Paternal and maternal major histocompatibility complex class I antigens are expressed co-dominantly by equine trophoblast.

Invasive equine trophoblast cells of the chorionic girdle express high levels of paternally inherited Major Histocompatibility Complex (MHC) class I antigens prior to migration into the endometrium to form the so-called endometrial cups. Three groups of experiments were performed to determine if maternally inherited MHC class I antigens are expressed on chorionic girdle cells. Results indicated that maternally and paternally inherited MHC class I antigens are co-dominantly expressed by cells of the invasive equine trophoblast, and therefore, that the expression of polymorphic equine MHC class I genes does not appear to be affected by genomic imprinting in this tissue. The demonstration that cells of the chorionic girdle were immunogenic supports the hypothesis that invasion of the maternal endometrium by chorionic girdle cells stimulates the production of anti-paternal alloantibodies normally observed in early horse pregnancy. The co-dominant expression of MHC class I antigens by invasive chorionic girdle cells has important implications for the mechanism of recognition of allogeneic fetal MHC class I antigens by the maternal immune system.