Regulatory T cells mediate maternal tolerance to the fetus

Pregnancy constitutes a major challenge to the maternal immune system, as it has to tolerate the persistence of paternal alloantigen. Although localized mechanisms contribute to fetal evasion from immune attack, maternal alloreactive lymphocytes persist. We demonstrate here an alloantigen-independent, systemic expansion of the maternal CD25+ T cell pool during pregnancy and show that this population contains dominant regulatory T cell activity. In addition to their function in suppressing autoimmune responses, maternal regulatory T cells suppressed an aggressive allogeneic response directed against the fetus. Their absence led to a failure of gestation due to immunological rejection of the fetus.

Natural killer cells and pregnancy

The fetus is considered to be an allograft that, paradoxically, survives pregnancy despite the laws of classical transplantation immunology. There is no direct contact of the mother with the embryo, only with the extraembryonic placenta as it implants in the uterus. No convincing evidence of uterine maternal T-cell recognition of placental trophoblast cells has been found, but instead, there might be maternal allorecognition mediated by uterine natural killer cells that recognize unusual fetal trophoblast MHC ligands.

Innate immunity of the newborn: basic mechanisms and clinical correlates

The fetus and newborn face a complex set of immunological demands, including protection against infection, avoidance of harmful inflammatory immune responses that can lead to pre-term delivery, and balancing the transition from a sterile intra-uterine environment to a world that is rich in foreign antigens. These demands shape a distinct neonatal innate immune system that is biased against the production of pro-inflammatory cytokines. This bias renders newborns at risk of infection and impairs responses to many vaccines. This Review describes innate immunity in newborns and discusses how this knowledge might be used to prevent and treat infection in this vulnerable population.

Defining postpartum uterine disease and the mechanisms of infection and immunity in the female reproductive tract in cattle

Uterine microbial disease affects half of all dairy cattle after parturition, causing infertility by disrupting uterine and ovarian function. Infection with Escherichia coli, Arcanobacterium pyogenes, and bovine herpesvirus 4 causes endometrial tissue damage. Toll-like receptors on endometrial cells detect pathogen-associated molecules such as bacterial DNA, lipids, and lipopolysaccharide (LPS), leading to secretion of cytokines, chemokines, and antimicrobial peptides. Chemokines attract neutrophils and macrophages to eliminate the bacteria, although persistence of neutrophils is associated with subclinical endometritis and infertility. Cows with uterine infections are less likely to ovulate because they have slower growth of the postpartum dominant follicle in the ovary, lower peripheral plasma estradiol concentrations, and perturbation of hypothalamic and pituitary function. The follicular fluid of animals with endometritis contains LPS, which is detected by the TLR4/CD14/LY96 (MD2) receptor complex on granulosa cells, leading to lower aromatase expression and reduced estradiol secretion. If cows with uterine disease ovulate, the peripheral plasma concentrations of progesterone are lower than those in normal animals. However, luteal phases are often extended in animals with uterine disease, probably because infection switches the endometrial epithelial secretion of prostaglandins from the F series to the E series by a phospholipase A2-mediated mechanism, which would disrupt luteolysis. The regulation of endometrial immunity depends on steroid hormones, somatotrophins, and local regulatory proteins. Advances in knowledge about infection and immunity in the female genital tract should be exploited to develop new therapeutics for uterine disease.

Immunology of placentation in eutherian mammals

The traditional way to study the immunology of pregnancy follows the classical transplantation model, which views the fetus as an allograft. A more recent approach, which is the subject of this Review, focuses on the unique, local uterine immune response to the implanting placenta. This approach requires knowledge of placental structure and its variations in different species, as this greatly affects the type of immune response that is generated by the mother. At the implantation site, cells from the mother and the fetus intermingle during pregnancy. Unravelling what happens here is crucial to our understanding of why some human pregnancies are successful whereas others are not.

