Expression of HLA-G in inflammatory bowel disease provides a potential way to distinguish between ulcerative colitis and Crohn's disease

In addition to being involved in nutrient uptake, the epithelial mucosa constitute the first line of defense against microbial pathogens. A direct consequence of this physiological function is a very complex network of immunological interactions that lead to a strong control of the mucosal immune balance. The dysfunction of immunological tolerance is likely to be a cause of inflammatory bowel disease (IBD), ulcerative colitis (UC) and Crohn's disease (CD). HLA-G is a non-classical major histocompatibility complex (HLA) class I molecule, which is highly expressed by human cytotrophoblast cells. These cells play a role in immune tolerance by protecting trophoblasts from being killed by uterine NK cells. Because of the deregulation of immune system activity in IBD, as well as the immunoregulatory role of HLA-G, we have analyzed the expression of HLA-G in intestinal biopsies of patients with UC and CD. Our study shows that the differential expression of HLA-G provides a potential way to distinguish between UC and CD. Although the reason for this differential expression is unclear, it might involve a different mechanism of immune regulation. In addition, we demonstrate that in the lamina propria of the colon of patients with UC, IL-10 is strongly expressed. In conclusion, the presence of HLA-G on the surface of intestinal epithelial cell in patients with UC lends support to the notion that this molecule may serve as a regulator of mucosal immune responses to antigens of undefined origin. Thus, this different pattern of HLA-G expression may help to differentiate between the immunopathogenesis of CD and UC.

HLA-G protein processing and transport to the cell surface

Data are presented on the intracellular trafficking of HLA-G protein, taking the unique features of this non-classical molecule into consideration: the existence of seven isoforms resulting from alternative splicing (HLA-G1 to G7), and reduced tail length compared with HLA class I antigens. Biochemical studies and analysis of viral strategies for escaping the host immune system led to the demonstration that (i) both the membrane-bound (HLA-G1) and the soluble (HLA-G5) forms of the molecule require peptide association for cell surface expression, using TAP-dependent or TAP-independent pathways; (ii) peptide loading onto the HLA-G protein plays a critical role in controlling the quality of the molecule reaching the cell surface; (iii) surface expression of truncated HLA-G molecules is possible, and (iv) HLA-G expression may be restricted to soluble HLA-G5. These data reveal that HLA-G presents specific cell trafficking pathways and strongly support the contention that the primary function of HLA-G is as of an inhibitor ligand for immune-competent cells.

Expanded polyglutamines in Caenorhabditis elegans cause axonal abnormalities and severe dysfunction of PLM mechanosensory neurons without cell death

Huntington's disease (HD) is a dominant neurodegenerative disease caused by polyglutamine (polyQ) expansion in the protein huntingtin (htt). HD pathogenesis appears to involve the production of mutated N-terminal htt, cytoplasmic and nuclear aggregation of htt, and abnormal activity of htt interactor proteins essential to neuronal survival. Before cell death, neuronal dysfunction may be an important step of HD pathogenesis. To explore polyQ-mediated neuronal toxicity, we expressed the first 57 amino acids of human htt containing normal [19 Gln residues (Glns)] and expanded (88 or 128 Glns) polyQ fused to fluorescent marker proteins in the six touch receptor neurons of Caenorhabditis elegans. Expanded polyQ produced touch insensitivity in young adults. Noticeably, only 28 +/- 6% of animals with 128 Glns were touch sensitive in the tail, as mediated by the PLM neurons. Similar perinuclear deposits and faint nuclear accumulation of fusion proteins with 19, 88, and 128 Glns were observed. In contrast, significant deposits and morphological abnormalities in PLM cell axons were observed with expanded polyQ (128 Glns) and partially correlated with touch insensitivity. PLM cell death was not detected in young or old adults. These animals indicate that significant neuronal dysfunction without cell death may be induced by expanded polyQ and may correlate with axonal insults, and not cell body aggregates. These animals also provide a suitable model to perform in vivo suppression of polyQ-mediated neuronal dysfunction.

