Cell-Mediated chapter 14 Effector responses HE CELL-MEDIATED AND HUMORAL BRANCHES OF the immune system assume different roles tecting the host. The effectors of the humoral branch are secreted antibodies, highly specific molecules that can bind and neutralize antigens on the surface of cells and in the extracellular spaces. The primary domain of anti- body protection lies outside the cell. If antibodies were the Big CtL Attacks Little Tumor Cell only agents of immunity, pathogens that managed to evade them and colonize the intracellular environment would Effector Responses escape the immune system. This is not the case. The princi pal role of cell-mediated immunity is to detect and eliminate a General Properties of Effector T Cells cells that harbor intracellular pathogens. Cell-mediated im a Cytotoxic T Cells munity also can recognize and eliminate cells, such as tumor lls, that have undergone genetic modifications so that they Natural Killer Cells xpress antigens not typical of normal cells Antibody-Dependent Cell-Mediated Cytotoxicity Both antigen-specific and-nonspecific cells can contri bute to the cell-mediated immune response. Specific cells in- Experimental Assessment of Cell-Mediated chude CD8 cytotoxic T lymphocytes(Tc cells or CTls)and Cytotoxicity cytokine-secreting CD4 TH cells that mediate delayed-type ypersensitivity(DTH). The discussion of DTH reactions and the role of CD4 T cells in their orchestration appears in Chapter 16. Nonspecific cells include nK cells and non- lymphoid cell types such as macrophages, neutrophils, and sinophils. The activity of both specific and nonspecific components usually depends on effective local concentra- Effector Responses tions of various cytokines. T cells, NK cells, and macrophages are the most important sources of the cytokines that organize The importance of cell-mediated immunity becomes evident and support cell-mediated immunity. Finally, although hu- when the system is defective. Children with DiGeorge syn noral and cell-mediated immunity have many distinctive drome, who are born without a thymus and therefore lack the features, they are not completely independent. Cells such T-cell component of the cell-mediated immune system, gen- as macrophages, NK cells, neutrophils, and eosinophils can erally are able to cope with infections of extracellular bacteria, use antibodies as receptors to recognize and target cells for but they cannot effectively eliminate intracellular pathogens killing. Also, chemotactic peptides generated by the activa- Their lack of functional cell-mediated immunity results in tion of complement in response to antigen-antibody com- repeated infections with viruses, intracellular bacteria, and plexes can contribute to assembling the cell types required for fungi. The severity of the cell-mediated immunodeficiency in a cell-mediated response these children is such that even the attenuated virus present in In the preceding chapters, various aspects of the humoral a vaccine, capable of only limited growth in normal individu and cell-mediated effector responses have been described. This als, can produce life-threatening infections. chapter addresses cytotoxic effector mechanisms mediated by Cell-mediated immune responses can be divided into two Tc cells, NK cells, antibody-dependent cell-mediated cytoto major categories according to the different effecto city(adcc), and the tions that are mobilized. One group comprises effector cells
■ Effector Responses ■ General Properties of Effector T Cells ■ Cytotoxic T Cells ■ Natural Killer Cells ■ Antibody-Dependent Cell-Mediated Cytotoxicity ■ Experimental Assessment of Cell-Mediated Cytotoxicity Big CTL Attacks Little Tumor Cell Cell-Mediated Effector Responses T - the immune system assume different roles in protecting the host. The effectors of the humoral branch are secreted antibodies, highly specific molecules that can bind and neutralize antigens on the surface of cells and in the extracellular spaces. The primary domain of antibody protection lies outside the cell. If antibodies were the only agents of immunity, pathogens that managed to evade them and colonize the intracellular environment would escape the immune system. This is not the case. The principal role of