Endothelial cells in chorionic fetal vessels of first trimester placenta express HLA-G

Using four different HLA-G-recognizing monoclonal antibodies (mAb), we investigated whether this nonclassical HLA class I molecule could be expressed in placental cell types other than extravillous cytotrophoblasts (evct) in which HLA-G has already been detected. Immunohistochemical analysis was performed on serial cryosections of first trimester placenta as well as on maternal decidual tissue. In addition to some proliferative evct, the recently described BFL.1 mAb also slightly stained some villous cytotrophoblast (vct) stem cells located near cell columns and cell islands, which until now have been considered as HLA-G negative. The same staining pattern was obtained with the 16G1 mAb raised against the soluble HLA-G isoform, whereas neither 87G nor HCA2 reacted with vct but did strongly label the invasive populations of evct, including interstitial and endovascular trophoblasts. Surprisingly, BFL.1 strongly and reproducibly stained endothelial cells in the fetal capillaries present in the mesenchymal core of the chorionic villi, whereas none of the other surrounding cellular components were stained. The same specific labeling was obtained, although with less intensity, with the three other HLA-G-recognizing mAb. In contrast, maternal endothelial cells present in spiral arteries of the decidua parietalis remained unstained. This unexpected cellular localization suggests that HLA-G may be present as a soluble form during the whole period of fetal vascularization and/or exert a nonimmunological function related to the endothelial cell type, in particular in the angiogenesis process which is highly active, until term, in chorionic villi.