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Birch pollinosis is often accompanied by adverse reactions to food due to pollen-allergen specific IgE cross-reacting with homologous food allergens. The tertiary structure of Pru av 1, the major cherry (Prunus avium) allergen, for example, is nearly identical with Bet v 1, the major birch (Betula verrucosa) pollen allergen. In order to define cross-reactive IgE epitopes, we generated and analysed mutants of Pru av 1 and Api g 1.0101, the major celery (Apium graveolens) allergen, by immunoblotting, EAST (enzyme allergosorbent test), CD and NMR spectroscopy. The mutation of Glu 45 to Trp 45 in the P-loop region, a known IgE epitope of Bet v 1, significantly reduced IgE binding to Pru av 1 in a subgroup of cherry-allergic patients. The backbone conformation of Pru av 1 wild-type is conserved in the three-dimensional structure of Pru av 1 Trp 45 , demonstrating that the side chain of Glu 45 is involved in a cross-reactive IgE epitope. Accordingly, for a subgroup of celery-allergic patients, IgE binding to the homologous celery allergen Api g 1.0101 was enhanced by the mutation of Lys 44 to Glu. The almost complete loss of IgE reactivity to the Pru av 1 Pro 112 mutant is due to disruption of its tertiary structure. Neither the mutation Ala 112 nor deletion of the Cterminal residues 155-159 influenced IgE binding to Pru av 1. In conclusion, the structure of the P-loop partially explains the crossreactivity pattern, and modulation of IgE-binding by site-directed mutagenesis is a promising approach to develop hypo-allergenic variants for patient-tailored specific immunotherapy.

Background: This study was performed to get further insights into antibody responses to cross–reactive carbohydrate determinants (CCD), including initial experiments to prove the biological activity of anti–CCD IgE. Earlier studies have shown that IgE specific for CCD occurs in about 25% of celery–allergic patients. The clinical significance of these antibody specificities is doubtful. Methods: Patient sera were selected on the basis of a positive case history of celery allergy and multiple binding to high molecular weight celery allergens on immunoblots. Specific IgE to native and heated celery tuber was determined by the enzyme allergosorbent test (EAST). N–glycans were purified after extensive digestion of specific glycoproteins, such as pineapple stem bromelain, bovine fibrin, and human IgG, and used as antigens in an IgE ELISA as well as in EAST and immunoblotting inhibition experiments. Dose–related histamine release was performed with BSA neoglycoproteins containing 3–4 units of the purified glycopeptides. Results: Seven celery–allergic patients were identified who clearly presented IgE against the N–glycan purified from bromelain which is a common structure within the plant kingdom. Chemical defucosylation showed that α1,3–fucose is a key structure for IgE binding. In patients with anti–CCD IgE, the maximal inhibition of celery EAST by the bromelain glycan ranged from 22 to 100%. Inhibition of celery immunoblots by preincubation of patient serum with this glycan led to a quenching of multiple bands at masses >40 kD. After linking the bromelain glycopeptide to BSA, a strong dose–related histamine release was obtained in a celery–allergic patient, occurring at lower concentrations than with the recombinant major protein allergen from celery, Api g 1. Conclusions: Our results demonstrate that IgE specific for CCD is common in celery–allergic patients, and can represent the major proportion of IgE against this food. α1,3–fucose was confirmed to be an essential part of the IgE epitope. Immunoblotting inhibition indicated the presence of this carbohydrate determinant on multiple glycoproteins in celery extract. Although histamine release was only performed in 1 patient, our data show that proteins carrying multiple glycan units can be biologically active in patients sensitized to CCD.

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