Function of the human liver in IgA homeostasis in plasma

Mucosal response of inactivated and recombinant COVID-19 vaccines in Congolese individuals

BACKGROUND

The efficacy of immunization against an airborne pathogen depends in part on its ability to induce antibodies at the major entry site of the virus, the mucosa. Recent studies have revealed that mucosal immunity is poorly activated after vaccination with messenger RNA vaccines, thus failing in blocking virus acquisition upon its site of initial exposure. Little information is available about the induction of mucosal immunity by inactivated and recombinant coronavirus disease 2019 (COVID-19) vaccines. This study aims to investigate this topic.

METHODS

Saliva and plasma samples from 440 healthy Congolese were collected including (1) fully vaccinated 2 month postvaccination with either an inactivated or a recombinant COVID-19 vaccine and (2) nonvaccinated control group. Total anti-severe acute respiratory syndrome coronavirus 2 receptor-binding domain IgG and IgA antibodies were assessed using in-house enzyme-linked immunosorbent assays for both specimens.

FINDINGS

Altogether, the positivity of IgG was significantly higher in plasma than in saliva samples both in vaccinated and nonvaccinated control groups. Inversely, IgA positivity was slightly higher in saliva than in plasma of vaccinated group. The overall IgG and IgA levels were respectively over 103 and 14 times lower in saliva than in plasma samples. We found a strong positive correlation between IgG in saliva and plasma also between IgA in both specimens (r = .70 for IgG and r = .52 for IgA). Interestingly, contrary to IgG, the level of salivary IgA was not different between seropositive control group and seropositive vaccinated group. No significant difference was observed between recombinant and inactivated COVID-19 vaccines in total IgG and IgA antibody concentration release 2 months postvaccination both in plasma and saliva.

CONCLUSION

Inactivated and recombinant COVID-19 vaccines in use in the Republic of Congo poorly activated mucosal IgA-mediated antibody response 2 months postvaccination.

Immunoglobulin Disorders and the Oral Cavity: A Narrative Review

The oral mucosa is a mechanical barrier against the penetration and colonization of microorganisms. Oral homeostasis is maintained by congenital and adaptive systems in conjunction with normal oral flora and an intact oral mucosa. Components contributing to the defense of the oral cavity include the salivary glands, innate antimicrobial proteins of saliva, plasma proteins, circulating white blood cells, keratinocyte products of the oral mucosa, and gingival crevicular fluid. General disturbances in the level of immunoglobulins in the human body may be manifested as pathological lesions in the oral mucosa. Symptoms of immunoglobulin-related general diseases such as mucous membrane pemphigoid (MMP), pemphigus vulgaris (PV), linear IgA bullous dermatosis (LABD), Epidermolysis Bullosa Aquisita (EBA), and Hyper-IgE syndrome (HIES) may appear in the oral cavity. In this review, authors present selected diseases associated with immunoglobulins in which the lesions appear in the oral cavity. Early detection and treatment of autoimmune diseases, sometimes showing a severe evolution (e.g., PV), allow the control of their dissemination and involvement of skin or other body organs. Immunoglobulin disorders with oral manifestations are not common, but knowledge, differentiation and diagnosis are essential for proper treatment.

Secretory immunity with special reference to the oral cavity

The two principal antibody classes present in saliva are secretory IgA (SIgA) and IgG; the former is produced as dimeric IgA by local plasma cells (PCs) in the stroma of salivary glands and is transported through secretory epithelia by the polymeric Ig receptor (pIgR), also named membrane secretory component (SC). Most IgG in saliva is derived from the blood circulation by passive leakage mainly via gingival crevicular epithelium, although some may be locally produced in the gingiva or salivary glands. Gut-associated lymphoid tissue (GALT) and nasopharynx-associated lymphoid tissue (NALT) do not contribute equally to the pool of memory/effector B cells differentiating to mucosal PCs throughout the body. Thus, enteric immunostimulation may not be the best way to activate the production of salivary IgA antibodies although the level of specific SIgA in saliva may still reflect an intestinal immune response after enteric immunization. It remains unknown whether the IgA response in submandibular/sublingual glands is better related to B-cell induction in GALT than the parotid response. Such disparity is suggested by the levels of IgA in submandibular secretions of AIDS patients, paralleling their highly upregulated intestinal IgA system, while the parotid IgA level is decreased. Parotid SIgA could more consistently be linked to immune induction in palatine tonsils/adenoids (human NALT) and cervical lymph nodes, as supported by the homing molecule profile observed after immune induction at these sites. Several other variables influence the levels of antibodies in salivary secretions. These include difficulties with reproducibility and standardization of immunoassays, the impact of flow rate, acute or chronic stress, protein loss during sample handling, and uncontrolled admixture of serum-derived IgG and monomeric IgA. Despite these problems, saliva is an easily accessible biological fluid with interesting scientific and clinical potentials.

