Maturation of antigen-presenting cells is compromised in HLA-G transgenic mice

Evaluation of HLA-G 14bp Ins/Del and +3142 C/G Polymorphisms in Type 1 Diabetes among South Indian Population

Background

Type 1 diabetes (T1D) is a multifactorial autoimmune disease, involving strong genetic components with familial predisposition. Human leukocyte antigen-G (HLA-G) is a non-classical HLA-class I molecule having several immunomodulatory functions. Polymorphisms in HLA-G are associated with several autoimmune diseases including T1D. This study aims to evaluate the association of HLA-G 14bp Ins/Del and +3142 C/G polymorphisms with T1D among the South Indian population.

Methods

The study was performed in a cohort of 123 T1D patients along with their 51 siblings and 126 parents. The association and linkage of HLA-G 14bp Ins/Del and +3142 C/G polymorphisms with T1D were analysed, and transmission disequilibrium test (TDT) was performed.

Results

Significantly increased frequencies of HLA-G 14bp Del/Del genotype (OR = 2.16, pc = 0.0302) and Del allele (OR = 1.71, pc = 0.0398) were observed in female patients compared to parents. Higher frequencies of DelDel/GG combined genotype (OR = 4.45, pc = 0.0049) and Del/G haplotype (OR = 2.91, pc = 0.0277) were observed in female patients compared to parents. TDT also revealed over-transmission of Del/G haplotype (25T vs 7UT; P = 0.0015) and a strong linkage disequilibrium between the studied polymorphisms.

Conclusion

This familial study shows the association of HLA-G 3'UTR 14bp Ins/Del polymorphism with the risk of T1D among the South Indian population, especially in females.

Leukocyte immunoglobulin-like receptor subfamily B: A novel immune checkpoint molecule at the maternal-fetal interface

Due to their crucial roles in embryo implantation, maternal-fetal tolerance induction, and pregnancy progression, immune checkpoint molecules (ICMs), such as programmed cell death-1, cytotoxic T-lymphocyte antigen 4, and T cell immunoglobulin mucin 3, are considered potential targets for clinical intervention in pregnancy complications. Despite the considerable progress on these molecules, our understanding of ICMs at the maternal-fetal interface is still limited. Identification of alternative and novel ICMs and the combination of multiple ICMs is urgently needed for deeply understanding the mechanism of maternal-fetal tolerance and to discover the causes of pregnancy complications. Leukocyte immunoglobulin-like receptor subfamily B (LILRB) is a novel class of ICMs with strong negative regulatory effects on the immune response. Recent studies have revealed that LILRB is enriched in decidual immune cells and stromal cells at the maternal-fetal interface, which can modulate the biological behavior of immune cells and promote immune tolerance. In this review, we introduce the structural features, expression profiles, ligands, and orthologs of LILRB. In addition, the potential mechanisms and functions mediated by LILRB for sustaining the maternal-fetal tolerance microenvironment, remodeling the uterine spiral artery, and induction of pregnancy immune memory are summarized. We have also provided new suggestions for further understanding the roles of LILRB and potential therapeutic strategies for pregnancy-related diseases.

Investigating Serum sHLA-G Cooperation With MRI Activity and Disease-Modifying Treatment Outcome in Relapsing-Remitting Multiple Sclerosis

Relapsing-remitting multiple sclerosis (RRMS) is a demyelinating disease in which pathogenesis T cells have a major role. Despite the unknown etiology, several risk factors have been described, including a strong association with human leukocyte antigen (HLA) genes. Recent findings showed that HLA class I-G (HLA-G) may be tolerogenic in MS, but further insights are required. To deepen the HLA-G role in MS inflammation, we measured soluble HLA-G (sHLA-G) and cytokines serum level in 27 patients with RRMS at baseline and after 12 and 24 months of natalizumab (NTZ) treatment. Patients were divided into high (sHLA-G>20 ng/ml), medium (sHLA-G between 10 and 20 ng/ml), and low (sHLA-G <10 ng/ml) producers. Results showed a heterogeneous distribution of genotypes among producers, with no significant differences between groups. A significant decrease of sHLA-G was found after 24 months of NTZ in low producers carrying the +3142 C/G genotype. Finally, 83.3% of high and 100% of medium producers were MRI-activity free after 24 months of treatment, compared to 63.5% of low producers. Of note, we did not find any correlation of sHLA-G with peripheral cell counts or cytokines level. These findings suggest that serum sHLA-G level may partly depend on genotype rather than peripheral inflammation, and that may have impacted on MRI activity of patients over treatment.

HLA-G 14bp Ins/Del Polymorphism, Plasma Level of Soluble HLA-G, and Association with IL-6/IL-10 Ratio and Survival of Glioma Patients

HLA-G is an immune checkpoint molecule with immunosuppressive and anti-inflammatory activities, and its expression and level of its soluble form (sHLA-G) may play an important role in tumor prognosis. The HLA-G 14bp ins/del polymorphism and the plasma level of soluble HLA-G (sHLA-G) were investigated by a polymerase chain reaction and ELISA, respectively, in 59 glioma patients. A significantly higher proportion of glioma patients had the 14 nt insert in both homozygous and heterozygous states compared to the control group. Glioma patients also had higher plasma levels of sHLA-G. Patients with methylated MGMT promoters had lower levels of sHLA-G than those with unmethylated MGMT promoters. The level of sHLA-G negatively correlated with the overall survival of patients. Glioblastoma patients who survived more than one year after diagnosis had lower levels of sHLA-G than those surviving less than one year. Patients with sHLA-G levels below the cut-off value of 40 U/mL survived significantly longer than patients with sHLA-G levels above 40 U/mL. The levels of sHLA-G were also negatively correlated with the level of IL-6 (p = 0.0004) and positively with IL-10/IL-6 (p = 0.046). Conclusion: The presence of the 14 nt insert in both homozygous and heterozygous states of the HLA-G 14bp ins/del polymorphism is more frequent in glioma patients and the elevated plasma levels of sHLA-G are negatively associated with their survival.

Functional evaluation of immunoregulatory molecules HLA-G, galectin-1, and IL-10 in people living with HIV

OBJECTIVES

Investigate polymorphisms and expressions of human leukocyte antigen-G (HLA-G), galectin-1 (Gal-1), and interleukin-10 (IL-10) in people living with HIV (PLHIV) with and without comorbidities to help understanding the mechanisms involved in triggering these disorders in PLHIV and in their prognosis.

