Polyoma virus infection after renal transplantation. Use of immunostaining as a guide to diagnosis

BACKGROUND: Polyoma virus infection is characterized by lymphocytic interstitial infiltrate in the kidney, and it mimics acute rejection. The purpose of this study is to estimate renal allograft outcome with this infection and characterize the lymphocytic infiltrates in polyoma virus-infected renal allografts. METHODS: Patients who had polyoma virus inclusions in renal allograft biopsies were identified. Other viral inclusions were excluded by immunohistochemistry. The lymphocytic infiltrates of six cases of polyoma virus infection were compared with six cases of definite acute rejection by immunostaining for T and B cells. RESULTS: There were 10 cases of polyoma virus infections in renal transplant recipients. Immunosuppressants consisted of mycophenolate mofetil with tacrolimus in eight cases and mycophenolate mofetil with cyclosporine in two. The median time of diagnosis of polyoma virus infection after transplantation was 9.5 months, and the time to graft failure after the diagnosis was 4 months. Reduced allograft survival was seen in patients who had polyoma virus infection. Immunostaining for T and B cells revealed marked increase in the B cells (CD20) in renal allografts with polyoma virus infection of 21% (range, 5-40%) compared with 6% (range, 0-10%) in those with acute rejection (P=0.039). Reduced cytotoxic T cells (TIA-1: median, 7%; range, 2-15%) were seen in polyoma virus-infected allografts compared with 24% (range, 15-30%) in those patients who had acute rejection (P=0.0159). CONCLUSION: Irreversible graft failure is more prevalent with polyoma virus infection. Enhanced immunosuppressants with mycophenolate mofetil with tacrolimus may play a role in the development of this infection. An increase in CD20 and a decrease in cytotoxic T cells in allografts is characteristic of polyoma virus infection.