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10.2217/17460794.2.2.183 © 2007 Future Medicine Ltd ISSN 1746-0794
REVIEW
Future Virol. (2007) 2(2), 183–192
183
Virology of the post-polio syndrome
Andreina Baj,
Salvatore Monaco,
Gianluigi Zanusso,
Elisa Dall’ora,
Laura Bertolasi &
Antonio Toniolo
†
†
Author for correspondence
Antonio Toniolo, MD
University of Insubria
Medical School, Laboratory of
Medical Microbiology,
Viale Borri 57, 21100
Varese, Italy
Tel.: +39 033 227 8309;
Fax: +39 033 226 0517;
antonio.toniolo@
uninsubria.it
Keywords: antivirals,
diagnosis, enteroviruses,
nucleic acid detection,
pathogenesis, picornaviruses,
viral antigens,
virus persistence
The three poliovirus serotypes (PVs) cause acute paralytic poliomyelitis. Decades after
being hit by polio, survivors may develop a condition known as post-polio syndrome (PPS).
PPS is characterized by extreme fatigue, progressing muscular weakness and chronic pain.
The pathogenesis is unclear and, thus, empirical therapies are employed. PVs are known to
be able to persist in infected host cells both in vitro and in vivo. The understanding of PV
genomes has made it possible to set up sensitive and specific molecular tests capable of
detecting minute amounts of virus in samples from PPS patients. Current data indicate that
complete PV genomes (or genomic fragments) remain present, decades after acute
paralysis, in the CNS of these patients. Virus persistence is hypothesized to bring about
chronic inflammation, immune-mediated injury and decreased expression of neurotrophic
factors. Establishing a pathogenetic link between PV persistence and PPS would be
extremely relevant to the development of an etiologic therapy aimed at virus eradication.
Post-polio syndrome
The three poliovirus serotypes (PVs) cause acute
paralytic poliomyelitis. Partial or fairly complete
neurological and functional recovery follows the
acute episode and a phase of neurological and
functional stability ensues. Post-polio syndrome
(PPS) may develop after 15–40 years of stability
in 20–78% of polio survivors. The syndrome is
characterized by new weakness, extensive fatigue,
progressing muscular weakness, new muscle
atrophy, chronic pain and cold intolerance
[1].
The pathogenesis of PPS is unclear, thus
empirical therapies are employed.
PPS is likely due to distal degeneration of
enlarged post-poliomyelitis motor units
[2]. Per-
sistence of PVs in the CNS has long been sus-
pected. Available data indicate that PV
persistence in PPS patients is a rather common
event
[3]. We will review this evidence to analyze
the possible link between PV persistence and dis-
ease progression. A perspective on the use of
antiviral drugs is also presented.
Biological properties & current
taxonomy of polioviruses
The three PV serotypes belong to the Enterovirus
genus of the Picornaviridae family. Virions are
nonenveloped icosahedral particles, 28 nm in
diameter. These agents are resistant to lipid sol-
vents and acidic pH. Based on neutralization tests,
three PV serotypes are distinguished. The genome
consists of a single-stranded, positive sense RNA
of approximately 7.4 kb, with a 22-aa virus-
encoded protein (3B, VPg) covalently linked to
the 5´ end
(Figure 1). The 5´ nontranslated region
(~740 nucleotides) has a complex secondary
structure representing the internal ribosome
entry site (IRES). The single open-reading frame
encodes a polyprotein of approximately 2200
amino acids that is processed to yield four capsid
proteins (VP1, -2, -3 and -4), a protease 2Apro,
proteins 2B and 2C, the 3BVPg precursor
(3AB), the major viral protease (3Cpro), and the
RNA-dependent RNA polymerase (3Dpol). The
3´ nontranslated region (
∼70 nucleotides) contains
a poly-A tail of variable length
[4].
