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

Abstract

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 Fc receptor FcgammaRIIb can 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. 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. 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. 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. These data argue against the involvement of FcgammaRIIb in the inhibition of CD40/IL-4-induced B-cell activation by IVIG.