Local Delivery of OncoVEXmGM-CSF Generates Systemic Antitumor Immune Responses Enhanced by Cytotoxic T-Lymphocyte–Associated Protein Blockade

<b>Purpose:</b> Talimogene laherparepvec, a new oncolytic immunotherapy, has been recently approved for the treatment of melanoma. Using a murine version of the virus, we characterized local and systemic antitumor immune responses driving efficacy in murine syngeneic models.<b>Experimental Design:</b> The activity of talimogene laherparepvec was characterized against melanoma cell lines using an <i>in vitro</i> viability assay. Efficacy of OncoVEX^mGM-CSF (talimogene laherparepvec with the mouse granulocyte-macrophage colony-stimulating factor transgene) alone or in combination with checkpoint blockade was characterized in A20 and CT-26 contralateral murine tumor models. CD8⁺ depletion, adoptive T-cell transfers, and Enzyme-Linked ImmunoSpot assays were used to study the mechanism of action (MOA) of systemic immune responses.<b>Results:</b> Treatment with OncoVEX^mGM-CSF cured all injected A20 tumors and half of contralateral tumors. Viral presence was limited to injected tumors and was not responsible for systemic efficacy. A significant increase in T cells (CD3⁺/CD8⁺) was observed in injected and contralateral tumors at 168 hours. <i>Ex vivo</i> analyses showed these cytotoxic T lymphocytes were tumor-specific. Increased neutrophils, monocytes, and chemokines were observed in injected tumors only. Importantly, depletion of CD8⁺ T cells abolished all systemic efficacy and significantly decreased local efficacy. In addition, immune cell transfer from OncoVEX^mGM-CSF-cured mice significantly protected from tumor challenge. Finally, combination of OncoVEX^mGM-CSF and checkpoint blockade resulted in increased tumor-specific CD8⁺ anti-AH1 T cells and systemic efficacy.<b>Conclusions:</b> The data support a dual MOA for OncoVEX^mGM-CSF that involves direct oncolysis of injected tumors and activation of a CD8⁺-dependent systemic response that clears injected and contralateral tumors when combined with checkpoint inhibition. <i>Clin Cancer Res; 23(20); 6190-202. ©2017 AACR</i>.