DESIGN

Here we evaluated the potential correlation between the genetic polymorphism and/or protein levels of HLA-G, Gal-1, and IL-10 with and without comorbidities of PLHIV.

METHODS

Two hundred HIV patients under antiretroviral treatment (83 with comorbidities and 117 without comorbidities) and 200 healthy individuals (controls) were genotyped, using PCR, for HLA-G 14-base pair polymorphism located at the 3' untranslated region in exon 8 insertion/insertion (Ins/Ins: low HLA-G expression) or deletion/deletion (Del/Del: high HLA-G expression). Soluble levels of HLA-G (sHLA-G), Gal-1, and IL-10 were quantified by enzyme-linked immunosorbet assay.

RESULTS

HIV patients without comorbidities exhibited higher frequency of 14-base pair Del/Del genotype than HIV patients with comorbidities. As expected, HIV patients Ins/Ins with and without comorbidities produced less sHLA-G than controls. However, HIV patients Del/Del with comorbidities expressed sHLA-G more than controls and HIV patients Del/Del without comorbidities. Interestingly, patients that showed low levels sHLA-G, and presence of comorbidities, exhibited high Gal-1 serum levels. However, an increase in soluble levels of IL-10 in PLHIV was observed when compared to controls, especially in the PLHIV group without comorbidities suggesting, a protective role of IL-10 in the development of comorbidities.

CONCLUSIONS

These data suggested that the high expression of sHLA-G and IL-10 or Gal-1 could be associated and could be associated with the development or not of comorbidities in PLHIV.

Skin Immunity and Tolerance: Focus on Epidermal Keratinocytes Expressing HLA-G

Although the role of epidermal cells in skin regeneration has been extensively documented, their functions in immunity and tolerance mechanisms are largely underestimated. The aim of the present review was to outline the state of knowledge on resident immune cells of hematopoietic origin hosted in the epidermis, and then to focus on the involvement of keratinocytes in the complex skin immune networks acting in homeostasis and regeneration conditions. Based on this knowledge, the mechanisms of immune tolerance are reviewed. In particular, strategies based on immunosuppression mediated by HLA-G are highlighted, as recent advances in this field open up perspectives in epidermis-substitute bioengineering for temporary and permanent skin replacement strategies.

Structural Characterization of the Interaction of Hypoxia Inducible Factor-1 with Its Hypoxia Responsive Element at the -964G > A Variation Site of the HLA-G Promoter Region

Human Antigen Leukocyte-G (HLA-G) gene encodes an immune checkpoint molecule that has restricted tissue expression in physiological conditions; however, the gene may be induced in hypoxic conditions by the interaction with the hypoxia inducible factor-1 (HIF1). Hypoxia regulatory elements (HRE) located at the HLA-G promoter region and at exon 2 are the major HIF1 target sites. Since the G allele of the −964G > A transversion induces higher HLA-G expression when compared to the A allele in hypoxic conditions, here we analyzed HIF1-HRE complex interaction at the pair-atom level considering both −964G > A polymorphism alleles. Mouse HIF2 dimer crystal (Protein Data Bank ID: 4ZPK) was used as template to perform homology modelling of human HIF1 quaternary structure using MODELLER v9.14. Two 3D DNA structures were built from 5′GCRTG’3 HRE sequence containing the −964G/A alleles using x3DNA. Protein-DNA docking was performed using the HADDOCK v2.4 server, and non-covalent bonds were computed by DNAproDB server. Molecular dynamic simulation was carried out per 200 ns, using Gromacs v.2019. HIF1 binding in the HRE containing −964G allele results in more hydrogen bonds and van der Waals contact formation than HRE with −964A allele. Protein-DNA complex trajectory analysis revealed that HIF1-HRE-964G complex is more stable. In conclusion, HIF1 binds in a more stable and specific manner at the HRE with G allele.

Human Keratinocytes Inhibit CD4+ T-Cell Proliferation through TGFB1 Secretion and Surface Expression of HLA-G1 and PD-L1 Immune Checkpoints

Human skin protects the body against infection and injury. This protection involves immune and epithelial cells, but their interactions remain largely unknown. Here, we show that cultured epidermal keratinocytes inhibit allogenic CD4⁺ T-cell proliferation under both normal and inflammatory conditions. Inhibition occurs through the secretion of soluble factors, including TGFB1 and the cell-surface expression of HLA-G1 and PD-L1 immune checkpoints. For the first time, we here describe the expression of the HLA-G1 protein in healthy human skin and its role in keratinocyte-driven tissue immunomodulation. The overexpression of HLA-G1 with an inducible vector increased the immunosuppressive properties of keratinocytes, opening up perspectives for their use in allogeneic settings for cell therapy.

Growth Hormone Stops Excessive Inflammation After Partial Hepatectomy, Allowing Liver Regeneration and Survival Through Induction of H2-Bl/HLA-G

BACKGROUND AND AIMS

Growth hormone (GH) is important for liver regeneration after partial hepatectomy (PHx). We investigated this process in C57BL/6 mice that express different forms of the GH receptor (GHR) with deletions in key signaling domains.

APPROACH AND RESULTS

PHx was performed on C57BL/6 mice lacking GHR (Ghr^-/- ), disabled for all GH-dependent Janus kinase 2 signaling (Box1^-/- ), or lacking only GH-dependent signal transducer and activator of transcription 5 (STAT5) signaling (Ghr391^-/- ), and wild-type littermates. C57BL/6 Ghr^-/- mice showed striking mortality within 48 hours after PHx, whereas Box1^-/- or Ghr391^-/- mice survived with normal liver regeneration. Ghr^-/- mortality was associated with increased apoptosis and elevated natural killer/natural killer T cell and macrophage cell markers. We identified H2-Bl, a key immunotolerance protein, which is up-regulated by PHx through a GH-mediated, Janus kinase 2-independent, SRC family kinase-dependent pathway. GH treatment was confirmed to up-regulate expression of the human homolog of H2-Bl (human leukocyte antigen G [HLA-G]) in primary human hepatocytes and in the serum of GH-deficient patients. We find that injury-associated innate immune attack by natural killer/natural killer T cell and macrophage cells are instrumental in the failure of liver regeneration, and this can be overcome in Ghr^-/- mice by adenoviral delivery of H2-Bl or by infusion of HLA-G protein. Further, H2-Bl knockdown in wild-type C57BL/6 mice showed elevated markers of inflammation after PHx, whereas Ghr^-/- backcrossed on a strain with high endogenous H2-Bl expression showed a high rate of survival following PHx.

