Quantitative study of the uptake of IgA by isolated rat hepatocytes

Effects of bilirubin ditaurate on biliary secretion of proteins and lipids: influence on the hepatic vesicle transport system

BACKGROUND

Several organic anions cause dissociation of biliary lipid secretion from bile acid secretion (uncoupling). As bile lipids originate from liver microsomes and are transported by carrier proteins and/or transcytotic vesicles, such a reduction of biliary lipid secretion may lead to cytosolic accumulation of vesicles. This study investigated whether bilirubin conjugate, a physiologically important organic anion, caused uncoupling and whether hepatic retention of compounds carried by transcytotic vesicles occurred subsequently, using bilirubin ditaurate, a synthetic commercially available compound.

METHODS

Cannulation of the bile duct and femoral vein was done in male Sprague-Dawley rats. Sodium taurocholate was infused intravenously at a constant rate of 100 nmol/min per 100 g bodyweight. Bilirubin ditaurate (50 nmol/min per 100 g bodyweight) was infused concomitantly, followed by periodical bile collection for analysis of lipids, total protein and immunoglobulin A.

RESULTS

Biliary bile acid secretion was not changed significantly by infusion of bilirubin ditaurate. In contrast, the secretion of cholesterol, phospholipids and immunoglobulin A was decreased by 57.3, 48.7 and 44.8%, respectively. The biliary cholesterol:phospholipid ratio was increased by 19%. Uncoupling was caused by bilirubin ditaurate and biliary immunoglobulin A secretion was decreased.

CONCLUSIONS

As immunoglobulin A is a major protein carried by intrahepatic transcytotic vesicles, uncoupling may involve impairment of intrahepatic vesicular transport. Also, a reduction of immunoglobulin A secretion into bile by organic anion-induced uncoupling may weaken biliary immunity.

Antibody against the human J chain inhibits polymeric Ig receptor-mediated biliary and epithelial transport of human polymeric IgA

To emphasize the requirement for a J chain in native polymeric immunoglobulins for their selective transport into exocrine secretions, IgG, purified from two different antisera specific for the human J chain, was shown to: (i) bind in vitro to human polymeric IgA (pIgA) by density gradient ultracentrifugation; (ii) inhibit binding in vitro of rat secretory component to human pIgA; (iii) inhibit hepatic transport of human pIgA into rat bile in vivo; and (iv) inhibit apical transcytosis of pIgA in vitro by polarized human polymeric immunoglobulin receptor (pIgR)-expressing Madin-Darby canine kidney cells. Inhibition of biliary transport increased with the molar ratio of anti-J chain antibodies against pIgA and their incubation time. Anti-J chain F(ab')2 and Fab fragments also inhibited biliary transport, excluding a role for phagocytic clearance or excessive size of the immune complexes. Anti-human-Fc alpha Fab, bound to human pIgA in complexes of larger size than those with anti-J chain Fab, did not inhibit biliary transport of human pIgA. Propionic acid-denatured human pIgA, although containing J chains, was very poorly transported into rat bile. Altogether, our data strongly support, now also by in vivo experiments, the crucial role of the J chain of native pIgA in its selective pIgR-mediated transport into secretions, as suggested long ago by in vitro data only. Recent data on J chain-knockout mice, with low IgA levels in bile and feces, cannot explain the role of the J chain in contributing to the secretory component/pIgR-binding site of normal pIgA, but otherwise agree with our study.

Altered hepatic transport of immunoglobulin A in mice lacking the J chain

We have created J chain knockout mice to define the physiologic role of the J chain in immunoglobulin synthesis and transport. The J chain is covalently associated with pentameric immunoglobulin (Ig) M and dimeric IgA and is also expressed in most IgG-secreting cells. J chain-deficient mice have normal serum IgM and IgG levels but markedly elevated serum IgA. Although polymeric IgA was present in the mutant mice, a larger proportion of their serum IgA was monomeric than was found in wild-type mouse serum. Bile and fecal IgA levels were decreased in J chain-deficient mice compared with wild-type mice, suggesting inefficient transport of J chain-deficient IgA by hepatic polymeric immunoglobulin receptors (pIgR). The pIgR-mediated transport of serum-derived IgA from wild-type and mutant mice was assessed in Madin-Darby canine kidney (MDCK) cells transfected with the pIgR. These studies revealed selective transport by pIgR-expressing MDCK cells of wild-type IgA but not J chain-deficient IgA. We conclude that although the J chain is not required for IgA dimerization, it does affect the efficiency of polymerization or have a role in maintaining IgA dimer stability. Furthermore, the J chain is essential for efficient hepatic pIgR transport of IgA.

