Francisella tularensis, the causative agent of
tularemia, is in the top category (category A) of potential agents of bioterrorism. The F. tularensis live
vaccine strain (LVS) is the only
vaccine currently available to protect against
tularemia; however, this unlicensed
vaccine is relatively toxic and provides incomplete protection against aerosolized F. tularensis, the most dangerous mode of transmission. Hence, a safer and more potent
vaccine is needed. As a first step toward addressing this need, we have constructed and characterized an attenuated version of LVS, LVS ΔcapB, both as a safer
vaccine and as a vector for the expression of recombinant F. tularensis
proteins. LVS ΔcapB, with a targeted deletion in a putative
capsule synthesis gene (capB), is antibiotic resistance marker free. LVS ΔcapB retains the immunoprotective
O antigen, is serum resistant, and is outgrown by parental LVS in human macrophage-like THP-1 cells in a competition assay. LVS ΔcapB is significantly attenuated in mice; the 50% lethal dose (LD(50)) intranasally (i.n.) is >10,000-fold that of LVS. Providing CapB in trans to LVS ΔcapB partially restores its virulence in mice. Mice immunized with LVS ΔcapB i.n. or intradermally (i.d.) developed humoral and cellular immune responses comparable to those of mice immunized with LVS, and when challenged 4 or 8 weeks later with a lethal dose of LVS i.n., they were 100% protected from illness and death and had significantly lower levels (3 to 5 logs) of LVS in the lung, liver, and spleen than
sham-immunized mice. Most importantly, mice immunized with LVS ΔcapB i.n. or i.d. and then challenged 6 weeks later by
aerosol with 10× the LD(50) of the highly virulent type A F. tularensis strain SchuS4 were significantly protected (100% survival after i.n. immunization). These results show that LVS ΔcapB is significantly safer than LVS and yet provides potent protective immunity against virulent F. tularensis SchuS4 challenge.