CONCLUSIONS

GH induction of H2-Bl expression is crucial for reducing innate immune-mediated apoptosis and promoting survival after PHx in C57BL/6 mice. Treatment with HLA-G may lead to improved clinical outcomes following liver surgery or transplantation.

Inhibition of iNKT Cells by the HLA-G-ILT2 Checkpoint and Poor Stimulation by HLA-G-Expressing Tolerogenic DC

Invariant Natural Killer T (iNKT) cells are a small and distinct population of T cells crucial in immunomodulation. After activation by alpha-GalactosylCeramide (αGC), an exogenic glycolipid antigen, iNKT cells can rapidly release cytokines to enhance specific anti-tumor activity. Several human clinical trials on iNKT cell-based anti-cancer are ongoing, however results are not as striking as in murine models. Given that iNKT-based immunotherapies are dependent mainly on antigen-presenting cells (APC), a human tolerogenic molecule with no murine homolog, such as Human Leucocyte Antigen G (HLA-G), could contribute to this discrepancy. HLA-G is a well-known immune checkpoint molecule involved in fetal-maternal tolerance and in tumor immune escape. HLA-G exerts its immunomodulatory functions through the interaction with immune inhibitory receptors such as ILT2, differentially expressed on immune cell subsets. We hypothesized that HLA-G might inhibit iNKT function directly or by inducing tolerogenic APC leading to iNKT cell anergy, which could impact the results of current clinical trials. Using an ILT2-transduced murine iNKT cell line and human iNKT cells, we demonstrate that iNKT cells are sensitive to HLA-G, which inhibits their cytokine secretion. Furthermore, human HLA-G⁺ dendritic cells, called DC-10, failed at inducing iNKT cell activation compared to their autologous HLA-G^‒ DCs counterparts. Our data show for the first time that the HLA-G/ILT2 ICP is involved in iNKT cell function modulation.

High plasma soluble levels of the immune checkpoint HLA-G molecule among bodybuilders

Introduction

Studies report that intense physical activity influences the down-regulation of immune function in athletes as well as the interaction between adipose tissue and the immune system.

Aim

This study aimed to compare the plasma soluble levels of the immune checkpoint HLA-G (sHLA-G) molecule with the fat mass and muscle mass index among 77 bodybuilders and 64 controls.

Results

The comparisons of the percentage of body fat (%BF) revealed that the groups of male and female bodybuilders showed a statistically significant reduction in the percentage of body fat when compared to their control group, (P <0.0001, for both comparisons). Regarding sHLA-G levels, the comparisons showed that the group of male bodybuilders had significantly higher sHLA-G levels compared to the group of female bodybuilders (P = 0.0011).

Conclusion

Our results showed that in bodybuilders with less body fat, the systemic levels of soluble HLA-G, an immunological molecule with recognized immunosuppressive function, are significantly higher and suggest that this immune mechanism may corroborate the immunosuppressive state in athletes undergoing intense and prolonged physical training.

Cytokine stimulation of the choriocarcinoma cell line JEG-3 leads to alterations in the HLA-G expression profile

The checkpoint molecule human leukocyte antigen (HLA)-G has restricted tissue expression, and plays a role in the establishment of maternal tolerance to the semi-allogenic fetus during pregnancy by expression on the trophoblast cells in the placenta. HLA-G exists in at least seven well-described mRNA isoforms, of which four are membrane-bound and three soluble. Regulation of the tissue expression of HLA-G and its isoforms is relatively unknown. Therefore, it is important to understand the regulation of HLA-G, and the HLA-G⁺ choriocarcinoma cell line JEG-3 is a widely used cellular model. We hypothesized that cytokines present in the microenvironment can regulate the HLA-G expression profile. In the present study, we systematically stimulated JEG-3 cells with various concentrations of IL-2, IL-4 IL-6, IL-10, IL-12, IL-15, IL-17A, TGF-β₁, TNF-α and IFN-γ1b. The results suggest that IFN-γ plays a role in maintenance of HLA-G expression, while IL-10 might be involved in regulation of the isoform profile.

The Tolerogenic Function of Regulatory T Cells in Pregnancy and Cancer

Regulatory T cells, a subpopulation of suppressive T cells, are potent mediators of self-tolerance and essential for the suppression of triggered immune responses. The immune modulating capacity of these cells play a major role in both transplantation, autoimmune disease, allergy, cancer and pregnancy. During pregnancy, low numbers of regulatory T cells are associated with pregnancy failure and pregnancy complications such as pre-eclampsia. On the other hand, in cancer, low numbers of immunosuppressive T cells are correlated with better prognosis. Hence, maternal immune tolerance toward the fetus during pregnancy and the escape from host immunosurveillance by cancer seem to be based on similar immunological mechanisms being highly dependent on the balance between immune activation and suppression. As regulatory T cells hold a crucial role in several biological processes, they may also be promising subjects for therapeutic use. Especially in the field of cancer, cell therapy and checkpoint inhibitors have demonstrated that immune-based therapies have a very promising potential in treatment of human malignancies. However, these therapies are often accompanied by adverse autoimmune side effects. Therefore, expanding the knowledge to recognize the complexities of immune regulation pathways shared across different immunological scenarios is extremely important in order to improve and develop new strategies for immune-based therapy. The intent of this review is to highlight the functional characteristics of regulatory T cells in the context of mechanisms of immune regulation in pregnancy and cancer, and how manipulation of these mechanisms potentially may improve therapeutic options.

Association of HLA-G 3'UTR polymorphisms and haplotypes with severe sepsis in a Brazilian population

BACKGROUND

The human leukocyte antigen G (HLA-G) is a molecule involved in immune system modulation, acting in the maintenance of a state of immune tolerance. Some polymorphisms in the HLA-G gene 3' untranslated region (3'UTR) were associated to distinct levels of HLA-G expression and to sepsis development. In the present study, haplotypes and polymorphisms of the HLA-G 3'UTR were analyzed in Brazilian septic patients.

