Co-localization of class II antigen and exogenous antigen in the rat enterocyte

Non-zero-sum microbiome immune system interactions

Fundamental asymmetries between the host and its microbiome in enzymatic activities and nutrient storage capabilities have promoted mutualistic adaptations on both sides. As a result, the enteric immune system has evolved so as not to cause a zero‐sum sterilization of non‐self, but rather achieve a non‐zero‐sum self‐reinforcing cooperation with its evolutionary partner the microbiome. In this review, we attempt to integrate the accumulated knowledge of immune—microbiome interactions into an evolutionary framework and trace the pattern of positive immune—microbiome feedback loops across epithelial, enteric nervous system, innate, and adaptive immune circuits. Indeed, the immune system requires commensal signals for its development and function, and reciprocally protects the microbiome from nutrient shortage and pathogen outgrowth. In turn, a healthy microbiome is the result of immune system curatorship as well as microbial ecology. The paradigms of host–microbiome asymmetry and the cooperative nature of their interactions identified in the gut are applicable across all tissues influenced by microbial activities. Incorporation of immune system influences into models of microbiome ecology will be a step forward toward defining what constitutes a healthy human microbiome and guide discoveries of novel host–microbiome mutualistic adaptations that may be harnessed for the promotion of human health.

Intracellular Localization of Microbial Transglutaminase and Its Influence on the Transport of Gliadin in Enterocytes

OBJECTIVE

Celiac disease (CD) is a systemic inflammatory disorder, characterized by the destruction of duodenal epithelium. The CD8 T cells involved are associated with cross-presentation. In addition to other factors, the rising prevalence of CD might be induced by microbial transglutaminase (mTG) an enzyme frequently used in food production that shares enzymatic and antigenic properties of tissue transglutaminase (TG2), the autoantigen in CD. We hypothesized that mTG and gliadin are transported into the endoplasmic reticulum (ER), indicating cross-presentation of both antigens.

METHODS

Apical incubation of duodenal biopsies from CD and control patients was performed with mTG alone or with mTG and simultaneously with Frazer's fraction. Evaluation was carried out by immunofluorescence and electron microscopy.

RESULTS

Approximately 6% to 9% of the intracellular mTG and gliadin were transported to the ER of enterocytes. RACE cells (Rapid uptake of Antigen into the Cytosol of Enterocytes) displayed an enhanced antigen uptake into a dilated ER. mTG strongly localized at the basolateral membrane and the lamina propria.

CONCLUSIONS

mTG and gliadin are transported to the ER of enterocytes and to a greater extent to the ER of RACE cells, suggesting cross-presentation of exogenous antigens. The strong localization of mTG at the basolateral membrane and the lamina propria may also indicate a potential antigenic interaction with cells of the immune system. Since mTG may not only been taken up with food stuffs but could also be released by bacteria within the intestinal microbiota, further investigations are needed regarding the role of mTG in CD pathogenesis.

Multivesicular bodies in intestinal epithelial cells: responsible for MHC class II-restricted antigen processing and origin of exosomes

In normal conditions intestinal epithelial cells (IECs) constitutively stimulate regulatory CD4(+) T cells. However, in Crohn's disease (CD), this major histocompatibility complex (MHC) class II-restricted antigen presentation results in stimulation of proinflammatory CD4(+) T cells. We hypothesized that these alternative functions might be mediated by differential sorting and processing of antigens into distinct MHC II-enriched compartments (MIICs). Accordingly, we analysed the endocytic pathways of lumenally applied ovalbumin (OVA) in IECs of the jejunum and ileum of wild-type (WT) and TNFDeltaARE/WT mice that develop a CD-resembling ileitis. Using quantitative reverse transcription polymerase chain reaction, we found that messenger RNA levels of interferon-gamma, tumour necrosis factor-alpha, interleukin-17 and interleukin-10 were significantly up-regulated in the inflamed ileum of TNFDeltaARE/WT mice, confirming CD-like inflammation. Fluorescence and immunoelectron microscopy revealed the presence of MHC II and invariant chain throughout the late endocytic compartments, with most molecules concentrated in the multivesicular bodies (MVB). OVA was targeted into MVB and, in contrast to other MIICs, accumulated in these structures within 120 min of exposure. The IEC-specific A33 antigen localized to internal vesicles of MVB and A33/class II-bearing exosomes were identified in intercellular spaces. Remarkably, the expression pattern of MHC II/invariant chain molecules and the trafficking of OVA were independent of mucosal inflammation and the specific region in the small intestine. MVB seem to be principally responsible for class II-associated antigen processing in IECs and to constitute the origin of MHC II-loaded exosomes. The distinctive functions of IECs in antigen presentation to CD4(+) T cells might arise as a result of differential processing within the MVB identified here.

