What role does endothelial infection play in SARS-CoV-2 infection?

Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) can cause inflammatory lung disease, including clot formation and hyper-permeability of the pulmonary vessels, leading to edema and bleeding in the lungs. Inflammation also affects other organs, mediated by the cytokine storm.

This inflammation is characterized by dysfunction of endothelial cells in several organs. The cause of this endothelialopathy is unknown. It could be due to direct infection of endothelial cells or to an indirect effect of cytokines.

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Integrin binding by SARS-CoV-2

Unlike previous coronaviruses pathogenic to humans, SARS-CoV-2 has a spike protein which is linked to host recognition and viral attachment via the angiotensin converting enzyme receptor 2 (ACE2). A unique three-residue RGD motif outside the ACE2 recognition site can allow the spike protein to bind to endothelial proteins called integrins, which bind to the RGD group.

In fact, the major integrin of endothelial cells, called αVβ3, is able to bind to several RGD-binding ligands. It also engages several extracellular matrix proteins, such as fibrinogen, fibronectin and vitronectin, via its binding pocket. These matrix proteins regulate cell adhesion, migration and proliferation, as well as angiogenesis.

This mutation could thus improve the binding of SARS-CoV-2 to the host cell and could be responsible for the high transmissibility of this virus compared to the previous ones, while also allowing multiple entry routes for the virus and promoting its dissemination within the host. by two receivers.

SARS-CoV-2 thus produces a marked disruption of the endothelial barrier, causing it to lose its integrity and producing a hyper-permeable state. This leads to shock and the rapid spread of the virus to major organs.

Endothelial infection with COVID-19

Endothelial cells are essential for several physiological processes, including the activation of immune cells, platelet aggregation and adhesion, leukocyte adhesion, and transmigration. They are also the target of many viruses, leading to multi-organ dysfunctions.

Some studies failed to show growth of the virus in endothelial cells, which was attributed to the lack of angiotensin-converting enzyme receptor 2 (ACE2) expression on these cells.

However, it can be argued that this is due to the intrinsic differences between the endothelial monolayer cultured in vitro and the endothelial lining of the blood vessels which handle circulating blood under shear stress; activation of endothelial cells by the high volume of cytokines; and close contact with the epithelial cells of the pulmonary capillaries.

Other researchers have reported that SARS-CoV-2 is found in association with the endothelial cell marker CD31 in the lungs, infected mice and non-human primates (PNH). More importantly, this finding has been identified in the lung tissue of people who have died from severe COVID-19.

Viral proteins have also been found in endothelial cells. Additionally, infected mice showed upregulated KRAS signaling pathways in lung tissue known to mediate cell activation and dysfunction. Experimental evidence shows that mouse endothelial cells are infected with SARS-CoV-2.

Although all endothelial cells express ACE2, not all are targets for the virus. Instead, it requires the co-expression of other host proteases such as the transmembrane serine protease TMPRSS2 or the cathepsins, which cleave the spike protein into its fusion conformation, allowing viral entry into the host cell via endocytosis. .

Endothelial cell damage

After viral entry into the endothelial cell, it begins to translate its proteins, to replicate and can directly induce cell damage and apoptosis. At the same time, endothelial cells activate T cells, although less so than other antigen presenting cells. In fact, endothelial cells activate only antigen-specific memory or effector T cells, not naive lymphocytes.

In doing so, endothelial cells can promote the destruction of infected cells by presenting viral proteins to CD8 T cells. In addition, endothelial cells in the microvasculature can cause CD4 memory or effector T cells to migrate through the endothelium. Antiviral cytokines, including interferon gamma (IFN-γ) can induce major histocompatibility complex (MHC) class I or class II molecules, costimulatory molecules that are generally required for T cell activation occur.

This means that the endothelial dysfunction caused by COVID-19 blocks the activation of lymphocytes via endothelial cells, causing an imbalance in the adaptive immune response.

Cytokine storm

The cytokine storm leads to some kind of overshoot, causing further endothelial dysfunction. These cytokines include interleukin-6 (IL-6) which stimulates endothelial cell secretion of pro-inflammatory mediators and complement activation, thereby further enhancing the degradation of the endothelial barrier.

Lymphocyte depletion often seen in COVID-19 could also be the result of excessive inflammation induced by endothelial cell damage. The reduced number of CD4 lymphocytes can cause an altered response to infection while stimulating further inflammation. Thus, the hyper-inflammatory response in severe and critical COVID-19 could be due to infection and endothelial cell dysfunction.

Loss of integrity of the endothelial barrier

Infection with SARS-CoV-2 causes immune dysfunction as well as extensive endothelial damage, in addition to coagulation defects and systemic microangiopathy. The unfavorable outcome of the disease is largely due to the increased vascular permeability secondary to the inflammation associated with the infection.

This hyper-permeability is associated with the leakage of cellular and non-cellular components of the blood into the small blood vessels of the lung, causing the alveoli to congestion with fluid. The patient drowns in fluid from leaking blood vessels, which can be life threatening by causing suffocation.

Hypercoagulability

At the same time, the coagulation cascades are deregulated, causing the formation of microthrombi throughout the circulation, as well as infiltration of leukocytes. Dysfunction of endothelial cells can cause further inflammation and leukocyte recruitment and adhesion.

Since endothelial cells express glycosaminoglycans and thrombomodulin on their cell surface, they inhibit the component of the coagulation cascade, thrombin, as well as a tissue factor protein inhibitor. Many relaxing factors such as nitric oxide (NO) and prostacyclin (PGI2) are also produced by these cells, thereby blocking the adhesion and migration of leukocytes and platelets, smooth muscle proliferation and exerting an effect anti-inflammatory and anti-apoptotic.

When endothelial cells are damaged by viral invasion, they cease to exert their anticoagulant effect, resulting in a thrombotic tendency which manifests as extensive microthrombi, hyaline membrane formation in the small arterioles of the lung, and diffuse alveolar injury.

High levels of D-dimers occur with this hypercoagulable state, leading to poor outcomes and higher mortality with COVID-19. Multiple procoagulant mechanisms are at work, from the exposure of tissue factor to coagulation factors in the blood to the loss of endothelial integrity and therefore to the activation of the intrinsic pathway of coagulation by the matrix described below. the endothelial cell layer, to the devastating release of van Willebrand Factor (vWF), due to endothelial dysfunction. This molecule acts to bridge platelets for aggregation and clot formation.

Infection of endothelial cells could be associated with viral invasion of adjacent tissues, i.e., smooth muscle cells of the arteries and cardiac myocytes.

Therapeutic implications

Thus, SARS-CoV-2 infection of endothelial cells could be an underlying cause of cardiovascular complications of COVID-19, including end-stage multi-organ dysfunction. It is plausible that endothelial cell apoptosis has been observed in patients who died from COVID-19, as well as microthrombi scattered in the pulmonary vascular bed as well as right ventricular dysfunction, are associated with direct infection of endothelial cells.

Binding of the spike protein to αVβ3 can be inhibited by the specific Vβ3 antagonist Cilengitide, an RGD tripeptide, which has a high affinity for this integrin and suppresses virus-endothelium binding at very low doses.

Other therapeutic strategies include serine protease inhibitors, renin-angiotensin-aldosterone system inhibitors, statins, heparin, corticosteroids and IL-6 inhibitors, all of which act at least in part through the stabilization and protection of endothelial integrity.

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