Attenuation markers of a candidate dengue type 2 vaccine virus, strain 16681 (PDK-53), are defined by mutations in the 5' noncoding region and nonstructural proteins 1 and 3

Safety and Immunogenicity of a Live Attenuated Tetravalent Dengue Vaccine Candidate in Flavivirus-Naive Adults: A Randomized, Double-Blinded Phase 1 Clinical Trial

BACKGROUND

Dengue viruses (DENVs) infect >300 million people annually, causing 96 million cases of dengue disease and 22 000 deaths [1]. A safe vaccine that protects against DENV disease is a global health priority [2].

METHODS

We enrolled 72 flavivirus-naive healthy adults in a phase 1 double-blinded, randomized, placebo-controlled dose-escalation trial (low and high dose) of a live attenuated recombinant tetravalent dengue vaccine candidate (TDV) given in 2 doses 90 days apart. Volunteers were followed for safety, vaccine component viremia, and development of neutralizing antibodies to the 4 DENV serotypes.

RESULTS

The majority of adverse events were mild, with no vaccine-related serious adverse events. Vaccinees reported injection site pain (52% vs 17%) and erythema (73% vs 25%) more frequently than placebo recipients. Low levels of TDV-serotype 2 (TDV-2), TDV-3, and TDV-4 viremia were observed after the first but not second administration of vaccine. Overall seroconversion rates and geometric mean neutralization titers after 2 doses were 84.2% and 54.1, respectively, for DENV serotype 1 (DENV-1); 92.1% and 292.8, respectively, for DENV-2; 86.8% and 32.3, respectively, for DENV-3; and 71.1% and 15.0, respectively, for DENV-4. More than 90.0% of high-dose recipients had trivalent or broader responses.

CONCLUSIONS

TDV was generally well tolerated, induced trivalent or broader neutralizing antibodies to DENV in most flavivirus-naive vaccinees, and is undergoing further development.

CLINICAL TRIALS REGISTRATION

NCT01110551.

Generation and preclinical immunogenicity study of dengue type 2 virus-like particles derived from stably transfected mosquito cells

Recent phase IIb/III trials of a tetravalent live attenuated vaccine candidate revealed a need for improvement in the stimulation of protective immunity against diseases caused by dengue type 2 virus (DENV-2). Our attempts to develop particulate antigens for possibly supplementing live attenuated virus preparation involve generation and purification of recombinant DENV-2 virus-like particles (VLPs) derived from stably (prM+E)-expressing mosquito cells. Two VLP preparations generated with either negligible or enhanced prM cleavage exhibited different proportions of spherical particles and tubular particles of variable lengths. In BALB/c mice, VLPs were moderately immunogenic, requiring adjuvants for the induction of strong virus neutralizing antibody responses. VLPs with enhanced prM cleavage induced higher levels of neutralizing antibody than those without, but the stimulatory activity of both VLPs was similar in the presence of adjuvants. Comparison of EDIII-binding antibodies in mice following two adjuvanted doses of these VLPs revealed subtle differences in the stimulation of anti-EDIII binding antibodies. In cynomolgus macaques, VLPs with enhanced prM cleavage augmented strongly neutralizing antibody and EDIII-binding antibody responses in live attenuated virus-primed recipients, suggesting that these DENV-2 VLPs may be useful as the boosting antigen in prime-boost immunization. As the levels of neutralizing antibody induced in macaques with the prime-boost immunization were comparable to those infected with wild type virus, this virus-prime VLP-boost regimen may provide an immunization platform in which a need for robust neutralizing antibody response in the protection against DENV-2-associated illnesses could be tested.

CD8+ T-cell Responses in Flavivirus-Naive Individuals Following Immunization with a Live-Attenuated Tetravalent Dengue Vaccine Candidate

We are developing a live-attenuated tetravalent dengue vaccine (TDV) candidate based on an attenuated dengue 2 virus (TDV-2) and 3 chimeric viruses containing the premembrane and envelope genes of dengue viruses (DENVs) -1, -3, and -4 expressed in the context of the attenuated TDV-2 genome (TDV-1, TDV-3, and TDV-4, respectively). In this study, we analyzed and characterized the CD8(+) T-cell response in flavivirus-naive human volunteers vaccinated with 2 doses of TDV 90 days apart via the subcutaneous or intradermal routes. Using peptide arrays and intracellular cytokine staining, we demonstrated that TDV elicits CD8(+) T cells targeting the nonstructural NS1, NS3, and NS5 proteins of TDV-2. The cells were characterized by the production of interferon-γ, tumor necrosis factor-α, and to a lesser extent interleukin-2. Responses were highest on day 90 after the first dose and were still detectable on 180 days after the second dose. In addition, CD8(+) T cells were multifunctional, producing ≥2 cytokines simultaneously, and cross-reactive to NS proteins of the other 3 DENV serotypes. Overall, these findings describe the capacity of our candidate dengue vaccine to elicit cellular immune responses and support the further evaluation of T-cell responses in samples from future TDV clinical trials.

Isolation and characterization of dengue virus serotype 2 from the large dengue outbreak in Guangdong, China in 2014

Dengue has been well recognized as a global public health threat, but only sporadic epidemics and imported cases were reported in recent decades in China. Since July 2014, an unexpected large dengue outbreak has occurred in Guangdong province, China, resulting in more than 40000 patients including six deaths. To clarify and characterize the causative agent of this outbreak, the acute phase serum from a patient diagnosed with severe dengue was subjected to virus isolation and high-throughput sequencing (HTS). Traditional real-time RT-PCR and HTS with Ion Torrent PGM detected the presence of dengue virus serotype 2 (DENV-2). A clinical DENV-2 isolate GZ05/2014 was obtained by culturing the patient serum in mosquito C6/36 cells. The complete genome of GZ05/2014 was determined and deposited in GenBank under the access number KP012546. Phylogenetic analysis based on the complete envelope gene showed that the newly DENV-2 isolate belonged to Cosmopolitan genotype and clustered closely with other Guangdong strains isolated in the past decade. No amino acid mutations that are obviously known to increase virulence or replication were identified throughout the genome of GZ05/2014. The high homology of Guangdong DENV-2 strains indicated the possibility of establishment of local DENV-2 circulation in Guangdong, China. These results help clarify the origin of this epidemic and predict the future status of dengue in China.

