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Activated monocytes augment TRAIL‐mediated cytotoxicity by human NK cells through release of IFN‐γ

Natural killer (NK) cells are innate lymphocytes that are able to directly kill tumor cells through different mechanisms including ligation of TNF‐related apoptosis‐inducing ligand (TRAIL) receptors. Zoledronic acid (ZA) is a bisphosphonate known to upregulate the expression of TRAIL on human γδ T cells. Here, we investigated whether exposure to ZA would upregulate TRAIL expression on human NK cells and augment their cytotoxicity against tumor cells. When cocultured with monocytes, treatment with ZA and IL‐2 resulted in a significant upregulation of TRAIL expression on human NK cells (p = 0.002). Consequently, ZA‐primed NK cells were significantly more cytotoxic against TRAIL sensitive tumor cells (p < 0.0001). In the presence of ZA and IL‐2, monocytes produced high levels of IFN‐γ; when cultured in the presence of neutralizing antibodies to IFN‐γ, TRAIL expression and TRAIL‐mediated cytotoxicity of NK cells were significantly reduced. Furthermore, in tumor‐bearing SCID/Beige mice, a significant delayed tumor progression and prolonged survival was observed after infusion of ZA‐primed NK cells compared with that observed in mice infused with unprimed NK cells. These findings represent a novel approach to potentiate TRAIL‐mediated apoptosis by adoptively infused NK cells that could improve the outcome in patients with cancer.