N-glycan in the variable region of monoclonal ACPA (CCP-Ab1) promotes the exacerbation of experimental arthritis

Objectives: The variable region of most ACPA IgG molecules in the serum of RA patients carries N-glycan (N-glycan(V)). To analyse the pathogenicity of N-glycan(V) of ACPAs, we analysed the pathogenicity of a monoclonal ACPA, CCP-Ab1, with or without N-glycan(V), which had been isolated from a patient with RA. Methods: CCP-Ab1 with no N-glycosylation site in the variable region (CCP-Ab1 N-rev) was generated, and antigen binding, the effect on in vitro differentiation of osteoclasts from bone marrow mononuclear cells of autoimmune arthritis-prone SKG mice (the cell size of TRAP(+) cells and bone resorption capacity) and the in vivo effect on the onset or exacerbation of autoimmune arthritis in SKG mice were evaluated in comparison with glycosylated CCP-Ab1. Results: Amino acid residues in citrullinated peptide (cfc1), which are essential for binding to CCP-Ab1 N-rev and original CCP-Ab1, were almost identical. The size of TRAP(+) cells was significantly larger and osteoclast bone resorption capacity was enhanced in the presence of CCP-Ab1, but not with CCP-Ab1 N-rev. This enhancing activity required the sialic acid of the N-glycan and Fc region of CCP-Ab1. CCP-Ab1, but not CCP-Ab1 N-rev, induced the exacerbation of experimental arthritis in the SKG mouse model. Conclusions: These data showed that N-glycan(V) was required for promoting osteoclast differentiation and bone resorption activity in both in vitro and in vivo assays. The present study demonstrated the important role of N-glycan(V) in the exacerbation of experimental arthritis by ACPAs.