Partial protection against Plasmodium vivax blood-stage infection in Saimiri monkeys by immunization with a recombinant C-terminal fragment of merozoite surface protein 1 in block copolymer adjuvant.

Merozoite surface protein 1 is a candidate for blood-stage vaccines against malaria parasites. We report here an immunization study of Saimiri monkeys with a yeast-expressed recombinant protein containing the C terminus of Plasmodium vivax merozoite surface protein 1 and two T-helper epitopes of tetanus toxin (yP2P30Pv20019), formulated in aluminum hydroxide (alum) and block copolymer P1005. Monkeys immunized three times with yP2P30Pv20019 in block copolymer P1005 had significantly higher prechallenge titers of immunoglobulin G (IgG) antibodies against the immunogen and asexual blood-stage parasites than those immunized with yP2P30Pv20019 in alum, antigen alone, or phosphate-buffered saline (PBS) (P < 0.05). Their peripheral blood mononuclear cell proliferative responses to immunogen stimulation 4 weeks after the second immunization were also significantly higher than those from the PBS control group (P < 0.05). Upon challenge with 100,000 asexual blood-stage parasites 5 weeks after the last immunization, monkeys immunized with yP2P30Pv20019 in block copolymer P1005 had prepatent periods longer than those for the control alone group (P > 0.05). Three of the five animals in this group also had low parasitemia (peak parasitemia, </=20 parasites/microliter of blood). Partially protected monkeys had significantly higher levels of prechallenge antibodies against the immunogen than those unprotected (P < 0.05). There was also a positive correlation between the prepatent period and titers of IgG antibodies against the immunogen and asexual blood-stage parasites and a negative correlation between accumulated parasitemia and titers of IgG antibodies against the immunogen (P < 0.05). These results indicate that when combined with block copolymer and potent T-helper epitopes, the yeast-expressed P2P30Pv20019 recombinant protein may offer some protection against malaria.