METHODS

The HLA-G 3'UTR was amplified by PCR, sequenced and eight polymorphisms were genotyped (the 14bp insertion/deletion, +3003T/C, +3010C/G, +3027A/C, +3035C/T, +3142G/C, +3187A/G and+3196C/G) in DNA samples from septic patients (with severe sepsis or septic shock) and controls. The haplotypes were inferred and association tests were performed through Chi square test and binary logistic regression.

RESULTS

The+3027AC genotype was associated asa risk factor to sepsis development (OR 3.17, P_Bonferroni 0.048). Further, the presence of the UTR-7 haplotype (OR 2.97, P_Bonferroni 0.018), and of 14bp-Ins_+3142G_+3187A haplotype (OR 2.39, P_Bonferroni 0.045) were associated with sepsis, conferring susceptibility.

CONCLUSION

Our data confirm an important role of HLA-G 3'UTR polymorphisms in the development of severe forms of sepsis (severe sepsis and septic shock). The genotyping of HLA-G genetic variants and haplotypes could be useful as a prediction tool of increased risk to severe sepsis.

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.

Therapeutic application of human leukocyte antigen-G1 improves atopic dermatitis-like skin lesions in mice

Human leukocyte antigen (HLA)-G is an immune checkpoint molecule that plays critical roles in immune response and in triggering inhibitory signaling to immune cells such as T cells, natural killer cells, and antigen-presenting cells. Thus, the application of HLA-G can be considered for treating immune response-related inflammatory disorders. We have previously reported that treatment with HLA-G1 and HLA-G2 ameliorates the joint swelling associated with collagen-induced arthritis of DBA/1 mice, an animal model for rheumatoid arthritis. In this study, we further investigated the effects of HLA-G1 on atopic dermatitis (AD), the most common inflammatory skin disorder. AD-like lesions were induced with the extract of the house dust mite Dermatophagoides farinae in NC/Nga mice. Continuous administration of HLA-G1 ameliorated the AD-like skin lesions in the mice. Furthermore, production of immunoglobulin E, interleukin (IL)-13, and IL-17A was significantly reduced in HLA-G1-treated mice, suggesting a Th2/Th17-mediated immune-inhibitory function of HLA-G1 in vivo. Our studies shed light on novel therapeutic strategies with recombinant HLA-G proteins for immune reaction-mediated chronic inflammatory disorders.

Immunological Properties of Corneal Epithelial-Like Cells Derived from Human Embryonic Stem Cells

Transplantation of ex vivo expanded corneal limbal stem cells (LSCs) has been the main treatment for limbal stem cell deficiency, although the shortage of donor corneal tissues remains a major concern for its wide application. Due to the development of tissue engineering, embryonic stem cells (ESCs)-derived corneal epithelial-like cells (ESC-CECs) become a new direction for this issue. However, the immunogenicity of ESC-CECs is a critical matter to be solved. In the present study, we explored the immunological properties of ESC-CECs, which were differentiated from ESCs. The results showed that ESC-CECs had a similar character and function with LSCs both in vitro and in vivo. In ESC-CECs, a large number of genes related with immune response were down-regulated. The expressions of MHC-I, MHC-II, and co-stimulatory molecules were low, but the expression of HLA-G was high. The ESC-CECs were less responsible for T cell proliferation and NK cell lysis in vitro, and there was less immune cell infiltration after transplantation in vivo compared with LSCs. Moreover, the immunological properties were not affected by interferon-γ. All these results indicated a low immunogenicity of ESC-CECs, and they can be promising in clinical use.

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.

HLA-G expression levels influence the tolerogenic activity of human DC-10

Human leukocyte antigen (HLA)-G is a non-classical HLA class I molecule with known immune-modulatory functions. Our group identified a subset of human dendritic cells, named DC-10, that induce adaptive interleukin-10-producing T regulatory type 1 (Tr1) cells via the interleukin-10-dependent HLA-G/ILT4 pathway. In this study we aimed at defining the role of HLA-G in DC-10-mediated Tr1 cell differentiation. We analyzed phenotype, functions, and genetic variations in the 3' untranslated region of the HLA-G locus of in vitro-differentiated DC-10 from 67 healthy donors. We showed that HLA-G expression on DC-10 is donor-dependent. Functional studies demonstrated that DC-10, independently of HLA-G expression, secrete interleukin-10 and negligible levels of interleukin-12. Interestingly, DC-10 with high HLA-G promote allo-specific anergic T cells that contain a significantly higher frequency of Tr1 cells, defined as interleukin-10-producing (P=0.0121) or CD49b(+)LAG-3(+) (P=0.0031) T cells, compared to DC-10 with low HLA-G. We found that the HLA-G expression on DC-10 is genetically imprinted, being associated with specific variations in the 3' untranslated region of the gene, and it may be finely tuned by microRNA-mediated post-transcriptional regulation. These data highlight the important role of HLA-G in boosting DC-10 tolerogenic activity and confirm that interleukin-10 production by DC-10 is necessary but not sufficient to promote Tr1 cells at high frequency. These new insights into the role of HLA-G in DC-10-mediated induction of Tr1 cells provide additional information for clinical use in Tr1- or DC-10-based cell therapy approaches.

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.

HLA-G Molecules in Autoimmune Diseases and Infections

Human leukocyte antigen (HLA)-G molecule, a non-classical HLA-Ib molecule, is less polymorphic when compared to classical HLA class I molecules. Human leukocyte antigen-G (HLA-G) was first detected on cytotrophoblast cells at the feto-maternal interface but its expression is prevalent during viral infections and several autoimmune diseases. HLA-G gene is characterized by polymorphisms at the 3' un-translated region and 5' upstream regulatory region that regulate its expression and are associated with autoimmune diseases and viral infection susceptibility, creating an unbalanced and pathologic environment. This review focuses on the role of HLA-G genetic polymorphisms, mRNA, and protein expression in autoimmune conditions and viral infections.

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.

