Oral Spore Vaccine Based on Live Attenuated Nontoxinogenic Bacillus anthracis Expressing Recombinant Mutant Protective Antigen

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Abstract
An attenuated nontoxinogenic nonencapsulated Bacillus anthracis spore vaccine expressing high levels of recombinant mutant protective antigen (PA), which upon subcutaneous immunization provided protection against a lethal B. anthracis challenge, was found to have the potential to serve also as an oral vaccine. Guinea pigs immunized per os with the recombinant spore vaccine were primed to B. anthracis vegetative antigens as well as to PA, yet only a fraction of the animals (30% to 50%) mounted a humoral response to all of these antigens. Protective immunity provided by per os immunization correlated with a threshold level of PA neutralizing antibody titers and was long-lasting. Protection conferred by per os immunization was attained when the vaccine was administered in the sporogenic form, which, unlike the vegetative cells, survived passage through the gastrointestinal tract. A comparison of immunization of unirradiated spores with immunization of γ-irradiated spores demonstrated that germination and de novo synthesis of PA were prerequisites for mounting an immune protective response. Oral immunization of guinea pigs with attenuated B. anthracis spores resulted in a characteristic anti-PA immunoglobulin isotype profile (immunoglobulin [G1 IgG1] versus IgG2), as well as induction of specific anti-PA secretory IgA, indicating development of mucosal immunity.