Redox Control of the Critical Immune Regulatory Enzyme Indoleamine 2,3-Dioxygenase by NADPH Oxidase 2-Derived Reactive Oxygen Species in Human Monocytes

The heme enzyme indoleamine 2,3-dioxygenase 1 (IDO1) is a critical immune regulatory enzyme capable of signalling for immune suppression in diverse pathological settings including chronic inflammation, infection and cancer. During inflammation, IDO1 is expressed within activated innate immune cells that also produce the reactive oxygen species (ROS) hydrogen peroxide (H 2 O 2 ) and hypochlorous acid (HOCl) by NADPH oxidase (Nox) and myeloperoxidase (MPO), respectively. We recently reported that H 2 O 2 inhibits recombinant IDO1 dioxygenase activity by engaging the enzyme’s heme peroxidase activity resulting in oxidative self-inactivation (JBC 2013;288:1548). To test if Nox-derived H 2 O 2 or MPO-derived HOCl also control cellular IDO1 activity, we studied IFN-γ-stimulated human monocytes that co-express IDO1, Nox and MPO. Stimulation of monocytes with PMA increased cellular production of H 2 O 2 and HOCl that coincided with the post-translational inhibition of IDO1 activity. The ability of PMA to inhibit IDO1 was abrogated by inhibition of Nox-derived H 2 O 2 production with the PKC inhibitor Bisindolylmaleimide or the flavoprotein oxidase inhibitor diphenyleniodonium, but not by inhibition of MPO-derived HOCl with 4-aminobenzoic acid hydrazide. IFN-γ-stimulated human monocytes not only expressed the phagocytic Nox2 isoform but also Nox4. While selective Nox4 inhibition reduced cellular H 2 O 2 production by ~50%, it did not reverse PMA’s inhibitory action on IDO1. Also, PMA did not inhibit IDO1 activity in IFN-γ-stimulated monocytes isolated from chronic granulomatous disease (CGD) patients expressing defective Nox2. These studies establish that Nox2-derived ROS control IDO1 activity in human innate immune cells and provide important new insights into the redox control of IDO1’s immune regulatory actions.