Characterization and complete genome sequences of high- and low- virulence variants of tick-borne encephalitis virus

Exploring the Susceptibility of C3H Mice to Tick-Borne Encephalitis Virus Infection: Implications for Co-Infection Models and Understanding of the Disease

Ticks and tick-borne diseases (TBDs) are increasingly recognized as a critical One Health concern. Tick-borne encephalitis (TBE), a severe neuro infection caused by the tick-borne encephalitis virus (TBEV), has emerged as a significant global public health threat. Laboratory animals, particularly mice, have played a pivotal role in advancing our understanding of TBD pathogenesis. Notably, BALB/c mice have been employed as models due to their heightened susceptibility to TBEV. However, the use of C3H mice, valued for other tick-borne pathogens, has remained unexplored for TBEV until now. This study aimed to assess the susceptibility of C3H mice to TBEV infection, laying the groundwork for future co-infection models involving TBEV and Borrelia. Experiments revealed that C3H mice are susceptible to TBEV infection through subcutaneous inoculation. While 102 PFU/mouse appeared necessary for full infection, 103 PFU/mouse induced consistent symptoms. However, subsequent assessment of ticks' acquisition of TBEV from infected mice met with limited success, raising questions about optimal infectious doses for natural infection. These findings suggest the potential of C3H mice for studying TBEV and co-infections with other pathogens, particularly Borrelia. Further exploration of the interplay between these pathogens, their transmission dynamics, and disease severity could enhance prevention and control strategies.

Rescue and in vitro characterization of a divergent TBEV-Eu strain from the Netherlands

Tick-borne encephalitis virus (TBEV) may cause tick-borne encephalitis (TBE), a potential life-threatening infection of the central nervous system in humans. Phylogenetically, TBEVs can be subdivided into three main subtypes, which differ in endemic region and pathogenic potential. In 2016, TBEV was first detected in the Netherlands. One of two detected strains, referred to as Salland, belonged to the TBEV-Eu subtype, yet diverged ≥ 2% on amino acid level from other members of this subtype. Here, we report the successful rescue of this strain using infectious subgenomic amplicons and its subsequent in vitro characterization by comparison to two well-characterized TBEV-Eu strains; Neudoerfl and Hypr. In the human alveolar epithelial cell line A549, growth kinetics of Salland were comparable to the high pathogenicity TBEV-Eu strain Hypr, and both strains grew considerably faster than the mildly pathogenic strain Neudoerfl. In the human neuroblastoma cell line SK-N-SH, Salland replicated faster and to higher infectious titers than both reference strains. All three TBEV strains infected primary human monocyte-derived dendritic cells to a similar extent and interacted with the type I interferon system in a similar manner. The current study serves as the first in vitro characterization of the novel, divergent TBEV-Eu strain Salland.

Identification of Host Factors Differentially Induced by Clinically Diverse Strains of Tick-Borne Encephalitis Virus

Tick-borne encephalitis virus (TBEV) is an important human arthropod-borne virus that causes tick-borne encephalitis (TBE) in humans. TBEV acutely infects the central nervous system (CNS), leading to neurological symptoms of various severity. No therapeutics are currently available for TBEV-associated disease. Virus strains of various pathogenicity have been described, although the basis of their diverse clinical outcome remains undefined. Work with infectious TBEV requires high-level biocontainment, meaning model systems that can recapitulate the virus life cycle are highly sought. Here, we report the generation of a self-replicating, noninfectious TBEV replicon used to study properties of high (Hypr) and low (Vs) pathogenic TBEV isolates. Using a Spinach2 RNA aptamer and luciferase reporter system, we perform the first direct comparison of Hypr and Vs in cell culture. Infectious wild-type (WT) viruses and chimeras of the nonstructural proteins 3 (NS3) and 5 (NS5) were investigated in parallel to validate the replicon data. We show that Hypr replicates to higher levels than Vs in mammalian cells, but not in arthropod cells, and that the basis of these differences map to the NS5 region, encoding the methyltransferase and RNA polymerase. For both Hypr and Vs strains, NS5 and the viral genome localized to intracellular structures typical of positive-strand RNA viruses. Hypr was associated with significant activation of IRF-3, caspase-3, and caspase-8, while Vs activated Akt, affording protection against caspase-mediated apoptosis. Higher activation of stress-granule proteins TIAR and G3BPI were an additional early feature of Vs but not for Hypr. These findings highlight novel host cell responses driven by NS5 that may dictate the differential clinical characteristics of TBEV strains. This highlights the utility of the TBEV replicons for further virological characterization and antiviral drug screening. IMPORTANCE Tick-borne encephalitis virus (TBEV) is an emerging virus of the flavivirus family that is spread by ticks and causes neurological disease of various severity. No specific therapeutic treatments are available for TBE, and control in areas of endemicity is limited to vaccination. The pathology of TBEV ranges from mild to fatal, depending on the virus genotype. Characterization of TBEV isolates is challenging due to the requirement for high-containment facilities. Here, we described the construction of novel TBEV replicons that permit a molecular comparison of TBEV isolates of high and low pathogenicity.

Guanine quadruplexes in the RNA genome of the tick-borne encephalitis virus: their role as a new antiviral target and in virus biology

We have identified seven putative guanine quadruplexes (G4) in the RNA genome of tick-borne encephalitis virus (TBEV), a flavivirus causing thousands of human infections and numerous deaths every year. The formation of G4s was confirmed by biophysical methods on synthetic oligonucleotides derived from the predicted TBEV sequences. TBEV-5, located at the NS4b/NS5 boundary and conserved among all known flaviviruses, was tested along with its mutated variants for interactions with a panel of known G4 ligands, for the ability to affect RNA synthesis by the flaviviral RNA-dependent RNA polymerase (RdRp) and for effects on TBEV replication fitness in cells. G4-stabilizing TBEV-5 mutations strongly inhibited RdRp RNA synthesis and exhibited substantially reduced replication fitness, different plaque morphology and increased sensitivity to G4-binding ligands in cell-based systems. In contrast, strongly destabilizing TBEV-5 G4 mutations caused rapid reversion to the wild-type genotype. Our results suggest that there is a threshold of stability for G4 sequences in the TBEV genome, with any deviation resulting in either dramatic changes in viral phenotype or a rapid return to this optimal level of G4 stability. The data indicate that G4s are critical elements for efficient TBEV replication and are suitable targets to tackle TBEV infection.

Delimitation of the Tick-Borne Flaviviruses. Resolving the Tick-Borne Encephalitis virus and Louping-Ill Virus Paraphyletic Taxa

The tick-borne flavivirus (TBFV) group contains at least 12 members where five of them are important pathogens of humans inducing diseases with varying severity (from mild fever forms to acute encephalitis). The taxonomy structure of TBFV is not fully clarified at present. In particular, there is a number of paraphyletic issues of tick-borne encephalitis virus (TBEV) and louping-ill virus (LIV). In this study, we aimed to apply different bioinformatic approaches to analyze all available complete genome amino acid sequences to delineate TBFV members at the species level. Results showed that the European subtype of TBEV (TBEV-E) is a distinct species unit. LIV, in turn, should be separated into two species. Additional analysis of TBEV and LIV antigenic determinant diversity also demonstrate that TBEV-E and LIV are significantly different both from each other and from the other TBEV subtypes. The analysis of available literature provided data on other virus phenotypic particularities that supported our hypothesis. So, within the TBEV+LIV paraphyletic group, we offer to assign four species to get a more accurate understanding of the TBFV interspecies structure according to the modern monophyletic conception.

