Anti-PD-L1 antibody direct activation of macrophages contributes to a radiation-induced abscopal response in glioblastoma

Background: Most glioblastomas recur near prior radiation treatment sites. Future clinical success will require achieving and optimizing an "abscopal effect,"whereby unirradiated neoplastic cells outside treatment sites are recognized and attacked by the immune system. Radiation combined with anti-programmed cell death ligand 1 (PD-L1) demonstrated modest efficacy in phase II human glioblastoma clinical trials, but the mechanism and relevance of the abscopal effect during this response remain unknown. Methods: We modified an immune-competent, genetically driven mouse glioma model (forced platelet derived growth factor [PDGF] expression?+?phosphatase and tensin homolog loss) where a portion of the tumor burden is irradiated (PDGF) and another unirradiated luciferase-expressing tumor (PDGF?+?luciferase) is used as a readout of the abscopal effect following systemic anti-PD-L1 immunotherapy. We assessed relevance of tumor neoepitope during the abscopal response by inducing expression of epidermal growth factor receptor variant III (EGFRvIII) (PDGF?+?EGFRvIII). Statistical tests were two-sided. Results: Following radiation of one lesion, anti-PD-L1 immunotherapy enhanced the abscopal response to the unirradiated lesion. In PDGF-driven gliomas without tumor neoepitope (PDGF?+?luciferase, n =?8), the abscopal response occurred via anti-PD-L1 driven, extracellular signal-regulated kinase-mediated, bone marrow-derived macrophage phagocytosis of adjacent unirradiated tumor cells, with modest survival implications (median survival 41 days vs radiation alone 37.5 days, P?=?0.03). In PDGF-driven gliomas with tumor neoepitope (PDGF?+?EGFRvIII, n =?8), anti-PD-L1 enhanced abscopal response was associated with macrophage and T-cell infiltration and increased survival benefit (median survival 36 days vs radiation alone 28 days, P?=?0.001). Conclusion: Our results indicate that anti-PD-L1 immunotherapy enhances a radiation- induced abscopal response via canonical T-cell activation and direct macrophage activation in glioblastoma.