The structure of the atypical killer cell immunoglobulin-like receptor, KIR2DL4

ITAM-based receptors in natural killer cells

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

Harnessing the potential of HLA-G in cancer therapy: advances, challenges, and prospects

Immune checkpoint blockades have been prized in circumventing and ablating the impediments posed by immunosuppressive receptors, reaching an exciting juncture to be an innovator in anticancer therapy beyond traditional therapeutics. Thus far, approved immune checkpoint blockades have principally targeted PD-1/PD-L1 and CTLA-4 with exciting success in a plethora of tumors and yet are still trapped in dilemmas of limited response rates and adverse effects. Hence, unveiling new immunotherapeutic targets has aroused immense scientific interest in the hope of expanding the clinical application of immune checkpoint blockades to scale new heights. Human leukocyte antigen-G (HLA-G), a non-classical major histocompatibility complex (MHC) class I molecule, is enriched on various malignant cells and is involved in the hindrance of immune effector cells and the facilitation of immunosuppressive cells. HLA-G stands out as a crucial next-generation immune checkpoint showing great promise for the benefit of cancer patients. Here, we provide an overview of the current understanding of the expression pattern and immunological functions of HLA-G, as well as its interaction with well-characterized immune checkpoints. Since HLA-G can be shed from the cell surface or released by various cells as free soluble HLA-G (sHLA-G) or as part of extracellular vesicles (EVs), namely HLA-G-bearing EVs (HLA-GEV), we discuss the potential of sHLA-G and HLA-GEV as predictive biomarkers. This review also addresses the advancement of HLA-G-based therapies in preclinical and clinical settings, with a focus on their clinical application in cancer.

Conserved biophysical compatibility among the highly variable germline-encoded regions shapes TCR-MHC interactions

T cells are critically important components of the adaptive immune system primarily responsible for identifying and responding to pathogenic challenges. This recognition of pathogens is driven by the interaction between membrane-bound T cell receptors (TCRs) and antigenic peptides presented on major histocompatibility complex (MHC) molecules. The formation of the TCR-peptide-MHC complex (TCR-pMHC) involves interactions among germline-encoded and hypervariable amino acids. Germline-encoded and hypervariable regions can form contacts critical for complex formation, but only interactions between germline-encoded contacts are likely to be shared across many of all the possible productive TCR-pMHC complexes. Despite this, experimental investigation of these interactions have focused on only a small fraction of the possible interaction space. To address this, we analyzed every possible germline-encoded TCR-MHC contact in humans, thereby generating the first comprehensive characterization of these largely antigen-independent interactions. Our computational analysis suggests that germline-encoded TCR-MHC interactions that are conserved at the sequence level are rare due to the high amino acid diversity of the TCR CDR1 and CDR2 loops, and that such conservation is unlikely to dominate the dynamic protein-protein binding interface. Instead, we propose that binding properties such as the docking orientation are defined by regions of biophysical compatibility between these loops and the MHC surface.

Local immune recognition of trophoblast in early human pregnancy: controversies and questions

The role of the maternal immune system in reproductive success in humans remains controversial. Here we focus on the events that occur in the maternal decidua during the first few weeks of human pregnancy, because this is the site at which maternal leukocytes initially interact with and can recognize fetal trophoblast cells, potentially involving allorecognition by both T cells and natural killer (NK) cells. NK cells are the dominant leukocyte population in first-trimester decidua, and genetic studies point to a role of allorecognition by uterine NK cells in establishing a boundary between the mother and the fetus. By contrast, definitive evidence that allorecognition by decidual T cells occurs during the first trimester is lacking. Thus, our view is that during the crucial period when the placenta is established, damaging T cell-mediated adaptive immune responses towards placental trophoblast are minimized, whereas NK cell allorecognition contributes to successful implantation and healthy pregnancy.

