Dengue-2 virus infection of human mononuclear cell lines and establishment of persistent infections

DENV and ZIKV infection: Species specificity and broad cell tropism

Nearly one-third of countries worldwide have reported cases of Dengue virus (DENV) and Zika virus (ZIKV) infections, highlighting the significant threat these viruses pose to global public health. As members of the Flavivirus genus within the Flaviviridae family, DENV and ZIKV have demonstrated the ability to infect a wide range of cell lines from multiple species in vitro. However, the range of susceptible animal models is notably limited, and field studies indicate that their capacity to infect host organisms is highly restricted, with a very narrow range of target cells in vivo. The virus's ability to hijack host cellular machinery plays a crucial role in determining its cellular and species specificity. In this review, we examine how DENV and ZIKV exploit host cells to facilitate their replication, offering new insights that could inform the development of antiviral drugs and therapeutic targets.

Possible effects of ancestry-related oxysterol-binding protein-like 10 genetic polymorphisms on dengue virus replication and anti-dengue immune response

Purpose

Oxysterol-binding protein-like 10 (OSBPL10) gene has been associated with reduced susceptibility to severe dengue in individuals of African descent. The aim of this study was to determine the possible effect of OSBPL10 on dengue virus (DENV) replication as well as the impact of African and European haplotypes of six OSBPL10 small nuclear polymorphisms (SNPs) on dengue multiplication and innate immune response.

Methods

We conducted gene knockdown experiments targeting OSBPL10 in THP-1 and Huh-7D12 cell lines, followed by a DENV-2 replication assay. Extracellular viral load was determined using qRT-PCR. To investigate the impact of SNPs haplotypes on viral replication and gene expression we cultured peripheral blood mononuclear cells (PBMC) from individuals with homozygous African and European haplotypes of OSBPL10 with DENV-2. Individual genotyping was performed using High Resolution Melt (HRM) analysis. The level of viral replication was assessed through plaque assay, while RT-PCR was employed to determine the expression levels of RXR-α, IFN-γ, IL-10 and IL-8 genes.

Results

In vitro OSBPL10 knockdown significantly reduced DENV-2 replication. Individuals carrying European haplotypes showed higher DENV titers along with elevated levels of RXR-α and IL-8 mRNA compared to those carrying African haplotypes, who exhibited lower viral loads alongside increased IFN-γ and IL-10 expression.

Conclusions

Our findings further explore the role of OSBPL10 in DENV multiplication, immune response to infection. The European haplotypes of OSBPL10 appear to increase DENV replication and promote RXR-α and IL-8 mRNA expression which correlates with the suppressive effect of these mediators on type I IFN, promoting viral replication and a deficient antiviral response. In contrast, the African haplotype showed a reduction in DENV replication and enhanced IFN-γ and IL-10 mRNA expression, which could be related to the better management of dengue infection and the low frequency of severe disease in this ethnic groupe.

Persistence of dengue serotype 2 viral RNA in blood cells of a returned traveler with dengue fever

Dengue virus (DENV) is one of the most significant vector-borne pathogens worldwide. In this report, we describe clinical features and laboratory detection of dengue in a 45-year-old traveler to Nicaragua on return home to the United States in 2019. Clinical presentation was mild, with rash, headache, and fatigue, with only low-grade transient fever. Infection dynamics were documented by serology and PCR of serially collected body fluids. DENV serotype 2 was detected in whole blood 1 day after symptoms emerged, with viral RNA isolated to the red cell fraction, and remained detectable through day 89. DENV-2 RNA was detected in serum only on day 4, and IgM was undetectable on day 4 but evident by day 13. Viral RNA was also detected in urine. This report of DENV-2 RNA persistence in blood cells but only transient appearance in serum, supports the potential diagnostic value of whole blood over serum for PCR and opportunity of an expanded testing window. Informed testing approaches can improve diagnostic accuracy and inform strategies that preserve individual and public health.

Macrophages derived from human induced pluripotent stem cells (iPSCs) serve as a high-fidelity cellular model for investigating HIV-1, dengue, and influenza viruses

Macrophages are important target cells for diverse viruses and thus represent a valuable system for studying virus biology. Isolation of primary human macrophages is done by culture of dissociated tissues or from differentiated blood monocytes, but these methods are both time consuming and result in low numbers of recovered macrophages. Here, we explore whether macrophages derived from human induced pluripotent stem cells (iPSCs)-which proliferate indefinitely and potentially provide unlimited starting material-could serve as a faithful model system for studying virus biology. Human iPSC-derived monocytes were differentiated into macrophages and then infected with HIV-1, dengue virus, or influenza virus as model human viruses. We show that iPSC-derived macrophages support the replication of these viruses with kinetics and phenotypes similar to human blood monocyte-derived macrophages. These iPSC-derived macrophages were virtually indistinguishable from human blood monocyte-derived macrophages based on surface marker expression (flow cytometry), transcriptomics (RNA sequencing), and chromatin accessibility profiling. iPSC lines were additionally generated from non-human primate (chimpanzee) fibroblasts. When challenged with dengue virus, human and chimpanzee iPSC-derived macrophages show differential susceptibility to infection, thus providing a valuable resource for studying the species-tropism of viruses. We also show that blood- and iPSC-derived macrophages both restrict influenza virus at a late stage of the virus lifecycle. Collectively, our results substantiate iPSC-derived macrophages as an alternative to blood monocyte-derived macrophages for the study of virus biology.

IMPORTANCE

Macrophages have complex relationships with viruses: while macrophages aid in the removal of pathogenic viruses from the body, macrophages are also manipulated by some viruses to serve as vessels for viral replication, dissemination, and long-term persistence. Here, we show that iPSC-derived macrophages are an excellent model that can be exploited in virology.

Clinical features and transmission risk analysis of dengue virus infections in Shenzhen, During 2014-2019

Dengue fever (DF) and dengue hemorrhagic fever (DHF) are among the most common tropical diseases affecting humans. To analyze the risk of clinical and transmission of DF/DHF in Shenzhen, the surveillance on patients of all-age patients with dengue virus (DENV) infections was conducted. Our findings revealed that the majority of DENV-infected patients are young to middle-aged males, and the development of the disease is accompanied by abnormal changes in the percentages of neutrophils, lymphocytes, and basophils. Demographic analysis revealed that these patients is concentrated in areas such as Futian District, which may be due to the higher mosquito density and temperature than that in other area. Subsequent, mosquito infection experiments confirmed that the effect of temperature shift on DENV proliferation and transmission. Not only that, constant temperatures can enhance the spread of DENV, even increase the risk of epidemic. Thus, the role of innate immune response should be highlighted in the prediction of severe severity of DENV-infected patients, and temperature should be taken into account in the prevention and control of DENV.

INTRODUCTION

Dengue fever (DF) and dengue hemorrhagic fever (DHF) are among the most common tropical diseases affecting humans, and which caused by the four dengue virus serotypes (DENV 1-4).

OBJECTIVES

To analyze the risk of clinical and transmission of DF/DHF in Shenzhen.

METHODS

The surveillance on patients of all-age patients with dengue virus (DENV) infections was conducted.

RESULTS

Our findings revealed that the majority of DENV-infected patients are young to middle-aged males, and the development of the disease is accompanied by abnormal changes in the percentages of neutrophils, lymphocytes, and basophils. Demographic analysis revealed that these patients is concentrated in areas such as Futian District, which may be due to the higher mosquito density and temperature than that in other area. Subsequent, mosquito infection experiments confirmed that the effect of temperature shift on DENV proliferation and transmission. Not only that, constant temperatures can enhance the spread of DENV, even increase the risk of epidemic.