Human enteroviruses (HEVs; at least 92 sero-
types) have recently been reclassified, based largely
on genome sequence and IRES structure
[5]. As
shown in
Table 1, the Enterovirus genus consists of
five species: PV, HEV-A, HEV-B, HEV-C and
HEV-D.
Mankind is assumed to be the only natural res-
ervoir of human enteroviruses. These agents are
able to persist for months in the environment
(e.g., sewage, surface and ground waters, espe-
cially in the presence of organic matter). Person-
to-person transmission occurs mainly by the
fecal–oral route. Thanks to vaccination campaigns
started in 1955, new poliomyelitis cases are now
reduced to less than 2000 per year worldwide.
PV receptors & tropism
The immunoglobulin (Ig) superfamily surface
molecule CD155 is known as human poliovirus
receptor (hPVR) and is believed to represent the
sole cellular binding molecule conferring suscep-
tibility to PVs
[6]. CD155 is a ligand for the
DNAX accessory molecule-1 (CD226) one of
the activating receptors of natural killer cells
[7].
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Simian homologs have been reported in mon-
keys (sPVR)
[8] and are the specific determinants
of PV susceptibility of anterior horn motor neu-
rons in these species
[9]. Though wild-type mice
are not susceptible to PVs, transgenic mice
expressing hPVR develop poliomyelitis-like
paralysis, especially upon intracerebral inocu-
lation
[10]. Additional determinants that regulate
PV expression in various cell types include
IRES-mediated translation
[11], the α/β inter-
feron response
[12], expression of intracellular
factors (e.g., eukaryotic translation initiation
factors 4G
[13]), the La autoantigen that stimu-
lates PV translation in rabbit reticulocyte lysates
and improves accuracy of initiation codon selec-
tion
[14], the poly(A)-binding protein that is a
target for the viral 2A protease
[15] and the prion
protein (PRNP) gene
[16].
In acute infection, the mode of CNS invasion
by PVs is still a contentious issue. Entry may
occur through several different routes:
• Transport across the blood–brain barrier
• Transport via infected leukocytes
• Retrograde axonal transport through peripheral
nerves
[2]
CD155-positive cells are thought to represent
the principal targets. Successful PV infection has
been demonstrated in cultured fetal human brain
cells
[17]. In this model, cells of the neuronal line-
age (but not glial cells) appeared to support both
acute and persistent infection. More recently, it
has been shown that human macrophages and
dendritic cells retain CD155 expression in vitro
and can be successfully infected by PV-1
[18].
This observation may help us understand one of
the entry modes of the virus into the CNS during
acute infection. It also suggests that inflamma-
tory cells can contribute to chronic CNS
infection by acting as virus reservoirs.
PV-induced cytopathology
In order to persist in infected hosts, viruses
tend to maintain apoptotic homeostasis and
evade the immune response. PVs are highly
cytolytic in permissive cells
[19]. Cell death
occurs through the direct damage of cell struc-
tures operated by both viral proteases and
structural viral proteins (e.g., cleavage of
microtubule proteins
[20] and leakage of
lysosomal contents). When PV reproduction is
hindered by restrictive conditions, cell death
occurs through the activation of apoptotic
pathways. Activation of caspases by
protease 3C, protein 2C and VP3 has been
documented
[21]. Viral replication and CNS
injury in PV-infected mice are associated with
the apoptotic response. Antiapoptotic mecha-
nisms are, however, activated in infected cells:
protein 2B hinders calcium-mediated signal-
ing, protein 3A inhibits cytokine secretion
[19]
Figure 1. Genomic RNA of polioviruses.
Genes and gene products are shown.
UTR: Untranslated region; VP: Viral protein.
