Expression of the apoptosis-inducing Fas ligand (FasL) in human first and third trimester placenta and choriocarcinoma cells

The Fas (Apo-1/CD95) ligand (Fast) belongs to the tumor necrosis factor family and acts through its receptor (FasR/Apo-1/CD95) to induce apoptosis in target cells. Fast is expressed in several immunologically privileged sites. Induction of apoptosis by Fast in invading lymphocytes acts as a mechanism of immune privilege and is important in preventing graft rejection. Furthermore, Fast is expressed in certain malignancies and it has been implicated as a possible key mechanism in immune privilege of these tumors. Since the invading placental trophoblast is another very important site with a privileged immune status, we investigated whether Fast is expressed in the normal and tumoral human placenta. For this purpose, mRNA was extracted from first and third trimester human placental samples as well as from JEG3 choriocarcinoma cells and reverse transcribed to obtain cDNAs. These were used as templates for PCR analysis of Fast expression, in which specific primers were employed to amplify an 853 bp fragment spanning the whole Fast coding region. A product of the appropriate length was amplified from normal placenta as well as from the choriocarcinoma cells. Expression of Fast protein was confirmed by Western Blot and was localized to trophoblast by immunohistochemistry using a Fast-specific antibody. Expression of Fast in the human placenta indicates that induction of apoptosis in lymphocytes by the invading trophoblast could be an important mechanism implicated in the immune tolerance of the fetal semi-allograft.