《免疫学》（英文版） Chapter 12 Cytokines

okInes chapter 12 HE DEVELOPMENT OF AN EFFECTIVE IMMUNE response involves lymphoid cells, inflammatory cells, and hematopoietic cells. The complex inter- actions among these cells are mediated by a group of pro- ins collectively designated cytokines to denote their role in cell-to-cell communication. Cytokines are low-molecular- weight regulatory proteins or glycoproteins secreted by ite blood cells and various other cells in the body in response to a number of stimuli. These proteins assist in reg cytokines possess direct effector functions of their owl me ulating the development of immune effector cells, and some This chapter focuses on the biological activity of cyto- Class I Cytokine Receptors ines, the structure of cytokines and their receptors, signal transduction by cytokine receptors, the role of cytokine abnormalities in the pathogenesis of certain diseases, and a Properties of Cytokines therapeutic uses of cytokines or their receptors. The impor Cytokine Receptors tant role of cytokines in the inflammatory response is a Cytokine Antagonists described in Chapter 15 a Cytokine Secretion by TH1 and TH2 Subsets a Cytokine-Related Diseases Properties of Cytokines a Therapeutic Uses of Cytokines and Their Receptors Cytokines bind to specific receptors on the membrane of Cytokines in Hematopoiesis target cells, triggering signal-transduction pathways that ultimately alter gene expression in the target cells(figu 12-la). The susceptibility of the target cell to a particular cytokine is determined by the presence of specific mem- brane receptors. In general, the cytokines and their receptor exhibit very high affinity for each other, with dissociatio constants ranging from 10 to 10M. Because their af- finities are so high, cytokines can mediate biological effects at picomolar concentrations the activity of numerous cells involved in the immune a particular cytokine may bind to receptors on the mem- response. For example, cytokines produced by activated TH brane of the same cell that secreted it, exerting autocrine cells can influence the activity of B cells, Tc cells, natural ction; it may bind to receptors on a target cell in close prox- killer cells, macrophages, granulocytes, and hematopoietic imity to the producer cell, exerting paracrine action; in a few stem cells, thereby activating an entire network of interact- cases, it may bind to target cells in distant parts of the body exerting endocrine action( Figure 12-1b). Cytokines regu Cytokines exhibit the attributes of pleiotropy, redun- late the intensity and duration of the immune response by dancy, synergy, antagonism, and cascade induction, which stimulating or inhibiting the activation, proliferation, and/ permit them to regulate cellular activity in a coordinated or differentiation of various cells and by regulating the secre- interactive way(Figure 12-2). a given cytokine that has tion of antibodies or other cytokines. As described later, different biological effects on different target cells has a binding of a given cytokine to responsive target cells gener- pleiotropic action. Two or more cytokines that mediate sim- ally stimulates increased expression of cytokine receptors ilar functions are said to be redundant; redundancy makes it and secretion of other cytokines, which affect other target difficult to ascribe a particular activity to a single cytokine cells in turn. Thus, the cytokines secreted by even a small Cytokine synergism occurs when the combined effect of two number of lymphocytes activated by antigen can influence cytokines on cellular activity is greater than the additive

■ Properties of Cytokines ■ Cytokine Receptors ■ Cytokine Antagonists ■ Cytokine Secretion by TH1 and TH2 Subsets ■ Cytokine-Related Diseases ■ Therapeutic Uses of Cytokines and Their Receptors ■ Cytokines in Hematopoiesis Class I Cytokine Receptors Cytokines T      response involves lymphoid cells, inflammatory cells, and hematopoietic cells. The complex inter￾actions among these cells are mediated by a group of pro￾teins collectively designated cytokinesto denote their role in cell-to-cell communication. Cytokines are low-molecular￾weight regulatory proteins or glycoproteins secreted by white blood cells and various other cells in the body in response to a number of stimuli. These proteins assist in reg￾ulating the development of immune effector cells, and some cytokines possess direct effector functions of their own. This chapter focuses on the biological activity of cyto￾kines, the structure of cytokines and their receptors, signal transduction by cytokine receptors, the role of cytokine abnormalities in the pathogenesis of certain diseases, and therapeutic uses of cytokines or their receptors. The impor￾tant role of cytokines in the inflammatory response is described in Chapter 15. Properties of Cytokines Cytokines bind to specific receptors on the membrane of target cells, triggering signal-transduction pathways that ultimately alter gene expression in the target cells (Figure 12-1a). The susceptibility of the target cell to a particular cytokine is determined by the presence of specific mem￾brane receptors. In general, the cytokines and their receptors exhibit very high affinity for each other, with dissociation constants ranging from 10–10 to 10–12 M. Because their af￾finities are so high, cytokines can mediate biological effects at picomolar concentrations. A particular cytokine may bind to receptors on the mem￾brane of the same cell that secreted it, exerting autocrine action; it may bind to receptors on a target cell in close prox￾imity to the producer cell, exerting paracrine action; in a few cases, it may bind to target cells in distant parts of the body, exerting endocrine action (Figure 12-1b). Cytokines regu￾late the intensity and duration of the immune response by stimulating or inhibiting the activation, proliferation, and/ or differentiation of various cells and by regulating the secre￾tion of antibodies or other cytokines. As described later, binding of a given cytokine to responsive target cells gener￾ally stimulates increased expression of cytokine receptors and secretion of other cytokines, which affect other target cells in turn. Thus, the cytokines secreted by even a small number of lymphocytes activated by antigen can influence the activity of numerous cells involved in the immune response. For example, cytokines produced by activated TH cells can influence the activity of B cells, TC cells, natural killer cells, macrophages, granulocytes, and hematopoietic stem cells, thereby activating an entire network of interact￾ing cells. Cytokines exhibit the attributes of pleiotropy, redun￾dancy, synergy, antagonism, and cascade induction, which permit them to regulate cellular activity in a coordinated, interactive way (Figure 12-2). A given cytokine that has different biological effects on different target cells has a pleiotropic action. Two or more cytokines that mediate sim￾ilar functions are said to be redundant; redundancy makes it difficult to ascribe a particular activity to a single cytokine. Cytokine synergism occurs when the combined effect of two cytokines on cellular activity is greater than the additive chapter 12

