Intrinsic susceptibility of mouse trophoblasts to natural killer cell-mediated attack in vivo

Visualization of microbes by 16S in situ hybridization in term and preterm placentas without intraamniotic infection

BACKGROUND

Numerous reports have documented bacteria in the placental membranes and basal plate decidua in the absence of immunopathology using histologic techniques. Similarly, independent metagenomic characterizations have identified an altered taxonomic makeup in association with spontaneous preterm birth. Here we sought to corroborate these findings by localizing presumptive intact bacteria using molecular histology within the placental microanatomy.

OBJECTIVE

Here we examined for microbes in term and preterm gestations using a signal-amplified 16S universal in situ hybridization probe set for bacterial rRNA, alongside traditional histologic methods of Warthin-Starry and Gram stains, as well as clinical culture methodologies. We further sought to differentiate accompanying 16S gene sequencing taxonomic profiles from germ-free (gnotobiotic) mouse and extraction and amplicon contamination controls.

STUDY DESIGN

Placentas were collected from a total of 53 subjects, composed of term labored (n = 4) and unlabored cesarean deliveries (n = 22) and preterm vaginal (n = 18) and cesarean deliveries (n = 8); a placenta from a single subject with clinical and histologic evident choriomanionitis was employed as a positive control (n = 1). The preterm cohort included spontaneous preterm birth with (n = 6) and without (n = 10) preterm premature rupture of membranes, as well as medically indicated preterm births (n = 10). Placental microbes were visualized using an in situ hybridization probe set designed against highly conserved regions of the bacterial 16S ribosome, which produces an amplified stable signal using branched DNA probes. Extracted bacterial nucleic acids from these same samples were subjected to 16S rRNA metagenomic sequencing (Illumina, V4) for course taxonomic analysis, alongside environmental and kit contaminant controls. A subset of unlabored, cesarean-delivered term pregnancies were also assessed with clinical culture for readily cultivatable pathogenic microbes.

RESULTS

Molecular in situ hybridization of bacterial rRNA enabled visualization and localization of low-abundance microbes after systematic high-power scanning. Despite the absence of clinical or histologic chorioamnionitis in 52 of 53 subjects, instances of 16S rRNA signal were confidently observed in 13 of 16 spontaneous preterm birth placentas, which was not significantly different from term unlabored cesarean specimens (18 of 22; P > .05). 16S rRNA signal was largely localized to the villous parenchyma and/or syncytiotrophoblast, and less commonly the chorion and the maternal intervillous space. In all term and unlabored cesarean deliveries, visualization of evident placental microbes by in situ hybridization occurred in the absence of clinical or histologic detection using conventional clinical cultivation, hematoxylin-eosin, and Gram staining. In 1 subject, appreciable villous bacteria localized to an infarction, where 16S microbial detection was confirmed by Warthin-Starry stain. In all instances, parallel sample principle coordinate analysis using Bray-Cutis distances of 16S rRNA gene sequencing data demonstrated consistent taxonomic distinction from all negative or potential contamination controls (P = .024, PERMANOVA). Classification from contaminant filtered data identified a distinct taxonomic makeup among term and preterm cohorts when compared with contaminant controls (false discovery rate <0.05).

CONCLUSION

Presumptively intact placental microbes are visualized as low-abundance, low-biomass and sparse populations within the placenta regardless of gestational age and mode of delivery. Their taxonomic makeup is distinct from contamination controls. These findings further support several previously published findings, including our own, which have used metagenomics to characterize low-abundance and low-biomass microbial communities in the placenta.

Uterine Natural Killer Cells: Functional Distinctions and Influence on Pregnancy in Humans and Mice

Our understanding of development and function of natural killer (NK) cells has progressed significantly in recent years. However, exactly how uterine NK (uNK) cells develop and function is still unclear. To help investigators that are beginning to study tissue NK cells, we summarize in this review our current knowledge of the development and function of uNK cells, and what is yet to be elucidated. We compare and contrast the biology of human and mouse uNK cells in the broader context of the biology of innate lymphoid cells and with reference to peripheral NK cells. We also review how uNK cells may regulate trophoblast invasion and uterine spiral arterial remodeling in human and murine pregnancy.

