IgE-binding and in vitro gastrointestinal digestibility of egg allergens in the presence of polysaccharides

Effect of lactic acid bacteria fermentation on cow milk allergenicity and antigenicity: A review

Cow milk is a major allergenic food. The potential prevention and treatment effects of lactic acid bacteria (LAB)-fermented dairy products on allergic symptoms have garnered considerable attention. Cow milk allergy (CMA) is mainly attributed to extracellular and/or cell envelope proteolytic enzymes with hydrolysis specificity. Numerous studies have demonstrated that LAB prevents the risk of allergies by modulating the development and regulation of the host immune system. Specifically, LAB and its effectors can enhance intestinal barrier function and affect immune cells by interfering with humoral and cellular immunity. Fermentation hydrolysis of allergenic epitopes is considered the main mechanism of reducing CMA. This article reviews the linear epitopes of allergens in cow milk and the effect of LAB on these allergens and provides insight into the means of predicting allergenic epitopes by conventional laboratory analysis methods combined with molecular simulation. Although LAB can reduce CMA in several ways, the mechanism of action remains partially clarified. Therefore, this review additionally attempts to summarize the main mechanism of LAB fermentation to provide guidance for establishing an effective preventive and treatment method for CMA and serve as a reference for the screening, research, and application of LAB-based intervention.

A Food Matrix Triggers a Similar Allergic Immune Response in BALB/c Mice Sensitized with Native, Denatured, and Digested Ovalbumin

The search for an animal model to evaluate the allergenic potential of processed food products is still ongoing. Both the sensitization to ovalbumin (OVA) in different structural states and the allergic response triggered after intragastric or food challenges were assessed. BALB/c mice were sensitized intraperitoneally to OVA (50 µg) in different structural states (native OVA, N-OVA; denatured OVA, D-OVA; formaldehyde- and lysine-treated OVA, FK-OVA; denatured OVA-FK, OVA-DFK; peptides from pepsin digestion, Pep-OVA). Anti-OVA-specific IgE responses were evaluated using ELISA. Anaphylactic signs and mMCP-1 serum levels were evaluated after intragastric (2.0 mg/OVA) and food (0.41 mg/OVA) challenges. IgE reactivities to N-OVA and D-OVA were similar among groups (p > 0.05). After the challenges, all OVA-sensitized mice developed mild to severe anaphylactic signs (p < 0.05 vs. control). Mice sensitized to N-OVA and D-OVA had the highest mMCP-1 serum levels after challenges (p < 0.05 vs. control). Allergic responses were similar despite the different OVA doses used for the challenges. The N-OVA-sensitized murine model of egg allergy proposed in the present study holds the potential for evaluating the impact of food matrix composition and processing on the threshold of egg-allergic responses.

Critical features of an in vitro intestinal absorption model to study the first key aspects underlying food allergen sensitization

New types of protein sources will enter our diet in a near future, reinforcing the need for a straightforward in vitro (cell-based) screening model to test and predict the safety of these novel proteins, in particular their potential risk for de novo allergic sensitization. The Adverse Outcome Pathway (AOP) for allergen sensitization describes the current knowledge of key events underlying the complex cellular interactions that proceed allergic food sensitization. Currently, there is no consensus on the in vitro model to study the intestinal translocation of proteins as well as the epithelial activation, which comprise the first molecular initiation events (ME1-3) and the first key event of the AOP, respectively. As members of INFOGEST, we have highlighted several critical features that should be considered for any proposed in vitro model to study epithelial protein transport in the context of allergic sensitization. In addition, we defined which intestinal cell types are indispensable in a consensus model of the first steps of the AOP, and which cell types are optional or desired when there is the possibility to create a more complex cell model. A model of these first key aspects of the AOP can be used to study the gut epithelial translocation behavior of known hypo- and hyperallergens, juxtaposed to the transport behavior of novel proteins as a first screen for risk management of dietary proteins. Indeed, this disquisition forms a basis for the development of a future consensus model of the allergic sensitization cascade, comprising also the other key events (KE2-5).

