Polio Virotherapy Targets the Malignant Glioma Myeloid Infiltrate with Diffuse Microglia Activation Engulfing the CNS.

BACKGROUND: Malignant gliomas commandeer dense inflammatory infiltrates with glioma-associated macrophages and microglia (GAMM) promoting immune suppression, -evasion, and tumor progression. Like all cells in the mononuclear phagocytic system, GAMM constitutively express the poliovirus receptor, CD155. Besides myeloid cells, CD155 is widely upregulated in the neoplastic compartment of malignant gliomas. Intratumor treatment with the highly attenuated rhino:poliovirus chimera, PVSRIPO, yielded long-term survival with durable radiographic responses in patients with recurrent glioblastoma (Desjardins et al. New England Journal of Medicine, 2018). This scenario raises questions about the contributions of myeloid vs. neoplastic cells to polio virotherapy of malignant gliomas.

METHODS: We investigated PVSRIPO immunotherapy in immunocompetent mouse brain tumor models with blinded, board-certified neuropathologist review, a range of neuropathological, immunohistochemical and immunofluorescence analyses, and RNAseq of the tumor region.

RESULTS: PVSRIPO treatment caused intense engagement of the GAMM infiltrate associated with substantial, but transient tumor regression. This was accompanied by marked microglia activation and proliferation in normal brain surrounding the tumor, in the ipsilateral hemisphere and extending into the contralateral hemisphere. There was no evidence for lytic infection of malignant cells. PVSRIPO-instigated 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 durable remissions.

CONCLUSIONS: Our work implicates GAMM as active drivers of PVSRIPO-induced antitumor inflammation and reveals profound and widespread neuroinflammatory activation of the brain-resident myeloid compartment by PVSRIPO.