No abstract

Circumsporozoite protein of the human malaria parasite Plasmodium falciparum (PfCSP) is the main target of antibodies that prevent the infection and disease. Protective antibodies recognize the central PfCSP domain, but our understanding of how parasite inhibition is associated with recognition of this domain and with the evolution of potent antibodies remains scattered. Here, we characterized the epitope specificity of 200 human monoclonal PfCSP antibodies. We show that the majority of PfCSP antibodies bind to NANP and NANP-like motifs with different preferences and define the molecular basis for recognition. Epitope cross-reactivity evolved with increasing antibody affinity around a conserved (N/D)P-NANP-N(V/A) core. High affinity to this motif, but not binding to NANP-like motifs, was associated with parasite inhibition and protection. Thus, NANP drives the development of potent PfCSP antibodies independently of their cross-reactivity profile, a finding with direct implications for the design of a second-generation PfCSP-based malaria vaccine. KEYWORDS Plasmodium falciparum, malaria vaccine, circumsporozoite protein, human antibodies, crystal structures, clonal selection, affinity maturation HIGHLIGHTS • The majority of human PfCSP antibodies recognize multiple epitopes • NANP affinity maturation drives the evolution of cross-reactive PfCSP antibodies • Preferential PfCSP antibody binding to a conserved core motif • High affinity not epitope specificity is associated with PfCSP antibody potency

Plasmodium sporozoites express circumsporozoite protein (CSP) on their surface, an essential protein that contains central repeating motifs. Antibodies targeting this region can neutralize infection, and the partial efficacy of RTS,S/AS01 – the leading malaria vaccine against P. falciparum (Pf) – has been associated with the humoral response against the repeats. Although structural details of antibody recognition of PfCSP have recently emerged, the molecular basis of antibody-mediated inhibition of other Plasmodium species via CSP binding remains unclear. Here, we analyze the structure and molecular interactions of potent monoclonal antibody (mAb) 3D11 binding to P. berghei CSP (PbCSP) using molecular dynamics simulations, X-ray crystallography, and cryoEM. We reveal that mAb 3D11 can accommodate all subtle variances of the PbCSP repeating motifs, and, upon binding, induces structural ordering of PbCSP through homotypic interactions. Together, our findings uncover common mechanisms of antibody evolution in mammals against the CSP repeats of Plasmodium sporozoites.

Malaria is a global health concern, and research efforts are ongoing to develop a superior vaccine to RTS,S/AS01. To guide immunogen design, we seek a comprehensive understanding of the protective humoral response against Plasmodium falciparum (Pf) circumsporozoite protein (PfCSP). In contrast to the well-studied responses to the repeat region and the C-terminus, the antibody response against the N-terminal domain of PfCSP (N-CSP) remains obscure. Here, we characterized the molecular recognition and functional efficacy of the N-CSP–specific monoclonal antibody 5D5. The crystal structure at 1.85-Å resolution revealed that 5D5 binds an α-helical epitope in N-CSP with high affinity through extensive shape and charge complementarity and the unusual utilization of an antibody N-linked glycan. Nevertheless, functional studies indicated low 5D5 binding to live Pf sporozoites and lack of sporozoite inhibition in vitro and in vivo. Overall, our data do not support the inclusion of the 5D5 N-CSP epitope into the next generation of CSP-based vaccines.