Cytokines in chronic inflammatory arthritis: VI. Analysis of the synovial cells involved in granulocyte-macrophage colony-stimulating factor production and gene expression in rheumatoid arthritis and its regulation by IL-1 and tumor necrosis factor-α

Granulocyte-macrophage CSF (GM-CSF) is a potent stimulator of macrophages and neutrophils and is produced by rheumatoid arthritis (RA) synovium. We now report studies that identify some of the synovial cells and cytokines responsible for local GM-CSF production and gene expression in RA. GM-CSF was assayed by ELISA in supernatants from cultured RA fibroblast-like synoviocytes stimulated with various cytokines (IL-1β, TNF-α, macrophage-CSF, IFN-γ IL-6, and TGF-β). Immunoreactive GM-CSF was detected in IL-1β and TNF-α-stimulated cultures, but not in cells cultured in medium or stimulated with any of the other cytokines. IL-1 and TNF-α had a synergistic effect on GM-CSF production. GM-CSF gene expression by fibroblast-like synoviocytes was analyzed by ribonuclease protection assay, Northern blot analysis, and in situ hybridization. Both IL-1β and TNF-α induced GM-CSF mRNA accumulation, with a maximum effect after 4 h of stimulation. We then studied GM-CSF production by macrophage-like synoviocytes (MLS) isolated from fresh synovial specimens by flow microfluorimetry. Fresh MLS spontaneously secreted the cytokine and exogenous IL-1β or TNF-α had no effect. After 1 wk in culture, additional stimulation with IL-1β or TNF-α was required for GM-CSF production. Finally, in situ hybridization performed on freshly isolated subpopulations of synovial cells, identified GM-CSF RNA transcripts in MLS.