Activation-Specific Metabolic Requirements

There has been increasing recognition of the importance of cellular metabolism and metabolic substrates for the function and differentiation of immune cells. In this study, for the first time to our knowledge, we investigate the metabolic requirements for production of IFN-g by freshly isolated NK cells. Primary murine NK cells mainly use mitochondrial oxidative phosphory-lation at rest and with short-term activation. Remarkably, we discovered significant differences in the metabolic requirements of murine NK cell IFN-g production depending upon the activation signal. Stimulation of NK cell IFN-g production was independent of glycolysis or mitochondrial oxidative phosphorylation when cells were activated with IL-12 plus IL-18. By contrast, stimulation via activating NK receptors required glucose-driven oxidative phosphorylation. Prolonged treatment with high-dose, but not low-dose, IL-15 eliminated the metabolic requirement for receptor stimulation. In summary, this study demonstrates that metabolism provides an essential second signal for induction of IFN-g production by activating NK cell receptors that can be reversed with prolonged high-dose IL-15 treatment. N atural killer cells are innate immune lymphocytes that provide a first line of defense against infection, particularly viruses, and can recognize and kill tumor cells that have downregulated self-MHC or express activating ligands (1–3). NK cell effector functions can be triggered by inflammatory cyto-kines, such as IL-12, IL-15, and IL-18, or by engaging germline-encoded activating NK receptors whose ligands are displayed by infected and/or tumor cells (3–5). In response, NK cells produce inflammatory cytokines, principally IFN-g, and kill target cells. Although many triggers of NK cell activation and subsequent NK cell effector responses have been well characterized, the metabolic fuels required to drive NK cell functional responses are largely unknown. Metabolism is the biochemical process used by cells to break down fuels for energy production (i.e., ATP) or to generate critical biomolecules. There are two primary and overlapping metabolic pathways for generating ATP from metabolic fuels, anaerobic gly-colysis and mitochondrial oxidative phosphorylation (OXPHOS) (6). Glucose fuels both pathways and is first metabolized in the cytoplasm via glycolysis to produce two molecules of pyruvate, for a net yield of two ATP molecules. Pyruvate can then be converted to lactate (anaerobic metabolism) or, in the presence of oxygen, transported into the mitochondria to fuel OXPHOS for a net yield of 30 + ATP molecules. Other metabolic substrates that can fuel mi-tochondrial OXPHOS include fatty acids (fatty acid oxidation) and glutamine (glutaminolysis). Metabolic pathways, including glycol-ysis, are attractive pharmacologic targets for disease …