A single residue in the Gc protein dictates neutralization sensitivity and virulence in SFTSV

Severe fever with thrombocytopenia syndrome virus (SFTSV), a World Health Organization (WHO) priority pathogen, causes hemorrhagic fever with up to 30% case fatality and lacks approved vaccines or therapeutics. Here, we isolated Gc-specific monoclonal antibodies H2, H4, and H12. H2 potently neutralized endemic Chinese SFTSV strains but failed against the heterologous NB24 strain. Mutational analysis identified Arg577 within Gc domain I as a critical residue essential for H2 binding. Reverse genetics demonstrated that Arg577 is both necessary for H2 binding and sufficient to confer neutralization susceptibility. Beyond immune recognition, residue 577 critically governed viral replication fitness and pathogenesis. In virulent HBMC, Arg577 was essential for efficient replication, assembly and overall viral fitness; in attenuated NB24 (native lysine), introducing Arg577 enhanced progeny production and virulence. Combining H2 with H12 overcame H2's narrow neutralization, achieving broad protection. Our study establishes Arg577 as a central regulator of SFTSV pathogenicity and informs dual-antibody therapy development.