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 ma mmalian pregnancy, reciprocal migration of cells is limited. To evalua te 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 We stern blotting and immunohistology. FasL mRNA and protein were detecte d 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 de cidual 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 th e uterus and then in the placenta, to deter trafficking of activated F as(+) immune cells between the mother and the fetus. To test whether t he absence of functional FasL affects pregnancy, uteroplacental units from homozygous matings of gld mice, a mutant strain lacking functiona l FasL, were examined. Extensive leukocytic infiltrates and necrosis a t the decidual-placental interface were observed from day 10 onward, r esorption sites were common, and small litters were delivered by gld m ice. These observations are consistent with the idea that FasL at the maternal-fetal interface protects the placenta against a maternal leuk ocytic influx that reduces fertility.