Identification of new citrulline-specific autoantibodies, which bind to human arthritic cartilage, by mass spectrometric analysis of citrullinated type II collagen.

Objective. To investigate type II collagen (CII) as a joint-specific target of the anti-citrullinated protein antibody (ACPA) response in rheumatoid arthritis (RA). Methods. Potential citrullinated neoepitopes were identified by high-resolution tandem mass spectrometry (MS/MS) of in vitro peptidylarginine deiminase 2 (PAD-2)-treated CII, and the relationship between citrullination and CII conformation was investigated by circular dichroism and conformation-dependent antibodies. Based on the MS analyses, synthetic peptides were designed and analyzed for serum IgG reactivity in the Epidemiological Investigation of RA (EIRA) case-control cohort of 1,949 RA patients and 278 healthy controls. Peptide-specific antibodies were purified from RA patient serum and used to stain RA cartilage specimens. Results. We described the conformation-dependent citrullination pattern of CII after PAD-2 treatment at room temperature and 37 degrees C and showed that CII could be citrullinated in its native triple-helical conformation. Screening of Arg and Cit pairs of synthetic peptides revealed new citrullinated B cell epitopes on CII. Antibodies directed to 2 proximal epitopes close to the C-terminus of the CII triple helix were recognized by autoantibodies in 21% and 17% of RA patients, respectively. Affinity-purified antibodies from RA sera directed to these 2 epitopes, but not antibodies directed to citrullinated alpha-enolase peptide 1, bound to RA cartilage. Conclusion. These findings suggest that cartilagedirected anticitrulline immunity contributes to the induction of joint inflammation in RA.