Ectopic Trophoblast Allografts in the Horse Resist Destruction by Secondary Immune Responses

Invasive trophoblast from Day 34 horse conceptuses survives in extrauterine sites in allogeneic recipients that are immunologically naive to donor major histocompatibility complex class I antigens. The ectopic trophoblast retains its in utero characteristics, including similar lifespan, physiologic effect of its secreted product (equine chorionic gonadotropin) upon the recipient's ovaries, and induction of host immune responses. Immunologic memory has not been considered previously in this experimental system. We hypothesized that primary exposure to ectopic trophoblast would affect the recipient's immune status such that the survival time of subsequent transplants would be altered. Secondary transplant lifespans could be shortened by destructive memory responses, as has been observed in ectopic trophoblast studies in rodents, or lengthened, as occurs when male skin grafts follow multiple syngeneic pregnancies in mice. Eight mares received two closely spaced trophoblast transplants. Both grafts for each recipient were obtained from conceptuses sired by the same stallion to provide consistency in histocompatibility antigen exposure. Donor stallions were major histocompatibility complex class I homozygotes. Cytotoxic antibody production was tracked to monitor recipients' immune responses to the transplants. Detection of serum equine chorionic gonadotropin was used as a proxy for transplant lifespan. There was no significant difference between the distributions of primary and secondary transplant lifespans, despite evidence of immunologic memory. These data demonstrate that secondary ectopic trophoblast transplants in horses do not experience earlier destruction or prolonged survival following immune priming of recipients. Mechanisms responsible for the eventual demise of the transplants remain unperturbed by secondary immune responses or chronic antigenic exposure.

Natural killer cells and regulatory T cells in early pregnancy loss

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

Immune balance at the foeto-maternal interface as the fulcrum of reproductive success

Viviparity has many evolutionary advantages but brings with it the problem of the semi-allogeneic foetus having to coexist with the mother for the duration of pregnancy. In species with haemochorial placentation this problem is particularly evident as foetal trophoblast cells are extensively intermingled with maternal tissue and are directly exposed to maternal blood. Fascinating adaptations on both the foetal and maternal side have allowed for this interaction to be re-directed away from an immune rejection response not only towards immunotolerance, but in fact towards actively supporting reproductive success. Recent data have shown that some of these remarkable adaptations are conserved between mice and humans. Thus, a subset of trophoblast cells that is directly exposed to the maternal uterine environment shares the feature of expressing an unusual antigen repertoire on their surface. Paternal antigens can be recognized by maternal immune cells, in particular uterine natural killer cells that express cognate receptors, to regulate the extensive remodelling events that take place at the implantation site. Detailed genetic dissection experiments in the mouse have further demonstrated the direct impact of antigenic dissimilarity on foetal growth. With the availability of inbred strains, in vitro culture systems of trophoblast stem cells, and in-depth genetic, genomic and epigenomic data the mouse will be a valuable model system to study the intricate immune crosstalk at the foeto-maternal boundary. These insights will pave the way towards unravelling the mutual and synergistic interactions between trophoblast and its surrounding maternal environment, and in doing so help understand pregnancy pathologies.

Maternal decidual macrophages inhibit NK cell killing of invasive cytotrophoblasts during human pregnancy

Human pregnancy is an immunological paradox. Semiallogeneic (fetal) placental cells (extravillous cytotrophoblasts [CTBs]) invade the uterine lining (decidua), which contains a unique decidual natural killer (dNK) cell population, identified by the cell surface phenotype CD56(bright) CD16(-) CD3(-) and CD14(+) CD206(+) macrophages (dMac). Previous reports suggested that human dNK cells are not a threat to the fetoplacental unit because they are anergic. In contrast, here we showed that purified and exogenously stimulated dNK cells are capable killers of cellular targets, including semiallogeneic CTBs. However, dMacs in the decidual leukocyte (DL) population restrained dNK killing through a transforming growth factor beta1 (TGF-beta1)-dependent mechanism. Our findings support a new model whereby dNK cells, capable of killing CTBs, are prevented from doing so by neighboring macrophages, thus protecting the fetal cells from NK cell attack. We speculate that this mechanism would inhibit dNK cell-mediated killing, even under conditions where high levels of cytokines may stimulate dNK cells, which could pose a threat to the developing placenta.

