Low molecular weight heparin modulates maternal immune response in pregnant women and mice with thrombophilia

PROBLEM

Thrombophilia is associated with pregnancy complications. Treatment with low molecular weight heparin (LMWH) improves pregnancy outcome, but the underlying mechanisms are not clear.

METHODS OF STUDY

We analyzed Treg frequency in blood from thrombophilic pregnancies treated with LMWH (n = 32) or untreated (n = 33) and from healthy pregnancies (n = 39) at all trimesters. Additionally, we treated pregnant wild-type, heterozygous and homozygous factor-V-Leiden (FVL) mice with LMWH or PBS and determined Treg frequency, pro-/anti-inflammatory cytokine levels and Caspase-3-activity in placenta and decidua.

RESULTS

Treg frequencies were increased in second and third trimester in LMWH-treated thrombophilic pregnancies compared to controls. Treg levels were comparable to those of normal pregnancies. Homozygous FVL mice had decreased decidual Tregs compared to wild-type mice. LMWH treatment normalized Tregs and was associated with increased decidual IL-10 mRNA. LMWH diminished Caspase-3-activity in mice of all genotypes.

CONCLUSION

We demonstrated anti-apoptotic and anti-inflammatory effects of LMWH in pregnant FVL mice. LMWH increased Treg levels in mice and humans, which suggests benefits of LMWH treatment for thrombophilic women during pregnancy.

Transcriptional profiling of human epithelial cells infected with plasmid-bearing and plasmid-deficient Chlamydia trachomatis

Chlamydia trachomatis is an obligate intracellular epitheliotropic bacterial pathogen of humans. Infection of the eye can result in trachoma, the leading cause of preventable blindness in the world. The pathophysiology of blinding trachoma is driven by multiple episodes of reinfection of conjunctival epithelial cells, producing an intense chronic inflammatory response resulting in submucosal tissue remodeling and scarring. Recent reports have shown that infection with trachoma organisms lacking the cryptic chlamydial plasmid is highly attenuated in macaque eyes, a relevant experimental model of human trachoma infection. To better understand the molecular basis of plasmid-mediated infection attenuation and the potential modulation of host immunity, we conducted transcriptional profiling of human epithelial cells infected with C. trachomatis plasmid-bearing (A2497) and plasmid-deficient (A2497P(-)) organisms. Infection of human epithelial cells with either strain increased the expression of host genes coding for proinflammatory (granulocyte-macrophage colony-stimulating factor [GM-CSF], macrophage colony-stimulating factor [MCSF], interleukin-6 [IL-6], IL-8, IL-1α, CXCL1, CXCL2, CXCL3, intercellular adhesion molecule 1 [ICAM1]), chemoattraction (CCL20, CCL5, CXCL10), immune suppression (PD-L1, NFKB1B, TNFAIP3, CGB), apoptosis (CASP9, FAS, IL-24), and cell growth and fibrosis (EGR1 and IL-20) proteins. Statistically significant increases in the levels of expression of many of these genes were found in A2497-infected cells compared to the levels of expression in A2497P(-)-infected cells. Our findings suggest that the chlamydial plasmid plays a focal role in the host cell inflammatory response to infection and immune avoidance. These results provide new insights into the role of the chlamydial plasmid as a chlamydial virulence factor and its contributions to trachoma pathogenesis.

Regulatory T-cells in pregnancy: historical perspective, state of the art, and burning questions

In this review, we first revisit the original concept of "suppressor T-cells" in pregnancy, put it in a historical perspective, and then highlight the main data that licensed its resurrection and revision into the concept of "regulatory T-cells" (Tregs) in pregnancy. We review the evidence for a major role of Tregs in murine and human pregnancy and discuss Treg interactions with dendritic and uterine natural killer cells, other players of maternal-fetal tolerance. Finally, we highlight what we consider as the most important questions in the field.

Endocrine factors modulating immune responses in pregnancy

How the semi-allogeneic fetus is tolerated by the maternal immune system remains a fascinating phenomenon. Despite extensive research activity in this field, the mechanisms underlying fetal tolerance are still not well understood. However, there are growing evidences that immune–immune interactions as well as immune–endocrine interactions build up a complex network of immune regulation that ensures fetal survival within the maternal uterus. In the present review, we aim to summarize emerging research data from our and other laboratories on immune modulating properties of pregnancy hormones with a special focus on progesterone, estradiol, and human chorionic gonadotropin. These pregnancy hormones are critically involved in the successful establishment, maintenance, and termination of pregnancy. They suppress detrimental maternal alloresponses while promoting tolerance pathways. This includes the reduction of the antigen-presenting capacity of dendritic cells (DCs), monocytes, and macrophages as well as the blockage of natural killer cells, T and B cells. Pregnancy hormones also support the proliferation of pregnancy supporting uterine killer cells, retain tolerogenic DCs, and efficiently induce regulatory T (Treg) cells. Furthermore, they are involved in the recruitment of mast cells and Treg cells into the fetal–maternal interface contributing to a local accumulation of pregnancy-protective cells. These findings highlight the importance of endocrine factors for the tolerance induction during pregnancy and encourage further research in the field.

