Transalveolar migration of leukocytes in response to LPS in vivo. Time course of leukocyte recruitment in BALF from male Mmp12^−/− and wild-type mice instilled intranasally with LPS (10 μg). Total number of leukocytes (A), neutrophils (B), and macrophages (C) in BALF are shown. Results are expressed as mean of 2 biological replicates (n = 2 animals per experiment) for 0-, 8-, 24-, and 48-hour time points and mean of 4 biological replicates (n = 2 animals per experiment) for 72-hour time point. Standard error bars are shown. P values calculated with Mann-Whitney nonparametric test. Secondary validation of the neutrophil influx was confirmed by MPO assay (P = .045; data not shown). (D) Tris-Tricine 15% SDS-PAGE gel analysis of MMP-12 cleavage of human monocyte chemoattractant proteins CCL2, -7, -8, and -13 with masses (m/z) obtained by MALDI-TOF MS. (E) Deconvolution of mass spectrometry data revealed that MMP-12 cleaved all 4 CC chemokines at position 4-5, a known inactivating cleavage.^11,22  Arrows indicate MMP-12 cleavage sites.