Epitope-dependent Functional Effects of Celiac Disease Autoantibodies on Transglutaminase 2

Enzyme-activating B-cell receptors boost antigen presentation to pathogenic T cells in gluten-sensitive autoimmunity

Autoantibodies against the enzyme transglutaminase 3 (TG3) are characteristic to the gluten-sensitive skin disorder dermatitis herpetiformis (DH), which is an extraintestinal manifestation of celiac disease. We here demonstrate that TG3-specific B cells can activate gluten-specific CD4+ T cells through B-cell receptor (BCR)-mediated internalization of TG3-gluten enzyme-substrate complexes. Stereotypic anti-TG3 antibodies using IGHV2-5/IGKV4-1 gene segments enhance the catalytic activity of TG3, and this effect translates into increased gluten presentation to T cells when such antibodies are expressed as BCRs. The crystal structure of TG3 bound to an IGHV2-5/IGKV4-1 Fab shows that antibody binding to a β-sheet in the catalytic core domain causes the enzyme to adopt the active conformation. This mechanism explains the production of stereotypic anti-TG3 autoantibodies in DH and highlights a role for TG3-specific B cells as antigen-presenting cells for gluten-specific T cells. Similar boosting effects of autoreactive BCRs could be relevant for other autoimmune diseases, including rheumatoid arthritis.

Selective activation of naïve B cells with unique epitope specificity shapes autoantibody formation in celiac disease

Many antibody responses induced by infection, vaccination or autoimmunity show signs of convergence across individuals with epitope-dependent selection of particular variable region gene segments and complementarity determining region 3 properties. However, not much is known about the relationship between antigen-specific effector cells and antigen-specific precursors present in the naïve B-cell repertoire. Here, we sought to address this relationship in the context of celiac disease, where there is a stereotyped autoantibody response against the enzyme transglutaminase 2 (TG2). By generating TG2-specific monoclonal antibodies from both duodenal plasma cells and circulating naïve B cells, we demonstrate a discord between the naïve TG2-specific repertoire and the cells that are selected for autoantibody production. Hence, the naïve repertoire does not fully reflect the epitope preference and gene usage observed for memory B cells and plasma cells. Instead, distinct naïve B cells that target particular TG2 epitopes appear to be selectively activated at the expense of TG2-binding B cells targeting other epitopes.

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.

Transglutaminase 2 has higher affinity for relaxed than for stretched fibronectin fibers

Transglutaminase 2 (TG2) plays a vital role in stabilizing extracellular matrix (ECM) proteins through enzymatic crosslinking during tissue growth, repair, and inflammation. TG2 also binds non-covalently to fibronectin (FN), an essential component of the ECM, facilitating cell adhesion, migration, proliferation, and survival. However, the interaction between TG2 and fibrillar FN remains poorly understood, as most studies have focused on soluble or surface-adsorbed FN or FN fragments, which differ in their conformations from insoluble FN fibers. Using a well-established in vitro FN fiber stretch assay, we discovered that the binding of a crosslinking enzyme to ECM fibers is mechano-regulated. TG2 binding to FN is tuned by the mechanical tension of FN fibers, whereby TG2 predominantly co-localizes to low-tension FN fibers, while fiber stretching reduces their affinity for TG2. This mechano-regulated binding relies on the proximity between the N-terminal β-sandwich and C-terminal β-barrels of TG2. Crosslinking mass spectrometry (XL-MS) revealed a novel TG2-FN synergy site within TG2's C-terminal β-barrels that interacts with FN regions located outside of the canonical gelatin binding domain, specifically FNI2 and FNIII14-15. Combining XL-MS distance restraints with molecular docking revealed the mechano-regulated binding mechanism between TG2 and modules FNI7-9 by which mechanical forces regulate TG2-FN interactions. This highlights a previously unrecognized role of TG2 as a tension sensor for FN fibers. This novel interaction mechanism has significant implications in physiology and mechanobiology, including how forces regulate cell adhesion, spreading, migration, phenotype modulation, depending on the tensional state of ECM fibers. Data are available via ProteomeXchange with identifier PXD043976.

