CHAPTER 9 Activation of T Lymphocytes
The goal of T cell activation is to generate, from a small pool of naive lymphocytes with predetermined receptors for any antigen, a large number of functional effector cells that can eliminate that antigen and a population of memory cells that remain for long periods to rapidly react against the antigen in case it is reintroduced. A fundamental characteristic of the T cell response, like all adaptive immune responses, is that it is highly specific for the antigen that elicits the response. Both the initial activation of naive T cells and the effector phases of T cell–mediated adaptive immune responses are triggered by recognition of the antigen by antigen receptors of T lymphocytes. In Chapter 6, we described the specificity of T cells for peptide fragments, derived from protein antigens, that are bound to and displayed by self major histocompatibility complex (MHC) molecules. In Chapter 7, we described the antigen receptors and other molecules of T cells that are involved in the activation of T cells by antigens. In this chapter, we describe the biology of T cell activation. We begin with a brief overview of T cell activation, discuss the role of costimulators and other signals provided by antigen-presenting cells (APCs) in T cell activation, and describe the sequence of proliferation and differentiation that occurs in the responses of CD4+ and CD8+ T cells to foreign antigens. The functions of differentiated effector cells in host defense are described in Chapter 10. Thus, Chapters 9 and 10 together cover the biology of T lymphocytes.
Overview of T Lymphocyte Activation
The initial activation of naive T lymphocytes occurs mainly in secondary lymphoid organs, through which these cells normally circulate and where they may encounter antigens presented by mature dendritic cells (Fig. 9-1). The immune system is designed to perform its functions of eliminating antigens only when needed, that is, when the system encounters pathogens. T lymphocytes with multiple specificities are generated in the thymus before antigen exposure. Naive T lymphocytes, which have not recognized and responded to antigens, circulate throughout the body in a resting state, and they acquire powerful functional capabilities only after they are activated. This activation of naive T lymphocytes occurs in specialized lymphoid organs, where the naive lymphocytes and APCs are brought together (see Chapter 2). Protein antigens that cross epithelial barriers or are produced in tissues are captured by dendritic cells and transported to lymph nodes. Antigens that enter the circulation may be captured by dendritic cells in the spleen. If these antigens are produced by microbes or administered with adjuvants (as in vaccines), the resulting innate immune response leads to the activation of dendritic cells and the expression of costimulators such as B7 proteins (described later in this chapter). Dendritic cells that have encountered microbes and internalized their antigens begin to mature and migrate to the T cell zones of draining lymph nodes. As discussed in Chapter 6, both naive T cells and mature dendritic cells are drawn to the T cell zones of secondary lymphoid organs by chemokines produced in these areas that engage the CCR7 chemokine receptor on the cells. By the time the mature dendritic cells reach the T cell areas, they display peptides derived from protein antigens on MHC molecules and also express costimulators. When a naive T cell of the correct specificity recognizes the peptide-MHC complexes and receives concomitant costimulatory signals from the dendritic cells, that naive lymphocyte is activated.
FIGURE 9–1 Activation of naive and effector T cells by antigen.
Antigens that are transported by dendritic cells to lymph nodes are recognized by naive T lymphocytes that recirculate through these lymph nodes. The T cells are activated to differentiate into effector and memory cells, which may remain in the lymphoid organs or migrate to nonlymphoid tissues. At sites of infection, the effector cells are again activated by antigens and perform their various functions, such as macrophage activation.
Antigen recognition and other activating stimuli induce several responses: cytokine secretion from the T cells; proliferation of the antigen-specific lymphocytes, leading to an increase in the numbers of cells in the antigen-specific clones (called clonal expansion); and differentiation of the naive cells into effector and memory lymphocytes (Fig. 9-2). In addition, the process of T cell activation is associated with characteristic changes in surface molecules, many of which play important roles in promoting the responses and in limiting them. Clonal expansion and differentiation proceed rapidly because of several positive feedback amplification mechanisms. For example, cytokines made by the activated T cells stimulate both T cell proliferation and differentiation into effector cells. In addition, activated T cells deliver signals back to the APCs, further enhancing their ability to activate T cells. At the same time, some surface molecules expressed on activated T cells as well as cytokines secreted by these cells have regulatory functions that serve to establish safe limits to the response. The steps in T cell responses and the nature of the positive and negative feedback loops are described later in the chapter.
FIGURE 9–2 Phases of T cell responses.
Antigen recognition by T cells induces cytokine (e.g., IL-2) secretion, particularly in CD4+ T cells, clonal expansion as a result of cell proliferation, and differentiation of the T cells into effector cells or memory cells. In the effector phase of the response, the effector CD4+ T cells respond to antigen by producing cytokines that have several actions, such as the recruitment and activation of leukocytes and activation of B lymphocytes, and CD8+ CTLs respond by killing other cells.
Effector T cells recognize antigens in lymphoid organs or in peripheral nonlymphoid tissues and are activated to perform functions that are responsible for the elimination of microbes and, in disease states, for inflammation and tissue damage. Whereas naive cells are activated mainly in lymphoid organs, differentiated effector cells may function in any tissue (see Fig. 9-1). The process of differentiation from naive to effector cells is associated with acquisition of the capacity to perform these specialized functions and the ability to migrate to any site of infection or inflammation. At these sites, the effector cells again encounter the antigen for which they are specific and respond in ways that serve to eliminate the source of the antigen. Effector T cells of the CD4+ helper lineage are classified into several subsets on the basis of their cytokine profiles and functions. Some of these differentiated helper cells express membrane molecules and secrete cytokines that activate (help) macrophages to kill phagocytosed microbes; others secrete cytokines that recruit leukocytes and thus stimulate inflammation; others enhance mucosal barrier functions; and yet others remain in lymphoid organs and help B cells to differentiate into cells that secrete antibodies. CD8+ cytotoxic T lymphocytes (CTLs), the effector cells of the CD8+ lineage, kill infected cells and tumor cells that display class I MHC–associated antigens.
Memory T cells that are generated by T cell activation are long-lived cells with an enhanced ability to react against the antigen. These cells are present in the recirculating lymphocyte pool and are abundant in mucosal tissues and the skin as well as in lymphoid organs. After a T cell response wanes, there are many more memory cells of the responding clone that persist than there were naive T cells before the response. These memory cells respond rapidly to subsequent encounter with the antigen and generate new effector cells that eliminate the antigen.
T cell responses decline after the antigen is eliminated by effector cells. This process of contraction is important for returning the immune system to a state of equilibrium, or homeostasis. It occurs mainly because the majority of antigen-activated effector T cells die by apoptosis. One reason for this is that as the antigen is eliminated, lymphocytes are deprived of survival stimuli that are normally provided by the antigen and by the costimulators and cytokines produced during inflammatory reactions to the antigen. It is estimated that more than 90% of the antigen-specific T cells that arise by clonal expansion die by apoptosis as the antigen is cleared.
With this overview, we proceed to a discussion of the signals required for T cell activation and the steps that are common to CD4+ and CD8+ T cells. We then describe effector and memory cells in the CD4+ and CD8+ lineages, with emphasis on subsets of CD4+ helper T cells and the cytokines they produce. We conclude with a discussion of the decline of immune responses.
Signals for T Lymphocyte Activation
The proliferation of T lymphocytes and their differentiation into effector and memory cells require antigen recognition, costimulation, and cytokines that are produced by the T cells themselves and by APCs and other cells at the site of antigen recognition. In this section, we will summarize the nature of antigens recognized by T cells and discuss specific costimulators and their receptors that contribute to T cell activation. Cytokines are discussed later in the chapter.
