Fibrin exposition on the surface of the remaining part of arterial thrombus arrests the thrombus growth

Introduction: A response to vessel injury is composed of three interconnected steps with: i) platelet aggregation at site of injury and thrombus build-up, ii) detachment of the intraluminal portion of the thrombus, iii) stabilization of the remaining part [1] [2] [3] [4] [5] [6] [7] [8]. While the molecular mechanism of the first two steps has been well unraveled, the process switching off platelet recruitment at later stage is still unknown. The aim of this work was to evaluate the role of fibrin in silencing the remaining part of the thrombus, beyond its stabilizing effect.

Method: We used WT mice and Fga^EK mice with no capacity for thrombin-mediated proteolysis of fibrinogen. Intravital microscopy was used to visualize thrombus dynamics in experimental models of FeCl₃- and mechanical-injuries. A tail-clip bleeding assay allowed to evaluate the bleeding time and the frequency of rebleeds. An in vitro flow based-assay was used to monitor platelet adhesion and thrombus formation.

Results: Intravital microscopy indicated that platelet attachment to a thrombus formed after FeCl₃-injury of the carotid or mechanical-injury of the aorta, stops after intraluminal part embolization, indicating that the thrombus becomes inactive, a stage we named the shutdown phase. Histological analysis showed that a fibrin shell covers the remaining patch during the shutdown phase, suggesting a role for fibrin in limiting platelet accumulation. This was best evidenced by the dynamics of thrombus formation in Fga^EK mice, unable to form fibrin, which consisted in repeated series of thrombus growthand detachment after vessel injury, with no reach of the shutdown phase. Concerning the mechanism, we showed in a flow-based assay, that a dense fibrin layer is not very thrombogenic and limits platelet recruitment to a couple of platelet layers. This result is in agreement with the observation that fibrin-rich thrombus modestly recruited circulating platelets compared to fibrin-poor thrombus highlighting the poor prothrombotic properties of fibrin during shutdown phase. In addition, fibrin lysis with rtPA during the shutdown phase resulted in an unexpected re-growth of the thrombus related to the release of clot bound thrombin.

Conclusion: In addition to the well-established role of fibrin in clot stabilization, these findings identify a novel role of fibrin in silencing the thrombus by limiting platelet accumulation on its remaining part after thrombus embolization.

 

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Conflict of Interest:

The authors declare no conflicts of interest.

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Publication History

Article published online:
13 February 2025