Uterine natural killer cells in the pregnant uterus

Rat placentation: an experimental model for investigating the hemochorial maternal-fetal interface

The rat possesses hemochorial placentation with deep intrauterine trophoblast cell invasion and trophoblast-directed uterine spiral artery remodeling; features shared with human placentation. Recognition of these similarities spurred the establishment of in vitro and in vivo research methods using the rat as an animal model to address mechanistic questions regarding development of the hemochorial placenta. The purpose of this review is to provide the requisite background to help move the rat to the forefront in placentation research.

Regional development of uterine decidualization: molecular signaling by Hoxa-10

Uterine decidualization, a key event in implantation, is critically controlled by stromal cell proliferation and differentiation. Although the molecular mechanism that controls this event is not well understood, the general consensus is that the factors derived locally at the site of implantation influence aspects of decidualization. Hoxa-10, a developmentally regulated homeobox transcription factor, is highly expressed in decidualizing stromal cells, and targeted deletion of Hoxa-10 in mice shows severe decidualization defects, primarily due to the reduced stromal cell responsiveness to progesterone (P(4)). While the increased stromal cell proliferation is considered to be an initiator of decidualization, the establishment of a full-grown functional decidua appears to depend on the aspects of regional proliferation and differentiation. In this regard, this article provides an overview of potential signaling mechanisms mediated by Hoxa-10 that can influence a host of genes and cell functions necessary for propagating regional decidual development.

Do molecular signals from the conceptus influence endometrium decidualization in rodents?

A critical period in establishing pregnancy occurs after the onset of implantation but before placental development. Evidence strongly suggests that abnormalities occurring during this period can result in pregnancy termination or in pre-eclampsia; the latter may lead to small-for-gestational-weight offspring that are likely to be unhealthy. Clearly, events occurring in the endometrium during the implantation process are crucial for proper fetal development and for optimal offspring health. In several mammalian species bi-directional communication between the conceptus and endometrium during implantation is required for successful pregnancy. Although different implantation and placentation modes occur in different mammalian species, common aspects of this bi-directional signaling may exist. The molecular signals from the trophoblast cells of the conceptus, which direct endometrial changes during implantation progression, are well known in some nonrodent species. Currently, we know little about such signaling in rodents during implantation progression, when the endometrium undergoes decidualization. This review focuses on data that support the hypothesis that paracrine signals from the rodent conceptus influence decidualization. Where possible, these findings are compared and contrasted with information currently known in other species that exhibit different implantation modes.

Pregnancy in the Brown Norway Rat: A Model for Investigating the Genetics of Placentation1

The placenta facilitates the exchange of nutrients and wastes in an effort to promote fetal development. Disruptions in the establishment of the placenta and its interactions with the maternal uterus are potential causes of pregnancy failure. In this study we investigated the pregnancy phenotype of two inbred rat strains: the Dahl Salt Sensitive (DSS) strain and the Brown Norway (BN) strain. The DSS strain is reported to have large litters, whereas the BN strain has small litters. Pregnant female rats of each strain were killed on various days of gestation. At the time of killing, the number of viable versus dead and/or resorbing conceptuses was determined. Placental tissues from viable conceptuses were collected and processed for biochemical and histologic analyses. The number of viable conceptuses at Days 8.5 and 18.5 of gestation was significantly greater in DSS versus BN rats. Additionally, the number of resorbing and/or dying conceptuses was significantly greater in the BN strain than in the DSS strain. Maternal responses to pregnancy and elements of placental and fetal development in DSS and BN rats differed. Immunohistologic analysis of placentation and gene expression profiles revealed that trophoblast cell invasion into the uterine mesometrial compartment was significantly less in the BN strain versus the DSS strain. In contrast, the uterine natural killer cell population was reciprocally expanded in the BN strain. The impairment in trophoblast cell invasion in BN rats was associated with a smaller junctional zone compartment of the chorioallantoic placenta. Collectively, the data indicate that BN rats exhibit a unique form of placentation and may represent an excellent model for investigating the genetics of placental development.

