Unrestrained activation of the
complement system till the terminal products, C5a and
C5b-9, plays a pathogenetic role in acute and chronic inflammatory diseases. In endothelial cells,
complement hyperactivation may translate into cell dysfunction, favoring
thrombus formation. The aim of this study was to investigate the role of the C5a/C5aR1 axis as opposed to
C5b-9 in inducing endothelial dysfunction and loss of antithrombogenic properties. In vitro and ex vivo assays with serum from patients with
atypical hemolytic uremic syndrome (aHUS), a prototype
rare disease of
complement-mediated microvascular
thrombosis due to genetically determined alternative pathway dysregulation, and cultured microvascular endothelial cells, demonstrated that the C5a/C5aR1 axis is a key player in endothelial thromboresistance loss. C5a added to normal human serum fully recapitulated the prothrombotic effects of aHUS serum. Mechanistic studies showed that C5a caused RalA-mediated exocytosis of
von Willebrand factor (vWF) and
P-selectin from Weibel-Palade bodies, which favored further vWF binding on the endothelium and platelet adhesion and aggregation. In patients with severe
COVID-19 who suffered from acute activation of
complement triggered by
severe acute respiratory syndrome coronavirus 2 infection, we found the same C5a-dependent pathogenic mechanisms. These results highlight C5a/C5aR1 as a common prothrombogenic effector spanning from genetic
rare diseases to
viral infections, and it may have clinical implications. Selective C5a/C5aR1 blockade could have advantages over C5 inhibition because the former preserves the formation of
C5b-9, which is critical for controlling
bacterial infections that often develop as comorbidities in severely ill patients. The ACCESS trial registered at www.clinicaltrials.gov as #NCT02464891 accounts for the results related to aHUS patients treated with
CCX168.