Extraspecific donkey-in-horse pregnancy as a model of early fetal death.

Transfer of donkey embryos to horse mares provides a useful model of early fetal death. Endometrial cups do not develop in this one type of extraspecific pregnancy and 80% of donkey fetuses are aborted between Days 80 and 100 of gestation in conjunction with abnormal implantation and an intense accumulation of leucocytes in the endometrium of the surrogate mare. Treatment of mares carrying donkey conceptuses with progestagen (allyl trenbolone) or purified horse chorionic gonadotrophin does not prevent abortion. However, passive immunization with serum from mares carrying intraspecific horse fetuses, or active immunization with donkey lymphocytes, causes a marked increase in fetal survival rate and the birth of live foals. Furthermore, both cytotoxic (rejection type) and immunoprotective maternal immune responses to the xenogeneic donkey fetus can be recalled in mares carrying repeated donkey-in-horse pregnancies. We suggest that the endometrial cup reaction in normal equine pregnancy provides a vital and temporally important antigenic stimulus which results in the mare mounting an immunoprotective response towards her allogeneic fetus in utero.