Therefore, as previously suggested [37], diversity in the -ctCSP epitope could significantly limit the breadth of antibody responses. indicated that this antibodies target two different sites on opposite faces of ctCSP. One site contains a polymorphic region (denoted -ctCSP) and has been previously characterized, whereas the second is a previously undescribed site around the conserved -sheet face of the ctCSP (denoted -ctCSP). Antibodies to the -ctCSP site exhibited broad reactivity with a diverse panel of ctCSP peptides whose sequences were derived from field isolates ofP.falciparumwhereas antibodies to the -ctCSP site showed very limited cross reactivity. Importantly, an antibody to the -site exhibited inhibition activity against malaria contamination 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 summary == The most advanced malaria vaccine candidate to date, RTS,S, is composed of the central repeat region, so called because it consists of repeats of an NANP amino-acid sequence, and the C-terminal domain name from thePlasmodium falciparumcircumsporozoite 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 targetPfCSP. Thus, further understanding of which antibodies are effective in the immune response toPfCSP is needed to facilitate design of next-generation malaria vaccines. While much is known about antibodies Glabridin to the NANP repeat region, the nature and efficacy of antibodies that target thePfCSP C-terminal domain name (ctCSP) is usually 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 differentP.falciparumisolates. Furthermore, these antibodies show protection Glabridin againstP.falciparuminfection 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-generationPfCSP-based malaria vaccines and therapeutics. == Introduction == Malaria continues to be a major global health priority with an estimated 229 million cases and 409,000 deaths in 2019 [1]. Increased resistance to antimalarial drugs has heightened concerns as widely used drugs like artemisinin, which is commonly used to combat chloroquine resistance, is increasingly failing [2,3]. The spread of drug resistance in regions of Africa is usually of particular concern since this continent accounted for 94% of estimated malaria cases and 94% of estimated deaths from malaria in 2019 [1]. To avoid the continued spread of resistance to antimalarial drugs and as a tool to limit overall malaria disease and death, the development of an effective vaccine against malaria has been prioritized [4,5]. Recent vaccine candidates targetPlasmodium falciparum, which accounted for an overwhelming majority of reported malaria infections in the WHO African Region in 2019 [1,6,7]. Currently, RTS,S/AS01 is the most advanced vaccine against malaria and has been recently approved by WHO for use in children in Africa. The vaccine is composed of a virus-like particle made up of 19 NANP repeats and the C-terminal region of the strain 3D7PfCSP linked to the hepatitis B surface antigen protein (HBsAg), and also includes unmodified HBsAg along with adjuvant AS01E [8]. The vaccine showed an efficacy of approximately 50% within the first 14 months of administration in phase 3 clinical trials in Africa. However, its efficacy decreased over time [6,9] and this decrease was attributed to waning titers of anti-CSP antibodies [10]. A related vaccine candidate, R21, is composed of the same HBsAg-CSP fusion without unmodified HBsAg and formulated with Matrix-M adjuvant [11]. In a recent phase 2 clinical trial, R21 was administered to children in Burkina Faso, with a 3-dose regimen at 4-week intervals prior to the malaria season and a Glabridin 4thdose one year later. Efficacy levels Glabridin of 74%, and 77%, in the low- and high-dose adjuvant groups respectively, were observed in a 12-month follow-up period following dose 3 [12]. However, only a single episode Rabbit polyclonal to SAC of malaria was reported in the control group.