Protection of mice against challenge with murine adenocarcinoma cells by naked DNA immunization with reverse transcriptase domain of rat TERT

TERT – telomerase reverse transcriptase – is well known tumor-associated antigen. Overexpression of TERT was detected in majority of tumor cells on different stages of malignant transformation. Several clinical trials of TERT based anti-cancer vaccines are currently ongoing, but the immune correlates of their antitumor activity are still unclear. The aim of the study was to induce the specific immune response against heterologous TERT by DNA immunization and evaluate the protective effect of immunization against challenge with syngenic TERT-expressing tumor cells in a mouse model. Expression optimized synthetic DNA for rat TERT or inactivated RT domain of TERT (RT-TERTin) was cloned into eukaryotic expression vector pVax1 and used to immunize BALB/c mice. Mice in groups of 5 received  plasmid DNA of rat TERT, RT-TERTin, or empty vector by intradermal injection followed by electroporation on days 1 (prime) and 21 (boost). Ten days after the boost, mice got two subcutaneous injections of 5x103 tumorigenic cells expressing rat TERT, RT-TERT or parental adenocarcinoma cells  labeled with luciferase. Tumor growth was followed by in vivo bioluminescence imaging (BLI) and morphometrically. Presence of metastasis in distal organs was assessed by ex vivo organ BLI, and confirmed by immunochemical staining. The tumor development was observer in all mice mock-immunized with empty vector. All the mice immunized with RT-TERTin got full protection against challenge from with tumor cells expressing TERT and RT-TERT, the grow of parental cells was limited comparing with vector group. The TERT immunized mice were rejecting TERT-expressing tumor cells, but developed tumors from RT-TERT expressing cells and parental cells. The formation of metastases in distal organs was restricted by TERT  and completely prevented by RT-TERTin immunization. The DNA immunization against inactivated RT domain of heterologous TERT provides full protection against tumor expressing the same TERT and grow limitation of tumor expressing endogenous TERT.