Diagnosis and treatment of celiac disease

The understanding of the pathogenesis of celiac disease has made huge advances in recent years. The disease is caused by an inappropriate immune response to dietary gluten proteins. This immune response is controlled by CD4⁺ T cells in the lamina propria that recognize gluten peptides in the context of disease predisposing HLA-DQ2 and HLA-DQ8 molecules.¹, ² These T cells are specific for proline- and glutamine-rich gluten peptides that are resistant to proteolysis and that have been become deamidated by the enzyme transglutaminase 2 (TG2). Strikingly, celiac disease patients produce antibodies to this same enzyme when exposed to dietary gluten. Here we discuss how the new insight in the pathogenesis has lead to development of new diagnostics and nourished research into novel treatments.     

Surface expression of transglutaminase 2 by dendritic cells and its potential role for uptake and presentation of gluten peptides to T cells

Celiac disease is a chronic small intestinal inflammation driven by gluten-reactive T cells of the intestinal mucosa. These T cells are HLA-DQ2 or -DQ8 restricted, and predominantly recognize gluten peptides that are deamidated by the enzyme transglutaminase 2 (TG2). Our recent results strongly suggest that duodenal CD11c(+) dendritic cells (DC) are directly involved in T cell activation in the celiac lesion. The aim of this study was to investigate whether surface-associated TG2 could be involved in receptor-mediated endocytosis of gluten peptides, a process that may contribute to the preferential recognition of deamidated peptides. We found that both monocyte-derived DC and local CD11c(+) DC in the duodenal mucosa expressed cell surface-associated TG2. As phenotypic characterization of CD11c(+) DC in the celiac lesion suggests that these cells may be derived from circulating monocytes, we used monocyte-derived DC in functional in vitro studies. Using a functional T cell assay, we obtained evidence that cell surface-associated TG2 is endocytosed by monocyte-derived DC. However, we were unable to obtain evidence for a role of surface TG2 in the loading and subsequent generation of deamidated gluten peptides in these cells.     

Responsive population dynamics and wide seeding into the duodenal lamina propria of transglutaminase-2-specific plasma cells in celiac disease

A hallmark of celiac disease is autoantibodies to transglutaminase 2 (TG2). By visualizing TG2-specific antibodies by antigen staining of affected gut tissue, we identified TG2-specific plasma cells in the lamina propria as well as antibodies in the subepithelial layer, inside the epithelium, and at the brush border. The frequency of TG2-specific plasma cells were found not to correlate with serum antibody titers, suggesting that antibody production at other sites may contribute to serum antibody levels. Upon commencement of a gluten-free diet, the frequency of TG2-specific plasma cells in the lesion dropped dramatically within 6 months, yet some cells remained. The frequency of TG2-specific plasma cells in the celiac lesion is thus dynamically regulated in response to gluten exposure. Laser microdissection of plasma cell patches, followed by antibody gene sequencing, demonstrated that clonal cells were seeded in distinct areas of the mucosa. This was confirmed by immunoglobulin heavy chain repertoire analysis of plasma cells isolated from individual biopsies of two untreated patients, both for TG2-specific and non-TG2-specific cells. Our results shed new light on the processes underlying the B-cell response in celiac disease, and the approach of staining for antigen-specific antibodies should be applicable to other antibody-mediated diseases.     

Galactosylation of Serum IgA1 <Emphasis Type="Italic">O</Emphasis>-Glycans in Celiac Disease

In celiac disease, gluten ingestion provokes small-bowel mucosal injury and production of IgA autoantibodies against transglutaminase 2 (TG2). It has been suggested that in celiac patients IgA could mediate the transepithelial passage of gluten peptides in a mechanism involving the transferrin receptor. As IgA1 with galactose-deficient O-linked glycans has elevated affinity for the transferrin receptor, we assessed whether total serum IgA1 and IgA1 anti-TG2 autoantibodies in celiac patients are aberrantly glycosylated. We report that males with celiac disease have higher total serum levels of galactose-deficient IgA1 than non-celiac males. Furthermore, O-glycans of the disease-specific TG2 IgA1 autoantibodies in celiac patients exhibited elevated galactose deficiency. A gluten-free diet had no effect on the total serum levels of galactose-deficient IgA1, whereas the amount of galactose-deficient anti-TG2 IgA1 decreased. Thus, the undergalactosylated IgA1 molecules are not pathognomonic for celiac disease, but galactose deficiency in IgA1 could be an aggravating factor.     

