The nonclassical class I molecule CD1d associates with the novel CD8 ligand gp180 on intestinal epithelial cells

The Mammary Gland in Mucosal and Regional Immunity

The mammary gland (MG) lacks a mucosa but is part of the mucosal immune system because of its role in passive mucosal immunity. The MG is not an inductive site for mucosal immunity. Rather, synthesis of immunoglobulin (Ig)A by plasma cells stimulated at distal inductive sites dominate in the milk of rodents, humans, and swine whereas IgG1 derived from serum predominates in ruminants. Despite the considerable biodiversity in the role of the MG, IgG passively transfers the maternal systemic immunological experience whereas IgA transfers the mucosal immunological experience. Although passive antibodies are protective, they and other lacteal constituents can be immunoregulatory. Immune protection of the MG largely depends on the innate immune system; the monocytes–macrophages group together with intraepithelial lymphocytes is dominant in the healthy gland. An increase in somatic cells (neutrophils) and various interleukins signal infection (mastitis) and a local immune response in the MG. The major role of the MG to mucosal immunity is the passive immunity supplied to the suckling neonate.

Characterizing CEACAM5 interaction with CD8α and CD1d in intestinal homeostasis

Normal intestinal epithelial cells (IECs) could act as non-professional antigen-presenting cells, selectively activating CD8(+)-suppressor T cells. An epithelial cell surface glycoprotein, gp180, recognized by monoclonal antibodies B9 and L12 was determined to be critical in this process. Purification and sequence analysis of mAb B9 reactive material revealed amino-acid sequence homology with CEACAM5. We demonstrate that CEACAM5 has properties attributed to gp180, such as CD8α binding and activation of CD8-associated Lck. CEACAM5 is the only CEACAM member interacting with CD1d through the B3 domain. Its N domain (recognized by B9) is required for CD8α binding. Removal of the N-domain glycosylated residues reduces B9 recognition, CD8α binding affinity, and activation of LcK. Therefore, conformational changes in CEACAM5 glycosylation site are critical for its interaction with CD8α. CEACAM5-activated CD8(+) T cells acquire the ability to suppress the proliferation of CD4(+) T cells in vitro in the presence of interleukin (IL)-15 or IL-7. We provide new insights into the role of CEACAM5 and define its specific immunoregulatory properties among the CEACAMs expressed on IECs. We suggest that unique set of interactions between CEACAM5, CD1d, and CD8 render CD1d more class I-like molecule, facilitating antigen presentation and activation of CD8(+)-suppressor regulatory T cells.

Microscopic colitis patients have increased proportions of Ki67(+) proliferating and CD45RO(+) active/memory CD8(+) and CD4(+)8(+) mucosal T cells

BACKGROUND

Collagenous colitis (CC) and lymphocytic colitis (LC) are chronic inflammatory bowel disorders of unknown etiology. This study investigated phenotypic characteristics of the mucosal lymphocytes in CC and LC.

METHODS

Lamina propria and intraepithelial lymphocytes (LPLs, IELs) isolated from mucosal biopsies from CC (n=7), LC (n=6), as well as LC or CC patients in histopathological remission, (LC-HR) (n=6) and CC-HR (n=4) and non-inflamed controls (n=10) were phenotypically characterized by four-color flow cytometry.

RESULTS

The proportions of CD8(+) IELs were increased in CC and LC (p<0.01) compared to controls. Increased proportions of CD45RO(+)CD8(+) IELs and LPLs were observed in LC and even more in CC patients (p<0.01). Both CC (p<0.05) and LC patients had elevated proportions of CD4(+)8(+) IELs and LPLs compared to controls. The proportions of CD45RO(+) cells were increased in CD4(+)8(+) IELs and LPLs (p<0.05) in CC and LC patients compared to controls. Both CC (p<0.05) and LC patients had higher proportions of Ki67(+)CD8(+) IELs and LPLs compared to controls. In contrast, decreased proportions of CD4(+) LPLs were observed in CC and LC as well as CD4(+) IELs in LC compared to controls. Increased proportions of Ki67(+)CD4(+) IELs and LPLs (p<0.05) were observed in CC and LC patients. CC-HR but not LC-HR patients demonstrated normalized proportions of both IELs and LPLs compared to CC and LC patients respectively.

CONCLUSION

LC and CC patients have differences in mucosal lymphocyte subsets, with increased proportions of Ki67(+) and CD45RO(+) CD8(+) and CD4(+)8(+) mucosal T cells.

Intestinal epithelial cells in inflammatory bowel diseases

The pathogenesis of inflammatory bowel diseases (IBDs) seems to involve a primary defect in one or more of the elements responsible for the maintenance of intestinal homeostasis and oral tolerance. The most important element is represented by the intestinal barrier, a complex system formed mostly by intestinal epithelial cells (IECs). IECs have an active role in producing mucus and regulating its composition; they provide a physical barrier capable of controlling antigen traffic through the intestinal mucosa. At the same time, they are able to play the role of non-professional antigen presenting cells, by processing and presenting antigens directly to the cells of the intestinal immune system. On the other hand, immune cells regulate epithelial growth and differentiation, producing a continuous bi-directional cross-talk within the barrier. Several alterations of the barrier function have been identified in IBD, starting from mucus features up to its components, from epithelial junctions up to the Toll-like receptors, and altered immune responses. It remains to be understood whether these defects are primary causes of epithelial damage or secondary effects. We review the possible role of the epithelial barrier and particularly describe the role of IECs in the pathogenesis of IBD.