CONCLUSION

1. Elevated levels of neutrophils, lymphocytes, basophils, and temperature are all significant risk factors for dengue transmission and pathogenesis; 2. Temperature increasing is associated with a higher risk of dengue transmission; 3. Fluctuations in temperature around 28 °C (28 ± 5 °C) would increase dengue transmission.

Infectivity and viability of dengue virus infected hepatocytes cocultured with peripheral blood mononuclear cells from a healthy subject

BACKGROUND Dengue virus (DENV) can infect and replicate in monocytes, resulting in antibody-dependent enhancement. The liver is the main target of DENV, and the infection mechanisms of DENV include direct cytopathic effects (CPEs) of the virus, mitochondrial dysfunction, and effect of cellular and humoral immune factors in the liver. This study was aimed to explore the infectivity of DENV and viability of human hepatocytes using Huh 7it-1 cells cocultured with peripheral blood mononuclear cells (PBMCs). METHODS Huh 7it-1 cells were infected with dengue virus serotype-2 (DENV-2) New Guinea C strain at multiplicity of infection of 0.5 and 1 FFU/cell, and cocultured in vitro with and without adherent PBMCs. The infectivity of DENV was assessed by immunoperoxidase staining. The viability of Huh 7it-1 cells was assessed using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT, a tetrazole) assay and trypan blue staining. Data were statistically analyzed by Shapiro–Wilk and analysis of variance for normality significances. RESULTS The result showed that addition of PBMCs to DENV-2 infected Huh 7it-1 cells decreased the infectivity of DENV (15–37%). DENV-2 infection decreased the viability of Huh 7it-1 cells (15.5–20.8%). Despite the decrease in infectivity of DENV, the addition of PBMCs increased the Huh 7it-1 cells viability (4.5–10.2%). CONCLUSIONS Addition of PBMCs to Huh 7it-1 cells that are infected with DENV-2 decreased the infectivity of DENV and increased Huh 7it-1 cells viability.

Assessment of Prolonged Dengue Virus Infection in Dermal Fibroblasts and Hair-Follicle Dermal Papilla Cells

Dengue virus (DENV)-mediated hair loss is one of the post-dengue fatigue syndromes and its pathophysiology remains unknown. Whether long-term or persistent infection with DENV in the scalp results in hair loss is unclear. In this study, we cultured human dermal fibroblasts (WS1 cells) and primary human hair-follicle dermal papilla cells (HFDPCs) in the long term with DENV-2 infection. The production of virion, the expression of inflammatory and anti-virus genes, and their signaling transduction activity in the infected cells were analyzed. DENV-2 NS3 protein and DENV-2 5′ UTR RNA were detected in fibroblasts and HFDPCs that were subjected to long-term infection with DENV-2 for 33 days. A significant amount of DENV-2 virion was produced by both WS1 cells and HFDPCs in the first two days of acute infection. The virion was also detected in WS1 cells that were infected in the long term, but HFDPCs failed to produce DENV-2 after long-term culture. Type I and type III interferons, and inflammatory cytokines were highly expressed in the acute phase of DENV infection in HFPDC and WS1 cells. However, in the long-term cultured cells, modest levels of anti-viral protein genes were expressed and we observed reduced signaling activity, which was correlated with the level of virus production changes. Long-term infection of DENV-2 downregulated the expression of hair growth regulatory factors, such as Rip1, Wnt1, and Wnt4. This in vitro study shows that the long-term infection with DENV-2 in dermal fibroblasts and dermal papilla cells may be involved with the prolonged-DENV-infection-mediated hair loss of post-dengue fatigue syndrome. However, direct evidence for viral replication in the human hair of a dengue victim or animal infection model is required.

Insight into the Tropism of Dengue Virus in Humans

In tropical and subtropical zones, arboviruses are among the major threats to human life, affecting a large number of populations with serious diseases. Worldwide, over three hundred million people are infected with dengue virus (DENV) every year as per the World Health Organization (WHO). DENV-mediated disease severity ranges from a mild fever to hemorrhagic fever and shock syndrome. Patients suffering from severe infection might experience multi-organ failure, cardiomyopathy and even encephalopathy, further complicating the disease pathogenesis. In life-threatening cases, DENV has been reported to affect almost all organs of the human body. In this review, we discuss the organ tropism of DENV in humans in depth as detected in various autopsy studies. Keeping in mind the fact that there is currently no DENV-specific antiviral, it is of utmost importance to achieve a vivid picture of the susceptible cells in humans which might help in designing antivirals against DENV, especially targeting those tissues in which infection might lead to life-threatening conditions.

Construction of a novel tetravalent dengue vaccine with a Salmonella Typhimurium bacterial ghost and evaluation of its immunogenicity and protective efficacy using a murine model

Efforts to develop a safe, effective, and affordable dengue vaccine have focused on providing simultaneous immunity against all four serotypes of the dengue virus (DENV). In the current study, Salmonella Typhimurium (ST) lysed by gene E activation was genetically constructed to deliver the envelope protein domain III (EDIII) of all four serotypes of DENV using a foreign antigen delivery and expression vector, pJHL184. Each DENV-EDIII protein expressed in the constructed strain was validated by immunoblot analysis. To assess the immunogenicity and protective efficacy of the constructs against dengue infection, BALB/c mice were injected once orally with either the individual ST-EDIII constructs or a mix of all four ST-EDIII constructs followed by intramuscular administration of the purified EDIII protein. Significantly elevated titers of EDIII-specific IgG, IgG1, and IgG2a were observed in the immunized mice (P < 0.01). Furthermore, lymphocyte proliferative activity and CD3⁺CD4⁺ T-cell subpopulations increased significantly in vitro in re-pulsed splenic T cells compared with those from non-immunized mice. In addition, a lower viral load was detected in the BG-EDIII vaccinated group after challenge with DENV-infected K562 cells. Collectively, the results demonstrate that DENV-EDIII expressed in the inactivated ST strain could induce robust humoral and cell-mediated immunity specific to the target antigen and could provide significant protective potential.

Hijacking the Host Immune Cells by Dengue Virus: Molecular Interplay of Receptors and Dengue Virus Envelope

Dengue virus (DENV) is one of the lethal pathogens in the hot climatic regions of the world and has been extensively studied to decipher its mechanism of pathogenesis and the missing links of its life cycle. With respect to the entry of DENV, multiple receptors have been recognized in different cells of the human body. However, scientists still argue whether these identified receptors are the exclusive entry mediators for the virus. Adding to the complexity, DENV has been reported to be infecting multiple organ types in its human host. Also, more than one receptor in a particular cell has been discerned to take part in mediating the ingress of DENV. In this review, we aim to discuss the different cells of the human immune system that support DENV infection and their corresponding receptors that DENV deploy to gain access to the cells.

