If one thing is for sure, it is that thrombosis is a prominent characteristic of COVID-19. Thromboinflammation due to SARS-CoV-2 infection hits young and old, symptomatic and asymptomatic, the healthy and the immunocompromised. In young, "healthy", asymptomatic cases or cases with so-called "mild symptoms", this is even further complicated by the fact that thrombosis goes undetected as opposed to severe cases that are admitted to hospital. In order to assess whether people who have had "mild" SARS-CoV-2 infection will actually recover, imaging and screening is required. This topic should raise enough concern to be part of long-term cohort studies.
I have been actively reporting on the prominence of thrombosis as the main feature of COVID since April 2020. Amidst the quest for vaccines to prevent infection with SARS-CoV-2, the thrombosis question must still be addressed. Firstly, because it is highly undesirable to create a generation of long-term cardiovascular, pulmonary and multiple organ damage due to COVID.
The one main question during this pandemic is: how should COVID's prominent thromboinflammation be treated or prevented?
1. COVID-19 is characterized by ongoing thrombotic events in spite of thromboprophylaxis and thrombolysis;
2. A close look at angiopathy in COVID-19: findings;
3. Loss of endothelial glycocalyx (eGC) thickness;
4. How COVID affects systemic systems and therefore poses a threat to the young and healthy as well as the immunocompromised;
5. Therapeutic restoration of the glycocalyx in COVID-19: an overlooked essential?
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| Some of the many mechanisms involved in COVID-19-thromboinflammation |
1. COVID-19 is characterized by ongoing thrombotic events in spite of thromboprophylaxis and thrombolysis
Thrombosis is such a stubborn complication of COVID-19, as there is a low response to adequate anticoagulation (Low Molecular Weight Heparin, enoxaparin 40-60 mg twice a day prophylaxis; or 1 mg/kg body weight twice a day for therapeutic treatment) and thrombolytic therapies (tissue Plasminogen Activator (tPA)). Inflammatory and prothrombotic changes in the arterial wall with the lack of lung perfusion are thought to cause diffuse arterial thrombosis in the lungs.
Outstanding parameters for determining the severity of COVID-thrombosis are the increase of platelet count, D-dimer, ferritin and LDH levels. While prothrombin time and fibrinogen levels are not extremely prolongated nor elevated, increase of platelet count is markedly high in some of the most severe cases (Thrombosis of pulmonary vasculature despite anticoagulation and thrombolysis: The findings from seven autopsies, Thrombosis update 2020 Dec; 1).
2. A close look at angiopathy in COVID-19: findings
Thrombus formation in the lungs shows underlying inflammatory abnormalities that are caused by the virus itself (SARS-CoV-2) and immune cells. Disseminated Intravascular Coagulation (DIC) is considered to be one of the causes of COVID-thrombosis in pulmonary vessels, the liver, kidneys and pelvic veins. The pulmonary vasculature prevails over deep vein thrombosis in the lower limbs (DVT). Seven autopsies found accumulation of fluid with surfactant, damaged epithelium, alveolar macrophages and neutrophils in the alveolar sacs. Focal epithelial proliferation with fused cells were found, as well as enlarged epithelial cells with SARS particles and pyroptosis in the alveoli (lung sacs). Hyaline membranes and fibrin were found in the alveolar space, some alveolar septa were infiltrated sclerotic. Thrombi in the capillaries were predominantly fibrotic.
Hypertrophy was found in the bronchi, while epithelium, mucus and macrophages were observed in the bronchial lumina. Dystrophy and necrosis were observed in the endothelium and muscle fibers, accompanied by fibrinoid necrosis, collagen particles and elastic fibers and infiltration of the vascular wall and perivascular space by plasma, neutrophils and lymphocytes.
In an untreated COVID patient, severe edema of the lungs, fibrin particles, hyaline membranes and pyroptosis of large alveolocytes were observed. Epithelial damage was accompanied by surfactant networks, macrophages and neutrophils in the alveoli. Advanced thrombosis was found in the pulmonary artery. Hemorrhages were observed in the brain (cerebral cortex, cerebellum and white matter of the hemispheres) and the lungs, stomach, colon, bladder and kidneys.
