Specificity, tolerance and developmental regulation of natural killer cells defined by expression of class I-specific Ly49 receptors

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.

Chimeric antigen receptor-natural killer cells: a promising sword against insidious tumor cells

Natural killer (NK) cells are a critical component of innate immunity, particularly in initial cancer recognition and inhibition of additional tumor growth or metastasis propagation. NK cells recognize transformed cells without prior sensitization via stimulatory receptors and rapidly eradicate them. However, the protective tumor microenvironment facilitates tumor escaping via induction of an exhaustion state in immune cells, including NK cells. Hence, genetic manipulation of NK cells for specific identification of tumor-associated antigens or a more robust response against tumor cells is a promising strategy for NK cells' tumoricidal augmentation. Regarding the remarkable achievement of engineered CAR-T cells in treating hematologic malignancies, there is evolving interest in CAR-NK cell recruitment in cancer immunotherapy. Innate functionality of NK cells, higher safety, superior in vivo maintenance, and the off-the-shelf potential move CAR-NK-based therapy superior to CAR-T cells treatment. In this review, we have comprehensively discussed the recent genetic manipulations of CAR-NK cell manufacturing regarding different domains of CAR constructs and their following delivery systems into diverse sources of NK cells. Then highlight the preclinical and clinical investigations of CAR-NK cells and examine the current challenges and prospects as an optimistic remedy in cancer immunotherapy.

Fucosylation of HLA-DRB1 regulates CD4+ T cell-mediated anti-melanoma immunity and enhances immunotherapy efficacy

Immunotherapy efficacy is limited in melanoma, and combinations of immunotherapies with other modalities have yielded limited improvements but also adverse events requiring cessation of treatment. In addition to ineffective patient stratification, efficacy is impaired by paucity of intratumoral immune cells (itICs); thus, effective strategies to safely increase itICs are needed. We report that dietary administration of L-fucose induces fucosylation and cell surface enrichment of the major histocompatibility complex (MHC)-II protein HLA-DRB1 in melanoma cells, triggering CD4+ T cell-mediated increases in itICs and anti-tumor immunity, enhancing immune checkpoint blockade responses. Melanoma fucosylation and fucosylated HLA-DRB1 associate with intratumoral T cell abundance and anti-programmed cell death protein 1 (PD1) responder status in patient melanoma specimens, suggesting the potential use of melanoma fucosylation as a strategy for stratifying patients for immunotherapies. Our findings demonstrate that fucosylation is a key mediator of anti-tumor immunity and, importantly, suggest that L-fucose is a powerful agent for safely increasing itICs and immunotherapy efficacy in melanoma.

MHC Class I–Dependent Shaping of the NK Cell Ly49 Receptor Repertoire Takes Place Early during Maturation in the Bone Marrow

MHC class I (MHC I) expression in the host influences NK cells in a process termed education. The result of this education is reflected in the responsiveness of NK cells at the level of individual cells as well as in the repertoire of inhibitory MHC I–specific receptors at the NK cell system level. The presence of MHC I molecules in the host environment gives rise to a skewed receptor repertoire in spleen NK cells where subsets expressing few (one or two) inhibitory receptors are expanded whereas subsets with many (three or more) receptors are contracted. It is not known whether this MHC I–dependent skewing is imposed during development or after maturation of NK cells. In this study, we tested the hypothesis that the NK cell receptor repertoire is shaped already early during NK cell development in the bone marrow. We used mice with a repertoire imposed by a single MHC I allele, as well as a C57BL/6 mutant strain with exaggerated repertoire skewing, to investigate Ly49 receptor repertoires at different stages of NK cell differentiation. Our results show that NK cell inhibitory receptor repertoire skewing can indeed be observed in the bone marrow, even during the earliest developmental steps where Ly49 receptors are expressed. This may partly be accounted for by selective proliferation of certain NK cell subsets, but other mechanisms must also be involved. We propose a model for how repertoire skewing is established during a developmental phase in the bone marrow, based on sequential receptor expression as well as selective proliferation.

Binary outcomes of enhancer activity underlie stable random monoallelic expression

Mitotically stable random monoallelic gene expression (RME) is documented for a small percentage of autosomal genes. We developed an in vivo genetic model to study the role of enhancers in RME using high-resolution single-cell analysis of natural killer (NK) cell receptor gene expression and enhancer deletions in the mouse germline. Enhancers of the RME NK receptor genes were accessible and enriched in H3K27ac on silent and active alleles alike in cells sorted according to allelic expression status, suggesting enhancer activation and gene expression status can be decoupled. In genes with multiple enhancers, enhancer deletion reduced gene expression frequency, in one instance converting the universally expressed gene encoding NKG2D into an RME gene, recapitulating all aspects of natural RME including mitotic stability of both the active and silent states. The results support the binary model of enhancer action, and suggest that RME is a consequence of general properties of gene regulation by enhancers rather than an RME-specific epigenetic program. Therefore, many and perhaps all genes may be subject to some degree of RME. Surprisingly, this was borne out by analysis of several genes that define different major hematopoietic lineages, that were previously thought to be universally expressed within those lineages: the genes encoding NKG2D, CD45, CD8α, and Thy-1. We propose that intrinsically probabilistic gene allele regulation is a general property of enhancer-controlled gene expression, with previously documented RME representing an extreme on a broad continuum.

Beyond Maternal Tolerance: Education of Uterine Natural Killer Cells by Maternal MHC Drives Fetal Growth

Reproductive immunology has moved on from the classical Medawar question of 60 years ago "why doesn't the mother reject the fetus?". Looking beyond fetal-maternal tolerance, modern reproductive immunology focuses on how the maternal immune system supports fetal growth. Maternal uterine natural killer (uNK) cells, in partnership with fetal trophoblast cells, regulate physiological vascular changes in the uterus of pregnant women and mice. These vascular changes are necessary to build the placenta and sustain fetal growth. NK cell functions in the uterus and elsewhere, including anti-viral and anti-tumour immunity mediated mostly by blood NK cells, are modulated by NK cell education, a quantifiable process that determines cellular activation thresholds. This process relies largely on interactions between self-MHC class I molecules and inhibitory NK cell receptors. By getting to know self, the maternal immune system sets up uNK cells to participate to tissue homeostasis in the womb. Placentation can be viewed as a form of natural transplantation unique in vertebrates and this raises the question of how uNK cell education or missing-self recognition affect their function and, ultimately fetal growth. Here, using combinations of MHC-sufficient and -deficient mice, we show that uNK cell education is linked to maternal and not fetal MHC, so that MHC-deficient dams produce more growth-restricted fetuses, even when the fetuses themselves express self-MHC. We also show that, while peripheral NK cells reject bone marrow cells according to the established rules of missing-self recognition, uNK cells educated by maternal MHC do not reject fetuses that miss self-MHC and these fetuses grow to their full potential. While these results are not directly applicable to clinical research, they show that NK education by maternal MHC-I is required for optimal fetal growth.

