Objective. To investigate the presence and role of interleukin-17 (IL-17) in rheumatoid arthritis (RA), and its regulation by antiinflammatory cytokines.Methods. The production of IL-17 was measured in supernatants of RA, osteoarthritis (OA), and normal synovial tissue pieces cultured ex vivo. Quantification of IL-17 was performed using a specific biologic assay. IL-17 gene expression was investigated by reverse transcriptase-polymerase chain reaction (RT-PCR)-techniques. Immunohistochemistry was used to evaluate the frequency of IL-17-positive cells in synovium.The secretion of IL-17 by synovium was measured in the presence of IL-4, IL-13, and IL-10. In addition, the contributions of exogenous and endogenous IL-17 to IL-6 production by RA synovium were studied.Results. Functional IL-17 was spontaneously produced by 16 of 18 RA (mean ؎ SEM 41.7 ؎ 11.4 units/ml), 2 of 12 OA (5.3 ؎ 4.5 units/ml), and 0 of 3 normal synovial explant cultures. IL-17 messenger RNA expression was demonstrated by RT-PCR in 4 of 5 RA and 0 of 3 OA synovial samples. By immunostaining of RA synovium, IL-17-producing cells were found in the T cell-rich area. Addition of both IL-4 and IL-13 completely inhibited the production of IL-17, whereas IL-10 had no effect. Addition of exogenous IL-17 to RA synovium resulted in an increase in IL-6 production, whereas that of a blocking anti-IL-17 antibody reduced production of IL-6.Conclusion. The T cell cytokine IL-17 was found to be highly produced by RA, but not by OA, synovium. Its production and function were down-regulated by IL-4 and IL-13. These results indicate that IL-17 contributes to the active, proinflammatory pattern that is characteristic of RA. Through the contribution of IL-17, some Th1-like T cells appear to mediate synovial inflammation.

It remains to be clarified whether dendritic cells (DC) reach the rheumatoid arthritis (RA) synovium, considered an ectopic lymphoid organ, as mature cells or undergo local maturation. We characterized by immunohistochemistry the DC subsets and used tonsils as a control. Immature and mature DC were defined by CD1a and DC-lysosome-associated membrane protein/CD83 expression, respectively. Immature DC were mainly detected in the lining layer in RA synovium. Mature DC were exclusively detected in the lymphocytic infiltrates. The DC-lysosome-associated membrane protein/CD1a ratio was 1.1 in RA synovium and 5.3 in tonsils, suggesting the relative accumulation of immature DC in RA synovium. We then focused on the expression of CCL20/CCR6 and CCL19/CCR7, CCL21/CCR7 chemokine/receptor complex, which control immature and mature DC migration respectively. A close association was observed between CCL20-producing cells and CD1a+ cells, suggesting the contribution of CCL20 to CCR6+ cell homing. Conversely, CCL21 and CCL19 expression was only detected in perivascular infiltrates. The association among CCL19/21-producing cells, CCR7 expression, and mature DC accumulation is in line with the roles of these chemokines in mature CCR7+ DC homing to lymphocytic infiltrates. The role of DC in disease initiation and perpetuation makes chemokines involved in DC migration a potential therapeutic target.

Macrophage inflammatory protein (MIP)-3α is a chemokine involved in the migration of T cells and immature dendritic cells. To study the contribution of proinflammatory cytokines and chemokines to the recruitment of these cells in rheumatoid arthritis (RA) synovium, we looked at the effects of the monocyte-derived cytokines ΙL-1β and TNF-α and the T cell-derived cytokine IL-17 on MIP-3α production by RA synoviocytes. Addition of ΙL-1β, IL-17, and TNF-α induced MIP-3α production in a dose-dependent manner. At optimal concentrations, ΙL-1β (100 pg/ml) was much more potent than IL-17 (100 ng/ml) and TNF-α (100 ng/ml). When combined at lower concentrations, a synergistic effect was observed. Conversely, the anti-inflammatory cytokines IL-4 and IL-13 inhibited MIP-3α production by activated synoviocytes, but IL-10 had no effect. Synovium explants produced higher levels of MIP-3α in RA than osteoarthritis synovium. MIP-3α-producing cells were located in the lining layer and perivascular infiltrates in close association with CD1a immature dendritic cells. Addition of exogenous IL-17 or ΙL-1β to synovium explants increased MIP-3α production. Conversely, specific soluble receptors for ΙL-1β, IL-17, and TNF-α inhibited MIP-3α production to various degrees, but 95% inhibition was obtained only when the three receptors were combined. Similar optimal inhibition was also obtained with IL-4, but IL-13 and IL-10 were less active. These findings indicate that interactions between monocyte and Th1 cell-derived cytokines contribute to the recruitment of T cells and dendritic cells by enhancing the production of MIP-3α by synoviocytes. The inhibitory effect observed with cytokine-specific inhibitors and Th2 cytokines may have therapeutic applications.