Bidirectional FcRn-dependent IgG transport in a polarized human intestinal epithelial cell line

Abstract

IntroductionSteady-state levels of IgG in the blood of adult mice, and likely all mammals, depend on IgG catabolism mediated in part by the MHC class I-related Fc receptor, FcRn (1). FcRn also mediates vectorial transport of IgG across certain epithelial barriers. In suckling mice and rats, intestinal absorption of maternal IgG from breast milk into the systemic circulation depends on FcRn (2). In humans, maternofetal transfer of IgG across the placenta also likely depends on FcRn (3). Thus, FcRn plays critical and well-documented roles in the regulation of IgG metabolism in adults and in the acquisition of humoral immunity in early life. These effects on the physiology of IgG in vivo result from the action of FcRn as an intracellular trafficking receptor (4).FcRn has been cloned from the rat, mouse, and human. The molecule is expressed as a heterodimer composed of a glycosylated heavy (α) chain (51 kDa in rodents and 40-45 kDa in humans) associated noncovalently with β2-microglobulin (β2M) (5). Binding of IgG to FcRn requires contact between solvent-exposed peptide sequences in the CH2 and CH3 domains of IgG and the α1 and α2 domains of FcRn, together with a single contact site in β2M (6-11). A hallmark of FcRn interaction with IgG is its pH dependence, showing high-affinity binding at acidic pH (pH ≤ 6.5) and weak or no binding at neutral pH (pH ≥ 7.0) (12, 13). FcRn is the only Fcγ receptor that exhibits MHC class I structure, and the only Fcγ receptor to exhibit pH dependency in ligand binding.The function of FcRn in the intestine of suckling mice and rats has been well documented (14). In neonatal mice and rats, FcRn is expressed at high levels by intestinal epithelial cells and mediates absorption of IgG by receptor-mediated transcytosis. FcRn expression in the neonatal rodent is developmentally downregulated, resulting in nearly complete loss of intestinal FcRn at the time of weaning (15)(16)(17) The MHC class I-related Fc receptor, FcRn, mediates the intestinal absorption of maternal IgG in neonatal rodents and the transplacental transport of maternal IgG in humans by receptor-mediated transcytosis. In mice and rats, expression of FcRn in intestinal epithelial cells is limited to the suckling period. We have recently observed, however, clear expression of FcRn in the adult human intestine, suggesting a function for FcRn in intestinal IgG transport beyond neonatal life in humans. We tested this hypothesis using the polarized human intestinal T84 cell line as a model epithelium. Immunocytochemical data show that FcRn is present in T84 cells in a punctate apical pattern similar to that found in human small intestinal enterocytes. Solute flux studies show that FcRn transports IgG across T84 monolayers by receptor-mediated transcytosis. Transport is bidirectional, specific for FcRn, and dependent upon endosomal acidification. These data define a novel bidirectional mechanism of IgG transport across epithelial barriers that predicts an important effect of FcRn on IgG function in immune surveillance and host de...

Net absorption of IgG via FcRn-mediated transcytosis across rat alveolar epithelial cell monolayers

“…The mRNA (ϳ1.6-kb) transcript we found for the ␣-subunit of rat FcRn is in concordance with that previously reported for rat FcRn mRNA of ϳ1.6 kb (29, 30) and human FcRn transcript of ϳ1.5 kb (7,31,32). A larger transcript of ϳ3.1 kb was also detected by Northern analysis (data not shown), which may be due to hybridization with an unidentified primary mRNA transcript of FcRn or with a partially spliced transcript.…”

“…IgG transport in other tissues, however, is not always absorptive. For example, an in vitro intestinal crypt T84 epithelial cell model exhibits ϳ3.6-fold greater ba transport of IgG compared with ab transport when studied with an upstream IgG concentration of 60 nM (7). These data suggest that rates, direction dependence, and associated mechanisms of IgG transport are tissue and/or cell specific.…”

“…2 , and IgY] of IgG transport in both the ab and ba directions, the presence of both ␣-and ␤-subunits of rat FcRn gene transcripts by RT-PCR analyses, and confirmation by Northern analyses of the expression of mRNA for ␣-subunit of rat FcRn. It has been shown that FcRn lacks binding affinity for IgY (7). Moreover, other Fc receptors (e.g., Fc␥RI or FcR␥II) have not been reported to be expressed in any epithelium to date.…”

Elucidation of intracellular recycling pathways leading to exocytosis of the Fc receptor, FcRn, by using multifocal plane microscopy

“…Through its ability to transport IgGs within and across cells, this Fc receptor is known to play a central role in the maintenance of serum IgG levels and in the trafficking of IgG molecules across cellular barriers (19)(20)(21)(22)(23)(24)(25)(26). The available data support the model (27) that IgGs are taken up by cells through fluid-phase pinocytosis.…”

Persorption Mechanisms of Luminal Antigenic Particulates via Apoptotic Epithelial Cells of Intestinal Villi into Systemic Blood Circulation in Orally Immunized Rats

“…The mRNA of the neonatal IgGbinding receptor (FcRn) gradually decreases until adulthood in rats [11], while FcRn is still expressed on intestinal epithelial cells in adult humans and mediates the transportation of IgG [5]. Furthermore, in adult rats, IgG is absorbed by intestinal epithelial cells [46].…”