Purification and characterisation of antigenic gliadins in coeliac disease

Liquid chromatography and mass spectrometry in food allergen detection

Food allergy is an important issue in the field of food safety because of the hazards for affected persons and the hygiene requirements and legal regulations imposed on the food industry. Consumer protection and law enforcement require suitable analytical techniques for the detection of allergens in foods. Immunological methods are currently preferred; however, confirmatory alternatives are needed. The determination of allergenic proteins by liquid chromatography and mass spectrometry has greatly advanced in recent years, and gel-free allergenomics is becoming a routinely used approach for the identification and quantitation of food allergens. The present review provides a brief overview of the principles of proteomic procedures, various chromatographic set ups, and mass spectrometry instrumentation used in allergenomics. A compendium of published liquid chromatography methods, proteomic analyses, typical marker peptides, and quantitative assays for 14 main allergy-causing foods is also included.

Determination of anti-omega-gliadin antibodies in serologic tests for coeliac disease

BACKGROUND

Serologic detection of coeliac disease in the general population or in subjects belonging to risk groups implies the use of a test with high efficiency, large-scale use, and low cost. The enzyme-linked immunosorbent assay (ELISA) technique is the most appropriate assay for performing this kind of studies. Even though anti-gliadin determination has been the test most frequently used as the first step in screening procedures, many false-positive results produced a low-specificity test. In a previous work a selective recognition of omega-gliadins, mainly by IgA antibodies, was observed. Results also indicated that omega-gliadins can be useful as antigens in serologic detection of coeliac disease. We therefore wanted to analyse the anti-gliadin antibody reactivity by using purified gliadins and to evaluate the actual performance of the anti-omega-gliadin antibody test.

METHODS

A population consisting of 105 coeliac patients, 81 healthy controls, and 73 subjects in a disease control group was analysed. Anti-endomysium (EMA), both IgG and IgA anti-omega-gliadins, and anti-tissue transglutaminase (tTG) antibodies were determined.

RESULTS

Concordant results, positive and negative, in the EMA and IgG and IgA anti-gliadin determinations were observed in 220 of 259 samples from the total population analysed. The three assays showed high efficiency, being 96.9%, 90.7%, and 91.1% for EMA and anti-omega-gliadins IgG and IgA, respectively. Anti-tTG determination was performed on 103 samples (69 controls and 34 coeliac patients), finding 4 false results (2 false positive and 2 false negative), whereas anti-omega-gliadins showed 10 false results (5 false negative and 5 false positive), 3 of which were coincident with anti-tTG determination. To compare the reactivity of anti-gliadin antibodies, alpha-, beta-, gamma- and omega-gliadins were isolated under non-denaturing conditions by acid preparative electrophoresis and cation-exchange fast protein liquid chromatography (FPLC) and used in an indirect ELISA test. The composition of these fractions was analysed by means of capillary electrophoresis, showing no cross-contamination among them.

CONCLUSIONS

The comparison of results using purified gliadins shows that omega-gliadins present a differential reactivity that has not previously been documented. Results using omega-gliadins isolated by either preparative electrophoresis or FPLC were similar. Tests using the purified omega-gliadin fraction present the best performance when anti-gliadin antibodies are evaluated.

The intestinal T cell response to alpha-gliadin in adult celiac disease is focused on a single deamidated glutamine targeted by tissue transglutaminase

The great majority of patients that are intolerant of wheat gluten protein due to celiac disease (CD) are human histocompatibility leukocyte antigen (HLA)-DQ2(+), and the remaining few normally express HLA-DQ8. These two class II molecules are chiefly responsible for the presentation of gluten peptides to the gluten-specific T cells that are found only in the gut of CD patients but not of controls. Interestingly, tissue transglutaminase (tTG)-mediated deamidation of gliadin plays an important role in recognition of this food antigen by intestinal T cells. Here we have used recombinant antigens to demonstrate that the intestinal T cell response to alpha-gliadin in adult CD is focused on two immunodominant, DQ2-restricted peptides that overlap by a seven-residue fragment of gliadin. We show that tTG converts a glutamine residue within this fragment into glutamic acid and that this process is critical for T cell recognition. Gluten-specific T cell lines from 16 different adult patients all responded to one or both of these deamidated peptides, indicating that these epitopes are highly relevant to disease pathology. Binding studies showed that the deamidated peptides displayed an increased affinity for DQ2, a molecule known to preferentially bind peptides containing negatively charged residues. Interestingly, the modified glutamine is accommodated in different pockets of DQ2 for the different epitopes. These results suggest modifications of anchor residues that lead to an improved affinity for major histocompatibility complex (MHC), and altered conformation of the peptide-MHC complex may be a critical factor leading to T cell responses to gliadin and the oral intolerance of gluten found in CD.

