The centromeric part of the human natural killer (NK) receptor complex: lectin-like receptor genes expressed in NK, dendritic and endothelial cells

Novel immune-related genes in the tumor microenvironment with prognostic value in breast cancer

Background

Breast cancer is one of the most frequently diagnosed cancers among women worldwide. Alterations in the tumor microenvironment (TME) have been increasingly recognized as key in the development and progression of breast cancer in recent years. To deeply comprehend the gene expression profiling of the TME and identify immunological targets, as well as determine the relationship between gene expression and different prognoses is highly critical.

Methods

The stromal/immune scores of breast cancer patients from The Cancer Genome Atlas (TCGA) were employed to comprehensively evaluate the TME. Then, TME characteristics were assessed, overlapping genes of the top 3 Gene Ontology (GO) terms and upregulated differentially expressed genes (DEGs) were analyzed. Finally, through combined analyses of overall survival, time-dependent receiver operating characteristic (ROC), and protein-protein interaction (PPI) network, novel immune related genes with good prognosis were screened and validated in both TCGA and GEO database.

Results

Although the TME did not correlate with the stages of breast cancer, it was closely associated with the subtypes of breast cancer and gene mutations (CDH1, TP53 and PTEN), and had immunological characteristics. Based on GO functional enrichment analysis, the upregulated genes from the high vs low immune score groups were mainly involved in T cell activation, the external side of the plasma membrane, and receptor ligand activity. The top GO terms of the upregulated DEGs from the high vs low immune score groups exhibited better prognosis in breast cancer; 15 of them were related to good prognosis in breast cancer, especially CD226 and KLRC4-KLRK1.

Conclusions

High CD226 and KLRC4-KLRK1 expression levels were identified and validated to correlate with better overall survival in specific stages or subtypes of breast cancer. CD226, KLRC4-KLRK1 and other new targets seem to be promising avenues for promoting antitumor targeted immunotherapy in breast cancer.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-021-07837-1.

HemITAM: A single tyrosine motif that packs a punch

Innate immune cells sense danger through a plethora of germline-encoded receptors that recognize pathogen-associated molecular patterns (PAMPs) or cellular molecules that are exposed only by stressed, infected, malignant, or dead cells. Many of these danger-sensing receptors belong to the C-type lectin-like superfamily (CLSF) and therefore are called C-type lectin-like receptors (CTLRs). Certain activating CTLRs, namely, CLEC-2, Dectin-1, DNGR-1, NKp80, and NKp65, which are encoded by genes that are clustered together in a subregion of the mammalian natural killer gene complex (NKC), use a single copy tyrosine signaling module termed the hemi-immunoreceptor tyrosine-based activation motif (hemITAM). These hemITAM-bearing CTLRs are present on myeloid cells and innate lymphocytes and stimulate various functions, such as phagocytosis, cytokine production, and cytotoxicity. Proximal signaling mechanisms involve the tyrosine phosphorylation of the hemITAM and the subsequent activation of the kinase Syk. Signaling and Syk recruitment by the hemITAM appear to be tuned by variable amino acids within or near the hemITAM, which give rise to differences in downstream signaling events and diverging functional outcomes among hemITAM-bearing receptors.

The evolution of the natural killer complex; a comparison between mammals using new high-quality genome assemblies and targeted annotation

Natural killer (NK) cells are a diverse population of lymphocytes with a range of biological roles including essential immune functions. NK cell diversity is in part created by the differential expression of cell surface receptors which modulate activation and function, including multiple subfamilies of C-type lectin receptors encoded within the NK complex (NKC). Little is known about the gene content of the NKC beyond rodent and primate lineages, other than it appears to be extremely variable between mammalian groups. We compared the NKC structure between mammalian species using new high-quality draft genome assemblies for cattle and goat; re-annotated sheep, pig, and horse genome assemblies; and the published human, rat, and mouse lemur NKC. The major NKC genes are largely in the equivalent positions in all eight species, with significant independent expansions and deletions between species, allowing us to propose a model for NKC evolution during mammalian radiation. The ruminant species, cattle and goats, have independently evolved a second KLRC locus flanked by KLRA and KLRJ, and a novel KLRH-like gene has acquired an activating tail. This novel gene has duplicated several times within cattle, while other activating receptor genes have been selectively disrupted. Targeted genome enrichment in cattle identified varying levels of allelic polymorphism between the NKC genes concentrated in the predicted extracellular ligand-binding domains. This novel recombination and allelic polymorphism is consistent with NKC evolution under balancing selection, suggesting that this diversity influences individual immune responses and may impact on differential outcomes of pathogen infection and vaccination.