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.

The role of KIR positive NK cells in diseases and its importance in clinical intervention

Natural killer (NK) cells are essential for the elimination of the transformed and cancerous cells. Killer cell immunoglobulin-like receptors (KIRs) which expressed by T and NK cells, are key regulator of NK cell function. The KIR and their ligands, MHC class I (HLA-A, B and C) molecules, are highly polymorphic and their related genes are located on 19 q13.4 and 6 q21.3 chromosomes, respectively. It is clear that particular interaction between the KIRs and their related ligands can influence on the prevalence, progression and outcome of several diseases, like complications of pregnancy, viral infection, autoimmune diseases, and hematological malignancies. The mechanisms of immune signaling in particular NK cells involvement in causing pathological conditions are not completely understood yet. Therefore, better understanding of the molecular mechanism of KIR-MHC class I interaction could facilitate the treatment strategy of diseases. The present review focused on the main characteristics and functional details of various KIR and their combination with related ligands in diseases and also highlights ongoing efforts to manipulate the key checkpoints in NK cell-based immunotherapy.

Transcriptional Regulation of Natural Killer Cell Development and Functions

Natural killer (NK) cells are the major lymphocyte subset of the innate immune system. Their ability to mediate anti-tumor cytotoxicity and produce cytokines is well-established. However, the molecular mechanisms associated with the development of human or murine NK cells are not fully understood. Knowledge is being gained about the environmental cues, the receptors that sense the cues, signaling pathways, and the transcriptional programs responsible for the development of NK cells. Specifically, a complex network of transcription factors (TFs) following microenvironmental stimuli coordinate the development and maturation of NK cells. Multiple TFs are involved in the development of NK cells in a stage-specific manner. In this review, we summarize the recent advances in the understandings of TFs involved in the regulation of NK cell development, maturation, and effector function, in the aspects of their mechanisms, potential targets, and functions.

Deciphering Natural Killer Cell Homeostasis

Natural killer (NK) cells have a central role within the innate immune system, eliminating virally infected, foreign and transformed cells through their natural cytotoxic capacity. Release of their cytotoxic granules is tightly controlled through the balance of a large repertoire of inhibitory and activating receptors, and it is the unique combination of these receptors expressed by individual cells that confers immense diversity both in phenotype and functionality. The diverse, yet unique, NK cell repertoire within an individual is surprisingly stable over time considering the constant renewal of these cells at steady state. Here we give an overview of NK cell differentiation and discuss metabolic requirements, intra-lineage plasticity and transcriptional reprogramming during IL-15-driven homeostatic proliferation. New insights into the regulation of NK cell differentiation and homeostasis could pave the way for the successful implementation of NK cell-based immunotherapy against cancer.

NK Cell-Based Immunotherapies in Cancer

With the development of technologies that can transform immune cells into therapeutic modalities, immunotherapy has remarkably changed the current paradigm of cancer treatment in recent years. NK cells are components of the innate immune system that act as key regulators and exhibit a potent tumor cytolytic function. Unlike T cells, NK cells exhibit tumor cytotoxicity by recognizing non-self, without deliberate immunization or activation. Currently, researchers have developed various approaches to improve the number and anti-tumor function of NK cells. These approaches include the use of cytokines and Abs to stimulate the efficacy of NK cell function, adoptive transfer of autologous or allogeneic ex vivo expanded NK cells, establishment of homogeneous NK cell lines using the NK cells of patients with cancer or healthy donors, derivation of NK cells from induced pluripotent stem cells (iPSCs), and modification of NK cells with cutting-edge genetic engineering technologies to generate chimeric Ag receptor (CAR)-NK cells. Such NK cell-based immunotherapies are currently reported as being promising anti-tumor strategies that have shown enhanced functional specificity in several clinical trials investigating malignant tumors. Here, we summarize the recent advances in NK cell-based cancer immunotherapies that have focused on providing improved function through the use of the latest genetic engineering technologies. We also discuss the different types of NK cells developed for cancer immunotherapy and present the clinical trials being conducted to test their safety and efficacy.

Synergized regulation of NK cell education by NKG2A and specific Ly49 family members

Mice lacking MHC class-I (MHC-I) display severe defects in natural killer (NK) cell functional maturation, a process designated as “education”. Whether self-MHC-I specific Ly49 family receptors and NKG2A, which are closely linked within the NK gene complex (NKC) locus, are essential for NK cell education is still unclear. Here we show, using CRISPR/Cas9-mediated gene deletion, that mice lacking all members of the Ly49 family exhibit a moderate defect in NK cell activity, while mice lacking only two inhibitory Ly49 members, Ly49C and Ly49I, have comparable phenotypes. Furthermore, the deficiency of NKG2A, which recognizes non-classical MHC-Ib molecules, mildly impairs NK cell function. Notably, the combined deletion of NKG2A and the Ly49 family severely compromises the ability of NK cells to mediate “missing-self” and “induced-self” recognition. Therefore, our data provide genetic evidence supporting that NKG2A and the inhibitory members of Ly49 family receptors synergize to regulate NK cell education.

MHC-I-induced signalling of various natural killer (NK) inhibitory receptors is critical for regulation NK cell education, but clear genetic evidence is still lacking. Here the authors generate multiple lines of mice differentially deficient in Ly49 family and/or NKG2A NK receptors, and find that self-MHCI specific Ly49 members and NKG2A synergize to regulate NK education.

Human Natural Killer Receptors, Co-Receptors, and Their Ligands

In the last 20 years, the study of human natural killer (NK) cells has moved from the first molecular characterizations of very few receptor molecules to the identification of a plethora of receptors displaying surprisingly divergent functions. We have contributed to the description of inhibitory receptors and their signaling pathways, important in fine regulation in many cell types, but unknown until their discovery in the NK cells. Inhibitory function is central to regulating NK-mediated cytolysis, with different molecular structures evolving during speciation to assure its persistence. More recently, it has become possible to characterize the NK triggering receptors mediating natural cytotoxicity, unveiling the existence of a network of cellular interactions between effectors of both natural and adaptive immunity. This unit reviews the contemporary history of molecular studies of receptors and ligands involved in NK cell function, characterizing the ligands of the triggering receptor and the mechanisms for finely regulating their expression in pathogen-infected or tumor cells. © 2018 by John Wiley & Sons, Inc.

