Expression of HLA G in human tumors is not a frequent event

Current Advance of Immune Evasion Mechanisms and Emerging Immunotherapies in Renal Cell Carcinoma

Renal cell carcinoma is a highly heterogeneous cancer group, and the complex microenvironment of the tumor provides appropriate immune evasion opportunities. The molecular mechanism of immune escape in renal cell carcinoma is currently a hot issue, focusing primarily on the major complex of histocompatibility, immunosuppressive cells, their secreted immunosuppressive cytokines, and apoptosis molecule signal transduction. Immunotherapy is the best treatment option for patients with metastatic or advanced renal cell carcinoma and combination immunotherapy based on a variety of principles has shown promising prospects. Comprehensive and in-depth knowledge of the molecular mechanism of immune escape in renal cell carcinoma is of vital importance for the clinical implementation of effective therapies. The goal of this review is to address research into the mechanisms of immune escape in renal cell carcinoma and the use of the latest immunotherapy. In addition, we are all looking forward to the latest frontiers of experimental combination immunotherapy.

HLA-G expression in melanomas

OBJECTIVE

Human leukocyte antigen G (HLA-G) is a non-classical HLA class I molecule involved in inducing tolerance at the feto-maternal interface and in escape of immune response by tumor cells. The aim of the study is to review the published literature on the expression of HLA-G in malignant melanomas and its clinicopathological and prognostic correlates.

METHODS

A systematic search was carried out in electronic databases. Studies dealing with HLA-G expression in surgically-removed human samples were retrieved and analyzed.

RESULTS

Of 1737 retrieved articles, 16 were included. The main themes regarded HLA-G expression in malignant melanocytic lesions, assessed by immunohistochemistry (IHC), soluble or molecular techniques, and its relationship with clinicopathological features, such as tumor thickness and malignant behavior. Overall significant HLA-G expression was found in 460/843 tumors (55%), and specifically in 251/556 melanomas (45%) evaluated with IHC, in 208/250 cases (83%) examined with soluble methods and in 13/23 melanoma lesions (57%) tested with polymerase chain reaction. Despite the correlation with parameters indicating an aggressive behavior, no studies demonstrated any prognostic value of HLA-G expression. Furthermore, uveal melanomas were constantly negative for this biomarker.

CONCLUSION

Overall, published data indicate that while HLA-G is involved in the interactions between melanomas and the immune system, it is unlikely to be the only factor to play such a role, therefore making it difficult to designate it as a prognostically relevant molecule. Evidence further suggests that HLA-G is not implicated in the immunobiology of uveal melanomas.

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

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

HLA-G Expression in Human Mesenchymal Stem Cells (MSCs) Is Related to Unique Methylation Pattern in the Proximal Promoter as well as Gene Body DNA

Multipotent human mesenchymal stem cells (MSCs) harbor clinically relevant immunomodulation, and HLA-G, a non-classical MHC class I molecule with highly restricted tissue expression, is one important molecule involved in these processes. Understanding of the natural regulatory mechanisms involved in expression of this elusive molecule has been difficult, with near exclusive reliance on cancer cell lines. We therefore studied the transcriptional control of HLA-G in primary isolated human bone marrow- (BM), human embryonic stem cell-derived (hE-), as well as placenta-derived MSCs (P-MSCs), and found that all 3 types of MSCs express 3 of the 7 HLA-G isoforms at the gene level; however, fibroblasts did not express HLA-G. Protein validation using BM- and P-MSCs demonstrated expression of 2 isoforms including a larger HLA-G-like protein. Interferon-γ (IFN-γ) stimulation upregulated both gene and protein expression in MSCs but not the constitutively expressing JEG-3 cell line. Most interestingly in human MSCs and placental tissue, hypomethylation of CpG islands not only occurs on the HLA-G proximal promoter but also on the gene body as well, a pattern not seen in either of the 2 commonly used choriocarcinoma cell lines which may contribute to the unique HLA-G expression patterns and IFN-γ-responsiveness in MSCs. Our study implicates the importance of using normal cells and tissues for physiologic understanding of tissue-specific transcriptional regulation, and highlight the utility of human MSCs in unraveling the transcriptional regulation of HLA-G for better therapeutic application.

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

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

Tumor-induced escape mechanisms and their association with resistance to checkpoint inhibitor therapy

Immunotherapy aims to activate the immune system to fight cancer in a very specific and targeted manner. Despite the success of different immunotherapeutic strategies, in particular antibodies directed against checkpoints as well as adoptive T-cell therapy, the response of patients is limited in different types of cancers. This attributes to escape of the tumor from immune surveillance and development of acquired resistances during therapy. In this review, the different evasion and resistance mechanisms that limit the efficacy of immunotherapies targeting tumor-associated antigens presented by major histocompatibility complex molecules on the surface of the malignant cells are summarized. Overcoming these escape mechanisms is a great challenge, but might lead to a better clinical outcome of patients and is therefore currently a major focus of research.

Tumor mechanisms of resistance to immune attack

The immune system plays a key role in the interactions between host and tumor. Immune selection pressure is a driving force behind the sculpting and evolution of malignant cancer cells to escape this immune attack. Several common tumor cell-based mechanisms of resistance to immune attack have been identified and can be broadly categorized into three main classes: loss of antigenicity, loss of immunogenicity, and creation of an immunosuppressive microenvironment. In this review, we will discuss in detail the relevant literature associated with each class of resistance and will describe the relevance of these mechanisms to human cancer patients. To conclude, we will outline the implications these mechanisms have for the treatment of cancer using currently available therapeutic approaches. Immunotherapy has been a successful addition to current treatment approaches, but many patients either do not respond or quickly become resistant. This reflects the ability of tumors to continue to adapt to immune selection pressure at all stages of development. Additional study of immune escape mechanisms and immunotherapy resistance mechanisms will be needed to inform future treatment approaches.

MHC/HLA Class I Loss in Cancer Cells

In this chapter I describe Tumour Immune Escape mechanisms associated with MHC/HLA class I loss in human and experimental tumours. Different altered HLA class-I phenotypes can be observed that are produced by different molecular mechanisms. Experimental and histological evidences are summarized indicating that at the early stages of tumour development there is an enormous variety of tumour clones with different MHC class I expression patterns. This phase is followed by a strong T cell mediated immune-selection of MHC/HLA class-I negative tumour cells in the primary tumour lesion. This transition period results in a formation of a tumour composed only of HLA-class I negative cells. An updated description of this process observed in a large variety of human tumors is included. In the second section I focus on MHC/HLA class I alterations observed in mouse and human metastases, and describe the generation of different tumor cell clones with altered MHC class I phenotypes, which could be similar or different from the original tumor clone. The biological and immunological relevance of these observations is discussed. Finally, the interesting phenomenon of metastatic dormancy is analyzed in association with a particular MHC class I negative tumor phenotype.

