Acute respiratory distress syndrome (ARDS) is defined as a syndrome of acute onset, with bilateral opacities on chest imaging and respiratory failure not caused by cardiac failure, leading to mild, moderate, or severe oxygenation impairment. The syndrome is most commonly a manifestation of sepsis-induced organ dysfunction, characterized by disruption of endothelial barrier integrity and diffuse lung damage. Imbalance between coagulation and inflammation is a predominant characteristic of ARDS, leading to extreme inflammatory response and diffuse fibrin deposition in vascular capillary bed and alveoli. Activated platelets, neutrophils, endothelial cells, neutrophil extracellular traps, microparticles, and coagulation proteases, participate in the complex process of immunothrombosis, which is a key event in ARDS pathophysiology. The present review is focused on the elucidation of immunothrombosis in ARDS and the potential therapeutic implications.

First, patients' serum Ang-2 levels are increased during severe sepsis and associated with disease severity. The strong relationship of serum Ang-2 with serum tumor necrosis factor-alpha suggests that the latter may participate in the regulation of Ang-2 production in sepsis. Second, inflammatory mediators reduce Ang-2 release from human lung microvascular endothelial cells, implying that this vascular bed may not be the source of increased Ang-2 in human sepsis.

Acute lung injury (ALI) and its most severe extreme the acute respiratory distress syndrome (ARDS) refer to increased-permeability pulmonary edema caused by a variety of pulmonary or systemic insults. ALI and in particular ARDS, are usually accompanied by refractory hypoxemia and the need for mechanical ventilation. In most cases, an exaggerated inflammatory and pro-thrombotic reaction to an initial stimulus, such as systemic infection, elicits disruption of the alveolo-capillary membrane and vascular fluid leak. The pulmonary endothelium is a major metabolic organ promoting adequate pulmonary and systemic vascular homeostasis, and a main target of circulating cells and humoral mediators under injury; pulmonary endothelium is therefore critically involved in the pathogenesis of ALI. In this review we will discuss mechanisms of pulmonary endothelial dysfunction and edema generation in the lung with special emphasis on the interplay between the endothelium, the immune and hemostatic systems, and highlight how these principles apply in the context of defined disorders and specific insults implicated in ALI pathogenesis.