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.

LRP-1 links post-translational modifications to efficient presentation of celiac disease-specific T cell antigens

Celiac disease (CeD) is an autoimmune disorder in which gluten-derived antigens trigger inflammation. Antigenic peptides must undergo site-specific deamidation to be presentable to CD4+ T cells in an HLA-DQ2 or -DQ8 restricted manner. While the biochemical basis for this post-translational modification is understood, its localization in the patient's intestine remains unknown. Here, we describe a mechanism by which gluten peptides undergo deamidation and concentration in the lysosomes of antigen-presenting cells, explaining how the concentration of gluten peptides necessary to elicit an inflammatory response in CeD patients is achieved. A ternary complex forms between a gluten peptide, transglutaminase-2 (TG2), and ubiquitous plasma protein α2-macroglobulin, and is endocytosed by LRP-1. The covalent TG2-peptide adduct undergoes endolysosomal decoupling, yielding the expected deamidated epitope. Our findings invoke a pathogenic role for dendritic cells and/or macrophages in CeD and implicate TG2 in the lysosomal clearance of unwanted self and foreign extracellular proteins.

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.

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.

Research advances in hydrogen-deuterium exchange mass spectrometry for protein epitope mapping

With the development of biomedical technology, epitope mapping of proteins has become critical for developing and evaluating new protein drugs. The application of hydrogen-deuterium exchange for protein epitope mapping holds great potential. Although several reviews addressed the hydrogen-deuterium exchange, to date, only a few systematic reviews have focused on epitope mapping using this technology. Here, we introduce the basic principles, development history, and review research progress in hydrogen-deuterium exchange epitope mapping technology and discuss its advantages. We summarize the main hurdles in applying hydrogen-deuterium exchange epitope mapping technology, combined with relevant examples to provide specific solutions. We describe the epitope mapping of virus assemblies, disease-associated proteins, and polyclonal antibodies as examples of pattern introduction. Finally, we discuss the outlook of hydrogen-deuterium exchange epitope mapping technology. This review will help researchers studying protein epitopes to gain a more comprehensive understanding of this technology.

B cell tolerance and antibody production to the celiac disease autoantigen transglutaminase 2

Autoantibodies to transglutaminase 2 (TG2) are hallmarks of celiac disease. To address B cell tolerance and autoantibody formation to TG2, we generated immunoglobulin knock-in (Ig KI) mice that express a prototypical celiac patient-derived anti-TG2 B cell receptor equally reactive to human and mouse TG2. We studied B cell development in the presence/absence of autoantigen by crossing the Ig KI mice to Tgm2-/- mice. Autoreactive B cells in Tgm2+/+ mice were indistinguishable from their naive counterparts in Tgm2-/- mice with no signs of clonal deletion, receptor editing, or B cell anergy. The autoreactive B cells appeared ignorant to their antigen, and they produced autoantibodies when provided T cell help. The findings lend credence to a model of celiac disease where gluten-reactive T cells provide help to autoreactive TG2-specific B cells by involvement of gluten-TG2 complexes, and they outline a general mechanism of autoimmunity with autoantibodies being produced by ignorant B cells on provision of T cell help.

Antibodies against neo-epitope of microbial and human transglutaminase complexes as biomarkers of childhood celiac disease

