Endothelial cells in chorionic fetal vessels of first trimester placenta express HLA-G

HLA-G gene polymorphism in human placentas: possible association of G*0106 allele with preeclampsia and miscarriage

Definite causes for several pathologies of pregnancy remain unknown. In light of several recent studies, however, diminished or aberrant HLA-G expression may be associated with certain complication of pregnancy and be linked to HLA-G polymorphism. We analyzed DNA from 60 normal placentas (controls), 140 placentas from miscarriage, 36 placentas from preeclampsia, 76 placentas from fetal hypotrophy, and 34 placentas with hypoxia for variations in coding regions (allelic groups G*0101 to G*0107) and the 14-bp deletion/insertion into the 3'-untranslated region. No statistically significant differences were observed in the distribution of allelic group between pathological placentas and controls with the exception of G*0106 allele frequency in preeclamptic compared with control placentas (21.2% and 6.6%, respectively). A greater frequency of this allele also was observed in the two subgroups of miscarriage and hypoxia compared with that in controls. In addition, presence of the 14-bp sequence was prominent in preeclampsia compared with controls (60.8% vs. 35%, respectively), and homozygotes with deletion were not detected in the pathology. The results suggest that the G*0106 allele, which is coupled with the presence of the 14-bp sequence, contributes and/or is a relevant marker in some specific complications of pregnancy, especially preeclampsia.

HLA-G and its role in implantation (review)

BACKGROUND

Human leukocyte antigen G (HLA-G) is thought to play a key role in implantation by modulating cytokine secretion to control trophopblastic cell invasion and to maintain a local immunotolerance.

METHOD OF STUDY

The literature is reviewed to provide a description of the genetic background, properties of the protein, and the function of HLA-G. Data are presented on potential clinical applications of HLA-G including the use of evaluation of HLA-G gene polymorphisms in the diagnosis of patients experiencing recurrent pregnancy loss and evaluation and testing of soluble HLA-G (sHLA-G) in embryo culture media for the selection of embryos for transfer after in vitro fertilization (IVF).

RESULTS

The literature supports a central role of HLA-G for successful implantation. Of couples experiencing recurrent pregnancy loss, 32% demonstrated the -1725G HLA-G polymorphism. Our data showed that when embryos were selected for transfer after IVF based on culture media concentrations of sHLA-G > or = 2 U/ml and good morphologic grade, a 65% pregnancy rate compared with a 0% pregnancy rate in those with <2 U/ml sHLA-G.

CONCLUSIONS

HLA-G is important for successful implantation in human beings. The HLA-G -725 promoter polymorphism is a risk factor for recurrent miscarriage. Measurement of sHLA-G in embryo culture media can help select embryos for transfer after IVF allowing fewer embryos to be transferred in an attempt to lower multiple gestation rates.

Soluble HLA-G molecules in follicular fluid: A tool for oocyte selection in IVF?

Currently, different approaches are used to select oocytes for in vitro fertilization (IVF) procedures, but they do not assure a significant association with the pregnancy outcome. Since several studies have proposed the expression of HLA-G antigens in early embryos to be a possible marker of elevated implantation rate, we have investigated the presence of soluble HLA-G molecules in 50 follicular fluids (FFs). The results have shown soluble HLA-G antigens (sHLA-G) in 19/50 (38%) FFs. Furthermore, we have related the presence of sHLA-G molecules in FFs to detection of the soluble antigens in culture supernatants of the corresponding fertilized oocyte, evidencing a significant relationship (p=1.3 x 10(-6); Fisher exact p-test). Specific ELISA and Western blot approaches identified both HLA-G5 and soluble HLA-G1 molecules in FFs while immunocytochemical analysis indicated polymorphonuclear-like and granulosa cells as responsible for production of sHLA-G1 and HLA-G5 molecules. In contrast, only sHLA-G1 antigens were detected in culture supernatants of fertilized oocytes. Overall, these results suggest a role for sHLA-G molecules in the ovulatory process and propose the FFs analysis for sHLA-G molecule presence as a useful tool for oocyte selection in IVF.

Human leukocyte antigen G expression: as a significant prognostic indicator for patients with colorectal cancer

Aberrant expression of human leukocyte antigen G (HLA-G) has been proposed to be involved in tumor escape mechanisms. It has been also proposed that detection of HLA-G might service as a potential biomarker for diagnosis or prediction of the clinical outcomes in ovarian and breast cancers, carcinoma of the lung and endometrial cancer. The aim of this current study is to determine if HLA-G is expressed in colorectal carcinomas and if the expression is associated with clinicopathological and prognostic data. The expression of HLA-G was investigated immunohistochemically in 201 patients with colorectal carcinomas. The correlation between HLA-G status, clinicopathological factors and the overall survival rate was analyzed. In this prospectively study, HLA-G protein expression was observed in 64.6% (130/201) of the primary site colorectal carcinomas, but not in the normal colorectal tissues or benign adenomas. HLA-G expression in the tumors was significantly correlated with the depth of invasion, histological grade, host immune response, lymph nodal metastasis and clinical stages of the disease (P=0.001, 0.0001, 0.002, 0.001 and 0.031, respectively). Patients with HLA-G positive tumors had a significantly shorter survival time than those patients with tumors that were HLA-G negative (P=0.0001). As well, in multivariate analysis, HLA-G demonstrated an independent prognostic factor (P=0.021, relative risk 3.14; 95% confidence interval, 1.34-8.10). Therefore, it can be gathered that HLA-G might serve as an independent prognostic factor for colorectal cancer patients.

