Isolation and characterization of secretory IgA (sIgA) and free secretory component (FSC) from rat bile

Purification and Characterisation of Badger IgA and Its Detection in the Context of Tuberculosis

European badgers are a wildlife reservoir of bovine tuberculosis in parts of Great Britain. Accurate diagnosis of tuberculosis in badgers is important for the development of strategies for the control of the disease. Sensitive serological tests for badger TB are needed for reasons such as cost and simplicity. Assay of mucosal IgA could be useful for diagnosing respiratory pathogens such as Mycobacterium bovis and for monitoring the response to mucosal vaccination. To develop an IgA assay, we purified secretory IgA from badger bile, identifying secretory component (SC), heavy chain (HC) and light chain (LC), at 66, 46 and 27 Kda, respectively, on the basis of size comparison with other species. Monoclonal antibodies (mAbs) were generated to purified IgA. We selected two for ELISA development. The detection limit of the IgA-specific mAbs was found to be approximately 20 ng/mL when titrated against purified badger bile. One monoclonal antibody specific for badger IgA was used to detect IgA in serum and tracheal aspirate with specificity to an immunodominant antigen of M. bovis. An M. bovis infection dose-dependent IgA response was observed in experimentally infected badgers. IgA was also detected by immunohistochemistry in the lungs of bTB-infected badgers. With further characterisation, these represent new reagents for the study of the IgA response in badgers.

[An immunological study of secreted human polymeric immunoglobulins' J-peptide tissue specificity]

Contents of J-peptide of secreted human polymeric immunoglobulins may vary considerably with different pathologies, reflecting the state of the adaptive immune system. In this work assessed the content of J-peptide in various tissues of healthy people to use as a baseline for studies related to the change in the content of J-peptide in pathologies.

Secretory component is cleaved by neutrophil serine proteinases but its epithelial production is increased by neutrophils through NF-kappa B- and p38 mitogen-activated protein kinase-dependent mechanisms

We previously showed that expression of polymeric immunoglobulin receptor (pIgR)/secretory component (SC), the epithelial receptor assuming transport of polymeric IgA in mucosal secretions, is strongly decreased in severe chronic obstructive pulmonary disease. Here, we evaluated in vitro the effects of polymorphonuclear neutrophil (PMN) mediators on pIgR/SC. On polyacrylamide gel electrophoresis analysis, soluble SC was rapidly cleaved by supernatants from phorbol-myristate-acetate-activated PMN, through a serine proteinase activity. Moreover, purified PMN serine proteinases also cleaved SC. Similarly, polymeric IgA was rapidly cleaved in monomers by neutrophil elastase, whereas secretory immunoglobulin A was relatively resistant to neutrophil elastase. Surface pIgR on human bronchial epithelial cells was also cleaved by serine proteinases, as shown by immunofluorescence. In contrast, pIgR/SC production by cultured epithelial cells (quantified by enzyme-linked immunosorbent assay) was significantly increased by supernatants from interleukin-8/formylmethionylleucylphenylalanine-activated PMN (122.6 +/- 17.3 versus 70.9 +/- 9 ng/mg protein, P < 0.01). Upregulation of pIgR/SC production by bronchial epithelial cells was abolished by nuclear factor kappa B- and p38 mitogen-activated protein kinase (MAPK) inhibitors. Moreover, supernatants from interleukin-8/formylmethionylleucylphenylalanine-activated PMN induced the phosphorylation of I kappa B-alpha and p38 MAPK in epithelial cells, independently of serine proteinases. Thus, PMN serine proteinases cleave pIgR/SC, whereas activated PMN induce an increased pIgR/SC expression through epithelial activation of nuclear factor kappa B and p38 MAPK pathways.

Polymeric IgA binding to the human pIgR elicits intracellular signalling, but fails to stimulate pIgR-transcytosis

The intracellular pathway of polymeric immunoglobulin receptor (pIgR) is governed by multiple signals that lead to constitutive transcytosis. In addition, in transfected polarized MDCK cells, polymeric immunoglobulin A (pIgA) binding stimulates rabbit pIgR-transcytosis, owing to phospholipase-C gamma 1 activation and increase of intracellular calcium. Transcytosis of rat pIgR across hepatocytes is similarly accelerated by pIgA injection. In contrast we show here that human Madrin-Darby Canine Kidney (pIgR)-transcytosis, in human Calu-3 and human pIgR-transfected MDCK cells, is not promoted by pIgA, as monitored by a continuous apical release of its secreted ectodomain. However, the incubation of cells expressing human or rabbit pIgR with pIgA induces a comparable IP3 production, and pIgR-transcytosis of either species is accelerated by the protein kinase C (PKC)-activator phorbol myristate acetate. Without pIgA, mimicking phospholipase-C activation by combining low concentrations of phorbol myristate acetate with ionomycin, or high concentrations of ionomycin alone, stimulates the rabbit, but not the human, pIgR transcytosis. These data suggest that the species difference in pIgA-induced pIgR-transcytosis does not stem from the defective production of second messengers, but from a different sensitivity of pIgR to intracellular calcium. Our results outline the danger of extrapolating to humans the abundant data obtained from mucosal vaccination of laboratory animals.

