Identification of a highly promiscuous and an HLA allele-specific T-cell epitope in the birch major allergen Bet v 1: HLA restriction, epitope mapping and TCR sequence comparisons

Background Allergen-specific CD4(+)T cells play an important regulatory rol e in atopic allergy. Objective To investigate the human leucocyte antigen (HLA) restriction and T-cell receptor (TCR) usage of allergen-specific T-cell clones (TCCs) that react with defined epitopes of Bet v 1, the major birch pollen allergen. Methods Five Bet v 1-specific TCCs derived from two birch pollen-allergic i ndividuals and specific for Bet v 1, were epitope-mapped with overlapping s ynthetic peptides. In addition, HLA-restriction and TCR CDR3 sequences were determined. Results Three TCCs reacted with a Bet v 1 peptide containing amino acid res idues 21-33 (BP21), the other two TCCs reacted with a minimal peptide compr ising residues 37-45 (BP37). Studies using neutralizing anti-HLA-monoclonal antibodies and HLA-typed APCs showed that the BP37-specific TCCs were rest icted by a HLA-DQA1*0301/DQB1*0603 heterodimer. In contrast, BP21 was recog nized in a highly promiscuous manner. TCCs recognizing this sequence were r estricted by HLA-DPB1*0201, a HLA-DQA1*0201/DQB1*0201 heterodimer, or HLA-D RB3*0101. Reverse transcription-polymerase chain reaction with primers for all known TCRAV and TCRBV gene segments, followed by CDR3 region sequencing , revealed the usage of five different TCRAV and four different TCRBV gene segments by the TCCs, as well as diversity in the joining region. All BP21- specific TCCs contained a negatively charged residue in their CDR3 alpha re gions, the CDR3 beta regions showed a high concentration of polar and OH-gr oup bearing residues. BP37-specific TCCs shared the amino acid combination LY in the middle of their CDR3 alpha regions, the CDR3 beta regions showed high concentration of OH-group bearing or charged residues. Conclusions This study shows the existence of a highly promiscuous T-cell e pitope in Bet v 1. The presence of additional T-cell epitopes in Bet v 1 ma y, however, hamper the clinical applicability of the epitope. Likewise, the diversity in TCR usage by T cells recognizing the epitope does not support the development of TCR-directed immunotherapy for birch pollen allergy.