Interactions of cell-surface galactosyltransferase with immunoglobulins

Detection of the activity of beta-1,4-galactosyltransferase (beta-1,4-GT) in suspensions of viable mouse hepatocytes, the human hepatoma cell line Hep G2, the human colonic adenocarcinoma cell line HT-29, the monocyte-like cell line U937, and human splenic B and T lymphocytes suggested the presence of beta-1,4-GT, in an enzymatically active form, on plasma membranes. The presence of beta-1,4-GT on cell surfaces was also indicated from the effect of trypsinization of live cells, which significantly reduced cell surface beta-1,4-GT activity, but did not affect the activity associated with cytoplasmic membranes. Furthermore, the presence of beta-1,4-GT on the cell surface was demonstrated by indirect immunofluorescence staining of cells with anti-beta-1,4-GT antibody. The detection of radioactivity in immunoglobulins (Ig) and their component chains after incubation with suspensions of intact cells in the presence of Mn2+ and UDP-[3H]-galactose, indicated that Ig molecules were galactosylated. In the absence of UDP-[3H]-galactose, beta-1,4-GT on cell surfaces, or immobilized on Sepharose-4B, formed stable complexes with galactose acceptors, including Ig. The efficiency of binding decreased in the order: J chain > alpha chain > mu chain > polymeric IgA2 > monomeric/polymeric IgA1 > IgM > IgG. Thus, beta-1,4-GT could act as a cell-surface receptor for Ig through a cation-dependent, lectin-like association of the beta-1,4-GT with the carbohydrate moieties of the Ig. This was confirmed by indirect surface immunofluorescence and radiolabeled ligand binding assays. The binding was inhibitable by EDTA, alpha-lactalbumin (in the presence of glucose), GlcNAc, or uridine 3',5'dialdehyde. At 37 degrees C, the apparent affinity constants and association rate constants of interaction between cell surface beta-1,4-GT on glutaraldehyde-fixed HT-29 and U937 cells and alpha 2 chain or monomeric IgA1 were in the range from 7.1 x 10(7) to 4.6 x 10(8) M-1 and from 1 x 10(5) to 3 x 10(6) M-1 s-1, respectively. The dissociation rate constants and half time of dissociation calculated from these data were in the range from 2.1 x 10(-2) to 5.0 x 10(-4) s-1 and from 33 to 1380 s, respectively. The number of alpha 2 or IgA1 molecules bound per HT-29 and U937 cell were in the range from 1.9 x 10(5) to 1.3 x 10(6). The binding of IgA by the cell surface beta-1,4-GT was not associated with internalization or the catabolic degradation of the ligand.

Uptake and degradation of filamentous actin and vitamin D-binding protein in the rat

Tissue uptake and degradation of 125I-tyramine-cellobiose-labelled filamentous actin, vitamin D-binding protein (DBP) and actin-DBP complex were studied in the rat. Actin and actin-DBP complex were cleared from plasma at a faster rate than was DBP. About 40% of injected actin was recovered in the liver between 10 and 30 min after administration. Of the total radioactivity recovered in the liver, about 35% and 40% was detected in parenchymal and endothelial cells respectively when labelled actin or DBP-actin complex was injected intravenously. When labelled DBP alone was injected, approx. 55% of the radioactivity recovered in liver was in the Kupffer cells. These results suggest that actin is targeting the DBP-actin complex to the endothelial and parenchymal liver cells. Filamentous actin was also taken up in large amounts and at a rapid rate in parenchymal as well as non-parenchymal liver cells in vitro. Our data indicate that the rat has a mechanism to clear actin and the DBP-actin complex from plasma and that both parenchymal and non-parenchymal liver cells are involved in this process.

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)

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.

Selective hepatobiliary transport of human polymeric IgA in mice

Both subclasses of human polymeric IgA (pIgA) were selectively transported from the serum into the bile of mice relative to human IgG or IgM. Removal of human pIgA from serum corresponded to the clearance kinetics shown for murine pIgA. The biliary pIgA was intact as determined by sucrose density gradient ultracentrifugation. This hepatic uptake was specific for the IgA isotype and occurred independently of receptors in the liver specific for glycoproteins that terminate with galactose or mannose moieties. Desialylation of human pIgA resulted in its rapid clearance from serum and subsequent deposition in the liver in a manner similar to most other desialylated serum glycoproteins. The desialylated pIgA present in bile was also an intact molecule; thus the asialoglycoprotein receptor may represent an additional mechanism for the transport of serum pIgA into bile.

