Affinity-matured homotypic interactions induce spectrum of PfCSP-antibody structures that influence protection from malaria infection

Abstract

The generation of high-quality antibody responses to PfCSP, the primary surface antigen of Plasmodium falciparum sporozoites, is paramount to the development of an effective malaria vaccine. Here we present an in-depth structural and functional analysis of a panel of potent antibodies encoded by the IGHV3-33 germline gene, which is among the most prevalent and potent antibody families induced in the anti-CSP immune response and targets the NANP repeat region. Cryo-EM reveals a remarkable spectrum of helical Fab-CSP structures stabilized by homotypic interactions between tightly packed Fabs, many of which correlate with somatic hypermutation. We demonstrate a key role of these mutated homotypic contacts for high avidity binding to CSP and in protection from P. falciparum malaria infection. These data emphasize the importance of anti-homotypic affinity maturation in the frequent selection of IGHV3-33 antibodies, advance our understanding of the mechanism(s) of antibody-mediated protection, and inform next generation CSP vaccine design.

Structural basis of epitope selectivity and potent protection from malaria by PfCSP antibody L9

“…S3, A and B). These findings provide strong evidence for affinity maturation to optimize antibody-antibody binding, which may in turn enhance CSP avidity and protective efficacy, as we have shown recently for multiple NPNA-specific IGHV3-33 mAbs (11).…”

“…Notably, these L9K residues (F28, R31, E68, H70) make no direct contacts with rsCSP (fig. S3; table S2), indicating that the minor repeat region facilitates antibody-antibody affinity maturation in the context of multiple adjacent NPNV motifs, as has been observed for extended NPNA repeats (11,18,19). L9 is one of the most potent anti-PfCSP mAbs and is currently undergoing clinical development as a monoclonal therapy for malaria prevention (17).…”

“…PfCSP, the primary surface antigen of P. falciparum sporozoites, is a major target for vaccines and mAbs as it is both highly conserved and critical for the initiation of malaria infection. PfCSP contains an immunodominant central repeat region composed of repeating four-amino-acid units, structurally defined as DPNA, NPNV, and NPNA (4)(5)(6)(7)(8)(9)(10)(11). These roughly define the junctional, minor repeat, and major repeat epitopes, respectively.…”

“…S1, table S1). In the cryo-EM map, we observe three tightly packed Fabs bound to a central rsCSP, with each Fab simultaneously interacting with the peptide and the adjacent Fab via homotypic interactions ( 8 , 11 , 18 , 19 ). In general, the complex is homogeneous and the density is well-resolved for each L9 variable region (Fv) as well as the rsCSP peptide (Fig.…”

“…S3, C and D). L9 also utilizes the strictly conserved IGHV3-33 germline residue W52 H in CDRH2, which in all structures of IGHV3-33 mAbs solved to date forms a critical CH-π interaction with P6 of the second NPNA repeat in the NPNA 2 epitope ( 7 , 8 , 11 , 20 ). However, in L9, this role is assumed by Y94 L , and W52 H principally acts to stabilize the Y94 L :P2 interaction through a π-π stacking interaction with the Y94 L side chain (Fig.…”