MEF2C regulates NK cell effector functions through control of lipid metabolism.

Natural killer (NK) cells are a critical first line of defense against viral infection. Rare mutations in a small subset of transcription factors can result in decreased NK cell numbers and function in humans, with an associated increased susceptibility to viral infection. However, our understanding of the specific transcription factors governing mature human NK cell function is limited. Here we use a non-viral CRISPR-Cas9 knockout screen targeting genes encoding 31 transcription factors differentially expressed during human NK cell development. We identify myocyte enhancer factor 2C (MEF2C) as a master regulator of human NK cell functionality ex vivo. MEF2C-haploinsufficient patients and mice displayed defects in NK cell development and effector function, with an increased susceptibility to viral infection. Mechanistically, MEF2C was required for an interleukin (IL)-2- and IL-15-mediated increase in lipid content through regulation of sterol regulatory element-binding protein (SREBP) pathways. Supplementation with oleic acid restored MEF2C-deficient and MEF2C-haploinsufficient patient NK cell cytotoxic function. Therefore, MEF2C is a critical orchestrator of NK cell antiviral immunity by regulating SREBP-mediated lipid metabolism.

The X-linked epigenetic regulator UTX controls NK cell-intrinsic sex differences.

Viral infection outcomes are sex biased, with males generally more susceptible than females. Paradoxically, the numbers of antiviral natural killer (NK) cells are increased in males. We demonstrate that while numbers of NK cells are increased in male mice, they display decreased effector function compared to females in mice and humans. These differences were not solely dependent on gonadal hormones, because they persisted in gonadectomized mice. Kdm6a (which encodes the protein UTX), an epigenetic regulator that escapes X inactivation, was lower in male NK cells, while NK cell-intrinsic UTX deficiency in female mice increased NK cell numbers and reduced effector responses. Furthermore, mice with NK cell-intrinsic UTX deficiency showed increased lethality to mouse cytomegalovirus. Integrative multi-omics analysis revealed a critical role for UTX in regulating chromatin accessibility and gene expression critical for NK cell homeostasis and effector function. Collectively, these data implicate UTX as a critical molecular determinant of sex differences in NK cells.

The transcription factor Fli1 restricts the formation of memory precursor NK cells during viral infection.

Natural killer (NK) cells are innate lymphocytes that possess traits of adaptive immunity, such as memory formation. However, the molecular mechanisms by which NK cells persist to form memory cells are not well understood. Using single-cell RNA sequencing, we identified two distinct effector NK cell (NKeff) populations following mouse cytomegalovirus infection. Ly6C- memory precursor (MP) NK cells showed enhanced survival during the contraction phase in a Bcl2-dependent manner, and differentiated into Ly6C+ memory NK cells. MP NK cells exhibited distinct transcriptional and epigenetic signatures compared with Ly6C+ NKeff cells, with a core epigenetic signature shared with MP CD8+ T cells enriched in ETS1 and Fli1 DNA-binding motifs. Fli1 was induced by STAT5 signaling ex vivo, and increased levels of the pro-apoptotic factor Bim in early effector NK cells following viral infection. These results suggest that a NK cell-intrinsic checkpoint controlled by the transcription factor Fli1 limits MP NK formation by regulating early effector NK cell fitness during viral infection.

Single-cell sequencing of human white adipose tissue identifies new cell states in health and obesity.

White adipose tissue (WAT) is an essential regulator of energy storage and systemic metabolic homeostasis. Regulatory networks consisting of immune and structural cells are necessary to maintain WAT metabolism, which can become impaired during obesity in mammals. Using single-cell transcriptomics and flow cytometry, we unveil a large-scale comprehensive cellular census of the stromal vascular fraction of healthy lean and obese human WAT. We report new subsets and developmental trajectories of adipose-resident innate lymphoid cells, dendritic cells and monocyte-derived macrophage populations that accumulate in obese WAT. Analysis of cell-cell ligand-receptor interactions and obesity-enriched signaling pathways revealed a switch from immunoregulatory mechanisms in lean WAT to inflammatory networks in obese WAT. These results provide a detailed and unbiased cellular landscape of homeostatic and inflammatory circuits in healthy human WAT.

