8536d_ch10_221 8/27/02 1: 37 PM Page 231 Mac 109 Mac 109: 1254_BpLGoldsby et al./Immunology 5e T-Cell Maturation. Activation, and Differentiation CHAPTER 10 Signal reception often leads to the generation within the of the zeta chain provide docking sites to which a protein cell of a "second messenger, "a molecule or ion that can tyrosine kinase called ZAP-70 attaches(step 2 in Figure diffuse to other sites in the cell and evoke changes 10-10)and becomes active. ZAP-70 then catalyzes the amples are cyclic nucleotides(cAMP, cGMP), calcium phosphorylation of a number of membrane-associated ion( Cazf), and membrane phospholipid derivatives adaptor molecules(step 3), which act as anchor points such as diacylglycerol (DAG)and inositol triphosphate for the recruitment of several intracellular signal transduction pathways. One set of pathways involves Protein kinases and protein phosphatases are activated form of the enzyme phospholipase C(PLC), which inhibited. Kinases catalyze the phosphorylation of target anchors to an adaptor molecule, is activated by residues(tyrosine, serine, or threonine)of key elements phosphorylation and cleaves a membrane phospholipid in many signal-transduction pathways. Phosphatases to generate second messengers. Another set activates catalyze dephosphorylation, reversing the effect of kinases. These enzymes play essential roles in many s Generation of multiple intracellular signals. Many ignal-transduction pathways of immunological interest. ignaling pathways are activated as a consequence of the Many signal transduction pathways involve the signal steps that occur in the initiation phase, as shown to the induced assembly of some components of the pathway right in Figure 10-10, and described below Molecules known as adaptor proteins bind specifically We shall consider several of the signaling pathways re- and simultaneously to two or more different molecules cruited by T-cell activation, but the overall process is quite with signaling roles, bringing them together and complex and many of the details will not be presented here The review articles suggested at the end of this chapter pro- gnals are amplified by enzyme cascades. Each enzyme in vide extensive coverage of this very active research area the cascade catalyzes the activation of many copies of the Phospholipase Cy(PLCy): PLCy is activated by phosphoryla next enzyme in the sequence, greatly amplifying the tion and gains access to its substrate by binding to a mem- signal at each step and offering many opportunities to brane-associated adaptor protein(Figure 10-11a). PLCY modulate the intensity of a signal along the way. hydrolyzes a phospholipid component of the membrane to generate inositol 1, 4, 5-triphosphate(IP3)and diacylglycerol The default setting for signal-transduction pathways is DAG). IP3 causes a rapid release of Ca" from the endoplas- OFE. In the absence of an appropriately presented signal, mic reticulum and opens Ca2* channels in the cell mem transmission through the pathway does not take place brane(Figure 10-11b). DAG activates protein kinase C,a multifunctional kinase that phosphorylates many different Multiple Signaling Pathways Are Initiated targets( Figure 10-1lc). by TCR Engagem Ca?t: Calcium ion is involved in an unusually broad range of processes, including vision, muscle contraction, and many The events that link antigen recognition by the T-cell recep- others. It is an essential element in many T-cell responses, in tor to gene activation echo many of the themes just reviewed. cluding a pathway that leads to the movement of a major The key element in the initiation of T-cell activation is the transcription factor, NFAT, from the cytoplasm into the nu recognition by the TCR of MHC-peptide complexes on cleus(Figure 10-1lb). In the nucleus, NFAT supports the antigen-presenting cells. transcription of genes required for the expression of the t- As described in Chapter 9, the TCR consists of a mostly cell growth-promoting cytokines IL-2, IL-4, and others extracellular ligand-binding unit, a predominantly intracel- Protein kinase C(PKC): This enzyme, which affects many lular signaling unit, the CD3 complex, and the homodimer of pathways, causes the release of an inhibitory molecule from s(zeta)chains. Experiments with knockout mice have shown the transcription factor NF-KB, allowing NF-KB to enter the that all of these components are essential for signal transduc- nucleus, where it promotes the expression of genes required tion. Two phases can be recognized in the antigen-mediated for T-cell activation(Figure 10-11c). NF-KB is essential for a induction of T-cell responses variety of T-cell responses and provides survival signals that Initiation. The engagement of MHC-peptide by the Tcr protect T cells from apoptotic death eads to clustering with CD4 or CD8 coreceptors as these The Ras/MaP kinase pathway: Ras is a pivotal component of coreceptors bind to invariant regions of the MHC a signal-transduction pathway that is found in many cell molecule(Figure 10-10). Lck, a protein tyrosine kinase types and is evolutionarily conserved across a spectrum of associated with the cytoplasmic tails of the coreceptors, eukaryotes from yeasts to humans. Ras is a small G protein is brought close to the cytoplasmic tails of the tCr whose activation by GTP initiates a cascade of protein ki- complex and phosphorylates the immunoreceptor nases known as the mitogen-activated protein kinase(MAP tyrosine-based activation motifs(ITAMs, described in kinase)pathway. As shown in Figure 10-12, phosphorylation Chapter 9). The phosphorylated tyrosines in the ITAMs of the end product of this cascade, MAP kinase(also called■ Signal reception often leads to the generation