The mechanisms controlling the recognition of tumor- and virus-infected cells by NKp46

Regulation of the Functions of Natural Cytotoxicity Receptors by Interactions with Diverse Ligands and Alterations in Splice Variant Expression

The natural cytotoxicity receptor (NCR) family is constituted by NKp46, NKp44, and NKp30 in humans, which are expressed mainly on natural killer (NK) cells and are encoded by the ncr1, ncr2, and ncr3 genes, respectively. NCRs have classically been defined as activating receptors that trigger cytotoxicity and cytokine responses by NK cells upon engaging with ligands on tumor cells. Several new findings, however, have challenged this model and identified alternative mechanisms regulating the function of NCRs. Recent reports indicate that ligand matters, since the interaction of NKp44 with distinct ligands on target cells can either activate or inhibit NK cells. Also, the NCRs have been found to interact with distinct specificities to various heparan sulfate glycosaminoglycans, which are complex polysaccharides found in extracellular matrix or on cell surface heparan sulfate proteoglycans (HSPGs). The NCRs can engage with HSPGs in trans as a co-ligand on the target cells or in cis on the NK cell surface to regulate receptor–ligand interactions and NK cell activation. A number of splice variants of ncr2 and ncr3 have also been identified, and a predominant expression of certain variants results in inhibitory signaling through NKp44 and NKp30. Several recent studies have found that the selective expression of some of these inhibitory splice variants can significantly influence outcome in the contexts of cancer, infection, and pregnancy. These findings establish that NCR functions are more diverse than originally thought, and better understanding of their splice variant expression profiles and ligand interactions are needed to establish their functional regulation in the context of human health.

Frequency and role of NKp46 and NKG2A in hepatitis B virus infection

Background and Aim

Natural Killer (NK) cells are involved in the control of viral infection. However, the role of NK cells in chronic hepatitis B (CHB) remains unclear. This study investigated the frequencies and roles of NK cells in CHB, with a focus on activating receptor NKp46 and inhibitory receptor NKG2A.

Patients/Method

Peripheral blood lymphocytes were obtained from 71 CHB patients and 37 healthy subjects (HS). The expressions of NKp46 and NKG2A were analyzed using flow cytometry. The role of NKp46-ligand was assessed using an in vitro co-culture system. Cytotoxicity and IFN-γ production in NK cells were evaluated using RT-PCR and flow cytometry.

Results

CHB patients were classified into treatment-naïve patients with low HBV DNA titer (CHB-L; n = 28), high HBV DNA titer (CHB-H; n = 24) by the cut-off level of serum HBV DNA 4 log copies/ml, and patients receiving nucleos(t)ide analogue (CHB-NA; n = 19). The expressions of NKp46 and NKG2A were higher in CHB-H than in HS/CHB-L/CHB-NA. HepG2.2.15 had higher NKp46-ligand expression than HepG2. When NK cells from HS were co-cultured with HepG2.2.15, inhibition of the NKp46 and NKp46-ligand interaction by anti-NKp46 antibody significantly reduced cytolysis of HepG2.2.15 and IFN-γ production. However, those reductions were not observed in co-culture with HepG2. Additionally, NK cells that highly expressed NKp46 also highly expressed NKG2A (NKp46^highNKG2A^high subset). The frequencies of NKp46^highNKG2A^high subset in CHB-H were higher than those in HS/CHB-L/CHB-NA. Among treatment-naïve CHB patients, the frequencies of NKp46^highNKG2A^high subset were positively correlated with serum ALT (P<0.01, r = 0.45) and HBV DNA (P<0.01, r = 0.59) levels. The expressions of Fas-L, STAT1, TRAIL and CD107a were higher and IFN-γ expression was lower in the NKp46^highNKG2A^high subset than in the other subsets.

Conclusion

The NKp46 and NKp46-ligand interaction contributes to NK cell activation. A novel NK cell subset, the NKp46^highNKG2A^high subset, may be associated with liver injury and HBV replication.

Modular Activating Receptors in Innate and Adaptive Immunity

Triggering of cell-mediated immunity is largely dependent on the recognition of foreign or abnormal molecules by a myriad of cell surface-bound receptors. Many activating immune receptors do not possess any intrinsic signaling capacity but instead form noncovalent complexes with one or more dimeric signaling modules that communicate with a common set of kinases to initiate intracellular information-transfer pathways. This modular architecture, where the ligand binding and signaling functions are detached from one another, is a common theme that is widely employed throughout the innate and adaptive arms of immune systems. The evolutionary advantages of this highly adaptable platform for molecular recognition are visible in the variety of ligand-receptor interactions that can be linked to common signaling pathways, the diversification of receptor modules in response to pathogen challenges, and the amplification of cellular responses through incorporation of multiple signaling motifs. Here we provide an overview of the major classes of modular activating immune receptors and outline the current state of knowledge regarding how these receptors assemble, recognize their ligands, and ultimately trigger intracellular signal transduction pathways that activate immune cell effector functions.

Avian influenza virus directly infects human natural killer cells and inhibits cell activity

Natural killer (NK) cell is a key component of innate immunity and plays an important role in host defense against virus infection by directly destroying infected cells. Influenza is a respiratory disease transmitted in the early phase of virus infection. Evasion of host innate immunity including NK cells is critical for the virus to expand and establish a successful acute infection. Previously, we showed that human influenza H1N1 virus infects NK cells and induces cell apoptosis, as well as inhibits NK cell activity. In this study, we further demonstrated that avian influenza virus also directly targeted NK cells as an immunoevasion strategy. The avian virus infected human NK cells and induced cell apoptosis. In addition, avian influenza virion and HA protein inhibited NK cell cytotoxicity. This novel strategy has obvious advantages for avian influenza virus, allowing the virus sufficient time to expand and subsequent spread before the onset of the specific immune response. Our findings provide an important clue for the immunopathogenesis of avian influenza, and also suggest that direct targeting NK cells may be a common strategy used by both human and avian influenza viruses to evade NK cell immunity.

Expression of NKp46 Splice Variants in Nasal Lavage Following Respiratory Viral Infection: Domain 1-Negative Isoforms Predominate and Manifest Higher Activity

The natural killer (NK) cell activating receptor NKp46/NCR1 plays a critical role in elimination of virus-infected and tumor cells. The NCR1 gene can be transcribed into five different splice variants, but the functional importance and physiological distribution of NKp46 isoforms are not yet fully understood. Here, we shed light on differential expression of NKp46 splice variants in viral respiratory tract infections and their functional difference at the cellular level. NKp46 was the most predominantly expressed natural cytotoxicity receptor in the nasal lavage of patients infected with four respiratory viruses: respiratory syncytia virus, adenovirus, human metapneumovirus, or influenza A. Expression of NKp30 was far lower and NKp44 was absent in all patients. Domain 1-negative NKp46 splice variants (i.e., NKp46 isoform d) were the predominantly expressed isoform in nasal lavage following viral infections. Using our unique anti-NKp46 mAb, D2-9A5, which recognizes the D2 extracellular domain, and a commercial anti-NKp46 mAb, 9E2, which recognizes D1 domain, allowed us to identify a small subset of NKp46 D1-negative splice variant-expressing cells within cultured human primary NK cells. This NKp46 D1-negative subset also showed higher degranulation efficiency in term of CD107a surface expression. NK-92 cell lines expressing NKp46 D1-negative and NKp46 D1-positive splice variants also showed functional differences when interacting with targets. A NKp46 D1-negative isoform-expressing NK-92 cell line showed enhanced degranulation activity. To our knowledge, we provide the first evidence showing the physiological distribution and functional importance of human NKp46 splice variants under pathological conditions.

