Chapter 4+5: Antigen Recognition in the Adaptive Immune System PDF

CHAPTER 4 Antigen Recognition in the Adaptive Immune System Structure of Lymphocyte Antigen Receptors and Development of Immune Repertoires ANTIGEN RECEPTORS OF LYMPHOCYTES, 81 Production of Diverse Antigen Receptors, 93 Antibodies, 82...

CHAPTER 4 Antigen Recognition in the Adaptive Immune System Structure of Lymphocyte Antigen Receptors and Development of Immune Repertoires ANTIGEN RECEPTORS OF LYMPHOCYTES, 81 Production of Diverse Antigen Receptors, 93 Antibodies, 82 Maturation and Selection of B Lymphocytes, 96 T Cell Receptors for Antigens, 89 Maturation and Selection of T Lymphocytes, 99 DEVELOPMENT OF IMMUNE REPERTOIRES, 90 SUMMARY, 101 Lymphocyte Development, 91 Antigen receptors serve critical roles in the mat- and has a unique receptor, different from the uration of lymphocytes from progenitors and receptors of all other clones. (Recall that a clone in all adaptive immune responses. In adaptive consists of a parent cell and its progeny.) The immunity, naive lymphocytes recognize anti- total number of distinct lymphocyte clones is gens to initiate responses, and effector T cells and very large, and this entire collection makes up antibodies recognize antigens to perform their the immune repertoire. Although each clone functions. of B lymphocytes or T lymphocytes recognizes a B and T lymphocytes express different different antigen, the antigen receptors transmit receptors that recognize antigens: mem- biochemical signals that are fundamentally the brane-bound antibodies on B cells and T same in all lymphocytes and are unrelated to cell receptors (TCRs) on T lymphocytes. specificity. These features of lymphocyte recog- The principal function of cellular receptors in nition and antigen receptors raise the following the immune system, as in other biologic sys- questions: tems, is to detect external stimuli (antigens, for How do the antigen receptors of lymphocytes the antigen receptors of the adaptive immune recognize extremely diverse antigens and system) and trigger responses of the cells on transmit activating signals to the cells? which the receptors are expressed. To recognize What are the differences in the recognition a large variety of different antigens, the antigen properties of antigen receptors on B cells and receptors of lymphocytes must be able to bind T cells? to and distinguish between many, often closely How is the vast diversity of receptor struc- related, chemical structures. Antigen receptors tures in the lymphocyte repertoire generated? are clonally distributed, meaning that each lym- The diversity of antigen recognition implies phocyte clone is specific for a distinct antigen the existence of many structurally different 79 80 CHAPTER 4 Antigen Recognition in the Adaptive Immune System B cell receptor T cell (antibody, Ig) receptor (TCR) Antigen presenting cell Membrane Ig Antigen MHC Antigen Igα Igβ TCR CD3 ζ Signal Signal transduction transduction Effector functions: complement fixation, Secreted phagocyte antibody binding Forms of Macromolecules (proteins, Mainly peptides displayed antigens polysaccharides, lipids, by MHC molecules recognized nucleic acids), on APCs small chemicals Conformational and Linear epitopes linear epitopes Diversity Each clone has a unique Each clone has a unique specificity; potential for >109 specificity; potential for distinct specificities >1011 distinct specificities Antigen Variable (V) regions of Variable (V) regions of recognition is heavy and light chains α and β chains of the TCR mediated by: of membrane Ig Signaling Proteins (Igα and Igβ) Proteins (CD3 and ζ) functions are associated with associated with the TCR mediated by: membrane Ig Effector Constant (C) regions of TCR does not perform functions are secreted Ig effector functions mediated by:  Antigen Receptors of Lymphocytes 81 FIGURE 4-1 Properties of antibodies and T cell antigen receptors (TCRs). Antibodies (also called immuno- globulins) may be expressed as membrane receptors or secreted proteins; TCRs only function as membrane receptors. When immunoglobulin (Ig) or TCR molecules recognize antigens, signals are delivered to the lympho- cytes by proteins associated with the antigen receptors. The antigen receptors and attached signaling proteins form the B cell receptor (BCR) and TCR complexes. Note that single antigen receptors are shown recognizing antigens, but signaling typically requires the binding of two or more receptors to adjacent antigen molecules. The important characteristics of these antigen-recognizing molecules are summarized. APCs, Antigen-presenting cells; Ig, immunoglobulin; MHC, major histocompatibility complex. antigen receptor proteins, more than can be types of molecules enables antibodies to rec- encoded in the inherited genome (germline). ognize diverse microbes and toxins in their Therefore, special mechanisms must exist for native form. In striking contrast, most T generating this diversity. cells see only peptides displayed on antigen- In this chapter, we describe the structures of presenting cells (APCs) bound to MHC mol- the antigen receptors of B and T lymphocytes and ecules. This specificity of T cells restricts their how these receptors recognize antigens. We also recognition to only cell-associated microbes discuss how the diversity of antigen receptors (see Chapter 3). is generated during the process of lymphocyte Antigen receptor molecules consist of development, thus giving rise to the repertoire regions (domains) involved in antigen of mature lymphocytes. The process of antigen- recognition—therefore varying between induced lymphocyte activation is described in clones of lymphocytes—and other regions later chapters. required for structural integrity and effec- tor functions—thus relatively conserved among all clones. The antigen-recognizing ANTIGEN RECEPTORS OF LYMPHOCYTES domains of the receptors are called variable The antigen receptors of B and T lymphocytes (V) regions, and the conserved portions are have several features that are important for the constant (C) regions. Even within each their functions in adaptive immunity (Fig. 4-1). V region, most of the sequence variability is Although these receptors have many similarities concentrated within short stretches, which in terms of structure and mechanisms of signal- are called hypervariable regions, or comple- ing, there are fundamental differences related to mentarity-determining regions (CDRs), be- the types of antigenic structures that B cells and cause they form the parts of the receptor that T cells recognize. bind antigens (i.e., they are complementary to Membrane-bound antibodies, which serve the shapes of antigens). By concentrating se- as the antigen receptors of B lymphocytes, quence variation in small regions of the recep- can recognize many types of chemical tor, it is possible to maximize the variability of structures, while most T cell antigen re- the antigen-binding part, while retaining the ceptors recognize only peptides bound to basic structure of the receptors. As discussed major histocompatibility complex (MHC) later, special mechanisms exist in develop- molecules. B lymphocyte antigen receptors ing lymphocytes to create genes that encode and the antibodies that B cells secrete are able different variable regions of antigen receptor to recognize the shapes, or conformations, of proteins in individual clones. macromolecules, including proteins, lipids, Antigen receptor chains are associated carbohydrates, and nucleic acids, as well as with invariant membrane proteins whose simpler, smaller chemical moieties. This broad function is to deliver intracellular signals specificity of B cells for structurally different following antigen recognition (see Fig. 4-1). 