Effects of DNA vaccine in murine malaria using a full-length cDNA library

In an attempt to develop a novel malaria vaccine, we constructed a full-length cDNA library from the erythrocytic-stage parasites of Plasmodium berghei ANKA strain using the plasmid vector pCE-FL, which is driven by an EF321 promoter and a CMV-IE enhancer. Here we report the initial trial to screen this library for DNA vaccine candidates against malaria parasite infection in mice. The library of P. berghei was divided into five groups, each representing 2,000 independent clones. Eight female BALB/c mice were injected with these subsets, with an initial injection directly into the spleen, followed by two subsequent intramuscular injections at 1-week intervals. As a control, the plasmid vector without any insert was used. Two weeks after the last injection, 50,000 infected erythrocytes were injected intraperitoneally. Unexpectedly, the survival rate of the vaccinated groups was lower than that of the control (p = 0.053, by Kaplan-Meyer method), suggesting that these DNA vaccines had adverse effects. There was no difference in parasitemia between the two groups. There was no difference between antibody titers before and after immunization in either group. Accelerated deaths in immunized mice occurred from 7 to 10 days after infection, when fur bristling, shivering and convulsions were observed. These observations suggested the possibility that the vaccination had an adverse effect on the cellular immunity that resulted in the development of severe malaria in BALB/c mice, which do not usually develop cerebral malaria.