HLA-G inhibits the allogeneic proliferative response

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.

The Role of HLA-G in Tumor Escape: Manipulating the Phenotype and Function of Immune Cells

Human leukocyte antigen-G (HLA-G) is a non-classical major histocompatibility complex class I (MHC I) molecule, and under physiological conditions, its expression is strictly restricted to the maternal–fetal interface and immune-privileged organs where HLA-G is expected to contribute to establishment and maintenance of immune tolerance. However, the expression of HLA-G has been found in various types of tumors, and the level of its expression frequently correlates with high-grade histology and poor prognosis, raising the possibility that it may play a negative role in tumor immunity. ILT2 and ILT4, present on a broad of immune cells, have been identified as the main receptors engaging HLA-G, and their interactions have been found to allow the conversion of effectors like NK cells and T cells to anergic or unresponsive state, activated DCs to tolerogenic state, and to drive the differentiation of T cells toward suppressive phenotype. Therefore, tumors can employ HLA-G to modulate the phenotype and function of immune cells, allowing them to escape immune attack. In this review, we discuss the mechanism underlying HLA-G expression and function, its role played in each step of the tumor-immunity cycle, as well as the potential to target it for therapeutic benefit.

Human Amniotic Epithelial Stem Cells: A Promising Seed Cell for Clinical Applications

Perinatal stem cells have been regarded as an attractive and available cell source for medical research and clinical trials in recent years. Multiple stem cell types have been identified in the human placenta. Recent advances in knowledge on placental stem cells have revealed that human amniotic epithelial stem cells (hAESCs) have obvious advantages and can be used as a novel potential cell source for cellular therapy and clinical application. hAESCs are known to possess stem-cell-like plasticity, immune-privilege, and paracrine properties. In addition, non-tumorigenicity and a lack of ethical concerns are two major advantages compared with embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs). All of the characteristics mentioned above and other additional advantages, including easy accessibility and a non-invasive application procedure, make hAESCs a potential ideal cell type for use in both research and regenerative medicine in the near future. This review article summarizes current knowledge on the characteristics, therapeutic potential, clinical advances and future challenges of hAESCs in detail.

Generation of hypoimmunogenic human pluripotent stem cells via expression of membrane-bound and secreted β2m-HLA-G fusion proteins

Allogeneic immune rejection is a major barrier for the application of human pluripotent stem cells (hPSCs) in regenerative medicine. A broad spectrum of immune cells, including T cells, natural killer (NK) cells, and antigen-presenting cells, which either cause direct cell killing or constitute an immunogenic environment, are involved in allograft immune rejection. A strategy to protect donor cells from cytotoxicity while decreasing the secretion of inflammatory cytokines of lymphocytes is still lacking. Here, we engineered hPSCs with no surface expression of classical human leukocyte antigen (HLA) class I proteins via beta-2 microglobulin (B2M) knockout or biallelic knockin of HLA-G1 within the frame of endogenous B2M loci. Elimination of the surface expression of HLA class I proteins protected the engineered hPSCs from cytotoxicity mediated by T cells. However, this lack of surface expression also resulted in missing-self response and NK cell activation, which were largely compromised by expression of β2m-HLA-G1 fusion proteins. We also proved that the engineered β2m-HLA-G5 fusion proteins were soluble, secretable, and capable of safeguarding low immunogenic environments by lowering inflammatory cytokines secretion in allografts. Our current study reveals a novel strategy that may offer unique advantages to construct hypoimmunogenic hPSCs via the expression of membrane-bound and secreted β2m-HLA-G fusion proteins. These engineered hPSCs are expected to serve as an unlimited cell source for generating universally compatible "off-the-shelf" cell grafts in the future.

TWIST1, MMP-21, and HLAG-1 co-overexpression is associated with ESCC aggressiveness

Esophageal squamous cell carcinoma (ESCC) is one of the most aggressive types of cancer, requiring reliable biomarkers for prognosis and therapeutic responsiveness. TWIST1, as an important factor responsible for metastasis of several cancers, is involved in tumor invasion and metastasis through indirectly regulation of MMP-21 expression. On the other hand, NF-ĸβ which is a regulator of HLAG-1 has direct interaction with TWIST1 protein. In this retrospective study we investigated the clinical significance of TWIST1, MMP-21, and HLAG-1 expression in ESCC, and the possible correlation between these genes and progression of the disease. The gene expression analyses of TWIST1, MMP-21, and HLA-G1 were performed by relative comparative real-time polymerase chain reaction in 58 ESCCs compared with corresponding margin-normal esophageal tissues. Significant overexpression of HLAG-1, TWIST1, and MMP-21 messenger RNA was observed in 22.4%, 41.4%, and 60.3% of tumor samples, respectively. Concomitant overexpression of TWIST1/MMP-21 and TWIST1/HLAG-1 were significantly correlated to each other in various clinicopathological features, including depth of tumor invasion, stage of tumor progression, lymphatic invasion, and grade of tumor cell differentiation ( P < 0.05). The current study is the first report of coexpression of TWIST1, MMP-21, and HLAG-1 in ESCC. Such findings suggest an oncogenic role for concomitant expression of these genes in ESCC invasion and metastasis.

