Citrullinated Proteins in Arthritis; their Presence in Joints and Effects on Immunogenicity

Lundberg, K.; Nijenhuis, S.; Vossenaar, E.; Venrooij, W. J.; Klareskog, L.; Harris, H. E.

Autoantibodies directed against citrulline‐containing proteins have an impressive specificity of nearly 100% in RA patients and a suggestive involvement in the pathogenesis. The targeted epitopes are generated by a post‐translational modification catalysed by the calcium‐dependent enzyme peptidyl arginine deaminase that converts the positively charged arginine to polar but uncharged citrullin. The aim of this study was to analyse the presence of citrulline in the joints at different time points of collagen‐induced arthritis in DA rats by immunohistochemistry and to investigate how immunogenicity and arthritogenicity was affected by citrullination of rat serum albumin (RSA) and collagen type II (CII). Our results indicate that citrulline could be detected in joints of arthritic animals, first appearance at the onset of disease and increasing as disease progressed into a chronic state. Unimmunized animals or time points before clinical signs of arthritis were negative. By morphology, we state that some infiltrating macrophages as well as the cartilage surface stain positive for citrulline, while the major source of citrullinated proteins appears to be fibrin depositions. A specific Cit‐RSA T‐cell response was observed in animals challenged by citrullinated RSA, no response was recorded when RSA was used as a stimulus. The IgG analysis reveals not only a response towards the modified protein but also cross‐reactivity to native RSA. No T‐cell or B‐cell response was noted in animals injected with unmodified RSA. Cit‐CII induced a disease with higher incidence and earlier onset than did the native counterpart. We conclude that, in contrast to the human disease, citrulline does not seem to appear before clinical signs. As inflammation proceeds, citrulline is detected specifically in the joints. All other organs investigated were negative. We also conclude that citrullination of a protein can break tolerance and increase its arthritogenic properties.