Characterization of monoclonal antibodies against duck Tembusu virus E protein: an antigen-capture ELISA for the detection of Tembusu virus infection.

The E protein of flaviviruses is the primary antigen that induces protective immunity, but a monoclonal antibody (mAb) against the E protein of duck Tembusu virus (DTMUV) has never been characterized. Six hybridoma cell lines secreting DTMUV anti-E mAbs were prepared and designated 2A5, 1F3, 1G2, 1B11, 3B6, and 4F9, respectively. An immunofluorescence assay indicated that the mAbs could specifically bind to duck embryo fibroblast (DEF) cells infected with DTMUV and that the E protein was distributed in the cytoplasm of the infected cells. Immunoglobulin isotyping differentiated the mAbs as IgG1 (1G2, 1B11, 4F9, 1F3, and 2A5) and IgG2b (3B6). The mAbs were used to identify three epitopes, A (2A5, 1F3, and 1G2), B (1B11 and 4F9), and C (3B6) on the E protein on the basis of a competitive binding assay. By using mAbs 1F3 and 3B6, we developed an antigen-capture enzyme-linked immunosorbent assay (AC-ELISA) to detect E antigen from clinical samples. The AC-ELISA did not react with other known pathogens, indicating that the mAbs are specific for DTMUV. Compared to RT-PCR, the specificity and sensitivity of the AC-ELISA was 94.1 % and 98.0 %, respectively. This AC-ELISA thus represents a sensitive and rapid method for detecting DTMUV infection in birds.

Duck tembusu virus and its envelope protein induce programmed cell death.

The cytopathic effect produced in cells infected with duck tembusu virus (DTMUV) suggests that this emerging virus may induce apoptosis in primary cultures of duck embryo fibroblasts (DEF). Here, we present evidence that DTMUV infection of cultured cells activates apoptosis and that the ability of DTMUV to induce apoptosis is not restricted to cell type because DTMUV-induced apoptosis in duck and mammalian host cells. We further investigated which viral components induce apoptosis in DTMUV-infected host cells. The major envelope glycoprotein (E) was investigated for its apoptotic activities in expressed cells. Transient expression of the E protein alone triggered apoptosis in DEF, Vero, and BHK cells. Expression of the E protein resulted in activation of caspase-3-like proteases in cultured cells. These results indicate that infection of cells with DTMUV or expression of DTMUV E protein alone induces apoptosis, providing the basis for future to define the molecules that play key roles in the fate of DTMUV-infected cells.

Muscovy duck reovirus p10.8 protein localizes to the nucleus via a nonconventional nuclear localization signal.

BACKGROUND: It was previously report that the first open reading frame of Muscovy duck reocvirus S4 gene encodes a 95-amino-acid protein, designed p10.8, which has no sequence similarity to other known proteins. Its amino acid sequence offers no clues about its function. RESULTS: Subcellular localization and nuclear import signal of p10.8 were characterized. We found that p10.8 protein localizes to the nucleus of infected and transfected cells, suggesting that p10.8 nuclear localization is not facilitated by viral infection or any other viral protein. A functional non-canonical nuclear localization signal (NLS) for p10.8 was identified and mapped to N-terminus residues 1-40. The NLS has the ability to retarget a large cytoplasmic protein to the nucleus. CONCLUSIONS: p10.8 imported into the nucleus might via a nonconventional signal nuclear signal.

A Conserved Epitope Mapped with a Monoclonal Antibody against the VP3 Protein of Goose Parvovirus by Using Peptide Screening and Phage Display Approaches.

BACKGROUND: Waterfowl parvovirus (WPV) infection causes high mortality and morbidity in both geese (Anser anser) and Muscovy ducks (Cairina moschata), resulting in significant losses to the waterfowl industries. The VP3 protein of WPV is a major structural protein that induces neutralizing antibodies in the waterfowl. However, B-cell epitopes on the VP3 protein of WPV have not been characterized. METHODS AND RESULTS: To understand the antigenic determinants of the VP3 protein, we used the monoclonal antibody (mAb) 4A6 to screen a set of eight partially expressed overlapping peptides spanning VP3. Using western blotting and an enzyme-linked immunosorbent assay (ELISA), we localized the VP3 epitope between amino acids (aa) 57 and 112. To identify the essential epitope residues, a phage library displaying 12-mer random peptides was screened with mAb 4A6. Phage clone peptides displayed a consensus sequence of YxRFHxH that mimicked the sequence 82Y/FNRFHCH88, which corresponded to amino acid residues 82 to 88 of VP3 protein of WPVs. mAb 4A6 binding to biotinylated fragments corresponding to amino acid residues 82 to 88 of the VP3 protein verified that the 82FxRFHxH88 was the VP3 epitope and that amino acids 82F is necessary to retain maximal binding to mAb 4A6. Parvovirus-positive goose and duck sera reacted with the epitope peptide by dot blotting assay, revealing the importance of these amino acids of the epitope in antibody-epitope binding reactivity. CONCLUSIONS AND SIGNIFICANCE: We identified the motif FxRFHxH as a VP3-specific B-cell epitope that is recognized by the neutralizing mAb 4A6. This finding might be valuable in understanding of the antigenic topology of VP3 of WPV.

