The HLA A*0201-restricted hTERT540-548 peptide is not detected on tumor cells by a CTL clone or a high-affinity T-cell receptor

Tumor-associated human telomerase reverse transcriptase [hTERT) is expressed in > 85% of human tumors but not in most normal cells. As a result, this antigen has received considerable attention from those interested in cancer immunotherapy. Specifically, there has been strong interest in MHC class I-associated peptides derived from hTERT because these are expressed on the cell surface and thus may enable the targeting of tumor cells. Much of this interest has focused on peptide 540-548, ILAKFLHWL, which was predicted to exhibit the strongest binding to the common HLA A*0201 presenting molecule. The hTERT(540-548) peptide is currently being assessed in therapeutic vaccination trials; however, there is controversy surrounding whether it is naturally processed and presented on the surface of neoplastic cells. Here, we generate two highly sensitive reagents to assess the presentation of hTERT(540-548) on tumor cells: (a) a CD8(+) CTL clone, and (b) a recombinant T-cell receptor (TCR) that binds with picomolar affinity and a half-life exceeding 14 h. This TCR enables the identification of individual HLA A2-hTERT(540-548) complexes on the cell surface. The use of both this TCR and the highly antigen-sensitive CTL clone shows that the hTERT(540-548) peptide cannot be detected on the surface of tumor cells, indicating that this peptide is not a naturally presented epitope. We propose that, in future, rigorous methods must be applied for the validation of peptide epitopes used for clinical applications.