C5a and C5aR1 are key drivers of microvascular platelet aggregation in clinical entities spanning from aHUS to COVID-19.

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 opposite to C5b-9 in inducing endothelial dysfunction and loss of anti-thrombogenic 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 of endothelial thromboresistance loss. C5a added to normal human serum, fully recapitulated the pro-thrombotic effects of aHUS serum. Mechanistic studies showed that C5a caused RalA-mediated exocytosis of 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 SARS-CoV-2 infection- we found the same C5a-dependent pathogenic mechanisms. These results highlight C5a/C5aR1 as a common pro-thrombogenic effector spanning from genetic rare diseases to viral infections, and may have clinical implications. Selective C5a/C5aR1 blockade could have advantages over C5 inhibition, since the former preserves the formation of C5b-9 that is critical to control bacterial infections that often develop as comorbidities in severely ill patients. (Clinicaltrials.gov identifier NCT02464891).