Sébastien Viel

T-bet and Eomes instruct the development of two distinct natural killer cell lineages in the liver and in the bone marrow

Mutually exclusive expression of T-bet and Eomes drives the development of distinct NK cell lineages with complementary functions.

The metabolic checkpoint kinase mTOR is essential for IL-15 signaling during the development and activation of NK 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.

TGF-β inhibits the activation and functions of NK cells by repressing the mTOR pathway

Blocking TGF-β signaling in natural killer cells enhances their metabolism and ability to kill tumor cells. Relieving NK cell suppression The immunosuppressive cytokine transforming growth factor–β (TGF-β) has beneficial effects when it resolves inflammation and prevents autoimmunity, but not when it inhibits antitumor immune responses. Viel et al. found that TGF-β signaling in mouse and human natural killer (NK) cells, cytotoxic cells that target tumor cells, inhibited the activation of the kinase mTOR, a central regulator of cellular metabolism and cytotoxic function. NK cells deficient in a TGF-β receptor subunit showed enhanced antitumor activity in mouse models of metastasis, suggesting that enhancing metabolism in NK cells may provide a therapeutic strategy to kill cancer cells. Transforming growth factor–β (TGF-β) is a major immunosuppressive cytokine that maintains immune homeostasis and prevents autoimmunity through its antiproliferative and anti-inflammatory properties in various immune cell types. We provide genetic, pharmacologic, and biochemical evidence that a critical target of TGF-β signaling in mouse and human natural killer (NK) cells is the serine and threonine kinase mTOR (mammalian target of rapamycin). Treatment of mouse or human NK cells with TGF-β in vitro blocked interleukin-15 (IL-15)–induced activation of mTOR. TGF-β and the mTOR inhibitor rapamycin both reduced the metabolic activity and proliferation of NK cells and reduced the abundances of various NK cell receptors and the cytotoxic activity of NK cells. In vivo, constitutive TGF-β signaling or depletion of mTOR arrested NK cell development, whereas deletion of the TGF-β receptor subunit TGF-βRII enhanced mTOR activity and the cytotoxic activity of the NK cells in response to IL-15. Suppression of TGF-β signaling in NK cells did not affect either NK cell development or homeostasis; however, it enhanced the ability of NK cells to limit metastases in two different tumor models in mice. Together, these results suggest that the kinase mTOR is a crucial signaling integrator of pro- and anti-inflammatory cytokines in NK cells. Moreover, we propose that boosting the metabolic activity of antitumor lymphocytes could be an effective strategy to promote immune-mediated tumor suppression.

Regulation of mouse NK cell development and function by cytokines

Natural Killer (NK) cells are innate lymphocytes with an important role in the early defense against intracellular pathogens and against tumors. Like other immune cells, almost every aspects of their biology are regulated by cytokines. Interleukin (IL)-15 is pivotal for their development, homeostasis, and activation. Moreover, numerous other activating or inhibitory cytokines such as IL-2, IL-4, IL-7, IL-10, IL-12, IL-18, IL-21, Transforming growth factor-β (TGFβ) and type I interferons regulate their activation and their effector functions at different stages of the immune response. In this review we summarize the current understanding on the effect of these different cytokines on NK cell development, homeostasis, and functions during steady-state or upon infection by different pathogens. We try to delineate the cellular sources of these cytokines, the intracellular pathways they trigger and the transcription factors they regulate. We describe the known synergies or antagonisms between different cytokines and highlight outstanding questions in this field of investigation. Finally, we discuss how a better knowledge of cytokine action on NK cells could help improve strategies to manipulate NK cells in different clinical situations.

Terminal NK cell maturation is controlled by concerted actions of T-bet and Zeb2 and is essential for melanoma rejection.

The transcription factor Zeb2 cooperates with T-bet to control NK cell maturation, viability, and exit from the bone marrow and is essential for rejection of melanoma lung metastasis.

