Heterogeneity of HLA-G genes identified by polymerase chain reaction/single strand conformational polymorphism (PCR/SSCP)

HLA-G in human reproduction: aspects of genetics, function and pregnancy complications

The non-classical human leukocyte antigen (HLA) class Ib genes, HLA-E, -G and -F, are located on chromosome 6 in the human major histocompatibility complex (MHC). HLA class Ib antigens resemble the HLA class Ia antigens in many ways, but several major differences have been described. This review will, in particular, discuss HLA-G and its role in human reproduction and in the human MHC. HLA-G seems to be important in the modulation of the maternal immune system during pregnancy and thereby the maternal acceptance of the semiallogenic fetus. Recent findings regarding aspects of HLA-G polymorphism, the possible significance of this polymorphism in respect to HLA-G function and certain complications of pregnancy (such as pre-eclampsia and recurrent spontaneous abortions (RSA)) are discussed together with possible importance to IVF. Finally, aspects of a possible role of HLA-G in organ transplantation and in inflammatory or autoimmune disease, and of HLA-G in an evolutionary context, are also briefly examined.

Genetic diversity among Borrelia strains determined by single-strand conformation polymorphism analysis of the ospC gene and its association with invasiveness

Lyme borreliosis (LB) is a tick-borne spirochetal infection caused by three Borrelia species: Borrelia afzelii, B. garinii, and B. burgdorferi sensu stricto. LB evolves in two stages: a skin lesion called erythema migrans and later, different disseminated forms (articular, neurological, cardiac.). Previous research based on analysis of ospC sequences allowed the definition of 58 groups (divergence of <2% within a group and >8% between groups). Only 10 of these groups include all of the strains isolated from disseminated forms that are considered invasive. The aim of this study was to determine whether or not invasive strains belong to restricted ospC groups by testing human clinical strains isolated from disseminated forms. To screen for ospC genetic diversity, we used single-strand conformation polymorphism (SSCP) analysis. Previously known ospC sequences from 44 different strains were first tested, revealing that each ospC group had a characteristic SSCP pattern. Therefore, we studied 80 disseminated-form isolates whose ospC sequences were unknown. Of these, 28 (35%) belonged to previously known invasive groups. Moreover, new invasive groups were identified: six of B. afzelii, seven of B. garinii, and one of B. burgdorferi sensu stricto. This study confirmed that invasive strains are not distributed among all 69 ospC groups but belong to only 24 groups. This suggests that OspC may be involved in the invasiveness of B. burgdorferi.

The Potential Role of HLA-G Polymorphism in Maternal Tolerance to the Developing Fetus

The mechanism by which the developing fetus survives the maternal immunity has eluded investigators and remains a central paradox in the field of stem cell research. If the mechanism can be defined, allogenic stem cells may find increased utility after transplantation. While several theories have been advanced, the differential expression of HLA antigens on trophoblasts has been the focus of many studies. Interestingly, an inverse relationship between HLA-G (nonclassical class I molecules) and class I MHC gene expression exists early in pregnancy. HLA-G transcripts are present in quite significant amounts in first-trimester placental tissue, particularly in the extravillous membranes, while the opposite occurs at term. This kind of expression is consistent with the theory that HLA-G might play a role in fetal protection. This could be consequent to either nonimmune (structural) or immune functions at the maternal-fetal interface. Current evidence suggest an immune function wherein HLA-G protects fetal cells from maternal uterine natural killer (NK) cells, which are found in large numbers within cells invading the trophoblasts. This effect has been attributed to maternal NK receptor alterations as well as inhibition of maternal NK cell traffic across the placenta. The recent identification of HLA-G polymorphism brings into play the potential role of these isoforms in fetal protection. Polymorphism may be associated with differential function or may effect linkage disequilibria with other HLA variants, providing fetal protection.

