Neurovirulence Test Research Articles

Comparative Study on MNVT of OPV Type I and III Reference Products in Different Periods

Widespread vaccination using the oral live attenuated polio vaccine (OPV) and Sabin strain inactivated vaccine (sIPV) have greatly reduced the incidence of polio worldwide. In the period post-polio, the virulence of reversion of the Sabin strain makes the use of OPV gradually becoming one of the major safety hazards. The verification and release of OPV has become the top priority. The monkey neurovirulence test (MNVT) is the gold standard for detecting whether OPV meets the criteria, which are recommended by the WHO and Chinese Pharmacopoeia. Therefore, we statistically analyzed the MNVT results of type I and III OPV at different stages: 1996-2002 and 2016-2022. The results show that the upper and lower limits and C value of the qualification standard of type I reference products in 2016-2022 have decreased compared with the corresponding scores in the 1996-2002 period. The upper and lower limit and C value of the qualified standard of type III reference products were basically the same as the corresponding scores in the 1996-2002. We also found significant differences in the pathogenicity of the type I and III in the cervical spine and brain, with the decreasing trend in the diffusion index of the type I and type III in the cervical spine and brain. Finally, two evaluation criteria were used to judge the OPV test vaccines from 2016 to 2022. The vaccines all met the test requirements under the evaluation criteria of the above two stages. Based on the characteristics of OPV, data monitoring was one of the most intuitive methods to judge changes in virulence.

Genetic and phenotypic stability of poliovirus shed from infants who received novel type 2 or Sabin type 2 oral poliovirus vaccines in Panama: an analysis of two clinical trials

Sabin strains used in oral poliovirus vaccines (OPV) can revert to virulence and, in rare instances, cause disease or generate vaccine-derived strains leading to outbreaks in areas of low immunisation coverage. A novel OPV2 (nOPV2) was designed to stabilise the viral genome against reversion and reduce recombination events that might lead to virulent strains. In this study, we evaluated the genetic and phenotypic stability of shed poliovirus following administration of one dose of monovalent OPV2 (mOPV2) or nOPV2 to infants aged 18-22 weeks. In two similarly designed clinical trials (NCT02521974 and NCT03554798) conducted in Panama, infants aged 18-22-weeks, after immunisation with three doses of bivalent OPV (types 1 and 3) and one dose of inactivated poliovirus vaccine, were administered one or two doses of mOPV2 or nOPV2. In this analysis of two clinical trials, faecally shed polioviruses following one dose of mOPV2 or nOPV2 were isolated from stools meeting predetermined criteria related to sample timing and viral presence and quantity and assessed for nucleotide polymorphisms using next-generation sequencing. A transgenic mouse neurovirulence test was adapted to assess the effect of the possible phenotypic reversion of shed mOPV2 and nOPV2 with a logistic regression model. Of the 91 eligible samples, 86 were able to be sequenced, with 72 evaluated in the transgenic mouse assay. Sabin-2 poliovirus reverts rapidly at nucleotide 481, the primary attenuation site in domain V of the 5' untranslated region of the genome. There was no evidence of neurovirulence-increasing polymorphisms in domain V of shed nOPV2. Reversion of shed Sabin-2 virus corresponded with unadjusted paralysis rates of 47·6% at the 4 log10 50% cell culture infectious dose (CCID50) and 76·7% at the 5 log10 CCID50 inoculum levels, with rates of 2·8% for 4 log10 CCID50 and 11·8% for 5 log10 CCID50 observed for shed nOPV2 samples. The estimated adjusted odds ratio at 4·5 log10 of 0·007 (95% CI 0·002-0·023; p<0·0001) indicates significantly reduced odds of mouse paralysis from virus obtained from nOPV2 recipients compared with mOPV2 recipients. The data indicate increased genetic stability of domain V of nOPV2 relative to mOPV2, with significantly lower neurovirulence of shed nOPV2 virus compared with shed mOPV2. While this vaccine is currently being deployed under an emergency use listing, the data on the genetic stability of nOPV2 will support further regulatory and policy decision-making regarding use of nOPV2 in outbreak responses. Bill & Melinda Gates Foundation.