Dengue virus 1 in Buenos Aires from 1999 to 2010: towards local spread

Dengue virus (DENV) is a public health problem representing the most important arthropod-borne viral disease in humans. In Argentina, Northern provinces have reported autochthonous cases since 1997, though these outbreaks have originated in bordering countries, where co-circulation of more than one serotype has been reported. In the last decade, imported dengue cases have been reported in Buenos Aires, the urban area of Argentina with the highest population density. In 2009, a dengue outbreak affected Buenos Aires and, for the first time, local transmission was detected. All cases of this outbreak were caused by DENV-1. In this report, we present the full-length sequences of 27 DENV-1 isolates, corresponding to imported cases of 1999–2000, as well as local and imported cases of the 2009 and 2010 outbreaks. We analyzed their phylogenetic and phylodynamic relationships and their global and local spread. Additionally, we characterized their genomic and phenotypic features. All cases belonged to DENV-1 genotype V. The most recent ancestor for this genotype was dated ∼1934, whereas that for the 2009 outbreak was dated ∼2007. The mean rates of nucleotide substitution were 4.98E-4 and 8.53E-4 subs./site/yr, respectively. We inferred an introduction from Paraguay in 1999–2000 and mainly from Venezuela during 2009–2010. Overall, the number of synonymous substitutions per synonymous site significantly exceeded the number of non-synonymous substitutions per site and 12 positively selected sites were detected. These analyses could contribute to a better understanding regarding spread and evolution of this pathogen in the Southern Cone of South America.

Investigating the efficacy of monovalent and tetravalent dengue vaccine formulations against DENV-4 challenge in AG129 mice

Dengue (DEN) is the most important mosquito-borne viral disease, with a major impact on global health and economics, caused by four serologically and distinct viruses termed DENV-1 to DENV-4. Currently, there is no licensed vaccine to prevent DEN. We have developed a live attenuated tetravalent DENV vaccine candidate (TDV) (formally known as DENVax) that has shown promise in preclinical and clinical studies and elicits neutralizing antibody responses to all four DENVs. As these responses are lowest to DENV-4 we have used the AG129 mouse model to investigate the immunogenicity of monovalent TDV-4 or tetravalent TDV vaccines, and their efficacy against lethal DENV-4 challenge. Since the common backbone of TDV is based on an attenuated DENV-2 strain (TDV-2) we also tested the efficacy of TDV-2 against DENV-4 challenge. Single doses of the tetravalent or monovalent vaccines elicited neutralizing antibodies, anti-NS1 antibodies, and cellular responses to both envelope and nonstructural proteins. All vaccinated animals were protected against challenge at 60 days post-immunization, whereas all control animals died. Investigation of DENV-4 viremias post-challenge showed that only the control animals had high viremias on day 3 post-challenge, whereas vaccinated mice had no detectable viremia. Overall, these data highlight the excellent immunogenicity and efficacy profile of our candidate dengue vaccine in AG129 mice.

Safety and immunogenicity of a recombinant live attenuated tetravalent dengue vaccine (DENVax) in flavivirus-naive healthy adults in Colombia: a randomised, placebo-controlled, phase 1 study

BACKGROUND

Dengue virus is the most serious mosquito-borne viral threat to public health and no vaccines or antiviral therapies are approved for dengue fever. The tetravalent DENVax vaccine contains a molecularly characterised live attenuated dengue serotype-2 virus (DENVax-2) and three recombinant vaccine viruses expressing the prM and E structural genes for serotypes 1, 3, and 4 in the DENVax-2 genetic backbone. We aimed to assess the safety and immunogenicity of tetravalent DENVax formulations.

METHODS

We undertook a randomised, double-blind, phase 1, dose-escalation trial between Oct 11, 2011, and Nov 9, 2011, in the Rionegro, Antioquia, Colombia. The first cohort of participants (aged 18-45 years) were randomly assigned centrally, via block randomisation, to receive a low-dose formulation of DENvax, or placebo, by either subcutaneous or intradermal administration. After a safety assessment, participants were randomly assigned to receive a high-dose DENVax formulation, or placebo, by subcutaneous or intradermal administration. Group assignment was not masked from study pharmacists, but allocation was concealed from participants, nurses, and investigators. Primary endpoints were frequency and severity of injection-site and systemic reactions within 28 days of each vaccination. Secondary endpoints were the immunogenicity of DENVax against all four dengue virus serotypes, and the viraemia due to each of the four vaccine components after immunisation. Analysis was by intention to treat for safety and per protocol for immunogenicity. Because of the small sample size, no detailed comparison of adverse event rates were warranted. The trial is registered with ClinicalTrials.gov, number NCT01224639.

FINDINGS

We randomly assigned 96 patients to one of the four study groups: 40 participants (42%) received low-dose vaccine and eight participants (8%) received placebo in the low-dose groups; 39 participants (41%) received high-dose vaccine, with nine (9%) participants assigned to receive placebo. Both formulations were well tolerated with mostly mild and transient local or systemic reactions. No clinically meaningful differences were recorded in the overall incidence of local and systemic adverse events between patients in the vaccine and placebo groups; 68 (86%) of 79 participants in the vaccine groups had solicited systemic adverse events compared with 13 (76%) of 17 of those in the placebo groups. By contrast, 67 participants (85%) in the vaccine group had local solicited reactions compared with five (29%) participants in the placebo group. Immunisation with either high-dose or low-dose DENVax formulations induced neutralising antibody responses to all four dengue virus serotypes; 30 days after the second dose, 47 (62%) of 76 participants given vaccine seroconverted to all four serotypes and 73 (96%) participants seroconverted to three or more dengue viruses. Infectious DENVax viruses were detected in only ten (25%) of 40 participants in the low-dose group and 13 (33%) of 39 participants in the high-dose group.

INTERPRETATION

Our findings emphasise the acceptable tolerability and immunogenicity of the tetravalent DENVax formulations in healthy, flavivirus-naive adults. Further clinical testing of DENVax in different age groups and in dengue-endemic areas is warranted.

FUNDING

Takeda Vaccines.

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.

Vaccine Development as a Means to Control Dengue Virus Pathogenesis: Do We Know Enough?

Dengue virus (DENV) is a mosquito-transmitted RNA virus responsible for 390 million infections each year and significant morbidity and mortality throughout tropical and subtropical regions of the world. Efforts to develop a DENV vaccine span 70 years and include the work of luminaries of the virus vaccine field. Although vaccines have been used to reduce the global health burden of other flaviviruses, the unique requirement for a single vaccine to protect against four different groups of dengue viruses, and the link between secondary infections and DENV disease pathogenesis, has limited success to date. In this review, we discuss several promising DENV vaccine candidates in clinical trials and assess how recent advances in understanding of DENV biology and immunity may expedite efforts toward the development of safe and effective vaccines.

