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.
Althout 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. It is associated with the capability of tumor cells to evade the immune control.
The HLA-G gene is located within the major histocompatibility complex on chromosome 6 (6p21.3) and is composed of 8 exons and 7 introns.
There are only 47 HLA-G alleles resulting in 15 different proteins, and 2 null alleles. This low polymorphism for the HLA-G gene contrasts with that of classical HLA class-I genes. Such allelic variations may modify the affinity of the protein for its receptors, or its recognition by antibodies.
CHEMICAL STRUCTURE AND IMAGES
The full transcript of this gene encodes a transmembrane protein with a molecular weight of 39 kDa. This protein has an extracellular part with three globular domains α1, α2 and α3 (encoded by exons 2, 3 and 4, respectively), a transmembrane region (encoded by exon 5) and a very short cytoplasmic tail of only 6 amino acids.
The primary transcript produced by transcription of the HLA-G gene can undergo alternative splicing that may yield 7 protein isoforms that all possess the α1 domain. The isoforms HLA-G1,-G2,-G3 and G4 are membrane-bound. The other isoforms are soluble.
(The immunosuppressive molecule HLA-G and its clinical implications, Crit Rev Clin Lab Sci. 2012)
Protein Aminoacids Percentage
Under physiological conditions, the production of HLA-G protein is restricted, to trophoblast, cornea, nail matrix, Beta-cells of the islets of Langerhans, and embryonic or mesenchymal stem cells.
Different cytokines, mainly interferons (IFN), (A specific interferon stimulated response element of the distal HLA-G promoter binds IFN-regulatory factor 1 and mediates enhancement of this nonclassical class I gene by IFN-beta, J Biol Chem.2001) interleukin-10 (IL-10) (The HLA-G genotype is associated with IL-10 levels in activated PBMCs. Immunogenetics, 2005) or epidermal growth factor (EGF) (Synthesis of beta(2)-microglobulin-free, disulphide-linked HLA-G5 homodimers in human placental villous cytotrophoblast cells. Immunology. 2007) can upregulate HLA-G expression, but may not induce it. On the other hand, HLA-G gene expression can be induced by glucocorticoids, (Glucocorticoid hormones upregulate levels of HLA-G transcripts in trophoblasts. Transplant Proc, 2001) or microenvironmental factors such as low oxygen tension or tryptophan (Regulatory role of tryptophan degradation pathway in HLA-G expression by human monocyte-derived dendritic cells. Mol Immunol. 2006) (Linking two immuno-suppressive molecules: indoleamine 2,3 dioxygenase can modify HLA-G cell-surface expression. 2005). HLA-G gene expression is further regulated by epigenetic mechanisms, which include DNA methylation and histone deacetylation.
In the past few years, HLA-G expression regulation by microRNAs (miRNAs) has been investigated. The miRNAs miRNA-148a, miRNA-148b, and miRNA- 152, causing the down-regulation of HLA-G expression. (Overexpression of miR-152 leads to reduced expression of human leukocyte antigen-G and increased natural killer cell mediated cytolysis in JEG-3 cells. Am J Obstet Gynecol, 2010).
HLA-G possesses the capability, common to HLA Class I molecules, to bind inhibitory receptors. Three HLA-G inhibitory receptors have been described to date: ILT2/ CD85j/LILRB1, ILT4/CD85d/LILRB2, and KIR2DL4/CD158d.
(The immunosuppressive molecule HLA-G and its clinical implications, Crit Rev Clin Lab Sci. 2012)
The known HLA-G receptors on immune cells are inhibitory receptors and most of the known functions of HLA-G are inhibitory and oriented towards immune inhibition and tolerance. They can be organized into three:
- The direct immuno-inhibitory functions through blocking effector cells
- The indirect immuno-inhibitory functions through regulatory cell generation
- The other functions of HLA-G that have immuno-inhibitory consequences
Direct immuno-inhibitory functions through blocking of effector cells
The first function of HLA-G is the direct inhibition of immune effector cells by engagement of their inhibitory receptors, which leads to target cell protection against cytolysis (cytolytic effector blockade), or impairment of an immune response by inhibition of one of its key contributors.
The membrane-bound complete HLA-G1 isoform inhibited the cytolytic function of uterine and peripheral NK cells. Trophoblast cells were protected by HLA-G from cytolysis by decidual and peripheral NK cells of semi-allogeneic or allogeneic origin. These findings also explained why classical HLA Class I-negative/HLA-G-positive fetal cells were not rejected by the mother’s immune system. This clearly established that in the context of pregnancy, HLA-G had a tolerogenic function.
It was demonstrated that through direct interaction with ILT2 or ILT4 inhibitory receptors, HLA-G inhibited the antigen-specific cytolytic function of cytotoxic α/β and γ/δ T lymphocytes, the alloproliferative response of CD4+ T cells, the on-going proliferation of NK and T cells, and the maturation of DCs.
Indirect immuno-inhibitory functions through regulatory cell generation
HLA-G possessed “long-term” tolerogenic functions as well, through the generation of regulatory/suppressor cells. Regulatory cells can be of various subsets that include APC, CD8+ T cells, and CD4+ T cells. The capability of HLA-G to induce regulatory T cells was investigated, and it was demonstrated that CD4+ and CD8+ T cells that had been stimulated in the presence of HLA-G not only lost their capability to respond to antigenic stimulation, but also differentiated into regulatory T cells capable of inhibiting the reactivity of other T cells.
