The Fc receptor for IgG expressed in the villus endothelium of human placenta ts Fc gamma RIIb2

To evaluate the potential role of human placental endothelial cells in the transport of IgG from maternal to fetal circulation, we studied Fc gamma re ceptor (Fc gammaR) expression by immunohistology and immunoblotting. Severa l pan-Fc gamma RII Abs that label the placental endothelium displayed a dis tribution pattern that correlated well with transport functions, being: int ense in the terminal villus and nil in the cord, In contrast, the MHC class 1-like IgG transporter, FcRn, and the classical Fc gamma RIIa were not exp ressed in transport-related endothelium of the placenta. Our inference, tha t Fc gamma RIIb 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 Fc gamma RIIb, These experiments showe d that the Fc gamma RII expressed in villus endothelium was the b2 isoform whose cytoplasmic tail is known to include a phosphotyrosyl-based motif tha t inhibits a variety of immune responses. We suggest that this receptor is perfectly positioned to transport IgG although as well it may scavenge immu ne complexes.