Thomas A. Brown

The human IgA system: A reassessment☆

In healthy adults the total daily production of secretory and serum IgA exceeds that of other immunoglobulin classes. Secretory and serum IgA display features of mutual independence: they are represented by molecules with different physiochemical and immunochemical properties and antibody activities and are produced by cells with different organ distributions. Secretory and serum IgA also exhibit different effector functions: interaction of secretory IgA with environmental antigens results in prevention of the penetration of such antigens by a variety of mechanisms. Although the function of polymeric serum IgA antibodies in certain animal species involves elimination of antigenic substances by noninflammatory means, the primary function of serum IgA remains unknown. It is proposed that in humans monomeric serum IgA (which prevents activation of the complement systems, inhibits phagocytosis and antibody-dependent cellular cytotoxicity) may protect endogenous antigens expressed on various cells and tissues by preventing their interaction with humoral and cellular immune mechanisms that may lead to tissue damage.

Preferential transport of IgA and IgA-immune complexes to bile compared with other external secretions

When radiolabeled dinitrophenyl-human serum albumin was injected intravenously in mice together with M315 IgA, labeled antigen was specifically transported into bile, but not into saliva, urine, milk or bronchial and intestinal secretions. Ultracentrifugal analysis showed that the antigen transported into bile was intact and partly complexed with IgA. The radioactivity that was present in other secretions regardless of M315 IgA, represented free and degraded fragments of antigen. M315 IgA alone was readily transported into bile, where it was detected at high titer by hemagglutination, but not into other secretions, apart from milk which contained only very low titers. The liver therefore appears to be singularly capable of transporting both free and complexed IgA into its secretion, the bile.

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.

Comparative Studies of the Biological Properties of Human IgA Subclasses

Abstract IgA-mediated effector functions are poorly defined mainly due to the unimpressive performance of IgA in classical assays of protective immunity (complement-mediated lysis and phagocytosis) and the paucity of clinical complications in patients with IgA deficiency. Nevertheless, the examination of specific antibody activity in different test systems has revealed that IgA1 and IgA2 antibodies may display unique defense properties. In complexes with corresponding antigens, IgA interferes with the inflammatory reactions initiated by complement-binding antibodies. Furthermore, secretory or serum IgA1 or IgA2 may interfere with bacterial adherence and inhibit NK cell activity by both specific antibody-dependent and -independent mechanisms. Naturally occurring and artificially induced (by various immunization routes) serum and secretory antibodies display marked IgA subclass specificities to certain groups of antigens. Thus, protein antigens of microbial or food origin evoke a predominant IgA1 response, while endotoxins and polysaccharides may induce both IgA1 and IgA2 responses, particularly in external secretions. Differences in the proportion of specific IgA1 or IgA2 antibodies in serum vs secretions to certain antigens (e.g. endotoxin) suggest that the production of IgA1 and IgA2 in these two compartments may be independently regulated

Expression and Immunogenicity of Hemagglutinin A from Porphyromonas gingivalis in an Avirulent Salmonella enterica Serovar Typhimurium Vaccine Strain

ABSTRACT Porphyromonas gingivalis is a major etiologic agent of periodontitis, a chronic inflammatory disease that ultimately results in the loss of the supporting tissues of the teeth. Previous work has demonstrated the usefulness of avirulent Salmonella enterica serovar Typhimurium strains as antigen delivery systems for protective antigens of pathogens that colonize or cross mucosal surfaces. In this study, we constructed and characterized a recombinantS. enterica serovar Typhimurium avirulent vaccine strain which expresses hemagglutinin A and carries no antibiotic resistance markers. HagA, a major virulence-associated surface protein, is a potentially useful immunogen that contains an antigenic epitope which, in humans, elicits an immune response that is protective against subsequent colonization by P. gingivalis. ThehagA gene, including its promoter, was cloned into a balanced-lethal Salmonella vector and transferred to the vaccine strain. Heterologous expression of HagA was demonstrated in both Escherichia coli JM109 and S. entericaserovar Typhimurium vaccine strain χ4072. The HagA epitope was present in its native configuration as determined by immunochemistry and immunoelectron microscopy. Purified recombinant HagA was recognized by sera from mice immunized with the S. enterica serovar Typhimurium vaccine strain. The HagA-specific antigen of the vaccine was also found to be recognized by serum from a periodontal patient. This vaccine strain, which expresses the functional hemagglutinin protein, induces a humoral immune response against HagA and may be useful for developing a protective vaccine against periodontal diseases associated with P. gingivalis.