Epithelia: lymphocyte interactions in the gut

The mucosal immune system is governed by a unique set of rules and regulations. The local microenvironment dictates the necessity for these differences. The intestinal epithelial cell (IEC) sits at the interface between an antigen-rich lumen and a lymphocyte-rich lamina propria (LP). The cross talk that occurs between these compartments serves to maintain intestinal homeostasis. IECs have the capacity to talk to LP lymphocytes, activating populations of unique regulatory T cells. These cells have the capacity to talk back to the epithelium, influencing epithelial cell growth and differentiation. This review looks at this cross talk and places it in the context of mucosal immunoregulation.

Candida-induced oral epithelial cell responses

OBJECTIVE

Oropharyngeal candidiasis (OPC), caused by Candida albicans, is the most common oral infection in HIV(+) persons. Oral epithelial cells are considered important for innate host defense against OPC with production of cytokines in response to C. albicans and the ability to inhibit Candida growth in vitro. The purpose of this study was to determine if Candida similarly induces cytokines by oral epithelial cells from HIV(+) persons, including those with OPC, as well as to determine if cytokines can influence the oral epithelial cell anti-Candida activity.

METHODS

Supernatants from oral epithelial cells from HIV(+) persons with and without OPC cultured with Candida were evaluated for cytokines by ELISA, or cytokines were added to the standard growth inhibition assay using epithelial cells from HIV(-) persons.

RESULTS

Results showed low Candida-induced epithelial cell cytokine production from HIV(+) persons, but with some elevated proinflammatory cytokines (TNF-alpha, IL-6) in those with OPC compared to those without OPC. The addition of specific proinflammatory or Th cytokines had no effect on oral epithelial cell anti-Candida activity in healthy HIV(-) persons.

CONCLUSION

These results suggest that oral epithelial cells from HIV(+) persons can contribute at some level to the oral cytokine milieu in response to Candida during OPC, but that cytokines do not appear to influence oral epithelial cell anti-Candida activity.

Importance of gastrointestinal ingestion and macromolecular antigens in the vein for oral tolerance induction

Oral administration of a certain dose of antigen can generally induce immunological tolerance against the same antigen. In this study, we showed the temporal appearance of ovalbumin (OVA) antigens in both portal and peripheral blood of mice after the oral administration of OVA. Furthermore, we detected 45,000 MW OVA in mouse serum 30 min after the oral administration of OVA. Based on this observation, we examined whether the injection of intact OVA into the portal or peripheral vein induces immunological tolerance against OVA. We found that the intravenous injection of intact OVA did not induce immunological tolerance but rather enhanced OVA-specific antibody production in some subclasses, suggesting that OVA antigens via the gastrointestinal tract but not intact OVA may contribute to establish immunological tolerance against OVA. Therefore, we examined the effects of digesting intact OVA in the gastrointestinal tract on the induction of oral tolerance. When mice were orally administered or injected into various gastrointestinal organs, such as the stomach, duodenum, ileum, or colon and boosted with intact OVA, OVA-specific antibody production and delayed-type hypersensitivity (DTH) response were significantly enhanced in mice injected into the ileum or colon, compared with orally administered mice. These results suggest that although macromolecular OVA antigens are detected after oral administration of OVA in tolerant-mouse serum, injection of intact OVA cannot contribute to tolerance induction. Therefore, some modification of macromolecular OVA in the gastrointestinal tract and ingestion may be essential for oral tolerance induction.

