Background Lupus nephritis (LN) remains the leading cause of mortality for SLE patients, and is associated with proteinuria and foot process effacement. In subsets of LN patients, B cell depletion therapies have been efficacious in lowering disease activity including glomerulopathy. The contributions of B cells to proteinuria and foot process effacement remain unknown. The development of a murine model of B-cell induced proteinuria and identification of pathogenic factors would enhance our understanding of immune-based glomerular diseases.
Objectives To create a model of B cell induced proteinuria and foot process effacement, and identify potential pathogenic cytokines capable of proteinuria.
Methods The B cell model antigen model hen egg lysozyme (HEL) was biotinylated, complexed to avidin and injected into mice. Naïve HEL-specific B cells were then adoptively transferred and proteinuria assessed. Kidneys were processed for immunofluorescence and scanning electron microscopy (SEM). Cultured podocyte membrane ruffling was assessed with DIC videomicroscopy. IL-4 expression in mice was achieved by hydrodynamically injecting murine IL-4 in the piggyBac vector system.
Results HEL embedded within the glomerular basement membrane (GBM) following IV injection. Proteinuria occurred after the transfer of naive HEL-specific B cells and associated with focal foot process effacement. No antibody or complement deposition was observed in proteinuric glomeruli. Intravital two-photon microscopy demonstrated that HEL-specific B cells arrested trafficking within glomeruli only in the presence of glomerular-localized HEL. This demonstrated that B cells were capable of inducing glomerular injury and proteinuria.
These data suggested cytokines secreted by activated B cells may be responsible for podocyte injury. Since foot process effacement is the histologic correlate of actin cytoskeletal rearrangement, we hypothesized that cytokines mediate podocyte injury through alterations in the actin cytoskeleton. We found that IL-4 induced unstable actin cytoskeletal changes leading to membrane ruffling. In addition, IL-4 generated foot process retractions on ex vivo fragments of renal cortex. Hydrodynamic DNA injection of wild-type mice with plasmid encoding IL-4 lead to proteinuria, which was reversed by JAK1/3 inhibition.
Conclusions We developed a novel model of B cell-induced proteinuria with focal foot process effacement. B cell derived cytokines such as IL-4 induced alterations in foot process morphology, leading to proteinuria. Transgenic IL-4 mice are associated with glomerulosclerosis independent of autoantibody production. We believe that IL-4 plays a direct role in LN by inducing podocyte injury through disruptions in the actin cytoskeleton. This has important implications in developing therapies to preserve podocyte function, thus limiting glomerular injury.
Acknowledgements A. Kim would like to thank the American College of Rheumatology/Rheumatology Research Foundation for funding.
Disclosure of Interest None declared