HSV-Induced Systemic Inflammation as an Animal Model for Behçet's Disease and Therapeutic Applications

Behçet’s disease (BD) affects multiple organs. It is mainly characterized by recurrent oral, skin, and genital aphthous ulcers, and eye involvement. Successful management of BD is increasing, although its etiology remains unclear. A number of etiologies have been proposed, including environmental, genetic, viral, and immunological factors. To understand its complex etiology and improve its management, animal models of BD have been used to enable more effective therapeutic applications with increased clinical significance. An herpes simplex virus (HSV) type 1-induced BD mouse model has shown disease characteristics similar to those seen in BD patients. An HSV-induced BD animal model has been used to test various therapeutic modalities. The applied modalities are several materials that are derived from natural products, conventional therapeutics, and possible biologics. In this review, we provided how they regulate inflammation in an HSV-induced BD model.

HLA-G peptide preferences change in transformed cells: impact on the binding motif

HLA-G is known for its strictly restricted tissue distribution. HLA-G expression could be detected in immune privileged organs and many tumor entities such as leukemia, multiple myeloma, and non-Hodgkin and Hodgkin’s lymphoma. This functional variability from mediation of immune tolerance to facilitation of tumor immune evasion strategies might translate to a differential NK cell inhibition between immune-privileged organs and tumor cells. The biophysical invariability of the HLA-G heavy chain and its contrary diversity in immunity implicates a strong influence of the bound peptides on the pHLA-G structure. The aim was to determine if HLA-G displays a tissue-specific peptide repertoire. Therefore, using soluble sHLA-G technology, we analyzed the K562 and HDLM-2 peptide repertoires. Although both cell lines possess a comparable proteome and recruit HLA-G-restricted peptides through the same peptide-loading pathway, the peptide features appear to be cell specific. HDLM-2 derived HLA-G peptides are anchored by an Arg at p1 and K562-derived peptides are anchored by a Lys. At p2, no anchor motif could be determined while peptides were anchored at pΩ with a Leu and showed an auxiliary anchor motif Pro at p3. To appreciate if the peptide anchor alterations are due to a cell-specific differential peptidome, we performed analysis of peptide availability within the different cell types. Yet, the comparison of the cell-specific proteome and HLA-G-restricted ligandome clearly demonstrates a tissue-specific peptide selection by HLA-G molecules. This exclusive and unexpected observation suggests an exquisite immune function of HLA-G.

Identification of novel microRNAs regulating HLA-G expression and investigating their clinical relevance in renal cell carcinoma

The non-classical human leukocyte antigen G (HLA-G) is expressed at a high frequency in renal cell carcinoma (RCC) and is associated with a higher tumor grade and a poor clinical outcome. This might be caused by the HLA-G-mediated inhibition of the cytotoxicity of T and NK cells. Therefore a selective targeting of HLA-G might represent a powerful strategy to enhance the immunogenicity of RCC lesions. Recent studies identified a number of HLA-G-regulating microRNAs (miRs) and demonstrated an inverse expression of some of these miRs with HLA-G in RCC in vitro and in vivo. However, it was postulated that further miRs might exist contributing to the tightly controlled selective HLA-G expression.By application of a miR enrichment assay (miTRAP) in combination with in silico profiling two novel HLA-G-regulatory miRs, miR-548q and miR-628-5p, were identified. Direct interactions of both miRs with the 3' untranslated region of HLA-G were confirmed with luciferase reporter gene assays. In addition, qPCR analyses and immunohistochemical staining revealed an inverse, expression of miR-628-5p, but not of miR-548q to the HLA-G protein in primary RCC lesions and cell lines. Stable overexpression of miR-548q and miR-628-5p caused a downregulation of HLA-G mRNA and protein. This leads in case of miR-548q to an enhanced NK cell-mediated HLA-G-dependent cytotoxicity, which could be reverted by ILT2 blockade suggesting a control of the immune effector cell activity at least by this miR. The identification of two novel HLA-G-regulatory miRs extends the number of HLA-G-relevant miRs tuning the HLA-G expression and might serve as future therapeutic targets.