Selective Amplification of the Genome Surrounding Key Placental Genes in Trophoblast Giant Cells

While most cells maintain a diploid state, polyploid cells exist in many organisms and are particularly prevalent within the mammalian placenta [1], where they can generate more than 900 copies of the genome [2]. Polyploidy is thought to be an efficient method of increasing the content of the genome by avoiding the costly and slow process of cytokinesis [1, 3, 4]. Polyploidy can also affect gene regulation by amplifying a subset of genomic regions required for specific cellular function [1, 3, 4]. This mechanism is found in the fruit fly Drosophila melanogaster, where polyploid ovarian follicle cells amplify genomic regions containing chorion genes, which facilitate secretion of eggshell proteins [5]. Here, we report that genomic amplification also occurs in mammals at selective regions of the genome in parietal trophoblast giant cells (p-TGCs) of the mouse placenta. Using whole-genome sequencing (WGS) and digital droplet PCR (ddPCR) of mouse p-TGCs, we identified five amplified regions, each containing a gene family known to be involved in mammalian placentation: the prolactins (two clusters), serpins, cathepsins, and the natural killer (NK)/C-type lectin (CLEC) complex [6-12]. We report here the first description of amplification at selective genomic regions in mammals and present evidence that this is an important mode of genome regulation in placental TGCs.

The characteristics of NK cells in Schistosoma japonicum-infected mouse spleens

Natural killer (NK) cells are classic innate immune cells that play roles in many types of infectious disease. Recently, some new characteristics of NK cells were discovered. In this study, C57BL/6 mice were infected with Schistosoma japonicum for 5-6 weeks and lymphocytes were isolated from the spleen to detect some of the NK cell characteristics by multiparametric flow cytometry. The results revealed that the S. japonicum infection induced a large amount of NK cells, although the percentage of NK cells was not increased significantly. At the same time, the results showed that infected mouse splenic NK cells expressed increased levels of CD25 and CD69 and produced more IL-2, IL-4, and IL-17 and less IFN-γ after stimulation with PMA and ionomycin. This meant that NK cells played a role in S. japonicum infection. Moreover, decreased NKG2A/C/E (CD94) expression levels were detected on the surface of NK cells from infected mouse spleens, which might serve as a NK cell activation mechanism. Additionally, high levels of IL-10, but not PD-1, were expressed on the infected mouse NK cells, which implied that functional exhaustion might exist in the splenic NK cells from S. japonicum-infected mice. Collectively, our results suggest that NK cells play important roles in the course of S. japonicum infection.

Lectin-like receptor Ly49s3 on dendritic cells contributes to the differentiation of regulatory T cells in the rat thymus

Naturally occurring regulatory T cells (nTregs), important for immune regulation and the maintenance of self-tolerance, develop in the thymus. The Hirosaki hairless rat (HHR), derived from the Sprague-Dawley rat (SDR), was shown to have decreased peripheral lymphocyte number, small thymus, and leukocyte infiltration in its dermis. In the HHR thymus, the medulla was underdeveloped and nTreg number was decreased. Array comparative genome hybridization revealed the deletion of an NK cell lectin-like receptor gene, Ly49s3, detecting MHC class I molecules on target cells, in the chromosome 4q42 region in HHRs. The gene was expressed in thymic conventional dendritic cells (cDCs) in SDRs, but not in HHRs. When CD4-single-positive or CD4(+)CD8(-)CD25(-) thymocytes were cultured with thymic cDCs, the expression of nTreg marker genes was lower when these cells were from HHRs than from SDRs, suggesting that HHR cDCs are deficient in the ability to induce and maintain nTreg differentiation. Expression of the genes was recovered when Ly49s3 was expressed on HHR thymic cDCs. Expression levels of MHC class II genes, presumably from cDCs, were parallel to those of nTreg marker genes in mixed-cell cultures. However, in the presence of an anti-MHC class I Ab, blocking interaction between Ly49s3 and MHC class I molecules, the expression of the former genes was upregulated, whereas the latter was downregulated. These results suggest that Ly49s3 contributes to nTreg regulation along with MHC class II molecules, whose effects alone are insufficient, and loss of Ly49s3 from thymic cDCs is the reason for the nTreg deficiency in HHRs.

