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 FcγRIIB and 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.