Recombinant vesicular stomatitis vaccine against Nipah virus has a favorable safety profile: Model for assessment of live vaccines with neurotropic potential.

Nipah virus (NiV) disease is a bat-borne zoonosis responsible for outbreaks with high lethality and is a priority for vaccine development. With funding from the Coalition of Epidemic Preparedness Innovations (CEPI), we are developing a chimeric vaccine (PHV02) composed of recombinant vesicular stomatitis virus (VSV) expressing the envelope glycoproteins of both Ebola virus (EBOV) and NiV. The EBOV glycoprotein (GP) mediates fusion and viral entry and the NiV attachment glycoprotein (G) is a ligand for cell receptors, and stimulates neutralizing antibody, the putative mediator of protection against NiV. PHV02 is identical in construction to the registered Ebola vaccine (Ervebo) with the addition of the NiV G gene. NiV ephrin B2 and B3 receptors are expressed on neural cells and the wild-type NiV is neurotropic and causes encephalitis in affected patients. It was therefore important to assess whether the NiV G alters tropism of the rVSV vector and serves as a virulence factor. PHV02 was fully attenuated in adult hamsters inoculated by the intramuscular (IM) route, whereas parental wild-type VSV was 100% lethal. Two rodent models (mice, hamsters) were infected by the intracerebral (IC) route with graded doses of PHV02. Comparator active controls in various experiments included rVSV-EBOV (representative of Ebola vaccine) and yellow fever (YF) 17DD commercial vaccine. These studies showed PHV02 to be more neurovirulent than both rVSV-EBOV and YF 17DD in infant animals. PHV02 was lethal for adult hamsters inoculated IC but not for adult mice. In contrast YF 17DD retained virulence for adult mice inoculated IC but was not virulent for adult hamsters. Because of the inconsistency of neurovirulence patterns in the rodent models, a monkey neurovirulence test (MNVT) was performed, using YF 17DD as the active comparator because it has a well-established profile of quantifiable microscopic changes in brain centers and a known reporting rate of neurotropic adverse events in humans. In the MNVT PHV02 was significantly less neurovirulent than the YF 17DD vaccine reference control, indicating that the vaccine will have an acceptable safety profile for humans. The findings are important because they illustrate the complexities of phenotypic assessment of novel viral vectors with tissue tropisms determined by transgenic proteins, and because it is unprecedented to use a heterologous comparator virus (YF vaccine) in a regulatory-enabling study. This approach may have value in future studies of other novel viral vectors.

Assessment of genetic changes and neurovirulence of shed Sabin and novel type 2 oral polio vaccine viruses

Sabin-strain oral polio vaccines (OPV) can, in rare instances, cause disease in recipients and susceptible contacts or evolve to become circulating vaccine-derived strains with the potential to cause outbreaks. Two novel type 2 OPV (nOPV2) candidates were designed to stabilize the genome against the rapid reversion that is observed following vaccination with Sabin OPV type 2 (mOPV2). Next-generation sequencing and a modified transgenic mouse neurovirulence test were applied to shed nOPV2 viruses from phase 1 and 2 studies and shed mOPV2 from a phase 4 study. The shed mOPV2 rapidly reverted in the primary attenuation site (domain V) and increased in virulence. In contrast, the shed nOPV2 viruses showed no evidence of reversion in domain V and limited or no increase in neurovirulence in mice. Based on these results and prior published data on safety, immunogenicity, and shedding, the nOPV2 viruses are promising alternatives to mOPV2 for outbreak responses.

Live Viral Vaccine Neurovirulence Screening: Current and Future Models.

Live viral vaccines are one of the most successful methods for controlling viral infections but require strong evidence to indicate that they are properly attenuated. Screening for residual neurovirulence is an important aspect for live viral vaccines against potentially neurovirulent diseases. Approximately half of all emerging viral diseases have neurological effects, so testing of future vaccines will need to be rapid and accurate. The current method, the monkey neurovirulence test (MNVT), shows limited translatability for human diseases and does not account for different viral pathogenic mechanisms. This review discusses the MNVT and potential alternative models, including in vivo and in vitro methods. The advantages and disadvantages of these methods are discussed, and there are promising data indicating high levels of translatability. There is a need to investigate these models more thoroughly and to devise more accurate and rapid alternatives to the MNVT.

