Synthesis of secretory component by rat hepatocytes in culture

Secretory immune system of the duck (Anas platyrhynchos). Identification and expression of the genes encoding IgA and IgM heavy chains

IgA has not previously been identified in waterfowl. Studies instead revealed physical and antigenic similarities between duck bile immunoglobulin (Ig) and serum IgM. Here, a differential screening approach was used to clone, from a duck spleen library, the cDNA encoding the heavy (H) chains of IgM and the Ig, identified here as IgA, occurring in duck secretions. Phylogenetic comparisons of inferred amino acid sequences of entire H chain constant (C) regions and of individual domains revealed that the duck mu chain was closest to chicken mu (54% overall identity), and duck alpha was closest to chicken alpha (50% identity). Comparison of the mu and alpha C regions revealed areas of up to 65% amino acid similarity within the C4 domains, accounting for the previously noted antigenic overlap of duck IgM and IgA. Messages for alpha and mu were detected in duck lymphoid organs but the alpha message was most abundant in the respiratory, alimentary and reproductive tracts. The alpha message first appeared around 14 days of age and reached adult levels of expression only at 35-50 days. The results indicate that the duck has a mucosal immune system which utilizes IgA; however, the delayed expression and secretion of duck IgA explains the susceptibility of ducklings to mucosal pathogens. Since the waterfowl are among the most primitive extant birds, the recognition of IgA in the duck supports the conclusion that IgA occurs throughout the class Aves and also existed in the common ancestors of birds and mammals.

Inflammatory status and preerythrocytic stages of malaria: role of the C-reactive protein

In the acquisition of protection against malaria, the role played by nonspecific factors, some being part of the cascade effect of cytokines, has to be considered. The C-reactive protein, a major acute phase reactant secreted by interleukin-1 stimulated hepatocytes, has an effect on the hepatic development of Plasmodia, both by preventing penetration of the sporozoite into the hepatocyte and by blocking parasite division through an antibody-like effect. This latter effect confirms the potential interest of targeting the uninuclear form of the parasite. Nevertheless, C-reactive Protein alone does not account for all the effects of the inflammatory response, other reactants from both serum and hepatocytes are also involved.

The liver and IgA: immunological, cell biological and clinical implications

Secretory immunoglobulin A is the characteristic and predominant immunoglobulin of the mucosal immune system; it participates in immunological protection at the level of mucous membrane surfaces. During the past 10 to 15 years, a great deal of experimental and clinical evidence has shown that the liver is very much involved in the sIgA system. In certain animals (rats, mice, rabbits), polymeric forms of IgA are efficiently cleared by the liver and transported into bile by a receptor-mediated vesicular pathway across hepatocytes. Taking advantage of this easily accessible pathway, investigators have defined many of the events in the external secretion of pIgA, including details about the synthesis and secretion of its receptor, secretory component. In the rat hepatocyte, secretory component is synthesized as a transmembrane glycoprotein and is expressed preferentially on the sinusoidal plasma membrane; circulating pIgA that binds to secretory component is internalized into endocytic vesicles and transported across the hepatocyte to the bile canalicular membrane, where the pIgA is released into bile as a soluble complex with a portion of the secretory component, the complex being secretory IgA. In some other animals (dog, guinea pig, sheep) as well as man, biliary epithelial cells, not hepatocytes, express secretory component and perform the transcytosis and secretion of pIgA into bile. In those species, much of the pIgA that reaches bile is synthesized locally in plasma cells that populate the biliary tree; this design is analogous to the release of sIgA into various mucosae in the body. The major biological functions ascribed to the secretion of IgA into bile are enhancement of immunological defense of the biliary and upper intestinal tracts and the clearance of harmful antigens from the circulation as IgA-antigen complexes. However, the importance of biliary IgA antibodies is largely unclarified, and man lacks the capacity for effective clearance of IgA-antigen complexes via the secretory component-mediated transhepatocellular pathway; whether this deficit contributes to the propensity for man to develop IgA immune complex diseases should be clarified. Among liver diseases, alcoholic disease is most closely linked to alterations in IgA metabolism. This association is manifested principally by the deposition of IgA along the sinusoids in the livers of the majority of alcoholics and in the renal mesangium of many.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Hepatic production of biliary IgM in the rat

Drainage of the thoracic duct resulted in a decrease in the IgA level in rat bile, but at the same time there was an increase in both the total IgM level and the specific IgM antibody activity to Escherichia coli 06 in the bile of animals immunized in the Peyer's patches with these bacteria. The increase in total IgM was significantly higher in animals immunized with the E. coli 06 than in unimmunized rats. The level of total IgG was not altered during the drainage. IgM antibodies to E. coli 04 given intravenously during lymph drainage did not appear in the bile, whereas specific IgM antibodies to E. coli 06 occurring after active immunization increased in the bile of the same animal. The data elucidate two aspects of the hepatic IgM turnover. First IgM could take the place of IgA in cases of IgA deficiency, and second the IgM might originate from intrahepatic production.

