Immune characteristics of plasmid DNA containing Plasmodium falciparum Pf70 gene fragment

Objective: To study the feasibility of Plasmodium falciparum Pf70 gene fragment as a component of DNA vaccine against the parasite infection. Methods: A 957bp Pf70 gene fragment was amplified by PCR using pGEX-Pf70 as template containing the gene fragment. The PCR product was inserted into an eucaryotic expression vector, pCMV-S, in the same reading frame with HBsAg gene in the vector to form a recombinant plasmid named pCMV-S-Pf70. Kunming mice were immunized with either purified plasmid pCMV-S-Pf70, or GST-Pf70 fusion protein purified using glutathione affinity chromatography. The immunizations were boosted twice at an interval of 14 days. Whole blood was collected from mice orbit on the day-7 after last boost and subjected to the assay of flow cytometry to determine the levels of CD4⁺ and CD8⁺ T lymphocytes. Results: Compared with no immunized mice, it was found that the number and the percentage of CD8⁺ T lymphocyte were obviously increased, whereas the percentage of CD4⁺ T lymphocyte was keeping unchanged and the number of this kind of T lymphocyte was slightly increased in the mice immunized with plasmid DNA. The humoral immune responses of the immunized mice with either plasmid pCMV-S-Pf70 or fusion protein GST-Pf70 were determined with ELISA. The result indicated that the titer of antibodies in the sera from DNA-immunized mice reached to 1:800 and that was much lower than what in the sera from the mice immunized with GST-Pf70 fusion protein (1:5000). Conclusion: Pf70 gene fragment may be a good candidate for P. falciiparum blood stage multiple DNA vaccine.