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

Detection of soluble HLA-G molecules in plasma and amniotic fluid

Although the cDNA sequence of HLA-G antigens is compatible with their expression as soluble molecules (sHLA-G), the determination of native sHLA-G levels in body fluids has not yet been described. The lack of this information is likely to reflect the difficulties in developing an assay suitable to measure sHLA-G antigens in the presence of soluble HLA-A, -B and -C (sHLA-I) antigens, since most of the available anti-HLA-G mAb do not detect soluble beta2-m associated HLA-G antigens or crossreact with sHLA-I antigens. Therefore, we have developed a two-step assay which eliminates the interference of classical HLA class I antigens. In the first step, the sample is depleted of sHLA-I antigens and of HLA-E antigens with mAb TP25.99. Then, HLA-G antigens are captured with mAb W6/32 and detected with anti-beta2-m mAb in ELISA. Utilizing this assay, sHLA-G antigen levels were measured in EDTA plasma from 92 controls with known HLA types, 28 women at delivery and the corresponding cord bloods and in 50 amniotic fluids. Mean sHLA-G plasma levels did not differ between males (24.9+/-3.0 SEM ng/ml; n=42) and females (20.1+/-2.1 SEM ng/ml; n = 50). However, sHLA-G levels in HLA-A11 positive probands (mean: 13.0+/-4.4 SEM ng/ml; n=12) were significantly (P<0.05) lower than in HLA-A11 negative ones (mean: 24.5+/-2.0 SEM ng/ml; n=80). sHLA-G levels in women at delivery (mean: 22.9+/-2.2 SEM ng/ml; n=28) were in the range of controls but were significantly (P<0.001) reduced in the corresponding cord bloods (mean: 13.8+/-1.5 SEM ng/ml; n=28). sHLA-G levels in amniotic fluids (mean: 15.5 + 1.0 SEM ng/ml; n=50) were significantly (P<0.001) lower than in plasma. sHLA-G levels were 5 and 11% of those of sHLA-I antigens in plasmas and amniotic fluids, respectively. Individual sHLA-G levels were not correlated with sHLA-I levels. SDS-PAGE analysis of plasma sHLA-G antigens revealed two molecular variants with a 35 kD and a 27 kD MW corresponding to the sizes of sHLA-G1 and -G2 isoforms. In conclusion, our study has shown that the two-step assay we have developed is reliable in measuring sHLA-G antigen levels. This assay will facilitate the analysis of the biological and clinical significance of sHLA-G antigens in plasma.

Molecular studies of trophoblast HLA-G: polymorphism, isoforms, imprinting and expression in preimplantation embryo

There is considerable interest in human HLA-G arising from the observation that it is expressed selectively on the surface of extravillous trophoblast, the fetal cell population directly in contact with the mother. We investigated several aspects of the molecular biology of this unusual molecule. Limited polymorphism at the nucleotide level, and even more restricted variation at the amino acid level, was found in our Caucasian population. A further unusual aspect of HLA-G is the occurrence of alternatively spliced mRNAs. Spliced messages that could give rise to either membrane-bound or soluble proteins have been reported and six of these alternative forms were detected in all first trimester and term placentae, highly purified villous and extravillous trophoblast and the cell lines, JEG-3 and 221-G. An additional novel splice variant involving loss of part of the 3'-untranslated region was observed with two alleles. Using a sensitive RNase protection assay higher levels of the membrane-bound RNAs as compared to the soluble forms were detected in first trimester and term placentae as well as in JEG-3. Contrary to previous findings our term samples taken from the maternal aspect showed higher levels of both mRNA species when compared to first trimester placenta. The question of imprinting was addressed through the detection of heterozygotes both in placental tissue and, more tellingly, in the purified trophoblast cells. There was no evidence of imprinting. In addition we did not find mRNA for HLA-G in human two to eight-cell embryos or in blastocyst or in sperm samples.

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.

Molecular and immunologic aspects of the nonclassical HLA class I antigen HLA-G: evidence for an important role in the maternal tolerance of the fetal allograft

PROBLEM

Human leukocyte antigen (HLA)-G is a major histocompatibility complex class I antigen, which is referred to as nonclassical because it displays a tissue-restricted distribution in the placenta, a reduced cytoplasmic domain, a limited polymorphism, and several isoforms. The HLA-G antigen is thought to play an essential role during pregnancy by protecting the semi-allogeneic fetus from recognition and destruction by maternal immune cells.

