Under selection pressure from pathogens, variable NK cell receptors that recognize polymorphic MHC class I, evolved convergently in different species of placental mammal. Unexpectedly, the killer cell immunoglobulin-like receptors (KIR) are shared by simian primates, including humans, and cattle but not by other species. Whereas much is known of human KIR genetics and genomics, knowledge of cattle KIR is limited to nine cDNA sequences. To facilitate comparison of the cattle and human KIR gene families, we determined the genomic location, structure and sequence of two cattle KIR haplotypes and defined KIR sequences of aurochs, the extinct wild ancestor of domestic cattle. Larger than its human counterpart, the cattle KIR locus evolved through successive duplications of a block containing ancestral KIR3DL and KIR3DX genes that existed before placental mammals. Comparison of two cattle KIR haplotypes and aurochs KIR show the KIR are polymorphic and the gene organization and content appears conserved. Of 18 genes 8 are functional and 10 were inactivated by point mutation. Selective inactivation of KIR3DL and activating receptor genes, leaves a functional cohort of one inhibitory KIR3DL, one activating KIR3DX and six inhibitory KIR3DX. Functional KIR diversity evolved from KIR3DX in cattle and from KIR3DL in simian primates. Although independently evolved, cattle and human KIR gene families share important function-related key properties, indicating that cattle KIR are NK cell receptors for cattle MHC class I. Combinations of KIR and MHC class I are the major genetic factors associated with human disease and merits investigation in cattle.
Under selection pressure from pathogens, variable NK cell receptors that recognize polymorphic MHC class I evolved convergently in different species of placental mammal. Unexpectedly, diversified killer cell Ig–like receptors (KIRs) are shared by simian primates, including humans, and cattle, but not by other species. Whereas much is known of human KIR genetics and genomics, knowledge of cattle KIR is limited to nine cDNA sequences. To facilitate comparison of the cattle and human KIR gene families, we determined the genomic location, structure, and sequence of two cattle KIR haplotypes and defined KIR sequences of aurochs, the extinct wild ancestor of domestic cattle. Larger than its human counterpart, the cattle KIR locus evolved through successive duplications of a block containing ancestral KIR3DL and KIR3DX genes that existed before placental mammals. Comparison of two cattle KIR haplotypes and aurochs KIR show the KIR are polymorphic and the gene organization and content appear conserved. Of 18 genes, 8 are functional and 10 were inactivated by point mutation. Selective inactivation of KIR3DL and activating receptor genes leaves a functional cohort of one inhibitory KIR3DL, one activating KIR3DX, and six inhibitory KIR3DX. Functional KIR diversity evolved from KIR3DX in cattle and from KIR3DL in simian primates. Although independently evolved, cattle and human KIR gene families share important function-related properties, indicating that cattle KIR are NK cell receptors for cattle MHC class I. Combinations of KIR and MHC class I are the major genetic factors associated with human disease and merit investigation in cattle.
Natural Killer (NK) cells play critical roles in immune defense and reproduction, yet remain the most poorly understood major lymphocyte population. Because their activation is controlled by a variety of combinatorially expressed activating and inhibitory receptors, NK cell diversity and function are closely linked. To provide an unprecedented understanding of NK cell repertoire diversity, we used mass cytometry to simultaneously analyze 35 parameters, including 28 NK cell receptors, on peripheral blood NK cells from five sets of monozygotic twins and twelve unrelated donors of defined HLA and killer cell immunoglobulin-like receptor (KIR) genotype. This analysis revealed a remarkable degree of NK cell diversity, with an estimated 6,000-30,000 phenotypic populations within an individual and >100,000 phenotypes in this population. Genetics largely determined inhibitory receptor expression, whereas activation receptor expression was heavily environmentally influenced. Therefore, NK cells may maintain self-tolerance through strictly regulated expression of inhibitory receptors, while using adaptable expression patterns of activating and costimulatory receptors to respond to pathogens and tumors. These findings further suggest the possibility that discrete NK cell subpopulations could be harnessed for immunotherapeutic strategies in the settings of infection, reproduction, and transplantation.
Allotypes of the natural killer (NK) cell receptor KIR3DL1 vary in both NK cell expression patterns and inhibitory capacity upon binding to their ligands, HLA-B Bw4 molecules, present on target cells. Using a sample size of over 1,500 human immunodeficiency virus (HIV)+ individuals, we show that various distinct allelic combinations of the KIR3DL1 and HLA-B loci significantly and strongly influence both AIDS progression and plasma HIV RNA abundance in a consistent manner. These genetic data correlate very well with previously defined functional differences that distinguish KIR3DL1 allotypes. The various epistatic effects observed here for common, distinct KIR3DL1 and HLA-B Bw4 combinations are unprecedented with regard to any pair of genetic loci in human disease, and indicate that NK cells may have a critical role in the natural history of HIV infection.
