Evidence for immune cell involvement in decidual spiral arteriole remodeling in early human pregnancy

Interstitial trophoblast cells: an enigmatic and variable component of the developing macaque placenta

The distribution of cytokeratin-positive interstitial trophoblast cells in the endometrium of the macaque during placental development was examined. Such cells are moderately abundant only from the 15th through the 22th day of pregnancy, although there is considerable individual variation. During this period of gestation, interstitial trophoblast cells are distributed in the perivascular stroma immediately surrounding spiral arteries, including coils of arteries already invaded by endovascular trophoblast. The interstitial trophoblast cells are not seen to directly intrude into the smooth muscle of the spiral arteries. Very few interstitial trophoblast cells are present from days 12 through 14 of gestation when the arteries are first invaded by endovascular trophoblast. Even fewer interstitial trophoblast cells are seen after day 30 of gestation. The brief time in gestation when interstitial trophoblast cells are abundant coincides with a time when spiral arteries are undergoing rapid modification into patent uteroplacental arteries. It is suggested that during this period, the interstitial trophoblast cells may facilitate changes in shape of the spiral arteries by alterations of the perivascular connective tissue, perhaps through interactions with other cellular constituents rather than by directly modifying the arteries per se.

Health during pregnancy and beyond: Fetal trophoblast cells as chief co-ordinators of intrauterine growth and reproductive success

Abstract Differentiation of extra-embryonic tissues and organs, notably the placenta, is vital for embryonic development and growth throughout gestation, starting from a few days after fertilization when the trophoblast cell lineage arises until parturition. In utero metabolic programming events may even extend the impact of placental function well into adulthood as they may predispose the offspring to common pathologies such as diabetes and cardiovascular disease. This review summarizes key steps that lead up to formation of a functional placenta. It highlights recent insights that have advanced our view of how early trophoblast expansion is achieved and how sufficient maternal blood supply to the developing fetus is secured. Exciting cumulative data have revealed the importance of a close cross-talk between the embryo proper and extra-embryonic trophoblast cells that involves extracellular matrix components in the establishment of a stem cell-like niche and proliferation compartment. Remarkably, placental function also relies on beneficial interactions between trophoblast cells and maternal immune cells at the implantation site. Our growing knowledge of the molecular mechanisms involved in trophoblast differentiation and function will help to devise informed approaches aimed at deciphering how placentation is controlled in humans as an essential process for reproductive success and long-term health.

Extravillous trophoblast and decidual natural killer cells: a remodelling partnership

BACKGROUND

During pregnancy, maternal uterine spiral arteries (SAs) are remodelled from minimal-flow, high-resistance vessels into larger diameter vessels with low resistance and high flow. Fetal extravillous trophoblasts (EVT) have important roles in this process. Decidual natural killer cells (dNK cells) are the major maternal immune component of the decidua and accumulate around SAs before trophoblast invasion. A role for dNK cells in vessel remodelling is beginning to be elucidated. This review examines the overlapping and dissimilar mechanisms used by EVT and dNK cells in this process and how this may mirror another example of tissue remodelling, namely cancer development.

METHODS

The published literature was searched using Pubmed focusing on EVT, dNK cells and SA remodelling. Additional papers discussing cancer development are also included.

RESULTS

Similarities exist between actions carried out by dNK cells and EVT. Both interact with vascular cells lining the SA, as well as with each other, to promote transformation of the SA. EVT differentiation has previously been likened to the epithelial-mesenchymal transition in cancer cells, and we discuss how dNK-EVT interactions at the maternal-fetal interface can also be compared with the roles of immune cells in cancer.

CONCLUSIONS

The combined role that dNK cells and EVT play in SA remodelling suggests that these interactions could be described as a partnership. The investigation of pregnancy as a multicellular system involving both fetal and maternal components, as well as comparisons to similar examples of tissue remodelling, will further identify the key mechanisms in SA remodelling that are required for a successful pregnancy.

The complement system at the embryo implantation site: friend or foe?

An inflammatory-like process and vascular remodeling represent the main changes that occur in decidua in the early phase of pregnancy. These changes are partly induced by trophoblast cells that colonize the decidua and are also contributed by the complement system, which can easily be activated as a result of tissue remodeling. Local control by several complement regulators including surface-bound and soluble molecules is critical to prevent complement-mediated tissue damage in normal pregnancy. C7 expressed on the endothelial cells (ECs) surface has been recognized as a novel complement regulator involved in the control of the proinflammatory effect of the terminal complement complex. The protective role of placental complement regulators in pregnancy is underscored by the recent finding of an association of preeclampsia with mutations in the genes encoding for some of these proteins. Complement components produced at feto-maternal interface serve an important function in placental development. C1q synthesized by decidual ECs and expressed on the cell surface is particularly important in this regard because it acts as a molecular bridge between endovascular trophoblast and ECs. C1q is also produced by extravillous trophoblast and is used to favor trophoblast migration through the decidua. Defective expression of C1q by trophoblast is associated with impaired trophoblast invasion of decidua and may have important implications in pregnancy disorders such as preeclampsia characterized by reduced vascular remodeling.

Macrophages infiltrate the human and rat decidua during term and preterm labor: evidence that decidual inflammation precedes labor

Preterm delivery is the leading cause of perinatal mortality and morbidity. Current tocolytics target myometrial contractions, a late step in the labor cascade. Identifying earlier events in parturition may lead to more effective therapeutic strategies. We hypothesized that inflammatory events in decidua (the maternal-fetal interface), characterized by leucocyte infiltration, are an early event during term and preterm labor (PTL). Leucocyte abundance in decidua of human pregnancies was quantified following term labor and PTL (idiopathic and infection associated), in conjunction with investigation of temporal inflammatory events in rat uterus during the perilabor period and in PTL induced by mifepristone. In human decidua, macrophage numbers were 4-fold higher in term labor (P < 0.01) and 2.5-fold higher in non-infection-associated PTL (P < 0.05) than in term nonlaboring samples. Neutrophil abundance was unchanged with labor but elevated in PTL with infection (5- to 53-fold increase; P < 0.01). T and NK cells were more abundant in idiopathic PTL than TL (P < 0.05). In rat, decidual macrophage infiltration increased 4.5-fold 12 h prior to labor and remained elevated during labor and early postpartum (P < 0.01). Decidual infiltration preceded that of the myometrium and was 4-fold higher (P < 0.01). In rat PTL, decidual macrophage numbers were also elevated (P < 0.01) and exceeded those of the myometrium (P < 0.05). These studies show for the first time that leucocytes infiltrate decidua during labor at term and preterm, supporting a role for leucocyte-derived inflammatory mediators in decidual activation. In the rat, this occurred prior to labor, suggesting it is an early event during parturition and thus a potential target for intervention.

Immunophenotype and cytokine profiles of rhesus monkey CD56bright and CD56dim decidual natural killer cells

The primate endometrium is characterized in pregnancy by a tissue-specific population of CD56(bright) natural killer (NK) cells. These cells are observed in human, rhesus, and other nonhuman primate decidua. However, other subsets of NK cells are present in the decidua and may play distinct roles in pregnancy. The purpose of this study was to define the surface marker phenotype of rhesus monkey decidual NK (dNK) cell subsets, and to address functional differences by profiling cytokine and chemokine secretion in contrast with decidual T cells and macrophages. Rhesus monkey decidual leukocytes were obtained from early pregnancy tissues, and were characterized by flow cytometry and multiplex assay of secreted factors. We concluded that the major NK cell population in rhesus early pregnancy decidua are CD56(bright) CD16(+)NKp30(-) decidual NK cells, with minor CD56(dim) and CD56(neg) dNK cells. Intracellular cytokine staining demonstrated that CD56(dim) and not CD56(bright) dNK cells are the primary interferon-gamma (IFNG) producers. In addition, the profile of other cytokines, chemokines, and growth factors secreted by these two dNK cell populations was generally similar, but distinct from that of peripheral blood NK cells. Finally, analysis of multiple pregnancies from eight dams revealed that the decidual immune cell profile is characteristic of an individual animal and is consistently maintained across successive pregnancies, suggesting that the uterine immune environment in pregnancy is carefully regulated in the rhesus monkey decidua.

