Soluble HLA-G expression and renal graft acceptance

Human leucocyte antigen-G: expression and function in airway allergic disease

Human leucocyte antigen-G (HLA-G) is a non-classical HLA class I molecule demonstrated originally in placental trophoblast cells. Recognition of the importance of HLA-G to the maternal immune accommodation of the semi-allogeneic fetus has led to investigations of its role in the suppression of immune responses and induction of tolerance. More recently, HLA-G has been shown to have increased expression in several immunological diseases including asthma and allergic rhinitis. The focus of this review is the potential role of HLA-G in immunological airway diseases.

HLA-G on peripheral blood CD4(+) T lymphocytes: a potential predictor for acute renal rejection

HLA-G Expression in grafts and serum has been shown to improve graft acceptance. However, its expression on peripheral blood lymphocytes (PBLs) during acute rejection (AR) remains unknown. In this study, we serially monitored HLA-G expression on CD4(+) and CD8(+) PBLs of 66 recipients undergoing renal transplantation using flow cytometry at different time points before and after transplantation, as well as during AR episode. In stable recipients, HLA-G expression on CD4(+) PBLs declined during the first week after transplantation and increased continuously with immunosuppressive therapy. Then, expression declined gradually after 1 month and remained at a higher level compared with pretransplantation. When AR occurred, HLA-G expression decreased significantly compared with the stable level. In three recipients suffering from recurrent rejection, it remained at a low level despite impact immunosuppressive treatment. With mix lymphocyte assay, HLA-G(+) CD4(+) T cells showed inhibitory role on proliferation of peripheral blood mononuclear cell. HLA-G expression on CD8(+) PBLs was almost undetectable at different time points in the recipients and healthy controls. Our results suggest that HLA-G on CD4(+) PBLs might provide a potential marker for the early diagnosis of renal AR and for the immunosuppressive status of recipients.

The role of HLA--G 14-bp polymorphism in allo-HSCT after short-term course MTX for GvHD prophylaxis

HLA-G molecules are HLA class Ib antigens characterized by tolerogenic and immunoinhibitory functions. The HLA-G 14-bp insertion/deletion (ins/del) polymorphism controls protein expression and seems to be implicated in both MTX treatment response and SCT outcome. The aim of our study is to evaluate the role of HLA-G 14 bp polymorphism in subjects affected by hematological malignancies undergoing allo-SCT and receiving MTX therapy for GvHD prophylaxis. We performed a retrospective analysis of HLA-G 14 bp polymorphism using a specific PCR in 47 recipients and in their respective donors, and evaluated the correlation with the incidence of aGvHD, OS and disease-free survival (DFS) after allo-SCT. We did not observe any correlation between this polymorphism and the risk of aGvHD occurrence. On the contrary, we found that the recipients with a 14 bp ins/14 bp ins genotype were characterized by a lower OS and DFS in univariate and multivariate analysis (OS=OR: 3.235; DFS=OR: 3.302). These data indicate a role for recipient HLA-G 14 bp polymorphism in allo-SCT immunotolerance status and follow-up.

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.

A simple method for identifying bone marrow mesenchymal stromal cells with a high immunosuppressive potential

BACKGROUND AIMS

The beneficial activity of mesenchymal stromal cells (MSC) in allogeneic hematopietic stem cell transplantation requires correct use in terms of cell dose and timing of infusion and the identification of biomarkers for selection. The immunosuppressive bone marrow (BM)-derived MSC (BM-MSC) functions have been associated with the production of soluble HLA-G molecules (sHLA-G) via interleukin (IL)-10. We have established a reliable method for evaluating BM-MSC HLA-G expression without the influence of peripheral blood mononuclear cells (PBMC).

METHODS

Thirteen BM-MSC from donors were activated with recombinant IL-10 or co-cultured with 10 different phytohemagglutinin (PHA)-treated PBMC (PHA-PBMC). Membrane-bound and sHLA-G expression was evaluated by flow cytometry and enzyme-linked immunosorbent assay (ELISA), respectively; lymphoproliferation was measured by (methyl-(3)H)thymidine.

RESULTS

The results demonstrated the ability of IL-10 to stimulate both membrane-bound and sHLA-G production by BM-MSC. The levels of HLA-G expression induced by IL-10 in BM-MSC were associated with the inhibition of PHA-PBMC proliferation (sHLA-G, P = 0.0008, r = 0.9308; membrane HLA-G, P = 0.0005, r = 0.9502).

CONCLUSIONS

We propose the evaluation of sHLA-G production in IL-10-treated BM-MSC cultures as a possible marker of immunoregulatory function.

