Aerosol prime-boost vaccination provides strong protection in outbred rabbits against virulent type A Francisella tularensis

Abstract

Tularemia, also known as rabbit fever, is a severe zoonotic disease in humans caused by the gram-negative bacterium Francisella tularensis (Ft). While there have been a number of attempts to develop a vaccine for Ft, few candidates have advanced beyond experiments in inbred mice. We report here that a prime-boost strategy with aerosol delivery of recombinant live attenuated candidate Ft S4ΔaroD offers significant protection (83% survival) in an outbred animal model, New Zealand White rabbits, against aerosol challenge with 248 cfu (11 LD50) of virulent type A Ft SCHU S4. Surviving rabbits given two doses of the attenuated strains by aerosol did not exhibit substantial post-challenge fevers, changes in erythrocyte sedimentation rate or in complete blood counts. At a higher challenge dose (3,186 cfu; 139 LD50), protection was still good with 66% of S4ΔaroD-vaccinated rabbits surviving while 50% of S4ΔguaBA vaccinated rabbits also survived challenge. Pre-challenge plasma IgG titers against Ft SCHU S4 corresponded with survival time after challenge. Western blot analysis found that plasma antibody shifted from predominantly targeting Ft O-antigen after the prime vaccination to other antigens after the boost. These results demonstrate the superior protection conferred by a live attenuated derivative of virulent F. tularensis, particularly when given in an aerosol prime-boost regimen.

Development, characterization and standardization of a nose-only inhalation exposure system for exposure of rabbits to small particle aerosols containingFrancisella tularensis

“…These data suggest that BHI-grown SCHU S4 is a more rigorous challenge for demonstrating protection in vaccine studies than MHB-grown SCHU S4. Considering that we have previously shown that attenuated derivatives of SCHU S4 outperform LVS and protect rabbits well against both morbidity and mortality at even higher challenge doses of SCHU S4 grown in BHI, this further illustrates the superior performance of these attenuated derivatives as potential vaccine candidates (4–6). The survival of LVS-vaccinated rabbits and consistent/higher SF from the aerosol characterization studies both argue that BHI should be used in pivotal Ft efficacy studies to achieve a rigorous, reproducible aerosol challenge sufficient to demonstrate the efficacy of potential vaccine candidates in animals.…”

“…When rabbit minute volume was plotted by aerosol exposure days, there was day-to-day variation as shown in Figure 5B and these differences were significant by one-way ANOVA using Brown-Forsythe’s test ( p <0.0001). In the midst of these studies, we had also evaluated inhalation of small particle aerosols containing attenuated strains as a potential means of delivery for live vaccines (4). Using the data acquired during those studies, we evaluated whether repeated aerosol exposures of rabbits in the nose-only tubes would alter respiratory function (Supplemental Figure 3).…”

“…The cause of death in the rabbits appeared to be severe septic shock and/or acute respiratory distress syndrome. Additionally, we have shown that attenuated strains of Ft can protect rabbits against aerosol challenge with virulent SCHU S4 (4–6). The degree of protection against morbidity and mortality is a function of the attenuated strain used, the number of vaccinations, and the route of vaccination.…”

“…The degree of protection against morbidity and mortality is a function of the attenuated strain used, the number of vaccinations, and the route of vaccination. An aerosol prime/boost vaccination regimen with one particular SCHU S4-derived recombinant strain, S4ΔaroD, protected 66-85% of rabbits against aerosolized SCHU S4 at robust challenge doses (20-200 LD 50 ) (4).…”

Development, Characterization, and Standardization of a Nose-Only Inhalation Exposure System for Exposure of Rabbits to Small-Particle Aerosols Containing Francisella tularensis

“…The cause of death in the rabbits appeared to be severe septic shock and/or acute respiratory distress syndrome. Additionally, we have shown that attenuated strains of F. tularensis can protect rabbits against aerosol challenge with virulent SCHU S4 (4)(5)(6). The degree of protection against morbidity and mortality is a function of the attenuated strain used, the number of vaccinations, and the route of vaccination.…”