Altered Cytokine Responses of Dengue-Specific CD4+T Cells to Heterologous Serotypes

Regional Variation of the CD4 and CD8 T Cell Epitopes Conserved in Circulating Dengue Viruses and Shared with Potential Vaccine Candidates

As dengue expands globally and many vaccines are under trials, there is a growing recognition of the need for assessing T cell immunity in addition to assessing the functions of neutralizing antibodies during these endeavors. While several dengue-specific experimentally validated T cell epitopes are known, less is understood about which of these epitopes are conserved among circulating dengue viruses and also shared by potential vaccine candidates. As India emerges as the epicenter of the dengue disease burden and vaccine trials commence in this region, we have here aligned known dengue specific T cell epitopes, reported from other parts of the world with published polyprotein sequences of 107 dengue virus isolates available from India. Of the 1305 CD4 and 584 CD8 epitopes, we found that 24% and 41%, respectively, were conserved universally, whereas 27% and 13% were absent in any viral isolates. With these data, we catalogued epitopes conserved in circulating dengue viruses from India and matched them with each of the six vaccine candidates under consideration (TV003, TDEN, DPIV, CYD-TDV, DENVax and TVDV). Similar analyses with viruses from Thailand, Brazil and Mexico revealed regional overlaps and variations in these patterns. Thus, our study provides detailed and nuanced insights into regional variation that should be considered for itemization of T cell responses during dengue natural infection and vaccine design, testing and evaluation.

Functional and transcriptional heterogeneity within the massively expanding HLADR+CD38+ CD8 T cell population in acute febrile dengue patients

IMPORTANCE

CD8 T cells play a crucial role in protecting against intracellular pathogens such as viruses by eliminating infected cells and releasing anti-viral cytokines such as interferon gamma (IFNγ). Consequently, there is significant interest in comprehensively characterizing CD8 T cell responses in acute dengue febrile patients. Previous studies, including our own, have demonstrated that a discrete population of CD8 T cells with HLADR+ CD38+ phenotype undergoes massive expansion during the acute febrile phase of natural dengue virus infection. Although about a third of these massively expanding HLADR+ CD38+ CD8 T cells were also CD69high when examined ex vivo, only a small fraction of them produced IFNγ upon in vitro peptide stimulation. Therefore, to better understand such functional diversity of CD8 T cells responding to dengue virus infection, it is important to know the cytokines/chemokines expressed by these peptide-stimulated HLADR+CD38+ CD8 T cells and the transcriptional profiles that distinguish the CD69+IFNγ+, CD69+IFNγ-, and CD69-IFNγ- subsets.

Viremia and Inflammatory Cytokines in Dengue: Interleukin-2 as a Biomarker of Infection, and Interferon-α and -γ as Markers of Primary versus Secondary Infection

The pathogenesis of Dengue virus (DENV) infection is complex and involves viral replication that may trigger an inflammatory response leading to severe disease. Here, we investigated the correlation between viremia and cytokine levels in the serum of DENV-infected patients. Between 2013 and 2014, 138 patients with a diagnosis of acute-phase DENV infection and 22 patients with a non-dengue acute febrile illness (AFI) were enrolled. Through a focus-forming assay (FFU), we determined the viremia levels in DENV-infected patients and observed a peak in the first two days after the onset of symptoms. A higher level of viremia was observed in primary versus secondary DENV-infected patients. Furthermore, no correlation was observed between viremia and inflammatory cytokine levels in DENV-infected patients. Receiver operating characteristic (ROC) curve analysis revealed that IL-2 has the potential to act as a marker to distinguish dengue from other febrile illnesses and is positively correlated with Th1 cytokines. IFN-α and IFN-γ appear to be potential markers of primary versus secondary infection in DENV-infected patients, respectively. The results also indicate that viremia levels are not the main driving force behind inflammation in dengue and that cytokines could be used as infection biomarkers and for differentiation between primary versus secondary infection.

Integrated systems immunology approach identifies impaired effector T cell memory responses as a feature of progression to severe dengue fever

BACKGROUND

Typical symptoms of uncomplicated dengue fever (DF) include headache, muscle pains, rash, cough, and vomiting. A proportion of cases progress to severe dengue hemorrhagic fever (DHF), associated with increased vascular permeability, thrombocytopenia, and hemorrhages. Progression to severe dengue is difficult to diagnose at the onset of fever, which complicates patient triage, posing a socio-economic burden on health systems.

METHODS

To identify parameters associated with protection and susceptibility to DHF, we pursued a systems immunology approach integrating plasma chemokine profiling, high-dimensional mass cytometry and peripheral blood mononuclear cell (PBMC) transcriptomic analysis at the onset of fever in a prospective study conducted in Indonesia.

RESULTS

After a secondary infection, progression to uncomplicated dengue featured transcriptional profiles associated with increased cell proliferation and metabolism, and an expansion of ICOS⁺CD4⁺ and CD8⁺ effector memory T cells. These responses were virtually absent in cases progressing to severe DHF, that instead mounted an innate-like response, characterised by inflammatory transcriptional profiles, high circulating levels of inflammatory chemokines and with high frequencies of CD4^low non-classical monocytes predicting increased odds of severe disease.

CONCLUSIONS

Our results suggests that effector memory T cell activation might play an important role ameliorating severe disease symptoms during a secondary dengue infection, and in the absence of that response, a strong innate inflammatory response is required to control viral replication. Our research also identified discrete cell populations predicting increased odds of severe disease, with potential diagnostic value.

Serological Evidence of Zika Virus Circulation in Burkina Faso

Zika virus (ZIKV) and dengue virus (DENV) are two closely related members of the Flaviviridae family, both transmitted by mosquitoes of the genus Aedes, and are among the arboviruses most at risk to human health. Burkina Faso has been facing an upsurge in DENV outbreaks since 2013. Unlike DENV, there is no serological evidence of ZIKV circulation in humans in Burkina Faso. The main objective of our study was to determine the seroprevalence of ZIKV and DENV in blood donors in Burkina Faso. A total of 501 donor samples collected in the two major cities of the country in 2020 were first tested by a competitive enzyme-linked immunosorbent assay to detect flavivirus antibodies. Positive sera were then tested using Luminex to detect ZIKV and DENV antibodies and virus-specific microneutralization tests against ZIKV were performed. The ZIKV seroprevalence was 22.75% in the donor samples and we found seropositivity for all DENV-serotypes ranging from 19.56% for DENV-1 to 48.86% for DENV-2. Molecular analyses performed on samples from febrile patients and Aedes aegypti mosquitoes between 2019 and 2021 were negative. Our study showed the important circulation of ZIKV and DENV detected by serology although molecular evidence of the circulation of ZIKV could not be demonstrated. It is essential to strengthen existing arbovirus surveillance in Burkina Faso and more broadly in West Africa by focusing on fevers of unknown origin and integrating vector surveillance to assess the extent of ZIKV circulation and identify the circulating strain. Further studies are needed to better understand the epidemiology of this virus in order to define appropriate prevention and response methods.

Immunoinformatics Aided Design and In-Vivo Validation of a Cross-Reactive Peptide Based Multi-Epitope Vaccine Targeting Multiple Serotypes of Dengue Virus

Dengue virus (DENV) is an arboviral disease affecting more than 400 million people annually. Only a single vaccine formulation is available commercially and many others are still under clinical trials. Despite all the efforts in vaccine designing, the improvement in vaccine formulation against DENV is very much needed. In this study, we used a roboust immunoinformatics approach, targeting all the four serotypes of DENV to design a multi-epitope vaccine. A total of 13501 MHC II binding CD4+ epitope peptides were predicted from polyprotein sequences of four dengue virus serotypes. Among them, ten conserved epitope peptides that were interferon-inducing were selected and found to be conserved among all the four dengue serotypes. The vaccine was formulated using antigenic, non-toxic and conserved multi epitopes discovered in the in-silico study. Further, the molecular docking and molecular dynamics predicted stable interactions between predicted vaccine and immune receptor, TLR-5. Finally, one of the mapped epitope peptides was synthesized for the validation of antigenicity and antibody production ability where the in-vivo tests on rabbit model was conducted. Our in-vivo analysis clearly indicate that the imunogen designed in this study could stimulate the production of antibodies which further suggest that the vaccine designed possesses good immunogenicity.

