Biology of Reproduction 67, 1367-1374 (2002)
© 2002 Society for the Study of Reproduction, Inc.
Do Nonhuman Primates Comprise Appropriate Experimental Models for Studying the Function of Human Leukocyte Antigen-G?1
Daudi K. Langata,b, and
Joan S. Hunt2,b,c
a Institute of Primate Research, Karen, Nairobi, Kenya
b Department of Anatomy and Cell Biology
c Department of Pathology and Laboratory Medicine, University of Kansas Medical Center, Kansas City, Kansas 66160-7400
 |
ABSTRACT
|
|---|
The expression and function of the human major histocompatibility complex (MHC) class Ia genes, human leukocyte antigen (HLA)-A, -B, and -C, is well-established; they are expressed in most nucleated cells and present endogenous peptides to CD8+ T cells. However, MHC class Ib genes are poorly characterized and have unknown functions. In humans, the best-characterized class Ib gene is HLA-G. This gene has a restricted tissue expression of the mRNA and a unique pattern of protein expression; it is expressed mainly in the extravillous cytotrophoblast cells in the placenta. The function of HLA-G is not clear, but its presence at the maternal-fetal interface suggests a role in protection of the semiallogeneic fetus. Whereas functional studies using in vitro models and transgenic mice provide useful insights regarding the potential function of this molecule, in vivo studies cannot be performed in humans. Nonhuman primates that are closely related to humans phylogenetically contain homologues of HLA-G. The MHC-G loci in nonhuman primates appear to have diverged from the human HLA-G. However, in the rhesus monkey (Macaca mulatta) and olive baboon (Papio anubis), a novel class Ia-related locus has been described. This gene encodes glycoproteins with characteristics that resemble those of HLA-G, including restricted tissue distribution, alternative splicing of mRNA, truncated cytoplasmic domain, and limited polymorphism. Thus, this molecule may be the functional homologue of HLA-G, and these two species may comprise appropriate models for elucidating the function of HLA-G.
immunology, placenta, pregnancy, syncytiotrophoblast, trophoblast
|
INTRODUCTION
|
|---|
The major histocompatibility complex (MHC) genes encode highly polymorphic glycoproteins that play a central role in immune recognition and regulation. This group of genes is best studied in humans and mice, where they have been divided into three classes: class I, class II, and class III [1]. Each class contains genes that encode proteins with immune and nonimmune functions. Class I genes are further subdivided into class Ia and class Ib. Class Ia genes have ubiquitous tissue distribution and are expressed in nearly all nucleated cells. A few tissues express low levels of class Ia genes, whereas some, including the syncytiotrophoblast and villous cytotrophoblast, do not express the class Ia genes, human leukocyte antigen (HLA)-A and HLA-B [2]. Class Ib genes have a more limited tissue distribution and unknown functions [3–7]. Recently, considerable attention has been focused on the class Ib genes because of their potential role in modulating immune functions in pregnancy. In this review, we examine the tissue expression and potential function of one of the class Ib genes, HLA-G, in humans and its homologues in nonhuman primates to identify models that could be used for functional studies of this molecule.
|
CLASS Ib GENE EXPRESSION IN HUMANS
|
|---|
In humans, the class Ib locus contains three expressed genes, HLA-E, HLA-F, and HLA-G [8]. Transfection experiments showed that HLA-E encoded a 41-kDa intracellular protein, HLA-F encoded a 40-kDa intracellular protein, and HLA-G encoded a 39-kDa, surface-expressed protein [9, 10]. Sequence analysis of the proteins encoded by these genes suggests they have a structure similar to that of class Ia MHC gene products, with the heavy chain associated with ß2-microglobulin [10, 11]. HLA-E is transcribed in virtually all tissues and cells that have been analyzed so far [12], but the HLA-E protein does not generally reach the cell surface unless it binds a nonamer peptide derived from the leader peptides of other class I molecules [13]. HLA-F mRNA and protein have been detected mainly in cells of the B-cell lineage, such as peripheral blood B cells and B-cell lines, and in tissues containing B cells, especially the adult tonsil and fetal liver [14]. HLA-G has generated considerable interest because of its unique properties, which include limited polymorphism, generation of alternatively spliced transcripts, restricted tissue distribution, and a truncated cytoplasmic domain in the glycoprotein [4–6]. This gene and its protein products are the subject of the current review.
|
HUMAN LEUKOCYTE ANTIGEN-G
|
|---|
Biochemical Structure
The HLA-G primary transcript is alternatively spliced into multiple transcripts in various human tissues (Fig. 1). The current tally is seven transcripts, including HLA-G1, which contains all eight exons that are typical of a class I MHC molecule. HLA-G2 excludes exon 3, whereas HLA-G3 does not contain exons 3 and 4 and HLA-G4 does not contain exon 4 [15, 16]. Two messages encoding soluble variants have also been described [17]. These transcripts, designated sHLA-G1 and sHLA-G2 (also called HLA-G5 and HLA-G6, respectively), contain a unique, 21-amino-acid peptide at the carboxyl terminus and are generated by retention of intron 4. A stop codon in codon 22 of this intron prevents read-through into the transmembrane-encoding exon 5. Recent reports also describe a new splice variant, HLA-G7, which has an open reading frame that extends into intron 2 and which may be translated into a soluble protein bearing the 
1 domain and two amino acids encoded by intron 2 [18].
