In vivo toxicity of a synthetic dodecapeptide from A gliadin in patients with coeliac disease

Development of a Sequence Searchable Database of Celiac Disease-Associated Peptides and Proteins for Risk Assessment of Novel Food Proteins

Celiac disease (CeD) is an autoimmune enteropathy induced by prolamin and glutelin proteins in wheat, barley, rye, and triticale recognized by genetically restricted major histocompatibility (MHC) receptors. Patients with CeD must avoid consuming these proteins. Regulators in Europe and the United States expect an evaluation of CeD risks from proteins in genetically modified (GM) crops or novel foods for wheat-related proteins. Our database includes evidence-based causative peptides and proteins and two amino acid sequence comparison tools for CeD risk assessment. Sequence entries are based on the review of published studies of specific gluten-reactive T cell activation or intestinal epithelial toxicity. The initial database in 2012 was updated in 2018 and 2022. The current database holds 1,041 causative peptides and 76 representative proteins. The FASTA sequence comparison of 76 representative CeD proteins provides an insurance for possible unreported epitopes. Validation was conducted using protein homologs from Pooideae and non-Pooideae monocots, dicots, and non-plant proteins. Criteria for minimum percent identity and maximum E-scores are guidelines. Exact matches to any of the 1,041 peptides suggest risks, while FASTA alignment to the 76 CeD proteins suggests possible risks. Matched proteins should be tested further by CeD-specific CD4/8+ T cell assays or in vivo challenges before their use in foods.

The molecular diversity of α-gliadin genes in the tribe Triticeae

Many of the unique properties of wheat flour are derived from seed storage proteins such as the α-gliadins. In this study these α-gliadin genes from diploid Triticeae species were systemically characterized, and divided into 3 classes according to the distinct organization of their protein domains. Our analyses indicated that these α-gliadins varied in the number of cysteine residues they contained. Most of the α-gliadin genes were grouped according to their genomic origins within the phylogenetic tree. As expected, sequence alignments suggested that the repetitive domain and the two polyglutamine regions were responsible for length variations of α-gliadins as were the insertion/deletion of structural domains within the three different classes (I, II, and III) of α-gliadins. A screening of celiac disease toxic epitopes indicated that the α-gliadins of the class II, derived from the Ns genome, contain no epitope, and that some other genomes contain much fewer epitopes than the A, S(B) and D genomes of wheat. Our results suggest that the observed genetic differences in α-gliadins of Triticeae might indicate their use as a fertile ground for the breeding of less CD-toxic wheat varieties.

Brachypodium distachyon as a model for defining the allergen potential of non-prolamin proteins

Epitope databases and the protein sequences of published plant genomes are suitable to identify some of the proteins causing food allergies and sensitivities. Brachypodium distachyon, a diploid wild grass with a sequenced genome and low prolamin content, is the closest relative of the allergen cereals, such as wheat or barley. Using the Brachypodium genome sequence, a workflow has been developed to identify potentially harmful proteins which may cause either celiac disease or wheat allergy-related symptoms. Seed tissue-specific expression of the potential allergens has been determined, and intact epitopes following an in silico digestion with several endopeptidases have been identified. Molecular function of allergen proteins has been evaluated using Gene Ontology terms. Biologically overrepresented proteins and potentially allergen protein families have been identified.

Dermatitis herpetiformis: from the genetics to the development of skin lesions

Dermatitis herpetiformis (DH) is a rare autoimmune disease linked to gluten sensitivity with a chronic-relapsing course. It is currently considered to be the specific cutaneous manifestation of celiac disease (CD). Both conditions are mediated by the IgA class of autoantibodies, and the diagnosis of DH is dependent on the detection of granular deposits of IgA in the skin. There is an underlying genetic predisposition to the development of DH, but environmental factors are also important. This paper describes these different factors and discusses the known mechanism that lead to the development of skin lesions.

Genetics and pathogenesis of coeliac disease

Coeliac disease is a common complex disease caused by a dietary intolerance to wheat gluten. Susceptibility is determined by both environmental and genetic factors. Coeliac disease results from complex interactions between the innate immune system, an adaptive T and B cell response and the mucosal barrier where inflammation is ultimately manifested. Genetic variants within the HLA region are well established, while variants outside of the HLA region have recently been identified. These variants are beginning to enhance our understanding of the immunology of the condition. This review focuses on the immunological pathogenesis of coeliac disease with special reference to the influence of genetic susceptibility on disease development.

