Activating Receptors for Self-MHC Class I Enhance Effector Functions and Memory Differentiation of NK Cells during Mouse Cytomegalovirus Infection

THEMIS2 Impairs Antitumor Activity of NK Cells by Suppressing Activating NK Receptor Signaling

NK cells are cytotoxic innate lymphocytes that play a critical role in antitumor immunity. NK cells recognize target cells by using a repertoire of activating NK receptors and exert the effector functions. Although the magnitude of activation signals through activating NK receptors controls NK cell function, it has not been fully understood how these activating signals are modulated in NK cells. In this study, we found that a scaffold protein, THEMIS2, inhibits activating NK receptor signaling. Overexpression of THEMIS2 attenuated the effector function of human NK cells, whereas knockdown of THEMIS2 enhanced it. Mechanistically, THEMIS2 binds to GRB2 and phosphorylated SHP-1 and SHP-2 at the proximity of activating NK receptors DNAM-1 and NKG2D. Knockdown of THEMIS2 in primary human NK cells promoted the effector functions. Furthermore, Themis2-deficient mice showed low metastatic burden in an NK cell-dependent manner. These findings demonstrate that THEMIS2 has an inhibitory role in the antitumor activity of NK cells, suggesting that THEMIS2 might be a potential therapeutic target for NK cell-mediated cancer immunotherapy.

ITAM-based receptors in natural killer cells

The ability of cells of the immune system to acquire features such as increased longevity and enhanced secondary responses was long thought to be restricted to cells of the adaptive immune system. Natural killer (NK) cells have challenged this notion by demonstrating that they can also gain adaptive features. This has been observed in both humans and mice during infection with cytomegalovirus (CMV). The generation of adaptive NK cells requires antigen-specific recognition of virally infected cells through stimulatory NK receptors. These receptors lack the ability to signal on their own and rather rely on adaptor molecules that contain ITAMs for driving signals. Here, we highlight our understanding of how these receptors influence the production of adaptive NK cells and propose areas in the field that merit further investigation.

Extrinsic and intrinsic drivers of natural killer cell clonality

Clonal expansion of antigen-specific lymphocytes is the fundamental mechanism enabling potent adaptive immune responses and the generation of immune memory. Accompanied by pronounced epigenetic remodeling, the massive proliferation of individual cells generates a critical mass of effectors for the control of acute infections, as well as a pool of memory cells protecting against future pathogen encounters. Classically associated with the adaptive immune system, recent work has demonstrated that innate immune memory to human cytomegalovirus (CMV) infection is stably maintained as large clonal expansions of natural killer (NK) cells, raising questions on the mechanisms for clonal selection and expansion in the absence of re-arranged antigen receptors. Here, we discuss clonal NK cell memory in the context of the mechanisms underlying clonal competition of adaptive lymphocytes and propose alternative selection mechanisms that might decide on the clonal success of their innate counterparts. We propose that the integration of external cues with cell-intrinsic sources of heterogeneity, such as variegated receptor expression, transcriptional states, and somatic variants, compose a bottleneck for clonal selection, contributing to the large size of memory NK cell clones.

Memory-like NK Cells Are a Critical Component of Vaccine-Induced Immunity to Trypanosoma cruzi Infection

Chagas disease by Trypanosoma cruzi infection is a major public health issue. The available therapeutic agents have limited efficacy and significant side effects. A reliable vaccine would reduce the threat of T. cruzi infections and prevent Chagas disease. Understanding the immune response to this infection would improve vaccine design. We previously demonstrated that adoptively transferred NK cells from mice immunized with highly attenuated T. cruzi, GFP-DDDHA strain, provided potent protection in naive recipients against secondary lethal challenge with various wild-type (WT) strains. To understand the importance of NK cells in protecting mice against T. cruzi infection, we performed an in-depth characterization of NK cell phenotype, responses, and memory-like traits during acute infections due to GFP-DDDHA and WT strains and in immunized mice during a recall response to a WT lethal challenge. NK cells robustly expanded and became more mature and cytolytic during the GFP-DDDHA strain immunization. NK cells in immunized mice responded more robustly after WT lethal challenge than during an acute primary WT infection. In addition, protection by immunization with the GFP-DDDHA strain is significantly weakened in NK cell-deficient mice and did not prevent parasitemia from WT lethal challenge, indicating that NK cells with memory-like traits were a critical component for early control of WT lethal challenge. Prior T. cruzi vaccine development studies have not included studies of this rapid NK response. These findings provide insights into overcoming existing challenges in developing a safe and effective vaccine to prevent this infection.