High diversification of CD94 by alternative splicing in New World primates

CD94 forms heterodimers with NKG2A, -C, or -E to constitute lectin-like natural killer cell receptors for MHC-E. Its structure differs from other C-type lectins in that the second α-helix is replaced by a loop that forms the interacting interface with the NKG2 molecules. Although CD94 has remained highly conserved mammals, several alternative splicing variants have been detected in some species. To evaluate the prevalence and significance of this phenomenon, we have cloned and sequenced CD94 cDNAs in six species of New World primates from the Cebidae and Atelidae families. Full-length sequences had a mean similarity of 96 % amongst New World primates and of 90 % to the human orthologue, with little variation in the residues interacting with NKG2 or MHC-E molecules. Despite this high conservation, a total of 14 different splice variants were identified, half of which were shared by two or more primate species. Homology-based modeling of the C-type lectin domain showed that most isoforms folded stably, although they had modifications that prevented its interaction with NKG2 and MHC-E. Two isoforms were predicted to replace the typical CD94 loop by a second α-helix, evidencing a domain fold transition from a CD94 structure to a canonical C-type lectin. These two structures were more similar to members of the CLEC lectin family than to the native CD94. Thus, CD94 has remained conserved in primates to maintain functional interactions with NKG2 and MHC-E, while at the same time has diversified by alternative splicing potentially providing additional functional scenarios.

Evolution of the C-type lectin-like receptor genes of the DECTIN-1 cluster in the NK gene complex

Pattern recognition receptors are crucial in initiating and shaping innate and adaptive immune responses and often belong to families of structurally and evolutionarily related proteins. The human C-type lectin-like receptors encoded in the DECTIN-1 cluster within the NK gene complex contain prominent receptors with pattern recognition function, such as DECTIN-1 and LOX-1. All members of this cluster share significant homology and are considered to have arisen from subsequent gene duplications. Recent developments in sequencing and the availability of comprehensive sequence data comprising many species showed that the receptors of the DECTIN-1 cluster are not only homologous to each other but also highly conserved between species. Even in Caenorhabditis elegans, genes displaying homology to the mammalian C-type lectin-like receptors have been detected. In this paper, we conduct a comprehensive phylogenetic survey and give an up-to-date overview of the currently available data on the evolutionary emergence of the DECTIN-1 cluster genes.

Inhibitory C-type lectin receptors in myeloid cells

C-type lectin receptors encoded by the natural killer gene complex play critical roles in enabling NK cell discrimination between self and non-self. In recent years, additional genes at this locus have been identified with patterns of expression that extend to cells of the myeloid lineage where many of the encoded inhibitory receptors have equally important functions as regulators of immune homeostasis. In the present review we highlight the roles of some of these receptors including recent insights gained with regard to the identification of exogenous and endogenous ligands, mechanisms of cellular inhibition and activation, regulated expression within different cellular and immune contexts, as well as functions that include the regulation of bone homeostasis and involvement in autoimmunity.

The CD94/NKG2 family of receptors: from molecules and cells to clinical relevance

Immune responses must be tightly regulated to avoid hyporesponsiveness on one hand or excessive inflammation and the development of autoimmunity (hyperresponsiveness) on the other hand. This balance is attained through the throttling of activating signals by inhibitory signals that ideally leads to an adequate immune response against an invader without excessive and extended inflammatory signals that promote the development of autoimmunity. The CD94/NKG2 family of receptors is composed of members with activating or inhibitory potential. These receptors are expressed predominantly on NK cells and a subset of CD8+ T cells, and they have been shown to play an important role in regulating responses against infected and tumorigenic cells. In this review, we discuss the current knowledge about this family of receptors, including ligand and receptor interaction, signaling, membrane dynamics, regulation of gene expression and their roles in disease regulation, infections, and cancer, and bone marrow transplantation.

