THEMES
Mucosal Immunity and Inflammation
III. The mucosal antigen barrier: cross talk with mucosal cytokines*

Mary H. Perdue

Intestinal Disease Research Program and Department of Pathology and Molecular Medicine, McMaster University, Hamilton, Ontario, Canada L8N3Z5


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
THE EPITHELIAL BARRIER
ANTIGEN HANDLING AND ORAL...
ENTEROPATHOGENS STIMULATE...
MUCOSAL RESPONSE TO STRESS
INTESTINAL HYPERSENSITIVITY
CONCLUSIONS
REFERENCES

We have known for many years that mucosal responses to antigens are regulated by immune cells and their molecular signals. More recently, it has become clear that epithelial cells also synthesize and secrete chemokines and cytokines. A sophisticated system of bidirectional cytokine signals is responsible for immune activation in the case of enteropathogens vs. immune suppression to food and commensal microbial antigens. A key factor in determining antigen handling is the route taken by antigens across the epithelial barrier. Cytokines and other mucosal messenger molecules play a critical role in the regulation of transepithelial antigen transport.

epithelium; permeability; inflammation; stress; intestinal hypersensitivity


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
THE EPITHELIAL BARRIER
ANTIGEN HANDLING AND ORAL...
ENTEROPATHOGENS STIMULATE...
MUCOSAL RESPONSE TO STRESS
INTESTINAL HYPERSENSITIVITY
CONCLUSIONS
REFERENCES

THE GASTROINTESTINAL MUCOSA is truly a unique environment when it comes to antigen handling. The antigenic load contained within the lumen is immense, and the mechanisms that have evolved to deal with this load are extremely complex. For example, the gut content of commensal microbes is ~1013 organisms, and it has been estimated that an individual consumes at least 2,500 kg of food antigens during a normal lifetime. Yet, for the most part, these foreign antigens do not elicit an inflammatory response. In addition, numerous pathogenic organisms are ingested. It is clear that a sophisticated system is required to discriminate between enteropathogens and harmless food molecules or antigens from commensal organisms. Only a single layer of epithelial cells separates the luminal contents from effector immune cells in the lamina propria. Intraepithelial leukocytes (IEL) are in even closer proximity to luminal antigens, being located within the epithelial layer just below the tight junctions that join adjacent epithelial cells. Regulation of the epithelial barrier is critical to the maintenance of the low reactivity state. Although there is now considerable information on the influence of immune cell cytokines and mediators on epithelial physiology, we are just beginning to understand the critical role of epithelial cells in regulating the response to ingested antigens, that is, in determining whether an immune response is activated or suppressed. Oral tolerance is the term used to refer to the state of unresponsiveness that exists for nonpathogenic antigens present within the gut lumen. This state is not a passive lack of response; rather, it is a specifically downregulated system essential for homeostasis. Over the past few years, much has been learned regarding mechanisms involved in regulating mucosal immunity, particularly the importance of bidirectional cytokine signals between epithelial cells and immune cells. These and other messenger molecules direct the routes and nature of transepithelial antigen transport and determine the consequences of antigen uptake into the mucosa.


    THE EPITHELIAL BARRIER
TOP
ABSTRACT
INTRODUCTION
THE EPITHELIAL BARRIER
ANTIGEN HANDLING AND ORAL...
ENTEROPATHOGENS STIMULATE...
MUCOSAL RESPONSE TO STRESS
INTESTINAL HYPERSENSITIVITY
CONCLUSIONS
REFERENCES

