B. anthracis is the causative agent of
anthrax. Pathogenesis is primarily mediated through the
exotoxins lethal factor and
edema factor, which bind protective
antigen (PA) to gain entry into the host cell. The current
anthrax vaccine (AVA,
Biothrax) consists of
aluminum-adsorbed cell-free filtrates of unencapsulated B. anthracis, wherein PA is thought to be the principle target of neutralization. In this study, we evaluated the efficacy of the natural adjuvant, C3d, versus
alum in eliciting an anti-PA humoral response and found that C3d conjugation to PA and
emulsion in
incomplete Freund's adjuvant (IFA) imparted superior protection from
anthrax challenge relative to PA in IFA or PA adsorbed to
alum. Relative to
alum-PA, immunization of mice with C3d-PA/IFA augmented both the onset and sustained production of PA-specific
antibodies, including
neutralizing antibodies to the receptor-binding portion (domain 4) of PA. C3d-PA/IFA was efficacious when administered either i.p. or s.c., and in adolescent mice lacking a fully mature B cell compartment. Induction of PA-specific
antibodies by C3d-PA/IFA correlated with increased efficiency of germinal center formation and plasma cell generation. Importantly, C3d-PA immunization effectively protected mice from intranasal challenge with B. anthracis spores, and was approximately 10-fold more effective than
alum-PA immunization or PA/IFA based on dose challenge. These data suggest that incorporation of C3d as an adjuvant may overcome shortcomings of the currently licensed
aluminum-based
vaccine, and may confer protection in the early days following acute
anthrax exposure.