HLA-G finally does something!

Progesterone regulates HLA-G gene expression through a novel progesterone response element

BACKGROUND

We have previously demonstrated that progesterone has a stimulatory effect on HLA-G gene expression. Because this effect was abolished by the anti-progestin, RU486, we hypothesize that this effect is through receptor-mediated up-regulation of the HLA-G gene. The objective of this study was to explore the molecular mechanisms of this effect.

METHODS

The transient transfection of a chloramphenicol acetyltransferase (CAT) construct containing a fragment of the HLA-G gene promoter into the JEG-3 choriocarcinoma cell line was performed. An electrophoretic mobility shift assay (EMSA) and a DNA fragment-binding enzyme-linked immunosorbent assay (ELISA) were carried out to locate a specific progesterone response element (PRE) in the HLA-G gene promoter region.

RESULTS

Progesterone treatment of JEG-3 cells transfected with the HLA-G gene promoter-CAT construct resulted in an increase of CAT synthesis, whereas RU486 blocked this transcriptional activation. A novel PRE-binding site sequence, with 60% homology to that of wild-type mouse mammary tumour virus (MMTV) PRE, was discovered in this region.

CONCLUSION

The effect of progesterone on HLA-G gene expression is through progesterone receptor (PR) activation, followed by binding to a novel PRE in the HLA-G promoter region. Therefore, one of the mechanisms of immunomodulation by progesterone during pregnancy may be through the regulation of HLA-G gene expression via this novel PRE.

Secretion of human leukocyte antigen-G by human embryos is associated with a higher in vitro fertilization pregnancy rate

OBJECTIVE

To investigate whether secretion of soluble human leukocyte antigen-G (HLA-G) by human embryos is associated with embryo development and IVF pregnancy outcome.

DESIGN

Retrospective study.

SETTING

In vitro fertilization program affiliated with a university research center.

PATIENT(S)

Infertile couples attending an IVF program were selected.

INTERVENTION(S)

Embryo culture conditioned medium (72 hours) from cases in which intracytoplasmic sperm injection was used for fertilization.

MAIN OUTCOME MEASURE(S)

Soluble HLA-G in embryo culture medium samples from IVF patients was assayed and associations between soluble HLA-G secretion and outcome measures were analyzed.

RESULT(S)

Two hundred seventy of 386 samples had detectable soluble HLA-G. Soluble HLA-G secretion was independent of embryo grade or patients' age. The cleavage rate of embryos secreting soluble HLA-G was significantly higher than that of those lacking it (blastomere number 6.71 +/- 0.09 vs. 5.86 +/- 0.22). The live birth rate from embryos with soluble HLA-G was significantly higher than that of those without (48.4% vs. 17.1%, chi(2) = 9.09). Combining soluble HLA-G detection and cleavage rate was most predictive of pregnancy.

CONCLUSION(S)

Our five conclusions are as follows: [1] embryonic secretion of soluble HLA-G protein is variable, [2] secretion of HLA-G is correlated with a higher cleavage rate, [3] secretion of HLA-G is associated with a higher pregnancy rate, [4] HLA-G secretion is a better independent predictor than cleavage rate alone, and [5] the combination of soluble HLA-G detection and high cleavage rate was the best predictor of outcome.

HLA-G protein concentrations in maternal serum and placental tissue are decreased in preeclampsia

OBJECTIVE

The aim of this study were to determine whether soluble human leukocyte antigen-G protein levels in serum and/or human leukocyte antigen protein in placental tissues differ between women with preeclampsia versus uncomplicated pregnancies. Study design human leukocyte antigen-G levels were determined with the use of a specific enzyme-linked immunosorbent assay in 20 subjects with preeclampsia and 14 normal control subjects.

RESULTS

Both serum and placental human leukocyte antigen-G levels were decreased significantly in the preeclampsia group (median, 0.026 microg/mL in serum; median, 0.026 microg/mg protein in placenta), in comparison with normal pregnant women (median, 0.093 microg/mL in serum; median, 0.088 microg/mg protein in placenta; P=.0112 and P=.0406, respectively). There was a significant correlation between serum and placental human leukocyte antigen-G levels (r=0.603; P=0.0002).

CONCLUSION

The reduced expression of placental human leukocyte antigen-G and reduced release of this protein into the maternal circulation in preeclampsia may alter the maternal-fetal immune relationship and thus be involved in the cause of this disorder.

Maternal-conceptus Cross Talk—A Review

The implantation process requires a functionally normal conceptus and a receptive endometrium, but also a communication link between them. This paracrine dialogue involves not only gonadal steroids but also a variety of other biologic molecules secreted by the conceptus and the reproductive tissues themselves in a communicative, interconnected network. The factors facilitating this dialogue includes chemokines, cytokines, adhesion molecules, growth factors. Recently, the soluble form of HLA-G and the vitamin D system have also been proposed as components of this cross-talk. Inherent among the factors involved is the fact that their effects are redundant and pleiotropic. Normal implantation and placentation are critical for a successful pregnancy. Therefore, a better understanding of the molecular mechanisms responsible for these processes will lead to the development of new regulating agents with novel diagnostic, biological and therapeutic potential for both facilitating and hindering a normal reproductive function.