A catalogue of Triticum monococcum genes encoding toxic and immunogenic peptides for celiac disease patients

The celiac disease (CD) is an inflammatory condition characterized by injury to the lining of the small-intestine on exposure to the gluten of wheat, barley and rye. The involvement of gluten in the CD syndrome has been studied in detail in bread wheat, where a set of “toxic” and “immunogenic” peptides has been defined. For wheat diploid species, information on CD epitopes is poor. In the present paper, we have adopted a genomic approach in order to understand the potential CD danger represented by storage proteins in diploid wheat and sequenced a sufficiently large number of cDNA clones related to storage protein genes of Triticum monococcum. Four bona fide toxic peptides and 13 immunogenic peptides were found. All the classes of storage proteins were shown to contain harmful sequences. The major conclusion is that einkorn has the full potential to induce the CD syndrome, as already evident for polyploid wheats. In addition, a complete overview of the storage protein gene arsenal in T. monococcum is provided, including a full-length HMW x-type sequence and two partial HMW y-type sequences.

A unified hypothesis of coeliac disease with implications for management of patients

This mini-review presents the research carried out within the context of two of the main hypotheses of the aetiology of coeliac disease. The enzymopathic hypothesis of the disease has been placed clearly as the underlying deficiency causing increased levels of toxic peptides, while the immunological hypothesis has been implicated in the pathogenesis of the disorder as the result of the action of undigested peptides in the small intestine. As a consequence, we are proposing a unified hypothesis of coeliac disease, which takes into account the actions of these undigested peptides through their direct cytotoxicity and their immunoactivity. At the same time, work aimed at defining some of these biologically active peptides, which could be said to be involved in the aetiopathogenesis of coeliac disease, will be reported. The review also focusses on the use of enzyme therapy for management of the disease, which when used in conjunction with the gluten-free diet, offers a safeguard against damage to the small intestine caused by small amounts of gluten.

The immune recognition of gluten in coeliac disease

Coeliac disease, the most common intestinal disorder of western populations, is an autoimmune enteropathy caused by an abnormal immune response to dietary gluten peptides that occurs in genetically susceptible individuals carrying the HLA-DQ2 or -DQ8 haplotype. Despite the recent progresses in understanding the molecular mechanisms of mucosal lesions, it remains unknown how increased amounts of gluten peptides can enter the intestinal mucosa to initiate the inflammatory cascade. Current knowledge indicates that different gluten peptides are involved in the disease process in a different manner, some fragments being 'toxic' and others 'immunogenic'. Those defined as 'toxic' are able to induce mucosal damage either when added in culture to duodenal endoscopic biopsy or when administered in vivo, while those defined as 'immunogenic' are able to specifically stimulate HLA-DQ2- or DQ8-restricted T cell clones isolated from jejunal mucosa or peripheral blood of coeliac patients. These peptides are able to trigger two immunological pathways: one is thought to be a rapid effect on the epithelium that involves the innate immune response and the other represents the adaptive immune response involving CD4+ T cells in the lamina propria that recognize gluten epitopes processed and presented by antigen presenting cells. These findings are the subject of the present review.

Gluten-free diet--what is toxic?

The cornerstone of treatment of coeliac disease is a gluten-free diet devoid of proteins from wheat, rye, barley and related cereals. Oats are tolerated by most patients with coeliac disease but are not totally innocent. There are considerable differences between individual patients with respect to clinical and mucosal responses to gluten challenge. In vitro and in vivo testing has identified synthetic peptides that are toxic to the coeliac small intestinal mucosa. This toxicity overlaps at least partly to the known epitopes that are recognised by small intestinal T-cells. However, the clinical significance of several of these epitopes is unclear, as is the maximum level of gluten intake that can be recommended to be safe for patients with coeliac disease. Future efforts may lead to better understanding of the disease processes as well as possible new therapeutic options.

Characterizing one of the DQ2 candidate epitopes in coeliac disease: A-gliadin 51-70 toxicity assessed using an organ culture system

OBJECTIVE

To investigate, using an organ culture system, in-vitro toxicity of region 51-70 of A-gliadin (SQQPYLQLQPFPQPQLPYSQ), a peptide overlapping some of the sequences recently characterized as DQ2-restricted T-cell epitopes in coeliac disease.

