The computational model placed the TCR and p MHC molecules on two apposed surfaces, and simulated their binding to each other when they were within a certain distance of each other.A threshold level of signaling was used to initiate forces that led to c SMAC formation.
The finding that CD28 and PKC-θ are strongly colocalized and segregated from most of the TCR accumulated in the core of the c SMAC (Yokosuka et al. This result splits the c SMAC into two distinct compartments, one that is more dynamic and may remain active in signaling. Elucidating the signaling function of the synapse, and the c SMAC in particular, has proven to be difficult because there are many competing effects and components at play, which makes it difficult to intuit mechanisms from experimental observations of a few variables. The computational models can also be used to design experiments that can sensitively discriminate between competing plausible hypotheses, which are consistent with all known experimental facts.In epithelial cells, these three molecules appear to interact with each other, are required for apical/basal polarity, and are localized to the basal/lateral membrane (Qin et al. Early in synapse formation, Scrb and Dlg are recruited to the synapse but at later time-points, they end up localized to the membrane distal to the synapse (Ludford-Menting et al. Inhibition of Scrb expression blocks motility and conjugate formation but the mechanism is not known. Although this pattern does not appear to be a universal feature of immunological synapses (Brossard et al. 2008), its distinct structure has served as a central focus of researchers in the field. The incorporation of mobile ligands into lipid bilayers has allowed the reorganization of surface receptors during T-cell activation to be visualized in real-time and with optimal resolution (Grakoui et al. In contrast, the reorganization of TCRs and integrins into p SMACs and c SMACs does not occur in systems using antibody coated glass (Bunnell et al. In addition, the highly sensitive TIRFM method allows individual molecules or small clusters of molecules to be imaged (Varma et al. Antigen induces the formation of small TCR microclusters on the order of 11–17 TCRs per microcluster (Varma et al. Although the TCR microclusters can also include molecules like CD2, CD4, CD8, and CD28, they exclude the tyrosine phosphatase CD45 (Varma et al. 2008), a step that may be important for TCR triggering (Choudhuri et al. Interestingly, LFA-1 also forms microclusters in the d SMAC that are segregated from the TCR microclusters (Kaizuka et al. Although both TCR and LFA-1 microclusters then move centripetally through the p SMAC (Kaizuka et al. In the case of the actin dependent LFA-1 clusters, one idea is that they disperse when they reach F-actin free c SMAC (Kaizuka et al. This model postulates that the c SMAC is a container that collects cargo that falls off the actinomyosin conveyor belt as it reaches the border of the p SMAC and the c SMAC.The bullseye pattern with the integrins, forming the outer ring surrounding the TCR that is concentrated in a central spot, is a highly recognized feature of the T-cell immunological synapse (Grakoui et al. Recent progress in understanding the formation of the immunological synapse has been facilitated by the use of artificial planar lipid bilayers as well as a method of imaging known as total interference reflection microscopy (TIRFM) (Campi et al. 2007), experiments using barriers placed in the bilayer (Mossman et al. In this model, because TCR clusters are actin independent, they can persist after falling into the c SMAC.Similar structures were detected in multifocal T cell-DC IS, in which CD80 and PKC-θ rich compartments were seen to be continuously dependent on dynamic TCR signaling (Tseng et al. These dynamic central compartments could account for signaling activity detected in the c SMAC in CD4 and CD8 T cells (Cemerski et al. Computational and theoretical models have been applied to complement experimental studies to help unravel the interplay of complex-competing effects in various aspects of T-cell signaling (Altan-Bonnet and Germain 2005; Cemerski et al. Such complementary computational and experimental investigations have aided the quest to understand the formation of and signaling in the immunological synapse.Shortly after the first report of the low levels of active signaling molecules in the c SMAC (Lee et al.