Insights into HLA-G Genetics Provided by Worldwide Haplotype Diversity

Human leukocyte antigen G (HLA-G) belongs to the family of non-classical HLA class I genes, located within the major histocompatibility complex (MHC). HLA-G has been the target of most recent research regarding the function of class I non-classical genes. The main features that distinguish HLA-G from classical class I genes are (a) limited protein variability, (b) alternative splicing generating several membrane bound and soluble isoforms, (c) short cytoplasmic tail, (d) modulation of immune response (immune tolerance), and (e) restricted expression to certain tissues. In the present work, we describe the HLA-G gene structure and address the HLA-G variability and haplotype diversity among several populations around the world, considering each of its major segments [promoter, coding, and 3′ untranslated region (UTR)]. For this purpose, we developed a pipeline to reevaluate the 1000Genomes data and recover miscalled or missing genotypes and haplotypes. It became clear that the overall structure of the HLA-G molecule has been maintained during the evolutionary process and that most of the variation sites found in the HLA-G coding region are either coding synonymous or intronic mutations. In addition, only a few frequent and divergent extended haplotypes are found when the promoter, coding, and 3′UTRs are evaluated together. The divergence is particularly evident for the regulatory regions. The population comparisons confirmed that most of the HLA-G variability has originated before human dispersion from Africa and that the allele and haplotype frequencies have probably been shaped by strong selective pressures.

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.

New insights into HLA-G mediated tolerance

Human Leukocyte Antigen G (HLA-G) is a nonclassical HLA class I molecule with well-characterized immunomodulatory activities. HLA-G was first described as a regulatory molecule that allows the fetus to elude the maternal immune response. In the last decade it has become evident that HLA-G is involved in modulating both innate and adaptive immune responses, in maintaining tolerance in autoimmune and inflammatory diseases and after transplantation, and in promoting immune escape in cancer and infectious diseases. HLA-G exerts its modulatory/regulatory functions directly by interacting with specific inhibitory receptors. The expression of HLA-G is finely tuned by genetic variations in the noncoding region of the locus. The recent discovery of dendritic cells-10 (DC-10) as naturally occurring HLA-G-expressing dendritic cells opens new perspectives in the identification of the molecular and cellular mechanisms underlying HLA-G-mediated tolerance. An overview on the HLA-G-mediated inhibition of innate and adaptive immune cells, on the genetic influence on HLA-G expression, and on HLA-G-expressing DC-10 is presented. Moreover, we discuss the central and critical role of DC-10 in the HLA-G-mediated tolerance.

Immunomodulatory properties of HLA-G in infectious diseases

HLA-G is a nonclassical major histocompatibility complex molecule first described at the maternal-fetal interface, on extravillous cytotrophoblasts. Its expression is restricted to some tissues in normal conditions but increases strongly in pathological conditions. The expression of this molecule has been studied in detail in cancers and is now also beginning to be described in infectious diseases. The relevance of studies on HLA-G expression lies in the well known inhibitory effect of this molecule on all cell types involved in innate and adaptive immunity, favoring escape from immune control. In this review, we summarize the features of HLA-G expression by type of infections (i.e, bacterial, viral, or parasitic) detailing the state of knowledge for each pathogenic agent. The polymorphism, the interference of viral proteins with HLA-G intracellular trafficking, and various cytokines have been described to modulate HLA-G expression during infections. We also discuss the cellular source of HLA-G, according to the type of infection and the potential role of HLA-G. New therapeutic approaches based on synthetic HLA-G-derived proteins or antibodies are emerging in mouse models of cancer or transplantation, and these new therapeutic tools may eventually prove useful for the treatment of infectious diseases.

In vivo evidence that secretion of HLA-G by immunogenic tumor cells allows their evasion from immunosurveillance

Human leukocyte antigen-G (HLA-G) expression by tumors has been evidenced in numerous malignancies in association with poor prognosis and resistance to immunotherapy in humans. Particularly, soluble form of HLA-G was measured at high concentrations in malignant effusions and plasma from cancer patients, and inhibits antitumor immune cells in vitro through interaction with immunoglobulin-like transcript (ILT) receptors. Nevertheless, in vivo study demonstrating that HLA-G secretion by tumor cells allows their escape from immunosurveillance remained to be established. Despite nondescribed murine homolog, direct functional interaction of HLA-G with murine paired immunoglobulin-like receptor (PIR)-B, ortholog of human ILT receptors, enables to investigate its role in vivo. Immunocompetent mice were injected either with syngeneic tumor cells co-expressing HLA-G5, the main soluble HLA-G isoform, and the conformation stabilizer human β2-microglubulin (hβ2m), or with hβ2m+ HLA-G5- tumor cells. hβ2m expressed at both tumor cell surface acted as a tumor antigen triggering a specific humoral response. Interestingly, although hβ2m+ HLA-G5- tumors were rejected, secreted HLA-G5 provided hβ2m+ HLA-G5+ tumors a protection against hβ2m-elicited immune rejection, enabling such immunogenic tumors to grow similarly to a poorly immunogenic tumor. HLA-G5 tumor expression was associated with local and peripheral immunosuppression, characterized by dampened anti-hβ2m B-cell response, quantitative and functional T-and B-cell defects, accumulation of myeloid-derived suppressor cells able to inhibit T-cell proliferation and reduced T- and B-cell tumor infiltrate. Our study provides the first in vivo proof that soluble HLA-G counteracts tumor rejection and reinforces the importance to consider HLA-G as a promising target to optimize current cancer immunotherapies.

HLA-G +3142 polymorphism as a susceptibility marker in two rheumatoid arthritis populations in Brazil

In this study, we sought to investigate the genetic influence of two HLA-G 3'-untranslated region (3'-UTR) polymorphisms - 14 bp (rs66554220) and +3142C>G (rs1063320) and their compounding haplotypes in susceptibility to rheumatoid arthritis (RA) in a two-region Brazilian study comprising of 539 patients and 489 controls. All subjects were polymerase chain reaction (PCR) genotyped for the referred polymorphisms and logistic regression models controlling for sex, city and age were performed. Homozygozity for the +3142G allele was associated with an increased risk of RA [odds ratio (OR) = 1.45, 95% confidence interval (CI) = 1.075-1.959, P(Bonf)  = 0.030], whereas no association was observed for the 14 bp polymorphism. Haplotype comparisons between patients and controls showed a decreased frequency of the delC haplotype in patients (OR = 0.70, 95% CI = 0.521-0.946, P(Bonf)  = 0.040), which remained significant in the rheumatoid factor (RF)-positive group (OR = 0.66, 95% CI = 0.482-0.900, P(Bonf)  = 0.018), but not in the RF-negative group. These results corroborate the hypothesis of an involvement of HLA-G in the susceptibility of RA. The +3142G allele is associated with haplotype lineages that share high identity and are regarded as low producers. The presence of the G allele in homozygosis could be responsible for a low HLA-G expression profile that could favor the triggering of RA.

