Normal murine and porcine embryos recruit NK cells to the uterus.

Decidual NK cells, indistinguishable from those found in lymphoid tissues, are present in cell suspensions prepared from maternal decidua of random-bred mice between Days 6.5 and 10.5 of first gestation. The stringency of the correlation between NK cells and normal embryos during successful pregnancy is unknown. Our previous finding that active NK cells were unable to mediate lysis of fresh embryonic tissues at any stage during gestation suggests that if NK cells play a functional role in normal pregnancy it would be a noncytolytic role. Before studies on the function of uterine NK cells were undertaken, evidence that the association of NK cells with normal embryos is widespread was sought by assessing NK cell activity in cell suspensions from decidua of syngeneically mated mice, from decidua of multiparous, random-bred mice, and from the endometrium of pigs during first pregnancy. Neither parity nor maternal-fetal compatibility changed the pattern of high levels of decidual NK cell activity early in pregnancy followed by decline. Porcine NK cell activity was not detected in uterine cells isolated from cycling pigs but in pregnant animals it gradually increased during the preattachment period and reached levels greater than those in blood, postattachment (Day 28). Some of this activity was hormone dependent but sustained increases in NK cell activity required the presence of an embryo. These studies demonstrate that the association of functional NK cells with normal embryos is widespread during early pregnancy.