METHOD OF STUDY

Alternative splicing of HLA-G mRNA was analyzed by Southern blot of reverse transcriptase-polymerase chain reaction products from trophoblasts of the first trimester of gestation and term placenta. The regulation of HLA-G gene expression was investigated by electrophoretic mobility shift assays using nuclear extracts from cells expressing different levels of HLA-G gene activity. Using polymerase chain reaction-single strand conformational polymorphism and sequencing, we studied HLA-G gene polymorphism in families from the Centre d'Etude du Polymorphisme Humain in Paris. To understand the function of the HLA-G molecule, cytotoxicity assays were carried out with peripheral blood mononuclear cells or polyclonal natural killer effectors cells from 30 different donors against HLA-G1 and HLA-G2 transfectants.

RESULTS

Four main aspects have been elucidated: 1) The primary transcript of the HLA-G gene is alternatively spliced into five main mRNA forms: HLA-G1 (full length), HLA-G2 (minus exon 3), which encodes a membrane-bound isoform associated with beta-2 microglobulin, HLA-G3 (minus exons 3 and 4), HLA-G4 (minus exon 4), and HLA-G5 (plus intron 4), which encodes a soluble form of the HLA-G antigen; 2) specific nuclear factors bind to an important regulatory element located more than 1.2 kb from the HLA-G gene. Three specific complexes are observed in cells that show HLA-G transcriptional activity and an additional factor that could correlate with the repression of HLA-G gene expression that is detected in natural killer cells; 3) we observed an important genomic polymorphism in exon 3 but a very low polymorphism at the protein level; 4) HLA-G1 and HLA-G2 transfectants clearly demonstrated that both HLA-G isoforms are capable of inhibiting natural killer lytic activity.

CONCLUSION

These results suggest that HLA-G acts as the public ligand for natural killer inhibitory receptors, thus protecting the fetus against maternal rejection.

Expression of allogeneic MHC class I antigens by transgenic mouse trophoblast does not interfere with the normal course of pregnancy

Mammalian embryos express paternal histocompatibility antigens which make them potential targets for maternal immune responses. Yet, the histoincompatible fetus survives and develops normally. Down regulation of classical MHC antigen expression by trophoblast cells which are in direct contact with maternal circulation has been repeatedly shown. The trophoblast cells are unable to function properly in antigen presentation and do not induce allogeneic rejection reactions. In the present study we have created transgenic mice that express an allogeneic class I transgene whose transcription is controlled by the transferrin receptor promoter. The expression patterns of the transgene product mice from a single transgenic line were studied in each of the typical placental subpopulations. The allogeneic class I antigen was expressed in the allantoic plate region of the trophoblast, and this expression was not restricted to the endothelial region but extended also to the spongiotrophoblast, as well as the major blood vessels and in the endodermal sinuses. In contrast to the normal class I expression, prominent levels of allogeneic H-2 antigens were detected in the labyrinthine trophoblast. The fetal resorption rate in females mated with these transgenic males was not higher then the normal rate, and the embryos survived and developed normally. These data imply that the unusual expression of allogeneic class I antigens in certain trophoblast subpopulations does not affect fetal development.

HLA expression on immature and mature human germ cells

Human leukocyte antigens (HLA) are membrane-bound glycoproteins encoded by the human major histocompatibility complex located on chromosome 6. They are known to function in immnunologic recognition and, with regard to reproduction, a number of non-immune functions have been proposed. Although the expression patterns of the major histocompatibility antigens have been extensively studied at the maternal fetal interface, there are still controversial reports on the expression of these molecules by human gametes and preimplantation stages. This brief review focuses on recent studies where the expression and distribution of HLA on human spermatogenic cells (spermatogonia, primary and secondary spermatocytes, spermatids, spermatozoa), primary and secondary oocytes, and preimplantation embryos have been investigated. These results, and their possible implications for the fertilization process and further embryonic development, will be presented.