Caco-2 Cell Response Induced by Peptides Released after Digestion of Heat-Treated Egg White Proteins

The heat treatment of food proteins induces structural modifications that influence their interaction with human fluids and cells. We aimed to evaluate the Caco-2 cell response induced by peptides produced after digestion of heat-treated egg white proteins. In vitro digestion of ovalbumin (OVA), ovomucoid (OM), and lysozyme (LYS), untreated or previously heated, was performed. The digestibility of proteins and the response of Caco-2 cells exposed to peptides (<10 kDa) generated during digestion were evaluated. Intact OVA and LYS persisted after the digestion of native proteins, whereas OM was completely hydrolysed. A heat treatment at 65 °C for 30 min did not alter the digestibility of OVA, whereas at 90 °C for 3 min, protein degradation was favoured. The digestibility of OM and LYS was not affected by heat treatment. Peptides derived from OVA and OM digestion induced IL-6 and IL-8 production. OVA and LYS digestion promoted the expression of Tslp, and Il6 and Il33, respectively. A heat treatment prior to OVA digestion reduced IL-6 production and Tslp expression. It was concluded that heat treatments can reduce the release of OVA-derived peptides, but not OM and LYS, with proinflammatory activity during digestion.

Gastrointestinal fate of food allergens and its relationship with allergenicity

Food allergens are closely related to their gastrointestinal digestion fate, but the changes in food allergens during digestion and related mechanisms are quite complicated. This review presents in detail digestion models for predicting allergenicity, the fates of food allergens in oral, gastric and duodenal digestion, and the applications of digestomics in mapping IgE-binding epitopes of digestion-resistant peptides. Moreover, this review highlights the structure-activity relationships of food allergens during gastrointestinal digestion. Digestion-labile allergens may share common structural characteristics, such as high flexibility, rendering them easier to be hydrolyzed into small fragments with decreased or eliminated allergenicity. In contrast, the presence of disulfide bonds, tightly wound α-helical structures, or hydrophobic domains in food allergens helps them resist gastrointestinal digestion, stabilizing IgE-binding epitopes, thus maintaining their sensitization. In rare cases, digestion leads to increased allergenicity due to exposure of new epitopes. Finally, the action of the food matrix and processing on the digestion and allergenicity of food allergens as well as the underlying mechanisms was overviewed. The food matrix can directly act on the allergen by forming complexes or new epitopes to affect its gastrointestinal digestibility and thereby alter its allergenicity or indirectly affect the allergenicity by competing for enzymatic cleavage or influencing gastrointestinal pH and microbial flora. Several processing techniques attenuate the allergenicity of food proteins by altering their conformation to improve susceptibility to degradation by digestive enzymes. Given the complexity of food components, the food itself rather than a single allergen should be used to obtain more accurate data for allergenicity assessment. PRACTICAL APPLICATION: The review article will help to understand the relationship between food protein digestion and allergenicity, and may provide fundamental information for evaluating and reducing the allergenicity of food proteins.

Epitope mapping and the effects of various factors on the immunoreactivity of main allergens in egg white

Egg white has high protein content and numerous biological/functional properties. However, reported allergenicity for some of the proteins in egg white is an issue that needs to be paid exclusive attention. A consideration of the structure of IgE epitopes and their sequences, as well as a comprehensive understanding of the effects of various processes on epitopes and the impact of the gastrointestinal tract on them, can help target such issues. The current study focuses on the identified IgE epitopes in egg white proteins and evaluation of the effects of the gastrointestinal digestion, carbohydrate moiety, food matrix, microbial fermentation, recombinant allergen, heat treatment, Maillard reaction and combination of various processes and gastrointestinal digestion on egg white allergenicity. Although the gastrointestinal tract reduces the immunoreactivity of native egg white proteins, some of the IgE epitope-containing fragments remain intact during the digestion process. It has been found that the gastrointestinal tract can have both positive and negative impacts on the IgE binding activities of egg white proteins. Elimination of the carbohydrate moiety leads to a reduction in the immunoreactivity of ovalbumin. But, such effects from the carbohydrate parts in the IgE binding activity need to be explored further. In addition, the interaction between the egg white proteins and the food matrix leads to various effects from the gastrointestinal tract on the digestion of egg white proteins and their subsequent immunoreactivity. Further on this matter, studies have shown that both microbial fermentation and Maillard reaction can reduce the IgE binding activities of egg white proteins. Also, as an alternate approach, the thermal process can be used to treat the egg white proteins, which may result in the reduction or increase in their IgE binding activities depending on the conditions used in the process. Overall, based on the reported data, the allergenicity levels of egg white proteins can be mitigated or escalated depending on the conditions applied in the processing of the food products containing egg white. So far, no practical solutions have been reported to eliminate such allergenicity.