A rapid immunization strategy with a live-attenuated tetravalent dengue vaccine elicits protective neutralizing antibody responses in non-human primates

Dengue viruses (DENVs) cause approximately 390 million cases of DENV infections annually and over 3 billion people worldwide are at risk of infection. No dengue vaccine is currently available nor is there an antiviral therapy for DENV infections. We have developed a tetravalent live-attenuated DENV vaccine tetravalent dengue vaccine (TDV) that consists of a molecularly characterized attenuated DENV-2 strain (TDV-2) and three chimeric viruses containing the pre-membrane and envelope genes of DENV-1, -3, and -4 expressed in the context of the TDV-2 genome. To impact dengue vaccine delivery in endemic areas and immunize travelers, a simple and rapid immunization strategy (RIS) is preferred. We investigated RIS consisting of two full vaccine doses being administered subcutaneously or intradermally on the initial vaccination visit (day 0) at two different anatomical locations with a needle-free disposable syringe jet injection delivery devices (PharmaJet) in non-human primates. This vaccination strategy resulted in efficient priming and induction of neutralizing antibody responses to all four DENV serotypes comparable to those elicited by the traditional prime and boost (2 months later) vaccination schedule. In addition, the vaccine induced CD4⁺ and CD8⁺ T cells producing IFN-γ, IL-2, and TNF-α, and targeting the DENV-2 NS1, NS3, and NS5 proteins. Moreover, vaccine-specific T cells were cross-reactive with the non-structural NS3 and NS5 proteins of DENV-4. When animals were challenged with DENV-2 they were protected with no detectable viremia, and exhibited sterilizing immunity (no increase of neutralizing titers post-challenge). RIS could decrease vaccination visits and provide quick immune response to all four DENV serotypes. This strategy could increase vaccination compliance and would be especially advantageous for travelers into endemic areas.

Challenges in reducing dengue burden; diagnostics, control measures and vaccines

Dengue is a major public health concern worldwide, with the number of infections increasing globally. The illness imposes the greatest economic and human burden on developing countries that have limited resources to deal with the scale of the problem. No cure for dengue exists; treatment is limited to rehydration therapy, and with vector control strategies proving to be relatively ineffective, a vaccine is an urgent priority. Despite the numerous challenges encountered in the development of a dengue vaccine, several vaccine candidates have shown promise in clinical development and it is believed that a vaccination program would be at least as cost-effective as current vector control programs. The lead candidate vaccine is a tetravalent, live attenuated, recombinant vaccine, which is currently in Phase III clinical trials. Vaccine introduction is a complex process that requires consideration and is discussed here. This review discusses the epidemiology, burden and pathogenesis of dengue, as well as the vaccine candidates currently in clinical development.

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.

Development of a live attenuated vaccine candidate against duck Tembusu viral disease

Duck Tembusu virus (DTMUV) is a newly emerging pathogenic flavivirus that is causing massive economic loss in the Chinese duck industry. To obtain a live vaccine candidate against the disease, the DTMUV isolate FX2010 was passaged serially in chicken embryo fibroblasts (CEFs). Characterization of FX2010-180P revealed that it was unable to replicate efficiently in chicken embryonated eggs, nor intranasally infect mice or shelducks at high doses of 5.5log10 tissue culture infectious doses (TCID50). FX2010-180P did not induce clinical symptoms, or pathological lesions in ducks at a dose of 5.5log10TCID50. The attenuation of FX2010-180P was due to 19 amino acid changes and 15 synonymous mutations. Importantly, FX2010-180P elicited good immune responses in ducks inoculated at low doses (3.5log10TCID50) and provided complete protection against challenge with a virulent strain. These results indicate that FX2010-180P is a promising candidate live vaccine for prevention of duck Tembusu viral disease.

A chimeric dengue virus vaccine using Japanese encephalitis virus vaccine strain SA14-14-2 as backbone is immunogenic and protective against either parental virus in mice and nonhuman primates

The development of a safe and efficient dengue vaccine represents a global challenge in public health. Chimeric dengue viruses (DENV) based on an attenuated flavivirus have been well developed as vaccine candidates by using reverse genetics. In this study, based on the full-length infectious cDNA clone of the well-known Japanese encephalitis virus live vaccine strain SA14-14-2 as a backbone, a novel chimeric dengue virus (named ChinDENV) was rationally designed and constructed by replacement with the premembrane and envelope genes of dengue 2 virus. The recovered chimeric virus showed growth and plaque properties similar to those of the parental DENV in mammalian and mosquito cells. ChinDENV was highly attenuated in mice, and no viremia was induced in rhesus monkeys upon subcutaneous inoculation. ChinDENV retained its genetic stability and attenuation phenotype after serial 15 passages in cultured cells. A single immunization with various doses of ChinDENV elicited strong neutralizing antibodies in a dose-dependent manner. When vaccinated monkeys were challenged with wild-type DENV, all animals except one that received the lower dose were protected against the development of viremia. Furthermore, immunization with ChinDENV conferred efficient cross protection against lethal JEV challenge in mice in association with robust cellular immunity induced by the replicating nonstructural proteins. Taken together, the results of this preclinical study well demonstrate the great potential of ChinDENV for further development as a dengue vaccine candidate, and this kind of chimeric flavivirus based on JE vaccine virus represents a powerful tool to deliver foreign antigens.

Genetic and phenotypic characterization of manufacturing seeds for a tetravalent dengue vaccine (DENVax)

Background

We have developed a manufacturing strategy that can improve the safety and genetic stability of recombinant live-attenuated chimeric dengue vaccine (DENVax) viruses. These viruses, containing the pre-membrane (prM) and envelope (E) genes of dengue serotypes 1–4 in the replicative background of the attenuated dengue-2 PDK-53 vaccine virus candidate, were manufactured under cGMP.

Methodology/Principal Findings

After deriving vaccine viruses from RNA-transfected Vero cells, six plaque-purified viruses for each serotype were produced. The plaque-purified strains were then analyzed to select one stock for generation of the master seed. Full genetic and phenotypic characterizations of the master virus seeds were conducted to ensure these viruses retained the previously identified attenuating determinants and phenotypes of the vaccine viruses. We also assessed vector competence of the vaccine viruses in sympatric (Thai) Aedes aegypti mosquito vectors.

Conclusion/Significance

All four serotypes of master vaccine seeds retained the previously defined safety features, including all three major genetic loci of attenuation, small plaques, temperature sensitivity in mammalian cells, reduced replication in mosquito cell cultures, and reduced neurovirulence in new-born mice. In addition, the candidate vaccine viruses demonstrated greatly reduced infection and dissemination in Aedes aegypti mosquitoes, and are not likely to be transmissible by these mosquitoes. This manufacturing strategy has successfully been used to produce the candidate tetravalent vaccine, which is currently being tested in human clinical trials in the United States, Central and South America, and Asia.

