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 as described in Figure 1. At the indicated days after challenge, cells were isolated from lung tissues and analyzed by flow cytometry. Panels depict (A) the percentage of CD4+CD43hiCD44hi cells, (B) the number of CD4+CD43hiCD44hi cells and (C) the number of CD8+CD43hiCD44hi cells. Data depict the mean and SD of 5 mice per group. Statistical significance was measured by one-way ANOVA with Bonferroni’s multiple comparison test and results are shown for p-only versus p/b (* P < 0.05, *** P < 0.001, ns, not significant).