VPg
740 7400
AAA
3´ UTR
3
D
3
C
3
B
3
A
2
C
2
B
2
A
VP
1
VP
3
VP
2
VP
4
5´ UTR
Nonstructural proteins
Structural proteins
Viral polymerase
Protease
Binds to 5´UTR (VPg)
Membrane permeabilization
Viral encapsidation
Viroporin
Protease
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and expression of tumor necrosis factor-related
apoptosis-inducing ligand receptor is down-
regulated
[21,22]. A specific antiapoptotic gene
product (protein L
•
) has been detected in the TO
strain of Theiler virus, a mouse-specific neuro-
tropic agent that causes persistent demyelinating
disease
[23]. Postmortem studies of poliomyelitis
patients performed over 50 years ago found
neuronal cytolysis in spinal anterior horns, poste-
rior hypothalamus, thalamic nuclei, putamen and
globus pallidus. Severe alterations in the mid-
brain reticular formation were observed even in
clinically mild cases
[24,25]. Neuronal cyto-
pathology has been documented in experimen-
tally infected mice
[26] and in rare postmortem
exams of PPS patients
[27].
Persistent PV infection (in vivo & in vitro)
For a virus to establish persistent infection, it
must be able to limit its cytopathic effects,
maintain its genome within host cells over time
and avoid elimination by the host immune sys-
tem. Chronic PV infection has been diagnosed
in individuals with profound immune defects
(either hypogammaglobulinemia or common
variable immunodeficiency)
[28–30]. Infected
patients have been shown to produce large
amounts of virus for years. Persistent infection
in vitro has been investigated, especially in
neuroblastoma cells
[31] and cultured fetal neu-
ronal cells
[17]. However, persistence is not a
peculiar property of PVs, but is in fact a rather
common event in enteroviral infections. For
instance, chronic infection of cultured human
glomerular mesangial and vascular endothelial
cells has been documented
[32,33]. Persistently
infected cells, in vitro, secrete a variety of
cytokines and growth factors that modulate cell
behavior (e.g., motility, adhesion and pro-
liferation) and stimulate the inflammatory
response
[34]. In our hands, infection with PV-1
of primary cultures of human skeletal
myoblasts was associated with the upregulation
of pro-inflammatory cytokines (interleukin
[IL]-1,-12, tumor necrosis factor-
β) and
cytokines supporting humoral immunity
(IL-4,-5 and -15), as well as with increased
expression and response to
α- and β-chemo-
kines
[Unpublished Observation]. Several of the above
cytokines are also upregulated in inflammatory
bowel diseases
[35] and in enteroviral infections
of human pancreatic islets
[36]. In persistently
infected cultures, only a small percentage of
cells appear to express viral antigens at any
given time and virus titers are usually low
(≤10
3
plaque forming units [PFU]/ml). Trans-
mission of the virus possibly occurs at intercel-
lular junctions. In culture, neuroblastoma cells
supporting persistent PV infection are charac-
terized by the expression of mutated forms of
CD155
[31]. In persistently infected cells,
immunofluorescence showed that expression of
viral proteins is localized into the nucleus
instead of the cytoplasmic compartment
[Unpub-
lished Observations]
. Thus, it appears that the differ-
ential cleavage of nucleus-localized proteins and
transcription factors (together with the global
repression of cellular transcription) could favor
PV persistence versus host-cell apoptosis
[19].
On the other hand, the high mutation rate of
positive-strand RNA viruses is known to pro-
duce the viral quasispecies (i.e., a group of
interactive variants) that play a fundamental
role in host adaptation and neurotropism
[37]. It
is also speculated that the innate defenses (e.g.,
the interferon system) reduce viral pathogenic-
ity by limiting the production of viral progeny,
and therefore the diversity of viral particles,
during virus spread to low-access sites, such as
the CNS
[38]. Genetic variation, together with
IL-10 production, may also represent an impor-
tant determinant of virus persistence in
Table 1. Taxonomy of the Enterovirus genus within the Picornaviridae family.
