Dengue viral RNA levels in peripheral blood mononuclear cells are associated with disease severity and preexisting dengue immune status

Single-cell RNA sequencing reveals the expansion of circulating tissue-homing B cell subsets in secondary acute dengue viral infection

The roles of antibodies secreted by subsets of B cells in dengue virus (DENV) infection have been extensively studied, yet, the contribution of tissue-homing B cells to antiviral immunity remains unclear. In this study, we performed a comprehensive analysis of B cell subpopulations in peripheral blood samples from DENV-infected patients using single-cell RNA-sequencing (scRNA-seq) datasets and flow cytometry. We showed that plasma cells (PCs) and plasmablasts (PBs) were the predominant B cell populations during the acute phase of secondary natural DENV infection, but not in convalescent phase nor in healthy controls. Interestingly, these cells expressed proliferation, adhesion, and tissue-homing genes, including SELPLG, a homing marker of the skin, the initial infected site of DENV. Flow cytometry analysis confirmed a significant upregulation of cell surface expression of a cutaneous lymphocyte-associated antigen (CLA) encoded by SELPLG in PCs and PBs, compared to naive and memory B cells from the same patients. The analysis of an independent single-cell B-cell receptor sequencing (scBCR-seq) dataset of DENV-infected patients revealed that the peripheral blood PCs and PBs exhibited the highest clonal expansion in secondary DENV infection compared to other B cell subsets. These clonally expanded cells also expressed the highest levels of tissue-homing genes, including SELPLG. In addition, by utilizing a public scRNA-seq dataset of SARS-CoV2 infection, we demonstrated the upregulation of several tissue-homing genes in PCs and PBs. Our study provides evidence for the potential roles of tissue-homing B cell subsets in the context of immune responses against viral infections in humans.

Generation and characterization of genetically and antigenically diverse infectious clones of dengue virus serotypes 1-4

Dengue is caused by four genetically distinct viral serotypes, dengue virus (DENV) 1-4. Following transmission by Aedes mosquitoes, DENV can cause a broad spectrum of clinically apparent disease ranging from febrile illness to dengue hemorrhagic fever and dengue shock syndrome. Progress in the understanding of different dengue serotypes and their impacts on specific host-virus interactions has been hampered by the scarcity of tools that adequately reflect their antigenic and genetic diversity. To bridge this gap, we created and characterized infectious clones of DENV1-4 originating from South America, Africa, and Southeast Asia. Analysis of whole viral genome sequences of five DENV isolates from each of the four serotypes confirmed their broad genetic and antigenic diversity. Using a modified circular polymerase extension reaction (CPER), we generated de novo viruses from these isolates. The resultant clones replicated robustly in human and insect cells at levels similar to those of the parental strains. To investigate in vivo properties of these genetically diverse isolates, representative viruses from each DENV serotype were administered to NOD Rag1^-/-, IL2rg^null Flk2^-/- (NRGF) mice, engrafted with components of a human immune system. All DENV strains tested resulted in viremia in humanized mice and induced cellular and IgM immune responses. Collectively, we describe here a workflow for rapidly generating de novo infectious clones of DENV - and conceivably other RNA viruses. The infectious clones described here are a valuable resource for reverse genetic studies and for characterizing host responses to DENV in vitro and in vivo.

Dengue virus co-opts innate type 2 pathways to escape early control of viral replication

Mast cell products and high levels of type 2 cytokines are associated with severe dengue disease. Group 2 innate lymphoid cells (ILC2) are type-2 cytokine-producing cells that are activated by epithelial cytokines and mast cell-derived lipid mediators. Through ex vivo RNAseq analysis, we observed that ILC2 are activated during acute dengue viral infection, and show an impaired type I-IFN signature in severe disease. We observed that circulating ILC2 are permissive for dengue virus infection in vivo and in vitro, particularly when activated through prostaglandin D2 (PGD2). ILC2 underwent productive dengue virus infection, which was inhibited through CRTH2 antagonism. Furthermore, exogenous IFN-β induced expression of type I-IFN responsive anti-viral genes by ILC2. PGD2 downregulated type I-IFN responsive gene and protein expression; and urinary prostaglandin D2 metabolite levels were elevated in severe dengue. Moreover, supernatants from activated ILC2 enhanced monocyte infection in a GM-CSF and mannan-dependent manner. Our results indicate that dengue virus co-opts an innate type 2 environment to escape early type I-IFN control and facilitate viral dissemination. PGD2 downregulates type I-IFN induced anti-viral responses in ILC2. CRTH2 antagonism may be a therapeutic strategy for dengue-associated disease.

