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

Abstract Purpose The limitation of hypofractionated radiation efficacy is due partly to the immunosuppressive tumor microenvironment. Indoleamine-2,3-dioxygenase 1 (IDO1) is an important regulator of tumor immune suppression. We evaluated the effects of IDO1 in hypofractionated radiation using a Lewis lung carcinoma (LLC) mouse model and tested whether IDO1 inhibition could sensitize those tumors to hypofractionated radiation. Methods and Materials Bilateral LLC tumors were established in C57BL/6 mice. Primary tumors were treated with 3 fractions of either 12-Gy or 6-Gy, and the IDO1 inhibitor INCB023843 was given starting on the first day of radiation. Plasma tryptophan (Trp) and kynurenine (Kyn) levels were quantified by liquid chromatography/tandem mass spectrometry. Tumor-infiltrating immune cells were isolated from the tumors, stained, and quantified by flow cytometry. Results The combination of INCB023843 and three 12-Gy fractions led to better tumor control and survival than radiation alone; INCB023843 plus three 6-Gy fractions had no benefit. IDO1 expression by tumor-infiltrating immune cells were increased by three 12-Gy doses and inhibited by the addition of INCB023843. Nearly all IDO1 + immune cells were also F4/80 + . Percentages of IDO1 + F4/80 + immune cells were drastically increased by three 12-Gy fractions and also by three 6-Gy fractions, but only INCB023843 combined with three 12-Gy fractions reduced those percentages. IDO1 + F4/80 + immune cells were further found to be CD11b + , Gr1-intermediate-expressing, CD206 – , and CD11c – , i.e., myeloid-derived suppressor cells (MDSCs). Three 12-Gy fractions also increased the percentages of tumor-infiltrating T regulatory cells and CD8 + T cells but adding INCB023843 did not affect those percentages. Conclusions 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.