Expression in Escherichia coli of Sin a 1, the major allergen from mustard

First electrochemical immunosensor for the rapid detection of mustard seeds in plant food extracts

This paper describes the first biosensor reported to date for the determination of mustard seed traces. The biosensor consists of an amperometric immunosensing platform able to sensitively and selectively determine Sin a 1 content, the major allergen of yellow mustard and the most abundant protein of these seeds. The immunosensing platform exploits the coupling of magnetic microbeads (MBs) modified with sandwich-type immune complexes, comprising polyclonal and monoclonal antibodies, selective to the target protein for its capturing and detection, respectively. In addition, a HRP-conjugated secondary antibody was used for enzymatic labelling of the monoclonal antibody, and amperometric transduction was made at screen-printed carbon electrodes (SPCEs) using the hydroquinone (HQ)/H2O2 system. The electrochemical immunosensor allows the simple and fast detection (a single 1-h incubation step) of Sin a 1 with a limit of detection of 0.82 ng mL-1 (20.5 pg of protein in 25 μL of sample) with high selectivity against structurally similar non-target allergenic proteins (such as Pin p 1 from pine nut). The developed immunoplatform was successfully used for the analysis of peanut, rapeseed, cashew, pine nut and yellow mustard extracts, giving only positive response for the yellow mustard extract with a Sin a 1 content, in full agreement with that provided by conventional ELISA methodology.

Detection and identification of allergens from Canadian mustard varieties of Sinapis alba and Brassica juncea

Currently, information on the allergens profiles of different mustard varieties is rather scarce. Therefore, the objective of this study was to assess protein profiles and immunoglobulin E (IgE)-binding patterns of selected Canadian mustard varieties. Optimization of a non-denaturing protein extraction from the seeds of selected mustard varieties was first undertaken, and the various extracts were quantitatively and qualitatively analyzed by means of protein recovery determination and protein profiling. The IgE-binding patterns of selected mustard seeds extracts were assessed by immunoblotting using sera from mustard sensitized and allergic individuals. In addition to the known mustard allergens—Sin a 2 (11S globulins), Sin a 1, and Bra j 1 (2S albumins)—the presence of other new IgE-binding protein bands was revealed from both Sinapis alba and Brassica juncea varieties. Mass spectrometry (MS) analysis of the in-gel digested IgE-reactive bands identified the unknown ones as being oleosin, β-glucosidase, enolase, and glutathione-S transferase proteins. A bioinformatic comparison of the amino acid sequence of the new IgE-binding mustard proteins with those of know allergens revealed a number of strong homologies that are highly relevant for potential allergic cross-reactivity. Moreover, it was found that Sin a 1, Bra j 1, and cruciferin polypeptides exhibited a stronger IgE reactivity under non-reducing conditions in comparison to reducing conditions, demonstrating the recognition of conformational epitopes. These results further support the utilization of non-denaturing extraction and analysis conditions, as denaturing conditions may lead to failure in the detection of important immunoreactive epitopes.

Structural stability and Sin a 1 anti-epitope antibody binding ability of yellow mustard (Sinapis alba L.) napin during industrial-scale myrosinase inactivation process

This study investigated the structural stability of yellow mustard (YM, Sinapis alba L.) napin and the changes of its Sin a 1 anti-epitope antibody-binding ability during myrosinase enzyme inactivation process. The food industry uses myrosinase-inactive non-pungent YM for uses beyond spice applications. Napin was isolated from seeds received from an industrial processor before (YM + M) and after (YM - M) myrosinase inactivation. Secondary and tertiary structural features and surface hydrophobicity parameters of napin were analyzed. The Sin a 1 content in YM seeds and the stability of Sin a 1-containing napin during simulated in vitro gastrointestinal (GI) digestion were determined by a non-competitive indirect enzyme-linked immunosorbent assay using the Sin a 1 anti-epitope antibody (AE-Ab) as the primary Ab. YM napin retained the dominant alpha-helical components of secondary and tertiary structure folds during this process. YM - M napin showed changes in hydrophobicity parameters of the molecules and binding ability of AE-Ab: 2.19 ± 0.48 g per 100 g of YM - M seeds vs. 1.49 ± 0.16 g per 100 g YM + M seeds. YM - M proteins were more susceptible for in vitro GI digestion and also showed a 30% reduction in AE-Ab binding ability upon digestion of napins. This suggests that the myrosinase inactivation process has induced the surface modification of napin, exposing Sin a 1 epitope, leading to an increase in AE-Ab binding. However, the epitope region of YM - M napin showed improved susceptibility for hydrolysis during GI digestion resulting in fewer available epitope regions, suggesting a possible reduction in napin immune reactivity.

Food processing and allergenicity

Food processing can have many beneficial effects. However, processing may also alter the allergenic properties of food proteins. A wide variety of processing methods is available and their use depends largely on the food to be processed. In this review the impact of processing (heat and non-heat treatment) on the allergenic potential of proteins, and on the antigenic (IgG-binding) and allergenic (IgE-binding) properties of proteins has been considered. A variety of allergenic foods (peanuts, tree nuts, cows' milk, hens' eggs, soy, wheat and mustard) have been reviewed. The overall conclusion drawn is that processing does not completely abolish the allergenic potential of allergens. Currently, only fermentation and hydrolysis may have potential to reduce allergenicity to such an extent that symptoms will not be elicited, while other methods might be promising but need more data. Literature on the effect of processing on allergenic potential and the ability to induce sensitisation is scarce. This is an important issue since processing may impact on the ability of proteins to cause the acquisition of allergic sensitisation, and the subject should be a focus of future research. Also, there remains a need to develop robust and integrated methods for the risk assessment of food allergenicity.

Quantitative detection of allergenic protein Sin a 1 from yellow mustard (Sinapis alba L.) seeds using enzyme-linked immunosorbent assay

Allergy to yellow mustard (YM; Sinapis alba L.) seed proteins has been reported and is currently seen as a constraint that hampers expansion of YM protein utilization. The most predominant allergenic protein of YM seed has been recognized as Sin a 1. In this study, Sin a 1 was purified ( S. alba var. Andante), rabbit polyclonal antibodies (pAb) specific to Sin a 1 were generated, and a sandwich enzyme-linked immunosorbent assay (S-ELISA) was developed to detect and quantify Sin a 1 from YM. The S-ELISA method using Sin a 1-pAb and its horseradish peroxidase conjugate resulted in a detection limit of 0.3 microg/mL for purified Sin a 1. The Sin a 1 contents of six YM lines were in the range of 0.82-2.94 mg/g when assayed by the developed S-ELISA method. The results showed that S-ELISA could distinguish Sin a 1 in YM seed-derived extracts rapidly and could be applied in controlling and/or monitoring of YM allergenic proteins.

Detection of sesame seed DNA in foods using real-time PCR

The detection of potentially allergenic foods, such as sesame seeds, in food products is a major concern for the food-processing industry. A real-time PCR method was designed to determine if sesame seed DNA is present in food products. The PCR reaction amplifies a 66-bp fragment of the sesame seed 2S albumin gene, which is detected with a sesame-specific, dual-labeled TaqMan probe. This reaction will not amplify DNA derived from other seeds present in baked goods, such as pumpkin, poppy, and sunflower seeds. Additionally, this assay will not cross-react with DNA from several tree nut species, such as almond, Brazil nut, cashew, hazelnut, and walnut, as well as four varieties of peanut. This assay is sensitive enough to detect 5 pg of purified sesame seed DNA, as well as sesame seed DNA in a spiked wheat cracker sample.

