Immunological memory and tolerance at the maternal-fetal interface: Implications for reproductive management of mares

The development of placentation that coincided with the evolution of mammals presented new challenges to the transmission of life from one generation to the next, particularly with regard to the possibility of maternal immunological recognition and destruction of the developing conceptus. The balance between immunity and tolerance dominates the immunological relationship between mother and fetus during mammalian pregnancy, and the focal point of this relationship lies at the interface between the trophoblast cells that comprise the outermost layer of the placenta and the maternal endometrial tissues. Immune memory and tolerance are two of the cardinal characteristics of the immune system. Immune memory is essential in preventing or lessening the effect of infections to the mother or conceptus, but may also be a threat to the semi-allogeneic tissues of the fetus and placenta. The mother must develop functional immune tolerance to her fetus, but at the same time retain her ability to combat infections while pregnant. To address this imperative, mammals have developed overlapping and independent mechanisms for evading maternal anti-fetal immune responses that could result in pregnancy loss. Studies of the unusual component of equine invasive trophoblast in the epitheliochorial placenta have illuminated aspects of immune memory and tolerance that have relevance to fertility in the horse and other mammalian species.