Processing stimulus dynamics by the NF-κB network in single cells

Cells at the site of an infection experience numerous biochemical signals that vary in amplitude, space, and time. Despite the diversity of dynamic signals produced by pathogens and sentinel cells, information-processing pathways converge on a limited number of central signaling nodes to ultimately control cellular responses. In particular, the NF-κB pathway responds to dozens of signals from pathogens and self, and plays a vital role in processing proinflammatory inputs. Studies addressing the influence of stimulus dynamics on NF-κB signaling are rare due to technical limitations with live-cell measurements. However, recent advances in microfluidics, automation, and image analysis have enabled investigations that yield high temporal resolution at the single-cell level. Here, we summarize the recent research which measures and models the NF-κB response to pulsatile and fluctuating stimulus concentrations, as well as different combinations and sequences of signaling molecules. Collectively, these studies show that the NF-κB network integrates external inflammatory signals and translates these into downstream transcriptional responses.

Themis2 regulates natural killer cell memory function and formation

Immunological memory is a hallmark of the adaptive immune system. Although natural killer (NK) cells are innate immune cells important for the immediate host defence, they can differentiate into memory NK cells. The molecular mechanisms controlling this differentiation are yet to be fully elucidated. Here we identify the scaffold protein Themis2 as a critical regulator of memory NK cell differentiation and function. Themis2-deficient NK cells expressing Ly49H, an activating NK receptor for the mouse cytomegalovirus (MCMV) antigen m157, show enhanced differentiation into memory NK cells and augment host protection against MCMV infection. Themis2 inhibits the effector function of NK cells after stimulation of Ly49H and multiple activating NK receptors, though not specific to memory NK cells. Mechanistically, Themis2 suppresses Ly49H signalling by attenuating ZAP70/Syk phosphorylation, and it also translocates to the nucleus, where it promotes Zfp740-mediated repression to regulate the persistence of memory NK cells. Zfp740 deficiency increases the number of memory NK cells and enhances the effector function of memory NK cells, which further supports the relevance of the Themis2-Zfp740 pathway. In conclusion, our study shows that Themis2 quantitatively and qualitatively regulates NK cell memory formation.

Cardinal features of immune memory in innate lymphocytes

The ability of vertebrates to 'remember' previous infections had once been attributed exclusively to adaptive immunity. We now appreciate that innate lymphocytes also possess memory properties akin to those of adaptive immune cells. In this Review, we draw parallels from T cell biology to explore the key features of immune memory in innate lymphocytes, including quantity, quality, and location. We discuss the signals that trigger clonal or clonal-like expansion in innate lymphocytes, and highlight recent studies that shed light on the complex cellular and molecular crosstalk between metabolism, epigenetics, and transcription responsible for differentiating innate lymphocyte responses towards a memory fate. Additionally, we explore emerging evidence that activated innate lymphocytes relocate and establish themselves in specific peripheral tissues during infection, which may facilitate an accelerated response program akin to those of tissue-resident memory T cells.

The CD16 and CD32b Fc-gamma receptors regulate antibody-mediated responses in mouse natural killer cells

Natural killer (NK) cells are innate lymphocytes capable of mediating immune responses without prior sensitization. NK cells express Fc-gamma receptors (FcγRs) that engage the Fc region of IgG. Studies investigating the role of FcγRs on mouse NK cells have been limited due to lack specific reagents. In this study, we characterize the expression and biological consequences of activating mouse NK cells through their FcγRs. We demonstrate that most NK cells express the activating CD16 receptor, and a subset of NK cells also expresses the inhibitory CD32b receptor. Critically, these FcγRs are functional on mouse NK cells and can modulate antibody-mediated responses. We also characterized mice with conditional knockout alleles of Fcgr3 (CD16) or Fcgr2b (CD32b) in the NK and innate lymphoid cell (ILC) lineage. NK cells in these mice did not reveal any developmental defects and were responsive to cross-linking activating NK receptors, cytokine stimulation, and killing of YAC-1 targets. Importantly, CD16-deficient NK cells failed to induce antibody-directed cellular cytotoxicity of antibody-coated B-cell lymphomas in in vitro assays. In addition, we demonstrate the important role of CD16 on NK cells using an in vivo model of cancer immunotherapy using anti-CD20 antibody treatment of B-cell lymphomas.

