Cellular metabolism and signaling pathways are key regulators to determine conventional T cell fate and function, but little is understood about the role of cell metabolism for natural killer T (NKT) cell survival, proliferation, and function. We found that NKT cells operate distinct metabolic programming from CD4 T cells. NKT cells are less efficient in glucose uptake than CD4 T cells with or without activation. Gene-expression data revealed that, in NKT cells, glucose is preferentially metabolized by the pentose phosphate pathway and mitochondria, as opposed to being converted into lactate. In fact, glucose is essential for the effector functions of NKT cells and a high lactate environment is detrimental for NKT cell survival and proliferation. Increased glucose uptake and IFN-γ expression in NKT cells is inversely correlated with bacterial loads in response to bacterial infection, further supporting the significance of glucose metabolism for NKT cell function. We also found that promyelocytic leukemia zinc finger seemed to play a role in regulating NKT cells’ glucose metabolism. Overall, our study reveals that NKT cells use distinct arms of glucose metabolism for their survival and function.

Reactive oxygen species (ROS) are byproducts of aerobic metabolism and contribute to both physiological and pathological conditions as second messengers. ROS are essential for activation of T cells, but how ROS influence NKT cells is unknown. In the current study, we investigated the role of ROS in NKT cell function. We found that NKT cells, but not CD4 nor CD8 T cells, have dramatically high ROS in the spleen and liver of mice but not in thymus or adipose tissues. Accordingly, ROS-high NKT cells exhibited increased susceptibility and apoptotic cell death with oxidative stress. High ROS in the peripheral NKT cells were primarily produced by NADPH oxidases and not mitochondria. We observed that sorted ROS-high NKT cells were enriched in NKT1 and NKT17 cells, while NKT2 cells were dominant in ROS-low cells. Furthermore, treatment of NKT cells with antioxidants lead to reduced frequencies of IFN-γ– and IL-17-expressing cells, indicating that ROS play a role in regulating the inflammatory function of NKT cells. The transcription factor promyelocytic leukemia zinc finger (PLZF) seemed to control the ROS levels. NKT cells from adipose tissues that do not express PLZF and those from PLZF haplo-deficient mice have low ROS. Conversely, ROS were highly elevated in CD4 T cells from mice ectopically expressing PLZF. Thus our findings demonstrate that PLZF controls ROS levels, which in turn governs the inflammatory function of NKT cells.

Iron has long been established as a critical mediator of T cell development and proliferation. However, the mechanisms by which iron controls CD4 T cell activation and expansion remain poorly understood. In this study, we show that stimulation of CD4 T cells from C57BL/6 mice not only decreases total and labile iron levels but also leads to changes in the expression of iron homeostatic machinery. Additionally, restraining iron availability in vitro severely inhibited CD4 T cell proliferation and cell cycle progression. Although modulating cellular iron levels increased IL-2 production by activated T lymphocytes, CD25 expression and pSTAT5 levels were decreased, indicating that iron is necessary for IL-2R–mediated signaling. We also found that iron deprivation during T cell stimulation negatively impacts mitochondrial function, which can be reversed by iron supplementation. In all, we show that iron contributes to activation-induced T cell expansion by positively regulating IL-2R signaling and mitochondrial function.

SUMMARY Kelch-like ECH-associated protein 1 (Keap1) and nuclear factor (erythroid-derived 2)-like 2 (Nrf2) proteins work in concert to regulate the levels of reactive oxygen species (ROS). The Keap1-Nrf2 antioxidant system also participates in T cell differentiation and inflammation, but its role in innate T cell development and functions remains unclear. We report that T cell-specific deletion of Keap1 results in defective development and reduced numbers of invariant natural killer T (NKT) cells in the thymus and the peripheral organs in a cell-intrinsic manner. The frequency of NKT2 and NKT17 cells increases while NKT1 decreases in these mice. Keap1-deficient NKT cells show increased rates of proliferation and apoptosis, as well as increased glucose uptake and mitochondrial function, but reduced ROS, CD122, and Bcl2 expression. In NKT cells deficient in Nrf2 and Keap1, all these phenotypic and metabolic defects are corrected. Thus, the Keap1-Nrf2 system contributes to NKT cell development and homeostasis by regulating cell metabolism.

Cellular metabolism is critical for generating energy and macromolecules for cell growth and survival. In recent years, the importance of metabolism in mediating T cell differentiation, proliferation, and function has been a hot topic of investigation. However, very little is known about metabolic regulation in invariant natural killer T (iNKT) cells. In this viewpoint, we will discuss what is currently known about immunometabolism in iNKT cells and how these findings relate to CD4 T cells.