Interferon gamma contributes to initiation of uterine vascular modification, decidual integrity, and uterine natural killer cell maturation during normal murine pregnancy

The dominant lymphocytes in human and murine implantation sites are transient, pregnancy-associated uterine natural killer (uNK) cells. These cells are a major source of interferon (IFN)-gamma. Implantation sites in mice lacking uNK cells (alymphoid recombinase activating gene [RAG]-2(-/)- common cytokine receptor chain gamma [gamma(c)](-/)-) or IFN-gamma signaling (IFN-gamma(-/)- or IFN-gammaRalpha(-/)-) fail to initiate normal pregnancy-induced modification of decidual arteries and display hypocellularity or necrosis of decidua. To investigate the functions of uNK cell-derived IFN-gamma during pregnancy, RAG-2(-/)-gamma(c)(-/)- females were engrafted with bone marrow from IFN-gamma(-/)- mice, IFN-gamma signal-disrupted mice (IFN-gammaRalpha(-/)- or signal transducer and activator of transcription [Stat]-1(-/)-), or from mice able to establish normal uNK cells (severe combined immunodeficient [SCID] or C57BL/6). Mated recipients were analyzed at midgestation. All grafts established uNK cells. Grafts from IFN-gamma(-/)- mice did not reverse host vascular or decidual pathology. Grafts from all other donors promoted modification of decidual arteries and decidual cellularity. Grafts from IFN-gammaRalpha(-/)- or Stat-1(-/)- mice overproduced uNK cells, all of which were immature. Grafts from IFN-gamma(-/)-, SCID, or C57BL/6 mice produced normal, mature uNK cells. Administration of murine recombinant IFN-gamma to pregnant RAG-2(-/)-gamma(c)(-/)- mice initiated decidual vessel modification and promoted decidual cellularity in the absence of uNK cells. These in vivo findings strongly suggest that uNK cell-derived IFN-gamma modifies the expression of genes in the uterine vasculature and stroma, which initiates vessel instability and facilitates pregnancy-induced remodeling of decidual arteries.

Direct evidence to support the role of HLA-G in protecting the fetus from maternal uterine natural killer cytolysis

HLA-G is a nonclassical major histocompatibility complex class I molecule selectively expressed on cytotrophoblasts at the feto-maternal interface, where it may play an important role in maternal tolerance of the fetus. We provide direct evidence under physiological conditions that supports the role of HLA-G in protecting cytotrophoblasts against natural killer (NK) cytolysis in 6 semiallogenic combinations of maternal uterine NK cells and their own trophoblast counterparts, as well as in 20 allogenic combinations of maternal uterine NK cells and trophoblasts from different mothers. We show that, in all cases studied, this HLA-G-mediated protection was abolished by treatment of cytotrophoblasts with an HLA-G-specific mAb. The HLA class I-negative K562 cell line transfected with the predominant HLA-G1 isoform results in similar protection and abolition from maternal uterine NK lysis. Because maternal uterine NK cells express killer inhibitory receptors for HLA-G, we conclude that their interactions contribute to the survival of the fetal semiallograft by confering immunological tolerance to its tissues.

Seminal plasma and male factor signalling in the female reproductive tract

In mammals, insemination results in the transmission of seminal factors that act, in the female reproductive tract, to promote sperm survival, to "condition" the female immune response to tolerate the conceptus and to organise molecular and cellular changes in the endometrium to facilitate embryo development and implantation. These events are initiated when signalling agents, including transforming growth factor-beta and other cytokines and prostaglandins secreted by seminal vesicle and prostate glands, interact with epithelial cells in the cervix and uterus to activate cytokine synthesis and to induce cellular and molecular changes resembling a classical inflammatory cascade. The consequences are the recruitment and activation of macrophages, granulocytes and dendritic cells, which have immune-regulatory and tissue-remodelling roles that culminate in improved endometrial receptivity to the implanting embryo. Cytokines elicited by seminal activation have embryotrophic properties and also contribute directly to the optimal development of the early embryo. This review summarises our current understanding of the physiology of responses to seminal plasma in the female reproductive tract and considers the evolutionary significance of seminal plasma in influencing female tissues to promote the success of pregnancy.

See no evil, hear no evil, do no evil: the lessons of immune privilege

Immune-mediated inflammation and allograft rejection are greatly reduced in certain organs, a phenomenon called 'immune privilege'. Immune privilege is well developed in three regions of the body: the eye, the brain and the pregnant uterus. Immune-mediated inflammation has devastating consequences in the eye and brain, which have limited capacity for regeneration. Likewise, loss of immune privilege at the maternal-fetal interface culminates in abortion in rodents. However, all three regions share many adaptations that restrict the induction and expression of immune-mediated inflammation. A growing body of evidence from rodent studies suggests that a breakdown in immune privilege contributes to multiple sclerosis, uveitis, corneal allograft rejection and possibly even immune abortion.

