Selective expansion of intraepithelial lymphocytes expressing the HLA-E-specific natural killer receptor CD94 in celiac disease

Interleukin-15, a master piece in the immunological jigsaw of celiac disease

BACKGROUND

The immune response causing celiac disease (CD) depends on the activation of intestinal CD4+ T cells by gluten-derived peptides presented by HLA-DQ2 or HLA-DQ8 molecules, the main genetic risk factor. However, additional factors are necessary to impair immune tolerance to dietary gluten, to stimulate intraepithelial lymphocytes (IEL) and to induce intestinal damage.

KEY MESSAGES

Current data point to a central role of interleukin-15 (IL-15). In situ and ex vivo studies indicate that IL-15 stimulates the accumulation and cytotoxic activation of CD8+ T IEL in active CD, and that of the malignant innate-like IEL in type II refractory CD (RCDII). Other studies show that IL-15 impairs the immunoregulatory control of effector T cells, notably CD8+. Recently, animal models have been designed to investigate the respective role of CD4+ T cells and IL-15 in CD. We discuss more particularly our results in such a model, which shows that IL-15 produced in excess in the intestine can cooperate with CD4+ T cells specific for a dietary antigen to trigger a celiac-like enteropathy. In this mouse model, CD4+ T cells activated by dietary ovalbumin secreted IL-2 which, along with IL-15, stimulated the expansion of noncognate intestinal cytotoxic CD8+ T cells containing large amounts of granzyme B. In the presence of IL-15, the latter cells did not respond to regulatory T cells, and accumulated in the intestine close to epithelial damage.

CONCLUSION

On the basis of these data, we propose that, in CD, gluten-specific CD4+ T cells synthesize cytokines that synergize with IL-15 to license the expansion and activation of cytotoxic IEL, which drive tissue damage. We suggest that IL-15 is a meaningful therapeutic target, notably in patients with RCDII in which malignant IEL can respond to IL-15 independently of signals provided by CD4+ T cells.

IL-15: a central regulator of celiac disease immunopathology

Interleukin-15 (IL-15) exerts many biological functions essential for the maintenance and function of multiple cell types. Although its expression is tightly regulated, IL-15 upregulation has been reported in many organ-specific autoimmune disorders. In celiac disease, an intestinal inflammatory disorder driven by gluten exposure, the upregulation of IL-15 expression in the intestinal mucosa has become a hallmark of the disease. Interestingly, because it is overexpressed both in the gut epithelium and in the lamina propria, IL-15 acts on distinct cell types and impacts distinct immune components and pathways to disrupt intestinal immune homeostasis. In this article, we review our current knowledge of the multifaceted roles of IL-15 with regard to the main immunological processes involved in the pathogenesis of celiac disease.

Gliadin peptides as triggers of the proliferative and stress/innate immune response of the celiac small intestinal mucosa

Celiac disease (CD) is a frequent inflammatory intestinal disease, with a genetic background, caused by gliadin-containing food. Undigested gliadin peptides induce innate and adaptive T cell-mediated immune responses. The major mediator of the stress and innate immune response to gliadin peptides (i.e., peptide 31-43, P31-43) is the cytokine interleukin-15 (IL-15). The role of epithelial growth factor (EGF) as a mediator of enterocyte proliferation and the innate immune response has been described. In this paper, we review the most recent literature on the mechanisms responsible for triggering the up-regulation of these mediators in CD by gliadin peptides. We will discuss the role of P31-43 in enterocyte proliferation, structural changes and the innate immune response in CD mucosa in cooperation with EGF and IL-15, and the mechanism of up-regulation of these mediators related to vesicular trafficking. We will also review the literature that focuses on constitutive alterations of the structure, signalling/proliferation and stress/innate immunity pathways of CD cells. Finally, we will discuss how these pathways can be triggered by gliadin peptide P31-43 in controls, mimicking the celiac cellular phenotype.

Gliadin peptides as triggers of the proliferative and stress/innate immune response of the celiac small intestinal mucosa

Celiac disease (CD) is a frequent inflammatory intestinal disease, with a genetic background, caused by gliadin-containing food. Undigested gliadin peptides induce innate and adaptive T cell-mediated immune responses. The major mediator of the stress and innate immune response to gliadin peptides (i.e., peptide 31–43, P31–43) is the cytokine interleukin-15 (IL-15). The role of epithelial growth factor (EGF) as a mediator of enterocyte proliferation and the innate immune response has been described. In this paper, we review the most recent literature on the mechanisms responsible for triggering the up-regulation of these mediators in CD by gliadin peptides. We will discuss the role of P31–43 in enterocyte proliferation, structural changes and the innate immune response in CD mucosa in cooperation with EGF and IL-15, and the mechanism of up-regulation of these mediators related to vesicular trafficking. We will also review the literature that focuses on constitutive alterations of the structure, signalling/proliferation and stress/innate immunity pathways of CD cells. Finally, we will discuss how these pathways can be triggered by gliadin peptide P31–43 in controls, mimicking the celiac cellular phenotype.

Lnk/Sh2b3 controls the production and function of dendritic cells and regulates the induction of IFN-γ-producing T cells

Dendritic cells (DCs) are proficient APCs that play crucial roles in the immune responses to various Ags and pathogens and polarize Th cell immune responses. Lnk/SH2B adaptor protein 3 (Sh2b3) is an intracellular adaptor protein that regulates B lymphopoiesis, megakaryopoiesis, and expansion of hematopoietic stem cells by constraining cytokine signals. Recent genome-wide association studies have revealed a link between polymorphism in this adaptor protein and autoimmune diseases, including type 1 diabetes and celiac disease. We found that Lnk/Sh2b3 was also expressed in DCs and investigated its role in the production and function of DC lineage cells. In Lnk(-/-) mice, DC numbers were increased in the spleen and lymph nodes, and growth responses of bone marrow-derived DCs to GM-CSF were augmented. Mature DCs from Lnk(-/-) mice were hypersensitive and showed enhanced responses to IL-15 and GM-CSF. Compared to normal DCs, Lnk(-/-) DCs had enhanced abilities to support the differentiation of IFN-γ-producing Th1 cells from naive CD4(+) T cells. This was due to their elevated expression of IL-12Rβ1 and increased production of IFN-γ. Lnk(-/-) DCs supported the appearance of IFN-γ-producing T cells even under conditions in which normal DCs supported induction of regulatory T cells. These results indicated that Lnk/Sh2b3 plays a regulatory role in the expansion of DCs and might influence inflammatory immune responses in peripheral lymphoid tissues.

