Secretion of pro-apoptotic intron 4-retaining soluble HLA-G1 by human villous trophoblast

The central role of natural killer cells in preeclampsia

Preeclampsia (PE) is a disease that is unique to pregnancy and affects multiple systems. It can lead to maternal and perinatal mortality. The precise etiology of PE is unclear. Patients with PE may have systemic or local immune abnormalities. A group of researchers has proposed that the immune communication between the fetus and mother is primarily moderated by natural killer (NK) cells as opposed to T cells, since NK cells are the most abundant immune cells in the uterus. This review examines the immunological roles of NK cells in the pathogenesis of PE. Our aim is to provide obstetricians with a comprehensive and updated research progress report on NK cells in PE patients. It has been reported that decidual NK (dNK) cells contribute to the process of uterine spiral artery remodeling and can modulate trophoblast invasion. Additionally, dNK cells can stimulate fetal growth and regulate delivery. It appears that the count or proportion of circulating NK cells is elevated in patients with or at risk for PE. Changes in the number or function of dNK cells may be the cause of PE. The Th1/Th2 equilibrium in PE has gradually shifted to an NK1/NK2 equilibrium based on cytokine production. An improper combination of killer cell immunoglobulin-like receptor (KIR) and human leukocyte antigen (HLA)-C may lead to insufficient activation of dNK cells, thereby causing PE. In the etiology of PE, NK cells appear to exert a central effect in both peripheral blood and the maternal-fetal interface. To maintain immune equilibrium both locally and systemically, it is necessary to take therapeutic measures directed at NK cells.

Human Leukocyte Antigens in Pregnancy and Preeclampsia

Preeclampsia is a pregnancy-induced hypertensive disorder, the pathophysiology of which includes underlying maternal cardiovascular disease, deficient spiral artery remodeling during placenta development, and inflammatory immune responses at the maternal-fetal interface. Human leukocyte antigens (HLA) are major histocompatibility complex molecules essential for the recognition of foreign antigens that is central to immune defense against pathogens and critical determinants for the immune system discriminating between self and non-self tissues, such as in transplantation. Pregnancy represents a naturally existing “transplantation”, where the maternal immune system must be immunologically tolerant to the developing fetus which is 50% allogeneic. It is then unsurprising that HLA also influence normal pregnancy and pregnancy complications including preeclampsia. Here we review the role of classical and non-classical HLA molecules in influencing normal physiologic function during pregnancy and describe the association of HLA with pathophysiology in preeclampsia.

Understanding human immunity in idiopathic recurrent pregnancy loss

Miscarriage, defined as the loss of a pregnancy before a viable gestation, affects 1 in 6 couples. Recurrent pregnancy loss (RPL), defined as two or more miscarriages, affects up to 1.9% of couples. The physical, psychological, and financial impact of miscarriage can be substantial. However, despite its multifactorial etiology, for up to 50% of couples a reason behind this condition cannot be identified, termed 'idiopathic RPL'. Much recent research has strived to understand this, with immune dysregulation being a source of particular interest. In this short review we summarize the current evidence on the complex role of the immune system both pre- and early post-conception in RPL. A key question is whether systemic peripheral blood markers, in particular natural killer cell and T cells, may be utilized to accurately predict and/ or diagnose those pregnancies at high risk of loss. Given the invasive nature of endometrial testing, identification of reliable peripheral immune biomarkers is particularly appealing. Clinical trials using potent immunomodulatory agents, including intravenous immunoglobulin, donor leukocyte immunization, and tumor necrosis factor (TNF)-α inhibitors, have been undertaken with the primary objective of preventing miscarriage in women with RPL. Standardisation of both diagnostic and prognostic immune cell testing assays is required to permit accurate identification of those women who may benefit from immunomodulation. Prompt clarification is required to meet the increasing expectation from couples and clinicians, as without these advancements women are at risk of exposure to potent immune-therapies and subsequent studies are at risk of failure, generating further controversy regarding the role of immune dysregulation in women with RPL. Through this review we highlight clear gaps in our current knowledge on immune activity in RPL.

Association of reduced maternal sHLA-G5 isoform levels and elevated TNF-α/IL-4 cytokine ratio with Recurrent Pregnancy Loss: A study on South Indian women

Inflammation is of critical importance in successful implantation during pregnancy. However, the establishment of maternal immune tolerance towards semi-allograft foetus is more exigent and is achieved predominantly by human leukocyte antigen-G (HLA-G) isoforms with a special emphasis on soluble HLA-G5 (sHLA-G5). Constant inflammation and lack of resolution by anti-inflammatory milieu, due to aberrant expression of critical immunoregulatory molecules such as sHLA-G5 and dysfunctional T helper cells 1 and 2 (Th1-Th2) cytokine shift, can lead to adverse pregnancy outcomes including recurrent pregnancy loss (RPL). Serum samples of 270 pregnant women (135 healthy parous and 135 with a history of RPL) were evaluated for the concentrations of sHLA-G5, interleukin-4 (IL-4) and tumour necrosis factor-alpha (TNF-α) using sandwich enzyme-linked immunosorbent assay (ELISA) and found elevated levels of sHLA-G5 and IL-4 in controls and higher TNF-α levels and TNF-α:IL-4 ratio in patients (P < .05). Stratified data analysis based on the time of sample collection, that is the first and second trimesters exhibited higher sHLA-G5 and IL-4 in both first and second trimesters in controls than patients, while they displayed lower levels concerning TNF-α and TNF-α:IL-4 ratio (P < .05). However, within patients and controls in the first or second trimesters, there was a significant variation concerning sHLA-G5 alone. Further, the outcome of pregnancies studied in the present investigation revealed a significant elevation in sHLA-G5 levels among women with successful pregnancies compared with women who experienced pregnancy loss, therefore, concluding the potential application of sHLA-G5 isoform as a marker in assisting improved pregnancy outcomes.