Ly49Q defines 2 pDC subsets in mice

Plasmacytoid dendritic cells (pDCs) play an important primary role for antiviral innate immunity by rapidly producing large amounts of type 1 interferon (IFN) upon viral infection. To study pDC biology, we generated a monoclonal antibody, termed 2E6, that recognizes pDCs. Molecular cloning of a cDNA encoding the 2E6 antigen revealed that it is a type II C-type lectin, Ly49Q, that consists of 247 amino acids with high homology to the natural killer (NK) receptor family Ly49, with an immunoreceptor tyrosine-based inhibitory motif in the cytoplasmic domain. Ly49Q is expressed on pDCs but not on NK cells or myeloid dendritic cells. B220+, CD11c+, CD11b– pDCs in bone marrow were divided into Ly49Q+ and Ly49Q– subsets. While both subsets produced IFN-α upon cytosine-phosphate-guanosine (CpG) and herpes simplex virus stimulation, Ly49Q– pDCs responded poorly to influenza virus. In addition, Ly49Q– pDCs produced inflammatory cytokines such as interleukin 6 (IL-6), IL-12, and tumor necrosis factor α (TNF-α) upon stimulation at lower levels than those produced by Ly49Q+ pDCs. In contrast to bone marrow, Ly49Q+ pDCs were only found in peripheral blood, lymph nodes, and spleen. These results indicate that Ly49Q is a specific marker for peripheral pDCs and that expression of Ly49Q defines 2 subsets of pDCs in bone marrow.

Imbalance of NKG2D and its inhibitory counterparts: how does tumor escape from innate immunity?

NK cells form a first line of defence against pathogens or host cells that are stressed or cancerous. NK cells express surface receptors that receive signals from the environment and determine their response to foreign or malignant cells. The effector functions of NK cells are regulated by integrated signals across the array of stimulatory and inhibitory receptors engaged upon interaction with target cell surface ligands. NKG2D is a peculiar activating receptor that is expressed as a disulphide-linked homodimer by all NK cells, alphabeta CD8(+) T cells, gammadeltaT cells and murine macrophages. It not only activates NK cells but also delivers co-stimulatory signals to CD8(+) T cells and gammadeltaT cells. The ligands of NKG2D are induced by cellular stress and are specifically expressed by some tumor cells. Recent studies reveal that the expression of MIC and ULBP on human tumor cells is sufficient to overcome the inhibitory effects of MHC class I expression on NK cell killing and indicate that NKG2D provides first line surveillance against stressed or abnormal cells that have been induced to express one of its ligands. However, malignant tumors develop means to control the expression of activating versus inhibitory receptors on immune cells and their ligands on tumor cell themselves in favor of tolerance. Modulating the balance between activating and inhibitory signals through NK cell receptors on NK cells may open a new approach to NK cell-based biotherapy for cancer and infectious diseases.

GATA-3 Is an Important Transcription Factor for Regulating Human NKG2A Gene Expression

CD94/NKG2A is an inhibitory receptor expressed by most human NK cells and a subset of T cells that recognizes HLA-E on potential target cells. To study the transcriptional regulation of the human NKG2A gene, we cloned a 3.9-kb genomic fragment that contains a 1.65-kb region upstream of the exon 1, as well as exon 1 (untranslated), intron 1 and exon 2. Using deletion mutants, we identified a region immediately upstream from the most upstream transcriptional initiation site that led to increased transcriptional activity from a luciferase reporter construct in YT-Indy (NKG2A positive) cells relative to Jurkat and K562 (both NKG2A negative) cells. We also localized a DNase I hypersensitivity site to this region. Within this 80-bp segment, we identified two GATA binding sites. Mutation of GATA binding site II (-2302 bp) but not GATA binding site I (-2332 bp) led to decreased transcriptional activity. Pull-down assays revealed that GATA-3 could bind oligonucleotide probes containing the wild type but not a mutated GATA site II. Using chromatin immunoprecipitation assays, we showed that GATA-3 specifically binds to the NKG2A promoter in situ in NKL and primary NK cells, but not in Jurkat T cells. Moreover, coexpression of human GATA-3 with an NKG2A promoter construct in K562 cells led to enhanced promoter activity, and transfection of NKL cells with small interfering RNA specific for GATA-3 reduced NKG2A cell surface expression. Taken together, our data indicate that GATA-3 is an important transcription factor for regulating NKG2A gene expression.