HLA-G protein expression in colorectal cancer evaluated by immunohistochemistry and western blot analysis: Its expression characteristics remain enigmatic

HLA-G protein expression could play a role in evasion of tumor immune surveillance. Accumulating evidence demonstrates that HLA-G is expressed in different types of malignancies, including colorectal cancer (CRC). The purpose of the current study was to further unravel whether HLA-G protein expression could play a role in immune evasion of CRC. Therefore, to firmly establish HLA-G protein expression, eight early passage human CRC cell lines and five human rectal cancer tissues were analyzed by western blot analysis. The results obtained by western blot analysis were compared with immunohistochemistry on tumor tissue sections of the same patient. Furthermore, multiple monoclonal antibodies (mAbs), 4H84, MEM-G/1 and 5A6G7, targeting HLA-G were used to unravel staining patterns. We showed that results obtained with immunohistochemistry did not correlate with protein expression detected by western blot analysis, using three different HLA-G targeting mAbs. Furthermore, with respect to the specificity of the mAbs employed, additional immune reactivity was detected using the mAbs MEM-G/1 and 5A6G7 in western blot analysis with K562 control cell lines overexpressing HLA-A2 or HLA-G, all tumor tissues and in two out of eight CRC cell lines. Based on the current study and our previously reported results, we conclude that claiming HLA-G plays a role in immune modulation of CRC seems premature, as results from anti-body based detection of HLA-G protein remain inconclusive. Until the time that detection of HLA-G is sensitive enough to detect all aspects of HLA-G expression in biological samples, rather than transfected cells or long time cultured cell lines, conclusions should be drawn with great care.

Regeneration of cervical reserve cell-like cells from human induced pluripotent stem cells (iPSCs): A new approach to finding targets for cervical cancer stem cell treatment

Cervical reserve cells are epithelial progenitor cells that are pathologically evident as the origin of cervical cancer. Thus, investigating the characteristics of cervical reserve cells could yield insight into the features of cervical cancer stem cells (CSCs). In this study, we established a method for the regeneration of cervical reserve cell-like properties from human induced pluripotent stem cells (iPSCs) and named these cells induced reserve cell-like cells (iRCs). Approximately 70% of iRCs were positive for the reserve cell markers p63, CK5 and CK8. iRCs also expressed the SC junction markers CK7, AGR2, CD63, MMP7 and GDA. While iRCs expressed neither ERα nor ERβ, they expressed CA125. These data indicated that iRCs possessed characteristics of cervical epithelial progenitor cells. iRCs secreted higher levels of several inflammatory cytokines such as macrophage migration inhibitory factor (MIF), soluble intercellular adhesion molecule 1 (sICAM-1) and C-X-C motif ligand 10 (CXCL-10) compared with normal cervical epithelial cells. iRCs also expressed human leukocyte antigen-G (HLA-G), which is an important cell-surface antigen for immune tolerance and carcinogenesis. Together with the fact that cervical CSCs can originate from reserve cells, our data suggested that iRCs were potent immune modulators that might favor cervical cancer cell survival. In conclusion, by generating reserve cell-like properties from iPSCs, we provide a new approach that may yield new insight into cervical cancer stem cells and help find new oncogenic targets.

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.

HLA-G: At the Interface of Maternal-Fetal Tolerance

During pregnancy, semiallogeneic fetal extravillous trophoblasts (EVT) invade the uterine mucosa without being rejected by the maternal immune system. Several mechanisms were initially proposed by Peter Medawar half a century ago to explain this apparent violation of the laws of transplantation. Then, three decades ago, an unusual human leukocyte antigen (HLA) molecule was identified: HLA-G. Uniquely expressed in EVT, HLA-G has since become the center of the present understanding of fetus-induced immune tolerance. Despite slow progress in the field, the last few years have seen an explosion in our knowledge of HLA-G biology. Here, we critically review new insights into the mechanisms controlling the expression and function of HLA-G at the maternal-fetal interface, and discuss their relevance for fetal tolerance.

HLA-E expression and its clinical relevance in human renal cell carcinoma

The non-classical human leukocyte antigen E (HLA-E) expression is frequently overexpressed in tumor diseases, transplants and virus-infected cells and represents an immunomodulatory molecule by binding to the receptors CD94/NKG2A, -B and -C on NK and T cells. Due to its immune suppressive features HLA-E expression might represent an important mechanism of tumors to escape immune surveillance.While an aberrant expression of the non-classical HLA-G antigen in human renal cell carcinoma (RCC) has been demonstrated to be associated with a worse outcome of patients and reduced sensitivity to immune effector cell-mediated cytotoxicity, the expression and function of HLA-E has not yet been analyzed in this tumor entity.Higher levels of HLA-E transcripts were detected in all RCC cell lines and tumor lesions, which were tested in comparison to normal kidney epithelium. Immunohistochemical staining of a tissue microarray (TMA) using the HLA-E-specific monoclonal antibody TFL-033 recognizing the cytoplasmic HLA-E α-chain as monomer revealed a heterogeneous HLA-E expression in RCC lesions with the highest frequency in chromophobe RCC when compared to other RCC subtypes. HLA-E expression did not correlate with the frequency of CD3+, CD4+, CD8+ and FoxP3+ immune cell infiltrations, but showed an inverse correlation with infiltrating CD56+ cells. In contrast to HLA-G, HLA-E expression in RCCs was not statistically significant associated with a decreased disease specific survival. These data suggest that HLA-E overexpression frequently occurs in RCC and correlates with reduced immunogenicity.