Immune tolerance at the maternal-placental interface in healthy pregnancy and pre-eclampsia

AIM

The objective of this review is to describe the immunological mechanisms which facilitate maternal tolerance at the maternal-placental interface, and to discuss how these mechanisms are disrupted in pre-eclampsia.

METHODS

A literature review was performed based on the analysis of papers available on PubMed. The most important and relevant studies regarding the immunological mechanisms which facilitate maternal tolerance in healthy pregnancy and pre-eclampsia are presented in this article.

RESULTS

The maternal-placental interface is the site where the immune tolerance begins and develops. Within the innate immunity, natural killer cells, macrophages and dendritic cells play a pivotal role in tolerance through regulation of inflammation. On the other hand, within the adaptive immunity, the correct increase of regulatory T cells is crucial for ensuring immune tolerance toward placental cells. Disturbances in maternal tolerance can lead to the appearance of pregnancy complications such as pre-eclampsia, which has a considerable impact on perinatal morbidity and mortality.

CONCLUSION

Our partial knowledge of immunological mechanisms involved in tolerance at the maternal-placental interface indicates that pre-eclampsia is characterized by alterations of this maternal immune tolerance, which could represent the origin of the disease.

O-GlcNAc Modification During Pregnancy: Focus on Placental Environment

Successful placentation is a key event for fetal development, which commences following embryo implantation into the uterine wall, eliciting decidualization, placentation, and remodeling of blood vessels to provide physiological exchange between embryo-fetus and mother. Several signaling pathways are recruited to modulate such important processes and specific proteins that regulate placental function are a target for the glycosylation with O-linked β-N-acetylglucosamine (O-GlcNAc), or O-GlcNAcylation. This is a reversible post-translational modification on nuclear and cytoplasmic proteins, mainly controlled by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). O-GlcNAcylation has been implicated as a modulator of proteins, both in physiological and pathological conditions and, more recently, O-GlcNAc has also been shown to be an important modulator in placental tissue. In this mini-review, the interplay between O-GlcNAcylation of proteins and placental function will be addressed, discussing the possible implications of this post-translational modification through placental development and pregnancy.

Soluble HLA-G Expression Inversely Correlates With Fetal Microchimerism Levels in Peripheral Blood From Women With Scleroderma

Women with scleroderma (SSc) maintain significantly higher quantities of persisting fetal microchimerism (FMc) from complete or incomplete pregnancies in their peripheral blood compared to healthy women. The non-classical class-I human leukocyte antigen (HLA) molecule HLA-G plays a pivotal role for the implantation and maintenance of pregnancy and has often been investigated in offspring from women with pregnancy complications. However data show that maternal HLA-G polymorphisms as well as maternal soluble HLA-G (sHLA-G) expression could influence pregnancy outcome. Here, we aimed to investigate the underlying role of maternal sHLA-G expression and HLA-G polymorphisms on the persistence of FMc. We measured sHLA-G levels by enzyme linked immunosorbent assay in plasma samples from 88 healthy women and 74 women with SSc. Male Mc was quantified by DYS14 real-time PCR in blood samples from 58 women who had previously given birth to at least one male child. Furthermore, eight HLA-G 5'URR/3'UTR polymorphisms, previously described as influencing HLA-G expression, were performed on DNA samples from 96 healthy women and 106 women with SSc. Peripheral sHLA-G was at lower concentration in plasma from SSc (76.2 ± 48.3 IU/mL) compared to healthy women (117.5 ± 60.1 IU/mL, p < 0.0001), independently of clinical subtypes, autoantibody profiles, disease duration, or treatments. Moreover, sHLA-G levels were inversely correlated to FMc quantities (Spearman correlation, p < 0.01). Finally, women with SSc had lower sHLA-G independently of the eight HLA-G 5'URR/3'UTR polymorphisms, although they were statistically more often homozygous than heterozygous for HLA-G polymorphism genotypes -716 (G/T), -201 (G/A), 14 bp (ins/del), and +3,142 (G/A) than healthy women. In conclusion, women with SSc display less sHLA-G expression independently of the eight HLA-G polymorphisms tested. This decreased production correlates with higher quantities of persisting FMc commonly observed in blood from SSc women. These results shed some lights on the contribution of the maternal HLA-G protein to long-term persistent fetal Mc and initiate new perspectives in this field.

Circulating fetal and total cell-free DNA, and sHLA-G in black South African women with gestational hypertension and pre-eclampsia

OBJECTIVE

Quantification of circulating fetal and total cell-free DNA (cfDNA) and soluble human leucocyte antigen (HLAG) in gestational hypertension and pre-eclampsia.

METHODS

Serum cfDNA were quantified in controls, pre-eclamptics, and gestational hypertensive patients using real-time qPCR. Soluble HLAG was measured by enzyme-linked immune-sorbent assay.

RESULTS

Serum fetal and total cfDNA levels were higher in pre-eclampsia compared with the controls and gestational hypertensives (p < 0.001), more so in severe compared with mild-to-moderate pre-eclampsia (p < 0.05). Soluble HLAG levels were lower in pre-eclamptics than controls and gestational hypertension (p < 0.05).

CONCLUSION

Circulating fetal and total cfDNA were increased, while soluble HLAG was decreased in pre-eclampsia.

Up-and-down immunity of pregnancy in humans

One part of the human placenta in early pregnancy is particularly important for local immunity: the decidua basalis, which is transformed endometrium located at the site of embryo implantation . This placental bed tissue contains both maternal uterine immune cells, including decidual natural killer (NK) cells, the dominant leukocyte population exhibiting a unique phenotype, and fetal extravillous trophoblast which comes into direct contact with maternal decidual cells . To establish a successful placental development and healthy pregnancy outcome, the maternal immune system must tolerate paternal antigens expressed by trophoblast cells yet remain efficient for clearing any local pathogen infection. This review deals mainly with decidual NK cells. A key element, among others, to achieve such dual functions is the direct interaction between activating and inhibitory receptors expressed by decidual NK cells and their specific ligands presented by trophoblast or other decidual cells. Depending whether maternal decidual cells and trophoblast are infected by viruses, the balance between activating and inhibitory receptor signals mediated by decidual NK cell-trophoblast cross-talk results in tolerance (healthy pregnancy) or specific killing (pathogen-infected cells).

