Interleukin-17-producing decidual CD4+ T cells are not deleterious for human pregnancy when they also produce interleukin-4

Background

Trophoblast expressing paternal HLA-C antigens resemble a semiallograft, and could be rejected by maternal CD4+ T lymphocytes. We examined the possible role in human pregnancy of Th17 cells, known to be involved in allograft rejection and reported for this reason to be responsible for miscarriages. We also studied Th17/Th1 and Th17/Th2 cells never investigated before. We defined for the first time the role of different Th17 subpopulations at the embryo implantation site and the role of HLA-G5, produced by the trophoblast/embryo, on Th17 cell differentiation.

Methods

Cytokine production by CD4+ purified T cell and T clones from decidua of normal pregnancy, unexplained recurrent abortion, and ectopic pregnancy at both embryo implantation site and distant from that site were analyzed for protein and mRNA production. Antigen-specific T cell lines were derived in the presence and in the absence of HLA-G5.

Results

We found an associated spontaneous production of IL-17A, IL-17F and IL-4 along with expression of CD161, CCR8 and CCR4 (Th2- and Th17-type markers) in fresh decidua CD4+ T cells during successful pregnancy. There was a prevalence of Th17/Th2 cells (producing IL-17A, IL-17F, IL-22 and IL-4) in the decidua of successful pregnancy, but the exclusive presence of Th17 (producing IL-17A, IL-17F, IL-22) and Th17/Th1 (producing IL-17A, IL-17F, IL-22 and IFN-γ) cells was found in the decidua of unexplained recurrent abortion. More importantly, we observed that Th17/Th2 cells were exclusively present at the embryo implantation site during tubal ectopic pregnancy, and that IL-4, GATA-3, IL-17A, ROR-C mRNA levels increased in tubal biopsies taken from embryo implantation sites, whereas Th17, Th17/Th1 and Th1 cells are exclusively present apart from implantation sites. Moreover, soluble HLA-G5 mediates the development of Th17/Th2 cells by increasing IL-4, IL-17A and IL-17F protein and mRNA production of CD4+ T helper cells.

Conclusion

No pathogenic role of decidual Th17 cells during pregnancy was observed. Indeed, a beneficial role for these cells was observed when they also produced IL-4. HLA-G5 could be the key feature of the uterine microenvironment responsible for the development of Th17/Th2 cells, which seem to be crucial for successful embryo implantation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12948-016-0039-y) contains supplementary material, which is available to authorized users.

HLA-G5 induces IL-4 secretion critical for successful pregnancy through differential expression of ILT2 receptor on decidual CD4⁺ T cells and macrophages

Successful pregnancy in humans has been associated with production of IL-4 by T cells at the feto-maternal interface. Soluble HLA-G5 produced by trophoblasts potentially controls the decidual T cell cytokine profile. We studied the effect of HLA-G5 on the cytokine profile of purified human macrophages and Ag-specific T cells in vitro. We demonstrated that HLA-G5 increased production of IL-12 by purified peripheral blood macrophages. Although IL-12 production by macrophages is known to induce IFN-γ production by CD4(+) T cells, HLA-G5 increased production of IL-4 but not IFN-γ by CD4(+) T cells after Ag presentation by macrophages. We found that this apparent paradox was due to the differential expression of the ILT2 HLA-G5 receptor on activated T cells and macrophages. This receptor was upregulated in the former and downregulated in the latter after Ag presentation and activation of both cell types. This observation was confirmed in situ, where decidual macrophages and T cells are continuously exposed to HLA-G5 produced locally and activated by trophoblast alloantigens. Freshly isolated decidua basalis macrophages expressed lower levels of ILT2 than peripheral blood macrophages from the same pregnant women. They did not spontaneously produce IL-12, whereas freshly isolated decidual CD4(+) T cells expressed high levels of activation markers (CD25, HLA-DR, and CD69) as well as ILT2 and spontaneously produced IL-4 but not IFN-γ. Therefore, HLA-G5 could be responsible, at least in part, via its interaction with ILT2, for decidual T cell IL-4 production, known to be crucial for successful pregnancy.

A novel antiangiogenic and vascular normalization therapy targeted against human CD160 receptor

A monoclonal anti-CD160 antibody inhibits the growth of new vessels in pathological ocular and tumor neoangiogenesis but not in healthy tissues.

