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Journal of NeuroVirology, 8(suppl. 2): 66–74, 2002
c
°2002 Taylor & Francis ISSN 1355–0284/02 $12.00+.00
DOI: 10.1080/135502802901068000
Pathogenesis of Semliki Forest virus encephalitis
John K Fazakerley
Centre for Infectious Diseases, University of Edinburgh, Summerhall, Edinburgh, United Kingdom
This article provides a review of the pathogenesis of Semliki Forest virus (SFV)
encephalitis. In mice, outcome of infection varies according to age of the mouse
and strain of the virus and can include acute encephalitis, subacute demyeli-
nating meningoencephalomyelitis, and persistent subclinical central nervous
system (CNS) infection. All strains of virus are virulent in mice infected <12
days of age. The L10 strain is also virulent in mice >14 days age, whereas the
A7(74) strain is avirulent. The genetic difference between these strains maps
to the nsp3 gene. For A7(74) virus, age-related virulence correlates with ability
of CNS neurons to replicate virus and undergo apoptotic cell death. Immature
developing neurons support complete virus replication but as neuronal pop-
ulations and circuits mature in the postnatal brain, virus infection becomes
progressively restricted and nonproductive. This restricted replication can be
overcome by gold I compounds, which may function by inducing neuronal
dedifferentiation to a state permissive for virus replication. Biochemical path-
ways associated with membrane biogenesis may be an important determinant
of this effect. Infection of some developing neuronal populations results in
apoptosis, whereas infection of mature neurons results in persistent infection.
An active type-I interferon system prevents virus spread in extraneural tissues.
An initial high-titer plasma viremia is controlled by immunoglobulin M (IgM)
antibodies. Virus enters the brain across cerebral endothelial cells and initiates
scattered foci of perivascular infection. The blood-brain barrier is disrupted.
Neurons and oligodendrocytes are the cell types most frequently infected. In-
fectivity in the brain can be eliminated by IgG antibodies, though an active
T-cell response is required for virus elimination. Lesions of inflammatory de-
myelination require the presence of CD8+T lymphocytes and probably result
from destruction by these cells of virally infected oligodendrocytes. Journal of
NeuroVirology (2002) 8(suppl. 2), 66–74.
Keywords: alphavirus; demyelination; encephalitis; Semliki Forest virus; SFV
Strains of SFV
Semliki Forest virus (SFV), an alphavirus of the
Togaviridae family, is naturally found in sub-Saharan
Africa where it is spread principally by Aedes
africanus and Aedes aegypti mosquitoes. SFV is
most closely related to Chikungunya, Getah, and Ma-
yaro alphaviruses. SFV was first isolated in 1942
from a squash of 130 female, Aedes abnormalis
mosquitoes captured in the Semliki Forest in Uganda
(Smithburn and Haddow, 1944). The natural ver-
tebrate host remains unknown but infections have
been documented in horses, monkeys, and man. The
Address correspondence to John K Fazakerley, Centre for Infec-
tious Diseases, University of Edinburgh, Summerhall, Edinburgh,
EH9 1QH, United Kingdom. E-mail: John.Fazakerley@ed.ac.uk
Received 19 August 2002; accepted 29 August 2002.
virus was associated with an epidemic of equine
encephalitis in Senegal (Robin et al, 1974). In a
1987 survey in the Central African Republic, SFV
was isolated from 22 patients with fever, severe per-
sistent headaches, myalgia, and arthralgia (Mathiot
et al, 1990). Virus was also isolated from locally
caught Aedes aegypti mosquitoes. There is one re-
ported case of the death of a scientist working with
the Osterrieth strain of the virus (Willems et al,
1979). The patient had neurological disease and at
postmortem examination, a typical viral meningoen-
cephalomyelitis was observed. SFV was isolated from
both the cerebrospinal fluid and the brain. It is known
that the scientist was working with virus super-
natant from baby hamster kidney (BHK) cells but
the route and dose of infection and the genotype
of this virus are unknown. Seroconversion in labo-
ratory workers is common (Willems et al, 1979). SFV
SFV encephalitis
JK Fazakerley 67
is relatively stable in aerosols of tissue homogenates
(Benbough, 1969), although aerosols of purified virus
are rapidly inactivated (de Jong et al, 1976).
