Dominant recognition by human CD8+ cytotoxic T lymphocytes of dengue virus nonstructural proteins NS3 and NS1.2a

A Prototype-Pathogen Approach for the Development of Flavivirus Countermeasures

Flaviviruses are a genus within the Flaviviridae family of positive-strand RNA viruses and are transmitted principally through mosquito and tick vectors. These viruses are responsible for hundreds of millions of human infections worldwide per year that result in a range of illnesses from self-limiting febrile syndromes to severe neurotropic and viscerotropic diseases and, in some cases, death. A vaccine against the prototype flavivirus, yellow fever virus, has been deployed for 85 years and is highly effective. While vaccines against some medically important flaviviruses are available, others have proven challenging to develop. The emergence and spread of flaviviruses, including dengue virus and Zika virus, demonstrate their pandemic potential. This review highlights the gaps in knowledge that need to be addressed to allow for the rapid development of vaccines against emerging flaviviruses in the future.

Dengue virus infection - a review of pathogenesis, vaccines, diagnosis and therapy

The transmission of dengue virus (DENV) from an infected Aedes mosquito to a human, causes illness ranging from mild dengue fever to fatal dengue shock syndrome. The similar conserved structure and sequence among distinct DENV serotypes or different flaviviruses has resulted in the occurrence of cross reaction followed by antibody-dependent enhancement (ADE). Thus far, the vaccine which can provide effective protection against infection by different DENV serotypes remains the biggest hurdle to overcome. Therefore, deep investigation is crucial for the potent and effective therapeutic drugs development. In addition, the cross-reactivity of flaviviruses that leads to false diagnosis in clinical settings could result to delay proper intervention management. Thus, the accurate diagnostic with high specificity and sensitivity is highly required to provide prompt diagnosis in respect to render early treatment for DENV infected individuals. In this review, the recent development of neutralizing antibodies, antiviral agents, and vaccine candidates in therapeutic platform for DENV infection will be discussed. Moreover, the discovery of antigenic cryptic epitopes, principle of molecular mimicry, and application of single-chain or single-domain antibodies towards DENV will also be presented.

Current Development and Challenges of Tetravalent Live-Attenuated Dengue Vaccines

Dengue is the most common arboviral disease caused by one of four distinct but closely related dengue viruses (DENV) and places significant economic and public health burdens in the endemic areas. A dengue vaccine will be important in advancing disease control. However, the effort has been challenged by the requirement to induce effective protection against all four DENV serotypes and the potential adverse effect due to the phenomenon that partial immunity to DENV may worsen the symptoms upon subsequent heterotypic infection. Currently, the most advanced dengue vaccines are all tetravalent and based on recombinant live attenuated viruses. CYD-TDV, developed by Sanofi Pasteur, has been approved but is limited for use in individuals with prior dengue infection. Two other tetravalent live attenuated vaccine candidates: TAK-003 by Takeda and TV003 by National Institute of Allergy and Infectious Diseases, have completed phase 3 and phase 2 clinical trials, respectively. This review focuses on the designs and evaluation of TAK-003 and TV003 vaccine candidates in humans in comparison to the licensed CYD-TDV vaccine. We highlight specific lessons from existing studies and challenges that must be overcome in order to develop a dengue vaccine that confers effective and balanced protection against all four DENV serotypes but with minimal adverse effects.

Dynamics of the Magnitude, Breadth and Depth of the Antibody Response at Epitope Level Following Dengue Infection

Dengue is a major public health problem in tropical and sub-tropical regions worldwide. Since the Zika epidemic and the increased co-circulation of other arboviruses, the serology-based diagnosis of dengue has become more problematic due to the high antigenic resemblance, especially among the flavivirus family. Therefore, a more comprehensive understanding of the diversity, specificity and temporal evolution of the antibody response following dengue infection is needed. In order to close this knowledge gap, we used a high-density peptide microarray of 9,072 linear peptides covering the entire proteome diversity of dengue, Zika, yellow fever and chikungunya viruses. The IgM and IgG antibody responses were measured against the designed microarray in symptomatic dengue infected individuals from an arbovirus endemic area in Peru and in overseas travelers returning to Belgium, as representatives of multiple-exposed and primary infections, respectively. Serum samples were collected longitudinally across four time points over the period of six months in Peru and over two time points in travelers. We show that epitopes eliciting the strongest flavivirus cross-reactive antibodies, in both primary and secondary infections were concentrated in the capsid, E, NS1, NS3 and NS5 proteins. The IgG antibody responses against NS1 and NS3 followed a rise-and-fall pattern, with peak titers between two to four weeks after onset of illness. The response to the E and NS5 proteins increased rapidly in the acute phase and was maintained at stable levels until at least 6 months after illness. A more scattered IgM antibody reactivity across the viral proteome was observed in the acute phase of the disease and that persisted through the 6-month window. The magnitude, breadth (i.e. number of unique epitopes targeted) and depth (i.e. number of epitope variants recognized) of the IgG response was higher in secondary infections compared to primary infections. For IgM antibodies, the magnitude of the response was higher in primary infected individuals whereas the breadth and depth of the response was lower in this group compared with the endemic subjects. Finally, through this arboviral proteome-wide epitope mapping, we were able to identify IgM and IgG dengue-specific epitopes which can be useful serological markers for dengue diagnosis and serostatus determination.

Identification and selection of immunodominant B and T cell epitopes for dengue multi-epitope-based vaccine

Dengue virus (DENV) comprises four serotypes (DENV1-4) which cause 390 million global infections with 500,000 hospitalizations and 25,000 fatalities annually. Currently, the only FDA approved DENV vaccine is the chimeric live-attenuated vaccine, Dengvaxia®, which is based on the yellow fever virus (YFV) genome that carries the prM and E genes of the respective DENV 1, 2, 3, and 4 serotypes. However, it has lower efficacies against serotypes DENV1 (51%) and DENV2 (34%) when compared with DENV3 (75%) and DENV4 (77%). The absence of T cell epitopes from non-structural (NS) and capsid (C) proteins of the yellow fever vaccine strain might have prevented Dengvaxia® to elicit robust cellular immune responses, as CD8⁺ T cell epitopes are mainly localized in the NS3 and NS5 regions. Multi-epitope-based peptide vaccines carrying CD4⁺, CD8⁺ T cell and B cell epitopes represent a novel approach to generate specific immune responses. Therefore, assessing and selecting epitopes that can induce robust B and T cell responses is a prerequisite for constructing an efficient multi-epitope peptide vaccine. Potent B and T cell epitopes can be identified by utilizing immunoinformatic analysis, but the immunogenicity of the epitopes have to be experimentally validated. In this review, we presented T cell epitopes that have been predicted by bioinformatic approaches as well as recent experimental validations of CD4⁺ and CD8⁺ T cell epitopes by ex-vivo stimulation of PBMCs with specific peptides. Immunoproteomic analysis could be utilized to uncover HLA-specific epitopes presented by DENV-infected cells. Based on various approaches, immunodominant epitopes capable of inducing strong immune responses could be selected and incorporated to form a universally applicable multi-epitope-based peptide dengue vaccine.