Recognition of Antigen
Antigen is always the necessary first signal for the activation of lymphocytes, ensuring that the resultant immune response remains specific for the antigen. Because CD4+ and CD8+ T lymphocytes recognize peptide-MHC complexes displayed by APCs, they can respond only to protein antigens or chemicals attached to proteins. In addition to the TCR recognizing peptides displayed by MHC molecules, several other T cell surface proteins participate in the process of T cell activation (see Fig. 7-9, Chapter 7). These include adhesion molecules, which stabilize the interaction of the T cells with APCs, and costimulators, which are described later. The nature of the biochemical signals delivered by antigen receptors and the role of these signals in the functional responses of the T cells are discussed in Chapter 7.
Activation of naive T cells requires recognition of antigen presented by dendritic cells. The reasons that dendritic cells are the most efficient APCs for initiation of T cell responses were discussed in Chapter 6. In lymphoid organs, dendritic cells present peptides derived from endocytosed protein antigens in association with class II MHC molecules to naive CD4+ T cells and peptides derived from cytosolic proteins displayed by class I MHC molecules to CD8+ T cells. CD4+ T cell–mediated immune reactions are elicited by protein antigens of microbes that are ingested by dendritic cells or by soluble protein antigens that are administered with adjuvants, in the case of vaccinations, and taken up by dendritic cells. These microbial or soluble antigens are internalized into vesicles by the dendritic cells, processed, and presented in association with class II MHC molecules. CD8+ T cell responses are induced by antigens that either are produced in the cytoplasm of dendritic cells (e.g., by viruses that infect these cells) or are ingested by dendritic cells, processed, and “cross-presented” on class I MHC molecules. Some chemicals introduced through the skin also elicit T cell reactions, called contact sensitivity reactions. Contact-sensitizing chemicals may tightly bind to or covalently modify self proteins, creating novel peptide determinants that are presented to CD4+ or CD8+ T cells.
Differentiated effector T cells can respond to antigens presented by cells other than dendritic cells. In humoral immune responses, B cells present antigens to helper T cells and are the recipients of activating signals from the helper cells (see Chapter 11); in cell-mediated immune responses, macrophages present antigens to and respond to T cells (see Chapter 10); and virtually any nucleated cell can present antigen to and be killed by CD8+ CTLs.
Role of Costimulators in T Cell Activation
The proliferation and differentiation of naive T cells require signals provided by molecules on APCs, called costimulators, in addition to antigen-induced signals (Fig. 9-3). The requirement for costimulatory signals was first suggested by the experimental finding that T cell antigen receptor engagement alone (e.g., with cross-linking anti-CD3 antibodies) resulted in much lower responses than those seen with antigens presented by activated APCs. This result indicated that APCs must express molecules in addition to antigen that are required for T cell activation. These molecules are called costimulators, and the “second signal” for T cell activation is called costimulation because it functions together with antigen (“signal 1”) to stimulate T cells. In the absence of costimulation, T cells that encounter antigens either fail to respond and die by apoptosis or enter a state of unresponsiveness called anergy (see Chapter 14).
FIGURE 9–3 Functions of costimulators in T cell activation.
A, The resting APC expresses few or no costimulators and fails to activate naive T cells. (Antigen recognition without costimulation may make T cells anergic; this phenomenon will be discussed in Chapter 14.) B, Microbes and cytokines produced during innate immune responses activate APCs to express costimulators, such as B7 molecules. The APCs then become capable of activating naive T cells. Activated APCs also produce cytokines such as IL-12, which stimulate the differentiation of naive T cells into effector cells.
The B7:CD28 Family of Costimulators
The best characterized costimulatory pathway in T cell activation involves the T cell surface receptor CD28, which binds the costimulatory molecules B7-1 (CD80) and B7-2 (CD86) expressed on activated APCs. CD28 was discovered when activating antibodies against human T cell surface molecules were screened for their ability to enhance T cell responses when added to the cells together with an activating anti-CD3 antibody (which was used as a mimic of antigen). The ligands for CD28 were discovered by screening DNA expression libraries for molecules that bound to CD28. The cloning of the genes encoding B7-1 and CD28 opened the way for a variety of experiments in mice that have clarified the role of these molecules and led to the identification of additional homologous proteins involved in T cell costimulation. For example, residual costimulatory activity of APCs from B7-1 knockout mice suggested the existence of additional costimulatory molecules, and homology-based cloning strategies led to the identification of the B7-2 molecule. The essential role of CD28 and B7-1 and B7-2 in T cell activation has been established not only by experiments with cross-linking antibodies but also by the severe T cell immune deficiency caused by knockout of these molecules in mice and by the ability of agents that bind to and block B7 to inhibit a variety of T cell responses. The development of therapeutic agents based on these principles is described later.
B7-1 and B7-2 are structurally similar integral membrane single-chain glycoproteins, each with two extracellular immunoglobulin (Ig)–like domains, although on the cell surface, B7-1 exists as a dimer and B7-2 as a monomer. CD28 is a disulfide-linked homodimer, each subunit of which has a single extracellular Ig domain. It is expressed on more than 90% of CD4+ T cells and on 50% of CD8+ T cells in humans (and on all naive T cells in mice).
The expression of B7 costimulators is regulated and ensures that T lymphocyte responses are initiated at the correct time and place. The B7 molecules are expressed mainly on APCs, including dendritic cells, macrophages, and B lymphocytes. They are absent or expressed at low levels on resting APCs and are induced by various stimuli, including microbial products that engage Toll-like receptors and cytokines such as interferon-γ (IFN-γ) produced during innate immune reactions to microbes. The induction of costimulators by microbes and by the cytokines of innate immunity promotes T cell responses to microbial antigens. This is an excellent illustration of the role of innate immune responses in enhancing adaptive immunity (see Chapter 4). In addition, activated T cells express CD40 ligand on their surface, which binds to CD40 expressed on APCs and delivers signals that enhance the expression of B7 costimulators on the APCs. This feedback loop serves to amplify T cell responses (described later). Of all potential APCs, mature dendritic cells express the highest levels of costimulators and, as a result, are the most potent stimulators of naive T cells. In Chapter 6, we mentioned the essential role of adjuvants in inducing primary T cell responses to protein antigens such as vaccines. Many adjuvants are products of microbes, or mimic microbes, and one of their major functions in T cell activation is to stimulate the expression of costimulators on APCs. Unactivated, or “resting,” APCs in normal tissues are capable of presenting self antigens to T cells, but because these tissue APCs express only low levels of costimulators, potentially self-reactive T cells that see the self antigens are not activated and may be rendered anergic (see Chapter 14). Regulatory T cells (see Chapter 14) are also dependent on B7:CD28-mediated costimulation for their generation and maintenance. It is possible that the low levels of B7 costimulators that are constitutively expressed by resting APCs are necessary to maintain regulatory T cells, which are important for tolerance to self antigens. The temporal patterns of expression of B7-1 and B7-2 differ; B7-2 is expressed constitutively at low levels and induced early after activation of APCs, whereas B7-1 is not expressed constitutively and is induced hours or days later.
