Increased Histone-DNA Complexes and Endothelial-Dependent Thrombin Generation in Severe COVID-19

Objective

Coagulopathy in severe COVID-19 is common but poorly understood. The purpose of this study was to determine how SARS-CoV-2 infection impacts histone levels, fibrin structure, and endogenous thrombin potential in the presence and absence of endothelial cells.

Approach

We studied individuals with SARS-CoV-2 infection and acute respiratory distress syndrome at the time of initiation of mechanical ventilation compared to healthy controls. Blood samples were assayed for levels of histone-DNA complexes. Confocal microscopy was used to evaluate fibrin structure in clots formed from recalcified plasma samples using fluorescently-labeled fibrinogen. Endogenous thrombin potential was measured by calibrated automated thrombin assays in the presence of tissue factor and phospholipid (PCPS) or cultured human endothelial cells.

Results

Circulating nucleosomes were elevated in the plasma of COVID-19 patients relative to healthy controls (n=6, each group). COVID-19 patient plasma thrombin generation was also altered. Despite having an increased endogenous thrombin potential, patient plasma samples exhibited prolonged lag times and times to peak thrombin in the presence of added tissue factor and PCPS. Strikingly different results were observed when endothelial cells were used in place of tissue factor and PCPS. Control plasma samples did not generate measurable thrombin (lag time >60 min); in contrast, plasma samples from COVID-19+ patients generated thrombin (mean lag time ∼20 min). Consistent with the observed alterations in thrombin generation, clots from COVID-19 subjects exhibited a denser fibrin network, thinner fibers and lower fibrin resolvability.

Conclusions

Elevated histones, aberrant fibrin formation, and increased endothelial-dependent thrombin generation in COVID-19 may contribute to coagulopathy.

HIGHLIGHTS

• Histone-DNA complexes are significantly elevated in the plasma of patients with severe SARS-CoV-2 infection.

• Measures of thrombin generation by calibrated automated thrombography and fibrin clots formedin situare altered in severe COVID-19.

• Plasma from COVID-19 patients promotes thrombin generation on cultured endothelial cells in the absence of added tissue factor or phospholipids.

• The additive effects of histones on thrombin generation and endothelial cell function may play a major role in the thrombotic complications observed in severe SARS-CoV-2 infection.