Transient inhibition of neutrophil functions enhances the antitumor effect of intravenously delivered oncolytic vaccinia virus

Oncolytic viruses (OVs) possess the unique ability to selectively replicate within tumor cells, leading to their destruction, while also reversing the immunosuppression within the tumor microenvironment and triggering an antitumor immune response. As a result, OVs have emerged as one of the most promising approaches in cancer therapy. However, the effective delivery of intravenously administered OVs faces significant challenges imposed by various immune cells within the peripheral blood, hindering their access to tumor sites. Notably, neutrophils, the predominant white blood cell population comprising approximately 50%-70% of circulating white cells in humans, show phagocytic properties. Our investigation revealed that the majority of oncolytic vaccinia viruses (VV) are engulfed and degraded by neutrophils in the bloodstream. The depletion of neutrophils using the anti-LY6G Ab (1-A8) resulted in an increased accumulation of circulating oncolytic VV in the peripheral blood and enhanced deposition at the tumor site, consequently amplifying the antitumor effect. Neutrophils heavily rely on PI3K signaling to sustain their phagocytic process. Additionally, our study determined that the inhibition of the PI3Kinase delta isoform by idelalisib (CAL-101) suppressed the uptake of oncolytic VV by neutrophils. This inhibition led to a greater presence of oncolytic VV in both the peripheral blood and at the tumor site, resulting in improved efficacy against the tumor. In conclusion, our study showed that inhibiting neutrophil functions can significantly enhance the antitumor efficacy of intravenous oncolytic VV. Neutrophils present in the peripheral blood play a crucial role in the uptake and degradation of oncolytic vaccinia viruses (VV) following intravenous administration. This process leads to a reduction in the circulating levels of oncolytic VV, ultimately impacting the availability of the virus to penetrate tumor tissues and exert its antitumor effects. Our research has determined that the depletion of neutrophils through Ab-mediated elimination or the inhibition of neutrophil activity using CAL-101, a specific inhibitor targeting PI3Kinase delta isoform, results in decreased uptake of oncolytic VV by neutrophils. Consequently, this leads to an increase in the viral load within peripheral plasma and tumor tissues, thereby enhancing the antitumor effect of intravenously administered oncolytic VV.image