[Transcytosis of immunoglobulin IgG]

Recent studies on cellular mechanism of transmission of maternal IgG are briefly reviewed. The mechanism has been studied in rodent intestine and yolk sac, where the role of neonate Fc receptors (FcRn) in the transcellular transport of IgG is now well understood. FcRn binds IgG on the cell surface (intestine) or in endosomes (yolk sac) at around pH 6.0. The IgG-FcRn complex is sorted into vesicles from the apical endosome, and transported basolaterally to release IgG extracellularly. In the human placenta, Fc gamma RIII and MHCI-related human Fc receptors (hFcRn) are expressed in the syncytiotrophoblast and are involved in the specific binding and transcytosis of IgG. Small GTP-binding proteins rab4 and/or rab5A are immunohistochemically localized on recycling and/or sorting endosomes, suggesting their involvement in forming vesicles transporting the IgG-FcR complex. Several other GTP-binding proteins and SNAREs are assumed to regulate this process, but remain to be identified. The cellular mechanism of IgG transcytosis, including endocytosis, sorting, fusion and exocytosis may be elucidated in terms of regulator proteins in near future.