Evolution and molecular interactions of major histocompatibility complex (MHC)-G, -E and -F genes

Classical HLA (Human Leukocyte Antigen) is the Major Histocompatibility Complex (MHC) in man. HLA genes and disease association has been studied at least since 1967 and no firm pathogenic mechanisms have been established yet. HLA-G immune modulation gene (and also -E and -F) are starting the same arduous way: statistics and allele association are the trending subjects with the same few results obtained by HLA classical genes, i.e., no pathogenesis may be discovered after many years of a great amount of researchers’ effort. Thus, we believe that it is necessary to follow different research methodologies: (1) to approach this problem, based on how evolution has worked maintaining together a cluster of immune-related genes (the MHC) in a relatively short chromosome area since amniotes to human at least, i.e., immune regulatory genes (MHC-G, -E and -F), adaptive immune classical class I and II genes, non-adaptive immune genes like (C2, C4 and Bf) (2); in addition to using new in vitro models which explain pathogenetics of HLA and disease associations. In fact, this evolution may be quite reliably studied during about 40 million years by analyzing the evolution of MHC-G, -E, -F, and their receptors (KIR—killer-cell immunoglobulin-like receptor, NKG2—natural killer group 2-, or TCR-T-cell receptor—among others) in the primate evolutionary lineage, where orthology of these molecules is apparently established, although cladistic studies show that MHC-G and MHC-B genes are the ancestral class I genes, and that New World apes MHC-G is paralogous and not orthologous to all other apes and man MHC-G genes. In the present review, we outline past and possible future research topics: co-evolution of adaptive MHC classical (class I and II), non-adaptive (i.e., complement) and modulation (i.e., non-classical class I) immune genes may imply that the study of full or part of MHC haplotypes involving several loci/alleles instead of single alleles is important for uncovering HLA and disease pathogenesis. It would mainly apply to starting research on HLA-G extended haplotypes and disease association and not only using single HLA-G genetic markers.

High-throughput virtual screening of small-molecule inhibitors targeting immune cell checkpoints to discover new immunotherapeutics for human diseases

Immunotherapy is widely used to treat various cancers, and the drugs used are called immune checkpoint (ICP) inhibitors. Overexpression of immune cell checkpoints is reported for other human diseases such as acute infections (malaria), chronic viral infection (HIV, hepatitis B virus, TB infections), allergy, asthma, neurodegeneration, and autoimmune diseases. Some mAbs (monoclonal antibodies) are available against ICPs, but they have side effects. Small molecule seems to be safer in comparison with mAbs. Three independent small-molecule inhibitor libraries consisting of 9466 compounds were screened against seven immune cell checkpoints by applying high-throughput virtual screening approach. A total of 13 ICP inhibitors were finalized based on docking, MM-GBSA scores, and ADME properties. Six compounds were selected for MD simulation, and then, rutin hydrate (targeting all seven immune cell checkpoints), amikacin hydrate (targeting six), and 6-hydroxyluteolin (targeting three) were found to be the best immune cell checkpoint inhibitors. These three potential inhibitors have shown the potential to activate human immune cells and thus may control the spread of human lifestyle or infectious diseases. Proposed inhibitors warrant the in vitro and in vivo validation to develop it as an immunotherapeutic.

