Mutual antagonism between interferon-gamma and tumor necrosis factor-alpha on fibroblast-like synoviocytes: Paradoxical induction of IFN-gamma and TNF-alpha receptor expression

We recently described mutual antagonism between IFN-gamma and TNF-alpha on human fibroblast-like synoviocytes (FLS). TNF-alpha inhibits IFN-gamma-induced HLA-DR expression and IFN-gamma blocks TNF-alpha-dependent synoviocyte proliferation, collagenase production, and GM-CSF secretion. To study the mechanism of antagonism we have analyzed the effect these factors on the expression of cytokine surface receptors.¹²⁵I-Labeled cytokine binding was measured on cultured FLS and the results were analyzed by Scatchard plots. Unstimulated synoviocytes expressed 9300 ± 1560 IFN-gamma binding sites per cell. A single class of high-affinity receptor was observed (K_d=4.5±2.5×10^–10M). TNF-alpha did not competitively inhibit¹²⁵I-IFN-gamma binding. When FLS were incubated with TNF-alpha (100 ng/ml), there was a paradoxical 49.5 ± 5.6% increase in the number of binding sites for IFN-gamma (P=0.001), with no change in theK_d. Unstimulated FLS also expressed 2850 ± 700 TNF-alpha receptors per cells, with a singleK_d consistent with the lower-affinity TNF-alpha receptor (7.4±0.2×10^–10M). IFN-gamma did not directly interfere with TNF-alpha binding. Preincubation of FLS with 100 U/ml of IFN-gamma resulted in a 28.9 ± 9.0% increase in TNF-alpha receptor expression (P<0.008), with no change in theK_d. Low levels of the soluble 55-kD TNF receptor were detected in FLS supernatants. IFN-gamma did not effect soluble TNF receptor production. These data are the first demonstration of IFN-gamma and TNF-alpha receptors on FLS and show that TNF-alpha and IFN-gamma increase the expression of each other's receptor. Therefore the mutual antagonism between these two cytokines must occur through a postreceptor mechanism.