Development of a highly effective vaccine or antibodies for prevention and ultimately elimination of malaria is urgently needed. Here, we report the isolation of a number of human monoclonal antibodies (mAbs) directed against the Plasmodium falciparum (Pf) circumsporozoite protein (CSP) from several subjects immunized with an attenuated whole sporozoite (SPZ) vaccine (Sanaria® PfSPZ Vaccine). Passive transfer of one of these antibodies, mAb CIS43, conferred high-level, sterile protection in two different mouse models of malaria infection. Stoichiometry and affinity of mAb CIS43 for PfCSP indicate two sequential multivalent binding events to six sites: the first 7-fold higher affinity binding event is to a unique “junctional” epitope positioned between the N-terminus and the central repeat domain of PfCSP. Moreover, mAb CIS43 prevented proteolytic cleavage of PfCSP on PfSPZ. Crystal structures of the CIS43 fragment antigen binding (Fab) in complex with the junctional epitope determined the molecular interactions of binding, revealed the epitope’s conformational flexibility, and defined NPN as the structural repeat motif. The demonstration that mAb CIS43 is highly effective for passive prevention of malaria has potential application for use in travelers, military personnel and elimination campaigns and identifies a new and conserved site of vulnerability on PfCSP for next generation rational vaccine design.

Highlights d The subdominant NVDP minor repeats of PfCSP are neutralizing epitopes d Potent mAbs with junctional and NANP cross-reactivity bind PfCSP in two steps d Cytotoxic PfCSP mAbs kill sporozoites in the liver d PfCSP mAbs prevent sporozoites from exiting liver sinusoids to infect hepatocytes

Highlights d Single-cell RNA-seq reveals two distinct B cell lineages d An alternative lineage contains CXCR3 + and atypical B cells d Alternative B cells are primed after primary vaccination and respond to boosters d Alternative B cells adopt a more atypical phenotype following repeated antigen exposure