Promotion of the Local Differentiation of Murine Th17 Cells by Synovial Macrophages During Acute Inflammatory Arthritis

Objective. To examine the generation of proinflammatory Th17 cells at the site of tissue inflammation and in draining lymph nodes using an interleukin-17 (IL17)-dependent model of acute inflammatory arthritis. Methods. Arthritis was elicited in mice by intra-articular injection of methylated bovine serum albumin (mBSA) into the knee and subcutaneous injection of IL-1 beta. Anti-IL-17 or control antibodies were administered during arthritis induction. Cytokine expression was evaluated by intracellular cytokine staining of synovial lymphocytes, by polymerase chain reaction analysis of RNA extracted from lymph node cells, and by enzyme-linked immunosorbent assay of cell culture supernatants. Th17 differentiation of naive CD4+ T cells was assessed in cocultures with macrophages from arthritic mice. Results. Anti-IL-17 antibody administered during acute arthritis markedly reduced disease, indicating that the model is IL-17 dependent. IL-17 messenger RNA (mRNA), but not protein, was detected in draining lymph node CD4+ T cells and preceded joint inflammation. In addition, mRNA for Th17 cell-stimulatory cytokines (transforming growth factor beta, IL-6) and Th17 cell-inhibitory cytokines (interferon-gamma, IL-4) was detected in lymph nodes following injection of mBSA and IL-1 beta. Th17 cells were clearly identified in the inflamed synovium at the peak of disease. Synovial macrophages supported Th17 cell generation from naive CD4+ T cell precursors stimulated via CD3 in vitro and produced high levels of IL-6. In contrast, peritoneal macrophages failed to induce Th17 cell differentiation and produced less IL-6. Conclusion. These results suggest that Th17 cell differentiation is initiated in draining lymph nodes but that IL-17-producing cells are restricted to the inflamed synovium, being generated in response to local cytokines produced by inflammatory macrophages.