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

Whole genome sequencing and phylogenetic analysis of dengue virus in Central Nepal from 2022 to 2023

BACKGROUND

In Nepal, dengue is an emerging disease of growing concern as outbreaks are increasing in both size and geographic reach and beginning to affect areas previously thought dengue-free. Dengue genomic surveillance has previously been limited within Nepal; however, with the increase in accessibility to sequencing technologies since the COVID-19 pandemic, it has recently become more feasible.

METHODS

This hospital-based retrospective study utilized banked samples from the 2022 and 2023 dengue seasons from Dhulikhel Hospital/Kathmandu University Hospital in Central Nepal. Next-generation sequencing was performed to obtain whole genome sequences of dengue virus which were analyzed phylogenetically using a maximum likelihood GTR + G model. Mutations were evaluated across viral particle region using the GISAID DengueServer.

RESULTS

We obtained 41 full-length sequences of DENV from 80 PCR-positive samples, including 24 sequences (58.5%) from 2022 and 17 sequences (41.5%) from 2023. We identified a shift in the majority serotype of our samples from DENV1 in 2022 to DENV3 in 2023, though 3 out of the 4 serotypes were identified in both years. Phylogenetic analysis revealed clusters within genotype III of DENV1 and genotype III of DENV3 closely related to strains from an outbreak of DENV in northern India in 2018-2019. DENV2 sequences fell into the cosmopolitan genotype IV-A1 and IV-B2 clades and were related to sequences from South and Southeast Asia and the USA, pointing to the global nature of dengue transmission. NS3 showed the highest frequency of mutation, whereas NS2B, NS4, NS5, and E were the most conserved. The most common mutations found were substitutions L17M and T20I in the 2 K peptide. A high number of mutations were observed in DENV3, followed by DENV2, with some mutations being unique to specific serotypes and others matching previously reported strains.

CONCLUSIONS

We identified possible clade shifts in the DENV1 and 2 populations and a rising prevalence of DENV3. Our study showed a high level of serotype diversity of DENV circulating in Central Nepal. Furthermore, our results indicate that DENV populations in Nepal are related to a geographically diverse set of sequences but are most strongly influenced by Indian strains of DENV.

Approaches of dengue control: vaccine strategies and future aspects

Dengue, caused by the dengue virus (DENV), affects millions of people worldwide every year. This virus has two distinct life cycles, one in the human and another in the mosquito, and both cycles are crucial to be controlled. To control the vector of DENV, the mosquito Aedes aegypti, scientists employed many techniques, which were later proved ineffective and harmful in many ways. Consequently, the attention shifted to the development of a vaccine; researchers have targeted the E protein, a surface protein of the virus and the NS1 protein, an extracellular protein. There are several types of vaccines developed so far, such as live attenuated vaccines, recombinant subunit vaccines, inactivated virus vaccines, viral vectored vaccines, DNA vaccines, and mRNA vaccines. Along with these, scientists are exploring new strategies of developing improved version of the vaccine by employing recombinant DNA plasmid against NS1 and also aiming to prevent the infection by blocking the DENV life cycle inside the mosquitoes. Here, we discussed the aspects of research in the field of vaccines until now and identified some prospects for future vaccine developments.

Orbivirus NS4 Proteins Play Multiple Roles to Dampen Cellular Responses

Non-structural protein 4 (NS4) of insect-borne and tick-borne orbiviruses is encoded by genome segment 9, from a secondary open reading frame. Though a protein dispensable for bluetongue virus (BTV) replication, it has been shown to counter the interferon response in cells infected with BTV or African horse sickness virus. We further explored the functional role(s) of NS4 proteins of BTV and the tick-borne Great Island virus (GIV). We show that NS4 of BTV or GIV helps an E3L deletion mutant of vaccinia virus to replicate efficiently in interferon-treated cells, further confirming the role of NS4 as an interferon antagonist. Our results indicate that ectopically expressed NS4 of BTV localised with caspase 3 within the nucleus and was found in a protein complex with active caspase 3 in a pull-down assay. Previous studies have shown that pro-apoptotic caspases (including caspase 3) suppress type I interferon response by cleaving mediators involved in interferon signalling. Our data suggest that orbivirus NS4 plays a role in modulating the apoptotic process and/or regulating the interferon response in mammalian cells, thus acting as a virulence factor in pathogenesis.

A fast-growing dengue virus mutant reveals a dual role of STING in response to infection

The four dengue viruses (DENVs) have evolved multiple mechanisms to ensure its survival. Among these mechanisms is the ability to regulate its replication rate, which may contribute to avoiding premature immune activation that limit infection dissemination: DENVs associated with dengue epidemics have shown slower replication rate than pre-epidemic strains. Correspondingly, wild-type DENVs replicate more slowly than their clinically attenuated derivatives. To understand how DENVs 'make haste slowly', we generated and screened for DENV2 mutants with accelerated replication that also induced high type-I interferon (IFN) expression in infected cells. We chanced upon a single NS2B-I114T amino acid substitution, in an otherwise highly conserved amino acid residue. Accelerated DENV2 replication damaged host DNA as mutant infection was dependent on host DNA damage repair factors, namely RAD21, EID3 and NEK5. DNA damage induced cGAS/STING signalling and activated early type-I IFN response that inhibited infection dissemination. Unexpectedly, STING activation also supported mutant DENV replication in infected cells through STING-induced autophagy. Our findings thus show that DENV NS2B has multi-faceted role in controlling DENV replication rate and immune evasion and suggest that the dual role of STING in supporting virus replication within infected cells but inhibiting infection dissemination could be particularly advantageous for live attenuated vaccine development.

Evolution of resistance to fluoroquinolones by dengue virus serotype 4 provides insight into mechanism of action and consequences for viral fitness

Drugs against flaviviruses such as dengue (DENV) and Zika (ZIKV) virus are urgently needed. We previously demonstrated that three fluoroquinolones, ciprofloxacin, enoxacin, and difloxacin, suppress replication of six flaviviruses. To investigate the barrier to resistance and mechanism(s) of action of these drugs, DENV-4 was passaged in triplicate in HEK-293 cells in the presence or absence of each drug. Resistance to ciprofloxacin was detected by the seventh passage and to difloxacin by the tenth, whereas resistance to enoxacin did not occur within ten passages. Two putative resistance-conferring mutations were detected in the envelope gene of ciprofloxacin and difloxacin-resistant DENV-4. In the absence of ciprofloxacin, ciprofloxacin-resistant viruses sustained a significantly higher viral titer than control viruses in HEK-293 and HuH-7 cells and resistant viruses were more stable than control viruses at 37 °C. These results suggest that the mechanism of action of ciprofloxacin and difloxacin involves interference with virus binding or entry.