Contrasting mode of evolution between the MHC class I genomic region and class II region in the three-spined stickleback (Gasterosteus aculeatus L.; Gasterosteidae: Teleostei)

Major histocompatibility complex (MHC) class I molecules display peptides on cell surfaces for subsequent T-cell recognition and are involved in the immune response against intracellular pathogens. In this study, a BAC library was created from a single three-spined stickleback and screened for clones containing MHC class I genes. In a 163.2-kb genomic sequence segment of a single clone, we identified three MHC class I genes in the same transcriptional orientation. Two class I genes are potentially expressed and functional. In one class I gene, the transmembrane region is missing and could therefore present a pseudogene. Alternatively, it presents a functional gene that encodes a soluble MHC class Ib molecule. Despite genomic similarities to the MHC class II region, which is characterized by interlocus recombination, we did not find any evidence for this kind of recombination in the class I genes. It thus seems that interlocus recombination may play a rather minor role in generating class I diversity in stickleback and that the class I region displays a higher genomic stability (i.e., lower local recombination rate). In addition, two non-MHC genes (Oct-2 beta and Na(+),K(+)-ATPasealpha3) have been identified in the analyzed class I region. The Oct-2 beta gene is a transcription factor that is expressed primarily in B lymphocytes, in activated T-cells, and in neuronal cells. The Na(+),K(+)-ATPasealpha3 gene is primarily expressed in the brain and heart and mediates catalytic activities. Both genes are located on the same linkage group together with the MHC class I genes in the zebra fish. In humans, however, homologues of Oct-2 beta and ATPasealpha3 lie outside the MHC region, which indicates that the concentration of immune genes found in mammalian genomes is a derived state.

Soluble HLA-G molecules increase during acute leukemia, especially in subtypes affecting monocytic and lymphoid lineages

Human leukocyte antigen G (HLA-G) molecules exhibit immunomodulatory properties corresponding to nonclassic class I genes of the major histocompatibility complex. They are either membrane-bound or solubly expressed during certain tumoral malignancies. Soluble human leukocyte antigen G (sHLA-G) molecules seem more frequently expressed than membrane-bound isoforms during hematologic malignancies, such as lymphoproliferative disorders. Assay of these molecules by enzyme-linked immunosorbent assay in patients suffering from another hematologic disorder (acute leukemia) highlights increased sHLA-G secretion. This increased secretion seems more marked in acute leukemia subtypes affecting monocytic and lymphoid lineages such as FABM4 and FABM5, as well as both B and T acute lymphoblastic leukemia (ALL). Moreover, this study uses in vitro cytokine stimulations and reveals the respective potential roles of granulocyte-macrophage colony-stimulating factor and interferon-gamma in increasing this secretion in FABM4 and ALL. Correlations between sHLA-G plasma level and clinical biologic features suggest a link between elevated sHLA-G level and 1) the absence of anterior myelodysplasia and 2) high-level leukocytosis. All these findings suggest that sHLA-G molecules could be a factor in tumoral escape from immune survey during acute leukemia.

Plasma soluble human leukocyte antigen G levels in asthmatic children

BACKGROUND

Human leukocyte antigen G (HLA-G) is a nonclassical major histocompatibility complex class I gene. HLA-G stimulates Th2 cytokine secretion by peripheral blood mononuclear cells. The role of soluble HLA-G (sHLA-G) in bronchial asthma is incompletely understood and the plasma level of sHLA-G in asthmatic children has not been investigated.

OBJECTIVE

It was the aim of this study to investigate the plasma level of sHLA-G in asthmatic children.

METHODS

Asthmatic (n = 53) and healthy children (n = 16) were included in the study. Levels of sHLA-G were determined in plasma using ELISA. Spirometry, total immunoglobulin E and eosinophil counts were obtained and skin testing done with a battery of 25 antigens with appropriate positive and negative controls.

RESULTS

No significant difference was observed in the plasma level of sHLA-G between the asthmatic and healthy children (p > 0.05). When we compared atopic asthmatics with healthy controls, we found significantly higher levels of sHLA-G in atopic asthmatics (p < 0.05). There was a significant difference in the peripheral blood eosinophil counts and total immunoglobulin E levels among the groups (p < 0.001).

CONCLUSION

Our study shows that plasma sHLA-G levels do not differ between asthmatic children and healthy controls. However, higher plasma levels of sHLA-G in atopic asthmatics may suggest a role for sHLA-G in atopy. Further investigations are required to better define the mechanism of the production and the role of sHLA-G molecules observed in patients with asthma.

Characterization of human leukocyte antigen-G (HLA-G) expression in endometrial adenocarcinoma

OBJECTIVES

The current study sought to determine if endometrial adenocarcinomas express human leukocyte antigen-G (HLA-G), an immune-regulatory protein, and if degree of expression correlates with the stage of carcinoma.

METHODS

Forty-four primary endometrial adenocarcinomas were tested using immunohistochemical staining with the 4H84 anti-HLA-G monoclonal antibody. Metastatic implants were not included. A subset of 10 samples was tested using RNA in situ hybridization to confirm the presence of HLA-G transcript. Results of staining were analyzed with respect to grade, tumor histology, and stage of disease. Spearman rank correlation was used to assess tumor grade, histology, and disease stage as a function of HLA-G protein staining. Receiver-operator characteristic (ROC) curve analysis was used to determine the feasibility of HLA-G protein staining as a clinical marker for advanced stage disease.

RESULTS

Immunohistochemical staining for HLA-G protein was seen in 55% (24/44) of primary site endometrial adenocarcinomas and localized to glandular but not stromal epithelium. RNA in situ hybridization confirmed the presence of transcript in the majority of samples tested and also localized to glandular epithelium. A significant correlation was seen with increasing HLA-G protein staining and increasing stage of endometrial cancer, P < 0.01. HLA-G was found to be a fair discriminator as a test for metastatic disease with an area under the ROC curve of 0.75 for metastatic versus non-metastatic disease.

CONCLUSIONS

HLA-G protein is expressed in a significant number of endometrial adenocarcinomas, in which it is localized to the glandular epithelium. HLA-G may serve as a clinical marker for the preoperative prediction of metastatic endometrial cancer.

Comparison of the expression of human leukocyte antigen (HLA)-G and HLA-E in women with normal pregnancy and those with recurrent miscarriage

Recurrent miscarriage affects 1% of all couples attempting pregnancy. Immunological factors are postulated to play a role in the aetiology of recurrent miscarriage because the fetus and placenta are immunologically different from the mother. In particular, altered expression of the, non-classical, class I histocompatibility leukocyte antigen (HLA) molecules has been postulated to play a role in the aetiology of recurrent miscarriage as the fetus and placenta are semi-allogenic to the mother. This study was conducted to examine whether altered expression of the non-classical class I HLA molecules, HLA-G and HLA-E, by cells at the maternofetal interface could play a role in the aetiology of recurrent miscarriage. First-trimester placental and decidual biopsies were obtained from 45 women with recurrent miscarriage and 17 gestation-matched normal controls. These biopsies were screened by immunohistochemistry for HLA-G and HLA-E and isotype-matched control antibodies. Staining was analysed by light microscopy and digital image analysis. In both recurrent miscarriage and normal pregnancy, HLA-G was localised to the extravillous trophoblast. There was no difference in the pattern of HLA-G expression between women with recurrent miscarriage and those with normal pregnancies. HLA-E was localised to the syncytiotrophoblast, villous mesenchymal cells, extravillous trophoblast and several decidual cell types, but staining for HLA-E appeared to be confined primarily to the cytoplasm. There was no difference in the pattern of HLA-E expression between women with recurrent miscarriage and those with normal pregnancies.

