Fetal endothelial cells in full-term placenta, but not in first trimester placenta, express FcγRIIb2 mRNA



In the human placenta, IgG-transport from maternal to fetal blood starts in the second trimester and continues through the third trimester. There are two cellular barriers for IgG-transport: the first and second barriers are syncytiotrophoblast and fetal endothelial cells (ECs), respectively. Sorting via FcRn is considered the IgG-transporter in the first barrier. The mechanism of IgG-transport in the second barrier is not fully understood. Recently, we reported a novel FcγRIIb-containing compartment that may serve as an IgG-transporter in the second barrier (Mol Biol Cell 13 (suppl) 548a, 2002). To further investigate the feasibility of the FcγRIIb-vesicle involvement in IgG-transport in placental ECs, we have studied FcR-expression in the developing human placenta by RT-PCR and direct sequencing analysis. First trimester and full-term placentas were used. We proved that the FcγRII expressed in villus ECs in full-term placenta was the b2 isoform. FcγRIIb2 mRNA expression could not be detected in first trimester placenta, while it was expressed in full-term placenta. In contrast, FcRn mRNA expression was detectable in first trimester placenta as well as in full-term placenta. These results seem consistent with our hypothesis that in first trimester placenta IgG is transcytosed via FcRn across the first barrier but IgG cannot be transported in the second barrier that does not express FcγRIIb2 and that the FcγRIIb2-positive compartment is critical for IgG-transport in the human placenta.