82 CHAPTER 4 Antigen Recognition in the Adaptive Immune System These signals, which are transmitted to the cy- on phagocytes and proteins of the comple- tosol and the nucleus, may cause a lympho- ment system. Thus, antibodies serve different cyte to divide, to differentiate, or in certain functions at different stages of humoral im- circumstances to die. Thus, the two functions mune responses: membrane-bound antibod- of lymphocyte receptors for antigen—specific ies on B cells recognize antigens to initiate the antigen recognition and signal transduction— responses, and secreted antibodies neutralize are mediated by different polypeptides. This and eliminate microbes and their toxins in again allows variability to be segregated in one the effector phase of humoral immunity. In set of molecules—the receptors themselves— cell-mediated immunity, the effector func- while leaving the conserved function of signal tion of microbe elimination is performed by T transduction in other, invariant proteins. The lymphocytes themselves and by other leuko- set of associated plasma membrane antigen cytes responding to the T cells. The antigen re- receptor and signaling molecules in B lym- ceptors of T cells are involved only in antigen phocytes is called the B cell receptor (BCR) recognition and T cell activation, and these complex, and in T lymphocytes it is called proteins are not secreted and do not mediate the T cell receptor (TCR) complex. When effector functions. antigen molecules bind to antigen receptors With this introduction, we describe next the of lymphocytes, the associated signaling pro- antigen receptors of lymphocytes, first antibodies teins of the receptor complexes are brought and then T cell receptors. into proximity. As a result, enzymes attached to the cytoplasmic portions of the signaling Antibodies proteins catalyze the phosphorylation of other An antibody molecule is composed of proteins. Phosphorylation triggers complex four polypeptide chains—two identical signaling cascades that culminate in the tran- heavy (H) chains and two identical light scriptional activation of many genes and the (L) chains—with each chain containing a production of numerous proteins that mediate variable region and a constant region (Fig. the responses of the lymphocytes. We return 4-2). The four chains are assembled to form a to the processes of T and B lymphocyte activa- Y-shaped molecule. Each light chain is attached tion in Chapters 5 and 7, respectively. to one heavy chain, and the two heavy chains are Antibodies exist in two forms—as mem- attached to each other, all by disulfide bonds. A brane-bound antigen receptors on B cells light chain is made up of one V and one C domain, or as secreted proteins—but TCRs exist and a heavy chain has one V and three or four only as membrane receptors on T cells. C domains. Each domain folds into a character- Secreted antibodies are present in the blood istic three-dimensional shape, called the immu- and mucosal secretions, where they function noglobulin (Ig) domain (see Fig. 4-2, D). An Ig to defend against microbes (i.e., they are the domain consists of two layers of a β-pleated sheet effector molecules of humoral immunity). held together by a disulfide bridge. The adjacent Antibodies are also called immunoglobulins strands of each β-sheet are connected by short, (Igs), referring to immunity-conferring proteins protruding loops; in the V regions of Ig molecules, with the characteristic slow electrophoretic these loops make up the three CDRs responsible mobility of globulins. Secreted antibodies for antigen recognition. Ig domains are present recognize microbial antigens and toxins by in many other proteins in the immune system, their variable domains, the same as the mem- as well as outside the immune system, and most brane-bound antigen receptors of B lympho- of these proteins are involved in responding to cytes. The constant regions of some secreted stimuli from the environment and from other antibodies have the ability to bind to other cells. All of these proteins are said to be mem- molecules that participate in the elimination bers of the immunoglobulin superfamily, and of antigens: these molecules include receptors A Secreted IgG B Membrane IgM Antigen- binding site Heavy Antigen- chain binding site N N N VH N N N S S VH S S N N S S VL S S CH1 S S S S S S S S VL S S S S CH1 S S CL S S S S Light S S S S chain CL S S Fab C C region S S Hinge S S CH2 Fc receptor/ S S complement S S CH2 binding sites Fc S S region S C 3 S H S S S S CH3 S S S S CH4 Tail piece C C Plasma membrane S S of B cells Disulfide bond Ig domain S S C C C Crystal structure of secreted IgG D Complementarity N determining region (CDR) loops VL CL 1 7 2 VH S 6 S 5 3 4 CH1 CH2 3b 3c C H3 C FIGURE 4-2 Structure of antibodies. Schematic diagrams of A, a secreted immunoglobulin G (IgG) molecule, and B, a molecule of a membrane-bound form of IgM, illustrating the domains of the heavy and light chains and the regions of the proteins that participate in antigen recognition and effector functions. N and C refer to the amino-terminal and carboxy-terminal ends of the polypeptide chains, respectively. C, The crystal structure of a se- creted IgG molecule illustrates the domains and their spatial orientation; the heavy chains are colored blue and red, the light chains are green, and carbohydrates are gray. D, The ribbon diagram of the Ig V domain shows the basic β-pleated sheet structure and the projecting loops that form the three CDRs. CDR, Complementarity determining region. (C, Courtesy of Dr. Alex McPherson, University of California, Irvine.) 84 CHAPTER 4 Antigen Recognition in the Adaptive Immune System they may have evolved from a common ances- humans, there are four subtypes of γ chain and tral gene. two of the α chain. Antibodies that contain dif- The antigen-binding site of an antibody ferent heavy chains belong to different classes, is composed of the V regions of both the or isotypes, and are named according to their heavy chain and the light chain, and the heavy chains (IgM, IgD, IgG, IgE, and IgA). Each core antibody structure contains two iden- isotype has distinct physical and biologic proper- tical antigen binding sites (see Fig. 4-2). Each ties and effector functions (Fig. 4-3). The anti- variable region of the heavy chain (called VH) gen receptors of naive B lymphocytes, which are or of the light chain (called VL) contains three mature B cells that have not encountered anti- hypervariable regions, or CDRs. Of these three, gen, are membrane-bound IgM and IgD. After the greatest variability is in CDR3, which is stimulation by antigen and helper T lympho- located at the junction of the V and C regions. As cytes, the antigen-specific B lymphocyte clone may be predicted from this variability, CDR3 is may expand and differentiate into progeny that also the portion of the Ig molecule that contrib- secrete antibodies. Some of the progeny of IgM utes most to antigen binding. and IgD expressing B cells may secrete IgM, and Functionally distinct portions of antibody other progeny of the same B cells may produce molecules were first identified based on frag- antibodies of other heavy-chain classes. This ments generated by proteolysis. The fragment change in Ig isotype production is called heavy- of an antibody that contains a whole light chain chain class (or isotype) switching; its mecha- (with its single V and C domains) attached to the nism and importance are discussed in Chapter V and first C domains of a heavy chain contains 7. Although heavy-chain C regions may switch the portion of the antibody required for antigen during humoral immune responses, each clone recognition and is therefore called Fab (fragment, of B cells maintains its specificity, because the V antigen-binding). The remaining heavy-chain C regions do not change. domains make up the Fc (fragment, crystalline) The two types of light chains, called κ and λ, region; this fragment tends to crystallize in solu- differ in their C regions. Each B cell expresses tion. In each Ig molecule, there are two identical either κ or λ but not both. Each type of light chain Fab regions that bind antigen and one Fc region may complex with any type of heavy chain in an that is responsible for most of the biologic activity antibody molecule, but unlike the heavy chains, and effector functions of the antibodies. (As dis- the two types of light chains have no functional cussed later, some types of antibodies exist as mul- differences. The light-chain class (κ or λ) also timers of two or five Ig molecules attached to one remains fixed throughout the life of each B cell another.) Between the Fab and Fc regions of most clone, regardless of whether or not heavy-chain antibody molecules is a flexible portion called the class switching has occurred. hinge region. The hinge allows the two antigen- binding Fab regions of each antibody molecule to Binding of Antigens by Antibodies move independent of each other, enabling them Antibodies are capable of binding a wide to simultaneously bind antigen epitopes that are variety of antigens, including macromole- separated from one another by varying distances. cules and small chemicals. The reason for this The C-terminal end of the heavy chain may is that the antigen-binding CDR loops of antibody be