Beyond C4d: the ultrastructural appearances of endothelium in ABO-incompatible renal allografts

Background. ABO incompatibility is no longer a barrier in kidney transplantation. C4d is frequently positive in ABOincompatible (iABO) biopsies without further signs of microcirculation injury. This phenomenon is assumed to represent graft accommodation. However, ultrastructural examination of glomerular and peritubular capillary endothelium might reveal subtle endothelial damage. Methods. We studied the ultrastructural appearance of the endothelium in 67 biopsies from 21 patients with iABO allografts and compared it with 20 patients (29 biopsies) with ABO-compatible (cABO) grafts with C4d positivity and 25 ABO-compatible control patients (25 biopsies) without serological or histological evidence of humoral rejection (C4d negative). Ten ultrastructural parameters indicative of chronic and acute glomerular and peritubular capillary damage in transmission electron microscopy (TEM) were semi-quantitatively graded and expressed in a sum score. Clinico-pathological data were compared as well as graft function at the time of biopsy and follow-up. Results. Ultrastructural parameters did not significantly differ between iABO and controls. In contrast, C4d-positive cABO had the highest TEM sum score (P = 0.001 versus iABO, P = 0.002 versus controls). The sum score did not differ between C4d-positive and C4d-negative iABO but did differ between patients with and without anti-HLA donor-specific antibodies (DSA). Graft function in iABO at the time of biopsy and at follow-up was similar to controls. Conclusions. Our ultrastructural observations support the concept of endothelial accommodation in iABO renal transplants. C4d positivity in the ABO-incompatible situation does not indicate injurious activation of the complement cascade