Summary Platelets play a central role in thrombosis, hemostasis, and inflammation. We show that activated platelets release inorganic polyphosphate (polyP), a polymer of 60-100 phosphate residues that directly bound to and activated the plasma protease factor XII. PolyP-driven factor XII-activation triggered release of the inflammatory mediator bradykinin by plasma kallikrein-mediated kininogen processing. PolyP increased vascular permeability and induced fluid extravasation in skin microvessels of mice. Mice deficient in factor XII or bradykinin receptors were resistant to polyP-induced leakage. PolyP initiated clotting of plasma via the contact pathway. Ablation of intrinsic coagulation pathway proteases factor XII and factor XI protected mice from polyP-triggered lethal pulmonary embolism. Targeting polyP with phosphatases interfered with procoagulant activity of activated platelets and blocked platelet-induced thrombosis in mice. Infusion of polyP restored defective plasma clotting of Hermansky-Pudlak Syndrome patients, which lack platelet polyP. The data identify polyP as a new class of mediator having fundamental roles in platelet-driven proinflammatory and procoagulant disorders.

Inorganic polyphosphate is an abundant component of acidocalcisomes of bacteria and unicellular eukaryotes. Human platelet dense granules strongly resemble acidocalcisomes, and we recently showed that they contain substantial amounts of polyphosphate, which is secreted upon platelet activation. We now report that polyphosphate is a potent hemostatic regulator, accelerating blood clotting by activating the contact pathway and promoting the activation of factor V, which in turn results in abrogation of the function of the natural anticoagulant protein, tissue factor pathway inhibitor. Polyphosphate was also found to delay clot lysis by enhancing a natural antifibrinolytic agent, thrombin-activatable fibrinolysis inhibitor. Polyphosphate is unstable in blood or plasma, owing to the presence of phosphatases. We propose that polyphosphate released from platelets or microorganisms initially promotes clot formation and stability; subsequent degradation of polyphosphate by blood phosphatases fosters inhibition of clotting and activation of fibrinolysis during wound healing.factor V ͉ platelets ͉ tissue factor pathway inhibitor ͉ acidocalcisomes ͉ dense granules P olyphosphate (polyP) is widely distributed in biology, being found in bacteria, fungi, plants, and animals (1). The biologic functions of polyP have been studied most extensively in prokaryotes and unicellular eukaryotes, in which high levels of polyP accumulate in acidic organelles known as acidocalcisomes. PolyP in these organisms can reach chain lengths of several hundred phosphate units. In unicellular organisms, polyP has been shown to play essential roles in stress responses and virulence (1, 2), although its function in higher eukaryotes, including man, has not been extensively investigated. Recently, we reported that dense granules of human platelets strongly resemble acidocalcisomes and contain millimolar levels of polyP (with chain lengths of Ϸ70-75 phosphate units), making acidocalcisomes the only known class of organelle conserved during evolution from bacteria to humans (3). Human platelets each have three to eight dense granules (also called ␦ granules), a type of secretory granule that contains serotonin, ADP, ATP, and PP i in addition to polyP. Patients with dense granule defects exhibit bleeding diatheses, underscoring the role of these secretory granules in hemostasis (4). Platelets contain Ϸ0.74 nmol polyP per 10 8 platelets, which is secreted after stimulation by platelet agonists such as thrombin (3). Therefore, released polyP could readily attain a concentration of 3 M in whole blood, and this could be far higher in platelet-rich thrombi. (Concentrations of polyP are expressed in this paper as phosphate monomer.)The importance of platelets in hemostasis suggested to us that polyP may play an important role in the blood clotting system. In this study, we found that polyP of the size released from activated platelets has a strongly net procoagulant effect, which is exerted at several levels in the clotting͞fibrinolysis system: PolyP activate...

Injury is the fourth leading cause of mortality worldwide, accounting for 9% of deaths globally (4.9 million people) in 2016 (ref. 1 ). Moreover, the burden is highest in individuals <50 years of age, among whom injury as a cause of death is second only to infectious diseases. Early preventable deaths after injury in civilian 2 and military 3 settings are primarily attributable to uncontrolled haemorrhage 2-8 , whereas later preventable deaths are typically due to hypercoagulability 9 . Consequently, there is intense interest worldwide in the pathogenesis of trauma-induced coagulopathy (TIC) to attenuate its adverse effects on the outcomes of seriously injured patients.Impaired coagulation following sudden death from injury has been observed for centuries 10 and, in the 1960s, the first clinical laboratory documentation of the temporal changes in coagulation following severe injury were documented 11 . However, early endogenous drivers of coagulopathy were not specifically investigated until 1982, when a case series of patients with major abdominal vascular injuries highlighted TIC as a common direct cause of early post-injury mortality: 89% of the deaths were bleeding-related, yet half occurred after mechanical control of bleeding sites -in other words, they were due to coagulopathy 12 . The remaining ongoing quagmire is the inability to distinguish between patients with exsanguinating injuries whose TIC is the result of metabolic failure (that is, who are bleeding because they are dying) from patients whose TIC is the cause of the ongoing blood loss (that is, who are dying because they are bleeding) 13 . Furthermore, not all patients with abnormalities in laboratory coagulation tests are bleeding 14 .Despite the long-term fascination with changes in coagulation resulting from shock and tissue injury 15 , there is no standard definition of TIC, which refers to abnormal coagulation capacity attributable to trauma. The term TIC was established during the Trans-Agency Consortium for Trauma Induced Coagulopathy Workshop conducted by the National Institutes of Health in April 2010 to describe the variety of phenomena that characterize this condition. TIC can manifest as a spectrum of phenotypes from hypocoagulation to hypercoagulation (fig. 1), as a function of several interactive factors, including (but not limited to) tissue injury, presence of shock and, in particular, time from injury (fig. 2).

The global pandemic of coronavirus disease 2019 (COVID-19) is associated with the development of acute respiratory distress syndrome (ARDS), which requires ventilation in critically ill patients. The pathophysiology of ARDS results from acute inflammation within the alveolar space and prevention of normal gas exchange. The increase in proinflammatory cytokines within the lung leads to recruitment of leukocytes, further propagating the local inflammatory response. A consistent finding in ARDS is the deposition of fibrin in the air spaces and lung parenchyma. COVID-19 patients show elevated D-dimers and fibrinogen. Fibrin deposits are found in the lungs of patients due to the dysregulation of the coagulation and fibrinolytic systems. Tissue factor (TF) is exposed on damaged alveolar endothelial cells and on the surface of leukocytes promoting fibrin deposition, while significantly elevated levels of plasminogen activator inhibitor 1 (PAI-1) from lung epithelium and endothelial cells create a hypofibrinolytic state. Prophylaxis treatment of COVID-19 patients with low molecular weight heparin (LMWH) is important to limit coagulopathy. However, to degrade pre-existing fibrin in the lung it is essential to promote local fibrinolysis. In this review, we discuss the repurposing of fibrinolytic drugs, namely tissue-type plasminogen activator (tPA), to treat COVID-19 associated ARDS. tPA is an approved intravenous thrombolytic treatment, and the nebulizer form has been shown to be effective in plastic bronchitis and is currently in Phase II clinical trial. Nebulizer plasminogen activators may provide a targeted approach in COVID-19 patients to degrade fibrin and improving oxygenation in critically ill patients. K E Y W O R D Sfibrin, fibrinolysis, plasminogen activator inhibitor 1, respiratory distress syndrome (adult), SARS virus, tissue plasminogen activatorThis is an open access article under the terms of the Creat ive Commo ns Attri bution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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