within the cell of a “second messenger,” a molecule or ion that can diffuse to other sites in the cell and evoke changes. Examples are cyclic nucleotides (cAMP, cGMP), calcium ion (Ca2), and membrane phospholipid derivatives such as diacylglycerol (DAG) and inositol triphosphate (IP3). ■ Protein kinases and protein phosphatases are activated or inhibited. Kinases catalyze the phosphorylation of target residues (tyrosine, serine, or threonine) of key elements in many signal-transduction pathways. Phosphatases catalyze dephosphorylation, reversing the effect of kinases. These enzymes play essential roles in many signal-transduction pathways of immunological interest. ■ Many signal transduction pathways involve the signal￾induced assembly of some components of the pathway. Molecules known as adaptor proteins bind specifically and simultaneously to two or more different molecules with signaling roles, bringing them together and promoting their combined activity. ■ Signals are amplified by enzyme cascades. Each enzyme in the cascade catalyzes the activation of many copies of the next enzyme in the sequence, greatly amplifying the signal at each step and offering many opportunities to modulate the intensity of a signal along the way. ■ The default setting for signal-transduction pathways is OFF. In the absence of an appropriately presented signal, transmission through the pathway does not take place. Multiple Signaling Pathways Are Initiated by TCR Engagement The events that link antigen recognition by the T-cell recep￾tor to gene activation echo many of the themes just reviewed. The key element in the initiation of T-cell activation is the recognition by the TCR of MHC-peptide complexes on antigen-presenting cells. As described in Chapter 9, the TCR consists of a mostly extracellular ligand-binding unit, a predominantly intracel￾lular signaling unit, the CD3 complex, and the homodimer of (zeta) chains. Experiments with knockout mice have shown that all of these components are essential for signal transduc￾tion. Two phases can be recognized in the antigen-mediated induction of T-cell responses: ■ Initiation. The engagement of MHC-peptide by the TCR leads to clustering with CD4 or CD8 coreceptors as these coreceptors bind to invariant regions of the MHC molecule (Figure 10-10). Lck, a protein tyrosine kinase associated with the cytoplasmic tails of the coreceptors, is brought close to the cytoplasmic tails of the TCR complex and phosphorylates the immunoreceptor tyrosine-based activation motifs (ITAMs, described in Chapter 9). The phosphorylated tyrosines in the ITAMs of the zeta chain provide docking sites to which a protein tyrosine kinase called ZAP-70 attaches (step 2 in Figure 10-10) and becomes active. ZAP-70 then catalyzes the phosphorylation of a number of membrane-associated adaptor molecules (step 3), which act as anchor points for the recruitment of several intracellular signal transduction pathways. One set of pathways involves a form of the enzyme phospholipase C (PLC), which anchors to an adaptor molecule, is activated by phosphorylation and cleaves a membrane phospholipid to generate second messengers. Another set activates small G proteins. ■ Generation of multiple intracellular signals. Many signaling pathways are activated as a consequence of the steps that occur in the initiation phase, as shown to the right in Figure 10-10, and described below. We shall consider several of the signaling pathways re￾cruited by T-cell activation, but the overall process is quite complex and many of the details will not be presented here. The review articles suggested at the end of this chapter pro￾vide extensive coverage of this very active research area. Phospholipase C (PLC): PLC is activated by phosphoryla￾tion and gains access to its substrate by binding to a mem￾brane-associated adaptor protein (Figure 10-11a). PLC hydrolyzes a phospholipid component of the membrane to generate inositol 1,4,5-triphosphate (IP3) and diacylglycerol (DAG). IP3 causes a rapid release of Ca2from the endoplas￾mic reticulum and opens Ca2 channels in the cell mem￾brane (Figure 10-11b). DAG activates protein kinase C, a multifunctional kinase that phosphorylates many different targets (Figure 10-11c). Ca2: Calcium ion is involved in an unusually broad range of processes, including vision, muscle contraction, and many others. It is an essential element in many T-cell responses, in￾cluding a pathway that leads to the movement of a major transcription factor, NFAT, from the cytoplasm into the nu￾cleus (Figure 10-11b). In the nucleus, NFAT supports the transcription of genes required for the expression of the T￾cell growth-promoting cytokines IL-2, IL-4, and others. Protein kinase C (PKC): This enzyme, which affects many pathways, causes the release of an inhibitory molecule from the transcription factor NF-B, allowing NF-B to enter the nucleus, where it promotes the expression of genes required for T-cell activation (Figure 10-11c). NF-B is essential for a variety of T-cell responses and provides survival signals that protect T cells from apoptotic death. The Ras/MAP kinase pathway: Ras is a pivotal component of a signal-transduction pathway that is found in many cell types and is evolutionarily conserved across a spectrum of eukaryotes from yeasts to humans. Ras is a small G protein whose activation by GTP initiates a cascade of protein ki￾nases known as the mitogen-activated protein kinase (MAP kinase) pathway. As shown in Figure 10-12, phosphorylation of the end product of this cascade, MAP kinase (also called T-Cell Maturation, Activation, and Differentiation CHAPTER 10 231 8536d_ch10_221 8/27/02 1:37 PM Page 231 Mac 109 Mac 109:1254_BJN:Goldsby et al. / Immunology 5e: