Anthrax vaccines primarily relying only on protective
antigen (PA), the cell binding component in
anthrax toxins provide incomplete protection when challenged with spores of virulent encapsulated Bacillus anthracis strains. Alternatively,
formaldehyde inactivated spores (FIS) or recombinant spore components generate anti-spore immune responses that inhibit the early stages of
infection and augment the PA protective efficacy. In the present study domain IV of PA was spliced with exosporium
antigen BxpB via a flexible G4S linker to generate a single functional
antigen r-PAbxpB that was further assessed for its protective efficacy against
anthrax toxins and spore
infection. Immunization of mice with r-PAbxpB elicited significantly high titer
antibodies comprising
IgG1:
IgG2a isotypes in 1:1 ratio, balanced up-regulation of both Th1 (
IL2,
IL12, IFN-γ) and Th2 (
IL4,
IL5,
IL10)
cytokines and high frequencies of CD4+ and CD8+ T cell subsets. The anti-r-PAbxpB
antibodies significantly enhanced spore phagocytosis, and killing within macrophages; inhibited their germination to vegetative cells and completely neutralized the
anthrax toxins as evidenced by the 100% protection in passive transfer studies. Active immunization with r-PAbxpB provided 100 and 83.3% protection in mice I.P. challenged with 5 × LD100 LD of toxins and 5 × 104 cfu/ml Ames spores, respectively while the
sham immunized group succumbed to
infection in 48 h. Therefore, the ability of r-PAbxpB to generate protective immune responses against both spores and toxin and provide significant protection suggests it as an efficient
vaccine candidate against B. anthracis
infection.