THE IMMUNE SYSTEM IN HEALTH AND DISEASE
`
`aa
`
`7
`
`
`
`
`
`CHARLES A JANEWAYePAUL TRAVERS
`MARK WALPORT
`MARK SHLOMCHIK
`
`
`
`Lassen - Exhibit 1040, p. 1
`
`Lassen - Exhibit 1040, p. 1
`
`

`

`immuno
`iologvye
`
`THE IMMUNE SYSTEM IN HEALTH AND DISEASE
`
`Charles A. Janeway, Jr.
`Yale University School of Medicine |
`is
`Paul Travers
`
`Anthony Nolan Research Institute, London
`®
`Mark Walport
`Imperial College Schoo! of Medicine, London
`S
`Mark J. Shlomchik
`
`Yale University School of Medicine
`
`Dp PUR
`ye
`Le
`
`AaazGAR,
`
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`& Franci®
`
`Lassen - Exhibit 1040, p. 2
`
`Lassen - Exhibit 1040, p. 2
`
`

`

`Vice President:
`Text Editors:
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`
`© 2001 by Garland Publishing.
`All rights reserved. No part of this publication may be reproduced,stored in a retrieval
`system or transmitted in any form or by any means—electronic, mechanical, photocopying,
`recording, or otherwise—withoutthe prior written permission of the copyright holder.
`
`Distributors:
`Inside North America: Garland Pubilshing, 29 West 35th Street,
`New York, NY 10001-2289.
`inside Japan. Nankodo Co. Ltd., 42-6, Hongo 3-Chrome, Bunkyo-ku,
`Tokyo, 113-8410, Japan.
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`
`ISBN 0 8153 3642 X (paperback) Garland
`ISBN 0 4430 7098 9 (paperback) Churchill Livingstone
`ISBN 0 4430 7089 7 (paperback) International Student Edition
`
`Library of Congress Cataloging-in-Publication Data
`Immunobiology : the immune system In health and disease / Charles A, Janeway, Jr. ...
`{et al,].-- 5th ed.
`p. em.
`Includes bibliographical references and index.
`ISBN 0-8153-3642-X (pbk.)
`1, Immunology. 2. Immunity. 1. Janeway, Chartes.If. Title.
`
`QR181 1454 2001
`616.07'9--de21
`
`2001016039
`
`Thla book was produced using QuarkXpress 4.11 and AdobeIllustrator 9.0
`
`Published by Garland Publishing, a memberof the Taylor & Francis Group,
`29 West 35th Street, New York, NY 10001-2299,
`
`Printed In the United States of America.
`1§ 141312 111098765432
`
`Lassen - Exhibit 1040, p. 3
`
`Lassen - Exhibit 1040, p. 3
`
`

`

`T
`
`
`
`ra
`
`CONTENTS
`
`PART!|AN INTRODUCTION TO IMMUNOBIOLOGY AND INNATE IMMUNITY
`
`1
`Basic Concepts in Immunology
`Chapter 1
`
`| Chapter 2—_Innate Immunity 35
`
`
`
`PART Il_|THE RECOGNITION OF ANTIGEN
`
`Chapter3
`Chapter4
`Chapter§
`
`Antigen Recognition by B-cell and T-cell Receptors
`The Generation of Lymphocyte Antigen Receptors
`Antigen Presentation to T Lymphocytes
`
`THE DEVELOPMENT OF MATURE LYMPHOCYTE RECEPTOR
`REPERTOIRES
`
`Chapter6
`
`Signaling Through immune System Receptors
`
`Chapter 7
`The Development and Survival of Lymphocytes
` -PARTIV| THE ADAPTIVE IMMUNE RESPONSE
`Chapter 8
`—T Cell-Mediated Immunity
`Chapter9
`The Humoral Immune Response
`Chapter 10 Adaptive immunity to Infection
`
`THE IMMUNE SYSTEMIN HEALTH AND DISEASE
`
`Failures of Host Defense Mechanisms
`Chapter 11
`Chapter 12 Allergy and Hypersensitivity
`Chapter 13 Autoimmunity and Transplantation
`Chapter 14 Manipulation of the Immune Response
`
`Afterword Evolution of the Immune System:Past, Present, and Future,
`by Charles A. Janeway,dr.
`
`Appendix!
`
`tmmunologists’ Toolbox
`
`Appendix Il
`
`CD Antigens
`
`Appendix Ill Cytokines and their Receptors
`
`Appendix IV Chemokines and their Receptors
`Appendix V_[mmunological Constants
`Biographies
`
`Glossary
`
`i
`
`93
`123
`155
`
`187
`
`221
`
`295
`341
`384
`
`425
`471
`501
`553
`
`597
`
`613
`
`661
`
`677
`
`680
`681
`682
`
`683
`
`js
`
`Index
`
`708
`
`Lassen - Exhibit 1040, p. 4
`
`Lassen - Exhibit 1040, p. 4
`
`