Antibodies in celiac disease: implications beyond diagnostics

Celiac disease is a multisystemic dietary, gluten-induced autoimmune disorder characterized by the presence of transglutaminase (TG) 2 serum autoantibodies. Distinct autoantibodies targeting members of the TG family (TG2, TG3 and TG6) are found deposited in small-bowel mucosa and in extraintestinal tissues affected by the disease. Serum autoantibodies against other self-antigens also emerge in untreated celiac disease patients. Although villous atrophy and crypt hyperplasia in small-bowel biopsy samples are still the gold standards in diagnostics, celiac disease-specific antibodies are widely used as diagnostic aids. Gluten-induced small-bowel mucosal T-cell response is the cornerstone in the pathogenesis of the disorder, but humoral immunity may also play a central role. This review article is focused on the autoantibodies that occur in the context of celiac disease. The article summarizes the diagnostic utility of different celiac-related antibodies and discusses their roles in the pathogenesis of the disease.     

A unifying hypothesis on the development of type 1 diabetes and celiac disease: gluten consumption may be a shared causative factor

This paper presents a hypothesis of the aetiology of the increasing incidence of type 1 diabetes (T1D). This together with the global increased incidence of celiac disease (CD) and that these increases cannot be explained by genetic factors suggest a common environmental factor for these two diseases. Even though enterovirus (EV) infections are believed to trigger T1D and gluten is the trigger of CD, the increasing intake of gluten containing products all over the world could be the trigger for both diseases directly and indirectly. It has been shown that the duration of exposure to gluten is related to the prevalence of T1D. It has also been shown that T1D patients at onset have an inflammatory reaction in the gut. Hence, early diagnose of CD followed by elimination of dietary gluten will lead to a decreased incidence of T1D.     

Experiences with animal models of dermatitis herpetiformis: a review

Dermatitis herpetiformis (DH) is caused by the consumption of gluten, which is also the trigger for celiac disease. DH is currently considered to be the skin manifestation of celiac disease, as both diseases have some degree of gluten-sensitive enteropathy. The human leukocyte antigens class II genes, DQ2 and DQ8, are tightly associated with both diseases, and there is an increased level of anti-gliadin antibodies in both diseases. Animal models of gluten sensitivity have been used to better understand the pathogenesis of both diseases. This paper describes these different models and discusses how certain elements of these models contribute to the development of DH.     

Experiences with animal models of dermatitis herpetiformis: A review

Dermatitis herpetiformis (DH) is caused by the consumption of gluten, which is also the trigger for celiac disease. DH is currently considered to be the skin manifestation of celiac disease, as both diseases have some degree of gluten-sensitive enteropathy. The human leukocyte antigens class II genes, DQ2 and DQ8, are tightly associated with both diseases, and there is an increased level of anti-gliadin antibodies in both diseases. Animal models of gluten sensitivity have been used to better understand the pathogenesis of both diseases. This paper describes these different models and discusses how certain elements of these models contribute to the development of DH.     

Anti-idiotypic response in mice expressing human autoantibodies

Celiac disease is an autoimmune illness characterized by intestinal mucosal injury and malabsorption precipitated by dietary exposure to gluten of some cereals. The immune response is based on both cellular and humoral components, although the former seem to be more important in the pathogenesis. The autoantibody response is directed at the enzyme tissue transglutaminase, tTG or TG2, which possibly play a role in the onset of the disease. In this study we sought to develop an animal model in which to analyze the immunological regulation and significance of anti-TG2 antibodies, by expressing specific human single-chain antibody fragments in mice using adeno-associated virus vectors. Upon vector injection in the skeletal muscles, high and persistent systemic levels of anti-TG2 antibodies were obtained. Mice injected with vectors encoding antibodies also recognizing rodent TG2, also developed a strong anti-idiotypic response. This finding raises the question of whether an anti-idiotypic response to anti-TG2 antibodies is a factor associated with celiac disease.     

Are xenogeneic anti-tissue transglutaminase antibodies the holy grail for celiac patients?

Celiac disease is an immune mediated disorder, the only one with a well-established origin, resulting from a permanent gluten intolerance. Although a gluten-free diet is currently the "safe" and appropriate therapy for celiac disease, this is not always an easy and simple option as "harmful" gluten may contaminate food during the processing and preparation phases. There are also further social pressures, which might be more pressing for young celiac patients, in following a strict gluten-free diet. Therefore, a new therapeutic approaches are sought which would permit celiacs to "peacefully" coexist with gluten. Presently, the most promising looks search for genetically modified wheat lacking toxic gluten peptides and the use of oral endopeptidases in attempt to curb gluten toxicity. Recently discovered role of anti-tissue transglutaminase antibodies in celiac pathogenesis has brought a prospect for a new hypothetical therapeutic approach, an oral immunization of celiacs with xenogeneic anti-tissue transglutaminase antibodies.     