Human T Lymphocytes Are Permissive for Dengue Virus Replication

Dengue virus (DV) infection can cause either a self-limiting flu-like disease or a threatening hemorrhage that may evolve to shock and death. A variety of cell types, such as dendritic cells, monocytes, and B cells, can be infected by DV. However, despite the role of T lymphocytes in the control of DV replication, there remains a paucity of information on possible DV-T cell interactions during the disease course. In the present study, we have demonstrated that primary human naive CD4⁺ and CD8⁺ T cells are permissive for DV infection. Importantly, both T cell subtypes support viral replication and secrete viable virus particles. DV infection triggers the activation of both CD4⁺ and CD8⁺ T lymphocytes, but preactivation of T cells reduces the susceptibility of T cells to DV infection. Interestingly, the cytotoxicity-inducing protein granzyme A is highly secreted by human CD4⁺ but not CD8⁺ T cells after exposure to DV in vitro Additionally, using annexin V and polycaspase assays, we have demonstrated that T lymphocytes, in contrast to monocytes, are resistant to DV-induced apoptosis. Strikingly, both CD4⁺ and CD8⁺ T cells were found to be infected with DV in acutely infected dengue patients. Together, these results show that T cells are permissive for DV infection in vitro and in vivo, suggesting that this cell population may be a viral reservoir during the acute phase of the disease.IMPORTANCE Infection by dengue virus (DV) causes a flu-like disease that can evolve to severe hemorrhaging and death. T lymphocytes are important cells that regulate antibody secretion by B cells and trigger the death of infected cells. However, little is known about the direct interaction between DV and T lymphocytes. Here, we show that T lymphocytes from healthy donors are susceptible to infection by DV, leading to cell activation. Additionally, T cells seem to be resistant to DV-induced apoptosis, suggesting a potential role as a viral reservoir in humans. Finally, we show that both CD4⁺ and CD8⁺ T lymphocytes from acutely infected DV patients are infected by DV. Our results raise new questions about DV pathogenesis and vaccine development.

Susceptibility and initial immune response of Tupaia belangeri cells to dengue virus infection

Dengue is an emerging disease of great public health significance worldwide. The lack of a suitable infection model has hampered dengue virus (DENV) pathogenesis study, and developing a suitable small animal model has been a long-standing challenge. The aim of this study was to develop a feasible experimental model of DENV infection using Tupaia belangeri. The susceptibility of tupaia to DENV infection and characteristics of its innate immune response were examined in vitro. We found that tupaia fibroblast cells support replication of DENV serotypes 1-4 with a linear increase in viral load 24-96h post-infection in both cells and culture supernatants. DENV-2 resulted in the highest viral growth among all serotypes. To characterize the innate immune response in tupaia cells during the early phase of DENV infection, we first evaluated the evolutionary relationship between tupaia Toll-like receptors (TLR1-9) and those of other mammalian species. Phylogenetic analysis showed that tupaia TLRs are evolutionarily much closer to human than they are to rodent. We next established an innate immune response measurement system by assessing the mRNA expression of TLR1-9 and four cytokines in DENV-infected tupaia cells. All serotypes induced the upregulation of TLR8 mRNA expression in infected tupaia cells. Silencing of TLR8 led to an increase in viral replication, indicating the existence of antiviral response through TLR8 on DENV infection. Although upregulation of IFN-β and IL-6 expression was only observed in DENV-1 infected cells and a significant suppression of TNF-α was observed in DENV-2 infected cells alone, IL-8 was upregulated in all DENV-1-4. Thus, this study demonstrates for the first time the susceptibility of tupaia cells to DENV infections and the role of TLR8 in the anti-viral response of tupaia cells to DENV. These findings demonstrate the potential utility of tupaia as a model for DENV research in the future.

Induction of Osteopontin by Dengue Virus-3 Infection in THP-1 Cells: Inhibition of the Synthesis by Brefelamide and Its Derivative

Osteopontin (OPN) is a multifunctional matricellular protein produced by a broad range of cells including osteoclasts, macrophages, T cells, endothelial cells, and vascular smooth muscle cells. OPN modulates various physiological and pathological events such as inflammation, wound healing, and bone formation and remodeling. Dengue virus (DENV) infection causes an increase in plasma OPN levels, which is correlated with the severity of symptoms and coagulation abnormalities. DENV infection also induces OPN gene expression in human macrophages. This study investigated the inhibitory effects of brefelamide and its methyl ether derivative on DENV-3 by measuring changes in OPN levels in human THP-1 and 293T cell lines infected at different multiplicities of infection and post-infection time points. OPN mRNA expression and viral RNA were detected by reverse transcriptase quantitative real-time PCR, whereas protein level was determined by enzyme-linked immunosorbent assay. We found that viral copy number was higher in 293T than in THP-1 cells. However, THP-1 constitutively expressed higher levels of OPN mRNA and protein, which were enhanced by DENV-3 infection. Brefelamide and its derivative suppressed OPN production in DENV-3 infected THP-1 cells; the effective doses of these compounds had no effect on uninfected cells, indicating low cytotoxicity. These results suggest that brefelamide and its methyl ether derivative have therapeutic effects in preventing inflammation, coagulopathy, and fibrinolysis caused by OPN upregulation induced by DENV-3 infection.

Challenges for the formulation of a universal vaccine against dengue

Dengue is rapidly becoming a disease of an escalating global public health concern. The disease is a vector-borne disease, transmitted by the bite of an Aedes spp. mosquito. Dynamic clinical manifestations, ranging from asymptomatic, flu-like febrile illness, dengue fever (DF) to dengue hemorrhagic fever (DHF) with or without dengue shock syndrome (DSS), make the disease one of the most challenging to diagnose and treat. DF is a self-limited illness, while DHF/DSS, characterized by plasma leakage resulting from an increased vascular permeability, can have severe consequences, including death. The pathogenesis of dengue virus infection remains poorly understood, mainly due to the lack of a suitable animal model that can recapitulate the cardinal features of human dengue diseases. Currently, there is no specific treatment or antiviral therapy available for dengue virus infection and supportive care with vigilant monitoring is the principle course of treatment. Since vector control programs have been largely unsuccessful in preventing outbreaks, vaccination seems to be the most viable option for prevention. There are four dengue viral serotypes and each one of them is capable of causing severe dengue. Although immunity induced by infection by one serotype is effective in protection against the homologous viral serotype, it only has a transient protective effect against infection with the other three serotypes. The meager cross protective immunity generated wanes over time and may even induce a harmful effect at the time of subsequent secondary infection. Thus, it is imperative to have a vaccine that can elicit equal and long-lasting immunity to all four serotypes simultaneously. Numerous tetravalent vaccines are currently either in the pipeline for clinical trials or under development. For those frontrunner tetravalent vaccines in clinical trials, despite good safety and immunogenicity profiles registered, issues such as imbalanced immune responses between serotypes and questions with regard to whether the optimum formulation have been identified remain unresolved. This review centers on these issues and offers strategies that may improve the tetravalent vaccine formulation.

Detection and sequencing of defective viral genomes in C6/36 cells persistently infected with dengue virus 2

Dengue virus is the most important arbovirus that affects humans, and it can establish persistent infections, especially in insect-derived cell cultures. Defective viral genomes have been implicated in the establishment and maintenance of persistent infections with several flaviviruses; however, there exists almost no information concerning defective dengue virus genomes. Here, we report the detection of defective dengue 2 virus genomes in persistently infected mosquito C6/36 cells. The defective viral genomes were detected at a low ratio compared with the wild-type genome. Deletions of approximately 147 residues (222-368) were found in the E protein, and these mainly affected domain III (73 %) of the protein; deletions of approximately 153 residues (4-156) and 228 residues (597-825) were found in the methyltransferase and polymerase domains, respectively, of the NS5 protein. The truncated versions of NS5 could be detected by western blot only in the protein extracts derived from persistently infected cells.

