Development of a Multivalent Subunit Vaccine against Respiratory Tularemia

Abstract Francisella tularensis (Ft); the causative agent of a fatal human disease tularemia is classified as a Category A Select Agent. No licensed vaccine is available for prevention of tularemia in the U.S.A. In this study, we used a novel Tobacco Mosaic Virus (TMV) based delivery platform for development of a fully protective multi-antigen subunit tularemia vaccine. Previously we have published that a trivalent TMV-conjugate vaccine confers 50% protection in immunized mice against respiratory Ft LVS challenge. In this study, we refined TMV-conjugate vaccine formulation to improve the level of protection in immunized C57BL/6 mice against respiratory tularemia. We developed a tetravalent vaccine by conjugating OmpA, DnaK, Tul4 and SucB proteins of Francisella to TMV. CpG adjuvant was also included in the vaccine formulation. C57BL/6 mice were immunized intranasally (i.n.) on days 0, 14 and 28 and challenged with 10LD100 of Ft LVS on day 49 post-primary immunization (PPI). Mice were monitored for survival, weight loss, antibody and recall responses, and duration of immune protection. 100% of immunized mice were protected against a 10LD100 i.n. challenge dose of Ft LVS. Mice vaccinated with TMV-tetravalent vaccine showed high levels of TH1 antibodies than those with trivalent vaccine formulation. The challenged mice exhibited significantly reduced bacterial burden in lungs, liver and spleen. Strong ex-vivo recall responses were observed in immunized mice as late as day 84 PPI and 80% of mice survived when challenged 163 days PPI. Collectively, this study demonstrates that tetravalent vaccine formulation provides complete protection, induces strong protective and memory immune responses against respiratory challenge with F. tularensis.