Identification of a Live Attenuated Vaccine Candidate for Tularemia Prophylaxis

Abstract

Francisella tularensis is the causative agent of a fatal human disease, tularemia. F. tularensis was used in bioweapon programs in the past and is now classified as a category A select agent owing to its possible use in bioterror attacks. Despite over a century since its discovery, an effective vaccine is yet to be developed. In this study four transposon insertion mutants of F. tularensis live vaccine strain (LVS) in Na/H antiporter (FTL_0304), aromatic amino acid transporter (FTL_0291), outer membrane protein A (OmpA)-like family protein (FTL_0325) and a conserved hypothetical membrane protein gene (FTL_0057) were evaluated for their attenuation and protective efficacy against F. tularensis SchuS4 strain. All four mutants were 100–1000 fold attenuated for virulence in mice than parental F. tularensis. Except for the FTL_0304, single intranasal immunization with the other three mutants provided 100% protection in BALB/c mice against intranasal challenge with virulent F. tularensis SchuS4. Differences in the protective ability of the FTL_0325 and FTL_0304 mutant which failed to provide protection against SchuS4 were investigated further. The results indicated that an early pro-inflammatory response and persistence in host tissues established a protective immunity against F. tularensis SchuS4 in the FTL_0325 immunized mice. No differences were observed in the levels of serum IgG antibodies amongst the two vaccinated groups. Recall response studies demonstrated that splenocytes from the FTL_0325 mutant immunized mice induced significantly higher levels of IFN-γ and IL-17 cytokines than the FTL_0304 immunized counterparts indicating development of an effective memory response. Collectively, this study demonstrates that persistence of the vaccine strain together with its ability to induce an early pro-inflammatory innate immune response and strong memory responses can discriminate between successful and failed vaccinations against tularemia. This study describes a live attenuated vaccine which may prove to be an ideal vaccine candidate for prevention of respiratory tularemia.

Repression of Inflammasome by Francisella tularensis during Early Stages of Infection

“…that generation of an early and enhanced innate immune response is in fact responsible for in vitro and in vivo killing of this mutant (37,41).…”

“…We reported that FTL_0325 suppresses proinflammatory cytokines primarily by interfering with NF-B signaling (37). We have also reported that FTL_0325 is required for both in vitro and in vivo growth, virulence in mice, and has a unique ability to induce an early production of TNF-␣ and IL-6 as early as day 1 post-infection in mice (41). In the present study we took advantage of this mutant to understand how F. tularensis regulates inflammasome repression.…”

“…The mice were sacrificed at days 1 and 3 post-inoculation, and the lung homogenates were prepared for quantitation of bacterial burden and IL-1␤ levels as previously published (41). All the animal procedures were performed in accordance with the Institutional Animal Care and Use Committees protocol approved by the New York Medical College.…”

Antioxidant Defenses of Francisella tularensis Modulate Macrophage Function and Production of Proinflammatory Cytokines

“…Mice were deeply anesthetized by intraperitoneal injection of ketamine/xylazine mixture and inoculated intranasally with 2 ϫ 10 3 cfu of sodBC mutant or F. tularensis LVS in a 20-l volume of PBS. Mice were sacrificed on days 1, 5, and 7 postinfection, and homogenates of lungs were prepared for quantitation of bacterial burden and pro-inflammatory cytokines following our previously published protocols (43,44).…”

Putting the Jigsaw Together - A Brief Insight Into the Tularemia

“…Despite the undisputed protective effect of the live vaccine strain, however, the vaccine was not licensed for human use due to difficulties with its standardization. To eliminate this problem researchers prepared targeted mutants of the LVS, the holarctica strain FSC 200, and the Schu S4 strain that are highly attenuated and are protective against challenge with virulent strains of F. tularensis [227][228][229]. Some of these may constitute a promising basis for the construction of a new live vaccine in the future.…”