Development of a Singleplex Real-Time Reverse Transcriptase PCR Assay for Pan-Dengue Virus Detection and Quantification

Dengue virus (DENV) infection is a significant global health problem. There are no specific therapeutics or widely available vaccines. Early diagnosis is critical for patient management. Viral RNA detection by multiplex RT-PCR using multiple pairs of primers/probes allowing the simultaneous detection of all four DENV serotypes is commonly used. However, increasing the number of primers in the RT-PCR reaction reduces the sensitivity of detection due to the increased possibility of primer dimer formation. Here, a one tube, singleplex real-time RT-PCR specific to DENV 3′-UTR was developed for the detection and quantification of pan-DENV with no cross reactivity to other flaviviruses. The sensitivity of DENV detection was as high as 96.9% in clinical specimens collected at the first day of hospitalization. Our assay provided equivalent PCR efficiency and RNA quantification among each DENV serotype. The assay’s performance was comparable with previously established real-time RT-PCR targeting coding sequences. Using both assays on the same specimens, our results indicate the presence of defective virus particles in the circulation of patients infected with all serotypes. Dual regions targeting RT-PCR enhanced the sensitivity of viral genome detection especially during the late acute phase when viremia rapidly decline and an incomplete viral genome was clinically evident.

Overview on Chikungunya Virus Infection: From Epidemiology to State-of-the-Art Experimental Models

Chikungunya virus (CHIKV) is currently one of the most relevant arboviruses to public health. It is a member of the Togaviridae family and alphavirus genus and causes an arthritogenic disease known as chikungunya fever (CHIKF). It is characterized by a multifaceted disease, which is distinguished from other arbovirus infections by the intense and debilitating arthralgia that can last for months or years in some individuals. Despite the great social and economic burden caused by CHIKV infection, there is no vaccine or specific antiviral drugs currently available. Recent outbreaks have shown a change in the severity profile of the disease in which atypical and severe manifestation lead to hundreds of deaths, reinforcing the necessity to understand the replication and pathogenesis processes. CHIKF is a complex disease resultant from the infection of a plethora of cell types. Although there are several in vivo models for studying CHIKV infection, none of them reproduces integrally the disease signature observed in humans, which is a challenge for vaccine and drug development. Therefore, understanding the potentials and limitations of the state-of-the-art experimental models is imperative to advance in the field. In this context, the present review outlines the present knowledge on CHIKV epidemiology, replication, pathogenesis, and immunity and also brings a critical perspective on the current in vitro and in vivo state-of-the-art experimental models of CHIKF.

Ivermectin Accelerates Circulating Nonstructural Protein 1 (NS1) Clearance in Adult Dengue Patients: A Combined Phase 2/3 Randomized Double-blinded Placebo Controlled Trial

BACKGROUND

Dengue is the most significant mosquito-borne viral disease; there are no specific therapeutics. The antiparasitic drug ivermectin efficiently inhibits the replication of all 4 dengue virus serotypes in vitro.

METHODS

We conducted 2 consecutive randomized, double-blind, placebo-controlled trials in adult dengue patients to evaluate safety and virological and clinical efficacies of ivermectin. After a phase 2 trial with 2 or 3 days of 1 daily dose of 400 µg/kg ivermectin, we continued with a phase 3, placebo-controlled trial with 3 days of 400 µg/kg ivermectin.

RESULTS

The phase 2 trial showed a trend in reduction of plasma nonstructural protein 1 (NS1) clearance time in the 3-day ivermectin group compared with placebo. Combining phase 2 and 3 trials, 203 patients were included in the intention to treat analysis (100 and 103 patients receiving ivermectin and placebo, respectively). Dengue hemorrhagic fever occurred in 24 (24.0%) of ivermectin-treated patients and 32 (31.1%) patients receiving placebo (P = .260). The median (95% confidence interval [CI]) clearance time of NS1 antigenemia was shorter in the ivermectin group (71.5 [95% CI 59.9-84.0] hours vs 95.8 [95% CI 83.9-120.0] hours, P = .014). At discharge, 72.0% and 47.6% of patients in the ivermectin and placebo groups, respectively had undetectable plasma NS1 (P = .001). There were no differences in the viremia clearance time and incidence of adverse events between the 2 groups.

CONCLUSIONS

A 3-day 1 daily dose of 400 µg/kg oral ivermectin was safe and accelerated NS1 antigenemia clearance in dengue patients. However, clinical efficacy of ivermectin was not observed at this dosage regimen.

Application of One-Step Reverse Transcription Droplet Digital PCR for Dengue Virus Detection and Quantification in Clinical Specimens

Detection and quantification of viruses in laboratory and clinical samples are standard assays in dengue virus (DENV) studies. The quantitative reverse transcription polymerase chain reaction (qRT-PCR) is considered to be the standard for DENV detection and quantification due to its high sensitivity. However, qRT-PCR offers only quantification relative to a standard curve and consists of several "in-house" components resulting in interlaboratory variations. We developed and optimized a protocol for applying one-step RT-droplet digital PCR (RT-ddPCR) for DENV detection and quantification. The lower limit of detection (LLOD95) and the lower limit of quantification (LLOQ) for RT-ddPCR were estimated to be 1.851 log10-copies/reaction and 2.337 log10-copies/reaction, respectively. The sensitivity of RT-ddPCR was found to be superior to qRT-PCR (94.87% vs. 90.38%, p = 0.039) while no false positives were detected. Quantification of DENV in clinical samples was independently performed in three laboratories showing interlaboratory variations with biases <0.5 log10-copies/mL. The RT-ddPCR protocol presented here could help harmonize DENV quantification results and improve findings in the field such as identifying a DENV titer threshold correlating with disease severity.