Differential Spatiotemporal Patterns of Galectin Expression are a Hallmark of Endotheliochorial Placentation

PROBLEM

Galectins influence the progress of pregnancy by regulating key processes associated with embryo-maternal cross talk, including angiogenesis and placentation. Galectin family members exert multiple roles in the context of hemochorial and epitheliochorial placentation; however, the galectin prolife in endotheliochorial placenta remains to be investigated.

METHOD OF STUDY

Here, we used immunohistochemistry to analyze galectin (gal)-1, gal-3 and gal-9 expression during early and late endotheliochorial placentation in two different species (dogs and cats).

RESULTS

We found that during early feline gestation, all three galectin members were more strongly expressed on trophoblast and maternal vessels compared to the decidua. This was accompanied by an overall decrease of gal-1, gal-3 and gal-9 expressions in late feline gestation. In canine early pregnancy, we observed that gal-1 and gal-9 were expressed strongly in cytotrophoblast (CTB) cells compared to gal-3, and no galectin expression was observed in syncytiotrophoblast (STB) cells. Progression of canine gestation was accompanied by increased gal-1 and gal-3 expressions on STB cells, whereas gal-9 expression remained similar in CTB and STB.

CONCLUSION

These data suggest that both the maternal and fetal compartments are characterized by a spatiotemporal regulation of galectin expression during endotheliochorial placentation. This strongly suggests the involvement of the galectin family in important developmental processes during gestation including immunemodulation, trophoblast invasion and angiogenesis. A conserved functional role for galectins during mammalian placental development emerges from these studies.

sHLA-G involved in the apoptosis of decidual natural killer cells following Toxoplasma gondii infection

This study aims to assess whether soluble HLA-G (sHLA-G) is involved in apoptosis of decidual natural killer (dNK) cells following Toxoplasma gondii infection. dNK cells or NK-92 cells were infected with T. gondii and co-cultured with trophoblast cells or BeWo cells. Infected co-cultured cells were treated without or with sHLA-G neutralizing antibody. Uninfected co-cultured cells were used as controls. Apoptosis of dNK cells were analyzed by flow cytometry and confocal microscope. Real-time PCR and Western blot were used to determine caspase 3 and caspase 8 expression. sHLA-G in supernatant were measured by enzyme-linked immunosorbent assay (ELISA). In infection groups, sHLA-G was increased, while dNK apoptosis proteins caspase 3 and caspase 8 were up-regulated, but significantly decreased in the presence of sHLA-G neutralizing antibody compared to controls. Under the situation of T. gondii-infected dNK cells co-cultured with trophoblast cells, the up-regulation of sHLA-G could induce dNK cells apoptosis which ultimately may contribute to the abnormal pregnancy outcomes with T. gondii infection.

Leptin promotes HLA-G expression on placental trophoblasts via the MEK/Erk and PI3K signaling pathways

INTRODUCTION

The development of the human haemochorial placenta requires complex regulatory mechanisms to protect invasive trophoblast cells from cytotoxic responses elicited by maternal immune cells. Leptin, the adipocyte derived hormone encoded by the Lep gene, is synthesized by placental trophoblasts and exerts pleiotropic effects on the immune system, including the promotion of inflammation and the activation of T cell responses.

METHODS

To address its possible involvement in the modulation of maternal immune responses during pregnancy, we investigated the effect of leptin on the expression of the class Ib histocompatibility antigen HLA-G as one of the chief immunosuppressive strategies used by trophoblast cells.

RESULTS

In vitro incubation of the trophoblast derived Swan 71 and JEG-3 cell lines with 25-50 ng/ml recombinant leptin significantly boosted HLA-G mRNA and protein expression, and this effect was abrogated upon pharmacological inhibition of the PI3K-Akt and MEK-Erk signaling pathways. A similar stimulatory effect of leptin was observed in term placental tissue explants, though 10-fold higher doses were required for stimulation. Further, JEG-3 cells treated with a leptin antisense oligodeoxynucleotide displayed decreased HLA-G expression levels, which were partially recovered by addition of stimulating doses of exogenous hormone. Immunofluorescence and qPCR analysis confirmed leptin biosynthesis in placental tissue, further showing that invasive extravillous trophoblast cells were a main source of this hormone during the first trimester of normal pregnancies.

DISCUSSION

Taken together, our results show that leptin acts as an autocrine/paracrine signal promoting HLA-G expression in placental trophoblasts suggesting an important role in the regulation of immune evasion mechanisms at the fetal maternal interface.

Maternal human leukocyte antigen-G (HLA-G) genetic variants associate with in utero mother-to-child transmission of HIV-1 in Black South Africans

A 14-bp insertion/deletion (indel) within the 3' untranslated region (3'UTR) that affects HLA-G expression has been associated with HIV-1 mother-to-child transmission (MTCT). However, other 3'UTR single nucleotide polymorphisms (SNPs) that influence HLA-G mRNA stability have been described but not analysed in the context of MTCT, and little is known about the role of HLA-G alleles. We examined HLA-G alleles and 3'UTR SNPs, including the 14-bp indel, in 216 mother-infant pairs from Johannesburg, South Africa. Mother-infant pairs were classified as HIV-1 non-transmitting (NT, n=144) or HIV-1 transmitting (TR, n=72) with either intrapartum (IP, n=29) or in utero (IU, n=19) infected infants. We found HLA-G allele, G(∗)01:01:02 (in strong linkage disequilibrium with the 14-bp insertion) and +3187G SNP were significantly over-represented in IU-TR mothers compared to NT mothers (P=0.036, OR=2.26; P=0.011, OR=2.96, respectively). These findings suggest that maternal HLA-G alleles and/or SNPs that might alter expression of HLA-G potentially influence IU HIV-1 MTCT.

