title: Structural analysis of a protective epitope of the Francisella tularensis O-polysaccharide.
abstract: Francisella tularensis (Ft), the Gram-negative facultative intracellular bacterium that causes tularemia, is considered a biothreat because of its high infectivity and the high mortality rate of respiratory disease. The Ft lipopolysaccharide (Ft LPS) is thought to be a main protective antigen in mice and humans, and we have previously demonstrated the protective effect of the Ft LPS-specific monoclonal antibody Ab52 in a mouse model of respiratory tularemia. Immunochemical characterization has shown that the epitope recognized by Ab52 is contained within two internal repeat units of the O-polysaccharide [O-antigen (OAg)] of Ft LPS. To further localize the Ab52 epitope and understand the molecular interactions between the antibody and the saccharide, we determined the X-ray crystal structure of the Fab fragment of Ab52 and derived an antibody-antigen complex using molecular docking. The docked complex, refined through energy minimization, reveals an antigen binding site in the shape of a large canyon with a central pocket that accommodates a V-shaped epitope consisting of six sugar residues, α-D-GalpNAcAN(1→4)-α-D-GalpNAcAN(1→3)-β-D-QuipNAc(1→2)-β-D-Quip4NFm(1→4)-α-D-GalpNAcAN(1→4)-α-D-GalpNAcAN. These results inform the development of vaccines and immunotherapeutic/immunoprophylactic antibodies against Ft by suggesting a desired topology for binding of the antibody to internal epitopes of Ft LPS. This is the first report of an X-ray crystal structure of a monoclonal antibody that targets a protective Ft B cell epitope.
authors: Department of Physiology and Biophysics, Boston University School of Medicine, Boston, MA 02118, USA.
authors: Michael J
authors: MJ
authors: Rynkiewicz
authors: Zhaohua
authors: Z
authors: Lu
authors: Julia H
authors: JH
authors: Hui
authors: Jacqueline
authors: J
authors: Sharon
authors: Barbara A
authors: BA
authors: Seaton
mesh_terms: D000911:Antibodies, Monoclonal / Q000737:chemistry*; D000936:Antigen-Antibody Complex / Q000737:chemistry; D002240:Carbohydrate Sequence; D018360:Crystallography, X-Ray; D000939:Epitopes; D005604:Francisella tularensis / Q000378:metabolism*; D007140:Immunoglobulin Fab Fragments / Q000737:chemistry*; D062105:Molecular Docking Simulation; D008969:Molecular Sequence Data; D019081:O Antigens / Q000737:chemistry* / Q000276:immunology; D011487:Protein Conformation
chemical_list: D000911:Antibodies, Monoclonal; D000936:Antigen-Antibody Complex; D000939:Epitopes; D007140:Immunoglobulin Fab Fragments; D019081:O Antigens