Complement-independent nephrotoxic serum nephritis in Munich Wistar rats. Immunologic and ultrastructural studies.

Immunologic mechanisms of proteinuria and ultrastructural alterations of the slit pore complex and glomerular charge barrier were investigated in Munich Wistar (MW) rats with nephrotoxic serum nephritis. Prior to disease induction, normal MW sera demonstrated 50% of the complement hemolytic activity compared with sera obtained from Sprague-Dawley rats. MW rats were sacrificed prior to, at onset (5 to 6 hours), and during maximal proteinuria (heterologous phase). Immunofluorescence revealed binding of rabbit antirat IgG antibodies in a linear pattern to the glomerular basement membrane (GBM) within 15 minutes postinjection. Complement deposition was not demonstrable in vivo in this model. Immediately after injection of nephrotoxic serum a decreased penetration of the GBM occurred, restricting ferritin to the level of the endothelium in in situ fixed glomeruli. GBM permeability to native ferritin did not increase despite areas of epithelial cell detachment, endothelial cell sloughing, and proteinuria between 2 and 24 hours postnephrotoxic serum injection. Colloidal iron initially decreased staining intensity between 6 and 8 hours, with a major decrease at 24 hours, indicating a loss in glomerular sialoprotein coincident with the onset of proteinuria. Polyethyleneimine (PEI) localization revealed an initial loss of anionic binding sites by 2 hours postinjection. At 6 hours peripheral capillary loops demonstrated only scattered, random polyethyleneimine-binding sites. Splitting of the lamina densa occurred at 24 hours with the exposure of previously undetected anionic binding sites within the lamina densa. Ultrastructurally, as early as 2 hours postnephrotoxic serum injection tissue perfused with tannic acid-glutaraldehyde showed epithelial membranes forming numerous pinocytotic vesicles. Blunting and retraction of foot processes caused displacement and stacking of slit diaphragms prior to the onset of proteinuria. Between 6 and 24 hours postinjection, slit diaphragms appeared to stretch and contract to compensate for epithelial cell retraction. Tangential sections showed neither alterations nor condensation products disrupting the isoporous substructure of the slit diaphragm 24 hours postnephrotoxic serum injection. Polymorphonuclear leukocytes were not found within capillary loops during the heterologous phase of nephrotoxic serum nephritis in MW rats. The absence of complement and polymorphonuclear leukocytes accompanying anti-GBM antibody deposition suggests that early epithelial cell injury and GBM charge alterations in MW rats are mediated by heterologous antibody via a complement-independent mechanism. The lower complement hemolytic activity in normal MW sera may explain the lack of complement involvement in renal lesions in this model of nephrotoxic serum nephritis. Loss of characteristic staining for both glomerular sialoprotein and discrete anionic sites in the GBM coincided with early epithelial cell alterations and occurred prior to the onset of measurable proteinuria.