Genetics of Natural Killer Cells in Human Health, Disease, and Survival

Natural killer (NK) cells have vital functions in human immunity and reproduction. In the innate and adaptive immune responses to infection, particularly by viruses, NK cells respond by secreting inflammatory cytokines and killing infected cells. In reproduction, NK cells are critical for genesis of the placenta, the organ that controls the supply of oxygen and nutrients to the growing fetus. Controlling NK cell functions are interactions of HLA class I with inhibitory NK cell receptors. First evolved was the conserved interaction of HLA-E with CD94:NKG2A; later established were diverse interactions of HLA-A, -B, and -C with killer cell immunoglobulin-like receptors. Characterizing the latter interactions is rapid evolution, which distinguishes human populations and all species of higher primate. Driving this evolution are the different and competing selections imposed by pathogens on NK cell-mediated immunity and by the constraints of human reproduction on NK cell-mediated placentation. Promoting rapid evolution is independent segregation of polymorphic receptors and ligands throughout human populations.

Immunoreceptor tyrosine-based inhibitory motif-dependent functions of an MHC class I-specific NK cell receptor

Natural killer (NK) cells express MHC class I (MHC-I)-specific receptors, such as Ly49A, that inhibit killing of cells expressing self-MHC-I. Self-MHC-I also "licenses" NK cells to become responsive to activating stimuli and regulates the surface level of NK-cell inhibitory receptors. However, the mechanisms of action resulting from these interactions of the Ly49s with their MHC-I ligands, particularly in vivo, have been controversial. Definitive studies could be derived from mice with targeted mutations in inhibitory Ly49s, but there are inherent challenges in specifically altering a single gene within a multigene family. Herein, we generated a knock-in mouse with a targeted mutation in the immunoreceptor tyrosine-based inhibitory motif (ITIM) of Ly49A that abolished the inhibitory function of Ly49A in cytotoxicity assays. This mutant Ly49A caused a licensing defect in NK cells, but the surface expression of Ly49A was unaltered. Moreover, NK cells that expressed this mutant Ly49A exhibited an altered inhibitory receptor repertoire. These results demonstrate that Ly49A ITIM signaling is critical for NK-cell effector inhibition, licensing, and receptor repertoire development.

Expression of CD226 is associated to but not required for NK cell education

DNAX accessory molecule-1 (DNAM-1, also known as CD226) is an activating receptor expressed on subsets of natural killer (NK) and T cells, interacts with its ligands CD155 or CD112, and has co-varied expression with inhibitory receptors. Since inhibitory receptors control NK-cell activation and are necessary for MHC-I-dependent education, we investigated whether DNAM-1 expression is also involved in NK-cell education. Here we show an MHC-I-dependent correlation between DNAM-1 expression and NK-cell education, and an association between DNAM-1 and NKG2A that occurs even in MHC class I deficient mice. DNAM-1 is expressed early during NK-cell development, precedes the expression of MHC-I-specific inhibitory receptors, and is modulated in an education-dependent fashion. Cd226^−/− mice have missing self-responses and NK cells with a normal receptor repertoire. We propose a model in which NK-cell education prevents or delays downregulation of DNAM-1. This molecule endows educated NK cells with enhanced effector functions but is dispensable for education.

CD226 is an activating receptor expressed in a co-varied manner with inhibitory receptors on natural killer (NK) cells, but whether CD226 is involved in NK cell education is unclear. Here the authors show that CD226 expression is plastic depending on the MHC environment and endows educated NK cells enhanced effector functions.

Differential Role of Hematopoietic and Nonhematopoietic Cell Types in the Regulation of NK Cell Tolerance and Responsiveness

Many NK cells express inhibitory receptors that bind self-MHC class I (MHC I) molecules and prevent killing of self-cells, while enabling killing of MHC I-deficient cells. But tolerance also occurs for NK cells that lack inhibitory receptors for self-MHC I, and for all NK cells in MHC I-deficient animals. In both cases, NK cells are unresponsive to MHC I-deficient cells and hyporesponsive when stimulated through activating receptors, suggesting that hyporesponsiveness is responsible for self-tolerance. We generated irradiation chimeras, or carried out adoptive transfers, with wild-type (WT) and/or MHC I-deficient hematopoietic cells in WT or MHC I-deficient C57BL/6 host mice. Unexpectedly, in WT hosts, donor MHC I-deficient hematopoietic cells failed to induce hyporesponsiveness to activating receptor stimulation, but did induce tolerance to MHC I-deficient grafts. Therefore, these two properties of NK cells are separable. Both tolerance and hyporesponsiveness occurred when the host was MHC I deficient. Interestingly, infections of mice or exposure to inflammatory cytokines reversed the tolerance of NK cells that was induced by MHC I-deficient hematopoietic cells, but not the tolerance induced by MHC I-deficient nonhematopoietic cells. These data have implications for successful bone marrow transplantation, and suggest that tolerance induced by hematopoietic cells versus nonhematopoietic cells may be imposed by distinct mechanisms.

Killer-cell immunoglobulin-like receptor genes and ligands and their role in hematologic malignancies

Natural killer (NK) cells are considered crucial for the elimination of emerging tumor cells. Effector NK-cell functions are controlled by interactions of inhibitory and activating killer-cell immunoglobulin-like receptors (KIRs) on NK cells with human leukocyte antigen (HLA) class I ligands on target cells. KIR and HLA are highly polymorphic genetic systems segregating independently, creating a great diversity in KIR/HLA gene profiles in different individuals. There is an increasing evidence supporting the relevance of KIR and HLA ligand gene background for the occurrence and outcome of certain cancers. However, the data are still controversial and the mechanisms of receptor-ligand mediated NK-cell action remain unclear. Here, the main characteristics and functions of KIRs and their HLA class I ligands are reviewed. In addition, we review the HLA and KIR correlations with different hematological malignancies and discuss our current understanding of the biological significance and mechanisms underlying these associations.

Activation of Natural Killer Cells by Probiotics

During the last decade, probiotics have been established to be important mediators of host immunity. Their effects on both innate and adaptive immunity have been documented in the literature. Although several reports have correlated different strains of bacteria as probiotics, their effects on immunity vary. Clearly, there is a complex interplay between various constituents of probiotics and the immune response in humans. The role of probiotics on natural killer (NK) cells in the gut has been the subject of a few reports. In this review, we summarize the reported findings on the role of probiotics in the activation of gut-associated NK cells and the response of NK cells to stimuli elicited by probiotics and their microenvironment. The effects of probiotics on the activation of NK cells and their secretion of immune factors (e.g., interferon-γ, tumor necrosis factor-α, interleukin-2, etc.) are discussed in regard to their clinical significance in various diseases. Current investigations are being pursued, in particular, on the role of probiotics-activated NK cells in promoting the adaptive immune response against pathogens.