cell-mediated immunity is to detect and eliminate cells that harbor intracellular pathogens. Cell-mediated immunity also can recognize and eliminate cells, such as tumor cells, that have undergone genetic modifications so that they express antigens not typical of normal cells. Both antigen-specific and -nonspecific cells can contribute to the cell-mediated immune response. Specific cells include CD8+ cytotoxic T lymphocytes (TC cells or CTLs) and cytokine-secreting CD4+ TH cells that mediate delayed-type hypersensitivity (DTH). The discussion of DTH reactions and the role of CD4+ T cells in their orchestration appears in Chapter 16. Nonspecific cells include NK cells and nonlymphoid cell types such as macrophages, neutrophils, and eosinophils. The activity of both specific and nonspecific components usually depends on effective local concentrations of various cytokines. T cells, NK cells, and macrophages are the most important sources of the cytokines that organize and support cell-mediated immunity. Finally, although humoral and cell-mediated immunity have many distinctive features, they are not completely independent. Cells such as macrophages, NK cells, neutrophils, and eosinophils can use antibodies as receptors to recognize and target cells for killing. Also, chemotactic peptides generated by the activation of complement in response to antigen-antibody complexes can contribute to assembling the cell types required for a cell-mediated response. In the preceding chapters, various aspects of the humoral and cell-mediated effector responses have been described. This chapter addresses cytotoxic effector mechanisms mediated by TC cells, NK cells, antibody-dependent cell-mediated cytotoxicity (ADCC), and the experimental assay of cytotoxicity. Effector Responses The importance of cell-mediated immunity becomes evident when the system is defective. Children with DiGeorge syndrome, who are born without a thymus and therefore lack the T-cell component of the cell-mediated immune system, generally are able to cope with infections of extracellular bacteria, but they cannot effectively eliminate intracellular pathogens. Their lack of functional cell-mediated immunity results in repeated infections with viruses, intracellular bacteria, and fungi. The severity of the cell-mediated immunodeficiency in these children is such that even the attenuated virus present in a vaccine, capable of only limited growth in normal individuals, can produce life-threatening infections. Cell-mediated immune responses can be divided into two major categories according to the different effector populations that are mobilized. One group comprises effector cells chapter 14
320 paRI I Immune Effector mechanisms that have direct cytotoxic activity. These effectors eliminate naive T cells) are able to respond to TCR-mediated signals foreign cells and altered self-cells by mounting a cytotoxic with little, if any co-stimulation. reaction that lyses their target. The various cytotoxic effector The reason for the different activation requirements of cells can be grouped into two general categories: one com- naive and activated t cells is an area of continuing research, prises antigen-specific cytotoxic T lymphocytes( CTls) and but some clues have been found. One is that many popula unspecific cells, such as natural killer(NK) cells and macro- tions of naive and effector T cells express different isoforms phages. The target cells to which these effectors are directed of CD45, designated CD45RA and CD45RO, which are pro include allogeneic cells, malignant cells, virus-infected cells, duced by alternative splicing of the Rna transcript of the and chemically conjugated cells. The other group is a sub- CD45 gene. This membrane molecule mediates TCR signal population of effector CD4* T cells that mediates delayed- transduction by catalyzing dephosphorylation of a tyrosine type hypersensitivity reactions(see Chapter 16). The next residue on the protein tyrosine kinases Lck and Fyn, activat- section reviews the general properties of effector T cells and ing these kinases and triggering the subsequent steps in T-cell how they differ from naive T cells. activation(see figures 10-10 and 10-11). The CD45RO iso- form, which is expressed on