FcRn-mediated antibody transport across epithelial cells revealed by electron tomography

Abstract

The Fc receptor of neonates (FcRn) mediates the apical-to-basolateral transcytosis of immunoglobulin G (IgG) across epithelial cells so that newborns are able to receive maternal antibodies. In this study Her et al. use electron tomography to visualize the transport of IgG during this process with a resolution of 4–6 nm. Individual FcRn ligands were identified inside intracellular organelles using a new gold enlargement technique compatible with high pressure frozen samples. The transcytosis pathways revealed show labelled-Fc moving through networks of vesicles as it migrates from the apical to basolateral surface. The neonatal Fc receptor (FcRn) transports maternal IgG across epithelial barriers1,2, thereby providing the fetus or newborn with humoral immunity before its immune system is fully functional. In newborn rats, FcRn transfers IgG from milk to blood by apical-to-basolateral transcytosis across intestinal epithelial cells. The pH difference between the apical (pH 6.0–6.5) and basolateral (pH 7.4) sides of intestinal epithelial cells facilitates the efficient unidirectional transport of IgG, because FcRn binds IgG at pH 6.0–6.5 but not at pH 7 or more1,2. As milk passes through the neonatal intestine, maternal IgG is removed by FcRn-expressing cells in the proximal small intestine (duodenum and jejunum); remaining proteins are absorbed and degraded by FcRn-negative cells in the distal small intestine (ileum)3,4,5,6. Here we use electron tomography to make jejunal transcytosis visible directly in space and time, developing new labelling and detection methods to map individual nanogold-labelled Fc within transport vesicles7 and simultaneously to characterize these vesicles by immunolabelling. Combining electron tomography with a non-perturbing endocytic label allowed us to conclusively identify receptor-bound ligands, resolve interconnecting vesicles, determine whether a vesicle was microtubule-associated, and accurately trace FcRn-mediated transport of IgG. Our results present a complex picture in which Fc moves through networks of entangled tubular and irregular vesicles, only some of which are microtubule-associated, as it migrates to the basolateral surface. New features of transcytosis are elucidated, including transport involving multivesicular body inner vesicles/tubules and exocytosis through clathrin-coated pits. Markers for early, late and recycling endosomes each labelled vesicles in different and overlapping morphological classes, revealing spatial complexity in endo-lysosomal trafficking.