IDO1 Inhibition Overcomes Radiation-Induced "Rebound Immune Suppression" by Reducing Numbers of IDO1-Expressing Myeloid-Derived Suppressor Cells in the Tumor Microenvironment.

In addition to its immune activation effects, hypofractionated radiation induced "rebound immune suppression" in the tumor microenvironment by activating and recruiting IDO1-expressing MDSCs in a dose-dependent manner. Adding an IDO1 inhibitor to hypofractionated radiation reduced the percentages of these cells, overcame the immune suppression, and sensitized LLC tumors to hypofractionated radiation.