A highly specific and sensitive sandwich blocking ELISA based on baculovirus expressed pseudorabies virus glycoprotein B

Improved detection sensitivity of anti-PRV variant antibodies through preparation of anti-gB and anti-gE monoclonal antibodies and development of blocking ELISAs

Since 2011, PRV has resurged in China and is characterized by a mutated strain with significant alterations in antigenicity and virulence. Therefore, we hypothesized that antibody detection kits based on classic PRV strains may have limitations in detecting PRV variants. For more sensitive antibody detection of PRV variants, two MABs targeting the gB and gE proteins were developed. IFA revealed that these MABs exhibited strong reactivity toward both classic and variant PRV strains. MAB-gE recognizes a novel conserved linear B-cell epitope (41PSAEVWD47), while MAB-gB recognizes a conformational B-cell epitope. The binding of both MABs was effectively inhibited in the PRV-positive pig blood samples. Accordingly, we established blocking-ELISAs to detect anti-PRV gB and gE antibodies, which achieved higher sensitivity than commercial kits. Moreover, the clinical serum samples results of our method and that of IFA were in high agreement, and our test results had a higher coincidence rate than that of a commercial kit. Assessing antibody levels by our methods at various times following immunization and challenge accurately reflected the trend of antibody-level changes and revealed the conversion to positive antibody status before the commercial kit. Our method is crucial for monitoring PRV infections, assessing immune responses, and controlling disease.

A fluorescent aptasensor for H5N1 influenza virus detection based-on the core-shell nanoparticles metal-enhanced fluorescence (MEF)

A fluorescent aptasensor system has been designed for the sensitive detection of recombinant hemagglutinin (rHA) protein of the H5N1 influenza virus in human serum. Guanine-richen anti-rHA aptamers by SELEX were immobilized on the surface of the Ag@SiO2 nanoparticles which performed as a metal-enhanced fluorescence (MEF) sensing platform. Thiazole orange (TO) was used as fluorescent tag which reported to the G-quadruplex secondary structural induced by aptamer-rHA binding event. In the absence of rHA protein, TO was free in the solution with almost no fluorescence emission. When rHA protein was added to the solution, the aptamer strand bound rHA protein to form a stable G-quadruplex complex, which can bind TO and excite the fluorescence emission of TO. Moreover, the excited-state TO captured by the G-quadruplex complex was forced to the surface of the Ag@SiO2 nanoparticles and could experience a surface plasmon resonance enhancement which can be transformed into more efficient fluorescence emission signals, therefore, the fluorescence signal of TO can be amplified largely. This system does not require covalent labeling with fluorophores to the aptamer and the background noise is very low. The detection of rHA protein of the H5N1 influenza virus could be operated both in aqueous buffer and human serum with the detection limit of 2 and 3.5ng/mL respectively. More important, the whole detection process can be finished in a PE tube within 30min, which makes it suitable as a self-contained diagnostic kit for H5N1 influenza virus point-of-care (POC) diagnostic.

Yellow fever virus envelope protein expressed in insect cells is capable of syncytium formation in lepidopteran cells and could be used for immunodetection of YFV in human sera

BACKGROUND

Yellow fever is an haemorrhagic disease caused by a virus that belongs to the genus Flavivirus (Flaviviridae family) and is transmitted by mosquitoes. Among the viral proteins, the envelope protein (E) is the most studied one, due to its high antigenic potencial. Baculovirus are one of the most popular and efficient eukaryotic expression system. In this study a recombinant baculovirus (vSynYFE) containing the envelope gene (env) of the 17D vaccine strain of yellow fever virus was constructed and the recombinant protein antigenicity was tested.

RESULTS

Insect cells infected with vSynYFE showed syncytium formation, which is a cytopathic effect characteristic of flavivirus infection and expressed a polypeptide of around 54 kDa, which corresponds to the expected size of the recombinant E protein. Furthermore, the recombinant E protein expression was also confirmed by fluorescence microscopy of vSynYFE-infected insect cells. Total vSynYFE-infected insect extracts used as antigens detected the presence of antibodies for yellow fever virus in human sera derived from yellow fever-infected patients in an immunoassay and did not cross react with sera from dengue virus-infected patients.

