Binding of gluten-derived peptides to the HLA-DQ2 (alpha1*0501, beta1*0201) molecule, assessed in a cellular assay

The human Major Histocompatibility Complex as a paradigm in genomics research

Since its discovery more than 50 years ago, the human Major Histocompatibility Complex (MHC) on chromosome 6p21.3 has been at the forefront of human genetic research. Here, we review from a historical perspective the major advances in our understanding of the nature and consequences of genetic variation which have involved the MHC, as well as highlighting likely future directions. As a consequence of its particular genomic structure, its remarkable polymorphism and its early implication in numerous diseases, the MHC has been considered as a model region for genomics, being the first substantial region to be sequenced and establishing fundamental concepts of linkage disequilibrium, haplotypic structure and meiotic recombination. Recently, the MHC became the first genomic region to be entirely re-sequenced for common haplotypes, while studies mapping gene expression phenotypes across the genome have strongly implicated variation in the MHC. This review shows how the MHC continues to provide new insights and remains in the vanguard of contemporary research in human genomics.

The pathogenesis of coeliac disease

Coeliac disease is a common condition and its prevalence in UK is now thought to be approximately 1:100. It is being diagnosed and treated more frequently as awareness at the primary care level has increased. Coeliac disease is a complex disorder and is frequently associated with other disease processes. The management of these patients needs to take on a holistic approach, whilst the physician needs to be aware of the rare complications. This article gives an up-to-date review of the literature written on the pathogenesis of coeliac disease. We have attempted to paint a picture from beginning to end, whilst clarifying the grey areas in between. General epidemiological factors are reviewed before looking at genetic risk factors. We assess the sensitivity and specificity of the investigative modalities available for clinical use and comment on optimum management of these patients thereafter. The future of coeliac disease looks promising for patients with several novel therapies on the horizon. Whilst further work is still needed to breed out the toxic epitopes from wheat, novel therapies may come from other areas such as the work aimed at restoring normal tolerance to gluten.

Coeliac disease: in vivo toxicity of the putative immunodominant epitope

BACKGROUND

Peptides from alpha-gliadins have been used to characterise the immunodominant coeliac toxic epitope. A peptide corresponding to amino acid residues 57-73 of A-gliadin causes peripheral blood mononuclear cells from coeliac patients to secrete interferon gamma (IFN-gamma); gluten specific small intestinal T cell clones proliferate in response to peptides corresponding to residues 57-68 and 62-75 of alpha-gliadins. We wished to investigate whether a peptide corresponding to residues 56-75 of alpha-gliadins exacerbates coeliac disease in vivo.

METHODS

Four adults with coeliac disease, all of whom were on a gluten free diet, underwent three challenges. Peptic-tryptic gliadin (PTG 1 g) served as a positive control. The test peptide and a negative control peptide were studied on separate occasions. The peptides were instilled into the duodenum and biopsies were taken before the infusion, and two, four, and six hours after commencing the infusions, using a Quinton hydraulic multiple biopsy capsule. Biopsy specimens were assessed blindly for villus height to crypt depth ratio (VH:CD), enterocyte cell height (ECH), and intraepithelial lymphocyte (IEL) count. We used the Mann-Whitney U test, with 95% confidence intervals, for statistical analysis.

RESULTS

VH:CD and ECH fell, and IEL increased significantly 4-6 hours after commencing infusions with both PTG and the test peptide in all subjects. The negative control peptide caused no significant changes to villus morphology, enterocyte height, or IEL count in any patient.

CONCLUSION

We have confirmed that the putative immunodominant epitope, a peptide corresponding to residues 56-75 of alpha-gliadins, exacerbates coeliac disease in vivo.

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 vivo toxicity of the putative immunodominant epitope

BACKGROUND

Peptides from alpha-gliadins have been used to characterise the immunodominant coeliac toxic epitope. A peptide corresponding to amino acid residues 57-73 of A-gliadin causes peripheral blood mononuclear cells from coeliac patients to secrete interferon gamma (IFN-gamma); gluten specific small intestinal T cell clones proliferate in response to peptides corresponding to residues 57-68 and 62-75 of alpha-gliadins. We wished to investigate whether a peptide corresponding to residues 56-75 of alpha-gliadins exacerbates coeliac disease in vivo.

METHODS

Four adults with coeliac disease, all of whom were on a gluten free diet, underwent three challenges. Peptic-tryptic gliadin (PTG 1 g) served as a positive control. The test peptide and a negative control peptide were studied on separate occasions. The peptides were instilled into the duodenum and biopsies were taken before the infusion, and two, four, and six hours after commencing the infusions, using a Quinton hydraulic multiple biopsy capsule. Biopsy specimens were assessed blindly for villus height to crypt depth ratio (VH:CD), enterocyte cell height (ECH), and intraepithelial lymphocyte (IEL) count. We used the Mann-Whitney U test, with 95% confidence intervals, for statistical analysis.

