Dengue fever as autochthonous infectious disease in Italy: Epidemiological, clinical and virological characteristics

BACKGROUND

Since August to November 2023, 82 cases of autochthonous or non-travel related Dengue virus (DENV) infection have been reported in Italy, highlighting a concerning trend of local transmission. We describe the clinical and laboratory findings of 10 autochthonous DENV in the metropolitan area of Rome admitted to the Lazzaro Spallanzani National Institute for Infectious Diseases.

METHOD AND RESULTS

Ten patients (3 males, 7 females; median age: 51) with classic dengue fever symptoms were admitted between August and November 2023. Laboratory tests confirmed dengue infection through DENV non-structural protein 1 and/or immunoglobulins (IgM/IgG) positive tests, moreover leukopenia, thrombocytopenia, elevated transaminases were detected. A subset of patients underwent extensive biological sampling, including real-time RT-PCR and immunofluorescence, to monitor DENV-RNA and antibody levels over 30 days. DENV-1 was detected in 8 patients and DENV-3 in 2. Upon admission specific IgM antibodies were found in 7 patients while IgG antibodies in 4 patients. DENV RNA was consistently detected in blood within the first 8 days but was less common in saliva and urine. No DENV RNA was detected after day 24.

CONCLUSION

These findings contribute to the understanding of the clinical course of DENV infection in a non-endemic setting as integrated epidemiological and clinical model to increase syndromic surveillance and timely diagnosis of DENV infections.

Comprehensive evaluation on progressive development strategies in DENV surveillance and monitoring infection rate among vector population

The elevated rise in dengue infection rate has been a health burden worldwide and it will continue to impact global health for years to come. Accumulated literature holds accountable the geographical expansion of the mosquito species transmitting the dengue virus DENV. The frequency of this viral disease outbreaks has increased rapidly in the recent years, owing to various geo-climatic and anthropological activities. Due to scarcity of any effective control measures, there has been a continuous traceable rise in mortality and morbidity rates. However, it has been reported that the spate of incidences is directly related to density of the virus infected vector (mosquito) population in a given region. In such a scenario, systems capable of detecting virus infected vector population would aid in estimating prediction of outbreak, as well as provide time to deploy suitable management strategies for vector control, and to break the vector-human transmission chain. This would also help in identifying areas, where much improvement is needed for vector management. To this context, we illustrate an exhaustive overview of both gold standards and as well as emerging advents for sensitive and specific mosquito population strategized viral detection technologies. We summarize the cutting-edge technologies and the challenges faced in pioneering to field application. Regardless the proven popularity of the gold standards for detection purpose, they offer certain limitations. Thus with the surge in the infection rate globally, approaches for development of newer advancements and technique upgradation to arrest the infection escalation and for early detection as a part of vector management should be prioritized.

Autochthonous Dengue Fever in 2 Patients, Rome, Italy

Since August 2023, outbreaks of dengue virus (DENV) infection have occurred in Italy. We report 2 autochthonous case-patients and their extended follow-up. Despite persistent DENV detected in blood by PCR, results for antigenomic DENV RNA were negative after day 5, suggesting that a 5-day isolation period is adequate to avoid secondary cases.

Development of HEK-293 Cell Lines Constitutively Expressing Flaviviral Antigens for Use in Diagnostics

Flaviviruses are important human pathogens worldwide. Diagnostic testing for these viruses is difficult because many of the pathogens require specialized biocontainment. To address this issue, we generated 39 virus-like particle (VLP)- and nonstructural protein 1 (NS1)-secreting stable cell lines in HEK-293 cells of 13 different flaviviruses, including dengue, yellow fever, Japanese encephalitis, West Nile, St. Louis encephalitis, Zika, Rocio, Ilheus, Usutu, and Powassan viruses. Antigen secretion was stable for at least 10 cell passages, as measured by enzyme-linked immunosorbent assays and immunofluorescence assays. Thirty-five cell lines (90%) had stable antigen expression over 10 passages, with three of these cell lines (7%) increasing in antigen expression and one cell line (3%) decreasing in antigen expression. Antigen secretion in the HEK-293 cell lines was higher than in previously developed COS-1 cell line counterparts. These antigens can replace current antigens derived from live or inactivated virus for safer use in diagnostic testing.

IMPORTANCE Serological diagnostic testing for flaviviral infections is hindered by the need for specialized biocontainment for preparation of reagents and assay implementation. The use of previously developed COS-1 cell lines secreting noninfectious recombinant viral antigen is limited due to diminished antigen secretion over time. Here, we describe the generation of 39 flaviviral virus-like particle (VLP)- and nonstructural protein 1 (NS1)-secreting stable cell lines in HEK-293 cells representing 13 medically important flaviviruses. Antigen production was more stable and statistically higher in these newly developed cell lines than in their COS-1 cell line counterparts. The use of these cell lines for production of flaviviral antigens will expand serological diagnostic testing of flaviviruses worldwide.

