Evidence That Pathogenic Transglutaminase 2 in Celiac Disease Derives From Enterocytes

Gluten dependent activation of CD4+ T cells by MHC class II-expressing epithelium

BACKGROUND AND AIMS

Intestinal epithelial cell (IEC) damage is a hallmark of celiac disease (CeD); however, its role in gluten-dependent T-cell activation is unknown. We investigated IEC-gluten-T cell interactions in organoid monolayers expressing human MHC class II (HLA-DQ2.5), which facilitates gluten antigen recognition by CD4+ T cells in CeD.

METHODS

Epithelial MHC class II (MHCII) was determined in active and treated CeD, and in non-immunized and gluten-immunized DR3-DQ2.5 transgenic mice, lacking mouse MHCII molecules. Organoid monolayers from DR3-DQ2.5 mice were treated with or without IFN-γ, and MHCII expression was evaluated by flow cytometry. Organoid monolayers and CD4+ T cell co-cultures were incubated with gluten, pre-digested, or not by elastase-producing Pseudomonas aeruginosa or its lasB mutant. T cell function was assessed based on proliferation, expression of activation markers, and cytokine release in the co-culture supernatants.

RESULTS

Active CeD patients and gluten-immunized DR3-DQ2.5 mice demonstrated epithelial MHCII expression. Organoid monolayers derived from gluten-immunized DR3-DQ2.5 mice expressed MHCII, which was upregulated by IFN-γ. In organoid monolayer-T cell co-cultures, gluten increased the proliferation of CD4+ T cells, expression of T cell activation markers, and the release of IL-2, IFN-γ, and IL-15 in co-culture supernatants. Gluten metabolized by P. aeruginosa, but not the lasB mutant, enhanced CD4+ T cell proliferation and activation.

CONCLUSIONS

Gluten antigens are efficiently presented by MHCII-expressing IECs, resulting in the activation of gluten-specific CD4+ T cells, which is enhanced by gluten pre-digestion with microbial elastase. Therapeutics directed at IECs may offer a novel approach for modulating both adaptive and innate immunity in CeD patients.

Non-Host Factors Influencing Onset and Severity of Celiac Disease

Celiac disease (CeD) is a chronic autoimmune condition driven by gluten ingestion in genetically predisposed individuals, resulting in inflammatory lesions in the proximal small intestine. Although the presence of specific HLA-linked haplotypes and gluten consumption are necessary for disease development, they alone do not account for the variable onset of CeD in susceptible individuals. This review explores the multifaceted role of non-host factors in CeD development, including dietary and microbial influences. We discuss clinical associations and observations highlighting the impact of these factors on disease onset and severity. Furthermore, we discuss studies in CeD-relevant animal models that offer mechanistic insights into how diet, the microbiome, and enteric infections modulate CeD pathogenesis. Finally, we address the clinical implications and therapeutic potential of understanding these cofactors offering a promising avenue for preventive and therapeutic interventions in CeD management.

A Look Into the Future: Are We Ready for an Approved Therapy in Celiac Disease?

As it appears that we are currently at the cusp of an era in which drugs that are new, re-purposed, or "supplements" will be introduced to the management of celiac disease, we need to reflect on whether the framework is set for celiac disease to be treated increasingly with pharmaceuticals as well as diet. This refers to reflecting on the rigor of current diagnostic practices; the limitations of the current standard of care, which is a gluten-free diet; and that we lack objective markers of disease severity. Investigating these issues will help us to identify gaps in technology and practices that could be critical for selecting patients with a well-defined need for an improved or alternative treatment. Both aspects, circumscribed limitations of the gluten-free diet and diagnostics helping to define celiac disease target groups, together with the guiding requirements by the responsible regulatory authorities, will contribute to defining the subgroups of patients with confirmed celiac disease eligible for distinct pharmacologic strategies. Because many patients with celiac disease are diagnosed in childhood, these aspects need to be differentially discussed for the pediatric setting. In this perspective, we aimed to describe these contextual issues and then looked ahead to the future. What might be the major challenges in celiac disease clinics in the coming years once drugs are an option alongside diet? And what will be the future objectives for researchers who further decipher the mucosal immunology of celiac disease? Speculating on the answers to these questions is as stimulating as it is fascinating to be part of this turning point.

New Insights on Genes, Gluten, and Immunopathogenesis of Celiac Disease

Celiac disease (CeD) is a gluten-induced enteropathy that develops in genetically susceptible individuals upon consumption of cereal gluten proteins. It is a unique and complex immune disorder to study as the driving antigen is known and the tissue targeted by the immune reaction can be interrogated. This review integrates findings gained from genetic, biochemical, and immunologic studies, which together have revealed mechanisms of gluten peptide modification and HLA binding, thereby enabling a maladapted anti-gluten immune response. Observations in human samples combined with experimental mouse models have revealed that the gluten-induced immune response involves CD4+ T cells, cytotoxic CD8+ T cells, and B cells; their cross-talks are critical for the tissue-damaging response. The emergence of high-throughput technologies is increasing our understanding of the phenotype, location, and presumably function of the gluten-specific cells, which are all required to identify novel therapeutic targets and strategies for CeD.