Recombinant HIV-1 vaccine candidates based on replication-defective flavivirus vector

Multiple approaches utilizing viral and DNA vectors have shown promise in the development of an effective vaccine against HIV. In this study, an alternative replication-defective flavivirus vector, RepliVax (RV), was evaluated for the delivery of HIV-1 immunogens. Recombinant RV-HIV viruses were engineered to stably express clade C virus Gag and Env (gp120TM) proteins and propagated in Vero helper cells. RV-based vectors enabled efficient expression and correct maturation of Gag and gp120TM proteins, were apathogenic in a sensitive suckling mouse neurovirulence test, and were similar in immunogenicity to recombinant poxvirus NYVAC-HIV vectors in homologous or heterologous prime-boost combinations in mice. In a pilot NHP study, immunogenicity of RV-HIV viruses used as a prime or boost for DNA or NYVAC candidates was compared to a DNA prime/NYVAC boost benchmark scheme when administered together with adjuvanted gp120 protein. Similar neutralizing antibody titers, binding IgG titers measured against a broad panel of Env and Gag antigens, and ADCC responses were observed in the groups throughout the course of the study, and T cell responses were elicited. The entire data demonstrate that RV vectors have the potential as novel HIV-1 vaccine components for use in combination with other promising candidates to develop new effective vaccination strategies.

Innovative in cellulo method as an alternative to in vivo neurovirulence test for the characterization and quality control of human live Yellow Fever virus vaccines: A pilot study

Live attenuated vaccines have proved to be mostly valuable in the prevention of infectious diseases in humans, especially in developing countries. The safety and potency of vaccine, and the consistency of vaccine batch-to-batch manufacturing, must be proven before being administrated to humans. For now, the tests used to control vaccine safety largely involve animal testing. For live viral vaccines, regulations require suppliers to demonstrate the absence of neurovirulence in animals, principally in non-human primates and mice. In a search to reduce the use of animals and embracing the 3Rs principles (Replacement, Reduction, Refinement in the use of laboratory animals), we developed a new Blood-Brain Barrier Minibrain (BBB-Minibrain) in cellulo device to evaluate the neuroinvasiveness/neurovirulence of live Yellow Fever virus (YFV) vaccines. A pilot study was performed using the features of two distinct YFV strains, with the ultimate goal of proposing a companion test to characterize YFV neurovirulence. Here, we demonstrate that the BBB-Minibrain model is a promising alternative to consider for future replacement of YFV vaccine in vivo neurovirulence testing (see graphical abstract).

A novel approach to a rabies vaccine based on a recombinant single-cycle flavivirus vector.

The RepliVax® vaccine (RV) platform is based on flavivirus genomes that are rationally attenuated by deletion. These single-cycle RV vaccine candidates targeting flavivirus pathogens have been demonstrated to be safe, highly immunogenic, and efficacious in animal models, including non-human primates. Here we show utility of the technology for delivery of a non-flavivirus immunogen by engineering several West Nile-based RV vectors to express full-length rabies virus G protein. The rabies virus G protein gene was incorporated in place of different West Nile structural protein gene deletions. The resulting RV-RabG constructs were demonstrated to replicate to high titers (8 log10 infectious particles/ml) in complementing helper cells. Following infection of normal cells, they provided efficient rabies virus G protein expression, but did not spread to surrounding cells. Expression of rabies virus G protein was stable and maintained through multiple rounds of in vitro passaging. A sensitive neurovirulence test in 2-3 day old neonatal mice demonstrated that RV-RabG candidates were completely avirulent indicative of high safety. We evaluated the RV-RabG variants in several animal models (mice, dogs, and pigs) and demonstrated that a single dose elicited high titers of rabies virus-neutralizing antibodies and protected animals from live rabies virus challenge (mice and dogs). Importantly, dogs were protected at both one and two years post-immunization, demonstrating durable protective immunity. The data demonstrates the potential of the RepliVax® technology as a potent vector delivery platform for developing vaccine candidates against non-flavivirus targets.

