Chimeric Japanese encephalitis virus/dengue 2 virus infectious clone: biological properties, immunogenicity and protection against dengue encephalitis in mice

Chimeric flavivirus causes vascular leakage and bone marrow suppression in a mouse model

Previously, we demonstrated the utility of a recombinant chimeric flavivirus (DV2ChimV), which carries the premembrane (prM) and envelope (E) genes of a type 2 DENV clinical (Thai) isolate on a backbone of Japanese encephalitis virus, for evaluating the protective efficacy of antidengue envelope antibodies both in vitro and in vivo. Here, to assess the potential use of this model for pathological studies, we aimed to characterize interferon-α/β-γ-receptor double-knockout mice (IFN-α/β/γR dKO mice) infected with DV2ChimV. Vascular leakage and bone marrow suppression are unique features of severe dengue. In the current model, DV2ChimV caused vascular leakage in the liver and intestine at the moribund stage. High levels of virus were detected in the bone marrow, and strong bone marrow suppression (i.e., disappearance of megakaryocytes and erythroblastic islets) was observed. These observations suggest that the DV2ChimV-infected mouse model mimics the vascular leakage and bone marrow suppression observed in human cases.

The pre membrane and envelope protein is the crucial virulence determinant of Japanese encephalitis virus

After sequence comparison, it was found that there are multiple amino acid mutations in pre-M and envelope (E) protein of Japanese encephalitis virus vaccine strain comparison with wild type (WT) strain SA14. It is generally acknowledged it is the mutations that have caused the virulence attenuation of vaccine strain, but lack of sufficient experimental evidences. For a better understanding of the mechanism of attenuation of Japanese encephalitis virus (JEV), in this study, we assessed whether prM/E is critical neurovirulence determinants of JEV with infectious cDNA clones technique. Substitutions prM/E of vaccine strain with that of WT SA14 did significantly increase the virulence of JEV to the similar level of wild type SA14, and simultaneously, replacement prM/E of JEV WT strain SA14 with that of vaccine strain SA14-14-2 decreased the virulence of JEV significantly to the similar level of vaccine stain. The results indicate that the prM/E protein is the crucial virulence determinant of Japanese encephalitis virus, although other proteins take part in the process to some extent.

Chimeric Japanese Encephalitis Virus SA14/SA14-14-2 Was Virulence Attenuated and Protected the Challenge of Wild-Type Strain SA14

The attenuated Japanese encephalitis virus (JEV) live vaccine SA14-14-2 prepared from wild-type (WT) strain SA14 was licensed to prevent Japanese encephalitis (JE) in 1989 in China. Many studies showed that the premembrane (prM) and envelope (E) protein were the crucial determinant of virulence and immunogenicity of JEV. So we are interested in whether the substitution of prM/E of JEV WT SA14 with those of vaccine strain SA14-14-2 could decrease neurovirulence and prevent the challenge of JEV WT SA14. Molecular clone technique was used to replace the prM/E gene of JEV WT strain SA14 with those of vaccine strain SA14-14-2 to construct the infectious clone of chimeric virus (designated JEV SA14/SA14-14-2), the chimeric virus recovered from BHK21 cells upon electrotransfection of RNA into BHK21 cells. The results showed that the recovered chimeric virus was highly attenuated in mice, and a single immunization elicited strong protective immunity in a dose-dependent manner. This study increases our understanding of the molecular mechanisms of neurovirulence attenuation and immunogenicity of JEV.

