Signaling via the T cell antigen receptor induces phosphorylation of Stat1on serine 727

The Stat1 transcription factor plays a pivotal role in both, the antiviral and antigrowth actions of interferons. Stat1 acquires the ability to bind D NA by becoming phosphorylated on Tyr(701). However, to effectively stimulat e gene transcription, it must also be phosphorylated on Ser(727). We show t hat engagement of T cell antigen receptor (TCR)/CD3 complex in either Jurka t cells or peripheral blood lymphocytes stimulates phosphorylation of Ser(7 27) but not Tyr(701) of Stat1. This process does not require the expression of tyrosine kinases Lck and Zap-70. Interestingly, pretreatment of T cells with the Src kinase inhibitor PP1 completely abrogated CD3-mediated serine phosphorylation of Stat1, whereas inhibitors to MEK1 and phosphatidylinosi tol 3-kinase had no effect. Phosphorylation of Ser(727) of Stat1 in T cells is not restricted to TCR/CD3 but also results when cells are stimulated vi a the costimulatory molecule CD28. The combination of CD3 and CD28 did not augment phosphorylation of Stat1 Ser(727). Surprisingly, Stat1-mediated tra nscriptional activity in response to IFN-alpha was enhanced with CD3 stimul ation, whereas CD3 alone had little effect. These findings suggest that Sta t1 is a signaling molecule in TCR signaling and may play a role in T cell f unction.