A family of MHC class I-like genes located in the vicinity of the mouse leukocyte receptor complex

Abnormal methylation of Mill1 gene regulates osteogenic differentiation involved in various phenotypes of skeletal fluorosis in rats and methionine intervention

Excessive fluoride intake can lead to skeletal fluorosis. Nutritional differences in the same fluoride-exposed environment result in osteosclerosis, osteoporosis, and osteomalacia. DNA methylation has been found to be involved in skeletal fluorosis and is influenced by environment and nutrition. In a previous study, we screened eight genes with differential methylation associated with various phenotypes of skeletal fluorosis. By combining gene functions, Mill1 gene was selected for subsequent experiments. First, we found that the Mill1 gene was hypomethylated and upregulated in osteosclerosis skeletal fluorosis, whereas it was hypermethylated and downregulated in osteoporosis/osteomalacia skeletal fluorosis. Similar results were obtained in the cell experiments. Subsequently, we validated the regulation of Mill1 gene methylation using DNMT1 and TET2 enzyme inhibitors. Furthermore, we knockdown and overexpression experiments confirmed its downregulation inhibited osteogenic differentiation, whereas osteogenic differentiation was promoted by its overexpression. These findings imply that abnormal methylation of the Mill1 gene triggered by fluoride under diverse nutritional conditions, regulates its expression and participates in osteogenic differentiation, potentially resulting in various phenotypes of skeletal fluorosis. Eventually, we use methionine for interventions both in vivo and in vitro. The results indicated that under normal nutrition and fluoride exposure followed by methionine intervention, the methylation levels of the Mill1 gene increased, whereas its high expression and enhanced osteogenic differentiation were restrained. This study offers a theoretical foundation for understanding the mechanism behind the various phenotypes of skeletal fluorosis through the perspective of DNA methylation and for employing nutrients to intervene in skeletal fluorosis.

Insulin sensor cells for the analysis of insulin secretion responses in single living pancreatic β cells

Investigation of the functions of insulin-secreting cells in response to glucose in single-living cells is essential for improving our knowledge on the pathogenesis of diabetes. Therefore, it is desired to develop a new convenient method that enables the direct detection of insulin secreted from single-living cells. Here, insulin-sensor-cells expressing a protein-based insulin-detecting probe immobilized on the extracellular membrane were developed to evaluate the insulin-secretion response in single-living pancreatic β cells. The protein-based insulin-detecting probe (NαLY) was composed of a bioluminescent protein (nano-luc), the αCT segment of the insulin receptor, L1 and CR domains of the insulin receptor, and a fluorescent protein (YPet). NαLY exhibited a bioluminescence resonance energy transfer (BRET) signal in response to insulin; thus, cells of Hepa1-6 line were genetically engineered to express NαLY on the extracellular membrane. The cells were found to act as insulin-sensor-cells, exhibiting a BRET signal in response to insulin. When the insulin-sensor-cells and pancreatic β cells (MIN6 cell line) were cocultured and stimulated with glucose, insulin-sensor-cells nearby pancreatic β cells showed the spike-shaped BRET signal response, whereas the insulin-sensor-cells close to one pancreatic β cell did not exhibit such signal response. However, all the insulin-sensor-cells showed a gradual increase in BRET signals, which were presumably attributed to the increase in insulin concentrations in the culture dish, confirming the function of these insulin-sensor-cells. Therefore, we demonstrated that heterogenetic insulin secretion in single-living pancreatic β cells could be measured directly using the insulin sensor cells.

Structure of MHC class I-like MILL2 reveals heparan-sulfate binding and interdomain flexibility

The MILL family, composed of MILL1 and MILL2, is a group of nonclassical MHC class I molecules that occur in some orders of mammals. It has been reported that mouse MILL2 is involved in wound healing; however, the molecular mechanisms remain unknown. Here, we determine the crystal structure of MILL2 at 2.15 Å resolution, revealing an organization similar to classical MHC class I. However, the α1-α2 domains are not tightly fixed on the α3-β₂m domains, indicating unusual interdomain flexibility. The groove between the two helices in the α1-α2 domains is too narrow to permit ligand binding. Notably, an unusual basic patch on the α3 domain is involved in the binding to heparan sulfate which is essential for MILL2 interactions with fibroblasts. These findings suggest that MILL2 has a unique structural architecture and physiological role, with binding to heparan sulfate proteoglycans on fibroblasts possibly regulating cellular recruitment in biological events.

