Immunohistochemical localization of PAI-1. In lung sections from control mice (A) and from mice infected with the bacterial mutant (B) PAI-1 staining was almost resctricted to the bronchiolar epithelium (arrows). Sections from mice infected with the ExoU-producing bacteria exhibited dark brown diaminobenzidine signals in intraalveolar fibrilar material reminiscent of hyaline membrane (arrows in C and E) and in type II pneumocytes (arrows in D). Original magnification: A-B, × 20; C, × 40; D and E, × 100. Photomicrographs are representative of sections from seven animals from each group.