Mutated HLA-G3 localizes to the cell surface but does not inhibit cytotoxicity of natural killer cells

HLA-G plays an important role in the induction of immune tolerance. Various attempts to produce good manufacturing practice levels of HLA-G as a therapeutic molecule have failed to date partly due to the complicated structure of full-length HLA-G1. Truncated HLA-G3 is simpler and easier to produce than HLA-G1 and contains the expected functional epitope in its only α1 monomorphic domain. In this study, we engineered the ER retrieval and retention signal on HLA-G3's cytoplasmic tail by replacing its RKKSSD motif with RAASSD. We observed that mutated HLA-G3 was highly expressed on the cell surface of transduced K562 cells but did not inhibit cytotoxicity of natural killer cells.

New evidence for balancing selection at the HLA-G locus in South Amerindians

HLA-G is a non-classical HLA (Human Leukocyte Antigen) molecule characterized by limited tissue distribution under normal physiological conditions and low variability at both DNA and protein levels. Several studies suggest that HLA-G could play a role, as an immunoregulatory molecule, in situations as diverse as transplantation, cancer, viral infections and inflammatory diseases. A total of 237 individuals from 21 South American tribes speaking nine different linguistic families were studied in relation to the 14 bp insertion/deletion polymorphism at the HLA-G gene. A consistent (seven in nine) excess of heterozygosity in samples classified by language was obtained. Our data supply evidences for balancing selection acting at the HLA-G 14 bp INDEL region. Enhanced fetal survival in a pathogen-rich environment may account for these findings.

β2-Microglobulin-free HLA-G activates natural killer cells by increasing cytotoxicity and proinflammatory cytokine production

Human leukocyte antigen-G (HLA-G) is a nonclassical HLA class-I molecule and plays a role in tissue specific immunoregulation. Many studies have addressed functional aspects of β2-microglobulin (β2m)-associated HLA-G1. β2m-free HLA-G has been found in human placental cytotrophoblasts and pancreatic β cells although its function remains unclear. In the present study, we investigated the function of β2m-free HLA-G by transfecting HLA-G1 and -G3 into human β2m deficient rat pancreatic β cell carcinoma (BRIN-BD11) cells. RT-PCR and western blots studies confirmed high expression of HLA-G1 and -G3 in -G1 and -G3 transfectants, respectively. HLA-G1 and -G3 were detected mainly in intracellular compartments of BRIN-BD11 transductants by confocal fluorescent microscopy and flow cytometry. Functional analysis revealed that β2m-free HLA-G promoted xenogeneic cytotoxic lysis of BRIN-BD11 cells by natural killer (NK) cells and increased production of IL-1β, TNF-α, and IFN-γ. Stimulation of cytotoxic lysis was impaired by blocking the MAPK and DNA-PKcs pathways in NK cells. Importantly, treatment with 33mAb, a KLR2DL4 receptor agonist, induced NK-mediated cytotoxic lysis of BRIN-BD11 cells transfected with a mock vector. Our data suggest that β2m-free HLA-G activates NK cells via engagement of KLR2DL4 receptors.

Full-length HLA-G1 and truncated HLA-G3 differentially increase HLA-E surface localization

Human leukocyte antigen (HLA)-E plays a role in immune tolerance induction and its transport to the cell surface is limited and dependent on the availability of HLA class I signal peptide. The role of HLA-G in regulating HLA-E surface localization remains controversial. The aim of our study was to clarify whether full-length and truncated HLA-G isoforms regulate HLA-E surface localization. Using a retroviral expression system and flow cytometric analysis, we found that surface HLA-E levels were significantly higher in HLA-G1 (34.1±4.4%, p<0.005) and -G3 (15.3±1.8%, p<0.04) versus empty vector (9.0±1.0%) transductants. Biotinylation and Western blot studies revealed HLA-E surface protein was increased by 4.5- and 1.3-fold in HLA-G1 and -G3 versus empty vector transductants. Although no significant differences in transcript and protein levels were detected between HLA-G1 and -G3 transductants, surface levels of HLA-G1 were 2.5-fold higher than HLA-G3 by flow cytometric analysis and Western blotting. Taken together, our data demonstrate that full-length HLA-G1 and truncated -G3 differentially increase HLA-E surface localization.

Reassessment of HLA-G isoform specificity of MEM-G/9 and 4H84 monoclonal antibodies

Human leukocyte antigen (HLA)-G is a non-classical HLA class I molecule thought to play a key role in maternal-fetal tolerance. Although initial studies suggested that HLA-G expression is restricted to extravillous cytotrophoblasts, expression was subsequently reported in a wide variety of other human tissues and tumor cells. However, consensus as to the validity of these collective findings has proven difficult because the antibodies used to define the temporal and spatial expression patterns of HLA-G remain incompletely characterized. The aim of our study was to reassess two of the most widely used HLA-G antibodies (MEM-G/9 and 4H84) in HLA-G-positive (JEG-3 and HLA-G transduced) and -negative (dermal fibroblast, mesenchymal stem cell, K562, and Jurkat) lines using flow cytometry, immunofluorescence, and western blotting. We found that MEM-G/9 recognized HLA-G3 by flow cytometry, indicating that its epitope is present on the α1 domain of HLA-G. Although 4H84 preferably recognized unfolded HLA-G-free chains, it showed strong non-specificity under certain methodological conditions.

Identification of a novel HLA-G+ regulatory population in blood: expansion after allogeneic transplantation and de novo HLA-G expression at graft-versus-host disease sites

BACKGROUND

The human leukocyte antigen-G (HLA-G) has been considered to be an important tolerogeneic molecule playing an essential role in maternal-fetal tolerance, which constitutes the perfect example of successful physiological immunotolerance of semi-allografts. In this context, we investigated the putative role of this molecule in the allogeneic hematopoietic cell transplantation setting.

DESIGN AND METHODS

The percentage of HLA-G(+) cells in peripheral blood of healthy donors and allo-transplanted patients was evaluated by flow cytometry. Their immunoregulatory and tolerogeneic properties were investigated in in vitro immunostimulatory and immunosuppression assays. Immunohistochemical analysis for HLA-G expression was performed in skin biopsies from allo-transplanted patients and correlated with the occurrence of graft-versus-host disease.

