To understand immunological aspects of osteoarthritis (OA), which has been considered a degenerative disease, we compared profiles of autoimmunity comprehensively between OA and rheumatoid arthritis (RA) using analysis of the chondrocytes' proteome.
Proteins extracted from normal articular chondrocytes were separated by two-dimensional electrophoresis. Western blotting (WB) was then used to detect antigenic protein spots, using 20 serum samples from either OA or RA patients. Mass fingerprinting was used for identification of the detected autoantigens. The identified proteins were prepared as recombinant fusion proteins with maltose binding protein (TPI-MBP) to confirm their antigenecity and to investigate the frequency of the autoantibodies, epitope localization, and clinical significance by ELISA and WB, using serum samples obtained from 93 patients with OA, 54 patients with RA, and 43 patients with SLE.
Sixty-two autoantigens were detected in responses to OA and RA serum samples, in which 19 protein spots were detected only in the OA group. One of these apparently OA-specific protein spots, detected in four out of 20 OA patients but not in RA patients, was identified as human triose phosphate isomerase (TPI). All four positive sera against this spot reacted to a fusion protein of TPI-MBP but not MBP alone. Also, TPI-MBP affinity-purified antibodies from these sera only reacted to the spot that was identified as TPI in the two-dimensional electrophoresis membrane. The frequencies of the anti-TPI IgG in the sera from OA, RA, and SLE patients were 24.5%, 5.6%, and 4.7%. Further frequencies of the autoantibody in synovial fluid from 29 OA patients and 19 RA patients were found to be 24.1% and 0%, respectively. All the positive samples were further confirmed by WB. In the epitope mapping using eight truncated recombinant TPI proteins, multiple epitopes were identified, one of which was recognized in more than 90% of the positive serum samples. Clinically, the X-ray grading was lower in the anti-TPI-positive OA group than that in the anti-TPI-negative group.
The overall profile of autoimmunity in OA differs from that in RA, which may reflect OA-specific pathological roles of autoimmunity. The autoantibodies to TPI, detected in OA predominantly and produced by the antigen-driven mechanism, would have potential as a diagnostic marker for OA.