Transcript Viral Virulence - University of California, Los Angeles

Viral Virulence
Relative nature of virulence
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La Crosse Virus
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Equal High
Virulence
(-)ve strand RNA virus
(Bunyaviridae)
Same family as Hantavirus, but is
arthropod-borne
neurotropic
Attenuated Clone B.5
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Reduced capacity to produce
viremia and cross the blood brain
barrier
Virulence revealed by intracerebral
innoculation, but only in suckling
mice
No difference in virulence with wt
strain when innoculated
subcutaneously in adult mice
Equal Low
Virulence
Attentuation
observed
Intracerebral infect ion
P FU per LD50
La Crosse
virus st rain
Subcu taneous infect ion
P FU per LD50
Suckling mice
Adult mice
Suckling mice
Adult mice
Virulent
Wild type
~1
~1
~1
>107
At tenuated
clone B.5
~1
>106
>105
>107
Choice of host and route
can dramatically influence
susceptibilty and resistance
Factor influencing Virulence
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Viral Factors
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Viral Strain
Route of Infection
Dose of Virus
Host Factors
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Species
Age
Genetic Susceptibility
SARS Mortality Rate
Ave.
95% CI
>60 years
55.0%
45.3 to 64.7%
<60 years
6.8%
4.0 to 9.6%
http://www.thelancet.com/journal/vol361/iss9368/full/llan.361.9368.early_online_publication.25595.1
Measures/Quantitation of Virulence
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Symptoms (e.g.)
 Paralysis (poliovirus)
 Jaundice (hepatitis)
 Rash (measles)
 Case/infection ratio
Death/Survival
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Study period
Wild type
1931-1954
0.7
~10,000
OPV
1961-1978
0.000062
1
# of IU/PFU per LD50 (50%
fatality in cohort)
Pathogenic lesions
Relative Neurovirulence
Score
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Virus
Paralyt ic rate
per 100
primary infections
Relative rates
Rabbit Myxoma Virus
1951
Virulence grade
Case fatality
Rate (%)
Mean survival
Time (days)
Percent
of isolates
I
II
IIIA
IIIB
IV
V
>99%
95%-99%
90%-95%
70%-90%
50%-70%
<50%
<13
14-16
17-22
23-28
29-50
NC
4%
18%
39%
25%
14%
1%
Biological control of wild rabbits:
co-evolution of viral virulence and host
resistance
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1859, 12 European rabbits were introduced into
an Australia farm; by 1928 more than a billion
rabbits (>500/sq.mile) were ruining agriculture
1950, rabbit myxoma virus (>99% mortality rate)
was introduced; by 1953, >95% of rabbit
population was eliminated, by 1955, rabbit
population began to increase.
Reasons:
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Virulence of rabbit myxoma virus decreased;
surviving rabbits developed increased resistance;
changes in vector activity (mosquitoes) decreased efficiency
of transmission
Measures/Quantitation of Virulence
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Symptoms (e.g.)
 Paralysis (poliovirus)
 Jaundice (hepatitis)
 Rash (measles)
 Case/infection ratio
Death/Survival

Pathogenic lesions
Relative Neurovirulence
Score
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# of IU/PFU per LD50 (50%
fatality in cohort)
Virus
Study period
Paralyt ic rate
per 100
primary infections
Wild type
1931-1954
0.7
~10,000
OPV
1961-1978
0.000062
1
Relative rates
Rabbit Myxoma Virus
1951
Virulence grade
Case fatality
Rate (%)
Mean survival
Time (days)
Percent
of isolates
I
II
IIIA
IIIB
IV
V
>99%
95%-99%
90%-95%
70%-90%
50%-70%
<50%
<13
14-16
17-22
23-28
29-50
NC
4%
18%
39%
25%
14%
1%
1959
Percent
of isolates
0%
1%
5%
10%
25%
59%
Experimental Manipulation of
Viral Virulence
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Passage in Animals
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Adaptation to survival in
host; may be tissue
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Passage in Cell Culture
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specific
VIRULENCE
Attentuation due to lack of
host immune response
Experimental Manipulation of
Viral Virulence
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Passage in Animals
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Adaptation to survival in host;
may be tissue specific
Passage in Cell Culture
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Yellow Fever
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Adaptation to neurovirulence
by intracerebral passaging
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SHIV (chimeric SimianHuman Immunodeficiency
Virus)
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Repeated passaging results in
severely pathogenic virus
(SHIV 89.6 to 89.6P) that
causes CD4 depletion and
death within 6 months
Attentuation due to lack of host
immune response
Vaccinia (small pox vaccine strain)
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MVA (Modified Vaccinia Ankara)
250 passages in Chick Embryonic
Fibroblast results in ability to infect
but not replicate in mammalian
cells
Results in loss of immune evasion
genes; more immunogenic vaccine
vector (?)
