Study of anti-dengue NS1 antibody by western blot

Magnitude and Functionality of the NS1-Specific Antibody Response Elicited by a Live-Attenuated Tetravalent Dengue Vaccine Candidate

BACKGROUND

Dengue virus (DENV) can cause life-threatening disease characterized by endothelial dysfunction and vascular leakage. DENV nonstructural protein 1 (NS1) induces human endothelial hyperpermeability and vascular leak in mice, and NS1 vaccination confers antibody-mediated protective immunity. We evaluated the magnitude, cross-reactivity, and functionality of NS1-specific IgG antibody responses in sera from a phase 2 clinical trial of Takeda's live-attenuated tetravalent dengue vaccine candidate (TAK-003).

METHODS

We developed an enzyme-linked immunosorbent assay to measure anti-DENV NS1 IgG in sera from DENV-naive or preimmune subjects pre- and postvaccination with TAK-003 and evaluated the functionality of this response using in vitro models of endothelial permeability.

RESULTS

TAK-003 significantly increased DENV-2 NS1-specific IgG in naive individuals, which cross-reacted with DENV-1, -3, and -4 NS1 to varying extents. NS1-induced endothelial hyperpermeability was unaffected by prevaccination serum from naive subjects but was variably inhibited by serum from preimmune subjects. After TAK-003 vaccination, all samples from naive and preimmune vaccinees completely abrogated DENV-2 NS1-induced hyperpermeability and cross-inhibited hyperpermeability induced by DENV-1, -3, and -4 NS1. Inhibition of NS1-induced hyperpermeability correlated with NS1-specific IgG concentrations. Postvaccination sera also prevented NS1-induced degradation of endothelial glycocalyx components.

CONCLUSION

We provide evidence for functional NS1-specific IgG responses elicited by a candidate dengue vaccine.

CLINICAL TRIALS REGISTRATION

NCT01511250.

Deciphering the Role of Bovine Viral Diarrhea Virus Non-Structural NS4B Protein in Viral Pathogenesis

Bovine viral diarrhea virus (BVDV) is a (+) ssRNA virus that belongs to the family Flaviviridae. BVDV is a significant animal pathogen causing substantial economic losses to the cattle industry worldwide through respiratory and gastrointestinal infections and abortion or birth of persistently infected calves. While the immunogenic profile of some of the BVDV proteins (i.e., E^rns, E2 and NS3) is well established during viral pathogenesis, very little information is available about most of BVDV's non-structural proteins in this regard. In recent times, the NS4B protein has emerged as an interesting target of diagnostic, vaccination and therapeutic value in viral infections of other members of the family Flaviviridae due to its key scaffold-like contribution in the viral replication complex. Although, BVDV-NS4B has a membrane topology alongside its role in induction of autophagosomes in vitro. However, information on its immunogenicity during BVDV pathogenesis and vaccination is scarce. To characterize the immunogenic profile of the NS4B, five cows were vaccinated with the live attenuated BVDV vaccine Bovela^® and blood samples were taken pre- and post-immunization for serum isolation. Virus neutralization assay (VNA) confirmed the presence of anti-BVDV antibodies in the sera of vaccinated cows. VNA also revealed pre-existing antibodies against BVDV in the pre-immunization sera of two cows. To identify BVDV-NS4B specific antibodies, the NS4B protein was expressed in mammalian cells by using the pCI-neo vector system. The sera from BVDV vaccinated cows were evaluated for the presence of BVDV-NS4B specific antibodies through western blot and indirect ELISA. Interestingly, t sera from cows with pre-existing immunity against BVDV were able to detect NS4B in western blot and ELISA, suggesting the presence of NS4B-specific antibodies. The obtained results provide the first indication of the immunogenic nature of BVDV-NS4B protein in sero-converted animals. These findings are consistent with the observation made for NS4B in other Flaviviridae members and confirm this protein as an interesting target with diagnostic, vaccination and therapeutic value.

Adaptive Immunity to Dengue Virus: Slippery Slope or Solid Ground for Rational Vaccine Design?