Various strains of virus have been used in lab-
oratories around the world and have been shown
experimentally to infect voles, mice, guinea pigs,
rabbits, and rats (Seamer et al, 1967a, 1967b; Bradish
et al, 1971). Different strains have been designated as
virulent or avirulent according to their virulence in
adult mice. The L10, V13, and Osterrieth strains and
the strain designated prototype are virulent (Bradish
et al, 1971; Glasgow et al, 1991). The A8, A7, A7(74),
and MRS MP 192/7 strains are avirulent (Bradish
et al, 1971; Henderson et al, 1970). Virulence in
mice is age-related because all strains are virulent
in neonatal and suckling mice. The L10 strain was
derived from the original isolate (Smithburn and
Haddow, 1944) by eight intracerebral passages
through adult and two through baby mouse brains
(Bradish et al, 1971). A7(74) was derived from the
AR2066 strain by seven passages through neonatal
mouse brain and colony selection on chick em-
bryo fibroblasts. AR2066 was isolated from Aedes
argenteopunctatus mosquitoes in Namancurra,
Mozambique (McIntosh et al, 1961). A panel of 15
E1 or E2 monoclonal antibodies raised to MRS MP
192/7 can distinguish between some of the virus
strains (Boere et al, 1984, 1986). Several series of
SFV mutants, including temperature-sensitive (ts)
mutants, have been described (Tan et al, 1969;
Keranen and Kaariainen, 1974; Hearne et al, 1987;
Barrett et al, 1980).
A number of strains including prototype, which
is related to L10, A7, and A7(74), have been cloned
and sequenced (Garoff et al, 1980; Takkinen, 1986;
Glasgow et al, 1994; Santagati et al, 1995, 1998;
Tarbatt et al, 1997; Tuittila et al, 2000). The molecu-
lar clone of the prototype strain, known as SFV4, has
been engineered to form a much used viral expres-
sion and vaccine system (Liljestrom and Garoff, 1991;
Tubulekas et al, 1997). Isolated SFV RNA is infec-
tious (Friedman et al, 1966). As with many positive-
stranded RNA viruses, studies of site-directed
mutants have proved to be a powerful approach to
mapping areas of the genome responsible for individ-
ual phenotypic characteristics. Among the SFV4, A7,
and A7(74) strains, there are multiple changes scat-
tered throughout the genome (Tuittila et al, 2000).
The difference in virulence between SFV4 and A7 or
A7(74) appears to be polygenic and changes in the
E2 gene, the 50untranslated region, and the carboxy-
terminal end of the nsP3 gene have been shown to
be determinants of virulence between these particu-
lar strains (Santagati et al, 1995, 1998; Tarbatt et al,
1997; Tuittila et al, 2000).
SFV replication
SFV is a relatively simple virus, it has an approx-
imately 12-kb genome of single-stranded positive-
sense RNA. Replication is via a negative-sense inter-
mediate that gives rise to full-length genomic RNA
and a subgenomic message, which represents the 30
third of the genomic RNA and contains the genes
for the viral structural proteins. Both the genomic
and subgenomic RNAs are capped and polyadeny-
lated. From 50to 30along the genomic RNA, there
are four nonstructural replicase proteins, designated
nsP1 to nsP4, followed by the capsid protein C and
the envelope glycoproteins E2, E3, and E1. Many
of the functions of the replicase proteins have been
elucidated. NsP1 is involved in capping the viral
RNAs and has methyl and guanylyl transferase ac-
tivities; it participates in initiation of negative-strand
RNA synthesis and targets the replicase complex to
membranes (Ahola and Kaariainen, 1995; Laakkonen
et al, 1994; Ahola et al, 1997; Peranen and Kaariainen,
1991). The large nsP2 protein has single-stranded
RNA-stimulated ATPase and GTPase activities, RNA
triphosphatase activity, RNA helicase activity, reg-
ulates synthesis of the 26S subgenomic RNA, is
involved in the cessation of negative-strand syn-
thesis, and contains a papain-like proteinase do-
main responsible for processing the nonstructural
polyprotein (Rikkonen et al, 1994; Vasiljeva et al,
2000; Gomez de Cedron et al, 1999; Suopanki et al,
1998; Merits et al, 2001). In infected cells, about half
the nsP2 synthesized is translocated to the nucleus
(Kujala et al, 1997; Peranen et al, 1990; Rikkonen
et al, 1992). The protein contains a nuclear-targeting
sequence that is present in all three cloned and se-
quenced strains of the virus. Disruption of this se-
quence has minimal effect on virus replication in
continuously cultured cells but severely attenuates
replication in mouse brain (Fazakerley et al, 2002).