Angiogenesis plays an essential role in several diseases of the eye and in the growth of solid tumors, but existing antiangiogenic therapies have limited benefits in several cases. We report the antiangiogenic effects of a monoclonal antibody, CL1-R2, in several animal models of neovascularization. CL1-R2 recognizes human CD160, a membrane receptor which is conserved in various mammal species. We show that CD160 is expressed on the endothelial cells of newly formed blood vessels in human colon carcinoma and mouse B16 melanoma but not in vessels of healthy tissues. CL1-R2 reduced fibroblast growth factor 2–induced neovascularization in the rabbit cornea, in a mouse model of oxygen-induced retinopathy, and in a mouse Matrigel plug assay. Treatment of B16 melanoma-bearing mice with CL1-R2 combined with cyclophosphamide chemotherapy caused regression of the tumor vasculature and normalization of the remaining vessels as shown by Doppler ultrasonography, intravital microscopy, and histology. These studies validate CD160 as a potential new target in cases of human pathological ocular and tumor neoangiogenesis that do not respond or become resistant to existing antiangiogenic drugs.

CD160: a unique activating NK cell receptor

Here we discuss CD160 an essential NK cell activating receptor that remains poorly understood. CD160 receptor exhibits a number of unique structural and functional characteristics that are not common to other killer immunoglobulin-like receptors that recognize major histocompatibility complex (MHC) class I molecules: (1) In addition to humans and mice, the cd160 gene is conserved in several other mammal species; (2) cd160 is located outside the NK gene complex and the Leukocyte Receptor Complex in humans; (3) CD160 expression is associated to the CD56(dim) CD16+ cytotoxic NK cell phenotype; (4) both human and mouse CD160 recognize MHC class Ia and Ib molecules; (5) unlike the other MHC class I-dependent activating NK receptors, CD160 is a glycosylphosphatidylinositol-anchored molecule with a single immunoglobulin-like domain, and does not bear immunoreceptor tyrosine-based activation motifs. Consequently, CD160 cannot signal by itself, requiring the recruitment of adaptor proteins. CD160 recruits phosphoinositide-3 kinase to trigger cytotoxicity and cytokine secretion; (6) specific engagement of NK CD160 receptor expressed by circulating NK cells produces proinflammatory cytokines IFN-γ, TNF-α, and, most notably, IL-6 and IL-8 as well as MIP1-β chemokine. The level of CD160-mediated IFN-γ production is always higher than the one observed after engagement of the CD16 receptor.

The human decidual NK-cell response to virus infection: what can we learn from circulating NK lymphocytes?

NK cells present in the peripheral blood respond rapidly to pathogens or pathogen-infected cells by various means including cytotoxicity and production of cytokines. Whether decidual NK (dNK) cells are able to play a similar role when the pregnant uterus is infected by viruses is still largely unknown. Decidual NK cells are generally considered as poorly cytotoxic when compared to their peripheral blood counterparts. However, we have recently demonstrated that freshly isolated dNK cells from healthy early pregnant uterus do have a cytotoxic potential mediated by the specific engagement of NKp46 activating receptor. We further found that the co-engagement of CD94/NKG2A inhibiting receptor drastically inhibits the cytolytic function of dNK. This latter observation suggests that in situ the CD94/NKG2A receptor interaction with its HLA-E specific ligand is a dominant negative regulatory mechanism that prevents unwanted dNK cell cytotoxicity in non-infected pregnant uterus. How do dNK cells behave when they are activated by virus-infected cells present at the maternal-fetal interface? Largely based on data obtained from circulating NK cells, this review briefly discusses the following questions: Does uterine viral infection promote decidual NK cell proliferative capacity in situ? Are dNK cells able to kill virus-infected autologous decidual target cells and thus limit the virus spreading to the fetus? Which viral-mediated signal(s) and molecular interactions may subvert inhibition of dNK cytotoxic potential? Does uterine viral infection promote decidual NK cell secretion of cytokines and chemokines that boost the anti-viral immune response?