Mucosal Vaccination: A Promising Alternative Against Flaviviruses

The Flaviviridae are a family of positive-sense, single-stranded RNA enveloped viruses, and their members belong to a single genus, Flavivirus. Flaviviruses are found in mosquitoes and ticks; they are etiological agents of: dengue fever, Japanese encephalitis, West Nile virus infection, Zika virus infection, tick-borne encephalitis, and yellow fever, among others. Only a few flavivirus vaccines have been licensed for use in humans: yellow fever, dengue fever, Japanese encephalitis, tick-borne encephalitis, and Kyasanur forest disease. However, improvement is necessary in vaccination strategies and in understanding of the immunological mechanisms involved either in the infection or after vaccination. This is especially important in dengue, due to the immunological complexity of its four serotypes, cross-reactive responses, antibody-dependent enhancement, and immunological interference. In this context, mucosal vaccines represent a promising alternative against flaviviruses. Mucosal vaccination has several advantages, as inducing long-term protective immunity in both mucosal and parenteral tissues. It constitutes a friendly route of antigen administration because it is needle-free and allows for a variety of antigen delivery systems. This has promoted the development of several ways to stimulate immunity through the direct administration of antigens (e.g., inactivated virus, attenuated virus, subunits, and DNA), non-replicating vectors (e.g., nanoparticles, liposomes, bacterial ghosts, and defective-replication viral vectors), and replicating vectors (e.g., Salmonella enterica, Lactococcus lactis, Saccharomyces cerevisiae, and viral vectors). Because of these characteristics, mucosal vaccination has been explored for immunoprophylaxis against pathogens that enter the host through mucosae or parenteral areas. It is suitable against flaviviruses because this type of immunization can stimulate the parenteral responses required after bites from flavivirus-infected insects. This review focuses on the advantages of mucosal vaccine candidates against the most relevant flaviviruses in either humans or animals, providing supporting data on the feasibility of this administration route for future clinical trials.

Current status and perspectives on vaccine development against dengue virus infection

Dengue virus (DENV) consists of four serotypes in the family Flaviviridae and is a causative agent of dengue fever, dengue hemorrhagic fever, and dengue shock syndrome. DENV is transmitted by mosquitoes, Aedes aegypti and A. albopictus, and is mainly observed in areas where vector mosquitoes live. The number of dengue cases reported by the World Health Organization increased more than 8-fold over the last two decades from 505,430 in 2000 to over 2.4 million in 2010 to 5.2 million in 2019. Although vaccine is the most effective method against DENV, only one commercialized vaccine exists, and it cannot be administered to children under 9 years of age. Currently, many researchers are working to resolve the various problems hindering the development of effective dengue vaccines; understanding of the viral antigen configuration would provide insight into the development of effective vaccines against DENV infection. In this review, the current status and perspectives on effective vaccine development for DENV are examined. In addition, a plausible direction for effective vaccine development against DENV is suggested.

[Dengue virus circulation in West Africa: An emerging public health issue]

Dengue is the most widespread arbovirosis in the world, with approximately 390 million cases per year, 96 millions of which have clinical manifestations and 25,000 deaths. In West Africa, the circulation of this virus in human populations was first reported in the 1960s in Nigeria. Clinical diagnosis of dengue in West Africa is made difficult by the existence of other diseases with similar clinical presentations. Biological diagnosis remains therefore the only alternative. This biological diagnosis requires high quality equipment and well-trained personnel, which are not always available in resource-limited countries. Thus, many cases of dengue fever are consistently reported as malaria, leading to mismanagement, which can have serious consequences on the health status of patients. It is therefore necessary to set up surveillance systems for febrile infections of unknown origin in Africa by strengthening the diagnostic capacities of national laboratories.

Altered Moesin and Actin Cytoskeleton Protein Rearrangements Affect Transendothelial Permeability in Human Endothelial Cells upon Dengue Virus Infection and TNF-α Treatment

It has been hypothesized that the host, viral factors, and secreted cytokines (especially TNF-α) play roles in the pathogenesis of secondary dengue infections. Mass spectrometry-based proteomic screening of cytoskeleton fractions isolated from human endothelial (EA.hy926) cells upon dengue virus (DENV) infection and TNF-α treatment identified 450 differentially altered proteins. Among them, decreased levels of moesin, actin stress fiber rearrangements, and dot-like formations of vinculin were observed with western blot analyses and/or immunofluorescence staining (IFA). In vitro vascular permeability assays using EA.hy926 cells, seeded on collagen-coated transwell inserts, showed low levels of transendothelial electrical resistance in treated cells. The synergistic effects of DENV infection and TNF-α treatment caused cellular permeability changes in EA.hy926 cells, which coincided with decreasing moesin levels and the production of abnormal organizations of actin stress fibers and vinculin. Functional studies demonstrated moesin overexpression restored transendothelial permeability in DENV/TNF-α-treated EA.hy926 cells. The present study improves the understanding of the disruption mechanisms of cytoskeleton proteins in enhancing vascular permeability during DENV infection and TNF-α treatment. The study also suggests that these disruption mechanisms are major factors contributing to vascular leakage in severe dengue patients.

Longitudinal Analysis of Dengue Virus-Specific Memory T Cell Responses and Their Association With Clinical Outcome in Subsequent DENV Infection

Memory T cells resulting from primary dengue virus (DENV) infection are hypothesized to influence the clinical outcome of subsequent DENV infection. However, the few studies involving prospectively collected blood samples have found weak and inconsistent associations with outcome and variable temporal trends in DENV-specific memory T cell responses between subjects. This study used both ex-vivo and cultured ELISPOT assays to further evaluate the associations between DENV serotype-cross-reactive memory T cells and severity of secondary infection. Using ex-vivo ELISPOT assays, frequencies of memory T cells secreting IFN-γ in response to DENV structural and non-structural peptide pools were low in PBMC from multiple time points prior to symptomatic secondary DENV infection and showed a variable response to infection. There were no differences in responses between subjects who were not hospitalized (NH, n=6) and those who were hospitalized with dengue hemorrhagic fever (hDHF, n=4). In contrast, responses in cultured ELISPOT assays were more reliably detectable prior to secondary infection and showed more consistent increases after infection. Responses in cultured ELISPOT assays were higher in individuals with hDHF (n=8) compared to NH (n=9) individuals before the secondary infection, with no difference between these groups after infection. These data demonstrate an association of pre-existing DENV-specific memory responses with the severity of illness in subsequent DENV infection, and suggest that frequencies of DENV-reactive T cells measured after short-term culture may be of particular importance for assessing the risk for more severe dengue disease.

Innate Lymphoid Cells Activation and Transcriptomic Changes in Response to Human Dengue Infection

Background

Dengue virus (DENV) infection has a global impact on public health. The clinical outcomes (of DENV) can vary from a flu-like illness called dengue fever (DF), to a more severe form, known as dengue hemorrhagic fever (DHF). The underlying innate immune mechanisms leading to protective or detrimental outcomes have not been fully elucidated. Helper innate lymphoid cells (hILCs), an innate lymphocyte recently discovered, functionally resemble T-helper cells and are important in inflammation and homeostasis. However, the role of hILCs in DENV infection had been unexplored.