Roles of HLA-G/KIR2DL4 in Breast Cancer Immune Microenvironment

Human leukocyte antigen (HLA)-G is a nonclassical MHC Class I molecule, which was initially reported as a mediator of immune tolerance when expressed in extravillous trophoblast cells at the maternal-fetal interface. HLA-G is the only known ligand of killer cell immunoglobulin-like receptor 2DL4 (KIR2DL4), an atypical family molecule that is widely expressed on the surface of NK cells. Unlike other KIR receptors, KIR2DL4 contains both an arginine–tyrosine activation motif in its transmembrane region and an immunoreceptor tyrosine-based inhibitory motif (ITIM) in its cytoplasmic tail, suggesting that KIR2DL4 may function as an activating or inhibitory receptor. The immunosuppressive microenvironment exemplified by a rewired cytokine network and upregulated immune checkpoint proteins is a hallmark of advanced and therapy-refractory tumors. Accumulating evidence has shown that HLA-G is an immune checkpoint molecule with specific relevance in cancer immune escape, although the role of HLA-G/KIR2DL4 in antitumor immunity is still uncharacterized. Our previous study had shown that HLA-G was a pivotal mediator of breast cancer resistance to trastuzumab, and blockade of the HLA-G/KIR2DL4 interaction can resensitize breast cancer to trastuzumab treatment. In this review, we aim to summarize and discuss the role of HLA-G/KIR2DL4 in the immune microenvironment of breast cancer. A better understanding of HLA-G is beneficial to identifying novel biomarker(s) for breast cancer, which is important for precision diagnosis and prognostic assessment. In addition, it is also necessary to unravel the mechanisms underlying HLA-G/KIR2DL4 regulation of the immune microenvironment in breast cancer, hopefully providing a rationale for combined HLA-G and immune checkpoints targeting for the effective treatment of breast cancer.

The Activating receptor KIR2DS2 bound to HLA-C1 reveals the novel recognition features of activating receptor

Killer cell immunoglobulin-like receptors (KIRs) are important receptors for regulating the killing of virus-infected or cancer cells of natural killer (NK) cells. KIR2DS2 can recognize peptides derived from hepatitis C virus (HCV) or global flaviviruses (such as dengue and Zika) presented by HLA-C*0102 to activate NK cells, and have shown promising results when used for cancer immunotherapy. Here, we present the complex structure of KIR2DS2 with HLA-C*0102 at a resolution of 2.5Å. Our structure reveals that KIR2DS2 can bind HLA-C*0102 and HLA-A*1101 in two different directions. Moreover, Tyr45 (in activating receptor KIR2DS2) and Phe45 (in inhibitory KIRs) distinguish the two different binding models and binding affinity between activating KIRs and inhibitory KIRs. The conserved "AT" motif of the peptide mediates recognition and determines the peptide specificity of recognition. These structural characteristic shed light on how KIRs activate NK cells and can provide a molecular basis for immunotherapy by NK cells.

Decidual Natural Killer Cells: A Good Nanny at the Maternal-Fetal Interface During Early Pregnancy

Decidual natural killer (dNK) cells are the tissue-resident and major subpopulation of NK cells at the maternal-fetal interface. It has been demonstrated that dNK cells play pivotal roles in pregnancy, including keeping maternal-fetal immune tolerance, promoting extravillous trophoblast (EVT) cell invasion, and driving uterine spiral artery remodeling. However, the molecular mechanisms haven't been elucidated until recent years. In this review, we systemically introduce the generation, subsets, and surface or soluble molecules of dNK cells, which are critical for maintaining the functions of dNK cells. Further, new functions of dNK cells including well-controlled cytotoxicity, immunosurveillance and immunotrophism supporting via the cell-cell interaction between dNK cells and EVT cells are mainly focused. The molecular mechanisms involved in these functions are also illustrated. Moreover, pregnancy-associated diseases caused by the dNK cells abnormalities are discussed. It will be important for future investigations about the mechanism of maintenance of pregnancy and parturition and potential clinical applications of dNK cells.

Structural plasticity of KIR2DL2 and KIR2DL3 enables altered docking geometries atop HLA-C

The closely related inhibitory killer-cell immunoglobulin-like receptors (KIR), KIR2DL2 and KIR2DL3, regulate the activation of natural killer cells (NK) by interacting with the human leukocyte antigen-C1 (HLA-C1) group of molecules. KIR2DL2, KIR2DL3 and HLA-C1 are highly polymorphic, with this variation being associated with differences in the onset and progression of some human diseases. However, the molecular bases underlying these associations remain unresolved. Here, we determined the crystal structures of KIR2DL2 and KIR2DL3 in complex with HLA-C*07:02 presenting a self-epitope. KIR2DL2 differed from KIR2DL3 in docking modality over HLA-C*07:02 that correlates with variabilty of recognition of HLA-C1 allotypes. Mutagenesis assays indicated differences in the mechanism of HLA-C1 allotype recognition by KIR2DL2 and KIR2DL3. Similarly, HLA-C1 allotypes differed markedly in their capacity to inhibit activation of primary NK cells. These functional differences derive, in part, from KIR2DS2 suggesting KIR2DL2 and KIR2DL3 binding geometries combine with other factors to distinguish HLA-C1 functional recognition.