The gastrointestinal tract is lined by a single layer of epithelial cells joined together at their apical poles by tight junctions. Tight junctions used to be thought of as passive structures, barricades to the paracellular pathway restricting passage of molecules beyond a relatively small size. However, studies over the past few years have documented that tight junctions are highly regulated gates that open and close in response to events in the lumen, signals from the lamina propria, and even messages from the epithelium itself. The transporting enterocytes are the major cell type in the epithelium. However, other cell types in the epithelium, such as goblet cells, enteroendocrine cells, and Paneth cells, act in concert with enterocytes to enhance the barrier properties of the epithelium to antigens. Paneth cells produce defensins (molecules with secretagogue properties) that target microorganisms; enteroendocrine cells secrete neuroendocrine molecules that can act in a paracrine fashion to alter epithelial properties; goblet cells synthesize mucus, a physical component of the barrier, and trefoil peptides needed for epithelial growth and repair. Epithelial cells are maintained on a scaffolding of myofibroblasts, interdigitating cells that secrete basement membrane components and also critical growth factors for the epithelium. In addition, ion secretory responses of the epithelium can be amplified severalfold by lipid mediators produced by myofibroblasts in response to secretagogues. The barrier properties of the epithelium are influenced by the enteric nervous system, with cholinergic neurotransmitters clearly facilitating enhanced passage of macromolecules through tight junctions (5). In addition, mucosal mast cells are strategically located beneath the basement membrane in contact with nerve fibers, providing the potential for these cells to regulate epithelial functions. Other immune cells present in the lamina propria include T and B lymphocytes, macrophages, eosinophils, neutrophils, and so forth. Immune cell cytokines, both proinflammatory interferon-gamma (IFN-gamma ) and tumor necrosis factor-alpha (TNF-alpha ), and anti-inflammatory interleukin (IL)-4 act on epithelial receptors to increase the permeability of tight junctions (4, 16). These and other cytokines have been shown to affect epithelial physiology, altering barrier and/or transport properties (Table 1).

                              
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Table 1.   Bidirectional cytokine signals between the epithelium and immune cells

Of course, the main role of epithelial enterocytes is absorption of nutrients. Vectorial transport of nutrients and ions (and therefore water) also depends on the epithelium being relatively tight, since back diffusion would reduce or negate the direction and force of the flow. However, when luminal concentrations of sugars are high, a degree of absorption takes place via the paracellular pathway in response to solvent drag. Although most nutrients are absorbed by epithelial transporters in the form of small molecules (such as sugars, amino acids, small peptides, and so forth), enterocytes can also sample intact protein antigens. It was shown many years ago that endosomal uptake of proteins occurs across the apical membrane of epithelial cells, and small quantities of intact protein exit the basal cell membrane after ~15-20 min. Most non-receptor-bound proteins absorbed by this route are degraded by lysosomal enzymes following fusion of endosomes with lysosomes. Binding to specific receptors expressed on the apical membrane protects proteins/peptides from degradation. Thus important growth factors such as epidermal growth factor (EGF) in salivary secretions and breast milk are able to gain access to the internal milieu of the body in an active form. However, certain enterotoxins, including verotoxin (also known as Shiga toxin, produced by enterohemorrhagic Escherichia coli), can make use of the endosomal route to gain access to the body (21).


    ANTIGEN HANDLING AND ORAL TOLERANCE
TOP
ABSTRACT
INTRODUCTION
THE EPITHELIAL BARRIER
ANTIGEN HANDLING AND ORAL...
ENTEROPATHOGENS STIMULATE...
MUCOSAL RESPONSE TO STRESS
INTESTINAL HYPERSENSITIVITY
CONCLUSIONS
REFERENCES

Intracellular processing of antigenic proteins within enterocytes appears to be important for oral tolerance. Bypassing this route may be detrimental. For example, enhanced intestinal epithelial permeability has been demonstrated in humans with inflammatory bowel disease (IBD), where it is thought to be a possible etiological factor. Theoretically, luminal antigens crossing the epithelium through leaky junctions bypass epithelial processing necessary for oral tolerance and activate resident immune cells in the lamina propria. A similar mechanism may be involved in the spontaneous gut inflammation that develops in the E-cadherin gene-deficient mouse. In these mice, specific sites of defective expression in the crypts are associated with local inflammation (9). However, it is clear that regulation of tight junctions occurs, and many factors can increase epithelial permeability acutely with no long-term consequences. Therefore, it would appear that chronic dysregulation of the epithelial barrier is required for disease. Although epithelial uptake of luminal antigens may be necessary for oral tolerance, invasion of epithelial cells by enteric pathogens results in secretion of chemokines that recruit cells involved in the inflammatory/immune response (10). Therefore, it is obvious that epithelial interactions with antigens are critical to the health and well-being of the host.

