Renal Cells from Patients with C3 Glomerulopathy Show a Strong Role in Complement Activation at the Kidney Tissue Level

Background: C3 glomerulopathy (C3G) is a rare renal disease characterized by hyperactivation of the complement pathway in serum, leading to C3 deposition in the kidney, resulting in a high progression to end-stage renal disease. Despite its clinical importance, C3G is poorly studied at the molecular level, delaying the identification of effective therapies. To investigate the contribution of the kidney to the pathogenesis, we studied renal cells isolated from the urine of patients with C3G. Methods: We isolated CD133-/CD24+ renal cells from the urine of five patients with C3G and five healthy subjects, characterized them by immunolabeling and FACS analysis, and performed RNA sequencing, biological pathways enrichment analysis, and network analysis. Additionally, we utilized CD133+/CD24+ renal progenitors from patients and healthy subjects (HS) to create renal spheroids and cultured them in serum (20%) from C3G patients and HS under baseline conditions and after complement depletion. Results: We identified 483 differentially expressed genes in renal cells from patients with C3G and healthy subjects. A significant modulation of the complement system in C3G renal cells emerged from the pathway analysis. Compared to HS, C3G renal cells seem to sustain the renal production of several complement components, such as C2, C3, C4A, C4B, CFB, and MASP1, which were highly upmodulated (FDR< 0.001 and LogFC>3). Moreover, we found the SERPINF2 gene, encoding for alpha-2-antiplasmin, which increases the complement activity, overexpressed (LogFC 4.33, FDR 0.009). It was also directly linked to IL1B gene, the central node of the inflammatory network. In immunofluorescence, the spheroids from C3G patients displayed strong expression of C3, regardless of the serum to which they were exposed (serum from HS, complement-depleted serum from C3G patients, and active serum), reinforcing the finding of local production observed in the transcriptomic analysis. Conclusion: Our findings strongly suggest that renal cells in C3G patients actively contribute to complement hyperactivation, challenging the notion of passive involvement. This insight may represent a breakthrough in understanding the molecular mechanisms of C3G and could pave the way for developing new therapeutic targets.