The four serotypes of dengue virus are the most widespread causes of arboviral disease, currently placing half of the human population at risk of infection. Pre-existing immunity to one dengue virus serotype can predispose to severe disease following secondary infection with a different serotype. The phenomenon of immune enhancement has complicated vaccine development and likely explains the poor long-term safety profile of a recently licenced dengue vaccine. Therefore, alternative vaccine strategies should be considered. This review summarises studies dissecting the adaptive immune responses to dengue virus infection and (experimental) vaccination. In particular, we discuss the roles of (i) neutralising antibodies, (ii) antibodies to non-structural protein 1, and (iii) T cells in protection and pathogenesis. We also address how these findings could translate into next-generation vaccine approaches that mitigate the risk of enhanced dengue disease. Finally, we argue that the development of a safe and efficacious dengue vaccine is an attainable goal.

The Good, the Bad, and the Shocking: The Multiple Roles of Dengue Virus Nonstructural Protein 1 in Protection and Pathogenesis

Dengue virus (DENV) is the most prevalent medically important mosquito-borne virus in the world. Upon DENV infection of a host cell, DENV nonstructural protein 1 (NS1) can be found intracellularly as a monomer, associated with the cell surface as a dimer, and secreted as a hexamer into the bloodstream. NS1 plays a variety of roles in the viral life cycle, particularly in RNA replication and immune evasion of the complement pathway. Over the past several years, key roles for NS1 in the pathogenesis of severe dengue disease have emerged, including direct action of the protein on the vascular endothelium and triggering release of vasoactive cytokines from immune cells, both of which result in endothelial hyperpermeability and vascular leak. Importantly, the adaptive immune response generates a robust response against NS1, and its potential contribution to dengue vaccines is also discussed.

Cooperation between CD4+ T Cells and Humoral Immunity Is Critical for Protection against Dengue Using a DNA Vaccine Based on the NS1 Antigen

Dengue virus (DENV) is spread through most tropical and subtropical areas of the world and represents a serious public health problem. At present, the control of dengue disease is mainly hampered by the absence of antivirals or a vaccine, which results in an estimated half worldwide population at risk of infection. The immune response against DENV is not yet fully understood and a better knowledge of it is now recognized as one of the main challenge for vaccine development. In previous studies, we reported that a DNA vaccine containing the signal peptide sequence from the human tissue plasminogen activator (t-PA) fused to the DENV2 NS1 gene (pcTPANS1) induced protection against dengue in mice. In the present work, we aimed to elucidate the contribution of cellular and humoral responses elicited by this vaccine candidate for protective immunity. We observed that pcTPANS1 exerts a robust protection against dengue, inducing considerable levels of anti-NS1 antibodies and T cell responses. Passive immunization with anti-NS1 antibodies conferred partial protection in mice infected with low virus load (4 LD₅₀), which was abrogated with the increase of viral dose (40 LD₅₀). The pcTPANS1 also induced activation of CD4⁺ and CD8⁺ T cells. We detected production of IFN-γ and a cytotoxic activity by CD8⁺ T lymphocytes induced by this vaccine, although its contribution in the protection was not so evident when compared to CD4⁺ cells. Depletion of CD4⁺ cells in immunized mice completely abolished protection. Furthermore, transfer experiments revealed that animals receiving CD4⁺ T cells combined with anti-NS1 antiserum, both obtained from vaccinated mice, survived virus infection with survival rates not significantly different from pcTPANS1-immunized animals. Taken together, results showed that the protective immune response induced by the expression of NS1 antigen mediated by the pcTPANS1 requires a cooperation between CD4⁺ T cells and the humoral immunity.

