Sensitizing potential of enzymatically cross-linked peanut proteins in a mouse model of peanut allergy

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).

Intestinal protein uptake and IgE-mediated food allergy

Food allergy is affecting 5-8% of young children and 2-4% of adults and seems to be increasing in prevalence. The cause of the increase in food allergy is largely unknown but proposed to be influenced by both environmental and lifestyle factors. Changes in intestinal barrier functions and increased uptake of dietary proteins have been suggested to have a great impact on food allergy. In this review, we aim to give an overview of the gastrointestinal digestion and intestinal barrier function and provide a more detailed description of intestinal protein uptake, including the various routes of epithelial transport, how it may be affected by both intrinsic and extrinsic factors, and the relation to food allergy. Further, we give an overview of in vitro, ex vivo and in vivo techniques available for evaluation of intestinal protein uptake and gut permeability in general. Proteins are digested by gastric, pancreatic and integral brush border enzymes in order to allow for sufficient nutritional uptake. Absorption and transport of dietary proteins across the epithelial layer is known to be dependent on the physicochemical properties of the proteins and their digestion fragments themselves, such as size, solubility and aggregation status. It is believed, that the greater an amount of intact protein or larger peptide fragments that is transported through the epithelial layer, and thus encountered by the mucosal immune system in the gut, the greater is the risk of inducing an adverse allergic response. Proteins may be absorbed across the epithelial barrier by means of various mechanisms, and studies have shown that a transcellular facilitated transport route unique for food allergic individuals are at play for transport of allergens, and that upon mediator release from mast cells an enhanced allergen transport via the paracellular route occurs. This is in contrast to healthy individuals where transcytosis through the enterocytes is the main route of protein uptake. Thus, knowledge on factors affecting intestinal barrier functions and methods for the determination of their impact on protein uptake may be useful in future allergenicity assessments and for development of future preventive and treatment strategies.

Enzymatic crosslinking and food allergenicity: A comprehensive review

Food allergy has become a major global public health concern. In the past decades, enzymatic crosslinking technique has been employed to mitigate the immunoreactivity of food allergens. It is an emerging non-thermal technique that can serve as a great alternative to conventional food processing approaches in developing hypoallergenic food products, owing to their benefits of high specificity and selectivity. Enzymatic crosslinking via tyrosinase (TYR), laccase (LAC), peroxidase (PO), and transglutaminase (TG) modifies the structural and biochemical properties of food allergens that subsequently cause denaturation and masking of the antigenic epitopes. LAC, TYR, and PO catalyze the oxidation of tyrosine side chains to initiate protein crosslinking, while TG initiates isopeptide bonding between lysine and glutamine residues. Enzymatic treatment produces a high molecular weight crosslinked polymer with reduced immunoreactivity and IgE-binding potential. Crosslinked allergens further inhibit mast cell degranulation due to the lower immunostimulatory potential that assists in the equilibration of T-helper (Th)1/Th2 immunobalance. This review provides an updated overview of the studies carried out in the last decade on the potential application of enzymatic crosslinking for mitigating food allergenicity that can be of importance in the context of developing hypoallergenic/non-allergenic food products.

Enterocytes in Food Hypersensitivity Reactions

Simple Summary

Hypersensitivity to food, affecting both animals and humans, is increasing. Until a decade ago, it was thought that enterocytes, the most abundant constituent of the intestinal surface mucosa layer, served only to absorb digested food and prevent foreign and non-digested substances from passing below the intestinal layer. Growing evidence supports the involvement of enterocytes in immunological responses. Here, we present a comprehensive review of the new roles of enterocytes in food hypersensitivity conducted in animal models in order to better understand complicated immune pathological conditions. In addition, resources for further work in this area are suggested, along with a literature overview of the specific roles of enterocytes in maintaining oral tolerance. Lastly, it will be beneficial to investigate the various animal models involved in food hypersensitivity to reach the needed momentum necessary for the complete and profound understanding of the mechanisms of the ever-growing number of food allergies in animal and human populations.

Abstract

Food hypersensitivity reactions are adverse reactions to harmless dietary substances, whose causes are hidden within derangements of the complex immune machinery of humans and mammals. Until recently, enterocytes were considered as solely absorptive cells providing a physical barrier for unwanted lumen constituents. This review focuses on the enterocytes, which are the hub for innate and adaptive immune reactions. Furthermore, the ambiguous nature of enterocytes is also reflected in the fact that enterocytes can be considered as antigen-presenting cells since they constitutively express major histocompatibility complex (MHC) class II molecules. Taken together, it becomes clear that enterocytes have an immense role in maintaining oral tolerance to foreign antigens. In general, the immune system and its mechanisms underlying food hypersensitivity are still unknown and the involvement of components belonging to other anatomical systems, such as enterocytes, in these mechanisms make their elucidation even more difficult. The findings from studies with animal models provide us with valuable information about allergic mechanisms in the animal world, while on the other hand, these models are used to extrapolate results to the pathological conditions occurring in humans. There is a constant need for studies that deal with this topic and can overcome the glitches related to ethics in working with animals.

