Publisher Summary This chapter discusses the impact of new information concerning IgA physiology on the immune system. IgA should not be considered only as an isotype providing specific humoral protection of mucosal surfaces but as an integral component of the entire immune system. An unusual structural feature of human IgA is the heterogeneity of the molecular forms with characteristic distribution in various body fluids. Though most IgA in serum displays a typical four-polypeptide chain structure of the basic molecule with two Q and two light (L) chains, external secretions contain dimeric and tetrameric, disulfide-linked molecules associated with additional polypeptides-J (joining) chain and secretory component (SC). IgA-producing plasma cells are distributed in various lymphoid and nonlymphoid tissues and are particularly preponderant in the lamina propria of the gut; in salivary, lacrimal, and lactating mammary glands; and in the human bone marrow. IgA occurs in different body fluids in predominantly polymeric or monomeric (plasma, cerebrospinal fluid) forms with a characteristic distribution of IgAl and IgAz molecules. Under normal conditions, an absolute majority of IgA-containing cells in secretory glands and tissues also contain J chain whereas such cells in, for example, normal bone marrow does not. Staining with fluorochrome-labeled anti-J chain is enhanced by the pretreatment of alcohol-fixed tissue sections with acid urea, which leads to the exposure of masked antigenic determinants of intracellular J chain. Specialized lymphoid tissues associated with mucosal surfaces play an essential role in the induction and regulation of generalized immune responses in external secretions.

Immunoglobulin A (IgA): molecular and cellular interactions involved in IgA biosynthesis and immune response.

The immune geography of IgA induction and function

The selective induction of antibodies in external secretions is desirable for the prevention of various systemic as well as predominantly mucosa-restricted infections. An enormous surface area of mucosal membranes is protected primarily by antibodies that belong, in many species, to the IgA isotype. Such antibodies are produced locally by large numbers of IgA-containing plasma cells distributed in subepithelial spaces of mucosal membranes and in the stroma of secretory glands. In humans and in some animal species, plasma-derived IgA antibodies do not enter external secretions in significant quantities and systemically administered preformed IgA antibodies would be of little use for passive immunization. Systemic administration of microbial antigens may boost an effective S-IgA immune response only in a situation whereby an immunized individual had previously encountered the same antigen by the mucosal route. Local injection of antigen in the vicinity of secretory glands is usually accompanied by an undesirable concomitant systemic response and frequently requires the addition of adjuvants that are unacceptable for administration in humans. Immunization routes that involve ingestion or possibly inhalation of antigens lead to the induction of not only local but also generalized immune responses manifested by the parallel appearance of S-Iga antibodies to ingested or inhaled antigens in secretions of glands distant from the site of immunization. Based on extensive studies in animal models as well as in humans, convincing evidence is available that antigen-sensitized and IgA-committed precursors of plasma cells from GALT are disseminated to the gut, other mucosa-associated tissues, and exocrine glands. However, due to the limited absorption of desired antigens from the gut lumen of orally immunized individuals, repeated large doses of antigens are required for an effective S-IgA response. Novel antigen delivery systems for the stimulation of such responses are currently being examined in several laboratories. Live attenuated or genetically manipulated bacteria expressing other microbial antigens have also been used for selective colonization of gut-associated lymphoid tissues. Unique antigen packaging and the use of adjuvants suitable for oral administration hold promise for an efficient antigen delivery to critical tissues in the intestine and deserve extensive exploration. The oral immunization route appears to have many advantages over systemic immunization.(ABSTRACT TRUNCATED AT 400 WORDS)

The common mucosal immune system and current strategies for induction of immune responses in external secretions

Transcytosis, the vesicular transport of macromolecules from one side of a cell to the other, is a strategy used by multicellular organisms to selectively move material between two environments without altering the unique compositions of those environments. In this review, we summarize our knowledge of the different cell types using transcytosis in vivo, the variety of cargo moved, and the diverse pathways for delivering that cargo. We evaluate in vitro models that are currently being used to study transcytosis. Caveolae-mediated transcytosis by endothelial cells that line the microvasculature and carry circulating plasma proteins to the interstitium is explained in more detail, as is clathrin-mediated transcytosis of IgA by epithelial cells of the digestive tract. The molecular basis of vesicle traffic is discussed, with emphasis on the gaps and uncertainties in our understanding of the molecules and mechanisms that regulate transcytosis. In our view there is still much to be learned about this fundamental process.