Methods

We performed flow cytometry to investigate the frequency and phenotype of hILCs in peripheral blood mononuclear cells from DENV-infected patients of different disease severities (DF and DHF), and at different phases (febrile and convalescence) of infection. Intracellular cytokine staining of hILCs from DF and DHF were also evaluated by flow cytometry after ex vivo stimulation. Further, the hILCs were sorted and subjected to transcriptome analysis using RNA sequencing. Differential gene expression analysis was performed to compare the febrile and convalescent phase samples in DF and DHF. Selected differentially expressed genes were then validated by quantitative PCR.

Results

Phenotypic analysis showed marked activation of all three hILC subsets during the febrile phase as shown by higher CD69 expression when compared to paired convalescent samples, although the frequency of hILCs remained unchanged. Upon ex vivo stimulation, hILCs from febrile phase DHF produced significantly higher IFN-γ and IL-4 when compared to those of DF. Transcriptomic analysis showed unique hILCs gene expression in DF and DHF, suggesting that divergent functions of hILCs may be associated with different disease severities. Differential gene expression analysis indicated that hILCs function both in cytokine secretion and cytotoxicity during the febrile phase of DENV infection.

Conclusions

Helper ILCs are activated in the febrile phase of DENV infection and display unique transcriptomic changes as well as cytokine production that correlate with severity. Targeting hILCs during early innate response to DENV might help shape subsequent immune responses and potentially lessen the disease severity in the future.

Characterization of the IL-17 and CD4+ Th17 Cells in the Clinical Course of Dengue Virus Infections

The aims of this study were to determine the involvement of interleukin 17 (IL-17) and IL-17-producing cells in dengue pathogenesis. Blood samples from dengue virus (DENV)-infected patients were collected on different days after the onset of symptoms. Patients were classified according to 1997 World Health Organization guidelines. Our study examined 152 blood samples from dengue fever (DF, n = 109) and dengue hemorrhagic fever (DHF, n = 43) patients and 90 blood samples from healthy controls (HC). High serum concentrations of IL-17A and IL-22 were also associated with DHF (IL-17A [DHF vs. DF, p < 0.01; DHF vs. HC, p < 0.0001]; IL-22 [DHF vs. DF, p < 0.05; DHF vs. HC, p < 0.0001]). Moreover, there was a positive correlation between serum levels of IL-17A and IL-23, a key cytokine that promotes IL-17-based immune responses (r = 0.4089, p < 0.0001). Consistent with the IL-17-biased immune response in DHF patients, we performed ex vivo activation of peripheral blood mononuclear cells (PBMCs) from DHF patients and flow cytometry analysis showed a robust IL-17-biased immune response, characterized by a high frequency of CD4+IL-17+ producing cells. Our results suggests IL-17-producing cells and their related cytokines can play a prominent role in this viral disease.

T lymphocyte responses to flaviviruses - diverse cell populations affect tendency toward protection and disease

Dengue virus (DENV), Yellow Fever virus, West Nile virus, Japanese encephalitis virus and Zika virus are medically important flaviviruses transmitted to humans by mosquitoes and circulate in overlapping geographic areas. Cross-reactive immune responses have been demonstrated among the flaviviruses, particularly the four DENV serotypes. The immunological imprint left by a flavivirus infection can therefore have profound effects on the responses to subsequent infections. In this review we summarize recent research focusing on T cell responses to DENV using clinical samples from prospective cohort studies in Asia. These data suggest that durability of different T cell populations after natural infection or vaccination is an important consideration for the outcome of subsequent flavivirus exposures and we argue for continued investigation in the context of longitudinal cohort studies.

Tick-Borne Encephalitis Virus: A Quest for Better Vaccines against a Virus on the Rise

Tick-borne encephalitis virus (TBEV), a member of the family Flaviviridae, is one of the most important tick-transmitted viruses in Europe and Asia. Being a neurotropic virus, TBEV causes infection of the central nervous system, leading to various (permanent) neurological disorders summarized as tick-borne encephalitis (TBE). The incidence of TBE cases has increased due to the expansion of TBEV and its vectors. Since antiviral treatment is lacking, vaccination against TBEV is the most important protective measure. However, vaccination coverage is relatively low and immunogenicity of the currently available vaccines is limited, which may account for the vaccine failures that are observed. Understanding the TBEV-specific correlates of protection is of pivotal importance for developing novel and improved TBEV vaccines. For affording robust protection against infection and development of TBE, vaccines should induce both humoral and cellular immunity. In this review, the adaptive immunity induced upon TBEV infection and vaccination as well as novel approaches to produce improved TBEV vaccines are discussed.

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

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

Identification of highly conserved, serotype-specific dengue virus sequences: implications for vaccine design

Background

The sequence diversity of dengue virus (DENV) is one of the challenges in developing an effective vaccine against the virus. Highly conserved, serotype-specific (HCSS), immune-relevant DENV sequences are attractive candidates for vaccine design, and represent an alternative to the approach of selecting pan-DENV conserved sequences. The former aims to limit the number of possible cross-reactive epitope variants in the population, while the latter aims to limit the cross-reactivity between the serotypes to favour a serotype-specific response. Herein, we performed a large-scale systematic study to map and characterise HCSS sequences in the DENV proteome.

Methods

All reported DENV protein sequence data for each serotype was retrieved from the NCBI Entrez Protein (nr) Database (txid: 12637). The downloaded sequences were then separated according to the individual serotype proteins by use of BLASTp search, and subsequently removed for duplicates and co-aligned across the serotypes. Shannon’s entropy and mutual information (MI) analyses, by use of AVANA, were performed to measure the diversity within and between the serotype proteins to identify HCSS nonamers. The sequences were evaluated for the presence of promiscuous T-cell epitopes by use of NetCTLpan 1.1 and NetMHCIIpan 3.2 server for human leukocyte antigen (HLA) class I and class II supertypes, respectively. The predicted epitopes were matched to reported epitopes in the Immune Epitope Database.

Results

A total of 2321 nonamers met the HCSS selection criteria of entropy < 0.25 and MI > 0.8. Concatenating these resulted in a total of 337 HCSS sequences. DENV4 had the most number of HCSS nonamers; NS5, NS3 and E proteins had among the highest, with none in the C and only one in prM. The HCSS sequences were immune-relevant; 87 HCSS sequences were both reported T-cell epitopes/ligands in human and predicted epitopes, supporting the accuracy of the predictions. A number of the HCSS clustered as immunological hotspots and exhibited putative promiscuity beyond a single HLA supertype. The HCSS sequences represented, on average, ~ 40% of the proteome length for each serotype; more than double of pan-DENV sequences (conserved across the four serotypes), and thus offer a larger choice of sequences for vaccine target selection. HCSS sequences of a given serotype showed significant amino acid difference to all the variants of the other serotypes, supporting the notion of serotype-specificity.

Conclusion

This work provides a catalogue of HCSS sequences in the DENV proteome, as candidates for vaccine target selection. The methodology described herein provides a framework for similar application to other pathogens.

Adaptive immune responses to primary and secondary dengue virus infections

Dengue is the leading mosquito-borne viral illness infecting humans. Owing to the circulation of multiple serotypes, global expansion of the disease and recent gains in vaccination coverage, pre-existing immunity to dengue virus is abundant in the human population, and secondary dengue infections are common. Here, we contrast the mechanisms initiating and sustaining adaptive immune responses during primary infection with the immune pathways that are pre-existing and reactivated during secondary dengue. We also discuss new developments in our understanding of the contributions of CD4⁺ T cells, CD8⁺ T cells and antibodies to immunity and memory recall. Memory recall may lead to protective or pathological outcomes, and understanding of these processes will be key to developing or refining dengue vaccines to be safe and effective.