Altered HLA-G transcription in pre-eclampsia is associated with allele specific inheritance: possible role of the HLA-G gene in susceptibility to the disease

Pre-eclampsia is a disorder of human pregnancy occurring in 5-10% of all births, and represents the leading cause of infant morbidity and mortality and maternal death. In pre-eclampsia, invasion of fetal trophoblasts into maternal arteries during early pregnancy is shallow or absent. Here we examined the hypothesis that HLA-G, a non-classical class I HLA expressed in cytotrophoblasts, may act as a key gene in pre-eclampsia. We analysed HLA-G at the level of transcription and genotyped a silent CAC-CAT polymorphism in exon 3 and a 14-bp insertion/deletion in the 3' untranslated region. A deficit in levels of the HLA-G3 transcript was observed in mild pre-eclampsia compared to normal placentas. The distribution of HLA-G polymorphisms was different between normal and pre-eclampsia samples. A correlation between the alteration in transcription of the HLA-G gene and certain HLA-G genotypes was also observed. Thus we provide the first evidence for a possible role of HLA-G in genetic susceptibility to, and pathogenesis of pre-eclampsia.

Soluble HLA-G protein secreted by allo-specific CD4+ T cells suppresses the allo-proliferative response: a CD4+ T cell regulatory mechanism

We recently reported that the nonclassical HLA class I molecule HLA-G was expressed in the endomyocardial biopsies and sera of 16% of heart transplant patients studied. The aim of the present report is to identify cells that may be responsible for HLA-G protein expression during the allogeneic reaction. Carrying out mixed lymphocyte cultures in which the responder cell population was depleted either in CD4(+) or CD8(+) T cells, we found that soluble HLA-G5 protein but not the membrane-bound HLA-G isoform was secreted by allo-specific CD4(+) T cells from the responder population, which suppressed the allogeneic proliferative T cell response. This inhibition may be reversed by adding the anti-HLA-G 87G antibody to a mixed lymphocyte culture. That may indicate a previously uncharacterized regulatory mechanism of CD4(+) T cell proliferative response.

[Predictive medicine: its future]

Predictive medicine or individualized preventive medicine, medicine based on the probabilities of carrying genes for susceptibility or resistance to a pathology, should be practiced with caution after a very strict evaluation of the risks-benefits.

HLA-G2, -G3, and -G4 isoforms expressed as nonmature cell surface glycoproteins inhibit NK and antigen-specific CTL cytolysis

HLA-G is a nonclassical MHC class I molecule that plays a major role in maternal-fetal tolerance. Four membrane-bound (HLA-G1 to -G4) and two soluble (HLA-G5, and -G6) proteins are generated by alternative splicing. Only HLA-G1 has been extensively studied in terms of both expression and function. We provide evidence here that HLA-G2, -G3, and -G4 truncated isoforms reach the cell surface of transfected cells, as endoglycosidase H-sensitive glycoproteins, after a 2-h chase period. Moreover, cytotoxicity experiments show that these transfected cells are protected from the lytic activity of both innate (NK cells) and acquired (CTL) effectors. These findings highlight the immunomodulatory role that HLA-G2, -G3, and -G4 proteins will assume during physiologic or pathologic processes in which HLA-G1 expression is altered.