The mechanisms that account for oral tolerance are still not completely understood, especially with respect to the role of the epithelium. The cytokine profile (whether proinflammatory Th1 or anti-inflammatory Th2) generated by mucosal T cells varies depending on the nature of the antigen and its processing and presentation. The key antigen-presenting cell may be the highly motile dendritic cell, as has been shown in the airways. Dendritic cells are present in large numbers in the gut mucosa and even within the epithelium (14). Their long cellular processes might be able to extend into the lumen and sample antigens and then transport these antigens to sites of interaction with T cells such as the local draining lymph nodes. However, in the airways where such a mechanism has been convincingly demonstrated, the properties of junctional structures in the multilayered epithelium may be quite different from those in the gut. To my knowledge, no one has documented extensions of dendritic cells penetrating through intestinal epithelial tight junctions. However, there is evidence for a role for dendritic cells in oral tolerance (26). M cells, specialized cells in the epithelium covering Peyer's patches, can certainly take up antigens and deliver them to professional antigen-presenting cells and T cells in the lymphoid patch (19). M cells transport particulate antigens such as bacteria and viruses and are undoubtedly important in the immune response to enteric microbes. Immune cell signals appear to be important for the development of M cells. There is considerable evidence that lymphoid follicles are a key inductive site for the generation of IgA-producing plasma cells that home to the intestinal lamina propria; enterocytes then transport IgA into the lumen via a membrane-bound receptor. However, an active role for M cells specifically in antigen presentation and oral tolerance has not been shown.

Enterocytes are capable of presenting antigens to T cells. It has long been known that enterocytes can be stimulated to express major histocompatibility (MHC) class II antigens, particularly during inflammation. IFN-gamma is an example of a proinflammatory cytokine that is a strong inducer of MHC II. It has been suggested that MHC presentation of antigen by enterocytes to IEL may be important for oral tolerance, with defective interactions in individuals predisposed to IBD, although there is no conclusive evidence for this hypothesis. However, with the use of CD1d (in association with gp180), a class Ib molecule, epithelial cells obtained from humans were shown to activate T cells, with cells from healthy humans preferentially stimulating CD8+ suppressor T cells, whereas cells from IBD patients stimulate CD4+ helper T cells (15). Regardless of the nature of the antigen-presenting cell responsible for oral tolerance, absorbed antigens in the circulation (plus other factors?) have tolerizing properties, since oral tolerance can be transferred to naive recipients in serum. In addition, cytokine signals, particularly transforming growth factor-beta (TGF-beta ), are clearly involved in the generation of oral tolerance (25).


    ENTEROPATHOGENS STIMULATE CYTOKINES SIGNALS
TOP
ABSTRACT
INTRODUCTION
THE EPITHELIAL BARRIER
ANTIGEN HANDLING AND ORAL...
ENTEROPATHOGENS STIMULATE...
MUCOSAL RESPONSE TO STRESS
INTESTINAL HYPERSENSITIVITY
CONCLUSIONS
REFERENCES

Complex cellular interactions and bidirectional messages occur in response to enteropathogens. Invasion of the enterocyte by pathogenic microbes, and in some cases by simply binding to epithelial cells, stimulates production of epithelial cytokines and chemokines [IL-8, TNF-alpha , lipid mediators, and so forth (Ref. 10, Table 1)], which recruit and activate inflammatory cells such as neutrophils. Neutrophils penetrate through the tight junctions of epithelial monolayers in response to bacterial products (13). In vivo, neutrophil recruitment to gastric mucosa in allergic inflammation was shown to be dependent at least in part on mast cell release of TNF-alpha (8). Microbial interactions with the apical membrane of epithelial cells can cause rearrangements of F-actin and associated proteins via phosphorylation of critical enzymes (myosin light-chain kinase, protein kinase C), leading to increased permeability of tight junctions (22, 27). The situation becomes even more complex if one takes into account the responses of resident immune cells. For example, cells of the monocyte/macrophage lineage are stimulated by microbial endotoxin and tripeptides (f-Met-Leu-Phe) to release potent mediators including TNF-alpha , nitric oxide, oxygen radicals, and so forth, which can contribute to enhanced epithelial leakiness (28). This is true for naive or blood-derived cells but not for gut macrophages, except in IBD where these cells are reactive to commensal bacteria (7). However, macrophages also synthesize IL-10 that may downregulate epithelial permeability by reducing production of T cell proinflammatory cytokines or possibly by acting directly on the epithelium. TGF-beta , the cytokine important in oral tolerance, acts directly on the epithelium to enhance its barrier function (16). Growth factors, such as EGF, insulin-like growth factor, and so forth, also have beneficial effects, and glucagon-like peptide-2, a recently identified intestinotrophic factor, appears to act specifically on intestinal epithelium to enhance its barrier properties (1).

