Figure 1.

Schematic representation of the maternal-fetal interface and its immunologicalplayers. Trophoblast cells (fetal) and endothelial cells (maternal) express sFlt1. The placental villi are shown in the right side of the image and decidua on the left. Invading trophoblast cells will encounter maternal complement system (C) in the decidua and in the intervillous space. Invading extravillous trophoblasts express HLA-C and HLA-G receptors (in yellow and orange, respectively), expression of HLA-G by villous trophoblast cells in the placenta decreases during the course of the pregnancy (7). Successful trophoblast invasion will extend to the vascular layers of myometrium and invade the uterine spiral arteries, where endovascular trophoblast cells (EvTC) will replace endothelial cells (E) causing remodulation and relaxation of the spiral artery to allow for non-turbulent high volume low pressure circulation into the intervillous space. Interstitial trophoblast cells (ITC) will remain in the maternal tissue creating tolerance of the fetal tissue in the maternal immune system. Hofbauer cells (HC) are the predominant immune cell population in the villi throughout placental development. The decidual immune cell population consists of macrophages (M), natural killer cells (NK), and populations of T-cells (T). Tolerance inducing Treg and Breg cells in particular are essential for a healthy pregnancy (8). Figure adapted from Lokki 2017 PhD thesis (9).