Defect in CEACAM family member expression in Crohn's disease IECs is regulated by the transcription factor SOX9

BACKGROUND

CEACAM1, CEACAM5, and CEACAM6 represent 3 of the CEACAM (carcinoembryonic antigen-related cell adhesion molecule) subfamily members expressed on intestinal epithelial cells (IECs). Deficiency in their expression, as seen in inflammatory bowel disease (IBD), results in the lack of activation of CD8+ regulatory T cells in the mucosa. Since CEACAM expression was shown to be regulated by the transcription factor SOX9, we sought to determine whether the defect in CEACAM expression in IBD was related to aberrant SOX9 expression.

METHODS

IECs and lamina propria lymphocytes (LPLs) were freshly isolated from colonic tissues. T84 and HT29 16E cells were cocultured with LPLs. SOX9 and CEACAM subfamily member expression was assessed by real-time polymerase chain reaction (PCR), Western blot, immunohistochemistry, and immunofluorescence.

RESULTS

In Crohn's disease (CD) but not in ulcerative colitis (UC), a significant reduction in mRNA and protein expression for CEACAM1 and 5 was noted; in contrast, no difference in SOX9 mRNA expression was seen. However, nuclear SOX9 immunostaining was increased in CD IECs. Furthermore, SOX9 protein was reduced in the cytoplasm of LPL-stimulated T84 and HT29 16E cells, while CEACAM5 expression was increased.

CONCLUSIONS

The defect in CEACAM family members in CD IECs appears to be related to the aberrant nuclear localization of SOX9. Changes in SOX9 expression in the CD mucosa relate to the local microenvironment and altered IEC:LPL crosstalk.

Intestinal epithelial cell proteome from wild-type and TNFDeltaARE/WT mice: effect of iron on the development of chronic ileitis

Environmental factors substantially contribute to the development of chronic intestinal inflammation in the genetically susceptible host. Nutritional components like iron may act as pro-oxidative mediators affecting inflammatory processes and cell stress mechanisms. To better characterize effects of dietary iron on epithelial cell responses under the pathological conditions of chronic intestinal inflammation, we characterized the protein expression profile (proteome) in primary intestinal epithelial cells (IEC) from iron-adequate and low-iron fed wild-type (WT) and TNFDeltaARE/WT mice. We performed all possible comparisons between the 4 groups according to genotype or diet. Histological analysis of iron-adequate fed TNFDeltaARE/WT mice (approximately 0.54 mg of iron/day) revealed severe ileal inflammation with a histopathology score of 8.3+/-0.91 (score range from 0-12). Interestingly, low-iron fed mice (approximately 0.03 mg of iron/day) were almost completely protected from the development of inflammatory tissue destruction (histopathology score of 2.30+/-0.73). In total, we identified 74 target proteins with significantly altered steady state expression levels in primary IEC using 2D-gel electrophoresis (2D SDS-PAGE) and peptide mass fingerprinting via MALDI-TOF mass spectrometry (MS). Interestingly, the overlap between the comparison of iron-adequate fed WT and TNFDeltaARE/WT mice (inflamed conditions) and the comparison between the iron-adequate and iron-low fed TNFDeltaARE/WT mice (absence of inflammation) revealed 4 contrarily regulated proteins including aconitase 2, catalase, intelectin 1 and fumarylacetoacetate hydrolase (FAH). These proteins are associated with energy homeostasis, host defense, oxidative and endoplasmic reticulum (ER) stress responses. In conclusion, the iron-low diet affected the epithelial cell proteome and inhibited the development of chronic intestinal inflammation, suggesting a critical role for nutritional factors in the pathogenesis of IBD.

Bacteria- and host-derived mechanisms to control intestinal epithelial cell homeostasis: implications for chronic inflammation

The genetic predisposition to deregulated mucosal immune responses and the concurrent prevalence of certain environmental triggers in developed countries are strong etiologic factors for the development of inflammatory bowel diseases in human subjects, including Crohn's disease and ulcerative colitis. Numerous clinical and experimental studies have shown that the intestinal microbes are critical for the initiation and progression of chronic intestinal inflammation. Activation of pattern recognition receptor signaling via members of the Toll-like receptor (TLR) and the nucleotide-binding oligomerization domain (NOD)-like families initiates inflammatory defense mechanisms that are required to alert and protect the host. Key inflammatory mechanisms such as nuclear transcription factor kappaB (NF-kappaB) activation and endoplasmic reticulum stress responses are controlled by a complex network of pathways that includes intrinsic feedback effectors and is targeted by immunosuppressive cytokines such as interleukin 10 (IL-10) and transforming growth factor (TGF)-beta. In the absence or after functional loss of these antiinflammatory feedback signals, physiological defense mechanisms may turn into pathological responses. The data discussed in the present review suggest that disturbances in the homeostasis between bacteria- and host-derived signals at the epithelial cell level lead to a break in the intestinal barrier function and to the development of mucosal immune disorders in genetically susceptible hosts.

CD1 expression on antigen-presenting cells

CD1 proteins present self and microbial glycolipids to CD 1-restricted T cells, or in the case of CD1d, to NKT cells. The CD1 family in humans consists of group I proteins CDla, CDlb, CDlc, and CDle and the group II protein CDld. Rodents express only CDld, but as CD1d is broadly expressed and traffics to all endosomal compartments, this single CD1 family member is thereby able to acquire antigens in many subcellular compartments. A complete understanding of the CD 1 family requires an appreciation of which cells express CD1 and how CD1 contributes to the unique function of each cell type. While group I CD 1 expression is limited to thymocytes and professional APCs, CD1d has a wider tissue distribution and can be found on many nonhematopoietic cells. The expression and regulation of CD1 are presented here with particular emphasis on the function of CD1 in thymocytes, B cells, monocytes and macrophages, dendritic cells (DCs), and intestinal epithelial cells (IECs). Altered expression of CD 1 in cancer, autoimmunity, and infectious disease is well documented, and the implication of CD 1 expression in these diseases is discussed.