Different probe molecules have been used to identify changes in epithelial barrier function. However, the size, shape, charge, and other properties of the probe will alter the data obtained. For example, decreased transepithelial resistance does not necessarily translate into increased permeability for all small probes, which theoretically cross the epithelium via the paracellular route, let alone for macromolecules. Inert probes such as mannitol and 51Cr-labeled EDTA have provided useful information on increases in permeability to small-sized molecules, and sugars that can be digested before uptake can indicate the location of the barrier defect (17). Results obtained in vivo may be different from those obtained in vitro due to the effects of blood flow. However, in vivo, increased permeability is important only if it results in pathophysiology as a result of penetration of a bioactive substance into the mucosa. Such substances include bacterial products (e.g., endotoxins, enterotoxins, superantigens) and other luminal macromolecular antigens capable of activating immune cells. Products of activated immune cells act on the epithelium and can amplify uptake of these molecules as well as bystander antigens. Therefore, epithelial permeability will continue to increase until protective factors restore the barrier by acting both on the epithelium itself and on immune cells to restore homeostasis. This scenario is likely to occur in response to acute irritation of the epithelium or other short-term disruptive events, including acute stress.


    MUCOSAL RESPONSE TO STRESS
TOP
ABSTRACT
INTRODUCTION
THE EPITHELIAL BARRIER
ANTIGEN HANDLING AND ORAL...
ENTEROPATHOGENS STIMULATE...
MUCOSAL RESPONSE TO STRESS
INTESTINAL HYPERSENSITIVITY
CONCLUSIONS
REFERENCES

Exposure of humans or rodents to acute stress can alter epithelial functions, stimulating secretion of ions, water, and mucus and enhancing epithelial permeability. Rats that have a cholinergic hypersensitivity appear to be particularly sensitive to the effects of stress (24). Recently reported studies in stressed mice demonstrated that acute stress alters brain expression of enzymes involved in the metabolism of ACh (10). However, the mucosal abnormalities in the gut after 2 h of restraint stress are relatively short lived, returning to normal within 24 h (P. R. Saunders and M. H. Perdue, unpublished observations). The barrier defect involves not only increased permeability to small inert probes such as mannitol and 51Cr-EDTA but also bioactive molecules, including a model protein antigen, horseradish peroxidase (HRP). HRP was demonstrated by electron microscopy to cross jejunal epithelium by both transcellular and paracellular routes (12). Atropine pretreatment restored epithelial barrier function in stressed rats. Mediators of the stress response include central and peripheral release of corticotropin-releasing hormone as well as cholinergic and adrenergic signals and mast cells (Refs. 6, 12, 23, and J. Santos and M. H. Perdue, unpublished observations). However, the long-term consequence of chronic stress-induced uptake of antigens on gastrointestinal inflammation and pathophysiology in animal models has not been reported. In humans, the intestinal effects of acute stress have also been shown to be mast cell mediated, and, in fact, the responses to stress were indistinguishable from those that occurred in hypersensitivity responses to food antigens in allergic individuals (23).


    INTESTINAL HYPERSENSITIVITY
TOP
ABSTRACT
INTRODUCTION
THE EPITHELIAL BARRIER
ANTIGEN HANDLING AND ORAL...
ENTEROPATHOGENS STIMULATE...
MUCOSAL RESPONSE TO STRESS
INTESTINAL HYPERSENSITIVITY
CONCLUSIONS
REFERENCES