View larger version (45K):
[in this window]
[in a new window]
|
FIG. 1. Comparison of the alternatively spliced transcripts derived from the HLA-G, Paan-AG, and Mamu-AG genes. a) The exon-intron organization of the HLA-G gene (HLA-6.0, acc. no. J03027 [9]) and the HLA-G alternatively spliced transcripts that have been described so far in humans. b and c) The alternatively spliced transcripts of Paan-AG gene (b) and Mamu-AG gene (c), which were isolated from the baboon and rhesus monkey placentas, respectively. The black boxes represent the exons that are translated to proteins, whereas the white boxes represent the exons that are not translated because of the presence of a premature stop codon (*). The boxes with dashed borders represent the exons that are spliced out in each cDNA. E1–E8, Exons 1 to 8; i1–i7, introns 1 to 8; 3'UT, 3'-untranslated region
|
|
The structure of each of these HLA-G protein isoforms has not been determined experimentally but may be predicted against known structures of HLA molecules. The full-length transcript, HLA-G1, contains all the amino acids that are important in formation of the tertiary structure of MHC class I molecules. These include the cysteine residues in the external domains of MHC class I, which form disulfide bridges that stabilize the tertiary structure of the molecule [9, 19]. In addition, the amino acid residues that contact ß2-microglobulin [20] and those that are important in peptide presentation, T-cell receptor, and CD8 coreceptor binding [19] are conserved [9]. HLA-G1 protein cross-reacts with W6/32, a monoclonal antibody that recognizes MHC class I heavy chain only in association with ß2-microglobulin [21]. The 1, 2, and 3 domains of soluble HLA-G1 are identical with those of the membrane-bound isoforms; thus, they may form similar tertiary structures.
The shorter isoforms of HLA-G are missing one or two domains as a result of alternative splicing. Hence, they cannot form structures similar to HLA-G1, because splicing eliminates the cysteine residues that are important in the formation of the tertiary structure in class I MHC molecules [19]. Some studies have suggested that HLA-G2 protein is present on the surfaces of transfected cells as a truncated class I molecule associated with ß2-microglobulin [22]. However, our own studies have shown that only the full-length HLA-G1 associates with ß2-microglobulin; HLA-G2 does not (Morales et al., personal communication). This is supported by the fact that of the 19 amino acids that make contact with ß2-microglobulin, four are located in the 2 domain [20], which is missing in HLA-G2 [15]. HLA-G2 proteins (both the soluble and membrane-bound isoforms) contain two external domains that strikingly resemble the two external domains of class II proteins, and it has been suggested that they may form homodimers or heterodimers that may act as surrogate class II molecules in cytotrophoblasts [15, 23]. As yet, however, the structures of these isoforms have not been determined.
Expression of HLA-G
HLA-G exhibits a rather unique and restricted pattern of expression in human tissues, both at the message and protein levels. HLA-G messages have been detected in different cell populations in both first-trimester and term placenta (summarized in Table 1). In addition, specific mRNAs have been detected in adult nonplacental tissues, including peripheral blood leukocytes, skin, spleen, thymus, prostate, testis, ovary, small intestine, eye, colon, heart, brain, lung, liver, and kidney, as well as in fetal tissues, including heart, lung, and kidney [24]. These studies showed that alternative splicing of the primary HLA-G transcript varied from tissue to tissue, with the placenta expressing all seven alternatively spliced transcripts but with some tissues, such as the thymus and keratinocyte cells, expressing three transcripts and others only one [24, 25].
The distribution of the HLA-G proteins has proved to be difficult to determine because of the high degree of homology between HLA-G and other class Ia and Ib molecules. Although a number of currently available monoclonal antibodies are specific for HLA-G, the results using these antibodies have been contradictory and inconsistent. Apart from the extravillous cytotrophoblast cells that are stained by all the HLA-G-specific antibodies, whether the other cell types in the placenta express HLA-G remains controversial [26]. Different results have been obtained by various researchers depending on the type of antibody, the method of tissue acquisition and fixation, and the immunohistochemical technique used. For instance, the antibody 16G1, which is specific for soluble HLA-G, labels the syncytiotrophoblast, whereas the antibody BFL.1 labels the endothelial cells of chorionic fetal blood vessels and the antibody 87G labels Hofbauer cells in paraformaldehyde-fixed tissue sections [26]. However, the antibody 4H84, which recognizes all HLA-G isoforms through an epitope located in the 1 domain of HLA-G [27], does not stain any of these cells; it only stains invasive extravillous cytotrophoblasts [28].
In general, it is accepted that HLA-G protein is expressed constitutively in vivo by all populations of extravillous cytotrophoblast cells in first-trimester placenta, including interstitial, endovascular, and placental giant cells (Table 1). In addition, interferon-, -ß, and -
were shown to enhance HLA-G mRNA and protein expression in transfected mouse fibroblasts despite the presence of a 16-base pair (bp) deletion in the enhancer A/interferon response element [29]. HLA-G has also been detected on the surface of human oocytes and preimplantation embryos [30], although earlier studies suggested that these tissues were class I negative [31]. Expression of HLA-G in male gametes is also controversial, with some reports demonstrating the presence of HLA-G mRNA and protein in immature and mature sperm cells but others indicating that these cells are completely class I negative (reviewed in [31]). Recent reports indicate that the extravillous cytotrophoblast cells, which circulate in maternal blood during pregnancy, express HLA-G [32]. Expression of HLA-G mRNA and protein in tumors and cell lines has also been reported, but these reports are contradictory and the issue still controversial [33]. The only nonreproductive tissue so far in which HLA-G protein has been detected is the thymus [25].
Soluble isoforms of HLA-G have been detected in amniotic fluid [27], syncytiotrophoblast (which is in contact with maternal blood of the intervillous space in paraformaldehyde-fixed first-trimester human placentas [34]), and circulating blood of pregnant women [35, 36]. The latter report [36] indicated that the isoforms of HLA-G circulating in maternal blood were either free sHLA-G heavy chains or soluble HLA-G2. Recently, it was shown that early embryos (obtained during in vitro fertilization procedures) secrete soluble HLA-G protein [37].
Function of HLA-G
The function of HLA-G is not known, and a number of recent reviews have discussed this subject extensively [5, 6, 11, 38]. HLA-G is an oligomorphic, multi-isoform molecule that can bind and present peptides similar to class Ia molecules. Studies have suggested that HLA-G may have multiple functions, including protection of the placenta from natural killer (NK) cells and activated cytotoxic T lymphocytes (CTL) and induction of Th2 cytokine secretion by decidual cells, and that soluble isoforms may act as specific immunosuppressors during pregnancy [6].
