Changes in NK cell activity during the estrous cycle and pregnancy in mice

Changes in NK cell activity during the estrous cycle and pregnancy in mice were studied by a 4 h 51Cr-release assay using YAC-1 cells as a target. NK activity both in the spleen and peripheral blood mononuclear cells showed a single fluctuation with a peak at met-estrous-2 during the estrous cycle. Splenic NK activity was suppressed in the early to mid-stages of pregnancy but increased sharply in the late stage. The activity declined thereafter, reaching the estrous level post-partum. NK activity in the peripheral blood showed a decrease throughout the entire pregnancy, but increased on Day 1 post-partum, returning to the estrous level thereafter. To elucidate the mechanisms involved in these variations of NK activity, NK-enriched and non-adherent cells were prepared from the spleens of mice at estrous and met-estrous-2 and those at early and late stages of pregnancy and then examined for NK activity. The results showed that there was no difference in the cytotoxic activity among these purified NK cells. Adherent cells purified from the spleens of mice in the early stage of pregnancy when co-cultured with the non-adherent NK cell fractions suppressed the NK cytotoxicity. These results strongly suggest the possibility that phagocytic and/or adherent cells may be involved in the regulation of splenic NK activity during the estrous cycle and pregnancy.