Noncontaminated dietary oats may hamper normalization of the intestinal immune status in childhood celiac disease

OBJECTIVES:

Life-long, strict gluten-free diet (GFD) is the only treatment for celiac disease (CD). Because there is still uncertainty regarding the safety of oats for CD patients, the aim was to investigate whether dietary oats influence the immune status of their intestinal mucosa.

METHODS:

Paired small intestinal biopsies, before and after >11 months on a GFD, were collected from children with CD who were enrolled in a randomized, double-blind intervention trial to either of two diets: standard GFD (GFD-std; n=13) and noncontaminated oat-containing GFD (GFD-oats; n=15). Expression levels of mRNAs for 22 different immune effector molecules and tight junction proteins were determined by quantitative reverse transcriptase (RT)-PCR.

RESULTS:

The number of mRNAs that remained elevated was higher in the GFD-oats group (P=0.05). In particular, mRNAs for the regulatory T cell (Treg) signature molecules interleukin-10 (IL-10) and transforming growth factor-β1 (TGF-β1), the cytotoxicity-activating natural killer (NK) receptors KLRC2/NKG2C and KLRC3/NKG2E, and the tight junction protein claudin-4 remained elevated. Between the two groups, most significant differences were seen for claudin-4 (P=0.003) and KLRC3/NKG2E (P=0.04).

CONCLUSIONS:

A substantial fraction of pediatric CD patients seem to not tolerate oats. In these patients, dietary oats influence the immune status of the intestinal mucosa with an mRNA profile suggesting presence of activated cytotoxic lymphocytes and Tregs and a stressed epithelium with affected tight junctions. Assessment of changes in levels of mRNA for claudin-4 and KLC3/NKG2E from onset to after a year on oats containing GFD shows promise to identify these CD patients.

IL-15: a central regulator of celiac disease immunopathology

Interleukin-15 (IL-15) exerts many biological functions essential for the maintenance and function of multiple cell types. Although its expression is tightly regulated, IL-15 upregulation has been reported in many organ-specific autoimmune disorders. In celiac disease, an intestinal inflammatory disorder driven by gluten exposure, the upregulation of IL-15 expression in the intestinal mucosa has become a hallmark of the disease. Interestingly, because it is overexpressed both in the gut epithelium and in the lamina propria, IL-15 acts on distinct cell types and impacts distinct immune components and pathways to disrupt intestinal immune homeostasis. In this article, we review our current knowledge of the multifaceted roles of IL-15 with regard to the main immunological processes involved in the pathogenesis of celiac disease.

Lnk prevents inflammatory CD8⁺ T-cell proliferation and contributes to intestinal homeostasis

The intracellular adaptor Lnk (also known as SH2B3) regulates cytokine signals that control lymphohematopoiesis, and Lnk(-/-) mice have expanded B-cell, megakaryocyte, and hematopoietic stem-cell populations. Moreover, mutations in the LNK gene are found in patients with myeloproliferative disease, whereas LNK polymorphisms have recently been associated with inflammatory and autoimmune diseases, including celiac disease. Here, we describe a previously unrecognized function of Lnk in the control of inflammatory CD8(+) T-cell proliferation and in intestinal homeostasis. Mature T cells from newly generated Lnk-Venus reporter mice had low but substantial expression of Lnk, whereas Lnk expression was downregulated during homeostatic T-cell proliferation under lymphopenic conditions. The numbers of CD44(hi) IFN-γ(+) CD8(+) effector or memory T cells were found to be increased in Lnk(-/-) mice, which also exhibited shortening of villi in the small intestine. Lnk(-/-) CD8(+) T cells survived longer in response to stimulation with IL-15 and proliferated even in nonlymphopenic hosts. Transfer of Lnk(-/-) CD8(+) T cells together with WT CD4(+) T cells into Rag2-deficient mice recapitulated a sign of villous abnormality. Our results reveal a link between Lnk and immune cell-mediated intestinal tissue destruction.

Crystal structure of Vδ1 T cell receptor in complex with CD1d-sulfatide shows MHC-like recognition of a self-lipid by human γδ T cells

The nature of the antigens recognized by γδ T cells and their potential recognition of major histocompatibility complex (MHC)-like molecules has remained unclear. Members of the CD1 family of lipid-presenting molecules are suggested ligands for Vδ1 TCR-expressing γδ T cells, the major γδ lymphocyte population in epithelial tissues. We crystallized a Vδ1 TCR in complex with CD1d and the self-lipid sulfatide, revealing the unusual recognition of CD1d by germline Vδ1 residues spanning all complementarity-determining region (CDR) loops, as well as sulfatide recognition separately encoded by nongermline CDR3δ residues. Binding and functional analysis showed that CD1d presenting self-lipids, including sulfatide, was widely recognized by gut Vδ1+ γδ T cells. These findings provide structural demonstration of MHC-like recognition of a self-lipid by γδ T cells and reveal the prevalence of lipid recognition by innate-like T cell populations.

Dietary gluten triggers concomitant activation of CD4+ and CD8+ αβ T cells and γδ T cells in celiac disease

Celiac disease is an intestinal autoimmune disease driven by dietary gluten and gluten-specific CD4(+) T-cell responses. In celiac patients on a gluten-free diet, exposure to gluten induces the appearance of gluten-specific CD4(+) T cells with gut-homing potential in the peripheral blood. Here we show that gluten exposure also induces the appearance of activated, gut-homing CD8(+) αβ and γδ T cells in the peripheral blood. Single-cell T-cell receptor sequence analysis indicates that both of these cell populations have highly focused T-cell receptor repertoires, indicating that their induction is antigen-driven. These results reveal a previously unappreciated role of antigen in the induction of CD8(+) αβ and γδ T cells in celiac disease and demonstrate a coordinated response by all three of the major types of T cells. More broadly, these responses may parallel adaptive immune responses to viral pathogens and other systemic autoimmune diseases.