Epitope Identification and Application for Diagnosis of Duck Tembusu Virus Infections in Ducks.

Duck Tembusu virus (DTMUV) causes substantial egg drop disease. DTMUV was first identified in China and rapidly spread to Malaysia and Thailand. The antigenicity of the DTMUV E protein has not yet been characterized. Here, we investigated antigenic sites on the E protein using the non-neutralizing monoclonal antibodies (mAbs) 1F3 and 1A5. Two minimal epitopes were mapped to 221LD/NLPW225 and 87YAEYI91 by using phage display and mutagenesis. DTMUV-positive duck sera reacted with the epitopes, thus indicating the importance of the minimal amino acids of the epitopes for antibody-epitope binding. The performance of the dot blotting assay with the corresponding positive sera indicated that YAEYI was DTMUV type-specific, whereas 221LD/NLPW225 was a cross-reactive epitope for West Nile virus (WNV), dengue virus (DENV), and Japanese encephalitis virus (JEV) and corresponded to conserved and variable amino acid sequences among these strains. The structure model of the E protein revealed that YAEYI and LD/NLPW were located on domain (D) II, which confirmed that DII might contain a type-specific non-neutralizing epitope. The YAEYI epitope-based antigen demonstrated its diagnostic potential by reacting with high specificity to serum samples obtained from DTMUV-infected ducks. Based on these observations, a YAEYI-based serological test could be used for DTMUV surveillance and could differentiate DTMUV infections from JEV or WNV infections. These findings provide new insights into the organization of epitopes on flavivirus E proteins that might be valuable for the development of epitope-based serological diagnostic tests for DTMUV.

Identification of a New Broadly Cross-reactive Epitope within Domain III of the Duck Tembusu Virus E Protein.

In 2010, a pathogenic flavivirus termed duck Tembusu virus (DTMUV) caused widespread outbreak of egg-drop syndrome in domesticated ducks in China. Although the glycoprotein E of DTMUV is an important structural component of the virus, the B-cell epitopes of this protein remains uncharacterized. Using phage display and mutagenesis, we identified a minimal B-cell epitope, 374EXE/DPPFG380, that mediates binding to a nonneutralizing monoclonal antibody. DTMUV-positive duck serum reacted with the epitope, and amino acid substitutions revealed the specific amino acids that are essential for antibody binding. Dot-blot assays of various flavivirus-positive sera indicated that EXE/DPPFG is a cross-reactive epitope in most flaviviruses, including Zika, West Nile, Yellow fever, dengue, and Japanese encephalitis viruses. These findings indicate that the epitope sequence is conserved among many strains of mosquito-borne flavivirus. Protein structure modeling revealed that the epitope is located in domain III of the DTMUV E protein. Together, these results provide new insights on the broad cross-reactivity of a B-cell binding site of the E protein of flaviviruses, which can be exploited as a diagnostic or therapeutic target for identifying, studying, or treating DTMUV and other flavivirus infections.

Characterization of monoclonal antibodies against duck hepatitis type 1 virus VP1 protein.

The VP1 protein of duck hepatitis type 1 virus (DHV-1), one of the major structural proteins, is able to induce neutralizing antibody in ducks, but a monoclonal antibody (mAb) against VP1 protein has never been characterized. Four hybridoma cell lines secreting anti DHV-1A VP1 mAbs were prepared and designated 2D9, 2D10, 5F7, and 3E8. Immunoglobulin subclass tests differentiated them as IgG1 (2D9 and 2D10) and IgG2b (5F7 and 3E8). Dot blot and western blotting assays showed that mAbs reacted with His-VP1 protein in a conformation-independent manner. Competitive binding assays indicated that mAbs delineated three epitopes, namely A, B and C, of VP1. Immunofluorescence assays indicated that mAbs could specifically bind to duck embryo fibroblast (DEF) cells infected with DHV-1A. mAbs 2D9, 2D10, and 5F7 had universal reactivity to heterologous DHV-1As tested in an antigen-capture ELISA, suggesting that they are highly conserved among DHV-1As. An antigen-capture ELISA could detect DHV-1A protein VP1 with a clear difference in absorbance values between the liver samples of DHV-1A- and mock-infected birds, indicating that the mAb capture ELISA is a useful method for the detection of DHV-1A infections.