Methods: Chimeras of Bet v 1.0101 and its homologue Api g 1.0101 were constructed. In each of the four chimeras, roughly one fourth of the surface residues of Api g 1.0101 were replaced by corresponding residues of Bet v 1.0101. The proteins were expressed in Escherichia coli and purified by chromatographic methods. Secondary structures were checked by CD-spectroscopy. IgE ELISA with Bet v 1.0101, Api g 1.0101 and the chimeras were performed with sera of 63 Bet v 1-sensitized birch pollen allergic patients. For inhibition ELISAs, chimeras were coated and inhibition was performed with the chimeras or Api g 1.0101.
Results: IgE binding to Api g 1.0101, Api-Bet-1, -2, -3 and -4 was observed for 22%, 81%, 79%, 70% and 38% of the sera, respectively. To assess the relevance of the grafted regions for IgE binding to Bet v 1, the amounts of IgE binding to the chimeras were compared with those to Api g 1.0101. Most of the sera recognised either three chimeras (39%) or all four chimeras (21%) better than Api g 1.0101. Only a minority of the sera showed increased binding to a single chimera. Inhibition ELISAs confirmed the presence of IgE specific for the grafted regions.
Conclusions: Our study indicates that the epitope recognition profile of Bet v 1-specific IgE is highly patient specific. Due to the different IgE binding patterns to Bet v 1, determined by binding of IgE to different chimeras, the existence of a single major IgE epitope on Bet v 1 can be excluded. Moreover, the Bet v 1-specific IgE repertoire is polyclonal and the IgE epitopes are distributed over the whole surface of Bet v 1.
This study was supported by grants P22559-B11 (CR) and SFB-F01802 (HB) from the Austrian Science Fund.