The truncated cytoplasmic tail of HLA-G serves a quality-control function in post-ER compartments

Autophagy-dependent regulation of MHC-I molecule presentation

The major histocompatibility complex (MHC) class I molecules present peptide antigens to MHC class I-restricted CD8+ T lymphocytes to elicit an effective immune response. The conventional antigen-processing pathway for MHC-I presentation depends on proteasome-mediated peptide generation and peptide loading in the endoplasmic reticulum by members of the peptide loading complex. Recent discoveries in this field highlight the role of alternative MHC-I peptide loading and presentation pathways, one of them being autophagy. Autophagy is a cell-intrinsic degradative pathway that ensures cellular homoeostasis and plays critical roles in cellular immunity. In this review article, we discuss the role of autophagy in MHC class I-restricted antigen presentation, elucidating new findings on the crosstalk of autophagy and ER-mediated MHC-I peptide presentation, dendritic cell-mediated cross-presentation and also mechanisms governing immune evasion. A detailed molecular understanding of the key drivers of autophagy-mediated MHC-I modulation holds promising targets to devise effective measures to improve T cell immunotherapies.

Neuronal MHC-I complex is destabilized by amyloid-β and its implications in Alzheimer's disease

BACKGROUNDS

The expression of major histocompatibility complex I (MHC-I) in neurons has recently been shown to regulate neurite outgrowth and synaptic plasticity. However, its contribution to neurodegenerative diseases such as Alzheimer's disease (AD) remains largely unknown.

METHODS

In this study, we investigated the relationship between impaired MHC-I-β2M complex and AD in vitro and human AD samples. Interaction between protein was identified by liquid chromatography-tandem mass spectrometry and confirmed by immunoprecipitation. Single-chain trimer of MHC-I-β2M was generated to study the effect of stabilization of MHC-I-β2M complex on NCAM1 signaling.

RESULTS

MHC-I is destabilized in the brains of AD patients and neuronal cells treated with oligomeric β-amyloid (Aβ). Specifically, Aβ oligomers disassemble the MHC-I-β2-microglobulin (β2M) complex, leading to reduced interactions with neural cell adhesion molecule 1 (NCAM1), a novel interactor of neuronal MHC-I, and decreased signaling. Inhibition of MHC-I-β2M complex destabilization by non-dissociable MHC-I-β2M-peptide complex restored MHC-I-NCAM1 signaling in neuronal cells.

CONCLUSIONS

The current study demonstrated that disruption of MHC-1-NCAM1 signaling by Aβ induced disassembly of MHC-I-β2M complex is involved in the pathophysiology of AD. Moreover, our findings suggest modulation of MHC-I stability may be a potential therapeutic target for restoring synaptic function in AD.

Intracellular trafficking of HLA-E and its regulation

Interest in MHC-E-restricted CD8+ T cell responses has been aroused by the discovery of their efficacy in controlling simian immunodeficiency virus (SIV) infection in a vaccine model. The development of vaccines and immunotherapies utilizing human MHC-E (HLA-E)-restricted CD8+ T cell response requires an understanding of the pathway(s) of HLA-E transport and antigen presentation, which have not been clearly defined previously. We show here that, unlike classical HLA class I, which rapidly exits the endoplasmic reticulum (ER) after synthesis, HLA-E is largely retained because of a limited supply of high-affinity peptides, with further fine-tuning by its cytoplasmic tail. Once at the cell surface, HLA-E is unstable and is rapidly internalized. The cytoplasmic tail plays a crucial role in facilitating HLA-E internalization, which results in its enrichment in late and recycling endosomes. Our data reveal distinctive transport patterns and delicate regulatory mechanisms of HLA-E, which help to explain its unusual immunological functions.

The mode of action of tapasin on major histocompatibility class I (MHC-I) molecules

Tapasin (Tsn) plays a critical role in antigen processing and presentation by major histocompatibility complex class I (MHC-I) molecules. The mechanism of Tsn-mediated peptide loading and exchange hinges on the conformational dynamics governing the interaction of Tsn and MHC-I with recent structural and functional studies pinpointing the critical sites of direct or allosteric regulation. In this review, we highlight these recent findings and relate them to the extensive molecular and cellular data that are available for these evolutionary interdependent proteins. Furthermore, allotypic differences of MHC-I with regard to the editing and chaperoning function of Tsn are reviewed and related to the mechanistic observations. Finally, evolutionary aspects of the mode of action of Tsn will be discussed, a short comparison with the Tsn-related molecule TAPBPR (Tsn-related protein) will be given, and the impact of Tsn on noncanonical MHC-I molecules will be described.

Prognostic significance of high circulating sHLA-G in ovarian carcinoma

HLA-G is a non-classical major histocompatibility complex class Ib molecule. Its expression has been described in various cancer types, including ovarian cancer. HLA-G molecule has been implicated in immune escape and in progression of ovarian tumor cells. Our goal was to assess if total soluble (s)HLA-G molecules or HLA-G5 and sHLA-G1 isoforms could be considered as circulating ovarian tumor biomarkers, we measured the concentration of these molecules in ovarian carcinoma patients stratified according with their clinicopathological parameters. sHLA-G, sHLA-G1 and HLA-G5 concentrations were dosed in plasma samples by sandwich-ELISA. The sHLA-G dimerization was analyzed after immunoprecipitation and SDS-PAGE migration. Total sHLA-G and sHLA-G1 levels were significantly represented in plasma of ovarian carcinoma patients compared to healthy controls. sHLA-G1 isoform concentration was highly represented in ovarian carcinoma compared to HLA-G5 isoforms. Additionally, high sHLA-G molecules have been found in aged patients, as well as in patients with advanced stages, and those with metastatic lymph nodes and those with distant metastasis. Elsewhere, sHLA-G monomers were highly represented in ovarian carcinoma patients compared to controls. sHLA-G plasmatic protein was highly represented in ovarian carcinoma. In effect, HLA-G might be considered as a new checkpoint molecule that could be used to assess progression and recurrence of the disease, thus placing it as a potential biomarker for advanced and complicated ovarian carcinoma.