Expression of angiogenic growth factors by uterine natural killer cells during early pregnancy

Remodeling of uterine spiral arteries is critical for the continuation of a successful pregnancy. Uterine natural killer (uNK) cells are the predominant leukocyte population in the early pregnant decidua, and a role for these cells in spiral artery remodeling in pregnancy has been suggested. Angiogenic growth factors were measured in isolated uNK and total (unseparated) decidual cells (8-10 or 12-14 weeks gestation, n=5 each gestational age) after culture for 48 h. Angiopoietin (Ang)1, placental growth factor, transforming growth factor-beta1 (TGF-beta1), and vascular endothelial growth factor (VEGF)-C were measured by enzyme-linked immunosorbent assay. Angiogenin, Ang2, fibroblast growth factor basic, intercellular adhesion molecule (ICAM)-1, keratinocyte growth factor (KGF), platelet-derived growth factor-BB, and VEGF-A were measured using a FASTQuant angiogenic growth factor multiplex protein assay. Levels of Ang2, ICAM-1, and KGF, secreted by the total decidual fraction, decreased with increasing gestational age. uNK levels of Ang2 and VEGF-C also decreased with increasing gestational age. At 8-10 weeks gestation, there was no difference in the level of Ang1, Ang2, TGF-beta1, and VEGF-C secreted by uNK cells and the total decidual fraction. At 12-14 weeks, uNK cells secreted significantly lower levels of VEGF-C than the total decidual fraction. Early pregnancy decidua is a major source of angiogenic growth factors whose levels decrease with increasing gestational age, suggesting that they may play a role in spiral artery remodeling. uNK cells appear to be a prominent source of Ang1, Ang2, TGF-beta1, and VEGF-C within the placental bed.

Uterine NK cells: active regulators at the maternal-fetal interface

Pregnancy presents an immunological conundrum because two genetically different individuals coexist. The maternal lymphocytes at the uterine maternal-fetal interface that can recognize mismatched placental cells are T cells and abundant distinctive uterine NK (uNK) cells. Multiple mechanisms exist that avoid damaging T cell responses to the fetus, whereas activation of uNK cells is probably physiological. Indeed, genetic epidemiological data suggest that the variability of NK cell receptors and their MHC ligands define pregnancy success; however, exactly how uNK cells function in normal and pathological pregnancy is still unclear, and any therapies aimed at suppressing NK cells must be viewed with caution. Allorecognition of fetal placental cells by uNK cells is emerging as the key maternal-fetal immune mechanism that regulates placentation.

Human uterine leukocytes and pregnancy

In human pregnancy, the embryo implants into the specialized mucosal wall of the uterus (decidua) and the placenta starts to form. Cells from the placenta (trophoblasts) invade into the uterine mucosa in order to open up maternal uterine arteries to ensure an adequate supply of blood to the developing fetus. The trophoblasts have a unique immunological phenotype compared to most cells especially with regard to their expression of major histocompatibility complex (MHC) antigens. On the other side of the interaction, the uterine mucosa (endometrium) differentiates in preparation for implantation. One of the changes that takes place is the appearance in the endometrium of a large number of maternal leukocytes in the final part of the menstrual cycle. If pregnancy ensues, these leukocytes continue to increase in number and are found in close contact with trophoblasts. The composition of this population of maternal immune cells is unusual compared to that seen at other mucosal sites. A lot of research has focused on whether maternal T-cell responses are suppressed or modified during pregnancy. Research has also concentrated on the specialized uterine natural killer (NK) cells, which are found in the decidua in large numbers during early pregnancy. These uterine NK cells have been shown to express receptors for trophoblast MHC antigens, but their role in pregnancy is still mysterious. The purpose of this review is to give an overview of what is known about the immunology at the implantation site and also to provide an update of some of the most recent findings in this field.

Natural killer cells in pregnancy and recurrent pregnancy loss: endocrine and immunologic perspectives

The endocrine system and the immune system interact closely during implantation and maintenance of pregnancy. One of the most striking examples of this communication is at the level of the decidua (endometrium of pregnancy). Here, under the influence of sex steroids, there is a dramatic increase of a unique population of lymphocytes, the uterine natural killer (uNK) cells, in early pregnancy. These cells derive predominantly from a subset of peripheral blood NK cells, which under hormonal influence gets recruited to the uterus. In mice, uNK cells play an important role in the development of placental vasculature. The role of these cells in human pregnancy is still not definitively established; however, they are believed to promote placental and trophoblast growth and provide immunomodulation at the maternal-fetal interface. In contrast to their presumptive role in the maintenance of a healthy pregnancy, uNK cells and peripheral NK cells are dysregulated in unexplained recurrent pregnancy loss. Herein, we review NK cell populations, their changes in number and function in altered endocrine environments during the menstrual cycle and pregnancy, the current data on their potential role in unexplained recurrent pregnancy loss, and mechanisms for potential therapies targeted to NK cell function for this enigmatic disorder.

