Tick-Borne Encephalitis Virus (TBEV), an Orthoflavivirus endemic in Europe and Asia, encodes non-structural (NS) proteins that modulate host antiviral responses, though their functions remain incompletely understood. In this thesis, I focused on two NS proteins—NS2B and NS4B—and identified novel host–virus interactions influencing infection outcomes.
NS2B induces an IKKε-dependent antiviral response
TBEV infection activates the ER stress–associated unfolded protein response (UPR), which can trigger interferon-stimulated genes (ISGs) via the IRE1α pathway. Among all viral proteins tested, only NS2B upregulated ISGs independently of IRE1α. I found that NS2B interacts with the kinase IKKε, leading to IRF3 activation and reduced viral replication. This antiviral effect persisted even with the enzymatically inactive NS2B3 S135A mutant. These findings reveal a new function of NS2B and suggest that IKKε may act as a sensor of viral proteins.
NS4B inhibits PKR-mediated stress granule formation
TBEV infection results in minimal stress granule (SG) formation (<10% of cells), indicating viral suppression of this antiviral pathway. I showed that NS4B blocks PKR activation by directly binding it through its cytoplasmic loop (C-loop), preventing eIF2α phosphorylation and SG assembly. Mutations in the C-loop weakened this inhibition, and TBEV mutants carrying these deletions restored SG formation. Thus, NS4B counteracts PKR-mediated antiviral signaling, promoting viral replication.
Conclusion
This work identifies dual host–virus interfaces: NS2B enhances antiviral signaling via IKKε, while NS4B suppresses PKR-induced SG formation. Together, these mechanisms illustrate how TBEV fine-tunes host responses to balance immune evasion and replication.