HLA-G is expressed by the glandular epithelium of peritoneal endometriosis but not in eutopic endometrium

BACKGROUND

HLA-G is a major histocompatability antigen with documented immune-regulatory function. Various epithelial cancers and tissue allografts have been noted to express HLA-G, which is postulated to aid in their escape from immunosurveillance. We evaluated peritoneal endometriosis and eutopic endometrium for the expression of HLA-G protein and gene transcript.

METHODS

Two experiments were performed: (i) archived tissue blocks from peritoneal endometriotic lesions (n = 15) and eutopic endometrium (n = 12) were evaluated for extent of protein immunostaining, and (ii) eutopic endometrial biopsies from women without (n = 17) and with (n = 24) endometriosis, and peritoneal endometriotic lesions (n = 14) were evaluated for presence of RNA transcript by in situ hybridization.

RESULTS

HLA-G protein localized in the glandular epithelium of 14 of 15 (93.3%) peritoneal endometriotic lesions, but not in stromal cells. HLA-G protein staining was absent in endometrial biopsies (n = 12). HLA-G gene transcript localized to the glandular epithelium in 13 of 14 (92.8%) peritoneal endometriotic lesions. HLA-G transcript was never observed in eutopic endometrium, regardless of cycle stage or whether from women with (n = 24) or without (n = 18) endometriosis.

CONCLUSIONS

HLA-G is expressed by endometriotic glandular epithelium but not by eutopic endometrium under normal conditions. Differential expression of HLA-G suggests that peritoneal inflammation or cellular stress may up-regulate mechanisms to promote ectopic endometrial survival.

Expression and localization of alpha-fetoprotein mRNA and protein in human early villous trophoblasts

Alpha-fetoprotein (AFP) is a major plasma protein produced during human fetal life. It is a good marker for several possible disorders affecting gestation. We previously reported that afp gene expression, which takes place mainly in yolk sac and fetal liver, also occurs in normal human placenta, specifically in early pregnancy. The aim of the present study was to determine the precise location of AFP synthesis sites within the placental villi. In situ hybridization and immunohistochemical experiments were performed on sections obtained from placentas of first-trimester and full-term pregnancies. We found that the pattern of afp gene expression was restricted to specific villous trophoblastic areas in early placentas. Both afp transcripts and AFP protein were mainly located in discontinuous regions, at junctions between two villi and at budding sites. In contrast, no AFP expression was detected in the cytotrophoblastic extravillous proliferative zone or in other placental cell types. According to the earlier studies, no AFP synthesis was detected in placental villous tissue from full-term pregnancies, using in situ hybridization and immunohistochemistry.

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.

Immunohistochemistry in human placental tissue--pitfalls of antigen detection

Because incongruous controversial staining results are a common phenomenon in the placenta, methodical investigations are important to prevent researchers from obtaining misleading results. While investigating dendritic cells (DC) at the human fetomaternal interface, we observed staining of endothelial cells (EC) in chorionic villi for CD83. Given the high specificity of this antigen for DC, this did not seem credible. Previous studies had revealed the same surprising staining pattern with human leukocyte antigen (HLA)-G antibodies. We therefore analyzed human placental EC staining more closely. Both CD83 and HLA-G antibodies were of the same mouse IgG2b isotype. We also observed EC staining with a panel of control antibodies of the IgG2b isotype. This suggests a high affinity of human placental capillaries for mouse IgG2b. Several commonly used techniques for blocking nonspecific binding of antibodies could not prevent this nonspecific EC staining. A new preincubation step with purified human IgG was introduced. This abolished any placental EC staining with CD83, HLA-G, and IgG2b isotype control antibodies, presumably by blocking Fc receptors, whereas specific staining patterns remained unchanged. Mouse antibody of the IgG2b isotype are bound nonspecifically by vascular endothelial cells in human placenta and this can be overcome by blocking with purified human IgG. This blocking procedure could also be appropriate for frozen tissues other than placenta in which Fc receptors are expressed.

Less HLA-G expression in Plasmodium falciparum-infected third trimester placentas is associated with more natural killer cells

During pregnancy, maternal immune tolerance of the fetal semi-allogeneic graft is partly the consequence of extravillous trophoblast HLA-G expression and its interaction with natural killer (NK) cells. Plasmodium falciparum malaria is frequently associated with maternal and fetal complications. Local HLA-G expression and the number of NK cells were evaluated immunohistochemically in P. falciparum-infected and uninfected placentas (15 each) collected in a seasonal malaria-hypoendemic area. In control placentas, HLA-G was almost always expressed in extravillous trophoblast whereas, in infected placentas, it was significantly more weakly expressed in extravillous trophoblast but was also detected in intervillous space macrophages. NK cells were evaluated in intervillous and intravillous spaces and in basal plate. NK cells were always more abundant in basal plate than in intervillous and intravillous spaces in infected or control placentas. For each area, more NK cells were seen in infected than control placentas. These data suggest that HLA-G down-regulation and more NK cells in placentas may be among the mechanisms involved in poor birth outcome associated with P. falciparum infection.