In vitro refolded napin-like protein of Momordica charantia expressed in Escherichia coli displays properties of native napin

Napins belong to the family of 2S albumin seed storage proteins and are shown to possess antifungal activity. Napins, in general, consist of two subunits (derived from single precursor) linked by disulphide bridges. Usually, reducing environment of the E. coli cytosol is not conducive for proper folding of heterodimeric proteins containing disulphide bridges. Present investigation reports for the first time expression of napin-like protein of Momordica charantia (rMcnapin) in E. coli and its in vitro refolding to produce biologically active protein. Full-length cDNA encoding napin-like protein (2S albumin) was isolated from M. charantia seeds by immunoscreening a cDNA expression library. The cDNA consisted of an open reading frame encoding a protein of 140 amino acid residues. The 36 amino acids at the N-terminus represent the signal and propeptide. The region encoding small and large chains of the M. charantia napin is separated by a linker of 8 amino acid residues. The region encoding napin (along with the linker) was PCR amplified, cloned into pQE-30 expression vector and expressed in E. coli. rMcnapin expressed as inclusion bodies was solubilized and purified by Ni2+-NTA affinity chromatography. The denatured and reduced rMcnapin was refolded by rapid dilution in an alkaline buffer containing glycerol and redox couple (GSH and GSSG). Refolded His-rMcnapin displayed similar spectroscopic properties as that of mature napin-like protein of M. charantia with 48.7% alpha-helical content. In addition, it also exhibited antifungal activity against T. hamatum with IC50 of 3 microg/ml. Refolded His-rMcnapin exhibited approximately 90% antifungal activity when compared with that of mature napin-like protein of M. charantia. Thus, a heterologous expression system and in vitro refolding conditions to obtain biologically active napin-like protein of M. charantia were established.

Expression in Escherichia coli and disulfide bridge mapping of PSC33, an allergenic 2S albumin from peanut

In this work, we describe the expression, purification, and disulfide mapping of the named 'peanut seed cDNA 33' (PSC33) peanut allergen. A variant of PSC33 (with N(63), E(64), Q(69) instead of D(63), Q(64), E(69)) has been identified in peanut by proteomic analysis of a highly IgE immunoreactive purification fraction. It is 92% homologous to Ara h 6. We raised monoclonal antibodies against PSC33 and amplified it by PCR from peanut leaf genomic DNA. PSC33 was intron-less and the two NEQ and DQE variants of PSC33 were equally amplified. Since expression of the natural PSC33 (DQE) gene was very low in Escherichia coli even with supplementation of rare codon tRNAs, a synthetic gene optimized for expression in E. coli of PSC33 (DQE) was introduced into a pET9-c vector. A high production of protein occurred in the inclusion bodies that was submitted to refolding using an additive-introduced stepwise dialysis protocol which consists in the gradual removal of the denaturing agent guanidine-HCl with controlled introduction of oxidized and reduced glutathione and l-arginine as a chemical chaperone. After reverse phase HPLC purification, 1mg of pure refolded protein (as assayed by MALDI-TOF mass spectrometry, mouse IgG immunoreactivity and circular dichroism) were obtained with every 100ml of bacterial culture. Trypsin and CNBr hydrolysis of the protein combined with MALDI-TOF mass spectrometry allowed us to assign disulfide bridges and show that the native and refolded proteins were identical. The four disulfides of canonical 2S albumins were conserved and the two supplementary cysteines of PSC33 were paired together.

A Recombinant Precursor of the Mustard Allergen Sin a 1 Retains the Biochemical and Immunological Features of the Heterodimeric Native Protein

BACKGROUND

Mustard has been an important cause of food allergy of increasing incidence in the last years. Sin a 1, a storage 2S albumin, is the most relevant allergen from this spice.

METHODS

Pichia pastoris has been used as host for the recombinant production of the precursor form of Sin a 1 (rproSin a 1). rproSin a 1 was purified to homogeneity by three chromatographic steps: gel filtration, anion exchange and reverse-phase HPLC. Molecular characterization was performed using Edman degradation, mass spectrometry, amino acid composition, and circular dichroism. Immunological properties were analyzed by immunoblotting, ELISA, and ELISA inhibition experiments.

