Targeted cleavage site mutations in the Gn precursor enable efficient generation of replication-competent rVSV-based surrogates for emerging nairoviruses.

Orthonairoviruses are rapidly emerging, tick-borne viruses including Crimean Congo hemorrhagic fever virus (CCHFV), a highly pathogenic virus requiring biosafety level 4 (BSL-4) containment. Recently discovered nairoviruses such as Yezo virus (YEZV) cause febrile illness and are spreading across East Asia. No vaccines or therapeutics exist for these emerging nairoviruses. Recombinant vesicular stomatitis virus (rVSV) systems are promising vaccine candidates for CCHFV and enable neutralization studies in lower containment laboratories; however, efficient rescue of rVSV expressing CCHFV glycoproteins has been technically challenging. Nairovirus glycoprotein precursor (GPC) processing requires cleavage by host protease subtilisin kexin isozyme-1 (SKI-1/S1P) to generate mature Gn, with frequent adaptive mutations observed in the RRLL cleavage site during rVSV-CCHFV rescue. Here, we investigated the role of PreGn cleavage in generating replication-competent rVSV-CCHFV. Targeted mutation of the SKI-1 cleavage site, resulting in uncleaved PreGn and immature Gn expression, markedly improved rVSV-CCHFV rescue efficiency when combined with Gc cytoplasmic tail truncation. This approach enabled robust generation of replication-competent rVSV-CCHFV across two genetically distinct strains (IbAr10200 and Turkey). To assess generalizability, we extended analysis to Hazara virus (HAZV) and YEZV. Unexpectedly, we efficiently rescued both rVSV-HAZV and rVSV-YEZV with intact cleavage sites, while cleavage mutations reduced their replication efficiency, indicating virus-specific requirements. Using human convalescent and animal sera, rVSV-CCHFV provided reliable neutralization assays with results comparable to authentic CCHFV under BSL-4 conditions. Animal and human CCHF-positive sera exhibited low, yet occasionally measurable, cross-neutralizing activity against VSV-HAZV. These findings define strategies for generating replication-competent rVSV vectors displaying nairovirus GPC and provide opportunities for studying neutralization in lower containment facilities.