effector T cells, associates with the TCR complex and its coreceptors, CD4 and CD8, much General Properties of Effector T Cells better than does the CD45RA isoform, which is expressed by naive T cells. Memory T cells have both isoforms, but the The three types of effectorTcells-CD4*, THl and TH2 cells, CD45RO is predominant. As a result, effector and memory and CD8* CTLs-exhibit several properties that set them T cells are more sensitive to TCR-mediated activation by a art from naive helper and cytotoxic T cells (Table 14-1). In peptide-MHC complex. They also have less stringent re particular, effector cells are characterized by their less strin quirements for co-stimulatory signals and therefore are able gent activation requirements, increased expression of cell- to respond to peptide -MHC complexes displayed on target adhesion molecules, and production of both membrane- cells or antigen-presenting cells that lack the co-stimulatory bound and soluble effector molecules B7 molecules The Activation Requirements Cell-Adhesion molecules facilitate of t cells differ TCR-Mediated Interactions T cells at different stages of differentiation may respond with CD2 and the integrin LFA-1 are cell-adhesion molecules on different efficiencies to signals mediated by the T-cell recep- the surfaces of T cells that bind, respectively, to LFA-3 and tor and may consequently require different levels of a second ICAMs(intracellular cell-adhesion molecules) on antigen set of co-stimulatory signals. As described in Chapter 10, presenting cells and various target cells(see Figure 9-13). The activation of naive T cells and their subsequent proliferation level of LFA-1 and CD2 is twofold to fourfold higher on and differentiation into effector T cells require both a pri- effector T cells than on naive T cells, enabling the effector mary signal, delivered when the TCR complex and CD4 T cells to bind more effectively to antigen-presenting cells or CD8 coreceptor interact with a foreign peptide-MHc and to various target cells that express low levels of ICAMs molecule complex, and a co-stimulatory signal, delivered by LFA-3 nteraction between particular membrane molecules on the As Chapter 9 showed, the initial interaction of an effector T cell and the antigen-presenting cell. In contrast, antigen T cell with an antigen-presenting cell or target cell is weak, experienced effector cells and memory cells(as opposed to allowing the TCr to scan the membrane for specific peptides TABLE 14-1 Comparison of naive and effector T cells Property Naive t cells Effector T cells stimulatory signal (CD28-B7 interaction) Required for activation equired for activation CD45RA CD45RO Cell-adhesion molecules(CD2 and LFA-1) fficking patterns HEVs' in secondary lymphoid tissue Tertiary lymphoid tissue inflammatory sites HEV= high endothelial venules, sites in blood vessel used by lymphocytes for extravasation
that have direct cytotoxic activity. These effectors eliminate foreign cells and altered self-cells by mounting a cytotoxic reaction that lyses their target. The various cytotoxic effector cells can be grouped into two general categories: one comprises antigen-specific cytotoxic T lymphocytes (CTLs) and nonspecific cells, such as natural killer (NK) cells and macrophages. The target cells to which these effectors are directed include allogeneic cells, malignant cells, virus-infected cells, and chemically conjugated cells. The other group is a subpopulation of effector CD4+ T cells that mediates delayedtype hypersensitivity reactions (see Chapter 16). The next section reviews the general properties of effector T cells and how they differ from naive T cells. General Properties of Effector T Cells The three types of effector T cells—CD4+ , TH1 and TH2 cells, and CD8+ CTLs—exhibit several properties that set them apart from naive helper and cytotoxic T cells (Table 14-1). In particular, effector cells are characterized by their less stringent activation requirements, increased expression of celladhesion molecules, and production of both membranebound and soluble effector molecules. The Activation Requirements of T Cells Differ T cells at different stages of differentiation may respond with different efficiencies to signals mediated by the T-cell