title: Mapping the epitopes of a neutralizing antibody fragment directed against the lethal factor of Bacillus anthracis and cross-reacting with the homologous edema factor.
abstract: The lethal toxin (LT) of Bacillus anthracis, composed of the protective antigen (PA) and the lethal factor (LF), plays an essential role in anthrax pathogenesis. PA also interacts with the edema factor (EF, 20% identity with LF) to form the edema toxin (ET), which has a lesser role in anthrax pathogenesis. The first recombinant antibody fragment directed against LF was scFv 2LF; it neutralizes LT by blocking the interaction between PA and LF. Here, we report that scFv 2LF cross-reacts with EF and cross-neutralizes ET, and we present an in silico method taking advantage of this cross-reactivity to map the epitope of scFv 2LF on both LF and EF. This method identified five epitope candidates on LF, constituted of a total of 32 residues, which were tested experimentally by mutating the residues to alanine. This combined approach precisely identified the epitope of scFv 2LF on LF as five residues (H229, R230, Q234, L235 and Y236), of which three were missed by the consensus epitope candidate identified by pre-existing in silico methods. The homolog of this epitope on EF (H253, R254, E258, L259 and Y260) was experimentally confirmed to constitute the epitope of scFv 2LF on EF. Other inhibitors, including synthetic molecules, could be used to target these epitopes for therapeutic purposes. The in silico method presented here may be of more general interest.
authors.affiliation: Unité de Biotechnologie des Anticorps, et des Toxines, Institut de Recherche Biomédicale des Armées, La Tronche, France.
authors.forename: Philippe
authors.initials: P
authors.lastname: Thullier
authors.forename: Arnaud
authors.initials: A
authors.lastname: Avril
authors.forename: Jacques
authors.initials: J
authors.lastname: Mathieu
authors.forename: Christian K
authors.initials: CK
authors.lastname: Behrens
authors.forename: Jean-Luc
authors.initials: JL
authors.lastname: Pellequer
authors.forename: Thibaut
authors.initials: T
authors.lastname: Pelat
mesh_terms: D000595:Amino Acid Sequence; D000907:Antibodies, Bacterial / Q000276:immunology; D057134:Antibodies, Neutralizing / Q000737:chemistry / Q000276:immunology*; D000915:Antibody Affinity; D000942:Antigens, Bacterial / Q000737:chemistry / Q000276:immunology*; D001408:Bacillus anthracis / Q000276:immunology*; D001427:Bacterial Toxins / Q000737:chemistry / Q000276:immunology*; D019295:Computational Biology; D003429:Cross Reactions / Q000276:immunology*; D004487:Edema / Q000276:immunology; D018604:Epitope Mapping / Q000379:methods; D000939:Epitopes / Q000737:chemistry / Q000276:immunology*; D007074:Immunoglobulin G / Q000276:immunology; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D016415:Sequence Alignment; D057127:Single-Chain Antibodies / Q000276:immunology
chemical_list: D000907:Antibodies, Bacterial; D057134:Antibodies, Neutralizing; D000942:Antigens, Bacterial; D001427:Bacterial Toxins; D000939:Epitopes; D007074:Immunoglobulin G; D057127:Single-Chain Antibodies; C030325:anthrax toxin