HLA-G is found in lipid rafts and can act as a signaling molecule

Reverse Signaling by MHC-I Molecules in Immune and Non-Immune Cell Types

Major histocompatibility complex (MHC) molecules are well-known for their role in antigen (cross-) presentation, thereby functioning as key players in the communication between immune cells, for example dendritic cells (DCs) and T cells, or immune cells and their targets, such as T cells and virus-infected or tumor cells. However, much less appreciated is the fact that MHC molecules can also act as signaling receptors. In this process, here referred to as reverse MHC class I (MHC-I) signaling, ligation of MHC molecules can lead to signal-transduction and cell regulatory effects in the antigen presenting cell. In the case of MHC-I, reverse signaling can have several outcomes, including apoptosis, migration, induced or reduced proliferation and cytotoxicity towards target cells. Here, we provide an overview of studies showing the signaling pathways and cell outcomes upon MHC-I stimulation in various immune and non-immune cells. Signaling molecules like RAC-alpha serine/threonine-protein kinase (Akt1), extracellular signal-regulated kinases 1/2 (ERK1/2), and nuclear factor-κB (NF-κB) were common signaling molecules activated upon MHC-I ligation in multiple cell types. For endothelial and smooth muscle cells, the in vivo relevance of reverse MHC-I signaling has been established, namely in the context of adverse effects after tissue transplantation. For other cell types, the role of reverse MHC-I signaling is less clear, since aspects like the in vivo relevance, natural MHC-I ligands and the extended downstream pathways are not fully known.The existing evidence, however, suggests that reverse MHC-I signaling is involved in the regulation of the defense against bacterial and viral infections and against malignancies. Thereby, reverse MHC-I signaling is a potential target for therapies against viral and bacterial infections, cancer immunotherapies and management of organ transplantation outcomes.

HLA class Ib in pregnancy and pregnancy-related disorders

The HLA class Ib genes, HLA-E, HLA-F, and HLA-G, were discovered long after the classical HLA class Ia genes. The elucidation of their functions had a modest beginning. However, their basic functions and involvement in pathophysiology and a range of diseases are now emerging. Although results from a range of studies support the functional roles for the HLA class Ib molecules in adult life, especially HLA-G and HLA-F have most intensively been, and were also primarily, studied in relation to reproduction and pregnancy. The expression of HLA class Ib proteins at the feto-maternal interface in the placenta seems to be important for the maternal acceptance of the semi-allogenic fetus. In contrast to the functions of HLA class Ia, HLA-G possesses immune-modulatory and tolerogenic functions. Here, we review an accumulating amount of data describing the functions of HLA class Ib molecules in relation to fertility, reproduction, and pregnancy, and a possible role for these molecules in certain pregnancy complications, such as implantation failure, recurrent spontaneous abortions, and pre-eclampsia. The results from different kinds of studies point toward a role for HLA class Ib, especially HLA-G, throughout the reproductive cycle from conception to the birth weight of the child.

Association of HLA-A and Non-Classical HLA Class I Alleles

The HLA-A locus is surrounded by HLA class Ib genes: HLA-E, HLA-H, HLA-G and HLA-F. HLA class Ib molecules are involved in immuno-modulation with a central role for HLA-G and HLA-E, an emerging role for HLA-F and a yet unknown function for HLA-H. Thus, the principal objective of this study was to describe the main allelic associations between HLA-A and HLA-H, -G, -F and -E. Therefore, HLA-A, -E, -G, -H and -F coding polymorphisms, as well as HLA-G UnTranslated Region haplotypes (referred to as HLA-G UTRs), were explored in 191 voluntary blood donors. Allelic frequencies, Global Linkage Disequilibrium (GLD), Linkage Disequilibrium (LD) for specific pairs of alleles and two-loci haplotype frequencies were estimated. We showed that HLA-A, HLA-H, HLA-F, HLA-G and HLA-G UTRs were all in highly significant pairwise GLD, in contrast to HLA-E. Moreover, HLA-A displayed restricted associations with HLA-G UTR and HLA-H. We also confirmed several associations that were previously found to have a negative impact on transplantation outcome. In summary, our results suggest complex functional and clinical implications of the HLA-A genetic region.

