Immunity, endothelial injury and complement-induced coagulopathy in COVID-19

Abstract

In December 2019, a novel coronavirus was isolated from the respiratory epithelium of patients with unexplained pneumonia in Wuhan, China. This pathogen, named severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), causes a pathogenic condition that has been termed coronavirus disease 2019 (COVID-19) and has reached pandemic proportions. As of 17 September 2020, more than 30 million confirmed SARS-CoV-2 infections have been reported in 204 different countries, claiming more than 1 million lives worldwide. Accumulating evidence suggests that SARS-CoV-2 infection can lead to a variety of clinical conditions, ranging from asymptomatic to life-threatening cases. In the early stages of the disease, most patients experience mild clinical symptoms, including a high fever and dry cough. However, 20% of patients rapidly progress to severe illness characterized by atypical interstitial bilateral pneumonia, acute respiratory distress syndrome and multiorgan dysfunction. Almost 10% of these critically ill patients subsequently die. Insights into the pathogenic mechanisms underlying SARS-CoV-2 infection and COVID-19 progression are emerging and highlight the critical role of the immunological hyper-response — characterized by widespread endothelial damage, complement-induced blood clotting and systemic microangiopathy — in disease exacerbation. These insights may aid the identification of new or existing therapeutic interventions to limit the progression of early disease and treat severe cases. This Review describes our current understanding of the pathogenic mechanisms involved in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and the progression of coronavirus disease 2019 (COVID-19), focusing on the immunological hyper-response and the induction of widespread endothelial damage, complement-associated blood clotting and systemic microangiopathy, as well as the effects of these processes on the kidney. The authors also discuss therapeutic interventions that currently hold most promise. Although most patients infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) experience mild to moderate symptoms that resolve after 6–10 days, almost 20% of patients develop severe illness characterized by atypical interstitial bilateral pneumonia and acute respiratory distress syndrome, with a high fatality rate. Accumulating evidence suggests that an unbalanced and unrestrained innate immune response, which comes at the expense of effective adaptive immunity, underpins the progression of coronavirus disease 2019 (COVID-19). In severe cases of COVID-19, massive endothelial dysfunction, widespread coagulopathy and complement-induced thrombosis can lead to the development of systemic microangiopathy and thromboembolism; these complications can be life-threatening and ultimately lead to multi-organ failure. The kidney is one of the main targets of COVID-19 complications, and abnormal kidney function is associated with a significantly increased risk of death in severely ill patients. Most repurposed anti-viral drugs have failed to improve clinical outcomes in COVID-19; by contrast, therapeutic interventions that target the host response, including the hyper-immune response, complement activation and systemic thrombosis, seem to be more promising approaches to the treatment of severe COVID-19.