receptor and may consequently require different levels of a second set of co-stimulatory signals. As described in Chapter 10, activation of naive T cells and their subsequent proliferation and differentiation into effector T cells require both a primary signal, delivered when the TCR complex and CD4 or CD8 coreceptor interact with a foreign peptide–MHC molecule complex, and a co-stimulatory signal, delivered by interaction between particular membrane molecules on the T cell and the antigen-presenting cell. In contrast, antigenexperienced effector cells and memory cells (as opposed to naive T cells) are able to respond to TCR-mediated signals with little, if any co-stimulation. The reason for the different activation requirements of naive and activated T cells is an area of continuing research, but some clues have been found. One is that many populations of naive and effector T cells express different isoforms of CD45, designated CD45RA and CD45RO, which are produced by alternative splicing of the RNA transcript of the CD45 gene. This membrane molecule mediates TCR signal transduction by catalyzing dephosphorylation of a tyrosine residue on the protein tyrosine kinases Lck and Fyn, activating these kinases and triggering the subsequent steps in T-cell activation (see figures 10-10 and 10-11). The CD45RO isoform, which is expressed on effector T cells, associates with the TCR complex and its coreceptors, CD4 and CD8, much better than does the CD45RA isoform, which is expressed by naive T cells. Memory T cells have both isoforms, but the CD45RO is predominant. As a result, effector and memory T cells are more sensitive to TCR-mediated activation by a peptide-MHC complex. They also have less stringent requirements for co-stimulatory signals and therefore are able to respond to peptide-MHC complexes displayed on target cells or antigen-presenting cells that lack the co-stimulatory B7 molecules. Cell-Adhesion Molecules Facilitate TCR-Mediated Interactions CD2 and the integrin LFA-1 are cell-adhesion molecules on the surfaces of T cells that bind, respectively, to LFA-3 and ICAMs (intracellular cell-adhesion molecules) on antigenpresenting cells and various target cells (see Figure 9-13). The level of LFA-1 and CD2 is twofold to fourfold higher on effector T cells than on naive T cells, enabling the effector T cells to bind more effectively to antigen-presenting cells and to various target cells that express low levels of ICAMs or LFA-3. As Chapter 9 showed, the initial interaction of an effector T cell with an antigen-presenting cell or target cell is weak, allowing the TCR to scan the membrane for specific peptides 320 PART III Immune Effector Mechanisms TABLE 14-1 Comparison of naive and effector T cells Property Naive T cells Effector T cells Co-stimulatory signal (CD28-B7 interaction) Required for activation Not required for activation CD45 isoform CD45RA CD45RO Cell-adhesion molecules (CD2 and LFA-1) Low High Trafficking patterns HEVs* in secondary lymphoid tissue Tertiary lymphoid tissues; inflammatory sites *HEV = high endothelial venules, sites in blood vessel used by lymphocytes for extravasation
Cell-Mediated Effector Responses CHAPTER 14 TABLE 14-2 Effector molecules produced by effector T cells Cell type Soluble effectors Membrane-bound effectors Cytotoxins(perforins and granzymes), IFN-Y, TNF-B Fas ligand( FASL) IL-2, IL-3, TNF-B, IFN-Y, GM-CSF(high) Tumor necrosis factorβ(TNFβ) IL-3, IL-4, IL-5, IL-6, IL-10, IL-13, GM-CSF (low) CD40 ligand presented by self-MHC molecules. If no peptide-MHC com- tumor cells)and in graft-rejection reactions. In general, plex is recognized by the effector cell, it will disengage from CTLs are CD8* and are therefore class I mhc restricted, al the aPC or target cell Recognition of a peptide-MHC com- though in rare instances CD4 class Il-restricted T cells have plex by the TCR, however, produces a signal that increases the been shown to function as CTls. since virtually all nucleated affinity of LFA-1 for ICAMs on the APC or target-cell cells in the body express class I MHC molecules, CTLs can membrane, prolonging the interaction between the cells. recognize and eliminate almost any altered body cell For example, THl effector cells remain bound to macro