The evolution of natural killer cell receptors

Natural killer (NK) cells are immune cells that play a crucial role against viral infections and tumors. To be tolerant against healthy tissue and simultaneously attack infected cells, the activity of NK cells is tightly regulated by a sophisticated array of germline-encoded activating and inhibiting receptors. The best characterized mechanism of NK cell activation is “missing self” detection, i.e., the recognition of virally infected or transformed cells that reduce their MHC expression to evade cytotoxic T cells. To monitor the expression of MHC-I on target cells, NK cells have monomorphic inhibitory receptors which interact with conserved MHC molecules. However, there are other NK cell receptors (NKRs) encoded by gene families showing a remarkable genetic diversity. Thus, NKR haplotypes contain several genes encoding for receptors with activating and inhibiting signaling, and that vary in gene content and allelic polymorphism. But if missing-self detection can be achieved by a monomorphic NKR system why have these polygenic and polymorphic receptors evolved? Here, we review the expansion of NKR receptor families in different mammal species, and we discuss several hypotheses that possibly underlie the diversification of the NK cell receptor complex, including the evolution of viral decoys, peptide sensitivity, and selective MHC-downregulation.

Prenatal Allospecific NK Cell Tolerance Hinges on Instructive Allorecognition through the Activating Receptor during Development

Little is known about how the prenatal interaction between NK cells and alloantigens shapes the developing NK cell repertoire toward tolerance or immunity. Specifically, the effect on NK cell education arising from developmental corecognition of alloantigens by activating and inhibitory receptors with shared specificity is uncharacterized. Using a murine prenatal transplantation model, we examined the manner in which this seemingly conflicting input affects NK cell licensing and repertoire formation in mixed hematopoietic chimeras. We found that prenatal NK cell tolerance arose from the elimination of phenotypically hostile NK cells that express an allospecific activating receptor without coexpressing any allospecific inhibitory receptors. Importantly, the checkpoint for the system appeared to occur centrally within the bone marrow during the final stage of NK cell maturation and hinged on the instructive recognition of allogeneic ligand by the activating receptor rather than through the inhibitory receptor as classically proposed. Residual nondeleted hostile NK cells expressing only the activating receptor exhibited an immature, anergic phenotype, but retained the capacity to upregulate inhibitory receptor expression in peripheral sites. However, the potential for this adaptive change to occur was lost in developmentally mature chimeras. Collectively, these findings illuminate the intrinsic process in which developmental allorecognition through the activating receptor regulates the emergence of durable NK cell tolerance and establishes a new paradigm to fundamentally guide future investigations of prenatal NK cell-allospecific education.

Genetic investigation of MHC-independent missing-self recognition by mouse NK cells using an in vivo bone marrow transplantation model

MHC-I-specific receptors play a vital role in NK cell-mediated "missing-self" recognition, which contributes to NK cell activation. In contrast, MHC-independent NK recognition mechanisms are less well characterized. In this study, we investigated the role of NKR-P1B:Clr-b (Klrb1:Clec2d) interactions in determining the outcome of murine hematopoietic cell transplantation in vivo. Using a competitive transplant assay, we show that Clr-b(-/-) bone marrow (BM) cells were selectively rejected by wild-type B6 recipients, to a similar extent as H-2D(b-/-) MHC-I-deficient BM cells. Selective rejection of Clr-b(-/-) BM cells was mitigated by NK depletion of recipient mice. Competitive rejection of Clr-b(-/-) BM cells also occurred in allogeneic transplant recipients, where it was reversed by selective depletion of NKR-P1B(hi) NK cells, leaving the remaining NKR-P1B(lo) NK subset and MHC-I-dependent missing-self recognition intact. Moreover, competitive rejection of Clr-b(-/-) hematopoietic cells was abrogated in Nkrp1b-deficient recipients, which lack the receptor for Clr-b. Of interest, similar to MHC-I-deficient NK cells, Clr-b(-/-) NK cells were hyporesponsive to both NK1.1 (NKR-P1C)-stimulated and IL-12/18 cytokine-primed IFN-γ production. These findings support a unique and nonredundant role for NKR-P1B:Clr-b interactions in missing-self recognition of normal hematopoietic cells and suggest that optimal BM transplant success relies on MHC-independent tolerance mechanisms. These findings provide a model for human NKR-P1A:LLT1 (KLRB1:CLEC2D) interactions in human hematopoietic cell transplants.

Natural killer cell regulation - beyond the receptors

Natural killer (NK) cells are lymphocytes that are important for early and effective immune responses against infections and cancer. In the last 40 years, many receptors, their corresponding ligands and signaling pathways that regulate NK cell functions have been identified. However, we now know that additional processes, such as NK cell education, differentiation and also the formation of NK cell memory, have a great impact on the reactivity of these cells. Here, we summarize the current knowledge about these modulatory processes.

NK cell self tolerance, responsiveness and missing self recognition

Natural killer (NK) cells represent a first line of defense against pathogens and tumor cells. The activation of NK cells is regulated by the integration of signals deriving from activating and inhibitory receptors expressed on their surface. However, different NK cells respond differently to the same stimulus, be it target cells or agents that crosslink activating receptors. The processes that determine the level of NK cell responsiveness have been referred to collectively as NK cell education. NK cell education plays an important role in steady state conditions, where potentially auto-reactive NK cells are rendered tolerant to the surrounding environment. According to the "tuning" concept, the responsiveness of each NK cell is quantitatively adjusted to ensure self tolerance while at the same time ensuring useful reactivity against potential threats. MHC-specific inhibitory receptors displayed by NK cells play a major role in tuning NK cell responsiveness, but recent studies indicate that signaling from activating receptors is also important, suggesting that the critical determinant is an integrated signal from both types of receptors. An important and still unresolved question is whether NK cell education involves interactions with a specific cell population in the environment. Whether hematopoietic and/or non-hematopoietic cells play a role is still under debate. Recent results demonstrated that NK cell tuning exhibits plasticity in steady state conditions, meaning that it can be re-set if the MHC environment changes. Other evidence suggests, however, that inflammatory conditions accompanying infections may favor high responsiveness, indicating that inflammatory agents can over-ride the natural tendency of NK cells to adjust to the steady state environment. These findings raise many questions such as whether viruses and tumor cells manipulate NK cell responsiveness to evade immune-recognition. As knowledge of the underlying processes grows, the possibility of modulating NK cell responsiveness for therapeutic purposes is becoming increasingly attractive, and is now under serious investigation in clinical studies.