Mouse models for studies of HLA-G functions in basic science and pre-clinical research

HLA-G was described originally as a tolerogenic molecule that allows the semiallogeneic fetus to escape from recognition by the maternal immune response. This review will discuss different steps in the study of HLA-G expression and functions in vivo, starting with analyses of expression of the HLA-G gene and its receptors in transgenic mice, and continuing with applications of HLA-G and its receptors in prevention of allograft rejection, transplantation tolerance, and controlling the development of infection. Humanized mouse models have been discussed for developing in vivo studies of HLA-G in physiological and pathological conditions. Collectively, animal models provide an opportunity to evaluate the importance of the interaction between HLA-G and its receptors in terms of its ability to regulate immune responses during maternal-fetal tolerance, survival of allografts, tumor-escape mechanisms, and development of infections when both HLA-G and its receptors are expressed. In addition, in vivo studies on HLA-G also offer novel approaches to achieve a reproducible transplantation tolerance and to develop personalized medicine to prevent allograft rejection.

HLA-G inhibition of NK-cell cytolytic function is uncoupled from tumor cell lipid raft reorganization

HLA-G is a non-classical HLA class I molecule with tolerogenic properties and restricted tissue distribution. The expression of HLA-G can be induced by tumors thus providing an efficient way to escape the anti-tumoral immune response. Although lipid rafts regulate diverse immunological mechanisms their relationship with HLA-G remains controversial. Our results show that HLA-G-mediated inhibition of both the interaction between NK and tumor cells, and of intracellular calcium flux in NK cells conjugated to their target cells were independent of lipid raft integrity. In addition, cytotoxicity assays indicated that HLA-G continued to efficiently inhibit NK-cell cytolytic function in several different tumor cells independently of lipid raft integrity. Confocal microscopy with 3D reconstruction combined with biochemical analysis showed that HLA-G was mainly localized outside the lipid rafts of tumor cells after cross-linking with specific antibody and remained excluded from lipid rafts during interaction with the ILT2 inhibitory receptor of NK cells. This study indicates that the inhibitory function of HLA-G is uncoupled from lipid raft organization, further distinguishing HLA-G from classical HLA molecules and providing novel information in the understanding of tumor immune escape mechanism mediated through HLA-G.

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.

Down-regulation of HLA-G Attenuates Cleavage Rate in Human Triploid Embryos

INTRODUCTION

HLA-G is a major histocompatibility complex (MHC), class Ib molecule that is selectively expressed at the fetal-maternal interface. It is thought to play a role in protecting the fetus from the maternal immune response. Interestingly, the preimplantation embryo development (Ped) gene product Qa-2 is also a mouse MHC class Ib protein that affects cleavage and division of preimplantation mouse embryos and subsequent embryonic survival. Data from many human in vitro fertilization (IVF) clinics suggest that the mouse Ped phenomenon also exists in human because embryos fertilized at the same time have different cleavage rates and consequently different IVF outcomes. As HLA-G is expressed in human early embryos, it is highly regarded as the functional homologue of Qa-2. Whether HLA-G expression is correlated with the cleavage rate of human embryos has great potential clinical value.

METHODS

In this study, 45 human early abnormal fertilized embryos (3 PN) from patients undergoing in vitro fertilization were used to test the effects of HLA-G knock-down via infection with adenovirus carrying its specific siRNA on the cleavage rate in a 2-day culture period. One-way ANOVA, Post hoc and Chi-square were used to compare groups. A p-value smaller than 0.05 was considered statistically significant.

RESULTS

Knocking-down HLA-G in human pre-implantation stage embryos resulted in a higher cell arrest rate and a slower cleavage rate.

CONCLUSION

The results from the present study suggested that HLA-G might play an important role in early human embryo development.

