Triggering receptors involved in natural killer cell-mediated cytotoxicity against choriocarcinoma cell lines

NK-92 cells change their phenotype and function when cocultured with IL-15, IL-18 and trophoblast cells

NK cell development is affected by their cellular microenvironment and cytokines, including IL-15 and IL-18. NK cells can differentiate in secondary lymphoid organs, liver and within the uterus in close contact with trophoblast cells. The aim was to evaluate changes in the NK cell phenotype and function in the presence of IL-15, IL-18 and JEG-3, a trophoblast cell line. When cocultured with JEG-3 cells, IL-15 caused an increase in the number of NKG2D+ NK-92 cells and the intensity of CD127 expression. IL-18 stimulates an increase in the amount of NKp44+ NK-92 cells and in the intensity of NKp44 expression by pNK in the presence of trophoblast cells. NK-92 cell cytotoxic activity against JEG-3 cells increased only in presence of IL-18. Data on changes in the cytotoxic activity of NK-92 cells against JEG-3 cells in the presence of IL-15 and IL-18 indicate the modulation of NK cell function both by the cytokine microenvironment and directly by target cells. IL-15 and IL-18 were present in conditioned media (CM) from 1st and 3rd trimester placentas. In the presence of 1st trimester CM and JEG-3 cells, NK-92 cells showed an increase in the intensity of NKG2D expression. In the presence of 3rd trimester CM and JEG-3 cells, a decrease in the expression of NKG2D by NK-92 cells was observed. Thus, culturing of NK-92 cells with JEG-3 trophoblast cells stimulated a pronounced change in the NK cell phenotype, bringing it closer to the decidual NK cell-like phenotype.

Natural Killer Cell Interactions With Myeloid Derived Suppressor Cells in the Tumor Microenvironment and Implications for Cancer Immunotherapy

The tumor microenvironment (TME) is a complex and heterogeneous environment composed of cancer cells, tumor stroma, a mixture of tissue-resident and infiltrating immune cells, secreted factors, and extracellular matrix proteins. Natural killer (NK) cells play a vital role in fighting tumors, but chronic stimulation and immunosuppression in the TME lead to NK cell exhaustion and limited antitumor functions. Myeloid-derived suppressor cells (MDSCs) are a heterogeneous group of myeloid cells with potent immunosuppressive activity that gradually accumulate in tumor tissues. MDSCs interact with innate and adaptive immune cells and play a crucial role in negatively regulating the immune response to tumors. This review discusses MDSC-mediated NK cell regulation within the TME, focusing on critical cellular and molecular interactions. We review current strategies that target MDSC-mediated immunosuppression to enhance NK cell cytotoxic antitumor activity. We also speculate on how NK cell-based antitumor immunotherapy could be improved.

Core 2 β1,6-N-acetylglucosaminyltransferases accelerate the escape of choriocarcinoma from natural killer cell immunity

Hyperglycosylated human chorionic gonadotropin (H-hCG) is secreted from choriocarcinoma and contains a core2 O-glycan formed by core2 β1,6-N-acetylglucosaminyl transferase (C2GnT). Choriocarcinoma is considered immunogenic as it is gestational and contains paternal chromosomal components. Here we examined the function of C2GnT in the evasion of choriocarcinoma cells from natural killer (NK) cell-mediating killing. We determined that C2GnT is highly expressed in malignant gestational trophoblastic neoplasms. C2GnT KO downregulates core2 O-glycan expression in choriocarcinoma cells, which are more efficiently killed by NK cells than control cells. C2GnT KO cell containing tumor necrosis factor-related apoptosis inducing ligand have lower viability than control cells. Additionally, poly-N-acetyllactosamine in core2 branched oligosaccharides on MHC class I-related chain A (MICA) and mucin1 (MUC1) is significantly reduced in C2GnT KO cells. Meanwhile, the cumulative survival rate of nude mice inoculated with C2GnT KO tumors was higher than that of the control group. These findings suggest that choriocarcinoma cells may escape NK cell-mediated killing via glycosylation of MICA and MUC1.

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C2GnT knockout reduces core2 O-glycan expression in choriocarcinoma cells

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C2GnT KO cells are more readily killed by NK cells

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C2GnT KO cells containing TRAIL have lower viability than controls

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MHC class I-related chain A and MUC1 glycosylation is reduced in C2GnT KO cells

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Survival rate of nude mice inoculated with C2GnT KO tumors is higher than controls

Innate immune responses in RNA viral infection

RNA viruses cause a multitude of human diseases, including several pandemic events in the past century. Upon viral invasion, the innate immune system responds rapidly and plays a key role in activating the adaptive immune system. In the innate immune system, the interactions between pathogen-associated molecular patterns and host pattern recognition receptors activate multiple signaling pathways in immune cells and induce the production of pro-inflammatory cytokines and interferons to elicit antiviral responses. Macrophages, dendritic cells, and natural killer cells are the principal innate immune components that exert antiviral activities. In this review, the current understanding of innate immunity contributing to the restriction of RNA viral infections was briefly summarized. Besides the main role of immune cells in combating viral infection, the intercellular transfer of pathogen and host-derived materials and their epigenetic and metabolic interactions associated with innate immunity was discussed. This knowledge provides an enhanced understanding of the innate immune response to RNA viral infections in general and aids in the preparation for the existing and next emerging viral infections.

Human natural killer cells mediate adaptive immunity to viral antigens

Adaptive immune responses are defined as antigen sensitization-dependent and antigen-specific responses leading to establishment of long-lived immunological memory. Although natural killer (NK) cells have traditionally been considered cells of the innate immune system, mounting evidence in mice and nonhuman primates warrants reconsideration of the existing paradigm that B and T cells are the sole mediators of adaptive immunity. However, it is currently unknown whether human NK cells can exhibit adaptive immune responses. We therefore tested whether human NK cells mediate adaptive immunity to virally encoded antigens using humanized mice and human volunteers. We found that human NK cells displayed vaccination-dependent, antigen-specific recall responses in vitro, when isolated from livers of humanized mice previously vaccinated with HIV-encoded envelope protein. Furthermore, we discovered that large numbers of cytotoxic NK cells with a tissue-resident phenotype were recruited to sites of varicella-zoster virus (VZV) skin test antigen challenge in VZV-experienced human volunteers. These NK-mediated recall responses in humans occurred decades after initial VZV exposure, demonstrating that NK memory in humans is long-lived. Our data demonstrate that human NK cells exhibit adaptive immune responses upon vaccination or infection. The existence of human memory NK cells may allow for the development of vaccination-based approaches capable of establishing potent NK-mediated memory functions contributing to host protection.

