Investigating the impact of natural killer (NK) cell metabolism on NK cell effector function

It is now known that metabolism, in addition to providing energy and biochemical building blocks, also regulates immune cell function. Over the last few years, we have increased our understanding of how NK cell metabolism impacts on murine NK cell function; however, nothing was known about human NK cells. In this study, we defined the metabolic changes that occur in human peripheral blood NK cells in response to overnight stimulation with either innate cytokines, IL-12 and IL-15, or IL-2, a cytokine generally associated with adaptive T cell responses. We also investigated NK cell metabolic changes after a more prolonged exposure (5 day) to IL-2. Analysis showed that NK cells upregulated glycolysis and oxidative phosphorylation (OxPHOS) in response cytokine. Many of the metabolic changes observed, particularly in the 5 day cultures were dependent on mTORC1. We also defined metabolic differences in important NK cell subsets. CD56 bright cells were more metabolically active compared to CD56 dim cells as they preferentially upregulated nutrient receptors and differed substantially in terms of their glucose metabolism. Our data showed that elevated levels of OxPHOS were required to support both cytotoxicity and IFNg production in NK cells and while elevated glycolysis was not required directly for NK cell degranulation, limiting the rate of glycolysis significantly impaired IFNg production by the CD56 bright subset of cells. \nWe also investigated how NK cell metabolism might be down-regulated after activation in chronic inflammatory conditions. We demonstrated that transforming growth factor beta (TGF-β), known to inhibit IFNγ production by NK cells, inhibited cytokine induced (18h) increases in OxPHOS, maximum respiration and glycolytic capacity of human NK cells. TGF-β-mediated suppression of NK cell metabolism or function did not involve the inhibition of mTORC1 in these experiments, and inhibition of the canonical TGF-β signaling pathway was able to restore almost all metabolic and functional responses. Disruption of cytokine induced changes in NK cell metabolism by prolonged exposure to either TGF-β or rapamycin resulted in dysfunctional NK cells suggesting that impaired metabolic changes could contribute to NK cell dysfunction during chronic inflammation. Interestingly, although the canonical signalling pathway was still the most active, TGF-β could suppress mTORC1 activity in these longer cultures. \nOur data support that human NK cells undergo metabolic changes in response to cytokine and that mTORC1 is important for sustained reprogramming of NK cell metabolism. Metabolism is important for human NK cell functions and may be dysregulated in pathological situations. These findings will help in the development of new ways to manipulate NK cell function during inflammation, virus infection or cancer.