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.

Induction of antibodies to the Plasmodium falciparum merozoite surface protein-1 (MSP1) by cross-priming with heterologous MSP1s.

The merozoite surface protein-1 (MSP1) of Plasmodium falciparum possesses intervening conserved and nonconserved sequences. The relative importance of these sequences in providing T cell help for Ab production was investigated in a series of cross-priming studies using homologous and heterologous parasite MSP1 proteins. Cross-priming with heterologous MSP1s was as efficient as homologous immunizations in inducing anti-MSP1 Abs. Similar to homologous immunization, cross-priming with heterologous MSP1s induced primarily Abs to conserved epitopes. The specificities of the Abs were also similar for the two immunization regimens. Studies were also performed with use of the C-terminal p42 fragment of MSP1 expressed in baculovirus (BVp42). When BVp42 was used either as the priming Ag followed by boosting with homologous (or heterologous) MSP1 or as the booster Ag after priming with homologous (or heterologous) MSP1, much lower anti-BVp42 Ab titers were produced compared with priming/boosting with homologous or heterologous MSP1s or BVp42 alone. Thus, immunization with the complete parasite MSP1 induced a dominant, conserved Th epitope(s) specific for anti-p42 Ab production, and such determinant(s) was either located outside the p42 region or was not provided by the BVp42 because of possible differences in the processing of parasite MSP1 vs BVp42. Our data provided a strong rationale to identify and include conserved Th epitope(s) in MSP1 vaccines. Furthermore, a MSP1 vaccine on the basis of the C-terminal p42 fragment may benefit by the inclusion of additional Th epitopes to achieve effective boosting in the field.

Regulation of antibody specificity to Plasmodium falciparum merozoite surface protein-1 by adjuvant and MHC haplotype.

An effective malaria vaccine must be capable of eliciting a protective immune response in individuals of diverse genetic makeup. In this report, we describe the co-regulation of immune responsiveness to growth-inhibitory Plasmodium falciparum merozoite surface protein-1 (MSP-1) epitopes by MHC-linked immune response genes and by the adjuvant used in MSP-1 vaccine formulations. When congenic mice differing in MHC haplotype were immunized with MSP-1 either in CFA or incorporated into a synthetic monophosphoryl lipid A (LA-15-PH)-liposome formulation, mice of different haplotypes produced anti-MSP-1 Abs capable of inhibiting P. falciparum growth. Mice of H-2b and H-2ja haplotypes produced Abs possessing high levels of inhibitory activity upon immunization with MSP-1 in LA-15-PH/liposomes whereas these haplotypes produced noninhibitory Abs when immunized with MSP-1 in CFA. Conversely, H-2d haplotype mice produced inhibitory Abs when immunized with MSP-1 in CFA but not when immunized with MSP-1 in LA-15-PH/liposomes. The LA-15-PH/liposome adjuvant was more effective than CFA in inducing growth-inhibitory Abs. The level of parasite growth inhibition observed for a particular mouse strain correlated with Ab titers against conserved, C-terminal MSP-1 epitopes, which appear to be important targets for Ab-mediated inhibition in mice immunized with both adjuvant formulations. Our results suggest that adjuvant formulation and MHC genes act in a reciprocal manner to control immune responsiveness to specific epitopes, and raise the possibility of manipulating genetically-controlled responsiveness to vaccine Ags by utilizing alternative adjuvants in vaccine formulations.