Rap1-mediated lymphocyte function-associated antigen-1 activation by the T cell antigen receptor is dependent on phospholipase C-gamma1.

The small GTPase, Rap1, is a potent activator of leukocyte integrins and enhances the adhesive activity of lymphocyte function-associated antigen-1 (LFA-1) when stimulated by the T cell receptor (TCR) or chemokines. However, the mechanism by which Rap1 is activated remains unclear. Here, we demonstrate that phospholipase C (PLC)-gamma1 plays a critical role in the signaling pathway leading to Rap1 activation triggered by the TCR. In Jurkat T cells, TCR cross-linking triggered persistent Rap1 activation, and SDF-1 (CXCL12) activated Rap1 transiently. A phospholipase C inhibitor, U73122, abrogated Rap1 activation triggered by both the TCR and SDF-1 (CXCL12). PLC-gamma1-deficient Jurkat T cells showed a marked reduction of TCR-triggered Rap1 activation and adhesion to intercellular adhesion molecule-1 (ICAM-1) mediated by LFA-1. In contrast, SDF-1-triggered Rap1 activation and adhesion were not affected in these cells. Transfection of these cells with an expression plasmid encoding PLC-gamma1 restored Rap1 activation by the TCR and the ability to adhere to ICAM-1, accompanied by polarized LFA-1 surface clustering colocalized with regulator of adhesion and polarization enriched in lymphoid tissues (RAPL). Furthermore, when expressed in Jurkat cells, CalDAG-GEFI, a calcium and diacylglycerol-responsive Rap1 exchange factor, associated with Rap1, and resulted in enhanced Rap1 activation and adhesion triggered by the TCR. Our results demonstrate that TCR activation of Rap1 depends on PLC-gamma1. This activity is likely to be mediated by CalDAG-GEFI, which is required to activate LFA-1.