RESULTS

We identified a CD14(+)HLA-G(pos) population with an HLA-DR(low) phenotype and decreased in vitro immunostimulatory capacity circulating in peripheral blood of healthy individuals. Naturally occurring CD14(+)HLA-G(pos) cells suppressed T-cell responses and exerted an immunotolerogenic action on T cells by rendering them hyporesponsive and immunosuppressive in vitro. After allogeneic hematopoietic cell transplantation, HLA-G(pos) cells increase in blood. Interestingly, besides an increase in CD14(+)HLA-G(pos) cells, there was also a pronounced expansion of CD3(+)HLA-G(pos) cells. Of note, CD3(+)HLA-G(pos) and CD14(+)HLA-G(pos) cells from transplanted patients were suppressive in in vitro lymphoproliferation assays. Furthermore, we found an upregulation of HLA-G expression in skin specimens from transplanted patients that correlated with graft-versus-host disease. Inflammatory cells infiltrating the dermis of transplanted patients were also HLA-G(pos).

CONCLUSIONS

We report the presence of naturally occurring HLA-G(pos) monocytic cells with in vitro suppressive properties. HLA-G expressing regulatory blood cells were found in increased numbers after allogeneic transplantation. Epithelial cells in skin affected by graft-versus-host disease revealed elevated HLA-G expression.

The tolerogenic interplay(s) among HLA-G, myeloid APCs, and regulatory cells

Myeloid antigen-presenting cells (APCs), regulatory cells, and the HLA-G molecule are involved in modulating immune responses and promoting tolerance. APCs are known to induce regulatory cells and to express HLA-G as well as 2 of its receptors; regulatory T cells can express and act through HLA-G; and HLA-G has been directly involved in the generation of regulatory cells. Thus, interplay(s) among HLA-G, APCs, and regulatory cells can be easily envisaged. However, despite a large body of evidence on the tolerogenic properties of HLA-G, APCs, and regulatory cells, little is known on how these tolerogenic players cooperate. In this review, we first focus on key aspects of the individual relationships between HLA-G, myeloid APCs, and regulatory cells. In its second part, we highlight recent work that gathers individual effects and demonstrates how intertwined the HLA-G/myeloid APCs/regulatory cell relationship is.

HLA-G expression is associated with metastasis and poor survival in the Balb/c nu/nu murine tumor model with ovarian cancer

Aberrant HLA-G expression is associated with tumor invasiveness and poor clinical prognosis; however, there is a lack of preclinical animal model to address whether HLA-G plays a causal role in the unfavorable prognosis of malignancies. In the current study, ovarian carcinoma cell lines (HO-8910 and Ovcar-3) were transfected with HLA-G gene. HLA-G expression was analyzed with western blot and flow cytometry. Transwell experiment was performed to analyze the cell migration and invasion capability and/or multicellular spheroid formation was investigated with the 3D culture assay in vitro. The effects of HLA-G expression for tumor cell organ metastasis and for mouse survival was analyzed with the Balb/c nu/nu mouse model. Our data showed that HO-8910-G and Ovcar-3-G cells are of higher invasion potential compared with the parental HO-8910 and Ovcar-3 cells. Multicellular spheroid formation exists only in HO-8910-G cells in a 3D culture assay. In Balb/c nu/nu mouse model, widespread metastasis was observed in mice xenografted with HO-8910-G cells, but not in the group with parental cells. Mouse survival was dramatically decreased in HO-8910-G and Ovcar-3-G xenografted mice than that with HO-8910 and Ovcar-3 cells, respectively. In summary, our study provided the first evidence that HLA-G expression is associated with tumor metastasis and with poor survival in an animal model with ovarian cancer.

Tolerogenic function of dimeric forms of HLA-G recombinant proteins: a comparative study in vivo

HLA-G is a natural tolerogenic molecule involved in the best example of tolerance to foreign tissues there is: the maternal-fetal tolerance. The further involvement of HLA-G in the tolerance of allogeneic transplants has also been demonstrated and some of its mechanisms of action have been elucidated. For these reasons, therapeutic HLA-G molecules for tolerance induction in transplantation are actively investigated. In the present study, we studied the tolerogenic functions of three different HLA-G recombinant proteins: HLA-G heavy chain fused to β2-microglobulin (B2M), HLA-G heavy chain fused to B2M and to the Fc portion of an immunoglobulin, and HLA-G alpha-1 domain either fused to the Fc part of an immunoglobulin or as a synthetic peptide. Our results demonstrate the tolerogenic function of B2M-HLA-G fusion proteins, and especially of B2M-HLA-G5, which were capable of significantly delaying allogeneic skin graft rejection in a murine in vivo transplantation model. The results from our studies suggest that HLA-G recombinant proteins are relevant candidates for tolerance induction in human transplantation.

HLA-G level on monocytoid dendritic cells correlates with regulatory T-cell Foxp3 expression in liver transplant tolerance

BACKGROUND

Human leukocyte antigen (HLA)-G is a nonclassical HLA class I molecule expressed as membrane-bound and soluble isoforms. Interaction of HLA-G with its receptor, immunoglobulin-like transcript 4 on dendritic cells (DCs) down-regulates their T-cell stimulatory ability.

METHODS

We examined expression of HLA-G, immunoglobulin-like transcript 4, other immune regulatory molecules (inducible costimulator ligand and glucocorticoid-induced tumor necrosis factor-related receptor ligand), and the activation marker CMRF44 on circulating monocytoid dendritic cell (mDC) and plasmacytoid dendritic cell by monoclonal antibody staining and flow cytometry. Three groups of stable liver transplant recipients: operationally tolerant (TOL), prospective immunosuppressive drug weaning, and maintenance immunosuppression (MI) were studied, together with healthy controls (HC). Serum HLA-G levels were measured by enzyme-linked immunosorbent assay.

RESULTS

In TOL patients, monocytoid dendritic cell (mDC) but not plasmacytoid dendritic cell expressed higher HLA-G than in MI patients or HC. In TOL patients, the incidence of CD4(+)CD25(hi)CD127(-) regulatory T cells (Treg) and the intensity of Treg forkhead box p3 (Foxp3) expression were significantly higher than in the MI group. HLA-G expression on circulating mDC correlated significantly with that of Foxp3 in the TOL group. There was no correlation between immunosuppressive drug (tacrolimus) dose or trough level and HLA-G expression or Treg frequency or Foxp3 expression. The incidence of patients with circulating HLA-G levels more than 100 ng/mL was highest in the TOL group, although statistical significance was not achieved.

