ISG20 Restricts BK Polyomavirus Infection and Engages in Reciprocal Regulation with Viral Large T Antigen

<i>BK polyomavirus</i> (BKPyV) causes severe urinary tract diseases, including BKPyV-associated nephropathy (BKPyVN) and ureteric stenosis, in immunocompromised individuals such as renal transplant recipients. Effective antiviral therapies for BKPyV infection remain an unmet clinical need. While the interferon-stimulated gene 20 (ISG20) exhibits broad-spectrum antiviral activity against RNA viruses, its role and mechanisms against DNA viruses are poorly defined. This study demonstrates, for the first time, potent antiviral activity of ISG20 against BKPyV. This restriction was observed with both endogenous levels of ISG20 and upon overexpression, and this effect was confirmed by ISG20 knockout and immunofluorescence imaging. We observed that ISG20 expression is dynamically regulated during BKPyV infection: it is upregulated both during early infection and by expression of the viral large T antigen (LT) alone. However, endogenous ISG20 expression becomes significantly suppressed during later stages of infection, coinciding with declining LT levels. The physical interaction between LT and both wild-type and mutant ISG20 suggests a potential viral strategy to sequester this restriction factor. These findings establish ISG20 as a novel host restriction factor against BKPyV and suggest that BKPyV employs LT-mediated mechanisms to evade or counteract ISG20's antiviral effects. Our results elucidate a complex biphasic interplay between BKPyV and host innate immunity, identifying ISG20 as a potential therapeutic target for BKPyV-associated diseases.