OBJECTIVE: To investigate the impact of type II collagen (CII)-reactive T cells on the Th1/Th2 cytokine balance in patients with rheumatoid arthritis (RA). METHODS: T cell proliferative responses to bovine CII were examined in synovial fluid mononuclear cells (SFMC) and peripheral blood mononuclear cells (PBMC) by mixed lymphocyte culture. CII-reactive T cell lines were generated from the SFMC and PBMC. Interferon-gamma (IFNgamma), interleukin-12 (IL-12), and IL-4 were measured by enzyme-linked immunosorbent assay in the SF, sera, and culture supernatants of PBMC and SFMC that had been stimulated with CII. RESULTS: The frequency of CII-reactive T cells was higher in the PBMC from RA patients than in that from osteoarthritis patients and healthy control subjects. In RA patients, CII-reactive T cells were more prevalent in SFMC than in PBMC. The mean level of IFNgamma and the ratio of IFNgamma to IL-4 were significantly higher in the culture supernatants of T cells stimulated with CII; these differences were more prominent in SFMC. Levels of IL-12 in the culture supernatants of SFMC and PBMC stimulated with CII were significantly higher than those in unstimulated supernatants. T cell responsiveness correlated well with the level of type 1 cytokines in culture supernatants from RA T cells stimulated with CII. In the CII-reactive cell lines, the increased production of IFNgamma was consistent with clonal expansion. CONCLUSION: CII-reactive T cells are more abundant in SFMC than in PBMC and are strongly associated with a shift toward Thl cytokine in the inflamed joints of RA patients. Our results suggest that a skewing toward type 1 cytokines by CII-reactive T cells may play an important role in the chronic inflammatory process of RA.