Implications of the polymorphism of HLA-G on its function, regulation, evolution and disease association

The HLA-G gene displays several peculiarities that are distinct from those of classical HLA class I genes. The unique structure of the HLA-G molecule permits a restricted peptide presentation and allows the modulation of the cells of the immune system. Although polymorphic sites may potentially influence all biological functions of HLA-G, those present at the promoter and 3′ untranslated regions have been particularly studied in experimental and pathological conditions. The relatively low polymorphism observed in the MHC-G coding region both in humans and apes may represent a strong selective pressure for invariance, whereas, in regulatory regions several lines of evidence support the role of balancing selection. Since HLA-G has immunomodulatory properties, the understanding of gene regulation and the role of polymorphic sites on gene function may permit an individualized approach for the future use of HLA-G for therapeutic purposes.

Dynamic expression of Qa-2 during acute graft rejection

Human leukocyte antigen (HLA)-G exhibits immunotolerogenicity and is related to allograft acceptance. Qa-2 is the murine homolog of HLA-G; it has structure and functions similar to those of HLA-G. We investigated the dynamic expression of Qa-2 in skin allografts by immunohistochemistry and on peripheral blood lymphocyte subsets by flow cytometry during the entire process of acute graft rejection (AGR) with a murine skin transplantation model to determine its relationship with the pathological changes of allografts and the influence of immunosuppressive therapy. In grafts without immunosuppressive treatment, Qa-2 did not exhibit obvious changes in syngeneic and allogeneic recipients. In contrast, with immunosuppressant-treated grafts, positive expression of Qa-2 was observed. It remained at high levels in the immunosuppressant-treated syngeneic group; however, it became weakly positive and even negative in infiltrating inflammatory cells as AGR advanced, but it remained strongly positive in other skin tissues throughout the AGR process. Qa-2 expression on CD4(+) and CD8(+) peripheral blood lymphocyte subsets remained stable at a normal level in the non-immunosuppressant-treated syngeneic group. Immunosuppressive treatment can also significantly upregulate Qa-2. In the allogeneic groups, decreased expression was observed when AGR was at histological grades 1 to 2 (well before gross rejection was observed). Qa-2 was upregulated again after the graft was rejected completely. The results suggest that the increase in Qa-2 may be attributed to the use of immunosuppressive treatments. Moreover, Qa-2 expression decreased significantly with AGR progression, suggesting that it may be a potential marker for predicting AGR, especially in the presence of immunosuppressive agents.

Monitoring the operationally tolerant liver allograft recipient

PURPOSE OF REVIEW

A fundamental goal in transplantation is the establishment of allograft function without ongoing immunosuppression. Robust allograft tolerance has been established in experimental transplantation models, whereas clinical operational tolerance has been described most frequently following human liver transplantation.

RECENT FINDINGS

Clinical assessment of tolerance has been limited to laboratory evaluation of organ function. Additional tools include graft monitoring through biopsy and blood sampling for biomarker analysis. Current biomarkers under assessment in recent years include dendritic cell subsets, regulatory T cells, antidonor antibodies, and gene polymorphisms. Emerging microarray analysis that is being prospectively validated will also be reviewed. A further tool in the characterization of the tolerant patient will be the accurate enrollment of such patients into a multicenter registry that will prospectively follow the natural history of the patient withdrawn from immunosuppression and help facilitate the entry of interested patients to mechanistic and immune monitoring trials. The International Solid Organ Transplant Tolerance Registry (www.transplant-tolerance.org) will be briefly described.

SUMMARY

Effective biomarker characterization of the operationally tolerant liver allograft recipient would allow earlier, well tolerated, prospective drug withdrawal with the goal of extending the potential benefits of drug minimization to an increasing number of patients in a more predictable fashion.

Heterogeneous pathways of maternal-fetal transmission of human viruses (review)

Several viruses can pass the maternal-fetal barrier, and cause diseases of the fetus or the newborn. Recently, however, it became obvious, that viruses may invade fetal cells and organs through different routes without acute consequences. Spermatozoa, seminal fluid and lymphocytes in the sperm may transfer viruses into the human zygotes. Viruses were shown to be integrated into human chromosomes and transferred into fetal tissues. The regular maternal-fetal transport of maternal cells has also been discovered. This transport might implicate that lymphotropic viruses can be released into the fetal organs following cellular invasion. It has been shown that many viruses may replicate in human trophoblasts and syncytiotrophoblast cells thus passing the barrier of the maternal-fetal interface. The transport of viral immunocomplexes had also been suggested, and the possibility has been put forward that even anti-idiotypes mimicking viral epitopes might be transferred by natural mechanisms into the fetal plasma, in spite of the selective mechanisms of apical to basolateral transcytosis in syncytiotrophoblast and basolateral to apical transcytosis in fetal capillary endothelium. The mechanisms of maternal-fetal transcytosis seem to be different of those observed in differentiated cells and tissue cultures. Membrane fusion and lipid rafts of high cholesterol content are probably the main requirements of fetal transcytosis. The long term presence of viruses in fetal tissues and their interactions with the fetal immune system might result in post partum consequences as far as increased risk of the development of malignancies and chronic pathologic conditions are discussed.