The role of natural killer cells in autoimmune liver disease: a comprehensive review

Natural Killer (NK) cells are important players of the innate arm of the immune system and provide an early defense against pathogens and tumor-transformed cells. Peripheral blood NK (PB-NK) cells were first identified because of their ability to spontaneously kill tumor-cell targets in vitro without the need for specific antigen priming, which is the reason that they were named 'natural killer' cells. The characterization of NK cells in human tissues and body organs represented another important step forward to better understand their physiology and physiopathology. In this regard, many reports revealed over the past decade a differential anatomic distribution of NK cell subsets in several sites such as the intestine, lung, cervix, placenta and liver as well as in secondary lymphoid organs such as spleen, lymph nodes and tonsils. Among all these tissues, the liver is certainly unique as its parenchyma contains an unusually high number of infiltrating immune cells with 30-50% of total lymphocytes being NK cells. Given the constant liver intake of non-self antigens from the gastrointestinal tract via the portal vein, hepatic NK (H-NK) cells must retain a certain degree of tolerance in the context of their immune-surveillance against dangers to the host. Indeed, the breakdown of the tolerogenic state of the liver-associated immune system has been shown to induce autoimmunity. However, the role of NK cells during the course of autoimmune liver diseases is still being debated mainly because a complete characterization of H-NK cells normally resident in healthy human liver has not yet been fully disclosed. Furthermore, the differences in phenotype and functions between human and mouse H-NK cells often preclude translation of results obtained from murine models into experimental approaches to be performed in humans. Here, we provide an extensive characterization of the phenotype of H-NK cells physiologically resident in the human liver by both mentioning data available in literature and including a set of original results recently developed in our laboratory. We then review our current knowledge in regard to the contribution of H-NK cells in regulating local immune homeostasis and tolerance as well as in inducing the development of liver autoimmunity.

Probiotics, prebiotics and immunomodulation of gut mucosal defences: homeostasis and immunopathology

Probiotics are beneficial microbes that confer a realistic health benefit on the host, which in combination with prebiotics, (indigestible dietary fibre/carbohydrate), also confer a health benefit on the host via products resulting from anaerobic fermentation. There is a growing body of evidence documenting the immune-modulatory ability of probiotic bacteria, it is therefore reasonable to suggest that this is potentiated via a combination of prebiotics and probiotics as a symbiotic mix. The need for probiotic formulations has been appreciated for the health benefits in "topping up your good bacteria" or indeed in an attempt to normalise the dysbiotic microbiota associated with immunopathology. This review will focus on the immunomodulatory role of probiotics and prebiotics on the cells, molecules and immune responses in the gut mucosae, from epithelial barrier to priming of adaptive responses by antigen presenting cells: immune fate decision-tolerance or activation? Modulation of normal homeostatic mechanisms, coupled with findings from probiotic and prebiotic delivery in pathological studies, will highlight the role for these xenobiotics in dysbiosis associated with immunopathology in the context of inflammatory bowel disease, colorectal cancer and hypersensitivity.

Mucosal cytokine response after short-term gluten challenge in celiac disease and non-celiac gluten sensitivity

OBJECTIVES

In celiac disease (CD), gluten induces both adaptive and innate immune responses. Non-celiac gluten sensitivity (NCGS) is another form of gluten intolerance where the immune response is less characterized. The aim of our study was to explore and compare the early mucosal immunological events in CD and NCGS.

METHODS

We challenged 30 HLA-DQ2(+) NCGS and 15 CD patients, all on a gluten-free diet, with four slices of gluten-containing bread daily for 3 days. Duodenal biopsy specimens were collected before and after challenge. The specimens were examined for cytokine mRNA by quantitative reverse transcriptase-PCR and for MxA-expression and CD3(+) intraepithelial lymphocytes (IELs) by immunohistochemistry and compared with specimens from untreated CD patients and disease controls.

RESULTS

In CD patients, tumor necrosis factor alpha (P=0.02) and interleukin 8 (P=0.002) mRNA increased after in vivo gluten challenge. The interferon gamma (IFN-γ) level of treated CD patients was high both before and after challenge and did not increase significantly (P=0.06). Four IFN-γ-related genes increased significantly. Treated and untreated CD patients had comparable levels of IFN-γ. Increased expression of MxA in treated CD patients after challenge suggested that IFN-α was activated on gluten challenge. In NCGS patients only IFN-γ increased significantly (P=0.03). mRNA for heat shock protein (Hsp) 27 or Hsp70 did not change in any of the groups. Importantly, we found that the density of IELs was higher in NCGS patients compared with disease controls, independent of challenge, although lower than the level for treated CD patients.

CONCLUSIONS

CD patients mounted a concomitant innate and adaptive immune response to gluten challenge. NCGS patients had increased density of intraepithelial CD3(+) T cells before challenge compared with disease controls and increased IFN-γ mRNA after challenge. Our results warrant further search for the pathogenic mechanisms for NCGS.

Natural cytotoxicity receptors: broader expression patterns and functions in innate and adaptive immune cells

Natural cytotoxicity receptors (NCRs) have been classically defined as activating receptors delivering potent signals to Natural Killer (NK) cells in order to lyze harmful cells and to produce inflammatory cytokines. Indeed, the elicitation of NK cell effector functions after engagement of NCRs with their ligands on tumor or virus infected cells without the need for prior antigen recognition is one of the main mechanisms that allow a rapid clearance of target cells. The three known NCRs, NKp46, NKp44, and NKp30, comprise a family of germ-line encoded Ig-like trans-membrane (TM) receptors. Until recently, NCRs were thought to be NK cell specific surface molecules, thus making it possible to easily distinguish NK cells from phenotypically similar cell types. Moreover, it has also been found that the surface expression of NKp46 is conserved on NK cells across mammalian species. This discovery allowed for the use of NKp46 as a reliable marker to identify NK cells in different animal models, a comparison that was not possible before due to the lack of a common and comprehensive receptor repertoire between different species. However, several studies over the recent few years indicated that NCR expression is not exclusively confined to NK cells, but is also present on populations of T as well as of NK-like lymphocytes. These insights raised the hypothesis that the induced expression of NCRs on certain T cell subsets is governed by defined mechanisms involving the engagement of the T cell receptor (TCR) and the action of pro-inflammatory cytokines. In turn, the acquisition of NCRs by T cell subsets is also associated with a functional independence of these Ig-like TM receptors from TCR signaling. Here, we review these novel findings with respect to NCR-mediated functions of NK cells and we also discuss the functional consequences of NCR expression on non-NK cells, with a particular focus on the T cell compartment.

HLA-E*0103X is associated with susceptibility to Pemphigus vulgaris

Non-classical human leucocyte antigen-E (HLA-E) mediates natural killer and CD8+ T-cell activity, suggesting a role in the regulation of autoimmunity. HLA-E*0103X/*0103X has been associated with Behcet's disease and HLA-E *0101/*0103X with childhood onset diabetes. We investigated HLA-E allele status in 52 Caucasian and Ashkenazi Jewish Pemphigus vulgaris (PV) patients and 51 healthy controls by restriction fragment length polymorphism-polymerase chain reaction and amplification refractory mutation system. Associations were determined via chi-square test, Fisher's exact test and logistical regression analysis. HLA-E outcomes included presumed homozygous *0101/*0101 or *0103X/*0103X genotype status or *0101/*0103X heterozygous status. PV did not significantly associate with either *0101/*0101 or *0101/*0103X genotypes. HLA-E*0103X/*0103X (presumed homozygote) is significantly increased in patients with PV versus controls (P = 0.0146, OR = 3.730, 95%CI = 1.241-11.213). Our data provide the first evidence that HLA-E*0103X is a marker for genetic risk in PV.

