Thurs 10302003

1
Viral Pathogenesis

• Thurs 10/30/2003

2
Viral Pathogenesis

• How a virus causes disease in its host
• Virulence the capacity of a virus to cause
  disease
• i.e. a measure of the severity of the disease and
  agent produces
• A highly pathogenic virus/disease does not
  necessarily have a severe presentation
• Involves a study of the virus replication
  strategy, the host immune system and the
  organs/tissues of the host

3
Virus Dissemination - Entry

• Common entry sites
• mucosal linings of the respiratory, alimentary,
  and urogenital tracts,
• conjunctiva/cornea of the eye
• Skin
• or, via a needle wound or bite

The respiratory route is probably the most common
From Flint et al Principles of Virology ASM Press
4
Entry via the respiratory tract

• Surface area of the human lungs 140 m2
• Many mechanical defences
• inc. the mucociliary blanket, (ciliated cells,
  mucus-secreting cells,
• Secretory IgA is very important in protecting the
  host
• Alveolar macrophages also very important
• Transmission is via
• Aerosols e.g. influenza
• Direct contact e.g. rhinovirus
• Infection may be localized, or may spread

5

• influenza neuraminidase (NA) may be important in
  digesting the mucus layer

From Flint et al Principles of Virology ASM
Press
6
Entry via the alimentary tract

• Viruses that enter via this route must be
  resistant to harsh environmental conditions
• The intestinal surface is covered with columnar
  villous epithelial cells, packed with microvilli.
  This is a formidable barrier, but adenovirus and
  calicivirus replicate here.

Examples of alimentary pathogens Systemic -
Enterovirus Reovirus Adenovirus Localized- Coron
avirus Rotavirus
Ciliary blunting very important in pathogenesis
7
Reoviruses - different routes of entry at
different sites in the host

• Reoviruses need to be proteolytically cleaved to
  rupture outer capsid during virus entry
• Respiratory strains must use the lysosomal
  hydrolases (e.g. cathepsins)
• Alimentary strains can use the gut proteases
  (trypsin, chymotrypsin)
• these viruses may enter via the plasma membrane,
  i.e are pH-independent

Membrane rupture is still unclear in both cases
8
M cells

• the carrier of secretory IgA via transcytosis.
• The Achilles heel of the gut?
• - Used by many pathogens for entry , eg reovirus,
  coronavirus, HIV (anal intercourse)

From Flint et al Principles of Virology ASM Press
9
Transcytosis of HIV

• The role of M cells in HIV invasion may (in
  reality) be questionable
• M-tropic HIV clearly transcytoses across the
  epithelium

10
Virus spread

• Infections can be localized, or can spread beyond
  the initial site of replication (a disseminated
  infection).
• With many organs involved the infection becomes
  systemic

From Flint et al Principles of Virology ASM Press
11
Mousepox - classic work of Fenner
The primary viremia often yields low virus titer,
but this can be followed by secondary viremia,
with high virus load
12
Polarized cells and virus spread

• At a cellular level directional release is very
  important for virus spread
• Apical release is back to where it all started
• Basolateral release is inwards away from lumenal
  defences
• Apical / basolateral targeting is mediated by
  virus glycoproteins that determine the site of
  budding
• classic e.g. -
• Influenza HA (apical)
• VSV G (basolateral)

tight junction
HA G
TGN
Modified from Flint et al Principles of Virology
ASM Press
13
Polarized cells, virus entry and the outcome of
virus infection

• Classic e.g. - influenza apical entry, VSV
  basolateral entry
• Herpes viruses are traditionally thought to enter
  and fuse at the cell surface
• However, in retinal epithelial cells HSV and CMV
  enter via endocytosis (but pH-independent)
• One idea is that the route of entry can determine
  latent vs. lytic replication

14
Role of epithelial cell polarity in Sendai virus
infection

• - Sendai virus (paramyxovirus)
• - wt virus buds apically - F (glycoprotein) is
  normally delivered to apical PM
• - Pantropic mutant (F1-R) has mutation(s) in the
  M (matrix protein) that disrupt the cell
  microtubule network and lead to a loss of
  polarity
• F now becomes uniformly distributed and virus can
  bud from the basolateral surface and spread
  into the host
• F1-R also had enhanced cleavability of F (see flu
  HA)

15
Epithelial cells and virus receptors

• Several virus receptors are cell adhesion
  molecules
• The receptors are theroretically not accessible
• May depend on epithelial trauma

From Spear Dev Cell 3462-464 (2002)
During adenovirus infection, an excess of the
fiber protein is released, which interacts with
the CAR receptor - disrupts epithelial integrity
and facilitates virus spread
From Walters et al Cell 110789-799 (2002)
16
Hematogenous spread
Disseminated infections often follow entry into
the bloodstream - hematogenous spread.