Evaluation of two artificial infection methods of live ticks as tools for studying interactions between tick-borne viruses and their tick vectors

Up to 170 tick-borne viruses (TBVs) have been identified to date. However, there is a paucity of information regarding TBVs and their interaction with respective vectors, limiting the development of new effective and urgently needed control methods. To overcome this gap of knowledge, it is essential to reproduce transmission cycles under controlled laboratory conditions. In this study we assessed an artificial feeding system (AFS) and an immersion technique (IT) to infect Ixodes ricinus ticks with tick-borne encephalitis (TBE) and Kemerovo (KEM) virus, both known to be transmitted predominantly by ixodid ticks. Both methods permitted TBEV acquisition by ticks and we further confirmed virus trans-stadial transmission and onward transmission to a vertebrate host. However, only artificial feeding system allowed to demonstrate both acquisition by ticks and trans-stadial transmission for KEMV. Yet we did not observe transmission of KEMV to mice (IFNAR^-/- or BALB/c). Artificial infection methods of ticks are important tools to study tick-virus interactions. When optimally used under laboratory settings, they provide important insights into tick-borne virus transmission cycles.

Transcriptomic Studies Suggest a Coincident Role for Apoptosis and Pyroptosis but Not for Autophagic Neuronal Death in TBEV-Infected Human Neuronal/Glial Cells

Tick-borne encephalitis virus (TBEV), a member of the Flaviviridae family, Flavivirus genus, is responsible for neurological symptoms that may cause permanent disability or death. With an incidence on the rise, it is the major arbovirus affecting humans in Central/Northern Europe and North-Eastern Asia. Neuronal death is a critical feature of TBEV infection, yet little is known about the type of death and the molecular mechanisms involved. In this study, we used a recently established pathological model of TBEV infection based on human neuronal/glial cells differentiated from fetal neural progenitors and transcriptomic approaches to tackle this question. We confirmed the occurrence of apoptotic death in these cultures and further showed that genes involved in pyroptotic death were up-regulated, suggesting that this type of death also occurs in TBEV-infected human brain cells. On the contrary, no up-regulation of major autophagic genes was found. Furthermore, we demonstrated an up-regulation of a cluster of genes belonging to the extrinsic apoptotic pathway and revealed the cellular types expressing them. Our results suggest that neuronal death occurs by multiple mechanisms in TBEV-infected human neuronal/glial cells, thus providing a first insight into the molecular pathways that may be involved in neuronal death when the human brain is infected by TBEV.

Long-term presence of tick-borne encephalitis virus in experimentally infected bank voles (Myodes glareolus)

Tick-borne encephalitis virus (TBEV) is a vector-borne pathogen that can cause serious neurological symptoms in humans. Across large parts of Eurasia TBEV is found in three traditional subtypes: the European, the Siberian and the Far-eastern subtype. Small mammalian animals play an important role in the transmission cycle as they enable the spread of TBEV among the vector tick population. To assess the impact of TBEV infection on its natural hosts, outbred bank voles (Myodes glareolus) were inoculated with one out of four European TBEV strains. Three of these TBEV strains were recently isolated in Germany. The forth one was the TBEV reference strain Neudörfl. Sampling points at 7, 14, 28, and 56 days post inoculation allowed the characterization of the course of infection. At each time point, six animals per strain were euthanized and eleven organ samples (brain, spine, lung, heart, small and large intestine, liver, spleen, kidney, bladder, sexual organ) as well as whole blood and serum samples were collected. The majority of bank voles (92/96) remained clinically unaffected after the inoculation with TBEV, but still developed a systemic infection during the first week, which transitioned to a viraemia and an infestation of the brain in some animals for the remainder of the first month. Viral RNA was found in whole blood samples of several animals (50/96), but only in a small fraction of the corresponding serum samples (4/50). From the whole blood, virus was successfully reisolated in cell culture until 14 days after inoculation. Less than five percent of all inoculated bank voles (4/96) displayed signs of distress in combination with a rapid weight loss and had to be euthanized prematurely. Overall, the recently isolated TBEV strains showed marked differences, such as a more frequent development of long-term viraemia and a higher detection rate of viral RNA in various organs, in comparison to the reference strain Neudörfl. Overall, our data suggest that the bank vole is a potential amplifying host in the TBEV transmission cycle and appears to be highly adapted to circulating TBEV strains.

Tick-Borne Encephalitis Virus Adaptation in Different Host Environments and Existence of Quasispecies

A highly virulent strain (Hypr) of tick-borne encephalitis virus (TBEV) was serially subcultured in the mammalian porcine kidney stable (PS) and Ixodes ricinus tick (IRE/CTVM19) cell lines, producing three viral variants. These variants exhibited distinct plaque sizes and virulence in a mouse model. Comparing the full-genome sequences of all variants, several nucleotide changes were identified in different genomic regions. Furthermore, different sequential variants were revealed to co-exist within one sample as quasispecies. Interestingly, the above-mentioned nucleotide changes found within the whole genome sequences of the new variants were present alongside the nucleotide sequence of the parental strain, which was represented as a minority quasispecies. These observations further imply that TBEV exists as a heterogeneous population that contains virus variants pre-adapted to reproduction in different environments, probably enabling virus survival in ticks and mammals.

Pathological modeling of TBEV infection reveals differential innate immune responses in human neurons and astrocytes that correlate with their susceptibility to infection

BACKGROUND

Tick-borne encephalitis virus (TBEV) is a member of the Flaviviridae family, Flavivirus genus, which includes several important human pathogens. It is responsible for neurological symptoms that may cause permanent disability or death, and, from a medical point of view, is the major arbovirus in Central/Northern Europe and North-Eastern Asia. TBEV tropism is critical for neuropathogenesis, yet little is known about the molecular mechanisms that govern the susceptibility of human brain cells to the virus. In this study, we sought to establish and characterize a new in vitro model of TBEV infection in the human brain and to decipher cell type-specific innate immunity and its relation to TBEV tropism and neuropathogenesis.

METHOD

Human neuronal/glial cells were differentiated from neural progenitor cells and infected with the TBEV-Hypr strain. Kinetics of infection, cellular tropism, and cellular responses, including innate immune responses, were characterized by measuring viral genome and viral titer, performing immunofluorescence, enumerating the different cellular types, and determining their rate of infection and by performing PCR array and qRT-PCR. The specific response of neurons and astrocytes was analyzed using the same approaches after enrichment of the neuronal/glial cultures for each cellular subtype.