An Historical Overview: The Discovery of How NK Cells Can Kill Enemies, Recruit Defense Troops, and More

Natural killer (NK) cells were originally defined as effector lymphocytes of innate immunity characterized by the unique ability of killing tumor and virally infected cells without any prior priming and expansion of specific clones. The "missing-self" theory, proposed by Klas Karre, the seminal discovery of the first prototypic HLA class I-specific inhibitory receptors, and, later, of the Natural Cytotoxicity Receptors (NCRs) by Alessandro Moretta, provided the bases to understand the puzzling behavior of NK cells. Actually, those discoveries proved crucial also for many of the achievements that, along the years, have contributed to the modern view of these cells. Indeed, NK cells, besides killing susceptible targets, are now known to functionally interact with different immune cells, sense pathogens using TLR, adapt their responses to the local environment, and, even, mount a sort of immunological memory. In this review, we will specifically focus on the main activating NK receptors and on their crucial role in the ever-increasing number of functions assigned to NK cells and other innate lymphoid cells (ILCs).

NKp44-NKp44 Ligand Interactions in the Regulation of Natural Killer Cells and Other Innate Lymphoid Cells in Humans

Natural Killer (NK) cells are potent cytotoxic cells belonging to the family of Innate Lymphoid Cells (ILCs). Their most characterized effector functions are directed to the control of aberrant cells in the body, including both transformed and virus-infected cells. NK cell-mediated recognition of abnormal cells primarily occurs through receptor-ligand interactions, involving an array of inhibitory and activating NK receptors and different types of ligands expressed on target cells. While most of the receptors have become known over many years, their respective ligands were only defined later and their impressive complexity has only recently become evident. NKp44, a member of Natural Cytotoxicity Receptors (NCRs), is an activating receptor playing a crucial role in most functions exerted by activated NK cells and also by other NKp44⁺ immune cells. The large and heterogeneous panel of NKp44 ligands (NKp44L) now includes surface expressed glycoproteins and proteoglycans, nuclear proteins that can be exposed outside the cell, and molecules that can be either released in the extracellular space or carried in extracellular vesicles. Recent findings have extended our knowledge on the nature of NKp44L to soluble plasma glycoproteins, such as secreted growth factors or extracellular matrix (ECM)-derived glycoproteins. NKp44L are induced upon tumor transformation or viral infection but may also be expressed in normal cells and tissues. In addition, NKp44-NKp44L interactions are involved in the crosstalk between NK cells and different innate and adaptive immune cell types. NKp44 expression in different ILCs located in tissues further extends the potential role of NKp44-NKp44L interactions.

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.

CXCR6+ NK Cells in Human Fetal Liver and Spleen Possess Unique Phenotypic and Functional Capabilities

Tissue-resident Natural Killer (NK) cells vary in phenotype according to tissue origin, but are typically CD56^bright, CXCR6⁺, and CD69⁺. NK cells appear very early in fetal development, but little is known about when markers of tissue residency appear during gestation and whether the expression of these markers, most notably the chemokine receptor CXCR6, are associated with differences in functional capability. Using multi-parametric flow cytometry, we interrogated fetal liver and spleen NK cells for the expression of a multitude of extracellular markers associated with NK cell maturation, differentiation, and migration. We analyzed total NK cells from fetal liver and spleen and compared them to their adult liver and spleen counterparts, and peripheral blood (PB) NK. We found that fetal NK cells resemble each other and their adult counterparts more than PB NK. Maturity markers including CD16, CD57, and KIR are lower in fetal NK cells than PB, and markers associated with an immature phenotype are higher in fetal liver and spleen NK cells (NKG2A, CD94, and CD27). However, T-bet/EOMES transcription factor profiles are similar amongst fetal and adult liver and spleen NK cells (T-bet⁻/EOMES⁺) but differ from PB NK cells (T-bet⁺EOMES⁻). Further, donor-matched fetal liver and spleen NK cells share similar patterns of expression for most markers as a function of gestational age. We also performed functional studies including degranulation, cytotoxicity, and antibody-dependent cellular cytotoxicity (ADCC) assays. Fetal liver and spleen NK cells displayed limited cytotoxic effector function in chromium release assays but produced copious amounts of TNFα and IFNγ, and degranulated efficiently in response to stimulation with PMA/ionomycin. Further, CXCR6⁺ NK cells in fetal liver and spleen produce more cytokines and degranulate more robustly than their CXCR6⁻ counterparts, even though CXCR6⁺ NK cells in fetal liver and spleen possess an immature phenotype. Major differences between CXCR6⁻ and + NK cell subsets appear to occur later in development, as a distinct CXCR6⁺ NK cell phenotype is much more clearly defined in PB. In conclusion, fetal liver and spleen NK cells share similar phenotypes, resemble their adult counterparts, and already possess a distinct CXCR6⁺ NK cell population with discrete functional capabilities.

