NK cells and surveillance in humans

Natural killer (NK) cells are part of the innate immune system and help to protect against infections and tumour transformation by eliminating affected cells. NK cells become activated upon target cell recognition through the integration of signals provided by both activating and inhibitory receptors. Ligands recognized by these receptors include major histocompatibility complex class I, stress-induced molecules, adhesion proteins and other molecules that are used by NK cells to identify cells to be killed. This recognition constitutes the basis of NK immunosurveillance, and its full understanding is important for therapeutic purposes, such as haploidentical bone marrow transplantation for haematological malignancies. Human NK cells are also found abundantly in the uterine decidua during early pregnancy. Besides a detrimental role in reaction to the semi-allogencic fetus, these uterine NK cells help to create a balanced environment for the conceptus, influencing important processes such as blastocyst implantation, trophoblast invasion and spiral artery development. This review summarizes the different aspects of human NK cell biology implicated in immunosurveillance.