Dengue is an emerging mosquito-borne disease present in an extensive area of the globe with an estimated risk exposure of half of the world’s population. Unfortunately, no specific treatment or vaccine is available to control this disease, which leads to approximately 20,000 casualties annually. The protective immune response against this pathogen consists of an important goal for the development of anti-dengue strategies. For years, the presence of neutralizing antibodies was believed to represent the major response for protection against dengue. However, a recent clinical trial showed that despite the induction of a balanced antibody response against all serotypes, vaccination had only a partial efficacy. In the present work, we aimed to elucidate the contribution of the cellular and humoral responses elicited by a DNA vaccine candidate encoding the non-structural 1 protein (NS1) from dengue virus. We observed that antibody as well as T cell responses are important for protection against dengue in a cooperative way. Our results demonstrated that an effective defense against virus was not achieved with antibodies or T cells alone, but rather with the combination of both responses. Therefore, we suggest that an ideal vaccine against dengue should induce both arms of the immune system.

The flavivirus NS1 protein: molecular and structural biology, immunology, role in pathogenesis and application as a diagnostic biomarker

The flavivirus nonstructural glycoprotein NS1 is an enigmatic protein whose structure and mechanistic function have remained somewhat elusive ever since it was first reported in 1970 as a viral antigen circulating in the sera of dengue-infected patients. All flavivirus NS1 genes share a high degree of homology, encoding a 352-amino-acid polypeptide that has a molecular weight of 46-55 kDa, depending on its glycosylation status. NS1 exists in multiple oligomeric forms and is found in different cellular locations: a cell membrane-bound form in association with virus-induced intracellular vesicular compartments, on the cell surface and as a soluble secreted hexameric lipoparticle. Intracellular NS1 co-localizes with dsRNA and other components of the viral replication complex and plays an essential cofactor role in replication. Although this makes NS1 an ideal target for inhibitor design, the precise nature of its cofactor function has yet to be elucidated. A plethora of potential interacting partners have been identified, particularly for the secreted form of NS1, with many being implicated in immune evasion strategies. Secreted and cell-surface-associated NS1 are highly immunogenic and both the proteins themselves and the antibodies they elicit have been implicated in the seemingly contradictory roles of protection and pathogenesis in the infected host. Finally, NS1 is also an important biomarker for early diagnosis of disease. In this article, we provide an overview of these somewhat disparate areas of research, drawing together the wealth of data generated over more than 40 years of study of this fascinating protein.

Approaches for the development of rapid serological assays for surveillance and diagnosis of infections caused by zoonotic flaviviruses of the Japanese encephalitis virus serocomplex

Flaviviruses are responsible for a number of important mosquito-borne diseases of man and animals globally. The short vireamic period in infected hosts means that serological assays are often the diagnostic method of choice. This paper will focus on the traditional methods to diagnose flaviviral infections as well as describing the modern rapid platforms and approaches for diagnostic antigen preparation.

Galactomannan thin films as supports for the immobilization of Concanavalin A and/or dengue viruses

The immobilization of the glucose/mannose-binding lectin from Concanavalia ensiformis seeds (ConA) onto a monolayer made of a galactomannan extracted from Leucaena leucocephala seeds (GML), which was adsorbed onto - amino-terminated surfaces, was investigated by means of ellipsometry and atomic force microscopy. The mean thickness of GML monolayer, which polysaccharide consists of linear 1→4-linked β-D-mannopyranosil units partially substituted at C-6 by α-D-galactopyranosyl units, amounted to (1.5±0.2) nm. ConA molecules adsorbed onto GML surfaces forming (2.0±0.5) nm thick layers. However, in the presence of mannose the adsorption failed, indicating that ConA binding sites were blocked by mannose and were no longer available for mannose units present in the GML backbone. The GML film was also used as support for the adsorption of three serotypes of dengue virus particles (DENV-1, DENV-2 and DENV-3), where DENV-2 formed the thickest film (4±2) nm. The adsorbed layer of DENV-2 onto ConA-covered GML surfaces presented mean thickness values similar to that determined for DENV-2 onto bare GML surfaces. The addition of free mannose units prevented DENV-2 adsorption onto ConA-covered GML films by ~50%, suggesting competition between virus and mannose for ConA binding sites. This finding suggests that if ConA is also adsorbed to GML surface and its binding site is blocked by free mannose, virus particles are able to recognized GML mannose unities substituted by galactose. Interactions between polysaccharides thin films, proteins, and viruses are of great relevance since they can provide basis for the development of biotechnological devices. These results indicate that GML is a potential polysaccharide for biomaterials development, as those could involve interactions between ConA in immune system and viruses.