The role of nsP3 is less well known. It is a phos-
phoprotein, with a role in the regulation of RNA
synthesis (Peranen et al, 1988; Wang et al, 1991).
NsP4 is the catalytic subunit of the RNA polymerase
(Keranen and Kaariainen, 1979). The polymerase
complex is associated with plasma membrane and
endo/lysosomes by specific interactions of nsP1 and
nsP2. SFV virions are enveloped and on budding
have a spike composed of three E1-E2 heterodimers.
Within the lipid envelope, the RNA is associated
with the capsid protein, which is in contact with the
C-terminal domain of the E2 protein. Budding of viri-
ons is driven by spike capsid interactions and does
not require the presence of full-length genomic RNA
(Suomalainen et al, 1992).
The receptor remains undetermined but virus en-
try and fusion are via the endosomal system and have
been much studied (Helenius et al, 1986). Fusion
of the viral and endosomal membranes is a func-
tion of the E1 glycoprotein and is dependent upon
the presence of cholesterol and reorganization of the
envelope spike at acid pH (Omar and Koblet, 1988;
Helenius et al, 1980; Kielian and Helenius, 1984;
Wahlberg and Garoff, 1992). Upon fusion, the capsid
is liberated into the cytoplasm. RNA replication is
SFV encephalitis
68 JK Fazakerley
associated with smooth membrane structures termed
cytopathic vacuoles (CPVs), types I and II (Grimley
et al, 1968; Grimley and Friedman, 1970; Kujala et al,
2001). These structures can be separated from other
subcellular structures on sucrose gradients, and bio-
chemical markers indicate they are derived from late
endosomes and lysosomes (Friedman et al, 1972;
Froshauer et al, 1988; Mehta et al, 1990; Kujala et al,
2001). Confocal microscopy and electronmicroscopy
(EM) studies reveal that the surfaces of CPV-I are cov-
ered by small invaginations termed spherules, which
contain the viral nsP’s and are the likely sites of vi-
ral RNA replication (Grimley et al, 1968; Kujala et al,
2001). In vitro studies of cells in continuous culture
have generally observed virus budding at the plasma
membrane. However, EM observations on brain sec-
tions from SFV-infected neonatal mice indicate that
in neurons, virions frequently bud into the lumen of
type-II vacuoles, which then fuse with the cell mem-
brane to release mature virions (Pathak et al, 1976).
In brain sections, clumps of electron-dense material
are observed throughout the cytoplasm early in in-
fection. These contain capsid protein and are proba-
bly RNA capsid complexes. They migrate to under-
lie membranes, including the plasma membrane at
sites of virus budding (Pathak et al, 1976; Pathak and
Webb, 1978, 1983a, 1988).
Neuroinvasion, CNS tropism,
and age-related virulence
Following intraperitoneal (IP) inoculation into adult
mice, SFV replicates in muscles, including skele-
tal, smooth, and cardiac muscle (Pusztai et al, 1971;
Amor et al, 1996). A high-titer plasma viremia is de-
tectable within 24 h. This usually falls after 48 h
and in the blood, virus is generally undetectable
by infectivity assay by 4 days (Pusztai et al, 1971;
Fazakerley et al, 1993). All strains that have been
studied (L10, A7, A7(74)) are neuroinvasive. The
much studied A7(74) and L10 strains have been
shown to replicate in and probably enter the brain
across cerebral endothelial cells (Pathak and Webb,
1974; SoiluHanninen et al, 1994), initiating perivas-
cular foci of infection (Fazakerley et al, 1993). It
is likely that other strains also enter the central
nervous system (CNS) by this route. Following in-
tranasal inoculation, infection of the brain via olfac-
tory nerves can also occur (Kaluza et al, 1987; Oliver
and Fazakerley, 1998). Once in the CNS, all strains
of the virus infect neurons and oligodendrocytes,
but not astrocytes, and only very rarely meningeal,
ependymal, or choroid plexus cells (Balluz et al,
1993; Pathak and Webb, 1983b).
