Cross-reactivity and epitope analysis of Pru a 1, the major cherry allergen

A high percentage of birch pollen allergic patients experiences food hypers ensivity after ingestion of fresh fruits and vegetables. The cross-reactivi ty of the major allergens of sweet cherry (Pru a 1), apple (Mal d 1), pear (Pyr c 1), celery tuber (Api g 1) and carrot (Dau c 1) is due to structural similarities which are reflected by high amino acid sequence identities wi th Bet v 1a, the major birch pollen allergen. Apart from a strong cross-rea ctivity to Bet v 1a, IgE inhibition experiments with Mal d 1, Pru a 1 and A pi g 1 demonstrated the presence of common and different epitopes among the tested food allergens. Secondary structure prediction of all investigated allergens indicated the presence of almost identical structural elements. I n particular, the 'P-loop' region is a common domain of the pollen related food allergens and of pathogenesis related proteins. To identify the IgE bi nding epitopes, five overlapping recombinant Pru a 1 fragments representing the entire amino acid sequence with lengths of approximately 60-120 residu es were investigated. Weak IgE binding capacity was measured exclusively wi th Pru a 1F4 (1-120) by immunoblotting, whereas none of the fragments showe d allergenicity in the rat basophil leukaemia cell mediator release assay. Site-directed mutagenesis experiments with Pru a 1 revealed that amino acid S112 is critical for IgE binding of almost all patients sera tested. This reduced IgE binding was also observed with a single point mutant of Bet v 1 a (S112P) and thus indicated serine 112 as an essential residue for preserv ing the structure of a cross-reactive IgE epitope. Moreover, two Pru a 1 mu tants with an altered 'P-loop' region, showed a lowered IgE binding capacit y for IgE from a subgroup of allergic patients. The investigation of essent ial features for preserving cross-reactive IgE-epitopes provides the struct ural basis for understanding the clinically observed cross-allergenicity be tween pollen and fruits. Moreover, non-anaphylactic allergen fragments or v ariants derived from the IgE-inducing pollen allergens may serve as useful tools for a new strategy of specific immunotherapy. (C) 1999 Elsevier Scien ce Ltd. All rights reserved.