Expression and function of NKp46 W32R: the human homologous protein of mouse NKp46 W32R (Noé)

Natural killer (NK) cells eradicate infected cells and tumors following the triggering of activating receptors, like the Natural Cytotoxicity Receptors (NCRs), which include NKp30, NKp44 and NKp46. NKp46 is the only NCR expressed in mice (mNKp46), and except for some Innate Lymphoid Cell (ILC) populations (ILC1/3 subsets), its expression is restricted to NK cells. Previously, a mouse named Noé was generated in which a random point mutation (W32R) impaired the cell surface expression of mNKp46. Interestingly, the Noé mice NK cells expressed twice as much of the transcription factor Helios, and displayed general non-NKp46 specific hyperactivity. We recently showed that the mNKp46 W32R (Noé) protein was expressed on the surface of various cells; albeit slowly and unstably, that it is aberrantly glycosylated and accumulates in the ER. Interestingly, the Tryptophan (Trp) residue in position 32 is conserved between humans and mice. Therefore, we studied here the human orthologue protein of mNKp46 W32R, the human NKp46 W32R. We demonstrated that NKp46 W32R is aberrantly glycosylated, accumulates in the ER, and is unstable on the cell surface. Furthermore, we showed that overexpression of NKp46 W32R or Helios resulted in augmented NK cell activation, which may be applied to boost NK activity for therapeutic applications.

Identification of an Activating Chicken Ig-like Receptor Recognizing Avian Influenza Viruses

Chicken Ig-like receptors (CHIRs) represent a multigene family encoded by the leukocyte receptor complex that encodes a variety of receptors that are subdivided into activating CHIR-A, inhibitory CHIR-B, and bifunctional CHIR-AB. Apart from CHIR-AB, which functions as an Fc receptor, CHIR ligands are unknown. In the current study, we used a panel of different BWZ.36 CHIR reporter cells to identify an interaction between specific CHIRs and avian influenza virus (AIV). The specificity of the CHIR-AIV interaction was further demonstrated using CHIR fusion proteins that bound to AIV-coated plates and were able to reduce the interaction of reporter cells with AIV. There was no difference in binding of CHIR to different AIV strains. Furthermore, CHIR fusion proteins reduced AIV-induced in vitro activation of NK cells obtained from lungs of AIV-infected animals, as judged by the lower frequency of CD107⁺ cells. Because the original CHIR reporter lines were generated based on sequence information about extracellular CHIR domains, we next identified a full-length CHIR that displayed similar binding to AIV. The sequence analysis identified this CHIR as a CHIR-A. Neuraminidase treatment of coated CHIR-human Ig proteins reduced binding of trimeric H5 proteins to CHIR. This suggests that the interaction is dependent on sialic acid moieties on the receptor. In conclusion, this article identifies AIV as a ligand of CHIR-A and describes the functional consequences of this interaction.

Natural Killer Receptor 1 Dampens the Development of Allergic Eosinophilic Airway Inflammation

The function of NCR1 was studied in a model of experimental asthma, classified as a type 1 hypersensitivity reaction, in mice. IgE levels were significantly increased in the serum of OVA immunized NCR1 deficient (NCR1gfp/gfp) mice in comparison to OVA immunized wild type (NCR1+/+) and adjuvant immunized mice. Histological analysis of OVA immunized NCR1gfp/gfp mice revealed no preservation of the lung structure and overwhelming peribronchial and perivascular granulocytes together with mononuclear cells infiltration. OVA immunized NCR+/+ mice demonstrated preserved lung structure and peribronchial and perivascular immune cell infiltration to a lower extent than that in NCR1gfp/gfp mice. Adjuvant immunized mice demonstrated lung structure preservation and no immune cell infiltration. OVA immunization caused an increase in PAS production independently of NCR1 presence. Bronchoalveolar lavage (BAL) revealed NCR1 dependent decreased percentages of eosinophils and increased percentages of lymphocytes and macrophages following OVA immunization. In the OVA immunized NCR1gfp/gfp mice the protein levels of eosinophils' (CCL24) and Th2 CD4+ T-cells' chemoattractants (CCL17, and CCL24) in the BAL are increased in comparison with OVA immunized NCR+/+ mice. In the presence of NCR1, OVA immunization caused an increase in NK cells numbers and decreased NCR1 ligand expression on CD11c+GR1+ cells and decreased NCR1 mRNA expression in the BAL. OVA immunization resulted in significantly increased IL-13, IL-4 and CCL17 mRNA expression in NCR1+/+ and NCR1gfp/gfp mice. IL-17 and TNFα expression increased only in OVA-immunized NCR1+/+mice. IL-6 mRNA increased only in OVA immunized NCR1gfp/gfp mice. Collectively, it is demonstrated that NCR1 dampens allergic eosinophilic airway inflammation.

Porcine CD3(+)NKp46(+) Lymphocytes Have NK-Cell Characteristics and Are Present in Increased Frequencies in the Lungs of Influenza-Infected Animals

The CD3⁻NKp46⁺ phenotype is frequently used for the identification of natural killer (NK) cells in various mammalian species. Recently, NKp46 expression was analyzed in more detail in swine. It could be shown that besides CD3⁻NKp46⁺ lymphocytes, a small but distinct population of CD3⁺NKp46⁺ cells exists. In this study, we report low frequencies of CD3⁺NKp46⁺ lymphocytes in blood, lymph nodes, and spleen, but increased frequencies in non-lymphatic organs, like liver and lung. Phenotypic analyses showed that the majority of CD3⁺NKp46⁺ cells coexpressed the CD8αβ heterodimer, while a minor subset expressed the TCR-γδ, which was associated with a CD8αα⁺ phenotype. Despite these T-cell associated receptors, the majority of CD3⁺NKp46⁺ lymphocytes displayed a NK-related phenotype (CD2⁺CD5⁻CD6⁻CD16⁺perforin⁺) and expressed mRNA of NKp30, NKp44, and NKG2D at similar levels as NK cells. Functional tests showed that CD3⁺NKp46⁺ lymphocytes produced IFN-γ and proliferated upon cytokine stimulation to a similar extent as NK cells, but did not respond to the T-cell mitogen, ConA. Likewise, CD3⁺NKp46⁺ cells killed K562 cells with an efficiency comparable to NK cells. Cross-linking of NKp46 and CD3 led to degranulation of CD3⁺NKp46⁺ cells, indicating functional signaling pathways for both receptors. Additionally, influenza A(H1N1)pdm09-infected pigs had reduced frequencies of CD3⁺NKp46⁺ lymphocytes in blood, but increased frequencies in the lung in the early phase of infection. Thus, CD3⁺NKp46⁺ cells appear to be involved in the early phase of influenza infections. In summary, we describe a lymphocyte population in swine with a mixed phenotype of NK and T cells, with results so far indicating that this cell population functionally resembles NK cells.

Red ginseng and vitamin C increase immune cell activity and decrease lung inflammation induced by influenza A virus/H1N1 infection

Objectives

Because red ginseng and vitamin C have immunomodulatory function and anti-viral effect, we investigated whether red ginseng and vitamin C synergistically regulate immune cell function and suppress viral infection.

Methods

Red ginseng and vitamin C were treated to human peripheral blood mononuclear cells (PBMCs) or sarcoma-associated herpesvirus (KSHV)-infected BCBL-1, and administrated to Gulo(−/−) mice, which are incapable of synthesizing vitamin C, with or without influenza A virus/H1N1 infection.

