Peptide immunisation of HLA-DR–transgenic mice permits the identification of a novel HLA-DRβ1*0101– and HLA-DRβ1*0401–restricted epitope from p53

Because of the central role of CD4⁺ T cells in antitumour immunity, the identification of the MHC class II–restricted peptides to which CD4⁺ T cells respond has become a priority of tumour immunologists. Here, we describe a strategy permitting us to rapidly determine the immunogenicity of candidate HLA-DR–restricted peptides using peptide immunisation of HLA-DR–transgenic mice, followed by assessment of the response in vitro. This strategy was successfully applied to the reported haemaglutinin influenza peptide HA(307–319), and then extended to three candidate HLA-DR–restricted p53 peptides predicted by the evidence-based algorithm SYFPEITHI to bind to HLA-DRβ1*0101 (HLA-DR1) and HLA-DRβ1*0401 (HLA-DR4) molecules. One of these peptides, p53(108–122), consistently induced responses in HLA-DR1– and in HLA-DR4–transgenic mice. Moreover, this peptide was naturally processed by dendritic cells (DCs), and induced specific proliferation in the splenocytes of mice immunised with p53 cDNA, demonstrating that immune responses could be naturally mounted to the peptide. Furthermore, p53(108–122) peptide was also immunogenic in HLA-DR1 and HLA-DR4 healthy donors. Thus, the use of this transgenic model permitted the identification of a novel HLA-DR–restricted epitope from p53 and constitutes an attractive approach for the rapid identification of novel immunogenic MHC class II–restricted peptides from tumour antigens, which can ultimately be incorporated in immunotherapeutic protocols.