Cancer Immune Therapy Edited by G. Stuhler and P. Walden ...

Tab. 1.3 hTERT-derived CTL epitopes
1.6Identification of the Telomerase Reverse Transcriptase (hTERT)
Epitope [Reference] Sequence Restriction element
I540 [28] ILAKFLHWL HLA-A2
R865 [32] RLVDDFLLV HLA-A2
K973 [36] KLFGVLRLK HLA-A3
V324 [37] VYAETKHFL HLA-A24
V461 [37] VYGFVRACL HLA-A24
Unlike most other tumor antigens, the expression of hTERTin tumor cells has been
linked to tumor growth and development. The expression of hTERT was shown to
contribute essentially to oncogenic transformation by permitting unlimited replicative
potential [38, 39]. In a landmark paper by Hahn, Counter, Weinberg and colleagues,
ectopic expression of hTERT in combination with two oncogenes (the simian
virus 40 large-T oncoprotein and an oncogenic allele of H-ras) resulted in direct tumorigenic
conversion of normal human epithelial and fibroblast cells, demonstrating
that disruption of the intracellular pathways regulated by large-T, oncogenic ras
and telomerase suffices to create a human tumor cell [38]. Moreover, inhibition of
telomerase activity in hTERT + tumor cells leads to telomere shortening and cell
death by apoptosis [40, 41]. This latter observation is critical in considering telomerase
as a tumor-associated antigen because it is already well-established that therapeutic
strategies targeting antigens not involved in tumor growth can result in the selection
of antigen-loss tumor mutants that are clinically progressive [6, 42].
hTERT-specific CTL have been generated ex vivo from cancer patients in multiple experimental
systems, suggesting that T cells specific to hTERT are neither fully deleted
nor irreversibly tolerized even in the setting of active neoplasia. In our series,
CTL from HLA-A2 patients with a variety of diseases, stages and treatment histories
were generated ex vivo and demonstrated hTERT-specific cytotoxicity in standard cytotoxicity
assays. In another experimental approach, hTERT-specific CTL were generated
ex vivo from cancer patients using autologous dendritic cells transduced with
hTERT mRNA [33]. These CTL were shown to kill primary human tumors in an antigen-specific
fashion. Finally, polyclonal anti-tumor cell T cells generated ex vivo from
patients following stimulation with autologous dendritic cells transduced with whole
tumor mRNA [34, 35] killed tumors but with multiple antigen specificities. For prostate
and renal cell carcinoma, a significant portion of the specific response of these
polyclonal T cell lines was against hTERT [34, 35].
A major concern in considering hTERTas a tumor antigen is the potential cytolysis of
rare normal cell types that express telomerase activity. Telomerase activity is absent in
nearly every major organ including heart, lung, liver, kidney and brain; however, hematopoietic
stem cells, activated lymphocytes, basal keratinocytes, gonadal cells and
certain epithelial cells have been reported to be telomerase-positive [31, 43±45]. In our
experiments, neither HLA-A2-restricted nor HLA-A3-restricted hTERT CTL lyse telomerase-positive
peripheral blood CD34 + cells, despite adequate target expression of
MHC class I[28]. Similar observations were made if CD34 + peripheral cells were first
9