High-resolution mass spectrometry unveils the molecular changes of ovalbumin induced by heating and their influence on IgE binding capacity.

Ovalbumin (OVA) is a food allergen whose allergenicity is modulated by heating. We aimed to establish a molecular connection between heat-induced structural modifications and the modulation of the IgE binding capacity of OVA. For this, we used model samples of heat-modified OVA with increasing complexity; glycated, aggregated, or glycated and aggregated. Using sera from egg-allergic individuals, we show that both aggregation and glycation strongly impacted IgE binding capacity, despite limited structural changes for glycated OVA. A molecular exploration at the amino acid level using high-resolution mass spectrometry revealed extensive cross-linking, mostly through disulfide and dehydroprotein bridges, and moderate but significant glycation. Structural modifications affected residues located within or at a few amino acids distance of known human linear IgE epitopes, such as C121, K123, S169, K190, K207, H332 and C368. We thus unveil key amino residues implicated in the changes in IgE binding of OVA induced by heating.

Critical structural elements for the antigenicity of wheat allergen LTP1 (Tri a 14) revealed by site-directed mutagenesis.

Lipid transfer proteins (LTPs) were identified as allergens in a large variety of pollens and foods, including cereals. LTPs belong to the prolamin superfamily and display an α-helical fold, with a bundle of four α-helices held together by four disulfide bonds. Wheat LTP1 is involved in allergic reactions to food. To identify critical structural elements of antibody binding to wheat LTP1, we used site-directed mutagenesis on wheat recombinant LTP1 to target: (i) sequence conservation and/or structure flexibility or (ii) each disulfide bond. We evaluated the modifications induced by these mutations on LTP1 secondary structure by synchrotron radiation circular dichroism and on its antigenicity with patient's sera and with mouse monoclonal antibodies. Disruption of the C28-C73 disulfide bond significantly affected IgE-binding and caused protein denaturation, while removing C13-C27 bond decreased LTP1 antigenicity and slightly modified LTP1 overall folding. In addition, we showed Lys72 to be a key residue; the K72A mutation did not affect global folding but modified the local 3D structure of LTP1 and strongly reduced IgE-binding. This work revealed a cluster of residues (C13, C27, C28, C73 and K72), four of which embedded in disulfide bonds, which play a critical role in LTP1 antigenicity.

Development of incurred chocolate bars and broth powder with six fully characterised food allergens as test materials for food allergen analysis.

The design and production of incurred test materials are critical for the development and validation of methods for food allergen analysis. This is because production and processing conditions, together with the food matrix, can modify allergens affecting their structure, extractability and detectability. For the ThRAll project, which aims to develop a mass spectrometry-based reference method for the simultaneous accurate quantification of six allergenic ingredients in two hard to analyse matrices. Two highly processed matrices, chocolate bars and broth powder, were selected to incur with six allergenic ingredients (egg, milk, peanut, soy, hazelnut and almond) at 2, 4, 10 and 40 mg total allergenic protein/kg food matrix using a pilot-scale food manufacturing plant. The allergenic activity of the ingredients incurred was verified using food-allergic patient serum/plasma IgE, the homogeneity of the incurred matrices verified and their stability at 4 °C assessed over at least 30-month storage using appropriate enzyme-linked immunosorbent assays (ELISA). Allergens were found at all levels from the chocolate bar and were homogenously distributed, apart from peanut and soy which could only be determined above 4 mg total allergenic ingredient protein/kg. The homogeneity assessment was restricted to analysis of soy, milk and peanut for the broth powder but nevertheless demonstrated that the allergens were homogeneously distributed. All the allergens tested were found to be stable in the incurred matrices for at least 30 months demonstrating they are suitable for method development.

Digestion differently affects the ability of native and thermally aggregated ovalbumin to trigger basophil activation.