Cytokines cHAPTER 12 277 (b) Cytokine-producing cell Autocrine action Paracrine action Nearby cell Gene Target cell Endocrine action Distant cell Biological effects JRE 12-1(a)Overview of the induction and function of es. (b)Most cytokines exhibit autocrine and/or paracrine action fewer exhibit endocrine action effects of the individual cytokines. In some cases, cytokines of cytokines, the chemokines, a group of low-molecular- exhibit antagonism; that is, the effects of one cytokine inhibit weight cytokines that affect chemotaxis and other aspects or offset the effects of another cytokine Cascade induction of leukocyte behavior. These molecules play an important occurs when the action of one cytokine on a target cell role in the inflammatory response and are described in induces that cell to produce one or more other cytokines, Chapter 15 which in turn may induce other target cells to produce other Because cytokines share many properties with hormones cytokines and growth factors, the distinction between these three The term cytokine encompasses those cytokines secreted classes of mediators is often blurred. All three are secreted by lymphocytes, substances formerly known as lympho- soluble factors that elicit their biological effects at picomolar kines, and those secreted by monocytes and macrophages, concentrations by binding to receptors on target cells substances formerly known as monokines. Although these growth factors tend to be produced constitutively, whereas other two terms continue to be used, they are misleading tokines and hormones are secreted in response to discrete because secretion of many lymphokines and monokines is stimuli, and secretion is short-lived, generally ranging from a not limited to lymphocytes and monocytes as these terms few hours to a few days. Unlike hormones, which generally imply, but extends to a broad spectrum of cells and types. act long range in an endocrine fashion, most cytokines act For this reason, the more inclusive term cytokine is preferred. over a short distance in an autocrine or paracrine fashion. In lany cytokines are referred to as interleukins, a name addition, most hormones are produced by specialized glands indicating that they are secreted by some leukocytes and act and tend to have a unique action on one or a few types of tar upon other leukocytes. Interleukins 1-25 have been identi- get cell. In contrast, cytokines are often produced by, and fied. There is that still other cytokines will bind to, a variety of cells be discovered and that the interleukin group will expand The activity of cytokines was first recognized in the mid further. Some cytokines are known by common names, 1960s, when supernatants derived from in vitro cultures of including the interferons and tumor necrosis factors. Re- lymphocytes were found to contain factors that could regulate cently gaining prominence is yet another another subgroup proliferation, differentiation, and maturation of allogeneic

effects of the individual cytokines. In some cases, cytokines exhibit antagonism; that is, the effects of one cytokine inhibit or offset the effects of another cytokine. Cascade induction occurs when the action of one cytokine on a target cell induces that cell to produce one or more other cytokines, which in turn may induce other target cells to produce other cytokines. The term cytokine encompasses those cytokines secreted by lymphocytes, substances formerly known as lympho￾kines, and those secreted by monocytes and macrophages, substances formerly known as monokines. Although these other two terms continue to be used, they are misleading because secretion of many lymphokines and monokines is not limited to lymphocytes and monocytes as these terms imply, but extends to a broad spectrum of cells and types. For this reason, the more inclusive term cytokine is preferred. Many cytokines are referred to as interleukins, a name indicating that they are secreted by some leukocytes and act upon other leukocytes. Interleukins 1–25 have been identi￾fied. There is reason to suppose that still other cytokines will be discovered and that the interleukin group will expand further. Some cytokines are known by common names, including the interferons and tumor necrosis factors. Re￾cently gaining prominence is yet another another subgroup of cytokines, the chemokines, a group of low-molecular￾weight cytokines that affect chemotaxis and other aspects of leukocyte behavior. These molecules play an important role in the inflammatory response and are described in Chapter 15. Because cytokines share many properties with hormones and growth factors, the distinction between these three classes of mediators is often blurred. All three are secreted soluble factors that elicit their biological effects at picomolar concentrations by binding to receptors on target cells. Growth factors tend to be produced constitutively, whereas cytokines and hormones are secreted in response to discrete stimuli, and secretion is short-lived, generally ranging from a few hours to a few days. Unlike hormones, which generally act long range in an endocrine fashion, most cytokines act over a short distance in an autocrine or paracrine fashion. In addition, most hormones are produced by specialized glands and tend to have a unique action on one or a few types of tar￾get cell. In contrast, cytokines are often produced by, and bind to, a variety of cells. The activity of cytokines was first recognized in the mid- 1960s, when supernatants derived from in vitro cultures of lymphocytes were found to contain factors that could regulate proliferation, differentiation, and maturation of allogeneic Cytokines CHAPTER 12 277 Gene activation Biological effects Signal Cytokine gene Inducing stimulus (a) Cytokine-producing cell Target cell Cytokine Receptor (b) Endocrine action Circulation Distant cell Paracrine action Nearby cell Autocrine action FIGURE 12-1 (a) Overview of the induction and function of cytokines. (b) Most cytokines exhibit autocrine and/or paracrine action; fewer exhibit endocrine action

278 paRT I Immune Effector mechanisms get Cell Effect Activatio Activated Tu cells PLEIOTROPY IL-4 Proliferation IFN-Y Thymocyte Activated TH cells Mast cel REDUNDANCY 9→出彐 Proliferation 12 Activated Tu cells B cell SYNERGY IL-4 Induces class switch to lge IL-5 Activated TH cells ANTAGONISM Activated TH cells IL-4 Blocks class switch to IgE induced by IL-4 IFN-Y IFN-Y TNF, IL-2, and Activated Tu cells B cell other cytokines FIGURE 12.2 Cytokine attributes of (a)pleiotropy, redundancy. synergy(synergism), antagonism, and(b) cascade induction immune-system cells. Soon after, it was discovered that pro- natans and the absence of well-defined assay systems for duction of these factors by cultured lymphocytes was induced individual cytokines. a great advance was made with the by activation with antigen or with nonspecific mitogens. Bio- development of gene-cloning techniques during the 1970s chemical isolation and purification of cytokines was ham- and 1980s, which made it possible to produce pure cytokines red because of their low concentration in culture super- by expressing the protein from cloned genes. The discovery of

immune-system cells. Soon after, it was discovered that pro￾duction of these factors by cultured lymphocytes was induced by activation with antigen or with nonspecific mitogens. Bio￾chemical isolation and purification of cytokines was ham￾pered because of their low concentration in culture super￾natants and the absence of well-defined assay systems for individual cytokines. A great advance was made with the development of gene-cloning techniques during the 1970s and 1980s, which made it possible to produce pure cytokines by expressing the protein from cloned genes. The discovery of 278 PART III Immune Effector Mechanisms Activated TH cells PLEIOTROPY Mast cell IL-4 Thymocyte B cell Activation Proliferation Differentiation Proliferation Target Cell Effect IL-4 Proliferation IL-2 IL-5 Activated TH cells REDUNDANCY IL-4 + IL-5 Activated TH cells SYNERGY Induces class switch to IgE Blocks class switch to IgE induced by IL-4 IL-4 IFN-γ Activated TH cells ANTAGONISM Proliferation B cell B cell B cell Activated TH cells Activated TH cells IFN-γ, TNF, IL-2, and other cytokines Macrophage IL-12 CASCADE INDUCTION IFN-γ (a) (b) FIGURE 12-2 Cytokine attributes of (a) pleiotropy, redundancy, synergy (synergism), antagonism, and (b) cascade induction