METHODS

Jejunal biopsies obtained from each of ten coeliac patients (eight treated, two untreated) and two non-coeliac patients were cultured in vitro for 18 h in the presence of A-gliadin amino acids 51-70 (200 microg/ml), organ culture medium only, peptic-tryptic digest of gliadin (1 mg/ml) or ovalbumin (1 mg/ml), the last two acting as positive and negative controls, respectively. Morphometric analysis involved measuring the cell height of 30 enterocytes, selected at random from the middle third of different villi for each section. Mean enterocyte cell heights (ECH) were compared with values for specimens cultured in medium alone. Levels of tissue transglutaminase antibody in biopsy supernatants were assessed by enzyme-linked immunosorbent assay (ELISA).

RESULTS

In eight of ten coeliac patients, A-gliadin 51-70 was significantly toxic, causing a 30% decrease in ECH when compared with medium alone. In two of ten subjects, the peptide did not show any toxic effect. In all ten cases, we found that both positive and negative controls worked as expected. The peptide was non-toxic in the non-coeliac individuals. Tissue transglutaminase antibody titre in the supernatant was not found to be related to mucosal damage.

CONCLUSIONS

We showed that the peptide corresponding to amino acids 51-70 of A-gliadin is characterized by in-vitro toxicity to the jejunal coeliac mucosa, correlating with recent findings of an immunological role of similar peptides. The lack of response in two of ten subjects suggests that this epitope may not be relevant in all cases of coeliac disease.

Coeliac disease

In the last few decades, the comprehension of epidemiological, pathogenic and clinical aspects of coeliac disease has increasingly improved. Serological screening studies on the general population have shown that the true coeliac disease prevalence in Europe is higher than previously reported. It has become clear that tissue transglutaminase has a crucial role in the pathogenesis of coeliac disease pathogenesis, and there is evidence that substitution of deamidated amino acidic residues at a critical position along the gliadin sequence dramatically increases immunological activation. The toxicity of many gluten epitopes has been investigated, so far, but recent studies have indicated the region 57-75 of alpha gliadin as a possible candidate epitope in the pathogenesis of coeliac disease. However, the wide heterogeneity of gliadin and glutenin molecules complicate any attempts to identify the toxic epitope, and the fascinating idea to produce detoxified grains will represent a great challenge in the near future. From a clinical point of view, there is now evidence of a broad spectrum of gluten conditions. Extra-intestinal signs, i.e., alopecia, unexplained neurological disorders, cryptic hypertransaminasaemia, increased red cell width, frequently constitute the only clinical manifestation at the diagnosis.

Coeliac disease--all questions answered?

Gliadin specific T cells in the small intestines of coeliac disease patients use the disease associated human leukocyte antigen-DQ2 molecules in their antigen recognition. In an exciting interplay with tissue transglutaminase, the immune system recognises modified gliadin peptides and mounts a phlogistic response. Moreover, the role for autoimmune phenomena and the mechanism of breaking of immunological tolerance remain elusive.

The changing face of celiac disease

The face of celiac disease has changed significantly over the past 50 years. With the advent of new noninvasive and more sensitive screening tools, it has become increasingly apparent that this disease presents in a heterogeneous fashion, with symptomatic disease only occurring in a small number of patients. Furthermore, great insights have been made into the disease's genetic and immunological components, thus increasing the medical community's understanding of the disease. The current gold standard for diagnosis is histological confirmation, and the cornerstone of therapy is lifelong elimination of gluten. Further advances in immunobiological techniques will most likely aid in earlier detection and commencement of the appropriate diet, thus preventing the development of associated complications.

Analysis and clinical effects of gluten in coeliac disease

The prolamin working group coordinates research on laboratory gluten analysis in food and on clinical evaluation of patient sensitivity to prolamins. As an observer organization to the Codex Alimentarius Commission, the group summarizes current data on analysis and effects of gluten in coeliac disease. All types of gliadin, the ethanol-soluble fraction of gluten, contain the coeliac-active factor. However, coeliac toxicity and immunogenicity (humoral and cellular) of various prolamins are not identical in coeliac patients. There are no conclusive data on the threshold of gluten sensitivity of coeliac patients. Information as to the long-term risk to coeliac patients exposed to small doses of gliadin is lacking. Therefore, every effort should be made to keep the diet of coeliac patients as gluten-free as possible. The prolamin group is currently evaluating a new enzyme-linked immunosorbent assay (ELISA) protocol for gluten analysis that could serve as a basis for further Codex regulations. The group recommends adherence to a single Codex limit for gluten-free foods. The current limit of 200 ppm gluten is questionable and requires reconsideration based on new information that will be available soon.