The choriocarcinoma cell line JEG-3 upregulates regulatory T cell phenotypes and modulates pro-inflammatory cytokines through HLA-G

An understanding of the interactions between immune cells and trophoblast cells, as well as choriocarcinoma cells, are of extreme importance in reproductive immunology and cancer immunology. In this study, we found that the human HLA-G-positive choriocarcinoma cell line JEG-3 upregulates CD4⁺CD25^hiCD127^lo T cells, increases the expression of HLA-G⁺CD4⁺ and CD8⁺ T cells, and decreases the expression of ILT2⁺ on CD4⁺ T cells in resting PBMCs after six days of co-culture. Expression of HLA-G on JEG-3 cells did not affect regulatory T cell phenotypes, but promoted modulation of pro-inflammatory cytokines IFN-γ, TNF-α and IL-17A. When JEG-3 cells were stimulated with rhIFN-γ prior to co-culture, CD4⁺HLA-G⁺ T cells were significantly increased, and IFN-γ and TNF-α elevated. Taken together, the results indicate that JEG-3 cells upregulate regulatory T cell phenotypes and modulate the level of pro-inflammatory cytokines, which might be important mechanisms in the tumor microenvironment and at the feto-maternal interface during pregnancy.

Inhibition of T Cell Alloreactivity by Bronchial Epithelium Is Impaired in Lung Transplant Recipients, Through Pathways Involving TGF-β, IL-10 and HLA-G

BACKGROUND

Bronchiolitis obliterans syndrome (BOS) after lung transplantation (LTx) results from bronchial epithelial cell (BECs) damages, thought to be orchestrated by T cells primed by antigen-presenting cell presenting alloantigens. In this cell cross-talk, BECs are also suspected to play a pivotal immunosuppressive role in T cell alloreactivity. We studied the immunomodulating role of BECs in a human ex vivo model of allogeneic T cell response, both in healthy subjects and LTx recipients.

METHODS

BECs from 35 LTx recipients (n = 22 stable, n = 13 BOS) and healthy controls (n = 25) were cultured as primary cell cultures. Their inhibitory capacities through the involvement of tolerogenic molecules (HLA-G, TGF-β, and IL-10) were tested on a mixed lymphocyte reaction between antigen-presenting cells and recipient T cells.

RESULTS

Control BECs inhibited T cell alloproliferation by a mean of 53 ± 7%. This inhibitory effect of BECs was significantly reduced in the stable LTx group (24 ± 8%, P = 0.009), but not in the BOS TxP group (53 ± 10%, P = 0.97). Neutralization of HLA-G, TGF-β, and IL-10 partially restored T cell alloproliferation, arguing for their involvement in the immunosuppressive effect of BECs. BECs culture supernatant from stable LTx patients with impaired BEC properties showed a skewed Th2-type secretion profile (high IL-4/IFN-γ ratio).

CONCLUSIONS

The inhibitory properties of BECs are dysregulated in stable LTx recipients, which could suggest their instrumental role in the initiation of BOS process and potential targeted therapies.

Soluble monomers, dimers and HLA-G-expressing extracellular vesicles: the three dimensions of structural complexity to use HLA-G as a clinical biomarker

The HLA-G molecule belongs to the family of nonclassical human leukocyte antigen (HLA) class I. At variance to classical HLA class I, HLA-G displays (i) a low number of nucleotide variations within the coding region, (ii) a high structural diversity, (iii) a restricted peptide repertoire, (iv) a limited tissue distribution and (v) strong immune-suppressive properties. The physiological HLA-G surface expression is restricted to the maternal-fetal interface and to immune-privileged adult tissues. Soluble forms of HLA-G (sHLA-G) are detectable in various body fluids. Cellular activation and pathological processes are associated with an aberrant or a neo-expression of HLA-G/sHLA-G. Functionally, HLA-G and its secreted forms are considered to be key players in the induction of short- and long-term tolerance. Thus, its unique expression profile and tolerance-inducing functions render HLA-G/sHLA-G an attractive biomarker to monitor the systemic health/disease status and disease activity/progression for clinical approaches in disease management and treatments. Here, we place emphasis on (i) the current status of the tolerance-inducing functions by HLA-G/sHLA-G, (ii) the current complexity to implement this molecule as a meaningful clinical biomarker regarding the three dimensions of structural diversity (monomers, dimers and HLA-G-expressing extracellular vesicles) with its functional implications, and (iii) novel and future approaches to detect and quantify sHLA-G structures and functions.

