Truncated HLA-G isoforms are retained in the endoplasmic reticulum and insufficiently provide HLA-E ligands

Role of HLA-G in tumors and upon COVID-19 infection

HLA-G expression of tumors and upon viral infections is involved in their immune escape leading to the evasion from both T and NK cell recognition. The underlying mechanisms of HLA-G expression in both pathophysiologic conditions are broad and range from genetic abnormalities to epigenetic, transcriptional and posttranscriptional regulation. This review summarizes the current knowledge of the frequency, regulation and clinical relevance of HLA-G expression upon neoplastic and viral transformation, its interaction with components of the microenvironment as well as its potential as diagnostic marker and/or therapeutic target. In addition, it discusses urgent topics, which have to be addressed in HLA-G research.

Harnessing the potential of HLA-G in cancer therapy: advances, challenges, and prospects

Immune checkpoint blockades have been prized in circumventing and ablating the impediments posed by immunosuppressive receptors, reaching an exciting juncture to be an innovator in anticancer therapy beyond traditional therapeutics. Thus far, approved immune checkpoint blockades have principally targeted PD-1/PD-L1 and CTLA-4 with exciting success in a plethora of tumors and yet are still trapped in dilemmas of limited response rates and adverse effects. Hence, unveiling new immunotherapeutic targets has aroused immense scientific interest in the hope of expanding the clinical application of immune checkpoint blockades to scale new heights. Human leukocyte antigen-G (HLA-G), a non-classical major histocompatibility complex (MHC) class I molecule, is enriched on various malignant cells and is involved in the hindrance of immune effector cells and the facilitation of immunosuppressive cells. HLA-G stands out as a crucial next-generation immune checkpoint showing great promise for the benefit of cancer patients. Here, we provide an overview of the current understanding of the expression pattern and immunological functions of HLA-G, as well as its interaction with well-characterized immune checkpoints. Since HLA-G can be shed from the cell surface or released by various cells as free soluble HLA-G (sHLA-G) or as part of extracellular vesicles (EVs), namely HLA-G-bearing EVs (HLA-GEV), we discuss the potential of sHLA-G and HLA-GEV as predictive biomarkers. This review also addresses the advancement of HLA-G-based therapies in preclinical and clinical settings, with a focus on their clinical application in cancer.

The regulation of ovary and conceptus on the uterine natural killer cells during early pregnancy

Uterine natural killer (uNK) cells are short-lived, terminally differentiated and the most abundant lymphocytes in the uterus which play a crucial role in the spiral arteriole modification and establishment of successful pregnancy. Dysregulation of uNK cells has been linked to gestational implications such as recurrent pregnancy loss, preeclampsia and fetal growth retardation. There is evidence showing that progesterone and estrogen can regulate the recruitment, proliferation, differentiation and function of uNK cells via direct action on intracellular nuclear receptors or through intermediary cells in the uterus during early pregnancy. As the deepening of related research in this field, the role of conceptus in such regulation has received extensive attention, it utilizes endocrine signaling (hCG), juxtacrine signaling (HLA-C, HLA-E, HLA-G) and paracrine signaling (cytokines) to facilitate the activities of uNK cells. In addition, under the influence of ovarian hormones, conceptus can increase expression of PIBF and HLA-G molecules to reduce cytotoxicity of uNK cells and promote angiogenesis. In this review, we aim to concentrate on the novel findings of ovarian hormones in the regulation of uNK cells, emphasize the regulatory role of conceptus on uNK cells and highlight the proposed issues for future research in the field.

Interactions between HLA-G and HLA-E in Physiological and Pathological Conditions

HLA-G and HLA-E are immunoregulatory molecules that belong to HLA-Ib family. The role of these molecules in the control of the immune response has been extensively analyzed, both in physiological and pathological conditions. We have here summarized data present in the literature regarding the interaction of these molecules in different settings. These data suggested that HLA-G and -E co-operate in physiological conditions (i.e., establishment of an immune tolerance at maternal/fetal interface during pregnancy), whereas their role in the course of tumors or autoimmune/inflammatory diseases may be different or even opposite. Future studies aimed at investigating the interaction between HLA-G and HLA-E will help to clarify mechanism(s) underlying the regulation of immune effector cells in health and disease.

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.