RESULTS

We overexpressed rproSin a 1 as a single polypeptide with both large and small chains linked by an internal processed fragment at high yield. The purified rproSin a 1 (>95%) was obtained as a monomeric and soluble protein. rproSin a 1 showed equivalent structural and immunological properties to natural heterodimeric Sin a 1 allergen. rproSin a 1 was recognized by 75% of the patients allergic to mustard. The inhibitory capacity of rproSin a 1 to the total allergenicity of mustard extracts varied from 13 to 83% in different patients, with a mean value of 54%.

CONCLUSIONS

rproSin a 1 is a good candidate to replace natural allergen in diagnosis protocols of mustard allergy. P. pastoris has been demonstrated to be a suitable expression system for the production of allergenic derivates of Sin a 1 that could be used for immunotherapy purposes in future.

Structural, biological, and evolutionary relationships of plant food allergens sensitizing via the gastrointestinal tract

The recently completed genome sequence of the model plant species Arabidopsis has been estimated to encode over 25,000 proteins, which, on the basis of their function, can be classified into structural and metabolic (the vast majority of plant proteins), protective proteins, which defend a plant against invasion by pathogens or feeding by pests, and storage proteins, which proved a nutrient store to support germination in seeds. It is now clear that almost all plant food allergens are either protective or storage proteins. It is also becoming evident that those proteins that trigger the development of an allergic response through the gastrointestinal tract belong primarily to two large protein superfamilies: (1) The cereal prolamin superfamily, comprising three major groups of plant food allergens, the 2S albumins, lipid transfer proteins, and cereal alpha-amylase/trypsin inhibitors, which have related structures, and are stable to thermal processing and proteolysis. They include major allergens from Brazil nut, peanuts, fruits, such as peaches, and cereals, such as rice and wheat; (2) The cupin superfamily, comprising the major globulin storage proteins from a number of plant species. The globulins have been found to be allergens in plant foods, such as peanuts, soya bean, and walnut; (3) The cyteine protease C1 family, comprising the papain-like proteases from microbes, plants, and animals. This family contains two notable allergens that sensitize via the GI tract, namely actinidin from kiwi fruit and the soybean allergen, Gly m Bd 30k/P34. This study describes the properties, structures, and evolutionary relationships of these protein families, the allergens that belong to them, and discusses them in relation to the role protein structure may play in determining protein allergenicity.

Mustard allergy confirmed by double-blind placebo-controlled food challenges: clinical features and cross-reactivity with mugwort pollen and plant-derived foods

BACKGROUND

Mustard IgE-mediated allergy is supposed to be a rare cause of food allergy, and its clinical features and cross-reactivities have not been fully elucidated.

METHODS

A prospective study was carried out, recruiting mustard allergic patients, and paired control subjects. A clinical questionnaire was administered, and skin-prick tests (SPT) with panels of aeroallergens and foods, serum extraction for in vitro tests and double-blind placebo-controlled food challenges (DBPCFC) were performed.

RESULTS

Thirty-eight mainly adult patients, with 10.5% reporting systemic anaphylaxis, were included in the study [age (mean +/- SD): 21.9 +/- 8.6 years]. DBPCFC were performed in 24 patients, being positive in 14 cases (58.3%). Patients with positive outcome showed significantly greater mustard SPT than those with negative outcome (8.2 +/- 3.7 vs 5.3 +/- 2.4 mm, P <0.05), and the receiver-operating characteristic (ROC) curve analysis yielded a cut-off value for mustard commercial SPT of 8 mm, with a specificity of 90% (95% CI, 55.5-98.3), and a sensitivity of 50% (95% CI, 23.1-76.9). A significant association between mustard hypersensitivity and mugwort pollen sensitization was found (97.4% of patients), with partial cross-reactivity demonstrated by UniCAP System inhibition assays. All patients showed sensitization to other members of Brassicaceae family, and cross-reactivity among them was also confirmed. Moreover, significant associations with nut (97.4%), leguminous (94.7%), corn (78.9%), and Rosaceae fruit (89.5%) sensitizations were also shown. Around 40% of these food sensitizations were symptomatic, including food-dependent exercise-induced anaphylaxis in six patients.

