B cell–deficient µMT mice were primed (p-only; triangles) or prime/boosted (p/b; squares) with D27-pLpxL, or left unvaccinated (unvac; circles), and then challenged intranasally with 200 MLD Y. pestis strain D27. At the time of challenge, the mice were treated with neutralizing mAb specific for IL-17 (aIL-17; solid symbols) or an isotype-matched control mAb (RIgG2a; open symbols). In parallel, sex- and age-matched naïve mice were challenged with same dose of strain D27 without mAb treatment (unvac nil). (A and B) Survival decreased significantly in prime/boost mice (B) but not in prime-only mice (A) treated with IL-17 mAb, as compared with mice treated with control mAb (P < 0.05 by log rank test; ns, not significant; n=17–25 mice per group; data pooled from 3 independent experiments). (C and D) Bacterial burden at day 4 after challenge. Neutralizing IL-17 modestly increased the bacterial burden in the lungs of prime-only mice, but did not significantly affect the pulmonary or hepatic burden in prime/boost mice (* P < 0.05 by Mann-Whitney test; ns, not significant; data are pooled from two independent experiments). The solid bar depicts the median and the broken line depicts the limit of detection.