Induction of protective antibodies in Saimiri monkeys by immunization with a multiple antigen construct (MAC) containing the Plasmodium vivax circumsporozoite protein repeat region and a universal T helper epitope of tetanus toxin.

Previous attempts in inducing protective immunity against Plasmodium vivax in human volunteers and nonhuman primates with recombinant circumsporozoite (CS) proteins have been unsuccessful, largely due to the failure of generating antibodies against the protective B epitope AGDR in the CS protein repeat region. We report here an immunization study in Saimiri monkeys with a multiple antigen construct (MAC) containing the P. vivax CS protein repeat region and a T helper epitope of tetanus toxin formulated in different adjuvants. Monkeys immunized three times with MAC in copolymer P1005, copolymer P1005 plus RaLPS, or MF-75 had titers of antibodies against CS repeat, sporozoites and the protective B epitope AGDR significantly higher than those immunized with MAC in alum or PBS (P < 0.05). Antibody levels in animals that received P1005 were maintained at high level for 7 months after the last immunization. Upon challenge with 10000 sporozoites 2 weeks after the last immunization, 75% (three of four) of monkeys from the alum group, 50% (three of six) of monkeys from the P1005 plus RaLPS group, 40% (two of five) of monkeys from the P1005 group, 33% (two of six) of monkeys from the MF-75 group, and 17% (one of six) of monkeys from the MAC alone group were fully protected. When immunized animals were challenged again with 30000 sporozoites 22 weeks after the last immunization. 40% (two of five) monkeys from the P1005 group were fully protected. The remaining (three) in this group developed low parasitemia (< 2000 parasites mm-3 of blood) after significantly longer prepatent period (P < 0.05). In addition, 17% (one of six) of monkeys each from the P1005 plus RaLPS and MF-75 groups were also fully protected. Protected animals had higher levels of prechallenge anti-AGDR antibody titers than unprotected (1933 vs 281 for the first challenge, P > 0.05; 21527 vs 196 for the rechallenge, P < 0.05). Anti-AGDR antibody titers were positively correlated with the prepatent period of infected animals (r = 0.42 for the first challenge, P > 0.05; r = 0.60 for the rechallenge, P < 0.05) and negatively correlated with the peak parasitemia (r = -0.39 for the first challenge, P < 0.05; r = 0.50 for the rechallenge, P < 0.05). The results suggested that when combined with the use of potent adjuvants and T helper epitopes, MAC subunit vaccines may potentially offer protection against malaria infection.