Umbilical cord blood CD34(+)cell-derived progeny produces human leukocyte antigen-G molecules with immuno-modulatory functions

Human umbilical cord blood units (UCBs) are an alternative source in allogeneic-stem-cell transplantation. Human leukocyte antigen (HLA)-G is a tolerogenic molecule with a possible implication in UCB immunoregulatory effect. HLA-G expression was observed in UCB myeloid and plasmacytoid dendritic cells; in contrast, CD34(+) cells did not produce this molecule. CD34(+) cells are primitive hematopoietic progenitor cells that are present in UCB and are necessary for long-term engraftment via production of immunoregulatory molecules and a hematopoietic progeny that supports cellular recovery. The role of these cells in UCB transplantation needs further evaluation of HLA-G expression in CD34(+) cells and their hematopoietic progeny. We confirmed the absence of HLA-G expression in CD34(+) cells, whereas CD34(+)-derived progeny secreted HLA-G molecules and expressed HLA-G mRNA in in vitro cultures. Furthermore, soluble HLA (sHLA)-G molecules purified from the culture supernatants of CD34(+)-derived progeny were able to suppress lymphoproliferative response in an HLA-G dose-dependent manner. Overall these results identify CD34(+)-derived hematopoietic progeny as producers of HLA-G molecules and support a role of this antigen as an immuno-modulatory factor in UCB.

Effects of fgf2 and oxygen in the bmp4-driven differentiation of trophoblast from human embryonic stem cells

Human embryonic stem cells (hESC) differentiate into trophoblast when treated with BMP4. Here we studied the effects of either low (4 % O(2), L) or atmospheric O(2) (20% O(2), A) in the presence and absence of FGF2 on H1 hESC cultured in presence of BMP4. Differentiation progressed from the periphery towards the center of colonies. It occurred most quickly in the absence of FGF2 and under A and was slowest in presence of FGF2 and under L. Chorionic gonadotrophin (CG) production required A while FGF2 suppressed progesterone synthesis under both A and L. FGF2 was then omitted while we examined trophoblast markers SSEA-1 and cytokeratin-7 and -8, whose expression also progressed inwards from the periphery of colonies and occurred more rapidly under A than L. By day 5, most cells outside central islands of Oct4-positive cells were positive for these antigens under both conditions and many also expressed HLA-G, a marker of extra-villous cytotrophoblast. Under A, but not L, CGalpha and CGbeta became prominent in GATA2-positive, peripherally located, multinucleated cells. In conclusion, BMP4 induced conversion of hESC exclusively towards trophoblast; FGF2 slowed differentiation, while O(2) accelerated this process and promoted syncytiotrophoblast formation.

Down-regulation of HLA-G boosted natural killer cell-mediated cytolysis in JEG-3 cells cultured in vitro

OBJECTIVE

To determine how decidual natural killer (NK) cells interact with fetal trophoblasts in vitro.

DESIGN

Prospective study.

SETTING

University hospitals and IVF units.

PATIENT(S)

Not applicable.

INTERVENTION(S)

Not applicable.

MAIN OUTCOME MEASURE(S)

An adenovirus vector containing small interfering RNA (siRNA) specifically targeting the human lymphocyte antigen-G (HLA-G) gene was constructed and applied to diminish HLA-G mRNA expression. The steady-state levels of HLA-G messenger RNA (mRNA) were then checked by reverse transcriptase-polymerase chain reaction (RT-PCR) and protein levels by Western blot analysis. The NK-mediated cell cytotoxicity in the siRNA treated cells was studied by application of a nonradioactive cytotoxicity assay.

RESULT(S)

Steady-state levels of HLA-G mRNA and protein were significantly diminished by the targeting siRNA. In cells where HLA-G expression was thus reduced, a significant increase in NK cell-mediated lysis occurred.

CONCLUSION(S)

These results indicate that the recombinant adenoviral vectors used were efficient tools for studying HLA-G function. More important, this study reveals an important immunoprotective function for HLA-G in controlling NK cell-mediated lysis of trophoblasts, cells whose role in mediating normal pregnancy is important.

Soluble HLA-G promotes Th1-type cytokine production by cytokine-activated uterine and peripheral natural killer cells

Soluble forms of HLA-G (sHLA-G) have been implicated in immune regulation. Fetal trophoblast cells are a prime source of HLA-G. Hence, an interaction between sHLA-G and uterine lymphocytes in the decidual tissues can easily be envisaged. These lymphocytes, when properly activated, are implicated in successful trophoblast invasion, placental maturation and maintenance of pregnancy. However, so far, no data are available on the effect of sHLA-G on the function and phenotype of these cells. Herein, we used a recombinant sHLA-G construct to determine the effect of sHLA-G on uterine lymphocyte cells present in endometrium at the time that it is optimally receptive to trophoblast invasion. In addition, we ascertained the effect of sHLA-G on peripheral lymphocytes. We found that upon co-culture with sHLA-G, proliferation of unfractionated IL-15-stimulated uterine mononuclear cells (UMCs) was inhibited. However, sHLA-G increased both interferon (IFN)-gamma and tumour necrosis factor (TNF)-alpha production by these cells. Vascular endothelial growth factor (VEGF) production was reduced. Notably, in contrast to membrane-bound HLA-G, sHLA-G did not affect the natural cytolytic activity of UMCs. Similarly, sHLA-G inhibited proliferation but stimulated pro-inflammatory cytokine production by cytokine-activated, unfractionated peripheral blood mononuclear cells (PBMCs). In addition, we showed that the overall inhibitory effect of sHLA-G on proliferation of the whole cell population could be ascribed to selective inhibition of CD4(+) T cells. In contrast, sHLA-G induced proliferation and IFN-gamma production by both uterine and peripheral natural killer (NK) cells. In conclusion, our data show that the sHLA-G modulates both UMC and PBMC function. sHLA-G, by promoting IFN-gamma production by uterine NK cells, may contribute to vascular remodelling of spiral arteries to allow for successful embryo implantation.

