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 sh own to express a major histocompatibility complex (MHC) class I-like F c receptor, or FcRn, which transports IgG in an apical to basolateral direction, Previous studies have suggested the possible expression of this receptor beyond the neonatal period within the liver, Since bile contains high levels of IgG, we sought to determine whether the FcRn w as functionally expressed by adult rat hepatocytes, Using primers spec ific for FcRn, which did not cross hybridize with MHC class I transcri pts, FcRn DNA was amplified by reverse transcriptase polymerase chain reaction from RNA of adult rat hepatocytes, This RNA contained functio nal FcRn transcripts as it encoded a beta(2)-microglobulin-associated cell surface protein as determined by immunoprecipitation of biotinyla ted cell surface proteins with a polyclonal anti-FcRn specific antiser um, Western blotting of hepatocyte canalicular (apical) and sinusoidal (basolateral) plasma membranes with an FcRn-specific monoclonal antib ody further confirmed the protein expression and suggested that FcRn w as enriched on the canalicular surface membranes, FcRn, on the surface of hepatocytes, was biologically 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 functionally expr essed outside of the neonatal period on the canalicular cell surface o f adult hepatocytes, This suggests that hepatocyte FcRn may bind lumin al IgG, providing a potential functional communication between parench ymal immune cells and bile.