Viral Virulence

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Viral Virulence
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Relative nature of virulence
Equal High Virulence
Equal Low Virulence

• La Crosse Virus
• (-)ve strand RNA virus (Bunyaviridae)
• Same family as Hantavirus, but is arthropod-borne
• neurotropic
• Attenuated Clone B.5
• 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

Attentuation observed
Choice of host and route can dramatically
influence susceptibilty and resistance
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Factor influencing Virulence

• Viral Factors
• Viral Strain
• Route of Infection
• Dose of Virus
• Host Factors
• Species
• Age
• Genetic Susceptibility

4
Measures/Quantitation of Virulence

• Symptoms (e.g.)
• Paralysis (poliovirus)
• Jaundice (hepatitis)
• Rash (measles)
• Case/infection ratio
• Death/Survival
• of IU/PFU per LD50 (50 fatality in cohort)
• Pathogenic lesions

Rabbit Myxoma Virus
1951
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Biological control of wild rabbits co-evolution
of viral virulence and host resistance

• 1859, 12 European rabbits were introduced into an
  Australia farm by 1928 more than a billion
  rabbits (gt500/sq.mile) were ruining agriculture
• 1950, rabbit myxoma virus (gt99 mortality rate)
  was introduced by 1953, gt95 of rabbit
  population was eliminated, by 1955, rabbit
  population began to increase.
• Reasons
• Virulence of rabbit myxoma virus decreased
• surviving rabbits developed increased resistance
  
• changes in vector activity (mosquitoes) decreased
  efficiency of transmission

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Measures/Quantitation of Virulence

• Symptoms (e.g.)
• Paralysis (poliovirus)
• Jaundice (hepatitis)
• Rash (measles)
• Case/infection ratio
• Death/Survival
• of IU/PFU per LD50 (50 fatality in cohort)
• Pathogenic lesions

Rabbit Myxoma Virus
1951
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Experimental Manipulation of Viral Virulence

• Passage in Cell Culture
• Attentuation due to lack of host immune response

• Passage in Animals
• Adaptation to survival in host may be tissue
  specific

VIRULENCE
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Experimental Manipulation of Viral Virulence

• Passage in Cell Culture
• Attentuation due to lack of host immune response
• Vaccinia (small pox vaccine strain)
• 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
• T-cell line adapted virus
• More neutralization sensitive than primary
  strains grown in fresh PBMCs

• Passage in Animals
• Adaptation to survival in host may be tissue
  specific
• Yellow Fever
• Adaptation to neurovirulence by intracerebral
  passaging
• SHIV (chimeric Simian-Human Immunodeficiency
  Virus)
• Repeated passaging results in severely pathogenic
  virus (SHIV 89.6 to 89.6P) that causes CD4
  depletion and death within 6 months

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Selection of Attentuated Viral Variants

• Temperature Sensitive Variants
• Antibody-resistant virus
• Some neutralization resistant viruses can have
  increased attentuation in vivo
• Neutralization resistance can also lead to
  increased virulence
• Mutagenized viruses and selection

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Study of Attentuated Viruses

• Variant viruses (wt. vs attentuated) should be
  genetically pure
• Variant viruses should differ by as little as
  possible
• Variant viruses should differ only under
  non-permissive conditions i.e. there should be
  culture or innoculation conditions where
  replication is comparable

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Comparative pathogenesis(Virulent vs attentuated
viruses)

• Portal of entry
• Upper vs lower respiratory tract for influenza
  ability to replicate in lower respiratory tract
  (higher temp.) results in increased pathogenictiy
• Viremia
• Most viremic can be most pathogenic (not always)
  (poliovirus strains)
• Ability to produce peripheral viremia may affect
  end-organ pathology (La Crosse vs Tahyna virus)
• Neural Spread
• Target Organ
• Tropism
• relative pathogenicity for different tissues
• Evasion of host immune responses

