β2-Integrin, LFA-1, and TCR/CD3 Synergistically Induce Tyrosine Phosphorylation of Focal Adhesion Kinase (pp125FAK) in PHA-Activated T Cells

Complete T cell activation requires not only a first signal via TCR/CD3 engagement but also a costimulatory signal through accessory receptors such as CD2, CD28, or integrins. Focal adhesion kinase, pp125FAK(FAK), was previously shown to be localized in focal adhesions in fibroblasts and to be involved in integrin-mediated cellular activation. Although signaling through β1- or β3-integrins induces tyrosine phosphorylation of FAK, there has been no evidence that activation of T cells through the β2-integrin, LFA-1, involves FAK. We report here that crosslinking of LFA-1 induces tyrosine phosphorylation of FAK in PHA-activated T cells. Moreover, cocrosslinking with anti-LFA-1 mAb and suboptimal concentration of anti-CD3 mAb markedly increases tyrosine phosphorylation of FAK in an antibody-concentration-dependent and time-kinetics-dependent manner compared with stimulation through CD3 alone, which correlates well with enhanced proliferation of PHA-activated T cells. Furthermore, LFA-1β costimulation with CD3 induces tyrosine phosphorylation ofSykassociated with FAK. These results indicate, for the first time, that signals mediated by LFA-1 can regulate FAK, suggesting that LFA-1-mediated T cell costimulation may be involved in T cell activation at least partially through FAK.