Identification and characterization of the antigenic site (epitope) on bovine β-lactoglobulin: common residues in linear and conformational epitopes

Animal-derived food allergen: A review on the available crystal structure and new insights into structural epitope

Immunoglobulin E (IgE)-mediated food allergy is a rapidly growing public health problem. The interaction between allergens and IgE is at the core of the allergic response. One of the best ways to understand this interaction is through structural characterization. This review focuses on animal-derived food allergens, overviews allergen structures determined by X-ray crystallography, presents an update on IgE conformational epitopes, and explores the structural features of these epitopes. The structural determinants of allergenicity and cross-reactivity are also discussed. Animal-derived food allergens are classified into limited protein families according to structural features, with the calcium-binding protein and actin-binding protein families dominating. Progress in epitope characterization has provided useful information on the structural properties of the IgE recognition region. The data reveals that epitopes are located in relatively protruding areas with negative surface electrostatic potential. Ligand binding and disulfide bonds are two intrinsic characteristics that influence protein structure and impact allergenicity. Shared structures, local motifs, and shared epitopes are factors that lead to cross-reactivity. The structural properties of epitope regions and structural determinants of allergenicity and cross-reactivity may provide directions for the prevention, diagnosis, and treatment of food allergies. Experimentally determined structure, especially that of antigen-antibody complexes, remains limited, and the identification of epitopes continues to be a bottleneck in the study of animal-derived food allergens. A combination of traditional immunological techniques and emerging bioinformatics technology will revolutionize how protein interactions are characterized.

Localization of G1A1a Allergenic Domain Destroyed by Thermal Processing

Glycinin is an important allergenic protein. A1a is the acidic chain of the G1 subunit in glycinin (G1A1a), and it has strong allergenicity. In this study, we used phage display technology to express the protein of G1A1a and its overlapping fragments and an indirect enzyme-linked immunosorbent assay (iELISA) to determine the antigenicity and allergenicity of the expressed protein. After three rounds of screening, it was determined that fragment A1a-2-B-I (151SLENQLDQMPRRFYLAGNQEQEFLKYQQEQG181) is the allergenic domain of G1A1a destroyed by thermal processing. In addition, three overlapping peptides were synthesized from fragments A1a-2-B-I, and a linear epitope was found in this domain through methods including dot blot and iELISA. Peptide 2 (157DQMPRRFYLANGNQE170) showed allergenicity, and after replacing it with alanine, it was found that amino acids D157, Q158, M159, and Y164 were the key amino acids that affected its antigenicity, while Q158, M159, R162, and N168 affected allergenicity.

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.

Glutamine and lysine as common residues from epitopes on α-lactalbumin and β-lactoglobulin from cow milk identified by phage display technology

Cow milk is an important source of food protein for children; however, it could lead to allergy, especially for infants. α-Lactalbumin (α-LA) and β-lactoglobulin (β-LG) from whey protein make up a relatively high proportion of milk proteins and have received widespread attention as major allergens in milk. However, few studies have identified the epitopes of both proteins simultaneously. In this study, ImmunoCAP and indirect ELISA were first used for detection of sIgE to screen sera from allergic patients with high binding capacity for α-LA and β-LG. Subsequently, the mimotopes was biopanned by phage display technology and bioinformatics and 17 mimic peptide sequences were obtained. Aligned with the sequences of α-LA or β-LG, we identified one linear epitope on α-LA at AA 11-26 and 5 linear epitopes on β-LG at AA 9-29, AA 45-57, AA 77-80, AA 98-101, and AA 121-135, respectively. Meanwhile, the 8 conformational epitopes and their distributions of α-LA and β-LG were located using the Pepitope Server. Finally, glutamine and lysine were determined as common AA residues for the conformational epitopes both on α-LA and β-LG. Moreover, we found the addition of mouse anti-human IgE during the biopanning process did not significantly affect the identification of the epitopes.

Effect of ferulic acid covalent conjugation on the functional properties and antigenicity of β-lactoglobulin

Binding to phenolics can improve the functional properties of proteins. Changes in structure, functional properties, and antigenicity of β-lactoglobulin (β-LG) after covalent conjugation with ferulic acid (FA) at different mass ratios were reported here. The results of SDS-PAGE and gel exclusion chromatography confirmed that covalent complexes were formed. When the mass ratio of β-LG and FA was 10:6, the binding content of FA was the highest. Fluorescence spectroscopy, UV-visible absorption spectrometry, and FTIR analysis showed that the structure of the complexes was more stretched compared to native β-LG. The addition of FA significantly improved the emulsifying property of β-LG. When the mass ratio was 10:6, the radical scavenging activities of DPPH and ABTS reached 65.06% and 88.22%, respectively, and the antigenicity of β-LG reduced by about 35%. This study provides novel β-LG-FA complexes in food systems to reduce the antigenicity of β-LG and improve functional properties.

