FIG. 4.

Flagellin promotes a protective response for intranasal infection with Yersinia pestis CO92. (a) Groups of 15 female BALB/c mice were immunized i.n. with 10 μg F1 antigen plus 1 μg flagellin (FliC) or PBS alone and boosted in an identical manner at 4 weeks. Plasma was collected 2 weeks postboost for analysis of antibody titers by ELISA (mean anti-F1 titer = 9.4 × 10⁵). One week later mice were challenged i.n. with a dose of Y. pestis CO92 equivalent to 100× the LD₅₀. Mice were monitored for 30 days postchallenge. (b) Groups of 10 antibody-deficient IgH^−/− mice were immunized i.n. with 10 μg F1 plus 1 μg FliC or PBS alone and boosted in an identical manner at 4 weeks. Mice were challenged 2 weeks later i.n. with a dose of Y. pestis equivalent to 155× the LD₅₀. (c and d) Groups of 10 female IFN-γ^−/− mice and wild-type C57BL/6 mice were immunized and boosted i.n. with 10 μg F1 plus 1 μg FliC. Plasma was collected 2 weeks postboost for analysis of antibody titers by ELISA (anti-F1 titer, ≥1 × 10⁶). One week later mice were challenged i.n. with a dose of Y. pestis equivalent to 150× the LD₅₀ and monitored for 16 days postchallenge.