Human genome search in celiac disease using gliadin cDNA as probe

Celiac disease is a wheat gliadin-promoted disorder that displays a complex genetic susceptibility associated with HLA-DQ2, and one or more unknown factor(s), possibly gliadin-like. The presence of mammalian proteins with partial gliadin similarity was suggested by transglutaminase-independent multi-tissue reactivity of gliadin-immunopurified antibodies from celiac patients. No non-plant sequence, however, was identified in gliadin peptide epitope searches of non-redundant and EST databanks via TBLASTN, BLASTP and FASTA, even at E values as high as 20. Therefore, an alpha-gliadin cDNA screen of human cDNA and genomic libraries was undertaken, an approach in keeping with positive human Northern and Southern analyses with the same probe. Four distinct cDNA clones were obtained, the most stringent of which (3.2 and 5.1 kb) were novel, and featured potential open reading frames with high gliadin domain II and domain IV homologies (BestFit quality scores >/=295 and 322, respectively, versus random value 126-127). Both were also homologous to ESTs. An additional 5' gliadin oligonucleotide screen identified the widely distributed cytoplasmic protein acyl coA hydrolase whose homology was restricted to the oligonucleotide probe (BestFit quality=215 versus 100 for random); and achaete-scute homologous protein, which displays particularly high gliadin domain II homology (BestFit quality 316 versus 111 for random). Genomic screening uncovered 16 positives, one of which was the ALR gene, whose similarity to three of gliadin's five domains (I, II and IV; BestFit quality 322-473 versus 121-154 for random) was remarkable. More extensive was novel genomic clone 2, with fragments hybridizing to cDNA probes approximating gliadin domains I, II+IV, V and the gliadin 5' untranslated region, and mapping by FISH to 19q13.11-13. 12. Two fragments were sequenced; one was exonic, as predicted by four different programs; and test oligonucleotides suggested widespread 4 and/or 2 kb mRNA expression, even at high stringency (t(m)-8.8 deg. C). Taken together, it is apparent that several genes with partial gliadin homology exist in the human genome. Many bear gliadin-like T-cell epitopes, are expressed in intestine and, like transglutaminase, are cytoplasmic. Glutamine to glutamic acid or other mutation within such epitopes followed by injury or infection-related release could explain enhanced disease susceptibility in affected families.

In vivo antigen challenge in celiac disease identifies a single transglutaminase-modified peptide as the dominant A-gliadin T-cell epitope

Celiac disease (CD) is an increasingly diagnosed enteropathy (prevalence, 1:200-1:300) that is induced by dietary exposure to wheat gliadins (as well as related proteins in rye and barley) and is strongly associated with HLA-DQ2 (alpha1*0501, beta1*0201), which is present in over 90% of CD patients. Because a variety of gliadin peptides have been identified as epitopes for gliadin-specific T-cell clones and as bioactive sequences in feeding studies and in ex vivo CD intestinal biopsy challenge, it has been unclear whether a 'dominant' T-cell epitope is associated with CD. Here, we used fresh peripheral blood lymphocytes from individual subjects undergoing short-term antigen challenge and tissue transglutaminase-treated, overlapping synthetic peptides spanning A-gliadin to demonstrate a transient, disease-specific, DQ2-restricted, CD4 T-cell response to a single dominant epitope. Optimal gamma interferon release in an ELISPOT assay was elicited by a 17-amino-acid peptide corresponding to the partially deamidated peptide of A-gliadin amino acids 57-73 (Q65E). Consistent with earlier reports indicating that host tissue transglutaminase modification of gliadin enhances gliadin-specific CD T-cell responses, tissue transglutaminase specifically deamidated Q65 in the peptide of A-gliadin amino acids 56-75. Discovery of this dominant epitope may allow development of antigen-specific immunotherapy for CD.

