The role of HLA-G in immunity and hematopoiesis

The non-classical HLA class I molecule HLA-G was initially shown to play a major role in feto–maternal tolerance. Since this discovery, it has been established that HLA-G is a tolerogenic molecule which participates to the control of the immune response. In this review, we summarize the recent advances on (1) the multiple structures of HLA-G, which are closely associated with their role in the inhibition of NK cell cytotoxicity, (2) the factors that regulate the expression of HLA-G and its receptors, (3) the mechanism of action of HLA-G at the immunological synapse and through trogocytosis, and (4) the generation of suppressive cells through HLA-G. Moreover, we also review recent findings on the non-immunological functions of HLA-G in erythropoiesis and angiogenesis.     

Inhibition of human Vγ9Vδ2 T-cell antitumoral activity through HLA-G: implications for immunotherapy of cancer

Vγ9Vδ2 T cells play a crucial role in the antitumoral immune response through cytokine production and cytotoxicity. Although the expression of the immunomodulatory molecule HLA-G has been found in diverse tumors, its impact on Vγ9Vδ2 T-cell functions remains unknown. Here we showed that soluble HLA-G inhibits Vγ9Vδ2 T-cell proliferation without inducing apoptosis. Moreover, soluble HLA-G inhibited the Vγ9Vδ2 T-cell production of IFN-γ induced by phosphoantigen stimulation. The reduction in Vγ9Vδ2 T-cell IFN-γ production was also induced by membrane-bound or soluble HLA-G expressed by tumor cell lines. Finally, primary tumor cells inhibited Vγ9Vδ2 T-cell proliferation and IFN-γ production through HLA-G. In this context, HLA-G impaired Vγ9Vδ2 T-cell cytotoxicity by interacting with ILT2 inhibitory receptor. These data demonstrate that HLA-G inhibits the anti-tumoral functions of Vγ9Vδ2 T cells and imply that treatments targeting HLA-G could optimize Vγ9Vδ2 T-cell-mediated immunotherapy of cancer.     

Detection of HLA-E and -G DNA alleles for population and disease studies

HLA-E and -G genes show a restricted polymorphism encoding for molecules whose variability is limited at the peptide binding site. Fourteen alleles that give rise to only three productive proteins for HLA-G (*0101, *0103 and *0104) and five alleles with three different proteins for HLA-E (*0101, *0102 and *0103) have been described. Expression of these molecules is low and found in many tissues for HLA-E; HLA-G protein is expressed in extravillous trophoblast cells and thymic epithelium. Molecular studies have shown how HLA-G and HLA-E bind to natural killer (NK) cells immunoglobulin and lectin-type inhibitory receptors. HLA-E may act as a sentinel of the cell; if classical class I and HLA-G are being expressed, HLA-E molecules may reach the cell surface and inhibit the lysis by NK cells. Most findings are consistent with the hypothesis that HLA-E and -G proteins may be tolerogenic molecules at either the T-cell receptor (TcR) (inflammation, graft rejection) or NK level, switching off cells which usually attack foreign (including foetus) or self (autoimmune) antigens. A low HLA-E and -G polymorphism is observed in humans, and their allele frequencies are mostly homogeneous in the populations tested so far. Many studies to detect these alleles are now being performed in isolated populations and also in pregnancy-associated pathologies. In the present paper, standard and detailed techniques to detect HLA-E and -G DNA polymorphism are reported and discussed. Received 14 July 1999; received after revision 25 August 1999; accepted 25 August 1999     

Natural killer cell activation by dendritic cells: balancing inhibitory and activating signals

Natural killer (NK) cells have originally been identified by their spontaneous cytolytic potential against tumor cells, which, however, might result from pre-activation due to prior pathogen exposure. Resting NK cells, on the contrary, require activation by bystander antigen-presenting cells to reach their full functional competence. In this review, we will summarize studies on how dendritic cells (DCs), the most potent type of antigen-presenting cell, communicate with human NK cells to activate them in secondary lymphoid organs and to integrate signals from activated NK cells at sites of inflammation for their own maturation. Furthermore, we will review aspects of the immunological synapse, which mediates this cross-talk. These studies provide the mechanistic understanding of how mature DCs can activate NK cells and survive to go on for the activation of adaptive immunity. This feature of DCs, to activate different waves of immune responses, could be harnessed for immunotherapies, including vaccinations.     

