CYLD suppresses infectious Bursal disease virus replication by deubiquitinating VP1

Cylindromatosis (CYLD), a deubiquitinase (DUB), removes K63-linked ubiquitin chains from idiosyncratic zymolytes, negatively regulating multifarious signaling channels, including NF-κ B, which is implicated in various cancers. Recently, the role of CYLD in antiviral immunity has gained attention, but its involvement in infectious Bursal diseases virus (IBDV) infection remains unclear. By coimmunoprecipitation combined with mass spectrometry analysis, we identified CYLD as a DUB bound to IBDV VP1. In IBDV-infected DF-1 cells, CYLD overexpression significantly inhibited IBDV replication, whereas CYLD knockout or knockdown enhanced it. Interestingly, this effect does not occur through the interferon pathway. At the same time, we also found that IBDV infection, as a trigger, can upregulate CYLD expression in both in vivo and in vitro environments. Further studies showed that CYLD specifically eliminated the K63-linked ubiquitin chains of VP1 and reduced VP1 polymerase activity, thereby inhibiting viral replication. Additionally, we identified C602 as a critical catalytic residue in CYLD's USP domain that affects its enzymatic activity and decides its interaction with VP1. Crucially, the viral core protein VP3 antagonized CYLD-mediated deubiquitination by disrupting the CYLD-VP1 interaction, thereby promoting viral replication. Our results demonstrate the significant function of CYLD in antiviral immunity and suggest it as a potential therapeutic target for IBDV infection, specifically through the VP3-CYLD-VP1 axis.