Anthrax is a zoonotic disease affecting most warm-blooded mammals. Primarily recognized as a disease of herbivores, it is caused by a spore-forming, rod-shaped bacterium, Bacillus anthracis. The disease has a worldwide distribution though, mostly of sporadic nature in developed countries due to effective vaccination and control measures. This feat was largely due to the introduction of the Sterne 34F2 live spore vaccine in the 1940s. While proving effective in controlling anthrax, the Sterne vaccine has a number of problems which, it is hoped, can be surmounted with the advent of recombinant peptides or DNA acellular alternatives. Such vaccines would overcome the issues of handling live B. anthracis during production and avoid batch to batch variation in content and immunogenicity and should improve the duration of immunity. Most importantly, these would also permit simultaneous anti-microbial treatment and vaccination of animals; allowing the early development of immunity in treated animals.
The principal virulence factors of B. anthracis are located on two plasmids pXO1 and pXO2. The pXO1 encodes the toxic factors; protective antigen (PA), lethal and oedema factors (LF and EF) respectively while pXO2 contains the encapsulation genes. The poly-?-D-glutamic acid capsule is poorly immunogenic and assists in post-infection dissemination of the organism. The capsule enables the anthrax bacilli to evade immune surveillance mechanisms and enter the circulatory system where it proliferates systemically. PA combines with LF, a zinc metalloprotease, to form lethal toxin (LT) that inactivates most mitogen-activated protein kinase kinases (MAPKK) and inflammasome-activating NLR1B leading to the impairment and death of susceptible macrophages. Oedema toxin (ET), formed by the binding of PA to EF a calmodulin dependent adenylate cyclase, disrupts fluid homeostasis across the host cell membranes.
The antigens PA, BclA (Bacillus collagen-like protein of anthracis) and FIS (formaldehyde inactivated spores), alone or in combination are known for their protective efficacy from previous studies in laboratory rodents. However, it remains to be elucidated if these vaccines or their combinations will elicit a protective immune response in goats against anthrax infections. In this study, these antigens in addition to various adjuvants were administered to goats in combinations. Also, we assessed the immunogenicity and efficacy of plasmid DNA vaccine encoding immunodominant domains of PA, LF and BclA using a heterologous DNA prime/protein boost approach. The aims were to assess the immunogenicity of these acellular vaccine candidates in goats and evaluate the protective capacity of the immune response in an in vivo A/J mouse passive protection model. Also, the immune response following simultaneous antibiotic treatment and immunization with either acellular or live spore vaccines were studied in goats. Attempts were made to compare the resulting immunity following booster vaccinations with the acellular or Sterne live spore vaccines.
Our findings indicated that the addition of FIS to recombinant PA and BclA vaccine candidates generated superior humoral immune responses compared to the recombinant peptides alone. Also, sera from goats vaccinated with the multi-component vaccine (rPA, rBclA and FIS) protected 73 % while the rPA+rBclA vaccinates sera protected 68 % of A/J mice against Sterne 34F2 spores in the passive protection test. Sera from goats primed with the plasmid DNA vaccine and boosted with FIS failed to protect any of the A/J mice in the challenge studies. However, caprine sera obtained following plasmid DNA vaccination and rPA and rBclA boosting protected just over 40 % of challenged mice.
Importantly, our results showed that the simultaneous administration of acellular vaccine candidates with antibiotics did not negate the development of crucial anti-toxin and anti-spore antibodies in goats. The immune responses from the latter did not differ from that induced in goats treated with acellular or Sterne live spore vaccines alone. The simultaneous administration of penicillin G with Sterne live spore vaccination, while not fully blocking the development of antibody titres, did obviate the production of antibodies in 60% of treated animals. In summary, these studies demonstrate the potential of utilizing a non-living vaccine candidate in the prevention and treatment of anthrax infections in a ruminant model.