Epigenetic regulation of natural killer cell memory

Immunological memory is the underlying mechanism by which the immune system remembers previous encounters with pathogens to produce an enhanced secondary response upon re-encounter. It stands as the hallmark feature of the adaptive immune system and the cornerstone of vaccine development. Classic recall responses are executed by conventional T and B cells, which undergo somatic recombination and modify their receptor repertoire to ensure recognition of a vast number of antigens. However, recent evidence has challenged the dogma that memory responses are restricted to the adaptive immune system, which has prompted a reevaluation of what delineates "immune memory." Natural killer (NK) cells of the innate immune system have been at the forefront of these pushed boundaries, and have proved to be more "adaptable" than previously thought. Like T cells, we now appreciate that their "natural" abilities actually require a myriad of signals for optimal responses. In this review, we discuss the many signals required for effector and memory NK cell responses and the epigenetic mechanisms that ultimately endow their enhanced features.

Phenotypic and Functional Characteristics of a Novel Influenza Virus Hemagglutinin-Specific Memory NK Cell

Immune memory represents the most efficient defense against invasion and transmission of infectious pathogens. In contrast to memory T and B cells, the roles of innate immunity in recall responses remain inconclusive. In this study, we identified a novel mouse spleen NK cell subset expressing NKp46 and NKG2A induced by intranasal influenza virus infection. These memory NK cells specifically recognize N-linked glycosylation sites on influenza hemagglutinin (HA) protein. Different from memory-like NK cells reported previously, these NKp46⁺ NKG2A⁺ memory NK cells exhibited HA-specific silence of cytotoxicity but increase of gamma interferon (IFN-γ) response against influenza virus-infected cells, which could be reversed by pifithrin-μ, a p53-heat shock protein 70 (HSP70) signaling inhibitor. During recall responses, splenic NKp46⁺ NKG2A⁺ NK cells were recruited to infected lung and modulated viral clearance of virus and CD8⁺ T cell distribution, resulting in improved clinical outcomes. This long-lived NK memory bridges innate and adaptive immune memory response and promotes the homeostasis of local environment during recall response.IMPORTANCE In this study, we demonstrate a novel hemagglutinin (HA)-specific NKp46⁺ NKG2A⁺ NK cell subset induced by influenza A virus infection. These memory NK cells show virus-specific decreased cytotoxicity and increased gamma interferon (IFN-γ) on reencountering the same influenza virus antigen. In addition, they modulate host recall responses and CD8 T cell distribution, thus bridging the innate immune and adaptive immune responses during influenza virus infection.

NK cell and ILC heterogeneity in colorectal cancer. New perspectives from high dimensional data

Innate lymphoid cells (ILCs) and tissue-resident natural killer (NK) cells ensure immunity at environmental interfaces and help maintain barrier integrity of the intestinal tract. This wide range of innate lymphocytes is able to provide fast and potent inflammatory responses that, when deregulated, have been associated with pathogenesis of inflammatory bowel disease (IBD) and colorectal cancer (CRC). While the presence of tumor-infiltrating NK cells is generally associated with a favorable outcome in CRC patients, emerging evidence reveals distinct roles for ILCs in regulating CRC pathogenesis and progression. Advances in next generation sequencing technology, and in particular of single-cell RNA-seq approaches, along with multidimensional flow cytometry analysis, have helped to deconvolute the complexity and heterogeneity of the ILC system both in homeostatic and pathological contexts. In this review, we discuss the protective and detrimental roles of NK cells and ILCs in the pathogenesis of CRC, focusing on the phenotypic and transcriptional modifications these cells undergo during CRC development and progression.

Natural Killer Cells: From Innate to Adaptive Features

Natural killer (NK) cells are innate lymphocytes that provide critical host defense against pathogens and cancer. Originally heralded for their early and rapid effector activity, NK cells have been recognized over the last decade for their ability to undergo adaptive immune processes, including antigen-driven clonal expansion and generation of long-lived memory. This review presents an overview of how NK cells lithely partake in both innate and adaptive responses and how this versatility is manifest in human NK cell-mediated immunity.

Virus-specific NK cell memory

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.

