A Chimeric Plasmodium falciparum Merozoite Surface Protein Vaccine Induces High Titers of Parasite Growth Inhibitory Antibodies

The C-terminal 19-kDa domain of Plasmodium falciparum merozoite surface protein 1 (PfMSP1(19)) is an established target of protective antibodies. However, clinical trials of PfMSP1(42), a leading blood-stage vaccine candidate which contains the protective epitopes of PfMSP1(19), revealed suboptimal immunogenicity and efficacy. Based on proof-of-concept studies in the Plasmodium yoelii murine model, we produced a chimeric vaccine antigen containing recombinant PfMSP1(19) (rPfMSP1(19)) fused to the N terminus of P. falciparum merozoite surface protein 8 that lacked its low-complexity Asn/Asp-rich domain, rPfMSP8 (Delta Asn/Asp). Immunization of mice with the chimeric rPfMSP1/8 vaccine elicited strong T cell responses to conserved epitopes associated with the rPfMSP8 (Delta Asn/Asp) fusion partner. While specific for PfMSP8, this T cell response was adequate to provide help for the production of high titers of antibodies to both PfMSP1(19) and rPfMSP8 (Delta Asn/Asp) components. This occurred with formulations adjuvanted with either Quil A or with Montanide ISA 720 plus CpG oligodeoxynucleotide (ODN) and was observed in both inbred and outbred strains of mice. PfMSP1/8-induced antibodies were highly reactive with two major alleles of PfMSP1(19) (FVO and 3D7). Of particular interest, immunization with PfMSP1/8 elicited higher titers of PfMSP1(19)-specific antibodies than a combined formulation of rPfMSP1(42) and rPfMSP8 (Delta Asn/Asp). As a measure of functionality, PfMSP1/8-specific rabbit IgG was shown to potently inhibit the in vitro growth of blood-stage parasites of the FVO and 3D7 strains of P. falciparum. These data support the further testing and evaluation of this chimeric PfMSP1/8 antigen as a component of a multivalent vaccine for P. falciparum malaria.