Recombinant HLA-G as Tolerogenic Immunomodulant in Experimental Small Bowel Transplantation

The non-classical MHC I paralogue HLA-G is expressed by cytotrophoblast cells and implicated with fetomaternal tolerance by downregulating the maternal adaptive and innate immune response against the fetus. HLA-G expression correlates with favorable graft outcome in humans and recently promising immunosuppressive effects of therapeutic HLA-G in experimental transplantation (skin allograft acceptance) were shown. Consequently, we examined this novel therapeutic approach in solid organ transplantation. In this study, therapeutic recombinant HLA-G5 was evaluated for the first time in a solid organ model of acute rejection (ACR) after orthotopic intestinal transplantation (ITX). Allogenic ITX was performed in rats (Brown Norway to Lewis) with and without HLA-G treatment. It was found that HLA-G treatment significantly reduced histologically proven ACR at both an early and late postoperative timepoint (POD 4/7), concomitant to a functionally preserved graft contractility at POD 7. Interestingly, graft infiltration by myeloperoxidase+ cells was significantly reduced at POD7 by HLA-G treatment. Moreover, HLA-G treatment showed an effect on the allogenic T-cell immune response as assessed by flow cytometry: The influx of recipient-derived CD8⁺ T-cells into the graft mesenteric lymphnodes at POD7 was significantly reduced while CD4⁺ populations were not affected. As a potential mechanism of action, an induction of T-reg populations in the mesenteric lymphnodes was postulated, but flow cytometric analysis of classical CD4⁺/CD25⁺/FoxP3⁺T_reg-cells showed no significant alteration by HLA-G treatment. The novel therapeutic approach using recombinant HLA-G5 reported herein demonstrates a significant immunosuppressive effect in this model of allogenic experimental intestinal transplantation. This effect may be mediated via inhibition of recipient-derived CD8⁺ T-cell populations either directly or by induction of non-classical T_reg populations.

Transcriptomics: A Step behind the Comprehension of the Polygenic Influence on Oxidative Stress, Immune Deregulation, and Mitochondrial Dysfunction in Chronic Kidney Disease

Chronic kidney disease (CKD) is an increasing and global health problem with a great economic burden for healthcare system. Therefore to slow down the progression of this condition is a main objective in nephrology. It has been extensively reported that microinflammation, immune system deregulation, and oxidative stress contribute to CKD progression. Additionally, dialysis worsens this clinical condition because of the contact of blood with bioincompatible dialytic devices. Numerous studies have shown the close link between immune system impairment and CKD but most have been performed using classical biomolecular strategies. These methodologies are limited in their ability to discover new elements and enable measuring the simultaneous influence of multiple factors. The “omics” techniques could overcome these gaps. For example, transcriptomics has revealed that mitochondria and inflammasome have a role in pathogenesis of CKD and are pivotal elements in the cellular alterations leading to systemic complications. We believe that a larger employment of this technique, together with other “omics” methodologies, could help clinicians to obtain new pathogenetic insights, novel diagnostic biomarkers, and therapeutic targets. Finally, transcriptomics could allow clinicians to personalize therapeutic strategies according to individual genetic background (nutrigenomic and pharmacogenomic). In this review, we analyzed the available transcriptomic studies involving CKD patients.

TLR3 Signaling Promotes the Induction of Unique Human BDCA-3 Dendritic Cell Populations

Conventional and plasmacytoid dendritic cells (cDCs and pDCs) are the two populations of DCs that can be readily identified in human blood. Conventional DCs have been subdivided into CD1c⁺, or blood dendritic cells antigen (BDCA) 1 and CD141⁺, or BDCA-3, DCs, each having both unique gene expression profiles and functions. BDCA-3 DCs express high levels of toll-like receptor 3 and upon stimulation with Poly I:C secrete IFN-β, CXCL10, and IL-12p70. In this article, we show that activation of human BDCA-3 DCs with Poly I:C induces the expression of activation markers (CD40, CD80, and CD86) and immunoglobulin-like transcript (ILT) 3 and 4. This Poly I:C stimulation results in four populations identifiable by flow cytometry based on their expression of ILT3 and ILT4. We focused our efforts on profiling the ILT4⁻ and ILT4⁺ DCs. These ILT-expressing BDCA-3 populations exhibit similar levels of activation as measured by CD40, CD80, and CD86; however, they exhibit differential cytokine secretion profiles, unique gene signatures, and vary in their ability to prime allogenic naïve T cells. Taken together, these data illustrate that within a pool of BDCA-3 DCs, there are cells poised to respond differently to a given input stimulus with unique output of immune functions.