Species-specific binding of human secretory component to SpsA protein of Streptococcus pneumoniae via a hexapeptide motif

SpsA, a pneumococcal surface protein belonging to the family of choline-binding proteins, interacts specifically with secretory immunglobulin A (SIgA) via the secretory component (SC). SIgA and free SC from mouse, rat, rabbit and guinea-pig failed to interact with SpsA indicating species-specific binding to human SIgA and SC. SpsA is the only pneumococcal receptor molecule for SIgA and SC as confirmed by complete loss of SIgA and SC binding to a spsA mutant. Analysis of recombinant SpsA fusion proteins showed that the binding domain is located in the N-terminal region of SpsA. By the use of different truncated N-terminal SpsA fusion proteins, the minimum binding domain was shown to be composed of 112 amino acids (residues 172-283). The sequence of this 112-amino-acids domain was used to spot synthesize 34 overlapping peptides, consisting of 15 amino acids each, with an offset of three amino acids on a cellulose membrane. One of the peptides reacted specifically with both SIgA and SC. By using a second membrane with immobilized synthetic peptides of decreasing length containing parts of the identified 15-amino-acid motif a hexapeptide, YRNYPT was identified as the binding motif for SC and SIgA. SpsA proteins with a size smaller than the assay-positive domain of 112 amino acids were able to inhibit the interaction of SIgA and pneumococci provided they contained the binding motif. The results indicated that the hexapeptide YRNYPT located in SpsA of pneumococcal strain type 1 (ATCC 33400) between amino acids 198 and 203 is involved in SIgA and SC binding. Because synthetic peptides containing only parts of the hexapeptide also assayed positive, these results further suggest that at least the amino acids YPT of the identified hexapeptide are critical for binding to SC and SIgA. Amino acid substitutions in the identified putative binding motif abolished SC-/SIgA-binding activity of the mutated SpsA protein, confirming the functional activity of this hexapeptide and the critical role of the amino acids YPT in SC and SIgA binding. Identification of this motif, which is highly conserved in SpsA protein among different serotypes, might contribute towards a new peptide based vaccine strategy.

Lack of SC/pIgR-mediated epithelial transport of a human polymeric IgA devoid of J chain: in vitro and in vivo studies

Three human polymeric IgA (pIgA) myeloma proteins of tetrameric size were compared for their J-chain content, their in vitro secretory component (SC)-binding ability, and their capacity to be transcytosed by polymeric immunoglobulin receptor (pIgR)-expressing epithelial cells in vitro and rat hepatocytes in vivo. One of the three pIgA preparations, pIgA-L, was shown to lack J chain and was unable to combine with purified free human and rat SC, whereas pIgA-G and pIgA-C contained J chain and combined readily with SC. Furthermore, pIgA-L was not transferred into rat bile after intravenous injection, and was hardly transported apically by polarized Madin-Darbey canine kidney cell monolayers expressing the human pIgR, whereas pIgA-G and pIgA-C were efficiently transported in both test systems. Together with our recent demonstration that antibodies to human J chain block the SC/pIgR-mediated epithelial transport of pIgA, these data unanimously confirm the proposed key role of J chain in the epithelial transport of polymeric immunoglobulins into exocrine secretions.

Primary rat lacrimal cells undergo acinar-like morphogenesis on reconstituted basement membrane and express secretory component under androgen stimulation

Single cells or small cell clusters, isolated from the rat lacrimal gland, were incubated on reconstituted basement membrane (matrigel) in a well-defined serum-free medium. During the first days of culture, cells reassociated and reorganized in structures resembling acini. These multicellular structures, maintained in culture for 2 weeks, consisted of well-polarized cuboidal cells surrounding a central lumen and exhibiting apically located microvilli. Myoepithelial cells were observed at the periphery of the acinar structures. Both in the native lacrimal and in the cultured aggregates, epithelial cells displayed strong immunoreactivity for cytokeratin 8, while myoepithelial cells were immunoreactive for vimentin and alpha-smooth muscle isoactin. These data indicate that the cultured aggregates closely mimic the in vivo architecture of lacrimal glands both by morphology and immunohistochemistry. We further demonstrated the presence of an intact androgen receptor and the ability of the cultured aggregates to respond to androgens with increased secretion of the secretory component. Comparable androgen responses were observed in lacrimal gland cultures of 5-week-old male and female rats. In conclusion, we report a morphologically and functionally differentiated culture system of primary rat lacrimal cells, in which androgen-regulated gene expression was observed. This culture model provides a unique experimental paradigm for studying the effects of hormones, cytokines, and growth factors on the morphogenesis, growth, and functional differentiation of lacrimal glands.