Targeted capture of genomic regions reduces sequencing cost while generating
higher coverage by allowing biomedical researchers to focus on specific loci
of interest, such as exons. Targeted capture also has the potential to
facilitate the generation of genomic data from DNA collected via saliva or
buccal cells. DNA samples derived from these cell types tend to have a lower
human DNA yield, may be degraded from age and/or have contamination from
bacteria or other ambient oral microbiota. However, thousands of samples
have been previously collected from these cell types, and saliva collection
has the advantage that it is a non-invasive and appropriate for a wide
variety of research.
We demonstrate successful enrichment and sequencing of 15 South African
KhoeSan exomes and 2 full genomes with samples initially derived from
saliva. The expanded exome dataset enables us to characterize genetic
diversity free from ascertainment bias for multiple KhoeSan populations,
including new exome data from six HGDP Namibian San, revealing substantial
population structure across the Kalahari Desert region. Additionally, we
discover and independently verify thirty-one previously unknown KIR
alleles using methods we developed to accurately map and call the highly
polymorphic HLA and KIR loci from exome capture data.
Finally, we show that exome capture of saliva-derived DNA yields sufficient
non-human sequences to characterize oral microbial communities, including
detection of bacteria linked to oral disease (e.g. Prevotella
melaninogenica). For comparison, two samples were sequenced using
standard full genome library preparation without exome capture and we found
no systematic bias of metagenomic information between exome-captured and
DNA from human saliva samples, collected and extracted using standard
procedures, can be used to successfully sequence high quality human exomes,
and metagenomic data can be derived from non-human reads. We find that
individuals from the Kalahari carry a higher oral pathogenic microbial load
than samples surveyed in the Human Microbiome Project. Additionally, rare
variants present in the exomes suggest strong population structure across
different KhoeSan populations.
Exomes; KhoeSan; Genetic diversity; Metagenomics; Microbiome
Monoclonal antibodies with specificity for HLA class I determinants of HLA were originally characterized using serological assays in which the targets were cells expressing 3-6 HLA class I variants. Because of this complexity, the specificities of the antibodies were defined indirectly by correlation. Here we use a direct binding assay, in which the targets are synthetic beads coated with one of 111 HLA class I variants, representing the full range of HLA-A, -B and -C variation. We studied one monoclonal antibody with monomorphic specificity (W6/32) and four with polymorphic specificity (MA2.1, PA2.1, BB7.2 and BB7.1) and compared the results with those obtained previously. W6/32 reacted with all HLA class I variants. MA2.1 exhibits high specificity for HLA-A*02, -B*57 and -B*58, but also exhibited cross-reactivity with HLA-A*11 and -B*15:16. At low concentration (1μg/ml) PA2.1 and BB7.2 were both specific for HLA-A*02 and -A*69, and at high concentration (50μg/ml) exhibited significant cross-reactions with HLA-A*68, -A*23, and -A*24. BB7.1 exhibits specificity for HLA-B*07 and -B*42, as previously described, but reacts equally well with HLA-B*81, a rare allotype defined some 16 years after the description of BB7.1. The results obtained with cell-based and bead-based assays are consistent and, in combination with amino acid sequence comparison, increase understanding of the polymorphic epitopes recognized by the MA2.1, PA2.1, BB7.2 and BB7.1 antibodies. Comparison of two overlapping but distinctive bead sets from two sources gave similar results, but the overall levels of binding were significantly different. Several weaker reactions were observed with only one of the bead sets.
HLA class I; monoclonal antibodies; epitope; polymorphism
Natural killer (NK) cells have roles in immunity and reproduction that are controlled by variable receptors that recognize MHC class I molecules. The variable NK cell receptors found in humans are specific to simian primates, where they have progressively co-evolved with MHC class I molecules. The emergence of MHC-C in hominids drove the evolution of a system of MHC-C receptors that is most elaborate in chimpanzees. In contrast, the human system appears to have been subject to different and competing selection pressures that have acted on its immunological and reproductive functions. We suggest that this compromise facilitated development of the bigger brains that enabled archaic and modern humans to migrate out-of-Africa and populate other continents.