New Developments in HLA-G in Cardiac Transplantation

Human Leukocyte Antigen-G (HLA-G) is a non-classical class 1b protein, whose gene is located on chromosome 6 (6p21.31). HLA-G inhibits the immune cells' cytotoxic activity by interacting with specific receptors on their membranes. Since it is a naturally occurring immune modulator, HLA-G has been investigated in transplantation. Indeed, a number of investigations reveal that HLA-G expression is influenced by genetic polymorphisms and in turn, those polymorphisms are associated with detrimental or beneficial outcomes in various pathological situations. The present review introduces the HLA-G molecule, the gene and its polymorphisms. It focuses on the expression of HLA-G and the role of polymorphisms primarily in heart transplant outcomes, secondarily in other transplant organs, as well as the role of the allograft and effect of medical therapy. We discuss the limitations in HLA-G transplant investigations and future directions. The immune inhibiting activity of HLA-G has a great deal of potential for its utilization in enhancing diagnostic, preventive and therapeutic strategies against rejection in the setting of transplantation.

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.

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.

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.

Inhibition of DNA methylation enhances HLA-G expression in human mesenchymal stem cells

Mesenchymal stem cells (MSCs) are immunosuppressive multipotent cells under investigation for potential therapeutic applications in regenerative medicine and prevention of graft-versus-host disease. Human leukocyte antigen (HLA)-G contributes to the immunomodulatory properties of MSCs. HLA-G expression in MSCs is very low and diminishes during in vitro expansion. Epigenetic regulation activates HLA-G expression in some cancer cell lines but not in MSCs. In the present study, adipose- and bone marrow-derived MSCs were exposed to the DNA demethylating agent 5-aza-2-deoxycytidine (5-aza-dC) and histone deacetylase inhibitor valproic acid (VPA) and HLA-G mRNA levels assessed using semi-quantitative reverse-transcription PCR. Exposure to 5-aza-dC resulted in HLA-G1 and -G3 upregulation in both early and late passage MSCs. VPA treatment did not induce HLA-G expression in both bone marrow and adipose derived MSCs. Our results provide the first evidence that HLA-G3 could be expressed in MSCs and that methylation-mediated repression is partly responsible for the observed low levels of HLA-G expression in MSCs. Our findings provide insight that treatment of MSCs with specific epigenetic regulatory modulators may improve their immunoregulatory capability for therapeutic applications.

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

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

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.

HLA-G as a tolerogenic molecule in transplantation and pregnancy

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

Major histocompatibility complex I mediates immunological tolerance of the trophoblast during pregnancy and may mediate rejection during parturition

During pregnancy in larger mammals, the maternal immune system must tolerate the fetus for months while resisting external infection. This tolerance is facilitated by immunological communication between the fetus and the mother, which is mediated by Major Histocompatibility Complex I (MHC I) proteins, by leukocytes, and by the cytokines secreted by the leukocytes. Fetal-maternal immunological communication also supports pregnancy by inducing physiological changes in the mother. If the mother “misunderstands” the signal sent by the fetus during pregnancy, the fetus will be miscarried or delivered preterm. Unlike any other maternal organ, the placenta can express paternal antigens. At parturition, paternal antigens are known to be expressed in cows and may be expressed in horses, possibly so that the maternal immune system will reject the placenta and help to expel it. This review compares fetal-maternal crosstalk that is mediated by the immune system in three species with pregnancies that last for nine months or longer: humans, cattle, and horses. It raises the possibility that immunological communication early in pregnancy may prepare the mother for successful expulsion of fetal membranes at parturition.

HLA-G is differentially expressed in thyroid tissues

BACKGROUND

HLA-G is a nonclassical major histocompatibility complex molecule that has well-recognized immunomodulatory properties. The expression of HLA-G in tumor cells has been considered to be detrimental, permitting tumor spreading and decreased survival. We evaluated the expression of HLA-G in histologically normal thyroid tissue, goiter, and benign and malignant thyroid tumors, and studied the relationship between HLA-G expression and patient clinical variables.

PATIENTS AND METHODS

The immunohistochemistry expression of HLA-G was performed on 72 specimens of papillary thyroid carcinoma (PTC), 19 follicular thyroid carcinomas (FTC), 22 follicular adenomas (FA), 22 colloid goiters (CG), and 14 histologically normal thyroid glands (NT). The percentage of HLA-G staining was graded from absent (-) to intense (+++).