3. Loss of endothelial glycocalyx (eGC) thickness
In the MYSTIC cohort study, COVID patients showed a 90% reduction in vascular density in the small capillaries. Loss of glycocalyx thickness of the endothelium is associated with severity. Notably, a 2019 study found the glycocalyx to be a therapeutic target in sepsis (The glycocalyx: a novel diagnostic and therapeutic target in sepsis, Critical Care 2019; 23: 16). In mechanically ventilated COVID patients, the decrease in vascular density was greater than in non-mechanically ventilated COVID patients, but both groups showed loss of vascular density. Density of microvessels > 10 µm did not differ in mechanically ventilated or non-mechanically ventilated COVID patients, showing that the small capillaries are the main affected site. Plasma syndecan-1, a marker for glycocalyx shedding, was increasingly elevated in patients with higher need for mechanical ventilation.
The vascular leakage-increasing Angiopoietin-2 (Angpt-2) was significantly increased in mechanically ventilated COVID patients. A 2019 study reports the therapeutic potential of Tie2 activation to promote endothelial glycocalyx restoration in human sepsis (Tie2 Activation Promotes Protection and Reconstitution of the Endothelial Glycocalyx in Human Sepsis, Journal of Thrombosis and Haemostasis 2019 Nov;119(11):1827-1838). The vasodilator and permeability factor VEGF-A was found to correlate with severity. ADAMTS13 levels (ADAMTS13 is the protease that cuts Ultra Large Von Willebrand Factor Multimers to protect against thrombosis) were significantly decreased in severe COVID, while Thrombomodulin (TM) and levels of shed ACE2 were markedly high. Plateletcrit (PCT = blood volume occupied by platelets) and Tumor Necrosis Factor-Alpha (TNF-α) were high in ventilated patients, while CRP, IL-6 and ferritin did not stand out as markers of severity (Microvascular dysfunction in COVID-19: the MYSTIC Study, Angiogenesis 2020 Oct 14: 1-13).
What can be gathered from the MYSTIC study, is:
- markers such as TNF-alpha, and elevation of plateletcrit should be monitored to estimate the severity of COVID-19;
- the VWF/ADAMTS13 ratio is a marker for severity, as increasing imbalance of VWF (elevation) to ADAMTS13 (loss) is associated with deterioration and endothelial disease;
- loss of glycocalyx is an outstanding marker for deterioration in COVID, associated with the loss of endothelial integrity;
- a therapeutic target to explore in COVID-19 is prevention of the heparanase-mediated loss of glycocalyx through a non-coagulant heparin fragment.
Immunopathological findings
Inflammatory infiltration of the vascular wall consists of CD45 lymphocytes, CD68 macrophages, CD61 megakaryocytes, CD3-, CD4- and CD8+ T-cells and CD15 neutrophils. Thrombosis of pulmonary artery branches is observed less frequently than thrombosis of the microvasculature. Therefore, it is essential to differentiate between pulmonary embolism and thrombosis in COVID.
Regardless of ARDS or the degree of viral pneumonia, the capillary is found to be destroyed, accompanied by fibrinoid necrosis. It is likely the loss of integrity of the vascular wall that initiates advanced thrombotic mechanisms in COVID. The virus itself damages the endothelium, followed by apoptosis which increases the loss of anticoagulation function. Cytotoxic upregulation of immune cells and endotheliocytes and activation of the complement system further endothelial damage.
Targeting the loss of pulmonary perfusion
It is hypothesized that loss of anticoagulant function is key in unmatched perfusion. This is a plausible explanation, as erratic pulmonary perfusion is consistent with persistence of thrombotic activity and failing thrombolytic therapies. Therefore, a therapy targeting pulmonary perfusion is proposed (Thrombosis
of pulmonary vasculature despite anticoagulation and thrombolysis: The
findings from seven autopsies, Thrombosis update 2020 Dec; 1).
4. How COVID affects systemic systems and therefore poses a threat to the young and healthy as well as the immunocompromised
From a hematologist's perspective, COVID is a systemic infection that encompasses cardiovascular, gastrointestinal, respiratory and hematopoietic systems. SARS-CoV-2 affects young individuals with no underlying disease and puts healthy young at risk for DIC (disseminated intravascular coagulopathy), myocarditis, venous thrombo-embolism (VTE) and lymphopenia.
Around 14 days after symptom onset, severe lymphopenia is reported in 83.2% of COVID cases, according to a cohort study involving 1099 patients. Progressive thrombocytopenia and leukopenia are reported to make for 26.2% and 33.7% of cases, showing about two weeks after symptom onset. Prominent lymphoplasmacytoid-lymphocytes with eccentric nuclei and basophilic cytoplasm and neutrophils with abnormal pseudo-Pelger nuclei and unusually enlarged vacuolated platelets were observed in severe COVID cases. Inflammatory cytokines contribute to lymphopenia.