The MILL (MHC-I-like located near the leukocyte receptor complex) family is a group of related nonclassical MHC-I molecules. Here the authors present the crystal structure of MILL2, which reveals an unusual interdomain flexibility, and show that MILL2 binds heparan sulfate on the surface of fibroblasts through a basic patch.

Comparative genomics of the NKG2D ligand gene family

NKG2D ligands (NKG2DLs) are a group of stress-inducible major histocompatibility complex (MHC) class I-like molecules that act as a danger signal alerting the immune system to the presence of abnormal cells. In mammals, two families of NKG2DL genes have been identified: the MIC gene family encoded in the MHC region and the ULBP gene family encoded outside the MHC region in most species. Some mammals have a third family of NKG2DL-like class I genes which we named MILL (MHC class I-like located near the leukocyte receptor complex). Despite the fact that MILL genes are more closely related to MIC genes than ULBP genes are to MIC genes, MILL molecules do not function as NKG2DLs, and their function remains unknown. With the progress of mammalian genome projects, information on the MIC, ULBP, and MILL gene families became available in many mammalian species. Here, we summarize such information and discuss the origin and evolution of the NKG2DL gene family from the viewpoint of host-pathogen coevolution.

Co-evolution with chicken class I genes

The concept of co-evolution (or co-adaptation) has a long history, but application at molecular levels (e.g., 'supergenes' in genetics) is more recent, with a consensus definition still developing. One interesting example is the chicken major histocompatibility complex (MHC). In contrast to typical mammals that have many class I and class I-like genes, only two classical class I genes, two CD1 genes and some non-classical Rfp-Y genes are known in chicken, and all are found on the microchromosome that bears the MHC. Rarity of recombination between the closely linked and polymorphic genes encoding classical class I and TAPs allows co-evolution, leading to a single dominantly expressed class I molecule in each MHC haplotype, with strong functional consequences in terms of resistance to infectious pathogens. Chicken tapasin is highly polymorphic, but co-evolution with TAP and class I genes remains unclear. T-cell receptors, natural killer (NK) cell receptors, and CD8 co-receptor genes are found on non-MHC chromosomes, with some evidence for co-evolution of surface residues and number of genes along the avian and mammalian lineages. Over even longer periods, co-evolution has been invoked to explain how the adaptive immune system of jawed vertebrates arose from closely linked receptor, ligand, and antigen-processing genes in the primordial MHC.

A gluten-free diet lowers NKG2D and ligand expression in BALB/c and non-obese diabetic (NOD) mice

The interplay between diet and immune parameters which could affect type 1 diabetes (T1D) pathogenesis is not sufficiently clarified. Intestinal up-regulation of the activating receptor natural killer group 2D (NKG2D) (CD314) and its ligands is a hallmark of coeliac disease. However, the direct effect of gluten on NKG2D expression is not known. We studied, by fluorescence activated cell sorter (lymphoid tissues) and reverse transcription–quantitative polymerase chain reaction (intestine and pancreatic islets), if a gluten-free diet (GF diet) from 4 weeks of age or a gluten-free diet introduced in breeding pairs (SGF diet), induced changes in NKG2D expression on DX5⁺(CD49b) natural killer (NK) cells, CD8⁺ T cells and in intestinal and islet levels of NKG2D and ligands in BALB/c and non-obese diabetic (NOD) mice. Gluten-free NOD mice had lower insulitis (P < 0·0001); reduced expression of NKG2D on DX5⁺ NK cells in spleen and auricular lymph nodes (P < 0·05); and on CD8⁺ T cells in pancreas-associated lymph nodes (P = 0·04). Moreover, the level of CD71 on DX5⁺ NK cells and CD8⁺ T cells (P < 0·005) was markedly reduced. GF and SGF mice had reduced expression of NKG2D and DX5 mRNA in intestine (P < 0·05). Differences in intestinal mRNA expression were found in mice at 8, 13 and 20 weeks. Intestinal expression of NKG2D ligands was reduced in SGF mice with lower expression of all ligands. In isolated islets, a SGF diet induced a higher expression of specific NKG2D ligands. Our data show that a gluten-free diet reduces the level of NKG2D and the expression of NKG2D ligands. These immunological changes may contribute to the lower T1D incidence associated with a gluten-free diet.

