Development and characterization of an in vitro model to study the cell biology of the human neonatal Fc receptor FcRn, in polarized epithelial cells

The human isoform of the MHC class-I related neonatal Fc receptor, FcRn, in contrast to the developmentally regulated expression along the neonatal rodent small intestine, is expressed in epithelial cells lining the small and large intestines of human adults. FcRn assembles as a heterodimer consisting of a heavy chain and β2-microglobulin (β₂m), which is essential for FcRn function. We observed that, in Madin-Darby canine kidney (MDCK) cells, the function of human FcRn (hFcRn) in mediating the bi-directional transport of IgG was significantly increased upon co-expression of the human isoform of β₂m. In MDCK cells, the presence of human β ₂m endowed upon hFcRn an enhanced ability to exit the endoplasmic reticulum and acquire mature carbohydrate side-chain modifications at steady-state, a faster kinetics of maturation, and augmented localization at the cell surface as a mature glycoprotein able to bind IgG. Although hFcRn with immature carbohydrate side-chain modifications was capable of exhibiting pH-dependent binding of IgG, only hFcRn with mature carbohydrate side-chain modifications was detected on the cell surface. These results show that hFcRn travels to the cell surface via the normal secretory pathway, and that the appropriate expression and function of hFcRn in MDCK cells depends upon the co-expression of human β ₂m. Based on these observations, an in vitro system was developed using MDCK cells which biochemically and cell biologically reconstitutes hFcRn. At steady-state, the cell surface distribution of hFcRn is strongly polarized to the basolateral membrane domain. This accurately reflects the steady-state distribution of hFcRn endogenously expressed in two human intestinal epithelial cell lines. The basolateral cell surface polarity of hFcRn correlates with the dominant apically directed transport of IgG. Transport in the opposite (apical to basolateral) direction is apparent, but only in the presence of a pH gradient favoring IgG-FcRn association. Restriction of hFcRn to the basolateral membrane requires an intact cytoplasmic tail. Ligand binding does not alter the cell surface distribution of hFcRn. On the basolateral surface, hFcRn exists as a dynamic pool that is rapidly and constituitively endocytosed. Thus, the strongly polarized cell surface distribution of hFcRn may dictate the dominant direction of IgG transport.