Phenotypic and genotypic characterization of dengue virus isolates differentiates dengue fever and dengue hemorrhagic fever from dengue shock syndrome

Dengue viruses (DENV) cause 50-100 million cases of acute febrile disease every year, including 500,000 reported cases of dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS). Viral factors have been proposed to influence the severity of the disease, but markers of virulence have never been identified on DENV. Three DENV serotype-1 isolates from the 2007 epidemic in Cambodia that are derived from patients experiencing the various clinical forms of dengue were characterized both phenotypically and genetically. Phenotypic characteristics in vitro, based on replication kinetics in different cell lines and apoptosis response, grouped isolates from DF and DHF patients together, whereas the virus isolate from a DSS patient showed unique features: a lower level of replication in mammalian cells and extensive apoptosis in mosquito cells. Genomic comparison of viruses revealed six unique amino acid residues in the membrane, envelope, and in non-structural genes in the virus isolated from the DSS patient.

DNA vaccines against dengue virus type 2 based on truncate envelope protein or its domain III

Two DNA vaccines were constructed encoding the ectodomain (domains I, II and III) of the DENV2 envelope protein (pE1D2) or only its domain III (pE2D2), fused to the human tissue plasminogen activator signal peptide (t-PA). The expression and secretion of recombinant proteins was confirmed in vitro in BHK cells transfected with the two plasmids, detected by immunofluorescence or immunoprecipitation of metabolically labeled gene products, using polyclonal and monoclonal antibodies against DENV2. Besides, results reveal that the ectodomain of the E protein can be efficiently expressed in vivo, in a mammalian system, without the prM protein that is hypothesized to act as a chaperonin during dengue infection. Balb/c mice were immunized with the DNA vaccines and challenged with a lethal dose of DENV2. All pE1D2-vaccinated mice survived challenge, while 45% of animals immunized with the pE2D2 died after infection. Furthermore, only 10% of pE1D2-immunized mice presented some clinical signs of infection after challenge, whereas most of animals inoculated with the pE2D2 showed effects of the disease with high morbidity degrees. Levels of neutralizing antibodies were significantly higher in pE1D2-vaccinated mice than in pE2D2-immunized animals, also suggesting that the pE1D2 vaccine was more protective than the pE2D2.

Dengue virus tropism in humanized mice recapitulates human dengue fever

Animal models of dengue virus disease have been very difficult to develop because of the virus' specificity for infection and replication in certain human cells. We developed a model of dengue fever in immunodeficient mice transplanted with human stem cells from umbilical cord blood. These mice show measurable signs of dengue disease as in humans (fever, viremia, erythema and thrombocytopenia), and after infection with the most virulent strain of dengue serotype 2, humanized mice showed infection in human cells in bone marrow, spleen and blood. Cytokines and chemokines were secreted by these human cells into the mouse bloodstream. We demonstrated that the pathology of dengue virus infection in these mice follows that reported in human patients, making this the first valid and relevant model for studying dengue fever pathogenesis in humans.

Cells in dengue virus infection in vivo

Dengue has been recognized as one of the most important vector-borne emerging infectious diseases globally. Though dengue normally causes a self-limiting infection, some patients may develop a life-threatening illness, dengue hemorrhagic fever (DHF)/dengue shock syndrome (DSS). The reason why DHF/DSS occurs in certain individuals is unclear. Studies in the endemic regions suggest that the preexisting antibodies are a risk factor for DHF/DSS. Viremia and thrombocytopenia are the key clinical features of dengue virus infection in patients. The amounts of virus circulating in patients are highly correlated with severe dengue disease, DHF/DSS. Also, the disturbance, mainly a transient depression, of hematological cells is a critical clinical finding in acute dengue patients. However, the cells responsible for the dengue viremia are unresolved in spite of the intensive efforts been made. Dengue virus appears to replicate and proliferate in many adapted cell lines, but these in vitro properties are extremely difficult to be reproduced in primary cells or in vivo. This paper summarizes reports on the permissive cells in vitro and in vivo and suggests a hematological cell lineage for dengue virus infection in vivo, with the hope that a new focus will shed light on further understanding of the complexities of dengue disease.

Dengue virus infection and virus-specific HLA-A2 restricted immune responses in humanized NOD-scid IL2rgammanull mice

Background

The lack of a suitable animal model to study viral and immunological mechanisms of human dengue disease has been a deterrent to dengue research.

Methodology/Principal Findings

We sought to establish an animal model for dengue virus (DENV) infection and immunity using non-obese diabetic/severe combined immunodeficiency interleukin-2 receptor γ-chain knockout (NOD-scid IL2rγ^null) mice engrafted with human hematopoietic stem cells. Human CD45⁺ cells in the bone marrow of engrafted mice were susceptible to in vitro infection using low passage clinical and established strains of DENV. Engrafted mice were infected with DENV type 2 by different routes and at multiple time points post infection, we detected DENV antigen and RNA in the sera, bone marrow, spleen and liver of infected engrafted mice. Anti-dengue IgM antibodies directed against the envelope protein of DENV peaked in the sera of mice at 1 week post infection. Human T cells that developed following engraftment of HLA-A2 transgenic NOD-scid IL2rγ^null mice with HLA-A2⁺ human cord blood hematopoietic stem cells, were able to secrete IFN-γ, IL-2 and TNF-α in response to stimulation with three previously identified A2 restricted dengue peptides NS4b 2353_(111–119), NS4b 2423_(181–189), and NS4a 2148_(56–64).

Conclusions/Significance

This is the first study to demonstrate infection of human cells and functional DENV-specific T cell responses in DENV-infected humanized mice. Overall, these mice should be a valuable tool to study the role of prior immunity on subsequent DENV infections.

Dengue virus infection induces upregulation of GRP78, which acts to chaperone viral antigen production

Dengue virus (DENV) pathogenesis is related to the host responses to viral infection within target cells, and therefore, this study assessed intracellular changes in host proteins following DENV infection. Two-dimensional gel electrophoresis and mass spectrometry identified upregulation of the host endoplasmic reticulum (ER) chaperone GRP78 in K562 cells following DENV infection, in the absence of virus-induced cell death. Upregulation of GRP78 in DENV-infected cells was confirmed by immunostaining and confocal microscopy and by Western blot analysis and was also observed in DENV-infected primary monocyte-derived macrophages, a natural target cell type for DENV infection. GRP78 was upregulated in both DENV antigen-positive and -negative cells in the DENV-infected culture, suggesting a bystander effect, with the highest GRP78 levels coincident with high-level DENV antigen production and infectious-virus release. Transfection of target cells to express GRP78 prior to DENV challenge did not affect subsequent DENV infection, but cleavage of GRP78 with the SubAB toxin, during an established DENV infection, yielded a 10- to 100-fold decrease in infectious-virus release, loss of intracellular DENV particles, and a dramatic decrease in intracellular DENV antigen. However, DENV RNA levels were unchanged, indicating normal DENV RNA replication but altered DENV antigen levels in the absence of GRP78. Thus, GRP78 is upregulated by DENV infection and is necessary for DENV antigen production and/or accumulation. This may be a common requirement for viruses such as flaviviruses that depend heavily on the ER for coordinated protein production and processing.

Molecular profiling of T-helper immune genes during dengue virus infection

In this study, we provide a comprehensive molecular profiling of the involvement of T- helper (Th) genes during dengue virus infection of different cell types. The Th gene profiles of three human cell types (monocytes, T-cells and hepatocytes) were analyzed simultaneously via array-based RT-PCR upon infection with dengue virus. Differential regulation of 41 Th genes was identified and of which 20 of those genes may contribute to immuno-pathogenesis of dengue virus infection by regulating inflammation, thrombocytopenia and vascular permeability. Among the strongly up-regulated genes were the RANTES, CC-CKR3, IRF4, CLEC2C, IL-6 and TLR6, which are potent inducer of inflammation and vascular permeability. Profiling genes obtained from this study may serve as potential biomarkers and the modulation of Th immune responses during dengue virus infection has important implications in disease outcome.

