Activation of Lyn, Blk, and Btk but not Syk in CD72-stimulated B lymphocytes

CD72 is a B cell-specific glycoprotein that has been shown to be important for activation of mature B cells. Previously we showed that some of the early signaling events, such as calcium mobilization and phospholipase- γ activation, were similar in B cell Ag receptor (BCR)- and CD72-stimulated B cells and that BCR- but not CD72-mediated early signaling events were locked by protein kinase A activation. The present report shows that CD72 ligation induces a variety of tyrosine-phosphorylated proteins, most of which were of the same molecular mass as those seen in anti-IgM-treated B cells, except for a 72-kDa protein. Further analysis showed that the tyrosine kinases lyn and blk were activated in CD72-ligated B cells. Interestingly, the non-src kinase syk was not activated in CD72-stimulated cells whereas the tec family kinase btk was in both CD72- and BCR-stimulated B cells. Furthermore, B cells from xid mice were unresponsive to CD72-induced proliferation, indicating an essential role for btk in CD72-induced signaling events. Surprisingly, tyrosine phosphorylation of phospholipase C-γ2 was normal in CD72-stimulated cells in spite of a lack of activation of syk. Furthermore, B cell proliferation through CD72 was blocked by the immunosuppressive agents cyclosporin A and FK506, indicating the important role for Ca²⁺-regulated a activation events similar to BCR-stimulated cells. We propose that btk can substitute for syk in inducing phospholipase C-γ2 tyrosine phosphorylation and initiating calcium mobilization in CD72- stimulated B lymphocytes.