Promoter methylation and mRNA expression of HLA-G in relation to HLA-G protein expression in colorectal cancer

Expression of human leukocyte antigen-G (HLA-G) is a suggested mechanism used by tumor cells to escape from host immune recognition and destruction. Advances in the field have made it evident that HLA-G is expressed in different types of malignancies including colorectal cancer (CRC). We analyzed HLA-G expression in 21 low passage CRC cell lines. The level of DNA methylation of the HLA-G gene and the presence of mRNA encoding HLA-G was measured. Moreover, HLA-G protein expression was determined by flow cytometry and immunohistochemistry (IHC). IHC was performed with three different monoclonal antibodies (mAbs) (4H84, MEM-G/1 and MEM-G/2). In addition, HLA-G protein expression was measured in matching primary tumor tissues. RNA analysis using RT-PCR followed by sequencing in 6 samples indicated strong homology of the PCR product with HLA-G3 in 5 samples. In accordance, in none of the cell lines, HLA-G1 expression was detected by flow-cytometry. Furthermore, no association between HLA-G DNA methylation patterns and HLA-G mRNA expression was observed. In addition, different immunohistochemical staining profiles among various anti-HLA-G mAbs were observed. In conclusion, the results of this study show that the HLA-G3 isoform was expressed in some of the CRC cell lines irrespective of the level of DNA methylation of HLA-G.

Trogocytic intercellular membrane exchanges among hematological tumors

Trogocytosis is the transfer of plasma membrane fragments and the molecules they contain between one donor and one acceptor/acquirer cell. Through trogocytosis, acceptor cells temporarily display and use cell-surface molecules they do not express themselves, but borrow from other cells. Here, we investigated whether liquid tumors possessed a trogocytic capability, if immune escape molecules could be acquired by tumor cells, transferred between cells of the same tumor, and if this could benefit the tumor as a whole.For this, we investigated trogocytosis in hematological cell lines and freshly isolated hematological tumor cells. We demonstrate that hematological tumor lines possess a trogocytic capability that allows them to capture membranes that contain the immune-inhibitory molecule HLA-G from allogeneic as well as from autologous sources. We further show that freshly isolated hematological tumor cells also possess these capabilities. This work reports for the first time the trogocytic capabilities of liquid tumor cells and introduces the notion of immune escape strategy sharing among tumor cells through trogocytosis of membrane-bound immune-inhibitory molecules.

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.

HLA-G expression is an independent predictor for improved survival in high grade ovarian carcinomas

Aberrant expression of human leukocyte antigens (HLA) class I has prognostic importance in various cancers. Here, we evaluated the prognostic value of classical (A/B/C) and nonclassical (G/E) HLA expression in 169 high grade epithelial ovarian cancer samples and linked that to clinicopathological characteristics and survival. Expression of HLA-A, -B/C, or -E was not correlated with survival. Survival was prolonged when tumours expressed HLA-G (P = 0.008) and HLA-G was an independent predictor for better survival (P = 0.011). In addition, HLA-G expression was associated with longer progression-free survival (P = 0.036) and response to chemotherapy (P = 0.014). Accordingly, high expression of HLA-G mRNA was associated with prolonged disease-free survival (P = 0.037) in 65 corresponding samples. Elevated serum-soluble HLA-G levels as measured by enzyme-linked immunosorbent assay in 50 matched patients were not correlated to HLA-G protein expression or gene expression nor with survival. During treatment, sHLA-G levels declined (P = 0.038). In conclusion, expression of HLA-G is an independent prognostic factor for improved survival in high grade epithelial ovarian cancer and a predictor for platinum sensitivity.

Synthetic HLA-G proteins for therapeutic use in transplantation

The human leukocyte antigen (HLA)-G is a tolerogenic molecule, whose expression by allografts is associated with better acceptance. An increasing interest in producing HLA-G as a clinical-grade molecule for therapy use is impaired by its complexity and limited stability. Our purpose was to engineer simpler and more stable HLA-G-derived molecules than the full-length HLA-G trimolecular complex that are also tolerogenic, functional as soluble molecules, and compatible with good manufacturing practice (GMP) production conditions. We present two synthetic molecules: (α3-L)x2 and (α1-α3)x2 polypeptides. We show their capability to bind the HLA-G receptor LILRB2 and their functions in vitro and in vivo. The (α1-α3)x2 polypeptide proved to be a potent tolerogenic molecule in vivo: One treatment of skin allograft recipient mice with (α1-α3)x2 was sufficient to significantly prolong graft survival, and four weekly treatments induced complete tolerance. Furthermore, (α1-α3)x2 was active as a soluble molecule and capable of inhibiting the proliferation of tumor cell lines, as does the full length HLA-G trimolecular complex. Thus, the synthetic (α1-α3)x2 polypeptide is a stable and simpler alternative to the full-length HLA-G molecule. It can be produced under GMP conditions, it functions as a soluble molecule, and it is at least as tolerogenic as HLA-G in vivo.

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.

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.

The prognostic role of classical and nonclassical MHC class I expression in endometrial cancer

The aim of this study was to investigate classical MHC class I and nonclassical MHC (human leukocyte antigen-G [HLA-G]) expression in a large cohort of patients with endometrial cancer, to determine the prognostic value of these cell surface markers and their relation with clinicopathological variables. Tissue microarrays containing epithelial endometrial carcinoma tissue from 554 patients were stained for classical and nonclassical MHC class I using the following monoclonal antibodies: 4H84 (anti-HLA-G), beta2-m (anti-beta-2-microglobulin) and HC-10 (MHC class I antigen heavy chain). Expression data were linked to known clinicopathological characteristics and survival. HLA-G upregulation and MHC class I downregulation in neoplastic cells was observed in 40% and 48%, respectively. Nonendometrioid tumor type, advanced stage disease (FIGO stage > or = II) and poorly or undifferentiated tumors were associated with MHC class I downregulation. Absence of HLA-G expression was independently associated with MHC class I downregulation. In univariate analysis, MHC class I downregulation was a predictor of worse disease-specific survival. Prognostic unfavorable tumor characteristics were correlated with downregulation of MHC class I expression in endometrial cancer cells. Furthermore, downregulated MHC class I has a negative impact on disease-specific survival, observed in a large cohort of patients with endometrial cancer. As there seems to be a relation between classical and nonclassical MHC class I molecules (HLA-G), further research is warranted to unravel this regulatory mechanism.

