Pollen-related food allergy: cloning and immunological analysis of isoforms and mutants of Mal d 1, the major apple allergen, and Bet v 1, the major birch pollen allergen

Background:Mal d 1, the major apple allergen, cross-reacts with IgE specifi c for the major birch pollen allergen, Bet v 1, and is responsible for birc h pollen related food allergy to apple. Isoforms of Bet v 1 showing minor s equence variations display different binding capacity for specific IgE anti bodies from allergic patients. Moreover, strain-dependent variation of alle rgenicity has been reported for apples. Objective: To investigate the occurrence of strain-dependent isoforms of Ma l d 1 which may differ in their allergenic potential, to obtain data on str uctures essential for binding of Mal d 1 to the antibody, and to gain insig hts into the structures responsible for its IgE cross-reactivity to Bet v 1 . Methods. The cDNA of Mal d from various apple strains was amplified by a PC R strategy based on conserved regions of known Mal d 1-sequences, and seque nced. Two major isoforms of Mal d 1 were expressed as recombinant proteins and purified, as were different variants of the major birch pollen allergen , Bet v 1. Together with already existing recombinant birch pollen and appl e allergens, these were subjected to allergenicity testing by IgE-immunoblo tting, enzyme allergo sorbent test and dose related mediator release. "Hot- spots" for IgE-reactivity a ere identified by site-directed mutagenesis. Results: Twelve Mal d 1-clones were sequenced from 7 apple varieties and co mpared to 3 known Mal d 1 sequences. The clones were clustered into two gro ups, each showing a high degree of sequence identity to one of the known se quences and specific differences to the third sequence. No strain-specific sequences were identified. In contrast, apple strains wi th reported differences in allergenicity showed different expression levels of the major allergen. Immunologic testing of recombinant allergens reveal ed high IgE binding capacity of 2 major isoforms, named GD26 and GS29, with a slightly higher IgE binding capacity of GD26. Moreover, the allergenicit y was similar to another rMal d 1 reported in the literature, representing the isoform divergent from our clones. Mutational analysis of our Mal d 1 a llergens identified serine in position 111 as essential for IgE binding. Al lergenicity was almost depleted by changing this residue into a proline. Mo reover, the corresponding serine residue, present in position 112 of Bet v 1, was in a similar manner crucial for the allergenicity of the birch polle n allergen. Conclusion: We conclude that divergent allergenicity of apple strains mainl y depends on different expression levels of the major allergen. Introductio n of a proline residue in position 111 of Mal d 1 and in position 112 of Be t v 1 led to a drastic reduction of allergenicity of both the pollen and th e food allergen, obviously also removing the cross-reactive epitope. Mutant s with reduced IgE-reactivity but maintained T-cell reactivity may represen t new candidates for a safer specific immunotherapy with reduced side-effec ts.