The Molecular and Functional Characteristics of HLA-G and the Interaction with Its Receptors: Where to Intervene for Cancer Immunotherapy?

Human leukocyte antigen G (HLA-G) mediates maternal-fetal immune tolerance. It is also considered an immune checkpoint in cancer since it may mediate immune evasion and thus promote tumor growth. HLA-G is, therefore, a potential target for immunotherapy. However, existing monoclonal antibodies directed against HLA-G lack sufficient specificity and are not suitable for immune checkpoint inhibition in a clinical setting. For this reason, it is essential that alternative approaches are explored to block the interaction between HLA-G and its receptors. In this review, we discuss the structure and peptide presentation of HLA-G, and its interaction with the receptors Ig-like transcript (ILT) 2, ILT4, and Killer cell immunoglobulin-like receptor 2DL4 (KIR2DL4). Based on our findings, we propose three alternative strategies to block the interaction between HLA-G and its receptors in cancer immunotherapy: (1) prevention of HLA-G dimerization, (2) targeting the peptide-binding groove of HLA-G, and (3) targeting the HLA-G receptors. These strategies should be an important focus of future studies that aim to develop immune checkpoint inhibitors to block the interaction between HLA-G and its receptors for the treatment of cancer.

HLA-G: Function, polymorphisms and pathology

HLA-G immune modulatory genes and molecules are presently being studied by a widespread number of research groups. In the present study, we do not aim to be exhaustive since the number of manuscripts published every year is overwhelming. Instead, our aim is pointing out facts about HLA-G function, polymorphism and pathology that have been confirmed by several different researchers, together with exposing aspects that may have been overlooked or not sufficiently remarked in this productive field of study. On the other hand, we question whether performing mainly studies on HLA-G and disease associations is going to give a clear answer in the future, since 40 years of study of classical HLA molecules association with disease has still given no definite answer on this issue.

Measuring Synthesis and Degradation of MHC Class I Molecules

Major histocompatibility complex (MHC) class I molecules function to present pathogen derived peptides to cytotoxic T cells and act as ligands for Natural Killer cells, thus alerting the immune system to the presence of invading pathogens. However, some MHC class I molecules, most notably HLA-B27, can be strongly associated with autoimmune diseases. In addition, the MHC class I pathway is a target for numerous viral evasion strategies Understanding not only the antigen presenting functions, but also the biosynthesis and the degradation pathways of MHC class I molecules has therefore become important in determining their role in pathogen and autoimmune related diseases. Here, we describe how using epitope tagged MHC class I molecules can aid in the analysis of MHC class I molecule biosynthesis and degradation as well as complementary studies using conventional conformationally specific antibodies. Coupled together with pharmacological manipulation which can target both biosynthetic and degradative pathways, this offers a powerful tool in analyzing MHC class I molecules.

Intercellular transfer of HLA-G: its potential in cancer immunology

Intercellular protein transfer between cancer cells and immune cells is a very common phenomenon that can affect different stages of host antitumor immune responses. HLA-G, a non-classical HLA class I antigen, has been observed to be widely expressed in various malignancies, and its immune-suppressive functions have been well recognised. HLA-G expression in cancer cells can directly mediate immune tolerance by interacting with inhibitory receptors such as ILT2 and ILT4 expressed on immune cells. Moreover, a network of multiple directional intercellular transfers of HLA-G among cancer cells and immune cells through trogocytosis, exosomes and tunnelling nanotubes provides malignant cells with an alternative ploy for antigen sharing and induces more complex heterogeneity, to modulate immune responses, ultimately leading to immune evasion, therapy resistance, disease progression and poor clinical outcome. Herein, we discuss the relative aspects of the intercellular transfer of HLA-G between tumor cells and immune cells and its potential use in tumor immunology research and translational cancer therapy.

HLA-G 14-bp polymorphism affects the age of onset in Type I Diabetes Mellitus

Type I diabetes mellitus (T1DM) is an organ-specific autoimmune disorder affecting the insulin-producing pancreatic cells. T1DM genetic association studies have so far revealed the involvement of more than 40 loci, with particularly strong associations for the human leucocyte antigens (HLA). Further to the well-established HLA class II associations, the immunomodulatory elements in the telomeric major histocompatibility complex locus, specifically nonclassical HLA class I, were also associated with T1DM, either in conferring susceptibility or by contributing to the overall pathogenesis. This study investigates the involvement of a 14-bp deletion polymorphism (rs371194629) at the 3' untranslated region of HLA-G in the context of T1DM and age of onset. The frequency of the polymorphism was determined in unrelated T1DM Cypriot patients and findings that emerge from this study show a strong association between the HLA-G 14-bp polymorphism and T1DM with respect to the age of onset. Specifically, the deletion/deletion (DEL/DEL) genotype was found to be associated with an early age of onset (P = 0.001), while the presence of the insertion allele (INS) was associated to a later age of onset (P = 0.0001), portraying a possible dominant effect over the deletion allele, a role in delaying disease onset and an overall involvement of HLA-G in the pathogenesis of type I diabetes mellitus.

