Expression and characterization of bluetongue virus serotype 21 VP7 antigen: C-terminal truncated protein has significantly reduced antigenicity

Establishment of an Immunological Method for Detection of Bluetongue Virus by Fluorescence-Linked Immunosorbent Assay

Bluetongue (BT) is a severe noncontagious infectious disease that occurs in sheep and wild ruminants but occasionally also in cattle and camels. The worldwide BT pandemic has had a significant impact on global livestock production. Rapid detection helps prevent outbreaks of bluetongue disease. Fluorescence-linked immunosorbent assay (FLISA) labeled with quantum dots (QDs) is typically used for detection due to its high sensitivity. There has been no reported detection of BT virus (BTV) using QD-based fluorescence immunoassays. In this study, monoclonal antibodies (MAbs) against BT were prepared by immunizing BALB/c mice with recombinant VP7 protein. Two MAbs with high sensitivity and specificity were selected as the detection antibody (2F11) and capture antibody (11B7). Then, the detection antibody was coupled with QDs to prepare QD-MAb fluorescence probes. Fluorescence-linked immunosorbent assay is highly specific, detecting only VP7 protein/BTV, and did not show any nonspecific reactions with other reoviruses. The detection limit of VP7 protein was 3.91 ng/mL using fluorescence-linked immunosorbent assay, with a coefficient of variation (CV) of less than 15%. The establishment of rapid, sensitive direct FLISA has potential for bluetongue virus detection and control of BT vaccine quality. IMPORTANCE Bluetongue virus causes the severe infectious disease BT. BTV has many serotypes, and there is no cross-protection among different serotypes. BT is listed as a notifiable animal infectious disease by the World Organisation for Animal Health (OIE) and occurs throughout the world, causing significant economic losses. The establishment of a fast and effective detection method is the key to controlling and preventing this disease. Current methods for detecting BTV mainly include reverse transcription-PCR (RT-PCR), enzyme-linked immunosorbent assays (ELISA), and immunochromatographic strips that are based on antigen-antibody recognition. Immunoassays are most commonly used because of their low cost, high specificity, and fast analysis, making them particularly useful for routine monitoring. These conventional detection strategies for BTV have some drawbacks. Recently, FLISA has been drawing attention due to its sensitivity, which is higher than traditional immunoassays. Fluorescence-linked immunosorbent assays (FLISA) using fluorescent materials as labels overcome ELISA's disadvantage of being time-consuming.

Soluble expression and purification of Bluetongue Virus Type 1 (BTV1) structure protein VP2 in Escherichia coli and its immunogenicity in mice

Background

The VP2 on the surface of the virus particle is the main structural protein of BTV, which can induce the host to produce neutralizing antibodies and play an important role in the antiviral immunity process. This study aimed to obtain the soluble VP2 and analyze its immunogenicity.

Methods

The gene encoding the full-length VP2 of BTV1 was amplified by PCR. The products from restriction enzyme digestion and ligase reaction between VP2 and vector pET-28a were transformed into E.coli DH5α. After PCR and sequencing detection, the positive plasmid PET28a-VP2 was transformed into E.coli BL21(DE3) and Rosetta(DE3) competent cells, expression induced by IPTG. The fusion protein was expressed in the optimized conditions with the induction of IPTG, purified by affinity chromatography and identified by SDS-PAGE and Western blotting. A total of 5 Balb/c mice aged 6–8 weeks were immunized with the fusion protein at a dose of 30 µg per mouse. Each mouse was immunized three times at an interval of 3 weeks.

Results

The recombinant plasmid PET28a-VP2 was successfully constructed. The expression strains were induced by 0.4 mmol/L IPTG at 16 °C for 10 h, and BTV1 VP2 was expressed in a soluble form. The purity of the recombinant VP2 protein (∼109 kDa) was about 90% in the concentration at 0.2 mg/ml afterpurification. The purified VP2 had good immunoreactivity with BTV1 positive serum. Taken together, thisstudy offered a route for producing soluble BTV VP2, which retains activity and immunogenicity, to bebeneficial to the research on developing BTV vaccine, and lay the foundation for further research on BTV.