NS1 Recombinant Proteins Are Efficiently Produced in Pichia pastoris and Have Great Potential for Use in Diagnostic Kits for Dengue Virus Infections

Dengue is one of the major diseases causing global public health concerns. Despite technological advances in vaccine production against all its serotypes, it is estimated that the dengue virus is responsible for approximately 390 million infections per year. Laboratory diagnosis has been the key point for the correct treatment and prevention of this disease. Currently, the limiting factor in the manufacture of dengue diagnostic kits is the large-scale production of the non-structural 1 (NS1) antigen used in the capture of the antibody present in the infected patients' serum. In this work, we demonstrate the production of the non-structural 1 protein of dengue virus (DENV) serotypes 1-4 (NS1-DENV1, NS1-DENV2, NS1-DENV3, and NS1-DENV4) in the methylotrophic yeast Pichia pastoris KM71H. Secreted recombinant protein was purified by affinity chromatography and characterized by SDS-PAGE and ELISA. The objectives of this study were achieved, and the results showed that P. pastoris is a good heterologous host and worked well in the production of NS1DENV 1-4 recombinant proteins. Easy to grow and quick to obtain, this yeast secreted ready-to-use proteins, with a final yield estimated at 2.8-4.6 milligrams per liter of culture. We reached 85-91% sensitivity and 91-93% specificity using IgM as a target, and for anti-dengue IgG, 83-87% sensitivity and 81-93% specificity were achieved. In this work, we conclude that the NS1 recombinant proteins are efficiently produced in P. pastoris and have great potential for use in diagnostic kits for dengue virus infections. The transformed yeast obtained can be used for production in industrial-scale bioreactors.

Horseradish peroxidase-labeled rabbit anti-non-structural protein 1 of dengue virus-2 for the diagnosis of dengue virus infections

BACKGROUND Early diagnosis of dengue virus (DENV) infection is essential for patient management and disease control. Detection of the antigen non-structural protein 1 (NS1) has been proven to provide early diagnosis of DENV infection. Thus, commercial NS1 antigen detection assays have been increasingly used and are becoming thetool of choice among clinicians to confirm DENV infection in Indonesia. METHODS To obtain anti-NS1 DENV antibody, NS1 protein (90 µg/ml) from the collection of the Department of Microbiology, Faculty of Medicine, Universitas Indonesia was injected into a rabbit. The anti-NS1 antibody from the rabbit was then labeled with horseradish peroxidase (HRP) using the periodate oxidation method. Sera were tested by enzyme-linked immunosorbent assay (ELISA) to detect NS1 from DENV-infected patients. RESULTS Serially diluted antibody labeled with HRP tested using the direct ELISA method showed the highest absorbance value at a 1:100 dilution (Mean [SD] = 1.35 [0.35]); even at a dilution as high as 1:3,200 (0.22 [0.15]), antibody labeled with HRP was able to detect the NS1 protein, although the absorbance value did not differ greatly from that of the negative control (0.13 [0.01]). CONCLUSIONS In an attempt to develop an NS1-based diagnostic test, polyclonal anti-NS1 DENV antibody was successfully produced as a diagnostic assay to determine the presence of DENV NS1 antigen in patients’ sera.

Association of high pressure and alkaline condition for solubilization of inclusion bodies and refolding of the NS1 protein from zika virus

Background

Proteins in inclusion bodies (IBs) present native-like secondary structures. However, chaotropic agents at denaturing concentrations, which are widely used for IB solubilization and subsequent refolding, unfold these secondary structures. Removal of the chaotropes frequently causes reaggregation and poor recovery of bioactive proteins. High hydrostatic pressure (HHP) and alkaline pH are two conditions that, in the presence of low level of chaotropes, have been described as non-denaturing solubilization agents. In the present study we evaluated the strategy of combination of HHP and alkaline pH on the solubilization of IB using as a model an antigenic form of the zika virus (ZIKV) non-structural 1 (NS1) protein.

Results

Pressure-treatment (2.4 kbar) of NS1-IBs at a pH of 11.0 induced a low degree of NS1 unfolding and led to solubilization of the IBs, mainly into monomers. After dialysis at pH 8.5, NS1 was refolded and formed soluble oligomers. High (up to 68 mg/liter) NS1 concentrations were obtained by solubilization of NS1-IBs at pH 11 in the presence of arginine (Arg) with a final yield of approximately 80% of total protein content. The process proved to be efficient, quick and did not require further purification steps. Refolded NS1 preserved biological features regarding reactivity with antigen-specific antibodies, including sera of ZIKV-infected patients. The method resulted in an increase of approximately 30-fold over conventional IB solubilization-refolding methods.

Conclusions

The present results represent an innovative non-denaturing protein refolding process by means of the concomitant use of HHP and alkaline pH. Application of the reported method allowed the recovery of ZIKV NS1 at a condition that maintained the antigenic properties of the protein.

Electronic supplementary material

The online version of this article (10.1186/s12896-018-0486-2) contains supplementary material, which is available to authorized users.

Recombinant Zika NS1 Protein Secreted from Vero Cells Is Efficient for Inducing Production of Immune Serum Directed against NS1 Dimer

Zika virus (ZIKV) is a mosquito-borne flavivirus that recently emerged in the South Pacific, Americas, and Caribbean islands, where the larger epidemics were documented. ZIKV infection in humans is responsible for neurological disorders and microcephaly. Flavivirus NS1 is a non-structural glycoprotein that is expressed on the cell surface and secreted as a hexameric lipoprotein particle. Intracellular NS1 exists as a dimer that is required for viral replication, whereas the secreted NS1 hexamer interacts with host factors, leading to pathophysiological conditions. In an effort to dispose of specific anti-ZIKV NS1 immune serum, Vero cells were transduced with a lentiviral vector containing the NS1 gene from an epidemic strain of ZIKV. We showed that stably transduced Vero/ZIKV NS1 cell clone was efficient in the secretion of recombinant NS1 oligomer. Immunization of adult rat with purified extracellular NS1 developed anti-ZIKV antibodies that specifically react with the NS1 dimer produced in human cells infected with African and Asian strains of ZIKV. The rat antibody against ZIKV NS1 dimer is a reliable biological tool that enables the immunological detection of secreted NS1 from host-cells infected with ZIKV.