Phenotype and distribution of dendritic cells in the porcine small intestinal and tracheal mucosa and their spatial relationship to epithelial cells

Dendritic cells (DC) as key mediators of tolerance and immunity perform crucial immunosurveillance functions at epithelial surfaces. In order to induce an immune response, the DC have to gain access to antigens present at the luminal surface of mucosal epithelia. The mechanisms of this process are still largely unclear. We have therefore analysed the distribution of DC in the porcine intestinal and respiratory mucosa and their spatial relationship to epithelial cells by immunohistology. Immunofluorescence analysis of cryosections taken from jejunal Peyer's patches and double-stained for DC and M cells (specialised for antigen uptake) have revealed that 35.2+/-3.9% of M cells are located directly adjacent to DC in the subepithelial domes, representing possible antigen transfer sites. In normal jejunal villi, a rare population of lamina propria DC extending cytoplasmic processes between enterocytes has been identified as a possible correlate for direct luminal antigen uptake. Like small intestinal DC, DC in the porcine trachea mostly co-express CD16 with MHC-II. Tracheal DC have been found at high densities both above and below the basement membrane (BM) of the tracheal epithelium, with 32.4 DC/mm BM and 23.0 DC/mm BM, respectively. The intraepithealial DC population forms a dense network, with many of the cytoplasmic processes being directed towards the tracheal lumen. Our morphological analyses indicate that DC at mucosal epithelial sites are ideally positioned for the uptake of luminal antigens.

Human small intestinal epithelial cells constitutively express the key elements for antigen processing and the production of exosomes

In humans, the small intestinal epithelial cells (IEC) have a high constitutive expression of MHC class II (MHC II), and contains lysosomes. The IEC also contains MHC II rich multivesicular compartments and has been shown to produce exosomes. This suggests a role for the IEC in antigen processing and presentation either directly or indirectly by the production of exosomes. However, the presence and localisation in the IEC of other key molecules involved in this process has not been studied previously. In the present work, we have investigated small intestinal biopsies from healthy adults and the HT29 IEC cell line with monoclonal antibodies against molecules involved in the antigen processing/presenting systems and molecules typically found on exosomes derived from professional APCs and IECs. Immunohistology was performed to study the expression and localisation of MHC II (HLA-DR), HLA-DM, MHC I (HLA-ABC), CD1d, Invariant chain, Lamp-1, CD68, CD63, B7.1, B7.2, ICAM-1, Cathepsin D/S/L and the IEC specific marker A33 in the IECs. We found that the IECs from the biopsies constitutively express MHC II, HLA-DM, MHC I, Invariant chain, Lamp-1, CD 68, CD63 and A33, and these markers were also found in the IFN-g treated HT-29 cells. All these molecules were found apically in the IECs of the biopsies, localised mainly in vesicular structures. Interestingly, in the baso-latereral area of the IEC, only MHC II, MHC I, Lamp 1, CD68, CD63 and A33 were found and also here with vesicular staining pattern which matches the molecules previously found on exosomes derived professional APCs and human IEC lines. CD1d, B7, ICAM-1, CD9 and cathepsin D and L were absent in the IEC compartment, but cathepsin S showed a relatively weak staining in the apical part of the IEC. The staining pattern and the morphological localisation of these markers suggest a prominent antigen processing/loading and trafficking compartment, and a possible baso-lateral release of exosomes in the normal human IEC.