Intestinal epithelial cell signalling and chronic inflammation: From the proteome to specific molecular mechanisms

Advancing knowledge regarding the cellular mechanisms of intestinal inflammation has led to a better understanding of the disease pathology in patients with inflammatory bowel disease (IBD) including Crohn's disease and ulcerative colitis. It has become clear from numerous studies that enteric bacteria are a critical component in the development and prevention/treatment of chronic intestinal inflammation. An emerging new paradigm suggests that changes in the homeostasis of bacteria- and host-derived signal transduction at the intestinal epithelial cell (IEC) level may lead to a break in barrier function and the development of adaptive immune disturbances. The functional loss of anti-inflammatory host-derived signals in the gut including the immunosuppressive cytokines Interleukin 10 (IL-10) and transforming growth factor (TGF)-beta are of high relevance to the pathogenesis of IBD. The development of analytical tools including two-dimensional (2D) high-resolution protein separation techniques and peptide mass fingerprinting via high-sensitivity mass-spectrometers (MS) allows the quantitative assessment of protein expression changes in disease-relevant cell types. By using these advanced methods, the characterization of the epithelial cell proteome from murine models of experimental colitis and human IBD patients identified novel disease-related mechanisms with respect to the regulation of the glucose-regulated endoplasmic reticulum stress response protein 78 (grp-78). In conclusion, the identification and functional analysis of differentially expressed proteins in purified intestinal target cell types will help to add important insights to the understanding of the molecular pathogenesis of these immune-mediated chronic intestinal disorders.

Intestinal epithelial cells from inflammatory bowel disease patients preferentially stimulate CD4+ T cells to proliferate and secrete interferon-gamma

Previous studies have suggested that intestinal epithelial cells (IECs) have the capacity to function as nonprofessional antigen presenting cells that in the normal state preferentially activate CD8+ T cells. However, under pathological conditions, such as those found in inflammatory bowel disease (IBD), persistent activation of CD4+ T cells is seen. The aim of this study was to determine whether the IBD IECs contribute to CD4+ T cell activation. Freshly isolated human IECs were obtained from surgical specimens of patients with or without IBD and cocultured with autologous or allogeneic peripheral blood T lymphocytes. Cocultures of normal T cells and IECs derived from IBD patients resulted in the preferential activation of CD4+ T cell proliferation that was associated with significant IFN-gamma, but not IL-2, secretion. Cytokine secretion and CD4+ T cell proliferation was inhibited by pretreatment of the IBD IECs with the anti-DR MAb L243. In contrast, normal IECs stimulated the proliferation and cytokine secretion by CD4+ T cells to a significantly lesser degree than IBD IECs. Furthermore, blockade of human leukocyte antigen-DR had a lesser effect in the normal IEC-CD4+ T cell cocultures. We conclude that IECs can contribute to the ongoing CD4+ T cell activation seen in IBD. We suggest that the apparent differences between the secreted levels of IFN-gamma indicate that it may play a dual role in intestinal homeostasis, in which low levels contribute to physiological inflammation whereas higher levels are associated with an uncontrolled inflammatory state.

Expression of nonclassical class I molecules by intestinal epithelial cells

It is well recognized that the nature of the immune response is different in the intestinal tract than in peripheral lymphoid organs. The immunologic tone of the gut-associated lymphoid tissue is one of suppression rather than active immunity, distinguishing pathogens from normal flora. Failure to control mucosal immune responses may lead to inflammatory diseases such as Crohn's disease (CD) and ulcerative colitis (UC) and celiac disease. It has been suggested that this normally immunosuppressed state may relate to unique antigen-presenting cells and unique T-cell populations. The intestinal epithelial cell (IEC) has been proposed to act as a nonprofessional antigen-presenting cell (APC). Previous studies have suggested that antigens presented by IECs result in the activation a CD8(+) regulatory T-cell subset in a nonclassical MHC I molecule restricted manner. We therefore analyzed the expression of nonclassical MHC I molecules by normal IECs and compared this to those expressed by inflammatory bowel disease (IBD) IECs. Normal surface IEC from the colon and, to a much lesser extent, the small bowel express nonclassical MHC I molecules on their surface. In contrast, mRNA is expressed in all intestinal epithelial cells. Surface IEC express CD1d, MICA/B, and HLA-E protein. In contrast, crypt IECs express less or no nonclassical MHC I molecules but do express mRNA for these molecules. Furthermore, the regulation of expression of distinct nonclassical class I molecules is different depending on the molecule analyzed. Interestingly, IECs derived from patients with UC fail to express any nonclassical MHC I molecules (protein and HLA-E mRNA). IECs from CD patients express HLA-E and MICA/B comparable to that seen in normal controls but fail to express CD1d. Thus, in UC there may be a failure to activate any nonclassical MHC I molecule restricted regulatory T cells that may result in unopposed active inflammatory responses. In CD only the CD1d-regulated T cells would be affected.