TAPBPR: a new player in the MHC class I presentation pathway

In order to provide specificity for T cell responses against pathogens and tumours, major histocompatibility complex (MHC) class I molecules present high-affinity peptides at the cell surface to T cells. A key player for peptide loading is the MHC class I-dedicated chaperone tapasin. Recently we discovered a second MHC class I-dedicated chaperone, the tapasin-related protein TAPBPR. Here, we review the major steps in the MHC class I pathway and the TAPBPR data. We discuss the potential function of TAPBPR in the MHC class I pathway and the involvement of this previously uncharacterised protein in human health and disease.

A Distinctive Cytoplasmic Tail Contributes to Low Surface Expression and Intracellular Retention of the Patr-AL MHC Class I Molecule

Chimpanzees have orthologs of the six fixed, functional human MHC class I genes. But, in addition, the chimpanzee has a seventh functional gene, Patr-AL, which is not polymorphic but contributes substantially to population diversity by its presence on only 50% of MHC haplotypes. The ancestral AL gene emerged long before the separation of human and chimpanzee ancestors and then subsequently and specifically lost function during human evolution, but was maintained in chimpanzees. Patr-AL is an alloantigen that participates in negative and positive selection of the T cell repertoire. The three-dimensional structure and the peptide-binding repertoire of Patr-AL and HLA-A*02 are surprisingly similar. In contrast, the expression of these two molecules is very different, as shown using specific mAbs and polyclonal Abs made against Patr-AL. Peripheral blood cells and B cell lines express low levels of Patr-AL at the cell surface. Higher levels are seen for 221-cell transfectants expressing Patr-AL, but in these cells a large majority of Patr-AL molecules are retained in the early compartments of the secretory pathway: mainly the endoplasmic reticulum, but also cis-Golgi. Replacing the cytoplasmic tail of Patr-AL with that of HLA-A*02 increased the cell-surface expression of Patr-AL substantially. Four substitutions distinguish the Patr-AL and HLA-A*02 cytoplasmic tails. Systematic mutagenesis showed that each substitution contributes changes in cell-surface expression. The combination of residues present in Patr-AL appears unique, but each individual residue is present in other primate MHC class I molecules, notably MHC-E, the most ancient of the functional human MHC class I molecules.

Non-classical HLA-class I expression in serous ovarian carcinoma: Correlation with the HLA-genotype, tumor infiltrating immune cells and prognosis

In our previous studies, we have shown that patients with serous ovarian carcinoma in advanced surgical stage disease have a particularly poor prognosis if they carry the HLA-A*02 genotype. This represent a stronger prognostic factor than loss or downregulation of the MHC class I heavy chain (HC) on tumor cells. In this study, we investigated the expression of the non-classical, immune tolerogenic HLA -G and -E on the tumor cells along with the infiltration of immune cells in the tumor microenvironment. FFPE primary tumors from 72 patients with advanced stages of serous adenocarcinoma and metastatic cells present in ascites fluid from 8 additional patients were included in this study. Both expression of HLA-G and aberrant expression of HLA-E were correlated to a significant worse prognosis in patients with HLA-A*02, but not with different HLA genotypes. Focal cell expression of HLA-G correlated to a site-specific downregulation of classical MHC class I HC products and aberrant HLA-E expression, showing a poor survival. HLA-G was more frequently expressed in metastatic cells than in primary tumor lesions and the expression of HLA-G inversely correlated with the frequency of tumor infiltrating immune cells. All these parameters can contribute together to identify and discriminate subpopulations of patients with extremely poor prognosis and can give them the opportunity to receive, and benefit of individually tailored treatments.

Biology of the immunomodulatory molecule HLA-G in human liver diseases

The non-classical human leukocyte antigen-G (HLA-G), plays an important role in inducing tolerance, through its immunosuppressive effects on all types of immune cells. Immune tolerance is a key issue in the liver, both in liver homeostasis and in the response to liver injury or cancer. It would therefore appear likely that HLA-G plays an important role in liver diseases. Indeed, this molecule was recently shown to be produced by mast cells in the livers of patients infected with hepatitis C virus (HCV). Furthermore, the number of HLA-G-positive mast cells was significantly associated with fibrosis progression. The generation of immune tolerance is a role common to both HLA-G, as a molecule, and the liver, as an organ. This review provides a summary of the evidence implicating HLA-G in liver diseases. In the normal liver, HLA-G transcripts can be detected, but there is no HLA-G protein. However, HLA-G protein is detectable in the liver tissues and/or plasma of patients suffering from hepatocellular carcinoma, hepatitis B or C, or visceral leishmaniasis and in liver transplant recipients. The cells responsible for producing HLA-G differ between diseases. HLA-G expression is probably induced by microenvironmental factors, such as cytokines. The expression of HLA-G receptors, such as ILT2, ILT4, and KIRD2L4, on liver cells has yet to be investigated, but these receptors have been detected on all types of immune cells, and such cells are present in liver. The tolerogenic properties of HLA-G explain its deleterious effects in cancers and its beneficial effects in transplantation. Given the key role of HLA-G in immune tolerance, new therapeutic agents targeting HLA-G could be tested for the treatment of these diseases in the future.

Heterologous expression of mutated HLA-G1 reduces alloreactivity of human dermal fibroblasts

Aim: To engineer a stable HLA-G molecule and evaluate its immunomodulatory properties in transgenic human dermal fibroblasts (HDFs). Materials & methods: A mutated HLA-G1 (mHLA-G1) molecule was generated by modifying the endoplasmic reticulum retrieval motif and 3′-untranslated region miRNA-binding sites of HLA-G1. Immunomodulatory properties of transgenic HDF-mHLA-G1 were evaluated in vitro. Results: Stable mHLA-G1 expressing HDF cells were successfully generated and flow cytometry analysis revealed that mHLA-G1 efficiently localized to the cell surface. Natural killer cell-mediated cytolysis of HDF-mHLA-G1/green fluorescent protein (GFP) was reduced by 73% compared with HDF-GDP. HDF-mHLA-G1/GFP decreased phytohemagglutinin-activated peripheral blood mononuclear cell proliferation by 30% versus HDF-GFP. Conclusion: We are the first to successfully create a human fibroblast source with reduced alloreactivity using a novel mHLA-G1 construct. This approach may be extended to other cell types including human embryonic stem cells for use in allogeneic transplantation for cell-based regenerative medicine applications.

