NKp44 expression, phylogenesis and function in non-human primate NK cells

The natural cytotoxicity receptor (NCR) genes in the family Felidae

Natural killer (NK) cells belong to the innate immune system. The germline-encoded natural killer cell receptors represent activating and inhibitory receptors regulating multiple NK cell activities. The natural cytotoxicity receptors (NCRs) are activating natural cytotoxicity triggering receptors 1, 2 and 3 (NKp46, NKp44, and NKp30), encoded by the genes NCR1, NCR2, and NCR3, respectively. NCRs may be expressed in different cell types engaged in mechanisms of innate and adaptive immunity. The family Felidae, comprising the domestic cat and a wide variety of free-ranging species represents a well-suited model for biomedical and evolutionary studies. We characterized the NCR1, NCR2 and NCR3 genes in a panel of felid species. We confirmed the presence of potentially functional genes NCR1, NCR2 and NCR3 in all species. All three genes are conserved within the family and are similar to other phylogenetically related mammalian families. The NCR1 and NCR2 phylogenetic trees based on both nucleotide and protein sequences corresponded to the current zoological taxonomy, with some exceptions suggesting effects of different selection pressures in some species. Highly conserved NCR3 sequences did not allow a robust phylogenetic analysis. Most interspecific differences both at the nucleotide and protein level were found in NCR2. Within species, the most polymorphic CDS was detected in NCR1. Selection analyses indicated the effects of purifying selection on individual amino acid sites in all three genes. In stray cats, a rather high intraspecific diversity was observed. This article is protected by copyright. All rights reserved.

The Natural Cytotoxicity Receptor NKp44 (NCR2, CD336) Is Expressed on the Majority of Porcine NK Cells Ex Vivo Without Stimulation

Natural killer (NK) cells have been studied extensively in humans and mice for their vital role in the vertebrate innate immune system. They are known to rapidly eliminate tumors or virus infected cells in an immune response utilizing their lytic properties. The natural cytotoxicity receptors (NCRs) NKp30 (NCR3), NKp44 (NCR2), and NKp46 (NCR1) are important mediators of NK-cell cytotoxicity. NKp44 expression was reported for NK cells in humans as well as in some non-human primates and found exclusively on activated NK cells. Previously, no information was available on NKp44 protein expression and its role in porcine lymphocytes due to the lack of species-specific monoclonal antibodies (mAbs). For this study, porcine-specific anti-NKp44 mAbs were generated and their reactivity was tested on blood and tissue derived NK cells in pigs of different age classes. Interestingly, NKp44 expression was detected ex vivo already on resting NK cells; moreover, the frequency of NKp44⁺ NK cells was higher than that of NKp46⁺ NK cells in most animals analyzed. Upon in vitro stimulation with IL-2 or IL-15, the frequency of NKp44⁺ NK cells, as well as the intensity of NKp44 expression at the single cell level, were increased. Since little is known about swine NK cells, the generation of a mAb (clone 54-1) against NKp44 will greatly aid in elucidating the mechanisms underlying the differentiation, functionality, and activation of porcine NK cells.

Influenza A Virus Hemagglutinin and Other Pathogen Glycoprotein Interactions with NK Cell Natural Cytotoxicity Receptors NKp46, NKp44, and NKp30

Natural killer (NK) cells are part of the innate immunity repertoire, and function in the recognition and destruction of tumorigenic and pathogen-infected cells. Engagement of NK cell activating receptors can lead to functional activation of NK cells, resulting in lysis of target cells. NK cell activating receptors specific for non-major histocompatibility complex ligands are NKp46, NKp44, NKp30, NKG2D, and CD16 (also known as FcγRIII). The natural cytotoxicity receptors (NCRs), NKp46, NKp44, and NKp30, have been implicated in functional activation of NK cells following influenza virus infection via binding with influenza virus hemagglutinin (HA). In this review we describe NK cell and influenza A virus biology, and the interactions of influenza A virus HA and other pathogen lectins with NK cell natural cytotoxicity receptors (NCRs). We review concepts which intersect viral immunology, traditional virology and glycobiology to provide insights into the interactions between influenza virus HA and the NCRs. Furthermore, we provide expert opinion on future directions that would provide insights into currently unanswered questions.

