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

Immune system and cholangiocytes: A puzzling affair in primary biliary cholangitis

Primary biliary cholangitis (PBC) is a cholestatic liver disease characterized by the destruction of the small and medium bile ducts. Its pathogenesis is still unknown. Despite the genome wide association study findings, the therapies targeting the cytokines pathway, tested so far, have failed. The concept of the biliary epithelium as a key player of the PBC pathogenesis has emerged over the last few years. It is now well accepted that the biliary epithelial cells (BECs) actively participate to the genesis of the damage. The chronic stimulation of BECs via microbes and bile changes the cell phenotype toward an active state, which, across the production of proinflammatory mediators, can recruit, retain, and activate immune cells. The consequent immune system activation can in turn damage BECs. Thus, the crosstalk between both innate and adaptive immune cells and the biliary epithelium creates a paracrine loop responsible for the disease progression. In this review, we summarize the evidence provided in literature about the role of BECs and the immune system in the pathogenesis of PBC. We also dissect the relationship between the immune system and the BECs, focusing on the unanswered questions and the future potential directions of the translational research and the cellular therapy in this area.

Advances in cholangiocyte immunobiology

Cholangiocytes, or bile duct epithelia, were once thought to be the simple lining of the conduit system comprising the intra- and extrahepatic bile ducts. Growing experimental evidence demonstrated that cholangiocytes are in fact the first line of defense of the biliary system against foreign substances. Experimental advances in recent years have unveiled previously unknown roles of cholangiocytes in both innate and adaptive immune responses. Cholangiocytes can release inflammatory modulators in a regulated fashion. Moreover, they express specialized pattern-recognizing molecules that identify microbial components and activate intracellular signaling cascades leading to a variety of downstream responses. The cytokines secreted by cholangiocytes, in conjunction with the adhesion molecules expressed on their surface, play a role in recruitment, localization, and modulation of immune responses in the liver and biliary tract. Cholangiocyte survival and function is further modulated by cytokines and inflammatory mediators secreted by immune cells and cholangiocytes themselves. Because cholangiocytes act as professional APCs via expression of major histocompatibility complex antigens and secrete antimicrobial peptides in bile, their role in response to biliary infection is critical. Finally, because cholangiocytes release mediators critical to myofibroblastic differentiation of portal fibroblasts and hepatic stellate cells, cholangiocytes may be essential in the pathogenesis of biliary cirrhosis.

Measurement of immunoglobulin concentrations in the feces of healthy dogs

Selective immunoglobulin A (IgA) deficiency is the most common primary immunodeficiency in humans and may be associated with chronic gastrointestinal disease. This observation has led to the suggestion that the high susceptibility of German shepherd dogs (GSD) to chronic enteropathies is related to a deficiency in mucosal IgA production. Relative deficiencies of IgA has been reported in the serum, saliva, tears, and feces of GSD both with and without alimentary disease; however, the findings of different studies are not consistent. The aim of this study was to confirm whether a relative deficiency of IgA exists in the feces of GSD. Feces were collected from healthy GSD (n = 209), Labrador retrievers (n = 96), beagles (n = 19), and miniature schnauzers (n = 32). Fecal IgA, IgM, and IgG were measured by capture enzyme-linked immunosorbent assays. Fecal IgG concentrations in the four breed groups were not significantly different. IgA concentrations were significantly greater in miniature schnauzers than in GSD (P = 0.0003) and Labradors (P = 0.0004) but not significantly different from those in beagles. IgM concentrations were significantly greater in miniature schnauzers than in GSD (P < 0.0001), Labradors (P < 0.0001), and beagles (P = 0.0098). These findings do not support the hypothesis that GSD have a relative deficiency in fecal IgA. The differences in immunoglobulin concentrations measured from a single defecation, between individuals of the same breed and between breeds, as well as the lack of an internal control molecule, make the determination of a normal reference range for all dogs impossible. Therefore, the usefulness of fecal immunoglobulin quantification for the assessment of intestinal immunoglobulin secretion in dogs is limited.

Immunoglobulins in nasal secretions of dog puppies from birth to six weeks of age

In order to investigate local immune defence mechanisms in the dog, the concentration of immunoglobulins (Ig) G, A and M in nasal secretions (NS) and serum of 42 healthy, neonatal Rottweiler puppies was determined. Ig were measured with a commercially available, dog-specific ELISA during the first six weeks of life. On average, IgG was the predominant Ig isotype during the first three days of life. The IgA:IgG ratio changed between weeks 1 and 3 due to markedly decreasing IgG concentrations. Between the fourth and sixth week, IgG predominated again. During the first week, only 21-39% of puppies had measurable amounts of IgM in NS, in week 2, this percentage increased to 69%. Marked differences between litters and between individual puppies within litters were found. No puppy diseased during the observation period and all developed normally.

