Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes the so-called coronavirus disease (COVID-19), which is characterized by a broad spectrum of clinical presentations ranging from asymptomatic patients to critically ill individuals with a high case fatality rate (1). The critical care community has increasingly recognized that cardiovascular and thrombotic complications are relatively common in COVID-19 (2). Indeed, direct involvement of the vascular endothelium was recently reported in a series of patients suffering from severe COVID-19 (3). The endothelial glycocalyx (eGC), which covers the luminal surface of endothelial cells, contributes to the maintenance of vascular homeostasis, whereas disruption of the eGC is observed early in critically ill patients and is associated with inferior outcomes (4, 5). Here, we investigated in translational human and cellular studies whether injury to the eGC can be found in critically ill patients with COVID-19 early after admission to the ICU. We collected plasma and serum from 19 adult individuals within 24 hours after invasive ventilation for acute respiratory distress syndrome and from 10 healthy human donors after written informed consent of patients or their legal representative. The first patient was admitted on March 19, 2020, and the observation period was until May 17, 2020. The median (interquartile range) observation duration was 47 (40–54) days. Baseline patient characteristics at study inclusion as well as a description of the further clinical course are summarized in Table 1. Organ failure was not restricted to the lungs, and multiorgan dysfunction was common both at inclusion and during the further clinical course. Surprisingly, global markers of endothelial injury such as Angpt-1 (angiopoietin-1)(control: 29 [26.2–30.9] ng/ml vs. COVID-19: 27.8 [23.4–36.2] ng/ml; P= 0.79) and Angpt-2 (control: 0.655 [0.336–1.113] ng/ml vs. COVID-19: 0.434 [0.035–1.338] ng/ml; P= 0.6) were unchanged in patients with COVID-19. In contrast, marked increases in the soluble form of the sTie2 (Tie2 receptor)(Figure 1A) and in syndecan-1 (Figure 1B)—indicating pathological shedding of transmembrane proteins involved in glycocalyx structure and processing—were observed. The key eGC sheddase Hpa-1 (heparanase-1) and its enzymatic activity were both not significantly increased (data not shown). To the contrary, the Hpa-1 counterpart, the protective Hpa-2 (heparanase-2), was pertinently reduced in all patients with COVID-19 (Figure 1C). Driven by this acquired Hpa-2 deficiency, the Hpa-1: Hpa-2 ratio was higher in patients with COVID-19 (P= 0.012; data not shown). Together, this indicates that critically ill patients with COVID-19 suffer from an acquired Hpa-2 deficiency that can contribute to the degradation of the eGC, maybe even before classical endothelial activation and injury. Next, eGC structure was analyzed in humans, employing sublingual sidestream darkfield (SDF) imaging. We quantified the size of the individual patients’ eGC using an indirect surrogate termed the perfused boundary region and found a decrease of perfused boundary region, indicating reduced eGC thickness in patients with COVID-19. To demonstrate that the deficiency of Hpa-2 is mechanistically involved in the degradation of the eGC, we used a microfluidic chamber with cultured endothelial cells (ECs) under flow that synthesize an intact and stable eGC under in vitro conditions. After stimulation with COVID-19 or control serum, the eGC was visualized by confocal microscopy followed by computerized three-dimensional reconstruction. Its thickness was then quantified by analyzing the heparan sulfate (HS)-positive area. We …