r 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