LACK OF HLA-G EXPRESSION IN A POPULATION OF EXTRAVILLOUS TROPHOBLAST IN NORMAL PLACENTA

HLA-G, one of the HLA class Ib genes, shows limited expression in placental trophoblasts, notably those at the maternal-fetal border, and is thought to be involved in protecting the baby from maternal immune rejection. Using HLA-G mono- clonal antibody 87 G in histochemical staining, we have investigated HLA-G expression in first, second and third trimester placenta from a normal pregnancy, and in third trimester placenta from preeclampsia. These studies showed that HLA-G was expressed only on extravillous trophoblast cells (EXT) at all stages of pregnancy in normal or preeclamptic placentas. However, a subset of EXTs that showed a complete lack of expression of HLA -G in normal placenta was found. These EXTs expressed cytokeratin only on the cell membrane and in cytoplasm not stained with eosin, whereas EXTs expressing HLA-G showed cytokeratin expression on the whole cell including the cytoplasm stained with eosin. In preeclamsia placentas, populations of EXTs lacking HLA-G expression were also found. Interestingly, their characteristics on staining anti-cytokeratin and eosin were similar to EXTs lacking HLA-G expression in normal placenta. Therefore, no significant differences in HLA-G expression were found between normal and preeclamptic placentas. EXT is classified to the intermediate differentiation stage from cytotrophoblast (CT) to syncytiotrophoblast (ST) and EXTs can be found at various stages of differentiation at any stage of placental development. It appears then that EXT differentiation may influence the expression of HLA-G. The existence of EXTs lacking HLA-G expression in normal placenta suggests that other molecules, independent of HLA-G, may function to keep a fetus from an undesirable maternal immune response