Evidence for Prescribed NK Cell Ly-49 Developmental Pathways in Mice

Previous studies of NK cell inhibitory Ly-49 genes showed their expression is stochastic. However, relatively few studies have examined the mechanisms governing acquisition of inhibitory receptors in conjunction with activating Ly-49 receptors and NK cell development. We hypothesized that the surface expression of activating Ly-49 receptors is nonrandom and is influenced by inhibitory Ly-49 receptors. We analyzed NK cell "clusters" defined by combinatorial expression of activating (Ly-49H and Ly-49D) and inhibitory (Ly-49I and Ly-49G2) receptors in C57BL/6 mice. Using the product rule to evaluate the interdependencies of the Ly-49 receptors, we found evidence for a tightly regulated expression at the immature NK cell stage, with the highest interdependencies between clusters that express at least one activating receptor. Further analysis demonstrated that certain NK clusters predominated at the immature (CD27⁺CD11b^-), transitional (CD27⁺CD11b⁺), and mature (CD27^-CD11b^-) NK cell stages. Using parallel in vitro culture and in vivo transplantation of sorted NK clusters, we discovered nonrandom expression of Ly-49 receptors, suggesting that prescribed pathways of NK cluster differentiation exist. Our data infer that surface expression of Ly-49I is an important step in NK cell maturation. Ki-67 expression and cell counts confirmed that immature NK cells proliferate more than mature NK cells. We found that MHC class I is particularly important for regulation of Ly-49D and Ly-49G2, even though no known MHC class I ligand for these receptors is present in B6 mice. Our data indicate that surface expression of both activating and inhibitory Ly-49 receptors on NK cell clusters occurs in a nonrandom process correlated to their maturation stage.

Type 1 innate lymphoid cells: Soldiers at the front line of immunity

Innate lymphoid cells (ILCs) are tissue-resident innate lymphocytes that have functions to protect the hosts against pathogens and that regulate tissue inflammation and homeostasis. ILC subsets rapidly produce particular cytokines in response to infection, inflammation, and tissue injury at the local environment. Type 1 ILCs (ILC1s) promptly and abundantly produce interferon (IFN)-γ but lack appreciable cytotoxic activity. ILC1s share many phenotypic, developmental, and functional characteristics with natural killer (NK) cells, which are circulating innate lymphocytes with potent natural cytotoxicity. However, recent studies have established ILC1s as distinct from NK cells. ILC1s predominantly reside in the liver—they initially were discovered as a liver-resident ILC subset—as well as in other lymphoid and non-lymphoid tissues. Accumulating evidence has demonstrated that ILC1s play an important and unique role in host protection and in immunomodulation in their resident organs. However, the pathophysiological role of tissue-resident ILC1s remains largely unclear. In this review, we summarize emerging evidence showing that ILC1s not only contribute to inflammation to protect against pathogens but also promote tissue protection and metabolism. We highlight a unique function of ILC1s in their resident tissues.

Clonal expansion of innate and adaptive lymphocytes

One of the hallmarks of the vertebrate adaptive immune system is the prolific expansion of individual cell clones that encounter their cognate antigen. More recently, however, there is growing evidence for the clonal expansion of innate lymphocytes, particularly in the context of pathogen challenge. Clonal expansion not only serves to amplify the number of specific lymphocytes to mount a robust protective response to the pathogen at hand but also results in selection and differentiation of the responding lymphocytes to generate a multitude of cell fates. Here, we summarize the evidence for clonal expansion in innate lymphocytes, which has primarily been observed in natural killer (NK) cells responding to cytomegalovirus infection, and consider the requirements for such a response in NK cells in light of those for T cells. Furthermore, we discuss multiple aspects of heterogeneity that both contribute to and result from the fundamental immunological process of clonal expansion, highlighting the parallels between innate and adaptive lymphocytes, with a particular focus on NK cells and CD8⁺ T cells.

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.

Spatial and temporal coordination of antiviral responses by group 1 ILCs

Group 1 innate lymphocytes consist of a phenotypically, spatially, and functionally heterogeneous population of NK cells and ILC1s that are engaged during pathogen invasion. We are only beginning to understand the context-dependent roles that different subsets of group 1 innate lymphocytes play during homeostatic perturbations. With a focus on viral infection, this review highlights the organization and regulation of spatially and temporally distinct waves of NK cell and ILC1 responses that collectively serve to achieve optimal viral control.

Innate lymphoid cell memory

Innate lymphoid cells (ILCs), including natural killer (NK) cells, ILC1s, ILC2s, ILC3s, and lymphoid tissue inducer (LTi) cells, comprise the first line of innate immune defense against pathogens and tumors. Over the past decade, accumulating evidence has demonstrated immunological memory in ILC subsets: for example, NK cells recall haptens, viruses, and cytokines; ILC1s recall haptens; and ILC2s recall cytokines. Although the development and functions of ILCs mirror those of T-cell subsets, ILC and T-cell memory exhibit both common characteristics and specific properties. Encouragingly, ILC memory has been found to confer benefits in long-term tumor control and vaccination, providing insight for novel memory ILC-based tumor immunotherapy and vaccine-development strategies. In this review, we discuss the evidence supporting ILC memory and present a comprehensive framework of the ILC memory system.

