Elsevier

Vaccine

Volume 21, Supplement 1, 1 April 2003, Pages S3-S10
Vaccine

Molecular aspects of TBE virus research

https://doi.org/10.1016/S0264-410X(02)00820-4Get rights and content

Introduction

Tick-borne encephalitis (TBE) virus is a member of the genus flavivirus in the flaviviridae family, which also comprises the genera pestivirus (including the animal pathogens classical swine fever virus, bovine viral diarrhea virus, and border disease virus) and hepacivirus (including the different genotypes of hepatitis C virus) [1]. These are small spherical enveloped viruses with a capsid composed of a single basic capsid protein and two (flavi and hepaci) or three (pesti) membrane-associated proteins. Most of the viruses of the genus flavivirus are transmitted to their vertebrate hosts by mosquitoes or ticks, and several of them are important human pathogens, such as yellow fever virus, Japanese encephalitis virus, the dengue viruses, West Nile virus, and TBE virus.

The positive-stranded RNA genome of TBE virus shares characteristic features with all of the other members of the flavivirus genus [2]. It has a length of approximately 11,000 nucleotides and contains a single long open reading frame (ORF) that is flanked by noncoding regions (NCRs) at the 5′- and 3′-ends. The 3′-NCR of TBE virus has been found to vary significantly in length from about 350 to 750 nucleotides due to a variable region inserted between the stop codon of the ORF and the 3′-terminal 350 nucleotides that form a highly conserved core element with characteristic sequence elements of potential functional importance [3], [4]. Secondary structure predictions of this region revealed well-defined structural features that are characteristic for TBE virus and others that are found in the genomes of flaviviruses in general [5], [6].

The genomic RNA is the only mRNA found in infected cells and the viral proteins are generated by the translation of a single polyprotein that is co- and post-translationally cleaved by a viral serine protease and cellular proteases (signalase and furin or furin-like) to yield the individual proteins. This includes the N-terminally located structural proteins C (capsid), prM/M (membrane), and E (envelope) as well as the nonstructural proteins NS1, NS2A, NS2B, NS3 (protease and helicase), NS4A, NS4B, and NS5 (RNA-dependent RNA polymerase). Virus assembly takes place in the endoplasmic reticulum and first leads to the formation of immature virions that contain the proteins C, E, and prM (Fig. 1). These particles are transported through the cellular secretory pathway and, shortly before release, prM is cleaved by furin or a similar enzyme in the acidic compartment of the trans-Golgi network to yield mature and fully infectious virions [7] (Fig. 1). The E protein has the dual function of receptor-binding and fusion activity and is the major viral antigen that induces neutralizing antibodies and a protective immune response. It is essential for viral entry into cells, a process involving receptor-mediated endocytosis and low pH-triggered, E protein-mediated fusion of the viral membrane with endosomal membranes, leading to the release of the viral genome into the cytoplasm [8]. The most likely function of the prM protein in immature virions is to prevent these low pH-triggered events from occurring already during exocytosis in the acidic TGN compartment.

Section snippets

Structure of protein E

Limited trypsin digestion of purified virions yields a C-terminally truncated soluble dimeric form of protein E (Fig. 1) [9] that has been crystallized and used for structure determination by X-ray crystallography [10]. Its atomic structure—determined to a resolution of 2.0 Å—was quite unusual compared to that of other viral envelope proteins because it does not form a spiky projection but instead is oriented parallel to the virion surface (Fig. 2). The two monomeric subunits are associated in a

Structure of virions and subviral particles

Electron micrographs of mature TBE virions reveal smooth-surfaced particles without spiky projections, consistent with the structural organization of the E protein as the major constituent of the virion surface. Cross-linking experiments have provided evidence that the protein E dimers in the virion envelope are not spatially separated but rather form a network of densely packed subunits [13]. Some of the binding sites of neutralizing antibodies apparently require this packing and are probably

Functions of protein E

Protein E has the dual function of mediating receptor interaction and low pH-induced fusion in the endosome after uptake by receptor-mediated endocytosis. Based on structural considerations [10] domain III has been implicated in receptor-binding because of its IgC-like fold and the involvement of such structures in many cellular protein-protein interactions. Furthermore, the lateral side of this domain differs significantly between mosquito- and tick-borne flaviviruses. A tight turn in the TBE

Molecular basis of virulence

In animal models, significant differences with respect to neurovirulence and/or neuroinvasiveness have been described for different strains of TBE viruses [30], [31], [32]. It has also been suggested that the TBE virus variant circulating in the Far-Eastern part of Russia might be more pathogenic for humans than that found in other endemic areas because the mortality rate in this region has been reported to be significantly higher (up to 20%) than, for instance, in Europe (0.5–1%) [33]. So far,