HLA Class Ib Molecules and Immune Cells in Pregnancy and Preeclampsia

Despite decades of research, the highly prevalent pregnancy complication preeclampsia, “the disease of theories,” has remained an enigma. Indeed, the etiology of preeclampsia is largely unknown. A compiling amount of studies indicates that the pathological basis involves a complex array of genetic predisposition and immunological maladaptation, and that a contribution from the mother, the father, and the fetus is likely to be important. The Human Leukocyte Antigen (HLA)-G is an increasing focus of research in relation to preeclampsia. The HLA-G molecule is primarily expressed by the extravillous trophoblast cells lining the placenta together with the two other HLA class Ib molecules, HLA-E and HLA-F. Soluble isoforms of HLA-G have been detected in the early endometrium, the matured cumulus–oocyte complex, maternal blood of pregnant women, in umbilical cord blood, and lately, in seminal plasma. HLA-G is believed to be involved in modulating immune responses in the context of vascular remodeling during pregnancy as well as in dampening potential harmful immune attacks raised against the semi-allogeneic fetus. In addition, HLA-G genetic variants are associated with both membrane-bound and soluble forms of HLA-G, and, in some studies, with preeclampsia. In this review, a genetic contribution from the mother, the father, and the fetus, together with the presence and function of various immune cells of relevance in pregnancy are reviewed in relation to HLA-G and preeclampsia.

Maternal HLA-C2 and 14 bp insertion in HLA-G is associated with recurrent implantation failure after in vitro fertilization treatment

The major rate-limiting step in in vitro fertilization (IVF) success appears to be the implantation of the semi-allogeneic embryo into the maternal endometrium. To determine possible risk factors of recurrent failure of embryos to implant, we investigated immunogenetic determinants as level of human leukocyte antigen (HLA) histocompatibility, frequency of killer-cell immunoglobulin-like receptors (KIR) and HLA-C alleles and HLA-G polymorphism. We DNA typed women with recurrent implantation failure (RIF) and their partners for classical HLA Class I, HLA Class II, HLA-G and KIR alleles and compared these results with couples with successful embryo implantation after their first IVF and normal fertile couples. No association was found between RIF and the degree of histocompatibility between partners or sharing of a specific antigen. Also, no significant difference in KIR haplotype or combination of HLA-C group and KIR was observed. We did find a higher frequency of HLA-C2 and a higher frequency of 14 base pair (bp) insertion in HLA-G in women with RIF. Therefore we conclude that the degree of histocompatibility between partners is not a determining factor for the occurrence of RIF. However, presence of the HLA-C2 allotype and the HLA-G allele with a 14 bp insertion is a significant risk factor.

HLA-G as a tolerogenic molecule in transplantation and pregnancy

HLA-G is a nonclassical HLA class I molecule. In allogeneic situations such as pregnancy or allograft transplantation, the expression of HLA-G has been related to a better acceptance of the fetus or the allograft. Thus, it seems that HLA-G is crucially involved in mechanisms shaping an allogeneic immune response into tolerance. In this contribution we focus on (i) how HLA-G is involved in transplantation and human reproduction, (ii) how HLA-G is regulated by genetic and microenvironmental factors, and (iii) how HLA-G can offer novel perspectives with respect to therapy.

Increased human leukocyte antigen-G expression at the maternal-fetal interface is associated with preterm birth

Abstract Objective: The maternal-fetal interface must modulate immune function to allow tolerance of fetal cells while still reacting to pathogens to suppress infection. Human leukocyte antigen-G (HLA-G) is a class Ib major histocompatibility complex protein involved in maternal-fetal tolerance. We posited that alterations in placental HLA-G expression predispose women to preterm birth. The aim of this study was to compare HLA-G expression in the maternal-fetal interface of term versus preterm human placentas.

METHODS

We performed a cross-sectional study of specimens from the basal plate of the human placenta from women enrolled in a tissue specimen and clinical data consortium. Immunohistochemistry with digital microscopic analysis was used to quantify HLA-G protein expression in the basal plate from preterm and term placentas.

RESULTS

Preterm birth <37 weeks occurred in 29.5% of 149 singleton pregnancies. HLA-G-positive cells occupied one-third of the basal plates, and the HLA-G-positive area was increased by 14% in placentas from preterm births than in those from term births (32.1% in term placentas versus 36.6% in preterm placentas).

CONCLUSION

Although HLA-G is required for maternal tolerance of the semi-allogeneic fetus, higher levels of HLA-G expression at the maternal-fetal interface is associated with preterm birth.

The many faces of human leukocyte antigen-G: relevance to the fate of pregnancy

Pregnancy is an immunological paradox, where fetal antigens encoded by polymorphic genes inherited from the father do not provoke a maternal immune response. The fetus is not rejected as it would be theorized according to principles of tissue transplantation. A major contribution to fetal tolerance is the human leukocyte antigen (HLA)-G, a nonclassical HLA protein displaying limited polymorphism, restricted tissue distribution, and a unique alternative splice pattern. HLA-G is primarily expressed in placenta and plays multifaceted roles during pregnancy, both as a soluble and a membrane-bound molecule. Its immunomodulatory functions involve interactions with different immune cells and possibly regulation of cell migration during placental development. Recent findings include HLA-G contributions from the father and the fetus itself. Much effort has been put into clarifying the role of HLA-G during pregnancy and pregnancy complications, such as preeclampsia, recurrent spontaneous abortions, and subfertility or infertility. This review aims to clarify the multifunctional role of HLA-G in pregnancy-related disorders by focusing on genetic variation, differences in mRNA stability between HLA-G alleles, differences in HLA-G isoform expression, and possible differences in functional activity. Furthermore, we highlight important observations regarding HLA-G genetics and expression in preeclampsia that future research should address.