Biosensors and rapid diagnostic tests on the frontier between analytical and clinical chemistry for biomolecular diagnosis of dengue disease: a review

The past decades have witnessed enormous technological improvements towards the development of simple, cost-effective and accurate rapid diagnostic tests for detection and identification of infectious pathogens. Among them is dengue virus, the etiologic agent of the mosquito-borne dengue disease, one of the most important emerging infectious pathologies of nowadays. Dengue fever may cause potentially deadly hemorrhagic symptoms and is endemic in the tropical and sub-tropical world, being also a serious threat to temperate countries in the developed world. Effective diagnostics for dengue should be able to discriminate among the four antigenically related dengue serotypes and fulfill the requirements for successful decentralized (point-of-care) testing in the harsh environmental conditions found in most tropical regions. The accurate identification of circulating serotypes is crucial for the successful implementation of vector control programs based on reliable epidemiological predictions. This paper briefly summarizes the limitations of the main conventional techniques for biomolecular diagnosis of dengue disease and critically reviews some of the most relevant biosensors and rapid diagnostic tests developed, implemented and reported so far for point-of-care testing of dengue infections. The invaluable contributions of microfluidics and nanotechnology encompass the whole paper, while evaluation concerns of rapid diagnostic tests and foreseen technological improvements in this field are also overviewed for the diagnosis of dengue and other infectious and tropical diseases as well.

The use of two-dimension electrophoresis to identify serum biomarkers from patients with dengue haemorrhagic fever

Dengue infection is a major public health problem affecting millions of people living in tropical countries. With no suitable vaccines and specific antiviral drugs, treatment for dengue is usually symptomatic and supportive. Early diagnosis and recognition of severe disease is therefore crucial for better management of the patient. Two-dimension electrophoresis was used to identify disease-associated proteins that can be used for diagnosis and as drug targets for treatment. Two markers, identified by mass spectrometry analysis as alpha1-antitrypsin and NS1 proteins were found to be upregulated in dengue fever (DF; n=10) and dengue haemorrhagic fever (DHF; n=10) patients compared with healthy individuals (n=8). Both alpha1-antitrypsin and NS1 proteins were overexpressed two-fold in DHF patients compared with DF patients. Our study suggests that alpha1-antitrypsin and NS1 protein could be used as biomarkers as early indicators of DHF risk among patients with suspected dengue infection.

Analysis of antibody response in human dengue patients from the Mexican coast using recombinant antigens

This study was undertaken to evaluate the feasibility of using recombinant dengue proteins to discriminate between acute dengue infections versus uninfected dengue samples. Dengue virus proteins E, NS1, NS3, and NS4B were cloned as fusion proteins and expressed in Escherichia coli. Recombinant products were tested in 100 serum samples obtained from acute dengue fever cases collected from 3 states of Mexico where dengue is endemic. Sera from 75 healthy individuals living in nonendemic areas for dengue were used as a control group. In sera from the dengue patients group, antibody responses to E protein were demonstrated in 91% of cases and NS1 protein was recognized to various extents (99%) within the first 7 days of infection. The antibody responses to NS3 and NS4B were frequently of low magnitude. Consistent negative antibody responses to all proteins were found in sera from the control group. These data suggest that the glutathione-S-transferase (GST)-dengue fusion proteins may be feasible antigens for a sensitive and specific serological assay.