RESULTS

HLA-G was faintly expressed in areas of hyperplasia in NT and CG. In PTC, FTC, and FA, the percentage of cell staining was significantly higher than in NT and CG (p<0.001 for each comparison). The tumor area with HLA-G expression was greater in FTC (p=0.0059) and PTC (p=0.0330) compared to FA. According to the magnitude of HLA-G staining, PTC tumors >1 cm exhibited increased HLA-G staining when compared to smaller tumors (p=0.03). Aggressive histologic subtypes of PTC have a higher median stained tumor area. No association was found between HLA-G expression and tumoral staging or patient disease-free survival.

CONCLUSIONS

The gradual increase of HLA-G expression from hyperplasia to carcinomas, and the association of strong HLA staining with some variables implicated in poor prognosis corroborate the unfavorable role of HLA-G in tumor thyroid cells, inhibiting cytotoxic immune system cells and facilitating tumor evasion and progression.

Switch from protective to adverse inflammation during influenza: viral determinants and hemostasis are caught as culprits

Influenza viruses cause acute respiratory infections, which are highly contagious and occur as seasonal epidemic and sporadic pandemic outbreaks. Innate immune response is activated shortly after infection with influenza A viruses (IAV), affording effective protection of the host. However, this response should be tightly regulated, as insufficient inflammation may result in virus escape from immunosurveillance. In contrast, excessive inflammation may result in bystander lung tissue damage, loss of respiratory capacity, and deterioration of the clinical outcome of IAV infections. In this review, we give a comprehensive overview of the innate immune response to IAV infection and summarize the most important findings on how the host can inappropriately respond to influenza.

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.

In vivo identification of an HLA-G complex as ubiquitinated protein circulating in exosomes

The nonclassical human leukocyte antigen-G (HLA-G) is a tolerogenic molecule that can be released to the circulation by expressing cells. This molecule can form dimers but some other complexed HLA-G forms have been proposed to be present in vivo. Here, we further characterized these other complexed HLA-G forms in vivo. Ascitic and pleural exudates from patients were selected based on positivity for HLA-G by ELISA. Complexed HLA-G was detected in exosomes, which indicates an intracellular origin of these forms. 2D-PAGE analysis of exudates and isolated exosomes showed that these high molecular weight complexes were more heterogeneous than the HLA-G1 expressed by cell cultures. Treatment with deglycosylating enzymes did not change the molecular weight of HLA-G complexes. Immunoblot analysis of exudates and exosomes with an anti-ubiquitin antibody showed that at least some of these structures correspond to ubiquitinated HLA-G. HLA-G ubiquitination could be reproduced in vitro in HLA-G1-transfected cell lines, although with a lower modified/nonmodified protein proportion than in exudates. In summary, we demonstrate new circulating HLA-G forms in vivo that open a new perspective in the study of HLA-G function and analysis.

HLA-G 3' untranslated region polymorphisms are associated with systemic lupus erythematosus in 2 Brazilian populations

OBJECTIVE

HLA-G has well recognized tolerogenic properties in physiological and nonphysiological conditions. The 3' untranslated region (3'UTR) of the HLA-G gene has at least 3 polymorphic sites (14-bpINS/DEL, +3142C/G, and +3196C/G) described as associated with posttranscriptional influence on messenger RNA production; however, only the 14-bpINS/DEL and +3142C/G sites have been studied in systemic lupus erythematosus (SLE).

METHODS

We investigated the HLA-G 3'UTR polymorphic sites (14-bpINS/DEL, +3003C/T, +3010C/G, +3027A/C, +3035C/T, +3142C/G, +3187A/G, and +3196C/G) in 190 Brazilian patients with SLE and 282 healthy individuals in allele, genotype, and haplotype analyses. A multiple logistic regression model was used to assess the association of the disease features with the HLA-G 3'UTR haplotypes.

RESULTS

Increased frequencies were observed of the 14-bpINS (p = 0.053), +3010C (p = 0.008), +3142G (p = 0.006), and +3187A (p = 0.013) alleles, and increased frequencies of the 14-bpINS-INS (p = 0.094), +3010 C-C (p = 0.033), +3142 G-G (p = 0.021), and +3187 A-A (p = 0.035) genotypes. After Bonferroni correction, only the +3142G (p = 0.05) and +3010C (p = 0.06) alleles were overrepresented in SLE patients. The UTR-1 haplotype (14-bpDEL/+3003T/+3010G/+3027C/+3035C/+3142C/+3187G/+3196C) was underrepresented in SLE (pcorr = 0.035).

CONCLUSION

These results indicate that HLA-G 3'UTR polymorphic sites, particularly +3142G and +3010C alleles, were associated with SLE susceptibility, whereas UTR-1 was associated with protection against development of SLE.