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.

Hepatobiliary transport of IgA in the golden Syrian hamster (Mesocricetus auratus)

Do hamsters, like rats, rabbits and mice, possess an hepatocyte 'IgA pump' whereby circulating plasma polymeric IgA (pIgA) is actively transported into bile, against a concentration gradient, via the polymeric Ig receptor or secretory component (SC)? Precipitating antisera, raised against rat Igs and serum proteins, and crossreacting with their hamster homologues, detected hamster SC by immunoelectrophoresis in bile, but not serum. Gel filtration of hamster bile indicated that free SC eluted between IgG and albumin, as for other mammals. Hamster bile IgA was pIgA, and was true secretory IgA (SIgA) by its reaction with anti-SC antiserum and by SDS-PAGE with reduction. Hamster serum IgA comprised both pIgA and IgA monomers. Mean bile-to-serum concentration ratios (B/S) for IgA, IgG, transferrin and albumin, measured by radial immunodiffusion, were 2.65, 0.019, 0.024, and 0.016, respectively, demonstrating strongly selective enrichment of bile in IgA. Human 125I-labelled dimeric IgA was injected into the circulation of five hamsters with cannulated bile ducts; 20% of the [125I]IgA (> 95% precipitable by trichloroacetic acid) was recovered in bile within 5 h, a figure close to that for mice, but smaller than that for rats and rabbits. The data suggest that bile significantly contributes to hamster intestinal SIgA, as shown for rats, rabbits and mice. This could be relevant to studies where hamsters are used as an experimental model for infection by the human intestinal pathogen, Clostridium difficile.

Interaction between streptococcal protein Arp and different molecular forms of human immunoglobulin A

Protein Arp, the IgA-binding protein of the group A Streptococcus, has affinity for the Fc-part of IgA. The binding between protein Arp and several different molecular forms of human IgA was characterized. It was found that protein Arp bound with higher affinity to uncomplexed forms of IgA than to complexed forms (secretory IgA, alpha 1-antitrypsin-IgA and alpha 1-microglobulin-IgA). Thus, the affinity constant was 2.0-5.9 x 10(8) M-1 for the binding to monomeric, dimeric, trimeric, and quadrimeric IgA, and 4.5-5.0 x 10(7) M-1 for binding to the complexed forms. Among the uncomplexed IgA-molecules, the affinity constant was in the same range for J chain-containing forms (dimeric, trimeric and quadrimeric IgA) as for forms without J chain (monomeric and a particular quadrimeric IgA devoid of J chain). Western blotting demonstrated that protein Arp bound exclusively to the alpha-chain of all IgA-forms. Several lines of evidence pointed to a localization of the binding site to the C alpha 3-domain. First, protein Arp did not bind to three N-terminal alpha-chain fragments which lacked a region corresponding to the C alpha 3-domain, including that form a four-chain myeloma IgA, naturally occurring in plasma. Second, the binding to dimeric and tri/quadrimeric IgA was partially blocked by an added secretory component, which has been suggested to bind to the C alpha 2- and C alpha 3-domains of the alpha-chain. Finally, alpha 1-antitrypsin and alpha 1-microglobulin, in the weakly binding IgA-complexes, have been shown to be linked to the C alpha 3-domain via the penultimate amino acid residue of the alpha-chain peptide, supporting the hypothesis of a localization of the binding site of protein Arp to the C alpha 3-domain.