Human T Cell Response to Dengue Virus Infection

DENV is a major public health problem worldwide, thus underlining the overall significance of the proposed Program. The four dengue virus (DENV) serotypes (1-4) cause the most common mosquito-borne viral disease of humans, with 3 billion people at risk for infection and up to 100 million cases each year, most often affecting children. The protective role of T cells during viral infection is well-established. Generally, CD8 T cells can control viral infection through several mechanisms, including direct cytotoxicity, and production of pro-inflammatory cytokines such as IFN-γ and TNF-α. Similarly, CD4 T cells are thought to control viral infection through multiple mechanisms, including enhancement of B and CD8 T cell responses, production of inflammatory and anti-viral cytokines, cytotoxicity, and promotion of memory responses. To probe the phenotype of virus-specific T cells, epitopes derived from viral sequences need to be known. Here we discuss the identification of CD4 and CD8 T cell epitopes derived from DENV and how these epitopes have been used by researchers to interrogate the phenotype and function of DENV-specific T cell populations.

T Cell Responses Induced by DNA Vaccines Based on the DENV2 E and NS1 Proteins in Mice: Importance in Protection and Immunodominant Epitope Identification

The importance of the cellular immune response against DENV has been increasingly highlighted in the past few years, in particular for vaccine development. We have previously constructed two plasmids, pE1D2, and pcTPANS1, encoding the envelope (E) ectodomain (domains I, II, and III) and the non-structural 1 (NS1) protein of dengue virus serotype 2 (DENV2), respectively. In the present work, we analyzed the induction of the cellular response in mice immunized with these DNA vaccines and identified the immunogenic peptides. Vaccinated BALB/c mice became protected against a lethal challenge of DENV2. Depletion of CD4+ cells in vaccinated animals almost completely abolished protection elicited by both vaccines. In contrast, a significant number of pE1D2- and pcTPANS1-immunized mice survived virus challenge after depletion of CD8+ cells, although some animals presented morbidity. To identify immunogenic peptides recognized by T cells, we stimulated splenocytes with overlapping peptide libraries covering the E and NS1 proteins and evaluated the production of IFN-γ by ELISPOT. We detected two and three immunodominant epitopes in the E and NS1 proteins, respectively, and four additional NS1-derived peptides after virus challenge. Characterization by intracellular cytokine staining (ICS) revealed that both CD4+ and CD8+ T cells were involved in IFN-γ and TNF-α production. The IFN-γ ICS confirmed reaction of almost all E-derived peptides before challenge and identified other epitopes after infection. All NS1-derived peptides were able to elicit IFN-γ production in CD4+ cells, while only a few peptides induced expression of this cytokine in CD8+ T lymphocytes. Interestingly, we observed an increase in the frequency of either CD4+ or CD8+ T cells producing TNF-α after immunization with the pE1D2 and challenge with DENV2, while lymphocytes from pcTPANS1-vaccinated animals maintained ordinary TNF-α production after virus infection. We also assessed the recognition of E and NS1 immunogenic peptides in C57BL/6 mice due to the difference in MHC haplotype expression. Two NS1-derived epitopes featured prominently in the IFN-γ response with cells from both animal strains. Overall, our results emphasize the importance of the T cell response involved in protection against dengue induced by E and NS1 based DNA vaccines.

A Modified mRNA Vaccine Targeting Immunodominant NS Epitopes Protects Against Dengue Virus Infection in HLA Class I Transgenic Mice

Dengue virus (DENV) induces strong T and B cell responses upon infection. Hence, it is difficult to determine the contribution of cell-mediated immunity alone in the long lasting protection against DENV infection and disease. Numerous CD4+ and CD8+ T cell epitopes have been identified, mainly in the non-structural proteins of DENV. Taking into account the immunogenicity and peptide sequence conservation among the different DENV serotypes, a minimal DENV antigen, called DENV1-NS, has been designed. This antigen is enriched in conserved and highly antigenic epitopes located in the NS3, NS4B, and NS5 regions of DENV1. To evaluate the ability of the DENV1-NS poly-epitope to express the antigenic peptides in the context of different HLA class I molecules, we established its in vivo immunogenicity by measuring, after DNA immunization and electroporation, the activation of DENV-specific CD8 T cells in transgenic mice expressing the human HLA-A*0201, -A*2402, -B*0702, and -B*3502 class I alleles. We then engineered a lipid nanoparticle (LNP) encapsulated modified mRNA vaccine encoding DENV1-NS and tested immunogenicity and protection in these human HLA class I transgenic mice, after transient blockade of the interferon (IFN) type I receptor. Significant protection was observed, after two injections of the mRNA vaccine. Collectively, these data strongly support the development of T cell-based vaccines targeting immunodominant T cell epitopes that generate potent virus-specific T cell responses conferring immunity against DENV infection.

Longitudinal Analysis of Memory B and T Cell Responses to Dengue Virus in a 5-Year Prospective Cohort Study in Thailand

Prior exposure to dengue virus (DENV) has a profound impact on the outcome of infection, which varies according to the interval between infections. Antibodies secreted by B cells and cytokines secreted by T cells are thought to contribute both to protective immunity against DENV and the pathogenesis of dengue disease. We analyzed peripheral blood mononuclear cells (PBMC) collected from Thai children over a 5-year prospective cohort study to define the dynamics of DENV-specific memory B and T cell responses and the impact of symptomatic or subclinical DENV infections. To measure B cell responses, PBMC were stimulated with IL-2 plus R848 and culture supernatants were tested for DENV-binding antibodies by ELISA. To measure T cell responses, PBMC were stimulated in dual-color ELISPOT assays with overlapping peptide pools of structural and non-structural proteins from the four DENV types. B cell responses were low to one or more DENV types prior to symptomatic infection and increased with reactivity to all four types after infection. Subjects who had a subclinical infection or who did not experience a DENV infection during the study period showed strong memory B cell responses to all four DENV types. T cell responses to DENV peptides demonstrated a cytokine hierarchy of IFN-γ > IL-2 > IFN-γ/IL-2. T cell responses were low or absent prior to secondary infections. The trends in T cell responses to DENV peptides over 3 year post-infection were highly variable, but subjects who had experienced a secondary DENV1 infection showed higher cytokine responses compared to subjects who had experienced a secondary DENV2 or subclinical infection. The longitudinal nature of our study demonstrates persistent memory B cell responses over years and a lasting but variable impact of secondary DENV infection on DENV-specific T cell responses.

Mouse Models of Heterologous Flavivirus Immunity: A Role for Cross-Reactive T Cells

Most of the world is at risk of being infected with a flavivirus such as dengue virus, West Nile virus, yellow fever virus, Japanese encephalitis virus, tick-borne encephalitis virus, and Zika virus, significantly impacting millions of lives. Importantly, many of these genetically similar viruses co-circulate within the same geographic regions, making it likely for individuals living in areas of high flavivirus endemicity to be infected with multiple flaviviruses during their lifetime. Following a flavivirus infection, a robust virus-specific T cell response is generated and the memory recall of this response has been demonstrated to provide long-lasting immunity, protecting against reinfection with the same pathogen. However, multiple studies have shown that this flavivirus specific T cell response can be cross-reactive and active during heterologous flavivirus infection, leading to the question: How does immunity to one flavivirus shape immunity to the next, and how does this impact disease? It has been proposed that in some cases unfavorable disease outcomes may be caused by lower avidity cross-reactive memory T cells generated during a primary flavivirus infection that preferentially expand during a secondary heterologous infection and function sub optimally against the new pathogen. While in other cases, these cross-reactive cells still have the potential to facilitate cross-protection. In this review, we focus on cross-reactive T cell responses to flaviviruses and the concepts and consequences of T cell cross-reactivity, with particular emphasis linking data generated using murine models to our new understanding of disease outcomes following heterologous flavivirus infection.

