Diagnosing Polyomavirus Nephropathy Without a Biopsy: Validation of the Urinary Polyomavirus-Haufen Test in a Proof-of-Concept Study Including Uromodulin Knockout Mice.

Background: Polyomavirus (PyV) 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 3-dimensional PyV aggregates in the urine, might provide crucial diagnostic information.
Methods: A multistep experimental design was used. The hypothesis was that PyV-haufen form within the kidneys under high concentrations of uromodulin, a kidney-specific protein and that PyV-haufen are, therefore, kidney-specific disease biomarkers.
Results: The first investigative step showed colocalization of uromodulin with aggregated PyV (1) in 10 kidneys with PyVN by immunohistochemistry, (2) in urine samples containing PyV-haufen by electron microscopy/immunogold labeling (n = 3), and (3) in urine samples containing PyV-haufen by immunoprecipitation assays (n = 4). In the in vitro experiments of the next step, only high uromodulin concentrations (≥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). In the third investigative step, none of 11 uromodulin-/- knockout mice (0%) with histologic signs of PyVN showed urinary PyV-haufen shedding, compared with 10 of 14 uromodulin+/+ wild-type mice (71%).
Conclusions: PyV-haufen form within kidneys under high uromodulin concentrations. Thus, PyV-haufen detected in the urine are specific biomarkers for intrarenal disease (ie, definitive PyVN).
Competing Interests: Potential conflicts of interest. All authors: No reported conflicts. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed.
(© The Author(s) 2024. Published by Oxford University Press on behalf of Infectious Diseases Society of America.)