THEMES
Current Concepts in Mucosal Immunity
IV. How epithelial transport of IgA antibodies relates to host defense*

Michael E. Lamm

Institute of Pathology, Case Western Reserve University, Cleveland, Ohio 44106

    ABSTRACT
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Abstract
Introduction
References

The humoral arm of the mucosal immune system is principally composed of locally synthesized polymeric IgA, whose Fc portion is adapted for binding to the polymeric immunoglobulin receptor that is expressed on the basolateral surface of mucosal epithelial cells, including enterocytes. This receptor mediates the endocytosis and transcytosis of polymeric IgA, which allows IgA to function in host defense at three anatomic levels in relation to mucosal epithelium: IgA antibodies in the lamina propria can bind antigens and excrete them through the epithelium into the lumen; antiviral IgA antibodies in transit through epithelial cells can inhibit virus production by an intracellular action; and IgA antibodies secreted into the lumen can prevent antigens and microbes from adhering to and penetrating the epithelium. The ways in which IgA antibodies function in mucous membranes provide challenging investigative opportunities for cell physiologists and cell biologists.

polymeric immunoglobulin receptor; epithelial transcytosis; mucosal defense

    INTRODUCTION
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Abstract
Introduction
References

IN KEEPING with its location at the interface between the body proper and the external environment and the anatomy of mucous membranes, the mucosal immune system manifests some distinctive features compared with the systemic immune system. This article focuses on IgA antibodies and how their role in host defense relates to a particular mechanism of transport through mucosal epithelium.

The bulk of the body's IgA antibodies, as well as the body's total pool of immunoglobulins, are produced by plasma cells in mucosal lamina propria, i.e., the loose connective tissue beneath the epithelium that lines a mucous membrane (2). Production of IgA is especially prominent in the intestinal tract because of its length, enormous surface area, and exposure to the antigens of microbes and food. Most of this IgA is destined for local secretion because of the existence of a particular receptor-mediated transport mechanism through the epithelium (15). The receptor, termed the polymeric immunoglobulin receptor (pIgR), binds to those molecules of immunoglobulin that are composed of more than a single basic four-chain immunoglobulin subunit, which is in turn composed of two heavy chains and two light chains. Such polymeric immunoglobulins include most of the locally synthesized IgA and all of the IgM. The pIgR is synthesized by enterocytes (and other mucosal epithelial and glandular cells) and is expressed on the basolateral plasma membrane, where it has an opportunity to bind locally produced IgA (and IgM) antibodies. The resulting complex of pIgR and IgA next undergoes endocytosis and vesicular transport to the apical surface of the enterocyte, where the pIgR is proteolytically cleaved between its external and intramembranous domains, thereby releasing IgA, still bound to the external domain of the pIgR, into the secretions. In contrast, immunoglobulins that are not polymeric, such as IgG, the major class of antibody in serum, have no physiological means of reaching the external secretions. They do not bind to the pIgR, and, like macromolecules generally, they are prevented from diffusing through the epithelium by the tight junctions that connect adjacent epithelial cells. Thus, although all classes of immunoglobulin are present in mucosal lamina propria, via either transudation from the bloodstream or synthesis by local plasma cells, only polymeric IgA and IgM are able to pass through the epithelial layer and enter the secretions to a significant extent.

In rodents there is another important route by which IgA can reach the intestinal lumen (3). Rodent hepatocytes express pIgR on the sinusoidal face (analogous to the basolateral surface of enterocytes) and thus are able to bind circulating polymeric IgA and endocytose and transport it to the canalicular face (analogous to the apical surface of enterocytes) for release into the bile. In humans, however, this route into the intestinal lumen is not applicable to circulating IgA, since human hepatocytes do not express pIgR.

By virtue of the pIgR-mediated transport mechanism, IgA antibodies can potentially combine with antigens in three anatomic compartments in relation to mucosal epithelium: 1) in the luminal secretions, 2) within the epithelial cells during transcytosis, and 3) in the lamina propria beneath the epithelium (Fig. 1).


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Fig. 1.   Compartments where immunoglobulin A (IgA) can potentially function in relation to mucosal epithelium. The lumen is above the layer of epithelial cells, which are interconnected by apical tight junctions. The lamina propria is below. Plasma cells (not drawn to scale) in the lamina propria secrete polymeric IgA. Cell A shows that polymeric IgA can be endocytosed at the basolateral surface (by pIgR), transcytosed, and secreted into the lumen, where it can combine with antigen (Ag) to form immune complexes (IgAIC). In cell B, which has been infected by a virus, it is suggested that a transcytotic vesicle containing IgA antibody to viral envelope protein can fuse with a post-Golgi vesicle containing newly synthesized envelope protein, which provides an opportunity for the antibody to disrupt production of new virus. In the lamina propria below cell C, IgA antibody combines with antigen. The immune complex is endocytosed (by pIgR) and transported intact across the cell and into the lumen.