Regulatory T cells: regulators of life

Pregnancy still represents one of the most fascinating paradoxical phenomena in science. Immediately after conception, the maternal immune system is challenged by the presence of foreign paternal antigens in the semen. This triggers mechanisms of recognition and tolerance that all together allow the embryo to implant and later the fetus to develop. Tolerance mechanisms to maintain pregnancy are of special interest as they defy the classical immunology rules. Several cell types, soluble factors, and immune regulatory molecules have been proposed to contribute to fetal tolerance. Within these, regulatory T cells (Treg) are one of the most studied immune cell populations lately. They are reportedly involved in fetal acceptance. Here, we summarize several aspects of Treg biology in normal and pathologic pregnancies focusing on Treg frequencies, subtypes, antigen specificity, and activity as well as on factors influencing Treg generation, recruitment, and function. This review also highlights the contribution of fetal Treg in tolerance induction and addresses the role of Treg in autoimmune diseases and infections during gestation. Finally, the potential of Treg as a predictive marker for the success of assisted reproductive techniques and for therapeutic interventions is discussed.

Intravenous immunoglobulin G modulates peripheral blood Th17 and Foxp3(+) regulatory T cells in pregnant women with recurrent pregnancy loss

PROBLEM

Th17 cells and Foxp3(+) regulatory T (Treg) cells have been proposed as new risk factors for recurrent pregnancy loss (RPL). Intravenous immunoglobulin G (IVIG) was reported to modulate various immune cells. In this study, we investigated the effect of IVIG on the levels of Th17 and Treg cells and pregnancy outcome in women with RPL.

METHOD OF STUDY

Thirty-seven pregnant women with RPL were enrolled in this study. All had cellular immune abnormality in preconceptional evaluation. Blood was drawn on the day of IVIG treatment and 1 week later from the study subjects during early pregnancy. The proportions of IL-17(+) and Foxp3(+) T cells were analyzed using flow cytometry.

RESULTS

Study population was divided into four groups (Q1-Q4) based on ascending order of the levels of Th17 and Foxp3(+) T cells. IVIG down-regulated Th17 cells in the highest quartile, Q4 (P = 0.001), and up-regulated CD4(+)  Foxp3(+) T cells in Q1 and Q2 (P = 0.025 and 0.029, respectively). In addition, Th17/CD4(+)  Foxp3(+) T cell ratio decreased in Q4 (P = 0.040). We also found a positive trend between successful pregnancy outcome and CD8(+)  IL-17(+) T cells before IVIG treatment (P = 0.05).

CONCLUSION

Intravenous immunoglobulin G treatment modulated imbalance of Th17 and Foxp3(+) Treg cells in pregnant RPL women with cellular immune abnormality.

Luteinizing hormone contributes to fetal tolerance by regulating adaptive immune responses

PROBLEM

Pregnancy hormones were proposed to be crucially involved in fetal tolerance. Recently, we showed that human chorionic gonadotropin (hCG) not only increases the number and activity of regulatory T cells (Treg) but also retains tolerogenic dendritic cells (DCs). Here, we investigate whether the highly homologous luteinizing hormone (LH) modulates Treg number and DC phenotype and thereby supports pregnancy.

METHOD OF STUDY

Abortion-prone females were treated with LH or PBS on different gestation days. Pregnancy outcome and the number and phenotype of Treg and DCs were evaluated in the periphery and locally.

RESULTS

We discovered that LH application completely prevented fetal rejection in abortion-prone females. This protective effect was associated with a Treg augmentation peripherally and locally. Moreover, LH reduced the number of total and mature DCs.

CONCLUSION

Our data suggest that LH, similar to hCG, is involved in the regulation of adaptive immune responses, thus contributing to fetal tolerance.

Is Infant Immunity Actively Suppressed or Immature?

Almost 7 million children under the age 5 die each year, and most of these deaths are attributable to vaccine-preventable infections. Young infants respond poorly to infections and vaccines. In particular, dendritic cells secrete less IL-12 and IL-18, CD8^pos T cells and NK cells have defective cytolysis and cytokine production, and CD4^pos T cell responses tend to bias towards a Th2 phenotype and promotion of regulatory T cells (T_regs). The basis for these differences is not well understood and may be in part explained by epigenetic differences, as well as immaturity of the infant's immune system. Here we present a third possibility, which involves active suppression by immune regulatory cells and place in context the immune suppressive pathways of mesenchymal stromal cells (MSC), myeloid-derived suppressor cells (MDSC), CD5^pos B cells, and T_regs. The immune pathways that these immune regulatory cells inhibit are similar to those that are defective in the infant. Therefore, the immune deficiencies seen in infants could be explained, in part, by active suppressive cells, indicating potential new avenues for intervention.