Transmitted by Aedes spp. mosquitoes found worldwide, dengue is the most important mosquito-borne viral disease in the world. The incidence of dengue has increased 30-fold over the past 50 years, and is now endemic in over 100 countries. Vaccination is believed to be one of the most effective strategies in dengue prevention. However, no vaccine is currently available, and prevention strategies to control mosquitoes in endemic areas have been insufficient in controlling dengue. We have developed a recombinant live-attenuated tetravalent vaccine against all four serotypes of dengue virus. This candidate vaccine is currently under human clinical evaluation. In this report, we provide information regarding our manufacturing strategy, and present details of the genetic and biological characterization of the master seed virus for each vaccine serotype. The study described here, our previously reported and ongoing pre-clinical studies, and current clinical trials will provide critical information to evaluate the safety and efficacy of the vaccine to protect humans against dengue.

Dengue structure differs at the temperatures of its human and mosquito hosts

We report on a conformational transition of dengue virus when changing the temperature from that present in its mosquito vectors to that of its human host. Using cryoelectron microscopy, we show that although the virus has a smooth surface, a diameter of ∼500 Å, and little exposed membrane at room temperature, the virions have a bumpy appearance with a diameter of ∼550 Å and some exposed membrane at 37 °C. The bumpy structure at 37 °C was found to be similar to the previously predicted structure of an intermediate between the smooth mature and fusogenic forms. As humans have a body temperature of 37 °C, the bumpy form of the virus would be the form present in humans. Thus, optimal dengue virus vaccines should induce antibodies that preferentially recognize epitopes exposed on the bumpy form of the virus.

The flavivirus NS1 protein: molecular and structural biology, immunology, role in pathogenesis and application as a diagnostic biomarker

The flavivirus nonstructural glycoprotein NS1 is an enigmatic protein whose structure and mechanistic function have remained somewhat elusive ever since it was first reported in 1970 as a viral antigen circulating in the sera of dengue-infected patients. All flavivirus NS1 genes share a high degree of homology, encoding a 352-amino-acid polypeptide that has a molecular weight of 46-55 kDa, depending on its glycosylation status. NS1 exists in multiple oligomeric forms and is found in different cellular locations: a cell membrane-bound form in association with virus-induced intracellular vesicular compartments, on the cell surface and as a soluble secreted hexameric lipoparticle. Intracellular NS1 co-localizes with dsRNA and other components of the viral replication complex and plays an essential cofactor role in replication. Although this makes NS1 an ideal target for inhibitor design, the precise nature of its cofactor function has yet to be elucidated. A plethora of potential interacting partners have been identified, particularly for the secreted form of NS1, with many being implicated in immune evasion strategies. Secreted and cell-surface-associated NS1 are highly immunogenic and both the proteins themselves and the antibodies they elicit have been implicated in the seemingly contradictory roles of protection and pathogenesis in the infected host. Finally, NS1 is also an important biomarker for early diagnosis of disease. In this article, we provide an overview of these somewhat disparate areas of research, drawing together the wealth of data generated over more than 40 years of study of this fascinating protein.

Gamma interferon (IFN-γ) receptor restricts systemic dengue virus replication and prevents paralysis in IFN-α/β receptor-deficient mice

We previously reported that mice lacking alpha/beta and gamma interferon receptors (IFN-α/βR and -γR) uniformly exhibit paralysis following infection with the dengue virus (DENV) clinical isolate PL046, while only a subset of mice lacking the IFN-γR alone and virtually no mice lacking the IFN-α/βR alone develop paralysis. Here, using a mouse-passaged variant of PL046, strain S221, we show that in the absence of the IFN-α/βR, signaling through the IFN-γR confers approximately 140-fold greater resistance against systemic vascular leakage-associated dengue disease and virtually complete protection from dengue-induced paralysis. Viral replication in the spleen was assessed by immunohistochemistry and flow cytometry, which revealed a reduction in the number of infected cells due to IFN-γR signaling by 2 days after infection, coincident with elevated levels of IFN-γ in the spleen and serum. By 4 days after infection, IFN-γR signaling was found to restrict DENV replication systemically. Clearance of DENV, on the other hand, occurred in the absence of IFN-γR, except in the central nervous system (CNS) (brain and spinal cord), where clearance relied on IFN-γ from CD8(+) T cells. These results demonstrate the roles of IFN-γR signaling in protection from initial systemic and subsequent CNS disease following DENV infection and demonstrate the importance of CD8(+) T cells in preventing DENV-induced CNS disease.

Immunogenicity and efficacy of chimeric dengue vaccine (DENVax) formulations in interferon-deficient AG129 mice

Formulations of chimeric dengue vaccine (DENVax) viruses containing the pre-membrane (prM) and envelope (E) genes of serotypes 1-4 expressed in the context of the attenuated DENV-2 PDK-53 genome were tested for safety, immunogenicity and efficacy in interferon receptor knock-out mice (AG129). Monovalent formulations were safe and elicited robust neutralizing antibody responses to the homologous virus and only limited cross-reactivity to other serotypes. A single dose of monovalent DENVax-1, -2, or -3 vaccine provided eighty or greater percent protection against both wild-type (wt) DENV-1 (Mochizuki strain) and DENV-2 (New Guinea C strain) challenge viruses. A single dose of monovalent DENVax-4 also provided complete protection against wt DENV-1 challenge and significantly increased the survival times after challenge with wt DENV-2. In studies using tetravalent mixtures, DENVax ratios were identified that: (i) caused limited viremia, (ii) induced serotype-specific neutralizing antibodies to all four DENV serotypes with different hierarchies, and (iii) conferred full protection against clinical signs of disease following challenge with either wt DENV-1 or DENV-2 viruses. Overall, these data highlight the immunogenic profile of DENVax, a novel candidate tetravalent dengue vaccine and the advantage of sharing a common attenuated genomic backbone among the DENVax monovalent vaccines that confer protection against homologous or heterologous virus challenge.