Toll-like receptor (TLR) expression and TLR-mediated cytokine/chemokine production by human uterine epithelial cells

The objective of this study was to examine the expression of toll-like receptors (TLRs) by the uterine epithelial cell line ECC-1 and to determine if stimulation of the expressed TLRs induces changes in cytokine and/or chemokine secretion. The expression of TLR1 to TLR9 by ECC-1 cells was demonstrated by reverse transcription polymerase chain reaction, with only TLR10 not being expressed. Stimulation of ECC-1 cells using agonists to TLR2, TLR4 and TLR5 induced the expression of the chemokines interleukin-8 (IL-8) and monocyte chemotactic protein-1 (MCP-1), as well as the pro-inflammatory cytokine IL-6, and occurred in a dose-dependent manner. In response to zymosan and flagellin, pathogen-associated molecular patterns (PAMP) that are recognized by TLR2 and TLR5 respectively, ECC-1 cells secreted significantly more IL-8, MCP-1 and IL-6 than in response to other TLR agonists. In contrast, agonists to TLR3, TLR7, and TLR9 had no effect on the secretion of the 13 cytokines or chemokines analysed. These results indicate that uterine epithelial cells are important sentinels of the innate immune system. Further it indicates that all but one of the known TLRs are expressed by ECC-1 cells and that stimulation through specific TLRs mediates changes in the expression of key chemokines and pro-inflammatory cytokines that aid in the defence of the uterus against potential pathogens.

Induction of regulatory T cells by physiological level estrogen

Naturally occurring CD4+CD25+ regulatory T cells (Treg) exert an important role in mediating maternal tolerance to the fetus during pregnancy, and this effect might be regulated via maternal estrogen secretion. Although estrogen concentration in the pharmaceutical range has been shown to drive expansion of CD4+CD25+ Treg cells, little is known about how and through what mechanisms E2 within the physiological concentration range of pregnancy affects this expansion. Using in vivo and in vitro mouse models in these experiments, we observed that E2 at physiological doses not only expanded Treg cell in different tissues but also increased expression of the Foxp3 gene, a hallmark for CD4+CD25+ Treg cell function, and the IL-10 gene as well. Importantly, our results demonstrate that E2, at physiological doses, stimulated the conversion of CD4+CD25- T cells into CD4+CD25+ T cells which exhibited enhanced Foxp3 and IL-10 expression in vitro. Such converted CD4+CD25+ T cells had similar regulatory function as naturally occurring Treg cells, as demonstrated by their ability to suppress naïve T cell proliferation in a mixed lymphocyte reaction. We also found that the estrogen receptor (ER) exist in the CD4+CD25- T cells and the conversion of CD4+CD25- T cells into CD4+CD25+ T cells stimulated by E2 could be inhibited by ICI182,780, a specific inhibitor of ER(s). This supports that E2 may directly act on CD4+CD25- T cells via ER(s). We conclude that E2 is a potential physiological regulatory factor for the peripheral development of CD4+CD25+ Treg cells during the implantation period in mice.

Engraftment of bone marrow from severe combined immunodeficient (SCID) mice reverses the reproductive deficits in natural killer cell-deficient tg epsilon 26 mice

A large, transient population of natural killer (NK) cells appears in the murine uterine mesometrial triangle during pregnancy. Depletion of uterine (u) NK cells, recently achieved using gene-ablated and transgenic mice, results in pathology. Pregnancies from matings of homozygous NK and T cell-deficient tg epsilon 26 mice have <1% of normal uNK cell frequency, no development of an implantation site-associated metrial gland, and an edematous decidua with vascular pathology that includes abnormally high vessel walls/lumens ratios. Fetal loss of 64% occurs midgestation and placentae are small. None of these features are seen in pregnant T cell-deficient mice. To confirm the role of the NK cell deficiency in these reproductive deficits, transplantation of tg epsilon 26 females was undertaken using bone marrow from B and T cell-deficient scid/scid donors. Engrafted pregnant females have restoration of the uNK cell population, induced metrial gland differentiation, reduced anomalies in the decidua and decidual blood vessels, increased placental sizes, and restoration of fetal viability at all gestational days studied (days 10, 12, and 14). Thus, uNK cells appear to have critical functions in pregnancy that promote decidual health, the appropriate vascularization of implantation sites, and placental size.

