New technology for development of host-targeted therapeutic vaccine against anthrax

Abstract Anthrax is a serious disease caused by Bacillus anthracis, a bacterium that can form spores for long-term survival and secrete anthrax toxins including protective antigen (PA), lethal factor (LF), and edema factor (EF). However, current anthrax postexposure treatments are inadequate at later stages of infection. Previously, our research have shown RNA inhibition (RNAi) on host cells silences anthrax toxin receptors (ATRs): tumor endothelial marker 8 (TEM8), capillary morphogenesis protein 2 (CMG2). This approach was protective against the cytotoxicity of anthrax toxins, indicating that ATR-targeted RNAi can be used as a therapy against anthrax. However, inefficient cytosolic delivery and toxicity of siRNA delivery vehicles limit the use siRNA as therapeutics. In this study, we have developed a detoxified anthrax edema toxin, which consists of PA and nontoxic N-terminal fragment of EF (EFn) conjugated with a peptide nona-D-arginine residues (EFn-9dR) to enable siRNA binding. The detoxified toxin was able to deliver specific siRNA to induce cmg2 gene silencing in different cell lines and C57BL/6 mice, and provide significant protection against anthrax lethal toxin challenge in vitro and in mice. Survived mice from toxin challenge were fully protected against lethal challenge with B. anthracis Sterne spores. The immune protective mechanism is mainly due to the significantly high serum neutralizing antibody response against anthrax toxins in these mice. These results suggest that detoxified anthrax toxin provides a tool for delivery of host-targeted siRNA into anthrax pathogenesis-associated host cells, and this new system could potentially be developed as a lifesaving postexposure therapeutic vaccine against anthrax.