Context matters: TH2 polarization resulting from pollen composition and not from protein-intrinsic allergenicity

Abstract

Background: Over 100 million people worldwide suffer from birch pollen allergy. However, identification of molecular determinants driving Th2-biased allergic sensitization to Bet v 1, the major birch pollen allergen, remains elusive.

Objective: Here, we examined whether Bet v 1 or the pollen matrix is responsible for activation of antigen-presenting cells and the subsequent Th2 polarization, relevant in the process of allergic sensitization.

Methods: The allergenicity of Bet v 1 and of birch pollen extract (BPE) was addressed by stimulation of murine and human dendritic cells and by in vivo monitoring of Th2 polarization. Further, Bet v 1 was depleted from BPE by immunoprecipitation in order to analyze its involvement in the occurrence of a Th2 response.

Results: The allergen alone did neither stimulate dendritic cells in vitro nor induced Th2 polarization in vivo, even in the presence of the natural LPS concentration determined in the BPE. In contrast, BPE was shown to activate dendritic cells and strongly promoted a Th2 polarization. Even upon immunization with Bet v 1-depleted BPE the amount of induced Th2 cells remained unaltered.

Conclusion: This finding indicates that the Th2-polarizing potential of BPE is Bet v 1 independent; therefore, sensitization to Bet v 1 is induced by an as-yet-undetermined pollen compound or mechanism in the pollen environment. These data suggest that sensitization is not exclusively linked to the intrinsic properties of individual proteins. These findings are relevant in understanding allergic sensitization towards pollen allergens and might pave the way for future prophylactic approaches.

FIG 1

Time-dependent activation of murine BMDCs stimulated with BPE or rBet v 1 +/− nLPS (a). Comparison of activation signal induced by rBet v 1 or nBet v 1 (b). Expression of maturation markers (c) and cytokine profile (d) of human moDCs stimulated with BPE. Error bars indicate mean and SEM (*P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001).

FIG 2

Flow cytometry gating strategy used to quantify CD4⁺ IL-4 GFP⁺ T-cells (a). Percentage of IL-4-expressing CD4⁺ T-cells of mice immunized with rBet v 1 or BPE (b). Comparison of nBet v 1-depleted and rBet v 1-replaced extracts with untreated BPE (c); *P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001. Schematic overview of nBet v 1-depletion of BPE using immunoprecipitation (d). mRNA expression of in vitro stimulated human naïve CD4⁺ T-cells with BPE (e).