Direct Infection of B Cells by Dengue Virus Modulates B Cell Responses in a Cambodian Pediatric Cohort

Dengue is an acute viral disease caused by dengue virus (DENV), which is transmitted by Aedes mosquitoes. Symptoms of DENV infection range from inapparent to severe and can be life-threatening. DENV replicates in primary immune cells such as dendritic cells and macrophages, which contribute to the dissemination of the virus. Susceptibility of other immune cells such as B cells to direct infection by DENV and their subsequent response to infection is not well defined. In a cohort of 60 Cambodian children, we showed that B cells are susceptible to DENV infection. Moreover, we show that B cells can support viral replication of laboratory adapted and patient-derived DENV strains. B cells were permissive to DENV infection albeit low titers of infectious virions were released in cell supernatants CD300a, a phosphatidylserine receptor, was identified as a potential attachment factor or receptor for entry of DENV into B cells. In spite of expressing Fcγ-receptors, antibody-mediated enhancement of DENV infection was not observed in B cells in an in vitro model. Direct infection by DENV induced proliferation of B cells in dengue patients in vivo and plasmablast/plasma cell formation in vitro. To summarize, our results show that B cells are susceptible to direct infection by DENV via CD300a and the subsequent B cell responses could contribute to dengue pathogenesis.

Temporally integrated single cell RNA sequencing analysis of PBMC from experimental and natural primary human DENV-1 infections

Dengue human infection studies present an opportunity to address many longstanding questions in the field of flavivirus biology. However, limited data are available on how the immunological and transcriptional response elicited by an attenuated challenge virus compares to that associated with a wild-type DENV infection. To determine the kinetic transcriptional signature associated with experimental primary DENV-1 infection and to assess how closely this profile correlates with the transcriptional signature accompanying natural primary DENV-1 infection, we utilized scRNAseq to analyze PBMC from individuals enrolled in a DENV-1 human challenge study and from individuals experiencing a natural primary DENV-1 infection. While both experimental and natural primary DENV-1 infection resulted in overlapping patterns of inflammatory gene upregulation, natural primary DENV-1 infection was accompanied with a more pronounced suppression in gene products associated with protein translation and mitochondrial function, principally in monocytes. This suggests that the immune response elicited by experimental and natural primary DENV infection are similar, but that natural primary DENV-1 infection has a more pronounced impact on basic cellular processes to induce a multi-layered anti-viral state.

Dengue Human Challenge Models allow for the analysis of host/virus interactions under highly controlled experimental conditions. However, it is unclear how close the immune response generated by an attenuated challenge virus compares to that generated by a naturally acquired DENV infection. In this study, we utilized single cell RNA sequencing to assess the immune response generated by both experimental and natural primary DENV-1 infections. This analysis suggests that the immune response elicited by experiential and natural primary DENV-1 infections are similar, but that natural DENV-1 infection has a more pronounced impact on basic cellular processes to induce a multi-layered anti-viral state.

Analysis of cell-associated DENV RNA by oligo(dT) primed 5' capture scRNAseq

Dengue is one of the most widespread vector-borne viral diseases in the world. However, the size, heterogeneity, and temporal dynamics of the cell-associated viral reservoir during acute dengue virus (DENV) infection remains unclear. In this study, we analyzed cells infected in vitro with DENV and PBMC from an individual experiencing a natural DENV infection utilizing 5’ capture single cell RNA sequencing (scRNAseq). Both positive- and negative-sense DENV RNA was detected in reactions containing either an oligo(dT) primer alone, or in reactions supplemented with a DENV-specific primer. The addition of a DENV-specific primer did not increase the total amount of DENV RNA captured or the fraction of cells identified as containing DENV RNA. However, inclusion of a DENV-specific cDNA primer did increase the viral genome coverage immediately 5’ to the primer binding site. Furthermore, while the majority of intracellular DENV sequence captured in this analysis mapped to the 5’ end of the viral genome, distinct patterns of enhanced coverage within the DENV polyprotein coding region were observed. The 5’ capture scRNAseq analysis of PBMC not only recapitulated previously published reports by detecting virally infected memory and naïve B cells, but also identified cell-associated genomic variants not observed in contemporaneous serum samples. These results demonstrate that oligo(dT) primed 5’ capture scRNAseq can detect DENV RNA and quantify virus-infected cells in physiologically relevant conditions, and provides insight into viral sequence variability within infected cells.

Phenotype and functionality of follicular helper T cells in patients with acute dengue infection

Background

The association of functionality and phenotype of follicular helper T cells (Tfh) with dengue virus (DENV) specific antibody responses and clinical disease severity has not been well studied.

Methods

We investigated the phenotype and functionality of Tfh cells and plasmablasts in adult patients (DF = 18, DHF = 22) with acute dengue (day 4 to 8 since onset of fever) of varying severity using multiparametric flowcytometry. The properties of Tfh cells were correlated with viraemia, disease severity, plasmablast responses and DENV-specific serum antibody responses. We further evaluated the kinetics of neutralizing antibodies (Neut50) throughout the course of illness in order to evaluate their association with clinical disease severity and viraemia.