Intestinal hypersensitivity reactions occur following ingestion of food antigens or infection with certain enteric parasites. Although such reactions are beneficial in eliminating parasites, reactions to normally innocuous food proteins serve no useful purpose to the host and appear to be completely inappropriate. Reactions to food antigens occur within minutes of ingestion of the offending food and include edema and hyperemia of the mucosa followed by ion secretion and increased motility. Immediate hypersensitivity reactions are initiated by antigen cross-linking of IgE antibodies bound to the surface of mucosal mast cells, releasing mediators that act on epithelial and smooth muscle receptors to induce functional changes. In rodent models, the intestinal ion secretory response to an antigen begins ~3 min after luminal antigen challenge. The rapidity of the response suggests that sensitization alters the nature of transepithelial antigen transport. This was verified in rats sensitized to HRP, in which, 2 min after luminal challenge, enterocytes contained a significantly greater amount of HRP compared with intestine from naive control rats or those sensitized to an irrelevant protein (2). In addition, the rate of transport of HRP across the cell was enhanced ~10-fold. Studies in mast cell-deficient Ws/Ws rats and normal littermate controls (3) showed that sensitization, but not the presence of mast cells, was required for the enhanced transepithelial transport of specific antigens. Subsequently, antigen-induced mast cell activation initiated an increase in tissue conductance and enhanced mucosal-to-serosal flux of HRP and the presence of HRP in the paracellular spaces and tight junctions. Taken together, these findings suggest that mast cell mediators (possibly interacting with nerves to release neurotransmitters) act on epithelial receptors to loosen tight junctions and facilitate passage of macromolecules between epithelial cells. The initial and mast cell-dependent phases of transepithelial antigen transport in sensitized rats are depicted in Fig. 1.


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Fig. 1.   Epithelial handling of antigen in intestinal hypersensitivity. The paracellular pathway is normally impermeable to intact protein antigens, which are taken up in small quantities by endocytosis and cross the epithelial barrier via the transcellular route. In sensitized rats, changes occur such that the initial phase (phase I) of transepithelial transport involves enhanced endosomal uptake and transport of specific antigen. Subsequent to antigen-induced activation of mast cells (phase II), the paracellular pathway becomes leaky to macromolecules due to the action of released molecular signals from immune (mast) cells and possibly nerves.

The studies described above suggest that the initial phase of antigen transport involves specific recognition at the level of epithelial cells, possibly mediated by immunoglobulin. Interactions between intestinal epithelium and immunoglobulins are recognized to play an important role in mucosal immunity. Transport of IgA across epithelial cells into the intestinal lumen is protective against pathogens, whereas epithelial uptake of IgG from the lumen is required for transfer of maternal immunoglobulins in colostrum to neonatal rodents. Both of these transport systems involve epithelial immunoglobulin receptors (pIgR and FcRn for IgA and IgG, respectively). FcRn has also been identified in human epithelial cells, but its role is not clear. In addition, IgE is present in intestinal secretions during food allergy reactions or following parasitic infection (18), suggesting the possibility of an epithelial receptor for luminally directed transport of IgE. However, to date, no evidence has been published describing an immunoglobulin-mediated system for importing noxious antigens into the body. Future publications will undoubtedly contain more information on the mechanisms that account for the enhanced antigen transport and role of cytokines in intestinal hypersensitivity reactions. Findings from this research may provide novel strategies for interfering with uptake of antigens into the body.


    CONCLUSIONS
TOP
ABSTRACT
INTRODUCTION
THE EPITHELIAL BARRIER
ANTIGEN HANDLING AND ORAL...
ENTEROPATHOGENS STIMULATE...
MUCOSAL RESPONSE TO STRESS
INTESTINAL HYPERSENSITIVITY
CONCLUSIONS
REFERENCES

An increasing number of studies are demonstrating that the intestinal epithelium is the critical interface in mucosal antigen handling. Cytokines in the local environment can influence the route of transepithelial antigen transport, that is, determine if an antigen takes the transcellular or the paracellular pathway. Although the transcellular route may be beneficial in most instances, resulting in oral tolerance, this may not necessarily be the case if the epithelial phenotype is altered, as in hypersensitivity. In addition, the epithelium acts as a sentinel to summon inflammatory cells when it is invaded by pathogens. Cytokines and chemokines play a key role in this multidirectional signaling. However, other molecules derived from nerves, myofibroblasts, endothelial cells, and the epithelium itself can alter the outcome or add amplification or inhibitory effects.

In animal models of spontaneous intestinal inflammation, the common features include a disrupted epithelial barrier and the requirement for microbial flora for full expression of inflammation. The mechanism responsible for the barrier defect can vary from abnormalities in molecular components of the tight junction to dysregulation of its function. It is clear that the balance of proinflammatory vs. anti-inflammatory cytokines is critical, but overproduction of either group can result in a disease if the dominant cytokine induces enhanced uptake of immunogenic molecules into the mucosa. As stated earlier, both IFN-gamma (Crohn's disease) and IL-4 (ulcerative colitis) are such cytokines. On the basis of this hypothesis, one would predict that defects or imbalance in the production of any of the critical factors or elements involved in the regulation or maintenance of the epithelial barrier would result in gut inflammation. Further studies will be required to determine if this postulate is correct.