The Celiac-Disease Superantigen Oligomerizes and Increases Permeability in an Enterocyte Cell Model

Celiac disease (CeD) is an autoimmune disorder triggered by gluten proteins, affecting approximately 1 % of the global population. The 33-mer deamidated gliadin peptide (DGP) is a metabolically modified wheat-gluten superantigen for CeD. Here, we demonstrate that the 33-mer DGP spontaneously assembles into oligomers with a diameter of approximately 24 nm. The 33-mer DGP oligomers present two main secondary structural motifs-a major polyproline II helix and a minor β-sheet structure. Importantly, in the presence of 33-mer DGP oligomers, there is a statistically significant increase in the permeability in the gut epithelial cell model Caco-2, accompanied by the redistribution of zonula occludens-1, a master tight junction protein. These findings provide novel molecular and supramolecular insights into the impact of 33-mer DGP in CeD and highlight the relevance of gliadin peptide oligomerization.

Tolerance-inducing therapies in coeliac disease - mechanisms, progress and future directions

Coeliac disease is an autoinflammatory condition caused by immune reactions to cereal gluten proteins. Currently, the only available treatment for the condition is a lifelong avoidance of gluten proteins in the diet. There is an unmet need for alternative therapies. Coeliac disease has a strong association with certain HLA-DQ allotypes (DQ2.5, DQ2.2 and DQ8), and these disease-associated HLA-DQ molecules present deamidated gluten peptides to gluten-specific CD4+ T cells. The gluten-specific CD4+ T cells are the drivers of the immune reactions leading to coeliac disease. Once established, the clonotypes of gluten-specific CD4+ T cells persist for decades, explaining why patients must adhere to a gluten-free diet for life. Given the key pathogenic role of gluten-specific CD4+ T cells, tolerance-inducing therapies that target these T cells are attractive for treatment of the disorder. Lessons learned from coeliac disease might provide clues for treatment of other HLA-associated diseases for which the disease-driving antigens are unknown. Thus, intensive efforts have been and are currently implemented to bring an effective tolerance-inducing therapy for coeliac disease. This Review discusses mechanisms of the various approaches taken, summarizing the progress made, and highlights future directions in this field.

Generation of circulating autoreactive pre-plasma cells fueled by naive B cells in celiac disease

Autoantibodies against the enzyme transglutaminase 2 (TG2) are characteristic of celiac disease (CeD), and TG2-specific immunoglobulin (Ig) A plasma cells are abundant in gut biopsies of patients. Here, we describe the corresponding population of autoreactive B cells in blood. Circulating TG2-specific IgA cells are present in untreated patients on a gluten-containing diet but not in controls. They are clonally related to TG2-specific small intestinal plasma cells, and they express gut-homing molecules, indicating that they are plasma cell precursors. Unlike other IgA-switched cells, the TG2-specific cells are negative for CD27, placing them in the double-negative (IgD-CD27-) category. They have a plasmablast or activated memory B cell phenotype, and they harbor fewer variable region mutations than other IgA cells. Based on their similarity to naive B cells, we propose that autoreactive IgA cells in CeD are generated mainly through chronic recruitment of naive B cells via an extrafollicular response involving gluten-specific CD4+ T cells.

Coeliac disease: the paradox of diagnosing a food hypersensitivity disorder with autoantibodies

Serum antibodies to the autoantigen transglutaminase 2 (TG2) are increasingly harnessed to diagnose coeliac disease. Diagnostic guidelines for children give recommendation for a no-biopsy-based diagnosis through detection of high amounts of IgA anti-TG2 antibodies in serum with confirmation of positivity in a separate blood sample by characteristic autoantibody-staining of tissue. While measurement of IgA anti-TG2 also is important in the diagnostic workup of adults, the adult guidelines still mandate examination of gut biopsies. This requirement might well change in the future, as might the necessity for confirming autoantibody positivity by tissue staining. The key role of autoantibody serology for diagnosis of coeliac disease is paradoxical. Coeliac disease was considered, and still can be considered, a food intolerance disorder where autoantibodies at face value are out of place. The immunological mechanisms underlying the formation of autoantibodies in response to gluten exposure have been dissected. This review presents the current insights demonstrating that the autoantibodies in coeliac disease are intimately integrated in the maladapted immune response to gluten.

Unraveling the Immunopathological Landscape of Celiac Disease: A Comprehensive Review

Celiac disease (CD) presents a complex interplay of both innate and adaptive immune responses that drive a variety of pathological manifestations. Recent studies highlight the role of immune-mediated pathogenesis, pinpointing the involvement of antibodies against tissue transglutaminases (TG2, TG3, TG6), specific HLA molecules (DQ2/8), and the regulatory role of interleukin-15, among other cellular and molecular pathways. These aspects illuminate the systemic nature of CD, reflecting its wide-reaching impact that extends beyond gastrointestinal symptoms to affect other physiological systems and giving rise to a range of pathological landscapes, including refractory CD (RCD) and, in severe cases, enteropathy-associated T cell lymphoma. The existing primary therapeutic strategy, a gluten-free diet (GFD), poses significant challenges, such as low adherence rates, necessitating alternative treatments. Emerging therapies target various stages of the disease pathology, from preventing immunogenic gluten peptide absorption to enhancing intestinal epithelial integrity and modulating the immune response, heralding potential breakthroughs in CD management. As the understanding of CD deepens, novel therapeutic avenues are emerging, paving the way for more effective and sophisticated treatment strategies with the aim of enhancing the quality of life of CD patients. This review aims to delineate the immunopathology of CD and exploring its implications on other systems, its complications and the development of novel treatments.