Effector functions of human decidual NK cells in healthy early pregnancy are dependent on the specific engagement of natural cytotoxicity receptors

Natural killer (NK) cells represent the major lymphocyte population in the decidua basalis of the human uterus during healthy early pregnancy. The activity of decidual NK (dNK) cells and their activation status are different from those of peripheral blood (PB)-NK cells; i.e. dNK cells exhibit a unique phenotype. Decidual NK cells have been defined as CD56(bright), CD16(neg), and more recently CD160(neg). They express a unique repertoire of NK cell receptors, identical among all donors tested. Decidual NK cells express in particular NKp46-, NKp30- and NKp44-activating receptors, contrasting with PB-NK cells which are devoid of NKp44-activating receptors. Specific engagement of each of these three so-called natural cytotoxicity receptors in dNK cells has important functional consequences in terms of cytokine, chemokine and angiogenic factor secretion as well as cytotoxic potential. Strikingly, and in contrast with PB-NK cells, engagement of NKp46- but not NKp30-activating receptor on freshly isolated dNK cells triggers cytotoxicity. Such cytotoxic potential of dNK cells is negatively controlled by NKG2A inhibitory receptor co-engagement. This suggests that in situ, dNK cells cannot kill trophoblast cells during normal pregnancy. Whether such NKG2A-mediated inhibition is abolished during pregnancies complicated by pathologies including viral infection is an interesting hypothesis that remains to be tested.

Soluble HLA-G in IVF/ICSI embryo culture supernatants does not always predict implantation success: a multicentre study

Several reports have described an association between the presence of soluble human leukocyte antigen G (sHLA-G) in human embryo culture supernatants (ES) and implantation success. However, not all studies agree with these findings. To further document this debate, a multicentre blinded study was performed to investigate, on a large number of IVF ES and ICSI ES, whether sHLA-G is a useful criterion for embryo selection before transfer. A total of 1405 ES from 355 patients were collected from three assisted reproductive technique (ART) centres and evaluated for their sHLA-G content in a single laboratory, using a chemiluminescence enzyme-linked immunosorbent assay. In only one centre was a significant association between sHLA-G-positive ES and successful implantation established (P = 0.0379), whereas no such association was observed in the other centres. It was found that the percentages and concentrations of sHLA-G-positive ES varied between centres, depending on culture media and ART conditions. The percentage of sHLA-G-positive ES was significantly higher in IVF ES than ICSI ES (P < 0.001 and P < 0.01 for two centres). These data demonstrate that substantial variations of sHLA-G content in ES occur between different ART centres, highlighting the influence of several technical parameters that differ from one centre to another.

[Immunity of pregnancy: novel concepts]

Pregnancy represents an immunological paradox, as underlined by the Nobel prize laureate Peter Medawar in the 1950s. This paradox is generating renewed interest with insights obtained in studies of pregnant mice and in ex vivo experiments performed with human cells and tissues. A number of molecular mechanisms have been discovered that prevent maternal placental immune effector cells located at the maternal-fetal interface from attacking fetus-derived cells. For example, maternal alloantibodies directed against paternal alloantigens expressed by the trophoblast are blocked by complement-inhibiting proteins, and maternal B cells specific for these paternal antigens are partially deleted Maternal antipaternal CD8+ cytotoxic T cells are inefficient, owing to the lack of HLA-A and HLA-B molecule expression on trophoblast target cells, together with the action of local immunosuppressive molecules, and transient tolerance of paternal alloantigens. NK cells present in the pregnant uterus and directed against fetus-derived trophoblast cells exhibit little if any cytotoxic potential. Interestingly, decidual NK cellltrophoblast interactions appear to play a physiological role in vascular uterine remodeling and in subsequent placental development. Most possible combinations of uterine NK KIR receptors and fetal HLA-C molecules expressed by the trophoblast are compatible with normal pregnancy, but the risk of severe preeclampsia appears to be far higher than normal when the mother's uterine NK cells do not express activating KIR (AA genotype) and when her fetus possesses group C2 HLA-C molecules.