Recent understanding of IBD pathogenesis: implications for future therapies

The inflammatory bowel diseases (IBD) are comprised of two major phenotypes, Crohn's disease (CD) and ulcerative colitis (UC). Research over the last couple of years has led to great advances in understanding the inflammatory bowel diseases and their underlying pathophysiologic mechanisms. From the current understanding, it is likely that chronic inflammation in IBD is due to aggressive cellular immune responses to a subset of luminal bacteria. Susceptibility to disease is thereby determined by genes encoding immune responses which are triggered by environmental stimuli. Based on extensive research over the last decade, there are several new and novel pathways and specific targets on which to focus new therapeutics. The following review summarizes the current view on the four basic tenets of the pathophysiological basis of IBD and its implications for therapies of IBD: genetics, immune dysregulation, barrier dysfunction and the role of the microbial flora.

Reduced CD1d Expression in Colonic Epithelium in Microscopic Colitis

CD1d is a member of a major histocompatibility complex class I-like molecule family. Its function may involve presenting microbial nonpeptide or lipidic antigens to T lymphocytes, therefore to serve as an important factor in normal mucosal immunity of the gastrointestinal tract. In this study, the expression level of CD1d in microscopic colitis (ie, collagenous and lymphocytic colitis) was examined, and compared with that in normal colonic mucosa. Formalin-fixed, paraffin-embedded colon biopsies with diagnosis of lymphocytic colitis (19 cases), collagenous colitis (6 cases), and no pathologic change (20 cases) were studied immunohistochemically using monoclonal antibodies against human CD1d, CD3, CD4, and CD8. CD1d staining in the epithelium and lamina propria was graded along a scale of 0 to 4. Intraepithelial CD3-positive lymphocytes were counted in an area of 300 epithelial cells for each specimen. The results show that CD1d was expressed in normal colonic epithelial cells, primarily on the basolateral membranes with a concentrated intracellular pool in the subnuclear region. The expression level was markedly reduced in both lymphocytic colitis (P<0.001) and collagenous colitis (P<0.001), along with a significant increase in the number of intraepithelial CD3/CD8 lymphocytes (P<0.001). These findings suggest that microscopic colitis is associated with decreased epithelial expression of CD1d, an important immunoregulatory molecule in the gastrointestinal tract.

Intestinal epithelial cell signalling and host-derived negative regulators under chronic inflammation: to be or not to be activated determines the balance towards commensal bacteria

Advancing knowledge regarding the cellular mechanisms of intestinal inflammation has led to a better understanding of the disease pathology in patients with chronic disorders of the gut including inflammatory bowel disease, coeliac disease, lymphocytic colitis and irritable bowel syndrome. An emerging new paradigm suggests that changes in the homeostasis of bacteria- and host-derived signal transduction at the epithelial cell level may lead to functional and immune disturbances of the intestinal epithelium. It has become clear from numerous studies that enteric bacteria are a critical component in the development and prevention/treatment of chronic intestinal inflammation. Signal-specific activation of mitogen-activated protein kinases (MAPK), interferon-regulated factors (IRF) and the transcription factor NF-kappaB through pattern recognition receptor signalling effectively induce inflammatory defence mechanisms. Unbalanced activation of these innate signalling pathways because of host genetic predispositions and/or the lack of adequate anti-inflammatory feedback mechanisms may turn a physiological response into a pathological situation including failure of bacterial clearance and development of chronic inflammation. Host-derived regulators from the immune and enteric nerve system crosstalk to the innate signalling network of the intestinal epithelium in order to shape the extent and duration of inflammatory processes.

Oral tolerance and inflammatory bowel disease

PURPOSE OF REVIEW

Oral tolerance refers to the ability of the mucosal immune system to actively inhibit systemic immune responses to fed antigens. Recently, clinical trials have used oral tolerance as a therapy for certain chronic inflammatory and autoimmune diseases such as multiple sclerosis and type I diabetes. Inflammatory bowel disease is now widely thought to be caused by the breakdown of oral tolerance through a combination of genetic and environmental factors. Therefore, it seems incongruous that clinicians would try to use oral tolerance therapy to alleviate the symptoms of inflammatory bowel disease. Yet, armed with the results of select animal models, trials have begun for oral tolerance therapy for Crohn's disease. This review will outline the recent advances in understanding oral tolerance, explore the relation between oral tolerance and inflammatory bowel disease, and comment on the likelihood of successful oral tolerance therapy for inflammatory bowel disease.

RECENT FINDINGS

The results of an oral tolerance trial in Crohn's disease patients in Israel have shown some promising results, whereas the results of studies of experimentally induced oral tolerance in patients with inflammatory bowel disease from the authors' laboratory have shown that feeding a neoantigen in an attempt to induce oral tolerance is not successful in patients with inflammatory bowel disease.

SUMMARY

The fundamental difference in the mechanisms of oral tolerance in mice and humans requires a more focused effort to understand the human mucosal immune system before oral tolerance therapy for autoimmune and chronic inflammatory disorders reaches its full potential.

Non-classical MHC class I molecules on intestinal epithelial cells: mediators of mucosal crosstalk

The mucosal immune environment consists of a complex combination of lymphoid cells, non-lymphoid cells, and lumenal bacteria. Signals from lumenal bacteria are constantly transmitted to the underlying tissues across the intestinal epithelial barrier. Intestinal epithelial cells (IECs) can sense these signals, integrate them, and interpret them for lamina propria lymphoid populations. One mechanism by which these signals are communicated is by the expression of non-classical major histocompatibility complex (MHC) class I molecules by IECs. Epithelial cells can express a surprising variety of non-classical MHC class I molecules. In some cases, IECs can act as non-professional antigen-presenting cells utilizing the expression of such non-classical MHC class I molecules to directly present bacterial antigens. In other cases, the expression of non-classical MHC class I molecules may act as a co-stimulatory molecule or adhesion molecule that can modify the mucosal immune response. Finally, the expression of these molecules on IECs can lead to a broad array of responses ranging from tolerance to inflammation. Overall, the IEC, via the expression of non-classical MHC class I molecules, is a central mediator of the constant crosstalk between the intestinal lumen and the mucosal immune system.