Envelope Protein Mutations L107F and E138K Are Important for Neurovirulence Attenuation for Japanese Encephalitis Virus SA14-14-2 Strain

The attenuated Japanese encephalitis virus (JEV) strain SA14-14-2 has been successfully utilized to prevent JEV infection; however, the attenuation determinants have not been fully elucidated. The envelope (E) protein of the attenuated JEV SA14-14-2 strain differs from that of the virulent parental SA14 strain at eight amino acid positions (E107, E138, E176, E177, E264, E279, E315, and E439). Here, we investigated the SA14-14-2-attenuation determinants by mutating E107, E138, E176, E177, and E279 in SA14-14-2 to their status in the parental virulent strain and tested the replication capacity, neurovirulence, neuroinvasiveness, and mortality associated with the mutated viruses in mice, as compared with those of JEV SA14-14-2 and SA14. Our findings indicated that revertant mutations at the E138 or E107 position significantly increased SA14-14-2 virulence, whereas other revertant mutations exhibited significant increases in neurovirulence only when combined with E138, E107, and other mutations. Revertant mutations at all eight positions in the E protein resulted in the highest degree of SA14-14-2 virulence, although this was still lower than that observed in SA14. These results demonstrated the critical role of the viral E protein in controlling JEV virulence and identified the amino acids at the E107 and E138 positions as the key determinants of SA14-14-2 neurovirulence.

Enterovirus A71 DNA-Launched Infectious Clone as a Robust Reverse Genetic Tool

Enterovirus A71 (EV-A71) causes major outbreaks of hand, foot and mouth disease, and is occasionally associated with neurological complications and death in children. Reverse genetics is widely used in the field of virology for functional study of viral genes. For EV-A71, such tools are limited to clones that are transcriptionally controlled by T7/SP6 bacteriophage promoter. This is often time-consuming and expensive. Here, we describe the development of infectious plasmid DNA-based EV-A71 clones, for which EV-A71 genome expression is under transcriptional control by the CMV-intermediate early promoter and SV40 transcriptional-termination signal. Transfection of this EV-A71 infectious DNA produces good virus yield similar to in vitro-transcribed EV-A71 infectious RNA, 6.4 and 5.8 log₁₀PFU/ml, respectively. Infectious plasmid with enhanced green fluorescence protein and Nano luciferase reporter genes also produced good virus titers, with 4.3 and 5.0 log₁₀ PFU/ml, respectively. Another infectious plasmid with both CMV and T7 promoters was also developed for easy manipulation of in vitro transcription or direct plasmid transfection. Transfection with either dual-promoter infectious plasmid DNA or infectious RNA derived from this dual-promoter clone produced infectious viral particles. Incorporation of hepatitis delta virus ribozyme, which yields precise 3’ ends of the DNA-launched EV-A71 genomic transcripts, increased infectious viral production. In contrast, the incorporation of hammerhead ribozyme in the DNA-launched EV-A71 resulted in lower virus yield, but improved the virus titers for T7 promoter-derived infectious RNA. This study describes rapid and robust reverse genetic tools for EV-A71.

Recovery of West Nile Virus Envelope Protein Domain III Chimeras with Altered Antigenicity and Mouse Virulence

Flaviviruses are positive-sense, single-stranded RNA viruses responsible for millions of human infections annually. The envelope (E) protein of flaviviruses comprises three structural domains, of which domain III (EIII) represents a discrete subunit. The EIII gene sequence typically encodes epitopes recognized by virus-specific, potently neutralizing antibodies, and EIII is believed to play a major role in receptor binding. In order to assess potential interactions between EIII and the remainder of the E protein and to assess the effects of EIII sequence substitutions on the antigenicity, growth, and virulence of a representative flavivirus, chimeric viruses were generated using the West Nile virus (WNV) infectious clone, into which EIIIs from nine flaviviruses with various levels of genetic diversity from WNV were substituted. Of the constructs tested, chimeras containing EIIIs from Koutango virus (KOUV), Japanese encephalitis virus (JEV), St. Louis encephalitis virus (SLEV), and Bagaza virus (BAGV) were successfully recovered. Characterization of the chimeras in vitro and in vivo revealed differences in growth and virulence between the viruses, within vivo pathogenesis often not being correlated within vitro growth. Taken together, the data demonstrate that substitutions of EIII can allow the generation of viable chimeric viruses with significantly altered antigenicity and virulence.