Innate immunity in the human female reproductive tract: antiviral response of uterine epithelial cells to the TLR3 agonist poly(I:C)

The objective of this study was to examine the expression of TLR by human primary uterine epithelial cells (UEC) and to determine whether exposure to the TLR agonist poly(I:C) would induce an antiviral response. The secretion of several cytokines and chemokines was examined as well as the mRNA expression of human beta-defensin-1 and -2 (HBD1 and HBD2), IFN-beta, and the IFN-beta-stimulated genes myxovirus resistance gene 1 and 2',5' oligoadenylate synthetase. The expression of TLR1-9 by UEC was demonstrated by RT-PCR, with only TLR10 not expressed. Stimulation of UEC with the TLR3 agonist poly(I:C) induced the expression of the proinflammatory cytokines TNF-alpha, IL-6, GM-CSF, and G-CSF, as well as the chemokines CXCL8/IL-8, CCL2/MCP-1, and CCL4/MIP-1beta. In addition, poly(I:C) exposure induced the mRNA expression of HBD1 and HBD2 by 6- and 4-fold, respectively. Furthermore, upon exposure to poly(I:C) UEC initiated a potent antiviral response resulting in the induction of IFN-beta mRNA expression 70-fold and myxovirus resistance gene 1 and 2',5' oligoadenylate synthetase mRNA expression (107- and 96-fold), respectively. These results suggest that epithelial cells that line the uterine cavity are sensitive to viral infection and/or exposure to viral dsRNA released from killed epithelial cells. Not only do UEC release proinflammatory cytokines and chemokines that mediate the initiation of an inflammatory response and recruitment of immune cells to the site of infection, but they also express beta-defensins, IFN-beta, and IFN-beta-stimulated genes that can have a direct inhibiting effect on viral replication.

Intrauterine infection and preterm labor

Preterm labor is defined as labor that begins before 37 completed weeks of pregnancy. More than 12% of infants born in the USA are preterm. At least 40% of preterm births are associated with intrauterine infection. Toll-like receptors (TLRs) are members of a family of cell-surface proteins responsible for recognition of a diverse spectrum of bacterial, viral and fungal pathogens. TLRs initiate the host innate (i.e. non-adaptive) immune response, inducing a proinflammatory cascade involving cytokines, chemokines, prostaglandins, and other effector molecules that result in the characteristic phenomena of labor, such as uterine contractions and rupture of fetal membranes. These cascades may also be activated by mechanisms that are not primarily infectious but are accompanied by inflammatory responses. Now that the molecular mechanisms linking infection and labor have been, to a large extent, elucidated, the challenge is to identify points of overlap with non-infectious causes of labor and to find intervention strategies that can minimize the negative impact of preterm delivery.

Intrauterine inflammation, insufficient to induce parturition, still evokes fetal and neonatal brain injury