Cytokines cHAPTER 12 characterized so far belong to one of four groups: the Cytokine bound hematopoietin family, the interferon family, the chemokine family, or the tumor necrosis factor family The structures of two members of the hematopoietin family, IL-2 and IL-4, are depicted in Figure 12-4. Although the amino acid sequences of these family members differ Add substrate considerably, all of them have a high degree of a-helical and measure structure and little or no B-sheet structure The molecules (a)Interleukin 2 36—42 strand 05 C-D Concentration of IL-12(pg/ml) FIGURE 12-3 ELISA assay of a cytokine. (a)The sample contain- ing the cytokine of interest is captured by specific antibody(blue) NHo COOH coated onto wells of a microtiter plate. A second specific antibody (blue), conjugated to an enzyme (E)such as horseradish peroxidase. forms a sandwich with the captured cytokine, immobilizing the en (b)Interleukin 4 yme in the microtiter well. A chromogenic substrate (S)is added. and the enzyme generates a color whose intensity is proportional to the amount of cytokine bound to the capture antibody. The optical density of this color produced by the unknown is compared with val ues on an appropriately determined standard curve. (b)The standard shown here is for human interleukin 12(IL-12). It is clear that assay is sufficiently sensitive to detect as little as 1 picogram of IL-12 /Part (b)courtesy of R&D Systems. cell lines whose growth depended on the presence of a partic lar cytokine provided researchers with the first simple assay systems. The derivation of monoclonal antibodies specific for each of the more important cytokines has made it possible to develop rapid quantitative immunoassays for each of them (Figure 12-3) FIGURE 12.4 Several representations of structures in the hemato. Cytokines Belong to Four Structural poietin family. (a) Left: Topographical representation of the primary Families structure of IL-2 showing a-helical regions(a and A-D)and connect ing chains of the molecule. Right: Proposed three-dimensional model Once the genes encoding various cytokines had been cloned, of IL-2. (b) Ribbon model of IL-4 deduced from x-ray crystallographic sufficient quantities of purified preparations became avail- analysis of the molecule In(a)and ( b)the a helices are shown in red able for detailed studies on their structure and function. and the B sheets in blue. The structures of other cytokines belonging to Cytokines generally have a molecular mass of less than the hematopoietin family are thought to be generally similar. /Part(b) 30 kDa. Structural studies have shown that the cytokines from/L Boulay and W.E. Paul, 1993, Curr Biol. 3: 573/

cell lines whose growth depended on the presence of a partic￾ular cytokine provided researchers with the first simple assay systems. The derivation of monoclonal antibodies specific for each of the more important cytokines has made it possible to develop rapid quantitative immunoassays for each of them (Figure 12-3). Cytokines Belong to Four Structural Families Once the genes encoding various cytokines had been cloned, sufficient quantities of purified preparations became avail￾able for detailed studies on their structure and function. Cytokines generally have a molecular mass of less than 30 kDa. Structural studies have shown that the cytokines characterized so far belong to one of four groups: the hematopoietin family, the interferon family, the chemokine family, or the tumor necrosis factor family. The structures of two members of the hematopoietin family, IL-2 and IL-4, are depicted in Figure 12-4. Although the amino acid sequences of these family members differ considerably, all of them have a high degree of -helical structure and little or no -sheet structure. The molecules Cytokines CHAPTER 12 279 Add enzyme￾conjugated secondary antibody Cytokine bound to antibody coated onto well Add substrate and measure color S E E E E S (a) (b) Concentration of IL-12 (pg/ml) Optical density 10 1 0.1 0.01 0.1 1 10 100 α S S 105 C N 133 6 114 30 36 42 73 80 96 52 58 D A C B C 105 B–C loop β strand C–D loop A–B loop COOH NH2 A C B' B D α P65 C58 (a) Interleukin 2 (b) Interleukin 4 A B C 2 1 D FIGURE 12-3 ELISA assay of a cytokine. (a) The sample contain￾ing the cytokine of interest is captured by specific antibody (blue) coated onto wells of a microtiter plate. A second specific antibody (blue), conjugated to an enzyme (E) such as horseradish peroxidase, forms a sandwich with the captured cytokine, immobilizing the en￾zyme in the microtiter well. A chromogenic substrate (S) is added, and the enzyme generates a color whose intensity is proportional to the amount of cytokine bound to the capture antibody. The optical density of this color produced by the unknown is compared with val￾ues on an appropriately determined standard curve. (b) The standard curve shown here is for human interleukin 12 (IL-12). It is clear that this assay is sufficiently sensitive to detect as little as 1 picogram of IL-12. [Part (b) courtesy of R&D Systems.] FIGURE 12-4 Several representations of structures in the hemato￾poietin family. (a) Left: Topographical representation of the primary structure of IL-2 showing -helical regions ( and A–D) and connect￾ing chains of the molecule. Right: Proposed three-dimensional model of IL-2. (b) Ribbon model of IL-4 deduced from x-ray crystallographic analysis of the molecule. In (a) and (b) the helices are shown in red and the sheets in blue. The structures of other cytokines belonging to the hematopoietin family are thought to be generally similar. [Part (b) from J. L. Boulay and W. E. Paul, 1993, Curr. Biol. 3:573.]

280 PART III Immune Effector Mechanisms VISUALIZING CONCEPTS immunity Cytokine promotion of activation, differentiation, proliferation, or cell death of T cells, B cells, macrophages. dendritic cells, NK cells, and other leukocytes. FIGURE12.5 Interaction of antigen with macrophages and the merous cytokines(blue arrows), generating a complex network of ubsequent activation of resting TH cells leads to release of nu- interacting cells in the immune response share a similar polypeptide fold, with four a-helical regions and differentiation, and the healing of wounds. Although the (A-D)in which the first and second helices and the third and immune response to a specific antigen may include the pro- fourth helices run roughly parallel to one another and are duction of cytokines, it is important to remember that connected by loops. cytokines act in an antigen-nonspecific manner. That is, they affect whatever cells they encounter that bear appropriate Cytokines Have Numerous Biological receptors and are in a physiological state that allows them Functions to respo Cytokines are involved in a staggeringly broad array of Although a variety of cells can secrete cytokines, the two biological activities including innate immunity, adaptive principal producers are the TH cell and the macrophage. immunity, inflammation, and hematopoiesis. Altogether, the Cytokines released from these two cell types activate an total number of proteins with cytokine activity easily exceeds entire network of interacting cells(Figure 12-5). Among the 100 and research continues to uncover new ones. Table 12-1 numerous physiologic responses that require cytokine in- summarizes the activities of some cytokines and places them volvement are development of cellular and humoral im- into functional groups. An expanded list of cytokines can be mune responses, induction of the inflammatory response, found in the Appendix. It should be kept in mind that most regulation of hematopoiesis, control of cellular proliferation of the listed functions have been identified from analysis of