Transcytosis: Crossing Cellular Barriers

Stimulated by a report on elevated IgA levels in nasopharyngeal carcinoma (NPC), we tested a total of 372 sera from patients with NPC, other carcinomas of head and neck or elsewhere, Burkitt's lymphoma (BL), infectious mononucleosis (IM) or healthy controls. The sera were titrated in indirect immunofluorescence tests for IgA antibodies to Epstein-Barr virus (EBV) capsid antigen (VCA) and to the diffuse (D) or restricted (R) components of the EBV-induced early antigen (EA) complex. The results proved NPC to be outstanding in that prior to therapy 93% of the patients tested revealed IgA antibodies to VCA and 73% to D, often at high titers which occasionally matched the corresponding IgG antibody levels. The EBV-specific IgA titers increased from stages I or II to stages III or IV; i.e. with the total tumor burden. Conversely, many of the NPC patients examined 2-6 years after initial therapy had only low levels of EBV-specific IgA or none at all, and the majority of those with high titers were known to have residual or recurrent disease. In contrast to untreated NPC patients, less than 5% of 73 patients with other carcinomas or of 76 healthy donors revealed VCA-specific IgA and even fewer EA-specific IgA; only 28% and 4% of 54 BL patients tested at admission had IgA antibodies to VCA and R, respectively, and 38% and 3% of 37 IM patients showed transient VCA- or D-specific IgA responses, all at generally low titers. While sera from untreated NPC patients often contained IgA antibodies also to herpes simplex type 1 virus, their incidence and range of low titers were similar to those obtained with sera from patients with other carcinomas or from healthy donors. It thus appears that the elevated IgA levels in NPC might be due to EBV-specific antibodies. Possible reasons for this unique response in NPC have been discussed.

Epstein-Barr virus-specific IgA serum antibodies as an outstanding feature of nasopharyngeal carcinoma.

The γ1A present in saliva and colostrum exists largely in the form of higher polymers, the major component of which has a sedimentation coefficient of 11S. The 11S γ1A in these fluids differs from the polymers found in normal and myeloma sera both immunologically and by the fact that their sedimentation coefficients are unaffected by disulfide bond reduction in the absence of urea. However, like other γ-globulins the 11S γ1A molecules consist of multiple polypeptide chains linked by disulfide bonds. Local synthesis of γ1A in the salivary gland has been shown by fluorescent and autoradiographic studies, although the fraction of the total salivary γ1A which is derived from local production is uncertain. No evidence of transport of intravenously administered I131-labeled 7S γ1A from serum to saliva was obtained. Immunological specificity has been demonstrated in the salivary and colostral γ1A. Whether that portion of the γ1A which is immunologically specific is a piece incorporated during the local synthesis of γ1A in the gland or is added by the epithelial cell in the process of transport remains to be determined. Antibody activity (isohemagglutinins) have been demonstrated in saliva and colostrum and have been shown to be of the γ1A-type. In both of these fluids activity is associated primarily with γ1A-polymers of 11S and 18S sizes. There appears to be an immunological system which is characteristic of certain external secretions. Its properties including the local production of a distinctive type of antibody separate it from the "systemic" system responsible for the production of circulating antibody. This system may play a significant role in the body's defense mechanisms against allergens and microorganisms.

