e TCR signal as discussed below and directly through the interactions of ADAP through Ena-VASP family members, which are involved directly in actin polymerization and contribute to regulation of Arp2/3 (Krause et al., 2000). SKAP-55 forms a complex with RIAM (Menasche et al., 2007), which binds to active Rap-1, a small G-protein related to Ras that is required for regulation of adhesion in lymphocytes. Activation of Rap-1 downstream of tyrosine kinase cascades requires a guanine nucleotide exchange factor (GEF) called C3G. TCR signaling activates C3G by a cascade involving Vav, a GEF for Rac (Krawczyk et al., 2002). Recent evidence suggests that Rac activates the WAVE2 complex, which activates Abl and CrkL-C3G to increase active Rap1 (Nolz et al., 2008). The Rap-1-RIAM complex then contributes to activation of talin, which directly binds to the β2 cytoplasmic domain in the critical hinge region to activate and cluster LFA-1 (Smith et al., 2005; Tadokoro et al., 2003; Wegener et al., 2007). This pathway accounts for the strong correlation of LFA-1 dependent cell-cell adhesion and talin accumulation at the interface.
Another connection to F-actin is observed in the APC. DC mediated antigen presentation is an active process that requires small G-proteins of the Rac family and intact F-actin (Al-Alwan et al., 2003; Benvenuti et al., 2004). This may be in part due to the interaction if ICAM-1 with the actin cytoskeleton, in part via ERM family members.
Norcross and Paul first discussed the idea of an immunological synapse with the main “synapse-like” features being a role for Ca2+ elevation, adhesion and directed secretion and polarity (Norcross, 1984). There are a handful of studies in the 1980’s and 90’s that provide quantitative data on cytoskeletal organization, secretory apparatus polarization and Ca2+ signaling in T cell conjugates with B cells e.g. (Geiger et al., 1982; Kupfer et al., 1986; Poenie et al., 1987), in addition to above mentioned studies on adhesion. The classical picture is of cell pairs with a T cell stably appended onto the often-larger target cells like an apse on a building-which is the origin of the term synapse. In one study, a distinct annular adhesive domain and central secretory domains were resolved by electron microscopy (Schmidt et al., 1988), which foreshadows the later work by Kupfer.
The molecular organization of these interfaces was not revealed until advances in imaging in the 1990’s led to the definition of discrete localization of TCR and LFA-1 and a more specific definition of immunological synapse was proposed (Dustin et al., 1998). Springer proposed in 1990 that LFA-1 and TCR would be segregated in adhesive interfaces because they are different sizes and it would be impossible for TCR to reach the MHC in a adhesive interface mediated by LFA-1 and it would be similarly difficult for LFA-1 to squeeze into an interface defined by the smaller CD2-CD58 adhesion system, which would in turn be ideal for the TCR (Springer, 1990). Springer further proposed that exclusion of large phosphatases like CD45 might be important for tyrosine phosphorylation down-stream of the tyrosine kinase cascade. This concept has been refined and tested and seems likely to be an important part of the signaling niche for the TCR (Choudhuri and van der Merwe, 2007; Varma et al., 2006). Springer didn’t speculate on the scale over which this segregation process would take place. The initial answers to this question were surprising. Kupfer examined fixed T cell-B cell conjugates and revealed a large central TCR cluster that was also rich in PKC-θ, defined as the central supramolecular activation cluster (cSMAC) (Monks et al., 1998)(Table 1). A ring of LFA-1-ICAM-1 interactions and talin surrounds this central cluster, defined as the peripheral supramolecular activation cluster (pSMAC). Each of these areas contained thousands of molecules in what was apparently a large non-covalent functional network-a supramolecular assembly. Kupfer later defined the distal supramolecular activation cluster as the outermost structure,