Cellular and molecular basis of antibody-dependent enhancement in human dengue pathogenesis

Dengue fever is gaining increased attention as a major global health problem. It occurs annually in 50–100 million people in more than 100 countries, and places half a million people at risk of life-threatening diseases: dengue hemorrhagic fever and dengue shock syndrome (DHF/DSS). The pathogenic mechanisms causing DHF/DSS are not clearly understood. This article reviews cellular and molecular mechanisms that might be responsible for the initiation of the pathogenic processes, including hypotheses for DHF/DSS, dengue-permissive target cells, putative dengue receptors, neutralizing and enhancing antibodies to dengue virus, mechanisms of vascular plasma leakage, innate immune response in dengue infection and antibody-dependent enhancement of dengue infection. While reviewing the literature, the article also gives the author’s opinion on perceived areas of importance for future research in human dengue pathogenesis.

Identification of dengue virus binding proteins using affinity chromatography

Several studies have identified putative dengue virus receptors using virus overlay protein binding assays (VOPBA) with some apparent success. Given that this technique relies upon the use of electrophoresis of proteins through polyacrylamide gels with varying amounts of protein denaturation, the physiological relevance of the proteins isolated is open to question. To address this issue a Sepharose 4B-dengue virus serotype 2-affinity column was constructed to selectively bind dengue virus binding proteins from HepG2 (liver) cell membrane preparations. Results show that GRP78, but not the 37/67 kDa high affinity laminin receptor, was specifically bound by the column. This result is consistent with earlier work and shows that while affinity chromatography may provide a useful adjunct to VOPBA based studies particularly in cases where proteins maybe sensitive to denaturation, proteins isolated by VOPBA can be physiologically relevant.

Peripheral blood mononuclear cells increase the permeability of dengue virus-infected endothelial cells in association with downregulation of vascular endothelial cadherin

Plasma leakage is one of the characteristic features of dengue haemorrhagic fever. The interaction among peripheral blood mononuclear cells (PBMCs), dengue virus and endothelial cells was analysed in vitro. Human umbilical vein endothelial cells (HUVECs) were infected with dengue-2 virus (DV-2) at an m.o.i. of 0.5 p.f.u. per cell. PBMCs were added to DV-2-infected HUVECs, and transendothelial electrical resistance (TEER) and transalbumin permeability were assessed. Dengue virus infection at an m.o.i. of 0.5 p.f.u. per cell alone did not decrease the TEER, but addition of PBMCs decreased the TEER, increased the albumin permeability and induced morphological changes of HUVECs. The extent of the decrease was more profound with adherent PBMCs than with non-adherent PBMCs. The expression of vascular endothelial cadherin (VE-cadherin) was examined using real-time RT-PCR and immunofluorescence. Addition of PBMCs to DV-2-infected HUVECs decreased the levels of mRNA transcripts and cell-surface expression of VE-cadherin. The results indicate that PBMCs increased the permeability of DV-2-infected HUVECs and that the increased permeability was concomitant with morphological change and the decrease in VE-cadherin expression. The results suggest that functional impairment of the DV-2-infected HUVEC monolayer was caused by interaction with PBMCs.

Monocytes, but not T or B cells, are the principal target cells for dengue virus (DV) infection among human peripheral blood mononuclear cells

A better understanding of the pathogenesis of dengue hemorrhagic fever and dengue shock syndrome requires the precise identification of dengue virus (DV) permissive target cells. To examine the relative DV permissiveness among cell subsets, we inoculated unfractionated human peripheral blood mononuclear cells with DV2-16681 in the presence or absence of pooled DV-immune human sera (PHS), and assessed infection with fluorescent dye labeled DV-specific monoclonal antibody and cell surface markers using flow cytometry. We found significantly higher levels of DV antigen staining on DV-infected than mock-infected primary monocytes (3.54 +/- 3.42% vs. 0.50 +/- 0.38%; P = 0.001). The magnitude of infection was markedly enhanced in the presence of highly diluted PHS (10.04 +/- 6.10% vs. 3.54 +/- 3.42%; P = 0.015). Under identical experimental conditions, primary T or B cells were not infected either with or without the addition of PHS (0.06 +/- 0.04% and 0.44 +/- 0.22% for T and B cells, respectively). Furthermore, depletion of CD14+ monocytes prior to DV inoculation abrogated the detection of infected cells, and the addition of monoclonal antibodies to either FcgammaRI (CD64) or FcgammaRII (CD32) led to a 50-70% reduction in antibody-dependent enhancement (ADE) of DV infection. Collectively, these results provide further support to the notion that primary monocytes and FcgammaRs expressed on these cells may be important in the initial steps of immune enhancement observed in some patients with natural DV infection. They also demonstrate that using modern experimental technology, DV infection, and neutralization and enhancement of DV infection can be easily assessed simultaneously in multiple cell types.

Characterization of retrovirus-based reporter viruses pseudotyped with the precursor membrane and envelope glycoproteins of four serotypes of dengue viruses

In this study, we successfully established retrovirus-based reporter viruses pseudotyped with the precursor membrane and envelope (PrM/E) proteins of each of the four serotypes of dengue viruses, which caused the most important arboviral diseases in this century. Co-sedimentation of the dengue E protein and HIV-1 core proteins by sucrose gradient analysis of the pseudotype reporter virus of dengue virus type 2, D2(HIVluc), and detection of HIV-1 core proteins by immunoprecipitation with anti-E monoclonal antibody suggested that dengue viral proteins were incorporated into the pseudotype viral particles. The infectivity in target cells, as assessed by the luciferase activity, can be inhibited by the lysosomotropic agents, suggesting a pH-dependent mechanism of entry. Amino acid substitutions of the leucine at position 107, a critical residue at the fusion loop of E protein, with lysine resulted in severe impairment in infectivity, suggesting that entry of the pseudotype reporter virus is mediated through the fusogenic properties of E protein. With more and more dengue viral sequences available from different outbreaks worldwide, this sensitive and convenient tool has the potential to facilitate molecular characterization of the PrM/E proteins of dengue field isolates.

Dengue virus (DV) replication in monocyte-derived macrophages is not affected by tumor necrosis factor alpha (TNF-alpha), and DV infection induces altered responsiveness to TNF-alpha stimulation

Tumor necrosis factor alpha (TNF-alpha) is believed to play a significant role in the pathogenesis of dengue virus (DV) infection, with elevated levels of TNF-alpha in the sera of DV-infected patients paralleling the severity of disease and TNF-alpha release being coincident with the peak of DV production from infected monocyte-derived macrophages (MDM) in vitro. Since macrophages are a primary cell target in vivo for DV infection, we investigated the potential antiviral role of TNF-alpha in regulating DV replication in MDM. While pretreatment of MDM with TNF-alpha had a minor inhibitory effect, addition of TNF-alpha to MDM with established DV infection had no effect on DV replication as measured by DV RNA levels or progeny virus production. Blocking endogenous TNF-alpha using short interfering RNA or inhibitory TNF-alpha antibodies also had no effect on infectious DV production or viral RNA synthesis. Together, these results demonstrate that DV replication in MDM is not affected by TNF-alpha. Additionally, normal cellular TNF-alpha signaling, measured by quantitation of TNF-alpha-induced stimulation of transcription from an NF-kappaB-responsive reporter plasmid or NF-kappaB protein nuclear translocation, was blocked in DV-infected MDM and Huh7 cells. Thus, DV replication in MDM is not affected by TNF-alpha, and infected cells do not respond normally to TNF-alpha stimulation. It is therefore unlikely that the increased production of TNF-alpha seen in DV infection directly effects DV clearance by reducing DV replication, and the ability of DV to alter TNF-alpha responsiveness highlights another example of viral subversion of cellular functions.