HLA-G 3'-UTR SNP and 14-bp deletion polymorphisms in Portuguese and Guinea-Bissau populations

In the HLA-G locus, the 3'-untranslated region (3'-UTR) begins in the mid exon 6, and ends in exon 8. The occurrence of a 14-bp deletion within exon 8, the only mutation known until now in the 3'-UTR, has been considered a risk factor for disease and allograft rejection. To describe the polymorphism within this region, direct sequencing analysis was performed on 120 DNA samples from Portugal and Guinea-Bissau. Results indicate that exon 8 is less conserved than the coding exons. Nine single nucleotide polymorphisms and the previously described 14-bp deletion were found within exon 8 of both populations. Molecular diversity was higher in the Guinean samples than in the Portuguese; however, little differentiation was found among the populations, suggesting that local selection on exon 8 sequence variation is absent. The screening for sequence motifs suggests that polymorphism on this region may be involved in HLA-G post-transcriptional regulation and, therefore, in phenotype variation.

Soluble total human leukocyte antigen class I and human leukocyte antigen-G molecules in kidney and kidney/pancreas transplantation

The expression of human leukocyte antigen (HLA)-G, a nonclassical HLA class I molecule, and its soluble forms (sHLA-G) are found to improve graft acceptance. In this study we investigated whether sHLA-G is the most biologically relevant molecule among all types of soluble HLA class I molecules for graft acceptance. We addressed this question in kidney-transplanted (n = 32) and kidney/pancreas-transplanted patients (n = 29). To this end we analyzed the levels of total soluble HLA class I (sHLA-I) in comparison to sHLA-G in 488 plasma samples procured before and serial after transplantation by specific enzyme-linked immunoabsorbent assay. Samples from 126 healthy individuals served as controls. Pretransplantation sHLA-I levels were significantly increased in patients (p < 0.001), whereas sHLA-G levels were in the range of those of healthy controls. Importantly, pretransplantation sHLA-I and sHLA-G levels did not differ between the two groups. Patients with biopsy-proven rejection (n = 15) revealed significantly lower sHLA-G levels before transplantation (mean +/- standard error of the mean, 12.9 +/- 1.8 vs. 20.1 +/- 1.9, p = 0.013) and after transplantation (p = 0.006, two-way analysis of variance) than patients without rejection (n = 46). In contrast, sHLA-I was slightly increased after but not before transplantation in patients with rejection (p < 0.05, two-way analysis of variance). Nonparametric determination analysis showed that pretransplantation levels of sHLA-G < 11.5 ng/ml (sensitivity, 60%; specificity, 80.4%) were related to rejection. Regarding antibody status, retransplantation, number of HLA mismatches, recipient age, and recipient body mass index, multivariate analysis showed that sHLA-G but not sHLA-I is an independent risk factor for graft rejection. Thus high levels of sHLA-G but not of sHLA-I seem to contribute to better graft acceptance after kidney or kidney/pancreas transplantation.

Isoforms of human leukocyte antigen-G and their inhibitory receptors in human kidney allograft acceptance

Novel therapeutic strategies such as the modulation of dendritic cell and T-cell function have exhibited great potential in clinical transplantation. Human leukocyte antigen (HLA)-G is a molecule that plays a significant role in establishing complex mechanisms to protect semiallogeneic fetuses from rejection by the maternal immune system. The unique characteristics of both cell-surface and soluble isoforms of HLA-G, the formation of disulfide-bonded dimers with the potential to augment inhibitory receptor signaling, and the function of HLA-G as a preferential ligand for the immunoglobulin-like transcript receptors make HLA-G very important in fundamental approaches for the modulation of immune responses to improve allogeneic graft survival in clinical transplantation. Experimental data from several groups as well as our data from experiments involving HLA-G-mediated human tolerogenic dendritic cells in vitro and receptor transgenic mice in vivo indicate that different isoforms of HLA-G have various immunomodulatory effects through the inhibitory receptors. This knowledge is crucial in understanding mechanisms of prolongation of allograft survival. The analyses of HLA-G isoforms and inhibitory receptors in patients with kidney allograft and the relationship among different isoforms of HLA-G, inhibitory receptors, their mediated immunoregulation, and graft acceptance or failure will be discussed here.

Regulation of T-cell immunity by leucocyte immunoglobulin-like receptors: innate immune receptors for self on antigen-presenting cells

Following recognition of microbial patterns, innate immune receptors provide a rapid innate response and trigger antigen-presenting cell maturation to instruct adaptive immune responses. Here we discuss a family of innate immune receptors for self - the leucocyte immunoglobulin-like receptors (LILRs). These LILRs exert powerful inhibitory effects on antigen-presenting cell phenotype and subsequent T-cell responses, and may act to constrain the effects of Toll-like receptor signalling. Despite their broad ligand specificity, differing affinities of LILRs for individual complexes of peptide-major histocompatibility complex can determine the nature of their effect on downstream immune responses. Expression and function of LILRs may be skewed in certain conditions such as cancer or human immunodeficiency virus infection, particularly by ectopic expression of human leucocyte antigen-G, a high-affinity LILR ligand. We discuss the relevance of LILR-mediated immune regulation across a range of scenarios from autoimmunity to transplant medicine, infection and cancer.

