Neutralizing antibodies and persistence of immunity following anthrax vaccination

Anthrax toxin consists of protective antigen (PA) and two toxic components, lethal factor (LF) and edema factor (EF). PA binds to mammalian cellular receptors and delivers the toxic components to the cytoplasm. PA is the primary antigenic component of the current anthrax vaccine. Immunity is due to the generation of antibodies that prevent the PA-mediated internalization of LF and EF. In this study, we characterized sera obtained from vaccinated military personnel. Anthrax vaccine is administered in a series of six injections at 0, 2, and 4 weeks and 6, 12, and 18 months, followed by annual boosters. The vaccination histories of the subjects were highly varied; many subjects had not completed the entire series, and several had not received annual boosters. We developed a simple colorimetric assay using alamarBlue dye to assess the antibody-mediated neutralization of LF-mediated toxicity to the J774A.1 murine macrophage cell line. Recently vaccinated individuals had high antibody levels and neutralizing activity. One individual who had not been boosted for 5 years had low immunoglobulin G antibody levels but a detectable neutralization activity, suggesting that this individual produced low levels of very active antibodies.