Challenges in Dengue Vaccines Development: Pre-existing Infections and Cross-Reactivity

Dengue is one of the most frequently transmitted mosquito-borne diseases in the world, which creates a significant public health concern globally, especially in tropical and subtropical countries. It is estimated that more than 390 million people are infected with dengue virus each year and around 96 million develop clinical pathologies. Dengue infections are not only a health problem but also a substantial economic burden. To date, there are no effective antiviral therapies and there is only one licensed dengue vaccine that only demonstrated protection in the seropositive (Immune), naturally infected with dengue, but not dengue seronegative (Naïve) vaccines. In this review, we address several immune components and their interplay with the dengue virus. Additionally, we summarize the literature pertaining to current dengue vaccine development and advances. Moreover, we review some of the factors affecting vaccine responses, such as the pre-vaccination environment, and provide an overview of the significant challenges that face the development of an efficient/protective dengue vaccine including the presence of multiple serotypes, antibody-dependent enhancement (ADE), as well as cross-reactivity with other flaviviruses. Finally, we discuss targeting T follicular helper cells (Tfh), a significant cell population that is essential for the production of high-affinity antibodies, which might be one of the elements needed to be specifically targeted to enhance vaccine precision to dengue regardless of dengue serostatus.

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.

An insight into dengue virus proteins as potential drug/vaccine targets

Dengue virus (DENV) is an arbovirus that belongs to family flaviviridae. Its genome is composed of a single stranded RNA molecule that encodes a single polyprotein. The polyprotein is processed by viral and cellular proteases to generate ten viral proteins. There are four antigenically distinct serotypes of DENV (DENV1, DENV2, DENV3 and DENV4), which are genetically related. Although protein variability is a major problem in dengue treatment, the functional and structural studies of individual proteins are equally important in treatment development. The data accumulated on dengue proteins are significant to provide detailed understanding of viral infection, replication, host-immune evasion and pathogenesis. In this review, we summarized the detailed current knowledge about DENV proteins.

Influence of chemokines on the endothelial permeability and cellular transmigration during dengue

During a pathogenic infection, an inflammatory process is triggered in which several inflammatory mediators, such as cytokines, chemokines, growth factors, complement system components, nitric oxide, and others induce integrity alteration on the endothelial barrier. Chemokines are responsible for regulating leukocyte trafficking under homeostatic conditions as well as activating immune system cells under inflammatory conditions. They are crucial molecules in the early stages of infection, leading to the recruitment of immune cells, namely neutrophils, monocytes, natural killer (NK) cells, natural killer T cells (NKT), dendritic cells (DC), T lymphocytes and all cells expressing chemokine receptors for inflammatory sites. Other functions, such as collagen production, tissue repair, a proliferation of hematopoietic precursors and angiogenesis, are also performed by these molecules. Chemokines, amongst inflammatory mediators, play a key role in dengue immunopathogenesis. Dengue fever is a disease caused by the dengue virus (DENV). It is characterized by a broad spectrum of clinical manifestations ranging from asymptomatic cases to mild and severe symptomatic ones. As for the latter, the appearance of hemorrhagic manifestations and changes in vascular permeability may lead the patient to develop cavitary effusions, organ involvement, and even death. As chemokines exert an influence on various homeostatic and inflammatory processes, acting vigorously on vascular endothelial activation and cell migration, the main purpose of this chapter is to discuss the influence of chemokines on the alteration of endothelial permeability and migration of T lymphocytes in DENV infection.

Aiming at the heart: the capsid protein of dengue virus as a vaccine candidate

INTRODUCTION

Dengue fever remains as a health problem worldwide. Although Dengvaxia®, was registered in several countries, the results after the immunization of people suggest an increase of risk in non-immune persons and children younger than 9 years old. No other vaccine is registered so far, thus the development of a safe and effective vaccine continues to be a priority for the WHO and the scientific community.

AREAS COVERED

This work reviews the structural and antigenic properties of the capsid protein of Dengue virus, along with results of studies performed to assess the immunogenicity and protective capacity in animals of vaccine candidates based on this protein.

EXPERT OPINION

The generation of a memory cellular immune response alone, after vaccination against Dengue virus, could be advantageous, as there would not be risk of increasing viral infectivity through sub-neutralizing antibodies. However, it is improbable to achieving sterilizing immunity. In this scenario, an infection could stablished but without the appearance of the severe disease. The cell-mediated immunity should keep the virus at bay. The capsid protein induces a protective immune response in animals without the induction of virus-binding antibodies. Vaccine candidates based on this protein could be an attractive strategy to induce protection against the severe Dengue disease.

T Lymphocytes as Measurable Targets of Protection and Vaccination Against Viral Disorders

Continuous epidemiological surveillance of existing and emerging viruses and their associated disorders is gaining importance in light of their abilities to cause unpredictable outbreaks as a result of increased travel and vaccination choices by steadily growing and aging populations. Close surveillance of outbreaks and herd immunity are also at the forefront, even in industrialized countries, where previously eradicated viruses are now at risk of re-emergence due to instances of strain recombination, contractions in viral vector geographies, and from their potential use as agents of bioterrorism. There is a great need for the rational design of current and future vaccines targeting viruses, with a strong focus on vaccine targeting of adaptive immune effector memory T cells as the gold standard of immunity conferring long-lived protection against a wide variety of pathogens and malignancies. Here, we review viruses that have historically caused large outbreaks and severe lethal disorders, including respiratory, gastric, skin, hepatic, neurologic, and hemorrhagic fevers. To observe trends in vaccinology against these viral disorders, we describe viral genetic, replication, transmission, and tropism, host-immune evasion strategies, and the epidemiology and health risks of their associated syndromes. We focus on immunity generated against both natural infection and vaccination, where a steady shift in conferred vaccination immunogenicity is observed from quantifying activated and proliferating, long-lived effector memory T cell subsets, as the prominent biomarkers of long-term immunity against viruses and their associated disorders causing high morbidity and mortality rates.

