ation was binding of talin to the cytoplasmic domain of LFA-1 (Smith et al., 2005). Talinis recruited to sites of LFA-1 interaction with ligands on APCs and this is a more sensitive process with respect to antigen dose than cytokine production or proliferation (Kupfer and Singer, 1989). The parallel field studying lymphocyte trafficking revealed that LFA-1 can also be acutely activated by G-protein coupled receptors that are associated with lymphocyte homing (Lawrence and Springer, 1991). Thus, LFA-1 regulation is a cornerstone in both body-wide navigation of lymphocytes through vascular interaction and the coordination of antigen recognition and strong, transient adhesion leading to the immunological synapse in tissues.
Specific signaling pathways are required for inside out signaling from TCR to LFA-1. The adapter molecule ADAP and its partner SKAP-55 have been shown to contribute to about half of TCR triggered adhesion (Peterson et al., 2001). This process is integrated with F-actin at both the level of the 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 accoun