Subunit vaccines against
anthrax based on recombinant protective
antigen (PA) potentially offer more consistent and less reactogenic
anthrax vaccines but require adjuvants to achieve optimal immunogenicity. This study sought to determine in a murine model of pulmonary
anthrax infection whether the
polysaccharide adjuvant
Advax or the innate immune adjuvant
murabutide alone or together could enhance PA immunogenicity by comparison to an
alum adjuvant. A single immunization with PA plus
Advax adjuvant afforded significantly greater protection against aerosolized Bacillus anthracis Sterne strain 7702 than three immunizations with PA alone.
Murabutide had a weaker adjuvant effect than
Advax when used alone, but when
murabutide was formulated together with
Advax, an additive effect on immunogenicity and protection was observed, with complete protection after just two doses. The combined adjuvant formulation stimulated a robust, long-lasting B-cell memory response that protected mice against an
aerosol challenge 18 months postimmunization with acceleration of the kinetics of the anamnestic
IgG response to B. anthracis as reflected by ∼4-fold-higher anti-PA
IgG titers by day 2 postchallenge versus mice that received PA with
Alhydrogel. In addition, the combination of
Advax plus
murabutide induced approximately 3-fold-less
inflammation than
Alhydrogel as measured by in vivo imaging of
cathepsin cleavage resulting from injection of ProSense 750. Thus, the combination of
Advax and
murabutide provided enhanced protection against inhalational
anthrax with reduced localized
inflammation, making this a promising next-generation
anthrax vaccine adjuvanting strategy.