Evaluation of the protective efficacy of recombinant protective antigen vaccine (GC1109)-immunized human sera using passive immunization in a mouse model

The protective efficacy of human sera from vaccinated individuals with a new recombinant protective antigen anthrax vaccine (GC1109) against lethal spore challenge was evaluated in a mouse model. Eighteen human sera were selected from the vaccinated individuals based on their toxin neutralizing assay (TNA) titer (ED₅₀ of 55 to 668). The selected sera were diluted and passively transferred to A/J mice and the mice were subsequently challenged with 100 × LD₅₀ of Bacillus anthracis Sterne spores. The correlation between the survival rate of passively immunized mice and the TNA ED₅₀ of transferred sera was presented (r = 0.873, P-value < 0.001). The estimated TNA titer for 50% survival rate against lethal challenge was 197 (95% confidence interval of 149 and 260). The result suggest that GC1109 is protective against exposure to B. anthracis and the TNA titer of vaccinated serum can be an indicator for protective efficacy.