Low-dose IL-2 induces CD56bright NK regulation of T cells via NKp44 and NKp46

Low-dose interleukin (IL)-2 has shown clinical benefits in patients with autoimmune and inflammatory diseases. Both regulatory T cells (Tregs ) and natural killer (NK) cells are increased in response to low-dose IL-2 immunotherapy. The role of regulatory T cells in autoimmune diseases has been extensively studied; however, NK cells have not been as thoroughly explored. It has not been well reported whether the increase in NK cells is purely an epiphenomenon or carries actual benefits for patients with autoimmune diseases. We demonstrate that low-dose IL-2 expands the primary human CD56bright NK cells resulting in a contact-dependent cell cycle arrest of effector T cells (Teffs ) via retention of the cycle inhibitor p21. We further show that NK cells respond via IL-2R-β, which has been shown to be significant for immunity by regulating T cell expansion. Moreover, we demonstrate that blocking NK receptors NKp44 and NKp46 but not NKp30 could abrogate the regulation of proliferation associated with low-dose IL-2. The increase in NK cells was also accompanied by an increase in Treg cells, which is dependent on the presence of CD56bright NK cells. These results not only heighten the importance of NK cells in low-dose IL-2 therapy but also identify key human NK targets, which may provide further insights into the therapeutic mechanisms of low-dose IL-2 in autoimmunity.

A subset of HLA-DP molecules serve as ligands for the natural cytotoxicity receptor NKp44

Natural killer (NK) cells can recognize virus-infected and stressed cells1 using activating and inhibitory receptors, many of which interact with HLA class I. Although early studies also suggested a functional impact of HLA class II on NK cell activity2,3, the NK cell receptors that specifically recognize HLA class II molecules have never been identified. We investigated whether two major families of NK cell receptors, killer-cell immunoglobulin-like receptors (KIRs) and natural cytotoxicity receptors (NCRs), contained receptors that bound to HLA class II, and identified a direct interaction between the NK cell receptor NKp44 and a subset of HLA-DP molecules, including HLA-DP401, one of the most frequent class II allotypes in white populations4. Using NKp44ζ+ reporter cells and primary human NKp44+ NK cells, we demonstrated that interactions between NKp44 and HLA-DP401 trigger functional NK cell responses. This interaction between a subset of HLA-DP molecules and NKp44 implicates HLA class II as a component of the innate immune response, much like HLA class I. It also provides a potential mechanism for the described associations between HLA-DP subtypes and several disease outcomes, including hepatitis B virus infection5-7, graft-versus-host disease8 and inflammatory bowel disease9,10.

Inhibition of the NKp44-PCNA Immune Checkpoint Using a mAb to PCNA

mAb-based blocking of the immune checkpoints involving the CTLA4-B7 and PD1-PDL1 inhibitory axes enhance T-cell-based adaptive immune responses in patients with cancer. We show here that antitumor responses by natural killer (NK) cells can be enhanced by a checkpoint-blocking mAb, 14-25-9, which we developed against proliferating cell nuclear antigen (PCNA). PCNA is expressed on the surface of cancer cells and acts as an inhibitory ligand for the NK-cell receptor, NKp44-isoform1. We tested for cytoplasmic- and membrane-associated PCNA by FACS- and ImageStream-based staining of cell lines and IHC of human cancer formalin fixed, paraffin embedded tissues. The mAb, 14-25-9, inhibited binding of chimeric NKp44 receptor to PCNA and mostly stained the cytoplasm and membrane of tumor cells, whereas commercial antibody (clone PC10) stained nuclear PCNA. NK functions were measured using ELISA-based IFNγ secretion assays and FACS-based killing assays. The NK92-NKp44-1 cell line and primary human NK cells showed increased IFNγ release upon coincubation with mAb 14-25-9 and various solid tumor cell lines and leukemias. Treatment with 14-25-9 also increased NK cytotoxic activity. In vivo efficacy was evaluated on patient-derived xenografts (PDX)-bearing NSG mice. In PDX-bearing mice, intravenous administration of mAb 14-25-9 increased degranulation (CD107a expression) of intratumorally injected patient autologous or allogeneic NK cells, as well as inhibited tumor growth when treated long term. Our study describes a mAb against the NKp44-PCNA innate immune checkpoint that can enhance NK-cell antitumor activity both in vitro and in vivo.