The CTL-mediated immune response can be divided into phages that display peptide-class II MHC complexes; TH2 ef- two phases, reflecting different aspects of the response. The fector cells remain bound to B cells that display peptide-class first phase activates and differentiates naive Tc cells into II MHC complexes; and CTl effector cells bind tightly to functional effector CTLs In the second phase, effector CTls virus-infected target cells that display peptide-class I MHc recognize antigen-class I MHC complexes on specific target com plexes cells, which leads them to destroy the target cells Effector T Cells Express a Variety Effector CTLs Are generated of Effector molecules from ctl Precursors Unlike naive T cells, effector T cells express certain effector Naive Tc cells are incapable of killing target cells and are there molecules, which may be membrane bound or soluble Table fore referred to as Ctl precursors( CTL-Ps)to denote their 14-2). The membrane-bound molecules belong to the tumor functionally immature state. Only after a CTL-P has been acti- necrosis factor (TNF) family of membrane proteins and vated will the cell differentiate into a functional CTl with include the Fas ligand( FASl)on CD8 CTLs, TNF-Bon THl cytotoxic activity. Generation of CTls from CTL-Ps appears to cells, and the CD40 ligand on TH2 cells. Each of the effector require at least three sequential signals(Figure 14-1 T-cell populations also secretes distinct panels of soluble effector molecules. CTLs secrete cytotoxins(perforins and An antigen-specific signal 1 transmitted by the TCR granzymes)as well as two cytokines, IFN-y and TNF-B As complex upon recognition of a peptide-class I MHC described in Chapter 12, the THl and TH2 subsets secrete largely nonoverlapping sets of cytokines Each of these membrane-bound and secreted molecules A co-stimulatory signal transmitted by the CD28-B7 plays an important role in various T-cell effector functions interaction of the CTL-P and the antigen-presenting The Fas ligand, perforins, and granzymes, for example, medi- Sle target-cell destruction by the CTL; membrane-bound. A signal induced by the interaction of IL-2 with the F-B and soluble IFN-y and GM-csF promote macro- h-affinity IL-2 receptor, resulting in proliferation and phage activation by the THl cell; and the membrane-bound differentiation of the antigen-activated CTl-P into CD40 ligand and soluble IL-4, IL-5, and IL-6 all play a role in effector Ctls B-cell activation by the TH2 cell Unactivated CTL-Ps do not express IL-2 or IL-2 receptors do not proliferate, and do not display cytotoxic activity. Anti Cytotoxic T Cells gen activation induces a CTL-P to begin expressing the IL-2 receptor and to a lesser extent IL-2, the principal cytokine CytotoxicT lymphocytes, or CTls, are generated by immune required for proliferation and differentiation of activated activation of T cytotoxic tc) cells. These effector cells have CTL-Ps into effector CTLs. In some cases, the amount of lytic capability and are critical in the recognition and elimi- IL-2 secreted by an antigen-activated CTL-P may be suffi nation of altered self-cells (e.g, virus-infected cells and cient to induce its own proliferation and differentiation; this
presented by self-MHC molecules. If no peptide-MHC complex is recognized by the effector cell, it will disengage from the APC or target cell. Recognition of a peptide-MHC complex by the TCR, however, produces a signal that increases the affinity of LFA-1 for ICAMs on the APC or target-cell membrane, prolonging the interaction between the cells. For example, TH1 effector cells remain bound to macrophages that display peptide–class II MHC complexes; TH2 effector cells remain bound to B cells that display peptide–class II MHC complexes; and CTL effector cells bind tightly to virus-infected target cells that display peptide–class I MHC complexes. Effector T Cells Express a Variety of Effector Molecules Unlike naive T cells, effector T cells express certain effector molecules, which may be membrane bound or soluble (Table 14-2). The membrane-bound molecules belong to the tumor necrosis factor (TNF) family of membrane proteins and include the Fas ligand (FASL) on CD8+ CTLs, TNF- on TH1 cells, and the CD40 ligand on