Influence of In Vitro IL-2 or IL-15 Alone or in Combination with Hsp 70 Derived 14-Mer Peptide (TKD) on the Expression of NK Cell Activatory and Inhibitory Receptors on Peripheral Blood T Cells, B Cells and NKT Cells

Previous studies from Multhoff and colleagues reported that plasma membrane Hsp70 acts as a tumour-specific recognition structure for activated NK cells, and that the incubation of NK cells with Hsp70 and/or a 14-mer peptide derived from the N-terminal sequence of Hsp70 (TKDNNLLGRFELSG, TKD, aa 450–463) plus a low dose of IL-2 triggers NK cell proliferation and migration, and their capacity to kill cancer cells expressing membrane Hsp70. Herein, we have used flow cytometry to determine the influence of in vitro stimulation of peripheral blood mononuclear cells from healthy individuals with IL-2 or IL-15, either alone or in combination with TKD peptide on the cell surface expression of CD94, NK cell activatory receptors (CD16, NK2D, NKG2C, NKp30, NKp44, NKp46, NKp80, KIR2DL4, DNAM-1 and LAMP1) and NK cell inhibitory receptors (NKG2A, KIR2DL2/L3, LIR1/ILT-2 and NKR-P1A) by CD3+CD56+ (NKT), CD3+CD4+, CD3+CD8+ and CD19+ populations. NKG2D, DNAM-1, LAMP1 and NKR-P1A expression was upregulated after the stimulation with IL-2 or IL-15 alone or in combination with TKD in NKT, CD8+ T cells and B cells. CD94 was upregulated in NKT and CD8+ T cells. Concurrently, an increase in a number of CD8+ T cells expressing LIR1/ILT-2 and CD4+ T cells positive for NKR-P1A was observed. The proportion of CD8+ T cells that expressed NKG2D was higher after IL-2/TKD treatment, when compared with IL-2 treatment alone. In comparison with IL-15 alone, IL-15/TKD treatment increased the proportion of NKT cells that were positive for CD94, LAMP1 and NKRP-1A. The more potent effect of IL-15/TKD on cell surface expression of NKG2D, LIR1/ILT-2 and NKRP-1A was observed in B cells compared with IL-15 alone. However, this increase was not of statistical significance. IL-2/TKD induced significant upregulation of LAMP1 in CD8+ T cells compared with IL-2 alone. Besides NK cells, other immunocompetent cells present within the fraction of peripheral blood mononuclear cells were influenced by the treatment with low-dose interleukins themselves or in combination with hsp70 derived (TKD) peptide.

Natural killer cell in the developing life

\Natural killer (NK) cells that provide first-line innate immune reactions against virus-infected and tumor cells have different roles in different body sites and in different stages. From the beginning of life, NK cells participate in many aspects of development, especially in a successful pregnancy and a healthy neonatal stage. This article reviews recent advances regarding the role of NK cells in implantation, placentation and immune tolerance during pregnancy as well as in the neonatal immune defense. The interactions between NK cells and other immune cells in each developmental stage are discussed.

NK cell receptor NKp46 regulates graft-versus-host disease

Hematopoietic stem cell transplantation (HSCT) is often the only curative treatment for a wide variety of hematologic malignancies. Donor selection in these diseases is crucial, given that transplanted cells can mediate not only the desired graft-versus-leukemia effect but also graft-versus-host disease (GVHD). Here, we demonstrate that in the absence of NKp46, a major killer receptor expressed by human and mouse natural killer (NK) cells, GVHD is greatly exacerbated, resulting in rapid mortality of the transplanted animals because of infection with commensal bacteria. Furthermore, we demonstrate that the exacerbated GVHD is the result of an altered ability of immune cells to respond to stimulation by immature dendritic cells. Because high and low expression of NKp46 on NK cells is observed in different individuals, our data indicate that choosing NKp46-high donors for the treatment of different hematologic malignancies might lead to better tumor eradication while minimizing GVHD.

TLR/NCR/KIR: Which One to Use and When?

By means of a complex receptor array, Natural killer (NK) cells can recognize variable patterns of ligands and regulate or amplify accordingly their effector functions. Such NK receptors include old, rather conserved, molecules, such as toll-like receptors (TLRs), which enable NK cells to respond both to viral and bacterial products, and newer and evolving molecules, such as killer Ig-like receptors and natural cytotoxicity receptors, which control NK cytotoxicity and are responsible for the elimination of virus-infected or tumor cells without damaging self-unaltered cells. In addition, to rapidly gain new functions NK cells also can acquire new receptors by trogocytosis. Thus, NK cells may have adapted their receptors to different functional needs making them able to play a key role in the modulation of critical events occurring in several compartments of human body (primarily in SLCs but also in decidua during pregnancy). In this review, we will discuss on how the various types of receptors can be used to address specific functions in different immunological contexts.

Role of the innate immunity in female reproductive tract

The mucosal immune system in the female reproductive tract (FRT) is well equipped to meet the sexually transmitted pathogens, allogeneic sperm, and the immunologically distinct fetus. Analysis of the FRT indicates that epithelial cells provide a physical barrier against pathogens and microbial infections as well as secretions containing anti-microbial peptides, cytokines, and chemokines which recruit and activate immune cells. Epithelial and immune cells confer protection in part through Toll-like receptors. The aim of this literature is to review the diverse components of the innate immune system, contributing to an exclusive protection system throughout the FRT.

Cytokine production by non-stimulated peripheral blood NK cells and lymphocytes in early-onset severe pre-eclampsia without HELLP

Preeclampsia involves an exacerbated maternal inflammatory response that suggests a possible role of innate immunity. NK cells can promote this kind of response through cytokine production and the expression of activating or inhibitory receptors. The aims of the present study were to explore cytokine production by peripheral blood mononuclear cells, as well as cytotoxic ability and receptor expression for HLA-E and HLA-G molecules in peripheral natural killer (NK) cells of women with early-onset severe preeclampsia without HELLP (hemolysis, elevated liver enzyme levels and a low platelet count) syndrome. The expression of the ILT2, KIRDL4, NKG2A, and NKG2C receptors and of cytotoxic activity was measured in non-stimulated NK cells, whereas the intracellular expression of IL-4, IL-10, IL-13, IL-12, IFNγ, TNF and VEGF, was assessed in non-stimulated peripheral blood mononuclear cells subsets using flow cytometry. Circulating soluble HLA-G was also determined by ELISA. The intracellular cytokines tested were significantly higher in NK cell subsets from severely preeclamptic women compared with the control group. On the other hand, the percentage of NK cells expressing NKG2A or NKG2C and the cytotoxic activity of NK cells were significantly higher in severely preeclamptic women. Furthermore, there was a significant correlation between urine protein concentration and soluble human leukocyte antigen G (soluble HLA-G) in serum. We conclude that patients with early-onset severe preeclampsia without HELLP syndrome have increased NK cell function related to cytokine production, cytotoxicity and expression of lectin-like receptors such as NKG2.