Identification of a gliadin T-cell epitope in coeliac disease: general importance of gliadin deamidation for intestinal T-cell recognition

Coeliac disease probably results from a T-cell response to wheat gliadin and is associated to HLA-DQ2. No gliadin epitopes recognized by intestinal T cells have yet been identified, limiting our understanding of the pathogenesis. Gut-lesion-derived DQ2-restricted T cells from coeliac disease patients were used to identify an epitope within a purified gamma-type gliadin. The structure of the epitope was characterized by mass spectrometry and verified by synthesis. The epitope (QPQQSFPEQQ) results from deamidation of a distinct glutamine in the native structure. This deamidation is important for binding to DQ2 and T-cell recognition. Other gut-derived T cells fail to recognize the epitope, although deamidation of unfractionated gliadin enhances the response of all gut-derived DQ2-restricted T cells isolated from several patients. Several DQ2-restricted T-cell epitopes exist, but for all of them deamidation of glutamine residues appears to be critical for creation of active epitopes. Native gliadin has few negatively charged residues but is very rich in glutamine. After deamidation gliadin becomes a rich source of DQ2 epitopes thus providing a link between DQ2, gliadin and coeliac disease. The necessity for modification may have general immunological relevance.

Identification of major rye secalins as coeliac immunoreactive proteins

Six distinct gamma- and omega-type secalins, together with two new low molecular mass glycoproteins, have been identified as the major coeliac immunoreactive proteins from a chloroform/methanol soluble extract from rye endosperm. These components were characterized by a combination of reverse-phase high-performance liquid chromatography, immunoblotting using a coeliac serum and microsequencing analysis. This allowed the identification of a group of secalins with different molecular masses according to their N-terminal amino-acid sequence: one omega-type secalin of 40 kDa (omega 1-40); three gamma-type secalins, one of 70 kDa (gamma-70) and two of 35 kDa (gamma-35); as well as two low molecular mass glycoproteins of 15 and 18 kDa, all exhibiting coeliac serum antigenicity. Moreover, four additional rye components, including two low molecular mass proteins, which did not react with coeliac sera, have also been identified. Analysis by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) of the three main purified coeliac immunogenic secalins, gamma-70, gamma-35 and omega 1-40, indicated molecular masses of 71457, 32240 and 39117 Da, respectively. The omega 1-40 secalin displays a significant absorption in the visible region which could be related to its peculiar low capacity to bind both coeliac sera antibodies and Coomassie brilliant blue dye.

Characterization of distinct alpha- and gamma-type gliadins and low molecular weight components from wheat endosperm as coeliac immunoreactive proteins

Distinct alpha- and gamma-type gliadins, as well as a few low molecular weight components have been identified as coeliac immunoreactive proteins from a chloroform/methanol extract from wheat endosperm. Characterization of these components involved the combination of reverse-phase high-performance liquid chromatography, immunoblotting following SDS-PAGE using a coeliac serum and microsequencing analysis. This has allowed the identification of a group of gliadins with different molecular weights, according to their N-terminal amino-acid sequence: five alpha-type gliadins of 31, 35, 38 and two of 45 kDa, one gamma 2-type gliadin of 40 kDa, two gamma 3-type gliadins of 31, and 50 kDa, and two gamma-type gliadins with an atypical gliadin N-terminal of 31, and 40 kDa, as well as a few unidentified low molecular weight components and three N-terminal blocked proteins, all exhibiting similar antigenicity.

Gamma-type gliadins cause secretion of prostaglandin E2 in patients with coeliac disease

Coeliac disease is induced by polypeptides in the prolamin fraction of wheat, termed gliadin. It has previously been demonstrated that the alpha-, the beta- and the gamma-gliadin fractions contain toxic components and it has furthermore been strongly indicated that alpha-type gliadins are toxic. Due to insufficient protein separation methods there has been no information as to whether also the gamma-type gliadins are injurious in coeliac disease. We have therefore purified one alpha-type (alpha-39) and two gamma-type gliadins (gamma-36 and gamma-47) in a preparative scale by a combination of different ion exchange chromatographies. The purity was analyzed by high performance liquid chromatography and by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate, while the typing was based on determination of N-terminal amino acid sequence. Six patients with coeliac disease in remission were included in the study. Each of the purified gliadins was given by an intestinal perfusion technique to two patients. The perfusion fluid was collected and analyzed for the concentration of prostaglandin E2 (PGE2) as a marker for a toxic effect. All patients reacted with increased PGE2 secretion. For the first time it is clearly demonstrated that gamma-type gliadins are active in coeliac disease.