RNA-binding proteins regulate the expression of the immune activating ligand MICB

The recognition of stress-induced ligands by the activating receptor NKG2D expressed on cytotoxic lymphocytes is crucial for the prevention and containment of various diseases and is also one of the best-studied examples of how danger is sensed by the immune system. Still, however, the mechanisms leading to the expression of the NKG2D ligands are far from being completely understood. Here, we use an unbiased and systematic RNA pull-down approach combined with mass spectrometry to identify six RNA-binding proteins (RBPs) that bind and regulate the expression of MICB, one of the major stress-induced ligands of NKG2D. We further demonstrate that at least two of the identified RBPs function during genotoxic stress. Our data provide insights into stress recognition and hopefully open new therapeutic venues.

Exceptionally high conservation of the MHC class I-related gene, MR1, among mammals

The major histocompatibility complex (MHC) class I-related gene, MR1, is a non-classical MHC class IA gene and is encoded outside the MHC region. The MR1 is responsible for activation of mucosal-associated invariant T (MAIT) cells expressing semi-invariant T cell receptors in the presence of bacteria, but its ligand has not been identified. A unique characteristic of MR1 is its high evolutionary conservation of the α1 and α2 domains corresponding to the peptide-binding domains of classical MHC class I molecules, showing about 90 % amino acid identity between human and mouse. To clarify the evolutionary history of MR1 and identify more critically conserved residues for the function of MR1, we searched for the MR1 gene using jawed vertebrate genome databases and isolated the MR1 cDNA sequences of marsupials (opossum and wallaby). A comparative genomic analysis indicated that MR1 is only present in placental and marsupial mammals and that the gene organization around MR1 is well conserved among analyzed jawed vertebrates. Moreover, the α1 and α2 domains, especially in amino acid residues presumably shaping a ligand-binding groove, were also highly conserved between placental and marsupial MR1. These findings suggest that the MR1 gene might have been established at its present location in a common ancestor of placental and marsupial mammals and that the shape of the putative ligand-binding groove in MR1 has been maintained, probably for presenting highly conserved component(s) of microbes to MAIT cells.

Immunogenetics of the NKG2D ligand gene family

NKG2D ligands (NKG2DLs) are a group of major histocompatibility complex (MHC) class I-like molecules, the expression of which is induced by cellular stresses such as infection, tumorigenesis, heat shock, tissue damage, and DNA damage. They act as a molecular danger signal alerting the immune system for infected or neoplastic cells. Mammals have two families of NKG2DL genes: the MHC-encoded MIC gene family and the ULBP gene family encoded outside the MHC region in most mammals. Rodents such as mice and rats lack the MIC family of ligands. Interestingly, some mammals have NKG2DL-like molecules named MILL that are phylogenetically related to MIC, but do not function as NKG2DLs. In this paper, we review our current knowledge of the MIC, ULBP, and MILL gene families in representative mammalian species and discuss the origin and evolution of the NKG2DL gene family.