Genus Species (No. of serotypes) Serotypes
Enterovirus Poliovirus (3) PV serotypes 1, 2, 3
Human enterovirus A (17) CVA serotypes 2, 3, 4, 5, 6, 7, 8, 10, 12, 14, 16; EV-71, 76, 89, 90, 91, 92
Human enterovirus B (56) CVB serotypes 1,2, 3,4,5,6, CVA9
Echo serotypes 1, 2, 3, 4, 5, 6, 7, 9, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21,
24, 25, 26, 27, 29, 30, 31, 32, 33, EV-69, 73, 74, 75, 77, 78, 79, 80, 81, 82, 83,
84, 85, 86, 87, 88, 93, 97, 98, 100, 101
Human enterovirus C (13) CVA serotypes 1, 11, 13, 17, 19, 20, 21, 22, 24, EV-95, 96, 99, 102
Human enterovirus D (3) EV-68, EV-70, EV-94
CVA: Coxsackievirus A group; CVB: Coxsackievirus B group; Echo: Echovirus; EV: Enterovirus; PV: Poliovirus.
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humans, but genomic changes of viral isolates
responsible for persistent infection have only
been documented in a few cases
[28]. With the
use of gene amplification and EM studies, it was
shown that PV may persist in the spinal cord of
paralyzed mice for over 1 year. Infectious virus’s,
however, could not be recovered, indicating that
PV replication was restricted. Inhibition of virus
replication was ascribed to suppressed synthesis
of plus-strand RNA
[39]. It has been concluded
that, as in coxsackievirus models, PV RNA may
persist in a double-stranded form that may be
responsible for the continuous activation of the
IFN system
[39].
Immune response to PV antigens
Immunity to poliomyelitis is largely dependent
upon neutralizing antibodies, both after natural
infection and after inoculation of inactivated
vaccines. The antigenic structure of PVs has
been studied primarily with the use of mono-
clonal antibodies in neutralization experiments
with escape mutants. Four major epitopes were
identified, designated as antigenic sites 1–4
(AgS1–S4)
[40]. In serotypes 1 and 2, AgS1 is
thought to include amino acids 1091–1102;
AgS2 consists of residues 2164–2166,
2168–2170, 2270, 1221–1224 and 1226; AgS3
includes amino acids 3058–3060, 3071 and
3073; and AgS4 is represented by residues 2072
and 3076. In serotype 3, AgS1 is composed of
the residues 1089–1100; AgS2 is thought to
consist of amino acids 2164, 2166–2167, 2172
and 1221–1224, 1226; AgS3 includes residues
3058–3060, 3071, 3073, and 1286–1290;
AgS4 is a complex of amino acids 2072, 3076–
3077 and 3079
[41]. Neutralizing IgA antibod-
ies responsible for intestinal immunity are in
the dimeric form required for recognition by
the polyimmunoglobulin receptor-mediating
secretory antibody transport at the mucosal
level
[42].
Though the protective role of cell-mediated
immunity against PVs and enteroviruses is still
debated, the presence of PV-specific CD4
T cells in individuals vaccinated against polio
has been demonstrated
[43]. These cells are
thought to participate in the induction of
humoral responses and in activating inflamma-
tion. Recently, it has been demonstrated in vac-
cinates that dendritic cells and macrophages
stimulate PV-specific CD4 and CD8 T-cell
responses in vitro. Both CD4 and CD8 cells
appeared to secrete IFN-
γ in response to
Table 2. Detection of PV genomes in samples from PPS patients and negative controls (patients with stable
poliomyelitis or other diseases).
No. PPS patients Negative controls Ref.
Sample Positive/total (n) Region
amplified
Serotypes (ID by
amplicon sequencing)
Stable
poliomyelitis
Other
diseases
24 CSF 3/24 5’UTR CVB-1, CVB-4 0/24 0/36 [52]
Serum 0/24 0/24 0/36
7 Spinal cord 3/7 5’UTR CVB-4 [52]
Cerebral cortex 0/7
40 CSF 0/40 5’UTR 0/17 [53]
4/40 3D pol PV-1 0/17
37 Peripheral blood
mononuclear
cells
0/37 5’UTR 0/5 [53]
7/37 3D pol 0/5
10 Muscle 0/10 5’UTR [53]
10 CSF 5/10 5’UTR PV-1, PV-2, PV-3 0/23 2/23 [54]
5/10 VP1 0/23 0/23
20 CSF 13/20 5’UTR PV-1, PV-2, PV-3 3/7 0/20 [3]
11/20 VP1 0/7 0/20
VP1–2A 0/7 0/20
12 CSF 12/12 5’ UTR PV-1, CVB-1 0/16 This
study
VP1 PV-1 0/16
CVB: Coxsackievirus B; CSF: Cerebrospinal fluid; PPS: Post–polio syndrome; PV: poliovirus; VP: Viral protein.
