In 2019, a new coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), caused a contagious disease known as coronavirus disease 2019 (COVID-19). The condition was declared to be a pandemic in March 2020 by the World Health Organization. ^{1 , 2} COVID-19 ranges from being asymptomatic in some cases ³ to causing mild symptoms in the majority of cases, including fever and upper respiratory and gastrointestinal tract symptoms. ^{4 , 5} However, it can progress to severe cases of pneumonia, multi-organ failure, and, ultimately, death. ^{6 – 8}

Akin to other coronaviruses, SARS-CoV-2 uses angiotensin-converting enzyme 2 (ACE2), which is a membrane-bound peptidase, as a gateway to enter the host cells ^{9 , 10} ; ACE2 expression was reported to be correlated with increased viral load in human cell lines ^{11 , 12} as well as rodents. ¹³

COVID-19 patients display a state of increased coagulability and thrombotic tendency. ^{14 , 15} Antithrombotic prophylaxis represents an essential part of the treatment plan to prevent the development of pulmonary embolism and deep vein thrombosis that develop in severe cases or cases with increased d-dimer. ^{16 – 18} However, despite antithrombotic prophylaxis, approximately 30% of patients with severe COVID-19 admitted to the intensive care unit have developed arterial and venous thrombotic events, increasing the mortality risk in these patients by 5.4 times. ¹⁹ Several mechanisms mediate the hypercoagulability of COVID-19 patients. ²⁰

Among these mechanisms is the exocytosis of Weibel-Palade bodies from the vascular endothelium upon SARS-CoV-2 invasion. ^{21 – 23} Weibel-Palade bodies are endothelial storage granules of mediators involved in inflammation and coagulation, primarily P-selectin and von Willebrand factor. ^{24 , 25} Weibel-Palade bodies contain several chemokines. ²⁶

Angiopoietins are members of a family of vascular growth factors that play crucial roles in angiogenesis by binding to their physiologic receptors, tyrosine kinase with immunoglobulin-like and epidermal growth factor (EGF)-like domains 1 (Tie-1) and Tie-2, where Tie-2 is the primary receptor. Angiopoietin-1 is primarily produced in pericytes and smooth muscle cells; however, large quantities are produced and released from platelets upon activation. ²⁷ It is critical for endothelial cell integrity, survival, migration, and the suppression of inflammatory gene expression. ^{28 , 29} Angiopoietin-2, on the other hand, is a context-dependent competitive antagonist of angiopoietin-1, promoting endothelial apoptosis and disrupting vascularization. ³⁰ Angiopoietin-2 is a chemokine stored in Weibel-Palade bodies, and its serum level is elevated in patients with severe COVID-19. ³¹ Parikh, suggests that disruption of the angiopoietin/Tie2 pathway contributes to vascular dyshomeostasis in sepsis. ³²

This study aimed to investigate serum levels of angiopoietin-1 in patients with severe COVID-19.

No significant difference was detected between patients with severe COVID-19 and the control subjects regarding age and sex. Therefore, demographic data are summarized in Table 2. ⁴⁵

Serum angiopoietin-1 level was significantly lower [14.52 (5.56) ng/ml] in the patients with severe COVID-19 than in the control subjects [30.56 (17.56) ng/ml], ( p-value <0.001) ( Table 3).

Serum angiopoietin-1 levels displayed diagnostic potential in differentiating between patients with severe COVID-19 and non-infected individuals according to the ROC curve ( Table 4 and Figure 1).

COVID-19 was hypothesized to be a vascular disease with endothelial damage or dysfunction, angiogenesis, and hypercoagulation. ³⁵ Endothelial cell integrity is crucial for protecting the thrombotic environment and developing inflammation in COVID-19 patients. Disruption of the endothelial thromboprotective role might lead to hypercoagulation, as primarily described in these patients. ^{14 , 15} In a prospective study, Smadja et al., showed that SARS-CoV-2 infection is accompanied by elevated levels of the soluble endothelial activation markers, namely, soluble E-selectin and angiopoietin-2, in critically ill hospitalized COVID-19 patients. ³¹

Angiopoietin-2, a context-dependent competitive antagonist of angiopoietin-1, is an essential regulator of endothelial homeostasis, angiogenesis, and proliferation through the angiopoietin/Tie-2 pathway. ³⁰ Elevated serum angiopoietin-2 levels have been reported in patients with sepsis or acute respiratory distress syndrome (ARDS). ³⁶ On the other hand, circulating angiopoietin-1 levels have been found to be lower in patients with sepsis-induced multiple organ dysfunction syndrome and to be correlated with the clinical course of the condition. ³⁷ Whitney et al., showed that the angiopoietin-2/angiopoietin-1 ratio is higher in septic patients with ARDS than in septic patients without ARDS, which in turn is higher than in the control subjects, suggesting that endothelial dysfunction mediated by alterations in angiopoietins levels is involved in the pathogenesis of ARDS in the course of extrapulmonary sepsis. ³⁸

In COVID-19, infected endothelial cells are stimulated to release angiopoietin-2 from the Weibel-Palade bodies. Once in circulation, angiopoietin-2 competitively inhibits angiopoietin-1, disrupting its thromboprotective role. ³⁹ Higgins et al., showed that diminished Tie2 signaling develops before overt disseminated intravascular coagulation and that exogenous angiopoietin-1 administration normalizes hypercoagulability in endotoxemic mice. This implies that the angiopoietin-1/Tie2 pathway is central in protecting against microvascular thrombus formation during sepsis, even without inflammation. ⁴⁰

In the present study, serum angiopoietin-1 levels were significantly lower in patients with severe COVID-19 than in the control group.

Abou-Arab et al., showed that the serum angiopoietin-2/angiopoietin-1 ratio is significantly higher in critically ill patients than in patients with severe COVID-19. However, serum angiopoietin-1 level was not significantly different between the two groups. ⁴¹ Compared to the present study, Abou-Arab et al., did not exclude patients with other comorbidities and did not compare serum angiopoietin-1 levels with healthy controls. ⁴¹ Similarly, Vassiliou et al., reported that serum angiopoietin-1 levels were not significantly different between survivors and non-survivors of critically ill COVID-19 patients admitted to the intensive care unit. ⁴²

As mentioned earlier, platelets produce and release a significant amount of angiopoietin-1. ²⁷ Thrombocytopenia is a common finding in severe COVID-19 infections, and it is associated with poor clinical outcomes. ⁴³ Thrombocytopenia in severe COVID-19 conditions was attributed to platelet apoptosis and platelet consumption by incorporation into microthrombi. ⁴⁴ Thus, lower serum angiopoietin-1 levels in patients with severe COVID-19 may be a consequence of thrombocytopenia.

In the present study, serum angiopoietin-1 showed good potential as a diagnostic marker to differentiate severe COVID-19 from non-infected individuals, with an AUC of 0.98, 100% specificity, and 97.8% sensitivity at a cut-off value of ≤21.05 ng/ml with a confidence interval of 0.94–1.00. On the other hand, Vassiliou et al., found that the serum angiopoietin-2, an angiopoietin-1 antagonist, is an excellent diagnostic marker to differentiate between severe and critically ill COVID-19 patients with an AUC of 0.86, with 77.3% specificity and 88.9% sensitivity at a cut-off value of ≥4 ng/ml with a confidence interval of 0.72–0.99. ⁴² This emphasizes the role of disturbed serum angiopoietin levels during COVID-19.

Patients with severe COVID-19 have lower serum angiopoietin-1 levels, and serum angiopoietin-1 levels have diagnostic potential for differentiating between patients with severe COVID-19 and non-infected individuals.