Comparative genomic analysis of mammalian NKG2D ligand family genes provides insights into their origin and evolution

NKG2D is a major activating receptor of natural killer cells. Its ligands are major histocompatibility complex (MHC) class I-like molecules whose expression is induced by cellular stresses such as infections and tumorigenesis. Humans have two families of NKG2D ligands (NKG2DL): MHC class I-related chains (MIC) encoded in the MHC and UL16-binding proteins (ULBP) encoded outside the MHC. By contrast, mice have only the latter family of ligands; instead, they have non-MHC-encoded MILL molecules that are closely related to MIC, but do not function as NKG2DL. To gain insights into the origin and evolution of MIC, ULBP, and MILL gene families, we conducted comparative genomic analysis of NKG2DL family genes in five mammalian species. In the opossum MHC, we identified a ULBP-like gene adjacent to a previously described MIC-like gene, suggesting that ULBP genes were originally encoded in the MHC. The opossum genome also contained a transcribed MILL-like gene in a region syntenic to the rodent regions encoding MILL molecules. These observations indicate that MIC-, ULBP-, and MILL-like genes emerged before the divergence of placental and marsupial mammals. Comparison of the human, cattle, rat, mouse, and opossum genomes indicates that after emigration from the MHC, ULBP genes underwent extensive duplications in each species. In mice, some of the ULBP genes appear to have been translocated telomerically on the same chromosome, forming a major cluster of existent NKG2DL genes.

Quantitative trait loci (QTL) analysis of longevity in C57BL/6J by DBA/2J (BXD) recombinant inbred mice

BACKGROUND AND AIMS

Genes associated with longevity have been identified using both single gene and genome-wide approaches in a variety of species. The aim of this study was to identify quantitative trait loci (QTLs) that influence longevity in male and female mice from twenty-three C57BL/6J by DBA/2J (BXD) recombinant inbred (RI) strains.

METHODS

Approximately 12 animals of each sex for each RI strain were maintained under standard conditions until natural death or moribundity criteria were met.

RESULTS

A number of life span-relevant loci previously reported on chromosomes (Chrs) 7, 8, 10 and 11 were confirmed. In addition, 5 previously unreported QTLs for mouse life span on Chrs 1, 2, 6, 11, and X were identified as significant and 3 QTLs on Chrs 5, 8, and 16 were suggestive.

CONCLUSIONS

Several QTLs were coincident in males and females although the modest correlation between male and female median lifespans and the identification of sex specific QTLs provide evidence that the genetic architecture underlying longevity in the sexes may differ substantially. The identification of multiple QTLs for longevity will provide valuable resources for both reductionist and integrationist research into mechanisms of life span determination.

Avian NK activities, cells and receptors

Natural killer (NK) activity has been examined in birds for over 30 years, but evidence that avian NK activity plays crucial roles in disease is only suggestive. In chickens, NK activity is mediated by TCR0 cells in the intestinal epithelium, but elsewhere subsets of alphabeta and gammadelta T cells (NKT cells) may be more important. There are few lectin-like NK receptor genes, located in the genomic region syntenic with the natural killer complex (NKC) as well as the major histocompatibility complex (MHC). In contrast, a huge number of Ig-like receptor genes are located in a region syntenic with the leukocyte receptor complex (LRC).

NKG2D is critical for NK cell activation in host defense against Pseudomonas aeruginosa respiratory infection

Pseudomonas aeruginosa is a major cause of nosocomial respiratory infections. The eradication of P. aeruginosa from the lung involves the orchestrated actions of the pulmonary epithelium and both resident and recruited immune cells. The NKG2D receptor is constitutively expressed on the surface of circulating and tissue-resident NK cells (and other cytotoxic lymphocytes), and is capable of controlling NK cell activation and production of cytokines, such as IFN-gamma via interactions with ligands expressed on the surface of stressed cells. Previously, we demonstrated that NKG2D mediates pulmonary clearance of P. aeruginosa. In the present study, we investigated the cellular and molecular mechanisms of NKG2D-mediated clearance of P. aeruginosa using a novel transgenic mouse model of doxycycline-inducible conditional expression of NKG2D ligands (retinoic acid early transcript 1, alpha) in pulmonary epithelial cells. NKG2D ligand expression in this model increased pulmonary clearance, cellular phagocytosis, and survival following P. aeruginosa respiratory infection. Additionally, NK cell sensitivity to ex vivo LPS stimulation was greater in lung cells isolated from naive transgenic mice administered doxycycline. We also showed that NK cells are the primary source of lymphocyte-derived IFN-gamma in response to P. aeruginosa respiratory infection. Significantly, we demonstrated that NKG2D is critical to the nonredundant IFN-gamma production by pulmonary NK cells following acute P. aeruginosa infection. These results represent the principal report of NKG2D-mediated activation of lung NK cells following respiratory infection with an opportunistic pathogen and further establish the importance of NKG2D in the host response against P. aeruginosa respiratory infection.