Heterelogous expression of mutated HLA-G decreases immunogenicity of human embryonic stem cells and their epidermal derivatives

Human embryonic stem cells (hESCs) are capable of extensive self-renewal and expansion and can differentiate into any somatic tissue, making them useful for regenerative medicine applications. Allogeneic transplantation of hESC-derived tissues from results in immunological rejection absent adjunctive immunosuppression. The goal of our study was to generate a universal pluripotent stem cell source by nucleofecting a mutated human leukocyte antigen G (mHLA-G) gene into hESCs using the PiggyBac transposon. We successfully generated stable mHLA-G(EF1α)-hESC lines using chEF1α promoter system that stably expressed mHLA-G protein during prolonged undifferentiated proliferation andin differentiated embryoid bodies as well as teratomas. Morphology, karyotype, and telomerase activity of mHLA-G expressing hESC were normal. Immunofluorescence staining and flow cytometry analysis revealed persistent expression of pluripotent markers, OCT-3/4 and SSEA-4, in undifferentiated mHLA-G(EF1α)-hESC. Nucleofected hESC formed teratomas and when directed to differentiate into epidermal precursors, expressed high levels of mHLA-G and keratinocyte markers K14 and CD29. Natural killer cell cytotoxicity assays demonstrated a significant decrease in lysis of mHLA-G(EF1a)-hESC targets relative to control cells. Similar results were obtained with mHLA-G(EF1α)-hESC-derived epidermal progenitors (hEEP). One way mixed T lymphocyte reactions unveiled that mHLA-G(EF1a)-hESC and -hEEP restrained the proliferative activity of mixed T lymphocytes. We conclude that heterologous expression of mHLA-G decreases immunogenicity of hESCs and their epidermal differentiated derivatives.

Controlling the Immunological Crosstalk during Conception and Pregnancy: HLA-G in Reproduction

In several years after its discovery in the placenta, the human leukocyte antigen (HLA) class Ib protein, HLA-G, was not given much attention, nor was it assigned great importance. As time has unraveled, HLA-G has proven to have distinctive functions and an unforeseen and possibly important role in reproduction. HLA-G is characterized mainly by its low polymorphism and restricted tissue distribution in non-pathological conditions. In fact, its expression pattern is primarily limited to extravillous cytotrophoblast cells at the maternal-fetal interface during pregnancy. Due to low polymorphism, almost the same protein is expressed by virtually all individuals. It is these unique features that make HLA-G differ from its highly polymorphic HLA class Ia counterparts, the HLA-A, -B, and -C molecules. Its function, seemingly diverse, is typically receptor-mediated, and involves interactions with a wide range of immune cells. As the expression of HLA-G primarily is limited to gestation, this has given rise to the hypothesis that HLA-G plays an important role in the immunological tolerance of the fetus by the mother. In keeping with this, it might not be surprising that polymorphisms in the HLA-G gene, and levels of HLA-G expression, have been linked to reproductive failure and pre-eclampsia. Based on recent studies, we speculate that HLA-G might be involved in mechanisms in reproductive immunology even before conception because HLA-G can be detected in the genital tract and in the blood of non-pregnant women, and is present in seminal fluid from men. In addition, HLA-G expression has been found in the pre-implanted embryo. Therefore, we propose that a combined contribution from the mother, the father, and the embryo/fetus is likely to be important. Furthermore, this review presents important aspects of HLA-G in relation to reproduction: from genetics to physiological effects, from pregnancy and pregnancy complications to a short discussion on future possible means of preventative measures and therapy.

The dual role of HLA-G in cancer

We here review the current data on the role of HLA-G in cancer based on recent findings of an unexpected antitumor activity of HLA-G in hematological malignancies. For the past decade, HLA-G has been described as a tumor-escape mechanism favoring cancer progression, and blocking strategies have been proposed to counteract it. Aside from these numerous studies on solid tumors, recent data showed that HLA-G inhibits the proliferation of malignant B cells due to the interaction between HLA-G and its receptor ILT2, which mediates negative signaling on B cell proliferation. These results led to the conjecture that, according to the malignant cell type, HLA-G should be blocked or conversely induced to counteract tumor progression. In this context, we will here present (i) the dual role of HLA-G in solid and liquid tumors with special emphasis on (ii) the HLA-G active structures and their related ILT2 and ILT4 receptors and (iii) the current knowledge on regulatory mechanisms of HLA-G expression in tumors.

HLA-G Expression Pattern: Reliable Assessment for Pregnancy Outcome Prediction

Because mothers and fathers are more or less dissimilar at multiple HLA loci, mother considers her fetus as a semi-allograft. Mother's immune system may recognize paternal HLA as foreign antigen and may develop anti-paternal HLA antibodies and cytotoxic T lymphocyte. There are some mechanisms that modulate maternal immune responses during pregnancy, in order to make uterus an immune privileged site. This immunosuppression is believed to be mediated, at least partly, by HLA-G, non-classical class I human leukocyte antigen (HLA) molecule that is strongly expressed in cytotrophoblast and placenta. The major HLA-G function is its ability to inhibit T and B lymphocytes, NK cells and antigen-presenting cells (APC).Since HLA-G is expressed strongly at the maternofetal interface and has an essential role in immunosuppression, HLA-G polymorphism and altered expression of HLA-G seems to be associated with some complications of pregnancy, such as pre-eclampsia, recurrent misscariage and failure in IVF.This perspective discusses recent findings about HLA-G genetics, function, expression and polymorphism; and focus on HLA-G role in the etiology of recurrent miscarriage.

