Enhancing Effect of IL-17 on IL-1-Induced IL-6 and Leukemia Inhibitory Factor Production by Rheumatoid Arthritis Synoviocytes and Its Regulation by Th2 Cytokines

IL-17 is a cytokine produced by CD4 T cells that activates the production of inflammatory mediators by synoviocytes. To study the contribution of soluble factors in the interaction between T cells and synoviocytes in rheumatoid arthritis (RA), we looked at the effect of IL-17 on these cells in the presence of cytokines classified as pro (IL-1)- and anti-inflammatory (IL-4, IL-13, IL-10). Both human rIL-1beta and rIL-17 induced IL-6 and leukemia inhibitory factor (LIF) production by synovial fibroblasts in a dose-dependent manner. After 7 days of culture, optimal concentrations of IL-1beta increased IL-6 (33-fold) and LIF (10-fold) production by synoviocytes, while IL-17 showed a lesser effect on IL-6 (17-fold) and LIF (4-fold) production. Using low concentrations of IL-17 and IL-1beta in combination, a synergistic effect was observed on the production of IL-6, whereas an additive effect was observed for LIF production. Production of biologically active IL-17 was demonstrated in RA synovium supernatants with the use of a blocking anti-IL-17 Ab. Both IL-4 and IL-13 had a modest stimulatory effect on IL-1- and IL-17-induced production of IL-6, but inhibited that of LIF. In contrast, IL-10 had a limited inhibitory effect on IL-6 production and no effect on that of LIF. These findings indicate that low levels of cytokines produced by monocytes (IL-1) and T cells (IL-17) can act together on synoviocytes. Thus, some RA synovium T cells producing IL-17 can activate mesenchymal cells leading to an increased proinflammatory pattern sensitive to Th2 cytokine regulation.