Autoantibody binding and unique enzyme-substrate intermediate conformation of human transglutaminase 3

Transglutaminase 3 (TG3), the autoantigen of dermatitis herpetiformis (DH), is a calcium dependent enzyme that targets glutamine residues in polypeptides for either transamidation or deamidation modifications. To become catalytically active TG3 requires proteolytic cleavage between the core domain and two C-terminal β-barrels (C1C2). Here, we report four X-ray crystal structures representing inactive and active conformations of human TG3 in complex with a TG3-specific Fab fragment of a DH patient derived antibody. We demonstrate that cleaved TG3, upon binding of a substrate-mimicking inhibitor, undergoes a large conformational change as a β-sheet in the catalytic core domain moves and C1C2 detaches. The unique enzyme-substrate conformation of TG3 without C1C2 is recognized by DH autoantibodies. The findings support a model where B-cell receptors of TG3-specific B cells bind and internalize TG3-gluten enzyme-substrate complexes thereby facilitating gluten-antigen presentation, T-cell help and autoantibody production.

The Oral Transglutaminase 2 Inhibitor ZED1227 Accumulates in the Villous Enterocytes in Celiac Disease Patients during Gluten Challenge and Drug Treatment

The enzyme transglutaminase 2 (TG2) plays a key role in celiac disease (CeD) pathogenesis. Active TG2 is located mainly extracellularly in the lamina propria but also in the villous enterocytes of the duodenum. The TG2 inhibitor ZED1227 is a promising drug candidate for treating CeD and is designed to block the TG2-catalyzed deamidation and crosslinking of gliadin peptides. Our aim was to study the accumulation of ZED1227 after oral administration of the drug. We studied duodenal biopsies derived from a phase 2a clinical drug trial using an antibody that detects ZED1227 when bound to the catalytic center of TG2. Human epithelial organoids were studied in vitro for the effect of ZED1227 on the activity of TG2 using the 5-biotin-pentylamine assay. The ZED1227-TG2 complex was found mainly in the villous enterocytes in post-treatment biopsies. The signal of ZED1227-TG2 was strongest in the luminal epithelial brush border, while the intensity of the signal in the lamina propria was only ~20% of that in the villous enterocytes. No signal specific to ZED1227 could be detected in pretreatment biopsies or in biopsies from patients randomized to the placebo treatment arm. ZED1227-TG2 staining co-localized with total TG2 and native and deamidated gliadin peptides on the enterocyte luminal surface. Inhibition of TG2 activity by ZED1227 was demonstrated in epithelial organoids. Our findings suggest that active TG2 is present at the luminal side of the villous epithelium and that inhibition of TG2 activity by ZED1227 occurs already there before gliadin peptides enter the lamina propria.

Expression of transglutaminase 2 in human gut epithelial cells: Implications for coeliac disease

BACKGROUND

Formation of complexes between transglutaminase 2 (TG2) and gluten can mechanistically explain why TG2 serves both as B-cell autoantigen and as an enzyme that creates deamidated gluten epitopes in coeliac disease (CeD). A model has been proposed where TG2 released from shed epithelial cells encounters high concentrations of dietary gluten peptides to form these TG2:gluten complexes. In this work we have characterised TG2 protein expression in gut epithelial cells in humans.

METHODS

Western blot analysis, immunofluorescence staining and mass spectrometry in combination with laser capture microdissection to gain spatial resolution were used to characterise TG2 expression in the epithelial cell layer of healthy and coeliac disease affected duodenum.

FINDINGS

TG2 is expressed in human duodenal epithelial cells, including cells in the apical region that are shed into the gut lumen. In untreated CeD the apical expression of TG2 is doubled. Enzymatically active TG2 is readily released from isolated human intestinal epithelial cells.

CONCLUSION

Shed epithelial cells are a plausible source of pathogenic TG2 enzyme in CeD. Increased epithelial TG2 expression and increased epithelial shedding in active CeD may reinforce action of luminal TG2 in this condition.

Human intestinal B cells in inflammatory diseases

The intestinal lumen contains an abundance of bacteria, viruses and fungi alongside ingested material that shape the chronically active intestinal immune system from early life to maintain the integrity of the gut epithelial barrier. In health, the response is intricately balanced to provide active protection against pathogen invasion whilst tolerating food and avoiding inflammation. B cells are central to achieving this protection. Their activation and maturation generates the body's largest plasma cell population that secretes IgA, and the niches they provide support systemic immune cell specialization. For example, the gut supports the development and maturation of a splenic B cell subset - the marginal zone B cells. In addition, cells such as the T follicular helper cells, which are enriched in many autoinflammatory diseases, are intrinsically associated with the germinal centre microenvironment that is more abundant in the gut than in any other tissue in health. In this Review, we discuss intestinal B cells and their role when a loss of homeostasis results in intestinal and systemic inflammatory diseases.

Human intestinal organoid models for celiac disease research

Celiac disease pathogenesis, in addition to immune cell component, encompasses pathogenic events also in the duodenal epithelium. In celiac disease patients, exposure to dietary gluten induces drastic changes in epithelial differentiation and elicit cellular response to inflammatory cytokines. The autoantigen in celiac disease, transglutaminase 2 (TG2) enzyme, has been also suggested to play its pathogenic gliadin deamidation event in the intestinal epithelium. Therefore in vitro epithelial cell-line models have been exploited in the past to study these pathogenic mechanisms, but they are hampered by their simplistic nature lacking proper cell-type composition and intestinal environ. Moreover, these cell models harbor many cancer-related mutations in tumor suppressor genes making them unsuitable for studying cell differentiation. Intestinal organoids provide a near-native epithelial cell model to study pathogenic agents and mechanisms related to celiac disease. Here we describe protocols to initiate and maintain celiac patient-derived organoid cultures and how to grow them in alternative ways allowing their exploitation in different kind of experiments.

