Patient-tailored cloning of allergens by phage display: peanut (Arachis hypogaea) profilin, a food allergen derived from a rare mRNA

Application of AllerCatPro 2.0 for protein safety assessments of consumer products

Foreign proteins are potentially immunogenic, and a proportion of these are able to induce immune responses that result in allergic sensitization. Subsequent exposure of sensitized subjects to the inducing protein can provoke a variety of allergic reactions that may be severe, or even fatal. It has therefore been recognized for some time that it is important to determine a priori whether a given protein has the potential to induce allergic responses in exposed subjects. For example, the need to assess whether transgene products expressed in genetically engineered crop plants have allergenic properties. This is not necessarily a straightforward exercise (as discussed elsewhere in this edition), but the task becomes even more challenging when there is a need to conduct an overall allergenicity safety assessment of complex mixtures of proteins in botanicals or other natural sources that are to be used in consumer products. This paper describes a new paradigm for the allergenicity safety assessment of proteins that is based on the use of AllerCatPro 2.0, a new version of a previously described web application model developed for the characterization of the allergenic potential of proteins. Operational aspects of AllerCatPro 2.0 are described with emphasis on the application of new features that provide improvements in the predictions of allergenic properties such as the identification of proteins with high allergenic concern. Furthermore, the paper provides a description of strategies of how AllerCatPro 2.0 can best be deployed as a screening tool for identifying suitable proteins as ingredients in consumer products as well as a tool, in conjunction with label-free proteomic analysis, for identifying and semiquantifying protein allergens in complex materials. Lastly, the paper discusses the steps that are recommended for formal allergenicity safety assessment of novel consumer products which contain proteins, including consideration and integration of predicted consumer exposure metrics. The article therefore provides a holistic perspective of the processes through which effective protein safety assessments can be made of potential allergenic hazards and risks associated with exposure to proteins in consumer products, with a particular focus on the use of AllerCatPro 2.0 for this purpose.

Peanut allergens: new consolidated findings on structure, characteristics, and allergome

Immunoglobulin E-mediated food allergy is the result of a complex pathomechanism. Factors contributing to the dysfunction of the immune system are the allergenic sources and the variable matrix effects arising from the processes involved in interaction with the gastrointestinal tract, the allergens themselves through their structural features, and the specific behavior of the individual immune system. The starting point for elucidating the pathomechanism of food allergy is the identification of allergens and the description of their structure. They are the basis for in vitro diagnostics as well as the development of immunotherapeutic drugs. With regard to Class I food allergy, peanut allergy affects by far the largest group of patients. 11 allergens have been identified in peanuts. Ara h 1, Ara h 3, and Ara h 4 belong to the cupin superfamily, Ara h 2, Ara h 6, and Ara h 7 to the prolamin superfamily; Ara h 5 (profilins) and Ara h 8 (superfamily of Bet v 1-homologous proteins) are associated with aeroallergens. Peanut lipid transfer proteins (LTP) and two peanut oleosins are listed as Ara h 9, Ara h 10, and Ara h 11 by the IUIS Allergen Nomenclature Subcommittee. Peanut agglutinin (PNA) and a third oleosin have been shown to possess allergenic properties. The effect of the above specified allergens has to be considered in the context of their matrix, which is influenced by processing factors.

Peanut allergens

Peanut allergens have the potential to negatively impact on the health and quality of life of millions of consumers worldwide. The seeds of the peanut plant Arachis hypogaea contain an array of allergens that are able to induce the production of specific IgE antibodies in predisposed individuals. A lot of effort has been focused on obtaining the sequences and structures of these allergens due to the high health risk they represent. At present, 16 proteins present in peanuts are officially recognized as allergens. Research has also focused on their in-depth immunological characterization as well as on the design of modified hypoallergenic derivatives for potential use in clinical studies and the formulation of strategies for immunotherapy. Detailed research protocols are available for the purification of natural allergens as well as their recombinant production in bacterial, yeast, insect, and algal cells. Purified allergen molecules are now routinely used in diagnostic multiplex protein arrays for the detection of the presence of allergen-specific IgE. This review gives an overview on the wealth of knowledge that is available on individual peanut allergens.