/pdf/characteristics-of-an-immune-system-common-to-certain-3skzszb2t2.pdf

Characteristics of an immune system common to certain external secretions

Studies of normal human lacrimal glands were performed by utilizing fluorescent antibody and ultrastructural techniques in order to examine immunoglobulin production and localization within the gland, and to investigate mechanisms of transport of these immunoglobulins from the gland. Throughout the interstitium of each gland, clusters of cells having the morphologic characteristics of plasma cells were evident. Fluorescent antibody staining showed these cells to contain predominantly IgA, with IgG or IgM occasionally present, and IgE only rarely demonstrable. Extracellular IgA staining was seen along basement membranes of acini, between acinar epithelial cells, and occasionally within acinar lumina. IgA was also occasionally seen intracellularly throughout acinar epithelial cells. IgG was diffusely evident throughout the interstitium but appeared particularly concentrated along the acinar basement membranes. Secretory component and lactoferrin were localized to the acinar epithelial cells, and secretory component could also be found in acinar intercellular spaces. The larger numbers of IgA-containing plasma cells relative to other immunoglobulin-specific plasma cells are consistent with the known proportions of immunoglobulins in tears, and suggest their local production within the lacrimal gland. A secretory mechanism involving intracellular transport of immunoglobulins by acinar epithelial cells is proposed.

Immunohistologic Studies of Human Lacrimal Gland: Localization of Immunoglobulins, Secretory Component and Lactoferrin

The middle ear washings and biopsy specimens of middle ear mucosa from 20 patients with secretory otitis media were examined for the distribution of major classes of immunoglobulin; presence and nature of antibody activity against mumps, measles, rubella and poliovirus; and immunologic localization of immunocompetent lymphoid cells. Appreciable amounts of secretory component, and γG, 7SγA, and 11S secretory γA immunoglobulin were regularly found in the ear fluids. The ratio of γG: γA immunoglobulin in the serum and middle ear fluids was approximately 10:1 and 10:4, respectively. Fluorescent antibody staining of mucosal tissues of middle ear demonstrated characteristic staining for secretory component in the surface epithelium, and the presence of γG and γA containing lymphoid tissue in the submucosa. Tissue culture studies demonstrated in vitro synthesis of γA and γG immunoglobulin and the secretory component in the mucosal tissues of middle ear. Specific antibody activity in the middle ear fluid against the viruses listed above was essentially limited to γA immunoglobulin, although such activity in the serum was predominantly associated with γG immunoglobulin. These data suggest the existence of a distinct secretory immune system in the middle ear. The implications of these observations may be applicable to the mechanism of protection, or pathogenesis of microbial infections in the middle ear.

Characteristics of Secretory Immune System in Human Middle Ear: Implications in Otitis Media

Complex formation between IgA and several serum proteins including albumin (1), βlipoproteins (2), haptoglobins (3), anti-hemophilic globulin (4) and in one case, to α1 glycoprotein (3) has been previously noted However, with the exception of albumin, the binding of these proteins to IgA myeloma proteins has not been regularly observed It has been reported (3) that 53% of IgA myeloma proteins as well as 74% of monoclonal IgM proteins complex with albumin Subsequently, a thorough study by Mannik (5) demonstrated that the binding of albumin by both IgA and IgM proteins involved covalent disulfide bonds Presumably, the binding site is located in the heavy chain and could involve the free SH group thought to be present in the α-chain (6), although this has not been directly demonstrated During the course of a study attempting to localize the site of albumin binding to IgA proteins, an antiserum to albumin was employed which also contained antibodies to α1 antitrypsin (α1AT) 2

The binding of alpha-1 antitrypsin to human IgA.

The secretory immune systems of the cow and sheep were found to be strikingly similar. The principal immunoglobulin in the saliva and colostrum of both species is a “fast” 7 S γ G1 protein which is low in carbohydrate. Although suggestive evidence was obtained for a 10 S secretory immunoglobulin, proof of its existence must await isolation and complete separation from the major 7 S γ G1 component. The secretory 7 S γ G1 is immunologically identical to the γ G1 of serum and in this study no evidence of a component analogous to the human secretory “piece” was detected.

Thomas B. Tomasi

Papers

Characteristics of an immune system common to certain external secretions

Immunohistologic Studies of Human Lacrimal Gland: Localization of Immunoglobulins, Secretory Component and Lactoferrin

Characteristics of Secretory Immune System in Human Middle Ear: Implications in Otitis Media

The binding of alpha-1 antitrypsin to human IgA.

Characterization of the Serum and Secretory Immune Systems of the Cow and Sheep