Postcapillary venule-like transformation of peritubular capillaries in acute renal allograft rejection. An...

To describe ultrastructural changes in renal peritubular capillaries during acute allograft rejection, biopsy specimens from five allografts with acute rejection were studied electron microscopically and compared with control specimens. The thickness, cross-sectional area, and luminal circumference of capillary endothelium were estimated morphometrically. Thickening of endothelial cells, loss of fenestration, increase in endothelial cell organelles, increased adherence and passage of lymphocytes and monocytes, and defects in the endothelial lining were demonstrated. The luminal circumference representing the size of the capillary was not changed. A balloonlike fragmentation of endothelial cells was sometimes observed around lymphocytes, suggesting cytotoxic injury to capillaries. The observations indicate that during rejection-induced endothelial activation, peritubular capillaries exhibit postcapillary venule-like transformation that enhances the influx of inflammatory cells into the kidney allograft.