Immunostimulation and the Promotion of Fertility

Abstract

One fact emerging from a number of studies of the maternal-fetal relationship is that the trophoblast—which forms the interface between the maternal and fetal circulations (see Figure 1)—is a privileged tissue with respect to transplantation rejection. Classic studies of Simmons and Russell (1962) showed that it is almost impossible to elicit a destructive reaction using transplanted allogeneic trophoblast. Recent striking experiments by Rossant and her colleagues (Clark et al., 1984; Rossant et al., 1982) and others (Fehilly et al., 1984; Meinecke-Tillman and Meinecke, 1984) emphasize the role of the trophoblast in protecting the fetus in situ. They have been able to produce viable adult interspecies chimeras by the construction of “Trojan horse” embryos. These reconstructed preimplantation embryos consist of an outer layer of trophectoderm that is syngeneic to the foster mother whose uterus is used for gestation. Trophectoderm is the layer that gives rise to the fetal part of the fetal-maternal interface in the placenta. The inner cell mass—which gives rise to the embryo proper—is all, or in part, derived from a xenogeneic embryo so that the resultant animal is completely or partly derived from a different species. This is only possible when the trophoblast is of the same species as the mother, as demonstrated by various control experiments. Although these studies do not elucidate what mechanisms are responsible for the protective role of the trophoblast, they certainly call attention to its existence.