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Patients with the acute respiratory distress syndrome (ARDS) have elevated levels of cell-free hemoglobin (CFH) in the air space, but the contribution of CFH to the pathogenesis of acute lung injury is unknown. In the present study, we demonstrate that levels of CFH in the air space correlate with measures of alveolar-capillary barrier dysfunction in humans with ARDS (r = 0.89, P < 0.001) and in mice with ventilator-induced acute lung injury (r = 0.89, P < 0.001). To investigate the specific contribution of CFH to ARDS, we studied the impact of purified CFH in the mouse lung and on cultured mouse lung epithelial (MLE-12) cells. Intratracheal delivery of CFH in mice causes acute lung injury with air space inflammation and alveolar-capillary barrier disruption. Similarly, in MLE-12 cells, CFH increases proinflammatory cytokine expression and increases paracellular permeability as measured by electrical cell-substrate impedance sensing. Next, to determine whether these effects are mediated by the iron-containing heme moiety of CFH, we treated mice with intratracheal hemin, the chloride salt of heme, and found that hemin was sufficient to increase alveolar permeability but failed to induce proinflammatory cytokine expression or epithelial cell injury. Together, these data identify CFH in the air space as a previously unrecognized driver of lung epithelial injury in human and experimental ARDS and suggest that CFH and hemin may contribute to ARDS through different mechanisms. Interventions targeting CFH and heme in the air space could provide a new therapeutic approach for ARDS.

Background Systemic blockade of Tissue Factor (TF) attenuates acute lung injury (ALI) in animal models of sepsis but the effects of global TF deficiency are unknown. Hypothesis We used mice with complete knockout of mouse TF and low levels (~1%) of human TF (LTF mice) to test the hypothesis that global TF deficiency attenuates lung inflammation in direct lung injury. Methods LTF mice were treated with 10 μg of lipopolysaccharide (LPS) or vehicle administered by direct intratracheal (IT) injection and studied at 24 hours. Results Contrary to our hypothesis, LTF mice had increased lung inflammation and injury as measured by bronchoalveolar lavage cell count (3.4 × 105 WT LPS versus 3.3 × 105 LTF LPS, p=0.947) and protein (493 μg/ml WT LPS versus 1014 μg/ml LTF LPS, p=0.006), proinflammatory cytokines (TNF-α, IL-10, IL-12, p<0.035 WT LPS versus LTF LPS) and histology compared to wild type mice. LTF mice also had increased hemorrhage and free hemoglobin in the airspace accompanied by increased oxidant stress as measured by lipid peroxidation products (F2-Isoprostanes and Isofurans). Conclusions These findings indicate that global TF deficiency does not confer protection in a direct lung injury model. Rather, TF deficiency causes increased intra-alveolar hemorrhage following LPS leading to increased lipid peroxidation. Strategies to globally inhibit tissue factor may be deleterious in patients with ALI.

Tissue factor (TF) initiates the extrinsic coagulation cascade in response to tissue injury, leading to local fibrin deposition. Low levels of TF in mice are associated with increased severity of acute lung injury (ALI) after intratracheal LPS administration. However, the cellular sources of the TF required for protection from LPS-induced ALI remain unknown. In the current study, transgenic mice with cell-specific deletions of TF in the lung epithelium or myeloid cells were treated with intratracheal LPS to determine the cellular sources of TF important in direct ALI. Cell-specific deletion of TF in the lung epithelium reduced total lung TF expression to 39% of wild-type (WT) levels at baseline and to 29% of WT levels after intratracheal LPS. In contrast, there was no reduction of TF with myeloid cell TF deletion. Mice lacking myeloid cell TF did not differ from WT mice in coagulation, inflammation, permeability, or hemorrhage. However, mice lacking lung epithelial TF had increased tissue injury, impaired activation of coagulation in the airspace, disrupted alveolar permeability, and increased alveolar hemorrhage after intratracheal LPS. Deletion of epithelial TF did not affect alveolar permeability in an indirect model of ALI caused by systemic LPS infusion. These studies demonstrate that the lung epithelium is the primary source of TF in the lung, contributing 60-70% of total lung TF, and that lung epithelial, but not myeloid, TF may be protective in direct ALI.Keywords: coagulation; fibrin; pulmonary; acute respiratory distress syndrome; alveolar capillary barrier permeability Clinical RelevanceThis study evaluates the role of tissue factor (TF) in direct acute lung injury using cell-specific genetic approaches. We identify the lung epithelium as the major source of TF in the airspace. Loss of epithelial TF increases lung injury, impairs coagulation, results in lung hemorrhage, and disrupts alveolar-capillary barrier permeability. These findings identify a potential new target for treatment of severe lung injury in humans.A pathologic hallmark of severe acute lung injury (ALI) in humans is intra-alveolar fibrin deposition, forming hyaline membranes lining the airspace (1). Activation of the tissue factor (TF) pathway is a major driver of coagulation in the airspace (2-5). We previously demonstrated a critical role for TF in protection from ALI caused by intratracheal LPS administration (2). LTF mice, which lack murine TF and express 1% of endogenous levels of human TF to overcome embryonic lethality, developed more severe ALI in response to intratracheal LPS than littermate controls. LTF mice had a local coagulation defect in the airspace, increased histologic lung injury, increased alveolar-capillary

These data suggest that HMEF is a novel, noninvasive method to accurately sample the distal airspace in patients with ARDS.

