Design and Evaluation of Chimeric Plasmodium falciparum Circumsporozoite Protein-Based Malaria Vaccines.

Efficacy data on two malaria vaccines, RTS,S and R21, targeting Plasmodium falciparum circumsporozoite protein ( Pf CSP), are encouraging. Efficacy may be improved by induction of additional antibodies to neutralizing epitopes outside of the central immunodominant repeat domain of Pf CSP. We designed four r Pf CSP-based vaccines in an effort to improve the diversity of the antibody response. We also evaluated P. falciparum merozoite surface protein 8 ( Pf MSP8) as a malaria-specific carrier protein as an alternative to hepatitis B surface antigen. We measured the magnitude, specificity, subclass, avidity, durability, and efficacy of vaccine-induced antibodies in outbred CD1 mice. In comparison to N-terminal- or C-terminal-focused constructs, immunization with near full-length vaccines, r Pf CSP (#1) or the chimeric r Pf CSP/8 (#2), markedly increased the breadth of B cell epitopes recognized covering the N-terminal domain, junctional region, and central repeat. Both r Pf CSP (#1) and r Pf CSP/8 (#2) also elicited a high proportion of antibodies to conformation-dependent epitopes in the C-terminus of Pf CSP. Fusion of Pf CSP to Pf MSP8 shifted the specificity of the T cell response away from Pf CSP toward Pf MSP8 epitopes. Challenge studies with transgenic Plasmodium yoelii sporozoites expressing Pf CSP demonstrated high and consistent sterile protection following r Pf CSP/8 (#2) immunization. Of note, antibodies to conformational C-terminal epitopes were not required for protection. These results indicate that inclusion of the N-terminal domain of Pf CSP can drive responses to protective, repeat, and non-repeat B cell epitopes and that Pf MSP8 is an effective carrier for induction of high-titer, durable anti- Pf CSP antibodies.