Induction of mucosal tolerance in Peyer's patch-deficient, ligated small bowel loops

To explore the requirement for M cells and the Peyer's patch (PP) in induction of oral tolerance and address the potential in vivo role of intestinal epithelial cells as nonprofessional APCs, we have attempted to induce tolerance in mice with ligated small bowel loops without M cells and Peyer's patches. A 2-centimeter section of vascularized small bowel was spliced away from the gut without disruption of the mesenteric attachments. We introduced OVA directly into the lumen of the loop prior to footpad immunization. By excising segments of bowel that contain PPs in some mice and segments without patches in others, we could study the necessity of the M cell and the underlying patch versus epithelial cells in induction of mucosal tolerance. We show that OVA-specific T cell proliferation and serum antibody responses are reduced in mice that have previously been given OVA both in PP-containing loops and in loops without patches. Furthermore, both high- and low-dose tolerance could be induced in the absence of PPs. Low-dose tolerance is associated with bystander suppression and requires IL-10, which indicates active suppression and the induction of regulatory cells. These data suggest that there is a critical role for components of the mucosal immune system other than PPs in antigen sampling and induction of oral tolerance.

Cytokine and chemokine production by human oral and vaginal epithelial cells in response to Candida albicans

Oropharyngeal and vaginal candidiases are the most common forms of mucosal fungal infections and are primarily caused by Candida albicans, a dimorphic fungal commensal organism of the gastrointestinal and lower female reproductive tracts. Clinical and experimental observations suggest that local immunity is important in host defense against candidiasis. Accordingly, cytokines and chemokines are present at the oral and vaginal mucosa during C. albicans infections. Since mucosal epithelial cells produce a variety of cytokines and chemokines in response to microorganisms and since C. albicans is closely associated with mucosal epithelial cells as a commensal, we sought to identify cytokines and/or chemokines produced by primary oral and vaginal epithelial cells and cell lines in response to C. albicans. The results showed that proinflammatory cytokines were produced by oral and/or vaginal epithelial cells at various levels constitutively with considerable interleukin-1alpha (IL-1alpha) and tumor necrosis factor alpha, but not IL-6, produced in response to C. albicans. In contrast, Th1-type (IL-12 and gamma interferon) and Th2-type-immunoregulatory (IL-10 and transforming growth factor beta) cytokines and the chemokines monocyte chemoattractant protein 1 and IL-8 were produced in low to undetectable concentrations with little additional production in response to C. albicans. Taken together, these results indicate that cytokines and chemokines are variably produced by oral and vaginal epithelial cells constitutively, as well as in response to C. albicans, and are predominated by proinflammatory cytokines.

Potential role for a carbohydrate moiety in anti-Candida activity of human oral epithelial cells

Candida albicans is both a commensal and a pathogen at the oral mucosa. Although an intricate network of host defense mechanisms are expected for protection against oropharyngeal candidiasis, anti-Candida host defense mechanisms at the oral mucosa are poorly understood. Our laboratory recently showed that primary epithelial cells from human oral mucosa, as well as an oral epithelial cell line, inhibit the growth of blastoconidia and/or hyphal phases of several Candida species in vitro with a requirement for cell contact and with no demonstrable role for soluble factors. In the present study, we show that oral epithelial cell-mediated anti-Candida activity is resistant to gamma-irradiation and is not mediated by phagocytosis, nitric oxide, hydrogen peroxide, and superoxide oxidative inhibitory pathways or by nonoxidative components such as soluble defensin and calprotectin peptides. In contrast, epithelial cell-mediated anti-Candida activity was sensitive to heat, paraformaldehyde fixation, and detergents, but these treatments were accompanied by a significant loss in epithelial cell viability. Treatments that removed existing membrane protein or lipid moieties in the presence or absence of protein synthesis inhibitors had no effect on epithelial cell inhibitory activity. In contrast, the epithelial cell-mediated anti-Candida activity was abrogated after treatment of the epithelial cells with periodic acid, suggesting a role for carbohydrates. Adherence of C. albicans to oral epithelial cells was unaffected, indicating that the carbohydrate moiety is exclusively associated with the growth inhibition activity. Subsequent studies that evaluated specific membrane carbohydrate moieties, however, showed no role for sulfated polysaccharides, sialic acid residues, or glucose- and mannose-containing carbohydrates. These results suggest that oral epithelial cell-mediated anti-Candida activity occurs exclusively with viable epithelial cells through contact with C. albicans by an as-yet-undefined carbohydrate moiety.