IMPORTANCE

The envelope (E) glycoprotein is the major protein present on the surface of flavivirus virions and is responsible for mediating virus binding and entry into target cells. Several viable West Nile virus (WNV) variants with chimeric E proteins in which the putative receptor-binding domain (EIII) sequences of other mosquito-borne flaviviruses were substituted in place of the WNV EIII were recovered, although the substitution of several more divergent EIII sequences was not tolerated. The differences in virulence and tissue tropism observed with the chimeric viruses indicate a significant role for this sequence in determining the pathogenesis of the virus within the mammalian host. Our studies demonstrate that these chimeras are viable and suggest that such recombinant viruses may be useful for investigation of domain-specific antibody responses and the more extensive definition of the contributions of EIII to the tropism and pathogenesis of WNV or other flaviviruses.

Dengue vaccines for travelers

Dengue is an arthropod-borne infection caused by a flavivirus and spread by the Aedes mosquitoes. Many of the countries where dengue is endemic are popular tourist destinations and the disease is an increasingly important problem encountered by international travelers. Personal protection against the day-feeding dengue vectors is problematic, indicating the urgent need for a dengue vaccine. This review discusses the challenges of vaccine development, current vaccine strategies and the prospects for the availability of a vaccine for travelers in the future. Cost-effectiveness studies will need to take into account many factors, including the attack rate of dengue in travelers, the proportion of travelers who will need hospitalization, the cost of altered travel itineraries, the cost of the vaccine, duration of travel, destination and season. To be licensed as a travelers' vaccine, vaccine trials must address safety, immunogenicity, duration of protection, schedules and boosters in adults (in particular in immunologically naive adults), trials that may differ from those conducted in endemic countries. Vaccine schedules with long intervals would be a major obstacle to the uptake of the vaccine by travelers. Enhanced reactogenicity or interference with immunization must be effectively excluded for travelers with prior or concurrent vaccination against other flaviviruses, such as yellow fever or Japanese encephalitis. Licensing dengue as a travelers' vaccine poses unique challenges beyond the development of a vaccine for the endemic population.

Evaluation of St. Louis encephalitis virus/dengue virus type 4 antigenic chimeric viruses in mice and rhesus monkeys

To develop a live attenuated virus vaccine against St. Louis encephalitis (SLE) virus, two antigenic chimeric viruses were generated by replacing the membrane precursor and envelope protein genes of dengue virus type 4 (DEN4) with those from SLE with or without a 30 nucleotide deletion in the DEN4 3' untranslated region of the chimeric genome. Chimeric viruses were compared with parental wild-type SLE for level of neurovirulence and neuroinvasiveness in mice and for safety, immunogenicity, and protective efficacy in rhesus monkeys. The resulting viruses, SLE/DEN4 and SLE/DEN4Delta30, had greatly reduced neuroinvasiveness in immunodeficient mice but retained neurovirulence in suckling mice. Chimerization of SLE with DEN4 resulted in only moderate restriction in replication in rhesus monkeys, whereas the presence of the Delta30 mutation led to over-attenuation. Introduction of previously described attenuating paired charge-to-alanine mutations in the DEN4 NS5 protein of SLE/DEN4 reduced neurovirulence in mice and replication in rhesus monkeys. Two modified SLE/DEN4 viruses, SLE/DEN4-436,437 clone 41 and SLE/DEN4-654,655 clone 46, have significantly reduced neurovirulence in mice and conferred protective immunity in monkeys against SLE challenge. These viruses may be considered for use as SLE vaccine candidates and for use as diagnostic reagents with reduced virulence.

Pathogenic flaviviruses

Haemorrhagic disease, encephalitis, biphasic fever, flaccid paralysis, and jaundice are typical manifestations of diseases in human beings after infections by mosquito-borne or tick-borne flaviviruses such as yellow fever, dengue, West Nile, St Louis encephalitis, Japanese encephalitis, tick-borne encephalitis, Kyasanur Forest disease, and Omsk haemorrhagic fever. Although the characteristics of these viruses are well defined, they are still unpredictable with increases in disease severity, unusual clinical manifestations, unexpected methods of transmission, long-term persistence, and the discovery of new species. This Seminar will compare the epidemiological and clinical features of the medically important flaviviruses, consider the effect of human activity on their evolution and dispersal, and draw attention to new findings and some of the unanswered questions, unresolved issues, and controversies that remain.

