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Department of Microbiology and Immunology and Sanders-Brown Center on Aging, University of Kentucky, Lexington, KY 40536
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-
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 blocked 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 activated 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-2 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 activation events similar to
BCR-stimulated cells. We propose that btk can substitute for syk in
inducing phospholipase C-2 tyrosine phosphorylation and initiating
calcium mobilization in CD72-stimulated B lymphocytes.
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