Proximal Tubule Cytoplasmic Fibrillary Inclusions Following Kidney Transplantation in a Patient With a Paraproteinemia

Pathological findings in the first biopsy specimen obtained on day 80 after transplantation. (A) Light microscopy shows engorged proximal tubular cells, whereas distal tubules and glomeruli are intact (periodic acid–Schiff; original magnification ×200). (B) Proximal tubular cells contain abundant cytoplasmic inclusion bodies, which stained negatively for periodic acid–Schiff (original magnification ×400). (C) Electron microscopy shows many enlarged lysosomes in the proximal tubular epithelium (original magnification ×6,000). (D) Proximal tubular cells show accumulation of fibrillary materials, composed of microtubular structures with an electron-lucent core and randomly arranged in longitudinal or cross-sectional arrays. Mitochondria were intact (original magnification ×15,000).

(A, B) Immunofluorescence examination of fibrillary materials in frozen sections of the second biopsy specimen obtained on day 180 after transplantation. Positive immunostaining for κ light chains was limited to (A) casts in the distal tubules and was not seen in the (A) proximal tubular epithelium. (B) No λ light chain deposition was detected ([A, B] original magnification ×200). (C-H) Electron micrographs of the (C, D) distal tubule and (E-H) glomerular tuft obtained from the second biopsy specimen on day 180 after transplantation. (E-H) Part of the tissue was retrieved from the paraffin block, deparaffinized, and reprocessed for electron microscopy because no glomeruli were included in the specimen for electron microscopy. (C) Casts in distal tubules contained abundant fibrillary materials, which were composed of (D) microtubular structures. Some fibrillary materials were also found in the cytoplasm of (D) vascular endothelial, (F) glomerular endothelial, (G) glomerular epithelial, and (H) mesangial cells.