Toward the Rational Design of a Malaria Vaccine Construct Using the MSP3 Family as an Example: Contribution of Immunogenicity Studies in Models

Abstract

Plasmodium falciparum merozoite surface protein 3 (MSP3), the target of antibodies that mediate parasite killing in cooperation with blood monocytes and are associated with protection in exposed populations, is a vaccine candidate under development. It belongs to a family of six structurally related genes. To optimize immunogenicity, we attempted to improve its design based on knowledge of antigenicity of various regions from the conserved C terminus of the six proteins and an analysis of the immunogenicity of "tailored" constructs. The immunogenicity studies were conducted in BALB/c and C57BL/6J mice, using MSP3 (referred to here as MSP3-1) as a model. Four constructs were designed in order to assess the effect of sequences flanking the 69-amino-acid region of MSP3-1 previously shown to be the target of biologically active antibodies. The results indicate major beneficial effects of removing (i) the subregion downstream from the 69-amino-acid sequence, since antibody titers increased by 2 orders of magnitude, and (ii) the upstream subregion which, although it defines a T-helper cell epitope, is not the target of antibodies. The construct, excluding both flanking sequences, was able to induce Th1-like responses, with a dominance of cytophilic antibodies. This led to design a multigenic construct based on these results, combining the six members of the MSP3 family. This new construction was immunogenic in mice, induced antibodies that recognized the parasite native proteins, and inhibited parasite growth in the functional antibody-dependent cellular inhibition assay, thus satisfying the preclinical criteria for a valuable vaccine candidate.

Toward the Rational Design of a Malaria Vaccine Construct Using the MSP3 Family as an Example: Contribution of Antigenicity Studies in Humans

“…That the antibody titers to MSP3 and other antigens remain very moderate in children as in adults implies that malarial antigens must contain regulatory regions to prevent the induction of high immune responses that could otherwise eradicate the parasite. Such regions were identified in the immunogenicity study (7), where it was found that removing the "e-f" region from MSP3.1 resulted in an increase by 2 orders of magnitude of antibodies to the ADCI-relevant "b-d" region. In this manner the immunity induced by a carefully designed immunogen should be more effective than natural exposure to the parasite.…”

“…Antibodies elicited by any member could still cross-react with other nonmutated members and mediate parasite inhibition. Within nonhomologous regions, sequence differences from one gene to the other may be related to the T-helper function, which was documented in MSP3.1 (7,32), each sequence being better fitted to a given major histocompatibility complex (MHC) class II subset. The conservation of the diversity would provide improved T-cell help in individuals with diverse MHC class II genetic backgrounds so as to generate the same essential antibodies directed to the conserved cross-reactive B-cell epitopes.…”

Production and Preclinical Evaluation of Plasmodium falciparum MSP-1 19 and MSP-3 11 Chimeric Protein, PfMSP-Fu 24

“…PfMSP-3 protein contains three blocks of four tandem heptad repeats based on the AXXAXX motif at the N terminus, a glutamic acid-rich domain, and a putative leucine zipper sequence at the C terminus (35). It has been demonstrated that high levels of cytophilic antibodies to PfMSP-3 are directed toward the conserved N-terminal region of PfMSP-3, which also contains T-helper epitopes (36,37).…”

Pre-Clinical Assessment of Novel Multivalent MSP3 Malaria Vaccine Constructs

“…six in total, was used as a mean to investigate the possibility that immunoregulatory regions were included in the constructs. Indeed, using the whole C-term region of MSP3.1 we observed previously that over immunization induced a sharp decrease in antibody titers as compared to those obtained after three immunizations, particularly in C57BL/6 mice [13]. This effect was found related to the inclusion of the “e–f” peptide region in the immunogen.…”