Measuring synthesis and degradation of MHC class I molecules

Major histocompatibility complex (MHC) class I molecules function to present pathogen-derived peptides to cytotoxic T cells or act as ligands for Natural Killer cells, thus alerting the immune system to the presence of invading pathogens. Furthermore MHC class I molecules can be strongly associated with autoimmune diseases. Therefore understanding not only the biosynthesis and the degradation pathways of MHC class I molecules has become important in determining their role in pathogen and autoimmune-related diseases. Here we describe how using epitope-tagged MHC class I molecules can aid in the analysis of MHC class I molecule biosynthesis and degradation and also complement studies using conventional conformationally specific antibodies. Coupled together with pharmacological manipulation which can target both biosynthetic and degradative pathways, this offers a powerful tool in analyzing MHC class I molecules.

The multi-faceted nature of HLA class I dimer molecules

The canonical role of major histocompatibility complex class I (MHCI) molecules in antigen presentation involves the recognition of a short peptide of intracellular origin, bound to the upper surface of the class I molecule, by CD8(+) T lymphocytes. Assembly and loading of the MHCI is a highly regulated, chaperone-mediated process and only when the fully folded MHCI molecule is correctly loaded with peptide is it released from the endoplasmic reticulum for trafficking to the cell surface. Current models of the interactions of MHCI molecules with their cognate receptors visualize them functioning as monomeric entities. However, in recent years, new data have revealed MHCI molecules with the ability to form disulphide-linked dimeric structures, with several distinct dimer entities being elucidated. We describe here three types of MHCI dimers; HLA-B27 dimers formed predominantly through the possession of an unpaired cysteine within the peptide-binding groove; HLA-G dimers, which form through a cysteine on its external surface; and a novel population we term redox-induced dimers, which can form between cysteine residues in the cytoplasmic tail domains. The characteristics of these dimeric MHCI molecules and their role in both normal immune responses and in disease pathogenesis are reviewed in this article.

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.

Human leukocyte antigen-G in cancer: are they clinically relevant?

The immunotolerant human leukocyte antigen (HLA)-G has direct inhibitory effects on natural killer cells (NK), dendritic cells (DC), T cells and can induce tolerant regulatory cells. The expression of HLA-G has been correlated with the tolerance of the fetus, the acceptance of organ transplants, and the immune escape of tumor cells and virus-infected cells. In the context of malignancies, aberrant induction of HLA-G expression has been extensively investigated and its clinical relevance has been postulated. We here focus on the HLA-G expression in malignancies and its relevance to tumor cell immune escaping and disease progression.

Human leukocyte antigen-G expression is associated with a poor prognosis in patients with esophageal squamous cell carcinoma

Human leukocyte antigen (HLA)-G inhibits functions of immune component cells and promotes malignant cells evading from antitumor immunity. We investigated the clinical relevance of HLA-G expression in esophageal squamous cell carcinoma (ESCC). In our study, HLA-G expression in 79 primary ESCC lesions and corresponding adjacent normal tissues were analyzed with immunohistochemistry. Soluble HLA-G (sHLA-G) in plasma was detected with enzyme-linked immunosorbent assay (ELISA) in 41 ESCC patients (including 19 case-matched lesions and plasma samples) and in 153 normal healthy controls. HLA-G expression was observed in 65.8% (52/79) of the ESCC lesions but not in adjacent normal esophageal tissues. HLA-G expression was more frequently observed in patients with advanced disease stage (III/IV vs. I/II, p = 0.01). Patients with HLA-G expression had a significantly worse survival, and HLA-G could be an independent prognostic factor. sHLA-G levels in plasma were significantly increased in patients compared to normal controls (median: 152.4 U/ml vs. 8.9 U/ml, p < 0.001). The area under receiver-operating characteristic (ROC) curve for sHLA-G in plasma was 0.992. However, no significant correlation was found between sHLA-G in plasma and clinical parameters studied. In conclusion, our findings indicated that HLA-G expression in ESCC is associated with poor survival and could be a prognostic indicator. Furthermore, increased levels of sHLA-G in plasma might be a useful preoperative biomarker for diagnosis.

Implications of the polymorphism of HLA-G on its function, regulation, evolution and disease association

The HLA-G gene displays several peculiarities that are distinct from those of classical HLA class I genes. The unique structure of the HLA-G molecule permits a restricted peptide presentation and allows the modulation of the cells of the immune system. Although polymorphic sites may potentially influence all biological functions of HLA-G, those present at the promoter and 3′ untranslated regions have been particularly studied in experimental and pathological conditions. The relatively low polymorphism observed in the MHC-G coding region both in humans and apes may represent a strong selective pressure for invariance, whereas, in regulatory regions several lines of evidence support the role of balancing selection. Since HLA-G has immunomodulatory properties, the understanding of gene regulation and the role of polymorphic sites on gene function may permit an individualized approach for the future use of HLA-G for therapeutic purposes.