RESULTS

VH:CD and ECH fell, and IEL increased significantly 4-6 hours after commencing infusions with both PTG and the test peptide in all subjects. The negative control peptide caused no significant changes to villus morphology, enterocyte height, or IEL count in any patient.

CONCLUSION

We have confirmed that the putative immunodominant epitope, a peptide corresponding to residues 56-75 of alpha-gliadins, exacerbates coeliac disease in vivo.

Investigation of the putative immunodominant T cell epitopes in coeliac disease

BACKGROUND

Coeliac disease (CD) is an enteropathy mediated by gluten specific T cells which secrete interferon gamma (IFN-gamma) when stimulated by gluten peptides presented by HLA-DQ2 or DQ8 molecules. Residues 62-75 of alpha(2) gliadin have been proposed as the immunodominant epitope in the majority of CD patients. Deamidation by tissue transglutaminase (tTG) of the glutamine (Q) at position 65 to glutamic acid (E) is essential for T cell stimulation.

AIMS

To investigate the antigenicity of this peptide and to establish whether its T cell activating properties can be downregulated by the formation of altered peptide ligands.

PATIENTS

Individuals with known CD.

METHODS

Peptide G4 corresponding to alpha(2) gliadin residues 62-75, Q-E65 and analogues, substituting each amino acid, except E65, in turn for alanine residues, were synthesised. Small intestinal biopsies were obtained from patients. Biopsies were cultured overnight with a peptic/tryptic digest of gliadin (PTG). Lymphocytes were cultured and restimulated with tTG treated PTG. A T cell line was cloned and clones tested for stimulation and IFN-gamma production in response to G4 and its analogues.

RESULTS

Some high activity clones were isolated with, for example, a stimulation index (SI) of 15 to G4 and secreting 327 pg/ml of IFN-gamma. Substitution of amino acids at several positions abolished or downregulated stimulation and IFN-gamma production.

CONCLUSIONS

Peptide G4 is highly immunogenic. Certain amino acid substitutions in peptide G4 abolish T cell reactivity while others are partial agonists which may have potential in immunomodulation in this condition.

In vitro presentation of gliadin-derived peptides by different cell lines

BACKGROUND

Gliadin peptide presentation and T-cell activation are critical events in the pathogenesis of celiac disease. Several studies have been performed to identify the toxic gliadin peptides but the complexity of the antigenic fraction makes this analysis difficult. In this work, an in vitro model for the analysis of gliadin peptide presentation is studied.

METHODS

The human cell lines U937 and THP-1 (monocytic), DUCAF and VAVY (immortalised B cells) and HT-29 and Caco-2 (intestinal epithelial cells) were incubated with biotin-labelled gliadin (bG). FITC-labelled streptavidin was used to detect biotinylated peptides at the cell surface by flow cytometry.

RESULTS

All cell lines tested showed a fluorescence signal derived from bG, that was highest when cells were stimulated with IFN-gamma for 48 h. Time course experiments performed using THP-1 cells showed that after 4-h incubation, almost a maximal signal can be reached. THP-1 cells incubated at 4 degrees C or after paraformaldehyde fixation showed a substantial signal reduction, suggesting that metabolic activity was necessary for the detection of the maximal fluorescence signal at the cell surface. The presence of HLA class II-bound biotinylated peptides was observed in cell lysates of THP-1 cells incubated with bG.

CONCLUSIONS

In all cell lines tested, a specific biotin-peptide-derived signal was observed. This was increased after IFN-gamma treatment and decreased after fixation or incubation at low temperature. The signal was higher in monocytic and B-cell lines than in the epithelial cell lines. The use of this procedure could be a useful tool to study the in vitro processing and presentation of naturally gliadin-derived peptides.

Celiac disease: strongly heritable, oligogenic, but genetically complex

Celiac disease, or gluten-sensitive enteropathy, is a small intestinal disorder which affects up to 1:250 people in the United States. Disease development has a strong genetic component, with a sibling relative risk (lambda(s)) of 30. One susceptibility locus is the MHC region, with a particular association with the HLA-DQ alleles DQA1*0501 and DQB1*0201. However, haplotype sharing studies suggest that genes within the MHC complex contribute no more than 40% to the sibling familial risk of disease. This means that the stronger genetic risk is likely to be conferred by a small number of non-HLA-linked genes. Genome-wide linkage studies, plus linkage and association studies of candidate loci have been used to try to identify these genes. However, these studies have either failed to detect loci, or produced inconsistent results. Such difficulties in identifying susceptibility genes are encountered when investigating any complex genetic disorder. Information from the Human Genome Project, coupled with new technology for high throughput single nucleotide polymorphism typing may help to identify the non-HLA determinants of celiac disease in the future.

