The Fc Receptor for IgG Expressed in the Villus Endothelium of Human Placenta Is FcgRIIb21

To evaluate the potential role of human placental endothelial cells in the transport of IgG from maternal to fetal circulation, we studied Fcg receptor (FcgR) expression by immunohistology and immunoblotting. Several pan-FcgRII Abs that label the pla-cental endothelium displayed a distribution pattern that correlated well with transport functions, being intense in the terminal villus and nil in the cord. In contrast, the MHC class 1-like IgG transporter, FcRn, and the classical FcgRIIa were not expressed in transport-related endothelium of the placenta. Our inference, that FcgRIIb was the likely receptor, we confirmed by analyzing purified placental villi, enriched in endothelium, by immunoblotting with a new Ab specific for the cytoplasmic tail of FcgRIIb. These experiments showed that the FcgRII expressed in villus endothelium was the b2 isoform whose cytoplasmic tail is known to include a phosphotyrosyl-based motif that inhibits a variety of immune responses. We suggest that this receptor is perfectly positioned to transport IgG although as well it may scavenge immune complexes. The Journal of Immunology, 2001, 166: 3882–3889. T he human placenta transports maternal IgG to an other-wise Ab-deficient fetus. This transport mechanism isthought to be an active process on the basis of two con-siderations: first, on the establishment late in gestation of a signif-icant fetal-to-maternal IgG concentration gradient; and second, on the selectivity of the process for IgG but not the other Ig molecules (1–3). The placental barrier between maternal and fetal circulator