anchored in the plasma membrane, as seen molecules can either come together to form clefts in B cell receptors, or it may terminate in a tail capable of accommodating small molecules or piece that lacks the membrane anchor so that the form more extended surfaces capable of accom- antibody is produced as a secreted protein. Light modating many larger molecules, including por- chains in Ig molecules are not attached to cell tions of proteins (Fig. 4-4). Antibodies bind to membranes. antigens by reversible, noncovalent interactions, There are five types of heavy chains, called µ, including hydrogen bonds, hydrophobic interac- δ, γ, ε, and α, which differ in their C regions; in tions, and charge-based interactions. The parts  Antigen Receptors of Lymphocytes 85 Isotype Serum Serum of Subtypes concentration half-life Secreted form Functions antibody (H chain) (mg/ml) (days) IgA IgA1,2 3.5 6 Mainly dimer, also Mucosal immunity (α1 or α2) monomer, Cα1 trimer Cα2 Cα3 J chain IgD None Trace 3 Monomer Naive B cell (δ) antigen receptor IgE None 0.05 2 Monomer Cε1 Defense against (ε) helminthic parasites, immediate Cε2 hypersensitivity Cε3 Cε4 IgG IgG1-4 13.5 23 Monomer Opsonization, (γ1, γ2, γ3 Cγ1 complement or γ4) activation, antibody- dependent cell- Cγ2 mediated cytotoxicity, Cγ3 neonatal immunity, feedback inhibition of B cells IgM None 1.5 5 Pentamer Cµ1 Cµ3 Naive B cell antigen (µ) Cµ4 receptor (monomeric Cµ2 form), complement activation J chain FIGURE 4-3 Features of the major isotypes (classes) of antibodies. This figure summarizes some important features of the major antibody isotypes of humans. Isotypes are classified on the basis of their heavy (H) chains; each isotype may contain either κ or λ light chain. The schematic diagrams illustrate the distinct shapes of the se- creted forms of these antibodies. Note that IgA consists of two subclasses, called IgA1 and IgA2, and IgG consists of four subclasses, called IgG1, IgG2, IgG3, and IgG4. Most of the opsonizing and complement fixation functions of IgG are attributable to IgG1 and IgG3. The domains of the heavy chains in each isotype are labeled. The plasma concentrations are average values in normal individuals. Ig, Immunoglobulin.  Antigen Receptors of Lymphocytes 89 antibodies. Monoclonal antibodies are now in Antigen Recognition by the TCR widespread use as therapeutic agents for many Both the α chain and the β chain of the TCR diseases in humans (Fig. 4-6). participate in specific recognition of MHC molecules and bound peptides (Fig. 4-8). T Cell Receptors for Antigens One of the remarkable features of T cell antigen The TCR, which recognizes peptide antigens recognition that has emerged from x-ray crystal- displayed by MHC molecules, is a membrane- lographic analyses of TCRs bound to MHC-peptide bound heterodimeric protein composed of complexes is that each TCR recognizes as few as an α chain and a β chain, each chain contain- one to three residues of the MHC-associated peptide. ing one variable (V) region and one constant The TCR recognizes antigen, but as with mem- (C) region (Fig. 4-7). The V and C regions are brane Ig on B cells, it is incapable of transmit- homologous to immunoglobulin V and C regions. ting signals to the T cell on its own. Associated In the V region of each TCR chain, there are three with the TCR is a complex of proteins, called the hypervariable, or complementarity-determining, CD3 and ζ proteins, which together with the TCR regions, each corresponding to a loop in the V make up the TCR complex (see Fig. 4-1). The CD3 domain. As in antibodies, CDR3 is the most vari- and ζ chains transmit some of the signals that able among different TCRs. are initiated when the TCR recognizes antigen. β chain N N α chain S S Vβ Vα Vβ Vα S S S S Cβ Cα S S Cβ S S Cα C C Disulfide bond S S Transmembrane S region Ig domain S Carbohydrate group FIGURE 4-7 Structure of the T cell antigen receptor (TCR). The schematic diagram of the αβ TCR (left) shows the domains of a typical TCR specific for a peptide-MHC complex. The antigen-binding portion of the TCR is formed by the Vα and Vβ domains. N and C refer to the amino-terminal and carboxy-terminal ends of the polypeptides. The ribbon diagram (right) shows the structure of the extracellular portion of a TCR as revealed by x-ray crystallog- raphy. Ig, Immunoglobulin; MHC, major histocompatibility complex. (From Bjorkman PJ: MHC restriction in three dimensions: a view of T cell receptor/ligand interactions. Cell 89:167-170, 1997. © Cell Press; with permission.)  Summary 101 these cells are called double-positive T cells not to enter the thymus.) The likely explanation (or double-positive thymocytes). for these distinct outcomes is that if the antigen Selection of Mature T Cells. Different clones of receptor of a T cell recognizes a self MHC–self double-positive T cells express different αβ TCRs. peptide complex with low avidity, the result is If the TCR of a T cell recognizes an MHC mol- positive selection, whereas high-avidity recogni- ecule in the thymus, which must be a self MHC tion leads to negative selection. High-avidity rec- molecule displaying a self peptide, and if the ognition occurs if the T cell expresses a TCR that interaction is of low or moderate affinity, this has a high affinity for that self peptide and if the T cell is selected to survive. T cells that do not self peptide is present in the thymus at a higher recognize an MHC molecule in the thymus die concentration than positively selecting peptides. by apoptosis; these T cells would not be useful If such a T cell were allowed to mature, anti- because they would be incapable of seeing MHC- gen recognition could lead to harmful immune displayed cell-associated antigens in that indi- responses against the self antigen in the periph- vidual. This preservation of self MHC–restricted ery, so the T cell must be eliminated. (i.e., useful) T cells is the process of positive As with B cells, the ability of T cells to rec- selection. During this process, T cells whose ognize foreign antigens relies on the generation TCRs recognize class I MHC–peptide complexes of a very diverse repertoire of clonal antigen preserve the expression of CD8, the coreceptor receptors. T cells that weakly recognize self that binds to class I MHC, and lose expression antigens in the thymus may strongly recognize of CD4, the coreceptor specific for class II MHC and respond to foreign microbial antigens in the molecules. Conversely, if a T cell recognizes class periphery. II MHC–peptide complexes, this cell maintains expression of CD4 and loses expression of CD8. SUMMARY Thus, what emerges are single-positive T cells (or single-positive thymocytes), which are either In the adaptive immune system, the molecules CD8+ class I MHC restricted or CD4+ class II MHC responsible for specific recognition of antigens restricted. During positive selection, the T cells are antibodies and T cell antigen receptors. also become functionally segregated: the CD8+ T Antibodies (also called immunoglobulins) may cells are capable of becoming CTLs on activation, be produced as membrane receptors of B lym- and the CD4+ cells are helper cells. phocytes and as proteins secreted by antigen- Immature, double-positive T cells whose recep- stimulated B cells that have differentiated into tors strongly recognize MHC-peptide complexes antibody-secreting plasma cells. Secreted anti- in the thymus undergo apoptosis. This is the pro- bodies are the effector molecules of humoral cess of negative selection, and it serves to elim- immunity, capable of neutralizing microbes inate T lymphocytes that could react in a harmful and microbial toxins and eliminating them by way against self proteins that are expressed in the activating various effector mechanisms. thymus. Some of these self proteins are present T cell receptors (TCRs) are membrane recep- throughout the body, and others are tissue pro- tors and are not secreted. teins that are expressed in thymic epithelial cells The core structure of antibodies consists of two by special mechanisms, as discussed in Chapter 9 identical heavy chains and two identical light in the context of self-tolerance. chains, forming a disulfide-linked complex. It may seem surprising that both positive Each chain consists of a variable (V) region, selection and negative selection are mediated which is the portion that recognizes antigen, by recognition of the same set of self MHC–self and a constant (C) region, which provides peptide complexes in the thymus. (Note that the structural stability