The Complement System Mediates COVID-19 Endothelial Cell Injury, Modifying Cell Permeability

Abstract

Background Coronavirus Disease 2019 (COVID-19) is a (SARS-CoV-2)-mediated respiratory disease. The severity of COVID-19 has been classified as mild, moderate, severe and critical per the WHO-China Joint Mission. Among a total of 72 314 case records from the Chinese Center for Disease Control and Prevention, approximately 81% of COVID-19 cases were defined as mild, 14% of as severe, and 5% as critical. The overall hospital mortality of COVID-19 cases is approximately 15-20%, but it is up to 40-49% among critical cases requiring intensive care unit admission. The complement system, is a key component of the innate immune response, consists of more than 50 soluble and membrane-bound proteins that provide defense against microbes and mediate inflammatory responses. The activation of the complement system involves a series of enzymatic transformations, where C3b acts as a mediator. The final step of the complement cascade is the formation of a membrane attack complex (MAC, C5b-9) on the plasma membrane (PM) of the cells. Several pathologies including thrombotic, inflammatory, autoimmune and age-related diseases are associated with abnormal complement activation. There is evidence linking the activation of the complement system and endothelial injury to organ damage and complications that increase the risk of mortality in COVID-19. We hypothesize that endothelial cell injury resulting from complement activation contributes to COVID-19-associated vascular injury and organ damage. Aims To identify differences in complement activation associated with mild and severe forms of COVID-19 To determine the role of differential complement activation by COVID-19 patient sera in mediating endothelial cell damage. Methodology Clinical information, and sera from SARS-CoV-2 positive patients with mild and severe COVID-19 were obtained from the Canadian COVID-19 Prospective Cohort Study (CANCOV) to identify complement activation and endothelial cell (EC) injury. Complement activation was determined using a previously established protocol (Noris et al., 2017). Briefly, primary human umbilical vein endothelial cells (HUVECs) were treated with adenosine diphosphate (ADP), followed by treatment with healthy control or COVID-19 patient serum. To confirm the biological activity of complement, we measured the deposition of C3b and C5b-9 on the plasma membrane of HUVECs via immunofluorescence (IF) on cells exposed to healthy or patient sera. In addition, a permeability assay using a transwell model was used to measure the changes in permeability induced by healthy or patient sera via complement activation. Briefly, HUVECs were seeded on the top part of the insert, allowing them to form a monolayer. Monolayer integrity was assessed by imaging cells stained with Hoechst (nuclear stain) and phalloidin (cytoskeleton stain) on the day of the experiment, as well as before and after incubation with healthy or patient sera. Results We were able to detect complement activation/deposition when exposing the cells to COVID-19 sera. As for C3b deposition, cells exposed to patient sera showed no difference compared to those treated with sera from healthy donors. However, we observed increased C5b-9 deposition on the PM of cells treated with patient sera compared to those treated with sera from healthy donors, with further increased deposition when cells are treated with deceased patient sera. We observed a trend demonstrating increased permeability when cells are treated with deceased COVID-19 patient sera compared to healthy donors. Cellular permeability is significantly lower in those cells treated sera from patients with mild or severe COVID-19. Conclusion There is a differential biological activity of complement on endothelial cells depending on the severity of COVID-19, this complement activation leads to an increase in cell permeability. While more research needs to be done, these results contribute to a better understanding of the consequences of complement activation on endothelium in COVID-19.