Characterization of Plaque Variants and the Involvement of Quasi-Species in a Population of EV-A71

Viral plaque morphologies in human cell lines are markers for growth capability and they have been used to assess the viral fitness and selection of attenuated mutants for live-attenuated vaccine development. In this study, we investigate whether the naturally occurring plaque size variation reflects the virulence of the variants of EV-A71. Variants of two different plaque sizes (big and small) from EV-A71 sub-genotype B4 strain 41 were characterized. The plaque variants displayed different in vitro growth kinetics compared to the parental wild type. The plaque variants showed specific mutations being present in each variant strain. The big plaque variants showed four mutations I97L, N104S, S246P and N282D in the VP1 while the small plaque variants showed I97T, N237T and T292A in the VP1. No other mutations were detected in the whole genome of the two variants. The variants showed stable homogenous small plaques and big plaques, respectively, when re-infected in rhabdomyosarcoma (RD) and Vero cells. The parental strain showed faster growth kinetics and had higher viral RNA copy number than both the big and small plaque variants. Homology modelling shows that both plaque variants have differences in the structure of the VP1 protein due to the presence of unique spontaneous mutations found in each plaque variant This study suggests that the EV-A71 sub-genotype B4 strain 41 has at least two variants with different plaque morphologies. These differences were likely due to the presence of spontaneous mutations that are unique to each of the plaque variants. The ability to maintain the respective plaque morphology upon passaging indicates the presence of quasi-species in the parental population.

Defeat Dengue and Zika Viruses With a One-Two Punch of Vaccine and Vector Blockade

Dengue virus (DENV) and Zika virus (ZIKV) are two mosquito-borne flaviviruses afflicting nearly half of the world population. Human infection by these viruses can either be asymptomatic or manifest as clinical diseases from mild to severe. Despite more cases are presented as self-limiting febrile illness, severe dengue disease can be manifested as hemorrhagic fever and hemorrhagic shock syndrome, and ZIKV infection has been linked to increased incidence of peripheral neuropathy Guillain-Barre syndrome and central neural disease such as microcephaly. The current prevention and treatment of these infectious diseases are either non-satisfactory or entirely lacking. Because DENV and ZIKV have much similarities in genomic and structural features, almost identical mode of mosquito-mediated transmission, and probably the same pattern of host innate and adaptive immunity toward them, it is reasonable and often desirable to investigate these two viruses side-by-side, and thereby devise common countermeasures against both. Here, we review the existing knowledge on DENV and ZIKV regarding epidemiology, molecular virology, protective immunity and vaccine development, discuss recent new discoveries on the functions of flavivirus NS1 protein in viral pathogenesis and transmission, and propose a one-two punch strategy using vaccine and vector blockade to overcome antibody-dependent enhancement and defeat Dengue and Zika viruses.

A Review on Dengue Vaccine Development

Dengue virus (DENV) has become a global health threat with about half of the world's population at risk of infection. Although the disease caused by DENV is self-limiting in the first infection, the antibody-dependent enhancement (ADE) effect increases the mortality in the second infection with a heterotypic virus. Since there is no specific efficient medicine in treatment, it is urgent to develop vaccines to prevent infection and disease progression. Currently, only a live attenuated vaccine, chimeric yellow fever 17D-tetravalent dengue vaccine (CYD-TDV), has been licensed for clinical use in some countries, and many candidate vaccines are still under research and development. This review discusses the progress, strengths, and weaknesses of the five types of vaccines including live attenuated vaccine, inactivated virus vaccine, recombinant subunit vaccine, viral vectored vaccine, and DNA vaccine.

Humanized Mice in Dengue Research: A Comparison with Other Mouse Models

Dengue virus (DENV) is an arbovirus of the Flaviviridae family and is an enveloped virion containing a positive sense single-stranded RNA genome. DENV causes dengue fever (DF) which is characterized by an undifferentiated syndrome accompanied by fever, fatigue, dizziness, muscle aches, and in severe cases, patients can deteriorate and develop life-threatening vascular leakage, bleeding, and multi-organ failure. DF is the most prevalent mosquito-borne disease affecting more than 390 million people per year with a mortality rate close to 1% in the general population but especially high among children. There is no specific treatment and there is only one licensed vaccine with restricted application. Clinical and experimental evidence advocate the role of the humoral and T-cell responses in protection against DF, as well as a role in the disease pathogenesis. A lot of pro-inflammatory factors induced during the infectious process are involved in increased severity in dengue disease. The advances in DF research have been hampered by the lack of an animal model that recreates all the characteristics of this disease. Experiments in nonhuman primates (NHP) had failed to reproduce all clinical signs of DF disease and during the past decade, humanized mouse models have demonstrated several benefits in the study of viral diseases affecting humans. In DENV studies, some of these models recapitulate specific signs of disease that are useful to test drugs or vaccine candidates. However, there is still a need for a more complete model mimicking the full spectrum of DENV. This review focuses on describing the advances in this area of research.

Beyond attachment: Roles of DC-SIGN in dengue virus infection

Dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN), a C-type lectin expressed on the plasma membrane by human immature dendritic cells, is a receptor for numerous viruses including Ebola, SARS and dengue. A controversial question has been whether DC-SIGN functions as a complete receptor for both binding and internalization of dengue virus (DENV) or whether it is solely a cell surface attachment factor, requiring either hand-off to another receptor or a co-receptor for internalization. To examine this question, we used 4 cell types: human immature dendritic cells and NIH3T3 cells expressing either wild-type DC-SIGN or 2 internalization-deficient DC-SIGN mutants, in which either the 3 cytoplasmic internalization motifs are silenced by alanine substitutions or the cytoplasmic region is truncated. Using confocal and super-resolution imaging and high content single particle tracking, we investigated DENV binding, DC-SIGN surface transport, endocytosis, as well as cell infectivity. DC-SIGN was found colocalized with DENV inside cells suggesting hand-off at the plasma membrane to another receptor did not occur. Moreover, all 3 DC-SIGN molecules on NIH3T3 cells supported cell infection. These results imply the involvement of a co-receptor because cells expressing the internalization-deficient mutants could still be infected.

West Nile Virus Temperature Sensitivity and Avian Virulence Are Modulated by NS1-2B Polymorphisms

West Nile virus (WNV) replicates in a wide variety of avian species, which serve as reservoir and amplification hosts. WNV strains isolated in North America, such as the prototype strain NY99, elicit a highly pathogenic response in certain avian species, notably American crows (AMCRs; Corvus brachyrhynchos). In contrast, a closely related strain, KN3829, isolated in Kenya, exhibits a low viremic response with limited mortality in AMCRs. Previous work has associated the difference in pathogenicity primarily with a single amino acid mutation at position 249 in the helicase domain of the NS3 protein. The NY99 strain encodes a proline residue at this position, while KN3829 encodes a threonine. Introduction of an NS3-T249P mutation in the KN3829 genetic background significantly increased virulence and mortality; however, peak viremia and mortality were lower than those of NY99. In order to elucidate the viral genetic basis for phenotype variations exclusive of the NS3-249 polymorphism, chimeric NY99/KN3829 viruses were created. We show herein that differences in the NS1-2B region contribute to avian pathogenicity in a manner that is independent of and additive with the NS3-249 mutation. Additionally, NS1-2B residues were found to alter temperature sensitivity when grown in avian cells.

West Nile virus (WNV) is a mosquito-borne virus that has caused outbreaks in humans in many regions of the world. Birds are the natural hosts for WNV. However, different strains of WNV cause different disease outcomes in birds. Here, we compared two WNV strains, one of which causes higher mortality and generates more virus in American crows than the other. Previous research has shown that this difference is due in large part to a difference between the two strains at a single amino acid in the NS3 gene; however, this difference does not completely explain the observed effect. Here we show that another region of the viral genome also affects disease outcomes in American crows, and changes the sensitivity of the virus to temperature when grown in bird cells. These findings help us to understand the genetic features that affect WNV infection and disease outcomes in its natural host. Detection of such features in new strains of WNV and related viruses could help to understand and predict future outbreaks.

