Immunosuppression is required for BK viremia and polyomavirus BK–associated nephropathy (PVAN) in kidney transplants (KTs), but the role of viral determinants is unclear. We examined BKV noncoding control regions (NCCR), which coordinate viral gene expression and replication. In 286 day–matched plasma and urine samples from 129 KT patients with BKV viremia, including 70 with PVAN, the majority of viruses contained archetypal (ww-) NCCRs. However, rearranged (rr-) NCCRs were more frequent in plasma than in urine samples (22 vs. 4%; P < 0.001), and were associated with 20-fold higher plasma BKV loads (2.0 × 104/ml vs. 4.4 × 105/ml; P < 0.001). Emergence of rr-NCCR in plasma correlated with duration and peak BKV load (R2 = 0.64; P < 0.001). This was confirmed in a prospective cohort of 733 plasma samples from 227 patients. For 39 PVAN patients with available biopsies, rr-NCCRs were associated with more extensive viral replication and inflammation. Cloning of 10 rr-NCCRs revealed diverse duplications or deletions in different NCCR subregions, but all were sufficient to increase early gene expression, replication capacity, and cytopathology of recombinant BKV in vitro. Thus, rr-NCCR BKV emergence in plasma is linked to increased replication capacity and disease in KTs.

Human polyomaviruses (HPyVs) are a growing challenge in immunocompromised patients in view of the increasing number of now 12 HPyV species and their diverse disease potential. Currently, histological evidence of disease is available for BKPyV causing nephropathy and haemorrhagic cystitis, JCPyV causing progressive multifocal leukoencephalopathy and occasionally nephropathy, MCPyV causing Merkel cell carcinoma and TSPyV causing trichodysplasia spinulosa, the last two being proliferative skin diseases. Here, the current role of HPyV in solid organ transplantation (SOT) was reviewed and recommendations regarding screening, monitoring and intervention were made. Pre-transplant screening of SOT donor or recipient for serostatus or active replication is currently not recommended for any HPyV. Post-transplant, however, regular clinical search for skin lesions, including those associated with MCPyV or TSPyV, is recommended in all SOT recipients. Also, regular screening for BKPyV replication (e.g. by plasma viral load) is recommended in kidney transplant recipients. For SOT patients with probable or proven HPyV disease, reducing immunosuppression should be considered to permit regaining of immune control. Antivirals would be desirable for treating proven HPyV disease, but are solely considered as adjunct local treatment of trichodysplasia spinulosa, whereas surgical resection and chemotherapy are key in Merkel cell carcinoma. Overall, the quality of the clinical evidence and the strength of most recommendations are presently limited, but are expected to improve in the coming years.

The human polyomavirus BK (BKV) causes nephropathy and hemorrhagic cystitis in kidney and bone marrow transplant patients, respectively. The anti-viral cidofovir (CDV) has been used in small case series but the effects on BKV replication are unclear, since polyomaviruses do not encode viral DNA polymerases. We investigated the effects of CDV on BKV(Dunlop) replication in primary human renal proximal tubule epithelial cells (RPTECs). CDV inhibited the generation of viral progeny in a dose-dependent manner yielding a 90% reduction at 40 lg/mL. Early steps such as receptor binding and entry seemed unaffected. Initial large T-antigen transcription and expression were also unaffected, but subsequent intra-cellular BKV DNA replication was reduced by >90%. Late viral mRNA and corresponding protein levels were also 90% reduced. In uninfected RPTECs, CDV 40 lg/mL reduced cellular DNA replication and metabolic activity by 7% and 11% in BrdU and WST-1 assays, respectively. BKV infection increased DNA replication to 142% and metabolic activity to 116%, respectively, which were reduced by CDV 40 lg/mL to levels of uninfected untreated RPTECs. Our results show that CDV inhibits BKV DNA replication downstream of large T-antigen expression and involves significant host cell toxicity. This should be considered in current treatment and drug development.

