Altered VWF:ADAMTS13 homeostasis is a target for therapeutic intervention in sickle cell disease

Sickle cell disease (SCD) is the most common inherited blood disorder, and it affects millions of people worldwide. In SCD, the primary event is polymerization of abnormal hemoglobin in the red blood cells (RBCs), leading to a cascade of intravascular and extravascular sequelae, including chronic hemolysis, a proinflammatory state, and up-regulation of endothelial biomarkers that lead to increased adhesiveness of RBCs and activated white blood cells and platelets.

Vaso-occlusive episodes (VOEs) are the pathognomonic clinical feature of SCD. The pathogenesis of VOE is complex and includes sickled erythrocyte adherence to the endothelium, leading to vessel injury, intimal proliferation, and vasoconstriction (1). This leads to prothrombotic and proinflammatory changes that cause additional red cells to adhere to the endothelium and also promotes the formation of platelet and fibrin rich microthrombi, which further potentiates microvascular occlusion. VOE subsequently triggers tissue ischemia–reperfusion injury, which potentiates endothelial dysfunction and oxidative stress, unleashing a systemic inflammatory response that can progress to acute chest syndrome and multiorgan dysfunction syndrome (2, 3). Despite most VOE being managed at home (4), VOE remains the most common cause of hospital admission for children and adults with SCD. Only three federal Food and Drug Administration therapies are approved to decrease the incidence rate of VOE in SCD (hydroxyurea, crizanlizumab, and l-glutamine). There are currently no targeted therapies to reduce pain and end-organ injury during acute VOE, and supportive therapies, such as hydration and opioid analgesia, remain the mainstay of therapy.

To address this therapeutic gap in preventing VOEs, Shi et al. (5) examined the role of the Von Willebrand factor (VWF) in sickle cell mice with VOE and the effect of ADAMTS13, a VWF-cleaving protease, on reducing inflammation, vaso-occlusion, and organ damage in a mouse model of sickle cell VOE. The possibility of a fourth agent that may provide multimodal synergistic therapy with a completely different mechanism of action is a potentially critical breakthrough.

Shi et al. (5) used intraperitoneal injection of tumor necrosis factor (TNF) to induce VOE in HbSS (Townes) mice. Recapitulating VOE in individuals with SCD, the experimental VOE was associated with increased plasma VWF levels as well as inflammatory infiltrates and vascular occlusions in multiple tissues. Vaso-occlusions in the liver and other organs showed increased staining for VWF, platelets, and fibrin. Taken together, these findings suggest that platelet–VWF thrombi are associated with VOE and provide biological plausibility for why thrombocytopenia is observed in mice with VOE.

To establish that VWF contributes directly to the pathogenesis of VOE, Shi et al. (5) generated HbSS mice lacking endothelial VWF and showed that these mice have reduced inflammatory infiltrates and vaso-occlusions in response to TNF challenge. Finally, the group pretreated HbSS mice with recombinant ADAMTS13 before TNF injection. ADAMTS13-pretreated mice had lower levels of plasma VWF, lower levels of inflammatory cytokines, and reduced VWF-positive thrombi and RBC vaso-occlusions in the liver and other organs. Importantly, treating HbSS mice after TNF injection also led to less vaso-occlusion, improvement in platelet count and hemoglobin levels, and reduced VOE-associated organ damage with improvement in Aspartate Aminotransferase (AST) and Alanine Aminotransferase (ALT) levels, reduced ischemic and inflammatory changes in the liver, and reduced injury to the kidneys. Together, these results suggest that the therapeutic use of ADAMTS13 could decrease the incidence of VTE or attenuate duration and severity of VOE.

Further support for the experiments conducted by Shi et al. (5) implicating the delicate balance between VWF and ADAMTS13 in VOE was provided by Rossato et al. (6). In a series of experiments, the team exposed HbSS mice to hypoxia followed by reoxygenation to mimic the ischemia and reperfusion of sickle VOE and noted that pre-treatment with recombinant human ADAMTS13 (rADAMTS13) prior to hypoxia–reperfusion led to less anemia and thrombocytopenia and lower lactate dehydrogenase levels (6). Lungs of rADAMTS13pre-treated HbSS mice also showed lower levels of endothelial adhesion molecules, such as Intercellular Adhesion Molecule 1 (ICAM-1), Vascular Adhesion Molecule 1 (VCAM1), and endothelin, and attenuated endothelial nitric oxide synthase levels (6). Hypoxia–reperfusion-induced vasculopathy, and inflammatory and ischemic changes in the kidneys and lungs were also attenuated by rADAMTS13 pre-treatment (6). Together, the experimental data from Shi et al. (5) and Rossato et al. (6) demonstrate that endothelial-VWF knockout and rADAMTS13 infusion prior to experimental VOE, prevent VOE in sickle cell mouse models. These preclinical studies provide compelling rationale for therapeutic strategies that aim to restore the altered homeostasic balance between VWF and ADAMTS13 in SCD.

Individuals with SCD have lower levels of ADAMTS13 activity (relative to VWF levels), with the lowest values during VOE (7). These observations suggest that increased VWF activity either plays an essential role in the pathobiology of VOE, or is merely increased in association with VOE and hemolysis. An imbalance between VWF (increased levels and activity) and ADAMTS13 (reduced activity) (Fig. 1) leads to a shift toward a hypercoagulable and proinflammatory state that is implicated in the pathogenesis of other microthrombotic disorders including cerebral malaria (8), preeclampsia (9), and COVID-19 (10). Current evidence suggests that the imbalance between VWF and ADAMTS13 in SCD is likely driven by the increased release of VWF from activated endothelium; a decrease in VWF cleavage by ADAMTS13 due to increased oxidative stress, free hemoglobin, and thrombospondin-1, or both (11).

Fig. 1.

Imbalance between VWF and ADAMTS13 in SCD. Inflammation and endothelial activation lead to increased VWF secretion and an increase in hyperadhesive VWF multimers. In SCD, cleavage of VWF by ADAMTS13 is reduced in the setting of oxidative stress and by increased plasma levels of free hemoglobin and thrombospondin-1. ADAMTS13 degradation by plasmin, thrombin, and leukocyte elastase has also been shown in other microthrombotic and inflammatory disorders. The VWF:ADAMTS13 imbalance leads to an increase in RBC vaso-occlusion through increased RBC trapping in VWF–platelet thrombi, promotes inflammation, and potentiates ischemia–reperfusion injury that can progress to systemic inflammation and end-organ injury in SCD VOE.

Based on significant preclinical data, the VWF–ADAMTS13 axis is implicated in ischemia–reperfusion injury. ADAMTS13 knockout mice develop larger myocardial infarcts (12) and brain infarcts (13) than wild-type mice, while VWF-deficient mice have smaller heart and brain infarcts (14, 15). In these models, recombinant ADAMTS13 infusion leads to reduced ischemia–reperfusion injury, smaller myocardial infarcts, and smaller infarct size after cerebral ischemia (12, 15). Taken together, these data make a compelling argument for rebalancing VWF:ADAMTS13 homeostasis to mitigate the thromboinflammatory response and VOE in SCD. A phase 1 trial of rADAMTS13 in SCD is currently recruiting (NCT03997760) (16). Based on the pleomorphic effects of VWF in inflammatory pain and vasculopathy in SCD (17), future studies will be needed to evaluate the efficacy of rADAMTS13 as a disease-modifying therapy to prevent and treat VOE.