Glutenin is involved in the gluten-driven mucosal T cell response

Gluten ingestion causes coeliac disease in susceptible individuals. Gluten is a heterogeneous mixture of glutenin and gliadin, the latter of which is considered responsible for disease induction. By combining high-performance liquid chromatography purification steps of gluten with a T cell bioassay and mass spectral analyses, we have identified a glutenin peptide (glt04 707-742) that activates T cells from the small intestine of a coeliac disease patient and results in the secretion of large amounts of IFN-gamma. The minimal T cell stimulatory core of the peptide (residues 724-734) is repetitively present in glutenin molecules. Moreover, it was observed that a large number of naturally occurring variants of this peptide are recognized by the T cells. These data suggest that the large heterogeneity of glutenin proteins dramatically increases the number of available T cell epitopes. Together, the results provide new insight into the nature of the gluten antigens that lead to coeliac disease and suggest that glutenin, next to gliadin-derived antigens, may be involved in the disease process.

In vitro screening of food peptides toxic for coeliac and other gluten-sensitive patients: a review

Experience gained through investigations on coeliac disease makes it possible to propose a screening method based on agglutination of isolated K562(S) cells to evaluate the occurrence in food protein of amino acid sequences that are able to adversely affect coeliac and related gluten-sensitive patients. The method consists of in vitro sequential peptic and tryptic digestion of food protein fractions under optimal pH, temperature and time conditions and in vitro incubation of the digest with K562(S) cells; the toxic potential is detected as an agglutination of K 562 (S) cells after a short incubation. Other in vitro test systems, including atrophic coeliac intestinal mucosa and rat fetal intestine, can be used to confirm the results obtained with the isolated cells. A fractionation step of the proteolytic digest on a sepharose-mannan column before exposure of the in vitro systems to the separated peptide fractions adds to the sensitivity of the method. This screening method is not only very useful to investigate action mechanisms in coeliac disease, but also to assess the safety of genetically-modified plant foods and novel foods for gluten-sensitive patients.

Anti-alpha-gliadin antibodies (AGA) in the serum of coeliac children and controls recognize an identical collection of linear epitopes of alpha-gliadin

Anti-gliadin antibodies can be found in the serum of patients with overt and subclinical coeliac disease, but also in that of some controls. The aim of the present study was to identify the linear epitopes of the alpha-gliadin molecule to which the humoral response is directed. Therefore, the IgG and IgA antibody reactivity against an overlapping set of synthetic peptides covering the entire sequence of alpha-gliadin was measured in the sera from patients with coeliac disease, from controls with elevated titres of anti-gliadin antibodies and from healthy children using an ELISA technique. The antibodies mainly recognize peptides derived from the N-terminal region of alpha-gliadin, containing the motif QPFXXQXPY. Reactivity was also detected against two other synthetic peptides, which do not contain this motif and represent a sequence encoded further to the C-terminal region of alpha-gliadin. Anti-gliadin antibodies in sera from patients with coeliac disease and from controls recognize the same linear epitopes. Thus, serological investigation of the specificity of these antibodies using a peptide ELISA does not allow discrimination between patients and controls.

Small intestinal T cells of celiac disease patients recognize a natural pepsin fragment of gliadin

Celiac disease is a common severe intestinal disease resulting from intolerance to dietary wheat gluten and related proteins. The large majority of patients expresses the HLA-DQ2 and/or DQ8 molecules, and gluten-specific HLA-DQ-restricted T cells have been found at the site of the lesion in the gut. The nature of peptides that are recognized by such T cells, however, has been unclear so far. We now report the identification of a gliadin-derived epitope that dominantly is recognized by intestinal gluten-specific HLA-DQ8-restricted T cells. The characterization of such epitopes is a key step toward the development of strategies to interfere in mechanisms involved in the pathogenesis of celiac disease.

