Involvement of circulating soluble HLA-G after liver transplantation in the low immunogenicity of hepatic allograft.

Graft rejection is a critical risk in solid-organ transplantation. To decrease such risk, an understanding of the factors involved in low immunogenicity of liver allografts could potentially make it possible to transfer this tolerogenic property to other transplanted organs. HLA-G, a natural physiological molecule belonging to the Human Leukocyte Antigen class (HLA) Ib family that induces tolerance, is associated with fewer rejections in solid-organ transplantation. In contrast to HLA-G, HLA antigen incompatibilities between donor and recipient can lead to rejection, except in liver transplantation. We compared HLA-G plasma levels and the presence of anti-HLA antibodies before and after LT to understand the low immunogenicity of the liver. We conducted a large prospective study that included 118 patients on HLA-G plasma levels during a 12-month follow-up and compared them to the status of anti-HLA antibodies. HLA-G plasma levels were evaluated by ELISA at seven defined pre- and post-LT time points. HLA-G plasma levels were stable over time pre-LT and were not associated with patient characteristics. The level increased until the third month post-LT, before decreasing to a level comparable to that of the pre-LT period at one year of follow-up. Such evolution was independent of biological markers and immunosuppressive treatment, except with glucocorticoids. An HLA-G plasma level ≤ 50 ng/ml on day 8 after LT was significantly associated with a higher rejection risk. We also observed a higher percentage of rejection in the presence of donor specific anti-HLA antibodies (DSA) and an association between the increase in HLA-G plasma levels at three months and the absence of DSA. The low immunogenicity of liver allografts could be related to early elevated levels of HLA-G, which lead, in turn, to a decrease in anti-HLA antibodies, opening potential new therapeutic strategies using synthetic HLA-G proteins.

HLA-G Is Widely Expressed by Mast Cells in Regions of Organ Fibrosis in the Liver, Lung and Kidney.

We previously demonstrated that mast cells expressing HLA-G are associated with regions of hepatitis C virus-induced liver fibrosis. Here, we aimed to determine whether HLA-G expression in mast cells is specific to viral etiology, the liver, or to the general process of fibrosis. We enumerated HLA-G+ cells and mast cells by the immunohistochemistry of (i) liver blocks from 41 cases of alcoholic cirrhosis, (ii) 10 of idiopathic pulmonary fibrosis (IPF), and (iii) 10 of renal fibrosis. The nature of the HLA-G+ cells was specified by multiplex immunofluorescence using software. More than half of all HLA-G+ cells were mast cells in fibrotic areas of alcoholic cirrhosis and IPF. In the kidneys, subjected to fibrosis, the HLA-G+ cells were indeed mast cells but could not be counted. Moreover, in certain cases of the liver and lung, we observed a number of cellular nodes, which were secondary or tertiary follicles, in which HLA-G was highly expressed by B lymphocytes. In conclusion, HLA-G+ mast cells could be observed in the fibrotic regions of all organs studied. Previous studies suggest a protective role for HLA-G+ mast cells against inflammation and fibrosis. The observed follicles with B lymphocytes that express HLA-G may also reinforce their antifibrotic role.

The anti-fibrotic role of mast cells in the liver is mediated by HLA-G and interaction with hepatic stellate cells.

BACKGROUND & AIMS: We have reported a significant association between HLA-G expression or the number of hepatic mast cells and liver fibrosis. Here, we investigated the role of HLA-G and mast cells in liver fibrosis, focusing, in particular, on interactions between human mast and stellate cells. METHODS: Human mast cells (HMC cell line, CD34-derived mast cells, or tissue-derived mast cells) were co-cultured with purified human hepatic stellate cells (HSCs), and collagen I production by HSCs was evaluated. Mast cells and HSCs were characterized by immunocytochemistry. Various conditions were tested: different times in direct or indirect contact, presence or absence of cytokines, addition or not of HLA-G, and presence or absence of specific protease inhibitors. RESULTS: The reciprocal interaction between HSCs and mast cells led to the attraction of mast cells to HSCs in vivo and in vitro, and to a significant decrease in collagen production, at all times of co-culture, following the direct or indirect contact of mast cells with HSCs alone or in the presence of TGF-ß, IFN-α or IL-10. We identified the diffusible factors involved in collagen I degradation as mast cell proteases. Moreover, HLA-G expression increased during the co-culture of HSCs and mast cells, with HLA-G acting on both mast cells and HSCs, to enhance collagen I degradation. CONCLUSIONS: Mast cells play a beneficial, anti-fibrotic role in liver fibrosis, via the HLA-G-mediated decrease of collagen I. These findings are consistent with high levels of cross-communication between mast cells and hepatic stellate cells and the role of HLA-G.