Various researchers have presented evidence suggesting the short isoforms of HLA-G may play a role in pregnancy modulation. One of the HLA-G alleles has a frame-shift mutation in exon 3 that results in amino acid substitutions in the second half of exon 3 and a stop codon at the beginning of exon 4, suggesting that the full-length HLA-G transcript may not be properly translated. Ober et al. [39] showed that an individual who was homozygous for this allele did not express HLA-G1 protein but did have successful pregnancies. Another report detailed a family of five siblings who were homozygous for this allele and had been delivered normally [40]. Three of the siblings were females who also had normal pregnancies. Thus, in the absence of the full-length HLA-G1, the shorter isoforms may perform the same function [41, 42], because mRNAs encoding the other isoforms of HLA-G were detected in these individuals [39, 40].
Expression and function of the shorter isoforms of HLA-G in vivo, however, have not been determined. Some studies have suggested that the shorter isoforms of HLA-G can be expressed on cell surfaces and can protect susceptible cells from NK and CTL lysis [42] and that the 1 domain of HLA-G, which is present in all isoforms, may be the ligand interacting with NK cells [22]. However, other reports suggest that the shorter isoforms do not reach the cell surface and, hence, may not have any function [43, 44]. Thus, the possible function of these isoforms is still controversial [33], and an animal model is necessary to enable in vivo experiments to be performed.
One of the strategies that has been used in the hope of developing an animal model is transgenesis. Several HLA-G transgenic mice are being used for functional studies of HLA-G (reviewed in [45]). The use of HLA-G transgenic mouse models enables certain types of experiments to be performed that cannot ethically be performed in humans. Other advantages include control of the genetic background of the experimental subjects, availability of tissue samples at all ages, and age-matched control subjects that do not express the transgene product. Experimental approaches that are feasible include grafting, use of infectious agents in vivo, and controlled matings with mice carrying other transgenes [45]. However, a number of differences between the human and murine reproductive parameters exist, including differences in the pattern of expression of endogenous MHC molecules in extraembryonic tissues and the length of gestation (21 days in the mouse, 40 wk in the human). In addition, binding of HLA-G to murine ß2-microglobulin and murine CD8 may be inadequate to mediate HLA-G functions ([45] and references therein). The mouse MHC locus is organized similar to that of the human and contains class I, II, and III regions [46, 47]. In contrast to those in the human placenta, class Ia genes are expressed in the spongiotrophoblast in the mouse placenta [7]. Recent evidence suggests that the mouse fetus may avoid immune destruction by controlling the maternal immune response rather than by reducing the placental transplantation antigen expression [7]. These differences place an important limitation on the interpretation of data from this species.
Because many nonhuman primates have homologues of both class Ia and class Ib MHC genes and because the reproductive system of most primates is similar to that of humans, nonhuman primates may offer suitable animal models to elucidate the function of HLA-G.
|
CLASS Ib GENE EXPRESSION IN NONHUMAN PRIMATES
|
|---|
Many nonhuman primates have homologues of HLA-A, HLA-B, and HLA-C in addition to class Ib homologues. The class Ib locus has not been extensively studied in nonhuman primates, and only limited information is available from a few species. The studies that have been performed have focused mainly on polymorphism and allelic diversity of MHC class I in nonhuman primates. Because of the interest generated by HLA-G in humans, its homologue in nonhuman primates (generally designated MHC-G) has also been widely studied in a number of these species. Comparison of the MHC-G genes in various species of primates, representing the Old World (Catarrhini) and New World monkeys (Platyrrhini), anthropoids, and humans, showed that this gene does not have the linear development throughout the postulated evolutionary pathway of primates [48]. Instead, it appears to have followed different evolutionary patterns in each group of species (Fig. 2), and recent studies have suggested that the evolution of the MHC-G gene may not support a functional role for the complete protein (reviewed in [49]). Figure 2 shows the evolutionary pattern of these genes. The clustering pattern of the MHC-G locus approximately mirrors the evolutionary pattern of the primate species, with the MHC-G from apes, Old World monkeys, and New World monkeys in different clusters. The New World primate MHC-G appears to have diverged from the MHC-G of the other species much earlier during evolution, and this gene has characteristics similar to those of class Ia (described below). The AG locus genes clustered with the A locus genes as expected, whereas the MHC-G of the Old World monkeys and the apes formed individual clusters (Fig. 2). The MHC-G loci of nonhuman primates are named according to the nomenclature convention recommended by Klein et al. [50], in which the first two letters of the genus and species names are combined to produce a taxonomic designation (Fig. 2).
View larger version (11K):
[in this window]
[in a new window]
|
FIG. 2. Phylogenetic tree depicting the evolutionary relationships between MHC-G and class Ia sequences from New and Old World monkeys, apes, and humans. Consensus sequences from exons 2 and 3 were aligned and a phylogenetic tree constructed using MEGALIGN Lasergene software (DNASTAR, Inc., Madison, WI). The length of each pair of branches represents the distance between the sequence pairs, whereas the units at the bottom of the tree indicate the number of substitution events. The locus names are based on the proposal by Klein et al. [50] in which the first two letters of the genus and species names are combined to form the taxa designation. Ceae, Cercopithecus aethiops (African green monkey); Gogo, Gorilla gorilla (gorilla); Mafa, Macaca fascicularis (crap-eating macaque); Mamu, Macaca mulatta (rhesus monkey); Paan, Papio anubis (olive baboon); Patr, Pan troglodytes (chimpanzee); Popy, Pongo pygmaeus (Orangutan); Saoe, Saquinus oedipus (cotton-top tamarin)
|
|
|
MHC-G IN OLD WORLD MONKEYS
|
|---|
The Old World monkeys (found in Asia and Africa) are grouped into one family, Cercopithecidae, which consists of two subfamilies, Colobinae and Cerpithecinae. To our knowledge, no reports of MHC-G studies involving members of the Colobinae subfamily have appeared. However, studies have shown that the MHC-G gene in the Cercopithecinae subfamily has a unique characteristic that sets it apart from MHC-G of the other primates. In this group, the MHC-G of the species that have been evaluated contains stop codons in a restricted area of exon 3 [51]. These species have homologues of class Ia (MHC-A and MHC-B) in addition to the other class Ib genes and pseudogenes. In this group, the MHC class I genes of the rhesus monkey (Macaca mulatta) and the baboon (Papio anubis) have been intensely studied, because these two species have been extensively used in biomedical research.