Expression of the immune-tolerogenic major histocompatibility molecule HLA-G in multiple sclerosis: implications for CNS immunity

HLA-G is a non-classical major histocompatibility complex (MHC) class I antigen with highly limited tissue distribution under non-pathological conditions. Although capable of acting as a peptide-presenting molecule, its strong immune-inhibitory properties identify HLA-G as a mediator of immune tolerance with specific relevance at immune-privileged sites such as trophoblast or thymus. To assess the role of HLA-G in CNS immunity, we investigated its expression in brain specimens from patients with multiple sclerosis (n = 11), meningitis (n = 2) and Alzheimer's disease (n = 2) and non-pathological CNS controls (n = 6). Furthermore, cultured human microglial cells and CSF of patients with multiple sclerosis and controls were assessed. Furthermore, CSF from MS patients and controls, as well as cultured human microglial cells were assessed. Using several HLA-G specific mAb and immunohistochemistry, HLA-G protein was found strongly expressed in brain specimens from patients with multiple sclerosis while it was rarely detectable in the non-pathological control specimens. In multiple sclerosis brain specimens, HLA-G immunoreactivity was observed in acute plaques, in chronic active plaques, in perilesional areas as well as in normal appearing white matter. In all areas microglial cells, macrophages, and in part endothelial cells were identified as the primary cellular source of expression. HLA-G was also found in other disease entities (meningitis, Alzheimer's specimens) where expression correlated to activation and MHC class II expression on microglial cells. Importantly, ILT2, a receptor for HLA-G, was also found in multiple sclerosis brain specimens thus emphasizing the relevance of this inhibitory pathway in vivo. HLA-G mRNA and protein expression and regulation could also be corroborated on cultured human microglial cells in vitro. Further, expression of HLA-G in the CSF of multiple sclerosis patients and controls was analysed by flow cytometry and ELISA. Monocytes represented the main source of cellular HLA-G expression in the CSF. Corresponding to the observations with the tissue specimens, CSF mean levels of soluble HLA-G were significantly higher in multiple sclerosis than in non-inflammatory controls (171 +/- 31 versus 39 +/- 10 U/ml; P = 0.0001). The demonstration of HLA-G and its receptor ILT2 on CNS cells and in areas of microglia activation implicate HLA-G as a contributor to the fundamental mechanisms regulating immune reactivity in the CNS. This pathway may act as an inhibitory feedback aimed to downregulate the deleterious effects of T-cell infiltration in neuroinflammation.

Erythroblasts secrete the nonclassical HLA-G molecule from primitive to definitive hematopoiesis

The initial steps of primitive hematopoiesis and endothelial vascular formation in the human embryo remain to be defined. Here, we report the identification of a novel marker, namely the nonclassical HLA-G class I molecule, which targets both primitive erythroid cells of the yolk sac and endothelial cells from developing vessels. Moreover, HLA-G was present in its soluble form in the erythropoietic lineage in all organs sustaining primitive to definitive erythropoiesis (ie, aorta-gonad-mesonephros, liver, spleen, and bone marrow). The alternatively spliced transcript coding the soluble HLA-G5 molecule was detected in erythroid cells. The corresponding intron 4–retaining 37-kDa HLA-G5 isoform was secreted from the erythroid progenitor stage to the reticulocyte but was lost in mature erythrocytes and in endothelial cells from differentiated vessels. This study constitutes the first description of an HLA class I antigen expression on the primitive erythroid lineage and provides a way of seeking both primitive and definitive erythropoiesis using HLA-G5. This new marker, previously known by its immunotolerogeneic properties, may be involved in erythroid differentiation, angiogenesis, or both.

The short cytoplasmic tail of HLA-G determines its resistance to HIV-1 Nef-mediated cell surface downregulation

During infection with the human immunodeficiency virus type 1 (HIV-1), selective downregulation of major histocompatibility complex (MHC) class I molecules by Nef protein allows infected cells to be protected from natural killer (NK) cell lysis and to escape the HIV-specific cytotoxic T-lymphocyte response. The nonclassical MHC class I molecule human leukocyte antigen (HLA)-G is mainly expressed in placental tissues and in thymic epithelial cells. Using chimeric molecules and flow cytometry, we show that in contrast with HLA-A2, the non classical MHC class I molecule HLA-G is resistant to Nef-induced cell surface downregulation solely because of the length of its intracytoplasmic domain. Moreover, confocal microscopy analysis indicates that Nef does not delocalize HLA-G molecules from the cell surface, whereas HLA-G molecules extended with the cytoplasmic tail of HLA-A2 accumulate intracellularly with Nef. Together, these data demonstrate that the short cytoplasmic tail of HLA-G confers resistance to Nef-induced downregulation and intracellular accumulation. This resistance may have functional consequences during the course of HIV infection.

Expression of HLA-G in human cornea, an immune-privileged tissue

Human leukocyte antigen (HLA)-G retains the capacity to modulate immune responses, favoring the establishment of tolerance in solid-tissue allotransplants. To better understand the mechanisms that promote corneal allograft survival, we investigated whether HLA-G was an immunoregulatory factor involved in corneal immunology. We therefore sought HLA-G expression in corneal tissues. Corneal transplantation consists in replacing the center of a diseased cornea with normal corneal tissue. Two corneal parts are not used in such surgery: diseased central corneal tissue and peripheral normal cornea. For this study, we used healthy corneas obtained from deceased donors and diseased corneas obtained from patients with pseudophakic bullous keratopathy or keratoconus who had undergone corneal transplantation. Immunohistochemical analysis carried out on the cryopreserved corneas showed a positive immunohistochemical staining with anti-HLA-G, anti-HLA-A, -B, and -C, and anti-HLA class I monoclonal antibodies. Staining was obtained for keratocytes, epithelial cells, and endothelial cells from both healthy and pathologic human corneas, revealing the presence of HLA class I proteins, including HLA-G. HLA-G transcripts were detected in normal cornea by reverse transcriptase-polymerase chain reaction with a classical pattern of alternative splicing. The detection of HLA-G protein in adult corneas leads to the conclusion that this protein may contribute to the maintenance of the privileged immune status of cornea.

Soluble HLA-G molecules are increased in lymphoproliferative disorders

The immunomodulatory properties of soluble human leukocyte antigen G (sHLA-G) explain its potential interest in malignancies. HLA-G frequently transcribed in lymphoproliferative disorders is rarely expressed at cell surface. In this article, we will demonstrate that the plasmatic level of soluble HLA-G was significantly increased in 70% of B chronic lymphocytic leukemia, 53% of non-Hodgkin B lymphoma (B-NHL), and 45% of T-NHL. To explain this variable secretion, the HLA-G secreting cell was searched and was identified as tumoral T4 lymphocytes only in one patient with Sezary syndrome. To approach the mechanisms involved in sHLA-G secretion, the potential role of cytokines has been studied in vitro on T lymphomas. A significant increase of sHLA-G level is observed after activation by cytokines associated with a small increase in the quantity of transcripts using real-time polymerase chain reaction, suggesting an involvement of both transcriptional and post-transcriptional mechanisms. Western Blot analysis reveals no evident variation of the protein expression whatever the conditions, suggesting a continuous secretion and a low intracellular storage. The frequency of the sHLA-G secretion associated to its inhibiting role on T cells and natural killer cells during tumoral lymphoid malignancies suggests a potential role of these molecules as escape mechanism from antitumoral response.