Do salivary antibodies reliably reflect both mucosal and systemic immunity?

Two major antibody classes operate in saliva: secretory IgA (SIgA) and IgG. The former is synthesized as dimeric IgA by plasma cells (PCs) in salivary glands and is exported by the polymeric Ig receptor (pIgR). Most IgG in saliva is derived from serum (mainly via gingival crevices), although some is locally produced. Gut-associated lymphoid tissue (GALT) and nasopharynx-associated lymphoid tissue (NALT) do not contribute equally to mucosal PCs throughout the body. Thus, enteric immunostimulation is an inadequate mode of stimulating salivary IgA antibodies, which are poorly associated with the intestinal SIgA response, for instance after enteric cholera vaccination. Nevertheless, the IgA response in submandibular/sublingual glands is better related to B cell induction in GALT than the parotid response. Such disparity is suggested by the elevated levels of IgA in submandibular secretions of AIDS patients, paralleling their highly upregulated intestinal IgA system. Moreover, in patients with active celiac disease, IgA antibodies to disease-precipitating gliadin are reliably represented in whole saliva but not in parotid secretion. Parotid SIgA may be more consistently linked to immune induction in palatine tonsils and adenoids (human NALT), as supported by the homing molecule profile of NALT-derived B cell blasts. Also several other variables influence the levels of antibodies in oral secretions. These include difficulties with reproducibility and standardization of immunoassays, the impact of flow rate, acute or chronic stress, protein loss during sample handling, and uncontrolled admixture of serum-derived IgG and monomeric IgA. Despite such problems, saliva remains an interesting biological fluid with great scientific and clinical potentials.

Mucosal immunoglobulins

Due to their vast surface area, the mucosal surfaces of the body represent a major site of potential attack by invading pathogens. The secretions that bathe mucosal surfaces contain significant levels of immunoglobulins (Igs), which play key roles in immune defense of these surfaces. IgA is the predominant antibody class in many external secretions and has many functional attributes, both direct and indirect, that serve to prevent infective agents such as bacteria and viruses from breaching the mucosal barrier. This review details current understanding of the structural and functional characteristics of IgA, including interaction with specific receptors (such as Fc(alpha)RI, Fc(alpha)/microR, and CD71) and presents examples of the means by which certain pathogens circumvent the protective properties of this important Ig.

Time-course changes of serum immunoglobulins (IgA, IgG, IgM) after liver transplantation for alcoholic cirrhosis

BACKGROUND

Serum immunoglubulin increase is a hallmark of liver disease. Serum IgA is specifically increased in alcoholic liver disease, which has been considered an IgA-associated disorder. No previous studies have been focused on the time-course changes of serum immunoglobulins (IgG, IgA, IgM) after liver transplantation.

AIM OF THE STUDY

To investigate the outcome of serum immunoglobulin levels in alcoholic cirrhosis after liver transplantation, with special focus on IgA values.

PATIENTS AND METHODS

A total of 18 patients, liver transplanted in our center because of alcoholic cirrhosis were included in the study. Serum immunoglobulins were assayed by nephelometry before transplantation and at different intervals after the procedure from the intraoperative period to more than a year after transplantation.

RESULTS

A rapid drop in IgA, IgG and IgM concentration was observed during the surgical procedure, particularly after donor liver reperfusion, and during the first days after transplantation. Mild transient hypogammaglobulinemia (IgG) was present. After a subsequent moderate re-increase, serum immunoglobulins (particularly IgA and IgG) remained stable within normal or near-normal limits during the following months after liver transplantation.

