Investigation of the role of uterine NK cells in initiation of spiral artery transformation in normal human pregnancy

Spiral arteries are the blood vessels in the womb that transport maternal blood to the placenta and baby. Early in a normal pregnancy these blood vessels undergo radical changes in their structure (termed 'transformation' or 'remodeling'). The spiral arteries become wider and the structure of the wall is altered so that the thick muscle which normally makes up the artery wall is lost. The end result of this process is that the spiral arteries are able to transport the extra blood and oxygen which is needed for the placenta and baby. These changes in the spiral arteries of the womb are necessary for a successful pregnancy outcome. If this process does not occur then complications such as late miscarriage, fetal growth restriction (small babies), premature labor and preeclampsia (high blood pressure, protein in urine and oedema in the mother) can occur. Despite its importance for a successful outcome, very little is known about the process by which the spiral arteries in the womb are altered in normal pregnancy. During the first half of pregnancy cells from the placenta (trophoblast cells) invade the spiral arteries and it was thought for a long time that the changes in the uterine spiral arteries were due to these cells. Recently, however, it has become apparent that there are initial changes in the structure of the vessel wall which occur before the placental trophoblast cells are present. Evidence from mouse pregnancy indicates that these initial changes are mediated by a specialist white blood cell type which is present in large numbers in the lining of the womb in early pregnancy; these cells are called uterine natural killer cells. Some of the changes that we see in the spiral artery walls are similar to those seen during the early stages of growth of new blood vessels from existing ones, a process known as angiogenesis. There has been a lot of work on angiogenesis in relation to cancer and so we know many of the factors that are involved in the different steps of this process. Therefore, we know which factors are likely to be involved in initiating the changes we see in the blood vessels of the uterus. In a previous study we have shown that the uterine natural killer cells are the major source of most of these key factors in the uterus. We have also shown that numbers of these cells are lower in the womb of women who have small babies or preeclampsia where the remodeling of the spiral arteries is deficient. Therefore, we want to further explore the role of this cell type in initiating remodeling of the uterine spiral arteries. We plan to do this using a combination of techniques. We will use tissue taken from the placenta and the underlying uterine lining during early pregnancy as well as models of vessel remodeling in the laboratory. Initially we will examine tissues from the uterus of normal pregnancies in order to determine precisely what changes occur in the structure of the spiral artery wall in the early stages of the remodeling process. We will then go on to use laboratory models of vessels and the component cells which make up the vessels and try to find out how the uterine natural killer cells have their effect. We hope that by improving our understanding of the processes which start and control spiral artery remodeling in normal pregnancy we will then be able to understand and try to treat pregnancies that become complicated.

Complete remodeling ('transformation' or 'physiological change') of uterine spiral arteries (SpAs) is crucial for successful pregnancy in humans. Failure of this process has been associated with important complications of pregnancy. In early pregnancy placental trophoblast (EVT) invade into the uterus and the SpAs. It is generally accepted that EVT is required for complete SpA remodeling and this has been the focus of most studies. However, it is clear that there is an EVT independent 'priming' of the SpAs with dilatation, permeability, endothelial swelling and separation of vascular smooth muscle cells, amongst other changes. We propose that this priming initiates SpA remodeling and that uterine natural killer (uNK) cells are intimately involved in this process. Uterine NK cells are abundant in decidua lining the uterus in early pregnancy and reduce in number when the SpA changes are fully developed. We suggest that the remodeling process in SpAs is similar to early stages of angiogenesis and uNK cells secrete a range of key angiogenic growth factors including VEGF-C, Ang2, TGF beta 1 and IFN gamma. Our aim is to examine in detail the priming of the SpAs due to uNK cells. We suggest that these factors disrupt cell junctions between vascular endothelial cells, between vascular smooth muscle cells and between endothelial cells and smooth muscle cells. We will test this hypothesis in vivo using immunohistochemistry of placental bed biopsies from the first half of pregnancy. We will also use three in vitro models (endothelial cells and aortic smooth muscle cells, chorionic plate artery, ex vivo uterine SpA) to directly test the effect of uNK cells on vessels. The proposed project will improve understanding of the mechanisms underlying successful SpA remodeling in normal pregnancy and improve understanding of deficient SpA remodeling in pregnancy complications, such as second trimester miscarriage, preterm labor, fetal growth restriction and preeclampsia.