The role of natural killer cells in the immune homeostasis of the maternal fetal interface

Successful conception, implantation and pregnancy require a complex and organized communication between the embryonal (allograft) and the maternal (host) immune system. Different leukocyte subsets have an important role in orchestrating the immune response at the fetal-maternal interface. There are certain similarities between the immune invasion of tumor cells and the physiological invasion of the trophoblastic cells of embryonic origin into the maternal decidua. The decidual natural killer cells are a special subset of natural killer cells and alongside with macrophages and dendritic cells, they are part of the innate immune system therefore they are the first immune cells contacting any intruder whether it is a tumor or embryonic tissue. Interestingly decidual natural killer cells not only do not eliminate invasive trophoblastic cells, but specifically promote their progression. Their angiogenic activity facilitates and coordinates local vascular remodeling of the forming placenta. In this article we review the different nature of trophoblastic cell and decidual natural killer cell interaction, the role of decidual natural killer cells in the vascularization and immune homeostasis of the decidua.