The Natural Cytotoxicity Receptors in Health and Disease

The Natural Cytotoxicity Receptors (NCRs), NKp46, NKp44, and NKp30, were some of the first human activating Natural Killer (NK) cell receptors involved in the non-MHC-restricted recognition of tumor cells to be cloned over 20 years ago. Since this time many host- and pathogen-encoded ligands have been proposed to bind the NCRs and regulate the cytotoxic and cytokine-secreting functions of tissue NK cells. This diverse set of NCR ligands can manifest on the surface of tumor or virus-infected cells or can be secreted extracellularly, suggesting a remarkable NCR polyfunctionality that regulates the activity of NK cells in different tissue compartments during steady state or inflammation. Moreover, the NCRs can also be expressed by other innate and adaptive immune cell subsets under certain tissue conditions potentially conferring NK recognition programs to these cells. Here we review NCR biology in health and disease with particular reference to how this important class of receptors regulates the functions of tissue NK cells as well as confer NK cell recognition patterns to other innate and adaptive lymphocyte subsets. Finally, we highlight how NCR biology is being harnessed for novel therapeutic interventions particularly for enhanced tumor surveillance.

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.

Splice variants of human natural cytotoxicity receptors: novel innate immune checkpoints

The natural cytotoxicity receptors (NCRs; NKp30, NKp44, and NKp46) were first defined as activating receptors on human NK cells that are important in recognition of and response to tumors. A flurry of recent research, however, has revealed that differential splicing can occur during transcription of each of the NCR genes, resulting in some transcripts that encode receptor isoforms with inhibitory functions. These alternative transcripts can arise in certain tissue microenvironments and appear to be induced by cytokines. Evidence indicates that some of the inhibitory NCRs are triggered by specific ligands, such as the interaction of the inhibitory isoform of NKp44 with PCNA on the surface of tumor cells. Here, we review the different NCR splice variants, cytokines that modulate their expression, their functional impacts on innate immune cells, and their differential expression in the contexts of cancer, pregnancy, and infections. The recent discovery of these inhibitory NCR isoforms has revealed novel innate immune checkpoints, many of which still lack defined ligands and clear mechanisms driving their expression. These NCR checkpoint pathways offer exciting potential therapeutic targets to manipulate innate immune functions under defined pathological conditions, such as cancer, pregnancy disorders, and pathogen exposure.

Haemagglutinin-neuraminidase from HPIV3 mediates human NK regulation of T cell proliferation via NKp44 and NKp46

HPIV3 is a respiratory virus causing airway diseases, including pneumonia, croup, and bronchiolitis, during infancy and childhood. Currently there is no effective vaccine or anti-viral therapy for this virus. Studies have suggested that poor T cell proliferation following HPIV3 infection is responsible for impaired immunological memory associated with this virus. We have previously demonstrated that NK cells mediate regulation of T cell proliferation during HPIV3 infection. Here we add to these studies by demonstrating that the regulation of T cell proliferation during HPIV3 infection is mediated via NK receptors NKp44 and NKp46 and involves the surface glycoprotein haemagglutinin-neuraminidase but not the fusion protein of the virus. These studies extend our knowledge of the regulatory repertoire of NK cells and provide mechanistic insights which may explain reoccurring failures of vaccines against this virus.