Results

Tfh cells (especially those producing IL-21 and co-expressing PD-1 and ICOS) were found to be significantly expanded (p < 0.0001) and highly activated in patients with DHF compared to those with DF. The frequency of Tfh cells significantly correlated with DENV-specific IgG, NS1-specific antibodies and Neut50 antibody titres in patients with DHF but not in those with DF. Although the Neut50 titres increased during the course of acute secondary DENV infection, they showed differences based on serotype. For instance, the Neut50 titres were significantly higher during the latter part of illness in patients with DF compared to DHF in DENV1 infection, while in DENV2, patients with DHF had significantly higher titres. The viral loads during early illness did not correlate with the subsequent rise in the Neut50 antibody titres during any time point of illness.

Conclusions

The expansion of Tfh cells is associated with DHF and DENV-specific IgG, NS1-specific and neutralizing antibodies. Neut50 titres did not associate with disease severity or viraemia at the point of first presentation during the febrile phase, but later titres do show differential association with severity in patients with DENV1 compared to DENV2.

Human Heavy Chain Antibody Genes Elicited in Thai Dengue Patients during DENV2 Secondary Infection

Dengue is one of the most serious mosquito-borne viral diseases occurring in humans. To combat the complexity of 4 antigenically distinct serotypes, the ideal vaccine for dengue should be able to stimulate cross-neutralizing antibodies. Recently, genetics-based immune responses have been studied to guide vaccine design against several viral pathogens. Despite a recent approval of dengue vaccine, information on genetics-based immune responses against dengue virus (DENV) is still limited. Consequently, we aimed to determine the profiles of immunoglobulin heavy chain genes from DENV2 infected patients. The immunoglobulin heavy chain variable region genes (IGHV) were amplified from peripheral blood mononuclear cells of DENV2 secondary infected patients in the acute, convalescence, and recovery phases. Antibody heavy chain genes were sequenced using next-generation sequencing, and analyzed to identify correlations with neutralizing and enhancing activities of the serum samples. IGHV1-69, 3-23, and 3-30 were frequently discovered in our Thai DENV2 infected patients. Our findings provide new data on the human B cell response during secondary DENV2 infections in Thai dengue patients that offer supportive information for dengue vaccine design and therapeutics development.

Altered monocyte response to the dengue virus in those with varying severity of past dengue infection

OBJECTIVE

We sought to investigate the differences in monocyte immune responses to the dengue virus (DENV) in those who previously had either severe disease (past SD) or non-severe dengue (past NSD) following a secondary dengue infection.

METHOD

Monocytes from healthy individuals who had either past SD (n = 6) or past NSD (n = 6) were infected at MOI one with all four DENV serotypes following incubation with autologous serum. 36-hours post infection, levels of inflammatory cytokines and viral loads were measured in the supernatant and expression of genes involved in viral sensing and interferon signaling was determined.

RESULTS

Monocytes of individuals with past SD produced significantly higher viral loads (p = 0.0426 and cytokines (IL-10 p = 0.008, IL-1β p = 0.008 and IL-6 p = 0.0411) when infected with DENV serotypes they were not immune to, compared to those who has past NSD. Monocytes of individuals with past SD also produced significantly higher viral loads (p = 0.022) and cytokines (IL-10 p < 0.0001, IL-1β < 0.0001 and IL-6 p < 0.0001) when infected with DENV serotypes they were previously exposed to, despite the monocytes being infected in the presence of autologous serum. A significant upregulation of NLRP3 (p = 0.005), RIG-I (0.0004) and IFNB-1 (0.01) genes were observed in those who had past SD compared to past NSD when infected with non-immune DENV serotypes.

CONCLUSION

Monocytes from those with past SD appear to show marked differences in viral loads, viral sensing and production of inflammatory mediators in response to the DENV, when compared to those who experienced past NSD, suggesting that initial innate immune responses may influence the disease outcome.

Detection of post-vaccination enhanced dengue virus infection in macaques: An improved model for early assessment of dengue vaccines

The need for improved dengue vaccines remains since the only licensed vaccine, Dengvaxia, shows variable efficacy depending on the infecting dengue virus (DENV) type, and increases the risk of hospitalization for severe dengue in children not exposed to DENV before vaccination. Here, we developed a tetravalent dengue purified and inactivated vaccine (DPIV) candidate and characterized, in rhesus macaques, its immunogenicity and efficacy to control DENV infection by analyzing, after challenge, both viral replication and changes in biological markers associated with dengue in humans. Although DPIV elicited cross-type and long-lasting DENV-neutralizing antibody responses, it failed to control DENV infection. Increased levels of viremia/RNAemia (correlating with serum capacity at enhancing DENV infection in vitro), AST, IL-10, IL-18 and IFN-γ, and decreased levels of IL-12 were detected in some vaccinated compared to non-vaccinated monkeys, indicating the vaccination may have triggered antibody-dependent enhancement of DENV infection. The dengue macaque model has been considered imperfect due to the lack of DENV-associated clinical signs. However, here we show that post-vaccination enhanced DENV infection can be detected in this model when integrating several parameters, including characterization of DENV-enhancing antibodies, viremia/RNAemia, and biomarkers relevant to dengue in humans. This improved dengue macaque model may be crucial for early assessment of efficacy and safety of future dengue vaccines.

