Expression of major histocompatibility complex (MHC) antigens on horse trophoblast

Antibodies to fetal major histocompatibility complex (MHC) antigens are routinely detected in the serum of pregnant mares some 2-4 weeks after formation of the endometrial cups at Day 36-38 after ovulation. Several experimental approaches were taken to determine whether paternal MHC antigens are expressed on horse placental tissues. First, absorption of anti-paternal MHC antisera with a large volume of endometrial cup cells removed antibody activity in only 2 of 4 experiments. Second, repeated immunization of horses with endometrial cup tissue recovered from a mare on Day 47 of pregnancy failed to induce the formation of anti-MHC antibodies. Third, a potent anti-MHC antiserum, raised in a pregnant mare which had previously received skin grafts from the MHC homozygous mating stallion, labelled chorionic girdle, but not normal allantochorion, when tested in an indirect immunoperoxidase labelling assay on tissues bearing the MHC antigens of the stallion. These results indicate that the rapidly dividing cells of the chorionic girdle, the progenitor tissue of the equine endometrial cups, express high levels of paternal MHC antigen, and may serve as the alloantigenic stimulus for cytotoxic antibody production by pregnant mares. Conversely, the mature, CG-secreting endometrial cup cells have a much reduced expression of paternal MHC antigen.