Development of a Microencapsulated Anthrax Spore Vaccine in the Mouse Model for Oral Administration in Wildlife

Abstract

Anthrax (Bacillus anthracis) is a zoonotic disease endemic to environments worldwide. Spores, the dormant form of the bacteria, can survive for decades in nature’s harshest environments and maintain their viability to cause disease. Outbreaks are common in free-ranging livestock and wildlife, thus making anthrax an economically and ecologically important disease. The currently available vaccine to protect livestock is a suspension of B. anthracis Sterne Strain 34F2 spores in saponin (Sterne vaccine). However, it is only available as a subcutaneous injection which is an impractical method of prevention for wildlife. Oral vaccination is the ideal method for free-ranging wildlife, but the Sterne vaccine has never been thoroughly evaluated for oral administration. The current study evaluated the antibody titers induced in mice by subcutaneous or oral vaccination with three different doses of the Sterne vaccine. Results presented here demonstrate that the Sterne vaccine is only effective as a subcutaneous injection and that an alternate oral anthrax vaccine formulation must be developed, therefore we evaluated the immunogenicity and in vitro protective abilities of a controlled release vehicle when orally administered in mice. Microencapsulated B. anthracis Sterne strain 34F2 spores (Sterne spores), coated with poly-L-lysine and a non-immunogenic eggshell precursor protein, vitelline protein B, stimulated antibody production after subcutaneous and oral vaccination. This antibody response was shown to be protective against in vitro anthrax toxin challenge encouraging further development of a new oral anthrax vaccine formulation. We also investigated the stability of these microcapsules as a function of the Sterne spore dose and coating formulation. An increased Sterne spore dose greatly improved the measured antibody titer following subcutaneous injection while modifying the ratio of poly-L-lysine to vitelline protein B exhibited dose response effects with respect to microcapsule stability in gastrointestinal environments. Taken together, these results suggest that microencapsulation of Sterne spores in a controlled delivery vehicle can enhance the immune response following oral vaccination, therefore promising efficient and protective oral vaccination of free-ranging livestock and wildlife.