Interleukin-33 stimulates formation of functional osteoclasts from human CD14+ monocytes

Interleukin (IL)-33 is a recently described pro-inflammatory cytokine. Here we demonstrate IL-33 as a regulator of functional osteoclasts (OCs) from human CD14⁺ monocytes. IL-33 stimulates formation of tartrate-resistant acid phosphatase (TRAP)⁺ multinuclear OCs from monocytes. This action was suppressed by anti-ST2 antibody, suggesting that IL-33 acts through its receptor ST2, but not by the receptor activator of NF-κB ligand (RANKL) decoy, osteoprotegerin, or anti-RANKL antibody. IL-33 stimulated activating phosphorylations of signaling molecules in monocytes that are critical for OC development. These included Syk, phospholipase Cγ2, Gab2, MAP kinases, TAK-1, and NF-κB. IL-33 also enhanced expression of OC differentiation factors including TNF-α receptor-associated factor 6 (TRAF6), nuclear factor of activated T cells cytoplasmic 1, c-Fos, c-Src, cathepsin K, and calcitonin receptor. IL-33 eventually induced bone resorption. This study suggests that the osteoclastogenic property of IL-33 is mediated through TRAF6 as well as the immunoreceptor tyrosine-based activation motif-dependent Syk/PLCγ pathway in human CD14⁺ monocytes.     

HLA-G in organ transplantation: towards clinical applications

HLA-G plays a particular role during pregnancy in which its expression at the feto–maternal barrier participates into the tolerance of the allogenic foetus. HLA-G has also been demonstrated to be expressed in some transplanted patients, suggesting that it regulates the allogenic response. In vitro data indicate that HLA-G modulates NK cells, T cells, and DC maturation through its interactions with various inhibitory receptors. In this paper, we will review the data reporting the HLA-G involvement of HLA-G in human organ transplantation, then factors that can modulate HLA-G, and finally the use of HLA-G as a therapeutic tool in organ transplantation.     

Mononuclear phagocyte-mediated antifungal immunity: the role of chemotactic receptors and ligands

Over the past two decades, fungal infections have emerged as significant causes of morbidity and mortality in patients with hematological malignancies, hematopoietic stem cell or solid organ transplantation and acquired immunodeficiency syndrome. Besides neutrophils and CD4⁺ T lymphocytes, which have long been known to play an indispensable role in promoting protective antifungal immunity, mononuclear phagocytes are now being increasingly recognized as critical mediators of host defense against fungi. Thus, a recent surge of research studies has focused on understanding the mechanisms by which resident and recruited monocytes, macrophages and dendritic cells accumulate and become activated at the sites of fungal infection. Herein, we critically review how a variety of G-protein coupled chemoattractant receptors and their ligands mediate mononuclear phagocyte recruitment and effector function during infection by the most common human fungal pathogens.     

Human adenosine deaminases ADA1 and ADA2 bind to different subsets of immune cells

At sites of inflammation and tumor growth, the local concentration of extracellular adenosine rapidly increases and plays a role in controlling the immune responses of nearby cells. Adenosine deaminases ADA1 and ADA2 (ADAs) decrease the level of adenosine by converting it to inosine, which serves as a negative feedback mechanism. Mutations in the genes encoding ADAs lead to impaired immune function, which suggests a crucial role for ADAs in immune system regulation. It is not clear why humans and other mammals possess two enzymes with adenosine deaminase activity. Here, we found that ADA2 binds to neutrophils, monocytes, NK cells and B cells that do not express CD26, a receptor for ADA1. Moreover, the analysis of CD4+ T-cell subset revealed that ADA2 specifically binds to regulatory T cells expressing CD39 and lacking the receptor for ADA1. Also, it was found that ADA1 binds to CD16? monocytes, while CD16+ monocytes preferably bind ADA2. A study of the blood samples from ADA2-deficient patients showed a dramatic reduction in the number of lymphocyte subsets and an increased concentration of TNF-α in plasma. Our results suggest the existence of a new mechanism, where the activation and survival of immune cells is regulated through the activities of ADA2 or ADA1 anchored to the cell surface.     

