Page 56 - TBE_Book_V2_2019

Chapter 2b: The molecular and antigenic structure of TBEV

Structure and Functions of E suggested that HS is not required for the
infection of CHO cells, and that one or more
The TBEV E protein has at least two essential other receptors are required for virus entry into
functions in the viral life cycle (Figure 2), these cells.
consistent with its prominent presentation at
the viral surface. It is responsible for inter- Since the structure determination of E, the
immunoglobulin-like domain III (Figure 1) has
actions with attachment factors and/or entry
been hypothesized to be a site of receptor
receptors at the plasma membrane of target
cells, and it mediates viral membrane fusion interactions not only for TBEV but for
1,69
after cellular uptake by receptor-mediated flaviviruses in general. This was primarily
based on the fact that a number of mutations
endocytosis. While TBEV membrane fusion has
affecting flavivirus virulence were concentrated
been studied in great detail, the search for viral
in this domain and that the so-called FG loop is
receptors is still quite elusive, reminiscent of
the situation described for flaviviruses in enlarged to contain an RGD sequence in some
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general. mosquito-borne flaviviruses , which is a
characteristic ligand-binding motif for members
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Cell attachment and receptors of the integrin family of cell surface receptors.
Experiments with recombinant domain III of
Several sets of experiments have provided Langat virus (a close relative of TBEV) have
evidence that TBEV can use negatively charged revealed that its addition to cells before
glycosaminoglycans (GAGs) such as heparan infection resulted in a somewhat decreased
sulfate (HS) as an attachment factor in certain virus growth, which was interpreted as evidence
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cells. 53,63 Passaging of a virus isolate from ticks that DIII is involved in receptor binding. So far,
in BHK-21 cells resulted in the accumulation of however, there is no information as to possible
mutations that were distributed over a large interaction partners of DIII at the cell surface
part of the upper and lateral surface of E and further efforts will be necessary to get a
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including each of the three domains (Figure 1). more complete picture of TBEV receptor
Importantly, these mutations resulted in an interactions.
increase of positive charges at the viral surface,
increasing its affinity for BHK-21 cells. Growth of In general, it is believed that flaviviruses may
use different receptors in different tissues of the
the mutant viruses, but not the wild type, could
various invertebrate and vertebrate host
be inhibited competitively by heparin, confirm-
species involved in natural transmission. It was
ing their adaptation and dependence on GAG-
binding for entry. The increased affinity for recently shown for several flaviviruses (but not
GAGs was associated with a decrease in yet for TBEV) that not only the E protein but
also lipids of the viral membrane may bind to
virulence in a mouse model and may be a 73,74
cellular lipid receptors. They normally
general principle for attenuating flaviviruses. A
recognize apoptotic cells and control their
connection between an increased binding to 75,76
GAGs and attenuation was also observed for removal by phagocytes. Hijacking these
64-66 receptors by viruses to gain access to cells has
viruses of the JEV serocomplex and the 17D 41
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strain of the live yellow fever vaccine. therefore been designated apoptotic mimicry.
Virus entry and membrane fusion
The role of GAGs in TBEV entry was investigated
in greater detail using mutant CHO cells that are
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deficient in the synthesis of GAGs. The presence of acidotropic agents such as
NH4Cl or bafilomycin A1 early in infection had a
Interestingly, while virus binding to these
strong inhibitory effect on the replication of
mutant cells was much lower than that to
normal CHO cells, no difference was observed in TBEV, consistent with virus uptake by receptor-
terms of cell infection. It was therefore mediated endocytosis and the importance of an
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