Vasculopathy in COVID-19

Abstract

COVID-19 is a primary respiratory illness that is frequently complicated by systemic involvement of the vasculature. Vascular involvement leads to an array of complications ranging from thrombosis to pulmonary edema secondary to loss of barrier function. This review will address the vasculopathy of COVID-19 with a focus on the role of the endothelium in orchestrating the systemic response to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. The endothelial receptor systems and molecular pathways activated in the setting of COVID-19 and the consequences of these inflammatory and prothrombotic changes on endothelial cell function will be discussed. The sequelae of COVID-19 vascular involvement at the level of organ systems will also be addressed, with an emphasis on the pulmonary vasculature but with consideration of effects on other vascular beds. The dramatic changes in endothelial phenotypes associated with COVID-19 has enabled the identification of biomarkers that could help guide therapy and predict outcomes. Knowledge of vascular pathogenesis in COVID-19 has also informed therapeutic approaches that may control its systemic sequelae. Because our understanding of vascular response in COVID-19 continues to evolve, we will consider areas of controversy, such as the extent to which SARS-CoV-2 directly infects endothelium and the degree to which vascular responses to SARS-CoV-2 are unique or common to those of other viruses capable of causing severe respiratory disease. This conceptual framework describing how SARS-CoV-2 infection affects endothelial inflammation, prothrombotic transformation, and barrier dysfunction will provide a context for interpreting new information as it arises addressing the vascular complications of COVID-19.

Graphical abstract

Figure 1.

Endothelial mechanisms that maintain vascular quiescence and vessel patency. The quiescent endothelium depicted in the center of the figure features a rich glycocalyx covering its inner surface (black lines) and is surrounded by pericytes (dark green) on its out surface. Circulating cells move freely within its lumen. The endothelium has several receptor systems that mediate constitutive cytoprotective signaling (green background). (Panel 1) Activation of Tie2 (blue) by multimeric angiopoitein-1 (orange). Clustering of Tie2 by angiopoieitin-1 results in phosphorylation of the Tie2 cytoplasmic tail. Stimulation of Tie2 also promotes barrier function via activation of Rac1. (Panel 2) Although ACE produces angiotensin II, which can stimulate inflammatory signaling, the resting endothelium expresses ACE2, which cleaves angiotensin II into Ang(1-7). Ang(1-7) binds to MAS1, resulting in cytoprotective signaling. (Panel 3) The endothelium expresses thrombomodulin (TM), which binds thrombin (IIa) and occludes its fibrin binding site, modifying its substrate specificity. TM is closely associated with the endothelial protein C receptor (EPCR), which binds protein C (PC) enabling it to be cleaved by thrombin and generate activated protein C (APC). APC is a potent anticoagulant that cleaves factors Va and VIIIa. It also cleaves protease-activated receptor 1 (PAR1) at a noncanonical cleavage site, stimulating cytoprotective signaling. Antithrombotic mechanisms are indicated with yellow background. (Panel 4) The endothelium expresses tissue factor pathway inhibitor-β (TFPIβ), which associates with the membrane surface via a glycosylphosphatidylinositol (GPI) anchor. TFPIβ binds to both factor VIIa (FVIIa) and factor Xa (FXa) via its kunitz domains (red), thereby inhibiting the ability of tissue factor (TF) to activate coagulation. (Panel 5): Fibrin is cleared from the vasculature by plasmin degradation. Endothelial cells secrete both tissue plasminogen activator (tPA) and urokinase plasminogen activation (uPA), which binds to the urinokinase plasminogen activator receptor (uPAR). Plasminogen activators convert plasminogen to plasmin, which cleaves fibrin, resulting in generation of fibrin degradation products (FDPs) and d-dimer. Plasminogen activator inhibitors 1 and 2 (PAI-1 and PAI-2) inhibit tPA and uPA. (Panel 6) Several surface proteins inhibit inappropriate complement activation. These include the type 1 membrane protein CD46 that inactivates C3b and C4b, the GPI-linked membrane protein CD55 (or complement decay-accelerating factor) that prevents formation of C3-convertase and C5-convertase, and the GPI-linked membrane protein CD59 that prevents C9 polymerization, thereby blocking formation of the membrane attack complex (MAC). (Panel 7) The endothelium also elaborates nitric oxide (NO) and PGI₂ to maintain platelets in a resting state. These mechanisms maintain blood flow as depicted in the schematic of a resting venule in the center of the figure: endothelium (pink rectangular cells), glycocalyx (black), platelets (beige), neutrophils (pink nucleated cells), monocytes (irregular cells with purple nucleus), and lymphocytes (blue). (Panel 8) Healthy endothelium is coated with a thick glycocalyx consisting of heparan sulfate and chondroitin sulfate attached to syndecan, hyaluronan, and glypican-1. This physical barrier buffers oncotic forces across the vessel wall and limits interaction with leukocytes and platelets. (Panel 9) Maintenance of endothelial cell barrier function (orange background) is an active process. Fluid and leukocyte extravasation is prevented by tight junctions containing junctional adhesion molecules and claudins and adherens junctions containing VE-cadherin. Activation of Tie2 by Angpt-1 or the sphingosine 1-phosphate receptor 1 (S1PR1) by sphingosine 1-phosphate (S1P) maintains cortical actin networks and promotes VE-cadherin adherens junctions at the cell surface. Cytoprotective signaling through EPCR and Tie2 inhibits activation of the inflammatory and prothrombotic transcription factor NF-κB. Laminar blood flow activates transcription factors KLF2 and KLF4, which promote maintenance of vascular tone via expression of nitric oxide synthase and C-natriuretic peptide (CNP) and reduce inflammation and thrombosis by increasing Tie2 and TM expression. Tonic bradykinin (BK) signaling through the constitutive bradykinin receptor 2 (B2R) and Angiotensin1-7 (Ang1-7) activation of the Mas1 receptor promotes vascular tone via NO synthase and suppressing inflammation. ACE, angiotensin-converting enzyme.