share a similar polypeptide fold, with four -helical regions (A–D) in which the first and second helices and the third and fourth helices run roughly parallel to one another and are connected by loops. Cytokines Have Numerous Biological Functions Although a variety of cells can secrete cytokines, the two principal producers are the TH cell and the macrophage. Cytokines released from these two cell types activate an entire network of interacting cells (Figure 12-5). Among the numerous physiologic responses that require cytokine in￾volvement are development of cellular and humoral im￾mune responses, induction of the inflammatory response, regulation of hematopoiesis, control of cellular proliferation and differentiation, and the healing of wounds. Although the immune response to a specific antigen may include the pro￾duction of cytokines, it is important to remember that cytokines act in an antigen-nonspecific manner. That is, they affect whatever cells they encounter that bear appropriate receptors and are in a physiological state that allows them to respond. Cytokines are involved in a staggeringly broad array of biological activities including innate immunity, adaptive immunity, inflammation, and hematopoiesis. Altogether, the total number of proteins with cytokine activity easily exceeds 100 and research continues to uncover new ones. Table 12-1 summarizes the activities of some cytokines and places them into functional groups. An expanded list of cytokines can be found in the Appendix. It should be kept in mind that most of the listed functions have been identified from analysis of 280 PART III Immune Effector Mechanisms VISUALIZING CONCEPTS Inflammation Macrophage Resting TH cell Adaptive immunity Cytokine promotion of macrophage activation Cytokine promotion of TH cell differentiation Hemato￾poiesis Cytokine promotion of activation, differentiation, proliferation, or cell death of T cells, B cells, macrophages, dendritic cells, NK cells, and other leukocytes. FIGURE 12-5 Interaction of antigen with macrophages and the subsequent activation of resting TH cells leads to release of nu￾merous cytokines (blue arrows), generating a complex network of interacting cells in the immune response.

Cytokines cHAPTER 12 TABLE 12.1 Functional groups of selected cytokines Cytokine Secreted by Targets and effects SOME CYTOKINES OF INNATE IMMUNITY Interleukin 1(IL-1) Monocytes, macrophages, Vasculature(inflammation); hypothalamus(fever) I iver(induction of acute phase proteins) Tumor necrosis Macrophages asculature(inflammation); liver(induction of acute phase Factor-a(TNF-a) proteins); loss of muscle, body fat (cachexia); induction of death in many cell types; neutrophil activation 12(L-12) Macrophages, dendritic cells NK cells; influences adaptive immunity (promotes TH1 subset) Interleukin 6(IL-6) Macrophages, endothelial cells Liver(induces acute phase proteins); influences adapt Interferon a(IFN-a) Macrophages Induces an antiviral state in most nucleated cells, increases mHc (This is a family of molecules class I expression; activates NK cells interferonβ(FNB) Fibroblasts Induces an antiviral state in most nucleated cells increases mHc class I expression; activates NK cells SOME CYTOKINES OF ADAPTIVE IMMUNITY Interleukin 2(IL-2) T cells AICD. NK cell activation and Interleukin 4 (IL-4) TH2 cells; mast cells Promotes TH2 differentiation; isotype switch to IgE Interleukin 25(IL-25) Unknown Induces secretion of TH2 cytokine profile Inhibits T-cell proliferation and effector functions; inhibits tor other cell type B-cell proliferation; promotes isotype switch to igE: Interferon y(IFN-y) THI cells; CD8 cells; NK cells Activates macrophages; increases expression MHC class I Many cytokines play roles in more than one functional category. Only the major cell types providing cytokines for the indicated activity are listed; other cell types may also have the capacity to synthesize the given cytokine. Also note that activated cells generally secrete greater amounts of cytokine than unactivated cells. the effects of recombinant cytokines, often at nonphysiologic antigen-activated lymphocytes. Another means of maintain concentrations, added individually to in vitro systems. In ing specificity may be a requirement for direct interaction vivo, however, cytokines rarely, if ever, act alone. Instead target cell is exposed to a milieu containing a mixture of trigger cytokine secretion, thus ensuring that effective concen- cytokines, whose combined synergistic or antagonistic ef- trations of the cytokine are released only in the vicinity of the fects can have very different consequences. In addition, intended target. In the case of the TH cell, a major producer of cytokines often induce the synthesis of other cytokines, re- cytokines, close cellular interaction occurs when the T-cell sulting in cascades of activity receptor recognizes an antigen-MHC complex on an appro- The nonspecificity of cytokines seemingly conflicts with priate antigen-presenting cell, such as a macrophage, den- the established specificity of the immune system. What keeps dritic cell, or B lymphocyte Cytokines secreted at the junction the nonspecific cytokines from activating cells in a nonspe- of these interacting cells reach high enough local concentra cific fashion during the immune response? One way in which tions to affect the target APC but not more distant cells. In specificity is maintained is by careful regulation of the ex- addition, the half-life of cytokines in the bloodstream or other pression of cytokine receptors on cells Often cytokine recep- extracellular fluids into wh ley are secreted is usually very tors are expressed on a cell only after that cell has interacted short, ensuring that they act for only a limited period of time with antigen. In this way cytokine activation is limited to and thus over a short distance

the effects of recombinant cytokines, often at nonphysiologic concentrations, added individually to in vitro systems. In vivo, however, cytokines rarely, if ever, act alone. Instead, a target cell is exposed to a milieu containing a mixture of cytokines, whose combined synergistic or antagonistic ef￾fects can have very different consequences. In addition, cytokines often induce the synthesis of other cytokines, re￾sulting in cascades of activity. The nonspecificity of cytokines seemingly conflicts with the established specificity of the immune system. What keeps the nonspecific cytokines from activating cells in a nonspe￾cific fashion during the immune response? One way in which specificity is maintained is by careful regulation of the ex￾pression of cytokine receptors on cells. Often cytokine recep￾tors are expressed on a cell only after that cell has interacted with antigen. In this way cytokine activation is limited to antigen-activated lymphocytes. Another means of maintain￾ing specificity may be a requirement for direct interaction between the cytokine-producing cell and the target cell to trigger cytokine secretion, thus ensuring that effective concen￾trations of the cytokine are released only in the vicinity of the intended target. In the case of the TH cell, a major producer of cytokines, close cellular interaction occurs when the T-cell receptor recognizes an antigen-MHC complex on an appro￾priate antigen-presenting cell, such as a macrophage, den￾dritic cell, or B lymphocyte. Cytokines secreted at the junction of these interacting cells reach high enough local concentra￾tions to affect the target APC but not more distant cells. In addition, the half-life of cytokines in the bloodstream or other extracellular fluids into which they are secreted is usually very short, ensuring that they act for only a limited period of time and thus over a short distance. Cytokines CHAPTER 12 281 TABLE 12-1 Functional groups of selected cytokines1 Cytokine* Secreted by** Targets and effects SOME CYTOKINES OF INNATE IMMUNITY Interleukin 1 (IL-1) Monocytes, macrophages, Vasculature (inflammation); hypothalamus (fever); endothelial cells, epithelial cells l iver (induction of acute phase proteins) Tumor Necrosis Macrophages Vasculature (inflammation); liver (induction of acute phase Factor- (TNF-) proteins); loss of muscle, body fat (cachexia); induction of death in many cell types; neutrophil activation Interleukin 12 (IL-12) Macrophages, dendritic cells NK cells; influences adaptive immunity (promotes TH1 subset) Interleukin 6 (IL-6) Macrophages, endothelial cells Liver (induces acute phase proteins); influences adaptive immunity (proliferation and antibody secretion of B cell lineage) Interferon (IFN-) Macrophages Induces an antiviral state in most nucleated cells; increases MHC (This is a family of molecules) class I expression; activates NK cells Interferon (IFN-) Fibroblasts Induces an antiviral state in most nucleated cells; increases MHC class I expression; activates NK cells SOME CYTOKINES OF ADAPTIVE IMMUNITY Interleukin 2 (IL-2) T cells T-cell proliferation; can promote AICD. NK cell activation and proliferation; B-cell proliferation Interleukin 4 (IL-4) TH2 cells; mast cells Promotes TH2 differentiation; isotype switch to IgE Interleukin 5 (IL-5) TH2 cells Eosinophil activation and generation Interleukin 25 (IL-25) Unknown Induces secretion of TH2 cytokine profile Transforming growth T cells, macrophages, Inhibits T-cell proliferation and effector functions; inhibits factor (TGF-) other cell types B-cell proliferation; promotes isotype switch to IgE; inhibits macrophages Interferon  (IFN-) TH1 cells; CD8+ cells; NK cells Activates macrophages; increases expression MHC class I and class II molecules; increases antigen presentation 1 Many cytokines play roles in more than one functional category. *Only the major cell types providing cytokines for the indicated activity are listed; other cell types may also have the capacity to synthesize the given cytokine. **Also note that activated cells generally secrete greater amounts of cytokine than unactivated cells.

