ling (Baetz et al., 1995). Stabilization of the pSMAC of CD4+ cytotoxic T lymphocyte (CTL) with a PKC-θ inhibitor quantified increased killing, the first evidence of for an advantage of an intact pSMAC (Beal et al., 2008). Structures similar to pSMACs also form between T cells in homotypic aggregates and are important for intensification of cytokine signals during T cell expansion (Sabatos et al., 2008). The symmetric immunological synapse may serve equally important functions in tolerance induction, priming and differentiation as discussed below.
Between chemokine dependent extravasation and MHC-peptide dependent immunological synapse formation T cells rapidly migrate in DC networks of T cell zones (Bajenoff et al., 2006). Rapid migration by T cells can be fully reconstituted with solid phase CCL21, a CCR7 ligand, in vitro (Woolf et al., 2007). This involved polarization and formation of a leading edge that takes the form of a flattened lamellipodium on CCL21 and ICAM-1 coated surfaces (Huang and Dustin, unpublished data). LFA-1 is not required for mobility in T cell zones (Woolf et al., 2007), but LFA-1 increases sensitivity to MHC-peptide complexes up to 100-fold (Bachmann et al., 1997) and ICAM-1 is required for antigen specific arrest in vivo (Scholer et al., 2008). The lamellipodium is a sensory structure and has been shown to be the most sensitive part of a T cell by ~40-fold in elegant studies with laser traps and anti-CD3 coated beads (Negulescu et al., 1996; Wei et al., 1999). Valitutti has also described the probing behavior of lymphocytes undergoing sustained signaling on APCs (Valitutti et al., 1995a). Another important experiment is the demonstration that TCR signaling is continually being renewed by new TCR-MHC-peptide interactions such that blocking the access to new MHC-peptide ligands with MHC antibodies immediately block signaling (Valitutti et al., 1995a).
Lamellipodia are coordinated by small G-proteins including Rac-1, which activates the WAVE2 complexes leading to dynamic actin in the periphery of nascent immunological synapses (Nolz et al., 2006). HS-1, a lymphocyte-specific cortactin is important for forming lamellipodia (Gomez et al., 2006). EVL, a hematopoietic cell member of the ENA-VASP family, is found at the tips of projections touching the APC (Lambrechts et al., 2000). Activation of these systems is not antigen specific and similar structures are induced during non-antigen specific engagement of adhesion molecule like LFA-1 by ICAM-1 (Smith et al., 2003) and CD2 by CD58 (or CD48 in the mouse) (Kaizuka et al., 2009). In response to TCR signals, the SLP-76 signaling module recruits and activates Vav, an exchange factor for Rac and Cdc42. Rac then activates WAVE2 and Cdc42 activates WASP. Both WAVE2 and WASP activate the Arp2/3 complex to generate branched actin networks. WAVE2 is associated with lamellipodial actin (Nolz et al., 2006), whereas WASP is associated with ventral projections such as podosomes and invadipodia (Carman et al., 2007). Behind the lamellipodium is the lamella, which has also been referred to as a focal zone in migrating T cells (Smith et al., 2005). These zones are rich in adhesion sites and integrin binding adapter talin, which is also a marker of the pSMAC. T cell may arrest migration and form a stable synapse, or alternatively, they may continue to migrate and integrate signals on the move. This rapid migration during antigen recognition contrasts with the stable synapse and has different functional consequences.
Rapid migration during antigen recognition is observed in T-DC interactions in vitro early stages of T cell interaction with low dose agonist stimuli in vivo (Henrickson et al., 2008) and with low potency TCR stimuli (Skokos et al., 2007). There are at least two explanations for these mobile junctions or “kinapses”(Dustin, 2008). Certain chemokinetic signals, such as CCL21 signals, can compete with antigen stop signals (Bromley et al., 2000). The ability of T cells to migrate for some period followed by arrest may relate to signaling thresholds or time dep