Genetic and environmental determinants of human NK cell diversity revealed by mass cytometry

Natural killer (NK) cells play critical roles in immune defense and reproduction, yet remain the most poorly understood major lymphocyte population. Because their activation is controlled by a variety of combinatorially expressed activating and inhibitory receptors, NK cell diversity and function are closely linked. To provide an unprecedented understanding of NK cell repertoire diversity, we used mass cytometry to simultaneously analyze 37 parameters, including 28 NK cell receptors, on peripheral blood NK cells from 5 sets of monozygotic twins and 12 unrelated donors of defined human leukocyte antigen (HLA) and killer cell immunoglobulin-like receptor (KIR) genotype. This analysis revealed a remarkable degree of NK cell diversity, with an estimated 6000 to 30,000 phenotypic populations within an individual and >100,000 phenotypes in the donor panel. Genetics largely determined inhibitory receptor expression, whereas activation receptor expression was heavily environmentally influenced. Therefore, NK cells may maintain self-tolerance through strictly regulated expression of inhibitory receptors while using adaptable expression patterns of activating and costimulatory receptors to respond to pathogens and tumors. These findings further suggest the possibility that discrete NK cell subpopulations could be harnessed for immunotherapeutic strategies in the settings of infection, reproduction, and transplantation.

Differential expression of the Ly49G(B6), but not the Ly49G(BALB), receptor isoform during natural killer cell reconstitution after hematopoietic stem cell transplantation

Inhibitory natural killer (NK) cell receptors specific for major histocompatibility complex class I (MHC-I) molecules include Ly49 receptors in mice and killer immunoglobulin-like receptors (KIR) in humans. The "licensing" or "arming" models imply that engagement of these receptors to self MHC-I molecules during NK cell development educates NK cells to be more responsive to cancer and viral infection. We recently reported that hematopoietic stem cell transplantation (HSCT) induced rapid and preferential expansion of functionally competent Ly49G(+), but not other Ly49 family, NK cells independent of NK cell licensing via Ly49-MHC-I interactions. We now extend these studies to evaluate expression of the two Ly49G receptor isoforms Ly49G(B6) and Ly49G(BALB), using mice with different MHC-I haplotypes that express one or both of the isoforms. NK cells from CB6F1 (H-2(bxd)) hybrid mice express two different alleles for Ly49G receptor, Ly49G(B6) and Ly49G(BALB). We found that CB6F1 mice had more Ly49G(B6+) NK cells than Ly49(BALB+) NK cells, and that only Ly49G(B6+) NK cells increased in relative numbers and in Ly49G mean fluorescence intensity values after HSCT similar to the B6 parental strain. We further observed that Ly49G(+) NK cells in BALB/c (H-2(d)) and BALB.B (H-2(b)) mice, which have the same background genes, recover slowly after HSCT, in contrast to Ly49G(+) NK cells in B6 (H-2(b)) recipients. The difference in expression of Ly49G(B6) relative to Ly49G(BALB) was linked to differences in the activity of the Pro1 promoter between the two alleles. Thus, we conclude that the Ly49G(B6) receptor dominates Ly49G expression on NK cells after HSCT in strains in which that allele is expressed. The data suggest that Ly49 allelic polymorphism within a particular Ly49 family member can differentially affect NK cell recovery after HSCT depending on the background genes of the recipient, not on the MHC-I haplotype.

Natural killer cell biology: an update and future directions

Natural killer (NK) cells constitute a minor subset of normal lymphocytes that initiate innate immune responses toward tumor and virus-infected cells. They can mediate spontaneous cytotoxicity toward these abnormal cells and rapidly secrete numerous cytokines and chemokines to promote subsequent adaptive immune responses. Significant progress has been made in the past 2 decades to improve our understanding of NK cell biology. Here we review recent discoveries, including a better comprehension of the "education" of NK cells to achieve functional competence during their maturation and the discovery of "memory" responses by NK cells, suggesting that they might also contribute to adaptive immunity. The improved understanding of NK cell biology has forged greater awareness that these cells play integral early roles in immune responses. In addition, several promising clinical therapies have been used to exploit NK cell functions in treating patients with cancer. As our molecular understanding improves, these and future immunotherapies should continue to provide promising strategies to exploit the unique functions of NK cells to treat cancer, infections, and other pathologic conditions.

The Glycophosphatidylinositol Anchor of the MCMV Evasin, m157, Facilitates Optimal Cell Surface Expression and Ly49 Receptor Recognition

The murine cytomegalovirus-encoded protein m157 is a cognate ligand for both inhibitory and activating receptors expressed by natural killer cells. Additionally, m157 is expressed on the surface of infected cells by a glycophosphatidylinositol (GPI) anchor. Although endogenous GPI-anchored proteins are known to be ligands for the NK cell receptor, NKG2D, the contribution of the GPI anchor for viral m157 ligand function is unknown. To determine whether the GPI anchor for m157 is dispensable for m157 function, we generated m157 variants expressed as transmembrane fusion proteins and tested cells expressing transmembrane m157 for the capacity to activate cognate Ly49 receptors. We found that the GPI anchor is required for high-level cell surface expression of m157, and that the transmembrane m157 ligand retains the capacity to activate reporter cells and NK cells expressing Ly49H, as well as Ly49I(129) reporter cells, but with reduced potency. Importantly, target cells expressing the transmembrane form of m157 were killed less efficiently and failed to mediate Ly49H receptor downregulation on fresh NK cells compared to targets expressing GPI-anchored m157. Taken together, these results show that the GPI anchor for m157 facilitates robust cell surface expression, and that NK cells are sensitive to the altered cell surface expression of this potent viral evasin.

The role of KIR genes and ligands in leukemia surveillance

The antileukemic potential of natural killer (NK) cells has been of rising interest in recent years. Interactions between inhibitory killer cell immunoglobulin-like receptors (KIR) and HLA class I ligands seem to be critically involved in the immunosurveillance process. It is also well established that mismatching of HLA class I-encoded KIR ligands in the setting of hematopoietic stem cell transplantation leads to allorecognition of leukemic cells by NK cells, which is in line with the concept of missing-self recognition. Recent data now suggest that KIR gene polymorphism constitutes another important parameter that needs to be taken into account for selection of suitable stem cell donors. Moreover, the role of KIR gene polymorphism for predisposition to leukemia is a current matter of debate. Here, we would like to review the role of KIR function and genetic polymorphism for recognition of leukemia and discuss the impact of these findings for developing novel concepts for NK cell-based immunotherapy strategies.