A specific interferon (IFN)-stimulated response element of the distal HLA-G promoter binds IFN-regulatory factor 1 and mediates enhancement of this nonclassical class I gene by IFN-beta

Type I interferons display a broad range of immunomodulatory functions. Interferon beta increases gene expression at the transcriptional level through binding of factors to the interferon-stimulated response element (ISRE) within the promoters of interferon-inducible genes, such as HLA class I. Despite mutation of the class I ISRE sequence within the nonclassical HLA-G class I gene promoter, we show that interferon beta enhances both transcription and cell surface expression of HLA-G in trophoblasts and amniotic and thymic epithelial cells that selectively express it in vivo. Deletion and mutagenesis analysis of a putative interferon-regulatory factor (IRF)-1 binding site within the HLA-G promoter show that HLA-G transactivation is mediated through an ISRE sequence 746 base pairs upstream from ATG, which is distinct from the interferon-responsive element described within proximal classical class I gene promoters. Electrophoretic mobility shift analysis and supershift analysis further demonstrate that interferon-responsive transcription factors, including IRF-1, specifically bind to the HLA-G ISRE. Our results provide evidence that IRF-1 binding to a functional ISRE within the HLA-G promoter mediates interferon beta-induced expression of the HLA-G gene. These observations are of general interest considering the implication of HLA-G in mechanisms of immune escape involved in fetal-maternal tolerance and other immune privilege situations.

Spontaneous retinopathy in HLA-A29 transgenic mice

Humans who have inherited the class I major histocompatibility allele HLA-A29 have a markedly increased relative risk of developing the eye disease termed birdshot chorioretinopathy. This disease affecting adults is characterized by symmetrically scattered, small, cream-colored spots in the fundus associated with retinal vasculopathy and inflammatory signs causing damage to the ocular structures, leading regularly to visual loss. To investigate the role of HLA-A29 in this disease, we introduced the HLA-A29 gene into mice. Aging HLA-A29 transgenic mice spontaneously developed retinopathy, showing a striking resemblance to the HLA-A29-associated chorioretinopathy. These results strongly suggest that HLA-A29 is involved in the pathogenesis of this disease. Elucidation of the role of HLA-A29 should be assisted by this transgenic model.

The Gln-Ala repeat transcriptional activator CA150 interacts with huntingtin: Neuropathologic and genetic evidence for a role in Huntington's disease pathogenesis

Huntington's disease (HD) is a neurodegenerative disease caused by polyglutamine expansion in the protein huntingtin (htt). Pathogenesis in HD appears to involve the formation of ubiquitinated neuronal intranuclear inclusions containing N-terminal mutated htt, abnormal protein interactions, and the aggregate sequestration of a variety of proteins (noticeably, transcription factors). To identify novel htt-interacting proteins in a simple model system, we used a yeast two-hybrid screen with a Caenorhabditis elegans activation domain library. We found a predicted WW domain protein (ZK1127.9) that interacts with N-terminal fragments of htt in two-hybrid tests. A human homologue of ZK1127.9 is CA150, a transcriptional coactivator with a N-terminal insertion that contains an imperfect (Gln-Ala)(38) tract encoded by a polymorphic repeat DNA. CA150 interacted in vitro with full-length htt from lymphoblastoid cells. The expression of CA150, measured immunohistochemically, was markedly increased in human HD brain tissue compared with normal age-matched human brain tissue, and CA150 showed aggregate formation with partial colocalization to ubiquitin-positive aggregates. In 432 HD patients, the CA150 repeat length explains a small, but statistically significant, amount of the variability in the onset age. Our data suggest that abnormal expression of CA150, mediated by interaction with polyglutamine-expanded htt, may alter transcription and have a role in HD pathogenesis.

