Mechanisms of SARS-CoV-2-induced lung vascular disease: potential role of complement

Abstract

The outbreak of COVID-19 disease, caused by SARS-CoV-2 beta-coronovirus, urges a focused search for the underlying mechanisms and treatment options. The lung is the major target organ of COVID-19, wherein the primary cause of mortality is hypoxic respiratory failure, resulting from acute respiratory distress syndrome, with severe hypoxemia, often requiring assisted ventilation. While similar in some ways to acute respiratory distress syndrome secondary to other causes, lungs of some patients dying with COVID-19 exhibit distinct features of vascular involvement, including severe endothelial injury and cell death via apoptosis and/or pyroptosis, widespread capillary inflammation, and thrombosis. Furthermore, the pulmonary pathology of COVID-19 is characterized by focal inflammatory cell infiltration, impeding alveolar gas exchange resulting in areas of local tissue hypoxia, consistent with potential amplification of COVID-19 pathogenicity by hypoxia. Vascular endothelial cells play essential roles in both innate and adaptive immune responses, and are considered to be "conditional innate immune cells" centrally participating in various inflammatory, immune pathologies. Activated endothelial cells produce cytokines/chemokines, dynamically recruit and activate inflammatory cells and platelets, and centrally participate in pro-thrombotic processes (thrombotic microangiopathies). Initial reports presented pathological findings of localized direct infection of vascular endothelial cells with SARS-CoV-2, yet emerging evidence does not support direct infection of endothelial or other vascular wall cell and thus widespread endothelial cell dysfunction and inflammation may be better explained as secondary responses to epithelial cell infection and inflammation. Endothelial cells are also actively engaged in a cross-talk with the complement system, the essential arm of innate immunity. Recent reports present evidence for complement deposition in SARS-CoV-2-damaged lung microcirculation, further strengthening the idea that, in severe cases of COVID-19, complement activation is an essential player, generating destructive hemorrhagic, capillaritis-like tissue damage, clotting, and hyperinflammation. Thus, complement-targeted therapies are actively in development. This review is intended to explore in detail these ideas.

Keywords: COVID 19; complement; endothelial cell; hypoxia; inflammation.

Fig. 1.

SARS-CoV-2 does not productively infect pulmonary vascular cells in vitro. Hypothetical scheme demonstrating potential interactions of SARS-CoV-2 and hypoxia in driving endothelial injury/dysfunction resulting in endothelial death, inflammation, and thrombosis.^–30

Fig. 2.

SARS-CoV-2 human airway and lung cellular tropism: the following references correspond to the references shown in Fig. 1.

Fig. 3.

Complement regulation on endothelial cells: C3, factor B (FB), and factor D (FD) are Alternative pathway proteins that are present in plasma. These proteins can be rapidly activated on susceptible surfaces. Endothelial cells express several different membrane bound complement regulatory proteins, including decay accelerating factor (CD55) and membrane cofactor protein (CD46). In addition, factor H is a soluble Alternative pathway regulator present in plasma that binds to glycosaminoglycans and sialic acid displayed on the surface of endothelial cells. These regulatory proteins inhibit complement activation at the levels of the C3 and C5 convertases. Another regulatory protein, CD59, is expressed on endothelial cells and blocks formation of C5b-9 (membrane attack complex, or MAC). C3a and C5a are soluble peptides generated during complement activation. They are rapidly inactivated by carboxypeptidases in serum. This system of regulatory proteins ordinarily protects endothelial cells from complement-mediated injury. In some diseases, however, impaired complement regulation on endothelial cells is associated with vascular injury.

Fig. 4.

Local/tissue complement activation in the lungs of COVID-19 patients: Immunohistochemistry analysis of pulmonary autopsy samples. Top left shows MASP-2 deposits localized to the interalveolar septa. Top right shows extensive deposition of C4d in the alveolar septa. Bottom left shows septal capillary distribution of C3d and bottom right shows similar capillary distribution for C5b-9. (Adapted from Magro et al.)

Fig. 5:

Co-localization of SARS-CoV-2 and complement activation: Immunohistochemistry analysis of autopsy samples. Top left shows deposition of C4d within the inter-alveolar septa. Top right using NUANCE Software the C4 image appears green and in bottom left the SARS-CoV-2 spike protein shows red. Bottom right is a merged image showing C4d and SARS-CoV-2 co-localization, yellow. (Adapted from Gao et al.)

Fig. 6.

Hypothetical pathway for complement-mediated inflammation of the pulmonary alveolus in COVID-19: (1) SARS-CoV-2 attaches to type II alveolar epithelial cell (AEC-II) receptor angiotensin-converting enzyme 2 (ACE2). (2) Complement activation is initiated upon recognition of viral glycans by lectins (e.g. collectin-11 and ficolin-1, which are secreted by AEC-II) complexed with MBL-associated serine proteases (MASPs) including MASP-2. Direct binding of MASP-2 to the N protein of SARS-CoV-2 has also been suggested to initiate lectin pathway activation (preprint: Gao et al.). (3) Complement deposition and MAC formation on AECs cause inflammasome activation and cell damage. (4) Release of complement C5a increases vascular permeability and recruitment/activation of polymorphs (PMN) and monocytes (MC) to the alveolus. (5) Monocytes differentiated into inflammatory macrophages (MΦ) overproduce pro-inflammatory cytokines in response to C3a and C5a stimulation. (6) Endothelial cell (EC) activation by C5a and MAC predisposes to thrombus formation, further enhanced through MBL recognition of viral particles in the vascular compartment leading to cleavage of thrombin and fibrinogen by MASPs. (Adapted from Polycarpou et al.)

Fig. 7.

Innate immune system profiling during hypoxia-driven PH: Innate immune system profiling during hypoxia-driven PH. (a) Comparative fold change analysis of proteins identified in both calf and steer samples and their respective changes in control versus PH conditions in the distal pulmonary artery (DPA). (b) Interactions of proteins identified in the complement and coagulation cascades are shown.

Fig. 8.

Activation of the complement cascade, as defined by deposition of C3d (terminal degradation fragment of C3 activation), is observed in a perivascular-specific pattern in the lungs of experimental animal PH models and idiopathic PAH (iPAH) humans. Sections were immunolabeled with C3d-specific mAb (red), which distinguishes tissue-bound C3d from the intact C3 or C3b, allowing assessment of tissue-specific activation of the complement cascade (18). PA: pulmonary artery; AW: airway. Scale, 100 mm.