Telomerase reverse transcriptase (540-548)

Human telomerase reverse transcriptase (hTERT), the catalytic subunit of telomerase, is an attractive target antigen for cancer immunotherapy due to its expression in the vast majority of human tumors. The first immunogenic peptide described from hTERT was the HLA-A2-restricted peptide hTERT540 (ILAKFLHWL). However, much discrepancy exists about the processing and presentation of this epitope on the surface of neoplastic cells. Originally, it was described that specific CTL can be generated in vitro and that such cells are able to kill a range of hTERT(+) tumor cell lines and primary tumors in a peptide-specific, HLA-A2-restricted fashion. Furthermore, it was described that vaccination of cancer patients with hTERT540 introduced functional antitumor CD8(+) T cells in patients. More recently, it was described that most patients with cancer have circulating hTERT540-specific CD8(+) T lymphocytes. In contrast, several other studies have concluded that hTERT540 is not presented on the surface of tumor cells and that immunization of cancer patients with hTERT540 leads to the introduction of specific T cells that do not recognize tumor cells in vivo. In the present commentary, we summarize these highly contradictive results about this potentially very important T-cell epitope. Furthermore, we describe novel data showing that naturally occurring immune responses against hTERT540 are, although rare, present in cancer patients and that such hTERT540-specific T cells are able to recognize and kill cancer cells. Hence, our data support the findings that hTERT540 peptide is presented by human tumors and can be a target for CTL-mediated tumor lysis.

Purpose: Telomerase is an attractive target antigen for cancer immunotherapies because it is expressed in >85% of human tumors but is rarely found in normal tissues. A HLA-A*0201-restricted T-cell epitope was previously identified within telomerase reverse transcriptase hTERT:540-548. This peptide was reported to induce CTL that recognized tumor cells and transfectants that endogenously expressed telomerase. Therefore, we initiated a clinical protocol to evaluate the therapeutic and immunological efficacy of this peptide. Experimental design: Fourteen patients with metastatic cancers were vaccinated with hTERT:540-548 emulsified in incomplete Freund's adjuvant. Results: In 7 patients, peripheral blood mononuclear cells collected after immunization recognized hTERT:540-548, whereas those collected before vaccination did not. However, none of these CTLs recognized tumors that endogenously expressed telomerase, and none of the patients had an objective clinical response. Several highly avid T-cell clones were generated that recognized T2 cells pulsed with

3. Identification of a new cytotoxic T-cell epitope p675 of human telomerase reverse transcriptase

Guichun Huang, Jian Geng, Rui Wang, Longbang Chen Cancer Biother Radiopharm. 2012 Nov;27(9):600-5. doi: 10.1089/cbr.2012.1193. Epub 2012 Aug 23.

Background: The identification of new tumor-associated antigens and epitopes is the prerequisite for cancer immunotherapy. The human telomerase catalytic subunit human telomerase reverse transcriptase (hTERT) is an attractive target for tumor therapy. The hTERT:540-548 peptide (p540, ILAKFLHWL) was previously identified as an HLA-A*0201-restricted T-cell epitope. However, other MHC-restricted epitopes in this antigen are urgently needed for an effective T-cell-based cancer treatment. The goal of this study was to identify other immunogenic peptides of hTERT restricted by HLA-A*0201 molecule, which accounts for about 15% of the Chinese population. Methods: Computer algorithms were used for epitope prediction, followed by analyzation on the plate of Discovery Studio 2.5. The candidate peptides were verified by T2-cell-binding assay, ELISPOT analysis, flow cytometry examination, and in vitro and in vivo tumor inhibition assay. Results: Six peptides were predicted by computer algorithms. Of these peptides, p540 and p675 (LLGASVLGL) were identified with lower binding free energies than the other four peptides, binding to HLA-A*02 more strongly, and elicited stronger IFN-γ secretion of patients' peripheral blood mononuclear cells. The CTL clone against p675 elicited a significant cancer cell inhibition effect HLA-A*0201 restrictly. Conclusions: We identified p675 as a new HLA-A*0201-restricted T-cell epitope, and the CTL clone against it could suppress tumor growth effectively.