Identification of an HLA-A*0201 restricted Bcl2-derived epitope expressed on tumors

A large number of human tumor-associated antigen-derived peptides have been identified that are recognized by CTLs in a MHC-I restricted fashion. The apoptosis inhibitory protein Bcl2 is overexpressed in many human cancers as part of their neoplastic phenotype. Since inhibition or loss of Bcl2 expression might impair tumor growth and survival, this protein may serve as a rational target for vaccine-induced CTL responses. By Western blot technique, we screened a panel of established human tumor cell lines for proteins involved in the apoptotic process. Two of eight tumor cell lines, a B lymphoma (Loukes) and a colon carcinoma (CCL220) cell line showed increased Bcl2 protein expression whereas the majority of tumor cell lines expressed proapoptotic proteins. Neither fibroblasts nor peripheral blood mononuclear cells showed Bcl2 expression. An HLA-A*0201 restricted CTL epitope was deduced in silica from the amino acid sequence of the Bcl2 protein and its binding affinity for HLA-A*0201 was confirmed using a biochemical binding assay. We here demonstrate that the 9-mer peptide Bcl285–93 induces specific CTL reactivity in immunized C57-A2Kb or -A2Db tg mice. These Bcl285–93 specific CTLs react with and lyse Bcl2-expressing human colon carcinoma CCL220 cells which have been transfected with a chimeric HLA-A*0201/H2-Kb DNA construct similar to that expressed in the transgenic mice. Based on these observations, we suggest that Bcl285–93 may be a target for immune therapy.