Fas/Fas ligand system mediates epithelial injury, but not pulmonary host defenses, in response to inhaled bacteria

Abstract

The Fas/Fas ligand (FasL) system has been implicated in alveolar epithelial cell apoptosis during pulmonary fibrosis and acute respiratory distress syndrome. However, Fas ligation can also lead to cell activation and cytokine production. The goal of this study was to determine the role of the Fas/FasL system in host defenses against Escherichia coli, Staphylococcus aureus, and Streptococcus pneumoniae. We administered bacteria by aerosolization into the lungs of Fas-deficient (lpr) mice and wild-type (C57BL/6) mice and measured bacterial clearance at 6 and 12 h. One hour prior to euthanasia, the mice received an intraperitoneal injection of human serum albumin (HSA) for alveolar permeability determinations. At all times after bacterial challenges, the lungs of the lpr mice contained similar or lower numbers of bacteria than those of the C57BL/6 mice. Alveolar permeability changes, as determined by bronchoalveolar lavage fluid HSA concentrations, were less severe in the lpr mice 6 h after the challenges. In response to E. coli, the lpr mice had significantly more polymorphonuclear leukocytes (PMN) and macrophage inflammatory protein 2 in the lungs, whereas histopathologic changes were less severe. In contrast, in response to the gram-positive cocci, the lpr animals had similar or lower numbers of PMN. We conclude that the Fas/FasL system contributes to the development of permeability changes and tissue injury during-gram negative bacterial pneumonia. The Fas/FasL system did not have a major role in the clearance of aerosolized bacteria from the lungs at the bacterial doses tested.

FIG. 1

: Bacterial clearance from the lungs of C57BL/6 (C57) and lpr mice, expressed as the percentage of bacterial recovery immediately after aerosol exposure. Bacterial deposition at time zero (T = 0) was 1.44 × 10⁶ CFU/lung for E. coli, 1.81 × 10⁸ CFU/lung for S. aureus, and 4.55 × 10⁴ CFU/lung for S. pneumoniae. There were six or more animals in each group at each observation time. Data are means and standard errors and are expressed as bacterial recovery (CFU) per gram of lung. Asterisks indicate a P value of <0.05.

FIG. 2

Total PMNs in the BALF at 0, 6, or 12 h following aerosolization of E. coli, S. aureus, or S. pneumoniae. There were six or more animals at each observation time. C57, C57BL/6. Data are means and standard errors. Asterisks indicate a P value of <0.05.

FIG. 3

MPO activities (units per milliliter per minute) in lung homogenates from C57BL/6 (C57) and lpr mice at 0, 6, and 12 h following aerosolization of E. coli, S. aureus, or S. pneumoniae. Data are means and standard errors.

FIG. 4

HSA concentrations in BALF from C57BL/6 (C57) and lpr mice at 0, 6, and 12 h following aerosolization of E. coli, S. aureus, or S. pneumoniae. HSA was injected intraperitoneally 1 h prior to euthanasia. Data are means and standard errors.

FIG. 5

Representative sections from the lungs of C57BL/6 mice immediately following E. coli aerosolization, showing normal histology (A and B); C57Bl/6 mice 6 h following E. coli aerosolization, showing dense neutrophilic infiltrates and thickening of the alveolar walls (C and D); and lpr mice 6 h following E. coli aerosolization, showing neutrophilic infiltrates and normal alveolar structures (E and F). Hematoxylin-eosin stain. Magnifications: A, C, and E, ×200; B, D, and F, ×400.