Diagnosing Polyomavirus Nephropathy without a Biopsy: Validation of the Urinary PyV-Haufen-Test in a Proof-of-Concept Study including Uromodulin Knock-out-Mice.

BACKGROUND: Polyomavirus nephropathy (PyVN) leads to kidney transplant dysfunction and loss. Since a definitive diagnosis requires an invasive kidney biopsy, a timely diagnosis is often hampered. In this clinical dilemma the PyV-haufen-test, centering around the detection of three-dimensional PyV aggregates in the urine, might provide crucial diagnostic information. METHODS: A multistep experimental design. Hypothesis: PyV-haufen form within the kidneys under high concentrations of uromodulin, a kidney specific protein; PyV-haufen are kidney-specific-disease-markers. RESULTS: Investigative step A showed colocalization of uromodulin with aggregated PyV (i) in ten kidneys with PyVN by immunohistochemistry, (ii) in urine samples containing PyV-haufen by electron microscopy/immunogold labeling (n = 3), and (iii) in urine samples containing PyV-haufen by immunoprecipitation assays (n = 4). Investigative step B: In in-vitro experiments only high uromodulin concentrations of ≥ 1.25 mg/mL aggregated PyV, as is expected to occur within injured nephrons. In contrast, in voided urine samples (n = 59) uromodulin concentrations were below aggregation concentrations (1.2 -19.6 µg/mL). Investigative step C: 0/11 (0%) uromodulin KO-/- mice with histologic signs of PyVN showed urinary PyV-haufen shedding compared to 10/14 (71%) WT+/+ mice. CONCLUSION: PyV-haufen form within kidneys under high uromodulin concentrations. Thus, PyV-haufen detected in the urine are specific biomarkers for intra-renal disease, i.e. definitive PyVN.

Ultrastructural Examination of Glomerular Fibrillary Deposits in Diabetic Nephropathy.

Glomerular fibrillary deposits have occasionally been reported in diabetic nephropathy, but no large-scale, ultrastructural evaluation of these deposits has been reported so far. Here, we report our study of glomerular non-Congophilic, DnaJ homolog subfamily B member 9 negative fibrillary deposits in diabetic nephropathy as characterized by transmission electron microscopy. Clinical data from 55 patients with biopsy-confirmed diabetic nephropathy and 18 healthy living donors were reviewed, and their biopsies were evaluated by light microscopy, immunofluorescence, and electron microscopy. Small fibrillary structures with a diameter of 10 ± 1 nm were present in all cases with diabetic nephropathy, regardless of the histologic class. In addition, glomerular fibrillary structures with a diameter of 23 ± 5 nm or 30 ± 7 nm were present in 35 cases. Interestingly, especially the small- and medium-sized fibrils, usually without apparent organization, were comparable with fibrils in fibrillary glomerulopathy. We conclude that glomerular fibrillary deposits occur far more commonly in renal biopsies of patients with diabetic nephropathy than generally considered. This is an important finding because their similarity to fibrils in fibrillary glomerulonephritis may complicate the histologic diagnostic process, especially in cases of overlapping clinical manifestations. Therefore, when encountering fibrillary deposits on electron microscopy, it is important to consider diabetic nephropathy as an alternative diagnosis.

Consensus Definitions of BK Polyomavirus Nephropathy in Renal Transplant Recipients for Clinical Trials.

BACKGROUND: BK polyomavirus (BKPyV) infection and BK polyomavirus nephropathy (BKPyVAN) are important causes of allograft dysfunction and premature allograft loss in renal transplant recipients. RESULTS AND DISCUSSION: Controlled clinical trials to evaluate new agents for prevention and treatment are needed but are hampered by the lack of outcome measures that accurately assess the effect of the intervention, are clinically relevant, and are acceptable from a regulatory perspective. METHODS: To facilitate consistent end points in clinical trials and to support clinical research and drug development, definitions of BKPyV infection and disease have been developed by the BK Disease Definitions Working Group of the Transplantation Associated Virus Infection Forum with the Forum for Collaborative Research, which consists of scientists, clinicians, regulators, and industry representatives. CONCLUSIONS: These definitions refine established principles of "proven" BKPyV disease and introduce a "probable" disease category that could be used in clinical trials to prevent or treat BKPyVAN in renal transplant recipients.

