The role of the liver in translocation of IgA into the gastrointestinal tract.

The liver plays a key role in the translocation of IgA into the upper gastrointestinal tract. The amount of IgA transported and the mechanisms involved, however, vary widely among species. In some, best defined in the rat, large amounts of polymeric IgA (pIgA) are cleared from the plasma by hepatocytes, which synthesize the polymeric immunoglobulin receptor, secretory component (SC), and express it on their sinusoidal plasma membranes. Circulating pIgA binds to SC, is internalized into endocytic vesicles and transported across the hepatocyte to the bile canalicular membrane, where the pIgA is released into bile in complex with a portion of the SC, i.e., secretory sIgA (sIgA). In some other species, including man, there is much less hepatic transport of circulating IgA, at least in part because SC is present only in biliary epithelium, and there is relatively more local synthesis of IgA within hepatobiliary tissues. On the other hand, certain IgA1 myeloma proteins appear to bind to and enter human hepatocytes via an asialoglycoprotein receptor. These species differences have implications for the biological significance of the biliary secretion of IgA, including the disposal of circulating IgA-antigen complexes into bile.