Defective negative regulation of antigen receptor signaling in Lyn-deficient B lymphocytes

Abstract

Background: To elucidate the role of the Src family kinase Lyn in B cell receptor (BCR) signaling, we and others previously generated lyn−/− mice and analyzed their B cell responses. Although the initiation of BCR signaling in lyn−/− B cells is delayed, BCR-induced ERK2 activation and proliferation are enhanced. As the co-receptors FcγRIIb1 and CD22 have been shown to be negative regulators of BCR signaling, we have now examined their functional roles in lyn−/− B cells.Results: B cells from lyn−/− mice have increased expression of the protein product of the early response gene egr-1, enhanced activation of Jun N-terminal kinase (JNK), and elevated calcium responses upon BCR cross-linking. Tyrosine phosphorylation of FcγRIIb1 in lyn−/− B cells was reduced but negative regulation of the BCR signal by FcγRIIb1 was only modestly impaired. In contrast, tyrosine phosphorylation of CD22 was greatly decreased in lyn−/− B cells, correlating with the inability of CD22 to downregulate the BCR-induced calcium response in these cells. Surprisingly, CD22 remains capable of regulating the ERK2 and JNK pathways in lyn−/− B cells, which may relate to the small residual increase in BCR-induced CD22 phosphorylation.Conclusions: BCR signal initiation and negative regulation by FcγRIIb1 is not critically dependent on Lyn. In contrast, Lyn plays a particularly important role in the tyrosine phosphorylation of CD22 and in the consequent inhibition of BCR-induced calcium influx. The net result of the Lyn deficiency in B cells is hyperresponsiveness to antigen stimulation, which may explain the autoimmunity observed in lyn−/− mice.