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

HLA-G - evolvement from a trophoblast specific marker to a checkpoint molecule in cancer, a narrative review about the specific role in breast- and gynecological cancer

Human leukocyte antigen G (HLA-G) is known as a non-classical molecule of the major histocompatibility complex class Ib and downregulates the mother's immune response against the fetus during pregnancy, thereby generating immune tolerance. Due to the latter effect, HLA-G is also referred to as an immune checkpoint molecule. Originally identified on extravillous trophoblasts, HLA-G is already known to induce immune tolerance at various stages of the immune response, for example through cell differentiation and proliferation, cytolysis and cytokine secretion. Because of these functions, HLA-G is involved in various processes of cancer progression, but a comprehensive review of the role of HLA-G in gynecologic cancers is lacking. Therefore, this review focuses on the existing knowledge of HLA-G in ovarian cancer, endometrial cancer, cervical cancer and breast cancer. HLA-G is predominantly expressed in cancer tissues adjacent to the extravillous trophoblast. Therefore, modulating its expression in the cancer target tissues of cancer patients could be a potential therapeutic approach to treat these diseases.

The Immune Modulation HLA-G*01:01:01 Full Allele Is Associated with Gastric Adenocarcinoma Development

The Human Leukocyte Antigen (HLA) system contains a set of genes involved at many levels in the innate and adaptive immune response. Among the non-classical HLA class I genes, HLA-G stands out for the numerous studies about its pivotal role in regulating/modulating immune responses. Also, its involvement in extravillous cytotrophoblast function, viral infections, autoimmunity, and cancer has been extensively documented. The present study explores for the first time the relationship between natural alleles of HLA-G, rather than STSs, SNPs, or partial gene polymorphisms, and the development of gastric adenocarcinoma, by analyzing the genetic profile of a cohort of 40 Spanish patients with this type of tumor using DNA extracted from paired biopsies of tumoral and adjacent non-tumoral gastric tissue. Our results reveal a significant statistical relationship between the presence of the HLA-G*01:01:01 allele and the development of gastric cancer, while other common alleles such as -G*01:04 or -G*01:05N did not demonstrate a significant correlation. Studying the involvement of HLA genes in the development of many diseases is relevant to understanding their pathophysiology. However, the absence of specific mechanisms underlying these associations suggests that investigating complete HLA natural alleles' extended haplotypes or complotypes may offer a more precise and valuable approach to elucidating the association of HLA with the pathogenesis of disease.

Preclinical and clinical studies of CAR-NK-cell therapies for malignancies

The development of chimeric antigen receptor T (CAR-T) cell therapy, a specific type of immunotherapy, in recent decades was a fantastic breakthrough for the treatment of hematological malignancies. However, difficulties in collecting normal T cells from patients and the time cost of manufacturing CAR-T cells have limited the application of CAR-T-cell therapy. In addition, the termination of related clinical trials on universal CAR-T cell therapy has made further research more difficult. Natural killer (NK) cells have drawn great attention in recent years. Chimeric antigen receptor-NK (CAR-NK) cell therapy is a promising strategy in the treatment of malignant tumors because of its lack of potential for causing graft-versus-host disease (GVHD). In this review, we will address the advances in and achievements of CAR-NK cell therapy.

Evolution and molecular interactions of major histocompatibility complex (MHC)-G, -E and -F genes

Classical HLA (Human Leukocyte Antigen) is the Major Histocompatibility Complex (MHC) in man. HLA genes and disease association has been studied at least since 1967 and no firm pathogenic mechanisms have been established yet. HLA-G immune modulation gene (and also -E and -F) are starting the same arduous way: statistics and allele association are the trending subjects with the same few results obtained by HLA classical genes, i.e., no pathogenesis may be discovered after many years of a great amount of researchers’ effort. Thus, we believe that it is necessary to follow different research methodologies: (1) to approach this problem, based on how evolution has worked maintaining together a cluster of immune-related genes (the MHC) in a relatively short chromosome area since amniotes to human at least, i.e., immune regulatory genes (MHC-G, -E and -F), adaptive immune classical class I and II genes, non-adaptive immune genes like (C2, C4 and Bf) (2); in addition to using new in vitro models which explain pathogenetics of HLA and disease associations. In fact, this evolution may be quite reliably studied during about 40 million years by analyzing the evolution of MHC-G, -E, -F, and their receptors (KIR—killer-cell immunoglobulin-like receptor, NKG2—natural killer group 2-, or TCR-T-cell receptor—among others) in the primate evolutionary lineage, where orthology of these molecules is apparently established, although cladistic studies show that MHC-G and MHC-B genes are the ancestral class I genes, and that New World apes MHC-G is paralogous and not orthologous to all other apes and man MHC-G genes. In the present review, we outline past and possible future research topics: co-evolution of adaptive MHC classical (class I and II), non-adaptive (i.e., complement) and modulation (i.e., non-classical class I) immune genes may imply that the study of full or part of MHC haplotypes involving several loci/alleles instead of single alleles is important for uncovering HLA and disease pathogenesis. It would mainly apply to starting research on HLA-G extended haplotypes and disease association and not only using single HLA-G genetic markers.

Association between exonic polymorphisms of human leukocyte antigen-G gene and non-segmental vitiligo in the Korean population

BACKGROUND

Vitiligo is a pigmentary skin disorder characterised by a chronic and progressive loss of melanocytes. Although several theories have been suggested to the pathogenesis of vitiligo, an autoimmune process leading to melanocyte destruction appears most likely. Human leukocyte antigen-G is a non-classic, major histocompatibility complex Class I molecule that plays an important role in the suppression of the immune response. Several recent studies have provided evidences that polymorphisms in the human leukocyte antigen-G gene might be related with autoimmune diseases.

OBJECTIVES

The aim of this study was to decide whether exonic single nucleotide polymorphisms in human leukocyte antigen-G contribute to the risk of developing non-segmental vitiligo in the Korean population.

METHODS

To evaluate the associations between exonic single nucleotide polymorphisms (rs1630223 [Ala5Ala] and rs12722477 [Leu134Ile]) of human leukocyte antigen-G and vitiligo, 244 patients with vitiligo and 398 healthy controls were recruited. Genotyping was performed using Fluidigm 192.24 Dynamic Array with EP1 (Fluidigm Corp., CA). The SNP type assay (Fluidigm Corp., CA), which employs allele-specifically designed fluorescences (FAM or VIC) primers and a common reverse primer was applied and the data were analysed using the EP1 single nucleotide polymorphisms genotyping analysis software to obtain genotype calls.

RESULTS

Two exonic single nucleotide polymorphisms (rs1630223 and rs12722477) exhibited significant associations with susceptibility and remained a statistically significant association following Bonferroni correction. These two single nucleotide polymorphisms were located within a block of linkage disequilibrium. Haplotypes G-C and A-A comprising rs1630223 and rs12722477 demonstrated a significant association with non-segmental vitiligo.

LIMITATIONS

The protein expression level of patients with vitiligo and controls was not studied and a replication study of the genetic association in an independent group was not managed.

CONCLUSION

Our results suggest that exonic human leukocyte antigen-G polymorphisms (rs1630223 and rs12722477) are associated with the development of non-segmental vitiligo.