Identification of allergenic epitopes destroyed by two processing technologies of glycinin A2 from soybean

BACKGROUND

Glycinin is one of the most highly allergenic proteins in soybeans, and G2 is one of the five allergenic subunits of glycinin. Compared with the alkaline chain, the acidic chain A2 of the G2 subunit has strong allergenicity. However, the precise epitopes of A2 and the epitopes destroyed during processing are still unknown.

RESULTS

In the present study, preparation of two specific antibodies damaged by processing and phage display techniques were applied to locate the antigenic epitopes of glycinin A2 polypeptide chains disrupted by two processing techniques (thermal processing and ultra-high pressure combined thermal processing). Bioinformatics methods were used to predict the possible epitopes of the A2 chain. The A2 chain and its overlapping segments were introduced into T7 phages and expressed on phage shell by phage display. An indirect enzyme-linked immunosorbent assay was used to screen for antigenic epitopes that had been disrupted by the two processing technologies. The results showed that the dominant antigenic region disrupted by processing was located mainly in the A2-3-B fragment. The reacting experiment with the serum of allergic patients showed that the A2-3-B fragment protein was not only an antigenic region, but also an allergenic region. The two processing technologies destroyed the allergenic epitopes of A2 chain, thereby reducing the allergenicity of protein. The amino acids where the dominant allergenic region disrupted by processing was located were: ²³³ AIVTVKGGLRVTAPAMRKPQQEEDDDDEEEQPQCVE²⁶⁸ .

CONCLUSION

Precise epitopes of the acidic chain A2 in glycinin were identified and epitopes destroyed in two common processing methods were also obtained. The application products of rapid detection of de-allergenicity effect of processed food can be developed according to the location of processed destruction allergenic region, which is of great significance with respect to preventing the occurrence of soybean allergenic diseases. © 2022 Society of Chemical Industry.

Isolation and allergenicity evaluation of glycated α-lactalbumin digestive products and identification of allergenic peptides

This study aimed to isolate and evaluate the allergenicity of glycated α-lactalbumin (ALA) digestive products and identify its allergenic peptides. The digestive products of native-, alone glycated- and ultrasound-assisted glycated ALA (ALA-D, ALA-gal-D, 100ALA-gal-D) were isolated into three fractions (F1, F2 and F3). High-resolution mass spectrometry showed that the digestion-resistant peptides of F2 and F3 mainly distributed in amino acid sequence (AA) 25-31, AA32-53, AA40-53, AA54-60, AA80-90, AA94-104. The allergenicity of the three fractions of glycated ALA was lower than that in ALA-D, indicating glycation of ALA could indeed reduce its allergenicity after digestion. Furthermore, most fractions isolated from high glycation-degree ALA had the lowest allergenicity. The IgG/IgE binding abilities of synthesized peptides indicated that AA94-104 firstly identified by us embodied the strongest allergenicity and might be the potential allergenic peptide. This will provide a theory for preparing hypoallergenic products based on the identified allergenic peptides.

High-Performance Thin-Layer Chromatography-Immunostaining as a Technique for the Characterization of Whey Protein Enrichment in Edam Cheese

Whey protein-enriched cheese can be produced by means of a high-temperature treatment of a part of the cheese milk. In this way, the nutritional quality of the resulting cheeses can be increased while resources are conserved. High-performance thin-layer chromatography-immunostaining (HPTLC-IS) using specific β-lactoglobulin (β-LG) antibodies was applied to study the implementation and stability of β-LG in two different sample sets of whey protein-enriched Edam model cheeses, including industrial-scale ones. Two methods were compared for the extraction of the proteins/peptides from the cheese samples. By applying tryptic hydrolysis directly from a suspended cheese sample instead of a supernatant of a centrifuged suspension, a better yield was obtained for the extraction of β-LG. When applying this method, it was found that selected epitopes in the tryptic β-LG peptides remain stable over the ripening period of the cheese. For four of the tryptic β-LG peptides detected by immunostaining, the amino acid sequence was identified using MALDI-TOF-MS/MS. One of the peptides identified was the semi-tryptic peptide VYVEELKPTP. A linear relationship was found between the content of this peptide in cheese and the proportion of high-heated milk in the cheese milk. β-LG enrichment factors of 1.72 (n = 3, sample set I) and 1.33 ± 0.19 (n = 1, sample set II) were determined for the cheese samples containing 30% high-heated milk compared to the non-enriched samples. The relative β-LG contents in the cheese samples with 30% high-heated milk were calculated to be 4.35% ± 0.39% (sample set I) and 9.11% ± 0.29% (sample set II) using a one-point calibration. It can be concluded that the HPTLC-IS method used is a suitable tool for the analysis of whey protein accumulation in cheese, being therefore potentially directly applicable on an industrial scale. For more accurate quantification of the whey protein content in cheese, an enhanced calibration curve needs to be applied.