Figure 2.

Inflammatory response to inhalation of COVID-19 in the pulmonary vasculature. Healthy lung: Depicted is a healthy lung (left) and a bronchiole (center) within the lung emphasizing the vasculature and alveolar sacs. A schematic of a histological cross section through an alveolar-capillary unit showing alveolar air sacs (white) lined by alveolar type 1 cells (purple) and separated from capillaries by a basement membrane (blue). The alveolar sac is studded with alveolar type 2 cells (yellow) and alveolar macrophages (green). The inset highlights the air-blood interface that is separated by the alveolar type 1 cell, basement membrane, and the capillary, from top to bottom. COVID lung: The left panel depicts inflammatory signaling through cytokine receptors (IL-1βR, TNFR, and IL-6R) and receptors of innate immunity (TLRs and NOD2) following SARS-CoV-2 infection. The right panel demonstrates sequelae of SARS-CoV-2 infection at the level of the alveolus. Deposition of debris and inflammatory products results in the formation of a hyaline membrane that lines the alveolar sac and impairs exchange (dark pink). Expression of leukocyte receptors results in monocytic infiltrates (purple) and neutrophil extravasation (light pink). Proliferation of alveolar type 2 cells is observed (yellow). Loss of barrier function results in capillary leak (tan). Loss of vascular integrity results in alveolar hemorrhage (red). Microvascular occlusion (red brown) and extravascular fibrin formation (brown) occur. Endothelial swelling and sluffing ensues. TLRs, toll-like receptors; TNFR, tumor necrosis factor receptor.

Figure 3.

Mechanisms by which SARS-CoV-2 induces the prothrombotic transformation of the endothelium. (A) In severe systemic SARS-CoV-2 infection, endothelilal cytoprotective mechanisms are lost. The glycocalyx is degraded by metalloproteases (MMPs) and heparanases (panel 1). Several cytoprotective signaling pathways are downregulated. Featured in panel 2 is loss of Tie2 signaling, which occurs following the release of angiopoietin-2 from endothelial stores. High concentrations of angiopoietin-2 displace angiopoietin-1, with loss of clustering and decreased Tie2 phosphorylation resulting in loss of cytoprotective signaling. The spike protein of SARS-CoV-2 binds to ACE2, resulting in its endocytosis and loss from the cell surface. This prevents the cleavage of angiotensin II into Ang(1-7). The cytoprotective signaling of Ang(1-7) is therefore lost, and instead, proinflammatory signaling through the angiotensin II type 1 receptor prevails (panel 3). Complement deposition in the setting of COVID-19 results in assembly of the MAC, which permeabilizes the endothelial cell membrane (panel 4). (B) SARS-CoV-2 contains (1) S protein, which binds ACE2 facilitating endocytosis, (2) membrane protein (M protein) and envelope protein (E protein), which reside in the viral membrane and (3) nucleocaspid protein that encapsulates SARS-CoV-2 single stranded RNA (ssRNA). SARS-CoV-2 elicits a prothrombotic transformation of the endothelium that results in occlusion of microcirculation by several mechanisms: (1) Loss of glycocalyx integrity provides SARS-CoV-2 better access to endothelial cell receptors. (2) Decreased cytoprotective signaling results in loss of barrier fortification and increased endothelial damage, dysfunction, and phosphatidylserine exposure (yellow outline). (3) Neutrophil-platelet aggregates are found in the circulation in COVID-19 and facilitate the release of NETs from neutrophils. (4) Activation of platelets and vWF release results in formation of platelet-rich thrombi. (5) Fibrin formation on the endothelium results from activation of both the intrinsic and extrinsic pathways. (6) Generation of TF-bearing microparticles (small purple circles) from activated macrophages also contributes to fibrin formation. (7) Endothelial-leukocyte interactions and loss of barrier function enable arrest and transmigration of leukocytes from the endothelium. (8) Permeabilization of membranes by complement deposition results in endothelial cell damage. (C) Several molecular processes contribute to the prothrombotic transformation of the endothelium. Upregulation of tissue factor in activated endothelium along with downregulation of TM and EPCR results in the activation of factor X to factor Xa (panel 1). Externalization of plasma membrane phosphatidylserine (yellow) facilitates assembly of the prothrombinase complex, resulting in the generation of thrombin from prothrombin (panel 2). Upregulation of leukocyte adhesion molecules including VCAM, ICAM, E-selectin, and P-selectin promotes the association of leukocytes with endothelium and facilitates their transmigration into surrounding tissue (panel 3). Endothelial dysfunction interferes with production of NO and PGI₂, and secretion of vWF from Weibel-Palade bodies (WPB, blue) facilitates recruitment of platelets to the endothelial surface. Platelets bind to vWF primarily via GPIbα of the GPIbα-GPIbβ-IX-V complex. Platelet-platelet aggregation is mediated largely through the binding of fibrinogen by integrin α_IIbβ₃ (panel 4). vWF, von Willebrand factor; NETs, neutrophil extracellular traps.