HLA-G1 co-expression boosts the HLA class I-mediated NK lysis inhibition

It is now acknowledged that the pattern of HLA-G expression is not restricted to extravillous cytotrophoblast cells, as several studies described HLA-G in HLA class I+ cells, such as thymic epithelial cells, cytokine-activated monocytes and some tumors. In these situations, HLA-G may provide an additional inhibitory signal to escape from NK cell-mediated cytotoxicity. Accordingly, the aim of this study was to define the behavior of HLA-G once it is co-expressed into an HLA-A, -B, -C and -E+ cell line. For this purpose, HLA-G1 cDNA was transfected into an HLA class I+ melanoma cell line which was used as a target towards freshly isolated peripheral blood NK cells. Cytotoxic experiments using either anti-HLA-G1 or anti-HLA-G1 inhibitory receptor mAb show that HLA-G1 boosts the HLA class I-mediated inhibition of polyclonal NK cells through interaction with ILT-2, which appears as the major HLA-G1 inhibitory receptor involved. Nevertheless, HLA-G1 is also able to inhibit the cytolytic activity of an ILT-2- NK clone which otherwise expresses another HLA-G1 inhibitory receptor belonging to the KIR103 gene family. In order to more precisely define the relative role exerted by HLA-G1 versus -E on polyclonal NK cells, antibody-blocking assays were carried out using either anti-HLA class I or anti-CD94/NKG2A. Results demonstrate that in the absence of HLA-G1, the naturally expressed HLA class I-mediated NK inhibition is predominantly exerted by HLA-E through binding with CD94/NKG2A. In contrast, once HLA-G1 is expressed, it becomes the major NK inhibitory ligand.

Heat shock and arsenite induce expression of the nonclassical class I histocompatibility HLA-G gene in tumor cell lines

The nonclassical histocompatibility class I gene HLA-G has a tissue-restricted expression. To explore mechanisms involved in HLA-G transcriptional regulation, we have investigated the effect of stress, including heat shock and arsenite treatment, on HLA-G expression in tumor cell lines. We show that stress induces an increase of the level of the different HLA-G alternative transcripts without affecting other MHC class I HLA-A, -B, -E, and -F transcripts. A heat shock element (HSE) that binds to heat shock factor 1 (HSF1) on stress conditions was further identified within the HLA-G promoter. Considering the ability of HLA-G to modulate the function of immunocompetent cells, we hypothesize a new feature of HLA-G as a signal regulating the immune response to stress.

HLA-G truncated isoforms can substitute for HLA-G1 in fetal survival

Pregnancy is considered as an immunologic paradox because the fetus can be viewed as a semiallograft by the mother's immune system. Among the different factors implicated in the maternal-fetal tolerance, a central role has been attributed to HLA-G. The primary HLA-G mRNA is alternatively spliced, encoding four membrane-bound isoforms (HLA-G1, -G2, -G3, and -G4), and three soluble forms (HLA-G5, -G6, and -G7). Whereas HLA-G1 is expressed on trophoblast cells, HLA-G2, -G3, and -G4 isoforms have been only identified as transcripts in trophoblast and term placentas. In this work, we first showed that these HLA-G transcripts are translated into proteins in first trimester cytotrophoblast cells. Then, using a target cell line transfected with HLA-G genomic DNA, we analyzed the functional implication of HLA-G isoforms expression on NK function. Our results show that not only HLA-G1, but also the other HLA-G truncated isoforms, can inhibit NK cytolysis and therefore contribute to immune privilege for the fetus.

The X1 box of HLA-G promoter is a target site for RFX and Sp1 factors

HLA-G gene regulation was investigated with regards to homologies among the pathways regulating both classical MHC class I and MHC class II gene expression. They include four conserved cis-acting regulatory elements located in the proximal promoter region referred to as the W/S/Z box, the X box that is comprised of the X1 and X2 halves, and the Y box with an inverted CCAAT site. The X1 box is the binding site for the ubiquitous RFX complex consisting of three subunits; the X2 box is bound by the X2BP/ATF/CREB family factors. The basic S-X-Y regulatory module interacts with CIITA, which is expressed constitutively in APCs, but may be inducible in others cell types by IFN-gamma. Within HLA-G gene promoter the only conserved motifs are S and X1 boxes. We thus investigated the binding capacity of the HLA-G X box in comparison to that of HLA-DRA and HLA-E. We demonstrate that X2 box mutations in HLA-G promoter affect the binding of ATF/CREB family factors and may privilege the X2 box to access by other shared factors. The X1 box is the target for RFX complex and an additional factor we identified as Sp1. We propose that the X region in the HLA-G gene promoter might participate to the combination of factors which play a role in HLA-G gene activation.