Regulation of Exacerbated Immune Responses in Human Peripheral Blood Cells by Hydrolysed Egg White Proteins

The anti-allergic potential of egg white protein hydrolysates (from ovalbumin, lysozyme and ovomucoid) was evaluated as their ability to hinder cytokine and IgE production by Th2-skewed human peripheral blood mononuclear cells (PBMCs), as well as the release of pro-inflammatory factors and generation of reactive oxygen species from Th1-stimulated peripheral blood leukocytes (PBLs). The binding to IgE of egg allergic patients was determined and the peptides present in the hydrolysates were identified. The hydrolysates with alcalase down-regulated the production of Th2-biased cytokines and the secretion of IgE to the culture media of Th2-skewed PBMCs, and they significantly neutralized oxidative stress in PBLs. The hydrolysates of ovalbumin and ovomucoid with pepsin helped to re-establish the Th1/Th2 balance in Th2-biased PBMCs, while they also inhibited the release of pro-inflammatory mediators and reduced oxidative stress in PBLs treated with inflammatory stimuli. The hydrolysates with alcalase, in addition to equilibrating Th2 differentiation, exhibited a low IgE-binding. Therefore, they would elicit mild allergic reactions while retaining T cell-stimulating abilities, which might correlate with an anti-allergic benefit.

Egg proteins as allergens and the effects of the food matrix and processing

Hen eggs are an important and inexpensive source of high-quality proteins in the human diet. Egg, either as a whole or its constituents (egg yolk and white), is a key ingredient in many food products by virtue of its nutritional value and unique functional properties, such as emulsifying, foaming, and gelling. Nevertheless, egg is also known because of its allergenic potential and, in fact, it is the second most frequent source of allergic reactions, particularly in children. This review deals with the structural or functional properties of egg proteins that make them strong allergens. Their ability to sensitize and/or elicit allergic reactions is linked to their resistance to gastroduodenal digestion, which ultimately allows them to interact with the intestinal mucosa where absorption occurs. The factors that affect protein digestibility, whether increasing it, decreasing it, or inducing a different proteolysis pattern, and their influence on their capacity to induce or trigger an allergic reaction are discussed. Special attention is paid to the effect of the food matrix and the processing practices on the capacity of egg proteins to modulate the immune response.

The protective role of the Bowman-Birk protease inhibitor in soybean lunasin digestion: the effect of released peptides on colon cancer growth

Lunasin is a naturally-occurring peptide demonstrating chemopreventive, antioxidant and anti-inflammatory properties. To exhibit these activities, orally ingested lunasin needs to survive proteolytic attack of digestive enzymes to reach target tissues in active form/s. Preliminary studies suggested the protective role of protease inhibitors, such as the Bowman-Birk inhibitor and Kunitz-trypsin inhibitor, against lunasin's digestion by both pepsin and pancreatin. This work describes in depth the behaviour of lunasin under conditions simulating the transit through the gastrointestinal tract in the absence or presence of soybean Bowman-Birk isoinhibitor 1 (IBB1) in both active and inactive states. By liquid chromatography coupled with tandem mass spectrometry (HPLC-MS/MS), the remaining lunasin at the end of gastric and gastro-duodenal phases was quantified. Protection against the action of pepsin was independent of the amount of IBB1 present in the analyzed samples, whereas an IBB1 dose-dependent protective effect against trypsin and chymotrypsin was observed. Peptides released from lunasin and inactive IBB1 were identified by MS/MS. The remaining lunasin and IBB1 as well as their derived peptides could be responsible for the anti-proliferative activity against colon cancer cells observed for the digests obtained at the end of simulated gastrointestinal digestion.