From the original perivascular foci, the virulent
L10 strain spreads rapidly around the brain, pro-
ducing a fatal panencephalitis (Figure 1) (Fazakerley
et al, 1993). This occurs irrespective of the age of
the mice and so rapidly that the immune response
Figure 1 Autoradiographic images showing distribution of SFV
RNA by in situ hybridization in brains of 3- to 4-week-old BALB/c
mice at 2, 3, 4, and 5 days post inoculation (IP). Mice were inocu-
lated with 5000 PFU of SFV A7(74) or L10. One group of mice also
received 10 mg of sodium aurothiomalate (GSTM) IP 3 to 4 hours
prior to virus infection. Infection with SFV A7(74) results in dis-
crete scattered foci of perivascular infection first apparent here at
4 days post infection. These foci do not enlarge with time even in
SCID mice (not shown). Prior treatment with GSTM results in rapid
spread of A7(74) infection to produce a panencephalitis similar to
that seen with the L10 strain of the virus. Modified from Fazakerley
et al (1993) and Scallan et al (1999), with permission.
has no time to intervene. In neonatal and suckling
mice, many neurons are destroyed. In adult mice, at
the time of death, most infected neurons have a rel-
atively normal morphology and the exact cause of
death is unclear. Destruction of a vital group of neu-
rons, toxic levels of cytokines, neuronal dysfunction,
or extraneural pathology are possibilities. In contrast,
the dynamics of A7(74) infection of the CNS varies
according to age of the mice. In neonatal mice, A7(74)
virus spreads rapidly around the brain and infection
is, as with the L10 strain, rapidly fatal. As the mice
age, the A7(74) virus is less able to spread in the CNS,
and there is a sharp age-related virulence: mice in-
fected at 12 days of age or less die, whereas those
infected at 14 days of age or more survive (Fleming,
1977; Oliver et al, 1997; Oliver and Fazakerley,
1998). In the mature adult mouse CNS, A7(74) virus
has a restricted replication and remains confined to
perivascular foci (Figure 1) (Pathak and Webb, 1978;
Fazakerley et al, 1993; Oliver et al, 1997). This age-
related virulence is not a function of the maturity
of specific immune responses, as this virus also re-
mains confined to small, predominantly perivascu-
lar foci in athymic nu/nu mice or mice with severe
combined immunodeficiency (SCID) (Fazakerley and
Webb, 1987b; Fazakerley et al, 1993; Amor et al,
1996).
The A7(74) age-related virulence is a function of
the maturity of CNS cells. In the first two weeks
after birth, in the mouse a number of major matura-
tional events are ongoing in the CNS, these include
SFV encephalitis
JK Fazakerley 69
Figure 2 Distribution of SFV A7(74) RNA (brown staining) by in
situ hybridization at 40 h post infection in the brains of mice in-
oculated (IP) at (A) P4, (B) P8, and (C) P12. In (A), virus can be
observed to infect columns of interconnected neurons. These are
broader in the frontal and fore- and hind-limb areas and narrower
in the occipital cortex, consistent with the known banding patterns
of these functional motor and sensory neuronal groupings (Purves
et al, 1992). At this age (P4), there is widespread axonogenesis
and synaptogenesis occurring in these interconnecting columns.