Soluble HLA-G in human placentas: synthesis in trophoblasts and interferon-gamma-activated macrophages but not placental fibroblasts

The HLA class Ib antigen, HLA-G, is highly expressed in early gestation placentas where it is believed to modulate maternal-fetal immunological interactions. In this study, soluble isoforms (sHLA-G) encoded by intron 4-retaining transcripts were identified in first trimester placentas by immunohistochemistry using a mAb specific for the C-terminus of sHLA-G. Immunoreactive sHLA-G protein was localized to trophoblast cells and to villous mesenchymal cells with the morphological features of macrophages. Reverse transcriptase polymerase chain reaction analysis which used primers specific for intron 4 and the 3' untranslated region of the HLA-G gene showed that transcripts encoding sHLA-G were present in the trophoblast-derived Jeg-3 cells as well as interferon-gamma-activated myelomonocytic U937 cells but were absent and uninducible in placental fibroblasts. These results indicate that placental sHLA-G is synthesized in trophoblast cells and activated placental macrophages and support the postulate that placenta-derived sHLA-G modulates maternal and fetal immune cell functions during pregnancy.

The nature of polymorphism of the HLA-DRB intron sequences is lineage specific

The sequence database of HLA-DRB genes is mainly derived from mRNA analysis or has focused exclusively on the polymorphism of the 2nd exon. Little is known about the non-coding sequences of the different DRB alleles which represent about 94% of the genes. In this study we have determined the sequence of the 3' 500 bp intron 1 fragment adjacent to exon 2 in all serologically defined HLA-DRB genes and their most frequent allelic subtypes. The intron sequences turned out to be highly polymorphic. Similar to the class I introns, this variability was not characterized by random point mutations but by a highly systematic diversity reflecting the lineage-specific relationship of the HLA-DR alleles. With a few exceptions in DRBI*15, 13 and 08 as well as DRB4 and 5, the variability mirrors the serological diversity. As well as delivering insight into the genetic relationship between the different DRB alleles, these sequences will provide an extremely valuable basis for developing advanced DRB sequencing strategies for clinical purposes.

HLA-G Isoforms Produced by Placental Cytotrophoblasts and Found in Amniotic Fluid Are Due to Unusual Glycosylation

The human placenta expresses HLA-G, a nonclassical (class Ib) MHC molecule that could play a central role in maternal tolerance of the semiallogeneic fetus. In this work, we report the production of a new mAb, 4H84, that specifically reacts with HLA-G in two formats: immunocytochemistry and immunoblotting. Immunolocalization experiments with 4H84 confirmed our previous finding that cytotrophoblasts within the uterine wall are the only cells in tissue sections of placenta that express the HLA-G protein. Additional experiments showed that both amniocytes and cytotrophoblasts in the amnion-chorion express this protein. Since multiple HLA-G transcripts have been described, we used immunoblotting to study the HLA-G isoforms produced by cytotrophoblasts in vitro and by the amnion-chorion in vivo. Cytotrophoblasts, their conditioned medium, and amniotic fluid samples contained heterodisperse immunoreactive bands (Mr 35,000–50,000). N-deglycosylation by peptide-N-glycosidase F digestion resolved these isoforms into two distinct bands. Cell samples contained primarily an Mr 37,000–42,000 protein, most likely encoded by the full-length mRNA. Conditioned medium and amniotic fluid contained a slightly smaller protein, most likely the secreted form lacking the transmembrane and cytoplasmic regions. Removal of polylactosamine chains by endo-β d-galactosidase digestion significantly reduced the electrophoretic mobility of the immunoreactive bands, suggesting that HLA-G, unlike class Ib molecules studied to date, carries N-acetyllactosamine units. These data show that Mr heterogeneity of HLA-G is due to its novel glycosylation, rather than to the translation of alternatively spliced mRNAs. We postulate that the unusual carbohydrate structures this molecule carries could interact with maternal immune cells and/or stabilize the molecule.

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.

The nature of polymorphism of the HLA class I non-coding regions and their contribution to the diversification of HLA