It is noted that the cytokine cascade in COVID, also called Cytokine Release Syndrome (COVID-CRS) or hypercytokinemia, follows a path similar to hemophagocytic lymphohistiocytosis (HLH) and Castleman. Interestingly, these hematologic syndromes share with COVID a tendency for thrombosis, hypercoagulation, hypotension and hypoxia (SARS-CoV-2 Infected Patient: from a Hematologist's Perspective, Mediterranean Journal of Hematology and Infectious Diseases 2020;12(1)). It can be confirmed that nasopharyngeal viral load (RNAaemia) correlates to levels of Interleukin-6 (IL-6) and disease severity. Serum levels of IL-6 relate to infection severity and viral load. Also, a significant positive correlation was found between viral load and IL-2R.
Patients at risk of vasculopathy are prone to endothelial damage due to COVID. Diabetes and hypertension are the main comorbities that increase the risk of severe disease, associated with endotheliopathy in COVID. Fasting blood sugar level (FPG) is a marker for severity and hypoxia risk. Endotheliopathy and platelet hyperactivation are marked by increased Von Willebrand Factor (VWF) antigen and thrombomodulin (TM) levels and low ADAMTS13 levels.
Inflammatory profile
The hematologist's perspective offers great insight into the inflammatory markers that should be monitored closely to predict COVID severity. Since SARS-CoV-1 (2003), it has been known that epithelial cell proliferation and macrophage induction in the lungs illustrate the inflammatory process that contributes to disease severity. An increased IL-6/IFN-γ ratio can be predictive of disease severity. Procalcitonin and C-reactive protein should be measured regularly. Procalcitonin levels were observed to be significantly elevated, from 4 to 8 times higher in severe SARS-CoV-2 infection. Severe cases can be further marked by dyspnea, lymphopenia, hypoalbuminemia, elevated alanine aminotransferase (ALA), lactate dehydrogenase, higher levels of IL-2R, IL-6, IL-10 and TNF-alpha.
From the hematologist's perspective, it is speculated that hyperferritinemia is not a common feature of COVID-19. However, iron availability favors viral replication in macrophages. Erythropoietin is known to regulate inflammatory cytokines and hepcidin, allowing iron absorption in the bone marrow. Iron redistribution could impair viral replication. Erythropoietin increases the red blood cell mass and has anti-apoptotic cytoprotective properties. As of April 2020, EPO has been shown to alleviate ARDS and to reduce damage through inhibition of NF-kB (Does recombinant human erythropoietin administration in critically ill COVID-19 patients have miraculous therapeutic effects?, Journal of Medical Virology Vol. 92, Issue 7, July 2020, P915-918). EPO has a potential negative effect, nevertheless, as this agent increases the risk of thrombosis in COVID-patients who are already at risk of developing thrombosis.
5. Therapeutic restoration of the glycocalyx in COVID-19: an overlooked essential?
It has been well known for 20 years or even more, that the endothelium is key in maintaining a healthy homeostasis in order to prevent sepsis and thrombosis. Thrombosis is an acknowledged complication of SARS-Coronaviruses (SARS-CoV-1, 2003 and SARS-CoV-2, 2019) and other highly thrombo-inflammatory pathogens. Sepsis is, in short, a toxic phase in which organs start to fail due to infection, inflammation, hypercoagulation and thrombosis. The glycocalyx exerts shear stress to maintain healthy blood flow. In COVID-19, microcirculation and shear stress are impaired. Sepsis, along with thrombosis, is a major contributor to deterioration in COVID.
Given the fact that endothelial damage marks the turnover to disease progression in COVID, glycocalyx integrity should be included in therapeutic treatment of COVID-19. Performing a search in PubMed, I used the keywords "Glycocalyx", "EGx" (Endothelial glycocalyx) and "restoration". The 2017 study "Therapeutic Restoration of Endothelial Glycocalyx in Sepsis, Journal of Pharmacology and Experimental Therapeutics 2017 Apr; 361(1): 115-121) proposes administration of Sulodexide (SDX), a heparin sulfate-like agent to restore endothelial integrity in sepsis.
A combination of antithrombotic therapy and endothelial glycocalyx restoration could offer new perspectives for an adequate treatment of COVID-19.