Differential down-modulation of HLA-G and HLA-A2 or -A3 cell surface expression following human cytomegalovirus infection

During pregnancy, the non-classical major histocompatibility complex (MHC) class I HLA-G molecule is specifically expressed in trophoblast cells at the materno-fetal interface and may exert a local control of the immune response against viral infections. Human cytomegalovirus (HCMV) infection, which is the major cause of congenital defects, encodes multiple glycoproteins (US2, US3, US6, US10 and US11) that interrupt the MHC class I pathway of antigen presentation. The effect of some of these unique short (US) proteins on HLA-G expression has been previously studied, but little is known about the modulation of HLA-G cell surface expression during the course of HCMV infection which ensures expression of all of these US proteins. Using flow cytometry analysis, HLA-G cell surface expression was evaluated in HCMV-infected U373-HLA-G transfectant cells and compared with the modulation of the endogenous classical HLA-A2 molecules. The results indicated that HCMV infection down-modulated HLA-G cell surface expression, but later after infection and to a lesser extent than HLA-A2. Using various HLA-G/HLA-A2 chimeras, we showed that the unique structure of HLA-G cytoplasmic tail was partly involved in the resistance of HLA-G to viral down-modulation. Such limited down-modulation of HLA-G may have functional consequences in term of innate immunity against congenital HCMV infection.

[The role of HLA-G expression in the embryo during implantation]

HLA-G is a non-classical major histocompatibility complex class I molecule, whose tissue distribution is mainly restricted to the placenta. HLA-G expression in the placenta is found mainly in extravillous cytotrophoblast that invades décidual tissue and maternal spiral arteries as well as villous cytotrophoblast (soluble form). Its function contributes to modulate local placental immunity during pregnancy: it is an immunosuppressive molecule inducing apoptosis of activated CD8(+) T cells and down-modulating CD4(+) T cell proliferation. HLA-G also modulates cytokine secretion of NK cells upon interaction with specific receptors. Soluble HLA-G1 may also contribute to the control of implantation.

Down-regulation of HLA-G1 cell surface expression in human cytomegalovirus infected cells

PROBLEM

Down-modulation of human leukocyte antigen (HLA)-G1 cell surface expression by human cytomegalovirus (HCMV) has only been studied in cellular models expressing independent unique short (US) recombinant proteins, but not in the context of viral infection. To explore the level of HLA-G1 cell surface expression after HCMV infection and to investigate the influence of US viral proteins, we infected HLA-G1 expressing cells by HCMV laboratory strains.

METHOD OF STUDY

Human U373-MG astrocytoma cells were transfected with HLA-G1 cDNA. Following HCMV infection, HLA-G1 cell surface expression of these transfectants was evaluated by flow cytometry and confocal microscopy, using an HLA-G specific monoclonal antibody, and compared with that of uninfected cells. US-deleted viruses were then used to evaluate the implication of US proteins.

RESULTS

Using flow cytometry, it was found that HCMV infection of U373-G1 cells decreased HLA-G1 cell surface expression. Similar results were obtained with two different HCMV strains, namely Towne and AD169. Two color confocal microscopy staining further confirmed such HLA-G down-modulation in HCMV-infected cells stained for immediate early (IE1/2) nuclear proteins expression. Infection of U373-G1 cells with US-deleted HCMV strain had no effect on the level of cell surface HLA-G1 expression, thus demonstrating the US dependency of the HCMV-mediated down-regulation of HLA-G1.

CONCLUSION

HCMV infection down-modulates HLA-G1 expression at the cell surface. This is likely to have functional consequences in case of HCMV uterine infection during pregnancy.

In vivo, RFX5 binds differently to the human leucocyte antigen-E, -F, and -G gene promoters and participates in HLA class I protein expression in a cell type-dependent manner

We analysed the regulation of human leucocyte antigen (HLA)-E, -F and -G genes, focusing on the SXY module, a promoter region that controls major histocompatibility complex (MHC) class II expression and participates in the expression of classical HLA class I molecules. It comprises the X1, X2 and Y boxes, bound by RFX, X2-BP/ATF/CREB and NFY factors, respectively. The complex recruits the master control factor CIITA. The SXY module is conserved in HLA-E and HLA-F gene promoters, whereas in the HLA-G promoter, the only conserved boxes are S and X1. Chromatin immunoprecipitation assays, performed on HLA-G positive and negative cell lines, demonstrated the in situ binding of RFX5 and CIITA to HLA-E and HLA-F, but not to HLA-G, promoters. In B cells from bare lymphocyte syndrome patients lacking RFX5 or CIITA, we observed lower steady-state levels of HLA-E and HLA-F transcripts but did not find any significant decrease in the cell-surface expression of HLA-E/classical HLA class I. In RFX5-deficient fibroblasts, the cell-surface expression of HLA molecules was decreased. RFX5 and CIITA are thus not involved in HLA-G expression and their importance for the surface expression of HLA-E/classical HLA class I molecules may vary depending on the cell type.

HLA-G and lymphoproliferative disorders

The immunomodulatory properties of the HLA-G molecule explain its relevance in malignancies. Our investigations in lymphoproliferative disorders show (i) a frequent and variable distribution of alternatively spliced HLA-G mRNA isoforms, (ii) a rare cell surface expression in diffuse large cell lymphomas with HLA class I loss in half of cases, and (iii) an increased serum level of sHLA-G in half of cases. The potential role of the microenvironment and/or tumoral process in HLA-G expression is discussed in the light of these data. HLA-G rather through its soluble isoform might provide a new way of immune evasion for lymphoid proliferations.

