ABSTRACT
CD28 is a coreceptor expressed on T lymphocytes. Signaling downstream of CD28 promotes multiple T cell functions such as proliferation, survival, and cytokine secretion. Adhesion to APCs is another function of T cells; however, little is known with regard to the role of CD28 in this process. Our previous studies have shown that CD28 inhibits T cell adhesion, but the underlying mechanism that mediates this process remains unknown. In the present study we discovered that signaling downstream of CD28 resulted in inhibition of Rap1 activity and decreased LFA-1-mediated adhesion. We showed that this was regulated by the recruitment of calcium-promoted Ras inactivator (CAPRI), a GTPase-activating protein, to the plasma membrane downstream of CD28 signaling. CAPRI trafficking to the plasma membrane was secondary to calcium influx and was mediated by its C2A and C2B domains. We conclude that CD28 inhibits Rap1-mediated adhesion by recruiting the protein CAPRI to the plasma membrane.