Novel interaction between proliferating cell nuclear antigen and HLA I on the surface of tumor cells inhibits NK cell function through NKp44

NK cell function is closely regulated by numerous inhibitory and activating receptors binding corresponding ligands on the surface of target cells, providing vital first line defenses against infections and cancer. NKp44, originally discovered as an activating NK cell receptor, was recently found to elicit inhibitory effects on NK cell effector function through recognition of cell surface PCNA. Other reports have pointed to potential associations between NKp44 and HLA I molecules, as well as HLA I and Damage Associated Molecular Pattern molecules (DAMPs) on the surface of tumor cells. In this report, we have identified novel interaction between HLA I and PCNA on the surface of human tumor cells by confocal microscopy and immunoprecipitation. In addition to previous reports, we show PCNA on the cell surface where novel association with HLA I does not require the presence of NKp44 expressing NK cells and occurs with endogenous PCNA. The association of HLA I and PCNA forms the inhibitory ligand for NKp44, resulting in inhibition of NK cell cytotoxicity. We further postulate NCR ligands are composed of DAMP molecules localized to the cell surface, colocalizing with HLA I, and potentially heparin sulfate proteoglycans.

[Immunological aspect of pregnancy]

Pregnancy is a temporary semi-allograft that survives for nine months. The importance of this event for the survival of the species justifies several tolerance mechanisms that are put into place at the beginning of pregnancy, some of which occur even at the time of implantation. The presence of multiple tolerance mechanisms and the richness of the means employed underline the central importance of the trophoblast. Understanding these mechanisms, and in particular, their integration into an overall scheme, enables the anomalies encountered in certain pathologies of pregnancy to be placed into context. Understanding these mechanisms and their interruption at the end of pregnancy should improve our understanding of disappointing results from current immunological treatments facilitate the implementation of new prophylactic and therapeutic strategies.

Origin, phenotype and function of human natural killer cells in pregnancy

During the early phases of pregnancy, natural killer (NK) cells are the predominant lymphoid cells in the human decidua. Here, rather than act as killers and/or drivers of inflammation, NK cells contribute to tissue building and remodeling and formation of new vessels due to the release of interleukin-8, vascular endothelial growth factor, stromal cell-derived factor-1 and interferon gamma-inducible protein-10. Here, we propose that the interaction of NK cells with CD14(+) myelomonocytic cells to promote induction of T regulatory cells plays a pivotal role in immunosuppression and tolerance towards the fetus allograft. Importantly, CD34(+) hematopoietic precursors are present in human decidua and may give rise to decidual NK cells. Defects in decidual NK cell generation, or in appropriate functional interactions with other cell types, could have major consequences for successful pregnancy.

Cloning, expression, and polymorphisms of natural killer cell receptor NCR1 in pigs

NCR1 (NKp46) is expressed on the surfaces of natural killer cells and recognizes hemagglutinin on the influenza virus. We cloned the NCR1 gene in pigs and found that porcine NCR1 was minimally expressed in the thymus, suggesting that NCR1 could be a useful marker of natural killer cells in pigs. We observed three nonsynonymous single nucleotide polymorphisms and one deletion of three nucleotides in the coding sequence of porcine NCR1; these may affect the function of NCR1. The polymorphisms detected here may be useful markers for breeding for influenza resistance in pigs.

Crosstalk between decidual NK and CD14+ myelomonocytic cells results in induction of Tregs and immunosuppression

Regulatory T cells (Tregs) are thought to play a major role in pregnancy by inhibiting the maternal immune system and preventing fetal rejection. In decidual tissues, NK cells (dNK) reside in close contact with particular myelomonocytic CD14(+) (dCD14(+)) cells. Here we show that the interaction between dNK and dCD14(+) cells results in induction of Tregs. The interaction is mediated by soluble factors as shown by transwell experiments, and the prominent role of IFN-gamma is revealed by the effect of a neutralizing monoclonal antibody. Following interaction with dNK cells, dCD14(+) cells express indoleamine 2,3-dioxygenase (IDO), which, in turn, induces Tregs. Notably, unlike peripheral blood NK (pNK) cells, dNK cells are resistant to inhibition by the IDO metabolite L-kynurenine. "Conditioned" dCD14(+) cells also may induce Tregs through transforming growth factor-beta (TGF-beta) production or CTLA-4-mediated interactions, as indicated by the effect of specific neutralizing Abs. Remarkably, only the interaction between dNK and dCD14(+) cells results in Treg induction, whereas other coculture combinations involving either NK or CD14(+) cells isolated from peripheral blood are ineffective. Our study provides interesting clues to understanding how the crosstalk between decidual NK and CD14(+) cells may initiate a process that leads to Treg induction and immunosuppression. Along this line, it is conceivable that an impaired function of these cells may result in pregnancy failure.

Tolerance to the foetal allograft?

In this review, we will detail the concept of tolerance and its history in reproductive immunology. We will then consider whether it applies to the foetal-maternal relationship and discuss the mechanisms involved in non-rejection of the foeto-placental unit.

Fetomaternal immunotolerance

Implantation of mammalian conceptus in uterine cavity is the result of evolutionary adaptation, through high level of physiological procedures to ensure its success. However the majority of pregnancy losses occur before or during implantation. It is expected that exploring and defining the molecular and physiological road map during the crucial time of implantation will enable us to decode and effectively treat fertility defects. Immunological, hormonal and molecular factors participate in the feto-maternal cross talk during implantation and designate the effectiveness of the process. The atypical expression of major histocompatibility complex and other protein-antigens, such as Fas/FasL and petformin in human trophoblast, the modified function of cellular constituents of the feto-maternal interface, as well as the specific role of some hormones and cytokines, represent substantive parameters of feto-maternal immunotolerance during implantation.

The developmental role of natural killer cells at the fetal-maternal interface

Natural killer (NK) cells have been extensively studied in their traditional roles in host defense against tumor or virally infected cells. Uterine NK cells are of 2 distinct subsets: endometrial NK (eNK) cells, found in the uterus during the menstrual cycle, and decidual NK (dNK) cells, found in the decidua during pregnancy. This review will explore the immunosurveillance and cytotoxicity profiles of NK cells, the inert nature of eNK cells, and the role of dNK cells as builders at the maternal-fetal interface that create a pregnancy-favorable environment by inducing angiogenesis, trophoblast invasion, and vascular remodeling.