Trials and Tribulations on the Path to Developing a Dengue Vaccine

Dengue is a rapidly expanding global health problem. Development of a safe and efficacious tetravalent vaccine along with strategic application of vector control activities represents a promising approach to reducing the global disease burden. Although many vaccine development challenges exist, numerous candidates are in clinical development and one has been tested in three clinical endpoint studies. The results of these studies have raised numerous questions about how we measure vaccine immunogenicity and how these readouts are associated with clinical outcomes in vaccine recipients who experience natural infection. In this review the authors discuss the dengue vaccine pipeline, development challenges, the dengue vaccine-immunologic profiling intersection, and research gaps.

Flavivirus RNA synthesis in vitro

Establishment of in vitro systems to study mechanisms of RNA synthesis for positive strand RNA viruses have been very useful in the past and have shed light on the composition of protein and RNA components, optimum conditions, the nature of the products formed, cis-acting RNA elements and trans-acting protein factors required for efficient synthesis. In this review, we summarize our current understanding regarding the requirements for flavivirus RNA synthesis in vitro. We describe details of reaction conditions, the specificity of template used by either the multi-component membrane-bound viral replicase complex or by purified, recombinant RNA-dependent RNA polymerase. We also discuss future perspectives to extend the boundaries of our knowledge.

Orthobunyavirus antibodies among humans in selected parts of the Rift Valley and northeastern Kenya

Ngari, Bunyamwera, Ilesha, and Germiston viruses are among the mosquito-borne human pathogens in the Orthobunyavirus genus, family Bunyaviridae, associated with febrile illness. Although the four orthobunyaviruses have been isolated from mosquito and/or tick vectors sampled from different geographic regions in Kenya, little is known of human exposure in such areas. We conducted a serologic investigation to determine whether orthobunyaviruses commonly infect humans in Kenya. Orthobunyavirus-specific antibodies were detected by plaque reduction neutralization tests in 89 (25.8%) of 345 persons tested. Multivariable analysis revealed age and residence in northeastern Kenya as risk factors. Implementation of acute febrile illness surveillance in northeastern Kenya will help to detect such infections.

Genome sequence analysis of in vitro and in vivo phenotypes of Bunyamwera and Ngari virus isolates from northern Kenya

Biological phenotypes of tri-segmented arboviruses display characteristics that map to mutation/s in the S, M or L segments of the genome. Plaque variants have been characterized for other viruses displaying varied phenotypes including attenuation in growth and/or pathogenesis. In order to characterize variants of Bunyamwera and Ngari viruses, we isolated individual plaque size variants; small plaque (SP) and large plaque (LP) and determined in vitro growth properties and in vivo pathogenesis in suckling mice. We performed gene sequencing to identify mutations that may be responsible for the observed phenotype. The LP generally replicated faster than the SP and the difference in growth rate was more pronounced in Bunyamwera virus isolates. Ngari virus isolates were more conserved with few point mutations compared to Bunyamwera virus isolates which displayed mutations in all three genome segments but majority were silent mutations. Contrary to expectation, the SP of Bunyamwera virus killed suckling mice significantly earlier than the LP. The LP attenuation may probably be due to a non-synonymous substitution (T858I) that mapped within the active site of the L protein. In this study, we identify natural mutations whose exact role in growth and pathogenesis need to be determined through site directed mutagenesis studies.

Dengue virus vaccine development

Dengue virus (DENV) is a significant cause of morbidity and mortality in tropical and subtropical regions, causing hundreds of millions of infections each year. Infections range from asymptomatic to a self-limited febrile illness, dengue fever (DF), to the life-threatening dengue hemorrhagic fever/dengue shock syndrome (DHF/DSS). The expanding of the habitat of DENV-transmitting mosquitoes has resulted in dramatic increases in the number of cases over the past 50 years, and recent outbreaks have occurred in the United States. Developing a dengue vaccine is a global health priority. DENV vaccine development is challenging due to the existence of four serotypes of the virus (DENV1-4), which a vaccine must protect against. Additionally, the adaptive immune response to DENV may be both protective and pathogenic upon subsequent infection, and the precise features of protective versus pathogenic immune responses to DENV are unknown, complicating vaccine development. Numerous vaccine candidates, including live attenuated, inactivated, recombinant subunit, DNA, and viral vectored vaccines, are in various stages of clinical development, from preclinical to phase 3. This review will discuss the adaptive immune response to DENV, dengue vaccine challenges, animal models used to test dengue vaccine candidates, and historical and current dengue vaccine approaches.

Mouse models to study dengue virus immunology and pathogenesis

The development of a compelling murine model of dengue virus (DENV) infection has been challenging, because DENV clinical isolates do not readily replicate or cause pathology in immunocompetent mice. However, research using immunocompromised mice and/or mouse-adapted viruses allows investigation of questions that may be impossible to address in human studies. In this review, we discuss the potential strengths and limitations of existing mouse models of dengue disease. Human studies are descriptive by nature; moreover, the strain, time, and sequence of infection are often unknown. In contrast, in mice, the conditions of infection are well defined and a large number of experimental parameters can be varied at will. Therefore, mouse models offer an opportunity to experimentally test hypotheses that are based on epidemiological observations. In particular, gain-of-function or loss-of-function models can be established to assess how different components of the immune system (either alone or in combination) contribute to protection or pathogenesis during secondary infections or after vaccination. In addition, mouse models have been used for pre-clinical testing of anti-viral drugs or for vaccine development studies. Conclusions based on mouse experiments must be extrapolated to DENV-infection in humans with caution due to the inherent limitations of animal models. However, research in mouse models is a useful complement to in vitro and epidemiological data, and may delineate new areas that deserve attention during future human studies.

Developing a dengue vaccine: progress and future challenges

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

Dengue vaccine: priorities and progress

Dengue transmission has increased considerably in the past 20 years. Currently, it can only be reduced by mosquito control; however, the application of vector-control methods are labor intensive, require discipline and diligence, and are hard to sustain. In this context, a safe dengue vaccine that confers long-lasting protection against infection with the four dengue viruses is urgently required. This review will discuss the requirements of a dengue vaccine, problems, and advances that have been made. Finally, new targets for research will be presented.

Comparison of full-length genomics sequences between dengue virus serotype 3, parental strain, and its derivatives, and B-cell epitopes prediction from envelope region

Biological markers are normally used to evaluate the candidate of live-attenuated dengue vaccines. D3V 16562 Vero 23 and D3V 16562 Vero 33 which were derivatives of D3V 16562, parental strain, showed the similar biological data. We used molecular techniques and computational tools to evaluate these derivatives. The nucleotide and amino acid sequences of the derivatives were compared to their parent. The secondary structures of untranslated regions and B-cell epitopes were predicted. The results showed that nucleotide substitutions mostly occurred in NS5 and NS5 of V2 was unusual because of amino acid change at 3349 (tryptophan →stop codon). The nucleotide substitutions in 5'UTR, prM, E, NS1, NS2A, NS3, and 3'UTR were 4, 1, 2, 2, 1, 3, and 2, respectively. The secondary structure of 5'UTR of V2 was different from P and V1. The secondary structure of 3'UTR of V2 was similar to P and certainly distinct from V1. Furthermore, B-cell epitopes prediction revealed that there were 21 epitopes of envelope and the interesting epitope was at position 297-309 because it was in domain III in which the neutralizing antibody is induced. For this study, the attenuation of derivatives was caused by the nucleotide substitutions in 5'UTR, 3'UTR, and NS5 regions. The genotypic data and B-cell epitope make the derivatives attractive for the chimeric and peptide DENV vaccine development.

