Local Delivery of OncoVEX mGM-CSF Generates Systemic Antitumor Immune Responses Enhanced by Cytotoxic T-Lymphocyte-Associated Protein Blockade.

Purpose: 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. Experimental Design: The activity of talimogene laherparepvec was characterized against melanoma cell lines using an in vitro 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. Results: 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. Ex vivo 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. Conclusions: 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. (C)2017 AACR.