Signaling and immune regulatory role of the dendritic cell immunoreceptor (DCIR) family lectins: DCIR, DCAR, dectin-2 and BDCA-2

This review focuses on a distinct family of dendritic cells (DC) expressing C-type lectins that include DC immunoreceptor (DCIR), DC immunoactivating receptor (DCAR), DC-associated C-type lectin (dectin)-2 and blood DC antigen (BDCA)-2. DCIR is a type II C-type lectin expressed on antigen presenting cells and granulocytes and acts as an inhibitory receptor via an intracellular immunoreceptor tyrosine-based inhibitory motif (ITIM). In contrast, DCAR has been identified as a molecule that forms a putative pair with DCIR. While both molecules share the highly homologous extracellular lectin domain, DCAR lacks the ITIM in its short cytoplasmic tail and acts as an activating receptor through association with the Fc receptor gamma chain. Two other lectins, dectin-2 and BDCA-2, are highly related to DCAR by similarities of their amino acid sequence, molecular structure and chromosomal localization. Although they also lack the ITIM, they are capable of transducing signal to regulate cellular functions positively or negatively. Here we propose to designate these four highly related molecules as the "DCIR family lectins" and discuss their signaling mechanisms, carbohydrate recognition, and other features that contribute to the function of DC to control immunity.

Human CD94 gene expression: dual promoters differing in responsiveness to IL-2 or IL-15

CD94 is a C-type lectin required for the dimerization of the CD94/NKG2 family of receptors, which are expressed on NK cells and T cell subsets. Little is known about CD94 gene expression and the elements that regulate CD94 transcription. In this study, we report that CD94 gene expression is regulated by distal and proximal promoters that transcribe unique initial exons specific to each promoter. This results in two species of transcripts; the previously described CD94 mRNA and a novel CD94C mRNA. All NK cells and CD94(+), CD8(+) alphabeta T cells transcribe CD94 mRNA. Stimulation of NK and CD8(+) alphabeta T cells with IL-2 or IL-15 induced the transcription of CD94C mRNA. The distal and proximal promoters both contain elements with IFN-gamma-activated and Ets binding sites, known as GAS/EBS. Additionally, an unknown element, termed site A, was identified in the proximal promoter. EMSA analyses showed that constitutive factors could bind to oligonucleotide probes containing each element. After treatment of primary NK cells with IL-2 or IL-15, separate inducible complexes could be detected with oligonucleotide probes containing either the proximal or distal GAS/EBS elements. These elements are highly conserved between mice and humans, which suggests that both species regulate CD94 gene expression via mechanisms that predate their evolutionary divergence.

CAM and NK Cells

It is believed that tumor development, outgrowth and metastasis are under the surveillance of the immune system. Although both innate and acquired immune systems play roles, innate immunity is the spearhead against tumors. Recent studies have revealed the critical role of natural killer (NK) cells in immune surveillance and that NK cell activity is considerably influenced by various agents, such as environmental factors, stress, foods and drugs. Some of these NK cell stimulants have been used in complementary and alternative medicine (CAM) since ancient times. Therefore, the value of CAM should be re-evaluated from this point of view. In this review, we overview the intimate correlation between NK cell functions and CAM agents, and discuss possible underlying mechanisms mediating this. In particular, neuro-immune crosstalk and receptors for CAM agents are the most important and interesting candidates for such mechanisms.

Carbohydrate and Non-Carbohydrate Ligands for the C-Type Lectin-Like Receptors of Natural Killer Cells. A Review

The superfamily of C-type animal lectins is defined by a sequence motif of the carbohydrate- recognition domains (CRDs) and comprises seven groups of molecules. The soluble proteins are group I proteoglycans, group III collectins, and group VII containing the isolated CRDs. Type I membrane proteins include group IV selectins and group VI macrophage receptors and related molecules. Type II membrane proteins are group II hepatic lectins and group V natural killer cell receptors. The latter group has recently attracted considerable attention of the biomedical community. These receptors are arranged at the surface of lymphocytes as homo- or heterodimers composed of two polypeptides consisting of N-terminal peptide tails responsible for signaling, transmembrane domain, neck regions of varying length, and C-terminal lectin-like domains (CTLDs). Since this group is evolutionarily most distant from the rest of C-type animal lectins, the sequence of the C-terminal ligand-binding domain has diversified to accommodate other ligands than calcium or carbohydrates. These domains are referred to as natural killer domains (NKDs) forming a large percentage of CTLDs in vertebrates. Here are summarized the data indicating that calcium, carbohydrates, peptides, and large proteins such as major histocompatibility complex (MHC) class I can all be ligands for NKDs. The wide range of ligands that can be recognized by NKDs includes some new, unexpected compounds such as signal peptide-derived fragments, heat shock proteins, or oxidized lipids. The biological importance of this extended range of recognition abilities is also discussed. A review with 134 references.

