Direct activation of the alternative complement pathway by SARS-CoV-2 spike proteins is blocked by factor D inhibition

Abstract

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is a highly contagious respiratory virus that can lead to venous/arterial thrombosis, stroke, renal failure, myocardial infarction, thrombocytopenia, and other end-organ damage. Animal models demonstrating end-organ protection in C3 deficient mice and evidence of complement activation in humans have led to the hypothesis that SARS-CoV-2 triggers complement-mediated endothelial damage, but the mechanism is unclear. Here, we demonstrate that SARS-CoV-2 spike protein (subunit 1 and 2), but not N protein, directly activates the alternative pathway of complement (APC). Complement dependent killing using the modified Ham test is blocked by either C5 or factor D inhibition. C3 fragments and C5b-9 are deposited on TF1PIGAnull target cells, and complement factor Bb is increased in the supernatant from spike protein treated cells. C5 inhibition prevents the accumulation of C5b-9 on cells, but not C3c; however, factor D inhibition prevents both C3c and C5b-9 accumulation. Addition of factor H mitigates the complement attack. In conclusion, SARS-CoV-2 spike proteins convert non-activator surfaces to activator surfaces by preventing the inactivation of the cell surface APC convertase. APC activation may explain many of the clinical manifestations (microangiopathy, thrombocytopenia, renal injury, and thrombophilia) of COVID-19 that are also observed in other complement-driven diseases such as atypical hemolytic uremic syndrome and catastrophic antiphospholipid antibody syndrome. C5 inhibition prevents accumulation of C5b-9 in vitro but does not prevent upstream complement activation in response to SARS-CoV-2 spike proteins.

Targeting complement cascade: an alternative strategy for COVID-19

“…Similarly, the clinical use of lampalizumab and its pharmacokinetic, pharmacodynamic, and biodistribution studies on the inhibition of complement component factor D has been reported with Cynomolgus Monkey (Le et al 2015). Further, the direct activation of alternative pathways by SARS-CoV-2 spike protein has been reported by Yu et al (2020). They have used ACH145951, an antagonist of factor D to eliminate the expression of alternative pathways by inhibiting factor D, thus resulting in the prevention of C3c and C5b-9 accumulation.…”

“…Coversin (Kuhn et al 2016) and Zilucoplan (Howard Jr et al 2020) are C5-specific complement inhibitors reported for their activity against anti-hemolytic and Severe Generalized Myasthenia Gravis (SGMS). Complement-mediatedhemolysis is also observed with SARS-CoV-2 patients by Yu et al (2020). Hence, Coversin and Zilucoplan, antagonists of C5 can be exploited for treating COVID-19 pathological conditions.…”

Update to coagulopathy in COVID-19: Manifestations and management

“…35 The SARS-CoV-2 spike protein activates complement in vitro via the alternative pathway. 36 One mechanism of microvascular thrombosis that may be specific to COVID-19 is the virus's affinity for angiotensin-converting enzyme 2, which is expressed on alveolar epithelial type II cells and various extrapulmonary tissues, including endothelial cells. SARS-CoV-2 has been shown to directly invade the endothelial cell.…”

The beneficial and pathogenic roles of complement in COVID-19

“…14 In another report utilizing an in vitro system, spike proteins 1 and 2 led to activation primarily of the alternative pathway on human cells. 13 These types of innate immune responses coupled with a rapid adaptive response lead to either no clinical signs of infection or a common cold-like or influenza-like syndrome. Viruses, as might be expected, have developed evasion tactics to limit complement activation.…”

“…For the sake of brevity, we have primarily referenced a limited number of reviews and original reports with a focus on the complement system. [2][3][4][5][6][7][8][9][10][11][12][13] ■ THE COMPLEMENT SYSTEM IN IMMUNITY AND DISEASE The complement system is a proteolytic cascade initiated by three pathways (classical, alternative, and lectin), each uniquely triggered to generate a potent, highly regulated, innate immune response and to set the stage for a prompt and decisive adaptive immune response: 1) membrane perturbation featuring C4band C3b-mediated opsonization/phagocytosis, as well as a lytic process mediated by the membrane attack complex (MAC, C5b-9), and 2) generation of a proinflammatory state largely mediated via the anaphylatoxins C3a and C5a. Using these same two general mechanisms, the complement system also facilitates the clearance of apoptotic material and cellular debris.…”