Boiling and Frying Peanuts Decreases Soluble Peanut (Arachis Hypogaea) Allergens Ara h 1 and Ara h 2 But Does Not Generate Hypoallergenic Peanuts

Peanut allergy continues to be a problem in most developed countries of the world. We sought a processing method that would alter allergenic peanut proteins, such that allergen recognition by IgE from allergic individuals would be significantly reduced or eliminated. Such a method would render accidental exposures to trace amounts of peanuts safer. A combination of boiling and frying decreased recovery of Ara h 1 and Ara h 2 at their expected MWs. In contrast, treatment with high pressures under varying temperatures had no effect on protein extraction profiles. Antibodies specific for Ara h 1, Ara h 2, and Ara h 6 bound proteins extracted from raw samples but not in boiled/fried samples. However, pre-incubation of serum with boiled/fried extract removed most raw peanut-reactive IgE from solution, including IgE directed to Ara h 1 and 2. Thus, this method of processing is unlikely to generate a peanut product tolerated by peanut allergic patients. Importantly, variability in individual patients' IgE repertoires may mean that some patients' IgE would bind fewer polypeptides in the sequentially processed seed.

An update on hypoallergenicity of peanut and soybean: where are we now?

Legumes are one of the major sources of proteins and positively correlate with the development of modern society. At the same time, unfortunately, they significantly contribute to the rising prevalence of food allergy.

Common food allergens and their IgE-binding epitopes

Food allergy is an adverse immune response to certain kinds of food. Although any food can cause allergic reactions, chicken egg, cow's milk, wheat, shellfish, fruit, and buckwheat account for 75% of food allergies in Japan. Allergen-specific immunoglobulin E (IgE) antibodies play a pivotal role in the development of food allergy. Recent advances in molecular biological techniques have enabled the efficient analysis of food allergens. As a result, many food allergens have been identified, and their molecular structure and IgE-binding epitopes have also been identified. Studies of allergens have demonstrated that IgE antibodies specific to allergen components and/or the peptide epitopes are good indicators for the identification of patients with food allergy, prediction of clinical severity and development of tolerance. In this review, we summarize our current knowledge regarding the allergens and IgE epitopes in the well-researched allergies to chicken egg, cow's milk, wheat, shrimp, and peanut.

Cross-reactivity of peanut allergens

Peanut seeds are currently widely used as source of human food ingredients in the United States of America and in European countries due to their high quality protein and oil content. This article describes the classification and molecular biology of peanut seed allergens with particular reference to their cross-reactivities. Currently, the IUIS allergen nomenclature subcommittee accepts 12 peanut allergens. Two allergens belong to the cupin and four to the prolamin superfamily, and six are distributed among profilins, Bet v 1-like proteins, oleosins, and defensins. Clinical observations frequently report an association of peanut allergy with allergies to legumes, tree nuts, seeds, fruits and pollen. Molecular cross-reactivity has been described between members of the Bet v 1-like proteins, the non-specific lipid transfer proteins, and the profilins. This review also addresses the less well-studied cross-reactivity between cupin and prolamin allergens of peanuts and of other plant food sources and the recently discovered cross-reactivity between peanut allergens of unrelated protein families.

Peanut allergens: an overview

Peanut is recognized as a potent food allergen producing one of the most frequent food allergies. This fact has originated the publication of an elevated number of scientific reports dealing with peanut allergens and, especially, the prevalence of peanut allergy. For this reason, the information available on peanut allergens is increasing and the debate about peanut allergy is always renewed. This article reviews the information currently available on peanut allergens and on the techniques used for their chemical characterization. Moreover, a general overview on the current biotechnological approaches used to reduce or eliminate peanut allergens is also provided.

