Porcine reproductive and respiratory syndrome virus NSP5 exploited UBE2L6 to promote viral replication via antagonising host RLRs and ISGylation.

Porcine reproductive and respiratory syndrome virus (PRRSV) inhibits the host innate immune response to promote its replication. The ubiquitin-proteasome system (UPS) and ISGylation both play roles in modulating host innate immunity. Within this process, ISG15-conjugating enzyme E2L6 (UBE2L6) functions as an E2 ubiquitin/ISG15-conjugating enzyme, which is crucial for the enzymatic cascades of UPS and ISGylation. However, the role of UBE2L6 during PRRSV infection remains unclear. Here, we report that UBE2L6 was up-regulated at both the transcript and protein levels during PRRSV infection. Overexpression of UBE2L6 facilitated PRRSV replication, whereas knockdown of UBE2L6 reduced viral replication. Mechanistically, UBE2L6 promoted the degradation of RIG-I and MDA5 protein expression via the ubiquitin-proteasome pathway and decreased ISGylation levels during PRRSV infection, thereby inhibiting the expression of type I interferons and interferon-stimulated genes (ISGs). In addition, UBE2L6 interacted with PRRSV NSP5 and stabilised the NSP5 protein. Together, PRRSV NSP5 and UBE2L6 further facilitated the degradation of RIG-I and MDA5 via the K48-linked ubiquitination pathway, ultimately facilitating PRRSV replication. Notably, UBE2L6 had minimal impact on RIG-I and MDA5 expression in the absence of PRRSV infection. In summary, UBE2L6 regulated host innate immunity and viral replication through its ubiquitination and ubiquitination-like functions. These findings provide novel insights into how PRRSV NSP5 exploits the host UPS to inhibit the innate immune response and deepen our understanding of the mechanism of host-virus interaction.