Granulomatous inflammation in BK polyomavirus-associated nephropathy.

Evolving BK polyomavirus-associated nephropathy (BKPyVAN) is characterized by tubulointerstitial inflammation that closely resembles acute T-cell-mediated allograft rejection if tubulitis is significant. The cellular composition of the inflammation varies during the course of BKPyVAN, and clusters of plasma cells may herald resolution of the infection. Less commonly, BKPyVAN can present with a predominance of histiocytes and granuloma formation. Granulomatous interstitial nephritis is uncommon in biopsies of either native or transplant kidneys. In both settings, this distinctive type of inflammatory response requires a systematic approach with careful clinicopathological assessment to determine its etiology. We present three patients with granulomatous BKPyVAN in the first year post-transplantation. These allograft biopsies at 4, 6, and 12 months post-transplant exemplify spontaneously resolving BKPyVAN, resolving infection after immunosuppression reduction, and early BKPyVAN, respectively. In immunosuppressed patients, BKPyVAN should be added to the relatively broad differential diagnosis of granulomatous tubulointerstitial nephritis.