Critical and differential roles of NKp46- and NKp30-activating receptors expressed by uterine NK cells in early pregnancy

In early human pregnancy, uterine decidual NK cells (dNK) are abundant and considered as cytokine producers but poorly cytotoxic despite their cytolytic granule content, suggesting a negative control of this latter effector function. To investigate the basis of this control, we examined the relative contribution to the cytotoxic function of different activating receptors expressed by dNK. Using a multicolor flow cytometry analysis, we found that freshly isolated dNK exhibit a unique repertoire of activating and inhibitory receptors, identical among all the donors tested. We then demonstrated that in fresh dNK, mAb-specific engagement of NKp46-, and to a lesser extent NKG2C-, but not NKp30-activating receptors induced intracellular calcium mobilization, perforin polarization, granule exocytosis and efficient target cell lysis. NKp46-mediated cytotoxicity is coactivated by CD2 but dramatically blocked by NKG2A coengagement, indicating that the dNK cytotoxic potential could be tightly controlled in vivo. We finally found that in dNK, mAb-specific engagement of NKp30, but not NKp46, triggered the production of IFN-gamma, TNF-alpha, MIP-1alpha, MIP-1beta, and GM-CSF proinflammatory molecules. These data demonstrate a differential, controlled role of NKp46- and NKp30-activating receptors expressed by dNK that could be critical for the outcome of pregnancy and the killing of uterine cells infected by pathogens.

CD160-activating NK cell effector functions depend on the phosphatidylinositol 3-kinase recruitment

CD160 NK cell-activating receptor is a glycosyl-phosphatidylinositol-anchored molecule that, upon specific engagement, triggers both cytotoxicity and a unique cytokine production [IFN-gamma, tumor necrosis factor-alpha (TNF-alpha) and IL-6] through an undefined signaling pathway. In the current study, we have identified several signaling molecules recruited after mAb-specific CD160 engagement in freshly isolated human circulating NK cells. Using confocal microscopy, we found that CD160 engagement induces the recruitment and co-localization of phosphorylated molecules with redistributed, capped CD160 at the cell surface. We then demonstrated that phosphatidylinositol 3-kinase (PI3K) signaling molecule is required for CD160-mediated cytotoxicity and cytokine release. First, we observed by confocal microscopy that engagement of CD160 induces its polarization and co-localization with PI3K. Second, we showed that pharmacological inhibitors of PI3K abrogate both CD160-mediated cytotoxicity and IFN-gamma, TNF-alpha and IL-6 cytokine release. We further found that CD160 engagement induced marked phosphorylation of Akt, as evidenced by western blotting. We identified additional CD160-mediated signaling molecules recruited downstream and upstream of PI3K. Both induction of phosphorylated ERK molecules after CD160-specific engagement and prevention of CD160-induced cytokine release by MEK pharmacological inhibitor indicate that ERK downstream pathway is implicated. Similarly, we identified that Syk molecule upstream of PI3K is involved in the signaling cascade mediated by CD160 engagement. Two different Syk-specific inhibitors blocked CD160-mediated cytokine release, and CD160-specific engagement induced the enhancement of phosphorylated Syk proteins. These data demonstrate that PI3K is a crucial signaling element for both effector functions of the CD160 NK cell-activating receptor.

Soluble HLA-G1 inhibits angiogenesis through an apoptotic pathway and by direct binding to CD160 receptor expressed by endothelial cells

HLA-G is a major histocompatibility complex class Ib molecule whose constitutive tissue distribution is restricted mainly to trophoblast cells at the maternal-fetal interface during pregnancy. In this study, we demonstrated the ability of the soluble HLA-G1 (sHLA-G1) isoform to inhibit fibroblast growth factor-2 (FGF2)-induced capillary-like tubule formation. Using a rabbit corneal neovascularization model, we further showed that sHLA-G1 inhibits FGF2-induced angiogenesis in vivo. We also demonstrated that sHLA-G1 induces endothelial cell apoptosis through binding to BY55/CD160, a glycosylphosphatidylinositolanchored receptor expressed by endothelial cells. Furthermore, we showed that the specific CL1-R2 anti-CD160 monoclonal antibody mimics sHLA-G1-mediated inhibition of endothelial cell tube formation and induction of apoptosis. Thus, the engagement of CD160 in endothelial cells may be essential for the inhibition of angiogenesis. sHLA-G1/CD160-mediated antiangiogenic property may participate in the vascular remodeling of maternal spiral arteries during pregnancy, and, given that we found that CD160 is strongly expressed in the vasculature of a murine tumor, it offers an attractive therapeutic target for preventing pathologic neovascularization.