HLA and melanoma: multiple alterations in HLA class I and II expression in human melanoma cell lines from ESTDAB cell bank

Altered HLA class I and class II cell surface expression has been reported in many types of malignancy and represents one of the major mechanism by which tumour cells escape from T lymphocytes. In this report, we review the results obtained from the study of constitutive and IFN-gamma-induced expression of HLA class I and II molecules in 91 human melanoma cell lines from the European Searchable Tumour Cell Line Database, and compare them with published data on HLA expression in other types of cancer. Various types of alterations in HLA class I cell surface expression were found in a high percentage (67%) of the studied cell lines. These alterations range from total to selective HLA class I loss and are associated with beta2-microglobulin gene mutations, transcriptional downregulation of HLA class I genes and antigen processing machinery components, or with the loss of heterozygosity in chromosome 6. The most frequently observed phenotype is selective downregulation of HLA-B locus, reversible after treatment with IFN-gamma. The expression of constitutive- or IFN-gamma induced-surface expression of at least one HLA class II locus is positive in 71.5% of the analysed cell lines. Four different HLA class II expression phenotypes were defined, and a positive correlation between the expression of class I and II molecules is discussed. More detailed information on the HLA expression patterns and others immunological characteristics of these melanoma cell lines can be found on the following website http://www.ebi.ac.uk/ipd/estdab .

Expression and regulation of non-classical HLA-G in renal cell carcinoma

Under physiological conditions, the non-classical major histocompatibility complex class Ib molecule human leukocyte antigen G (HLA-G) is selectively expressed in placental trophoblasts, thymus and cornea. In pathological situations, HLA-G expression was frequently found in tumour cells of distinct origin, thereby allowing these tumour cells to escape immune surveillance. Although HLA-G expression occurs at a relatively high frequency in renal cell carcinoma (RCC) of the clear cell subtype, the molecular mechanisms of its aberrant expression in RCC has not yet been determined. Therefore, the constitutive and cytokine-mediated HLA-G expression as well as its mode of regulation was investigated. In addition to HLA-G-specific mRNA expression, membrane-bound and soluble/shed HLA-G protein was determined. Eight of 14 RCC cell lines analysed (57%) exhibited HLA-G-specific transcripts, whereas only 6 of 14 RCC cell lines (43%) expressed HLA-G protein, suggesting a post-transcriptional control of HLA-G in some cases. Treatment of RCC cell lines with either interferon-gamma or interleukin-10, respectively, increased HLA-G-specific mRNA and protein in six of eight HLA-G(+) RCC lines (75%), but not in HLA-G(-) RCC cells. A 5'-aza-2-deoxycytidine (5-Aza-dC)-mediated demethylation of the HLA-G promoter DNA resulted in an enhanced HLA-G expression in four of six RCC cell lines, whereas a de novo induction of HLA-G was only observed in one HLA-G(-) RCC cell line on treatment with 5-Aza-dC. Thus, there exist multiple mechanisms controlling HLA-G expression in RCC, which might also have an impact on the development of RCC-specific immunotherapies.

Epigenetic changes within the promoter region of the HLA-G gene in ovarian tumors

Background

Previous findings have suggested that epigenetic-mediated HLA-G expression in tumor cells may be associated with resistance to host immunosurveillance. To explore the potential role of DNA methylation on HLA-G expression in ovarian cancer, we correlated differences in HLA-G expression with methylation changes within the HLA-G regulatory region in an ovarian cancer cell line treated with 5-aza-deoxycytidine (5-aza-dC) and in malignant and benign ovarian tumor samples and ovarian surface epithelial cells (OSE) isolated from patients with normal ovaries.

Results

A region containing an intact hypoxia response element (HRE) remained completely methylated in the cell line after treatment with 5-aza-dC and was completely methylated in all of the ovarian tumor (malignant and benign) samples examined, but only variably methylated in normal OSE samples. HLA-G expression was significantly increased in the 5-aza-dC treated cell line but no significant difference was detected between the tumor and OSE samples examined.

Conclusion

Since HRE is the binding site of a known repressor of HLA-G expression (HIF-1), we hypothesize that methylation of the region surrounding the HRE may help maintain the potential for expression of HLA-G in ovarian tumors. The fact that no correlation exists between methylation and HLA-G gene expression between ovarian tumor samples and OSE, suggests that changes in methylation may be necessary but not sufficient for HLA-G expression in ovarian cancer.

Immune-refractory cancers and their little helpers--an extended role for immunetolerogenic MHC molecules HLA-G and HLA-E?

There is strong evidence to support a role for non-classical MHC class I (class Ib) molecules, most notably HLA-E and HLA-G in tumour immune escape. In this article, we summarize the current knowledge on their expression, regulation and functional relevance in various malignancies, particularly brain tumours. Special emphasis is devoted to the phenomenon that these tolerogenic molecules are expressed by non-transformed cells that are found in close neighborhood to tumour cells representing either parenchymal cells or immune cells attracted to the tumour microenvironment. Here they may act as "natural" or "inducible" suppressors of anti-tumoural immune responses. We thus speculate about the role of HLA-G expressing T cells, a novel population of natural regulatory cells that was identified recently. It is suggested that various cell types within a tumour cooperate in order to inhibit anti-tumour immunity-and that immunetolerogenic HLA-G may play a major role in this context.

HLA-G in the skin--friend or foe?

Skin is the largest organ of the human body that harbors a robust and versatile immune surveillance system. Whereas impaired immune function of the skin enables tumor growth, excessive immune activation results in different inflammatory diseases of the skin. HLA-G is a non-classical MHC class I molecule that was initially described to provide immunotolerogenic signals. In this context, HLA-G was mainly investigated as a mechanism that tumor cells employ to evade and inhibit host immune response. Expression of HLA-G in several inflammatory conditions in the skin implicated that the (dys)function of this molecule may also underlie excessive immune stimulation observed in these disorders. This review focuses on the functionality of HLA-G in the skin and summarizes available data obtained from studies performed in skin cancer and inflammatory dermatoses.

Structure, expression and function of HLA-G in renal cell carcinoma

Tumors have developed different strategies to escape from immune cell recognition which include the downregulation or loss of the classical HLA class I antigens as well as aberrant expression of non-classical HLA antigens like HLA-G. Abnormalities in MHC class surface expression have also been described in renal cell carcinoma (RCC) and represent mechanisms to avoid elimination by immune effector cells. We here review the structure/polymorphism, mRNA and protein expression profile of HLA-G in RCC and corresponding normal kidney epithelium, its mode of regulation and its functional consequences on immune responses. A heterogeneous constitutive HLA-G mRNA and/or protein expression was found in both RCC lesions and RCC cell lines, whereas normal kidney epithelium totally lack HLA-G mRNA and protein expression. In comparison to other tumor entities, the frequency of HLA-G expression is relatively high in RCC. Since HLA-G expression is lost during cultivation of RCC cells, the tumor microenvironment and/or endothelium appear to be involved in the regulation of HLA-G expression in this disease. HLA-G expression could be transcriptionally upregulated in RCC by interferons, IL-10 and gangliosides. Silencing of HLA-G expression in RCC is often associated with methylation of the HLA-G promoter which could be reverted by the treatment with demethylating agents. Functional studies using natural killer cells, lymphokine activated killer cells as well as antigen-specific CD8+ cytotoxic T lymphocytes demonstrated that HLA-G expression prevents lysis of RCC cells by these different immune effector cells. In contrast, HLA-G-negative normal kidney cells as well as HLA-G-negative RCC cells were not recognized by NK and T cells. Thus, HLA-G represents one important immune escape mechanism of human RCC which has an impact on the design of T and NK cell-based immunotherapies in this disease.