Intestinal epithelial cells secrete exosome-like vesicles

BACKGROUND & AIMS

Given the observations that intestinal epithelial cells (IECs) can present antigens to CD4(+) T lymphocytes and that professional antigen-presenting cells secrete exosomes (antigen-presenting vesicles), we hypothesized that IECs may secrete exosomes carrying molecules implicated in antigen presentation, which may be able to cross the basement membrane and convey immune information to noncontiguous immune cells.

METHODS

Human IEC lines HT29-19A and T84-DRB1*0401/CIITA were grown on microporous filters. Release of exosomes under basal or inflammatory conditions was evaluated in conditioned apical and basolateral media after differential ultracentrifugations. Morphologic and biochemical characterization of exosomes was performed using immunoelectron microscopy, Western blotting, and matrix-assisted laser desorption ionization-time of flight mass spectrometry.

RESULTS

The intestinal cell lines released 30-90-nm-diameter vesicles from the apical and basolateral sides, and this release was significantly increased in the presence of interferon gamma. MHC class I, MHC class II, CD63, CD26/dipeptidyl-peptidase IV, and A33 antigen were present in epithelial-derived exosomes. CONCLUSIONS; Human IEC lines secrete exosomes bearing accessory molecules that may be involved in antigen presentation. These data are consistent with a model in which IECs may influence antigen presentation in the mucosal or systemic immune system independent of direct cellular contact with effector cells.

Oral tolerance

The ability of the mucosal immune system to distinguish between harmful and harmless antigens is essential for mounting protective immune responses and preventing the induction of mucosal pathology yet the basis for this remains unclear. As fed antigen can also exert systemic effects understanding oral tolerance and priming will also have important consequences for therapy and vaccination. Here we will not only review the increasing amount of information about the potential mechanisms of oral tolerance and priming but also attempt to shed some light on how differences in the uptake and handling i.e. 'the journey' of orally administered antigen may promote these mechanisms.

MHC class II expression and antigen presentation by human endometrial cells

It has been demonstrated previously that mixed cell suspensions from the female reproductive tract consisting of human epithelial and stromal cells were capable of presenting foreign antigen to autologous T cells. There have been, however, no reported studies examining antigen presentation by isolated epithelial cells from the human female reproductive tract. It is now shown that freshly isolated epithelial cells from the uterine endometrium constitutively express MHC class II antigen and that class II was upregulated on cultured epithelium by interferon gamma (IFNgamma). Using a highly purified preparation, it was demonstrated that these epithelial cells were able to process and present tetanus toxoid recall antigen driving autologous T cell proliferation. Cells isolated from the basolateral sub-epithelium stroma were also potent antigen presenting cells in this model system. Thus, isolated endometrial epithelial cells were able to directly process and present antigen to T cells and may be responsible for the transcytosis and delivery of antigen to professional antigen presenting cells found in the sub-epithelial stroma.

Uptake and presentation of antigen to T cells by primary colonic epithelial cells in normal and diseased states

BACKGROUND & AIMS

The immunoregulatory properties of primary colonic epithelial cells (CECs) have not been defined. The ability of CECs from wild-type and interleukin 2-deficient (IL-2(-/-)) mice to take up a complex protein antigen and present peptides via MHC molecules to T cells was assessed and contrasted with that of primary small intestinal epithelial cells (SIECs).

METHODS

Uptake of fluorescein isothiocyanate (FITC)-labeled ovalbumin (FITC-OVA) by CECs and SIECs from wild-type and IL-2(-/-) mice was measured by flow cytometry. The effect of disrupting cytoskeleton organization and metabolic activity of CEC on antigen uptake was assessed. An OVA/I-A(b)-specific CD4(+) T-cell line transfected with an NFAT-lacZ reporter gene construct was used to evaluate the ability of CECs and SIECs as well as CECs from healthy and colitic IL-2(-/-) mice to present antigen to T cells.