Human decidual NK cells: unique phenotype and functional properties -- a review

Human decidual NK cells are massively recruited at the site of embryonic implantation (decidua basalis). They differ in many ways from their peripheral blood NK cell counterparts in terms of gene expression, phenotype and functionality. The major subpopulation of decidual NK cells is CD56(bright) whereas the minor subset is CD56(dim), contrasting with the peripheral blood NK cells whose major subpopulation is CD56(dim). Decidual NK cell cytolytic function is much reduced despite the presence of several activating receptors and the essential machinery required for lysis. Decidual NK cells produce a number of cytokines that are not normally secreted by peripheral blood NK cells. Human decidual NK cell potential functions at the maternal-fetal interface are not yet clearly established but several hypotheses are being evaluated, including control of extravillous invasion, control of uterine vascular remodeling, and local anti-viral activity.

HLA class I/NK cell receptor interaction in early human decidua basalis: possible functional consequences

Human decidual NK (dNK) cells differ from their peripheral blood (PB)-NK counterparts. The major subset of PB-NK is CD56dim, CD16+, CD160+ (highly cytolytic), whereas the major subpopulation of dNK is CD56bright, CD16- and CD160- (high cytokine producer). Extravillous cytotrophoblast invading the decidua basalis in early pregnancy expresses the polymorphic HLA-C, and nonpolymorphic HLA-E and HLA-G molecules that can interact with specific HLA class I-dependent dNK receptors, including the immunoglobulin-like KIRs, the lectin-like CD94/NKG2 and the CD160 receptors. There is no clear evidence thus far that dNK cells kill trophoblast cells. Instead they are able to secrete cytokines which are likely to be beneficial for the placental development, maternal uterine spiral arteries remodeling, and the antiviral immune response.

Cutting edge: engagement of CD160 by its HLA-C physiological ligand triggers a unique cytokine profile secretion in the cytotoxic peripheral blood NK cell subset

CD160 is an Ig-like activating NK cell receptor expressed on the majority of circulating NK cells. This population corresponds to the nonproliferating, highly cytolytic, CD56dimCD16+ subset. CD160 engagement by HLA-C molecules mediates cytotoxic function. In this study, we report that upon specific activation by the physiological ligand HLA-C, or Ab cross-linking, CD160+ peripheral blood NK cells produce IFN-gamma, TNF-alpha, and IL-6. This unique CD160-mediated cytokine production differs from the one observed after CD16 engagement whose expression is also restricted to the CD56dim cytotoxic NK cell subset. As already reported for the CD160-mediated cytotoxic effector function, CD160-mediated cytokine production by peripheral blood-NK cells is negatively controlled by the killer Ig-like receptor CD158b. Thus, the CD160 receptor represents a unique triggering surface molecule expressed by cytotoxic NK cells that participates in the inflammatory response and determines the type of subsequent specific immunity.

Engagement of CD160 receptor by HLA-C is a triggering mechanism used by circulating natural killer (NK) cells to mediate cytotoxicity

Circulating human natural killer (NK) lymphocytes have been functionally defined by their ability to exert cytotoxic activity against MHC class I-negative target cell lines, including K562. Therefore, it was proposed that NK cells recognized the "missing self." We show here that the Ig-like CD160 receptor expressed by circulating CD56(dim+) NK cells or IL-2-deprived NK cell lines is mainly involved in their cytotoxic activity against K562 target cells. Further, we report that HLA-C molecules that are constitutively expressed by K562 trigger NK cell lysis through CD160 receptor engagement. In addition, we demonstrate, with recombinant soluble HLA-Cw3 and CD160 proteins, direct interaction of these molecules. We also find that CD158b inhibitory receptors partially interfere with CD160-mediated cytotoxicity, whereas CD94CD159a and CD85j have no effect on engagement with their respective ligands. Thus, CD160HLA-C interaction constitutes a unique pathway to trigger NK cell cytotoxic activity.

Secretion of pro-apoptotic intron 4-retaining soluble HLA-G1 by human villous trophoblast

One major materno-fetal interface in the human placenta is constituted by the syncytiotrophoblast, in contact with maternal blood of the intervillous space, which derives from differentiation and fusion of the villous cytotrophoblast (vct). In the present work, we purified vct from term placenta by depleting HLA class I- and class II-positive cells. We found by RT-PCR that both soluble intron 4-retaining HLA-G1 (sHLA-G1) and HLA-G2 isoforms were transcribed in purified vct. Using different HLA-G-specific mAb, we demonstrated by intracellular flow cytometry, Western blotting and ELISA, that sHLA-G1 but no soluble HLA class Ia molecule was secreted by vct. We then purified sHLA-G1 from vct culture supernatant and found that it exhibited an unusual glycosylation pattern. Finally, we showed that such trophoblast-derived sHLA-G1 triggered specific apoptosis of activated CD8+ T cells. Taken together, these results demonstrated that vct did secrete functional sHLA-G1 in primary culture and suggested that, in vivo, sHLA-G1 might be an important immunomodulatory molecule controlling the activity of maternal immune effector CD8+ cells circulating in the blood that immerses chorionic villi.