Antibodies to Escherichia coli and anti-adhesive activity in paired serum, hepatic and gall bladder bile samples

Human bile contains a mixture of immunoglobulins excreted through the liver and produced in the biliary tract. This study examines the specific antibody activity of the biliary immunoglobulins against Escherichia coli antigens. Paired samples of serum, hepatic bile, and gall bladder bile were obtained from 23 patients with gallstones and five patients with healthy gall bladders. Antibody activity against E. coli antigens was found in all the sera and most of the bile samples. The levels of IgA, IgM, IgG, and secretory component (SC)-combined antibodies were lower in bile than in serum. Selective treatment of IgA by the liver was suggested by the finding of a correlation between the serum and the bile IgA antibody activity. IgG antibodies were only found in inflamed gall bladders. The bile was shown to have antibacterial activity against E. coli, i.e. an ability to inhibit the attachment to epithelial cells, but the inhibitory activity was not restricted to the immunoglobulin fraction of the bile.

Relationship between secretory IgA, IgA-containing (C3-fixing) circulating immune complexes, and complement components (C3, C4) in patients with obstructive jaundice

Serum secretory IgA, IgA-containing circulating immune complexes (IgA-CIC), complement components, and major immunoglobulins were measured in patients with biliary tract stones and/or tumors of the biliary tract or pancreas. The levels of secretory IgA and total IgA were increased in patients with and without obstructive jaundice. The levels of both C3 and C4 were significantly higher in patients with or without obstructive jaundice than in healthy controls. In patients with obstructive jaundice the increased levels of secretory IgA, total IgA, and IgA-CIC were correlated with the increase of C3 but not with that of C4.

The liver in the IgA secretory immune system. Dogs, but not rats and rabbits, are suitable models for human studies

The liver transport of polymeric IgA (pIgA) from plasma into bile and the immunohistochemical distribution of secretory component (SC) in the liver were studied in dogs, and compared to those in humans, rats, and rabbits. Results were as follows: (i) according to bile and serum protein concentrations and specific activities, plasma pIgA in dogs, like in humans, is transported into bile approximately 10 times more efficiently than albumin, as compared to 320 and 1060 times in rabbits and rats, respectively. (ii) Only approximately 1% of an i.v. dose of [125I]pIgA is transported into bile over 8 hr in dogs, like in humans, as compared to approximately 50% over 3 hr in rats and rabbits. These results agree with much smaller daily fractional catabolic rates of intravascular pIgA in dogs (0.28) and humans (0.48) than in rats (24.0). (iii) Total bile IgA contributes daily about 1.5 mg per kg to intestinal pIgA in dogs, a figure similar in humans (0.8 mg per kg) but much smaller than in rats (38 mg per kg) and rabbits (35 mg per kg). (iv) Biliary obstruction in dogs, like in humans, results only in minor and late increases in serum pIgA levels, contrasting with greater than 8-fold increases within 24 hr in rats and rabbits. (v) Unlike in rats and rabbits, SC in dog liver as well as in human liver cannot be detected in hepatocytes although clearly present in bile duct cells. To conclude: (i) major species differences in plasma-to-bile transport of pIgA exist, most probably related to species differences in the ability of hepatocytes to synthetize SC.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in IgA following varying degrees of biliary obstruction in the rat

Polymeric IgA is rapidly transported from blood to bile by the rat liver. The effect of varying degrees of biliary obstruction on this transport process was studied. IgA concentrations were measured by radioimmunoassay. Serum IgA concentrations increased progressively, and IgA output in bile declined with increasing bile duct obstruction. The decline in bile IgA output was explained by both diminished bile flow and decreased concentrations of IgA in bile. Very little polymeric IgA was present in normal rat serum. In contrast, using column chromatography on Ultrogel AcA 22, increases in serum IgA associated with cholestasis were shown to be due to increments in polymeric IgA. Serum IgA was a more sensitive indicator of cholestasis than was serum alkaline phosphatase. IgA and secretory component were found, using indirect immunofluorescence, surrounding bile canaliculi and on or adjacent to the hepatocyte plasma membrane lining the sinusoids. With biliary obstruction, staining for IgA and secretory component intensified markedly near the bile canaliculi. We conclude that: (a) polymeric IgA must be efficiently removed from serum by the normal rat liver; (b) even minimal cholestasis impairs IgA output into bile, and (c) impairment of IgA transport during cholestasis appears to occur at or near the canalicular membrane.

