Bet v 1 and homologous food allergens are similarly processed by antigen-presenting cells but differ in T cell reactivity

Background

Various plant foods, e.g. apple and celery, express proteins that are homologues of the major birch-pollen allergen Bet v 1, e.g Mal d 1 and Api g 1. The proteins have 63% and 72% sequence similarity with Bet v 1 and share with it a common 3-dimensional structure. Despite this great molecular similarity, Bet v 1 is the only one among its homologues with the ability to sensitise atopic individuals. The aim of this study was to assess whether differences in the uptake and processing by antigen-presenting cells and in the presentation to T cells could be responsible for Bet v 1's ability to sensitise.

Methods

Uptake of allergens by PBMC, surface binding to and degradation by monocyte-derived dendritic cells (mdDC) were assessed. Peptides derived from digestion of Bet v 1, Mal d 1 and Api g 1 by endo-lysosomal extracts were analysed by mass spectrometry. Epitope-specificity of allergen-specific T cell lines from birch pollen-allergic individuals with associated food-allergies was mapped using synthetic 12-mer peptides. Binding of allergen-derived peptides by HLA class II molecules was analysed in silico.

Results

Significant differences were found neither in surface binding, in the kinetics of uptake by PBMC, the intracellular degradation by mdDC nor in the degradation by endo-lysosomal extracts. An immunodominant T cell epitope was found only in Bet v 1, but could not be referred to preferential binding to the most common HLA class II molecules.

Conclusion

The ability of Bet v 1 to sensitise is not conferred by differential antigen-processing but might stem from differences in T cell reactivity and the diverse routes of uptake of the aeroallergen Bet v 1 and the food allergens.