CONCLUSIONS

Liver transplantation for alcoholic cirrhosis is followed by a decrease in serum IgA, IgG and IgM. In the short term, low IgG levels may be observed. In the long term, serum levels of IgA and IgG become normal or near-normal.

Increased serum concentration of IgA2 subclass and IgA2/IgA1 ratio: specific markers of chronic alcoholic abuse?

Enhanced serum IgA concentrations are common in alcoholic liver cirrhosis, but functional differences between IgA subclasses and their relation with interleukin-6 (IL-6) have not been described. Distinct immunoregulatory mechanisms may exist that selectively affect one subclass. This possibility prompted us to investigate the distribution of IgA1 and IgA2 subclasses in the serum of 25 heavy alcohol drinkers (alcohol: 80 to 200 g per day) without clinical disorders, in comparison with 35 patients affected by alcoholic liver cirrhosis, 29 viral hepatitis patients and 33 social drinkers as a control group. Mean (+/- SD) IgA2 concentration (0.56 +/- 0.31 g/l) was significantly increased (p < 0.01) in heavy alcohol drinkers, with an IgA2/IgA1 ratio of 0.33 +/- 0.12, while the mean total IgA concentration was similar to the control group. Mean IgA1 and IgA2 concentrations were significantly increased (p < 0.001) in alcoholic liver cirrhosis patients (6.13 +/- 4.52 g/l and 1.83 +/- 1.93 g/l respectively, with an IgA2/IgA1 ratio of 0.32 +/- 0.19) and viral hepatitis patients (3.66 +/- 2.59 g/l and 0.69 +/- 0.67 g/l respectively, with an IgA2/IgA1 ratio of 0.21 +/- 0.14) High serum IL-6 concentrations (34 +/- 33 ng/l) were correlated with elevated IgA1 and IgA2 concentrations only in patients with alcoholic liver cirrhosis. IgA2 subclass and IgA2/IgA1 ratio could therefore be used as markers of chronic alcohol abuse directly related to the extent and duration of the alcohol abuse and the effectiveness of alcohol withdrawal.

Immunoglobulin A and interleukin 6 form a positive secretory feedback loop: a study of normal subjects and alcoholic cirrhotics

Patients with alcoholic liver cirrhosis (ALC) have high serum levels and spontaneous in vitro production of immunoglobulin (Ig) A. Deposits of IgA are also found in liver sinusoids. Increased interleukin 6 (IL-6) production is another feature of this disease. This study shows a linear correlation between increased lipopolysaccharide (LPS)-induced IL-6 production and increased spontaneous IgA and IgG secretion by peripheral blood mononuclear cells (PBMCs). PBMCs and purified monocytes isolated from healthy control subjects and patients with ALC contain elevated IL-6 messenger RNA levels and produce IL-6 in response to stimulation with soluble polymeric IgA (p-IgA) or attached monomeric IgA (m-IgA) but not with soluble m-IgA. The addition of monospecific antibody to human IL-6 inhibits spontaneous IgA production by PBMC. This inhibition is more pronounced in patients with ALC. These data provide evidence that IgA, possibly by attachment to cells possessing Fc alpha receptors and secreting IL-6, is involved in the production of this major mediator and the amplification of Ig secretion. Circulating IgA and IgA deposits could therefore initiate a process of autoamplification implicated in the development of hypergammaglobulinemia in ALC.

The presence and measurement of secretory component in human bile and blood

Five monoclonal antibodies which recognized three separate epitopes on the free secretory component molecule were produced using free secretory component obtained from human colostrum. Two-site immunoradiometric assays were developed to measure free secretory component and secretory IgA. Monoclonal antibody M9 was used on coated plates as the capture antibody. Monoclonal antibody M7 was used as the labelled signal antibody for the assay of free secretory component and a commercially available monoclonal anti-IgA antibody was used as the labelled signal antibody for the assay of secretory IgA. Free secretory component was found in human serum and bile. In serum, its concentration was raised in patients with high serum alkaline phosphatase due to liver disorders but not in patients with high serum alkaline phosphatase due to non-liver disorders. In bile from bile duct drains collected during the first week after liver transplantation, free secretory component was found in concentrations of up to 33 mg/l, in vast excess of that found in bile from gallstone patients (up to 0.3 mg/l). Bile from gallstone patients but not from liver transplant patients produced proteolytic degradation of free secretory component when incubated in vitro. The finding of large amounts of free secretory component, the free cleaved fragment of the polymeric IgA receptor in human bile, further supports the existence of the blood to bile transhepatocytic pathway in humans.