Use of Complete Eluted Peptide Sequence Data from HLA-DR and -DQ Molecules to Predict T Cell Epitopes, and the Influence of the Nonbinding Terminal Regions of Ligands in Epitope Selection

In diseases with a strong association with an HLA haplotype, identification of relevant T cell epitopes may allow alteration of the pathologic process. In this report we use a reverse immunogenetic approach to predict possible HLA class II-restricted T cell epitopes by using complete pool sequencing data. Data from HLA-DR2(B1*1501), -DR3(B1*0301), -DQ2(A1*0501, B1*0201), and -DQ8(A1*0301, B1*0302) alleles were used by a computer program that searches a candidate protein to predict ligands with a relatively high probability of being processed and presented. This approach successfully identified both known T cell epitopes and eluted single peptides from the parent protein. Furthermore, the program identified ligands from proteins in which the binding motif of the HLA molecule was unable to do so. When the information from the nonbinding N- and C-terminal regions in the pool sequence was removed, the ability to predict several ligands was markedly reduced, particularly for the HLA-DQ alleles. This suggests a possible role for these regions in determining ligands for HLA class II molecules. Thus, the use of complete eluted peptide sequence data offers a powerful approach to the prediction of HLA-DQ and -DR peptide ligands and T cell epitopes.

Relation between gliadin structure and coeliac toxicity

Gliadin, the alcohol-soluble protein fraction of wheat, contains the factor toxic for coeliac patients. The numerous components of gliadin can be classified according to their primary structure into omega 5-, omega 1,2-, alpha- and gamma-type. Both omega-types have almost entirely repetitive amino acid sequences consisting of glutamine, proline and phenylalanine. alpha- and gamma-type gliadins contain four and five different domains, respectively, and are homologous within the domains III and V. Unique for each alpha- and gamma-type is domain I, which consists mostly of repetitive sequences rich in glutamine, proline and aromatic amino acids. Coeliac toxicity of gliadin is not destroyed by digestion with gastropancreatic enzymes. In vivo testing established the toxicity of alpha-type gliadins and in vitro testing of gliadin peptides revealed that domain I of alpha-type gliadins is involved in activating coeliac disease. The sequences -Pro-Ser-Gln-Gln- and -Gln-Gln-Gln-Pro- were demonstrated to be common for toxic gliadin peptides. Most of the in vivo and in vitro studies of synthetic peptides confirmed the importance of one or both of these sequences. Cultivated hexaploid, tetraploid and diploid wheat species do not differ significantly in potential toxic sequences of alpha-type gliadins.

In vitro hydrolysis of gliadin and casein peptides: secondary defect in coeliac disease shown by organ culture

BACKGROUND

Small-intestinal organ culture was used as an in vitro model of coeliac disease, studying biopsy specimens from patients with coeliac disease, cow's milk allergy, and controls.

METHODS

Organ culture incubations were done using the pure gliadin peptide B3144 (amino acid sequences 3-56 of alpha-type gliadins) and a control peptide from casein (amino acid sequences 152-193 of alpha s1-casein). The importance of using negative controls was stressed by non-specific tissue damage. By reversed-phase high-performance liquid chromatography of organ culture supernatants, 27 specimens were further investigated.

RESULTS

There was good retrieval of peptide calibration peaks after culture. Qualitative and quantitative evaluation of chromatography runs showed degradation of at least 29% of B3144 and 37% of Cas-P. Normal mucosa (controls and coeliac patients on a gluten-free diet) was able to hydrolyse peptide fractions completely, whereas incubation with damaged mucosa (coeliac disease, cow's milk allergy) left initial peptides.

CONCLUSION

It is concluded, using a pure single gliadin peptide, that deficient peptide hydrolysis in coeliac disease was a secondary event.

In vitro toxicity of gluten peptides in coeliac disease assessed by organ culture

BACKGROUND

We have used organ culture to investigate the in vitro toxicity of three oligopeptides corresponding to amino acids 31-49 (peptide A), 202-220 (peptide B), and 3-21 (peptide C) of A-gliadin, Frazer's fraction III (FFIII), and ovalbumin.

METHODS

Eight to 14 jejunal biopsy specimens were obtained from each of 8 treated and 7 untreated coeliac patients and 5 normal controls and cultured for 18 h in organ culture with test peptide (1 mg/ml) or medium alone. Mean enterocyte cell heights (ECH) were compared with paired values for specimens grown in medium alone.

RESULTS

A significant reduction in the mean of the ECH values for each of the patient groups was observed with peptide A and FFIII in both treated (p = 0.01 and 0.02, respectively) and untreated (p = 0.03 and 0.01) coeliac patients when compared with tissue incubated with medium alone. No significant changes in the mean ECH value were noted in any of the patient groups in tissue incubated with peptide B, peptide C, or ovalbumin as compared with those with medium alone.