The Icarus Flight of Perinatal Stem and Renal Progenitor Cells Within Immune System

Our immune system actively fights bacteria and viruses, and it must strike a delicate balance between over- and under-reaction, just like Daedalus and Icarus in Greek mythology, who could not escape their imprisonment by flying too high or too low. Both human amniotic epithelial and mesenchymal stromal cells and the conditioned medium generated from their culture exert multiple immunosuppressive activities. They have strong immunomodulatory properties that are influenced by the types and intensity of inflammatory stimuli present in the microenvironment. Notably, very recently, the immunomodulatory activity of human adult renal stem/progenitor cells (ARPCs) has been discovered. ARPCs cause a decrease in Tregs and CD3+ CD4- CD8- (DN) T cells in the early stages of inflammation, encouraging inflammation, and an increase in the late stages of inflammation, favoring inflammation quenching. If the inflammatory trigger continues, however, ARPCs cause a further increase in DN T cells to avoid the development of a harmful inflammatory state. As in the flight of Daedalus and Icarus, who could not fly too high or too low to not destroy their wings by the heat of the sun or the humidity of the sea, in response to an inflammatory environment, stem cells seem to behave by paying attention to regulating T cells in the balance between immune tolerance and autoimmunity. Recognizing the existence of both suppressive and stimulatory properties, and the mechanisms that underpin the duality of immune reaction, will aid in the development of active immunotherapeutic approaches that manipulate the immune system to achieve therapeutic benefit.

HLA-G and Other Immune Checkpoint Molecules as Targets for Novel Combined Immunotherapies

HLA-G is an HLA-class Ib molecule that is involved in the establishment of tolerance at the maternal/fetal interface during pregnancy. The expression of HLA-G is highly restricted in adults, but the de novo expression of this molecule may be observed in different hematological and solid tumors and is related to cancer progression. Indeed, tumor cells expressing high levels of HLA-G are able to suppress anti-tumor responses, thus escaping from the control of the immune system. HLA-G has been proposed as an immune checkpoint (IC) molecule due to its crucial role in tumor progression, immune escape, and metastatic spread. We here review data available in the literature in which the interaction between HLA-G and other IC molecules is reported, in particular PD-1, CTLA-4, and TIM-3, but also IDO and TIGIT. Clinical trials using monoclonal antibodies against HLA-G and other IC are currently ongoing with cancer patients where antibodies and inhibitors of PD-1 and CTLA-4 showed encouraging results. With this background, we may envisage that combined therapies using antibodies targeting HLA-G and another IC may be successful for clinical purposes. Indeed, such immunotherapeutic protocols may achieve a better rescue of effective anti-tumor immune response, thus improving the clinical outcome of patients.

HLA-G in Allergy: Does It Play an Immunoregulatory Role?

Allergy is an inflammatory process determined by a cascade of immune events characterized by T-helper 2 lymphocytes polarization leading to interleukin-4 upregulation, IgE secretion, and mast cell and eosinophil activation. HLA-G molecules, both in membrane-bound and in soluble forms, are known to play a key immunoregulatory role and their involvement in allergic diseases is supported by increasing literature data. HLA-G expression and secretion is specifically induced in peripheral blood mononuclear cells of allergic patients after in vitro incubation with the causal allergen. Elevated levels of soluble HLA-G molecules are detected in serum of patients with allergic rhinitis correlating with allergen-specific IgE levels, clinical severity, drug consumption and response to allergen-specific immunotherapy. HLA-G genetic polymorphisms confer susceptibility to allergic asthma development and high levels of soluble HLA-G molecules are found in plasma and bronchoalveolar lavage fluid of patients with allergic asthma correlating with allergen-specific IgE levels. Interestingly, allergic pregnant women have lower plasma sHLA-G levels than non-allergic women during the 3^rd trimester of pregnancy and at delivery. Finally, in allergic patients with atopic dermatitis HLA-G molecules are expressed by T cells, monocytes-macrophages and Langerhans cells infiltrating the dermis. Although at present is difficult to completely define the role of HLA-G molecules in allergic diseases, it may be suggested that they are specifically expressed and secreted by immune cells during the allergic reaction in an attempt to suppress allergic inflammation.

HLA-G Is Widely Expressed by Mast Cells in Regions of Organ Fibrosis in the Liver, Lung and Kidney

We previously demonstrated that mast cells expressing HLA-G are associated with regions of hepatitis C virus-induced liver fibrosis. Here, we aimed to determine whether HLA-G expression in mast cells is specific to viral etiology, the liver, or to the general process of fibrosis. We enumerated HLA-G⁺ cells and mast cells by the immunohistochemistry of (i) liver blocks from 41 cases of alcoholic cirrhosis, (ii) 10 of idiopathic pulmonary fibrosis (IPF), and (iii) 10 of renal fibrosis. The nature of the HLA-G⁺ cells was specified by multiplex immunofluorescence using software. More than half of all HLA-G⁺ cells were mast cells in fibrotic areas of alcoholic cirrhosis and IPF. In the kidneys, subjected to fibrosis, the HLA-G⁺ cells were indeed mast cells but could not be counted. Moreover, in certain cases of the liver and lung, we observed a number of cellular nodes, which were secondary or tertiary follicles, in which HLA-G was highly expressed by B lymphocytes. In conclusion, HLA-G⁺ mast cells could be observed in the fibrotic regions of all organs studied. Previous studies suggest a protective role for HLA-G⁺ mast cells against inflammation and fibrosis. The observed follicles with B lymphocytes that express HLA-G may also reinforce their antifibrotic role.

HLA-G gene editing in tumor cell lines as a novel alternative in cancer immunotherapy

Cancer immunotherapies based mainly on the blockade of immune-checkpoint (IC) molecules by anti-IC antibodies offer new alternatives for treatment in oncological diseases. However, a considerable proportion of patients remain unresponsive to them. Hence, the development of novel clinical immunotherapeutic approaches and/or targets are crucial.W In this context, targeting the immune-checkpoint HLA-G/ILT2/ILT4 has caused great interest since it is abnormally expressed in several malignancies generating a tolerogenic microenvironment. Here, we used CRISPR/Cas9 gene editing to block the HLA-G expression in two tumor cell lines expressing HLA-G, including a renal cell carcinoma (RCC7) and a choriocarcinoma (JEG-3). Different sgRNA/Cas9 plasmids targeting HLA-G exon 1 and 2 were transfected in both cell lines. Downregulation of HLA-G was reached to different degrees, including complete silencing. Most importantly, HLA-G - cells triggered a higher in vitro response of immune cells with respect to HLA-G + wild type cells. Altogether, we demonstrated for the first time the HLA-G downregulation through gene editing. We propose this approach as a first step to develop novel clinical immunotherapeutic approaches in cancer.

Targeting human leukocyte antigen G with chimeric antigen receptors of natural killer cells convert immunosuppression to ablate solid tumors

Background

Immunotherapy against solid tumors has long been hampered by the development of immunosuppressive tumor microenvironment, and the lack of a specific tumor-associated antigen that could be targeted in different kinds of solid tumors. Human leukocyte antigen G (HLA-G) is an immune checkpoint protein (ICP) that is neoexpressed in most tumor cells as a way to evade immune attack and has been recently demonstrated as a useful target for chimeric antigen receptor (CAR)-T therapy of leukemia by in vitro studies. Here, we design and test for targeting HLA-G in solid tumors using a CAR strategy.

