multiply in the
`extracellular spaces of the body, and most intracellular pathogens spread by
`moving from cell to cell through the extracellular fluids. The extracellular
`spaces are protected by the humoral immune response, in which antibodies
`produced by B cells cause the destruction of extracellular microorganisms
`and prevent the spread of intracellular infections. The activation of B cells
`and their differentiation into antibody-secreting plasma cells (Fig. 9.1) is
`triggered by antigen and usually requires helper T cells. The term ‘helper T cell'
`is often used to mean a cell from the TH2 class of CD4 T cells (see Chapter 8),
`but a subset of THI cells can also help in B—cell activation. In this chapter we
`Will therefore use the term helper T cell to mean any armed effector CD4
`T cell that can activate a B cell. Helper T cells also control isoty'pe switching
`and have a role in initiating somatic hypermutation of antibody variable
`V-region genes, molecular processes that were described in Chapter 4.
`
`I
`
`Antibodies contribute to immunity in three main ways [see Fig. 9.1). To enter
`cells. viruses and intracellular bacteria bind to specific molecules on the target
`cell surface. Antibodies that bind to the pathogen can prevent this and are
`Salt! to neutralize the pathogen. Neutralization by antibodies is also important
`1'1 Preventing bacterial toxins from entering cells. Antibodies protect against
`bmteris that multiply outside cells mainly by facilitating uptake of the
`Pathogen by phagocytlc cells that are specisiizerl to destroy ingested bacteria.
`Antibodies do this In either of two ways. In the first, bound antibodies coating
`the pathogen are recognized by Ft: receptors on phegocytic cells that bind to
`
`
`
`Lassen — Exhibit 1040, p. 5
`
`Lassen - Exhibit 1040, p. 5
`
`

`

`Chapter 9: The Humoral Immune Response
`
`Fig. 9.1 The humoral immune
`response is mediated by antibody
`molecules that are secreted by
`plasma cells. Antigen that binds to the
`B-cell antigen receptor signals B cells
`andfits. at the same time, internalized
`and processed into peptides that
`activate armed helper T cells. Signals
`from the bound antigen and from the
`helper T cell induce the B cell to
`proliferate and differentiate into a
`plasma cell secreting specific antibody
`(top two panels). These antibodies
`protect the host from Infection in three
`main ways. They can inhibit the toxic
`effects or infectivity of pathogens by
`binding to them: this is termed
`neutralization (bottom left panel). By
`coating the pathogens, they can enable
`accessory cells that recognize the Fc
`portions of arrays of antibodies to ingest
`and kill the pathogen, a process called
`opsonization (bottom center panel).
`Antibodies can also trigger activation of
`the complement system. Complement
`proteins can strongly enhance
`opsonization. and can directly kill some
`bacterial cells (bottom right panel).
`
`
`
`Antibody prevents
`bacterial adherence
`
`Antibody promotes
`phagocl/‘OSiS
`
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`the antibody constant C region (see Section 4-18). Coating the surface of a
`pathogen to enhance phagocytosis is called opsonization. Alternatively,
`antibodies binding to the surface of a. pathogen can activate the proteins of
`the complement system, which was described in Chapter 2. Complement
`activation results in complement proteins being bound to the pathogen su r-
`face. and these opsonize the pathogen by binding complement receptors on
`phagocytes. Other complement components recruit phagocytic cells to the
`site of infection. and the terminal components of complement can lyse certain
`microorganisms directly by forming pores in their membranes. Which effector
`mechanisms are engaged in a particular response is determined by the isotype
`or class of the antibodies produced.
`
`In the first part of this chapter we will describe the interactions of B cells with
`helper T cells that lead to the production of antibodies, the affinity maturation
`of this antibody response, the isotype switching that confers functional
`diversity. and the generation of memory 3 cells that provide Iong-lasring
`immunity to reinfection. In the rest of the chapter we will discuss in detail the
`mechanisms whereby antibodies contain and eliminate infections.
`
`
`
`Lassen — Exhibit 1040, p. 6
`
`Lassen - Exhibit 1040, p. 6
`
`