Key findings

Red ginseng and vitamin C increased the expression of CD25 and CD69 of PBMCs and natural killer (NK) cells. Co-treatment of them decreased cell viability and lytic gene expression in BCBL-1. In Gulo(−/−) mice, red ginseng and vitamin C increased the expression of NKp46, a natural cytotoxic receptor of NK cells and interferon (IFN)-γ production. Influenza infection decreased the survival rate, and increased inflammation and viral plaque accumulation in the lungs of vitamin C-depleted Gulo(−/−) mice, which were remarkably reduced by red ginseng and vitamin C supplementation.

Conclusions

Administration of red ginseng and vitamin C enhanced the activation of immune cells like T and NK cells, and repressed the progress of viral lytic cycle. It also reduced lung inflammation caused by viral infection, which consequently increased the survival rate.

Identification of putative novel O-glycosylations in the NK killer receptor Ncr1 essential for its activity

Natural killer (NK) cells kill tumor and virus-infected cells using activating NK cell receptors. One of the major NK-activating receptors is NKp46 and its mouse ortholog Ncr1. NKp46/Ncr1 is expressed exclusively on NK cells and on a subset of innate lymphoid cells. NKp46/Ncr1 was shown to be involved in a myriad of pathologies and immunological settings. Specifically, NKp46/Ncr1 was shown to interact with the viral hemagglutinin (HA) protein and with an unknown tumor/cellular ligand. NKp46 and Ncr1 are structurally similar; however, they are substantially different in their glycosylation patterns. Although the human NKp46 carries both O- and N-glycosylations that are essential for its activity, the mouse Ncr1 was predicted to have N-linked glycosylations only. Here we discovered using prediction algorithms and high-performance liquid chromatography analysis that Ncr1 carries two putative novel O-glycosylations, one of which (Thr 225) is conserved in NKp46. We next used surface plasmon resonance, biochemical, mutational and functional in vitro and in vivo assays to demonstrate that the putative O-glycosylations of Ncr1 are critical for its function.

NKp46 Clusters at the Immune Synapse and Regulates NK Cell Polarization

Natural killer (NK) cells play an important role in first-line defense against tumor and virus-infected cells. The activity of NK cells is tightly regulated by a repertoire of cell surface expressed inhibitory and activating receptors. NKp46 is a major NK cell-activating receptor that is involved in the elimination of target cells. NK cells form different types of synapses that result in distinct functional outcomes: cytotoxic, inhibitory, and regulatory. Recent studies revealed that complex integration of NK receptor signaling controls cytoskeletal rearrangement and other immune synapse-related events. However, the distinct nature by which NKp46 participates in NK immunological synapse formation and function remains unknown. In this study, we determined that NKp46 forms microclusters structures at the immune synapse between NK cells and target cells. Over-expression of human NKp46 is correlated with increased accumulation of F-actin mesh at the immune synapse. Concordantly, knock-down of NKp46 in primary human NK cells decreased recruitment of F-actin to the synapse. Live cell imaging experiments showed a linear correlation between NKp46 expression and lytic granules polarization to the immune synapse. Taken together, our data suggest that NKp46 signaling directly regulates the NK lytic immune synapse from early formation to late function.

Expression, Function, and Molecular Properties of the Killer Receptor Ncr1-Noé

NK cells kill various cells using activating receptors, such as the natural cytotoxicity receptors (NCRs). NKp46 is a major NCR and is the only NCR expressed in mice (denoted Ncr1). Using Ncr1-deficient mice (Ncr1(gfp/pfp)) we demonstrated that Ncr1 controls various pathologies, and that in its absence Ncr1-related functions are impaired. In 2012, another Ncr1-related mouse was generated, named Noé, in which a random mutation, W32R, in position 32, impaired the Ncr1-Noé cell surface expression. Interestingly, in the Noé mice, Ncr1-dependent deficiencies were not observed. Additionally, the Noé-NK cells were hyperactivated, probably due to increased Helios expression, and the Noé mice demonstrate increased clearance of influenza and murine CMV. In contrast, in the Ncr1(gfp/pfp) mice infection with influenza was lethal and we show in the present study no difference in murine CMV infection between Ncr1(gfp/pfp) and wild-type (WT) mice. Because the foremost difference between the Noé and Ncr1(gfp/gfp) mice is the presence of a mutated Ncr1-Noé protein, we studied its properties. We show that Ncr1-Noé and various other Ncr1 mutants in position 32 can be expressed on the surface, albeit slowly and unstably, and that ligand recognition and function of the various Ncr1-Noé is similar to the WT Ncr1. We further show that the glycosylation pattern of Ncr1-Noé is aberrant, that the Ncr1-Noé proteins accumulate in the endoplasmic reticulum, and that the expression of Ncr1-Noé proteins, but not WT Ncr1, leads to increased Helios expression. Thus, we suggest that the NK hyperactivated phenotype observed in the Noé mice might result from the presence of the Ncr1-Noé protein.

Lysis of HIV-1-infected autologous CD4+ primary T cells by interferon-alpha-activated NK cells requires NKp46 and NKG2D

OBJECTIVE

Autologous HIV-1-infected CD4 primary T cells (aHIVCD4) have been shown to be largely resistant to natural killer (NK)-cell-mediated lysis because of viral strategies of immune evasion. We have previously shown that a preactivation of NK cells with plasmacytoid dendritic cells can significantly augment lysis of aHIVCD4 through a mechanism dependent on interferon-alpha (IFN-α).

DESIGN

The goal of the present study is to identify the specific NK-activating receptors involved in NK lysis of aHIVCD4 following IFN-α activation.

METHODS

Peripheral blood mononuclear cells (PBMC) were incubated with aHIVCD4 to induce the secretion of endogenous levels of IFN-α and drive NK activation. We then utilized a standard chromium lysis assay to assess the degree of IFN-α-activated lysis of aHIVCD4 in the presence or absence of masking antibodies to a panel of NK-activating receptors and co-receptors.

RESULTS

Direct recognition of HIV-1-infected, but not uninfected, autologous CD4 primary T cells by PBMC induced the secretion IFN-α (median 2280 pg/ml, P < 0.001, n = 9) that, in turn, activated NK cells (P < 0.001, n = 12) and significantly increased their cytolytic potential against aHIVCD4 (P < 0.01, n = 12). The masking of NKp46 (P < 0.01, n = 8) and NKG2D (P < 0.05, n = 8), but not 2B4, NTBA, NKp30 or NKp44, significantly reduced IFN-α-activated lysis of aHIVCD4.

CONCLUSIONS

Taken together, these results demonstrate that endogenous levels of IFN-α secreted by plasmacytoid dendritic cells induce NK cells to lyse aHIVCD4 via the engagement of NKp46 and NKG2D.

Role of NK cells in immunotherapy and virotherapy of solid tumors

Although natural killer (NK) cells are endowed with powerful cytolytic activity against cancer cells, their role in different therapies against solid tumors has not yet been fully elucidated. Their interactions with various elements of the tumor microenvironment as well as their possible effects in contributing to and/or limiting oncolytic virotherapy render this potential immunotherapeutic tool still difficult to exploit at the bedside. Here, we will review the current literature with the aim of providing new hints to manage this powerful cell type in future innovative therapies, such as the use of NK cells in combination with new cytokines, specific mAbs (inducing ADCC), Tyr-Kinase inhibitors, immunomodulatory drugs and/or the design of oncolytic viruses aimed at optimizing the effect of NK cells in virotherapy.