Development and clinical evaluation of a highly accurate dengue NS1 rapid test: from the preparation of a soluble NS1 antigen to the construction of an RDT

Early diagnosis of dengue virus (DENV) is important. There are numerous products on the market claiming to detect DENV NS1, but these are not always reliable. In this study, a highly sensitive and accurate rapid diagnostic test (RDT) was developed using anti-dengue NS1 monoclonal antibodies. A recombinant NS1 protein was produced with high antigenicity and purity. Monoclonal antibodies were raised against this purified NS1 antigen. The RDT was constructed using a capturing (4A6A10, Kd=7.512±0.419×10(-9)) and a conjugating antibody (3E12E6, Kd=7.032±0.322×10(-9)). The diagnostic performance was evaluated with NS1-positive clinical samples collected from various dengue endemic countries and compared to SD BioLine Dengue NS1 Ag kit. The constructed RDT exhibited higher sensitivity (92.9%) with more obvious diagnostic performance than the commercial kit (83.3%). The specificity of constructed RDT was 100%. The constructed RDT could offer a reliable point-of-care testing tool for the early detection of dengue infections in remote areas and contribute to the control of dengue-related diseases.

Dengue NS1 and prM antibodies increase the sensitivity of acute dengue diagnosis test and differentiate from Japanese encephalitis infection

Accurate and early diagnosis of dengue infection is essential for dengue case management. In outbreak conditions, it is essential to include two different tests to diagnose dengue and the choice depends on the number of days after the onset of illness in which the sample is collected. During the laboratory diagnosis of dengue in late acute and convalescent phase by MAC-ELISA, it is necessary to rule out possible cross reactions of closely related flavivirus, such as Japanese encephalitis virus which is commonly co-circulating. In the present investigation, the usefulness of dengue virus NS1 and prM antibodies in diagnosing and differentiating dengue from Japanese encephalitis infection was assessed using samples collected during out-breaks. It was shown here that, detection of antibodies against dengue NS1 and prM proteins increases the sensitivity of dengue diagnosis until 15days. Moreover, detection of antibodies against both proteins was able to differentiate dengue from Japanese encephalitis infection.

High-level expression of functionally active dengue-2 non-structural antigen 1 production in Escherichia coli

Detection of nonstructural protein (NS1) is an important diagnostic marker during acute phase of dengue infection. Not only for diagnostic purpose, the protein had important role in vaccine design as well, as a candidate for studying virus assembly and maturation. Various researchers employed different expression systems and strategies for recombinant NS1 protein production. Attempts to express NS1 protein in prokaryotic and yeast expression system result in formation of insoluble protein which needs to undergo refolding to attain native structural and functional forms. Here, we report the production of soluble NS1 protein in E. coli by using appropriate vector and employing suitable culture conditions to maximize protein production. Proteins were purified using metal affinity chromatography. SDS-PAGE and western blot analysis reveal the native structure of NS1 protein. Solid phase ELISA using the recombinantly expressed antigen with positive and negative dengue samples showed that the expressed protein retains its antigenic and immunological properties. To our knowledge, this is the first report on the successful production of functionally active recombinant dengue-2 NS1 protein production without undergoing any in vitro posttranslational modification process.

Dengue virus type 4 in Niterói, Rio de Janeiro: the role of molecular techniques in laboratory diagnosis and entomological surveillance

In Niterói, state of Rio de Janeiro, dengue virus type 4 (DENV-4) was isolated for the first time in March 2011. We analysed the laboratory findings of the first cases and evaluated the use of molecular techniques for the detection of DENV-4 in Aedes aegypti that were field-caught. Conventional reverse transcriptase-polymerase chain reaction (RT-PCR) and Simplexa™ Dengue real-time RT-PCR confirmed DENV-4 infection in all cases. Additionally, DENV-4 was confirmed in a female Ae. aegypti with 1.08 x 10(3) copies/mL of virus, as determined by quantitative real-time RT-PCR. This is the first time the Simplexa™ Dengue real-time assay has been used for the classification of cases of infection and for entomological investigations. The use of these molecular techniques was shown to be important for the surveillance of dengue in humans and vectors.

Comprehensive Mapping Antigenic Epitopes of NS1 Protein of Japanese Encephalitis Virus with Monoclonal Antibodies

Japanese encephalitis virus (JEV) non-structural protein 1 (NS1) contributes to virus replication and elicits protective immune responses during infection. JEV NS1-specific antibody responses could be a target in the differential diagnosis of different flavivirus infections. However, the epitopes on JEV NS1 are poorly characterized. The present study describes the full mapping of linear B-cell epitopes in JEV NS1. We generated eleven NS1-specific monoclonal antibodies from mice immunized with recombinant NS1. For epitope mapping of monoclonal antibodies, a set of 51 partially-overlapping peptides covering the entire NS1 protein were expressed with a GST-tag and then screened using monoclonal antibodies. Through enzyme-linked immunosorbent assay (ELISA), five linear epitope-containing peptides were identified. By sequentially removing amino acid residues from the carboxy and amino terminal of peptides, the minimal units of the five linear epitopes were identified and confirmed using monoclonal antibodies. Five linear epitopes are located in amino acids residues (5)AIDITRK(11), (72)RDELNVL(78), (251)KSKHNRREGY(260), (269)DENGIVLD(276), and (341)DETTLVRS(348). Furthermore, it was found that the epitopes are highly conserved among JEV strains through sequence alignment. Notably, none of the homologous regions on NS1 proteins from other flaviviruses reacted with the MAbs when they were tested for cross-reactivity, and all five epitope peptides were not recognized by sera against West Nile virus or Dengue virus. These novel virus-specific linear B-cell epitopes of JEV NS1 would benefit the development of new vaccines and diagnostic assays.