HLA-G, pre-eclampsia, immunity and vascular events

Pre-eclampsia, one of the main complications in pregnancy, is characterised by shallow cytotrophoblast invasion of decidua as well as by vascular endothelial cell dysfunction, leading to a poor perfusion of placenta. A striking feature of pre-eclamptic pregnancies is that expression of HLA-G protein is reduced in term placentas compared with normal pregnancy. How such HLA-G deficient expression may be related to the pre-eclamptic pathology is unknown. Here, we review the major structural characteristics of HLA-G and some of its functions that have been recently characterised. Soluble HLA-G1 isoform down-regulates both CD8(+) and CD4(+) T cell reactivity. HLA-G also modulates innate immunity by binding to several NK and/or decidual receptors, inducing particular cytokine secretion. HLA-G was shown to be less susceptible to human cytomegalovirus-derived US protein down-modulation. Finally, soluble HLA-G1 down-regulates endothelial cell proliferation and migration. In view of these different HLA-G properties, we will briefly discuss how defective HLA-G function may contribute to the low trophoblast invasion and vascular abnormalities observed in pre-eclamptic placentas.

Analysis of HLA-G expression in malignant hematopoetic cells from leukemia patients

It has been suggested that HLA-G antigens may provide tumor cells with an effective immune escape mechanism. So far mostly solid tumors have been analyzed; HLA-G antigen was only exceptionally detected. To further examine HLA-G expression, patients were chosen with different forms of leukemia: AML (25), CML (4), ALL (9), CLL (8), HCL (2) and NHL (3). Using flow cytometry with three HLA-G specific mAbs (87G, 01G and MEM-G/9), western blotting with two specific mAbs (4H84 and MEM-G/1) and RT-PCR, neither HLA-G antigen nor mRNA for any HLA-G isoform was detected. These results strongly suggest that HLA-G antigen is not expressed in freshly isolated human leukemia cells and therefore is not involved in their escape from immune attack.

Soluble HLA-G1 at the materno-foetal interface--a review

HLA-G differs from the other MHC class I genes. This includes a unique promoter region, a restricted constitutive tissular distribution, the translation of different membrane-bound and soluble isoforms, a shortened cytoplasmic tail and a minimal polymorphim. Soluble HLA-G1 is an immunosuppressive molecule inducing apoptosis of activated CD8(+) T cells and down-modulating CD4(+) T cell proliferation. Soluble HLA-G1 may also contribute to the control of implantation.

HLA-G gene repression is reversed by demethylation

The HLA-G molecule plays an important role in immune tolerance, protecting the fetus from maternal immune attack, and probably contributes to graft tolerance and tumor escape from the host immune system. HLA-G expression is tightly regulated and involves mechanisms acting in part at the transcriptional level. Nevertheless, almost all regulatory sequences that govern constitutive and inducible HLA class I gene transcription are disrupted in the HLA-G gene promoter, suggesting an unusual regulatory process. In further investigating the molecular mechanisms of HLA-G gene activation, we evaluated the influence of epigenetic mechanisms on seven HLA-G-negative cell lines that exhibit various phenotypes. Exposure of cells to histone deacetylase inhibitors, or to the demethylating agent 5-aza-2'-deoxycytidine, revealed that HLA-G gene transcription is inhibited by DNA methylation. Reversal of methylation-mediated repression may directly induce HLA-G cell-surface expression, supporting the idea that HLA-G might be activated by such a mechanism during malignancy, inflammation, and allogenic reactions.

Induction of HLA-G-restricted human cytomegalovirus pp65 (UL83)-specific cytotoxic T lymphocytes in HLA-G transgenic mice

The non-classical major histocompatibility complex class I molecule HLA-G is expressed mainly by extravillous trophoblasts at the materno-foetal interface. HLA-G has been found to bind endogenously processed nonameric peptides but its function as a restriction element for a cytotoxic T cell response to viruses with tropism for trophoblastic cells has never been demonstrated. In this study, candidate viral peptides derived from human cytomegalovirus (HCMV) pp65 (UL83), which stabilized the HLA-G molecule on HLA-G-transfected T2 cells, were identified. The specific anti-pp65 cytotoxic T lymphocyte (CTL) response restricted by HLA-G in triple transgenic mice (HLA-G, human beta2m, human CD8alpha) was then investigated by injection of dendritic cells loaded with synthetic pp65-derived peptides or by infection with canarypox virus expressing pp65. Results showed that CTLs from HLA-G mice have the capacity to kill target cells either infected with recombinant vaccinia viruses expressing pp65 or loaded with specific pp65-derived peptides using HLA-G as an antigen-presenting molecule. It was also demonstrated that these HLA-G-restricted pp65-specific T cells are able to kill the human astrocytoma cell line U373, which was transfected with HLA-G and infected with HCMV. Moreover, using HLA-G tetramers refolded with a synthetic pp65-derived peptide, peptide-specific CD8(+) cells restricted by HLA-G have been detected in vivo. These findings provide the first evidence that HLA-G can select anti-HCMV-restricted CTLs in vivo, although the potency of this cytolytic response is limited (20-25 %). The weak HLA-G-restricted anti-HCMV response is probably due to HLA-G-mediated inhibitory signals on the development of an antiviral CTL response.

Mild acid treatment induces cross-reactivity of 4H84 monoclonal antibody specific to nonclassical HLA-G antigen with classical HLA class I molecules

Mild acid treatment by releasing beta(2)m and antigenic peptides leaves human leukocyte antigen (HLA) class I free heavy chains attached to the cell surface. Acid treatment thus allows detection of the cell surface class I antigens by monoclonal antibodies (mAbs) specific to HLA-free heavy chains. We found that acid treatment also enables detection of the cell surface non-classical HLA-G class I antigen with mAbs specific for HLA-G free heavy chains, including 4H84 mAb recognizing all isoforms. Furthermore, we found that 4H84 mAb, but not other mAbs specific to HLA-G free heavy chains, binds to the surface of 8 out of 16 acid-treated leukemia cell lines. Nevertheless, HLA-G antigen is not present in any of these leukemia cells. This was demonstrated by failure to detect any antigen with 4H84 mAb in immunoblotting as well as by inability to detect HLA-G mRNA by RT-PCR. The antigen recognized by 4H84 mAb in some acid treated leukemia cells was identified by immunoprecipitation as a 45 kDa protein. A number of observations indicate that 45 kDa proteins are none other than classical class I heavy chains. Acid treatment thus induces the ability of the 4H84 mAb to recognize some classical HLA class I molecules. Remarkably, 4H84 determinant on HLA-G is linear but corresponding determinant present on some partially folded classical HLA class I free heavy chains is conformational. In view of the unexpected cross-reactivity, detection of HLA-G with this mAb must be carefully evaluated to avoid false detection.

