The authors examined the efficacy of
Bacillus anthracis protective antigen (PA) combined with adjuvants as
vaccines against an
aerosol challenge of virulent
anthrax spores in rhesus macaques. Adjuvants tested included i)
aluminum hydroxide (
Alhydrogel), ii)
saponin QS-21 and iii)
monophosphoryl lipid A (MPL) in
squalene/
lecithin/
Tween 80 emulsion (SLT). Animals were immunized once with either 50 micrograms of recombinant PA plus adjuvant, or with
Anthrax Vaccine Adsorbed (AVA), the licensed human
anthrax vaccine. The serological response to PA was measured by
enzyme linked
immunosorbent assay. Lymphocyte proliferation and serum neutralization of in vitro lethal toxin cytotoxicity were also assayed. In all
vaccine groups, anti-PA
IgM and
IgG titers peaked at 2 weeks and 4-5 weeks postimmunization, respectively. Five weeks postimmunization, animals in all
vaccine groups demonstrated PA-specific lymphocyte proliferation and sera that neutralized in vitro cytotoxicity. Six weeks after immunization, the animals were challenged by
aerosol with approximately 93 LD50 of virulent
anthrax spores. Animals were bled daily for 1 week to monitor
bacteremia, and deaths were recorded. Anti-PA ELISA titers in all groups of immunized animals were substantially increased 2 weeks after challenge. One dose of each
vaccine provided significant protection (> 90%) against
inhalation anthrax in the rhesus macaques.