Dengue virus type 2: replication and tropisms in orally infected Aedes aegypti mosquitoes

BACKGROUND

To be transmitted by its mosquito vector, dengue virus (DENV) must infect midgut epithelial cells, replicate and disseminate into the hemocoel, and finally infect the salivary glands, which is essential for transmission. The extrinsic incubation period (EIP) is very relevant epidemiologically and is the time required from the ingestion of virus until it can be transmitted to the next vertebrate host. The EIP is conditioned by the kinetics and tropisms of virus replication in its vector. Here we document the virogenesis of DENV-2 in newly-colonized Aedes aegypti mosquitoes from Chetumal, Mexico in order to understand better the effect of vector-virus interactions on dengue transmission.

RESULTS

After ingestion of DENV-2, midgut infections in Chetumal mosquitoes were characterized by a peak in virus titers between 7 and 10 days post-infection (dpi). The amount of viral antigen and viral titers in the midgut then declined, but viral RNA levels remained stable. The presence of DENV-2 antigen in the trachea was positively correlated with virus dissemination from the midgut. DENV-2 antigen was found in salivary gland tissue in more than a third of mosquitoes at 4 dpi. Unlike in the midgut, the amount of viral antigen (as well as the percent of infected salivary glands) increased with time. DENV-2 antigen also accumulated and increased in neural tissue throughout the EIP. DENV-2 antigen was detected in multiple tissues of the vector, but unlike some other arboviruses, was not detected in muscle.

CONCLUSION

Our results suggest that the EIP of DENV-2 in its vector may be shorter that the previously reported and that the tracheal system may facilitate DENV-2 dissemination from the midgut. Mosquito organs (e.g. midgut, neural tissue, and salivary glands) differed in their response to DENV-2 infection.

MCP-1, a highly expressed chemokine in dengue haemorrhagic fever/dengue shock syndrome patients, may cause permeability change, possibly through reduced tight junctions of vascular endothelium cells

Vascular leakage, one hallmark of dengue haemorrhagic fever (DHF) and dengue shock syndrome, has been linked to the mediators secreted from cells in the circulatory system. In this study, extremely high expression levels of monocyte chemoattractant protein-1 (MCP-1) were found in the plasma of DHF patients compared with low MCP-1 expression levels in the plasma of enterovirus 71-infected patients. It was also found that MCP-1 expression was induced in dengue virus 2 (DV2)-infected monocytes and lymphocytes, but not in liver or endothelial cells. Exposing monolayers of human umbilical vein endothelial cells (HUVECs) to recombinant human MCP-1 (rhMCP-1) or to the culture supernatant of DV2-infected human monocytes increased the vascular permeability of the cells. MCP-1-neutralizing monoclonal antibody only partially prevented monolayer permeability change. Consistently, the distribution of the tight junction protein ZO-1 on the cellular membranes of HUVECs was disrupted by rhMCP-1 or by the conditioned medium of DV2-infected monocytes. In summary, it was found that the increased permeability and disrupted tight junctions of human vascular endothelium cells were effected through a mechanism partially dependent on MCP-1, which was secreted by DV2-infected monocytes and lymphocytes.

Dengue virus nonstructural protein NS5 induces interleukin-8 transcription and secretion

Elevated circulating levels of chemokines have been reported in patients with dengue fever and are proposed to contribute to the pathogenesis of dengue disease. To establish in vitro models for chemokine induction by dengue 2 virus (DEN2V), we studied a variety of human cell lines and primary cells. DEN2V infection of HepG2 and primary dendritic cells induced the production of interleukin-8 (IL-8), RANTES, MIP-1alpha, and MIP-1beta, whereas only IL-8 and RANTES were induced following dengue virus infection of HEK293 cells. Chemokine secretion was accompanied by an increase in steady-state mRNA levels. No chemokine induction was observed in HEK293 cells treated with poly(I:C) or alpha interferon, suggesting a direct effect of virus infection. To determine the mechanism(s) involved in the induction of chemokine production by DEN2V, individual dengue virus genes were cloned into plasmids and expressed in HEK293 cells. Transfection of a plasmid expressing NS5 or a dengue virus replicon induced IL-8 gene expression and secretion. RANTES expression was not induced under these conditions, however. Reporter assays showed that IL-8 induction by NS5 was principally through CAAT/enhancer binding protein, whereas DEN2V infection also induced NF-kappaB. These results indicate a role for the dengue virus NS5 protein in the induction of IL-8 by DEN2V infection. Recruitment and activation of potential target cells to sites of DEN2V replication by virus-induced chemokine production may contribute to viral replication as well as to the inflammatory components of dengue virus disease.

Virus Overlay Protein Binding Assay (VOPBA) Reveals Serotype Specific Heterogeneity of Dengue Virus Binding Proteins on HepG2 Human Liver Cells

OBJECTIVE

This study sought to investigate the presence of dengue virus binding proteins expressed on the surface of HepG2 cells and to determine if there were serotype specific differences in binding.

METHODS

HepG2 cell membrane proteins were extracted and separated by SDS-PAGE, transferred to nitrocellulose membranes and incubated with dengue virus serotypes 2, 3 and 4 under varying hybridization conditions. The positions of dengue virus binding proteins were established with a pan-specific anti-dengue virus monoclonal antibody.

RESULTS

Dengue virus binding proteins were seen at approximately 78-80, 90, 98, and 102 kD for dengue serotype 2, 90, 130 and 182 kD for dengue serotype 3, and 90 and 130 kD for dengue serotype 4. Binding of the serotypes 3 and 4 was significantly altered by the hybridization conditions, while serotype 2 was affected to a lesser extent.

CONCLUSIONS

The virus overlay assay used here provides further evidence that there is a serotype specific component regulating the entry of the dengue virus into cells. Given that several virus binding proteins are seen for each serotype, multiple proteins may be required to facilitate the entry of the virus into some cell types.

Apoptosis in dengue virus infected liver cell lines HepG2 and Hep3B

While both in vivo and in vitro evidence has suggested that liver cells undergo apoptosis in response to dengue virus infection, little is known about the mechanism of induction. Given that the p53 tumour suppressor gene is a key mediator of apoptosis, we sought to define the role of this gene in response to dengue virus infection. After infection, a p53 wild type liver cell line (HepG2) showed changes consistent with apoptosis including alterations of cell morphology, cellular detachment and DNA laddering. However, p53 was neither up-regulated, nor showed any evidence of complexing with dengue virus proteins as determined by immunoprecipitation. Infection of a p53 null liver cell line (Hep3B) also produced changes consistent with the induction of apoptosis. While the profile of the cells undergoing apoptosis in each cell line was similar as determined by flow cytometry, the absolute levels were markedly different with up to 90% of Hep3B cells undergoing apoptosis compared to only 20% of HepG2 cells at day 5 post infection. By day 7, all Hep3B infected cells were dead. In contrast, it proved possible to culture dengue virus infected HepG2 cells for 3 months. Viral progeny released from the p53 null cell line were nine-fold higher per attached cell than from the p53 wild type cell line. These results suggest that, while induction of apoptosis in liver cells is mediated by a non-p53 regulated pathway, p53 may play a role in restricting the level of viral progeny to below a critical level at which apoptosis is triggered.

