T-cell receptor antagonists induce Vav phosphorylation by selective activation of Fyn kinase

T cell receptor (TCR) antagonists inhibit antigen-induced T cell activation and by themselves fail to induce phenotypic changes associated with T cell activation. However, we have recently shown that TCR antagonists are induc ers of antigen-presenting cell (APC)-T cell conjugates. The signaling pathw ay associated with this cytoskeleton-dependent event appears to involve tyr osine phosphorylation and activation of Vav. In this study, we investigated the role played by the protein tyrosine kinases Fyn. Lck. and ZAP-70 in an tagonist-induced signaling pathway. Antagonist stimulation increased tyrosi ne phosphorylation and kinase activity of Fyn severalfold, whereas little o r no increase in Lck and ZAP-70 activity was observed. Second, TCR stimulat ion of Lck(-), Fyn(hi) Jurkat cells induced strong tyrosine phosphorylation of Vav. In contrast, minimal increase in tyrosine phosphorylation of Vav w as observed in Lck(hi), Fyn(lo) Jurkat cells. Finally. study of T cells fro m a Fyn-deficient TCR transgenic mouse also showed that Fyn was required fo r tyrosine phosphorylation and activation of Vav induced by both antagonist and agonist peptides. The deficiency in Vav phosphorylation in Fyn-deficie nt T cells was associated with a defect in the formation of APC-T cell conj ugates when T cells were stimulated with either agonist or antagonist pepti de. We conclude from these results that Vav is a selective substrate for Fy n, especially under conditions of low-affinity TCR-mediated signaling, and that this signaling pathway involving Fyn, Vav, and Rac-1 is required for t he cytoskeletal reorganization that leads to T cell-APC conjugates and the formation of the immunologic synapse.