Pinnipeds, marine carnivores, diverged from terrestrial carnivores ~45 million years ago, before their adaptation to marine environments. This lifestyle change exposed pinnipeds to different microbiota and pathogens, with probable impact on their MHC class I genes. Investigating this question, genomic sequences were determined for 71 MHC class I variants: 27 from harbor seal and 44 from gray seal. These variants form three MHC class I gene lineages, one comprising a pseudogene. The second, a candidate nonclassical MHC class I gene, comprises a nonpolymorphic transcribed gene related to dog DLA-79 and giant panda Aime-1906. The third is the diversity lineage, which includes 62 of the 71 seal MHC class I variants. All are transcribed, and they minimally represent six harbor and 12 gray seal MHC class I genes. Besides species-specific differences in gene number, seal MHC class I haplotypes exhibit gene content variation and allelic polymorphism. Patterns of sequence variation, and of positions for positively selected sites, indicate the diversity lineage genes are the seals’ classical MHC class I genes. Evidence that expansion of diversity lineage genes began before gray and harbor seals diverged is the presence in both species of two distinctive sublineages of diversity lineage genes. Pointing to further expansion following the divergence are the presence of species-specific genes and greater MHC class I diversity in gray seals than harbor seals. The elaboration of a complex variable family of classical MHC class I genes in pinnipeds contrasts with the single, highly polymorphic classical MHC class I gene of dog and giant panda, terrestrial carnivores.
Pinniped; MHCclass I; Evolution; Polymorphism
The Caribbean basin is home to some of the most complex interactions in recent history among previously diverged human populations. Here, we investigate the population genetic history of this region by characterizing patterns of genome-wide variation among 330 individuals from three of the Greater Antilles (Cuba, Puerto Rico, Hispaniola), two mainland (Honduras, Colombia), and three Native South American (Yukpa, Bari, and Warao) populations. We combine these data with a unique database of genomic variation in over 3,000 individuals from diverse European, African, and Native American populations. We use local ancestry inference and tract length distributions to test different demographic scenarios for the pre- and post-colonial history of the region. We develop a novel ancestry-specific PCA (ASPCA) method to reconstruct the sub-continental origin of Native American, European, and African haplotypes from admixed genomes. We find that the most likely source of the indigenous ancestry in Caribbean islanders is a Native South American component shared among inland Amazonian tribes, Central America, and the Yucatan peninsula, suggesting extensive gene flow across the Caribbean in pre-Columbian times. We find evidence of two pulses of African migration. The first pulse—which today is reflected by shorter, older ancestry tracts—consists of a genetic component more similar to coastal West African regions involved in early stages of the trans-Atlantic slave trade. The second pulse—reflected by longer, younger tracts—is more similar to present-day West-Central African populations, supporting historical records of later transatlantic deportation. Surprisingly, we also identify a Latino-specific European component that has significantly diverged from its parental Iberian source populations, presumably as a result of small European founder population size. We demonstrate that the ancestral components in admixed genomes can be traced back to distinct sub-continental source populations with far greater resolution than previously thought, even when limited pre-Columbian Caribbean haplotypes have survived.
Latinos are often regarded as a single heterogeneous group, whose complex variation is not fully appreciated in several social, demographic, and biomedical contexts. By making use of genomic data, we characterize ancestral components of Caribbean populations on a sub-continental level and unveil fine-scale patterns of population structure distinguishing insular from mainland Caribbean populations as well as from other Hispanic/Latino groups. We provide genetic evidence for an inland South American origin of the Native American component in island populations and for extensive pre-Columbian gene flow across the Caribbean basin. The Caribbean-derived European component shows significant differentiation from parental Iberian populations, presumably as a result of founder effects during the colonization of the New World. Based on demographic models, we reconstruct the complex population history of the Caribbean since the onset of continental admixture. We find that insular populations are best modeled as mixtures absorbing two pulses of African migrants, coinciding with the early and maximum activity stages of the transatlantic slave trade. These two pulses appear to have originated in different regions within West Africa, imprinting two distinguishable signatures on present-day Afro-Caribbean genomes and shedding light on the genetic impact of the slave trade in the Caribbean.