CONCLUSIONS

Mustard allergy is a not-uncommon disorder that can induce severe reactions. Significant associations with mugwort pollinosis and several plant-derived food allergies are demonstrated, suggesting a new mustard-mugwort allergy syndrome. A relationship between this syndrome and food-dependent exercise-induced anaphylaxis is also reported.

Improving diagnostic tests for food allergy with recombinant allergens

Food allergy is one important manifestation of atopic allergy. Primary food allergy mainly affects young children (class I), whereas adults frequently develop food allergy as a consequence of an inhalant sensitization (class II). At present, the diagnostic instrument for proving class II food allergy is not satisfactory. Skin tests as well as serological tests are in general neither very specific nor highly sensitive because they depend on food extracts, which differ in their content of individual allergens, vary between manufacturers, and even between different batches. Since the presence of food allergen-specific IgE antibodies does not always correlate with clinical symptoms against the respective food, oral provocation tests (ideally double-blind placebo-controlled food challenges) have to be performed to validate serological diagnosis or skin tests. However, oral provocation tests are connected to several practical problems and include a specific risk for the allergic patient. Applying DNA technology, up to 40 food allergens have been produced in recombinant form, which implies standardized quality and unlimited quantity of the respective proteins. Hence, such molecules might be used to solve problems of clinical and molecular allergology in diagnosis, research, and therapy of class II food allergies. First experiments with recombinant food allergens in this respect appear very promising.

Characterization of allergenic food proteins for improved diagnostic methods

PURPOSE OF REVIEW

Diagnostic decision points for food allergen-specific serum IgE concentration and for skin prick test results have been established for several foods, reducing the requirement for food challenges in a number of patients. Many patients, however, still need to undergo oral food challenges because their food-specific IgE level is in the undefined range. In addition, diagnostic decision points could not be established for several foods. It appears that measurement of serum specific IgE concentrations to individual allergens is superior to determination of specific IgE to the crude food extract containing allergenic and nonallergenic proteins. This review will outline recent advantages in characterization of food allergens as well as the relevance of this knowledge for use in recently developed protein microarray technology.

RECENT FINDINGS

Protein microarrays have been developed to profile allergen-specific IgE antibodies from human serum with the advantage of screening hundreds of allergens in parallel using minute amounts of blood. This technology, however, requires prior knowledge of the proteins to be studied. The identification and characterization of clinically relevant allergens have increased dramatically within the last several years. Relevant new allergens have been identified, especially in tree nuts and seeds. Interestingly, most of these allergens belong to the same family of seed storage proteins. In addition, known food allergens have been further characterized and IgE-binding sites have been determined. Moreover, 'informative' peptides shown to be predictive for the persistence of food allergy have been identified.

SUMMARY

The combination of food allergen characterization and protein or peptide microarray technology will enable us to develop improved diagnostic tools in food allergy.

Prospective study of mustard allergy: first study with double-blind placebo-controlled food challenge trials (24 cases)

BACKGROUND

Mustard allergy accounts for 1.1% of food allergies in children. However, double-blind placebo-controlled food challenge trials (DB PCFCs) have not yet been proposed.

OBJECTIVE

To carry out DB PCFCs to determine the real frequency of mustard allergy in patients sensitized to mustard.

METHODS

A prospective study was conducted in 30 subjects aged 3-20 years presenting positive prick tests to ground mustard seeds (Brassica nigra), mustard flour (B. juncea), metabisulfite-free strong mustard seasoning (B. juncea) and a commercialized allergenic extract (B. nigra). Twenty-seven subjects were screened for mustard-specific immunoglobulin E (IgE). PCFCs were carried out either DB or single blind (SB) with up to 1340 mg of metabisulfite-free seasoning.