Myeloma expression systems

Myeloma expression systems have been utilized successfully for the production of various recombinant proteins. In particular, myeloma cell lines have been exploited to express a variety of different antibodies for diagnostic applications as well as in the treatment of various human diseases. The use of myeloma cells for antibody production is advantageous because they are professional immunoglobulin-secreting cells and are able to make proper post-translational modifications. Proper glycosylation has been shown to be important for antibody function. Advances in genetic engineering and molecular biology techniques have made it possible to isolate murine and human variable regions of almost any desired specificity. Antibodies and antibody variants produced in myeloma cells have been extremely helpful in elucidating the amino acid residues and structural motifs that contribute to antibody function. Because of their domain nature, immunoglobulin genes can be easily manipulated to produce chimeric or humanized antibodies. These antibodies are less immunogenic in humans and also retain their specificity for antigen and biologic properties. In addition, novel proteins in which antibodies are fused to non-immunoglobulin sequences as well as secretory IgA have been produced in myeloma cells.

Relative deficiency in IgA production by duodenal explants from German shepherd dogs with small intestinal disease

Matched samples of serum, saliva and tears were collected from four groups of dogs; two of the groups were German shepherd dogs (GSDs) either with (Group 1) or without (Group 4) a variety of small intestinal disorders; the remaining two groups were dogs of other breeds, again with (Group 2) or without (Group 3) small intestinal disease. Capture ELISAs were used to measure IgG, IgM, IgA and albumin concentrations within these samples; intestinal humoral immune status of clinical cases was assessed by quantifying immunoglobulin production from duodenal explant cultures.There were no significant differences in IgG, IgM or IgA concentrations in serum, saliva or tears between the different groups of dog. Moreover, no significant differences were noted between groups for IgG, IgM and IgA salivary and tear secretory indices. IgA production by 24-h explant cultures was significantly lower in GSDs compared with non-GSDs with small intestinal disease (groups 1 and 2, respectively), but the numbers of lamina propria IgA(+) plasma cells in duodenal biopsies were not different between groups. These results suggest that there may be a relative deficiency in local IgA secretion in GSDs with small intestinal enteropathies, which is not reflected in either serum IgA concentrations, or in secretion at unaffected mucosal sites. It remains to be determined whether such a deficiency is a breed-related primary defect, or whether it arises secondary to the pathological processes within the intestinal mucosa.

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 (IgG) transport, transmitting immunity from mother to young, and IgG protection, making IgG the longest surviving of all plasma proteins. Reflecting its role as transport receptor (termed FcRn, for neonatal rat intestine, the tissue from which it was first cloned), FcRB is expressed antenatally in the rabbit, mouse and rat fetal yolk sac and in human placental syncytiotrophoblasts, and neonatally in the intestinal epithelium of mice and rats. Reflecting its role as protection receptor (FcRp), FcRB is expressed in the vascular endothelium throughout life, where it protects IgG from the on-going catabolic activities of this tissue. FcRB detected in hepatocytes was hypothesized to mediate transport of IgG from serum to bile, thus potentially extending the transport expression (FcRn) of this receptor beyond the perinatal period. Our results show serum-to-bile transport of IgG to be unaffected in mice functionally deleted for FcRB. Accordingly, the hypothesis is rejected that FcRB functions as transport receptor (FcRn) in liver. The default conclusion is that FcRB in hepatocytes functions as FcRp, serving to protect IgG from catabolism in hepatocytes that accompanies the endocytic activity 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, paralleling the endocytic activities of the associated tissues.