`

`
`
`B-ceil activation by armed helper T cells
`
`
`[Smell-activation by armedr’hElper T cells.
`
`The surface immunoglobulin that serves as the B-cell antigen receptor (BCR)
`has two roles in B-cell activation. First, like the antigen receptor on T cells, it
`transmits signals directly to the cell's interior when it binds antigen (see
`section 6-1). Second, the B-cell antigen receptor delivers the antigen to intra-
`cellular sites where it is degraded and returned to the B-cell surface as peptides
`bound to MHC class II molecules (see Chapter 5). The peptide:MHC class II
`complex can be recognized by antigen—specific armed helper T cells, stimu-
`letting them to make proteins that, in turn. cause the B cell to proliferate and
`its progeny to differentiate into antibody-secreting cells. Some microbial
`antigens can activate B cells directly in the absence of T-cell help. The ability
`of B cells to respond directly to these antigens provides a rapid response to
`many important bacterial pathogens. However, somatic hypermutation and
`switching to certain immunoglobulin isotypes depend on the interaction of
`antigen—stimulated B cells with helper T cells and other cells in the peripheral
`lymphoid organs. Antibodies induced by microbial antigens alone are there-
`fore less variable and less functionally versatile than those induced with
`T-cell help.
`
`9-1
`
`The humoral immune response is initiated when B cells that bind
`antigen are signaled by helperT cells or by certain microbial
`antigens alone.
`
`It is a general rule in adaptive immunity that naive antigen-specific lympho-
`cytes are difficult to activate by antigen alone. Naive T cells require a
`co—stimulatory signal from professional antigen—presenting cells; naive B
`cells require accessory signals that can come either from an armed helper T
`cell or, in some cases, directly from microbial constituents.
`
`Antibody responses to protein antigens require antigen—specific T-cell help.
`B cells can receive help from armed helper T cells when antigen bound by
`surface immunoglobulin is internalized and returned to the cell surface as
`peptides bound to MHC class II molecules. Armed helper T cells that recognize
`the peptide:MHC complex then deliver activating signals to the B cell. Thus,
`protein antigens binding to B cells both provide a specific signal to the B cell
`by cross-linking its antigen receptors and allow the B cell to attract antigen-
`specific T-cell help. These antigens are unable to induce antibody responses
`in animals or humans who lack T cells, and they are therefore known as
`thymus—dependent or TD antigens (Fig. 9.2, top two panels).
`
`The B-cell co-receptor complex of CD19:CD21:CD81 [see Section 6—8) can
`greatly enhance B-cell responsiveness to antigen. CD21 (also known as
`complement receptor 2, CR2) is a receptor for the complement fragment
`C3d (see Section 2-11). When mice are immunized with hen egg lysozyme
`Coupled to three linked molecules of the complement fragment C3dg, the
`modified lysozyme induces antibody without added adjuvant at doses up to
`10.000 times smaller than unmodified hen egg lysozyme. Whether binding of
`CD21 enhances B-cell responsiveness by increasing B-cell signaling, by
`inducing co-stimulatory molecules on the B cell, or by increasing the receptor-
`mediured uptake of antigen, is not yet known. As we will see later in this
`Chapter. antibodies already bound to antigens can activate the complement
`si’Stem, thus coating the antigen with C3d and producing a more potent
`antigen, Which in turn leads to more efficient B-cell activation and antibody
`Production.
`
`
`
`Fig. 9.2 A second signal ls required
`for B-cell activation by either thymus-
`dependent or thymus-independent
`antigens. The first signal required for
`B-cell activation is delivered through
`its antigen receptor (top panel). For
`thymus-dependent antigens, the second
`signal is delivered by a helper T cell that
`recognizes degraded fragments of the
`antigen as peptides bound to MHC class
`II molecules on the B-cell surface
`(center panel); the interaction between
`CD40 ligand (CD40L) on the T cell and
`CD40 on the B cell contributes an
`essential part of this second signal.
`For thymus-independent antigens. the
`second signal can be delivered by the
`antigen itself (lower panel), or by non-
`ihymus-derived accessory cells (not
`shown).
`
`
`
`Lassen — Exhibit 1040, p. 7
`
`Lassen - Exhibit 1040, p. 7
`
`

