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Mice in which the gene that encodes the receptor (R) for leukemia inhibitory factor (LIF) has been deleted show abnormal growth and development of the placenta. This indicates that LIF plays an important role in placental development. The expression of LIF-R and LIF was examined in human trophoblast and decidua using in situ hybridization and immunocytochemistry. LIF-R mRNA and immunoreactivity was localized in villous and extravillous trophoblast throughout pregnancy, and in endothelial cells of the fetal villi. Strong expression of mRNA encoding LIF was detected in decidual leukocytes, which are abundant at the implantation site. Extravillous trophoblast, which invades the maternal decidua, therefore expresses LIF-R as it moves past decidual leukocytes, which express LIF mRNA. The effect of LIF on cultured human trophoblast was examined in vitro. Recombinant human LIF had no effect on [3H]thymidine incorporation by purified extravillous trophoblast, nor on expression of integrins alpha1, alpha5, or beta1 by isolated trophoblast. These results identify fetal endothelial cells and all cells of the trophoblast lineage as targets for the action of LIF in human placenta. Although its effects on trophoblast are not yet clear, LIF appears to mediate interactions between maternal decidual leukocytes and invading trophoblast. LIF may also play a critical role in controlling angiogenesis in the placental villi, since human fetal endothelial cells express LIF-R, and mice lacking a functional LIF receptor gene show altered vascular development in the placenta.

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Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a classical haematopoietic cytokine which has been implicated in placental growth and development. In this study, we investigated the production of GM-CSF in human first trimester pregnancy by the predominant uterine lymphocyte population of decidual CD56+ NK cells (large granular lymphocytes) and the factors that influence this production using enzyme-linked immunosorbent assays (ELISAs) and bioassays, supplemented by immunocytochemistry. We have also investigated and compared production of GM-CSF by human first trimester trophoblast and by JEG-3 and JAR choriocarcinoma cells. Our data show that appreciable amounts of GM-CSF are produced in first trimester maternal decidua and that a significant component of this secretion was from decidual large granular lymphocytes (LGL). Production of GM-CSF by LGL was constitutive and considerably greater than that of freshly isolated peripheral blood leukocytes. GM-CSF secretion by decidual LGL could be enhanced by co-culture on a monolayer of decidual stromal cells, and could also be increased in a dose-dependent manner by stimulation with interleukin-1 (IL-1) or IL-2. IL-4, IL-6, tumour necrosis factor alpha (TNF alpha), transforming growth factor beta (TGF beta), interferon alpha (IFN alpha) and IFN gamma individually had no effect on GM-CSF secretion, although IL-4, TGF beta and IFN alpha all inhibited the action of IL-2. IFN gamma had no effect on the IL-2-induced GM-CSF secretion, but did antagonize the action of IL-1. Normal human first trimester trophoblast was also found to produce GM-CSF, although no production whatsoever was seen by JEG-3 or JAR choriocarcinoma cells. These results suggest that GM-CSF from uterine lymphocytes, and from trophoblast itself, may influence placental growth and development in both a paracrine and an autocrine manner.

The main population of lymphocytes found in the human decidua during early pregnancy are NK-like cells with a distinctive phenotype, CD56bright CD16- CD3-. These cells are in close association with invading trophoblast that may be their in vivo target. We have examined three aspects of decidual NK function in vitro: cytotoxicity, proliferation, and cytokine production. The functional assays indicate uterine lymphocytes differ fundamentally from both PBL and even from classical circulating NK cells. Their role in the establishment of normal pregnancy remains unknown.