HLA-G remains a mystery

In this brief summary, we argue that many widely held beliefs about HLA-G are questionable. Recent research has led to a re-evaluation of many of the characteristics that were thought to make HLA-G unusual among the MHC class I molecules. First, contrary to reports suggesting that the gene encoding HLA-G exhibits marked polymorphism in some human populations, recent data have shown that the HLA-G gene has comparatively little polymorphism - a feature that might allow it to be expressed in the placenta without causing rejection by the maternal immune system. Second, although truncated forms of HLA-G are generated in the placenta, most of them are unlikely to have significant biological effects as they do not reach the cell surface. Third, the hypothesis that a major role of HLA-G is to prevent attack of the placenta by maternal natural killer cells is now the subject of renewed scrutiny. Finally, there is little evidence that the induction of expression of HLA-G is a major mechanism by which tumor cells avoid immune attack. HLA-G has once again become as mysterious as when it was discovered: an MHC class I molecule expressed at a challengingly extraordinary site--the immunologically uneasy interface between mother and fetus.

HLA-G has a concentration-dependent effect on the generation of an allo-CTL response

Human leucocyte antigen (HLA) -G is expressed on trophoblast cells during pregnancy, suggesting a role in protection of the semiallogeneic fetus. Published data suggest that HLA-G protects a cell against natural killer cell lysis. It has been hypothesized that HLA-G may also protect the fetus by preventing allo-cytotoxic T lymphocyte (CTL) responses. To test this hypothesis, we assayed the effects of various concentrations of purified HLA-G on CTL response in a mixed lymphocyte culture (MLC) system. We found that concentrations > or =0.1 microg/ml of HLA-G suppressed the allo-CTL response by 30-100% over the control, but, paradoxically, concentrations of 0.01-0.05 microg/ml of HLA-G augmented the allo-CTL response by 25-50% over the control. Concentrations < or = 0.001 microg/ml HLA-G had no effect. Addition of HLA-G to preprimed allo-CTL effector cells did not affect their killing ability. Allo-CTL suppressive doses of HLA-G induced a T helper type 2 (Th2) cytokine response, whereas allo-CTL-enhancing doses of HLA-G induced a Th1-type cytokine response. HLA-G purified from first-trimester placenta does not affect allo-proliferative responses nor does it alter the percentage of CD4+ or CD8+ T cells in MLCs. These findings support a potential role for HLA-G-mediated suppression of allo-CTL formation in normal pregnancies. In addition, the effects observed at lower concentrations of HLA-G may have interesting implications for the condition of pre-eclampsia in which concentrations of this HLA class I molecule are reduced.

The short forms of HLA-G are unlikely to play a role in pregnancy because they are not expressed at the cell surface

HLA-G is a nonclassical class I MHC molecule of unknown function expressed on human invasive trophoblast. In trophoblast cells, HLA-G mRNA is alternatively spliced into a variety of forms which are predicted to encode a full length membrane-bound form, three short membrane-bound isoforms and two soluble isoforms. The aim of this study was to determine which of these protein isoforms are translated, which are expressed on the cell surface and which are secreted. Artificial cDNAs encoding the isoforms were generated by PCR mutagenesis, ligated to an epitope tag and transfected into a human cell line capable of expressing MHC class I. Protein products of appropriate sizes were detected in cells transfected with cDNAs encoding all membrane-bound forms, but surface biotinylation studies indicated that only full length membrane-bound HLA-G was present at the cell surface. Full length HLA-G was also detected by surface antibody binding and flow cytometry. Soluble HLA-G1 was detected in cells transfected with the appropriate cDNA only after treatment with monensin, which inhibits transport of glycoproteins through the Golgi apparatus. These results suggest that full length HLA-G, but not short HLA-G isoforms can be expressed on the surface of human cells and that soluble HLA-G is rapidly secreted. Thus, it is likely that the full length membrane-bound and soluble forms of HLA-G are the only biologically active forms to which the mother is exposed.

Polymorphism in the regulatory region located more than 1.1 kilobases 5' to the start site of transcription, the promoter region, and exon 1 of the HLA-G gene