Interactions between HLA class I molecules and killer-cell immunoglobulin-like receptors (KIR) control natural killer cell (NK) functions in immunity and reproduction. Encoded by genes on different chromosomes, these polymorphic ligands and receptors correlate highly with disease resistance and susceptibility. Although studied at low-resolution in many populations, high-resolution analysis of combinatorial diversity of HLA class I and KIR is limited to Asian and Amerindian populations with low genetic diversity. At the other end of the spectrum is the West African population investigated here: we studied 235 individuals, including 104 mother-child pairs, from the Ga-Adangbe of Ghana. This population has a rich diversity of 175 KIR variants forming 208 KIR haplotypes, and 81 HLA-A, -B and -C variants forming 190 HLA class I haplotypes. Each individual we studied has a unique compound genotype of HLA class I and KIR, forming 1–14 functional ligand-receptor interactions. Maintaining this exceptionally high polymorphism is balancing selection. The centromeric region of the KIR locus, encoding HLA-C receptors, is highly diverse whereas the telomeric region encoding Bw4-specific KIR3DL1, lacks diversity in Africans. Present in the Ga-Adangbe are high frequencies of Bw4-bearing HLA-B*53:01 and Bw4-lacking HLA-B*35:01, which otherwise are identical. Balancing selection at key residues maintains numerous HLA-B allotypes having and lacking Bw4, and also those of stronger and weaker interaction with LILRB1, a KIR-related receptor. Correspondingly, there is a balance at key residues of KIR3DL1 that modulate its level of cell-surface expression. Thus, capacity to interact with NK cells synergizes with peptide binding diversity to drive HLA-B allele frequency distribution. These features of KIR and HLA are consistent with ongoing co-evolution and selection imposed by a pathogen endemic to West Africa. Because of the prevalence of malaria in the Ga-Adangbe and previous associations of cerebral malaria with HLA-B*53:01 and KIR, Plasmodium falciparum is a candidate pathogen.
Natural killer cells are white blood cells with critical roles in human health that deliver front-line immunity against pathogens and nurture placentation in early pregnancy. Controlling these functions are cell-surface receptors called KIR that interact with HLA class I ligands expressed on most cells of the body. KIR and HLA are both products of complex families of variable genes, but present on separate chromosomes. Many HLA and KIR variants and their combinations associate with resistance to specific infections and pregnancy syndromes. Previously we identified basic components of the system necessary for individual and population survival. Here, we explore the system at its most genetically diverse by studying the Ga-Adangbe population from Ghana in West Africa. Co-evolution of KIR receptors with their HLA targets is ongoing in the Ga-Adangbe, with every one of 235 individuals studied having a unique set of KIR receptors and HLA class I ligands. In addition, one critical combination of receptor and ligand maintains alternative forms that either can or cannot interact with their ‘partner.’ This balance resembles that induced by malfunctioning variants of hemoglobin that confer resistance to malaria, a candidate disease for driving diversity and co-evolution of KIR and HLA class I in the Ga-Adangbe.
IgA is the predominant immunoglobulin isotype in mucosal tissues and external secretions, playing important roles both in defense against pathogens and in maintenance of commensal microbiota. Considering the complexity of its interactions with the surrounding environment, IgA is a likely target for diversifying or positive selection. To investigate this possibility, the action of natural selection on IgA was examined in depth with six different methods: CODEML from the PAML package and the SLAC, FEL, REL, MEME and FUBAR methods implemented in the Datamonkey webserver. In considering just primate IgA, these analyses show that diversifying selection targeted five positions of the Cα1 and Cα2 domains of IgA. Extending the analysis to include other mammals identified 18 positively selected sites: ten in Cα1, five in Cα2 and three in Cα3. All but one of these positions display variation in polarity and charge. Their structural locations suggest they indirectly influence the conformation of sites on IgA that are critical for interaction with host IgA receptors and also with proteins produced by mucosal pathogens that prevent their elimination by IgA-mediated effector mechanisms. Demonstrating the plasticity of IgA in the evolution of different groups of mammals, only two of the eighteen selected positions in all mammals are included in the five selected positions in primates. That IgA residues subject to positive selection impact sites targeted both by host receptors and subversive pathogen ligands highlights the evolutionary arms race playing out between mammals and pathogens, and further emphasizes the importance of IgA in protection against mucosal pathogens.