RESULTS

We showed that infection of human neuronal/glial cells mimicked three major hallmarks of TBEV infection in the human brain, namely, preferential neuronal tropism, neuronal death, and astrogliosis. We further showed that these cells conserved their capacity to mount an antiviral response against TBEV. TBEV-infected neuronal/glial cells, therefore, represented a highly relevant pathological model. By enriching the cultures for either neurons or astrocytes, we further demonstrated qualitative and quantitative differential innate immune responses in the two cell types that correlated with their particular susceptibility to TBEV.

CONCLUSION

Our results thus reveal that cell type-specific innate immunity is likely to contribute to shaping TBEV tropism for human brain cells. They describe a new in vitro model for in-depth study of TBEV-induced neuropathogenesis and improve our understanding of the mechanisms by which neurotropic viruses target and damage human brain cells.

Optimization of a Quantitative Real-Time RT-PCR Technique for Evaluation of Concentration of Genomic +RNA of Tick-Borne Encephalitis Virus

Background. The specific detection of genomic/template +RNA and replicative –RNA of tick-born encephalitis virus (TBEV) is necessary to study the mechanisms of viral replication in the cells of reservoir and accidental hosts. However, the current approaches of quantitative reverse transcription – polymerase chain reaction (qRT-PCR) are rather focused on the detection of total viral RNA load in the sample. Thus, the significant optimization is necessary both for RT-PCR and for RNA copy number standard preparation.Aims. To develop the set of standard samples of synthetic +RNA of TBEV and to optimize qRT-PCR for quantification of genomic +RNA of the virus.Materials and methods. Fragment of the genomic +RNA of TBEV was synthesized using pTZ57R-T\A plasmid vector with embedded T7 promoter and T7 RNA polymerase. The DNA contamination was removed using RNase-free DNase I treatment followed by additional RNA purification step. Reverse transcription was performed using specific antisense primer 11154R 5`- AGCGGGTGTTTTTCCG-3` and qPCR detection was used according to the modified procedure of M. Schwaiger and P. Cassinotti (2003).Results. As a result of the amplification of standard samples, the concentration of positive polarity ТBEV RNA, carried out in five independent repetitions on different days, the correlation coefficient R2 between the quantification cycle and the concentration of the standard sample was 0.99, and the efficiency of PCR was 100 %. The coefficient of variation in assessing the inter-test accuracy of determination averaged 2.8 %.Conclusions. Optimized qRT-PCR procedure and set of +RNA standards allow to determine the concentration of genomic +RNA of TBEV in routine laboratory practice.

Tick-borne encephalitis virus inhibits rRNA synthesis and host protein production in human cells of neural origin

Tick-borne encephalitis virus (TBEV), a member of the genus Flavivirus (Flaviviridae), is a causative agent of a severe neuroinfection. Recently, several flaviviruses have been shown to interact with host protein synthesis. In order to determine whether TBEV interacts with this host process in its natural target cells, we analysed de novo protein synthesis in a human cell line derived from cerebellar medulloblastoma (DAOY HTB-186). We observed a significant decrease in the rate of host protein synthesis, including the housekeeping genes HPRT1 and GAPDH and the known interferon-stimulated gene viperin. In addition, TBEV infection resulted in a specific decrease of RNA polymerase I (POLR1) transcripts, 18S and 28S rRNAs and their precursor, 45-47S pre-rRNA, but had no effect on the POLR3 transcribed 5S rRNA levels. To our knowledge, this is the first report of flavivirus-induced decrease of specifically POLR1 rRNA transcripts accompanied by host translational shut-off.

Tick-borne encephalitis virus (TBEV) is a causative agent of a severe human neuroinfection that threatens Europe and Asia. Little is known about the interaction of this neurotropic virus with neural cells, even though this may be important to better understand why or how TBEV can cause high pathogenicity in humans, especially following neural cell infection. Here, we showed that TBEV induced host translational shut-off in cells of neural origin. In addition, TBEV interfered also with the expression of host ribosomal RNAs. Interestingly, the transcriptional shut-off was documented for rRNA species transcribed by RNA polymerase I (18S rRNA, 28S rRNA and their precursor 45-47S pre-rRNA), but not for RNA polymerase III rRNA transcripts (5S rRNA). Artificial inhibition of host translation using cycloheximide resulted in the decrease of all rRNA species. Based on these data, TBEV seems to specifically target transcription of RNA polymerase I. These new findings further increase our understanding of TBEV interactions with a key target cell type.

Tick-borne encephalitis virus: molecular determinants of neuropathogenesis of an emerging pathogen

Tick-borne encephalitis virus (TBEV) is a zoonotic agent causing severe encephalitis. The transmission cycle involves the virus, the Ixodes tick vector, and a vertebrate reservoir, such as small mammals (rodents, or shrews). Humans are accidentally involved in this transmission cycle. Tick-borne encephalitis (TBE) has been a growing public health problem in Europe and Asia over the past 30 years. The mechanisms involved in the development of TBE are very complex and likely multifactorial, involving both host and viral factors. The purpose of this review is to provide an overview of the current literature on TBE neuropathogenesis in the human host and to demonstrate the emergence of common themes in the molecular pathogenesis of TBE in humans. We discuss and review data on experimental study models and on both viral (molecular genetics of TBEV) and host (immune response, and genetic background) factors involved in TBE neuropathogenesis in the context of human infection.

Adventitious Virus Detection in Cells by High-Throughput Sequencing of Newly Synthesized RNAs: Unambiguous Differentiation of Cell Infection from Carryover of Viral Nucleic Acids

The use of high-throughput sequencing (HTS) to identify viral contamination of biological products is extremely sensitive and provides a broad range of detection. Nevertheless, viral sequences identified can also be inert. Examples include contamination resulting from reagents or the presence of inactivated viruses in animal-derived components of the cell culture medium. We therefore developed a method that relies on the sequencing of newly synthesized RNAs, an unequivocal sign of the presence of a transcriptionally active virus. This improvement in the specificity of viral testing increases the acceptability of HTS as a standard test for cells used in manufacturing biologicals and in biotherapies.

The use of high-throughput sequencing (HTS) to identify viruses in biologicals differs from current molecular approaches, since its use enables an unbiased approach to detection without the need to design specific primers to preamplify target sequences. Its broad range of detection and analytical sensitivity make it an important tool to ensure that biologicals are free from adventitious viruses. Similar to other molecular methods, however, identification of viral sequences in cells by HTS does not prove viral infection, since this could reflect carryover of inert viral sequences from reagents or other sources or the presence of transcriptionally inactive cellular sequences. Due to the broad range of detection associated with HTS, the above can potentially be perceived as a drawback for the testing of pharmaceutical biological products using this method. In order to avoid the identification of inert viral sequences, we present a methodology based on metabolic RNA labeling and sequencing, which enables the specific identification of newly synthesized viral RNAs in infected cells, resulting in the ability to unambiguously distinguish active infection by DNA or RNA viruses from inert nucleic acids. In the present study, we report the ability to differentiate Vero cells acutely infected by a single-stranded positive-sense RNA virus (tick-borne encephalitis virus) from cells which have been in contact with nonreplicating virus particles. Additionally, we also found a laboratory contamination by the squirrel monkey retrovirus of our Vero cell line, which was derived from an Old World (African green) monkey, a type of contamination which until now has been identified only in cells derived from primates from the New World.