Non-classical transcriptional regulation of HLA-G: an update

Human leucocyte antigen-G (HLA-G) plays a key role in maternal–foetal tolerance and allotransplantation acceptance and is also implicated in tumour escape from the immune system. The modulation of HLA-G expression can prove to be very important to therapeutic goals in some pregnancy complications, transplantation, cancer and possibly autoimmune diseases. In spite of substantial similarities with classical HLA-class I genes, HLA-G is characterized by a restricted tissue-specific expression in non-pathological situations. HLA-G expression is mainly controlled at the transcriptional level by a unique gene promoter when compared with classical HLA-class I genes, and at the post-transcriptional level including alternative splicing, mRNA stability, translation and protein transport to the cell surface. We focus on the characteristics of the HLA-G gene promoter and the factors which are involved in HLA-G transcriptional modulation. They take part in epigenetic mechanisms that control key functions of the HLA-G gene in the regulation of immune tolerance.

Immunohistochemical study of HLA-G expression in lung transplant recipients

Human leukocyte antigen-G (HLA-G), a nonclassical HLA class I protein, promotes immune tolerance of solid-organ allografts, yet its role in lung transplantation (LTx) is unknown. We examined the expression of HLA-G in lung allografts through immunohistochemistry by a cross-sectional study of 64 LTx recipients, classified into four groups (stable patients, acute rejection [AR], bronchiolitis obliterans syndrome [BOS] and symptomatic viral shedders). A marked expression of HLA-G in bronchial epithelial cells (BEC) was frequently observed in stable recipients (n = 18/35 [51%]), but not in patients with AR (n = 14) or with BOS (n = 8). HLA-G was also expressed by 4 of 7 symptomatic viral shedders. In addition, HLA-G-positive patients from the stable group (n = 35) experienced lower incidence of resistant AR and/or BOS during long-term follow-up, as compared with their HLA-G-negative counterparts. Finally, in vitro data showed that interferon-gamma, a cytokine present in lung allograft microenvironment, upregulated HLA-G mRNA and protein expression in primary cultured human BEC. We conclude that HLA-G expression in the bronchial epithelium of lung allograft is elevated in some LTx recipients in association with their functional stability, suggesting a potential role of HLA-G as a tolerance marker.

A study of HLA-G polymorphism and linkage disequilibrium in renal transplant patients and their donors

The role of the Major Histocompatibility Complex (MHC) in transplantation immunology is widely known. Incompatibilities associated with Human Leukocyte Antigens (HLA) between donors and recipients are related to poorer prognosis in allograft acceptance and survival, often resulting in rejection episodes. HLA-A, HLA-B, HLA-DR and HLA-DQ compatibility are widely studied in clinical transplants but few studies investigated the influence of non-classical HLA loci, such as HLA-G, a non-classical class I HLA gene located at 6p21.31 in the MHC region, i.e. 300 kb telomeric to HLA-A. MHC region genes are characterized by extreme polymorphism as well as strong positive linkage disequilibrium (LD) between HLA loci (alleles). LD studies related to MHC region provide investigators with a tool to assess candidate genes with an at-risk HLA haplotype, with implications for allograft transplants, human reproduction and disease susceptibility. Many studies reported striking LD between HLA-G and HLAA alleles and also between HLA-G and HLA Class II alleles, but the biologicalimplications for these findings are not clear yet. DNA sequencing methodology was used to determine HLA-G (exons 2 and 3) polymorphisms from 52 patients who underwent kidney transplantation and their donors. It is the purpose of this study to investigate the influence of HLA-G polymorphism in a set of kidney transplants and the occurrence of rejection episodes. It was observed that pairs with 2 HLA-G matches presented a lower risk of rejection occurrence than those pairs with 0 or 1 match. It was also observed that subjects whose genotype presented one synonymous substitution (S) in one HLA-G allele in the HLA-G0101 group of alleles and another allele with a non-synonymous substitution (N/S) on HLA-G0103, HLA-G010401, HLA-G010403 or HLA-G0105N alleles, apparently had a greater chance of rejection episodes. Additionally, HLA-G, as well as HLA-A, -B e -DR, compatibility may also be important for allograft acceptance (rejection probability lower than 0.09%). Besides, heterozygous S/NS patients had a five times greater chance of rejection than S/S and NS/NS patients. Some haplotypes found in the present study were already described in literature: A01-G01B (010102 or 0106), A03-G0101A, A23-G010401, A26-G01B, A31-G0103, A02-G0101A, A24-G0101A, A33-G0103, A68-G01B. We have also described LD between HLA-G alleles and HLA class II DRB1 allelic groups and found significant LD between DRB104-G01B, DRB113-G010401, DRB114-G010108, DRB115-G0103, DRB103-G0101A and DRB103-G01B.