Effect of thermal injury on transfer of IR22 IgA myeloma protein into bile in the rat

We previously observed a 75-90% decrease in concentration of biliary IgA after thermal injury to rat skin. Decrease in biliary IgA might result from an alteration in supply of polymeric IgA delivered to the hepatocyte or from an alteration in hepatocyte transfer of polymeric IgA into bile. In the present study, we examined the transfer of intravenously administered 125I-IgA into bile. Purified IR22 rat IgA myeloma protein consisting of both monomeric and polymeric IgA was labelled with 125I. Sprague-Dawley rats (140-180 g) received a 20-30% body surface area scald-burn or sham treatment. The bile duct was cannulated 18-24 h later and 125I-IgA preparations were injected into the tail vein. Bile was collected under light ether anesthesia for 3 h. In rats injected with 125I-IR22 IgA myeloma protein there were no significant differences in total, TCA-precipitable, or immunoprecipitable radioactivity in bile from burn-injured or sham-treated animals. On Bio-Gel A-1.5 m gel permeation, the radioactivity in bile from sham-treated animals eluted in the region of polymeric IgA as expected; the radioactivity in the bile from burn-injured animals eluted equally in the same regions as polymeric IgA and monomeric IgA. In sham-treated rats injected with isolated polymeric IgA only, bile contained primarily polymeric IgA. In burn-injured rats injected with polymeric IgA only, bile contained a mixture of polymeric IgA and monomeric IgA. These findings suggest that hepatocyte processing of polymeric IgA is altered after thermal injury, resulting in the transformation of some polymeric IgA into its monomeric form.

Thiol groups and other chemical characteristics of rat monoclonal immunoglobulin A

1. Three aspects of the monoclonal rat IgA IR22 were investigated: its free thiol groups, its hinge structure and its glycopeptides. 2. Exposed thiol groups, 1.5 per IgA dimer, were located in the Fc region, and buried thiol groups, 3.2 per IgA dimer, were located in the Fd region. 3. Rat IgA was cleaved to Fab fragments by pepsin, but, unlike mouse IgA, it was not cleaved at the hinge region by papain. 4. The contents of GlcNAc indicated that there were two N-linked glycopeptides in the Fab region, and one in the Fc or tail regions.

Ontogeny of the receptor for polymeric immunoglobulins in rat hepatocytes

Based on in vitro experiments measuring daily secretion rates in the culture media of rat hepatocytes and in vivo experiments using pulse labeling of intracellular precursors, the present study examines the ontogenic expression of the polymeric immunoglobulin receptor and secretory component by hepatocytes during growth. Our data indicate that hepatocytes from infant and suckling rats (day 5, 15) cultured in serum-free and hormone-free conditions only secreted trace amounts of secretory component. Beginning on day 20, basal secretion rate showed a marked upsurge with a 10-fold increase by day 35. The addition of dexamethasone (10(-7) mol/L) to the culture media enhanced by 2.5-fold the basal secretion of secretory component by hepatocytes from 20-, 25-, and 35-day-old rats, while addition of insulin to the media had no effect. The response to dexamethasone was dose-dependent (10(-5), 10(-6), 10(-7) mol/L) and specific. In vivo pulse labeling of receptor precursors in hepatocytes from 40-day-old rats allowed the identification of three intracellular forms: a 105-kilodalton peptide and a 116-120-kilodalton mature doublet. In 13-day-old rats, three immature precursors were detected: a 105-kilodalton peptide and a high molecular weight doublet of 185-190 kilodaltons. Sucklings (13 days) treated with corticosterone showed a pattern of precursors similar to controls. These findings support the following conclusions: (a) hepatocytes from infant and suckling rats synthesize and process immature receptor precursors whose expression is unaffected by corticosterone treatment, and (b) active secretion of secretory component is initiated at weaning independently from humoral and hormonal factors while the magnitude of its production by the liver is under the control of glucocorticoids.

Anti-cholera toxin IgA-, IgG- and IgM-secreting cells in various rat lymphoid tissues after repeated intestinal or parenteral immunizations

Single antibody-secreting spot-forming cells (SFC) of the 3 main isotypes were counted in lymphoid cells from the gut lamina propria (LP), Peyer's patches (PP), mesenteric nodes (MN) and spleen (SP) of rats immunized 2-6 times intraduodenally (ID) or intraperitoneally (IP) with cholera toxin (CT). Responses for all isotypes peaked in all tissues after 4 ID- or IP-immunizations at much larger values than previously reported, and significantly decreased thereafter, except in LP and PP after IP-injections, where IgA- or IgG-SFC, but not IgM-SFC, only appeared or increased after 6 IP-doses. The highest IgA-SFC numbers (17% of tested cells) were in LP after 4 ID-doses. The isotype ratio was IgA greater than IgG greater than IgM in LP and PP after ID-injections, but IgG greater than IgA greater than IgM in MN and SP after both ID- and IP-routes. The isotype dispersion was much larger in LP and PP than in MN and SP. Our data show that at least 4 IP CT-doses were required to only elicit a few IgG- and almost no IgA-SFC in LP and PP, outlining the need for intestinal CT-immunizations to induce strong mucosal IgA-SFC responses. We also show good systemic responses elicited both by enteral and parenteral routes, and the small contribution of PP in total SFC, particularly after parenteral immunizations.