Dengue virus (DENV) is responsible for the most widespread arboviral disease affecting humans. A pre-existing suboptimal immunity to DENV is accepted as being the major risk factor for severe dengue. Thus, if vaccination does not elicit optimal DENV-specific immunity, a vaccine might, instead, increase the risk of severe dengue in vaccinated individuals, as seen with the only licensed vaccine (Dengvaxia) in children naïve to DENV at vaccination. It is thus crucial to assess dengue vaccine safety at the earliest development stages, ideally in the preclinical stage. The dengue macaque model has been used to assess preclinical efficacy of dengue vaccines, with post-challenge DENV replication as the sole efficacy endpoint. However, this model had not predicted the Dengvaxia-associated safety signals. Here we characterized, in macaques, a dengue purified and inactivated vaccine (DPIV) candidate for its immunogenicity and efficacy/safety. Using a multiparameter approach, including characterization of viral replication and biomarkers relevant to dengue/severe dengue in humans, we were able to detect vaccine-associated safety signals in this model. While these results enabled us to discontinue at an early stage the DPIV development, this improved dengue macaque model may also be instrumental for early assessment of efficacy/safety of future dengue vaccines.

Find the right sample: A study on the versatility of saliva and urine samples for the diagnosis of emerging viruses

BACKGROUND

The emergence of different viral infections during the last decades like dengue, West Nile, SARS, chikungunya, MERS-CoV, Ebola, Zika and Yellow Fever raised some questions on quickness and reliability of laboratory diagnostic tests for verification of suspected cases. Since sampling of blood requires medically trained personal and comprises some risks for the patient as well as for the health care personal, the sampling by non-invasive methods (e.g. saliva and/ or urine) might be a very valuable alternative for investigating a diseased patient.

MAIN BODY

To analyse the usefulness of alternative non-invasive samples for the diagnosis of emerging infectious viral diseases, a literature search was performed on PubMed for alternative sampling for these viral infections. In total, 711 papers of potential relevance were found, of which we have included 128 in this review.

CONCLUSIONS

Considering the experience using non-invasive sampling for the diagnostic of emerging viral diseases, it seems important to perform an investigation using alternative samples for routine diagnostics. Moreover, during an outbreak situation, evaluation of appropriate sampling and further processing for laboratory analysis on various diagnostic platforms are very crucial. This will help to achieve optimal diagnostic results for a good and reliable case identification.

Virus-inclusive single-cell RNA sequencing reveals the molecular signature of progression to severe dengue

A fraction of the 400 million people infected with dengue annually progresses to severe dengue (SD). Yet, there are currently no biomarkers to predict disease progression. We profiled the landscape of host transcripts and viral RNA in thousands of single blood cells from dengue patients prior to progressing to SD. We discovered cell type-specific immune activation and candidate predictive biomarkers. We also determined preferential virus association with specific cell populations, particularly naive B cells and monocytes. We explored immune activation of bystander cells, clonality and somatic evolution of adaptive immune repertoires, as well as viral genomics. This multifaceted approach could advance understanding of pathogenesis of any viral infection, map an atlas of infected cells, and promote the development of prognostics.

Dengue virus (DENV) infection can result in severe complications. However, the understanding of the molecular correlates of severity is limited, partly due to difficulties in defining the peripheral blood mononuclear cells (PBMCs) that contain DENV RNA in vivo. Accordingly, there are currently no biomarkers predictive of progression to severe dengue (SD). Bulk transcriptomics data are difficult to interpret because blood consists of multiple cell types that may react differently to infection. Here, we applied virus-inclusive single-cell RNA-seq approach (viscRNA-Seq) to profile transcriptomes of thousands of single PBMCs derived early in the course of disease from six dengue patients and four healthy controls and to characterize distinct leukocyte subtypes that harbor viral RNA (vRNA). Multiple IFN response genes, particularly MX2 in naive B cells and CD163 in CD14⁺ CD16⁺ monocytes, were up-regulated in a cell-specific manner before progression to SD. The majority of vRNA-containing cells in the blood of two patients who progressed to SD were naive IgM B cells expressing the CD69 and CXCR4 receptors and various antiviral genes, followed by monocytes. Bystander, non-vRNA–containing B cells also demonstrated immune activation, and IgG1 plasmablasts from two patients exhibited clonal expansions. Lastly, assembly of the DENV genome sequence revealed diversity at unexpected sites. This study presents a multifaceted molecular elucidation of natural dengue infection in humans with implications for any tissue and viral infection and proposes candidate biomarkers for prediction of SD.