T Cell Immunity to Zika and Dengue Viral Infections

The 2015-2016 Zika virus (ZIKV) outbreak in the Americas has brought this relatively unknown flavivirus into the spotlight as the etiologic agent of congenital Zika syndrome in newborns and, potentially, of Guillain-Barré syndrome in adults. ZIKV shares a high degree of amino acid sequence homology with the closely related flavivirus Dengue virus (DENV). Due to the paucity of research investigating T cell-mediated immune responses to ZIKV, the precise roles of T cells in protection against and pathogenesis of ZIKV infection are unclear. Nevertheless, knowledge gained from studies of T cell responses to other flaviviruses, such as the four DENV serotypes, provides a framework for investigating and understanding anti-ZIKV T cell responses. In this review, we summarize our current understanding of how T cells respond to primary and secondary infections with DENV and ZIKV. A thorough understanding of the role of ZIKV-specific and DENV cross-reactive T cells in controlling infection is essential for developing ZIKV- and DENV-specific vaccines with maximal safety and efficacy.

HLA-directed bioinformatics approach for genome-wide mapping of dengue CTL epitopes

Aim: In silico approach was used to predict cytotoxic T lymphocyte (CTL) epitopes from the proteome of all four dengue serotypes. Methods: The immune epitope database analysis resource was used to analyze the CTL epitopes of dengue serotypes. The prediction of epitopes was done against nine high frequency HLA class I alleles occurring worldwide. Results: A total of 2784 epitopes were predicted from all four dengue virus proteomes. Immune epitope database analysis resource tool predicted 202 epitopes as positive for immunogenecity. A total of 39 of 257 consensus epitopes predicted were present in all four serotypes. This study identified nine new class I-restricted epitopes. Conclusion: Mapping of these potentially immunogenic dengue epitopes paves the way for future investigation of their utility as vaccine candidates to prevent or treat dengue virus infections.

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 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.

A novel immunization approach for dengue infection based on conserved T cell epitopes formulated in calcium phosphate nanoparticles

Dengue virus (DV) is the etiologic agent of dengue fever, the most significant mosquito-borne viral disease in humans. Most DV vaccine approaches are focused on generating antibody mediated responses; one such DV vaccine is approved for use in humans but its efficacy is limited. While it is clear that T cell responses play important role in DV infection and subsequent disease manifestations, fewer studies are aimed at developing vaccines that induce robust T cells responses. Potent T cell based vaccines require 2 critical components: the identification of specific T cell stimulating MHC associated peptides, and an optimized vaccine delivery vehicle capable of simultaneously delivering the antigens and any required adjuvants. We have previously identified and characterized DV specific HLA-A2 and -A24 binding DV serotypes conserved epitopes, and the feasibility of an epitope based vaccine for DV infection. In this study, we build on those previous studies and describe an investigational DV vaccine using T cell epitopes incorporated into a calcium phosphate nanoparticle (CaPNP) delivery system. This study presents a comprehensive analysis of functional immunogenicity of DV CaPNP/multipeptide formulations in vitro and in vivo and demonstrates the CaPNP/multipeptide vaccine is capable of inducing T cell responses against all 4 serotypes of DV. This synthetic vaccine is also cost effective, straightforward to manufacture, and stable at room temperature in a lyophilized form. This formulation may serve as an effective candidate DV vaccine that protects against all 4 serotypes as either a prophylactic or therapeutic vaccine.

Identification of putative unique immunogenic ZIKV and DENV1-4 peptides for diagnostic cellular based tests

Since the re-emergence of Zika virus in 2014 and subsequent association with microcephaly, much work has focused on the development of a vaccine to halt its spread throughout the world. The mosquito vector that transmits this virus is widespread and responsible for the spread of other arboviridae including Dengue. Current diagnostic methods rely on serologic testing that are complicated by cross reactivity and therefore unable to distinguish Zika from Dengue infection in the absence of virus isolation. We performed an in silico analysis to identify potential epitopes that may stimulate a unique T-lymphocyte response to distinguish prior infection with Zika or Dengue. From this analysis, we not only identified epitopes unique to Zika and Dengue, but also identified epitopes unique to each Dengue serotype. These peptides contribute to a pool of peptides identified for vaccine development that can be tested in vitro to confirm immunogenicity, absence of homology and global population coverage. The current lack of accurate diagnostic testing hampers our ability to understand the scope of the epidemic, implications for vaccine implementation and complications related to monoinfection and co-infection with these two closely related viruses.

Complexity of Human Antibody Response to Dengue Virus: Implication for Vaccine Development

The four serotypes of dengue virus (DENV) are the leading cause of arboviral diseases in humans. Decades of efforts have made remarkable progress in dengue vaccine development. Despite the first dengue vaccine (dengvaxia from Sanofi Pasteur), a live-attenuated tetravalent chimeric yellow fever-dengue vaccine, has been licensed by several countries since 2016, its overall moderate efficacy (56.5–60.8%) in the presence of neutralizing antibodies during the Phase 2b and 3 trials, lower efficacy among dengue naïve compared with dengue experienced individuals, and increased risk of hospitalization among young children during the follow-up highlight the need for a better understanding of humoral responses after natural DENV infection. Recent studies of more than 300 human monoclonal antibodies (mAbs) against DENV have led to the discovery of several novel epitopes on the envelope protein recognized by potent neutralizing mAbs. This information together with in-depth studies on polyclonal sera and B-cells following natural DENV infection has tremendous implications for better immunogen design for a safe and effective dengue vaccine. This review outlines the progress in our understanding of mouse mAbs, human mAbs, and polyclonal sera against DENV envelope and precursor membrane proteins, two surface proteins involved in vaccine development, following natural infection; analyses of these discoveries have provided valuable insight into new strategies involving molecular technology to induce more potent neutralizing antibodies and less enhancing antibodies for next-generation dengue vaccine development.

