M egalin/gp330 is an endocytic receptor that internalizes multiple ligands including apolipoproteins E (apo E) and B100 (apo B). Megalin is the main antigenic target in passive Heymann nephritis (pHN), where it binds circulating autoantibodies leading to the formation of subepithelial immune deposits (ID)-the hallmark of pHN. Apo E and apo B were found recently to accumulate within these IDs, and evidence was provided that their lipids may undergo peroxidation, causing glomerular basement membrane damage and proteinuria. Here we investigated if ID-forming antimegalin IgG can inhibit the binding and internalization of apo E-betaVLDL (very low density lipoprotein) by megalin, and lead to their accumulation within IDs. By immunoelectron microscopy, apo E and apo B were detected in clathrin-coated pits and multivesicular bodies of podocytes in control rats, suggesting that the uptake of lipoproteins is a constitutive function of the glomerular epithelium. When pHN was induced by intravenous injection of antimegalin IgG, apo E and apo B were found within IDs by immunofluorescence and immunoelectron microscopy. Bound antibodies eluted from glomeruli of rats with pHN were found to inhibit the binding and internalization of apo E-enriched betaVLDL by megalin. These results indicate that pHN-inducing antimegalin IgG is capable of interfering with the uptake of lipoproteins by megalin in vivo during the formation of IDs.