Dengue virus potentially promotes migratory responses on endothelial cells by enhancing pro-migratory soluble factors and miRNAs

The most life-threatening effect of the Dengue virus (DENV) infection is an acute destabilization of the microvascular endothelial cell (MEC) barrier leading to plasma leakage, hypovolemic shock and haemorrhage. However, the underlying cellular mechanisms responsible for the dysfunction of MECs are not well understood. To identify potential cellular processes altered during DENV infection of MECs, expression profiles of cytokines/growth factors and microRNAs were measured by Luminex assay and next generation sequencing, respectively. Synchronously DENV2-infected MECs increase the secretion of IL-6, IL-8, FGF-2, GM-CSF, G-CSF, TGF-α, GRO, RANTES, MCP-1 and MCP-3. Conditioned media of infected MECs increased the migration of non-infected MECs. Furthermore, six miRNAs deregulated at 24 hpi were predicted to regulate host genes involved in cell migration and vascular developmental processes such as angiogenesis. These in silico analyses provide insights that support that DENV promotes an acute migratory phenotype in MECs that contributes to the vascular destabilization observed in severe dengue cases.

Multi-color fluorescent reporter dengue viruses with improved stability for analysis of a multi-virus infection

Reporter virus is a versatile tool to visualize and to analyze virus infections. However, for flaviviruses, it is difficult to maintain the inserted reporter genes on the viral genome, limiting its use in several studies that require homogeneous virus particles and several rounds of virus replication. Here, we showed that flanking inserted GFP genes on both sides with ribosome-skipping 2A sequences improved the stability and the consistency of their fluorescent signals for dengue-virus-serotype 2 (DENV2) reporter viruses. The reporter viruses can infect known susceptible mammalian cell lines and primary CD14+ human monocytes. This design can accommodate several fluorescent protein genes, enabling the generation of multi-color DENV2-16681 reporter viruses with comparable replication capabilities, as demonstrated by their abilities to maintain their fluorescent intensities during co-infections and to exclude superinfections regardless of the fluorescent tags. The reported design of multi-color DENV2 should be useful for high-throughput analyses, single-cell analysis, and characterizations of interference and superinfection in animal models.

Magnitude of viremia, antigenemia and infection of circulating monocytes in children with mild and severe dengue

Dengue is a major public health problem in tropical regions around the world. Viral and immune host factors determine the clinical courses of the infection. We analyzed the dynamics of viremia (by real-time polymerase chain reactions), antigenemia (through detection of the viral non-structural protein [NS]-1 by enzyme-linked immunosorbent assays) and the frequency of virus-infected peripheral blood mononuclear cells (PBMCs) (by multiparametric flow cytometry) in children with primary or secondary dengue virus (DENV) infection in mild to severe cases. Additionally, we evaluated the association of these factors with clinical severity and laboratory parameters. The levels of viremia and antigenemia peaked during the early days of illness and these viral parameters were correlated (rho=0.37, P=0.003). Circulating monocytes were the most naturally infected subset within the PBMCs population, with kinetics similar to those of viremia and antigenemia. The levels of viremia and antigenemia were higher in children with primary infections than in those with secondary infections (P≤0.04). Although there were no associations between the three evaluated factors and clinical severity, the levels of plasma NS1 and the frequency of dengue virus-infected monocytes correlated with prolonged coagulation times. In short, the viremia, antigenemia and infected monocytes were detected early and were not related to clinical severity. The magnitude of antigenemia and infected circulating monocytes was associated with coagulation disorders.

Insights into the molecular evolution of Dengue virus type 4 in Puerto Rico over two decades of emergence

Dengue has emerged globally as a major human health problem since the 1950s and is now the most important arboviral disease of humans, infecting nearly 400 million people annually. While some cases are asymptomatic, others can develop a febrile illness (dengue fever) or even progress to severe and fatal dengue. Dengue is caused by any of 4 closely related but distinct viruses, known as Dengue virus serotype 1 to 4 (DENV-1 to DENV-4) which are maintained in endemic transmission to humans in large urban centers of the tropics by Aedes mosquitoes. Since the early 1960s, Puerto Rico, a major metropolitan center in the Caribbean, has experienced increasingly larger and clinically more severe epidemics following the introduction of all four dengue serotypes. The first dengue hemorrhagic fever epidemic in 1986, and a particularly severe outbreak in 1998 were dominated by novel DENV-4 strains that evolved in Puerto Rico, replacing earlier strains and spreading throughout the region. Sequence characterization of 54 complete DENV-4 genomes and their comparative evolution against 74 previously published viral sequences from the region over several decades shows that DENV-4 strains from these periods were genetically distinct based on unique changes in the envelope and non-structural genes. Their replacement of earlier strains in Puerto Rico progressed rapidly, suggesting that strong natural selection played a role in their fixation. This study confirms that DENVs evolve through rapid lineage turnover driven in part by natural selection and genetic drift.

Dendritic cells in dengue virus infection: targets of virus replication and mediators of immunity

Dendritic cells (DCs) are sentinels of the immune system and detect pathogens at sites of entry, such as the skin. In addition to the ability of DCs to control infections directly via their innate immune functions, DCs help to prime adaptive B- and T-cell responses by processing and presenting antigen in lymphoid tissues. Infected Aedes aegypti or Aedes albopictus mosquitoes transmit the four dengue virus (DENV) serotypes to humans while probing for small blood vessels in the skin. DENV causes the most prevalent arthropod-borne viral disease in humans, yet no vaccine or specific therapeutic is currently licensed. Although primary DENV infection confers life-long protective immunity against re-infection with the same DENV serotype, secondary infection with a different DENV serotype can lead to increased disease severity via cross-reactive T-cells or enhancing antibodies. This review summarizes recent findings in humans and animal models about DENV infection of DCs, monocytes, and macrophages. We discuss the dual role of DCs as both targets of DENV replication and mediators of innate and adaptive immunity, and summarize immune evasion strategies whereby DENV impairs the function of infected DCs. We suggest that DCs play a key role in priming DENV-specific neutralizing or potentially harmful memory B- and T-cell responses, and that future DC-directed therapies may help induce protective memory responses and reduce dengue pathogenesis.