Monitoring NK cell activity in patients with hematological malignancies

Natural killer (NK) cells are lymphocytes of the innate immune system that can recognize and kill various types of malignant cells. Monitoring the activity of peripheral NK cells in patients affected by hematological malignancies may provide prognostic information or unveil ongoing tumor-specific immune responses. Moreover, further insights into the biology of NK cells might also promote the development of novel strategies for stimulating their anticancer activity. Here, we review the main methods to monitor phenotypic and functional NK cell properties in cancer patients, focusing on individuals affected by multiple myeloma, a hematological malignancy currently treated with immunomodulatory drugs.

Alteration of Natural Killer cell phenotype and function in obese individuals.

Obesity is associated with increased cancer rates and higher susceptibility to infections. The adipose tissue of obese individuals is inflammatory and may negatively impact on innate and adaptive immunity in a systemic way. Here, we explored the phenotype and function of peripheral Natural Killer (NK) cells of patients in correlation with their body mass index (BMI). We found that high BMI was associated with an increased activation status of peripheral NK cells, as measured by surface levels of CD69 and levels of granzyme-B. However, these activated NK cells had an impaired capacity to degranulate or to produce cytokines/chemokines when exposed to tumor cell lines deficient in MHC-I expression or coated with antibodies. This suggests that chronic stimulation of NK cells during obesity may lead to their incapacity to respond normally and eliminate target cells, which could contribute to the greater susceptibility of obese individuals to develop cancers or infectious diseases.

T-bet and Eomes govern differentiation and function of mouse and human NK cells and ILC1

T‐bet and Eomes are T‐box transcription factors that drive the differentiation and function of cytotoxic lymphocytes such as NK cells. Their DNA‐binding domains are highly similar, suggesting redundant transcriptional activity. However, while these transcription factors have different patterns of expression, the phenotype of loss‐of‐function mouse models suggests that they play distinct roles in the development of NK cells and other innate lymphoid cells (ILCs). Recent technological advances using reporter mice and conditional knockouts were fundamental in defining the regulation and function of these factors at steady state and during pathological conditions such as various types of cancer or infection. Here, we review these recent developments, focusing on NK cells as prototypical cytotoxic lymphocytes and their development, and also discuss parallels between NK cells and T cells. We also examine the role of T‐bet and Eomes in human NK cells and ILC1s. Considering divergent findings on mouse and human ILC1s, we propose that NK cells are defined by coexpression of T‐bet and Eomes, while ILC1s express only one of these factors, either T‐bet or Eomes, depending on the tissue or the species.

Back to the drawing board: Understanding the complexity of hepatic innate lymphoid cells

Recent studies of immune populations in nonlymphoid organs have highlighted the great diversity of the innate lymphoid system. It has also become apparent that mouse and human innate lymphoid cells (ILCs) have distinct phenotypes and properties. In this issue of the European Journal of Immunology, Harmon et al. [Eur. J. Immunol. 2016. 46: 2111–2120] characterized human hepatic NK‐cell subsets. The authors report that hepatic CD56bright NK cells resemble mouse liver ILC1s in that they express CXCR6 and have an immature phenotype. However, unlike mouse ILC1s, they express high levels of Eomes and low levels of T‐bet, and upon stimulation with tumor cells, secrete low amounts of cytokines. These unexpected findings further support the differences between human and mouse immune populations and prompt the study of the role of hepatic ILC subsets in immune responses.

High mTOR activity is a hallmark of reactive natural killer cells and amplifies early signaling through activating receptors

NK cell education is the process through which chronic engagement of inhibitory NK cell receptors by self MHC-I molecules preserves cellular responsiveness. The molecular mechanisms responsible for NK cell education remain unclear. Here, we show that mouse NK cell education is associated with a higher basal activity of the mTOR/Akt pathway, commensurate to the number of educating receptors. This higher activity was dependent on the SHP-1 phosphatase and essential for the improved responsiveness of reactive NK cells. Upon stimulation, the mTOR/Akt pathway amplified signaling through activating NK cell receptors by enhancing calcium flux and LFA-1 integrin activation. Pharmacological inhibition of mTOR resulted in a proportional decrease in NK cell reactivity. Reciprocally, acute cytokine stimulation restored reactivity of hyporesponsive NK cells through mTOR activation. These results demonstrate that mTOR acts as a molecular rheostat of NK cell reactivity controlled by educating receptors and uncover how cytokine stimulation overcomes NK cell education.