Regulation of systemic metabolism by tissue-resident immune cell circuits.

Recent studies have demonstrated that tissue homeostasis and metabolic function are dependent on distinct tissue-resident immune cells that form functional cell circuits with structural cells. Within these cell circuits, immune cells integrate cues from dietary contents and commensal microbes in addition to endocrine and neuronal signals present in the tissue microenvironment to regulate structural cell metabolism. These tissue-resident immune circuits can become dysregulated during inflammation and dietary overnutrition, contributing to metabolic diseases. Here, we review the evidence describing key cellular networks within and between the liver, gastrointestinal tract, and adipose tissue that control systemic metabolism and how these cell circuits become dysregulated during certain metabolic diseases. We also identify open questions in the field that have the potential to enhance our understanding of metabolic health and disease.

Infection induces tissue-resident memory NK cells that safeguard tissue health.

Tissue health is dictated by the capacity to respond to perturbations and then return to homeostasis. Mechanisms that initiate, maintain, and regulate immune responses in tissues are therefore essential. Adaptive immunity plays a key role in these responses, with memory and tissue residency being cardinal features. A corresponding role for innate cells is unknown. Here, we have identified a population of innate lymphocytes that we term tissue-resident memory-like natural killer (NKRM) cells. In response to murine cytomegalovirus infection, we show that circulating NK cells were recruited in a CX3CR1-dependent manner to the salivary glands where they formed NKRM cells, a long-lived, tissue-resident population that prevented autoimmunity via TRAIL-dependent elimination of CD4+ T cells. Thus, NK cells develop adaptive-like features, including long-term residency in non-lymphoid tissues, to modulate inflammation, restore immune equilibrium, and preserve tissue health. Modulating the functions of NKRM cells may provide additional strategies to treat inflammatory and autoimmune diseases.

ILC1 Confer Early Host Protection at Initial Sites of Viral Infection.

Infection is restrained by the concerted activation of tissue-resident and circulating immune cells. Whether tissue-resident lymphocytes confer early antiviral immunity at local sites of primary infection prior to the initiation of circulating responses is not well understood. Furthermore, the kinetics of initial antiviral responses at sites of infection remain unclear. Here, we show that tissue-resident type 1 innate lymphoid cells (ILC1) serve an essential early role in host immunity through rapid production of interferon (IFN)-γ following viral infection. Ablation of Zfp683-dependent liver ILC1 lead to increased viral load in the presence of intact adaptive and innate immune cells critical for mouse cytomegalovirus (MCMV) clearance. Swift production of interleukin (IL)-12 by tissue-resident XCR1+ conventional dendritic cells (cDC1) promoted ILC1 production of IFN-γ in a STAT4-dependent manner to limit early viral burden. Thus, ILC1 contribute an essential role in viral immunosurveillance at sites of initial infection in response to local cDC1-derived proinflammatory cytokines.

Mouse cytomegalovirus-experienced ILC1s acquire a memory response dependent on the viral glycoprotein m12.

Innate lymphoid cells (ILCs) are tissue-resident sentinels that are essential for early host protection from pathogens at initial sites of infection. However, whether pathogen-derived antigens directly modulate the responses of tissue-resident ILCs has remained unclear. In the present study, it was found that liver-resident type 1 ILCs (ILC1s) expanded locally and persisted after the resolution of infection with mouse cytomegalovirus (MCMV). ILC1s acquired stable transcriptional, epigenetic and phenotypic changes a month after the resolution of MCMV infection, and showed an enhanced protective effector response to secondary challenge with MCMV consistent with a memory lymphocyte response. Memory ILC1 responses were dependent on the MCMV-encoded glycoprotein m12, and were independent of bystander activation by proinflammatory cytokines after heterologous infection. Thus, liver ILC1s acquire adaptive features in an MCMV-specific manner.

Type 1 Innate Lymphoid Cells Protect Mice from Acute Liver Injury via Interferon-γ Secretion for Upregulating Bcl-xL Expression in Hepatocytes.

