CD28 co-stimulatory regimes differ in their dependence on phosphatidylinositol 3-kinase: Common co-signals induced by CD80 and CD86

Abstract

CD80 (B7-1) and CD86 (B7-2) ligation of CD28 provide co-stimulatory signals required for optimal lymphokine production in response to TCRζ-CD3 ligation. CD28 binds to several intracellular proteins including phosphatidylinositol 3-kinase (PI3-kinase), the tyrosine kinase ITK and the growth factor receptor-bound protein/Son of Sevenless (GRB-2/SOS) complex. Previously, we showed that TCRζ-CD3 and CD28 co-stimulation required PI3-kinase binding to the pYMNM motif of the cytoplasmic domain of the co-receptor. In this study, we have investigated whether CD28-associated PI3-kinase is required for CD80 and CD86 co-stimulation, as well as in co-signaling that involves different primary signals (i.e. TCRζ-CD3 versus phorbol ester/ionomycin). In the presence of anti-CD3, ligation of CD28 by both CD80 and CD86 was found to induce 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 signaling pathway leading to IL-2 production. By contrast, CD28 mediated co-stimulation involving receptor ligation plus phorbol ester/ionomycin induced IL-2 independent of PI3-kinase binding to CD28. These data indicate that TCRζ-CD3-dependent CD80 and CD86 co-signaling requires PI3-kinase binding to the CD28pYMNM motif, while phorbol ester and ionomycin can bypass this requirement in CD28 co-stimulation.