Albumin-Keratin Casts Obstruct Renal Tubular and Vascular Lumens Following Kidney Ischemia.

INTRODUCTION: We have recently reported that in ischemic acute kidney injury (AKI), red blood cell (RBC) trapping causes fluid extravasation from the congested capillary circulation and subsequent toxic injury to the surrounding tubules. The goal of this study was to identify the cause of the obstruction leading to RBC trapping. METHODS: Studies were performed in Sprague Dawley rats and human kidneys. Rats underwent warm arterial clamp ischemia for 45 minutes with 0 to 24 hours of reperfusion. New blood entering the kidney upon reperfusion of the kidney was tracked using Evans blue dye. At harvest, kidneys were fixed and histological analysis performed. Vascular and tubular casts from 4 human and 3 rat kidneys were isolated using laser dissection microscopy and their contents determined by mass spectrometry shotgun proteomics. RESULTS: New blood entering the kidney appeared to be impeded in large renal veins by a homogeneous substance which stained blue in trichrome sections. Scanning electron microscopy revealed the substance to be a solid that conformed the shape of the encased RBCs. A similar substance was observed in the lumens of many tubules and small capillaries. Proteomic analysis identified albumin and keratins as major proteins present in the casts, and this was confirmed using immunohistochemistry. CONCLUSION: Our data indicate that albumin-keratin casts obstruct both vascular and tubular structures in the kidney following a period of ischemia. These casts may be the primary cause of vascular obstruction post-ischemia and likely lead to RBC trapping and associated acute tubular injury.