Optimization of Dengue-3 recombinant NS1 protein expression in E. coli and in vitro refolding for diagnostic applications

Dengue non-structural protein (NS1) is known to be protective antigen and also has immense application for serodiagnosis. Several serodiagnostic assays available for dengue viral infection are dependent on tissue culture-grown viral proteins. This task is unsafe, laborious, more expensive that makes it unsuitable for routine large-scale production. Although bacterial expression is relatively simple and easy for recombinant protein expression, it is more challenging to make NS1 protein with native structural and immunological features using bacterial expression system. We have successfully developed a method leading to the purification and refolding of recombinant dengue virus type 3 (DENV3) NS1. The gene encoding NS1 was amplified and cloned in pET28a (+) vector. In order to increase the purity of the recombinant NS1, the transgene was engineered to carry 6× Histidine tags at both N and C-terminal ends. The recombinant construct (pETNS1) was transformed into E. coli Rosetta-gami cells and the expression conditions viz IPTG concentration, media type, temperature, and harvest time were optimized. The size of the expressed protein was found to be ~45 kDa and the authenticity of the expressed protein was confirmed using anti-His and anti-NS1 monoclonal antibodies. The NS1 protein was purified under denaturing conditions, to attain the native conformation, NS1 protein was in vitro refolded and dialyzed. The refolded NS1 protein was detected by commercial Immuno chromatographic strip and NS1 specific monoclonal antibodies. IgM antibody capture ELISA was performed using refolded recombinant NS1 protein which recognized the IgM antibodies in dengue-positive samples of acute phase of infection. Our result suggests that rNS1 protein has immense diagnostic potential and can be used in developing point of care diagnostic assays.

Production of recombinant nonstructural 1 protein in Escherichia coli for early detection of Japanese encephalitis virus infection

Japanese encephalitis is a major public health problem in South-East Asia and Western Pacific countries. The recombinant nonstructural 1 (rNS1) protein of Japanese encephalitis virus is a potential diagnostic as well as vaccine candidate. Developments of cost-effective and simple culture media as well as appropriate culture conditions are generally favourable for large-scale production of recombinant proteins. The effects of medium composition and cultivation conditions on the production of rNS1 protein were investigated in shake flask culture as well as batch cultivation of Escherichia coli. Further, the fed-batch process was also carried out for high cell density cultivation (HCDC) of E. coli expressing rNS1 protein. Isopropyl-β-d-thiogalactopyranoside (IPTG) was used to induce the expression of rNS1 protein at ∼ 13 g dry cell weight per litre of culture. The final dry cell weight after fed-batch cultivation was ∼ 17 g l(-1) . The Inclusion bodies were isolated and purified through affinity chromatography to give a final product yield of ∼ 142 mg l(-1) . The reactivity of purified protein was confirmed by Western blotting and Enzyme linked immunosorbent assay. These results show that rNS1 protein may be used as a diagnostic reagent or for further prophylactic studies. This approach of producing rNS1 protein in E. coli with high yield may also offer promising method for production of other viral recombinant proteins.

Production of mouse granulocyte-macrophage colony-stimulating factor by gateway technology and transgenic rice cell culture

To establish a production platform for recombinant proteins in rice suspension cells, we first constructed a Gateway-compatible binary T-DNA destination vector. It provided a reliable and effective method for the rapid directional cloning of target genes into plant cells through Agrobacterium-mediated transformation. We used the approach to produce mouse granulocyte-macrophage colony-stimulating factor (mGM-CSF) in a rice suspension cell system. The promoter for the αAmy3 amylase gene, which is induced strongly by sugar depletion, drove the expression of mGM-CSF. The resulting recombinant protein was fused with the αAmy3 signal peptide and was secreted into the culture medium. The production of rice-derived mGM-CSF (rmGM-CSF) was scaled up successfully in a 2-L bioreactor, in which the highest yield of rmGM-CSF was 24.6 mg/L. Due to post-translational glycosylation, the molecular weight of rmGM-CSF was larger than that of recombinant mGM-CSF produced in Escherichia coli. The rmGM-CSF was bioactive and could stimulate the proliferation of a murine myeloblastic leukemia cell line, NSF-60.

Expression and immunoaffinity purification of recombinant dengue virus 2 NS1 protein as a cleavable SUMOstar fusion

Dengue virus (DENV) encoded nonstructural one (NS1) is a 352 amino acid protein that exists in multiple oligomeric states and is conserved within the flavivirus family. Although NS1 has been heavily researched for its diagnostic utility, there is a gap in the understanding of its role in a range of viral processes, including replication and development of clinical pathologies such as vascular leakage. Many of these functions involve unknown interactions with viral and host proteins. This study describes the generation of a mouse monoclonal antibody (mAb 56.2) that reacts with NS1 from DENV1 and 2, and the expression of recombinant SUMOstar-tagged DENV2 NS1 (DENV2 S∗-NS1) in baculovirus. This is the first time dengue NS1 has been produced as a SUMOstar fusion with the S∗-tag increasing protein solubility and secretion compared with a non-S∗-tagged NS1 construct. The protein was readily purified using a mAb 56.2 immunoaffinity column and untagged NS1 was obtained by treatment with tobacco etch virus protease to remove the S∗-tag. Size exclusion chromatography and glycosylation assays showed that both secreted S∗-NS1, and cleaved NS1, are hexameric and glycosylated, and will be useful tools in elucidating dengue NS1 protein interactions and functions.

