A novel CSP C-terminal epitope targeted by an antibody with protective activity against Plasmodium falciparum

Potent and durable vaccine responses will be required for control of malaria caused by Plasmodium falciparum (Pf). RTS,S/AS01 is the first, and to date, the only vaccine that has demonstrated significant reduction of clinical and severe malaria in endemic cohorts in Phase 3 trials. Although the vaccine is protective, efficacy declines over time with kinetics paralleling the decline in antibody responses to the Pf circumsporozoite surface protein (PfCSP). Although most attention has focused on antibodies to repeat motifs on PfCSP, antibodies to other regions may play a role in protection. Here, we expressed and characterized seven monoclonal antibodies to the C-terminal domain of CSP (ctCSP) from volunteers immunized with RTS,S/AS01. Competition and crystal structure studies indicated that the antibodies target two different sites on opposite faces of ctCSP. One site contains a polymorphic region (denoted alpha-ctCSP) and has been previously characterized, whereas the second is a previously undescribed site on the conserved beta-sheet face of the ctCSP (denoted beta-ctCSP). Antibodies to the beta-ctCSP site exhibited broad reactivity with a diverse panel of ctCSP peptides whose sequences were derived from field isolates of P. falciparum whereas antibodies to the alpha-ctCSP site showed very limited cross reactivity. Importantly, an antibody to the beta-site demonstrated inhibition activity against malaria infection in a murine model. This study identifies a previously unidentified conserved epitope on CSP that could be targeted by prophylactic antibodies and exploited in structure-based vaccine design. Author summaryThe most advanced malaria vaccine candidate to date, RTS,S, is composed of the central repeat region, so called because it consists of repeats of the NANP amino-acid sequence, and the C-terminal domain from the Plasmodium falciparum circumsporozoite protein (PfCSP). RTS,S is about 50% effective against the liver stage of the malaria parasite, but its efficacy decreases over time, concomitant with waning of antibodies that target PfCSP. Thus, further understanding of which antibodies are effective in the immune response to PfCSP is needed to facilitate design of next-generation malaria vaccines. While much is known about antibodies to the NANP repeat region, the nature and efficacy of antibodies that target the PfCSP C-terminal domain (ctCSP) is underexplored. Here, we characterize antibodies against ctCSP that were derived from volunteers in a phase 2a trial of RTS,S with a fractional dose regimen. We find that some antibodies bind to a previously identified polymorphic site on ctCSP, but others bind to a novel site that is highly conserved across different P. falciparum isolates. Furthermore, these antibodies show protection against P. falciparum infection in a mouse model. Thus, a previously unidentified and conserved site on ctCSP can be targeted by antibodies and will aid in design of more effective next-generation PfCSP-based malaria vaccines and therapeutics.