PaRt I Immune Effector Mechanisms RECEPTOR FAMILY LIGANDS Cytokine Receptors (a)Immunoglobulin superfamil As noted already, to exert their biological effects, cytokines receptors must first bind to specific receptors expressed on the mem- IL- brane of responsive target cells. Because these receptors are M-CSF expressed by many types of cells, the cytokines can affect a diverse array of cells. Biochemical characterization of cyto- kine receptors initially progressed at a very slow pace because their levels on the membrane of responsive cells is quite low. As with the cytokines themselves, cloning of the genes encod- ing cytokine receptors has led to rapid advances in the iden tification and characterization of these receptors (b) Class I Cytokine Receptors Fall Within Five Families (hematopoietin) Receptors for the various cytokines are quite diverse struc IL-2 IL- turally, but almost all belong to one of five families of recep- IL-3 IL-4 GM-CSF tor proteins( Figure 12-6) 自自> Conser -5 G-CSF Immunoglobulin superfamily receptors IL-6 IL-7 LIF Class I cytokine nown as the IL-9 CNTF hematopoietin IL-11 I L-12 Prolactin Class ll cytokine receptor family (also known as the interferon receptor family (c)Class Il cytokine receptors Many of the cytokine-binding receptors that function in nd hematopoietic systems belong to the cla eceptor family. The members of this receptor fami have conserved amino acid sequence motifs in the extracellu lar domain consisting of four positionally conserved cysteine residues(CCCC)and a conserved sequence of tryptophan- serine-(any amino acid)-tryptophan-serine(wSXWS, where X is the nonconserved amino acid). The receptors for all the (d)TNF receptors cytokines classified as hematopoietins belong to the class I TNFα cytokine receptor family, which also is called the hematopoi- CD40 etin receptor family. The class ll cytokine receptors possess Nerve growth factor (NGF) the conserved CCCC motifs, but lack the wSXWS motif pre- sent in class I cytokine receptors. Initially only the three interferons, a, B, and y, were thought to be ligands for these receptors. However, recent work has shown that the IL-10 receptor is also a member of this group Another feature common to most of the hematopoietin (class I cytokine) and the class lI cytokine receptor families is (e)Chemokine receptors ANTES PF4 MCAF FIGURE 12. Schematic diagrams showing the structural features NAP-2 that define the five types of receptor proteins to which most cytokines bind. The receptors for most of the interleukins belong to the clas tokine receptor family. C refers to conserved cysteine

Cytokine Receptors As noted already, to exert their biological effects, cytokines must first bind to specific receptors expressed on the mem￾brane of responsive target cells. Because these receptors are expressed by many types of cells, the cytokines can affect a diverse array of cells. Biochemical characterization of cyto￾kine receptors initially progressed at a very slow pace because their levels on the membrane of responsive cells is quite low. As with the cytokines themselves, cloning of the genes encod￾ing cytokine receptors has led to rapid advances in the iden￾tification and characterization of these receptors. Cytokine Receptors Fall Within Five Families Receptors for the various cytokines are quite diverse struc￾turally, but almost all belong to one of five families of recep￾tor proteins (Figure 12-6): ■ Immunoglobulin superfamily receptors ■ Class I cytokine receptor family (also known as the hematopoietin receptor family) ■ Class II cytokine receptor family (also known as the interferon receptor family) ■ TNF receptor family ■ Chemokine receptor family Many of the cytokine-binding receptors that function in the immune and hematopoietic systems belong to the class I cytokine receptor family. The members of this receptor family have conserved amino acid sequence motifs in the extracellu￾lar domain consisting of four positionally conserved cysteine residues (CCCC) and a conserved sequence of tryptophan￾serine-(any amino acid)-tryptophan-serine (WSXWS, where X is the nonconserved amino acid). The receptors for all the cytokines classified as hematopoietins belong to the class I cytokine receptor family, which also is called the hematopoi￾etin receptor family. The class II cytokine receptors possess the conserved CCCC motifs, but lack the WSXWS motif pre￾sent in class I cytokine receptors. Initially only the three interferons, , , and , were thought to be ligands for these receptors. However, recent work has shown that the IL-10 receptor is also a member of this group. Another feature common to most of the hematopoietin (class I cytokine) and the class II cytokine receptor families is 282 PART III Immune Effector Mechanisms RECEPTOR FAMILY LIGANDS (a) Immunoglobulin superfamily receptors IL-1 M-CSF C-Kit S S S S S S (b) Class I cytokine receptors (hematopoietin) IL-2 IL-3 IL-4 IL-5 IL-6 IL-7 IL-9 IL-11 IL-12 IL-13 IL-15 GM-CSF G-CSF OSM LIF CNTF Growth hormone Prolactin Conserved cysteines WSXWS (c) Class II cytokine receptors (interferon) IFN-α IFN-β IFN-γ IL-10 C C C C (d) TNF receptors C1 C3 C2 C1 C3 C2 C1 C3 C2 C1 C3 C2 TNF-α TNF-β CD40 Nerve growth factor (NGF) FAS (e) Chemokine receptors IL-8 RANTES MIP-1 PF4 MCAF NAP-2 G-protein FIGURE 12-6 Schematic diagrams showing the structural features that define the five types of receptor proteins to which most cytokines bind. The receptors for most of the interleukins belong to the class I cytokine receptor family. C refers to conserved cysteine.