HLA-G Molecules: from Maternal–Fetal Tolerance to Tissue Acceptance

Over the past few years, HLA-G, the non-classical HLA class I molecule, has been the center of investigations that have led to the description of its specific structural and functional properties. Although located in the HLA class I region of chromosome six, the HLA-G gene may be distinguished from other HLA class I genes by its low polymorphism and alternative splicing that generates seven HLA-G proteins, whose tissue-distribution is restricted to normal fetal and adult tissues that display a tolerogeneic function toward both innate and acquired immune cells. We review these points, with special emphasis on the role of HLA-G in human pathologies, such as cancer, viral infection, and inflammatory diseases, as well as in organ transplantation.

Detection of HLA-G on human extravillous cytotrophoblast and skeletal muscle with a new monoclonal antibody MEM-G/1

Using immunohistochemistry with the newly available monoclonal antibody MEM-G/1 the reaction patterns on frozen and formaldehyde-fixed paraffin-embedded sections on human placentas, lymph nodes, skeletal muscles, and kidney and liver allografts were compared. HLA-G (a nonclassical major histocompatibility complex class I molecule that is assumed to influence the immune response during pregnancy and some pathological conditions) was found within human extravillous cytotrophoblast but not within villous cytotrophoblast and placental mesenchymal tissue. No HLA-G expression on human lymph nodes, tonsils, and kidney and liver allografts was demonstrated. However, HLA-G expression was observed in all samples of skeletal muscle. The binding capacity of monoclonal antibody MEM-G/1 provides new possibilities to study physiological and pathophysiological roles of HLA-G in humans.

Do nonhuman primates comprise appropriate experimental models for studying the function of human leukocyte antigen-G?

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.

Effect of matrigel on human extravillous trophoblasts differentiation: modulation of protease pattern gene expression

The human placenta is characterized by extensive trophoblast invasion of the uterus. Indeed, extravillous cytotrophoblast cells invade the decidua and the upper third of uterine spiral arteries in the myometrium. This invasion is reflected in situ by the expression of specific markers. In order to study this invasion process, we have established an in vitro culture model of human extravillous trophoblast isolated from first trimester chorionic villi. The aim of this study was to investigate the effect of a composite matrix, the Matrigel required for the culture of this homogenous population of extravillous trophoblasts (EVCT), on their in vitro differentiation. The effect of Matrigel was studied on different markers characterized by immunocytochemistry and by real-time polymerase chain reaction assay of transcripts. In addition, the expression of 12 different matrix metalloproteases and their inhibitors were investigated. We show that human extravillous cytotrophoblasts acquire an invasive phenotype on Matrigel associated with a specific pattern of protease gene expression. This in vitro model will be of interest to study the cellular mechanisms involved in abnormal trophoblast invasion observed in poor placentation and preeclampsia.

Viewpoint on the functionality of the human leukocyte antigen-G null allele at the fetal-maternal interface

The description of healthy individuals homozygous for the human leukocyte antigen-G (HLA-G) null allele raised doubts about the role of HLA-G in fetal-maternal tolerance. In light of recent results, we discuss this point by considering the potential activity of this null allele that might, indeed, produce functional truncated HLA-G molecules. In this context, we have recently described that, like the full-length HLA-G1, the HLA-G2, -G3, and -G4 truncated isoforms may be expressed at the cell surface and may modulate both innate and acquired immune responses.

A soluble isoform of the rhesus monkey nonclassical MHC class I molecule Mamu-AG is expressed in the placenta and the testis

The nonclassical MHC class I locus HLA-G is expressed primarily in the placenta, although other sites of expression have been noted in normal and pathological situations. In addition, soluble HLA-G isoforms have been detected in the serum of pregnant and nonpregnant women as well as men. The rhesus monkey placenta expresses a novel nonclassical MHC class I molecule Mamu-AG, which has features remarkably similar to those of HLA-G. We determined that the rhesus placenta expresses Mamu-AG mRNA (Mamu-AG5), retaining intron 4 as previously noted in HLA-G5. Immunostaining experiments with Ab 16G1 against the soluble HLA-G5 intron 4 peptide demonstrated that an immunoreactive protein(s) was present in the syncytiotrophoblasts of the chorionic villi of the rhesus placenta, within villous cytotrophoblasts, and occasionally within cells of the villous stroma. The Mamu-AG5 mRNA was readily detected in rhesus testis (although not in ejaculated sperm). Whereas an Ab against membrane-bound Mamu-AG stained few cells, primarily in the interstitium of the testis, there was consistent immunostaining for Mamu-AG5 in cells within the seminiferous tubules, which was corroborated by localization of Mamu-AG mRNA by in situ hybridization. While primary spermatocytes were negative, Sertoli cells, spermatocytes, and spermatids were consistently positive for 16G1 immunostaining. The specific recognition of the soluble Mamu-AG isoform was confirmed by Western blotting of Mamu-AG5 expressed in heterologous cells. The results demonstrate that a soluble nonclassical MHC class I molecule is expressed in the rhesus monkey placenta and testis, and confirm and extend the unique homology between HLA-G and the rhesus nonclassical molecule Mamu-AG.

Specific activation of the non-classical class I histocompatibility HLA-G antigen and expression of the ILT2 inhibitory receptor in human breast cancer

The HLA-G molecule is a non-classical HLA class I antigen selectively expressed by trophoblastic cells that invade the maternal decidua during human pregnancy. HLA-G is believed to contribute to tolerance of the semi-allogeneic fetus by inhibiting maternal immune responses. Similarly, HLA-G expression in tumour cells may favour their escape from host immune surveillance. This study investigated HLA-G expression in human mammary tumours. Immunohistochemical analysis of cryo-preserved and paraffin-embedded breast tissue biopsies, using two HLA-G-specific antibodies, revealed that unlike non-cancerous breast tissue in the vicinity of the tumour, 14 out of 36 breast cancer lesions selectively expressed HLA-G. HLA-G expression was significantly more frequent in lesions that were highly infiltrated by host immune cells, thus correlating HLA-G activation with inflammation. Further histological and double-staining immunofluorescence analysis attributed HLA-G expression mainly to tumour epithelial cells and to subsets of infiltrating CD68+ and CD8+ cells. RT-PCR analysis suggested that HLA-G was activated at the transcriptional level in breast tumours. The presence of ILT2 (Ig-like transcript 2) killing inhibitory receptors known to interact with HLA-G was also demonstrated in host immune cells that infiltrate breast cancer lesions. These results indicate that HLA-G is up-regulated at high frequencies in human breast cancer, where it may impair efficient anti-tumour immunity.