Tricking the balance: NK cells in anti-cancer immunity

Natural Killer (NK) cells are classically considered innate immune effector cells involved in the first line of defense against infected and malignant cells. More recently, NK cells have emerged to acquire properties of adaptive immunity in response to certain viral infections such as expansion of specific NK cell subsets and long-lasting virus-specific responses to secondary challenges. NK cells distinguish healthy cells from abnormal cells by measuring the net input of activating and inhibitory signals perceived from target cells through NK cell surface receptors. Acquisition of activating ligands in combination with reduced expression of MHC class I molecules on virus-infected and cancer cells activates NK cell cytotoxicity and release of immunostimulatory cytokines like IFN-γ. In the cancer microenvironment however, NK cells become functionally impaired by inhibitory factors produced by immunosuppressive immune cells and cancer cells. Here we review recent progress on the role of NK cells in cancer immunity. We describe regulatory factors of the tumor microenvironment on NK cell function which determine cancer cell destruction or escape from immune recognition. Finally, recent strategies that focus on exploiting NK cell anti-cancer responses for immunotherapeutic approaches are outlined.

Oncolysis by paramyxoviruses: multiple mechanisms contribute to therapeutic efficiency

Oncolytic paramyxoviruses include some strains of Measles, Mumps, Newcastle disease, and Sendai viruses. All these viruses are well equipped for promoting highly specific and efficient malignant cell death, which can be direct and/or immuno-mediated. A number of proteins that serve as natural receptors for oncolytic paramyxoviruses are frequently overexpressed in malignant cells. Therefore, the preferential interaction of paramyxoviruses with malignant cells rather than with normal cells is promoted. Due to specific genetic defects of cancer cells in the interferon (IFN) and apoptotic pathways, viral replication has the potential to be promoted specifically in tumors. Viral mediation of syncytium formation (a polykaryonic structure) promotes intratumoral paramyxo-virus replication and spreading, without exposure to host neutralizing antibodies. So, two related processes: efficient intratumoral infection spread as well as the consequent mass malignant cell death, both are enhanced. In general, the paramyxoviruses elicit strong anticancer innate and adaptive immune responses by triggering multiple danger signals. The paramyxoviruses are powerful inducers of IFN and other immuno-stimulating cytokines. These viruses efficiently promote anticancer activity of natural killer cells, dendritic cells, and cytotoxic T lymphocytes. Moreover, a neuraminidase (sialidase), a component of the viral envelope of Newcastle Disease, Mumps, and Sendai viruses, can cleave sialic acids on the surface of malignant cells thereby unmasking cancer antigens and exposing them to the immune system. These multiple mechanisms contribute to therapeutic efficacy of oncolytic paramyxovi-ruses and are responsible for encouraging results in preclinical and clinical studies.

Targeting natural killer cell reactivity by employing antibody to NKp46: implications for type 1 diabetes

Natural killer (NK) cells belong to the innate lymphoid cells. Their cytotoxic activity is regulated by the delicate balance between activating and inhibitory signals. NKp46 is a member of the primary activating receptors of NK cells. We previously reported that the NKp46 receptor is involved in the development of type 1 diabetes (T1D). Subsequently, we hypothesized that blocking this receptor could prevent or hinder disease development. To address this goal, we developed monoclonal antibodies for murine NKp46. One mAb, named NCR1.15, recognizes the mouse homologue protein of NKp46, named Ncr1, and was able to down-regulate the surface expression of NKp46 on primary murine NK cells following antibody injection in vivo. Additionally, NCR1.15 treatments were able to down-regulate cytotoxic activity mediated by NKp46, but not by other NK receptors. To test our primary assumption, we examined T1D development in two models, non-obese diabetic mice and low-dose streptozotocin. Our results show a significantly lower incidence of diabetic mice in the NCR1.15-treated group compared to control groups. This study directly demonstrates the involvement of NKp46 in T1D development and suggests a novel treatment strategy for early insulitis.

Natural killer cells: a coherent model for their functional role in Mycobacterium tuberculosis infection

Tuberculosis is still a leading cause of bacterial infection worldwide, with an estimate of over two billion people latently infected with Mycobacterium tuberculosis (MTB). A delicate interplay between MTB and the host's innate and acquired immune system can influence the outcome of the infection, which ranges from pathogen elimination to the establishment of a latent infection or a progressive disease. Although the host cell-mediated adaptive immune response is of vital importance in the control of MTB infection, growing evidence indicates that innate immune cells may greatly influence the outcome of the interaction between the bacterium and the host. Among the cell populations likely to play a role in the host immune response to MTB, natural killer (NK) cells have recently attracted considerable interest. This review is dedicated to dissecting the role of NK cells in immunity to tuberculosis, reporting the most relevant findings and providing a working model of the possible contribution of NK cells in early and late events associated with MTB infection.

Early responses of natural killer cells in pigs experimentally infected with 2009 pandemic H1N1 influenza A virus

Natural killer (NK) cells are important players in the innate immune response against influenza A virus and the activating receptor NKp46, which binds hemagglutinin on the surface of infected cells, has been assigned a role in this context. As pigs are natural hosts for influenza A viruses and pigs possess both NKp46⁻ and NKp46⁺ NK cells, they represent a good animal model for studying the role of the NKp46 receptor during influenza. We explored the role of NK cells in piglets experimentally infected with 2009 pandemic H1N1 influenza virus by flow cytometric analyses of cells isolated from blood and lung tissue and by immunostaining of lung tissue sections. The number of NKp46⁺ NK cells was reduced while NKp46⁻ NK cells remained unaltered in the blood 1–3 days after infection. In the lungs, the intensity of NKp46 expression on NK cells was increased during the first 3 days, and areas where influenza virus nucleoprotein was detected were associated with increased numbers of NKp46⁺ NK cells when compared to uninfected areas. NKp46⁺ NK cells in the lung were neither found to be infected with influenza virus nor to be undergoing apoptosis. The binding of porcine NKp46 to influenza virus infected cells was verified in an in vitro assay. These data support the involvement of porcine NKp46⁺ NK cells in the local immune response against influenza virus.

Influenza virus uses its neuraminidase protein to evade the recognition of two activating NK cell receptors

Natural Killer (NK) cells play a central role in the defense against viral infections and in the elimination of transformed cells. The recognition of pathogen-infected and tumor cells is controlled by inhibitory and activating receptors. We have previously shown that among the activating (killer) NK cell receptors the natural cytotoxicity receptors, NKp44 and NKp46, interact with the viral hemagglutinin (HA) protein expressed on the cell surface of influenza-virus-infected cells. We further showed that the interaction between NKp44/NKp46 and viral HA is sialic-acid dependent and that the recognition of HA by NKp44 and NKp46 leads to the elimination of the infected cells. Here we demonstrate that the influenza virus developed a counter-attack mechanism and that the virus uses its neuraminidase (NA) protein to prevent the recognition of HA by both the NKp44 and NKp46 receptors, resulting in reduced elimination of the infected cells by NK cells.

