Functional Role of Endosomes in Transcytosis of Immunoglobulin G

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To understand the mechanism of acquiring passive immunity, selective transcytosis of immunoglobulin G (IgG) in the rat yolk sac epithelial cell was studied by the fluorescence ratiometry and immunocyto-chemistry with using a confocal laser scanning microscope (CLSM).1.The internal pH of the sorting endosome was measured after endocytic labeling with fluorescently labeled dextran followed by the dual-fluorescence ratiometry using CLSM,The pH was 6.1 in good coincidence with the pH 6.0 previously found to be optimalfor the formation of IgG-Fcreceptor complex (IgG-FcR) in vitro.2. The intracellular transport pathways of transferrin (Tf) and IgG were examined and immunocytochemically compared with the localization of GTR-binding proteins rab4, rab 5A and rab8. The rab proteins except for rab8 were colocalized with Tf and/or IgG in the sorting and/or recycling endosomes. 3. The immunocytochemical examination was repeated after chasing Tf and IgG for 15-30 min in the Presence of an antibiotics brefeldin-A (10muM). Tf was accumulated to recycling endosomes (rab4 positive), while IgG was found in sorting endosomes (rab4/rab5A positive) and IgG-carrying transcytotic vesicles (rab4/rab5A positive, rab8 negative). Transcytosis of igG was apparently little affected by brefeldin-A.These results confirm the idea that IgG-FcR is transcytosed through the mildly acidic endosomes including sorting endosomes and IgG-FcR carrying vesicles. It is suggested that rab4 and rab5A are involved in regulation of fusion/fission events on the sorting endosome. A clathrin-and COPs-independent mechanism of sorting and transport of IgG-FcR is also suggested.