This connectivity is progressively completed between P4 and P10
and is associated with a progressive curtailment in the number of
columns infected by the virus (Band C). In the animals inoculated
at P12 (C), only cells in deep layer VI of the cortex, the cingulated
gyrus, and the corpus callosum are infected (arrowheads). Modi-
fied from Oliver et al (1997), with permission.
axonogenesis, synaptogenesis, gliogenesis, and
myelination (reviewed by Fazakerley, 2001). The
A7(74) strain of SFV is able to replicate productively
in highly active maturing neurons undergoing
axonogenesis and synaptogenesis (Figure 2). As
these processes are completed, a number of major
changes occur that restrict virus replication (Pathak
and Webb, 1978; Fazakerley et al, 1993; Oliver
et al, 1997; Oliver and Fazakerley, 1998). Restriction
may be linked to shutdown of metabolic processes
required by the virus, for example, production and
transport of smooth membrane vesicles (Oliver
et al, 1997) or to up-regulation of inhibitory pro-
cesses, for example, antiapoptotic genes (Scallan
et al, 1997). Of relevance here is the changed
course of events following treatment of infected
mice with gold compounds. Treatment of adult
mice with sodium aurothiomalate and other gold I
compounds results in conversion of this normally
avirulent infection to a virulent infection (Allner
et al, 1974; Bradish et al, 1975; Mehta and Webb,
1982; Gates et al, 1984; Scallan and Fazakerley,
1999). The gold salt is transported to the CNS where
it induces smooth membrane production in neurons
Figure 3 A discrete lesion of inflammatory demyelination in a
white matter tract in the cerebellum. SFV A7(74), 18 days post
infection (IP), 5-week-old BALB/c mouse.
(Pathak and Webb, 1983a). In mice treated with gold
compounds, A7(74) virus replication is no longer
restricted in neurons but is productive and spreads
rapidly around the brain (Figure 1), resulting in a
panencephalitis characteristic of that observed with
the virulent L10 virus (Pathak and Webb, 1983a;
Scallan and Fazakerley, 1999).
Infection of neurons in the neonate can result in
death of specific neuronal populations by apopto-
sis (Allsopp et al, 1998), whereas infection of ma-
ture (adult) neurons is generally nondestructive and
can result in virus persistence (Fazakerley et al, 1993;
Amor et al, 1996). In the normal course of postnatal
development of the CNS, as immature neurons of the
neonatal mouse brain make their connections, they
become more resistant to apoptotic death, whereas
those neurons that do not form correct connections
die by this process (reviewed by Fazakerley, 2001).
In culture, SFV A7(74) and L10 both kill continu-
ously growing cell lines by apoptosis, but expres-
sion of antiapoptotic genes prolongs survival (Scallan
et al, 1997). SFV L10 and A7(74) also kill primary cul-
tures of mouse embryonic sensory neurons by apop-
tosis (Allsopp et al, 1998), but A7(74) does not de-
stroy primary cultures of neurons from adult mice
whereas L10 does. For renewable cell populations,
apoptotic cell death upon infection can be consid-
ered to be a highly successfully, altruistic antiviral
defense mechanism. For mature, nonrenewable, vital
cell populations such as neurons this would not be
the case, and selective pressures may have dictated
that mature neurons are a specialized case and do not
readily undergo apoptosis (Allsopp and Fazakerley,
2000; Fazakerley and Allsopp, 2001).
Taken together, the foregoing studies suggest the
following course of events: In the developing mouse
brain, both strains of SFV, L10 and A7(74), repli-
cate efficiently in immature neurons. It is likely that
SFV encephalitis
70 JK Fazakerley
these cells contain suitable membranes and biochem-
ical pathways for complete virus replication. Given
their susceptibility to apoptosis, some infected pop-
ulations of neurons undergo apoptotic cell death. As
neuronal circuits and cells mature in the first 2 post-
natal weeks, neuronal physiology changes such that
A7(74) replication becomes restricted with viral RNA
replication and protein synthesis but no, or minimal,
virus budding and spread of infection. These more
mature neurons are also less susceptible to apop-
tosis and the result, in the absence of specific im-
munity (SCID mice), is a persistent nondestructive
neuronal infection. Given that gold compounds re-
lieve the restriction on viral replication and induce
smooth membrane synthesis, and given the require-
ment for suitable membranes for virus replication
(CPVs, spherules) and budding, it seems likely that
a change in membrane synthesis, or associated bio-
chemical pathways, may be the age-related change
responsible for switching the outcome of A7(74) in-
fection from productive and destructive to restricted
and persistent. Given that replication of the L10 strain
is not restricted in mature neurons, the genetic locus
responsible for this phenotypic difference between
the L10 and A7(74) strains may be informative as to
the mechanisms involved. This difference appears to
reside in the carboxy-terminus domain of the nsP3
protein (Tuittila et al, 2000), but the function of this
remains unknown. It also remains unknown how SFV
triggers the apoptotic response in cells, including
neurons, and what changes occur on neuronal matu-
ration, at least in some populations of neurons, that
reduce susceptibility to apoptosis.