12
Comparative pathogenesis(Virulent vs attentuated
viruses)

• Portal of entry
• Upper vs lower respiratory tract for influenza
  ability to replicate in lower respiratory tract
  (higher temp.) results in increased pathogenictiy
• Viremia
• Most viremic can be most pathogenic (not always)
  (poliovirus strains)
• Ability to produce peripheral viremia may affect
  end-organ pathology (La Crosse vs Tahyna virus)
• Neural Spread
• Target Organ
• Tropism
• relative pathogenicity for different tissues
• Evasion of host immune responses

Log10 PFU per ml
Log10 PFU per mg brain
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
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Comparative pathogenesis(Virulent vs attentuated
viruses)

• Portal of entry
• Upper vs lower respiratory tract for influenza
  ability to replicate in lower respiratory tract
  (higher temp.) results in increased pathogenictiy
• Viremia
• Most viremic can be most pathogenic (not always)
• Neural Spread
• 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
• relative pathogenicity for different tissues
• Evasion of host immune responses

avirulent
virulent
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Comparative pathogenesis(Virulent vs attentuated
viruses)
Bunyavirus

• Portal of entry
• Upper vs lower respiratory tract for influenza
  ability to replicate in lower respiratory tract
  (higher temp.) results in increased pathogenictiy
• Viremia
• Most viremic can be most pathogenic (not always)
• Neural Spread
• 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
• Bunyavirus neurotropism
• Neurotropism neuroinvasiveness
• Poliovirus enterotropism vs neurotropism
• Tropism
• relative pathogenicity for different tissues
• Evasion of host immune responses

LD50 based on IC injection
Log10 PFU per mg brain
Poliovirus
15
Comparative pathogenesis(Virulent vs attentuated
viruses)
HIV

• Portal of entry
• Upper vs lower respiratory tract for influenza
  ability to replicate in lower respiratory tract
  (higher temp.) results in increased pathogenictiy
• Viremia
• Most viremic can be most pathogenic (not always)
• Neural Spread
• 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
• Bunyavirus neurotropism
• Poliovirus enterotrpism vs neurotropism
• Tropism
• relative pathogenicity for different tissues
• Evasion of host immune responses

(Late)
(Early)
(50)
Clinical AIDS
Sexual Transmission
16
Comparative pathogenesis(Virulent vs attenuated
viruses)
Alveolar macs

• Portal of entry
• Upper vs lower respiratory tract for influenza
  ability to replicate in lower respiratory tract
  (higher temp.) results in increased pathogenictiy
• Viremia
• Most viremic can be most pathogenic (not always)
• Neural Spread
• 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
• Bunyavirus neurotropism
• Poliovirus enterotrpism vs neurotropism
• Tropism
• relative pathogenicity for different tissues
• Evasion of host immune responses
• LCMV (Clone 13 vs Armstrong strain)
• 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
• Armstrong strain leads to immunizing infection
  and viral clearance
• SIV/HIV (wt vs Dnef)
• Also, Sidney Blood Bank cohort example

Virus titer
Viremia (Log10 RNA copies/ml)
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Genetic Determinants of Virulence

• 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
• Consideration for recombinant life-virus vaccine
  devlopment
• 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)

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Virulence Genes of Cellular Origin

• Virokines
• Mimic the action of cytokines increases host
  cell proliferation and virus production
• Viroceptors
• Cytokine decoys
• Ab or Complement scavenger

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Virulence Genes of Cellular Origin

• Pox Viruses
• VCP (Vaccinia Complement Control Protein)
• Abrogates complement mediated atttack on viral
  infected cells
• Homolog of C4-BP that inactivates C4b, a critical
  player in the complement cascade
• TNF Viroceptors
• 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
• TH2 cytokine which suppresses Th1 (cell-mediated)
  immunity
• Mousepox engineered to express IL-4 becomes
  extremely virulent (Super-pox)

• Herpesviruses
• gE/gI glycoprotein can act as Fc receptors
  prevent effector functions of antiviral
  antibodies produced by the host

20
Science 2001 Jan 26291(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.
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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?? Bioterr
orism agent??
Mousepox-sensitive (BALB/c)
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

• IL-4 is a Type 2 cytokine
• enhances humoral immunity