Mechanisms of the generation and maintenance of neutralizing, toxin-specific responses to anthrax vaccination

Current anthrax vaccination methods are predicated on the idea that virtually all individuals vaccinated will produce protective levels of IgG both directed against the primary immunogen, protective antigen (PA), as well as capable of neutralizing anthrax toxin in vitro. In order to generate and maintain protective anti-PA IgG, the U.S. anthrax vaccination, Anthrax Vaccine Adsorbed (AVA), is given in five priming doses as well as yearly boosters. However, a fraction of individuals who have received a full priming series and have been vaccinated within the year do not possess measurable anti-PA IgG (89/1356, 6.6%). In addition, of those that do possess anti-PA IgG, almost one-third (361/1267, 28.5%) do not neutralize toxin in vitro better than unvaccinated controls. Therefore, determining both the mechanism for the generation and maintenance of protective anti-PA IgG as well as the characteristics of anti-PA IgG that determine neutralization capacity is critical. Plasma concentrations of anti-PA IgG are positively associated with number of vaccinations received and negatively associated with time since most recent vaccination and African American race (5, 6). Here, we first demonstrate that in vitro toxin neutralization is not only a function of the magnitude of anti-PA IgG, but also of the avidity, IgG subclass, and domain specificity of anti-PA IgG. We further show that multiple factors are associated with the generation and maintenance of anti-PA IgG levels, including the MHC class II locus and the maintenance of antigen-specific memory B cell populations. We also demonstrate that two proposed mechanisms of inadequate anti-PA IgG production, general vaccine hyporesponsiveness and racial differences in PA receptor expression, are not associated with the humoral immune response to AVA. In the era of rapid adjuvant development and individualized medicine, we conclude that future anthrax vaccinations and protective approaches should be formulated in order to produce high avidity anti-PA IgG1 for specific neutralizing epitopes or select domains of PA. In addition, the vaccination schedule should be tailored for individuals based on demographic data, memory B cell response, and potentially MHC class II genotype.