Differences in epitope recognition, isotype and titer of antisera to Plasmodium falciparum merozoite surface protein 4 raised by different modes of DNA or protein immunization.

Plasmodium falciparum merozoite surface protein 4 (MSP4) is being developed as a component of a subunit vaccine against asexual stages of malaria. Three DNA constructs were produced that induced expression of MSP4 either in the cytoplasm of transfected cells or secreted from cells under the control of the human tissue plasminogen activator (TPA) signal or the native P. falciparum MSP4 signal. Only the construct containing the TPA signal induced detectable antibodies in mice, although gene expression was demonstrated in all constructs and MSP4 was shown to be secreted using either signal by in vitro transient transfection of COS cells. Two recombinant MSP4 proteins that encoded the same sequence as the plasmid DNA were produced in E. coli (EcMSP4-His) and S. cerevisiae (yMSP4-His) and used to raise antibodies in mice. Comparison of the antibodies elicited by these various antigen formulations showed differences in titer, isotype and epitope recognition. The titer of antibodies induced by DNA vaccination was lower than that induced by yMSP4-His, which in turn was lower than that induced by EcMSP4-His. The isotype profiles of the antibodies were also different, the plasmid DNA induced predominantly IgG(2a) responses whereas the two proteins induced predominantly IgG(1) responses. The antibodies induced by DNA and yMSP4-His recognized predominantly the C-terminal epidermal growth factor (EGF)-like domain of the protein, whereas EcMSP4-His induced antibodies recognizing all domains of the protein equally. The antibodies induced by DNA vaccination were directed almost extensively to conformational epitopes so that reactivity with native MSP4 was abolished after disulfide bonds in the protein were disrupted. Antibodies induced by recombinant proteins recognized linear epitopes as well and reactivity to native MSP4 was preserved after reduction and alkylation of parasite proteins.