Tissue transglutaminase (tTG) and microbial transglutaminase (mTG) cross-link gliadins to form complexes that expose immunogenic neo-epitopes to produce tTG and mTG-neo-epitope antibodies. The aim of this study was to test the diagnostic performance of antibodies against non-complexed and complexed forms of transglutaminases, to correlate their activities to the intestinal damage and to explore age group dependency in celiac disease (CD). A total of 296 children with untreated CD and 215 non-celiac disease controls were checked by in-house enzyme-linked immunosorbent assays detecting immunoglobulin (Ig)A, IgG or combined detection of IgA and IgG (check) against tTG, AESKULISA® tTG New Generation (tTG-neo) and mTG-neo (RUO), IgA and IgG antibodies against deamidated gliadin peptide (DGP) and human IgA anti-endomysium antibodies (EMA) using AESKUSLIDES® EMA. Intestinal pathology was graded according the revised Marsh criteria, and age dependencies of the antibody activities were analysed. Using cut-offs estimated from receiver operating characteristic (ROC) curves, the highest area under curve (AUC) of the TG assays was 0·963 for tTG-neo check, followed by tTG check (0·962) when the diagnosis was based on enteric mucosal histology. tTG-neo check was the most effective to reflect the intestinal abnormalities in CD (r = 0·795, P < 0·0001). High levels of anti-mTG-neo IgG and anti-tTG-neo IgG appeared in the earlier age groups, as compared to anti-tTG IgG (P < 0·001). Considering antibody diagnostic performance based on AUC, enteric damage reflection and predictability at an early age, the anti-neo tTG check was the most effective diagnostic biomarker for pediatric CD. The mTG neo check might represent a new marker for CD screening, diagnosis and predictability.

Efficient T cell-B cell collaboration guides autoantibody epitope bias and onset of celiac disease

B cells play important roles in autoimmune diseases through autoantibody production, cytokine secretion, or antigen presentation to T cells. In most cases, the contribution of B cells as antigen-presenting cells is not well understood. We have studied the autoantibody response against the enzyme transglutaminase 2 (TG2) in celiac disease patients by generating recombinant antibodies from single gut plasma cells reactive with discrete antigen domains and by undertaking proteomic analysis of anti-TG2 serum antibodies. The majority of the cells recognized epitopes in the N-terminal domain of TG2. Antibodies recognizing C-terminal epitopes interfered with TG2 cross-linking activity, and B cells specific for C-terminal epitopes were inefficient at taking up TG2-gluten complexes for presentation to gluten-specific T cells. The bias toward N-terminal epitopes hence reflects efficient T-B collaboration. Production of antibodies against N-terminal epitopes coincided with clinical onset of disease, suggesting that TG2-reactive B cells with certain epitope specificities could be the main antigen-presenting cells for pathogenic, gluten-specific T cells. The link between B cell epitopes, antigen presentation, and disease onset provides insight into the pathogenic mechanisms of a T cell-mediated autoimmune condition.

Extra-Intestinal Manifestations of Coeliac Disease in Children: Clinical Features and Mechanisms

Celiac disease (CD) is a systemic autoimmune disease due to a dysregulated mucosal immune response to gluten and related prolamines in genetically predisposed individuals. It is a common disorder affecting ~1% of the general population, its incidence is steadily increasing. Changes in the clinical presentation have become evident since the 80s with the recognition of extra-intestinal symptoms like short stature, iron deficiency anemia, altered bone metabolism, elevation of liver enzymes, neurological problems. Recent studies have shown that the overall prevalence of extra-intestinal manifestations is similar between pediatric and adult population; however, the prevalence of specific manifestations and rate of improvement differ in the two age groups. For instance, clinical response in children occurs much faster than in adults. Moreover, an early diagnosis is decisive for a better prognosis. The pathogenesis of extra-intestinal manifestations has not been fully elucidated yet. Two main mechanisms have been advanced: the first related to the malabsorption consequent to mucosal damage, the latter associated with a sustained autoimmune response. Importantly, since extra-intestinal manifestations dominate the clinical presentation of over half of patients, a careful case-finding strategy, together with a more liberal use of serological tools, is crucial to improve the detection rate of CD.