CD28 signals work in cooperation with antigen recognition to initiate the responses of naive T cells. CD28 engagement leads to the activation of several signaling pathways, some of which may amplify signals from the TCR complex, and others may be independent of but parallel to TCR-induced signals (Fig. 9-4). The cytoplasmic tail of CD28 includes a tyrosine-containing motif that after phosphorylation can recruit the regulatory subunit of phosphatidylinositol 3-kinase (PI3-kinase). The CD28 tail also contains two proline-rich motifs, one of which can bind the Tec family tyrosine kinase Itk, and the other binds to the Src family kinase Lck. CD28 ligated by its B7 ligands can activate PI3-kinase and the Akt kinase and also facilitates the activation of the Ras/ERK MAP kinase pathway. PI3-kinase, as discussed in Chapter 7, creates phosphatidylinositol trisphosphate (PIP3) moieties on the inner leaflet of the plasma membrane that can contribute to the recruitment and activation of the Itk tyrosine kinase, the phospholipase PLCγ, and another kinase called PDK1. PDK1 phosphorylates and activates Akt. Akt in turn phosphorylates a number of targets, inactivating proapoptotic proteins and activating antiapoptotic factors, thus contributing to increased cell survival. CD28 also provides an independent pathway for the activation of the Vav exchange factor and the subsequent activation of the Rac/JNK MAP kinase pathway. In addition, CD28 signals have been shown to induce NF-κB binding to a site in the IL-2 gene promoter, called the CD28 response element, that is not activated by TCR-mediated signals. The net result of these signaling pathways is the increased expression of antiapoptotic proteins such as Bcl-2 and Bcl-XL, which promote survival of T cells; increased metabolic activity of T cells; enhanced proliferation of the T cells; production of cytokines such as IL-2; and differentiation of the naive T cells into effector and memory cells (see Fig. 9-4). Previously activated effector and memory T cells are less dependent on costimulation by the B7:CD28 pathway than are naive cells. This property of effector and memory cells enables them to respond to antigens presented by various APCs that may reside in nonlymphoid tissues and may express no or low levels of B7. For instance, the differentiation of CD8+ T cells into effector CTLs requires costimulation, but effector CTLs can kill other cells that do not express costimulators.
FIGURE 9–4 Mechanisms of T cell stimulation by CD28.
CD28 engagement induces signaling pathways that enhance or work together with TCR signals to stimulate the expression of survival proteins, cytokines, and cytokine receptors, to promote cell proliferation, and to induce differentiation toward effector and memory cells. These differentiation events may be secondary to the increased clonal expansion and may also involve increased production of various transcription factors.
Numerous receptors homologous to CD28 and their ligands homologous to B7 have been identified, and these proteins regulate T cell responses both positively and negatively (Fig. 9-5). Following the demonstration of the importance of B7 and CD28, several other proteins structurally related to B7-1 and B7-2 or to CD28 were identified by homology-based gene cloning. A surprising conclusion has emerged that some of the members of the B7:CD28 families are involved in T cell activation (and are thus costimulators) and others are critical inhibitors of T cells (and have sometimes been called coinhibitors). The costimulatory receptor other than CD28 whose function is best understood is ICOS (inducible costimulator, CD278). Its ligand, called ICOS-L (CD275), is expressed on dendritic cells, B cells, and other cell populations. ICOS plays an essential role in T cell–dependent antibody responses, particularly in the germinal center reaction. It is required for the development and activation of follicular helper T cells, which provide critical activating signals to B cells in germinal centers (see Chapter 11).
FIGURE 9–5 The B7 and CD28 families.
The known B7 family ligands expressed on APCs and CD28 family receptors expressed on T cells are shown, with their expression patterns and likely major functions. Other inhibitory receptors have been defined, such as BTLA, but these are not homologous to CD28 and are therefore not shown here.
The outcome of T cell activation is influenced by a balance between engagement of activating and inhibitory receptors of the CD28 family. The inhibitory receptors of the CD28 family are CTLA-4 (cytotoxic T lymphocyte antigen 4, so called because this molecule was the fourth protein identified in a search for molecules expressed in CTLs) and PD-1 (programmed death 1). (The names of these two proteins do not accurately reflect their distribution or function.) The concept that a balance between activating and inhibitory receptors controls the magnitude of responses in the immune system was mentioned in Chapter 4 in the context of natural killer (NK) cells (see Fig. 4-6, Chapter 4). A similar idea is applicable to responses of T and B lymphocytes, although the receptors involved are quite different. Because the inhibitory receptors CTLA-4 and PD-1 are involved in the phenomenon of tolerance, and abnormalities in their expression or function cause autoimmune diseases, we will discuss them in more detail in Chapter 14 in the context of tolerance and autoimmunity. Suffice it to say here that CD28 and CTLA-4 provide an illustrative example of two receptors that recognize the same ligands (the B7 molecules) but have opposite functional effects on T cell activation. CTLA-4 is a high-affinity receptor for B7, and it has been postulated that it is engaged when B7 levels on APCs are low (as on resting APCs displaying self antigens or APCs that are no longer exposed to microbes, after the microbes are cleared and the innate immune response subsides). CD28 has a 20- to 50-fold lower affinity for B7, and it may be engaged when B7 levels are relatively high (e.g., on exposure to microbes and innate immune responses). According to this model, the level of B7 expression on APCs influences the relative engagement of CD28 or CTLA-4, and this in turn determines if responses are initiated or terminated. Once engaged, CTLA-4 may competitively inhibit access of CD28 to B7 molecules on APCs, remove B7 from the surface of APCs, or deliver inhibitory signals that block activating signals from the TCR and CD28.
Other Costimulatory Pathways
Many other T cell surface molecules, including CD2 and integrins, have been shown to deliver costimulatory signals in vitro, but their physiologic role in promoting T cell activation is less clear than that of the CD28 family. We have discussed the functions of CD2 family proteins in Chapter 7 and of integrins in Chapter 3. Several other receptors that belong to the large tumor necrosis factor (TNF) receptor (TNFR) superfamily and their ligands, which are homologous to TNF, have been shown to stimulate and to inhibit T cells under various experimental conditions. The roles of these proteins in controlling normal and pathologic immune responses remain areas of active investigation.
The interaction of CD40L on T cells with CD40 on APCs enhances T cell responses by activating the APCs. CD40 ligand (CD40L) is a TNF superfamily membrane protein that is expressed primarily on activated T cells, and CD40 is a member of the TNF receptor superfamily expressed on B cells, macrophages, and dendritic cells. The functions of CD40 in activating macrophages in cell-mediated immunity and activating B cells in humoral immune responses are described in Chapters 10 and 11, respectively. Activated helper T cells express CD40L, which engages CD40 on the APCs and activates the APCs to make them more potent by enhancing their expression of B7 molecules and secretion of cytokines such as IL-12 that promote T cell differentiation (Fig. 9-6). This phenomenon is sometimes called licensing because activated T cells license APCs to become more powerful stimulators of immune responses. Thus, the CD40 pathway indirectly amplifies T cell responses by inducing costimulators on APCs, but CD40L does not function by itself as a costimulator for T cells.
FIGURE 9–6 Role of CD40 in T cell activation.
Naive T cells are activated by peptide-MHC complexes on activated APCs. Antigen recognition by T cells together with costimulation (not shown in the figure) induces the expression of CD40 ligand (CD40L) on the activated T cells. CD40L engages CD40 on APCs and may stimulate the expression of B7 molecules and the secretion of cytokines that activate T cells. Thus, CD40L on the T cells makes the APCs “better” APCs, and thus promotes and amplifies T cell activation.
