Tacrolimus (FK506) is one of the principal immunosuppressive agents used after solid organ transplantations to prevent allograft\nrejection. Chronic renal injury induced by tacrolimus is characterized by linear fibrosis in the medullary rays; however, the early\nmorphologic findings of acute tacrolimus nephrotoxicity are not well characterized. Kidney injury molecule-1 (KIM-1) is a specific\ninjury biomarker that has been proven to be useful in the diagnosis of mild to severe acute tubular injury on renal biopsies. This\nstudy was motivated by a patient with acute kidney injury associated with elevated serum tacrolimus levels in whomKIM-1 staining\nwas present only in proximal tubules located in the medullary rays in the setting of otherwise normal light, immunofluorescent,\nand electron microscopy. We subsequently evaluated KIM-1 expression in 45 protocol and 39 indicated renal transplant biopsies\nto determine whether higher serum levels of tacrolimus were associated with acute segment specific injury to the proximal tubule,\nas reflected by KIM-1 staining in the proximal tubules of the cortical medullary rays. The data suggest that tacrolimus toxicity\npreferentially affects proximal tubules in medullary rays and that this targeted injury is a precursor lesion for the linear fibrosis\nseen in chronic tacrolimus toxicity.
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