IgD promotes pannus formation by activating Wnt5A-Fzd5-CTHRC1-NF-κB signaling pathway in FLS of CIA rats and the regulation of IgD-Fc-Ig fusion protein

Rheumatoid arthritis (RA) is a systemic autoimmune disease characterized by joint inflammation, synovial hyperplasia, cartilage degeneration, bone erosion, and pannus. Immunoglobulin D (IgD) plays an important role in autoimmune diseases although the content of it in vivo is low. Increased concentrations of anti-IgD autoantibodies have been detected in many RA patients. IgD-Fc-Ig fusion protein is constructed by connecting human IgD Fc domain and IgG1 Fc domain, which specifically block the IgD/ IgDR pathway and regulate the function of cells expressing IgDR to treat RA. The expression levels of Wnt5A and Frizzled 5 are higher in RA synovial tissue specimens. The complex of Wnt5A-Fzd5-LRP5/6-CTHRC1 promotes the expression of hypoxia inducible factor-1α by activating nuclear factor kappa-B (NF-κB), leading to high expression of VEGF and participating in angiogenesis. VEGF is the strongest angiogenic factor found so far. Here, we aimed to explore whether IgD participates in synovitis by binding to IgDR and regulating the activation of Wnt5A-Fzd5-CTHRC1-NF-κB signaling pathway in fibroblast synovial cells (FLSs), whether IgD-Fc-Ig fusion protein inhibits VEGF production in FLS of CIA and explore mechanism. We found that IgDR is expressed on MH7A and FLS. IgD promotes VEGF expression by activating Wnt5A-Fzd5-CTHRC1-NF-κB signaling pathway in MH7A and FLS. After activation of Fzd5 with Wnt5A, IgD-Fc-Ig reduced VEGF-A level in the culture supernatant of MH7A stimulation by IgD. The expressions of CTHRC1, Fzd5, p-P65 and VEGF in MH7A and FLSs were down-regulated after IgD-Fc-Ig treatment. IgD-Fc-Ig suppressed the combination of CTHRC1 and Fzd5 as well. By using the animal model, we demonstrated that IgD-Fc-Ig suppress ankle CTHRC1 and Fzd5 production resulted in inhibition of index of joint inflammation of CIA rats, which were consistent with vitro results. Conclusively, IgD-Fc-Ig inhibits IgD and Wnt5A-induced angiogenesis and joint inflammation by suppressing the combination of CTHRC1 and Fzd5. Our results show that IgD-Fc-Ig exerts its suppressive effect on IgD and Wnt5A by Wnt5A-Fzd5-CTHRC1-NF-κB signaling pathway.