Celiac disease

Increased awareness of the high prevalence of celiac disease and the degree of underdiagnosis has led to calls for routine screening in certain groups. The potential consequences of underdiagnosis have been the subject of several studies. Where there is a delay in diagnosis, there may be an increased risk of associated autoimmune diseases. Alternatively, there may be marked neurological complications, or fetal growth retardation where pregnancy preceeds diagnosis. Once a diagnosis is obtained, treatment may become easier with further evidence that oats are well tolerated by most people with celiac disease. Screening will be facilitated by the development of highly sensitive and specific enzyme-linked immunosorbent assays for tissue transglutaminase. Further insights into the genetics of the condition have been gained with the discovery of a new human leukocyte antigen susceptibility type, plus a possible non-human-leukocyte antigen susceptibility gene.

Quantitative analysis of peptide-MHC class II interaction

The tremendous progress in the field of basic immunology and immunochemistry made in the last decade has significantly advanced our understanding of antigen processing and presentation by MHC class I and II proteins. In this review different techniques to study peptide interaction with MHC class II molecules are summarized and their impact on the elucidation of quantitative parameters, like affinities or kinetic data, is discussed. A recently introduced method based on synthetic combinatorial peptide libraries allows to quantify the binding contribution of each amino acid residue in a class II ligand and is presented in more detail. As this knowledge is fundamental for current investigations in modern medicine, e.g. for novel immune system based therapy concepts, further aspects like the design of new high affinity MHC class II ligands and the prediction of peptide antigens are discussed.

Expression of interleukin-4 and interferon-gamma in the small bowel of patients with dermatitis herpetiformis and isolated gluten-sensitive enteropathy

Although possessing a morphologically similar small bowel abnormality to patients with isolated gluten-sensitive enteropathy (GSE), patients with dermatitis herpetiformis (DH) have few gastrointestinal symptoms and exhibit blistering skin lesions and cutaneous IgA deposits. To determine whether clinical discrepancies between these gluten-sensitive conditions might be the result of different patterns of small bowel cytokine expression, duodenal biopsies were obtained from eight DH patients and nine isolated GSE patients. Biopsies were evaluated for interleukin-4 (IL-4) and interferon-gamma (IFN-gamma) expression by reverse-transcriptase polymerase chain reaction (message) and immunohistochemistry (protein). In DH patients, most of whom had no gut symptoms, IFN-gamma mRNA expression was significantly less than in isolated GSE patients with symptomatic gut disease. Conversely, IL-4 mRNA expression in DH patients was greater than that found among isolated GSE patients. These findings suggest that the different clinical phenotypes of gluten sensitivity may be caused by variation in cytokine expression in the small bowel response to gluten.

A non-toxic analogue of a coeliac-activating gliadin peptide: a basis for immunomodulation?

BACKGROUND

A-gliadin residues 31-49 (peptide A) binds to HLA-DQ2 and is toxic to coeliac small bowel. Analogues of this peptide, which bind to DQ2 molecules but are non-toxic, may be a potential route to inducing tolerance to gliadin in patients with coeliac disease.

METHODS

Toxicity was investigated with small bowel organ culture in six patients with untreated coeliac disease, four with treated coeliac disease and six controls. Analogue peptides comprised alanine substituted variants of peptide A at L31 (peptide D), P36 (E), P38 (F), P39 (G) and P42 (H).

RESULTS

Peptides D and E were toxic in biopsies from some patients. Peptides F, G and H were not toxic.

CONCLUSIONS

Peptide F, which binds to DQ2 more strongly than peptide A, is not toxic in patients with coeliac disease in-vitro; this could be an initial step towards investigation of the induction of tolerance to gliadin in patients affected by coeliac disease.

Celiac disease

There continues to be much research interest in celiac disease, with well over 200 papers published in the year under review, covering a multitude of areas from population screening to the molecular immunology of disease pathogenesis. The high prevalence of "silent" disease again has been demonstrated, and there is continued emphasis on the increasingly wide recognized range of associated conditions and extraintestinal manifestations. The case for the safety of oats is further strengthened by a study involving patients with dermatitis hepetiformis. One of the most significant scientific advances has been the discovery of a potential role for tissue transglutaminase, recently found to be the autoantigen of antiendomysial antibodies, in disease pathogenesis, by enhancing gliadin peptide class II binding and consequent T-cell activation. However, the very central role of T cells has been thrown into doubt with an elegant study that assessed the effect of blocking costimulation in T-cell activation.