RESULTS

Uptake of FITC-OVA by CECs is concentration dependent, is not saturated at physiologic concentrations, and requires metabolically active cells. CECs from IL-2(-/-) mice take up significantly more antigen than those from wild-type mice. CECs are more efficient APCs than SIECs, and antigen-pulsed CECs from IL-2(-/-) mice induce the highest levels of T-cell activation.

CONCLUSIONS

Primary CECs are efficient APCs for CD4 MHC class II-restricted T cells. Antigen uptake and presentation is up-regulated in animals prone to develop intestinal inflammation.

Antigen processing and presentation by intestinal epithelial cells - polarity and complexity

The mechanisms by which gut-associated lymphoid tissue (GALT) maintains a balance between oral tolerance and active immune response in the face of exposure to high antigen concentrations remains a central question in mucosal immunity. Here, Robert Hershberg and colleagues discuss the evidence that human intestinal epithelial cells function as antigen-presenting cells (APCs) capable of regulating T-cell responses in the intestinal mucosa

Mucosal immunity and gastrointestinal antigen processing

The intestine is the largest lymphoid organ in the body by virtue of lymphocyte numbers and quantity of immunoglobulin produced. This is largely related to the enormous antigen load to which these cells are exposed on a daily basis. However, despite this, the mucosa-associated lymphoid tissue appears to be regulated by unique mechanisms, and this is reflected in specific phenomena (oral tolerance, controlled or physiologic inflammation) as well as unusual lymphoid populations (intraepithelial lymphocytes) that respond to alternative pathways of activation. This, coupled with the existence of novel antigen-presenting cells (intestinal epithelial cells) sets the scene for distinct immune responses. It is these distinct regulatory factors that support immunosuppression or tolerance rather than active immunity at a site juxtaposed to the external environment. This review defines these novel interactions and suggests how alteration in normal function may result in allergic or inflammatory responses. A clearer understanding of mucosal immunoregulation may lead to new therapeutic approaches for these diseases.

The intestinal epithelial cell: processing and presentation of antigen to the mucosal immune system

The immunologic tone of the intestinal tract is one of suppressed or highly regulated responses. While there are several components (intrinsic and extrinsic to the gut-associated lymphoid tissue) responsible for this immunologically suppressed tone, the intestinal epithelial call (IEC) has been proposed as a key player in this process. IECs can take up and process antigen but distinct surface molecules and restriction elements allow them to present these antigens to unique regulatory T cells. These include the expression of the class Ib molecule CD1d as well as a novel CD8 ligand, gp180. These molecules come together to activate a subpopulation of CD8+ regulatory cells whose function is to suppress immune responses in an antigen non-specific fashion most likely through cognate interactions. This form of regulation may be unique to the gut-associated lymphoid tissue which is consistent with the unusual demands upon this part of the immune system.

Vectorial function of major histocompatibility complex class II in a human intestinal cell line

This study explores the expression and the function of major histocompatibility complex class II in the intestinal epithelial cell line CaCo-2, which has been widely used as a model for the human gastrointestinal epithelium. Human leucocyte antigen (HLA)-DR expression on CaCo-2 cells is induceable by interferon-gamma (IFN-gamma), but responsiveness to IFN-gamma is dependent on cell differentiation and IFN-gamma availability at the basolateral cell surface. HLA-DR expression is concentrated in apical cytoplasmic vesicles and on the basolateral cell surface. Invariant chain is expressed in apical vesicles but is absent from the cell surface. Immunoprecipitation studies show a slow rate of dissociation of HLA-DR from Ii. Double labelling shows some overlap between HLA-DR expression and basolateral endosomal markers but no overlap with apical endosomal markers. Functional studies show processing and presentation of lysozyme endocytosed from the basolateral, but not apical surfaces. CaCo-2 cells may provide a useful model with which to dissect the antigen-processing pathways in polarized epithelial cells. The regulated access of antigens taken up from the gut lumen to the processing compartments may prevent overloading the immune system with antigens derived from normal gut contents.