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PV antigens and were shown to recognize and
kill targets infected with PV-1 via the per-
forin/granzyme B pathway
[43]. Thus, immu-
nity to PV vaccination appears to not only
involve the production of polyclonal neutraliz-
ing antibodies but also the stimulation of long-
term CD4 and CD8 memory/cytotoxic T-cell
responses. T-cell epitopes appear to concentrate
in conserved regions of capsid proteins
[44].
Finally, the inflammatory response triggered by
enteroviral infections depends on the expres-
sion of Toll-like receptors on target cells that
may contribute to chronic inflammation
[45].
Though a strong protective response occurs in
all enteroviral infections, these agents are now
viewed as an emerging cause of persistent viral
replication and chronic disease
[46].
Pathogenetic views of
post-polio syndrome
A number of pathways have been proposed to
explain the pathological changes associated with
PPS
[2]. As summarized in Figure 2, the most
widely accepted hypothesis is distal degeneration
of axonal sprouts in the enlarged motor units that
are known to develop in patients recovering from
acute paralytic poliomyelitis (Wiechers and Hub-
bell hypothesis). Loss of entire motor units is also
possible. The following processes are taken as
contributing or triggering factors:
• The normal aging process, overuse myopathy
and disuse muscular atrophy. It is possible that,
with increasing age, the physiological loss of
motor neurons is especially significant in
patients with an already reduced number of
motor units. Overuse is known to produce
increased weakness in partially denervated skel-
etal muscle
[47]. Disuse was shown to be asso-
ciated with muscle hypotrophy and weakness
in normal individuals. These consequences are
enhanced in PPS patients
[48].
• The viral persistence hypothesis. PV is an
extremely lytic virus, yet persistence has been
observed in animal and cell culture models as
well as in immunodeficient humans. Small
perimysial or perivascular lymphocytic infil-
trates have been observed in the muscle biopsy
of PPS patients, raising the possibility of
chronic active myositis
[49]. Lymphocytic infil-
trates, neuronal loss and active gliosis have
been reported in the spinal cord in deceased
PPS patients
[50]. Intrathecal production of
anti-PV antibodies (a possible sign of active
intrathecal infection) has been documented
by some investigators
[51], but could not be
confirmed
[3].
In support of the hypothesis, several studies
have reported the presence of portions of the PV
genome in tissues and in the cerebrospinal fluid
(CSF) samples of PPS patients. The data are
summarized in
Table 2. The prevalence of posi-
tive CSF samples ranged from 10 to 65% in dif-
ferent studies. Amplified fragments have been
partially sequenced, thus allowing the presump-
tive identification of the three PV serotypes in
different patients. In a few cases, presumptive
identification of cerebral blood volume-1 and -4
has been obtained. Serum samples and muscle
tissue were consistently negative. Peripheral
blood mononuclear cells were positive in seven
of 37 patients (19%). We studied a limited series
of patients diagnosed as having PPS according to
the criteria of the European Federation of Neuro-
logical Societies
[1]. PV genome fragments
(5´UTR and VP1 regions) were detected in all
cases (12 of 12). Amplicons were directly
sequenced. Upon alignment with deposited
enteroviral sequences, most cases were compatible
with infection by PV type-1.
Taken together, the reported findings indicate
that portions of PV genomes may persist for
years in the CNS of PPS patients. However, the
link between persistent infection and the slowly
progressive course of PPS remains unclear.