CONCLUSIONS

These results suggest that peptide A is toxic in vitro to the jejunal mucosa of both treated and untreated coeliac patients, correlating with recent findings that this peptide exacerbates coeliac disease in vivo.

[Spelt wheat and celiac disease]

Spelt wheat (Triticum spelta L.) has not been investigated for the toxicity on coeliac disease patients until now. Because clinical studies are out of considerations for ethical reasons, spelt wheat and coeliac-active bread wheat (Triticum aestivum L.) were compared by the analysis of N-terminal sequences of alpha-gliadins, which have been proposed to be responsible for the toxic effect. The gliadin fractions of the spelt wheats 'Roquin' and 'Schwabenkorn' and of the bread wheat 'Rektor' were preparatively separated by RP-HPLC and major alpha-gliadin components were then compared by N-terminal sequence analysis. The results did not reveal any significant difference between spelt and bread wheats within the first 25 positions. For the determination of sequences further from the N-terminus, the gliadin fractions of the spelt wheats were hydrolyzed with pepsin and trypsin. The resulting peptides were successively separated by gel permeation chromatography and RP-HPLC. Those peptides derived from the N-terminal part of alpha-gliadins were identified by reference peptides isolated previously from bread wheat [this journal 194: 229 (1992)]. Retention times upon RP-HPLC and amino acid compositions of corresponding peptides confirmed the identity of spelt and bread wheat concerning the N-terminal sequences of alpha-gliadins from position 3 to 56. For these reasons, it can be concluded that spelt wheat is a coeliac-toxic cereal and has to be avoided by coeliac patients.

Cytokine mRNA expression in the mucosa of treated coeliac patients after wheat peptide challenge

This study investigated the presence of mRNA coding for interferon gamma (IFN gamma), tumour necrosis factor alpha (TNF alpha), and interleukins 2 (IL2) and 6 (IL6), in the mucosa of four coeliac patients in remission who had been challenged with either gliadin or synthetic gliadin oligopeptides. Jejunal biopsy specimens from these patients, taken before and at two, four, and six hours after challenge, were hybridised with specific 35S-labelled DNA oligonucleotide probes. The lamina propria of all the patients contained significantly increased numbers of cytokine mRNA expressing cells four hours after challenge with gliadin or an oligopeptide corresponding to amino acids 31-49 of A-gliadin (peptide A). No significant changes were seen with the peptides corresponding to aminoacids 202-220 (peptide B) or 3-21 (peptide C) of A-gliadin, with the exception of one patient who showed a significant increase in the number of TNF alpha mRNA expressing cells four hours after challenge with peptide B. In vivo studies in coeliac disease have shown that significant histological changes occur in the mucosa of treated coeliac patients four hours after challenge with either gliadin or peptide A. These findings suggest that the histological changes seen previously in the mucosa of coeliac patients after wheat peptide challenge may be caused by increased expression of cytokines within the mucosa.

The precipitating factor in coeliac disease

In recent years, remarkable progress has been made in the elucidation of cereal protein structure and its relation to coeliac toxicity. Gluten proteins of wheat can be classified according to their primary structure into high-, medium- and low-molecular-weight (HMW, MMW, LMW) groups. Each of these groups contains two or three different protein types having partly homologous, partly unique, structural elements: chi- and gamma-type HMW subunits of glutenin (HMW group), omega 5 and omega 1,2-type gliadins (MMW group) and alpha-type gliadins, gamma-type gliadins and LMW subunits of glutenin (LMW group). Numerous proteins from the same type do exist with only a few modifications of the amino-acid sequence. The structure of the HMW and LMW group proteins can be divided into three and five domains, respectively. Most typical for each type and unique for cereals are the glutamine- and proline-rich domains containing repetitive sequences (HMW group: domain B; LMW group: domain I). omega-type gliadins consist almost entirely of repetitive sequences. Rye and barley, closely related to wheat, have protein types homologous to those of wheat. Early investigations showed that wheat gluten and, in particular, the alcohol-soluble gliadin fraction contained the factor toxic for coeliac patients. Equivalent protein fractions of rye, barley and probably oats were also considered to be toxic. The effects of toxic proteins were not destroyed by digestion with pepsin, trypsin and pancreatin. In-vivo (instillation) testing established the toxicity of alpha-type gliadins, and in-vitro (organ culture) testing of gliadin peptides demonstrated that the N-terminal region (domain I) of alpha-type gliadins is involved in activating coeliac disease. The longest sequences common for toxic peptides were found to be -Pro-Ser-Gln-Gln- and -Gln-Gln-Gln-Pro-. Various in-vitro tests and two in-vivo studies on synthetic peptides support the importance of one or both of these sequences. They do not occur in non-toxic food proteins and are characterized by their ability to form a beta-turn conformation. Although these sequences are probably not sufficient for toxicity in themselves, and other amino-acid residues are additionally required, they could serve as the starting point for further investigation.

