Efficacious, nontoxigenic Bacillus anthracis spore vaccines based on strains expressing mutant variants of lethal toxin components.

We reported previously on the development of a Bacillus anthracis vaccine strain expressing high levels of recombinant protective antigen (rPA) [Cohen et al., Infec Immun 2000;68(8):4549-58]. To further explore the potential of the B. anthracis platform, we generated several attenuated strains expressing lethal toxin components PA and LF, which are biologically inactive, yet retain their antigenic properties. A single injection of 5 x 10(7) spores of one of these strains, carrying PA mutation at a site involved in effector translocation (residues 313-314) was shown to resemble wild type PA in inducing production of high levels of anti-PA neutralizing antibodies and producing effective protective immunity for 12 months. Long-term protection and persistence of functional antibody titers was observed after the gradual elimination of spores from guinea pig tissues 3 months after injection and in the measurable absence of bacteria in tissues. The mutant toxin components could, thus be an effective alternatives to their native counterparts when presented to the immune system in context of a live B. anthracis strain. These live vaccine prototypes may serve as a platform for future multi-component vaccines.