Microglia retard dengue virus-induced acute viral encephalitis

Patients with dengue virus (DENV) infection may also present acute viral encephalitis through an unknown mechanism. Here, we report that encephalitic DENV-infected mice exhibited progressive hunchback posture, limbic seizures, limbic weakness, paralysis, and lethality 7 days post-infection. These symptoms were accompanied by CNS inflammation, neurotoxicity, and blood-brain barrier destruction. Microglial cells surrounding the blood vessels and injured hippocampus regions were activated by DENV infection. Pharmacologically depleting microglia unexpectedly increased viral replication, neuropathy, and mortality in DENV-infected mice. In microglia-depleted mice, the DENV infection-mediated expression of antiviral cytokines and the infiltration of CD8-positive cytotoxic T lymphocytes (CTLs) was abolished. DENV infection prompted the antigen-presenting cell-like differentiation of microglia, which in turn stimulated CTL proliferation and activation. These results suggest that microglial cells play a key role in facilitating antiviral immune responses against DENV infection and acute viral encephalitis.

Dengue virus specific dual HLA binding T cell epitopes induce CD8+ T cell responses in seropositive individuals

Dengue virus infects an estimated 300 million people each year and even more are at risk of becoming infected as the virus continues to spread into new areas. Despite the increase in viral prevalence, no anti-viral medications or vaccines are approved for treating or preventing infection. CD8+ T cell responses play a major role in viral clearance. Therefore, effective vaccines that induce a broad, multi-functional T cell response with substantial cross-reactivity between all virus serotypes can have major impacts on reducing infection rates and infection related complications. Here, we took an immunoproteomic approach to identify novel MHC class I restricted T cell epitopes presented by dengue virus infected cells, representing the natural and authentic targets of the T cell response. Using this approach we identified 4 novel MHC-I restricted epitopes: 2 with the binding motif for HLA-A24 molecules and 2 with both HLA-A2 and HLA-A24 binding motifs. These peptides were able to activate CD8+ T cell responses in both healthy, seronegative individuals and in seropositive individuals who have previously been infected with dengue virus. Importantly, the dual binding epitopes activated pre-existing T cell precursors in PBMCs obtained from both HLA-A2+ and HLA-A24+ seropositive individuals. Together, the data indicate that these epitopes are immunologically relevant T cell activating peptides presented on infected cells during a natural infection and therefore may serve as candidate antigens for the development of effective multi-serotype specific dengue virus vaccines.

The Complexity of a Dengue Vaccine: A Review of the Human Antibody Response

Dengue is the most prevalent mosquito-borne viral disease worldwide. Yet, there are no vaccines or specific antivirals available to prevent or treat the disease. Several dengue vaccines are currently in clinical or preclinical stages. The most advanced vaccine is the chimeric tetravalent CYD-TDV vaccine of Sanofi Pasteur. This vaccine has recently cleared Phase III, and efficacy results have been published. Excellent tetravalent seroconversion was seen, yet the protective efficacy against infection was surprisingly low. Here, we will describe the complicating factors involved in the generation of a safe and efficacious dengue vaccine. Furthermore, we will discuss the human antibody responses during infection, including the epitopes targeted in humans. Also, we will discuss the current understanding of the assays used to evaluate antibody response. We hope this review will aid future dengue vaccine development as well as fundamental research related to the phenomenon of antibody-dependent enhancement of dengue virus infection.

Clinical outcome and genetic differences within a monophyletic Dengue virus type 2 population

The exact mechanisms of interplay between host and viral factors leading to severe dengue are yet to be fully understood. Even though previous studies have implicated specific genetic differences of Dengue virus (DENV) in clinical severity and virus attenuation, similar studies with large-scale, whole genome screening of monophyletic virus populations are limited. Therefore, in the present study, we compared 89 whole genomes of DENV-2 cosmopolitan clade III isolates obtained from patients diagnosed with dengue fever (DF, n = 58), dengue hemorrhagic fever (DHF, n = 30) and dengue shock syndrome (DSS, n = 1) in Singapore between July 2010 and January 2013, in order to determine the correlation of observed viral genetic differences with clinical outcomes. Our findings showed no significant difference between the number of primary and secondary infections that progressed to DHF and DSS (p>0.05) in our study cohort. Despite being highly homogenous, study isolates possessed 39 amino acid substitutions of which 10 substitutions were fixed in three main groups of virus isolates. None of those substitutions were specifically associated with DHF and DSS. Notably, two evolutionarily unique virus groups possessing C-P43T+NS1-S103T+NS2A-V83I+NS3-R337K+ NS3-I600T+ NS5-P136S and NS2A-T119N mutations were exclusively found in patients with DF, the benign form of DENV infections. Those mutants were significantly associated with mild disease outcome. These observations indicated that disease progression into DHF and DSS within our patient population was more likely to be due to host than virus factors. We hypothesize that selection for potentially less virulent groups of DENV-2 in our study cohort may be an evolutionary adaptation of viral strains to extend their survival in the human-mosquito transmission cycle.

Elucidating the role of T cells in protection against and pathogenesis of dengue virus infections

Dengue viruses (DENV) cause significantly more human disease than any other arbovirus, with hundreds of thousands of cases leading to severe disease in thousands annually. Antibodies and T cells induced by primary infection with DENV have the potential for both positive (protective) and negative (pathological) effects during subsequent DENV infections. In this review, we summarize studies that have examined T-cell responses in humans following natural infection and vaccination. We discuss studies that support a role for T cells in protection against and those that support a role for the involvement of T cells in the pathogenesis of severe disease. The mechanisms that lead to severe disease are complex, and T-cell responses are an important component that needs to be further evaluated for the development of safe and efficacious DENV vaccines.