Through recognition of HLA class I, killer cell immunoglobulin-like receptors (KIR) modulate NK cell functions in human immunity and reproduction. Although a minority of HLA-A and –B allotypes are KIR ligands, HLA-C allotypes dominate this regulation, because they all carry either the C1 epitope recognized by KIR2DL2/3 or the C2 epitope recognized by KIR2DL1. The C1 epitope and C1-specific KIR evolved first, followed several million years later by the C2 epitope and C2-specific KIR. Strong, varying selection pressure on NK cell functions drove the diversification and divergence of hominid KIR, with six positions in the HLA class I binding site of KIR being targets for positive selection. Introducing each naturally occurring residue at these positions into KIR2DL1 and KIR2DL3, produced 38 point mutants that were tested for binding to 95 HLA- A, -B and –C allotypes. Modulating specificity for HLA-C is position 44, whereas positions 71 and 131 control cross reactivity with HLA-A*11:02. Dominating avidity modulation is position 70, with lesser contributions from positions 68 and 182. KIR2DL3 has lower avidity and broader specificity than KIR2DL1. Mutation can increase the avidity and change the specificity of KIR2DL3, whereas KIR2DL1 specificity was resistant to mutation and its avidity could only be lowered. The contrasting inflexibility of KIR2DL1 and adaptability of KIR2DL3 fits with C2-specific KIR having evolved from C1-specific KIR, and not vice versa. Substitutions restricted to activating KIR all reduced the avidity of KIR2DL1 and KIR2DL3, further evidence that activating KIR function often becomes subject to selective attenuation.
Natural Killer Cells; MHC; Epitopes; Comparative Immunology/Evolution
Human killer cell immunoglobulin-like receptors (KIR) recognize A3/11, Bw4, C1 and C2 epitopes carried by mutually exclusive subsets of HLA-A, B, and C allotypes. Chimpanzee and orangutan have counterparts to HLA-A, B, and C, and KIR that recognize the A3/11, Bw4, C1 and C2 epitopes, either individually or in combination. Because rhesus macaque has counterparts of HLA-A and B, but not HLA-C, we expected that rhesus KIR would better recognize HLA-A and B, than HLA-C. Comparison of the interactions of nine rhesus KIR3D with 95 HLA isoforms, showed the KIR have broad specificity for HLA-A, B, and C, but vary in avidity. Considering both the strength and breadth of reaction, HLA-C was the major target for rhesus KIR, followed by HLA-B, then HLA-A. Strong reactions with HLA-A were restricted to the minority of allotypes carrying the Bw4 epitope, whereas strong reactions with HLA-B partitioned between allotypes having and lacking Bw4. Contrasting to HLA-A and B, every HLA-C allotype bound to the nine rhesus KIR. Sequence comparison of high- and low-binding HLA allotypes revealed the importance of polymorphism in the helix of the α1 domain and the peptide-binding pockets. At peptide position 9, nonpolar residues favor binding to rhesus KIR, whereas charged residues do not. Contrary to expectation, rhesus KIR bind more effectively to HLA-C, than to HLA-A and B. This property is consistent with MHC-C having evolved in hominids to be a generally superior ligand for KIR than MHC-A and MHC-B.
KIR receptors; MHC; rhesus macaque; NK cells
Natural killer (NK) cells are a population of lymphocytes that function in both immune defense and reproduction. Diversifying NK cell phenotype and function are interactions between NK cell receptors and major histocompatibility complex (MHC) class I ligands. As a consequence of strong and variable selection these ligand-receptor systems are polymorphic, rapidly evolving, and considerably species-specific. Counterparts to the human system of HLA class I ligands and killer cell immunoglobulin-like receptors (KIR) are present only in apes and Old World monkeys. HLA-C, the dominant ligand for human KIR and the only polymorphic HLA class I expressed by trophoblast, is further restricted to humans and great apes. Even then, the human system appears qualitatively different from that of chimpanzees, in that it has evolved a genetic balance between particular groups of receptors and ligands that favor reproductive success and other groups of receptors and ligands that have been correlated with disordered placentation. Human populations that have survived successive episodes of epidemic disease and population bottlenecks maintain a breadth of diversity for KIR and HLA class I, implying that loss of such diversity disfavors long-term survival of a human population.
HLA; natural killer cells; balancing selection; evolution; immunity; reproduction
Whole genome comparisons identified introgression from archaic to modern humans. Our analysis of highly polymorphic HLA class I, vital immune system components subject to strong balancing selection, shows how modern humans acquired the HLA-B*73 allele in west Asia through admixture with archaic humans called Denisovans, a likely sister group to the Neandertals. Virtual genotyping of Denisovan and Neandertal genomes identified archaic HLA haplotypes carrying functionally distinctive alleles that have introgressed into modern Eurasian and Oceanian populations. These alleles, of which several encode unique or strong ligands for natural killer cell receptors, now represent more than half the HLA alleles of modern Eurasians and also appear to have been later introduced into Africans. Thus, adaptive introgression of archaic alleles has significantly shaped modern human immune systems.
