Proteolytic profiling of human plasma reveals an immunoactive complement C3 fragment

\ua9 The Author(s) 2025.Dysregulated proteolysis is central to autoimmune pathogenesis. The complement cascade, a major protease network, generates fragments that modulate immunity and tissue injury. We developed a scalable blood plasma N-terminomics workflow that markedly expands detection of proteolytic events in vitro and in vivo. Applied to 143 systemic lupus erythematosus (SLE) patients, Multi-Omics Factor Analysis (MOFA) linked N-terminal signatures to immunological and clinical heterogeneity. This revealed a previously unrecognized complement fragment, C3-LHF1, encompassing the C345C domain and rivaling, based on intensity detected by mass spectrometry, the abundance of canonical fragments like C3a and C3b. C3-LHF1 associated with renal function and remission in lupus nephritis, and exhibited dual functions: inhibiting classical and lectin complement pathways and acting as a partial IL6ST (gp130) agonist, independent of IL6Rα. In human kidney organoids, C3-LHF1 induced JAK/STAT3 signaling, amplified TNFα-driven CXCL10 secretion, and reduced podocyte marker expression, suggesting a role in tissue remodeling. These findings reveal unanticipated complexity in complement-mediated signaling and provide a comprehensive atlas of protein N-termini in human plasma, which enables discovery of novel immunoregulatory mechanisms and therapeutic targets in inflammatory disease