Ectopic transplantation of equine invasive trophoblast

A system for transplanting invasive equine trophoblast (i.e., chorionic gir dle) to ectopic sites has been developed as a means to study the differenti ation of this tissue and to assess maternal immune responses to the concept us tissue in a site outside the uterus. Chorionic girdle was isolated from Day 33 to 34 conceptuses and surgically placed into the vulvar mucosa or su bdermal skin of recipient mares. Biopsy specimens of the graft sites for im munohistochemical staining were taken at weekly or biweekly intervals after grafting. Serum samples were collected from each recipient and tested for antibody to donor major histocompatibility complex (MHC) class I antigens u sing the lymphocyte microcytotoxicity assay. Transplanted trophoblast cells expressed differentiation markers associated with invading chorionic girdl e and endometrial cup cells. The transplanted trophoblast cells were also l abeled by an antibody to eCG. Strong cellular and humoral immune responses to the transplanted tissue were mounted by the recipients, similar to those occurring during normal equine pregnancy. Despite these responses, the inv asive trophoblast transplants survived for at least 28 days after grafting and downregulated MHC class I antigens, as do the mature endometrial cup ce lls in equine pregnancy. These findings suggest that invasive equine tropho blast has the capacity to differentiate fully in equine nonuterine tissues, and that it can evade maternal immune responses independent of the physiol ogical state of pregnancy and in sites other than the uterus.