, 1994; Ritchie & Waldor, 2005; Mann et al., 2007). Also present on the surface of Y. pestis is the highly immunogenic F1 capsular antigen which composes a proteinaceous capsule (Meyer et al., 1974a, b; Friedlander et al., 1995). The expression of the F1 antigen is find more temperature regulated and encoded by the

caf operon on the pFra plasmid (Chen & Elberg, 1977; Galyov et al., 1990). The capsule is synthesized in large quantities (Davis et al., 1996) and allows Y. pestis to be antiphagocytic and prevents adhesion to epithelial cells (Williams et al., 1972; Liu et al., 2006). Currently, there is no approved plague vaccine for human use in the United States. The killed whole cell-based vaccine (Plague vaccine, USP) was discontinued in 1999 because it does not protect against pneumonic plague (Heath et al., 1998), the

most likely learn more disease route for use of Y. pestis as a bioweapon. The recombinant F1-LcrV fusion protein was demonstrated to be protective in an animal model of pneumonic plague (Powell et al., 2005). However, adding to the difficulties of developing a successful vaccine, the LcrV antigen is very heterogeneous across Yersinia species (Anisimov et al., 2007). Live vaccines offer exposure to the full antigenic spectrum from a pathogen and would not be subject to the limitations encountered with vaccine development based on a limited set of recombinant proteins. This strategy has been used in preventing infectious diseases by many pathogens (Agin et al., 2005; Feunou et al., 2008; Pasquali et al., 2008), but the only human-approved, live bacterial vaccine currently available for research

purposes in the U.S. is the attenuated LVS strain of Francisella tularensis (Isherwood et al., 2005). Based on an attenuated Pgm− strain, the live EV76 vaccine against Y. pestis is protective against pneumonic plague and induces a high antibody titer (Byvalov et al., 1984), but its use has been discontinued due to chronic infections and adverse reactions (Meyer et al., 1974a, b; Welkos et al., 2002). The use of genetically engineered attenuated pathogens as vaccines, on the other hand, offers the potential to circumvent such deleterious side effects. In the current Tenofovir work, we show that a ΔyscN Y. pestis mutant is highly attenuated in mice but also protects them against lethal doses of the fully virulent CO92 strain in a subcutaneous (s.c.) model of plague. The fully virulent CO92 parent strain (Doll et al., 1994), ΔyscN mutant (Swietnicki et al., 2011), and CO92 pLcr− (USAMRIID collection) strains of Y. pestis were maintained on sheep blood agar plates or in heart infusion (HI) broth. When growth occurred at 37 °C, HI broth was supplemented with either 2.5 mM CaCl2 or 20 mM MgCl2 and 20 mM sodium oxalate (MOX), as indicated.