Flow cytometric immunophenotyping of cancer cells in effusion specimens: Diagnostic and research applications

Flow cytometry (FCM) immunophenotyping is frequently used as an ancillary technique for the diagnosis of hematological malignancies or for measurement of DNA content. In recent years, we applied FCM to the diagnosis of metastatic adenocarcinoma and malignant mesothelioma in effusions. We established a panel of antibodies that allows for rapid and effective differentiation between epithelial cells, mesothelial cells, and leukocytes. FCM was subsequently used for quantitative analysis of integrin subunits. Recently, we studied different parameters of the immune response, including HLA molecules and chemokine receptors, using this method. Our data suggest that FCM is an effective method for the characterization of cancer cells in clinical effusion specimens in both the diagnostic and research setting, and that this method is comparable to immunohistochemistry in terms of sensitivity and specificity, with the additional advantage of providing quantitative data. This review discusses previous work in this area and the future potential of this method in the characterization of tumor cells in serous effusions.

Altered expression of cellular membrane molecules of HLA-DR, HLA-G and CD99 in cervical intraepithelial neoplasias and invasive squamous cell carcinoma

OBJECTIVE

The aim of this study was to investigate the role of HLA-DR, HLA-G and CD99 during cervical carcinogenesis and to examine the prognostic significance of these protein expressions in invasive squamous cell carcinoma (SCC).

METHODS

Using specific antibodies for HLA-DR, HLA-G and CD99, we examined protein expressions in 19 normal cervix, 15 mild dysplasia (CIN I), 22 moderate dysplasia (CIN II), 23 severe dysplasia (CIN III), and 34 invasive squamous cell carcinoma by immunohistochemistry. And we detected the expression of Ki67 in the same specimens.

RESULTS

None of normal cervix and CINs except three cases of CIN III expressed HLA-DR. HLA-DR expression increased progressively with the grade of the tumor, and significant differences could be observed between grade 1 and grade 2 (P<0.01) and between grade 1 and grade 3 (P<0.05). In all normal epithelial control samples, HLA-G expression was seen in ectocervical squamous and endocervical columnar epithelium and the staining was strong and uniform. Only a small proportion of CINs and SCCs showed reduced expression of HLA-G. Compared with the results in the control samples, CINs and SCCs showed significantly reduced expression of HLA-G (P<0.001). SCCs showed significantly increased expression of CD99 when compared with normal cervix and CINs (P<0.05). Ki67 was expressed in all specimens. Significant differences were observed between CINs and normal cervix (P<0.001) and SCCs and controls (P<0.001), but no significant differences could be observed between SCCs and CINs. None of the expressions of these proteins was associated with any of clinicopathological parameters.

CONCLUSIONS

These results indicate that increased expression of HLA-DR and CD99 may be related to the evolution of cervical cancer. All protein expressions were not associated with clinicopathological parameters.

Expression of HLA-G in malignant mesothelioma and clinically aggressive breast carcinoma

The aim of the present study was to evaluate HLA-G expression in breast carcinoma and malignant mesothelioma (MM). Malignant breast carcinoma effusions (46) and corresponding solid tumors (39) and 104 MM (26 effusions, 78 solid tumors) were analyzed using immunohistochemistry (IHC). HLA-G protein and mRNA expression were further studied using immunoblotting (IB) and RT-PCR. HLA-ABC expression was analyzed using flow cytometry (FCM). IHC showed predominantly focal HLA-G expression in 12 of 46 (26%) breast carcinoma effusions and 16 of 39 (41%) solid lesions. In MM, 20 of 78 (26%) solid lesions and 14 of 26 (54%) effusions were focally HLA-G positive. Expression in MM was higher in effusions (p=0.008). IB showed more frequent HLA-G expression in MM compared with breast carcinoma effusions, while RT-PCR showed HLA-G mRNA expression in both tumors. FCM showed conserved HLA-ABC expression in 15 of 15 effusions. Breast cancer patients with HLA-G-positive tumor cells had shorter disease-free survival (mean 37 vs 85, median 25 vs 31 months), though not significantly (p=0.14). In conclusion, HLA-G is focally expressed in MM and breast carcinoma, while HLA-ABC expression is conserved. However, the up-regulated expression of HLA-G in MM effusions and its possible association with shorter disease-free survival in advanced stage of breast carcinoma suggest a possible role in immune response evasion in some tumors.

HLA-G gene activation in tumor cells involves cis-acting epigenetic changes

The tissue distribution of HLA-G molecules is broader than originally reported in trophoblastic cells. On the basis of numerous studies, HLA-G is also expressed in malignant tumors and involved in tumor immune escape. The mechanisms of HLA-G gene regulation differ from those of classical HLA class I genes and involve epigenetic processes. Here, we provide additional evidence on the influence of DNA demethylation on HLA-G activation. We also analyze the 5' regulatory region of HLA-G in 2 cellular models, melanoma (FON, M8) and choriocarcinoma (JEG-3, JAR), either expressing HLA-G transcripts or not. The data strongly suggest that HLA-G is silenced as a result of CpG site hypermethylation within a 5' regulatory region encompassing 450 bp upstream of the start codon, whereas it is activated upon demethylation. This result correlates with the acetylation status of histones within this region and the putative locus control region located at -1.2 kb. cis-acting epigenetic changes and the fact that demethylating agents activate HLA-G expression at least 5 days following treatment should be taken into account in epigenetic cancer therapies.

HLA-G expression in effusions is a possible marker of tumor susceptibility to chemotherapy in ovarian carcinoma

OBJECTIVE

We recently showed that the levels of secreted human leukocyte antigen-G (HLA-G), a nonclassical MHC class I antigen, are significantly elevated in malignant effusions in ovarian carcinoma compared to benign ones. The objective of this study was to evaluate the expression and clinical role of HLA-G in effusions and corresponding solid tumors from patients diagnosed with advanced-stage ovarian carcinoma.