Exposure to prenatal inflammation is a known risk factor for long term neurobehavioral disorders including cerebral palsy, schizophrenia, and autism. Models of systemic inflammation during pregnancy have demonstrated an association with an immune response an adverse neurobehavioral outcomes for the exposed fetus. Yet, the most common route for an inflammatory exposure to a fetus is from intrauterine inflammation as occurs with chorioamnionitis. The aims of this study were to assess the effect of intrauterine inflammation on fetal and neonatal brain development and to determine if the gestational age of exposure altered the maternal or fetal response to inflammation. CD-1 timed pregnant mice on embryonic day 15 (E15) and E18.5 were utilized for this study. Dams were randomized to receive intrauterine infusion of lipopolysaccharide (LPS, 50 μg/dam) or normal saline. Different experimental groups were used to assess both acute and long-term outcomes. For each gestational age and each treatment group, fetal brains, amniotic fluid, maternal serum and placentas were collected 6h after intrauterine infusion. Rates of preterm birth, maternal morbidity and litter size were assessed. IL6 levels were assayed in maternal serum and amniotic fluid. An immune response was determined in the fetal brains and placentas by QPCR. Cortical cultures were performed to assess for fetal neuronal injury. Gene expression changes in postnatal day 7 brains from exposed and unexposed pups were determined. In the preterm period, low dose LPS resulted in a 30% preterm birth rate. Litter sizes were not different between the groups at either gestational age. IL6 levels were not significantly increased in maternal serum at either gestational time period. Low dose LPS increased IL6 levels in the amniotic fluid from exposed dams in the term but not preterm period. Regardless of gestational age of exposure, low dose intrauterine LPS activated an immune response in the placenta and fetal brain. Exposure to intrauterine LPS significantly decreased dendritic counts in cortical cultures from both the preterm and term period. Exposure to intrauterine inflammation altered gene expression patterns in the postnatal brain; this effect was dependent on gestational age of exposure. In conclusion, intrauterine inflammation, even in the absence of preterm parturition, can evoke fetal brain injury as evidence by alterations in cytokine expression and neuronal injury. Despite an absent or limited maternal immune response in low dose intrauterine inflammation, the immune system in the placenta is activated which is likely sufficient to induce a fetal immune response and subsequent brain injury. Changes in the fetal brain lead to changes in gene expression patterns into the neonatal period. Subclinical intrauterine inflammation can lead to fetal brain injury and is likely to be mechanistically associated with long term adverse outcomes for exposed offspring.

On the nature and function of human uterine granular lymphocytes

The 'foetal allograft' concept of the maternal-foetal relationship is currently being questioned. This is partly due to the increasing awareness of uterine large granular lymphocytes, a subset of natural killer cells found in the human endometrium at the time of placentation. Analogous cells are present in several species. The possible role of these cells in the regulation of the trophoblast-maternal interface is discussed here by Ashley King and Yung Wai Loke.

Maternal high-fat diet disturbs uteroplacental hemodynamics and increases the frequency of stillbirth in a nonhuman primate model of excess nutrition

Prepregnancy maternal obesity confers an increased risk of stillbirth, but the mechanisms are unknown. Maternal obesity is associated with placental inflammation. We considered that maternal diet may predispose to the increased risk of placental inflammation and stillbirth. We hypothesized that a chronic high-fat diet (HFD) is associated with abnormal uteroplacental circulation and placental inflammation. Here we used a nonhuman primate model to determine the effect of chronic HFD on the uterine and placental hemodynamics, placental histology, and inflammation in a prospective, observational study of 24 Japanese macaques. Overall, there was a statistically significant (38-56%) reduction in uterine volume blood flow from HFD animals, whether they were lean or obese. Consumption of a HFD, independent of obesity, increased placental inflammatory cytokines and the expression of Toll-like receptor 4. We show that HFD consumption by obese mothers with hyperinsulinemia also reduced volume blood flow on the fetal side of the placenta and significantly increased the frequency of both placental infarctions and stillbirth. These results suggest that a HFD, independent of obesity, decreases uterine volume blood flow. Maternal obesity and insulin resistance further exacerbates the placental dysfunction and results in an increased frequency of stillbirth.

Gestation stage-dependent intrauterine trophoblast cell invasion in the rat and mouse: novel endocrine phenotype and regulation

Trophoblast cell invasion into the uterine wall is characteristic of hemochorial placentation. In this report, we examine trophoblast cell invasion in the rat and mouse, the endocrine phenotype of invasive trophoblast cells, and aspects of the regulation of trophoblast cell invasion. In the rat, trophoblast cells exhibit extensive interstitial and endovascular invasion. Trophoblast cells penetrate through the decidua and well into the metrial gland, where they form intimate associations with the vasculature. Trophoblast cell invasion in the mouse is primarily interstitial and is restricted to the mesometrial decidua. Both interstitial and endovascular rat trophoblast cells synthesize a unique set of prolactin (PRL)-like hormones/cytokines, PRL-like protein-A (PLP-A), PLP-L, and PLP-M. Invading mouse trophoblast cells also possess endocrine activities, including the expression of PLP-M and PLP-N. The trafficking of natural killer (NK) cells and trophoblast cells within the mesometrial uterus is reciprocal in both the rat and mouse. As NK cells disappear from the mesometrial compartment, a subpopulation of trophoblast cells exit the chorioallantoic placenta and enter the decidua. Furthermore, the onset of interstitial trophoblast cell invasion is accelerated in mice with a genetic deficiency of NK cells, Tg epsilon 26 mice, implicating a possible regulatory role of NK cells in trophoblast cell invasion. Additionally, the NK cell product, interferon-gamma (IFNgamma), inhibits trophoblast cell outgrowth, and trophoblast cell invasion is accelerated in mice with a genetic deficiency in the IFNgamma or the IFNgamma receptor. In summary, trophoblast cells invade the uterine wall during the last week of gestation in the rat and mouse and possess a unique endocrine phenotype, and factors present in the uterine mesometrial compartment modulate their invasive behavior.