Characterization of an IgA receptor from group B streptococci: specificity for serum IgA

Some strains of group B streptococci express a cell surface protein which binds IgA. This report describes some properties of such an IgA receptor and compares it with a previously described IgA receptor from group A streptococci. The group B receptor was released in an almost pure form from bacteria incubated at elevated pH, and could be isolated by IgA-Sepharose affinity chromatography. The sequence of the N-terminal 19 amino acid residues was unique. The receptor preferentially binds IgA of human origin, as shown in immunoblotting experiments with purified IgA from nine different species. The affinity constant of the purified receptor for serum IgA was determined to be 3.5 x 10(8) M-1, but for secretory IgA it was too low to allow determination. This result indicates that secretory component and/or J chain interferes with the binding of IgA to this type of bacterial receptor, which may be one of the physiological functions of these polypeptides. A reduction in affinity was also observed for another complexed form of IgA, alpha 1-microglobulin-IgA. The group B receptor is antigenically unrelated to the IgA receptor from group A streptococci (protein Arp), but competitive inhibition experiments indicate that they bind to the same region in IgA. The implications of these findings, and the biological role of bacterial IgA receptors, are discussed.

Stimulation of secretory IgA and secretory component of immunoglobulins in small intestine of rats treated with Saccharomyces boulardii

Saccharomyces boulardii (S.b.) is largely used in Western European countries for the treatment of acute infectious enteritis and antibiotic-induced gastrointestinal disorders. To study the mechanisms of the protective effect of S.b. against enteral pathogen infection, we assessed the response of the intestinal secretion of secretory IgA (s-IgA) and of the secretory component of immunoglobulins (SC) to oral administration of high doses (0.5 mg/g body weight, three times per day) of S.b. cells in growing rats. S.b. cells (biological activity: 2.8 x 10(9) viable cells/100 mg) were administered daily by gastric intubation to weanling rats from day 14 until day 22 postpartum. Control groups received either 0.9% saline or ovalbumin following the same schedule. Expressed per milligram of cell protein, SC content was significantly increased in crypt cells isolated from the jejunum (48.5% vs saline controls, P less than 0.05) as it was in the duodenal fluid (62.8% vs saline controls, P less than 0.01) of rats treated with S.b. Oral treatment with S.b. had no effect on the secretion of SC by the liver. In the duodenal fluid of rats treated with S.b. cells, the mean concentration of s-IgA was increased by 56.9% (P less than 0.01) over the concentration of s-IgA measured in saline controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Neutralization of cholera toxin by rat IgA secretory antibodies induced by a free synthetic peptide

Secretory immunoglobulin A (sIgA) is the major immunoglobulin in the bile of several species. They contribute to local immune defences of the gut. The protection against cholera toxin (CT) is due to the presence of specific sIgA in the bile and in the gut. We have already reported that oral administration of the peptide corresponding to the sequence 50-75 of cholera toxin B subunit elicits serum antibodies neutralizing CT activity, and that IgA and local protection are observed in the intestine of P50-75 orally immunized mice. In this study, we demonstrate the potential of this synthetic peptide as immunogen without carrier or adjuvant, not only in a strain known to be sensitive to CT, but also in an outbred one. Furthermore, this peptide stimulates the mucosal immunity, since we show that P50-75 induced-sIgA purified from rats bile and serum, are capable of neutralizing CT activity in the in vivo intestinal ligated loop test.

Chemotactic properties of rat immunoglobulins and immune complexes

The effect of rat immunoglobulins and immune complexes on the locomotor function of rat polymorphonuclear leukocytes (PMN) was investigated in vitro. Rat immunoglobulin G1 (IgG1), IgG2a, IgG2b, and IgA monoclonal antibodies specific for the dinitrophenyl hapten were used. Both monomeric and polymeric IgA showed chemotactic activity in a dose-dependent manner. IgG1 and IgG2b also induced a dose-dependent locomotor response of PMN, but the nature of the induced migration was chemokinetic (enhancing random migration). IgG2a was chemotactic and induced maximal migration at a relatively low concentration. IgG1- and IgG2b-immune complexes induced stronger migration than antibody alone; however, IgA- and IgG2a-immune complexes did not. IgA was shown to modify the chemotactic movement of PMN induced by N-formylmethionyl-leucyl-phenylalanine (FMLP). In the presence of both IgA and FMLP in the lower chamber, the migration towards suboptimal concentrations of FMLP was enhanced. By contrast, IgA in the upper chamber decreased migration towards the optimal or higher concentrations of FMLP. These findings suggest that IgA may work synergistically with luminal chemoattractants to mobilize PMN to the locus of infection on the mucosal surface. In addition, the intense activity of IgG2a alone and IgG1- or IgG2b-immune complexes in inducing PMN migration may play an important role in inflammatory processes. The data indicate that immunoglobulins have a direct effect on PMN mobility.

