title: Predicting HIV-1 broadly neutralizing antibody epitope networks using neutralization titers and a novel computational method.
abstract: BACKGROUND
  Recent efforts in HIV-1 vaccine design have focused on immunogens that evoke potent neutralizing antibody responses to a broad spectrum of viruses circulating worldwide. However, the development of effective vaccines will depend on the identification and characterization of the neutralizing antibodies and their epitopes. We developed bioinformatics methods to predict epitope networks and antigenic determinants using structural information, as well as corresponding genotypes and phenotypes generated by a highly sensitive and reproducible neutralization assay.282 clonal envelope sequences from a multiclade panel of HIV-1 viruses were tested in viral neutralization assays with an array of broadly neutralizing monoclonal antibodies (mAbs: b12, PG9,16, PGT121 - 128, PGT130 - 131, PGT135 - 137, PGT141 - 145, and PGV04). We correlated IC50 titers with the envelope sequences, and used this information to predict antibody epitope networks. Structural patches were defined as amino acid groups based on solvent-accessibility, radius, atomic depth, and interaction networks within 3D envelope models. We applied a boosted algorithm consisting of multiple machine-learning and statistical models to evaluate these patches as possible antibody epitope regions, evidenced by strong correlations with the neutralization response for each antibody.
  
  
  RESULTS
  We identified patch clusters with significant correlation to IC50 titers as sites that impact neutralization sensitivity and therefore are potentially part of the antibody binding sites. Predicted epitope networks were mostly located within the variable loops of the envelope glycoprotein (gp120), particularly in V1/V2. Site-directed mutagenesis experiments involving residues identified as epitope networks across multiple mAbs confirmed association of these residues with loss or gain of neutralization sensitivity.
  
  
  CONCLUSIONS
  Computational methods were implemented to rapidly survey protein structures and predict epitope networks associated with response to individual monoclonal antibodies, which resulted in the identification and deeper understanding of immunological hotspots targeted by broadly neutralizing HIV-1 antibodies.
authors.forename: Mark C
authors.initials: MC
authors.lastname: Evans
authors.forename: Pham
authors.initials: P
authors.lastname: Phung
authors.forename: Agnes C
authors.initials: AC
authors.lastname: Paquet
authors.forename: Anvi
authors.initials: A
authors.lastname: Parikh
authors.forename: Christos J
authors.initials: CJ
authors.lastname: Petropoulos
authors.forename: Terri
authors.initials: T
authors.lastname: Wrin
authors.affiliation: Monogram Biosciences Inc,, 345 Oyster Point Blvd,, South San Francisco, CA 94080, USA. mojganhd@yahoo.com.
authors.forename: Mojgan
authors.initials: M
authors.lastname: Haddad
mesh_terms: D000911:Antibodies, Monoclonal / Q000737:chemistry / Q000235:genetics / Q000276:immunology / Q000378:metabolism; D057134:Antibodies, Neutralizing / Q000737:chemistry / Q000235:genetics / Q000276:immunology* / Q000378:metabolism; D001666:Binding Sites, Antibody / Q000235:genetics / Q000276:immunology; D019295:Computational Biology / Q000379:methods*; D000939:Epitopes / Q000737:chemistry* / Q000235:genetics / Q000276:immunology* / Q000378:metabolism; D015483:HIV Antibodies / Q000737:chemistry / Q000235:genetics / Q000276:immunology* / Q000378:metabolism; D015699:HIV Envelope Protein gp120 / Q000276:immunology; D015497:HIV-1 / Q000276:immunology*; D006801:Humans; D009500:Neutralization Tests
chemical_list: D000911:Antibodies, Monoclonal; D057134:Antibodies, Neutralizing; D000939:Epitopes; D015483:HIV Antibodies; D015699:HIV Envelope Protein gp120