Role of secretory IgA, secretory component, and eosinophils in mucosal inflammation

Eosinophils and their products are important in the pathophysiology of allergic inflammation in mucosal tissues. Secretory component (SC) bound to IgA mediates transepithelial transport of IgA. As another biological activity of SC, we have reported that secretory IgA (sIgA) and SC preferentially activate human eosinophils. When eosinophils were stimulated with immobilized sIgA, degranulation and superoxide production were greater than when stimulated with serum IgA. In contrast, neutrophils responded similarly to sIgA and serum IgA. Superoxide production by eosinophils stimulated with cytokines was enhanced synergistically by immobilized SC, while SC showed no effect on neutrophil activation. Eosinophil superoxide production stimulated with sIgA was abolished by anti-CD18 mAb, suggesting that beta2 integrins might be crucial for this reaction. There are several reports that SC and sIgA may play important roles in regulating eosinophil functions in vivo in diseases associated with mucosal eosinophilia and in various allergic diseases. It is speculated that eosinophils in the mucosa are activated by SC or sIgA, and that subsequent degranulation and superoxide production are induced.     

Both Fc alpha domains of human IgA are involved in in vitro interaction between secretory component and dimeric IgA

The sites of interaction between 125I labelled human secretory component (SC) and dimeric IgA were located by studying the inhibitory effect of various antibodies to IgA. Several Fab' fragments were isolated from three sera of hyperimmunized rabbits. The specificity of these different antibody preparations, as determined by a RIA inhibition test or by ELISA, showed that two were directed against both domains of Fc alpha, two against C alpha 2, two against C alpha 3 and one against Fd alpha. A monoclonal antibody against C alpha 3 was also used. The results indicate that both the C alpha 2 and C alpha 3 domains are equally and independently involved in the interaction between SC and dimeric IgA.     

Enumeration of the antigenic determinants of human secretory component (SC) and secretory IgA (sIgA)

The number of antigenic determinants of human SC and sIgA was determined with specific anti-SC antibodies isolated from a pool of two hyperimmunized sheep. The Fab' antibody fragments were prepared and [125I] labelled, while pure SC and pure sIgA, isolated from colostrum, were labelled with 131I. The [125I] Fab' antibodies were added, in very large molar excess, to the [131I] antigens at various ratios. The Fab'-Ag complexes were separated from the antibody excess by gel-filtration. The highest Fab'/Ag molar ratios of the complexes, which correspond to the maximal number of accessible antigenic determinants, were calculated. We found at least 16 sites (16.6 +/- 1.36) on free SC while only 12 (12.27 +/- 0.51) were located on sIgA. These results confirm the existence of a significant number of hidden determinants of the SC subunit of sIgA and establish that about 1/4 of the SC molecule is implied in this binding.     

Rat bile as a convenient source of secretory IgA and free secretory component

Rat hepatic bile contains three proteins as major constituents: secretory IgA (SIgA), free secretory component (FSC) and albumin. Traces of α-macroglo-bulin, transferrin, IgG and IgM are also detectable. The bile duct daily pours between 5–12 mg each of SIgA and FSC into the rat duodenum. The origin and function of these proteins in bile may represent important clues in the understanding. of the SIgA system.     

Immunohistochemical study of secretory component, secretory IgA and carcinoembryonic antigen in large bowel carcinomas

The epithelium of 41 large bowel carcinomas was scored immunohistochemically on a semiquantitative basis for the presence of secretory component (SC), secretory IgA, and carcinoembryonic antigen (CEA). Both the tumour and the adjacent "transitional mucosa" were evaluated. The various immunofluorescence scores obtained, the histological grades of the tumours, their Dukes' stages, and the plasma CEA levels were subjected to non-parametric correlation analyses. Tumour SC was positively correlated with histological tumour grade and inversely related to Dukes' stage. Tumour secretory IgA generally showed a pattern similar to that of SC. Tumour CEA showed no correlation with any of the other parameters. The contents of SC and secretory IgA in the transitional mucosa were negatively correlated with Dukes' stage and plasma CEA. Whether the variations observed in the epithelial cell markers reflected primary events in the malignant development or secondary alterations is unknown. Nevertheless, especially the amount of tumour SC may turn out to be of prognostic value.     

Inhibition of microbial IgA proteases by human secretory IgA and serum

Microbial IgA proteases cleave human serum IgA1 immunoglobulin, but human secretory IgA is resistant to hydrolysis. We have found this resistance to be due to an inhibition of protease activity that is mediated by the Fab region of secretory IgA. The IgA proteases of the genus Neisseria are more sensitive to inhibition than is the protease of Streptococcus sanguis. There is also a serum inhibitor of Neisseria proteases that co-chromatographs with IgG. Monoclonal (myeloma) human IgG proteins and plasma protease inhibitors such as alpha-1-antitrypsin and alpha-2-macroglobulin do not inhibit. Human sera do not contain inhibitor to S. sanguis protease activity. We conclude that microbial IgA proteases are subject to inhibition by IgA in secretions and IgG in serum, and this activity is most consistent with being an anti-enzyme antibody. The insensitivity of S. sanguis IgA protease to inhibition is unexplained but provides further evidence that the IgA proteases are structurally diverse.     