Mamu-G and Mamu-AG in Rhesus Monkey
In the rhesus monkey, the HLA-G orthologue Mamu-G has been shown to be a pseudogene [52]. However, transcripts from this gene were predicted to encode a peptide with an 1 domain and a truncated 2 domain, consistent with the analysis of genomic DNA of other Old World monkeys that demonstrated a stop codon in exon 3 (encoding the 2 domain) but not in exon 2 (encoding the 1 domain) [51]. In contrast, further analysis of cDNA from the rhesus monkey placenta resulted in the identification of a novel class Ib locus, Mamu-AG [53]. This gene is closely related to the A locus (Fig. 2) but encodes glycoproteins with all the characteristics of HLA-G, including restricted tissue distribution, alternative splicing of the mRNA, truncated cytoplasmic domain, and limited diversity (Table 2) [53].
Mamu-AG transcripts have been detected in the placenta, amniotic membranes, and a number of nonplacental tissues, including the kidney, spleen, eye, brain, lung, spinal cord, heart, testis, ovary, and adrenal glands [54]. However, the highest level of expression of Mamu-AG mRNA is consistently in the placenta and amniotic membranes, with expression in the other tissues being low. In contrast to HLA-G expression in human placenta, Mamu-AG mRNA and protein were strongly expressed in villous syncytiotrophoblast, whereas the villous cytotrophoblast was negative [55]. Mamu-AG protein was also detected in extravillous cytotrophoblast cells invading maternal vessels and decidua at the implantation site during early pregnancy [56], which is a pattern similar to that of HLA-G protein.
Recent studies have more carefully evaluated rhesus monkey tissues for Mamu-AG mRNAs that might encode soluble protein(s), and the rhesus monkey placenta was found to express a variant of Mamu-AG mRNA (sMamu-AG1) that retains intron 4, as previously noted in human sHLA-G1 [17] and baboon sPaan-AG1 [57] (Fig. 1), and that would encode a soluble isoform ([58]; T.G. Golos, personal communication, see database entry AY059404). The sMamu-AG1 transcript was also present in amnion, testis, spleen, thyroid, and ovary, but in other rhesus tissues, it was either absent (adrenal, lymph nodes, skeletal muscle, skin, heart, lungs, liver, and pancreas) or barely detectable (kidney, ovary, and intestine) [58].
Immunostaining experiments with the antibody 16G1 against sHLA-G1 intron 4 peptide demonstrated that immunoreactive proteins were present in the syncytiotrophoblast of the chorionic villi of the rhesus monkey placenta, within the villous cytotrophoblasts, and occasionally, within cells of the villous stroma [58], which is a pattern similar to that obtained with human and baboon placenta [34, 57, 59, 60]. These results confirm and extend the unique homology between HLA-G and the novel class Ib molecules, Paan-AG and Mamu-AG.
Paan-AG in Olive Baboon
The only other nonhuman primate in which the class Ib AG locus has been described is the olive baboon. The baboon AG locus, Paan-AG, was first identified in baboon term placenta in 1999 [61] and was shown to be similar to Mamu-AG. Two Paan-AG alleles were isolated from baboon term placental cDNA, and comparison of these alleles with other baboon class I genes suggested that the AG locus may have been derived from a class Ia A locus [61]. Recent studies in our laboratory have demonstrated that Paan-AG mRNA is alternatively spliced in a manner similar to HLA-G [57, 60]. We identified alternatively spliced transcripts homologous to HLA-G1, -G2, -G3, and -G4 from baboon first-trimester and term placentas (Fig. 1). In addition, we identified a transcript that retains intron 4 and encodes soluble Paan-AG1 glycoprotein, similar to soluble HLA-G1 (Table 2). Two transcripts that have a truncated exon 3 and that encode a glycoprotein with the 1 domain and a truncated 2 domain were also detected [57, 60].
We screened a number of baboon tissues for expression of Paan-AG messages and proteins and found that Paan-AG1 messages and Paan-AG messages with a truncated exon 3 were present in all the tissues tested, including the placenta, decidua, cycling endometrium, kidney, liver, spleen, thymus, lung, skeletal muscle, and heart, similar to HLA-G [57, 60]. However, the shorter, alternatively spliced transcripts were detected only in the placenta and decidua, not in other tissues. We generated isoform-specific polyclonal antibodies to Paan-AG1, Paan-AG2, and soluble Paan-AG1 glycoproteins and used these antibodies to determine whether these transcripts are translated in baboon tissues. These studies are still on-going, but preliminary results show that soluble Paan-AG1 and membrane-anchored Paan-AG1 are strongly expressed in the villous syncytiotrophoblast in both first-trimester and term placenta [59, 60]. The two proteins are also expressed by the extravillous cytotrophoblast cells at the basal plate. However, no Paan-AG2 protein has been detected in the baboon placenta.
Comparison of the predicted amino acid sequences of the proteins derived from the full-length transcripts encoded by the AG locus with class Ia amino acid sequences showed that the amino acids essential for formation of the tertiary structure of class I MHC molecules and interaction with ß2-microglobulin are conserved in Paan-AG1 and Mamu-AG1 [53, 60, 61]. Both proteins also cross-react with W6/32, a monoclonal antibody that recognizes HLA class I heavy chain in association with ß2-microglobulin [55, 62]. Thus, the AG locus encoded proteins may have a tertiary structure similar to that of MHC class I molecules. The predicted amino acid sequence of the shorter isoforms and soluble isoforms are also similar to their human counterparts, suggesting that they may form similar tertiary structures, but no experimental data are currently available.