`

`Chapter 9: The Humoral Immune Response
`
`Although armed peptide-specific helper T cells are required for 1316“,
`responses to protein antigens. many microbial constituents. such as hostel-1&1?
`
`polysaccharides. can induce antibody production in the absence of he] of
`
`'1‘ cells. These microbial antigens are known as thymus-independent or Ti:
`
`antigens because they induce antibody responses in individuals who have no-
`
`T lymphocytes. The second signal required to activate antibody production-'3
`to Tl antigens is either provided directly by recognition ot‘a common microbial
`constituent [see Fig. 9.2, bottom panei) or by a nontltymus-derlved accessuw‘
`cell in conjunction with massive cross-linking of B-ceil receptor . which
`would occur when a B cell binds repeating epitopes on the bacterial cell.
`Thymus-independent antibody responses provide some protection against
`extracellular bacteria, and we will return to them later.
`
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`Armed helperT cells activate B cells that recognize the same
`antigen.
`
`T—ceil dopeiident antibody responses require the activation of B cells by helper
`T cells that respond to the same antigen; this is called linked recognition. This
`means that before B cells can be induced to make antibody to an infecting
`pathogen, a C134 T cell specific for peptides from this pathogen must first be
`activated to produce the appropriate armed helper T cells. This presumably
`occurs by interaction with an antigen-presenting dendritic cell (see Section
`8-1). Although the epitope recognized by the armed helper T cell must there-
`fore he linked to that recognized by the B cell. the two cells need not recognize
`identical epitopes. Indeed, we saw in Chapter 5 that ‘1‘ cells can recognize
`internal peptides that are quite distinct from the surface cpitopes on the same
`protein recognized by B cells. For more complex nature! antigens, such as
`viruses. the T cell and the B cell might not even recognize the same protein.
`It is, however, crucial that the peptide recognized by the T cell be a physical part
`of the antigen recognized by the B cell. which can thus produce the appropriate
`peptide after internalization of the antigen bound to its B-ccll receptors.
`
`For example. by recognizing an epitope on a viral protein coat. a B cell can
`internalize a complete virus particle. After internalization. the virus particle is
`degraded. and peptides from internal viral proteins as well as coat proteins
`can be displayed by MHC class ll molecules on the B-ccll surface. Helper
`T cells that have been primed earlier in an infection by macrophages or
`dendritic cells presentng these internal peptides can then activate the B cell
`to make antibodies that recognize the coat protein (Fig. 9.3).
`
`The specific activation of the B cell by a T cell sensitized to the same antigen
`or pathogen depends on the ability of the antigen-specific B cell to concentrate
`the appropriate peptide on its surface MHC class it molecules. 3 cells that
`bind a particular antigen are up to 10,000 times more efficient at displaying
`peptide fragments of that antigen on their MHC class II molecules than are
`B cells that do not bind the antigen. Armed helper T cells will thus help only
`those B cells whose receptors bind an antigen containing the peptide they
`recognize.
`
`derived from viral proteins, including must recognize epitopes oi the same
`
`Fig. 9.3 B cells and helper T cells
`molecular complex in order to
`interact. An epitope on a viral coat
`protein is recognized by the surface
`immunoglobulin on a B cell and the virus
`is internalized and degraded. Peptides
`
`internal proteins, are returned to the
`B-cell surface bound to MHC class II
`molecules (see Chapter 5). Here, these
`complexes are recognized by helper T
`cells. which help to activate the B cells to
`produce antibody against the coat protein.
`
`Lassen — Exhibit 1040, p. 8
`
`Lassen - Exhibit 1040, p. 8
`
`