Cutting edge: soluble HLA-G1 triggers CD95/CD95 ligand-mediated apoptosis in activated CD8+ cells by interacting with CD8

The nonpolymorphic soluble HLA-G1 (sHLA-G1) isoform has been reported to be secreted by trophoblast cells at the materno-fetal interface, suggesting that it may act as immunomodulator during pregnancy. In this paper, we report that affinity-purified beta2-microglobulin-associated sHLA-G1 triggered apoptosis in activated, but not resting CD8+ peripheral blood cells. We demonstrate by Western blotting that sHLA-G1 enhanced CD95 ligand expression in activated CD8+ cells. Cytotoxicity was inhibited by preincubation of the cells with a CD95 antagonist mAb (ZB4) or a soluble recombinant CD95-Fc, indicating that apoptosis is mediated through the CD95/CD95 ligand pathway. Finally, we show that such sHLA-G1-induced apoptosis depends on the interaction with CD8 molecules, with cell death being blocked by various CD8 mAbs.

Comparative reactivity of different HLA-G monoclonal antibodies to soluble HLA-G molecules

Different HLA-G monoclonal antibodies (mAbs) were first evaluated for their capability to identify soluble HLA-G (sHLA-G) in ELISA. Three of them, namely 87G, BFL.1 and MEM-G/9, when used as coating mAbs together with W6/32 capture mAb, identified beta2-microglobulin (beta2m)-associated-sHLA-G but not soluble HLA-B7 (sHLA-B7) in cell culture supernatants from transfected cells. By comparison, the anti-HLA class I mAb 90 did recognize both sHLA-G and sHLA-B7. By using these HLA-G mAbs, sHLA-G was identified in amniotic fluids as well as in culture supernatants of first trimester and term placental explants but not in cord blood. Intron 4-retaining sHLA-G isoforms were identified in some amniotic fluids by the use of an intron 4-specific mAb (16G1). Reactivity of these different HLA-G mAbs was then compared to determine their respective binding sites on soluble and membrane-bound HLA-G. Using both ELISA and flow cytometry analysis, we showed that they did not compete with each other, which suggested that they did not recognize the same determinants. Finally, we report that two mAbs directed against the alpha1 domain of HLA class I heavy chain (mAb 90 and YTH 862) did compete with 87G, therefore demonstrating that this latter mAb recognized an epitope localized on this external domain of HLA-G.

The full length HLA-G1 and no other alternative form of HLA-G is expressed at the cell surface of transfected cells

In contrast to HLA class Ia, the HLA-G class Ib transcripts can be alternativeley spliced to yield several isoforms including four potentially membrane-bound variants, namely HLA-G1, -G2, -G3 and G4. It is so far unclear whether each of these splice variants lacking one or two external domains is properly translated and expressed at the cell surface. We used targeted Enhanced Green Fluorescence Protein (EGFP)-HLA-G fusion cDNA to track HLA-G isoform expression in living murine (L-human beta2m) and human (JAR) transiently transfected cells. It was demonstrated that the four HLA-G1, -G2, -G3, and -G4 isoforms were translated in these transfectants by the means of (i) Western blotting analysis, using an anti-EGFP mAb; (ii) intracellular double labeling flow cytometry analysis, using the EGFP natural fluorescence and phycoerythrin-labeled HCA2 anti-HLA-G mAb; and (iii) immunocytochemistry on isolated acetone fixed transfectants with the use of different anti-HLA-G mAbs. Cell surface flow cytometry analysis using the HCA2 mAb revealed that only the HLA-G1 isoform was expressed as a membrane-bound protein. Two color confocal microscopy performed on fixed, permeabilized cells further showed that the EGFP green fluorescence co-localized with anti-calnexin rhodamine fluorescence in the four HLA-G isoform transfectants but only in HLA-G1 transfectant was the green EGFP fluorescence also detectable at the outer part of the cells, suggesting that the HLA-G2, -G3, and G4 were retained in the endoplasmic reticulum. Such intracellular retention of the three shorter forms of HLA-G suggest that they may play a role in regulating cell surface expression either of the full length HLA-G1 form or of HLA-E.