Reducing the Allergenicity of α-Lactalbumin after Lipid Peroxidation

This study analyzed the effect of lipid peroxidation using 2,2'-azobis(2-amidinopropane)dihydrochloride (AAPH) and acrolein on the in vitro and in vivo allergenicity of α-lactalbumin (α-La). The structure of oxidized α-La was evaluated by sodium dodecyl sulfate polyacrylamide gel electrophoresis, fluorescence spectroscopy, and circular dichroism, whereas the changes in the allergenic properties were evaluated. Lipid peroxidation induced changes to the structural properties that might destroy and/or mask α-La epitopes. In comparison to native α-La, oxidation complexes caused a decrease in the immunoglobulin E (IgE) binding capacity, as observed via immunoblotting. Moreover, the capacity to release mediators and cytokines from KU812 cells was also greatly reduced. In vivo, oxidation with AAPH and acrolein caused a significant reduction in IgE, IgG, IgG1, mast cell protease 1, and plasma histamine, along with the reduction of mast surface c-Kit⁺ and FcεRI⁺ expression. Therefore, these results indicate that oxidation via AAPH and acrolein can potentially reduce the allergenicity of α-La, which can help with the better understanding of the changes in allergenicity of milk allergen by lipid peroxidation.

Are Physicochemical Properties Shaping the Allergenic Potency of Plant Allergens?

This review searched for published evidence that could explain how different physicochemical properties impact on the allergenicity of food proteins and if their effects would follow specific patterns among distinct protein families. Owing to the amount and complexity of the collected information, this literature overview was divided in two articles, the current one dedicated to protein families of plant allergens and a second one focused on animal allergens. Our extensive analysis of the available literature revealed that physicochemical characteristics had consistent effects on protein allergenicity for allergens belonging to the same protein family. For example, protein aggregation contributes to increased allergenicity of 2S albumins, while for legumins and cereal prolamins, the same phenomenon leads to a reduction. Molecular stability, related to structural resistance to heat and proteolysis, was identified as the most common feature promoting plant protein allergenicity, although it fails to explain the potency of some unstable allergens (e.g. pollen-related food allergens). Furthermore, data on physicochemical characteristics translating into clinical effects are limited, mainly because most studies are focused on in vitro IgE binding. Clinical data assessing how these parameters affect the development and clinical manifestation of allergies is minimal, with only few reports evaluating the sensitising capacity of modified proteins (addressing different physicochemical properties) in murine allergy models. In vivo testing of modified pure proteins by SPT or DBPCFC is scarce. At this stage, a systematic approach to link the physicochemical properties with clinical plant allergenicity in real-life scenarios is still missing.

Polyphenol-oxidase-catalyzed cross-linking of Ara h 2: reaction sites and effect on structure and allergenicity

BACKGROUND

Peanut is among the most common of food allergies, and one of its allergens is Ara h 2. A previous study revealed that this allergen was recognized by serum immunoglobulin E (IgE) in over 90% of a peanut-allergic patient population. Enzymatic cross-linking is a popular processing method used to tailor food functionality, such as antigenicity.

RESULT

The cross-linking reactions of Ara h 2 were catalyzed by polyphenol oxidase (PPO), and the relevant reaction sites were identified using mass spectrometry and StavroX software. Two pairs of intramolecular cross-linking peptides and two intermolecular cross-linking peptides were found. Intramolecular cross-linking was speculated to occur between ARG¹³¹ (amino acids 116-131) and TYR⁶⁵ (amino acids 63-80) and between TYR⁶⁰ (amino acids 56-62) and ARG⁹² (amino acids 92-102); the intermolecular cross-linking sites were ARG³¹ with TYR⁸⁴ or TYR⁸⁹ and TYR⁶⁵ or TYR⁷² with ARG⁹² or ARG¹⁰² . Three out of four cross-linking peptides were found in α-helices, and destruction of this secondary structure resulted in a loose tertiary structure. Although seven linear allergen epitopes were involved in cross-linking, the IgE binding capacity of protein changed slightly, while its sensitization potential decreased in mouse model.

