Citrullinated proteins in arthritis: presence in joints and effects on immunogenicity

Autoantibodies (Ab) directed against citrulline (Cit)-containing proteins have a specificity of nearly 100% in rheumatoid arthritis (RA) patients. The presence of these markers early in disease, even before clinical onset, and the observation that these autoantibodies are produced locally in the pannus suggest an involvement in the pathogenesis. The targeted epitopes are generated by deimination, a post-translational modification catalyzed by the enzyme peptidyl arginine deiminase.

Our aim was to analyze the presence of Cit-proteins and fibrinogen in the joints at different time points in collagen-induced arthritis (CIA) in rats and to investigate how citrullination of an autoantigen affects its immunogenicity.

Synovial tissue sections from DA rats were stained for expression of citrulline and fibrinogen. Lew1AV1 rats were immunized with Cit-rat serum albumin (RSA) or unmodified RSA, and antibody and T-cell responses were evaluated.

Citrulline was detected in arthritic joints from disease onset and increased expression was noted as disease progressed into a more chronic state. Naïve rats or time points before arthritis onset were negative for citrulline-specific staining. Infiltrating cells, as well as the cartilage surface, stained positive for citrulline, although the major source of citrullinated proteins appeared to be fibrin depositions. A specific Cit-RSA T-cell response was observed in animals challenged by Cit-RSA. In contrast, no response was recorded when RSA was used as a stimulus. The IgG response revealed not only a response toward the modified protein but also cross-reactivity to the unmodified form of RSA.

In CIA, joint inflammation precedes the presence of Cit-proteins and citrullination increases immunogenicity of an autoantigen. Our results suggest that citrullination is induced by inflammation and might be contributing to the development of autoreactive T and B cells.