Effect of preeclampsia serum on human uterine spiral artery smooth muscle cell apoptosis in a coculture model with cytotrophoblasts

BACKGROUND/AIMS

To investigate cytotrophoblast (CTB) invasive ability and human uterine spiral artery smooth muscle cell (HUSASMC) apoptosis in a coculture model with serum from preeclamptic pregnancies.

METHODS

Transwell migration assay was used to detect the invasive ability of CTBs. Cocultured CTBs and HUSASMCs were incubated with normal or preeclamptic serum for 24 h. Monocultures of CTBs and HUSASMCs were treated identically to the cocultures and served as controls. HUSASMC viability and apoptosis rates were determined by MTT and annexin V-FITC assays. The expressions of Fas ligand (FasL) mRNA in CTBs and Fas mRNA in HUSASMCs were detected by RT-PCR. The expression of the Fas protein in HUSASMCs was detected by Western blotting.

RESULTS

In a model of CTBs cocultured with HUSASMCs, preeclamptic serum effectively decreased the invasive ability and FasL mRNA expression of the CTBs. Preeclampsia serum also increased HUSASMC viability, decreased their apoptotic rate, and decreased the expression of Fas mRNA and protein.

CONCLUSION

The abnormal invasive ability of CTBs and decreased expression of the Fas/FasL system may be directly involved in the defective remodeling of the uterine spiral arteries during preeclampsia. Furthermore, the decrease in HUSASMC apoptosis may be related to the abnormal expression of Fas/FasL.

Dynamic shift from CD85j/ILT-2 to NKG2D NK receptor expression pattern on human decidual NK during the first trimester of pregnancy

During the first trimester of human pregnancy, Natural Killer (NK) cells of the maternal uterine mucosa (e.g. decidua) have a unique phenotype and are involved in crucial physiological processes during pregnancy. We investigated whether modifications of the NK receptor repertoire occur during the first trimester of pregnancy. We found significantly decreased expression of KIR2DL1/S1 and KIR2DL2/L3/S2 receptors, NKp30 and NKp44 activatory receptors, and the CD85j (ILT-2) inhibitory receptor. We also observed significantly increased expression of the NKG2D activatory receptor at the decidual NK cell surface. By flow cytometry, we further highlighted an evolution of NK subsets between 8 and 12 weeks of gestation, with a shift from the KIR2DL1/S1⁺/KIR2DL2/L3/S2⁺ subset towards the double negative subset, coupled with a decrease of the CD85j⁺/NKG2D⁻ subset in favour of the CD85j⁻/NKG2D⁺ subset. Furthermore, cell surface expression of NK receptor ligands, including CD85j and NKG2D ligands, has been characterized by flow cytometry on decidual immune CD14⁺ and CD3⁺ cells. HLA-G, the high affinity ligand of CD85j, was detected on both cell types. In contrast, NKG2D ligands ULBP-2 ULBP-3 and MICA/B were not expressed on CD14⁺ and CD3⁺ cells, however a variable expression of ULBP-1 was observed. The ligand expression of KIR2DL1/S1 and KIR2DL2/L3/S2 was also analyzed: the HLA-C molecule was expressed at a low level on some CD14⁺ cells whereas it was not detected on CD3⁺ cell surface. NK receptor ligands are known to be also expressed on the invading placental trophoblast cells. Thus, the phenotypic evolutions of decidual NK cells described in this present study may preserve their activation/inhibition balance during the first trimester of pregnancy.

Pro-inflammatory cytokine-stimulated first trimester decidual cells enhance macrophage-induced apoptosis of extravillous trophoblasts

OBJECTIVE

As human blastocyst-derived extravillous trophoblasts (EVTs) invade the early decidua, they are positioned to interact with immune cells and resident decidual cells, and remodel spiral arteries into high capacity vessels that increase blood flow to the developing fetal-placental unit. Shallow EVT invasion elicits incomplete vascular transformation and reduces uteroplacental blood flow that presages adverse pregnancy outcomes. Excess macrophages in the decidua induce EVT apoptosis via tumor necrosis factor-alpha (TNF-α) secretion. Our previous observation that pro-inflammatory cytokines enhance neutrophil and macrophage activator granulocyte-macrophage colony-stimulating factor (GM-CSF) expression in first trimester decidual cells is now extended to include: (1) the specific macrophage activator M-CSF; (2) macrophage activation and subsequent enhancement of EVT apoptosis by both GM-CSF and M-CSF.

STUDY DESIGN

Quantitative reverse transcription-polymerase chain reaction and enzyme-linked immunosorbent assay assessed M-CSF expression in first trimester decidual cells incubated with interleukin-1 beta (IL-1β) or TNF-α. Peripheral monocyte-derived macrophages pre-incubated with conditioned media from decidual cell cultures were co-cultured with a first trimester EVT cell line, HTR-8/SVneo cells. Macrophage activation was examined and EVT apoptosis evaluated by DNA fragmentation, caspase activation and cell membrane asymmetry.

RESULTS

IL-1β or TNF-α significantly enhanced M-CSF expression in first trimester decidual cells. The conditioned media from these cultures activates macrophages, which promote caspase 3/7-dependent EVT apoptosis with antibodies against GM-CSF or M-CSF blocking this effect.

CONCLUSIONS

Pro-inflammatory cytokines increases synthesis of M-CSF in first trimester decidual cells. Both GM-CSF and M-CSF activate macrophages, which initiate caspase-dependent EVT apoptosis.

New phenotypic aspects of the decidual spiral artery wall during early post-implantation mouse pregnancy

During pregnancy the walls of decidual spiral arteries (SAs) undergo clinically important structural modifications crucial for embryo survival/growth and maternal health. However, the mechanisms of SA remodeling (SAR) are poorly understood. Although an important prerequisite to this understanding is knowledge about the phenotype of SA muscular wall prior to and during the beginning of mouse SAR, this remains largely unexplored and was the main aim of this work. Using histological and immunohistochemical techniques, this study shows for the first time that during early mouse gestation, from embryonic day 7.5 (E7.5) to E10.5, the decidual SA muscular coat is not a homogeneous structure, but consists of two concentric layers. The first is a largely one cell-thick sub-endothelial layer of contractile mural cells (positive for α-smooth muscle actin, calponin and SM22α) with pericyte characteristics (NG2 positive). The second layer is thicker, and evidence is presented that it may be of the synthetic/proliferative smooth muscle phenotype, based on absence (α-smooth muscle actin and calponin) or weak (SM22α) expression of contractile mural cell markers, and presence of synthetic smooth muscle characteristics (expression of non-muscle Myosin heavy chain-IIA and of the cell proliferation marker PCNA). Importantly, immunohistochemistry and morphometrics showed that the contractile mural cell layer although prominent at E7.5-E8.5, becomes drastically reduced by E10.5 and is undetectable by E12.5. In conclusion, this study reveals novel aspects of the decidual SA muscular coat phenotype prior to and during early SAR that may have important implications for understanding the mechanisms of SAR.