Adaptation of yellow fever virus 17D to Vero cells is associated with mutations in structural and non-structural protein genes

Serial passaging of yellow fever virus 17D in Vero cells was employed to derive seed material for a novel inactivated vaccine, XRX-001. Two independent passaging series identified a novel lysine to arginine mutation at amino acid 160 of the envelope protein, a surface-exposed residue in structural domain I. A third passage series resulted in an isoleucine to methionine mutation at residue 113 of the NS4B protein, a central membrane spanning region of the protein which has previously been associated with Vero cell adaptation of other mosquito-borne flaviviruses. These studies confirm that flavivirus adaptation to growth in Vero cells can be mediated by structural or non-structural protein mutations.

Replacement of conserved or variable sequences of the mosquito-borne dengue virus 3' UTR with homologous sequences from Modoc virus does not change infectivity for mosquitoes

The genus Flavivirus includes both vector-borne and no known vector (NKV) species, but the molecular determinants of transmission mode are not known. Conserved sequence differences between the two groups occur in 5' and 3' UTRs. To investigate the impact of these differences on transmission, chimeric genomes were generated, in which UTRs, UTRs+capsid, or the upper 3' UTR stem-loop of mosquito-borne dengue virus (DENV) were replaced with homologous regions from NKV Modoc virus (MODV); the conserved pentanucleotide sequence (CPS) was also deleted from the DENV genome. Virus was not recovered following transfection of these genomes in three different cell types. However, DENV genomes in which the CPS or variable region (VR) of the 3' UTR were replaced with MODV sequences were recovered and infected Aedes aegypti mosquitoes with similar efficiencies to DENV. These results demonstrate that neither vector-borne CPS nor VR is required for vector-borne transmission.

Efficacy of a tetravalent chimeric dengue vaccine (DENVax) in Cynomolgus macaques

Three tetravalent formulations of chimeric dengue (DENVax) viruses containing the pre-membrane and envelope genes of serotypes 1-4 expressed by the attenuated DENV-2 PDK-53 genome were tested for safety, immunogenicity, and efficacy in cynomolgus macaques (Macaca fascicularis). Subcutaneous injection of the DENVax formulations was well-tolerated. Low levels of viremia of only one of the four vaccine viruses were detected yet virus neutralizing antibody titers were induced against all four dengue virus serotypes after one or two administrations of vaccine. All animals immunized with the high-dose formulation were protected from viremia, and all immunized animals were completely protected from DENV-3 and DENV-4 challenge. A lower dose of DENVax formulation partially protected animals from DENV-1 or DENV-2 challenge. In contrast, all control animals developed high levels of viremia for multiple days after challenge with DENV 1-4. This study highlights the immunogenicity and efficacy of the tetravalent DENVax formulations in nonhuman primates.

Development and clinical evaluation of multiple investigational monovalent DENV vaccines to identify components for inclusion in a live attenuated tetravalent DENV vaccine

The Laboratory of Infectious Diseases at the National Institute of Allergy and Infectious Diseases, National Institutes of Health has been engaged in an effort to develop a safe, efficacious, and affordable live attenuated tetravalent dengue vaccine (LATV) for more than ten years. Numerous recombinant monovalent DENV vaccine candidates have been evaluated in the SCID-HuH-7 mouse and in rhesus macaques to identify those candidates with a suitable attenuation phenotype. In addition, the ability of these candidates to infect and disseminate in Aedes mosquitoes had also been determined. Those candidates that were suitably attenuated in SCID-HuH-7 mice, rhesus macaques, and mosquitoes were selected for further evaluation in humans. This review will describe the generation of multiple candidate vaccines directed against each DENV serotype, the preclinical and clinical evaluation of these candidates, and the process of selecting suitable candidates for inclusion in a LATV dengue vaccine.

Replacement of the 3' untranslated variable region of mosquito-borne dengue virus with that of tick-borne Langat virus does not alter vector specificity

The four major flavivirus clades are transmitted by mosquitoes, ticks, directly between vertebrates or directly between arthropods, respectively, but the molecular determinants of mode of transmission in flaviviruses are unknown. To assess the role of the UTRs in transmission, we generated chimeric genomes in which the 5' UTR, capsid and/or 3' UTR of mosquito-borne dengue virus serotype 4 (rDENV-4) were replaced, separately or in combination, with those of tick-borne Langat virus (rLGTV). None of the chimeric genomes yielded detectable virus following transfection. Replacement of the variable region (VR) in the rDENV-4 3' UTR with that of rLGTV generated virus rDENV-4-rLGTswapVR, which showed lower replication than its wild-type parents in mammalian but not mosquito cells in culture and was able to infect mosquitoes in vivo. Neither rDENV-4 nor rDENV-4-rLGTswapVR could infect larval Ixodes scapularis ticks immersed in virus, while rLGTV was highly infectious via this route.

The neurovirulence and neuroinvasiveness of chimeric tick-borne encephalitis/dengue virus can be attenuated by introducing defined mutations into the envelope and NS5 protein genes and the 3' non-coding region of the genome

Tick-borne encephalitis (TBE) is a severe disease affecting thousands of people throughout Eurasia. Despite the use of formalin-inactivated vaccines in endemic areas, an increasing incidence of TBE emphasizes the need for an alternative vaccine that will induce a more durable immunity against TBE virus (TBEV). The chimeric attenuated virus vaccine candidate containing the structural protein genes of TBEV on a dengue virus genetic background (TBEV/DEN4) retains a high level of neurovirulence in both mice and monkeys. Therefore, attenuating mutations were introduced into the envelope (E(315)) and NS5 (NS5(654,655)) proteins, and into the 3' non-coding region (Delta30) of TBEV/DEN4. The variant that contained all three mutations (vDelta30/E(315)/NS5(654,655)) was significantly attenuated for neuroinvasiveness and neurovirulence and displayed a reduced level of replication and virus-induced histopathology in the brains of mice. The high level of safety in the central nervous system indicates that vDelta30/E(315)/NS5(654,655) should be further evaluated as a TBEV vaccine.

Construction of a dengue virus type 4 reporter replicon and analysis of temperature-sensitive mutations in non-structural proteins 3 and 5

Replicon systems have been useful to study mechanisms of translation and replication of flavivirus RNAs. In this study, we constructed a dengue virus 4 replicon encoding a Renilla luciferase (R_luc) reporter, and six single-residue substitution mutants were generated: L128F and S158P in the non-structural protein (NS) 3 protease domain gene, and N96I, N390A, K437R and M805I in the NS5 gene. The effects of these substitutions on viral RNA translation and/or replication were examined by measuring R_luc activities in wild-type and mutant replicon RNA-transfected Vero cells incubated at 35, 37 and 39 °C. Our results show that none of the mutations affected translation of replicon RNAs; however, L128F and S158P of NS3 at 39 °C, and N96I of NS5 at 37 and 39 °C, presented temperature-sensitive (ts) phenotypes for replication. Furthermore, using in vitro methyltransferase assays, we identified that the N96I mutation in NS5 exhibited a ts phenotype for N7-methylation, but not for 2′-O-methylation.