IMPORTANCE The use of high-throughput sequencing (HTS) to identify viral contamination of biological products is extremely sensitive and provides a broad range of detection. Nevertheless, viral sequences identified can also be inert. Examples include contamination resulting from reagents or the presence of inactivated viruses in animal-derived components of the cell culture medium. We therefore developed a method that relies on the sequencing of newly synthesized RNAs, an unequivocal sign of the presence of a transcriptionally active virus. This improvement in the specificity of viral testing increases the acceptability of HTS as a standard test for cells used in manufacturing biologicals and in biotherapies.

Virus RNA Load in Patients with Tick-Borne Encephalitis, Slovenia

We determined levels of tick-borne encephalitis (TBE) virus (TBEV) RNA in serum samples obtained from 80 patients during the initial phase of TBE in Slovenia. For most samples, levels were within the range of 3-6 log₁₀ copies RNA/mL. Levels were higher in female patients than in male patients, but we found no association between virus load and several laboratory and clinical parameters, including severity of TBE. However, a weak humoral immune response was associated with a more severe disease course, suggesting that inefficient clearance of virus results in a more serious illness. To determine whether a certain genetic lineage of TBEV had a higher virulence potential, we obtained 56 partial envelope protein gene sequences by directly sequencing reverse transcription PCR products from clinical samples of patients. This method provided a large set of patient-derived TBEV sequences. We observed no association between phylogenetic clades and virus load or disease severity.

Phylogenetics of tick-borne encephalitis virus in endemic foci in the upper Rhine region in France and Germany

Objective

Tick-borne encephalitis (TBE) caused by the tick-borne encephalitis virus (TBEV) is the most important tick-borne arboviral disease in Europe and Asia. The Upper Rhine Valley is thought to be the very western border of TBEV distribution in Europe. The aim of our study was to identify natural foci and isolate TBEV from ticks, to determine the prevalence of TBEV in local tick populations and to study the phylogenetic relatedness of circulating TBEV strains in this region.

Material and methods

Ticks were collected between 2016, 2017 and 2018 by flagging. TBEV was isolated from collected ticks and phylogenetic analyses were performed. Minimal infection rates (MIR) of the collected ticks were calculated.

Results

At 12 sampling sites, a total of 4,064 Ixodes ticks were collected in 2016 and 2017 –(and one single collection 2018). 953 male, 856 female adult ticks and 2,255 nymphs were identified. The MIR rates were 0,17% (1/595) for Schiltach (Germany) and 0,11% (1/944) for Foret de la Robertsau (France), respectively. Overall, the three newly described TBEV strains, isolated in the years 2016 and 2017 from the Upper Rhine Valley have no close phylogenetic relation and show a genetic relationship with strains from eastern Europe. The 2018 TBEV strain from Aubachstrasse (Germany), however, is closely related to the TBEV found in Schiltach (Germany).

Conclusion

In conclusion, we demonstrate, to our knowledge for the first time, the phylogenetic relations of the newly isolated TBEV strains on both sides of the upper Rhine river.

Tick-Borne Encephalitis Virus: A Structural View

Tick-borne encephalitis virus (TBEV) is a growing health concern. It causes a severe disease that can lead to permanent neurological complications or death and the incidence of TBEV infections is constantly rising. Our understanding of TBEV’s structure lags behind that of other flaviviruses, but has advanced recently with the publication of a high-resolution structure of the TBEV virion. The gaps in our knowledge include: aspects of receptor binding, replication and virus assembly. Furthermore, TBEV has mostly been studied in mammalian systems, even though the virus’ interaction with its tick hosts is a central part of its life cycle. Elucidating these aspects of TBEV biology are crucial for the development of TBEV antivirals, as well as the improvement of diagnostics. In this review, we summarise the current structural knowledge on TBEV, bringing attention to the current gaps in our understanding, and propose further research that is needed to truly understand the structural-functional relationship of the virus and its hosts.

Viral Determinants of Virulence in Tick-Borne Flaviviruses

Tick-borne flaviviruses have a global distribution and cause significant human disease, including encephalitis and hemorrhagic fever, and often result in neurologic sequelae. There are two distinct properties that determine the neuropathogenesis of a virus. The ability to invade the central nervous system (CNS) is referred to as the neuroinvasiveness of the agent, while the ability to infect and damage cells within the CNS is referred to as its neurovirulence. Examination of laboratory variants, cDNA clones, natural isolates with varying pathogenicity, and virally encoded immune evasion strategies have contributed extensively to our understanding of these properties. Here we will review the major viral determinants of virulence that contribute to pathogenesis and influence both neuroinvasiveness and neurovirulence properties of tick-borne flaviviruses, focusing particularly on the envelope protein (E), nonstructural protein 5 (NS5), and the 3′ untranslated region (UTR).

Tick-borne encephalitis virus induces chemokine RANTES expression via activation of IRF-3 pathway

Background

Tick-borne encephalitis virus (TBEV) is one of the most important flaviviruses that targets the central nervous system (CNS) and causes encephalitides in humans. Although neuroinflammatory mechanisms may contribute to brain tissue destruction, the induction pathways and potential roles of specific chemokines in TBEV-mediated neurological disease are poorly understood.

Methods

BALB/c mice were intracerebrally injected with TBEV, followed by evaluation of chemokine and cytokine profiles using protein array analysis. The virus-infected mice were treated with the CC chemokine antagonist Met-RANTES or anti-RANTES mAb to determine the role of RANTES in affecting TBEV-induced neurological disease. The underlying signaling mechanisms were delineated using RANTES promoter luciferase reporter assay, siRNA-mediated knockdown, and pharmacological inhibitors in human brain-derived cell culture models.

Results

In a mouse model, pathological features including marked inflammatory cell infiltrates were observed in brain sections, which correlated with a robust up-regulation of RANTES within the brain but not in peripheral tissues and sera. Antagonizing RANTES within CNS extended the survival of mice and reduced accumulation of infiltrating cells in the brain after TBEV infection. Through in vitro studies, we show that virus infection up-regulated RANTES production at both mRNA and protein levels in human brain-derived cell lines and primary progenitor-derived astrocytes. Furthermore, IRF-3 pathway appeared to be essential for TBEV-induced RANTES production. Site mutation of an IRF-3-binding motif abrogated the RANTES promoter activity in virus-infected brain cells. Moreover, IRF-3 was activated upon TBEV infection as evidenced by phosphorylation of TBK1 and IRF-3, while blockade of IRF-3 activation drastically reduced virus-induced RANTES expression.

Conclusions

Our findings together provide insights into the molecular mechanism underlying RANTES production induced by TBEV, highlighting its potential importance in the process of neuroinflammatory responses to TBEV infection.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0665-9) contains supplementary material, which is available to authorized users.