Celiac disease: an immunological jigsaw

Celiac disease (CD) is a chronic enteropathy induced by dietary gluten in genetically predisposed people. The keystone of CD pathogenesis is an adaptive immune response orchestrated by the interplay between gluten and MHC class II HLA-DQ2 and DQ8 molecules. Yet, other factors that impair immunoregulatory mechanisms and/or activate the large population of intestinal intraepithelial lymphocytes (IEL) are indispensable for driving tissue damage. Herein, we summarize our current understanding of the mechanisms and consequences of the undesirable immune response initiated by gluten peptides. We show that CD is a model disease to decipher the role of MHC class II molecules in human immunopathology, to analyze the mechanisms that link tolerance to food proteins and autoimmunity, and to investigate how chronic activation of IEL can lead to T cell lymphomagenesis.

Recent advances in the development of new treatments for celiac disease

INTRODUCTION

Celiac disease is a common autoimmune condition induced by dietary gluten in genetically susceptible individuals. So far, the only available treatment for the disorder is a lifelong strict gluten-free diet, because of which small intestinal histological changes recover and symptoms disappear. However, gluten-free dieting is restrictive, and nutritionally less than optimal, and gluten is difficult to avoid.

AREAS COVERED

With improving insight into the pathogenesis of celiac disease, several possible drug targets have been suggested. The new strategies include degradation of gluten intraluminally, reduction of mucosal permeability, inhibition of the transglutaminase 2 enzyme, blocking antigen presentation by HLA-DQ2 or HLA-DQ8, modulation of the immune responses of many cytokines, and vaccination.

EXPERT OPINION

Non-dietary treatment options are warranted either as adjunctive therapy together with dieting or to replace the gluten-free diet. The key question is whether the envisaged novel drug is able to prevent gluten-induced small intestinal mucosal injury as efficiently as a strict gluten-free diet, alleviating symptoms and signs of the disease. Furthermore, the gluten dose that can be detoxified, if at all, must be established. The new drug should also be as safe as dietary treatment. Several novel treatment options are under development.

Immunologic characteristics of intrarenal T cells: trafficking of expanded CD8+ T cell β-chain clonotypes in progressive lupus nephritis

OBJECTIVE

To better define the immunologic character of the T cell infiltrate in lupus nephritis.

METHODS

We performed double immunohistochemical staining and clonotypic T cell receptor (TCR) β-chain sequencing in multiple anatomic regions isolated by laser-capture microdissection from renal biopsy samples.

RESULTS

Systemic lupus erythematosus (SLE) kidneys have a variably patterned and often extensive infiltrate of predominantly clonally expanded T cells of CD4 and CD8 lineages. CD4+ T cells were prominent in nearly two-thirds of SLE biopsy samples and were distributed as broad periglomerular aggregates or intermixed with CD8+ T cells forming periglomerular caps. Sequencing of the TCR from periglomerular regions showed a predominance of clonally expanded T cells. The CD8+ T cells, which were present in all biopsy samples, often adhered to Bowman's capsule and infiltrated the tubular epithelium. They exhibited features that suggest participation in an adaptive immune response: differentiation into CD28(null) memory-effector phenotype, trafficking of the same expanded clonotype to different regions of the kidney and to the peripheral blood, and clonal persistence for years in repeat biopsy samples. CD8+ T cell tubulitis was especially associated with progressive changes.

CONCLUSION

The immunologic characteristics of the infiltrating CD4+ and CD8+ T cells in the lupus kidney indicate that they have the potential to mediate injury, which may be relevant to development of progressive renal failure. Whereas the oligoclonality of the CD4+ T cell infiltrate is consistent with the paradigm of SLE as a class II major histocompatibility complex-associated autoimmune disease, the finding of CD8+ T cell clonality and trafficking implies participation in a distinct systemic adaptive immune response.

Intraepithelial lymphocytes in celiac disease immunopathology

Celiac disease is a T cell-mediated immune disorder induced by dietary gluten that is characterized by the development of an inflammatory anti-gluten CD4 T cell response, anti-gluten antibodies, and autoantibodies against tissue transglutaminase 2 and the activation of intraepithelial lymphocytes (IELs) leading to the destruction of the intestinal epithelium. Intraepithelial lymphocytes represent a heterogeneous population of T cells composed mainly of cytotoxic CD8 T cells residing within the epithelial layer, whose main role is to maintain the integrity of the epithelium by eliminating infected cells and promoting epithelial repair. Dysregulated activation of IELs is a hallmark of CD and is critically involved in epithelial cell destruction and the subsequent development of villous atrophy. In this review, we compare and contrast the phenotype and function of human and mouse small intestinal IELs under physiological conditions. Furthermore, we discuss how conditions of epithelial distress associated with overexpression of IL-15 and non-classical MHC class I molecules induce cytotoxic IELs to become licensed killer cells that upregulate activating NKG2D and CD94/NKG2C natural killer receptors, acquiring lymphokine killer activity. Pathways leading to dysregulated IEL activation could eventually be targeted to prevent villous atrophy and treat patients who respond poorly to gluten-free diet.

Future treatment strategies for celiac disease

INTRODUCTION

Ingestion of dietary gluten in wheat, rye and barley by celiac patients leads to small-bowel mucosal villous atrophy, crypt hyperplasia and massive inflammation, often coupled with clinical symptoms and signs. Currently, the only effective treatment is a strict life-long gluten-free diet excluding all gluten-containing food products. In view of the extremely restricted nature of the diet, new treatment options would clearly be desirable.

AREAS COVERED

The improved understanding of celiac disease pathogenesis has enabled researchers to suggest alternative strategies to treat the disorder. This review covers current approaches toward developing an appropriate drug and discusses the possible problems associated with these approaches.

EXPERT OPINION

Phase II clinical trials are already ongoing to test the efficacy of novel alternative treatments for celiac disease. Before any of the candidates can enter Phase III trials, however, researchers must develop novel reliable non-invasive surrogate markers for intestinal injury and disease activity which also accurately reflect patient-related outcomes. Any novel medication for celiac disease should be as effective and safe as the gluten-free diet, and this constitutes a challenge for drug development. It is thus likely that the first medication entering the market will be supplementary to rather than substitute the gluten-free diet.

