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Articles in PresS, published online ahead of print August 15, 2001
Am J Physiol Renal Physiol, 10.1152/ajprenal.0164.2001
Submitted on May 25, 2001
Accepted on August 6, 2001
1 Department of Internal Medicine, Juntendo University School of Medicine, Division of Nephrology, 2-1-1 Hongo, Bunkyo-ku, Tokyo, Japan; Juntendo University School of Medicine, Atopy (Allergy) Research Center, 2-1-1 Hongo, Bunkyo-ku, Tokyo, Japan
2 Juntendo University School of Medicine, Atopy (Allergy) Research Center, 2-1-1 Hongo, Bunkyo-ku, Tokyo, Japan
3 Department of Internal Medicine, Juntendo University School of Medicine, Division of Nephrology, 2-1-1 Hongo, Bunkyo-ku, Tokyo, Japan
* To whom correspondence should be addressed. E-mail: yasu{at}med.juntendo.ac.jp.
In the kidney, proteins filtered through glomeruli are reabsorbed by endocytosis along the proximal tubules to avoid renal loss of large amounts of proteins. Indeed, various proteins such as albumin, ß2- microglobulin and several hormones are reabsorbed via receptor-mediated endocytosis in the proximal tubules. Recently, neonatal Fc receptor (FcRn) that is involved in the transport of IgG across several epithelial and endothelial cells was reported to be expressed in renal proximal tubular epithelial cells. It was suggested that FcRn may play an important role in the reabsorption of IgG from the tubular fluid. However, to date there has been no direct evidence for receptor-mediated endocytosis of IgG in the renal proximal tubular epithelial cells in humans. To explore physiological roles of FcRn in the proximal tubules, we examined IgG transport using the human renal proximal tubular epithelial cells (RPTECs). FcRn was expressed in RPTECs and physically associated with ß2- microglobulin, preserving the capacity of specific pH-dependent IgG binding. Both cell surface and intracellular constitutive expression of FcRn was detected with a specific antibody to FcRn by immunofluorescence. Human IgG was bound to the cell surface of RPTECs in a pH-dependent manner. The human IgG transport assay revealed that receptor-mediated transepithelial transport of intact IgG in RPTECs is bidirectional and that it requires the formation of acidified intracellular compartments. Using double immunofluorescence, the internalized human IgG was marked in cytoplasm of RPTECs and co-localized with FcRn. These data define the mechanisms of FcRn-associated IgG transport in the RPTEC monolayers. It was suggested that the intact pathway for human IgG transepithelial transport may avoid lysosomal degradation of IgG.
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