Field-isolate recombinant tick-borne encephalitis viruses define reporter-stability guidelines for antiviral screening in flaviviruses

As arthropod-borne viruses continue to threaten populations globally, there is a pressing need for experimental systems that enable rapid antiviral discovery. Reverse-genetics platforms producing recombinant reporter orthoflaviviruses have been developed to address this gap. Here, we present two new recombinant tick-borne encephalitis viruses (TBEVrec) generated on a European-subtype Haselmuehl Tiho1 isolate backbone. A reporter gene, either eGFP or Nluc, was inserted in the capsid-coding region of the genome downstream of the capsid RNA regulatory signal and separated from the complete viral polyprotein by a 2A self-cleaving peptide. TBEVrec was better rescued using the circular polymerase extension reaction (CPER) than with the infectious subgenomic amplicon (ISA) method. TBEVrec replicated efficiently in relevant human cell lines, with comparable replication to wild-type TBEV in a neuronal cell line and moderately reduced titers and RNA levels in immune-derived cell lines. Using either eGFP or Nluc, we illustrate how TBEVrec enabled high-content RNAi screening, highlighting Nucleolin and PRKD1 as potential TBEV host factors, and drug screening on a benchtop plate reader. Nanopore sequencing of the eGFP insert revealed that the reporter is excised without affecting flanking regions. Comparative analysis of eGFP and Nluc further shows that this instability is time- and cell type-dependent, and that Nluc is comparatively more stable. From these observations, we outline safeguards and design principles that are broadly applicable both to the rescue of existing constructs and to the design of future recombinant reporter virus platforms. HIGHLIGHTSO_LIGFP and Nluc TBEV reporters built from a field isolate. C_LIO_LIDetermination of GFP reporter excision borders. C_LIO_LINanoluc reporter shows greater stability than GFP. C_LIO_LIRescue and early-passaging conditions improve reporter stability. C_LIO_LINCL and PRKD1 are candidate host factors for TBEV replication. C_LI