The Molecular and Functional Characteristics of HLA-G and the Interaction with Its Receptors: Where to Intervene for Cancer Immunotherapy?

Human leukocyte antigen G (HLA-G) mediates maternal-fetal immune tolerance. It is also considered an immune checkpoint in cancer since it may mediate immune evasion and thus promote tumor growth. HLA-G is, therefore, a potential target for immunotherapy. However, existing monoclonal antibodies directed against HLA-G lack sufficient specificity and are not suitable for immune checkpoint inhibition in a clinical setting. For this reason, it is essential that alternative approaches are explored to block the interaction between HLA-G and its receptors. In this review, we discuss the structure and peptide presentation of HLA-G, and its interaction with the receptors Ig-like transcript (ILT) 2, ILT4, and Killer cell immunoglobulin-like receptor 2DL4 (KIR2DL4). Based on our findings, we propose three alternative strategies to block the interaction between HLA-G and its receptors in cancer immunotherapy: (1) prevention of HLA-G dimerization, (2) targeting the peptide-binding groove of HLA-G, and (3) targeting the HLA-G receptors. These strategies should be an important focus of future studies that aim to develop immune checkpoint inhibitors to block the interaction between HLA-G and its receptors for the treatment of cancer.

HLA-G Neo-Expression on Tumors

HLA-G is known to modulate the immune system activity in tissues where physiological immune-tolerance is necessary (i.e., maternal-fetal interface, thymus, and cornea). However, the frequent neo-expression of HLA-G in many cancer types has been previously and extensively described and is correlated with a bad prognosis. Despite being an MHC class I molecule, HLA-G is highly present in tumor context and shows unique characteristics of tissue restriction of a Tumor Associated Antigen (TAA), and potent immunosuppressive activity of an Immune CheckPoint (ICP). Consequently, HLA-G appears to be an excellent molecular target for immunotherapy. Although the relevance of HLA-G in cancer incidence and development has been proven in numerous tumors, its neo-expression pattern is still difficult to determine. Indeed, the estimation of HLA-G's actual expression in tumor tissue is limited, particularly concerning the presence and percentage of the new non-canonical isoforms, for which detection antibodies are scarce or inexistent. Here, we summarize the current knowledge about HLA-G neo-expression and implication in various tumor types, pointing out the need for the development of new tools to analyze in-depth the HLA-G neo-expression patterns, opening the way for the generation of new monoclonal antibodies and cell-based immunotherapies.

The microstructure in the placenta is influenced by the functional diversity of HLA-G allelic variants

The main expression sites of HLA-G are human extravillous trophoblast cells. The interaction of HLA-G with uterine NK cells promotes their maturation and differentiation into decidual NK (dNK) cells. dNK cells secrete chemokines, cytokines, and proangiogenic factors in favor of a vascular remodeling and an immune suppressive microenvironment of the decidua. HLA-G is the most polymorphic member of the oligomorphic non-classical HLA molecule family; yet, the impact of polymorphic differences is not comprehensively understood. sHLA-G levels in embryo culture medium correlate with successful pregnancy; however, it remains questionable if HLA-G allelic diversity impacts on the outcome of dNK cell development. We utilized synthetic sHLA-G*01:01, 01:03, and 01:04 molecules and transduced K652/mHLA-G*01:01, 01:03, and 01:04 cells to study the biological interaction between HLA-G alleles and primary NK cells of human term placenta. Despite its low frequency, HLA-G*01:04 and not the most prevalent allele HLA-G*01:01 appear to be strong catalysts of dNK cell proliferation. Concluding, this study illustrates novel insights into the impact and binding efficiency of the three most common variants of HLA-G on primary placental NK cells.

