Human Type II Fcγ Receptors Inhibit B Cell Activation by Interacting with the p21 -dependent Pathway

Abstract

Co-ligation of antigen receptors and type II Fcγ receptors (FcγRIIb) on B cells interrupts signal transduction and ultimately inhibits antibody production. We have identified p52 Shc in the FcγRIIb1-specific immunoprecipitates isolated from the membrane fraction of BL41 Burkitt lymphoma cells following B cell receptor-FcγRIIb1 co-ligation. The insolubilized synthetic peptide representing the phosphorylated form of the tyrosine-based inhibitory motif of FcγRIIb also binds Shc from the lysates of activated but not from resting BL41 cells. This suggests that the binding does not depend on the interaction of FcγRIIb1-phosphotyrosine with the SH2 domain of Shc. Tyr phosphorylation of FcγRIIb1-associated Shc is low, indicating an impaired function. Shc is implicated in regulating p21ras activation; thus, we have compared p21ras activities in BL41 cells treated in different ways. p21ras activity is reduced when B cell receptor and FcγRIIb1 are co-ligated. p21ras couples protein-tyrosine kinase-dependent events to the Ser/Thr kinase-mediated signaling pathway leading to the activation of mitogen-activated protein kinases (MAPK). Our results show that B cell receptor-FcγRIIb1 co-cross-linking partially inhibits mitogen-activated protein kinase activity. We conclude that FcγRIIb1-dependent inhibition of human B cell activation may be based on interrupting signal transduction between protein-tyrosine kinases and the p21ras/mitogen-activated protein kinase-dependent activation pathway. Co-ligation of antigen receptors and type II Fcγ receptors (FcγRIIb) on B cells interrupts signal transduction and ultimately inhibits antibody production. We have identified p52 Shc in the FcγRIIb1-specific immunoprecipitates isolated from the membrane fraction of BL41 Burkitt lymphoma cells following B cell receptor-FcγRIIb1 co-ligation. The insolubilized synthetic peptide representing the phosphorylated form of the tyrosine-based inhibitory motif of FcγRIIb also binds Shc from the lysates of activated but not from resting BL41 cells. This suggests that the binding does not depend on the interaction of FcγRIIb1-phosphotyrosine with the SH2 domain of Shc. Tyr phosphorylation of FcγRIIb1-associated Shc is low, indicating an impaired function. Shc is implicated in regulating p21ras activation; thus, we have compared p21ras activities in BL41 cells treated in different ways. p21ras activity is reduced when B cell receptor and FcγRIIb1 are co-ligated. p21ras couples protein-tyrosine kinase-dependent events to the Ser/Thr kinase-mediated signaling pathway leading to the activation of mitogen-activated protein kinases (MAPK). Our results show that B cell receptor-FcγRIIb1 co-cross-linking partially inhibits mitogen-activated protein kinase activity. We conclude that FcγRIIb1-dependent inhibition of human B cell activation may be based on interrupting signal transduction between protein-tyrosine kinases and the p21ras/mitogen-activated protein kinase-dependent activation pathway.