CONCLUSIONS

The E protein expressed by the recombinant baculovirus in insect cells is antigenically similar to the wild protein and it may be useful for different medical applications, from improved diagnosis of the disease to source of antigens for the development of a subunit vaccine.

Expression of a secreted version of the hemagglutinin-neuraminidase glycoprotein of Newcastle disease virus

The hemagglutinin-neuraminidase (HN) glycoprotein of Newcastle disease virus (NDV) constitutes, together with the fusion glycoprotein, the main surface antigen of this avian pathogen, which causes a highly contagious disease, relevant economically worldwide. The purpose of this work was to obtain the HN glycoprotein as a soluble antigen in culture supernatants of recombinant baculovirus-infected Spodoptera frugiperda (Sf9) cells and to evaluate its application to the development of a recombinant enzyme-linked immunosorbent assay (rELISA) for the analysis of chicken sera. A transfer vector for baculovirus containing the sequence of a melittin signal peptide was constructed and the sequence coding for HN protein without its own signal peptide was cloned. The recombinant protein was secreted and recovered easily from the culture medium of Sf9-infected cells. The recombinant protein was evaluated as antigen for ELISA coating the plates with the recovered HN using 79 positive and 142 negative samples. The Cohen kappa value resulted 0.91, indicating excellent agreement between the rELISA and the hemagglutinin inhibition tests. The rELISA was also compared with a commercial ELISA, finding high levels of agreement between both assays. The present results show that the cloning strategy developed yielded the HN protein free in the cell culture supernatant and that the recombinant protein retained its reactivity with anti-NDV HN antibodies in chicken sera.

Development of epitope-blocking ELISA for universal detection of antibodies to human H5N1 influenza viruses

BACKGROUND

Human infections with highly pathogenic H5N1 avian influenza viruses have generally been confirmed by molecular amplification or culture-based methods. Serologic surveillance has potential advantages which have not been realized because rapid and specific serologic tests to detect H5N1 infection are not widely available.

METHODOLOGY/PRINCIPAL FINDINGS

Here we describe an epitope-blocking ELISA to detect specific antibodies to H5N1 viruses in human or animal sera. The assay relies on a novel monoclonal antibody (5F8) that binds to an epitope comprising amino acid residues 274-281 (CNTKCQTP) in the HA1 region of H5 hemagglutinin. Database search analysis of publicly available sequences revealed that this epitope is conserved in 100% of the 163 H5N1 viruses isolated from humans. The sensitivity and specificity of the epitope-blocking ELISA for H5N1 were evaluated using chicken antisera to multiple virus clades and other influenza subtypes as well as serum samples from individuals naturally infected with H5N1 or seasonal influenza viruses. The epitope-blocking ELISA results were compared to those of hemagglutinin inhibition (HI) and microneutralization assays. Antibodies to H5N1 were readily detected in immunized animals or convalescent human sera by the epitope-blocking ELISA whereas specimens with antibodies to other influenza subtypes yielded negative results. The assay showed higher sensitivity and specificity as compared to HI and microneutralization.

CONCLUSIONS/SIGNIFICANCE

The epitope-blocking ELISA based on a unique 5F8 mAb provided highly sensitive and 100% specific detection of antibodies to H5N1 influenza viruses in human sera.

Development of real-time polymerase chain reaction assays for rapid detection and differentiation of wild-type pseudorabies and gene-deleted vaccine viruses