Isolation and characterization of a Far-Eastern strain of tick-borne encephalitis virus in China

Tick-borne encephalitis virus (TBEV) is a leading cause of human neurological infection in many parts of Europe and Asia. Although several TBEV isolates have been reported, current understanding of the biological characteristics of a Chinese strain is limited. In this study, a Far-Eastern strain of TBEV designated WH2012 was isolated in northern China. Its genome has been sequenced and found to be closely related to other Chinese TBEV isolates. Human cell lines of neural origin exposed to WH2012 showed cytopathic effects and WH2012 replicated most efficiently in human neuroblastoma cells SK-N-SH. In addition, WH2012 possessed a pathogenic potential in the mouse model, characterized by inducing a complete paralysis in the hindlimbs with a fatal outcome. We herein describe the first data regarding biological properties of TBEV from China. This study may help future research on pathogenic mechanisms of the neurological disease induced by TBEV infection in China.

Nucleoside inhibitors of tick-borne encephalitis virus

Tick-borne encephalitis virus (TBEV) is a leading cause of human neuroinfections in Europe and Northeast Asia. There are no antiviral therapies for treating TBEV infection. A series of nucleoside analogues was tested for the ability to inhibit the replication of TBEV in porcine kidney cells and human neuroblastoma cells. The interactions of three nucleoside analogues with viral polymerase were simulated using advanced computational methods. The nucleoside analogues 7-deaza-2'-C-methyladenosine (7-deaza-2'-CMA), 2'-C-methyladenosine (2'-CMA), and 2'-C-methylcytidine (2'-CMC) inhibited TBEV replication. These compounds showed dose-dependent inhibition of TBEV-induced cytopathic effects, TBEV replication (50% effective concentrations [EC50]of 5.1 ± 0.4 μM for 7-deaza-2'-CMA, 7.1 ± 1.2 μM for 2'-CMA, and 14.2 ± 1.9 μM for 2'-CMC) and viral antigen production. Notably, 2'-CMC was relatively cytotoxic to porcine kidney cells (50% cytotoxic concentration [CC50] of ∼50 μM). The anti-TBEV effect of 2'-CMA in cell culture diminished gradually after day 3 posttreatment. 7-Deaza-2'-CMA showed no detectable cellular toxicity (CC50 > 50 μM), and the antiviral effect in culture was stable for >6 days posttreatment. Computational molecular analyses revealed that compared to the other two compounds, 7-deaza-2'-CMA formed a large cluster near the active site of the TBEV polymerase. High antiviral activity and low cytotoxicity suggest that 7-deaza-2'-CMA is a promising candidate for further investigation as a potential therapeutic agent in treating TBEV infection.

Full genome sequences and molecular characterization of tick-borne encephalitis virus strains isolated from human patients

Tick-borne encephalitis virus (TBEV) causes tick-borne encephalitis (TBE), one of the most important human neuroinfections across Eurasia. Up to date, only three full genome sequences of human European TBEV isolates are available, mostly due to difficulties with isolation of the virus from human patients. Here we present full genome characterization of an additional five low-passage TBEV strains isolated from human patients with severe forms of TBE. These strains were isolated in 1953 within Central Bohemia in the former Czechoslovakia, and belong to the historically oldest human TBEV isolates in Europe. We demonstrate here that all analyzed isolates are distantly phylogenetically related, indicating that the emergence of TBE in Central Europe was not caused by one predominant strain, but rather a pool of distantly related TBEV strains. Nucleotide identity between individual sequenced TBEV strains ranged from 97.5% to 99.6% and all strains shared large deletions in the 3' non-coding region, which has been recently suggested to be an important determinant of virulence. The number of unique amino acid substitutions varied from 3 to 9 in individual isolates, but no characteristic amino acid substitution typical exclusively for all human TBEV isolates was identified when compared to the isolates from ticks. We did, however, correlate that the exploration of the TBEV envelope glycoprotein by specific antibodies were in close proximity to these unique amino acid substitutions. Taken together, we report here the largest number of patient-derived European TBEV full genome sequences to date and provide a platform for further studies on evolution of TBEV since the first emergence of human TBE in Europe.

Sequencing of a tick-borne encephalitis virus from Ixodes ricinus reveals a thermosensitive RNA switch significant for virus propagation in ectothermic arthropods

Tick-borne encephalitis virus (TBEV) is a flavivirus with major impact on global health. The geographical TBEV distribution is expanding, thus making it pivotal to further characterize the natural virus populations. In this study, we completed the earlier partial sequencing of a TBEV pulled out of a pool of RNA extracted from 115 ticks collected on Torö in the Stockholm archipelago. The total RNA was sufficient for all sequencing of a TBEV genome (Torö-2003), without conventional enrichment procedures such as cell culturing or suckling mice amplification. To our knowledge, this is the first time that the genome of TBEV has been sequenced directly from an arthropod reservoir. The Torö-2003 sequence has been characterized and compared with other TBE viruses. In silico analyses of secondary RNA structures formed by the two untranslated regions revealed a temperature-sensitive structural shift between a closed replicative form and an open AUG accessible form, analogous to a recently described bacterial thermoswitch. Additionally, novel phylogenetic conserved structures were identified in the variable part of the 3'-untranslated region, and their sequence and structure similarity when compared with earlier identified structures suggests an enhancing function on virus replication and translation. We propose that the thermo-switch mechanism may explain the low TBEV prevalence often observed in environmentally sampled ticks. Finally, we were able to detect variations that help in the understanding of virus adaptations to varied environmental temperatures and mammalian hosts through a comparative approach that compares RNA folding dynamics between strains with different mammalian cell passage histories.

Tick-borne encephalitis virus delays interferon induction and hides its double-stranded RNA in intracellular membrane vesicles

Tick-borne encephalitis virus (TBEV) (family Flaviviridae, genus Flavivirus) accounts for approximately 10,000 annual cases of severe encephalitis in Europe and Asia. Here, we investigated the induction of the antiviral type I interferons (IFNs) (alpha/beta IFN [IFN-alpha/beta]) by TBEV. Using strains Neudörfl, Hypr, and Absettarov, we demonstrate that levels of IFN-beta transcripts and viral RNA are strictly correlated. Moreover, IFN induction by TBEV was dependent on the transcription factor IFN regulatory factor 3 (IRF-3). However, even strain Hypr, which displayed the strongest IFN-inducing activity and the highest RNA levels, substantially delayed the activation of IRF-3. As a consequence, TBEV can keep the level of IFN transcripts below the threshold value that would permit the release of IFN by the cell. Only after 24 h of infection have cells accumulated sufficient IFN transcripts to produce detectable amounts of secreted IFNs. The delay in IFN induction appears not to be caused by a specific viral protein, since the individual expressions of TBEV C, E, NS2A, NS2B, NS3, NS4A, NS4B, NS5, and NS2B-NS3, as well as TBEV infection itself, had no apparent influence on specific IFN-beta induction. We noted, however, that viral double-stranded RNA (dsRNA), an important trigger of the IFN response, is immunodetectable only inside intracellular membrane compartments. Nonetheless, the dependency of IFN induction on IFN promoter stimulator 1 (IPS-1) as well as the phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF2alpha) suggest the cytoplasmic exposure of some viral dsRNA late in infection. Using ultrathin-section electron microscopy, we demonstrate that, similar to other flaviviruses, TBEV rearranges intracellular membranes. Virus particles and membrane-connected vesicles (which most likely represent sites of virus RNA synthesis) were observed inside the endoplasmic reticulum. Thus, apparently, TBEV rearranges internal cell membranes to provide a compartment for its dsRNA, which is largely inaccessible for detection by cytoplasmic pathogen receptors. This delays the onset of IFN induction sufficiently to give progeny particle production a head start of approximately 24 h.