Sdmg1 is a component of secretory granules in mouse secretory exocrine tissues

Sdmg1 is a conserved eukaryotic transmembrane protein that is mainly expressed in the gonads where it may have a role in mediating signaling between somatic cells and germ cells. In this study we demonstrate that secretory exocrine cells in the pancreas, salivary gland, and mammary gland also express Sdmg1. Furthermore, we show that Sdmg1 expression is up‐regulated during pancreas development when regulated secretory granules start to appear, and that Sdmg1 colocalizes with secretory granule markers in adult pancreatic acinar cells. In addition, we show that Sdmg1 co‐purifies with secretory granules during subcellular fractionation of the pancreas and that Sdmg1 and the secretory granule marker Vamp2 are localized to distinct subdomains in the secretory granule membrane. These data suggest that Sdmg1 is a component of regulated secretory granules in exocrine secretory cells and that the developmental regulation of Sdmg1 expression is related to a role for Sdmg1 in post‐Golgi membrane trafficking. Developmental Dynamics 238:223–231, 2009. © 2008 Wiley‐Liss, Inc.     

Activation of complement by human serum IgA, secretory IgA and IgA1 fragments

Activation of the complement (C) system by human IgA was studied. Both subclasses of IgA, IgA1 and IgA2, and secretory IgA were shown to activate C, as determined by deposition of C3 on glutaraldehyde-activated microwells coated with IgA. The activation of the C system occurred in the presence of MgEGTA and not in D-deficient serum. In addition to C3, deposition of properdin (P) but not of C4 was detected. These results indicate that C activation, as determined by measuring deposition of C3 and P, occurred by the alternative pathway (AP). The data further show that the major part of the hinge region, which is deleted in IgA2 as compared with IgA1 and which forms the major structural difference between the two subclasses, is not involved in C activation. Reduction and alkylation destroyed the ability of IgA to activate C, as has also been demonstrated for IgG. In order to define the C activating region of the IgA molecule, several fragments of IgA1 were tested. The four-chain molecules F(ab')2 and F(abc)2 were shown to activate the AP. No activation was observed with the two-chain fragments Fab and Fc. The Fc fragment of IgA also did not activate the CP, as does the Fc fragment of IgG. This indicates that activation of the AP of C by IgA is dependent on the presence of the F(ab')2 fragment. In conclusion: human IgA does activate C by the AP. This activation requires an intact F(ab')2 fragment.     

Recombinant expression of polymeric IgA: incorporation of J chain and secretory component of human origin.

Mucosal J (joining) chain-expressing IgA immunocytes produce dimeric IgA that is actively transported by the epithelial polymeric Ig receptor (pIgR) to exocrine secretions. Release of secretory IgA (SIgA) occurs by cleavage of the covalently linked pIgR ectodomain, also known as bound secretory component. We have identified the human J-chain cDNA sequence through database screening, and isolated it from B cells for recombinant expression. Co-expression of this cDNA with an alpha heavy chain and a lambda light chain in Chinese hamster ovary (CHO) cells resulted in a mixture of recombinant monomeric and dimeric IgA in culture supernatants. This dimeric IgA was transported by the pIgR-mediated mechanism in vitro. Furthermore, expression of the human pIgR ectodomain together with the dimeric IgA, resulted in production of complete SIgA by the CHO cells. These results demonstrated that co-expression of the necessary polypeptide components allows a single mammalian cell to produce SIgA. Development of production systems for human antigen-specific recombinant SIgA may be important for applications in passive mucosal vaccination.     

Intracellular storage of IgA and secretory component in carcinomas of the female breast

Summary This study reports ten cases of mammary carcinoma with intracytoplasmic lumina and inclusion bodies visible by light microscopy; five tumors were classified as lobular in type and four as different forms of infiltrating ductular carcinomas. One tumor showed a lobular growth in combination with ductular structures. For the identification of the inclusion bodies, indirect immunofluorescence on paraffin embedded material was performed, which revealed IgA as well as secretory component within the intracytoplasmic lumina. It was concluded that the production of secretory component and the uptake of IgA is possible even in carcinomas without glandular structures, and that immunomorphology should supplement histological and histochemical evaluation in order to define the contents of intracytoplasmic lumina.     

Molecular state of secretory component in human sera

The secretory component (SC) of human sera is never found free but always bound to IgA. The complex molecule is extremely similar to the main form of secretory IgA found in secretions, i.e. (IgA)₂/J/SC. Indeed, the in vitro behavior of these two proteins is identical: gel filtration, immunochemical reactivity and bonds involved. The overall molecular structure must also be the same since, in both cases, this complex molecule is held together by covalent bonds. The absence of all forms of SC in sera (free SC, sIgM, oligomeric IgA-bound SC) except for (IgA)₂/J/SC is not in favor of passive transfer and rather suggests an active and selective transfer mechanism, possibly different from those generally accepted in mucosal or glandular cells. The origin of the serum sIgA will be discussed.     