Uses of phage display in agriculture: a review of food-related protein-protein interactions discovered by biopanning over diverse baits

This review highlights discoveries made using phage display that impact the use of agricultural products. The contribution phage display made to our fundamental understanding of how various protective molecules serve to safeguard plants and seeds from herbivores and microbes is discussed. The utility of phage display for directed evolution of enzymes with enhanced capacities to degrade the complex polymers of the cell wall into molecules useful for biofuel production is surveyed. Food allergies are often directed against components of seeds; this review emphasizes how phage display has been employed to determine the seed component(s) contributing most to the allergenic reaction and how it has played a central role in novel approaches to mitigate patient response. Finally, an overview of the use of phage display in identifying the mature seed proteome protection and repair mechanisms is provided. The identification of specific classes of proteins preferentially bound by such protection and repair proteins leads to hypotheses concerning the importance of safeguarding the translational apparatus from damage during seed quiescence and environmental perturbations during germination. These examples, it is hoped, will spur the use of phage display in future plant science examining protein-ligand interactions.

Measurement of Ara h 1-, 2-, and 3-specific IgE antibodies is useful in diagnosis of peanut allergy in Japanese children

BACKGROUND

Food challenges are time-consuming, expensive, and not always possible to perform. Therefore, new tools to diagnose food allergy are desired. The aim was to evaluate IgE antibodies to peanut allergens in the diagnosis of peanut allergy in Japanese children using ImmunoCAP(®) and IgE immunoblotting.

METHODS

The study included 2-13-yr-old consecutive patients (n = 57) referred to our specialist clinic for investigation of current peanut allergy using food challenge. All children had a previous doctor's diagnosis of peanut allergy and were on elimination diet. Serum samples were analyzed for IgE reactivity to peanut, recombinant (r) Ara h 1, 2, 3, 5, 8, and 9. IgE immunoblotting (n = 23) was performed using extracts from raw and roasted peanut.

RESULTS

Twenty-six of the children failed (allergic group), and 31 passed the peanut challenge (tolerant group). The rAra h 2 ImmunoCAP test was superior in its ability to differentiate between children in the allergic and tolerant groups with a sensitivity and specificity of 88% and 84%, respectively (cutoff, 0.35 kU(A)/l). The combination of rAra h 1, 2, and 3 resulted in a higher specificity (94%) when IgE to all of them was the criteria for positivity. ImmunoCAP generally showed a good agreement with immunoblotting using both raw and roasted peanut for IgE reactivity to Ara h 1, 2, and 3.

CONCLUSIONS

Measurement of IgE antibodies to rAra h 1, 2, and 3 is useful in the diagnosis of peanut allergy and in the investigation of reactions to raw and roasted peanut.

Identification of a new IgE-binding epitope of peanut oleosin that cross-reacts with buckwheat

Peanut and buckwheat induce a severe allergic reaction, anaphylaxis, which is considered to be mediated by immunoglobulin E (IgE). We identified in this study a new IgE-binding epitope of the peanut allergen that cross-reacted with buckwheat. The phosphate-buffered saline-soluble fraction of buckwheat inhibited the binding between IgE and the peanut allergen. A cross-reactive peptide was isolated from the α-chymotrypsin hydrolysate of peanut. Based on the amino acid sequence and mass spectrometric analysis data, the peptide was identified as Ser-Asp-Gln-Thr-Arg-Thr-Gly-Tyr (SDQTRTGY); this sequence is identical to amino acids 2-9 in the N-terminal hydrophilic domain of oleosin 3 which is located on the surface of the lipid storage body. Synthetic SDQTRTGY was found to bind with IgE in the sera of all eight peanut-allergic patients tested. Since many foods of plant origin contain oleosin, the possibility of an anaphylactic cross-reaction in allergic patients should always be considered.

Patterns of sensitization to peanut allergen components in Taiwanese Preschool children

BACKGROUND/PURPOSE

Peanut allergy is very common in Western countries, although it is seldom encountered in Eastern countries. Peanuts are comprised of at least 11 components, but the contribution to clinical symptoms by each component in each individual is not known. This study investigated the distributions of sensitivity to peanut allergen components among Taiwanese children who were sensitized to peanuts and followed the evolution of sensitization patterns to these components.