Methods

We developed a novel CAR strategy using natural killer (NK) cell as effector cells, featuring enhanced cytolytic effect via DAP12-based intracellular signal amplification. A single-chain variable fragment (scFv) against HLA-G is designed as the targeting moiety, and the construct is tested both in vitro and in vivo on four different solid tumor models. We also evaluated the synergy of this anti-HLA-G CAR-NK strategy with low-dose chemotherapy as combination therapy.

Results

HLA-G CAR-transduced NK cells present effective cytolysis of breast, brain, pancreatic, and ovarian cancer cells in vitro, as well as reduced xenograft tumor growth with extended median survival in orthotopic mouse models. In tumor coculture assays, the anti-HLA-G scFv moiety promotes Syk/Zap70 activation of NK cells, suggesting reversal of the HLA-G-mediated immunosuppression and hence restoration of native NK cytolytic functions. Tumor expression of HLA-G can be further induced using low-dose chemotherapy, which when combined with anti-HLA-G CAR-NK results in extensive tumor ablation both in vitro and in vivo. This upregulation of tumor HLA-G involves inhibition of DNMT1 and demethylation of transporter associated with antigen processing 1 promoter.

Conclusions

Our novel CAR-NK strategy exploits the dual nature of HLA-G as both a tumor-associated neoantigen and an ICP to counteract tumor spread. Further ablation of tumors can be boosted when combined with administration of chemotherapeutic agents in clinical use. The readiness of this novel strategy envisions a wide applicability in treating solid tumors.

B Cells Control Mucosal-Associated Invariant T Cell Responses to Salmonella enterica Serovar Typhi Infection Through the CD85j HLA-G Receptor

Mucosal-associated invariant T (MAIT) cells are an innate-like population of T cells that display a TCR Vα7.2+ CD161+ phenotype and are restricted by the nonclassical MHC-related molecule 1 (MR1). Although B cells control MAIT cell development and function, little is known about the mechanisms underlying their interaction(s). Here, we report, for the first time, that during Salmonella enterica serovar Typhi (S. Typhi) infection, HLA-G expression on B cells downregulates IFN-γ production by MAIT cells. In contrast, blocking HLA-G expression on S. Typhi-infected B cells increases IFN-γ production by MAIT cells. After interacting with MAIT cells, kinetic studies show that B cells upregulate HLA-G expression and downregulate the inhibitory HLA-G receptor CD85j on MAIT cells resulting in their loss. These results provide a new role for HLA-G as a negative feedback loop by which B cells control MAIT cell responses to antigens.

Late-onset intrauterine growth restriction and HHV-6 infection: A pilot study

Late-onset Intrauterine growth restriction (IUGR) refers to impaired growth and development of the fetus, characterized by placental morphological abnormalities that affect the fetus's supply of nutrients. Human leukocyte antigen-G (HLA-G) is physiologically expressed during pregnancy, but decreased in normal placenta during the last weeks of gestation possibly inducing childbirth. Several viruses involved in congenital infection, such as herpesviruses, exploit HLA-G expression as an immune-escape mechanism. To date, despite different congenital herpetic infections having been associated with late IUGR, no direct implication of Human herpesvirus 6 (HHV-6) infection has been reported. We evaluated HLA-G expression and HHV-6 infection in 11 placentas from late-onset IUGR newborns and 11 placentas from uncomplicated pregnancies by histopathological and immunohistochemistry analysis. We found higher levels of HLA-G expression and HHV-6 presence in IUGR placenta samples compared with control placenta samples. We report HHV-6 staining in IUGR placenta samples, characterized by high HLA-G expression. These preliminary data suggest a possible involvement of HHV-6 infection in HLA-G deregulation that might affect vessel remodeling and prevent the correct pregnancy outcome in the IUGR condition.

The role of CD71+ erythroid cells in the regulation of the immune response

Complex regulation of the immune response is necessary to support effective defense of an organism against hostile invaders and to maintain tolerance to harmless microorganisms and autoantigens. Recent studies revealed previously unappreciated roles of CD71⁺ erythroid cells (CECs) in regulation of the immune response. CECs physiologically reside in the bone marrow where erythropoiesis takes place. Under stress conditions, CECs are enriched in some organs outside of the bone marrow as a result of extramedullary erythropoiesis. However, the role of CECs goes well beyond the production of erythrocytes. In neonates, increased numbers of CECs contribute to their vulnerability to infectious diseases. On the other side, neonatal CECs suppress activation of immune cells in response to abrupt colonization with commensal microorganisms after delivery. CECs are also enriched in the peripheral blood of pregnant women as well as in the placenta and are responsible for the regulation of feto-maternal tolerance. In patients with cancer, anemia leads to increased frequency of CECs in the peripheral blood contributing to diminished antiviral and antibacterial immunity, as well as to accelerated cancer progression. Moreover, recent studies revealed the role of CECs in HIV and SARS-CoV-2 infections. CECs use a full arsenal of mechanisms to regulate immune response. These cells suppress proinflammatory responses of myeloid cells and T-cell proliferation by the depletion of ʟ-arginine by arginase. Moreover, CECs produce reactive oxygen species to decrease T-cell proliferation. CECs also secrete cytokines, including transforming growth factor β (TGF-β), which promotes T-cell differentiation into regulatory T-cells. Here, we comprehensively describe the role of CECs in orchestrating immune response and indicate some therapeutic approaches that might be used to regulate their effector functions in the treatment of human conditions.

First immunotherapeutic CAR-T cells against the immune checkpoint protein HLA-G

BACKGROUND

CAR-T cells immunotherapy is a breakthrough in the treatment of hematological malignancies such as acute lymphoblastic leukemia (ALL) and B-cell malignancies. However, CAR-T therapies face major hurdles such as the lack of tumor-specific antigen (TSA), and immunosuppressive tumor microenvironment sometimes caused by the tumorous expression of immune checkpoints (ICPs) such as HLA-G. Indeed, HLA-G is remarkable because it is both a potent ICP and a TSA. HLA-G tumor expression causes immune escape by impairing innate and adaptive immune responses and by inducing a suppressive microenvironment. Yet, to date, no immunotherapy targets it.

METHODS

We have developed two anti-HLA-G third-generation CARs based on new anti-HLA-G monoclonal antibodies.

RESULTS

Anti-HLA-G CAR-T cells were specific for immunosuppressive HLA-G isoforms. HLA-G-activated CAR-T cells polarized toward T helper 1, and became cytotoxic against HLA-G+ tumor cells. In vivo, anti-HLA-G CAR-T cells were able to control and eliminate HLA-G+ tumor cells. The interaction of tumor-HLA-G with interleukin (IL)T2-expressing T cells is known to result in effector T cell functional inhibition, but anti-HLA-G CAR-T cells were insensitive to this inhibition and still exerted their function even when expressing ILT2. Lastly, we show that anti-HLA-G CAR-T cells differentiated into long-term memory effector cells, and seemed not to lose function even after repeated stimulation by HLA-G-expressing tumor cells.

