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 Anotherbody, 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.