Maternal immunological recognition of pregnancy in equids.

There is little evidence for maternal immunological recognition of pregnancy in most species with the striking exception of the members of the genus Equus. Almost all mares make strong cytotoxic antibody responses to paternally inherited fetal antigens by Day 60 of gestation. Most of these responses are directed against antigens of the Major Histocompatibility Complex (MHC), which constitutes the primary immunogenetic barrier to successful organ transplantation. The source of fetal MHC antigens in the pregnant mare appears to be the specialized trophoblast cells of the chorionic girdle region of the developing placenta. These cells invade the endometrium between Days 36 and 38 after ovulation to form the endometrial cups. The progenitor girdle cells express high levels of paternal MHC antigens, while the non-invasive trophoblast cells of the allantochorion and the differentiated trophoblast cells in the mature endometrial cups do not. This expression of MHC antigens by the chorionic girdle cells is unusual for a trophoblast tissue, and differs from most forms of trophoblast studied in other species. The maternal anti-fetal antibody responses of equine pregnancy do not adversely affect fetal development, nor are they required for successful pregnancy. However, it is possible that fetal loss of an immunological nature could occur in rare cases when the immunoregulatory mechanisms of pregnancy break down.