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The BK polyomavirus exhibits tropism for the renal tubular epithelium, where it establishes latent infection. Vigorous immunosuppression of renal allograft recipients can lead to reactivation of the infection and the development of polyomavirus-associated nephropathy (PVAN). Clinically, gradually decreasing renal function, viremia and viruria are observed several months after transplantation; allograft failure occurs in 1-10% of patients. Definitive diagnosis requires an allograft biopsy. Histologically, viral replication results in tubular epithelial cell enlargement, karyomegaly and nuclear inclusion bodies. The cytopathic changes are often associated with lysis of tubular epithelial cells, denudation of the basement membrane and an interstitial inflammatory response. The involvement is multifocal; distal nephron segments are more severely affected than proximal segments. Changes observed during light microscopy are suggestive but not pathognomonic for PVAN, and the diagnosis must be confirmed by adjunct studies. Adjunct studies consist of immunohistochemistry on paraffin sections using an antibody to the SV40 large T antigen, or electron microscopy of infected tubular epithelial cells (virions 40 nm in diameter). PVAN manifests in three histologic patterns: pattern A, viral cytopathic changes with no or only minimal inflammation; pattern B, cytopathic and cytolytic lesions with interstitial inflammation; or pattern C, predominantly interstitial fibrosis and tubular atrophy, with variable cytopathic and inflammatory changes. These patterns correlate with clinical outcomes.

Donor biopsies are very useful as zero-hour biopsies establishing baseline information for comparison with subsequent transplant biopsies. As none of the histological variables and scores provides perfect prediction, preimplantation biopsy results have to be interpreted in the context of all available donor and recipient information.

The main causes of the late dysfunction of renal allografts are chronic rejection and chronic transplant nephropathy. Both are clinicopathologic entities, with a similar clinical presentation, but different histologic appearances. Chronic rejection is characterized by the presence of alloantigeninduced lesions (transplant arteriopathy and transplant glomerulopathy), and chronic transplant nephropathy by nonspecific sclerosing changes. The incidence of transplant arteriopathy and transplant glomerulopathy is relatively low. Electron microscopy (EM) may overcome the limitations in the histologic diagnosis of chronic rejection, because it verifies alloantigen-induced chronic microvasculopathy in the peritubular capillaries (transplant capillaropathy), and identifies transplant glomerulopathy more precisely than does light microscopy. To assess the value of EM in chronic rejection diagnosis, a retrospective search for transplant capillaropathy and transplant glomerulopathy was performed in a consecutive series of 91 biopsies performed >6 months after implantation (median: 26 months, range 6 -186) and the diagnoses were reclassified on the basis of the ultrastructural findings. The definitions used were: transplant capillaropathy: a peritubular capillary profile with seven or more circumferential basement membrane layers, or at least three profiles with five or six circumferential layers; ultrastructurally verified transplant glomerulopathy: thickening of the capillary wall in at least three loops in consequence of the widening of the subendothelial space by abnormal basement membrane material, and the formation of a new layer(s) of basal lamina; and chronic rejection: the presence of transplant capillaropathy and/or transplant glomerulopathy and/or transplant arteriopathy. Histologically, chronic transplant nephropathy, chronic rejection, chronic cyclosporine nephrotoxicity, glomerulonephritis, acute rejection, "suspicious" for acute rejection, and "others" were diagnosed in 37%, 34%, 21%, 19%, 57%, 30%, and 5% of the specimens, respectively. The results of EM increased the diagnosis of chronic rejection to 69% of the cases, and decreased chronic transplant nephropathy to 15%. The individual incidence of transplant capillaropathy and transplant glomerulopathy was 79% and 57%, respectively, and their cumulative incidence was 92%. Five biopsies exhibited merely transplant arteriopathy. A late dysfunction typically had more than one cause; the most frequent combination was chronic rejection and acute rejection. In conclusion, the EM search for transplant capillaropathy and transplant glomerulopathy doubled the frequency of the diagnosis of chronic rejection. Currently, the evaluation of renal allograft biopsies from recipients with a late dysfunction relies on standard light microscopy. Because light microscopy per se proved to be insensitive in the diagnosis of chronic rejection, incorporation of EM into the evaluation of late dysfunction biopsies is strongly recommended. The long-term survival of renal allografts is ...