The Alveolar Epithelium can Initiate the Extrinsic Coagulation Cascade through Expression of Tissue Factor

Abstract

RATIONALE The alveolar compartment is a pro-coagulant, anti- fibrinolytic environment in acute lung injury (ALI) and the acute respiratory distress syndrome (ARDS).

OBJECTIVES To test the hypothesis that the alveolar epithelium can initiate intra-alveolar coagulation by expressing active tissue factor (TF). METHODS: Using an in vitro cell surface TF assay and TF ELISA we measured the activity and production of TF in cultured alveolar epithelial (A549) cells following exposure to cytomix (TNF-& [alpha], IL-1β, IFN-γ). TF gene transcription was measured by semi-quantitative RT-PCR. Immunohistochemistry for TF was done on lung sections of ARDS patients compared to controls. TF protein levels were measured by ELISA in undiluted pulmonary edema fluid from ALI/ARDS patients compared to control patients with hydrostatic pulmonary edema.

MAIN RESULTS TF activity, mRNA and protein levels increased in A549 cells after stimulation with cytomix. In addition, increased TF activity was measured on A549 cells following incubation with pulmonary edema fluid from ALI/ARDS patients. Immunohistochemistry for TF in human lung tissue from ARDS patients showed prominent TF staining in alveolar epithelial cells as well as intra- alveolar macrophages and hyaline membranes. Additionally, TF antigen levels and TF procoagulant activity were very high in the pulmonary edema fluid of ALI/ARDS patients compared to plasma, and higher plasma levels were associated with mortality.

CONCLUSIONS The alveolar epithelium is capable of modulating intra- alveolar coagulation through upregulation of TF following exposure to inflammatory stimuli and may contribute to intra-alveolar fibrin deposition in ARDS.