HLA-G in reproduction: studies on the maternal-fetal interface

For more than a decade, investigators have known that membrane-bound and soluble isoforms of the HLA class Ib molecule, HLA-G, are present at the maternal-fetal interface. Although it is clear that extravillous cytotrophoblast cells are major producers, other cells may also contribute. Recent studies in our laboratory raised the question of whether soluble isoforms might reach the maternal and/or fetal blood circulation. A capture enzyme-linked immunoabsorbent assay (ELISA) identified soluble HLA-G (sHLA-G) in maternal blood throughout pregnancy but failed to detect sHLA-G in cord sera. Further studies suggested that the circulating proteins may be either free heavy chain (sHLA-G1 and/or sHLA-G2) or exclusively sHLA-G2. To study the potential function(s) of the soluble isoforms to modulate local or systemic immunity in mothers, we generated recombinant sHLA-G1 and -G2 in both prokaryotic and eukaryotic systems. Preliminary experiments conducted using DNA microarray analysis suggest that sHLA-G is capable of modulating gene expression in blood mononuclear leukocytes. Potential local targets were also identified; decidual and placental macrophages but not trophoblast cells contained mRNA encoding two of the known receptors for HLA-G, ILT2 and ILT4. Collectively, the studies are consistent with the hypothesis that sHLA-G produced at the maternal-fetal interface targets to the cells of the monocyte/macrophage lineage and modulates their functions for the benefit of pregnancy.     

HCMV感染对人THP-1细胞HLA-G及其受体表达的影响

目的:研究人巨细胞病毒(HCMV)感染对人THP-1细胞人类白细胞抗原G(HLA-G)异构体及其受体表达的影响探讨HLA-G在HCMV逃逸宿主免疫应答中的作用。方法:HCMV Towne株感染THP-1细胞后,采用RT-PCR和Western blot检测HLA-G异构体mRNA和蛋白水平,流式细胞术检测THP-1细胞HLA-G及其表面受体ILT2、ILT4的表达,ELISA检测细胞培养上清中IL-10及可溶性HLA-G(sHLA-G)水平,同时检测细胞存活率。结果:HCMV感染后细胞未出现明显凋亡,细胞存活率高。HCMV感染THP-1细胞1 d后HLA-G1、-G3、-G4和-G5的mRNA表达明显上调,HLA-G1和HLA-G5的蛋白表达明显上调。THP-1细胞HLA-G、ILT2和ILT4的表达在感染1 d后明显上调。sHLA-G水平在感染1 d后显著升高,与对照组比较差异有统计学意义(P0.01)。THP-1细胞培养上清液IL-10水平在感染1 d后明显上调,与对照组比较,差异有统计学意义(P0.05)。结论:HCMV感染THP-1细胞能诱导HLA-G异构体的差异表达,以HLA-G1和HLA-G5为主,且上调其表面受体ILT2/ILT4的表达。同时,HCMV感染能诱导THP-1细胞分泌IL-10。该研究为进一步探讨HCMV逃避机体免疫应答的机制提供实验依据。     

HLA-G expressing DC-10 and CD4 T cells accumulate in human decidua during pregnancy

Multiple mechanisms underlie the surprising willingness of mothers to tolerate the semi-allogeneic fetal tissues during pregnancy. Chief among these is the expression of the HLA-G molecules that has been largely demonstrated to be responsible for reprogramming the local maternal immune response towards tolerance. We recently identified a subset of tolerogenic dendritic cells, DC-10 that secrete high amounts of IL-10 and express high levels of HLA-G and its ligand ILT4. DC-10 are present in the peripheral blood and are essential in inducing adaptive regulatory T cells. We investigated the presence of DC-10 and HLA-G-expressing CD4⁺ T cells in human decidua in the first trimester of pregnancy. Results showed that these cells are highly represented in human decidua as compared to the peripheral blood. This is the first report describing decidual DC-10 and CD4⁺HLA-G⁺ T cells, strongly suggesting that they may accumulate or be induced at the fetal maternal interface to promote tolerance.     

Cytometry-based analysis of HLA-G functions according to ILT2 expression

HLA-G was described as a molecule inhibiting NK and T cells functions through its receptor, ILT2. However, most functional studies of HLA-G were so far performed on heterogeneous immune populations and regardless of ILT2 expression. This may lead to an underestimation of the effect of HLA-G. Thus, considering the immune subpopulations sensitive to HLA-G remained an important issue in the field. Here we present a new cytometry assay to evaluate HLA-G effects on both NK and CD8⁺ T cell cytotoxic functions.Using flow cytometry allows for the comparison of HLA-G function on multiple subsets and multiple functions in the same time. In particular, we sharpen the analysis by specifically studying the immune subpopulations expressing HLA-G receptor ILT2. We focused our work on: IFN-gamma production and cytotoxicity (CD107a expression) by CD8⁺ T cells and NK cells expressing or not ILT2. We compared the expression of these markers in presence of target cells, expressing or not HLA-G1, and added a blocking antibody to reverse HLA-G inhibition.This new method allows for the discrimination of cell subsets responding and non-responding to HLA-G1 in one tube. We confirm that HLA-G-specifically inhibits the ILT2⁺ CD8⁺ T cell and ILT2⁺ NK cell subsets but not ILT2-negative ones. By blocking HLA-G/ILT2 interaction using an anti-ILT2 antibody we restored the cytotoxicity level, corroborating the specific inhibition of HLA-G1. We believe that our methodology enables to investigate HLA-G immune functions easily and finely towards other immune cell lineages or expressing other receptors, and might be applied in several pathological contexts, such as cancer and transplantation.     