`

`The Humoral Immune
`Response
`
`
`
`Manyofthebacteria that cause infectious disease in humans multiply in the
`extracellular spaces of the body, and mostintracellular pathogens spread by
`moving from cell to cell through the extracellular fluids. The extracellular
`spacesare protected by the humoral immuneresponse,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 plasmacells (Fig. 9.1) is
`triggered by antigen and usually requires helper T cells. The term ‘helperT cell’
`is often used to meana cell from the Ty2 class of CD4 T cells (see Chapter 8),
`but a subset of Tyz1 cells can also helpin B-cell activation. In this chapter we
`will therefore use the term helperT cell to mean any armed effector CD4
`T cell that can activate a B cell. Helper T cells also control isotype switching
`and have a role in initiating somatic hypermutation of antibody variable
`V-region genes, molecular processes that were described in Chapter4.
`Antibodies contribute to immunity in three main ways(see Fig. 9.1). To enter
`Cells, viruses and intracellular bacteria bind to specific moleculeson the target
`Cell surface. Antibodies that bind to the pathogen can preventthis and are
`_ Said to neutralize the pathogen. Neutralization by antibodies is also important
`in preventing bacterial toxins from enteringcells. Antibodies protect against
`bacteria that multiply outside cells mainly by facilitating uptake of the
`Pathogenby phagocytic cells that are specialized to destroy ingested bacteria.
`Antibodies do this in either of two ways. In the first, bound antibodies coating
`the pathogenare recognized by Fc receptors on phagocytic cells that bind to
`
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`
`Lassen - Exhibit 1040, p. 5
`
`Lassen - Exhibit 1040, p. 5
`
`

`

`Chapter9: The Humoral Immune Response
`
`Fig. 9.1 The humoral immune
`responseis mediated by antibody
`molecules that are secreted by
`plasmacells. Antigen that binds to the
`B-cell antigen receptor signals B cells
`andis, at the sametime,internalized
`andprocessedinto 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
`plasmacell secreting specific antibody
`(top lwo panels). These antibodies
`protect the host from Infection in three
`main ways, They caninhibit the toxic
`effects orinfectivity of pathogens by
`binding to them: this is termed
`neutralization (bottom left panel). By
`coating the pathogens, they can enable
`accessory calls that recognize the Fc
`portions of arrays of antibodies to ingest
`andkill the pathogen, a process called
`opsonization (bottom center panel).
`Antibodles canalso trigger activation of
`the complement system. Complement
`proteins can strongly enhance
`opsonization, and can directly kill some
`bacterial cells (bottom right panel).
`
`a
`
`oshe
`
`Antibody prevents
`bacterial adherence
`
`Antibody promotes
`1
`phagocytosis
`
`Antibody activates complement,
`which enhances opsonization
`and lyses some bacteria
`
`ae
`
`
`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 complementproteins being boundto the pathogen sur-
`face, and these opsonize the pathogenby binding complementreceptors on
`phagocytes. Other complement components recruit phagocytic cells to the
`site of infection, and the terminal components of complementcanlyse certain
`microorganismsdirectly by forming pores in their membranes. Whicheffector
`mechanisms are engaged in a particular responseis determinedbytheisotype
`orclass of the antibodies produced.
`
`i
`|
`|
`|
`
`a
`
`ll
`In the first part of this chapterwe will describe the interactions of B cells with
`helperT cells that lead to the production of antibodies, the affinity maturation i
`of this antibody response, the isotype switching that confers functional
`
`diversity, and the generation of memoryBcells that provide long-lasting f
`immunity to reinfection.In the rest of the chapter we will discussin detail the
`!
`mechanisms whereby antibodies contain and eliminate infections.
`int
`|
`
`
`
`Lassen - Exhibit 1040, p. 6
`
`Lassen - Exhibit 1040, p. 6
`
`