HLA-G5 induces IL-4 secretion critical for successful pregnancy through differential expression of ILT2 receptor on decidual CD4⁺ T cells and macrophages

Successful pregnancy in humans has been associated with production of IL-4 by T cells at the feto-maternal interface. Soluble HLA-G5 produced by trophoblasts potentially controls the decidual T cell cytokine profile. We studied the effect of HLA-G5 on the cytokine profile of purified human macrophages and Ag-specific T cells in vitro. We demonstrated that HLA-G5 increased production of IL-12 by purified peripheral blood macrophages. Although IL-12 production by macrophages is known to induce IFN-γ production by CD4(+) T cells, HLA-G5 increased production of IL-4 but not IFN-γ by CD4(+) T cells after Ag presentation by macrophages. We found that this apparent paradox was due to the differential expression of the ILT2 HLA-G5 receptor on activated T cells and macrophages. This receptor was upregulated in the former and downregulated in the latter after Ag presentation and activation of both cell types. This observation was confirmed in situ, where decidual macrophages and T cells are continuously exposed to HLA-G5 produced locally and activated by trophoblast alloantigens. Freshly isolated decidua basalis macrophages expressed lower levels of ILT2 than peripheral blood macrophages from the same pregnant women. They did not spontaneously produce IL-12, whereas freshly isolated decidual CD4(+) T cells expressed high levels of activation markers (CD25, HLA-DR, and CD69) as well as ILT2 and spontaneously produced IL-4 but not IFN-γ. Therefore, HLA-G5 could be responsible, at least in part, via its interaction with ILT2, for decidual T cell IL-4 production, known to be crucial for successful pregnancy.

IFPA Senior Award Lecture: Reproductive immunology in perspective--reprogramming at the maternal-fetal interface

Involvement of the maternal and fetal immune systems in the events of pregnancy was generally overlooked by reproductive biologists until the mid-twentieth century when many landmark explorations were reported. Now, more than half a century later, it is well understood that with the initiation of pregnancy, immune cells in mammalian uteri are reprogrammed, losing their cytotoxic potential and providing an immunosuppressive environment suitable for harboring the genetically different fetus. We propose that it is the placenta that is mainly responsible for this conversion and maintenance throughout pregnancy. Studies in our laboratory indicate that trophoblast-derived soluble HLA-G has a subtle but well defined role in programming uterine placental macrophages, a potentially destructive immune cell population. Thus, placental HLA-G plays a critical role in assuring that the developing fetus emerges unscathed at parturition.

Promoter sequences confirm the three different evolutionary lineages described for HLA-G

HLA-G alleles follow a different pattern of polymorphism generation that those of the HLA classical I alleles. However, this polymorphism maintenance could have an evolutionary specific pathways based on non coding regions as introns, 14 bp deletion/insertion (exon 8) or promoter regions. For this reason, a systematic sequencing study of HLA-G promoter region was done in 36 individuals with a total of 15 different alleles. From the 12 sequences obtained, 7 were new sequences and not previously described. Results show that the sequences have three different patterns of evolution confirming the results obtained in the rest of the sequence regions (exons, introns and 3'UTR) where three different lineages were established. Only one of these lineages includes the non-human primate promoter sequences suggesting the possibility of this lineage could come directly from non-human primates while the other two could be generated after the speciation. More non-human primates MHC-G promoter sequences must be obtained to confirm this hypothesis. Expression and functional assays could be done considering the differences obtained in the promoter regions involving the HLA-G function (mRNA expression, isoforms).

Immunomodulatory molecules are released from the first trimester and term placenta via exosomes

The semiallogenic fetus is tolerated by the maternal immune system through control of innate and adaptive immune responses. Trophoblast cells secrete nanometer scale membranous particles called exosomes, which have been implicated in modulation of the local and systemic maternal immune system. Here we investigate the possibility that exosomes secreted from the first trimester and term placenta carry HLA-G and B7 family immunomodulators. Confocal microscopy of placental sections revealed intracellular co-localization of B7-H1 with CD63, suggesting that B7-H1 associates with subcellular vesicles that give rise to exosomes. First trimester and term placental explants were then cultured for 24 h. B7H-1 (CD274), B7-H3 (CD276) and HLA-G5 were abundant in pelleted supernatants of these cultures that contained microparticles and exosomes; the latter, however, was observed only in first trimester pellets and was nearly undetectable in term explant-derived pellets. Further purification of exosomes by sucrose density fractionation confirmed the association of these proteins specifically with exosomes. Finally, culture of purified trophoblast cells in the presence or absence of EGF suggested that despite the absence of HLA-G5 association with term explant-derived exosomes, it is present in exosomes secreted from mononuclear cytotrophoblast cells. Further, differentiation of cytotrophoblast cells reduced the presence of HLA-G5 in secreted exosomes. Together, the results suggest that the immunomodulatory proteins HLA-G5, B7-H1 and B7-H3, are secreted from early and term placenta, and have important implications in the mechanisms by which trophoblast immunomodulators modify the maternal immunological environment.

The immunosuppressive molecule HLA-G and its clinical implications

Human leukocyte antigen G (HLA-G) is a non-classical major histocompatibility complex (MHC) class I molecule that, through interaction with its receptors, exerts important tolerogenic functions. Its main physiological expression occurs in placenta where it seems to participate in the maternal tolerance toward the fetus. HLA-G has been studied as a marker of pregnancy complications such as abortion or pre-eclapmsia. Although HLA-G is not expressed in most adult tissues, its ectopic expression has been observed in some diseases such as viral infections, autoimmune disorders, and especially cancer. HLA-G neo-expression in cancer is associated with the capability of tumor cells to evade the immune control. In this review, we will summarize HLA-G biology and how it participates in these physiopathological processes. Special attention will be paid to its role as a diagnostic tool and also as a therapeutic target.

