N-glycan in the monoclonal ACPA, CCP-Ab1 the variable region 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-glycanV). To analyze the pathogenicity of N-glycanV of ACPAs, we analyzed the pathogenicity of a monoclonal ACPA, CCP-Ab1, with or without N-glycanV, 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 to glycosylated CCP-Ab1.

RESULTS: Amino acid residues in citrullinated peptide (cfc-1), 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-glycanV 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-glycanV in the exacerbation of experimental arthritis by ACPAs.