Circulating Biomarkers of COVID-19-Triggered Endotheliopathy: From Conjecture to Certainty

Background: Clinical and analytical data on patients suffering from coronavirus disease-2019 (COVID-19) indicate that endothelial damage plays a key role in the pathophysiology of the disease and is responsible for the pulmonary complications and the thrombotic microangiopathy affecting multiple organs, which contribute directly to mortality (Ackerman et al. N Engl J Med 2020). Detection of biomarkers of endothelial injury in circulating blood may provide critical diagnostic and prognostic information on the disease course (Goshua et al. Lancet Haematology 2020). Endothelial injury is also a cornerstone of pathobiology in other septic and potentially life-threatening inflammatory syndromes.

Objectives: To identify circulating markers of endothelial damage in COVID-19 patients, and compare their levels with those observed in other septic syndromes.

Methods: Plasma samples from non-critically ill patients with confirmed COVID-19 pneumonia (positive nasopharyngeal swab and confirmatory radiological chest imaging) requiring admission (n=42) were collected during the first 36h of hospitalization. Endothelial damage was evaluated by measuring in plasma: i) markers of endothelial function and activation (sVCAM-1, VWF, ADAMTS-13 activity, Protein C and α2-antiplasmin as a marker of fibrinolysis); ii) heparan sulfate (HS) levels, as indicators of endothelial glycocalyx degradation and loss of endothelial barrier function; and iii) C5b9 deposits on endothelial cells in culture, and soluble C5b9 (sC5b9) levels, to measure complement activation. Circulating dsDNA was analyzed as an indicator of the presence of neutrophil extracellular traps (NETs). ELISA tests were used for sVCAM-1, Protein C, HS, and sC5b9 levels. ADAMTS-13 activity was evaluated by FRETS. VWF, Protein C, and α2-antiplasmin were measured at the Atellica COAG 360 (Siemens Healthineers). C5b9 deposits were assessed by immunofluorescence and dsDNA levels by Quant-iT PicoGreen assay kit. Results were compared with those obtained in healthy donors (controls, n=45), and patients with non-infectious systemic inflammatory response syndrome (NI-SIRS, n=8) and septic shock (SS, n=8).

Results: Levels of sVCAM-1 were significantly higher in COVID-19 patients vs. controls, NI-SIRS and SS (159±12 vs. 79±4, 57±8 and 80±10 ng/mL, respectively, p<0.005) (Mean±SDM). VWF was elevated in COVID-19 patients vs. controls (240±26 vs. 96±5%, p<0.001), with similar values in NI-SIRS (271±40%), and significantly reduced vs. SS (476±43%, p<0.001). HS levels in COVID-19 patients were twice those detected in controls (1669±174 vs. 839±36 ng/mL, p=0.001), but they did not differ from those in NI-SIRS (1372±368 ng/mL), and were significantly lower than in SS (3677±880 ng/mL, p<0.001 vs COVID-19). Regarding complement activation, deposits of C5b9 on endothelial cells were significantly increased vs. controls (2-fold, p<0.01), with no notable differences vs. NI-SIRS (3±1-fold) and significantly lower than in SS (8±2-fold, p<0.001). Remarkably, sC5b9 levels were much more elevated in COVID-19 patients (1064±120 vs. 204±11 ng/mL, p<0.001), and no significant differences were observed vs. NI-SIRS (902±160 ng/mL) or SS (958±180 ng/mL). Also of note, presence of NETs was significantly elevated in the plasma of COVID-19 patients vs. controls (16±1.3 vs. 2±0.3 ng/ml, p<0.001), but similar to NI-SIRS (19±5 ng/mL) and clearly inferior to SS (33±6 ng/mL, p<0.001) (Figure). Importantly and in contrast, ADAMTS-13 activity, Protein C, and α2-antiplasmin values were within the normal range in COVID-19 patients.

Conclusions: Our data clearly demonstrate the presence of endothelial stress products in the circulating blood of non-critically ill COVID-19 patients. These biomarkers of endothelial injury are suggestive indicators of different aspects of the disease: specifically, release of acute phase reactants, degradation of the endothelial cell glycocalyx, and activation of the complement system. Furthermore, this profile of biomarkers in COVID-19 appears specific, with a differential behavior in comparison with septic shock, in which endothelial damage is also known to be critical. Additional studies are needed to validate these biomarkers as diagnostic and prognostic tools of the endothelial complications in COVID-19 patients, both in early disease and later, as well as supporting specific forms of therapeutic intervention.