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.

Natural killer cell receptors

Natural killer (NK) cells are an important arm of the innate immune response that are directly involved in the recognition and lysis of virus-infected and tumor cells. Such function is under the control of a complex array of germline-encoded receptors able to deliver either inhibitory or activating signals. The majority of inhibitory receptors expressed by NK cells are major histocompatibility complex (MHC) class I-specific and display clonal and stochastic distribution on the cell surface. Thus, a given NK cell expresses at least one self class I inhibitory receptor. Under normal conditions, the strength of inhibitory signals delivered by multiple interactions always overrides the activating signals, resulting in NK cell self-tolerance. Under certain pathological conditions, such as viral infections or tumor transformation, the delicate balance of inhibition versus activation is broken, resulting in downregulation or loss of MHC class I expression. In general, the degree of inhibition induced by class I-specific receptors is proportional to the amount of these molecules on the cell surface. Thus, in transformed cells, this inhibition can be overridden by the triggering signal cascades, leading to cell activation. The majority of triggering receptors expressed by NK cells belong to the multichain immune recognition receptor (MIRR) family and use separate signal-transducing polypeptides similar to those used by other immune receptors such as the T-cell antigen receptor, the B-cell antigen receptor and other receptors expressed by myeloid cells. Inhibitory receptors are not members of the MIRR family but they are relevant for a better understanding the exquisite equilibrium and regulatory crosstalk between positive and negative signals.

Regulatory role of NKp44, NKp46, DNAM-1 and NKG2D receptors in the interaction between NK cells and trophoblast cells. Evidence for divergent functional profiles of decidual versus peripheral NK cells

During the first trimester of pregnancy NK cells represent >50% of the lymphoid cells present in the human decidua where they reside in close contact with trophoblast cells. Because in decidual tissues NK cell activation and function may be induced by this interaction, we analyzed the cellular ligands recognized by activating NK receptors expressed on trophoblast cells. We show that these cells primarily express the NKp44 and DNAM-1 ligands and that interaction between these ligands and their corresponding receptors results in NK cell triggering. While activated peripheral blood NK (pNK) cells lysed the trophoblast cell lines JAR and JEG3, decidual NK (dNK) cells did not. On the other hand, they released VEGF, SDF-1, IP10 and large amounts of IL-8. Interaction with K562 target cells was exploited to induce optimal NK cell triggering, allowing a parallel, quantitative assessment of both cytolytic activity and cytokine production elicited by dNK cells. While dNK cells were unable to kill K562 even at high effector:target (E:T) ratios, they released large amounts of IL-8 also at low E:T ratios, a scenario compatible with dNK trophoblast cells interaction occurring within decidual tissues.

Clinical implication of natural killer cells and reproduction

The regulation of natural killer (NK) cells in the peripheral blood and endometrial layers has been associated with reproductive immunopathology such as recurrent spontaneous abortions (RSA), infertility of implantation failures, or pre-eclampsia. The placenta has a complex anatomical structure and different subsets of NK cells with various functional roles can directly interact with trophoblasts. NK cell subpopulations and their functions, putative roles of NK cells in peripheral blood and endometrium are reviewed in relation to RSA and infertility. An increase in NK cell numbers and /or activity in pre- or post-conceptional period in women with RSA or infertility with multiple implantation failures are a significant clinical concern. In addition, immuno-phenotypic characteristics of NK cells in these women support the changes for their increased activity status. Further studies are needed to explore underlying mechanism of NK cells in RSA, infertility, and other reproductive immunopathologies. Possible neurological and hormonal control of NK cells and NK cell interaction with various leukocyte populations need further investigation in women with reproductive failures.

O papel do complexo principal de histocompatibilidade na fisiologia da gravidez e na patogênese de complicações obstétricas

Este trabalho tem por objetivo discutir a estrutura e função dos Antígenos Leucocitários Humanos (HLA), seus métodos de detecção, nomenclatura e os mecanismos imunopatológicos que o associam com a fisiologia da gestação e morbidades obstétricas. Sabe-se que o equilíbrio imunológico entre mãe e concepto é imprescindível na manutenção da gravidez. Moléculas do HLA - notadamente o HLA-G expresso na interface materno-fetal - exercem função importante na tolerância imunológica materna, evitando rejeição fetal e algumas complicações obstétricas. Além disso, o HLA permeia diversas etapas do desenvolvimento conceptual como clivagem, formação do trofoblasto e implantação. Para revisão, foram pesquisados os bancos de dados MEDLINE e LILACS, utilizando os descritores: "HLA antigens"; "pregnancy"; "embryonic development"; "pregnancy complication"; "abortion, habitual"; "pre-eclampsia". O conhecimento sobre a influência do HLA na gravidez é necessário para melhor manejo da gestação e patologias obstétricas auto-imunes, favorecendo intervenções precoces e terapêutica específica, reduzindo a morbimortalidade materna e perinatal.

Expressions of natural cytotoxicity receptors and NKG2D on decidual natural killer cells in patients having spontaneous abortions

OBJECTIVE

To investigate the relation between expressions of NKp46, NKp44, NKp30, and NKG2D receptors on decidual natural killer cells (dNK) and human spontaneous abortion (SA).

DESIGN

Case control study.

SETTING

University hospital.

PATIENT(S)

Twenty-one patients with normal chromosome abortuses and 25 healthy early pregnant women.

INTERVENTION(S)

Detected the cytolytic activity of dNK to K562 target cells; measured the expressions of NCRs and NKG2D on dNK.

MAIN OUTCOME MEASURE(S)

Cytotoxicity of dNK and expressions of NKp46, p44, p30, and NKG2D receptors on CD56(bright)CD16(-)/CD56(dim)CD16(+) dNK subsets were detected by flow cytometry. The relations between dNK cytotoxicity and the expression of the receptors on two dNK subsets were investigated.

