Generation and Characterization of Antinonstructural Protein 1 Monoclonal Antibodies and Development of Diagnostics for Dengue Virus Serotype 2.

Dengue virus (DENV) circulates in tropical and subtropical areas around the world, where it causes high morbidity and mortality. There is no effective treatment of infection, with supportive care being the only option. Furthermore, early detection and diagnosis are important to facilitate clinical decisions. In this study, seven monoclonal antibodies (mAbs) recognizing nonstructural protein 1 (NS1) of DENV were generated by hybridoma techniques. These antibodies can be divided into two groups: serotype-specific (DB6-1, DB12-3, and DB38-1) and nonspecific (consisting of antibodies DB16-1, DB20-6, DB29-1, and DB41-2). The B-cell epitopes of DB20-6 and DB29-1 were identified by phage display and site-directed mutagenesis, and its binding motif, WXXWGK, was revealed to correspond to amino acid residues 115-120 of the DENV-2 NS1 protein. A diagnostic platform, consisting of a serotype-specific capture antibody and a complex detection antibody, exhibited a detection limit of about 1 ng/mL, which is sufficient to detect NS1 in clinical serum samples from dengue patients. This diagnostic platform displayed better specificity and sensitivity than two examined commercial NS1 diagnostic platforms. In summary, our results indicate that these newly generated mAbs are suitable for detection of NS1 protein of DENV-2 in clinical samples.

Establishment and Comparison of Two Different Diagnostic Platforms for Detection of DENV1 NS1 Protein.

Dengue virus (DENV) infection is currently at pandemic levels, with populations in tropical and subtropical regions at greatest risk of infection. Early diagnosis and management remain the cornerstone for good clinical outcomes, thus efficient and accurate diagnostic technology in the early stage of the disease is urgently needed. Serotype-specific monoclonal antibodies (mAbs) against the DENV1 nonstructural protein 1 (NS1), DA12-4, DA13-2, and DA15-3, which were recently generated using the hybridoma technique, are suitable for use in diagnostic platforms. Immunofluorescence assay (IFA), enzyme-linked immunosorbent assay (ELISA) and Western blot analysis further confirmed the serotype specificity of these three monoclonal antibodies. The ELISA-based diagnostic platform was established using the combination of two highly sensitive mAbs (DA15-3 and DB20-6). The same combination was also used for the flow cytometry-based diagnostic platform. We report here the detection limits of flow cytometry-based and ELISA-based diagnostic platforms using these mAbs to be 0.1 and 1 ng/mL, respectively. The collected clinical patient serum samples were also assayed by these two serotyping diagnostic platforms. The sensitivity and specificity for detecting NS1 protein of DENV1 are 90% and 96%, respectively. The accuracy of our platform for testing clinical samples is more advanced than that of the two commercial NS1 diagnostic platforms. In conclusion, our platforms are suitable for the early detection of NS1 protein in DENV1 infected patients.

Cellulose-based diagnostic devices for diagnosing serotype-2 dengue fever in human serum.

Here, two types of cellulose-based in vitro diagnostic devices are demonstrated for the diagnosis of dengue virus infection in both buffer system and human serum: 1) paper-based ELISA for providing the semiquantitative information of the disease activity of serotype-2 dengue fever to healthcare persons (i.e., monitoring the disease activity with a specific serotype in single patients); 2) lateral flow immunoassays to screen for infection with serotype-2 dengue fever (i.e., rapid YES or NO diagnosis prepared for large populations, in terms of global public health). Paper-based ELISA (specific to serotype-2 dengue fever), which builds off of our previous studies and a revised previous ELISA procedure, owns multiple advantages: 1) high sensitivity (about 40 times higher than the current ELISA-based approaches, due to our therapeutic-based monoclonal antibody) and specificity (specific to dengue virus serotype-2 nonstructural protein-1 antigens); 2) tiny amount of sample and reagent used for single tests; 3) short operating duration (i.e., rapid diagnostic device); and, 4) inexpensiveness (appropriate for use in all developing and underdeveloped nations of the world). Due to the higher sensitivity and shorter operating duration of paper-based ELISA (compared with conventional ELISA, and lateral flow immunoassays also performed in this study), this study has not only been able to perform the diagnosis of dengue virus serotype-2 nonstructural protein-1 antigens in both buffer system and human serum but also to evaluate dengue virus serotype-2 envelope proteins in the buffer system, thus successfully achieving the first such use of these proteins as the target antigen for the development of diagnostic tools. These results provide a more comprehensive understanding for the genesis of dengue fever diagnostic tools (through antibody-antigen recognition).

Vaccinia mature virus fusion regulator A26 protein binds to A16 and G9 proteins of the viral entry fusion complex and dissociates from mature virions at low pH.