Evaluation of a commercial Dengue NS1 enzyme-linked immunosorbent assay for early diagnosis of dengue infection

PURPOSE

Dengue is one of the most serious mosquito-borne viral infections affecting tropical and subtropical countries in the world. Since there is no immunoprophylactic or specific antiviral therapy available, timely and rapid diagnosis plays a vital role in patient management and implementation of control measures. This paper evaluates a commercially available NS1 antigen capture ELISA vis-a-vis SD bioline Dengue NS1 antigen test for early detection of dengue virus.

MATERIALS AND METHODS

To evaluate a commercial NS1 antigen detection kit vis-a-vis SD bioline Dengue NS1 antigen test, a total of 91 clinical samples were tested. Virological investigations with regard to dengue virus, viz. NS1 antigen capture ELISA (Panbio, Australia), SD bioline Dengue NS1 antigen test, RT-PCR and virus isolation were performed.

RESULTS

Out of 91 samples, 24 (26%) were positive by NS1 antigen capture ELISA, 15 (16%) by SD bioline Dengue NS1 antigen test and 11(12%) positive by RT-PCR analysis. The RT-PCR-positive samples were further subjected to virus isolation and resulted in three isolates. The results of the Panbio NS1 antigen capture ELISA, SD bioline Dengue NS1 antigen test, RT-PCR and virus isolation were correlated among themselves.

CONCLUSIONS

The present study comprehensively established the utility of NS1 antigen ELISA in early diagnosis of dengue infection.

Dengue fever: diagnosis and treatment

Dengue fever is a common tropical infection. This acute febrile illness can be a deadly infection in cases of severe manifestation, causing dengue hemorrhagic shock. In this brief article, I will summarize and discuss the diagnosis and treatment of this disease. For diagnosis of dengue, most tropical doctors make use of presumptive diagnosis; however, the definite diagnosis should be based on immunodiagnosis or viral study. Focusing on treatment, symptomatic and supportive treatment is the main therapeutic approach. The role of antiviral drugs in the treatment of dengue fever has been limited, but is currently widely studied.

Comparison of three commercially available dengue NS1 antigen capture assays for acute diagnosis of dengue in Brazil

Background

Dengue is associated with explosive urban epidemics and has become a major public health problem in many tropical developing countries, including Brazil. The laboratory diagnosis of dengue can be carried out using several approaches, however sensitive and specific assays useful to diagnose in the early stage of fever are desirable. The flavivirus non-structural protein NS1, a highly conserved and secreted glycoprotein, is a candidate protein for rapid diagnosis of dengue in endemic countries.

Methodology/Principal Findings

We aimed to evaluate the potential use of 3 commercial kits in a panel of 450 serum samples for early diagnosis of dengue in Brazil. The PanBio Early ELISA (PanBio Diagnostics) showed a sensitivity of 72.3% (159/220) and a specificity of 100%, while the sensitivity of the Platelia™ NS1 assay (Biorad Laboratories) was 83.6% (184/220). However, the highest sensitivity (89.6%; 197/220) was obtained by using the NS1 Ag Strip (Biorad Laboratories). A lower sensitivity was observed in DENV-3 cases by all 3 kits. Serum positive by virus isolation were more often positive than cases positive by RT-PCR by all three assays and a higher detection rate was observed during the first four days after the onset of the symptoms. The presence or absence of IgM showed no influence in the confirmation by the pan-E Early ELISA (P = 0,6159). However, a higher confirmation by both Platelia™ NS1 (Biorad) and Dengue NS1 Ag Strip (Biorad) in the absence of IgM was statistically significant (P<0,0001 and P = 0,0008, respectively). Only the Platelia™ NS1 test showed a higher sensitivity in confirming primary infections than secondary ones.

Conclusions/Significance

The results indicate that commercial kits of dengue NS1 antigen are useful for the laboratory diagnosis of acute primary and secondary dengue. It can be used in combination with the MAC-ELISA for case detection and as screening test to complement viral isolation.

Dengue is the one of the most prevalent arthropod-borne viral diseases in tropical regions of the world. Manifestations may vary from asymptomatic to potentially fatal complications. Laboratorial diagnosis is essential to diagnose dengue and differentiate it from other diseases. Dengue virus non-structural protein 1 (NS1) may be used as a marker of acute dengue virus infection. Our results, based in the comparison of three NS1 antigen capture assays available, have shown that this approach is reliable for the early diagnosis of dengue infections, especially in the first four days after the onset of the symptoms. A lower sensitivity was observed in DENV-3 cases. Serum positive by virus isolation were more often detected than those positive by RT-PCR by all three assays. Only the Platelia™ NS1 test showed a higher sensitivity in confirming primary infections than secondary ones. In conclusion, NS1 antigen capture commercial kits are useful for diagnosis of acute primary and secondary dengue infections and, in endemic countries where secondary infections are expected to occur, may be used in combination with MAC-ELISA to increase the overall sensitivity of both tests.

Evaluation of nonstructural 1 antigen assays for the diagnosis and surveillance of dengue in Singapore

Early and accurate diagnosis of dengue is imperative for disease surveillance, which helps in the control of dengue in endemic countries. In this study, we evaluated the performance of three commercially available dengue nonstructural 1 (NS1) antigen assays (Bio-Rad Platelia™ Dengue NS1 Antigen ELISA, PanBio Dengue Early ELISA, and Bio-Rad Dengue NS1 Antigen Strip test) and compared them with reverse-transcription polymerase chain reaction (RT-PCR) and other commercially available serological assays for the diagnosis of dengue. The analysis showed RT-PCR to be the most sensitive and specific (100%) diagnostic method during the first 3 days of fever. The overall sensitivity of dengue NS1 antigen assays within the same period was 81.7%, indicating their potential role as a cost-effective and convenient alternative method to RT-PCR for the diagnosis of dengue fever in a primary healthcare setting. However, reduced sensitivity in detecting secondary dengue infections was one of the drawbacks of dengue NS1 antigen assays. Nonetheless, it remains a useful assay for the early detection of dengue and hence could play an important role in routine surveillance efforts to control dengue outbreaks in Singapore.