Intracellular receptor sorting during endocytosis: comparative immunoelectron microscopy of multiple receptors in rat liver

Using double-label quantitative immunoelectron microscopy on ultrathin cryosections of rat liver, we have compared the endocytotic pathways of the receptors for asialoglycoprotein (ASGP-R), mannose-6-phosphate ligands (MP-R), and polymeric IgA (IgA-R). All three were found within the Golgi complex, along the entire plasma membrane, in coated pits and vesicles, and within a compartment of uncoupling of receptors and ligand ( CURL ). The receptors occurred randomly at the cell surface, in coated pits and vesicles. Within CURL tubules ASGP-R and MP-R were colocalized , but IgA-R and ASGP-R displayed dramatic microheterogeneity. Thus, in addition to its role in uncoupling and sorting recycling receptor from ligand, CURL serves as a compartment to segregate recycling receptor (e.g. ASGP-R) from receptor involved in transcytosis (e.g. IgA-R).

Immunohistochemical localization of secretory component in the liver of guinea pigs and dogs versus rats, rabbits, and mice

Secretory component (SC) was localized in the liver of guinea pigs, dogs, rabbits, rats, and mice. In rabbits, rats, and mice SC localized predominantly in bile canaliculi and on hepatocyte sinusoidal membranes but was doubtful in cholangiocytes . In dogs and guinea pigs SC-staining was not detected in/on hepatocytes and canaliculi but was strong in/on cholangiocytes , as reported for humans. In guinea pigs IgA biliary output was small (0.23 mg/kg/day), as for dogs and humans, and below IgG output (1.4 mg), in contrast to rats, whose IgA biliary output (38 mg/kg/day) was much larger than IgG output (2mg). Biliary obstruction in guinea pigs induced only minor increases in serum IgA (+ 26% over 24 h), as reported for dogs and humans, in contrast to rats (+ 800% over 24h) and rabbits. Hepatocyte SC expression correlates with IgA hepatobiliary excretion, being low in guinea pigs, dogs, and humans but high in rats, rabbits, and mice.

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.

Immunoglobulins in rabbit hepatic bile: selective secretion of IgA and IgM and active plasma-to-bile transfer of polymeric IgA

Proteins in hepatic bile from cannulated rabbits were analysed by gel filtration, electrophoresis, density gradient ultracentrifugation, and immunochemical methods. Bile-to-serum concentration ratios (no. = 8) demonstrated that IgA and free secretory component (SC) were major constituents of rabbit bile. Relative concentration ratios, obtained by dividing the bile to serum level ratio for each protein by the corresponding ratio for albumin, were 310, 1.6, 1.0, 0.82, and 0.54 for IgA, IgM, albumin, transferrin, and IgG respectively, suggesting that both IgA and IgM are selectively secreted into bile. Ligation of the bile duct (no. = 5) led to a selective increase of the serum levels of IgA and SC, the latter as secretory IgA. After intravenous injection (no. = 4) of human polymeric IgA, 37-69% of the injected dose was recovered in bile after 3 h, in contrast to 4-5% for human monomeric IgA. The active transfer of both rabbit and human polymeric IgA into rabbit bile occurs via the same hepatic SC-mediated transport process as that described for the rat.

Expression of a monoclonal antibody-defined liver-associated antigen in normal rat hepatocytes and hepatocellular carcinoma cells

In order to examine changes in the expression of normal cellular antigens during hepatocarcinogenesis, we have developed and characterized a monoclonal antibody detecting an antigen which is particularly associated with adult rat hepatocytes. The antibody showed no reactivity with a range of freshly-prepared normal syngeneic adult rat cell types, but showed strong reactivity with hepatocytes derived from several rat strains in antibody binding tests. Immunoperoxidase staining of sections of normal rat liver showed that the antigen was distributed throughout hepatocytes within the liver, but did not stain cells other than hepatocytes. The antigen detected by this monoclonal antibody was not detectable on a number of transplanted and primary dimethylaminoazobenzene-induced hepatomas. Preliminary data indicated that this monoclonal antibody may be suitable for use in flow cytofluorimetric sorting of mixed populations of normal and tumour cells, allowing the investigation of cell phenotypes during hepatocarcinogenesis.