The genetics of pemphigus

Pemphigus vulgaris (PV) is an autoimmune blistering disorder with a complex etiology involving an interplay of genetic as well as environmental factors, most of which remain unknown. Despite the identification of several human leukocyte antigen (HLA) alleles as risk factors for disease, no other non-HLA genes have clearly been implicated in disease susceptibility. Newer candidate gene and whole-genome approaches are needed to illuminate the full palate of genetic risk elements in PV. Based on this information, genetic-based tools can be expected to provide a scientific rationale for future clinical decision-making by physicians and facilitate an era of personalized medicine.

The light and dark sides of intestinal intraepithelial lymphocytes

The intraepithelial lymphocytes (IELs) that reside within the epithelium of the intestine form one of the main branches of the immune system. As IELs are located at this critical interface between the core of the body and the outside environment, they must balance protective immunity with an ability to safeguard the integrity of the epithelial barrier: failure to do so would compromise homeostasis of the organism. In this Review, we address how the unique development and functions of intestinal IELs allow them to achieve this balance.

Integration of genetic and immunological insights into a model of celiac disease pathogenesis

Celiac disease (CD) is a gluten-sensitive enteropathy that develops in genetically susceptible individuals by exposure to cereal gluten proteins. This review integrates insights from immunological studies with results of recent genetic genome-wide association studies into a disease model. Genetic data, among others, suggest that viral infections are implicated and that natural killer effector pathways are important in the pathogenesis of CD, but most prominently these data converge with existing immunological findings that CD is primarily a T cell-mediated immune disorder in which CD4(+) T cells that recognize gluten peptides in the context of major histocompatibility class II molecules play a central role. Comparison of genetic pathways as well as genetic susceptibility loci between CD and other autoimmune and inflammatory disorders reveals that CD bears stronger resemblance to T cell-mediated organ-specific autoimmune than to inflammatory diseases. Finally, we present evidence suggesting that the high prevalence of CD in modern societies may be the by-product of past selection for increased immune responses to combat infections in populations in which agriculture and cereals were introduced early on in the post-Neolithic period.

Coeliac disease and gluten sensitivity

Coeliac disease (CD) is a systemic immune-mediated disorder elicited by gluten in genetically susceptible individuals. The common factor for all patients with CD is the presence of a variable combination of gluten-dependent clinical manifestations, specific autoantibodies (anti-tissue transglutaminase/anti-endomysium), HLA-DQ2 and/or DQ8 haplotypes and different degrees of enteropathy. Recently, gluten sensitivity has received much interest, although the limits and possible overlap between gluten sensitivity and CD remain poorly defined. At present, a number of morphological, functional and immunological disorders that are lacking one or more of the key CD criteria (enteropathy, associated HLA haplotypes and presence of anti-transglutaminase two antibodies) but respond to gluten exclusion are included under the umbrella of gluten sensitivity. The possible immunological mechanisms underlying these conditions are discussed. Emphasis is given to specific autoantibodies as markers of the coeliac spectrum and to the hypothesis that innate epithelial stress can exist independently from adaptive intestinal immunity in gluten sensitivity.

[Histopathology of coeliac disease]

Coeliac disease is a common disease, affecting 1% of the population. Clinical manifestations are multiple. The diagnosis requires serologic testing and a duodenal biopsy that shows the characteristic findings of intraepithelial lymphocytosis, crypt hyperplasia and villous atrophy, and a positive response to a gluten-free diet. In most patients, the histological diagnosis is easily established. Pitfalls in the pathological diagnosis include a poorly orientated biopsy specimen, either an inadequate biopsy sampling in patients with patchy villous atrophy and the other causes of villous atrophy. A non-response to the gluten-free diet needs to reassess first, the initial diagnosis, second to be sure of the gluten-free diet adherence, and third, to exclude malignant complications such as refractory celiac disease or enteropathy-associated T-cell lymphoma.

CD8⁺ T cells with an intraepithelial phenotype upregulate cytotoxic function upon influenza infection in human lung

The human lung T cell compartment contains many CD8⁺ T cells specific for respiratory viruses, suggesting that the lung is protected from recurring respiratory infections by a resident T cell pool. The entry site for respiratory viruses is the epithelium, in which a subset of lung CD8⁺ T cells expressing CD103 (αE integrin) resides. Here, we determined the specificity and function of CD103⁺CD8⁺ T cells in protecting human lung against viral infection. Mononuclear cells were isolated from human blood and lung resection samples. Variable numbers of CD103⁺CD8⁺ T cells were retrieved from the lung tissue. Interestingly, expression of CD103 was seen only in lung CD8⁺ T cells specific for influenza but not in those specific for EBV or CMV. CD103⁺ and influenza-reactive cells preferentially expressed NKG2A, an inhibitor of CD8⁺ T cell cytotoxic function. In contrast to CD103⁻CD8⁺ T cells, most CD103⁺CD8⁺ cells did not contain perforin or granzyme B. However, they could quickly upregulate these cytotoxic mediators when exposed to a type I IFN milieu or via contact with their specific antigen. This mechanism may provide a rapid and efficient response to influenza infection, without inducing cytotoxic damage to the delicate epithelial barrier.

Gluten-dependent enteropathy and atypical human leukocyte antigen alleles

The risk for developing celiac disease is associated with the major histocompatibility complex class II human leukocyte antigen DQ2 and DQ8. We retrospectively reviewed the medical records of 127 consecutive cases of adult-onset celiac disease evaluated at a single United States center to determine the distribution of the associated human leukocyte antigen DQA1 and DQB1 alleles. The median patient age of diagnosis was 41 (range, 16-81) years. Ninety-five adults underwent human leukocyte antigen DQ typing. Eighty patients were DQ2 positive, 24 were DQ8 positive, and 11 were DQ2 and DQ8 positive. Four patients carried the uncommon, low-risk haplotype DQ2.2 (DQA1*02 and DQB1*02) without DQA1*05. Two patients did not carry human leukocyte antigen DQ2 or DQ8. All of the patients with atypical human leukocyte antigen DQ responded to a gluten-free diet. Although the majority of patients carry the human leukocyte antigen DQ2 or DQ8, gluten-dependent enteropathy periodically presents in adults with low-risk alleles.

The enteropathy associated with common variable immunodeficiency: the delineated frontiers with celiac disease

OBJECTIVES

The enteropathy associated with common variable immunodeficiency (CVID) is poorly characterized, and its possible relationships with well-defined causes of enteropathy, such as celiac sprue (CS), remain debated. We aimed to assess the clinical and histopathological features of the enteropathy associated with CVID.