JE Nakayama/JE SA14-14-2 virus structural region intertypic viruses: biological properties in the mouse model of neuroinvasive disease

A molecular clone of Japanese encephalitis (JE) virus Nakayama strain was used to create intertypic viruses containing either the 5'-C-prM-E or the prM-E region of the attenuated JE SA14-14-2 virus in the JE Nakayama background. These two intertypic JE viruses, JE-X/5'CprME(S) and JE-X/prME(S), respectively, generally resembled the parental JE virus in cell culture properties. Similar to virus derived from the JE Nakayama molecular clone (JE-XJN), JE-X/prME(S) was highly neuroinvasive and neurovirulent for young adult mice, whereas JE-X/5'CprME(S) was attenuated for neuroinvasiveness and only partially attenuated for neurovirulence. Immunization of young mice with JE-X/5'CprME(S) virus elicited neutralizing antibodies against JE Nakayama virus and conferred protection against encephalitis following challenge with JE Nakayama virus. The sequence of the JE-X/5'CprME(S) virus differed from that of JE-X/prME(S) virus at two nucleotides in the 5' UTR, 3 amino acid positions in the capsid protein, 4 positions in the prM protein and 1 in the envelope protein. For JE-X/prME(S) virus, the 4 differences in prM and the single substitution in the envelope represented reversions to the sequence of JE Nakayama virus. Overall, this study reveals that molecular determinants associated with the prM-E region of the attenuated JE SA14-14-2 virus are insufficient by themselves to confer an attenuation phenotype upon JE Nakayama virus. This suggests a role for determinants in the 5' UTR and/or the capsid protein of the JE SA 14-14-2 virus genome in influencing the virulence properties of the JE Nakayama virus in the mouse model.

Vaccine candidates for dengue virus type 1 (DEN1) generated by replacement of the structural genes of rDEN4 and rDEN4Delta30 with those of DEN1

Background

Antigenic chimeric viruses have previously been generated in which the structural genes of recombinant dengue virus type 4 (rDEN4) have been replaced with those derived from DEN2 or DEN3. Two vaccine candidates were identified, rDEN2/4Δ30(ME) and rDEN3/4Δ30(ME), which contain the membrane (M) precursor and envelope (E) genes of DEN2 and DEN3, respectively, and a 30 nucleotide deletion (Δ30) in the 3' untranslated region of the DEN4 backbone. Based on the promising preclinical phenotypes of these viruses and the safety and immunogenicity of rDEN2/4Δ30(ME) in humans, we now describe the generation of a panel of four antigenic chimeric DEN4 viruses using either the capsid (C), M, and E (CME) or ME structural genes of DEN1 Puerto Rico/94 strain.

Results

Four antigenic chimeric viruses were generated and found to replicate efficiently in Vero cells: rDEN1/4(CME), rDEN1/4Δ30(CME), rDEN1/4(ME), and rDEN1/4Δ30(ME). With the exception of rDEN1/4(ME), each chimeric virus was significantly attenuated in a SCID-HuH-7 mouse xenograft model with a 25-fold or greater reduction in replication compared to wild type DEN1. In rhesus monkeys, only chimeric viruses with the Δ30 mutation appeared to be attenuated as measured by duration and magnitude of viremia. rDEN1/4Δ30(CME) appeared over-attenuated since it failed to induce detectable neutralizing antibody and did not confer protection from wild type DEN1 challenge. In contrast, rDEN1/4Δ30(ME) induced 66% seroconversion and protection from DEN1 challenge. Presence of the Δ30 mutation conferred a significant restriction in mosquito infectivity upon rDEN1/4Δ30(ME) which was shown to be non-infectious for Aedes aegypti fed an infectious bloodmeal.

Conclusion

The attenuation phenotype in SCID-HuH-7 mice, rhesus monkeys, and mosquitoes and the protective immunity observed in rhesus monkeys suggest that rDEN1/4Δ30(ME) should be considered for evaluation in a clinical trial.