METHODS

Effusions (= 148), corresponding primary tumors (= 66), and metastatic lesions (= 122) were analyzed using immunohistochemistry with an anti-HLA-G monoclonal antibody.

RESULTS

HLA-G was detected in cancer cells in 49/148 (33%) effusions, 33/66 (50%) primary tumors, and 59/122 (48%) solid metastases. These differences did not reach statistical significance. Expression in effusions and solid metastases significantly correlated (P = 0.029). HLA-G expression in tumor cells was significantly lower in effusions obtained during or following chemotherapy (P = 0.038). The presence of HLA-G-positive tumor cells in effusions obtained prior to the institution of chemotherapy correlated with better overall survival (P = 0.042). HLA-G expression in primary tumors and solid metastases did not correlate with any of the clinicopathologic parameters studied.

CONCLUSIONS

HLA-G is expressed in a significant number of ovarian carcinomas at all anatomic sites. The reduced expression of HLA-G in post-chemotherapy effusions and its correlation with improved survival may be related to preferential susceptibility of HLA-G-expressing cells at this site. Our findings suggest a new role for HLA-G as a prognostic indicator in advanced-stage ovarian cancer in effusions.

The role of HLA-G for protection of human renal cell-carcinoma cells from immune-mediated lysis: implications for immunotherapies

HLA-G as a non-classical MHC class I molecule exhibits a limited tissue distribution and exerts multiple immune regulatory functions including the induction of immune tolerance. In addition, HLA-G has been detected in some tumors of different histology and therefore may represent a novel immune escape mechanism of tumor cells. Despite the immunogenicity of renal cell carcinoma (RCC), outgrowth of tumor cells occurs which might be attributable to abrogation of efficient anti-tumor responses. We here review the potential role of HLA-G in RCC immunology, the HLA-G expression pattern and its functional consequences on immune responses. A heterogenous constitutive and interferon-gamma inducible HLA-G mRNA and protein expression was found in RCC cell lines and tumor lesions, but not in autologous normal kidney epithelium. HLA-G transcription and protein expression was detected at a high frequency in primary RCC lesions and RCC cell lines. Functional studies performed with alloreactive natural and lymphokine activated killer cells as well as antigen-specific CD8+ T cell populations demonstrated that HLA-G expression inhibits lysis of RCC cells by these different immune effector cells, whereas HLA-G- normal kidney cells were recognized. Thus, aberrant HLA-G expression might participate in evasion of these tumor cells from immunosurveillance.

HLA-G in skin cancer: a wolf in sheep's clothing?

Despite well-defined and immunogenic tumor antigens, and even in the presence of tumor antigen-specific cytotoxic cells, the immune system does not appear to be very effective in eradicating cells that have undergone malignant transformation. Tumor cells, even though invading and representing a threat, are not truly "foreign" but autologous cells that have become transformed in a subtle way, enabling them to escape the host immune system. Melanoma, and to less extent nonmelanoma, skin cancers have developed different strategies to circumvent host immunosurveillance. HLA-G is one of the molecules implicated in cancer immunescape. This review will concentrate on induction and expression of this nonclassical class I molecule in different skin cancer types presenting existing experimental evidence on this topic.

Exosomes bearing HLA-G are released by melanoma cells

Tumor cells release membrane vesicles, named exosomes, capable of specific cytotoxic T-lymphocyte activation by transferring tumor antigens to dendritic cells. By contrast, the nonclassical human leucocyte antigen (HLA)-G class I molecule displays immunotolerant properties and can be ectopically expressed by tumor cells, thereby allowing their escape from immunosurveillance. We describe here that a melanoma cell line, named Fon, established from an HLA-G-positive melanoma biopsy, spontaneously expressed high levels of the HLA-G1 membrane-bound isoform. Exosomes released by Fon cells were purified and analyzed both for their density on sucrose gradient and their protein composition by Western blotting and flow cytometry. Besides the expression of well-described proteins such as Lamp-2, notably, these melanoma-derived exosomes bore HLA-G1. In addition, exosomes harboring HLA-G1 were secreted by the HLA-G-negative M8 melanoma cells transfected with the HLA-G1 cDNA. Thus, the presence of tolerogenic HLA-G molecules on melanoma-derived exosomes may provide a novel way for tumors to modulate host's immune response.

Altered expression of nonclassical HLA class Ib antigens in human renal cell carcinoma and its association with impaired immune response

An optimal antitumoral immune response requires the activation of both CD8(+) and CD4(+) T lymphocytes by the peptide antigen presentation via the human leukocyte antigen (HLA) class I and class II molecules, respectively. Downregulation or loss of HLA molecules has been found in human renal cell carcinoma (RCC) and provides a strategy of these tumors to evade T-cell mediated immunosurveillance. In addition, a tumor-specific upregulation of HLA-G has been recently described in RCC, which also leads to an impaired immune response. We here summarize the frequency of the constitutive and/or interferon-gamma (IFN-gamma) inducible expression of nonclassical HLA class Ib antigens in RCC cell lines, surgically removed RCC lesions and normal kidney epithelium, the molecular characteristics of HLA-G expression, and its role in immune recognition.

HLA-G in melanoma: can the current controversies be solved?

The potential role of HLA-G in tumor immune escape has stimulated interest in the analysis of HLA-G antigens in malignant cells. Malignant melanoma is the tumor which has been mostly analyzed for HLA-G expression. Results obtained by seven groups of investigators about HLA-G expression in 108 melanoma cell lines have been concordant. HLA-G mRNA has been found in about 50% of the cell lines tested, whereas HLA-G protein has been found in less than 1% of the cell lines analyzed. In contrast, results obtained from six groups of investigators about HLA-G protein expression in 133 melanoma lesions have been conflicting. The possible causes of these conflicting results as well as the reasons for the discrepancy in HLA-G expression between cultured melanoma cell lines and surgically removed lesions have been discussed. Lastly, data about the potential clinical relevance of HLA-G expression in melanoma has been reviewed. The available data in the literature strongly suggest that progress in this exciting research area would greatly benefit from experiments to solve the current controversies in the field.