The function of tissue transglutaminase in celiac disease

Celiac disease is a chronic small bowel disorder caused by an abnormal immune response to an array of epitopes of wheat gluten and related proteins of rye and barley in genetically susceptible individuals who express the HLA-DQ2/-DQ8 haplotype. Gluten peptides are efficiently presented by celiac disease-specific HLA-DQ2- and HLA-DQ8-positive antigen presenting cells to CD4(+) T-cells that, once activated, drive a T helper cell type 1 response leading to the development of the typical celiac lesion-villous atrophy, crypt hyperplasia and intraepithelial and lamina propria infiltration of inflammatory cells. Tissue transglutaminase (tTG) is a calcium dependent ubiquitous enzyme which catalyses posttranslational modification of proteins and is released from cells during inflammation. tTG is suggested to exert at least two crucial roles in celiac disease: as a deamidating enzyme, that can enhance the immunostimulatory effect of gluten, and as a target autoantigen in the immune response. Since glutamine-rich gliadin peptides are excellent substrates for tTG, and the resulting deamidated and thus negatively charged peptides have much higher affinity for the HLA-DQ2 and HLA-DQ8 molecules, the action of tTG is believed to be a key step in the pathogenesis of celiac disease. This review is focused on the function of tTG in celiac disease, although it also deals with novel advances in tTG-based therapies.     

The gluten syndrome: A neurological disease

Hypothesis: Gluten causes symptoms, in both celiac disease and non-celiac gluten-sensitivity, by its adverse actions on the nervous system.Many celiac patients experience neurological symptoms, frequently associated with malfunction of the autonomic nervous system. These neurological symptoms can present in celiac patients who are well nourished. The crucial point, however, is that gluten-sensitivity can also be associated with neurological symptoms in patients who do not have any mucosal gut damage (that is, without celiac disease).Gluten can cause neurological harm through a combination of cross reacting antibodies, immune complex disease and direct toxicity. These nervous system affects include: dysregulation of the autonomic nervous system, cerebella ataxia, hypotonia, developmental delay, learning disorders, depression, migraine, and headache.If gluten is the putative harmful agent, then there is no requirement to invoke gut damage and nutritional deficiency to explain the myriad of the symptoms experienced by sufferers of celiac disease and gluten-sensitivity. This is called “The Gluten Syndrome”.     

Is celiac disease an autoimmune disorder?

Celiac disease, which results from an immune reaction to ingested cereal gluten proteins, has several autoimmune features. In particular, celiac disease patients produce highly disease specific IgA and IgG autoantibodies to tissue transglutaminase when they are on a gluten-containing diet, and they have small intestinal intraepithelial lymphocytes which can mediate direct cytotoxicity of enterocytes expressing MIC molecules in an antigen non-specific manner. Similar to typical autoimmune disorders, celiac disease has a multifactorial aetiology with complex genetics, and several autoimmune diseases are commonly presented by patients with celiac disease. Much has been learned about the immunology of celiac disease in recent years, and there is overwhelming evidence that the immune response to gluten is central to the pathogenesis. In light of this, the many autoimmune phenomena associated with celiac disease are thought-provoking, and they challenge us to rethink the boundaries between autoimmunity and immunopathology.     

Gene, gut and schizophrenia: the meeting point for the gene-environment interaction in developing schizophrenia

Both schizophrenia and celiac disease involve a genetic component. Several lines of evidence have shown a genetic relationship between these two conditions. Celiac disease is characterized by damage to the microscopic finger-like projections called villi, which line the small intestine and play a significant role in digestion, due to an inflammatory condition caused by a reaction to wheat gluten or related rye and barley proteins. Celiac disease represents not only malabsorption leading to a poor nutritional condition but also an alteration of gut permeability. Individuals with a history of a childhood celiac condition have a raised risk of developing schizophrenia. Psychotic symptoms often occur in adult celiac disease. It can be hypothesized that apart from malnutrition, the meeting point for the gene-environment interaction may be an alteration in gut permeability, in which the gut may lose its capacity to block exogenous psychosis-causing substances that may enter the body thus causing the development of schizophrenia and other mental conditions. To support this hypothesis, the conditional test was conducted to look at the combined effect of the CLDN5 gene involved in forming permeability barriers and the DQB1 gene that has been found to be associated with celiac disease. The results demonstrate that these two genes possibly work together in conferring a susceptibility to schizophrenia.     

