The subepithelial immune deposits of Dorus Zadel Black (DZB) rats with
mercury-induced
membranous nephropathy consist of
autoantibodies directed to
laminin P1 and of
complement. The animals develop massive
proteinuria within 10-14 days which is associated with obliteration of foot processes of glomerular visceral epithelial cells (GVEC), or podocytes. Previous studies indicate that these
autoantibodies are probably not the sole mediator of
proteinuria and GVEC damage. In this study we investigated whether circulating or macrophage-derived
cytokines can contribute to the GVEC changes as detected in vivo. In vivo at the height of the
proteinuria, increased intraglomerular IFN-gamma immunoreactivity was found. In diseased rats a five-fold increase in intraglomerular macrophages was found, but we could not detect intraglomerular IFN-alpha, IFN-beta,
IL-1 beta or tumour
necrosis factor-alpha (
TNF-alpha) by using immunohistology. Subsequently, we exposed cultured GVEC to these
cytokines to investigate their cytotoxic effects on several physiological and structural parameters. IFN-gamma and
IL-4 were the only
cytokines that exerted toxic effects, resulting in a rapidly decreased transepithelial resistance of confluent monolayers, which was closely associated with altered immunoreactivity of the
tight junction protein ZO-1.
IL-4 also affected
vimentin and
laminin immunoreactivity. IFN-gamma and
IL-4 only interfered with monolayer integrity when added to the basolateral side of the GVEC, indicating specific (receptor-mediated) effects. Only
IL-4 decreased the viability of the cells, and treated monolayers demonstrated an increased passage of the 44-kD
protein horseradish peroxidase. From our experiments we concluded that IFN-gamma subtly affected monolayer integrity at the level of the tight junctions, and that
IL-4 additionally induced cell death. We hypothesize that the toxic effects of the
cytokines IFN-gamma and
IL-4 as seen with cultured podocytes are necessary together with the
autoantibodies, for the ultimate induction of
proteinuria in
mercury nephropathy in the DZB rat.