DISTRIBUTION OF FC-GAMMA RECEPTORS ON TROPHOBLAST DURING HUMAN PLACENTAL DEVELOPMENT - AN IMMUNOHISTOCHEMICAL AND IMMUNOBLOTTING STUDY

Expression of Fcgamma receptors on human placental trophoblast was inv estigated by immunostaining and immunoblotting using a panel of Fcgamm a receptor monoclonal antibodies (mAb). Fcgamma receptors typical of o ther cell types were not detected on syncytiotrophoblast in term place ntae when transplancental IgG transport was maximal. Unexpectedly, how ever, and by contrast with term, all Fcgamma receptor III mAb tested b ound to first trimester placental syncytiotrophoblast by immunostainin g. Reactivity was relatively restricted and varied between specimens. Fcgamma receptor III products of 41,000-45,000 and 49,000-52,000 MW we re consistently detected on first trimester tropboblast membranes by i mmunoblotting and levels of these products were greatly reduced follow ing treatment with phosphatidylinositol-specific phospholipase C, sugg esting that the early trophoblast Fcgamma receptor III is glycosyl-pho sphatidylinositol (GPI) linked. The mAb Leu-11b behaved differently to other anti-Fcgamma receptor III mAb examined. By immunostaining, Leu- 11b bound to syncytiotrophoblast at term and detected both syncytiotro phoblast and underlying cytotrophoblast in the first trimester. In add ition to the GPI-anchored Fcgamma receptor III in first trimester, Leu -11b also detected a 74,000 MW component on both first trimester and t erm trophoblast membranes by immunoblotting. Thus trophoblast appears to express a GPI-anchored Fcgamma receptor III in first trimester but not term placentae. With the exception of the 74,000 MW Leu-11b-define d product whose function is unclear, currently available Fcgamma recep tor mAb appear to be incapable of detecting the protein involved in Ig G transport during the later stages of gestation.