Overcoming Tumor Resistance to Oncolyticvaccinia Virus with Anti-PD-1-Based Combination Therapy by Inducing Antitumor Immunity in the Tumor Microenvironment

The tumor microenvironment (TME) comprises different types of immune cells, which limit the therapeutic efficacy of most drugs. Although oncolytic virotherapy (OVT) boosts antitumor immunity via enhanced infiltration of tumor-infiltrated lymphocytes (TILs), immune checkpoints on the surface of tumors and TILs protect tumor cells from TIL recognition and apoptosis. OVT and immune checkpoint blockade (ICB)-based combination therapy might overcome this issue. Therefore, combination immunotherapies to modify the immunosuppressive nature of TME and block immune checkpoints of immune cells and tumors are considered. In this study, cancer-favoring oncolytic vaccinia virus (CVV) and anti-programmed cell death protein-1 (anti-PD-1) were used to treat mouse colorectal cancer. Weekly-based intratumoral CVV and intraperitoneal anti-PD-1 injections were performed on Balb/c mice with subcutaneous CT26 tumors. Tumor volume, survival curve, and immunohistochemistry-based analysis demonstrated the benefit of co-treatment, especially simultaneous treatment with CVV and anti-PD-1. Infiltration of CD8(+)PD-1(+)T-cells showed correlation with these results. Splenocytes enumeration also suggested CD4(+)and CD8(+)T-cell upregulation. In addition, upregulated CD8, PD-1, and CD86 messenger RNA expression was observed in this combination therapy. Therefore, CVV+anti-PD-1 combination therapy induces antitumor immunity in the TME, overcoming the rigidity and resistance of the TME in refractory cancers.