Differentiation of type 1 T regulatory cells (Tr1) by tolerogenic DC-10 requires the IL-10-dependent ILT4/HLA-G pathway

Type 1 T regulatory (Tr1) cells suppress immune responses in vivo and in vitro and play a key role in maintaining tolerance to self- and non-self-antigens. Interleukin-10 (IL-10) is the crucial driving factor for Tr1 cell differentiation, but the molecular mechanisms underlying this induction remain unknown. We identified and characterized a subset of IL-10-producing human dendritic cells (DCs), termed DC-10, which are present in vivo and can be induced in vitro in the presence of IL-10. DC-10 are CD14(+), CD16(+), CD11c(+), CD11b(+), HLA-DR(+), CD83(+), CD1a(-), CD1c(-), express the Ig-like transcripts (ILTs) ILT2, ILT3, ILT4, and HLA-G antigen, display high levels of CD40 and CD86, and up-regulate CD80 after differentiation in vitro. DC-10 isolated from peripheral blood or generated in vitro are potent inducers of antigen-specific IL-10-producing Tr1 cells. Induction of Tr1 cells by DC-10 is IL-10-dependent and requires the ILT4/HLA-G signaling pathway. Our data indicate that DC-10 represents a novel subset of tolerogenic DCs, which secrete high levels of IL-10, express ILT4 and HLA-G, and have the specific function to induce Tr1 cells.

Symposium: innovative techniques in human embryo viability assessment. Soluble human leukocyte antigen-G and pregnancy success

Non-invasive methods of assessing embryo quality are critical for pregnancy success following IVF or intracytoplasmic sperm injection (ICSI). The addition of new non-invasive morphological and biochemical analyses may further improve pregnancy success, allowing the transfer of a single embryo, thereby reducing the risks involved in multiple births following IVF/ICSI. The presence of a protein, soluble human leukocyte antigen-G (sHLA-G), in embryo cultures has been suggested as a way to non-invasively predict embryo quality and pregnancy success, especially when used in conjunction with current embryo quality assessment methods. Detection of sHLA-G in embryo culture medium has been correlated with pregnancy success in 12 studies, but three studies were not able to detect sHLA-G. This is a review of the literature on sHLA-G detection in IVF/ICSI, and reasons are proposed for the reported discrepancies, as well as guidelines for reporting of data in future studies. Furthermore, it is suggested that the use of an HLA-G transgenic mouse model would advance understanding of the mechanism of action of sHLA-G in preimplantation embryos and its correlation to embryo health and viability. Research on a mouse model, combined with clinical studies, should enable the development of a fast and reliable method for utilizing sHLA-G detection to improve pregnancy success after IVF/ICSI.

Expression of major histocompatibility complex class I proteins and their antigen processing chaperones in mouse embryonic stem cells from fertilized and parthenogenetic embryos

Embryonic stem (ES) cells are pluripotent cells with the potential to differentiate into cells or tissues that may be used for transplantation therapy. Parthenogenetic ES (pES) cells have been recently derived from both mouse and human oocytes and hold promise as a cell source that is histocompatible to the oocyte donor. Because of the importance of major histocompatibility complex (MHC) antigens in mediating tissue rejection or acceptance, we examined levels of mRNA and protein expression of MHC class I proteins, as well as several MHC class I antigen processing and presentation chaperones in mouse ES cells derived from both fertilized (fES) and parthenogenetic (pES) embryos. We found that H-2K, Qa-2, TAP1, TAP2, and tapasin mRNAs were all expressed at low levels in undifferentiated and differentiating ES cells and were significantly upregulated in response to interferon-gamma (IFN-gamma) treatment following 14 days of differentiation. Likewise, expression of H-2K(b) and H-2K(k) proteins were upregulated to detectable levels by IFN-gamma after 14 days of differentiation, but Qa-2 protein expression remained low or absent. We also found that MHC class I, TAP1, TAP2, and tapasin mRNAs were all expressed at very low levels in ES cells compared with T cells, suggesting transcriptional regulation of these genes in ES cells. Calnexin, a chaperone molecule involved in other pathways than MHC expression, had mRNA levels that were similar in ES cells and T cells and was not upregulated by IFN-gamma in ES cells. Overall, ES cells derived from fertilized embryos and parthenogenetic embryos displayed remarkably similar patterns of gene expression at the mRNA and protein levels. The similarity between the fES and pES cell lines with regard to expression of MHC class I and antigen-processing machinery provides evidence for the potential usefulness of pES cells in transplantation therapy.

