BACKGROUND: In contrast to other Bet v 1-related food allergens, the major carrot allergen, Dau c 1, has been suggested to induce food allergy independently from Bet v 1. As T cells are crucial in the sensitization process, we sought to characterize the T-cell response to Dau c 1 and its cross-reactivity with Bet v 1.
METHODS: Dau c 1-specific T-cell lines (TCL) and clones (TCC) established from PBMC of birch pollen-allergic patients with carrot allergy were analyzed for reactivity to Bet v 1, epitope specificity, allergen-induced cytokine secretion, and expression of integrins α4β7 and α4β1, critical for gut and lung homing, respectively. mRNA expression of GATA3 and Tbet was analyzed in sorted CD3+ CD4+ CFSElow cells proliferating upon stimulation of PBMC with Dau c 1 or Bet v 1. Dau c 1 was incubated with endolysosomal proteases, and the resulting fragments were identified by mass spectrometry. RESULTS: Among 14 distinct T-cell-activating regions, Dau c 1139-153 was recognized by 55% of the patients. Only 6 of 15 (40%) Dau c 1-specific TCL and 9 of 21 (43%) TCC reacted with Bet v 1. Bet v 1-nonreactive TCC were mainly Th1-like and showed a higher expression of the integrin β7 and a significantly lower expression of the integrin β1 than Bet v 1-positive TCC. A Th1-like response was also detected in Dau c 1-reactive CD3+ CD4+ CFSElow cells. Full-length Dau c 1 was still detectable after 48 h of endolysosomal degradation. Proteolytic fragments of Dau c 1 matched its T-cell-activating regions. CONCLUSION: Dau c 1 displays several characteristics of sensitizing allergens, namely a major T-cell-activating region, low susceptibility to endolysosomal degradation, and induction of a Bet v 1-independent T-cell response. These cellular insights confirm that the major carrot allergen has a special status among Bet v 1-related food allergens.