European Patent in Immunoncology: From Immunological Principles of Implantation to Cancer Treatment

The granted European patent EP 2 561 890 describes a procedure for an immunological treatment of cancer. It is based on the principles of the HLA-supported communication of implantation and pregnancy. These principles ensure that the embryo is not rejected by the mother. In pregnancy, the placenta, more specifically the trophoblast, creates an “interface” between the embryo/fetus and the maternal immune system. Trophoblasts do not express the “original” HLA identification of the embryo/fetus (HLA-A to -DQ), but instead show the non-classical HLA groups E, F, and G. During interaction with specific receptors of NK cells (e.g., killer-immunoglobulin-like receptors (KIR)) and lymphocytes (lymphocyte-immunoglobulin-like receptors (LIL-R)), the non-classical HLA groups inhibit these immunocompetent cells outside pregnancy. However, tumors are known to be able to express these non-classical HLA groups and thus make use of an immuno-communication as in pregnancies. If this occurs, the prognosis usually worsens. This patent describes, in a first step, the profiling of the non-classical HLA groups in primary tumor tissue as well as metastases and recurrent tumors. The second step comprises tailored antibody therapies, which is the subject of this patent. In this review, we analyze the underlying mechanisms and describe the currently known differences between HLA-supported communication of implantation and that of tumors.

Conservation, Extensive Heterozygosity, and Convergence of Signaling Potential All Indicate a Critical Role for KIR3DL3 in Higher Primates

Natural killer (NK) cell functions are modulated by polymorphic killer cell immunoglobulin-like receptors (KIR). Among 13 human KIR genes, which vary by presence and copy number, KIR3DL3 is ubiquitously present in every individual across diverse populations. No ligand or function is known for KIR3DL3, but limited knowledge of expression suggests involvement in reproduction, likely during placentation. With 157 human alleles, KIR3DL3 is also highly polymorphic and we show heterozygosity exceeds that of HLA-B in many populations. The external domains of catarrhine primate KIR3DL3 evolved as a conserved lineage distinct from other KIR. Accordingly, and in contrast to other KIR, we show the focus of natural selection does not correspond exclusively to known ligand binding sites. Instead, a strong signal for diversifying selection occurs in the D1 Ig domain at a site involved in receptor aggregation, which we show is polymorphic in humans worldwide, suggesting differential ability for receptor aggregation. Meanwhile in the cytoplasmic tail, the first of two inhibitory tyrosine motifs (ITIM) is conserved, whereas independent genomic events have mutated the second ITIM of KIR3DL3 alleles in all great apes. Together, these findings suggest that KIR3DL3 binds a conserved ligand, and a function requiring both receptor aggregation and inhibitory signal attenuation. In this model KIR3DL3 resembles other NK cell inhibitory receptors having only one ITIM, which interact with bivalent downstream signaling proteins through dimerization. Due to the extensive conservation across species, selection, and other unusual properties, we consider elucidating the ligand and function of KIR3DL3 to be a pressing question.