The successful eradication of pseudorabies in U.S. domestic swine was accomplished through the use of glycoprotein E (gE) deleted modified live virus vaccines and an accompanying gE differential enzyme-linked immunosorbent assay (ELISA). Yet, pseudorabies virus (PRV) was established in feral swine in the United States, becoming a potential reservoir of PRV for infection of domestic swine and other native wildlife. A critical need for the current PRV surveillance program in the United States is the rapid detection of PRV infection. For this reason, a set of 2 real-time polymerase chain reaction (PCR) assays by using TaqMan chemistry was developed and evaluated for their capability in the detection and differentiation of field and vaccine strains of PRV. PCR primers and probes were designed for gB and gE genes of PRV, respectively. The newly developed PRV-specific real-time PCR assays could detect all wild-type PRV isolates from diagnostic submissions and differentiate them from vaccine strains. The analytical sensitivity of the assays was approximately 0.1 plaque-forming units per reaction. The assays were highly specific for PRV, because no positive results were obtained from testing other common swine viral pathogens and other animal herpesviruses. The results of testing samples from domestic and feral swine and from bovine showed that the real-time PCR assays are more sensitive than gel-based PCR. These results demonstrated the potential application of the developed real-time PCR assays as a differential test for rapid and specific detection of PRV in domestic and feral swine, as well as nonporcine species that can be infected with PRV and serve as carriers.

Secreted expression of the classical swine fever virus glycoprotein Erns in yeast and application to a sandwich blocking ELISA

E(rns) is an envelope glycoprotein of classical swine fever virus (CSFV) with RNase activity. The purpose of this study was to produce an active E(rns) for further applications using the yeast secreted expression system. The E(rns) gene was cloned into the expression vector pGAPZalphaC which was introduced into Pichia pastoris. Expression of E(rns) protein in culture supernatant was confirmed by Western blot analysis using both the monoclonal antibody against CSFV E(rns) and CSFV-positive swine serum. The yeast-expressed E(rns) (yE(rns)) was shown to have N-linked glycosylation and to form homodimer of 74 kDa molecules. All monomer, homodimer, and deglycosylated forms of yE(rns) demonstrated intrinsic ribonuclease activity and a clear preference for uridine-rich sequence. A direct sandwich blocking enzyme-linked immunosorbent assay (ELISA) based on the yE(rns) was developed with a high sensitivity and specificity. The yE(rns) which possesses enzymatic activity and retains antigenicity may provide a useful material for developing a diagnostic kit.

Nipah virus glycoprotein: production in baculovirus and application in diagnosis

A method for serological diagnosis of Nipah virus (NiV) is described. DNA encoding truncated G protein of NiV was cloned into the pFastBac HT vector, and the fusion protein to His-tag was expressed in insect cells by recombinant baculovirus. The resulting His-G recombinant fusion protein was purified by affinity chromatography and used as the coating antigen for serological testing by indirect enzyme-linked immunosorbant assay (ELISA). When tested against a panel of swine serum samples, the recombinant G protein-based ELISA successfully discriminated all 40 samples previously determined to be serum neutralizing test (SNT) positive from 11 SNT negatives samples. The data show that the recombinant G protein exhibits the antigenic epitopes and conformation necessary for specific antigen-antibody recognition. The main advantage of the recombinant G protein-based NiV ELISA compared to an ELISA using whole virus antigen is the use of a single antigenic protein instead of inactivated whole virus which is required to be prepared under high risk and cost. This test is suitable for routine diagnosis of NiV and also for epidemiological surveys as it allows highly reliable testing of a large number of sera rapidly.

Expression of pseudorabies virus gE epitopes in Pichia pastoris and its utilization in an indirect PRV gE-ELISA

Pseudorabies virus glycoprotein E (PRV gE) has been recognized as a suitable diagnostic antigen for pseudorabies. In order to produce gE antigen in large quantities and at low cost, a gene fragment encoding PRV gE epitopes was expressed in Pichia pastoris expression system. SDS-PAGE and Western blotting revealed that the expression product was two recombinant proteins, approximately 38 and 32 kDa, in the culture supernatant of P. pastoris integrant 72 h after induction. Protein concentration assay showed the expression product amounted to 106.7 mg/l, accounting for 66.67% of total culture supernatant proteins. An indirect PRV gE-ELISA was then established by using the recombinant expression product as a coating antigen. Cross-reactivity assay showed that this antigen was PRV specific. Reproducibility experiment displayed good consistency. Comparison of detection results of 348 field serum samples between PRV gE-ELISA and a commercially available PRV diagnostic kit showed there was no significant difference between these two methods (P > 0.05).