The AG locus therefore appears to be a functional homologue of HLA-G, and comparison of the HLA-G, Mamu-AG, and Paan-AG gene sequences shows that they share many similarities (Table 2). Thus, the rhesus monkey and the baboon may be nearly ideal models for functional studies to elucidate the role of HLA-G at the maternal-fetal interface. Baboons are abundant in Africa and breed with high fertility rates (>80%) in captivity. Details of baboon reproductive patterns also are available, because these animals have been used extensively in studies on reproduction [63]. The rhesus monkey is found in Asia, and currently, most primate research centers in the world keep colonies of rhesus monkeys for biomedical research. In addition, details of class I expression in the rhesus monkey placenta and immune cells at the maternal-fetal interface are available [64]. Both species, in addition to other nonhuman primates, have been used as models for infectious diseases, organ transplantation, and the mechanism of immune pathology following infection, because their immune systems are similar to that of humans [reviewed in 65 and references therein].
|
MHC-G IN NEW WORLD MONKEYS
|
|---|
One of the characteristics of HLA-G that sets it apart from other class I genes is a 23-bp deletion in position 161–183 on intron 2 [9]. This deletion has been observed in all the MHC-G genes of the Cercopithecidae and Pongidae families that have been studied so far [48, 51]. However, the MHC-G in New World monkeys does not contain this deletion [48]. In this group, the MHC genes of the cotton-top tamarin (Saquinus oedipus), a New World primate found in South and Central America, have been extensively studied. The MHC-G (Saoe-G) in this species exhibits relatively more alleles, with nonsynonymous substitutions located in the T-cell receptor, NK receptor, and the antigen-binding site [48]. This species does not have HLA-A or -B homologues [66]. Thus, in the absence of class Ia molecules (except for the HLA-C-related one), Saoe-G could be the antigen-presenting molecule [48]. So far, no reports have detailed the pattern of expression of this gene in this species, because all these studies have been performed using genomic DNA. However, the characteristics of Saoe-G described above indicate that this species cannot be used as a model for functional studies of HLA-G.
|
MHC-G IN ANTHROPOIDS
|
|---|
In the anthropoids (Pongidae), MHC-G appears to be relatively monomorphic, although only a few species have been tested. The low polymorphism observed in the MHC-G of gorilla (Gorilla gorilla), chimpanzee (Pan troglodytes), and pigmy chimpanzee (P. paniscus) was similar to that observed in HLA-G [48]. It is only the MHC-G of orangutan (Pongo pygmaeus) that shows five protein alleles that affect both the T-cell receptor and antigen-binding site [48]. The MHC-G mRNA of the Pongidae species was also found to be alternatively spliced. However, in contrast to HLA-G, only three alternatively spliced transcripts, MHC-G1, -G2, and -G3, were detected [67]. MHC-G4 and transcripts encoding the soluble protein isoforms were not detected in these species. No expression studies of this gene have been done as yet in this group of nonhuman primates. Although the MHC-G in this group appears to be similar to HLA-G, the anthropoids are an endangered species in the wild and are generally not available for research, which limits their potential as animal models.
|
SUMMARY
|
|---|
The function of HLA-G has not been elucidated, yet evidence is accumulating in support of the idea that both membrane-bound and soluble forms of HLA-G exert multiple functions on immune cells [5, 6]. Ethical considerations do not allow in vivo experiments to be performed in humans, and the extent to which in vitro experiments and mouse models can reflect the in vivo situation is limited. Therefore, animal models with reproductive systems similar to that of the human would be ideal for performing experiments to elucidate the function of these molecules. The apes, which are closest to humans in the evolutionary tree (Fig. 2), are generally endangered species and are not readily available for experimentation. The MHC-G in New World monkeys appears to play a role similar to MHC class Ia in humans, whereas MHC-G in Old World monkeys is inactivated [48]. However, the new AG locus that has been described in the rhesus monkey and olive baboon presents a good alternative, because this locus encodes a protein with characteristics similar to those of HLA-G, suggesting these two species can serve as appropriate animal models to elucidate the function of HLA-G. Both species have a monodiscoid, hemochorial placenta that is similar, both developmentally and structurally, to human placenta.
Studies are currently on-going to further characterize the AG locus products and the function of these molecules in these two species. Future studies include experiments to test whether introduction of isoform-specific anti-AG antibodies will interfere with an on-going pregnancy or with implantation. Data using such functional experiments may be extrapolated to humans given the similarity between HLA-G and the AG locus as well as the similarity in the immune responses of humans, rhesus monkeys, and olive baboons.
|
ACKNOWLEDGMENTS
|
|---|
The authors thank Dr. Thaddeus Golos (Wisconsin Regional Primate Research Center, Madison, WI) for his helpful comments on the manuscript.
|
FOOTNOTES
|
|---|
1 Supported by a CONRAD Twinning grant with the Institute of Primate Research, Nairobi, Kenya, the CONRAD program (CICCR), and the University of Kansas Medical Center Research Institute, Kansas City, USA. 
2 Correspondence: Joan S. Hunt, Dept. of Anatomy and Cell Biology, University of Kansas Medical Center, 3901 Rainbow Boulevard, Kansas City, KS 66160-7400. FAX: 913 588 7180; jhunt{at}kumc.edu
Received: 14 March 2002.
First decision: 11 April 2002.
Accepted: 2 May 2002.
