Inhibition of B cell receptor-mediated activation of primary human B cells by coengagement of CD19 and FcγRIIb with Fc-engineered antibodies

The humoral immune response requires antigen-specific B cell activation and subsequent terminal differentiation into plasma cells. Engagement of B cell antigen receptor (BCR) on mature B cells activates an intracellular signaling cascade, including calcium mobilization, which leads to cell proliferation and differentiation. Coengagement by immune complex of BCR with the inhibitory Fc receptor FcγRIIb, the only IgG receptor expressed on B cells, inhibits B cell activation signals through a negative feedback loop. We now describe antibodies that mimic the inhibitory effects of immune complex by high-affinity coengagement of FcγRIIb and the BCR coreceptor complex on human B cells. We engineered the Fc domain of an anti-CD19 antibody to generate variants with up to ∼430-fold greater affinity to FcγRIIb. Relative to native IgG1, the FcγRIIb binding-enhanced (IIbE) variants strongly inhibited BCR-induced calcium mobilization and viability in primary human B cells. Inhibitory effects involved phosphorylation of SH2-containing inositol polyphosphate 5-phosphatase (SHIP), which is known to be involved in FcγRIIb-induced negative feedback of B cell activation by immune complex. Coengagement of BCR and FcγRIIb by IIbE variants also overcame the anti-apoptotic effects of BCR activation. The use of a single antibody to suppress B cell functions by coengagement of BCR and FcγRIIb may represent a novel approach in the treatment of B cell-mediated autoimmune diseases.