T cell activation at the immunological synapse: Vesicles emerge for later signaling

Signaling at the immunological synapse (IS) between a T cell and an antigenpresenting cell is initiated by the proximal tyrosine kinases Lck and z chain-associated protein kinase of 70 kD (ZAP-70) after engagement of the T cell receptor (TCR) by an antigen peptide-loaded major histocompatibility complex. Activation of these kinases leads to the formation of small protein aggregates (microclusters) within the IS, which contain kinases and adaptor proteins important for signal propagation. Src homology 2 (SH2) domain-containing leukocyte phosphoprotein of 76 kD (SLP-76), which is cytosolic, and LAT, which is membrane-associated, are key adaptor proteins phosphorylated by TCR-associated ZAP-70, and they form microclusters at the IS. Microclusters of either protein move during T cell activation; however, how they come in contact with each other to propagate signals remains unclear. A study addressed this question by monitoring the origin and kinetics of SLP-76 or LAT microclusters. Whereas the majority of LAT formed microclusters at the IS, a fraction of LAT resided on mobile, intracellular vesicles beneath the IS. Although SLP-76 microclusters formed just below the plasma membrane at the IS and moved centripetally, LAT-containing subsynaptic vesicles moved more rapidly, but slowed when in contact with SLP-76 microclusters. This interaction of vesicular LAT with SLP-76 microclusters coincided with the phosphorylation of LAT on key tyrosines that mediate its interaction with the SLP-76-associated adaptor protein GADS. Together, these data provide new insight into signaling at the IS and suggest that the endosomal network likely participates in T cell signaling.