The role of the Brambell receptor (FcRB) in liver: protection of endocytosed immunoglobulin G (IgG) from catabolism in hepatocytes rather than transport of IgG to bile

The Brambell receptor (FcRB) mediates functions of both immunoglobulin G (I gG) transport, transmitting immunity from mother to young, and IgG protecti on, making IgG the longest surviving of all plasma proteins. Reflecting its role as transport receptor (termed FcRn, for neonatal rat intestine, the t issue from which it was first cloned), FcRB is expressed antenatally in the rabbit, mouse and rat fetal yolk sac and in human placental syncytiotropho blasts, and neonatally in the intestinal epithelium of mice and rats. Refle cting its role as protection receptor (FcRp), FcRB is expressed in the vasc ular endothelium throughout life, where it protects IgG from the on-going c atabolic activities of this tissue. FcRB detected in hepatocytes was hypoth esized to mediate transport of IgG from serum to bile, thus potentially ext ending the transport expression (FcRn) of this receptor beyond the perinata l period. Our results show serum-to-bile transport of IgG to be unaffected in mice functionally deleted for FcRB. Accordingly, the hypothesis is rejec ted that FcRB functions as transport receptor (FcRn) in liver. The default conclusion is that FcRB in hepatocytes functions as FcRp, serving to protec t IgG from catabolism in hepatocytes that accompanies the endocytic activit y of these cells. We conclude that there remains to date no evidence of an FcRn-like transport function of the Brambell receptor beyond the perinatal period, after which the FcRp function of the receptor predominates, paralle ling the endocytic activities of the associated tissues.