A specific immune transcriptomic profile discriminates chronic kidney disease patients in predialysis from hemodialyzed patients

BACKGROUND

Chronic kidney disease (CKD) patients present a complex interaction between the innate and adaptive immune systems, in which immune activation (hypercytokinemia and acute-phase response) and immune suppression (impairment of response to infections and poor development of adaptive immunity) coexist. In this setting, circulating uremic toxins and microinflammation play a critical role. This condition, already present in the last stages of renal damage, seems to be enhanced by the contact of blood with bioincompatible extracorporeal hemodialysis (HD) devices. However, although largely described, the cellular machinery associated to the CKD- and HD-related immune-dysfunction is still poorly defined. Understanding the mechanisms behind this important complication may generate a perspective for improving patients outcome.

METHODS

To better recognize the biological bases of the CKD-related immune dysfunction and to identify differences between CKD patients in conservative (CKD) from those in HD treatment, we used an high-throughput strategy (microarray) combined with classical bio-molecular approaches.

RESULTS

Immune transcriptomic screening of peripheral blood mononuclear cells (1030 gene probe sets selected by Gene-Ontology) showed that 275 gene probe sets (corresponding to 213 genes) discriminated 9 CKD patients stage III-IV (mean±SD of eGFR: 32.27+/-14.7 ml/min) from 17 HD patients (p<0.0001, FDR=5%). Seventy-one genes were up- and 142 down-regulated in HD patients. Functional analysis revealed, then, close biological links among the selected genes with a pivotal role of PTX3, IL-15 (up-regulated in HD) and HLA-G (down-regulated in HD). ELISA, performed on an independent testing-group [11 CKD stage III-IV (mean±SD of eGFR: 30.26±14.89 ml/min) and 13 HD] confirmed that HLA-G, a protein with inhibition effects on several immunological cell lines including natural killers (NK), was down-expressed in HD (p=0.04). Additionally, in the testing-group, protein levels of CX3CR1, an highly selective chemokine receptor and surface marker for cytotoxic effector lymphocytes, resulted higher expressed in HD compared to CKD (p<0.01).

CONCLUSION

Taken together our results show, for the first time, that HD patients present a different immune-pattern compared to the un-dialyzed CKD patients. Among the selected genes, some of them encode for important biological elements involved in proliferation/activation of cytotoxic effector lymphocytes and in the immune-inflammatory cellular machinery. Additionally, this study reveals new potential diagnostic bio-markers and therapeutic targets.

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.

Human leukocyte antigen-G is frequently expressed in glioblastoma and may be induced in vitro by combined 5-aza-2'-deoxycytidine and interferon-γ treatments: results from a multicentric study

Human leukocyte antigen-G (HLA-G) is a nonclassical major histocompatibility complex (MHC) class I molecule involved in immune tolerance processes, playing an important role in the maintenance of the semi-allogeneic fetus. Although HLA-G expression is restricted in normal tissues, it is broadly expressed in malignant tumors and may favor tumor immune escape. We analyzed HLA-G protein and mRNA expression in tumor samples from patients with glioblastoma collected in France, Denmark, and Brazil. We found HLA-G protein expression in 65 of 108 samples and mRNA in 20 of 21 samples. The absence of HLA-G protein expression was associated with a better long-term survival rate. The mechanisms underlying HLA-G gene expression were investigated in glioma cell lines U251MG, D247MG, and U138MG. Induction of HLA-G transcriptional activity was dependent of 5-aza-2'-deoxycytidine treatment and enhanced by interferon-γ. HLA-G protein expression was observed in U251MG cells only. These cells exhibited a permissive chromatin state at the HLA-G gene promoter and the highest levels of induced HLA-G transcriptional activity following 5-aza-2'-deoxycytidine treatment. Several antigen-presenting machinery components were up-regulated in U251MG cells after demethylating and IFN-γ treatments, suggesting an effect on the up-regulation of HLA-G cell surface expression. Therefore, because of its role in tumor tolerance, HLA-G found to be expressed in glioblastoma samples should be taken into consideration in clinical studies on the pathology and in the design of therapeutic strategies to prevent its expression in HLA-G-negative tumors.

