A Case of BK Virus Nephropathy with Strong C4d Deposition in a Renal Allograft Recipient

C4d deposition in peritubular capillaries in renal allograft biopsies is a significant marker for diagnosis of antibody-mediated rejection. However, it is unclear whether C4d deposition could be derived from BK virus infection. We present a case of BK virus nephropathy with strong C4d deposition 10 months after kidney transplantation. The diagnosis of BK virus nephropathy was missed out, whereas strong C4d deposition was noted in the first biopsy and therefore anti-rejection therapy was started. The deterioration of renal function led to a evaluate the possibility of BK virus nephropathy with second graft biopsy and further studies of BK virus replication status. Second graft biopsy revealed BK virus nephropathy without rejection. Finally, discontinuation of immunosuppressants and addition of anti-viral therapy for BK virus resulted in recovery of renal function, despite development of pancytopenia and subsequent fungal infection after leflunomide therapy. As in this case, initial focal pathologic changes from BK virus nephropathy could be overlooked by light microscopy. In addition, even though C4d positivity in peritubular capillaries is a good marker for diagnosis of antibody-mediated rejection, the meticulous examinations of the localization of C4d is needed, considering BK virus activates complement pathways and therefore leads to deposition of C4d mainly in tubular basement membrane. Based on our case of BK virus nephropathy with strong C4d deposition, we suggest that C4d deposition could be derived from BK virus nephropathy and therefore, it should be differentiated from acute antibodymediated rejection in a renal allograft recipient.