| Tularemia is an infectious disease caused by the Gram-negative facultative intracellular bacterium Francisella tularensis (Ft). Respiratory infection causes the most severe clinical manifestations including septic shock and death. Although the disease is effectively treated with antibiotics, the potential use of antibiotic resistant strains as biological weapons has generated interest in identifying vaccine candidates and developing alternate therapies. Unfortunately, the Ft live vaccine strain (LVS), which is partially protective in humans but pathogenic in mice, is not currently licensed due to safety concerns.;The studies described herein are part of an effort by our laboratory to produce and evaluate monoclonal antibodies against Ft as treatment for respiratory tularemia. A set of monoclonal antibodies generated from LVS immunized mice by hybridoma and recombinant DNA technologies, and shown to cross-react with clinical Ft isolates, were characterized for antigen-binding specificity. Seven antibodies to five Ft proteins and ten antibodies to Ft lipopolysaccharide (LPS) were identified. Further characterization of six anti-LPS antibodies showed that all bound to the polysaccharide O side chain (0-antigen) and did not react with a LVS O-antigen deficient mutant. Nucleotide sequence analysis showed that the light chain variable regions of three of these antibodies are encoded by the same variable kappa and joining kappa genes, suggesting that the three antibodies bind to the same O-antigen epitope and that the light chain might play a major role in their antigen specificity.;Other studies in our laboratory had shown that IgG2a is the best isotype at conferring protection in a mouse model of respiratory tularemia. Therefore, four IgG2a anti-Ft 0-antigen antibodies, Ab3, Ab52, Ab54, and the commercially available FB11, were compared for epitope specificity and in vivo efficacy in the mouse model. Ab3, Ab52 and Ab54 bound to internal epitopes of the O-antigen molecule whereas FB11 bound, with the highest affinity/avidity, to its terminal non-reducing end. Furthermore, the end-binder showed superior efficacy at protecting mice against lethal intranasal challenge with LVS. These results suggest that end-binding anti-O-antigen antibodies are potentially more desirable than internal-binding antibodies as therapeutics against Francisella tularensis. |