METHODS

We enrolled 29 preschool children (age=2.11±1.36 years) who were sensitized to peanuts above class 3. Serum was analyzed for specific immunoglobulin E (IgE) antibodies to recombinant Ara h 1, Ara h 2, Ara h 3, Ara h 8, and Ara h 9. Allergen component-specific IgE ≥0.35 kU(A)/L was defined as positive. Eighteen children were retested 22.64±5.1 months later. Peanut allergy symptoms were recorded from detailed questionnaires.

RESULTS

The percentages of children sensitized to Ara h 1, 2, 3, 8, and 9 were, respectively, 51.8%, 65.5%, 62.1%, 13.8%, and 24.1%. Regarding changing patterns of peanut component sensitization at follow-up, children with clinical symptoms to peanuts had persistent elevations of Ara h 2-specific IgE: 12.6±1.01 up to 34.15±19.4 kU(A)/L; p=0.144. In contrast, Ara h 2 concentrations decreased significantly in children without clinical symptoms. Ara h 8 and 9 were nonspecific for children with or without symptoms.

CONCLUSION

Ara h 1, Ara h 2, and Ara h 3 were major components contributing to peanut sensitization in Taiwanese children. Ara h 2 was probably the most important component that contributed to clinical symptoms and remained steady in children who had peanut allergy.

Design and screening of M13 phage display cDNA libraries

The last decade has seen a steady increase in screening of cDNA expression product libraries displayed on the surface of filamentous bacteriophage. At the same time, the range of applications extended from the identification of novel allergens over disease markers to protein-protein interaction studies. However, the generation and selection of cDNA phage display libraries is subjected to intrinsic biological limitations due to their complex nature and heterogeneity, as well as technical difficulties regarding protein presentation on the phage surface. Here, we review the latest developments in this field, discuss a number of strategies and improvements anticipated to overcome these challenges making cDNA and open reading frame (ORF) libraries more readily accessible for phage display. Furthermore, future trends combining phage display with next generation sequencing (NGS) will be presented.

2-D DIGE analysis of the proteome of extracts from peanut variants reveals striking differences in major allergen contents

Over the last decade, an increasing prevalence of peanut allergies was observed worldwide. Peanuts are meanwhile categorized among the most dangerous food allergens. This is particularly relevant since peanut-derived ingredients are widely used in industrial food production. To minimize the problem of hidden food allergens causing severe anaphylactic reactions, pre-packaged food containing peanut components needs to be classified according to European ruling since 2005. Food companies search for strategies to reduce the allergenicity of peanut-derived food additives either by genetically altering the allergen content or by identifying peanut varieties with low levels of major allergens. In our study, we focused on peanut extracts from Indonesia that apparently contain lower levels of the major Arachis hypogaea allergen 1 (Ara h 1). Basic extracts of Virginia-type and Indonesian peanuts were compared by 1- and 2-DE. We identified more than hundred individual components in these extracts by MS and provide a high-resolution allergen map that also includes so far unknown fragments of major peanut allergens. The reduced level of Ara h 1 associated with a significantly lower abundance of the most potent peanut allergen Ara h 2 in various Indonesian peanuts was also confirmed by Western blotting with monoclonal antibodies and sera of allergic patients.

The non-specific lipid transfer protein, Ara h 9, is an important allergen in peanut

BACKGROUND

Plant food allergy in the Mediterranean area is mainly caused by non-specific lipid transfer proteins (nsLTP). The aim of this study was to characterize peanut nsLTP in comparison with peach nsLTP, Pru p 3, and assess its importance in peanut allergy.