For many years it has been appreciated that IgA antibodies in the intestinal (and other mucosal) secretions provide a first line of immunological defense against microbial pathogens by helping to prevent them from adhering to and penetrating the mucosal epithelium (9). Early on in support of this concept, correlations were made clinically and experimentally between the content of specific IgA antibodies in the luminal secretions and resistance to challenge by a given pathogen. More recently, it has been possible to show experimentally with a number of mucosal pathogens, including enteric pathogens, that IgA antibody acting as the sole immunologically specific agent can mediate effective resistance to microbial challenge (1, 13, 14, 21). Such considerations have provided much of the rationale for attempts to develop oral vaccines capable of stimulating effective mucosal IgA antibody responses against infectious agents. Historically, the best example is oral vaccination against polio virus.

    INTRAEPITHELIAL CELL NEUTRALIZATION OF VIRUSES

In addition to the traditional concept of IgA antibody action in the mucosal secretions described above, evidence has emerged in recent years to support a potentially important locus of IgA action inside mucosal epithelial cells. This possibility arises from the obligatory, pIgR-mediated transepithelial route IgA follows to enter the secretions. Might it be possible for transcytosing IgA antibody with specificity for a component of an intracellular pathogen like a virus to interfere with the replication cycle of the microbe? Acting in this fashion, IgA would be providing a second line of immune defense behind a primary front in the luminal secretions. Parenthetically, an intracellular function such as this one for antibody is a somewhat heretical proposal, in that thinking in immunology has long envisioned that antibodies only interact with antigens (including microbes) extracellularly, leaving the responsibility for dealing with intracellular pathogens to the cell-mediated immune system.

The initial experiments supporting the feasibility of an intracellular antimicrobial action on the part of IgA antibodies in the course of being transcytosed through mucosal epithelium were done in two-chambered culture vessels containing polarized monolayers of epithelial cells that expressed pIgR on the basolateral surface. The cells were infected from the apical surface with a virus, and IgA monoclonal antibody to viral envelope protein was then added below the basolateral cell surface, from which it could be endocytosed via the pIgR and transcytosed. Under these conditions, and with controls to exclude an extracellular action, the production of Sendai virus, a parainfluenza virus, and influenza virus was specifically inhibited by the antibody (11, 12). In further support of an intracellular action, in this experimental setup it has also been possible to demonstrate microscopically that transcytosing IgA antibody is in fact able to access a subcellular compartment that contains viral envelope protein. Double-label immunofluorescence was initially used, and more recently double-label immunoelectron microscopy was used (Fig. 2).


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Fig. 2.   Colocalization of Sendai virus HN protein and specific IgA antibody in a saponin-permeabilized epithelial cell. Madin-Darby canine kidney (MDCK) cell monolayers were infected apically with Sendai virus, and anti-HN IgA antibody was added basolaterally. The large 15-nm gold particles label IgA and the small 5-nm gold particles label HN protein. The bar (0.25 µm) overlies the nucleus (H. Fujioka and M. Mazanec, unpublished results).

The concept of intraepithelial cell neutralization of viruses by transcytosing IgA antibody also has support from studies in vivo. For example, IgA monoclonal antibody to inner capsid protein given systemically to mice is able to inhibit intestinal epithelial infection by rotavirus (4). In an analogous fashion, systemically administered IgA monoclonal antibody to the E2 surface spike protein of mouse hepatitis virus is able to inhibit virus production in the livers of infected mice (D. Huang and M. Mazanec, unpublished results). In both of these situations, the inference is that IgA antibodies being transcytosed across intestinal epithelial cells and hepatocytes, respectively, are acting intracellularly to interfere with virus production.

These results on antiviral actions of IgA antibodies should not be construed to suggest that all (or even most) IgA antibodies to components of viruses that infect epithelial cells are likely to be able to mediate intracellular neutralization. The ability of a transcytosing IgA antibody to be effective in this regard probably depends on a number of factors, including the particular viral component and antigenic determinant to be recognized and the structure and replication cycle of a given virus, which collectively will determine the likelihood of a transcytosing vesicle containing IgA antibody encountering a particular viral constituent at a susceptible point. For example, a viral envelope glycoprotein that is synthesized in the rough endoplasmic reticulum and processed through the Golgi apparatus, trans-Golgi network, and apical endosomes (Fig. 1) should stand a better chance of encountering transcytosing IgA antibody and being a target for intracellular neutralization than an internal viral constituent that is synthesized on ribosomes that are free in the cytoplasm and not so connected to vesicular trafficking pathways.