`

`B-cell activation by armed helperT cells
`
`
`_B-cellactivation by armedhelperTcells.
`
`The surface immunoglobulinthatservesas the B-cell antigen receptor (BCR)
`has two roles in B-cell activation.First, like the antigen receptor onTcells,it
`transmits signals directly to the cell's interior when it binds antigen (see
`Section 6-1), Second,the B-cell antigen receptordelivers the antigento intra-
`cellular sites whereit is degraded and returnedto the B-cell surface as peptides
`bound to MHCclassII molecules (see Chapter 5). The peptide:MHCclassII
`complex can be recognized by antigen-specific armed helperT cells, stimu-
`Jating them to makeproteins 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. Theability
`of B cells to responddirectly to these antigens provides a rapid response to
`many important bacterial pathogens. However, somatic hypermutation and
`switching to certain immunoglobulin isotypes depend onthe interaction of
`antigen-stimulatedB cells with helper T cells and othercells in the peripheral
`lymphoid organs. Antibodies induced by microbial antigensaloneare there-
`fore less variable and less functionally versatile than those induced with
`T-cell help.
`
`
`
`9-1 The humoral immuneresponseisinitiated whenBcells that bind
`antigen are signaled by helper T 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 comeeither from an armed helper T
`cell or, in some cases, directly from microbial constituents.
`
`Antibody responsesto 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 MHCclassII molecules. ArmedhelperT cells that recognize
`the peptide: MHC complex then deliver activating signals to the B cell. Thus,
`protein antigens bindingto B cells both providea specific signal to the B cell
`by cross-linking its antigen receptors andallow theBcell to attract antigen-
`specific T-cell help. These antigens are unable to induce antibody responses
`Fig. 9.2 A secondsignalIs required
`for B-cell activation by elther thymus-
`in animals or humans wholack T cells, and they are therefore knownas
`dependentor thymus-independent
`thymus-dependentor TD antigens(Fig. 9.2, top two panels).
`antigens. Thefirst signal required for
`B-cell activation is delivered through
`The B-cell co-receptor complex of CD19:CD21:CD81 (see Section 6-8) can
`its antigen receptor (top panel). For
`greatly enhance B-cell responsiveness to antigen. CD21 (also known as
`thymus-dependent antigens, the second
`complementreceptor 2, CR2) is a receptor for the complement fragment
`signal is delivered by a helper T cell that
`C3d (see Section 2-11), When mice are immunized with hen egg lysozyme
`recognizes dagraded fragments of the
`coupled to three linked molecules of the complementfragment C3dg, the
`antigen as peptides bound to MHC class
`Il molecules on the B-cell surface
`modified lysozyme induces antibody without added adjuvant at doses up to
`10,000 times smaller than unmodified hen egg lysozyme. Whetherbindingof
`(center panel); the interaction between
`CD21 enhances B-cell responsiveness by increasing B-cell signaling, by
`CD40ligand (CD40L) on the T cell and
`CD40 onthe B cell contributes an
`inducing co-stimulatory moleculesontheBcell, or by increasing the receptor-
`essential part of this second signal.
`Mediated uptake of antigen, is not yet known. As wewill see later in this
`For thymus-independent antigens, the
`chapter, antibodies already bound to antigenscan activate the complement
`second signal can be delivered by the
`System, thus coating the antigen with C3d and producing a more potent
`antigen itself (lower panel), or by non-
`antigen, which in turn leads to moreefficient B-cell activation and antibody
`thymus-derived accessory cells (not
`Production.