Soluble HLA-G: purification from eukaryotic transfected cells and detection by a specific ELISA

PROBLEM

The detection of soluble forms of human leukocyte antigen-G molecule (sHLA-G) at the maternal-fetal interface suggest that sHLA-G may play a role during pregnancy. To study the potential functions of sHLA-G, we developed a procedure to detect and produce such HLA-G isoforms.

METHOD OF STUDY

Transfected cell lines expressing either sHLA-G1s cDNA (JAR-G1s) or an sHLA-G monochain DNA (Fox-G-mono) containing extracellular domains of HLA-G linked to the human beta 2-microglobulin were used. Specific sHLA-G enzyme-linked immunosorbent assay (ELISA), using anti-HLA-G monoclonal antibodies (mAbs) (87G and BFL.1) as coating antibodies and the biotinylated HLA class I mAb, W6/32, to reveal the bound molecules, was then developed.

RESULTS

To assess the specificity of the ELISA, we tested cell culture supernatants from the trophoblast-derived JEG-3 cell line and the HLA-G1s-transfected JAR cells, and we detected sHLA-G in both supernatants. sHLA-G monochain was also detected by ELISA in transfected cell supernatants using the conformational mAb, W6/32, showing that the conformation of sHLA-G monochain was proper. Using the same ELISA, sHLA-G was detected in various samples of amniotic fluid. To test the potential role of sHLA-G, sHLA-G has been purified by immunoaffinity columns, using W6/32 mAb, from culture supernatants of HLA-G1s or sHLA-G monochain-transfected cells.

CONCLUSION

These important tools will be useful both for the detection of sHLA-G in various biological fluids and in functional tests.

Placental HLA-G protein expression in vivo: where and what for?

In contrast to HLA-A and -B class Ia genes that are down-regulated in human trophoblast cells, HLA-G class Ib molecules are expressed in the placenta throughout gestation. In addition to extravillous cytotrophoblast that invade the decidua basalis essentially, HLA-G was also observed in endothelial cells of fetal vessels in the chorionic villi as well as in amnion cells and amniotic fluid. Both membrane-bound and soluble HLA-G isoforms have been detected. In view of the recently published functional data showing that HLA-G: (i) has the capability to bind and present peptides; (ii) is recognized by at least three different killing inhibitory receptors; and (iii) is a regulator of HLA-E expression, we can predict that such functions are likely to be exerted by extravillous cytotrophoblast. Of particular importance will be the anti-viral function of HLA-G at this materno-fetal interface, knowing that HLA-G was shown to be expressed by thymic medullary epithelial cells. In addition to these immunological functions, due to its presence on chorionic fetal endothelial cells, we hypothesize that HLA-G could also be a regulator of chorionic villous angiogenesis. Finally, soluble HLA-G isoforms may act as specific immunosuppressors during pregnancy.

Absence of imprinting of HLA class Ia genes leads to co-expression of biparental alleles on term human trophoblast cells upon IFN-gamma induction

Human trophoblast cells have developed various efficient regulatory mechanisms to prevent cell surface expression of the classical HLA-A, -B, and (but not always) -C class I molecules. This allows them to escape maternal alloimmune attack during pregnancy. However, recent results have demonstrated that such a lack of expression could be reversed in villous cytotrophoblast cells purified from term placenta by in vitro IFN-gamma treatment. In this context, we investigated whether both maternal and paternal HLA class Ia antigens were co-dominantly expressed in such trophoblast cells. Using polymerase chain reaction sequence-specific primers for HLA-A and HLA-C alleles, we detected transcripts of both paternal and maternal origins, showing that these genes were not affected by genomic imprinting, at least in term placenta. After in vitro IFN-gamma treatment, the polymorphic HLA-A and HLA-B antigens of both parental origins become detectable at the cell surface, as assessed by flow cytometry and/or complement-dependent microtoxicity test. Appearance of paternal antigens on trophoblast cells upon IFN-gamma induction raises the question of the in vivo biological consequences of this phenomena, in term of materno-fetal tolerance and in particular of a potential allogeneic cytotoxic immune response.