886 Stereotactic Radiosurgery Reprograms Macrophage, Microglia, and T Cell Populations in the Glioblastoma Immune Microenvironment

Abstract

INTRODUCTION: Daily ionizing radiation, the first-line treatment for glioblastoma, 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. METHODS: Syngeneic GL261 (3x105 cells/mouse) or SB28 glioblastoma cells (3 x 104 cells/mouse) were implanted into the frontal lobe of immunocompetent C57BL/6J mice. 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 or at the time of euthanasia after monitoring for survival. 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 in GL261 tumors , however, there were very few changes in the lymphoid compartment of SB28 tumors . 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 in SB28 tumors . CONCLUSIONS: Single-fraction SRS 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.