After decades of mouse and human research, we now know that NK cells have unique properties including memory. Although initially described as MHC unrestricted killers, NK cells have several families of receptors that directly recognize MHC including Ly49 receptors in the mouse and killer immunoglobulin-like receptors (KIR) in humans. The strength of this signal is determined by polymorphisms in NK cell inhibitory receptor genes and their MHC ligands inherited on different chromosomes. Inhibitory receptors protect “self” expressing normal tissue from being killed by NK cells and protecting against autoimmunity. Therefore, for NK cells to kill and produce cytokines they must encounter activating receptor ligands in the context of “missing self” that occurs with some viral infections and malignant transformation. The second property of inhibitory receptors is to educate or license NK cells to acquire function. This is best demonstrated in the mouse and in humans by enhanced function on self inhibitory receptor expressing NK cells when in a host expressing cognate ligate. In contrast, NK cells without inhibitory receptors or with non-self inhibitory receptors are relatively hyporesponsive. The basic biology of NK cells in response to cytokines, education, and viruses will translate into strategies to manipulate NK cells for therapeutic purposes.
It is 14 years since the IMGT/HLA database was first released, providing the HLA community with a searchable repository of highly curated HLA sequences. The HLA complex is located within the 6p21.3 region of human chromosome 6 and contains more than 220 genes of diverse function. Of these, 21 genes encode proteins of the immune system that are highly polymorphic. The naming of these HLA genes and alleles and their quality control is the responsibility of the World Health Organization Nomenclature Committee for Factors of the HLA System. Through the work of the HLA Informatics Group and in collaboration with the European Bioinformatics Institute, we are able to provide public access to these data through the website http://www.ebi.ac.uk/imgt/hla/. Regular updates to the website ensure that new and confirmatory sequences are dispersed to the HLA community and the wider research and clinical communities. This article describes the latest updates and additional tools added to the IMGT/HLA project.
Alternative lysine and methionine residues at position 44 in the D1 domain determine the specificities of human lineage III killer cell immunoglobulin-like receptors (KIR) for the C1 and C2 epitopes of HLA-C. KIR having glutamate 44 are also present in orangutans (Popy2DLB) and chimpanzees (Pt-2DL9) but notably absent from humans. Popy2DLB exhibits broad specificity for both the C1 and C2 epitopes, whereas Pt-2DL9 has narrow specificity for C2. Mutation of phenylalanine 45 in Popy2DLB to the cysteine residue present in Pt-2DL9 was sufficient to narrow the Popy2DLB specificity to be like that of Pt-2DL9. In contrast, replacement of cysteine 45 in Pt-2DL9 by phenylalanine had no effect on its C2 specificity, but reduced the avidity. In a similar manner, replacement of phenylalanine 45 with cysteine in Popy2DLA, which has lysine 44 and recognizes C1, maintained this specificity while reducing avidity. Position 45 is exceptionally variable, exhibiting twelve residues that distinguish KIR of different lineages and species. Our study demonstrates the potential for variation at position 45 to modulate KIR avidity and specificity for HLA-C. The various effects of position 45 mutation are consistent with a model in which a Popy2DLB-like receptor, having glutamate 44 and broad specificity for C1 and C2, facilitated the evolution of the C2 epitope from the C1 epitope and C2-specific KIR from C1-specific KIR. With the acquisition of C2 and C2-specific receptors, the selection against this broadly specific receptor led to its loss from the human line and narrowing of its specificity on the chimpanzee line.
KIR receptors; MHC; Non-human primates; Evolution
Variegated expression of variable NK cell receptors for polymorphic MHC class I broadens the range of an individual’s NK cell response, and the capacity for populations and species to survive disease epidemics and population bottlenecks. On evolutionary time-scales this component of immunity is exceptionally dynamic, unstable and short-lived, being dependent upon co-evolution of ligands and receptors subject to varying, competing selection pressures. Consequently these systems of variable NK cell receptors are largely species-specific and have recruited different classes of glycoprotein, even within the primate order of mammals. Such disparity helps explain substantial differences in NK cell biology between humans and animal models, for which the population genetics is largely ignored. KIR3DL1/S1, that recognizes the Bw4 epitope of HLA-A and –B and is the most extensively studied of the variable NK cell receptors, exemplifies how variation in all possible parameters of function is recruited to diversify the human NK cell response.