CONCLUSION

We report for the first time that HLA-G, which is both a TSA and an ICP, constitutes a valid target for CAR-T cell therapy to specifically target and eliminate both tumor cells and HLA-G+ suppressive cells.

Role and expression of non-classical human leukocyte antigen-G in renal transplanted allografts

BACKGROUND

The non-classical class I molecule human leukocyte antigen-G (HLA-G) has great potential to modulate the immune response. However, the mechanism underlying HLA-G induction remains unknown. Therefore, this study aimed to determine the factors that induce HLA-G expression on proximal tubular epithelial cells (pTECs) in renal transplanted allografts in vivo and in vitro.

METHODS

This study included 40 adult Japanese patients with renal allografts (35 and five patients with kidneys from living and deceased donors, respectively) who survived for at least 1 year. We evaluated HLA-G1/5 expression using an immunofluorescence method and investigated the induction of HLA-G expression in primary cultured human pTECs by cytokines and immunosuppressants.

RESULTS

The HLA-G expression was identified in the perinuclear region or on the basement membrane of pTECs of renal biopsy tissue in 12 (30%) of 40 patients at 2-4 weeks and at 1 year following transplantation. A reduction of 30% in the estimated glomerular filtration rate was lower in the HLA-G-positive group than that of the negative group (p = 0.016). Cox proportional hazard models also demonstrated that HLA-G1/5 expression on pTECs was an independent predictor of improved renal allograft function (hazard ratio, 0.189; 95% CI 0.041-0.850, p = 0.030). Interferon-beta was the most powerful inducer of HLA-G expression in vitro, whereas the immunosuppressants everolimus, tacrolimus, cyclosporin, and dexamethasone did not induce any expression.

CONCLUSION

Unlike immunosuppressants, acquired HLA-G expression might confer long-term renal preservation effects in renal transplanted allografts.

Structural Modeling and Molecular Dynamics of the Immune Checkpoint Molecule HLA-G

HLA-G is considered to be an immune checkpoint molecule, a function that is closely linked to the structure and dynamics of the different HLA-G isoforms. Unfortunately, little is known about the structure and dynamics of these isoforms. For instance, there are only seven crystal structures of HLA-G molecules, being all related to a single isoform, and in some cases lacking important residues associated to the interaction with leukocyte receptors. In addition, they lack information on the dynamics of both membrane-bound HLA-G forms, and soluble forms. We took advantage of in silico strategies to disclose the dynamic behavior of selected HLA-G forms, including the membrane-bound HLA-G1 molecule, soluble HLA-G1 dimer, and HLA-G5 isoform. Both the membrane-bound HLA-G1 molecule and the soluble HLA-G1 dimer were quite stable. Residues involved in the interaction with ILT2 and ILT4 receptors (α3 domain) were very close to the lipid bilayer in the complete HLA-G1 molecule, which might limit accessibility. On the other hand, these residues can be completely exposed in the soluble HLA-G1 dimer, due to the free rotation of the disulfide bridge (Cys42/Cys42). In fact, we speculate that this free rotation of each protomer (i.e., the chains composing the dimer) could enable alternative binding modes for ILT2/ILT4 receptors, which in turn could be associated with greater affinity of the soluble HLA-G1 dimer. Structural analysis of the HLA-G5 isoform demonstrated higher stability for the complex containing the peptide and coupled β2-microglobulin, while structures lacking such domains were significantly unstable. This study reports for the first time structural conformations for the HLA-G5 isoform and the dynamic behavior of HLA-G1 molecules under simulated biological conditions. All modeled structures were made available through GitHub (https://github.com/KavrakiLab/), enabling their use as templates for modeling other alleles and isoforms, as well as for other computational analyses to investigate key molecular interactions.

HLA-G Neo-Expression on Tumors

HLA-G is known to modulate the immune system activity in tissues where physiological immune-tolerance is necessary (i.e., maternal-fetal interface, thymus, and cornea). However, the frequent neo-expression of HLA-G in many cancer types has been previously and extensively described and is correlated with a bad prognosis. Despite being an MHC class I molecule, HLA-G is highly present in tumor context and shows unique characteristics of tissue restriction of a Tumor Associated Antigen (TAA), and potent immunosuppressive activity of an Immune CheckPoint (ICP). Consequently, HLA-G appears to be an excellent molecular target for immunotherapy. Although the relevance of HLA-G in cancer incidence and development has been proven in numerous tumors, its neo-expression pattern is still difficult to determine. Indeed, the estimation of HLA-G's actual expression in tumor tissue is limited, particularly concerning the presence and percentage of the new non-canonical isoforms, for which detection antibodies are scarce or inexistent. Here, we summarize the current knowledge about HLA-G neo-expression and implication in various tumor types, pointing out the need for the development of new tools to analyze in-depth the HLA-G neo-expression patterns, opening the way for the generation of new monoclonal antibodies and cell-based immunotherapies.

The Role of HLA-G in Human Papillomavirus Infections and Cervical Carcinogenesis

Human leukocyte antigen (HLA)-G, a non-classical HLA-class I molecule, has a low polymorphism frequency, restricted tissue distribution and immunoinhibitory property. HLA-G expression in tumor cells and cells chronically infected with virus may enable them to escape from host immune surveillance. It is well-known that the HLA-G molecule is a novel biomarker and potential therapeutic target that is relevant in various types of cancers, but its role in cervical cancer has not been fully explored. In this review, we aim to summarize and discuss the immunologic role of the HLA-G molecule in the context of HPV infections and the process of cervical cancer carcinogenesis. A better understanding of the potential impact of HLA-G on the clinical course of persistent HPV infections, cervical epithelial cell transformation, tumor growth, recurrence and metastasis is needed to identify a novel diagnostic/prognostic biomarker for cervical cancer, which is critical for cervical cancer risk screening. In addition, it is also necessary to identify HLA-G-driven immune mechanisms involved in the interactions between host and virus to explore novel immunotherapy strategies that target HLA-G/immunoglobulin-like transcript (ILT) immune checkpoints.

Expression of membrane-bound human leucocyte antigen-G in systemic sclerosis and systemic lupus erythematosus

Human leucocyte antigen-G (HLA-G) is a nonclassical class I major histocompatibility complex (MHC) molecule characterized by complex immunoregulatory and tolerogenic functions. Membrane-bound HLA-G is expressed on the surface of different cell populations in both physiological and pathological conditions. Systemic sclerosis (SSc) is a multisystem autoimmune disease characterized by widespread tissue fibrosis, vascular lesions and immunological alterations. Systemic lupus erythematosus is the prototypic systemic autoimmune disease affecting virtually any organ system, such as skin, joints, central nervous system, or kidneys. In SSc and SLE patients, the membrane expression of HLA-G on monocytes (0.88 ± 1.54 and 0.43 ± 0.75, respectively), CD4+ (0.42 ± 0.78 and 0.63 ± 0.48, respectively), CD8+ (2.65 ± 3.47 and 1.29 ± 1.34, respectively) and CD4+ CD8+ double-positive cells (13.87 ± 15.97 and 3.79 ± 3.11, respectively) was significantly higher than in healthy controls (0.12 ± 0.07; 0.01 ± 0.01; 0.14 ± 0.20 and 0.32 ± 0.38, respectively) (p < 0.0001). Our results show that in SSc and SLE the membrane expression of HLA-G by different subpopulations of peripheral blood mononuclear cells (PBMC) is increased, suggesting a potential role of HLA-G molecules in the complex immunological pathogenesis of these two autoimmune disorders.