`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 Bea
`responsesto protein antigens, many microbial constituents, suchas bacteria
`polysaccharides, can induce antibody productionin the absence of helpac
`T cells, These microbial antigens are known as thymus-independent or TL
`antigens because they induce antibody responsesin individuals who haven.
`T lymphocytes, The second signal required to activate antibody Production.
`toT! antigensis eitherprovideddirectly by recognition ofacommon Microbja
`constituent(see Fig. 9.2, bottom panel) or by a nonthymus-derived accesso,
`cell in conjunction with massive cross-linking of B-cell receptors, which
`would occur when aBcell binds repeating epitopes on the bacterial cell.
`Thymus-independent antibody responses provide someprotection againge
`extracellular bacteria, and we will return to them later.
`
`t¢v
`
`
`
`9-2. Armed helperTcells activate B cells that recognize the same
`antigen.
`
`derived from viral proteins,including must recognize epitopes of the same
`
`T-cell dependentantibodyresponsesrequire the activationof B cells by helper
`T cells that respond to the sameantigen;thisis called linked recognition. This
`meansthat before B cells can be induced to make antibody to an infecting
`pathogen, a CD4T cell specific for peptides from this pathogen mustfirst be
`activated to produce the appropriate armed helperT cells. This presumably
`occurs by interaction with an antigen-presenting dendritic cell (see Section
`8-1). Although the epitope recognized by the armed helperT cell mustthere-
`fore be linked to that recognized by theBcell, the twocells need notrecognize
`identical epitopes, Indeed, we saw in Chapter 5 that Tcells can recognize
`internal peptides thatare quite distinctfrom the surface epitopes on the same
`protein recognized by B cells. For more complex natural 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 bea physical part
`ofthe antigen recognized bytheB cell, which can thus produce the appropriate
`peptideafter internalizationof the antigen boundto its B-cell receptors.
`For example, by recognizing an epitope on a viral pratein coat, a B cell can
`internalize a completevirus particle. After internalization,thevirusparticleis
`degradedand peptides from internal viral proteins as well as coat proteins
`can be displayed by MHCclassII molecules on the B-cell surface. Helper
`T cells that have been primedearlier in an infection by macrophages or
`dendritic cells presenting these internal peptides can thenactivate the B cell
`to make antibodies that recognize the coat protein (Fig. 9.3).
`The specific activation oftheB cell bya T cell sensitized to the same antigen
`or pathogen dependsontheability of the antigen-specific B cell to concentrate
`the appropriate peptide on its surface MHC class II molecules. B cells that
`bind a particular antigen are up to 10,000 times moreefficient at displaying
`peptide fragmentsof that antigen on their MHCclassII molecules than are
`B cells that do not bind the antigen. Armed helperT cells will thus help only
`those B cells whose receptors bind an antigen containing the peptide they
`recognize.
`
`i i ¢
`
`|dbe s
`
`|n h
`
`ia tT L dJ i
`
`jq 4[1v Y|
`
`SSeeer
`
`
`Fig. 9.3 B cells and helper T cells
`internal proteins, are returned to the
`molecular complex in order to
`B-cell surface bound to MHCclass|!
`interact. An epitope on a viral coat
`molecules (see Chapter 5). Here, these
`protein is recognized by the surface
`complexes are recognized by helper T
`immunoglobulin on aBcell and the virus
`cells, which help to activate the B cells to
`is internalized and degraded. Peptides
`produce antibady against the coatprotein.
`
`Lassen - Exhibit 1040, p. 8
`
`3t
`
`Lassen - Exhibit 1040, p. 8
`
`