Severe Dengue Prognosis Using Human Genome Data and Machine Learning

Dengue has become one of the most important worldwide arthropod-borne diseases. Dengue phenotypes are based on laboratorial and clinical exams, which are known to be inaccurate.

OBJECTIVE

We present a machine learning approach for the prediction of dengue fever severity based solely on human genome data.

METHODS

One hundred and two Brazilian dengue patients and controls were genotyped for 322 innate immunity single nucleotide polymorphisms (SNPs). Our model uses a support vector machine algorithm to find the optimal loci classification subset and then an artificial neural network (ANN) is used to classify patients into dengue fever or severe dengue.

RESULTS

The ANN trained on 13 key immune SNPs selected under dominant or recessive models produced median values of accuracy greater than 86%, and sensitivity and specificity over 98% and 51%, respectively.

CONCLUSION

The proposed classification method, using only genome markers, can be used to identify individuals at high risk for developing the severe dengue phenotype even in uninfected conditions.

SIGNIFICANCE

Our results suggest that the genetic context is a key element in phenotype definition in dengue. The methodology proposed here is extendable to other Mendelian based and genetically influenced diseases.

Dengue haemorrhagic fever: a job done via exosomes?

Dengue fever is one of those unique diseases where host immune responses largely determine the pathogenesis and its severity. Earlier studies have established the fact that dengue virus (DENV) infection causes haemorrhagic fever and shock syndrome, but it is not directly responsible for exhibiting these clinical symptoms. It is noteworthy that clinically, vascular leakage syndrome does not develop for several days after infection despite a robust innate immune response that elicits the production of proinflammatory and proangiogenic cytokines. The onset of hyperpermeability in severe cases of dengue disease takes place around the time of defervescence and after clearance of viraemia. Extracellular vesicles are known to carry biological information (mRNA, miRNA, transcription factors) from their cells of origin and have emerged as a significant vehicle for horizontal transfer of stress signals. In dengue virus infection, the relevance of exosomes can be instrumental since the majority of the immune responses in severe dengue involve heavy secretion and circulation of pro-inflammatory cytokines and chemokines. Here, we present an updated review which will address the unique and puzzling features of hyperpermeability associated with DENV infection with a special focus on the role of secreted extracellular vesicles.

Plectin is Required for Trans-Endothelial Permeability: A Model of Plectin Dysfunction in Human Endothelial Cells After TNF-α Treatment and Dengue Virus Infection

The clinical sign of dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS) in humans is increased vascular permeability. Virus-specific factors and host factors, including secreted cytokines and especially TNF-α, are suggested as having roles in the pathogenesis of these conditions. Proteomic analysis with MS is performed in membrane fraction isolated from human endothelial cells (EA.hy926) upon DENV infection and TNF-α treatment. In the 451 altered proteins that are identified, decreased plectin expression is revealed by Western blot analysis, while immunofluorescence staining (IFA) shows actin stress fiber rearrangement and decreased VE-cadherin in treated EA.hy926 cells. In vitro vascular permeability assay was used to determine transepithelial electrical resistance (TEER) in EA.hy926 cells seeded on collagen-coated Transwell inserts. The low level of TEER, the low expression of plectin and VE-cadherin, and the unusual organization of actin stress fiber are found to be correlated with increased membrane permeability in DENV2 and TNF-α-treated EA.hy926 cells. Similar results are observed when using siRNA knockdown plectin in mock EA.hy926 cells. This study provides better understanding of the role that disruption of cytoskeleton linker protein plays in increased vascular permeability, and suggests these factors as major contributors to vascular leakage in DHF/DSS patients.

Chagas Disease in Southern Coastal Ecuador: Coinfections with Arboviruses and a Comparison of Serological Assays for Chagas Disease Diagnosis

Occurrence of Chagas disease and arbovirus coinfections is unknown, despite the vast co-endemic areas throughout the Americas. This study examined the proportion of individuals positive for Trypanosoma cruzi and coinfections with dengue, chikungunya, and Zika viruses in Machala, Ecuador (January 2014-December 2015). Chagas seropositivity was evaluated with five commercially available assays. Dengue infections were identified by nonstructural protein 1 rapid test and enzyme linked immunosorbent assay (ELISA), immunoglobulin M ELISA, and reverse transcription PCR (RT-PCR); chikungunya and Zika infections were identified by RT-PCR. Of 658 individuals, six were positive for T. cruzi (0.91%), including one T. cruzi/dengue coinfection and one T. cruzi/chikungunya/dengue coinfection. The clinical manifestations of coinfected individuals corresponded to severe dengue and dengue with warning signs, respectively. We observed discrepant results by using the Hemagen Chagas kit and the rapid test Chagas Detect Plus (false positives: 3.9% and 15.4%), highlighting the need to assess diagnostic assays in geographic regions with distinct taxonomic units of T. cruzi.

CD4 T cell responses to flaviviruses

Flaviviruses pose an increasing threat to global health with their potential to cause severe disease in millions of people. Protective and long-lived immunity is closely linked to the generation of CD4 T cells, which provide B cell help and support high affinity neutralizing antibody responses. Research performed during the last years revealed important new insights into the antigen specificities and diverse effector functions of CD4 T cell responses to flaviviruses. Moreover, the identification of mechanisms involved in the regulation of T cell specificity and function provides significant advances in our understanding of how durable protective immunity is established. Here, we summarize what is known about human CD4 T cell responses to flaviviruses, with a special emphasis on CD4 T cells that provide direct help to B cells producing neutralizing and protective antibodies. We review recent progress in the identification of epitope sites in the context of the atomic structures of flavivirus proteins and highlight specific influences that shape the human CD4 T cell response in the context of infection or vaccination. Finally, we discuss challenges facing vaccine efforts to generate appropriate CD4 T cell responses, as well as recent strategies to enhance T cell-mediated antibody responses.

Cross-Reactivity and Anti-viral Function of Dengue Capsid and NS3-Specific Memory T Cells Toward Zika Virus

Zika virus (ZIKV), a flavivirus with homology to dengue virus (DENV), is spreading to areas of DENV hyper-endemicity. Heterologous T cell immunity, whereby virus-specific memory T cells are activated by variant peptides derived from a different virus, can lead to enhanced viral clearance or diminished protective immunity and altered immunopathology. In mice, CD8+ T cells specific for DENV provide in vivo protective efficacy against subsequent ZIKV infection. In humans, contrasting studies report complete absence or varying degrees of DENV/ZIKV T cell cross-reactivity. Moreover, the impact of cross-reactive T cell recognition on the anti-viral capacity of T cells remains unclear. Here, we show that DENV-specific memory T cells display robust cross-reactive recognition of ZIKV NS3 ex vivo and after in vitro expansion in respectively n = 7/10 and n = 9/9 dengue-immune individuals tested. In contrast, cross-reactivity toward ZIKV capsid is low or absent. Cross-reactive recognition of DENV or ZIKV NS3 peptides elicits similar production of the anti-viral effector mediators IFN-γ, TNF-α, and CD107a. We identify 9 DENV/ZIKV cross-reactive epitopes, 7 of which are CD4+ and 2 are CD8+ T cell epitopes. We also show that cross-reactive CD4+ and CD8+ T cells targeting novel NS3 epitopes display anti-viral effector potential toward ZIKV-infected cells, with CD8+ T cells mediating direct lyses of these cells. Our results demonstrate that DENV NS3-specific memory T cells display anti-viral effector capacity toward ZIKV, suggesting a potential beneficial effect in humans of pre-existing T cell immunity to DENV upon ZIKV infection.