Alterations in maternal and fetal heart functions accompany failed spiral arterial remodeling in pregnant mice

OBJECTIVE

Our goal was to define mechanisms that protect murine pregnancies deficient in spiral arterial remodeling from hypertension, hypoxia, and intrauterine growth restriction.

STUDY DESIGN

Microultrasound analyses were conducted on virgin, gestation day 2, 4, 7, 9, 10, 12, 14, 16, 18, and postpartum BALB/c (wild type) mice and BALB/c-Rag2(-/-)/Il2rg(-/-) mice, an immunodeficient strain lacking spiral arterial remodeling.

RESULTS

Rag2(-/-)/Il2rg(-/-) dams had normal spiral arterial flow velocities, greatly elevated uterine artery flow velocities between gestational day 10-16 and smaller areas of placental flow from gestational day 14 to term than controls. Maternal heart weight and output increased transiently. Conceptus alterations included higher flow velocities in the umbilical-placental circulation that became normal before term and bradycardia persistent to term.

CONCLUSION

Transient changes in maternal heart weight and function accompanied by fetal circulatory changes successfully compensate for deficient spiral arterial modification in mice. Similar compensations may contribute to the elevated risk for cardiovascular diseases seen in women and their children who experience preeclamptic pregnancies.

Arteriolar reactivity in lymphocyte-deficient mice

Mounting evidence suggests that lymphocytes have the capacity to contribute to the regulation of systemic circulatory control. We postulated that T and natural killer (NK) cells could modify basal microvascular activity under physiologically normal conditions. In situ intravital microscopy of mouse cremaster vasculature was used to evaluate arteriolar reactivities to the vasoconstrictors angiotensin II (ANG II) and phenylephrine (Phe) and the vasodilators acetylcholine (ACh) and adenosine (Ado) in normal [+/+; wild type (WT)] and genetically immunodeficient (T−B−NK+ or T−B−\NK−) C57BL/6 and BALB/c mice, strain backgrounds with differentially polarized T cell cytokine production. Immunodeficient mice tended to have smaller baseline and maximal diameters of third-order cremaster arterioles than their congenic WT partners. In C57BL/6, baseline diameters were similar in T-B− mice without or with NK cells; in BALB/c, baseline diameters were larger in T-B-NK− mice than in T−B−NK+ mice. Thus, at baseline, lymphocytes tended to promote vasodilation, except BALB/c NK cells, which mediated mild vasoconstriction. The presence of NK cells suppressed dilations to Ado in both strains, to ACh in the C57BL/6 strain, and dilatory responses to ANG II in C57BL/6 and to Phe in BALB/c. In the BALB/c strain, the presence of T and B cells promoted vasodilatory responses to Ado, attenuated dilations to low ACh concentrations, and exaggerated dilation and constriction responses to ANG II. Thus, under agonist challenge, NK cells generally promote constriction, whereas influences of T and B cells depend upon the stimulus. Therefore, lymphocytes or their products have physiological influences on microvascular arteriolar reactivity.

Natural killer cells direct hemochorial placentation by regulating hypoxia-inducible factor dependent trophoblast lineage decisions

Natural killer (NK) cells are recruited into the uterine stroma during establishment of the hemochorial placenta and are proposed regulators of uterine spiral artery remodeling. Failures in uterine spiral artery remodeling are linked to diseases of pregnancy. This prompted an investigation of the involvement of NK cells in placentation. NK cell depletion decreased the delivery of proangiogenic factors and delayed uterine spiral artery development, leading to decreased oxygen tension at the placentation site, stabilized hypoxia-inducible factor 1A protein, and redirected trophoblast differentiation to an invasive phenotype. Trophoblast cells replaced the endothelium of uterine spiral arteries extending the depth of the placental vascular bed and accelerating vessel remodeling. Hypoxia-regulated trophoblast lineage decisions, including expansion of invasive trophoblast, could be reproduced in vitro by using rat trophoblast stem cells and were dependent on hypoxia-inducible factor signaling. We conclude that NK cells guide hemochorial placentation through controlling a hypoxia-sensitive adaptive reflex regulating trophoblast lineage decisions.

Mechanism of maternal vascular remodeling during human pregnancy

Remodeling of maternal spiral arteries by invasion of extravillous trophoblast (EVT) is crucial for an adequate blood supply to the fetus. EVT cells that migrate through the decidual tissue destroy the arterial muscular lining from the outside (interstitial invasion), and those that migrate along the arterial lumen displace the endothelium from the inside (endovascular invasion). Numerous factors including cytokines/growth factors, chemokines, cell adhesion molecules, extracellular matrix-degrading enzymes, and environmental oxygen have been proposed to stimulate or inhibit the differentiation/invasion of EVT. Nevertheless, it is still difficult to depict overall pictures of the mechanism controlling perivascular and endovascular invasion. Potential factors that direct interstitial trophoblast towards maternal spiral artery are relatively high oxygen tension in the spiral artery, maternal platelets, vascular smooth muscle cells, and Eph/ephrin system. On the other hand, very little is understood about endovascular invasion except for the involvement of endothelial apoptosis in this process. Only small numbers of molecules such as polysialylated neural cell adhesion molecules and CCR1 have been suggested as specific markers for the endovascular trophoblast. Therefore, an initial step to approach the mechanisms for endovascular invasion could be more detailed molecular characterization of the endovascular trophoblast.

Interaction between uterine natural killer cells and extravillous trophoblast cells: effect on cytokine and angiogenic growth factor production

BACKGROUND

Uterine natural killer (uNK) cells are a major source of cytokines and angiogenic growth factors (AGFs), with AGF levels decreasing and cytokine levels increasing with gestational age. The factors that regulate AGF and cytokine secretion are unclear but may involve interactions between uNK cells and extravillous trophoblast (EVT) cells. We hypothesize that uNK cell interaction with EVT cells alters their cytokine and AGF secretion.

METHODS

Ex vivo co-cultures of uNK cells with either EVT (irradiated or fresh) or villous cytotrophoblast (CTB; control cell type) cells isolated from the same patients at 8-10 or 12-14 weeks gestational age (n = 10 each group) were established. Co-cultures were established with either direct contact between the different cell types or with the cells separated by a 0.4 µm filter. AGFs and cytokines were measured in cell culture supernatants using multiplex analysis (FAST Quant) or ELISA.

RESULTS

Secretion of angiopoietin-1 (P < 0.006) and vascular endothelial growth factor-C (P < 0.001) by uNK cells was lower when these cells were co-cultured, either directly or indirectly, with both trophoblast cell types at both gestational ages tested compared with when cultured alone. In contrast, interleukin (IL)-6 (P < 0.0001), IL-8 (P < 0.0001) and transforming growth factor-β1 (P < 0.002) were decreased only in direct uNK/EVT and uNK/CTB co-culture conditions at 8-10 and 12-14 weeks gestational age.

CONCLUSIONS

AGF and cytokine secretion was reduced after co-culture of uNK cells and both EVT and CTB cells. It remains unclear whether uNK cell AGF and cytokine production was reduced after co-culture with trophoblast cells (EVT or CTB) or whether trophoblast cell (EVT or CTB) AGF and cytokine production was reduced after co-culture with uNK cells. Local production of AGFs and cytokines in the placental bed may be lowered when uNK cells come in direct contact with EVT cells.