CONCLUSION

Exploring the structural change of Ara h 2 after cross-linking is beneficial in further understanding the influence of structure on sensitization. This result indicated the future possibility of precision processing on structure of proteins to improve their properties. © 2019 Society of Chemical Industry.

Different thermal processing effects on peanut allergenicity

BACKGROUND

Peanut allergy is one of the most common food allergies worldwide. Studies have shown that the incidence of peanut allergies in Western-born Asians is higher than that in Asia-born Asians. Notably, Europeans and Americans mostly eat roasted peanuts, whereas Asians mostly eat boiled or fried peanuts.

RESULTS

BALB/c mice were sensitized using purified protein from raw, roasted or boiled peanuts, then fed the same by oral gavage. The relevant allergic reactions were studied using BALB/c mice model, including a rat basophilic leukemia (RBL) cell model, simulated gastric fluid experiments, and ultraviolet (UV) and circular dichroism (CD) spectral analysis. Serological studies showed increased levels of immunoglobulin E, interleukin-4 and interleukin-5, and pathological studies showed mast cell degranulation and inflammatory changes in jejunal tissues, with an increase in thymic stromal lymphopoietin (TSLP) gene expression in all treatment groups compared with the control group (phosphate-buffered saline). Compared with the raw peanut group, sera from the roasted peanut group produced a significant increase in RBL β-hexosaminidase A release in vitro, and roasted peanuts showed increased resistance to digestion in simulated gastric fluid experiments. Ultraviolet and CD spectral analyses showed that the roasting and boiling processes altered the structure of the major peanut allergens, which may have contributed to the differences observed in peanut allergenicity.

CONCLUSION

Our findings indicate that peanut allergies are related to peanut thermal processing methods. In our mouse model, the raw, roasted and boiled peanuts elicited different degrees of allergic response. Compared with raw peanut, roasted peanuts show a higher allergenicity, whereas the boiled peanuts show a lower allergenicity. © 2018 Society of Chemical Industry.

Electrophoretic Separation of Very Large Molecular Weight Proteins in SDS Agarose

Very large proteins (subunit sizes, >200 kDa) are difficult to electrophoretically separate on polyacrylamide gels. A SDS vertical agarose gel system has been developed that has vastly improved resolving power for very large proteins. Proteins with molecular masses between 200 and 4000 kDa can be clearly separated. Inclusion of a reducing agent in the upper reservoir buffer and use of a large pore-sized agarose have been found to be key technical procedures for obtaining optimum protein migration and resolution.

Cross-linking of shrimp tropomyosin catalyzed by transglutaminase and tyrosinase produces hypoallergens for potential immunotherapy

Transglutaminase or tyrosinase treatment reduces tropomyosin allergenicity and produces potential hypoallergens for immunotherapy.

The Maillard Reaction Reduced the Sensitization of Tropomyosin and Arginine Kinase from Scylla paramamosain, Simultaneously

The Maillard reaction was established to reduce the sensitization of tropomyosin (TM) and arginine kinase (AK) from Scylla paramamosain, and the mechanism of the attenuated sensitization was investigated. In the present study, the Maillard reaction conditions were optimized for heating at 100 °C for 60 min (pH 8.5) with arabinose. A low level of allergenicity in mice was shown by the levels of allergen-specific antibodies, and more Th1 and less Th2 cells cytokines produced and associated transcription factors with the Maillard reacted allergen (mAllergen). The tolerance potency in mice was demonstrated by the increased ratio of Th1/Th2 cytokines. Moreover, mass spectrometry analysis showed that some key amino acids of IgE-binding epitopes (K₁₁₂, R₁₂₅, R₁₃₃ of TM; K₃₃, K₁₁₈, R₂₀₂ of AK) were modified by the Maillard reaction. The Maillard reaction with arabinose reduced the sensitization of TM and AK, which may be due to the masked epitopes.

The Role of Dietary Phenolic Compounds in Protein Digestion and Processing Technologies to Improve Their Antinutritive Properties

Digestion is the key step for delivering nutrients and bioactive substances to the body. The way different food components interact with each other and with digestive enzymes can modify the digestion process and affect human health. Understanding how food components interact during digestion is essential for the rational design of functional food products. Plant polyphenols have gained much attention for the bioactive roles they play in the human body. However, their strong beneficial effects on human health have also been associated with a negative impact on the digestion process. Due to the generally low absorption of phenolic compounds after food intake, most of the consumed polyphenols remain in the gastrointestinal tract, where they then can exert inhibitory effects on enzymes involved in the degradation of saccharides, lipids, and proteins. While the inhibitory effects of phenolics on the digestion of energy-rich food components (saccharides and lipids) may be regarded as beneficial, primarily in weight-control diets, their inhibitory effects on the digestion of proteins are not desirable for the reason of reduced utilization of amino acids. The effect of polyphenols on protein digestion is reviewed in this article, with an emphasis on food processing methods to improve the antinutritive properties of polyphenols.