`

` r|1I f
`
`it-
`
`B-cell activation by armed helper T cells
`
`
`
`Lassen — Exhibit 1040, p. 9
`
`1,199.4 Protein antigens attached to
`:1
`'Iéowgaccherlde antigens allow T cells
`a” help potysacoharlde-epeclflo
`3‘3 cello. Haemophitue influenza type
`B vaccine Is a conlugete at bacterial
`' tyasccharlde and the tetanus toxoid
`fmteln. The B cell recognizes and binds
`,
`e polysaccneride, Intemallzes and
`upgrades the whole conjugate and then
`
`displays toxoid-derived peptides on
`surface MHC class II molecules. Helper
`T cells generated in response to earlier
`vaccination against the toxoid recognize
`the complex on the B-cell surface and
`activate the B cell to produce anti-
`polysaccharide antibody. This antibody
`can then protect against infection with
`H. influenzae type B.
`
`
`
`il‘he requirement for linked recognition has important consequences for
`the regulation and manipulation of the humoral immune response. One is
`Iii-lat linked recognition helps ensure self tolerance, as will be described in
`Chapter 13. An important application of linked recognition is in the design of
`vaccines. such as that used to immunize infants against Haemophilus
`Influences type B. This bacterial pathogen can infect the lining of the brain,
`called the meninges, causing meningitis and, in severe cases, neurological
`damage or death. Protective immunity to this pathogen is mediated by anti-
`bodies against its capsular polysaccharide. Although adults make very
`effective thymus—independent responses to these polysaccharide antigens,
`such responses are weak in the immature immune system of the infant. To
`make an effective vaccine for use in infants, therefore, the polysaccharide is
`linked chemically to tetanus toxoid, a foreign protein against which infants
`are routinely and successfully vaccinated (see Chapter 14). B cells that bind
`the polysaccharide component of the vaccine can be activated by helper
`'1' cells specific for peptides of the linked toxoid (Fig. 9.4).
`
`Linked recognition was originally discovered through studies of the production
`of antibodies to haptens (see Appendix 1, Section A-l). Haptens are small
`chemical groups that cannot elicit antibody responses on their own because
`they cannot cross-link B-cell receptors and they cannot recruit T-cell help.
`When coupled at high density to a carrier protein, however, they become
`iInmunogenic, because the protein will carry multiple hapten groups that
`can now cross-link B-cell receptors. in addition, T-cell dependent responses
`are possible because T cells can be primed to peptides derived from the
`protein. Coupling of a hapten to a protein is responsible for the allergic
`Tesponses shown by many people to the antibiotic penicillin, which reacts
`With host proteins to form a coupled hapten that can stimulate an antibody
`IESponse, as we will learn in Chapter 12.
`
`3-3
`
`Antigenic peptides bound to self MHc class II molecules trigger
`armed helperT cells to make membrane-bound and secreted
`molecules that can activate a B cell.
`
`_
`
`killed helper T cells activate B cells when they recognize the appropriate
`PeptideMHC class II complex on the B-cell surface (Fig. 9.5). As with armed
`.Tl-ll cells acting on macrophages, recognition of peptidezMHC class II
`melexcs on B cells triggers armed helper T cells to synthesize both cell-
`““1161 and secreted effector molecules that synergize in activating the B cell.
`One particularly important T—cell effector molecule is a membrane-bound
`molecule of the tumor necrosis factor (TNF) family known as CD40 ligand
`[013401. also known as CD154) because it binds to the B-cell surface molecule
`CD40. CD40 is a member of the 'I‘NF-receptor family of cytokine receptors
`[‘99 Section 8-20) however,
`it does not contain a 'death domain.‘ It is
`illl'otved in directing all phases of the B-cell response. Binding of CD40 by
`twill helps to drive the resting B cell into the cell cycle and is essential for
`'Eell responses to thymus-dependent antigens.
`
`Lassen - Exhibit 1040, p. 9
`
`

