Expression of major histocompatibility complex (MHC) class 1 molecules on early trophoblast

The expression of class I Major Histocompatibility Complex (MHC) molecules by early trophoblast of intraspecific horse and donkey, interspecific mule and extraspecific donkey-in-horse conceptuses was determined using a rat monoclonal antibody (MAC 291) in a peroxidase anti-peroxidase immunohistochemical technique. Most non-invasive allantochorion of horse, donkey and mule conceptuses did not express class I MHC molecules at any stage of gestation except in small isolated patches of pseudostratified trophoblast lying adjacent to the openings of endometrial glands. In contrast, MHC class I molecules were expressed strongly on horse chorionic girdle cells at Days 33 and 34 of gestation, just prior to their invasion. However, class I MHC was down-regulated with the differentiation of these girdle cells into mature gonadotrophin-secreting endometrial cup cells between Days 40 and 45 so that by Days 55-65, class I molecules were no longer detected on endometrial cups. Similarly, all endometrial cups originating from 3 intraspecific donkey conceptuses at Days 41, 59 and 82 and 2 interspecific mule conceptuses at Days 46 and 47 were negative for class I molecules. A total of 7 extraspecific donkey-in-horse pregnancies, in which no endometrial cups from and implantation is abnormal, were established by embryo transfer. The chorionic girdle recovered from a single donkey-in-horse conceptus at Day 35 of gestation stained strongly for MHC class I molecules. Later in gestation (Days 73-91) and in contrast to the other forms of equine pregnancy examined, most unimplanted, failing donkey allantochorion was strongly stained for MHC class I molecules and had large numbers of lymphocytes in the adjacent endometrial stroma. The hypothesis is raised that the mechanisms that normally suppress the expression of MHC class I molecules by the epithelial trophoblast layer of the equine placenta can only function if the apical surface of the cells is in close and stable contact with other tissues such as the endometrial epithelium.