endent down-regulation of CCR7. This gives the T cells flexibility in looking at different amounts of antigen on DCs-a few DCs with high amounts of MHC-peptide can stop T cells whereas many DCs with few antigen can sustain signaling until the T cells resets its sensitivity and can arrest (Henrickson et al., 2008). Weaker MHC-peptide never overcome the chemokinetic signals and continually migrate in DC networks (Skokos et al., 2007). T cells encountering these weaker ligands cannot arrest in the steady state and only integrate signals through kinapses. This mode of signal integration is sufficient to induce an alternative mode of tolerance induction when the antigen is presented on many DCs through scavenger receptor DEC-205. The failure of some weaker self-antigens to stop T cells may contribute the failure of tolerance induction and the development of autoimmuity (Zehn and Bevan, 2006). In autoimmunity the combination of strong innate signals and weak TCR signals can lead to robust responses, although there are differences in the kinetics of T cell release from lymph nodes and fitness of the cells that may limit memory generation autoimmunity (Zehn and Bevan, 2006). Thus, a migrating T cell sensing MHC-peptide complexes at its leading edge can integrate signals and make decisions about tolerance to abundant antigens, but not rare self-antigens.
How do T cells form a synapse? T cells stopped by MHC-peptide complexes remained highly dynamics with prominent cycles of extension and retraction around a pivot point (Dustin et al., 1997; Valitutti et al., 1995a). Studies on planar bilayers reveal what appears to be an early spreading, receptor engagement and contractile process that form the cSMAC over a period of 5 minutes and which is F-actin dependent (Grakoui et al., 1999). These results also led to speculation that myosin II based contraction might also be involved because of the substantial decreases in contact size and the speed of receptor cluster movement (Dustin and Cooper, 2000; Grakoui et al., 1999). Similar results were obtained in live cell-cell systems in which TCR in small clusters moved to the center to form the cSMAC (Krummel et al., 2000). However, these authors eventually rejected the hypothesis that myosin II is required for this contraction and transport process because knockdown of myosin IIA did not appear to eliminate the ability to form a synapse with an APC (Jacobelli et al., 2004). More recent studies in the planar bilayer model actually supported the hypothesis that myosin IIA is important for microcluster transport and signal amplification in that system and for signal amplification in cell-cell models (Ilani et al., 2009). It is possible that there are alternative mechanisms to form a cSMAC in cell-cell systems that involve directed vesicular transport, which would be myosin II independent (Das et al., 2004). Nonetheless, the extension of an APC embracing lamellipodium is observed in cell-cell systems (Antón et al., 2002; Tskvitaria-Fuller et al., 2003). In Jurkat T leukemia cells, spreading on anti-CD3 coated surfaces form a well organized F-actin ring at early time points, and that this ring appear to break up after a contractile phase (Bunnell et al., 2001). These studies suggest that the initial contact expansion is mediated by F-actin driven protrusion, which then shift to a more dynamic extension-retraction process during sustained signaling. In fact the pivoting process observed with solid phase MHC-peptide complexes suggest that the stop was based on loss of persistent polarity. What could account for the ability of cells to rapidly transition from migration to arrest?
Quantitative analysis of TCR dynamics and cell spreading provided insight into how the stable and dynamic characteristics of the synapse are reconciled. Observations of contact areas on planar bilayers with total internal reflection fluorescence microscopy (TIRFM) reveal that the dSMAC is a radial lamellipodium. First, TCR in cells form synapses over a period of 30 minutes and even after cSMAC formation, sub-micron TCR clus