Cytokines CHAPTER 12 283 multiple subunits, often including one subunit that binds a)GM-CSF receptor subfamily(common p subunit) specific cytokine molecules and another that mediates signal transduction Note however that these two functions are not GM-CSF always confined to one subunit or the other. Engagement of all of the class I and class ll cytokine receptors studied to date has been shown to induce tyrosine phosphorylation of the receptor through the activity of protein tyrosine kinases closely associated with the cytosolic domain of the receptors Subfamilies of Class I Cytokine Receptors Have Signaling Subunits in Common GM-CSFRo IL-3R Several subfamilies of class i cytokine receptors have beer identified, with all the receptors in a subfamily having identical signal-transducing subunit Figure 12-7 schemati cally illustrates the members of three receptor subfamilies, named after GM-cSe. IL-2 and IL-6 The sharing of signal-transducing subunits among recep- tors explains the redundancy and antagonism exhibited by (b)IL-6 Receptor subfamily(common gp130 subunit) some cytokines. Consider the GM-CSF receptor subfamily. which includes the receptors for IL-3, IL-5, and GM-CSF(see CNTF LIF/OSM Figure 12-7a) Each of these cytokines binds to a unique low affinity, cytokine-specific receptor consisting of an a subunit only. All three low-affinity subunits can associate noncova- IL-11 ntly with a common signal-transducing B subunit. The re- sulting dimeric receptor not only exhibits increased affinity for the cytokine but also can transduce a signal across the membrane after binding the cytokine(Figure 12-8a) Inter- estingly, IL-3, IL-5, and GM-CSF exhibit considerable redun- dancy. IL-3 and GM-CSF both ac cells and progenitor cells, activate monocytes, and induce megakaryocyte differentiation. All three of these cytokines induce eosinophil proliferation and basophil degranulation with release of histamine Since the receptors for IL-3, IL-5, and GM-CSF share a CNTFR common signal-transducing B subunit, each of these cyto- kines would be expected to transduce a similar activation nal, accounting for the redundancy among their biological effects(Figure 12-8b). In fact, all three cytokines induce the same patterns of protein phosphorylation. Furthermore, IL-3 and GM-CSF exhibit antagonism; IL-3 binding has been p130 0130 shown to be inhibited by GM-CSE, and conversely, binding IL-2 receptor subfamily(common y subunit IL-15 FIGURE 12-7 Schematic diagrams of the three subfamilies of class I cytokine receptors. All members of a subfamily have a common sig nal-transducing subunit(blue), but a unique cytokine-specific subunit IL-15Ro In addition to the conserved cysteines(double black lines)and WSXWS motifs(red lines) that characterize class I cytokine receptors, immu- noglobulin-like domains are present in some of these receptors. CNTF ciliary neurotrophic factor: LIF/OSM= leukemia-inhibitory factor/ IL2BB正7R9R oncostatin /Adapted from K. Sugamura et al, 1996, Annu. Rev. Im- munol. 14: 179 IL-4R

multiple subunits, often including one subunit that binds specific cytokine molecules and another that mediates signal transduction. Note, however, that these two functions are not always confined to one subunit or the other. Engagement of all of the class I and class II cytokine receptors studied to date has been shown to induce tyrosine phosphorylation of the receptor through the activity of protein tyrosine kinases closely associated with the cytosolic domain of the receptors. Subfamilies of Class I Cytokine Receptors Have Signaling Subunits in Common Several subfamilies of class I cytokine receptors have been identified, with all the receptors in a subfamily having an identical signal-transducing subunit. Figure 12-7 schemati￾cally illustrates the members of three receptor subfamilies, named after GM-CSF, IL-2, and IL-6. The sharing of signal-transducing subunits among recep￾tors explains the redundancy and antagonism exhibited by some cytokines. Consider the GM-CSF receptor subfamily, which includes the receptors for IL-3, IL-5, and GM-CSF (see Figure 12-7a). Each of these cytokines binds to a unique low￾affinity, cytokine-specific receptor consisting of an subunit only. All three low-affinity subunits can associate noncova￾lently with a common signal-transducing subunit. The re￾sulting dimeric receptor not only exhibits increased affinity for the cytokine but also can transduce a signal across the membrane after binding the cytokine (Figure 12-8a). Inter￾estingly, IL-3, IL-5, and GM-CSF exhibit considerable redun￾dancy. IL-3 and GM-CSF both act upon hematopoietic stem cells and progenitor cells, activate monocytes, and induce megakaryocyte differentiation. All three of these cytokines induce eosinophil proliferation and basophil degranulation with release of histamine. Since the receptors for IL-3, IL-5, and GM-CSF share a common signal-transducing subunit, each of these cyto￾kines would be expected to transduce a similar activation sig￾nal, accounting for the redundancy among their biological effects (Figure 12-8b). In fact, all three cytokines induce the same patterns of protein phosphorylation. Furthermore, IL-3 and GM-CSF exhibit antagonism; IL-3 binding has been shown to be inhibited by GM-CSF, and conversely, binding Cytokines CHAPTER 12 283 GM-CSF IL-3 IL-5 ββ β (a) GM-CSF receptor subfamily (common β subunit) (c) IL-2 receptor subfamily (common γ subunit) IL-6 CNTF IL-11 (b) IL-6 Receptor subfamily (common gp130 subunit) CNTFR IL-2Rβ IL-2Rβ IL-2Rα IL-15Rα IL-7R IL-9R γ γγ γ IL-15 IL-9 IL-7 IL-4R γ IL-2 IL-4 GM-CSFRα IL-3R IL-5R LIF/OSM gp130 gp130 gp130 gp130 FIGURE 12-7 Schematic diagrams of the three subfamilies of class I cytokine receptors. All members of a subfamily have a common sig￾nal-transducing subunit (blue), but a unique cytokine-specific subunit. In addition to the conserved cysteines (double black lines) and WSXWS motifs (red lines) that characterize class I cytokine receptors, immu￾noglobulin-like domains are present in some of these receptors. CNTF = ciliary neurotrophic factor; LIF/OSM = leukemia-inhibitory factor/ oncostatin. [Adapted from K. Sugamura et al., 1996, Annu. Rev. Im￾munol. 14:179.]