Survival in acute myeloid leukemia is associated with NKp44 splice variants

NKp44 is a receptor encoded by the NCR2 gene, which is expressed by cytokine-activated natural killer (NK) cells that are involved in anti-AML immunity. NKp44 has three splice variants corresponding to NKp44ITIM+ (NKp44-1) and NKp44ITIM- (NKp44-2, and NKp44-3) isoforms. RNAseq data of AML patients revealed similar survival of NKp46+NKp44+ and NKp46+NKp44- patients. However, if grouped according to the NKp44 splice variant profile, NKp44-1 expression was significantly associated with poor survival of AML patients. Moreover, activation of PBMC from healthy controls showed co-dominant expression of NKp44-1 and NKp44-3, while primary NK clones show more diverse NKp44 splice variant profiles. Cultured primary NK cells resulted in NKp44-1 dominance and impaired function associated with PCNA over-expression by target cells. This impaired functional phenotype could be rescued by blocking of NKp44 receptor. Human NK cell lines revealed co-dominant expression of NKp44-1 and NKp44-3 and showed a functional phenotype that was not inhibited by PCNA over-expression. Furthermore, transfection-based overexpression of NKp44-1, but not NKp44-2/NKp44-3, reversed the endogenous resistance of NK-92 cells to PCNA-mediated inhibition, and resulted in poor formation of stable lytic immune synapses. This research contributes to the understanding of AML prognosis by shedding new light on the functional implications of differential splicing of NKp44.

Regulation of natural cytotoxicity receptors by heparan sulfate proteoglycans in -cis: A lesson from NKp44

NKp44 (NCR2) is a distinct member of natural cytotoxicity receptors (NCRs) family that can induce cytokine production and cytolytic activity in human NK cells. Heparan sulfate proteoglycans (HSPGs) are differentially expressed in various normal and cancerous tissues. HSPGs were reported to serve as ligands/co-ligands for NKp44 and other NCRs. However, HSPG expression is not restricted to either group and can be found also in NK cells. Our current study reveals that NKp44 function can be modulated through interactions with HSPGs on NK cells themselves in -cis rather than on target cells in -trans. The intimate interaction of NKp44 and the NK cell-associated HSPG syndecan-4 (SDC4) in -cis can directly regulate membrane distribution of NKp44 and constitutively dampens the triggering of the receptor. We further demonstrate, that the disruption of NKp44 and SDC4 interaction releases the receptor to engage with its ligands in -trans and therefore enhances NKp44 activation potential and NK cell functional response.

Natural cytotoxicity receptors and their ligands

Natural killer (NK) cells are innate lymphoid cells (ILCs) that participate to the clearance of pathogen-infected cells and tumour cells. NK cells and subsets of ILCs express the natural cytotoxicity receptors (NCRs) NKp46, NKp44 and NKp30 at their surface. NCRs have been shown to recognize a broad spectrum of ligands ranging from viral-, parasite- and bacterial-derived ligands to cellular ligands; however, the full identification of NCR ligands remains to be performed and will undoubtedly contribute to a better understanding of NK cell and ILC biology.

Focused examination of the intestinal lamina propria yields greater molecular insight into mechanisms underlying SIV induced immune dysfunction

Background

The Gastrointestinal (GI) tract is critical to AIDS pathogenesis as it is the primary site for viral transmission and a major site of viral replication and CD4⁺ T cell destruction. Consequently GI disease, a major complication of HIV/SIV infection can facilitate translocation of lumenal bacterial products causing localized/systemic immune activation leading to AIDS progression.

Methodology/Principal Findings

To better understand the molecular mechanisms underlying GI disease we analyzed global gene expression profiles sequentially in the intestine of the same animals prior to and at 21 and 90d post SIV infection (PI). More importantly we maximized information gathering by examining distinct mucosal components (intraepithelial lymphocytes, lamina propria leukocytes [LPL], epithelium and fibrovascular stroma) separately. The use of sequential intestinal resections combined with focused examination of distinct mucosal compartments represents novel approaches not previously attempted. Here we report data pertaining to the LPL. A significant increase (±1.7-fold) in immune defense/inflammation, cell adhesion/migration, cell signaling, transcription and cell division/differentiation genes were observed at 21 and 90d PI. Genes associated with the JAK-STAT pathway (IL21, IL12R, STAT5A, IL10, SOCS1) and T-cell activation (NFATc1, CDK6, Gelsolin, Moesin) were notably upregulated at 21d PI. Markedly downregulated genes at 21d PI included IL17D/IL27 and IL28B/IFNγ3 (anti-HIV/viral), activation induced cytidine deaminase (B-cell function) and approximately 57 genes regulating oxidative phosphorylation, a critical metabolic shift associated with T-cell activation. The 90d transcriptome revealed further augmentation of inflammation (CXCL11, chitinase-1, JNK3), immune activation (CD38, semaphorin7A, CD109), B-cell dysfunction (CD70), intestinal microbial translocation (Lipopolysaccharide binding protein) and mitochondrial antiviral signaling (NLRX1) genes. Reduced expression of CD28, CD4, CD86, CD93, NFATc1 (T-cells), TLR8, IL8, CCL18, DECTIN1 (macrophages), HLA-DOA and GPR183 (B-cells) at 90d PI suggests further deterioration of overall immune function.