Figure 2. Current pathogenetic views of the
post-polio syndrome.
Pathogenic factors in the post-polio syndrome
and/or
- Loss of entire motor units
- Distal degeneration of axonal
sprouts in enlarged motor units
Distal degeneration hypothesis:
Cofactors:
- Aging: loss of motor neurons
- Overuse: muscle degeneration
- Disuse: muscle atrophy
Persistent viral replication:
- Virus-induced cell damage
- Continuous expression of
viral antigens
- Immunomediated injury
- Chronic inflammatory response
- Reduced production of
neurotrophic factors
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It has been speculated that the detection of
viral genomes in the CSF is merely incidental to
PPS and that virus persistence has no pathological
effect on neural cells. The majority of authors
have found no PV genomic sequences in the CSF
of control patients
[3,52–54, and Unpublished Data].
Persistence of PV genomes in the CNS is
hypothesized as causing chronic inflammatory
responses and producing pathological changes,
mediated either by cytotoxic or immune-mediated
injury. The decreased expression of neurotrophic
factors has also been invoked in PPS as well as in
other degenerative motor neuron diseases
[55,56].
At present, however, there is no evidence to sug-
gest a link between the persistence of genomic PV
sequences and the ‘new symptoms proper’ of PPS.
Virus persistence is likely to have been established
in the early phase of the disease and virus
sequences are also present in the ‘stable stage’.
Diagnostic virology
The profound knowledge of picornaviral genomes
that has been accumulating over the last decades
has been summarized into a single database
[101].
These and other bioinformatics data have allowed
significant progress to be made in the taxonomy
of these agents
[5] and have paved the way to set-
ting up sensitive and specific molecular tests that
are currently used to investigate the etiological
role of enteroviruses in disorders of the CNS,
heart and pancreas
[57–59]. While no role for
enteroviruses has been obtained in cases of lateral
amyotrophic sclerosis
[60], molecular tools have
repeatedly demonstrated the presence of PV
genome fragments in a variable proportion of PPS
patients. These tools are now almost ready for
practical diagnostics and will shortly allow com-
plete sequencing of the persisting PV genome
fragments. Sequence data are expected to reveal
Table 3. Potential antiviral treatments against enteroviral infections.
Target/action Compound Activity Ref.
Antiviral state Interferon-α Side effects. Not clinically evaluated
Virus neutralization Immunoglobulins Normal HuIg have been used in infected
neonates and immunocompromized hosts
with controversial results
Capsid-function inhibitors Disoxaril Acts synergistically with enviroxime to inhibit
PV-1 in cell culture
[67]
Pleconaril Broad spectrum and potent anti-EV and
anti-RV activity
Documented clinical activity in acute
poliovirus infection
[62]
Pirodavir Prototype of a novel class of broad-spectrum
antipicornavirus compounds. Weak activity
against PVs
[63]
BTA188 Activity against PV-1 strain Chat [63]
Inhibitors of genomic RNA
synthesis
Enviroxime Acts synergistically with disoxaril to inhibit
PV-1 in cell culture
[67]
Ribavirin Reduces infectious PV production by up to
10–5th-fold in vitro. Antiviral effect
correlates with mutagenic activity
[65]
siRNA Acts by silencing targeted genes through
RNase degradation of the siRNA-bound
sequence
[68]
5-nitrocytidine triphosphate Acts by inhibiting PV RNA-dependent RNA
polymerase
[66]
3C protease inhibitors Rupintrivir Broad spectrum anti-EV activity, even if
initiated several hours after infection.
[64]
Inhibition of ubiquitin-
proteasome pathways
PDTC Active against CVB and PV. Copper and zinc
are required for activity. PDTC treatment
leads to accumulation of short-lived proteins
in infected cells. The inhibitory effect may be
due to modulation of ubiquitination.