Although type 1 innate lymphoid cells (ILC1s) have been originally found as liver-resident ILCs, their pathophysiological role in the liver remains poorly investigated. Here, we demonstrated that carbon tetrachloride (CCl4) injection into mice activated ILC1s, but not natural killer (NK) cells, in the liver. Activated ILC1s produced interferon-γ (IFN-γ) and protected mice from CCl4-induced acute liver injury. IFN-γ released from activated ILC1s promoted the survival of hepatocytes through upregulation of Bcl-xL. An activating NK receptor, DNAM-1, was required for the optimal activation and IFN-γ production of liver ILC1s. Extracellular adenosine triphosphate accelerated interleukin-12-driven IFN-γ production by liver ILC1s. These findings suggest that ILC1s are critical for tissue protection during acute liver injury.

No time to die: Epigenetic regulation of natural killer cell survival.

NK cells are short-lived innate lymphocytes that can mediate antigen-independent responses to infection and cancer. However, studies from the past two decades have shown that NK cells can acquire transcriptional and epigenetic modifications during inflammation that result in increased survival and lifespan. These findings blur the lines between the innate and adaptive arms of the immune system, and suggest that the homeostatic mechanisms that govern the persistence of innate immune cells are malleable. Indeed, recent studies have shown that NK cells undergo continuous and strictly regulated adaptations controlling their survival during development, tissue residency, and following inflammation. In this review, we summarize our current understanding of the critical factors regulating NK cell survival throughout their lifespan, with a specific emphasis on the epigenetic modifications that regulate the survival of NK cells in various contexts. A precise understanding of the molecular mechanisms that govern NK cell survival will be important to enhance therapies for cancer and infectious diseases.

Adipose-Resident Group 1 Innate Lymphoid Cells Promote Obesity-Associated Insulin Resistance.

Innate lymphoid cells (ILCs) function to protect epithelial barriers against pathogens and maintain tissue homeostasis in both barrier and non-barrier tissues. Here, utilizing Eomes reporter mice, we identify a subset of adipose group 1 ILC (ILC1) and demonstrate a role for these cells in metabolic disease. Adipose ILC1s were dependent on the transcription factors Nfil3 and T-bet but phenotypically and functionally distinct from adipose mature natural killer (NK) and immature NK cells. Analysis of parabiotic mice revealed that adipose ILC1s maintained long-term tissue residency. Diet-induced obesity drove early production of interleukin (IL)-12 in adipose tissue depots and led to the selective proliferation and accumulation of adipose-resident ILC1s in a manner dependent on the IL-12 receptor and STAT4. ILC1-derived interferon-γ was necessary and sufficient to drive proinflammatory macrophage polarization to promote obesity-associated insulin resistance. Thus, adipose-resident ILC1s contribute to obesity-related pathology in response to dysregulated local proinflammatory cytokine production.

Leukemia inhibitory factor protects against graft-versus-host disease while preserving graft-versus-leukemia activity.

Graft-versus-host disease (GVHD) remains a major complication after allogeneic hematopoietic stem cell transplantation, a widely used therapy for hematologic malignancies and blood disorders. Here, we report an unexpected role of cytokine leukemia inhibitory factor (LIF) in protecting against GVHD development. Administrating recombinant LIF protein (rLIF) protects mice from GVHD-induced tissue damage and lethality without compromising the graft-versus-leukemia activity, which is crucial to prevent tumor relapse. We found that rLIF decreases the infiltration and activation of donor immune cells and protects intestinal stem cells to ameliorate GVHD. Mechanistically, rLIF downregulates IL-12-p40 expression in recipient dendritic cells after irradiation through activating STAT1 signaling, which results in decreased major histocompatibility complex II levels on intestinal epithelial cells and decreased donor T-cell activation and infiltration. This study reveals a previously unidentified protective role of LIF for GVHD-induced tissue pathology and provides a potential effective therapeutic strategy to limit tissue pathology without compromising antileukemic efficacy.

Natural Killer Cell Memory.

Natural killer (NK) cells have historically been considered short-lived cytolytic cells that can rapidly respond against pathogens and tumors in an antigen-independent manner and then undergo cell death. Recently, however, NK cells have been shown to possess traits of adaptive immunity and can acquire immunological memory in a manner similar to that of T and B cells. In this review, we discuss evidence of NK cell memory and the mechanisms involved in the generation and survival of these innate lymphocytes.