The non-classic Human Leucocyte Antigen class Ib molecule, HLA-G, is expressed on the invasive, extra-villous cytotrophoblast in human placenta. HLA-G protects against natural killer (NK)-cell-mediated lysis and may modulate the secretion of cytokines. Aberrant expression of HLA-G has been reported in certain disorders of pregnancy. We have studied the DNA sequences of the putative regulatory region located more than 1.1 kilobases 5' from the start site of transcription (a 244 bp HindIII/EcoRI fragment) of the HLA-G gene and of the promoter region to detect any possible polymorphism. We detected one nucleotide substitution in the HindIII/ EcoRI region and one in the promoter region in the alleles G*01012, G*01013, G*0104, and G*0105N compared to G*01011. Several nucleotide substitutions were detected in the region between the 1.1 kb 5' regulatory region and the promoter region. Alleles can be divided into two groups based on the detected polymorphism. The nucleotide substitutions may have implications for the binding of nuclear factors to the regulatory regions. To our knowledge this is the first study of any polymorphism in the 5'-flanking sequences to the HLA-G gene. Further studies are needed to elucidate the exact impact of the detected polymorphism on levels or developmental regulation of HLA-G expression.

Little Evidence of HLA-G mRNA Polymorphism in Caucasian or Afro-Caribbean Populations

HLA-G is a nonclassical class I MHC molecule of unknown function expressed on human trophoblast. The level of polymorphism at the HLA-G locus is of considerable importance, since the paternally inherited gene product is exposed to the maternal immune system during pregnancy. However, previous studies of HLA-G polymorphism using genomic DNA samples have produced conflicting results. Our aim was to investigate polymorphism in trophoblast HLA-G mRNA from pregnancies in ten Caucasian and twelve Afro-Caribbean women by RT-PCR. A similar PCR protocol was also applied to umbilical cord blood genomic DNA from two Caucasian and two Afro-Caribbean neonates. Caucasian cDNA yielded only two different sequences: G*01011, and one containing a previously reported synonymous substitution. Afro-Caribbean samples yielded these sequences as well as one previously reported conservative (leucine-to-isoleucine) substitution. PCR amplification from genomic DNA samples from both populations using previously published primer pairs generated sequences containing multiple substitutions, many of which were nonsynonymous. More than two sequences were produced from genomic DNA from each individual. In contrast, amplification from the same genomic DNA using new primers complementary to exons of the HLA-G gene yielded the same few sequences generated from cDNA. These results suggest that polymorphism at the HLA-G locus is extremely limited in Caucasian and Afro-Caribbean populations. This suggests that spurious polymorphism has been reported in African Americans due to the use of intron-complementary PCR primers on genomic DNA samples. The monomorphic nature of HLA-G may allow trophoblast to carry out the immunological functions of class I-bearing tissues without compromising successful pregnancy.

HLA-G polymorphisms: ethnic differences and implications for potential molecule function

PROBLEM

Human leukocyte antigen (HLA)-G is uniquely expressed on extravillous cytotrophoblasts of the placenta and is postulated to be a mediator of maternal immune tolerance. Although it was originally considered to be nonpolymorphic, variations of the HLA-G DNA sequence have been reported, and a limited number of HLA-G alleles been defined.

METHOD OF STUDY

The HLA-G wild-type sequence was compared with HLA-A2 with regard to the conservation of functionally essential parts of classical HLA-I molecules. HLA-G polymorphisms were analyzed under the aspect of ethnic differences, site, and consequences for postulated molecule functions.

RESULTS

HLA-G exhibits a high degree of conservation relative to HLA-A2 in functionally relevant sites of HLA-class I molecules. However, polymorphic sites in HLA-G and classical HLA loci are not congruent.

CONCLUSION

The type and localization of HLA-G polymorphisms suggest that different parts of HLA-G molecule underlie different selective constraints.

HLA-G polymorphisms and allele frequencies in Caucasians

HLA-G, a nonclassical class I MHC molecule, is uniquely expressed on extravillous cytotrophoblasts of the maternal-fetal interface and is suggested to be essential for establishment of maternal-fetal immune tolerance. Although the level of polymorphism in HLA-G has originally been considered low, number, nature and site of polymorphisms seem to vary between different ethnic populations. We investigated HLA-G polymorphisms in a population of German and Croatian origin by SSCP-analysis and direct sequencing as well as RFLP analysis for presence of the 1597delC mutation. HLA-A alleles associated with the different HLA-G alleles were determined by SSP PCR-typing. In Caucasians, HLA-G exhibits a low degree of polymorphism on the amino-acid level and only slightly higher variability on the nucleotide level. In 264 independent chromosomes, 4 HLA-G alleles on the level of amino acid polymorphisms and an additional 6 variations of nucleotide sequences could be identified. The null-allele G*0105N was present at an allele frequency of 2.3%, which is higher than initially suggested for Caucasians but lower than in Hispanics and African-Americans. Furthermore, some HLA-G alleles exhibit strong linkage disequilibrium with HLA-A.