    EXCRETORY IMMUNE FUNCTION

A third line of defense mediated by IgA antibodies in relation to mucosal epithelium is suggested by the following considerations. Since mucosal epithelium is not totally impermeable to penetration by macromolecules (5), intact antigens or their fragments can to a small extent gain access to the lamina propria, especially in the intestine, where the luminal contents are charged with constituents of food and with microflora. This constant exposure is supplemented by periodic bouts of infection, which can result in the presence of intact microbes and their antigens in the lamina propria. Accordingly, the IgA antibodies that are continuously secreted from local plasma cells can be expected to have ready opportunities to bind diverse antigens that happen to be in the microenvironment (Fig. 1).

Studies in vitro have demonstrated that soluble immune complexes composed of protein antigens and specific IgA antibodies can be endocytosed and transported without degradation through polarized epithelial cells by the same mechanism and route utilized by free IgA (7). Furthermore, this process can be extended to antigens as large as a virus (6). Evidence for an analogous function of IgA in vivo derives in part from work with the rodent liver that takes advantage of its ability to transport polymeric IgA from blood to bile via hepatocyte pIgR; the rodent liver can also transport immune complexes composed of soluble antigen (16-18) or virus (6) bound to IgA antibody. Recent evidence suggests that IgA antibody in vivo can also transport antigen from lamina propria across intestinal epithelium (J. Robinson and M. Lamm, unpublished results), which would be more relevant to the human situation. Thus, IgA antibodies appear to be capable of providing an "excretory" immune system that can rid the body of foreign substances and minimize exposure to a potentially harmful burden of local or systemic immune complexes. This excretory function of the mucosal immune system can perhaps be viewed as analogous to the lungs excreting CO2 or the kidneys excreting urea.

    IGA AND MUCOSAL INFLAMMATION

The portion of the immunoglobulin molecule not directly involved in binding antigen, i.e., the Fc portion, is in general concerned with activating and mediating secondary phenomena that can lead to the production or release of inflammatory mediators such as histamine and products of the complement cascade. Compared to IgG, IgM, and IgE, antibodies of the IgA class are much less proinflammatory and can even be considered anti-inflammatory (8). IgA antibodies thus contribute to the general downimmunoregulatory tone of the mucosal immune system (20), which has the dual role of protecting against ubiquitous foreign substances and microbes, while at the same time not subjecting the mucosa to undue inflammation. In a sense, the limited ability of the Fc portion of IgA to activate pro-inflammatory phenomena can be viewed as part of the evolutionary focusing of the secondary functions of IgA on the ability to bind to the pIgR, which provides a mechanism for transport across mucosal epithelium. In this regard, it is interesting that certain cytokines, like interferon-gamma , that may be present during bouts of local inflammation have the ability to upregulate the expression of epithelial pIgR (10, 19, 22), which increases the capacity for IgA transport and thus enhances IgA function.

The challenge of more fully understanding the mechanisms underlying mucosal IgA antibody function, including the implications of the interactions of IgA with mucosal epithelium, should prove increasingly attractive to investigators in a number of disciplines beyond immunology and infectious disease, including cell physiology and cell biology.

    ACKNOWLEDGEMENTS

Research in the author's laboratory is supported by National Institutes of Health Grants AI-26449 and AI-36359.

    FOOTNOTES

* Fourth in a series of invited articles on Current Concepts in Mucosal Immunity.

Address for reprint requests: M. E. Lamm, Institute of Pathology, Case Western Reserve Univ., 2085 Adelbert Rd., Cleveland, OH 44106.

    REFERENCES
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Abstract
Introduction
References

1.   Blanchard, T. G., S. J. Czinn, R. Maurer, W. D. Thomas, G. Soman, and J. G. Nedrud. Urease-specific monoclonal antibodies prevent Helicobacter felis infection in mice. Infect. Immun. 63: 1394-1399, 1995[Abstract].

2.   Brandtzaeg, P. Distribution and characteristics of mucosal immunoglobulin-producing cells. In: Handbook of Mucosal Immunology, edited by P. L. Ogra, J. Mestecky, M. E. Lamm, W. Strober, J. R. McGhee, and J. Bienenstock. San Diego, CA: Academic, 1994, p. 251-262.

3.   Brown, W. R., and T. M. Kloppel. The liver and IgA: immunological, cell biological and clinical implications. Hepatology 9: 763-784, 1989[Medline].

4.   Burns, J. W., M. Siadat-Pajouh, A. A. Krishnaney, and H. B. Greenberg. Protective effect of rotavirus VP6-specific IgA monoclonal antibodies that lack neutralizing activity. Science 272: 104-107, 1996[Abstract].