A functional role for soluble HLA-G antigens in immune modulation mediated by mesenchymal stromal cells

BACKGROUND

It has been suggested that soluble factors produced by bone marrow (BM) mesenchymal stromal cells (MSC) play a fundamental role in mediating immune modulation. HLA-G antigens (Ag) are major histocompatibility complex (MHC) class Ib molecules characterized by a limited polymorphism and a splicing mechanism that regulates the production of membrane-bound and soluble isoforms. Interleukin-10 (IL-10) cytokine is one of the main up-modulators of soluble HLA-G Ag (sHLA-G) production by CD14+ peripheral blood monocyte cells and increased IL-10 levels are reported to be associated with MSC immune modulation.

METHODS

We investigated, by specific enzyme-linked immunosorbent assay (ELISA), the possible role of sHLA-G molecules in the inhibition of the peripheral blood mononuclear cell (PBMC) response to phytohemagglutinin (PHA) mediated by MSC from different sources.

RESULTS

There was a significant correlation between the presence of increased levels of sHLA-G and IL-10 in the MSC/PBMC/PHA culture supernatants and lymphoproliferative inhibition. Neutralizing experiments performed with monoclonal Ab directed against HLA-G and IL-10 molecules confirmed the inhibitory ability of sHLA-G Ag. Furthermore, exogenous IL-10 induced sHLA-G molecule secretion by MSC alone in a polymorphic way, while a longitudinal analysis confirmed the loss of MSC inhibitory functions in relation to in vitro MSC aging.

DISCUSSION

Overall the results obtained suggest a functional role for sHLA-G molecules in inhibiting the PBMC response mediated by MSC. Moreover, the ability of IL-10 to induce sHLA-G Ag production by MSC alone could be proposed as a marker of MSC functional ability.

Myocardial HLA-G reliably indicates a low risk of acute cellular rejection in heart transplant recipients

BACKGROUND

Human leukocyte antigen-G (HLA-G), a non-classical MHC I protein with restricted tissue expression, plays an essential role in immune tolerance and has been negatively associated with acute and chronic rejection after heart transplantation. We assessed myocardial HLA-G expression in adult heart transplant patients in an attempt to determine the value of this protein in identifying patients with a low risk of acute cellular rejection.

METHODS

Two groups of patients were included in this study. Group A (non-rejecting) included 29 patients who had no moderate or severe rejection episodes (ISHLT Grade <2R) post-transplant. Group B (rejecting) included 38 patients with at least two moderate or severe rejection episodes (Grade >or=2R) within a 1-year period. Expression of HLA-G in three myocardial biopsies post-transplant from each patient was determined through immunohistochemical staining.

RESULTS

In Group A, 86% of patients had HLA-G(+) biopsies compared with 11% of patients in Group B (p < 0.001; sensitivity 86%, specificity 87%). Whereas 60% of non-rejecting HLA-G(+) patients had at least two positive biopsies, all rejecting HLA-G(+) patients had only one positive biopsy.

CONCLUSIONS

There is a significant negative association between myocardial HLA-G expression and acute cellular rejection after cardiac transplantation. Detection of HLA-G appears to reliably indicate a low risk of developing moderate or severe allograft rejection and may, subsequently, allow for a reduced immunosuppressive regimen.

Beyond the increasing complexity of the immunomodulatory HLA-G molecule

Human leukocyte antigen G (HLA-G) is a nonclassic major histocompatibility complex (MHC) class I molecule that functions as an immunomodulatory molecule capable of protecting fetal tissues from the maternal immune system. The relevance of HLA-G in other contexts was investigated soon afterward. Numerous studies have sought (and some have shown) the relevance of HLA-G in pathologic conditions, such as transplantation, autoimmunity, and cancer and hematologic malignancies. One of the main goals of the current research on HLA-G is now to use it in the clinic, either for diagnosis or as a therapeutic tool/target. For this, precise knowledge on the nature and functions of HLA-G is critical. We highlight here what we consider are recent key basic findings on the immunomodulatory function of HLA-G. These strengthen the case for considering HLA-G as clinically relevant.

HLA-G: from biology to clinical benefits

The relevance of the nonclassical human leukocyte antigen (HLA) class I molecule HLA-G in human physiological and pathological contexts has been the center of intense investigation. In light of the recent advances, we report here the clinical implications of HLA-G as a tolerogenic molecule promoting uterine implantation of the embryo or acceptance of solid allografts while allowing the evasion of tumors or viruses from the immune response. These recent findings are important in terms of clinical benefits at both diagnostic and therapeutic levels.