Fetus specific T cell modulation during fertilization, implantation and pregnancy

Recently there is an increasing interest in aspects of a more specific immunoregulation during pregnancy. Understanding these mechanism might have a broader application not only for reproductive immunology but also in general for biology and medicine. Especially the induction, already before conception, of feto-specific T cells with a possibly regulatory function gives a biological explanation of local immunotolerance at the maternal fetal interface, supporting the epidemiological evidence of a feto/paternal-specific immuneregulation. Understanding the expression of specific HLA-classes on trophoblast and the crosstalk of these antigens with various cell types, specifically modulated in the decidua, resulting in the secretion of cytokines and (angiogenic) chemokines has given us a more and more detailed understanding of this regulation. This regulation could be induced by fetal cells circulating in the mother (microchimerism) and from the interaction with fetal subcellular fractions as exosomes, but also from paternal antigens present in seminal fluid. Molecular interaction between paternal and fetal antigens and receptors in endometrium and the decidua are discussed. This review highlights besides uNK cells, especially the function of CD4+ and CD8+ T cells with a regulatory function in the context of recurrent miscarriage and pre-eclampsia. Besides HLA, also male-specific minor histocompatibility antigens and the genetic background for these pregnancy complications are discussed.

Circulating CD56+ cells of diabetic women show deviated homing potential for specific tissues during and following pregnancy

BACKGROUND

Human uterine natural killer (uNK) cells, the dominant lymphocytes in early pregnancy decidua, are important for spiral arterial remodelling. uNK cells are thought to arise from circulating CD56(bright) NK cells that egress into decidualizing endometrium. Both incomplete spiral arterial modification and aberrant NK cell function have been linked with pre-eclampsia, a syndrome that is more prevalent in diabetic women. Since previous in vitro studies have shown that changes in decidual endothelium induced by type 1 diabetes (T1D) reduce its interactions with circulating leucocytes, we hypothesized that diabetes additionally has direct effects on circulating CD56(+) NK cells that impair their decidual homing potential.

METHODS

Serial blood samples were collected from control, T1D and T2D pregnant women throughout and after pregnancy. In vitro adhesion under shear forces was used to assay the functional capacity of circulating leucocytes and of CD56(+) cells to adhere to endothelium in cryostat sections of gestation day (gd) 7 normal mouse decidua, pancreas and lymph node.

RESULTS

Fewer CD56(+) cells from diabetic compared with control women adhered to normal decidual endothelium. The CD56(+) cell/total cell adhesion ratio was also lower in diabetics. More diabetic CD56(+) cells adhered to pancreatic endothelium and their proportion was greater than for controls. Neither absolute nor proportional adhesion of CD56(+) cells to lymph node endothelium differed between diabetics and controls.

CONCLUSIONS

The CD56(+) cell adhesion patterns of T1D and T2D women differ from those of non-diabetic women and support the hypothesis that diabetes impairs mechanisms that could be used by CD56(+) cells for egress into decidua.

Deep trophoblast invasion and spiral artery remodelling in the placental bed of the chimpanzee

Deep trophoblast invasion is usually considered to be a unique feature of human placentation as compared to other primates. Because of the occasional occurrence of preeclampsia in great apes, which in the human is associated with impaired deep invasion, this uniqueness may be questioned. The availability of two well-documented pregnant chimpanzee uteri in the Hubrecht Collection (Museum für Naturkunde, Berlin) allowed us to evaluate the extent of trophoblast invasion in this species. By adjusting currently used protocols, we obtained successful immunohistochemical staining for cytokeratin and α-actin, as well as Ulex europaeus agglutinin 1 (UEA1) lectin staining, in this archival material. In both specimens interstitial trophoblast invasion had occurred in both decidua and myometrium. Because of a lack of published data on fetal growth for this species, fetal sizes (7cm and 13cm) could not be strictly related to gestational ages and thus be compared with the time-course of human trophoblast invasion. However, since the earlier specimen did not show any endovascular trophoblast invasion in spiral arteries - in contrast to pregnant human uteri with equivalent fetal sizes - endovascular migration seems to begin at a different gestational age in the chimpanzee. In the later specimen endovascular trophoblast was associated with spiral artery remodelling in the inner myometrium, and this invasion was extended to include a radial artery, which at that stage still showed relatively intact vascular smooth muscle and elastic lamina. We conclude that invasion depth and spiral artery remodelling are basically similar in chimpanzees and humans, although the seemingly different time of onset may have implications for uteroplacental oxygen supply and fetal development.

The "Great Obstetrical Syndromes" are associated with disorders of deep placentation

Defective deep placentation has been associated with a spectrum of complications of pregnancy including preeclampsia, intrauterine growth restriction, preterm labor, preterm premature rupture of membranes, late spontaneous abortion, and abruptio placentae. The disease of the placental vascular bed that underpins these complications is commonly investigated with targeted biopsies. In this review, we critically evaluate the biopsy technique to summarize the salient types of defective deep placentation, and propose criteria for the classification of defective deep placentation into 3 types based on the degree of restriction of remodeling and the presence of obstructive lesions in the myometrial segment of the spiral arteries.

Do uterine natural killer (uNK) cells contribute to female reproductive disorders?

The most abundant immune cells in the uterine decidua around the time of implantation and early placental development are the uterine natural killer (uNK) cells. Altered numbers of uNK cells have been associated with several human reproductive disorders, including recurrent miscarriage, recurrent implantation failure, uterine fibroids, sporadic miscarriage, fetal growth restriction and preeclampsia. Understanding of the function of uNK cells in non-pregnant and pregnant endometrium is now increasing; the potential contribution of altered numbers and function of uNK cells to reproductive disorders is the focus of this review.

Two unique human decidual macrophage populations

Several important events occur at the maternal-fetal interface, including generation of maternal-fetal tolerance, remodeling of the uterine smooth muscle and its spiral arteries and glands, and placental construction. Fetal-derived extravillous trophoblasts come in direct contact with maternal decidual leukocytes. Macrophages represent ∼20% of the leukocytes at this interface. In this study, two distinct subsets of CD14(+) decidual macrophages (dMs) are found to be present in first-trimester decidual tissue, CD11c(HI) and CD11c(LO). Gene expression analysis by RNA microarray revealed that 379 probes were differentially expressed between these two populations. Analysis of the two subsets revealed several clusters of coregulated genes that suggest distinct functions for these subsets in tissue remodeling, growth, and development. CD11c(HI) dMs express genes associated with lipid metabolism and inflammation, whereas CD11c(LO) dMs express genes associated with extracellular matrix formation, muscle regulation, and tissue growth. The CD11c(HI) dMs also differ from CD11c(LO) dMs in their ability to process protein Ag and are likely to be the major APCs in the decidua. Moreover, these populations each secrete both proinflammatory and anti-inflammatory cytokines that may contribute to the balance that establishes fetal-maternal tolerance. Thus, they do not fit the conventional M1/M2 categorization.

Persistence of decidual NK cells and KIR genotypes in healthy pregnant and preeclamptic women: a case-control study in the third trimester of gestation

BACKGROUND

Natural Killer (NK) cells are the most abundant lymphocytes in the decidua during early gestation. The interactions of NK cells with the extravillous cytotrophoblast have been associated with a normal spiral artery remodeling process, an essential event for a successful pregnancy. Recent data indicate that alterations in the amount of decidual NK (dNK) cells contribute to the development of preeclampsia (PE). Moreover, genetic studies suggest that Killer Immunoglobulin-like Receptors (KIR) expressed in dNK cells influence the susceptibility to PE. Although dNK cells have been well characterized during early pregnancy, they have been scarcely studied in the third trimester of gestation. The aim of this work was to characterize dNK cells at the last trimester of gestation and to analyze the KIR genotype of healthy and PE women.