Inhibition of human gamma delta [corrected] T-cell antitumoral activity through HLA-G: implications for immunotherapy of cancer

Vγ9Vδ2 T cells play a crucial role in the antitumoral immune response through cytokine production and cytotoxicity. Although the expression of the immunomodulatory molecule HLA-G has been found in diverse tumors, its impact on Vγ9Vδ2 T-cell functions remains unknown. Here we showed that soluble HLA-G inhibits Vγ9Vδ2 T-cell proliferation without inducing apoptosis. Moreover, soluble HLA-G inhibited the Vγ9Vδ2 T-cell production of IFN-γ induced by phosphoantigen stimulation. The reduction in Vγ9Vδ2 T-cell IFN-γ production was also induced by membrane-bound or soluble HLA-G expressed by tumor cell lines. Finally, primary tumor cells inhibited Vγ9Vδ2 T-cell proliferation and IFN-γ production through HLA-G. In this context, HLA-G impaired Vγ9Vδ2 T-cell cytotoxicity by interacting with ILT2 inhibitory receptor. These data demonstrate that HLA-G inhibits the anti-tumoral functions of Vγ9Vδ2 T cells and imply that treatments targeting HLA-G could optimize Vγ9Vδ2 T-cell-mediated immunotherapy of cancer.

Fetus specific T cell modulation during fertilization, implantation and pregnancy

Recently there is an increasing interest in aspects of a more specific immunoregulation during pregnancy. Understanding these mechanism might have a broader application not only for reproductive immunology but also in general for biology and medicine. Especially the induction, already before conception, of feto-specific T cells with a possibly regulatory function gives a biological explanation of local immunotolerance at the maternal fetal interface, supporting the epidemiological evidence of a feto/paternal-specific immuneregulation. Understanding the expression of specific HLA-classes on trophoblast and the crosstalk of these antigens with various cell types, specifically modulated in the decidua, resulting in the secretion of cytokines and (angiogenic) chemokines has given us a more and more detailed understanding of this regulation. This regulation could be induced by fetal cells circulating in the mother (microchimerism) and from the interaction with fetal subcellular fractions as exosomes, but also from paternal antigens present in seminal fluid. Molecular interaction between paternal and fetal antigens and receptors in endometrium and the decidua are discussed. This review highlights besides uNK cells, especially the function of CD4+ and CD8+ T cells with a regulatory function in the context of recurrent miscarriage and pre-eclampsia. Besides HLA, also male-specific minor histocompatibility antigens and the genetic background for these pregnancy complications are discussed.

Evaluation of HLA-G5 plasmatic levels during pregnancy and relationship with the 14-bp polymorphism

PROBLEM

Plasmatic HLA-G levels increase during pregnancy, but the contribution of each different isoform has not been elucidated yet.

METHOD OF STUDY

HLA-G5 was analyzed by ELISA in 19 controls, 79 women in the first 8 weeks of pregnancy and in nine women monthly until delivery. Genotyping for the 14-bp polymorphism was performed by PCR amplification of exon 8.

RESULTS

HLA-G5 was detected in plasma from 80% of pregnant women. The levels did not change during pregnancy, and there were no differences compared to control non-pregnant women. There was a high interindividual variation that was maintained throughout the pregnancy. The presence of +14-bp allele was associated with HLA-G5 positivity. Pregnant women who were heterozygotic to 14-bp polymorphism had significantly higher levels of HLA-G5 compared to -14 bp/-14-bp homozygotic.

CONCLUSION

Plasmatic HLA-G5 levels do not change during pregnancy and its concentration depends on 14-bp polymorphism.

The importance of HLA-G expression in embryos, trophoblast cells, and embryonic stem cells

The nonclassical HLA-G molecule is a trophoblast-specific molecule present in almost every pregnancy. It differs from classical HLA class I molecules by the low degree of allelic variants and the high diversity of protein structures. HLA-G is reported to be a tolerogenic molecule that acts on cells of both innate and adaptive immunity. At the maternal-fetal interface HLA-G seems to be responsible largely for the reprogramming of local maternal immune response. This review will focus on the HLA-G gene expression profile in pregnancy, in preimplantation embryos, and in human embryonic stem cells with emphasis on the structural diversity of the HLA-G protein and its potential functional and diagnostic implications.

Expression profiles of peripheral CD160+ lymphocytes during the course of healthy human pregnancy

PROBLEM

CD160 receptor is expressed by natural killer (NK) and T-cell subsets, and after activation, it could enhance cytotoxicity or pro-inflammatory cytokine production on NK cells. Here, we investigated the phenotype of peripheral CD160+ cells during healthy pregnancy.

METHOD OF STUDY

We analyzed the expression of CD69 activation marker, gamma/delta TCR, and NKG2A or NKG2D NK cell receptors on CD160+ lymphocytes of non-pregnant and healthy pregnant women at four different stages of pregnancy by flow cytometry.

RESULTS

In our hands, CD160 receptor-positive lymphocytes were present during pregnancy; however, they had different characteristics depending on gestational age. During implantation, CD160+ cells showed low activation rate, decreased NK receptor expression while 40% of Vδ2 + T cells expressed CD160 receptor. In turn, all the above parameters increased as pregnancy proceeds.

CONCLUSION

Our results indicate that CD160+ lymphocytes could be able to play a role in the maintenance of healthy pregnancy.

HLA-G and immune evasion in cancer cells

Acquisition of novel gene products or new antigens in cancer cells elicits a host immune response that results in selection pressure for tumor clones to evade immunosurveillance. Similar to maternal-fetal tolerance and allotransplantation acceptance, upregulation of HLA-G expression has been found as one of the mechanisms that are programmed in cancer cells. HLA-G expression is frequently detected in a wide variety of human cancers and its protein levels negatively correlate with poor clinical outcome. The immune inhibitory effect can be achieved by binding of HLA-G molecules to the immunoglobulin-like inhibitory receptors that are expressed on the immunocompetent cells at all stages of the immune response. This review summarizes recent studies of HLA-G expression in human cancer, with a special focus on the molecular mechanisms that underlie how HLA-G molecules facilitate tumor cell evasion of the host immune response, and presents new directions for developing HLA-G-based diagnosis/therapeutics.