Novel intragraft regulatory lymphoid structures in kidney allograft tolerance.

Intragraft events thought to be relevant to the development of tolerance are here subjected to a comprehensive mechanistic study during long-term spontaneous tolerance that occurs in C57BL/6 mice that receive life sustaining DBA/2 kidneys. These allografts rapidly develop periarterial Treg-rich organized lymphoid structures (TOLS) that form in response to class II but not to class I MHC disparity and form independently of lymphotoxin α and lymphotoxin ß receptor pathways. TOLS form in situ in the absence of lymph nodes, spleen, and thymus. Distinctive transcript patterns are maintained over time in TOLS including transcripts associated with Treg differentiation, T cell checkpoint signaling, and Th2 differentiation. Pathway transcripts related to inflammation are expressed in early stages of accepted grafts but diminish with time, while B cell transcripts increase. Intragraft transcript patterns at one week posttransplant distinguish those from kidneys destined to be rejected, that is, C57BL/6 allografts into DBA/2 recipients, from those that will be accepted. In contrast to inflammatory tertiary lymphoid organs (iTLOs) that form in response to chronic viral infection and transgenic Lta expression, TOLS lack high endothelial venules and germinal centers. TOLS represent a novel, pathogenetically important type of TLO that are in situ markers of regulatory tolerance.

Renal comorbidities in collapsing variant focal segmental glomerulosclerosis: more than a coincidence?

BACKGROUND: Collapsing focal segmental glomerulosclerosis (FSGS) has various underlying etiologies and often leads to renal failure. The impact of biopsy-proven renal comorbidities in promoting collapsing glomerulopathy (CG) has not been systematically evaluated in large comparative studies. Those data are reported here. METHODS: Biopsies with the initial diagnosis of CG in native (n = 321) or transplant kidneys (n = 30) were identified in the University of North Carolina nephropathology database (1 January 2011 to 1 January 2016). Two cohorts were defined: 'sole' CG without and 'accompanied' CG with significant morphologic renal comorbidities. Tip-variant FSGS (T-FSGS) and time-matched biopsies served as control cohorts for comparative analyses. RESULTS: CG was significantly more common in native (4.4%) and transplant biopsies (4.1%) compared with T-FSGS (0.7 and <0.1%, respectively, difference versus CG P < 0.01). 'Associated' disease was significantly more common in CG (native: 151/321; 47.0%, transplant: 21/30; 70%, P < 0.05) versus T-FSGS (native: 14/51; 27.5%, transplant: exceptional; all differences versus CG P < 0.05). In native biopsies with 'accompanied' CG but not in control groups, stenosing vasculopathies including thrombotic microangiopathies were significantly more prevalent (P < 0.01). In transplants, the high incidence of 'accompanied' CG was linked to de novo diseases, mainly rejection and vascular injury. In native kidneys, membranous glomerulopathies were prevalent in 'accompanied' T-FSGS (36%) and CG (14%) (difference versus time-matched controls P < 0.01 and P < 0.05, respectively); they were uncommon in transplants. CONCLUSIONS: CG but not T-FSGS shows a high rate of comorbidities, with prominent vasculopathies presumably driving 'ischemic' CG-specific glomerular injury and also the disease course. These findings facilitate future studies into therapy, prognosis and reversibility of 'accompanied' CG.

The 2018 Banff Working Group classification of definitive polyomavirus nephropathy: A multicenter validation study in the modern era.