The sequence database of HLA class I genes is mainly derived from mRNA analysis. Little is known about the non-coding sequences of the different class I alleles. In this study we have determined the sequence of the 1st through 3rd introns of the majority of HLA-A and -B alleles. The few published sequences emerged to contain substantial errors. The introns turned out to be highly polymorphic with a variability of 14.6% in the 1st intron decreasing to 6.2% in the 3rd intron. Against all expectations, this variability is not characterised by random point mutations but by a highly systematic diversity reflecting the ancestral relationship of the HLA alleles. The variability is arrested on the level of the serological diversity. The striking conservation within each ancestral lineage suggests that point mutations have been negatively selected. This finding could be explained by the evolutionary pressure on base order, promoting the potential to extrude single-strand stem-loops from supercoiled duplex DNA, which is believed to be important for combination. Moreover, the GC content was found to be as high as 78% in the 1st and 2nd introns and 55% in the 3rd intron. These CpG islands are directly involved in the exchange of short stretches of DNA in unequal crossing-over events. Additionally, conversion between different class I sequences is facilitated by regions of strong homology, stabilizing the pairing of variable regions. All these observations indicate the potential of a substantial contribution of introns to the recombinational activity of class I genes. The exclusive clustering of CpG islands in the 1st and 2nd introns restricts the gene conversion events to the regions of the 2nd and 3rd exons and therefore protects the conservation of the 5 flanking region and the 3 part of the gene. Since there are less diversification forces acting on introns they may be more conserved in a trans-species manner than exons. Therefore, they could provide the answer for the controversy regarding intra- or trans-species evolution.

Co-dominant expression of the HLA-G gene and various forms of alternatively spliced HLA-G mRNA in human first trimester trophoblast

Genes may be silenced at the transcriptional level by 'genomic imprinting' in such a way that only one of the parental alleles is expressed. Imprinting may be tissue-specific and in some cases it seems also to be time-dependent during development. The phenomenon has been studied in pre- and post-implantation developmental processes. Animal studies of genomic imprinting of major histocompatibility complex (MHC) antigens in the placenta have shown discordant results. To address this issue in the human placenta, we examined the expression of the non-classical human leukocyte antigen (HLA) class I gene, HLA-G. Genomic imprinting of the HLA-G locus could have implications for the interaction in the feto-maternal relationship. Restriction Fragment Length Polymorphism (RFLP), allele-specific amplification and Single Strand Conformation Polymorphism (SSCP) analysis followed by DNA sequencing were performed on Reverse Transcription (RT) Polymerase Chain Reaction (PCR) products of HLA-G mRNA to examine the expression of maternal and paternal alleles. Our results demonstrate that HLA-G is co-dominantly expressed in first trimester trophoblast cells. A "new" non-synonymous base substitution in exon 4 was detected. We also investigated the different alternatively spliced forms of HLA-G mRNA in first trimester trophoblast and found the full-length transcript to be the far most abundant.

HLA class I expression on the materno-fetal interface

PROBLEM

The conditions that permit the genetically dissimilar (haplo-non-identical) human fetus to evade rejection by its mothers immune system have been the subject of intense interest for several years. As the placental cells, which are in contact with maternal blood or tissue, are devoid of HLA class II antigens, the interest has focused on the expression of HLA class I molecules.

METHOD OF STUDY

Recent findings on the constitutive, transcriptional, and translational expression of HLA class I molecules on anatomically and morphologically different subpopulations of trophoblast cells will form the basis of this article.

RESULTS

The expression of HLA class I molecules in the trophoblast cells, forming the materno-fetal junctional zone is inhomogeneous. It differs depending on the differentiation and location of trophoblast cells within the placenta and furthermore on the stage of gestation. On the transcriptional level HLA-A, -B, -C, -E, and -G could be detected on individual trophoblast populations, whereas only HLA-C and HLA-G seem to be translated to protein.

CONCLUSIONS

The expression of HLA class I antigens by trophoblast cells is not simply suppressed. Instead, less polymorphic HLA-G and HLA-C antigens are carefully selected from the class I multigene family. This gives rise to the assumption that these two HLA class I molecules play an important role in the maintenance of pregnancy.

Identification of a thymic epithelial cell subset sharing expression of the class Ib HLA-G molecule with fetal trophoblasts

HLA-G is the only class I determinant of the major histocompatibility complex (MHC) expressed by the trophoblasts, the fetal cells invading the maternal decidua during pregnancy. A unique feature of this nonclassical HLA molecule is its low polymorphism, a property that has been postulated to play an important role in preventing local activation of maternal alloreactive T and natural killer cells against the fetus. Yet, the mechanisms by which fetal HLA-G can be recognized as a self-MHC molecule by the maternal immune system remain unclear. Here we report the novel observation that HLA-G is expressed in the human thymus. Expression is targeted to the cell surface of thymic medullary and subcapsular epithelium. Thymic epithelial cell lines were generated and shown to express three alternatively spliced HLA-G transcripts, previously identified in human trophoblasts. Sequencing of HLA-G1 transcripts revealed a few nucleotide changes resulting in amino acid substitutions, all clustered within exon 3 of HLA-G, encoding for the alpha2 domain of the molecule. Our findings raise the possibility that maternal unresponsiveness to HLA-G-expressing fetal tissues may be shaped in the thymus by a previously unrecognized central presentation of this MHC molecule on the medullary epithelium.