Immature NK cells suppress dendritic cell functions during the development of leukemia in a mouse model

To analyze the mechanisms by which cancer cells escape from hosts' immune surveillance, we investigated the changes in immune status during the progression of leukemia induced by injecting mice with WEHI-3B cells. In the bone marrow (BM) of leukemic mice, only DX5(+)CD3(-) cells were continuously increased, despite the progression of leukemia. In addition, DX5(+)CD3(-) cells were rapidly increased in peripheral blood (PB) 20 days after inoculation. We also found that myeloid dendritic cells (DCs) expressing low levels of I-A(d) and having low allo-T cell stimulatory activity were markedly increased in PB and spleen. The increase in DX5(+) cells in BM was thought to be induced by soluble factors from leukemic cells. DX5(+) cells from leukemic mice were CD3(-), B220(-), Gr-1(-), CD14(-), CD94(-), Ly-49C/F(-), asialo GM1(+), CD25(+), CD122(+), Thy-1(bright), and c-kit(dim) and showed low killing activity against YAC-1 cells, suggesting that those DX5(+) cells were immature NK cells. NK cells from leukemic PB down-regulated the expression of I-A(d) on DCs, an effect mediated by TGF-beta. Moreover, these NK cells significantly suppressed the allo-T cell stimulatory activity of DCs, an effect requiring cell-to-cell contact between NK cells and DCs and thought to involve CD25. Importantly, NK cells from leukemic PB inhibited generation of autotumor-specific CTL induced by DCs in primary MLR or by DC immunization. In conclusion, we identified circulating immature NK cells with immunosuppressive activities. These cells may be important for understanding the involvement of the host immune system during the development of leukemia.

Hoxa-10 deficiency alters region-specific gene expression and perturbs differentiation of natural killer cells during decidualization

Uterine decidualization, a key event for successful implantation, is critically controlled by stromal cell proliferation and differentiation. One hallmark event of decidualization is the acquisition of stromal cell polyploidy through terminal differentiation at the anti-mesometrial pole of the implantation site. Hoxa-10, a developmentally regulated homeobox transcription factor, is highly expressed in decidualizing stromal cells, and targeted deletion of Hoxa-10 in mice shows severe decidualization defects, primarily due to reduced stromal cell responsiveness to progesterone. However, the underlying molecular mechanism by which Hoxa-10 regulates this process remains largely unknown. Here, we show that Hoxa-10 deficiency confers diminished core cell cycle activity during stromal cell proliferation without disturbing polyploidy, suggesting that these events depend on local regulators that impact cell cycle machinery. To further address this question, we compared global gene expression profiles in uteri of wild-type and Hoxa-10(-/-) mice after inducing decidualization. Our studies show two major aspects of decidualization downstream of Hoxa-10. First, Hoxa-10 deficiency results in the aberrant region-specific expression of cyclin-dependent kinase-4 (cdk4) and -6 (cdk6), growth differentiation factor 10 (Gdf10), hepatocyte growth factor (Hgf) and Snail2. Second, Hoxa-10 deficiency compromises natural killer (NK) cell differentiation without altering trafficking of NK precursor cells during decidualization. Collectively, the results provide evidence that Hoxa-10 influences a host of genes and cell functions necessary for propagating normal decidual development during the post-implantation period.

Interleukin-11 signaling is required for the differentiation of natural killer cells at the maternal-fetal interface