`

` ' 1 a
`
`B-cell activation by armed helperT cells
`
`
`
`Lassen - Exhibit 1040, p. 9
`
`
`
`‘fia. 9-4 Protein antigens attached to
`polysaccharide antigensallow T cells
`4o help polysaccharide-specific
`‘gcells. Haemophilus influenzaetype
`“p vaccine is a conjugateof bacterial
`0 glysaccharide and the tetanus toxoid
`j notein. The B ceil recognizes and binds
`the polysaccharide,Intemalizes and
`degrades the whole conjugate and then
`
`displays toxoid-derived peptides on
`surface MHC classII molecules. Helper
`T calls generated in responseto 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.
`
`The requirement for linked recognition has important consequences for
`the regulation and manipulation of the humoral immuneresponse. Oneis
`that linked recognition helps ensureself tolerance, as will be described in
`Chapter 13. An importantapplication oflinked recognition is in the design of
`vaccines, such as that used to immunize infants against Haemophilus
`influenzaetype B. This bacterial pathogen can infect the lining ofthe 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 responsesto these polysaccharide antigens,
`such responses are weak in the immature immunesystem of the infant. To
`make an effective vaccine for use in infants, therefore, the polysaccharideis
`linked chemically to tetanus toxoid, a foreign protein against which infants
`are routinely and successfully vaccinated (see Chapter14). B cells that bind
`the polysaccharide component of the vaccine can be activated by helper
`T cells specific for peptides of the linked toxoid (Fig. 9.4).
`
`Linked recognition wasoriginally discovered through studies of the production
`of antibodies to haptens (see Appendix I, Section A-1). Haptens are small
`chemical groups that cannotelicit antibody responses on their own because
`they cannotcross-link B-cell receptors and they cannot recruit T-cell help.
`When coupled at high density to a carrier protein, however, they become
`immunogenic, because the protein will carry multiple hapten groups that
`can nowcross-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
`tesponse, as wewill learn in Chapter12.
`
`$3
`
`Antigenic peptides boundto self MHCclassIl moleculestrigger
`armed helper T cells to make membrane-bound and secreted
`moleculesthat can activate a B cell.
`
`Armed helper T cells activate B cells when they recognize the appropriate
`Peptide:MHCclass II complex on the B-cell surface (Fig. 9.5), As with armed
`Tal cells acting on macrophages, recognition of peptide:MHC class II
`Complexes on B cells triggers armed helper T cells to synthesize both cell-
`ound and secreted effector molecules that synergizein 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
`CD40L, also knownas CD154) becauseit binds to the B-cell surface molecule
`CD40, CD40 is a memberof the TNF-receptorfamily of cytokine receptors
`(See Section 8-20) however, it does not contain a ‘death domain.’ It is
`Wolved in directing all phases of the B-cell response. Binding of CD40 by
`CDao. helps to drive the resting B cell into the cell cycle andis essential for
`“cell responses to thymus-dependentantigens.
`
`Lassen - Exhibit 1040, p. 9
`
`