|
REFERENCES
|
|---|
-
Klein J. Natural History of the Major Histocompatibility Complex. New York: John Wiley and Sons; 1986
-
Goodfellow PN, Barnstable CJ, Bodmer WF, Snary D, Crumpton MJ. Expression of HLA system antigens on placenta. Transplantation 1976 22:595-603[Medline]
-
Hunt JS, Hutter H. Current theories on protection of the fetal semiallograft. In: Hunt JS (ed.), HLA and the Maternal-Fetal Relationship. Austin, TX: R.G. Landes; 1996: 27–50
-
Hunt JS. Major histocompatibility complex antigens in reproduction. In: Glasser J, Aplin J, Giudice L, Tabibzadeh S (eds.), The Endometrium. London: Harwood Academic Publishers; 2001: 392–402
-
Le Bouteiller P, Blaschitz A. The functionality of HLA-G is emerging. Immunol Rev 1999 167:233-244[CrossRef][Medline]
-
Le Bouteiller P, Solier C, Proll J, Aquerre-Girr M, Fournel S, Lenfant F. Placental HLA-G protein expression in vivo: where and what for?. Hum Reprod Update 1999 5:223-233[Abstract/Free Full Text]
-
Bainbridge D. Evolution of mammalian pregnancy in the presence of the maternal immune system. Rev Reprod 2000 5:67-74[Abstract]
-
Geraghty DE. Structure of the HLA class I region and expression of its resident genes. Curr Opin Immunol 1993 5:3-7[CrossRef][Medline]
-
Geraghty DE, Koller BH, Orr HT. A human major histocompatibility complex class I gene that encodes a protein with a shortened cytoplasmic segment. Proc Natl Acad Sci U S A 1987 84:9145-9149[Abstract/Free Full Text]
-
Shimizu Y, Geraghty DE, Koller BH, Orr HT, DeMars R. Transfer and expression of three cloned human non-HLA-A, -B, -C class I genes in mutant lymphoblastoid cells. Proc Natl Acad Sci U S A 1988 85:227-231[Abstract/Free Full Text]
-
O'Callaghan CA, Bell JI. Structure and function of the human MHC class Ib molecules HLA-E, HLA-F, and HLA-G. Immunol Rev 1998 163:129-138[CrossRef][Medline]
-
Koller BH, Geraghty DE, Shimizu Y, DeMars R, Orr HT. HLA-E: a novel class I gene expressed in resting T lymphocytes. J Immunol 1988 141:897-904[Abstract]
-
Braud V, Jones EY, McMichael A. The human major histocompatibility complex class Ib molecule HLA-E binds signal sequence-derived peptides with primary anchor residues at positions 2 and 9. Eur J Immunol 1997 27:1164-1169[Medline]
-
Wainwright SD, Biro PA, Holmes CA. HLA-F is a predominantly empty, intracellular TAP-associated MHC class Ib protein with a restricted expression pattern. J Immunol 2000 164:319-328[Abstract/Free Full Text]
-
Ishitani A, Geraghty DE. Alternative splicing of HLA-G transcripts yields proteins with primary structures resembling both class I and class II antigens. Proc Natl Acad Sci U S A 1992 89:3947-3951[Abstract/Free Full Text]
-
Kirszenbaum M, Moreau P, Gluckman E, Dausset J, Carosella E. An alternatively spliced form of HLA-G mRNA in human trophoblasts and evidence for the presence of HLA-G transcripts in adult lymphocytes. Proc Natl Acad Sci U S A 1994 91:4209-4213[Abstract/Free Full Text]
-
Fujii T, Ishitani A, Geraghty DE. A soluble form of HLA-G antigen is encoded by a messenger ribonucleic acid containing intron 4. J Immunol 1994 153:5516-5524[Abstract]
-
Paul P, Adrian-Cabestre F, Ibrahim EC, Lefebvre S, Khalil-Daher I, Vazeux G, Moya-Quiles RM, Bermond F, Dausset J, Carosella ED. Identification of HLA-G7 as a new splice variant of the HLA-G mRNA and expression of soluble HLA-G5, -G6, and -G7 transcripts in human transfected cells. Hum Immunol 2000 61:1138-1149[CrossRef][Medline]
-
Bjorkman PJ, Saper MA, Samraoui B, Bennett WS, Strominger JL, Wiley DC. Structure of the human class I histocompatibility antigen, HLA-A2. Nature 1987 329:506-512[CrossRef][Medline]
-
Tysoe-Calnon A, Grundy JE, Perkins SJ. Molecular comparisons of the ß2-microglobulin binding sites in class I major histocompatibility complex chains and proteins of related sequences. Biochem J 1991 277:359-369
-
Bulmer JN, Johnson PM. Antigen expression by trophoblast populations in the human placenta and their possible immunological relevance. Placenta 1985 6:127-140[CrossRef][Medline]
-
Rouas-Freiss N, Marchal RE, Kirszenbaum M, Dausset J, Carosella ED. The 1 domain of HLA-G1 and HLA-G2 inhibits cytotoxicity induced by natural killer cells: is HLA-G the public ligand for natural killer cell inhibitory receptors?. Proc Natl Acad Sci U S A 1997 94:5249-5254[Abstract/Free Full Text]
-
Audus KL, Soares MJ, Hunt JS. Characteristics of the fetal/maternal interface with potential usefulness in the development of future immunological and pharmacological strategies. J Pharmacol Exp Ther 2002 301:402-409[Abstract/Free Full Text]
-
Onno M, Guillaudeux T, Amiot L, Renard I, Drenou B, Hirel B, Girr M, Semana G, Le Bouteiller P, Fauchet R. The HLA-G gene is expressed at a low mRNA level in different human cells and tissues. Hum Immunol 1994 41:79-86[CrossRef][Medline]
-
Crisa L, McMaster MT, Ishii JK, Fisher SJ, Salomon DR. Identification of a thymic epithelial cell subset sharing expression of class Ib HLA-G molecule with fetal trophoblasts. J Exp Med 1997 186:289-298[Abstract/Free Full Text]
-
Blaschitz A, Hutter H, Leitner V, Pilz S, Wintersteiger R, Dohr G, Sedlmayr P. Reaction patterns of monoclonal antibodies to HLA-G in human tissues and on cell lines: a comparative study. Hum Immunol 2000 61:1074-1085[CrossRef][Medline]