Dendritic cell immunoactivating receptor, a novel C-type lectin immunoreceptor, acts as an activating receptor through association with Fc receptor gamma chain

An increasing number of C-type lectin receptors are being discovered on dendritic cells, but their signaling abilities and underlying mechanisms require further definition. Among these, dendritic cell immunoreceptor (DCIR) induces negative signals through an inhibitory immunoreceptor tyrosine-based inhibitory motif (ITIM) in its cytoplasmic tail. Here we identify a novel C-type lectin receptor, dendritic cell immunoactivating receptor (DCAR), whose extracellular lectin domain is highly homologous to that of DCIR. DCAR is expressed similarly in tissues to DCIR, but its short cytoplasmic portion lacks signaling motifs like ITIM. However, a positively charged arginine residue is present in the transmembrane region of the DCAR, which may explain its association with Fc receptor gamma chain and its stable expression on the cell surface. Furthermore, cross-linking of DCAR in the presence of gamma chain activates calcium mobilization and tyrosine phosphorylation of cellular proteins. These signals are mediated by the immunoreceptor tyrosine-based activating motif (ITAM) of the gamma chain. Thus, DCAR is closely related to DCIR, but it introduces activating signals into antigen-presenting cells through its physical and functional association with ITAM-bearing gamma chain. The identification of this activating immunoreceptor provides an example of signaling via a dendritic cell-expressed C-type lectin receptor.

NKG2D ligands: unconventional MHC class I-like molecules exploited by viruses and cancer

Our best teachers in revealing the importance of immune pathways are viruses and cancers that have subverted the most prominent pathways to escape from immune recognition. Viruses and cancer impair antigen presentation by classical MHC class I to escape adaptive immunity. The activating receptor NKG2D and its MHC class I-like ligands are other recently defined innate and adaptive immune pathways exploited by viruses and cancer. This review discusses recent advances in the understanding of how NKG2D, expressed on innate immune cells including natural killer cells, gammadelta+ T cells and macrophages, and adaptive immune cells such as CD8+ T cells, recognize stress-induced, MHC class I-like, self-ligands. Moreover, we describe how viruses and cancer have developed strategies to evade this recognition pathway.

Urochordates and the origin of natural killer cells: identification of a CD94/NKR-P1-related receptor in blood cells of Botryllus

Transplantation immunity based on the recognition of MHC molecules is well described in vertebrates. Vertebrates, however, do not undergo transplantation reaction naturally. The phylogenetically closest group in which transplantation reactions can occur is the Urochordata. Therefore, these animals occupy a key position for understanding the evolution of the vertebrate immune system. When screening for genes differentially expressed during allorecognition in Botryllus schlosseri, we isolated a gene coding for a type II transmembrane protein with a C-type lectin-binding domain and close similarity to vertebrates CD94 and NKR-P1. Here we show that the gene, BsCD94-1, is differentially regulated during allorecognition and that a subpopulation of blood cells carries the corresponding receptor on its cell surface. Southern blot analysis with DNA from individual colonies and intronless BsCD94-1 probe reveal variation between individuals at the genomic level. CD94 in vertebrates is one of the markers for natural killer cells and binds to MHC class I molecules. Natural killer cells play a major role in recognition and elimination of allogeneic cells. Their evolutionary origin, however, remained unknown. The results presented here indicate that the elaboration of the vertebrate immune system may have its roots in an ancestral population of cells in the urochordate blood.