Innate immune control of HIV

Mounting evidence suggests a role for innate immunity in the early control of HIV infection, before the induction of adaptive immune responses. Among the early innate immune effector cells, dendritic cells (DCs) respond rapidly following infection aimed at arming the immune system, through the recognition of viral products via pattern recognition receptors. This early response results in the potent induction of a cascade of inflammatory cytokines, intimately involved in directly setting up an antiviral state, and indirectly activating other antiviral cells of the innate immune system. However, epidemiologic data strongly support a role for natural killer (NK) cells as critical innate mediators of antiviral control, through the recognition of virally infected cells through a network of receptors called the killer immunoglobulin-like receptors (KIRs). In this review, the early events in innate immune recognition of HIV, focused on defining the biology underlying KIR-mediated NK-cell control of HIV viral replication, are discussed.

The role of natural killer cells in hematopoietic stem cell transplantation

Natural killer (NK) cells are important elements of innate immunity, and a large body of evidence supports the significant role of NK in immune surveillance against infections and tumors. Regulation of cytotoxic activity is mediated through activating and inhibitory receptors expressed on the cell surface. NK cells are key players of allogeneic hematopoietic stem cell transplantation (allo-SCT), and previous studies showed the beneficial effect of NK alloreactivity in prevention of relapse, especially in the setting of haploidentical SCT. Biology of human NK cells is an area of active research. Exploitation of the molecular mechanisms regulating NK maturation, tolerance to self, and NK-mediated cytotoxicity will help in the development of innovative NK cell immunotherapy methods.

Mouse NK cell-mediated rejection of bone marrow allografts exhibits patterns consistent with Ly49 subset licensing

Natural killer (NK) cells can mediate the rejection of bone marrow allografts and exist as subsets based on expression of inhibitory/activating receptors that can bind MHC. In vitro data have shown that NK subsets bearing Ly49 receptors for self-MHC class I have intrinsically higher effector function, supporting the hypothesis that NK cells undergo a host MHC-dependent functional education. These subsets also play a role in bone marrow cell (BMC) allograft rejection. Thus far, little in vivo evidence for this preferential licensing across mouse strains with different MHC haplotypes has been shown. We assessed the intrinsic response potential of the different Ly49(+) subsets in BMC rejection by using β2-microglobulin deficient (β2m(-/-)) mice as donors. Using congenic and allogeneic mice as recipients and depleting the different Ly49 subsets, we found that NK subsets bearing Ly49s, which bind "self-MHC" were found to be the dominant subset responsible for β2m(-/-) BMC rejection. This provides in vivo evidence for host MHC class I-dependent functional education. Interestingly, all H2(d) strain mice regardless of background were able to resist significantly greater amounts of β2m(-/-), but not wild-type BMC than H2(b) mice, providing evidence that the rheostat hypothesis regarding Ly49 affinities for MHC and NK-cell function impacts BMC rejection capability.

All is fair in virus-host interactions: NK cells and cytomegalovirus

The infection of mice with mouse cytomegalovirus (MCMV) as a model of human cytomegalovirus (HCMV) infection has been particularly informative in elucidating the role of innate and adaptive immune response mechanisms during infection. Millions of years of co-evolution between cytomegaloviruses (CMV) and their hosts has resulted in numerous attempts to overwhelm each other. CMVs devote many genes to modulating the host natural killer (NK) cell response and NK cells employ many strategies to cope with CMV infection. While focusing on these attack-counterattack measures, this review will discuss several novel mechanisms of immune evasion by MCMV, the role of Ly49 receptors in mediating resistance to MCMV, and the impact of the initial NK cell response on the shaping of adaptive immunity.

Inhibitory receptors specific for MHC class I educate murine NK cells but not CD8αα intestinal intraepithelial T lymphocytes

The engagement of inhibitory receptors specific for major histocompatibility complex class I (MHC-I) molecules educates natural killer (NK) cells, meaning the improvement of the response of activation receptors to subsequent stimulation. It is not known whether inhibitory MHC-I receptors educate only NK cells or whether they improve the responsiveness of all cell types, which express them. To address this issue, we analyzed the expression of inhibitory MHC-I receptors on intestinal intraepithelial lymphocytes (iIELs) and show that T-cell receptor (TCR)-αβ CD8αα iIELs express multiple inhibitory receptors specific for MHC-I molecules, including CD94/NKG2A, Ly49A, and Ly49G2. However, the presence of MHC-I ligand for these receptors did not improve the response of iIELs to activation via the TCR. The absence of iIEL education by MHC-I receptors was not related to a lack of inhibitory function of these receptors in iIELs and a failure of these receptors to couple to the TCR. Thus, unlike NK cells, iIELs do not undergo an MHC-I-guided education process. These data suggest that education is an NK cell-specific function of inhibitory MHC-I receptors.

NK cell receptor/H2-Dk-dependent host resistance to viral infection is quantitatively modulated by H2q inhibitory signals

The cytomegalovirus resistance locus Cmv3 has been linked to an epistatic interaction between two loci: a Natural Killer (NK) cell receptor gene and the major histocompatibility complex class I (MHC-I) locus. To demonstrate the interaction between Cmv3 and H2^k, we generated double congenic mice between MA/My and BALB.K mice and an F₂ cross between FVB/N (H-2^q) and BALB.K (H2^k) mice, two strains susceptible to mouse cytomegalovirus (MCMV). Only mice expressing H2^k in conjunction with Cmv3^MA/My or Cmv3^FVB were resistant to MCMV infection. Subsequently, an F₃ cross was carried out between transgenic FVB/H2-D^k and MHC-I deficient mice in which only the progeny expressing Cmv3^FVB and a single H2-D^k class-I molecule completely controlled MCMV viral loads. This phenotype was shown to be NK cell–dependent and associated with subsequent NK cell proliferation. Finally, we demonstrated that a number of H2^q alleles influence the expression level of H2^q molecules, but not intrinsic functional properties of NK cells; viral loads, however, were quantitatively proportional to the number of H2^q alleles. Our results support a model in which H-2^q molecules convey Ly49-dependent inhibitory signals that interfere with the action of H2-D^k on NK cell activation against MCMV infection. Thus, the integration of activating and inhibitory signals emanating from various MHC-I/NK cell receptor interactions regulates NK cell–mediated control of viral load.