Apart from CNS disease, another interesting prop-
erty of some strains, for example, A7 but not L10, is
their ability to cross the mouse placenta and induce
abortions (Atkins et al, 1982; Milner and Marshall,
1984). The A7 ts22 mutant is teratogenic, inducing
skeletal, skin, and neural tube defects in develop-
ing fetuses (Mabruk et al, 1988, 1989). This is of
interest given the teratogenic effects of the related
rubella virus. As with most RNA viruses, SFV read-
ily generates defective interfering particles on high-
multiplicity passage in culture (Bruton and Kennedy,
1976; Barrett et al, 1984). In a series of experiments,
defective interfering (DI) particles administered ei-
ther intraperitoneally or intranasally at the same time
as virulent virus have been shown to inhibit virus
replication, delay the time of death, completely pro-
tect animals, or convert the infection from lethal to
persistent (Dimmock and Kennedy, 1978; Barrett and
Dimmock, 1984; Atkinson et al, 1986).
Role of immune response
Infection of SCID mice with SFV A7(74) results in
persistent CNS infection without any apparent neu-
ronal loss (Amor et al, 1996). In contrast, in im-
munocompetent mice, virus is eliminated from the
CNS. This comparison demonstrates two important
points, firstly there is no, or at most only rare, direct
A7(74) virus-induced death of infected cells in the
adult mouse brain, and secondly, specific immune
responses are required to eliminate this infection. In
immunocompetent mice, infectious virus cannot be
detected by plaque assay after day 8, by the more sen-
sitive ICLD50 after day 11, and by in situ hybridiza-
tion after day 14 (Suckling et al, 1978; Jagelman et al,
1978; Fazakerley et al, 1993). Reverse transcriptase–
polymerase chain reaction (RT-PCR) indicates that vi-
ral RNA can be detected at later time points (Donnelly
et al, 1997). After the clearance of detectable infectiv-
ity, from day 14 onwards, immunocompetent mice
develop lesions of inflammatory demyelination in
the white matter. These lesions are apparent through-
out the CNS (Suckling et al, 1978; Kelly et al, 1982).
No demyelinating lesions are observed in SCID mice
(Amor et al, 1996), despite CNS virus persistence.
Specific immune responses therefore have a role both
in clearing this infection and in the generation of
lesions of demyelination.
The roles of different effector functions of the im-
mune response have been determined over the years
by studying naturally arising mutant mice, genet-
ically engineered mice, mice treated with general
or selective immunosuppressive regimens, and by
adoptive transfer experiments. Following IP inocu-
lation of virus, plasma levels of type-I interferons
parallel those of the viremia (Bradish et al, 1975). In
mice with a genetic deletion of functional type-I inter-
feron receptors (Muller et al, 1994), IP infection with
SFV is fatal within 48 h, with widespread infection
of many organs and tissues (JKF, unpublished data).
Type-I interferons are therefore crucial in protecting
against widespread SFV infection in numerous tis-
sues and their absence leads to widespread infection
and death. The role of other cytokines has not been
investigated in detail, but interleukin (IL)-1α, IL-1β,
IL-3, IL-6, IL-10, and tumor necrosis factor (TNF)α
can be readily detected in the CNS following SFV
A7(74) infection (Morris et al, 1997).