A role for the MHC class I-like Mill molecules in nutrient metabolism and wound healing

MHC class I family members serve multiple functions beyond antigen presentation. We provide insight into the structure, expression and function of the Mill subfamily. This family includes two surface glycoproteins, Mill1 and Mill2. Protein sequences for Mill1 and Mill2 are most highly related to the NKG2D ligands, MICA and MICB, but neither of them bound to NKG2D. Computer-based protein modelling indicated that hereditary haemochromatosis protein (HFE), a molecule involved in iron uptake, was most similar. Mill1 and Mill2 were observed on cycling thymocytes, proliferating smooth muscle cells and fibroblasts. Using soluble Mill proteins, we found evidence for a soluble ligand in serum. Like HFE, the Mill family may be involved in nutrient metabolism. Skin was one of the only three organs found to express transcripts for both Mill1 and Mill2. Addition of antibodies specific for Mill2 to wounded skin enhanced healing. Our results suggest a role for the Mill proteins in cellular metabolism, with possible therapeutic significance.

Evaluation of human major histocompatibility complex class I chain-related A as a potential target for tumor imaging

To investigate the feasibility of using human major histocompatibility complex class I chain-related A (MICA) as a target for tumor imaging diagnosis, 10C6, a monoclonal antibody (mAb) that specifically recognizes MICA in vitro, was labeled with (99m)Tc (technetium) and administered into mice bearing MICA-positive human ovarian epithelial carcinoma line SKOV3. Measurement of organ-specific radioactivity showed that tumor accumulated radioactivity continuously, while the uptake in the other organs decreased over time. Scintigram showed that the tumor became clearly visible at 24h post-injection of radio-labeled 10C6 mAb. These results suggest that MICA is a promising target for tumor imaging and mAb 10C6 may be used clinically for early tumor diagnosis.

Characterization of the opossum immune genome provides insights into the evolution of the mammalian immune system

The availability of the first marsupial genome sequence has allowed us to characterize the immunome of the gray short-tailed opossum (Monodelphis domestica). Here we report the identification of key immune genes, including the highly divergent chemokines, defensins, cathelicidins, and Natural Killer cell receptors. It appears that the increase in complexity of the mammalian immune system occurred prior to the divergence of the marsupial and eutherian lineages approximately 180 million years ago. Genomes of ancestral mammals most likely contained all of the key mammalian immune gene families, with evolution on different continents, in the presence of different pathogens leading to lineage specific expansions and contractions, resulting in some minor differences in gene number and composition between different mammalian lineages. Gene expansion and extensive heterogeneity in opossum antimicrobial peptide genes may have evolved as a consequence of the newborn young needing to survive without an adaptive immune system in a pathogen laden environment. Given the similarities in the genomic architecture of the marsupial and eutherian immune systems, we propose that marsupials are ideal model organisms for the study of developmental immunology.

MHC Class I-Like MILL Molecules Are β2-Microglobulin-Associated, GPI-Anchored Glycoproteins That Do Not Require TAP for Cell Surface Expression

MILL (MHC class I-like located near the leukocyte receptor complex) is a family of MHC class I-like molecules encoded outside the MHC, which displays the highest sequence similarity to human MICA/B molecules among known class I molecules. In the present study, we show that the two members of the mouse MILL family, MILL1 and MILL2, are GPI-anchored glycoproteins associated with beta2-microglobulin (beta2m) and that cell surface expression of MILL1 or MILL2 does not require functional TAP molecules. MILL1 and MILL2 molecules expressed in bacteria could be refolded in the presence of beta2m, without adding any peptides. Hence, neither MILL1 nor MILL2 is likely to be involved in the presentation of peptides. Immunohistochemical analysis revealed that MILL1 is expressed in a subpopulation of thymic medullary epithelial cells and a restricted region of inner root sheaths in hair follicles. The present study provides additional evidence that MILL is a class I family distinct from MICA/B.