Immune Responses to Dengue and Zika Viruses-Guidance for T Cell Vaccine Development

Despite numerous efforts to identify the molecular and cellular effectors of the adaptive immunity that induce a long-lasting immunity against dengue or Zika virus infection, the specific mechanisms underlying such protective immunity remain largely unknown. One of the major challenges lies in the high level of dengue virus (DENV) seroprevalence in areas where Zika virus (ZIKV) is circulating. In the context of such a pre-existing DENV immunity that can exacerbate ZIKV infection and disease, and given the lack of appropriate treatment for ZIKV infection, there is an urgent need to develop an efficient vaccine against DENV and ZIKV. Notably, whereas several ZIKV vaccine candidates are currently in clinical trials, all these vaccine candidates have been designed to induce neutralizing antibodies as the primary mechanism of immune protection. Given the difficulty to elicit simultaneously high levels of neutralizing antibodies against the different DENV serotypes, and the potential impact of pre-existing subneutralizing antibodies induced upon DENV infection or vaccination on ZIKV infection and disease, additional or alternative strategies to enhance vaccine efficacy, through T cell immunity, are now being considered. In this review, we summarize recent discoveries about cross-reactive B and T cell responses against DENV and ZIKV and propose guidelines for the development of safe and efficient T cell vaccines targeting both viruses.

Serum Cytokine Profiles in Patients with Dengue Fever at the Acute Infection Phase

Background

Dengue virus (DENV) is transmitted by mosquito and has been circulating in Guangdong, China, for over 30 years. Dengue infection causes mild to severe disease symptoms in human. Cytokine profiles were suggested to be crucial especially during the acute stage in the dengue infection.

Aim

To determine the cytokine profiles at the acute stage in patients with primary or secondary dengue infection in Guangzhou city in the 2014 outbreak.

Methods

We investigated 23 inflammatory cytokines in serum collected from dengue-infected patients and analyzed their correlations with their clinical indexes.

Results

The concentrations of CXCL9, IP-10, CXCL11, IL-8, IL-10, and CCL2 in serum were significantly higher in the groups of DENV-infected patients during the first two weeks than those of control group while CCL17 and CXCL5 showed lower expression level in the patients. Among these cytokines, CXCL9, CCL17, and CXCL5 showed statistical difference between the groups of primary and secondary infections. The platelet count and lactate dehydrogenase were correlated with the level of CCL17 and MIP-1α/CXCL5, respectively, in the group of secondary infection.

Conclusions

We determined the cytokine profiles in serum of the patients during the 2014 dengue outbreak. The expression of specific cytokines was associated with the secondary infection.

Immune Response to Dengue and Zika

Flaviviruses such as dengue (DENV), yellow fever (YFV), West Nile (WNV), and Zika (ZIKV) are human pathogens of global significance. In particular, DENV causes the most prevalent mosquito-borne viral diseases in humans, and ZIKV emerged from obscurity into the spotlight in 2016 as the etiologic agent of congenital Zika syndrome. Owing to the recent emergence of ZIKV as a global pandemic threat, the roles of the immune system during ZIKV infections are as yet unclear. In contrast, decades of DENV research implicate a dual role for the immune system in protection against and pathogenesis of DENV infection. As DENV and ZIKV are closely related, knowledge based on DENV studies has been used to prioritize investigation of ZIKV immunity and pathogenesis, and to accelerate ZIKV diagnostic, therapeutic, and vaccine design. This review discusses the following topics related to innate and adaptive immune responses to DENV and ZIKV: the interferon system as the key mechanism of host defense and viral target for immune evasion, antibody-mediated protection versus antibody-dependent enhancement, and T cell-mediated protection versus original T cell antigenic sin. Understanding the mechanisms that regulate the balance between immune-mediated protection and pathogenesis during DENV and ZIKV infections is critical toward development of safe and effective DENV and ZIKV therapeutics and vaccines.

Understanding the Human T Cell Response to Dengue Virus

Our understanding of how T cells respond to dengue virus has greatly advanced in the last decade but important questions still remain unanswered. Dengue virus infection elicits a broad anti-viral T cell response with NS3, NS4b and NS5 being the main targets for CD8+ T cells, which dominate the response while the structural proteins capsid, envelope and the secreted protein NS1 are the preferential targets for CD4+ T cells. Upon T cell activation during acute dengue infection, dengue-specific T cells acquire expression of the skin-homing marker cutaneous associated antigen (CLA) and they can be found at high frequencies in the skin of infected patients. This suggests that the skin represents an important site for the immuno surveillance of dengue virus. The immunoprotective role of skin-homing dengue-specific T cells, their potential involvement in pathological skin manifestations and their long-term persistence as tissue resident T cells to provide immediate onsite protection are open questions that we are currently investigating. The contribution of pre-existing dengue-specific T cells towards protective immunity and/or immunopathology during secondary dengue infection remains a major knowledge gap. The evidence supporting these opposing outcomes and our current understanding of the characteristics of the human T cell response to dengue virus will be discussed.

Development of Peptide Vaccines in Dengue

Dengue is one of the most important arboviral infections worldwide, infecting up to 390 million people and causing 25,000 deaths annually. Although a licensed dengue vaccine is available, it is not efficacious against dengue serotypes that infect people living in South East Asia, where dengue is an endemic disease. Hence, there is an urgent need to develop an efficient dengue vaccine for this region. Data from different clinical trials indicate that a successful dengue vaccine must elicit both neutralizing antibodies and cell mediated immunity. This can be achieved by designing a multi-epitope peptide vaccine comprising B, CD8+ and CD4+ T cell epitopes. As recognition of T cell epitopes are restricted by human leukocyte antigens (HLA), T cell epitopes which are able to recognize several major HLAs will be preferentially included in the vaccine design. While peptide vaccines are safe, biocompatible and cost-effective, it is poorly immunogenic. Strategies to improve its immunogenicity by the use of long peptides, adjuvants and nanoparticle delivery mechanisms are discussed.

Dengue vaccine-induced CD8+ T cell immunity confers protection in the context of enhancing, interfering maternal antibodies

Declining levels of maternal antibodies were shown to sensitize infants born to dengue-immune mothers to severe disease during primary infection, through the process of antibody-dependent enhancement of infection (ADE). With the recent approval for human use of Sanofi-Pasteur's chimeric dengue vaccine CYD-TDV and several vaccine candidates in clinical development, the scenario of infants born to vaccinated mothers has become a reality. This raises 2 questions: will declining levels of maternal vaccine-induced antibodies cause ADE; and, will maternal antibodies interfere with vaccination efficacy in the infant? To address these questions, the above scenario was modeled in mice. Type I IFN-deficient female mice were immunized with live attenuated DENV2 PDK53, the core component of the tetravalent DENVax candidate currently under clinical development. Pups born to PDK53-immunized dams acquired maternal antibodies that strongly neutralized parental strain 16681, but not the heterologous DENV2 strain D2Y98P-PP1, and instead caused ADE during primary infection with this strain. Furthermore, pups failed to seroconvert after PDK53 vaccination, owing to maternal antibody interference. However, a cross-protective multifunctional CD8+ T cell response did develop. Thus, our work advocates for the development of dengue vaccine candidates that induce protective CD8+ T cells despite the presence of enhancing, interfering maternal antibodies.