and, in heavy chains, per- thymus can contain only self MHC molecules forms the effector functions of antibodies. The and self peptides; microbial peptides are concen- V region of one heavy chain and of one light trated in peripheral lymphoid tissues and tend chain together form the antigen-binding site, 102 CHAPTER 4 Antigen Recognition in the Adaptive Immune System and thus the core structure has two identical During their maturation, lymphocytes are se- antigen-binding sites. lected to survive at several checkpoints; only T cell receptors consist of an α chain and a cells with complete functional antigen recep- β chain. Each chain contains one V region tors are preserved and expanded. In addition, and one C region, and both chains partici- T lymphocytes are positively selected to rec- pate in the recognition of antigens, which for ognize peptide antigens displayed by self MHC most T cells are peptides displayed by MHC molecules and to ensure that the recognition molecules. of the appropriate type of MHC molecule The V regions of immunoglobulin (Ig) and matches the coreceptor preserved. TCR molecules contain hypervariable seg- Immature lymphocytes that strongly recog- ments, also called complementarity-determin- nize self antigens are negatively selected and ing regions (CDRs), which are the regions of prevented from completing their maturation, contact with antigens. thus eliminating cells with the potential of re- The genes that encode antigen receptors acting in harmful ways against self tissues. consist of multiple segments separated in the germline and brought together during matu- REVIEW QUESTIONS ration of lymphocytes. In B cells, the Ig gene segments undergo recombination as the cells 1. What are the functionally distinct domains (re- mature in the bone marrow, and in T cells, the gions) of antibody and TCR molecules? What TCR gene segments undergo recombination features of the amino acid sequences in these during maturation in the thymus. regions are important for their functions? Receptors of different specificities are gen- 2. What are the differences in the types of anti- erated in part by different combinations of gens recognized by antibodies and TCRs? V, D, and J gene segments. The process of 3. What mechanisms contribute to the diversity recombination introduces variability in the of antibody and TCR molecules? Which of nucleotide sequences at the sites of recom- these mechanisms contributes the most to the bination by adding or removing nucleo- diversity? tides from the junctions. The result of this 4. What are some of the checkpoints during lym- introduced variability is the development phocyte maturation that ensure survival of the of a diverse repertoire of lymphocytes, in useful cells? which clones of cells with different antigen 5. What is the phenomenon of negative selec- specificities express receptors that differ in tion, and what is its importance? sequence and recognition, and most of the differences are concentrated at the regions Answers to and discussion of the Review Questions of gene recombination. are available at https://studentconsult.inkling.com.  CHAPTER 5 T Cell–Mediated Immunity Activation of T Lymphocytes by Cell-Associated Antigens PHASES OF T CELL RESPONSES, 104 Secretion of Cytokines and Expression ANTIGEN RECOGNITION AND COSTIMULATION, 107 of Cytokine Receptors, 117 Recognition of MHC-Associated Peptides, 107 Clonal Expansion, 119 Role of Adhesion Molecules in T Cell Responses, 110 Differentiation of Naive T Cells into Effector Cells, 120 Role of Costimulation in T Cell Activation, 110 Development of Memory T Lymphocytes, 120 Stimuli for Activation of CD8+ T Cells, 112 MIGRATION OF T LYMPHOCYTES IN CELL-MEDIATED BIOCHEMICAL PATHWAYS OF T CELL ACTIVATION, 113 IMMUNE REACTIONS, 121 FUNCTIONAL RESPONSES OF T LYMPHOCYTES TO Decline of the Immune Response, 126 ANTIGEN AND COSTIMULATION, 117 SUMMARY, 126 T lymphocytes perform multiple functions in do not possess intrinsic mechanisms for de- defending against infections by various kinds of stroying the microbes, especially in the cytosol. microbes. A major role for T lymphocytes is in cell- Even some phagocytosed microbes within mac- mediated immunity, which provides defense rophages can escape into the cytosol and evade against infections by intracellular microbes. In sev- the microbicidal mechanisms of the vesicular eral types of infections, microbes may find a haven compartment. T cells kill the infected cells, thus inside cells, from where they must be eliminated eliminating the reservoir of infection. by cell-mediated immune responses (Fig. 5-1). In addition to cell-mediated immunity, T lym- Many microbes are ingested by phagocytes as phocytes also play important roles in defense part of the early defense mechanisms of innate against microbes that replicate outside cells, immunity, but some of these microbes have including several types of bacteria, fungi, and evolved to resist the microbicidal activities of helminthic parasites. Some T cells induce inflam- phagocytes. Many pathogenic intracellular matory responses rich in activated leukocytes bacteria and protozoa are able to survive, and that are particularly efficient at killing extracellu- even replicate, in the vesicles of phagocytes. lar microbes. We discuss these T cell subsets and In such infections, T cells stimulate the ability their functions in Chapter 6. Other populations of of macrophages to kill the ingested microbes. T cells help B cells to produce antibodies as part Some microbes, notably viruses, are able to in- of humoral immune responses (see Chapter 7). fect and replicate inside a wide variety of cells, Most of the functions of T lymphocytes— and parts of the life cycles of the viruses take activation of phagocytes, killing of infected place in the cytosol. These infected cells often cells, and help for B cells—require that the T 103 104 CHAPTER 5 T Cell–Mediated Immunity Intracellular microbes Examples A Phagocyte Phagocytosed microbes Intracellular bacteria: that survive within Mycobacteria phagolysosomes Listeria monocytogenes Microbes that escape Legionella pneumophila from phagolysosomes Fungi: into cytoplasm Cryptococcus neoformans Protozoa: Leishmania Trypanosoma cruzi B Nonphagocytic cell (e.g., epithelial cell) Viruses: All Virus Cellular Rickettsiae: receptor All for virus Protozoa: Plasmodium falciparum Cryptosporidium parvum Microbes that infect nonphagocytic cells FIGURE 5-1 Types of intracellular microbes combated by T cell–mediated immunity. A, Microbes may be ingested by phagocytes and may survive within vesicles (phagolysosomes) or escape into the cytosol, where they are not susceptible to the microbicidal mechanisms of the phagocytes. B, Viruses may infect many cell types, including nonphagocytic cells, and replicate in the nucleus and cytosol of the infected cells. Rickettsiae and some protozoa are obligate intracellular parasites that reside in non-phagocytic cells. lymphocytes interact with other cells, which effector T cells that have specialized functions may be phagocytes, infected host cells, or B and the ability to eliminate diverse microbes? lymphocytes. Furthermore, the initiation of T What molecules are produced by T lympho- cell responses requires that the cells recognize cytes that mediate their communications with antigens displayed by dendritic cells, which other cells, such as macrophages, B lympho- capture antigens and concentrate them in lym- cytes, and other leukocytes? phoid organs. Thus, T lymphocytes work by After describing here how T cells recognize communicating with other cells. Recall that and respond to the antigens of cell-associated the specificity of T cells for peptides displayed microbes, in Chapter 6 we discuss how these T by major histocompatibility complex (MHC) cells function to eliminate the microbes. molecules ensures that the T cells can see and respond only to antigens associated with other cells (see Chapters 3 and 4). This chapter dis- PHASES OF T CELL RESPONSES cusses the way in which T lymphocytes are acti- Naive T lymphocytes recognize antigens vated by recognition of cell-associated antigens in the peripheral (secondary) lymphoid and other stimuli. We address the following organs, which initiates proliferation of the questions: T cells and their differentiation into effec- What signals are needed to activate T lympho- tor and memory cells, and the effector cells cytes, and what cellular receptors are used to perform their functions when they are acti- sense and respond to these signals? vated