Functions of ectopically transplanted invasive horse trophoblast

The invasive and fully antigenic trophoblast of the chorionic girdle portion of the equine fetal membranes has the capacity to survive and differentiate after transplantation to ectopic sites. The objectives of this study were to determine i) the survival time of ectopically transplanted allogeneic trophoblast cells in non-pregnant recipient mares, ii) whether equine chorionic gonadotropin (eCG) can be delivered systemically by transplanted chorionic girdle cells, and iii) whether eCG delivered by the transplanted cells is biologically active and can suppress behavioral signs associated with estrus. Ectopically transplanted chorionic girdle survived for up to 105 days with a mean lifespan of 75 days (95% confidence interval 55-94) and secreted sufficient eCG for the hormone to be measurable in the recipients' circulation. Immunohistochemical labeling of serial biopsies of the transplant sites and measurement of eCG profiles demonstrated that graft survival was similar to the lifespan of equine endometrial cups in normal horse pregnancy. The eCG secreted by the transplanted cells induced corpora lutea formation and sustained systemic progesterone levels in the recipient mares, effects that are also observed during pregnancy. This in turn caused suppression of estrus behavior in the recipients for up to 3 months. Thus, ectopically transplanted equine trophoblast provides an unusual example of sustained viability and function of an immunogenic transplant in a recipient with an intact immune system. This model highlights the importance of innate immunoregulatory capabilities of invasive trophoblast cells and describes a new method to deliver sustained circulating concentrations of eCG in non-pregnant mares.

Murine trophoblast cells induce NK cell interferon-gamma production through KLRK1

Murine models suggest that natural killer (NK) cells are important for normal implantation site development, in part, through the production of interferon gamma (IFNG). As KLRK1 (NKG2D) is expressed on human and murine uterine NK (uNK) cells, we examined the role of KLRK1 in the interaction between murine trophoblasts and NK cells. Flow cytometric analysis revealed that both murine trophoblast stem (TS) cells and differentiated trophoblast giant cells expressed the KLRK1 ligand retinoic acid early transcript 1, or RAET1. Coculture of activated NK cells with either TS cells or giant cells led to the production of IFNG, as measured by ELISA. In addition, coculture with TS cells led to the downregulation of KLRK1. Both responses were inhibited by soluble KLRK1 ligand, but not by irrelevant protein. Further studies demonstrated the presence of KLRK1 ligand on uterine cells derived from either virgin or pregnant mice, although uterine RAET1 protein expression was upregulated in vitro by progesterone, but not estradiol. We suggest that the interaction of KLRK1 and RAET1 may be involved in IFNG production by uNK cells, and thus, this receptor-ligand pair may contribute to successful murine implantation site development.

Gender-dependent survival of allogeneic trophoblast stem cells in liver

In view of the well-known phenomenon of trophoblast immune privilege, trophoblast stem cells (TSCs) might be expected to be immune privileged, which could be of interest for cell or gene therapies. Yet in the ectopic sites tested so far, TSC transplants fail to show noticeable immune privilege and seem to lack physiological support. However, we show here that after portal venous injection, green fluorescent protein (GFP)-labeled TSCs survive for several months in the livers of allogeneic female but not male mice. Gonadectomy experiments revealed that this survival does not require the presence of ovarian hormones but does require the absence of testicular factors. By contrast, GFP-labeled allogeneic embryonic stem cells (ESCs) are reliably rejected; however, these same ESCs survive when mixed with unlabeled TSCs. The protective effect does not require immunological compatibility between ESCs and TSCs. Tumors were not observed in animals with either successfully engrafted TSCs or coinjected ESCs. We conclude that in a suitable hormonal context and location, ectopic TSCs can exhibit and confer immune privilege. These findings suggest applications in cell and gene therapy as well as a new model for studying trophoblast immunology and physiology.