Role of NK cells in influenza infection

Within days after infection, natural killer (NK) cells are recruited to the lungs and play an essential role in the immune response against influenza infection. Through interactions with the virus itself, as well as viral-infected cells, NK cells secrete a variety of cytokines and can contain viral replication by killing infected cells early after influenza infection. However, the virus has means of evading NK cell responses, including escaping NK cell recognition through mutation of the viral hemagglutinin (HA) protein, regulating HA levels, and by directly infecting and destroying NK cells. Although much of our understanding of NK cell role in influenza infection has come from animal models, there is increasing information from human infection. Studies conducted during the 2009 H1N1 pandemic provided much needed information on the importance of NK cells during human infection and suggest that NK lymphopenia may correlate with increased disease severity. However, more information on how different influenza virus subtypes influence NK cell levels and activities, the role of the different NK cell receptors in infection, and the impact of NK cells on human infection, particularly in high risk populations is needed.

Recognition of tumors by the innate immune system and natural killer cells

In recent years, roles of the immune system in immune surveillance of cancer have been explored using a variety of approaches. The roles of the adaptive immune system have been a major emphasis, but increasing evidence supports a role for innate immune effector cells such as natural killer (NK) cells in tumor surveillance. Here, we discuss some of the evidence for roles in tumor surveillance of innate immune cells. In particular, we focus on NK cells and other immune cells that express germline-encoded receptors, often labeled NK receptors. The impact of these receptors and the cells that express them on tumor suppression is summarized. We discuss in detail some of the pathways and events in tumor cells that induce or upregulate cell-surface expression of the ligands for these receptors, and the logic of how those pathways serve to identify malignant, or potentially malignant cells. How tumors often evade tumor suppression mediated by innate killer cells is another major subject of the review. We end with a discussion on some of the implications of the various findings with respect to possible therapeutic approaches.

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.

Homo-oligomerization of the activating natural killer cell receptor NKp30 ectodomain increases its binding affinity for cellular ligands

The natural cytotoxicity receptors, comprised of three type I membrane proteins NKp30, NKp44, and NKp46, are a unique set of activating proteins expressed mainly on the surface of natural killer (NK) cells. Among these, NKp30 is a major receptor targeting virus-infected cells, malignantly transformed cells, and immature dendritic cells. To date, only few cellular ligands of NKp30 have been discovered, and the molecular details of ligand recognition by NKp30 are poorly understood. Within the current study, we found that the ectodomain of NKp30 forms functional homo-oligomers that mediate high affinity binding to its corresponding cellular ligand B7-H6. Notably, this homo-oligomerization is strongly promoted by the stalk domain of NKp30. Based on these data, we suggest that homo-oligomerization of NKp30 in the plasma membrane of NK cells, which might be favored by IL-2-dependent up-regulation of NKp30 expression, provides a way to improve recognition and lysis of target cells by NK cells.

NKp46 regulates allergic responses

Natural killer (NK) cells are cytotoxic cells that are able to rapidly kill viruses, tumor cells, parasites, bacteria, and even cells considered "self". The activity of NK cells is controlled by a fine balance of inhibitory and activating signals mediated by a complex set of different receptors. However, the function of NK cells is not restricted only to the killing of target cells, NK cells also possess other properties such as the secretion of proangiogenic factors during pregnancy. Here, we demonstrate another unique NK-cell activity, namely the regulation of T-cell mediated allergic responses, which is dependent on the NK-cell specific receptor NKp46 (Ncr1 in mice). Using mice in which the Ncr1 gene has been replaced with a green fluorescent protein, we demonstrate reduced delayed-type hypersensitivity and airway hypersensitivity. Interestingly, we show that this reduction in airway hypersensitivity is due to differences in the stimulation of T cells resulting in an altered cytokine profile.

Effects of vitamin D3, calcipotriol and FTY720 on the expression of surface molecules and cytolytic activities of human natural killer cells and dendritic cells

We describe here the effects of three drugs that are either approved or have the potential for treating multiple sclerosis (MS) patients through the in vitro activities of human natural killer (NK) cells and dendritic cells (DCs). Our results indicate that 1,25(OH)₂D₃, the biologically active metabolite vitamin D₃, calcipotriol and FTY720 augment IL-2-activated NK cell lysis of K562 and RAJI tumor cell lines as well as immature (i) and mature (m) DCs, with variable efficacies. These results are corroborated with the ability of the drugs to up-regulate the expression of NK cytotoxicity receptors NKp30 and NKp44, as well as NKG2D on the surfaces of NK cells. Also, they down-regulate the expression of the killer inhibitory receptor CD158. The three drugs down-regulate the expression of CCR6 on the surface of iDCs, whereas vitamin D₃ and calcipotriol tend to up-regulate the expression of CCR7 on mDCs, suggesting that they may influence the migration of DCs into the lymph nodes. Finally, vitamin D₃, calcipotriol and FTY720 enhance NK17/NK1 cell lysis of K562 cells, suggesting that a possible mechanism of action for these drugs is via activating these newly described cells. In conclusion, our results show novel mechanisms of action for vitamin D₃, calcipotriol and FTY720 on cells of the innate immune system.

A soluble fragment of the tumor antigen BCL2-associated athanogene 6 (BAG-6) is essential and sufficient for inhibition of NKp30 receptor-dependent cytotoxicity of natural killer cells

Immunosurveillance of tumor cells depends on NKp30, a major activating receptor of human natural killer (NK) cells. The human BCL2-associated athanogene 6 (BAG-6, also known as BAT3; 1126 amino acids) is a cellular ligand of NKp30. To date, little is known about the molecular details of this receptor ligand system. Within the current study, we have located the binding site of NKp30 to a sequence stretch of 250 amino acids in the C-terminal region of BAG-6 (BAG-6(686-936)). BAG-6(686-936) forms a noncovalent dimer of 57-59 kDa, which is sufficient for high affinity interaction with NKp30 (KD < 100 nM). As our most important finding, BAG-6(686-936) inhibits NKp30-dependent signaling, interferon-γ release, and degranulation of NK cells in the presence of malignantly transformed target cells. Based on these data, we show for the first time that BAG-6(686-936) comprises a subdomain of BAG-6, which is sufficient for receptor docking and inhibition of NKp30-dependent NK cell cytotoxicity as part of a tumor immune escape mechanism. These molecular insights provide an access point to restore tumor immunosurveillance by NK cells and to increase the efficacy of cellular therapies.

The expression of the beta cell-derived autoimmune ligand for the killer receptor nkp46 is attenuated in type 2 diabetes

NK cells rapidly kill tumor cells, virus infected cells and even self cells. This is mediated via killer receptors, among which NKp46 (NCR1 in mice) is prominent. We have recently demonstrated that in type 1 diabetes (T1D) NK cells accumulate in the diseased pancreas and that they manifest a hyporesponsive phenotype. In addition, we found that NKp46 recognizes an unknown ligand expressed by beta cells derived from humans and mice and that blocking of NKp46 activity prevented diabetes development. Here we investigated the properties of the unknown NKp46 ligand. We show that the NKp46 ligand is mainly located in insulin granules and that it is constitutively secreted. Following glucose stimulation the NKp46 ligand translocates to the cell membrane and its secretion decreases. We further demonstrate by using several modalities that the unknown NKp46 ligand is not insulin. Finally, we studied the expression of the NKp46 ligand in type 2 diabetes (T2D) using 3 different in vivo models and 2 species; mice and gerbils. We demonstrate that the expression of the NKp46 ligand is decreased in all models of T2D studied, suggesting that NKp46 is not involved in T2D.