Targeted mutagenesis as a rational approach to dengue virus vaccine development

The recombinant dengue virus type 4 (rDEN4) vaccine candidate, rDEN4Delta30, was found to be highly infectious, immunogenic and safe in human volunteers. At the highest dose (10(5) PFU) evaluated in volunteers, 25% of the vaccinees had mild elevations in liver enzymes that were rarely seen at lower doses. Here, we describe the generation and selection of additional mutations that were introduced into rDEN4Delta30 to further attenuate the virus in animal models and ultimately human vaccinees. Based on the elevated liver enzymes associated with the 10(5) PFU dose of rDEN4Delta30 and the known involvement of liver infection in dengue virus pathogenesis, a large panel of mutant viruses was screened for level of replication in the HuH-7 human hepatoma cell line, a surrogate for human liver cells and selected viruses were further analyzed for level of viremia in SCID-HuH-7 mice. It was hypothesized that rDEN4Delta30 derivatives with restricted replication in vitro and in vivo in HuH-7 human liver cells would be restricted in replication in the liver of vaccinees. Two mutations identified by this screen, NS3 4995 and NS5 200,201, were separately introduced into rDEN4Delta30 and found to further attenuate the vaccine candidate for SCID-HuH-7 mice and rhesus monkeys while retaining sufficient immunogenicity in rhesus monkeys to confer protection. In humans, the rDEN4Delta30-200,201 vaccine candidate administered at 10(5) PFU exhibited greatly reduced viremia, high infectivity and lacked liver toxicity while inducing serum neutralizing antibody at a level comparable to that observed in volunteers immunized with rDEN4Delta30. Clinical studies of rDEN4Delta30-4995 are ongoing.

Targeted mutagenesis as a rational approach to dengue virus vaccine development

The recombinant dengue virus type 4 (rDEN4) vaccine candidate, rDEN4Delta30, was found to be highly infectious, immunogenic and safe in human volunteers. At the highest dose (10(5) PFU) evaluated in volunteers, 25% of the vaccinees had mild elevations in liver enzymes that were rarely seen at lower doses. Here, we describe the generation and selection of additional mutations that were introduced into rDEN4Delta30 to further attenuate the virus in animal models and ultimately human vaccinees. Based on the elevated liver enzymes associated with the 10(5) PFU dose of rDEN4Delta30 and the known involvement of liver infection in dengue virus pathogenesis, a large panel of mutant viruses was screened for level of replication in the HuH-7 human hepatoma cell line, a surrogate for human liver cells and selected viruses were further analyzed for level of viremia in SCID-HuH-7 mice. It was hypothesized that rDEN4Delta30 derivatives with restricted replication in vitro and in vivo in HuH-7 human liver cells would be restricted in replication in the liver of vaccinees. Two mutations identified by this screen, NS3 4995 and NS5 200,201, were separately introduced into rDEN4Delta30 and found to further attenuate the vaccine candidate for SCID-HuH-7 mice and rhesus monkeys while retaining sufficient immunogenicity in rhesus monkeys to confer protection. In humans, the rDEN4Delta30-200,201 vaccine candidate administered at 10(5) PFU exhibited greatly reduced viremia, high infectivity and lacked liver toxicity while inducing serum neutralizing antibody at a level comparable to that observed in volunteers immunized with rDEN4Delta30. Clinical studies of rDEN4Delta30-4995 are ongoing.

A single point mutation in nonstructural protein NS2 of bovine viral diarrhea virus results in temperature-sensitive attenuation of viral cytopathogenicity

For Bovine viral diarrhea virus (BVDV), the type species of the genus Pestivirus in the family Flaviviridae, cytopathogenic (cp) and noncytopathogenic (ncp) viruses are distinguished according to their effect on cultured cells. It has been established that cytopathogenicity of BVDV correlates with efficient production of viral nonstructural protein NS3 and with enhanced viral RNA synthesis. Here, we describe generation and characterization of a temperature-sensitive (ts) mutant of cp BVDV strain CP7, termed TS2.7. Infection of bovine cells with TS2.7 and the parent CP7 at 33 degrees C resulted in efficient viral replication and a cytopathic effect. In contrast, the ability of TS2.7 to cause cytopathogenicity at 39.5 degrees C was drastically reduced despite production of high titers of infectious virus. Further experiments, including nucleotide sequencing of the TS2.7 genome and reverse genetics, showed that a Y1338H substitution at residue 193 of NS2 resulted in the temperature-dependent attenuation of cytopathogenicity despite high levels of infectious virus production. Interestingly, TS2.7 and the reconstructed mutant CP7-Y1338H produced NS3 in addition to NS2-3 throughout infection. Compared to the parent CP7, NS2-3 processing was slightly decreased at both temperatures. Quantification of viral RNAs that were accumulated at 10 h postinfection demonstrated that attenuation of the cytopathogenicity of the ts mutants at 39.5 degrees C correlated with reduced amounts of viral RNA, while the efficiency of viral RNA synthesis at 33 degrees C was not affected. Taken together, the results of this study show that a mutation in BVDV NS2 attenuates viral RNA replication and suppresses viral cytopathogenicity at high temperature without altering NS3 expression and infectious virus production in a temperature-dependent manner.

Phase 1 trial of the dengue virus type 4 vaccine candidate rDEN4{Delta}30-4995 in healthy adult volunteers

rDEN4Delta30-4995 is a live attenuated dengue virus type 4 (DENV4) vaccine candidate specifically designed as a further attenuated derivative of the rDEN4Delta30 parent virus. In a previous study, 5 of 20 vaccinees who received 10(5) plaque-forming units (PFU) of rDEN4Delta30 developed a transient elevation of the serum alanine aminotransferase (ALT) level and an asymptomatic maculopapular rash developed in 10 of 20. In the current study, 28 healthy adult volunteers were randomized to receive 10(5) PFU of rDEN4Delta30-4995 (20) or placebo (8) as a single subcutaneous injection. The vaccine was safe, well-tolerated, and immunogenic. An asymptomatic generalized maculopapular rash and elevations in ALT levels were observed in 10% of the rDEN4Delta30-4995 vaccinees. None of the rDEN4Delta30-4995 vaccinees became viremic, yet 95% developed a four-fold or greater increase in neutralizing antibody titers. Thus, rDEN4Delta30-4995 was demonstrated to be safe, highly attenuated, and immunogenic. However, an asymptomatic localized erythematous rash at the injection site was seen in 17/20 rDEN4Delta30-4995 vaccinees. Therefore, alternative DENV4 vaccine strains were selected for further clinical development.

Identification of genetic determinants of a tick-borne flavivirus associated with host-specific adaptation and pathogenicity

Tick-borne flaviviruses are maintained in nature in an enzootic cycle involving a tick vector and a vertebrate host. Thus, the virus replicates in two disparate hosts, each providing selective pressures that can influence virus replication and pathogenicity. To identify viral determinants associated with replication in the individual hosts, plaque purified Langat virus (TP21pp) was adapted to growth in mouse or tick cell lines to generate two virus variants, MNBp20 and ISEp20, respectively. Virus adaptation to mouse cells resulted in four amino acid changes in MNBp20 relative to TP21pp, occurring in E, NS4A and NS4B. A comparison between TP21pp and ISEp20 revealed three amino acid modifications in M, NS3 and NS4A of ISEp20. ISEp20, but not MNBp20, was attenuated following intraperitoneal inoculation of mice. Following isolation from mice brains, additional mutations reproducibly emerged in E and NS3 of ISEp20 that were possibly compensatory for the initial adaptation to tick cells. Thus, our data implicate a role for E, M, NS3, NS4A and NS4B in host adaptation and pathogenicity of tick-borne flaviviruses.