Development of Dengue type-2 virus replicons expressing GFP reporter gene in study of viral RNA replication

Insertion of green fluorescent protein (GFP) encoding-gene into virus genes has provided a valuable tool for flavivirus research. This study aimed to develop dengue virus (DENV) replicons expressing GFP reporter that would provide a fast in vitro system to analyze functional roles of specific DENV sequences in viral replication. Two classes of recombinant replicon constructs were generated; one was a RNA-launched replicon with a GFP gene directly inserted into a full-length DENV genome (FL-DENV/GFP), and the other consisted of 4 types of DNA-launched DENV subgenomic replicons with GFP replacement at various structural genes (Δ-DENV/GFP). The FL-DENV/GFP resulted in GFP expression in transfected cells with no viable DENV being recovered from the transfection. The Δ-DENV/GFP constructs with partial structural gene deletion (ΔC-, ΔCprM/M-, ΔprM/M-, or ΔE-) expressed bright and long lasting GFP. The GFP expression intensity in living cells correlated well with the level of RNA replication. Various mutations in the 5'noncoding region of DENV-2 previously shown to be important genetic determinants for virus replication and mouse virulence were incorporated into the 5 different replicon constructs. Characterizations of 29 mutants demonstrated that these replicons can provide a useful platform for a quick and powerful in vitro system to analyze genetic determinants of DENV replication. These constructs can also be useful for development of vectors expressing foreign genes for various researches.

Role of arginine-56 within the structural protein VP3 of foot-and-mouth disease virus (FMDV) O1 Campos in virus virulence

FMDV O1 subtype undergoes antigenic variation under diverse growth conditions. Of particular interest is the amino acid variation observed at position 56 within the structural protein VP3. Selective pressures influence whether histidine (H) or arginine (R) is present at this position, ultimately influencing in vitro plaque morphology and in vivo pathogenesis in cattle. Using reverse genetics techniques, we have constructed FMDV type O1 Campos variants differing only at VP3 position 56, possessing either an H or R (O1Ca-VP3-56H and O1Ca-VP3-56R, respectively), and characterized their in vitro phenotype and virulence in the natural host. Both viruses showed similar growth kinetics in vitro. Conversely, they had distinct temperature-sensitivity (ts) and displayed significantly different pathogenic profiles in cattle and swine. O1Ca-VP3-56H was thermo stable and induced typical clinical signs of FMD, whereas O1Ca-VP3-56R presented a ts phenotype and was nonpathogenic unless VP3 position 56 reverted in vivo to either H or cysteine (C).

Development of DENVax: a chimeric dengue-2 PDK-53-based tetravalent vaccine for protection against dengue fever

Dengue. virus infection is the leading arboviral cause of disease worldwide. A vaccine is being developed based on the attenuated DEN-2 virus, DEN-2 PDK-53. In this review, we summarize the characteristics of the parent DEN-2 PDK-53 strain as well as the chimeric viruses containing the prM and E genes of DEN-1, DEN-3 or DEN-4 virus in the genetic backbone of the DEN-2 PDK-53 virus (termed DENVax). Tetravalent DENVax formulations containing cloned, fully sequenced isolates of the DEN-2 PDK-53 virus and the three chimeras have been evaluated for safety and efficacy in preclinical animal models. Based on the safety, immunogenicity and efficacy in preclinical studies, Phase 1 clinical testing of DENVax has been initiated.

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.

Differential accumulation of genetic and phenotypic changes in Venezuelan equine encephalitis virus and Japanese encephalitis virus following passage in vitro and in vivo

The requirement to replicate in both vertebrate and invertebrate hosts is thought to limit the introduction of genetic changes into the genome of arboviruses. Serial passage under laboratory conditions will overcome this limitation allowing for genetic changes to be introduced and affecting the virulence of the virus for animals. In the studies detailed here, the consequence of removing the restriction of alternate replication was demonstrated to be different depending on the virus. Passing Venezuelan equine encephalitis virus in tissue culture cells, eggs or mice resulted in up to 11 nucleotide or amino acid changes but no significant change in the virulence of the virus for mice. Passing Japanese encephalitis virus (JEV) under the identical conditions resulted in as many as 22 nucleotide or amino acid changes that often resulted in improved survival probabilities. For JEV, most genetic changes along with the attenuated phenotype were selected within 5 passes.

Evolution of attenuating mutations in dengue-2 strain S16803 PDK50 vaccine and comparison of growth kinetics with parent virus

A live-attenuated dengue-2 virus strain S16803 vaccine candidate that is immunogenic and safe in humans was derived by 50 passages in primary dog kidney (PDK) cells. To identify mutations associated with attenuation of the dengue-2 PDK50 vaccine strain, we determined the nucleotide changes that arose during PDK passage of the dengue-2 virus. Thirteen mutations distinguished the PDK50 virus from low-passage parent resulting in amino acid substitutions in the premembrane (E89G), envelope (E202K, N203D), nonstructural proteins NS1 (A43T), NS2A (L181F), NS2B (I26V), and NS4B (I/T108T, L112F). In addition, the PDK50 virus contained a C to T change of nucleotide 57 in the 5' non-coding region and four silent mutations of nucleotides 591, 987, 6471, and 8907. An infectious PDK50 cDNA clone virus was produced and characterized for growth kinetics in monkey (LLC-MK(2), Vero) and mosquito (C6/36) cells. Identification of mutations in the vaccine strain and availability of an infectious clone will permit systematic analysis of the importance of individual or collective mutations on attenuation of dengue virus.

Subversion of innate defenses by the interplay between DENV and pre-existing enhancing antibodies: TLRs signaling collapse

Background

The phenomenon of antibody dependent enhancement as a major determinant that exacerbates disease severity in DENV infections is well accepted. While the detailed mechanism of antibody enhanced disease severity is unclear, evidence suggests that it is associated with both increased DENV infectivity and suppression of the type I IFN and pro-inflammatory cytokine responses. Therefore, it is imperative for us to understand the intracellular mechanisms altered during ADE infection to decipher the mechanism of severe pathogenesis.

Methodology/Principal Findings

In this present work, qRT-PCR, immunoblotting and gene array analysis were conducted to determine whether DENV-antibody complex infection exerts a suppressive effect on the expression and/or function of the pathogen recognition patterns, focusing on the TLR-signaling pathway. We show here that FcγRI and FcγRIIa synergistically facilitated entry of DENV-antibody complexes into monocytic THP-1 cells. Ligation between DENV-antibody complexes and FcR not only down regulated TLRs gene expression but also up regulated SARM, TANK, and negative regulators of the NF-κB pathway, resulting in suppression of innate responses but increased viral production. These results were confirmed by blocking with anti-FcγRI or anti-FcγRIIa antibodies which reduced viral production, up-regulated IFN-β synthesis, and increased gene expression in the TLR-dependent signaling pathway. The negative impact of DENV-ADE infection on the TLR-dependent pathway was strongly supported by gene array screening which revealed that both MyD88-dependent and –independent signaling molecules were down regulated during DENV-ADE infection. Importantly, the same phenomenon was seen in PBMC of secondary DHF/DSS patients but not in PBMC of DF patients.