Structure and flexibility of the extracellular region of the PirB receptor

Murine paired immunoglobulin receptor B (PirB) and its human ortholog leukocyte immunoglobulin-like receptor B2 (LILRB2) are widely expressed inhibitory receptors that interact with a diverse set of extracellular ligands and exert functions ranging from down-regulation of immune responses to inhibition of neuronal growth. However, structural information that could shed light on how PirB interacts with its ligands is lacking. Here, we report crystal structures of the PirB ectodomain; the first full ectodomain structure for a LILR family member, at 3.3-4.5 Å resolution. The structures reveal that PirB's six Ig-like domains are arranged at acute angles, similar to the structures of leukocyte immunoglobulin-like receptor (LILR) and killer-cell immunoglobulin-like receptor (KIR). We observe that this regular arrangement is followed throughout the ectodomain, resulting in an extended zigzag conformation. In two out of the five structures reported here, the repeating zigzag is broken by the first domain that can adopt two alternative orientations. Quantitative binding experiments revealed a 9 μm dissociation constant for PirB-myelin-associated glycoprotein (MAG) ectodomain interactions. Taken together, these structural findings and the observed PirB-MAG interactions are compatible with a model for intercellular signaling in which the PirB extracellular domains, which point away from the cell surface, enable interaction with ligands in trans.

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

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

Production of Recombinant Killer Immunoglobulin-Like Receptors for Crystallography and Luminex-Based Assays

The killer immunoglobulin-like receptors (KIR) are a highly diverse family of cell-surface receptors that are of importance to the effector function of Natural Killer cells. KIR have been implicated in the detection and clearance of malignantly transformed cells and in the immune-control of viruses including HIV, HCV and CMV. Recently, the mismatching of donor and recipient KIR has been demonstrated to improve success of hematopoietic stem cell transplantation treatments of leukemias. Due to the high degree of diversity amongst the KIR, a number of strategies are required for the production of recombinant protein for medical, biochemical and structural applications. Each of these strategies has advantages and limitations and is suitable for different subsets of the KIR and their intended use. Here we describe the preparation of these proteins for crystallography and the novel adaptation of tetramer production for this protein family that is suitable for a number of assays including single-antigen bead binding by Luminex. These methods are intended to provide comprehensive details for the production and characterization of each KIR and to be broadly applicable to other cell surface receptors of the immune system.

Natural Killer Cell Interactions with Classical and Non-Classical Human Leukocyte Antigen Class I in HIV-1 Infection

Natural killer (NK) cells are effector lymphocytes of the innate immune system that are able to mount a multifaceted antiviral response within hours following infection. This is achieved through an array of cell surface receptors surveilling host cells for alterations in human leukocyte antigen class I (HLA-I) expression and other ligands as signs of viral infection, malignant transformation, and cellular stress. This interaction between HLA-I ligands and NK-cell receptor is not only important for recognition of diseased cells but also mediates tuning of NK-cell-effector functions. HIV-1 alters the expression of HLA-I ligands on infected cells, rendering them susceptible to NK cell-mediated killing. However, over the past years, various HIV-1 evasion strategies have been discovered to target NK-cell-receptor ligands and allow the virus to escape from NK cell-mediated immunity. While studies have been mainly focusing on the role of polymorphic HLA-A, -B, and -C molecules, less is known about how HIV-1 affects the more conserved, non-classical HLA-I molecules HLA-E, -G, and -F. In this review, we will focus on the recent progress in understanding the role of non-classical HLA-I ligands in NK cell-mediated recognition of HIV-1-infected cells.

KIR, LILRB and their Ligands' Genes as Potential Biomarkers in Recurrent Implantation Failure

