Uterine NK cells and macrophages in pregnancy.

The presence of immune cells in the placental bed is important for both mother and child. Although various immune cells can be found in the placental bed, such as regulatory T cells and dendritic cells, uterine NK cells and macrophages are the most prominent immune cells in the placental bed in early pregnancy. uNK cell and macrophage numbers in the placental bed decrease in the third trimester. These cells seem to be specifically adapted for their function and environment. uNK cells do not show cytotoxic activity, but are producers of cytokines, growth factors and many other factors. uNK cell function is regulated by inhibitory and activating receptors binding to HLA class I on trophoblast cells. uNK cells are also involved in regulating trophoblast invasion. Macrophages mainly show an M2-like phenotype and also produce cytokines and various other factors. They are important in phagocytosis of various cells and cell debris in the placental bed. Both cell types are also involved in angiogenesis and spiral artery remodeling in the placental bed. In this review we will elaborate on the most important functions of uNK cells and macrophages in the placental bed in humans. We will also discuss animal models, since they may provide clues for function of uNK cells and macrophages in humans.