Citrullination of Collagen Type II and Epitope-Specific Antibody Responses in Rheumatoid Arthritis and Osteoarthritis: A Comparative Analysis

Citrullination of various proteins including the major structural protein of articular cartilage, collagen type II (COL2), has been indicated to play a role in rheumatoid arthritis (RA). Humoral responses against citrullinated COL2 epitopes have also been examined in patients with another common arthritic disorder, osteoarthritis (OA), but have not been shown to be present in a substantial proportion of patients in the studied groups. To further investigate this issue, we assessed anti-COL2 epitope responses in patients with OA and RA, with particular focus on immune responses to the citrullinated form of one of the main COL2 epitopes associated with RA. This research was combined with experiments aiming to define the involvement of COL2 citrullination in these two prevalent arthritic disorders by evaluation of the presence of citrulline in normal hyaline cartilage and in purified joint COL2 from RA and OA patients. Enzyme-linked immunosorbent assays (ELISA) were performed to determine the levels of antibodies specific to COL2 epitopes (C1, U1, J1 and citrullinated C1 epitope) in blood plasma of RA patients, OA patients and control individuals that do not suffer from arthritis disorder. The relative level of citrullination of COL2 purified from normal (healthy), OA and RA hyaline cartilage was assessed by direct ELISA. Immunofluorescent staining with anti-citrulline antibody was performed on histological sections of normal cartilage. Our results demonstrate significant reactivity to COL2 and three main COL2 epitopes, including citrullinated form of COL2 (358–369) epitope, in patients with OA. Collectively, more than 90% of the patients in the OA cohort showed reactivity against COL2 (358–369) epitope and/or citrullinated COL2 (358–369), which has not been observed in previous studies. A novel finding regarding COL2 citrullination of the present study is the determined tendency for higher relative level of citrullination of COL2 from arthritic patients compared to COL2 samples from individuals that do not suffer arthritic disorder. A tendency for increased relative anti-citrulline immunoreactivity for blood plasma proteins of RA patients compared to control individuals and OA patients was detected. Positive anti-citrulline staining was observed in the perichondrium and in the extracellular matrix surrounding chondrocytes in normal cartilage. Overall, our results indicate that citrullination is a physiological process that may become altered or enhanced during joint disease leading to bone destruction.