[69]
CVB: Coxsackievirus B; EV: Enterovirus; PDTC: Pyrrolidine dithiocarbamate; PV: Poliovirus; RV: Rhinovirus; siRNA: Small interfering RNA.
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whether complete or defective genomes are
present and how these agents can persist in hosts
that have developed strong immune responses as
judged, at least, by neutralizing antibody assays.
Anti-enteroviral drugs
Since enteroviruses may be responsible for a vari-
ety of life-threatening CNS and heart diseases, a
number of anti-enteroviral drugs are under
development. None have yet been approved for
clinical use. The reappearance of poliomyelitis in
countries unable to meet the eradication goals set
by the WHO, along with the emergence of PPS
in developed countries, has enhanced this interest
[40,61]. As summarized in Table 3, investigated
drugs are directed against different steps of enter-
ovirus replication: early virus–cell interactions
(receptor binding and/or uncoating), genomic
RNA synthesis, proteolytic cleavage of the viral
polyprotein, ubiquitin–proteasome pathways,
Executive summary
Poliomyelitis
• Polioviruses (PVs) belong to the Enterovirus genus and cause acute paralytic poliomyelitis, a disease that affects millions of people.
• Poliomyelitis cases are now reduced to less than 2000 per year worldwide thanks to the effective vaccines developed by Jonas Salk
and Albert Sabin.
The post-polio syndrome
• Post-polio syndrome (PPS) is a condition that strikes polio survivors anytime from 15–40 years after recovery from the initial attack.
New symptoms include fatigue, muscle weakness, muscle and joint pain, muscular atrophy and cold intolerance. PPS is produced
by further weakening of the muscles previously damaged by polio as well as of healthy muscle groups deteriorating after years of
overuse in an effort to compensate for paralytic units. In severe cases, survivors must again use the artificial lung.
• Like polio itself, there is no known cure for PPS.
• Currently, there are 10–20 million living polio sufferers worldwide.
Poliovirus persistent infection
• Chronic PV infection has been documented in individuals with profound immune defects; these patients may produce large
amounts of virus for years. Persistent infection has also been investigated in vitro and in mice. Persistent infection of cultured cells
leads to the increased expression of pro-inflammatory and other cytokines/growth factors. Transmission of the virus possibly
occurs at intercellular junctions.
• In acute infection, virus eradication is largely dependent upon neutralizing antibodies, but CD4 and CD8 T-cell responses have also
been detected. Toll-like receptors on target cells may contribute to the inflammatory response.
• Although a strong protective response occurs in infected people, different enteroviruses are now viewed as an emerging cause of
chronic disease.
Pathogenetic views of PPS
• Distal degeneration of axons in enlarged motor units and/or the loss of entire motor units is the critical pathogenetic event. Two
mechanisms contribute: aging, overuse myopathy and disuse muscular atrophy are viewed as co-factors; PV persistence has been
suggested by molecular, immunological and histopathological studies. So far, however, infectious PV has not been isolated from
PPS patients.
Molecular evidence for PV persistence in PPS patients
• Molecular methods have allowed amplification tests capable of detecting minute amounts of PV genomes to be set up. Among
PPS patients, the reported prevalence of positives ranges from 10 to 65% in different studies. PV genome fragments have been
shown to persist for decades in cerebrospinal fluid.
• Our studies succeeded in detecting PV genome fragments in 12 out of 12 cases and to attribute the majority of them to
type-1 PV.
Anti-enteroviral drugs
• A number of anti-enteroviral drugs are under development but none has been approved for clinical use.
• Of interest are a series of capsid inhibitors, protease inhibitors and inhibitors of the 3D RNA polymerase.
Conclusion & future perspective
• PPS is now the most prevalent motor neuron disease in developed countries. The process does not stop spontaneously.
• The disease requires combined treatments, but current therapies are of limited help.
• Since a variety of anti-enteroviral drugs may soon become available, the time seems ripe to re-consider the viral hypothesis of PPS
on the basis of molecular virology. Viral persistence is a slow process that may allow the administration of
prolonged therapies.
