ABSTRACT
Background: Severe COVID-19 infection is associated with a wide spectrum of clinical manifestations, leading to systemic thromboinflammation and multiorgan dysfunction. The primary cause of multiorgan damage is widespread endothelial injury, leading to microangiopathy and organ ischemia. The molecular mechanisms by which ischemic endothelial cells causes microvascular obstruction remains ill defined. Aim(s): Identification of distinct microvascular occlusion mechanisms in COVID-19. Method(s): The microvasculature of multiple organs from patients dying from COVID-19, myocardial infarction or stroke were analyzed by H&E, immunohistochemistry, SEM and CLEM. Animal models of gut ischemia and stroke were also examined. Intravital confocal microscopy examined endothelial injury and microvascular obstruction mechanisms mediated by platelets, red cells and fibrin. Result(s): We demonstrate the existence of a distinct microvascular hemostatic mechanism mediated by hemolyzed red blood cells (RBC), independent of platelets and fibrin. Extensive RBC hemolysis was apparent in the microvasculature of COVID-19 patients and in humans with major organ ischemia, leading to widespread microvascular obstruction. This RBC hemostatic mechanism was triggered by organ ischemia and associated with localized accumulation of hemolyzed RBCs at sites of endothelial necroptosis. RBC hemolysis was impaired in animals lacking the necroptosis mediator, MLKL or the C9 component of complement, indicating the involvement of cell intrinsic and extrinsic membrane lytic processes. Intravital microscopy revealed that the RBC hemostatic mechanism was triggered by the fragmentation of lyzed RBCs and the deposition of RBC membranes on the surface of dying endothelial cells, forming an endovascular sealant that prevents interstitial bleeding. Conclusion(s): Our studies demonstrate the existence of a previously unidentified microvascular hemostatic mechanism mediated by hemolyzed RBCs. Dysregulation of this hemostatic mechanism is linked to microvascular obstruction and bleeding in COVID-19 and ischemic diseases.