Modulation of HLA-G expression in human thymic and amniotic epithelial cells

Expression of the nonclassical HLA class I antigen, HLA-G, is tightly regulated. HLA-G physiologic expression is mostly restricted to some placental and thymic cell types. Only few established cell lines express HLA-G in vitro. Cytokine-induced expression of HLA-G is hardly observed and also depends on the cell lineage. We assessed expression and cytokine regulation of HLA-G in primary cultures derived from human thymus and amnion epithelial cells, which also express HLA-G in vivo. We show that HLA-G cell surface expression is maintained, but decreases gradually, in primary cultures derived from human thymus and amnion epithelial cells. We also show that IFN-gamma re-induces HLA-G cell surface expression and upregulates classical class I gene expression in both primary cultures and in a thymus derived cell line. We further show that IFN-gamma also upregulates levels of HLA-G transcripts in TEC primary cultures. This study provides evidence that IFN-gamma induction of HLA-G expression occurs in the human amnion and the thymus, and is mediated at the transcriptional level in these tissues. These results also suggest a role for the microenvironment in regulating HLA-G in vivo gene expression in the thymus and amnion membrane.

Analysis of the role of HLA-G in preeclampsia

Preeclampsia (PE) is a multisystem disorder of human pregnancy, occurring in 5%-10% of all population births and represents the leading cause of both fetal and maternal morbidity and mortality in pregnancy. Although the disorder only becomes clinically apparent late in pregnancy, the underlying pathology indicates that invasion of fetal trophoblasts into maternal spiral arteries during early pregnancy is shallow or absent in PE. A large number of epidemiologic studies have been carried out and they demonstrate that the disorder is highly heritable and occurs with a high incidence in all populations. Studies have shown that PE is largely under genetic control, but the mode of its inheritance remains unclear. Genetic studies have been carried out using both large scale linkage analysis and candidate gene approaches; however, the genetic mechanisms underlying the disorder have yet to be determined. We focus on the potential role of HLA-G, a nonclassical class I HLA located on chromosome 6, which appears to be a key component in trophoblast invasion. We examine the hypothesis that HLA-G may have a key role in both genetic susceptibility to, and pathogenesis of, PE.

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

The nonclassical HLA-G primary transcript is alternatively spliced to generate several mRNAs that have the capacity to encode four membrane bound isoforms, namely HLA-G1, -G2, -G3, and -G4 and two soluble isoforms HLA-G5 and -G6. We aimed at defining the capacity of full length and truncated soluble HLA-G transcripts to be translated in human cell lines. Our study of HLA-G alternative transcripts in various human tissues led us to identify a new splice variant of the HLA-G mRNA, named G7, in which open reading frame continues in intron 2. Due to the presence of a stop codon within intron 2, HLA-G7 transcripts retain the capacity to be translated as soluble truncated HLA-G proteins bearing the alpha1 domain linked to two specific aminoacids encoded by intron 2. Expression vectors containing cDNAs encoding HLA-G5, -G6, and -G7 isoforms were transfected into human cell lines. The presence of translated HLA-G5, -G6, and -G7 proteins was detected in protein extracts of transfected cells by Western blot and immunoprecipitation, but only the full length HLA-G5 soluble isoform could be clearly detected as a secreted protein in both transfected cells supernatants and body fluids.

