417. Induction of Potent Cytotoxic and Antitumor Activity By a Highly Optimized hTERT DNA Vaccine

Abstract

Immunotherapy of cancer through induction of anti-tumor cellular immunity has recently re-emerged as an important therapy for the treatment of nonresponsive cancers. Human telomerase reverse transcriptase (hTERT) is highly expressed in more than 85% of human tumors, with little or no expression in normal somatic cells. Expression of hTERT correlates well with telomerase activity and is associated with tumor cell growth and contributes crucially to the long-term survival of tumor cells. In addition, targeting hTERT may have the potential to eliminate cancer stem cells as recent studies have suggested that cancer stem or stem-like cells express hTERT. Immunological analysis supports that hTERT is a widely applicable target recognized by T cells and can be potentially studied as a broad cancer immune therapeutic, or as a unique line of defense against tumor recurrence. There remains an urgent need to develop more potent hTERT vaccines.Here, we developed a synthetic highly optimized hTERT DNA vaccine (phTERT) to express full-length hTERT. When delivered by electroporation, phTERT elicited robust hTERT-specific CD8 response (the total response in phTERT-immunized mice was 1817±211 SFU/106splenocytes). Epitope mapping results showed that 9 out of 26 matrix pools exhibited more than 50 spots, indicating phTERT could elicit a broad range of T-cell immune responses. The average percentage of CD107a, IFN-γ and TNF-α secreting cells from the total CD8+ T cell population were approximately 0.8%, 1.2% and 1.8%, respectively. Moreover, vaccination with phTERT slows the tumor growth in HPV16-associated tumor-bearing mice. Six out of ten phTERT-immunized mice still survived thirty-nine days post tumor implantation while all mice in naive group were dead or euthanized. In vivo cytotoxicity assay confirmed that phTERT-induced CD8 cells exhibited specific CTL activity and were capable of eliminating hTERT-pulsed target cells. In addition, the ability of phTERT to overcome tolerance was evaluated in a NHP model, whose TERT protein is 96% homologous to that of hTERT. Immunized monkeys exhibited robust (average of 1834 SFU/106PBMCs), diverse (multiple immunodominant epitopes) IFN-γ responses and antigen-specific perforin release (average of 332 SFU/106 PBMCs), suggesting phTERT breaks tolerance and induces potent cytotoxic responses in this human relevant model. Lastly, we demonstrated that co-immunization with a rhesus macaque IL-12 plasmid as an adjuvant could further increase the vaccine-induced responses for about two fold in NHP. Taken together, these findings support that this synthetic EP-delivered DNA phTERT may have a role as a broad therapeutic cancer vaccine candidate.