The inhibitory Fc receptors function to regulate the antigendriven activation and expansion of lymphocytes. In B cells, the Fc␥RIIB1 is a potent inhibitor of B cell antigen receptor (BCR) signaling when coligated to the BCR by engagement of antigen-containing immune complexes. Inhibition is mediated by the recruitment of the inositol phosphatase, SHIP, to the Fc␥RIIB1 phosphorylated tyrosine-based inhibitory motif (ITIM). Here we show that BCR-independent aggregation of the Fc␥RIIB1 transduces an ITIM-and SHIP-independent proapoptotic signal that is dependent on members of the c-Abl tyrosine kinase family. These results define a novel Abl family kinase-dependent Fc␥RIIB1 signaling pathway that functions independently of the BCR in controlling antigen-driven B cell responses.B cell antibody responses are initiated by the binding of multivalent antigens to the B cell antigen receptor (BCR) 2 (1, 2). Engagement of the BCR triggers signal cascades that lead to the proliferation of the B cells and their differentiation into both long and short lived antibody-secreting plasma cells and memory B cells. The antigen-driven expansion and differentiation of B cells is a carefully balanced process that ensures adequate levels of circulating protective antibody and memory B cells and simultaneously avoids excessive deleterious production of antibody such as occurs in autoimmune diseases. The low affinity inhibitory receptor for IgG, Fc␥RIIB1, has emerged as a key regulator of B cells responses (3). Although B cells express members of the recently identified extended family of FcR homologs these proteins do not appear to bind Fcs and thus do not function as FcRs (4). During an immune response, once antibody levels reach a critical point resulting in the formation of antigen-and antibody-containing immune complexes (ICs), the simultaneous binding of ICs to the BCR and Fc␥RIIB1 inhibits BCR signaling. Coligation of the Fc␥RIIB1 and the BCR by ICs leads to the phosphorylation of the Fc␥RIIB1 by the Src family kinase Lyn on the tyrosines in the immunoreceptor tyrosine-based inhibitory motif (ITIM), resulting in the recruitment of the inositol phosphatase, SHIP, which inhibits the BCR trigger Ca 2ϩ mobilization and B cell proliferation (3). The inhibition of Ca 2ϩ mobilization and proliferation appears to proceed through two separable pathways, one requiring the phosphatase activity of SHIP to hydrolyze PI3K-generated phosphatidylinositol trisphosphate preventing Btk and phospholipase C␥ activation (5) and a second requiring SHIP to recruit the Ras GAP-binding protein p62 dok that functions to inhibit extracellular signal-regulated kinase activation (6). The physiological significance of the role of the Fc␥RIIB1 in regulating BCR signaling is demonstrated by the phenotype of Fc␥RIIB1-deficient mice that show susceptibility to induced autoimmune diseases and to spontaneous autoimmune disease in certain strains (7).In addition to its inhibitory activity when coligated to the BCR, the Fc␥RIIB1, when aggregated to itself, propagates a...