Induction of biologically active antibodies in mice, rabbits, and monkeys by Plasmodium falciparum EBA-175 region II DNA vaccine

Background: Plasmodium falciparum merozoites bind to and invade human eryth rocytes via specific erythrocyte receptors. This establishes the erythrocyt ic stage of the parasite life cycle that causes clinical disease resulting in 2-3 million deaths per year. We tested the hypothesis that a Plasmodium falciparum ligand, EBA-175 region II (RII), which binds its erythrocyte rec eptor glycophorin A during invasion, can be used as an immunogen to induce antibodies that block the binding of RII to erythrocytes and thereby inhibi t parasite invasion of erythrocytes. Accordingly, we immunized mice, rabbit s, and monkeys with DNA plasmids that encoded the 616 amino acid RII. Materials and Methods: DNA vaccine plasmids that targeted the secretion of recombinant RII protein with and without the universal T-cell helper epitop es P2P30 were used to immunize mice, rabbits, and Aotus monkeys. RII specif ic antibodies were assessed by IFA, ELISA, blocking of native [S-35] labele d EBA-175 binding to human erythrocytes, and growth inhibition assays, all in vitro. Results: The RII DNA plasmids were highly immunogenic as measured by ELISA and IFA. The anti-RII antibodies blocked the binding of native EBA-175 to e rythrocytes, and resetting of erythrocytes on COS-7 cells expressing RII. M ost important, murine and rabbit anti-RII antibodies inhibited the invasion of merozoites into erythrocytes. We immunized nonhuman primates and showed that the RII-DNA plasmids were immunogenic and well tolerated in these mon keys. Monkeys were challenged with parasitized erythrocytes; one of three m onkeys that received RII DNA plasmid was protected from fulminant disease. After challenge with live parasites, anti-RII antibody titers were boosted in the immunized monkeys. Conclusions: By proving the hypothesis that anti-RII antibodies can block m erozoite invasion of erythrocytes, these studies pave the way for the clini cal evaluation of EBA-175 as a receptor-blockade vaccine.