Investigation into predominant peptide and potential allergenicity of ultrasonicated β-lactoglobulin digestion products

The effect of ultrasonicated β-lactoglobulin on the allergenic potential of predominant peptide was studied in vitro digestion. Gastrointestinal (GI) digestion of ultrasonicated β-lg was fractionated into four fractions, which have different molecular weight and allergenic potentials. The lowest allergenicity of fraction was produced by ultrasonicated β-Lg after GI digestion, depending on the changes in the structure of β-Lg by ultrasonic and the promotion of its proteolysis, resulting in the production of numerous small peptides with significantly reduced IgE activity and basophil histamine release. Mass spectrometry analysis showed that ultrasonic can promote the further hydrolysis of large intermediate peptides, Y42, L54, L57/L58, L95, L104/F105, L122 were target residues that became more available to protease by the pretreatment of ultrasonic, thus have a smaller molecular weight with reduced allergenic potential. Ultrasonic processing of milk products alone could reduce the risk of an allergenic reaction in milk allergy patients to some extent.

Location of destroyed antigenic sites of Gly m Bd 60 K after three processing technologies

Gly m Bd 60 K, which is the α subunit of β-conglycinin, is a major soybean (Glycine max) allergen. We used high hydrostatic pressure (HHP), thermal techniques, and glycation to treat β-conglycinin, which can effectively reduce the antigenicity of β-conglycinin. β-conglycinin was used to immunize New Zealand rabbits, and the antiserum had a titer > 1: 1 × 105 and an IC50 of 2.254 μg/mL. β-conglycinin was subjected to HHP, thermal techniques, and glycation and mixed with rabbit antiserum against β-conglycinin to obtain the site-specific antiserum. The overlapping gene fragments of Gly m Bd 60 K were amplified by polymerase chain reaction (PCR), then cloned into a T7 phage vector and packaged in vitro, the recombinant T7 phages were constructed. Indirect ELISA (iELISA) was used to locate the destroyed antigenic sites and, after three rounds of segment expression and identification, the C2-1 and C2-2 fragments were identified as destroyed antigenic sites of Gly m Bd 60 K. Allergenicity analysis showed that the C2-1 and C2-2 fragments reacted with allergic patients' serum, which indicated that the destroyed sites were allergic sites.

Identification of the linear immunodominant epitopes in the β subunit of β-conglycinin and preparation of epitope antibodies

β-conglycinin is one of the major allergens in soybean protein. The purpose of this study was to predict and to identify the major linear epitopes of the β subunit of β-conglycinin. Potential linear epitopes were predicted and confirmed by three immunoinformatics tools combined with the Immune Epitope Database (IEDB). Ten potential epitope peptides were synthesized by Fmoc (9-fluorenylmethoxycarbonyl) solid phase peptide synthesis and were validated by the indirect competitive enzyme-linked immunosorbent assay (ic-ELISA) using sera from soybean allergic patients. Polyclonal antibodies, which were prepared by immunizing rabbits with synthesized peptides, were used to confirm their binding ability with β-conglycinin through western blot and dot blot assays. The results showed that 10 peptides were screened as the main epitopes for the β subunit of β-conglycinin. All 10 peptides (P1-P10) presented IgG binding activity, and P2 and P6 were also validated as IgE binding peptides. Moreover, the results of dot blot showed that P5 and P8 might be located inside the protein molecule. Western blot indicated that most of polyclonal antibodies were bound effectively to the β subunit of β-conglycinin. In addition, few polyclonal antibodies exhibited an immune cross-reaction with the α and α' subunits.

Identification of antigenic sites destructed by high hydrostatic pressure (HHP) of the β subunit of β-conglycinin

β-conglycinin is one of the most allergenic proteins, and its constituent subunits α', α, and β are all potential allergens to humans. In the present study, we concentrated on the destructed antigenic sites of β subunit of β-conglycinin after high hydrostatic pressure (HHP) treatment. In this paper, the overlapping gene fragments of the β subunit of β-conglycinin were amplified by polymerase chain reaction (PCR) and cloned into T7 phage vectors. After being packaged in vitro, the recombinant T7 phage was constructed, and the overlapping fragments of the β subunit were displayed on the phage surface. The recombinant phages that expressed the overlapping fragments of the β subunit were used to react with specific antiserum by indirect ELISA to identify the HHP destructed antigenic sites. After three rounds of expression and identification, we used synthetic peptide technology to identify that the obtained fragment was a conformational epitope. We further confirmed that HHP treatment changed the conformational structure of β-conglycinin, which reduced the antigenicity of the protein.

