Specific presence of intracellular citrullinated proteins in rheumatoid arthritis synovium - Relevance to antifilaggrin autoantibodies

Objective. To investigate the presence of citrullinated proteins in the syn ovial membrane of patients with rheumatoid arthritis (RA) and controls, and to analyze a possible relationship with antifilaggrin autoantibody (AFA) r eactivity. Methods. Synovial biopsy samples were obtained from 88 consecutive patients undergoing needle arthroscopy for knee synovitis associated with RA (n = 3 6), spondylarthropathy (n = 35), osteoarthritis (n = 9), or other diagnoses (n = 8). Tissue sections were stained with 2 different anticitrulline poly clonal antibodies and an antifilaggrin monoclonal antibody (mAb). The pheno type of citrulline-positive cells and the colocalization with affinity-puri fied AFA were investigated by double immunofluorescence on frozen sections. Results. Studies with the first antibody showed that citrulline is expresse d intracellularly in the lining and sublining layers of RA synovial tissue. Staining with the second antibody, monospecific for proteins containing mo dified citrulline, and with anti-inducible nitric oxide synthetase confirme d the presence of citrullinated proteins rather than free citrulline in the synovium. Citrulline-positive cells were detected in 50% of the RA patient s (18 of 36) but in none of the controls (0 of 52). The anticitrulline reac tivity colocalized with affinity-purified AFA reactivity, although staining s with the antifilaggrin mAb indicated the absence of filaggrin in the syno vium. Conclusion. Intracellular citrullinated proteins, which are not recognized by an antifilaggrin mAb, are expressed in RA but not in control synovium. T he high specificity of this finding and the colocalization with AFA reactiv ity boost the interest in citrullinated proteins as possible triggers of au toimmune responses in RA. Moreover, this is the first description of a spec ific histologic marker for RA synovium.