Identification of Host Factors Differentially Induced by Clinically Diverse Strains of Tick-Borne Encephalitis Virus.

Tick-borne encephalitis virus (TBEV) is an important human arthropod-borne virus that causes tick-borne encephalitis (TBE) in humans. TBEV acutely infects the central nervous system (CNS), leading to neurological symptoms of various severity. No therapeutics are currently available for TBEV-associated disease. Virus strains of various pathogenicity have been described, although the basis of their diverse clinical outcome remains undefined. Work with infectious TBEV requires high-level biocontainment, meaning model systems that can recapitulate the virus life cycle are highly sought. Here, we report the generation of a self-replicating, noninfectious TBEV replicon used to study properties of high (Hypr) and low (Vs) pathogenic TBEV isolates. Using a Spinach2 RNA aptamer and luciferase reporter system, we perform the first direct comparison of Hypr and Vs in cell culture. Infectious wild-type (WT) viruses and chimeras of the nonstructural proteins 3 (NS3) and 5 (NS5) were investigated in parallel to validate the replicon data. We show that Hypr replicates to higher levels than Vs in mammalian cells, but not in arthropod cells, and that the basis of these differences map to the NS5 region, encoding the methyltransferase and RNA polymerase. For both Hypr and Vs strains, NS5 and the viral genome localized to intracellular structures typical of positive-strand RNA viruses. Hypr was associated with significant activation of IRF-3, caspase-3, and caspase-8, while Vs activated Akt, affording protection against caspase-mediated apoptosis. Higher activation of stress-granule proteins TIAR and G3BPI were an additional early feature of Vs but not for Hypr. These findings highlight novel host cell responses driven by NS5 that may dictate the differential clinical characteristics of TBEV strains. This highlights the utility of the TBEV replicons for further virological characterization and antiviral drug screening.