Primary acute renal failure ("acute tubular necrosis") in the transplanted kidney: morphology and pathogenesis.

"Acute tubular necrosis" (ATN) in the transplanted kidney, when properly differentiated from other causes of acute renal failure, appears to be a relatively benign condition. It has been widely assumed to be pathologically identical to ATN in the native kidney, but its histopathologic features have not been studied in detail. Because immunosuppressive therapy with cyclosporine adds an additional layer of complexity to the morphologic changes observed, in the present study we have confined our observations to patients immunosuppressed with steroids and azathioprine. Thirteen renal allograft biopsies from patients with ATN and 5 biopsies from patients with normal allograft function were compared with the previously obtained series of 57 native kidney ATN biopsies and 20 control biopsies. Both qualitative and quantitative differences between transplant and native kidney ATN were found. Compared with native kidney ATN, transplant ATN showed significantly less thinning and absence of proximal tubular brush border and less variation in size and shape of cells in individual tubular cross-sections. There were also significantly fewer casts and less dilatation of Bowman's space and a significantly greater number of polarizable crystals presumed to be oxalate in transplant ATN. In native kidney ATN the tubular injury sites were mostly characterized by desquamation of individual epithelial cells leaving areas of bare basement membrane (the "non-replacement" phenomenon). In transplant ATN, sites of tubular injury, although rare and affecting only short tubular segments, were characterized by the actual presence of identifiable necrotic tubular cells, a finding seldom seen in native kidney ATN. There also was a greater interstitial infiltrate of mononuclear inflammatory cells in transplant ATN compared to native kidney ATN. Electron microscopic studies of 9 transplant ATN biopsies showed a mild reduction in proximal tubular brush border compared with controls but this alteration was significantly less than that observed in native kidney ATN. There was no significant alteration in proximal or distal basolateral infoldings and this contrasted sharply with the marked reduction in basolateral infoldings of the plasma membrane observed in native kidney ATN. Disintegrated necrotic cells were found by electron microscopy in transplant ATN whereas these were not observed in native kidney ATN. There were significantly more cells with apoptosis (shrinkage necrosis) in transplant ATN than in native kidney ATN. There were significantly more cells with apoptosis (shrinkage necrosis) in transplant ATN than in native kidney ATN. On the other hand, there were significantly greater numbers of "non-replacement" sites in the distal tubules in native kidney ATN compared to transplant ATN.(ABSTRACT TRUNCATED AT 400 WORDS)