`

` Fig. 9.5 Armed helperT cells stimulate the proliferation and
`
`
`
`then the differentiation of antigen-binding B cells. The
`specific interaction of an antigen-binding B cell with an armed
`helper T cell leads to the expression of the B-cell stimulatory
`
`molecule CD40ligand (CD40L) on the helper T-cell surface ang
`to the secretion of the B-cell stimulatory cytokinesIL-4, IL-5, ang
`IL-6, which drive the proliferation and differentiation of the B cel
`into antibody-secreting plasmacells.
`
`
`
`
`
`B cells are stimulated to proliferate in vitro whenthey are exposed to a mixture
`ofartificially synthesized CD40L andthe cytokine interleukin-4 (IL-4). IL-4 is
`
`also made by armed Ty,2 cells when they recognize their specific ligand on
`
`the B-cell surface, and IL-4 and CD40L are thoughtto synergize in driving the
`
`clonal expansion that precedes antibody production invivo. IL-4 is secreted
`
`in a polar fashion by the T},2 cell andis directed atthe site of contact with the
`B cell (Fig. 9.6) so that it acts selectively on the antigen-specitic target B cell.
`
`
`Fig. 9.6 When an armed helperT cell
`encounters an antigen-binding B cell,
`
`it becomespolarized and secretes
`
`IL-4 and other cytokinesat 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
`
`CD40ligand (CD40L), which binds to
`
`CD40 on theBcell. As shownin the top
`left panel, the tight junction formed
`cyloskeletal
`betweenthe cells upon antigen-specific
`
`binding seemsto be sealed byaring of
`
`adhesion molecules, with LFA-1 on the
`
`T cell interacting with ICAM-1 on the
`
`B ceil (see Fig. 8.30). The cytoskeleton
`
`becomespolarized, as revealed by the
`
`relocation of the cytoskeletal protein
`
`talin (stained redin right center panel},
`
`to the point of cell-cell contact, and the
`
`secretory apparatus (the Golgi
`
`apparatus)is reoriented by the cyto-
`
`skeleton 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
`
`showsIL-4 (stained green) confined to
`
`the space betweentheBcell and the
`
`helper T cell. MTOC, microtubule-
`
`organizing center. Photographs
`courtesy of A. Kupfer.
`
`protein talin
`
`
`
`
`
`Lassen - Exhibit 1040, p.
`
`10
`
`Lassen - Exhibit 1040, p. 10
`
`

