Radiotherapy Enhances the Oncolytic Efficacy of the Novel Oncolytic Herpesvirus VG161 and Amplifies Its Antitumor Immunity in Breast Cancer

VG161 is an oncolytic HSV‐1 with an ICP34.5 deletion and armed with multiple immunomodulatory factors. The absence of ICP34.5 restricts viral replication in neurons while conferring tumor specificity. However, this modification attenuates viral replication in tumor cells compared to wild‐type HSV‐1. Radiotherapy (RT) has been reported to promote viral replication and exert an immune‐priming effect. Based on this, we hypothesize that RT could potentiate both the VG161 replication and its antitumor efficacy in breast cancer (BC). Our findings suggest that the optimal regimen for combining VG161 with RT involves administering 5 Gy irradiation 6 h after VG161 infection, which ensures that RT maximally promotes VG161 replication in BC. This enhancement in VG161 replication is mediated by the upregulation of GADD34 and HVEM induced by RT. Moreover, RT augments the expression of immunostimulatory transgenes carried by VG161 and immunogenic cell death of BC cells. In vivo, VG161 combined with RT demonstrates superior antitumor efficacy compared with either monotherapy in BC. Mechanistic investigations reveal that this combination therapy increases the abundances of tumor‐infiltrating lymphocytes and elicits potent systemic antitumor immunity that inhibits local tumors and regresses abscopal metastases. The observed synergistic effect of VG161 and RT encourages further clinical translation.