Emerging topics and new perspectives on HLA-G

Following the Fifth International Conference on non-classical HLA-G antigens (HLA-G), held in Paris in July 2009, we selected some topics which focus on emerging aspects in the setting of HLA-G functions. In particular, HLA-G molecules could play a role in: (1) various inflammatory disorders, such as multiple sclerosis, intracerebral hemorrhage, gastrointestinal, skin and rheumatic diseases, and asthma, where they may act as immunoregulatory factors; (2) the mechanisms to escape immune surveillance utilized by several viruses, such as human cytomegalovirus, herpes simplex virus type 1, rabies virus, hepatitis C virus, influenza virus type A and human immunodeficiency virus 1 (HIV-1); and (3) cytokine/chemokine network and stem cell transplantation, since they seem to modulate cell migration by the downregulation of chemokine receptor expression and mesenchymal stem cell activity blocking of effector cell functions and the generation of regulatory T cells. However, the immunomodulatory circuits mediated by HLA-G proteins still remain to be clarified.     

<Emphasis Type="Italic">Yersinia pseudotuberculosis</Emphasis> supports Th17 differentiation and limits de novo regulatory T cell induction by directly interfering with T cell receptor signaling

Adaptive immunity critically contributes to control acute infection with enteropathogenic Yersinia pseudotuberculosis; however, the role of CD4⁺ T cell subsets in establishing infection and allowing pathogen persistence remains elusive. Here, we assessed the modulatory capacity of Y. pseudotuberculosis on CD4⁺ T cell differentiation. Using in vivo assays, we report that infection with Y. pseudotuberculosis resulted in enhanced priming of IL-17-producing T cells (Th17 cells), whereas induction of Foxp3⁺ regulatory T cells (Tregs) was severely disrupted in gut-draining mesenteric lymph nodes (mLNs), in line with altered frequencies of tolerogenic and proinflammatory dendritic cell (DC) subsets within mLNs. Additionally, by using a DC-free in vitro system, we could demonstrate that Y. pseudotuberculosis can directly modulate T cell receptor (TCR) downstream signaling within naïve CD4⁺ T cells and Tregs via injection of effector molecules through the type III secretion system, thereby affecting their functional properties. Importantly, modulation of naïve CD4⁺ T cells by Y. pseudotuberculosis resulted in an enhanced Th17 differentiation and decreased induction of Foxp3⁺ Tregs in vitro. These findings shed light to the adjustment of the Th17-Treg axis in response to acute Y. pseudotuberculosis infection and highlight the direct modulation of CD4⁺ T cell subsets by altering their TCR downstream signaling.     

The importance of HLA-G expression in embryos,trophoblast cells,and embryonic stem cells

The nonclassical HLA-G molecule is a trophoblast-specific molecule present in almost every pregnancy. It differs from classical HLA class I molecules by the low degree of allelic variants and the high diversity of protein structures. HLA-G is reported to be a tolerogenic molecule that acts on cells of both innate and adaptive immunity. At the maternal–fetal interface HLA-G seems to be responsible largely for the reprogramming of local maternal immune response. This review will focus on the HLA-G gene expression profile in pregnancy, in preimplantation embryos, and in human embryonic stem cells with emphasis on the structural diversity of the HLA-G protein and its potential functional and diagnostic implications.     

Biology of HLA-G in cancer: a candidate molecule for therapeutic intervention?

Although the expression of the non-classical HLA class I molecule HLA-G was first reported to be restricted to the fetal–maternal interface on the extravillous cytotrophoblasts, the distribution of HLA-G in normal tissues appears broader than originally described. HLA-G expression was found in embryonic tissues, in adult immune privileged organs, and in cells of the hematopoietic lineage. More interestingly, under pathophysiological conditions HLA-G antigens may be expressed on various types of malignant cells suggesting that HLA-G antigen expression is one strategy used by tumor cells to escape immune surveillance. In this article, we will focus on HLA-G expression in cancers of distinct histology and its association with the clinical course of diseases, on the underlying molecular mechanisms of impaired HLA-G expression, on the immune tolerant function of HLA-G in tumors, and on the use of membrane-bound and soluble HLA-G as a diagnostic or prognostic biomarker to identify tumors and to monitor disease stage, as well as on the use of HLA-G as a novel therapeutic target in cancer.     

