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We studied arthritis-related autoantibodies in 136 patients with knee osteoarthritis (OA) and 67 age- and sex-matched healthy individuals in the Shanghai District of China. Serum antibody titers for recombinant fusion proteins of cartilage intermediate layer protein (CILP) and YKL-39 were analyzed using enzyme-linked immunosorbent assay (ELISA) and Western blot. Serum antibody titers against recombinant osteopontin (OPN) and cyclic citrullinated peptide (CCP) antibodies were measured also using ELISA. Anti-CILP antibodies were detected in 25/136 OA patients but only 1/67 controls. Anti-YKL-39, anti-OPN, and anti-CCP antibodies were detected in 9/136, 11/136, and 7/136 of the OA patients, respectively, and 0/67 controls. There was rarely overlap of these antibodies in a single patient, suggesting distinct antigen specificity in each case. The antibodies were detected in patients with OA of grades II and III but not grade IV. The prevalence of autoantibodies to various arthritis-related proteins in early-stage knee OA supports the involvement of a specific immune response in initial cartilage degeneration in OA. [ABSTRACT FROM AUTHOR]