METHODS

Peanut-allergic patients from Spain (n=32) were included on the basis of a positive case history and either a positive skin prick test or specific IgE to peanut. For comparison, sera of 41 peanut-allergic subjects from outside the Mediterranean area were used. Natural Ara h 9 and two isoforms of recombinant Ara h 9, expressed in Pichia pastoris, were purified using a two-step chromatographic procedure. Allergen characterization was carried out by N-terminal sequencing, circular dichroism (CD) spectroscopy, immunoblotting, IgE inhibition tests and basophil histamine release assays.

RESULTS

Compared with natural peanut nsLTP, the recombinant proteins could be purified in high amounts from yeast supernatant (> or =45 mg/L). The identity of the proteins was verified by N-terminal amino acid sequencing and with rabbit nsLTP-specific antibodies. CD spectroscopy revealed similar secondary structures for all preparations and Pru p 3. The Ara h 9 isoforms showed 62-68% amino acid sequence identity with Pru p 3. IgE antibody reactivity to rAra h 9 was present in 29/32 Spanish and 6/41 non-Mediterranean subjects. Recombinant Ara h 9 showed strong cross-reactivity to nPru p 3 and similar IgE-binding capacity as nAra h 9. The two Ara h 9 isoforms displayed similar IgE reactivity. In peanut-allergic patients with concomitant peach allergy, Ara h 9 showed a weaker allergenic potency than Pru p 3 in histamine release assays.

CONCLUSIONS

Ara h 9 is a major allergen in peanut-allergic patients from the Mediterranean area. Ara h 9 is capable of inducing histamine release from basophils, but to a lesser extent than Pru p 3.

Functional cloning by phage display

This review focusses on the isolation of proteins from genomic or cDNA expression products libraries displayed on phage. The use of phage display is highlighted for the characterization of binding proteins with diverse biological functions. Phage display is compared with another strategy, the yeast two-hybrid method. The combination of both strategies is especially powerful to eliminate false positives and to get information on the biochemical functions of proteins.

Identification of wheat gliadins as an allergen family related to baker's asthma

BACKGROUND

Flour is still one of the most common causes of occupational asthma worldwide. Thus far, little is known about the relevant allergens causing baker's asthma. Therefore the reliability of current diagnostic procedures is insufficient. Only few of the suspected causative wheat allergens have been hitherto characterized on the molecular level.

OBJECTIVE

The aim was to identify and characterize unknown wheat allergens related to baker's asthma to improve the reliability of diagnostic procedures.

METHODS

A wheat pJuFo cDNA phage display library was created and screened for IgE binding to wheat proteins with pooled sera from patients with baker's asthma. After identifying an alphabeta-gliadin, the frequency of sensitization was investigated by means of ELISA screening of 153 bakers' sera with the recombinant alphabeta-gliadin. Furthermore, the allergenicity of native total gliadin (alphabeta, gamma, omega) was analyzed by means of ImmunoCAP.

RESULTS

One cDNA clone was identified as an alphabeta-gliadin. Serum IgE antibodies to the recombinant allergen were found in 12% of bakers with occupational asthma. Of the asthmatic bakers, 33% showed sensitization to native total gliadin; 4% of them had negative results on routine IgE testing with wheat extract.

CONCLUSIONS

Gliadins represent a newly discovered family of inhalable allergens in baker's asthma. This finding demonstrates that water-insoluble proteins might also represent causative allergens.

Food allergen detection methods and the challenge to protect food-allergic consumers

The detection of allergenic ingredients in food products has received increased attention from the food industry and legislative and regulatory agencies over recent years. This has resulted in the improvement of measures aimed at the protection of food-allergic consumers. The controlled production of food products and control activities executed by food inspection agencies rely on the availability of methods capable of detecting traces of allergenic ingredients. The development of such methods faces a multitude of analytical challenges. Those challenges will be identified and discussed in this review. Furthermore, future developments and trends in analytical methodology as applied to the detection of food allergens are reported.

Cross-reactive and species-specific immunoglobulin E epitopes of plant profilins: an experimental and structure-based analysis

BACKGROUND

Profilins are cross-reactive plant allergens responsible for multiple pollen sensitization and pollen-associated food allergy. While it is assumed that profilins from different species are immunologically equivalent, some studies suggest partial or even lacking IgE cross-reactivity between certain profilins.

