In this regard it is important to appreciate that although ITAM sequences are conserved, they are not identical, ITAM Multiplicity in the Immune System and hence may be able to associate with distinct mole-An intriguing feature of many receptors that participate cules or associate with the same effectors with differin immune responses is the presence of multiple sub- ent affinities. Third, multiple ITAMs could potentially units and/or motifs that mediate signal transduction. negatively regulate signaling by immune receptors, One of the best studied of these signaling motifs is the depending on the extent or specific pattern of ITAM ITAM (immune-receptortyrosine-basedactivationmotif) phosphorylation (Figure 1C). For example, monopho-(Reth, 1989). Virtually all receptors that utilize ITAMs sphorylated ITAMs could directly recruit inhibitory profor signal transduction contain multiple copies of these teins, such as phosphatases, to the receptor complex. motifs. These include some Fc receptors (Ravetch and The tyrosine phosphatases SHP-1, SHP-2, and SHIP Kinet, 1991), activating NK receptors (Lanier et al., 1998), associate with a sequence (YxxL) that closely resembles PIR-A (paired immunoglobulin-like receptor-A)(Kuba- a monophosphorylated ITAM (referred to as an immunegawa et al., 1999), the B cell antigen receptor (BCR) receptor tyrosine-based inhibitory motif [ITIM]) present (Kurosaki, 1999), and the T cell antigen receptor (TCR) in several inhibitory coreceptors, including Fc RIIB,(Klausner et al., 1990). Depending on the particular cell CD22, and KIRs. The recruitment of tyrosine phosphatype and state of maturation, signals mediated through tases to coreceptors that contain ITIMs has been shown ITAMs can regulate cell survival, cell death, develop- to negatively regulate signaling through ITAM-conment, or effector functions. taining activating receptors (Vivier and Daeron, 1997). ITAMs consist of semiconserved sequences of amino Alternatively, ITAMs could act to sequester effector molacids that contain two appropriately spaced tyrosines ecules in their inactive forms, making such molecules (YXXL/IX 6–8 YXXL/I; where X denotes nonconserved resi- unavailable to other receptors, as has been suggested dues)(Reth, 1989). Following receptor engagement, for FcεRI (Torigoe et al., 1998). The mechanisms by phosphorylation of ITAM tyrosine residues by Src family which ITAM multiplicity may regulate immune function kinases represents one of the earliest events in the sig- are not mutually exclusive, and each may be critical, naling cascade (reviewed in Weiss, 1993; Wange and depending on the cell type. Samelson, 1996; Rudd, 1999). In general, phosphorylation of both tyrosines within an ITAM (diphosphorylation) ITAM Multiplicity and the TCR is thought to be essential for signaling, as this is required The TCR contains several distinct signaling subunits for efficient recruitment of the tandem SH2 domain con- with up to ten ITAMs distributed among these chains. taining protein tyrosine kinases Syk and ZAP-70 to the Although the clonotypic (TCR and TCR) chains are receptor complex. Activation of Syk and/or ZAP-70 re- required for antigen recognition, they do not directly sults in the recruitment and phosphorylation of proteins participate in signal transduction. Instead, TCR signalthat couple immune receptors to downstream signaling ing is mediated by the invariant chains of the TCR: pathways. CD3-,-,-ε, and chain. Each of the CD3 components While the importance of ITAMs for signal transduction contains a single ITAM within its cytoplasmic tail, and is clear, it remains uncertain why immune receptors con- because there are two CD3ε chains in each …