Influence of population diversity on neurovirulence potential of plaque purified L-Zagreb variants

BackgroundDespite continuing research efforts, determinants of mumps virus virulence are still largely unknown. One of consequences of this is difficulty in striking a balance between efficacy and safety of live attenuated mumps vaccines. Among mumps vaccine strains associated with occurrence of postvaccinal aseptic meningitis is L-Zagreb, developed by further attenuation of vaccine strain L-3. Starting from an archived L-Zagreb sample with suboptimal neuroattenuation score, we isolated different viral variants and compared their genetic and phenotypic properties, in investigation of neurovirulence markers. MethodsSix different L-Zagreb variants were isolated by plaque purification. Their neurovirulent status was determined by rat-based neurovirulence test; population structure was determined by deep sequencing. ResultsWe isolated one well neuroattenuated viral variant, two marginally neuroattenuated, and three insufficiently neuroattenuated. No genetic markers of neurovirulence could be identified. None of variants had detectable amounts of defective interfering particles. Two characteristics set insufficiently neuroattenuated variants apart from less-neurovirulent ones: elevated variability level in regions 1293−3314, 5363−7773 and 9382−11657, and/or elevated number of mutations present in frequencies ≥1%. The most neurovirulent variants possessed both of these features. ConclusionsDistinctive heterogeneity profiles were obtained for insufficiently neuroattenuated L-Zagreb variants. No markers that would discriminate between marginally and well neuroattenuated variants were identified. The findings of this study may serve as a guideline during development of an improved L3/L-Zagreb vaccine strain.

Isolation and Characterization of Vaccine-Derived Polioviruses, Relevance for the Global Polio Eradication Initiative.

Stool specimens were collected from children with acute flaccid paralysis (AFP) and their contacts, and viral isolation was performed according to standard procedures. If the specimens tested positive for poliovirus, then intratypic differentiation (ITD) methods were performed on the viral isolates to determine whether the poliovirus isolates were wild or of vaccine origin, these include a poliovirus diagnostic ITD real-time PCR method and a vaccine-derived poliovirus (VDPV) screening real-time PCR method.Viral RNA was extracted from the poliovirus isolates by using the QIAamp Mini Viral RNA Extraction Kit (Qiagen) and was used for RT-PCR amplification by the standard method. The entire VP1 region of the poliovirus isolates was amplified by RT-PCR with primers that flanked the VP1-coding region. After purification of the PCR products by the QIAquick Gel Extraction Kit (Qiagen), the amplicons were bidirectionally sequenced with the ABI PRISM 3130 Genetic Analyzer (Applied Biosystems). A neurovirulence test of polioviruses isolates was carried out using PVR-Tg21 mice that expressed the human poliovirus receptor (CD155). And the temperature sensitivities of polioviruses isolates were assayed on monolayer RD cells in 24-well plates as described.

Immunogenicity of Novel Mumps Vaccine Candidates Generated by Genetic Modification

Mumps is a highly contagious human disease, characterized by lateral or bilateral nonsuppurative swelling of the parotid glands and neurological complications that can result in aseptic meningitis or encephalitis. A mumps vaccination program implemented since the 1960s reduced mumps incidence by more than 99% and kept the mumps case numbers as low as hundreds of cases per year in the United States before 2006. However, a large mumps outbreak occurred in vaccinated populations in 2006 and again in 2009 in the United States, raising concerns about the efficacy of the vaccination program. Previously, we have shown that clinical isolate-based recombinant mumps viruses lacking expression of either the V protein (rMuVΔV) or the SH protein (rMuVΔSH) are attenuated in a neurovirulence test using newborn rat brains (P. Xu et al., Virology 417:126-136, 2011, http://dx.doi.org/10.1016/j.virol.2011.05.003; P. Xu et al., J. Virol. 86:1768-1776, 2012, http://dx.doi.org/10.1128/JVI.06019-11) and may be good candidates for vaccine development. In this study, we examined immunity induced by rMuVΔSH and rMuVΔV in mice. Furthermore, we generated recombinant mumps viruses lacking expression of both the V protein and the SH protein (rMuVΔSHΔV). Analysis of rMuVΔSHΔV indicated that it was stable in tissue culture cell lines. Importantly, rMuVΔSHΔV was immunogenic in mice, indicating that it is a promising candidate for mumps vaccine development.

