Radiation attenuates prostate tumor antiviral responses to vesicular stomatitis virus containing IFNβ, resulting in pronounced antitumor systemic immune responses

Vesicular stomatitis virus (VSV) expressing IFNb induces apoptosis in multiple tumor models while maintaining an excellent safety profile. VSV-IFNb is oncoselective due to permissive replication in cells with an altered IFN pathway. The human VSV-IFNb (hIFNb) vector is currently used in clinical trials as a standalone therapy; however, we hypothesized that oncolytic virotherapy might be more effective when used in combination with radiotherapy (RT). We investigated the synergistic effects of RT and VSV-hIFNb in the subcutaneous PC3 and orthotopic LNCaP prostate xenograft models and a syngeneic RM9 prostate tumor model. VSV-IFNb combined with RT amplified tumor killing for PC3 and LNCaP xenografts, and RM9 tumors. This was attributed to the induction of proapoptotic genes leading to increased VSV-IFNb infection and replication, VSV expression, and oncolysis. In the RM9 tumors, combination therapy resulted in a robust antitumor immune response. Treated RM9 tumor-bearing mice demonstrated an increase in CD8^þ and CD4^þ T-cell numbers, 100% resistance to tumor rechallenge, and reduced resistance to reimplantation challenge with CD8^þ knockdown. RT enhanced the activity of VSV-mediated oncolysis via attenuation of the innate antiviral response, resulting in increased VSV replication and the generation of an adaptive immune response earmarked by an increase in CD8^þ lymphocyte numbers and antitumor activity. Local tumor irradiation combined with VSV-IFNb affects tumor cell death through direct and systemic activity in conjunction with pronounced antitumor immunity.