by the same antigens in peripheral How are the few naive T cells specific for any tissues or lymphoid organs (Fig. 5-2). Naive T microbe converted into the large number of cells express antigen receptors and co-receptors  DC 105 Induction of response CD4+ CD8+ Antigen T cells T cells recognition in lymphoid organs CD8+ T cell CD4+ T cells expansion and effector (CTLs) differentiation T cells Differentiated effector T cells Naive enter circulation T cell Migration of effector T cells and other leukocytes to site of antigen Effector T cells encounter antigens in peripheral tissues Cells with intracellular microbes Activation of Cytokines effector T cells T cell effector functions Leukocyte activation CTL killing of (inflammation); infected cell phagocytosis and killing of microbes FIGURE 5-2 Induction and effector phases of cell-mediated immunity. Induction of response: Naive CD4+ T cells and CD8+ T cells recognize peptides that are derived from protein antigens and presented by dendritic cells (DCs) in peripheral lymphoid organs. The T lymphocytes are stimulated to proliferate and differentiate into effector cells, many of which enter the circulation. Some of the activated CD4+ T cells remain in the lymph node, migrate into follicles, and help B cells to produce antibodies (shown in Fig. 5-13). Migration of effector T cells and other leukocytes to site of antigen: effector T cells and other leukocytes migrate through blood vessels in peripheral tissues by binding to endothelial cells that have been activated by cytokines produced in response to infection in these tissues. T cell effector functions: CD4+ T cells recruit and activate phagocytes to destroy microbes, and CD8+ cytotoxic T lymphocytes (CTLs) kill infected cells. 106 CHAPTER 5 T Cell–Mediated Immunity Antigen Cytokine Proliferation Differentiation Effector functions recognition secretion and cytokine receptor CD4+: Activation of macrophages, expression B cells, other Effector T cell cells (CD4+ helper or CD8+ CTL) CD8+: Killing of infected “target cells”; Naive macrophage APC T cell IL-2R (CD4+ activation or CD8+) Cytokines (e.g., IL-2) Memory T cell (CD4+ or CD8+) Lymphoid organs Peripheral tissues FIGURE 5-3 Steps in the activation of T lymphocytes. Naive T cells recognize major histocompatibility complex (MHC)–associated peptide antigens displayed on antigen-presenting cells and other signals (not shown). The T cells respond by producing cytokines, such as interleukin-2 (IL-2), and expressing receptors for these cytokines, leading to an autocrine pathway of cell proliferation. The result is expansion of the clone of T cells that are spe- cific for the antigen. Some of the progeny differentiate into effector cells, which serve various functions in cell- mediated immunity, and memory cells, which survive for long periods. Other changes associated with activation, such as the expression of various surface molecules, are not shown. APC, Antigen-presenting cell; CTL, cytotoxic T lymphocyte; IL-2R, interleukin-2 receptor. that function in recognizing cells harboring increase in the number of antigen-specific lym- microbes, but these cells are incapable of per- phocytes, a process called clonal expansion. forming the effector functions required for elim- The activated lymphocytes undergo the process inating the microbes. Differentiated effector cells of differentiation, which results in the conver- are capable of performing these functions, which sion of naive T cells into a population of effector they do in lymphoid organs and in peripheral, T cells which function to eliminate microbes. nonlymphoid tissues. In this chapter we focus Many of the effector T cells leave the lym- on the responses of naive T cells to antigens. phoid organs, enter the circulation, and mi- The development of effector T lymphocytes and grate to any site of infection, where they can their functions in cell-mediated immunity are eradicate the infection. Some effector T cells described in Chapter 6, and the roles of helper T may remain in the lymph node, where they cells in antibody responses in Chapter 7. function to eradicate infected cells at that site The responses of naive T lymphocytes to or provide signals to B cells that promote anti- cell-associated microbial antigens consist of body responses against the microbes. a series of sequential steps that result in an Some of the progeny of the T cells that have increase in the number of antigen-specific T proliferated in response to antigen develop cells and the conversion of naive T cells to into memory T cells, which are long-lived effector and memory cells (Fig. 5-3). and functionally inactive, circulate for months One of the earliest responses is the secretion of or years, and are ready to respond rapidly to cytokines and increased expression of recep- repeated exposure to the same microbe. tors for various cytokines. As effector T cells eliminate the infectious agent, Some cytokines stimulate the proliferation of the stimuli that triggered T cell expansion and the antigen-activated T cells, resulting in a rapid differentiation also are eliminated. As a result,  Antigen Recognition and Costimulation 107 most of the cells in the greatly expanded clone of Adhesion molecules strengthen the binding of antigen-specific lymphocytes die, returning the T cells to APCs. system to a resting state, with only memory cells Molecules called costimulators, which are remaining from the immune response. expressed on APCs after encounter with mi- This sequence of events is common to both crobes, bind to costimulatory receptors on CD4+ and CD8+ T lymphocytes, although there the naive T cells thus promoting responses to are important differences in the properties and infectious pathogens. effector functions of CD4+ and CD8+ cells, as dis- Cytokines amplify the T cell response and di- cussed in Chapter 6. rect it along various differentiation pathways. Naive and effector T cells have differ- The roles of these molecules in T cell responses ent patterns of circulation and migration to antigens are described next. Cytokines are dis- through tissues, which are critical for their cussed mainly in Chapter 6. different roles in immune responses. As dis- cussed in previous chapters, naive T lympho- Recognition of MHC-Associated Peptides cytes constantly recirculate through peripheral The T cell receptor for antigen (the TCR) lymphoid organs searching for foreign protein and the CD4 or CD8 coreceptor together antigens. The antigens of microbes are trans- recognize complexes of peptide antigens ported from the portals of entry of the microbes and MHC molecules on APCs, and this rec- to the same regions of peripheral lymphoid organs ognition provides the initiating, or first, sig- through which naive T cells recirculate. In these nal for T cell activation (Fig. 5-5). The TCRs organs, the antigens are processed and displayed expressed on all CD4+ and CD8+ T cells consist by MHC molecules on dendritic cells, the antigen- of an α chain and a β chain, both of which par- presenting cells (APCs) that are the most efficient ticipate in antigen recognition (see Chapter 4, stimulators of naive T cells (see Chapter 3). When Fig. 4-7). (A small subset of T cells expresses a T cell recognizes antigen, it is transiently arrested TCRs composed of γ and δ chains, which do not on the dendritic cell and it initiates an activation recognize MHC-associated peptide antigens.) program. Following activation and differentia- The TCR of a T cell specific for a foreign (e.g., tion, the cells may leave the lymphoid organ and microbial) peptide recognizes the displayed pep- migrate preferentially to the inflamed tissue, the tide and simultaneously recognizes residues of original source of the antigen. The control of this the MHC molecule located around the peptide- directed migration is discussed later in this chapter. binding cleft. Every mature MHC-restricted T With this overview, we proceed to a descrip- cell expresses either CD4 or CD8, both of which tion of the stimuli required for T cell activation are called coreceptors because they bind to the and regulation. We then describe the biochemical same MHC molecules that the TCR binds and signals that are generated by antigen recognition are required for initiation of signaling from the and the biologic responses of the lymphocytes. TCR complex. At the time when the TCR is rec- ognizing the peptide-MHC complex, CD4 or CD8 ANTIGEN RECOGNITION AND recognizes the class II or class I MHC molecule, respectively, at a site separate from