RESULT(S)

The patients with SA expressed higher proportion of NKp44 on CD56(bright)CD16(-)dNK and higher proportions of NKp46 and NKp44 on CD56(dim)CD16(+)dNK with statistical significance. The expressions of NKp44 on CD56(bright)CD16(-)dNK and NKp46 on CD56(dim)CD16(+)dNK correlated with dNK cytotoxicity positively (r(2) = 0.677 and r(2) = 0.634, respectively). The expression of NKp46 on CD56(bright)CD16(-)dNK was much higher than that on CD56(dim)CD16(+)dNK.

CONCLUSION(S)

Enhanced dNK cytotoxicity due to increased NKp46 and NKp44 expressions may undergo the susceptibility to SA. Both CD56(bright)CD16(-)dNK and CD56(dim)CD16(+) dNK may be involved in lysing of the target cells.

Immune cells in uteroplacental tissues throughout pregnancy: a brief review

In a brief introduction, this review states why the presence of immune cells at the interface poses problems for an immunologist (Medawar paradigm). Different types of placentation are then discussed, and the various interactions with leukocytes, the extreme being with the equids where a certain degree of 'attack' is often seen. The limits of animal models when dealing with the human situation are emphasized. It is then stated why the various phases of pregnancy are different, and an analysis made of the cellular movements at the implantation, peri-implantation, immediate post-implantation and resorption windows in rodents. Details of the cellular components involved are given, as are hints for the human situation. The Th1/Th2 paradigm is described, with clinical examples, and its limits. Thus, the newly appraised dual role of natural killer (NK) cells is discussed, with examples in rodents and in humans (pre-eclampsia, implantation failure, abortion systems). Clinical data on the IL-12/IL-18/NK tripod and implantation failure in humans are detailed.

DAP12: an adapter protein with dual functionality

Expressed predominantly on myeloid and natural killer (NK) cells, DAP12 is an adapter protein that can associate with a variety of receptors. To date, DAP12 has predominantly been characterized as an adapter protein that activates various myeloid and NK cell effector functions; however, recent findings have demonstrated that DAP12 can also inhibit myeloid functions. Here we review the dual functionality of DAP12 and present evidence that DAP12 can suppress as well as activate NK cells.

Coordinated acquisition of inhibitory and activating receptors and functional properties by developing human natural killer cells

The stages of human natural killer (NK) cell differentiation are not well established. Culturing CD34(+) progenitors with interleukin 7 (IL-7), IL-15, stem cell factor (SCF), FLT-3L, and murine fetal liver cell line (EL08.1D2), we identified 2 nonoverlapping subsets of differentiating CD56(+) cells based on CD117 and CD94 (CD117(high)CD94(-) and CD117(low/-)CD94(+) cells). Both populations expressed CD161 and NKp44, but differed with respect to NKp30, NKp46, NKG2A, NKG2C, NKG2D, CD8, CD16, and KIR. Only the CD117(low/-) CD94(+) population displayed cytotoxicity and interferon-gamma production. Both populations arose from a single CD34(+)CD38(-) Lin(-) cell and their percentages changed over time in a reciprocal fashion, with CD117(high)CD94(-) cells predominating early and decreasing due to an increase of the CD117(low/-)CD94(+) population. These 2 subsets represent distinct stages of NKcell differentiation, since purified CD117(high) CD94(-) cells give rise to CD117(low/-)CD94(+) cells. The stromal cell line (EL08.1D2) facilitated the transition from CD117(high)CD94(-) to CD117(low/-)CD94(+) via an intermediate phenotype (CD117(low)CD94(low/-)). EL08.1D2 also maintained the mature phenotype, preventing the reversion of CD117(low/-)CD94(+) cells to the intermediate (CD117(low)CD94(low/-)) phenotype. An analogous population of CD56(+)CD117(high)CD94(-) cells was found in cord blood. The identified stages of NK-cell differentiation provide evidence for coordinated acquisition of HLA-specific inhibitory receptors (ie, CD94/NKG2A) and function in developing human NK cells.

Lethal influenza infection in the absence of the natural killer cell receptor gene Ncr1

The elimination of viruses and tumors by natural killer cells is mediated by specific natural killer cell receptors. To study the in vivo function of a principal activating natural killer cell receptor, NCR1 (NKp46 in humans), we replaced the gene encoding this receptor (Ncr1) with a green fluorescent protein reporter cassette. There was enhanced spread of certain tumors in 129/Sv but not C57BL/6 Ncr1(gfp/gfp) mice, and influenza virus infection was lethal in both 129/Sv and C57BL/6 Ncr1(gfp/gfp) mice. We noted accumulation of natural killer cells at the site of influenza infection by tracking the green fluorescent protein. Our results demonstrate a critical function for Ncr1 in the in vivo eradication of influenza virus.

HLA-G and immune tolerance in pregnancy

Multiple mechanisms underlie the surprising willingness of mothers to tolerate genetically different fetal tissues during pregnancy. Chief among these is the choice of HLA-G, a gene with few alleles, rather than the highly polymorphic HLA-A and -B genes, for expression by the placental cells that interface directly with maternal blood and tissues. Novel aspects of this major histocompatibility complex class Ib gene include alternative splicing to permit production of membrane and soluble isoforms, deletions that dampen responses to interferons, and a shortened cytoplasmic tail that affects expression at the cell surface. Placental cells migrating into the maternal uterus synthesize both membrane and soluble isoforms, which interact with inhibitory receptors on leukocytes such as ILT2 and ILT4. Cytotoxic T lymphocytes either die or reduce production of one of their major coreceptor/activator cell surface molecules, CD8; natural killer cells are immobilized and mononuclear phagocytes are programmed into suppressive modes characterized by high production of anti-inflammatory cytokines. The idea that placental HLA-G proteins facilitate semiallogeneic pregnancy by inhibiting maternal immune responses to foreign (paternal) antigens via these actions on immune cells is now well established, and the postulate that the recombinant counterparts of these proteins may be used as powerful tools for preventing immune rejection of transplanted organs is gaining in popularity.