`

`
`
` Fig. 9.5 Armed helper T cells stimulate the proliferation and
` molecule CD40 ligand (CD40L) on the helper T-cell surface and
`
`to the secretion of the B-cell stimulatory cytokines |L-4, IL—5, and
`then the differentiation of antigen-binding B cells. The
`
`lL-6, which drive the proliferation and differentiation of the B cell
`specific interaction of an antigen-binding B cell with an armed
`helper T cell leads to the expression of the B-cell stimulatory
`into antibody-secreting plasma cells.
`
`
`
`B cells are stimulated to proliferate in Vitro when they are exposed to a mixture
`of artificially synthesized CD40L and the cytokine interleukin—4 (IL—4]. lL-4 is
`also made by armed T112 cells when they recognize their specific ligand 0n
`the Bach surface, and lL-4 and CU40L are thought to synergize in driving the
`clonal expansion that precedes antibody production in viz/0. IL—4 is secreted
`in a polar fashion by the T112 cell and is directed at the site of contact with the
`B cell (Fig. 9.6) so that it acts selectively on the antigen-specific target B cell.
`
`protein talin
`
`
`
`
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`
`Fig. 9.6 When an armed helper T cell
`encounters an antigen-binding B cell.
`it becomes polarized and secretes
`lL-4 and other cytokines at the point
`of cell—cell contact. On binding antigen
`on the B cell through its T-cell receptor,
`the helper T cell is induced to express
`CD40 ligand (CD40L), which binds to
`CD40 on the B cell. As shown in the top
`left panel, the tight junction formed
`between the cells upon antigen-specific
`binding seems to be sealed by a ring of
`adhesion molecules, with LFA-1 on the
`T cell interacting with |CAM-1 on the
`B cell (see Fig. 8.30). The cytoskeleton
`becomes polarized, as revealed by the
`relocation of the cytoskeletal protein
`talin (stained red in right center panel),
`to the point of cell-cell contact, and the
`secretory apparatus (the Golgi
`apparatus) is reoriented by the cyto—
`skeieton toward the point of contact
`with the B cell. As shown in the bottom
`panels, cytokines are released at the
`point of contact. The bottom right panel
`shows lL-4 (stained green) confined to
`the space between the B cell and the
`helper T cell. MTOC, microtubule-
`organizing center. Photographs
`courtesy of A. Kupfer.
`
`
`
`Lassen — Exhibit 1040, p.
`
`10
`
`Lassen - Exhibit 1040, p. 10
`
`