HIV
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T-cell line adapted virus
More neutralization sensitive
than primary strains grown in
fresh PBMCs
Selection of Attentuated Viral Variants
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Temperature Sensitive Variants
Antibody-resistant virus
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Some neutralization resistant viruses can
have increased attentuation in vivo
Neutralization resistance can also lead to
increased virulence
Mutagenized viruses and selection
Study of Attentuated Viruses
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Variant viruses (wt. vs attentuated) should be
genetically pure
Variant viruses should differ by as little as
possible
Variant viruses should differ only under “nonpermissive” conditions; i.e. there should be
culture or innoculation conditions where
replication is comparable
Comparative pathogenesis
(Virulent vs attentuated viruses)
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Portal of entry
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Upper vs lower respiratory tract for
influenza; ability to replicate in lower
respiratory tract (higher temp.) results
in increased pathogenictiy
Viremia
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Most viremic can be most pathogenic
(not always) (poliovirus strains)
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Ability to produce peripheral viremia
may affect end-organ pathology (La
Crosse vs Tahyna virus)
Neural Spread
Target Organ
Tropism
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relative pathogenicity for different
tissues
Evasion of host immune responses
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Portal of entry
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Upper vs lower respiratory tract for
influenza; ability to replicate in lower
respiratory tract (higher temp.) results
in increased pathogenictiy
Viremia
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Most viremic can be most pathogenic
(not always) (poliovirus strains)
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Ability to produce peripheral viremia
may affect end-organ pathology (La
Crosse vs Tahyna virus)
Neural Spread
Target Organ
Tropism
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relative pathogenicity for different
tissues
Evasion of host immune responses
Log10 PFU per
mg brain
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Log10 PFU per ml
Comparative pathogenesis
(Virulent vs attentuated viruses)
Tahyna virus actually replicates better than La Crosse
virus in brain, but inability to produce fatal encephalitis
after subcutaneous injection is due to lack of replication
in periphery
Comparative pathogenesis
(Virulent vs attentuated viruses)
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Spr ead of Infe ction
(Ne ur oblas tom a Ce lls )
120
Virulent (wt)
Avir ul ent ( RV 194-2)
100
Percent Infect ed Cells
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Portal of entry
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Upper vs lower respiratory tract
for influenza; ability to replicate in
lower respiratory tract (higher
temp.) results in increased
pathogenictiy
Viremia
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Most viremic can be most
pathogenic (not always)
Neural Spread
60
40
0
0
2
4
6
8
10
Log2 Viral Dilut ions
avirulent
virulent
IM injection of wt vs avirulent strain of
rabies virus (e.g. MAR variant RV 194-2)
results in equal speed of spread to
CNS, but once there, spreads more
slowly to contiguous neurons
Target Organ
Tropism
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relative pathogenicity for different
tissues
Evasion of host immune responses
80
20
Spr ead of Infe ction
(Baby Ham s te r Kide ne y-21 C ells )
120
Virulent (wt)
Avir ul ent ( RV 194-2)
100
Percent INf ceted Cells
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80
60
40
20
0
0
2
4
Log2 Viral Dilut ions
6
8
10
Comparative pathogenesis
(Virulent vs attentuated viruses)
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Portal of entry
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Upper vs lower respiratory tract for
influenza; ability to replicate in lower
respiratory tract (higher temp.) results
in increased pathogenictiy
Viremia
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Most viremic can be most pathogenic
(not always)
Neural Spread
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Target Organ
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IM injection of wt vs avirulent strain of rabies
virus (e.g. MAR variant RV 194-2) results in
equal speed of spread to CNS, but once there,
spreads more slowly to contiguous neurons
Bunyavirus
Bunyavirus neurotropism
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Neurotropism = neuroinvasiveness
Poliovirus enterotropism vs neurotropism
Tropism
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relative pathogenicity for different
tissues
Evasion of host immune responses
LD50 based
on IC injection
Log10 PFU per
mg brain
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Poliovirus
Ty pe 1
pol iovirus strain
TCD50
pe r ml
En te rotropism
TCD50 pe r po
ID50
Neurotropism
TCD50 pe r ic
PD50
Viru len t Mahone y
(C NS suspension )
106
103.3
(m on ke ys)
101.9
Attenuated LSc
(Tissue
cul ture
flui d)
107.6
~104
(hu mans)
>107.6
Comparative pathogenesis
(Virulent vs attentuated viruses)
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Portal of entry
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Upper vs lower respiratory tract for
influenza; ability to replicate in lower
respiratory tract (higher temp.) results
in increased pathogenictiy
Viremia
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Most viremic can be most pathogenic
(not always)
Neural Spread
