Primer: histopathology of polyomavirus-associated nephropathy in renal allografts

This short article is an introduction to the histology of damage to transplanted kidneys caused by active BK virus infection. Aimed at trainee pathologists and nephrologists wishing to enhance their understanding of the histological basis of the diseases they manage, this beautifully illustrated paper describes the key features, differential diagnosis and clinical implications of this increasingly frequent complication. 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. Key Points The incidence of polyomavirus-associated nephropathy (PVAN) of renal allografts has increased since the introduction of new-generation immunosuppressive regimens PVAN is caused by the ubiquitous BK virus, which establishes latent infection in urothelium and renal tubular epithelium; latent BK virus can be reactivated by immunosuppression Diagnosis of PVAN requires renal biopsy; histopathological features include enlargement of tubular epithelial cells and dev