Expression of functionally active FcRn and the differentiated bidirectional transport of IgG in human placental endothelial cells

The mechanism of selective transport of the immunoglobulins G from the placental struma to che lumen of the fetal blood vessels has not been elucidated yet. It was postulated that the specific transport as well as the regulation of IgG level ill the blood, involves the MHC class I related receptor FcRn for the Fc domain of IgG. We questioned whether human placental endothelial cells (HPEC) express FcRn and, if present, whether it is in a functionally active form. The experiments were performed on cultured HPEC and as positive control, human trophoblastic (JEG3) and mouse endothelial cells (SVEC) were used. Expression of FcRn, was demonstrated by indirect immunofluorescence and RT-PCR. The role of FcRn was assessed by quantifying the transcellular transport of [I-125]-hIgG or [I-125)-rF(ab')(2) fragments from the apical to basolateral surface, and in the reverse direction of HPEC grown on filters in a double chamber system. The intracellular pathway of FcRn or IgG was examined by electron microscopy using the proteins adsorbed to 5 nm and 20 nm colloidal Sold particles, respectively. The results showed that: (a) FcRn is expressed by human placental endothelial cells, in a functionally active form; ib) transcytosis of IgG in HPEC is a rime-dependent process that cakes place preferentially from the basolateral to the apical compartment; and (c) both IgG and FcRn colocalize in an intracellular endocytic compartment, chloroquine sensitive. Together these data suggest that the regulation of IgG level by endothelial cells may result from interplay between salvaging, exocytosis, and transcytosis of the molecules. One can assume that IgG that does not bind to FcRn may be destined For destruction, and this would explain the mechanism by which IgG homeostasis is maintained. (C) American Society for Histocompatibility and Immunogenetics, 2001. Published by Elsevier Science Inc.