The NKG2D-activating receptor mediates pulmonary clearance of Pseudomonas aeruginosa

The NKG2D-activating receptor is expressed on cytotoxic lymphocytes and interacts with ligands expressed on the surface of cells stressed by pathogenic and nonpathogenic stimuli. In this study, we investigated the physiologic importance of NKG2D receptor-ligand interactions in response to acute pulmonary Pseudomonas aeruginosa infection. P. aeruginosa infection increased the expression of mouse NKG2D ligands (Rae1) in airway epithelial cells and alveolar macrophages in vivo and also increased the cell surface expression of human NKG2D ligands (ULBP2) on airway epithelial cells in vitro. NKG2D receptor blockade with a specific monoclonal antibody inhibited the pulmonary clearance of P. aeruginosa. NKG2D receptor blockade also resulted in decreased production of Th1 cytokines and nitric oxide in the lungs of P. aeruginosa-infected mice. Additionally, NKG2D receptor blockade reduced the epithelial cell sloughing that accompanies P. aeruginosa infection. Macrophage phagocytosis and bronchoalveolar lavage cellularity were not different in P. aeruginosa-infected mice with and without NKG2D receptor blockade. These results demonstrate the importance of NKG2D-mediated immune activation in the clearance of acute bacterial infection and suggest that epithelial cell-lymphocyte interactions mediate pulmonary cytokine production, epithelial cell integrity, and bacterial clearance.

A transmembrane-anchored rat RAE-1-like transcript as a ligand for NKR-P2, the rat ortholog of human and mouse NKG2D

NKG2D is a potent activating immunoreceptor that has been extensively characterized in NK cells and shown to lend costimulatory functions in CD8(+) T cells under certain conditions. The list is growing of ligands for NKG2D, which are distantly related to MHC class I molecule and are often up-regulated during cellular distress. Here, we describe the cloning of a novel (and incidentally the first) ligand for NKR-P2, the rat ortholog of human and mouse NKG2D, termed as rat RAE-1-like transcript (RRLT). The newly identified ligand is homologous to mouse RAE-1 family of proteins, but differs from them in being transmembrane anchored. The protein localizes to cell surface, and is expressed in a variety of tumor cell lines and tissues. Ectopic expression of the ligand induces NK cell activation, and renders the target cells susceptible to NK cell killing. In addition, the role of RRLT-NKR-P2 interaction in tumor killing has been demonstrated.

Reconstructing an ancestral mammalian immune supercomplex from a marsupial major histocompatibility complex

The first sequenced marsupial genome promises to reveal unparalleled insights into mammalian evolution. We have used the Monodelphis domestica (gray short-tailed opossum) sequence to construct the first map of a marsupial major histocompatibility complex (MHC). The MHC is the most gene-dense region of the mammalian genome and is critical to immunity and reproductive success. The marsupial MHC bridges the phylogenetic gap between the complex MHC of eutherian mammals and the minimal essential MHC of birds. Here we show that the opossum MHC is gene dense and complex, as in humans, but shares more organizational features with non-mammals. The Class I genes have amplified within the Class II region, resulting in a unique Class I/II region. We present a model of the organization of the MHC in ancestral mammals and its elaboration during mammalian evolution. The opossum genome, together with other extant genomes, reveals the existence of an ancestral "immune supercomplex" that contained genes of both types of natural killer receptors together with antigen processing genes and MHC genes.