The Autoimmune Regulator (Aire) transactivates HLA-G gene expression in thymic epithelial cells

The Autoimmune Regulator (Aire) protein coordinates the negative selection of developing thymocytes by inducing the expression of hundreds of tissue-specific antigens within the thymic medulla, which is also a primary site of the expression of the immune checkpoint HLA-G molecule. Considering the immunomodulatory properties of Aire and HLA-G, and considering that the role of the constitutive thymus expression of HLA-G has not been elucidated, we studied the effect of AIRE cDNA transfection on HLA-G expression in 4D6 thymic cells and in the HLA-G-positive JEG-3 choriocarcinoma cells. Aire promoted the transactivation of HLA-G gene by increasing the overall transcription, inducing the transcription of at least G1 and G2/G4 isoforms, and incrementing the occurrence and distribution of intracellular HLA-G protein solely in 4D6 thymic cells. Luciferase-based assays and chromatin immunoprecipitation experiments performed in 4D6 cells revealed that Aire targeted at least two regions within the 5'-untranslated regulatory region (5'-URR) extending 1·4 kb from the first ATG initiation codon. The interaction occurs independently of three putative Aire-binding sites. These results indicate that the Aire-induced upregulation of HLA-G in thymic cells is likely to act through the interaction of Aire with specific HLA-G 5'-URR DNA-binding factors. Such a multimeric transcriptional complex might operate in the thymus during the process of promiscuous gene expression.

A role for both HLA-F and HLA-G in reproduction and during pregnancy?

The human major histocompatibility complex includes a group of non-classical HLA class I genes, HLA-E, -F and -G. While nearly all focus since the discovery of these class Ib molecules have been on basic biochemistry and molecular biology of HLA-G and HLA-E, as well as their expression patterns, functions in immune modulation and during pregnancy, and also possible implications in a range of diseases, in infertility and pregnancy complications, HLA-F has nearly been ignored. However, recent discoveries show that HLA-F can be expressed as both open conformers binding to a number of KIRs on primarily NK cells, as well as peptide-bound HLA-F binding to ILT2 and ILT4. Furthermore, a number of reports indicate a possible involvement of HLA-F in viral infections, in cancer immunology, and in fertility and reproduction, which may initiate more interest in this rather unknown HLA class I molecule. In this short review, we focus on recent discoveries that indicate a functional role for HLA-F in reproduction and during pregnancy, and the role of HLA-F in relation to HLA-G.

Human Hepatocytes and Differentiated Adult-Derived Human Liver Stem/Progenitor Cells Display In Vitro Immunosuppressive Properties Mediated, at Least in Part, through the Nonclassical HLA Class I Molecule HLA-G

One of the main challenges in liver cell therapy (LCT) is the induction of a tolerogenic microenvironment to promote graft acceptance in the recipient. Little is known about the immunomodulatory potential of the hepatic cells used in liver cell therapy. In this work, we wanted to evaluate the immunosuppressive properties of human hepatocytes and adult-derived human liver stem/progenitor cells (ADHLSCs), as well as the potential involvement of the immunomodulatory molecule HLA-G. We demonstrated that both cell types were capable of inhibiting the proliferative response of PBMCs to an allogenic stimulus and that the immune inhibitory potential of ADHLSCs, although lower than that of hepatocytes, increased after hepatogenic differentiation. We demonstrated that liver cells express HLA-G and that the immune inhibition pattern was clearly associated to its expression. Interestingly, HLA-G expression increased after the third step of differentiation, wherein oncostatin M (OSM) was added. A 48 hr treatment with OSM was sufficient to induce HLA-G expression in ADHLSCs and result in immune inhibition. Surprisingly, blocking HLA-G partially reversed the immune inhibition mediated by hepatocytes and differentiated ADHLSCs, but not that of undifferentiated ADHLSCs, suggesting that additional immune inhibitory mechanisms may be used by these cells. In conclusion, we demonstrated that both hepatocytes and ADHLSCs present immunomodulatory properties mediated, at least in part, through HLA-G, which can be upregulated following hepatogenic differentiation or liver cell pretreatment with OSM. These observations open up new perspectives for the induction of tolerance following LCT and for potential therapeutic applications of these liver cells.

Analysis of HLA-G expression in renal tissue in lupus nephritis: a pilot study

BACKGROUND

The study aimed to investigate whether HLA-G antigen is expressed in the kidneys of patients affected by lupus nephritis (LN) and whether its detection in renal biopsies could be adopted as a marker of treatment response and prognosis.

METHODS

Thirty renal biopsies from patients with LN were selected and analyzed through immunohistochemistry. Laboratory and clinical data were retrospectively collected at baseline, 6 and 12 months and at the latest clinical appointment. A number of patients (63.3%) were treated with rituximab (RTX) +/- methylprednisolone in the induction phase. The expression of HLA-G in glomeruli, tubules and infiltrating cells was examined and compared between lupus patients who achieved either complete or partial renal response and those who did not respond to treatment.

RESULTS

HLA-G staining was observed in the glomeruli of 20 of 30 samples from patients with LN. The expression of the antigen was detected in podocytes, along glomerular capillary walls, on parietal glomerular epithelial cells and within the juxtaglomerular apparatus. Seventy per cent of patients whose glomeruli expressed HLA-G achieved partial or complete response at 6 months and 75% at the latest available follow up compared with 30% and 40%, respectively, of those who did not show any expression. The pattern of staining in tubules and infiltrating cells was highly variable precluding any clinical correlation.

CONCLUSION

This study demonstrates that HLA-G is expressed in renal tissue in LN. Our retrospective data suggest that its expression could correlate with response to treatment.

Effects of everolimus and HLA-G on cellular proliferation and neutrophil adhesion in an in vitro model of cardiac allograft vasculopathy

Human leukocyte antigen-G (HLA-G) expression is modulated by immunosuppressant use and is associated with lower incidence of graft rejection and cardiac allograft vasculopathy (CAV). We examined whether everolimus induces HLA-G expression and inhibits human coronary artery smooth muscle cell (HCASMC) proliferation, a critical event in CAV. Also, we examined whether TNFα-stimulated neutrophil adhesion is inhibited by HLA-G on human coronary artery endothelial cells (HCAECs). HLA-G expression in HCASMCs following everolimus treatment was determined by western-blot densitometric analysis. HCASMCs proliferation following incubation with recombinant HLA-G was determined by automated cell counter detecting 2-10 µm particles. Assessment of recombinant HLA-G on neutrophil adhesion to HCAECs in response to TNF-α induced-injury was determined by nonstatic adhesion assays. HLA-G expression was upregulated in HCASMCs following everolimus exposure (1000 ng/ml; P < .05). HLA-G (500, 1000 ng/ml; both P < .05) reduced HCASMC proliferation and inhibited TNFα-stimulated neutrophil adhesion to endothelial cells at all concentrations (0.1-1 ng/ml; all P < .001). Our study reveals novel regulation of HLA-G by everolimus, by demonstrating HLA-G upregulation and subsequent inhibition of HCASMC proliferation. HLA-G is a potent inhibitor of neutrophil adhesion to HCAECs. Findings support HLA-G's importance and potential use in heart transplantation for preventative therapy or as a marker to identify patients at high risk for developing CAV.

