Polyomavirus-associated nephropathy in renal transplantation: critical issues of screening and management.

Polyomavirus-associated nephropathy (PVAN) is an emerging disease in renal transplant patients with variable prevalence of 1-10% and graft loss up to 80%. BK virus (BKV) is the primary etiologic agent, but JC virus (JCV) and possibly simian virus SV40 may account for some cases. Intense immunosuppression is viewed as the most important risk factor. However, the preferential manifestation in renal transplants as compared to other allografts or to autologous kidneys of other organ transplants suggests that organ determinants and immunologic factors synergize: Renal tubular epithelial cells and their compensatory proliferation to restore tubular integrity after immunologic, ischemic or toxic injury may provide the critical cellular milieu supporting polyomavirus replication while immune control is impaired due to maintenance immunosuppression, anti-rejection treatment and HLA-mismatches. Patient determinants (older age, male gender, seronegative recipient), and viral factors (genotype, serotype) may have a contributory role. The definitive diagnosis of PVAN requires allograft biopsy which is, however, challenged by (i) limited sensitivity due to (multi-)focal involvement (sampling errors); (ii) varying presentations with cytopathic-inflammatory and/or fibrotic/scarring patterns; (iii) coexisting acute rejection which is difficult to differentiate, but impacts on intervention strategies. Screening for polyomavirus replication in the urine and in the plasma complements allograft biopsy by high sensitivity and allows for noninvasive monitoring. Thus, we suggest a terminology similar to invasive fungal diseases where viruria ("decoy cells") defines patients at risk ("possible PVAN") who should be evaluated for plasma viral load. Increasing BK viremia (>10,000 copies/mL) or urine VP-1 mRNA (>6.5x10(5) copies/ng total RNA) load defines "presumptive PVAN" for which an intervention of reducing immunosuppression should be considered even if the diagnosis could not be confirmed by allograft biopsy ("definitive PVAN"). The response to intervention should be monitored using plasma DNA or urine mRNA load.