Anti-PD-L1 antibody direct activation of macrophages contributes to an abscopal response in murine brain tumors

Immunotherapy approaches for glioblastoma multiforme have been thus far largely unsuccessful, suggesting unappreciated complexity in glioma biology and immunology. The intra-tumoral heterogeneity of these intrinsic brain tumors results in therapies killing only a subset of the tumor cells; therefore, therapeutic success will require achieving and optimizing an “abscopal effect” where tumor cells not specifically targeted are recognized and attacked as bystanders by the immune system. We have modified an immune-competent, genetically-driven mouse glioma model where a portion of the tumor burden is treated and another untreated portion is used as a readout of therapeutic efficacy. We find that following radiation of one lesion, anti-PD-L1 therapy enhances the abscopal response (macrophages and T-cells) to the un-irradiated lesion. In gliomas with few baseline T-cells, the anti-PD-L1-enhanced abscopal response occurs as an anti-PD-L1-driven, macrophage-mediated, and ERK-dependent increase in phagocytosis of tumor cells. Our results indicate that combined radiation and anti-PD-L1 therapy for gliomas results in peripherally-derived macrophages being responders in tumors with few baseline T-cells in the microenvironment. We established Proneural (PN) gene set-enriched gliomas with luciferase expression by injecting 2x10e5 RCAS vector producing DF-1 cells bearing RCAS-PDGF and RCAS-Cre vectors into the cortex of adult Ntva Ink4a/Arf-/- Ptenflox/flox mice with an additional Lox-Stop-Lox luciferase cassette. Here, the second vector RCAS-Cre activates the luciferase gene and is selected for with simultaneous deletion of Pten (allowing for positive selection of luciferase expressing tumor cells). We also generated a CL/MES-gene set enriched glioma by injecting 2x10e5 RCAS vector producing DF-1 cells bearing RCAS-PDGF and RCAS-Cre vectors into the cortex of adult Ntva Ink4a/Arf-/- Ptenflox/flox mice with Lox-Stop-Lox EGFRvIII. EGFR Normal = Proneural (PN) gene set-enriched gliomas EGFR Tumor = CL/MES-gene set enriched glioma