Identification of IgE-binding peptides in hen egg ovalbumin digested in vitro with human and simulated gastroduodenal fluids

The digestibility of the major egg allergen ovalbumin (OVA, Gal d 2) with human and simulated digestive fluids was assessed. Degradation of OVA was faster when treated with human fluids, particularly following duodenal digestion, leading to gastrointestinal digests with lower IgE binding. Gastric digestion with both systems yielded 52 identical cleavage sites and a similar peptide pattern with 47 peptides in common. Subsequent duodenal digestion showed that the human fluid released fewer and shorter peptides. Several high-frequency IgE-binding epitopes were detected among the fragments of molecular mass lower than 3 kDa identified in the digests: OVA (141-154) and OVA (164-176) in the gastrointestinal digests produced with human fluids; and OVA (125-134), OVA (159-172), OVA (141-154), OVA (188-198), OVA (326-336), and OVA (370-385) in the gastrointestinal digests produced with simulated fluids. The high binding frequency of the fragment OVA (370-385), which reacted with 80% of the sera from allergic patients used, was noteworthy.

Influence of the carbohydrate moieties on the immunoreactivity and digestibility of the egg allergen ovomucoid

BACKGROUND

Ovomucoid (OM) has two carbohydrate chains on each of the first and second domains and one in the third. The contribution of the covalently bound carbohydrate chains to the overall OM allergenicity is controversial. Another aspect directly related with the immunological properties of OM that has not been studied in depth is the importance of the carbohydrate chains on its digestibility.

OBJECTIVE

The aim of the study was to assess the involvement of the carbohydrate moieties of OM in its digestibility and allergenic properties.

METHODS

IgE-binding and basophil activation by glycosylated and enzymatically deglycosylated OM (dOM) were compared using blood from egg-allergic patients. The peptides obtained after digestion using a physiologically relevant model were identified by RP-HPLC-MS/MS and the IgE-binding of the resulting fragments was evaluated by DOT-Blot.

RESULTS

No structural changes were observed after deglycosylation of OM. 80% of the patients showed lower IgE binding to dOM as compared with OM and, in some patients, IgE reactivity could not be inhibited by pre-incubation with dOM. A subtle reduction in the percentage of activated basophils was observed when incubated with dOM as compared to OM. Following simulated digestion, dOM was more extensively degraded than OM, particularly during the gastric phase and both, OM and dOM, yielded, after the duodenal phase, immunoreactive fragments that were totally or partially coincident with previously described epitopes.

CONCLUSION & CLINICAL RELEVANCE

this work demonstrated an enhanced IgE reactivity towards carbohydrate containing OM in some egg-allergic patients that could be attributed to cross-sensitization or sensitization to the glycosylated components. The carbohydrate chains contributed to an increased resistance to proteolysis, and thus, to its allergenic potency. Evaluation of the products of digestion of OM and dOM revealed the presence of high-frequency IgE-binding epitopes that could remain linked by disulphide bonds.

In vitro digestibility of bovine β-casein with simulated and human oral and gastrointestinal fluids. Identification and IgE-reactivity of the resultant peptides

Stability during digestion is considered an important feature in determining the allergenicity of food proteins. This study aimed to provide an immunological characterisation of the digestion products of the major cow's milk allergen β-casein (β-CN) produced by in vitro orogastrointestinal hydrolysis with simulated and human digestive fluids. β-CN was unaffected by oral digestion, but quickly broke down during the early stages of gastric digestion. The degradation with human fluids was faster than that with commercial enzymes. There were similarities in the peptide patterns of the hydrolysates produced in both models, showing 20 peptides in common after gastric digestion. After gastroduodenal digestion, the human fluids gave less numerous and shorter peptides. The IgE binding of most of the individual sera used to the hydrolysates produced with simulated and human fluids increased at the end of the gastric phase and decreased when the duodenal digestion was completed. Two IgE-binding synthetic peptides: β-CN (57-68) and β-CN (82-93), which matched fragments released by β-CN following in vitro digestion with simulated and human fluids, consisted of the most immunoreactive areas of the protein. The similarities found between the in vitro simulated digestion system and that using human digestive fluids suggest that the former would provide a reasonably good estimation of the potential allergenicity of protein digests.