Inhibition of PD-L1/PD-1 Checkpoint Increases NK Cell-Mediated Killing of Melanoma Cells in the Presence of Interferon-Beta

<b>Background/Objectives:</b> Melanoma is a highly malignant skin tumor with poor response to conventional chemotherapeutic regimens. Melanoma cells induce cytotoxic T cell-mediated immune responses, and immunotherapy has significantly improved survival rates for patients with advanced disease. <b>Methods</b>: Here, we investigate NK cell-mediated melanoma cell killing and its regulation by PD-L1/PD-1 blockade and IFNβ. Four melanoma cell lines were used in this study. To evaluate NK cell cytotoxicity, cells were exposed to NK cells with or without IFNβ. The calcein release assay was used to measure cell death, while specific inhibitors and siRNA silencing were applied to determine the contribution of individual effector pathways. <b>Results</b>: NK cells were able to kill melanoma cells with sensitivity to killing varying between different cell lines. Cytotoxic effects were mainly mediated through activation of the TRAIL signaling cascade. In cell lines with low sensitivity to NK cell killing, expression of PD-L1 was noted and killing by NK cells could be significantly increased by inhibition of the PD-L1/PD-1 checkpoint. Killing of melanoma cells could be further increased by incubation of NK cells with IFNβ. <b>Conclusions</b>: Our results point to a role of NK cells in the killing of melanoma cells and a potential clinical benefit of a combination therapy of IFNβ and anti-PD-1 antibody.