Reproductive failure in humans is a very important social and economic problem, because nowadays women decide to conceive later in life and delay motherhood. Unfortunately, with increasing age they have less chance for natural fertilization and maintenance of pregnancy. Many of them need assisted reproductive technology. Approximately 10% of women after in vitro fertilization-embryo transfers experience recurrent implantation failure (RIF). Multiple factors may contribute to RIF, including oocyte and sperm quality, parental chromosomal anomalies, genetic or metabolic abnormalities of the embryo, poor uterine receptivity, immunological disturbances in the implantation site, and some gynecologic pathologies such as endometriosis, uterine fibroids, hydrosalpinx and endometrial polyps. Moreover, the procedure of in vitro fertilization itself could adversely influence the implantation. Nowadays, many studies are focused on the role of natural killer (NK) cells in normal and pathologic pregnancy because NK cells constitute the dominant cell population in the endometrium and they come in close contact with the allogeneic extravillous trophoblast cells in early pregnancy decidua. The majority of these cells are of CD56^bright phenotype. These cells can express killer immunoglobulin-like receptors (KIRs), which, upon recognition of HLA class I molecules (HLA-C and HLA-G) on trophoblasts, may either stimulate or inhibit NK cells to produce soluble factors, and display low cytotoxicity necessary for maintenance of the allogeneic embryo and fetus in the next steps of pregnancy. Moreover, some members of the leukocyte immunoglobulin-like receptor (LILR) family, also named ILT (immunoglobulin-like transcript), are present in the human placenta. LILRB1 (ILT2) was described mainly on stromal cells, while LILRB2 (ILT4), in addition to stromal cells, was also found around vessels in the smooth muscle layer. In this review we focus on the possible role of polymorphism of KIR, LILRB and their ligands (HLA-C, HLA-G) in susceptibility to recurrent implantation failure, which could serve as diagnostic biomarkers of this disease.

Uterine Natural Killer Cells: Functional Distinctions and Influence on Pregnancy in Humans and Mice

Our understanding of development and function of natural killer (NK) cells has progressed significantly in recent years. However, exactly how uterine NK (uNK) cells develop and function is still unclear. To help investigators that are beginning to study tissue NK cells, we summarize in this review our current knowledge of the development and function of uNK cells, and what is yet to be elucidated. We compare and contrast the biology of human and mouse uNK cells in the broader context of the biology of innate lymphoid cells and with reference to peripheral NK cells. We also review how uNK cells may regulate trophoblast invasion and uterine spiral arterial remodeling in human and murine pregnancy.

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.

Killer Immunoglobulin-Like Receptor Profiles Are not Associated with Risk of Amoxicillin-Clavulanate-Induced Liver Injury in Spanish Patients

Natural killer cells are an integral part of the immune system and represent a large proportion of the lymphocyte population in the liver. The activity of these cells is regulated by various cell surface receptors, such as killer Ig-like receptors (KIR) that bind to human leukocyte antigen (HLA) class I ligands on the target cell. The composition of KIR receptors has been suggested to influence the development of specific diseases, in particularly autoimmune diseases, cancer and reproductive diseases. The role played in idiosyncratic drug-induced liver injury (DILI) is currently unknown. In this study, we examined KIR gene profiles and HLA class I polymorphisms in amoxicillin-clavulanate (AC) DILI patients in search for potential risk associations. One hundred and two AC DILI patients and 226 controls were genotyped for the presence or absence of 16 KIR loci, including the two pseudogenes 2DP1 and 3DP1. No significant differences were found in the distribution of individual KIRs between patients and controls, which were comparable to previously reported KIR data from ethnically similar cohorts. The 21.6 and 21.2% of the patients and controls, respectively, were homozygous haplotype A carriers, while 78.4 and 78.8%, respectively, contained at least one B haplotype (Bx). The genotypes translated into 27 (AC DILI) and 46 (controls) different gene profiles, with 19 being present in both groups. The most frequent Bx gene profile containing KIRs 2DS2, 2DL2, 2DL3, 2DP1, 2DL1, 3DL1, 2DS4, 3DL2, 3DL3, 2DL4, and 3PD1 was present in 16% of the DILI patients and 14% of the controls. The distribution of HLA class I epitopes did not differ significantly between AC DILI patients and controls. The most frequent receptor-ligand combinations in the DILI patients were 2DL3 + epitope C1 (67%) and 3DL1 + Bw4 motif (67%), while 2DL1 + epitope C2 (69%) and 3DL1 + Bw4 motif (69%) predominated in the controls. This is to our knowledge the first analysis of KIR receptor-HLA ligand associations in DILI, although our findings do not support evidence of these genetic variations playing a major role in AC DILI development.