Development of recombinant NS1-NS3 antigen based indirect ELISA for detection of bluetongue antibodies in sheep

Bluetongue is an insect borne (Culicoides) viral disease of small ruminants. The virus blankets the globe with a wide serotypic variation, numbered from 1 to 28. In India 21 different serotypes have been reported to be circulating across the various agro-climatic zones of the country. Non-structural proteins (NSPs) of bluetongue virus have always remained ideal target for differentiation of infected from vaccinated animals. The current study is an extrapolation of our previous work where a novel fusion construct comprising of bluetongue viral segment NS1 and NS3 was successfully cloned, expressed, purified with an efficient strategy for its suitable implementation as a diagnostic antigen. In this study, the applicability of the fusion construct has been further evaluated and optimised for field applicability. The fusion construct used in an ELISA platform projected a relative diagnostic sensitivity and specificity of 98.1% and 95.5% respectively against a pre-established test panel. The rNS1-NS3 ELISA showed substantially good agreement with the commercial BTV antibody detection kit. Finally, the study brings together the diagnostic capability of two NSPs, which can be a handy tool for sero-surveillance of bluetongue.

Production and Easy One-Step Purification of Bluetongue Recombinant VP7 from Infected Sf9 Supernatant for an Immunoenzymatic Assay (ELISA)

Bluetongue (BT) is non-contagious, vector-borne viral disease of domestic and wild ruminants, transmitted by midges (Culicoides spp.) and is caused by Bluetongue virus (BTV). BTV is the type species of the Orbivirus genus within the Reoviridae family and possesses a genome consisting of 10 double-stranded RNA segments encoding 7 structural and 4 nonstructural proteins. Viral Protein 7 (VP7) is the major sera group-specific protein and is a good antigen candidate for immunoenzymatic assays for the BT diagnosis. In our work, BTV-2 recombinant VP7 (BTV-2 recVP7), expressed in Spodoptera frugiperda (Sf9) cells using a baculovirus system, was produced and purified by affinity chromatography from the supernatant of infected cell culture. The use of the supernatant allowed us to obtain a high quantity of recombinant protein with high purity level by an easy one-step procedure, rather than the multistep purification from the pellet. RecVP7-BTV2 was detected using a MAb anti-BTV in Western blot and it was used to develop an immunoenzymatic assay.

Solubilisation and purification of recombinant bluetongue virus VP7 expressed in a bacterial system

Bluetongue virus (BTV) is an Orbivirus that has a profound economic impact due to direct loss of livestock as well as movement bans in an attempt to prevent the spread of the disease to susceptible areas. BTV VP7, along with VP3, forms the inner capsid core of the virus where it acts as the barrier between the outer layer and the inner core housing the genetic material. Purification of BTV VP7 has proven to be problematic and expensive mainly due to its insolubility is several expression systems. To overcome this, in this paper we present a protocol for the solubilisation of BTV VP7 from inclusion bodies expressed in E.coli, and subsequent purification using nickel affinity chromatography. The purified protein was then characterised using native PAGE, far ultraviolet circular dichroism (far-UV CD) and intrinsic fluorescence and found to have both secondary and tertiary structure even in the presence of 5 M urea. Both tertiary and secondary structure was further shown to be to be maintained at least to 42 °C in 5 M urea.

Preparation and Characterization of a Monoclonal Antibody Against the Core Protein VP7 of the 25th Serotype of Bluetongue Virus

Bluetongue virus (BTV) is a member of the genus Orbivirus, within the family Reoviridae. The VP7 protein of BTV is used for developing group-specific serological assays. To prepare monoclonal antibody (MAb) against VP7 of the 25th serotype BTV, the RNA S7 encoding VP7 was cloned into prokaryotic expression vectors pET-28a (+) and pGEX-6P-1 to generate recombinant plasmids. The recombinant protein VP7 was expressed in Escherichia coli BL21 (DE3), respectively. The results of SDS-PAGE revealed that the VP7 was expressed and the molecular mass of recombinant fusion protein pET-28a (+)/VP7 and pGEX-6P-1/VP7 was approximately 44 kDa and 64 kDa, respectively. The Western blot analysis indicated that the recombinant VP7 possessed good immunoreactivity. After purification, pET-28a (+)/VP7 was used to immunize BALB/c mice, while pGEX-6P-1/VP7 was used to screen for well-to-well MAb-secreting hybridomas. The hybridoma cell line 3H7 against recombinant VP7 that secreted MAbs was obtained. The isotype of 3H7 was identified as IgG1. The purification of recombinant VP7 protein and the monoclonal antibody will have potential applications on competitive ELISA format for BT-specific serum detection method.