BackgroundIt is unclear how to identify which patients at risk for acute respiratory distress syndrome (ARDS) will develop this condition during critical illness. Elevated microparticle (MP) concentrations in the airspace during ARDS are associated with activation of coagulation and in vitro studies have demonstrated that MPs contribute to acute lung injury, but the significance of MPs in the circulation during ARDS has not been well studied. The goal of the present study was to test the hypothesis that elevated levels of circulating MPs could prospectively identify critically ill patients who will develop ARDS and that elevated circulating MPs are associated with poor clinical outcomes.MethodsA total of 280 patients with platelet-poor plasma samples from the prospective Validating Acute Lung Injury biomarkers for Diagnosis (VALID) cohort study were selected for this analysis. Demographics and clinical data were obtained by chart review. MP concentrations in plasma were measured at study enrollment on intensive care unit (ICU) day 2 and on ICU day 4 by MP capture assay. Activation of coagulation was measured by plasma recalcification (clot) times.ResultsARDS developed in 90 of 280 patients (32%) in the study. Elevated plasma MP concentrations were associated with reduced risk of developing ARDS (odds ratio (OR) 0.70 per 10 μM increase in MP concentration, 95% CI 0.50–0.98, p = 0.042), but had no significant effect on hospital mortality. MP concentration was greatest in patients with sepsis, pneumonia, or aspiration as compared with those with trauma or receiving multiple blood transfusions. MP levels did not significantly change over time. The inverse association of MP levels with ARDS development was most striking in patients with sepsis. After controlling for age, presence of sepsis, and severity of illness, higher MP concentrations were independently associated with a reduced risk of developing ARDS (OR 0.69, 95% CI 0.49–0.98, p = 0.038). MP concentration was associated with reduced plasma recalcification time.ConclusionsElevated levels of circulating MPs are independently associated with a reduced risk of ARDS in critically ill patients. Whether this is due to MP effects on systemic coagulation warrants further investigation.Electronic supplementary materialThe online version of this article (doi:10.1186/s13054-017-1700-7) contains supplementary material, which is available to authorized users.

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-α (TNF-α) upregulates TF in the injured lung, and there is emerging evidence that another cytokine, interferon-γ (IFN-γ) also modulates expression. The effects of TNF and IFN on regulation of TF were studied in alveolar epithelial A549 cells. In addition, potential mechanisms of modulation of TF expression by the two cytokines were analyzed with the hypothesis that IFN-γ acts synergistically with TNF-α to upregulate alveolar epithelial TF through modulation of TNFR expression. TNF-α but not IFN-γ treatment increased TF mRNA, protein and cell surface TF activity. The combination of IFN-γ and TNF-α treatment augmented the effects of TNF-α on TF upregulation and also increased release of procoagulant microparticles (MPs) from A549 cells. IFN-γ modulated expression of both TNF-α receptors, studies utilizing neutralizing antibodies against the two TNF receptors showed the TF effects were mediated primarily through augmenting TNFR1-dependent cellular responses. These findings have important implications for regulation of fibrin formation in the lung in the setting of acute inflammation.

Tissue factor (TF) is a critical mediator of direct acute lung injury (ALI) with global TF deficiency resulting in increased airspace inflammation, alveolar-capillary permeability, and alveolar hemorrhage after intra-tracheal lipopolysaccharide (LPS). In the lung, TF is expressed diffusely on the lung epithelium and intensely on cells of the myeloid lineage. We recently reported that TF on the lung epithelium, but not on myeloid cells, was the major source of TF during intra-tracheal LPS-induced ALI. Because of a growing body of literature demonstrating important pathophysiologic differences between ALI caused by different etiologies, we hypothesized that TF on myeloid cells may have distinct contributions to airspace inflammation and permeability between direct and indirect causes of ALI. To test this, we compared mice lacking TF on myeloid cells (TF∆mye, LysM.Cre+/−TFflox/flox) to littermate controls during direct (bacterial pneumonia, ventilator-induced ALI, bleomycin-induced ALI) and indirect ALI (systemic LPS, cecal ligation and puncture). ALI was quantified by weight loss, bronchoalveolar lavage (BAL) inflammatory cell number, cytokine concentration, protein concentration, and BAL procoagulant activity. There was no significant contribution of TF on myeloid cells in multiple models of experimental ALI, leading to the conclusion that TF in myeloid cells is not a major contributor to experimental ALI.