Selective transport of polymeric immunoglobulin A in bile. Quantitative relationships of monomeric and polymeric immunoglobulin A, immunoglobulin M, and other proteins in serum, bile, and saliva

In 17 adults, serum, hepatic bile, and saliva samples were analyzed for their sedimentation profile of IgA and secretory component (SC), and for their concentrations of albumin, orosomucoid, transferrin, IgG, IgA, alpha 2-macroglobulin (alpha 2M), IgM, and SC. Polymeric IgA(p-IgA) averaged 13% (50-700 micrograms/ml) of total IgA in serum, 70% (43-88%) in bile, and 93% (74-98%) in saliva. Most of the p-IgA in bile sedimented with SC, which also occurred free (8-44%), and with IgM. In bile, albumin (155-1,485 micrograms/ml) was the predominant protein, followed by IgG (32-480 micrograms/ml), and total IgA (37-209 micrograms/ml). In saliva, p-IgA (72-902 micrograms/ml) predominated, followed by albumin (16-385 micrograms/ml) and IgG (9-178 micrograms/ml). Secretion-to-serum albumin-relative concentration ratios (S/S-ARCR = 1 for albumin) in bile averaged 22 for p-IgA, 1.91 for IgM, 1.28 for monomeric IgA (m-IgA), 0.70 for IgG, and 0.57 for alpha 2M, indicating for p-IgA, IgM, and to a lesser extent for m-IgA, a selective excretion into bile. In saliva, a 16-fold greater selective excretion of p-IgA (mean S/S-ARCR = 354) was found. Labeled m- and p-IgA were injected intravenously into five patients. Specific activities indicated that for p-IgA 50% was serum derived in bile, as compared with 2% in saliva, and to 85% for m-IgA in bile. In the patient with the highest excretion of 125I-p-IgA in bile, only 2.8% of the injected dose was recovered in bile within 24 h after injection. Compared with rats and rabbits, the serum-to-bile transport of p-IgA in humans is much smaller.

High serum levels of secretory IgA in liver disease: possible liver origin of the circulating secretory component

Patients with liver disease frequently display unexplained elevations of serum secretory IgA (sIgA). The sIgA levels in various liver diseases were compared to various biochemical or clinical parameters. Patients with primary biliary cirrhosis, biliary tract obstruction, or acute hepatitis displayed highest sIgA levels. In chronic parenchymal liver disease sIgA levels correlated strongly with serum alkaline phosphatase (r = 0.79), leucine aminopeptidase (r = 0.83), and direct bilirubin levels (r = 0.63), but not with prothrombin time, aminopyrine breath test, or presence of portacaval shunting. In acute hepatitis sIgA correlated best with serum glutamic oxaloacetic transaminase (r = 0.69) but not with bilirubin; in four patients with fulminant hepatitis, sIgA fell rapidly together with all liver enzymes and prothrombin time; it rose quickly again in one patient when parenchymal regeneration occurred. These results suggest a hepatobiliary origin of the serum sIgA in liver disease. In acute hepatitis the persistence of hepatocytes seems necessary for maintaining high serum sIgA levels, suggesting a possible hepatocyte origin of the secretory component.

Quantitative study of the uptake of IgA by isolated rat hepatocytes

Recently dispersed rat hepatocytes showed high capacity for uptake of monomeric and polymeric human IgA by simple absorptive endocytosis. Maximum uptake exceeded 15 million monomer units per cell. The asialo-glycoprotein receptor was not involved in this process. J chain-containing polymers were on the average taken up somewhat better than monomers and substantially better than J chain-deficient polymers. This result would agree with a partial involvement of the secretory component (SC) in the absorptive uptake of IgA. Different receptor mechanism might hence explain the hepatic transport of IgA from blood to bile in the rat and perhaps be involved in the catabolism of IgA. After binding to the plasma membrane of the rat hepatocyte, most of the human IgA was internalized in endocytic vesicles. A considerable proportion of radiolabelled IgA apparently reached the lysosomes after internalization and became degraded.