Protective B cell epitopes of Francisella tularensis O-polysaccharide targeted by mouse and human antibodies (166.22)

Abstract

Abstract Francisella tularensis (Ft), the gram-negative bacterium that causes tularemia, is considered a potential bioterrorism agent. Antibodies to Ft lipopolysaccharide (LPS) are protective against respiratory tularemia in mouse models, and we have previously described mouse monoclonal antibodies (MAbs) to non-overlapping terminal and internal epitopes of the Ft LPS O-polysaccharide (OAg). In the current study, we used competition ELISA with oligosaccharides of defined OAg repeat length, x-ray crystallography and docking studies to map the epitope of the terminal-binding MAb FB11 to one OAg tetrasaccharide repeat at the non-reducing end, and the epitope of the internal-binding MAb Ab52 to six sugar residues, spanning two OAg repeats, each repeat contributing three residues. Both FB11 and Ab52 conferred or prolonged survival in a mouse model of respiratory tularemia against Ft type B live vaccine strain (LVS) or the highly virulent Ft type A strain SchuS4, respectively. The two MAbs were successfully used as reporters, in competition ELISA, to demonstrate the presence of antibodies to both terminal and internal Ft OAg epitopes in the sera of tularemia patients. These results indicate that two Ft OAg repeats are necessary and sufficient for antibody binding to both terminal and internal protective OAg epitopes, which may inform the design of vaccines for tularemia. Furthermore, the FB11 and Ab52 MAbs could be used to monitor protective B cell responses in tularemia vaccine trials.