HLA-G protein up-regulation in primary cutaneous lymphomas is associated with interleukin-10 expression in large cell T-cell lymphomas and indolent B-cell lymphomas

Primary cutaneous lymphomas (CLs) constitute a spectrum of diseases characterized by a clonal accumulation of lymphocytes in the skin. Most CLs display a T(h)2 cytokine profile, including expression of interleukin-10 (IL-10). Because the up-regulation of HLA-G, a nonclassical class Ib molecule inducible by IL-10, might account for the immunescape of the malignant clone, HLA-G and IL-10 expression has been investigated in 45 cases of primary CL (10 of B-cell and 35 of T-cell origin) with quantitative polymerase chain reaction (PCR) and immunohistochemistry. HLA-G message was present in all cutaneous B-cell (CBCL) and T-cell (CTCL) lymphomas evaluated. Immunohistochemistry revealed HLA-G protein expression in 23 (51%) of 45 cases (7 of 10 CBCL, 16 of 35 CTCL). While in CBCL mostly indolent types displayed HLA-G positivity, in CTCL HLA-G expression was associated with high-grade histology and advanced stage of the disease. Except for neoplastic and infiltrating lymphocytes, other cells such as macrophages and dendritic cells showed HLA-G immunoreactivity. Furthermore, IL-10 protein expression was demonstrated in 16 (73%) of 22 HLA-G(+) cases, which correlated with HLA-G protein presence (P <.001). HLA-G up-regulation together with IL-10 expression in CL might additionally contribute to the evasion of immunosurveillance and facilitate the transition from low- to high-grade lymphomas.

[HLA molecules, immunity and gestation]

During pregnancy, the fetus develops particularly efficient molecular regulatory mechanisms to prevent possible maternal anti-paternal alloimmune response and avoid viral spreading from maternal tissue. Among the different mechanisms, there has been noted a selective expression of HLA molecules on trophoblast cells: the absence of HLA class II and of polymorphic HLA-A and HLA-B expression but presence of both non polymorphic HLA-G and HLA-E class Ib as well as of HLA-C class Ia. The functional consequences of such a particular pattern of HLA expression in gestation are examined here.

Is antigen presentation the primary function of HLA-G?

Human histocompatibility leukocyte antigen (HLA)-G is an antigen-presenting molecule. This review discusses the possibility that this might not be its primary function. HLA-G indeed modulates innate immunity by interacting with immunoglobulin-like receptors and by regulating HLA-E expression and its subsequent interaction with CD94/NKG2 receptors. HLA-G also down-modulates both CD8(+) and CD4(+) T-cell responsiveness.

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.

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.

Reaction patterns of monoclonal antibodies to HLA-G in human tissues and on cell lines: a comparative study

We compared the immunohistochemical reaction patterns of HLA-G-specific antibodies 87G, 4H84, G233, 16G1, and BFL.1 on human placentas under three different preparative conditions and on cryosections of other human tissues. Human and murine cell lines, either naturally expressing or transfected with HLA-G, were analyzed for their reaction patterns by immunocytochemistry and flow cytometry. Antibodies HCA2, TP25.99, W6/32 to classical HLA class I, anti-beta(2)-m and various non-HLA-G expressing cell lines were used as controls. The binding ability of the antibodies depends on the histotechnical procedure used. 4H84 and HCA2 bind to HLA-G despite aldehyde fixation and also paraffin embedding. 87G does not bind HLA-G in studies involving fixation with aldehydes. G233 labels HLA-G in aldehyde fixed but not paraffin embedded tissues. By immunocytochemistry HLA-G2 is merely detected with antibodies 4H84 and HCA2. MAb 16G1 binds to HLA-Gsol transfected cell lines only. The HLA-G specificity of mAb BFL.1 was considered as doubtful because it failed to react with most of the HLA-G transfected cell lines. Binding of 87G to the surface of monocytes or U-937 cells stimulated with IFN-gamma and GM-CSF is an Fc-receptor mediated phenomenon.

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.

Soluble HLA-G circulates in maternal blood during pregnancy

OBJECTIVE

Soluble isoforms of the HLA class Ib gene HLA-G have been identified at the maternal-fetal interface. Because soluble forms of other HLA class I antigens modulate T-cell reactivity and induce cellactivated apoptosis, our goal was to determine whether soluble HLA-G circulates in maternal or fetal blood and to identify the specific isoform.

STUDY DESIGN

Capture enzyme-linked immunosorbent assays with mouse monoclonal antibodies directed toward an epitope present on all isoforms of soluble HLA-G were constructed to identify soluble HLA-G in 44 serum samples from nonpregnant control subjects, 129 serum samples from pregnant women, and 10 samples of term cord blood. Distinguishing between soluble HLA-G1, which is composed of heavy chains complexed with light chains (beta(2)-microglobulin), and soluble HLA-G2, which consists only of heavy chains, was achieved by substituting a monoclonal antibody that requires beta(2)-microglobulin for binding (W6/32) in the capture phase of the enzyme-linked immunosorbent assay.

RESULTS

Capture enzyme-linked immunosorbent assays with mouse anti-soluble HLA-G showed that soluble HLA-G was present at all stages of gestation and that levels of soluble HLA-G were statistically significantly higher in serum samples from pregnant women than in serum samples from nonpregnant women. In contrast, W6/32 failed to detect soluble HLA-G in serum samples from pregnant women. Cord serum samples did not contain detectable soluble HLA-G.

CONCLUSION

Collectively, the data indicate that pregnancy is characterized by the presence of soluble HLA-G circulating in maternal blood and strongly suggest that the major isoform is soluble HLA-G2.