HLA-G and IL-10 expression in human cancer—different stories with the same message

Immune evasion in cancer may result from structural and functional alterations of human leukocyte antigen (HLA) class I molecules and/or local release of immunosuppressive cytokines, such as interleukin (IL)-10. In lung cancer, both of these mechanisms seem to often take place, resulting in the impaired tumor recognition and the progression of the disease. In primary cutaneous lymphomas on the other side, the shift towards immunosuppressive T helper (Th)2 cytokine profile and the secretion of IL-10 appears to occur more frequently than the loss of HLA class I molecules. In addition to down-regulation of HLA class I expression, IL-IO appears to be one of the factors responsible for the up-regulation of HLA-G, another molecule involved in the immunescape. It is possible that the expression of HLA-G itself may account for induction of Th2-skewing state and the production of IL-10, thence establishing a vicious circle of immune abrogation in cancer. This article reviews the current literature on this topic and provides new insights into the role of HLA-G and IL-10 in cancer.

Analysis of HLA-G expression in malignant hematopoetic cells from leukemia patients

It has been suggested that HLA-G antigens may provide tumor cells with an effective immune escape mechanism. So far mostly solid tumors have been analyzed; HLA-G antigen was only exceptionally detected. To further examine HLA-G expression, patients were chosen with different forms of leukemia: AML (25), CML (4), ALL (9), CLL (8), HCL (2) and NHL (3). Using flow cytometry with three HLA-G specific mAbs (87G, 01G and MEM-G/9), western blotting with two specific mAbs (4H84 and MEM-G/1) and RT-PCR, neither HLA-G antigen nor mRNA for any HLA-G isoform was detected. These results strongly suggest that HLA-G antigen is not expressed in freshly isolated human leukemia cells and therefore is not involved in their escape from immune attack.

HLA-G gene repression is reversed by demethylation

The HLA-G molecule plays an important role in immune tolerance, protecting the fetus from maternal immune attack, and probably contributes to graft tolerance and tumor escape from the host immune system. HLA-G expression is tightly regulated and involves mechanisms acting in part at the transcriptional level. Nevertheless, almost all regulatory sequences that govern constitutive and inducible HLA class I gene transcription are disrupted in the HLA-G gene promoter, suggesting an unusual regulatory process. In further investigating the molecular mechanisms of HLA-G gene activation, we evaluated the influence of epigenetic mechanisms on seven HLA-G-negative cell lines that exhibit various phenotypes. Exposure of cells to histone deacetylase inhibitors, or to the demethylating agent 5-aza-2'-deoxycytidine, revealed that HLA-G gene transcription is inhibited by DNA methylation. Reversal of methylation-mediated repression may directly induce HLA-G cell-surface expression, supporting the idea that HLA-G might be activated by such a mechanism during malignancy, inflammation, and allogenic reactions.

Mild acid treatment induces cross-reactivity of 4H84 monoclonal antibody specific to nonclassical HLA-G antigen with classical HLA class I molecules

Mild acid treatment by releasing beta(2)m and antigenic peptides leaves human leukocyte antigen (HLA) class I free heavy chains attached to the cell surface. Acid treatment thus allows detection of the cell surface class I antigens by monoclonal antibodies (mAbs) specific to HLA-free heavy chains. We found that acid treatment also enables detection of the cell surface non-classical HLA-G class I antigen with mAbs specific for HLA-G free heavy chains, including 4H84 mAb recognizing all isoforms. Furthermore, we found that 4H84 mAb, but not other mAbs specific to HLA-G free heavy chains, binds to the surface of 8 out of 16 acid-treated leukemia cell lines. Nevertheless, HLA-G antigen is not present in any of these leukemia cells. This was demonstrated by failure to detect any antigen with 4H84 mAb in immunoblotting as well as by inability to detect HLA-G mRNA by RT-PCR. The antigen recognized by 4H84 mAb in some acid treated leukemia cells was identified by immunoprecipitation as a 45 kDa protein. A number of observations indicate that 45 kDa proteins are none other than classical class I heavy chains. Acid treatment thus induces the ability of the 4H84 mAb to recognize some classical HLA class I molecules. Remarkably, 4H84 determinant on HLA-G is linear but corresponding determinant present on some partially folded classical HLA class I free heavy chains is conformational. In view of the unexpected cross-reactivity, detection of HLA-G with this mAb must be carefully evaluated to avoid false detection.

HLA-G Molecules: from Maternal–Fetal Tolerance to Tissue Acceptance

Over the past few years, HLA-G, the non-classical HLA class I molecule, has been the center of investigations that have led to the description of its specific structural and functional properties. Although located in the HLA class I region of chromosome six, the HLA-G gene may be distinguished from other HLA class I genes by its low polymorphism and alternative splicing that generates seven HLA-G proteins, whose tissue-distribution is restricted to normal fetal and adult tissues that display a tolerogeneic function toward both innate and acquired immune cells. We review these points, with special emphasis on the role of HLA-G in human pathologies, such as cancer, viral infection, and inflammatory diseases, as well as in organ transplantation.

Major histocompatibility complex abnormalities in non-Hodgkin lymphomas

An optimal antitumoral immune response requires the participation of both CD8 and CD4 T lymphocytes, which are activated by peptide antigen presentation via human leucocyte antigen (HLA) class I and class II molecules respectively. Loss of HLA molecules has been observed in different malignancies, and provides a mechanism for escape from immune surveillance. Furthermore, HLA-G, a class Ib molecule, is considered to be an immune tolerance-inducing molecule. HLA-G expression on tumour cells could provide a further mechanism for immune escape. To determine the frequency and the pattern of HLA defects in non-Hodgkin lymphomas (NHL), HLA expression was prospectively studied in 614 NHL cases, using flow cytometry. Furthermore, HLA-G expression was tested in 50 cases, including 20 cases selected on the basis of their defective HLA class I expression. In 64 cases (10.4%), lymphomatous cells exhibited lower HLA class I mean fluorescence intensity compared with reactive cells. Their characteristics were (1) the diversity of histological entities; (2) the significant frequency of relapse or transformation; (3) the increased incidence of high-grade NHL compared with low-grade; and (4) the severity of the class I defect in 50% of the cases, mainly in high-grade NHL. A defect in HLA-DR expression was always associated with a severe class I defect (12 cases; 2%). The HLA-G protein was detected in three class I defective cases. These HLA alterations frequently appeared as a secondary event at relapse or at transformation, suggesting a direct role in lymphomagenesis.