Dissection of cytotoxic and helper T cell responses

Cytotoxic (CD8⁺) and helper (CD4⁺) T cells play a crucial role in resolving infections by intracellular pathogens. The development of technologies to visualize antigen-specific T cell responses in mice and men over the past decade has allowed a dissection of the formation of adaptive T cell immunity. This review gives a brief overview of the currently used detection techniques and possible future additions. Furthermore, we discuss our current understanding of the formation of antigen-specific T cell responses, with particular attention to the similarities and differences in CD4⁺ and CD8⁺ T cell responses, the functional heterogeneity within responder T cell pools and the regulation of CD8⁺ T cell responses by dendritic cells and CD4⁺ helper T cells. Received 16 June 2005; received after revision 2 August 2005; accepted 15 August 2005     

HLA-G protein processing and transport to the cell surface

Data are presented on the intracellular trafficking of HLA-G protein, taking the unique features of this non-classical molecule into consideration: the existence of seven isoforms resulting from alternative splicing (HLA-G1 to G7), and reduced tail length compared with HLA class I antigens. Biochemical studies and analysis of viral strategies for escaping the host immune system led to the demonstration that (i) both the membrane-bound (HLA-G1) and the soluble (HLA-G5) forms of the molecule require peptide association for cell surface expression, using TAP-dependent or TAP-independent pathways; (ii) peptide loading onto the HLA-G protein plays a critical role in controlling the quality of the molecule reaching the cell surface; (iii) surface expression of truncated HLA-G molecules is possible, and (iv) HLA-G expression may be restricted to soluble HLA-G5. These data reveal that HLA-G presents specific cell trafficking pathways and strongly support the contention that the primary function of HLA-G is as of an inhibitor ligand for immune-competent cells. Received 4 June 2002; accepted 2 July 2002 RID="*" ID="*"Corresponding author.     

Blockage of the NLRP3 inflammasome by MCC950 improves anti-tumor immune responses in head and neck squamous cell carcinoma

The NLRP3 inflammasome is a critical innate immune pathway responsible for producing active interleukin (IL)-1β, which is associated with tumor development and immunity. However, the mechanisms regulating the inflammatory microenvironment, tumorigenesis and tumor immunity are unclear. Herein, we show that the NLRP3 inflammasome was over-expressed in human HNSCC tissues and that the IL-1β concentration was increased in the peripheral blood of HNSCC patients. Additionally, elevated NLRP3 inflammasome levels were detected in tumor tissues of Tgfbr1/Pten 2cKO HNSCC mice, and elevated IL-1β levels were detected in the peripheral blood serum, spleen, draining lymph nodes and tumor tissues. Blocking NLRP3 inflammasome activation using MCC950 remarkably reduced IL-1β production in an HNSCC mouse model and reduced the numbers of myeloid-derived suppressor cells (MDSCs), regulatory T cells (Tregs) and tumor-associated macrophages (TAMs). Moreover, inhibiting NLRP3 inflammasome activation increased the numbers of CD4⁺ and CD8⁺ T cells in HNSCC mice. Notably, the numbers of exhausted PD-1⁺ and Tim3⁺ T cells were significantly reduced. A human HNSCC tissue microarray showed that NLRP3 inflammasome expression was correlated with the expression of CD8 and CD4, the Treg marker Foxp3, the MDSC markers CD11b and CD33, and the TAM markers CD68 and CD163, PD-1 and Tim3. Overall, our results demonstrate that the NLRP3 inflammasome/IL-1β pathway promotes tumorigenesis in HNSCC and inactivation of this pathway delays tumor growth, accompanied by decreased immunosuppressive cell accumulation and an increased number of effector T cells. Thus, inhibition of the tumor microenvironment through the NLRP3 inflammasome/IL-1β pathway may provide a novel approach for HNSCC therapy.     

Dynamics of the interaction of γδ T cells with their neighbors in vivo

γδ T cells are a diverse component of the immune system in humans and mice with presumably important but still largely unknown functions. Understanding the dynamic interaction of γδ T cells with their neighbors should help to understand their physiological role. This review addresses recent advances and strategies to visualize the dynamic interactions of γδ T cells with their neighbors in vivo. Current knowledge regarding the dynamic contacts of tissue resident γδ T cells and epithelial cells, but also of the communication between circulating γδ T cells and DCs, monocytes and FoxP3⁺ regulatory T cells is revisited with emphasis on the role of γδ T cell motility.     

Mechanisms of NK cell activation: CD4+ T cells enter the scene

Natural killer (NK) cells are innate lymphocytes involved in immunosurveillance through their cytotoxic activity and their capacity to secrete inflammatory cytokines. NK cell activation is necessary to initiate effector functions and results from a complex series of molecular and cellular events. We review here the signals that trigger NK cells and discuss recent findings showing that, besides antigen-presenting cells, T cells can play a central role in the initiation of NK cell activation in lymph nodes.