Assessment of requirements for IL-15 and IFN regulatory factors in uterine NK cell differentiation and function during pregnancy

In mouse and human, precursors of NK cell lineage home to decidualizing uteri. To assess the requirement for IL-15, an essential cytokine for NK differentiation in lymphoid tissue, on uterine NK (uNK) cell differentiation, implantation sites from IL-15(-/-) mice were analyzed histologically. IL-15(-/-) implantation sites had no uNK cells, no spiral-artery modification, and lacked the decidual integrity found in normal mice. IL-15(-/-) recipients of C57BL/6 marrow displayed similar pathology. However, implantation sites from recombination-activating gene-2(-/-)gamma(c)(-/-) (alymphoid) recipients of IL-15(-/-) marrow showed normal uNK cells, modified spiral arteries, and well-developed decidua basalis. Deletion of the IFN-regulatory factor (IRF)-1, but not IRF-2 (factors important in peripheral NK cell differentiation) limited but did not prevent uNK cell development. In situ hybridization localized IRF-1 largely to placental trophoblast cells. IRF-1(-/-) marrow transplanted into recombination-activating gene-2(-/-)gamma(c)(-/-) displayed competence for full uNK cell differentiation. IL-15 mRNA expression at implantation sites of IRF-1(-/-) and C57BL/6 was similar, suggesting that, unlike in bone marrow and spleen, IRF-1 does not regulate IL-15 in the pregnant uterus. Terminal differentiation of uNK cells was not promoted in pregnant IRF-1(-/-) mice by 5-day infusion of murine rIL-15, suggesting that IRF-1 deficiency rather than IL-15 deficiency limits uNK cell differentiation in these mice. Further, IRF-1 regulates placental growth, birth weight, and postnatal growth of offspring. These studies indicate that uNK cell development and maturation share some aspects with NK cell development in other tissues, but also display distinctive tissue-specific regulation.

A brief review of recent data on some cytokine expressions at the materno-foetal interface which might challenge the classical Th1/Th2 dichotomy

Focussing attention on cytokines at the materno-foetal interface represents one of the major advances made in the field. This owes much to the visionary views of Tom Wegmann, and to the changes brought about in the field by immunotrophism and Th1/Th2 paradigms. We review these briefly and also point out some emerging problems.However, a certain number of newly discovered cytokines do not fit into the classical Th1/Th2 dichotomy. Yet, by their capacity to activate or downregulate NK cells, by their action on adhesion molecules, and by their regulatory effects on the vascularisation process, they are of possible interest within the materno-foetal relationship. Therefore, as a first step, we have undertaken a systematic study of the expression of IL-11, IL-12, IL-13, IL-15, IL-16, IL-17 and IL-18 in the uterus, the peri-implantation embryo, and later on decidual and placental tissues throughout pregnancy. These cytokines were detected in every case, with, in each case, a precise localisation, which will be detailed, and which indeed suggests important regulatory functions, especially during implantation. In some cases, as will be shown in the peri-implantation uterus, those cells are perfectly expressed by uterine GMG-NK-like cells. Comparative ELISAs and quantitative RT-PCR have been or are being conducted, but already the expression patterns that are observed, and the very precise window of appearance that is observed for some of the GMG NK-like cells, either around or in the implanting embryo, as well as the complexity of the respective distributions, strongly suggest that, as useful as it certainly was for a while, the Th1/Th2 paradigm must now be considered as an over-simplification. Rather, the existing data point to sequential windows and are suggestive of a system where an extreme complexity is allied to very precise timing and tuning. They also suggest that the materno-foetal relationship is not simply maternal tolerance of a foreign tissue, but a series of intricate mutual cytokine interactions governing selective immune regulation and also control of the adhesion and vascularisation processes during this dialogue.