Human Herpes simplex 1 virus infection of endometrial decidual tissue-derived MSC alters HLA-G expression and immunosuppressive functions

OBJECTIVES

Mesenchymal stromal/stem cells have immunosuppressive functions. Our previous results demonstrated that one of the players of this immunomodulation can be ascribed to the Human Leukocyte Antigen-G. HLA-G, a non classical HLA class I antigen, is involved in immune tolerance during pregnancy, organ transplantation, autoimmune and inflammatory diseases. In this study we wanted to verify whether human endometrial decidual tissue derived (EDT)-MSC could express HLA-G. Additionally we assessed the permissivity to Human Herpesvirus infections, using HSV-1 as a model, and the possible effect on EDT-MSC immunosuppressive functions towards peripheral blood mononuclear cell (PBMC) proliferation.

METHODS

We analyzed immune-inhibitory functions and HLA-G expression in human EDT-MSC before and after HSV-1 infection.

RESULTS

We observed that EDT-MSC express HLA-G molecules, that partly are responsible for the immune-inhibitory functions of EDT-MSC towards PBMC proliferation. EDT-MSC are permissive for a productive infection by HSV-1, that decreases HLA-G expression and affects EDT-MSC immune-inhibitory functions.

CONCLUSIONS

We demonstrate that EDT-MSC are susceptible to HSV-1 infection, that reduces HLA-G expression and their immune-inhibitory function. These data could have a clinical implication in the use of EDT-MSC as an immunosuppressant, in particular in steroid-refractory GvHD after allogeneic hematopoietic stem cell transplantation and in autoimmune diseases.

Evaluation of membrane-bound and soluble forms of human leucocyte antigen-G in systemic sclerosis

Systemic sclerosis (SSc) is a complex disease characterized by immune dysregulation, extensive vascular damage and widespread fibrosis. Human leucocyte antigen-G (HLA-G) is a non-classic class I major histocompatibility complex (MHC) molecule characterized by complex immunomodulating properties. HLA-G is expressed on the membrane of different cell lineages in both physiological and pathological conditions. HLA-G is also detectable in soluble form (sHLA-G) deriving from the shedding of surface isoforms (sHLA-G1) or the secretion of soluble isoforms (HLA-G5). Several immunosuppressive functions have been attributed to both membrane-bound and soluble HLA-G molecules. The plasma levels of sHLA-G were higher in SSc patients (444·27 ± 304·84 U/ml) compared to controls (16·74 ± 20·58 U/ml) (P < 0·0001). The plasma levels of transforming growth factor (TGF)-β were higher in SSc patients (18 937 ± 15 217 pg/ml) compared to controls (11 099 ± 6081 pg/ml; P = 0·003), and a significant correlation was found between TGF-β and the plasma levels of total sHLA-G (r = 0·65; P < 0·01), sHLA-G1 (r = 0·60; P = 0·003) and HLA-G5 (r = 0·47; P = 0·02). The percentage of HLA-G-positive monocytes (0·98 ± 1·72), CD4+ (0·37 ± 0·68), CD8+ (2·05 ± 3·74) and CD4+ CD8+ double-positive cells (14·53 ± 16·88) was higher in SSc patients than in controls (0·11 ± 0·08, 0·01 ± 0·01, 0·01 ± 0·01 and 0·39 ± 0·40, respectively) (P < 0·0001). These data indicate that in SSc the secretion and/or shedding of soluble HLA-G molecules and the membrane expression of HLA-G by peripheral blood mononuclear cells (PBMC) is clearly elevated, suggesting an involvement of HLA-G molecules in the immune dysregulation of SSc.

The genetic diversity within the 1.4 kb HLA-G 5' upstream regulatory region moderately impacts on cellular microenvironment responses

The HLA-G 5'URR extending 1.4 kb from the ATG presents a unique set of regulatory elements among HLA genes. Several variable sites have been described that coincide with or are close to these elements, thus HLA-G 5'URR polymorphism might influence the HLA-G expression level. We cloned the ten most frequent HLA-G 5'URR haplotypes to evaluate their activity on a luciferase reporter gene in HLA-G⁺ cell lines (JEG-3/choriocarcinoma and FON⁺/melanoma). We also investigated associations between the plasma HLA-G (sHLA-G) levels and the HLA-G 5'URR variability in 157 healthy individuals. Cell lines were transfected with pGL3-Basic vector constructions containing HLA-G 5'URR sequences. The G010101a (in JEG-3) and G010101b (in FON⁺) haplotypes exhibited higher promoter activity, whereas the G010101d (in JEG-3) and G010102a (in FON⁺) haplotypes exhibited lower promoter activity. In the presence of HLA-G inducers (interferon-β and progesterone) or repressors (cyclopamine) HLA-G promoter activity was modulated, but certain haplotypes exhibited differential responses. No strict association was observed between plasma sHLA-G levels and the 5'URR haplotypes or genotypes; however, the G010101b haplotype was underrepresented among HLA-G-negative plasmas. Therefore, the HLA-G 5'URR polymorphism may have an impact on the modulation of HLA-G gene expression, but alone provides a limited predictive value for sHLA-G levels in vivo.

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.

Correlation of serum sHLA-G levels with cyst stage in patients with cystic echinococcosis: is it an immune evasion strategy?

Patients with cystic echinococcosis (CE) can harbour cysts for years or even decades, apparently without effect of the immune system on the metacestode. Although several immune evasion mechanisms by echinococcal cysts have been described, it is unclear whether the human leucocyte antigen (HLA) system plays a role in the susceptibility or resistance to CE in humans. HLA-G molecules are known to exert a suppressive action on dendritic cells maturation and on natural killer (NK) cells functions, therefore hampering T-cell responses and NK cytolysis. HLA-G plays an important role in immune tolerance, is involved in foetus and in allotransplant tolerance, and may be involved in tumoral and viral immune evasion. In this study, we assessed the presence and levels of soluble HLA-G (sHLA-G) in patients with CE using a commercial ELISA kit to determine whether host's HLA-G may have a role in the course of human CE.

Soluble HLA-G and HLA-E Levels in Bone Marrow Plasma Samples Are Related to Disease Stage in Neuroblastoma Patients

The role of nonclassical HLA-class Ib molecules HLA-G and HLA-E in the progression of Neuroblastoma (NB), the most common pediatric extracranial solid tumor, has been characterized in the last years. Since BM infiltration by NB cells is an adverse prognostic factor, we have here analyzed for the first time the concentration of soluble (s)HLA-G and HLA-E in bone marrow (BM) plasma samples from NB patients at diagnosis and healthy donors. sHLA-G and sHLA-E are present in BM plasma samples, and their levels were similar between NB patients and controls, thus suggesting that these molecules are physiologically released by resident or stromal BM cell populations. This hypothesis was supported by the finding that sHLA-G and sHLA-E levels did not correlate with BM infiltration and other adverse prognostic factors (MYCN amplification and age at diagnosis). In contrast, BM plasma levels of both molecules were higher in patients with metastatic disease than in patients with localized NB, thus suggesting that concentration of these molecules might be correlated with disease progression. The prognostic role of sHLA-G and sHLA-E concentration in the BM plasma for NB patients will be evaluated in future studies, by analyzing the clinical outcome of the same NB patients at follow-up.