The Immunobiology and Pathogenesis of Celiac Disease

Among human leukocyte antigen (HLA)-associated disorders, celiac disease has an immunopathogenesis that is particularly well understood. The condition is characterized by hypersensitivity to cereal gluten proteins, and the disease lesion is localized in the gut. Still, the diagnosis can be made by detection of highly disease-specific autoantibodies to transglutaminase 2 in the blood. We now have mechanistic insights into how the disease-predisposing HLA-DQ molecules, via presentation of posttranslationally modified gluten peptides, are connected to the generation of these autoantibodies. This review presents our current understanding of the immunobiology of this common disorder that is positioned in the border zone between food hypersensitivity and autoimmunity.

Immunopathogenesis and environmental triggers in coeliac disease

Coeliac disease (CD) is a frequent immune enteropathy induced by gluten in genetically predisposed individuals. Its pathogenesis has been extensively studied and CD has emerged as a model disease to decipher how the interplay between environmental and genetic factors can predispose to autoimmunity and promote lymphomagenesis. The keystone event is the activation of a gluten-specific immune response that is driven by molecular interactions between gluten, the indispensable environmental factor, HLA-DQ2/8, the main predisposing genetic factor and transglutaminase 2, the CD-specific autoantigen. The antigluten response is however not sufficient to induce epithelial damage which requires the activation of cytotoxic CD8+ intraepithelial lymphocytes (IEL). In a plausible scenario, cooperation between cytokines released by gluten-specific CD4+ T cells and interleukin-15 produced in excess in the coeliac gut, licenses the autoimmune-like attack of the gut epithelium, likely via sustained activation of the Janus kinase-signal transducer and activator of transcription (JAK/STAT) pathway in IEL. Demonstration that lymphomas complicating CD arise from IEL that have acquired gain-of-function JAK1 or STAT3 mutations stresses the key role of this pathway and explains how gluten-driven chronic inflammation may promote this rare but most severe complication. If our understanding of CD pathogenesis has considerably progressed, several questions and challenges remain. One unsolved question concerns the considerable variability in disease penetrance, severity and presentation, pointing to the role of additional genetic and environmental factors that remain however uneasy to untangle and hierarchize. A current challenge is to transfer the considerable mechanistic insight gained into CD pathogenesis into benefits for the patients, notably to alleviate the gluten-free diet, a burden for many patients.

Type 2 Transglutaminase in Coeliac Disease: A Key Player in Pathogenesis, Diagnosis and Therapy

Type 2 transglutaminase (TG2) is the main autoantigen in coeliac disease (CD), a widespread inflammatory enteropathy caused by the ingestion of gluten-containing cereals in genetically predisposed individuals. As a consequence, serum antibodies to TG2 represent a very useful marker in CD diagnosis. However, TG2 is also an important player in CD pathogenesis, for its ability to deamidate some Gln residues of gluten peptides, which become more immunogenic in CD intestinal mucosa. Given the importance of TG2 enzymatic activities in CD, several studies have sought to discover specific and potent inhibitors that could be employed in new therapeutical approaches for CD, as alternatives to a lifelong gluten-free diet. In this review, we summarise all the aspects regarding TG2 involvement in CD, including its enzymatic reactions in pathogenesis, the role of anti-TG2 antibodies in disease management, and the exploration of recent strategies to reduce deamidation or to use transamidation to detoxify gluten.

Pivotal Role of Inflammation in Celiac Disease

Celiac disease (CD) is an immune-mediated enteropathy triggered in genetically susceptible individuals by gluten-containing cereals. A central role in the pathogenesis of CD is played by the HLA-restricted gliadin-specific intestinal T cell response generated in a pro-inflammatory environment. The mechanisms that generate this pro-inflammatory environment in CD is now starting to be addressed. In vitro study on CD cells and organoids, shows that constant low-grade inflammation is present also in the absence of gluten. In vivo studies on a population at risk, show before the onset of the disease and before the introduction of gluten in the diet, cellular and metabolic alterations in the absence of a T cell-mediated response. Gluten exacerbates these constitutive alterations in vitro and in vivo. Inflammation, may have a main role in CD, adding this disease tout court to the big family of chronic inflammatory diseases. Nutrients can have pro-inflammatory or anti-inflammatory effects, also mediated by intestinal microbiota. The intestine function as a crossroad for the control of inflammation both locally and at distance. The aim of this review is to discuss the recent literature on the main role of inflammation in the natural history of CD, supported by cellular fragility with increased sensitivity to gluten and other pro-inflammatory agents.

Transglutaminase 2 affinity and enzyme-substrate intermediate stability as determining factors for T-cell responses to gluten peptides in celiac disease

The adaptive immune response of celiac disease (CeD) involves presentation of gluten peptides to CD4⁺ T cells by transglutaminase 2 (TG2) specific B cells. This B‐cell/T‐cell crosstalk is facilitated by involvement of TG2:gluten peptide complexes that act principally in the form of enzyme‐substrate intermediates. Here, we have addressed how gluten peptide affinity and complex stability in the presence of secondary substrates affect the uptake of TG2:gluten peptide complexes by TG2‐specific B cells and the activation of gluten‐specific T cells. We studied affinity of various gluten peptides for TG2 by biochemical assay, and monitored uptake of gluten peptides by TG2‐specific B cells by flow cytometry. Crosstalk between TG2‐specific B cells and gluten‐specific T cells was assayed with transfectants expressing antigen receptors derived from CeD patients. We found that gluten peptides with high TG2 affinity showed better uptake by TG2‐specific B cells. Uptake by B cells, and subsequent activation of T cells, was negatively affected by polyamines acting as secondary TG2 substrates. These results show that affinity between gluten peptide and TG2 governs the selection of T‐cell epitopes via enhanced uptake of TG2:gluten complexes by TG2‐specific B cells, and that exogenous polyamines can influence the CeD immune responses by disrupting TG2:gluten complexes.

