The Combination of Immunosuppression and Carrier Cells Significantly Enhances the Efficacy of Oncolytic Poxvirus in the Pre-Immunized Host

Pre-existing anti-poxvirus immunity in cancer patients presents a severe barrier to poxvirus-mediated oncolytic virotherapy. We have explored strategies of immunosuppression (IS) and/or immune evasion for efficient delivery of an oncolytic vaccinia virus (vvDD) to tumors in the pre-immunized mice. Transient IS using immunosuppressive drugs, including tacrolimus, mycophenolate mofetil and methylprednisolone sodium succinate, have been used successfully in organ transplantation. This drug cocktail alone did not enhance viral recovery from subcutaneous tumor after systemic viral delivery. Using B cell knockout mice, we confirmed that the neutralizing antibodies played a significant role in preventing poxvirus infection. Using a MC38 peritoneal carcinomatosis (PC) model, we found that the combination of IS and tumor cells as carriers led to the most effective viral delivery, viral replication and viral spread inside the tumor mass. We found that our immunosuppressive drug cocktail facilitated recruitment of tumor-associated macrophages and conversion into an immunosuppressive M2 phenotype (IL-10(hi)/IL-12(low)) in the tumor microenvironment. A combination of IS and carrier cells led to significantly prolonged survival in the tumor model. These results demonstrated the feasibility of treating pre-vaccinated patients with peritoneal carcinomatosis using an oncolytic poxvirus and a combined immune intervention strategy.