Activated NK and T cells may also temporarily behave as regulatory suppressor cells and inhibit immune responses through HLA-G after they have acquired HLA-G-containing membranes from cells in their vicinity by the mechanism of trogocytosis. HLA G, by interacting with ILT receptors present on APCs, induces their differentiation into myeloid suppressive cells, that are characterized by a strong ability to suppress various T cell functions.
Other functions with immuno-inhibitory consequences
Other HLA-G functions are less known; one can cite the capability of sHLA-G to induce apoptosis of NK cells and CTLs, the capability of soluble and membrane-bound HLA-G to influence the expression and release of IFN-γ by NK cells, the capability of soluble and membrane-bound HLA-G to induce the upregulation of inhibitory receptors including its own, and the anti-angiogenic function of sHLA-G through CD160 binding.
It is important to cite the recently described capability of HLA-G to modulate cytokine and chemokine receptors. In T cells, it was first demonstrated that sHLA-G differentially inhibited the expression of CCR2, CXCR3 and CXCR5 in CD4+ T cells, CXCR3 in CD8+ T cells, Th1 clones, and TCR Vdelta2/gamma9 T cells, and upregulated CXCR4 expression in TCR Vdelta2/gamma9 T cells. In addition, sHLA-G differentially modulates chemotaxis and cytokine/chemokine secretion in NK cells.
HLA-G in pregnancy
Preimplantation development is the first critical step in early human development. It covers the time period from before fertilization to implantation.
(The immunosuppressive molecule HLA-G and its clinical implications, 2012)
Each stage is accompanied by striking changes in protein syntheses, metabolism, and morphology.
HLA-G has a tolerogenic role in maternal-fetal tolerance.
The spatial and temporal HLA-G expression during preimplantation development and its value as a diagnostic marker for the prediction of successful implantation of the embryo after assisted reproduction techniques.
HLA-G is variably expressed during the critical stages of preimplantation development as mRNA as well as a protein; this tempts to speculate that HLA-G would fulfil an essential function in the embryo survival.
The patient’s age, number of transferred embryos, morphological grading of embryos and sHLA-G status were independent predictors of pregnancy by multivariate analysis. The morphological scoring system was found to be the best followed by sHLA-G testing for the selection of the most promising embryos capable of implantation and survival. This implies that sHLA-G might be considered as a second parameter if a choice has to be made between embryos of equal morphological quality.
INVOLVEMENT IN DISEASE
Apart from placenta, HLA-G expression is not detected in most healthy tissues. However, it has been shown to be expressed in allograft after transplantation (HLA-G in organ transplantation: towards clinical applications.Cell Mol Life Sci. 2011) and in diverse pathological conditions, such as autoimmune diseases (rheumatoid arthritis, LES, multiple sclerosis) or viral infections (cytomegalovirus, HIV-1, influenza A virus, H1N1 influenza virus).
One of the most noticeable and striking findings of HLA-G research was that some tumors have the capability to neo-express this molecule (Soluble human leukocyte antigen-G serum level is elevated in melanoma patients and is further increased by interferon-alpha immunotherapy.Cancer. 2001). Given the immune-inhibitory functions of HLA-G, this expression could constitute a mechanism used by cancer cells to evade immune surveillance.
Because soluble HLA-G (sHLA-G) is released into the circulation, its measurement in body fluids can be a valuable tool in the management of these diseases.
HLA-G in pathological pregnancies
HLA-G expression is lower in placental tissues obtained from women with recurrent spontaneous abortion (RSA) and pre-eclampsia (Gene expression in chorionic villous samples at 11 weeks of gestation in women who develop pre-eclampsia later in pregnancy: implications for screening.Prenat Diagn. 2011). Both complications may be caused by an inadequate immune tolerance at the maternal-fetal interface that leads to an immune response directed at the fetus.
Low levels of HLA-G in pre-eclampsia may indicate that trophoblasts lacking HLA-G should constitute targets for a maternal immune attack and/or be unable to invade maternal spiral arteries.
On the other hand, increased sHLA-G levels are found in women with HELLP (Hemolysis, Elevated Liver enzymes, Low Platelet count) syndrome and in women delivered preterm due to intrauterine activation (uncontrolable labor, rupture of fetal membranes, cervical insufficiency).
HLA-G has been studied in depth in this context and may now be of diagnostic utility in the case of infertility or pregnancy complications. Similarly, the study of HLA-G ectopic expression in adult tissues in different diseases has vastly improved the knowledge of these pathologies, particularly autoimmune diseases, viral infections, or cancer. In these situations, the analysis of HLA-G in biological samples can help to understand the disease and has been shown to be of use as a prognostic factor.
Given HLA-G’s immune-inhibitory properties, its measurement may help to choose a therapeutic orientation where HLA-G may be targeted. Indeed, it would be of interest to induce HLA-G expression in infertile patients to increase implantation, or in autoimmune disease patients to turn down the auto-immune reaction, whereas it would be important to suppress its immune-suppressive expression in cancer.