Wheat Amylase Trypsin Inhibitors Aggravate Intestinal Inflammation Associated with Celiac Disease Mediated by Gliadin in BALB/c Mice

Celiac disease (CD) is an autoimmune intestinal disorder caused by the ingestion of gluten in people who carry the susceptible gene. In current celiac disease research, wheat gluten is often the main target of attention, neglecting the role played by non-gluten proteins. This study aimed to describe the effects of wheat amylase trypsin inhibitors (ATI, non-gluten proteins) and gliadin in BALB/c mice while exploring the further role of relevant adjuvants (cholera toxin, polyinosinic: polycytidylic acid and dextran sulfate sodium) intervention. An ex vivo splenocyte and intestinal tissue were collected for analysis of the inflammatory profile. The consumption of gliadin and ATI caused intestinal inflammation in mice. Moreover, the histopathology staining of four intestinal sections (duodenum, jejunum, terminal ileum, and middle colon) indicated that adjuvants, especially polyinosinic: polycytidylic acid, enhanced the villi damage and crypt hyperplasia in co-stimulation with ATI and gliadin murine model. Immunohistochemical results showed that tissue transglutaminase and IL-15 expression were significantly increased in the jejunal tissue of mice treated with ATI and gliadin. Similarly, the expression of inflammatory factors (TNF-α, IL-1β, IL-4, IL-13) and Th1/Th2 balance also showed that the inflammation response was significantly increased after co-stimulation with ATI and gliadin. This study provided new evidence for the role of wheat amylase trypsin inhibitors in the pathogenesis of celiac disease.

Features of ZED1227: The First-In-Class Tissue Transglutaminase Inhibitor Undergoing Clinical Evaluation for the Treatment of Celiac Disease

ZED1227 is a small molecule tissue transglutaminase (TG2) inhibitor. The compound selectively binds to the active state of TG2, forming a stable covalent bond with the cysteine in its catalytic center. The molecule was designed for the treatment of celiac disease. Celiac disease is an autoimmune-mediated chronic inflammatory condition of the small intestine affecting about 1–2% of people in Caucasian populations. The autoimmune disease is triggered by dietary gluten. Consumption of staple foods containing wheat, barley, or rye leads to destruction of the small intestinal mucosa in genetically susceptible individuals, and this is accompanied by the generation of characteristic TG2 autoantibodies. TG2 plays a causative role in the pathogenesis of celiac disease. Upon activation by Ca²⁺, it catalyzes the deamidation of gliadin peptides as well as the crosslinking of gliadin peptides to TG2 itself. These modified biological structures trigger breaking of oral tolerance to gluten, self-tolerance to TG2, and the activation of cytotoxic immune cells in the gut mucosa. Recently, in an exploratory proof-of-concept study, ZED1227 administration clinically validated TG2 as a “druggable” target in celiac disease. Here, we describe the specific features and profiling data of the drug candidate ZED1227. Further, we give an outlook on TG2 inhibition as a therapeutic approach in indications beyond celiac disease.

Review article: therapeutic targets for the pharmacologic management of coeliac disease-the future beyond a gluten-free diet

BACKGROUND

Coeliac disease (CeD) is an immune-mediated small bowel enteropathy resulting from dietary gluten exposure. Presently, the only effective treatment is adoption of a gluten-free diet (GFD), although strict adherence is challenging to maintain, and inadvertent gluten exposures are inevitable for most patients. Hence, there is substantial interest in drug development in CeD and multiple novel therapies are under investigation.

AIMS

To review existing and upcoming clinical trial programmes for pharmacologic agents for CeD.

METHODS

A narrative review was performed, informed by a search of MEDLINE, Embase, the Cochrane CENTRAL Library and clinicaltrials.gov.

RESULTS

We summarise the pathophysiology of CeD and the specific steps that are potentially amenable to pharmacologic treatment. We evaluate the evidence supporting existing and future drug targets, including trials of peptidases, gluten sequestrants, tight junction regulators, anti-transglutaminase 2 therapies, immune tolerizing agents, advanced biologics and small molecules, and microbiome-targeted strategies. We highlight unique considerations for conducting CeD trials, including identifying appropriate study populations, assessing results in the context of a gluten challenge, and interpreting CeD-specific clinical and histologic outcomes. Understanding these factors is crucial for accurately appraising the evidence. Finally, we outline what the future of CeD therapy may hold with the introduction of pharmacotherapies.

CONCLUSIONS

There is a need for pharmacologic options for CeD, either used adjunctively with a GFD for accidental or intentional gluten exposures or for refractory disease. Multiple promising agents are in development, and these trials are likely to lead to approvals for the first generation of pharmacologic agents for CeD within the next 5 years.

