Structural Analysis of a Protective Epitope of the Francisella tularensis O-Polysaccharide

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• 作者：Michael J. Rynkiewicz ; Zhaohua Lu ; Julia H. Hui ; Jacqueline Sharon ; Barbara A. Seaton
• 刊名：Biochemistry
• 出版年：2012
• 出版时间：July 17, 2012
• 年：2012
• 卷：51
• 期：28
• 页码：5684-5694
• 全文大小：400K
• 年卷期：v.51,no.28(July 17, 2012)
• ISSN：1520-4995

文摘

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鈥揳ntigen 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, 伪-d-GalpNAcAN(1鈫?)-伪-d-GalpNAcAN(1鈫?)-尾-d-QuipNAc(1鈫?)-尾-d-Quip4NFm(1鈫?)-伪-d-GalpNAcAN(1鈫?)-伪-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.