Refolded dengue virus type 2 NS1 protein expressed in Escherichia coli preserves structural and immunological properties of the native protein

The dengue virus NS1 protein has been shown to be a protective antigen under different experimental conditions but the recombinant protein produced in bacterial expression systems is usually not soluble and loses structural and immunological features of the native viral protein. In the present study, experimental conditions leading to purification and refolding of the recombinant dengue virus type 2 (DENV-2) NS1 protein expressed in Escherichia coli are described. The refolded recombinant protein was recovered as heat-stable soluble dimers with preserved structural features, as demonstrated by spectroscopic methods. In addition, antibodies against epitopes of the NS1 protein expressed in eukaryotic cells recognized the refolded protein expressed in E. coli but not the denatured form or the same protein submitted to a different refolding condition. Collectively, the results demonstrate that the recombinant NS1 protein preserved important conformation and antigenic determinants of the native virus protein and represents a valuable reagent either for the development of vaccines or for diagnostic methods.

Super induction of dengue virus NS1 protein in E. coli

The non-structural protein 1 (NS1) of dengue virus is a useful target for diagnostics of dengue infection since the protein is abundantly circulating in blood during the acute phase of the disease. Prior work has established that secreted NS1 levels in plasma correlates with viremia levels and hence can also be used to diagnose patients at the risk for developing dengue hemorrhagic fever. Thus detection of non-structural dengue antigens may be of benefit for an early rapid diagnosis of dengue infection due to its long half life in the blood. Here we describe a simple and efficient method for the expression of NS1 in Escherichia coli, which could potentially be used to develop monoclonal and bispecific antibodies for point of care diagnostics. E. coli codon optimized synthetic full-length NS1 gene of dengue serotype 1 (DEN-1) was successfully cloned and expressed in very high-level as inclusion bodies. The NS1 protein was successfully affinity purified and refolded as a recombinant NS1 (rNS1) protein in E. coli and yield was 230–250 mg/L of bacterial culture. The rNS1 protein was used to immunize mice for hybridoma development. The polyclonal antiserum from animals immunized with this rNS1 protein was found to specifically recognize the rNS1, thus demonstrating the immunogenic nature of the protein. The rNS1 protein purified from E. coli could be useful for developing a sensitive serum diagnostic assay to monitor dengue outbreaks.

Proteomic Analysis of Endothelial Cell Autoantigens Recognized by Anti-Dengue Virus Nonstructural Protein 1 Antibodies

We previously showed the occurrence of autoimmune responses in dengue virus (DV) infection, which has potential implications for the pathogenesis of dengue hemorrhagic syndrome. In the present study, we have used a proteomic analysis to identify several candidate proteins on HMEC-1 endothelial cells recognized by anti-DV nonstructural protein 1 (NS1) antibodies. The target proteins, including ATP synthase β chain, protein disulfide isomerase, vimentin, and heat shock protein 60, co-localize with anti-NS1 binding sites on nonfixed HMEC-1 cells using immunohistochemical double staining and confocal microscopy. The cross-reactivity of anti-target protein antibodies with HMEC-1 cells was inhibited by NS1 protein pre-absorption. Furthermore, a cross-reactive epitope on NS1 amino acid residues 311–330 (P311–330) was predicted using homologous sequence alignment. The reactivity of dengue hemorrhagic patient sera with HMEC-1 cells was blocked by synthetic peptide P311–330 pre-absorption. Taken together, our results identify putative targets on endothelial cells recognized by anti-DV NS1 antibodies, where NS1 P311–330 possesses the shared epitope.

Development of an antigen capture immunoassay based on monoclonal antibodies specific for dengue virus serotype 2 nonstructural protein 1 for early and rapid identification of dengue virus serotype 2 infections

The dengue virus (DENV) has four distinct serotypes (DENV1, DENV2, DENV3, and DENV4) that require differentiation for effective prevention of morbid diseases. The recently developed DENV1-specific NS1 antigen capture enzyme-linked immunosorbent assay (ELISA) based on the monoclonal antibodies (MAbs) that recognize distinct epitopes on nonstructural protein 1 (NS1) of a specific DENV serotype is convenient and cost-effective, but assays have not yet been developed for DENV serotypes 2 to 4. This paper describes the development and validation of a DENV2-specific NS1 antigen capture ELISA by selection and optimization of the pair of well-characterized MAbs that recognized epitopes specific for DENV2 NS1 from a large panel of MAbs. The DENV2 NS1 ELISA displayed exclusive sensitivity with the DENV2 serotype and did not cross-react with the other three DENV serotypes. The sensitivity and specificity of the DENV2 NS1 ELISA were 83.3% (25/30) and 100% (504/504) when used to test 30 acute-phase serum samples from patients infected with DENV2 identified by virus isolation or reverse transcription-PCR serotyping and 504 serum samples from healthy individuals, respectively. The specificity of this assay was also evaluated using a panel of serum samples which were positive for DENV1, other flaviviruses, and nonflaviviruses; no cross-reactions were observed in these clinical samples. The DENV2 NS1 ELISA was eightfold more sensitive than a commercially available serotype-cross-reactive NS1 ELISA (Panbio Diagnostics, Brisbane, Australia) when the two assays were used to test the DENV2-infected cell culture supernatants in parallel. The Panbio NS1 ELISA displayed variation in sensitivity between DENV serotypes. The DENV2-specific NS1 antigen capture ELISA can be used as a tool for the rapid identification of DENV2 infections.