Conclusions/Significance

Our present work demonstrates the mechanism by which DENV uses pre-existing immune mediators to defeat the principal activating pathway of innate defense resulting in suppression of an array of innate immune responses. Interestingly, this phenomenon specifically occurred during the severe form of DENV infection but not in the mild form of disease.

Dengue is the most common vector-borne viral disease in humans, with 50–100 million infections per year. The severity of dengue ranges from an acute febrile illness, DF, to a life-threatening vascular leakage syndrome with or without shock, DHF/DSS. Determinants of these syndromes are mainly host factors including non protective but cross reactive antibodies which are known as preexisting enhancing antibodies. These antibodies enhance disease severity through increasing the virus infected cell mass and facilitating intracellular virus replication. Here we demonstrate that DENV exploits preexisting subneutralizing antibodies to defeat the pathogen recognition system and to down regulate the TLR signaling pathway resulting in suppression of an array of innate immune responses. Furthermore, we also show that this phenomenon specifically occurs in the severe form of dengue but not in the mild form of disease.

Study on the determinants of suckling mice neurovirulence of dengue 2 virus

pDVWS501 was a genomic-length cDNA clone of dengue 2 virus, through which infectious virus (MON501) could be rescued. MON501 was neurovirulent in mice, whose E residues 62 and 203 were Lys and Asn, respectively. Two genomic-length cDNA clones (TB62 and TB203) were constructed by pointed mutation of pDVWS501 with OL-PCR, E62 of TB62 and E203 of TB203 were converted to Glu and Asp, respectively. RNA transcripts of pDVWS501, TB62 and TB203 were produced in vitro and electroporated into BHK-21 cells. The cultures were collected after 7 days and used as inoculum to infect C6/36 cells. The existence of rescued dengue viruses in the culture was proved by RT-PCR, and the typical cytopathic effect (CPE) of C6/36 caused by dengue virus emerged after 2-5 days' inoculation. Sequence analysis further confirmed the existence of recovered and recombinant DEN2 viruses, whose 5' termini had an additional non-virus nucleotides "G", while the 3' terminal sequences remained the same as natural. The neurovirulence of three viruses was evaluated in 1-day-old mice by the intracerebral route with 10(5)-10(2) TCID50. Compared with MON501 group, the number of infected mice with the signs of encephalitis in HFT62 and HFT203 groups was less, and the surviving time was longer. The properties of these mutants demonstrated that E62 and E203 are determinants of suckling mice neurovirulence.

Identification of mutations in a candidate dengue 4 vaccine strain 341750 PDK20 and construction of a full-length cDNA clone of the PDK20 vaccine candidate

Dengue 4 virus strain 341750 serially passaged 20 times in primary dog kidney (PDK) cells was shown to have reduced infectivity for rhesus monkeys but was immunogenic and protected the monkeys from challenge with low passage parent dengue 4 virus. The dengue 4 PDK20 virus was also shown to be attenuated for human volunteers. We compared the genomic nucleotide sequences of low passage parent and PDK20 attenuated vaccine strains and identified 11 nucleotide (nt) substitutions in the PDK20 genome. Five mutations caused amino acid changes in viral proteins E (N366N/S), NS1 (E146Q), NS4B (S/L112L and A240V), and NS5 (F/L790L). Silent mutations occurred in genes encoding NS1 (nt 2609), NS3 (nt 6113, 6230 and 6239) and NS5 (nt 8081 and 8588). A full-length cDNA clone of the dengue 4 strain 341750 PDK20 was constructed and RNA transcripts of the clone were infectious in monkey kidney (LLC-MK(2)) and Aedes albopictus (C6/36) cells. The sequence analysis and availability of an infectious clone provide molecular tools to investigate the basis for the attenuation of dengue 4 virus.

Characterization of recombinant dengue-2 virus derived from a single nucleotide substitution in the 5' noncoding region

Variants of wild-type dengue serotype 2 (DEN-2) virus containing nucleotide substitutions at positions 14, 15, or 57 in the 5′ NCR were constructed by PCR-mediated site-directed mutagenesis. All three viruses containing a single point substitution demonstrated attenuation phenotype as evidenced by decreases replication and plaque size in cell culture assay. All three variants were less neurovirulent in newborn mice compared to the wild type. The mutants were immunogenic in adult mice immunogenicity and maintained stable replication characteristics following passage in mice. The variant viruses were competent for replication in Aedes aegypi mosquito vector, albeit at lower levels of infection and dissemination in the mosquito than the wild-type Den-2 16681 virus. Although all of the viruses, including the wild type, were found transmissible in mosquito life cycles, they were found subsequentially decreased in efficiency of infection, transmission, and dissemination rates along the mosquito generations and all of them remained genetically stable.

Viral determinants in the NS3 helicase and 2K peptide that promote West Nile virus resistance to antiviral action of 2',5'-oligoadenylate synthetase 1b

The interferon-inducible 2',5'-oligoadenylate synthetase 1b (Oas1b) protein inhibits West Nile virus (WNV) infection by preventing viral RNA (vRNA) accumulation in infected cells. Serial passage of WNV in Oas1b-expressing mouse cells selected a virus variant with improved growth capacity. Two major amino acid substitutions were identified in this Oas1b-resistant WNV variant: NS3-S365G in the ATPase/helicase domain of NS3 and 2K-V9M in the C-terminal segment of NS4A. To assess their effect on antiviral activity of Oas1b, the NS3 and 2K mutations were engineered into an infectious WNV cDNA clone. The NS3 mutation alters requirement of ATP for ATPase activity and attenuates Oas1b-mediated suppression of vRNA accumulation. However, growth of NS3-mutant virus remains impaired in Oas1b-expressing cells. Only the 2K-V9M mutation efficiently rescued viral growth by promoting vRNA replication. Thus, WNV resistance to Oas1b antiviral action could be attributed to the 2K-V9M substitution with a potential role of NS3-S365G through rescue of vRNA accumulation.

Control of dengue virus translation and replication

Dengue poses an increasing threat to public health worldwide. Studies conducted over the past several decades have improved our knowledge of the mechanisms of dengue virus translation and replication. New methodologies have facilitated advances in our understanding of the RNA elements and viral and host factors that modulate dengue virus replication and translation. This review integrates research findings and explores future directions for research into the cellular and molecular mechanisms of dengue virus infection. Lessons learned from dengue virus will inform approaches to other viruses and expand our understanding of the ways in which viruses co-opt host cells during the course of infection. In addition, knowledge about the molecular mechanisms of dengue virus translation and replication and the role of host cell factors in these processes will facilitate development of antiviral strategies.