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.     

The adaptive immune response in celiac disease

Compared to other human leukocyte antigen (HLA)-associated diseases such as type 1 diabetes, multiple sclerosis, and rheumatoid arthritis, fundamental aspects of the pathogenesis in celiac disease are relatively well understood. This is mostly because the causative antigen in celiac disease-cereal gluten proteins-is known and the culprit HLA molecules are well defined. This has facilitated the dissection of the disease-relevant CD4+ T cells interacting with the disease-associated HLA molecules. In addition, celiac disease has distinct antibody responses to gluten and the autoantigen transglutaminase 2, which give strong handles to understand all sides of the adaptive immune response leading to disease. Here we review recent developments in the understanding of the role of T cells, B cells, and antigen-presenting cells in the pathogenic immune response of this instructive disorder.     

Analysis of the binding of gluten T-cell epitopes to various human leukocyte antigen class II molecules

Celiac disease is a prevalent disorder of the small intestine that is caused by an inflammatory reaction to dietary gluten in genetically susceptible individuals. More than 90% of patients express the HLA-DQ2 molecule, whereas DQ8 is carried by most of the remaining patients. DQ2- and DQ8-mediated presentation of gluten peptides to CD4+ T cells is a key event in the pathogenesis of the disease. The association of celiac disease with these human leukocyte antigen (HLA) molecules is explained by a preferential binding of gluten peptides to these HLA molecules, although the actual data on this in the literature are scarce. The objective of this study was to test this hypothesis. A panel of peptides representing DQ2-restricted gluten T-cell epitopes was tested for binding to various HLA class II molecules using various experimental approaches. The results demonstrate that the gluten T-cell epitopes mainly bind to the DQ2 molecule.     

Refractory celiac disease: from bench to bedside

Refractory celiac disease is defined by the persistence of symptoms of malnutrition and intestinal villous atrophy for more than 6-12?months despite strict gluten-free diet in celiac patients. Diagnosis of this rare condition is made after excluding other causes of chronic small intestinal inflammation and villous atrophy and inadvertent intake of gluten. Over the past 15?years, multidisciplinary approaches have been developed to assess the mechanism of resistance to the diet, and two distinct entities have been delineated. Type II refractory celiac disease (RCD) can be defined as a low-grade intraepithelial lymphoma. RCD II is characterised by a massive accumulation of abnormal IEL that display an aberrant hybrid NK/T cell phenotype, contain clonal T cell rearrangement(s) and can mediate a cytolytic attack of the gut epithelium. This condition has a severe prognosis, largely due to the frequent transformation of RCDII IEL into overt aggressive enteropathy-type-associated T cell lymphoma. In contrast, in type I RCD, intestinal lymphocytes have a normal phenotype, and this generally milder condition remains often difficult to differentiate from uncomplicated CD except for the resistance to gluten-free diet (GFD). Several mechanisms may underlie resistance to gluten. Herein, we review the distinctive characteristics of RCD I and RCD II, the mechanisms underlying the onset of resistance to GFD, the risk of developing high grade lymphoma and possible clues to improve their treatment.     

Important Lessons Derived from Animal Models of Celiac Disease

Several animal models have been recently developed to recapitulate various components of the complex process that is celiac disease. In addition to the increasing diversity of murine models there are now monkey models of celiac disease. Mouse strains and protocols have been developed that are now just beginning to address the complex interactions among the innate and adaptive immune responses to gluten, as well as gluten-dependent autoimmunity in celiac disease. The most important conclusion that these models have provided us with so far is that while all three components (innate gluten sensitivity, adaptive gluten sensitivity, and autoimmunity) are independent phenomena, all are necessary for celiac disease to develop.     

Important lessons derived from animal models of celiac disease

Several animal models have been recently developed to recapitulate various components of the complex process that is celiac disease. In addition to the increasing diversity of murine models there are now monkey models of celiac disease. Mouse strains and protocols have been developed that are now just beginning to address the complex interactions among the innate and adaptive immune responses to gluten, as well as gluten-dependent autoimmunity in celiac disease. The most important conclusion that these models have provided us with so far is that while all three components (innate gluten sensitivity, adaptive gluten sensitivity, and autoimmunity) are independent phenomena, all are necessary for celiac disease to develop.