Recombinant Human IgG1-Hexamer Reduces Pathogenic Autoantibodies in the K/BxN Mouse Model of Arthritis Independent of FcRn

Arthritis in K/BxN mice is provoked by pathogenic autoantibodies to glucose-6-phosphate isomerase (G6PI), which is a ubiquitously expressed enzyme that is present in cells, in the circulation and on the articular cartilage. When G6PI autoantibodies (auto-Abs) deposit on the articular cartilage of K/BxN mice, arthritis ensues due to the activation of various components of the innate immune system. Recent studies have investigated the in vivo efficacy of recombinant fragment-crystallizable (Fc) protein-based therapeutics. Many of the recombinant Fc proteins that have been evaluated have shown to have a protective effect in mouse models of arthritis, such as the K/BxN serum transfer model. More recently, rFc-µTP-L309C, a recombinant human IgG1-Fc with an additional point mutation at position L309C fused to the human IgM tail-piece to form a hexamer, has been shown to ameliorate the arthritis in K/BxN mice. Additional studies have shown that rFc-µTP-L309C has multiple effects that work together to ameliorate the arthritis, including inhibition of neutrophil migration into the joint, inhibition of IL-1β production, downregulation of Th1 and Th17 cells and increases in T regulatory cells and IL-10. In this work, rFc-µTP-L309C was shown to effectively prevent arthritis in the K/BxN serum transfer model and significantly down-regulate inflammatory cytokines/chemokines and ameliorate the arthritis in the endogenous K/BxN model. This amelioration of the arthritis was mediated by a significant decrease in antibody levels. Interestingly, this effect seems to be independent from the neonatal Fc receptor (FcRn). rFc-µTP-L309C was shown to specifically inhibit G6PI autoantibody secretion from B-cells with a concomitant increase in TGFβ and decrease in B-cell activating factor (BAFF). These new findings suggests that rFc-µTP-L309C may provide a therapeutic benefit for any antibody-mediated autoimmune disease through its effects on B cells.