Flaviviruses, an expanding threat in public health: focus on dengue, West Nile, and Japanese encephalitis virus

The flaviviruses dengue, West Nile, and Japanese encephalitis represent three major mosquito-borne viruses worldwide. These pathogens impact the lives of millions of individuals and potentially could affect non-endemic areas already colonized by mosquito vectors. Unintentional transport of infected vectors (Aedes and Culex spp.), traveling within endemic areas, rapid adaptation of the insects into new geographic locations, climate change, and lack of medical surveillance have greatly contributed to the increase in flaviviral infections worldwide. The mechanisms by which flaviviruses alter the immune and the central nervous system have only recently been examined despite the alarming number of infections, related deaths, and increasing global distribution. In this review, we will discuss the expansion of the geographic areas affected by flaviviruses, the potential threats to previously unaffected countries, the mechanisms of pathogenesis, and the potential therapeutic interventions to limit the devastating consequences of these viruses.

NK Cells during Dengue Disease and Their Recognition of Dengue Virus-Infected cells

The innate immune response, in addition to the B- and T-cell response, plays a role in protection against dengue virus (DENV) infection and the degree of disease severity. Early activation of natural killer (NK) cells and type-I interferon-dependent immunity may be important in limiting viral replication during the early stages of DENV infection and thus reducing subsequent pathogenesis. NK cells may also produce cytokines that reduce inflammation and tissue injury. On the other hand, NK cells are also capable of inducing liver injury at early-time points of DENV infection. In vitro, NK cells can kill antibody-coated DENV-infected cells through antibody-dependent cell-mediated cytotoxicity. In addition, NK cells may directly recognize DENV-infected cells through their activating receptors, although the increase in HLA class I expression may allow infected cells to escape the NK response. Recently, genome-wide association studies have shown an association between MICB and MICA, which encode ligands of the activating NK receptor NKG2D, and dengue disease outcome. This review focuses on recognition of DENV-infected cells by NK cells and on the regulation of expression of NK cell ligands by DENV.

Developing a dengue vaccine: progress and future challenges

Dengue is an expanding public health problem in the tropics and subtropical areas. Millions of people, most from resource-constrained countries, seek treatment every year for dengue-related disease. Despite more than 70 years of effort, a safe and efficacious vaccine remains unavailable. Antidengue antiviral drugs also do not exist despite attempts to develop or repurpose drug compounds. Gaps in the knowledge of dengue immunology, absence of a validated animal or human model of disease, and suboptimal assay platforms to measure immune responses following infection or experimental vaccination are obstacles to drug and vaccine development efforts. The limited success of one vaccine candidate in a recent clinical endpoint efficacy trial challenges commonly held beliefs regarding potential correlates of protection. If a dengue vaccine is to become a reality in the near term, vaccine developers should expand development pathway explorations beyond those typically required to demonstrate safety and efficacy.

Low levels of antibody-dependent enhancement in vitro using viruses and plasma from dengue patients

Background

The majority of dengue patients infected with any serotype of dengue virus (DENV) are asymptomatic, but the remainder may develop a wide spectrum of clinical symptoms, ranging from mild dengue fever (DF) to severe dengue hemorrhagic fever (DHF). Severe cases occur more often in patients who experience a secondary infection with a different virus serotype. A phenomenon called antibody-dependent enhancement (ADE) has been proposed to explain the onset of these severe cases, but the exact mechanism of ADE remains unclear.

Methodology/Principal Finding

Virus neutralization and ADE assays were performed using ultracentrifugation supernatants of acute-phase sera from patients with secondary infections or human monoclonal antibodies (HuMAbs) as anti-DENV antibodies. Virus sources included infectious serum-derived viruses from the ultracentrifugation precipitates, laboratory-culture adapted DENV, or recombinant DENVs derived from patient sera. In contrast to the high levels of ADE observed with laboratory virus strains, low ADE was observed with autologous patient-derived viruses, when patient sera were used to provide the antibody component in the ADE assays. Similar results were obtained using samples from DF and DHF patients. Recombinant-viruses derived from DHF patients showed only minor differences in neutralization and ADE activity in the presence of HuMAbs or plasma derived from the same DHF patient.

Conclusion/Significance

Serum or plasma taken from patients during the acute phase of a secondary infection showed high levels of ADE, but no neutralization activity, when assayed in the presence of laboratory-adapted virus strains. By contrast, serum or plasma from the same patient showed high levels of neutralization activity but failed to induce significant ADE when the assays were performed with autologous virus. These results demonstrate the significance of the virus source when measuring ADE. They also suggest that repeated passage of DENV in cell culture has endowed it with the capacity to induce high levels of ADE.