`

`B-cell activation by armed helper T cells
`
`The combination of B-cell receptor and CD40 ligation, along with IL—4 and
`other signals derived from direct T-cell contact, leads to B-cell proliferation.
`Some of these contact signals have recently been elucidated. They involve
`other TNF/TNF—receptor family members, including CD30 and CD30 ligand
`and BLyS (B lymphocyte stimulator) and its receptor on B cells, TACI. After
`several rounds of proliferation, B cells can further differentiate into antibody-
`secreting plasma cells. Two additional cytokines, IL-5 and IL-6, both secreted
`by helper T cells, contribute to these later stages of B-cell activation.
`
`9-4
`
`lsotype switching requires expression of CD40L by the helperT cell
`and is directed by cytokines.
`
`Antibodies are remarkable not only for the diversity of their antigen—binding
`Sites but also for their versatility as effector molecules. The specificity of an
`antibody response is determined by the antigen-binding site, which consists
`of the two variable V domains, V1.1 and VL; however, the effector action of the
`antibody is determined by the isotype of its heavy-chain C region (see Section
`4.15). A given heavy-chain V domain can become associated with the
`C region of any isotype through the process of isotype switching (see Section
`4—16). We will see later in this chapter how antibodies of each isotype
`contribute to the elimination of pathogens. The DNA rearrangements that
`underlie isotype switching and confer this functional diversity on the
`humoral immune response are directed by cytokines, especially those
`released by armed effector CD4 T cells.
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`All naive B cells express cell-surface IgM and IgD, yet IgM makes up less than
`10% of the immunoglobulin found in plasma, where the most abundant
`isotype is IgG. Much of the antibody in plasma has therefore been produced
`by B cells that have undergone isotype switching. Little IgD antibody is
`produced at any time, so the early stages of the antibody response are
`dominated by IgM antibodies. Later, IgG and IgA are the predominant
`isotypes, with IgE contributing a small but biologically important part of the
`response. The overall predominance of IgG results, in part, from its longer
`lifetime in the plasma (see Fig. 4.16).
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`lsotype switching does not occur in individuals who lack functional CD40L,
`which is necessary for productive interactions between B cells and helper
`T cells; such individuals make only small amounts of IgM antibodies in
`response to thymus-dependent antigens and have abnormally high levels of
`IgM in their plasma. These IgM antibodies may be induced by thymus-
`independent antigens expressed by the pathogens that chronically infect
`these patients, who suffer from severe humoral immunodeficiency, as we will
`see in Chapter 11.
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`Most of what is known about the regulation of isotype switching by helper
`T cells has come from experiments in which mouse B cells are stimulated
`with bacterial lipopolysaccharide (LPS) and purified cytokines in vitro. These
`experiments show that different cytokines preferentially induce switching to
`different isotypes. Some of these cytokines are the same as those that drive
`B-cell proliferation in the initiation of a B-cell response. In the mouse, IL-4
`preferentially induces switching to IgG1 and IgE, whereas transforming
`growth factor (TGFHS induces switching to IgG2b and IgA. THZ cells make
`both of these cytokines as well as IL—5, which induces IgA secretion by cells
`that have already undergone switching. Although TH] cells are relatively poor
`initiators of antibody responses, they participate in isotype switchng by
`releasing interferon (IFN)-y, which preferentially induces switching to IgGZa
`and IgG3. The role of cytokines in directing B cells to make the different
`antibody isotypes is summarized in Fig. 9.7.
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`Ind
`eii
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`ire
`i is
`on
`he
`ed
`he
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`311'
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`Immunodeficiency
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`Hyper IgM
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`Lassen — Exhibit 1040, p. 11
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`Lassen - Exhibit 1040, p. 11
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`-
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`Chapter 9: The Humoral Immune Response
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`Fig. 9.7 Different cytokines induce
`switching to different isotypes. The
`individual cytokines induce (violet) or
`inhibit (red) production of certain
`isotypes. Much of the inhibitory effect is
`probably the result of directed switching
`to a different isotype. These data are
`drawn from experiments with mouse
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`cells.
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`Cytoicines induce isotype switching by stimulating the formation and Splicing
`of mRNA transcribed from the switch recombination sites that lie 5' to each
`heavy-chain C gene [see Fig. 4.20]. When activated B cells are exposed to [1,-4.
`for example. transcription front a site upstream of the switch regions oi Cd
`and C, can be detected a day or two before switching occurs (Fig. 9.8}. Recent
`data suggest that the production of a spliced switch transcript has a role in
`directing switching, but the mechanism is not yet clear. Each of the cytokines
`that induces switching seems to induce transcription from the switch regions
`of two different heavy-chain C genes, promoting specific recombination to
`one or other of these genes only. Such a directed mechanism is supported by
`the observation that individual B cells frequently undergo switching to the
`same C gene on both chromosomes, even though the antibody heavy chain is
`only being expressed from one of the chromosomes. Thus, helper T cells
`regulate both the production of antibody by B cells and the isotype that
`determines the effector function of the antibody.
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`Fig. 9.8 lsotype switching is preceded
`by transcriptional activation of heavy-
`chaln C-region genes. Resting naive
`B cells transcribe the p. and 6 genes at a
`low rate, giving rise to surface lgM and
`lgD. Bacterial lipopolysaccharide (LPS),
`which can activate B cells independently
`of antigen, induces lgM secretion. In the
`presence of IL-4. however, C.” and Cs
`are transcribed at a low rate, presaging
`switches to |th and lgE production.
`The transcripts originate before the
`5’ end of the region to which switching
`occurs, and do not code for protein.
`Similarly. TGF-ii gives rise to Cm, and
`Ca transcripts and drives switching to
`lgGZb and lgA. It is not known what
`determines which of the two trans-
`criptionally activated heavy-chain
`C genes undergoes switching. Arrows
`indicate transcription. The figure shows
`isotype switching in the mouse.
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`Lassen — Exhibit 1040, p. 12
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`Lassen - Exhibit 1040, p. 12
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`

