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Degradation of Neutrophil Extracellular Traps and its impact on thrombolysis

Final Report Summary - NET-LYSIS (Degradation of Neutrophil Extracellular Traps and its impact on thrombolysis)

Summary report: “NET-Lysis”

Thrombosis is a major cause of morbidity and mortality. Thrombosis is the formation of a blood clot within arteries or veins. Blood clots eventually occlude blood vessels and thus obstruct the supply of organs with oxygen and nutrients leading to organ damage. Frequent diseases precipitated by thrombosis are myocardial infarction, stroke, and pulmonary embolism.

Blood clot formation is driven by the aggregation of blood platelets and by the formation of fibrin strands. Fibrin serves as a scaffold and stabilizes blood clots. Conventional antithrombotic drugs target platelet aggregation and fibrin formation to disintegrate blood clots. The goal of this thrombolytic therapy is to reestablish the blood supply to organs and thus ameliorate e.g. myocardial infarction or stroke. However present thrombolytic therapies are inefficient and new strategies are urgently needed.

Current basic medical research indicates that neutrophils are participating in blood clot formation in addition to platelets and fibrin. Neutrophils are the predominant white blood cells and mainly known for their protective function against bacterial and fungal infection. It is now known that neutrophils are actively recruited to sites of blood clot formation. We could recently show that neutrophils release their DNA during blood clotting to promote thrombosis. DNA is very long molecule, which is stored in a compact form within the nucleus. In order to release its DNA, the nucleus of neutrophils is disassembled and compact DNA is loosened to single DNA filaments. Neutrophils then actively release DNA fibers, which form web-like structures outside of the cell. These structures resemble the appearance of spider-webs and are termed Neutrophil Extracellular Traps (NETs). During thrombosis, NETs support the formation of blood clots by binding platelets and activating fibrin formation. As a consequence, NETs accelerate thrombosis and blood clots are more stable in the presence of NETs. Within thrombi, NETs interact with fibrin and appear to be a previously unrecognized scaffold for blood clots.

It is therefore possible that efficient thrombolysis requires degradation of platelet aggregates and fibrin as well as NETs. We speculated that DNases in plasma degrade NET-DNA within blood clots, ameliorate thrombosis, and promote thrombolysis.

To test our hypothesis, we analyzed acute thrombotic microangiopathies (TMAs). TMAs are a heterogeneous group of life-threatening conditions characterized by disseminated microvascular thrombosis with thrombocytopenia, fragmentation of erythrocytes and ischemic organ damage. We have previously identified markers of NETs in TMAs of various pathophysiological causes including thrombocytopenic purpura, hemolytic uremic syndrome and tumor-associated TMA. We now hypothesized that timely and efficient removal of NETs is required to prevent excessive thrombus formation and investigated whether disseminated microvascular thrombosis in acute TMAs is associated with a defect in the degradation of NETs.

We could show that NETs generated in vitro were efficiently degraded by plasma from healthy donors. However, NETs remained stable after exposure to plasma from TMA patients. The inability to degrade NETs was linked to a reduced DNase activity in TMA plasma. Plasma DNase1, which is the predominant DNA-degrading enzyme in circulation, was required for efficient NET-degradation and TMA plasma showed decreased levels of this enzyme. Supplementation of TMA plasma with recombinant human DNase1 restored NET-degradation activity. Our data indicates that DNase1-mediated degradation of NETs is impaired in patients with TMAs.

The role of plasma DNases in thrombosis is, as of yet, poorly understood. Reduced plasma DNase1 activity may cause the persistence of pro-thrombotic NETs and thus promote microvascular thrombosis in TMA patients and potentially other thrombotic diseases. Furthermore, plasma DNases could serve as new drug candidates for thrombolytic therapy and improve the treatment of patients with thrombosis in future.

We have now established a laboratory to continue this line of research. To follow our work, please visit