Vaccination with lipopolysaccharide isolated from the live vaccine strain of Francisella tularensis protects against murine tularemia

A monoclonal antibody (mAb) prepared against the lipopolysaccharide (LPS) of Francisella tularensis was inoculated into mice 2 h prior to challenge with 1000 LD₅₀ F. tularensis, the live vaccine strain (LVS). Mice treated with 2-mg mAb survived the challenge inoculum. Successful passive transfer of protection by the anti-LPS monoclonal led us to explore the active vaccine potential of LPS extracted from F. tularensis strain LVS. Mice immunized with LVS-LPS generated Ab that recognized phenol water extracted LPS from Francisella, as well as whole F. tularensis LVS bacteria. Ab did not crossreact with Escherichia coli, Yersinia enterocolitica, or Brucella abortus, but did react with the virulent SCHU-4 strain of F. tularensis. LVS-LPS immunized mice were protected from challenge with 10³ LD₅₀ F tularensis LVS inoculated intraperitoneally and 10⁷ F. tularensis LVS inoculated intradermally, but were not protected against lethal challenge with an unrelated gram-negative organism, E. coli BORT.