THE ROLE OF BIOTECHNOLOGY IN DENGUE VIRAL INFECTION : Effort for The Development of A Dengue Vaccine

Introduction: Dengue fever and Dengue Haemorrhagic Fever (DF/DHF) are caused by dengue viruses (DENV). There are four antigenically related but distinct DENV serotypes (DENV-1 through DENV-4). Humans are the amplifying vertebrate host and Aedes mosquitos are the primarily mosquito vector as well as the reservoir of infection. DENV infection causes a spectrum of diseases, ranging from asymptomatic infections to infections complicated by haemorrhagic shock and death (Yoksan, 2008). Epidemiology: WHO (2008) estimates that about 2.5 billion people or 40% of the world’s population live in areas where there is a risk of dengue transmission. About 500,000 cases of dengue’s severest form (DHF/DSS) occur annually, resulting in about 24,000 deaths, mostly among children. Tropical and subtropical areas of South East Asia and Latin America are the hardest hit by dengue infection. Although dengue rarely occurs in the continental United States, it is endemic in Puerto Rico and Hawaii, a U.S. territory. Mosquitos capable of transmitting the virus have been found in the U.S. (in Florida, Georgia, Louisiana, Alabama, Mississippi, and Texas) over the last 10 years (Cano and Bannister, 2001; CDC, 2009; Science News, 2010). Facing this problem, vaccination is the answer. Vaccine Development: Dengue is an expanding public health problem, and an effective vaccine remains elusive. Significant influence of sequential infections with different dengue virus serotypes on the severity of disease can be viewed in terms of beneficial and detrimental effects of the heterologous immunity. A more complete understanding of these effects is likely to be critical for predicting optimal vaccine – induced immune responses, with the aim of protecting the global population from emerging infectious disease threats (Rothman, 2004). In 1980, Mahidol University committed to develop a live-attenuated tetravalent DENV vaccine. They were subjected to general safety test and monkey neurovirulence tests in accordance with the U.S. FDA requirements. (Yoksan 2008) Results: All vaccine recipients developed either a mild or no adverse reaction to the vaccine, the immunogenicity data were discussed. The current strategy of creating tetravalent DENV vaccine formulation can lead to an unbalanced immune response. This is attributed to viral interference that apparently comes into play when three monovirulent vaccine viruses DENV-1, DENV-2, DENV-4 are mixed with DENV-3 to create a tetravalent formulation (Yoksan, 2008). Summary: More research is needed on a priority basis to work out the viral interference factor in order to make the production of a tetravalent vaccine out of the attenuated DENV-3 candidate vaccine strain a success. Key words: Dengue Haemorrhagic Fever, Shock and death, tetravalent dengue vaccine, viral interference factor.

Characterization of the RepliVax platform for replication-defective flavivirus vaccines

RepliVax, a novel replication-defective vaccine platform has recently been described as a suitable means of generating potent vaccines targeting flaviviruses. In this study, we directly compared attenuation, immunogenicity and efficacy of several prototype RepliVax constructs to available, well characterized live attenuated (LAV) and inactivated (INV) flavivirus vaccine controls in mice and hamsters. Other important aspects of general mechanisms and properties of RepliVax vaccines were also studied. The prototypes were found to be nonpathogenic in sensitive suckling mouse neurovirulence tests, and highly immunogenic and efficacious in mice and hamsters, with evidence that immunogenicity can be comparable to LAV controls in terms of both magnitude and durability of response. Our data also suggest that choice of inoculation route can be beneficial for maximizing RepliVax immunogenicity. Additionally, different vaccine constructs can be administered as cocktail formulations without compromising immunogenicity of individual components. RepliVax constructs were determined to induce a Th1 biased immune response, similar to LAVs, and different from INV inducing a Th2 type response. The results presented validate the utility of the RepliVax platform for development of novel flavivirus vaccines.