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.

Evaluation of an NS1 antigen detection for diagnosis of acute dengue infection in patients with acute febrile illness

Diagnosis of dengue infection during the febrile stage has been challenging. We evaluated the accuracy of NS1 protein detection in diagnosing dengue infection in patients presenting with acute febrile illness in Bangkok, Thailand. Of the 235 subjects presented with fever of unknown source within 5 days, 132 (56.2%) were male with the median age of 17.8 (range, 3-52) years. The median duration of fever was 4 (range, 1-5) days. One hundred seventy-one (72.8%) patients had dengue infection, of which 158 (92.4%) were secondary infections. The sensitivity of NS1 Ag test was 63.2% (95% confidence interval [CI], 55.7-70.0), and the specificity was 98.4% (95% CI, 91.7-99.7). The positive and negative predictive values were 99.0% and 52.5%, respectively. The immune complex dissociation by acid treatment increase sensitivity from 63.2% to 72%. In an endemic area, Plateliatrade mark NS1 Ag test has limited sensitivity but very high specificity for diagnosis of dengue infection in patients with acute febrile illness.

Secreted Expression and Purification of Dengue 2 Virus Full-length Nonstructural Glycoprotein NS1 in Pichia. pastoris

The dengue 2 virus (DEN-2) RNA (NGC strain) was used as a substrate to produce DNA clones of the full-length NS1 genes via reverse transcriptase synthesis of cDNA followed by polymerase chain reaction amplification of the NS1 region. Products were cloned into pPICZalphaB vector for sequencing and into Pichia pastoris for expression. A recombinant protein with a molecular size of approximately 80 KDa was secreted into the supernatant from the yeast cells when induced with methanol. The expressed protein was able to bind with mouse polyclonal antibody or NS1-specific monoclonal antibody of dengue 2 virus. Purified NS1-poly(His)-tagged fusion protein was obtained from the expressed product by passing through a metal-chelating affinity chromatographic (MCAC) column. The study also verified that our purified rNS1 protein retained its antigenicity. High-level production of the rNS1 protein up to 70 mg/l indicates that P. pastoris is an efficient expression system for dengue virus full-length NS1 glycoprotein.

The recombinant nonstructural polyprotein NS1 of porcine parvovirus (PPV) as diagnostic antigen in ELISA to differentiate infected from vaccinated pigs

To differentiate pigs infected with porcine parvovirus (PPV) from those vaccinated with inactivated whole-virus vaccine, an enzyme-linked immunosorbent assay (ELISA) based on detection of a nonstructural polyprotein 1 (NS1) was developed. A threshold of 0.23 optical density units was established and the assay had high specificity (100), sensitivity (88), accuracy (90) and positive predictive value (100) using haemagglutination inhibition as the standard method. A reproducibility test revealed that the coefficients of variation of sera within-plates and between-run were less than 10%. The assay showed no cross-reactivity with antibodies to porcine reproductive respiratory syndrome virus, pseudorabies virus, foot and mouth disease virus, Actinobacillus pleuropneumoniae, Toxoplasma or Chlamydia. Sera obtained from pigs infected with PPV reacted with recombinant NS1 protein in the ELISA. Sera from pigs vaccinated with inactivated whole virus did not recognize this protein in the ELISA. In contrast, antibodies against PPV whole virus were present in both PPV-infected and vaccinated animals. Serum conversion against NS1 was first detected 10 days after infection and NS1-specific antibodies were detectable up to half a year post infection. In conclusion, the PPV-NS1 ELISA can differentiate PPV-infected versus inactivated PPV-vaccinated pigs and could be applied in disease diagnosis and surveillance.

NS1 protein secretion during the acute phase of West Nile virus infection

The West Nile virus (WNV) nonstructural protein NS1 is a protein of unknown function that is found within, associated with, and secreted from infected cells. We systematically investigated the kinetics of NS1 secretion in vitro and in vivo to determine the potential use of this protein as a diagnostic marker and to analyze NS1 secretion in relation to the infection cycle. A sensitive antigen capture enzyme-linked immunosorbent assay (ELISA) for detection of WNV NS1 (polyclonal-ACE) was developed, as well as a capture ELISA for the specific detection of NS1 multimers (4G4-ACE). The 4G4-ACE detected native NS1 antigens at high sensitivity, whereas the polyclonal-ACE had a higher specificity for recombinant forms of the protein. Applying these assays we found that only a small fraction of intracellular NS1 is secreted and that secretion of NS1 in tissue culture is delayed compared to the release of virus particles. In experimentally infected hamsters, NS1 was detected in the serum between days 3 and 8 postinfection, peaking on day 5, the day prior to the onset of clinical disease; immunoglobulin M (IgM) antibodies were detected at low levels on day 5 postinfection. Although real-time PCR gave the earliest indication of infection (day 1), the diagnostic performance of the 4G4-ACE was comparable to that of real-time PCR during the time period when NS1 was secreted. Moreover, the 4G4-ACE was found to be superior in performance to both the IgM and plaque assays during this time period, suggesting that NS1 is a viable early diagnostic marker of WNV infection.

Immunoglobulin M enzyme-linked immunosorbent assay using recombinant polypeptides for diagnosis of dengue

We demonstrate that a mixture of four recombinant dengue virus E polypeptides corresponding to the N-terminal region of the envelope protein from all serotypes substitutes for standard antigens in two immunoglobulin M enzyme-linked immunosorbent assay formats with 100% concordance, making these polypeptides a useful and accessible reagent for serological diagnosis of dengue in endemic countries.

