A general strategy to enhance immunogenicity of low-affinity HLA-A2.1-associated peptides: implication in the identification of cryptic tumor epitopes

Low-affinity MHC class I-associated cryptic epitopes derived from self prot eins overexpressed in a wide variety of human tumors or derived from antige ns of viruses exhibiting a high mutation rate, could be interesting candida tes for tumor and virus immunotherapy, respectively. However, identificatio n of low-affinity MHC-associated epitopes comes up against their poor immun ogenicity. Here we describe an approach that enhances immunogenicity of non immunogenic low-affinity HLA-A2.1-binding peptides. It consists of modifyin g their sequence by introducing a tyrosine in the first position (P1Y). P1Y substitution enhances affinity of HLA-A2.1-associated peptides without alt ering their antigenic specificity. In fact, P1Y variants of ten nonimmunoge nic low-affinity peptides exhibited a 2.3- to 55-fold higher binding affini ty and/or stabilized the HLA-A2.1 for at least 2 h more than the correspond ing native peptides. More importantly, P1Y variants efficiently triggered i n vivo native peptide-specific CTL which also recognized the corresponding naturally processed epitope. The possibility for generating CTL against any low-affinity HLA-A2.1-associated peptide provides us with the necessary to ol for the identification of cryptic tumor and virus epitopes which could b e used for peptide-based immunotherapy.