Fas ligand is positioned in mouse uterus and placenta to prevent trafficking of activated leukocytes between the mother and the conceptus.

Despite intimate juxtaposition of maternal and fetal tissues during mammalian pregnancy, reciprocal migration of cells is limited. To evaluate the postulate that cell traffic is restricted by expression of Fas ligand (FasL) in the uterus and placenta, FasL mRNA was identified by using reverse transcription-PCR, and FasL protein was identified by Western blotting and immunohistology. FasL mRNA and protein were detected at all stages tested (gestation days (g.d.) 6-18). At g.d. 6 to 10, immunoreactive FasL was prominent in glandular epithelial cells and decidual cells. Between g.d. 12 and 14, expression shifted to placental trophoblast cells bordering maternal blood spaces and fetal placental endothelial cells. Thus, FasL is appropriately positioned, first in the uterus and then in the placenta, to deter trafficking of activated Fas+ immune cells between the mother and the fetus. To test whether the absence of functional FasL affects pregnancy, uteroplacental units from homozygous matings of gld mice, a mutant strain lacking functional FasL, were examined. Extensive leukocytic infiltrates and necrosis at the decidual-placental interface were observed from day 10 onward, resorption sites were common, and small litters were delivered by gld mice. These observations are consistent with the idea that FasL at the maternal-fetal interface protects the placenta against a maternal leukocytic influx that reduces fertility.