Influenza virus infection but not H1N1 influenza virus immunization is associated with changes in peripheral blood NK cell subset levels

The innate immune system constitutes the first line of defense against viral agents, and NK cells seem to have an important protective role during the early phases of influenza virus infections. We decided to assess the levels of NK and NKT lymphocytes and the expression levels of different membrane receptors (NKp44, NKp46, NKG2A, killer cell immune-like receptor [KIR] 3DL1/DS1, KIR2DL1/DS1, and CD161) in peripheral blood samples of patients with influenza (n = 17) and healthy individuals immunized against this virus (seasonal and [H1N1]pdm2009 influenza vaccines; n = 15 and 12, respectively). Blood samples were obtained from all individuals, and NK and NKT cell subsets were analyzed by multiparametric flow cytometry. We found that the patients with severe influenza (n = 9) showed significant increases in the percentages of NKp46(+) NKp44(+) NK cells and the proportions of NK and NKT lymphocytes expressing KIR2DL1 and KIR3DL1 and reductions in the percentages of NKp46(+) NKp44(-) NK cells compared to those in the healthy controls (n = 27). In contrast, influenza immunization, against either the seasonal or the pandemic H1N1 virus, was not associated with important changes in the levels of NK and NKT lymphocytes or the expression levels of the different receptors by these cells. Our data suggest that severe influenza is associated with important and complex alterations on NK cells, which might contribute to the pathogenesis of this condition.

Neuraminidase-mediated, NKp46-dependent immune-evasion mechanism of influenza viruses

Natural killer (NK) cells play an essential role in the defense against influenza virus, one of the deadliest respiratory viruses known today. The NKp46 receptor, expressed by NK cells, is critical for controlling influenza infections, as influenza-virus-infected cells are eliminated through the recognition of the viral hemagglutinin (HA) protein by NKp46. Here, we describe an immune-evasion mechanism of influenza viruses that is mediated by the neuraminidase (NA) protein. By using various NA blockers, we show that NA removes sialic acid residues from NKp46 and that this leads to reduced recognition of HA. Furthermore, we provide in vivo and in vitro evidence for the existence of this NA-mediated, NKp46-dependent immune-evasion mechanism and demonstrate that NA inhibitors, which are commonly used for the treatment of influenza infections, are useful not only as blockers of virus budding but also as boosters of NKp46 recognition.

Structural insights into activation of antiviral NK cell responses

Natural killer (NK) cells are key components of innate immune responses, providing surveillance against cells undergoing tumorigenesis or infection, by viruses or internal pathogens. NK cells can directly eliminate compromised cells and regulate downstream responses of the innate and acquired immune systems through the release of immune modulators (cytokines, interferons). The importance of the role NK cells play in immune defense was demonstrated originally in herpes viral infections, usually mild or localized, which become severe and life threatening in NK-deficient patients . NK cell effector functions are governed by balancing opposing signals from a diverse array of activating and inhibitory receptors. Many NK receptors occur in paired activating and inhibitory isoforms and recognize major histocompatibility complex (MHC) class I proteins with varying degrees of peptide specificity. Structural studies have made considerable inroads into understanding the molecular mechanisms employed to broadly recognize multiple MHC ligands or specific pathogen-associated antigens and the strategies employed by viruses to thwart these defenses. Although many details of NK development, signaling, and integration remain mysterious, it is clear that NK receptors are key components of a system exquisitely tuned to sense any dysregulation in MHC class I expression, or the expression of certain viral antigens, resulting in the elimination of affected cells.

Activating natural cytotoxicity receptors of natural killer cells in cancer and infection

Natural killer (NK) cells are central players in the vertebrate immune system that rapidly eliminate malignantly transformed or infected cells. The natural cytotoxicity receptors (NCRs) NKp30, NKp44, and NKp46 are important mediators of NK cell cytotoxicity, which trigger an immune response on recognition of cognate cellular and viral ligands. Tumour and viral immune escape strategies targeting these receptor-ligand systems impair NK cell cytotoxicity and promote disease. Therefore, a molecular understanding of the function of the NCRs in immunosurveillance is instrumental to discovering novel access points to combat infections and cancer.

Control of human viral infections by natural killer cells

Natural killer (NK) cells are effector cells of the innate immune system and are important in the control of viral infections. Their relevance is reflected by the multiple mechanisms evolved by viruses to evade NK cell-mediated immune responses. Over recent years, our understanding of the interplay between NK cell immunity and viral pathogenesis has improved significantly. Here, we review the role of NK cells in the control of four important viral infections in humans: cytomegalovirus, influenza virus, HIV-1, and hepatitis C virus.

Differential lung NK cell responses in avian influenza virus infected chickens correlate with pathogenicity

Infection of chickens with low pathogenicity avian influenza (LPAI) virus results in mild clinical signs while infection with highly pathogenic avian influenza (HPAI) viruses causes death of the birds within 36-48 hours. Since natural killer (NK) cells have been shown to play an important role in influenza-specific immunity, we hypothesise that NK cells are involved in this difference in pathogenicity. To investigate this, the role of chicken NK-cells in LPAI virus infection was studied. Next activation of lung NK cells upon HPAI virus infection was analysed. Infection with a H9N2 LPAI virus resulted in the presence of viral RNA in the lungs which coincided with enhanced activation of lung NK cells. The presence of H5N1 viruses, measured by detection of viral RNA, did not induce activation of lung NK cells. This suggests that decreased NK-cell activation may be one of the mechanisms associated with the enhanced pathogenicity of H5N1 viruses.

Virus-mediated inhibition of natural cytotoxicity receptor recognition

Natural killer (NK) cells are a part of the innate immune system that functions mainly to kill transformed and infected cells. Their activity is controlled by signals derived from a panel of activating and inhibitory receptors. The natural cytotoxicity receptors (NCRs): NKp30, NKp44, and NKp46 (NCR1 in mice) are prominent among the activating NK cell receptors and they are, notably, the only NK-activating receptors that are able to recognize pathogen-derived ligands. In addition, the NCRs also recognize cellular ligands, the identity of which remains largely unknown. In this review, we summarize the current knowledge regarding viruses that are recognized by the NCRs, focusing on the diverse immune-evasion mechanisms employed by viruses to escape this detection. We also discuss the unique role the NCRs have in regulating NK cell activity with particular emphasis on the in vivo function of NKp46/NCR1.

Natural killer p46High expression defines a natural killer cell subset that is potentially involved in control of hepatitis C virus replication and modulation of liver fibrosis

Natural killer (NK) cells play a role in the early control and natural course of hepatitis C virus (HCV) infection. NK cell function is regulated by a multitude of receptors, including activating NKp46 receptor. However, reports on NKp46 in hepatitis C are controversial. Therefore, we investigated the hepatic recruitment and function of NKp46(+) NK cells, considering differential surface expression of NKp46 resulting in NKp46(High) and NKp46(Dim) subsets. Intra- and extrahepatic NK-cell subsets from HCV-infected patients were characterized by flow cytometry. Cytotoxic activity and interferon-gamma (IFN-γ) secretion were studied using K-562, P815, and primary hepatic stellate cells as targets. Anti-HCV activity of NK-cell subsets was studied using the replicon system. Density of NKp46 surface expression clearly segregated NKp46(Dim) and NKp46(High) subsets, which differed significantly with respect to the coexpression of maturation markers and NK-cell receptors. More important, NKp46(High) NK cells showed a higher cytolytic activity and stronger IFN-γ secretion than NKp46(Dim) NK cells. Accordingly, NKp46(High) NK cells efficiently blocked HCV replication in vitro. Blocking experiments confirmed an important role for the NKp46 receptor. Furthermore, we found an intrahepatic accumulation of NKp46(High) NK cells. Of note, high cytolytic activity of NKp46(High) NK cells was also confirmed in the intrahepatic NK-cell population, and the frequency of intrahepatic NKp46(High) NK cells was inversely correlated with HCV-RNA levels and fibrosis stage.