Activation of rat complement by soluble and insoluble rat IgA immune complexes

The ability of rat monoclonal IgA, specific for 2,4-dinitrophenyl (DNA), to activate the complement (C) system of the rat was investigated using aggregated IgA or IgA immune complexes (IC). IgA was coated onto a solid phase, and tested for its capacity to bind C3 upon incubation at 37 degrees C in normal rat serum (NRS) in the presence of Mg-EGTA. Binding of C3 was observed dependent on the dose of dimeric (d-), polymeric (p-) and secretory IgA tested. In contrast, little C3 fixation was observed in this system with monomeric (m-) rat IgA or with mouse m- and d-IgA (MOPC315). Soluble and insoluble rat IgA IC were prepared using dinitrophenylated rat serum albumin (DNP8RSA) as antigen (Ag), and assessed for C activation. It was shown that insoluble IC (immune precipitates; IP) containing m-, d- or pIgA of rat origin activate the alternative pathway of rat C, as demonstrated by their capacity to induce C consumption in NRS in the presence of Mg-EGTA. When p- and m-IgA IP were compared for their capacity to activate C, it was found that p-IgA activated C four times as efficiently as m-IgA IP (at 2 mg/ml). Soluble rat IgA IC were prepared in an excess of DNP8RSA, fractionated by gel filtration on Sepharose 6B, and analyzed for C activation and antibody (Ab)/Ag ratio. In contrast to m-IgA IP, soluble m-IgA did not activate C. On the other hand soluble d-IgA IC activated C dependent on their concentration and size: at a concentration of 0.1 mg/ml high-molecular weight d-IgA IC with a high Ab/Ag ratio were four times as efficient as low-molecular weight IC with a low Ab/Ag ratio, and twice as efficient as IP prepared at equivalence. To demonstrate the induction by IgA of the assembly of the terminal membrane attack complex, trinitrophenyl (TNP)-conjugated rat red blood cells (TNP-RRBC) coated with d- or p-IgA were shown to be lysed in NRS in the presence of Mg-EGTA. No lysis of m-IgA-coated TNP-RRBC was observed. The results in this study demonstrate that both soluble and insoluble rat IgA IC activate the alternative pathway of homologous rat C. Alternative pathway activation by soluble rate IgA IC is dependent on the size of the IC. The degree of polymerization of the IgA Ab itself also influences C activation.

Selectivity, specificity and kinetics of the androgen regulation of the ocular secretory immune system

The purpose of the present investigation was to determine the selectivity, specificity and kinetics of the androgen regulation of the ocular secretory immune system. To examine the mucosal selectivity of hormone action, tears, saliva, respiratory and intestinal secretions, urine and serum were collected from orchiectomized rats following four days of treatment with saline or testosterone. Androgen administration significantly increased the tear levels of IgA and free SC. In contrast, this combined hormone response was not duplicated in other mucosal secretions or serum. Consequently, it appears that the androgen control of the ocular secretory immune system is unique to the eye. To evaluate androgenic specificity, orchiectomized rats were treated for four days with saline, testosterone, 5 alpha-androstan-17 beta-o1 ('5 alpha'), 4-estren-7 alpha-methyl-17 beta-o1-3-one ('4-E') or the synthetic analogue, danazol. Results demonstrated that testosterone and '4-E', a potent androgen, both stimulated the accumulation of tear IgA and free SC, compared to amounts in tears of saline-treated controls. The weak androgen '5 alpha' induced a slight rise in tear free SC, but not IgA, levels, whereas danazol had no effect on the ocular secretory immune system. These findings indicate that androgen structure is of critical importance in modulation of mucosal immunity in the eye. Lastly, to assess the kinetics of hormone action on tear IgA and free SC, orchiectomized rats were acutely (0, 1.5, 4, 7, 12, or 24 hours) or chronically (0, 4, 7, 11, 14, or 17 days) exposed to testosterone. Acute androgen exposure for less than 12 hours did not elicit any change in the tear content of IgA or free SC. In comparison, chronic androgen treatment induced a progressive and dramatic rise in the levels of both tear proteins. By 17 days of hormone exposure, the IgA and free SC concentrations in tears had increased by 14- and 18-fold amounts, respectively, relative to amounts in placebo-treated controls. Of interest, the temporal pattern of accumulation of tear IgA and free SC were different. Overall, these results show prolonged, but not abbreviated, exposure to androgens has a profound impact on the IgA and free SC profile in tears.