RESULTS

The mean diameter of the wheal induced by prick tests with the allergenic extract was lower (n.s.) than that induced by the native mustard products: 5.8 mm (1.5-15) vs 6.9 mm (0.5-18) for B. nigra ground seeds, 7.8 mm (1-20) for B. juncea flour and 9.7 mm (3-20) for the strong mustard seasoning. The diameter of the wheal induced by the allergenic extract was significantly different from that induced by the mustard seasoning (P < 0.005). The mean of mustard specific-IgE values was 8.7 KU/l (0.35-72.4). Seven of 30 food challenges were considered positive. Mean prick test results in the positive and negative PCFC subgroups were 5.5 mm vs 5.9 mm for the commercialized extract, 10.9 mm vs 5.8 mm for B. nigra ground seeds (P < 0.01), 9.9 mm vs 7.1 mm for B. juncea flour (n.s. P > 0.25) and 11.5 mm vs 9.1 mm for the metabisulfite-free mustard seasoning (n.s. P > 0.1). Mean specific IgE values determined by CAP system radioallergosorbent test (Phadebas Pharmacia) were higher but not significantly so (P > 0.25) in the subgroup with mustard allergy (12.3 K/l vs 7.6 KU/l).

CONCLUSIONS

About 23.3% of the sensitized subjects were allergic to a routine dose of mustard. Positive prick tests and the presence of specific IgE were not predictive. SB PCFC or DB PCFC is required before recommending avoidance diets.

New plant-origin food allergens

Plant-origin foods, especially nuts and seeds, are the most important sources of food allergic reactions. An important characteristic is the quantitative and qualitative variability of their content in allergenic molecules, depending on plant growth, ripening, environmental stresses or industrial processing. In this review we will focus on newly identified allergens. Recent research have characterized and extensively studied their biochemistry, structure and immunological properties.

Precursor of the inactive 2S seed storage protein from the Indian mustard Brassica juncea is a novel trypsin inhibitor. Charaterization, post-translational processing studies, and transgenic expression to develop insect-resistant plants

A number of trypsin inhibitor (TI) genes have been used to generate insect-resistant plants. Here we report a novel trypsin inhibitor from Indian mustard Brassica juncea (BjTI) that is unique in being the precursor of a 2S seed storage protein. The inhibitory activity is lost upon processing. The predicted amino acid sequence of the precursor based on the B. juncea 2S albumin (Bj2S) gene cloned and sequenced in this laboratory (Bj2Sc; GenBank(TM) accession number ) showed a soybean-TI active site-like motif GPFRI at the expected processing site. The BjTI was found to be a thermostable Kunitz type TI that inhibits trypsin at a molar ratio of 1:1. The 20-kDa BjTI was purified from midmature seeds and found to be processed in vitro to 9- and 4-kDa subunits upon incubation with seed extract. The Bj2Sc sequence was expressed in Escherichia coli pET systems as the inhibitor precursor. The radiolabeled gene product was expressed in vitro in a coupled transcription-translation system and showed the expected processing into subunits. Two in vitro expressed pre-2S proteins, mutated at Gly and Asp residues, were processed normally to mature subunits, showing thereby no absolute requirement of Gly and Asp residues for processing. Finally, the 2S gene was introduced into tobacco and tomato plants. Third generation transgenics expressing BjTI at 0.28-0.83% of soluble leaf proteins showed remarkable resistance against the tobacco cutworm, Spodoptera litura. This novel TI can be used in transforming seed crops for protection to their vegetative parts and early seed stages, when insect damage is maximal; as the seeds mature, the TI will be naturally processed to the inactive storage protein that is safe for consumption.

Proteomics of allergens

The present state of proteomics research is generally outlined and the character of allergenic compounds briefly elucidated. The principles of experimental approaches to isolation, purification, identification and characterization of allergens and to monitoring of their biological activity are described, with emphasis on the most modern methods. Selected examples are given for illustration and important results are summarized in tables.

