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Interferon-gamma (IFN-gamma) induces the enzyme indoleamine dioxygenase (IDO) in a variety of human cell types. Furthermore, tumor necrosis factor-alpha (TNF-alpha) and interleukin-1 (IL-1) synergistically increase IFN-induced IDO activity. Inasmuch as cytokines can upregulate cytokine receptor expression, one mechanism of cytokine synergy may be at the level of receptor expression. To test the hypothesis that this mechanism of IDO regulation is active in epithelial cells, HeLa cells were treated with IFN-gamma, TNF-alpha, or IL-1beta to determine optimal cytokine concentrations and time for maximal cytokine receptor expression. Flow cytometric analysis with antibodies to receptors for IFN-gamma, TNF-alpha, or IL-1beta indicated that each cytokine upregulated expression of the other cytokine receptors by 4 h, with maximal expression observed between 16 and 20 h after cytokine treatment. Furthermore, increases in IFN-gamma receptors (IFNGR) induced by IL-1beta were found to be dependent on NF-kappaB transactivation. To determine if increases in IFNGR expression alone contributes to synergistic IDO induction, cells were stimulated with IL-1beta to upregulate receptor expression, and the NF-kappaB concentration was allowed to return to basal levels. When treated with IFN-gamma, enhanced Stat1 signaling and IDO induction were still observed, indicating that increased cytokine receptor expression contributes to synergistic increases in IDO activity.

Upregulation of IFN-γ Receptor Expression by Proinflammatory Cytokines Influences IDO Activation in Epithelial Cells