LST1 and NCR3 expression in autoimmune inflammation and in response to IFN-gamma, LPS and microbial infection

Many genes in the central region of the major histocompatibility complex (MHC) encode proteins involved in immune and inflammatory responses. In this study, we have further characterized two genes in the MHC class IV region, leucocyte-specific transcript (LST) 1 and natural cytotoxicity-triggering receptor 3 (NCR3) (also known as 1C7 and natural killer (NK)p30). The specific function of LST1 is not known, although expression analysis and functional data suggest an immunomodulatory role. The LST1 gene undergoes extensive alternative splicing, giving rise to both membrane-bound (encoded by exon 3) and soluble isoforms. The NCR3 protein is involved in NK-mediated cytotoxicity and plays a role in NK/dendritic cell crosstalk. Expression of these genes was examined, by real-time reverse transcriptase-polymerase chain reaction, in autoimmune-induced inflammation, specifically rheumatoid-arthritis-affected blood and synovium, and in response to stimulation with inflammatory mediators and bacterial agents. The expression of LST1, specifically splice variants encoding soluble isoforms and NCR3, was increased in rheumatoid-arthritis-affected blood and synovium and was associated with more severe inflammation in the synovium. Furthermore, both genes were significantly up-regulated in response to lipopolysaccharide, interferon (IFN)-gamma and bacterial infection. These findings suggest that NCR3 and soluble isoforms of LST1 may play a role in inflammatory and infectious diseases.

Innate and adaptive immunity in female genital tract: cellular responses and interactions

The mucosal immune system in the female reproductive tract (FRT) has evolved to meet the unique requirements of dealing with sexually transmitted bacterial and viral pathogens, allogeneic spermatozoa, and the immunologically distinct fetus. Analysis of the FRT indicates that the key cells of the innate and adaptive immune systems are present and functionally responsive to antigens. Acting through Toll-like receptors in the Fallopian tubes, uterus, cervix, and in the vagina, epithelial cells, macrophages, natural killer cells, and neutrophils confer protection through the production of chemokines and cytokines, which recruit and activate immune cells, as well as bactericidal and virucidal agents, which confer protection at times when adaptive immunity is downregulated by sex hormones to meet the constraints of procreation. The overall goal of this paper is to define the innate immune system in the FRT and, where possible, to define the regulatory influences that occur during the menstrual cycle that contribute to protection from and susceptibility to potential pathogens. By understanding the nature of this protection and the ways in which innate and adaptive immunity interact, these studies provide the opportunity to contribute to the foundation of information essential for ensuring reproductive health.

Paradoxic inhibition of human natural interferon-producing cells by the activating receptor NKp44

Natural killer (NK) cell-mediated cytotoxicity is triggered by multiple activating receptors associated with the signaling adaptor protein DNAX activation protein 12/killer cell-activating receptor-associated protein (DAP12/KARAP). Here, we show that one of these receptors, NKp44, is present on a subset of natural interferon-producing cells (IPCs) in tonsils. NKp44 expression can also be induced on blood IPCs after in vitro culture with interleukin 3 (IL-3). Crosslinking of NKp44 does not trigger IPC-mediated cytotoxicity but, paradoxically, inhibits interferon alpha (IFN-alpha) production by IPCs in response to cytosine-phosphate-guanosine (CpG) oligonucleotides. We find that IPCs in tonsils are in close contact with CD8+ T cells and demonstrate that a subset of memory CD8+ T cells produces IL-3. Therefore, IL-3-mediated induction of NKp44 on IPCs may be an important component of the ongoing crosstalk between the innate and adaptive immune response that allows memory CD8+ T cells to control the IPC response to virus.

The HLA-G*0105N null allele induces cell surface expression of HLA-E molecule and promotes CD94/NKG2A-mediated recognition in JAR choriocarcinoma cell line

HLA-G is a non-classical HLA class Ib molecule primarily expressed in trophoblast cells, and is thought to play a key role in the induction of materno-fetal tolerance during pregnancy. In addition, the HLA-G gene provides a suitable leader sequence peptide capable of binding to HLA-E. However, the existence of placentas homozygous for the HLA-G*0105N null allele suggests that HLA-G1 might not be essential for fetal survival. To investigate whether expression of the HLA-G*0105N allele supports HLA-E cell surface expression, we transfected the HLA-G*0105N gene into JAR trophoblast cells. Flow cytometry analysis showed that HLA-G*0105N-transfected cells express surface HLA-E to a similar extent as the unmutated HLA-G gene, whereas HLA-G1 cell surface expression was undetectable. Using the NKL cell line in a standard (51)Cr release assay, the HLA-E molecules were found to inhibit natural killer lysis, through a mechanism partially dependent on CD94/NKG2A-mediated recognition.

Human choriocarcinoma cell resistance to natural killer lysis due to defective triggering of natural killer cells

The trophoblast, the outermost layer of the human placenta, lacks expression of the classical human leukocyte antigen (HLA) class I molecules. This prevents allorecognition by T cells but raises the question of what protects the trophoblast from natural killer (NK) cells. In a previous study, we have shown that choriocarcinoma cell (CC) resistance to NK lysis was mainly independent of HLA class I molecules. In the present study, we postulated that CC may prevent activation of NK cells by failing to stimulate their triggering receptors (TR). To test this hypothesis, we evaluated the lysis of JAR and JEG-3 CC after effective cross-linking and activation of NK cells by means of lectins or antibodies. Our results show that NK-resistant CC were sensitive to lysis by unstimulated peripheral blood lymphocytes in the presence of phytohemagglutin (PHA), to antibody-dependent cell cytotoxicity in presence of anti-Tja antibodies, and to monoclonal antibody redirected killing using anti-TR antibodies anti-CD16 and anti-CD244/2B4. Finally, CC fail to express CD48, the ligand for CD244/2B4. These results indicate that the resistance of CC to lysis results primarily from defective NK cell activation, at least partially due to the lack of expression of ligands, such as CD48, involved in the triggering of NK cells.

HLA-G Molecules: from Maternal–Fetal Tolerance to Tissue Acceptance

Over the past few years, HLA-G, the non-classical HLA class I molecule, has been the center of investigations that have led to the description of its specific structural and functional properties. Although located in the HLA class I region of chromosome six, the HLA-G gene may be distinguished from other HLA class I genes by its low polymorphism and alternative splicing that generates seven HLA-G proteins, whose tissue-distribution is restricted to normal fetal and adult tissues that display a tolerogeneic function toward both innate and acquired immune cells. We review these points, with special emphasis on the role of HLA-G in human pathologies, such as cancer, viral infection, and inflammatory diseases, as well as in organ transplantation.