A single N-linked glycosylation site in the Japanese encephalitis virus prM protein is critical for cell type-specific prM protein biogenesis, virus particle release, and pathogenicity in mice

The prM protein of Japanese encephalitis virus (JEV) contains a single potential N-linked glycosylation site, N(15)-X(16)-T(17), which is highly conserved among JEV strains and closely related flaviviruses. To investigate the role of this site in JEV replication and pathogenesis, we manipulated the RNA genome by using infectious JEV cDNA to generate three prM mutants (N15A, T17A, and N15A/T17A) with alanine substituting for N(15) and/or T(17) and one mutant with silent point mutations introduced into the nucleotide sequences corresponding to all three residues in the glycosylation site. An analysis of these mutants in the presence or absence of endoglycosidases confirmed the addition of oligosaccharides to this potential glycosylation site. The loss of prM N glycosylation, without significantly altering the intracellular levels of viral RNA and proteins, led to an approximately 20-fold reduction in the production of extracellular virions, which had protein compositions and infectivities nearly identical to those of wild-type virions; this reduction occurred at the stage of virus release, rather than assembly. This release defect was correlated with small-plaque morphology and an N-glycosylation-dependent delay in viral growth. A more conservative mutation, N15Q, had the same effect as N15A. One of the four prM mutants, N15A/T17A, showed an additional defect in virus growth in mosquito C6/36 cells but not human neuroblastoma SH-SY5Y or hamster BHK-21 cells. This cell type dependence was attributed to abnormal N-glycosylation-independent biogenesis of prM. In mice, the elimination of prM N glycosylation resulted in a drastic decrease in virulence after peripheral inoculation. Overall, our findings indicate that this highly conserved N-glycosylation motif in prM is crucial for multiple stages of JEV biology: prM biogenesis, virus release, and pathogenesis.

Rapid subtyping of tick-borne encephalitis virus isolates using multiplex RT-PCR

Tick-borne encephalitis virus, an emerging pathogen in several countries in Europe and Asia, has been divided into three subtypes (European, Siberian and Far Eastern). These subtypes are associated with different severities of the disease. For that reason, early determination of the subtype in a clinical sample or in ticks removed from a patient in areas of co-circulation of two or three subtypes is of high importance. The development of a simple method of multiplex RT-PCR for rapid and easy subtyping of tick-borne encephalitis virus isolates is reported to fill this requirement. The method is based on the unique combination of oligonucleotide primers hybridizing with subtype-specific "signature" positions of the sequence encoding the viral envelope protein. The developed multiplex RT-PCR also appears to be a useful method in studies focused on the molecular-epidemiology of tick-borne encephalitis virus.

Humoral and cellular immune response to RNA immunization with flavivirus replicons derived from tick-borne encephalitis virus

A new vaccination principle against flaviviruses, based on a tick-borne encephalitis virus (TBEV) self-replicating noninfectious RNA vaccine that produces subviral particles, has recently been introduced (R. M. Kofler, J. H. Aberle, S. W. Aberle, S. L. Allison, F. X. Heinz, and C. W. Mandl, Proc. Natl. Acad. Sci. USA 7:1951-1956, 2004). In this study, we evaluated the potential of the self-replicating RNA vaccine in mice in comparison to those of live, attenuated vaccines and a formalin-inactivated whole-virus vaccine (ImmunInject). For this purpose, mice were immunized using gene gun-mediated application of the RNA vaccine and tested for CD8+ T-cell responses, long-term duration, neutralizing capacity, and isotype profile of specific antibodies and protection against lethal virus challenge. We demonstrate that the self-replicating RNA vaccine induced a broad-based, humoral and cellular (Th1 and CD8+ T-cell response) immune response comparable to that induced by live vaccines and that it protected mice from challenge. Even a single immunization with 1 microg of the replicon induced a long-lasting antibody response, characterized by high neutralizing antibody titers, which were sustained for at least 1 year. Nevertheless, it was possible to boost this response further by a second injection with the RNA vaccine, even in the presence of a concomitant CD8+ T-cell response. In this way it was possible to induce a balanced humoral and cellular immune response, similar to infection-induced immunity but without the safety hazards of infectious agents. The results also demonstrate the value of TBEV replicon RNA for inducing protective long-lasting antiviral responses.

[Phylogenetic analysis and pathogenicity of tick-borne encephalitis virus from Japan and far-east Russia]

Phylogenetic analysis of tick-borne encephalitis (TBE) virus revealed that Hokkaido strain of TBE virus evolved several hundreds years ago in far-east Russia. TBE virus strains in Irkutsk area were identified as Siberian subtype of TBE virus. BHK-cell adapted mutant of TBE virus showed lower neuro-invasive virulence in mice than parent virus. The mutant carried one amino acid substitution in envelope protein which resulted in increase of positive charge of the protein. The mutant-infected mice showed lower virus titers in bloods and spleens than the parent-infected mice. Infectious c-DNA clone of TBE virus Hokkaido strain was successfully generated and was applied to examine the neurovirulence in mice. One amino acid change in envelope protein and 2 amino acid changes in Ns5 protein showed a synergistic effect on reduced neurovirulence in mice.

Molecular epidemiology of tick-borne encephalitis virus in Ixodes ricinus ticks in Lithuania

In Lithuania, 171-645 serologically confirmed cases of tick-borne encephalitis occurred annually [Mickiene et al. (2001): Eur J Clin Microbiol Infect Dis 20:886-888] in 1993-1999, and the tick-borne encephalitis virus (TBEV) seroprevalence in the general population was found previously to be 3.0% [Juceviciene et al. (2002): J Clin Virol 25:23-27]. To assess the risk for TBEV virus infection in Lithuania and to characterize the agent a panel of 3,234 ticks combined into 436 pools [Juceviciene et al., 2005] were tested for presence of TBEV RNA by a nested RT-PCR targeting at the NS5 gene. Six pools were confirmed positive and the prevalence of the infected ticks was 0.2% (if one tick per pool [Juceviciene et al., 2005] was considered positive) and the proportion of positive tick pools was 1.4%. The prevalence of the infected ticks in the Panevezys, Siauliai, and Radviliskis regions (in central Lithuania) was 0.1%, 0.4%, and 1.7% corresponding with a higher TBE disease burden in these regions. The 252-nucleotide NS5-region amplicons, and a longer sequence (737 nucleotides) obtained from one sample from the PrM-E gene region, were sequenced. Phylogenetic analysis of the latter showed that all western type TBEV PrM-E sequences, including the Lithuanian strains, were monophyletic, showed no clustering and had very little variation. The NS5 sequences, although identical within one locality, did not show any mutations common to strains from the two Lithuanian regions, nor could any geographical clustering be found among western type TBEV strains from other areas.

