[Clinical aspects of human polyomaviruses in renal transplantation]

A THREAT FOR RENAL ALLOGRAFT: Human polyomavirus infections (BK virus, JC virus), known for the past 30 years, were considered as common in renal transplantation until the recently reported studies describing the responsibility of BKv (and less JCv) in the occurrence of tubulo-interstitial nephritis in around 5% of renal transplant recipients, with worsening of the renal function leading to graft failure in 10 to 45% of infected patients. Their description coincided with the use of new immunosuppressors (tacrolimus and mycophenolate mofetil) without, however, their responsibility clearly incriminated. EARLY DIAGNOSIS FOR EFFICIENT TREATMENT: The presence of cells infected by the polyomavirus ("decoy cells") in the urine and the detection of BKv or JCv DNA by PCR in the plasma and urine are viral replication markers which strongly suggest the possibility of a polyomavirus nephropathy. TWO CLINICAL VARYING FORMS: Polyomavirus infection is frequent and often asymptomatic. The diagnosis requires the detection of large nucleus "decoy cells" in fresh urine. Polyomavirus renal allograft disease is characterised by the association of decoy cells and renal failure related to a tubulo-interstitial nephropathy and the presence of DNA of the virus in the plasma. The diagnosis requires identification of intra-nuclear viral inclusions in epithelial cells using immunohistochemistry, in situ hybridisation, or electron microscopy techniques. A DIFFICULT DIAGNOSIS: Confusion between interstitial nephritis and acute cellular rejection is the major risk leading to therapeutic error. Risk factors include over-immunosuppression and/or treatment of rejection episodes which could increase viral replication as well as the emergence of mutant BKv strains at the origin of tubulo-interstitial nephritis, leading to acute and chronic dysfunction of the renal transplantation.