Biology of the immunomodulatory molecule HLA-G in human liver diseases.

The non-classical human leukocyte antigen-G (HLA-G), plays an important role in inducing tolerance, through its immunosuppressive effects on all types of immune cells. Immune tolerance is a key issue in the liver, both in liver homeostasis and in the response to liver injury or cancer. It would therefore appear likely that HLA-G plays an important role in liver diseases. Indeed, this molecule was recently shown to be produced by mast cells in the livers of patients infected with hepatitis C virus (HCV). Furthermore, the number of HLA-G-positive mast cells was significantly associated with fibrosis progression. The generation of immune tolerance is a role common to both HLA-G, as a molecule, and the liver, as an organ. This review provides a summary of the evidence implicating HLA-G in liver diseases. In the normal liver, HLA-G transcripts can be detected, but there is no HLA-G protein. However, HLA-G protein is detectable in the liver tissues and/or plasma of patients suffering from hepatocellular carcinoma, hepatitis B or C, or visceral leishmaniasis and in liver transplant recipients. The cells responsible for producing HLA-G differ between diseases. HLA-G expression is probably induced by microenvironmental factors, such as cytokines. The expression of HLA-G receptors, such as ILT2, ILT4, and KIRD2L4, on liver cells has yet to be investigated, but these receptors have been detected on all types of immune cells, and such cells are present in liver. The tolerogenic properties of HLA-G explain its deleterious effects in cancers and its beneficial effects in transplantation. Given the key role of HLA-G in immune tolerance, new therapeutic agents targeting HLA-G could be tested for the treatment of these diseases in the future.

Immunomodulatory properties of HLA-G in infectious diseases.

HLA-G is a nonclassical major histocompatibility complex molecule first described at the maternal-fetal interface, on extravillous cytotrophoblasts. Its expression is restricted to some tissues in normal conditions but increases strongly in pathological conditions. The expression of this molecule has been studied in detail in cancers and is now also beginning to be described in infectious diseases. The relevance of studies on HLA-G expression lies in the well known inhibitory effect of this molecule on all cell types involved in innate and adaptive immunity, favoring escape from immune control. In this review, we summarize the features of HLA-G expression by type of infections (i.e, bacterial, viral, or parasitic) detailing the state of knowledge for each pathogenic agent. The polymorphism, the interference of viral proteins with HLA-G intracellular trafficking, and various cytokines have been described to modulate HLA-G expression during infections. We also discuss the cellular source of HLA-G, according to the type of infection and the potential role of HLA-G. New therapeutic approaches based on synthetic HLA-G-derived proteins or antibodies are emerging in mouse models of cancer or transplantation, and these new therapeutic tools may eventually prove useful for the treatment of infectious diseases.

Expression of HLA-G by mast cells is associated with hepatitis C virus-induced liver fibrosis.

BACKGROUND & AIMS: Infection with hepatitis C virus is a worldwide health problem. An inadequate Th2 cytokine response promotes the fibrosis-cirrhosis fate. Immune-modulating molecules favoring a Th2 profile, such as HLA-G molecules of the HLA class Ib family, may play a role in chronic hepatitis. HLA-G contributes to the escape of tumors, and their involvement in viral infections has been increasingly described. The aim of this work was to study the expression of HLA-G in the liver, its cellular source and its regulation in cases of chronic C hepatitis. METHODS: HLA-G cells in blocks of liver derived from patients infected with HCV were labeled by immunohistochemistry and enumerated. Double immunofluorescence allowed the identification of the cellular source. HLA-G secretion by a human mast cell line was quantified by ELISA after various stimulations. After treatment with IFN-α, real-time PCR was performed to determine the kinetics of cytokine expression profiles, followed by heat map clustering analysis. RESULTS: The number of HLA-G+ cells was significantly associated with the area of fibrosis. For the first time, we identify the HLA-G+ cells as being mast cells. HLA-G secretion was significantly induced in human mast cells stimulated by IL-10 or interferons of class I. The transcriptome of the secretome of this cell line stimulated by IFN-α revealed that (i) the HLA-G gene is upregulated late, and that (ii) T lymphocytes and NK cells are recruited. CONCLUSIONS: These findings suggest an autocrine loop in the genesis of HCV liver fibrosis, based on mast cells expressing HLA-G.

