Oncolytic virus therapy is being evaluated in clinical trials for human glioma. While it is widely assumed that the patient’s immune response to the virus infection limits the therapy’s utility, investigations into the specific cell type(s) involved in this response have been performed using non-specific pharmacological inhibitors or allogeneic models with compromised immunity. To identify the immune cells that participate in clearing an oncolytic infection in glioma, we used flow cytometry and immunohistochemistry to immunophenotype an orthotopic glioma model in immunocompetent mice after Myxoma virus (MYXV) administration. These studies revealed a large resident microglia and macrophage population in untreated tumours, and robust monocyte, T and NK cell infiltration 3 days following MYXV infection. To determine the role on the clinical utility of MYXV therapy for glioma, we used a combination of knockout mouse strains and specific immunocyte ablation techniques. Collectively, our experiments identify an important role for tumour-resident myeloid cells and overlapping roles for recruited NK and T cells in the clearance and efficacy of oncolytic MYXV from gliomas. Using a cyclophosphamide regimen to achieve lymphoablation prior and during MYXV treatment, we prevented treatment-induced peripheral immunocyte recruitment and, surprisingly, largely ablated the tumour-resident macrophage population. Virotherapy of CPA-treated animals resulted in sustained viral infection within the glioma as well as a substantial survival advantage. This study demonstrates that resistance to MYXV virotherapy in syngeneic glioma models involves a multi-faceted cellular immune response that can be overcome with CPA-mediated lymphoablation.