Analysis of HLA-G expression in breast cancer tissues

Among the different mechanisms by which cancer can elude the immune system, alterations in the expression of human leukocyte antigen (HLA) class I molecules on tumor cells may play a crucial role by impairing the HLA molecules interaction with T and natural killer (NK) cells specific receptors. More recently, aberrant expression of HLA-G has been described in different tumor tissues in addition to HLA class I downregulation. The HLA-G molecule is a nonclassical HLA class I antigen selectively expressed by trophoblast and thymic epithelial cells. Several studies reported that the HLA-G function might represent an additional mechanism of tumor immune escape, mainly inhibiting NK and cytotoxic T-cell activity. Here we report the analysis of HLA-G expression both at RNA level by reverse transcriptase-polymerase chain reaction and at protein level by Western blot and immunohistochemistry in 25 breast cancer patient tissues. The aim of this study was to elucidate the HLA-G gene expression pattern in breast tumor tissues and correlate it with HLA class I alterations. Our results demonstrated that HLA-G molecules expression was never found even in a group of patients revealing HLA class I total loss, and that HLA-G is not expressed in breast cancer tissue with a low-tumor grade (G1-G2) and minimal stromal contamination.

IFN-gamma protects short-term ovarian carcinoma cell lines from CTL lysis via a CD94/NKG2A-dependent mechanism

IFN-gamma regulates the immunogenicity of target cells by increasing their expression of HLA class I molecules. This facilitates the T cell receptor-mediated recognition by CD8(+) T cells but decreases target cell sensitivity to lysis by NK cells due to engagement of inhibitory NK receptors. In this study, short-term tumor cell lines from patients with advanced ovarian carcinomas were established. We demonstrate the paradoxical finding that IFN-gamma treatment of these short-term ovarian carcinoma cell lines (OVACs) resulted in resistance of tumor cells to lysis by peptide- and allospecific CD8(+) T cells. Blocking experiments revealed that this phenomenon was dependent on enhanced inhibitory signalling via CD94/NKG2A receptors expressed on the effector cells. This was associated with increased expression of HLA-E mRNA and HLA-G at the protein level in IFN-gamma-treated OVACs. Furthermore, pulsing of untreated OVACs with the leader sequence peptide of HLA-G protected these cells from lysis by CTLs, thus mimicking the inhibitory effect of IFN-gamma. This study provides evidence that CD94/NKG2A receptors play an important role in regulating T cell activity against tumors and shows that IFN-gamma modulation of target cells may shift the balance of triggering and inhibitory signals to T cells, turning off their cytolytic activity.

HLA-G immunoreactivity is specific for intermediate trophoblast in gestational trophoblastic disease and can serve as a useful marker in differential diagnosis

HLA-G is a nonclassical MHC class I antigen that has been shown to be a specific marker for normal intermediate trophoblast (IT). In this study HLA-G immunoreactivity assessed with an HLA-G specific antibody (4H84) was detected in all 14 cases of choriocarcinoma, 14 placental site trophoblastic tumors, 13 epithelioid trophoblastic tumors, 16 placental site nodules, and nine exaggerated placental sites. In contrast, HLA-G immunoreactivity was not detected in 34 nontrophoblastic uterine neoplasms. HLA-G immunoreactivity was present in all the IT cells of exaggerated placental sites and placental site trophoblastic tumors and in 70-100% of IT cells in placental site nodules and epithelioid trophoblastic tumors. The pattern of distribution of HLA-G in different subpopulations of IT confirms the relationship of various trophoblastic lesions to different types of IT (exaggerated placental site and placental site trophoblastic tumor to implantation site IT and placental site nodule and epithelioid trophoblastic tumor to chorionic-type IT) and suggests that choriocarcinoma is related to villous-type IT because the majority of mononucleate cells in this neoplasm were HLA-G immunoreactive. In conclusion, HLA-G immunoreactivity appears to be specific for IT in gestational trophoblastic disease and can serve as a useful marker in the differential diagnosis of these lesions.

HLA expression in uveal melanoma: there is no rule without some exception

A decrease in the expression of HLA antigens is considered a characteristic of tumor progression and is considered an important tumor-escape mechanism. In general, HLA Class I expression is even further decreased on metastases. Tumor cells that loose their HLA Class I antigens become less susceptible to lysis by specific T cells, but may become more sensitive to Natural Killer cells. Loss of HLA Class I can be observed at different levels, i.e. total loss of Class I, loss of expression of one locus or one haplotype, or even one specific allele. We studied HLA expression on human uveal melanoma and observed that loss of expression of a locus or one or more alleles is a common phenomenon. However, in contrast with the commonly accepted paradigm, loss of HLA Class I expression on the uveal melanoma was not associated with tumor cell escape and a worse survival, but with a better survival of the patients involved. We hypothesize that this is due to the route of metastases formation: in uveal melanoma, spreading of metastases is purely hematogeneous, and it is quite possible that NK-cell mediated surveillance of tumor cells in the blood is the underlying mechanism. This is supported by our finding that metastases of uveal melanoma have a high HLA Class I expression, leading to our conclusion that uveal melanoma is an exception to the general rule regarding HLA Class I expression in tumor immunology.

A functional role of HLA-G expression in human gliomas: an alternative strategy of immune escape

HLA-G is a nonclassical MHC molecule with highly limited tissue distribution that has been attributed chiefly immune regulatory functions. Glioblastoma is paradigmatic for the capability of human cancers to paralyze the immune system. To delineate the potential role of HLA-G in glioblastoma immunobiology, expression patterns and functional relevance of this MHC class Ib molecule were investigated in glioma cells and brain tissues. HLA-G mRNA expression was detected in six of 12 glioma cell lines in the absence of IFN-gamma and in 10 of 12 cell lines in the presence of IFN-gamma. HLA-G protein was detected in four of 12 cell lines in the absence of IFN-gamma and in eight of 12 cell lines in the presence of IFN-gamma. Immunohistochemical analysis of human brain tumors revealed expression of HLA-G in four of five tissue samples. Functional studies on the role of HLA-G in glioma cells were conducted with alloreactive PBMCs, NK cells, and T cell subpopulations. Expression of membrane-bound HLA-G1 and soluble HLA-G5 inhibited alloreactive and Ag-specific immune responses. Gene transfer of HLA-G1 or HLA-G5 into HLA-G-negative glioma cells (U87MG) rendered cells highly resistant to direct alloreactive lysis, inhibited the alloproliferative response, and prevented efficient priming of cytotoxic T cells. The inhibitory effects of HLA-G were directed against CD8 and CD4 T cells, but appeared to be NK cell independent. Interestingly, few HLA-G-positive cells within a population of HLA-G-negative tumor cells exerted significant immune inhibitory effects. We conclude that the aberrant expression of HLA-G may contribute to immune escape in human glioblastoma.