Fas ligand expression by maternal decidual cells is negatively correlated with the abundance of leukocytes present at the maternal-fetal interface

The abundance of leukocytes at the maternal-fetal interface could influence the fate and invasion of extravillous trophoblasts. However, the mechanism(s) involved in determining the number of leukocytes present at the maternal-fetal interface as well as the nature of the interactions between invading fetal trophoblasts, maternal leukocytes and decidual cells are not well understood. In the present studies, we examined Fas ligand (FasL)/Fas expression at the maternal-fetal interface in human placental tissues of early pregnancy by immunohistochemistry. The types of cells and their localization were also characterized by specific cell markers (cytokeratin, vimentin and CD45 for trophoblast, decidual cells and leukocytes, respectively). The cells undergoing apoptosis and specific apoptotic trophoblasts were detected by TUNEL assay and M30 cytoDEATH immunostaining, respectively. Using single or double immunostaining, we found that FasL expression in decidual cells was negatively correlated with the number of Fas-expressing leukocytes in the same region. Furthermore, the density of leukocytes had an inverse relationship with the number of interstitial trophoblasts present in the same area. We observed also that extravillous trophoblasts are viable despite expressing Fas and being in close proximity to decidual cell-derived FasL. These data support our hypothesis that maternal decidual cell-derived FasL may be involved in preventing the recruitment of Fas-bearing leukocytes at the maternal-fetal interface through apoptosis induction by Fas/FasL interaction, thereby promoting trophoblast invasion.