-
McMaster MT, Zhou Y, Shorter S, Kapasi K, Geraghty DE, Lim KH, Fisher S. HLA-G isoforms produced by placental cytotrophoblast and found in amniotic fluid are due to unusual glycosylation. J Immunol 1998 160:5922-5928[Abstract/Free Full Text]
-
McMaster MT, Librach CL, Zhou Y, Lim KH, Janatpour MJ, DeMars R, Kovats S, Damsky C, Fisher S. Human placental HLA-G expression is restricted to differentiated cytotrophoblasts. J Immunol 1995 154:3771-3778[Abstract]
-
Chu W, Yang Y, Geraghty DE, Hunt JS. Interferons enhance HLA-G mRNA and protein in transfected mouse fibroblasts. J Reprod Immunol 1999 42:1-15[CrossRef][Medline]
-
Jurisicova A, Casper RF, MacLusky NJ, Mills GB, Librach CL. HLA-G expression during preimplantation human embryo development. Proc Natl Acad Sci U S A 1996 93:161-165[Abstract/Free Full Text]
-
Hutter H, Dohr G. HLA expression on immature and mature human germ cells. J Reprod Immunol 1998 38:101-122[CrossRef][Medline]
-
Van Wijk IJ, Griffioen S, Tjoa ML, Mulders MAM, van Vugt JMG, Loke YW, Oudejans CBM. HLA-G expression in trophoblast cells circulating in maternal peripheral blood during early pregnancy. Am J Obstet Gynecol 2001 184:991-997[CrossRef][Medline]
-
Bainbridge D, Ellis S, Le Bouteiller P, Sargent I. HLA-G remains a mystery. Trends Immunol 2001 22:548-552[CrossRef][Medline]
-
Chu W, Fant ME, Geraghty DE, Hunt JS. Soluble HLA-G in human placentas: synthesis in trophoblast and interferon-gamma-activated macrophages but not placental fibroblasts. Hum Immunol 1998 59:435-442[CrossRef][Medline]
-
Pfeiffer KA, Rebmann V, Passler M, van der Ven K, van der Ven H, Krebs D, Grosse-Wilde H. Soluble HLA levels in early pregnancy after in vitro fertilization. Hum Immunol 2000 61:559-564[CrossRef][Medline]
-
Hunt JS, Jadhav L, Chu W, Geraghty D, Ober C. Soluble HLA-G circulates in maternal blood during pregnancy. Am J Obstet Gynecol 2000 183:682-688[CrossRef][Medline]
-
Fuzzi B, Rizzo R, Criscuoli L, Noci I, Melchiorri L, Scarselli B, Bencini E, Menicucci A, Baricordi OR. HLA-G expression in early embryos is a fundamental prerequisite for the obtainment of pregnancy. Eur J Immunol 2002 32:311-315[CrossRef][Medline]
-
Carosella ED, Moreau P, Aractingi S, Rouas-Freiss N. HLA-G: a shield against inflammatory aggression. Trends Immunol 2001 22:553-555[CrossRef][Medline]
-
Ober C, Aldrich C, Rosinsky B, Robertson A, Walker MA, Willadsen S, Verp MS, Geraghty DE, Hunt JS. HLA-G1 protein expression is not essential for fetal survival. Placenta 1998 19:127-132[CrossRef][Medline]
-
Castro MJ, Morales P, Rojo-Amigo R, Martinez-Lazo J, Allende L, Varela P, Garcia-Berciano M, Guillen-Perales J, Arnaiz-Villena A. Homozygous HLA-G*0105N healthy individuals indicate that membrane-anchored HLA-G1 molecule is not necessary for survival. Tissue Antigens 2000 56:232-239[CrossRef][Medline]
-
Menier C, Riteau B, Dausset J, Carosella ED, Rouas-Freiss N. HLA-G truncated isoforms can substitute for HLA-G1 I fetal survival. Hum Immunol 2000 61:1118-1125[CrossRef][Medline]
-
Riteau B, Rouas-Freiss N, Menier C, Paul P, Dausset J, Carosella ED. HLA-G2, -G3, and -G4 isoforms expressed as nonmature cell surface glycoproteins inhibit NK and antigen-specific CTL cytolysis. J Immunol 2001 166:5018-5026[Abstract/Free Full Text]
-
Bainbridge DRJ, Ellis SA, Sargent II. The short forms of HLA-G are unlikely to play a role in pregnancy because they are not expressed at the cell surface. J Reprod Immunol 2000 47:1-6[CrossRef][Medline]
-
Mallet V, Proll J, Solier C, Aguerre-Girr M, De Rossi M, Loke YW, Lenfant F, Le Bouteiller P. The full-length HLA-G1 and no other alternative form of HLA-G is expressed at the cell surface of transfected cells. Hum Immunol 2000 61:212-224[CrossRef][Medline]
-
Schmidt CM. Studies on HLA-G transgenic mice. In: Hunt JS (ed.), HLA and the Maternal-Fetal Relationship. Austin, TX: R.G. Landes; 1996: 109–132
-
Trowsdale J. "Both man and beast": comparative organization of MHC genes. Immunogenetics 1995 41:1-17[CrossRef][Medline]
-
Gunther E, Walter L. Comparative aspects of rat, mouse and human MHC class I gene regions. Cytogenet Cell Genet 2000 91:107-112[CrossRef][Medline]
-
Arnaiz-Villena A, Morales P, Gomez-Casado E, Castro MJ, Varela P, Rojo-Amigo R, Martinez-Laso J. Evolution of MHC-G in primates: a different kind of molecule for each group of species. J Reprod Immunol 1999 43:111-125[CrossRef][Medline]
-
Arnaiz-Villena A, Martinez-Lazo J, Castro MJ, Morales P, Moscoso J, Varela P, Gomez-Casado E, Allende L. The evolution of the MHC-G gene does not support a functional role for the complete protein. Immunol Rev 2001 183:65-75[CrossRef][Medline]
-
Klein J, Bontrop RE, Dawkins RL, Erlich HA, Gyllensten UB, Heise ER, Jones PP, Parham P, Wakeland EK, Watkins DI. Nomenclature for the major histocompatibility complexes of different species: a proposal. Immunogenetics 1990 31:217-219[Medline]
-
Castro MJ, Morales P, Fernandez-Soria V, Suarez B, Recio MJ, Alvarez M, Martin-Villa M, Arnaiz-Villena A. Allelic diversity at the primate MHC-G locus: exon 3 bears stop codons in all Cerpithecinae sequences. Immunogenetics 1996 43:327-336[Medline]
-
Boyson JE, Iwanaga KK, Golos TG, Watkins DI. Identification of the rhesus monkey HLA-G orthologue. Mamu-G is a pseudogene. J Immunol 1996 157:5428-5437[Abstract]
