Interferon-γ and tumor necrosis factor-α act synergistically to up-regulate tissue factor in alveolar epithelial cells

Fibrin deposition mediated through activation of tissue factor (TF) in the airspace is central to the pathogenesis of acute lung injury. Defining the mechanisms of TF regulation in the lung is critical to understanding pulmonary fibrin formation. Tumor necrosis factor-alpha (TNF-alpha) up-regulates TF in the injured lung, and there is emerging evidence that another cytokine, interferon-gamma (IFN-gamma), also modulates expression. The effects of TNF-alpha and IFN-gamma on regulation of TF were studied in alveolar epithelial A549 cells. In addition, potential mechanisms of modulation of TF expression by the 2 cytokines were analyzed with the hypothesis that IFN-gamma acts synergistically with TNF-alpha to up-regulate alveolar epithelial TF through modulation of TNF receptor (TNFR) expression. TNF-alpha but not IFN-gamma treatment increased TF mRNA, protein, and cell surface TF activity. The combination of IFN-gamma and TNF-alpha treatment augmented the effects of TNF-alpha on TF up-regulation and also increased release of procoagulant microparticles (MPs) from A549 cells. IFN-gamma modulated expression of both TNF-alpha receptors. Studies utilizing neutralizing antibodies against the two TNF receptors showed that the TF effects were mediated primarily through augmentation of TNFR1-dependent cellular responses. These findings have important implications for regulation of fibrin formation in the lung in the setting of acute inflammation.