A versatile 3D tissue matrix scaffold system for tumor modeling and drug screening

Most of the anticancer drug candidates entering preclinical trials fail to be approved for clinical applications. The following are among the main causes of these failures: studying molecular mechanisms of cancer development, identifying therapeutic targets, and testing drug candidates using inappropriate tissue culture models, which do not recapitulate the native microenvironment where the cancer cells originate. It has become clear that three-dimensional (3D) cell cultures are more biologically and clinically relevant than 2D models. The spatial and mechanical conditions of 3D cultures enable the cancer cells to display heterogeneous growth, assume diverse phenotypes, express distinct gene and protein products, and attain metastatic potential and resistance to drugs that are reminiscent of tumors in humans. However, the current 3D culture systems using synthetic polymers or selected components of the extracellular matrix (ECM) are defective (particularly the biophysical and biochemical properties of the native ECM) and remain distant to optimally support the signaling cue-oriented cell survival and growth. We introduce a reconstitutable tissue matrix scaffold (TMS) system fabricated using native tissue ECM, with tissue-like architecture and resilience. The structural and compositional properties of TMS favor robust cell survival, proliferation, migration, and invasion in culture and vascularized tumor formation in animals. The combination of porous and hydrogel TMS allows compartmental culture of cancerous and stromal cells, which are distinguishable by biomarkers. The response of the cancer cells grown on TMS to drugs well reflects animal and clinical observations. TMS enables more biologically relevant studies and is suitable for preclinical drug screening.

Cloning, Expression, and the Effects of Processing on Sarcoplasmic-Calcium-Binding Protein: An Important Allergen in Mud Crab

Shellfish allergy is a prevalent, long-lasting disorder usually persisting throughout life. However, the allergen information is incomprehensive in crab. This study aimed to identify a novel allergen in crab, show its potential in diagnosis and reduce the allergenicity by food processing. A 21-kDa protein was purified from Scylla paramamosain and confirmed as sarcoplasmic calcium binding protein (SCP) by matrix-assisted laser desorption ionization-time-of-flight/time-of-flight mass spectrometry (MALDI-TOF/TOF-MS). Total RNA was isolated from crab muscle, and a rapid amplification of cDNA was performed to obtain an ORF of 579 bp that coded for 193 amino acid residues. According to the results of circular dichroism analysis and ELISA assay, the recombinant SCP (rSCP) expressed in Escherichia coli showed similar physicochemical and immunoreactive properties to native SCP (nSCP). Additionally, the extensive cross reactivity of SCP among different species and the bidirectional IgE cross-reactivity between nSCP and rSCP were detected by iELISA. The allergenicity of rSCP was reduced via Maillard reaction or enzymatic cross-linking reaction, which was confirmed by the results of scanning electron microscopy, dot blot, and digestion assay. A straightforward and reproducible way was developed to obtain high yields of rSCP that maintains structural integrity and full IgE reactivity, which could compensate the low specific IgE-titers of most patient sera for future diagnosis. Furthermore, the Maillard reaction and enzymatic cross-linking reaction were effective approaches for the production of hypoallergenic seafood.

Allergenicity and Oral Tolerance of Enzymatic Cross-Linked Tropomyosin Evaluated Using Cell and Mouse Models

The enzymatic cross-linking of proteins to form high-molecular-weight compounds may alter their sensitization potential. The IgG-/IgE-binding activity, digestibility, allergenicity, and oral tolerance of cross-linked tropomyosin with tyrosinase (CTC) or horseradish peroxidase (CHP) were investigated. ELISA results demonstrated CTC or CHP reduced its IgE-binding activity by 34.5 ± 1.8 and 63.5 ± 0.6%, respectively. Compared with native tropomyosin or CTC, CHP was more easily digested into small fragments; CHP decreased the degranulation of RBL-2H3 cells and increased endocytosis by dendritic cells. CHP can induce oral tolerance and reduce allergenicity in mice by decreasing IgE and IgG1 levels in serum, the production of T-cell cytokines, and the percentage composition of dendritic cells. These findings demonstrate CHP has more potential of reducing the allergenicity than CTC via influencing the morphology of protein, changing the original method of antigen presentation, modulating the Th1/Th2 immunobalance, and inducing the oral tolerance of the allergen tropomyosin.

Ara h 2 cross-linking catalyzed by MTGase decreases its allergenicity

Ara h 2 cross-linking involving linear allergen epitopes significantly decreases its allergenicity.