-
Boyson JE, Iwanaga KK, Golos TG, Watkins DI. Identification of a novel MHC class I gene, Mamu-AG, expressed in the placenta of a primate with an inactivated G locus. J Immunol 1997 159:3311-3321[Abstract]
-
Slukvin II, Watkins DI, Golos TG. Tissue distribution of the mRNA for a rhesus monkey major histocompatibility complex class Ib molecule, Mamu-AG. Tissue Antigens 1999 53:282-291[CrossRef][Medline]
-
Slukvin II, Boyson JE, Watkins DI, Golos TG. The rhesus monkey analogue of human lymphocyte antigen-G is expressed primarily in villous syncytiotrophoblast. Biol Reprod 1998 58:728-738[Abstract/Free Full Text]
-
Slukvin II, Lunn DP, Watkins DI, Golos TG. Placental expression of the nonclassical MHC class I molecule, Mamu-AG at implantation in the rhesus monkey. Proc Natl Acad Sci U S A 2000 97:9104-9109[Abstract/Free Full Text]
-
Langat DK, Morales PJ, Fazleabas AT, Mwenda JM, Hunt JS. Transcripts derived from the nonclassical major histocompatibility complex gene, Paan-AG, are expressed in baboon placentas. FASEB J 2001 15:370 (abstract 282.12)
-
Ryan AF, Grendell RG, Golos TG. A soluble variant of the rhesus monkey nonclassical MHC class I molecule, Mamu-AG, is expressed in the placenta and the testis. J Soc Gynecol Invest 2002 9:229A (abstract 503)
-
Langat DK, Morales PJ, Fazleabas AT, Mwenda JM, Hunt JS. Baboon placentas express alternatively spliced transcripts and proteins encoded by the nonclassical gene, Paan-AG. FASEB J 2002 16:A1234 (abstract 930.2)
-
Langat DK, Morales PJ, Fazleabas AT, Mwenda JM, Hunt JS. Baboon placentas express soluble and membrane-bound Paan-AG proteins encoded by alternatively spliced transcripts of the class Ib major histocompatibility complex gene, Paan-AG. Immunogenetics 2002 54:164-173[CrossRef][Medline]
-
Boyson JE, Iwanaga KK, Urvater JA, Hughes AL, Golos TG, Watkins DI. Evolution of a new nonclassical MHC class I locus in two Old World primate species. Immunogenetics 1999 49:86-98[CrossRef][Medline]
-
Stern PL, Beresford N, Friedman CI, Stevens VC, Risk JM, Johnson PM. Class I-like MHC molecules expressed by baboon placental syncytiotrophoblast. J Immunol 1987 138:1088-1091[Abstract/Free Full Text]
-
Stevens VC. Some reproductive studies I the baboon. Hum Reprod Update 1997 3:533-540[Abstract/Free Full Text]
-
Slukvin II, Boyson JE, Watkins DI, Golos TG. A nonhuman primate model for maternal-fetal immune tolerance. Transplant Proc 1999 31:1844-1846[CrossRef][Medline]
-
Bontrop RE. Nonhuman primates: essential partners for biomedical research. Immunol Rev 2001 183:5-9[CrossRef][Medline]
-
Watkins DI, Chen ZW, Hughes AL, Evans MG, Tedder TF, Letvin NL. Evolution of the MHC class I genes of a New World primate from ancestral homologues of human nonclassical genes. Nature 1990 346:60-63[CrossRef][Medline]
-
Castro MJ, Morales P, Martinez-Lazo J, Allende L, Rojo-Amigo R, Gonzalez-Hevilla M, Varela P, Moscoso J, Garcia-Berciano M, Arnaiz-Villena A. Lack of MHC-G4 and soluble (-G5 and -G6) isoforms in the higher primates, Pongidae. Hum Immunol 2000 26:1164-1168
-
Hunt JS, Fishback JL, Andrews GK, Wood GW. Expression of class I HLA genes by trophoblast cells: analysis by in situ hybridization. J Immunol 1988 140:1293-1299[Abstract]
-
Blaschitz A, Lenfant F, Mallet V, Hartmann M, Bensussan A, Geraghty DE, Le Bouteiller P, Dohr G. Endothelial cells in chorionic fetal vessels of first trimester placenta express HLA-G. Eur J Immunol 1997 27:3380-3388[Medline]
-
Loke YW, King A, Burrows T, Gardner L, Bowen M, Hiby S, Howlett S, Holmes N, Jacobs D. Evaluation of trophoblast HLA-G antigen with a specific monoclonal antibody. Tissue Antigens 1997 50:135-146[Medline]
-
Yelavarthi KK, Fishback JL, Hunt JS. Analysis of HLA-G mRNA in human placental and extraplacental membrane cells by in situ hybridization. J Immunol 1991 146:2847-2854[Abstract]
-
Hunt JS, Fishback JL, Chumbley G, Loke YW. Identification of class I mRNA in human first-trimester trophoblast cells by in situ hybridization. J Immunol 1990 144:4420-4425[Abstract]
-
Loke YW, King A. Human Implantation: Cell Biology and Immunology. Cambridge, U.K.: Cambridge University Press; 1995
-
Houlihan JM, Biro PA, Fergar-Payne A, Simpson KL, Holmes CH. The human amnion is a site of MHC class Ib expression: evidence for the expression of HLA-E and HLA-G. J Immunol 1995 154:5665-5674[Abstract]
-
Hammer A, Hutter H, Blaschitz A, Manhert W, Hartmann M, Uchanska-Ziegler B, Ziegler A, Dohr G. Amnion epithelial cells, in contrast to trophoblast cells, express all classical HLA class I molecules together with HLA-G. Am J Reprod Immunol 1997 37:161-171
This article has been cited by other articles:
|
|
|
|
 
L. A. Guethlein, A. M. Older Aguilar, L. Abi-Rached, and P. Parham
Evolution of Killer Cell Ig-Like Receptor (KIR) Genes: Definition of an Orangutan KIR Haplotype Reveals Expansion of Lineage III KIR Associated with the Emergence of MHC-C
J. Immunol.,
July 1, 2007;
179(1):
491 - 504.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
T. V. F. Hviid
HLA-G in human reproduction: aspects of genetics, function and pregnancy complications
Hum. Reprod. Update,
May 1, 2006;
12(3):
209 - 232.
[Abstract]
[Full Text]
[PDF]
|
|
|
TOP
ABSTRACT
INTRODUCTION
