Prime-boost using Separate Oncolytic Viruses in Combination with Checkpoint Blockade Improves Anti-tumor Therapy

The anti-tumor effects associated with oncolytic virus therapy are mediated significantly through immune-mediated mechanisms which depends both on the type of virus and the route of delivery. Here, we show that intra-tumoral (i.t.) oncolysis by Reovirus induced the priming of a CD8+, Th1-type anti-tumor response. In contrast, systemically delivered VSV expressing a cDNA library of melanoma antigens (VSV-ASMEL) promoted a potent anti-tumor CD4+ Th17 response. Therefore, we hypothesised that combining the Reovirus-induced CD8+ T cell response, with the VSV-ASMEL CD4+ Th17 helper response, would produce enhanced anti-tumor activity. Consistent with this, priming with i.t. Reovirus, followed by an intra-venous VSV-ASMEL Th17 boost, significantly improved survival of mice bearing established subcutaneous (s.c.) B16 melanoma tumors. We also show that combination of either therapy alone with anti-PD-1 immune checkpoint blockade augmented both the Th1 response induced by systemically delivered Reovirus in combination with GM-CSF, and also the Th17 response induced by VSV-ASMEL. Significantly, anti-PD-1 also uncovered an anti-tumor Th1 response following VSV-ASMEL treatment that was not seen in the absence of checkpoint blockade. Finally, the combination of all three treatments (priming with systemically delivered Reovirus, followed by double boosting with systemic VSV-ASMEL and anti-PD-1) significantly enhanced survival, with long-term cures, compared to any individual, or double, combination therapies, associated with strong Th1 and Th17 responses to tumor antigens. Our data show that it is possible to generate fully systemic, highly effective anti-tumor immunovirotherapy by combining oncolytic viruses, along with immune checkpoint blockade, to induce complimentary mechanisms of anti-tumor immune responses.