Isolation and manipulation of mouse trophoblast stem cells

The isolation of stable trophoblast stem (TS) cell lines from early mouse embryos has provided a useful cell culture model to study trophoblast development. TS cells are derived from pre-implantation blastocysts or from the extraembryonic ectoderm of early post-implantation embryos. The derivation and maintenance of mouse TS cells is dependent upon continuous fibroblast growth factor (FGF) signaling. Gene expression analysis, differentiation in culture, and chimera formation show that TS cells accurately model the mouse trophoblast lineage. This unit describes how to derive, maintain, and manipulate TS cells, including DNA transfection and chimera formation.

NK cell tolerance and the maternal-fetal interface

Natural killer (NK) cells play a fundamental role in the innate immune response through their ability to secrete cytokines and kill target cells without prior sensitization. These effector functions are central to NK cell anti-viral and anti-tumor abilities. Due to their cytotoxic nature, it is vital that NK cells have the capacity to recognize normal self-tissue and thus prevent their destruction. In addition to their role in host defense, NK cells accumulate at the maternal-fetal interface and are thought to play a critical role during pregnancy. The close proximity of uterine NK (uNK) cells to fetal trophoblast cells of the placenta would seemingly lead to catastrophic consequences, as the trophoblast cells are semi-allogeneic. A fundamental enigma of pregnancy is that the fetal cells constitute an allograft but, in normal pregnancies, they are in effect not perceived as foreign and are not rejected by the maternal immune system. Although the mechanisms involved in achieving NK cell tolerance are becoming increasingly well-defined, further clarification is required, given the clinical implications of this work in the areas of infection, transplantation, cancer and pregnancy. Herein, we discuss several mechanisms of NK cell tolerance and speculate as to how they may apply to uNK cells at the maternal-fetal interface.

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.

Maintenance of mouse trophoblast stem cell proliferation by TGF-β/activin

Mouse trophoblast stem (TS) cells can be grown indefinitely in vitro with FGF4 and embryonic fibroblast conditioned media (EFCM). Here, we report that the active protein components of EFCM include TGF-beta and the related factor activin, and that long-term continuous TS cell proliferation is possible in media supplemented with only serum, FGF4, and TGF-beta. As trophoblasts are an epithelial cell type, the promotion of TS cell proliferation represents an unusual function for TGF-beta and activin since TGF-beta in particular is well known as an inhibitor of nontransformed epithelial cell proliferation. Our data suggest that constitutive FGF signaling in TS cells selectively inhibits the ability of TGF-beta to repress c-myc expression, a central component of the TGF-beta cytostatic transcriptional response previously observed to be lost in other epithelial cell types upon oncogenic Ras transformation.

Connexin31-deficient trophoblast stem cells: a model to analyze the role of gap junction communication in mouse placental development

The connexin (Cx) expression during placental development in rodents is subject to exacting spatiotemporal regulation. Following implantation, Cx31 characterizes the early trophoblast cell lineage and is expressed by the spongiotrophoblast during placental development until birth. Inactivation of the Cx31 gene results in a transient placental dysmorphogenesis with an imbalance in the trophoblast cell lineage differentiation in favor to giant cells [Dev. Biol. 231 (2001) 334]. In this study, we show that trophoblast stem (TS) cells exhibit the same connexin expression found in trophoblast cell lineage differentiation. Undifferentiated TS cells exclusively express Cx31 protein and Cx31.1 transcripts. Upon differentiation of TS cells, placental-specific Cx26 and Cx43 are induced. Cx31 knockout TS cells revealed an accelerated differentiation process to giant cells compared to controls, indicated by an overall shift in expression of connexins and marker genes such as Mash2, Pl-1, and Tpbpa. Moreover, proliferation was significantly reduced in Cx31 knockout TS cells upon differentiation. Both wild type and Cx31 knockout TS cells are able to invade and erode host vessels when injected into nude mice. We conclude that during trophoblast cell lineage differentiation, the Cx31 gap junction channel is involved in maintaining the proliferative diploid trophoblast cell population.