Immunological relationship between the mother and the fetus

The immunological relationship between the mother and the fetus is a bi-directional communication determined on the one hand by fetal antigen presentation and on the other hand by recognition of and reaction to these antigens by the maternal immune system. There is evidence now that immunological recognition of pregnancy is important for the maintenance of gestation, and that inadequate recognition of fetal antigens might result in failed pregnancy. In contrast to HLA-A and -B Class I genes that are downregulated in human trophoblast cells, nonpolymorphic Class I molecules, e.g., HLA-G Class Ib, are expressed in extravillous cytotrophoblast and also in endothelial cells of fetal vessels in the chorionic villi as well as in amnion cells and amniotic fluid. The trophoblast does not induce transplantation immunity and resists NK- and CTL-mediated lysis in vitro. According to our present knowledge, HLA-G presents antigens for gamma/delta T cells and at the same time defends the trophoblast from cytotoxic effector mechanisms. Since polymorphic MHC is absent from the trophoblast, presentation of fetally derived antigens is unlikely to be MHC restricted. gamma/delta T cells recognize a distinct group of ligands with a smaller receptor repertoire than alpha/beta T cells. Most gamma/delta T cells recognize unprocessed foreign antigens without MHC. In the decidua gamma/delta TCR-positive cells significantly increase in number and the majority of decidual gamma/delta T cells are in an activated form due to recognition of conserved mammalian molecules on the trophoblast. Following recognition of fetally derived antigens, the immune system reacts with the setting in of a wide range of protective mechanisms. Many observations suggest that pregnancy is associated with an altered TH1/TH2 balance. Maternal immune response is biased toward humoral immunity and away from cell-mediated immunity that could be harmful to the fetus. Cytokines of maternal origin act on placental development. On the other hand, antigen expression on the placenta determines maternal cytokine pattern. Normal human pregnancy is characterized by low peripheral NK activity, and increased NK activity seems to play a role in spontaneous abortions of unknown etiology. In early human pregnancy the majority of uterine lymphocytes are CD56(bright) granulated NK cells, which do not express CD16 or CD3. In rodents and humans, uterine NK cells are under hormonal control. In early pregnancy they are enriched at sites where fetal trophoblast infiltrates the decidua. The dynamics of the appearance of uterine NK cells suggest that one of the functions of these cells is control of placentation. Another protective mechanism operating in favor of pregnancy is progesterone-dependent immunomodulation. Due to stimulation by fetally derived antigens, pregnancy lymphocytes develop progesterone receptors and in the presence of progesterone produce a mediator (PIBF) that, through altering the cytokine balance, inhibits NK activity and exerts an antiabortive effect in mice.

The role of gamma/delta T cells in the feto-maternal relationship

Polymorphic MHC is absent from the trophoblast, therefore, it resists NK as well as CTL-mediated lysis in vitro. Activated gamma / delta TCR positive cells are significantly enriched in the decidua as well as in peripheral blood of healthy pregnant women. Human peripheral gamma / delta lymphocytes preferentially express the V gamma 9/V delta 2 TCR, whereas those of the decidua use the V delta 1 chain. These subpopulations are functionally polarized, the former being Th1, the latter Th2. Potentially cytotoxic V delta 2+ lymphocytes recognize HLA-E on the trophoblast via the CD94/NKG2A receptor, which induces an inhibitory signal, thus potentially inhibiting Th1 type cytokine production.

HLA-G polymorphisms and molecule function--questions and more questions--a review

Human leukocyte antigen (HLA)-G is a non-classical HLA-class I antigen which is predominantly expressed on invasive trophoblastic cells and is postulated to be a mediator of maternal-fetal tolerance. HLA-G interacts with NK cells, can present nonamer peptides and binds CD8 in an analogous manner to classical HLA-I. The HLA-G protein exists in soluble and membrane-bound isoforms generated through alternative splicing. Although initially considered to be non-polymorphic, variations of the HLA-G DNA sequence have been reported which led to the definition of a limited number of HLA-G alleles including the Null-allele G*0105N. Whereas the HLA-G DNA sequence shows a high degree of conservation in positions which are essential for classical HLA-I molecule functions, polymorphic sites in HLA-G are not congruent with sites of high nucleotide variability in classical HLA. The identification of two females with recurrent spontaneous abortions who are homozygous for the G*0105N Null-allele re-opens the discussion about the role of HLA-G in pregnancy and underlines the need of a systematic analysis of the different hypotheses of HLA-G function in vivo.

Mechanisms of action of major-histocompatibility-complex-linked genes affecting reproduction

PROBLEM

To provide insight into the mechanisms of action of the major-histocompatibility-complex (MHC)-linked genes affecting reproduction.

METHOD OF STUDY

The data were obtained using a variety of cellular and molecular techniques in experimental animals and from population genetic studies in humans.

RESULTS

In the mouse, the preimplantation embryonic development (Ped) locus, whose functional gene is Q9, regulates fast and slow cleavage of the early embryo. There is also evidence for a growth and reproduction complex (Grc)-like region from serologic, molecular, and cytogenetic studies. In the human, the human leukocyte antigen (HLA)-G gene has been associated with an increased rate of embryonic cleavage in those embryos that express the HLA-G antigen. Sharing of HLA antigens in couples has been associated with recurrent spontaneous abortions, gestational trophoblastic tumors, and unexplained infertility. Detailed mapping studies showed that the genes responsible are not the HLA genes themselves, but genes closely linked to the HLA-DR-DQ-B genes. The HLA region genes can interact epistatically with the C3 allele of transferrin to increase the incidence of fetal loss. In the rat, the Grc region, which is closely linked to the MHC, has been associated with embryonic loss, growth defects, and susceptibility to chemical carcinogens. The Grc can interact epistatically with the tail anomaly lethal (Tal) gene or the hood restriction (Hre) gene to enhance these effects.

CONCLUSIONS

There are two basic mechanisms for the effects of MHC-linked genes on reproduction and development: individual gene effects (Ped [Q9], HLA-G) and extended genetic effects (MHC-linked genes in the rat [Grc] and in the human). The nature of these genetic effects, particularly the MHC-linked effects, can also provide some insight into the different theories of human origins: These effects are most consistent with the monogenic theory.