OMP and plague: One week after the last booster, mice were challenged via the s.c. route with 500 LD50 (2.5 × 104 CFU) of the Y. pestisΔcaf1 [336] mutant (Ail, Omp, and Pla antigens) or 15 LD50 (i.n., 7500 CFU) of the same mutant in the pneumonic plague model.