Abstract
The neonatal Fc receptor (FcRn) transports IgG across epithelial cell barriers to provide maternal antibodies to offspring and serves as a protection receptor by rescuing endocytosed IgG and albumin from lysosomal degradation. Here we describe the generation of polarized Madin–Darby canine kidney (MDCK) cells expressing rat FcRn (rFcRn) to investigate the potential requirement for ligand bivalency in FcRn-mediated transport. The rFcRn-MDCK cells bind, internalize and bidirectionally transcytose the bivalent ligands IgG and Fc across polarized cell monolayers. However, they cannot be used to study FcRn-mediated transport of the monovalent ligand albumin, as we observe no specific binding, internalization or transcytosis of rat albumin. To address whether ligand bivalency is required for transport, the ability of rFcRn to transcytose and recycle wild-type Fc homodimers (wtFc; two FcRn-binding sites) and a heterodimeric Fc (hdFc; one FcRn-binding site) was compared. We show that ligand bivalency is not required for transcytosis or recycling, but that wtFc is transported more efficiently than hdFc, particularly at lower concentrations. We also demonstrate that hdFc and wtFc have different intracellular fates, with more hdFc than wtFc being trafficked to lysosomes and degraded, suggesting a role for avidity effects in FcRn-mediated IgG transport.
Highlights
The plasma membranes of epithelial cell barriers are segregated into two spatially and functionally distinct domains that serve as a means for complex organisms
In the course of conducting new experiments with the previously described rat FcRn (rFcRn)-green fluorescent protein (GFP)-Madin–Darby canine kidney (MDCK) cell line, we discovered that we could not replicate some of the published properties of the cell line [26]
It had been reported that the rFcRn-GFP-MDCK cells functioned in transport of Fc when ligand was added at both permissive and nonpermissive pH values for the rFcRn–IgG interaction and that rFcRn-GFP fluorescence underwent a striking redistribution upon addition of ligand at either pH [25]
Summary
The plasma membranes of epithelial cell barriers are segregated into two spatially and functionally distinct domains that serve as a means for complex organisms. A requirement for the formation of the oligomeric ribbon is a homodimeric Fc or IgG molecule capable of bridging between FcRn proteins on opposing membrane faces Consistent with this hypothesis, previous studies demonstrated that a heterodimeric Fc molecule (hdFc), which contains one FcRn-binding chain and one non-FcRn-binding chain, is less efficiently transcytosed across neonatal mouse intestine [12] and exhibits a shorter serum half-life [13] than a wild-type homodimeric Fc molecule (wtFc). These results indicate that two FcRnbinding sites on a ligand are required for the purposeful movement of vesicles containing FcRn–ligand complexes through the transcytotic and protection pathways. These studies support a model in which FcRn acts as a protection receptor for a monomeric protein, albumin, as well as for dimeric IgG and Fc ligands
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