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

Interferon-γ (IFN-γ) induces the enzyme indoleamine dioxygenase (IDO) in a variety of human cell types. Furthermore, tumor necrosis factor-α (TNF-α) 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-γ, TNF-α, or IL-1β to determine optimal cytokine concentrations and time for maximal cytokine receptor expression. Flow cytometric analysis with antibodies to receptors for IFN-γ, TNF-α, or IL-1β 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-γ receptors (IFNGR) induced by IL-1β were found to be dependent on NF-κB transactivation. To determine if increases in IFNGR expression alone contributes to synergistic IDO induction, cells were stimulated with IL-1β to upregulate receptor expression, and the NF-κB concentration was allowed to return to basal levels. When treated with IFN-γ, enhanced Stat1 signaling and IDO induction were still observed, indicating that increased cytokine receptor expression contributes to synergistic increases in IDO activity.