Vaccinia mature virus enters cells through either endocytosis or plasma membrane fusion, depending on virus strain and cell type. Our previous results showed that vaccinia virus mature virions containing viral A26 protein enter HeLa cells preferentially through endocytosis, whereas mature virions lacking A26 protein enter through plasma membrane fusion, leading us to propose that A26 acts as an acid-sensitive fusion suppressor for mature virus (S. J. Chang, Y. X. Chang, R. Izmailyan R, Y. L. Tang, and W. Chang, J. Virol. 84:8422-8432, 2010). In the present study, we investigated the fusion suppression mechanism of A26 protein. We found that A26 protein was coimmunoprecipitated with multiple components of the viral entry-fusion complex (EFC) in infected HeLa cells. Transient expression of viral EFC components in HeLa cells revealed that vaccinia virus A26 protein interacted directly with A16 and G9 but not with G3, L5 and H2 proteins of the EFC components. Consistently, a glutathione S-transferase (GST)-A26 fusion protein, but not GST, pulled down A16 and G9 proteins individually in vitro. Together, our results supported the idea that A26 protein binds to A16 and G9 protein at neutral pH contributing to suppression of vaccinia virus-triggered membrane fusion from without. Since vaccinia virus extracellular envelope proteins A56/K2 were recently shown to bind to the A16/G9 subcomplex to suppress virus-induced fusion from within, our results also highlight an evolutionary convergence in which vaccinia viral fusion suppressor proteins regulate membrane fusion by targeting the A16 and G9 components of the viral EFC complex. Finally, we provide evidence that acid (pH 4.7) treatment induced A26 protein and A26-A27 protein complexes of 70 kDa and 90 kDa to dissociate from mature virions, suggesting that the structure of A26 protein is acid sensitive.

Vaccinia virus A25 and A26 proteins are fusion suppressors for mature virions and determine strain-specific virus entry pathways into HeLa, CHO-K1, and L cells.

Mature vaccinia virus enters cells through either fluid-phase endocytosis/macropinocytosis or plasma membrane fusion. This may explain the wide range of host cell susceptibilities to vaccinia virus entry; however, it is not known how vaccinia virus chooses between these two pathways and which viral envelope proteins determine such processes. By screening several recombinant viruses and different strains, we found that mature virions containing the vaccinia virus A25 and A26 proteins entered HeLa cells preferentially through a bafilomycin-sensitive entry pathway, whereas virions lacking these two proteins entered through a bafilomycin-resistant pathway. To investigate whether the A25 and A26 proteins contribute to entry pathway specificity, two mutant vaccinia viruses, WRDeltaA25L and WRDeltaA26L, were subsequently generated from the wild-type WR strain. In contrast to the WR strain, both the WRDeltaA25L and WRDeltaA26L viruses became resistant to bafilomycin, suggesting that the removal of the A25 and A26 proteins bypassed the low-pH endosomal requirement for mature virion entry. Indeed, WRDeltaA25L and WRDeltaA26L virus infections of HeLa, CHO-K1, and L cells immediately triggered cell-to-cell fusion at a neutral pH at 1 to 2 h postinfection (p.i.), providing direct evidence that viral fusion machinery is readily activated after the removal of the A25 and A26 proteins to allow virus entry through the plasma membrane. In summary, our data support a model that on vaccinia mature virions, the viral A25 and A26 proteins are low-pH-sensitive fusion suppressors whose inactivation during the endocytic route results in viral and cell membrane fusion. Our results also suggest that during virion morphogenesis, the incorporation of the A25 and A26 proteins into mature virions may help restrain viral fusion activity until the time of infections.

Disulfide bond formation at the C termini of vaccinia virus A26 and A27 proteins does not require viral redox enzymes and suppresses glycosaminoglycan-mediated cell fusion.

Vaccinia virus A26 protein is an envelope protein of the intracellular mature virus (IMV) of vaccinia virus. A mutant A26 protein with a truncation of the 74 C-terminal amino acids was expressed in infected cells but failed to be incorporated into IMV (W. L. Chiu, C. L. Lin, M. H. Yang, D. L. Tzou, and W. Chang, J. Virol 81:2149-2157, 2007). Here, we demonstrate that A27 protein formed a protein complex with the full-length form but not with the truncated form of A26 protein in infected cells as well as in IMV. The formation of the A26-A27 protein complex occurred prior to virion assembly and did not require another A27-binding protein, A17 protein, in the infected cells. A26 protein contains six cysteine residues, and in vitro mutagenesis showed that Cys441 and Cys442 mediated intermolecular disulfide bonds with Cys71 and Cys72 of viral A27 protein, whereas Cys43 and Cys342 mediated intramolecular disulfide bonds. A26 and A27 proteins formed disulfide-linked complexes in transfected 293T cells, showing that the intermolecular disulfide bond formation did not depend on viral redox pathways. Finally, using cell fusion from within and fusion from without, we demonstrate that cell surface glycosaminoglycan is important for virus-cell fusion and that A26 protein, by forming complexes with A27 protein, partially suppresses fusion.