Regulatory T cells (Treg) expand during pregnancy and are present at the fetal-maternal interface at very early stages in pregnancy. The migration mechanisms of Treg to the pregnant uterus are still unclear. Human chorionic gonadotropin (hCG) is secreted by the blastocyst immediately after fertilization and has chemoattractant properties. Therefore, we sought to analyze whether hCG secreted by early trophoblasts attracts Treg to the uterus and hence contributes to maternal tolerance toward the fetus. Decidua and placenta tissue samples from patients having spontaneous abortions or ectopic pregnancies were employed to evaluate Treg and hCG levels. Age-matched samples from normal pregnant women served as controls. We further performed in vitro studies with primary first trimester trophoblast cells and a choriocarcinoma cell line (JEG-3) aiming to evaluate the ability of secreted hCG to attract Treg. Patients having miscarriages or ectopic pregnancy presented significantly decreased hCG mRNA and protein levels associated with decreased Foxp3, neuropilin-1, IL-10, and TGF-β mRNA levels as compared with normal pregnant women. Using migration assays we demonstrated that Treg were attracted by hCG-producing trophoblasts or choriocarcinoma cells. Treg migration toward cells transfected with hCG expression vectors confirmed the chemoattractant ability of hCG. Our data clearly show that hCG produced by trophoblasts attracts Treg to the fetal-maternal interface. High hCG levels at very early pregnancy stages ensure Treg to migrate to the site of contact between paternal Ags and maternal immune cells and to orchestrate immune tolerance toward the fetus.

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, 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 dendritic cells and efficiently induce regulatory T cells. Furthermore, they are involved in the recruitment of mast cells and regulatory T 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.

The mechanisms underlying immune tolerance during pregnancy are poorly understood. In this regard, Treg seem to play an important role in mediating maternal tolerance to the fetus. We proposed a crucial role of T regulatory cells (Treg) in avoiding immunological rejection of the fetus after observing diminished number and function of Treg in abortion‐prone mice. We further confirmed the protective role of Treg during pregnancy by transferring pregnancy‐induced Treg into abortion‐prone mice, which prevented rejection. Here, we analyzed the mechanisms involved in Treg‐mediated protection. As expected, Treg therapy prevented abortion, while expanding the peripheral and thymic Treg population. Surprisingly, the decidual levels of the Th1 cytokines IFN‐γ and TNF‐α were not diminished after therapy. Interestingly, the mRNA levels of leukemia inhibitory factor, TGF‐β and heme oxygenase‐1 at the fetal‐maternal interface were dramatically up‐regulated after Treg transfer, while the levels of indolamine 2,3‐dioxygenase remained unchanged. Our data suggest that Treg treatment can not prevent T cell infiltration or high Th1 levels but is able to create a privileged tolerant microenvironment at the fetal‐maternal interface, further shedding light onto the molecular mechanisms involved in pregnancy tolerance.

Key Points: EVs cause accumulation of activated maternal platelets within the placenta, resulting in a thromboinflammatory response and PE. Activated maternal platelets cause NLRP3-inflammasome activation in trophoblast cells via ATP release and purinergic signaling.