Complex expression of natural killer receptor genes in single natural killer cells

Human natural killer (NK) cells express several inhibitory and non-inhibitory NK receptors per cell. Understanding the expression patterns of these receptor genes in individual cells is important to understanding their function. Using a single-cell reverse transcription-polymerase chain reaction (RT-PCR) method, we analysed the expression of nine NK receptor genes in 38 resting CD56+ NK cells from peripheral blood of normal donors. We observed highly diverse patterns of receptor expression in these cells. No NK receptor is expressed universally in every CD56+ NK cell. The expressed receptor types per cell varied from two to eight. We specifically analysed the distribution of inhibitory (DL) and non-inhibitory (DS) killer immunoglobulin-like receptors (KIR). The frequency of individual receptor expression varied from 26% for 2DS2 to 68% for both 2DL1 and 2DL4. A comparison of the coexpression of DL and DS receptors showed a significant association in the expression of 2DL2 and 2DS2 (chi2=16.6; P<0.001) genes but no association between 2DL1 and 2DS1 or between 3DL1 and 3DS1 genes. Coexpression analysis of the 2DL1 and 2DL2 genes in 2DL4+ and 2DL4- cells showed a strong association in 2DL4+ but not in 2DL4- cells, suggesting a differential effect of the 2DL4 gene on the expression of 2DL1 and 2DL2 genes. Single-cell RT-PCR is a powerful tool to study multiple receptor gene expression ex vivo in individual NK cells and provides information about the expression pattern of KIR receptors that may suggest mechanisms of gene expression responsible for generation of the KIR repertoire.

Differential expression of inhibitory and activating CD94/NKG2 receptors on NK cell clones

Natural killer cells are known to express a variety of surface receptors involved in HLA class I monitoring. It is thus of interest to investigate the clonal distribution and relative expression levels of activating versus inhibitory NK receptors. We have developed a quantitative real-time reverse transcription-polymerase chain reaction (RT-PCR) assay designed to determine specific and absolute mRNA levels for NKG2-A/B, -C, -E, -F, -H and NKG2-D. When analyzing NK cell clones derived from a single donor we found differential expression of inhibitory (NKG2-A/B) versus triggering (NKG2-C and potentially -E, -F, -H) NK receptor chains. The generation of the splice variants NKG2-E and -H seemed to occur at a constant ratio. We further compared NKG2 transcript levels to surface receptor expression as monitored by flow cytometric analysis and to NK cell cytotoxicity as detected by reverse ADCC: a clear correlation was observed. Thus, the data obtained reveal a substantial variability in the NKG2 repertoire among NK cell subpopulations, which is likely to affect the sensitivity and reactivity towards the ligand HLA-E.

Structure and function of major histocompatibility complex (MHC) class I specific receptors expressed on human natural killer (NK) cells

Natural killer (NK) cells express receptors that are specific for MHC class I molecules. These receptors play a crucial role in regulating the lytic and cytokine expression capabilities of NK cells. In humans, three distinct families of genes have been defined that encode for receptors of HLA class I molecules. The first family identified consists of type I transmembrane molecules belonging to the immunoglobulin (Ig) superfamily and are called killer cell Ig-like receptors (KIR). A second group of receptors belonging to the Ig superfamily, named ILT (for immunoglobulin like transcripts), has more recently been described. ILTs are expressed mainly on B, T and myeloid cells, but some members of this group are also expressed on NK cells. They are also referred to as LIRs (for leukocyte Ig-like receptor) and MIRs (for macrophage Ig-like receptor). The ligands for the KIR and some of the ILT receptors include classical (class Ia) HLA class I molecules, as well as the nonclassical (class Ib) HLA-G molecule. The third family of HLA class I receptors are C-type lectin family members and are composed of heterodimers of CD94 covalently associated with a member of the NKG2 family of molecules. The ligand for most members is the nonclassical class I molecule HLA-E. NKG2D, a member of the NKG2 family, is expressed as a homodimer, along with the adaptor molecule DAP10. The ligands of NKG2D include the human class I like molecules MICA and MICB, and the recently described ULBPs. Each of these three families of receptors has individual members that can recognize identical or similar ligands yet signal for activation or inhibition of cellular functions. This dichotomy correlates with particular structural features present in the transmembrane and intracytoplasmic portions of these molecules. In this review we will discuss the molecular structure, specificity, cellular expression patterns, and function of these HLA class I receptors, as well as the chromosomal location and genetic organization.