    FOOTNOTES

* Third in a series of invited articles on Mucosal Immunity and Inflammation.

Address for reprint requests and other correspondence: M. H. Perdue, HSC, 3N5C, McMaster Univ., 1200 Main St. West, Hamilton, ON, Canada L8N3Z5 (E-mail: perdue{at}mcmaster.ca).


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
THE EPITHELIAL BARRIER
ANTIGEN HANDLING AND ORAL...
ENTEROPATHOGENS STIMULATE...
MUCOSAL RESPONSE TO STRESS
INTESTINAL HYPERSENSITIVITY
CONCLUSIONS
REFERENCES

1.   Benjamin, M. A., D. M. McKay, and M. H. Perdue. Glucagon-like peptide-2 (GLP-2) reduces intestinal epithelial permeability of both the paracellular and transcellular pathway (Abstract). Gastroenterology 114: A1129, 1998.

2.   Berin, C. B., A. J. Kiliaan, J. A. Groot, J. A. M. Taminiau, and M. H. Perdue. Rapid transepithelial antigen transport: impact of sensitization and the hypersensitivity reaction. Gastroenterology 113: 856-864, 1997[Medline].

3.   Berin, M. C., A. J. Kiliaan, P. C. Yang, J. Groot, Y. Kitamura, and M. H. Perdue. The influence of mast cells on pathways of antigen transport in rat intestine. J. Immunol. 161: 2561-2566, 1998[Abstract/Free Full Text].

4.   Berin, M. C., P. C. Yang, L. Ciok, S. Waserman, and M. H. Perdue. Role for IL-4 in macromolecular transport across human intestinal epithelium. Am. J. Physiol. 276 (Cell Physiol. 45): C1046-C1052, 1999[Abstract/Free Full Text].

5.   Bijlsma, P. B., A. J. Kiliaan, G. Scholten, M. Heyman, J. A. Groot, and J. A. M. Taminiau. Carbachol, but not forskolin, increases mucosal-to-serosal transport of intact protein in rat ileum in vitro. Am. J. Physiol. 271 (Gastrointest. Liver Physiol. 34): G147-G155, 1996[Abstract/Free Full Text].

6.   Castagliuolo, I., J. T. LaMont, B. Qui, K. S. M. Fleming, S. T. Bhaskar, S. T. Nilulasson, C. Kornetsky, and C. Pothoulakis. Acute stress causes mucin release from rat colon: role of CRF and mast cells. Am. J. Physiol. 271 (Gastrointest. Liver Physiol. 34): G884-G892, 1996[Abstract/Free Full Text].

7.   Duchmann, R., I. Kaiser, E. Hermann, W. Mayet, K. Ewe, and K. H. M. Zumbuschenfelde. Tolerance exists towards resident intestinal flora but is broken in active inflammatory bowel disease (IBD). Clin. Exp. Immunol. 102: 448-448, 1995[Medline].

8.   Furuta, G. T., A. Schmidt-Choudhury, M. Y. Wang, Z. S. Wang, L. Lu, R. I. Furlano, and B. K. Wershil. Mast cell-dependent tumor necrosis factor-alpha production participates in allergic gastric inflammation in mice. Gastroenterology 113: 1560-1569, 1997[Medline].

9.   Hermiston, M. L., and J. I. Gordon. Inflammatory bowel disease and adenomas in mice expressing a dominant negative N-cadherin. Science 270: 1203-1207, 1995[Abstract].

10.   Kagnoff, M. F. Epithelial cells as sensors for microbial infection. J. Clin. Invest. 100: 6-10, 1997[Free Full Text].

11.   Kaufer, D., A. Friedman, E. Seidman, and H. Soreg. Acute stress facilitates long-lasting changes in cholinergic gene expression. Nature 393: 373-377, 1998[Medline].

12.   Kiliaan, A. J., P. R. Saunders, P. B. Bijlsma, C. M. Berin, J. A. M. Taminiau, A. Groot, and M. H. Perdue. Stress stimulates transepithelial macromolecular uptake in rat jejunum. Am. J. Physiol. 275 (Gastrointest. Liver Physiol. 38): G1037-G1044, 1998[Abstract/Free Full Text].