Cholera toxin neutralization: a comparison of purified serum IgG and biliary secretory IgA antibodies

Rats were immunized three times with cholera toxin via the intraintestinal or intravenous route, and their respective biliary secretory IgA (sIgA) or serum IgG antibodies were affinity-purified on a cholera toxin immunoabsorbent. On a molar basis, the sIgA antibodies were roughly seven-fold more efficient than IgG antibodies in neutralizing cholera toxin in the ligated intestinal loop assay. Various explanations for this difference in neutralizing capacity are proposed.

Biochemical characterization of chicken secretory component

Human dimeric IgA was injected i.v. into chickens whose bile was then collected. The human IgA which had been transported across the hepatocytes of the chicken was subsequently purified from the bile and shown to be associated with a protein of 80 kDa and pI 4.6, which reacted with rabbit antisera to chicken bile proteins but not with an antiserum to human alpha chain. The chicken bile protein thus has functional and biochemical properties similar to those of mammalian secretory component.

Subcellular localization of transferrin protein and iron in the perfused rat liver. Effect of Triton WR 1339, digitonin and temperature

The subcellular localization of 3H-labelled 59Fe-loaded transferrin accumulated by the liver has been studied by means of cell fractionation techniques. More than 96% of the 59Fe present in the liver of rats perfused with 59Fe-labelled transferrin is recovered in the parenchymal cells. Rat livers were perfused with 10 micrograms/ml 3H-labelled 59Fe-saturated transferrin, homogenized separated in nuclear (N), mitochondrial (M), light mitochondrial (L), microsomal (P) and supernatant (S) fractions; M, L and P fractions were further analysed by isopycnic centrifugation in sucrose gradients. 3H label distributes essentially around densities of 1.13-1.14 g/ml overlapping to a large extent with the distribution of galactosyltransferase, the marker enzyme of the Golgi complex. However, after treatment with low concentrations of digitonin the 3H label dissociates from galactosyltransferase and is shifted to higher densities, suggesting an association of transferrin with cholesterol-rich endocytic vesicles which could derive from the plasma membrane. 59Fe is mostly found in the supernatant fraction largely in the form of ferritin, as indicated by its reaction with antiferritin antibodies. In the mitochondrial fraction the density distribution of 59Fe suggests an association with lysosomes and/or mitochondria. In contrast to the lysosomal enzyme cathepsin B, the density distribution of 59Fe was only slightly affected by pretreatment of the rats with Triton WR 1339, suggesting its association with the mitochondria. At 15 degrees C, 59Fe and 3H labels are recovered together in low-density endocytic vesicles. On the basis of our results we suggest that, at low extracellular transferrin concentration, iron uptake by the liver involves endocytosis of the transferrin protein. The complex is interiorized in low-density acidic vesicles where iron is released. The iron passes into the cytosol, where it is incorporated into ferritin and into the mitochondria. The iron-depleted transferrin molecule would then be returned to the extracellular medium during the recycling of the plasma membrane.

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)

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

The liver efficiently transports dimeric immunoglobulin A (dIgA) from blood to bile in a direct, nonlysosomal pathway involving smooth-surfaced vesicles. Secretory component (SC), the plasma membrane receptor for dIgA, is released into bile still bound to its ligand by disulfide bridges. Rabbit IgG antirat SC binds specifically to plasma membrane SC, yet the biliary secretion of anti-SC is markedly lower than that of dIgA, suggesting that the IgG antibodies utilize an alternate transhepatocellular pathway. Uptake of commercially available antihuman SC conjugated to horseradish peroxidase was examined by quantitative electron microscopic immunocytochemistry using primary rat hepatocyte monolayer cultures. Coincubation with human polymeric IgA, rabbit antiserum to rat SC, free human SC, human secretory IgA, and rat bile, all significantly suppressed uptake of anti-SC-horseradish peroxidase, thus demonstrating the specificity of the labeled antibody. Coated vesicles accounted for greater than 70% of the total uptake of either the anti-SC-horseradish peroxidase preparation or colloidal gold-labeled IgG antirat SC. Both compounds could also be observed in other structures associated with the degradative pathway, i.e., multivesicular bodies and lysosomes. Moreover, the extent to which 125I-anti-SC was degraded was significantly greater than that of 125I-dIgA. These data demonstrate that dIgA and anti-SC utilize different intracellular pathways, with anti-SC undergoing lysosomal degradation.