Recombinant pronapin precursor produced in Pichia pastoris displays structural and immunologic equivalent properties to its mature product isolated from rapeseed

2S albumin storage proteins from rapeseed (Brassica napus), called napins, consist of two different polypeptide chains linked by disulphide bridges, which are derived by proteolytic cleavage from a single precursor. The precursor form of the napin BnIb (proBnIb) has been cloned using a PCR strategy and sequenced. The amino-acid sequence deduced from the clone includes 31 residues of the small chain and 75 of the large chain, which are connected by the peptide Ser-Glu-Asn. Expression of the cDNA encoding proBnIb has been carried out in the methylotrophic yeast Pichia pastoris. The induced protein was secreted to the extracellular medium at a yield of 80 mg.L(-1) of culture and was purified by means of size-exclusion chromatography and reverse phase-HPLC. Recombinant proBnIb appeared properly folded as its molecular and spectroscopic properties were equivalent to those of the mature heterodimeric protein. As 2S albumin storage proteins from Brassicaceae have been shown to be type I allergy inducers, the immunological activity of the recombinant proBnIb was analysed as a measure of its structural integrity. The immunological properties of the recombinant precursor and the natural napin were indistinguishable by immunoblotting and ELISA inhibition using polyclonal antisera and sera of patients allergic to mustard and rapeseed. In conclusion, the recombinant expression of napin precursors in P. pastoris has been shown to be a successful method for high yield production of homogeneous and properly folded proteins whose polymorphism and complex maturation process limited hitherto their availability.

Isolation of food allergens

The identification of food allergens is a priority in the management of food allergy, because of the need to obtain standardized extracts and pure allergens for diagnosis and therapy. It is thus important to develop methods for purification of allergenic molecules in order to study their biological and immunological characteristics. Protocols for protein extraction from foods and for allergen purification are reviewed in this paper. We report published methods for extraction of allergens from either animal and vegetable foods and detailed purification methodologies including ion-exchange, gel filtration and reversed-phase chromatography of well known allergens.

Recombinant food allergens

Allergenic (glyco)proteins are the elicitors of food allergies and can cause acute severe hypersensitivity reactions. Recombinant food allergens are available in standardised quantity and constant quality. Therefore, they offer new perspectives to overcome current difficulties in the diagnosis, treatment and investigation of food allergies. This review summarises the expression strategies and characteristics of more than 40 recombinant food allergens that have been produced until today. Their IgE-binding properties are compared to those of their natural counterparts, in addition their application as diagnostic tools, the generation of hypoallergenic recombinant isoforms and mutants for therapeutic purposes, the determination of epitopes and cross-reactive structures are described.

Molecular and biochemical classification of plant-derived food allergens

Molecular biology and biochemical techniques have significantly advanced the knowledge of allergens derived from plant foods. Surprisingly, many of the known plant food allergens are homologous to pathogenesis-related proteins (PRs), proteins that are induced by pathogens, wounding, or certain environmental stresses. PRs have been classified into 14 families. Examples of allergens homologous to PRs include chitinases (PR-3 family) from avocado, banana, and chestnut; antifungal proteins such as the thaumatin-like proteins (PR-5) from cherry and apple; proteins homologous to the major birch pollen allergen Bet v 1 (PR-10) from vegetables and fruits; and lipid transfer proteins (PR-14) from fruits and cereals. Allergens other than PR homologs can be allotted to other well-known protein families such as inhibitors of alpha-amylases and trypsin from cereal seeds, profilins from fruits and vegetables, seed storage proteins from nuts and mustard seeds, and proteases from fruits. As more clinical data and structural information on allergenic molecules becomes available, we may finally be able to answer what characteristics of a molecule are responsible for its allergenicity.

Mustard allergy in children

BACKGROUND

Mustard allergy is not well known. This study aimed to assess its clinical features and other associated allergies, and to define skin prick tests (SPT), specific IgE, and dose response by oral food challenge.