• When more virological data becomes available, clinical trials should be designed to evaluate the effects of long-term
administration of antivirals in virus-positive patients.
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future science group
and silencing viral genes through small inter-
fering RNA. Of particular interest are a series of
capsid inhibitors (pleconaril, pirodavir and
BTA188)
[62,63], protease inhibitors such as
rupintrivir
[64], and inhibitors of the 3D RNA
polymerase (ribavirin and 5-nitrocytidine
triphosphate)
[65,66].
Conclusion
PPS is now the most prevalent motor neuron
disease in developed countries. It is rarely fatal,
but most studies have documented a pro-
gressive decline in muscular strength and qual-
ity of life
[2]. The process does not stop
spontaneously. The disease requires combined
treatments but current therapies are of limited
help. The best-documented clinical trial based
on the use of normal Ig preparations showed
only marginal effects
[70]. Reliable methods for
PV detection and a detailed understanding of
PV persistence are required in order to develop
new immunological and/or antiviral treatments
for reducing the progression of neural and
muscular damage.
Future perspective
Since a variety of anti-enteroviral drugs are under
development, the time seems ripe to reconsider
the viral hypothesis of PPS on the basis of mole-
cular virology results. Better understanding of PV
persistence is required. Viral persistence is a slow
process that may allow the administration of pro-
longed therapies. It is known that infected cells
respond to the replicating viral genome by pro-
ducing a variety of cytokines, growth factors,
adhesion molecules, surface receptors and anti-
apoptotic mediators. In persisting CNS infec-
tions, exogenous genomes may propagate bi-
directionally along axons and synapses to previ-
ously uninfected tissues, thereby expanding the
initial insult. It is now possible to use sensitive
molecular methods to detect PV genome frag-
ments in patients manifesting PPS symptoms.
Thus, clinical trials could be designed to evaluate
the effect of long-term administration of antivirals
in virus-positive patients. Clinical methods are
already available to assess the treatment outcome
[70] and these can be supported by measurements
of viral load in CSF during therapy.
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Websites
101. Picornavirus information
www.picornaviridae.com
Affiliations
•Andreina Baj, MD
University of Insubria Medical School,
Laboratory of Medical Microbiology, Viale Borri
57, 21100 Varese, Italy
Tel.: +39 033 227 8585;
Fax: +39 033 226 0517;
andreina.baj@uninsubria.it
•Salvatore Monaco
, MD
University of Verona, Department of
Neurological and Visual Sciences, Policlinico GB
Rossi, Piazzale Scuro 10, 37134 Verona, Italy
Tel.: +39 045 812 4461;
Fax: +39 045 585 933;
salvatore.monaco@mail.univr.it
• Gianluigi Zanusso
, MD
University of Verona, Department of
Neurological and Visual Sciences, Policlinico GB
Rossi, Piazzale Scuro 10, 37134 Verona, Italy
Tel.: +39 045 807 4922;
Fax: +39 045 585 933;
gianluigi.zanusso@univr.it
• Elisa Dall’Ora
, MD
University of Verona, Department of
Neurological and Visual Sciences, Policlinico GB
Rossi, Piazzale Scuro 10, 37134 Verona, Italy
Tel.: +39 045 807 4922;
Fax: +39 045 585 933;
elisa.dallora@azosp.vr.it
• Laura Bertolasi
, MD
University of Verona, Department of
Neurological and Visual Sciences, Policlinico GB
Rossi, Piazzale Scuro 10, 37134 Verona, Italy
Tel.: +39 045 807 4922;
Fax: +39 045 585 933;
laura.bertolasi@azosp.vr.it
• Antonio Toniolo
, MD
University of Insubria Medical School,
Laboratory of Medical Microbiology,
Viale Borri 57, 21100 Varese, Italy
Tel.: +39 033 227 8309;
Fax: +39 033 226 0517;
antonio.toniolo@uninsubria.it
