AHR Agonist ITE Boosted PD1 Antibody’s Effects by Inhibiting Myeloid-Derived Cells Suppressive Cells in an Orthotopic Mouse Glioma Model

<b>Background:</b> Glioblastoma is "cold". Consequently, immune checkpoint blockade therapy has failed to improve patients' survival, which is negatively correlated with patients' peripheral MDSC counts. AHR is known to mediate immune-suppressive functions of certain tryptophan metabolites such as kynurenine; yet, there lack of reports on how AHR agonists affect glioma immunity. <b>Methods/Objectives</b>: We hypothesized that ITE could synergize with PD1 antibody as AHR is one major node of immune-suppressive pathways, and tested it using an immune-competent mouse glioma model. <b>Results</b>: The combination of ITE+PD1 antibody glioma MDSC was significantly reduced, along with increased infiltration of the CD4-CD8+ and CD4+CD8+ T cells, leading to extended mouse survival. ITE treatment alone significantly reduces the infiltration of CD11b+Ly6G+Ly6Clo cells, namely PMN-MDSCs, and neutrophils, while the combination with PD1 antibody significantly reduces all MDSCs plus neutrophils. The presence of ITE inhibits the expression of IL11 and the in vitro induction of MDSCs from mouse PBMCs by IL11. The identification of the ITE-AHR-IL11-MDSC pathway provides more mechanistic insights into AHR's effects. The fact that ITE, which is otherwise immune-suppressive, can activate immunity in glioma indicates that searching for drugs targeting AHR should go beyond antagonists.