TH2 cells. Each of the effector T-cell populations also secretes distinct panels of soluble effector molecules. CTLs secrete cytotoxins (perforins and granzymes) as well as two cytokines, IFN- and TNF-. As described in Chapter 12, the TH1 and TH2 subsets secrete largely nonoverlapping sets of cytokines. Each of these membrane-bound and secreted molecules plays an important role in various T-cell effector functions. The Fas ligand, perforins, and granzymes, for example, mediate target-cell destruction by the CTL; membrane-bound TNF- and soluble IFN- and GM-CSF promote macrophage activation by the TH1 cell; and the membrane-bound CD40 ligand and soluble IL-4, IL-5, and IL-6 all play a role in B-cell activation by the TH2 cell. Cytotoxic T Cells Cytotoxic T lymphocytes, or CTLs, are generated by immune activation of T cytotoxic (TC) cells. These effector cells have lytic capability and are critical in the recognition and elimination of altered self-cells (e.g., virus-infected cells and tumor cells) and in graft-rejection reactions. In general, CTLs are CD8+ and are therefore class I MHC restricted, although in rare instances CD4+ class II–restricted T cells have been shown to function as CTLs. Since virtually all nucleated cells in the body express class I MHC molecules, CTLs can recognize and eliminate almost any altered body cell. The CTL-mediated immune response can be divided into two phases, reflecting different aspects of the response. The first phase activates and differentiates naive TC cells into functional effector CTLs. In the second phase, effector CTLs recognize antigen–class I MHC complexes on specific target cells, which leads them to destroy the target cells. Effector CTLs Are Generated from CTL Precursors Naive TC cells are incapable of killing target cells and are therefore referred to as CTL precursors (CTL-Ps) to denote their functionally immature state. Only after a CTL-P has been activated will the cell differentiate into a functional CTL with cytotoxic activity. Generation of CTLs from CTL-Ps appears to require at least three sequential signals (Figure 14-1): ■ An antigen-specific signal 1 transmitted by the TCR complex upon recognition of a peptide–class I MHC molecule complex ■ A co-stimulatory signal transmitted by the CD28-B7 interaction of the CTL-P and the antigen-presenting cell ■ A signal induced by the interaction of IL-2 with the high-affinity IL-2 receptor, resulting in proliferation and differentiation of the antigen-activated CTL-P into effector CTLs Unactivated CTL-Ps do not express IL-2 or IL-2 receptors, do not proliferate, and do not display cytotoxic activity. Antigen activation induces a CTL-P to begin expressing the IL-2 receptor and to a lesser extent IL-2, the principal cytokine required for proliferation and differentiation of activated CTL-Ps into effector CTLs. In some cases, the amount of IL-2 secreted by an antigen-activated CTL-P may be sufficient to induce its own proliferation and differentiation; this Cell-Mediated Effector Responses CHAPTER 14 321 TABLE 14-2 Effector molecules produced by effector T cells Cell type Soluble effectors Membrane-bound effectors CTL Cytotoxins (perforins and granzymes), IFN-, TNF- Fas ligand (FASL) TH1 IL-2, IL-3, TNF-, IFN-, GM-CSF (high) Tumor necrosis factor (TNF-) TH2 IL-3, IL-4, IL-5,IL-6, IL-10,IL-13,GM-CSF (low) CD40 ligand
322 PART I Immune Effector Mechanisms Class II mhc CD4 THl cell Co-stimulatory signal IL-2 HI cell Class I mhc IL-2R. Activation arget cell Ta CTL-P g-activated CTL-P Proliferation Effector cytotoxic function FIGURE 14-1 Generation of effector CTLS. Upon interaction with additional IL-2 secreted by TH1 cells resulting from antigen activation antigen-class I MHC complexes on appropriate target cells, CTL-Ps be- and proliferation of CD4"' T cells. In the subsequent effector phase, CTLS gin to express IL-2 receptors (IL-2R) and lesser amounts of IL-2 Prolif- destroy specific target cells. eration and differentiation of antigen-activated CTL-Ps generally require is particularly true of memory CTL-Ps, which have lower the immune response is rapidly terminated, lessening the activation requirements than naive cells do( figure 14-2a). likelihood of nonspecific tissue damage from the inflamma