Mucosal Immunization with AttenuatedSalmonella entericaSerovar Typhi Expressing Protective Antigen of Anthrax Toxin (PA83) Primes Monkeys for Accelerated Serum Antibody Responses to Parenteral PA83 Vaccine

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Abstract
Salmonella entericag serovar Typhi vaccine strain CVD 908-htrA was genetically engineered for stable plasmidbased expression of protective antigen of anthrax toxin (PA83) fused with the export protein ClyA (ClyA-PA83). The priming potential of CVD 908-htrA expressing ClyA-PA83 was assessed in 12 rhesus and 20 cynomolgus macaques that were immunized mucosally (i.e., intranasally) on days 0 and 14. A parenteral booster with purified PA83 plus alum was given to rhesus macaques on days 42 and 225; cynomolgus monkeys received a booster with either PA or licensed anthrax vaccine (BioThrax; Emergent Biosolutions) only one time, 3 months after priming. Monkeys primed with S. Typhi expressing ClyA-PA83 developed high levels of serum toxin-neutralization activity (TNA) antibodies (50% effective dose [ED50],>1.3 × 103), 7 days after receipt of the booster, whereas unprimed controls lacked serumTNA(ED50, 0). In nonhuman primates, the success of this anthrax vaccine strategy based on heterologous mucosal priming followed by a parenteral subunit vaccine booster paves the way for clinical trials.