Autoimmunity against YKL-39, a human cartilage derived protein, in patients with osteoarthritis.

Our previous study revealed that some patients with rheumatoid arthritis (RA) possessed autoantibodies to YKL-39, a cartilage related protein. We investigated whether patients with osteoarthritis (OA) also displayed autoimmunity to YKL-39.Autoantibodies to recombinant YKL-39 as well as human cartilage glycoprotein-39 were detected by ELISA and Western blotting. The tested serum samples were derived from 117 patients with OA, 94 patients with RA, and 2 groups of 50 arthropathy-free healthy donors who matched the OA and RA groups for age and sex. We determined autoepitopes on YKL-39 using 3 overlapping fragments of YKL-39 (designated F1, F2, F3). T cell proliferation response to YKL-39 was analyzed using the 3H-thymidine incorporation assay.Autoantibodies to YKL-39 were detected in 13 (11.1%) patients with OA and 11 (11.8%) with RA. In the epitope mapping, all the 3 fragments of YKL-39 were found to carry autoepitopes, but F1 was recognized most frequently. Proliferative responses of peripheral blood mononuclear cells against YKL-39 were detected in 6 (46%) of the 13 OA patients who were positive for the anti-YKL-39 autoantibodies and in 2 (17%) of the 11 antibody positive RA patients.These results show that autoimmunity to YKL-39 in patients with OA was present at equal or somewhat higher frequency than in patients with RA. The cellular and humoral immune responses to YKL-39 may be involved in the pathological process of OA as well as RA.