Scientific Publications by FDA Staff

Clin Vaccine Immunol 2010 Jun;17(6):895-903

Comparison of Three Anthrax Toxin Neutralization Assays.

Ngundi MM, Meade BD, Lin TL, Tang WJ, Burns DL

Different types of anthrax toxin neutralization assays have been utilized to measure the antibody levels elicited by anthrax vaccines in both nonclinical and clinical studies. In this study, we sought to determine whether three commonly used toxin neutralization assays-a J774A.1, a RAW 264.7 and a CHO cell-based assay-yield comparable estimates of neutralization activities for sera obtained after vaccination with anthrax vaccines composed of recombinant protective antigen (rPA). In order to compare among assays, sera were assayed alongside a common reference serum and neutralization titers were expressed relative to the reference in each assay. Analysis of sera from rabbits immunized with multiple doses of rPA vaccine showed that, for later bleeds, quantitative agreement between assays was good, however for early bleeds, some heterogeneity in relative neutralization estimates was observed. Analysis of serum samples from rabbits, non-human primates and humans immunized with rPA vaccine showed that relative neutralization estimates obtained in the different assays agreed to varying extents depending on the species of origin of the sera examined. We identified differences in magnitude of Fc receptor-mediated neutralization associated with the J774A.1 and RAW 264.7 cell-based assays which may account for some of the species dependence of the assays. The differences in relative neutralization estimates among the assays were relatively small and always less than 2.5 fold. However, because toxin neutralization assays will likely be used to establish efficacy of new anthrax vaccines, our findings should be considered when interpreting assay output.