Further studies of the in vitro activity of synthetic gliadin peptides in coeliac disease

Studies of in vitro activity of synthetic peptides derived from the A-gliadin structure were carried out using assays based on cultures of foetal chick intestinal mucosa and on incubation with rat liver lysosomes. The peptide corresponding to residues 11-19, displayed very high activity in the chick intestinal assay, but was only weakly active in the lysosomal assay. Peptide 9-19 was highly active in the chick intestinal assay but was only mildly active in the lysosomal assay. Peptide 8-19 was still appreciably active in both assays. The results on this group of peptides suggest the importance of residues 8-12 to activity and possibly also of a N-terminal glutamine residue. The peptide 213-227, found in a sub-fraction of fraction 9, was only weakly active in both assays, indicating that the PSQQ motif was not solely responsible for toxicity. Thus, as the peptide 208-219 was shown previously to be active in the chick intestinal assay, it is likely that the 208-212 region of this peptide is of prime importance in conferring activity. The results show, for the first time, that a nonapeptide from the N-terminal region of A-gliadin is very active in an in vitro model of toxicity in coeliac disease.

Wheat peptide challenge in coeliac disease

The exact nature of the cereal moiety that exacerbates coeliac disease is unknown. In-vitro studies have implicated both the N-terminal and far C-terminal domains of one of the wheat prolamins, A-gliadin. Peptides within these regions may act as epitopes that trigger immune events leading to enteropathy. We synthesized three peptides corresponding to amino-acids 3-21, 31-49, and 202-220 of A-gliadin. Four patients with coeliac disease were challenged by intraduodenal infusion of 1 g of gliadin or 200 mg of the synthetic peptides. Jejunal biopsies were taken before and at hourly intervals for 6 h after the infusion. Morphometric variables were measured and intraepithelial lymphocytes counted. Significant histological changes occurred in the small intestinal mucosa after challenge with a synthetic peptide corresponding to amino acids 31-49 of A-gliadin. The N-terminal peptide, residues 3-21 of A-gliadin, did not cause histological changes in any of the patients. In one of the four patients, minor histological changes following challenge with the peptide corresponding to residues 202-220 of A-gliadin were seen. Our results suggest that the oligopeptide corresponding to aminoacids 31-49 of A-gliadin is toxic in vivo, but there is no evidence of toxicity of the far N-terminal peptide, residues 3-21. The C-terminal peptide 202-220 may contain an epitope to which patients with coeliac disease display variable sensitivity. Since the oligopeptide corresponding to amino-acids 31-49 of A-gliadin is recognised by HLA DQ2-restricted T cells, the observed effects may be due to immune activation within the intestinal mucosa.

Coeliac disease: characterisation of monoclonal antibodies raised against a synthetic peptide corresponding to amino acid residues 206-217 of A-gliadin

A dodecapeptide of A-gliadin, which shares amino acid homologies with the E1b protein of adenovirus 12, was used to produce murine monoclonal antibodies. Five monoclonal antibodies were produced and were screened by enzyme linked immunosorbant assay, immunodot assay, and immunoblotting. The antibodies were tested against whole wheat gliadin and its alpha, beta, gamma, and omega subfractions, and the prolamins of rye, barley, oats, maize, millet, rice, and sorghum. Four of the five antibodies cross reacted with one or more of the coeliac non-toxic cereals--maize, millet, sorghum, and rice. The monoclonal antibody that did not cross react with these non-toxic cereals, did not recognize Frazer's fraction III, a peptic-tryptic digest of wheat gluten which is known to be toxic. The results suggest that the A-gliadin dodecapeptide shares a region of homology with cereals that do not exacerbate coeliac disease. This study does not support the hypothesis that prior infection with adenovirus 12 is a precipitating factor in coeliac disease.