Characterization of severe fever with thrombocytopenia syndrome virus Japanese isolate YG1 strain quasispecies using reverse genetics approaches

Three amino acid mutations have been identified in the isolated YG1 strain of severe fever thrombocytopenia syndrome virus (SFTSV), Gn (Y328H) accounts for 26.9% of the virus in patients blood, Gc (R624W) and L (N1891K) those are minor. To investigate viral properties caused by each mutation, we rescued viruses with one-three mutations. Mutations Y328H and R624W in GP increased the cell fusion activity and plaque size. Theses enhancement was more pronounced for both Y328H and R624W. The pseudotyped vesicular stomatitis virus coated with the SFTSV GP Y328H mutant showed lower infectivity in Vero E6 cells, which was compensated for by the additional R624W mutation. In the process of adaptation for virus with Y328H, the R624W mutation may be acquired. Moreover, only the viruses with the N1891K mutation in L showed significant CPE and the CPE was inhibited by the pan-caspase inhibitor, suggesting that caspase-dependent cell death occurred. Programmed cell death associated molecules caspase-1 and caspase-3 were induced in both CPE inducing and wild-type virus-infected cells. Furthermore, infection with the wild-type virus suppressed actinomycin D-induced cell death. These results suggest that SFTSV infected cells initiate programmed cell death, whereas wildt-ype virus may inhibit cell death. Furthermore, the N1891K mutation in L virus was outcompeted by a 10-fold less wild-type virus in Vero E6 cells indicating that it was not advantageous for viral survival in Vero E6 cells. Thus the quasispecies composition of SFTSV appeared to be influenced by propagative environment. ImportanceThis study shows information on viral pathogenesis by analyzing quasispecies derived from one fatal case of severe fever with thrombocytopenia syndrome virus (SFTSV) infection. Observation with recombinant SFTSV altered Gn and Gc suggests that combining mutations may increase the viability of mutant viruses, selecting viruses to create a suitable population for propagation. The N1891K mutation in L protein of SFTSV was related to CPE appearance. On the other hand, wild-type virus which is major population in patient infection was suppressive for cell death. It was suggested that SFTSV has a mechanism to escape cell death for the prolonged viral propagation in infected cells. Although the mechanism is still unknown, it has been suggested that RNA virus polymerase might be involved in the regulation of cell death. This study proposed the mechanism underlying the adaptation to the environment and survival of virus as quasispecies.