Immunodominance changes as a function of the infecting dengue virus serotype and primary versus secondary infection

Dengue virus (DENV) is the causative agent of dengue fever (DF). This disease can be caused by any of four DENV serotypes (DENV1 to -4) which share 67 to 75% sequence homology with one another. The effect of subsequent infections with different serotypes on the T cell repertoire is not fully understood. We utilized mice transgenic for human leukocyte antigens (HLA) lacking the alpha/beta interferon (IFN-α/β) receptor to study responses to heterologous DENV infection. First, we defined the primary T cell response to DENV3 in the context of a wide range of HLA molecules. The primary DENV3 immune response recognized epitopes derived from all 10 DENV proteins, with a significant fraction of the response specific for structural proteins. This is in contrast to primary DENV2 infection, in which structural proteins are a minor component of the response, suggesting differential antigen immunodominance as a function of the infecting serotype. We next investigated the effect of secondary heterologous DENV infection on the T cell repertoire. In the case of both DENV2/3 and DENV3/2 heterologous infections, recognition of conserved/cross-reactive epitopes was either constant or expanded compared to that in homologous infection. Furthermore, in heterologous infection, previous infection with a different serotype impaired the development of responses directed to serotype-specific but not conserved epitopes. Thus, a detrimental effect of previous heterotypic responses might not be due to dysfunctional and weakly cross-reactive epitopes dominating the response. Rather, responses to the original serotype might limit the magnitude of responses directed against epitopes that are either cross-reactive to or specific for the most recently infecting serotype.

IMPORTANCE

DENV transmission occurs in more than 100 countries and is an increasing public health problem in tropical and subtropical regions. At present, no effective antiviral therapy or licensed vaccine exists, and treatment is largely supportive in nature. Disease can be caused by any of the four DENV serotypes (DENV1 to -4), which share a high degree of sequence homology with one another. In this study, we have addressed the question of how the T cell repertoire changes as a function of infections with different serotypes and of subsequent heterologous secondary infections. This is of particular interest in the field of dengue viruses, in which secondary infections with different DENV serotypes increase the risk of severe disease. Our results on the evolution of the immune response after primary and secondary infections provide new insights into HLA-restricted T cell responses against DENV relevant for the design of a vaccine against DENV.

Recombinant dengue 2 virus NS3 protein conserves structural antigenic and immunological properties relevant for dengue vaccine design

The NS3 protein is a multifunctional non-structural protein of flaviviruses implicated in the polyprotein processing. The predominance of cytotoxic T cell lymphocytes epitopes on the NS3 protein suggests a protective role of this protein in limiting virus replication. In this work, we studied the antigenicity and immunogenicity of a recombinant NS3 protein of the Dengue virus 2. The full-length NS3 gene was cloned and expressed as a His-tagged fusion protein in Escherichia coli. The pNS3 protein was purified by two chromatography steps. The recombinant NS3 protein was recognized by anti-protease NS3 polyclonal antibody and anti-DENV2 HMAF by Western Blot. This purified protein was able to stimulate the secretion of high levels of gamma interferon and low levels of interleukin-10 and tumor necrosis factor-α in mice splenocytes, suggesting a predominantly Th-1-type T cell response. Immunized BALB/c mice with the purified NS3 protein showed a strong induction of anti-NS3 IgG antibodies, essentially IgG2b, as determined by ELISA. Immunized mice sera with recombinant NS3 protein showed specific recognition of native dengue protein by Western blotting and immunofluorescence techniques. The successfully purified recombinant protein was able to preserv the structural and antigenic determinants of the native dengue protein. The antigenicity shown by the recombinant NS3 protein suggests its possible inclusion into future DENV vaccine preparations.

T-cell immunity to infection with dengue virus in humans

Dengue virus (DENV) is the etiologic agent of dengue fever, the most significant mosquito-borne viral disease in humans. Up to 400 million DENV infections occur every year, and severity can range from asymptomatic to an acute self-limiting febrile illness. In a small proportion of patients, the disease can exacerbate and progress to dengue hemorrhagic fever and/or dengue shock syndrome, characterized by severe vascular leakage, thrombocytopenia, and hemorrhagic manifestations. A unique challenge in vaccine development against DENV is the high degree of sequence variation, characteristically associated with RNA viruses. This is of particular relevance in the case of DENV since infection with one DENV serotype (primary infection) presumably affords life-long serotype-specific immunity but only partial and temporary immunity to other serotypes in secondary infection settings. The role of T cells in DENV infection and subsequent disease manifestations is not fully understood. According to the original antigenic sin theory, skewing of T-cell responses induced by primary infection with one serotype causes less effective response upon secondary infection with a different serotype, predisposing to severe disease. Our recent study has suggested an HLA-linked protective role for T cells. Herein, we will discuss the role of T cells in protection and pathogenesis from severe disease as well as the implications for vaccine design.

Evasion of the human innate immune system by dengue virus

Dengue virus is a worldwide health problem, with billions of people at risk annually. Dengue virus causes a spectrum of diseases, namely dengue fever, dengue hemorrhagic fever and dengue shock syndrome with the latter two being linked to death. Understanding how dengue is able to evade the immune system and cause enhanced severity of disease is the main topics of interest in the Fernandez-Sesma laboratory at Mount Sinai School of Medicine. Using primary human immune cells, our group investigates the contribution of dengue virus-specific proteins to the evasion of innate immunity by this virus and the host factors that the virus interacts with in order to evade immune recognition and to establish infection in humans. Here, we review recent findings from our group as well as published data from other groups regarding immune modulation by dengue virus.

Enhanced humoral and HLA-A2-restricted dengue virus-specific T-cell responses in humanized BLT NSG mice

Dengue is a mosquito-borne viral disease of humans, and animal models that recapitulate human immune responses or dengue pathogenesis are needed to understand the pathogenesis of the disease. We recently described an animal model for dengue virus (DENV) infection using humanized NOD-scid IL2rγ(null) mice (NSG) engrafted with cord blood haematopoietic stem cells. We sought to further improve this model by co-transplantation of human fetal thymus and liver tissues into NSG (BLT-NSG) mice. Enhanced DENV-specific antibody titres were found in the sera of BLT-NSG mice compared with human cord blood haematopoietic stem cell-engrafted NSG mice. Furthermore, B cells generated during the acute phase and in memory from splenocytes of immunized BLT-NSG mice secreted DENV-specific IgM antibodies with neutralizing activity. Human T cells in engrafted BLT-NSG mice secreted interferon-γ in response to overlapping DENV peptide pools and HLA-A2 restricted peptides. The BLT-NSG mice will allow assessment of human immune responses to DENV vaccines and the effects of previous immunity on subsequent DENV infections.

