The human placenta is a semi-allogeneic tissue whose growth and development requires tolerance by the maternal immune system. Placental cells come in close contact with maternal blood and uterine tissues yet are able to evade immune recognition during the course of a normal pregnancy. The maternal immune system faces a challenge during pregnancy: to maintain tolerance toward foreign fetal alloantigens while simultaneously staging a response to potential pathogens at the maternal-fetal interface. The mechanisms through which placental cells evade maternal immune recognition are poorly understood, particularly in the context of human pregnancy. The uterine lining, called decidua, is a particularly understudied and important microenvironment, because it is the interface where placental cells called extravillous trophoblast (EVT) come in close contact with maternal immune cells, of which decidual natural killer (dNK) cells are the most abundant. EVT are highly invasive cells which are required for proper remodeling of the maternal uterine lining, including vascular remodeling which leads to establishment of maternal blood flow to the placenta. Interactions between placental EVT and decidual leukocytes are known to facilitate maternal vascular remodeling by EVT and limit the extent of EVT invasion into the uterine wall. Indeed, problems in preterm birth could result from inappropriate responses by dNK cells. Unfortunately, interactions between dNK and trophoblasts are difficult to study in an ongoing pregnancy, due to lack of access to the decidual compartment, where these important interactions occur. While animal models have offered some insights into these processes, they do not accurately model human placentation and pregnancy. This proposal aims to evaluate the decidual cell population in both term and preterm birth in a systematic, detailed manner, then to combine this knowledge with the latest technologies in regenerative medicine in order to develop in vitro models for the study of dNK-EVT interactions. Over the past few years, our collaborative team of investigators has established optimized methods for differentiation of pluripotent stem cells into both NK cells and EVT. We now propose to generate matched maternal and placental induced pluripotent stem cells (iPSC), and differentiate these cells into dNK cells and EVT, respectively, in order to model interactions between these two cell types. Successful completion of this proposal will establish a reproducible and manipulatable model system for studying interactions between the placenta and maternal immune system and has the potential to lead to identification of mechanisms through which abnormalities in these interactions increase the risk of idiopathic spontaneous preterm birth.