Conclusions/Significance

The reported transcriptional signatures provide significant new details on the molecular pathology of HIV/SIV induced GI disease and provide new opportunity for future investigation.

Lessons from chimpanzee-based research on human disease: the implications of genetic differences

Assertions that the use of chimpanzees to investigate human diseases is valid scientifically are frequently based on a reported 98-99% genetic similarity between the species. Critical analyses of the relevance of chimpanzee studies to human biology, however, indicate that this genetic similarity does not result in sufficient physiological similarity for the chimpanzee to constitute a good model for research, and furthermore, that chimpanzee data do not translate well to progress in clinical practice for humans. Leading examples include the minimal citations of chimpanzee research that is relevant to human medicine, the highly different pathology of HIV/AIDS and hepatitis C virus infection in the two species, the lack of correlation in the efficacy of vaccines and treatments between chimpanzees and humans, and the fact that chimpanzees are not useful for research on human cancer. The major molecular differences underlying these inter-species phenotypic disparities have been revealed by comparative genomics and molecular biology - there are key differences in all aspects of gene expression and protein function, from chromosome and chromatin structure to post-translational modification. The collective effects of these differences are striking, extensive and widespread, and they show that the superficial similarity between human and chimpanzee genetic sequences is of little consequence for biomedical research. The extrapolation of biomedical data from the chimpanzee to the human is therefore highly unreliable, and the use of the chimpanzee must be considered of little value, particularly given the breadth and potential of alternative methods of enquiry that are currently available to science.

Proliferating cell nuclear antigen is a novel inhibitory ligand for the natural cytotoxicity receptor NKp44

NK cells play an important role in the early immune response to cancer. The NKp44 activating receptor is the only natural cytotoxicity receptor that is expressed exclusively by primate NK cells, yet its cellular ligands remain largely unknown. Proliferating cell nuclear Ag (PCNA) is overexpressed in cancer cells. In this study, we show that the NKp44 receptor recognizes PCNA. Their interaction inhibits NK cell function through NKp44/ITIM. The physical interaction of NKp44 and PCNA is enabled by recruitment of target cell PCNA to the NK immunological synapse. We demonstrate that PCNA promotes cancer survival by immune evasion through inhibition of NKp44-mediated NK cell attack.

Involvement of activating NK cell receptors and their modulation in pathogen immunity

Natural Killer (NK) cells are endowed with cell-structure-sensing receptors providing inhibitory protection from self-destruction (inhibitory NK receptors, iNKRs, including killer inhibitory receptors and other molecules) and rapid triggering potential leading to functional cell activation by Toll-like receptors (TLRs), cytokine receptors, and activating NK cell receptors including natural cytotoxicity receptors (NCRs, i.e., NKp46, NKp46, and NKp44). NCR and NKG2D recognize ligands on infected cells which may be endogenous or may directly bind to some structures derived from invading pathogens. In this paper, we address the known direct or indirect interactions between activating receptors and pathogens and their expression during chronic HIV and HCV infections.