Primary vaccination with low dose live dengue 1 virus generates a proinflammatory, multifunctional T cell response in humans

The four dengue virus serotypes (DENV-1-DENV-4) have a large impact on global health, causing 50-100 million cases of dengue fever annually. Herein, we describe the first kinetic T cell response to a low-dose DENV-1 vaccination study (10 PFU) in humans. Using flow cytometry, we found that proinflammatory cytokines, IFNγ, TNFα, and IL-2, were generated by DENV-1-specific CD4(+) cells 21 days post-DENV-1 exposure, and their production continued through the latest time-point, day 42 (p<0.0001 for all cytokines). No statistically significant changes were observed at any time-points for IL-10 (p = 0.19), a regulatory cytokine, indicating that the response to DENV-1 was primarily proinflammatory in nature. We also observed little T cell cross-reactivity to the other 3 DENV serotypes. The percentage of multifunctional T cells (T cells making ≥ 2 cytokines simultaneously) increased with time post-DENV-1 exposure (p<0.0001). The presence of multifunctional T cells together with neutralizing antibody data suggest that the immune response generated to the vaccine may be protective. This work provides an initial framework for defining primary T cell responses to each DENV serotype and will enhance the evaluation of a tetravalent DENV vaccine.

Recombinant nucleocapsid-like particles from dengue-2 induce functional serotype-specific cell-mediated immunity in mice

The interplay of different inflammatory cytokines induced during dengue virus infection plays a role in either protection or increased disease severity. In this sense, vaccine strategies incorporating whole virus are able to elicit both functional and pathological responses. Therefore, an ideal tetravalent vaccine candidate against dengue should be focused on serotype-specific sequences. In the present work, a new formulation of nucleocapsid-like particles (NLPs) obtained from the recombinant dengue-2 capsid protein was evaluated in mice to determine the level of protection against homologous and heterologous viral challenge and to measure the cytotoxicity and cytokine-secretion profiles induced upon heterologous viral stimulation. As a result, a significant protection rate was achieved after challenge with lethal dengue-2 virus, which was dependent on CD4(+) and CD8(+) cells. In turn, no protection was observed after heterologous challenge. In accordance, in vitro-stimulated spleen cells from mice immunized with NLPs from the four dengue serotypes showed a serotype-specific response of gamma interferon- and tumour necrosis factor alpha-secreting cells. A similar pattern was detected when spleen cells from dengue-immunized animals were stimulated with the capsid protein. Taking these data together, we can assert that NLPs constitute an attractive vaccine candidate against dengue. They induce a functional immune response mediated by CD4(+) and CD8(+) cells in mice, which is protective against viral challenge. In turn, they are potentially safe due to two important facts: induction of serotype specific cell-mediated immunity and lack of induction of antiviral antibodies. Further studies in non-human primates or humanized mice should be carried out to elucidate the usefulness of the NLPs as a potential vaccine candidate against dengue disease.

Conserved MHC class I-presented dengue virus epitopes identified by immunoproteomics analysis are targets for cross-serotype reactive T-cell response

Dengue fever and dengue hemorrhagic fever are significant global public health problems, and understanding the overall immune response to infection will contribute to appropriate management of the disease and its potentially severe complications. Live attenuated and subunit vaccine candidates, which are under clinical evaluation, induce primarily an antibody response to the virus and minimal cross-reactive T-cell responses. Currently, there are no available tools to assess protective T-cell responses during infection or after vaccination. In this study, we utilize an immunoproteomics process to uncover novel HLA-A2-specific epitopes derived from dengue virus (DV)-infected cells. These epitopes are conserved, and we report that epitope-specific cytotoxic lymphocytes (CTLs) are cross-reactive against all 4 DV serotypes. These epitopes have potential as new informational and diagnostic tools to characterize T-cell immunity in DV infection and may serve as part of a universal vaccine candidate complementary to current vaccines in trial.

T-cell responses to dengue virus in humans

Dengue virus (DENV) is a leading cause of morbidity and mortality in most tropical and subtropical areas of the world. Dengue virus infection induces specific CD4+CD8– and CD8+CD4– T cells in humans. In primary infection, T-cell responses to DENV are serotype cross-reactive, but the highest response is to the serotype that caused the infection. The epitopes recognized by DENV-specific T cells are located in most of the structural and non-structural proteins, but NS3 is the protein that is most dominantly recognized. In patients with dengue hemorrhagic fever (DHF) caused by secondary DENV infection, T cells are highly activated in vivo. These highly activated T cells are DENV-specific and oligoclonal. Multiple kinds of lymphokines are produced by the activated T cells, and it has been hypothesized that these lymphokines are responsible for induction of plasma leakage, one of the most characteristic features of DHF. Thus, T-cells play important roles in the pathogenesis of DHF and in the recovery from DENV infection.

Insights into HLA-restricted T cell responses in a novel mouse model of dengue virus infection point toward new implications for vaccine design

The frequency of dengue virus (DENV) infection has increased dramatically in the last few decades, and the lack of a vaccine has led to significant morbidity and mortality worldwide. To date, a convenient murine system to study human T cell responses to DENV has not been available. Mice transgenic for HLA are widely used to model human immune responses, and it has been shown that mouse-passaged DENV is able to replicate to significant levels in IFN-α/βR(-/-) mice. To cover a wide range of HLA phenotypes, we backcrossed IFN-α/βR(-/-) mice with HLA A*0201, A*0101, A*1101, B*0702, and DRB1*0101-transgenic mice. A DENV proteome-wide screen identified a total of 42 epitopes across all HLA-transgenic IFN-α/βR(-/-) strains tested. In contrast, only eight of these elicited responses in the corresponding IFN-α/βR(+/+) mice. We were able to identify T cell epitopes from 9 out of the 10 DENV proteins. However, the majority of responses were derived from the highly conserved nonstructural proteins NS3 and NS5. The relevance of this model is further demonstrated by the fact that most of the epitopes identified in our murine system are also recognized by PBMC from DENV-exposed human donors, and a dominance of HLA B*0702-restricted responses has been detected in both systems. Our results provide new insights into HLA-restricted T cell responses against DENV, and we describe in this study a novel murine model that allows the investigation of T cell-mediated immune mechanisms relevant to vaccine design.