Measurement of IgG, IgM and IgA concentrations in canine serum, saliva, tears and bile

Capture ELISAs, for canine IgG, IgM, IgA and albumin, were developed and used to analyse immunoglobulin (Ig) concentrations in both serum and secretions. Matched samples of serum, saliva and tears were taken from 31 dogs, assigned to two groups based on age, whilst bile samples were obtained from nine adult dogs at post-mortem. Serum and tear IgA concentrations were significantly lower in dogs < or = 12 months of age compared with dogs > 12 months of age (p = 0.006 and 0.045, respectively). There was no significant correlation between serum and secretory Ig levels, with the single exception of serum and tear IgM concentrations (rp = 0.553, p = 0.004). IgG and IgM concentrations were significantly correlated in matched tear and saliva samples (IgG: rp = 0.470, p = 0.023; IgM: rp = 0.651, p < 0.0001). Albumin concentrations were significantly correlated with IgG, but not IgM or IgA, in both saliva and tears (saliva, rp = 0.581, p = 0.004; tears, rp = 0.843, p < 0.0001) whilst IgA and IgM concentrations were significantly correlated with each other in both secretions (saliva, rp = 0.644, p = 0.001; tears, rp = 0.555, p = 0.009). Significantly, more Ig of all classes was secreted into saliva than tears as calculated by a secretory index. A large diurnal and day-to-day variation was observed in Ig concentrations in serial saliva and tear samples taken from a further four dogs. Serum Ig concentrations are therefore, poor indicators of mucosal secretion in this species and significant intra-individual variation exists in secretory Ig levels. Both findings should be taken into account in future studies of canine mucosal immunoglobulins.

Elevation of plasma immunoglobulin A in the spontaneously hypertensive rat

Prior studies describe deficiencies of T-cell-mediated immunity in the spontaneously hypertensive rat (SHR) strain of Okamoto and Aoki. This report describes an alteration of humoral immunity: elevation of the plasma concentration of immunoglobulin (Ig) A and of circulating IgA autoantibodies to single-stranded DNA, double-stranded DNA, and thyroglobulin. The increased plasma IgA levels are evident in prehypertensive SHR, hence not secondary to the hypertension, and they result mainly from increments in polymeric IgA. Plasma IgA content also varied concordantly with the level of systolic blood pressure as influenced by age (older > younger) and gender (male > female) in both the SHR and control Wistar-Kyoto rat strains. Strain differences in plasma IgG or IgM were not observed. Studies of peripheral blood lymphocytes indicate that increased production of IgA is one mechanism for the increment in plasma content. The number of blood lymphocytes capable of producing IgA in vitro in response to the mitogen lipopolysaccharide is increased in SHR. When cultured in the absence or presence of lipopolysaccharide, peripheral blood lymphocytes of SHR secrete more IgA in vitro than do cells of the control strain. No significant strain differences in biliary or renal excretion of IgA were observed. The observed alterations of IgA in the SHR either are causative factors in the development of the hypertension or are the products of an epiphenomenon in which IgA and blood pressure are affected separately, but in parallel, by causative factors related to rat strain, age, and gender.

Expression of the polymeric immunoglobulin receptor by cultured aged rat hepatocytes

The Fischer rat shows an age-related loss of both hepatic blood to bile transport and secretory component-specific binding sites for polymeric immunoglobulin (Ig) A. This age-related loss of hepatic IgA receptor function is also shown by cultured hepatocytes. It is reported here that compared with young cells, binding and uptake of 125I-polymeric IgA by cultured hepatocytes was markedly reduced in cells from senescent animals. In addition, cells from old animals showed markedly diminished secretion of secretory component determined by enzyme-linked immunosorbent assay and expression of polymeric immunoglobulin receptor determined by incorporation of 35S-labeled amino acid and fluorography. It is suggested that the age-related decrease in IgA receptor-mediated transport from serum to bile results, at least in part, from decreased expression and secretion of total hepatic secretory component.

Defense system in the biliary tract against bacterial infection

Bacteria can invade the biliary tract by ascending from the duodenum and via the hematogenous route from the hepatic portal venous blood. The sphincter of Oddi, situated at the junction of the biliary tract and the upper gastrointestinal tract, forms an effective mechanical barrier to duodenal reflex and ascending bacterial infection. Conversely, Kupffer cells and the tight junctions between hepatocytes help prevent bacteria and toxic metabolites from entering the hepatobiliary system from the portal circulation. The continuous flushing action of bile and the bacteriostatic effects of bile salts keeps the biliary tract sterile under normal conditions. Secretory immunoglobulin A (sIgA), the predominant immunoglobulin in the bile, and mucus excreted by the biliary epithelium probably function as antiadherence factors, preventing microbial colonization. When barrier mechanisms break down, as in surgical or endoscopic sphincterotomy and with insertion of biliary stents, pathogenic bacteria enter the biliary system at high concentrations and take up residence on any foreign bodies. Intrabiliary pressure is a key factor in the development of cholangitis. Chronic biliary obstruction raises the intrabiliary pressure. This adversely influences the defensive mechanisms such as the tight junctions, Kupffer cell functions, bile flow, and sIgA production in the system, resulting in a higher incidence of septicemia and endotoxemia in these patients. Knowledge of biliary defense against infection is still quite primitive. Unclear are the roles of sIgA in the bile, mechanism of bacterial adhesion to the biliary epithelium, Kupffer cell function in biliary obstruction, and the antimicrobial activity of bile salts.