Therapeutic Costimulatory Blockade
New therapeutic agents are being developed for suppression of injurious immune responses on the basis of the understanding of these costimulatory pathways (Fig. 9-7). CTLA-4–Ig, a fusion protein consisting of the extracellular domain of CTLA-4 and the Fc portion of human IgG, binds to B7-1 and B7-2 and blocks the B7:CD28 interaction. The reason for use of the extracellular domain of CTLA-4 rather than of CD28 to block B7 molecules is that CTLA-4 binds to B7 with a 20- to 50-fold greater affinity, as mentioned before. Attachment of the Fc portion of IgG increases the in vivo half-life of the protein. CTLA-4-Ig is an approved therapy for rheumatoid arthritis, and clinical trials are currently assessing its efficacy in the treatment of transplant rejection, psoriasis, and Crohn’s disease. Antibodies that block the inhibitory receptors CTLA-4 and PD-1 are in clinical trials for the immunotherapy of tumors. As one might predict from the role of CTLA-4 in maintaining self-tolerance, blocking of this inhibitory receptor induces autoimmune reactions in some patients. Inhibitors of the CD40L:CD40 pathway are also in clinical trials for transplant rejection and chronic inflammatory autoimmune diseases.
Functional Responses of T Lymphocytes
The earliest responses of antigen-stimulated T cells include changes in the expression of various surface molecules as well as the secretion of cytokines and the expression of cytokine receptors. These are followed by proliferation of the antigen-specific cells, driven in part by the secreted cytokines, and then by differentiation of the activated cells into effector and memory cells. In the remainder of this chapter, we will describe each of these steps, their underlying mechanisms, and their functional consequences.
Changes in Surface Molecules During T Cell Activation
After activation, there are characteristic changes in the expression of various surface molecules in T cells, which are best defined in CD4+ helper cells (Fig. 9-8). Many of the molecules that are expressed in activated T cells are also involved in the functional responses of the T cells. Some of the functionally important molecules induced on activation are the following.
FIGURE 9–8 Changes in surface molecules after T cell activation.
A, The approximate kinetics of expression of selected molecules after activation of T cells by antigens and costimulators are shown. The illustrative examples include a transcription factor (c-Fos), a cytokine (IL-2), and surface proteins. These proteins are typically expressed at low levels in naive T cells and are induced by activating signals. CTLA-4 (not shown) is induced 1 to 2 days after activation. The kinetics are estimates and will vary with the nature of the antigen, its dose and persistence, and the type of adjuvant. B, The major functions of selected surface molecules are shown and described in the text.
IL-2 Secretion and IL-2 Receptor Expression
Cytokines play critical roles in adaptive immune responses; in such responses, the major sources of cytokines are T cells, especially (but not exclusively) CD4+ helper T cells. The most important cytokine produced by T cells early after activation, often within 2 to 4 hours after recognition of antigen and costimulators, is interleukin-2 (IL-2), and this is described here. The cytokines secreted by effector cells are described in Chapter 10, when we discuss the functions of effector CD4+ T cells.
IL-2 is a growth, survival, and differentiation factor for T lymphocytes and plays a major role in the regulation of T cell responses by virtue of its crucial role in the maintenance of regulatory T cells. Because of its ability to support proliferation of antigen-stimulated T cells, IL-2 was originally called T cell growth factor (TCGF). It acts on the same cells that produce it or on adjacent cells (i.e., it functions as an autocrine or paracrine cytokine).
IL-2 is produced mainly by CD4+ T lymphocytes. Activation of T cells by antigens and costimulators stimulates transcription of the IL-2 gene and synthesis and secretion of the protein. IL-2 production is rapid and transient, starting within 2 to 3 hours of T cell activation, peaking at about 8 to 12 hours, and declining by 24 hours. CD4+ T cells secrete IL-2 into the immunologic synapse formed between the T cell and APC (see Chapter 7). IL-2 receptors on T cells also tend to localize to the synapse, so that the cytokine and its receptor reach sufficiently high local concentrations to initiate cellular responses.
Secreted IL-2 is a 14- to 17-kD glycoprotein that folds into a globular protein containing four α helices (Fig. 9-9). It is the prototype of the four–α-helical cytokines that interact with type I cytokine receptors (see Chapter 7).
FIGURE 9–9 Structure of IL-2 and its receptor.
The crystal structure of IL-2 and its trimeric receptor shows how the cytokine interacts with the three chains of to the receptor.
(Reproduced from Wang X, M Rickert, and KC Garcia. Structure of the quaternary complex of interleukin-2 with its α, β and γc receptors. Science 310:1159-1163, 2005, with the permission of the publishers. Courtesy of Drs. Patrick Lupardus and K. Christopher Garcia, Stanford University School of Medicine, Palo Alto, California.)
Functional IL-2 receptors are transiently expressed on activation of naive and effector T cells; regulatory T cells always express IL-2 receptors. The IL-2 receptor (IL-2R) consists of three noncovalently associated proteins including IL-2Rα (CD25), IL-2/15Rβ (CD122), and γc (CD132). Of the three chains, only IL-2Rα is unique to the IL-2R. IL-2 binds to the α chain alone with low affinity, and this does not lead to any detectable cytoplasmic signaling or biologic response. IL-2/15Rβ, which is also part of the IL-15 receptor, contributes to IL-2 binding and engages JAK3-STAT5–dependent signal transduction pathways (see Chapter 7). The γ chain is shared with a number of cytokine receptors, including those for IL-4, IL-7, IL-9, IL-15, and IL-21, and is therefore called the common γ chain (γc). Even though the γc is not directly involved in binding IL-2, its association with the receptor complex is required for high-affinity IL-2 binding and for full activation of signal transduction pathways. The IL-2Rβγc complexes are expressed at low levels on resting T cells (and on NK cells) and bind IL-2 with a Kd of approximately 10−9 M (Fig. 9-10). Expression of IL-2Rα and, to a lesser extent, of IL-2Rβ is increased on activation of naive CD4+ and CD8+ T cells. Cells that express IL-2Rα and form IL-2Rαβγc complexes can bind IL-2 more tightly, with a Kd of approximately 10−11 M, and growth stimulation of such cells occurs at a similarly low IL-2 concentration. IL-2, produced in response to antigen stimulation, is a stimulus for induction of IL-2Rα, providing a mechanism by which T cell responses amplify themselves. CD4+ regulatory T cells (see Chapter 14) express the full IL-2R complex and are thus poised to respond to the cytokine. Chronic T cell stimulation leads to shedding of IL-2Rα, and an increased level of shed IL-2Rα in the serum is used clinically as a marker of strong antigenic stimulation (e.g., acute rejection of a transplanted organ).
FIGURE 9–10 Regulation of IL-2 receptor expression.
Resting (naive) T lymphocytes express the IL-2Rβγ complex, which has a moderate affinity for IL-2. Activation of the T cells by antigen, costimulators, and IL-2 itself leads to expression of the IL-2Rα chain and high levels of the high-affinity IL-2Rαβγ complex.
Functions of IL-2
The biology of IL-2 is fascinating because it plays critical roles in both promoting and controlling T cell responses and functions (Fig. 9-11).