• The presence of virus in the blood viremia
• Either free in the blood or within lympocytes
• Virus moves from the epithelium to the blood via
  lymphatic system

Active viremia is produced by virus
replication Passive viremia is caused by
injection, without replication
From Flint et al Principles of Virology ASM Press
17
Viral virulence

• The ability of a virus to cause disease in an
  infected host
• A virulent strain causes significant disease
• An avirulent or attenuated strain causes no, or
  reduced, disease
• Virulence depends on
• dose
• virus strain
• inoculation route
• host factors

Virulence is a relative property Quantitation of
viral virulence (eg LD50) can be used, but not to
compare different viruses
18
Alteration of viral virulence - I

• Viral genes that affect virulence may
• 1) affect the ability of the virus to replicate
• 2) enable the virus to spread in the host or
  between hosts
• 3) defeat the hosts defense mechanism
• 4) produce gene products that are directly toxic

Mutations in these genes may have little or no
effect on replication in cell culture - they are
non-essential genes
e.g HSV-1 DNA polymerase affects replication in
terminally differentiated neurons, but not in
cell culture
From Flint et al Principles of Virology ASM Press
19
Alteration of viral virulence - II

• In terminally differentiated neurons, there are
  limited pools of dNTPs for DNA viruses to
  replicate efficiently
• HSV encodes enzymes such as thymidine kinase, and
  ribonucleotide reductase, which are important in
  all cells, but especially in neurons
• Mutations in TK and RR can severely effect
  neurotropism

20
Alteration of viral virulence - III

• Non-coding sequences can affect virulence
• 3 serotypes of the Sabin poliovirus (attenuated
  vaccine strain) are attenuated in 5 non-coding
  region
• affects ability to replicate in neurons
• affects translation of mRNA in cultured neuronal
  cells (but not other cell types)
• Attenuated viruses do not replicate efficiently
  in the gut, so less virus is spread to other sites

21
Alteration of viral virulence - IV

• Genes that modify the hosts defense
• A diverse array of viral proteins that sabotage
  the innate and adaptive immune system
• virokines - secreted proteins that mimic e.g.
  cytokines
• viroreceptors - homologs of host receptors or
  cell-surface immune molecules
• Mostly in large DNA viruses, esp. poxviruses
• HSV-1 - encodes Fc receptor (gE/gI) - evades
  complement lysis
• Herpesviruses and encode proteins (gC in HSV-1)
  that bind the C3 component of complement
• Inhibition of apoptosis

22
Alteration of viral virulence - V

• Genes that enable the virus to spread
• e.g. a single amino acid change in glycoprotein D
  of HSV-1 affects the ability of the virus to
  spread in neurons
• mechanism unknown?
• Either direct effect on entry in neuronal tissue
  ,
• Or secondary effect via the immune system
• Bunyavirus shows a similar effect with its G
  protein
• Spread of poliovirus may simply be due to the
  relative level of viremia following initial
  infection in the gut
• 1 in 150 human cases of polio cause paralysis

23
A viral exotoxin

• NSP4 protein of rotavirus appears to act as a
  viral enterotoxin
• Triggers a signal transduction pathway in the
  intestinal mucosa, leading to elevated Ca2 and
  potentiates chloride secretion gt diarrhea

• Fundamentally different mechanism to baterial
  toxins e.g. cholera

24
Injury induced by viruses

• Direct cpe
• - apoptosis
• - blockage of host protein synthesis, membrane
  traffic, depolymerization of the cytoskeleton,
  syncytia formation
• Indirect effects
• LCMV - infection of newborn mice gt persistent
  infection of cells in the growth hormone
  producing cells of the pituitary gland gt
  reduction (2-fold) in secretion of growth hormone
  gt death within 2-4 weeks

25
Immunopathology

• Most virus-induced pathology is caused by the
  hosts immune system
• The price paid for elimination of an infection
• Most immunopathology caused by T cells (CD8 CTL)
• Experiments with LCMV in knockout mice clearly
  show that tissue damage during infection requires
  CD8 T cells
• Liver damage by hepatitis B virus also depends on
  CD8 T cells
• Myocarditis due to coxsackie B virus requires
  CD8 T cells, in particular perforin (the major
  cytotoxic protein)

26
Antibody enhancement of infection

• Dengue hemorrhagic fever
• Normally self-limiting / asymptomatic
• 4 serotypes (do not cross-protect)
• Upon infection with second serotype,
  non-protective antibodies bind the virus and
  facilitate uptake via Fc-receptors in peripheral
  blood monocytes (receptor-negative)
• Infected monocytes ? proinflammatory cytokines ?
  T cells ?? more cytokines - leads to
  plasma leakage and hemorrhage ?? shock and death

Anti-HIV antibodies can also exacerbate infection
27
Injury caused by free radicals

• Superoxide (O2-) and nitric oxide (NO)
• NO is produced in abundance in virus-infected
  tissue during inflammation as part of the innate
  immune response
• Produced by interferon-inducible nitric oxide
  synthase (iNOS)
• Inhibits viral replication
• At low levels NO is not damaging and is
  protective, but high concentrations or prolonged
  production causes tissue damage

28
Further reading

• Chapter 17 of Flint et al
• Nathanson Viral Pathogenesis