Site of catabolism of autologous and heterologous IgA in non-human primates

Because of similarities between the human and monkey immune systems, we considered the monkey a suitable model for studies on the catabolism of various molecular forms of IgA, for which little information is available. The residualizing label dilactitol-[125I]tyramine was coupled to monkey (Macaca fuscata) IgA and IgG, as well as to human monomeric and polymeric myeloma IgA1 and IgA2 proteins. When labelled proteins were injected intravenously into monkeys, the non-metabolizable radioiodinated tracer accumulated at the cellular site of protein degradation, allowing identification of the catabolic sites. To determine the uptake of injected proteins by various tissues, monkeys were sacrificed 6-7 days after injection of labelled proteins, when blood-associated radioactivity was less than or equal to 10% of the injected dose, as measured by plasma clearance. When monkey or human monomeric IgA, as well as human polymeric IgA, irrespective of subclass, was administered to monkeys, the liver showed the greatest tissue uptake relative to total dose injected and to organ weight, and the highest acid soluble radioactivity (degraded protein). Although both hepatocytes and non-parenchymal liver cells were involved in IgA uptake, the hepatocytes were more active. Therefore, it appears that the liver is the major site of uptake and catabolism of IgA in monkeys and possibly in humans.

Antigenic mimicry of an immunoglobulin A epitope by a hepatitis B virus cell attachment site

The preS(21-47) sequence of the hepatitis B virus (HBV) envelope protein is involved in binding of the virus to cell receptors. A protein similarity search revealed a partial homology between this sequence and a segment of the human immunoglobulin A (IgA) heavy chain constant region, suggesting that the cell attachment site for HBV might be located on secretory component representing a receptor for polymeric IgA. Data presented herein do not support this hypothesis but provide evidence for immunological cross-reactivity between IgA and the preS(21-47) region of the HBV env protein.

Hepatocellular processing of endocytosed proteins

In conclusion, proteins of hepatobiliary transport utilize receptor-mediated endocytosis and intracellular vesicles and rely on functionally dynamic microtubules for their transport by hepatocytes. The many diverse transport pathways in hepatocytes reflect the many functions served by the uptake of various proteins from the blood. The mechanisms of sorting of ligands and their receptors in endosomes and the factors that regulate the intracellular transport pathways are not yet known. Future investigations in this area promise to yield many exciting discoveries about the hepatocellular processing of proteins.

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.

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 asialoglycoprotein receptor-mediated binding of human polymeric immunoglobulin A

In the rat, asialoorosomucoid and rat dimeric immunoglobulin A are both taken up by hepatocytes via receptor-mediated endocytosis. The fate of these two proteins, however, differs significantly. Rat dimeric IgA is taken up into smooth vesicles, transported to the bile canaliculus and secreted intact into the bile, whereas asialoglycoproteins are internalized via coated vesicles and transported to lysosomes for degradation. Recently, several studies both in the rat and in cultured human hepatoma cells have suggested that the receptor for asialoglycoproteins may play a role in the hepatic uptake and processing of human polymeric IgA. Using receptor-binding techniques, we have provided quantitative data for the competition of human monomeric, polymeric and secretory IgA with asialoorosomucoid for its receptor on liver plasma membrane preparations from rat, monkey and man. Some IgA molecules required desialylation with neuraminidase to enhance markedly their efficacy for asialoorosomucoid inhibition. Quantitatively, human IgA molecules showed an affinity for the ASOR receptor similar to that for asialoceruloplasmin. Rat dimeric IgA does not compete for this binding site. We conclude that human IgA can compete with ligands for the asialoglycoprotein receptor of rat, monkey and human liver. This receptor may provide an alternative pathway for the hepatic processing of IgA and IgA immune complexes when secretory component-mediated uptake is not available as in the monkey and man, particularly under pathological conditions where serum IgA concentrations accumulate to abnormally high levels.

