Early tests on an experimental anthrax vaccine suggest that it could protect against anthrax attacks by bioterrorists. The vaccine, which spurred the immune system to attack anthrax's deadly toxin and a close cousin of the microbe, expands the list of alternatives to the current vaccine, which is less than ideal for widespread use.
The deadliest cases of anthrax begin when people inhale dormant bacterial cells called spores. When these come to life, they spread like wildfire and pump out two toxins that cripple immune cells. The only currently available anthrax vaccine, anthrax vaccine adsorbed (AVA), is given primarily to soldiers. But it's difficult to manufacture reliably and sometimes causes headaches, sore muscles, fever, chills, and nausea. It's also slow: Six shots over 18 months are needed to boost immunity enough to protect people from inhalational anthrax.
AVA stops infections by provoking the immune system to make antibodies that block the toxins. Biochemist Julia Wang of Harvard Medical School in Boston and her colleagues figured that a vaccine that also triggered direct attacks on the bacteria might speed up protection with fewer side effects. So they chemically linked a toxin protein called protective antigen (PA), which is the key ingredient of the current vaccine and leading candidate vaccines in the pipeline, to a polypeptide called PGA that coats anthrax bugs.
Mice injected just three times with this combination vaccine developed a powerful contingent of antibodies. In test tubes, the antibodies bound to PA and blocked it from killing cultured hamster cells. The mice also developed antibodies against PGA, and their immune cells destroyed more than 80% of a close cousin of anthrax in test tube experiments. The vaccine completely protected mice from lethal doses of anthrax toxin, according to results published online this week in the Proceedings of the National Academy of Sciences.
Despite these encouraging results, microbiologist Michèle Mock of the Pasteur Institute in Paris says that the acid test is whether the vaccine protects the mice against actual infections by virulent anthrax. (The researchers were unable to gain access to the handful of labs in the United States where live animals can be infected with the deadliest strains of anthrax.) But microbiologist Phil Baker of the National Institute of Allergy and Infectious Diseases, which funded the work, says, "it's a very important observation ... whose relevance remains to be determined."