Tolerance of the fetal allograft enables the human conceptus to implant itself into the maternal uterus and survive and grow there. This tolerance phenomenon remains largely obscure, notably because it appears to be controlled by multiple mechanisms. CD95 ligand (CD95-L), which can trigger death of CD95-positive cells by apoptosis, may participate in inducing anti-fetus-sensitized CD95-positive T lymphocytes to enter apoptosis. Using immunohistochemistry (first trimester and term placentae), FACS assays (term placenta) and RT-PCR assays (term placenta), the presence of CD95-L protein and mRNA has been shown in crude placental tissue preparations and isolated placental cells. Among the latter, CD95-L expression was detected in trophoblastic cells, fetal blood cells (mRNA only) and also the Hofbauer macrophages. No CD95-L was detected in fibroblasts or fetal endothelial cells. Thus trophoblastic cells, Hofbauer macrophages, and perhaps also fetal blood cells could form a sequential barrier blocking maternal activated defence cells bearing CD95 molecules.
