Transferrin Receptor 1 Mediates Lysosomal Accumulation of Galactose-Deficient IgA1 in Mesangial Cells, Contributing to IgA Nephropathy

Background: The deposition of Gd-IgA1 in the mesangium is a pathological characteristic of IgA nephropathy (IgAN), but the mechanism remains unclear. Methods: We used CRISPR/Cas9 to screen for potential membrane proteins mediating the binding of mesangial cells to Gd-IgA1 and identified transferrin receptor 1 (TfR1) as one of the candidates. In vitro, we overexpressed and knocked down TfR in mesangial cells and explored its influence on Gd-IgA1 binding with mesangial cells. In vivo, we overexpressed TfR in BALB/c nude mice via AAV9, injected with ICG, HC-IgA1-ICG, and Gd-IgA1-ICG (i.v.), and then monitored the deposition intensity of IgA1 in kidneys by intravital fluorescent imaging. We also validated our finding on renal tissues from patients with IgAN. TfR expression and localization were detected. Results: We observed that Gd-IgA1 showed significantly reduced binding to mesangial cells in the TfR knockdown group compared to controls. Conversely, Gd-IgA1 demonstrated significantly increased binding to mesangial cells in the TfR overexpression group compared to controls. We demonstrated that Gd-IgA1 deposition in kidneys was significantly enhanced in TfR overexpression mice, and Gd-IgA1 was found co-localized with TfR and the lysosomal marker lamp1 in renal. To validate our finding, we performed immunohistochemical staining and revealed that TfR expression was significantly increased in glomeruli of patients with IgAN compared to para-cancercontrols, and TfR was found co-localized with IgA. Transmission electron microscopy (TEM) and immunoelectron microscopy (IEM) revealed the presence of IgA1 protein accumulation in the lysosomes of mesangial cells in patients with IgAN. Conclusion: Our research elucidates that TfR in mesangial cells promotes the binding and endocytosis of Gd-IgA1, leading to intracellular aggregation in lysosomes and have potential novel strategies for IgAN therapy.