The role of enterocytes in the intestinal barrier function and antigen uptake

The intestinal epithelium is a critical interface between the organism and its environment. The cell polarity and structural properties of the enterocytes, limiting the amount of antigen reaching the epithelial surface, form the basis of the integrity of the epithelium. However, apart from their participation in digestive processes, the enterocytes perform more than just a passive barrier function. The resistance of the tight junctions regulates the paracellular transport of antigens. Furthermore, the enterocytes take up and process antigens, involving two functional pathways. In the major pathway, enzymes in the lysosomes degrade the antigens. In the minor direct transcytotic pathway, the antigens are not degraded and are released into the interstitial space. Moreover, the enterocytes can present processed antigens directly to T cells and are often directly involved in immune processes. In inflammatory conditions, the properties of the epithelial barrier and the outcome of the immune response to luminal antigens can be changed.

Molecular basis for the high affinity interaction between the thymic leukemia antigen and the CD8alphaalpha molecule

The mouse thymic leukemia (TL) Ag is a nonclassical MHC class I molecule that binds with higher affinity to CD8alphaalpha than CD8alphabeta. The interaction of CD8alphaalpha with TL is important for lymphocyte regulation in the intestine. Therefore, we studied the molecular basis for TL Ag binding to CD8alphaalpha. The stronger affinity of the TL Ag for CD8alphaalpha is largely mediated by three amino acids on exposed loops of the conserved alpha3 domain. Mutant classical class I molecules substituted with TL Ag amino acids at these positions mimic the ability to interact with CD8alphaalpha and modulate lymphocyte function. These data indicate that small changes in the alpha3 domain of class I molecules potentially can have profound physiologic consequences.

Altered expression of CD1d molecules and lipid accumulation in the human hepatoma cell line HepG2 after iron loading

Iron overload in the liver may occur in clinical conditions such as hemochromatosis and nonalcoholic steatohepatitis, and may lead to the deterioration of the normal liver architecture by mechanisms not well understood. Although a relationship between the expression of ICAM-1, and classical major histocompatibility complex (MHC) class I molecules, and iron overload has been reported, no relationship has been identified between iron overload and the expression of unconventional MHC class I molecules. Herein, we report that parameters of iron metabolism were regulated in a coordinated-fashion in a human hepatoma cell line (HepG2 cells) after iron loading, leading to increased cellular oxidative stress and growth retardation. Iron loading of HepG2 cells resulted in increased expression of Nor3.2-reactive CD1d molecules at the plasma membrane. Expression of classical MHC class I and II molecules, ICAM-1 and the epithelial CD8 ligand, gp180 was not significantly affected by iron. Considering that intracellular lipids regulate expression of CD1d at the cell surface, we examined parameters of lipid metabolism in iron-loaded HepG2 cells. Interestingly, increased expression of CD1d molecules by iron-loaded HepG2 cells was associated with increased phosphatidylserine expression in the outer leaflet of the plasma membrane and the presence of many intracellular lipid droplets. These data describe a new relationship between iron loading, lipid accumulation and altered expression of CD1d, an unconventional MHC class I molecule reported to monitor intracellular and plasma membrane lipid metabolism, in the human hepatoma cell line HepG2.

Natural Killer T Cells in Mucosal Homeostasis

The mucosal-associated lymphoid tissues (MALT), including the gut-associated lymphoid tissues, are a tightly regulated environment. In fact, it might be stated that on the basis of studies from animal models of inflammatory bowel disease (IBD), the major means of peripheral regulation of immune responses in the intestine is not necessarily from processes such as deletion or anergy, but more likely from the controls imposed upon responses due to the activities of a variety of regulatory subsets of cells. One type of regulatory cellular subset that has recently gained attention is the subset of T cells that are associated with CD1d-restricted responses. Recently, CD1d-restricted T cells have been increasingly appreciated to play a significant role in mucosal tissues of the intestine and lung, for example. Insights from these studies have clearly elevated these cells to particular importance in the regulation of a variety of infectious and inflammatory conditions, such as those associated with idiopathic IBD. In this review, we focus on recent observations on the characteristics of CD1d-restricted pathways in mucosal compartments, after a brief introduction into the biology of CD1d and CD1d-restricted T cells.

Activation of a Unique Population of CD8+T Cells by Intestinal Epithelial Cells

Intestinal epithelial cells may play a role in the regulation of immune responses toward luminal antigens. We show that a subset of CD8(+) T cells undergoes oligoclonal expansion in the intestinal mucosa, probably through interaction with a unique complex expressed on epithelial cells, formed by a CEA subfamily member (gp180) and CD1d. This subset, which is regulatory in vitro, may play a role in the control of intestinal immune responses toward luminal antigens. A lack of expansion of these CD8(+) regulatory T cells, probably related to the defective expression of the gp180/CD1d complex, is observed in inflammatory bowel disease.

