Uterine natural killer cells in species with epitheliochorial placentation.

The epitheliochorial placenta represents the least intimate association between maternal and fetal tissues. The best known examples of this form of placentation are the domestic livestock species. Current information on the nature and proposed functions of uterine lymphocyte populations in ruminants (sheep and cattle), horses and pigs is presented. In ruminants unusual gamma delta T cells may play a role in mid to late gestation. During normal horse pregnancy, fetally derived endometrial cup cells invade the uterine stroma and are destroyed by maternal leukocytes midway through gestation. Natural killer (NK) cells or lymphokine-activated killer cells may be involved in this process, but the presence of these cell types in the equine uterus has yet to be established. The pig is similar to the human and rodent in that NK-like cells are present in the uterine stroma and seem to play a role in pregnancy. These cells are activated during early placental development and may be important in early interactions between the conceptus and the maternal immune system. The contribution of pregnancy-associated uterine lymphocytes to successful gestation has not been established. However, a common theme among these species is that the presence of the genetically foreign conceptus seems to activate uterine lymphocytes and to redirect their activities towards the promotion of fetal survival.