284 paRI I Immune Effector mechanisms of GM-CSF has been shown to be inhibited by IL-3. Since the signal-transducing B subunit is shared between the re L-3 L-5 GM-CSF ceptors for these two cytokines, their antagonism is due to Low-affinity receptors Exterior competition for a limited number of B subunits by the Membrane cytokine-specific a subunits of the receptors( Figure 12-8c) Interior A similar situation is found among the IL-6 receptor sub- family, which includes the receptors for IL-6, IL-11, leukemia- L阝 subunit inhibitory factor (LIF), oncostatin M(OSM), and ciliary neurotrophic factor(CNTF)(see Figure 12-7b). In this case High-affinit a common signal-transducing subunit called gp130 associ- ates with one or two different cytokine-specific subunits. LIF and osM. which must share certain structural features both bind to the same a subunit. As expected, the cytokines that bind to receptors in this subfamily display overlapping bio- logical activities: IL-6, OSM, and LiF induce synthesis of acute-phase proteins by liver hepatocytes and differentiation of myeloid leukemia cells into macrophages; IL-6, LIE, and CNTF affect neuronal development, and IL-6, IL-ll, and OSM stimulate megakaryocyte maturation and platelet pro duction. The presence of gp130 in all receptors of the IL-6 subfamily explains their common signaling pathways as well as the binding competition for limited gp130 molecules tha is observed among these cytokines A third signal-transducing subunit defines the IL-2 recep tor subfamily, which includes receptors for IL-2, IL-4, IL-7, GM-CSF IL-9, and IL-15(see Figure 12-7c). The IL-2 and the IL-15 receptors are heterotrimers, consisting of a cytokine-specific a chain and two chains-B and y-responsible for signal transduction. The IL-2 receptor y chain functions as the sig nal-transducing subunit in the other receptors in this sub- family, which are all dimers. Recently, it has been shown that congenital X-linked severe combined immunodeficiency (XSCID)results from a defect in the y-chain gene, which maps to the X chromosome. The immunodeficiencies ob served in this disorder are due to the loss of all the cytokine functions mediated by the IL-2 subfamily receptors The IL-2R Is One of the Most Thoroughly Studied Cytokine Receptors Because of the central role of IL-2 and its receptor in the FIGURE 12-8 Interactions between cytokine-specific subunits and a clonal proliferation of T cells, the IL-2 receptor has received common signal-transducing subunit of cytokine receptors.(a)Sche- olete trimeric receptor comprises three distinct subunits- and GM-CSF. The cytokine-specific subunits exhibit low-affinity binding the a, B, and y chains. The p and y chains belong to the class and cannot transduce an activation signal. Noncovalent association of I cytokine receptor family, containing the characteristic CCCC each subunit with a common B subunit yields a high-affinity dimeric re- Figure 12-7c) B subunit, allows the generation of cytokine-specific signals despite the The IL-2 receptor occurs in three forms that exhibit dif- generation of the same signal by the different cytokine receptors shown. ferent affinities for IL-2: the low-affinity monomeric IL-2Ro, ( c)Competition of ligand-binding chains of different receptors for a the intermediate-affinity dimeric IL-2RBy, and the high- common subunit can produce antagonistic effects between cytokines affinity trimeric IL-2RaBy(Figure 12-9). Because the a Here binding of IL-3 by a subunits of the IL-3 receptor allows them to chain is expressed only by activated T cells, it is often referred out-compete a chains of the GM-CSF receptor for B subunits. / Part (a) to as the tAC(T-cell activation)antigen A monoclonal anti- adapted from T Kishimoto et al., 1992, Science 258: 593

of GM-CSF has been shown to be inhibited by IL-3. Since the signal-transducing subunit is shared between the re￾ceptors for these two cytokines, their antagonism is due to competition for a limited number of subunits by the cytokine-specific subunits of the receptors (Figure 12-8c). A similar situation is found among the IL-6 receptor sub￾family, which includes the receptors for IL-6, IL-11, leukemia￾inhibitory factor (LIF), oncostatin M (OSM), and ciliary neurotrophic factor (CNTF) (see Figure 12-7b). In this case, a common signal-transducing subunit called gp130 associ￾ates with one or two different cytokine-specific subunits. LIF and OSM, which must share certain structural features, both bind to the same subunit. As expected, the cytokines that bind to receptors in this subfamily display overlapping bio￾logical activities: IL-6, OSM, and LIF induce synthesis of acute-phase proteins by liver hepatocytes and differentiation of myeloid leukemia cells into macrophages; IL-6, LIF, and CNTF affect neuronal development, and IL-6, IL-11, and OSM stimulate megakaryocyte maturation and platelet pro￾duction. The presence of gp130 in all receptors of the IL-6 subfamily explains their common signaling pathways as well as the binding competition for limited gp130 molecules that is observed among these cytokines. A third signal-transducing subunit defines the IL-2 recep￾tor subfamily, which includes receptors for IL-2, IL-4, IL-7, IL-9, and IL-15 (see Figure 12-7c). The IL-2 and the IL-15 receptors are heterotrimers, consisting of a cytokine-specific chain and two chains— and —responsible for signal transduction. The IL-2 receptor  chain functions as the sig￾nal-transducing subunit in the other receptors in this sub￾family, which are all dimers. Recently, it has been shown that congenital X-linked severe combined immunodeficiency (XSCID) results from a defect in the -chain gene, which maps to the X chromosome. The immunodeficiencies ob￾served in this disorder are due to the loss of all the cytokine functions mediated by the IL-2 subfamily receptors. The IL-2R Is One of the Most Thoroughly Studied Cytokine Receptors Because of the central role of IL-2 and its receptor in the clonal proliferation of T cells, the IL-2 receptor has received intensive study. As noted in the previous section, the com￾plete trimeric receptor comprises three distinct subunits— the , , and  chains. The and  chains belong to the class I cytokine receptor family, containing the characteristic CCCC and WSXWS motifs, whereas the chain has a quite different structure and is not a member of this receptor family (see Figure 12-7c). The IL-2 receptor occurs in three forms that exhibit dif￾ferent affinities for IL-2: the low-affinity monomeric IL-2R, the intermediate-affinity dimeric IL-2R, and the high￾affinity trimeric IL-2R (Figure 12-9). Because the chain is expressed only by activated T cells, it is often referred to as the TAC (T-cell activation) antigen. A monoclonal anti- 284 PART III Immune Effector Mechanisms β α High-affinity receptors β α β α α Low-affinity receptors β subunit α α Exterior Membrane Interior IL-3 IL-5 GM-CSF (a) α Same signal from all three α α (b) IL-3 GM-CSF α α α α α α β β β β β β αα α α α α (c) FIGURE 12-8 Interactions between cytokine-specific subunits and a common signal-transducing subunit of cytokine receptors. (a) Sche￾matic diagram of the low-affinity and high-affinity receptors for IL-3, IL-5, and GM-CSF. The cytokine-specific subunits exhibit low-affinity binding and cannot transduce an activation signal. Noncovalent association of each subunit with a common subunit yields a high-affinity dimeric re￾ceptor that can transduce a signal across the membrane. (b) Associa￾tion of cytokine-specific subunits with a common signaling unit, the subunit, allows the generation of cytokine-specific signals despite the generation of the same signal by the different cytokine receptors shown. (c) Competition of ligand-binding chains of different receptors for a common subunit can produce antagonistic effects between cytokines. Here binding of IL-3 by subunits of the IL-3 receptor allows them to out-compete chains of the GM-CSF receptor for subunits. [Part (a) adapted from T. Kishimoto et al., 1992, Science 258:593.]