The costimulatory signal upregulation is associated with Th1 bias at the maternal-fetal interface in human miscarriage

PROBLEM To evaluate whether the association of the costimulatory signal regulation with T helper 1/T helper 2 (Th1/Th2) bias at maternal-fetal interface in human pregnancy loss. METHOD OF STUDY The expression of CD80 and CD86 in decidual tissues and CD28 and cytotoxic T-lymphocyte antigen-4 (CTLA-4) in the decidual T cells was compared between normal early pregnancy and miscarriage by qPCR and Western blot. The cytokine production in decidual T cells was performed by flow cytometry. The correlation of costimulatory molecule expression with Th1/Th2 cytokines was analyzed. RESULTS The CD80 mRNA and protein expression showed no significant difference between normal pregnancy and miscarriage. An increase in the expression of CD28 and CD86 was accompanied by a decrease in the expression of CTLA-4 in miscarriage in comparison with the early pregnancy. The higher expression of interleukin (IL)-2 and interferon-γ (IFN-γ), and lower expression of IL-4 and IL-10 in the decidual T cells were present in miscarriage. A correlation analysis showed a significant positive correlation of CD86 and CD28 expression with the Th1 cytokine production (IL-2 and IFN-γ), a significant negative correlation of CTLA-4 expression with the Th1 cytokine production. CONCLUSION The upregualtion of costimulatory signals on T cells might form an abnormal immune microenvironment, a shift to Th1 responses, at maternal-fetal interface, which leads to human miscarriage.     

Mobilizing dendritic cells for tolerance by engagement of immune inhibitory receptors for HLA-G

The presence of dendritic cells (DC) in the maternal decidua has pointed to a biologic role of antigen-presenting cell in maternal-fetal interaction. The expression of immune inhibitory receptors on DC opens the intriguing possibility that these types of receptors are directly involved in maturation/activation of DC and modulate their function. We show that the triggering of the murine inhibitory receptor paired immunoglobulin-like receptor-B by cross-linking or by human leukocyte antigen (HLA)-G tetramer resulted in the modulation of DC function and prolongation of allogeneic graft survival. In addition, we found that the engagement of human inhibitory receptor ILT4 by its natural ligand, HLA-G, alters maturation of human DC. In this study, we examined the mechanisms for the modulation of antigen-presenting cells by HLA-G. These findings have established an important link between HLA-G and immune inhibitory receptor regulation in vivo and in vitro, thereby placing HLA-G in the inhibitory pathway.     

Expression of membrane-bound HLA-G at the maternal-fetal interface is not associated with pregnancy maintenance among patients with idiopathic recurrent pregnancy loss

Recurrent pregnancy loss (RPL) has many known aetiologies. However, using current diagnostic testing, a large fraction of recurrent pregnancy losses remain unexplained. Many of these may have immune underpinnings. HLA-G is a non-classical MHC class I product whose fairly restricted expression at the maternal-fetal interface suggests a role in successful embryonic implantation and/or subsequent pregnancy maintenance. The study of immune-mediated RPL should be enhanced by comparing groups of idiopathic RPL patients with normal fetal chromosomes to RPL patients with known chromosomal abnormalities in their index pregnancies. We hypothesized that if alteration of HLA-G expression at the maternal-fetal interface were associated with immune-mediated RPL, such changes might be detectable using these comparisons. HLA-G protein expression at the maternal-fetal interface in maternal and gestational age-matched women with history of idiopathic RPL and normal male fetuses were compared with expression in RPL patients with known fetal trisomy 16 in their index pregnancy. We detected no significant quantitative differences in the levels of HLA-G between these groups of RPL patients. Within the limitations of this study, we conclude that HLA-G expression is not a major immunological determinant of pregnancy maintenance among patients with idiopathic RPL.     

HLA-G and mother–child perinatal HIV transmission

Transplacental passage is a well-known phenomenon in HIV infection and immune responses at the maternal-fetal interface play a critical role in perinatal mother-to-child HIV transmission (MCHT). The high expression of HLA-G at the maternal-fetal interface and its role in mediating immune tolerance suggest that it could play an important role in MCHT. We investigated the role of HLA-G polymorphism in perinatal HIV transmission in 348 ART naïve mother–child pairs enrolled in a mother–child HIV transmission cohort, established in Nairobi, Kenya in 1986.     

Pseudomonas aeruginosa Quorum Sensing Molecule N-(3-Oxododecanoyl)-l-Homoserine-Lactone Induces HLA-G Expression in Human Immune Cells

HLA-G is a nonclassical class I human leukocyte antigen (HLA) involved in mechanisms of immune tolerance. The objective of this study was to determine whether N-(3-oxododecanoyl)-l-homoserine lactone (3O-C₁₂-HSL), a quorum sensing molecule produced by Pseudomonas aeruginosa, could modify HLA-G expression to control the host immune response. We evaluated the ability of 3O-C₁₂-HSL to induce HLA-G expression in primary immune cells, monocytes (U937 and THP1), and T-cell lines (Jurkat) in vitro and analyzed the cellular pathway responsible for HLA-G expression. We studied the HLA-G promoter with a luciferase assay and interleukin-10 (IL-10) and p38/CREB signaling with enzyme-linked immunosorbent assay and immunofluorescence, respectively. We observed that 3O-C₁₂-HSL is able to induce HLA-G expression in human monocytes and T cells. We showed that the induction of HLA-G by 3O-C₁₂-HSL is p38/CREB and IL-10 dependent. 3O-C₁₂-HSL treatment is able to arrest only the U937 cell cycle, possibly due to the peculiar expression of the ILT2 receptor in the U937 cell line. Our observations suggest HLA-G as a mechanism to create a protected niche for the bacterial reservoir, similar to the role of HLA-G molecules during viral infections.