Histidine at position 1042 of the p150 region of a KRT live attenuated rubella vaccine strain is responsible for the temperature sensitivity

The Japanese live attenuated KRT rubella vaccine strain has a temperature sensitivity (ts) phenotype. The objective of this study is to identify the region responsible for this phenotype. Genomic sequences of the KRT strain and the wild-type strain (RVi/Matsue.JPN/68) with the non-ts phenotype were investigated and reverse genetic systems (RG) for these strains were developed. The ts phenotype of KRT varied drastically on replacement of the p150 gene (encoding a methyltransferase and a nonstructural protease). Analysis of four chimeric viruses showed the region responsible for the ts phenotype to be located between Bsm I and Nhe I sites (genome position 2803-3243). There were two amino acid differences at positions 1007 and 1042. Mutations were introduced into the KRT cDNA clone, designated G1007D, H1042Y and G1007D-H1042Y. H1042Y and G1007D-H1042Y grew well at a restrictive temperature with a 100-fold higher titer than G1007D and the KRT strain, but a 10-fold lower titer than RVi/Matsue.JPN/68. Since the growth of H1042Y was not completely the same as that of the wild-type strain at the restrictive temperature, we also assessed whether other genomic regions have an additive effect with H1042Y on the ts phenotype. H1042Y-RViM SP having structural proteins of RVi/Matsue.JPN/68 grew better than H1042Y, similar to RVi/Matsue.JPN/68. Thus, we concluded that one mutation, of the histidine at position 1042 of p150, was essential for the ts phenotype of the KRT strain, and structural proteins of KRT had an additive effect with H1042Y on the ts phenotype.

Reverse genetics and the study of dengue virus

Dengue virus infection causes the most important arthropod-borne disease of humans. Currently, there are no dengue vaccines or antiviral therapies in clinical use, although their development is a global health priority. Using a technique known as ‘reverse genetics’, the dengue virus RNA genome can be manipulated, either by the introduction of specific mutations or the deletion and/or substitution of entire genes. This has led to the production of novel recombinant viruses that have potential as vaccines and the production of noninfectious viral subgenomes (termed replicons) useful for drug screening. Reverse genetics is also an invaluable tool for studying the role of dengue virus RNA elements and proteins in replication and pathogenesis. This review describes the contribution of reverse genetics to dengue virus research to date, highlighting the potential use of this technology in the development of effective control measures against dengue in the future.

High temperatures activate local viral multiplication and cell-to-cell movement of Melon necrotic spot virus but restrict expression of systemic symptoms

The infection of melon plants by Melon necrotic spot virus (MNSV) and the development of necrotic disease symptoms are a seasonal occurrence in Japan, which take place between winter and early summer, but not during mid-summer. In this paper we investigate the effect of three different temperatures (15, 20, and 25 degrees C) on the local and systemic expression of MNSV in melon plants. Previously, the incidence of plants expressing systemic symptoms caused by MNSV and other viruses was found to be greater at temperatures less than 20 degrees C. In this study, our temperature-shift experiments support previous studies that found the expression of systemic symptoms increases as temperature falls from 25 to 20 degrees C and decreases as temperature rises from 20 to 25 degrees C. However, MNSV replication in melon cells and local viral movement within leaves following the inoculation of melon protoplasts or cotyledons were more frequent at 25 degrees C than at 15 or 20 degrees C.

Characterization of a small plaque variant of West Nile virus isolated in New York in 2000

A small-plaque variant (SP) of West Nile virus (WNV) was isolated in Vero cell culture from kidney tissue of an American crow collected in New York in 2000. The in vitro growth of the SP and parental (WT) strains was characterized in mammalian (Vero), avian (DF-1 and PDE), and mosquito (C6/36) cells. The SP variant replicated less efficiently than did the WT in Vero cells. In avian cells, SP growth was severely restricted at high temperatures, suggesting that the variant is temperature sensitive. In mosquito cells, growth of SP and WT was similar, but in vivo in Culex pipiens (L.) there were substantial differences. Relative to WT, SP exhibited reduced replication following intrathoracic inoculation and lower infection, dissemination, and transmission rates following oral infection. Analysis of the full length sequence of the SP variant identified sequence differences which led to only two amino acid substitutions relative to WT, prM P54S and NS2A V61A.

Claire M, Elkins R, Collins PL, Murphy BR, Schaap-Nutt A. Recombinant human parainfluenza virus type 2 vaccine candidates containing a 3' genomic promoter mutation and L polymerase mutations are attenuated and protective in non-human primates

Previously, we identified several attenuating mutations in the L polymerase protein of human parainfluenza virus type 2 (HPIV2) and genetically stabilized those mutations using reverse genetics [Nolan SM, Surman S, Amaro-Carambot E, Collins PL, Murphy BR, Skiadopoulos MH. Live-attenuated intranasal parainfluenza virus type 2 vaccine candidates developed by reverse genetics containing L polymerase protein mutations imported from heterologous paramyxoviruses. Vaccine 2005;39(23):4765-74]. Here we describe the discovery of an attenuating mutation at nucleotide 15 (15(T-->C)) in the 3' genomic promoter that was also present in the previously characterized mutants. We evaluated the properties of this promoter mutation alone and in various combinations with the L polymerase mutations. Amino acid substitutions at L protein positions 460 (460A or 460P) or 948 (948L), or deletion of amino acids 1724 and 1725 (Delta1724), each conferred a temperature sensitivity (ts) phenotype whereas the 15(T-->C) mutation did not. The 460A and 948L mutations each contributed to restricted replication in the lower respiratory tract of African green monkeys, but the Delta1724 mutation increased attenuation only in certain combinations with other mutations. We constructed two highly attenuated viruses, rV94(15C)/460A/948L and rV94(15C)/948L/Delta1724, that were immunogenic and protective against challenge with wild-type HPIV2 in African green monkeys and, therefore, appear to be suitable for evaluation in humans.

Development of a live attenuated dengue virus vaccine using reverse genetics

There are four serotypes of dengue (DEN1-DEN4) virus that are endemic in most areas of Southeast Asia, Central and South America, and other subtropical regions. The number of cases of severe disease associated with DEN virus infection is growing because of the continued spread of the mosquito vector, Aedes aegypti, which transmits the virus to humans. Infection with DEN virus can result in an asymptomatic infection, a febrile illness called dengue fever (DF), and the very severe disease called dengue hemorrhagic fever/dengue shock syndrome (DHF/DSS). Currently, a licensed vaccine is not available. However, a tetravalent vaccine is urgently needed to prevent DF and DHF/DSS, the latter of which occurs predominantly in partially immune individuals. A live attenuated, tetravalent DEN virus vaccine candidate has been generated using reverse genetics that is able to provide immunity to each of the four serotypes of DEN. Attenuation has been achieved by generating recombinant DEN (rDEN) viruses which are modified by deletion or, alternatively, by antigenic chimerization between two related DEN viruses using the following two strategies: 1) introduction of an attenuating 30 nucleotide deletion (Delta30) mutation into the 3' untranslated region of DEN1 and DEN4; and 2) replacement of structural proteins of the attenuated rDEN4Delta30 vaccine candidate with those from DEN2 or DEN3. Attenuation of the four monovalent vaccine candidates has been achieved for rhesus monkeys or humans and an immunogenic tetravalent vaccine candidate has been formulated. The level of attenuation of each dengue vaccine component can be increased, if needed, by introduction of additional attenuating mutations that have been well characterized.

