Development of Recombinant Domains of Protective Antigen of Bacillus anthracis and Evaluation of their Immune Response in Mouse Model for Use as Vaccine Candidates for Anthrax

Bacillus anthracis, the causative agent of anthrax is considered as the most important biological warfare agent. This Gram-positive, spore forming bacterium has three modes of infection i.e. cutaneous, inhalational and gastrointestinal in human. The principal virulence factors of this bacterium consist of an anti-phagocytic capsule composed of poly-D-glutamic acid and a secreted tripartite bacterial toxin composed of protective antigen (PA), lethal factor (LF) and edema factor (EF). PA is the pivotal protein of the anthrax toxin complex and immune response to PA is central to protection against B. anthracis. In this study, overlapping portions of four different domains of PA were cloned and expressed. The recombinant proteins were purified and used for immunization in mice. The ELISA results showed that all the domains elicited high antibody titres in vaccinated animals. However domain PAD3-4 showed the highest immune response against PA. Among the IgG subtypes, IgG1 response was predominant in all the immunized groups followed by IgG2. This indicated the induction of Th2 type immune responses against all the recombinant protein vaccine candidates. The study showed that the individual domains have also the potential as vaccine candidates for anthrax.