PERSPECTIVE
Coagulation Abnormalities in Acute Lung Injury and Sepsis

Edward Abraham

Division of Pulmonary Sciences and Critical Care Medicine, University of Colorado Health Sciences Center, Denver, Colorado

Intraalveolar and intravascular fibrin deposition is frequently found in the setting of acute lung injury (ALI) or acute respiratory distress syndrome (ARDS) (1, 2). Fibrin deposits enhance inflammatory response by increasing vascular permeability, activating endothelial cells to produce proinflammatory cytokines and other mediators, inducing the accumulation of activated neutrophils, and modulating immunoregulatory responses in the lung. Infusions of endotoxin produce fibrin deposits in the lung and other organs, and these widespread intravascular alterations are postulated to contribute to multiple organ dysfunction in sepsis (3, 4, 5). Fan and colleagues now show that hemorrhagic shock can potentiate the effects of small doses of endotoxin, creating a procoagulant milieu in the lungs, in which reactive oxygen intermediates (ROI) and tumor necrosis factor- (TNF-) produced by alveolar macrophages have important roles (6).

Procoagulant activity is enhanced in the lungs of patients with ARDS (7, 8). Levels of tissue factor, a pivotal mediator in the extrinsic coagulation pathway, are increased in ARDS bronchoalveolar lavage (BAL) specimens, and tissue factor appears to occupy an important role in the ARDS-associated procoagulant state (9, 10). In addition, fibrinolytic processes are inhibited in ALI, as shown by increased BAL concentrations of plasminogen activator inhibitor 1 (PAI-1), and may also contribute to fibrin generation in ALI (11). In experimental models of endotoxemia, early increases in the anticoagulant tissue plasminogen activator are rapidly followed by sustained elevations in PAI-1, leading to a prolonged antifibrinolytic state (12).

	Coagulation Activation and Modulation

Recent experimental studies in humans and nonhuman primates demonstrate that activation of coagulation after endotoxemia, bacterial infusion, or administration of proinflammatory cytokines, such as TNF-, is driven primarily by the extrinsic pathway (13, 14). Regulatory mechanisms that prevent coagulation from being generalized under normal conditions involve antithrombin III (ATIII), protein C, protein S, and tissue factor pathway inhibitor (TFPI) (Figure 1). Each of these inhibitory mechanisms is presently being investigated as a therapeutic intervention to improve outcome from severe sepsis.

View larger version (26K): [in this window] [in a new window]   Figure 1.   The intrinsic and extrinsic coagulation pathways and the points at which coagulation inhibitors exert their action. Inhibitor effects: TFPI, purple concaved rectangle; ATIII, red concaved rectangle; APC, yellow concaved rectangle.

Tissue factor occupies a central position in the extrinsic coagulation pathway (14, 15). Tissue factor is highly thrombogenic, and, under normal conditions, only minute amounts are exposed to the circulating blood. However, under pathophysiologic conditions, alveolar macrophages, neutrophils, and endothelial cells can express tissue factor on their surface, thereby leading to development of a coagulopathy (15, 16). Endotoxin, activated complement, and cytokines, such as TNF- or interleukin-1, can all upregulate tissue factor expression (14, 17, 18). Infusion of endotoxin or bacteria in primates results in activation of coagulation directly dependent on tissue factor activity, since antitissue factor antibodies completely prevent such sepsis-associated coagulopathies (14, 19).

Exposed tissue factor binds and activates factor VII, which cleaves factor X to Xa. Factor Xa converts prothrombin to thrombin, activating factor V. Activated factor V is a potent cofactor for factor Xa, and enhances the ability of factor Xa to generate thrombin. TFPI regulates extrinsic pathway activity by suppressing the activity of tissue factor/ VIIa/Xa complexes (20). The role that TFPI plays in regulating coagulation cascades in ARDS or sepsis is incompletely understood. Plasma levels of TFPI are not generally diminished in patients with disseminated intravascular coagulation (DIC) (21). However, because there are different pools of TFPI, of which the largest is endothelial associated, plasma levels may not provide an accurate reflection of TFPI reserve. Administration of TFPI improves survival and organ function when given to rabbits with septic peritonitis or to baboons infused with endotoxin or Escherichia coli (22, 23). A recent small Phase II human study also suggested that TFPI administration could reduce mortality associated with sepsis.

ATIII inhibits activated proteases, including factors IXa, Xa, and thrombin (24). Plasma levels of ATIII decrease in experimental and clinical sepsis and inversely correlate with survival in these settings (25). Reconstituting ATIII to supraphysiologic concentrations may offer protection against DIC and related morbidity. Evidence suggests that administering ATIII to septic patients may improve survival (26). A large clinical trial is presently investigating this hypothesis.

Protein C is a vitamin K-dependent protein, which is activated by the thrombin-thrombomodulin complex on endothelial cells (27). Activated protein C (APC), with the cofactor protein S, cleaves and inactivates the procoagulant activity of activated factors V and VIII. Low plasma levels of protein C predict poor clinical outcome in patients with DIC or sepsis (28). Case studies in purpura fulminans due to meningococcemia, in which DIC plays an important role in contributing to morbidity and mortality, suggest that administration of protein C can improve outcome (29). A limited clinical study also suggested benefit when APC was given to septic patients.

