Auto-antibodies to post-translationally modified proteins in osteoarthritis.

OBJECTIVE: Autoantibodies (AutoAbs) have been observed in osteoarthritis (OA) with broad antigenicity, although their prevalence and role remain unclear. Post-translational modification (PTMs) of proteins (oxidation, carbamylation, citrullination) is associated with synovitis and can lead to AutoAb development. Given the prevalence of synovitis, we explored whether AutoAbs to PTM-antigens are common in OA compared with rheumatoid arthritis (RA). METHODS: Serum (n = 895) was obtained from healthy controls, OA and RA patients; and arthritic synovial fluid (SF, n = 290). ELISAs were used to quantify anti-citrullinated peptide (ACPA), anti-carbamylated protein (anti-CarP), anti-oxidized collagen (anti-ROS-CI/CII) antibodies. RESULTS: In sera, positivity for PTM-antigens AutoAbs was observed at a lower frequency in OA with 64.1% (95%CI: 57.2-70.1%) more ACPA+ and 29.8% (21.0-37.3%) more anti-CarP + patients in RA (both P < 0.0001). Levels of ACPA, anti-CarP were also lower in OA (P < 0.0001). Anti-ROS-CII positivity was lower in OA compared to RA (16.6%, 4.8-28.6%) less frequent, P = 0.033) but not anti-native-CII. There was no impact of age/gender on AutoAbs associations with diseases either looking at positivity or levels. In SF, OA patients were often ACPA+ (45.9%) although less frequently than in RA (P = 0.004). Anti-CarP were rarely observed (<5% all samples). All collagen AutoAbs were more frequent in RA compared to OA (all P < 0.010) but only levels of anti-CII and anti-ROS-CII were significantly higher in they RA (P < 0.050). CONCLUSION: Although the frequency of AutoAbs for PTM proteins were lower in OA sera compared to RA, a higher proportion of OA SF were positive. The relative retention of AutoAbs in the OA joint requires further investigation.

Pitfalls in the detection of citrullination and carbamylation.

Carbamylation and citrullination are both post-translational modifications against which (auto)antibodies can be detected in sera of rheumatoid arthritis (RA) patients. Carbamylation is the chemical modification of a lysine into a homocitrulline, whereas citrullination is an enzymatic conversion of an arginine into a citrulline. It is difficult to distinguish between the two resulting amino acids due to similarities in structure. However, differentiation between citrulline and homocitrulline is important to understand the antigens that induce antibody production and to determine which modified antigens are present in target tissues. We have observed in literature that conclusions are frequently drawn regarding the citrullination or carbamylation of proteins based on reagents that are not able to distinguish between these two modifications. Therefore, we have analyzed a wide spectrum of methods and describe here which method we consider most optimal to distinguish between citrulline and homocitrulline. We have produced several carbamylated and citrullinated proteins and investigated the specificity of (commercial) antibodies by both ELISA and western blot. Furthermore, detection methods based on chemical modifications, such as the anti-modified citrulline-"Senshu" method and also mass spectrometry were investigated for their capacity to distinguish between carbamylation and citrullination. We observed that some antibodies are able to distinguish between carbamylation and citrullination, but an overlap in reactivity is often present in the commercially available anti-citrulline antibodies. Finally, we conclude that the use of mass spectrometry is currently essential to differentiate between citrullinated and carbamylated proteins present in complex biological samples.