Objective: To study the regulation of granulocyte macrophage colony stimulating factor (GM-CSF) production by human articular chondrocytes which may contribute to the local GM-CSF production encountered in rheumatoid joints. This growth factor induces human macrophages to migrate and proliferate, improves their accessory function and increases the expression of HLA-DR antigens on macrophages and macrophage-like synoviocytes.

Methods: GM-CSF was assayed by ELISA and a bioassay in cell and organ culture supernatants from human articular chondrocytes, by in situ hybridization, Northern blot analysis and affinity chromatography.

Results: Both interleukin 1 (IL-1) and tumor necrosis factor-alpha (TNF-alpha) synergistically or additively stimulated chondrocytes to produce significant amounts of immunoreactive and bioactive GM-CSF with maximum values of 2928 pg/ml (p < 0.0001 both for IL-1 and/or TNF-alpha vs baseline). Affinity chromatography using specific monoclonal antibodies for human GM-CSF resulted in the purification of a chondrocyte derived 22-23 kDa protein. In situ hybridization demonstrated that the number of chondrocytes that expressed GM-CSF mRNA correlated well to the amount of GM-CSF secreted into the cultures. Transforming growth factor beta (TGF-beta) and to a lesser extent interferon-gamma (IFN-gamma) were able to decrease GM-CSF production induced by IL-1 and/or TNF-alpha. In contrast, basic fibroblast growth factor (FGF) in combination with IL-1 strongly increased GM-CSF secretion up to 8.5-fold. IFN-gamma, IL-6, TGF-beta, bFGF and IL-8 given alone failed to induce chondrocytes to produce GM-CSF. Steroids and low concentrations of cyclooxygenase inhibitors in general suppressed cytokine induced GM-CSF production.

Conclusion: Our data demonstrate that both proinflammatory cytokines IL-1 and TNF-alpha induce an immunoreactive and biologically active GM-CSF by human articular chondrocytes that appears to be downregulated by TGF-beta and upregulated by FGF. GM-CSF produced locally by cartilage cells may be an important cytokine involved in the activation and proliferation of pannus cells, that can be modulated by interactions with cytokines present in the inflamed joints, thus possibly contributing to the chronic infiltration and destruction of cartilage in inflammatory joint diseases.