Figure 4.

Mechanism of endothelial barrier function and disruption in COVID-19. (A) The alveolar-capillary interface regulates gas exchange, performs essential barrier functions, maintains blood flow and hemostasis, and controls leukocyte trafficking. Noxious stimuli including cell death, leukocyte activation, protease release, thrombosis, and hypoxia induce endothelial inflammation, which promotes fluid and cellular extravasation that when unchecked can lead to pulmonary edema and ventilation/perfusion mismatch. (B) Severe COVID-19 is associated with cleavage of endothelial cell receptors from the cell surface. Proteases such as matrix metalloproteinase (MMPs), disintegrin and metalloproteinases (ADAMs), and serine proteases promote loss of barrier function directly via cleavage of junctional molecules such as VE-cadherin or indirectly by cleaving cytoprotective receptors such as Tie2 or thrombomodulin (TM) from the cell surface. Transendothelial migration of neutrophils and monocytes is a major source of these proteases. Cleavage of the glycocalyx may induce vascular leak via generation of hyaluronic acid fragments which activate CD44 to promote endothelial permeability. (C) Vasoactive molecules upregulated in COVID-19 may compromise endothelial barrier function and dysregulate vascular tone. Thrombin cleavage of PAR1 results in phosphorylation of myosin light chain (MLC), leading to reorganization of the actin cytoskeleton into contractile stress fibers. Endothelial barrier destabilization is further promoted by activation of integrin β₁ by Angpt-2 secreted from Weibel-Palade bodies (WPB). Induction of bradykinin receptor 1 (B1R) during inflammation, combined with excessive kallikrein-kinin activation and the persistence of bradykinin and its breakdown products (eg, Des-Arg[9]-BK) due to ACE2 deficiency may result in excessive vasodilation and edema. (D) Cytokines such as IL-1β, IL-6, and TNFα, DAMPs, PAMPs, and VEGF present in plasma during severe COVID-19 result in reorganization of VE-cadherin away from junctional sites, promoting vascular leak. VEGF induces VE-cadherin phosphorylation, targeting the protein for internalization. Cytokine activation of NF-kB and JAK/STAT3 results in upregulation of leukocyte adhesion proteins such as E-selectin, VCAM-1, and ICAM-1, which mediate leukocyte recruitment and transendothelial migration. (E) Microvascular thrombosis further impairs tissue oxygenation, which activates hypoxia-inducible factor (HIF) to express VEGF, which directly promotes vascular permeability, and Angpt-2 and VE-PTP, which synergize to block Tie2 signaling. Loss of laminar flow and high shear stress from thrombosis or dysregulation of vascular tone inhibits cytoprotective KLF2/4 signaling, which further suppresses the vasodilatory capacity of the endothelium by inhibiting NO synthase and C-natriuretic peptide and decreasing Tie2 mRNA. AT1R, Angiotensin receptor 1; CNP, C-type natriuretic peptide; mRNA, messenger RNA; TEM, transendothelial migration; VEGF, vascular endothelial growth factor.

Figure 5.

Vascular complications of COVID-19. Due to widespread endothelial dysfunction and thrombosis, COVID-19 has the ability to affect nearly any organ. Pulmonary disease, specifically ARDS, may be driven in large part by vascular dysfunction and microvascular thrombosis. Increased thrombotic events, most notably venous thromboembolism but also myocardial infarction and stroke, have been reported. Vascular dysfunction may contribute neurologic, renal, and dermatologic manifestations of COVID-19, as well as post-COVID sequelae such as multisystem inflammatory syndrome in children (MIS-C).