13.   Madara, J. L. Pathobiology of neutrophil interactions with intestinal epithelia. Aliment. Pharmacol. Ther. S3: 57-62, 1997.

14.   Maric, I., P. G. Holt, M. H. Perdue, and J. Bienenstock. Class II MHC antigen (Ia)-bearing dendritic cells in the epithelium of the rat intestine. J. Immunol. 156: 1408-1408, 1996[Abstract].

15.   Mayer, L. Current concepts in mucosal immunity. I. Antigen presenation in the intestine: new rules and regulations. Am. J. Physiol. 274 (Gastrointest. Liver Physiol. 37): G7-G9, 1998[Abstract/Free Full Text].

16.   McKay, D. M., and A. W. Baird. Cytokine regulation of epithelial permeability and ion transport. Gut 44: 283-289, 1999[Free Full Text].

17.   Meddings, J. B., and I. Gibbons. Discrimination of site-specific alterations in gastrointestinal permeability in the rat. Gastroenterology 114: 83-92, 1998[Medline].

18.   Negrao-Correa, D., L. S. Adams, and R. G. Bell. Intestinal transport and catabolism of IgE. A major blood-independent pathway of IgE dissemination during a Trichinella spiralis infection of rats. J. Immunol. 157: 4037-4037, 1996[Abstract].

19.   Neutra, M. R. Role of M cells in transepithelial transport of antigens and pathogens to the mucosal immune system. Am. J. Physiol. 274 (Gastrointest. Liver Physiol. 37): G785-G791, 1998[Abstract/Free Full Text].

20.   Perdue, M. H., and D. M. McKay. Epithelium. In: Allergy and Allergic Diseases: The New Mechanisms and Therapeutics. Totowa, NJ: Humana, 1998, p. 281-303.

21.   Philpott, D. J., C. A. Ackerley, A. J. Kiliaan, M. A. Karmali, M. H. Perdue, and P. M. Sherman. Translocation of verotoxin-1 across T84 monolayers: mechanism of bacterial toxin penetration of epithelium. Am. J. Physiol. 273 (Gastrointest. Liver Physiol. 36): G1349-G1358, 1997[Abstract/Free Full Text].

22.   Philpott, D. J., D. M. McKay, W. Mak, M. H. Perdue, and P. M. Sherman. Signal transduction pathways involved in enterohemorrhagic Escherichia coli-induced alterations in T84 epithelial permeability. Infect. Immun. 66: 1680-1687, 1998[Abstract/Free Full Text].

23.   Santos, J., E. Saperas, C. Nogueiras, M. Mourelle, M. Antolin, A. Cadahia, and J. R. Malagelada. Release of mast cell mediators into the jejunum by cold pain stress in humans. Gastroenterology 114: 640-648, 1998[Medline].

24.   Saunders, P. R., U. Kosecka, D. M. McKay, and M. H. Perdue. Acute stressors stimulate ion secretion and increase epithelial permeability in rat intestine. Am. J. Physiol. 267 (Gastrointest. Liver Physiol. 30): G794-G794, 1994[Abstract/Free Full Text].

25.   Strobel, S., and A. M. Mowat. Immune responses to dietary antigens: oral tolerance. Immunol. Today 19: 173-181, 1998[Medline].

26.   Viney, J. L., A. M. Mowat, J. M. O'Malley, E. Williamson, and N. Fanger. Expanding dendritic cells in vivo enhances the induction of oral tolerance. J. Immunol. 160: 653-658, 1998.

27.   Yuhan, R., A. Koutsouris, S. D. Savkovic, and G. Hecht. Enteropathogenic Escherichia coli-induced myosin light chain phosphorylation alters intestinal epithelial permeability. Gastroenterology 113: 1873-1882, 1997[Medline].

28.   Zareie, M., D. M. McKay, G. G. Kovarik, and M. H. Perdue. Monocyte/macrophages evoke epithelial dysfunction: indirect role of tumor necrosis factor-alpha . Am. J. Physiol. 275 (Cell Physiol. 44): C932-C939, 1998[Abstract/Free Full Text].


Am J Physiol Gastroint Liver Physiol 277(1):G1-G5
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