Chimeric West Nile/dengue virus vaccine candidate: preclinical evaluation in mice, geese and monkeys for safety and immunogenicity

A live attenuated virus vaccine is being developed to protect against West Nile virus (WN) disease in humans. Previously, it was found that chimeric West Nile/dengue viruses (WN/DEN4 and WN/DEN4Delta30) bearing the membrane precursor and envelope protein genes of WN on a backbone of dengue type 4 virus (DEN4) with or without a deletion of 30 nucleotides (Delta30) in the 3' noncoding region of the DEN4 part of the chimeric genome were attenuated and efficacious in mice and monkeys against WN challenge. Here, we report the generation of a clinical lot of WN/DEN4Delta30 virus and its further preclinical evaluation for safety and immunogenicity in mice, geese and monkeys. The vaccine candidate had lost neuroinvasiveness in highly sensitive immunodeficient mice inoculated intraperitoneally and had greatly reduced neurovirulence in suckling mice inoculated intracerebrally (IC). Compared to the wild-type WN parent, the chimeric virus was highly restricted in replication in both murine and human neuroblastoma cells as well as in brains of suckling mice. The WN/DEN4Delta30 virus failed to infect geese, indicating that chimerization of WN with DEN4 completely attenuated WN for this avian host. This observation suggests that the WN/DEN4 chimeric viruses would be restricted in their ability to be transmitted from vaccinees to domestic or wild birds. In monkeys, the WN/DEN4Delta30 vaccine candidate was highly immunogenic despite its low level of replication with undetectable viremia. Furthermore, the WN/DEN4Delta30 vaccine virus was safe and readily induced neutralizing antibodies against WN in monkeys immune to each of the four serotypes of dengue virus. These studies confirm the attenuation of WN/DEN4Delta30 for non-human primates, including dengue-immune monkeys, and demonstrate both a highly restricted replication (>10(8)-fold decrease) in the brain of mice inoculated IC and an absence of infectivity for birds, findings that indicate this vaccine should be safe for both the recipient and the environment.

A tick-borne Langat virus mutant that is temperature sensitive and host range restricted in neuroblastoma cells and lacks neuroinvasiveness for immunodeficient mice

Langat virus (LGT), the naturally attenuated member of the tick-borne encephalitis virus (TBEV) complex, was tested extensively in clinical trials as a live TBEV vaccine and was found to induce a protective, durable immune response; however, it retained a low residual neuroinvasiveness in mice and humans. In order to ablate or reduce this property, LGT mutants that produced a small plaque size or temperature-sensitive (ts) phenotype in Vero cells were generated using 5-fluorouracil. One of these ts mutants, clone E5-104, exhibited a more than 10(3)-fold reduction in replication at the permissive temperature in both mouse and human neuroblastoma cells and lacked detectable neuroinvasiveness for highly sensitive immunodeficient mice. The E5-104 mutant possessed five amino acid substitutions in the structural protein E and one change in each of the nonstructural proteins NS3 and NS5. Using reverse genetics, we demonstrated that a Lys(46)-->Glu substitution in NS3 as well as a single Lys(315)-->Glu change in E significantly impaired the growth of LGT in neuroblastoma cells and reduced its peripheral neurovirulence for SCID mice. This study and our previous experience with chimeric flaviviruses indicated that a decrease in viral replication in neuroblastoma cells might serve as a predictor of in vivo attenuation of the neurotropic flaviviruses. The combination of seven mutations identified in the nonneuroinvasive E5-104 mutant provided a useful foundation for further development of a live attenuated TBEV vaccine. An evaluation of the complete sequence of virus recovered from brain of SCID mice inoculated with LGT mutants identified sites in the LGT genome that promoted neurovirulence/neuroinvasiveness.

Comparison of live and inactivated tick-borne encephalitis virus vaccines for safety, immunogenicity and efficacy in rhesus monkeys

Three antigenic chimeric live attenuated tick-borne encephalitis virus (TBEV) vaccine candidates were compared for level of replication in murine and human neuroblastoma cells, for neurovirulence and neuroinvasiveness in mice, and for safety, immunogenicity and efficacy in rhesus monkeys. Two chimeric viruses were generated by replacing the membrane precursor and envelope protein genes of dengue type 4 virus (DEN4) with the corresponding genes of a Far Eastern TBEV, Sofjin strain, in the presence (TBEV/DEN4Delta30) or absence (TBEV/DEN4) of a 30 nucleotide deletion (Delta30) in the 3' noncoding region of the DEN4 part of the chimeric genome. A third chimeric TBEV vaccine candidate was based on the antigenically distant, but naturally attenuated Langat virus (LGT). Chimerization of LGT with DEN4 resulted in decreased neurovirulence and neuroinvasiveness in mice and highly restricted viremia in rhesus monkeys. Also, the LGT/DEN4 chimera was highly restricted in replication in both murine and human neuroblastoma cells. In contrast, TBEV/DEN4 and TBEV/DEN4Delta30 were neither attenuated for neurovirulence in the mice nor restricted in replication in the neuroblastoma cells. However, both were highly attenuated for neuroinvasiveness in mice. TBEV/DEN4 replicated to moderately high titer in rhesus monkeys (mean peak viremia=10(3.1)PFU/ml) indicating that the TBEV/DEN4 chimerization had only a modest, if any, attenuating effect in monkeys. However, the addition of the Delta30 mutation to TBEV/DEN4 greatly attenuated the chimeric virus for rhesus monkeys (mean peak viremia=10(0.7)PFU/ml) and induced a higher level of antibody against the TBEV than did LGT/DEN4. A single dose of either highly attenuated TBEV/DEN4Delta30 or LGT/DEN4 vaccine candidate or three doses of an inactivated TBEV vaccine were efficacious in monkeys against wild-type LGT challenge. These results indicate that both TBEV/DEN4Delta30 and LGT/DEN4 are safe and efficacious in rhesus monkeys and should be further evaluated as vaccine candidates for use in humans.