The alpha1 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?

We have investigated the protective role of the membrane-bound HLA-G1 and HLA-G2 isoforms against natural killer (NK) cell cytotoxicity. For this purpose, HLA-G1 and HLA-G2 cDNAs were transfected into the HLA class I-negative human K562 cell line, a known reference target for NK lysis. The HLA-G1 protein, encoded by a full-length mRNA, presents a structure similar to that of classical HLA class I antigens. The HLA-G2 protein, deduced from an alternatively spliced transcript, consists of the alpha1 domain linked to the alpha3 domain. In this study we demonstrate that (i) HLA-G2 is present at the cell surface as a truncated class I molecule associated with beta2-microglobulin; (ii) NK cytolysis, observed in peripheral blood mononuclear cells and in polyclonal CD3(-) CD16(+) CD56(+) NK cells obtained from 20 donors, is inhibited by both HLA-G1 and HLA-G2; this HLA-G-mediated inhibition is reversed by blocking HLA-G with a specific mAb; this led us to the conjecture that HLA-G is the public ligand for NK inhibitory receptors (NKIR) present in all individuals; (iii) the alpha1 domain common to HLA-G1 and HLA-G2 could mediate this protection from NK lysis; and (iv) when transfected into the K562 cell line, both HLA-G1 and HLA-G2 abolish lysis by the T cell leukemia NK-like YT2C2 clone due to interaction between the HLA-G isoform on the target cell surface and a membrane receptor on YT2C2. Because NKIR1 and NKIR2, known to interact with HLA-G, were undetectable on YT2C2, we conclude that a yet-unknown specific receptor for HLA-G1 and HLA-G2 is present on these cells.

[Fetomaternal tolerance: role of HLA-G molecule in the protection of the fetus against maternal natural killer activity]

HLA-G is a non-classical major histocompatibility complex class I molecule selectively expressed on extravillous trophoblast cells at the fetal-maternal interface. HLA-G may play an important role in maintaining maternal immune tolerance of the semi-allogenic fetus. In this study, we demonstrate for the first time the protective role of HLA-G during pregnancy. Indeed, cytotrophoblast cells of the fetus are resistant to lytic activity by maternal decidual natural killer cells. In order to precisely characterize the immunological functions of HLA-G products, we have investigated the protective role of the membrane-bound HLA-G1 and HLA-G2 isoforms against NK cell cytotoxicity. For this purpose, HLA-G1 and HLA-G2 cDNAs were transfected into the HLA-class I negative human K562 cell line. We demonstrate that both HLA-G1 and HLA-G2 transfectants inhibit NK cytolysis observed in peripheral blood from 25 donors (males and females). This led us to the conjecture that HLA-G is the public ligand for natural killer inhibitory receptors present in all individuals.

PCR identification of class I major histocompatibility complex genes transcribed in mouse blastocyst and placenta

We used an RT-PCR based strategy to amplify, clone and sequence MHC class I genes transcribed in the blastocyst and placenta of BALB/c mice. The PCR primers used were capable of amplifying many novel class I sequences from genomic DNA. By comparing the resulting sequence data with known class I sequences, we identified a number of different class I genes transcribed in these tissues. These include H2-K, -D, -L and a novel sequence in blastocysts, and H2-K, -D, -L, -D2, -T9, -T13, -T17, -T18, -M2 and three additional novel sequences in placenta. We postulate that some members of this spectrum of blastocyst and placentally-expressed MHC class Ib genes may act together at the maternal-fetal interface in ways that are important for a successful pregnancy.