The genetic basis for the association of the 8.1 ancestral haplotype (A1, B8, DR3) with multiple immunopathological diseases

An individual's major histocompatibility complex (MHC) ancestral haplotype (AH) is the clearest single determinant of susceptibility to MHC associated immunopathological disease, as it defines the alleles carried at all loci in the MHC. However, the direct effects of any of the 150-200 genes that constitute the MHC are difficult to determine since recombination only occurs at defined hotspots. This review concerns the 8.1 AH (HLA-A1, C7, B8, C4AQ0, C4B1, DR3, DQ2), which is carried by most Caucasians with HLA-B8. It is associated with accelerated human immunodeficiency virus (HIV) disease, and susceptibility to insulin-dependent diabetes mellitus (IDDM), systemic lupus erythematosus, dermatitis herpetiformis, common variable immunodeficiency and IgA deficiency, myasthenia gravis and several other conditions. We have mapped susceptibility genes for HIV, IDDM and myasthenia gravis to the central MHC between HLA-B and the tumour necrosis factor or complement genes. Here we consider which of the remaining 8.1-associated diseases are more closely associated with HLA-DR3 and/or DQ2. Several candidate genes in the central MHC have the potential to modulate immune or inflammatory responses in an antigen-independent manner, as is seen in studies of cultured cells from healthy carriers of the 8.1 AH. Hence these genes may act as a common co-factor in the diverse immunopathological conditions associated with the 8.1 AH.

Review article: coeliac disease and its management

Coeliac disease (CD) is a disorder of the small intestine, characterized by villous atrophy, due to an intolerance to dietary gluten in genetically susceptible individuals, which responds to gluten withdrawal. The underlying immunological mechanisms causing the disorder are still being worked out. In recent years a wide range of clinical presentations has become increasingly apparent, as has a lengthening list of associated conditions. Severe malabsorption with steatorrhoea and profound weight loss is seen infrequently, perhaps as a result of earlier diagnosis and the recognition of 'silent' and 'latent' disease. The prevalence of CD as judged by population screening with, in particular, anti-endomysial antibodies, appears to be much higher than that found with clinically apparent cases. There are a variety of well recognized complications, the commonest probably being osteopenia and osteoporosis. The marked increased risk of lymphoma can be avoided by a strict gluten-free diet. Follow-up of patients needs to be lifelong with prompt investigation of new symptoms and blood test abnormalities.

Increased HLA-DQ2-affinity of a synthetic gliadin peptide by acid-induced deamidation of glutamine residues

Presentation of antigenic gliadin peptides by the HLA-DQ2 molecule is considered as a key event in celiac disease pathogenesis. Chemical deamidation of the side chains of glutamine residues might have a strong influence on gliadin peptide binding to the DQ2 molecule. Glutamine deamidation of A-gliadin peptide (45-56) under acidic conditions corresponding to the gastric environment was studied using RP-HPLC, Edman degradation, capillary electrophoresis and electrospray mass spectrometry. Deamidation resulted in peptides with increased DQ2-affinities as assessed in a cell-free binding assay.

Measurement of gluten using a monoclonal antibody to a coeliac toxic peptide of A-gliadin

BACKGROUND

Future European Community regulations will require a sensitive and specific assay for measurement of coeliac toxic gluten proteins in foods marketed as gluten-free. To avoid spurious cross reactions with non-toxic proteins, specific antibodies and target antigens are required. A synthetic 19 amino acid peptide of A gliadin has been shown to cause deterioration in the morphology of small intestinal biopsy specimens of coeliac patients in remission.

AIMS

To develop an assay for detection of gluten in foods, based on measurement of a known toxic peptide.

METHODS

A monoclonal antibody raised against the toxic A gliadin peptide, with a polyclonal anti-unfractionated gliadin capture antibody, was used to develop a double sandwich enzyme linked immunosorbent assay (ELISA) for the measurement of gluten in foods.

RESULTS

Standard curves for gliadin and for rye, barley, and oat prolamins were produced. The sensitivity of the assay was 4 ng/ml of gliadin, 500 ng/ml for rye prolamins, and 1000 ng/ml for oat and barley prolamins. The assay could detect gluten in cooked foods, although at reduced sensitivity. Prolamins from coeliac non-toxic rice, maize, millet, and sorghum did not cross react in the assay. A variety of commercially available gluten-free foods were analysed; small quantities of gluten were detected in some products.

CONCLUSION

The assay may form the basis of a sensitive method for measurement of gluten in foods for consumption by patients with coeliac disease.