Human uterine natural killer cells.

The human uterine mucosa is infiltrated by large numbers of CD56+ natural killer (NK) cells which are particularly abundant around the time of implantation and during early pregnancy. These NK cells have the phenotype CD56bright CD16-mCD3- and the characteristic morphology of large granular lymphocytes. The NK cells are in close association with the trophoblast cells which invade into the uterus. The expression of HLA class I antigens, HLA-G and HLA-C, by these trophoblast cells raises the possibility that maternal NK cells can recognise and respond to the fetal trophoblast cells. Thus, the maternal-fetal interaction, and hence reproductive success, may depend on an NK allorecognition system.