Neutrophil Extracellular Traps Promote Platelet-Driven Contraction of Inflammatory Blood Clots via Local Generation of Endogenous Thrombin and Softening of the Fibrin Network.

Immunothrombosis can substantially affect the course and outcomes of severe infections and immune-mediated diseases. While inflammatory thrombi are neutrophil-rich, impact of neutrophils on clot contraction, a key modulator of thrombus stability and obstructiveness, was unknown. This study investigated how neutrophils and neutrophil extracellular traps (NETs) affect the rate and extent of platelet-driven clot contraction. Isolated human neutrophils were stimulated with phorbol-12-myristate-13-acetate (PMA) to induce NETosis, confirmed by fluorescence microscopy and scanning electron microscopy. Thrombin-induced clots, formed from whole blood or platelet-rich plasma, were supplemented with non-activated or PMA-activated neutrophils. Clot contraction kinetics and viscoelasticity were analyzed. PMA-activated neutrophils significantly enhanced the rate and final extent of clot contraction compared to controls. This promoting effect was abolished by deoxyribonuclease (DNAse) I, confirming that it was mediated by NETs embedded in the fibrin network. The factor Xa inhibitor rivaroxaban also abrogated this effect, indicating a role for NET-induced endogenous thrombin generation and platelet hyperactivation. Thromboelastography revealed that NETs made clots softer and more deformable. We conclude that activated neutrophils promote clot contraction via NETs embedded in the fibrin network, which enhance platelet contractility via endogenous thrombin production and increase clot deformability, suggesting that inflammatory thrombosis may require treatments addressing this enhanced contractility.