Steps of the tick-borne encephalitis virus replication cycle that affect neuropathogenesis

Tick-borne encephalitis virus (TBEV) is an important human pathogen that causes severe neurological illness in large areas of Europe and Asia. The neuropathogenesis of this disease agent is determined by its capacity to enter the central nervous system (CNS) after peripheral inoculation ("neuroinvasiveness") and its ability to replicate and cause damage within the CNS ("neurovirulence"). TBEV is a small, enveloped flavivirus with an unsegmented, positive-stranded RNA genome. Mutations affecting various steps of its natural replication cycle were shown to influence its neuropathogenic properties. This review describes experimental approaches and summarizes results on molecular determinants of neurovirulence and neuroinvasiveness that have been identified for this virus. It focuses on molecular mechanisms of three particular steps of the viral life cycle that have been studied in some detail for TBEV and two closely related tick-borne flaviviruses (Louping ill virus (LIV) and Langat virus (LGTV)), namely (i) the envelope protein E and its role in viral attachment to the cell surface, (ii) the 3'-noncoding region of the genome and its importance for viral RNA replication, and (iii) the capsid protein C and its role in the assembly process of infectious virus particles. Mutations affecting each of these three molecular targets significantly influence neuropathogenesis of TBEV, particularly its neuroinvasiveness. The understanding of molecular determinants of TBEV neuropathogenesis is relevant for vaccine development, also against other flaviviruses.

Size heterogeneity in the 3' noncoding region of South American isolates of yellow fever virus

The 3' noncoding region (3' NCR) of flaviviruses contains secondary and tertiary structures essential for virus replication. Previous studies of yellow fever virus (YFV) and dengue virus have found that modifications to the 3' NCR are sometimes associated with attenuation in vertebrate and/or mosquito hosts. The 3' NCRs of 117 isolates of South American YFV have been examined, and major deletions and/or duplications of conserved RNA structures have been identified in several wild-type isolates. Nineteen isolates (designated YF-XL isolates) from Brazil, Trinidad, and Venezuela, dating from 1973 to 2001, exhibited a 216-nucleotide (nt) duplication, yielding a tandem repeat of conserved hairpin, stem-loop, dumbbell, and pseudoknot structures. YF-XL isolates were found exclusively within one subclade of South American genotype I YFV. One Brazilian isolate exhibited, in addition to the 216-nt duplication, a deletion of a 40-nt repeated hairpin (RYF) motif (YF-XL-DeltaRYF). To investigate the biological significance of these 3' NCR rearrangements, YF-XL-DeltaRYF and YF-XL isolates, as well as other South American YFV isolates, were evaluated for three phenotypes: growth kinetics in cell culture, neuroinvasiveness in suckling mice, and ability to replicate and produce disseminated infections in Aedes aegypti mosquitoes. YF-XL-DeltaRYF and YF-XL isolates showed growth kinetics and neuroinvasive characteristics comparable to those of typical South American YFV isolates, and mosquito infectivity trials demonstrated that both types of 3' NCR variants were capable of replication and dissemination in a laboratory-adapted colony of A. aegypti.

Emergence of attenuated West Nile virus variants in Texas, 2003

In order to understand how West Nile virus (WNV) has evolved since its introduction into North America, we have studied the genetic and phenotypic variation among WNV isolates collected in various areas during consecutive transmission seasons. The present report describes for the first time phenotypic changes occurring in the North American WNV population. Several isolates collected in Texas during 2003 display a small plaque (sp) and temperature sensitive (ts) phenotype, as well as reduced replication in cell culture, in comparison to isolates collected in 2002 and New York in 1999. Studies of mouse neuroinvasiveness/neurovirulence also indicate that several of these isolates were attenuated in neuroinvasiveness, but not for neurovirulence. The complete genome and deduced amino acid sequences of several of these isolates have been determined in order to map the mutations responsible for this phenotypic variation. These data indicate microevolution of WNV and the emergence of isolates exhibiting phenotypic variation.

Tick-borne virus diseases of human interest in Europe

Several human diseases in Europe are caused by viruses transmitted by tick bite. These viruses belong to the genus Flavivirus, and include tick-borne encephalitis virus, Omsk haemorrhagic fever virus, louping ill virus, Powassan virus, Nairovirus (Crimean-Congo haemorrhagic fever virus) and Coltivirus (Eyach virus). All of these viruses cause more or less severe neurological diseases, and some are also responsible for haemorrhagic fever. The epidemiology, clinical picture and methods for diagnosis are detailed in this review. Most of these viral pathogens are classified as Biosafety Level 3 or 4 agents, and therefore some of them have been classified in Categories A-C of potential bioterrorism agents by the Centers for Disease Control and Prevention. Their ability to cause severe disease in man means that these viruses, as well as any clinical samples suspected of containing them, must be handled with specific and stringent precautions.

Flavivirus Immunization with Capsid-Deletion Mutants: Basics, Benefits, and Barriers

The flaviviruses comprise a number of arthropod-transmitted human disease agents that cause significant and increasing health threats in major parts of the world. The development of new vaccines is of vital importance, but the stringent need for safety, efficacy and cost-effectiveness together with the problems associated with the specific immune pathogenesis of some flavivirus infections impose significant challenges to innovative vaccine research. Using tick-borne encephalitis virus (TBEV) as a model, the viral capsid protein gene was recently identified as a novel target for generating flavivirus vaccines. This approach can be applied to produce either attenuated strains that can serve as live vaccines or to make a new type of a genetic vaccine consisting of non-infectious RNA replicons from which subviral particles are synthesized in vivo. Flaviviruses are small, enveloped viruses with an unsegmented positive-stranded RNA genome encoding a single polyprotein that is cleaved into the individual viral proteins. The specific introduction of various deletions and other mutations into the genomic segment coding for the capsid protein C and the biochemical and immunological characterization of the resulting mutants in cell culture and an animal model have revealed remarkable properties of this building block of the nucleocapsid and yielded information that opened the way for new vaccine approaches. In this review the in vitro and in vivo findings with various capsid deletion mutants of TBEV are summarized and discussed in the context of recent structural and biochemical data obtained for protein C of various flaviviruses. Potential benefits of this new strategy for generating flavivirus vaccines as well as hurdles that still have to be overcome are discussed in comparison to conventional or other experimental approaches. Capsid-deletion mutants can be used to rationally design safe and effective vaccine strains or to create new vaccines that combine advantages of genetic vaccination, conventional inactivated, and live vaccines.