Aberrant human leucocyte antigen-G expression and its clinical relevance in hepatocellular carcinoma

The clinical relevance of human leucocyte antigen-G (HLA-G) has been postulated in malignancies. Hepatocellular carcinoma (HCC) is a major contributor to cancer incidence and mortality worldwide; however, potential roles of HLA-G in HCC remain unknown. In the current study, HLA-G expression in 219 primary HCC lesions and their adjacent non-tumourous samples was analysed with immunohistochemistry. Correlations among HLA-G expression and various clinical parameters were evaluated. Meanwhile, functional analysis of transfected cell surface HLA-G expression on NK cell cytolysis was performed in vitro. HLA-G expression was observed in 50.2% (110/219) of primary HCC lesions, and undetectable in corresponding adjacent normal liver tissues. HLA-G expression was found in 37.8%, 41.9% and 71.4% of stage I, II and III HCC lesions, respectively. Data revealed that HLA-G expression in HCC was strongly correlated to advanced disease stage (I versus II, P= 0.882; I versus III, P= 0.020; II versus III, P= 0.037). HLA-G expression was also more frequently observed in elder patients (≥median 52 years, 57.5%versus 43.4%, P= 0.004). Meanwhile, plasma soluble HLA-G in HCC patients was significantly higher than that in normal controls (median, 92.49U/ml versus 9.29U/ml, P= 0.000). Functional assay showed that HLA-G expression in transfected cells could dramatically decrease the NK cell cytolysis (P= 0.036), which could be markedly restored by the blockade of HLA-G (P= 0.004) and its receptor ILT2 (P= 0.019). Our finding indicated that HLA-G expression was strongly correlated to advanced disease stage, and more frequently observed in elder patients. Its relevance to HCC progression might be result from the inhibition of NK cell cytolysis.

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.

Inhibitory NK receptor recognition of HLA-G: regulation by contact residues and by cell specific expression at the fetal-maternal interface

The non-classical HLA-G protein is distinguished from the classical MHC class I molecules by its expression pattern, low polymorphism and its ability to form complexes on the cell surface. The special role of HLA-G in the maternal-fetal interface has been attributed to its ability to interact with specific receptors found on maternal immune cells. However this interaction is restricted to a limited number of receptors. In this study we elucidate the reason for this phenomenon by comparing the specific contact residues responsible for MHC-KIR interactions. This alignment revealed a marked difference between the HLA-G molecule and other MHC class I molecules. By mutating these residues to the equivalent classical MHC residues, the HLA-G molecule regained an ability of interacting with KIR inhibitory receptors found on NK cells derived either from peripheral blood or from the decidua. Functional NK killing assays further substantiated the binding results. Furthermore, double immunofluorescent staining of placental sections revealed that while the conformed form of HLA-G was expressed in all extravillous trophoblasts, the free heavy chain form of HLA-G was expressed in more distal cells of the column, the invasion front. Overall we suggest that HLA-G protein evolved to interact with only some of the NK inhibitory receptors thus allowing a control of inhibition, while permitting appropriate NK cell cytokine and growth factor production necessary for a viable maternal fetal interface.

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.

Isoforms of human leukocyte antigen-G and their inhibitory receptors in human kidney allograft acceptance

Novel therapeutic strategies such as the modulation of dendritic cell and T-cell function have exhibited great potential in clinical transplantation. Human leukocyte antigen (HLA)-G is a molecule that plays a significant role in establishing complex mechanisms to protect semiallogeneic fetuses from rejection by the maternal immune system. The unique characteristics of both cell-surface and soluble isoforms of HLA-G, the formation of disulfide-bonded dimers with the potential to augment inhibitory receptor signaling, and the function of HLA-G as a preferential ligand for the immunoglobulin-like transcript receptors make HLA-G very important in fundamental approaches for the modulation of immune responses to improve allogeneic graft survival in clinical transplantation. Experimental data from several groups as well as our data from experiments involving HLA-G-mediated human tolerogenic dendritic cells in vitro and receptor transgenic mice in vivo indicate that different isoforms of HLA-G have various immunomodulatory effects through the inhibitory receptors. This knowledge is crucial in understanding mechanisms of prolongation of allograft survival. The analyses of HLA-G isoforms and inhibitory receptors in patients with kidney allograft and the relationship among different isoforms of HLA-G, inhibitory receptors, their mediated immunoregulation, and graft acceptance or failure will be discussed here.

Non-classical transcriptional regulation of HLA-G: an update

Human leucocyte antigen-G (HLA-G) plays a key role in maternal–foetal tolerance and allotransplantation acceptance and is also implicated in tumour escape from the immune system. The modulation of HLA-G expression can prove to be very important to therapeutic goals in some pregnancy complications, transplantation, cancer and possibly autoimmune diseases. In spite of substantial similarities with classical HLA-class I genes, HLA-G is characterized by a restricted tissue-specific expression in non-pathological situations. HLA-G expression is mainly controlled at the transcriptional level by a unique gene promoter when compared with classical HLA-class I genes, and at the post-transcriptional level including alternative splicing, mRNA stability, translation and protein transport to the cell surface. We focus on the characteristics of the HLA-G gene promoter and the factors which are involved in HLA-G transcriptional modulation. They take part in epigenetic mechanisms that control key functions of the HLA-G gene in the regulation of immune tolerance.

Evolution of the opossum major histocompatibility complex: evidence for diverse alternative splice patterns and low polymorphism among class I genes

The opossum major histocompatibility complex (MHC) shares a similar organization with that of non-mammals while containing a diverse set of class I genes more like that of eutherian (placental) mammals. There are 11 class I loci in the opossum MHC region, seven of which are known to be transcribed. The previously described Monodelphis domestica (Modo)-UA1 and Modo-UG display characteristics consistent with their being classical and non-classical class I genes, respectively. Here we describe the characteristics of the remaining five transcribed class I loci (Modo-UE, -UK, -UI, -UJ and -UM). All five genes have peptide-binding grooves with low or no polymorphism, contain unpaired cysteines with the potential to produce homodimer formation and display genomic organizational features that would be unusual for classical class I loci. In addition, Modo-UJ and -UM were expressed in alternatively spliced mRNA forms, including a potentially soluble isoform of Modo-UJ. Thus, the MHC region of the opossum contains a single class I gene that is clearly classical and six other class I genes each with its own unique characteristics that probably perform roles other than or in addition to antigen presentation.