Interleukin-11 (IL-11) is a multifunctional hematopoietic growth factor that has been implicated in the control of reproduction. Studies on IL-11 receptor-alpha (IL-11R alpha)-deficient mice showed that female mice are infertile due to defective decidualization. In this report, we evaluated the development of decidual cells, immune cells, and the vasculature associated with the implantation site of IL-11R alpha-deficient mice; with the aim of better understanding the nature of the fertility defect. Messenger RNAs for decidual differentiation, such as decidual prolactin-related protein and prolactin-like protein-J are expressed in the IL-11R alpha mutant. However, the number of decidual cells expressing these genes is decreased in the mutant compared with the wild-type control. Although, trophoblast cells differentiate and express placental lactogen-I in the IL-11R alpha-deficient uterine environment, they fail to progress and expand in number. Defects in the organization of the decidual vasculature were also apparent in the IL-11R alpha mutant uterus. The most dramatic effect of IL-11 signaling was on the hematopoietic environment of the uterine decidua. Differentiated/perforin-expressing uterine natural killer (NK) cells were virtually absent from implantation sites of IL-11R alpha mutant mice. NK cell precursors were capable of homing to the IL-11R alpha-deficient uterus and a known regulator of NK cell differentiation; IL-15 was expressed in the IL-11R alpha mutant uterus. Splenic NK cells from IL-11R alpha mutant mice were also able to respond to IL-15 in vitro. Thus, the defect in NK precursor cell maturation was not intrinsic to the NK precursor cells but was dependent upon the tissue environment. In summary, IL-11 signaling is required for decidual-specific maturation of NK cells.

The granzyme B inhibitor, PI-9, is differentially expressed during placental development and up-regulated in hydatidiform moles

The intracellular serpin Proteinase Inhibitor-9 (PI-9) is a potent inhibitor of the cytotoxic lymphocyte (CL) proteinase granzyme B, a major effector molecule used by CLs to induce target cell apoptosis. PI-9 is produced by CLs to protect against mis-directed granzyme B. However, PI-9 expression has also been reported in immune privileged tissues. In the present study, cell-specific expression of PI-9 in placental tissue of various gestational ages was examined by immunohistochemistry. PI-9 is highly expressed by the extravillous trophoblasts that have invaded the decidua, and this high expression is maintained throughout pregnancy. Similar levels were also observed in proliferative villous cytotrophoblasts. Syncytial trophoblasts generally do not produce PI-9 to a significant level until the last few weeks of pregnancy. The villous stroma contains mixed populations of PI-9 positive and negative cells throughout pregnancy, with highest expression during the second trimester. Compared to first trimester placentas, syncytial trophoblasts of partial and complete hydatidiform moles showed marked up-regulation of PI-9. Examination of choriocarcinoma cell lines also demonstrated a very high level of PI-9 is produced by these cells, which may provide protection from granzyme B-mediated apoptosis. The cell-specific expression of PI-9 in the placenta is consistent with a function in the maintenance of immune privilege, and it is proposed that up-regulated expression of PI-9 in gestational trophoblastic diseases contributes to disease pathogenesis via immune evasion.

Prolactin-like protein-A is a functional modulator of natural killer cells at the maternal-fetal interface

Natural killer (NK) cells are the predominant lymphocytes present in healthy rodent and human implantation sites. In the rat, the expansion, differentiation and subsequent migration of NK cells away from the developing chorioallantoic placenta coincide with the expression of a novel pregnancy- and trophoblast cell-specific cytokine, prolactin (PRL)-like protein A (PLP-A). PLP-A specifically binds to uterine NK cells but does not appear to utilize receptor systems for PRL. In the present report, we show that PLP-A interactions with NK cells are not mediated by receptors utilized by known modulators of NK cell function, including interleukin-2, interleukin-7, interleukin-12, and interleukin-15 (IL-15). Uterine NK cells respond to PLP-A or IL-15 with an increase in intracellular calcium mobilization. In contrast, PLP-A, unlike IL-15, effectively suppresses the ability of NK cells to produce interferon-gamma (IFNgamma), a key mediator of NK cell function. Placental PLP-A expression is reciprocal to mesometrial decidua expression of IFNgamma. Increased expression of PLP-A by the placenta coincides with the decline of IFNgamma content in the mesometrial decidua adjacent to the placenta. In summary, trophoblast cell-derived PLP-A contributes to the regulation of NK cells at the maternal-fetal interface to ensure appropriate embryonic growth and development.

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.