Signal initiation in T-cell receptor microclusters.

Although dynamic imaging technologies have provided important insights into the underlying processes responsible for T-cell activation, the processes that link antigen recognition to downstream signaling remain poorly defined. Converging lines of inquiry indicate that T-cell receptor (TCR) microclusters are the minimal structures capable of directing effective TCR signaling. Furthermore, imaging studies have determined that these structures trigger the assembly of oligomeric signaling scaffolds that contain the adapters and effectors required for T-cell activation. Existing models of T-cell activation accurately explain the sensitivity and selectivity of antigen recognition. However, these models do not account for important properties of microclusters, including their peripheral formation, size, and movement on the actin cytoskeleton. Here we examine how lipid rafts, galectin lattices, and protein scaffolds contribute to the assembly, function, and fate of TCR microclusters within immune synapses. Finally, we propose a 'mechanical segregation' model of signal initiation in which cytoskeletal forces contribute to the lateral segregation of molecules and cytoskeletal scaffolds provide a template for microclusters assembly.