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Natural killer (NK) cells are critical to immune surveillance against infections and cancer. Their role in immune surveillance requires that NK cells are present within tissues in a quiescent state. Mechanisms by which NK cells remain quiescent in tissues are incompletely elucidated. The transcriptional repressor BACH2 plays a critical role within the adaptive immune system, but its function within innate lymphocytes has been unclear. Here, we show that BACH2 acts as an intrinsic negative regulator of NK cell maturation and function. BACH2 is expressed within developing and mature NK cells and promotes the maintenance of immature NK cells by restricting their maturation in the presence of weak stimulatory signals. Loss of BACH2 within NK cells results in accumulation of activated NK cells with unrestrained cytotoxic function within tissues, which mediate augmented immune surveillance to pulmonary cancer metastasis. These findings establish a critical function of BACH2 as a global negative regulator of innate cytotoxic function and tumor immune surveillance by NK cells.

The bZIP transcription factor Nfil3 (also known as E4BP4) is required for the development of natural killer (NK) cells and type 1 innate lymphoid cells (ILC1s). We find that Nfil3 plays a critical role in the development of other mucosal tissue-associated innate lymphocytes. Type 3 ILCs (ILC3s), including lymphoid tissue inducer (LTi)–like cells, are severely diminished in both numbers and function in Nfil3-deficient mice. Using mixed bone marrow chimeric mice, we demonstrate that Nfil3 is critical for normal development of gut-associated ILC3s in a cell-intrinsic manner. Furthermore, Nfil3 deficiency severely compromises intestinal innate immune defense against acute bacterial infection with Citrobacter rodentium and Clostridium difficile. Nfil3 deficiency resulted in a loss of the recently identified ILC precursor, yet conditional ablation of Nfil3 in the NKp46+ ILC3 subset did not perturb ILC3 numbers, suggesting that Nfil3 is required early during ILC3 development but not for lineage maintenance. Lastly, a marked defect in type 2 ILCs (ILC2s) was also observed in the lungs and visceral adipose tissue of Nfil3-deficient mice, revealing a general requirement for Nfil3 in the development of all ILC lineages.

Development of the natural killer (NK) cell lineage is dependent on the transcription factor Nfil3 (or E4BP4), which is thought to act downstream of IL-15 signaling. Nfil3-deficient mice lack NK cells, whereas other lymphocyte lineages (B, T, and NKT cells) remain largely intact. We report the appearance of Ly49H-expressing NK cells in Nfil3−/− mice infected with mouse cytomegalovirus (MCMV) or recombinant viruses expressing the viral m157 glycoprotein. Nfil3−/− NK cells at the peak of antigen-driven expansion were functionally similar to NK cells from infected wild-type mice with respect to IFN-γ production and cytotoxicity, and could comparably produce long-lived memory NK cells that persisted in lymphoid and nonlymphoid tissues for >60 d. We demonstrate that generation and maintenance of NK cell memory is an Nfil3-independent but IL-15–dependent process. Furthermore, specific ablation of Nfil3 in either immature NK cells in the bone marrow or mature peripheral NK cells had no observable effect on NK cell lineage maintenance or homeostasis. Thus, expression of Nfil3 is crucial only early in the development of NK cells, and signals through activating receptors and proinflammatory cytokines during viral infection can bypass the requirement for Nfil3, promoting the proliferation and long-term survival of virus-specific NK cells.

The emergence of the adaptive immune system took a toll in the form of pathologies mediated by self-reactive cells. Regulatory T cells (T reg cells) exert a critical brake on responses of T and B lymphocytes to self- and foreign antigens. Here, we asked whether T reg cells are required to restrain NK cells, the third lymphocyte lineage, whose features combine innate and adaptive immune cell properties. Although depletion of T reg cells led to systemic fatal autoimmunity, NK cell tolerance and reactivity to strong activating self- and non-self–ligands remained largely intact. In contrast, missing-self responses were increased in the absence of T reg cells as the result of heightened IL-2 availability. We found that IL-2 rapidly boosted the capacity of NK cells to productively engage target cells and enabled NK cell responses to weak stimulation. Our results suggest that IL-2–dependent adaptive-innate lymphocyte cross talk tunes NK cell reactivity and that T reg cells restrain NK cell cytotoxicity by limiting the availability of IL-2.

Activation and expansion of T and B lymphocytes and myeloid cells are controlled by Foxp3+ regulatory T cells (T reg cells), and their deficiency results in a fatal lympho- and myeloproliferative syndrome. A role for T reg cells in the homeostasis of innate lymphocyte lineages remained unknown. Here, we report that T reg cells restrained the expansion of immature CD127+ NK cells, which had the unique ability to up-regulate the IL2Rα (CD25) in response to the proinflammatory cytokine IL-12. In addition, we observed the preferential accumulation of CD127+ NK cells in mice bearing progressing tumors or suffering from chronic viral infection. CD127+ NK cells expanded in an IL-2–dependent manner upon T reg cell depletion and were able to give rise to mature NK cells, indicating that the latter can develop through a CD25+ intermediate stage. Thus, T reg cells restrain the IL-2–dependent CD4+ T cell help for CD127+ immature NK cells. These findings highlight the adaptive control of innate lymphocyte homeostasis.