The binding of human milk lactoferrin to immunoglobulin A

It was recognized that in human milk some amounts of lactoferrin (LF) were naturally bound to secretory IgA (sIgA). Since not only secretory component (SC) but also LF was released from sIgA by disulfide bond cleavage, it is conceivable that LF is naturally bound to IgA as well as SC. An in vitro binding test to LF and IgA was performed and the binding was confirmed by the use of an IgA-Sepharose 4B affinity column.

Rat liver membrane secretory component is larger than free secretory component in bile: evidence for proteolytic conversion of membrane form to free form

Secretory component is a receptor for polymeric immunoglobulins on epithelial cells and hepatocytes that facilitates transport of polymeric immunoglobulins into external secretions. Little is known about the transcellular migration of secretory component-polymeric IgA complexes or the membrane forms of secretory component. We therefore examined rat bile and liver membranes to identify and compare the various molecular species of secretory component. Bile or liver membrane proteins were electrophoresed in sodium dodecyl sulfate-polyacrylamide gels and electrophoretically transferred to nitrocellulose membranes. Protein profiles on blots were probed with antisecretory component antiserum, and the immunoreactive bands were visualized by indirect immunoperoxidase staining. Bile collected in the presence of proteolytic inhibitors showed an immunoreactive doublet band (Mr = 82,000 and 78,000) in the molecular weight range of free secretory component. By contrast, free secretory component in bile collected in the absence of proteolytic inhibitors and purified by affinity chromatography migrated as a single protein with an Mr = 70,000. Both components of the free secretory component doublet bound dimeric IgA when blots were probed with human dimeric IgA. Crude liver membranes prepared in the presence of proteolytic inhibitors showed two immunoreactive secretory component-containing bands, Mr = 107,000 and 99,000, whereas membranes prepared without proteolytic inhibitors showed two smaller immunoreactive bands; one of these proteolytically severed proteins comigrated with the 82,000-dalton free secretory component in bile. These results indicate that membrane forms of secretory component are present in rat liver. The observations that the membrane secretory component is larger than biliary free secretory component and yields biliary SC-like forms of secretory component upon proteolysis support the hypothesis that free secretory component in bile is a proteolytic product of larger liver membrane-associated secretory component.

Evolution of low molecular weight immunoglobulins--III. The immunoglobulin of chicken bile--not an IgA

Studies on comparison of antigenic relationships between the immunoglobulin of chicken bile--the so-called chicken IgA--and human, porcine, as well as murine IgA were performed by a series of double-antibody and solid-phase RIA systems using H chain-specific anti-chicken biliary Ig sera and alpha chain-specific anti-human IgA sera produced in rabbits and in carp. It was not possible to demonstrate a significant immunological cross-reactivity of the Ig of chicken bile to the Ig's of IgA class. Besides, we could not find an Ig type similar to this chicken biliary Ig in pooled normal sera or in the crude Ig fractions precipitated by (NH4)2SO4 of several vertebrate species (man, swine, buzzard, duck, goose, kestrel, kite, three tortoise species, frog, carp and perch); or in the bile of duck, goose, two snake species and frog. Only pooled chicken normal serum contained this biliary Ig type. Accordingly, the chicken biliary Ig can no longer be considered a precursor of mammalian IgA. This Ig of chicken bile represents a unique Ig class of birds and is to be called IgB (dominant biliary Ig of some birds).

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.

Selective excretion of IgA in rat bronchial secretions: lack of significant contribution from plasma IgA

Concentrated rat bronchial washings (BW) were analyzed by gel-filtration and immunochemical methods. BW contained mainly albumin, transferrin and IgG. Free secretory component and secretory IgA were identified in BW; the BW-IgA had the same three sedimentation coefficients, i.e. +/- 11 S, 13 S, 15 S by sucrose density gradient ultracentrifugation, as rat milk and rat bile IgA; the three peaks were secretory IgA. Compared to serum, and relatively to albumin, BW were significantly enriched in IgA, although much less than rat bile. Purified polyclonal rat polymeric 125I-IgA was injected intravenously into normal rats, and into rats with bile duct ligation or partial hepatectomy, which decrease the liver plasma-to-bile transfer of IgA. BW were then collected, one or four hours later, to assess the recovery of the 125I-IgA in BW and to estimate the contribution of serum IgA to BW-IgA. Very little 125I-IgA (less than 0.2%) was recovered in all BW. The specific activity, measured only in the rat with the highest recovery in BW, was 20 times lower in BW than in serum. The data demonstrate that rat serum IgA does not contribute significantly to IgA in BW.