Transglutaminase 2 and Transglutaminase 2 Autoantibodies in Celiac Disease: a Review.

Celiac disease is a common inflammatory disorder with a prevalence of 1-2 % in which a distinct dietary wheat, rye, and barley component, gluten, induces small-bowel mucosal villous atrophy, crypt hyperplasia, and inflammation. The small-bowel mucosal damage can be reversed by a strict lifelong gluten-free diet, which is currently the only effective treatment for the condition. A key player in the pathogenetic process leading to the enteropathy is played by a protein called transglutaminase 2 (TG2), which is able to enzymatically modify gluten-derived gliadin peptides. The TG2-catalyzed deamidation of the gliadin peptides results in their increased binding affinity to the disease-predisposing human leukocyte antigen (HLA) DQ2 and DQ8 molecules, thus enabling a strong immune response to be launched. Blocking the enzymatic activity of TG2 has thus been suggested as a suitable novel pharmacological approach to treat celiac disease. By virtue of its transamidation capacity, TG2 is also able to cross-link gliadin peptides to itself, this resulting in the generation of TG2-gliadin peptide complexes whose presence might provide an explanation for the generation of the TG2 autoantibodies characteristic of celiac disease. Due to their excellent specificity for the disorder, the TG2-targeted autoantibodies are widely used in the diagnostics as a first-line test to select patients for gastrointestinal endoscopy. More recently, it has come to be appreciated that these autoantibodies and also the TG2-specific B cells might play an active role in the disease pathogenesis. In this review, we assess the role of TG2, TG2-specific B cells, and autoantibodies in celiac disease.

Ex vivo Culture of Duodenal Biopsies from Patients with Dermatitis Herpetiformis Indicates that Transglutaminase 3 Antibody Production Occurs in the Gut.

Coeliac disease and dermatitis herpetiformis (DH) are characterized by autoantibodies targeting transglutaminase (TG)2 and TG3, respectively. Previous studies show that TG2 antibodies are produced in the gut and can be assessed in organ culture of small-intestinal biopsies from patients with coeliac disease. Thus far, no studies have investigated TG3 antibodies in organ culture of biopsies from patients with DH, or exploited the method in DH. The aim of this study was to investigate TG3 and TG2 antibody responses in serum and small-intestinal biopsies from patients with DH with active disease, and from those in remission. The majority of patients with DH were negative for both serum and organ culture medium TG2-targeting antibodies. Surprisingly, patients with active DH secreted TG3 antibodies into the culture medium despite seronegativity. In patients secreting high levels of TG3 antibodies into the culture medium, we also detected TG3-antibody-positive cells in the small-intestinal mucosa. These findings suggest that TG3 antibodies can be investigated in the organ culture system and that their secretion occurs in the small intestine, especially in active DH.

Transglutaminase as a therapeutic target for celiac disease.

INTRODUCTION: The only current treatment for celiac disease is a strict gluten-free diet. The ubiquitous presence of gluten in groceries, however, makes the diet burdensome and difficult to maintain, and alternative treatment options are thus needed. Here, the important role of transglutaminase 2 (TG2) in the pathogenesis of celiac disease makes it an attractive target for drug development. AREAS COVERED: The present paper gives an overview of TG2 and addresses its significance in the pathogenesis of celiac disease. Moreover, the article summarizes preclinical studies performed with TG2 inhibitors and scrutinizes issues related to this therapeutic approach. EXPERT OPINION: Activation of TG2 in the intestinal mucosa is central in celiac disease pathogenesis and researchers have therefore suggested TG2 inhibitors as a potential therapeutic approach. However, a prerequisite for such a drug is that it should be specific for TG2 and not affect the activity of other members of the transglutaminase family. Such compounds have already been introduced and tested in vitro, but a major obstacle to further development is the lack of a well-defined animal model for celiac disease. Nonetheless, with encouraging results in preclinical studies clinical trials with TG2 inhibitors are eagerly awaited.

Impaired epithelial integrity in the duodenal mucosa in early stages of celiac disease.

The small-bowel mucosal damage characteristic of celiac disease (CD) develops from normal villus morphology to inflammation and finally to villus atrophy with crypt hyperplasia. Patients with early stage CD may already suffer from abdominal symptoms before the development of villus atrophy. Although epithelial junctional integrity is compromised in overt disease, the appearance of such changes in early phases of the disorder is not known. We investigated whether alterations in epithelial junction protein expression occur already in early stage CD with normal mucosal morphology, and whether this correlates with inflammation indicators and clinical symptoms. The study involved 10 patients with early stage and 10 patients with overt villus atrophy that were followed yearly according to the study protocol. As controls, 20 nonceliac subjects were included. The expression of junction proteins (occludin, claudin 3, zonula occludens 1, and E-cadherin) was studied in small-intestinal biopsies using immunohistochemistry and Western blot. The correlation between junctional proteins and mucosal morphology, autoantibodies, the number of intraepithelial lymphocytes (IELs), and gastrointestinal symptoms was assessed. The expression of all junction proteins was already decreased in early stage CD when compared with nonceliac controls (P < 0.05). Junction protein expression correlated positively with mucosal villus morphology and negatively with the number of IELs, the intensity of small-intestinal autoantibody deposits, and serum autoantibodies. The expression of claudin 3 showed a negative correlation with diarrheal score (R = -0.314, P = 0.04). These findings show that the mucosal epithelial integrity is disrupted already in early stage CD before the disorder progresses to full-blown enteropathy.