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.

Secretory component-dependent binding of immunoglobulin A in the rat, monkey and human: a comparison of intestine and liver

The source and significance of immunoglobulin A in bile remains controversial. In the rat, and several other species, immunoglobulin A is transported through hepatocytes by a specific receptor, secretory component. In humans, immunohistochemical methods have indicated a distinct lack of receptors for immunoglobulin A on hepatocytes. Binding assays with 125I-immunoglobulin A and membranes from hepatocytes and intestinal cells of the rat display secretory component-dependent binding. Primate intestinal cells also show secretory component-specific binding of immunoglobulin A. Primate liver, on the other hand, does not show immunoglobulin A binding mediated by the polymeric immunoglobulin receptor.

Characteristics of serum IgA and liver IgA deposits in alcoholic liver disease

Patients with alcoholic liver disease frequently reveal an increase in IgA serum concentration and IgA deposits in a continuous pattern along hepatic sinusoids. We investigated whether the hepatic IgA deposits are a passive reflection of changes in concentration or composition of IgA in the circulation, or represent a distinct effect of alcohol on the liver. Forty-one patients with alcoholic liver disease (daily alcohol intake at least 50 gm for more than five consecutive years) were compared with 41 patients with nonalcoholic liver disease. Patients in both groups were matched for serum IgA and histopathological changes in the liver biopsy. IgA deposits in the liver were found in 78% of the alcoholic patients and in 12% of the nonalcoholic patients. The presence of deposits was not related to histopathological changes in the liver or to the serum IgA concentration. In serum IgA subclass distribution, alcoholic patients differed from nonalcoholic patients by a slight but significant shift to IgA2; in contrast, the hepatic IgA deposits in alcoholic patients were almost of the IgA1 subclass. Serum secretory component (which is an equivalent of serum secretory IgA) was elevated in both alcoholic and nonalcoholic patients; patients with a liver biopsy revealing hepatitis showed the highest level. In contrast, the hepatic deposits did not contain secretory component. We conclude that the continuous deposits of IgA along liver sinusoids are not a passive reflection of changes in concentration or composition of circulating IgA, but may represent a distinct effect of alcohol on the liver related to the role of this organ in IgA metabolism.

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.

Hepatobiliary transport of plasma IgA in the mouse: contribution to clearance of intravascular IgA

Labeled monomeric and polymeric (pIgA) mouse monoclonal IgA were injected intravenously into mice which were either sequentially bled for plasma turnover studies of IgA, or cannulated at their common bile duct, with excluded gallbladder, for quantitation of plasma-to-bile transport of pIgA. Our data show that mice do display a relatively high rate of biliary transport of plasma pIgA (22-28% of the injected 125I-labeled pIgA over 3 h), which accounts for approximately 90% of the total amount of pIgA (8.8 mg/kg/day) daily delivered by hepatic bile into the duodenal fluid of this species. However, in mice the absolute biliary output of pIgA does not exceed that of IgG (9.5 mg/kg/day) and the kinetics of the hepatobiliary transport of plasma pIgA appear to be slower than in the rat. Furthermore, as plasma survival studies of 125I-labeled pIgA yielded a plasma turnover of pIgA averaging 20.6 mg/kg/day, it can be approximated that the hepatobiliary pathway contributes for only 38% to the elimination of intravascular pIgA from mouse plasma, a figure to be compared to 89.8% in the rat and approximately 8.9% in man. We conclude that internal catabolism plays a dominant role in the clearance of intravascular pIgA in the mouse which appears as a model intermediate between rats and humans. Supporting this conclusion, serum pIgA two days after common bile duct ligation in 6 mice was increased by 2.5-fold vs. greater than 14-fold in ligated rats and 1.1-fold in humans with complete biliary obstruction.

Secretory component: the polymeric immunoglobulin receptor. What's in it for the gastroenterologist and hepatologist?