METHODS

Decidual samples were obtained during Caesarean section from control (n = 10) and PE (n = 9) women. Flow cytometric analysis of CD3, CD56, CD16 and CD9 was used to characterize and quantify dNK cells in both groups. Cell surface markers from decidual leukocytes were compared with PBMC from healthy donors.KIR genotyping was performed in genomic DNA (control, n = 86; PE, n = 90) using PCR-SSP.

RESULTS

The results indicate that dNK cells persist throughout pregnancy. They represented 20% of total leukocytes in control and PE groups, and they expressed the same cell surface markers (CD3-, CD56+, CD16- and CD9+) as dNK in the first trimester of gestation. There were no significant differences in the percentage of dNK cells between control and PE groups. The analysis of KIR gene frequencies and genotypes was not statistically different between control and PE groups. The ratio of activating to inhibitory genes indicated that the overall inhibitory balance (0.2-0.5) was more frequent in the PE group (control, 31.3% vs PE, 45.5%), and the activating balance (0.6-1.1) was more frequent in the control group (control, 68.6% vs PE, 54.4%). However this difference was not significant.

CONCLUSION

We demonstrated the persistence of dNK cells in PE and control women at the third trimester of pregnancy; these dNK cells had a similar phenotype to those found during early pregnancy. The predominance of a KIR inhibitory balance in the PE group could be associated to the physiopathology of PE.

Deep placentation

Deep placentation in human pregnancy is realised by deep invasion of the placental bed by the extravillous trophoblast, involving the decidua and the inner (junctional zone) myometrium. Interstitial invasion of the stroma and endovascular trophoblast invasion of the spiral arteries both occur. Deep endovascular trophoblast invasion into the myometrial segments of spiral arteries is important for proper placental functioning. Before this extended vascular invasion begins, decidua-associated vascular remodelling, which includes swelling and disorganisation of the vascular smooth muscle, occurs during a period of rising placental oxygen. This early remodelling step may accommodate the progressively increasing maternal blood flow to the developing placenta. The subsequent trophoblast-associated remodelling step enhances and stabilises the widening of the vessels, whereas the vascular smooth muscle and elastic lamina are replaced by a fibrinoid matrix with embedded trophoblast. Defective deep remodelling contributes to placental malfunctioning in complications of pregnancy.

Natural killer cell-triggered vascular transformation: maternal care before birth?

Natural killer (NK) cells are found in lymphoid and non-lymphoid organs. In addition to important roles in immune surveillance, some NK cells contribute to angiogenesis and circulatory regulation. The uterus of early pregnancy is a non-lymphoid organ enriched in NK cells that are specifically recruited to placental attachment sites. In species with invasive hemochorial placentation, these uterine natural killer (uNK) cells, via secretion of cytokines, chemokines, mucins, enzymes and angiogenic growth factors, contribute to the physiological change of mesometrial endometrium into the unique stromal environment called decidua basalis. In humans, uNK cells have the phenotype CD56(bright)CD16(dim) and they appear in great abundance in the late secretory phase of the menstrual cycle and early pregnancy. Gene expression studies indicate that CD56(bright)CD16(dim) uterine and circulating cells are functionally distinct. In humans but not mice or other species with post-implantation decidualization, uNK cells may contribute to blastocyst implantation and are of interest as therapeutic targets in female infertility. Histological and genetic studies in mice first identified triggering of the process of gestation spiral arterial modification as a major uNK cell function, achieved via interferon (IFN)-γ secretion. During spiral arterial modification, branches from the uterine artery that traverse the endometrium/decidua transiently lose their muscular coat and ability to vasoconstrict. The expression of vascular markers changes from arterial to venous as these vessels dilate and become low-resistance, high-volume channels. Full understanding of the vascular interactions of human uNK cells is difficult to obtain because endometrial time-course studies are not possible in pregnant women. Here we briefly review key information concerning uNK cell functions from studies in rodents, summarize highlights concerning human uNK cells and describe our preliminary studies on development of a humanized, pregnant mouse model for in vivo investigations of human uNK cell functions.

IFPA Gabor Than Award lecture: Transformation of the spiral arteries in human pregnancy: key events in the remodelling timeline

During human pregnancy, the uterine spiral arteries are progressively remodelled to form dilated conduits lacking maternal vasomotor control. This phenomenon ensures that a constant supply of blood is delivered to the materno-fetal interface at an optimal velocity for nutrient exchange. Conversion of a tonic maternal arteriole composed of multiple layers of vascular smooth muscle, elastin and numerous other extracellular matrix components, into a highly dilated yet durable vessel, requires tight regulatory control and the coordinated actions of multiple cell types. Initial disruption of the vascular wall, characterised by foci of endothelial cell loss, and separation and misalignment of vascular smooth muscle cells (VSMC), is coincident with an influx of uterine natural killer (uNK) cells and macrophages. uNK cells are a source of angiogenic growth factors and matrix degrading proteases, thus they possess the capacity to initiate changes in VSMC phenotype and instigate extracellular matrix catabolism. However, complete vascular cell loss, mediated in part by apoptosis and dedifferentiation, is only achieved following colonisation of the arteries by extravillous trophoblast (EVT). EVT produce a variety of chemokines, cytokines and matrix degrading proteases, enabling them to influence the fate of other cells within the placental bed and complete the remodelling process. The complex interplay of cell-cell and cell-matrix interactions required for effective vascular transformation will be examined, with a particular focus on the role of (i) uNK cells and (ii) the enzyme matrix metalloproteinase-12 (MMP-12). Parallels with remodelling events occurring in other vascular beds will also be drawn.

Decidualisation and angiogenesis

The timing of decidualisation and vascular processes during the implantation period is of paramount importance for the development of a receptive endometrium suitable for implantation. The endometrium transforms during the secretory phase into a well-vascularised receptive tissue characterised by increased vascular permeability, oedema, proliferation and differentiation of stromal cells into decidual cells, invasion of leucocytes, vascular remodelling and angiogenesis. Decidualisation continues in the presence of conception and an influx of immune cells, trophoblasts and vascular adaptation will occur. Vascular changes include spiral artery remodelling, angiogenesis and the induction of angiogenic factors. Disturbances in uterine blood supply are associated with first-trimester miscarriages and third-trimester perinatal morbidity and mortality caused by pre-eclampsia and foetal growth restriction. This article assesses decidual vascular changes during human implantation, and evaluates the involvement of angiogenesis in the pathogenesis of miscarriages, pre-eclampsia and intrauterine growth restriction.

Remodelling at the maternal–fetal interface: relevance to human pregnancy disorders

In human pregnancy, successful placentation and remodelling of the uterine vasculature require the integration of a number of stages, which are crucial for a healthy pregnancy. As the demands of the developing fetus for nutrients and oxygen increase, the capacity of the maternal blood vessels to supply this must be altered radically, with deficiencies in this process implicated in a number of dangerous pregnancy complications. The complex signalling networks that regulate these tightly co-ordinated events are becoming clearer as more studies of early pregnancy are performed. It is the aim of this review to draw together our knowledge of events that occur to facilitate a successful pregnancy ranging from the preparation for implantation, through the invasion and differentiation of the trophoblast and the regulation of these processes by other cells within the decidual environment, to the active role that the trophoblast and maternal immune cells play in facilitating the remodelling of the uterine spiral arteries. The events involved in a healthy pregnancy will then be compared to aberrant placentation and remodelling, which are characteristics of many pregnancy disorders, and recent advances in detection of abnormal placental development will also be discussed.

