A Francisella tularensis Schu S4 Purine Auxotroph Is Highly Attenuated in Mice but Offers Limited Protection against Homologous Intranasal Challenge

Background: Francisella tularensis is a Gram-negative coccobacillus that causes the febrile illness tularemia. Subspecies that are pathogenic for humans include those comprising the type A (subspecies tularensis) or type B (subspecies holarctica) biovars. An attenuated live vaccine strain (LVS) developed from a type B isolate has previously been used to vaccinate at-risk individuals, but offers limited protection against high dose (> 1000 CFUs) challenge with type A strains delivered by the respiratory route. Due to differences between type A and type B F. tularensis strains at the genetic level, it has been speculated that utilization of an attenuated type A strain as a live vaccine might offer better protection against homologous respiratory challenge compared with LVS. Here, we report the construction and characterization of an unmarked Delta purMCD mutant in the highly virulent type A strain Schu S4. Methodology/Principal Findings: Growth of Schu S4 DpurMCD was severely attenuated in primary human peripheral blood monocyte-derived macrophages and in the A549 human lung epithelial cell line. The Schu S4 DpurMCD mutant was also highly attenuated in mice when delivered via either the intranasal or intradermal infection route. Mice vaccinated intranasally with Schu S4 DpurMCD were well protected against high dose intradermal challenge with virulent type A or type B strains of F. tularensis. However, intranasal vaccination with Schu S4 DpurMCD induced tissue damage in the lungs, and conferred only limited protection against high dose Schu S4 challenge delivered by the same route. The level of protection observed was similar to that conferred following vaccination with wild-type LVS or the analogous LVS DpurMCD mutant. Conclusions/Significance: Collectively, these results argue that development of the next generation live attenuated vaccine for Francisella should be based on use of the less pathogenic type B biovar rather than the more reactogenic type A biovar.