High levels of von Willebrand factor with reduced specific activities in hospitalized patients with or without COVID-19.

The COVID-19 pandemic is often accompanied by severe respiratory illness and thrombotic complications. Von Willebrand Factor (VWF) levels are highly elevated in this condition. However, limited data are available on the qualitative activity of VWF in COVID-19. We measured plasma VWF levels quantitatively (VWF antigen) and qualitatively (ristocetin-induced platelet agglutination, glycoprotein IbM (GPIbM) binding, and collagen binding). Consistent with prior reports, VWF antigen levels were significantly elevated in hospitalized patients with or without COVID-19. The GPIbM and collagen binding activity-to-antigen ratios were significantly reduced, consistent with qualitative changes in VWF in COVID-19. Of note, critically ill hospitalized patients without COVID-19 had similar reductions in VWF activity-to-antigen ratios as patients with COVID-19. Our data suggest that qualitative changes in VWF in COVID-19 may not be specific to COVID-19. Future studies are warranted to determine the mechanisms responsible for qualitative changes in VWF in COVID-19 and other critical illnesses.• VWF levels were increased in COVID-19 compared to healthy controls.• VWF activity-to-antigen ratios were decreased in COVID-19 compared to healthy controls.• There were no differences in VWF activity-to-antigen ratios between hospitalized patients with or without COVID-19.• These findings are consistent with qualitative changes in VWF in systemic inflammation which are not specific to COVID-19.• Future studies are needed to define possible roles of changes in conformation or multimer length in the qualitative changes in VWF in systemic inflammation.

The Activity of von Willebrand Factor in Hospitalized Patients with COVID-19

Infection with the SARS-CoV-2 virus, or COVID-19, is associated with an increased risk of thrombosis at all levels of the vascular tree including the microvasculature, arteries and veins. However, the mechanisms involved in the heterogenic thrombotic presentations in COVID-19 are poorly understood. One potential contributor to the prothrombotic state in COVID-19 is von Willebrand Factor (vWF), a multimeric glycoprotein produced by endothelial cells and platelets that is unfolded and activated by shear stress or artificially with ristocetin. vWF has been implicated in pathogenesis of arterial clots including stroke, consistent with its activation by shear. vWF has also been implicated in thrombotic thrombocytopenic purpura, a subtype of microvascular thrombosis. Elevated vWF has even been associated with increased risk of venous thrombosis, a classically low shear environment. The purpose of this study was to determine whether vWF had a higher propensity to become activated in COVID-19, suggestive of an altered configuration. We hypothesized that vWF from patients with COVID-19 would be more active than healthy controls. Discarded whole blood samples were collected from hospitalized patients diagnosed with COVID-19;blood from healthy donors was collected via venipuncture. Patient and control plasma were rapidly thawed at 37 ? then transferred to de-identified containers to blind performing scientist. Samples were processed in a duplicate manner adding 30 ?L of plasma to a solution containing 200 x103/?L lyophilized platelets. We measured vWF activation using two methods: the first involved agglutination with an intermediate concentration of ristocetin (between 0.6 - 0.7 mg/mL) to achieve less than 40% of normal agglutination. The second method involved detection of A1-12 antibody binding with ELISA. The A1-12 antibody binds to the N-terminus of the A1 subunit, a location that is typically obscured by the A2 domain and thus is only accessible with vWF is unfolded. All measured values were compared to normal pooled plasma (NPP) processed in the same experiment. Notably all measures of activation of vWF were significantly higher in COVID-19 patients compared to healthy controls. The primary slope of the agglutination was the most significantly elevated when compared to controls (162 verses 106 % of NPP, p = 0.002). The absolute amount of vWF was also significantly increased in patients compared to controls (148 verses 100% of NPP, p < 0.001). These preliminary data are suggestive of increased activity of vWF in COVID-19 infectionin addition to an increased absolute level. vWF may be a significant pathway that becomes dysregulated during the immune response to SARS-CoV-2. This study is limited by its use of discarded blood that has undergone one freeze-thaw cycle. Further work is needed to correlate these results to clinical outcomes.

Microvascular thrombosis: experimental and clinical implications.

A significant amount of clinical and research interest in thrombosis is focused on large vessels (eg, stroke, myocardial infarction, deep venous thrombosis, etc.); however, thrombosis is often present in the microcirculation in a variety of significant human diseases, such as disseminated intravascular coagulation, thrombotic microangiopathy, sickle cell disease, and others. Further, microvascular thrombosis has recently been demonstrated in patients with COVID-19, and has been proposed to mediate the pathogenesis of organ injury in this disease. In many of these conditions, microvascular thrombosis is accompanied by inflammation, an association referred to as thromboinflammation. In this review, we discuss endogenous regulatory mechanisms that prevent thrombosis in the microcirculation, experimental approaches to induce microvascular thrombi, and clinical conditions associated with microvascular thrombosis. A greater understanding of the links between inflammation and thrombosis in the microcirculation is anticipated to provide optimal therapeutic targets for patients with diseases accompanied by microvascular thrombosis.