| Summary: | We report an anti-SV40 positive collecting duct (CD) cell carcinoma in a renal allograft 4.5 years after transplantation that developed during smoldering BK virus (BKV) nephropathy. In the allograft biopsy six months after transplantation documenting BK nephropathy, anti-SV40 positivity co-localized with the CD marker CK-HMW and proliferation marker MIB-1, suggesting that BKV preferentially infects CD cells and causes them to proliferate. In previous reports on BKV related renal malignancies, immunohistochemistry also pointed to a CD origin of the tumors. Six additional cases of BKV induced renal tumors were found in four CD carcinoma case-reports and one large case-series of “usual” CD carcinomas, supporting the hypothesis that BKV specifically induce CD cell tumor formation. In view of the rarity of BKV induced tumors a second hit might be needed to induce malignancy. In our case the use of lithium—which also causes CD cell proliferation—at the time of BKV infection may have provided this second hit. Keywords: Kidney transplantation, BK virus, Collecting duct cell carcinoma
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