Regulatory T cells: peace keepers in the gut

The mucosa-associated lymphoid tissue (MALT) has the task of protecting the host from pathogens while maintaining the integrity of the gut. Immune responses are tightly regulated such that there is tolerance of nonpathogenic bacteria as well as dietary antigens present in the intestinal lumen. The failure to control these responses leads to a disruption in tolerance, which has been proposed as one mechanism involved in the development of inflammatory bowel disease (IBD). Different mechanisms are involved in the control of immune responses in the intestinal tract, including active suppression by regulatory T cells. Distinct subsets of regulatory T cells coexist in the intestinal mucosa, which is a fertile environment for their growth. Most of these are defined by their phenotype and/or their ability to produce regulatory cytokines such as interleukin-10 and transforming growth factor-beta A lack of activation and/or expansion of regulatory cells could play a role in the uncontrolled inflammation seen in IBD. Regulatory T cells may be activated by cytokines, and their inductive phase may be antigen-driven. There are limited data relating to the true surface interactions regulating the activation of these cells. Most of the CD4 regulatory T cells (Tr1, Th3, and CD4 CD25+) are thought to interact with dendritic cells. Subsets of regulatory T cells (such as CD8 TrE cells) may recognize antigens presented by intestinal epithelial cells. A better understanding of the mechanisms by which these regulatory T cells are expanded and/or activated in the intestinal mucosa may provide clues as how to use them as a novel therapeutic tool in the treatment of patients with IBD.

Accessory cell function of airway epithelial cells

Oei, Erwin, Thomas Kalb, Prarthana Beuria, Matthieu Allez, Atsushi Nakazawa, Miyuki Azuma, Michael Timony, Zanetta Stuart, Houchu Chen, and Kirk Sperber. Accessory cell function of airway epithelial cells.

We previously demonstrated that airway epithelial cells (AECs) have many features of accessory cells, including expression of class II molecules CD80 and CD86 and functional Fcγ receptors. We have extended these studies to show that freshly isolated AECs have mRNA for cathepsins S, V, and H [proteases important in antigen (Ag) presentation], invariant chain, human leukocyte antigen (HLA)-DM-α and HLA-DM-β, and CLIP, an invariant chain breakdown product. A physiologically relevant Ag, ragweed, was colocalized with HLA-DR in AECs, and its uptake was increased by granulocyte-macrophage colony-stimulating factor and IFN-γ treatments, which had no effect on CD80 and CD86 expression. We demonstrate the presence of other costimulatory molecules, including B7h and B7-H1, on AECs and the increased expression of B7-H1 on AECs after treatment with granulocyte-macrophage colony-stimulating factor and IFN-γ. Finally, we compared T cell proliferation after allostimulation with AECs and dendritic cells (DCs). The precursor frequency of peripheral blood T cells responding to AECs was 0.264% compared with 0.55% for DCs. DCs stimulated CD45RO+, CD45RA+, CCR7+and CCR7−CD4+, and CD8+T cells, whereas AECs stimulated only CD45RO+, CD45RA−, CCR7−, CD4+, and CD8+T cells. There was no difference in cytokine production, type of memory T cells stimulated (effector vs. long-term memory), or apoptosis by T cells cocultured with AECs and DCs. The localization of AECs exposed to the external environment may make them important in the regulation of local immune responses.

Delineation of a CD1d-restricted antigen presentation pathway associated with human and mouse intestinal epithelial cells

BACKGROUND & AIMS

CD1d, a major histocompatibility complex (MHC) class I-related molecule that is responsible for the presentation of glycolipid antigens to subsets of natural killer T (NK-T) cells, is expressed by intestinal epithelial cells (IECs). However, CD1d-restricted antigen presentation has not yet been examined on IECs.

METHODS

A mouse intestinal epithelial cell line (MODE-K), a human epithelial cell line (T84), T84 cells transfected with CD1d and/or MHC class II, and freshly isolated human IECs were examined for their ability to present model glycolipid antigens to NK-T cells as defined by interleukin (IL)-2 or IL-4 secretion.

RESULTS

MODE-K and freshly isolated human IECs exhibited dose-dependent, CD1d-restricted presentation of the functional glycolipid antigen, alpha-galactosylceramide (alpha GalCer), to the mouse NK-T cell hybridoma, DN32.D3. The human IEC line, T84, mainly presented alpha GalCer when transfected with human CD1d. Presentation of alpha GalCer by CD1d-transfected T84 cells (T84d) to DN32.D3 cells was greater along the basal surface in comparison with the apical surface. Induction of the MHC class II antigen presentation machinery by cotransfecting T84d with the MHC class I transactivator (CIITA) did not alter this polarity of presentation. Neither MODE-K nor T84 cells transfected with CD1d, CD1d plus CIITA, or CD1d plus HLA-DR were able to present glycolipid antigens requiring intracellular processing. The MODE-K cell line could also present alpha GalCer to primary mouse NK-T cells.

CONCLUSIONS

CD1d is expressed functionally on IECs with a polarity of presentation (basal > apical) predicting a role in presentation of mucosal glycolipid antigens to local CD1d-restricted T cells.

Expansion of CD8+ T cells with regulatory function after interaction with intestinal epithelial cells

BACKGROUND & AIMS

Regulatory T cells play a role in the control of immune responses in the intestinal mucosa and their absence may predispose to inflammatory bowel disease (IBD). We have previously shown that T cells activated by intestinal epithelial cells (IECs) are suppressive in function. Our goal was to characterize the phenotype and function of T cells proliferating after interaction with IECs.