Chicken CD1 genes are located in the MHC: CD1 and endothelial protein C receptor genes constitute a distinct subfamily of class-I-like genes that predates the emergence of mammals

Mammals have several major histocompatibility complex (MHC) class-I-like genes. Although some of them are assumed to have originated before the emergence of mammals, the origin of class-I-like genes is poorly understood. We analyzed here the recently released chicken draft genome sequence and identified two families of class-I-like genes: CD1 and PROCR (the gene for the endothelial protein C receptor). Chickens have two CD1 genes, designated CD1.1 and CD1.2, located in tandem approximately 840 bp apart from each other. Chicken CD1.1 and CD1.2 are neither group 1- nor group 2-like, indicating that the two groups of CD1 emerged in a mammalian lineage. Although the database provides no information as to their chromosomal localization, we found that chicken CD1 genes are located adjacent to the previously characterized MHC B system contig on chromosome 16. We confirmed the linkage of CD1 to the B system by dual-color fluorescence in situ hybridization. Chickens have a single copy of PROCR. Among known class-I-like genes, PROCR is most closely related to CD1, indicating that CD1 and PROCR constitute a distinct subfamily of class-I-like genes that predates the emergence of mammals.

Features of MHC and NK gene clusters

When comparing the immune genome to the genome in general, a higher prevalence for association with disease is the only genetic feature significant in immune genes as a group. However, some genetic features, such as marked levels of polymorphism and gene duplication, are present in subsets of immune genes, namely the Major Histocompatibility Complex (MHC) and Natural Killer (NK) cell receptor gene complexes. In this review, we discuss features of MHC and NK receptor gene clusters, their epistatic interactions, and the impact of both on association to disease.

Hagfish leukocytes express a paired receptor family with a variable domain resembling those of antigen receptors

Jawed vertebrates are equipped with TCR and BCR with the capacity to rearrange their V domains. By contrast, jawless vertebrates, represented by hagfish and lampreys, apparently lack such receptors. We describe in this study a family of hagfish genes carrying a single V-type domain resembling those of TCR/BCR. This multigene family, which we call agnathan paired receptors resembling Ag receptors (APAR), is expressed in leukocytes and predicted to encode a group of membrane glycoproteins with organizations characteristic of paired Ig-like receptors, consisting of activating and inhibitory forms. APAR has a J region in its V-type domain, and its V and J regions are encoded in a single exon. Thus, APAR is a member of the emerging families of diversified, innate immune-type receptors with TCR/BCR-like V-type domains and has many of the features expected for a primordial TCR/BCR-like receptor. The extracellular domain of APAR may be descended from a V-type domain postulated to have acquired recombination signal sequences in a jawed vertebrate lineage.

Comparative genomics of major histocompatibility complexes

The major histocompatibility complex (MHC) is a gene dense region found in all jawed vertebrates examined to date. The MHC contains a high percentage of immune genes, in particular genes involved in antigen presentation, which are generally highly polymorphic. The region plays an important role in disease resistance. The clustering of MHC genes could be advantageous for co-evolution or regulation, and its study in many species is desirable. Even though some linkage of MHC genes is apparent in all gnathostomes, the genomic organization can differ greatly by species, suggesting rapid evolution of MHC genes after divergence from a common ancestor. Previous reviews of comparative MHC organization have been written when relatively fragmentary sequence and mapping data were available on many species. This review compares maps of MHC gene orders in commonly studied species, where extensive sequencing has been performed.

Activation of natural killer cells: underlying molecular mechanisms revealed

Natural killer (NK) cells, the third major lymphocyte population, are important effector cells against certain infections and tumours. They have also been implicated as a link between innate and adaptive immune responses. In recent years, much attention has been paid to the NK cell inhibitory receptors and their interaction with major histocompatibility complex class I molecules on target cells. This review summarizes recent findings on regulation of NK cell activity with an emphasis on NK cell stimulatory receptors. A particular emphasis is devoted to the receptor NKG2D that is expressed on all NK cells.