The primary function of the SC-pIg system is to secrete pIgs into various external secretions. The cellular mechanism responsible for this transport is schematically depicted in Figure 5. Polymeric immunoglobulin A, which is synthesized by plasma cells that are part of the mucosa-associated lymphoid tissue, gains access to the SC on the abluminal surface of epithelial cells by diffusion from sites of synthesis in mucosae or enters the blood circulation and is cleared, largely by hepatic transport, into bile. The pIgA binds to SC on the abluminal surface of the epithelial cells (and probably hepatocytes) initially by noncovalent interactions that are saturable, reversible, and specific for pIgA and IgM. Subsequently, covalent interaction between SC and its ligand occurs to a variable degree in different species. The SC-IgA complex is endocytosed by the epithelial cell or hepatocyte and is transported across the cell into the external secretions by a microtubule-dependent vesicular transport mechanism. At some point during the transport, the complex is rendered soluble by proteolytic cleavage of the membrane-associated SC molecule to release the soluble sIgA into the gland lumen or the canaliculus. In the intestinal lumen, SC helps protect the sIgA molecule from proteolytic degradation. The sIgA may play a major role in the mucosal defense against pathogenic organisms or harmful antigens. The SC-pIg system differs from many of the other known receptor-ligand interactions in several important ways. First, the synthesis or expression of the receptor (SC), or both, are not regulated by the concentration of the ligand. Second, SC probably is not dissociated from its ligand or recycled to the cell surface as it is secreted in complex with its ligand (pIg) into the external secretions. Third, the interaction of pIgs with their receptor does not function to regulate an intracellular process, but results in transcellular transport of the ligand, which acts in the external environment. Fourth, after initial noncovalent, reversible binding between the receptor and its ligand, the interaction becomes covalent by the formation of disulfide linkages between SC and the pIg. Finally, SC is initially inserted into the abluminal domain of epithelial cells as an integral membrane protein and subsequently is proteolytically cleaved to a soluble molecule which is secreted by the cell. Thus, in contrast to many cell-surface receptor-ligand interactions in which the ligand is ultimately degraded and the receptor is conserved, the SC-pIgA interaction results in partial proteolytic degradation of the receptor and conservation of the ligand.(ABSTRACT TRUNCATED AT 400 WORDS)

Biliary proteins

The study of biliary proteins has grown enormously in the last 10 years. Although much has been recently learned about the nature, origin and hepatobiliary transport of these proteins, little is known of their function in bile or their effect on its physical state. This review will focus on description of the proteins and mechanisms by which they are secreted into bile.

Secretion of immunoglobulins and plasma proteins from the jejunal mucosa. Transport rate and origin of polymeric immunoglobulin A

Parameters of secretion of IgA and several other plasma proteins from the jejunal mucosa were investigated in 11 individuals who had a normal distribution of Ig-containing cells in the lamina propria and in one patient who was totally deficient in jejunal IgA and IgM plasmacytes. Jejunal samples were collected during segmental gut perfusion. The following results were obtained: (a) The secretion of polymeric IgA (p-IgA, mean equals 217 micrograms/40 cm per min) exceeded those of albumin (132 micrograms), IgG (35 micrograms), and monomeric IgA (m-IgA, 15 micrograms, or 6.4% of total IgA). About 35% of IgA was IgA2 in the jejunal secretion, compared with approximately 23% in serum. This closely corresponds to the 35 and 24% of IgA2 plasmocytes in jejunal mucosa and peripheral lymph nodes, respectively. (b) For each protein, a relative coefficient of excretion (RCE) was calculated (jejunum to serum concentration ratio expressed relative to that of albumin). RCEs of 1.41 for orosomucoid, 1.0 for albumin, 0.83 for IgG, and 0.74 for IgE and, in the deficient patient, of 0.64 for m-IgA and 0.016 for IgM were obtained. This was inversely related to the molecular weight of these proteins and indicated their predominantly passive transport into the jejunum. However, in normal individuals, the RCE of transferrin (approximately 1.11 greater than 1, P greater than 0.05), alpha 2-macro globulin (approximately 0.77), m-IgA (approximately 1.98), and p-IgA (approximately 218) exceeded the value expected from simple seepage from plasma, thus pointing to an additional role of either local gut synthesis and/or active transepithelial transport. (c) Approximately 98% of p-IgA, approximately 99% of IgM, and approximately 68% of m-IgA in jejunal secretions were derived from local production in the gut wall, as determined by 125I-p-IgA specific activities and/or by comparison between the RCE values of the deficient patient to the values of controls. Therefore, the jejunal production of p-IgA (approximately 312 mg/d per 40 cm vs. approximately 54 mg/d from bile) contributes the majority of upper intestinal IgA in humans. The active transport of plasma p-IgA across the intestinal mucosa (approximately 0.08 mg/40 cm per kg per d) contributes less than 2% of the total amount of p-IgA (4.5 mg/kg per d) that is cleared daily from plasma.

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.