The mechanism of reduced IgG/IgE-binding of β-lactoglobulin by pulsed electric field pretreatment combined with glycation revealed by ECD/FTICR-MS

Bovine β-lactoglobulin (β-Lg) is a major allergen existing in milk and causes about 90% of IgE-mediated cow's milk allergies. Previous studies showed that pulsed electric field (PEF) treatment could partially unfold the protein, which may contribute to the improvement of protein glycation. In this study, the effect of PEF pretreatment combined with glycation on the IgG/IgE-binding ability and the structure of β-Lg was investigated. The result showed that PEF pretreatment combined with glycation significantly reduced the IgG and IgE binding abilities, which was attributed to the changes of secondary and tertiary structure and the increase in glycation sites and degree of substitution per peptide (DSP) value determined by electron capture dissociation Fourier transform ion cyclotron resonance mass spectrometry (ECD/FTICR-MS). Unexpectedly, glycation sites (K47, K91 and K135) added by two mannose molecules were identified in glycated β-Lg with PEF pretreatment. Moreover, the results indicated that PEF pretreatment at 25 kV cm^-1 for 60 μs promoted the reduction of IgG/IgE-binding capacity by increasing the glycation degree of β-Lg, whereas single PEF treatment under the same conditions markedly enhanced the IgG/IgE-binding ability by partially unfolding the structure of β-Lg. The results suggested that ECD/FTICR-MS could help us to understand the mechanism of reduction in the IgG/IgE-binding of β-Lg by structural characterization at the molecular level. Therefore, PEF pretreatment combined with glycation may provide an alternative method for β-Lg desensitization.

Reducing the allergenic capacity of β-lactoglobulin by covalent conjugation with dietary polyphenols

To help produce hypoallergenic food, this study investigated reducing the allergenicity and improving the functional properties of bovine β-lactoglobulin (βLG) by covalent conjugation with (-)-epigallo-catechin 3-gallate (EGCG) and chlorogenic acid (CA). The covalent bond between the polyphenols and the amino acid side-chains in βLG was confirmed by MALDI-TOF-MS and SDS-PAGE. Structural analysis by fluorescence spectroscopy, circular dichroism (CD) and Fourier transform infrared (FTIR) indicated that the covalent conjugate of EGCG and CA led to the changed protein structure of βLG. Western blot analysis and enzyme-linked immunosorbent assay indicated that conjugation of βLG with these polyphenols was effective in reducing the IgE-binding capacity of βLG. The conjugates maintained the retinol-binding activity without denaturation the protein and enhanced the thermal stability with high antioxidant activity. The study provides an innovative approach to producing hypoallergenic food.

Mechanism of Reduction in IgG and IgE Binding of β-Lactoglobulin Induced by Ultrasound Pretreatment Combined with Dry-State Glycation: A Study Using Conventional Spectrometry and High-Resolution Mass Spectrometry

Bovine β-lactoglobulin (β-Lg) is one of major allergens in cow's milk. Previous study showed that ultrasound treatment induced the conformational changes of β-Lg and promoted the glycation in aqueous solutions, which is, however, less efficient compared with dry-state. In this work, the effect of ultrasound pretreatment combined with dry-state glycation on the IgG and IgE binding of β-Lg was studied. Dry-state glycation with mannose after ultrasound pretreatment at 0-600 W significantly reduced the IgG and IgE binding of β-Lg, with the lowest values observed at 400 W. The decrease in the IgG and IgE binding of β-Lg was attributed to the increase in glycation extent and the changes of secondary and tertiary structure, which reflected in the increase of UV absorbance, α-helix and β-sheet contents, as well as the decrease of intrinsic fluorescence intensity, surface hydrophobicity, β-turn, and random coil contents. Moreover, ultrasound pretreatment promoted the reduction of IgG and IgE binding abilities by improving glycation, reflecting in the increase of the glycation sites and the degree of substitution per peptide (DSP) value determined by Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Ultrasound pretreatment at 400 W showed the most significantly enhanced glycation extent. Besides, the results suggested FTICR-MS could provide insights into the glycation at molecular level, which was conducive to the understanding of the mechanism of the reduction in the IgG and IgE binding of β-Lg. Therefore, ultrasound pretreatment combined with dry-state glycation may be a promising method for β-Lg hyposensitization.