Human chorionic gonadotropin enhances trophoblast-epithelial interaction in an in vitro model of human implantation

Embryo implantation, which is an absolute requirement for reproduction, starts with blastocyst apposition to the uterine endometrium, followed by attachment to the endometrial surface epithelium. Recent clinical studies reported an increase in implantation and pregnancy rates among women receiving intrauterine human chorionic gonadotropin (hCG) prior to embryo transfer suggesting that, at least in some cases, female infertility is a result of inadequate secretion of hCG. In this study, we characterized the effect of hCG on trophoblast-epithelial interaction by further developing our recently described in vitro model of implantation. Here, we confirmed hCG increased attachment of trophoblast to epithelial cells, using a single-cell trophoblast-epithelial coculture system in addition to a blastocyst-like spheroid-epithelial coculture system. Furthermore, we discovered that the source and concentration was pivotal; the first preparation of hCG affected 2 molecules related to implantation, MUC16 and osteopontin, while the second preparation required additional cytokines to mimic the effects. Using this system, we can develop a comprehensive knowledge of the cellular and gene targets of hCG and other factors involved in embryo apposition and implantation and potentially increase the number of therapeutic targets for subfertile patients.

The Endocrine Milieu and CD4 T-Lymphocyte Polarization during Pregnancy

Acceptance of the fetal semi-allograft by the mother's immune system has become the focus of intensive research. CD4+ T cells are important actors in the establishment of pregnancy. Th1/Th2 paradigm has been expanded to include CD4+ regulatory T (Treg) and T helper 17 (Th17) cells. Pregnancy hormones exert very significant modulatory properties on the maternal immune system. In this review, we describe mechanisms by which the endocrine milieu modulates CD4 T cell polarization during pregnancy. We first focused on Treg and Th17 cells and on their importance for pregnancy. Secondly, we review the effects of pregnancy hormones [progesterone (P4) and estradiol (E2)] on immune cells previously described, with a particular attention to human chorionic gonadotropin (hCG). The importance of Treg cells for pregnancy is evidenced. They are recruited before implantation and are essential for pregnancy maintenance. Decreased number or less efficient Treg cells are implicated in fertility disorders. As for Th17 cells, the few available studies suggest that they have a negative impact on fertility. Th17 frequency is increased in infertile patients. With the combination of its pro-effects on Th2 and Treg cells and anti-effects on Th1 and Th17 cells, P4 contributes to establishment of a favorable environment for pregnancy. E2 effects are more dependent on the context but it seems that E2 promotes Treg and Th2 cells while it inhibits Th1 cells. hCG positively influences activities of Treg and uterine natural killer cells. This embryo signal is an essential actor for the success of pregnancy, both as the endocrine factor regulating P4 secretion by the ovarian corpus luteum, but also as a paracrine agent during implantation as well as an angiogenic and immunologic mediator during the course of gestation. Luteinizing hormone (LH) immune properties begin to be studied but its positive impact on Treg cells suggests that LH could be a considerable immunomodulator in the mouse.

Natural killer cells and regulatory T cells in early pregnancy loss

Survival of the allogeneic embryo in the uterus depends on the maintenance of immune tolerance at the maternal-fetal interface. The pregnant uterus is replete with activated maternal immune cells. How this immune tolerance is acquired and maintained has been a topic of intense investigation. The key immune cells that predominantly populate the pregnant uterus are natural killer (NK) cells. In normal pregnancy, these cells are not killers, but rather provide a microenvironment that is pregnancy compatible and supports healthy placentation. In placental mammals, an array of highly orchestrated immune elements to support successful pregnancy outcome has been incorporated. This includes active cooperation between maternal immune cells, particularly NK cells, and trophoblast cells. This intricate process is required for placentation, immune regulation and to remodel the blood supply to the fetus. During the past decade, various types of maternal immune cells have been thought to be involved in cross-talk with trophoblasts and in programming immune tolerance. Regulatory T cells (Tregs) have attracted a great deal of attention in promoting implantation and immune tolerance beyond implantation. However, what has not been fully addressed is how this immune-trophoblast axis breaks down during adverse pregnancy outcomes, particularly early pregnancy loss, and in response to unscheduled inflammation. Intense research efforts have begun to shed light on the roles of NK cells and Tregs in early pregnancy loss, although much remains to be unraveled in order to fully characterize the mechanisms underlying their detrimental activity. An increased understanding of host-environment interactions that lead to the cytotoxic phenotype of these otherwise pregnancy compatible maternal immune cells is important for prediction, prevention and treatment of pregnancy maladies, particularly recurrent pregnancy loss. In this review, we discuss relevant information from experimental and human models that may explain the pregnancy disrupting roles of these pivotal sentinel cells at the maternal-fetal interface.

