CD28 COSTIMULATORY REGIMES DIFFER IN THEIR DEPENDENCE ON PHOSPHATIDYLINOSITOL 3-KINASE - COMMON COSIGNALS INDUCED BY CD80 AND CD86

CD80 (B7-1) and CD86 (B7-2) ligation of CD28 provide co-stimulatory si gnals required for optimal lymphokine production in response to TCR ze ta-CD3 ligation, CD28 binds to several intracellular proteins includin g phosphatidylinositol 3-kinase (PI3-kinase), the tyrosine kinase ITK and the growth factor receptor-bound protein/Son of Sevenless (GRB-2/S OS) complex, Previously, we showed that TCR zeta-CD3 and CD28 co-stimu lation required PI3-kinase binding to the pYMNM motif of the cytoplasm ic domain of the co-receptor, In this study, we have investigated whet her CD28-associated PI3-kinase is required for CD80 and CD86 co-stimul ation, as well as in co-signaling that involves different primary sign als (i.e. TCR zeta-CD3 versus phorbol ester/ionomycin), In the presenc e of anti-CD3, ligation of CD28 by both CD80 and CD86 was found to ind uce PI3-kinase recruitment and IL-2 production, Furthermore, mutations at Y-191 and M-194 within the pYMNM motif blocked the ability of both ligands to induce IL-2, CD80 and CD86 therefore share a common signal ing pathway leading to IL-2 production. By contrast, CD28 mediated co- stimulation involving receptor ligation plus phorbol ester/ionomycin i nduced IL-2 independent of PI3-kinase binding to CD28, These data indi cate that TCR zeta-CD3-dependent CD80 and CD86 co-signaling requires P I3-kinase binding to the CD28pYMNM motif, while phorbol ester and iono mycin can bypass this requirement in CD28 co-stimulation.