Cytotoxic T lymphocyte recognition of HLA-G in mice

Several features of HLA-G's sequence and expression pattern distinguish HLA-G from its classical counterparts. These features, including HLA-G's limited polymorphism and its expression at the maternal-fetal interface, have been used as a basis for suggesting a distinct functional role for this nonclassical class I HLA molecule. On the other hand, published data do demonstrate that HLA-G has much in common with its classical counterparts. It associates with beta 2-microglobulin and cytosolic peptides, it binds to CD8, and its presence can inhibit NK-cell-mediated lysis of HLA-G-bearing target cells. To develop a model in which HLA-G's function could be more thoroughly studied, we produced several HLA-G-expressing transgenic mouse strains. We report here the results of skin graft experiments which show that nontransgenic mice reject HLA-G-expressing transgenic murine skin as foreign and that this rejection is associated with the presence in the recipient of lymphocytes capable of specifically lysing HLA-G-expressing cells. In addition, experiments are described which demonstrate that HLA-G transgenic mice recognize HLA-G as a "self" molecule. Together the reported data demonstrate that HLA-G is capable of stimulating an HLA-G-restricted CTL response, that HLA-G molecules can serve as target molecules in lytic interactions with CTLs, and that HLA-G is involved in education of the lymphocytic repertoire of HLA-G transgenic mice.

HLA-G gene polymorphism segregation within CEPH reference families

HLA-G, a nonclassical HLA class I antigen, presents tissue-restricted expression on human trophoblasts and may play an important role in immune tolerance of mother-versus-fetus. In this work we have demonstrated extensive HLA-G genomic polymorphism within three CEPH reference families, by PCR-SSCP analysis and direct sequencing. Among six unrelated parents we assigned eight HLA-G alleles, seven of which are new. We observed the segregation of HLA-G alleles of heterozygous parents among their offspring that matched the segregation of the HLA class I haplotypes. Only one of the mutations observed was found to be nonsynonymous indicating low polymorphism of the HLA-G molecule.

Biparental alleles of HLA-G are co-dominantly expressed in the placenta

HLA-G is the only major histocompatibility complex molecule expressed in the human placenta and thus has been considered to be necessary for maintenance of pregnancy. We investigated whether HLA-G expression is regulated in a parent-of-origin allele-specific manner. Of six first trimester and three third trimester placentas, three first trimester and two third trimester placentas showed heterozygosity at the PstI polymorphic site in the 3'-untraslated region. Reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed biallelic expression of HLA-G in all the informative cases, indicating that HLA-G is not imprinted during the gestational period, at least at the transcriptional level. As HLA-G has been postulated to be polymorphic not only at the DNA sequence level but also at the peptide level, co-dominant expression of the gene suggests that each parental allele is involved in the allogenic response during pregnancy.

Allelic diversity at the primate Mhc-G locus: exon 3 bears stop codons in all Cercopithecinae sequences

Twenty-seven major histocompatibility complex (Mhc)-G exon 2, exon 3, and exon 2 and 3 allelic sequences were obtained together with 12 different intron 2 sequences. Homo sapiens, Pan troglodytes, Pan paniscus, Gorilla gorilla, Pongo pygmaeus, Macaca fascicularis, Macaca mulatta, and Cercopithecus aethiops individuals were studied. Polymorphism does not follow the classical pattern of three hypervariable regions per domain and is found in all species studied; exon 3 (equivalent to the alpha 2 protein domain) shows stop codons in the Cercopithecinae group but not in the Pongidae and human groups. Dendrograms show that cotton top tamarin (Saguinus oedipus) Mhc-G sequences are closer to Homo sapiens and Pongidae than to Cercopithecinae, probably due to the stop codons existing at exon 3 of the latter. There is a clear trans-species evolution of allelism in Cercopithecinae and also in exon 2 of all the other apes studied, but a generation of allelism within each species may be present on exon 3 sequences. This discrepancy may be due to the preferential use of exon 2 over exon 3 at the mRNA splicing level within each species in order to obtain the appropriate functional G product. Mhc-G intron 2 shows conserved motifs in all species studied, particularly a 23 base pair deletion between positions 161 and 183 which is locus specific, and some of the invariant residues, important for peptide presentation, conserved in classical class I molecules from fish and reptiles to humans were not found in Mhc-G alleles; the intron 2 dendrogram also shows a particular pattern of allelism within each species. In summary, Mhc-G has substantial differences from other classical class I genes: polymorphism patterns, tissue distribution, gene structure, splicing variability, and probably an allelism variability within each species at exon 3. The G proteins may also be different. This indicates that the Mhc-G function may not be peptide presentation to the clonotypic T-cell receptor.