Engagement of CD160 receptor by HLA-C is a triggering mechanism used by circulating natural killer (NK) cells to mediate cytotoxicity

Circulating human natural killer (NK) lymphocytes have been functionally defined by their ability to exert cytotoxic activity against MHC class I-negative target cell lines, including K562. Therefore, it was proposed that NK cells recognized the "missing self." We show here that the Ig-like CD160 receptor expressed by circulating CD56(dim+) NK cells or IL-2-deprived NK cell lines is mainly involved in their cytotoxic activity against K562 target cells. Further, we report that HLA-C molecules that are constitutively expressed by K562 trigger NK cell lysis through CD160 receptor engagement. In addition, we demonstrate, with recombinant soluble HLA-Cw3 and CD160 proteins, direct interaction of these molecules. We also find that CD158b inhibitory receptors partially interfere with CD160-mediated cytotoxicity, whereas CD94CD159a and CD85j have no effect on engagement with their respective ligands. Thus, CD160HLA-C interaction constitutes a unique pathway to trigger NK cell cytotoxic activity.

Enhancement of natural killer cell activation and antibody-dependent cellular cytotoxicity by interferon-alpha and interleukin-12 in vaginal mucosae Sivmac251-infected Macaca fascicularis

We studied the innate immune system of Cynomolgus monkeys (Macaca fascicularis) experimentally infected via the vaginal mucosae with a virulent simian immunodeficiency virus isolate SIVmac251. Animals were evaluated for their natural killer (NK) cell activity, and for their antibody-dependent cellular cytotoxicity. NK cells from SIVmac251-infected macaques show impaired NK cell activity compared to cells from uninfected animals. Subsequent treatment of NK cells with interferon-a (IFN-alpha) or interleukin-12 (IL-12) alone partially restored the NK activity. However, either treatment of NK cells with both IFN-alpha and IL-12 completely reversed the impairment of cytotoxicity induced by simian immunodeficiency virus (SIV) infection. Incubation of NK cells from infected but not from uninfected monkeys with IFN-alpha and IL-12 for 8 days increased the percentage of CD16+/CD56+ cells twofold to five-fold and enhanced antibody-dependent cellular cytotoxicity (ADCC) activity. Thus IFN-alpha and IL-12 greatly enhance both the NK cell and ADCC activities of peripheral blood cells from SIVmac251-infected animals and increase the number of NK cells in longer term culture. The combined effect of IFN-alpha and IL-12 in enhancing NK cell activity may provide a novel therapeutic approach for the restoration of depressed NK cell activity observed in human immunodeficiency virus (HIV)-infected patients.

MICA triggering signal for NK cell tumor lysis is counteracted by HLA-G1-mediated inhibitory signal

MICA, a highly glycosylated membrane-anchored cell-surface MHC Class I-related chain, has recently been reported to activate NK cell cytolytic responses in epithelial tumors. Tumor cells may escape from NK lysis by counteracting NK cytotoxicity activating signals with inhibitory ones. Among the molecules that mediate an NK inhibitory signal, HLA-G1, a non-classical MHC Class I antigen, is of particular interest. HLA-G1 is ectopically expressed in various tumors, including melanoma and constitutes the major NK inhibitory ligand in the M8 melanoma cell line when coexpressed with HLA-A, -B, -C and -E molecules. We have evaluated the balance between 2 powerful signals that affect NK cell tumor lysis, one inhibitory and the other one activating, respectively HLA-G1 and MICA. For this purpose, we transfected the M8 melanoma cell line, which spontaneously expresses MICA, with HLA-G1 cDNA, using it as a target for the NKL effector. We carried out cytotoxicity assays, using antibodies that disrupt interactions between the MICA and HLA-G1 ligands and their respective NK effector counterparts, the NKG2D activating and ILT2 inhibitory receptors. Results showed that 1) MICA expressed in the M8 melanoma cell line triggered NK cell tumor lysis and 2) HLA-G1 coexpression mediated the inhibition of NK cytotoxicity by mitigating the MICA activating signal. HLA-G1 expression in a tumor cell line in which MICA is switched on would therefore appear to be a powerful way to turn off NK cells, supporting the emerging idea that the balance between positive and negative NK cytolysis signals critically influences tumor progression.

A brief review of recent data on some cytokine expressions at the materno-foetal interface which might challenge the classical Th1/Th2 dichotomy

Focussing attention on cytokines at the materno-foetal interface represents one of the major advances made in the field. This owes much to the visionary views of Tom Wegmann, and to the changes brought about in the field by immunotrophism and Th1/Th2 paradigms. We review these briefly and also point out some emerging problems.However, a certain number of newly discovered cytokines do not fit into the classical Th1/Th2 dichotomy. Yet, by their capacity to activate or downregulate NK cells, by their action on adhesion molecules, and by their regulatory effects on the vascularisation process, they are of possible interest within the materno-foetal relationship. Therefore, as a first step, we have undertaken a systematic study of the expression of IL-11, IL-12, IL-13, IL-15, IL-16, IL-17 and IL-18 in the uterus, the peri-implantation embryo, and later on decidual and placental tissues throughout pregnancy. These cytokines were detected in every case, with, in each case, a precise localisation, which will be detailed, and which indeed suggests important regulatory functions, especially during implantation. In some cases, as will be shown in the peri-implantation uterus, those cells are perfectly expressed by uterine GMG-NK-like cells. Comparative ELISAs and quantitative RT-PCR have been or are being conducted, but already the expression patterns that are observed, and the very precise window of appearance that is observed for some of the GMG NK-like cells, either around or in the implanting embryo, as well as the complexity of the respective distributions, strongly suggest that, as useful as it certainly was for a while, the Th1/Th2 paradigm must now be considered as an over-simplification. Rather, the existing data point to sequential windows and are suggestive of a system where an extreme complexity is allied to very precise timing and tuning. They also suggest that the materno-foetal relationship is not simply maternal tolerance of a foreign tissue, but a series of intricate mutual cytokine interactions governing selective immune regulation and also control of the adhesion and vascularisation processes during this dialogue.