Antibody Responses to Transglutaminase 3 in Dermatitis Herpetiformis: Lessons from Celiac Disease

Dermatitis herpetiformis (DH) is the skin manifestation of celiac disease, presenting with a blistering rash typically on the knees, elbows, buttocks and scalp. In both DH and celiac disease, exposure to dietary gluten triggers a cascade of events resulting in the production of autoantibodies against the transglutaminase (TG) enzyme, mainly TG2 but often also TG3. The latter is considered to be the primary autoantigen in DH. The dynamics of the development of the TG2-targeted autoimmune response have been studied in depth in celiac disease, but the immunological process underlying DH pathophysiology is incompletely understood. Part of this process is the occurrence of granular deposits of IgA and TG3 in the perilesional skin. While this serves as the primary diagnostic finding in DH, the role of these immunocomplexes in the pathogenesis is unknown. Intriguingly, even though gluten-intolerance likely develops initially in a similar manner in both DH and celiac disease, after the onset of the disease, its manifestations differ widely.

Changes in Non-Deamidated versus Deamidated Epitope Targeting and Disease Prediction during the Antibody Response to Gliadin and Transglutaminase of Infants at Risk for Celiac Disease

Celiac disease (CeD) is a conditional autoimmune disorder with T cell-mediated immune response to gluten coupled with antibody production to gliadin and the self-protein tissue transglutaminase (TG2). TG2 contributes to the CeD pathomechanism by deamidating gliadin, thereby generating more immunogenic peptides. Anti-gliadin antibodies may appear before the autoantibody production. The scope of this study was to dissect these early antibody responses by investigating serum samples collected during the PreventCD prospective double-blind study, where infants with high CeD risk were randomized to 200 mg daily gluten intake or placebo from 4 to 6 months of age, followed by frequent blood testing on regular gluten consumption in both groups. After primary gluten intake, children with or without later CeD produced IgA and IgG antibodies which preferentially recognized non-deamidated gliadin peptides. At CeD development with anti-TG2 seroconversion, there was a significant increase in the antibody reaction toward deamidated gliadin peptides (DGP), with maturation in the binding strength for the PEQPFP gamma-gliadin core peptide. The earliest produced autoantibodies targeted TG2’s celiac epitope 2. Our results reveal a qualitative change in the gliadin-directed humoral immune response at the time when anti-TG2 antibodies appear, but anti-DGP antibodies in the absence of anti-TG2 antibodies are not disease-predictive.

Single-cell approaches to dissect adaptive immune responses involved in autoimmunity: the case of celiac disease

Single-cell analysis is a powerful technology that has found widespread use in recent years. For diseases with involvement of adaptive immunity, single-cell analysis of antigen-specific T cells and B cells is particularly informative. In autoimmune diseases, the adaptive immune system is obviously at play, yet the ability to identify the culprit T and B cells recognizing disease-relevant antigen can be difficult. Celiac disease, a widespread disorder with autoimmune components, is unique in that disease-relevant antigens for both T cells and B cells are well defined. Furthermore, the celiac disease gut lesion is readily accessible allowing for sampling of tissue-resident cells. Thus, disease-relevant T cells and B cells from the gut and blood can be studied at the level of single cells. Here we review single-cell studies providing information on such adaptive immune cells and outline some future perspectives in the area of single-cell analysis in autoimmune diseases.

Intestinal anti-tissue transglutaminase IgA deposits as a complementary method for the diagnostic evaluation of celiac disease in patients with low-grade histological lesions

Evaluating the usefulness of intestinal anti-transglutaminase IgA (anti-TG2 IgA) deposits detection as a complementary or decision-supporting tool in the diagnosis of celiac disease (CD) in patients with low degree of enteropathy. Small intestinal biopsies (SIB) were performed from 2008 to 2017 in patients on suspicion of CD (positive CD serology and/or symptoms) referred to our Pediatric Gastroenterology Unit. We determined anti-TG2 IgA deposits by using double immunofluorescence in all the patients in whom Marsh 0 or Marsh1 was detected in the conventional histological study and in a random selection of patients with clearly positive serology and histological Marsh 2-3 lesion. 75 pediatric patients were split into 3 groups according to the final diagnosis: 1) 13 children with a Marsh 0 or 1, negative CD serology and final non-CD diagnosis;none presented intestinal anti-TG2 IgA deposits; 2) 15 potential CD cases (Marsh 0 or 1 and CD-associated antibodies), detecting anti-TG2 IgA deposits in 12; on follow-up, another biopsy performed in 11/15 showed villi atrophy in 7 and a Marsh 2 lesion in two of them, patients being finally diagnosed as CD cases; and 3) 47 children with Marsh 2-3 histological lesion and final CD diagnosis; all of them had intestinal anti-TG2 IgA deposits. Anti-TG2 deposits are a useful complementary tool for CD diagnosis in pediatric population with digestive pathologies suggestive of CD. It is especially helpful in those with low grade lesion, in which anti-TG2 deposits are predictive of the development of more severe lesions on follow-up.

Emergence of an adaptive immune paradigm to explain celiac disease: a perspective on new evidence and implications for future interventions and diagnosis

INTRODUCTION

Recent patient studies have shown that gluten-free diet is less effective in treating celiac disease than previously believed, and additionally patients remain vulnerable to gluten-induced acute symptoms and systemic cytokine release. Safe and effective pharmacological adjuncts to gluten-free diet are in preclinical and clinical development. Clear understanding of the pathogenesis of celiac disease is critical for drug target identification, establishing efficacy endpoints and to develop non-invasive biomarkers suitable to monitor and potentially diagnose celiac disease.

