PHOSPHATIDYLINOSITOL 3-KINASE ACTIVATION IN NORMAL HUMAN B-LYMPHOCYTES - DIFFERENTIAL SENSITIVITY OF GROWTH AND DIFFERENTIATION TO WORTMANNIN

A variety of signals, mediated via either the B cell Ag receptor (BCR) , or non-BCR molecules such as CD40 or cytokine receptors, have been s hown to be crucial for the regulation of B cell survival, growth, and differentiation. Although it is clear that a variety of signaling path ways can be activated in B cells in a stimulus-dependent manner, it re mains unknown whether differential activation of these signaling pathw ays is the underlying mechanism controlling B cell fate, i.e., growth vs differentiation. Initial studies reported here indicated that stimu lation of highly purified peripheral blood B cells with the polyclonal B cell activators Staphylococcus aureus and CD40 ligand (CD40L) resul ted in the rapid induction of phosphatidylinositol 3-kinase (PI 3-kina se) activity. Moreover, pretreatment of B cells with wortmannin, a spe cific inhibitor of PI 3-kinase, resulted in a complete block in induct ion of PI 3-kinase activity. The effects of wortmannin, as well as a s econd PI 3-kinase inhibitor, LY294002, on the induction of both B cell growth and differentiation were therefore investigated. Although thes e PI 3-kinase inhibitors variably inhibited B cell DNA synthesis in a stimulus-dependent manner, both drugs effected a near-complete block o f the ability of each of these stimuli to induce Ig production. Furthe rmore, separation of B cells into naive IgD(+) and postswitch IgD(-) B cells failed to reveal differential sensitivity of these populations to wortmannin. These results suggest that activation of PI 3-kinase, o r other wortmannin- and LY294002-sensitive targets, is a crucial event that occurs during the differentiation of normal human B lymphocytes. The differential sensitivity of B cell responses to inhibitors of PI 3-kinase supports the notion that distinct signal transduction pathway s are involved in differentiation vs proliferation of normal human B l ymphocytes.