Soluble HLA-G molecules and HLA-G 14-base pair polymorphism after allogeneic hematopoietic cell transplantation

HLA-G 14-base pair (bp) polymorphism and soluble human leukocyte antigen G were previously reported to be implicated in allogeneic hematopoietic cell transplantation (allo-HSCT) outcome. However, soluble HLA-G blood levels and the 14-bp insertion-deletion polymorphism were separately assessed in the context of allo-HSCT. The aim of the present study was to examine the influence of the 14-bp insertion/deletion polymorphism of the HLA-G gene together with the soluble HLA-G plasma levels on allo-HSCT complications. We investigated the possible impact of HLA-G 14-bp polymorphism together with the pretransplantation and posttransplantation concentration of soluble HLA-G in 59 patients undergoing allo-HSCT. No association was found between the HLA-G 14-bp polymorphism, the soluble HLA-G level and acute graft-versus-host disease (GvHD), disease recurrence, or death. In contrast with previous reports the present data suggest a weak or negligible involvement of both 14-bp polymorphism on HLA-G gene and sHLA-G concentration in posttransplantation complications such as acute or chronic GvHD, relapse, or death.

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.

Multimeric structures of HLA-G isoforms function through differential binding to LILRB receptors

The non-classical Human leukocyte antigen G (HLA-G) differs from classical HLA class I molecules by its low genetic diversity, a tissue-restricted expression, the existence of seven isoforms, and immuno-inhibitory functions. Most of the known functions of HLA-G concern the membrane-bound HLA-G1 and soluble HLA-G5 isoforms, which present the typical structure of classical HLA class I molecule: a heavy chain of three globular domains α1-α2-α3 non-covalently bound to β-2-microglobulin (B2M) and a peptide. Very little is known of the structural features and functions of other HLA-G isoforms or structural conformations other than B2M-associated HLA-G1 and HLA-G5. In the present work, we studied the capability of all isoforms to form homomultimers, and investigated whether they could bind to, and function through, the known HLA-G receptors LILRB1 and LILRB2. We report that all HLA-G isoforms may form homodimers, demonstrating for the first time the existence of HLA-G4 dimers. We also report that the HLA-G α1-α3 structure, which constitutes the extracellular part of HLA-G2 and HLA-G6, binds the LILRB2 receptor but not LILRB1. This is the first report of a receptor for a truncated HLA-G isoform. Following up on this finding, we show that the α1-α3-Fc structure coated on agarose beads is tolerogenic and capable of prolonging the survival of skin allografts in B6-mice and in a LILRB2-transgenic mouse model. This study is the first proof of concept that truncated HLA-G isoforms could be used as therapeutic agents.

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.

CD86+ or HLA-G+ can be transferred via trogocytosis from myeloma cells to T cells and are associated with poor prognosis

The transfer of membrane proteins between cells during contact, known as trogocytosis, can create novel cells with a unique phenotype and altered function. We demonstrate that trogocytosis is more common in multiple myeloma (MM) than chronic lymphocytic leukemia and Waldenstrom macroglobulinaemia; that T cells are more probable to be recipients than B or natural killer cells; that trogocytosis occurs independently of either the T-cell receptor or HLA compatibility; and that after trogocytosis, T cells with acquired antigens can become novel regulators of T-cell proliferation. We screened 168 patients with MM and found that CD86 and human leukocyte antigen G (HLA-G) were antigens commonly acquired by T cells from malignant plasma cells. CD3+ CD86acq+ and CD3+ HLA-Gacq+ cells were more prevalent in bone marrow than peripheral blood samples. The presence of either CD86 or HLA-G on malignant plasma cells was associated with a poor prognosis. CD38++ side population cells expressed HLA-G, suggesting that these putative myeloma stem cells could generate immune tolerance. HLA-G+ T cells had a regulatory potency similar to natural Tregs, thus providing another novel mechanism for MM to avoid effective immune surveillance.

The immunosuppressive molecule HLA-G and its clinical implications

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

CD8(+) Tregs in autoimmunity: learning "self"-control from experience

Autoreactive CD8(+) regulatory T cells (Tregs) play important roles as modulators of immune responses against self, and numerical and functional defects in CD8(+) Tregs have been linked to autoimmunity. Several subsets of CD8(+) Tregs have been described. However, the origin of these T cells and how they participate in the natural progression of autoimmunity remain poorly defined. We discuss several lines of evidence suggesting that the autoimmune process itself promotes the development of autoregulatory CD8(+) T cells. We posit that chronic autoantigenic exposure fosters the differentiation of non-pathogenic autoreactive CD8(+) T cells into antigen-experienced, memory-like autoregulatory T cells, to generate a "negative feedback" regulatory loop capable of countering pathogenic autoreactive effectors. This hypothesis predicts that approaches capable of boosting autoregulatory T cell memory will be able to blunt autoimmunity without compromising systemic immunity.

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.