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M-tropic
IM injection of wt vs avirulent strain of rabies
virus (e.g. MAR variant RV 194-2) results in
equal speed of spread to CNS, but once there,
spreads more slowly to contiguous neurons
Target Organ
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HIV
CD4+
CD4+
CD4+
Bunyavirus neurotropism
Poliovirus enterotrpism vs neurotropism
Tropism
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relative pathogenicity for different
tissues
Evasion of host immune responses
T-tropic
CCR5+ CXCR4+
Primary T cells
CCR5+
CXCR4+
Macrophages
T cell Line
(Early)
(Late)
Sexual
Transmission
(~50%)
Clinical AIDS
Comparative pathogenesis
(Virulent vs attenuated viruses)
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Target Organ
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IM injection of wt vs avirulent strain of rabies virus
(e.g. MAR variant RV 194-2) results in equal speed of
spread to CNS, but once there, spreads more slowly
to contiguous neurons
Alveolar macs
Virus titer
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Portal of entry
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Upper vs lower respiratory tract for influenza;
ability to replicate in lower respiratory tract
(higher temp.) results in increased
pathogenictiy
Viremia
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Most viremic can be most pathogenic (not
always)
Neural Spread
Bunyavirus neurotropism
Poliovirus enterotrpism vs neurotropism
Tropism
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relative pathogenicity for different tissues
Evasion of host immune responses
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LCMV (Clone 13 vs Armstrong strain)
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Clone 13 replicates better/faster in
macrophages; rapid destruction of macs
leads to atttenuation of antigen
presentation, suppression of immune
response and thus results in viral escape
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Armstrong strain leads to immunizing
infection and viral clearance
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SIV/HIV (wt vs Dnef)
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Also, Sidney Blood Bank cohort example
Viremia
(Log10 RNA copies/ml)
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Genetic Determinants of Virulence
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Mutant vs wt Clones
Attenuation or virulence can be due to changes in viral proteins or
UTR of viral genomes
Generally, increased number of mutations is correlated with
increased attenuation and reduced chance of reversion
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Consideration for recombinant life-virus vaccine devlopment
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SIV Dnef
Reversion to virulence does not necessarily require back
mutation; compensatory mutations in same protein (or even
different proteins) is possible
Attenuating mutations are generally host range alterations
(replication is affected only in some tissues, or cells)
Virulence Genes of Cellular Origin
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Virokines
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Mimic the action of cytokines; increases
host cell proliferation and virus production
Viroceptors
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Cytokine decoys
Ab or Complement scavenger
Virulence Genes of Cellular Origin
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Pox Viruses
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VCP (Vaccinia Complement
Control Protein)
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TNF Viroceptors
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Abrogates complement mediated
atttack on viral infected cells
Homolog of C4-BP that inactivates
C4b, a critical player in the
complement cascade
TNF is proinflammatory cytokine that
activates immune networks
Soluble TNF-receptor homology
encoded by poxviruses can TNF
secreted by host cell and dampen
subsequent immune response
(IL-4): “Super-”pox
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TH2 cytokine which suppresses Th1
(cell-mediated) immunity
Mousepox engineered to express IL-4
becomes extremely virulent (“Super”pox)
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Herpesviruses
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gE/gI glycoprotein can act
as Fc receptors; prevent
effector functions of antiviral
antibodies produced by the
host
Science 2001 Jan 26;291(5504):585
AUSTRALIA:
Engineered Mouse Virus Spurs Bioweapon Fears
Elizabeth Finkel
MELBOURNE, AUSTRALIA--The surprising virulence of a virus genetically altered to reduce rodent
infestations in Australia has raised alarm over whether such research could be hijacked to produce
biological weapons. In an unusual twist, those sounding the alarm are not environmental activists but
the scientists themselves. Despite their warning, it's not clear whether the unexpected result, which
turned a vector into a potent killer, could be duplicated in viruses that affect humans. But scientists say
it should serve as a warning to the community to be more aware of the potentially harmful
consequences of their work.
% Mortality in Immunized mice
Can Vaccinia (the attenuated version of smallpox
that is used in smallpox vaccine) engineered to
express IL-4 become a super smallpox--overcome
people who has already been vaccinated??
Wt
IL-4+
Balb/c
0
60
C57BL/6
0
60
Bioterrorism agent??
Immunized with attenuated
wt mousepox strain
Mousepox-IL-4+
IL-4 is a Type 2 cytokine
 enhances humoral immunity
Mousepox-sensitive
(BALB/c)
Mousepox-sensitive
(BALB/c)
Mousepox-resistant
(C57BL/6))
Mousepox-resistant
(C57BL/6))
Early activation of virus-specifc CTLs
Production of high levels of type 1 cytokines
IL-2, IL-12, IFN-g and TNF-a