Temporal and regional regulation of major histocompatibility complex class I expression at the bovine...

In most mammals trophoblast cells do not express major histocompatibility complex (MHC) antigens. This probably protects the placenta from immune attack. We have used immunohistochemistry to study the ontogeny of MHC class I expression by bovine trophoblast and endometrial epithelial cells. The interplacentomal, placentomal arcade and placentomal villous/crypt regions were studied. In the interplacentomal region a substantial proportion of trophoblast cells were class I positive from the sixth month on and about half of the endometrial epithelium was class I positive throughout pregnancy. In the arcade region trophoblast class I expression was first observed in the sixth month, increased slowly and peaked at term. Here there was no endometrial epithelial class I expression until term and then only a small percentage of cells were positive. In contrast, in the placentomal villous/crypt region both trophoblast and endometrial epithelium were class I negative throughout gestation. This study shows that cattle have extensive trophoblast class I expression. Moreover class I expression on placentomal, cryptal endometrial epithelium is shut down. Because binucleate trophoblast cells migrate and fuse with endometrial epithelial cells, total shut down of class I expression in areas of intimate interdigitation may be critical for avoidance of immunological rejection.