CONCLUSIONS

Higher HLA-G expression on circulating mDC in TOL recipients compared with MI or HC, suggests a possible role of HLA-G in immune regulation possibly mediated by enhanced host Treg Foxp3 expression.

Role of HLA-G in tumor escape through expansion of myeloid-derived suppressor cells and cytokinic balance in favor of Th2 versus Th1/Th17

The expression of HLA-G by malignant cells has been proposed as a tumor escape mechanism from immunosurveillance. However, although the inhibitory effect of HLA-G on antitumoral immune effectors has been documented in vitro, it remains to be resolved in vivo. In this context, the development of an animal model is now a priority to establish the proof of concept that an HLA-G(+) tumor cell develops and tolerizes the host antitumor immune response in vivo. In the present study, we provide the first in vivo evidence of such a role by a xenotumor model in mice based on the interactions between human HLA-G and the murine paired immunoglobulin-like receptor-B (PIR-B). We demonstrate that human tumor cells expressing HLA-G grow in an immunocompetent host by affecting both innate and adaptive immunity. Expansion of blood myeloid-derived CD11b(+)Gr1(+)PIR-B(+) suppressor cells, loss of peripheral T cells, and cytokinic balance in favor of Th2 versus Th1/Th17 constitute the main mechanisms by which HLA-G promotes tumor expansion. These data demonstrate for the first time that HLA-G plays a crucial role in in vivo tumor evasion. Finally, blocking HLA-G function by a specific Ab inhibits the in vivo development of the tumor, offering a new innovative therapeutic strategy in cancer.

Emerging topics and new perspectives on HLA-G

Following the Fifth International Conference on non-classical HLA-G antigens (HLA-G), held in Paris in July 2009, we selected some topics which focus on emerging aspects in the setting of HLA-G functions. In particular, HLA-G molecules could play a role in: (1) various inflammatory disorders, such as multiple sclerosis, intracerebral hemorrhage, gastrointestinal, skin and rheumatic diseases, and asthma, where they may act as immunoregulatory factors; (2) the mechanisms to escape immune surveillance utilized by several viruses, such as human cytomegalovirus, herpes simplex virus type 1, rabies virus, hepatitis C virus, influenza virus type A and human immunodeficiency virus 1 (HIV-1); and (3) cytokine/chemokine network and stem cell transplantation, since they seem to modulate cell migration by the downregulation of chemokine receptor expression and mesenchymal stem cell activity blocking of effector cell functions and the generation of regulatory T cells. However, the immunomodulatory circuits mediated by HLA-G proteins still remain to be clarified.

HLA-G in organ transplantation: towards clinical applications

HLA-G plays a particular role during pregnancy in which its expression at the feto-maternal barrier participates into the tolerance of the allogenic foetus. HLA-G has also been demonstrated to be expressed in some transplanted patients, suggesting that it regulates the allogenic response. In vitro data indicate that HLA-G modulates NK cells, T cells, and DC maturation through its interactions with various inhibitory receptors. In this paper, we will review the data reporting the HLA-G involvement of HLA-G in human organ transplantation, then factors that can modulate HLA-G, and finally the use of HLA-G as a therapeutic tool in organ transplantation.

The role of HLA-G in immunity and hematopoiesis

The non-classical HLA class I molecule HLA-G was initially shown to play a major role in feto-maternal tolerance. Since this discovery, it has been established that HLA-G is a tolerogenic molecule which participates to the control of the immune response. In this review, we summarize the recent advances on (1) the multiple structures of HLA-G, which are closely associated with their role in the inhibition of NK cell cytotoxicity, (2) the factors that regulate the expression of HLA-G and its receptors, (3) the mechanism of action of HLA-G at the immunological synapse and through trogocytosis, and (4) the generation of suppressive cells through HLA-G. Moreover, we also review recent findings on the non-immunological functions of HLA-G in erythropoiesis and angiogenesis.

Peroxynitrite-dependent killing of cancer cells and presentation of released tumor antigens by activated dendritic cells

Dendritic cells (DCs), essential for the initiation and regulation of adaptive immune responses, have been used as anticancer vaccines. DCs may also directly trigger tumor cell death. In the current study, we have investigated the tumoricidal and immunostimulatory activities of mouse bone marrow-derived DCs. Our results indicate that these cells acquire killing capabilities toward tumor cells only when activated with LPS or Pam3Cys-SK4. Using different transgenic mouse models including inducible NO synthase or GP91 knockout mice, we have further established that LPS- or Pam3Cys-SK4-activated DC killing activity involves peroxynitrites. Importantly, after killing of cancer cells, DCs are capable of engulfing dead tumor cell fragments and of presenting tumor Ags to specific T lymphocytes. Thus, upon specific stimulation, mouse bone marrow-derived DCs can directly kill tumor cells through a novel peroxynitrite-dependent mechanism and participate at virtually all levels of antitumor immune responses, which reinforces their interest in immunotherapy.

Recent advances on the non-classical major histocompatibility complex class I HLA-G molecule

The human leukocyte antigen (HLA)-G non-classical major histocompatibility complex (MHC) class I molecule was originally described in first-trimester trophoblasts at the fetal-maternal interface in 1990. Eight years later, the First International Conference on this molecule was inaugurated by Prof Jean Dausset, recipient of the Nobel Prize in Medicine. The Fifth International Conference on HLA-G, held in Paris on July 2009, began with a tribute to Prof Jean Dausset who left us recently. This conference was co-chaired by Dr Edgardo D. Carosella and Prof Hans Grosse-Wilde, included 57 oral presentations and was attended by approximately 140 delegates from 16 countries. We summarize here the major advances on the HLA-G molecule that were reported, including findings on its biological activity and characterization of new mechanisms of action, notably through mesenchymal stem cells and regulatory cells, and the previously unexplored role of HLA-G on immune cells such as gammadelta T-cells and B lymphocytes. Furthermore, the role of HLA-G during pregnancy was revisited and its impact in pathologies such as cancer, autoimmune disorders and transplantation was further extended.