Neurovirulence tests of three 17D yellow fever vaccine strains

The results of tests of the neurovirulence of three yellow fever vaccine preparations of different lineages are presented. Two preparations that have been used to make vaccines of acceptable safety and efficacy gave very similar results. A third preparation from the Robert Koch Institute, designated 168-73, was proposed as a reference preparation for the mouse potency assay in 1985 by WHO, but has been more often used as a reference in the monkey neurovirulence test. In the test described here 168-73 was of lower virulence than either of the other two preparations.

Alternative methods and strategies to reduce, refine, and replace animal use for human vaccine post-licensing safety testing: state of the science and future directions

NICEATM and ICCVAM convened an international workshop to review the state of the science of human and veterinary vaccine potency and safety testing methods, and to identify opportunities to advance new and improved methods that can further reduce, refine, and replace animal use (the 3Rs). Six topics were addressed in detail by speakers and workshop participants and are reported in a series of reports. This workshop report, the fifth in the series, addresses methods and strategies for human vaccine post-licensing safety testing that can reduce, refine, and replace animal use. It also provides recommendations for priority research and other activities necessary to advance the development and/or implementation of 3Rs methods for human vaccine post-licensing safety testing. Workshop participants agreed that future 3Rs activities should give highest priority to vaccine safety tests that (1) use the most animals per test and for which many vaccine lots are tested annually, (2) produce high variability and/or require frequent repeat tests, (3) are associated with severe animal pain and distress, and/or (4) involve nonhuman primates. Based on these criteria, safety tests for diphtheria, pertussis, oral polio, and tetanus vaccines were identified as the highest priorities. Recommended priority research, development, and validation activities included (1) expanding use of the transgenic mouse model for oral polio vaccine, and (2) developing alternatives to the monkey neurovirulence test for preclinical safety and lot release neurovirulence testing of mumps vaccines. Implementation of the workshop recommendations is expected to advance alternative in vitro methods for human vaccine post-licensing safety testing that will benefit animal welfare while ensuring continued production of safe human vaccines and protection of human and animal health.

Toward replacement of the monkey neurovirulence test in vaccine safety testing

For live, attenuated vaccines derived from neurotropic wild-type viruses, regulatory authorities require neurovirulence safety testing, typically using monkeys, to assure the absence of residual neurotoxicity. Ethical concerns surrounding the use of nonhuman primates in product testing, coupled with questions over its predictive value, has resulted in a concerted effort to replace monkey-based neurovirulence safety testing with more informative, validated alternative methods that include the use of lower animal species (e.g., mice and rats) and/or in vitro assays such as mutation analysis by PCR and restriction enzyme cleavage (MAPREC). MAPREC is a WHO-approved screening tool to assess reversion to neurovirulence of oral poliovirus vaccine (OPV). Monitoring the genetic consistency of OPV lots by identification and quantification of the mutational profile using the recently developed technology of massively parallel sequencing (MPS) also holds promise not only as a replacement for nonhuman primate testing of OPV lots but for other vaccines for which animal-based tests are currently performed as a measure of manufacturing consistency and freedom of adventitious virus contamination. In many cases, the greatest hurdle to availability of such alternative methods has been the process rather than the science. This report summarizes the current status of alternative methods of neurovirulence safety testing, both those validated and those currently in development.

Massively parallel sequencing for monitoring genetic consistency and quality control of live viral vaccines