Dengue vaccine candidates in development

Each of the DENV serotypes can cause the full spectrum of dengue illness. Epidemiological studies have implicated preexisting heterotypic DENV antibody as a risk factor for more severe disease upon secondary DENV infection. For these reasons, a successful DENV vaccine must protect against all four DENV serotypes. Live attenuated DENV vaccine candidates are the furthest along in development and clinical evaluation. Two live attenuated tetravalent vaccine candidates are in Phase 2 clinical trials in DENV endemic regions. Numerous other vaccine candidates including inactivated whole virus, recombinant subunit protein, DNA and virus-vectored vaccines are also under development. Those DENV vaccine candidates that have been evaluated in preclinical animal models or in clinical trials will be discussed.

Structural and functional studies of the promoter element for dengue virus RNA replication

The 5' untranslated region (5'UTR) of the dengue virus (DENV) genome contains two defined elements essential for viral replication. At the 5' end, a large stem-loop (SLA) structure functions as the promoter for viral polymerase activity. Next to the SLA, there is a short stem-loop that contains a cyclization sequence known as the 5' upstream AUG region (5'UAR). Here, we analyzed the secondary structure of the SLA in solution and the structural requirements of this element for viral replication. Using infectious DENV clones, viral replicons, and in vitro polymerase assays, we defined two helical regions, a side stem-loop, a top loop, and a U bulge within SLA as crucial elements for viral replication. The determinants for SLA-polymerase recognition were found to be common in different DENV serotypes. In addition, structural elements within the SLA required for DENV RNA replication were also conserved among different mosquito- and tick-borne flavivirus genomes, suggesting possible common strategies for polymerase-promoter recognition in flaviviruses. Furthermore, a conserved oligo(U) track present downstream of the SLA was found to modulate RNA synthesis in transfected cells. In vitro polymerase assays indicated that a sequence of at least 10 residues following the SLA, upstream of the 5'UAR, was necessary for efficient RNA synthesis using the viral 3'UTR as template.

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.

Treatment of AG129 mice with antisense morpholino oligomers increases survival time following challenge with dengue 2 virus

Objectives

To determine the antiviral activity of phosphorodiamidate morpholino oligomers (PMO) and peptide-conjugated PMO (PPMO) in AG129 mice infected with dengue 2 virus (DENV-2).

Methods

Antisense PMO and PPMO were designed against the 5′ terminal region (5′SL) or the 3′-cyclization sequence region (3′CS) of DENV genomic RNA and administered to AG129 mice before and/or after infection with DENV-2. In addition, cell culture evaluations designed to determine optimum PPMO length, and pharmacokinetic and toxicity analysis of PPMO were also carried out.

Results

Mock-treated AG129 mice lived for 9–17 days following intraperitoneal (ip) infection with 10⁴–10⁶pfu of DENV-2 (strain New Guinea C). Intraperitoneal administration of 5′SL or 3′CS PPMO before and after DENV infection produced an increase in the average survival time of up to 8 days. Animals receiving only post-infection PPMO treatment did not benefit significantly. Cell culture studies showed that PPMO of 22–24 bases long produced substantially higher DENV titre reductions than did PPMO that were either shorter or longer. Pharmacokinetic and toxicology analysis with non-infected animals showed that nine consecutive once‐daily ip treatments of 10 mg/kg PPMO resulted in high concentrations of PPMO in the liver and caused little impact on overall health.

Conclusions

The data indicate that PPMO had considerable antiviral efficacy against DENV-2 in the AG129 mouse model and that PPMO treatment early in the course of an infection was critical to extending the survival times of DENV-2-infected mice in the AG129 model system.

Clinical isolates of dengue virus with distinctive susceptibility to nitric oxide radical induce differential gene responses in THP-1 cells

In the present study, 10 clinical isolates of dengue virus were selected according to their susceptibility to the inhibitory effect of nitric oxide radical, NO. Five of them are nitric oxide-susceptible viruses while the other five are nitric oxide-resistant viruses. These isolates were investigated to identify genetic factors that are responsible for the different phenotypes. Due to the evidence showing that NO suppresses DENV RNA polymerase activity, we, therefore, hypothesized that the RdRp domain of NS5 may responsible for NO inhibition. To answer this question, sequences of NS5 gene of NO-susceptible viruses and NO-resistant viruses were compared. We found that these two groups of viruses contain different amino acid sequence at position 621 to 646 in the active site of NS5. These data suggested that response to the inhibitory effect of nitric oxide radical may, at least in part, be regulated by NS5. The effect of these two different phenotypes of viruses on host cells was studied using cDNA array screening. The cDNA array analysis demonstrated that the nitric oxide-resistant group had a stronger influence on host cells since it induced changes in the expression of a greater number of genes than did the nitric oxide-susceptible group, 97 genes versus 71 genes, respectively. The NO-resistant virus also stimulated cytokines known to be virulent factors, such as IL 6, IL 8, RANTES, and the inflammatory factors. In conclusion, our data demonstrated that dengue viruses isolated from patients show genotypic and phenotypic differences which may correlate with virulence.

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.

Prospects for a dengue virus vaccine

The number of cases of severe dengue disease continues to grow in endemic areas of southeast Asia, Central and South America, and other subtropical regions. Children bear the greatest burden of disease, and the development of an effective vaccine remains a global public health priority. A tetravalent vaccine is urgently needed and must be effective against all four dengue virus serotypes, be cost-effective and provide long-term protection. In this Review we discuss the unique immunological concerns in dengue virus vaccine development and the current prospects for the development of an acceptable vaccine, a goal that is likely to be reached in the near future.

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.

Substitution or deletion mutations between nt 54 and 70 in the 5' non-coding region of dengue type 2 virus produce variable effects on virus viability

A C57U nucleotide mutation in a predicted RNA stem structure (nt 11-16/56-61) of the 5' non-coding region (5'NCR) of dengue 2 (DEN-2) 16681 virus is partially attenuating, but unstable during serial passage of certain candidate DEN-2 PDK-53-based vaccine viruses containing this mutation. Here, 11 different mutations (one or more point substitution and/or deletion) between nt 54 and 70 in the 5'NCR of the pD2/IC-30P-A (16681) infectious clone are described. Four mutants were infectious. Three mutants with single point substitutions replicated well in cell culture and exhibited variable neurovirulence in mice. Constructs containing multiple substitutions or any deletions failed to produce infectious viruses. Unexpectedly, a double C57U+G58C mutant replicated as efficiently as D2/IC-30P-A virus, and was more neurovirulent for newborn ICR mice. Thus, despite its predicted additional disruption of the RNA stem structure, the engineered contiguous secondary G58C mutation caused reversion of the partially attenuated phenotype caused by the 5'NCR-C57U mutation.

