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Research Centre for Gastroenterology, Institute of Cell and Molecular Science, Barts and The London, Queen Mary's School of Medicine and Dentistry, University of London, London, UK
* To whom correspondence should be addressed. E-mail: i.r.sanderson{at}qmul.ac.uk.
| ABSTRACT |
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| Introduction |
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We believe that signaling across the epithelium is important not only in health but also in the treatment of disease. The primary therapy for children with Crohn's disease in the United Kingdom is treatment with enteral feeds. Randomized control studies showed in the 1980s (1,2) that an elemental diet was as effective in inducing remission as high-dose steroids with the added benefit that it did not suppress linear growth. More recent studies in children demonstrated a rapid fall in immune markers of disease activity, with IL-6 falling within 3 d of starting the diet (3). Changes in nutritional status, such as weight or skinfold thickness were not detectable until after 2 wk. It is established that it was not a change in nutritional body status that resulted in remission of inflammation but a rapid effect, most likely within the intestine itself. Although there are many mechanisms by which enteral feeds may have their activity, we think it possible that 1 of them is by radically altering the luminal environment to such an extent that it varies the signals from the intestinal epithelium to the mucosal immune system.
Genes expressed in the intestinal epithelial cell can be divided into 2 groups. There are the intrinsic proteins, which include those that are important for epithelial function, such as brush border proteins, solute transporters, and brush border enzymes. Proteins necessary for the function of any cell type such as histones, enzymes in the metabolic pathways, and cytoskeletal proteins are also part of this group. The proteins in the second group act as signals between the enterocyte and other cells of the intestine. These include surface molecules such as major histocompatibility complex (MHC) class II and secreted proteins including chemokines or insulin-like growth factor (IGF)-binding proteins.
This latter group of signaling proteins enables the epithelial cell to orchestrate events in the intestine. In our research group we have hypothesized that the expression of signaling molecules by the epithelial cell is regulated by changes in the intestinal lumen. By this means, the intestinal lumen acts through the epithelium to alter indirectly events in the intestine, particularly those of the mucosal immune system. It is also possible that the signaling molecules released have wider effects than on the local immune response.
This article first presents evidence that altering the expression of signaling genes in the enterocyte affects the mucosal immune system. Second, it describes how changes in the intestinal lumen (influenced by diet) alter the expression of these genes. The effects of the lumen on epithelial cell gene expression can, therefore, be considered as an afferent limb in this process; the effect of the epithelial cell on the mucosal immune system is the efferent limb.
The efferent limb
Evidence for the effect of epithelial cell gene expression on the mucosal immune system has come from the ability to selectively alter the expression of genes in the intestinal epithelial cell by transgenic techniques. We have used chemokine expression by the epithelium as a model to show that the epithelium can orchestrate the mucosal immune system. The chemokine IL-8, which in humans results in recruitment of neutrophils, was the first identified chemotatic cytokine. However, IL-8 is not expressed in the mouse. To examine the effects of chemokines on the mucosal immune system, a system was developed whereby the chemokine macrophage inflammatory protein-2 (MIP-2), whose effects are very similar to those of IL-8 in humans, was linked to a FABPI (fatty acid binding protein of the intestine) promoter (4). The promoter is active only in the epithelial cells of the small intestine and proximal colon. A construct was developed in which the FABPI promoter and MIP-2 cDNA were linked to an intron and a polyadenylation site. This construct was injected into mouse oocytes. The epithelium from the first generation of the founder was shown to express MIP-2 mRNA. Analysis showed effects both on neutrophil and on lymphocyte recruitment. The transgenic mice had increased myeloperoxidase activity in the lamina propria of the small intestine and proximal colon (Fig. 1), where the FABPI promoter is active. These findings are indicative of neutrophil infiltration, which was confirmed by histology. In contrast, there was no increase in myeloperoxidase activity and neutrophil recruitment in the distal colon, liver, and spleen, where the FABPI promoter is inactive.
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Further analysis of the immune system in the MIP-2 transgenic mice demonstrated that the small intestine had increased lymphocyte infiltration in addition to neutrophils. Lymphocyte numbers in the lamina propria were significantly increased, and there was also a doubling of the numbers of intraepithelial lymphocytes. The increase of intraepithelial lymphocytes was caused by an increase in
ß-lymphocytes and in 
-lymphocytes. Further examination of the receptors on the surface of the intraepithelial lymphocytes showed that they expressed the CXCR2, which is the receptor responsible for MIP-2 activity. Therefore, these experiments show that altering the expression of only 1 chemokine in the epithelium has marked effects on both lymphocyte and neutrophil function. However, changes in the intestinal lumen may affect many chemokines as well as other cytokines that alter immune function. It is likely, therefore, that the changes in gene expression in the epithelium have far-reaching effects on the rest of the mucosal immune system.