AREAS COVERED

The role and clinical effects of CD4+ T cells directed against deamidated gluten in the context of an "adaptive immune paradigm" are reviewed. Alternative hypotheses of gluten toxicity are discussed and contrasted. In the context of recent patient studies, implications of the adaptive immune paradigm for future strategies to prevent, diagnose, and treat celiac disease are outlined.

EXPERT OPINION

Effective therapeutics for celiac disease are likely to be approved and necessitate a variety of new clinical instruments and tests to stratify patient need, monitor remission, and confirm diagnosis in uncertain cases. Sensitive assessments of CD4+ T cells specific for deamidated gluten are likely to play a central role in clinical management, and to facilitate research and pharmaceutical development.

TG2-gluten complexes as antigens for gluten-specific and transglutaminase-2 specific B cells in celiac disease

A hallmark of celiac disease is the gluten-dependent production of antibodies specific for deamidated gluten peptides (DGP) and the enzyme transglutaminase 2 (TG2). Both types of antibodies are believed to result from B cells receiving help from gluten-specific CD4⁺ T cells and differentiating into antibody-producing plasma cells. We have here studied the collaboration between DGP- and TG2-specific B cells with gluten-specific CD4⁺ T cells using transgenic mice expressing celiac patient-derived T-cell and B-cell receptors, as well as between B-cell transfectants and patient-derived gluten-specific T-cell clones. We show that multivalent TG2-gluten complexes are efficient antigens for both TG2-specific and DGP-specific B cells and allow both types of B cells to receive help from gluten-specific T cells of many different specificities.

Microbial Transglutaminase Is a Very Frequently Used Food Additive and Is a Potential Inducer of Autoimmune/Neurodegenerative Diseases

Microbial transglutaminase (mTG) is a heavily used food additive and its industrial transamidated complexes usage is rising rapidly. It was classified as a processing aid and was granted the GRAS (generally recognized as safe) definition, thus escaping full and thorough toxic and safety evaluations. Despite the manufacturers claims, mTG or its cross-linked compounds are immunogenic, pathogenic, proinflammatory, allergenic and toxic, and pose a risk to public health. The enzyme is a member of the transglutaminase family and imitates the posttranslational modification of gluten, by the tissue transglutaminase, which is the autoantigen of celiac disease. The deamidated and transamidated gliadin peptides lose their tolerance and induce the gluten enteropathy. Microbial transglutaminase and its complexes increase intestinal permeability, suppresses enteric protective pathways, enhances microbial growth and gliadin peptide's epithelial uptake and can transcytose intra-enterocytically to face the sub-epithelial immune cells. The present review updates on the potentially detrimental side effects of mTG, aiming to interest the scientific community, induce food regulatory authorities' debates on its safety, and protect the public from the mTG unwanted effects.

Propofol Suppresses Microglia Inflammation by Targeting TGM2/NF-κB Signaling

Background

Propofol is a known intravenous hypnotic drug used for induction and maintenance of sedation and general anesthesia. Emerging studies also reveal a neuroprotective effect of propofol in diverse diseases of neuronal injuries via modulating microglia activation. In this study, we aimed to uncover the downstream targets of propofol in this process.

Methods

RNA sequencing analysis to identify genes implicated in the propofol-mediated neuroprotective effect. Quantitative real-time PCR, enzyme-linked immunosorbent assay, and Western blotting analysis were performed to analyze inflammatory gene expression, cytokine levels, and TGM2. BV2 cells and primary microglia were used for functional verification and mechanism studies.

Results

The multifunctional enzyme transglutaminase 2 (TGM2) was identified as a putative functional mediator of propofol. TGM2 was significantly upregulated in lipopolysaccharide- (LPS-) primed BV2 cells. Genetic silencing of TGM2 abolished LPS-induced microglial activation. Notably, gain-of-function experiments showed that the proinflammatory effects of TGM2 were dependent on its GTP binding activity instead of transamidase activity. Then, TGM2 was revealed to activate the NF-κB signaling pathway to facilitate microglial activation. Propofol can inhibit TGM2 expression and NF-κB signaling in BV2 cells and primary microglia. Ectopic expression of TGM2 or constitutively active IKKβ (CA-IKKβ) can compromise propofol-induced anti-inflammatory effects.

Conclusions

Our findings suggest that TGM2-mediated activation of NF-κB signaling is an important mechanism in the propofol-induced neuroprotective effect that prevents microglial activation.

Interplay Between Gluten, HLA, Innate and Adaptive Immunity Orchestrates the Development of Coeliac Disease

Several environmental, genetic, and immune factors create a "perfect storm" for the development of coeliac disease: the antigen gluten, the strong association of coeliac disease with HLA, the deamidation of gluten peptides by the enzyme transglutaminase 2 (TG2) generating peptides that bind strongly to the predisposing HLA-DQ2 or HLA-DQ8 molecules, and the ensuing unrestrained T cell response. T cell immunity is at the center of the disease contributing to the inflammatory process through the loss of tolerance to gluten and the differentiation of HLA-DQ2 or HLA-DQ8-restricted anti-gluten inflammatory CD4⁺ T cells secreting pro-inflammatory cytokines and to the killing of intestinal epithelial cells by cytotoxic intraepithelial CD8⁺ lymphocytes. However, recent studies emphasize that the individual contribution of each of these cell subsets is not sufficient and that interactions between these different populations of T cells and the simultaneous activation of innate and adaptive immune pathways in distinct gut compartments are required to promote disease immunopathology. In this review, we will discuss how tissue destruction in the context of coeliac disease results from the complex interactions between gluten, HLA molecules, TG2, and multiple innate and adaptive immune components.