Viral and host factors in early hepatitis C virus infection

Since 1980, the Transfusion-transmitted Viruses Study (TTVS) (1974-1980) has continued to maintain its computerized database and stored sera to enable ongoing study of new transfusion events since the 1970s. Most recently, we have used this resource to study parameters of acute hepatitis C virus (HCV) infection among 94 donor-recipient pairs in which there was transmission. In addition, frequent recipient observations permitted further characterization of the early phase of the infection's course. Donor RNA load ranged from 3.7 to 3,160,000 IU/mL. Onset of recipient viremia was judged from a total of 67 sera collected during the 4th through 8th days posttransfusion; only 2 of the 67 sera were still RNA nonreactive by that time. The recipients' latent periods to an alanine aminotransferase (ALT) elevation of > or =90 IU/L ranged from 6 to 112 days (median, 46 days) and was shorter with higher donor RNA levels. Descriptors of the recipient's illness showed several strongly positive and negative correlations. The latent period tended to be shorter in the 37% of cases that were clinically overt. Attributes of donors with genotypes 1 and non-1 and subtypes 1a and 1b did not differ significantly. Recipients with genotype 1 strains had shorter latent intervals than non-1 strains. On multivariate analysis, latent period was significantly associated (negatively) only with the highest ALT level during the first 120 days of follow-up (P = .014). In conclusion, host factors are more important determinants of acute HCV infection dynamics than virus-associated factors.

A custom-designed recombinant multiepitope protein as a dengue diagnostic reagent

Currently, dengue fever is the most important re-emerging mosquito-borne viral disease, with the major proportion of the target population residing in the developing countries of the world. In endemic areas, potentially fatal secondary dengue infections, characterized by high anti-dengue IgG antibody titers, are most common. Most currently available commercial dengue diagnostic kits rely on the use of whole virus antigens and are consequently associated with false positives due to serologic cross-reactivity, high cost of antigen production, and biohazard risk. This has prompted the need to develop an alternate antigen to replace the whole virus antigen in diagnostic tests. We have designed and expressed a novel recombinant protein antigen by assembling key immunodominant linear IgG-specific dengue virus epitopes, chosen on the basis of pepscan analysis, phage display, and computer predictions. The recombinant dengue multiepitope protein was expressed to high levels in Escherichia coli, purified in a single step, yielding >25 mg pure protein per liter culture. We developed an in-house enzyme-linked immunosorbent assay (ELISA) to detect anti-dengue antibodies in a panel of 20 patient sera using the purified recombinant dengue multiepitope protein as the capture antigen. The ELISA results were in excellent agreement with those obtained using a commercially available diagnostic test, Dengue Duo rapid strip test from PanBio, Australia. The high epitope density, careful choice of epitopes, and the use of E. coli system for expression, coupled to simple purification, jointly have the potential to lead to the development of an inexpensive diagnostic test with a high degree of sensitivity and specificity.

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.

Development and evaluation of an enzyme-linked immunosorbent assay for quantifying antibodies to Japanese encephalitis virus nonstructural 1 protein to detect subclinical infections in vaccinated horses

Antibodies to Japanese encephalitis virus (JEV) nonstructural 1 (NS1) protein constitute a marker of natural JEV infection among populations vaccinated with inactivated JE vaccine. In Japan, with few recent human JE cases, the natural infection rate is critical to evaluate the necessity of continuing JE vaccination. A sensitive immunochemical staining method for detecting NS1 antibodies in individuals naturally and subclinically infected with JEV was previously established. Here, an enzyme-linked immunosorbent assay (ELISA) to detect NS1 antibodies in equine sera was developed and evaluated as an alternative to immunostaining. By this method, NS1 antigens contained in culture fluids from cells stably transfected with the NS1 and NS2A genes were captured by a rabbit anti-NS1 polyclonal antibody. Three nanograms per well of NS1 antigen, corresponding to 1:2 to 1:8 dilutions of the culture fluid, was sufficient for testing. ELISA values were obtained by a single-serum dilution (1:100), which correlated with ELISA titers obtained by an endpoint method. Under a tentative cutoff value (0.122) statistically calculated from NS1 antibody levels of horses in an area where JEV is not endemic, a high level of qualitative agreement (85.3%) was obtained between the ELISA and immunostaining methods. A significant correlation coefficient (0.799; P < 0.001) was also obtained between the two methods. Three experimentally infected horses seroconverted no later than 13 to 23 days postinfection, whereas 4 field horses infected during an epizootic remained positive for NS1 antibodies for at least 40 weeks. Our results indicate that the ELISA used here was sufficiently sensitive to detect subclinical infections in vaccinated equine populations.

Potential application of nonstructural protein NS1 serotype-specific immunoglobulin G enzyme-linked immunosorbent assay in the seroepidemiologic study of dengue virus infection: correlation of results with those of the plaque reduction neutralization test

An NS1 serotype-specific indirect enzyme-linked immunosorbent assay (ELISA) was developed to differentiate primary and secondary dengue virus infections and serotypes of primary dengue virus infection. For this report, we carried out retrospective seroepidemiologic studies on serum samples collected from residents of Liuchiu Hsiang, Pingtung County, an isolated island in southern Taiwan during 1997-1998. The results demonstrated that good correlation existed between dengue virus NS1 serotype-specific immunoglobulin G (IgG) ELISA and dengue virus plaque reduction neutralization test (PRNT). Our data suggested that NS1 serotype-specific IgG ELISA could replace PRNT for seroepidemiologic studies to differentiate Japanese encephalitis and dengue virus infections and for dengue virus serotyping.