METHODS

Irradiated human IECs, isolated from normal resection specimens, were cultured with carboxy fluorescein succinimidyl ester (CFSE) labeled T cells. Flow cytometric analysis of T cells was performed at days 5-10. CD8+ T cells proliferating in culture with IECs were sorted and added to suppressive assays.

RESULTS

The precursor frequency of T cells proliferating in response to IECs ranged from 0.3%-0.9%. Several subpopulations were shown to proliferate (CD8+CD28-/CD8+CD28+/CD4+CD25+), but one population (CD8+CD28-CD101+CD103+) appeared to be dependent on contact with the CD8 ligand gp180. After sorting, culture in the presence of interleukin (IL)-7 and IL-15 allowed for the generation of cell lines. IEC-activated CD8+ T cells, but not nonactivated CD8+ T cells, were suppressive in function. Suppression belonged to the CD101+CD103+ subset of IEC-activated CD8+ T cells and appeared to require cell contact. CD8+ lamina propria T cells also showed suppressive function, suggesting the presence of CD8+ regulatory T cells in the mucosa.

CONCLUSIONS

IECs are able to induce the proliferation of a small fraction of CD8+ peripheral T cells. The CD8+CD28- subset of IEC-activated CD8+ T cells, which express CD101 and CD103, interacts with IECs through gp180 and has regulatory function.

A non-class I MHC intestinal epithelial surface glycoprotein, gp180, binds to CD8

The activation of CD8(+) T cells by normal intestinal epithelial cells in antigen-specific or allogeneic mixed cell culture systems has significant implications for the modulation of mucosal immune responses due to the fact that these T cells appear to have regulatory rather than cytolytic activity. A 180-kDa glycoprotein (gp180) has been identified and shown to be important in CD8(+) T cell activation by intestinal epithelial cells. In this study, we examine, in further detail, the role that the CD8 molecule plays in this interaction. It has been previously shown that monoclonal antibodies against gp180 inhibited the activation of CD8-associated p56(lck) in T cells. Although indirectly suggested by these data, there was no evidence that the activation of this protein tyrosine kinase was a direct result of gp180 interacting with the CD8 molecule. In this study, we document that soluble gp180 is able to bind to CD8-Fc fusion proteins and is absorbed by human CD8 alpha but not CD4 transfected murine T cells and that this interaction is dependent upon carbohydrate on the gp180 molecule. Furthermore, the sites used for binding by gp180 are distinct from those used by the conventional CD8 ligand, class I MHC. Thus, gp180 appears to be a novel CD8 ligand that plays an important role in the activation of CD8-associated kinases and of CD8(+) T cells.

Food allergy. Mechanisms, diagnosis, and management in children

We eat approximately two to three tons of food in our lifetime, but most people do not have an adverse reaction to foods. Many people believe that they have an allergic reaction to foods; however, the actual incidence confirmed by history and challenges suggests a prevalence rate closer to 2% to 8% in young infants and less than 2% in adults.

CD8+CD28- T cells: certainties and uncertainties of a prevalent human T-cell subset

Human peripheral blood CD8+ T cells comprise cells that are in different states of differentiation and under the control of complex homeostatic processes. In a number of situations ranging from chronic inflammatory conditions and infectious diseases to ageing, immunodeficiency, iron overload and heavy alcohol intake, major phenotypic changes, usually associated with an increase in CD8+ T cells lacking CD28 expression, take place. CD8+CD28- T cells are characterized by a low proliferative capacity to conventional stimulation in vitro and by morphological and functional features of activated/memory T cells. Although the nature of the signals that give origin to this T-cell subset is uncertain, growing evidence argues for the existence of an interplay between epithelial cells, molecules with the MHC-class I fold and CD8+ T cells. The possibility that the generation of CD8+CD28- T cells is the combination of TCR/CD3zeta- and regulatory factor-mediated signals as a result of the sensing of modifications of the internal environment is discussed.

Regulation of CD8 expression in mast cells by exogenous or endogenous nitric oxide

We recently reported a novel CD8 molecule on rat alveolar macrophages and peritoneal mast cells (PMC). However, little is known about the regulation of CD8 expression and function on these cells. We investigated the regulation of CD8 expression on PMC by NO, because NO can regulate inflammatory responses and also because anti-CD8 Ab stimulates inducible NO synthase and NO production by PMC and alveolar macrophages. Ligation of CD8alpha on PMC with Ab (OX8) induced CD8alpha mRNA expression after 3-6 h, and flow cytometry demonstrated that OX8 treatment increased CD8alpha protein expression compared with PMC treated with isotype control IgG1. To test whether NO mediates the up-regulation of CD8alpha, we used the NO donor S-nitrosoglutathione (500 microM) and NO synthase inhibitors (N(G)-monomethyl-L-arginine and N(G)-nitro-L-arginine methyl ester; 100 microM). S-nitrosoglutathione up-regulated both mRNA and protein expression of CD8alpha in PMC compared with that in sham-treated cells, while NO synthase inhibitors down-regulated OX8 Ab-induced CD8alpha expression. To investigate how NO regulates CD8 expression on PMC, we examined the cGMP-dependent pathway using 8-bromo-cGMP (2 mM) and the guanylate cyclase inhibitor, 1H-oxadiazoloquinoxalin-1-one (20 microM). 8-Bromo-cGMP up-regulated CD8 expression, whereas 1H-oxadiazoloquinoxalin-1-one down-regulated its expression. Thus, ligation of CD8 up-regulates CD8 expression on PMC, a response mediated at least in part by NO through a cGMP-dependent pathway. The significance of this up-regulation of CD8alpha on mast cells (MC) is unclear, but since ligation of CD8 on MC with OX8 Ab can alter gene expression and mediator secretion, up-regulation of CD8 may enhance the MC response to natural ligation of this novel form of CD8.