Manipulation of cell surface macromolecules by flaviviruses

Cell surface macromolecules play a crucial role in the biology and pathobiology of flaviviruses, both as receptors for virus entry and as signaling molecules for cell–cell interactions in the processes of vascular permeability and inflammation. This review examines the cell tropism and pathogenesis of flaviviruses from the standpoint of cell surface molecules, which have been implicated as receptors in both virus–cell as well as cell–cell interactions. The emerging picture is one that encompasses extensive regulation and interplay among the invading virus, viral immune complexes, Fc receptors, major histocompatibility complex antigens, and adhesion molecules.

American genotype structures decrease dengue virus output from human monocytes and dendritic cells

The dengue virus type 2 structures probably involved in human virulence were previously defined by sequencing the complete genome of both American and Southeast (SE) Asian genotype templates in patient serum (K. C. Leitmeyer et al., J. Virol. 73:4738-4747, 1999). We have now evaluated the effects of introducing a mutation in the envelope glycoprotein (E) gene and/or replacement of 5'- and 3'-nontranslated regions on dengue virus replication in human primary cell cultures. A series of chimeric infectious clones were generated containing different combinations of American and SE Asian genotype sequences. Some of the chimeric viruses had altered plaque morphology in mammalian cells; however, they replicated at similar rates in mosquito cells as measured by quantitative reverse transcription-PCR and plaque assay. Although susceptibility to virus infection varied from donor to donor in experiments using human macrophage and dendritic cells, we were able to measure consistent differences in viral RNA output per infected cell. Using this measurement, we demonstrated that the chimeric virus containing the E mutation had a lower virus output compared to the parental infectious clone. A larger reduction in virus output was observed for the triple mutant and the wild-type, American genotype virus from which chimeric inserts were derived. It appears that the three changes function synergistically, although the E mutation alone gives a lower output compared to the 5'- and 3'-terminal mutations. The data suggest that these changes may be responsible for decreased dengue virus replication in human target cells and for virulence characteristics during infection.

Supernatants from dengue virus type-2 infected macrophages induce permeability changes in endothelial cell monolayers

The ability of dengue virus-infected human monocyte-derived macrophages to induce permeability changes in primary human umbilical vein endothelial cells was investigated. Supernatants from dengue virus type 2-infected monocyte-derived macrophages increased permeability in human umbilical vein endothelial cell monolayers without inducing endothelial cell infection. Production of permeabilising activity from monocyte-derived macrophages occurred after the peak of progeny virus release. TNF-alpha, a known inducer of endothelial cell permeability, was released from dengue virus infected monocyte-derived macrophages but its release did not coincide with release of endothelial cell permeabilising activity. Permeability induction was enhanced by pre-incubation with supernatants from infected monocyte-derived macrophages harvested at the time of peak release of TNF-alpha and infectious virus. Thus, supernatants from dengue virus-infected monocyte-derived macrophages contain factors that increase human umbilical vein endothelial cell permeability, but this is not accompanied by endothelial cell infection or directly correlated with release of dengue virus or TNF-alpha from monocyte-derived macrophages. This model system can be used for further in vitro analysis of mechanisms that may relate to capillary leakage and the development of dengue haemorrhagic fever/dengue shock syndrome.

Detection of dengue virus replication in peripheral blood mononuclear cells from dengue virus type 2-infected patients by a reverse transcription-real-time PCR assay

While dengue virus is thought to replicate in mononuclear phagocytic cells in vivo, attempts to detect it in peripheral blood mononuclear cells (PBMC) by virus isolation or antigen detection have had variable and generally low rates. In this study, we developed a reverse transcription (RT)-real-time PCR assay to quantify positive- and negative-sense RNA of dengue virus type 2 within the cells. The assay includes an RT step using either sense or antisense primer followed by a real-time PCR step using the designed primers and probe, which target a capsid region highly conserved in dengue virus type 2 strains. It can be used to monitor the dynamic change of intracellular dengue virus RNA species during the course of infection. When this assay is employed in quantification of dengue virus RNA species in PBMC from 10 patients infected with dengue virus type 2, both positive- and negative-sense dengue RNA can be detected, indicating that dengue virus is actively replicating in PBMC in vivo. Moreover, the amounts of negative-sense dengue virus RNA in PBMC correlate very well with the viral load of dengue virus in plasma, suggesting that quantification of negative-sense dengue virus RNA in PBMC may provide another indicator of dengue virus replication in vivo. Use of this convenient, sensitive, and accurate method of quantification in clinical samples from patients with different disease severity would further our understanding of the pathogenesis of dengue.

Non Fc receptor-mediated infection of human macrophages by dengue virus serotype 2

Four human monocyte-derived macrophage membrane proteins, with apparent molecular masses of 27, 45, 67 and 87 kDa, were identified as possible receptors for dengue virus serotype 2 (DEN-2) (Mexican isolate 200787/1983), based on affinity chromatography, immunofluorescence, virus overlay protein-binding assays and Western blotting. Additionally, mouse polyclonal antibodies raised against each of the four proteins were capable of partially inhibiting in vitro DEN-2 infection of monocyte-macrophages, thus supporting the notion of a role for such proteins as DEN-2 receptors. Parallel studies were carried out using the human promonocytic U-937 cell line, both as undifferentiated cells and as monocyte-like phorbol myristate acetate (PMA)-differentiated cells, as target cells. Whereas interaction between DEN-2 and undifferentiated U-937 cells was almost negligible, PMA-differentiated U-937 cells were shown to harbour putative receptors (with molecular masses of 45 and 67 kDa) for DEN-2, similar to those found in human monocyte-derived macrophages. To our knowledge, this is the first report that describes putative receptors for DEN-2 in primary cultures of human macrophages.

MIP-1 alpha and MIP-1 beta induction by dengue virus

Dengue virus (DV) infection can result in either a mild febrile illness known as dengue fever (DF) or a life-threatening disease called dengue hemorrhagic fever (DHF). DHF is more prevalent in patients undergoing secondary DV infection. This observation has led to the hypothesis that DHF may be the result of immune reactions to the secondary DV infection; an event termed immunopathology. Two cellular factors, MIP-1 alpha and MIP-1 beta, have been found to be induced by infection with DV. MIP-1 induction by DV infection was observed in a myelomonocytic cell line, as well as in peripheral blood mononuclear cells isolated from a dengue naive donor. MIP-1 induction was not due to factors secreted by infected cells. In fact, replication-competent virus was required to induce MIP-1. Evidence is also provided that MIP-1 genes are expressed in patients with dengue disease. It is hypothesized that these chemokines may have roles in the immunopathology of dengue infections and may contribute to fever and bone marrow suppression observed in patients with DV infections.

Chemical mutagenesis of dengue virus type 4 yields mutant viruses which are temperature sensitive in vero cells or human liver cells and attenuated in mice

A recombinant live attenuated dengue virus type 4 (DEN4) vaccine candidate, 2ADelta30, was found previously to be generally well tolerated in humans, but a rash and an elevation of liver enzymes in the serum occurred in some vaccinees. 2ADelta30, a non-temperature-sensitive (non-ts) virus, contains a 30-nucleotide deletion (Delta30) in the 3' untranslated region (UTR) of the viral genome. In the present study, chemical mutagenesis of DEN4 was utilized to generate attenuating mutations which may be useful in further attenuation of the 2ADelta30 candidate vaccine. Wild-type DEN4 2A virus was grown in Vero cells in the presence of 5-fluorouracil, and a panel of 1,248 clones were isolated. Twenty ts mutant viruses were identified that were ts in both simian Vero and human liver HuH-7 cells (n = 13) or only in HuH-7 cells (n = 7). Each of the 20 ts mutant viruses possessed an attenuation phenotype, as indicated by restricted replication in the brains of 7-day-old mice. The complete nucleotide sequence of the 20 ts mutant viruses identified nucleotide substitutions in structural and nonstructural genes as well as in the 5' and 3' UTRs, with more than one change occurring, in general, per mutant virus. A ts mutation in the NS3 protein (nucleotide position 4995) was introduced into a recombinant DEN4 virus possessing the Delta30 deletion, thereby creating rDEN4Delta30-4995, a recombinant virus which is ts and more attenuated than rDEN4Delta30 virus in the brains of mice. We are assembling a menu of attenuating mutations that should be useful in generating satisfactorily attenuated recombinant dengue vaccine viruses and in increasing our understanding of the pathogenesis of dengue virus.

