RBIO-04. STEREOTACTIC RADIOSURGERY REPROGRAMS MACROPHAGE, MICROGLIA, AND CD8+ T CELL POPULATIONS IN THE GLIOBLASTOMA IMMUNE MICROENVIRONMENT

Abstract

Abstract INTRODUCTION Fractionated radiotherapy is a first-line treatment for glioblastoma, but daily ionizing radiation prevents durable immune infiltration of the tumor microenvironment. Hypofractionated radiotherapy is used to treat glioblastomas at recurrence, but the impact of hypofractionated radiotherapy on the glioblastoma immune microenvironment is incompletely understood. Here we define immune microenvironment changes across multiple immunocompetent intracranial mouse models of glioblastoma after treatment with ionizing radiation mimicking stereotactic radiosurgery (SRS) in humans. METHODS Syngeneic GL261 (3x105 cells/mouse) or SB28 glioblastoma cells (3x104 cells/mouse) were implanted into the frontal lobe of immunocompetent C57BL/6J mice (18 mice/arm x 4 arms). Intracranial bioluminescence was used to assess glioblastoma growth. After tumor engraftment, glioblastomas were treated with conformal SRS (18Gy/1Fx) or sham. Glioblastomas were collected for histologic, single-cell, or molecular analyses 5 days after treatment (6 mice/arm) or at the time of euthanasia after monitoring for survival (12 mice/arm). Glioblastoma immune microenvironment responses were assessed using (1) H&E, (2) single cell mass cytometry (CyTOF) or IHC to define or validate immune cell changes, respectively, or (3) multiplexed cytokine assays to elucidate molecular mechanisms reprograming the glioblastoma immune microenvironment in response to SRS. RESULTS SRS attenuated glioblastoma growth and prolonged survival compared to sham treatment in both immunocompetent intracranial mouse models (GL261: 14 days versus 27 days, p< 0.001, SB28: 19 days versus 22 days, p=0.001). CyTOF showed SRS decreased immunosuppressive macrophage infiltration and increased microglia or CD8+ T cell infiltration of the glioblastoma immune microenvironment. Histologic analyses validated T cell and microglia infiltration after SRS. Glioblastoma cytokine analysis revealed inhibition of pro-tumor/anti-inflammatory cytokines (IL6, LIF) after SRS. CONCLUSIONS Single-fraction SRS durably reprograms glioblastoma macrophage, microglia, and CD8+ T cell populations in preclinical models, suggesting SRS or inhibition of pro-tumor/anti-inflammatory mechanisms underlying the immunosuppressive glioblastoma microenvironment represent immunogenic therapies that may offer a benefit to patients with glioblastoma.