Expression of endothelial selectin ligands on human leukocytes following dive

The fact that impaired endothelial-dependent vasodilatation after scuba diving often occurs without visible changes in the endothelial layer implies its biochemical origin. Since Lewisx(CD15) and sialyl-Lewisx(CD15s) are granulocyte and monocyte carbohydrate antigens recognized as ligands by endothelial selectins, we assumed that they could be sensitive markers for impaired vasodilatation following diving. Using flow cytometry, we determined the CD15 and CD15s peripheral blood mononuclear cells of eight divers, 30 mins before and 50 mins after a single dive to 54 m for 20 mins bottom time. The number of gas bubbles in the right heart was monitored by ultrasound. Gas bubbles were seen in all eight divers, with the average number of bubbles/cm2 1.9+/-1.9. The proportion of CD15+monocytes increased 2-fold after the dive as well as the subpopulation of monocytes highly expressing CD15s. The absolute number of monocytes was slightly, but not significantly, increased after the dive, whereas the absolute number of granulocytes was markedly elevated (up to 61%). There were no significant correlations between bubble formation and CD15+monocyte expression (r=-0.56; P=0.17), as well as with monocytes highly expressing CD15s (r=0.43; P=0.29). This study suggests that biochemical changes induced by scuba diving primarily activate existing monocytes rather than increase the number of monocytes at a time of acute arterial endothelial dysfunction.

MicroRNA expression in preimplantation mouse embryos from Ped gene positive compared to Ped gene negative mice

PURPOSE

The mouse preimplantation embryo development (Ped) gene product, Qa-2, influences the rate of preimplantation embryonic development and overall reproductive success. Here we investigated the expression pattern of two microRNAs, miR-125a and miR-125b, known to be involved in development in lower organisms, in preimplantation embryos from the two-cell, four-cell, eight-cell, morula, and blastocyst stages of development from the congenic B6.K1 (Ped negative) and B6.K2 (Ped positive) strains of mice.

METHOD

B6.K1 and B6.K2 congenic mice differ only in the absence (B6.K1) or presence (B6.K2) of the genes encoding Qa-2 protein. We analyzed the expression of miR-125a and miR-125b in B6.K1 and B6.K2 preimplantation embryos by using real-time PCR.

RESULT

We found no variability in miR-125b expression at any developmental stage in both strains. However, miR-125a expression increased during development in both strains and was ten times higher in Ped negative (B6.K1) embryos than in Ped positive (B6.K2) embryos by the blastocyst stage of development.

CONCLUSION

Our results show that the absence of the Ped gene profoundly affects the level of a miRNA (miR-125a) known to regulate early development. The implication is that miR-125a is likely involved in the regulation of timing of early development in mice.

HLA-G: from biology to clinical benefits

The relevance of the nonclassical human leukocyte antigen (HLA) class I molecule HLA-G in human physiological and pathological contexts has been the center of intense investigation. In light of the recent advances, we report here the clinical implications of HLA-G as a tolerogenic molecule promoting uterine implantation of the embryo or acceptance of solid allografts while allowing the evasion of tumors or viruses from the immune response. These recent findings are important in terms of clinical benefits at both diagnostic and therapeutic levels.