Clonal Expansion of T Cells
T cell proliferation in response to antigen recognition is mediated primarily by a combination of signals from the antigen receptor, costimulators, and autocrine growth factors, primarily IL-2. The cells that recognize antigen produce IL-2 and also preferentially respond to it, ensuring that the antigen-specific T cells are the ones that proliferate the most. The result of this proliferation is clonal expansion, which generates the large number of cells required to eliminate the antigen from a small pool of naive antigen-specific lymphocytes. Before antigen exposure, the frequency of naive T cells specific for any antigen is 1 in 105 to 106 lymphocytes. After microbial antigen exposure, the frequency of all CD8+ T cells specific for that microbe may increase to about 1 in 3 to 1 in 10, representing a >50,000-fold expansion of antigen-specific CD8+ T cells, and the number of specific CD4+ cells increases to 1 in 100 to about 1 in 1000 lymphocytes (Fig. 9-12). Studies in mice first showed this tremendous expansion of the antigen-specific population in some acute viral infections and, remarkably, it occurred within as little as 1 week after infection. Equally remarkable was the finding that during this massive antigen-specific clonal expansion, “bystander” T cells not specific for the virus did not proliferate. The expansion of T cells specific for Epstein-Barr virus and human immunodeficiency virus (HIV) in acutely infected humans is also on this order of magnitude. This conclusion has been reached by analyses of antigen-specific T cell responses in humans, using either fluorescent multimers of MHC molecules loaded with particular peptides or intracellular cytokine stains of T cells stimulated with peptides derived from these viruses (see Appendix IV).
FIGURE 9–12 Clonal expansion of T cells.
The numbers of CD4+ and CD8+ T cells specific for microbial antigens and the expansion and decline of the cells during immune responses are illustrated. The numbers are approximations based on studies of model microbial and other antigens in inbred mice.
Many of the progeny of the antigen-stimulated cells differentiate into effector cells. Because there are important differences in effector cells of the CD4+ and CD8+ lineages, these are described separately below. Effector cells are short-lived, and the numbers of antigen-specific T cells rapidly decline as the antigen is eliminated. After the immune response subsides, the surviving memory cells specific for the antigen number on the order of 1 in 104.
Differentiation of CD4+ T Cells into TH1, TH2, and TH17 Effector Cells
Effector cells of the CD4+ lineage are characterized by their ability to express surface molecules and to secrete cytokines that activate other cells (B lymphocytes, macrophages, and dendritic cells). Whereas naive CD4+ T cells produce mostly IL-2 on activation, effector CD4+ T cells are capable of producing a large number and variety of cytokines that have diverse biologic activities.
There are three distinct subsets of CD4+ T cells, called TH1, TH2, and TH17, that function in host defense against different types of infectious pathogens and are involved in different types of tissue injury in immunologic diseases (Fig. 9-13). A fourth population, called follicular helper T cells, is important in antibody responses and is described in Chapter 11. Regulatory T cells are another distinct population of CD4+ T cells. Their function is to control immune reactions to self and foreign antigens, and they are described in Chapter 14 in the context of immunologic tolerance. Although these subsets are identifiable in immune reactions (and can often be generated in cell culture), many effector CD4+ T cells produce various combinations of cytokines or only some of the cytokines characteristic of a particular subset and are not readily classifiable into separable populations. Whether these populations with mixed or limited cytokine patterns are intermediates in the development of the polarized effector cells or are themselves fixed populations is not known. It is also clear that some of these differentiated T cells may convert from one population into another by changes in activation conditions. The extent and significance of such “plasticity” are topics of active research.
FIGURE 9–13 Properties of TH1, TH2, and TH17 subsets of CD4+ helper T cells.
Naive CD4+ T cells may differentiate into distinct subsets of effector cells in response to antigen, costimulators, and cytokines. The columns to the right list the major differences between the best-defined subsets.
Properties of TH1, TH2, and TH17 Subsets
Elucidation of the development, properties, and functions of subsets of effector CD4+ T cells has been one of the most impressive accomplishments of immunology research. It was appreciated many years ago that host responses to different infections varied greatly, as did the reactions in different immunologic diseases. For instance, the immune reaction to intracellular bacteria like Mycobacterium tuberculosis is dominated by activated macrophages, whereas the reaction to helminthic parasites consists of IgE antibody production and the activation of eosinophils. Along the same lines, in many chronic autoimmune diseases, tissue damage is caused by inflammation with accumulation of neutrophils, macrophages, and T cells, whereas in allergic disorders, the lesions contain abundant eosinophils along with other leukocytes. The realization that all these phenotypically diverse immunologic reactions are dependent on CD4+ T cells raised an obvious question: How can the same CD4+ cells elicit such different responses? The answer, as we now know, is that CD4+ T cells consist of subsets of effector cells that produce distinct sets of cytokines, elicit quite different reactions, and are involved in host defense against different microbes as well as in distinct types of immunologic diseases. The first subsets that were discovered were called TH1 and TH2 (so named because they were the first two subsets identified). It was subsequently found that some inflammatory diseases that were thought to be caused by TH1-mediated reactions were clearly not dependent on this type of T cell, and this realization led to the discovery of TH17 cells (called TH17 because their characteristic cytokine is IL-17). In the next section, we describe the properties of these subsets and how they develop from naive T cells. We will return to their cytokine products, effector functions, and roles in cell-mediated immunity in Chapter 10.
The defining characteristics of differentiated subsets of effector cells are the cytokines they produce, the transcription factors they express, and epigenetic changes in cytokine gene loci. These characteristics of TH1, TH2, and TH17 cells are described below.
The signature cytokines produced by the major CD4+ T cell subsets are IFN-γ for TH1 cells; IL-4, IL-5, and IL-13 for TH2 cells; and IL-17 and IL-22 for TH17 cells (see Fig. 9-13). The cytokines produced by these T cell subsets determine their effector functions and roles in diseases. The cytokines also participate in the development and expansion of the respective subsets (described later). In addition, these subsets of T cells differ in the expression of adhesion molecules and receptors for chemokines and other cytokines, which are involved in the migration of distinct subsets to different tissues (see Chapter 10).
Development of TH1, TH2, and TH17 Subsets
Differentiated TH1, TH2, and TH17 cells all develop from naive CD4+ T lymphocytes, mainly in response to cytokines present early during immune responses, and differentiation involves transcriptional activation and epigenetic modification of cytokine genes. The process of differentiation, which is sometimes referred to as polarization of T cells, can be divided into induction, stable commitment, and amplification (Fig. 9-14). Cytokines act on antigen-stimulated T cells to induce the transcription of cytokine genes that are characteristic of differentiation toward each subset. With continued activation, epigenetic changes occur so that the genes encoding that subset’s cytokines are more accessible for activation, and genes that encode cytokines not produced by that subset are rendered inaccessible. Because of these changes, the differentiating T cell becomes progressively committed to one specific pathway. Cytokines produced by any given subset promote the development of this subset and inhibit differentiation toward other CD4+ subpopulations. Thus, positive and negative feedback loops contribute to the generation of an increasingly polarized population of effector cells.
FIGURE 9–14 Development of TH1, TH2, and TH17 subsets.
Cytokines produced early in the innate or adaptive immune response to microbes promote the differentiation of naive CD4+ T cells into TH1, TH2, or TH17 cells by activating transcription factors that stimulate production of the cytokines of each subset (the early induction step). Progressive activation leads to stable changes in the expressed genes (commitment), and cytokines promote the development of each population and suppress the development of the other subsets (amplification). These principles apply to all three major subsets of CD4+ effector T cells.