`

`ind
`ell
`
`ire
`tis
`on
`he
`ed
`he
`all.
`
`B-cell activation by armedhelperT cells
`
`The combination of B-cell receptor and CD40ligation, 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 roundsofproliferation, B cells can furtherdifferentiate into antibody-
`secreting plasmacells. 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
`
`Isotype switching requires expression of CD40L bythe helperT cell
`and is directed by cytokines.
`
`Antibodies are remarkable notonlyforthe diversity of their antigen-binding
`sites but also for their versatility as effector molecules. The specificity of an
`antibody responseis determinedby the antigen-bindingsite, which consists
`of the two variable V domains, V;; and V,; however, the effector action of the
`antibody is determinedbythe isotypeof 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 throughthe processof 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 rearrangementsthat
`underlie isotype switching and confer this functional diversity on the
`humoral immune response are directed by cytokines, especially those
`released by armedeffector CD4T cells.
`All naiveB cells express cell-surface IgM and IgD, yet IgM makesupless than
`10% of the immunoglobulin found in plasma, where the most abundant
`isotype is IgG. Muchof the antibody in plasma has therefore been produced
`by B cells that have undergone isotype switching. Little IgD antibodyis
`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 importantpartof the
`response. The overall predominanceof IgG results, in part, from its longer
`lifetime in the plasma(see Fig. 4.16),
`
`Immunodeficiency
`
`Hyper IgM
`
`Isotype 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-dependentantigens and have abnormally highlevels 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 wewill
`see in Chapter11.
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`Most of what is known aboutthe regulation of isotype switching by helper
`T cells has come from experiments in which mouseB cells are stimulated
`with bacterial lipopolysaccharide (LPS) and purified cytokines in vitro. These
`experiments show thatdifferent cytokines preferentially induce switching to
`different isotypes. Some of these cytokines are the sameas those that drive
`B-cell proliferation in the initiation of a B-cell response. In the mouse,IL-4
`preferentially induces switching to IgGl and IgE, whereas transforming
`growth factor (TGF)-B induces switching to IgG2b and IgA. Ty2 cells make
`both of these cytokines as well as IL-5, which inducesIgA secretion by cells
`that have already undergone switching. Although Tj] cells are relatively poor
`initiators of antibody responses, they participate in isotype switching by
`releasing interferon (IFN)-y, which preferentially induces switching to IgG2a
`and IgG3. Therole of cytokines in directing B cells to make the different
`antibodyisotypes is summarizedin Fig. 9.7.
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`Lassen - Exhibit 1040, p. 11
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`Lassen - Exhibit 1040, p. 11
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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. Muchof the inhibitory effectis
`probably the result of directed switching
`to a different isotype. These data are
`drawn from experiments with mouse
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`Chapter9: The Humoral Immune Response
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`Cytokines induceisotype switching by stimulating the formation and splicing
`of mRNAtranscribed from the switch recombination sites thatlie 5’ to each
`heavy-chain C gene(see Fig. 4,20). When activated B cells are exposed to [L-4,
`for example, transcription froma site upstream of the switch regions of Cy
`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 mechanismis notyetclear. Eachofthe cytokines
`that induces switching seemsto induce transcription from the switch regions
`of two different heavy-chain C genes, promoting specific recombination to
`oneorother of these genes only. Such a directed mechanism is supported by
`the observation thatindividual B cells frequently undergo switching to the
`same C gene on both chromosomes, even thoughthe antibody heavy chainis
`only being expressed from one of the chromosomes. Thus, helper Tcells
`regulate both the production of antibody by B cells and the isotype that
`determinesthe effector function of the antibody,
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`Fig. 9.8 Isotype switching is preceded
`by transcriptional activation of heavy-
`chain C-region genes.Resting naive
`B cells transcribe the j, and 8 genes at a
`low rate, giving rise to surface IgM and
`IgD. Bacterial lipopolysaccharide (LPS),
`which can activate B cells independently
`of antigen, induces IgM secretion. in the
`presenceof IL-4, however, Cy; and Ce
`are transcribed ata low rate, presaging
`switches to IgG1 and IgE production.
`Thetranscripts originate before the
`5’ endof the region to which switching
`occurs, and do not codefor protein.
`Simllarly, TGF-B gives rise to Cyzp and
`Ca transcripts and drives switching to
`igG2b andIgA.It is not known what
`determines which ofthe twotrans-
`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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`B-cell activation by armed helperT cells
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`9-5
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` Antigen-binding B cells are trappedin the T-cell zone of secondary
`lymphoid tissues and are activated by encounter with armed helper
`T cells.
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`Oneof the most puzzling features of the antibody responseis how an antigen-
`specific B cell manages to encountera 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 | 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 I in 108 and 1 in 10!*. Achieving such an encounteris a far more
`difficult challenge than getting effector T cells activated, because,in the latter
`case, only one of the twocells 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 Chapter8), the answer
`seemstolie in the antigen-specific trapping of migrating lymphocytes.
`Whenan antigen is introduced into an animal, it is captured and processed
`by professional antigen-presentingcells, 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
`veryefficiently. This trapping clearly involves the specific antigen receptor on
`the T cell, althoughit is stabilized by the activation of adhesion molecules
`and chemokinesas we learnedin 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 lymphoidtissue by a similar mechanism.
`On encountering antigen, migrating antigen-bindingBcells are arrested by
`the activation of adhesion molecules and the engagement of chemokine
`receptors such as CCR7, a receptor for MIP-3B 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 andactivated to helper status in the T-cell zones, and whenB cells
`migrate into lymphoid tissue through high endothelial venules theyfirst
`enter these same T-cell zones. Most ofthe B cells move quickly through the
`T-cell zone into the B-cell zone (the primary follicle), but those B cells that
`have boundantigen are trapped. Thus, antigen-bindingB cells are selectively
`trappedin 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
`lymphocytewill proliferate for several days to constitute the first phase of the
`primary humoral immuneresponse.
`After several days, the primary focusofproliferation beginsto involute. Many
`of the lymphocytes comprising the focus undergo apoptosis. However, some
`of the proliferating B cells differentiate into antibody-synthesizing plasma
`cells and migrate to the red pulp ofthe spleen or the medullary cords ofthe
`lymph node. The differentiation of a B cell into a plasmacell is accompanied
`by many morphological changes thatreflectits commitmentto the production
`of large amounts ofsecreted antibody. The properties of resting B cells and
`Plasmacells are compared in Fig, 9.10. Plasma cells have