High level of soluble HLA-G in amniotic fluid is correlated with congenital transmission of Toxoplasma gondii.

The expression of human leukocyte antigen (HLA)-G on cytotrophoblast cells contributes to maternal-fetal tolerance. Soluble forms of HLA-G (sHLA-G) can be detected in amniotic fluid (AF) and a decrease of sHLA-G is known to be correlated to fetal loss. In this work we investigated the role of sHLA-G in the transplacental passage of the protozoan parasite Toxoplasma gondii, responsible for congenital toxoplasmosis in about 30% of fetuses when primary infection (PI) occurs during pregnancy. We determined the sHLA-G concentration in 61 AF from women with PI and 24 controls. Our results showed higher sHLA-G levels in AF from PI than in controls (p<0.001). Moreover sHLA-G level from congenitally infected fetuses (n=12) was higher than in fetus in whom congenital infection was ruled out (n=49, p<0.05). These data suggest that sHLA-G could participate in immunomodulation necessary to avoid fetal loss due to Toxoplasma infection, but that over-expression could favor congenital transmission.

Suppressive properties of human CD4+CD25+ regulatory T cells are dependent on CTLA-4 expression.

It has been demonstrated that T cells with regulatory properties are present within the peripheral blood CD4(+)CD25(+) T cell compartment. Here, we describe an original method to purify human CD4(+)CD25(+)CD152(+) T lymphocytes as living cells by forcing the exportation of CTLA-4 molecules stored in intracellular vesicules at the cell surface. By doing so, we demonstrate that CD4(+)CD25(+) T cells contain a smaller and more homogeneous population enriched in cells with in vitro regulatory activity. Moreover, we show that this enrichment in regulatory T cells is associated with an increased expression of Foxp3 and that CD4(+)CD25(+)CD152(+) T lymphocytes display a much stronger suppressive activity in controlling in vitro proliferation of alloantigen-specific T cells than CD4(+)CD25(+)CD152(-) T lymphocytes purified in parallel. Lastly, by purifying such cells expressing CTLA-4, we demonstrate that indeed CTLA-4 is involved in CD4(+)CD25(+)CD152(+) T cell regulatory activity, while suppressive cytokines are not.

Stimulation of peripheral blood and intestinal mucosa cells by synthetic CpG oligodeoxynucleotides.

The breakdown of tolerance to autologous bacterial flora has been implicated as a major factor contributing to the initiation and perpetuation of chronic inflammation in inflammatory bowel diseases (IBD). To test whether bacterial DNA is at the origin of inflammation in IBD, we have examined the response of lamina propria (LPMC) or peripheral mononuclear cells (PBMC) and purified T cells from IBD patients and control patients to stimulations with a set of oligodeoxynucleotides (ODNs) characterized by the presence or absence of cytosine-guanosine dinucleotides (CpG) and/or 3' poly-guanosine (poly-G) extension. Furthermore we have evaluated the costimulatory activities of these ODNs on T cells activated via CD2 or CD3 pathway. We demonstrated that CpG ODNs induce higher proliferation of LPMC from inflammatory intestinal mucosa compared to healthy mucosa. We confirmed that CpG ODNs do not directly costimulate peripheral blood T cells activated by CD3 pathway. Finally, we revealed that CpG or non-CpG ODNs with 3' poly-G extension inhibit completely CD2 activation of purified PB or LP T-cells whereas only CpG ODNs without poly-G extension enhance proliferation and IFN-gamma production of PB T cells stimulated by CD2 pathway only in presence of NK and NK T cells. Our data suggest that NK T cells may be the primary target of ODNs and play a crucial role in indirect T-cell activation by ODN.