Degradative intracellular transport of antisecretory component in cultured hepatocytes. An alternate pathway for the immunoglobulin A receptor

Studies on the relationships of IgA to human liver. IgA deposition in non-alcoholic liver diseases

An investigation was conducted to clarify the relationships of IgA to the human liver. Immunocytochemical studies were performed on biopsy specimens from patients with cirrhosis and chronic hepatitis without any apparent history of alcohol abuse. The results showed that 1) a large amount of IgA is associated with the sinusoidal surface of hepatocytes, endothelial cells and Kupffer cells, 2) this IgA contains J chain and can form a complex with secretory component, and 3) this mainly belongs to the IgA1 subclass, 4) IgA in vesicles within hepatocytes and Kupffer cells is always associated with acid phosphatase activity, and 5) IgA containing vesicles within ductular epithelial cells always lack such enzyme activity. We conclude that 1) the IgA bound to the surface of hepatocytes, sinus endothelial cells and Kupffer cells is polymeric IgA1 uncomplexed with SC, and 2) this IgA occasionally enters these cells, and may be degraded in the lysosomes. 3) Polymeric IgA combines with SC in the ductular epithelium and may be secreted into bile. These findings suggest that J chain-linked polymeric IgA bound to the surface of hepatocytes and Kupffer cells has a certain pathological significance in liver diseases and might be involved in the clearance of excess IgA from the circulation.

Intracellular processing of human vs. rat immunoglobulin A in the rat liver

It is well established that in the rat, rat dimeric IgA is transported from blood to bile across rat liver parenchymal cells via a series of minute smooth membrane-limited vesicles. This pathway is unique from that taken by a number of other ligands, which are internalized for degradation, in that there appears to be little involvement of coated vesicles, multivesicular bodies and lysosomes. The transmembrane receptor for IgA, secretory component, is not recycled but is secreted in part with the ligand into bile and must be produced continuously within the liver cell. Several recent studies have suggested that the receptor for asialoglycoproteins, as well as the structures involved in its processing, may play an important role in IgA processing. It was noted, however, that in all of these studies human polymeric IgA1 was used in the rat model. Using purified rat and human IgA preparations, we have demonstrated by light and quantitative electron microscopic autoradiography, as well as by certain biochemical procedures, that the two ligands are processed quite differently from one another in the rat. Human IgA disappears from the plasma at a slower rate and is much less efficiently transported into the bile. In addition, up to as much as 30% of the human IgA is diverted to the lysosomal pathway. This diversion of human polymeric IgA may be related to either the association of a serine-linked oligosaccharide at the hinge region of the human polymeric IgA1 or that large polymers, often found in human IgA preparations may initiate secretory component receptor aggregation, which in turn, interferes with the normal physiological processing of the IgA molecule.