Soluble human leukocyte antigen-G isoforms in maternal plasma in early and late pregnancy

PROBLEM

Human Leukocyte Antigen (HLA)-G is a class Ib gene located in the human major histocompatibility complex (MHC). Several lines of investigation indicate that the HLA-G molecule is involved in the maternal acceptance of the semi-allogenic fetus during pregnancy and in the development of tolerance. Expression of soluble HLA-G (sHLA-G) is positively correlated with successful in vitro fertilization (IVF) treatments, and aberrant expression of HLA-G in certain complications of pregnancy, such as pre-eclampsia and spontaneous abortion, has been reported. The main purpose of this study was to investigate the levels of different soluble HLA-G isoforms in maternal plasma in early and late pregnancy.

METHOD OF STUDY

Soluble HLA-G (sHLA-G) can be detected in maternal blood, and in this study, two different isoforms of sHLA-G, namely sHLA-G1 generated by shedding of membrane-bound HLA-G1 and HLA-G generated by specific HLA-G transcripts, have been investigated early [median of 16.4 weeks of gestation (GW)] and late (median: 38.9 GW) in pregnancy in an original cohort of 580 pregnant Caucasian women.

RESULTS

Lower concentrations of sHLA-G1 were found late in pregnancy (>32 GW) in a group of women with severe pre-eclampsia compared with controls with uncomplicated pregnancies (P = 0.029, P(C) = 0.09; Mann-Whitney; Logistic regression analysis: P = 0.024, OR = 0.920, 95% CI: 0.855-0.989). However, this was not the case with HLA-G5, and significantly more of the cases with severe pre-eclampsia had detectable plasma HLA-G5 compared with that of the control group (P = 0.013, P(C) = 0.04; Mann-Whitney). Similar findings were not observed in women with gestational hypertension or existing hypertension continuing into pregnancy. Furthermore, there was a trend toward lower maternal plasma sHLA-G1 in a group of women with premature birth (<37 GW) compared with that of the control group (P = 0.028, P(C) = 0.17; Mann-Whitney). On the contrary, HLA-G5 was lower in the control group compared with that in the premature group (P = 0.004, P(C) = 0.02; Mann-Whitney).

CONCLUSION

This study shows in line with other published studies that a high, detectable soluble HLA-G concentration in maternal plasma or serum is not mandatory for a successful pregnancy. However, complications during pregnancy, such as (severe) pre-eclampsia, spontaneous abortion, IUGR, and premature birth, are associated with a low or undetectable level of soluble HLA-G in the maternal blood circulation. Also, this study indicates that sHLA-G1 is the interesting soluble HLA-G isoform in pre-eclampsia, and that low or undetectable levels of HLA-G5 at the end of pregnancy seem to be associated with an uncomplicated normal pregnancy, whereas in severe pre-eclampsia and possibly other pregnancy complications, such as preterm birth and IUGR, the level of HLA-G5 is higher.

Differences in the soluble HLA-G blood serum concentration levels in patients with ovarian cancer and ovarian and deep endometriosis

PROBLEM

The relationship between endometriosis and cancer has been widely discussed in the literature but is still not well clarified. Perhaps significantly, soluble human leukocyte antigen-G (sHLA-G) has been identified in the microenvironment of both ovarian cancer and endometrioma. The aim of this study has been to evaluate the sHLA-G levels in the blood sera of women with deep endometriosis and ovarian endometrioma over the course of the menstrual cycle and to compare to the levels of sHLA-G in the blood sera of women with ovarian cancer.

METHOD OF STUDY

In our study, we examined the blood sera obtained from 123 patients operated on because of ovarian cancer (65 cases), ovarian endometrioma (30 cases), and deep endometriosis (28 cases). We decided to compare the levels of sHLA-G in patients with endometriosis to those found in patients with ovarian cancer with respect to the menstrual cycle phases. The sHLA-G concentration level was measured by enzyme-linked immunosorbent assay kit.

RESULTS

The level of sHLA-G concentration in the blood serum of patients with deep endometriosis fluctuates over the course of the menstrual cycle, and during the proliferative and secretory phases, it remains at a high level comparable to that found in patients with ovarian cancer. By contrast, the level of sHLA-G concentration in the blood serum of patients with ovarian endometrioma fluctuates minimally over the course of the different menstrual cycle phases and, as in patients with ovarian cancer, it remains at high level during the proliferative phase.

CONCLUSION

sHLA-G blood serum concentration levels would seem to provide important information regarding the degree of immune system regulation disturbance in both ectopic endometrial cells and the cancer cell suppressive microenvironment.

Fetomaternal immunotolerance

Implantation of mammalian conceptus in uterine cavity is the result of evolutionary adaptation, through high level of physiological procedures to ensure its success. However the majority of pregnancy losses occur before or during implantation. It is expected that exploring and defining the molecular and physiological road map during the crucial time of implantation will enable us to decode and effectively treat fertility defects. Immunological, hormonal and molecular factors participate in the feto-maternal cross talk during implantation and designate the effectiveness of the process. The atypical expression of major histocompatibility complex and other protein-antigens, such as Fas/FasL and petformin in human trophoblast, the modified function of cellular constituents of the feto-maternal interface, as well as the specific role of some hormones and cytokines, represent substantive parameters of feto-maternal immunotolerance during implantation.