Current and emerging therapies for coeliac disease

Coeliac disease is a common enteropathy that occurs in genetically susceptible individuals in response to the ingestion of gluten proteins present in wheat, rye and barley. Currently, the only available treatment for the condition is a strict, life-long gluten-free diet that, despite being safe and often effective, is associated with several challenges. Due to the high cost, particularly restrictive nature and perception of decreased quality of life associated with the diet, some patients are continuously exposed to gluten, which prevents an adequate disease control. Moreover, a subgroup of patients does not respond to the diet adequately, and healing of the small-bowel mucosa can be incomplete. Thus, there is a need for alternative treatment forms. The increasingly understood pathogenetic process of coeliac disease has enabled the identification of various targets for future therapies. Multiple investigational therapies ranging from tolerogenic to immunological approaches are in the pipeline, and several drug candidates have entered phase II/III clinical trials. This Review gives a broad overview of the different investigative treatment modalities for coeliac disease and summarizes the latest advances in this field.

An updated overview on celiac disease: from immuno-pathogenesis and immuno-genetics to therapeutic implications

INTRODUCTION

Celiac disease (CD) is an autoimmune enteropathy triggered by ingestion of gluten. While presenting many similarities with other autoimmune diseases, celiac disease is unique in that the external trigger, gluten, and the genetic background necessary for disease development (HLA DQ2/DQ8) are well described. The prevalence of celiac disease is dramatically increasing over the years and new epidemiologic data show changes regarding age of onset and symptoms. A better understanding of CD-pathogenesis is fundamental to highlight the reasons of this rise of celiac diagnoses.

AREAS COVERED

In this review we describe CD-pathogenesis by dissecting all the components necessary to lose tolerance to gluten (ingestion of gluten, genetic predisposition, loss of barrier function and immune response). Additionally, we also highlight the role that microbiome plays in celiac disease as well as new proposed therapies and experimental tools.

EXPERT OPINION

Prevalence of autoimmune diseases is increasing around the world. As a result, modern society is strongly impacted by a social and economic burden. Given the unique characteristics of celiac disease, a better understanding of its pathogenesis and the factors that contribute to it may shed light on other autoimmune diseases for which external trigger and genetic background are not known.

Gliadin Sequestration as a Novel Therapy for Celiac Disease: A Prospective Application for Polyphenols

Celiac disease is an autoimmune disorder characterized by a heightened immune response to gluten proteins in the diet, leading to gastrointestinal symptoms and mucosal damage localized to the small intestine. Despite its prevalence, the only treatment currently available for celiac disease is complete avoidance of gluten proteins in the diet. Ongoing clinical trials have focused on targeting the immune response or gluten proteins through methods such as immunosuppression, enhanced protein degradation and protein sequestration. Recent studies suggest that polyphenols may elicit protective effects within the celiac disease milieu by disrupting the enzymatic hydrolysis of gluten proteins, sequestering gluten proteins from recognition by critical receptors in pathogenesis and exerting anti-inflammatory effects on the system as a whole. This review highlights mechanisms by which polyphenols can protect against celiac disease, takes a critical look at recent works and outlines future applications for this potential treatment method.

The gliadin p31-43 peptide: Inducer of multiple proinflammatory effects

Coeliac disease (CD) is the prototype of an inflammatory chronic disease induced by food. In this context, gliadin p31-43 peptide comes into the spotlight as an important player of the inflammatory/innate immune response to gliadin in CD. The p31-43 peptide is part of the p31-55 peptide from α-gliadins that remains undigested for a long time, and can be present in the small intestine after ingestion of a gluten-containing diet. Different biophysical methods and molecular dynamic simulations have shown that p31-43 spontaneously forms oligomeric nanostructures, whereas experimental approaches using in vitro assays, mouse models, and human duodenal tissues have shown that p31-43 is able to induce different forms of cellular stress by driving multiple inflammatory pathways. Increased proliferative activity of the epithelial cells in the crypts, enterocyte stress, activation of TG2, induction of Ca²⁺, IL-15, and NFκB signaling, inhibition of CFTR, alteration of vesicular trafficking, and activation of the inflammasome platform are some of the biological effects of p31-43, which, in the presence of appropriate genetic susceptibility and environmental factors, may act together to drive CD.

Innate and adaptive immunity in celiac disease

PURPOSE OF REVIEW

The current review is prompted by recent studies indicating that adaptive immunity could be sufficient to explain rapid onset symptoms as well as many chronic effects of gluten in celiac disease.

RECENT FINDINGS

Gluten re-exposure in treated celiac disease drives a coordinated systemic cytokine release response implicating T-cell activation within 2 h. Instead of direct effects of gluten on innate immunity, long lasting memory CD4+ T cells activated within 2 h of ingesting gluten or injecting purified gluten peptides now appear to be responsible for acute digestive symptoms. In addition, memory B cells and plasma cells specific for gluten and transglutaminase 2, rather than innate immune cells, are the preferred antigen-presenting cells for gluten in the gut. A variety of innate immune stimuli such as transient infections and local intestinal microbiome, not necessarily gluten itself, may contribute to disease initiation and transition to overt intestinal mucosal injury. Gluten-specific adaptive immunity in the gut and blood are now shown to be closely linked, and systemic cytokine release after gluten provides an additional explanation for extraintestinal manifestations of celiac disease.

SUMMARY

Clinical studies utilizing cytokines as new biomarkers for gluten immunity promise to improve understanding of clinical effects of gluten, accelerate therapeutics development, and augment diagnosis.