BK polyomavirus (BKPyV) infection represents a major problem in transplantation, particularly for renal recipients developing polyomavirus‐associated nephropathy (PyVAN). The possibility that BKPyV may also be oncogenic is not routinely considered. Twenty high‐grade renourinary tumors expressing polyomavirus large T antigen in the entirety of the neoplasm in 19 cases, including the metastases in six, have been reported in transplant recipients with a history of PyVAN or evidence of BKPyV infection. Morphological and phenotypical features consistent with inactivation of the tumor suppressors pRB and p53 were found in the bladder tumors, suggesting a carcinogenesis mechanism involving the BKPyV large tumor oncoprotein/antigen. The pathogenesis of these tumors is unclear, but given the generally long interval between transplantation and tumor development, the risk for neoplasms after BKPyV infections may well be multifactorial. Other elements potentially implicated include exposure to additional exogenous carcinogens, further viral mutations, and cell genomic instability secondary to viral integration, as occurs with the Merkel cell PyV–associated carcinoma. The still scarce but increasingly reported association between longstanding PyVAN and renourinary neoplasms requires a concerted effort from the transplant community to better understand, diagnose, and treat the putative association between the BKPyV and these neoplasms.

The immunomodulatory drug leflunomide is frequently used for treating polyomavirus-associated nephropathy, yet its antiviral mechanism is unclear. We characterized the effects of the active leflunomide metabolite A771726 (LEF-A) on the polyomavirus BK (BKV) life cycle in human renal tubular epithelial cells. LEF-A at 10 g/ml reduced the extracellular BKV load by 90% (IC 90 ) but with significant host cytostatic effects. BKV genome replication, late protein expression, and virion assembly and release were inhibited with visible disruption of the nuclear replication architecture. Both host cell and antiviral effects were largely reversed by uridine addition, implicating nonspecific pyrimidine depletion as the major anti-BKV mechanism of leflunomide.

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Antiviral drugs with specific activity against polyomavirus replication have not been developed in the past. This deficiency has become fully apparent with the emergence of polyomavirus-associated nephropathy in kidney-transplant recipients, with a prevalence rate of up to 10%. In most cases, high BK virus replication in tubular epithelial cells causes significant cytopathology, leading to permanently impaired renal allograft function and return to hemodialysis within 6-60 months. In 5-10% of allogenic bone marrow/hematopoietic stem cell transplant recipients, high-level BK virus replication in the ureter/bladder mucosa has been associated with postengraftment hemorrhagic cystitis, which appears to involve significant immunopathology. Thus, in view of the increasing clinical need, a number of drugs have been studied in small case series. We review the antiviral strategies explored to date and specifically discuss available in vivo and in vitro data on cidofovir, leflunomide, fluoroquinolones and intravenous immunoglobulins, regarding mechanism, administration, dosing and outcome and provide a perspective on future therapy options.

Polyomavirus BK (BKPyV) infects most people subclinically during childhood and establishes a lifelong infection in the renourinary tract. In most immunocompetent individuals, the infection is completely asymptomatic, despite frequent episodes of viral reactivation with shedding into the urine. In immunocompromised patients, reactivation followed by high-level viral replication can lead to severe disease: 1-10% of kidney transplant patients develop polyomavirus-associated nephropathy (PyVAN) and 5-15% of allogenic hematopoietic stem cell transplant patients develop polyomavirus-associated haemorrhagic cystitis (PyVHC). Other conditions such as ureteric stenosis, encephalitis, meningoencephalitis, pneumonia and vasculopathy have also been associated with BKPyV infection in immunocompromised individuals. Although BKPyV has been associated with cancer development, especially in the bladder, definitive evidence of a role in human malignancy is lacking. Diagnosis of PyVAN and PyVHC is mainly achieved by quantitative PCR of urine and plasma, but also by cytology, immunohistology and electron microscopy. Despite more than 40 years of research on BKPyV, there is still no effective antiviral therapy. The current treatment strategy for PyVAN is to allow reconstitution of immune function by reducing or changing the immunosuppressive medication. For PyVHC, treatment is purely supportive. Here, we present a summary of the accrued knowledge regarding BKPyV.