In general, though, most activated CTL-Ps require addi- tory response tional IL-2 produced by proliferating THl cells to proliferate The role of THl cells in the generation of CTls from naive and differentiate into effector CTLs. The fact that the IL-2 CTL-Ps is not completely understood, and it is unlikely that a receptor is not expressed until after a CTL-P has been acti- THl cell and CTl-P interact directly. However, IL-2 and vated by antigen plus a class I MHc molecule favors the stimulation are important in the transformation of naive clonal expansion and acquisition of cytotoxicity by only the CTL-Ps into effector cells, and THl cells can be mediators antigen-specific CTL-Ps in the provision of these essential requirements. As shown The proliferation and differentiation of both antigen- in Figure 14-2b, the interaction of helper cells with antigen activated THl cells and CTL-Ps depend on IL-2. In IL-2 presenting cells can result in production of IL-2 by the TH1 knockout mice, the absence of IL-2 has been shown to abol- cell. The paracrine action of this cytokine on nearby naive ish CTL-mediated cytotoxicity. After clearance of antigen, CTL-Ps whose TCRs are engaged can cause them to prolifer- the level of IL-2 declines, which induces THl cells and Ctls ate and differentiate into active Ctls. Additionally, thl can to undergo programmed cell death by apoptosis. In this way, induce the up-regulation of co-stimulatory molecules on the
is particularly true of memory CTL-Ps, which have lower activation requirements than naive cells do (Figure 14-2a). In general, though, most activated CTL-Ps require additional IL-2 produced by proliferating TH1 cells to proliferate and differentiate into effector CTLs. The fact that the IL-2 receptor is not expressed until after a CTL-P has been activated by antigen plus a class I MHC molecule favors the clonal expansion and acquisition of cytotoxicity by only the antigen-specific CTL-Ps. The proliferation and differentiation of both antigenactivated TH1 cells and CTL-Ps depend on IL-2. In IL-2 knockout mice, the absence of IL-2 has been shown to abolish CTL-mediated cytotoxicity. After clearance of antigen, the level of IL-2 declines, which induces TH1 cells and CTLs to undergo programmed cell death by apoptosis. In this way, the immune response is rapidly terminated, lessening the likelihood of nonspecific tissue damage from the inflammatory response. The role of TH1 cells in the generation of CTLs from naive CTL-Ps is not completely understood, and it is unlikely that a TH1 cell and CTL-P interact directly. However, IL-2 and costimulation are important in the transformation of naive CTL-Ps into effector cells, and TH1 cells can be mediators in the provision of these essential requirements. As shown in Figure 14-2b, the interaction of helper cells with antigenpresenting cells can result in production of IL-2 by the TH1 cell. The paracrine action of this cytokine on nearby naive CTL-Ps whose TCRs are engaged can cause them to proliferate and differentiate into active CTLs. Additionally, TH1 can induce the up-regulation of co-stimulatory molecules on the 322 PART III Immune Effector Mechanisms IL-2 APC Class II MHC + antigen CD4 IL-2R TH1 cell TH1 cell TH1 cell IL-2 Co-stimulatory signal Activation Proliferation, differentiation Target cell CTL-P CD8 CTL Class I MHC + antigen IL-2R CD3 Ag-activated CTL-P Co-stimulatory signal IL-2R expression Proliferation Effector cytotoxic function – + – – + IL-2 expression – + + – – ± ± FIGURE 14-1 Generation of effector CTLs. Upon interaction with antigen–class I MHC complexes on appropriate target cells, CTL-Ps begin to express IL-2 receptors (IL-2R) and lesser amounts of IL-2. Proliferation and differentiation of antigen-activated CTL-Ps generally require additional IL-2 secreted by TH1 cells resulting from antigen activation and proliferation of CD4+ T cells. In the subsequent effector phase, CTLs destroy specific target cells