Polyomavirus nephropathy (PVN) remained inadequately classified until 2018 when the Banff Working Group published a new 3-tier morphologic classification scheme derived from in-depth statistical analysis of a large multinational patient cohort. Here we report a multicenter "modern-era" validation study that included 99 patients with definitive PVN transplanted post January 1, 2009 and followed the original 2018 study design. Results validate the PVN classification, that is, the 3 PVN disease classes predicted clinical presentation, allograft function, and outcome independent of therapeutic intervention. PVN class 1 compared to classes 2 and 3 was diagnosed earlier (16.9 weeks posttransplant [median], P = .004), and showed significantly better function at 24 months postindex biopsy (serum creatinine 1.75 mg/dl, geometric mean, vs class 2: P = .037, vs class 3: P = .013). Class 1 presented during long-term follow-up with a low graft failure rate: 5% class 1, vs 30% class 2, vs 50% class 3 (P = .009). Persistent PVN was associated with an increased risk for graft failure (and functional decline in class 2 at 24 months postdiagnosis; serum creatinine with persistence: 2.48 mg/dL vs 1.65 with clearance, geometric means, P = .018). In conclusion, we validate the 2018 Banff Working Group PVN classification that provides significant clinical information and enhances comparative data analysis.

Intragraft gene expression in native kidney BK virus nephropathy versus T cell-mediated rejection: Prospects for molecular diagnosis and risk prediction.

Novel tools are needed to improve diagnostic accuracy and risk prediction in BK virus nephropathy (BKVN). We assessed the utility of intragraft gene expression testing for these purposes. Eight hundred genes were measured in 110 archival samples, including a discovery cohort of native kidney BKVN (n = 5) vs pure T cell-mediated rejection (TCMR; n = 10). Five polyomavirus genes and seven immune-related genes (five associated with BKVN and two associated with TCMR) were significantly differentially expressed between these entities (FDR < 0.05). These three sets of genes were further evaluated in samples representing a spectrum of BK infection (n = 25), followed by a multicenter validation cohort of allograft BKVN (n = 60) vs TCMR (n = 10). Polyomavirus 5-gene set expression reliably distinguished BKVN from TCMR (validation cohort AUC = 0.992), but the immune gene sets demonstrated suboptimal diagnostic performance (AUC ≤ 0.720). Within the validation cohort, no significant differences in index biopsy gene expression were identified between BKVN patients demonstrating resolution (n = 35), persistent infection (n = 14) or de novo rejection (n = 11) 6 months following a standardized reduction in immunosuppression. These results suggest that, while intragraft polyomavirus gene expression may be useful as an ancillary diagnostic for BKVN, assessment for concurrent TCMR and prediction of clinical outcome may not be feasible with current molecular tools.

The Banff 2019 Kidney Meeting Report (I): Updates on and clarification of criteria for T cell- and antibody-mediated rejection.

The XV. Banff conference for allograft pathology was held in conjunction with the annual meeting of the American Society for Histocompatibility and Immunogenetics in Pittsburgh, PA (USA) and focused on refining recent updates to the classification, advances from the Banff working groups, and standardization of molecular diagnostics. This report on kidney transplant pathology details clarifications and refinements to the criteria for chronic active (CA) T cell-mediated rejection (TCMR), borderline, and antibody-mediated rejection (ABMR). The main focus of kidney sessions was on how to address biopsies meeting criteria for CA TCMR plus borderline or acute TCMR. Recent studies on the clinical impact of borderline infiltrates were also presented to clarify whether the threshold for interstitial inflammation in diagnosis of borderline should be i0 or i1. Sessions on ABMR focused on biopsies showing microvascular inflammation in the absence of C4d staining or detectable donor-specific antibodies; the potential value of molecular diagnostics in such cases and recommendations for use of the latter in the setting of solid organ transplantation are presented in the accompanying meeting report. Finally, several speakers discussed the capabilities of artificial intelligence and the potential for use of machine learning algorithms in diagnosis and personalized therapeutics in solid organ transplantation.

BK polyomavirus nephropathy with systemic viral spread: Whole genome sequencing data from a fatal case of BKPyV infection.

