Establishment of a Reverse Genetic System of Severe Fever with Thrombocytopenia Syndrome Virus Based on a C4 Strain

Severe fever with thrombocytopenia syndrome virus(SFTSV)is an emerging tick-borne bunyavirus that causes hemorrhagic fever-like disease(SFTS)in humans with a case fatality rate up to 30%.To date,the molecular biology involved in SFTSV infection remains obscure.There are seven major genotypes of SFTSV(C1-C4 and J1-J3)and previously a reverse genetic system was established on a C3 strain of SFTSV.Here,we reported successfully establishment of a reverse genetics system based on a SFTSV C4 strain.First,we obtained the 5’-and 3’-terminal untranslated region(UTR)sequences of the Large(L),Medium(M)and Small(S)segments of a laboratory-adapted SFTSV C4 strain through rapid amplification of cDNA ends analysis,and developed functional T7 polymerase-based L-,M-and S-segment minigenome assays.Then,fulllength cDNA clones were constructed and infectious SFTSV were recovered from co-transfected cells.Viral infectivity,growth kinetics,and viral protein expression profile of the rescued virus were compared with the laboratory-adapted virus.Focus formation assay showed that the size and morphology of the foci formed by the rescued SFTSV were indistinguishable with the laboratory-adapted virus.However,one-step growth curve and nucleoprotein expression analyses revealed the rescued virus replicated less efficiently than the laboratory-adapted virus.Sequence analysis indicated that the difference may be due to the mutations in the laboratory-adapted strain which are more prone to cell culture.The results help us to understand the molecular biology of SFTSV,and provide a useful tool for developing vaccines and antivirals against SFTS.