Allergen-driven HLA-G expression and secretion in peripheral blood mononuclear cells from allergic rhinitis patients

BACKGROUND

It has been reported that soluble HLA-G serum levels are increased in patients with pollen-induced allergic rhinitis and decrease after immunotherapy. However, no functional study has been conducted so far. The aim of this study was to evaluate the membrane expression and secretion of HLA-G molecules in peripheral blood mononuclear cells from allergic rhinitis patients after in vitro incubation with the causal allergen.

METHODS AND RESULTS

Twenty-two allergic rhinitis patients and ten healthy subjects were enrolled. Membrane HLA-G expression was determined by flow cytometry and soluble HLA-G in culture supernatant was determined by immunoenzymatic assay. HLA-G expression was detected in CD4+ (T-helper-2) cells and monocytes after in vitro stimulation with the causal allergen but not with non specific stimuli and non causal allergens. Accordingly, the release of soluble HLA-G in culture supernatant occurred only after the stimulation with the causal allergen. Collectively, these results were confirmed by Western blot analysis.

CONCLUSIONS

The present study provides the first in vitro evidence that in allergic patients HLA-G expression and secretion is specifically induced by the causal allergen. These data may add new insights into the pathogenetic mechanisms underlying allergic inflammation and allergen specific immunotherapy.

HLA-G coding region and 3'untranslated region (3'UTR) in two Chinese Han populations

In this study, exons 2-4 and 3'untranslated region (3'UTR) of human leukocyte antigen (HLA)-G gene were investigated for 201 and 104 healthy unrelated Han samples recruited from Hunan Province, southern China and central Inner Mongolia Autonomous Region, northern China, respectively, using sequence-based typing and cloning methods. Totally 12 HLA-G alleles in the coding region, 9 variable sites in 3'UTR, 8 3'UTR haplotypes and 15 HLA-G extended haplotypes (EHs) incorporating the coding region and 3'UTR were observed. Very strong linkage disequilibrium (LD) was observed between HLA-A and HLA-G, and between HLA-G coding region and 3'UTR in each population (all global P=0.0000). Seven HLA-A-G haplotypes showed significant LD in both populations. Three HLA-G alleles in the coding region, 4 polymorphic sites in the 3'UTR, 3 3'UTR haplotypes and 4 HLA-G EHs differed significantly in their distributions between the 2 Chinese Han populations (all P≤0.0001). There was evidence for balancing selection acting on HLA-G 3'UTR positions +3010, +3142 and +3187 in the two populations. The NJ dendrograms demonstrated the existence of two basic HLA-G lineages and indicated that, HLA-G*01:01:01, the most common HLA-G allele, formed a separate lineage from other alleles. Our results shed new lights into HLA-G genetics among Chinese Han populations. The findings reported here are of importance for future studies related to post-transcriptional regulation of HLA-G allelic expression and the potential role of HLA-G in disease association in populations of Chinese ancestry.

Immunomodulation of classical and non-classical HLA molecules by ionizing radiation

Radiotherapy has been employed for the treatment of oncological patients for nearly a century, and together with surgery and chemotherapy, radiation oncology constitutes one of the three pillars of cancer therapy. Ionizing radiation has complex effects on neoplastic cells and on tumor microenvironment: beyond its action as a direct cytotoxic agent, tumor irradiation triggers a series of alterations in tumoral cells, which includes the de novo synthesis of particular proteins and the up/down-regulation of cell surface molecules. Additionally, ionizing radiation may induce the release of "danger signals" which may, in turn lead to cellular and molecular responses by the immune system. This immunomodulatory action of ionizing radiation highlights the importance of the combined use (radiotherapy plus immunotherapy) for cancer healing. Major histocompatibility complex antigens (also called Human Leukocyte Antigens, HLA in humans) are one of those molecules whose expression is modulated after irradiation. This review summarizes the modulatory properties of ionizing radiation on the expression of HLA class I (classical and non-classical) and class II molecules, with special emphasis in non-classical HLA-I molecules.

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.

Co-expression of ILT4/HLA-G in human non-small cell lung cancer correlates with poor prognosis and ILT4-HLA-G interaction activates ERK signaling

Non-small cell lung cancer (NSCLC) is the most common malignant tumor in the world, of which prognosis is generally poor due to insufficient mechanistic understanding. To explore the molecular pathogenesis of NSCLC, the co-expression of immunoglobulin-like transcript 4 (ILT4) and its ligand human leukocyte antigen G (HLA-G) in NSCLC tissues and cells were investigated. Here, we detected the expression of ILT4 and HLA-G in 81 tumor specimens from primary NSCLC patients, and we found that co-expression of ILT4/HLA-G was significantly associated with regional lymph node involvement, advanced stages, and the overall survival of patients. In NSCLC cell lines, HLA-G expression increased/decreased accordingly when ILT4 was up-/down-regulated, and ILT4 expression increased in a concentration-dependent manner via the stimulation of HLA-G fusion protein. Interestingly, HLA-G fusion protein could also up-regulate the phospho-ERK1/2 expression, which means the activation of extracellular signal-regulated kinase (ERK) signaling. All in all, our results indicate that the ILT4-HLA-G interaction might play an important role in NSCLC progression. Identification of ILT4 and HLA-G expression may provide an indicator to predict prognosis and guide prevention and treatment of NSCLC.

Human Leukocyte Antigen-G (HLA-G) Expression in Cancers: Roles in Immune Evasion, Metastasis and Target for Therapy

Aberrant induction of human leukocyte antigen-G (HLA-G) expression has been observed in various malignancies and is strongly associated with tumor immune escape, metastasis and poor prognosis. To date, great achievements have been made in understanding the underlying mechanisms of HLA-G involved in tumor progression. HLA-G could lead to tumor evasion by inhibition of immune cell cytolysis, differentiation and proliferation and inhibition of cytokine production, induction of immune cell apoptosis, generation of regulatory cells and expansion of myeloid-derived suppressive cells and by impairment of chemotaxis. Moreover, HLA-G could arm tumor cells with a higher invasive and metastatic potential with the upregulation of tumor-promoting factor expression such as matrix metalloproteinases (MMPs), indicating that ectopic HLA-G expression could render multiple effects during the progression of malignancies. In this review, we summarized the mechanisms of HLA-G involved in promoting tumor cell immune escaping, metastasis and disease progression. Special attention will be paid to its significance as an attractive therapeutic target in cancers.

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.

