An Intracellular C3-CFB Complement Axis Reprograms Tubular Metabolism and Chromatin to Drive Kidney Fibrosis 2307504

Introduction Complement is often viewed as liver-derived and extracellular, yet many non-hepatic cells generate intracellular complement. Proximal tubular epithelial cells (TECs) drive kidney injury and fibrosis and express complement, but how de novo TEC complement controls metabolism, chromatin, and fibrogenesis is unknown. We asked whether a TEC-intrinsic complement axis links inflammatory signaling to metabolic and epigenetic reprogramming after injury. Methods Mouse TEC injury models were combined with confocal microscopy, Seahorse metabolic profiling, single-nucleus and bulk RNA-seq, CUT&Tag, MNase-seq, and EMSA. C3 and CFB were perturbed using conditional C3 deletion, cell-permeable CFB inhibitors, and structure-guided CFB mutants informed by AlphaFold2/GraphSite modeling. Human kidney single-cell and bulk transcriptomes were analyzed for conservation and clinical relevance. Results IL-1β—NF-κB/c-JUN induced C3 and CFB in injured TECs. C3 accumulated in mitochondria and co-localized with CFB, which processed C3, rewired metabolism, and promoted profibrotic programs. CFB also accumulated in nuclei; CUT&Tag/EMSA and modeling defined a DNA-binding motif and pocket in CFB. Nuclear CFB cleaved histones, blocked by CFB inhibitors, and MNase-seq showed increased nucleosome protection with CFB inhibition. Conditional C3 deletion or intracellular CFB inhibition reduced TEC metabolism and attenuated post-injury interstitial fibrosis in mice. In human kidney datasets, TEC C3-CFB co-expression associated with injury signatures and inversely correlated with kidney function. Conclusion A TEC-intrinsic C3—CFB complement axis links inflammatory cytokine signaling to mitochondrial metabolism, chromatin state, and fibrosis after kidney injury. Nuclear CFB acts as a DNA-binding protease that modulates histones and nucleosomes. Targeting intracellular, rather than systemic, complement may limit inflammatory kidney disease while preserving host defense.