Surface density of CD27 and CD11b subdivides mouse natural killer (NK) cells into 4 subsets: CD11b^lowCD27^low, CD11b^lowCD27^high, CD11b^highCD27^high, and CD11b^highCD27^low. To determine the developmental relationship between these 4 subsets, we used several complementary approaches. First, we took advantage of NDE transgenic mice that express enhanced green fluorescent protein (EGFP) and diphtheria toxin receptor specifically in NK cells. Diphtheria toxin injection leads to a transient depletion of NK cells, allowing the monitoring of the phenotype of developing EGFP⁺ NK cells after diphtheria toxin injection. Second, we evaluated the overall proximity between NK-cell subsets based on their global gene profile. Third, we compared the proliferative capacity of NK-cell subsets at steady state or during replenishment of the NK-cell pool. Fourth, we performed adoptive transfers of EGFP⁺ NK cell subsets from NDE mice into unirradiated mice and followed the fate of transferred cells. The results of these various experiments collectively support a 4-stage model of NK-cell maturation CD11b^lowCD27^low → CD11b^lowCD27^high → CD11b^highCD27^high → CD11b^highCD27^low. This developmental program appears to be associated with a progressive acquisition of NK-cell effector functions.

Blocking TGF-β signaling in natural killer cells enhances their metabolism and ability to kill tumor cells.

The kinase mTOR is important in lymphocyte bioenergetics but has not been examined in natural killer cells. Walzer and colleagues demonstrate that mTOR is key to sustaining the proliferation and effector function of these cells. Interleukin 15 (IL-15) controls both the homeostasis and the peripheral activation of natural killer (NK) cells. The molecular basis for this duality of action remains unknown. Here we found that the metabolic checkpoint kinase mTOR was activated and boosted bioenergetic metabolism after exposure of NK cells to high concentrations of IL-15, whereas low doses of IL-15 triggered only phosphorylation of the transcription factor STAT5. mTOR stimulated the growth and nutrient uptake of NK cells and positively fed back on the receptor for IL-15. This process was essential for sustaining NK cell proliferation during development and the acquisition of cytolytic potential during inflammation or viral infection. The mTORC1 inhibitor rapamycin inhibited NK cell cytotoxicity both in mice and humans; this probably contributes to the immunosuppressive activity of this drug in different clinical settings.

Several recent publications have focused on the newly described interactions between natural-killer (NK) cells and dendritic cells (DCs). Activated NK cells induce DC maturation either directly or in synergy with suboptimal levels of microbial signals. Immature DCs appear susceptible to autologous NK-cell-mediated cytolysis while mature DCs are protected. NK-cell-induced DC activation is dependent on both tumor necrosis factor-α (TNF-α)/interferon-γ (IFN-γ) secretion and a cell-cell contact involving NKp30. In vitro, interleukin-12 (IL-12)/IL-18, IL-15, and IFN-α/β production by activated DCs enhance, in turn, NK-cell IFN-γ production, proliferation, and cytotoxic potential, respectively. In vivo, NK-cell/DC interactions may occur in lymphoid organs as well as in nonlymphoid tissues, and their consequences are multiple. By inducing DC activation, NK-cell activation induced by tumor cells can indirectly promote antitumoral T-cell responses. Reciprocally, DCs activated through Toll-like receptors (TLRs) induce potent NK-cell activation in antiviral responses. Thus, DCs and NK cells are equipped with complementary sets of receptors that allow the recognition of various pathogenic agents, emphasizing the role of NK-cell/DC crosstalk in the coordination of innate and adaptive immune responses.