Role of two outer membrane antigens in the induction of protective immunity against Francisella tularensis strains of different virulence

A crude outer membrane preparation from Francisella tularensis live vaccine strain was used to immunise mice. Immunised mice were completely protected from a F. tularensis challenge. We evaluated the role of two major outer membrane antigens in the induction of protective immunity, namely lipopolysaccharide and an outer membrane protein FopA. We presented FopA to the immune system using an aromatic amino acid dependent Salmonella typhimurium as a vector. Although mice mounted an immune response to cloned FopA no significant protection was induced. However, lipopolysaccharide-immunised mice were completely protected from a F. tularensis live vaccine strain challenge. No increase in LD(50) was observed using F. tularensis Schu4 as the challenge strain, although there was a significant increase in time to death. These data question the validity of the murine F, tularensis live vaccine strain model.