5.   Curtis, G. H., and D. G. Gall. Macromolecular transport by rat gastric mucosa. Am. J. Physiol. 262 (Gastrointest. Liver Physiol. 25): G1033-G1040, 1992[Abstract/Free Full Text].

6.   Gan, Y.-J., J. Chodosh, A. Morgan, and J. W. Sixbey. Epithelial cell polarization is a determinant in the infectious outcome of immunoglobulin A-mediated entry by Epstein-Barr virus. J. Virol. 71: 519-526, 1997[Abstract].

7.   Kaetzel, C. S., J. K. Robinson, K. R. Chintalacharuvu, J.-P. Vaerman, and M. E. Lamm. The polymeric immunoglobulin receptor (secretory component) mediates transport of immune complexes across epithelial cells: a local defense function of IgA. Proc. Natl. Acad. Sci. USA 88: 8796-8800, 1991[Abstract].

8.   Kilian, M., and M. W. Russell. Function of mucosal immunoglobulins. In: Handbook of Mucosal Immunology, edited by P. L. Ogra, J. Mestecky, M. E. Lamm, W. Strober, J. R. McGhee, and J. Bienenstock. San Diego, CA: Academic, 1994, p. 127-137.

9.   Lamm, M. E. Interaction of antigens and antibodies at mucosal surfaces. Annu. Rev. Microbiol. 51: 311-340, 1997[Medline].

10.   Loman, S., J. Radl, H. M. Jansen, T. A. Out, and R. Lutter. Vectorial transcytosis of dimeric IgA by the Calu-3 human lung epithelial cell line: upregulation by IFN-gamma . Am. J. Physiol. 272 (Lung Cell. Mol. Physiol. 16): L951-L958, 1997[Abstract/Free Full Text].

11.   Mazanec, M. B., C. L. Coudret, and D. R. Fletcher. Intracellular neutralization of influenza virus by IgA anti-HA monoclonal antibodies. J. Virol. 69: 1339-1343, 1995[Abstract].

12.   Mazanec, M. B., C. S. Kaetzel, M. E. Lamm, D. Fletcher, and J. G. Nedrud. Intracellular neutralization of virus by immunoglobulin A antibodies. Proc. Natl. Acad. Sci. USA 89: 6901-6905, 1992[Abstract].

13.   Mazanec, M. B., J. G. Nedrud, and M. E. Lamm. Immunoglobulin A monoclonal antibodies protect against Sendai virus. J. Virol. 61: 2624-2626, 1987[Medline].

14.   Michetti, P., M. J. Mahan, J. M. Slauch, J. J. Mekalanos, and M. R. Neutra. Monoclonal secretory immunoglobulin A protects mice against oral challenge with the invasive pathogen Salmonella typhimurium. Infect. Immun. 60: 1786-1792, 1992[Abstract].

15.   Mostov, K. E. Transepithelial transport of immunoglobulins. Annu. Rev. Immunol. 12: 63-84, 1994[Medline].

16.   Peppard, J., E. Orlans, A. W. R. Payne, and E. Andrew. The elimination of circulating complexes containing polymeric IgA by excretion in the bile. Immunology 42: 83-90, 1981[Medline].

17.   Russell, M. W., T. A. Brown, and J. Mestecky. Role of serum IgA. Hepatobiliary transport of circulating antigen. J. Exp. Med. 153: 968-976, 1981[Abstract].

18.   Socken, D. J., E. S. Simms, B. R. Nagy, M. M. Fisher, and B. J. Underdown. Secretory component-dependent hepatic transport of IgA antibody-antigen complexes. J. Immunol. 127: 316-319, 1981[Abstract/Free Full Text].

19.   Sollid, L. M., D. Kvale, P. Brandtzaeg, G. Markussen, and E. Thorsby. Interferon-gamma enhances expression of secretory component, the epithelial receptor for polymeric immunoglobulins. J. Immunol. 138: 4303-4306, 1987[Abstract/Free Full Text].

20.   Weiner, H. L. Oral tolerance: immune mechanisms and treatment of autoimmune diseases. Immunol. Today 19: 335-343, 1997.

21.   Winner, L., III, J. Mack, R. Weltzin, J. J. Mekalanos, J.-P. Kraehenbuhl, and M. Neutra. New model for analysis of mucosal immunity: intestinal secretion of specific monoclonal immunoglubulin A from hybridoma tumors protects against Vibrio cholerae infection. Infect. Immun. 59: 977-982, 1991[Medline].

22.   Youngman, K. R., C. Fiocchi, and C. S. Kaetzel. Inhibition of IFN-gamma activity in supernatants from stimulated human intestinal mononuclear cells prevents upregulation of the polymeric Ig receptor in an intestinal epithelial cell line. J. Immunol. 153: 675-681, 1994[Abstract/Free Full Text].


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