Preparation and characterization of the affinity and neutralization abilities of human monoclonal antibodies against bunyavirus SFTSV glycoprotein C

Severe fever with thrombocytopenia syndrome (SFTS) caused by severe fever with thrombocytopenia syndrome virus (SFTSV) has become a persistent threat to public health. Since neutralizing monoclonal antibody therapy can invoke an immediate and effective passive immunity, endeavours made to discover the therapeutic monoclonal antibodies should be worthwhile. This study aimed to screen and purify the human monoclonal antibodies (hmAbs) against SFTSV glycoprotein C and characterize the properties of the hmAbs.

Human monoclonal antibodies against SFTSV glycoprotein C (Gc) were prepared from the convalescent SFTS patients’ lymphocytes using the phage display technology. A single-chain variable fragment antibody library against SFTSV Gc was successfully constructed, and two specific single-chain variable fragment antibodies were screened and expressed as hmAbs immunoglobulin G (IgG) (hIgG4 and hIgG5).

Enzyme-linked immunosorbent assay, Western blot and immunofluorescence assay test results confirmed that both hIgG4 and hIgG5 specifically bound to SFTSV Gc antigen and intact SFTSV virions. The neutralization test showed that both hmAbs exhibited broad-spectrum neutralizing activity against three SFTSV strains with different genotypes (A, E and F) in vitro. By analyzing the RNA level of M segment by reverse transcription-polymerase chain reaction, the SFTSVs’ RNA levels were decreased, specifically, the RNA of SFTS virus JS3, SFTS virus JS4 and Phlebovirus JS2010-014 was reduced by 79%, 80% and 87% respectively for hIgG4, and the value was 88%, 87% and 89% for hIgG5. According to the observation of the cytopathic effect, the minimal protective concentration of hIgG4 and hIgG5 was 62.5 μg/mL and 31.25 μg/mL respectively.

hIgG4 and hIgG5 possess potent neutralizing effects, showing great potential as candidate therapeutic hmAbs for SFTS treatment.