Soluble human leucocyte antigen-G molecules in peripheral blood haematopoietic stem cell transplantation: a specific role to prevent acute graft-versus-host disease and a link with regulatory T cells

Haematopoietic stem cell transplantation is often complicated by the life-threatening graft-versus-host disease (GVHD) which consists of an allogeneic reaction of the graft cells against the host organs. The aim of this study was to investigate the putative involvement of soluble human leucocyte antigen (sHLA) class I molecules, and particularly sHLA-G molecules, in the occurrence and/or prevention of acute GVHD (aGVHD) in allogeneic peripheral blood stem cell (PSC) transplantation. Whole sHLA class I molecules seem to be involved in aGVHD pathogenesis because detection of a high concentration of these molecules in the first month post allograft is correlated with aGVHD occurrence. Conversely, a high level of sHLA-G molecules before and after allograft could indicate good prognosis in PSC allograft transplantation. sHLA-G molecules seem to be involved in aGVHD prevention, not only because they are enriched in plasma of patients without aGVHD, but also because: (i) a positive correlation has been found between sHLA-G level and CD4+ CD25+ CD152+ natural regulatory T cell (T(reg)) frequency in the blood of transplanted patients; and (ii) the presence of CD4+ CD25+ CD152+ natural T(reg) is correlated with increased sHLA-G expression in in vitro mixed leucocyte reaction cultures. Altogether, these results support the immunomodulatory function of sHLA-G molecules that might create a regulatory network together with the natural T(reg) to foster the induction of a tolerogenic environment and improve PSC transplantation favourable outcome.

Human leukocyte antigen-G expression after kidney transplantation is associated with a reduced incidence of rejection

HLA-G is a non-classic Human Leukocyte Antigen (HLA-G) Class I of low polymorphism and restricted tissue distribution that displays tolerogenic functions. In heart transplantation and in combined liver/renal allograft transplantation, the expression of HLA-G has been associated with a lower incidence of acute graft rejection episodes and absence of chronic dysfunction. Since the expression of HLA-G in renal biopsies has been investigated only in few patients who received a combined kidney and liver transplant, in this study we performed a cross-sectional study, systematically comparing the expression of HLA-G in post-transplanted renal grafts, stratifying patients according to the presence or absence of rejection.

PATIENTS AND METHODS

Seventy-three renal specimens (10 with acute rejection and 13 with chronic allograft nephropathy, and 50 with no signs of rejection) were immunohistochemically evaluated for HLA-G expression.

RESULTS

In the group as a whole, HLA-G molecules were detected in 40 cases (54.8%). Among specimens that presented HLA-G expression, 2 out of 40 (5%) exhibited acute rejection, 2 (5%) exhibited chronic allograft nephropathy, and the remaining 36 (90%) exhibited no signs of rejection. The comparison between patients with rejection and those without rejection showed that the expression of HLA-G was significantly increased in specimens exhibiting no signs of rejection (p<0.0001). Considering only patients with acute rejection, 8 out of 10 patients showed no HLA-G expression in their kidney biopsies when compared to patients exhibiting no signs of rejection and absence of HLA-G was observed in 14 out of 50 (p=0.0032). Similarly, considering only patients with chronic allograft nephropathy, absence of HLA-G expression was observed in 11 out of 13 specimens, whereas in patients without rejection absence of HLA-G was observed in 14 out of 50 (p=0.003). Therapy with tacrolimus was significantly associated with the expression of HLA-G and a better graft prognosis.

CONCLUSIONS

Our results suggest that HLA-G expression in the kidney allograft and the use of tacrolimus are associated with a lower frequency of acute renal rejection and chronic allograft nephropathy.

The human leucocyte antigen-G 14-basepair polymorphism correlates with graft-versus-host disease in unrelated bone marrow transplantation for thalassaemia

The presence of the 14-bp insertion polymorphism of the human leucocyte antigen (HLA)-G gene (HLA-G) promotes immune tolerance through increased synthesis of HLA-G molecules. We investigated this polymorphism in a large cohort of 53 thalassaemia patients transplanted from an unrelated donor. Sixteen patients (30.2%) homozygous for the 14-bp deletion had a higher risk of developing acute graft-versus-host disease (aGvHD) than patients homozygous for the 14-bp insertion (-14-bp/-14-bp vs +14-bp/+14-bp: Relative Risk = 15.0; 95% confidence interval 1.59-141.24; P = 0.008). Therefore, the 14-bp polymorphism could be an important predictive factor for aGvHD following bone marrow transplantation.

Tolerogenic functions of human leukocyte antigen G: from pregnancy to organ and cell transplantation

Over the past few years, the number of publications concerning the human leukocyte antigen (HLA)-G molecule, its functions, and its pathological implications has greatly increased, largely exceeding those focusing simply on fetal-maternal activity. The role of this molecule in other situations of tolerance such as transplantation, tumor dissemination, or virus infections has also been reported. In this paper, we focus our attention on the relevance of HLA-G in transplantation in the light of recent data associating regulatory T cells and HLA-G, and which provide evidence of the role of HLA-G in improving graft acceptance.