Natural Killer Cells; MHC; Comparative Immunology/Evolution; Antigens/Peptides/Epitopes
In placental mammals, natural killer (NK) cells are a population of lymphocytes that make unique contributions to immune defence and reproduction, functions essential for survival of individuals, populations and species. Modulating these functions are conserved and variable NK-cell receptors that recognize epitopes of major histocompatibility complex (MHC) class I molecules. In humans, for example, recognition of human leucocyte antigen (HLA)-E by the CD94:NKG2A receptor is conserved, whereas recognition of HLA-A, B and C by the killer cell immunoglobulin-like receptors (KIRs) is diversified. Competing demands of the immune and reproductive systems, and of T-cell and NK-cell immunity—combined with the segregation on different chromosomes of variable NK-cell receptors and their MHC class I ligands—drive an unusually rapid evolution that has resulted in unprecedented levels of species specificity, as first appreciated from comparison of mice and humans. Counterparts to human KIR are present only in simian primates. Observed in these species is the coevolution of KIR and the four MHC class I epitopes to which human KIR recognition is restricted. Unique to hominids is the emergence of the MHC-C locus as a supplier of specialized and superior ligands for KIR. This evolutionary trend is most highly elaborated in the chimpanzee. Unique to the human KIR locus are two groups of KIR haplotypes that are present in all human populations and subject to balancing selection. Group A KIR haplotypes resemble chimpanzee KIR haplotypes and are enriched for genes encoding KIR that bind HLA class I, whereas group B KIR haplotypes are enriched for genes encoding receptors with diminished capacity to bind HLA class I. Correlating with their balance in human populations, B haplotypes favour reproductive success, whereas A haplotypes favour successful immune defence. Evolution of the B KIR haplotypes is thus unique to the human species.
natural killer cells; major histocompatibility complex; balancing selection
Cerebral malaria is a major, life-threatening complication of Plasmodium falciparum malaria, and has very high mortality rate. In murine malaria models, natural killer (NK) cell responses have been shown to play a crucial role in the pathogenesis of cerebral malaria. To investigate the role of NK cells in the developmental process of human cerebral malaria, we conducted a case-control study examining genotypes for killer immunoglobulin-like receptors (KIR) and their human leukocyte antigen (HLA) class I ligands in 477 malaria patients. We found that the combination of KIR2DL3 and its cognate HLA-C1 ligand was significantly associated with the development of cerebral malaria when compared with non-cerebral malaria (odds ratio 3.14, 95% confidence interval 1.52–6.48, P = 0.00079, corrected P = 0.02). In contrast, no other KIR-HLA pairs showed a significant association with cerebral malaria, suggesting that the NK cell repertoire shaped by the KIR2DL3-HLA-C1 interaction shows certain functional responses that facilitate development of cerebral malaria. Furthermore, the frequency of the KIR2DL3-HLA-C1 combination was found to be significantly lower in malaria high-endemic populations. These results suggest that natural selection has reduced the frequency of the KIR2DL3-HLA-C1 combination in malaria high-endemic populations because of the propensity of interaction between KIR2DL3 and C1 to favor development of cerebral malaria. Our findings provide one possible explanation for KIR-HLA co-evolution driven by a microbial pathogen, and its effect on the global distribution of malaria, KIR and HLA.
NK cells play an important role in early defense against pathogens. Killer immunoglobulin-like receptors (KIR) are a diverse family of activating and inhibitory receptors expressed on human NK cells. Some inhibitory KIRs recognize human leukocyte antigen (HLA) class I molecules as their ligands. The KIR loci exhibit presence or absence polymorphism, and thus, some individuals lack particular KIR-HLA receptor-ligand pairs, which affects their NK cell responses. We herein show that presence of both KIR2DL3 and its cognate HLA-C1 ligand in malaria patients was strongly associated with the development of human cerebral malaria. This result suggests that NK cells from the patients carrying both KIR2DL3 and HLA-C1 exhibit functional responses that facilitate development of cerebral malaria. In addition, the gene frequency of the KIR2DL3 and HLA-C1 combination was found to be significantly lower in populations with high-endemic malaria. These observations suggest that the combination of KIR2DL3 and HLA-C1 has decreased in malaria high-endemic populations under selection from cerebral malaria, a major life-threatening complication of Plasmodium falciparum malaria.
