 ligands in supported planar bilayers is based on diffusion trapping of receptors and ligands in microdomains with multiple BCR and ligands. This mode of clustering is likely driven by F-actin dependent membrane fluctuations and adhesion, perhaps accounting for the role of F-actin in enhancing B cell recognition of solid phase ligands (Carrasco et al., 2004; Tolar et al., 2008). For T cells, the F-actin requirement seems to involve both forming microclusters and then formation of signaling complexes at the TCR clusters. This is based on the observation that many TCR microclusters are stable after F-actin depolymerization, but signaling is still terminated even though the receptor clusters persist. All antigen receptors engage in signaling that activates actin polymerization via the Rac and Cdc42 small G-proteins leading to activation of the WAVE2 complexes and WASp (Barda-Saad et al., 2005; Nolz et al., 2006). In the absence of WAVE2 the lamellipodium at the periphery of T cell contacts with anti-CD3 coated surfaces is lost (Nolz et al., 2006). This does not alter that ability of these cells to initiate early TCR signaling in response to solid phase anti-CD3, but more work is needed to determine if this alters sensitivity to MHC-peptide ligands as expected.
The recent focus on microclusters raises questions about cSMAC-associated signals. Is the cSMAC just for signal termination or is it a site of TCR or other relevant signaling? Is the cSMAC even one compartment? Kupfer defined the cSMAC based on accumulation of TCR and PKC-θ. Dynamic studies using fluorescence recovery after photobleaching (FRAP) revealed that TCR-MHC-peptide interactions are particularly stable in the cSMAC (Grakoui et al., 1999) and that the TCR microclusters that converge to form the cSMAC appear to fuse together into one stable structure on planar bilayers (Varma et al., 2006). This structure is partly shed by the T cell if it breaks the synapse and migrates or wanes over 10’s of minutes in stable synapses. Is this highly stabilized structu