Attenuating characteristics of DEN-2 PDK53 in flavivirus-naïve peripheral blood mononuclear cells

A live-attenuated DEN-2 virus, DEN-2 strain 16681-PDK53, has been found to be attenuated for both humans and mice with an unknown mechanism. To partially answer this question, responses of flavivirus-naïve primary human PBMC to infection with attenuated DEN-2 PDK53 (D2/IC-VV45R) virus and its parental, virulent DEN-2 16681 virus (D2/IC-30P-A) were investigated at the cellular and genetic levels using cDNA array analysis. Both DEN-2 viruses produced similar replication kinetics in flavivirus-naïve PBMC. In contrast, virulent DEN-2 virus caused a higher percentage of apoptotic death. A macro-array analysis showed that the virulent D2/IC-30P-A virus induced changes in the expression of a greater number of genes than did the attenuated D2/IC-VV45R virus, 31 genes versus 19 genes, respectively, by 24 h post-infection. Interestingly, both viruses stimulated cytokines known to be virulence factors for DEN virus infection, such as IL-1beta, IL-6, IL-8, IL-10, MIP-1beta, and MIP-1alpha. The virulent virus additionally up-regulates immune suppression factors and down-regulates immune activator and growth factors. In conclusion, our data demonstrated that D2-PDK53 effected less change in PBMC than D2-16681 in terms of observable cellular effect and expression of cytokine and chemokine related genes.

Dengue virus (DENV) antibody-dependent enhancement of infection upregulates the production of anti-inflammatory cytokines, but suppresses anti-DENV free radical and pro-inflammatory cytokine production, in THP-1 cells

The immunopathogenesis of dengue haemorrhagic fever and dengue shock syndrome is thought to be mediated by a variety of host factors. Enhancing antibodies are one of the key regulating molecules. These antibodies, via antibody-dependent enhancement (ADE) of infection, are able to facilitate dengue virus (DENV) growth in Fc-bearing host cells. The mechanism of ADE-enhanced DENV production is believed to be mediated through increasing the infected-cell mass. In the present work, the effect of ADE infection was explored further, focusing on the post-entry events of ADE infection. It was hypothesized that the higher virus production in ADE infection compared with DENV infection may be due to the ability of this infection pathway to suppress key antiviral molecules. Therefore, the influence of ADE infection on pro- and anti-inflammatory cytokines, including interleukin-12 (IL-12), gamma interferon (IFN-gamma), tumour necrosis factor alpha (TNF-alpha), IL-6 and IL-10, was investigated and it was found that DENV infection via the Fc receptor-mediated pathway was able to suppress the transcription and translation of IL-12, IFN-gamma and TNF-alpha. In contrast, infection via this route facilitated expression and synthesis of the anti-inflammatory cytokines IL-6 and IL-10. Moreover, this study demonstrates that the ADE infection pathway also suppresses an innate anti-DENV mediator, nitric oxide radicals, by disrupting the transcription of the iNOS gene transcription factor, IRF-1, and blocking the activation of STAT-1. In conclusion, ADE infection not only facilitates the entry process, but also modifies innate and adaptive intracellular antiviral mechanisms, resulting in unrestricted DENV replication in THP-1 cells.

Cell-specific adaptation of two flaviviruses following serial passage in mosquito cell culture

West Nile Virus (WNV) is a mosquito-borne flavivirus that was introduced into the U.S. in the New York City area in 1999. Despite its successful establishment and rapid spread in a naive environment, WNV has undergone limited evolution since its introduction. This evolutionary stability has been attributed to compromises made to permit alternating cycles of viral replication in vertebrate hosts and arthropod vectors. Outbreaks of a close relative of WNV, St. Louis encephalitis virus (SLEV), occur in the U.S. periodically and are also characterized by limited genetic change overtime. We measured both phenotypic and genotypic changes in WNV and SLEV serially passaged in mosquito cell culture in order to clarify the role of an individual host cell type in flavivirus adaptation and evolution. Genetic changes in passaged WNV and SLEV were minimal but led to increased relative fitness and replicative ability of the virus in the homologous cell line C6/36 mosquito cells. Similar increases were not measured in the heterologous cell line DF-1 avian cells. These phenotypic changes are consistent with the concept of cell-specific adaptation in flaviviruses.

Biological properties of chimeric West Nile viruses

Recently, we have described a lineage 2 attenuated WN virus suitable for the development of a live WN vaccine. To design vaccine candidates with an improved immunogenicity, we assembled an infectious clone of the NY99 strain and created several chimeric constructs with reciprocal exchanges of structural protein genes between attenuated W956 and virulent NY99 and investigated their biological properties. Our data indicated that, while the growth rates of NY99 and chimeric viruses in tissue culture are determined primarily by properties of the structural proteins, determinants responsible for a highly cytopathic phenotype of NY99 or lack thereof for W956 are located within the nonstructural protein region of the WN genome. The high virulence of NY99 and the attenuated phenotype of W956 were found to be associated with determinants in the nonstructural region. Chimeric viruses carrying the NY99 structural proteins were attenuated in neuroinvasiveness and demonstrated an immunogenicity superior to W956.

Engineering RNA interference-based resistance to dengue virus type 2 in genetically modified Aedes aegypti

Mosquitoes (Aedes aegypti) were genetically modified to exhibit impaired vector competence for dengue type 2 viruses (DENV-2). We exploited the natural antiviral RNA interference (RNAi) pathway in the mosquito midgut by constructing an effector gene that expresses an inverted-repeat (IR) RNA derived from the premembrane protein coding region of the DENV-2 RNA genome. The A. aegypti carboxypeptidase A promoter was used to express the IR RNA in midgut epithelial cells after ingestion of a bloodmeal. The promoter and effector gene were inserted into the genome of a white-eye Puerto Rico Rexville D (Higgs' white eye) strain by using the nonautonomous mariner MosI transformation system. A transgenic family, Carb77, expressed IR RNA in the midgut after a bloodmeal. Carb77 mosquitoes ingesting an artificial bloodmeal containing DENV-2 exhibited marked reduction of viral envelope antigen in midguts and salivary glands after infection. DENV-2 titration of individual mosquitoes showed that most Carb77 mosquitoes poorly supported virus replication. Transmission in vitro of virus from the Carb77 line was significantly diminished when compared to control mosquitoes. The presence of DENV-2-derived siRNAs in RNA extracts from midguts of Carb77 and the loss of the resistance phenotype when the RNAi pathway was interrupted proved that DENV-2 resistance was caused by a RNAi response. Engineering of transgenic A. aegypti that show a high level of resistance against DENV-2 provides a powerful tool for developing population replacement strategies to control transmission of dengue viruses.