Cytokines CHAPTER 12 285 Intermediate Low affinity strated that one of the first events after the interaction of ity IL-2R IL-2R with one of these receptors is a series of pre tein tyrosine phosphorylations. While these results were ini- tially puzzling, they were explained when a unifying model emerged from studies of the molecular events triggered by binding of interferon gamma(IFN-y)to its receptor,a mem- ber of the class ll family 夏 IFN-y was originally discovered because of its ability to induce cells to block or inhibit the replication of a wide vari- ety of viruses. Antiviral activity is a property it shares with IFN-a and IFN-B. However, unlike these other interferons, IL-2Ra FN-y plays a central role in many immunoregulatory IL2Rβ processes, including the regulation of mononuclear phago- ytes, B-cell switching to certain IgG classes, and the support Affinity constant (L):107M 8M or inhibition of the development of TH-cell subsets. The dis overy of the major signaling pathway invoked by interaction of IFN-y with its receptor led to the realization that signal 109M 10-11 transduction through most, if not all, class I and class II ytokine receptors involves the following steps, which are the Cells Activated CD4+and CD8+Tcells basis of a unifying signaling model(Figure 12-10) Resting t cells Activated b cells Cow numbers) The cytokine receptor is composed of separate subunits, an a chain required for cytokine binding and for signal FIGURE 12.9 Comparison of the three forms of the IL-2 receptor. transduction and a p chain necessary for signaling but with only a minor role in binding Signal transduction is mediated by the p and y chains, but all three chains are required for high-affinity binding of IL-2 a Different inactive protein tyrosine kinases are associated with different subunits of the receptor. The a chain of the receptor is associated with a novel family of protein tyrosine kinases, the Janus kinase (JAK)*family. The body, designated anti-TAC, which binds to the 55-kDa a association of the JAK and the receptor subunit occurs chain, is often used to identify IL-2Ro on cells Signal trans spontaneously and does not require the binding of duction by the IL-2 receptor requires both the B and y cytokine. However, in the absence of cytokine, JAKs lack chains, but only the trimeric receptor containing the a chain protein tyrosine kinase activity as well binds IL-2 with high affinity. Although the y chain Cytokine binding induces the association of the two appears to be constitutively expressed on most lymphoid separate cytokine receptor subunits and activation of the cells, expression of the a and B chains is more restricted and receptor-associated JAKs. The ability of IFN-y, which is markedly enhanced after antigen has activated resting binds to a class ll cytokine receptor, to bring about the lymphocytes. This phenomenon ensures that only antigen activated CD4* and CD8 T cells will express the high association of the ligand-binding chains of its receptor has been directly demonstrated by x-ray crystallographic affinity IL-2 receptor and proliferate in response to physio udies, as shown in Figure 12-11 logic levels of Il-2. Activated t cells 5 10 high-affinity receptors and ten times as many low-. Activated JAKs create docking sites for the STAT affinity receptors. NK cells express the B and y subunits con- transcription factors by phosphorylation of specific tyrosine stitutively, accounting for their ability to bind IL-2 with an residues on cytokine receptor subunits. Once receptor intermediate affinity and to be activated by IL-2. associated JAKs are activated, they phosphorylate specific tyrosines in the receptor subunits of the Engaged Cytokine Receptors Activate Signaling Pathways While some important cytokine receptors lie outside the class I and class lI families, the majority are included within The Roman god Janus had two faces these two families. As mentioned previously, class I and class catalytic site that, when activated, has Il cytokine receptors lack signaling motifs(e.g, intrinsic chemists, wearied by the multitude of different protein kinases that have been tyrosine kinase domains). Yet, early observations demon- scovered, claim JAK means just Another

body, designated anti-TAC, which binds to the 55-kDa chain, is often used to identify IL-2R on cells. Signal trans￾duction by the IL-2 receptor requires both the and  chains, but only the trimeric receptor containing the chain as well binds IL-2 with high affinity. Although the  chain appears to be constitutively expressed on most lymphoid cells, expression of the and chains is more restricted and is markedly enhanced after antigen has activated resting lymphocytes. This phenomenon ensures that only antigen￾activated CD4+ and CD8+ T cells will express the high￾affinity IL-2 receptor and proliferate in response to physio￾logic levels of IL-2. Activated T cells express approximately 5  103 high-affinity receptors and ten times as many low￾affinity receptors. NK cells express the and  subunits con￾stitutively, accounting for their ability to bind IL-2 with an intermediate affinity and to be activated by IL-2. Engaged Cytokine Receptors Activate Signaling Pathways While some important cytokine receptors lie outside the class I and class II families, the majority are included within these two families. As mentioned previously, class I and class II cytokine receptors lack signaling motifs (e.g., intrinsic tyrosine kinase domains). Yet, early observations demon￾strated that one of the first events after the interaction of a cytokine with one of these receptors is a series of pro￾tein tyrosine phosphorylations. While these results were ini￾tially puzzling, they were explained when a unifying model emerged from studies of the molecular events triggered by binding of interferon gamma (IFN-) to its receptor, a mem￾ber of the class II family. IFN- was originally discovered because of its ability to induce cells to block or inhibit the replication of a wide vari￾ety of viruses. Antiviral activity is a property it shares with IFN- and IFN-. However, unlike these other interferons, IFN- plays a central role in many immunoregulatory processes, including the regulation of mononuclear phago￾cytes, B-cell switching to certain IgG classes, and the support or inhibition of the development of TH-cell subsets. The dis￾covery of the major signaling pathway invoked by interaction of IFN- with its receptor led to the realization that signal transduction through most, if not all, class I and class II cytokine receptors involves the following steps, which are the basis of a unifying signaling model (Figure 12-10). ■ The cytokine receptor is composed of separate subunits, an chain required for cytokine binding and for signal transduction and a chain necessary for signaling but with only a minor role in binding. ■ Different inactive protein tyrosine kinases are associated with different subunits of the receptor. The chain of the receptor is associated with a novel family of protein tyrosine kinases, the Janus kinase (JAK)* family. The association of the JAK and the receptor subunit occurs spontaneously and does not require the binding of cytokine. However, in the absence of cytokine, JAKs lack protein tyrosine kinase activity. ■ Cytokine binding induces the association of the two separate cytokine receptor subunits and activation of the receptor-associated JAKs. The ability of IFN-, which binds to a class II cytokine receptor, to bring about the association of the ligand-binding chains of its receptor has been directly demonstrated by x-ray crystallographic studies, as shown in Figure 12-11. ■ Activated JAKs create docking sites for the STAT transcription factors by phosphorylation of specific tyrosine residues on cytokine receptor subunits. Once receptor￾associated JAKs are activated, they phosphorylate specific tyrosines in the receptor subunits of the Cytokines CHAPTER 12 285 Subunit composition: Dissociation constant (Kd ): Cells expressed by: IL-2Rβ IL-2Rγ IL-2Rα IL-2Rβ IL-2Rγ IL-2Rα 10−9M 10−11M 10−8M Affinity constant (Ka): 107M 1011M 108M NK cells Resting T cells (low numbers) Activated CD4+ and CD8+ T cells Activated B cells (low numbers) β γ α α β γ Intermediate affinity IL-2R High affinity IL-2R Low affinity IL-2R FIGURE 12-9 Comparison of the three forms of the IL-2 receptor. Signal transduction is mediated by the and  chains, but all three chains are required for high-affinity binding of IL-2. *The Roman god Janus had two faces. Kinases of the Janus family have two sites, a binding site at which they link with the cytokine receptor subunit and a catalytic site that, when activated, has protein tyrosine kinase activity. Some bio￾chemists, wearied by the multitude of different protein kinases that have been discovered, claim JAK means Just Another Kinase.