Radiotherapy, Temozolomide, and Antiprogrammed Cell Death Protein 1 Treatments Modulate the Immune Microenvironment in Experimental High-Grade Glioma

Abstract BACKGROUND The lack of immune synergy with conventional chemoradiation could explain the failure of checkpoint inhibitors in current clinical trials for high-grade gliomas (HGGs). OBJECTIVE To analyze the impact of radiotherapy (RT), Temozolomide (TMZ) and antiprogrammed cell death protein 1 (αPD1) (as single or combined treatments) on the immune microenvironment of experimental HGGs. METHODS Mice harboring neurosphere /CT-2A HGGs received RT (4 Gy, single dose), TMZ (50 mg/kg, 4 doses) and αPD1 (100 μg, 3 doses) as monotherapies or combinations. The influence on survival, tumor volume, and tumor-infiltrating immune cells was analyzed. RESULTS RT increased total T cells ( P = .0159) and cluster of differentiation (CD)8 + T cells ( P = .0078) compared to TMZ. Lymphocyte subpopulations resulting from TMZ or αPD1 treatment were comparable with those of controls. RT reduced M2 tumor-associated macrophages/microglia ( P = .0019) and monocytic myeloid derived suppressor cells (mMDSCs, P = .0003) compared to controls. The effect on mMDSC was also seen following TMZ and αPD1 treatment, although less pronounced ( P = .0439 and P = .0538, respectively). Combining RT with TMZ reduced CD8 + T cells ( P = .0145) compared to RT alone. Adding αPD1 partially mitigated this effect as shown by the increased CD8 + T cells/Tregs ratio, even if this result failed to reach statistical significance ( P = .0973). Changing the combination sequence of RT, TMZ, and αPD1 did not alter survival nor the immune effects. CONCLUSION RT, TMZ, and αPD1 modify the immune microenvironment of HGG. The combination of RT with TMZ induces a strong immune suppression which cannot be effectively counteracted by αPD1.