Location of secretory component on the Fc edge of dimeric IgA1 reveals insight into the role of secretory IgA1 in mucosal immunity

Secretory immunoglobulin A (SIgA) is the most prevalent antibody in the human body and a first line of defense in mucosal immunity. We located secretory component (SC) relative to dimeric IgA1 (dIgA1) within the SIgA1 structure using the constrained modeling of solution scattering and analytical ultracentrifugation data. The extended solution structure of dIgA1 is largely preserved within SIgA1. From conformational searches of SC locations, the best-fit SC models within SIgA1 show that SC is extended along the outermost convex edge of the Fc dimer in dIgA1. The topology of our SIgA1 structure reveals that it is able to bind to one FcαRI receptor molecule. SC binding to the Fc dimer confers protection to SIgA1 by the masking of proteolytically susceptible surface sites from bacterial proteases in the harsh environment of the mucosa. The models support a “zipper-like” unfolding of SC upon dIgA1 in the formation and transportation of SIgA1 into the mucosa.     

The stoichiometry of J chain in human secretory dimeric IgA

Dimeric human secretory IgA was completely reduced with mercaptoethanol and alkylated with [14C]iodoacetamide. The component polypeptide chains were separated by high performance gel filtration in 5 M guanidine HCl into two fractions: one containing secretory component (SC) + heavy (H) chains; and the second containing light (L) + J chains. L and J chains were subsequently separated by polyacrylamide gel electrophoresis in sodium dodecyl sulfate (SDS) or in alkaline urea. Calculations of the J chain stoichiometry in the dimeric secretory IgA (S-IgA) molecule were based on: the measurement of the ratio of radioactivities of SC + H chain and L + J chain-fractions or L chain- and J chain-fractions; the known stoichiometry of SC, H and L chains; and the known number of half-cystine residues in the component polypeptide chains of S-IgA molecule. The data demonstrated that one molecule of dimeric S-IgA contains approx. one J chain.     

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

Secretory IgA (sIgA) and free secretory component (SC) have been purified from rat bile and compared to sIgA and free SC from rat and human milk. In rat bile and milk, sIgA exists in a series of polymers with S rates of 11,13 and 15 and molecular weights of 405,000, 540,000 and 690,000, respectively. These three forms have the same basic chain composition (light chains, -chains and SC) when reduced and analysed by polyacrylamide gel electrophoresis in SDS. Rat -chains and SC were slightly smaller than their human counterparts. SC was covalently linked to IgA in purified biliary sIgA; the latter apparently contained molecules of SC in different configurations, as revealed by antigenic analyses with anti-free-SC and anti-bound-SC antisera. There was antigenic identity between bile and milk for both FSC and sIgA. The existence of configurational determinants of SC in both sIgA and FSC was serologically demonstrated. The amino acid composition of rat bile free SC compared well to that of bovine, canine, rabbit and human free SC.     

Immunoreactive Elastin in benign breast tissues

Summary Immunohistological localisation of elastin was achieved by means of the peroxidase-antiperoxidase method after preliminary trypsinisation of sections from 48 benign breast biopsies. The procedure allows retrospective examination of routinely formalin-fixed, paraffin-embedded breast tissue. In general the immunolocalisation of elastin showed a close microanatomical correlation with the fibres demonstrable in sections from the same blocks by standard elastic-fibre stains. Discrepancies between elastic-fibre stains and elastin immunoreactivity appear to relate to the enhanced avidity of the antibody for immature elastin. In this way sites of recent synthesis of elastin were demonstrated in the inner zone of the periductal elastica, sclerosing adenosis, and in the internal elastic lamina of breast arteries which displayed reduplication of the internal elastic lamina or intimal proliferation.     

In vitro synthesis of immunoglobulins, secretory component, complement and lysozyme by human gastrointestinal tissues. II. Pathological tissues.

An in vitro culture technique has been used to study synthesis of proteins by biopsies of human gastrointestinal mucosa which were obtained at endoscopy or surgery from patients with biliary gastritis, atrophic gastritis, peptic ulcer, gastric cancer, coeliac disease, Crohn's disease and ulcerative colitis. As in normal mucosa, immunoglobulin synthesis was found in all sites, but marked increases, especially in IgG, were seen in biliary gastritis and ulcerative colitis. In untreated coeliac disease, synthesis of IgG and IgM was increased. Synthesis of complement components did not differ from that found in normal mucosa. Increased lysozyme synthesis was seen in Crohn's disease. This study shows that useful information may be acquired from short-term culture studies of the small biopsies obtained with fibre optic endoscopes.