Antibodies to envelope glycoprotein of dengue virus during the natural course of infection are predominantly cross-reactive and recognize epitopes containing highly conserved residues at the fusion loop of domain II

The antibody response to the envelope (E) glycoprotein of dengue virus (DENV) is known to play a critical role in both protection from and enhancement of disease, especially after primary infection. However, the relative amounts of homologous and heterologous anti-E antibodies and their epitopes remain unclear. In this study, we examined the antibody responses to E protein as well as to precursor membrane (PrM), capsid, and nonstructural protein 1 (NS1) of four serotypes of DENV by Western blot analysis of DENV serotype 2-infected patients with different disease severity and immune status during an outbreak in southern Taiwan in 2002. Based on the early-convalescent-phase sera tested, the rates of antibody responses to PrM and NS1 proteins were significantly higher in patients with secondary infection than in those with primary infection. A blocking experiment and neutralization assay showed that more than 90% of anti-E antibodies after primary infection were cross-reactive and nonneutralizing against heterologous serotypes and that only a minor proportion were type specific, which may account for the type-specific neutralization activity. Moreover, the E-binding activity in sera of 10 patients with primary infection was greatly reduced by amino acid replacements of three fusion loop residues, tryptophan at position 101, leucine at position 107, and phenylalanine at position 108, but not by replacements of those outside the fusion loop of domain II, suggesting that the predominantly cross-reactive anti-E antibodies recognized epitopes involving the highly conserved residues at the fusion loop of domain II. These findings have implications for our understanding of the pathogenesis of dengue and for the future design of subunit vaccine against DENV as well.

Circulating human antibodies against dengue NS1 protein: potential of recombinant D2V-NS1 proteins in diagnostic tests

The dengue virus (DV) causes one of the most important arthropod-borne human viral diseases throughout the tropical and subtropical countries. However, the morbidity and mortality of DV infections could be reduced with an early hospitalization care and a rapid risk identification of developing the dengue haemorrhagic fever (DHF). The nonstructural glycoprotein 1 (NS1) has been pointed as a reagent for immune-assay diagnostic test optimization. To evaluate this potential, recombinant DV2-NS1 proteins (rNS1) were produced from Escherichia coli (NS1EC) and insect cells (NS1IC) expression. The tests were performed by analysis of a human serum panel reacted against different rNS1 forms. The results demonstrated high correspondence between the DV positive sera and the assay results using native or refolded forms of either NS1IC or NS1EC. Also, the IgG and IgM anti-rNS1 level profiles showed distinct distribution, depending on protein form and disease status. However, the IgM anti-rNS1 reactions did not show sensibility to detect the DV in primary infections. The data obtained from the paired serum samples reactivity comparison suggested a heterogeneous human immune response and absence of correspondence between the IgG and IgM profile levels. Moreover, a patient with negative reference test could be detected by specific IgG anti-rNS1 assays presented here. Therefore, these results sustain the usefulness of dengue nonstructural proteins, in particular the NS1, in diagnostic tests as a complementary reagent.

Trends in dengue diagnosis

The conventional diagnosis of dengue virus infections includes the detection of the virus in serum or tissue samples, both by isolation in culture or through detection of specific viral molecules (genome RNA or dengue antigens) and detection of specific anti-dengue antibodies (serology). Isolation of dengue virus provides the most direct and conclusive approach to diagnosis, despite the demand for high-level equipment, technical skills and manpower. However, it is useless in early diagnosis because several days are required to isolate and classify the virus. Serology, despite being simpler, is not able to afford an accurate early diagnosis in primary infections because 4-5 days are required for the immune system to produce a sufficient amount of antibodies. Moreover, it leads to misleading results in secondary infections owing to cross-reactivity among serotype-specific antibodies and with other flavivirus antibodies. The RT-PCR and other PCR-based techniques are fast, serotype-discriminating, more sensitive and easier to carry out than conventional nucleic-acid hybridisation, but are handicapped by easy sample contamination and high technological demands. Recently, advances in bioelectronics have generated commercial kits and new techniques for detection of dengue antibodies and RNA, based on biosensor technology. Most of them are rapid, easy to operate, reusable, cheap, sensitive and serotype-specific. Nevertheless, their accuracy is still questionable because most still lack validation and standardisation. This review summarises and describes the techniques currently employed and anticipated in the near future for diagnosis of dengue disease.