West Nile virus-induced bax-dependent apoptosis

The mechanism of cell death induced by West Nile virus (WNV), a causative agent of human febrile syndrome and encephalitis, was investigated. WNV-infected K562 and Neuro-2a cells manifested the typical features of apoptosis, including cell shrinkage, chromatin condensation and subdiploid DNA content by flow cytometry. DNA fragmentation into nucleosomal size and changes in outer cell membrane phospholipid composition were also observed in K562 cells. UV-inactivated virus failed to induce the above-mentioned characteristics, suggesting that viral replication may be required for the induction of apoptosis by WNV. Additionally, signals involved in WNV-induced apoptosis are associated with the up-regulation of bax gene expression.

Dengue virus binding to human leukocyte cell lines: receptor usage differs between cell types and virus strains

Monocyte macrophages (Mphi) are thought to be the principal target cells for the dengue viruses (DV), the cause of dengue fever and hemorrhagic fever. Cell attachment is mediated by the virus envelope (E) protein, but the host-cell receptors remain elusive. Currently, candidate receptor molecules include proteins, Fc receptors, glycosaminoglycans (GAGs) and lipopolysaccharide binding CD14-associated molecules. Here, we show that in addition to Mphi, cells of the T- and B-cell lineages, and including cells lacking GAGs, can bind and become infected with DV. The level of virus binding varied widely between cell lines and, notably, between virus strains within a DV serotype. The latter difference may be ascribable to one or more amino acid differences in domain II of the E protein. Heparin had no significant effect on DV binding, while heparinase treatment of cells in all cases increased DV binding, further supporting the contention that GAGs are not required for DV binding and infection of human cells. In contrast to a recent report, we found that lipopolysaccharide (LPS) had either no effect or enhanced DV binding to, and infection of, various human leukocyte cell lines, while in all virus-cell combinations, depletion of Ca(2+)/Mg(2+) enhanced DV binding. This argues against involvement of beta(2) integrins in virus-host cell interactions, a conclusion in accord with the demonstration of three virus binding membrane proteins of < 75 kDa. Collectively, the results of this study question the purported exclusive importance of the E protein domain III in DV binding to host cells and point to a far more complex interaction between various target cells and, notably, individual DV strains.

Infection of human cells by dengue virus is modulated by different cell types and viral strains

Although prior studies have investigated cellular infection by dengue virus (DV), many have used highly passaged strains. We have reassessed cellular infection by DV type 2 (DV2) using prototype and low-passage isolates representing genotypes from different geographic areas. We observed marked variation in the susceptibility to infection among cell types by different DV2 strains. HepG2 hepatoma cells were susceptible to infection by all DV2 strains assayed. Although the prototype strain generated higher titers of secreted virus than the low-passage isolates, this difference did not correspond to positive- or negative-strand viral RNA levels and thus may reflect variation in efficiency among DV2 isolates to translate viral proteins or package and/or secrete virus. In contrast, human foreskin fibroblasts were susceptible to the prototype and low-passage Thai isolates but not to five Nicaraguan strains tested, as reflected by the absence of accumulation of negative-strand viral RNA, viral antigen, and infectious virus. A similar pattern was observed with the antibody-dependent pathway of infection. U937 and THP-1 myeloid cells and peripheral blood monocytes were infected in the presence of enhancing antibodies by the prototype strain but not by low-passage Nicaraguan isolates. Again, the barrier appeared to be prior to negative-strand accumulation. Thus, depending on the cell type and viral isolate, blocks that limit the production of infectious virus in vitro may occur at distinct steps in the pathway of cellular infection.

Dengue viral infections; pathogenesis and epidemiology

Dengue viral infections affect up to 100 million individuals per year. Dengue haemorrhagic fever is a clinical form of disease characterised by intravascular fluid loss. There has been a marked increase in the incidence of this form of the disease over the last few decades, associated with significant mortality, particularly in the paediatric population. A number of theories relating to the pathogenesis of dengue haemorrhagic fever exist that have evolved from the analysis of the epidemiology of this disease. Virological and immunopathological factors are both important but the exact mechanisms for the disease are unknown.

Modulation of Dengue virus infection in human cells by alpha, beta, and gamma interferons

A role for interferon (IFN) in modulating infection by dengue virus (DV) has been suggested by studies in DV-infected patients and IFN receptor-deficient mice. To address how IFN modulates DV type 2 infection, we have assayed IFN-alpha, -beta, and -gamma for the ability to enhance or diminish antibody-independent and antibody-dependent cell infection using a competitive, asymmetric reverse transcriptase-mediated PCR (RT-PCR) assay that quantitates positive and negative strands of viral RNA, a flow cytometric assay that measures viral antigen, and a plaque assay that analyzes virion production. Our data suggest that IFN-alpha and -beta protect cells against DV infection in vitro. Treatment of hepatoma cells with IFN-alpha or -beta decreases viral RNA levels greater than 1, 000-fold, the percentage of cells infected 90 to 95%, and the amount of infectious virus secreted 150- to 100,000-fold. These results have been reproduced with several cell types and viral strains, including low-passage isolates. In contrast, IFN-gamma has a more variable effect depending on the cell type and pathway of infection. Quantitative RT-PCR experiments indicate that IFN inhibits DV infection by preventing the accumulation of negative-strand viral RNA.

Study of Dengue virus infection in SCID mice engrafted with human K562 cells

Here we report that severe combined immunodeficient (SCID) mice engrafted with human K562 cells (K562-SCID mice) can be used as an animal model to study dengue virus (DEN) infection. After intratumor injection into K562 cell masses of PL046, a Taiwanese DEN-2 human isolate, the K562-SCID mice showed neurological signs of paralysis and died at approximately 2 weeks postinfection. In addition to being detected in the tumor masses, high virus titers were detected in the peripheral blood and the brain tissues, indicating that DEN had replicated in the infected K562-SCID mice. In contrast, the SCID mice were resistant to DEN infection and the mock-infected K562-SCID mice survived for over 3 months. These data illustrate that DEN infection contributed directly to the deaths of the infected K562-SCID mice. Other serotypes of DEN were also used to infect the K562-SCID mice, and the mortality rates of the infected mice varied with different challenge strains, suggesting the existence of diverse degrees of virulence among DENs. To determine whether a neutralizing antibody against DEN in vitro was also protective in vivo, the K562-SCID mice were challenged with DEN-2 and received antibody administration at the same time or 1 day earlier. Our results revealed that the antibody-treated mice exhibited a reduction in mortality and a delay of paralysis onset after DEN infection. In contrast to K562-SCID, the persistently DEN-infected K562 cells generated in vitro invariably failed to be implanted in the mice. It seems that in the early stage of implantation, a gamma interferon activated, nitric oxide-mediated anti-DEN effect might play a role in the innate immunity against DEN-infected cells. The system described herein offers an opportunity to explore DEN replication in vivo and to test various antiviral protocols in infected hosts.