CD3+CD4low and CD3+CD8low are induced by HLA-G: novel human peripheral blood suppressor T-cell subsets involved in transplant acceptance

HLA-G is a tolerogenic molecule whose detection in sera and within allografted tissues is associated with better graft acceptance. HLA-G mediates T-cell differentiation into suppressor cells, which are thought to promote tolerance. Here, we investigated such T cells phenotypically and functionally and assessed their clinical relevance in the peripheral blood of patients who have undergone transplantation. Our results demonstrate that HLA-G expressed by antigen-presenting cells or present as soluble protein down-regulates the expression of CD4 and CD8 on allostimulated T cells at both transcriptional and posttranslational levels. These CD3(+)CD4(low) and CD3(+)CD8(low) T-cell subsets are characterized by an increased proportion of cells expressing CD45RA and HLA-DR, and a decreased number of cells expressing CD62L. In addition, these HLA-G-induced CD3(+)CD4(low) and CD3(+)CD8(low) subpopulations are Foxp3-negative suppressor T cells whose function involves IL-10. Biologic relevance came from analysis of patients who underwent transplantation, with high HLA-G plasma concentrations associated with better graft survival. Peripheral blood from these patients contains increased levels of IL-10 concomitantly to an enhanced representation of CD3(+)CD4(low) and CD3(+)CD8(low) T cells compared with HLA-G-negative patients who underwent transplantation and healthy individuals. These data define novel immunosuppressive subpopulations of peripheral blood T cells induced by HLA-G with potent implications in peripheral tolerance.

Soluble HLA-G: Are they clinically relevant?

HLA-G is a non-classical HLA-class Ib molecule with multiple immunoregulatory properties. Its main function in physiological conditions is to abrogate maternal NK cell activity against foetal tissue and to establish immune tolerance at maternal-foetal interface. HLA-G is expressed not only as a membrane bound molecule on the surface of cells, but also as a soluble moiety in body fluids. The major isoforms of HLA-G present in serum are soluble HLA-G1 and HLA-G5 which are generated by shedding or proteolytic cleavage of the membrane bound isoform and by secretion of a soluble isoform, respectively. Here we review the data about soluble HLA-G (sHLA-G) serum levels in different pathological conditions, including immune-mediated disorders, transplantation and malignancies. In particular, we focus on sHLA-G expression and function in human neuroblastoma, a pediatric tumor, with special emphasis on a novel potential immuno escape mechanism utilized by NB to instruct monocytes to produce and release sHLA-G. Finally, the potential clinical relevance of sHLA-G serum levels is discussed.

HLA-G in B-chronic lymphocytic leukaemia: clinical relevance and functional implications

HLA-G appears to be involved in regulatory functions counteracting the cellular immune response of T and NK cells by several pathways. We here summarize the HLA-G expression patterns in leukaemia with emphasis on the clinical relevance of this expression for disease progression. Especially in patients with B-chronic lymphocytic leukaemia (B-CLL) the HLA-G expression on B-CLL cells was strongly associated with a reduced treatment-free survival. The corresponding immunological parameters point to a broad immunosuppression in these patients. Thus, HLA-G seems to contribute to the impaired immune response in B-CLL supporting disease progression.

Expression and release of soluble HLA-E is an immunoregulatory feature of endothelial cell activation

Human leukocyte antigen (HLA)-E belongs, with HLA-G and HLA-F, to the non-classic major histocompatibility complex (MHC) class I (Ib) molecules, broadly defined by a limited polymorphism and a restricted pattern of cellular expression. In contrast to HLA-G, the expression and function of HLA-E and HLA-F in physiologic and pathologic processes remain poorly established. In the present study, we show that HLA-E protein expression in normal human nonlymphoid organs is mainly restricted to endothelial cells (ECs). HLA-E is also basally expressed by B and T lymphocytes, natural killer (NK) cells and by macrophages. We demonstrate that tumor necrosis factor alpha (TNFalpha), interleukin-1beta (IL-1beta), and interferon gamma (IFNgamma) up-regulate the cell-surface expression of HLA-E on ECs in vitro and induce the release of soluble HLA-E (sHLA-E). HLA-E up-regulation protects IFN-gamma-activated ECs from NK-mediated cell lysis, while sHLA-E protects bystander cells. Finally, sHLA-E is not detected in normal sera, and increased serum levels correlate with disease activity in patients with antineutrophil cytoplasmic antibody-associated systemic vasculitis. Thus, HLA-E expression and release of sHLA-E are features of EC activation and emphasize immunoregulatory functions of the endothelium. The present identification of soluble HLA-E molecules may have important implications in understanding the pathogenesis of immune-mediated vascular diseases and for the diagnosis and monitoring of patients.