CD1 expression is differentially regulated by microglia, macrophages and T cells in the central nervous system upon inflammation and demyelination

Expression of CD1 by microglia, macrophages and T cells was investigated ex vivo. In the healthy central nervous system (CNS), resident microglia, macrophages and T cells express levels of CD1 significantly lower than that expressed by splenic macrophages and T cells. During experimental autoimmune encephalomyelitis (EAE), CD1 expression by microglia and the number of CD1+ microglia increase. Macrophages and T cells strongly upregulate CD1 expression in the CNS, but not in the spleen. Whereas the function of CD1 expressed by T cells remains unclear, the expression by microglia and macrophages provides the CNS with a (glyco)lipidic-presenting molecule in an inflammatory and demyelinating environment.

Epithelial cell antigen presentation

For decades intestinal epithelial cells were thought of as passive barriers to luminal contents, cells involved in nutrient absorption and electrolyte secretion. Studies during the past 10 to 15 years have changed that concept as our understanding of mucosal immunity has evolved and as we have come to understand that the regulation of this system is unique in terms of cellular interactions and factors produced. The intestinal epithelial cell has moved to the forefront of these studies where it has been shown to be an active participant in mucosal immunoregulation and inflammation. Results of the earliest studies suggested that epithelial cells might be involved in immune regulation because they expressed a series of cell surface molecules that correlate with classical antigen presentation (class I and II MHC molecules). Later on, the expression of nonclassical class I molecules was identified on these cells as well, raising the possibility of novel forms of interactions with unique cell populations. This was followed by the observation that epithelial cells secrete cytokines and chemokines, which not only regulate mucosal immune responses but also regulate inflammatory responses. By such processes, the IEC has been proposed as being a bridge between innate and adaptive immunity. The recent description of Toll-like receptors on IECs adds further support to this concept. Clearly there is growing appreciation of the multifaceted role that the IEC plays in the gut. The work on IECs in the past year has helped to refine this role.

Staphylococcal enterotoxin B induces potent cytotoxic activity by intraepithelial lymphocytes

In food poisoning, Staphylococcus aureus secretes staphylococcal enterotoxin B (SEB), a superantigen that causes intense T-cell proliferation and cytotoxicity. The effects of SEB on lytic activity by human intestinal intraepithelial lymphocytes (IEL) were investigated. Jejunal IEL, from morbidly obese individuals undergoing gastric bypass operations, were tested for SEB-induced cytotoxicity against C1R B-lymphoblastoid cells, HT-29 adenocarcinoma cells, or CD1d-transfected cells using the 51Cr-release assay. Fas and Fas ligand expression were detected by immunofluorescence and flow cytometry and soluble ligand by enzyme-linked immunosorbent assay (ELISA). In the presence of SEB, IEL became potently cytotoxic against C1R cells and interferon-gamma (IFN-gamma)-precultured HT-29 cells, causing 55+/-10% and 31+/-6% lysis, respectively, greater than that by phytohaemagglutinin (PHA)-, interleukin-2 (IL-2)-, or anti-T-cell receptor (TCR)-activated IEL. SEB-stimulated peripheral blood (PB) CD8+ T cells lysed similar numbers of C1R cells but fewer HT-29 cells (53+/-13% and 8+/-5%, respectively). IEL killing of C1R cells involved interaction of major histocompatibility complex (MHC) class II with TCR, CD2 with CD58, and CD11a with CD54, and was perforin mediated. SEB-induced IEL lysis of HT-29 cells, in contrast, was caused by an unknown target cell structure, not MHC class II or CD1d, and resulted from a combination of perforin and Fas-mediated events. The potent cytotoxic activities of IEL promoted by SEB utilize two different mechanisms, depending on the surface receptors expressed by the target cells.

Early nutrition and the development of immune function in the neonate

The present review will concentrate on the development of the gut-associated lymphoid tissue and the role of early nutrition in promoting immune function. The intestine is the largest immune organ in the body, and as such is the location for the majority of lymphocytes and other immune effector cells. The intestine is exposed to vast quantities of dietary and microbial antigens, and is the most common portal of entry for pathogens, some of which are potentially lethal. The development of normal immune function of the intestine is therefore vital for survival, and is dependent on appropriate antigen exposure and processing, and also an intact intestinal barrier. In early life innate mechanisms of defence are probably more important than active or adaptive mechanisms in responding to an infectious challenge, since the healthy neonate is immunologically naïve (has not seen antigen) and has not acquired immunological memory. During this period maternal colostrum and milk can significantly augment resistance to enteric infections. The mechanisms of enhancing disease resistance are thought to be passive, involving a direct supply of anti-microbial factors, and active, by promoting the development of specific immune function. A tolerance response to dietary and non-invasive antigens is generally induced in the gut. However, it must also be able to mount an adequate immune response to ensure clearance of foreign antigens. It is now recognized that regulation of tolerance and active immune responses is critical to health, and failure to regulate these responses can lead to recurrent infections, inflammatory diseases and allergies. The education of the immune system in early life is thought to be critical in minimizing the occurrence of these immune-based disorders. During this phase of development maternal milk provides signals to the immune system that generate appropriate response and memory. One factor that has been proposed to contribute to the increase in the incidence of immune-based disorders, e.g. atopic diseases in Western countries, is thought to be the increased prevalence of formula-feeding.

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.