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`B-cell activation by armed helper T cells
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`9-5
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`Antigen-binding B cells are trapped in the T-cell zone of secondary
`lymphoid tissues and are activated by encounter with armed helper
`T cells.
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`One of the most puzzling features of the antibody response is how an antigen—
`Specific B cell manages to encounter a helper T cell with an appropriate
`antigen specificity. This question arises because the frequency of naive
`lymphocytes specific for any given antigen is estimated to be between 1 in
`10,000 and 1 in 1,000,000. Thus. the chance of an encounter between a
`T lymphocyte and a B lymphocyte that recognize the same antigen should be
`between 1 in 108 and 1 in 1012. Achieving such an encounter is a far more
`difficult challenge than getting effector T cells activated, because, in the latter
`case, only one of the two cells involved has specific receptors. Moreover,
`T cells and B cells mostly occupy quite distinct zones in peripheral lymphoid
`tissue (see Fig. 1.8). As in naive T-cell activation (see Chapter 8]. the answer
`seems to lie in the antigen-specific trapping of migrating lymphocytes.
`
`When an antigen is introduced into an animal. it is captured and processed
`by professional antigen-presenting cells, especially the dendritic cells that
`migrate from the tissues into the T-cell zones of local lymph nodes.
`Recirculating naive T cells pass by such cells continuously and those rare
`T cells whose receptors bind peptides derived from the antigen are trapped
`very efficiently. This trapping clearly involves the specific antigen receptor on
`the T cell. although it is stabilized by the activation of adhesion molecules
`and chemokines as we learned in Sections 8-3 and 8-4. Ingenious experiments
`using mice transgenic for rearranged immunoglobulin genes show that, in
`the presence of the appropriate antigen, B cells with antigen-specific receptors
`are also trapped in the T-cell zones of lymphoid tissue by a similar mechanism.
`On encountering antigen, migrating antigen-binding B cells are arrested by
`the activation of adhesion molecules and the engagement of chemokine
`receptors such as CCR7, a receptor for MIP-Sl} and SLC.
`
`Trapping of B cells in the T-cell zones provides an elegant solution to the
`problem posed at the beginning of this section. T cells are themselves
`trapped and activated to helper status in the T—cell zones, and when B cells
`migrate into lymphoid tissue through high endothelial venules they first
`enter these same T—cell zones. Most of the B cells move quickly through the
`T-cell zone into the B-cell zone (the primary follicle), but those B cells that
`have bound antigen are trapped. Thus, antigen-binding B cells are selectively
`trapped in precisely the correct location to maximize the chance of encount-
`ering a helper T cell that can activate them. Interaction with armed helper
`T cells activates the B cell to establish a primary focus of clonal expansion
`(Fig. 9.9). Here, at the border between T—cell and B-cell zones, both types of
`lymphocyte will proliferate for several days to constitute the first phase of the
`primary humoral immune response.
`
`After several days, the primary focus of proliferation begins to involute. Many
`of the lymphocytes comprising the focus undergo apoptosis. However, some
`of t