Potential Protective Mechanisms of Tissue Factor in Acute Lung Injury

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Acute Respiratory Distress Syndrome (ARDS) is a common cause of acute respiratory failure that carries a high mortality rate and has no beneficial targeted therapies. It is a pressing problem for Veterans and current estimates suggest that more Veterans will die from ARDS each year than will die from lung cancer. Despite this, there are no pharmacologic therapies for ARDS. New insights into the pathogenesis are needed to deepen our understanding of the underlying mechanisms that lead to ARDS as well as to develop novel therapeutics. A major pathologic feature of ARDS is activation of the Tissue Factor (TF) pathway of coagulation in the airspace. TF is an integral membrane protein that both initiates the extrinsic coagulation cascade and serves several non-coagulant functions including promoting cell adhesion and migration through interactions with integrin proteins. For decades, procoagulant pathways, including the TF pathway, have been implicated as a mechanism of injury in ARDS. However, a number of clinical trails of anti-coagulants have not shown a benefit with this approach. One potential explanation is that TF in the airspace is protective in ARDS. Our preliminary in vivo and in vitro data show that loss of lung epithelial, but not macrophage, TF causes loss of epithelial barrier integrity, decreased cell surface β1 integrin and abnormal cell adhesion. Conversely, overexpression of TF in alveolar epithelial type II cells restores barrier integrity. In addition, treatment with recombinant factor VII (rFVII), the primary TF ligand, into the airspace attenuates permeability in a mouse model of ARDS. Together, these observations represent a major shift in understanding the role of the TF pathway in ARDS, a pathway that has almost universally been considered harmful. The goals of this proposal are to test the novel hypotheses that (1) loss of lung epithelial TF leads to loss of epithelial barrier integrity through disruption of integrin proteins and (2) upregulation of TF in the airspace maintain epithelial barrier integrity in a direct lung injury model. Therefore, local delivery of rFVII to the airspace could represent a unique therapeutic approach for the prevention and treatment of ARDS. Using novel transgenic mice with alveolar epithelial type II cell specific deletion or overexpression of TF we will define the in vivo cell specific mechanism of TF effects on lung epithelial barrier integrity. We will also use a novel TF knockout lung epithelial cell line to study the molecular interactions between TF and β1 integrin and the role of TF in regulating epithelial barrier integrity by expressing a library of novel TF mutants. Finally we will use a clinically relevant human model of ARDS (ex vivo perfused human lung) to test the therapeutic potential of rFVII to tighten the epithelial barrier in human ARDS. Our experimental design is both mechanistic and translational and utilizes novel transgenic mouse models, cell lines as well a human lung injury model to both understand the mechanism of TF effects in the lung as well as to test the therapeutic potential of rFVII. This proposal takes advantage of the unique skills and resources available at Vanderbilt including state of the art computational modeling as well as adhesion complex dynamics measurements thus ensuring that results from these studies will advance the field and provide the necessary foundation for development of targeted therapeutics for Veterans with ARDS.