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Immunotherapy with polio:rhinovirus recombinant (PVSRIPO) has shown evidence of efficacy in a phase I clinical trial for recurrent GBM, resulting in durable radiographic responses and 21% long-term survival at 36 months. Ongoing research aims to enhance the clinical response rate by resolving the mechanisms of action and therapy resistance in vivo, thereby devising more effective therapies. Mouse glioma (CT2A) cells were intracranially implanted (day 0) in transgenic mice carrying poliovirus receptor CD155, and treated with intratumor PVSRIPO (5×105 pfu; day 6) to dissect early and late events following therapy. A blinded pathological review of 45 post-treatment tumors was performed. On day 8, a histological response, featured by tumor dissociation and shrinkage, with inflammation and microglia enrichment in the treated hemisphere, was common in PVSRIPO group (6/7) compared to controls (0/4). However, the response rate fell over time (7/12 on day 12; 1/7 on day 15) and the therapy was overcome by aggressive tumor regrowth. RNAseq was performed and Gene Set Enrichment Analysis of the tumor microenvironment revealed an acute type-I interferon (IFN)-related inflammation, correlating with the histological findings of profound proinflammatory engagement of microglia (Iba1+) widespread in the treated hemisphere. Microglia proliferation (Ki67+) was observed in the treated hemisphere, likely resulting from PVSRIPO infection, in CT2A and B16 intracranial models. This suggests an association of adaptive antitumor immunity—elicited by immediate intratumor type-I IFN-dominant inflammation—with tumor regression. Thus, buttressing type-I IFN directed antitumor CD8+T cell immunity, e.g. with blockade of the PD1:PD-L1 immune checkpoint, might contribute to tumor remission. Indeed, combination therapy with αPD-L1 antibody in the CT2A model showed longer median survival and higher long-term remission rate compared to monotherapy alone; CD8 T cell depletion can completely abrogate this efficacy with this therapy combination, confirming the role of anti-tumor immunity in this approach.

EXTH-77. POLIO VIROTHERAPY OF MURINE BRAIN TUMORS INDUCES MICROGLIA PROLIFERATION AND INFLAMMATION THAT IS POTENTIATED BY IMMUNE CHECKPOINT BLOCKADE