METHODS

Our study investigated 36 children with positive mustard SPT. The diagnosis of mustard allergy was based on open or single-blind, placebo-controlled food challenge (SBPCFC). We compared the subjects to 22 controls.

RESULTS

The initial clinical features were atopic dermatitis (51.8%), and urticaria and/or angioedema (37%). Fifteen children were allergic (positive SBPCFC) and 21 children were nonallergic (negative SBPCFC). Symptoms after mustard ingestion started under 3 years of age in 53.3% of the subjects. There was no significant difference in the food allergies and associated inhalant allergen sensitizations between the two groups. In the allergic group, the mean wheal diameter for mustard SPT was 8.8 mm and the median concentration of mustard serum (s) IgE 14.8 kU/l. The mean cumulative reactive dose were 153 mg.

CONCLUSIONS

Allergic reactions to mustard started early in life. Clinical symptoms were not severe in children. Mustard should be included in screening tests of food allergy in children.

Food allergens

A food allergen may be defined as a substance that reacts with IgE antibodies, induces allergic sensitisation or induces allergic reactions. Some allergens only induce allergic sensitisation but do not provoke symptoms, while others bind IgE but do not induce mast cell degranulation. There is no common structure that can predict whether a given antigen may be a strong food allergen. A complete food allergen, e.g. fish parvalbumin, is capable of stimulating the immune system to produce IgE antibodies, and degranulate mast cells upon subsequent contact. The reason(s) for why some patients with IgE to ovalbumin tolerate eggs, and why some react on one occasion but not on another, are mostly unclear, but may be related to changes in gut permeability induced by other food substances or by gastro-intestinal inflammation prior to the allergen contact. IgE antibodies to fruit or vegetables often show cross-reactivity, due to carbohydrate structures. These cross-reactive glycans have been designated cross-reactive carbohydrate determinants (CCD). Anti-CCD antibodies are highly cross-reactive. The antibodies do not have clinical significant because CCD-containing foods are usually well-tolerated by patients with IgE antibodies to CCD. These IgE antibodies may cause confusion in relation to allergy diagnosis.

Detection, isolation and complete amino acid sequence of an aeroallergenic protein from rapeseed flour

BACKGROUND

Seed proteins have been found to cause hypersensitivity by ingestion or inhalation. Rapeseed flour was responsible for allergic symptoms in a patient, who develops into allergy to mustard spice.

OBJECTIVE

To determine the presence of allergenic proteins in rapeseed flour, and analyse the structure of the main component and its crossreactivity with the mustard allergen.

METHODS

SDS-PAGE (sodium dodecyl sulfate-polyacrylamide gel electrophoresis) and subsequent immunoblotting with a serum from a rapeseed allergic patient were performed to detect IgE-binding proteins. Proteolytic digestions and high performance liquid chromatography were used to obtain the peptides from the allergenic BnIII napin from rapeseed flour. Automatic Edman degradations were carried out to determine their amino acid sequences, which were compared with other sequences in nucleotide and amino acid sequence databases. Crossreactivity assays were carried out by ELISA inhibition using sera from a rapeseed allergic patient and from patients allergic to mustard.

RESULTS

The 2S albumins of rapeseed were recognized by the serum from a patient allergic to this seed. The most abundant isoform of the allergenic napins, BnIII, was used for structural and immunological analysis. The protein consists of two different chains of 9.5 and 4.5 kDa. Their complete amino acid sequences were determined. The protein exhibited structural relationships with other napin-like storage proteins from seeds. IgE and IgG crossreactivity between rapeseed and mustard allergens was also demonstrated. Considering the structural and immunological data, certain polypeptide regions are suggested to be involved in the allergenicity of these proteins.

CONCLUSIONS

Rapeseed contains 2S storage proteins which may cause allergy in hypersensitive individuals. These proteins exhibit great sequence similarity with 2S albumins from different seeds. Crossreactivity between mustard and rapeseed flours can be explained by sequence homology.