A double-edged kinase Lyn: A positive and negative regulator for antigen receptor-mediated signals

B cells from young lyn(-/-) mice are hyperresponsive to anti-IgM-induced proliferation, suggesting involvement of lyn in negative regulation of B cell antigen receptor (BCR)-mediated signaling. Here we show that tyrosine phosphorylation of Fc gamma RIIB aid CD22 coreceptors, which are important for feedback suppression of BCR-induced signaling, was severely impaired in lyn(-/-) B cells upon their coligation with the BCR. Hypophosphorylation on tyrosine residues of these molecules resulted in failure of recruiting the tyrosine phosphatase SHP-1 and inositol phosphatase SHIP, SH2-containing potent inhibitors of BCR-induced B cell activation, to the coreceptors. Consequently, lyn(-/-) B cells exhibited defects in suppressing BCR-induced Ca2+ influx and proliferation. Thus, Lyn is critically important in tyrosine phosphorylation of the coreceptors, which is required for feedback suppression of B cell activation.