USARMIID study uses anthrax capsule to protect against attack

A study using the capsule of the disease-causing anthrax bacterium protected monkeys from deadly infection, which represents the first successful use of non-toxin vaccine to protect monkeys from the disease.

Bacillus anthracis, the anthrax-causing bacterium, produces three main components - lethal toxin, edema toxin and the capsule. During anthrax infection, the bacterium invades the host and increases its concentrations. The capsule surrounds the bacterium to prevent its ingestion and to protect it from the white blood cells. In using the capsule as a vaccine during the study, anti-capsule antibody responses occurred in rabbits and monkeys.

"This is the first non-toxin anthrax vaccine shown to be protective in monkeys," Arthur M. Friedlander, a scientist with the U.S. Army Medical Research Institute of Infectious Diseases and the senior author of the study, said. "In addition, this new capsule vaccine is expected to work against possible vaccine-resistant strains of anthrax, as well as in recipients whose immune systems may not respond to protective antigen alone."

Current vaccines against anthrax in humans are based on a protective antigen, which is a component of anthrax toxins. While PA does confer protection alone, researchers, including Friedlander, have concerns about the reliance on a single antigen. Capsules from bacteria have been used in licensed vaccines for other diseases, including those for types of meningitis and pneumonia.

In the study, rabbits were not protected against a high aerosol challenge dose, though a significant number of monkeys that received the capsule vaccine survived. The results of the study suggest that adding a capsule to a protective antigen to create a vaccine of multiple components could enhance and broaden protection. The next step, according to Friedlander, would be to do a larger study in monkeys looking at different capsule vaccine doses.