Clinical correlates of human leucocyte antigen (HLA)-G in systemic sclerosis

Human leucocyte antigen (HLA)-G has a tolerogenic function and could play a role in the pathogenesis of immune-mediated diseases, including systemic sclerosis (SSc). The aim of this study was to evaluate HLA-G serum expression (sHLA-G) and the HLA-G gene 14 base pairs (bp) insertion/deletion (del(-)/del(+)) polymorphism in patients with Ssc, to search for possible associations with clinical and laboratory variables. sHLA-G was measured by enzyme-linked immunosorbent assay (ELISA) in sera from 77 patients with SSc and 32 healthy donors (HD); the 14 bp del(-)/del(+) polymorphism was evaluated by polymerase chain reaction (PCR) amplification of peripheral blood mononuclear cells (PBMC) genomic DNA. Receiver operating characteristics (ROC) analysis identified the HLA-G cut-off that best discriminated dichotomized clinical and serological variables, that was subsequently employed to subdivide SSc patients into HLA-G high (HLA-G(+)) and low (HLA-G(-)) profile groups. sHLA-G were not statistically different between SSc patients and HD, nor between distinct SSc autoantibody subsets. Subdividing SSc patients by HLA-G positivity or negativity yielded significant differences for the modified Rodnan skin score (mRss) (P = 0.032), 'general' (P = 0.031) and 'kidney' (P = 0.028) Medsger severity scores (MSS) and disease activity index, and especially Δ heart/lung (P = 0.005). A worse 'general' MSS (P = 0.002) and Δ heart/lung (P = 0.011) were more frequent in the low sHLA-G group. These two variables and mRss were associated with sHLA-G levels at logistic regression analysis. Treatment had no influence on sHLA-G. Moreover, a higher frequency of scleredema was detected in the del(+)/del(+) than the del(-)/del(+) group (P = 0.04). These data suggest modulatory effects of sHLA-G on SSc. Prospective studies are needed to investigate a role in predicting the disease course.

Human Parthenogenetic Embryonic Stem Cell-Derived Neural Stem Cells Express HLA-G and Show Unique Resistance to NK Cell-Mediated Killing

Parent-of-origin imprints have been implicated in the regulation of neural differentiation and brain development. Previously we have shown that, despite the lack of a paternal genome, human parthenogenetic (PG) embryonic stem cells (hESCs) can form proliferating neural stem cells (NSCs) that are capable of differentiation into physiologically functional neurons while maintaining allele-specific expression of imprinted genes. Since biparental ("normal") hESC-derived NSCs (N NSCs) are targeted by immune cells, we characterized the immunogenicity of PG NSCs. Flow cytometry and immunocytochemistry revealed that both N NSCs and PG NSCs exhibited surface expression of human leukocyte antigen (HLA) class I but not HLA-DR molecules. Functional analyses using an in vitro mixed lymphocyte reaction assay resulted in less proliferation of peripheral blood mononuclear cells (PBMC) with PG compared with N NSCs. In addition, natural killer (NK) cells cytolyzed PG less than N NSCs. At a molecular level, expression analyses of immune regulatory factors revealed higher HLA-G levels in PG compared with N NSCs. In line with this finding, MIR152, which represses HLA-G expression, is less transcribed in PG compared with N cells. Blockage of HLA-G receptors ILT2 and KIR2DL4 on natural killer cell leukemia (NKL) cells increased cytolysis of PG NSCs. Together this indicates that PG NSCs have unique immunological properties due to elevated HLA-G expression.

Clinical relevance of miR-mediated HLA-G regulation and the associated immune cell infiltration in renal cell carcinoma

In human tumors of distinct origin including renal cell carcinoma (RCC), the non-classical human leukocyte antigen G (HLA-G) is frequently expressed, thereby inhibiting the cytotoxic activity of T and natural killer (NK) cells. Recent studies demonstrated a strong post-transcriptional gene regulation of the HLA-G by miR-152, -148A, -148B and -133A. Standard methods were applied to characterize the expression and function of HLA-G, HLA-G-regulatory microRNAs (miRs) and the immune cell infiltration in 453 RCC lesions using a tissue microarray and five RCC cell lines linking these results to clinical parameters. Direct interactions with HLA-G regulatory miRs and the HLA-G 3' untranslated region (UTR) were detected and the affinities of these different miRs to the HLA-G 3'-UTR compared. qPCR analyses and immunohistochemical staining revealed an inverse expression of miR-148A and -133A with the HLA-G protein in situ and in vitro. Stable miR overexpression caused a downregulation of HLA-G protein enhancing the NK and LAK cell-mediated cytotoxicity in in vitro CD107a activation assays revealing a HLA-G-dependent cytotoxic activity of immune effector cells. A significant higher frequency of CD3⁺/CD8⁺ T cell lymphocytes, but no differences in the activation markers CD69, CD25 or in the presence of CD56⁺, FoxP3⁺ and CD4⁺ immune cells were detected in HLA-G⁺ compared to HLA-G^- RCC lesions. This could be associated with higher WHO grade, but not with a disease-specific survival. These data suggest a miR-mediated control of HLA-G expression in RCC, which is associated with a distinct pattern of immune cell infiltration.

The Role of HLA-G Molecule and HLA-G Gene Polymorphisms in Tumors, Viral Hepatitis, and Parasitic Diseases

Considering that the non-classical HLA-G molecule has well-recognized tolerogenic properties, HLA-G expression is expected to be deleterious when present in tumor cells and in cells chronically infected by viruses, whereas HLA-G expression is expected to be advantageous in autoimmune disorders. The expression of HLA-G on tissue or peripheral blood cells, the levels of soluble HLA-G and polymorphic sites along the gene have been studied in several disorders. In this study, we revised the role of the molecule and polymorphic sites along the HLA-G gene in tumors, viral hepatitis, and parasitic disorders. Overall, several lines of evidence clearly show that the induction of HLA-G expression in tumors has been associated with worse disease outcome and disease spread. In addition, the few studies conducted on hepatitis and parasitic disorders indicate that HLA-G may contribute to disease pathogenesis. Few isolated polymorphic sites, primarily located at the coding or 3′ untranslated HLA-G region, have been evaluated in these disorders, and a complete HLA-G typing together with the study of gene regulatory elements may further help on the understanding of the influence of the genetic background on disease susceptibility.

Human leucocyte antigen-G (HLA-G) and its murine functional homolog Qa2 in the Trypanosoma cruzi Infection

Genetic susceptibility factors, parasite strain, and an adequate modulation of the immune system seem to be crucial for disease progression after Trypanosoma cruzi infection. HLA-G and its murine functional homolog Qa2 have well-recognized immunomodulatory properties. We evaluated the HLA-G 3′ untranslated region (3′UTR) polymorphic sites (associated with mRNA stability and target for microRNA binding) and HLA-G tissue expression (heart, colon, and esophagus) in patients presenting Chagas disease, stratified according to the major clinical variants. Further, we investigated the transcriptional levels of Qa2 and other pro- and anti-inflammatory genes in affected mouse tissues during T. cruzi experimental acute and early chronic infection induced by the CL strain. Chagas disease patients exhibited differential HLA-G 3′UTR susceptibility allele/genotype/haplotype patterns, according to the major clinical variant (digestive/cardiac/mixed/indeterminate). HLA-G constitutive expression on cardiac muscle and colonic cells was decreased in Chagasic tissues; however, no difference was observed for Chagasic and non-Chagasic esophagus tissues. The transcriptional levels of Qa2 and other anti and proinflammatory (CTLA-4, PDCD1, IL-10, INF-γ, and NOS-2) genes were induced only during the acute T. cruzi infection in BALB/c and C57BL/6 mice. We present several lines of evidence indicating the role of immunomodulatory genes and molecules in human and experimental T. cruzi infection.