Human uterine leukocytes and pregnancy

In human pregnancy, the embryo implants into the specialized mucosal wall of the uterus (decidua) and the placenta starts to form. Cells from the placenta (trophoblasts) invade into the uterine mucosa in order to open up maternal uterine arteries to ensure an adequate supply of blood to the developing fetus. The trophoblasts have a unique immunological phenotype compared to most cells especially with regard to their expression of major histocompatibility complex (MHC) antigens. On the other side of the interaction, the uterine mucosa (endometrium) differentiates in preparation for implantation. One of the changes that takes place is the appearance in the endometrium of a large number of maternal leukocytes in the final part of the menstrual cycle. If pregnancy ensues, these leukocytes continue to increase in number and are found in close contact with trophoblasts. The composition of this population of maternal immune cells is unusual compared to that seen at other mucosal sites. A lot of research has focused on whether maternal T-cell responses are suppressed or modified during pregnancy. Research has also concentrated on the specialized uterine natural killer (NK) cells, which are found in the decidua in large numbers during early pregnancy. These uterine NK cells have been shown to express receptors for trophoblast MHC antigens, but their role in pregnancy is still mysterious. The purpose of this review is to give an overview of what is known about the immunology at the implantation site and also to provide an update of some of the most recent findings in this field.