Endotheliopathy in COVID-19 and Sickle Cell Disease

Background: This thesis examines endotheliopathy in two distinct disease settings; COVID-19 and Sickle Cell Disease (SCD). Coagulation activation and endothelial cell (EC) dysfunction are now well-known features of COVID-19, however our understanding of EC activation in acute and convalescent disease has evolved substantially since the pandemic began. SCD is associated with coagulation activation, recurrent episodes of painful ‘crisis’ and a risk of stroke in up to 10% of children. Recent data point to novel roles for EC activation and von Willebrand Factor (VWF) in both COVID-19 and SCD pathogenesis. 

Methods: Acute and convalescent COVID-19 adults were recruited from St James’s and Beaumont Hospitals. A paediatric SCD cohort was recruited from Children’s Health Ireland (CHI) at Crumlin. Plasma levels of EC biomarkers were measured, including VWF, with VWF multimers examined by gel electrophoresis. Levels and activity of ADAMTS13 were evaluated. Coagulation parameters were assessed, including D-dimer and Factor VIII (FVIII) levels and thrombin generation (TG). The ability of patient plasma to activate human umbilical vein endothelial cells (HUVEC) and promote angiogenesis was evaluated using ex vivo HUVEC stimulation and tube formation assays, respectively. 

Results: Deranged coagulation parameters, including elevated D-dimer and Fibrinogen levels were common in acute COVID-19 and predictive of clinical outcome. Patients with severe COVID-19 demonstrated prolonged, extreme VWF elevations, abnormal VWF multimers and an increased VWF-ADAMTS13 ratio. Both increased synthesis and reduced VWF clearance contributed to elevated levels. HUVEC stimulation with COVID19 plasma resulted in increased EC activation and tube formation. In convalescent COVID-19 patients, elevated D-dimer, VWF and FVIII levels were noted in 25%-30%. Increased VWF and decreased ADAMTS13 levels resulted in a similar (but less marked) disturbance of the VWF-ADAMTS13 axis to that observed in acute COVID-19. In SCD, significant elevations in EC biomarkers were noted in sickle ‘crisis’, compared with ‘steady-state’, especially in children with acute chest syndrome (ACS). In steadystate, chronically transfused children had reduced VWF and FVIII levels, reduced TG and normal VWF multimers compared with children on hydroxycarbamide. 

Conclusions: Acute COVID-19 confers a state of marked EC activation and a pro-thrombotic environment. A proportion of convalescent COVID-19 patients demonstrate sustained endotheliopathy, which may play a role in post-COVID thrombotic events and/or Long COVID syndrome. In paediatric SCD, acute EC activation is a feature of sickle crisis, especially ACS. Our findings highlight favourable effects of blood transfusion on red cell and EC health compared with hydroxycarbamide.