getic barriers, similar to recent models for the role of lateral mechanical forces in integrin activation (Zhu et al., 2008).
The specific adapters that link TCR to F-actin are not well described. The complex may include NCK, SLP-76, ADAP, SKAP55, EVL, Vav, Rac, Cdc42 and WASP certainly may play a role once signaling is initiated (Barda-Saad et al., 2005; Bubeck Wardenburg et al., 1998; Krause et al., 2000). It has also been suggested that ZAP-70 may play a role through binding of ezrin (Ilani et al., 2007). It appears that TCR signaling in microclusters ceases when the microcluster reaches the actin depleted cSMAC. Since activated TCR appear equipped to generate their own dynamic actin focus, the suppression of this mode of actin dymanics in the cSMAC is likely to be active, rather than a passive dissipation of lamellar actin. Less is known about the basal interactions of TCR with actin that might be available to stabilize a cluster early in the triggering process. This is likely a complex situation. An example of the potential complexity can be seen in interaction networks recently compiled as the integrin adhesome (Zaidel-Bar et al., 2007).
Once the ITAMs are exposed Src family kinases can then initiate phosphorylation by diffusion into the same domain (Douglass and Vale, 2005)(Fig 2). Exclusion of CD45 in the nascent foci would increase the half-life of the phosphorylated ITAM allowing sufficient time for ZAP-70 recruitment (Varma et al., 2006). This cluster may interact more strongly with LAT so that ZAP-70 could phosphorylate LAT and Itk to initiate PLC-g recruitment and initiation of Ca2+ signaling, PKC activation and Ras-GRP recruitment. Integrin microclusters will be interspersed with the TCR microclusters. Integrin dependent signaling will generate additional diacylglycerol for RasGRP by activating phospholipase D and phosphatidic acid phosphatase (Mor et al., 2007). Electron micrographs of membrane sheets suggest that immunoreceptor rich and LAT rich islands only partially mix (Lillemeier et al., 2006; Wilson et al., 2001). The force needed to induce island mixing may be generated by periodic myosin II based contraction. This may account for the role of myosin IIA in the signal amplification from Lck, which is normally activated after myosin IIA knock-down, to ZAP-70 and LAT, which are only weakly activated in myosin IIA knock-down T cells (Ilani et al., 2009).
The TCR triggering process in microclusters and how this is sustained in a synapse has relevance to T cell decision-making. T cell differentiation is controlled in large part by cytokines and small molecules from APC. There are two independent reports that propose distinct mechanisms for the synapse to influence differentiation. One is the concentration of interferon-γ receptors in the synapse leading to increased T helper 1 (Th1) cell development (Maldonado et al., 2004). IL-4 counteracts this through Stat6 (Maldonado et al., 2004). The other is the control of polarity networks leading to asymmetric cell division to control memory-effector decisions (Chang et al., 2007; Ludford-Menting et al., 2005). A particular challenge is in understanding how the microcluster-based mechanism sustains signaling from small numbers of ligand to drive differentiation. If each microcluster can sustain signaling for 2 minutes as the microclusters translocates from the periphery to the cSMAC. How is signaling sustained with only 10 MHC-peptide complexes? An interesting observation is that although the actin depleted zone at the center of the synapse is always present, the size of the cSMAC is linearly dependent upon the MHC-peptide density such that no TCR cSMAC if formed at the lower limit of MHC-peptide density. Under these conditions our model would posit that MHC-peptide ligand are recycled expensively by long term serial triggering at the F-actin rich cSMAC-pSMAC boundary. This fits with notions of serial triggering initially propose by Valittuti and Lanzavechia (Valitutti et al., 1995b), but would induce very little if any TCR down-regulation because all receptors a