Are transglutaminase 2 inhibitors able to reduce gliadin-induced toxicity related to celiac disease? A proof-of-concept study.

PURPOSE: Celiac disease is an autoimmune-mediated enteropathy characterized by adaptive and innate immune responses to dietary gluten in wheat, rye and barley in genetically susceptible individuals. Gluten-derived gliadin peptides are deamidated by transglutaminase 2 (TG2), leading to an immune response in the small-intestinal mucosa. TG2 inhibitors have therefore been suggested as putative drugs for celiac disease. In this proof-of-concept study we investigated whether two TG2 inhibitors, cell-impermeable R281 and cell-permeable R283, can prevent the toxic effects of gliadin in vitro and ex vivo. METHODS: Intestinal epithelial Caco-2 cells were treated with peptic-tryptic-digested gliadin (PT-gliadin) with or without TG2 inhibitors and thereafter direct toxic effects (transepithelial resistance, cytoskeletal rearrangement, junction protein expression and phoshorylation of extracellular-signal-regulated kinase 1/2) were determined. In an organ culture of celiac-patient-derived small-intestinal biopsies we measured secretion of TG2-autoantibodies into the culture medium and the densities of CD25- and interleukin (IL) 15-positive cells, forkhead box P3 (FOXP3)-positive regulatory T cells (Tregs) and Ki-67-positive proliferating crypt cells. RESULTS: Both TG2 inhibitors evinced protective effects against gliadin-induced detrimental effects in Caco-2 cells but the cell-impermeable R281 seemed slightly more potent. In addition, TG2 inhibitor R281 modified the gluten-induced increase in CD25- and IL15-positive cells, Tregs and crypt cell proliferation, but had no effect on antibody secretion in celiac-patient-derived biopsies. CONCLUSIONS: Our results suggest that TG2 inhibitors are able to reduce certain gliadin-induced effects related to responses in vitro and ex vivo.

RhoB is associated with the anti-angiogenic effects of celiac patient transglutaminase 2-targeted autoantibodies.

Celiac patient-derived anti-transglutaminase 2 (TG2) antibodies disturb several steps in angiogenesis, but the detailed molecular basis is not known. Therefore, we here analyzed by microarray technology the expression of a set of genes related to angiogenesis and endothelial cell biology in order to identify factors that could explain our previous data related to vascular biology in the context of celiac disease. To this end, in vitro models using human umbilical vein endothelial cells (HUVECs) or in vivo models of angiogenesis were used. A total of 116 genes were analyzed after treatment with celiac patient autoantibodies against TG2. Compared to treatment with control IgA celiac patient, total IgA induced a consistent expression change of 10 genes, the up-regulation of four and down-regulation of six. Of these genes the up-regulated RhoB was selected for further studies. RhoB expression was found to be up-regulated at both messenger RNA and protein level in response to celiac patient total IgA as well as anti-TG2-specific antibody derived from a celiac patient. Interestingly, down-regulation of RhoB by specific small interfering RNA treatment in endothelial cells could rescue the deranged endothelial length and tubule formation caused by celiac disease autoantibodies. RhoB function is controlled by its post-translational modification by farnesylation. This modification of RhoB required for its correct function can be prevented by the cholesterol lowering drug simvastatin, which was also able to abolish the anti-angiogenic effects of celiac anti-TG2 autoantibodies. Taken together, our results would suggest that RhoB plays a key role in the response of endothelial cells to celiac disease-specific anti-TG2 autoantibodies.

In vitro models for gluten toxicity: relevance for celiac disease pathogenesis and development of novel treatment options.

In genetically predisposed individuals, dietary gluten in wheat, rye and barley triggers celiac disease, a systemic autoimmune disorder hallmarked by an extensive small-bowel mucosal immune response. The current conception of celiac disease pathogenesis is that it involves components of both innate and adaptive immunity whose activation typically leads to small-bowel villous atrophy with crypt hyperplasia. Currently, the only effective treatment for celiac disease is a strict lifelong gluten-free diet excluding all wheat-, rye- and barley-containing food products. During the diet, the clinical symptoms improve and the small-bowel mucosal damage recovers, while re-introduction of gluten into the diet leads to re-appearance of the symptoms and deterioration of the small-bowel mucosal architecture. In view of the restricted nature of the diet, alternative treatment is warranted. Improved understanding of the molecular basis of celiac disease has enabled researchers to suggest other therapeutic approaches. Although there is no animal model reproducing all features of celiac disease, the use of in vitro approaches including a variety of cell lines and the celiac patient small-bowel mucosal biopsy organ culture has generated knowledge about pathogenesis of celiac disease. In these culture systems, gluten induces different effects that can be quantified, thus also enabling studies concerning the efficacy of candidate therapeutic compounds for celiac disease. This review describes the intestinal epithelial cell models, celiac patient T-cell lines and clones, as well as the small-bowel mucosal organ culture methods widely used in studies of celiac disease, and summarizes the major findings obtained with these systems.