CONCLUSIONS

NKp46(High) expression defines a specific NK-cell subset that may be involved in both the suppression of HCV replication and HCV-associated liver damage underpinning the role of NK cells in the immunopathogenesis of HCV.

Super-resolution imaging of remodeled synaptic actin reveals different synergies between NK cell receptors and integrins

Natural killer (NK) cells secrete lytic granules to directly kill virus-infected or transformed cells and secrete cytokines to communicate with other cells. Three-dimensional super-resolved images of F-actin, lytic granules, and IFN-γ in primary human NK cells stimulated through different activating receptors reveal that both IFN-γ and lytic granules accumulated in domains where the periodicity of the cortical actin mesh at the synapse opened up to be penetrable. Ligation of some activating receptors alone (eg, CD16 or NKG2D) was sufficient to increase the periodicity of the actin mesh, but surprisingly, ligation of others (eg, NKp46 or CD2) was not sufficient to induce cortical actin remodeling unless LFA-1 was coligated. Importantly, influenza virus particles that can be recognized by NK cells similarly did not open the actin mesh but could if LFA-1 was coligated. This leads us to propose that immune cells using germline-encoded receptors to directly recognize foreign proteins can use integrin recognition to differentiate between free pathogens and pathogen-infected cells that will both be present in blood. This distinction would not be required for NK cell receptors, such as NKG2D, which recognize host cell-encoded proteins that can only be found on diseased cells and not pathogens.

Unlike natural killer (NK) p30, natural cytotoxicity receptor NKp44 binds to multimeric α2,3-NeuNAc-containing N-glycans

Natural cytotoxicity receptor 2 (NCR2 or natural killer (NK)p44) and NCR3 (NKp30) bind to heparin and heparin sulfate; however, other natural ligands have yet to be identified. We previously reported that NCR1 (NKp46) can bind to multimeric NeuNAc-containing N-glycans and sulfated glycans. In this study, we investigated whether NKp44 and NKp30 can bind to NeuNAc-containing glycans using their common recombinant extracellular domain tagged with 6×His (NKp44-H6 and NKp30-H6). NKp44-H6, but not NKp30-H6, bound multimeric sialyl Lewis X expressing transferrin secreted by HepG2 cells (HepTF) with a K(d) of 420 nM. Competitive and direct binding assays revealed that NKp44-H6 mainly recognizes α2,3-NeuNAc residues on non-reducing ends of N-glycans on HepTF. Moreover, site-directed mutants of NKp44-H6, such as R47Q, R55Q, R92Q, R95Q, K103Q, and R106Q, had reduced binding to α2,3-sialylated N-glycans. These results suggest that NKp44 binds to α2,3-sialylated N-glycans through ionic interactions, and that these binding sites might have some overlap with heparin binding sites.

The stalk domain and the glycosylation status of the activating natural killer cell receptor NKp30 are important for ligand binding

The natural cytotoxicity receptors are a unique set of activating proteins expressed mainly on the surface of natural killer (NK) cells. The human natural cytotoxicity receptor family comprises the three type I membrane proteins NKp30, NKp44, and NKp46. Especially NKp30 is critical for the cytotoxicity of NK cells against different targets including tumor, virus-infected, and immature dendritic cells. Although the crystal structure of NKp30 was recently solved (Li, Y., Wang, Q., and Mariuzza, R. A. (2011) J. Exp. Med. 208, 703-714; Joyce, M. G., Tran, P., Zhuravleva, M. A., Jaw, J., Colonna, M., and Sun, P. D. (2011) Proc. Natl. Acad. Sci. U.S.A. 108, 6223-6228), a key question, how NKp30 recognizes several non-related ligands, remains unclear. Therefore, we investigated the parameters that impact ligand recognition of NKp30. Based on various NKp30-hIgG1-Fc fusion proteins, which were optimized for minimal background binding to cellular Fcγ receptors, we identified the flexible stalk region of NKp30 as an important but so far neglected module for ligand recognition and related signaling of the corresponding full-length receptor proteins. Moreover, we found that the ectodomain of NKp30 is N-linked glycosylated at three different sites. Mutational analyses revealed differential binding affinities and signaling capacities of mono-, di-, or triglycosylated NKp30, suggesting that the degree of glycosylation could provide a switch to modulate the ligand binding properties of NKp30 and NK cell cytotoxicity.

Modeling within-host dynamics of influenza virus infection including immune responses

Influenza virus infection remains a public health problem worldwide. The mechanisms underlying viral control during an uncomplicated influenza virus infection are not fully understood. Here, we developed a mathematical model including both innate and adaptive immune responses to study the within-host dynamics of equine influenza virus infection in horses. By comparing modeling predictions with both interferon and viral kinetic data, we examined the relative roles of target cell availability, and innate and adaptive immune responses in controlling the virus. Our results show that the rapid and substantial viral decline (about 2 to 4 logs within 1 day) after the peak can be explained by the killing of infected cells mediated by interferon activated cells, such as natural killer cells, during the innate immune response. After the viral load declines to a lower level, the loss of interferon-induced antiviral effect and an increased availability of target cells due to loss of the antiviral state can explain the observed short phase of viral plateau in which the viral level remains unchanged or even experiences a minor second peak in some animals. An adaptive immune response is needed in our model to explain the eventual viral clearance. This study provides a quantitative understanding of the biological factors that can explain the viral and interferon kinetics during a typical influenza virus infection.

Influenza, commonly referred to as the flu, is a contagious respiratory illness caused by influenza virus infections. Although most infected subjects with intact immune systems are able to clear the virus without developing serious flu complications, the mechanisms underlying viral control are not fully understood. In this paper, we address this question by developing mathematical models that include both innate and adaptive immune responses, and fitting them to experimental data from horses infected with equine influenza virus. We find that the innate immune response, such as natural killer cell-mediated infected cell killing and interferon's antiviral effect, can explain the first rapid viral decline and subsequent second peak viremia, and that the adaptive immune response is needed to eventually clear the virus. This study improves our understanding of influenza virus dynamics and may provide more information for future research in influenza pathogenesis, treatment, and vaccination.

Elucidating the mechanisms of influenza virus recognition by Ncr1

Natural killer (NK) cells are innate cytotoxic lymphocytes that specialize in the defense against viral infection and oncogenic transformation. Their action is tightly regulated by signals derived from inhibitory and activating receptors; the later include proteins such as the Natural Cytotoxicity Receptors (NCRs: NKp46, NKp44 and NKp30). Among the NCRs, NKp46 is the only receptor that has a mouse orthologue named Ncr1. NKp46/Ncr1 is also a unique marker expressed on NK and on Lymphoid tissue inducer (LTI) cells and it was implicated in the control of various viral infections, cancer and diabetes. We have previously shown that human NKp46 recognizes viral hemagglutinin (HA) in a sialic acid-dependent manner and that the O-glycosylation is essential for the NKp46 binding to viral HA. Here we studied the molecular interactions between Ncr1 and influenza viruses. We show that Ncr1 recognizes influenza virus in a sialic acid dependent manner and that N-glycosylation is important for this binding. Surprisingly we demonstrate that none of the predicted N-glycosilated residues of Ncr1 are essential for its binding to influenza virus and we thus conclude that other, yet unidentified N-glycosilated residues are responsible for its recognition. We have demonstrated that N glycosylation play little role in the recognition of mouse tumor cell lines and also showed the in-vivo importance of Ncr1 in the control of influenza virus infection by infecting C57BL/6 and BALB/c mice knockout for Ncr1 with influenza.