OBJECTIVE

We aimed to obtain a semi-quantitative assessment of the contributions of conserved and species-specific epitopes to IgE binding of plant profilins.

METHODS

We compared model structures of profilins from timothy, mugwort, celery and bell pepper with crystal structures of birch and latex profilins. We predicted potential conformational epitopes that consisted of contiguous patches of at least 20% surface-exposed residues. Celery and timothy profilins were purified from their natural sources, and profilins from birch, mugwort, bell pepper and latex were expressed in Escherichia coli. The structural integrity of all purified proteins was confirmed by circular dichroism spectroscopy. IgE ELISAs and ELISA inhibitions using sera from 22 profilin-sensitized allergic patients were carried out.

RESULTS

Peptide backbone conformations of all six profilins were highly similar. Nine variable epitopes and two containing high proportions of conserved residues were predicted. IgE from all sera bound to all tested profilins and the amounts were highly correlated. However, IgE inhibition experiments revealed that up to 60% of total IgE binding was mediated by species-specific epitopes. The extent of cross-reactivity among profilins from timothy, birch, latex and celery was greater than cross-reactivity to mugwort and bell pepper profilins. This pattern was mirrored by sequence similarities among one of the predicted variable epitopes. Patients with IgE to cross-reactive epitopes displayed allergic reactions to a greater number of plant foods than patients having IgE directed to species-specific epitopes.

CONCLUSION

The large extent of cross-reactivity among plant profilins justifies using a single profilin for diagnosis. However, the fine specificity of IgE directed to variable epitopes may influence the clinical manifestation of profilin sensitization.

Screening the allergenic repertoires of wheat and maize with sera from double-blind, placebo-controlled food challenge positive patients

BACKGROUND

Food allergy to wheat and maize is an increasing factor of deterioration of life quality, especially childhood and can, in rare cases, even induce anaphylaxis. Although omega-5 gliadin from wheat and maize lipid transfer protein have been characterized as major cereal allergens on the molecular level, the list of food allergens is far to be complete.

METHODS

To identify the IgE-binding repertoires of wheat and maize we screened respective cDNA libraries displayed on phage surface with sera from patients with a confirmed food allergy. The study included six patients with a positive double-blind, placebo-controlled food challenge (DBPCFC) to wheat, nine patients with a positive DBPCFC to maize, and six patients with anaphylactic reactions after ingestion of wheat.

RESULTS

The enriched sequences encoding IgE-binding proteins showed heterogeneous repertoires for both, wheat and maize. The selected wheat repertoire yielded 12, the maize repertoire 11 open reading frames. Among these we identified allergens belonging to already characterized allergens families, such as gliadin, profilin and beta-expansin. Besides, we found novel proteins with high cross-reactive potential, such as thioredoxins, as well as sequences that had so far not been related to cereal allergy at all. The IgE-binding capacity of some selected proteins was evaluated in vitro and cross-reactivity was demonstrated by competition ELISA.

CONCLUSION

With regard to the heterogeneity of the characterized sequences as well as to the biochemical nature of the new allergens detected we conclude that wheat and maize-related food allergy is more complex than so far anticipated.

Analysis of the composition of an immunoglobulin E reactive high molecular weight protein complex of peanut extract containing Ara h 1 and Ara h 3/4