Ovalbumin (OVA), a major allergen from hen's egg albumen, tends to aggregate when heated. Depending on the balance of attractive and repulsive interactions, heat-induced OVA aggregates have various morphologies, which differ in digestibility. In the context of food allergy to egg, we investigated the ability of native and thermally aggregated OVA as well as their digests to induce the degranulation of a humanized rat basophil leukemia (RBL) cell line, which was sensitized with a pool of sera from egg-allergic children. Native and two thermally aggregated OVA forms were digested in vitro using a gastrointestinal digestion model based on the INFOGEST harmonized protocol including a final degradation with jejunal brush border membranes (BBM) enzymes. The course of digestion was monitored by the OPA method and by RP-HPLC. Digestibility was OVA small aggregates>OVA large aggregates>>native OVA and BBM peptidases only significantly hydrolyzed small-sized peptides from gastro-duodenal digests of the aggregates. The degranulation ability of the native OVA slightly changed during the gastric phase but mostly decreased during the duodenal digestion with no further change with BBM digestion. The degranulation ability of aggregates, which was significantly lower than the ability of native OVA, was not significantly affected by digestion. Digestibility and ability to induce basophil degranulation can thus not be straightforward linked.

Relative reactivity to egg white and yolk or change upon heating as markers for baked egg tolerance.

BACKGROUND: Hen's egg food allergy is frequent in childhood and phenotypically heterogeneous. Some children can tolerate extensively heated egg. We investigated whether individual relative responses could differentiate children who tolerate baked egg. METHODS: Reactivities to raw, pasteurized or hard-boiled egg (E), egg white (EW), and egg yolk (EY) fractions were tested by skin prick test (SPT) in 54 egg-allergic children. IgE-sensitization to EW and EY was determined by ImmunoCAP and IgE-binding to EW and 8 EW proteins and to EY and 4 EY sub-fractions by ELISA. Population heterogeneity was assessed by hierarchical ascending classification upon individual variations of reactivity and links between classifications and clinical features by analyzing the contingency tables. RESULTS: All children had positive SPT to raw E and raw EW and 72% to raw EY. Heating decreased SPT-reactivity for some children, pasteurization being less effective than hard-boiling. Children were classed into three classes from relative SPT-reactivity to raw fractions, two from variations of SPT-reactivity with each thermal processing or EW/EY ratio of sensitization, and four from their sensitization pattern. Classifications according to heating were found independent of each other. SPT variations with hard-boiling, IgE-sensitization (ratio or pattern) were linked to allowance by the physicians of egg in baked products. CONCLUSIONS: Egg-allergic children were often both sensitized to EY and EW, and heterogeneous patterns of relative responses were evidenced. Irrespective of age and level of sensitization, a low EW/EY ratio or SPT getting null with hard-boiling was found in children allowed to eat baked egg.

Acid-Hydrolyzed Gliadins Worsen Food Allergies through Early Sensitization.

SCOPE: Food allergies result from a complex immune response involving both innate and adaptive immune cells. Major proteins of wheat flour, gliadins, appear to be important allergens, and their characteristics can influence the allergic response. This study investigates the immune reaction when developing a food allergy to gliadins in native, deamidated, or hydrolyzed forms. METHODS: The immune response after one or two intraperitoneal sensitizations and after oral challenge with each gliadin form is analyzed. RESULTS: Results demonstrate that deamidated gliadins induce a stronger allergic reaction compared to native gliadins. Moreover, deamidation induces an earlier increase in intestinal permeability associated with more pronounced Th2 and Th17 polarizations together with an influx of antigen-presenting cells, especially cDC2. CONCLUSION: Altogether, Results indicate that industrial processes such as deamidation or hydrolysis influences food allergenicity through immune modulation and helps us to develop tools to determine how these processes can influence this reaction and encourage or decrease allergic reactions.

How Proteins Aggregate Can Reduce Allergenicity: Comparison of Ovalbumins Heated under Opposite Electrostatic Conditions.

Heated foods are recommended for avoiding sensitization to food proteins, but depending on the physicochemical conditions during heating, more or less unfolded proteins aggregate differently. Whether the aggregation process could modulate allergenicity was investigated. Heating ovalbumin in opposite electrostatic conditions led to small (A-s, about 50 nm) and large (A-L, about 65 µm) aggregates that were used to sensitize mice. The symptoms upon oral challenge and rat basophil leukemia degranulation with native ovalbumin differed on the basis of which aggregates were used during the sensitization. Immunoglobulin-E (IgE) production was significantly lower with A-s than with A-L. Although two common linear IgE-epitopes were found, the aggregates bound and cross-linked IgE similarly or differently, depending on the sensitizing aggregate. The ovalbumin aggregates thus displayed a lower allergenic potential when formed under repulsive rather than nonrepulsive electrostatic conditions. This further demonstrates that food structure modulates the immune response during the sensitization phase with some effects on the elicitation phase of an allergic reaction and argues for the need to characterize the aggregation state of allergens.