METHODS

The medical files of 50 CVID patients with gastrointestinal symptoms were analyzed retrospectively. Histological, phenotypic, and molecular analysis of intestinal endoscopic specimens was centrally performed.

RESULTS

Chronic diarrhea was the most frequent gastrointestinal symptom (92%), and biological evidence of malabsorption was observed in 54% of patients. Chronic gastritis associated or not with pernicious anemia and microscopic colitis were the most frequently observed histopathological features in gastric and colonic mucosa, respectively. Small-bowel biopsies available in 41 patients showed moderate increase in intestinal intraepithelial lymphocytes in 31 patients (75.6%) and villous atrophy in 21 patients (51%). Distinctive features from CS were a profound depletion in plasma cells and follicular lymphoid hyperplasia. Presence of peripheral blood CD8+ hyperlymphocytosis was predictive of intestinal intraepithelial hyperlymphocytosis. Intravenous (i.v.) immunoglobulin (Ig) therapy had no effect on enteropathy-related symptoms. Gluten-free diet improved only two out of 12 patients with villous atrophy, whereas all patients (7/7) responded to steroid therapy.

CONCLUSIONS

Several distinctive features differentiate CVID enteropathy from other causes of enteropathy including CS. Replacement i.v. Ig therapy is insufficient to improve gastrointestinal symptoms. Steroids are effective in reducing inflammation and restoring mucosal architecture.

Flow cytometry of intestinal intraepithelial lymphocytes in celiac disease

This article reviews the multiple uses of flow cytometry in the diagnosis, monitoring and research of celiac disease, the most prevalent chronic autoimmune gastrointestinal disease. The phenotyping of intraepithelial lymphocytes (IELs) is of clinical relevance in the diagnosis of the disease given the characteristic features of elevated CD3+ IELs (αβ and γδ TcR) and the decrease in CD3- IELs. IEL biomarkers are also useful in the assessment of the response to the gluten-free diet and, importantly, in the diagnosis of the severe complications of celiac disease: refractory celiac disease and enteropathy-associated T-cell lymphoma. Novel applications of flow cytometry for the detection of anti-transglutaminase antibodies (a validated biomarker of celiac disease) and of gluten (the triggering antigen of the autoimmune process) are also discussed. The assessment of diagnostic and prognostic biomarkers by flow cytometry in celiac disease is performed routinely in a growing number of centers and it is an example of the versatility of this technique and its applicability to the research and clinical study of solid tissues.

Gliadin peptide P31-43 localises to endocytic vesicles and interferes with their maturation

BACKGROUND

Celiac Disease (CD) is both a frequent disease (1:100) and an interesting model of a disease induced by food. It consists in an immunogenic reaction to wheat gluten and glutenins that has been found to arise in a specific genetic background; however, this reaction is still only partially understood. Activation of innate immunity by gliadin peptides is an important component of the early events of the disease. In particular the so-called "toxic" A-gliadin peptide P31-43 induces several pleiotropic effects including Epidermal Growth Factor Receptor (EGFR)-dependent actin remodelling and proliferation in cultured cell lines and in enterocytes from CD patients. These effects are mediated by delayed EGFR degradation and prolonged EGFR activation in endocytic vesicles. In the present study we investigated the effects of gliadin peptides on the trafficking and maturation of endocytic vesicles.

METHODS/PRINCIPAL FINDINGS

Both P31-43 and the control P57-68 peptide labelled with fluorochromes were found to enter CaCo-2 cells and interact with the endocytic compartment in pulse and chase, time-lapse, experiments. P31-43 was localised to vesicles carrying early endocytic markers at time points when P57-68-carrying vesicles mature into late endosomes. In time-lapse experiments the trafficking of P31-43-labelled vesicles was delayed, regardless of the cargo they were carrying. Furthermore in celiac enterocytes, from cultured duodenal biopsies, P31-43 trafficking is delayed in early endocytic vesicles. A sequence similarity search revealed that P31-43 is strikingly similar to Hrs, a key molecule regulating endocytic maturation. A-gliadin peptide P31-43 interfered with Hrs correct localisation to early endosomes as revealed by western blot and immunofluorescence microscopy.

CONCLUSIONS

P31-43 and P57-68 enter cells by endocytosis. Only P31-43 localises at the endocytic membranes and delays vesicle trafficking by interfering with Hrs-mediated maturation to late endosomes in cells and intestinal biopsies. Consequently, in P31-43-treated cells, Receptor Tyrosine Kinase (RTK) activation is extended. This finding may explain the role played by gliadin peptides in inducing proliferation and other effects in enterocytes from CD biopsies.

Celiac disease: how complicated can it get?

In the small intestine of celiac disease patients, dietary wheat gluten and similar proteins in barley and rye trigger an inflammatory response. While strict adherence to a gluten-free diet induces full recovery in most patients, a small percentage of patients fail to recover. In a subset of these refractory celiac disease patients, an (aberrant) oligoclonal intraepithelial lymphocyte population develops into overt lymphoma. Celiac disease is strongly associated with HLA-DQ2 and/or HLA-DQ8, as both genotypes predispose for disease development. This association can be explained by the fact that gluten peptides can be presented in HLA-DQ2 and HLA-DQ8 molecules on antigen presenting cells. Gluten-specific CD4⁺ T cells in the lamina propria respond to these peptides, and this likely enhances cytotoxicity of intraepithelial lymphocytes against the intestinal epithelium. We propose a threshold model for the development of celiac disease, in which the efficiency of gluten presentation to CD4⁺ T cells determines the likelihood of developing celiac disease and its complications. Key factors that influence the efficiency of gluten presentation include: (1) the level of gluten intake, (2) the enzyme tissue transglutaminase 2 which modifies gluten into high affinity binding peptides for HLA-DQ2 and HLA-DQ8, (3) the HLA-DQ type, as HLA-DQ2 binds a wider range of gluten peptides than HLA-DQ8, (4) the gene dose of HLA-DQ2 and HLA-DQ8, and finally,(5) additional genetic polymorphisms that may influence T cell reactivity. This threshold model might also help to understand the development of refractory celiac disease and lymphoma.