Patr-AL is an expressed, non-polymorphic MHC class I gene carried by ∼50% of chimpanzee MHC haplotypes. Comparing Patr-AL+ and Patr-AL- haplotypes showed Patr-AL defines a unique 125kb genomic block flanked by blocks containing classical Patr-A and pseudogene Patr-H. Orthologous to Patr-AL are polymorphic orangutan Popy-A and the 5′ part of human pseudogene HLA-Y, carried by ∼10% of HLA haplotypes. Thus the AL gene alternatively evolved in these closely related species to become classical, non-classical and non-functional. Although differing by 30 amino acid substitutions in the peptide-binding α1 and α2 domains, Patr-AL and HLA-A*0201 bind overlapping repertoires of peptides; the overlap being comparable to that between the A*0201 and A*0207 subtypes differing by one substitution. Patr-AL thus has the A02 supertypic peptide-binding specificity. Patr-AL and HLA-A*0201 have similar three-dimensional structures, binding peptides in similar conformation. Although comparable in size and shape, the B and F specificity pockets of Patr-AL and HLA-A*0201 differ in both their constituent residues and contacts with peptide anchors. Uniquely shared by Patr-AL, HLA-A*0201, and other members of the A02 supertype are the absence of serine at position 9 in the B pocket and the presence of tyrosine at position 116 in the F pocket. Distinguishing Patr-AL from HLA-A*02 is an unusually electropositive upper face on the α2 helix. Stimulating PBMC from Patr-AL- chimpanzees with B cells expressing Patr-AL, produced potent alloreactive CD8 T cells with specificity for Patr-AL and no crossreactivity toward other MHC class I, including HLA-A*02. PBMC from Patr-AL+ chimpanzees are tolerant of Patr-AL.
Variable interaction between the Bw4 epitope of HLA-B and the polymorphic KIR3DL1/S1 system of inhibitory and activating NK cell receptors diversifies the development, repertoire formation and response of human NK cells. KIR3DL1*004, a common KIR3DL1 allotype, in combination with Bw4+, HLA-B slows progression of HIV infection to AIDS. Analysis here of KIR3DL1*004 membrane traffic in NK cells shows this allotype is largely misfolded but stably retained in the endoplasmic reticulum, where it binds to the chaperone calreticulin and does not induce the unfolded protein response. A small fraction of KIR3DL1*004 folds correctly and leaves the endoplasmic reticulum to be expressed on the surface of primary NK and transfected NKL cells, in a form that can be triggered to inhibit NK cell activation and secretion of interferon-γ. Consistent with this small proportion of correctly-folded molecules, trace amounts of MHC Class I co-immunoprecipitated with KIR3DL1*004. There was no indication of any extensive intracellular interaction between unfolded KIR3DL1*004 and cognate Bw4+ HLA-B. A similarly limited interaction of Bw4 with KIR3DL1*002, when both were expressed by the same cell, was observed despite the efficient folding of KIR3DL1*002 and its abundance on the NK cell surface. Several positions of polymorphism modulate KIR3DL1 abundance at the cell surface, differences that do not necessarily correlate with the potency of allotype function. In this context our results suggest the possibility that the effect of Bw4+ HLA-B and KIR3DL1*004 in slowing progression to AIDS is mediated by interaction of Bw4+ HLA-B with the small fraction of cell surface KIR3DL1*004.
Interactions between killer immunoglobulin-like receptors (KIRs) and their HLA-A, -B, and -C ligands diversify the functions of human natural killer cells. Consequently, combinations of KIR and HLA genotypes affect resistance to infection and autoimmunity, success of reproduction and outcome of hematopoietic cell transplantation. HLA-C, with its C1 and C2 epitopes, evolved in hominids to be specialized KIR ligands. The system’s foundation was the C1 epitope, with C2 a later addition, by several million years. The human inhibitory receptor for C1 is encoded by KIR2DL2/3, a gene having two divergent allelic lineages: KIR2DL2 is a B KIR haplotype component and KIR2DL3 an A KIR haplotype component. Although KIR2DL2 and KIR2DL3 exhibit quantitative differences in specificity and avidity for HLA-C, they qualitatively differ in their genetics, functional effect, and clinical influence. This is due to linkage disequilibrium between KIR2DL2 and KIR2DS2, a closely related activating receptor that was selected for lost recognition of HLA-C.
killer cells; natural; killer cell immunoglobulin-like receptor; receptor–ligand interaction; disease association; structure–function relationship
In primates and cattle two ancient killer-cell immunoglobulin-like receptor (KIR) lineages independently evolved to become diverse NK cell receptors. In mice, KIR genes were sidelined to the X chromosome, a possible consequence of pathogen-mediated selection on the receptor for IgA-Fc. In humans, KIR uniquely form two omnipresent haplotype groups (A and B), postulated to play complementary and necessary roles in immune defense and reproduction. The basis of KIR3DL1/S1 polymorphism is three ancient lineages maintained by long-term balancing selection and present in all human populations. Conserved and variable NK cell receptors produce structurally diverse NK cell receptor repertoires within a defined range of missing-self response.