Red Spot Lesions in the Duodenal Bulb Are a Highly Specific Endoscopic Sign of Celiac Disease: A Prospective Study

We have recognized red spot lesions (RSLs) in the duodenal bulb in children with celiac disease (CD) and believe they may represent an underappreciated and distinct endoscopic sign of CD. A total of 171 pediatric patients undergoing esophagogastroduodenoscopy with duodenal biopsy for symptoms consistent with CD were prospectively recruited. There were 75 patients who met criteria for CD and the remaining 96 patients served as symptomatic controls. As compared to endoscopic markers frequently mentioned in literature, RSLs had comparable sensitivity, specificity, positive predictive value, and negative predictive value of 31%, 94%, 80%, and 64%, respectively. If RSLs are noted during endoscopy in a patient with gastrointestinal symptoms that might be the result of CD, then sufficient duodenal biopsies to make the diagnosis of CD should be obtained.

A Functional Idiotype/Anti-Idiotype Network Is Active in Genetically Gluten-Intolerant Individuals Negative for Both Celiac Disease-Related Intestinal Damage and Serum Autoantibodies

An unbalance between Abs that recognize an autoantigen (idiotypes; IDs) and Igs that bind such Abs (anti-IDs) is considered a functional event in autoimmune disorders. We investigated the presence of an ID/anti-ID network in celiac disease (CD), a condition in which antitissue transglutaminase 2 (TG2) Abs are suspected to contribute to CD pathogenesis. To characterize the ID side, we reproduced by in vitro yeast display the intestine-resident Abs from CD and control patients. These TG2-specific IDs were used to identify potential anti-IDs in the serum. We observed elevated titers of anti-IDs in asymptomatic patients with predisposition to CD and demonstrated that anti-ID depletion from the serum restores a detectable humoral response against TG2. Our study provides an alternative approach to quantify CD-related autoantibodies in cases that would be defined "negative serology" with current diagnostic applications. Therefore, we suggest that developments of this technology could be designed for perspective routine tests.

Anti-type 2 transglutaminase antibodies as modulators of type 2 transglutaminase functions: a possible pathological role in celiac disease

Auto-antibodies to the ubiquitous enzyme type-2 transglutaminase (TG2) are a specific hallmark of celiac disease (CD), a widely diffused, multi-factorial disease, affecting genetically predisposed subjects. In CD an inflammatory response, at the intestinal level, is triggered by diet consumption of gluten-containing cereals. Intestinal mucosa displays various degrees of atrophy and hyperplasia, with consequent global intestinal dysfunction and other relevant extra-intestinal symptoms. Through deamidation of specific glutamines of gluten-derived gliadin peptides, TG2 strongly enhances gliadin immunogenicity. In addition, TG2 cross-linking activity may generate complexes between TG2 itself and gliadin peptides, and these complexes seem to cause the auto-immune response by means of an apten-carrier-like mechanism of antigen presentation. Anti-TG2 antibodies can be early detected in the intestinal mucosa of celiac patients and are also abundantly present into the serum, thus potentially reaching other organs and tissues by blood circulation. Recently, the possible pathogenetic role of auto-antibodies to TG2 in CD has been investigated. Here, we report an overview about the genesis of these antibodies, their specificity, their modulating ability toward TG2 enzymatic or non-enzymatic activities and their biological effects exerted by interacting with extracellular TG2 or with cell-surface TG2. We also discuss the auto-immune response occurring in CD against other TG members (i.e. type 3 and type 6) and analyze the occurrence of anti-TG2 antibodies in other auto-immune CD-related diseases. Data now available let us to suppose that, even if antibodies to TG2 do not represent the triggering molecules in CD, they could be important players in disease progression and manifestations.

Autoantibodies in the Extraintestinal Manifestations of Celiac Disease

Increased antibody reactivity towards self-antigens is often indicative of a disruption of homeostatic immune pathways in the body. In celiac disease, an autoimmune enteropathy triggered by the ingestion of gluten from wheat and related cereals in genetically predisposed individuals, autoantibody reactivity to transglutaminase 2 is reflective of the pathogenic role of the enzyme in driving the associated inflammatory immune response. Autoantibody reactivity to transglutaminase 2 closely corresponds with the gluten intake and clinical presentation in affected patients, serving as a highly useful biomarker in the diagnosis of celiac disease. In addition to gastrointestinal symptoms, celiac disease is associated with a number of extraintestinal manifestations, including those affecting skin, bones, and the nervous system. Investigations of these manifestations in celiac disease have identified a number of associated immune abnormalities, including B cell reactivity towards various autoantigens, such as transglutaminase 3, transglutaminase 6, synapsin I, gangliosides, and collagen. Clinical relevance, pathogenic potential, mechanism of development, and diagnostic and prognostic value of the various identified autoantibody reactivities continue to be subjects of investigation and will be reviewed here.