Analysis of the hepatobiliary transport of IgA with monoclonal anti-idiotype and anti-allotype antibodies.

The processing and fate of mixed immune complexes is influenced by the antibody isotypes present. The hepatobiliary transport of mixed immune complexes containing the mouse IgA myeloma protein J558 and corresponding monoclonal IgG or IgM anti-J558 idiotype or monoclonal IgG anti-mouse IgA allotype antibodies has been studied. The anti-idiotype or anti-allotype antibodies were radiolabeled and injected into mice with or without mouse polymeric IgA (J558). IgG anti-idiotype antibodies to J558 IgA were selectively transported into bile by J558 IgA. This process occurred with a radiolabeled Fab preparation of the IgG anti-idiotype and was inhibitable with IgA of an irrelevant antigenic specificity. Thus, polymeric IgA influenced the fate of IgA-IgG idiotype-anti-idiotype serum immune complexes. A monoclonal anti-idiotype antibody of the IgM isotype (D8-3) was not selectively transported into bile by itself or as an IgA-IgM complex. A monoclonal IgG antibody (CB5-6) to a mouse allotype determinant in the Fc portion of IgA was not selectively transported into bile. This anti-allotype monoclonal antibody inhibited the hepatobiliary transport of 125I-polymeric J558 IgA and therefore appeared to directly or indirectly block the site in the Fc region of IgA recognized by the hepatic receptor.

Evidence for a Common Mucosal Immune System in Humans

The finding that ingestion of antigens results in the selection induction of IgA antibodies in external secretions suggests that antigen sensitizes Peyers patch lymphoid cells, which migrate to mucosal sites and generate local secretory IgA (S-IgA) antibody responses. Evidence for a common mucosal immune system in humans has been scanty because of the difficulty in demonstrating migratory behavior of Peyers patch cells. In the present study, peripheral blood mononuclear cells (PBMC) from human volunteers who had ingested capsules containing killed Streptococcus mutans were assayed for spontaneous antibody-producing cells. Four of five volunteers exhibited circulating IgA-producing cells within 7 days and reached maximum responses by days 10-12. One IgA-deficient subject exhibited IgM responses with identical kinetics. Pokeweed-mitogen-stimulated PBMC produced anti-S. mutans antibodies predominantly of the IgA isotype. Significant S-IgA anti-S. mutans antibodies were detected in saliva and tears by day 14, and the antibodies reached maximum titers by 3 weeks. No changes in serum anti-S. mutans antibodies were noted. The IgA-deficient subject produced salivary secretory IgM antibodies. These results suggest that, after antigen ingestion, peripheral blood contains antigen-specific precursors of IgA plasma cells and that their presence precedes the appearance of S-IgA antibodies in external secretions. Therefore, these experiments provide further support for the existence of a common mucosal immune system in humans.

Effects of Endogenous Gut LPS on Cells of the Secretory Immune System

Lipopolysaccharide (LPS), the major constituent of the outer membrane of gram-negative bacteria, is a potent biostimulant for lymphoreticular cells of the host’s immune system (14). The major source of LPS in mammalian species is the indigenous gram-negative microflora of the gastrointestinal tract, of which Bacteroides species becomes the predominant genera during the first few weeks of life. The first major lymphoid tissue affected by gut LPS is the gut-associated lymphoreticular tissue (GALT), which is mainly represented by distinct lymphoid nodules, termed Peyer’s patches (PP), along the small intestine. The GALT or PP are major inductive sites for IgA responses to ingested environmental antigens and intestinal pathogens.