Characterization of IL-17A-producing cells in celiac disease mucosa

Celiac disease (CD) is a gluten-sensitive enteropathy associated with a marked infiltration of the mucosa with IFN-gamma-secreting Th1 cells. Recent studies have shown that a novel subset of T cells characterized by expression of high levels of IL-17A, termed Th17 cells, may be responsible for pathogenic effects previously attributed to Th1 cells. In this study, we characterized the expression of IL-17A-producing cells in CD. By real-time PCR and ELISA, it was shown that expression of IL-17A RNA and protein is more pronounced in active CD biopsy specimens in comparison with inactive CD and normal mucosal biopsy specimens. Flow cytometry confirmed that IL-17A is overproduced in CD mucosa and that CD4(+) and CD4(+)CD8(+) cells were major sources. The majority of IL-17A-producing CD4(+) and CD4(+)CD8(+) cells coexpressed IFN-gamma but not CD161. The addition of a peptic-tryptic digest of gliadin to ex vivo organ cultures of duodenal biopsy specimens taken from inactive CD patients enhanced IL-17A production by both CD4(+) and CD4(+)CD8(+) cells. Because we previously showed that IL-21, a T cell-derived cytokine involved in the control of Th17 cell responses, is overproduced in CD, we next assessed whether IL-17A expression is regulated by IL-21. Blockade of IL-21 activity by a neutralizing IL-21 Ab reduced IL-17A expression in cultures of active CD and peptic-tryptic digest of gliadin-treated CD biopsy specimens. In conclusion, our data show that IL-17A is increased in CD and is produced by cells that also make IFN-gamma.

Tissue-mediated control of immunopathology in coeliac disease

Coeliac disease is an inflammatory disorder with autoimmune features that is characterized by destruction of the intestinal epithelium and remodelling of the intestinal mucosa following the ingestion of dietary gluten. A common feature of coeliac disease and many organ-specific autoimmune diseases is a central role for T cells in causing tissue destruction. In this Review, we discuss the emerging hypothesis that, in coeliac disease, intestinal tissue inflammation--induced either by infectious agents or by gluten--is crucial for activating T cells and eliciting their tissue-destructive effector functions.

Celiac disease: from pathogenesis to novel therapies

Celiac disease has become one of the best-understood HLA-linked disorders. Although it shares many immunologic features with inflammatory bowel disease, celiac disease is uniquely characterized by (1) a defined trigger (gluten proteins from wheat and related cereals), (2) the necessary presence of HLA-DQ2 or HLA-DQ8, and (3) the generation of circulating autoantibodies to the enzyme tissue transglutaminase (TG2). TG2 deamidates certain gluten peptides, increasing their affinity to HLA-DQ2 or HLA-DQ8. This generates a more vigorous CD4(+) T-helper 1 T-cell activation, which can result in intestinal mucosal inflammation, malabsorption, and numerous secondary symptoms and autoimmune diseases. Moreover, gluten elicits innate immune responses that act in concert with the adaptive immunity. Exclusion of gluten from the diet reverses many disease manifestations but is usually not or less efficient in patients with refractory celiac disease or associated autoimmune diseases. Based on the advanced understanding of the pathogenesis of celiac disease, targeted nondietary therapies have been devised, and some of these are already in phase 1 or 2 clinical trials. Examples are modified flours that have been depleted of immunogenic gluten epitopes, degradation of immunodominant gliadin peptides that resist intestinal proteases by exogenous endopeptidases, decrease of intestinal permeability by blockage of the epithelial ZOT receptor, inhibition of intestinal TG2 activity by transglutaminase inhibitors, inhibition of gluten peptide presentation by HLA-DQ2 antagonists, modulation or inhibition of proinflammatory cytokines, and induction of oral tolerance to gluten. These and other experimental therapies will be discussed critically.

A deregulated immune response to gliadin causes a decreased villus height in DQ8 transgenic mice

Celiac disease (CD) is an enteropathy triggered by gluten and mediated by CD4+ T cells. A complete understanding of CD immunopathogenesis has been hindered due to the lack of adequate in vivo models. Here, we explored the effect of the inhibition of COX by indomethacin in wheat gliadin-sensitized transgenic mice expressing the HLA-DQ8 heterodimer, a molecule associated with CD. Treated mice showed a gliadin-specific immune response with a significant reduction of villus height, not linked to crypt hyperplasia and to expansion of intraepithelial T cells. Notably, treated mice showed increased numbers of CD25+ and apoptotic cells in the lamina propria, whereas high basal levels of IFN-gamma secretion, along with a reduced gliadin-specific IL-2 expression were detected in MLN. Biochemical assessment of the lesion revealed increased mRNA of Lamb3 and Adamts2, encoding for ECM proteins, and enhanced activities of metalloproteinases MMP1, 2 and 7. We conclude that an intestinal sensitivity to gliadin, in connection with COX inhibition, caused a decreased villus height in DQ8 tg mice. The lesion was induced by a deregulated mucosal cell immunity to gliadin, thus triggering activation of a specific ECM protein pathway responsible for lamina propria remodeling.

Involvement of interleukin-15 and interleukin-21, two γ-chain-related cytokines, in celiac disease

Celiac disease (CD), an enteropathy caused by dietary gluten in genetically susceptible individuals, is histologically characterized by villous atrophy, crypt cell hyperplasia, and increased number of intra-epithelial lymphocytes. The nature of CD pathogenesis remains unclear, but recent evidence indicates that both innate and adaptive immune responses are necessary for the phenotypic expression and pathologic changes characteristic of CD. Extensive studies of molecules produced by immune cells in the gut of CD patients have led to identification of two cytokines, namely interleukin (IL)-15 and IL-21, which are thought to play a major role in orchestrating the mucosal inflammatory response in CD. Here we review the current knowledge of the expression and function of IL-15 and IL-21 in CD.

Stratifying risk for celiac disease in a large at-risk United States population by using HLA alleles

BACKGROUND & AIMS

Susceptibility to celiac disease (CD) is related to HLA-DQ2 and DQ8 alleles and the heterodimers they encode. The objective of this study was to stratify risk for CD on the basis of HLA-DQ genotype.

METHODS

DNA from 10,191 subjects who are at risk for CD was analyzed for HLA-DQ haplotypes. Individuals with CD were identified as those who tested positive for anti-endomysial immunoglobulin A (EMA+) in an immunofluorescence assay.

RESULTS

Samples homozygous for DQ2.5 (HLA-DQA1 05-DQB1 02) or DQ2.2/DQ2.5 (HLA-DQA1 05-DQB1 02 and HLA-DQA1 0201-DQB1 02) comprised 5.38% of the total; 28.28% of these were EMA+ (95% confidence interval [CI], 24.55-32.26). Of the samples that were DQ2.5 heterozygous (HLA-DQA1 05-DQB1 02); 9.09% were EMA+ (95% CI, 7.82-10.51). Among samples in which HLA-DQ8 (HLA-DQA1 03-DQB1 0302) was detected, 8.42% of homozygotes (95% CI, 3.71-15.92) and 2.11% of heterozygotes (95% CI, 1.43-3.00) were EMA+. Samples with DQ2.2/DQ8 or DQ2.5/DQ8 comprised 5.08% of the total, and 11.78% of these were EMA+ (95% CI, 9.13-14.87). HLA-DQ2 and HLA-DQ8 were absent in 4283 samples (42.03% of the total); 0.16% of these samples were EMA+ (95% CI, 0.07-0.34).