Ovarian insufficiency and early pregnancy loss induced by activation of the innate immune system

We describe a murine model of early pregnancy failure induced by systemic activation of the CD40 immune costimulatory pathway. Although fetal loss involved an NK cell intermediate, it was not due to lymphocyte-mediated destruction of the fetus and placenta. Rather, pregnancy failure resulted from impaired progesterone synthesis by the corpus luteum of the ovary, an endocrine defect in turn associated with ovarian resistance to the gonadotropic effects of prolactin. Pregnancy failure also required the proinflammatory cytokine TNF-alpha and correlated with the luteal induction of the prolactin receptor signaling inhibitors suppressor of cytokine signaling 1 (Socs1) and Socs3. Such links between immune activation and reproductive endocrine dysfunction may be relevant to pregnancy loss and other clinical disorders of reproduction.

T-bet regulates the terminal maturation and homeostasis of NK and Valpha14i NKT cells

Natural killer (NK) and CD1d-restricted Valpha14i natural killer T (NKT) cells play a critical early role in host defense. Here we show that mice with a targeted deletion of T-bet, a T-box transcription factor required for Th1 cell differentiation, have a profound, stem cell-intrinsic defect in their ability to generate mature NK and Valpha14i NKT cells. Both cell types fail to complete normal terminal maturation and are present in decreased numbers in peripheral lymphoid organs of T-bet(-/-) mice. T-bet expression is regulated during NK cell differentiation by NK-activating receptors and cytokines known to control NK development and effector function. Our results identify T-bet as a key factor in the terminal maturation and peripheral homeostasis of NK and Valpha14i NKT cells.

The immunological problem of pregnancy: 50 years with the hope of progress. A tribute to Peter Medawar

This article is a tribute to the late Sir Peter Brian Medawar for his pivotal contribution in 1953 to the establishment of the field of Reproductive Immunology, with a brief and selective assessment of the progress made during the ensuing 50 years towards elucidation of the mechanisms responsible for the paradoxical survival of the conceptus as an intra-uterine allograft within the immunologically competent genetically alien female host. Medawar's succinct and stimulating theories have been central throughout the whole of this time and his basic conclusion, that the single most important factor ensuring the success of gestation is the anatomical separation of the fetus from its mother, remains substantially valid to this day. The extent to which other factors are of significance, particularly those relating to the relative roles of maternal adaptive and innate immune responses to the developing feto-placental unit, has yet to be fully defined.

T-bet is required for optimal production of IFN-gamma and antigen-specific T cell activation by dendritic cells

IFN-gamma is well known as the signature cytokine of CD4+ T helper 1, CD8+, and natural killer cells, but recent studies demonstrate that antigen-presenting cells, in particular dendritic cells (DCs), are another potent source for this proinflammatory cytokine. T-bet, a transcription factor that controls IFN-gamma expression in CD4+ T cells, was reported recently to be expressed in human monocytes and myeloid DCs. In this study we investigate the role of T-bet in this important cell type. The development, differentiation, and activation of bone marrow and splenic DCs were unimpaired in mice lacking T-bet. However, T-bet was essential for the optimal production of IFN-gamma by both CD8alpha+ and CD8alpha- DCs. T-bet-deficient DCs were significantly impaired in their capacity to secrete IFN-gamma after both stimulation with IL-12 alone or in combination with IL-18. Further, T-bet-/- DCs were impaired in their ability to activate the T helper 1 program of adoptively transferred antigen-specific T cells in vivo. The rapid up-regulation of T-bet by IFN-gamma in DCs coupled with a function for DC-derived IFN-gamma in T cell activation may constitute a positive feedback loop to maximize type 1 immunity.