*Purpose: This study aimed to investigate the pathological characteristics of BK polyomavirus-associated nephropathy with glomerular involvement in kidney transplant recipients.

*Methods: Forty-four patients with glomerular BK polyomavirus (BKPyV) infection were included for analysis. Immunohistochemical (IHC) staining was performed on paraffin sections using monoclonal mouse anti-SV40 large T antigen antibody.

*Results: In BKPyV-infected glomeruli, the glomerular parietal epithelial cells (GPECs) were swollen, hyperchromatic, and enlarged, with an increased nuclear to cytoplasm (N/C) ratio and smudgy basophilic intra-nuclear viral inclusions. IHC staining revealed the distribution of BKPyV involvement in GPECs, podocytes, and shedding cells within Bowman’s space (Figure 1). Banff g1 glomerulitis was found in 43.2% of the biopsies. Mononuclear inflammatory cells were found mainly in the glomerular capillary wall, and occasionally among the proliferating GEPCs. Peri-glomerular fibrosis was the most common chronic change (65.9%). Notably, BKPyV affected GEPC proliferation and caused crescent formation (7 biopsies, 15.9%). Three biopsies exhibited fibrous crescents and the absence of viral inclusions. The other 4 biopsies exhibited cellular and fibro-cellular crescents, with viral cytopathic changes and positive IHC staining in the proliferative GPECs (Figure 2). Electron microscopy showed viral particles in both GPECs and podocytes. Most BKPyV-infected GPECs were degenerative, with mitochondrial swelling, endoplasmic reticulum expansion, and multi-layered membranous structure formation. Twelve (27.3%) patients received repeat biopsies within 1.6 to 39.5 months (median: 13.5 months), but none revealed persistent glomerular BKPyV infection.

*Conclusions: BKPyVAN with glomerular involvement presents distinct glomerular changes.

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