Novel vaccine approaches

The currently available commercial TBE vaccines are inactivated whole virus vaccines adsorbed onto aluminum hydroxide (see Barrett and Kunz, this issue). Based on the cloning and sequencing of the viral genome, the knowledge of the atomic structure of the major protective immunogen (protein E) and the availability of recombinant expression systems, as well as the construction of infectious clones, a number of alternative new vaccine approaches have been investigated in recent years. A detailed

Molecular epidemiology

Flaviviruses have been subdivided into several serocomplexes based on cross-neutralization analysis with polyclonal immune sera [52] and more recently by phylogenetic analysis using either partial or complete genomic sequences [53], [54], [55], [56]. Several genetic lineages have been identified that consist of viruses transmitted by mosquitoes, ticks, and those without known arthropod vector [1]. The amino acid sequence identity in the E protein between each of the genetic lineages, which in

Summary and conclusions

The first phase of research on TBE virus research primarily focussed on its natural transmission cycle, its geographical distribution, the disease it causes in humans, and its impact on public health. Vaccine development was a major achievement, and the currently available vaccines, which are based on technological innovations for virus propagation in cell culture and its purification on a large industrial scale, proved to be highly effective for preventing TBE in humans (see Kunz and Barrett,

Acknowledgements

I thank Steven Allison for helpful advice and critically reading the manuscript.

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References (66)

  • K. Venugopal et al.

    Towards a new generation of flavivirus vaccines

    Vaccine

    (1994)
  • C. Schmaljohn et al.

    Evaluation of tick-borne encephalitis DNA vaccines in monkeys

    Virology

    (1999)
  • T.P. Monath et al.

    Clinical proof of principle for ChimeriVax: recombinant live, attenuated vaccines against flavivirus infections

    Vaccine

    (2002)
  • M.S. Marin et al.

    Phylogeny of TYU, SRE, and CFA virus: different evolutionary rates in the genus flavivirus

    Virology

    (1995)
  • V.N. Bakhvalova et al.

    Tick-borne encephalitis virus strains of Western Siberia

    Virus Res.

    (2000)
  • H. Holzmann et al.

    Molecular epidemiology of tick-borne encephalitis virus: cross-protection between European and Far-Eastern subtypes

    Vaccine

    (1992)
  • N. Chiba et al.

    Protection against tick-borne encephalitis virus isolated in Japan by active and passive immunization

    Vaccine

    (1999)
  • van Regenmortel CMFDHL, Beal MHV, editors. Family flaviviridae. Virus taxonomy: seventh report of the International...
  • Lindenbach BD, Charles MR. Flaviviridae: the viruses and their replication. In: David M, Knipe PMH, editors. Virology....
  • S. Rauscher et al.

    Secondary structure of the 3′-noncoding region of flavivirus genomes: comparative analysis of base pairing probabilities

    RNA

    (1997)
  • V. Proutski et al.

    Secondary structure of the 3′ untranslated region of flaviviruses: similarities and differences

    Nucl. Acids Res.

    (1997)
  • K. Stadler et al.

    Proteolytic activation of tick-borne encephalitis virus by furin

    J. Virol.

    (1997)
  • F.X. Heinz et al.

    The flavivirus envelope protein E: isolation of a soluble form from tick-borne encephalitis virus and its crystallization

    J. Virol.

    (1991)
  • F.A. Rey et al.

    The envelope glycoprotein from tick-borne encephalitis virus at 2 Å resolution

    Nature

    (1995)
  • C.W. Mandl et al.

    Antigenic structure of the flavivirus envelope protein E at the molecular level, using tick-borne encephalitis virus as a model

    J. Virol.

    (1989)
  • H. Holzmann et al.

    Characterization of monoclonal antibody-escape mutants of tick-borne encephalitis virus with reduced neuroinvasiveness in mice

    J. Gen. Virol.

    (1997)
  • F.X. Heinz et al.

    Chemical crosslinking of tick-borne encephalitis virus and its subunits

    J. Gen. Virol.

    (1980)
  • S.L. Allison et al.

    Synthesis and secretion of recombinant tick-borne encephalitis virus protein E in soluble and particulate form

    J. Virol.

    (1995)
  • J. Schalich et al.

    Recombinant subviral particles from tick-borne encephalitis virus are fusogenic and provide a model system for studying flavivirus envelope glycoprotein functions

    J. Virol.

    (1996)
  • Lindenbach BD, Charles MR. Flaviviridae: the viruses and their replication. In: David M, Knipe PMH, editors. Virology....
  • D.G. Maldov et al.

    Tick-borne encephalitis virus interaction with the target cells

    Arch. Virol.

    (1992)
  • E.V. Protopopova et al.

    Isolation of a cellular receptor for tick-borne encephalitis virus using anti-idiotypic antibodies

    Vopr. Virusol.

    (1997)
  • J. Kopecky et al.

    A putative host cell receptor for tick-borne encephalitis virus identified by anti-idiotypic antibodies and virus affinoblotting

    Intervirology

    (1999)
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