A bird's eye view of NK cell receptor interactions with their MHC class I ligands

The surveillance of target cells by natural killer (NK) cells utilizes an ensemble of inhibitory and activating receptors, many of which interact with major histocompatibility complex (MHC) class I molecules. NK cell recognition of MHC class I proteins is important developmentally for the acquisition of full NK cell effector capacity and during target cell recognition, where the engagement of inhibitory receptors and MHC class I molecules attenuates NK cell activation. Human NK cells have evolved two broad strategies for recognition of human leukocyte antigen (HLA) class I molecules: (i) direct recognition of polymorphic classical HLA class I proteins by diverse receptor families such as the killer cell immunoglobulin-like receptors (KIRs), and (ii) indirect recognition of conserved sets of HLA class I-derived peptides displayed on the non-classical HLA-E for recognition by CD94-NKG2 receptors. In this review, we assess the structural basis for the interaction between these NK receptors and their HLA class I ligands and, using the suite of published KIR and CD94-NKG2 ternary complexes, highlight the features that allow NK cells to orchestrate the recognition of a range of different HLA class I proteins.

Co-evolution of NK receptors and HLA ligands in humans is driven by reproduction

Allogeneic individuals co-exist during pregnancy in eutherian mammals. Maternal and fetal cells intermingle at the site of placental attachment in the uterus, where the arteries are remodeled to supply the fetus with oxygen and nutrients. This access by placental cells to the maternal supply line determines the growth and birth weight of the baby and is subject to stabilizing selection. Invading placental trophoblast cells express human leukocyte antigen class I ligands (HLA-E, HLA-G, and HLA-C) for receptors on maternal uterine natural killer (NK) and myelomonocytic cells, CD94/NKG2, leukocyte immunoglobulin-like receptor (LILR), and killer immunoglobulin receptor (KIR). Of these, only the KIR/HLA-C system is highly polymorphic. Different combinations of maternal KIR and fetal HLA-C variants are correlated with low birth weight and pre-eclampsia or high birth weight and obstructed labor, the two extremes of the obstetric dilemma. This situation has arisen because of the evolution of bipedalism and subsequently, in the last million years, larger brains. At this point, the human system began to reach a balance between KIR A and KIR B haplotypes and C1 and C2 epitopes of HLA-C alleles that reflects a functional compromise between the competing demands of immunity and reproduction.

Possible Role of HLA-G, LILRB1 and KIR2DL4 Gene Polymorphisms in Spontaneous Miscarriage

The KIR2DL4 receptor and its ligand HLA-G are considered important for fetal-maternal immune tolerance and successful pregnancy. The absence of a particular variant of KIR2DL4 might be a bad prognostic factor for pregnancy outcome. However, it could be compensated by the presence of the respective LILRB1 allele. Therefore, we investigated the KIR2DL4, LILRB1 and HLA-G polymorphisms in 277 couples with spontaneous abortion and 219 control couples by HRM, PCR-SSP and RFLP methods. We found a protective effect of women’s heterozygosity in −716 HLA-G (p = 0.0206) and LILRB1 (p = 0.0131) against spontaneous abortion. Surprisingly, we observed more 9A/10A genotypes of KIR2DL4 gene carriers in the group of male partners from the miscarriage group in comparison to the men from the control group (p = 0.0288). Furthermore, there was no association of women’s KIR2DL4 polymorphism with susceptibility to spontaneous abortion. Multivariate analysis indicated that women’s −716 HLA-G and LILRB1 and men’s KIR2DL4 9A/10A are important in terms of the protection or susceptibility to miscarriage, respectively (p = 0.00968). In conclusion, a woman’s heterozygosity in HLA-G and LILRB1 might be an advantage for a success of reproduction, but the partner’s heterozygosity in 9A/10A KIR2DL4 alleles might not.

Description of the novel KIR2DL4*035 allele identified using high-throughput sequencing

A newly identified allele of the KIR2DL4 natural killer cell receptor for human leukocyte antigen (HLA) class I.