Detection of immune-complex-dissociated nonstructural-1 antigen in patients with acute dengue virus infections

Accurate and timely diagnosis of dengue virus (DEN) infections is essential for the differential diagnosis of patients with febrile illness and hemorrhagic fever. In the present study, the diagnostic value of a newly developed immune-complex dissociated nonstructural-1 (NS-1) antigen dot blot immunoassay (DBI) was compared to a commercially available DEN antigen detection kit (denKEY Blue kit; Globio Co., Beverly, Mass.) and a reverse transcription-PCR (RT-PCR) kit. Serial serum or plasma samples (n = 181) obtained from 55 acute DEN-infected patients were used. In samples obtained from 32 of these 55 DEN-infected patients, viral RNA could be detected by RT-PCR. DEN antigen was detected in only 10 of these 55 patient samples by using the denKEY kit. When these samples were treated with acid to release the immune-complex-associated NS-1 antigen for detection by DBI, 43 of these 55 patients were found to be positive for DEN NS-1 antigen. In nondissociated samples, 22 of these patients were found to be positive by the DBI. In the presence of DEN-specific immunoglobulin M antibodies, both viral RNA and DEN (NS-1) antigen could be detected. The number of positive samples identified by RT-PCR and DBI from these patients with primary DEN infections varied between 28 and 78%. In secondary DEN infections, the number of samples that tested positive by the DBI after immune-complex dissociation (DIS-DBI) was 25% higher than the number of samples that tested positive by RT-PCR and was 35% higher than that determined by nondissociated antigen (NDIS-DBI) detection. We conclude that the denKEY kit has limited diagnostic value for acute DEN infections compared to the RT-PCR and the NDIS-DBI and DIS-DBI methods. We clearly demonstrate that in secondary DEN infections the dissociation of NS-1 immune complexes is essential for early diagnosis of DEN infections.

Immunoassay targeting nonstructural protein 5 to differentiate West Nile virus infection from dengue and St. Louis encephalitis virus infections and from flavivirus vaccination

West Nile virus (WNV) is an emerging flavivirus that has caused frequent epidemics since 1996. Besides natural transmission by mosquitoes, WNV can also be transmitted through blood transfusion and organ transplantation, thus heightening the urgency of development of a specific and rapid serologic assay of WNV infection. The current immunoassays lack specificity because they are based on detection of antibodies against WNV structural proteins and immune responses to structural proteins among flaviviruses cross-react to each other. Here, we describe microsphere immunoassays that detect antibodies to nonstructural proteins 3 and 5 (NS3 and NS5). In contrast to immunoassays based on viral envelope and NS3 proteins, the NS5-based assay (i) reliably discriminates between WNV infections and dengue virus or St. Louis encephalitis virus infections, (ii) differentiates between flavivirus vaccination and natural WNV infection, and (iii) indicates recent infections. These unique features of the NS5-based immunoassay will be very useful for both clinical and veterinary diagnosis of WNV infection.

Comparative immunological recognition of proteins from New Guinea "C" dengue virus type 2 prototype and from a dengue virus type 2 strain isolated in the State of Rio de Janeiro, Brazil

The protein profiles of the New Guinea "C" dengue virus type 2 (DENV-2)prototype and those of a Brazilian DENV-2 isolated in the State of Rio de Janeiro in 1995 were compared. SDS-PAGE analysis showed that the virus from Rio de Janeiro expresses NS5 (93.0 kDa), NS3 (66.8 kDa) E (62.4 kDa) and NS1 (41.2 kDa) proteins differently from the New Guinea "C" virus. The immunoblot revealed specificity and antigenicity for the NS3 protein from DENV-2 Rio de Janeiro mainly in primary infections, convalescent cases, and in secondary infections in both cases and only antigenicity for E and NS1 proteins for both viruses in primary and secondary infections.