BK polyomavirus (BKPyV) infections with multi-organ involvement are rare. Here, we report for the first time whole genome sequencing data from a patient with systemic BKPyV disease. She presented post stem cell transplantation with graft-vs-host disease, suffered from profound immunosuppression, and developed fatal BKPyV disease of kidneys, lungs, and pancreas. The lytic infection was caused by an episomal BKPyV-Ib strain with canonical structural and receptor encoding gene sequences. However, DNA from all infected tissue sites showed diverse BKPyV-NCCR rearrangements (rr-NCCR) involving the P, Q, and R domains, while largely sparing O and S, carrying initiation sites for early and late BKPyV gene transcripts crucial for viral replication and assembly. Common to all rr-NCCR variants was a break point in Q (position 17-27) that can form the nidus for double DNA strand break formation and gene rearrangements. Metastatic clonal BKPyV spread from kidneys to other organs was not detected. We hypothesize that lack of immune surveillance and a specific NCCR break point promote profound gene rearrangements of NCCR-P, Q, and R with alterations of regulatory feedback loops. As a result, viral replication and pathogenicity are enhanced leading to severe, often fatal systemic disease not caused by the common archetypical BKPyV strains.

The two-faced nature of BK polyomavirus: lytic infection or non-lytic large-T-positive carcinoma.

In immunocompromised patients, reactivation of latent BK polyomavirus (BKPyV) can cause disease with lytic infections of the kidneys and the lower urinary tract. Emerging evidence also links BKPyV to oncogenesis and high-grade intrarenal and transitional cell carcinomas. These neoplasms strongly express polyomavirus large-T antigen as a defining feature; that is, they are 'large-T-positive carcinomas'. Such neoplasms arise in immunocompromised patients, typically in renal allograft recipients, and preferentially in tissues harbouring latent BKPyV. In recent articles in this journal, it was shown that tumour cells harbour replication-incompetent clonal BKPyV. The virus can be truncated and randomly integrated into the genome, and/or it can be mutated in an episomal state. Truncation and/or deletions in the BKPyV non-coding control region can hamper late viral gene expression, replication, and cell lysis, while facilitating overexpression of early genes, including that encoding large-T. Biologically active fusion proteins or alterations in human tumour suppressor or promoter function have not been described so far, making uncontrolled large-T gene expression in non-lytically infected cells a prime suspect for neoplastic transformation. Current concepts of BKPyV-induced disease, including recent reports from this journal, are discussed, and evolving paradigms of BKPyV-associated oncogenesis are highlighted. © 2018 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

The Banff Working Group Classification of Definitive Polyomavirus Nephropathy: Morphologic Definitions and Clinical Correlations.

Polyomavirus nephropathy (PVN) is a common viral infection of renal allografts, with biopsy-proven incidence of approximately 5%. A generally accepted morphologic classification of definitive PVN that groups histologic changes, reflects clinical presentation, and facilitates comparative outcome analyses is lacking. Here, we report a morphologic classification scheme for definitive PVN from the Banff Working Group on Polyomavirus Nephropathy, comprising nine transplant centers in the United States and Europe. This study represents the largest systematic analysis of definitive PVN undertaken thus far. In a retrospective fashion, clinical data were collected from 192 patients and correlated with morphologic findings from index biopsies at the time of initial PVN diagnosis. Histologic features were centrally scored according to Banff guidelines, including additional semiquantitative histologic assessment of intrarenal polyomavirus replication/load levels. In-depth statistical analyses, including mixed effects repeated measures models and logistic regression, revealed two independent histologic variables to be most significantly associated with clinical presentation: intrarenal polyomavirus load levels and Banff interstitial fibrosis ci scores. These two statistically determined histologic variables formed the basis for the definition of three PVN classes that correlated strongest with three clinical parameters: presentation at time of index biopsy, serum creatinine levels/renal function over 24 months of follow-up, and graft failure. The PVN classes 1-3 as described here can easily be recognized in routine renal biopsy specimens. We recommend using this morphologic PVN classification scheme for diagnostic communication, especially at the time of index diagnosis, and in scientific studies to improve comparative data analysis.