Strong Clonal Relatedness between Serum and Gut IgA despite Different Plasma Cell Origins

Mucosal antigens induce generation of lamina propria plasma cells (PCs) that secrete dimeric immunoglobulin A (IgA) destined for transport across the epithelium. In addition, blood contains monomeric IgA. To study the relationship between mucosal and systemic antibody responses, we took advantage of celiac disease patient samples for isolation of gut PCs as well as serum IgA and IgG reactive with a gluten-derived peptide or the autoantigen transglutaminase 2. Proteomic analysis of serum IgA revealed antigen-specific V-gene preferences, which matched those found in gut PCs. Further, gut PC CDR-H3 sequences were abundant in serum IgA but also detectable in serum IgG. Our data indicate that the same B cell clones that give rise to gut PCs also contribute to the serum antibody pool. However, serum IgA antibodies had a molecular composition distinct from that of IgA antibodies secreted in the gut, suggesting that individual B cell clones give rise to different PC populations.

High-Definition Mapping of Four Spatially Distinct Neutralizing Epitope Clusters on RiVax, a Candidate Ricin Toxin Subunit Vaccine

RiVax is a promising recombinant ricin toxin A subunit (RTA) vaccine antigen that has been shown to be safe and immunogenic in humans and effective at protecting rhesus macaques against lethal-dose aerosolized toxin exposure. We previously used a panel of RTA-specific monoclonal antibodies (MAbs) to demonstrate, by competition enzyme-linked immunosorbent assay (ELISA), that RiVax elicits similar serum antibody profiles in humans and macaques. However, the MAb binding sites on RiVax have yet to be defined. In this study, we employed hydrogen exchange-mass spectrometry (HX-MS) to localize the epitopes on RiVax recognized by nine toxin-neutralizing MAbs and one nonneutralizing MAb. Based on strong protection from hydrogen exchange, the nine MAbs grouped into four spatially distinct epitope clusters (namely, clusters I to IV). Cluster I MAbs protected RiVax's α-helix B (residues 94 to 107), a protruding immunodominant secondary structure element known to be a target of potent toxin-neutralizing antibodies. Cluster II consisted of two subclusters located on the "back side" (relative to the active site pocket) of RiVax. One subcluster involved α-helix A (residues 14 to 24) and α-helices F-G (residues 184 to 207); the other encompassed β-strand d (residues 62 to 69) and parts of α-helices D-E (154 to 164) and the intervening loop. Cluster III involved α-helices C and G on the front side of RiVax, while cluster IV formed a sash from the front to back of RiVax, spanning strands b, c, and d (residues 35 to 59). Having a high-resolution B cell epitope map of RiVax will enable the development and optimization of competitive serum profiling assays to examine vaccine-induced antibody responses across species.

Transglutaminase 2 in human diseases

Transglutaminase 2 (TG2) is an inducible transamidating acyltransferase that catalyzes Ca(2+)-dependent protein modifications. In addition to being an enzyme, TG2 also serves as a G protein for several seven transmembrane receptors and acts as a co-receptor for integrin β1 and β3 integrins distinguishing it from other members of the transglutaminase family. TG2 is ubiquitously expressed in almost all cell types and all cell compartments, and is also present on the cell surface and gets secreted to the extracellular matrix via non-classical mechanisms. TG2 has been associated with various human diseases including inflammation, cancer, fibrosis, cardiovascular disease, neurodegenerative diseases, celiac disease in which it plays either a protective role, or contributes to the pathogenesis. Thus modulating the biological activities of TG2 in these diseases will have a therapeutic value.