The role of tapasin in MHC class I protein trafficking in embryos and T cells

Preimplantation mouse embryos express both classical (class Ia) and nonclassical (class Ib) MHC class I proteins, and yet are not rejected by the maternal immune system. Although the function of the embryonic MHC class Ia proteins is unknown, one MHC class Ib protein, Qa-2, the product of the preimplantation embryo development (Ped) gene, actually enhances reproductive success. Similar in structure to MHC class Ia proteins, Qa-2 protein is a trimer of the alpha (heavy) chain, beta(2) microglobulin and a bound peptide. Studies on the folding, assembly and trafficking of MHC class Ia molecules to the cell surface have revealed this process to be dependent on multiple protein chaperone molecules, but information on the role of chaperone molecules in Qa-2 expression is incomplete. Here, we report the detection of mRNA for four chaperone molecules (TAP1, TAP2, calnexin and tapasin) in preimplantation embryos. We then focused on the role of the MHC-dedicated chaperone, tapasin, on Qa-2 protein expression. First, we demonstrated that tapasin protein is expressed by preimplantation embryos. Then, we used tapasin knockout mice to evaluate the role of tapasin in Qa-2 protein expression on both T cells and preimplantation embryos. We report here that optimal cell surface expression of Qa-2 is dependent on tapasin in both T cells and preimplantation embryos. Identification of the molecules involved in regulation of MHC class I protein expression in early embryos is an important first step in gaining insight into mechanisms of escape of embryos from destruction by the maternal immune system.

Preimplantation embryo development (Ped) gene copy number varies from 0 to 85 in a population of wild mice identified as Mus musculus domesticus

The preimplantation embryo development (Ped) gene regulates the rate of preimplantation embryonic cleavage division and subsequent embryo survival. In the mouse, the Ped gene product is Qa-2 protein, a nonclassical MHC class I molecule encoded by four tandem genes, Q6/Q7/Q8/Q9. Most inbred strains of mice have all four genes on each allelic chromosome, making a total of eight Qa-2 encoding genes, but there are a few strains that are missing all eight genes, defining a null allele. Mouse strains with the presence of the Qa-2 encoding genes express Qa-2 protein and produce embryos with a faster rate of preimplantation embryonic development and a greater chance of embryo survival compared to mouse strains with the null allele. There is extensive evidence that the human homolog of Qa-2 is HLA-G. HLA-G in humans, like Qa-2 in mice, is associated with enhanced reproductive success. The human population is an outbred population. Therefore, for a better comparison to the human population, we undertook an investigation of the presence of the genes encoding Qa-2 in an outbred population of mice. We used Real-Time Quantitative PCR to quantify the number of Qa-2 encoding genes in a population of 32 wild mice identified as Mus musculus domesticus both by morphologic assessment and by PCR analysis of their DNA. We found great variability in the number of Qa-2 encoding genes in the wild mice tested. The wild mouse with the highest number of Qa-2 encoding genes had 85 such genes, whereas we discovered one wild mouse without any Qa-2 encoding genes. Evolutionary implications of a range of Qa-2 encoding gene numbers in the wild mouse population are discussed, as well as the relevance of our findings to humans.

Activation of T cells by cross-linking Qa-2, the ped gene product, requires Fyn

PROBLEM

Qa-2, the product of the Ped (preimplantation development) gene, regulates the rate of cell division of preimplantation mouse embryos by an unknown mechanism. Due to the limited availability of preimplantation embryos, T cells were used as a model system to assess the possible roles of Fyn and Lck, and two downstream effectors, PI-3 kinase and Akt, in Qa-2 induced cell proliferation.

METHOD OF STUDY

Resting T cells were stimulated to proliferate by treating with mouse anti-Qa-2 antibody, cross-linking with anti-mouse immunoglobulin, and adding PMA. The effects of kinase inhibitors on this proliferation were studied. Co-immunoprecipitates of T-cell lysates were analyzed for possible associations between Qa-2 and Fyn or Lck. Fyn knockout mice (Fyn-/-) were used to determine whether Fyn is required for T-cell activation induced by cross-linking Qa-2.

RESULTS

An inhibitor of Src family kinases and inhibitors of PI-3 kinase and Akt suppressed proliferation of resting T cells induced by cross-linking Qa-2. Fyn, but not Lck, co-immunoprecipitated with Qa-2. Fyn-/- T cells failed to proliferate in response to Qa-2 cross-linking.

CONCLUSION

Fyn, PI-3 kinase, and Akt are required for the activation of T cells by cross-linking Qa-2.