Zika virus oncolytic activity requires CD8+ T cells and is boosted by immune checkpoint blockade.

Glioblastoma multiforme (GBM) is a fatal human cancer in part because GBM stem cells are resistant to therapy and recurrence is inevitable. Previously, we demonstrated Zika virus (ZIKV) targets GBM stem cells and prevents death of mice with gliomas. Here, we evaluated the immunological basis of ZIKV-mediated protection against GBM. Introduction of ZIKV into the brain tumor increased recruitment of CD8+ T and myeloid cells to the tumor microenvironment. CD8+ T cells were required for ZIKV-dependent tumor clearance because survival benefits were lost with CD8+ T cell depletion. Moreover, while anti-PD-1 antibody monotherapy moderately improved tumor survival, when coadministered with ZIKV, survival increased. ZIKV-mediated tumor clearance also resulted in durable protection against syngeneic tumor rechallenge, which also depended on CD8+ T cells. To address safety concerns, we generated an immune-sensitized ZIKV strain, which was effective alone or in combination with immunotherapy. Thus, oncolytic ZIKV treatment can be leveraged by immunotherapies, which may prompt combination treatment paradigms for adult patients with GBM.