CD40/CD40L is required for resolution of pulmonary inflammation. Bacterial pneumonia was induced in WT mice and CD40L^−/− mice by i.t. instillation of viable K. pneumoniae (1.5 × 10⁷ CFUs/mouse). Some WT mice received Fab fragments of blocking antibodies against P-selectin (clone RB40.34, 50 µg/mouse), CD40 (clone HM40-3, 50 µg/mouse i.v.), or isotype control 2 d after induction of pneumonia. (A) The percentage of apoptotic neutrophils in the BAL was analyzed by flow cytometry (data pooled from two independent experiments with four to six mice in each group; Mann-Whitney test was used for statistical analysis, *, P < 0.05). (B) The concentration of the cytokine IL-10 in the BAL fluid was analyzed by ELISA (data pooled from two independent experiments with four to six mice in each group; Mann-Whitney test was used for statistical analysis, *, P < 0.05). (C and D) 14 d after induction of pneumonia, (C) lung collagen content (estimated by measuring the absorbance of oxidized hydroxyproline in lung tissue homogenisates) and (D) lung compliance were analyzed (data pooled from three independent experiments with four to five mice in each group; Mann-Whitney test was used for statistical analysis, *, P < 0.05). Bacterial pneumonia was induced in WT mice by i.t. instillation of viable K. pneumoniae (1.5 × 10⁷ CFUs/mouse). Mice received blocking antibodies against CD40 (clone HM40-3, 50 µg/mouse i.v.) isotype control before induction of pneumonia. (E–G) At days 3 and 5, the number of (E) neutrophils and (F) T reg cells in the BAL fluid, and (G) the percentage of apoptotic neutrophils in the BAL were analyzed by flow cytometry (data pooled from two independent experiments with four to five mice in each group; Mann-Whitney test was used for statistical analysis, *, P < 0.05). (H) The concentration of IL-10 in the BAL was analyzed by ELISA (data pooled from two independent experiments with four to five mice in each group; Mann-Whitney test was used for statistical analysis, *, P < 0.05).