Oncolytic herpes simplex viruses (oHSVs) have demonstrated efficient lytic replication in human glioblastoma tumors using immunodeficient mouse models, but early-phase clinical trials have reported few complete responses. Potential reasons for the lack of efficacy are limited vector potency and the suppressive glioma tumor microenvironment (TME). Here we compare the oncolytic activity of two HSV-1 vectors, a KOS-strain derivative KG4:T124 and an F-strain derivative rQNestin34.5v.1, in the CT2A and GL261N4 murine syngeneic glioma models. rQNestin34.5v1 generally demonstrated a greater in vivo viral burden compared to KG4:T124. However, both vectors were rapidly cleared from CT2A tumors, while virus remained ensconced in GL261N4 tumors. Immunological evaluation revealed that the two vectors induced similar changes in immune cell recruitment to either tumor type at 2 days after infection. However, at 7 days after infection, the CT2A microenvironment displayed the phenotype of an untreated tumor, while GL261N4 tumors exhibited macrophage and CD4+/CD8+ T cell accumulation. Furthermore, the CT2A model was completely resistant to virus therapy, while in the GL261N4 model rQNestin34.5v1 treatment resulted in enhanced macrophage recruitment, impaired tumor progression, and long-term survival of a few animals. We conclude that prolonged intratumoral viral presence correlates with immune cell recruitment, and both are needed to enhance anti-tumor immunity.
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