Nearly 45 years after the discovery of the first two human polyomaviruses BK and JC, their life-long persistence and mechanisms of pathogenesis remain poorly understood and efficient antiviral treatments are severely lacking. In this review, we sought to provide an update on recent advances in understanding the life cycle of these two viruses, particularly focusing on their interaction with the host immune system and pathogenesis. We have also discussed novel treatment approaches and highlighted areas of future research.

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International guidelines define a BK virus (BKV) load of >4 log 10 copies/ml as presumptive of BKV-associated nephropathy (BKVN) and a cutoff for therapeutic intervention. To investigate whether BKV DNA loads (BKVL) are comparable between laboratories, 2 panels of 15 and 8 clinical specimens (urine, whole blood, and plasma) harboring different BKV genotypes were distributed to 20 and 27 French hospital centers in 2013 and 2014, respectively. Although 68% of the reported results fell within the acceptable range of the expected result ؎0.5 log 10 , the interlaboratory variation ranged from 1.32 to 5.55 log 10 . Polymorphisms specific to BKV genotypes II and IV, namely, the number and position of mutations in amplification target genes and/or deletion in standards, arose as major sources of interlaboratory disagreements. The diversity of DNA purification methods also contributed to the interlaboratory variability, in particular for urine samples. Our data strongly suggest that (i) commercial external quality controls for BKVL assessment should include all major BKV genotypes to allow a correct evaluation of BKV assays, and (ii) the BKV sequence of commercial standards should be provided to users to verify the absence of mismatches with the primers and probes of their BKV assays. Finally, the optimization of primer and probe design and standardization of DNA extraction methods may substantially decrease interlaboratory variability and allow interinstitutional studies to define a universal cutoff for presumptive BKVN and, ultimately, ensure adequate patient care.T he emergence of BK virus-associated nephropathy (BKVN) as a major cause of graft dysfunction and loss in kidney transplant recipients (KTR) arises from the use of highly potent immunosuppressive drugs (1-3). This is a growing medical problem as the population of transplant recipients continues to increase. In Europe and the United States, the number of kidney transplantations has increased up to 50% in the last 20 years (www.kidney .niddk.nih.gov and http://www.era-edta.org). BKV reactivation or reinfection occurs in 40 to 50% of KTR, followed by BKVN in 6.6% of KTR at 5 years posttransplant, ultimately leading to graft dysfunction and loss in up to 50% of cases (4). The diagnosis of BKVN is based on the documentation of viral cytopathic effects observed in tubular epithelial cells accompanied by inflammatory cell infiltration after renal biopsy (5, 6). Immunohistochemistry with SV40 staining is the gold standard for diagnosing definitive BKVN (7). Nevertheless, in the early stages of BKVN, kidney allograft biopsy results may be falsely negative at an estimated rate of 10 to 30% (8). Prospective studies showed that high BKV viruria usually precedes viremia by 4 to 12 weeks, with a sustained BKV viremia above the threshold of 4 log 10 copies/ml defined as presumptive of BKVN, with a positive predictive value of Ͼ80% (9, 10). These studies showed that BKVN can be effectively and safely prevented using a preemptive reduction in immunosuppression (11,12). There...