Intrinsic genetic instability of RNA viruses may lead to the accumulation of revertants during manufacture of live viral vaccines, requiring rigorous quality control to ensure vaccine safety. Each lot of oral poliovirus vaccine (OPV) is tested for neurovirulence in animals and also for the presence of neurovirulent revertants. Mutant analysis by PCR and restriction enzyme cleavage (MAPREC) is used to measure the frequency of neurovirulent mutations at the 5' untranslated region (UTR) of the viral genome that correlate with the level of neurovirulence determined by the monkey neurovirulence test. However, MAPREC can only monitor mutations at a few genomic loci and miss mutations at other sites that could adversely affect vaccine quality. Here we propose to use massively parallel sequencing (MPS) for sensitive detection and quantification of all mutations in the entire genome of attenuated viruses. Analysis of vaccine samples and reference preparations demonstrated a perfect agreement with MAPREC results. Quantitative MPS analysis of validated reference preparations tested by MAPREC produced identical results, suggesting that the method could take advantage of the existing reference materials and be used as a replacement for the MAPREC procedure in lot release of OPV. Patterns of mutations present at a low level in vaccine preparations were characteristic of seed viruses used for their manufacture and could be used for identification of individual batches. This approach may represent the ultimate tool for monitoring genetic consistency of live viral vaccines.

Safety evaluation of chimeric Langat/Dengue 4 flavivirus, a live vaccine candidate against tick‐borne encephalitis

The chimeric flavivirus LGT/DEN4 containing prM and E genes of naturally attenuated Langat virus with remaining sequence derived from low neuroinvasive Dengue 4 virus was previously produced and assessed as a candidate for live vaccine against tick-borne encephalitis (TBE) [Pletnev and Men (1998): Proc Natl Acad Sci USA 95:1746-1751; Pletnev et al. (2000): Virology 274:23-31; Pletnev et al. (2001): J Virol 75:8259-8267; Wright et al. (2008): Vaccine 26:882-890]. In this article we compared two animal species: mice and monkeys, in order to select most sensitive models for safety evaluation of new vaccine candidates against TBE. Direct neurovirulence in suckling mice, neuroinvasiveness upon peripheral inoculation, rate of virus multiplication and expansion in CNS and its ability to persist in the central nervous system (CNS) were studied in adult mice; virological and pathomorphological examination of the CNS and visceral organs after intrathalamic virus inoculation was selected as a safety neurovirulence test in monkeys. The chimera was substantially less virulent in both animal models compared to the Absettarov strain of TBE virus. LGT/DEN4 was highly attenuated in suckling and adult mice with no evidence of viral persistence in CNS. In contrast to the mouse model, the chimera was able to reproduce in the CNS of monkeys to moderate titers, caused pathomorphological lesions in two and even illness in one of four animals, and was registered in simian brain on the 30th day post-infection. The presented data show that tests in mice solely might not be a sufficient model for safety testing of chimeric viruses.

Neuropathogenesis and Neurovirulence of Live Flaviviral Vaccines in Monkeys

The stated goal of the work by Maximova et al. ([12][1]) was to evaluate the safety of a new live vaccine candidate for tick-borne encephalitis (TBE), the chimeric virus TBEV/DEN4Δ30, in a monkey neurovirulence test (MNVT) ([7][2], [25][3]). In the first part of the study, 22 monkeys were

Construction and biological properties of yellow fever 17D/dengue type 1 recombinant virus

Dengue virus, a mosquito-borne flavivirus, is one of the most formidable public health threats in tropical and subtropical regions. As yet, there is no licensed vaccine to protect against the disease. A chimeric yellow fever (YF) 17D/dengue (DEN) type 1 virus was constructed by replacing the pre-membrane and envelope genes of YF 17D virus with those from DEN 1 VeMir95 virus, a Venezuelan isolate. The chimeric YF 17D/DEN 1 VeMir95 virus was regenerated from full-length infectious clones stably propagated in Escherichia coli by transfection of Vero cells with in vitro transcribed RNA. The chimeric virus proliferated efficiently in Vero cells ( approximately 6.6 log(10) plaque-forming units/ml). The chimeric virus was not neurovirulent to 3-week-old Swiss Webster mice inoculated by the intracerebral route, in contrast to the YF 17DD vaccine strain that was lethal for 90% of the mice. The YF 17D/DEN 1 virus at Passage 6 was more attenuated for rhesus monkeys than the YF 17DD commercial vaccine after intracerebral inoculation according to the standard neurovirulence test. This virus is a potential candidate to be included in a tetravalent DEN vaccine formulation. The availability of the cloned cDNA allows further structure/function studies on the viral envelope.