	Interactions between TNF- and Coagulation

TNF- induces the expression of tissue factor and downregulates thrombomodulin on endothelial surfaces, thereby enhancing coagulation (12, 18). Local injection of TNF- leads to deposition of fibrin (30). Infusion of TNF- into humans produces a procoagulant state, accompanied by inhibition of fibrinolysis (31, 32). In particular, administration of TNF- to control subjects led to activation of the extrinsic, but not intrinsic, coagulation pathway, with increases in circulating levels of factor Xa and prothrombin fragment F₁₊₂. Although TNF- injection into humans induced a brief increase in fibrinolytic activity, as shown by increases in plasminogen activator and D-dimer levels, increases in PAI-1 were evident starting one hour after TNF- administration, resulting in overall inhibition of fibrinolysis at later time points.

The results of Fan and associates (6) are consistent with those described previously and demonstrate a central role for TNF- in inducing pulmonary procoagulant activity in their model of hemorrhagic shock followed by intratracheal endotoxin. Anti-TNF- treatment prevented lipopolysaccharide (LPS)-induced increases in tissue factor and PAI-1 expression in the lungs. Interestingly, the TNF- blockade did not modulate endotoxin-induced accumulation of neutrophils into the lung, but it did appear to diminish neutrophil activation, as assessed by chemiluminescence assays. Such results, showing that the migration of neutrophils to the lungs occurs by mechanisms distinct from those inducing their activation, are consistent with experiments in models of endotoxemia-induced ALI (33).

	Interrelationships between ROI and Coagulation

Both hemorrhage and endotoxemia result in increased production of ROI (34, 35), an effect expected to be enhanced by the combination of the two pathophysiologic insults, such as in the experiments of Fan and coworkers. ROI have been demonstrated to be increased in patients with ARDS (36). ROI also appear to have an important role in initiating inflammatory cascades leading to ALI (34, 35).

ROI are involved in the activation of the transcriptional regulatory factor nuclear factor-B (NF-B) in many, but not all, cell types (38). Exposure to increased levels of ROI can lead to enhanced translocation of NF-B to the nucleus and upregulated expression of NF-B-dependent genes. In experimental models of hemorrhage or endotoxemia, administration of ROI scavengers, such as pyrrolidine dithiocarbamate (PDTC) or N-acetyl cysteine (NAC), or inhibition of the ROI-generating enzyme, xanthine oxidase, decrease NF-B activation in the lungs (35, 38, 39). Recent data from our laboratory show that xanthine oxidase blockade prevents increases in NF-B-dependent proinflammatory cytokines, such as TNF- and macrophage inflammatory peptide-2 (MIP-2), which are normally seen in lung neutrophils after hemorrhage (40). However, even though xanthine oxidase inhibition diminishes neutrophil activation, there is no effect on the accumulation of neutrophils in the lungs after hemorrhage.

Transcriptional activation of the human tissue factor gene in monocytic cells exposed to LPS is mediated by binding of NF-B heterodimers to a B site in the tissue factor promoter (41). Incubation of human monocytes with either PDTC or NAC blocks LPS-induced expression of tissue factor (42, 43). Similarly, increases in myocardial tissue factor levels produced by ischemia/reperfusion are abolished by oxygen radical scavengers (44). The studies by Fan and colleagues (6) indicate that hemorrhage induces a state where the lungs are primed to upregulate tissue factor through a pathway initiated by ROI generation, in which NF-B activation and TNF- expression play pivotal roles. Such data therefore suggest a mechanism by which ROI may initiate and potentiate ALI through inducing a tissue factor-dependent procoagulant state.

	How Important Are Coagulation Abnormalities in ALI?

Despite the fact that endotoxemia, hemorrhage, or exposure to proinflammatory cytokines such as TNF- can lead to a procoagulant state, several important questions remain. Although tissue factor generation and widespread fibrin deposition accompany ALI, we still don't know how important these factors are in modulating the development and progression of ARDS. Accumulation of fibrin may enhance pulmonary inflammation or may simply be a result of the proinflammatory state that accompanies ALI and not substantially contribute to lung damage. As mentioned previously, experimental models suggest that coagulation abnormalities do contribute in important ways to endotoxemia- or sepsis-induced organ dysfunction. In those studies, interventions to correct the procoagulant state were provided either before or shortly after infusion of bacteria or endotoxin. Such experiments indicate that coagulation abnormalities and fibrin deposition are indeed important in initiating organ system dysfunction, at least in acute models where DIC plays a major role. However, the importance of coagulation alterations and fibrin deposition has not been explored well in settings where the progression of infection is more indolent and thus closer to most clinical situations. Similarly, it is presently unknown if interventions that affect coagulation will be beneficial when ALI is already present. We will have to wait for the results of ongoing clinical trials with TFPI, ATIII, or APC to know if modulation of coagulation with such agents can improve outcome from ARDS and sepsis.

	Footnotes

Address correspondence to: Edward Abraham M.D., Division of Pulmonary Sciences and Critical Care Medicine, University of Colorado Health Sciences Center, Box C272, 4200 E. Ninth Avenue, Denver, CO 80262.

(Received in original form February 2, 2000).

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