Pseudolinear C4d deposits in a hereditary glomerulopathy caused by a rare NC1 collagen-4-alpha-5 missense mutation: a "new disease entity"?

C4d positive glomerulopathies with pseudolinear capillary wall deposits caused by basement membrane (GBM) remodeling have sporadically been reported in renal transplants. Here we describe the case of a hypertensive 60 year-old male with a 5 month history of nephrotic range proteinuria in the setting of normal serum creatinine, complement and ANA levels. Work-up showed MGUS (IgG/kappa restricted). A diagnostic renal biopsy to search for monoclonal gammopathy of renal significance demonstrated thickened glomerular capillary walls with strong pseudolinear complement factor C4d deposits by immunofluorescence microscopy (IF); all other IF studies including stains for Col4A3 were unrevealing with only minor abnormalities seen for Col4A5. The strong and unusual C4d staining of undetermined direct diagnostic significance triggered additional electron microscopic studies uncovering marked structural GBM changes suggestive of a hereditary nephropathy. Further genetic testing revealed a very rare X-linked single missense mutation in the NC1 domain of Col4A5 (exon 51) with a single amino acid substitution (COL4A5 p.A1581S) that has thus far not been reported in hereditary nephropathies. Our case provides further support for pseudolinear glomerular C4d deposits as general markers of GBM remodeling, in our case an unexpected hereditary nephropathy in an older male. Pseudolinear C4d: a general signpost for architectural GBM disturbance and a stimulus for in-depth studies including electron microscopy.

Clinical Features and Outcomes of a Racially Diverse Population with Fibrillary Glomerulonephritis.

BACKGROUND: Fibrillary glomerulonephritis is characterized by randomly arranged fibrils, approximately 20 nm in diameter by electron microscopy. Patients present with proteinuria, hematuria and kidney insufficiency, and about half of the reported patients progress to end-stage kidney disease within 4 years. The dependence of patient characteristics and outcomes on race has not been explored. In this study, we describe a cohort of patients with fibrillary glomerulonephritis and compare their clinical characteristics and outcomes with those of patients previously described. METHODS: The University of North Carolina (UNC) Nephropathology Database was used to retrospectively identify patients diagnosed with fibrillary glomerulonephritis between 1985 and 2015. Of these patients, those treated at UNC were selected. Their demographic and clinical characteristics - including signs and symptoms, comorbidities, laboratory values, treatments and outcomes - were compared with those of patients described earlier. RESULTS: Among the 287 patients identified, 42 were treated at the UNC Kidney Center. When compared to earlier cohorts, a higher frequency of black race, hepatitis C virus (HCV) infection and use of hemodialysis were noted in both black and HCV-positive patients. Autoimmune diseases, infections and malignancies were frequently observed, present in over half of all cases. CONCLUSION: According to this study, fibrillary glomerulonephritis represents a secondary glomerular disease process (associated with autoimmune disease, infection or malignancy) in many cases and hence screening is essential. As the screening for comorbidities increased over time, more underlying causes were identified. We noted a high frequency of HCV among black patients, suggesting a possible causative association. Treatment of underlying disease is essential for patients for the best outcome.

Glomerular C4d deposits can mark structural capillary wall remodelling in thrombotic microangiopathy and transplant glomerulopathy: C4d beyond active antibody-mediated injury: a retrospective study.