CMV and natural killer cells: shaping the response to vaccination

Cytomegaloviruses (CMVs) are highly prevalent, persistent human pathogens that not only evade but also shape our immune responses. Natural killer (NK) cells play an important role in the control of CMV and CMVs have in turn developed a plethora of immunoevasion mechanisms targeting NK cells. This complex interplay can leave a long-lasting imprint on the immune system in general and affect responses toward other pathogens and vaccines. This review aims to provide an overview of NK cell biology and development, the manipulation of NK cells by CMVs and the potential impact of these evasion strategies on responses to vaccination.

Memory responses of natural killer cells

Natural killer (NK) cells have traditionally been classified as a cellular component of the innate immune system, given their ability to rapidly produce effector cytokines and kill infected or transformed cells without prior exposure. More recently, NK cells have been shown to possess features of adaptive immunity such as clonal expansion, longevity, and robust recall responses. NK cell memory can be broadly divided into two categories: antigen-specific and antigen-independent. In the first case, exposure to certain viral or hapten stimuli endows NK cells with antigen-specific immunological memory, similar to T and B cells. In the second case, exposure of NK cells to specific cytokine milieus can imprint long-lasting changes on effector functions, resulting in antigen-independent memory-like NK cells. In this review, we discuss the various conditions that promote generation of these two categories of memory NK cells, and the mechanistic requirements underlying these processes.

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.

Mouse cytomegalovirus encoded immunoevasins and evolution of Ly49 receptors - Sidekicks or enemies?

Cytomegaloviruses (CMVs) have dedicated a large portion of their genome towards immune evasion targeting many aspects of the host immune system, particularly NK cells. However, the host managed to cope with the infection by developing multiple mechanisms to recognize viral threat and counterattack it, thus illustrating never-ending evolutionary interplay between CMV and its host. In this review, we will focus on several mechanisms of NK cell evasion by mouse CMV (MCMV), the role of host inhibitory and activating Ly49 receptors involved in the virus control and acquisition of adaptive features by NK cells as a consequence of MCMV infection.

Expression of KIR2DS1 by decidual natural killer cells increases their ability to control placental HCMV infection

The combination of the activating killer cell Ig-like receptor 2DS1 (KIR2DS1) expressed by maternal decidual natural killer cells (dNK) and the presence of its ligand, the HLA-C allotype HLA-C2, expressed by fetal trophoblasts, reduces the risk of developing pregnancy complications. However, no molecular or cellular mechanism explains this genetic correlation. Here we demonstrate that KIR2DS1+ dNK acquired higher cytotoxic function than KIR2DS1- dNK when exposed to human cytomegalovirus (HCMV)-infected decidual stromal cells (DSC), particularly when DSCs express HLA-C2. Furthermore, dNK were unable to degranulate or secrete cytokines in response to HCMV-infected primary fetal extravillous trophoblasts. This emphasizes the immunological challenge to clear placental viral infections within the immune-privileged placenta. Activation of dNK through KIR2DS1/HLA-C2 interaction increases their ability to respond to placental HCMV infection and may limit subsequent virus-induced placental pathology. This mechanism is directly related to how KIR2DS1 expressed by dNK reduces development of severe pregnancy complications such as miscarriages and preterm delivery.

About Training and Memory: NK-Cell Adaptation to Viral Infections

Viral infections continuously challenge and shape our immune system. Due to their fine antigen recognition ability, adaptive lymphocytes protect against pathogen reencounter by generating specific immunological memory. Innate cells such as macrophages also adapt to pathogen challenge and mount resistance to reinfection, a phenomenon termed trained immunity. As part of the innate immunity, natural killer (NK) cells can display rapid effector functions and play a crucial role in the control of viral infections, especially by the β-herpesvirus cytomegalovirus (CMV). CMV activates the NK-cell pool by inducing proinflammatory signals, which prime NK cells, paralleling macrophage training. In addition, CMV dramatically shapes the NK-cell repertoire due to its ability to trigger specific NK cell-activating receptors, and enables the expansion and persistence of a specific NK-cell subset displaying adaptive and memory features. In this chapter, we will discuss how different signals during CMV infection contribute to NK-cell training and acquisition of classical memory properties and how these events can impact on reinfection and cross-resistance.