the peptide- COSTIMULATION binding cleft. As discussed in Chapter 3, when The initiation of T cell responses requires protein antigens are ingested by APCs from the multiple receptors on the T cells recogniz- extracellular milieu into vesicles, these anti- ing ligands on APCs (Fig. 5-4). gens are processed into peptides that are dis- The T cell receptor (TCR) recognizes MHC- played by class II MHC molecules. In contrast, associated peptide antigens. protein antigens present in the cytosol are pro- CD4 or CD8 coreceptors on the T cells recog- cessed by proteasomes into peptides displayed by nize MHC molecules on the APC and help the class I MHC molecules. Thus, CD4+ and CD8+ T TCR complex to deliver activating signals. cells recognize antigens from different cellular 108 CHAPTER 5 T Cell–Mediated Immunity A Receptors and signaling Ligands of class II molecules of CD4+ lymphocyte MHC expressing APC Signal CD4 transduction Peptide TCR Class II Antigen MHC recognition CD3 ITAM ζ Signal CD28 B7-1/B7-2 transduction CTLA-4 B7-1/B7-2 ITIM PD-1 PD-L1/PD-L2 Adhesion LFA-1 ICAM-1 B Surface Function Ligand molecules of T lymphocytes Name Expressed on CD3 Signal None transduction by TCR complex ζ Signal None transduction by TCR complex CD4 Signal Class II MHC Antigen transduction presenting cells CD8 Signal Class I MHC All nucleated transduction cells CD28 Signal B7-1/B7-2 Antigen transduction presenting cells (costimulation) CTLA-4 Inhibition B7-1/B7-2 Antigen presenting cells PD-1 Inhibition PD-L1/PD-L2 Antigen presenting cells, tissue cells, tumor cells LFA-1 Adhesion, ICAM-1 Antigen signal presenting cells, transduction endothelium  Antigen Recognition and Costimulation 109 FIGURE 5-4 Receptors and ligands involved in T cell activation. A, Major surface molecules of CD4+ T cells involved in the activation of these cells and their corresponding ligands on antigen-presenting cells. CD8+ T cells use most of the same molecules, except that the TCR recognizes peptide-class I MHC complexes, and the core- ceptor is CD8, which recognizes class I MHC. CD3 is composed of three polypeptide chains, δ, ε, and γ, arranged in two pairs (δε and γε); we show CD3 as three chains. Immunoreceptor tyrosine-based activation motifs (ITAMs) are the regions of signaling proteins that are phosphorylated on tyrosine residues and become docking sites for other tyrosine kinases (see Fig. 5-10). Immunoreceptor tyrosine-based inhibitory motifs (ITIMs) are the regions of signaling proteins that are sites for tyrosine phosphatases that counteract actions of ITAMs. B, Important proper- ties of major surface molecules of T cells involved in functional responses. Cytokines and cytokine receptors are not listed here. The functions of most of these molecules are described in this chapter; the role of CTLA-4 and PD-1 in shutting off T cell responses is described in Chapter 9. LFA-1 is an integrin involved in leukocyte binding to endothelium and other cells. APC, Antigen-presenting cell; ICAM-1, intercellular adhesion molecule 1; LFA-1, leukocyte function–associated antigen 1; MHC, major histocompatibility complex; PD-1, programmed death-1; TCR, T cell receptor. compartments. The TCR and its coreceptor need Class II MHC to be engaged simultaneously to initiate the T APC cell response, and multiple TCRs likely need to be triggered for T cell activation to occur. Once these conditions are achieved, the T cell begins Antigen CD3 z its activation program. recognition X The biochemical signals that lead to T cell activation are triggered by a set of proteins linked to the TCR that are part of the TCR T cell CD4 complex and by the CD4 or CD8 coreceptor TCR (see Fig. 5-5). In lymphocytes, antigen recogni- Lck tion and subsequent signaling are performed by different sets of molecules. The TCR αβ het- ITAM erodimer recognizes antigens, but it is not able to transmit biochemical signals to the interior of the Signal cell. The TCR is noncovalently associated with a transduction complex of transmembrane signaling molecules including three CD3 proteins and a protein called the ζ chain. The TCR, CD3, and ζ chain make up FIGURE 5-5 Antigen recognition and signal trans- the TCR complex. Although the α and β TCRs duction during T cell activation. Different T cell must vary among T cell clones in order to recog- molecules recognize antigen and deliver biochemical nize diverse antigens, the signaling functions of signals to the interior of the cell as a result of antigen TCRs are the same in all clones, and therefore the recognition. The CD3 and ζ proteins are noncovalently CD3 and ζ proteins are invariant among different attached to the T cell receptor (TCR) α and β chains by T cells. The mechanisms of signal transduction by interactions between charged amino acids in the trans- these proteins of the TCR complex are discussed membrane domains of these proteins (not shown). The later in the chapter. figure illustrates a CD4+ T cell; the same interactions are involved in the activation of CD8+ T cells, except that T cells can also be activated experimentally by the coreceptor is CD8 and the TCR recognizes a pep- molecules that bind to the TCRs of many or all tide–class I MHC complex. APC, Antigen-presenting clones of T cells, regardless of the peptide-MHC cell; ITAM, immunoreceptor tyrosine-based activation specificity of the TCR. These polyclonal activators motifs; MHC, major histocompatibility complex. of T cells include antibodies specific for the TCR 110 CHAPTER 5 T Cell–Mediated Immunity or associated CD3 proteins, polymeric carbo- Role of Costimulation in T Cell Activation hydrate-binding proteins such as phytohemag- The full activation of T cells depends on glutinin (PHA), and certain microbial proteins, the recognition of costimulators on APCs in including staphylococcal enterotoxins, which addition to antigen (Fig. 5-6). We have previ- are called superantigens. Polyclonal activators ously referred to costimulators as second signals often are used as experimental tools to study T for T cell activation (see Chapters 2 and 3). The cell responses, and in clinical settings to test for T name costimulator derives from the fact that cell function or to prepare metaphase spreads for these molecules provide stimuli to T cells that karyotyping (analyzing chromosomes). Micro- function together with stimulation by antigen. bial superantigens may cause systemic inflam- The best-defined costimulators for T cells are matory disease by inducing excessive cytokine two related proteins called B7-1 (CD80) and B7-2 release from many T cells. (CD86), both of which are expressed on APCs and whose expression is increased when the APCs X encounter microbes. These B7 proteins are recog- Role of Adhesion Molecules in T Cell nized by a receptor called CD28, which is expressed Responses on most T cells. Different members of the B7 Adhesion molecules on T cells recognize and CD28 families serve to stimulate or inhibit their ligands on APCs and stabilize the immune responses (Fig. 5-7). The binding of CD28 binding of the T cells to the APCs. Most TCRs on T cells to B7 on the APCs generates signals in bind the peptide-MHC complexes for which the T cells that work together with signals gener- they are specific with low affinity. To induce a ated by TCR recognition of antigen presented by response, the binding of T cells to APCs must be MHC proteins on the same APCs. CD28-mediated stabilized for a sufficiently long period to achieve signaling is essential for the responses of naive the necessary signaling threshold. This stabiliza- T cells; in the absence of CD28:B7 interactions, tion function is performed by adhesion mole- antigen recognition by the TCR is insufficient for cules on the T cells that bind to ligands expressed T cell activation. The requirement for costimula- on APCs. The most important of these adhesion tion ensures that naive T lymphocytes are acti- molecules belong to the family of heterodimeric vated fully by microbial antigens and not by (two-chain) proteins called integrins. The major harmless foreign substances or by self antigens, T cell integrin involved in binding to APCs is because, as stated previously, microbes stimulate leukocyte function–associated antigen 1 (LFA- the expression of B7 costimulators on APCs. 1), whose ligand on APCs is called intercellular A protein called ICOS (inducible costimulator), adhesion molecule 1 (ICAM-1). which is related to CD28 and also expressed on On resting naive T cells, which are cells that T cells, plays an important role in the develop- have