Edited by: Joel LeMaoult, Commissariat a l’Energie Atomique et aux Energies Alternatives, France
Reviewed by: Raymond John Steptoe, University of Queensland, Australia; Shomyseh Sanjabi, Gladstone Institutes, USA
Specialty section: This article was submitted to Immunological Tolerance, a section of the journal Frontiers in Immunology
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The HLA-G molecule is a member of the non-classical HLA class I family. Its surface expression is physiologically restricted to the maternal–fetal interface and to immune privileged adult tissues. Despite the restricted tissue expression, HLA-G is detectable in body fluids as secreted soluble molecules. A unique feature of HLA-G is the structural diversity as surface expressed and as secreted molecules. Secreted HLA-G can be found in various body fluids either as free soluble HLA-G or as part of extracellular vesicles (EVs), which are composed of various antigens/ligands/receptors, bioactive lipids, cytokines, growth factors, and genetic information, such as mRNA and microRNA. Functionally, HLA-G and its secreted forms are considered to play a crucial role in the network of immune-regulatory tolerance mechanisms, preferentially interacting with the cognate inhibitory receptors LILRB1 and LILRB2. The HLA-G mediated tolerance is described in processes of pregnancy, inflammation, and cancer. However, almost all functional and clinical implications of HLA-G
HLA-G is a non-classical HLA class I molecule. It is a potent suppressive molecule that impairs effector functions of immune cells belonging to the innate and adaptive immune system. Under physiological conditions, its surface expression is restricted to the maternal–fetal interface and to immune privileged adult tissues (
A unique feature of HLA-G is that it exists in multiple structures, either expressed on the cell surface or in a secreted form. These different forms can mainly be attributed to alternative splicing of the primary transcript and differential association with β2-microglobulin (β2m). Four isoforms (HLA-G1, G2, G3, and G4) are membrane-expressed and three isoforms express either intron 4 (HLA-G5 and -G6) or intron 2 (HLA-G7) but lack the transmembrane and cytoplasmic domains, resulting in their secretion. With the exception of HLA-G3 (
Regarding function, HLA-G and the soluble counterparts preferentially exert their immune modulating or suppressing functions by interaction with the two inhibitory receptors, leukocyte immunoglobulin-like receptor subfamily B member 1 (LILRB1) and LILRB2. LILRB1 is expressed on subpopulations of T-cells, B-cells, and Natural Killer (NK) cells. Monocytes/macrophages/dendritic cells (DC) express both receptors. These two receptors distinguish between β2m-associated and β2m-free HLA-G: LILRB1 interacts with HLA-G molecules associated to β2m, whereas LILRB2 specifically recognizes β2m-free HLA-G (
Based on the functionality of receptors and their expression profile, membrane-expressed and soluble forms of HLA-G molecules are involved in immune regulation in pregnancy, inflammation, and cancer. Thus, HLA-G can be considered as an immune checkpoint molecule (
Extracellular vesicles are phospholipid bilayer-enclosed vesicles, which are released by most cell types, including immune cells, tumor cells, stroma cells, trophoblast cells, and adult and embryonic stem cells (
Different cell types release differently assembled EV. Furthermore, it is tempting to speculate that even individual cells release different EV types. Importantly, the cell of origin controls the molecular composition and cargo (
The composition of EVs is responsible for the biodistribution, for the interaction of EVs to target cells or to extracellular matrix. Membrane fusion of EVs to target cells allows the transfer of bioactive molecules, including, e.g., CCR5 (
Besides membrane fusion, EVs can be internalized by different pathways including phagocytosis, clathrin- and caveolin-mediated endocytosis, or micropinocytosis (
The communication and immune modulation by EVs take place among cells within same entity or between different types of cells. Various effector cells of the innate and adaptive immune system, including T cells and NK cells, antigen presenting cells (APCs), and mast cells have been reported to donate or to acquire ligand/receptor/genetic information
Without any doubt, the neo-ectopic expression of HLA-G molecules either on the surface of tumor cells or released as soluble forms can be considered a critical factor for cancer progression. Albeit high blood levels of soluble forms of HLA-G have concordantly been related to cancer, the prognostic relevance of soluble HLA-G in the blood has not always been established as an independent marker in terms of disease progression and survival (
Secreted HLA-G-expressing EVs (Table
