Ligation of CD38 inhibits proliferation and induces apoptosis of human immature B cells, but the molecular mechanisms underlying this function are unknown. We found that CD38 dimerization with the specific mAbs T16 and IB4 induces rapid and transient tyrosine phosphorylation of several intracellular proteins in the immature B cell lines RS4;11, REH, 380, Nalm6, and OP-1. This effect could be markedly reduced by incubating cells with the tyrosine kinase inhibitors genistein, staurosporine, and herbimycin A. CD38 dimerization induced tyrosine phosphorylation of the protein kinase syk and increased syk kinase activity. CD38 dimerization also induced tyrosine phosphorylation of phospholipase C-gamma and of the p85 subunit of phosphatidylinositol 3-kinase (PI 3-K). The latter was accompanied by a distinct increase in PI 3-kinase activity in the immunoprecipitates obtained with an anti-phosphotyrosine Ab. In contrast to the signaling triggered by surface Ig engagement in B lymphocytes, CD38 ligation did not appear to induce tyrosine phosphorylation of the src-like protein tyrosine kinases lyn, fyn, and btk, or of vav- and ras-GTPase-activating protein, nor did it induce detectable changes in cytosolic CA2+ concentrations. CD38 signaling also differed from cytokine-induced signaling in that it did not cause tyrosine phosphorylation of Jak1 and Jak2. Finally, CD38 ligation did not inhibit IL-3-induced tyrosine phosphorylation of Jak2. These results identify CD38 as a cell surface receptor with signal transduction properties activated by dimerization. Induction of signal transduction by CD38 ligation implies the existence of a yet unidentified natural ligand of CD38.

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