BNIP3- and BNIP3L-Mediated Mitophagy Promotes the Generation of Natural Killer Cell Memory.

Natural killer (NK) cells are innate lymphocytes that possess traits of adaptive immunity, such as clonal expansion, contraction, and generation of long-lived "memory" cells, processes poorly understood at the molecular level. Here, we found that as proliferating NK cells accumulated dysfunctional mitochondria during viral infection, a protective mitophagy pathway was induced during the contraction phase to promote their survival in a reactive oxygen species (ROS)-dependent manner. Inhibition of mechanistic target of rapamycin (mTOR) or activation of AMP-activated protein kinase (AMPK) during the contraction-to-memory phase transition of the antiviral response increased autophagic activity and enhanced memory NK cell numbers through an Atg3-dependent mechanism. Furthermore, we demonstrated a temporally regulated role for mitophagy-inducing proteins BCL2/adenovirus E1B 19-kDa interacting protein 3 (BNIP3) and BNIP3-like (BNIP3L) in the generation of robust NK cell memory. Thus, our study reveals the functional importance of mitophagy during the dynamic response of these cytolytic innate lymphocytes.

True Detective: Unraveling Group 1 Innate Lymphocyte Heterogeneity.

Innate lymphoid cells (ILCs) consist of a heterogeneous family of lymphocytes that regulate tissue homeostasis and can contribute to pathology in mice and humans. Mammalian group 1 ILCs are defined by the production of interferon (IFN)-γ and the functional dependence on the transcription factor T-bet. While recent studies demonstrate that group 1 ILCs consist of circulating mature natural killer (NK) cells and tissue-resident ILC1s, the functional, phenotypic, and developmental properties that distinguish these two cell lineages are often confusing and difficult to dissect. In this review, we critically evaluate the current knowledge of mammalian group 1 ILC heterogeneity and propose new inclusive nomenclature to clarify the roles of ILC1s and NK cells during homeostasis and disease.

NK Cell Responses Redefine Immunological Memory.

Immunological memory has traditionally been regarded as a unique trait of the adaptive immune system. Nevertheless, there is evidence of immunological memory in lower organisms and invertebrates, which lack an adaptive immune system. Despite their innate ability to rapidly produce effector cytokines and kill virally infected or transformed cells, NK cells also exhibit adaptive characteristics such as clonal expansion, longevity, self-renewal, and robust recall responses to antigenic or nonantigenic stimuli. In this review, we highlight the intracellular and extracellular requirements for memory NK cell generation and describe the emerging evidence for memory precursor NK cells and their derivation.

Send it, receive it, quick erase it: A mouse model to decipher chemokine communication.

A method to precisely determine which cells respond to chemokines in vivo is currently lacking. A novel class of dual fluorescence reporter mice could help identify cells that produce and/or sense a given chemokine in vitro and in vivo (Rodrigo et al. 2024. J. Exp. Med.https://doi.org/10.1084/jem.20231814).

Suppression of adaptive NK cell expansion by macrophage-mediated phagocytosis inhibited by 2B4-CD48.

Infection of mice by mouse cytomegalovirus (MCMV) triggers activation and expansion of Ly49H+ natural killer (NK) cells, which are virus specific and considered to be "adaptive" or "memory" NK cells. Here, we find that signaling lymphocytic activation molecule family receptors (SFRs), a group of hematopoietic cell-restricted receptors, are essential for the expansion of Ly49H+ NK cells after MCMV infection. This activity is largely mediated by CD48, an SFR broadly expressed on NK cells and displaying augmented expression after MCMV infection. It is also dependent on the CD48 counter-receptor, 2B4, expressed on host macrophages. The 2B4-CD48 axis promotes expansion of Ly49H+ NK cells by repressing their phagocytosis by virus-activated macrophages through inhibition of the pro-phagocytic integrin lymphocyte function-associated antigen-1 (LFA-1) on macrophages. These data identify key roles of macrophages and the 2B4-CD48 pathway in controlling the expansion of adaptive NK cells following MCMV infection. Stimulation of the 2B4-CD48 axis may be helpful in enhancing adaptive NK cell responses for therapeutic purposes.