A progesterone-induced blocking factor corrects high resorption rates in mice treated with antiprogesterone.

Earlier we showed that because of the presence of functional progesterone receptors, lymphocytes of healthy pregnant women produced an immunomodulatory protein in the presence of progesterone, whereas those of nonpregnant persons did not. Progesterone-treated murine pregnancy lymphocytes release a similar factor. The present study reveals the biologic significance of this finding. Treatment of BALB/c mice that were 8 days pregnant with a progesterone receptor blocker (RU 486) resulted in 100% resorption of the fetuses. Simultaneous administration of the supernatant from progesterone-treated murine pregnancy spleen cells restored the resorption rate to the original 6% observed in untreated control animals. These data suggest that functional lymphocytic progesterone binding sites are needed for the maintenance of normal pregnancy. Because of the blockage of progesterone receptors and the consequent inability of the lymphocytes to produce the progesterone-induced blocking factor, abortion is initiated by immune factors. The fact that administration of the preformed blocking factor counteracted the effect of antiprogesterone treatment suggests that progesterone-mediated immunosuppression is needed for the maintenance of normal gestation.