Effective natural killer (NK) cell responses against virally infected cells are regulated by NK cell receptors that specifically recognize target cells. In the current study, we validated the specific interaction taking place between NK cell receptors and MHC class I molecules on the surface of infected cells, resulting in resistance to cytomegalovirus. Genetic dissection of this mechanism of interaction revealed that the NK cell response occurs exclusively through the triggering of the activating Ly49P receptor by the MHC class I H2-D^k molecule. We observed, in this context, that NK cells were incapable of clearing the virus when target cells also expressed MHC class I H2^q molecules, which strongly and quantitatively inhibit NK cells. Our findings reveal that the interplay between inhibitory and activating NK cell receptors and their MHC class I ligands generate signals that shape the outcome of infection.

Comprehensive analysis of transcript start sites in ly49 genes reveals an unexpected relationship with gene function and a lack of upstream promoters

Comprehensive analysis of the transcription start sites of the Ly49 genes of C57BL/6 mice using the oligo-capping 5′-RACE technique revealed that the genes encoding the “missing self” inhibitory receptors, Ly49A, C, G, and I, were transcribed from multiple broad regions in exon 1, in the intron1/exon2 region, and upstream of exon -1b. Ly49E was also transcribed in this manner, and uniquely showed a transcriptional shift from exon1 to exon 2 when NK cells were activated in vitro with IL2. Remarkably, a large proportion of Ly49E transcripts was then initiated from downstream of the translational start codon. By contrast, the genes encoding Ly49B and Q in myeloid cells, the activating Ly49D and H receptors in NK cells, and Ly49F in activated T cells, were predominantly transcribed from a conserved site in a pyrimidine-rich region upstream of exon 1. An ∼200 bp fragment from upstream of the Ly49B start site displayed tissue-specific promoter activity in dendritic cell lines, but the corresponding upstream fragments from all other Ly49 genes lacked detectable tissue-specific promoter activity. In particular, none displayed any significant activity in a newly developed adult NK cell line that expressed multiple Ly49 receptors. Similarly, no promoter activity could be found in fragments upstream of intron1/exon2. Collectively, these findings reveal a previously unrecognized relationship between the pattern of transcription and the expression/function of Ly49 receptors, and indicate that transcription of the Ly49 genes expressed in lymphoid cells is achieved in a manner that does not require classical upstream promoters.

Activation or tolerance of natural killer cells is modulated by ligand quality in a nonmonotonic manner

Natural killer (NK) cells extend important immune resistance in vertebrates by lysing infected and tumor cells. A fine balance between opposing signals generated by a diverse set of stimulatory and inhibitory NK-cell receptors determines the fate of target cells interacting with the NK cells. We have developed a mathematical model involving membrane proximal initial signaling events that provides novel mechanistic insights into how activation of NK cells is modulated by the half-life of receptor-ligand interaction and ligand concentrations. We show that strong stimulatory ligands produce digital activation, whereas weaker stimulatory ligands can mediate inhibition by strengthening the signals generated by inhibitory ligands, as indicated in experiments in knockout mice. We find under certain conditions, counterintuitively, inhibitory receptors can help mediate activation instead of inhibition. Mechanistic insights gained from NK-cell signaling can facilitate understanding of complex signaling responses that occur due to cross talk between dueling signaling pathways in other cell types.

Mice lacking Ly49E show normal NK cell development and provide evidence for probabilistic expression of Ly49E in NK cells and T cells

Ly49E is an unusual member of the Ly49 family that is expressed on fetal NK cells, epithelial T cells, and NKT cells, but not on resting adult NK cells. Ly49E(bgeo/bgeo) mice in which the Ly49E gene was disrupted by inserting a β-geo transgene were healthy, fertile, and had normal numbers of NK and T cells in all organs examined. Their NK cells displayed normal expression of Ly49 and other NK cell receptors, killed tumor and MHC class I-deficient cells efficiently, and produced normal levels of IFN-γ. In heterozygous Ly49E(+/bgeo) mice, the proportion of epidermal T cells, NKT cells, and IL-2-activated NK cells that expressed Ly49E was about half that found in wild-type mice. Surprisingly, although splenic T cells rarely expressed Ly49E, IL-2-activated splenic T cells from Ly49E(bgeo/bgeo) mice were as resistant to growth in G418 as NK cells and expressed similar levels of β-geo transcripts, suggesting that disruption of the Ly49E locus had increased its expression in these cells to the same level as that in NK cells. Importantly, however, the proportion of G418-resistant heterozygous Ly49E(+/bgeo) cells that expressed Ly49E from the wild-type allele was similar to that observed in control cells. Collectively, these findings demonstrate that Ly49E is not required for the development or homeostasis of NK and T cell populations or for the acquisition of functional competence in NK cells and provide compelling evidence that Ly49E is expressed in a probabilistic manner in adult NK cells and T cells.

Analyses of HLA-C–specific KIR repertoires in donors with group A and B haplotypes suggest a ligand-instructed model of NK cell receptor acquisition

To determine the influence of KIR and HLA class I polymorphism on human NK cell repertoires, 32 different clonotypes representing all possible combinations of 4 inhibitory KIR and NKG2A were analyzed by multicolor flow cytometry. In donors homozygous for the common group A KIR haplotype, a significant influence of HLA-C ligands was seen: KIR repertoires were dominated by clonotypes expressing a single KIR for the respective cognate ligand, either the C1-specific KIR2DL3 or C2-specific KIR2DL1. In contrast, in donors possessing the polymorphic group B haplotypes, a similar adaptation to cognate HLA-C was lacking. We suggest that this discrepancy is largely the result of a suppressive effect of the group B–specific KIR2DL2 on the frequency of KIR2DL1+ NK cells. In functional assays, KIR2DL2 not only recognized C1 but also C2 ligands, showing overlapping specificity with KIR2DL1. Moreover, using an NK cell differentiation assay we show sequential acquisition of KIR2DL2 before KIR2DL1 on developing NK cells. Together, these observations are compatible with a ligand-instructed model of NK cell education, in which recognition of HLA class I by an inhibitory receptor (KIR2DL2) suppresses subsequent expression of a second receptor (KIR2DL1) of related specificity. Importantly, the ligand-instructed model fits to the observed KIR repertoires in both broad KIR haplotype groups.

Natural killer cell education and tolerance

Natural killer (NK) cells play a key role in the immune response to certain infections and malignancies by direct cytolysis of infected or transformed cells and by secretion of potent immune mediators. NK cells express an array of activating receptors that recognize self-molecules. If not restrained by inhibitory receptors recognizing major histocompatibility complex (MHC) class I proteins on the surface of self cells, NK cells are able to kill normal, healthy cells. Not all NK cells express inhibitory receptors for self-MHC class I; thus, other tolerance mechanisms are necessary to prevent NK cell-mediated autoimmunity. Here we review the major mechanisms of NK cell education and tolerance.