Enzyme-linked immunosorbent assay specific to Dengue virus type 1 nonstructural protein NS1 reveals circulation of the antigen in the blood during the acute phase of disease in patients experiencing primary or secondary infections

During flavivirus infection in vitro, nonstructural protein NS1 is released in a host-restricted fashion from infected mammalian cells but not vector-derived insect cells. In order to analyze the biological relevance of NS1 secretion in vivo, we developed a sensitive enzyme-linked immunosorbent assay (ELISA) to detect the protein in the sera of dengue virus-infected patients. The assay was based on serotype 1 NS1-specific mouse and rabbit polyclonal antibody preparations for antigen immunocapture and detection, respectively. With purified dengue virus type 1 NS1 as a protein standard, the sensitivity of our capture ELISA was less than 1 ng/ml. When a panel of patient sera was analyzed, the NS1 antigen was found circulating from the first day after the onset of fever up to day 9, once the clinical phase of the disease is over. The NS1 protein could be detected even when viral RNA was negative in reverse transcriptase-PCR or in the presence of immunoglobulin M antibodies. NS1 circulation levels varied among individuals during the course of the disease, ranging from several nanograms per milliliter to several micrograms per milliliter, and peaked in one case at 50 microg/ml of serum. Interestingly, NS1 concentrations did not differ significantly in serum specimens obtained from patients experiencing primary or secondary dengue virus infections. These findings indicate that NS1 protein detection may allow early diagnosis of infection. Furthermore, NS1 circulation in the bloodstream of patients during the clinical phase of the disease suggests a contribution of the nonstructural protein to dengue virus pathogenesis.

Antibody to the nonstructural protein NS1 of Japanese encephalitis virus: potential application of mAb-based indirect ELISA to differentiate infection from vaccination

An indirect enzyme-linked immunosorbent assay (ELISA) was developed to detect and differentiate the antibody responses to Japanese encephalitis (JE) virus nonstructural protein NS1 between infected and vaccinated individuals. The results showed that all convalescent sera from JE patients contained NS1-specific IgG antibodies, while 65 and 40% of these sera showed detectable NS1-specific IgM and IgA antibodies, respectively. Specificity analysis showed that NS1-specific IgM and IgA antibodies from JE patients do not cross-react to dengue virus NS1 glycoprotein, while IgG antibodies from 10% of JE patients showed significant cross-reaction to dengue virus NS1 glycoprotein. To differentiate infection from vaccination, the immune sera from 24 children vaccinated with inactivated JE vaccine were analyzed. The data showed that none of these immune sera had detectable NS1-specific IgG antibodies. The results demonstrated the potential application of JE NS1-specific indirect ELISA to differentiate infection from vaccination.

Dengue NS1-specific antibody responses: isotype distribution and serotyping in patients with Dengue fever and Dengue hemorrhagic fever

To understand the antibody responses to dengue (DEN) nonstructural 1 (NS1) glycoprotein and their roles in protective immunity or pathogenesis of dengue fever (DF) and dengue hemorrhagic fever (DHF), we have analyzed the NS1-speccific IgM, IgA and IgG antibodies from patients with DF and DHF. An isotype-specific, indirect enzyme-linked immunosorbent assay (ELISA) was established by coating a NS1-specific monoclonal antibody (MAb), D2/8-1, to capture soluble NS1 antigens secreted in the culture supernatants of Vero cells infected with DEN virus. We observed strong anti-NS1 antibody responses in all of the convalescent sera of patients with DF and DHF. Similar NS1-specific isotypic and serotypic antibody responses were found in the sera from DF and DHF patients. The results showed that all DEN infections induced significant NS1-specific IgG, whereas 75% and 60% of primary DF patients vs. 40% and 90% of secondary DF patients produced IgM and IgA antibodies, respectively. Specificity analysis showed that DEN NS1-specific IgG and IgA antibodies cross-react strongly to Japanese encephalitis (JE) virus NS1 glycoprotein, whereas DEN NS1-specific IgM antibodies do not cross-react to JE virus NS1 glycoprotein at all. The serotype specificity of NS1-specific IgM, IgA and IgG were found to be 80%, 67% and 75% for primary infections, and 50%, 22% and 30% for secondary infections in positive samples of DF patients. Similar pattern was found in DHF patients. The results showed that all of the DF and DHF patients produced significant NS1-specific antibodies. We did not observe direct correlation between the anti-NS1 antibody responses and DHF because sera from patients with DF and DHF showed similar anti-NS1 antibody responses.

