Improved immunogenicity and efficacy of the recombinant 19-kilodalton merozoite surface protein 1 by the addition of oligodeoxynucleotide and aluminum hydroxide gel in a murine malaria vaccine model.

Vaccination of mice with yeast-secreted Plasmodium yoelii-derived 19-kilodalton merozoite surface protein 1 (yMSP1(19)) has been shown to afford protection from challenge with a lethal strain of P. yoelii. Sterile immunity can be achieved when MSP1(19) is emulsified in Freund adjuvant but not when it is adsorbed to aluminum hydroxide gel (alum). Because complete Freund adjuvant is not an acceptable adjuvant for use in humans, alternative adjuvants must be identified for formulating MSP1(19) as a vaccine for use in humans. To determine whether oligodeoxynucleotides with CpG motifs (ODN), reported to be a powerful new class of adjuvants, could enhance the immunogenicity of yMSP1(19), C57BL/6 mice were vaccinated either with yMSP1(19) formulated with Freund adjuvant, with alum, or with ODN plus alum and challenged intravenously with P. yoelii 17XL asexual blood-stage parasites. Adsorption of immunogen and adjuvant to alum was optimized by adjusting buffer (phosphate versus acetate) and pH. We found that the adjuvant combination of ODN plus alum with yMSP1(19), injected intraperitoneally (i.p.), increased immunoglobulin G (IgG) yMSP1(19)-specific antibody production 12-fold over Freund adjuvant given i.p., 3-fold over Freund adjuvant given subcutaneously (s.c.), 300-fold over alum given i.p., and 48-fold over alum given s.c. The predominant antibody isotype in the group receiving alum-ODN-yMSP1(19) was IgG1. Increased antibody levels correlated to protection from a challenge with P. yoelii 17XL. Supernatant cytokine levels of gamma interferon in yMSP1(19)-stimulated splenocytes were dramatically elevated in the alum-ODN-yMSP1(19) group. Interleukin-10 (IL-10) levels were also elevated; however, no IL-5 was detected. The cytokine profile, as well as the predominant IgG1 antibody isotype, suggests the protective immune response was a mixed Th1/Th2 response.