Mature natural killer cells reset their responsiveness when exposed to an altered MHC environment

Some mature natural killer (NK) cells cannot be inhibited by major histocompatibility complex (MHC) I molecules, either because they lack corresponding inhibitory receptors or because the host lacks the corresponding MHC I ligands for the receptors. Such NK cells nevertheless remain self-tolerant and exhibit a generalized hyporesponsiveness to stimulation through activating receptors. To address whether NK cell responsiveness is set only during the NK cell differentiation process, we transferred mature NK cells from wild-type (WT) to MHC I–deficient hosts or vice versa. Remarkably, mature responsive NK cells from WT mice became hyporesponsive after transfer to MHC I–deficient mice, whereas mature hyporesponsive NK cells from MHC I–deficient mice became responsive after transfer to WT mice. Altered responsiveness was evident among mature NK cells that had not divided in the recipient animals, indicating that the cells were mature before transfer and that alterations in activity did not require cell division. Furthermore, the percentages of NK cells expressing KLRG1, CD11b, CD27, and Ly49 receptors specific for H-2^b were not markedly altered after transfer. Thus, the functional activity of mature NK cells can be reset when the cells are exposed to a changed MHC environment. These findings have important implications for how NK cell functions may be curtailed or enhanced in the context of disease.

Current perspectives of natural killer cell education by MHC class I molecules

From the early days of natural killer (NK) cell research, it was clear that MHC genes controlled the specificity of mouse NK cell-dependent responses, such as the ability to reject transplanted allogeneic bone marrow and to kill tumour cells. Although several mechanisms that are involved in this 'education' process have been clarified, most of the mechanisms have still to be identified. Here, we review the current understanding of the processes that are involved in NK cell education, including how the host MHC class I molecules regulate responsiveness and receptor repertoire formation in NK cells and the signalling pathways that are involved.

Activating and inhibitory receptors of natural killer cells

Natural killer (NK) cells are potent immune effector cells that can respond to infection and cancer, as well as allowing maternal adaptation to pregnancy. In response to malignant transformation or pathogenic invasion, NK cells can secrete cytokine and may be directly cytolytic, as well as exerting effects indirectly through other cells of the immune system. To recognize and respond to inflamed or infected tissues, NK cells express a variety of activating and inhibitory receptors including NKG2D, Ly49 or KIR, CD94-NKG2 heterodimers and natural cytotoxicity receptors, as well as co-stimulatory receptors. These receptors recognize cellular stress ligands as well as major histocompatibility complex class I and related molecules, which can lead to NK cell responses. Importantly, NK cells must remain tolerant of healthy tissue, and some of these receptors can also prevent activation of NK cells. In this review, we describe the expression of prominent NK cell receptors, as well as expression of their ligands and their role in immune responses. In addition, we describe the main signaling pathways used by NK cell receptors. Although we now appreciate that NK cell biology is more complicated than first thought, there are still facets of their biology that remain unclear. These will be highlighted and discussed in this review.

Inhibitory Ly49 receptors on mouse natural killer cells

The Ly49 receptors, which are expressed in a stochastic manner on subsets of murine natural killer (NK) cells, T cells, and other cells, are encoded by the Klra gene family and include receptors with either inhibitory or activating function. All of the inhibitory Ly49 receptors are characterized by an immunoreceptor tyrosine-based inhibitory motif in their cytoplasmic domain, which upon phosphorylation recruits tyrosine or lipid phosphatases to dampen signals transmitted through other activating receptors. Most of the inhibitory Ly49 receptors recognize polymorphic epitopes on major histocompatibility complex (MHC) class I proteins as ligands. Here, we review the polymorphism, ligand specificity, and signaling capacity of the inhibitory Ly49 receptors and discuss how these molecules regulate NK cell development and function.

"Self"-reflection by KIR

Significant debate exists over the proposed mechanisms by which NK receptor expression coordinates with the acquisition of function and titration of responsiveness, a process called NK-cell education. In this issue of Blood, Andersson and colleagues provide new insights into the mechanisms underlying NK-cell repertoire formation and the modulation of NK-cell function.

KIR acquisition probabilities are independent of self-HLA class I ligands and increase with cellular KIR expression

Inhibitory killer cell immunoglobulin-like receptors (KIRs) preserve tolerance to self and shape the functional response of human natural killer (NK) cells. Here, we have evaluated the influence of selection processes in the formation of inhibitory KIR repertoires in a cohort of 44 donors homozygous for the group A KIR haplotype. Coexpression of multiple KIRs was more frequent than expected by the product rule that describes random association of independent events. In line with this observation, the probability of KIR acquisition increased with the cellular expression of KIRs. Three types of KIR repertoires were distinguished that differed in frequencies of KIR- and NKG2A-positive cells but showed no dependency on the number of self-HLA class I ligands. Furthermore, the distribution of self- and nonself-KIRs at the cell surface reflected a random combination of receptors rather than a selection process conferred by cognate HLA class I molecules. Finally, NKG2A was found to buffer overall functional responses in KIR repertoires characterized by low-KIR expression frequencies. The results provide new insights into the formation of inhibitory KIR repertoires on human NK cells and support a model in which variegated KIR repertoires are generated through sequential and random acquisition of KIRs in the absence of selection.

Probing natural killer cell education by Ly49 receptor expression analysis and computational modelling in single MHC class I mice

Murine natural killer (NK) cells express inhibitory Ly49 receptors for MHC class I molecules, which allows for “missing self” recognition of cells that downregulate MHC class I expression. During murine NK cell development, host MHC class I molecules impose an “educating impact” on the NK cell pool. As a result, mice with different MHC class I expression display different frequency distributions of Ly49 receptor combinations on NK cells. Two models have been put forward to explain this impact. The two-step selection model proposes a stochastic Ly49 receptor expression followed by selection for NK cells expressing appropriate receptor combinations. The sequential model, on the other hand, proposes that each NK cell sequentially expresses Ly49 receptors until an interaction of sufficient magnitude with self-class I MHC is reached for the NK cell to mature. With the aim to clarify which one of these models is most likely to reflect the actual biological process, we simulated the two educational schemes by mathematical modelling, and fitted the results to Ly49 expression patterns, which were analyzed in mice expressing single MHC class I molecules. Our results favour the two-step selection model over the sequential model. Furthermore, the MHC class I environment favoured maturation of NK cells expressing one or a few self receptors, suggesting a possible step of positive selection in NK cell education. Based on the predicted Ly49 binding preferences revealed by the model, we also propose, that Ly49 receptors are more promiscuous than previously thought in their interactions with MHC class I molecules, which was supported by functional studies of NK cell subsets expressing individual Ly49 receptors.