CXCL1 controls neutrophil recruitment, bacterial clearance, and survival in pneumococcal lung infection. WT and Cxcl1^−/− mice were infected intratracheally with S pneumoniae 6303 (5 × 10⁴ CFU; A-C), A66.1 (2 × 10⁵ CFU; D), WU2 (5 × 10⁷ CFU; E) strains or PBS (control). (A) Total number of neutrophils in BALF. (B) The bacterial burden in the BALF and spleen was quantitated at 48 hours postinfection. (C-E) Survival was monitored up to 8 days. A Kaplan-Meier plot is used to show survival of mice from each group. (F-G) C57BL/6J, A/J, and BALB/cJ mice were treated with anti-CXCL1/KC mAb or IgG i.p. at 24 and 2 hours before infection and infected intratracheally with S pneumoniae 6303 (5 × 10⁴ CFU). (F) Neutrophil counts in BALF were measured through the diff-quick method. (G) The bacterial burden in the lungs was quantitated. (H-J) Bone marrow reconstituted mice were infected intratracheally with S pneumoniae 6303 (5 × 10⁴ CFU). Neutrophil count in BALF (H) and bacterial burden in BALF (I) and lungs (J) were quantitated at 48 hours postinfection (n = 5-6 mice/infection group; n = 10 mice/survival; n = 3 mice/control group). All experiments were performed 3 times, with the exception of the survival and chimera studies, which were performed twice. Statistical significance was determined by unpaired t-test (A,F), Mann-Whitney (B,G), log-rank (C-E), and 1-way ANOVA (followed by Bonferroni’s post hoc comparisons; H-J). *P < .05; **P < .01.