Structural Basis of IgE Binding to α- and γ-Gliadins: Contribution of Disulfide Bonds and Repetitive and Nonrepetitive Domains.

Wheat products cause IgE-mediated allergies. The present study aimed to decipher the molecular basis of α- and γ-gliadin allergenicity. Gliadins and their domains, the repetitive N-terminal and the nonrepetitive C-terminal domains, were cloned and expressed in Escherichia coli. Their secondary structures and their IgE binding capacity were compared with those of natural proteins before and after reduction/alkylation. Allergenicity was evaluated with sera from patients who had a wheat food allergy or baker's asthma. The secondary structures of natural and recombinant proteins were slightly different. Compared with natural gliadins, recombinant proteins retained IgE binding but with reduced reactivity. Reduction/alkylation decreased IgE binding for both natural and recombinant gliadins. Although more continuous epitopes were identified in the N-terminal domains of α- and γ-gliadins, both the N-terminal and C-terminal domains contributed to IgE binding. As for other members of the prolamin superfamily, disulfide bonds appear to be of high importance for IgE binding.

Meta-analysis of IgE-binding allergen epitopes.

IgE-binding epitopes are related to allergic symptoms by eliciting degranulation of special cells and release of molecules that trigger the hypersensitivity reaction. Little is known about what characterises allergen IgE-binding epitopes, although advances in analytical methods have led to the identification of a large number of them. To assess if a binary classification of allergen regions into epitopes or non-epitopes may accurately reflect biological reality, we computed the fraction of allergen amino acids that are involved in epitopes. A relationship between this fraction and the increasing number of literature references was modelled. Due to the wide variety of methods that are used in the literature, a peak in the number of matches between an allergen sequence and its epitopes confirms their validity. Accordingly, our graphical representation of positive assays along sequences provides an overview of epitope localisation, which should help to highlight major positions for IgE binding to allergens.

Wheat gliadins modified by deamidation are more efficient than native gliadins in inducing a Th2 response in Balb/c mice experimentally sensitized to wheat allergens.

SCOPE: Wheat gluten proteins such as gliadins constitute major food allergens. Gluten can be modified industrially by deamidation which increases its solubility and enhances its use as a food ingredient. Sensitization to deamidated gluten has been reported to cause severe allergic reactions with anaphylaxis. The aim of this study was therefore to compare the sensitization and elicitation potentials of native (NG) and deamidated (DG) gliadins. The reactivity pattern of mice IgE was also compared with that of DG-allergic patients. METHODS AND RESULTS: The ability of DG to sensitize Balb/c mice using intra-peritoneal administration with aluminium hydroxide as an adjuvant, and to elicit an allergic response after a challenge, was tested in comparison with NG. Mice sensitized with DG secreted higher levels of total IgE, IL-4, gliadin-specific IgE and IgG1 than mice sensitized with NG. By contrast, mice sensitized with NG produced higher levels of gliadin-specific IgG2a and INFγ. After a challenge, histamine levels were higher in mice sensitised with DG. CONCLUSIONS: DG can sensitize mice much more efficiently than NG. Moreover, this mouse model of allergy to DG revealed an IgE reactivity pattern against purified gliadins which was very similar to that of DG-allergic patients.

Intestinal translocation capabilities of wheat allergens using the Caco-2 cell line.

Because intestinal absorption of food protein can trigger an allergic reaction, the effect of wheat proteins on intestinal epithelial cell permeability was evaluated and the abilities of these proteins in native or pepsin-hydrolyzed state to cross the epithelial cell monolayer were compared. Enterocytic monolayers were established by culturing Caco-2 cells, a model of enterocytes, on permeable supports that separate the apical and basal compartments. Proteins were added into the apical compartment, and the transepithelial resistance (TER) was measured; proteins that crossed the cell monolayer were detected in the basal medium by ELISA. Wheat proteins did not alter the cell monolayer. TER and Caco-2 cell viability were conserved, and the passage of dextran was prevented. Native and pepsin-hydrolyzed forms of omega5-gliadin and lipid transfer proteins were detected in the basal medium. The results suggest that these two major allergens in food allergy to wheat were able to cross the cell monolayer by the transcellular route.

Perceptual interactions between characteristic notes smelled above aqueous solutions of odorant mixtures.

