Identification of citrullinated cellular fibronectin in synovial fluid from patients with rheumatoid arthritis

Cellular fibronectin (cFn) has been implicated in the pathogenesis of rheumatoid arthritis (RA), and we previously demonstrated the presence of citrullinated cFn in rheumatoid synovial tissues. The present study aimed to investigate whether citrullinated cFn can be detected in the plasma or synovial fluid of RA patients.

Twenty-five rheumatoid arthritis synovial fluid (RASF), seven osteoarthritis synovial fluid (OASF) and 12 plasma samples from RA patients were examined. Citrullination of cFn was determined by immunoprecipitation (IP), western blotting and enzyme-linked immunosorbent assay (ELISA), in which peptidyl-citrulline within cFn was detected using a specific anti-cFn monoclonal antibody in combination with anti-modified citrulline antibody after chemical modification.

Levels of citrullination associated with cFn, as determined by ELISA, were significantly higher in RASF than in OASF samples. IP and western blotting detected citrullinated cFn in RASF but not in plasma samples from RA patients. Levels of total cFn were elevated in RASF compared with OASF, and 24 out of 25 RASF samples were positive for anti-CCP antibody. However, no correlation was observed between levels of citrullinated cFn and those of total cFn or anti-CCP antibody in RASF. On the other hand, a significant positive correlation was observed between the levels of matrix metalloproteinase-3 (MMP-3) and cFn citrullination in RASF.

Citrullinated cFn appears to be produced within the affected joint and might be involved in the pathogenesis of rheumatoid synovitis.