4-1BB agonist antibody treatment induces a population of KLRG1+ T cells that infiltrate melanoma tumors. We investigated the origin and function of these cells, as well as their place within established T cell paradigms. We find that these T cells, particularly the CD4 lineage, represent a novel phenotype characterized by enhanced, multipotent cytotoxicity. Distinct from described polarities, this T cell phenotype is driven by the T-box transcription factor Eomesodermin. Formation of this phenotype requires 4-1BB signaling on both T and antigen-presenting cells and the resulting production of the cytokines IL-27, IL-15, and IL-10. Furthermore, we find CD4+ T cells bearing the signature features of this phenotype in the livers of mice infected with both bacterial and viral intracellular pathogens, suggesting a role for these cells in infectious immunity. These T cells constitute a novel phenotype that resolves multiple questions associated with 4-1BB activation, including how 4-1BB enhances tumor-specific cytotoxicity and how 4-1BB can promote tumor immunity while repressing autoimmunity.

Although natural killer (NK) cells are classified as innate immune cells, recent studies demonstrate that NK cells can become long-lived memory cells and contribute to secondary immune responses. The precise signals that promote generation of long-lived memory NK cells are unknown. Using cytokine receptor-deficient mice, we show that interleukin-12 (IL-12) is indispensible for mouse cytomegalovirus (MCMV)-specific NK cell expansion and generation of memory NK cells. In contrast to wild-type NK cells that proliferated robustly and resided in lymphoid and nonlymphoid tissues for months after MCMV infection, IL-12 receptor–deficient NK cells failed to expand and were unable to mediate protection after MCMV challenge. We further demonstrate that a STAT4-dependent IFN-γ–independent mechanism contributes toward the generation of memory NK cells during MCMV infection. Understanding the full contribution of inflammatory cytokine signaling to the NK cell response against viral infection will be of interest for the development of vaccines and therapeutics.

N 6-methyladenosine (m6A) is the most prevalent posttranscriptional modification on RNA. NK cells are the predominant innate lymphoid cells that mediate antiviral and antitumor immunity. However, whether and how m6A modifications affect NK cell immunity remain unknown. Here, we discover that YTHDF2, a well-known m6A reader, is upregulated in NK cells upon activation by cytokines, tumors, and cytomegalovirus infection. Ythdf2 deficiency in NK cells impairs NK cell antitumor and antiviral activity in vivo. YTHDF2 maintains NK cell homeostasis and terminal maturation, correlating with modulating NK cell trafficking and regulating Eomes, respectively. YTHDF2 promotes NK cell effector function and is required for IL-15–mediated NK cell survival and proliferation by forming a STAT5–YTHDF2 positive feedback loop. Transcriptome-wide screening identifies Tardbp to be involved in cell proliferation or survival as a YTHDF2-binding target in NK cells. Collectively, we elucidate the biological roles of m6A modifications in NK cells and highlight a new direction to harness NK cell antitumor immunity.

NK cells express a limited number of germline-encoded receptors that identify infected or transformed cells, eliciting cytotoxicity, effector cytokine production, and in some circumstances clonal proliferation and memory. To maximize the functional diversity of NK cells, the array and expression level of surface receptors vary between individual NK cell “clones” in mice and humans. Cytomegalovirus infection in both species can expand a population of NK cells expressing receptors critical to the clearance of infected cells and generate a long-lived memory pool capable of targeting future infection with greater efficacy. Here, we discuss the pathways and factors that regulate the generation and maintenance of effector and memory NK cells and propose how this understanding may be harnessed therapeutically.

Natural killer (NK) cells are innate lymphocytes that exhibit adaptive features, such as clonal expansion and memory, during viral infection. Although activating receptor engagement and proinflammatory cytokines are required to drive NK cell clonal expansion, additional stimulatory signals controlling their proliferation remain to be discovered. Here, we describe one such signal that is provided by the adrenergic nervous system, and demonstrate that cell-intrinsic adrenergic signaling is required for optimal adaptive NK cell responses. Early during mouse cytomegalovirus (MCMV) infection, NK cells up-regulated Adrb2 (which encodes the β2-adrenergic receptor), a process dependent on IL-12 and STAT4 signaling. NK cell–specific deletion of Adrb2 resulted in impaired NK cell expansion and memory during MCMV challenge, in part due to a diminished proliferative capacity. As a result, NK cell-intrinsic adrenergic signaling was required for protection against MCMV. Taken together, we propose a novel role for the adrenergic nervous system in regulating circulating lymphocyte responses to viral infection.