Entry sites for oral vaccines and drugs: A role for M cells, enterocytes and dendritic cells?

M cells have long been considered as the unique entry site of macromolecules and pathogens in the intestine, allowing delivery to antigen-presenting cells in the Peyer's patches. Therefore, antigen formulation for the development of oral vaccines has been based on administration of antigens in the form of live replicating pathogens or soluble antigen vectorized into biodegradable microspheres. However, progress in the understanding of the biology of dendritic cells, as well as identification of their localization at different sites of the intestine, suggest that they may capture antigen directly from the lumen of mucosal tissues or from epithelial cells of the intestine. Besides, a role for the absorptive epithelium in antigen presentation through both classical or non-classical MHC elements suggests that PP may not be the exclusive inductive site of the immune response in the gut. Thus, depending on the nature of the antigen (soluble or infectious) there may be different sites of antigen entry through the intestine, and each site may have distinct efficiency to promote a protective immune response, depending on the presence and function of dendritic cells. Cross talk between M cells, epithelial cells and dendritic cells may play an important role in determining the outcome of tolerance versus immunity.

The role of enterocytes in gut dysfunction

Stem cells in the intestinal epithelium give rise to enterocytes, goblet cells, enteroendocrine cells, and Paneth cells. Each of these cell lines plays a role in cytoprotection of the intestinal mucosa. In particular, it has been demonstrated that mature enterocytes can act as antigen presenting cells. Parenteral and enteral nutrition are used to nourish critically ill patients. However, these regimens are unfortunately associated with gut atrophy. Glutamine, the preferred intestinal nutrient, reverses this gut atrophy and plays a key role in maintaining the barrier function of the gut. Specific nutrients (putrescine, spermidine, spermine) have been used to modulate intestinal adaption. In addition, ornithine has been shown to act as a regulator of intestinal adaption. In this review, we discuss the relationship between the biology of enterocytes and failure of the gut barrier.

Current concepts in mucosal immunity. I. Antigen presentation in the intestine: new rules and regulations

The recognition that immune responses in the intestine differ from those seen systemically has led to a search for novel pathways involved in mucosal immunoregulation. One cell type that has surfaced as a prime candidate for such a regulatory role is the intestinal epithelial cell. A number of laboratories have documented that intestinal epithelial cells sample luminal antigens and process and present these to primed T cells. However, several unique features have emerged, making their potential role as antigen-presenting cells a critical part of mucosal homeostasis.

Review article: Local and systemic regulation of mucosal immunity

An increased interest in mucosal immunity has stemmed from the identification of novel T-cell populations and developments in oral vaccines and oral tolerance. The development of physiological inflammation is antigen driven. Upon recognition of antigen, the lamina propria (LP) is populated with lymphocytes and activated peripheral cells acquire the capacity to home to the gut. Antigen entry to the gut is via follicle-associated epithelium or M cells. The antigen now interacts with macrophages or CD4+ cells, and go on to the Peyer's patch where B cells undergo a transforming growth factor beta (TGF-beta)-mediated isotope switch to immunoglobulin (Ig) A. TGF-beta may also play a role in the development of oral tolerance. The intestinal epithelium is seen as the site for the activation of CD8+ suppressor cells. Controlled inflammation is therefore explained by the interaction of LP lymphocytes and intra-epithelial cells (IEC). IECs are capable of extending processes that express regulatory surface molecules coupled with antigen processed from luminal uptake. CD8+ T-cell activation is favoured over CD4+ T-cell activation due to the size of the antigenic binding peptide. The result is suppressed inflammation. Other antigen-presenting cells (APCs) and dendritic cells may also contribute to local immunosuppression.