Endovascular trophoblast and preeclampsia: A reassessment

Since the earliest report on impaired spiral artery remodelling in preeclamptic human pregnancies, numerous studies have been devoted to possible mechanisms of impaired trophoblast invasion. A better knowledge of early uteroplacental blood flow has provided a physiological context for the processes of spiral artery invasion and associated remodelling, revealing a closely timed relationship between increasing flow and early steps in vascular remodelling. Concerning the impaired trophoblast invasion in preeclampsia, it has also to be considered that impaired invasion not only concerns invasion depth per se, but also the extension of this deep invasion from the central towards the more lateral spiral arteries of the placental bed. Since also in preeclampsia the very central spiral arteries may be normally invaded, the existence of such spatial gradient provides a further dimension to the problem. A practical consequence is that frequently used rodent models, which show invasion of two or three spiral arteries only, may be less useful for studying this particular aspect of the disease. Amongst non-human primates, baboons and rhesus monkeys are 'shallow invaders', and only in some of the great apes deep trophoblast invasion and associated spiral artery remodelling occurs. A better knowledge of the evolutionary history of deep invasion and its possible selective benefit might ultimately improve our understanding of failed deep invasion and impaired spiral artery remodelling in preeclampsia.

Interferon gamma contributes to preimplantation embryonic development and to implantation site structure in NOD mice

BACKGROUND

Pre-eclampsia, a syndrome usually accompanied by incomplete spiral arterial modification, occurs at an increased frequency in diabetic women. Hyperglycemia in non-obese type 1 diabetic (NOD) mice impairs gestational spiral arterial remodeling despite high local levels of interferon gamma (Ifng), the triggering cytokine in mice. Pregnancies in NOD.Ifng(-/-) mice were assessed to investigate this issue.

METHODS

Fecundity was assessed using the breeding history, flushing of preimplantation embryos and histological and morphometric studies of implantation sites in normoglycemic (n-) and hyperglycemic (d-) females of NOD.Ifng(-/-) and NOD genotypes.

RESULTS

NOD.Ifng(-/-) but not NOD mice are mostly infertile. In NOD.Ifng(-/-), copulation often does not result in a post-implantation pregnancy. Defective fertilization and delayed preimplantation development limit n-NOD.Ifng(-/-) fertility, and both mechanisms are exacerbated by hyperglycemia. At mid-gestation, implantation sites in n-NOD.Ifng(-/-) and n-NOD mice are histologically similar. However, in d-NOD.Ifng(-/-), there is minimal development of spiral arteries, hypertrophy of the myometrial region containing uterine Natural Killer (uNK) cells and a deficit in cytoplasmic granule formation in the uNK cells.

CONCLUSIONS

Ifng contributes to the success of fertilization and to the rate of preimplantation mouse embryo development in normogylcemic and hyperglycemic pregnancies. A physiological role for this cytokine in human preimplantation development merits investigation.

Preeclampsia is associated with increased cytotoxic T-cell capacity to paternal antigens

OBJECTIVE

During an uncomplicated pregnancy the conceptus is a semiallogeneic entity in which rejection is prevented by suppression of the maternal immune system. We hypothesized that this suppression is disturbed in patients with preeclampsia and that a maternal immune response to fetal (foreign/paternal) antigens in the fetal-maternal interface may be responsible for local inflammation, with subsequent endothelial dysfunction and systemic disease.

STUDY DESIGN

Blood samples were obtained from 14 women with preeclampsia (cases), 14 gestational-age and parity-matched women with uncomplicated pregnancies (controls), and their partners. We determined the partner-specific cytotoxic T-lymphocyte precursor frequency (CTLpf) and the CTLpf directed to unrelated partners with uncomplicated pregnancies. We measured the CTLpf in peripheral blood mononuclear cells (PBMCs) from cases and controls using limited-dilution assays. In addition, proliferation was tested in a mixed-lymphocyte culture (MLR).

RESULTS

The partner-specific CTLpf was significantly higher in cases compared with controls (median, 183 [15-338] vs 67 [9-232] per million PBMCs, P = .02). In contrast, in women with uncomplicated pregnancies, the partner-specific CTLpf was down-regulated compared with the CTLpf directed to an unrelated partner who fathered uncomplicated pregnancies (P = .02). No difference was found in partner-specific MLR response between cases and controls.

CONCLUSION

These results suggest that women with preeclampsia have a higher cytotoxic T-cell response to paternal antigens compared with pregnant controls. This insufficiently suppressed immune response may eventually lead to the development of preeclampsia.

Expression of the vasoactive proteins AT1, AT2, and ANP by pregnancy-induced mouse uterine natural killer cells

Angiotensin II receptor type 1 (AT1) activation leads to vasoconstriction and type 2 receptor (AT2) leads to vasodilation. Atrial natriuretic peptide (ANP) antagonizes the effects of AT1. In human and murine pregnancies, uterine natural killer (uNK) cells closely associate with decidual blood vessels. Protein localization of AT1, AT2, and ANP to mouse uNK cells was examined between gestation days (gds) 6 and 12, the interval of uNK cell expansion. Percentages of uNK cells expressing AT1 or AT2 changed between gd6 and gd10. Atrial natriuretic peptide did not localize to uNK cells at gd6 or 8, but did colocalize to uNK cells at gd10 and 12, times immediately after spiral arterial modification. This is the first report of AT1, AT2, and ANP expression in uterine immune cells. Expression of these molecules suggests that uNK cells have the potential to contribute to the changes in blood pressure that occur between days 5 and 12 of pregnancy in mice.

Caspase activation is not required for villous cytotrophoblast fusion into syncytiotrophoblasts

The villous trophoblast renews itself by fusion of individual stem cells (cytotrophoblasts, CT) with a functional syncytium (syncytiotrophoblast, ST). The literature indicates that fusion occurs with limited activation (proteolytic cleavage) of caspase-8 in CT and is inhibited either by blocking caspase-8 synthesis or inhibiting activation with a caspase-8-specific inhibitor, zIETD. We challenge part of this evidence: inhibition of differentiation with caspase-8 inhibitors. Br-cAMP-stimulated differentiation of isolated CT into multinucleated syncytia in culture is not blocked with three different low molecular weight inhibitors of caspase-8: broad caspase inhibitors zVAD-fmk and qVD-OPh and the caspase-8-specific inhibitor zIETD-fmk. Syncytialization was determined by desmoplakin staining of intracellular boundaries surrounding >2 nuclei and by diffusion within fused cells of long-lived cytoplasmic staining from half of original CT to the unstained half. Differentiation of isolated CT into hCGβ-secreting syncytiotrophoblast was also not blocked by the inhibitors nor was upregulation of hCGβ secretion blocked in ST-stripped and regenerated 5 day explant cultures. The ratio of CT to ST nuclei present was also not changed in explant cultures by caspase inhibitors. The effectiveness of caspase inhibitors was demonstrated by their ability to completely block TNFα-induced apoptosis. We conclude that activation of caspases in general, and caspase-8 in particular, is not required for villous CT differentiation into ST. However, another role of intact caspase-8 (proform) in CT differentiation remains possible.

