Expression of allogeneic MHC class I antigens by transgenic mouse trophoblast does not interfere with the normal course of pregnancy

Mammalian embryos express paternal histocompatibility antigens which make t hem potential targets for maternal immune responses. Yet, the histoincompat ible fetus survives and develops normally. Down regulation of classical MHC antigen expression by trophoblast cells which are in direct contact with m aternal circulation has been repeatedly shown. The trophoblast cells are un able to function properly in antigen presentation and do not induce allogen eic rejection reactions. In the present study we have created transgenic mi ce that express an allogeneic class I transgene whose transcription is cont rolled by the transferrin receptor promoter. The expression patterns of the transgene product mice from a single transgenic line were studied in each of the typical placental subpopulations. The allogeneic class I antigen was expressed in the allantoic plate region of the trophoblast, and this expre ssion was not restricted to the endothelial region but extended also to the spongiotrophoblast, as well as the major blood vessels and in the endoderm al sinuses. In contrast to the normal class I expression, prominent levels of allogeneic H-2 antigens were detected in the labyrinthine trophoblast. T he fetal resorption rate in females mated with these transgenic males was n ot higher then the normal rate, and the embryos survived and developed norm ally. These data imply that the unusual expression of allogeneic class I an tigens in certain trophoblast subpopulations does not affect fetal developm ent.