Dengue virus-specific CD4+ and CD8+ T lymphocytes target NS1, NS3 and NS5 in infected Indian rhesus macaques

Every year, Dengue virus (DENV) infects approximately 100 million people. There are currently several vaccines undergoing clinical studies, but most target the induction of neutralizing antibodies. Unfortunately, DENV infection can be enhanced by subneutralizing levels of antibodies that bind virions and deliver them to cells of the myeloid lineage, thereby increasing viral replication (termed antibody-dependent enhancement [ADE]). T lymphocyte-based vaccines may offer an alternative that avoids ADE. The goal of our study was to describe the cellular immune response generated after primary DENV infection in Indian rhesus macaques. We infected eight rhesus macaques with 10⁵ plaque-forming units (PFU) of DENV serotype 2 (DENV2) New Guinea C (NGC) strain, and monitored viral load and the cellular immune response to the virus. Viral replication peaked at day 4 post-infection and was resolved by day 10. DENV-specific CD4+ and CD8+ T lymphocytes targeted nonstructural (NS) 1, NS3 and NS5 proteins after resolution of peak viremia. DENV-specific CD4+ cells expressed interferon-gamma (IFN-γ) along with tumor necrosis factor-alpha (TNF-α), interleukin-2 (IL-2), and macrophage inflammatory protein-1 beta (MIP-1β). In comparison, DENV-specific CD8+ cells expressed IFN-γ in addition to MIP-1β and TNF-α and were positive for the degranulation marker CD107a. Interestingly, a fraction of the DENV-specific CD4+ cells also stained for CD107a, suggesting that they might be cytotoxic. Our results provide a more complete understanding of the cellular immune response during DENV infection in rhesus macaques and contribute to the development of rhesus macaques as an animal model for DENV vaccine and pathogenicity studies.

Antibody recognition of the dengue virus proteome and implications for development of vaccines

Dengue is a mosquito-borne infection caused by four distinct serotypes of dengue virus, each appearing cyclically in the tropics and subtropics along the equator. Although vaccines are currently under development, none are available to the general population. One of the main impediments to the successful advancement of these vaccines is the lack of well-defined immune correlates of protection. Here, we describe a protein microarray approach for measuring antibody responses to the complete viral proteome comprised of the structural (capsid, membrane, and envelope) and nonstructural (NS1, NS2A, NS2B, NS3, NS4A, NS4B, and NS5) components of all four dengue virus serotypes (1 to 4). We examined rhesus macaques vaccinated with tetravalent vaccines consisting of live-attenuated virus (LAV) or purified inactivated virus (PIV), followed by boosting with LAV and challenging with wild-type dengue virus. We detected temporal increases in antibodies against envelope proteins in response to either vaccine, while only the PIV/LAV vaccination strategy resulted in anticapsid antibodies. In contrast to results from vaccination, naïve macaques challenged with wild-type viruses of each serotype demonstrated a balanced response to nonstructural and structural components, including responses against the membrane protein. Our results demonstrate discriminating details concerning the nature of antibody responses to dengue virus at the proteomic level and suggest the usefulness of this information for vaccine development.

Differential in vivo clearance and response to secondary heterologous infections by H2(b)-restricted dengue virus-specific CD8+ T cells

Cytotoxic T lymphocytes (CTL) are hypothesized to play a role in clearance during primary dengue virus (DENV) infections, and contribute to immunopathology during secondary heterologous infections in humans. We previously reported skewed T-cell responses to secondary DENV infection in BALB/c (H-2(d)) mice, reproducing characteristics of human DENV infection. To set the stage for using widely available transgenic and knockout mice, we extended these studies to identify DENV-specific T-cell responses in C57BL/6 (H-2(b)) mice. We identified dominant CD8+ T-cell responses to H-2D(b)-restricted epitopes on the DENV NS4a (aa 249-265) and NS5 (aa 521-537) proteins. High frequencies of IFN-γ- and TNF-α-producing T cells directed at both epitopes were detected following primary infection with all four DENV serotypes, and were augmented by secondary DENV infections. In vivo cytotoxicity assays demonstrated rapid clearance of target cells pulsed with the NS4a peptide; in contrast, NS5 peptide-pulsed target cells were poorly cleared in vivo. These data characterize two H-2(b)-restricted T-cell epitopes displaying divergent in vivo function. These results should facilitate further studies of the in vivo effects of DENV-specific T cells, including the use of genetically modified mouse strains.

Meta-analysis of all immune epitope data in the Flavivirus genus: inventory of current immune epitope data status in the context of virus immunity and immunopathology

A meta-analysis was performed in order to inventory the immune epitope data related to viruses in the genus Flavivirus. Nearly 2000 epitopes were captured from over 130 individual Flavivirus-related references identified from PubMed and reported as of September 2009. This report includes all epitope structures and associated immune reactivity from the past and current literature, including: the epitope distribution among pathogens and related strains, the epitope distribution among different pathogen antigens, the number of epitopes defined in human and animal models of disease, the relationship between epitopes identified in different disease states following natural (or experimental) infection, and data from studies focused on candidate vaccines. We found that the majority of epitopes were defined for dengue virus (DENV) and West Nile virus (WNV). The prominence of DENV and WNV data in the epitope literature is likely a reflection of their overall worldwide impact on human disease, and the lack of vaccines. Conversely, the relatively smaller number of epitopes defined for the other viruses within the genus (yellow fever and Japanese encephalitis virus) most likely reflects the presence of established prophylaxis and/or their more modest impact on morbidity and mortality globally. Through this work we hope to provide useful data to those working in the area of Flavivirus research.

