CpG oligodeoxynucleotide enhances immunity against blood-stage malaria infection in mice parenterally immunized with a yeast-expressed 19 kDa carboxyl-terminal fragment of Plasmodium yoelii merozoite surface protein-1 (MSP1(19)) formulated in oil-based Montanides.

The 19kDa carboxyl-terminal fragment of Plasmodium yoelii merozoite surface protein-1 (MSP1(19)), an analog of the leading falciparum malaria vaccine candidate, induces protective immunity to challenge infection when formulated with complete/incomplete Freund's adjuvant (CFA/IFA), an adjuvant unsuitable for use in humans. In this study, we investigate Montanide ISA51 and Montanide ISA720 as well as CpG oligodeoxynucleotide (ODN) as adjuvants for induction of immunity to MSP1(19). Mice immunized with MSP1(19) adjuvanted with Montanide ISA51 were protected even though some mice experienced low-grade parasitemia before resolving the infection. Mice immunized with MSP1(19) adjuvanted with Montanide ISA720 showed delayed patent parasitemia with all mice ultimately succumbing to infection. Interestingly, when the synthetic CpG ODN 1826 was included in either Montanide formulation, mice were completely protected with no parasites detected in the blood. MSP1(19)-specific antibodies in MSP1(19)-immunized mice adjuvanted with Montanide ISA51 or Montanide ISA720 showed predominantly IgG1 antibody and low levels of IgG2a. CpG ODN 1826 significantly enhanced both IgG1 and IgG2a antibody responses in Montanide ISA51-adjuvanted mice but significantly enhanced only the IgG2a antibody response in Montanide ISA720-adjuvanted mice. To investigate the relative roles of antibody and CD4(+) T cells in protection, MSP1(19)-immunized mice adjuvanted with Montanide ISA720 and CpG ODN 1826 were depleted of CD4(+) T cells just prior to challenge. Results showed that three of nine immunized/T cell depleted mice died following infection. These results suggest that antibody and CD4(+) T cells are critical for protection following immunization with MSP1(19) adjuvanted with Montanide and CpG ODN and that the formulation of a human malaria vaccine candidate in Montanide ISA720 or ISA51 together with human compatible CpG ODN would be useful for improving efficacy.