Rapid Publication A Major Histocompatibility Complex Class I-related Fc Receptor for IgG on Rat Hepatocytes

Intestinal epithelial cells of the neonatal rat and mouse have been shown to express a major histocompatibility complex (MHC) class I-like Fc receptor, or FcRn, which transports IgG in an apical to basolateral direction. Previous studies have suggested the possible expression of this receptor be-yond the neonatal period within the liver. Since bile contains high levels of IgG, we sought to determine whether the FcRn was functionally expressed by adult rat hepatocytes. Using primers specific for FcRn, which did not cross hybridize with MHC class I transcripts, FcRn DNA was amplified by reverse transcriptase polymerase chain reaction from RNA of adult rat hepatocytes. This RNA contained functional FcRn transcripts as it encoded a fi2-microglobulin-associ-ated cell surface protein as determined by immunoprecipita-tion of biotinylated cell surface proteins with a polyclonal anti-FcRn specific antiserum. Western blotting of hepato-cyte canalicular (apical) and sinusoidal (basolateral) plasma membranes with an FcRn-specific monoclonal anti-body further confirmed the protein expression and sug-gested that FcRn was enriched on the canalicular surface membranes. FcRn, on the surface of hepatocytes, was bio-logically functional as it bound Fc fragments of IgG at pH 6.0 but not 8.0, which is the same pH dependence observed for FcRn in rat neonatal enterocytes. Thus, FcRn is func-tionally expressed outside of the neonatal period on the can-alicular cell surface of adult hepatocytes. This suggests that hepatocyte FcRn may bind luminal IgG, providing a poten-tial functional communication between parenchymal im-mune cells and bile. (J. Clin. Invest. 1995. 95:2397-2402.) Key words: bile * liver * immunosurveillance * Kupffer cells * lymphocytes * Fc recepto