Placental apoptosis in health and disease

Apoptosis, programmed cell death, is an essential feature of normal placental development but is exaggerated in association with placental disease. Placental development relies upon effective implantation and invasion of the maternal decidua by the placental trophoblast. In normal pregnancy, trophoblast apoptosis increases with placental growth and advancing gestation. However, apoptosis is notably exaggerated in the pregnancy complications, hydatidiform mole, pre-eclampsia, and intrauterine growth restriction (IUGR). Placental apoptosis may be initiated by a variety of stimuli, including hypoxia and oxidative stress. In common with other cell-types, trophoblast apoptosis follows the extrinsic or intrinsic pathways culminating in the activation of caspases. In contrast, the formation of apoptotic bodies is less clearly identified, but postulated by some to involve the clustering of apoptotic nuclei and liberation of this material into the maternal circulation. In addition to promoting a favorable maternal immune response, the release of this placental-derived material is thought to provoke the endothelial dysfunction of pre-eclampsia. Widespread apoptosis of the syncytiotrophoblast may also impair trophoblast function leading to the reduction in nutrient transport seen in IUGR. A clearer understanding of placental apoptosis and its regulation may provide new insights into placental pathologies, potentially suggesting therapeutic targets.

Trophoblast- and vascular smooth muscle cell-derived MMP-12 mediates elastolysis during uterine spiral artery remodeling

During the first trimester of pregnancy, the uterine spiral arteries are remodeled, creating heavily dilated conduits that lack maternal vasomotor control but allow the placenta to meet an increasing requirement for nutrients and oxygen. To effect permanent vasodilatation, the internal elastic lamina and medial elastin fibers must be degraded. In this study, we sought to identify the elastolytic proteases involved in this process. Primary first-trimester cytotrophoblasts (CTBs) derived from the placenta exhibited intracellular and membrane-associated elastase activity; membrane-associated activity was primarily attributable to matrix metalloproteinases (MMP). Indeed, Affymetrix microarray analysis and immunocytochemistry implicated MMP-12 (macrophage metalloelastase) as a key mediator of elastolysis. Cultured human aortic smooth muscle cells (HASMCs) exhibited constitutive membrane-associated elastase activity and inducible intracellular elastase activity; these cells also expressed MMP-12 protein. Moreover, a specific inhibitor of MMP-12 significantly reduced CTB- and HASMC-mediated elastolysis in vitro, to 31.7 ± 10.9% and 23.3 ± 8.7% of control levels, respectively. MMP-12 is expressed by both interstitial and endovascular trophoblasts in the first-trimester placental bed and by vascular SMCs (VSMCs) in remodeling spiral arteries. Perfusion of isolated spiral artery segments with CTB-conditioned medium stimulated MMP-12 expression in medial VSMCs. Our data support a model in which trophoblasts and VSMCs use MMP-12 cooperatively to degrade elastin during vascular remodeling in pregnancy, with the localized release of elastin peptides and CTB-derived factors amplifying elastin catabolism.

Lymphatics in the human endometrium disappear during decidualization

BACKGROUND

The mammalian placenta plays a central role in maternal tolerance of the semi-allogeneic fetus and fluid balance between the maternal and fetal compartments. The lymphatics play a role in both these function. The aim of this study was to describe the distribution of lymphatic vessels in human decidua, with particular focus on the lymphatics that surround remodelling spiral arteries during decidualization and trophoblast invasion.

METHODS

Placental bed and non-placental bed (decidua parietalis) biopsies were obtained from 41 women undergoing elective termination of pregnancy at 6-18 weeks gestational age as well as placental bed biopsies from 5 women undergoing elective Caesarean section at term. In addition to routine haematoxylin and eosin staining, double immunohistochemical labelling was performed on serial 3-µm sections to identify lymphatic vessels in conjunction with one of the following: blood vessels, smooth muscle, epithelial and trophoblast cells or proliferating cells. Representative photomicrographs of all sections were obtained from a total of 273 areas (46 samples, average 6 range 3-15 areas per sample). Descriptive findings of the organization of lymphatics in human placental bed and decidua parietalis were made from a total of 1638 images.

RESULTS

Lymphatic vessels positive for podoplanin were abundant in non-decidualized hypersecretory endometrium at all stages of gestation. By contrast, the decidua was nearly always devoid of lymphatics. In some samples, structures that appeared to be regressing lymphatics could be observed at the boundary between non-decidualized hypersecretory and decidualized endometrium. Lymphatic vessels were notably absent from the vicinity of spiral arteries that were surrounded by decidualized stromal cells. Lymphatic vessels in non-decidualized hypersecretory endometrium appeared larger and more elongated as gestation progressed. Proliferating lymphatic vascular endothelial cells were identified in both large vessels, and in streaks of D2-40 positive cells that could have been newly forming lymphatic vessels. Placental bed lymphatics exhibited limited and variable staining with LYVE-1 at all stages of pregnancy apart from term.

CONCLUSIONS

We have made novel observations on lymphatics in the placental bed and their relationship with other structures throughout pregnancy. Endometrial stromal cell decidualization results in a loss of lymphatics, with this phenomenon being particularly apparent around the spiral arteries.

Pre-eclampsia: fitting together the placental, immune and cardiovascular pieces

The success of pregnancy is a result of countless ongoing interactions between the placenta and the maternal immune and cardiovascular systems. Pre-eclampsia is a serious pregnancy complication that arises from multiple potential aberrations in these systems. The pathophysiology of pre-eclampsia is established in the first trimester of pregnancy, when a range of deficiencies in placentation affect the key process of spiral artery remodelling. As pregnancy progresses to the third trimester, inadequate spiral artery remodelling along with multiple haemodynamic, placental and maternal factors converge to activate the maternal immune and cardiovascular systems, events which may in part result from increased shedding of placental debris. As we understand more about the pathophysiology of pre-eclampsia, it is becoming clear that the development of early- and late-onset pre-eclampsia, as well as intrauterine growth restriction (IUGR), does not necessarily arise from the same underlying pathology.

Embryonic stem cells as models of trophoblast differentiation: progress, opportunities, and limitations

While the determination of the trophoblast lineage and the facilitation of placental morphogenesis by trophoblast interactions with other cells of the placenta are crucial components for the establishment of pregnancy, these processes are not tractable at the time of human implantation. Embryonic stem cells (ESCs) provide an embryonic surrogate to derive insights into these processes. In this review, we will summarize current paradigms which promote trophoblast differentiation from ESCs, and potential opportunities for their use to further define signals directing morphogenesis of the placenta following implantation of the embryo into the endometrium.

Vascular-leukocyte interactions: mechanisms of human decidual spiral artery remodeling in vitro

Transformation of uterine spiral arteries is critical for healthy human pregnancy. We recently proposed a role for maternal leukocytes in decidual spiral artery remodeling and suggested that matrix metalloprotease (MMP) activity contributed to the destruction of the arterial wall. In the current study we used our first trimester placental-decidual co-culture (PDC) model to define the temporal relationship and test the mechanistic aspects of this process. PDC experiments were assessed by image analysis over a six-day time-course for degree of vascular transformation and leukocyte distribution around progressively remodeled arterioles. We observed rapid transformation in PDCs associated with loss of vascular smooth muscle cells, widening of the vessel lumen, and significant accumulation of uterine Natural Killer cells and macrophages within the vascular wall (P < 0.001) before trophoblast presence in the vessel lumens. These events did not occur in decidua-only cultures. Active MMP-9 was detected in leukocytes and vascular cells of remodeling arterioles, and inhibition of MMP-2/9 activity in PDC resulted in failure of decidual vascular remodeling compared with vehicle-treated PDCs. Apoptosis of vascular cells, macrophage-mediated phagocytosis, and vascular smooth muscle cell dedifferentiation contributed to the remodeling observed. The PDC model indicates that placental presence is required to initiate decidual spiral artery remodeling but that uterine Natural Killer cells and macrophages mediate the early stages of this process at the cellular level.

