Suppression of IL-4-and CD40-induced B-lymphocyte activation by intravenous immunoglobulin is not mediated through the inhibitory IgG receptor FcγRIIb

Background: Intravenous immunoglobulin (IVIG) has been used extensively in the treatment of autoimmune and allergic diseases, but the precise mechanism behind its efficacy remains unclear. Ligation of the low-affinity IgG Fe receptor FcgammaRIIb ran inhibit B-lymphocyte activation. Our laboratory has shown that IVIG suppresses proliferation and IgE production by human B cells stimulated with IL-4 and anti-CD40 antibodies. Objective: We sought to determine whether the regulatory action of IVIG is mediated through binding FcgammaRIIb, phosphorylation of the receptor, and induction of phosphatases, including SH2-containing inositol-5'-phosphatase. Methods: All experiments, were performed on human tonsillar B cells. Phenotyping was performed by means of flow cytometry. Cells were cultured with IL-4 and anti-CD40 antibodies with or without IVIG (10 mg/mL), and FCgammaRIIb receptor activation and phosphorylation were measured by means of Western blot analysis. Results: FcgammaRIIb was the predominant isoform of Fcgamma receptor expressed on tonsillar B cells, and preincubation with IVIG failed to block binding of FcgammaRIIb antibody. Anti-FcgammaRIIb antibodies did not reverse inhibition of B-cell proliferation or IgE production by IVIG. Treatment of stimulated B lymphocytes with IVIG for 1 to 60 minutes did not change the global protein tyrosine phosphorylation pattern, except for tyrosine phosphorylation of an unidentified 30-kd protein. We directly examined tyrosine phosphorylation of FcgammaRIIb and its downstream-associated phosphatase, SH2-containing inositol-5'-phosphatase. Both remained unchanged after IVIG treatment, as did other related phosphatases. Conclusion: These data argue against the involvement of FcgammaRIIb in the inhibition of CD40/IL-4-induced B-cell activation by IVIG.