Peptide-receptive Major Histocompatibility Complex Class I Molecules Cycle between Endoplasmic Reticulum and cis-Golgi in Wild-type Lymphocytes

Prior to binding to a high affinity peptide and transporting it to the cell surface, major histocompatibility complex class I molecules are retained inside the cell by retention in the endoplasmic reticulum (ER), recycling through the ER-Golgi intermediate compartment and possibly the cis-Golgi, or both. Using fluorescence microscopy and a novel in vitro COPII (ER-to-ER-Golgi intermediate compartment) vesicle formation assay, we find that in both lymphocytes and fibroblasts that lack the functional transporter associated with antigen presentation, class I molecules exit the ER and reach the cis-Golgi. Intriguingly, in wild-type T1 lymphoma cells, peptide-occupied and peptide-receptive class I molecules are simultaneously exported from ER membranes with similar efficiencies. Our results suggest that binding of high affinity peptide and exit from the ER are not coupled, that the major histocompatibility complex class I quality control compartment extends into the Golgi apparatus under standard conditions, and that peptide loading onto class I molecules may occur in post-ER compartments.

A homodimeric complex of HLA-G on normal trophoblast cells modulates antigen-presenting cells via LILRB1

In healthy individuals, the non-classical MHC molecule HLA-G is only expressed on fetal trophoblast cells that invade the decidua during placentation. We show that a significant proportion of HLA-G at the surface of normal human trophoblast cells is present as a disulphide-linked homodimer of the conventional beta(2)m-associated HLA-I complex. HLA-G is a ligand for leukocyte immunoglobulin-like receptors (LILR), which bind much more efficiently to dimeric HLA-G than to conventional HLA-I molecules. We find that a LILRB1-Fc fusion protein preferentially binds the dimeric form of HLA-G on trophoblast cells. We detect LILRB1 expression on decidual myelomonocytic cells; therefore, trophoblast HLA-G may modulate the function of these cells. Co-culture with HLA-G(+) cells does not inhibit monocyte-derived dendritic cell up-regulation of HLA-DR and costimulatory molecules on maturation, but did increase production of IL-6 and IL-10. Furthermore, proliferation of allogeneic lymphocytes was inhibited by HLA-G binding to LILRB1/2 on responding antigen-presenting cells (APC). As HLA-G is the only HLA-I molecule that forms beta(2)m-associated dimers with increased avidity for LILRB1, this interaction could represent a placental-specific signal to decidual APC. We suggest that the placenta is modulating maternal immune responses locally in the uterus through HLA-G, a trophoblast-specific, monomorphic signal present in almost every pregnancy. See accompanying commentary: (http://dx.doi.org/10.1002/eji.200737515).

Trogocytosis-based generation of suppressive NK cells

Trogocytosis is a fast uptake of membranes and associated molecules from one cell by another. Trogocytosis between natural killer (NK) cells and tumors is already described, but the functional relevance of NK-tumor targets material exchange is unclear. We investigated whether the immunosuppressive molecule HLA-G that is commonly expressed by tumors in vivo and known to block NK cytolytic function, could be transferred from tumor cells to NK cells, and if this transfer had functional consequences. We show that activated NK cells acquire HLA-G1 from tumor cells, and that upon this acquisition, NK cells stop proliferating, are no longer cytotoxic, and behave as suppressor cells. Such cells can inhibit other NK cells' cytotoxic function and protect NK-sensitive tumor cells from cytolysis. These data are the first demonstration that trogocytosis of HLA-G1 can be a major mechanism of immune escape that acts through effector cells made to act as suppressor cells locally, temporarily, but efficiently. The broader consequences of membrane sharing between immune and non-immune cells on the function of effectors and the outcome of immune responses are discussed.

Bap31 enhances the endoplasmic reticulum export and quality control of human class I MHC molecules

The assembly of class I MHC molecules and their export from the endoplasmic reticulum (ER) is governed by chaperones and accessory proteins. We present evidence that the putative cargo receptor protein Bap31 participates in the transport and the quality control of human class I molecules. Transfection of the human adenocarcinoma cell line HeLa with yellow fluorescent protein-Bap31 chimeras increased surface levels of class I in a dose-dependent manner, by as much as 3.7-fold. The increase in surface class I resulted from an increase in the rate of export of newly synthesized class I molecules to the cell surface and from an increase in the stability of the exported molecules. We propose that Bap31 performs quality control on class I molecules in two distinct phases: first, by exporting peptide-loaded class I molecules to the ER/Golgi intermediate compartment, and second, by retrieving class I molecules that have lost peptides in the acidic post-ER environment. This function of Bap31 is conditional or redundant, because we find that Bap31 deficiency does not reduce surface class I levels. Overexpression of the Bap31 homolog, Bap29, decreases surface class levels in HeLa, indicating that it does not substitute for Bap31.

Immune regulation by pretenders: cell-to-cell transfers of HLA-G make effector T cells act as regulatory cells

Trogocytosis is the uptake of membrane fragments from one cell by another and has been described for immune cells in mice and humans. Functional consequences of trogocytosis are emerging, but a dramatic immune function has still to be associated with it. Here we show that some resting, and most activated, CD4+ and CD8+ T cells acquire immunosuppressive HLA-G1 from antigen-presenting cells (APCs) in a few minutes. Acquisition of HLA-G through membrane transfers does not change the real nature of the T cells but immediately reverses their function from effectors to regulatory cells. These regulatory cells can inhibit allo-proliferative responses through HLA-G1 that they acquired. These data demonstrate that trogocytosis of HLA-G1 leads to instant generation of a new type of regulatory cells, which act through cell-surface molecules they temporarily display but do not express themselves. Such regulatory cells whose existence is most likely limited in space and time might constitute an "emergency" immune suppression mechanism used by HLA-G-expressing tissues to protect themselves against immune aggression. In addition, T cells acquire from HLA-G-expressing APCs their HLA-G-dependent capability to induce the slower differentiation of regulatory cells that act independently of HLA-G. These data re-emphasize the significance of HLA-G expression in normal and pathologic situations.

