Natural killer cell activity from pregnant subjects is modulated by RU 486

Natural killer cells form an integral component of the body's innate immune system. Natural killer cell activity is reduced during pregnancy, especially in the latter half. To investigate the role progesterone may play in immunomodulating natural killer cell activity during pregnancy, we evaluated the effect of RU 486 on natural killer cells isolated from pregnant subjects. Natural killer cell activity was measured with an 18-hour, Chromium 51 release, microcytotoxicity assay with K-562 cells as target cells. We demonstrated that RU 486, in a concentration range from 5 to 40 mumol/L, augmented natural killer activity threefold to fivefold over baseline. This augmentation of activity was suppressed to baseline by the addition of excess progesterone. The addition of hydrocortisone resulted in an insignificant reduction in this augmented activity. This study suggests that progesterone may play a role as an immunomodulating factor in maternal acceptance of the fetal allograft.