Natural killer cell receptors in the horse: evidence for the existence of multiple transcribed LY49 genes

In rodents, the Ly49 family encodes natural killer (NK) receptors interacting with classical MHC class I molecules, whereas the corresponding receptors in primates are members of the killer cell immunoglobulin-like receptor (KIR) family. Recent evidence indicates that the cattle, domestic cat, dog, and pig have a single LY49 and multiple KIR genes, suggesting that predominant NK receptors in most non-rodent mammals might be KIR. Here, we show that the horse has at least six LY49 genes, five with an immunoreceptor tyrosine-based inhibition motif (ITIM) and one with arginine in the transmembrane region. Interestingly, none of the horse KIR-like cDNA clones isolated by library screening encoded molecules likely to function asNK receptors; four types of clones were KIR-Ig-like transcript (KIR-ILT) hybrids and contained premature stop codons and/or frameshift mutations, and two putative allelic sequences predicting KIR3DL molecules had mutated ITIM. To our knowledge, this is the first report suggesting that non-rodent mammals may use LY49 as NK receptors for classical MHC class I. We also show that horse spleen expresses ILT-like genes with unique domain organizations. Radiation hybrid mapping and fluorescence in situ hybridization localized horse LY49 and KIR/ILT genes to chromosomes 6q13 and 10p12, respectively.

Thermodynamic analysis of degenerate recognition by the NKG2D immunoreceptor: not induced fit but rigid adaptation

The homodimeric immunoreceptor NKG2D drives the activation of effector cells following engagement of diverse, conditionally expressed MHC class I-like protein ligands. NKG2D recognition is highly degenerate in that a single surface on receptor monomers binds pairs of distinct surfaces on each structurally divergent ligand, simultaneously accommodating multiple nonconservative ligand allelic or isoform substitutions. In contrast to TCR-pMHC and other NK receptor-ligand interactions, thermodynamic and kinetic analyses of four NKG2D-ligand pairs (MIC-A*001, MIC-B*005, ULBP1, and RAE-1beta) reported here show that the relative enthalpic and entropic terms, heat capacity, association rates, and activation energy barriers are comparable to typical, rigid protein-protein interactions. Rather than "induced-fit" binding, NKG2D degeneracy is achieved using distinct interaction mechanisms at each rigid interface.

Roles of the NKG2D immunoreceptor and its ligands

According to present concepts, innate immunity is regulated by receptors that determine danger levels by responding to molecules that are associated with infection or cellular distress. NKG2D is, perhaps, the best characterized receptor that is associated with responses to cellular distress, defined as transformation, infection or cell stress. This review summarizes recent findings that concern NKG2D, its ligands, its signalling properties and its role in disease, and provides a framework for considering how the induction of immune responses can be regulated by cellular responses to injury.

Genomic organization of the mammalian MHC

The Human Genome Project transformed the quest of more than 50 years to understand the major histocompatibility complex (Mhc). The sequence of the Mhc from human and mouse, together with a large amount of sequence and mapping information from several other species, allows us to draw general conclusions about the organization and origin of this crucial part of the immune system. The Mhc is a mosaic of stretches formed by conserved and nonconserved genes. Surprisingly, of the approximately 3.6-Mb Mhc, the stretches that encode the class I and class II genes, which epitomize the Mhc, are the least conserved part, whereas the approximately 1.7-Mb stretches that encode at least 115 other genes are highly conserved. We summarize the available data to answer the questions (a) What is the Mhc? and (b) How can we define it in a general, not species-specific, way? Knowing what is essential and what is incidental helps us understand the fundamentals of the Mhc, and defining the species differences makes the model organisms more useful.

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.

In vivo immunogenetics: from MIC to RAET1 loci

The major histocompatibility complex (MHC) comprises approximately one thousandth of the genome and encompasses its most polymorphic members. This diversity enables the MHC, at the population level, to counteract the extraordinarily diverse microbiological threats. Reviewed here are two separate sets of MHC class I genes: MIC and RAET1. Whilst the former are encoded within the MHC (6p21.3), the latter are located on the opposite arm of the same chromosome (6q24.2-q25.3). Differing from the prototypical class I genes in structure, transcription, diversity and potential function, they both exemplify the versatility of the MHC fold, despite convergence onto a single ligand, the activatory C-type lectin-like receptor, NKG2D. Why the immune system uses two distinct gene families to interact with a unique ligand remains a fascinating question. To answer this question, the reader will be chronologically exposed to the field whilst following a single thread, i.e. genomics and gene diversity.