BCR-ABL Specific Sequences Are Required To Prevent Ph⁺ Acute Lymphoblastic Leukemia in Syngeneic Mice by DNA Vaccination.

Although 70 – 80 % of children with childhood acute lymphoblastic leukemia (ALL) can be cured by poly-chemo-therapy, the prognosis of patients with Philadelphia chromosome positive (Ph⁺) ALL remains poor. Therefore, new relapse prevention strategies are needed for patients with Ph⁺ ALL during remission. We have shown previously, that vaccination of mice with leukemia cell lines modified to express costimulatory molecules and cytokines induce a systemic immunity against the syngeneic BCR-ABL^p185 expressing cell line BM185. However, the difficulties to culture and transfect human leukemia cells limit the clinical application of leukemia cell based vaccines. Thus, we evaluated the pre-immunization of mice with DNA based vaccines subsequently challenged by the cell line BM185. Ballistic transfer of minimalistic immunogenically defined gene expression (MIDGE) vectors encoding a BCR-ABL^p185 fusion specific peptide or GM-CSF were used for in vivo transfection of murine skin. In addition, we used double stem-loop immunomodulators (dSLIM), containing three CpG-motifs as non-specific immune adjuvant. We provide survival data that shows specific immunization and protection of mice which received the complete vaccine BCR-ABL/GM-CSF/dSLIM. Mean tumor-free survival (p=0.019) and overall survival (p=0.008) were significantly longer compared to non-vaccinated mice and 26.7% survived and never developed leukemia. In contrast, tumor-free and overall survival of mice immunized with either dSLIM or GM-CSF alone or both dSLIM and GM-CSF was not significantly longer compared to non-vaccinated mice. Similarly, substitution of BCR-ABL by irrelevant TEL-AML1 sequences abolished the vaccine efficacy of the BCR-ABL/GM-CSF/dSLIM vaccine. The biometrical data were confirmed by CTL assays which showed that specific lysis was significantly higher after vaccination with BCR-ABL/GM-CSF/dSLIM compared to GM-CSF/dSLIM (p<0.05) and to naïve mice (p<0.005). Previously, we have shown in T-cell depletion studies that CD8+ T cells were the effector cells in the BM185 cell vaccine model and we suggest that CD8+ T cells also play an essential role in the BM185 DNA vaccine model. Together, we provide biometrical and functional data that DNA-based vaccination with BCR-ABL^p185 fusion specific sequences, GM-CSF and dSLIM can protect mice against a lethal Ph⁺ ALL challenge.