The afferent limb
In the preceding section we gave evidence that the epithelium can orchestrate the events of the mucosal immune system. It is the purpose of this section to show that alterations in the intestinal lumen can affect the expression of these genes. In our laboratory we examined 3 sets of genes in intestinal epithelium (6) and studied how luminal factors can alter their expression. These included the MHC Class II complex, the IGF-binding protein complex, and the chemokines IL-8 and monocyte chemotactic protein-1 (MCP-1).
The MHC complex is the structure by which antigen is presented to the T-cell receptor on T lymphocytes. In the epithelial cell of the mouse, it is not expressed until after weaning. Our earlier experiments showed that the timing of its expression was altered by the time of weaning. Furthermore, weaning the mice onto an elemental diet prevented the expression of MHC class II and its associated genes (7), including the invariant chain (Ii chain) (except for some very limited expression at very late time points). The class II transactivator (CIITA) is a nuclear protein whose expression is both necessary and sufficient for MHC class II and Ii chain expression (8). Later experiments demonstrated that weaning up-regulated CIITA expression in mouse intestinal epithelium (9). This was the first demonstration that changes in diet could regulate the nuclear proteins that control gene expression in the intestinal epithelium. Interestingly, CIITA is present in 3 isotypes in mouse intestinal epithelium, and experiments showed that weaning induced only 1 of them, CIITA IV (Fig. 2). CIITA III increased slowly independent of diet, and its expression correlated with the late weak expression in elemental diet fed mice, mentioned earlier.
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Butyrate levels therefore reflect events in the intestinal lumen, and we hypothesized that their concentrations may alter epithelial cell signaling. We therefore examined the effects of butyrate on the expression of IL-8 and MCP-1 (11,12). Increasing the concentration of sodium butyrate increased IL-8 secretion while it simultaneously decreased MCP-1 expression. These effects were seen in resting epithelial cell lines but were much more marked in cells that had been stimulated with a proinflammatory agent such as LPS or IL-1ß (Fig. 3). We also examined the effect of butyrate on IGF binding protein expression (13,14). Unlike the chemokines, IGF binding proteins (IGFBPs) do not require proinflammatory stimulation for expression, but they are constitutively secreted. Their function is to modulate the actions of IGFs, in addition to an inherent mild antiproliferative action on cells. Butyrate reversibly up-regulates IGFBP-2 and down-regulates IGFBP-3 (Fig. 4). Distinct IGF binding proteins have different affinities and differentially alter the bioavailability of IGF-I and IGF-II (14).
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Myofibroblast interaction
The signaling mechanisms described in the article thus far have assumed that proteins expressed by the enterocyte interact unchanged with the cells of the immune system of the intestine. However, this is not the case. In vitro experiments coincubating myofibroblasts with Caco-2 cells greatly enhanced the chemotactic properties of the cultured supernatant. Analysis of this effect was based on the expression of matrix metalloproteinases (MMPs) by the myofibroblasts that acted on an inactive chemokine precurser. Only after this molecule had been cleaved into the active chemokine NAP-2 (20) was full activity observed (Fig. 8).
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It is likely that inulin and oligofructose significantly alter the luminal environment. Our experiments examine the hypothesis that changes in the intestinal lumen can alter the expression of molecules in the enterocyte that direct the mucosal immune system. The intestinal epithelium acts as a relay for transducing the information of the intestinal environment to the mucosal immune system. Such a relay is also seen in surface receptors that signal the presence of microbial-associated molecular patterns through the epithelium (22). This mechanism has advantages over other forms of immune surveillance in the gut that require the breach of the mucosal barrier. Such breaches can be manipulated by invading organisms to enter the body. The classic example of this is the polio virus, which enters the intestine through the M cell to the immune system of Peyer's patch, which is designed as a sampling system of the mucosal environment.
| FOOTNOTES |
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2 Supported by the following research grants: NIH (AI43472, DK47753, DK43351, DK40561, DK 33506) and the Medical Research Council (48475). ![]()
3 Author disclosure: I. R. Sanderson, support for travel to conference provided by ORAFTI, and an honorarium of
250 was paid to his university research account. ![]()
4 Abbreviations used: CIITA, class II transactivator; FABPI, fatty acid binding protein of the intestine; GALT, gut-associated lymphoid tissue; IGF, insulin-like growth factor; IGFBP, Insulin-like growth factor binding protein; MHC, major histocompatibility complex; MIP-2, macrophage inflammatory protein-2; MMP, matrix metalloproteinase; PBP, platelet basic protein; TSA, trichostatin A. ![]()
| LITERATURE CITED |
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