HLA-G, -E, -F preworkshop: tools and protocols for analysis of non-classical class I genes transcription and protein expression

Non-classical MHC class I HLA-E, -F, and -G molecules differ from classical class I histocompatibility antigens by specific patterns of transcription, protein expression, and immunological functions. Restriction of the expression pattern of these non-classical antigens may play a key role in modulation of immune responses during pregnancy and diseases but remains to be additionally defined. A specific component of the second International Conference on HLA-G and the 13th HLA-G Histocompatibility Workshop will be dedicated to the analysis of transcription and expression of non-classical class I genes in normal and pathological tissues. In a first step, referred to as the preworkshop, we here report the analysis and conclusions of a working group which was constituted to gather and validate optimal reagents and protocols allowing RT-PCR analysis of HLA-E, -F, -G transcript levels and flow cytometry and immunochemistry analysis of HLA-G expression in cells and tissues. As a result of this work, use of specific primers and probes detecting alternative transcripts of HLA-E, -F, and G have been validated in transfected cells expressing differential pattern of HLA class I antigens. Analysis of the specificity and affinity of collected antibodies has allowed definition of reagents to be proposed for immunochemistry and flow cytometry analysis of HLA-G expression in normal and pathological tissues during the workshop. This work has allowed constitution of an extended workshop group which is now initiating analysis of non-classical class I transcription and expression in various cells and tissues, a collective contribution that will additionally refine our view of the expression of these antigens in normal and pathological situations.

Implication of HLA-G molecule in heart-graft acceptance

HLA-G found in five of 31 heart-transplant recipients was associated with a decrease of acute and chronic rejection episodes.

Behavioral alterations associated with apoptosis and down-regulation of presenilin 1 in the brains of p53-deficient mice

Presenilin 1 (PS1) expression is repressed by the p53 tumor suppressor. As shown herein, wild-type PS1 is an effective antiapoptotic molecule capable of significantly inhibiting p53-dependent and p53-independent cell death. We analyzed, at the functional and molecular levels, the brains of p53 knockout mice. Surprisingly, we found that lack of p53 expression induces apoptotic brain lesions, accompanied by learning deficiency and behavioral alterations. p53-deficient mice show an unexpected overexpression of p21(waf1) with subsequent down-regulation of PS1 in their brains. This process is progressive and age-dependent. These data indicate that the p53 pathway, besides affecting tumor suppression, may play a major role in regulating neurobehavioral function and cell survival in the brain.

HLA-G promotes immune tolerance

HLA-G is a non-classical major histocompatibility complex class I molecule that differs from the classical HLA-A, -B and -C molecules by (i) alternative splicing of mRNAs encoding for at least four membrane-bound and two soluble HLA-G isoforms, (ii) a limited polymorphism, and (iii) a tissue-restricted distribution. Studies over the past few years have elucidated the function of HLA-G demonstrating inhibition of both NK cell- and T cell-mediated cytolysis. Furthermore, aside from its expression during pregnancy, we have shown that HLA-G is also expressed in solid tumor cells (i.e. human melanoma cell lines and ex vivo melanoma biopsies). Here we present a review of the current state of knowledge of the immunotolerant functions of HLA-G and their implications in materno-fetal tolerance and tumor immunosurveillance.

A linkage disequilibrium map of the MHC region based on the analysis of 14 loci haplotypes in 50 French families

A sample of 100 individuals from 50 French families of known pedigrees were typed for 14 loci of the HLA region (DPB1, DQB1, DQA1, DRB1, DRB3, 4, 5, C4B, C4A, Bf, C2, TNFa, TNFb, B, Cw, A). Linkage disequilibrium in each pair of loci was investigated by an exact test using a Markov chain algorithm. The results indicate no disequilibrium between DPB1 and the other loci, whereas the other class II genes are all significantly linked to each other. Linkage disequilibrium is also detected between some pairs of class I and class II-class I loci despite the long physical distance separating the loci (e.g. A-B, Cw-DRB1). On the other hand, some contiguous loci of the class III region are found to be in equilibrium with each other. Several hypotheses including selection, but also unequal allelic diversity at different MHC loci are discussed to explain this complex pattern of linkage disequilibrium.