Vascular events in viral hemorrhagic fevers: a comparative study of dengue and hantaviruses

Viral hemorrhagic diseases are a group of systemic viral infections with worldwide distribution and are significant causes of global mortality and morbidity. The hallmarks of viral hemorrhagic fevers are plasma leakage, thrombocytopenia, coagulopathy and hemorrhagic manifestations. The molecular mechanisms leading to plasma leakage in viral hemorrhagic fevers are not well understood. A common theme has emerged in which a complex interplay between pathogens, host immune response, and endothelial cells leads to the activation of endothelial cells and perturbation of barrier integrity. In this article, two clinically distinct viral hemorrhagic fevers caused by dengue viruses and hantaviruses are discussed to highlight their similarities and differences that may provide insights into the pathogenesis and therapeutic approach.

Immunopathogenesis Versus Protection in Dengue Virus Infections

Dengue viruses (DENV) are mosquito-borne viruses that cause significant morbidity. The existence of four serotypes of DENV with partial immunologic cross-reactivity creates the opportunity for individuals to experience multiple acute DENV infections over the course of their lifetimes. Research over the past several years has revealed complex interactions between DENV and the human innate and adaptive immune systems that can have either beneficial or detrimental influences on the outcome of infection. Further studies that seek to distinguish protective from pathological immune responses in the context of natural DENV infection as well as clinical trials of candidate DENV vaccines have an important place in efforts to control the global impact of this re-emerging viral disease.

Utility of humanized BLT mice for analysis of dengue virus infection and antiviral drug testing

Dengue virus (DENV) is the cause of a potentially life-threatening disease that affects millions of people worldwide. The lack of a small animal model that mimics the symptoms of DENV infection in humans has slowed the understanding of viral pathogenesis and the development of therapies and vaccines. Here, we investigated the use of humanized "bone marrow liver thymus" (BLT) mice as a model for immunological studies and assayed their applicability for preclinical testing of antiviral compounds. Human immune system (HIS) BLT-NOD/SCID mice were inoculated intravenously with a low-passage, clinical isolate of DENV-2, and this resulted in sustained viremia and infection of leukocytes in lymphoid and nonlymphoid organs. In addition, DENV infection increased serum cytokine levels and elicited DENV-2-neutralizing human IgM antibodies. Following restimulation with DENV-infected dendritic cells, in vivo-primed T cells became activated and acquired effector function. An adenosine nucleoside inhibitor of DENV decreased the circulating viral RNA when administered simultaneously or 2 days postinfection, simulating a potential treatment protocol for DENV infection in humans. In summary, we demonstrate that BLT mice are susceptible to infection with clinical DENV isolates, mount virus-specific adaptive immune responses, and respond to antiviral drug treatment. Although additional refinements to the model are required, BLT mice are a suitable platform to study aspects of DENV infection and pathogenesis and for preclinical testing of drug and vaccine candidates. IMPORTANCE Infection with dengue virus remains a major medical problem. Progress in our understanding of the disease and development of therapeutics has been hampered by the scarcity of small animal models. Here, we show that humanized mice, i.e., animals engrafted with components of a human immune system, that were infected with a patient-derived dengue virus strain developed clinical symptoms of the disease and mounted virus-specific immune responses. We further show that this mouse model can be used to test preclinically the efficacy of antiviral drugs.

Therapeutic antibodies as a treatment option for dengue fever

Dengue fever is the most prevalent mosquito-borne viral disease globally with about 100 million cases of acute dengue annually. Severe dengue infection can result in a life-threatening illness. In the absence of either a licensed vaccine or antiviral drug against dengue, therapeutic antibodies that neutralize dengue virus (DENV) may serve as an effective medical countermeasure against severe dengue. However, therapeutic antibodies would need to effectively neutralize all four DENV serotypes. It must not induce antibody-dependent enhancement of DENV infection in monocytes/macrophages through Fc gamma receptor (FcγR)-mediated phagocytosis, which is hypothesized to increase the risk of severe dengue. Here, we review the strategies and technologies that can be adopted to develop antibodies for therapeutic applications. We also discuss the mechanism of antibody neutralization in the cells targeted by DENV that express Fc gamma receptor. These studies have provided significant insight toward the use of therapeutic antibodies as a potentially promising bulwark against dengue.

The role of endothelial activation in dengue hemorrhagic fever and hantavirus pulmonary syndrome

The loss of the endothelium barrier and vascular leakage play a central role in the pathogenesis of hemorrhagic fever viruses. This can be caused either directly by the viral infection and damage of the vascular endothelium, or indirectly by a dysregulated immune response resulting in an excessive activation of the endothelium. This article briefly reviews our knowledge of the importance of the disruption of the vascular endothelial barrier in two severe disease syndromes, dengue hemorrhagic fever and hantavirus pulmonary syndrome. Both viruses cause changes in vascular permeability without damaging the endothelium. Here we focus on our understanding of the virus interaction with the endothelium, the role of the endothelium in the induced pathogenesis, and the possible mechanisms by which each virus causes vascular leakage. Understanding the dynamics between viral infection and the dysregulation of the endothelial cell barrier will help us to define potential therapeutic targets for reducing disease severity.