CpG methylation patterns in the 5' part of the nonclassical HLA-G gene in peripheral blood CD34+ cells and CD2+ lymphocytes

A dominant goal of research focused on the nonclassical human leukocyte antigen G (HLA-G) gene is to understand the molecular mechanism involved in its limited expression. In the present report, we examined DNA methylation as a potential regulatory mechanism of HLA-G transcription in two cell types of the adult lymphomyeloid lineage: CD2+ lymphocytes express several mRNA isoforms while transcripts are undetectable in CD34+ hematopoietic cells. The methylation status of 63 CpG sites in the promoter and in the 5' CpG island was established using bisulfite-treated genomic DNA sequencing. Methylation was first analyzed by the direct sequencing of bisulfite-treated and amplified products. The general patterns of CpG methylation in the 5' part of the gene were found to be similar for CD34+ cells and CD2+ lymphocytes: the distribution of methylation was not uniform across the 63 CpG sites. In the promoter region, both CpG dinucleotides were partially or fully methylated whereas in the CpG island, several CpG sites were totally demethylated. Unexpectedly, in HLA-G positive CD2+ lymphocytes, a great number of CpG dinucleotides displayed a higher frequency of methylation relative to that found in CD34+ cells. However, the sequence analysis of cloned products revealed that the molecules have different methylation patterns which suggests that the HLA-G gene is differentially expressed in CD2+ cells. Our results suggest that methylation is not the sole mechanism that achieves the repression of HLA-G transcription in immature CD34+ cells.

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.

Distribution of HLA-G alternative mRNAs including soluble forms in normal lymphocytes and in lymphoid cell-derived leukemia

The non-classical HLA-G gene is the only class I antigen expressed in trophoblasts at the maternofetal interface. In placenta, the HLA-G gene produces several alternatively spliced isoforms encoding bound-membrane proteins (G1, G2, G3 and G4) lacking, respectively, exon 7; exons 7 and 3; exons 7, 3 and 4, and exons 7 and 4. In addition, two isoforms (G1s and G2s) containing an intron 4 sequence are able to encode soluble antigens. We have recently reported that the HLA-G gene is transcriptionally active in lymphocytes and is not transcribed in CD34+ cells, polynuclear cells or monocytes. To investigate the functional significance of the different isoforms in lymphocytes, we studied their distribution in normal T and B lymphocytes and in malignant lymphoid cells by using the RT-PCR technique followed by hybridization with exon-specific probes and sequencing assays. In transcriptionally active lymphocytes, the HLA-G primary transcript is the major form and is differentially spliced in B and T lymphocytes: (i) G1s is found in several samples of T and B cells whereas G2s is only transcribed in T lymphocytes, (ii) the G4 isoform is never detected in B lymphocytes. In addition, we have shown that HLA-G is inactive in some samples of lymphocytes. Our data suggest that HLA-G transcription is regulated at the initiation level and at the subsequent splicing. These two levels of regulation may be dysregulated in some cases of T-ALL and CLL. The potential functions of the HLA-G alternative forms in lymphocytes, such as peptide binding and modulation of the immune response, are discussed.

Detection of membrane-bound HLA-G translated products with a specific monoclonal antibody

A monomorphic anti-HLA-G monoclonal antibody (mAb) was obtained by immunization of HLA-B27/human beta 2-microglobulin double-transgenic mice with transfected murine L cells expressing both HLA-G and human beta 2-microglobulin. This mAb, designated BFL.1, specifically recognizes, by flow cytometry analysis, the immunizing HLA-G-expressing cells, whereas it does not bind to parental untransfected or to HLA-B7- and HLA-A3-transfected L cells, suggesting that it distinguishes between classical HLA-A and -B and nonclassical HLA-G class I molecules. This was further assessed by the absence of BFL.1 reactivity with a number of human cell lines known to express classical HLA class I proteins. In addition, we showed that the BFL.1 mAb also labels HLA-G-naturally-expressing JEG-3 and HLA-G-transfected JAR human choriocarcinoma cell lines as well as a subpopulation of first-trimester placental cytotrophoblast cells. Further biochemical studies were performed by immunoprecipitation of biotinylated membrane lysates: BFL.1, like the monomorphic W6/32 mAb, immunoprecipitated a 39-kDa protein in HLA-G-expressing cell lines, a size corresponding to the predicted full-length HLA-G1 isoform. However, in contrast to W6/32, which immunoprecipitates both classical and nonclassical HLA class I heavy chains, BFL.1 mAb does not recognize the class Ia products. Such a mAb should be a useful tool for analysis of HLA-G protein expression in various normal and pathological human tissues and for determination of the function(s) of translated HLA-G products.