A recombinant baculovirus 42-kilodalton C-terminal fragment of Plasmodium falciparum merozoite surface protein 1 protects Aotus monkeys against malaria

The immunogenicity and protective efficacy of baculovirus recombinant polypeptide based on the Plasmodium falciparum merozoite surface protein 1 (MSP-1) has been evaluated in Aotus lemurinus griseimembra monkeys. The MSP-1-based polypeptide, BVp42, corresponds to the 42-kDa C-terminal processing fragment of the precursor molecule. Immunization of Aotus monkeys with BVp42 in complete Freund's adjuvant resulted in high antibody titers against the immunogen as well as parasite MSP-1. Fine specificity studies indicated that major epitopes recognized by these antibodies localize to conserved determinants of the 19-kDa C-terminal fragment derived from cleavage of the 42-kDa processing fragment. Effective priming of MSP-1-specific T cells was also demonstrated in lymphocyte proliferation assays. All three Aotus monkeys immunized with BVp42 in complete Freund's adjuvant showed evidence of protection of protection against blood-stage challenge with P. falciparum. Two animals were completely protected, with only one parasite being detected in thick blood films on a single days after injection. The third animal had a modified course of infection, controlling its parasite infection to levels below detection by thick blood smears for an extended period in comparison with adjuvant control animals. All vaccinated, protected Aotus monkeys produced antibodies which inhibited in vitro parasite growth, indicating that this assay may be a useful correlate of protective immunity and that immunity induced by BVp42 immunization is mediated, at least in part, by a direct effect of antibodies against the MSP-1 C-terminal region. The high level of protection obtained in these studies supports further development of BVp42 as a candidate malaria vaccine.

Immunogenicity of the C-terminal 19-kDa fragment of the Plasmodium falciparum merozoite surface protein 1 (MSP1), YMSP1(19) expressed in S. cerevisiae

The immunogenicity of the C-terminal 19-kDa fragment of Plasmodium falciparum MSP1 expressed in yeast as a nonfusion product, YMSP1(19), was studied. Immunization with YMSP1(19) in rabbits induced high titers of Abs specific for native conformational epitopes on parasite MSP1. In mice, immunogenicity was dependent on the mouse strain and the adjuvant formulation. This suggests that different adjuvants may alter the immunogenicity of MSP1(19) in a genetically diverse population. Although YMSP1(19) induced anti-MSP1 Abs, they did not inhibit in vitro parasite growth. This contrasts with the strong inhibitory activities of Abs produced against a recombinant MSP1(42) (BVp42), which contains the entire MSP1(19) coding sequence. Further analyses showed that YMSP1(19) was the target of the inhibitory, anti-BVp42 Abs because YMSP1(19) could completely block binding of anti-BVp42 Abs to parasite MSP1 or BVp42. Moreover, depletion of YMSP1(19)-specific Abs completely abolished the parasite inhibitory activities of anti-BVp42 sera. Anti-YMSP1(19) sera did not block the inhibitory activities of anti-BVp42 sera, suggesting that inhibitory epitopes were not in close structural proximity with noninhibitory epitopes. The finding that YMSP1(19) possessed inhibitory epitopes but induced anti-MSP1 Abs that were not inhibitory suggests that although the T-epitope(s) produced by immunization with YMSP1(19) could provide help for Ab production, it did not induce an effective inhibitory Ab response. We hypothesize that the nature/specificity of T helper epitopes on MSP1 may be crucial in efficient induction of biologically relevant and/or protective Abs.

Immunogenicity of the C-terminal 19-kDa fragment of the Plasmodium falciparum merozoite surface protein 1 (MSP1), YMSP1(19) expressed in S. cerevisiae

The immunogenicity of the C-terminal 19-kDa fragment of Plasmodium falciparum MSP1 expressed in yeast as a nonfusion product, YMSP1(19), was studied. Immunization with YMSP1(19) in rabbits induced high titers of Abs specific for native conformational epitopes on parasite MSP1. In mice, immunogenicity was dependent on the mouse strain and the adjuvant formulation. This suggests that different adjuvants may alter the immunogenicity of MSP1(19) in a genetically diverse population. Although YMSP1(19) induced anti-MSP1 Abs, they did not inhibit in vitro parasite growth. This contrasts with the strong inhibitory activities of Abs produced against a recombinant MSP1(42) (BVp42), which contains the entire MSP1(19) coding sequence. Further analyses showed that YMSP1(19) was the target of the inhibitory, anti-BVp42 Abs because YMSP1(19) could completely block binding of anti-BVp42 Abs to parasite MSP1 or BVp42. Moreover, depletion of YMSP1(19)-specific Abs completely abolished the parasite inhibitory activities of anti-BVp42 sera. Anti-YMSP1(19) sera did not block the inhibitory activities of anti-BVp42 sera, suggesting that inhibitory epitopes were not in close structural proximity with noninhibitory epitopes. The finding that YMSP1(19) possessed inhibitory epitopes but induced anti-MSP1 Abs that were not inhibitory suggests that although the T-epitope(s) produced by immunization with YMSP1(19) could provide help for Ab production, it did not induce an effective inhibitory Ab response. We hypothesize that the nature/specificity of T helper epitopes on MSP1 may be crucial in efficient induction of biologically relevant and/or protective Abs.