Peritubular capillary C4d (ptc-C4d) usually marks active antibody-mediated rejection, while pseudolinear glomerular capillary C4d (GBM-C4d) is of undetermined diagnostic significance, especially when seen in isolation without concurrent ptc-C4d. We correlated GBM-C4d with structural GBM abnormalities and active antibody-mediated rejection in 319 renal transplant and 35 control native kidney biopsies. In kidney transplants, ptc-C4d was associated with GBM-C4d in 97% by immunofluorescence microscopy (IF) and 61% by immunohistochemistry (IHC; P < 0.001). Transplant glomerulopathy correlated with GBM-C4d (P < 0.001) and presented with isolated GBM-C4d lacking ptc-C4d in 69% by IF and 40% by IHC. Strong isolated GBM-C4d was found post year-1 in repeat biopsies with transplant glomerulopathy. GBM-C4d staining intensity correlated with Banff cg scores (rs = 0.45, P < 0.001). Stepwise exclusion and multivariate logistic regression corrected for active antibody-mediated rejection showed significant correlations between GBM duplication and GBM-C4d (P = 0.001). Native control biopsies with thrombotic microangiopathies demonstrated GBM-C4d in 92% (IF, P < 0.001) and 35% (IHC). In conclusion, pseudolinear GBM-C4d staining can reflect two phenomena: (i) structural GBM changes with duplication in native and transplant kidneys or (ii) active antibody-mediated rejection typically accompanied by ptc-C4d. While ptc-C4d is a dynamic 'etiologic' marker for active antibody-mediated rejection, isolated strong GBM-C4d can highlight architectural glomerular remodelling.

C4d-expressing glomerulopathy and proteinuria post transplantation of a too-big-for-size mismatched kidney allograft: An unusual case with good outcome
.

A 5-year-old severely growth-retarded child with tubulointerstitial, oliguric end-stage renal disease received an adult-size kidney transplant. Three years post grafting under standard triple immunosuppression (mycophenolate mofetil, tacrolimus, and prednisone) de novo nephrotic range proteinuria without the nephrotic syndrome developed. Graft function was normal (serum creatinine: 0.2 - 0.3 mg/dL), there were no donor-specific HLA antibodies (DSA), and the urine sediment was inactive. Two biopsies collected 3 and 4 years post-transplantation showed severe glomerular capillary wall remodeling and associated pseudolinear C4d staining as morphologic correlates for the proteinuria. Changes resembled those seen in so-called "size-mismatch transplant glomerulopathies". There was no evidence of a glomerulonephritis, acute or chronic rejection including transplant glomerulopathy, interstitial fibrosis, peritubular capillary C4d deposits, or multilamination of peritubular capillary basement membranes. The glomerular changes were not detected in the implantation zero-hour biopsy or the recipient's native renal biopsy. At the end of follow-up 64 months post transplantation, proteinuria persisted at subnephrotic levels in the setting of stable graft function and undetectable DSAs. This unique case adds to the list of causes of nonrejection-associated post-transplant proteinuria. It demonstrates for the first time that a too-large-for-body-size mismatched graft is associated with a presumably sheer stress-induced C4d expressing glomerulopathy, severe proteinuria, and favorable outcome.
.

The oncogenic potential of BK-polyomavirus is linked to viral integration into the human genome.

It has been suggested that BK-polyomavirus is linked to oncogenesis via high expression levels of large T-antigen in some urothelial neoplasms arising following kidney transplantation. However, a causal association between BK-polyomavirus, large T-antigen expression and oncogenesis has never been demonstrated in humans. Here we describe an investigation using high-throughput sequencing of tumour DNA obtained from an urothelial carcinoma arising in a renal allograft. We show that a novel BK-polyomavirus strain, named CH-1, is integrated into exon 26 of the myosin-binding protein C1 gene (MYBPC1) on chromosome 12 in tumour cells but not in normal renal cells. Integration of the BK-polyomavirus results in a number of discrete alterations in viral gene expression, including: (a) disruption of VP1 protein expression and robust expression of large T-antigen; (b) preclusion of viral replication; and (c) deletions in the non-coding control region (NCCR), with presumed alterations in promoter feedback loops. Viral integration disrupts one MYBPC1 gene copy and likely alters its expression. Circular episomal BK-polyomavirus gene sequences are not found, and the renal allograft shows no productive polyomavirus infection or polyomavirus nephropathy. These findings support the hypothesis that integration of polyomaviruses is essential to tumourigenesis. It is likely that dysregulation of large T-antigen, with persistent over-expression in non-lytic cells, promotes cell growth, genetic instability and neoplastic transformation.