Cell type | EVs source | Potential target cell response | Function/mechanism | Clinical relevance | Reference |
---|---|---|---|---|---|
Melanoma | Tolerance-inducing effect of melanoma derived HLA-G-bearing EVs on immune cells | Potential induction of inhibitory signaling by HLA-G1-bearing EVs |
Unknown clinical relevance | ( |
|
Kidney cancer | Inhibitory effect of HLA-G-bearing EVs on monocyte differentiation into mature DCs and reduced T cell proliferation | Inhibitory effect of HLA-G1-bearing EVs on monocyte differentiation and their maturation to DCs | Suppression of immune effector cells by HLA-G1-bearing EVs, leading to disease progression | ( |
|
Breast cancer | Modulation of immune effector functions by circulating HLA-G-bearing EVs | Unknown function | Association of high circulating amounts of HLA-G-bearing EVs to disease progression | ( |
|
Trophoblast | Modulation of immune effector functions by cytotrophoblast-derived HLA-G5-bearing EVs | Unknown function | Unknown clinical relevance, but potential biomarker for pregnancy-related disorders | ( |
|
Mesenchymal stem/stromal cells (MSCs) | Induction of tolerance between graft and host immune cells by MSCs-derived EVs | Immunomodulation by synergistic additive effect of HLA-G, IL-10, and TGFβ | Potential therapeutic option for patients with therapy–refractory GvHD using MSC-derived HLA-G-bearing EVs | ( |
The first
Very recently, we established the prognostic relevance of HLA-G-bearing EVs for neoadjuvant chemotherapy-treated (NACT) breast cancer patients for the first time (
Further underlining the functional relevance of HLA-G-bearing EVs in cancer, a recent study demonstrated that (i) EVs released by renal cancer stem cells carry HLA-G with a HLA-G1 typical molecular weight, (ii) these HLA-G-bearing EVs impair the differentiation of monocytes to mature DCs, and (iii) the presence of these DCs reduces the T cell proliferation. Thus, HLA-G-bearing EVs mediate inhibitory effects on monocyte differentiation and their maturation to DCs (
At the maternal–fetal interface, HLA-G and its soluble forms are expressed on both sides, on extravillous trophoblast cells lining the placenta and on tolerance-inducing DCs (DC-10) being enriched in the first trimester decidua (
Similar to trophoblast cells, mesenchymal stem/stromal cells (MSCs) express surface-expressed and soluble forms of HLA-G, which are involved in the suppression of T and NK cell functions (
Currently, the known functions of HLA-G are restricted to receptors expressed on the surface of effector cells of the innate and adaptive immune system. In this way, HLA-G inhibits the cytolytic function of NK cells (
Membrane fusion of EVs to target cells can represent a possible mode of how HLA-G can be transferred to target cells. In this context, it is noteworthy that a cellular translocation of HLA-G from APCs to activated T cells (
Independent of the pathway, internalization of HLA-G-bearing EVs provides the opportunity for HLA-G to participate in yet unknown intracellular pathways. Interestingly, both soluble HLA-G5 and shed HLA-G1 have been reported to be bound by the transiently expressed KIR2DL4 receptor and to be endocytosed into early endosomes of NK cells (
Regarding the role of HLA-G in diagnosis, prognosis, and treatment, the cell-specific signature of HLA-G-bearing EVs may not only provide information about the potential target cells and about its potential interplay of the cognate receptor/ligand on target cells but also about the cells producing these EVs (
It is well established that tumor cells, cytotrophoblast cells, and MSCs secret HLA-G-bearing EVs in addition to non-vesicular soluble HLA-G. All of these cell types are highly capable of promoting immune tolerance and tissue remodeling. Mechanisms and functional consequences of HLA-G-bearing EVs and their specific contribution to the biology of these cells have yet to be determined. So far, the classical concept of HLA-G function is based on the interaction of HLA-G with receptors being expressed on the cell surface membrane. EVs, however, may serve as a ticket for HLA-G to interact directly with cells or to enter into the inside of cells. The internalization of HLA-G may introduce new pathways or yet unknown cognate receptors, by which HLA-G contributes to intracellular communication. In that way, HLA-G-bearing EVs are likely to represent an important element in the biology of HLA-G.
VR: concept and design, drafting of manuscript. VR and PH: critical revision of the manuscript for important intellectual points, supervision. LK, FN, BW, LM, VR, and PH: drafting of manuscript.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
This work was supported by IFORES (D/107-81080), University of Duisburg-Essen, and by Deutsche Krebshilfe (109816).
This work was partly supported by research Ph.D. scholarship funding from CAPES Foundation, Ministry of Education of Brazil, Brasília – DF 70.040-020, Brazil.