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.

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.

Hormonal influence on the secretory immune system of the eye: endocrine impact on the lacrimal gland accumulation and secretion of IgA and IgG

The objective of the current investigation was to explore the processes underlying the androgen control of tear IgA and to determine whether hormone exposure also modifies tear IgG content. In addition, studies evaluated the impact of diabetes on the androgen regulation of secretory immunity in the eye. Tears and lacrimal glands were collected from age-matched, adult male rats, which had undergone hypophysectomy, selective ablation of the anterior pituitary, streptozotocin-induced diabetes, sham-surgery and/or orchiectomy and had been exposed to vehicle or physiological amounts of testosterone for varying periods of time. Our findings demonstrated that testosterone administration selectively increased the accumulation of IgA, but not IgG, in tears and lacrimal glands of orchiectomized rats. This hormone effect was associated with a 2-fold enhancement of the IgA transfer from lacrimal tissue to tears; IgA movement was against a gradient. In contrast, androgen exposure had no significant influence on the lacrimal gland/tear transfer of IgG, which was down a 90-fold gradient. Testosterone action on the lacrimal gland appeared to involve an increase in IgA production, but not a consistent alteration in the total number of IgA-containing cells. Similarly, androgen exposure had no impact on the population of IgG-containing lymphocytes in lacrimal tissue. Of interest, ablation of the anterior or entire pituitary in orchiectomized rats, which procedure inhibits testosterone-induced stimulation of tear IgA levels, significantly reduced the total number of IgA-containing cells in the lacrimal gland. Induction of diabetes by streptozotocin injection to orchiectomized rats resulted in diminished tear IgA content and decreased numbers of lacrimal IgA-positive lymphocytes, but did not prevent the testosterone-associated rise in IgA antibody content. In summary, our findings demonstrate that androgens increase the lacrimal gland production and secretion of IgA, but not IgG.

Receptor-mediated binding and uptake of immunoglobulin A by human liver

We have studied the molecular mechanisms of the binding and uptake of secretory and serum immunoglobulin A (IgA) of both subclasses (1 and 2) and molecular forms (monomer and polymer) by the particulate fraction of human liver homogenate and by a human hepatoma cell line (HepG2). Inhibition by asialoorosomucoid and the requirement for the presence of calcium indicated that the binding of secretory IgA and polymeric IgA1 was mediated by the asialoglycoprotein receptor. Secretory component, which functions as a receptor for polymeric IgA in several animal species, was detected in the epithelial cells of bile ducts, but not in hepatocytes. Secretory IgA and all molecular forms and subclasses of serum IgA were bound by HepG2 cells, which do not express secretory component. The requirement for the presence of calcium, the presence of a terminal galactose residue in IgA, and the molecular weight of the major plasma membrane protein responsible for binding (41,700 daltons) indicated the involvement of asialoglycoprotein receptor. Immunoglobulin A proteins bound by HepG2 cells were endocytosed and catabolized.

The transport and metabolism of bovine IgM

IgM is present in cows milk, is able to bind secretory component (SC), has a purported role as a secretory immunoglobulin in other species and has been identified with various antibody functions in cows milk. To determine the origin of cows milk IgM, we administered extrinsic 131I-IgM to lactating cows with cannulated bile and parotid ducts and studied the kinetics of its disappearance from serum and its appearance in milk, bile and parotid saliva for 60 hr post-injection. Pentameric IgM appeared to require a long equilibration time and disappeared from serum with a T1/2 of 40 hr. The transport of IgM into bile also appeared biphasic. Results showed that no 131I-IgM was transported intact into parotid saliva and that most radioactivity in milk and bile after 6 hr was in the form of low mol. wt, TCA-precipitable fragments rather than of the size of a pentamer. During the first 24 hr only 0.83% of the administered dose reached the milk in pentameric form, nevertheless, isotope dilution calculations indicated that nearly all milk IgM was derived from serum. During a 12 hr collection period, corresponding to one milking, greater than 200 mg of serum IgM is secreted in milk. During the first 24 hr, only 0.70% of the administered IgM reached the bile as a pentamer. It was calculated that 50% of the pentameric IgM in bile, 3 hr after administration, was serum-derived. Twenty-five per cent of the IgM appearing in bile and ca 10% of the IgM appearing milk, becomes associated with secretory component. A hypothesis to explain the degradation associated with this inefficient transport mechanism is presented.