Following IP inoculation of SFV, A7(74) BALB/c
mice produce a rapid and neutralizing serum im-
munoglobulin M (IgM) antibody response followed
rapidly by an IgG2a response and a much slower IgG1
response (Fazakerley et al, 1993). During the nor-
mal course of infection in immunocompetent mice,
the blood-brain barrier is leaky from days 4 to 10,
allowing passage of immunoglobulins (Parsons and
Webb, 1982a; SoiluHanninen et al, 1994). Antibody-
producing plasma cells can be detected in the brain
and intrathecal antibodies can be detected for months
post infection (Parsons and Webb, 1984). SCID mice,
which have no serum antiviral antibodies, have both
a persistent viremia and a persistent CNS infection
(Amor et al, 1996), whereas nu/nu mice control the
viremia but have a persistent CNS infection. The
nu/nu mice produce serum antiviral IgM but not IgG
(Suckling et al, 1982), suggesting that this may be
SFV encephalitis
JK Fazakerley 71
controlling the viremia but unable to eliminate the
CNS infection. That this is the case is confirmed by
resolution of the persistent viremia but not the CNS
infection on transfer of day 7 nu/nu mouse serum to
infected SCID mice (Amor et al, 1996). In contrast,
transfer of high-titer IgG sera from immunocompe-
tent BALB/c mice to SCID mice was able to abolish
both the viremia and titers of infectious virus from the
CNS (Amor et al, 1996). Whether this was complete
eradication of all viral sequences was not checked
by more sensitive techniques such as RT-PCR. Anti-
bodies are highly effective in protecting mice from
challenge with a lethal dose of SFV. A number of
B-cell epitopes have been mapped on the viral glyco-
proteins (Boere et al, 1984; Snijders et al, 1991) and
even a single monoclonal antibody directed to the
E2 envelope glycoprotein has been shown to protect
mice against a virulent infection (Boere et al, 1983).
Furthermore, a non-neutralizing anti-E2 monoclonal
has been shown to be able to clear A7(74) virus from
persistently infected SCID mouse brains (Amor et al,
1996). Combinations of linear B-cell epitopes and
B- and T-cell epitopes have been tested as potential
vaccine candidates (Snijders et al, 1991).
Following IP infection of BALB/c mice with SFV
A7(74), perivascular and infiltrating mononuclear
cells are apparent in the CNS from 3 days onwards
(Parsons and Webb, 1982b). Histopathological stud-
ies show that the areas of inflammatory infiltrates
correspond to the areas of infection (Subak-Sharpe
et al, 1993). Perivascular cuffs are a prominent fea-
ture of the neuropathology and are maximal be-
tween 7 and 10 days. Cerebral endothelial cells up-
regulate adhesion molecule expression after infection
(SoiluHanninen et al, 1997; Morris et al, 1997), and
some of these, ependymal cells, meningeal cells, most
infiltrating mononuclear cells and some parenchy-
mal cells with a dendritic morphology, are major
histocompatibility complex (MHC)-I+(Morris et al,
1997). The parenchymal MHC-I+cells form a network
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CD45/B220+B cells are also present in the inflamma-
tory lesions, including those showing demyelination.
Mice infected with SFV have good delayed-type
hypersensitivity responses to the virus (Kraaijeveld
et al, 1983), and T-cell epitopes for some hap-
lotypes have been mapped (Snijders et al, 1991,
1992a, 1992b). Adoptive-transfer studies in SCID and
nu/nu mice and studies in mice immunosuppressed
with total body irradiation, cyclophosphamide, cy-
closporine, or cycloleucine all demonstrate that
T-cell responses are pathogenic and are required to
generate the lesions of demyelination (Fazakerley
and Webb, 1987a, 1987c; Amor and Webb, 1987;
Amor et al, 1996). Depletion of CD8+cells with a
monoclonal antibody greatly reduces demyelination,
whereas depletion of CD4+cells reduces the extent
of inflammation but not of demyelination (Subak-
Sharpe et al, 1993). Given the requirement for CD8+
cells for demyelination and the tropism of A7(74)
virus for oligodendrocytes, the most likely mecha-
nism of demyelination would seem to be a CD8+
T cell–mediated destruction of virally infected oligo-
dendrocytes, though this remains to be proven. Mice
infected with SFV A7(74) generate CNS antigen-
specific autoantibodies, are more sensitive to the
induction of experimental allergic encephalomyeli-
tis, and have T cells that cross-react between vi-
ral and CNS epitopes (Amor and Webb, 1988;
Mokhtarian and Swoveland, 1987), though any role
for autoimmune responses in the generation of
SFV A7(74)-induced demyelinating lesions remains
unclear.
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