Soluble HLA-G and HLA-G1 Expressing Antigen-Presenting Cells Inhibit T-Cell Alloproliferation through ILT-2/ILT-4/FasL-Mediated Pathways

HLA-G is a tolerogenic molecule involved in maternal-fetal tolerance and in allograft acceptance. Soluble HLA-G proteins are present at high levels in plasma from transplanted patients who better accept their graft. In addition, infiltrating mononuclear cells expressing HLA-G can be detected within grafted tissues. To define the role of these HLA-G proteins in preventing graft rejection, we investigated the ability of HLA-G1 expressing antigen presenting cells (APC) and of soluble HLA-G proteins (i.e., HLA-G5 and shed HLA-G1) to inhibit T-cell alloproliferation and analyzed the molecules involved in such inhibition. Results demonstrated that both membrane-bound and soluble HLA-G proteins inhibited T-cell alloproliferation. This inhibition involved engagement of immunoglobulinlike transcript (ILT)-2 and ILT-4 receptors by HLA-G. Moreover, blocking Fas ligand (FasL) reversed HLA-G mediated inhibition, demonstrating that the Fas/FasL pathway is also recruited by HLA-G to exert its immunosuppressive function on T cells. These data highlight the role played by HLA-G in better graft acceptance status observed in transplanted patients with HLA-G(+) grafted cells and high HLA-G plasma levels. Evidence to support such role in vivo was provided by the capacity of purified HLA-G5 from the plasma of the transplanted patient to suppress T-cell alloresponses.

New insights into the pathophysiology of gestational diabetes mellitus: possible role of human leukocyte antigen-G

Diabetes can develop in up to 10% of pregnant women who have not previously had the condition. This condition which usually begins in the second half of the pregnancy is called gestational diabetes mellitus (GDM). In most cases, all diabetic symptoms disappear following delivery. However, women with GDM have an increased risk of developing type 2 diabetes mellitus (DM) later in life, especially if they were overweight before the pregnancy. The cause of GDM is unknown. Although hormones present in the pregnancy, especially human placental lactogen, are thought to be responsible for the development of this condition, many questions remain to be answered. It is still not known why GDM develops in a subgroup of pregnant women. It may be possible that events leading to the development of GDM are triggered by an antigenic load which is the fetus itself. Human leukocyte antigen-G (HLA-G) expression that functions to protect the fetus from immune attack by down-regulating cytotoxic T cell responses to fetal trophoblast antigens is postulated to protect the islet cells of the pancreatic tissue also. HLA-G and nuclear factor-kappaB (NF-kappaB) interaction is suggested to be central in the events leading to GDM development. An analogy between the development of DM in some transplant patients and GDM development in a proportion of pregnancies is postulated, so that an antigenic load triggers the diabetogenic process. Further support of this hypothesis with new studies may lead to the possibility that recombinant HLA-G can be used for the prevention of diabetes in high risk patients.

Post-transplant soluble MICA and MICA antibodies predict subsequent heart graft outcome

The objective of this retrospective study was to evaluate the role of MICA in heart graft acceptance. Pre- and post-transplant sera from 31 patients were evaluated for MICA antibodies by cytotoxicity on recombinant cell lines and soluble MICA (sMICA) concentrations by ELISA. The results demonstrated that the patients with post-transplant anti-MICA antibodies were at a high risk for the development of severe acute rejection (AR) (p<0.03; OR=8.5). However, the presence of post-transplant sMICA was found to be associated with functioning grafts without AR episodes (p<0.03, OR=7.9). In this preliminary survey, the negative association of sMICA with AR was found to be in the absence of MICA antibodies. Further research is needed to clarify the role of sMICA in allograft acceptance. Post-transplant evaluation of humoral immune response to MICA and the measure of sMICA in patient's sera may provide a good predictor of AR.

sHLA-G levels in the monitoring of immunosuppressive therapy and rejection following heart transplantation

The aim of this study was to further determine the immediate influence, over a 12-h period, after the initiation of daily immunosuppressive treatment on the serum levels of sHLA-G in heart transplant patients during the post-transplant period (1 month). It was found that there are two patterns of patients in term of the changes observed in their levels of sHLA-G in response to the immunosuppressive treatment. One group (group A) showed no changes on sHLA-G while the other group (group B) a significant rise in sHLA-G levels was observed at 2 to 4 h post dose. Interestingly, it was observed that the patients in group B have better prognosis of acceptance of the heart graft than those of group A. On the other hand it was found that the patients with high levels of sHLA-G (77.3+/-34.8 ng/ml) in pre-transplant sera have a better prognosis of acceptance of the heart graft than those with low sHLA-G levels (9.7+/-7.1 ng/ml). In conclusion, both the intensity of changes of sHLA-G levels induced by immunosuppression and basal levels in pre-transplant could be used in the monitoring of the immunosuppression as well as the heart transplant evolution.