Soluble total human leukocyte antigen class I and human leukocyte antigen-G molecules in kidney and kidney/pancreas transplantation

The expression of human leukocyte antigen (HLA)-G, a nonclassical HLA class I molecule, and its soluble forms (sHLA-G) are found to improve graft acceptance. In this study we investigated whether sHLA-G is the most biologically relevant molecule among all types of soluble HLA class I molecules for graft acceptance. We addressed this question in kidney-transplanted (n = 32) and kidney/pancreas-transplanted patients (n = 29). To this end we analyzed the levels of total soluble HLA class I (sHLA-I) in comparison to sHLA-G in 488 plasma samples procured before and serial after transplantation by specific enzyme-linked immunoabsorbent assay. Samples from 126 healthy individuals served as controls. Pretransplantation sHLA-I levels were significantly increased in patients (p < 0.001), whereas sHLA-G levels were in the range of those of healthy controls. Importantly, pretransplantation sHLA-I and sHLA-G levels did not differ between the two groups. Patients with biopsy-proven rejection (n = 15) revealed significantly lower sHLA-G levels before transplantation (mean +/- standard error of the mean, 12.9 +/- 1.8 vs. 20.1 +/- 1.9, p = 0.013) and after transplantation (p = 0.006, two-way analysis of variance) than patients without rejection (n = 46). In contrast, sHLA-I was slightly increased after but not before transplantation in patients with rejection (p < 0.05, two-way analysis of variance). Nonparametric determination analysis showed that pretransplantation levels of sHLA-G < 11.5 ng/ml (sensitivity, 60%; specificity, 80.4%) were related to rejection. Regarding antibody status, retransplantation, number of HLA mismatches, recipient age, and recipient body mass index, multivariate analysis showed that sHLA-G but not sHLA-I is an independent risk factor for graft rejection. Thus high levels of sHLA-G but not of sHLA-I seem to contribute to better graft acceptance after kidney or kidney/pancreas transplantation.

Nitric oxide produces HLA-G nitration and induces metalloprotease-dependent shedding creating a tolerogenic milieu

Human leucocyte antigen G (HLA-G) is a tolerogenic molecule that protects the fetus from maternal immune attack, may favour tumoral immunoescape and is up-regulated in viral and inflammatory diseases. The aim of this work was to discover if nitric oxide (NO) could affect HLA-G expression or function because NO is an important modulator of innate and adaptive immunity. For this purpose HLA-G expression and function were analysed following treatment with a NO donor or a peroxynitrite donor in various cell lines expressing HLA-G either spontaneously or upon transfection. Results showed NO-dependent nitration of both cellular and soluble HLA-G protein, but not all HLA-G moieties underwent nitration. Endogenous biosynthesis of NO by both U-937-HLA-G1 and M8-HLA-G5 stable transfectants also caused HLA-G nitration. The NO decreased total HLA-G cellular protein content and expression on the cell surface, while increasing HLA-G shedding into the culture medium. This effect was post-transcriptional and the result of metalloprotease activity. By contrast, NO pretreatment did not affect HLA-G capability to suppress NK cytotoxicity and lymphocyte proliferation. Our studies show that NO regulates the availability of HLA-G molecules without modifying their biological activities.

Application of cryo-compatible antibodies to human placenta paraffin sections

The hepes-glutamic acid buffer-mediated organic solvent protection effect (HOPE) -fixation and paraffin embedding technique has been described to expand possibilities for immuno-labellings due to low denaturation of proteins. In this study, the issue was addressed as to whether the HOPE technique could be a useful tool in placenta tissue-based studies when only cryo-compatible antibodies are available. Such antibodies can be used on cryostat sections only, giving results of considerably inferior morphological detail as compared to routinely fixed paraffin embedded tissue sections. Commercially available, only cryo-compatible, monoclonal antibodies against a conformational epitope of HLA-G (clone MEM-G/9) and leukocyte differentiation antigens CD56, CD163 and CD34 III were selected and applied to frozen sections, routinely formalin-fixed and HOPE-fixed paraffin sections. All tested antibodies immunolocalized their antigen on cryo sections and on HOPE-fixed but not formalin-fixed paraffin sections. The HOPE technique provides an excellent preservation of protein antigenicity together with well presented morphological details in paraffin embedded placenta tissues. The detection of native or conformation-dependent epitopes in paraffin sections expands the immunolocalization possibilities in placenta research and reproductive immunology.

Synthesis of beta(2)-microglobulin-free, disulphide-linked HLA-G5 homodimers in human placental villous cytotrophoblast cells

Human leucocyte antigen-G (HLA-G) is a natural immunosuppressant produced in human placentas that binds differently to the inhibitory leucocyte immunoglobulin-like receptors LILRB1 (ILT2) and LILRB2 (ILT4) according to its biochemical structure. To predict the binding functions of the HLA-G5 soluble isoform synthesized in placental villous cytotrophoblast (vCTB) cells, we investigated structural features of this protein. Biochemical and immunological studies showed that vCTB cell HLA-G5 heavy (H)-chain proteins are disulphide-bonded homodimers unassociated with beta(2)-microglobulin (beta2m) light-chain proteins. Although comparatively low levels of beta2m messenger RNA (mRNA) were identified by real-time reverse transcription-polymerase chain reaction, immunoprecipitation studies failed to detect beta2m protein even when specific mRNA was doubled by transduction of a lentivirus-beta2m complementary DNA into vCTB cells. No abnormalities were identified in the translational start codon of vCTB cell beta2m mRNA and differentiation into syncytium did not promote beta2m synthesis. The failure of vCTB cells to exhibit beta2m in vitro was paralleled by a lack of detectable beta2m in vCTB cells in vivo. Lack of the beta2m protein could be the result of low levels of beta2m transcripts or of as yet unidentified translational defects. Experiments with recombinant ectodomains of LILRB indicate that beta2m-free HLA-G binds strongly to LILRB2, a receptor that is expressed by macrophages. This potentially immunosuppressive cell type is abundant in the pregnant uterus. Thus, our findings are consistent with the postulate that the natural beta2m-free homodimeric form of HLA-G5 synthesized in primary vCTB cells could comprise a particularly effective tolerogenic molecule at the maternal-fetal interface.