TCR-independent cytokine stimulation induces non-MHC-restricted T cell activity and is negatively regulated by HLA class I

Recent evidence suggests that the functional status of T cells activated independently from their TCR differs substantially from classical MHC-restricted T cells. Here, we show that TCR-independent, short-term stimulation via the common gamma-chain of the IL-2/IL-15 receptor induces non-MHC-restricted cytotoxicity and sustained cytokine secretion in purified CD4+ or CD8+ T cells. NK-like cytotoxicity is directed against MHC class I-negative targets and can be inhibited by classical and non-classical HLA class I molecules. Known inhibitory receptors, such as CD85j (ILT2) and leukocyte-associated Ig-like receptor-1, are not responsible for this HLA-mediated inhibition. NK-like cytotoxicity can be costimulated by NKG2D (CD314) triggering, but 2B4 (CD244) and DNAM-1 (CD226) are not involved. NK-like T cells display an activated phenotype and secrete various cytokines, including IFN-gamma, TNF-alpha, IL-5, IL-13 and MIP-1beta. Under normal conditions, HLA class I-mediated inhibition may function as a safety mechanism to prevent unbalanced cytokine production and effector killing mechanisms by T cells that were activated independently from their TCR. Non-MHC-restricted activity represents a functional status rather than a property of distinct T cell subpopulations. Thus, cytokine-induced, non-MHC-restricted T cells may be relevant in immune responses against tumors showing aberrant MHC expression through their capacities of cytokine production and direct tumor cell eradication.

Soluble HLA-G Molecules Induce Apoptosis in Natural Killer Cells

Membrane-bound human leukocyte antigen-G (HLA-G) molecules are primarily expressed by cytotrophoblasts of the fetus. They are thought to protect the fetus from immunologic attack by the maternal immune system and have recently been associated with transplantation graft acceptance. In addition, soluble HLA-G molecules (sHLA-G) have been shown to play a role in the success of pregnancies, but are upregulated in certain cancers. However, the exact mechanism for this regulation has remained elusive. The aim of this study was to examine the mechanism by which sHLA-G interact with natural killer (NK) cells in vitro. sHLA-G effectively blocked NK lysis of target cells via fracticide killing of NK cells by apoptosis. These studies support the protective role of sHLA-G in immunologic reactions by interacting with NK cells, thus providing a regulatory function.

Comparison of the expression of human leukocyte antigen (HLA)-G and HLA-E in women with normal pregnancy and those with recurrent miscarriage

Recurrent miscarriage affects 1% of all couples attempting pregnancy. Immunological factors are postulated to play a role in the aetiology of recurrent miscarriage because the fetus and placenta are immunologically different from the mother. In particular, altered expression of the, non-classical, class I histocompatibility leukocyte antigen (HLA) molecules has been postulated to play a role in the aetiology of recurrent miscarriage as the fetus and placenta are semi-allogenic to the mother. This study was conducted to examine whether altered expression of the non-classical class I HLA molecules, HLA-G and HLA-E, by cells at the maternofetal interface could play a role in the aetiology of recurrent miscarriage. First-trimester placental and decidual biopsies were obtained from 45 women with recurrent miscarriage and 17 gestation-matched normal controls. These biopsies were screened by immunohistochemistry for HLA-G and HLA-E and isotype-matched control antibodies. Staining was analysed by light microscopy and digital image analysis. In both recurrent miscarriage and normal pregnancy, HLA-G was localised to the extravillous trophoblast. There was no difference in the pattern of HLA-G expression between women with recurrent miscarriage and those with normal pregnancies. HLA-E was localised to the syncytiotrophoblast, villous mesenchymal cells, extravillous trophoblast and several decidual cell types, but staining for HLA-E appeared to be confined primarily to the cytoplasm. There was no difference in the pattern of HLA-E expression between women with recurrent miscarriage and those with normal pregnancies.