Cell-Mediated Effector Responses cHAPTER 14 32 Y Virus-infected dendritic cell FIGURE 14-2 Proliferation of memory CTL-Ps may not require help activation. (b)A TH cell may provide the IL-2 necessary for proliferation from TH cells.(a)Antigen-activated memory CTL-Ps appear to secrete suf- of an antigen-activated naive CTL-P when it binds to the same APC as the ficient IL-2 to stimulate their own proliferation and differentiation into effec CTL-P. Also, TH cells may alter the behavior of APCs in a number of way tor CTLs. They also may not require the CD28-B7 co-stimulatory signal for such as increasing the display of co-stimulatory molecules by the APC. surface of antigen-presenting cells. In this manner, THl cells the tetramer become fluorescently labeled(Figure 14-3) help CTL-P division and differentiation by causing the gen- Using flow cytometry, it is then possible to determine the eration of adequate levels of co-stimulation. proportion of cells in a population that have TCRs specific for a particular antigen by counting the number of fluores- CD8 CTLs Can be tracked with mHc cently labeled cells in a cell population. This very sensitive Tetramer Technology approach can detect antigen-specific T cells even when their frequency in the CD8t population is as low as 0.1%. Further MHC tetramers are laboratory-generated complexes of four more one can directly measure the increase in antigen MHC class I molecules bound to a specific peptide and specific CD8* T cells in response to exposure to pathogens linked to a fluorescent molecule. a given MHC-tetramer- such as viruses or cancer-associated antigens. In a related peptide complex binds only CD8 T cells that have TCRs application, researchers infected mice with vesicular stomati- specific for the particular peptide-MHC complex that makes tis virus(VSv) and systematically examined the distribution up the tetramer. Thus, when a particular tetramer is added to of CD8* cells specific for a VSV-derived peptide-MHC com a cell population containing T cells(spleen cells or lymph- plex throughout the entire body. This study demonstrated ode cells, for example), cells that bear TCRs specific for that during acute infection with VSV, the distribution of
surface of antigen-presenting cells. In this manner, TH1 cells help CTL-P division and differentiation by causing the generation of adequate levels of co-stimulation. CD8+ CTLs Can Be Tracked with MHC Tetramer Technology MHC tetramers are laboratory-generated complexes of four MHC class I molecules bound to a specific peptide and linked to a fluorescent molecule. A given MHC-tetramer– peptide complex binds only CD8+ T cells that have TCRs specific for the particular peptide-MHC complex that makes up the tetramer. Thus, when a particular tetramer is added to a cell population containing T cells (spleen cells or lymphnode cells, for example), cells that bear TCRs specific for the tetramer become fluorescently labeled (Figure 14-3). Using flow cytometry, it is then possible to determine the proportion of cells in a population that have TCRs specific for a particular antigen by counting the number of fluorescently labeled cells in a cell population. This very sensitive approach can detect antigen-specific T cells even when their frequency in the CD8+ population is as low as 0.1%. Furthermore, one can directly measure the increase in antigenspecific CD8+ T cells in response to exposure to pathogens such as viruses or cancer-associated antigens. In a related application, researchers infected mice with vesicular stomatitis virus (VSV) and systematically examined the distribution of CD8+ cells specific for a VSV-derived peptide-MHC complex throughout the entire body. This study demonstrated that during acute infection with VSV, the distribution of Cell-Mediated Effector Responses CHAPTER 14 323 (a) IL-2 Memory CTL-P B7 CD28 CTL (b) Virus-infected dendritic cell Naive CTL-P TH1 IL-2 CTL Virus-infected dendritic cell 1 2 1 2 1 2 FIGURE 14-2 Proliferation of memory CTL-Ps may not require help from TH cells. (a) Antigen-activated memory CTL-Ps appear to secrete sufficient IL-2 to stimulate their own proliferation and differentiation into effector CTLs. They also may not require the CD28-B7 co-stimulatory signal for activation. (b) A TH cell may provide the IL-2 necessary for proliferation of an antigen-activated naive CTL-P when it binds to the same APC as the CTL-P. Also, TH cells may alter the behavior of APCs in a number of ways, such as increasing the display of co-stimulatory molecules by the APC