Differential expression of human leukocyte antigen-G (HLA-G) messenger RNAs and proteins in normal human prostate and prostatic adenocarcinoma

Human leukocyte antigen-G (HLA-G) is a major histocompatibility complex class Ib gene expressed in normal organs and in some tumors. The glycoproteins encoded by this gene are best known for their immunosuppressive properties. Because isoform-specific expression of HLA-G in male reproductive organs has not been reported, we investigated HLA-G1, -G2, -G5, -G6 mRNAs and proteins in four-to-five samples of normal prostate glands, prostates with benign prostatic hyperplasia and prostate adenocarcinomas using RT-PCR and immunohistochemistry. All tissues contained HLA-G1, -G2, -G5 and -G6 specific mRNAs, but only HLA-G5 protein was detectable. In normal prostate glands, HLA-G5 protein was prominent in the cytoplasm of tubuloglandular epithelia and in glandular secretions. Staining was reduced in samples of benign prostatic hyperplasia but remained localized to the cytoplasm of glandular epithelia and secretions. In prostatic adenocarcinomas, HLA-G5 protein was detectable mainly in the secretions. Thus, HLA-G5 but not HLA-G1, -G2 or -G6 is produced in the normal prostate and is present in prostatic secretions. In addition, normal cellular localization is disturbed in benign and malignant prostatic adenocarcinomas. The results are consistent with this molecule may influencing female immune receptivity to sperm and suggest that such immunosuppression could be disturbed in men with prostatic adenocarcinomas.

Selective export of HLA-F by its cytoplasmic tail

MHC class I molecules exit the endoplasmic reticulum (ER) by an unknown mechanism. Although a selective export mechanism has been proposed for the anterograde transport of class I, a motif responsible for export has never been identified. Although classical class I molecules lacking their cytoplasmic tail are expressed on the cell surface, we found that HLA-F was entirely dependent on its cytoplasmic tail for export from the ER. Two known export motifs were recognizable in HLA-F. A C-terminal valine residue functioned in ER export and interacted with coat complex (COP)II, while an RxR motif also played an important role in anterograde transport and bound to 14-3-3 proteins. This divergent trafficking of HLA-F implicates an alternative function for HLA-F, independent of loading with peptides in the ER.

The class I HLA repertoire of pancreatic islets comprises the nonclassical class Ib antigen HLA-G

Selective expression of the human class Ib HLA molecule HLA-G in immunologically protected sites and its function in the inhibition of NK and T-cell effector functions support an important role of this molecule in immunoregulation. Here, we demonstrate that HLA-G is constitutively expressed in the endocrine compartment of the human pancreas. Surface expression of this HLA determinant in endocrine cells is regulated in response to growth and inflammatory stimuli. Furthermore, we provide evidence that HLA-G expressed in this tissue may associate with a subset of insulin-containing granules and may be shuttled to the cell surface in response to secretory stimuli. Thus, HLA-G presentation by endocrine cells may be regulated in concert with their secretory activity. These results identify the expression of a major histocompatibility complex locus with putative regulatory functions in human pancreatic islets, a finding with potentially important implications for the progression of autoimmunity as well as for the establishment of transplant tolerance to this tissue.

Lack of tyrosine 320 impairs spontaneous endocytosis and enhances release of HLA-B27 molecules

Several lines of evidence suggest that endocytosis of MHC class I molecules requires conserved motifs within the cytoplasmic domain. In this study, we show, in the C58 rat thymoma cell line transfected with HLA-B27 molecules, that replacement of the highly conserved tyrosine (Tyr320) in the cytoplasmic domain of HLA-B27 does not hamper cell surface expression of beta2-microglobulin H chain heterodimers or formation of misfolded molecules. However, Tyr320 replacement markedly impairs spontaneous endocytosis of HLA-B27. Although wild-type molecules are mostly internalized via endosomal compartments, Tyr320-mutated molecules remain at the plasma membrane in which partial colocalization with endogenous transferrin receptors can be observed, also impairing their endocytosis. Finally, we show that Tyr320 substitution enhances release of cleaved forms of HLA-B27 from the cell surface. These studies show for the first time that Tyr320 is most likely part of a cytoplasmic sorting motif involved in spontaneous endocytosis and shedding of MHC class I molecules.

Modo-UG, a marsupial nonclassical MHC class I locus

Modo-UG is a class I gene located in the MHC of the marsupial Monodelphis domestica, the gray, short-tailed opossum. Modo-UG is expressed as three alternatively spliced mRNA forms, all of which encode a transmembrane form with a short cytoplasmic tail that lacks phosphorylation sites typically found in classical class I molecules. The three alternative mRNAs would encode a full-length form, an isoform lacking the alpha2 domain, and one lacking both alpha2 and alpha3 domains. Genotyping both captive-bred and wild M. domestica from different geographic regions revealed no variation in the residues that make up Modo-UG's peptide-binding groove. Modo-UG's low polymorphism is contrasting to that of a nearby class I locus, Modo-UA1, which has a highly polymorphic peptide-binding region. Absence of functional polymorphism in Modo-UG is therefore not a general feature of opossum class I genes but the result of negative selection. Modo-UG is the first MHC linked marsupial class I to be described that appears to clearly have nonclassical features.