Objective: To examine the ability of fibroblast-like synoviocytes in rheumatoid arthritis (RA) to produce interleukin-7 (IL-7) and IL-15, and the ability of these cytokines to induce the proliferation of synovium-infiltrating T cells.
Methods: Messenger RNA (mRNA) and protein levels of IL-7 and IL-15 in synovial tissue cells and fibroblast cell lines were determined by reverse transcriptase-polymerase chain reaction and enzyme-linked immunosorbent assay, respectively. T cell-enriched populations from RA synovial tissues were isolated by deleting adherent cells after a 14-hour incubation in plastic dishes or by expanding T cells during a 14-day incubation of tissue cells with IL-2 alone, and their proliferative responses to IL-7, IL-15, and IL-2 were measured by 3H-thymidine incorporation.
Results: Freshly isolated cells from RA synovial tissues more strongly expressed mRNA for both IL-7 and IL-15 compared with the cells from osteoarthritis tissues, and could spontaneously release greater amounts of these cytokine proteins in culture. Fibroblast cell lines prepared from RA patients were able to produce large amounts of IL-15 and small amounts of IL-7 at both the transcriptional and protein levels, and their cytokine production was significantly elevated when stimulated with IL-1 and tumor necrosis factor alpha. Purified synovial tissue macrophages spontaneously released IL-15 but not IL-7, and synovial T cells did not produce either cytokine. IL-7 and IL-15, similar to IL-2, stimulated the proliferation of synovial tissue T cells from RA patients; IL-7 was less potent than IL-15 or IL-2.
Conclusion: These results indicated that fibroblast-like synoviocytes are an important source of the cytokines with IL-2-like activity, IL-15 and IL-7, in RA joints, and that IL-15 may be mainly responsible for local T cell activation and expansion in the presence of deficient IL-2 production by T cells.