Date Published: June 22, 2015
Publisher: Public Library of Science
Author(s): Sukalyani Banik, Ahd Ahmed Mansour, Ragavan Varadharajan Suresh, Sherri Wykoff-Clary, Meenakshi Malik, Alison A. McCormick, Chandra Shekhar Bakshi, Ashlesh K Murthy.
Francisella tularensis is a facultative intracellular pathogen, and is the causative agent of a fatal human disease known as tularemia. F. tularensis is classified as a Category A Biothreat agent by the CDC based on its use in bioweapon programs by several countries in the past and its potential to be used as an agent of bioterrorism. No licensed vaccine is currently available for prevention of tularemia. In this study, we used a novel approach for development of a multivalent subunit vaccine against tularemia by using an efficient tobacco mosaic virus (TMV) based delivery platform. The multivalent subunit vaccine was formulated to contain a combination of F. tularensis protective antigens: OmpA-like protein (OmpA), chaperone protein DnaK and lipoprotein Tul4 from the highly virulent F. tularensis SchuS4 strain. Two different vaccine formulations and immunization schedules were used. The immunized mice were challenged with lethal (10xLD100) doses of F. tularensis LVS on day 28 of the primary immunization and observed daily for morbidity and mortality. Results from this study demonstrate that TMV can be used as a carrier for effective delivery of multiple F. tularensis antigens. TMV-conjugate vaccine formulations are safe and multiple doses can be administered without causing any adverse reactions in immunized mice. Immunization with TMV-conjugated F. tularensis proteins induced a strong humoral immune response and protected mice against respiratory challenges with very high doses of F. tularensis LVS. This study provides a proof-of-concept that TMV can serve as a suitable platform for simultaneous delivery of multiple protective antigens of F. tularensis. Refinement of vaccine formulations coupled with TMV-targeting strategies developed in this study will provide a platform for development of an effective tularemia subunit vaccine as well as a vaccination approach that may broadly be applicable to many other bacterial pathogens.
Francisella tularensis is the causative agent of a fatal human disease known as tularemia [1,2]. Francisella is divided into two species; tularensis and philomiragia [3,4]. There are four subspecies of F. tularensis: tularensis (also known as F. tularensis type A), novicida, mediasiatica and holarctica (also known as F. tularensis type B) . Of these, only F. tularensis subsp. tularensis and subsp. holarctica can cause disease in immunocompetent humans. F. tularensis SchuS4 strain is a representative of F. tularensis subspecies tularensis. F tularensis has been classified as Tier 1 Category A Select Agent, which is the highest priority category on the list of CDC among other potential Biothreat agents . The bioweapon potential of F. tularensis is on account of its extreme virulence, low infectious dose, ease of aerosol dissemination and capacity to cause severe illness and death in a very short period of time . No licensed vaccine is currently available in the USA for prevention of tularemia [8,9]. Considering the bioweapon potential of F. tularensis and repercussions of 2001 anthrax attack in the USA, there has been an increased interest in development of vaccine and effective countermeasures against bioterror agents. An ideal solution for prevention of tularemia occurring naturally or consequent to the use of Francisella as a bioweapon or an act of bioterrorism is to develop a safe and effective vaccine capable of inducing long lasting protection in a relatively short period of time .
The possibility of using F. tularensis as a bioterror agent has renewed attention towards F. tularensis research and to develop a licensable vaccine for effective prevention of tularemia. The tularemia vaccine development research has largely been focused on development of live attenuated or inactivated tularemia vaccines. However, concerns about their efficacy and safety have halted the progress. A recent study confirms this notion and reported a variant of F. tularensis LVS which is 100 times lethal than the standard ATCC strain indicating that, as feared, F. tularensis LVS may revert back to its virulent form . Recombinant subunit vaccines obviously have potential safety advantages over inactivated or live attenuated vaccines.