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Isolation, kinetic analysis, and structural characterization of an antibody targeting the Bacillus anthracis major spore surface protein BclA

Authors

  • Stewart D. Nuttall,

    Corresponding author
    1. CSIRO Division of Materials Science and Engineering, 343 Royal Pde, Parkville, Victoria, 3052, Australia
    • CSIRO Materials Science and Engineering, 343 Royal Pde, Parkville, Victoria, 3052, Australia;
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  • Michelle L. Wilkins,

    1. CSIRO Division of Materials Science and Engineering, 343 Royal Pde, Parkville, Victoria, 3052, Australia
    Current affiliation:
    1. Michelle L. Wilkins current address is CSIRO Livestock Industries, Australian Animal Health Laboratory, Victoria, 3220, Australia
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  • Victor A. Streltsov,

    1. CSIRO Division of Materials Science and Engineering, 343 Royal Pde, Parkville, Victoria, 3052, Australia
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  • Luisa Pontes-Braz,

    1. CSIRO Division of Materials Science and Engineering, 343 Royal Pde, Parkville, Victoria, 3052, Australia
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  • Olan Dolezal,

    1. CSIRO Division of Materials Science and Engineering, 343 Royal Pde, Parkville, Victoria, 3052, Australia
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  • Hung Tran,

    1. DSTO Human Protection and Performance Division, 506 Lorimer Street, Fishermans Bend, Melbourne, Victoria 3207, Australia
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  • Chun-Qiang Liu

    Corresponding author
    1. DSTO Human Protection and Performance Division, 506 Lorimer Street, Fishermans Bend, Melbourne, Victoria 3207, Australia
    • CSIRO Materials Science and Engineering, 343 Royal Pde, Parkville, Victoria, 3052, Australia;
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Abstract

One method of laboratory- or field-based testing for anthrax is detection of Bacillus anthracis spores by high-affinity, high specificity binding reagents. From a pool of monoclonal antibodies, we selected one such candidate (A4D11) with high affinity for tBclA, a truncated version of the B. anthracis exosporium protein BclA. Kinetic analysis utilising both standard and kinetic titration on a Biacore biosensor indicated antibody affinities in the 300 pM range for recombinant tBclA, and the A4D11 antibody was also re-formatted into scFv configuration with no loss of affinity. However, assays against B. anthracis and related Bacilli species showed limited binding of intact spores as well as significant cross-reactivity between species. These results were rationalized by determination of the three-dimensional crystallographic structure of the scFv-tBclA complex. A4D11 binds the side of the tBclA trimer, contacting a face of the antigen normally packed against adjacent trimers within the exosporium structure; this inter-spore interface is highly conserved between Bacilli species. Our results indicate the difficulty of generating a high-affinity antibody to differentiate between the highly conserved spore structures of closely related species, but suggest the possibility of future structure-based antibody design for this difficult target. Proteins 2011. © 2011 Wiley-Liss, Inc.

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