Objective Antibodies against citrullinated proteins (ACPA) are a characteristic of rheumatoid arthritis (RA). Carbamylation is a different type of post translational modification, where a Lysine amino acid is converted into a homocitrullin. Recently we identified antibodies binding to carbamylated proteins (anti-CarP) in a subgroup of RA patients. In ACPA negative RA patients anti-CarP antibodies associate with joint damage.

The Aim of this study was to determine whether these anti-CarP antibodies are present in animal models of arthritis.

Methods Collagen induced arthritis (CIA) was induced in DBA/1 (n = 29) and C57Bl/6 (n = 20) mice by immunisation with type II collagen in CFA. Arthritis severity was monitored using a clinical scoring system. Non-immunised animals (n = 9) served as negative controls. After disease onset serum was harvested and antibody levels were determined by ELISA. The specificity of our anti-CarP. ELISA was validated using dotblots.

Results Whereas no anti-CarP antibodies could be detected in non-immunised DBA/1 mice, anti-CarP total Ig was present in 93% of the arthritic mice. Of those mice 39% had IgG1 and 79% had IgG2a anti-CarP antibodies. Antibodies to citrullinated proteins could not be detected. The levels of mouse collagen-specific IgG2a correlated with the clinical score. However, the levels of the different anti-CarP isotypes did not. Around 60% of the immunised C56Bl/6 mice developed arthritis. Anti-CarP IgG2c could be detected in 55% of those mice and could not be detected in the mice that did not get CIA. Anti-CarP IgG1 was detected in 28% of the arthritic mice. Interestingly, mouse collagen specific IgG2c antibodies were detected in 100% of the immunised C57Bl/6 mice. Dotblot analysis, using carbamylated and non-modified proteins confirmed the ELISA results regarding the specificity of the antibodies for homocitrulline containing proteins.

Conclusions Mice with CIA have antibodies to carbamylated proteins and their presence associated with disease development. All immunised mice have anti-mouse CII antibodies, indicating that the presence of anti-CarP antibodies could be a disease specific marker for arthritis in mice. Further studies will be required to determine the role of anti-CarP in the pathogenesis of arthritis.