not previously recognized and been acti- ment and function of follicular helper T cells dur- vated by antigen, the LFA-1 integrin is in a low- ing germinal center responses (see Chapter 7). affinity state. Antigen recognition by a T cell Another set of molecules that participate increases the affinity of that cell’s LFA-1. There- in T cell responses are CD40 ligand (CD40L, fore, once a T cell sees antigen, it increases the or CD154) on activated T cells and CD40 on strength of its binding to the APC presenting APCs. These molecules do not directly enhance that antigen, providing a positive feedback loop. T cell activation. Instead, CD40L expressed on Thus, integrin-mediated adhesion is critical for an antigen-stimulated T cell binds to CD40 on the ability of T cells to bind to APCs displaying APCs and activates the APCs to express more microbial antigens. Integrins also play an impor- B7 costimulators and to secrete cytokines (e.g., tant role in directing the migration of effector T interleukin-12 (IL-12)) that enhance T cell dif- cells and other leukocytes from the circulation ferentiation. Thus, the CD40L-CD40 interaction to sites of infection. This process is described in promotes T cell activation by making APCs bet- Chapter 2 and later in this chapter. ter at stimulating T cells.  Antigen Recognition and Costimulation 111 Antigen recognition T cell response CD28 "Resting" (costimulator- deficient) APC Naive No response T cell or tolerance Activation of APCs by microbes, innate immune response Effector and B7 IL-2 memory T cells CD28 Activated APC: increased expression of costimulators, secretion of cytokines T cell proliferation and differentiation Cytokines (e.g., IL-12) FIGURE 5-6 Role of costimulation in T cell activation. Resting antigen-presenting cells (APCs), which have not been exposed to microbes or adjuvants, may present peptide antigens, but they do not express costimula- tors and are unable to activate naive T cells. T cells that recognize antigen without costimulation may become unresponsive (tolerant) to subsequent exposure to antigen. Microbes, as well as cytokines produced during innate immune responses to microbes, induce the expression of costimulators, such as B7 molecules, on the APCs. The B7 costimulators are recognized by the CD28 receptor on naive T cells, providing signal 2. In conjunction with antigen recognition (signal 1), this recognition initiates T cell responses. Activated APCs also produce cytokines that stimulate the differentiation of naive T cells into effector cells. IL, Interleukin. The role of costimulation in T cell activation the immune system into responding to purified explains an observation mentioned in earlier protein antigens in a vaccine as if these proteins chapters. Protein antigens, such as those used in were parts of infectious microbes. vaccines, fail to elicit T cell–dependent immune The increasing understanding of costimulators responses unless these antigens are administered has led to new strategies for inhibiting harmful with substances that activate APCs, especially immune responses. Agents that block B7:CD28 dendritic cells. Such substances are called adju- interactions are used in the treatment of rheu- vants, and they function mainly by inducing matoid arthritis, other inflammatory diseases, the expression of costimulators on APCs and by and graft rejection, and antibodies that block stimulating the APCs to secrete cytokines that CD40:CD40L interactions are being tested in activate T cells. Most adjuvants are products of inflammatory diseases and to treat graft rejection. microbes (e.g., killed mycobacteria, which is X often used in experimental studies) or substances Inhibitory Receptors of T Cells that mimic microbes, and they bind to pattern Inhibitory receptors are critical for limit- recognition receptors of the innate immune sys- ing and terminating immune responses. tem, such as Toll-like receptors and NOD-like Two important inhibitory receptors—CTLA-4 receptors (see Chapter 2). Thus, adjuvants trick and PD-1—are structurally related to CD28 (see  Functional Responses of T Lymphocytes to Antigen and Costimulation 117 PKC is activated by diacylglycerol, which, like also initiates a distinct set of signals that comple- 1 IP3, is generated by PLC-mediated hydro- ment TCR signals. lysis of membrane inositol lipids. PKCθ acts Lymphocyte activation is also associ- through adaptor proteins recruited to the TCR ated with a profound change in metabolic complex to activate NF-κB. NF-κB exists in pathways. In naive (resting) T cells, low levels the cytoplasm of resting T cells in an inactive of glucose are taken up and used to generate form, bound to an inhibitor called IκB. TCR- energy in the form of ATP, by mitochondrial oxi- induced signals downstream of PKCθ activate dative phosphorylation. Upon activation, glucose a kinase that phosphorylates IκB and targets it uptake increases markedly, and the cells switch for destruction. As a result, NF-κB is released to aerobic glycolysis. This process generates less and moves to the nucleus, where it promotes ATP but facilitates the synthesis of more amino the transcription of several genes. acids, lipids, and other molecules that provide T cell receptor signal transduction also involves building blocks for organelles and for producing a lipid kinase called phosphatidylinositol-3 new cells. As a result, it is possible for activated (PI-3) kinase, which phosphorylates the mem- T cells to more efficiently manufacture the cel- brane phospholipid PIP2 to generate PIP3. PIP3 lular constituents that are needed for their rapid is required for the activation of a number of tar- increase in size and for producing daughter cells. gets, including a serine-threonine kinase called Akt, or protein kinase B, which has many roles, Having described the stimuli and biochemi- including stimulating expression of antiapop- cal pathways in T cell activation, we now discuss totic proteins and thus promoting survival of how T cells respond to antigens and differentiate antigen-stimulated T cells. The PI-3 kinase/Akt into effector cells capable of combating microbes. pathway is triggered not only by the TCR but also by CD28 and IL-2 receptors. Closely linked to the Akt pathway is mTOR (mammalian tar- FUNCTIONAL RESPONSES OF get of rapamycin), a serine-threonine kinase T LYMPHOCYTES TO ANTIGEN that is involved in stimulating protein transla- tion and promoting cell survival and growth. A AND COSTIMULATION drug that binds to and inactivates mTOR—ra- The recognition of antigen and costimulators by T pamycin—is used to treat graft rejection. cells initiates an orchestrated set of responses that The various transcription factors that are culminate in the expansion of the antigen-specific induced or activated in T cells, including NFAT, clones of lymphocytes and the differentiation of AP-1, and NF-κB, stimulate transcription and the naive T cells into effector cells and memory subsequent production of cytokines, cytokine cells (see Fig. 5-3). Many of the responses of T cells receptors, cell cycle inducers, and effector mol- are mediated by cytokines that are secreted by the ecules such as CD40L (see Fig. 5-9). All of these T cells and act on the T cells themselves and on signals are initiated by antigen recognition, many other cells involved in immune defenses. because binding of the TCR and coreceptors to Each component of the biologic responses of T peptide-MHC complexes is necessary to initiate cells is discussed next. signaling in T cells. As stated earlier, recognition of costimulators, such as B7 molecules, by their receptor CD28 is Secretion of Cytokines and Expression essential for full T cell responses. The biochemi- of Cytokine Receptors cal signals transduced by CD28 on binding to In response to antigen and costimulators, T B7 costimulators are less well defined than are lymphocytes, especially CD4+ T cells, rap- TCR-triggered signals. CD28 engagement likely idly secrete the cytokine IL-2. Cytokines are amplifies some TCR signaling pathways that are a large group of proteins that function as media- triggered by antigen recognition (signal 1) and tors of immunity and inflammation. We have 118 CHAPTER 5 T Cell–Mediated Immunity Costimulator (B7) IL-2Rβγc CD28 complex IL-2Rβγc T cell activation Resting (naive) by antigen T cell + costimulator APC Low-affinity IL-2R Secretion of IL-2 IL-2 (Kd ~ 10-9 M) IL-2Rαβγc complex IL-2Rαβγc Expression of IL-2Rα chain; formation of high-affinity IL-2Rαβγ complex High-affinity IL-2R (Kd ~ 10-11 