CONCLUSIONS

High-resolution, sequence-specific oligonucleotide probe typing with 35 DQA1-specific and 37 DQB1-specific probes of DNA from more than 10,000 subjects was used to stratify risk of CD in an at-risk U.S. population. DQ2 homozygosity (DQ2.5/DQ2.2+2.5) increased risk for CD, estimated by the rate of EMA positivity, compared with the entire sample population and other DQ genotypes. These data suggest a quantitative relationship between the type/proportion of DQ heterodimers and the risk of CD and identify potential immunotherapeutic targets.

Interleukin 21 up-regulates perforin-mediated cytotoxic activity of human intra-epithelial lymphocytes

Human intra-epithelial lymphocytes (IELs) are predominantly T-cell receptor-alphabeta(+) (TCR-alphabeta(+)) CD8(+) CD45RO(+) memory T cells located between intestinal epithelial cells. They respond to a greater extent to stimulation with interleukin (IL)-15 than to CD3/TCR triggering, suggesting that they react to the cytokine milieu in their local environment rather than to cognate antigen. A newly described member of the gammac cytokine family, IL-21, has potent antitumor effects. As IELs resemble lymphocytes infiltrating neoplastic lesions, their response to IL-21 may be relevant in vivo. Here, IL-21 was shown to increase perforin-mediated cytotoxicity and serine esterase release by IELs. This IL-21-mediated up-regulation occurred without changes in IEL survival or cell division. Interestingly, the effects of IL-21 occurred without increased phosphorylation of signal transducer and activator of transcription (STAT)1, STAT3, STAT4, STAT5, extracellular signal-regulated kinase (ERK), or p38. IL-21 had no effect on Fas ligand (FL)- or tumour necrosis factor-alpha (TNF-alpha)-mediated cytotoxicity, but it down-regulated IL-15-stimulated expression of CD25 and CD94, indicating that it has both positive and negative actions. This functional profile is unique to human IELs, emphasizing that they are a distinct compartment of lymphocytes and that IL-21 may promote their role in tumour immunosurveillance.

Coeliac disease

Coeliac disease is a chronic inflammatory disorder of the small bowel induced in genetically susceptible people by the irritant gluten and possibly other environmental cofactors. The disorder is characterised by a diverse clinical heterogeneity that ranges from asymptomatic to severely symptomatic, and it manifests with frank malabsorption, an increased morbidity attributable to the frequent association with autoimmune disorders and increased mortality resulting from the emergence of T-cell clonal proliferations that predispose the patient to enteropathy-type T-cell lymphoma. Our understanding of the molecular basis for this disorder has improved and enabled the identification of targets for new therapies, although a strict gluten-free diet remains the mainstay of safe and effective treatment. In this Seminar we critically reassess the clinical and diagnostic aspects of this disease and new perspectives in its pathogenesis and treatment.

Cytosolic PLA2 is required for CTL-mediated immunopathology of celiac disease via NKG2D and IL-15

IL-15 and NKG2D promote autoimmunity and celiac disease by arming cytotoxic T lymphocytes (CTLs) to cause tissue destruction. However, the downstream signaling events underlying these functional properties remain unclear. Here, we identify cytosolic phospholipase A₂ (cPLA₂) as a central molecule in NKG2D-mediated cytolysis in CTLs. Furthermore, we report that NKG2D induces, upon recognition of MIC⁺ target cells, the release of arachidonic acid (AA) by CTLs to promote tissue inflammation in association with target killing. Interestingly, IL-15, which licenses NKG2D-mediated lymphokine killer activity in CTLs, cooperates with NKG2D to induce cPLA₂ activation and AA release. Finally, cPLA₂ activation in intraepithelial CTLs of celiac patients provides an in vivo pathophysiological dimension to cPLA₂ activation in CTLs. These results reveal an unrecognized link between NKG2D and tissue inflammation, which may underlie the emerging role of NKG2D in various immunopathological conditions and define new therapeutic targets.

Innate T cell responses in human gut

One arm of the gut-associated immune system is represented by a vast collection of T lymphocytes which participate in the subtle interplay between innate and adaptive immune mechanisms and maintain homeostasis at the main body external surface. Mounting data are providing exciting new insight into the innate-like mechanisms which enable intestinal T cells to rapidly sense local conditions and which broaden the spectrum of their functions and regulation at this strategic location. Herein we discuss how innate-like T cell recognition by unconventional T cell subsets and expression of innate NK receptors might modulate immune T cell responses in the human normal or diseased intestine.

Aberrant extrathymic T cell receptor gene rearrangement in the small intestinal mucosa: a risk factor for coeliac disease?

BACKGROUND

Coeliac disease is a small intestine enteropathy caused by permanent intolerance to wheat gluten. Gluten intake by patients with coeliac disease provokes a strong reaction by intestinal intraepithelial lymphocytes (IELs), which normalises on a gluten-free diet.

AIM

To investigate whether impaired extrathymic T cell maturation and/or secondary T cell receptor (TCR) gene recombination in IELs are features of coeliac disease which could contribute to the failure of establishing tolerance to gluten.

METHODS

Expression levels of the four splice-forms of recombination activating gene-1 (RAG1) mRNA and preT alpha-chain (preTalpha) mRNA were determined in IEL-subsets of children with coeliac disease and controls. Frequencies of RAG1 expressing IELs were determined by immunomorphometry.

RESULTS

In controls, the RAG1-1A/2 splice-form selectively expressed outside the thymus, was dominant and expressed in both mature (TCR(+)) and immature (CD2(+)CD7(+)TCR(-)) IELs ( approximately 8 mRNA copies/18S rRNA U). PreTalpha was expressed almost exclusively in CD2(+)CD7(+)TCR(-) IELs ( approximately 40 mRNA copies/18S rRNA U). By contrast, RAG1 and preTalpha mRNA levels were low in patients with coeliac disease compared to controls, both with active disease and with inactive, symptom-free disease on a gluten-free diet (p values <0.01 for mature and <0.05 for immature IELs). Similarly, the frequencies of RAG1+ IELs were significantly lower in patients with coeliac disease compared to controls (p<0.001).

CONCLUSIONS

Patients with coeliac disease appear to have an impaired capacity for extrathymic TCR gene rearrangement. This is an inherent feature, which probably plays a pivotal role in the failure to efficiently downregulate the T cell response to gluten.