Recombinant, live-attenuated tetravalent dengue virus vaccine formulations induce a balanced, broad, and protective neutralizing antibody response against each of the four serotypes in rhesus monkeys

Three tetravalent vaccine (TV) formulations of previously described monovalent dengue (DEN) virus vaccine candidates were compared to a tetravalent formulation of wild-type DEN viruses (T-wt) for replication in SCID mice transplanted with human liver cells (SCID-HuH-7) or for replication and immunogenicity in rhesus monkeys. TV-1 consists of recombinant DEN1, -2, -3, and -4, each with a 30-nucleotide deletion in the 3' untranslated region (Delta30). TV-2 consists of rDEN1Delta30, rDEN4Delta30, and two antigenic chimeric viruses, rDEN2/4Delta30 and rDEN3/4Delta30, both also bearing the Delta30 mutation. TV-3 consists of rDEN1Delta30, rDEN2Delta30, rDEN4Delta30, and a 10-fold higher dose of rDEN3/4Delta30. TV-1 and TV-2 were attenuated in SCID-HuH-7 mice with minimal interference in replication among the virus components. TV-1, -2, and -3 were attenuated in rhesus monkeys as measured by duration and peak of viremia. Each monkey immunized with TV-1 and TV-3 seroconverted to the four DEN components by day 28 with neutralization titers ranging from 1:52 to 1:273 and 1:59 to 1:144 for TV-1 and TV-3, respectively. TV-2 induced low antibody titers to DEN2 and DEN3, but a booster immunization after 4 months increased the neutralizing antibody titers to greater than 1:100 against each serotype and elicited broad neutralizing activity against 19 of 20 DEN subtypes. A single dose of TV-2 induced protection against wild-type DEN1, DEN3, and DEN4 challenge, but not DEN2. However, two doses of TV-2 or TV-3 induced protection against DEN2 challenge. Two tetravalent formulations, TV-2 and TV-3, possess properties of a successful DEN vaccine and can be considered for evaluation in clinical trials.

Introduction of mutations into the non-structural genes or 3' untranslated region of an attenuated dengue virus type 4 vaccine candidate further decreases replication in rhesus monkeys while retaining protective immunity

A dengue virus vaccine candidate, rDEN4Delta30, has been previously reported to be safe and immunogenic in humans, but a subset of vaccinees developed asymptomatic rash, elevation of liver enzymes and/or mild neutropenia. In the current study, mutations that had previously been shown to reduce replication of DEN4 virus in suckling mice and/or in SCID mice engrafted with human liver cells (SCID-HuH-7 mice) were introduced into rDEN4Delta30 in an attempt to further attenuate this virus. Three of the five resulting modified rDEN4Delta30 viruses showed decreased replication in SCID-HuH-7 mice relative to rDEN4Delta30. Moreover, in rhesus monkeys, two of the modified rDEN4Delta30 viruses showed a decrease in replication relative to rDEN4Delta30 while generating levels of neutralizing antibody similar to rDEN4Delta30 virus. All of the modified rDEN4Delta30 viruses completely protected immunized rhesus monkeys from challenge with wild-type DEN4 virus. Based on their attenuation for both human liver cells and rhesus monkeys, two of the modified rDEN4Delta30 vaccine candidates are currently being prepared for use in clinical trials. The application of these attenuating mutations to flavivirus vaccine development is discussed.

Vaccine candidates derived from a novel infectious cDNA clone of an American genotype dengue virus type 2

Background

A dengue virus type 2 (DEN-2 Tonga/74) isolated from a 1974 epidemic was characterized by mild illness and belongs to the American genotype of DEN-2 viruses. To prepare a vaccine candidate, a previously described 30 nucleotide deletion (Δ30) in the 3' untranslated region of DEN-4 has been engineered into the DEN-2 isolate.

Methods

A full-length cDNA clone was generated from the DEN-2 virus and used to produce recombinant DEN-2 (rDEN-2) and rDEN2Δ30. Viruses were evaluated for replication in SCID mice transplanted with human hepatoma cells (SCID-HuH-7 mice), in mosquitoes, and in rhesus monkeys. Neutralizing antibody induction and protective efficacy were also assessed in rhesus monkeys.

Results

The rDEN2Δ30 virus was ten-fold reduced in replication in SCID-HuH-7 mice when compared to the parent virus. The rDEN-2 viruses were not infectious for Aedes mosquitoes, but both readily infected Toxorynchites mosquitoes. In rhesus monkeys, rDEN2Δ30 appeared to be slightly attenuated when compared to the parent virus as measured by duration and peak of viremia and neutralizing antibody induction. A derivative of rDEN2Δ30, designated rDEN2Δ30-4995, was generated by incorporation of a point mutation previously identified in the NS3 gene of DEN-4 and was found to be more attenuated than rDEN2Δ30 in SCID-HuH-7 mice.

Conclusions

The rDEN2Δ30 and rDEN2Δ30-4995 viruses can be considered for evaluation in humans and for inclusion in a tetravalent dengue vaccine.

Microevolution and virulence of dengue viruses

The evolution of dengue viruses has had a major impact on their virulence for humans and on the epidemiology of dengue disease around the world. Although antigenic and genetic differences in virus strains had become evident, it is mainly due to the lack of animal models of disease that has made it difficult to detect differences in virulence of dengue viruses. However, phylogenetic studies of many different dengue virus samples have led to the association between specific genotypes (within serotypes) and the presentation of more or less severe disease. Currently, dengue viruses can be classified as being of epidemiologically low, medium, or high impact; i.e., some viruses may remain in sylvatic cycles of little or low transmissibility to humans, others produce dengue fever (DF) only, and some genotypes have been associated with the potential to cause the more severe dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS) in addition to DF. Although the factors that contribute to dengue virus epidemiology are complex, studies have suggested that specific viral structures may contribute to increased replication in human target cells and to increased transmission by the mosquito vector; however, the immune status and possibly the genetic background of the host are also determinants of virulence or disease presentation. As to the question of whether dengue viruses are evolving toward virulence as they continue to spread throughout the world, phylogenetic and epidemiological analyses suggest that the more virulent genotypes are now displacing those that have lower epidemiological impact; there is no evidence for the transmission of antigenically aberrant, new strains.

Patterns of epistasis in RNA viruses: a review of the evidence from vaccine design

Epistasis results when the fitness effects of a mutation change depending on the presence or absence of other mutations in the genome. The predictions of many influential evolutionary hypotheses are determined by the existence and form of epistasis. One rich source of data on the interactions among deleterious mutations that has gone untapped by evolutionary biologists is the literature on the design of live, attenuated vaccine viruses. Rational vaccine design depends upon the measurement of individual and combined effects of deleterious mutations. In the current study, we have reviewed data from 29 vaccine-oriented studies using 14 different RNA viruses. Our analyses indicate that (1) no consistent tendency towards a particular form of epistasis exists across RNA viruses and (2) significant interactions among groups of mutations within individual viruses occur but are not common. RNA viruses are significant pathogens of human disease, and are tractable model systems for evolutionary studies--we discuss the relevance of our findings in both contexts.

Molecularly engineered live-attenuated chimeric West Nile/dengue virus vaccines protect rhesus monkeys from West Nile virus

Two molecularly engineered, live-attenuated West Nile virus (WN) vaccine candidates were highly attenuated and protective in rhesus monkeys. The vaccine candidates are chimeric viruses (designated WN/DEN4) bearing the membrane precursor and envelope protein genes of WN on a backbone of dengue 4 virus (DEN4) with or without a deletion of 30 nucleotides (Delta 30) in the 3' noncoding region of DEN4. Viremia in WN/DEN4- infected monkeys was reduced 100-fold compared to that in WN- or DEN4-infected monkeys. WN/DEN4-3'Delta 30 did not cause detectable viremia, indicating that it is even more attenuated for monkeys. These findings indicate that chimerization itself and the presence of the Delta 30 mutation independently contribute to the attenuation phenotype for nonhuman primates. Despite their high level of attenuation in monkeys, the chimeras induced a moderate-to-high titer of neutralizing antibodies and prevented viremia in monkeys challenged with WN. The more attenuated vaccine candidate, WN/DEN4-3'Delta 30, will be evaluated first in our initial clinical studies.