Cellular and molecular regulation of spiral artery remodelling: lessons from the cardiovascular field

A number of important changes take place in the maternal uterine vasculature during the first few weeks of pregnancy resulting in increased blood flow to the intervillous space. Vascular endothelial and smooth muscle cells are lost from the spiral arteries and are replaced by fetal trophoblast cells. Failure of the vessels to remodel sufficiently is a common feature of pregnancy pathologies such as early pregnancy loss, intrauterine growth restriction and pre-eclampsia. There is evidence to suggest that some vascular changes occur prior to trophoblast invasion, however, in the absence of trophoblasts remodelling of the spiral arteries is reduced. Until recently our knowledge of these events has been obtained from immunohistochemical studies which, although extremely useful, can give little insight into the mechanisms involved. With the development of more complex in vitro models a picture of events at a cellular and molecular level is beginning to emerge, although some caution is required in extrapolating to the in vivo situation. Trophoblasts synthesise and release a plethora of cytokines and growth factors including members of the tumour necrosis factor family. Studies suggest that these factors may be important in regulating the remodelling process by inducing both endothelial and vascular smooth muscle cell apoptosis. In addition, it is evident from studies in other vascular beds that the structure of the vessel is influenced by factors such as flow, changes in the composition of the extracellular matrix, the phenotype of the vascular cells and the local immune cell environment. It is the aim of this review to present our current knowledge of the mechanisms involved in spiral artery remodelling and explore other possible pathways and cellular interactions that may be involved, informed by studies in the cardiovascular field.

Uterine NK cells, spiral artery modification and the regulation of blood pressure during mouse pregnancy

Reproductive success in mammals involves coordinated changes in the immune and cardiovascular as well as in the neuroendocrine and reproductive systems. This review addresses studies that identify potential links for NK cells and T cells with the local and systemic cardiovascular adaptations of pregnancy. The studies reviewed have utilized immunohistochemisty and in vivo analyses of vascular parameters by ultrasound, chronic monitoring of hemodynamics via radiotelemetric recording and intravital microscopy. At the uterine level, functional subsets of uterine natural killer cells were identified. These included subsets expressing molecules important for vasoregulation, in addition to those previously identified for angiogenesis. Spiral arteries showed conducted responses that could account for conceptus control of vasoactivity and mouse gestational blood pressure 5-phase pattern. Vascular immunology is an emerging transdisciplinary field, critical for both reproductive immunology and cardiovascular disease.

Changes in endovascular trophoblast invasion and spiral artery remodelling at term in a transgenic preeclamptic rat model

As a follow-up to our previous study which revealed a surprisingly deeper endovascular trophoblast (ET) invasion on day 18 in a transgenic preeclamptic (PE) rat model (hAngiotensinogen female symbol x hRenin male symbol) compared to non-PE controls, we examined further changes in ET invasion and associated spiral artery (SA) remodelling at term (day 21). PE transgenic rats and non-PE reversely mated (RM) transgenic rats were compared to normal SD rats (C). Sections were stained to visualize trophoblast, fibrinoid, vascular smooth muscle (VSM) and endothelium. SA were evaluated in three depth levels in the mesometrial triangle (MT) using the KS-400 image analysis system. In separate transgenic rats, Doppler ultrasound was performed in uterine arteries, and the resistance indices (RI) were calculated. Although for the whole MT differences in ET invasion were no longer significant between the PE and C, indicating a partial catching up in C rats, there was still significantly more ET in the deepest level in the PE group as compared to the C and RM groups. At the same time the SA walls in PE rats contained significantly more fibrinoid (versus RM and C) and VSM (versus C). In all SA cross-sections, re-endothelialisation was prominent, but significantly different between PE and C group. The Doppler results showed a significantly lower RI in the arcuate uterine artery of the PE group compared to the C group. There was no evidence of elimination of deeply invaded ET at term, previously considered as a possible mechanism for restriction of vascular remodelling in human PE. The differences in vascular remodelling, previously described on day 18 by histology and Doppler data, were maintained on day 21, but there was extensive endothelial repair in the three groups. Atherosis-like lesions were observed in the three groups, most frequently in the RM group, but were never associated with placental infarcts.

Spiral arterial remodeling is not essential for normal blood pressure regulation in pregnant mice

Maternal cardiovascular adaptations occur in normal pregnancy, systemically, and within the uterus. In humans, gestational control of blood pressure is clinically important. Transient structural remodeling of endometrial spiral arteries normally occurs in human and mouse pregnancies. In mice, this depends on uterine natural killer cell function. Using normal and immune-deficient mice, we asked whether spiral artery remodeling critically regulates gestational mean arterial pressure and/or placental growth. Radiotelemetric transmitters were implanted in females and hemodynamic profiles to a dietary salt challenge and to pregnancy were assessed. Implantation sites from noninstrumented females were used for histological morphometry. Both normal and immune-deficient mice had normal sensitivity to salt and showed similar 5-phase gestational patterns of mean arterial pressure correlating with stages of placental development, regardless of spiral artery modification. After implantation, mean arterial pressure declined during the preplacental phase to reach a midgestation nadir. With gestation day 9 opening of placental circulation, pressure rose, reaching baseline before parturition, whereas heart rate dropped. Heart rate stabilized before parturition. Placental sizes deviated during late gestation when growth stopped in normal mice but continued in immune-deficient mice. As an indication of the potential for abnormal hemodynamics, 2 pregnant females delivering dead offspring developed late gestational hypertension. This study characterizes a dynamic pattern of blood pressure over mouse pregnancy that parallels human gestation. Unexpectedly, these data reveal that spiral artery remodeling is not required for normal gestational control of blood pressure or for normal placental growth.

The immunomodulatory roles of macrophages at the maternal-fetal interface

Macrophages are versatile cells that play a central role in innate and adaptive immunity and participate in a wide variety of biological processes. In the uterine decidua, macrophages represent a major leukocyte subset throughout pregnancy. Here, decidual macrophages exert an immunosuppressive phenotype characterized by abundant production of interleukin (IL)-10 and indoleamine 2,3-dioxygenase activity. Their polarized cytokine secretion pattern has recently been classified as an M2 phenotype. These features of decidual macrophages favor maternal immune tolerance to semiallogenic fetus. In addition, macrophages cooperate with trophoblast cells during the early stages of human pregnancy to support uterine vasculature remodeling by removing apoptotic cells and through the production of proteases that degrade the extracellular matrix. In the peripartum period, macrophages also participate in the regulation of cervical ripening and the initiation of parturition through the production of proinflammatory cytokines and prostaglandin E(2) (PGE(2)). Aberrant activity of uterine macrophages is linked to the pathogenesis of preeclampsia and preterm delivery. Here, we review the immunomodulatory roles of decidual macrophages during pregnancy.

Using ultrasonography to define fetal-maternal relationships: moving from humans to mice

Ultrasound scanning is a noninvasive, accurate, and cost-effective method to create images of the female reproductive tract clinically and in research. Ultrasonography is particularly valuable for studying the dynamic relationships among mother, placenta, and fetus during pregnancy because this modality does not disturb the ongoing course of gestation. Importantly, the complex vascular changes in the mother induced by pregnancy and the vascular system generated to support placental function can be assessed quantitatively and functionally by ultrasonography. Many mouse models are available that address aspects of human placental function and dysfunction, but high-quality microultrasound technology suitable for use in pregnant mice has become widely available only recently. This technical advance now enables real-time recording of maternal-fetal interactions in pregnant rodents. The ability to perform microultrasonic analyses of parameters such as uterine arterial remodeling, hemodynamic changes, placental development, and fetal growth in mice now permits research that uses the same imaging platform as that for human patients. This capability will enhance the translation of information derived from rodent studies to the clinic.