Unmethylated CpG dinucleotide motifs present in bacterial genomes or synthetic
oligodeoxynucleotides (ODNs) serve as strong immunostimulatory agents in mice, monkeys and humans. We determined the adjuvant effect of murine
CpG ODN 1826 on the immunogenicity and protective efficacy of the Saccharomyces cerevisiae-expressed 19-kDa C-terminal region of
merozoite surface protein 1 (yMSP1(19)) of the murine
malaria parasite Plasmodium yoelii. We found that in C57BL/6 mice, following sporozoite challenge, the degree of protective immunity against
malaria induced by yMSP1(19) in a formulation of
Montanide ISA 51 (ISA) plus
CpG ODN 1826 was similar or superior to that conferred by yMSP1(19) emulsified in complete
Freund's adjuvant (CFA/
incomplete Freund's adjuvant). In total, among mice immunized with yMSP1(19), 22 of 32 (68.7%) with ISA plus CpG 1826, 0 of 4 (0%) with CFA/
incomplete Freund's adjuvant, 0 of 4 (0%) with CpG 1826 mixed with ISA (no yMSP1(19)), and 0 of 11 (0%) with CpG 1826 alone were completely protected against development of erythrocytic stage
infection after sporozoite challenge. The adjuvant effect of
CpG ODN 1826 was manifested as both significantly improved complete protection from
malaria (defined as the absence of detectable erythrocytic form parasites) (P = 0.007, chi square) and reduced parasite burden in infected mice. In vivo depletions of
interleukin-12 and
gamma interferon cytokines and CD4+ and CD8+ T cells in vaccinated mice had no significant effect on immunity. On the other hand,
immunoglobulin G (
IgG) isotype levels appeared to correlate with protection. Inclusion of
CpG ODN 1826 in the yMSP1(19) plus ISA
vaccine contributed towards the induction of higher levels of
IgG2a and
IgG2b (Th1 type)
antibodies, suggesting that
CpG ODN 1826 caused a shift towards a Th1 type of immune response that could be responsible for the higher degree of protective immunity. Our results indicate that this potent adjuvant formulation should be further evaluated for use in clinical trials of recombinant
malarial vaccine candidates.