HLA-G-mediated inhibition of antigen-specific cytotoxic T lymphocytes

In the present study, we demonstrate that the non-classical MHC class I molecule HLA-G impairs specific cytolytic T cell functions in addition to its well-established inhibition of NK lysis. The antigen-specific cytotoxic T lymphocyte (CTL) response analyzed was mediated by CD8(+) T cells specific for the influenza virus matrix epitope, M58-66, presented by HLA-A2. The transfection of HLA-G1 cDNA in target cells carrying the M58-66 epitope reduced their lysis by these virus-specific CTL. This HLA-G-mediated inhibition of antigen-specific CTL lysis was (i) peptide dose dependent, (ii) reversed by blocking HLA-G with a specific mAb and (iii) still observed despite the blockade of HLA-E/CD94/NKG2A interaction. By inhibiting both CTL and NK functions, HLA-G appears to have an extensive role in immune tolerance.

SIAH-1 promotes apoptosis and tumor suppression through a network involving the regulation of protein folding, unfolding, and trafficking: identification of common effectors with p53 and p21(Waf1)

We have previously described biological model systems for studying tumor suppression in which, by using H-1 parvovirus as a selective agent, cells with a strongly suppressed malignant phenotype (KS or US) were derived from malignant cell lines (K562 or U937). By using cDNA display on the K562/KS cells, 15 cDNAs were now isolated, corresponding to genes differentially regulated in tumor suppression. Of these, TSAP9 corresponds to a TCP-1 chaperonin, TSAP13 to a regulatory proteasome subunit, and TSAP21 to syntaxin 11, a vesicular trafficking molecule. The 15 cDNAs were used as a molecular fingerprint in different tumor-suppression models. We found that a similar pattern of differential regulation is shared by activation of p53, p21(Waf1), and the human homologue of Drosophila seven in absentia, SIAH-1. Because SIAH-1 is differentially expressed in the various models, we characterized it at the protein and functional levels. The 32-kDa, mainly nuclear protein encoded by SIAH-1, can induce apoptosis and promote tumor suppression. These results suggest the existence of a common mechanism of tumor suppression and apoptosis shared by p53, p21(Waf1), and SIAH-1 and involving regulation of the cellular machinery responsible for protein folding, unfolding, and trafficking.

HLA-G expression in human melanoma cells: protection from NK cytolysis

Expression of the non-classical HLA-G class I antigen is physiologically restricted to a limited number of tissues including trophoblasts, and is thought to play a role in establishing tolerance of the fetus by the maternal immune system. We investigated whether ectopic expression of HLA-G could also be detected in tumor cells and confer them the ability to escape immune cytotoxic responses. High levels of all alternatively spliced HLA-G transcripts could be detected in melanoma cells by RT-PCR. Analysis of biopsies from a melanoma patient revealed a higher HLA-G transcription level in skin metastasis as compared to healthy skin, while specific amplification of the HLA-G5 transcript was only observable in the tumor. HLA-G protein expression could also be detected in two melanoma cell lines. HLA-G-positive tumors inhibit cytotoxic lysis by the NK cell line YT2C2-PR. This inhibition is not observed with B-EBV cell lines bearing matched class I specificities, and is thought to occur through interaction of HLA-G with inhibitory receptors that are distinct from known KIRs interacting with HLA-E or classical class I molecules. Together, these results confirm that HLA-G expression at the surface of tumor cells can participate in the evasion of antitumoral immune responses and favor tumor progression.