Human NK cell recognition of target cells in the prism of natural cytotoxicity receptors and their ligands

The matter of the pathogen- and cancer-associated ligands recognized by the Natural Cytotoxicity Receptors (NCRs) has been a subject of intense research ever since the identification of the NCRs more than 12 years ago by Alessandro and Lorenzo Moretta: NKp46 in 1997, NKp44 in 1998, and finally NKp30 in 1999. Expression patterns recognized by NCRs include pathogen-derived, pathogen-induced, and cancer-associated cellular 'self' ligands. Pathogen-exposed cells may exhibit both types of pathogen-associated ligands. Transformed cells, in contrast, exhibit only 'self' ligands which are derived from both the intracellular- and membrane-associated milieu of self molecules. These expression patterns allow for NCR-based NK cell discrimination between healthy and affected cells, in the realms of both pathogenic infection and potential tumorigenesis. The focus of this review is on the current knowledge regarding the identities of NCR ligands and the type of target cells expressing these ligands.

Direct recognition of Fusobacterium nucleatum by the NK cell natural cytotoxicity receptor NKp46 aggravates periodontal disease

Periodontitis is a common human chronic inflammatory disease that results in the destruction of the tooth attachment apparatus and tooth loss. Although infections with periopathogenic bacteria such as Porphyromonas gingivalis (P. gingivalis) and Fusobacterium nucleatum (F. nucleatum) are essential for inducing periodontitis, the nature and magnitude of the disease is determined by the host's immune response. Here, we investigate the role played by the NK killer receptor NKp46 (NCR1 in mice), in the pathogenesis of periodontitis. Using an oral infection periodontitis model we demonstrate that following F. nucleatum infection no alveolar bone loss is observed in mice deficient for NCR1 expression, whereas around 20% bone loss is observed in wild type mice and in mice infected with P. gingivalis. By using subcutaneous chambers inoculated with F. nucleatum we demonstrate that immune cells, including NK cells, rapidly accumulate in the chambers and that this leads to a fast and transient, NCR1-dependant TNF-α secretion. We further show that both the mouse NCR1 and the human NKp46 bind directly to F. nucleatum and we demonstrate that this binding is sensitive to heat, to proteinase K and to pronase treatments. Finally, we show in vitro that the interaction of NK cells with F. nucleatum leads to an NCR1-dependent secretion of TNF-α. Thus, the present study provides the first evidence that NCR1 and NKp46 directly recognize a periodontal pathogen and that this interaction influences the outcome of F. nucleatum-mediated periodontitis.

Altered dendritic cell-natural killer interaction in Kenyan sex workers resistant to HIV-1 infection

BACKGROUND

Natural killer (NK) cells are members of the innate immune system that play an important role in the defense against viral infection. They are also involved in the regulation of adaptive immune responses through cytokine secretion and the interaction with antigen-presenting cells. However, their role in HIV infection is only partially understood.

OBJECTIVE

Here we studied the phenotype and function of NK cells of highly HIV-exposed but seronegative (HESN) uninfected commercial sex workers from Kenya who can be epidemiologically defined as relatively resistant to HIV infection.

DESIGN

The purpose of this study was to gain insight into the role of NK cells in mediating resistance to HIV-1. This information can be used to better understand protection from infection which can be used for informing future design of effective prophylactics and therapeutics for HIV.

METHODS

Whole blood samples were collected from study participants and isolated NK cells and dendritic cells were used in assays for phenotyping and cell function.

RESULTS

Activated NK cells from resistant women killed autologous immature dendritic cells more efficiently and also secreted more interferon (IFN)-γ than those of uninfected, susceptible women. Interestingly, NK cells from HIV-resistant women were significantly more effective in inducing secretion of IL-12 in immature dendritic cells.

CONCLUSIONS

These data suggest that an altered NK cell-dendritic cell interaction plays an important role in the protection from infection with HIV-1.

Recognition and prevention of tumor metastasis by the NK receptor NKp46/NCR1

NK cells employ a variety of activating receptors to kill virally infected and tumor cells. Prominent among these receptors are the natural cytotoxicity receptors (NCRs) (NKp30, NKp44, and NKp46), of which only NKp46 has a mouse ortholog (NCR1). The tumor ligand(s) of NKp46/NCR1 is still unknown, but it was shown that the human NKp46 and the mouse NCR1 are involved in tumor eradication both in vitro and in vivo. Whether any of the NK activating receptors is involved in the prevention of tumor metastasis is unknown. To address this question, we studied the activity of the NK cell receptor NKp46/NCR1 in two spontaneous metastasis models, the B16F10.9 melanoma (B16) and the Lewis lung carcinoma (D122) in the NCR1 knockout mouse that was generated by our group, in various in vitro and in vivo assays. We demonstrated that all B16 and D122 tumors, including those generated in vivo, express an unknown ligand(s) for NKp46/NCR1. We have characterized the properties of the NKp46/NCR1 ligand(s) and demonstrated that NKp46/NCR1 is directly involved in the killing of B16 and D122 cells. Importantly, we showed in vivo that NKp46/NCR1 plays an important role in controlling B16 and D122 metastasis. Thus, to our knowledge, in this study we provide the first evidence for the direct involvement of a specific NK killer receptor in preventing tumor metastasis.

Innate immunity in multiple sclerosis white matter lesions: expression of natural cytotoxicity triggering receptor 1 (NCR1)

Background

Pathogenic or regulatory effects of natural killer (NK) cells are implicated in many autoimmune diseases, but evidence in multiple sclerosis (MS) and its murine models remains equivocal. In an effort to illuminate this, we have here analysed expression of the prototypic NK cell marker, NCR1 (natural cytotoxicity triggering receptor; NKp46; CD335), an activating receptor expressed by virtually all NK cells and therefore considered a pan-marker for NK cells. The only definitive ligand of NCR1 is influenza haemagglutinin, though there are believed to be others. In this study, we investigated whether there were differences in NCR1⁺ cells in the peripheral blood of MS patients and whether NCR1⁺ cells are present in white matter lesions.

Results

We first investigated the expression of NCR1 on peripheral blood mononuclear cells and found no significant difference between healthy controls and MS patients. We then investigated mRNA levels in central nervous system (CNS) tissue from MS patients: NCR1 transcripts were increased more than 5 times in active disease lesions. However when we performed immunohistochemical staining of this tissue, few NCR1⁺ NK cells were identified. Rather, the major part of NCR1 expression was localised to astrocytes, and was considerably more pronounced in MS patients than controls. In order to further validate de novo expression of NCR1 in astrocytes, we used an in vitro staining of the human astrocytoma U251 cell line grown to model whether cell stress could be associated with expression of NCR1. We found up-regulation of NCR1 expression in U251 cells at both the mRNA and protein levels.

Conclusions

The data presented here show very limited expression of NCR1⁺ NK cells in MS lesions, the majority of NCR1 expression being accounted for by expression on astrocytes. This is compatible with a role of this cell-type and NCR1 ligand/receptor interactions in the innate immune response in the CNS in MS patients. This is the first report of NCR1 expression on astrocytes in MS tissue: it will now be important to unravel the nature of cellular interactions and signalling mediated through innate receptor expression on astrocytes.