M) IL-2–induced T cell proliferation FIGURE 5-11 Role of interleukin-2 and IL-2 receptors in T cell proliferation. Naive T cells express the low- affinity IL-2 receptor (IL-2R) complex, made up of the β and γC chains (γC designates common γ chain, so called because it is a component of receptors for several cytokines). On activation by antigen recognition and costimu- lation, the cells produce IL-2 and express the α chain of the IL-2R (CD25), which associates with the β and γC chains to form the high-affinity IL-2 receptor. Binding of IL-2 to its receptor initiates proliferation of the T cells that recognized the antigen. APC, Antigen-presenting cell. already discussed cytokines in innate immune The receptor for IL-2 is a three-chain molecule. responses, which are produced mainly by den- Naive T cells express two signaling chains but do dritic cells and macrophages (see Chapter 2). In not express the α chain (CD25) that enables the adaptive immunity, cytokines are secreted by T receptor to bind IL-2 with high affinity. Within cells, mainly CD4+ cells. Because most of these hours after activation by antigens and costimu- cytokines are produced by effector T cells and lators, the T cells produce the third chain of the serve diverse roles in host defense, we describe receptor, and now the complete IL-2 receptor is them in Chapter 6 when we discuss the effector able to bind IL-2 strongly. Thus, IL-2 produced mechanisms of cell-mediated immunity. by antigen-stimulated T cells preferentially binds IL-2 is produced within 1 to 2 hours after acti- to and acts on the same T cells, an example of vation of CD4+ T cells. Activation also increases autocrine cytokine action. the expression of the high-affinity IL-2 recep- The principal functions of IL-2 are to tor, thus rapidly enhancing the ability of the stimulate the survival and proliferation of T cells to bind and respond to IL-2 (Fig. 5-11). T cells, resulting in an increase in the number  Functional Responses of T Lymphocytes to Antigen and Costimulation 119 Clonal Contraction expansion (homeostasis) Number of microbe-specific T cells 106 104 CD8+ T cells Memory Infection CD4+ 102 T cells 7 14 200 Days after infection FIGURE 5-12 Expansion and decline of T cell responses. The numbers of CD4+ and CD8+ T cells specific for various antigens, and the clonal expansion and contraction during immune responses, are illustrated. The numbers are approximations based on studies of model microbial and other antigens in inbred mice; in humans, the num- bers of lymphocytes are about 1000-fold greater. of the antigen-specific T cells; because of these The magnitude of clonal expansion is remark- actions, IL-2 was originally called T cell growth able, especially for CD8+ T cells. Before infec- factor. IL-2 also is essential for the maintenance tion, the frequency of CD8+ T cells specific for of regulatory T cells and thus for controlling any one microbial protein antigen is about 1 in immune responses, as we discuss in Chapter 9. 105 or 1 in 106 lymphocytes in the body. At the CD8+ T lymphocytes that recognize antigen peak of some viral infections, possibly within a and costimulators do not appear to secrete large week after the infection, as many as 10% to 20% amounts of IL-2, but these lymphocytes prolifer- of all the lymphocytes in the lymphoid organs ate prodigiously during immune responses. Anti- may be specific for that virus. This means that gen recognition and costimulation may be able the numbers of cells in antigen-specific clones to drive the proliferation of CD8+ T cells, or IL-2 have increased by more than 10,000-fold, with may be provided by CD4+ helper T cells. an estimated doubling time of about 6 hours. Several features of this clonal expansion are sur- Clonal Expansion prising. First, this enormous expansion of T cells T lymphocytes activated by antigen and specific for a microbe is not accompanied by a costimulation begin to proliferate within detectable increase in bystander cells that do not 1 or 2 days, resulting in expansion of anti- recognize that microbe. Second, even in infec- gen-specific clones (Fig. 5-12). This expansion tions with complex microbes that contain many quickly provides a large pool of antigen-specific protein antigens, a majority of the expanded lymphocytes from which effector cells can be clones are specific for only a few, often less than generated to combat infection. five, immunodominant peptides of that microbe. 120 CHAPTER 5 T Cell–Mediated Immunity The expansion of CD4+ T cells appears to be large family of proteins structurally related to the 100-fold to 1000-fold less than that of CD8+ cytokine tumor necrosis factor (TNF). The CD40L cells. This difference may reflect differences in gene is transcribed in CD4+ T cells in response the functions of the two types of T cells. CD8+ to antigen recognition and costimulation, and CTLs are effector cells that kill infected cells by so CD40L is expressed on activated helper T direct contact, and many CTLs may be needed cells (see Fig. 5-9). It binds to its receptor, CD40, to kill large numbers of infected cells. By con- which is expressed mainly on macrophages, B trast, each CD4+ effector cell secretes cytokines lymphocytes, and dendritic cells. Engagement of that activate many other effector cells, so a CD40 stimulates these cells, and thus CD40L is an relatively small number of cytokine producers important participant in the activation of macro- may be sufficient. phages and B lymphocytes by helper T cells (see Chapters 6 and 7). The interaction of CD40L on T cells with CD40 on dendritic cells increases the Differentiation of Naive T Cells into expression of costimulators on these APCs and Effector Cells the production of T cell–stimulating cytokines, Some of the progeny of antigen-stimulated, thus providing a positive feedback (amplification) proliferating T cells differentiate into effec- mechanism for APC-induced T cell activation. tor cells whose function is to eradicate infections. This process of differentiation is the Development of Memory T Lymphocytes result of changes in gene expression, such as the A fraction of antigen-activated T lympho- activation of genes encoding cytokines (in CD4+ cytes differentiates into long-lived memory T cells) or cytotoxic proteins (in CD8+ CTLs). It cells. These cells are a pool of lymphocytes that begins in concert with clonal expansion, and are induced by microbes and are waiting for the differentiated effector cells appear within 3 or infection to return. We do not know what fac- 4 days after exposure to microbes. Effector cells tors determine whether the progeny of antigen- of the CD4+ lineage acquire the capacity to pro- stimulated lymphocytes will differentiate into duce different sets of cytokines. The subsets of effector cells or memory cells. Memory cells have T cells that are distinguished by their cytokine several important characteristics. profiles are named Th1, Th2, and Th17 (Fig. Memory cells survive even after the infection 5-13). Many of these cells leave the peripheral is eradicated and antigen is no longer present. lymphoid organs and migrate to sites of infec- Certain cytokines, including IL-7 and IL-15, tion, where their cytokines recruit other leu- which are produced by stromal cells in tissues, kocytes that destroy the infectious agents. The may serve to keep memory cells alive and cy- development and functions of these effector cells cling slowly. are described in Chapter 6, when we discuss cell- Memory T cells may be rapidly induced to mediated immunity. Other differentiated CD4+ T produce cytokines or kill infected cells on cells remain in the lymphoid organs and migrate encountering the antigen that they recognize. into lymphoid follicles, where they help B lym- These cells do not perform any effector func- phocytes to produce antibodies (see Chapter 7). tions until they encounter antigen, but once Effector cells of the CD8+ lineage acquire the activated, they respond much more vigorously ability to kill infected cells; their development and rapidly than do naive lymphocytes. and function are also described in Chapter 6. Memory T cells can be found in lymphoid or- CD4+ helper T cells activate phagocytes gans, in various peripheral tissues, especially and B lymphocytes through the action of mucosa and skin, and in the circulation. They plasma membrane proteins and by secreted can be distinguished from naive and effector cytokines (Fig. 5-14). The most important cell cells by several criteria (see Chapter 1). A subset surface protein inv