Salivary and serum antibodies in patients with Yersinia enterocolitica infection

The systemic and secretory antibody response in patients with yersiniosis was studied by measuring Yersinia antibodies of various isotypes (IgG, IgA and IgM) and the total concentrations of the corresponding Ig classes in serum and saliva. Specific antibody activities of IgG (IgG antibody concentration divided by IgG concentration) were almost identical in serum and saliva of all patients although the pair of values varied from patient to patient. Almost all salivary IgG of these patients was therefore probably a transudate from the blood. Specific antibody activities of IgA and of IgM, on the other hand, varied independently in serum and saliva. Occasional great differences between serum and saliva values indicate that most of the salivary IgA and IgM (more than 90%) is produced locally at least in some individuals. The local anti-Yersinia response was restricted to IgA in some individuals, to IgM in others, whilst yet other patients produced salivary antibodies of both isotypes.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on IgA serum and bile levels in rats

Serum IgA is actively transported from blood to bile against a concentration gradient in the liver by the binding of dimeric IgA to secretory component, endocytosis and transport to the bile canaliculus by vesicles. As 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been shown to elicit hepatotoxicity, the effects of TCDD on rat serum and bile IgA levels were investigated. Rats were orally administered 50 micrograms TCDD/kg body weight in 95% corn oil: 5% acetone. At days 5, 10, 15, 20 and 30 after treatment, rats were anesthetized and a cannula inserted into the bile duct for collection of bile. In addition, blood was drawn, and, after euthanasia, the liver and thymus weights were recorded. Enzyme-linked immunosorbent assay (ELISA) techniques were employed to determine IgA in serum and bile and IgG levels in serum. Rocket immunoelectrophoresis was carried out to support ELISA results. It was found that serum IgA increased with time while serum IgG remained unchanged. In addition, while serum IgA levels were increasing, there was a concomitant decrease in biliary IgA. Thymus and liver weight changes were also observed. The data indicate that TCDD affects hepatic clearance of serum dimeric IgA and suggests that liver damage may be reflected by increased serum levels of IgA.

Low level transport of IgA to bile via the asialoglycoprotein receptor

The rat and rabbit transport IgA from blood to bile by a highly efficient transcellular pathway mediated by secretory component (SC). Other mammals do not express SC on liver hepatocytes, but they do transport a small amount of IgA to bile. In the first part of this study, human polymeric IgA was radiolabeled and depleted of SC binding activity by successive affinity adsorption. Transport of this preparation intact to rat bile was 4%, but was reduced to 2% when 50 mg unlabeled asialoglycoprotein was preadministered. The 2% decline corresponds to the percent of asialo-orosomucoid diverted to bile from the lysosomal pathway. In guinea-pigs, missorting of asialo-orosomucoid intact to bile was 10% of the injected dose. Transport of normal human IgA to bile was 1-2%, even though guinea-pigs do not express SC in the liver. Excess unlabeled asialofetuin reduced the transport of asialo-orosomucoid by 10-fold and IgA by 6-fold. This demonstrates that the asialoglycoprotein receptor can mediate transport of IgA to bile in small amounts, but that this transport may be only a biological artifact resulting from limited fidelity of intracellular protein sorting.

Gut-liver interactions in the IgA system

Recent experimental work in rodents has demonstrated that the liver can function as an 'IgA-pump', transporting polymeric IgA from serum to bile via a secretory-component mediated uptake at the surface of the hepatic parenchymal cell. Studies are surveyed which indicate that, although this process occurs in man, it is to a strikingly lesser degree, and interruption of this pathway does not explain the high serum IgA levels often seen in chronic liver disease: the explanation for this lies in enhanced synthesis of IgA, and diminished polymeric IgA catabolism unrelated to hepatobiliary transport.

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.

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.

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)