LIGATION OF CD28 RECEPTOR BY B7 INDUCES FORMATION OF D-3 PHOSPHOINOSITIDES IN T-LYMPHOCYTES INDEPENDENTLY OF T-CELL RECEPTOR CD3 ACTIVATION/

The co-stimulatory role of B7/CD28 interactions is important in promot ing T cell activation. Very little is known about the intracellular ev ents that follow CD28 engagement although recent evidence has implicat ed coupling of CD28 to a protein tyrosine kinase signal transduction p athway. In this study we have investigated the putative role of D-3 ph osphoinositides as mediators of CD28 receptor signaling, since phospho inositide (PI) 3-kinase, the enzyme responsible for D-3 phosphoinositi de formation, is a known substrate for protein tyrosine kinases associ ated with certain T cell surface receptors such as CD4 and interleukin -2 receptor. The lipid products of PI 3-kinase activity have been sugg ested to play a role in mitogenic signaling and growth regulation in o ther cells. Chinese hamster ovary cells (CHO) previously transfected w ith B7 cDNA, induced time-dependent elevation above basal levels of ph osphatidylinositol(3,4)-bisphosphate (PtdIns(3,4)P2) and PtdIns(3,4,5) P3, while parental CHO cells that did not express B7 had no effect on these lipids. Moreover, the elevation of these same lipids by CD3 liga tion was potentiated in an additive manner by CHO-B7+ but not by CHO-B 7- cells. CHO-B7+ and CHO-B7- cells did not activate phospholipase C a s evidenced by their inability to modulate basal or CD3-induced change s in the levels of phosphatidic acid or D-4 and D-5 phosphoinositides. These data imply that PI 3-kinase but not phospholipase C, may be an important signal transduction molecule with respect to CD28-mediated c o-stimulation and T cell activation following ligation by B7.