There are several important general features of T cell subset differentiation.
With this background, we proceed to a description of the signals for and mechanisms of development of each subset.
TH1 Differentiation
TH1 differentiation is driven mainly by the cytokines IL-12 and IFN-γ and occurs in response to microbes that activate dendritic cells, macrophages, and NK cells (Fig. 9-15). The differentiation of antigen-activated CD4+ T cells to TH1 effectors is stimulated by many intracellular bacteria, such as Listeria and mycobacteria, and by some parasites, such as Leishmania, all of which infect dendritic cells and macrophages. It is also stimulated by viruses and by protein antigens administered with strong adjuvants. A common feature of these infections and immunization conditions is that they elicit innate immune reactions that are associated with the production of certain cytokines, including IL-12, IL-18, and type I interferons. All these cytokines promote TH1 development; of these, IL-12 is probably the most potent. Knockout mice lacking IL-12 are extremely susceptible to infections with intracellular microbes. IL-18 synergizes with IL-12, and type I interferons may be important for TH1 differentiation in response to viral infections, especially in humans. Other microbes stimulate NK cells to produce IFN-γ, which is itself a strong TH1-inducing cytokine and also acts on dendritic cells and macrophages to induce more IL-12 secretion. Once TH1 cells have developed, they secrete IFN-γ, which promotes more TH1 differentiation and thus strongly amplifies the reaction. In addition, IFN-γ inhibits the differentiation of naive CD4+ T cells to the TH2 and TH17 subsets, thus promoting the polarization of the immune response in one direction. T cells may further enhance cytokine production by dendritic cells and macrophages, by virtue of CD40 ligand (CD40L) on activated T cells engaging CD40 on the APCs and stimulating IL-12 secretion.
FIGURE 9–15 Development of TH1 cells.
IL-12 produced by dendritic cells and macrophages in response to microbes, including intracellular microbes, and IFN-γ produced by NK cells (all part of the early innate immune response to the microbes) activate the transcription factors T-bet, STAT1, and STAT4, which stimulate the differentiation of naive CD4+ T cells to the TH1 subset. IFN-γ produced by the TH1 cells amplifies this response and inhibits the development of TH2 and TH17 cells.
IFN-γ and IL-12 stimulate TH1 differentiation by activating the transcription factors T-bet, STAT1, and STAT4 (see Fig. 9-15). T-bet, a member of the T-box family of transcription factors, is considered to be the master regulator of TH1 differentiation. T-bet expression is induced in naive CD4+ T cells in response to antigen and IFN-γ. IFN-γ activates the transcription factor STAT1, which in turn stimulates expression of T-bet. T-bet then promotes IFN-γ production through a combination of direct transcriptional activation of the IFN-γ gene and by inducing chromatin remodeling of the IFN-γ locus. The ability of IFN-γ to stimulate T-bet expression and the ability of T-bet to enhance IFN-γ transcription set up a positive amplification loop that drives differentiation of T cells toward the TH1 phenotype. IL-12 contributes to TH1 commitment by binding to receptors on antigen-stimulated CD4+ T cells and activating the transcription factor STAT4, which further enhances IFN-γ production. Mice deficient in IL-12, IL-12 receptor, T-bet, or STAT4 cannot mount effective TH1 responses to infections, and humans with genetic deficiencies in the IL-12R signaling pathway have impaired responses to infections with several kinds of intracellular bacteria.
TH2 Differentiation
TH2 differentiation is stimulated by the cytokine IL-4 and occurs in response to helminths and allergens (Fig. 9-16). Helminths and allergens cause chronic T cell stimulation, often without the strong innate immune responses that are required for TH1 differentiation. Thus, TH2 cells may develop in response to microbes and antigens that cause persistent or repeated T cell stimulation without much inflammation or the production of pro-inflammatory cytokines that drive TH1 and TH17 responses. The differentiation of antigen-stimulated T cells to the TH2 subset is dependent on IL-4, which raises an interesting question: Because differentiated TH2 cells are the major source of IL-4 during immune responses to protein antigens, where does the IL-4 come from before TH2 cells develop? In some situations, such as helminthic infections, IL-4 produced by mast cells and, possibly, other cell populations, such as basophils recruited into lymphoid organs and eosinophils, may contribute to TH2 development. Another possibility is that antigen-stimulated CD4+ T cells secrete small amounts of IL-4 from their initial activation. If the antigen is persistent and present at high concentrations, the local concentration of IL-4 gradually increases. If the antigen also does not trigger inflammation with attendant IL-12 production, the result is increasing differentiation of T cells to the TH2 subset. Once TH2 cells have developed, the IL-4 they produce serves to amplify the reaction and inhibits the development of TH1 and TH17 cells.
FIGURE 9–16 Development of TH2 cells.
IL-4 produced by activated T cells themselves or by mast cells and eosinophils, especially in response to helminths, activates the transcription factors GATA-3 and STAT6, which stimulate the differentiation of naive CD4+ T cells to the TH2 subset. IL-4 produced by the TH2 cells amplifies this response and inhibits the development of TH1 and TH17 cells.
IL-4 stimulates TH2 development by activating the transcription factor STAT6, and STAT6, together with TCR signals, induces expression of GATA-3 (see Fig. 9-16). GATA-3 is a transcription factor that acts as a master regulator of TH2 differentiation, enhancing expression of the TH2 cytokine genes IL-4, IL-5, and IL-13, which are located in the same genetic locus. GATA-3 works by directly interacting with the promoters of these genes and also by causing chromatin remodeling, which opens up the locus for accessibility to other transcription factors. This is similar to the way in which T-bet influences IFN-γ expression. GATA-3 functions to stably commit differentiating cells toward the TH2 phenotype, enhancing its own expression through a positive feedback loop. Furthermore, GATA-3 blocks TH1 differentiation by inhibiting expression of the signaling chain of the IL-12 receptor. Knockout mice lacking IL-4, STAT6, or GATA-3 are deficient in TH2 responses.
TH17 Differentiation
The development of TH17 cells is stimulated by pro-inflammatory cytokines produced in response to bacteria and fungi (Fig. 9-17). Various bacteria and fungi act on dendritic cells and stimulate the production of cytokines including IL-6, IL-1, and IL-23. Engagement of the lectin-like receptor Dectin-1 on dendritic cells by fungal products is a signal for the production of these cytokines. The combination of cytokines that drive TH17 cell development may be produced not only in response to particular microbes, such as fungi, but also when cells infected with various bacteria and fungi undergo apoptosis and are ingested by dendritic cells. IL-23 may be more important for the proliferation and maintenance of TH17 cells than for their induction. TH17 differentiation is inhibited by IFN-γ and IL-4; therefore, strong TH1 and TH2 responses tend to suppress TH17 development. A surprising aspect of TH17 differentiation is that TGF-β, which is produced by many cell types and is an anti-inflammatory cytokine (see Chapter 14), promotes the development of proinflammatory TH17 cells when other mediators of inflammation, such as IL-6 or IL-1, are present. Some experimental results indicate that TGF-β does not directly stimulate TH17 development but is a potent suppressor of TH1 and TH2 differentiation and thus removes the inhibitory effect of these two subsets and allows the TH17 response to develop under the influence of IL-6 or IL-1. According to this idea, the action of TGF-β in promoting TH17 responses is indirect. TH17 cells produce IL-21, which may further enhance their development, providing an amplification mechanism.
FIGURE 9–17 Development of TH17 cells.