Dengue virus-reactive CD8+ T cells mediate cross-protection against subsequent Zika virus challenge

Zika virus (ZIKV) and dengue virus (DENV) are antigenically related flaviviruses that share cross-reactivity in antibody and T cell responses, and co-circulate in increasing numbers of countries. Whether pre-existing DENV immunity can cross-protect or enhance ZIKV infection during sequential infection of the same host is unknown. Here, we show that DENV-immune Ifnar1^−/− or wild-type C57BL/6 mice infected with ZIKV have cross-reactive immunity to subsequent ZIKV infection and pathogenesis. Adoptive transfer and cell depletion studies demonstrate that DENV-immune CD8⁺ T cells predominantly mediate cross-protective responses to ZIKV. In contrast, passive transfer studies suggest that DENV-immune serum does not protect against ZIKV infection. Thus, CD8⁺ T cell immunity generated during primary DENV infection can confer protection against secondary ZIKV infection in mice. Further optimization of current DENV vaccines for T cell responses might confer cross-protection and prevent antibody-mediated enhancement of ZIKV infection.

Dengue virus-specific antibody and CD8⁺ T cells that cross-react with Zika virus have been described. Here, the authors establish a functionally protective role for cross-reactive dengue virus-specific CD8⁺ T cells during challenge with Zika virus.

Immune correlates of protection for dengue: State of the art and research agenda

Dengue viruses (DENV1-4) are mosquito-borne flaviviruses estimated to cause up to ∼400 million infections and ∼100 million dengue cases each year. Factors that contribute to protection from and risk of dengue and severe dengue disease have been studied extensively but are still not fully understood. Results from Phase 3 vaccine efficacy trials have recently become available for one vaccine candidate, now licensed for use in several countries, and more Phase 2 and 3 studies of additional vaccine candidates are ongoing, making these issues all the more urgent and timely. At the "Summit on Dengue Immune Correlates of Protection", held in Annecy, France, on March 8-9, 2016, dengue experts from diverse fields came together to discuss the current understanding of the immune response to and protection from DENV infection and disease, identify key unanswered questions, discuss data on immune correlates and plans for comparison of results across assays/consortia, and propose a research agenda for investigation of dengue immune correlates, all in the context of both natural infection studies and vaccine trials.

Serum from dengue virus-infected patients with and without plasma leakage differentially affects endothelial cells barrier function in vitro

Background

Although most of cases of dengue infections are asymptomatic or mild symptomatic some individuals present warning signs progressing to severe dengue in which plasma leakage is a hallmark.

Methodology/Principal findings

The present study used Electric Cell-substrate Impedance Sensing (ECIS^®) which allows for electrical monitoring of cellular barrier function measuring changes in Transendothelial Electric Resistance (TEER) to investigate the parameters associated with dengue induced leakage. Three groups of individuals were tested: dengue-positives with plasma leakage (leakage), dengue-positives without plasma leakage (no leakage), and dengue-negatives (control). Data show that TEER values of human umbilical vein endothelial cells (HUVECs) was significantly lower after incubation with serum from subjects of the leakage group in comparison to the no leakage or control groups. The serum levels of CXCL1, EGF, eotaxin, IFN-γ, sCD40L, and platelets were significantly decreased in the leakage group, while IL-10, IL-6, and IP-10 levels were significantly increased. We also found a strong correlation between TEER values and augmented levels of IP-10, GM-CSF, IL-1α, and IL-8, as well as decreased levels of CXCL1 and platelets.

Conclusions/Significance

The present work shows that the magnitude of the immune response contributes to the adverse plasma leakage outcomes in patients and that serum components are important mediators of changes in endothelial homeostasis during dengue infections. In particular, the increased levels of IP-10 and the decreased levels of CXCL1 and platelets seem to play a significant role in the disruption of vascular endothelium associated with leakage outcomes after DENV infection. These findings may have important implications for both diagnostic and therapeutic approaches to predict and mitigate vascular permeabilization in those experiencing the most severe clinical disease outcomes after dengue infection.

Host-Virus Interaction of ZIKA Virus in Modulating Disease Pathogenesis

The Zika virus (ZIKV) is a newly emerging pathogen that has resulted in a worldwide epidemic. It primarily spreads either through infected Aedes aegypti or Aedes albopictus mosquitos leading to severe neurological disorders such as microcephaly and Guillain-Barré syndrome in susceptible individuals. The mode of ZIKV entry into specific cell types such as: epidermal keratinocytes, fibroblasts, immature dendritic cells (iDCs), and stem-cell-derived human neural progenitors has been determined through its major surface envelope glycoprotein. It has been known that oligosaccharides that are covalently linked to viral envelope proteins are crucial in defining host-virus interactions. However, the role of sugars/glycans in exploiting host-immune mechanisms and aiding receptor-mediated virus entry is not well defined. Therefore, this review focuses on host-pathogen interactions to better understand ZIKV pathogenesis.

Innate immune receptors drive dengue virus immune activation and disease

Dengue is a worldwide disease with 400 million annual infections that can lead to septic shock and viral hemorrhagic fever with internal bleeding. These symptoms are the result of uncontrolled immune activation. Macrophages and dendritic cells are the main target of dengue virus (DENV) and the cellular source of cytokines associated with this immune activation. Macrophages and dendritic cells express several innate immune receptors that have been implicated in DENV immune activation, of which, CLEC5A, RIG-I and MDA5 are most important. Notably, activation of these receptors have profound effects on adaptive immune responses against DENV. This review will focus on how innate immune receptors drive DENV immune activation by inducing inflammatory cytokines and by activating adaptive immune responses.

Dengue: knowledge gaps, unmet needs, and research priorities

Dengue virus is a mosquito-borne pathogen that causes up to about 100 million cases of disease each year, placing a major public health, social, and economic burden on numerous low-income and middle-income countries. Major advances by investigators, vaccine developers, and affected communities are revealing new insights and enabling novel interventions and approaches to dengue prevention and control. Such research has highlighted further questions about both the basic understanding of dengue and efforts to develop new tools. In this report, the third in a Series on dengue, we discuss existing approaches to dengue diagnostics, disease prognosis, surveillance, and vector control in low-income and middle-income countries, as well as potential consequences of vaccine introduction. We also summarise current knowledge and recent insights into dengue epidemiology, immunology, and pathogenesis, and their implications for understanding natural infection and current and future vaccines.

Genetic diversity and evolution of dengue virus serotype 3: A comparative genomics study

Dengue virus serotype 3 (DENV-3), one of the four serotypes of Dengue viruses, is geographically diverse. There are five distinct genotypes (I-V) of DENV-3. Emerging strains and lineages of DENV-3 are increasingly being reported. Availability of genomic data for DENV-3 strains provides opportunity to study its population structure. Complete genome sequences are available for 860 strains of four genotypes (I, II, III and V) isolated worldwide and were analyzed using population genetics and evolutionary approaches to map landscape of genomic diversity. DENV-3 population is observed to be stratified into five major subpopulations. Genotype I and II formed independent subpopulations while genotype III is subdivided into three subpopulations (GIII-a, GIII-b and GIII-c) and is therefore heterogeneous. Genotypes I, II and GIII-a subpopulations comprise of Asian strains whereas GIII-c comprises of American strains. GIII-b subpopulation includes mainly of American strains along with a few strains from Sri Lanka. Genetic admixture is predominantly observed in Sri Lankan strains of genotype III and all strains of genotype V. Inter-genotype recombination was observed to occur in non-structural region of several Asian strains whereas extent of recombination was limited in American strains. Significant positive selection was found to be operational on all genes and observed to be the main driving force of genetic diversity. Positive selection was strongly operational on the branches leading to Asian genotypes and helped to delineate the genetic differences between Asian and American lineages. Thus, inter-genotype recombination, migration and adaptive evolution are the major determinants of evolution of DENV-3.