Peanuts (Arachis hypogaea) contain some of the most potent food allergens. In recent years an increasing prevalence of peanut allergies both in children and adults has been observed in the USA and in Europe. In vitro identification and characterization of allergens including those from peanut have been frequently performed by Western blotting. However this method may alter the immunoglobulin E (IgE) antibody reactivity since the proteins are denatured by detergent treatment and/or reduction of disulfide bonds by reducing reagents and does not answer the question how peanut allergens interact with the human digestive apparatus and immune system. Size exclusion chromatography of peanut extract shows that approximately 90% of the total protein content is eluted as one peak in the exclusion volume with a molecular mass of over 200 kDa. The proteins of this fraction were analyzed by blue-native polyacrylamide gel electrophoresis (PAGE), immunoblotting, two-dimensional PAGE and Western blotting. A complex of Ara h 1 (Acc. no. P43237), Ara h 3/4 (AAM46958), Ara h 3 (AAC63045), Ara h 4 (AF086821), Gly 1 (AAG01363) and iso-Ara h 3 (AAT39430) was identified using patients' IgE and allergen-specific monoclonal antibodies; N-terminal sequencing and matrix-assisted laser desorption/ionisation-time of flight analysis verified these findings. A comparison of the peanut allergen sequences of Ara h 3/4, Ara h 3, Ara h 4 and peanut trypsin inhibitor (AF487543) and the proteins Gly 1 and iso-Ara h 3, not yet described as allergens, leads to the conclusion that these proteins are isoallergens of each other. It was shown that these isoallergens are post-translationally cleaved and held together by disulfide bonds in accordance to the 11S plant seed storage proteins signature.

Isolation and characterization of natural Ara h 6: evidence for a further peanut allergen with putative clinical relevance based on resistance to pepsin digestion and heat

Peanut allergy is a significant health problem because of its prevalence and the potential severity of the allergic reaction. The characterization of peanut allergens is crucial to the understanding of the mechanism of peanut allergy. Recently, we described cloning of the peanut allergen Ara h 6. The aim of this study was isolation and further characterization of nAra h 6. We purified nAra h 6 from crude peanut extract using gel filtration and anion exchange chromatography. The preparation was further characterized by two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) with subsequent immunoblotting. Stability of nAra h 6 was studied by an in vitro digestibility assay as well as by resistance against thermal processing. Sequencing of nAra h 6 identified the N-terminal amino acid sequence as MRRERGRQGDSSS. Further results clearly demonstrated stability of nAra h 6 against pepsin digestion and heating. Immunoglobulin G (IgE) binding analysis and its biological activity shown by RBL 25/30-test of natural Ara h 6 supported the importance of this peanut allergen. Investigation of nAra h 6 revealed evidence for a further peanut allergen with putative clinical relevance based on resistance to pepsin digestion and heat.

Ara h 8, a Bet v 1–homologous allergen from peanut, is a major allergen in patients with combined birch pollen and peanut allergy

BACKGROUND

We recently described patients with soybean allergy mainly mediated by cross-reactivity to birch pollen allergens. A majority of those patients were reported to have peanut allergy.

OBJECTIVE

We sought to study the occurrence of peanut allergy in patients allergic to birch pollen and characterized the Bet v 1-homologous peanut allergen Ara h 8.

METHODS

Recombinant Ara h 8 was cloned with degenerated primers and expressed in Escherichia coli. Nine Swiss and 11 Dutch patients with peanut and birch pollen allergy and a positive double-blind, placebo-controlled food challenge result to peanut were investigated for IgE reactivity to birch pollen and purified peanut allergens and cross-reactivity between birch and peanut. Ara h 8 stability against digestion and roasting was assessed by means of RAST inhibition. The IgE cross-linking potency of Ara h 8 was tested on the basis of basophil histamine release.

RESULTS

During double-blind, placebo-controlled food challenge, all patients experienced symptoms in the oral cavity, progressing to more severe symptoms in 40% of patients. CAP-FEIA detected recombinant (r) Ara h 8-specific IgE in 85%. IgE binding to Ara h 8 was inhibited by Bet v 1 in peanut extract immunoblotting and in RAST inhibition. In EAST inhibition recombinant rAra h 8 inhibited IgE binding to peanut in 4 of 7 tested patient sera. Antipeanut response was dominated by Ara h 8 in 12 of 17 tested patients. Furthermore, our results demonstrate a low stability of Ara h 8 to roasting and no stability to gastric digestion. Basophil histamine release with rAra h 8 was more than 20% in 5 of 7 tested sera.