The murine family of gut-restricted class Ib MHC includes alternatively spliced isoforms of the proposed HLA-G homolog, "blastocyst MHC"

The gastrointestinal tract is populated by a multitude of specialized immune cells endowed with receptors for classical (class Ia) and nonclassical (class Ib) MHC proteins. To identify class I products that engage these receptors and impact immunity/tolerance, we studied gut-transcribed class Ib loci and their polymorphism in inbred, outbred, and wild-derived mice. Intestinal tissues enriched in epithelial cells contained abundant transcripts of ubiquitously expressed and preferentially gut-restricted Q and T class I loci. The latter category included the "blastocyst Mhc" gene, H2-Bl, and its putative paralog, Tw5. Expression of H2-Bl was previously detected only at the maternal/fetal interface, where it was proposed to induce immune tolerance via interactions with CD94/NKG2A receptors. Analysis of coding region polymorphism performed here revealed two major alleles of H2-Bl with conserved residues at positions critical for class I protein folding and peptide binding. Both divergent alleles are maintained in outbred and wild mice under selection for fecundity and pathogen resistance. Surprisingly, we found that alternative splicing of H2-Bl mRNA in gut tissues is prevalent and allele-specific. It leads to strain-dependent expression of diverse repertoires of canonical and noncanonical transcripts that may give rise to distinct ligands for intestinal NK cell, T cell, and/or intraepithelial lymphocyte receptors.

Differential Expression of Alternatively Spliced Transcripts of HLA-G in Human Preimplantation Embryos and Inner Cell Masses

It has been reported that preimplantation human embryos secrete HLA-G, and the levels may be predictive of their ability to implant. However, it is not known which of the membrane-bound (HLA-G 1-4) and soluble (HLA-G 5-6) alternatively spliced forms are present, nor the developmental stage at which they appear. Therefore, we have investigated HLA-G mRNA isoform expression on single embryos at the two-, four-, six-, and eight-cell, morula, and blastocyst stages. The percentage of embryos expressing each HLA-G isoform mRNA increased with developmental stage, but contrary to expectation, HLA-G5 mRNA was not detected in single two- to eight-cell embryos and was only expressed by 20% of morulae and blastocysts. Similarly, soluble HLA-G6 mRNA was not detected until the blastocyst stage and then in only one-third of embryos. In contrast, labeling with MEM G/9 Ab (specific for HLA-G1 and -G5) was observed in 15 of 20 two- to eight-cell embryos and 5 of 5 blastocysts. This disparity between mRNA and protein may be due to HLA-G protein remaining from maternal oocyte stores produced before embryonic genome activation and brings into question the measurement of soluble HLA-G for clinical evaluation of embryo quality. Although HLA-G is expressed in the preimplantation embryo, later it is primarily expressed in the invasive trophoblast of the placenta rather than the fetus. Therefore, we have investigated whether down-regulation of HLA-G first occurs in the inner cell mass (precursor fetal cells) of the blastocyst and, in support of this concept, have shown the absence HLA-G1 and -G5 protein and mRNA.

HLA-G in human reproduction: aspects of genetics, function and pregnancy complications

The non-classical human leukocyte antigen (HLA) class Ib genes, HLA-E, -G and -F, are located on chromosome 6 in the human major histocompatibility complex (MHC). HLA class Ib antigens resemble the HLA class Ia antigens in many ways, but several major differences have been described. This review will, in particular, discuss HLA-G and its role in human reproduction and in the human MHC. HLA-G seems to be important in the modulation of the maternal immune system during pregnancy and thereby the maternal acceptance of the semiallogenic fetus. Recent findings regarding aspects of HLA-G polymorphism, the possible significance of this polymorphism in respect to HLA-G function and certain complications of pregnancy (such as pre-eclampsia and recurrent spontaneous abortions (RSA)) are discussed together with possible importance to IVF. Finally, aspects of a possible role of HLA-G in organ transplantation and in inflammatory or autoimmune disease, and of HLA-G in an evolutionary context, are also briefly examined.

The HLA-G*0105N null allele induces cell surface expression of HLA-E molecule and promotes CD94/NKG2A-mediated recognition in JAR choriocarcinoma cell line

HLA-G is a non-classical HLA class Ib molecule primarily expressed in trophoblast cells, and is thought to play a key role in the induction of materno-fetal tolerance during pregnancy. In addition, the HLA-G gene provides a suitable leader sequence peptide capable of binding to HLA-E. However, the existence of placentas homozygous for the HLA-G*0105N null allele suggests that HLA-G1 might not be essential for fetal survival. To investigate whether expression of the HLA-G*0105N allele supports HLA-E cell surface expression, we transfected the HLA-G*0105N gene into JAR trophoblast cells. Flow cytometry analysis showed that HLA-G*0105N-transfected cells express surface HLA-E to a similar extent as the unmutated HLA-G gene, whereas HLA-G1 cell surface expression was undetectable. Using the NKL cell line in a standard (51)Cr release assay, the HLA-E molecules were found to inhibit natural killer lysis, through a mechanism partially dependent on CD94/NKG2A-mediated recognition.