Diffuse Microglia Inflammation is Associated with Anti-Tumor Efficacy in Polio Virotherapy for Glioblastoma

INTRODUCTION: Glioblastomas (GBM) harbor abundant inflammatory infiltrates with glioma-associated macrophages and microglia (GAMM) actively promoting tumor progression. Intratumor virotherapy with the highly attenuated rhino:poliovirus chimera, PVSRIPO, causes sublethal infection and activation of GAMM via the constitutively expressed human poliovirus receptor, CD155. METHODS: We optimized immune-competent CT2A murine glioblastoma models for blinded neuropathological analysis of therapy responses. Samples (n = 55) were collected in a time series after PVSRIPO or mock intratumor infusion for systemic review of histological features related to therapy. Immune cells were defined both by morphology and immunohistochemical (IHC) staining patterns. Multiplex IHC and RNA sequencing were performed to study the immune infiltrate and PD1/PD-L1 immune checkpoint expression. The role of the PD1:PD-L1 axis in restricting the antitumor efficacy of PVSRIPO was tested using survival studies. RESULTS: PVSRIPO treatment caused substantial, but transient tumor regression with intense engagement of the GAMM infiltrate. This was accompanied by diffuse microglia activation and inflammtion in the CNS surrounding the tumor, extending to the ipsilateral and even the contralateral hemispheres. PVSRIPO-instigated pervasive microglia activation occurred against a backdrop of sustained innate antiviral inflammation, associated with induction of the PD-L1 immune checkpoint on GAMM. Combining PVSRIPO with PD1/PD-L1 blockade led to more durable remissions in CT2A model. CONCLUSIONS: Our work implicates GAMM as active drivers of PVSRIPO induced antitumor inflammation and reveals profound and widespread neuroinflammatory activation of the CNS-resident myeloid compartment by PVSRIPO, which is closely associated with tumor regression.