Haufen image

(A) Renal parenchyma: Immunohistochemical stain with an antibody directed against the outer polyomavirus capsid protein VP-1. This stain marks virions (in contrast to antibodies directed against polyomavirus large T antigen that is found within nuclei during viral replication). Illustrated here (thick arrow) are densely packed "cast-like" intratubular viral aggregates (brown staining reaction; 20× original magnification).
(B) Voided urine sample: Intratubular viral aggregates can be excreted from the kidney and detected in the urine as biomarkers of BKPyVAN. Such a urinary biomarker, termed urinary polyomavirus-Haufen, is illustrated here. Polyomavirus-Haufen consist of densely packed polyomaviruses forming tight, three-dimensional, cast-like aggregates (arrow points at individual virions). Negative staining urine electron microscopy (uranyl acetate); 80,000× original magnification.