Twenty-two experienced panelists rated odor intensity of aqueous solutions of citral, octen-1-ol-3, and hexanal. The panel assessed unmixed components and mixtures (9 binary and 4 ternary). In sensory sessions dedicated to mixtures (n = 6), evaluation was focused on one target odor, presented at a fixed concentration. All components had lower odor intensity on mixed presentations. In many cases, information obtained from simpler systems was not extended to complex mixtures. In a mixture, the competition between odorant molecules on qualitative aspects (dominance/suppression) imbalanced components contribution, anticipated from the quantitative distribution. Hexanal appeared to be the potentially weaker odorant in paired combinations, whereas octen-1-ol-3 had a lower relative impact on ternary systems. Suppression of the odor of octen-1-ol-3 and a concomitant increase in the odor of hexanal was common to all ternary mixtures. Reciprocal inhibition of octen-1-ol-3 and citral odors through perceptual interactions was suspected. Mutual suppression is suspected to have eased the perception of hexanal intensity.

Flavor perception and aroma release from model dairy desserts.

Six model dairy desserts, with three different textures and two sucrose levels, were equally flavored with a blend of four aroma compounds [ethyl pentanoate, amyl acetate, hexanal, and (E)-2-hexenal] and evaluated by a seven person panel in order to study whether the sensory perception of the flavor and the aroma release during eating varied with the textural characteristics or the sweetness intensity of the desserts. The sensory perception was recorded by the time intensity (TI) method, while the in vivo aroma release was simultaneously measured by the MS-nose. Considering the panel as a whole, averaged flavor intensity increased with sucrose level and varied with the texture of the desserts. Depending on the aroma compound, the averaged profile of in vivo aroma release varied, but for each aroma compound, averaged aroma release showed no difference with the sucrose level and little difference with the texture of the desserts. Perceptual sweetness-aroma interactions were the main factors influencing perception whatever the texture of the desserts.

The nature of the apolar phase influences the structure of the protein emulsifier in oil-in-water emulsions stabilized by bovine serum albumin. A front-surface fluorescence study.

Proteins are widely used as emulsifiers in food emulsions. Model emulsions, designed to study emulsifying properties of proteins and their conformation at the interfaces often contain a hydrocarbon as apolar phase instead of natural triglycerides as found in food products. Yet, some results indicate that the protein conformation at the interface depends on the nature of the apolar phase. Front-surface fluorescence spectroscopy was used to evidence differences in the structure of bovine serum albumin (BSA) adsorbed at the interface of emulsions prepared with different apolar phases: an hydrocarbon (n-dodecane), a synthetic medium-chain triglyceride (miglyol) and a natural vegetable oil (sunflower oil). Emulsions had similar size distributions of oil droplets. Front-surface fluorescence emission spectra of tryptophanyl residues of the protein (Trp) in emulsions, creams and serums varied as a function of the nature of hydrophobic phase. In emulsions and creams, wavelength of the maximum fluorescence intensities was blue-shifted as compared to the BSA solution. The shift was larger in creams than in emulsions and in samples containing dodecane than with the other apolar phases. Fourth derivative spectra of emulsions and creams exhibited two peaks assigned, respectively, to Trp located in hydrophilic and hydrophobic environments. The peaks were slightly red-shifted in the presence of sunflower oil as compared to miglyol and dodecane and the relative intensity of the "hydrophobic peak" was higher in dodecane. The effects were greater in creams than in emulsions. Fluorescence intensity of Trp was the highest in the serums of emulsions prepared with dodecane as compared to serums issued from sunflower oil and miglyol emulsions. Thus, proportion of adsorbed protein was lower in dodecane emulsions than with the other apolar phases. These results evidence that the mean environment of Trp was more hydrophobic in emulsions and creams than in solutions due to a displacement of some of the Trp of the protein to a more hydrophobic environment. Dodecane had the greatest impact on Trp environment (more hydrophobic) followed by miglyol and then by sunflower oil. This is likely due to differences in the conformation of the protein at the hydrocarbon-water interface as compared to the triacylglycerol-water ones. In addition, sunflower oil provoked a large decrease of Trp fluorescence intensity in emulsions and creams as compared to miglyol or dodecane. This could be due to contaminant quenchers in the oil or to interactions of the unsaturated fatty chains with the protein inducing quenching of the Trp. These observations should be related to the physical properties of the apolar phase and its molecular organization and interactions with the protein at the interface.