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Structural basis for antibody recognition of the NANP repeats in Plasmodium falciparum circumsporozoite protein

  1. Ian A. Wilsona,f,1
  1. aDepartment of Integrative Structural and Computational Biology, The Scripps Research Institute, La Jolla, CA 92037;
  2. bPATH’s Malaria Vaccine Initiative, PATH Center for Vaccine Innovation and Access, Washington, DC 20001;
  3. cAtreca Inc., Redwood City, CA 94063;
  4. dDepartment of Microbiology and Immunology, Stanford University, Stanford, CA 94305;
  5. eMalaria Research Institute, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD 21205;
  6. fThe Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, CA 92037
  1. Contributed by Ian A. Wilson, October 21, 2017 (sent for review September 7, 2017; reviewed by Brendan S. Crabb and Wim Hol)

Significance

The Plasmodium falciparum circumsporozoite protein (CSP) has been studied for decades as a potential immunogen, but little structural information is available on how antibodies recognize the immunodominant NANP repeats within CSP. The most advanced vaccine candidate is RTS,S, which includes multiple NANP repeats. Here, we analyzed two functional antibodies from an RTS,S trial and determined the number of repeats that interact with the antibody Fab fragments using isothermal titration calorimetry and X-ray crystallography. Using negative-stain electron microscopy, we also established how the antibody binds to the NANP repeat region in a recombinant CSP construct. The structural features outlined here provide a rationale for structure-based immunogen design to improve upon the efficacy of the current RTS,S vaccine.

Abstract

Acquired resistance against antimalarial drugs has further increased the need for an effective malaria vaccine. The current leading candidate, RTS,S, is a recombinant circumsporozoite protein (CSP)-based vaccine against Plasmodium falciparum that contains 19 NANP repeats followed by a thrombospondin repeat domain. Although RTS,S has undergone extensive clinical testing and has progressed through phase III clinical trials, continued efforts are underway to enhance its efficacy and duration of protection. Here, we determined that two monoclonal antibodies (mAbs 311 and 317), isolated from a recent controlled human malaria infection trial exploring a delayed fractional dose, inhibit parasite development in vivo by at least 97%. Crystal structures of antibody fragments (Fabs) 311 and 317 with an (NPNA)3 peptide illustrate their different binding modes. Notwithstanding, one and three of the three NPNA repeats adopt similar well-defined type I β-turns with Fab311 and Fab317, respectively. Furthermore, to explore antibody binding in the context of P. falciparum CSP, we used negative-stain electron microscopy on a recombinant shortened CSP (rsCSP) construct saturated with Fabs. Both complexes display a compact rsCSP with multiple Fabs bound, with the rsCSP–Fab311 complex forming a highly organized helical structure. Together, these structural insights may aid in the design of a next-generation malaria vaccine.

Footnotes

  • ?1To whom correspondence should be addressed. Email: wilson{at}scripps.edu.
  • Author contributions: D.O., F.Z., A.B.W., C.R.K., and I.A.W. designed research; D.O., J.L.T., Y.F.-G., and A.B.W. performed research; U.W.-R., C.F.O., D.E., J.G., W.V., and C.R.K. contributed new reagents/analytic tools; D.O., J.L.T., U.W.-R., C.F.O., D.E., J.G., W.V., Y.F.-G., F.Z., A.B.W., C.R.K., and I.A.W. analyzed data; and D.O., A.B.W., C.R.K., and I.A.W. wrote the paper.

  • Reviewers: B.S.C., Burnet Institute; and W.H., University of Washington.

  • Conflict of interest statement: W.V. and D.E. are employees of and own equity in Atreca, Inc.

  • Data deposition: The X-ray structure factors and coordinates have been deposited in the Protein Data Bank [PDB ID codes 6AXK for Fab311-(NPNA)3 and 6AXL for Fab317-(NPNA)3]. The EM reconstructions and maps are deposited in the Electron Microscopy Data Bank [accession codes EMD-7068 (rsCSP + Fab311) and EMD-7069 (rsCSP + Fab317)].

  • This article contains supporting information online at www.danielhellerman.com/lookup/suppl/doi:10.1073/pnas.1715812114/-/DCSupplemental.

This open access article is distributed under Creative Commons Attribution License 4.0 (CC BY).

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