INTERFERON-GAMMA (IFN-GAMMA) AND IL-4 EXPRESSED DURING MERCURY-INDUCED MEMBRANOUS NEPHROPATHY ARE TOXIC FOR CULTURED PODOCYTES

The subepithelial immune deposits of Dorus Zadel Black (DZB) rats with mercury-induced membranous nephropathy consist of autoantibodies dire cted to laminin P1 and of complement. The animals develop massive prot einuria within 10-14 days which is associated with obliteration of foo t processes of glomerular visceral epithelial cells (GVEC), or podocyt es. Previous studies indicate that these autoantibodies are probably n ot the sole mediator of proteinuria and GVEC damage. In this study we investigated whether circulating or macrophage-derived cytokines can c ontribute to the GVEC changes as detected in vivo. In vivo at the heig ht of the proteinuria, increased intraglomerular IFN-gamma immunoreact ivity was found. In diseased rats a five-fold increase in intraglomeru lar 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 GVEG to these cytokines to investigate their cytotoxic effects on several phys iological 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 protei n 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-mediat ed) effects. Only IL-4 decreased the viability of the cells, and treat ed monolayers demonstrated an increased passage of the 44-kD protein h orseradish peroxidase. From our experiments we concluded that IFN-gamm a subtly affected monolayer integrity at the level of the tight juncti ons, and that IL-4 additionally induced cell death. We hypothesize tha t the toxic effects of the cytokines IFN-gamma and IL-4 as seen with c ultured podocytes are necessary together with the autoantibodies, for the ultimate induction of proteinuria in mercury nephropathy in the DZ B rat.