Human Dengue antibodies against structural and nonstructural proteins

Antibodies against dengue virus type 2 and 4 proteins in acute-phase sera of 10 primary and 10 secondary dengue fever and dengue hemorrhagic fever patients were studied by Western blotting. In the first group the immune response was barely detectable, while in the second group more proteins were detected, with a very strong reaction. Anti-NS1 and -NS3 antibodies were detected mainly in secondary cases. Anti-E, -NS3, and -NS5 antibodies were detected in a high number of cases. The possibility of implementing early diagnostic assays for antigen detection is suggested.

An antigen capture enzyme-linked immunosorbent assay reveals high levels of the dengue virus protein NS1 in the sera of infected patients

We describe the development of a capture enzyme-linked immunosorbent assay for the detection of the dengue virus nonstructural protein NS1. The assay employs rabbit polyclonal and monoclonal antibodies as the capture and detection antibodies, respectively. Immunoaffinity-purified NS1 derived from dengue 2 virus-infected cells was used as a standard to establish a detection sensitivity of approximately 4 ng/ml for an assay employing monoclonal antibodies recognizing a dengue 2 serotype-specific epitope. A number of serotype cross-reactive monoclonal antibodies were also shown to be suitable probes for the detection of NS1 expressed by the remaining three dengue virus serotypes. Examination of clinical samples demonstrated that the assay was able to detect NS1 with minimal interference from serum components at the test dilutions routinely used, suggesting that it could form the basis of a useful additional diagnostic test for dengue virus infection. Furthermore, quantitation of NS1 levels in patient sera may prove to be a valuable surrogate marker for viremia. Surprisingly high levels of NS1, as much as 15 microg/ml, were found in acute-phase sera taken from some of the patients experiencing serologically confirmed dengue 2 virus secondary infections but was not detected in the convalescent sera of these patients. In contrast, NS1 could not be detected in either acute-phase or convalescent serum samples taken from patients with serologically confirmed primary infection. The presence of high levels of secreted NS1 in the sera of patients experiencing secondary dengue virus infections, and in the context of an anamnestic antibody response, suggests that NS1 may contribute significantly to the formation of the circulating immune complexes that are suspected to play an important role in the pathogenesis of severe dengue disease.

Precise location of sequential dengue virus subcomplex and complex B cell epitopes on the nonstructural-1 glycoprotein

The reactions of a panel of 34 mouse monoclonal antibodies (MAbs) specific for the dengue-2 virus nonstructural-1 glycoprotein (NS1), were analysed using 174 overlapping synthetic nonameric peptides covering the entire sequence. Using this methodology, four epitopes were identified. One pair of MAbs, which defined a dengue-2/4 virus subcomplex epitope (24C: amino acids 299-309) using native NS1 proteins, showed the same reaction pattern with synthetic peptides containing the corresponding NS1 sequences of each virus serotype. One amino acid substitution, present in the sequences from the dengue-1/3 virus subcomplex abrogated almost all reaction by these MAbs. A dengue complex epitope (LX1: amino acids 111-121) was also located and peptides containing the sequences of each serotype were shown to contain only antigenically silent amino-acid substitutions. In contrast, MAbs which defined a dengue type-specific epitope (LD2: amino acids 25-33) and another dengue subcomplex epitope (24A: amino acids 61-69) failed to show the same reaction profiles using peptides of each serotype, suggesting that these determinants were partially dependent upon conformation. The LX1 epitope is a good candidate for further trials aimed at generating cross-protective immune responses to these viruses without the risk of antibody-dependent enhancement.