Immunohistochemical localization of fibrin. Lung sections from control mice (A) and PA103 ΔexoU-infected mice (B) exhibited minimal infiltration of inflammatory cells in interalveolar septum (arrow in B) and barely detectable fibrin staining. In sections from mice infected with the ExoU-producing bacteria, dark brown stained fibrin was detected all over the lung parenchyma (C; asterisk). Photomicrographs D-F exhibit higher magnifications of sections from PA103-infected mice. Arrows point to fibrin deposition in interalveolar septum (D) and inside blood vessels (E and F). Note in F, the huge edema in interalveolar space (asterisks). Original magnifications: A-C, × 20; D-F, × 100. Photomicrographs are representative of sections from seven animals from each group.