Soluble HLA-G molecule. An alternatively spliced HLA-G mRNA form candidate to encode it in peripheral blood mononuclear cells and human trophoblasts

The HLA-G nonclassic MHC class I gene expressed at the maternal-fetal interface may be involved in cell protection against NK cell lysis. HLA-G mRNA is observed in different adult or fetal human cells and exhibits four alternative forms: HLA-G1, HLA-G2, HLA-G3, and HLA-G4 lacking, respectively, exon 7; exons 7 and 3, exons 7, 3, and 4; exons 7 and 4. Because exon 5 encodes the transmembrane domain of the HLA-G antigen, none of these transcripts could give a soluble form as detected in supernatant of trophoblasts. In this report, we describe an additional alternatively spliced for of HLA-G transcript (HLA-G5) present in adult PBMCs and first-trimester trophoblasts that contains intron 4. Moreover, as with all other transcripts, HLA-G5, is devoid of exon 7. Its relative frequency is, respectively, approximately 1:8 and approximately 1:26 in adult PBMCs and first-trimester trophoblasts. The presence of intron 4 generates a stop codon that excludes transmembrane region (exon 5) of the HLA-G molecule and therefore might produce a soluble antigen. These results are discussed with regard to data on soluble forms of classic HLA antigens and the possible role of HLA-G.

Evidence for the presence of the alternatively spliced HLA-G mRNA forms in human mononuclear cells from peripheral blood and umbilical cord blood

The HLA-G monomorphic, nonclassic HLA class I gene encodes the molecule that is the only major histocompatibility complex antigen expressed on cytotrophoblasts of placenta. This restricted expression on fetal tissue that is in contact with maternal tissue suggests that HLA-G products may play a role in maternofetal tolerance. We previously have demonstrated in first-trimester human trophoblasts a new alternatively spliced form of HLA-G mRNA lacking exon 4 (HLA-G4) and weak expression of HLA-G1 copy mRNA in adult peripheral blood lymphocytes. By using exon-specific HLA-G primers, we demonstrate in this work the presence of three additional alternatively spliced forms of HLA-G mRNA in human peripheral mononuclear cells (HLA-G2, HLA-G3, and HLA-G4). In umbilical cord blood, HLA-G transcription was observed at very low level and only three alternatively spliced forms were detected (HLA-G1, HLA-G2, and HLA-G3). In contrast, we did not revealed any HLA-G transcript in CD34+ fraction of cord blood cells.

An alternatively spliced form of HLA-G mRNA in human trophoblasts and evidence for the presence of HLA-G transcript in adult lymphocytes

The HLA-G monomorphic, nonclassical class I gene encodes the major histocompatibility complex (MHC) molecule, which is the only MHC antigen expressed on cytotrophoblast cells of placenta. In this work, we have investigated expression of the HLA-G gene in fetal tissues and adult peripheral blood cells by using a sensitive hot-start reverse transcriptase PCR technique. PCR amplification with HLA-G primers specific for exon 3 has enabled us to demonstrate an alternatively spliced form of HLA-G mRNA present in fetal first trimester trophoblasts and lacking exon 4 (HLA-G.3-5). This low abundance transcript (approximately 1:200) in comparison to full-length mRNA may encode the protein that excludes the alpha 3 domain and by conformational changes may present a different ability to bind to peptides. Moreover, expression of the HLA-G transcript was found in adult peripheral lymphocytes and equally in B- and T-cell populations. These results are discussed in the context of the fetal-maternal relationship presented by HLA-G gene products.