Molecular determinants of STEC-HUS: from complement activation to microvascular thrombosis.

Shiga-like toxin-producing E. coli-induced hemolytic uremic syndrome (STEC-HUS) is a rare but severe disease characterized by microangiopathic hemolysis, thrombocytopenia, and renal failure. No specific therapy is available, and long-term complications are common. Growing evidence indicates that STEC-HUS is associated with excessive complement activation, however the impact on disease pathogenesis is still debated. This study investigated the involvement of the three complement pathways in STEC-HUS. We analyzed 37 patients during the acute phase and 24 patients after hospital discharge. Ex-vivo assays with patient sera and cultured microvascular endothelial cells demonstrated that acute-phase sera triggered abnormal C3 and C5b-9 deposition, leading to increased cell surface expression of vWF and P-selectin, which in turn promoted thrombus formation on endothelial cells. The factor B inhibitor iptacopan, but not inhibitors of classical or lectin pathways, effectively blocked complement deposition and prevented thrombus formation, highlighting the alternative pathway as a driver of complement dysregulation and microvascular thrombosis in acute STEC-HUS. Additionally, we observed persistent complement activation in a substantial subset of patients studied after hospital discharge, as indicated by abnormal C5b-9 formation. Notably, most of these patients had not achieved full remission, showing at least one hematologic abnormality and/or elevated serum creatinine. These findings emphasize the pivotal role of complement overactivation in STEC-HUS pathogenesis and support the potential of alternative pathway inhibitors as promising therapeutic options. Moreover, our results underscore the potential of the ex-vivo tests as valuable tools for monitoring complement activity and clotting abnormalities over time, possibly facilitating detection and management of disease sequelae.