The effect of a progesterone-induced immunologic blocking factor on NK-mediated resorption

Transfer of spleen cells from poly (I)-poly(C12U)-treated Balb/c mice to 6.5-days-pregnant Balb/c mice significantly increased the resorption rate (P less than 0.01) from 11% to 48%. The supernatant of progesterone-treated spleen cells from pregnant C3H mice abrogated the effect of spleen cell transfer. We conclude that by blocking NK activity in vivo, the progesterone-induced blocking factor favors the maintenance of pregnancy.