A Type I IFN-Inducing Oncolytic Virus Improves NK Cell-Mediated Killing of Tumor Cells In Vitro Through Multiple Mechanisms.

Natural killer (NK) cell adoptive immunotherapy is a promising therapeutic approach in which NK cells perform targeted lysis of tumor cells. Oncolytic viruses are also effective cancer therapeutic agents due to their ability to selectively target and kill tumor cells. Combination therapies that integrate NK cells and oncolytic viruses have been shown to enhance tumor killing compared to individual treatment strategies alone. Using in vitro expanded human NK cells (PM21-NK cells), we tested the relative ability of tumor cells infected with WT parainfluenza virus 5 (PIV5), which is a poor inducer of type 1 interferon (IFN-I), versus PIV5 P/V gene mutant, which is a strong inducer of IFN-I synthesis, to modulate NK cell activities. Both WT and P/V mutant viruses were capable of infecting PM21-NK cells and caused extensive cytopathic effects. Co-culturing of PM21-NK cells with virus-infected tumor cells resulted in spread of WT PIV5 to naïve NK cells, but NK cells were protected from spread of the P/V mutant virus by IFN-I induction. Direct treatment of PM21-NK cells with IFN-I or media from P/V-virus-infected tumor cells enhanced NK cell cytotoxicity, at least in part due to upregulation of the death ligand, TRAIL. IFN-I-treated PM21-NK cells also showed a decrease in IFN-γ secretion, a cytokine we have previously shown to reduce PM21-NK cell tumor killing. Our results highlight multiple mechanisms by which an IFN-I-inducing oncolytic virus can enhance NK-cell-mediated killing of target virus-infected and uninfected tumor cells.