The concept of allergen-associated molecular patterns (AAMP)

For proteins to become allergenic, they need to acquire features enabling them to induce B cell activation and isotype switch to IgE production. Crosslinking of the B-cell receptor (BCR) is the most efficient way to productively activate B-cells. The IgE-crosslinking capability of allergens is equally crucial in the effector phase of immediate type allergy. Antigens, which acquire enhanced crosslinking capacity by oligomerization, aggregation, or the expression of repetitive epitopes may therefore gain allergenic potency. The accumulated evidence for repetitive epitope display by allergens suggests the existence of allergen-associated molecular patterns.

Crosslinking of peanut allergen Ara h 2 by polyphenol oxidase: digestibility and potential allergenicity assessment

BACKGROUND

Peanut is one of the eight major food allergens. Its allergen, Ara h 2, can be recognized by over 90% of serum IgE samples from peanut-allergic patients. Therefore, reducing the allergenicity of Ara h 2 is especially important.

RESULTS

In the present study, polyphenol oxidase (PPO), a protein cross-linking reaction catalyst that acts on tyrosine residue, was used to modify Ara h 2. After crosslinking, the microstructure, digestibility, IgG binding capability and IgE binding capability of Ara h 2 were analyzed. Cross-linking decreased the potential allergenicity of Ara h 2 by masking the allergen epitope, while the antigenicity of Ara h 2 changed slightly. After crosslinking, the apparent diameter of Ara h 2 was altered from 300 to 1700 nm or 220 nm, indicating that polymerization could either be inter- or intramolecular. Regarding digestibility, crosslinked Ara h 2 was relatively more easily digested by gastric fluid compared with the untreated Ara h 2, but much more difficult in the intestinal fluid.

CONCLUSION

The crosslinking reaction catalyzed by PPO, as a non-thermal process, may be beneficial for avoiding food allergy. The reaction could mask allergen epitopes, decreasing the allergenicity of Ara h 2. © 2015 Society of Chemical Industry.

Assessment of the sensitizing capacity and allergenicity of enzymatic cross-linked arginine kinase, the crab allergen

SCOPE

The enzymatic cross-linking of an allergen by food processing may alter its sensitization potential. In this study, the IgE-binding activity and allergenicity of cross-linked thermal polymerized arginine kinase (CL-pAK) were investigated.

METHODS AND RESULTS

The IgE-binding activity and stability of CL-pAK were analyzed by immunological and proteomics methods. The sensitization and potency to induce oral tolerance of CL-pAK were tested using in vivo assays and a cell model. According to the results of inhibition of ELISA, the half inhibitory concentration of AK after cross-linking changed from 1.13 to 228.36 μg/mL. The results of in vitro digestion demonstrated that CL-pAK showed more resistance to gastrointestinal digestion than native AK. Low allergenicity and capacity to induce oral tolerance in mice were shown by the sera levels of AK-specific antibodies and T-cell cytokine production. Exposure of RBL-2H3 cells to CL-pAK compared with AK, resulted in lower levels of mast degranulation and histamine.

CONCLUSION

Enzymatic cross-linking with thermal polymerization of AK by tyrosinase and caffeic acid had high potential in mitigating IgE-binding activity and allergenicity, which were influenced by altering the molecular and immunological features of the shellfish protein.

Peanut protein structure, polyphenol content and immune response to peanut proteins in vivo are modulated by laccase

Laccase cross-linking of peanut protein causes changes in the protein structure, phenolic composition and immunological properties of the treated peanut protein.

Allergen composition analysis and allergenicity assessment of Chinese peanut cultivars

Peanut (Arachis hypogaea) is among the eight major food allergens in the world. Several attempts have been made to decrease or eliminate the allergenicity of peanut. Systemic screening of thousands of peanut cultivars may identify peanut with low allergenicity. In this study, the allergen compositions of 53 Chinese peanut cultivars were characterized, and their allergenicity to sera IgE of Chinese patients and in a mouse model was assessed. Contents of total protein and allergens were quantified by SDS-PAGE and densitometry analysis on gel. Although the contents of allergens broadly varied among cultivars, they were related to one another. The IgE binding capacity of cultivars was tested by ELISA, and their allergenicity was further evaluated in a mouse model by oral sensitization. Results showed that the allergenicity of peanut was affected by allergen composition rather than a single allergen. Peanut cultivars with low allergenicity may contain more Ara h 3/4 (24 kDa), Ara h 2 and less Ara h 3/4 (43, 38, and 36 kDa), Ara h 6. Screening based on allergen composition would facilitate the identification of low-allergenic peanut.