Expression of the Neonatal Fc Receptor FcRn Is Not Required for the Amelioration of Murine ITP by IVIg or a Monoclonal Antibody

Abstract 2529

There are several theories as to the mechanism of intravenous immunoglobulin (IVIg) in the treatment of autoimmune diseases such as immune thrombocytopenia (ITP). One prominent theory involves accelerated pathogenic autoantibody clearance by saturation of the neonatal Fc Receptor (FcRn). FcRn is an IgG receptor, and FcRn within endosomes binds endocytosed IgG and diverts IgG from degradation. In the treatment of ITP, it has been theorized that high concentrations of IVIg saturate FcRn, reducing the ability of the pathogenic anti-platelet antibodies to bind FcRn, increasing their catabolism, and thus rapidly decreasing their serum concentration which results in a decrease in their ability to induce platelet clearance. Mice lacking FcRn have only 20–30% of the level of endogenous IgG as compared with wild-type mice and have an accelerated clearance rate of both endogenous and injected IgG. We have utilized these mice in a murine model of ITP to help understand the role, if any, that FcRn plays in IVIg's therapeutic activity.

Both IVIg (2 g/kg body weight) and monoclonal antibodies to CD44 (2 mg/kg body weight; please see submitted abstract ID# 29504, “Amelioration of murine immune thrombocytopenia by CD44 antibodies: a potential therapy for ITP?”) successfully ameliorate murine ITP. To definitively determine if FcRn is required for the acute amelioration of ITP by these two therapeutics, we employed FcRn deficient mice in the murine ITP model. Here, we demonstrate that FcRn deficient mice (B6.129X1-Fcgrttm1Dcr/DcrJ) injected with an anti-platelet antibody exhibit a slightly more profound degree of thrombocytopenia than wild-type mice. FcRn deficient mice treated with IVIg or the anti-CD44 antibody KM114 (at a 3 log fold lower dosage than IVIg) were protected from ITP to the same extent as wild-type mice.

FcRn has an absolute requirement for the protein β₂ microglobulin (β₂M) to be functionally expressed. Specifically, β₂M deficient mice do not possess functional FcRn and also show low endogenous IgG levels and increased clearance of IgG. To verify and substantiate the results found with FcRn deficient mice, we next employed β₂M deficient mice in the murine ITP model and found that β₂M deficient mice (B6.129P2-B2mtm1Unc/J) treated with IVIg or KM114 were also protected from ITP to the same extent as wild-type mice.

These data suggest that for both high dose IVIg as well as low dose monoclonal CD44 antibody treatment in an acute ITP model, FcRn expression is dispensable.