IL-1 and IL-6 produced by APCs and transforming growth factor-β (TGF-β) produced by various cells activate the transcription factors RORγt and STAT3, which stimulate the differentiation of naive CD4+ T cells to the TH17 subset. IL-23, which is also produced by APCs, especially in response to fungi, stabilizes the TH17 cells. TGF-β may promote TH17 responses indirectly by suppressing TH1 and TH2 cells, both of which inhibit TH17 differentiation (not shown in the figure). IL-21 produced by the TH17 cells amplifies this response.
The development of TH17 cells is dependent on the transcription factors RORγt and STAT3 (see Fig. 9-17). TGF-β and the inflammatory cytokines, mainly IL-6 and IL-1, work cooperatively to induce the production of RORγt, a transcription factor that is a member of the retinoic acid receptor family. RORγt is a T cell–restricted protein encoded by the RORC gene, so sometimes the protein may be referred to as RORc. The inflammatory cytokines, notably IL-6, activate the transcription factor STAT3, which functions with RORγt to drive the TH17 response. Mutations in the gene encoding STAT3 are the cause of a rare human immune deficiency disease called Job’s syndrome because patients present with multiple bacterial and fungal abscesses of the skin, resembling the biblical punishments visited on Job. These patients have defective TH17 responses.
TH17 cells appear to be especially abundant in mucosal tissues, particularly of the gastrointestinal tract, suggesting that the tissue environment influences the generation of this subset, perhaps by providing high local concentrations of TGF-β and other cytokines. This observation also suggests that TH17 cells may be especially important in combating intestinal infections and in the development of intestinal inflammation. The development of TH17 cells in the gastrointestinal tract is also dependent on the local microbial population.
The functions of differentiated effector cells of the CD4+ lineage are mediated by surface molecules, primarily CD40 ligand, and by secreted cytokines. We will describe the cytokines produced by differentiated CD4+ effector cells and their functions in Chapter 10.
Differentiation of CD8+ T Cells into Cytotoxic T Lymphocytes
The activation of naive CD8+ T cells requires antigen recognition and second signals, but the nature of the second signals may be different from those for CD4+ cells. We have previously described the role of dendritic cells in presenting antigens to and costimulating naive CD8+ cells.
The full activation of naive CD8+ T cells and their differentiation into functional CTLs and memory cells may require the participation of CD4+ helper cells. In other words, helper T cells can provide second signals for CD8+ T cells. The requirement for helper cells may vary according to the type of antigen exposure. In the setting of a strong innate immune response to a microbe, if APCs are directly infected by the microbe, or if cross-presentation of microbial antigens is efficient, CD4+ T cell help may not be required. CD4+ helper T cells may be required for CD8+ T cell responses to latent viral infections, organ transplants, and tumors, all of which tend to elicit relatively weak innate immune reactions. The varying importance of CD4+ T cells in the development of CTL responses is illustrated by studies with mice that lack helper T cells. In these mice, some viral infections fail to generate effective CTLs or CD8+ memory cells and are not eradicated, whereas other viruses do stimulate effective CTL responses. A lack of CD4+ T cell helper function is the accepted explanation for the defects in CTL generation seen in individuals infected with HIV, which infects and eliminates only CD4+ T cells.
Helper T cells may promote CD8+ T cell activation by several mechanisms (Fig. 9-18). Helper T cells may secrete cytokines that stimulate the differentiation of CD8+ T cells. Antigen-stimulated helper T cells express CD40 ligand (CD40L), which binds to CD40 on APCs and activates (“licenses”) these APCs to make them more efficient at stimulating the differentiation of CD8+ T cells.
FIGURE 9–18 Role of helper T cells in the differentiation of CD8+ T lymphocytes.
CD4+ helper T cells promote the development of CD8+ CTLs by secreting cytokines that act directly on the CD8+ cells (A) or by activating APCs to become more effective at stimulating the differentiation of the CD8+ T cells (B).
Differentiation of CD8+ T cells into effector CTLs involves acquisition of the machinery to perform target cell killing. The most specific feature of CTL differentiation is the development of membrane-bound cytoplasmic granules that contain proteins, including perforin and granzymes, whose function is to kill other cells (described in Chapter 10). In addition, differentiated CTLs are capable of secreting cytokines, mostly IFN-γ, that function to activate phagocytes. The molecular events in CTL differentiation involve transcription of genes encoding these effector molecules. Two transcription factors that are required for this program of new gene expression are T-bet (which we discussed in relationship to TH1 differentiation earlier) and eomesodermin, which is structurally related to T-bet.
Development of Memory T Cells
T cell–mediated immune responses to an antigen usually result in the generation of memory T cells specific for that antigen, which may persist for years, even a lifetime. Thus, memory cells provide optimal defense against pathogens that are prevalent in the environment and may be repeatedly encountered. The success of vaccination is attributed in large part to the ability to generate memory cells on initial antigen exposure. Edward Jenner’s classic experiment of successful vaccination of a child against smallpox is a demonstration of a memory response. Despite the importance of this historic observation, many fundamental questions about the generation of memory cells have still not been answered.
As expected, the size of the memory pool is proportional to the size of the naive antigen-specific population. Memory cells may develop from effector cells along a linear pathway, or effector and memory populations follow divergent differentiation and are two alternative fates of lymphocytes activated by antigen and other stimuli (Fig. 9-19). The mechanisms that determine whether an individual antigen-stimulated T cell will become a short-lived effector cell or enter the long-lived memory cell pool are not established. The signals that drive the development of memory cells are also not established. One possibility is that memory cells contain transcription factors that are different from those present in effector cells, and the nature of the transcription factors influences the choice of differentiation pathway.
FIGURE 9–19 Development of memory T cells.
In response to antigen and costimulation, naive T cells differentiate into effector and memory cells. Some of the phenotypic markers of these cell populations are shown in A. A, According to the linear model of memory T cell differentiation, most effector cells die and some survivors develop into the memory population. B, According to the branched differentiation model, effector and memory cells are alternative fates of activated T cells.
Properties of Memory T Cells
Memory T cells have several characteristics that are responsible for their survival and rapid activation.
The most reliable phenotypic markers for memory T cells appear to be the surface expression of the IL-7 receptor and a protein of unknown function called CD27 and the absence of markers of naive and recently activated T cells (see Table 2-3, Chapter 2; and Fig. 9-19). In humans, most naive T cells express the 200-kD isoform of a surface molecule called CD45 that contains a segment encoded by an exon designated A. This CD45 isoform can be recognized by antibodies specific for the A-encoded segment and is therefore called CD45RA (for “restricted A”). In contrast, most memory T cells express a 180-kD isoform of CD45 in which the A exon RNA has been spliced out; this isoform is called CD45RO. However, this way of distinguishing naive from memory T cells is not perfect, and interconversion between CD45RA+ and CD45RO+ populations has been documented.
Both CD4+ and CD8+ memory T cells are heterogeneous and can be subdivided into subsets based on their homing properties and functions. Central memory T cells express the chemokine receptor CCR7 and L-selectin and home mainly to lymph nodes. They have a limited capacity to perform effector functions when they encounter antigen, but they undergo brisk proliferative responses and generate many effector cells on antigen challenge. Effector memory T cells, on the other hand, do not express CCR7 or L-selectin and home to peripheral sites, especially mucosal tissues. On antigenic stimulation, effector memory T cells produce effector cytokines such as IFN-γ or rapidly become cytotoxic, but they do not proliferate much. This effector subset, therefore, is poised for a rapid response to a repeated exposure to a microbe, but complete eradication of the infection may also require large numbers of effectors generated from the central memory T cells. It is unclear if all memory T cells can be classified into central and effector memory cells.