Molecular mechanisms controlling constitutive and IFN-gamma-inducible HLA-G expression in various cell types

HLA-G molecule is thought to play a major role in down-regulating the maternal immune response by inhibiting NK and T cell cytolytic activities. We examined the molecular regulatory mechanisms that may control the restricted expression pattern of the HLA-G gene. We first analyzed protein interactions between nuclear extracts from the HLA-G-positive JEG-3 choriocarcinoma and the HLA-G-negative NK-like YT2C2 cell lines to a 244 bp regulatory element located 1.2 kb from the HLA-G gene, previously shown to direct HLA-G expression in transgenic mouse placenta. This allowed characterization of cell-specific DNA-protein interactions that could account for differential cell-specific expression of the HLA-G gene. In particular two DNA-protein complexes were exclusively observed in YT2C2, suggesting that this HLA-G regulatory element is a target for putative cell-specific repressor factors. We further mapped nuclear factor binding sites to a 70 bp fragment in the upstream region of the regulatory element. We then investigated the effect of IFN-gamma on HLA-G gene expression. HLA-G cell surface expression was enhanced by IFN-gamma treatment in JEG-3 and U937 cell lines and peripheral blood monocytes while no effect was observed in tera-2 teratocarcinoma cell line. HLA-G transcriptional activity was increased only in JEG-3 and U937 cell lines. Activity of the 1.4-kb HLA-G promoter region was unchanged after IFN-gamma treatment in JEG-3 and Tera-2. These results suggest that both post-transcriptional and transcriptional mechanisms implicating IFN-responsive regulatory sequences outside the 1.4 kb-region are involved in IFN-gamma gene activation of the HLA-G gene.

HLA-G inhibits the allogeneic proliferative response

HLA-G is a non-classical MHC class I molecule expressed at the feto/maternal interface where it plays a role in materno-fetal tolerance by inhibiting NK cells. Expression of killing inhibitory receptors capable of interacting with HLA-G on T lymphocytes led us to hypothesize that HLA-G molecules could also modulate T cell responses, analyzed here in the context of the allogeneic proliferative response. Using LCL-HLA-G transfectants as stimulators of T cells present among peripheral mononuclear cells and K562-HLA-G1 transfectants as inhibitors in a classical mixed lymphocyte reaction, we showed that HLA-G is able to inhibit T cell allo-proliferation. These findings provide new insight into the role of HLA-G in preventing allograft rejection.

Role of HLA-G versus HLA-E on NK function: HLA-G is able to inhibit NK cytolysis by itself

Recent studies have shown that endogenous HLA-E molecules are stabilized on the cell surface upon the expression of HLA-G which contains within its leader sequence, a nonapeptide capable of binding with the HLA-E/beta2m complex. Since HLA-E was found to be the major ligand for the CD94/NKG2A inhibitory receptor, we determined the role of HLA-G versus HLA-E on NK lysis inhibition. We showed that K562 cells transfected with HLA-G1 cDNA are protected from NK lysis by direct interaction between HLA-G1 and killing inhibitory receptor(s). This NK lysis inhibition is not dependent on HLA-E expression, since no HLA-E protein was detected on K562 cells; HLA-G1 is therefore able to inhibit NK lysis by itself.

IL-10 selectively induces HLA-G expression in human trophoblasts and monocytes

HLA-G plays an essential role in feto-maternal tolerance by inhibiting lysis by maternal NK cells. The factors that allow tissue-specific activation of HLA-G gene expression in trophoblasts remain to be characterized. We investigated the potential effect of IL-10, a cytokine which is secreted in placenta, on HLA-G gene transcription in trophoblasts. Using Northern blot, RNase protection assay and RT-PCR analysis, we demonstrated that IL-10 enhances steady-state levels of HLA-G transcription in cultured trophoblast cells. We further tested the effect of IL-10 on HLA-G gene transcription and protein expression in peripheral blood monocytes, showing that IL-10 can up-regulate HLA-G cell surface expression in this cell type. This effect of IL-10 is selective, since classical MHC class I products and MHC class II are down-regulated in monocytes following IL-10 treatment. Induction of HLA-G expression by IL-10 on monocytes may thus play a role in down-regulation of the immune response. We propose that IL-10 secretion by trophoblasts during pregnancy may also influence the HLA class I expression pattern at the feto-maternal barrier, thus protecting the fetus from rejection. This should be taken into consideration in the design of treatment for pathologies of pregnancy.