A Vector Expressing PML-RARα Fused to GM-CSF Is an Effective DNA Vaccine for Inducing Specific Immune Response to APL Cells.

Despite combinations of different therapeutic strategies have significantly increased survival, acute leukemia is still not curable. To further improve outcome, specific immunotherapy might be one of the best choice. DNA vaccines have been showed leading to strong and persistent cell-mediated and humoral immune response to the antigen encoded by the plasmid. However, little data exist regarding the DNA vaccines in acute leukemia, there are few studies reported that PML-RARα DNA vaccines were developed, but the host immune response were weakly, due to the weak immunogenicity of tumor antigens. In order to improve the effect of DNA vaccine for acute promyelocytic leukemia (APL) therapy, we have used a full-length human GM-CSF (hGM-CSF) sequence fused to PML-RARα breakpoint-drived sequence and develop a vector coexpressing PML-RARα gene and hGM-CSF gene, which was expected to to promote T cells response in host. PML-RARα fusion gene segment and the hGM-CSF gene were amplified from NB4 cells or pORF-hGM-CSF plasmid. Both PCR products were cloned into PIRES plasmid respectively to construct a recombinant plasmid PML-RARα-IRES-hGM-CSF. The recombinant plasmids were then transfected into K562 or A549 cells respectively. The expression of the PML-RARα/GM-CSF mRNA and protein in transfected cells were identified by RT-PCR, dot blotting, ELISA and Western-Blot respectively. By in vivo assays, BALB/c mice were vaccinated at 6–8 week of age with a total of 200 μg DNA in normal saline, injected into two sites in the quadriceps muscles on day 0, 7 and 21. The plasmid containing the same PML-RARα segment and blank plasmid served as controls. Two weeks after the final DNA boost, both PML-RARα/GM-CSF mRNA and protein, serum INF-γ and anti-NB4 cells specific cytotoxicity of splenocytes following 7 days of stimulation in vitro with freeze thawing NB4 cells and recombinant human IL-2 were assessed by ELISA and LDH assays. The results showed that the sequence of the fragments inserted in multi-clone site (MCS) A and MCS B of PIRES plasmid were absolutely correct by double restriction enzyme cutting analysis (Xba I/Sal I) and sequence analysis, the PML-RARα/GM-CSF mRNA and protein could be identified in transfected K562 or A549 cells and in mice quadriceps muscles. The level of serum INF-γ and cytotoxicity of splenocytes against NB4 cells from immunized mice was significant increased than that from control groups. In conclusions, the vector expressing PML-RARα and hGM-CSF was successfully constructed, which can more effective immune response and anti-APL cells effect in animal models than that from plasmid containing single PML-RARα segment. It could be farther used in the research as PML-RARα DNA vaccine for APL.