Memory T cells are also heterogeneous in terms of cytokine profiles. For example, some CD4+ memory T cells may be derived from precursors before commitment to the TH1, TH2, or TH17 phenotype, and when activated by re-exposure to antigen and cytokines, they can differentiate into any of these subsets. Other memory T cells may be derived from fully differentiated TH1, TH2, or TH17 effectors and retain their respective cytokine profiles on reactivation. Memory CD8+ T cells may also exist that maintain some of the phenotypic characteristics of differentiated CTLs.
Decline of T Cell Responses
Elimination of antigen leads to contraction of the T cell response, and this decline is responsible for maintaining homeostasis in the immune system. There are several reasons that the response declines. As the antigen is eliminated and the innate immune response associated with antigen exposure abates, the signals that normally keep activated lymphocytes alive and proliferating are no longer active. As mentioned earlier, costimulation and growth factors like IL-2 stimulate expression of the antiapoptotic proteins Bcl-2 and Bcl-XL in the activated lymphocytes, and these proteins keep cells viable. As the level of costimulation and the amount of available IL-2 decrease, the levels of antiapoptotic proteins in the cells drop. At the same time, growth factor deprivation activates sensors of cellular stress (such as the BH3-only protein Bim), which trigger the mitochondrial pathway of apoptosis and are no longer opposed by the antiapoptotic proteins (see Fig. 14-7, Chapter 14). The net result of these changes is that most of the cells that were produced by activation die and the generation of newly activated cells declines, so the pool of antigen-activated lymphocytes contracts.
There has been much interest in the possibility that various regulatory mechanisms play a role in the normal contraction of immune responses. Such mechanisms might include the inhibitory receptors CTLA-4 and PD-1, apoptosis induced by death receptors of the TNF receptor superfamily (such as TNFRI and Fas), and regulatory T cells. However, it is still not established that these control mechanisms are essential for the normal decline of most immune responses.
Summary
T Cell Activation and Proliferation
Boyman O, Letourneau S, Krieg C, Sprent J. Homeostatic proliferation and survival of naive and memory T cells. European Journal of Immunology. 2009;39:2088-2094.
Jenkins MK, Chu HH, McLachlan JB, Moon JJ. On the composition of the pre-immune repertoire of T cells specific for peptide–major histocompatibility complex ligands. Annual Review of Immunology. 2010;28:275-294.
Von Andrian UH, Mackay CR. T-cell function and migration. New England Journal of Medicine. 2000;343:1020-1034.
Costimulation: B7, CD28, and More
Chen L. Co-inhibitory molecules of the B7-CD28 family in the control of T-cell immunity. Nature Reviews Immunology. 2004;4:336-347.
Croft M. Co-stimulatory members of the TNFR family: keys to effective T cell immunity? Nature Reviews Immunology. 2003;3:609-620.
Greenwald RJ, Freeman GJ, Sharpe AH. The B7 family revisited. Annual Review of Immunology. 2005;23:515-548.
Keir ME, Butte MJ, Freeman GJ, Sharpe AH. PD-1 and its ligands in tolerance and immunity. Annual Review of Immunology. 2008;26:677-704.
Watts TH. TNF/TNFR family members in costimulation of T cell responses. Annual Review of Immunology. 2005;23:23-68.
Huse M, Quann EJ, Davis MM. Shouts, whispers and the kiss of death: directional secretion in T cells. Nature Immunology. 2008;9:1105-1111.
Malek TR. The biology of interleukin-2. Annual Review of Immunology. 2008;26:453-479.
Rochman Y, Spolski R, Leonard WJ. New insights into the regulation of T cells by γc family cytokines. Nature Reviews Immunology. 2009;9:169-173.
Differentiation of CD4+ T Cells into Subsets of Effector Cells: TH1, TH2, and TH17
Amsen D, Spilanakis CG, Flavell RA. How are TH1 and TH2 cells made? Current Opinion in Immunology. 2009;21:153-160.
Annunziato F, Romagnani S. Heterogeneity of human effector CD4+ T cells. Arthritis Research and Therapy. 2009;11:257-264.
Bettelli E, Korn T, Oukka M, Kuchroo VK. Induction and effector functions of TH17 cells. Nature. 2008;453:1051-1057.
Dong C. TH17 cells in development: an updated view of their molecular identity and genetic programming. Nature Reviews Immunology. 2008;8:337-348.
Korn T, Bettelli E, Oukka M, Kuchroo VK. IL-17 and TH17 cells. Annual Review of Immunology. 2009;27:485-517.
Lee GR, Kim ST, Spilianakis CG, Fields PE, Flavell RA. T helper cell differentiation: regulation by cis elements and epigenetics. Immunity. 2006;24:369-379.
Locksley RM. Nine lives: plasticity among helper T cell subsets. Journal of Experimental Medicine. 2009;206:1643-1646.
McGeachy MJ, Cua DJ. Th17 cell differentiation: the long and winding road. Immunity. 2008;28:445-453.
Murphy KM, Reiner SL. The lineage decisions of helper T cells. Nature Reviews Immunology. 2002;2:933-944.
Murphy KM, Stockinger B. Effector T cell plasticity: flexibility in the face of changing circumstances. Nature Immunology. 2010;11:674-680.
Paul WE, Zhu J. How are TH2 responses initiated and amplified? Nature Reviews Immunology. 2010;10:225-235.
Placek K, Coffre M, Maiella S, Bianchi E, Rogge L. Genetic and epigenetic networks controlling T helper cell 1 differentiation. Immunology. 2009;127:155-162.
Steinman L. A brief history of TH17, the first major revision in the TH1/ TH2 hypothesis of T cell–mediated tissue damage. Nature Medicine. 2007;13:139-145.
Wilson CB, Rowell E, Sekimata M. Epigenetic control of T-helper cell differentiation. Nature Reviews Immunology. 2009;9:91-105.
Zhu J, Yamane H, Paul WE. Differentiation of effector CD4 T cell populations. Annual Review of Immunology. 2010;28:445-489.
Castellino F, Germain RN. Cooperation between CD4+ and CD8+ T cells: when, where, and how. Annual Review of Immunology. 2006;24:519-540.
Masopust D, Vezys V, Wherry EJ, Ahmed R. A brief history of CD8 T cells. European Journal of Immunology. 2007;37:S103-S110.
Prlic M, Williams MA, Bevan MJ. Requirements for CD8 T-cell priming, memory generation, and maintenance. Current Opinion in Immunology. 2007;19:315-319.
Gourley TS, Wherry EJ, Masopust D, Ahmed R. Generation and maintenance of immunological memory. Seminars in Immunology. 2004;16:323-333.
Sallusto F, Geginat J, Lanzavecchia A. Central memory and effector memory T cell subsets: function, generation, and maintenance. Annual Review of Immunology. 2004;22:745-763.
Sallusto F, Lanzavecchia A. Heterogeneity of CD4+ memory T cells: functional modules for tailored immunity. European Journal of Immunology. 2009;39:2076-2082.
Schluns KS, Lefrancois L. Cytokine control of memory T cell development and survival. Nature Reviews Immunology. 2003;3:269-279.