Natural cytotoxicity of NC-2+ cells against the growth and metastasis of WEHI-164 fibrosarcoma

We have previously reported a new receptor (NC-2) for natural cytotoxicity (NC) on murine leucocytes, identified by monoclonal antibody D9 (mAb D9). Pretreatment of mouse spleen cells with different concentrations of mAb D9 in vitro blocked NC against WEHI-164, whereas natural killing (NK) activity against YAC-1 was unaffected. This paper reports the immune surveillance against the growth of WEHI-164 tumour cells in mice by NC-2(+) Cells. The kinetics of in vivo reduction in NC activity were investigated by treating BALB/c and (CBA × C57BL/6) F1 mice with a single injection of 40 μg of mAb D9 and monitoring splenic NC activity by (51) Cr-release assay at intervals from 24 h to 3 weeks. Control mice were injected with OKT8 irrelevant antibody. Results showed a significant (P < 0.05) reduction in splenic NC activity within 24 h which persisted for up to 1 week. Similar results were also obtained when (CBA × C57BL/6) F1 mice were employed (P<0.001). In vivo tumour studies were undertaken to investigate the role of NC-2(+) cells in surveillance against tumour growth and metastasis of the WEHI-164 fibrosarcoma. When syngeneic BALB/c mice were injected with 40 μg of mAb D9 and then challenged with 5 × 10(5) WEHI-164 cells, results showed significantly increased growth rate of the transplanted WEHI-164 fibrosarcoma and tumour nodules in the lungs of animals, when compared to control mice with normal NC activity. Our data support an innate surveillance in metastasis and growth of WEHI-164 fibrosarcoma in mice.

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.

CD8+ NK cells are predominant in chimpanzees, characterized by high NCR expression and cytokine production, and preserved in chronic HIV-1 infection

HIV-1 infection in humans results in an early and progressive NK cell dysfunction and an accumulation of an "anergic" CD56- CD16+ NK subset, which is characterised by low natural cytotoxicity receptor expression and low cytokine producing capacity. In contrast to humans, chimpanzee NK cells do not display a distinguishable CD56(bright) and CD56(dim) subset but, as shown here, could be subdivided into functionally different CD8+ and CD8- subsets. The CD8+ NK cells expressed significantly higher levels of triggering receptors including NKp46 and, upon in vitro activation, produced more IFN-gamma, TNF-alpha and CD107 than their CD8- counterparts. In addition, chimpanzee CD8- NK cells had relatively high levels of HLA-DR expression, suggestive of an activated state. Killing inhibitory receptors were expressed only at low levels; however, upon in vitro stimulation, they were up-regulated in CD8+ but not in CD8- NK cells and were functionally capable of inhibiting NKp30-triggered killing. In contrast to HIV-1-infected humans, infected chimpanzees maintained their dominant CD8+ NK cell population, with high expression of natural cytotoxicity receptors.

Comparative analysis of NK-cell receptor expression and function across primate species: Perspective on antiviral defenses

Natural killer (NK) cells are lymphoid effectors that are involved in the innate immune surveillance against infected and/or tumor cells. Their function is under the fine-tuning control of cell surface receptors that display either inhibitory or activating function and in healthy condition, mediate self-tolerance. It is known that inhibitory receptors are characterized by clonal and stochastic distribution and are extremely sensible to any modification, downregulation or loss of MHC class I surface expression that are induced in autologous cells upon viral infection or cancer transformation. This alteration of the MHC class I expression weakens the strength of the inhibitory receptor-induced interaction, thus resulting in a prompt triggering of NK cell function, which ends up in the inhibition of tumor progression and proliferation of pathogen-infected cells. Thus, the inhibitory function of NK cells is only one face of the coin, since NK-cell activation is controlled by different arrays of activating receptors that finally are involved in the induction of cytolysis and/or cytokine release. Interestingly, the inhibitory NK-cell receptors that are involved in dampening NK cell-mediated responses evolved during speciation in different, often structurally unrelated surface-expressed molecules, all using a conserved signaling pathway. In detail, during evolution, the inhibitory receptors that assure the recognition of MHC class I molecules, originate in, at least, three different ways. This ended up in multigene families showing marked structural divergences that coevolved in a convergent way with the availability of appropriate MHC ligand molecules.

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. Our laboratory has 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. Only in the last ten years has it become possible to characterize the NK triggering receptors mediating natural cytotoxicity, leading to an appreciation of the existence of a cellular interaction network between effectors of both natural and adaptive immunity. This report 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.