Replication of oral BK virus in human salivary gland cells.

BK polyomavirus (BKPyV) is the most common viral pathogen among allograft patients. Increasing evidence links BKPyV to the human oral compartment and to HIV-associated salivary gland disease (HIVSGD). To date, few studies have analyzed orally derived BKPyV. This study aimed to characterize BKPyV isolated from throat wash (TW) samples from HIVSGD patients. The replication potential of HIVSGD-derived clinical isolates HIVSGD-1 and HIVSGD-2, both containing the noncoding control region (NCCR) architecture OPQPQQS, were assessed and compared to urine-derived virus. The BKPyV isolates displayed significant variation in replication potential. Whole-genome alignment of the two isolates revealed three nucleotide differences that were analyzed for a potential effect on the viral life cycle. Analysis revealed a negligible difference in NCCR promoter activity despite sequence variation and emphasized the importance of functional T antigen (Tag) for efficient replication. HIVSGD-1 encoded full-length Tag, underwent productive infection in both human salivary gland cells and kidney cells, and expressed viral DNA and Tag protein. Additionally, HIVSGD-1 generated DNase-resistant particles and by far surpassed the replication potential of the kidney-derived isolate in HSG cells. HIVSGD-2 encoded a truncated form of Tag and replicated much less efficiently. Quantitation of infectious virus, via the fluorescent forming unit assay, suggested that HIVSGD BKPyV had preferential tropism for salivary gland cells over kidney cells. Similarly, the results suggested that kidney-derived virus had preferential tropism for kidney cells over salivary gland cells. Evidence of HIVSGD-derived BKPyV oral tropism and adept viral replication in human salivary gland cells corroborated the potential link between HIVSGD pathogenesis and BKPyV.

Polyomaviruses and disease: is there more to know than viremia and viruria?

PURPOSE OF REVIEW: Polyomavirus nephropathy (PVN) mainly caused by BK virus (BKV) remains the most common productive viral infection of the kidney. Over the past decade, clinical interest often focused on BK viremia and viruria as the diagnostic mainstays of patient management. The purpose of this review is to discuss viral nephropathy in the context of BK viremia and viruria and new strategies to optimize diagnostic accuracy and patient management. The emerging roles of polyomaviruses in oncogenesis, salivary gland disease, and post-bone marrow transplantation as well as novel Polyomavirus strains are highlighted. RECENT FINDINGS: Areas of investigation include proposals by the Banff working group on the classification of PVN and studies on PVN progression and resolution, including the role cellular immune responses may play during reconstitution injury. New noninvasive strategies to optimize the diagnosis of PVN, that is, the urinary 'polyomavirus-haufen' test and mRNA expression levels for BKV in the urine, hold great promise to accurately identify patients with viral nephropathy. Tools are now available to separate 'presumptive' from 'definitive' disease in various patient cohorts including individuals post-bone marrow transplantation. Recent observations also point to a currently underrecognized role of polyomaviruses in oncogenesis post-transplantation and salivary gland disease in patients with HIV-AIDS. SUMMARY: This review summarizes recent studies on PVN and the significance of the BKV strain in disease. Current paradigms for patient management post-(renal) transplantation are discussed in the setting of new observations. Issues that still require clarification and further validation are highlighted.

Renal allograft biopsies: a guide of ins and outs for best results.

Renal allograft biopsies remain the best diagnostic tool to investigate the type and degree of graft injury, provide therapeutic and prognostic information and to assess the extent of irreversible chronic organ damage - if done right. This review highlights pertinent aspects relevant not only for collecting optimal tissue samples but also for rendering diagnoses. Pathologists and clinicians are provided with "take home messages" and practical tips what to do, what to avoid and what to keep in mind.