Live Attenuated Mutants of Francisella tularensis Protect Rabbits against Aerosol Challenge with a Virulent Type A Strain

Abstract

c Francisella tularensis, a Gram-negative bacterium, is the causative agent of tularemia. No licensed vaccine is currently available for protection against tularemia, although an attenuated strain, dubbed the live vaccine strain (LVS), is given to at-risk laboratory personnel as an investigational new drug (IND). In an effort to develop a vaccine that offers better protection, recombinant attenuated derivatives of a virulent type A strain, SCHU S4, were evaluated in New Zealand White (NZW) rabbits. Rabbits vaccinated via scarification with the three attenuated derivatives (SCHU S4 ⌬guaBA, ⌬aroD, and ⌬fipB strains) or with LVS developed a mild fever, but no weight loss was detected. Twenty-one days after vaccination, all vaccinated rabbits were seropositive for IgG to F. tularensis lipopolysaccharide (LPS). Thirty days after vaccination, all rabbits were challenged with aerosolized SCHU S4 at doses ranging from 50 to 500 50% lethal doses (LD 50 ). All rabbits developed fevers and weight loss after challenge, but the severity was greater for mock-vaccinated rabbits. The ⌬guaBA and ⌬aroD SCHU S4 derivatives provided partial protection against death (27 to 36%) and a prolonged time to death compared to results for the mock-vaccinated group. In contrast, LVS and the ⌬fipB strain both prolonged the time to death, but there were no survivors from the challenge. This is the first demonstration of vaccine efficacy against aerosol challenge with virulent type A F. tularensis in a species other than a rodent since the original work with LVS in the 1960s. The ⌬guaBA and ⌬aroD SCHU S4 derivatives warrant further evaluation and consideration as potential vaccines for tularemia and for identification of immunological correlates of protection.

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.…”

“…One argument against using the rabbit for tularemia vaccine studies had been that vaccination with LVS extended time to death but not survival (5, 6, 10, 21). Our own prior studies are included in those findings, including the observation that oral inoculation or inhalation of LVS did extend rabbit time to death more than subcutaneous inoculation (5).…”

“…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.…”

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.…”

Tularemia — Serious Zoonotic Disease

“…The name of the bacterium comes from the name of the researcher, Dr. Edward Francis, dealing with these pathogens (McCoy and Chapin 1912). Although the micro-organism is pathogenic to 190 species of "animals", clinical symptoms occur mainly in lagomorphs and rodents (Glinski and Kostro 2005, Reed et al 2014). The natural reservoirs are murine, muskrats, water rats, ground squirrels, voles and rabbits (Rastawicki and Jagielski 2005, Osiak et al 2006).…”