A protective role for dengue virus-specific CD8+ T cells

Infection with one of the four serotypes of dengue virus (DENV1-4) can result in a range of clinical manifestations in humans, from dengue fever to the more serious dengue hemorrhagic fever/dengue shock syndrome. Although T cells have been implicated in the immunopathogenesis of secondary infections with heterologous DENV serotypes, the role of T cells in protection against DENV is unknown. In this study, we used a mouse-passaged DENV2 strain, S221, to investigate the role of CD8(+) T cells in the immune response to primary DENV infection. S221 did not replicate well in wild-type mice, but did induce a CD8(+) T cell response, whereas viral replication and a robust CD8(+) T cell response were observed after infection of IFN-alpha/betaR(-/-) mice. Depletion of CD8(+) T cells from IFN-alpha/betaR(-/-) mice before infection resulted in significantly higher viral loads compared with undepleted mice. Mapping the specificity of the CD8(+) T cell response led to the identification of 12 epitopes derived from 6 of the 10 DENV proteins, with a similar immunodominance hierarchy observed in wild-type and IFN-alpha/betaR(-/-) mice. DENV-specific CD8(+) T cells produced IFN-gamma, TNF-alpha, expressed cell surface CD107a, and exhibited cytotoxic activity in vivo. Finally, immunization with four of the immunodominant CD8(+) T cell epitopes enhanced viral clearance. Collectively, our results reveal an important role for CD8(+) T cells in the host defense against DENV and demonstrate that the anti-DENV CD8(+) T cell response can be enhanced by immunization, providing rationale for designing DENV-specific vaccines that induce cell-mediated immunity.

Scientific consultation on cell mediated immunity (CMI) in dengue and dengue vaccine development

Dengue is a re-emerging arboviral disease of great public health importance. Limited understanding of protective immune responses against dengue has hampered advancement of dengue vaccine candidates. Demonstrating an immunological correlate of protection has been limited to associating quantitative neutralizing antibody titers with clinical outcomes following infection. There have been a number of studies investigating the role of cell mediated immunity (CMI) in natural infections and these have demonstrated roles in both virus clearance and potentiating disease. Vaccine developers have extended the exploratory study of CMI in natural infection to the study of dengue vaccine recipients. Primary infections and monovalent vaccine administration generates dengue type-specific T-cell responses. Secondary infection, vaccination of flavivirus primed individuals, or administration of multivalent vaccine candidates results in broad, cross-reactive T-cell responses, similar to the broadening of antibody patterns. However, the precise function of CMI in protection or disease pathology remains ill-defined and, at present, there is no evidence to suggest that CMI can be utilized as a correlate of protection. Nonetheless, the study of CMI in natural infection and following vaccine administration should continue in an attempt to improve the understanding of dengue immunopathology, vaccine candidate immunogenicity, and potential correlates of protection.

Protective and enhancing HLA alleles, HLA-DRB1*0901 and HLA-A*24, for severe forms of dengue virus infection, dengue hemorrhagic fever and dengue shock syndrome

Background

Dengue virus (DV) infection is one of the most important mosquito-borne diseases in the tropics. Recently, the severe forms, dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS), have become the leading cause of death among children in Southern Vietnam. Protective and/or pathogenic T cell immunity is supposed to be important in the pathogenesis of DHF and DSS.

Methodology/Principal Findings

To identify HLA alleles controlling T cell immunity against dengue virus (DV), we performed a hospital-based case control study at Children's Hospital No.2, Ho Chi Minh City (HCMC), and Vinh Long Province Hospital (VL) in Southern Vietnam from 2002 to 2005. A total of 211 and 418 patients with DHF and DSS, respectively, diagnosed according to the World Health Organization (WHO) criteria, were analyzed for their characteristic HLA-A, -B and -DRB1 alleles. Four hundred fifty healthy children (250 from HCMC and 200 from VL) of the same Kinh ethnicity were also analyzed as population background. In HLA class I, frequency of the HLA-A*24 showed increased tendency in both DHF and DSS patients, which reproduced a previous study. The frequency of A*24 with histidine at codon 70 (A*2402/03/10), based on main anchor binding site specificity analysis in DSS and DHF patients, was significantly higher than that in the population background groups (HCMC 02-03 DSS: OR = 1.89, P = 0.008, DHF: OR = 1.75, P = 0.033; VL 02-03 DSS: OR = 1.70, P = 0.03, DHF: OR = 1.46, P = 0.38; VL 04-05 DSS: OR = 2.09, P = 0.0075, DHF: OR = 2.02, P = 0.038). In HLA class II, the HLA-DRB1*0901 frequency was significantly decreased in secondary infection of DSS in VL 04-05 (OR = 0.35, P = 0.0025, Pc = 0.03). Moreover, the frequency of HLA-DRB1*0901 in particular was significantly decreased in DSS when compared with DHF in DEN-2 infection (P = 0.02).

Conclusion

This study improves our understanding of the risk of HLA-class I for severe outcome of DV infection in the light of peptide anchor binding site and provides novel evidence that HLA-class II may control disease severity (DHF to DSS) in DV infection.

Dengue has become one of the most common viral diseases transmitted by infected mosquitoes (with any of the four dengue virus serotypes: DEN-1, -2, -3, or -4). It may present as asymptomatic or illness, ranging from mild to severe disease. Recently, the severe forms, dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS), have become the leading cause of death among children in Southern Vietnam. The pathogenesis of DHF/DSS, however, is not yet completely understood. The immune response, virus virulence, and host genetic background are considered to be risk factors contributing to disease severity. Human leucocyte antigens (HLA) expressed on the cell surface function as antigen presenting molecules and those polymorphism can change individuals' immune response. We investigated the HLA-A, -B (class I), and -DRB1 (class II) polymorphism in Vietnamese children with different severity (DHF/DSS) by a hospital-based case-control study. The study showed persons carrying HLA-A*2402/03/10 are about 2 times more likely to have severe dengue infection than others. On the other hand, HLA-DRB1*0901 persons are less likely to develop DSS with DEN-2 virus infection. These results clearly demonstrated that HLA controlled the susceptibility to severe forms of DV infection.

Cross-reactive memory CD8(+) T cells alter the immune response to heterologous secondary dengue virus infections in mice in a sequence-specific manner

Dengue virus is the causative agent of dengue fever and the more-severe dengue hemorrhagic fever (DHF). Human studies suggest that the increased risk of DHF during secondary infection is due to immunopathology partially mediated by cross-reactive memory T cells from the primary infection. To model T cell responses to sequential infections, we immunized mice with different sequences of dengue virus serotypes and measured the frequency of peptide-specific T cells after infection. The acute response after heterologous secondary infections was enhanced compared with the acute or memory response after primary infection. Also, the hierarchy of epitope-specific responses was influenced by the specific sequence of infection. Adoptive-transfer experiments showed that memory T cells responded preferentially to the secondary infection. These findings demonstrate that cross-reactive T cells from a primary infection alter the immune response during a heterologous secondary infection.