Maternal CD4⁺ and CD8⁺ T cell tolerance towards a fetal minor histocompatibility antigen in T cell receptor transgenic mice

Tolerance of the maternal immune system in pregnancy is important for successful pregnancy because the semiallogeneic fetus may be subject to antifetal responses. We examined maternal tolerance to the fetus using a murine system in which a model paternally inherited antigen, ovalbumin (OVA), is expressed exclusively in the fetus and placenta. By employing T cell receptor (TCR) transgenic mice specific for major histocompatibility complex class I- or class II-restricted epitopes of OVA (OT-I and OT-II) as mothers, we investigated the fate of fetus-specific CD8⁺ and CD4⁺ T cells, respectively, during gestation. Both OVA-specific CD8⁺ and CD4⁺ T cells displayed an activated phenotype in the peripheral lymphoid tissues of OVA-bred OT-I and OT-II mice, consistent with their encounter of fetal antigen. Whereas a small percentage of OVA-specific CD4⁺ T cells were deleted in the periphery and thymus of OVA-bred OT-II mice, with evidence of TCR downregulation in the remaining T cells, deletion and TCR downregulation were not observed in OVA-bred OT-I mice. Both CD4⁺ and CD8⁺ T cells upregulated inducible costimulator expression in response to the fetal antigen, but only CD4⁺ T cells consistently upregulated the inhibitory receptors programmed cell death 1 and cytotoxic T lymphocyte antigen-4. More regulatory T cells (Tregs) were present in pregnant OVA-bred than in WT-bred OT-II mice, revealing that Tregs expanded specifically in response to the fetal antigen. These data indicate that several mechanisms tolerize fetal antigen-specific maternal CD4⁺ T cells, whereas tolerance of fetal antigen-specific CD8⁺ T cells is less effective. The importance of these mechanisms is underscored by the finding that fetal loss occurs in OVA-bred OT-I but not OT-II mice.

Fetal-maternal alignment of regulatory T cells correlates with IL-10 and Bcl-2 upregulation in pregnancy

Transplacental immune regulation refers to the concept that during pregnancy, significant cross-talk occurs between the maternal and fetal immune system with potential long-term effects for both the mother and child. In this study, we made the surprising observation that there is a strong correlation of peripheral blood regulatory T (Treg) cells between the mother and the fetus. In contrast, there is no significant Treg cell correlation between paternal fetal dyads (pairs), suggesting that the specific context of pregnancy, rather than the genetic parental similarity to the fetus, is responsible for this correlation. Gene microarray analysis of Treg cells identified a typical IL-10-dependent signature in maternal and fetal Treg cells. In addition, a direct correlation of serum IL-10 protein levels between maternal fetal dyads was observed. Furthermore, we show that maternal serum IL-10 levels correlate with serum estradiol and estriol, implicating hormonal involvement in this alignment. Interestingly, we show that Treg cells possess higher expression of IL-10 receptor α and that Treg cell IL-10 receptor α expression directly correlates with their Bcl-2 expression. Indeed, in vitro data in both humans and mice demonstrate that IL-10 upregulates Bcl-2 specifically in Treg cells but not non-Treg cells. Our results provide evidence for transplacental regulation of cellular immunity and suggest that IL-10 may influence Treg cell homeostasis through its effect on Treg cell Bcl-2 expression. These novel findings have important implications on immune tolerance in pregnancy and beyond in areas of autoimmunity, allergy, and transplantation.

Adaptive immune responses during pregnancy

It has long been believed that there is no immune interaction between mother and conceptus during pregnancy. This concept changed after evidence was provided that the maternal immune system is aware of the semiallogeneic conceptus and develops strategies to tolerate it. Since then, finely regulated mechanisms of active tolerance toward the fetus have been described. This Special Issue of the American Journal of Reproductive Immunology deals with these mechanisms. It begins with the description of minor histocompatibility antigens in the placenta; it further goes through adaptive immune responses toward paternal fetal antigens, mostly concentrating on regulatory T cells and molecules modulating the Th1/Th2 balance. The participation of antibody-producing B cells in normal and pathological pregnancies is also discussed. This introductory chapter resumes the concepts presented throughout the Issue and discusses the clinical applications raised from these concepts.