Anthrax is considered a major bioterrorism threat because the spores of Bacillus anthracis are very hardy, easy to disperse, and potentially easy to obtain; anthrax vaccination is considered a critical component of the defense against this agent. Currently, there is one US-licensed anthrax vaccine and new vaccines are under development. The major antigen in this preparation is believed to be a protein known as protective antigen (PA) and research in animal models has indicated that antibodies to PA can protect against B. anthracis. Efforts are under way to develop and standardize improved quality control tests for anthrax vaccines and to evaluate and standardize assays that can be used to evaluate the immune response in humans and relevant animal models. Quality control: Improved laboratory tests to assess the safety, purity, and potency of anthrax vaccines will enhance the control of the licensed product and expedite licensure of new generation immunogens. Currently available methods assess the potency of anthrax vaccines by evaluating protection from a lethal challenge with virulent spores; the goals of the project are to develop non-lethal animal methods that do not require viable bacteria, and possibly in vitro methods that eliminate the need for laboratory animals. Development and evaluation are in progress of a mouse model in which the immune response to specific components of the vaccine is measured. FY 2001 activities: 1) Performance of immunoassays that assess the PA-binding and lethal toxin-neutralizing properties of antibodies is being evaluated and optimized. 2) These assays have been used to evaluate the immune response of two strains of mice to varying doses of licensed anthrax vaccine. 3) Preparation, under contract, of a mouse reference anti-PA serum is in progress. This activity required the laboratory to complete a pilot study to determine an appropriate dose and schedule for immunization. Clinical serologic assays: Development and standardization of appropriate immunologic assays is essential to the evaluation of experimental anthrax vaccines, since efficacy trials in humans will be impossible to conduct because of the low incidence of disease caused by these organisms and the toxicity of these agents that preclude human challenge studies. Thus information obtained from animal challenge studies will have to be translated into correlates of protection in humans. CBER has initiated studies to assist with the development, validation, and international standardizationof assays to assess the immune response to vaccination. FY 2001 activities: 1) To date, the tissue culture based assay has been used to measure lethal toxin-neutralizing antibodies on a limited number of human sera; 2) enzyme immunoassays to measure human antibodies to PA are under initial development.
