This study aims to establish an indirect ELISA method for detecting RVFV antibodies u-sing recombinant proteins of Rift Valley fever virus(RVFV)Gn protein Ⅱ-Ⅲ structural domains as the encapsulated antigen which was expressed by the Escherichia coli(E.coli)expression sys-tem.The gene sequences encoding the Ⅱ and Ⅲ subdomains of RVFV Gn protein were inserted in-to pET-30a(+)to construct the recombinant plasmid pET-RVFV Gn-D Ⅱ-Ⅲ.After transforma-tion of the recombinant plasmid into DE3(BL21)competent cells,the recombinant Gn-D Ⅱ-Ⅲ protein was induced with IPTG and purified using affinity chromatography.An indirect ELISA method for the detection of RVFV antibodies was developed using purified recombinant protein as coating antigen and SPA-HRP as the enzyme-labelled secondary antibody.Western blot analysis confirmed that the RVFV Gn-D Ⅱ-Ⅲ protein was successfully expressed.The optimal expression conditions for RVFV Gn-D Ⅱ-Ⅲ protein were induced with 0.8 mmol/L IPTG at 37 ℃ for 5 h.The Gn-D Ⅱ-Ⅲ protein was purified using affinity chromatography with a purity of 91.9％,and the purified protein was used as the encapsulated antigen to develop an ELISA assay for RVFV anti-bodies.The specificity evaluation showed that the method specifically detected RVFV-positive sera and did not cross-react with sera positive for West Nile virus(WNV),Ebola virus(EBOV),Mar-burg virus(MARV)and tick-borne encephalitis virus(TBEV).When the RVFV Gn-D Ⅲ-Ⅲ posi-tive serum was diluted to 6 400 times,the test result still showed positive results,demonstrating the method had good sensitivity.The repeatability evaluation results indicated that the variation co-efficients for both intra-and inter-batch responses was less than 10％,indicating that the method had good repeatability.In conclusion,the RVFV Gn-D Ⅱ-Ⅲ protein was successfully expressed u-sing the E.coli expression system.The purified recombinant Gn-D Ⅱ-Ⅲ protein was used as the encapsulated antigen to develop an indirect ELISA assay for RVFV antibodies,which provides a preliminary basis for the diagnosis of RVF and the research and development of RVF vaccines.

This study aims to establish an indirect ELISA method for detecting RVFV antibodies u-sing recombinant proteins of Rift Valley fever virus(RVFV)Gn protein Ⅱ-Ⅲ structural domains as the encapsulated antigen which was expressed by the Escherichia coli(E.coli)expression sys-tem.The gene sequences encoding the Ⅱ and Ⅲ subdomains of RVFV Gn protein were inserted in-to pET-30a(+)to construct the recombinant plasmid pET-RVFV Gn-D Ⅱ-Ⅲ.After transforma-tion of the recombinant plasmid into DE3(BL21)competent cells,the recombinant Gn-D Ⅱ-Ⅲ protein was induced with IPTG and purified using affinity chromatography.An indirect ELISA method for the detection of RVFV antibodies was developed using purified recombinant protein as coating antigen and SPA-HRP as the enzyme-labelled secondary antibody.Western blot analysis confirmed that the RVFV Gn-D Ⅱ-Ⅲ protein was successfully expressed.The optimal expression conditions for RVFV Gn-D Ⅱ-Ⅲ protein were induced with 0.8 mmol/L IPTG at 37 ℃ for 5 h.The Gn-D Ⅱ-Ⅲ protein was purified using affinity chromatography with a purity of 91.9％,and the purified protein was used as the encapsulated antigen to develop an ELISA assay for RVFV anti-bodies.The specificity evaluation showed that the method specifically detected RVFV-positive sera and did not cross-react with sera positive for West Nile virus(WNV),Ebola virus(EBOV),Mar-burg virus(MARV)and tick-borne encephalitis virus(TBEV).When the RVFV Gn-D Ⅲ-Ⅲ posi-tive serum was diluted to 6 400 times,the test result still showed positive results,demonstrating the method had good sensitivity.The repeatability evaluation results indicated that the variation co-efficients for both intra-and inter-batch responses was less than 10％,indicating that the method had good repeatability.In conclusion,the RVFV Gn-D Ⅱ-Ⅲ protein was successfully expressed u-sing the E.coli expression system.The purified recombinant Gn-D Ⅱ-Ⅲ protein was used as the encapsulated antigen to develop an indirect ELISA assay for RVFV antibodies,which provides a preliminary basis for the diagnosis of RVF and the research and development of RVF vaccines.

The PCR-amplified severe fever with thrombocytopenia syndrome virus(SFTSV)Gn-DⅢ-Ⅲ gene was inserted into the pET-30a(+)prokaryotic expression vector to generate the re-combinant plasmid pET-SFTSV-Gn-D Ⅲ-Ⅲ.The plasmid was transformed into E.coli BL21(DE3)for Gn-DⅢ-m protein expression and the expression conditions were optimized.The Gn-DⅢ-Ⅲ protein purified with Ni-NTA column affinity chromatography was applied as the captured antigen to establish an indirect ELISA method for the detection of SFTSV antibody.The results demonstrated that the recombinant plasmid pET-SFTSV-Gn-D Ⅲ-Ⅲ was successfully constructed as identified by PCR and sequencing.The recombinant protein SFTSV Gn-D m-Ⅲ was soluble ex-pression in E.coli under the optimal induction conditions of 0.4 mmol/L IPTG at 25 ℃ for 4 h,and the protein purity was 91.77％after purification by Ni-NTA column.The optimal reaction con-ditions for the indirect ELISA of SFTSV antibody were as follows:coating antigen concentration(5 μg/mL),primary antibody(incubation at 37 ℃ for 1.5 h),and secondary antibody(diluted 1:10 000 and incubated at 37 ℃ for 1 h).The established method had no cross-reactivity with Rift Valley fever virus(RVFV),Ebola virus(EBOV),and tick-borne encephalitis virus(TBEV)posi-tive sera.The method had a high sensitivity,with P/N＞2.1 for SFTSV-positive sera diluted to 81920.Coefficients of variation for intra-and inter-batch reactions were less than 10％.Detection of four SFTSV-infected human clinical serum samples showed the serum samples from patients in re-mission were tested as positive(P/N＞2.1),while serum samples from patients with multiple or-gan failure were detected as negative(P/N＜2.1).The results indicated that the SFTSV Gn-D Ⅲ-Ⅲ protein was successfully expressed and purified,and it was used as the coating protein to estab-lish an indirect ELISA assay for SFTSV antibody,which possesses good specificity,sensitivity and reproducibility.This method might be applied to detect human SFTSV clinical serum samples.