Morphological differentiation between rejection and cyclosporin nephrotoxicity in renal allografts

In a prospective study of renal dysfunction in 60 consecutive allograft recipients treated with cyclosporin and prednisolone routine renal biopsies at one week and one month after transplantation, as well as for all episodes of renal dysfunction, were performed. The one year graft survival of this group was 88%. In a retrospective clinical analysis of these patients 35 episodes of dysfunction due to rejection, defined by a response to antirejection treatment alone, and 30 episodes due to cyclosporin nephrotoxicity, defined by a response to reduction in cyclosporin dose alone, were identified. The morphological findings from these biopsies were compared with 20 samples from routine biopsies taken from patients with stable renal function. All patients diagnosed as having rejection had a diffuse, interstitial mononuclear cell infiltrate (32 of 35) or arteritis (19 of 35), or both. In contrast, focal mononuclear cell infiltrates were common in both patients with nephrotoxicity and those with stable function (17 of 30 and 14 of 20, respectively). There were no important differences between biopsies from those with nephrotoxicity and those with stable function, except that arteriolar hyalinosis was considerably more common in the nephrotoxic patients than in those with stable function. Many patients with stable function were, in retrospect, in a state of stable mild nephrotoxicity. In our experience rejection should only be diagnosed when there is at least a diffuse interstitial infiltrate or an arteritis. Focal mononuclear cell infiltrates do not denote rejection. The development of arteriolar lesions in the absence of rejection is indicative of nephrotoxicity.