Functional Expression of the Human Neonatal Fc-receptor, HFcRn, in Isolated Cultured Human Syncytiotrophoblasts

Materno-fetal IgG transfer in the mature human placenta involves transport across the syncytiotrophoblast (STB) and fetal endothelial cell layer. The MHC class I-related Fc gamma-receptor (hFcRn) localized in STB as well as in endothelial cells is involved in overall IgG transfer from the maternal into the fetal circulation. Functional hFcRn is a heterodimer of a transmembrane alpha-chain and beta2-microglobulin. To establish the basis for future studies to unravel the mechanism of IgG transport in STB, we investigated hFcRn alpha-chain and beta2-microglobulin expression in cytotrophoblasts (CTB) isolated from human term placentae and cultured in vitro under conditions where differentiation into multinuclear STB takes place (>or=48 h). Northern blot analysis demonstrated up-regulation of alpha-chain mRNA after 48 h of in vitro cultivation. Likewise, hFcRn alpha-chain and beta2-microglobulin were at the limit of detection by immunofluorescence microscopy in CTB immediately after isolation, but their expression increased upon STB formation. hFcRn alpha-chain co-localized with beta2-microglobulin in multinuclear STB and formed a functional, i.e. low pH IgG binding, receptor as shown by affinity isolation. The in vitro differentiated STB exhibited specific, low pH-dependent IgG binding to the plasma membrane. In conclusion, these cultures can now be applied to study the role of hFcRn in IgG transport and trafficking in STB cultures in vitro.