A novel principle of attenuation for the development of new generation live flavivirus vaccines

The genus Flavivirus includes a number of important human pathogens that impose major health problems in large regions of the world. The emergence of flaviviruses in new geographic regions (e.g., West Nile virus in North America) and rapid socioeconomic changed in many developing countries where flaviviruses such as dengue virus and Japanese encephalitis virus and endemic demand the development of new vaccines against these diseases. Using tick-borne encephalitis virus as a model we have established a new method to generate attenuated flavivirus strains that may be useful for generating cost-effective and safe live vaccines. This method relies on the specific introduction of deletions into one of the structural proteins, the capsid protein C. These deletions remove parts or all of an internal stretch of hydrophobic amino acid residues that probably is involved in virion assembly. We observed that remarkably long deletions were tolerated, yielding viable viral mutants that were highly attenuated in the mouse model but efficiently induced protective immunity. Biochemical analyses suggested that attenuation was caused by an assembly defect of infectious virions but the mutants produced ample amounts of non-infections subviral particles. The generation of viable mutants with deletions longer that 16 amino acid residues depended on additional, spontaneously emerging mutations within protein C that increased the hydrophobicity of the mutant protein. Although the second-site mutations increased infectivity, they did not restore neuroinvasiveness. Mouse experiments demonstrated excellent safety and immunogenicity profiles for these mutants.

Tick-borne flaviviruses

Tick-borne encephalitis (TBE), one of the most dangerous neuroinfections in Europe and Asia, is caused by tick-borne encephalitis virus (TBEV) and currently involves approximately 11,000 human cases annually, mostly in Russia. This chapter describes the main problems associated with the epidemiology, ecology, pathogenesis, and control of this disease. We have attempted to review the factors that influence the incidence and distribution of TBE, and to discuss possible reasons for the different clinical manifestations including most commonly observed asymptomatic infections, fever forms, acute encephalitis, and the less frequently registered biphasic milk fever and chronic encephalitis. Epidemiologic data concerning the other tick-borne flaviviruses, namely Louping ill virus, Langat virus, and Powassan virus that also produce encephalitis on a smaller scale, are also presented. Here we describe the history and current epidemiological role of Omsk hemorrhagic fever virus and Kyasanur forest disease virus, two viruses that are genetically closely related to TBEV, but produce hemorrhagic fever instead of encephalitis, and provide possible explanations for these differences. The other viruses in the tick-borne flavivirus group are also included despite the fact that they do not play an essential epidemiologic role in humans. This chapter contains a brief history of vaccination against TBE including the trials with live attenuated vaccine and reviews the modern trends in development of vaccine virus strains.

Mimicking live flavivirus immunization with a noninfectious RNA vaccine

Flaviviruses are human pathogens of world-wide medical importance. They have recently received much additional attention because of their spread to new regions (such as West Nile virus to North America), highlighting their potential as newly emerging disease agents. Using tick-borne encephalitis virus, we have developed and evaluated in mice a new genetic vaccine based on self-replicating but noninfectious RNA. This RNA contains all of the necessary genetic information for establishing its replication machinery in the host cell, thus mimicking a natural infection. However, genetic modifications in the region encoding the capsid protein simultaneously prevent the assembly of infectious virus particles and promote the secretion of noninfectious subviral particles that elicit neutralizing antibodies. These characteristics demonstrate that a new generation of flavivirus vaccines can be designed that stimulate the same spectrum of innate and specific immune responses as a live vaccine but have the safety features of an inactivated vaccine.

A BHK-21 cell culture-adapted tick-borne encephalitis virus mutant is attenuated for neuroinvasiveness

We derived the baby hamster kidney (BHK)-21 cell culture-adapted, tick-borne encephalitis (TBE) virus mutant. To reveal the pathogenicity of the TBE virus, we compared the pathogenicity of the mutant (Oshima Cl-1) and parental (Oshima 5-10) virus in mouse model. The neurovirulence of mutant in mice was identical to that of parent. However, the level of neuroinvasiveness was higher for parent than for mutant. The degrees of viremia and virus titers in the spleen were lower in mice that were inoculated subcutaneously (s.c.) with mutant than in mice that received parent. Unlike parent, mutant was rarely detected in the brains of s.c. inoculated mice. Genetic analysis revealed that mutant had single amino acid substitutions in each of the E and NS5 proteins compared with parent. Furthermore, while mutant infection of BHK-21 cells was inhibited by glycosaminoglycans (GAGs), this was not the case for parent. In summary, the BHK-21-cell-adapted mutant virus showed reduced neuroinvasiveness in mice due to low-level induction of viremia. The attenuation process involved a single amino acid change in the E protein, which may have resulted in the rapid clearance of the virus due to its high affinity for negatively charged molecules in vivo.

Development of a quantitative real-time RT-PCR assay with internal control for the laboratory detection of tick borne encephalitis virus (TBEV) RNA

BACKGROUND

Tick borne encephalitis virus (TBEV), is a human flavivirus causing tick borne encephalitis (TBE), a viral infection of the central nervous system endemic in Europe and Asia.

OBJECTIVES

To develop a reverse transcription polymerase chain reaction (RT-PCR) assay based on quantitative real-time RT-PCR technology (TaqMan) for detection and quantification of TBEV RNA. The test includes an internal control (IC) to avoid false negative results.

STUDY DESIGN

The system was established and validated using wild-type (WT) non-infectious synthetic RNA representing a fragment of the 3' non-coding region of the TBEV genome. In addition, synthetic RNA differing from the WT synthetic RNA by a unique probe binding region was used as IC to monitor the overall efficiency of the RT-PCR.

RESULTS

The analytical sensitivity of the assay was at least ten copies of the TBEV synthetic transcript in presence of 50 copies of the IC. Successful amplification was obtained for different strains within the TBEV complex (Hypr, Hochosterwitz, Laibach, Elsass=Alsace, ZZ9, Wladiwostok). Among 14 serum and 21 cerebrospinal fluid (CSF) samples obtained from 28 patients with clinical suspicion of TBEV 1 CSF sample tested positive for TBEV RNA. In addition, no TBEV RNA could be detected in blood samples obtained from three vaccinated people 1 and 3 days post-vaccination. Thus indicating that a positive result is unlikely to be caused by recent vaccination.

CONCLUSIONS

A quantitative, highly sensitive and specific real-time RT-PCR assay has been developed for the detection of TBEV RNA. Inclusion of an IC is important to monitor the possible occurrence of false-negative results caused by the presence of inhibitory factors. This assay should be an important asset for the routine laboratory detection of TBEV RNA.

Tick-borne encephalitis

Tick-borne encephalitis (TBE) is one of the most dangerous human infections occurring in Europe and many parts of Asia. The etiological agent Tick-borne encephalitis virus (TBEV), is a member of the virus genus Flavivirus, of the family Flaviviridae. TBEV is believed to cause at least 11,000 human cases of encephalitis in Russia and about 3000 cases in the rest of Europe annually. Related viruses within the same group, Louping ill virus (LIV), Langat virus (LGTV) and Powassan virus (POWV), also cause human encephalitis but rarely on an epidemic scale. Three other viruses within the same group, Omsk hemorrhagic fever virus (OHFV), Kyasanur Forest disease virus (KFDV) and Alkhurma virus (ALKV), are closely related to the TBEV complex viruses and tend to cause fatal hemorrhagic fevers rather than encephalitis. This review describes the clinical manifestations associated with TBEV infections, the main molecular-biological properties of these viruses, and the different factors that define the incidence and severity of disease. The role of ticks and their local hosts in the emergence of new virus variants with different pathogenic characteristics is also discussed. This review also contains a brief history of vaccination against TBE including trials with live attenuated vaccine and modern tendencies in developing of vaccine virus strains.