Mapping and Role of the CD8+ T Cell Response During Primary Zika Virus Infection in Mice

CD8+ T cells may play a dual role in protection against and pathogenesis of flaviviruses, including Zika virus (ZIKV). We evaluated the CD8+ T cell response in ZIKV-infected LysMCre+IFNARfl/fl C57BL/6 (H-2b) mice lacking the type I interferon receptor in a subset of myeloid cells. In total, 26 and 15 CD8+ T cell-reactive peptides for ZIKV African (MR766) and Asian (FSS13025) lineage strains, respectively, were identified and validated. CD8+ T cells from infected mice were polyfunctional and mediated cytotoxicity. Adoptive transfer of ZIKV-immune CD8+ T cells reduced viral burdens, whereas their depletion led to higher tissue burdens, and CD8-/- mice displayed higher mortality with ZIKV infection. Collectively, these results demonstrate that CD8+ T cells protect against ZIKV infection. Further, this study provides a T cell competent mouse model for investigating ZIKV-specific T cell responses.

Characterization of Human CD8 T Cell Responses in Dengue Virus-Infected Patients from India

Epidemiological studies suggest that India has the largest number of dengue virus infection cases worldwide. However, there is minimal information about the immunological responses in these patients. CD8 T cells are important in dengue, because they have been implicated in both protection and immunopathology. Here, we provide a detailed analysis of HLA-DR⁺ CD38⁺ and HLA-DR^- CD38⁺ effector CD8 T cell subsets in dengue patients from India and Thailand. Both CD8 T cell subsets expanded and expressed markers indicative of antigen-driven proliferation, tissue homing, and cytotoxic effector functions, with the HLA-DR⁺ CD38⁺ subset being the most striking in these effector qualities. The breadth of the dengue-specific CD8 T cell response was diverse, with NS3-specific cells being the most dominant. Interestingly, only a small fraction of these activated effector CD8 T cells produced gamma interferon (IFN-γ) when stimulated with dengue virus peptide pools. Transcriptomics revealed downregulation of key molecules involved in T cell receptor (TCR) signaling. Consistent with this, the majority of these CD8 T cells remained IFN-γ unresponsive even after TCR-dependent polyclonal stimulation (anti-CD3 plus anti-CD28) but produced IFN-γ by TCR-independent polyclonal stimulation (phorbol 12-myristate 13-acetate [PMA] plus ionomycin). Thus, the vast majority of these proliferating, highly differentiated effector CD8 T cells probably acquire TCR refractoriness at the time the patient is experiencing febrile illness that leads to IFN-γ unresponsiveness. Our studies open novel avenues for understanding the mechanisms that fine-tune the balance between CD8 T cell-mediated protective versus pathological effects in dengue.

IMPORTANCE

Dengue is becoming a global public health concern. Although CD8 T cells have been implicated both in protection and in the cytokine-mediated immunopathology of dengue, how the balance is maintained between these opposing functions remains unknown. We comprehensively characterized CD8 T cell subsets in dengue patients from India and Thailand and show that these cells expand massively and express phenotypes indicative of overwhelming antigenic stimulus and tissue homing/cytotoxic-effector functions but that a vast majority of them fail to produce IFN-γ in vitro Interestingly, the cells were fully capable of producing the cytokine when stimulated in a T cell receptor (TCR)-independent manner but failed to do so in TCR-dependent stimulation. These results, together with transcriptomics, revealed that the vast majority of these CD8 T cells from dengue patients become cytokine unresponsive due to TCR signaling insufficiencies. These observations open novel avenues for understanding the mechanisms that fine-tune the balance between CD8-mediated protective versus pathological effects.

Protective Role of Cross-Reactive CD8 T Cells Against Dengue Virus Infection

Infection with one of the four dengue virus serotypes (DENV1-4) presumably leads to lifelong immunity against the infecting serotype but not against heterotypic reinfection, resulting in a greater risk of developing Dengue Hemorrhagic Fever/Dengue Shock Syndrome (DHF/DSS) during secondary infection. Both antibodies and T cell responses have been implicated in DHF/DSS pathogenesis. According to the T cell-based hypothesis termed “original antigenic sin,” secondary DENV infection is dominated by non-protective, cross-reactive T cells that elicit an aberrant immune response. The goal of our study was to compare the roles of serotype-specific and cross-reactive T cells in protection vs. pathogenesis during DENV infection in vivo. Specifically, we utilized IFN-α/βR^−/− HLA*B0702 transgenic mice in the context of peptide vaccination with relevant human CD8 T cell epitopes. IFN-α/βR^−/− HLA*B0702 transgenic mice were immunized with DENV serotype 2 (DENV2)-specific epitopes or variants found in any of the other three serotypes (DENV1, DENV3 or DENV4), followed by challenge with DENV. Although cross-reactive T cell responses were lower than responses elicited by serotype-specific T cells, immunization with either serotype-specific or variant peptide epitopes enhanced viral clearance, demonstrating that both serotype-specific and cross-reactive T cells can contribute to protection in vivo against DENV infection.

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Serotype-cross-reactive CD8 T cells elicit a polyfunctional immune response similar to the response elicited by serotype-specific CD8 T cells.

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Serotype cross-reactive CD8 T cells play a role in protection against DENV infection in a HLA-B*0702 Transgenic IFN-α/βR^−/− mouse model.

There are four major subtypes (serotypes) of the mosquito-borne Dengue virus. Infection with a first serotype is generally asymptomatic, but secondary infection with a different serotype is capable of causing severe disease. T cells previously exposed to a first serotype and which produce an immune response to a second serotype are said to be cross-reactive. Using a mouse model engineered with human T cell features, we characterized the cross-reactive T cell response to live dengue virus serotypes and viral protein fragments. Our results suggested cross-reactive T cells contribute to control of and protection against infection by a second dengue serotype, rather than leading to more severe disease.

Altered immune response of immature dendritic cells following dengue virus infection in the presence of specific antibodies

Dengue virus (DENV) replication is known to prevent maturation of infected dendritic cells (DCs) thereby impeding the development of adequate immunity. During secondary DENV infection, dengue-specific antibodies can suppress DENV replication in immature DCs (immDCs), however how dengue-antibody complexes (DENV-IC) influence the phenotype of DCs remains elusive. Here, we evaluated the maturation state and cytokine profile of immDCs exposed to DENV-ICs. Indeed, DENV infection of immDCs in the absence of antibodies was hallmarked by blunted upregulation of CD83, CD86 and the major histocompatibility complex molecule HLA-DR. In contrast, DENV infection in the presence of neutralizing antibodies triggered full DC maturation and induced a balanced inflammatory cytokine response. Moreover, DENV infection under non-neutralizing conditions prompted upregulation of CD83 and CD86 but not HLA-DR, and triggered production of pro-inflammatory cytokines. The effect of DENV-IC was found to be dependent on the engagement of FcγRIIa. Altogether, our data show that the presence of DENV-IC alters the phenotype and cytokine profile of DCs.

Paper-based point-of-care testing for diagnosis of dengue infections

Dengue endemic is a serious healthcare concern in tropical and subtropical countries. Although well-established laboratory tests can provide early diagnosis of acute dengue infections, access to these tests is limited in developing countries, presenting an urgent need to develop simple, rapid, and robust diagnostic tools. Point-of-care (POC) devices, particularly paper-based POC devices, are typically rapid, cost-effective and user-friendly, and they can be used as diagnostic tools for the prompt diagnosis of dengue at POC settings. Here, we review the importance of rapid dengue diagnosis, current dengue diagnostic methods, and the development of paper-based POC devices for diagnosis of dengue infections at the POC.