BK Nephritis

This biopsy is from a patient with ESRD due to polycystic kidney disease. He is s/p deceased donor renal transplant 11 months prior to this biopsy. He was noted to have a steadily rising creatinine from baseline of 1.3 mg/dl up to 4.6 mg/dl at biopsy. Note the severe interstitial inflammation with nuclear atypia and severe arteriosclerosis (Figure 1, H&E 100x). There are markedly atypical nuclei including an apoptotic body (arrowhead) seen in the second image (Figure 2, 400x). The third image shows an SV40 IHC stain for polyoma virus that is positive in many nuclei (Figure 3, IHC SV40 100x). The polyclonal antibody raised against polyoma cross-reacts with both BK and JC viruses, but in renal transplants is almost always BK. After the biopsy was reported, a markedly elevated BK viremia was found.