The use of "altered
peptide ligands" (APL),
epitopes designed for exerting increased immunogenicity as compared with native determinants, represents nowadays one of the most utilized strategies for overcoming immune tolerance to
self-antigens and boosting anti-
tumor T cell-mediated immune responses. However, the actual ability of APL-primed T cells to cross-recognize natural
epitopes expressed by
tumor cells remains a crucial concern. In the present study, we show that
CAP1-6D, a superagonist analogue of a carcinoembriyonic
antigen (CEA)-derived
HLA-A*0201-restricted
epitope widely used in clinical setting, reproducibly promotes the generation of low-affinity CD8(+) T cells lacking the ability to recognized CEA-expressing
colorectal carcinoma (CRC) cells. Short-term T cell cultures, obtained by priming peripheral blood mononuclear cells from
HLA-A*0201(+) healthy donors or CRC patients with
CAP1-6D, were indeed found to heterogeneously cross-react with saturating concentrations of the native
peptide CAP1, but to fail constantly lysing or recognizing through IFN- gamma release CEA(+)CRC cells. Characterization of anti-CAP1-6D T cell avidity, gained through
peptide titration, CD8-dependency assay, and staining with mutated tetramers (D227K/T228A), revealed that anti-CAP1-6D T cells exerted a differential interaction with the two CEA
epitopes, i.e., displaying high affinity/CD8-independency toward the APL and low affinity/CD8-dependency toward the native
CAP1 peptide. Our data demonstrate that the efficient detection of
self-antigen expressed by
tumors could be a feature of high avidity CD8-independent T cells, and underline the need for extensive analysis of
tumor cross-recognition prior to any clinical usage of APL as anti-
cancer vaccines.