CONCLUSIONS

Peanut allergy might be mediated in a subgroup of our patients by means of cross-reaction of Bet v 1 with the homologous peanut allergen Ara h 8.

Cloning allergens via phage display

Although an impressive list of allergenic structures has been elucidated during the last decade by classical cloning methods, the size of the repertoire of molecular structures able to elicit allergic reactions is still unknown. Selective enrichment of cDNA libraries displayed on phage surface with serum IgE from allergic individuals combined with robotic-based high-throughput screening technology has proved to be extremely successful for the rapid isolation of allergens. The basic concept of linking the phenotype, expressed as gene product displayed on the phage coat, to its genetic information integrated into the phage genome, creates fusion proteins covalently associated with the infectious particle itself. Therefore, cDNA libraries displayed on phage surface can be screened for the presence of specific clones using the discriminative power of affinity purification. The selection of IgE-binding clones involves the enrichment of phage binding to serum IgE immobilised to a solid phase during consecutive rounds of affinity selection. As a consequence of the physical linkage between genotype and phenotype, sequencing of the DNA of the integrated section of the phage genome can readily elucidate the amino acid sequence of the surface-displayed allergen. In spite of some biological limitations imposed by Escherichia coli as expression host, phage surface display technology has strongly contributed to the rapid isolation of a vast variety of IgE-binding structures.

Confirmation of the allergenic peanut protein, Ara h 1, in a model food matrix using liquid chromatography/tandem mass spectrometry (LC/MS/MS)

Enzymatic digestion of total protein along with liquid chromatography/tandem mass spectrometry (LC/MS/MS) was used to confirm the presence of a major peanut allergen in food. Several peptides obtained from the enzymatic digestion of the most abundant peanut allergen, Ara h 1, were identified as specific peptide biomarkers for peanut protein. Using ice cream as a model food matrix, a method was developed for the detection of the allergen peptide biomarkers. A key component of the method was the use of molecular mass cutoff filters to enrich the Ara h 1 in the protein extracts. By applying the method to ice cream samples containing various levels of peanut protein, levels as low as 10 mg/kg of Ara h 1 could routinely be detected. This method provides an unambiguous means of confirming the presence of the peanut allergen, Ara h 1, in foods and can easily be modified to detect other food allergens.

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.

Current understanding of cross-reactivity of food allergens and pollen

Pollen-allergic patients frequently present allergic symptoms after ingestion of several kinds of plant-derived foods. The majority of these reactions is caused by four distinct cross-reactive structures that are present in birch pollen. Proteins that share common epitopes with Bet v 1, the major birch pollen allergen, occur in pollens of several tree species: apples, stone fruits, celery, carrot, nuts, and soybeans. Approximately 70% of our patients who are allergic to birch pollen may experience symptoms after consumption of foods from these groups. In contrast, two minor allergenic structures-profilins and cross-reactive carbohydrate determinants (CCD)-that sensitize approximately 10-20% of all pollen-allergic patients are also present in grass pollen and weed pollen. Moreover, IgE-binding proteins related to the birch pollen minor allergen Bet v 6 have been found in many vegetable foods such as apple, peach, orange, lychee fruit, strawberry, persimmon, zucchini, and carrot. Frequently, the occurrence of cross-reactive IgE antibodies is not correlated with the development of clinical food allergy. In particular, the clinical relevance of sensitization to CCD is doubtful. Generally, pollen-related allergens tend to be more labile during heating procedures and in the digestive tract compared to allergens from classical allergenic foods such as peanut. However, recent DBPCFC studies have shown that both cooked celery and roasted hazelnuts still pose an allergenic risk for pollen-sensitized subjects. Since pathogenesis-related proteins share several common features with allergens and both the Bet v 1 and the Bet v 6-related food allergens are defense-related proteins, approaches to introduce such proteins as a measure to protect plants against diseases should be performed with caution as they may increase the allergenicity of these crops.