Virology Lecture 4 The Herpesviruses Dr Mike Kotiw Ref: Sherris p503518

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Virology Lecture 4 The HerpesvirusesDr Mike
KotiwRef Sherris p503-518
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Lecture aims

• To be aware of the members of the herpesviridae
• To be able to describe the general structure of
  Herpes Simplex viruses
• To be able to describe how Herpes Simplex viruses
  are classified
• To be able to describe how a Herpes Simplex virus
  may be diagnosed
• To understand the pathogenesis of Herpes Simplex
  viruses

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The herpesviruses

• Name from the Greek word Herpein meaning to
  creep
• 1950s, Burnet demonstrated HSV latency following
  a primary infection
• Weller (1954) found VZV from chicken pox and
  zoster were the same causal agent.
• Over 100 Herpesviruses have been isolated
• There is at least one for most animal species
• There are 8 known human Herpesviruses
• The herpesviridae family is divided into 3
  sub-families based on host range, cell tropism
  and growth rates

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The Herpesvirinae

• Alphaherpesvirinae
• may grow in a number of cell types, rapid and
  commonly destroy host cells
• Simplexvirus human herpesvirus 1, 2 (HSV-1,
  HSV-2)
• Varicellovirus human herpesvirus 3 (VZV)
• Betaherpesvirinae
• Tend to grow slowly, have restricted cell type
  range
• Cytomegalovirus human herpesvirus 5 (HCMV)
• Roseolovirus human herpesvirus 6, 7 (HHV-6,
  HHV-7)
• Gammaherpesvirinae
• Tend to grow slowly and often immortalize
  lymphoid cells of host
• Lymphocryptovirus human herpesvirus 4 (EBV)
• Rhadinovirus human herpesvirus 8 (HHV-8) 8
  (HHV-8)

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Herpesvirus characteristicsGeneral

• Characteristically large genomes
• may be 180- gt gt200kbp DNA
• Are complex viruses
• may code for more than 30 viral proteins
• Genetically diverse
• all are similar in terms of virus structure and
  genome organization
• All herpesviruses appear much the same in
  electron micrographs

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Herpesviruses Family Classification

• Size 180-200nm
• Envelope present
• Tegument Protein-filled region between capsid
  and envelope
• Capsid Icosahedral, 95-105nm diameter 162
  hexagonal capsomeres
• Core DNA bound around protein
• Genome Linear dsDNA 130-235kbp
• Replication Nuclear
• Assembly Nuclear

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HerpesvirusesGenome arrangement

• All herpesvirus genomes have a unique long (UL)
  and a unique short (US) region
• The unique sequences bound by inverted repeats
• Repeats allow rearrangements of the unique
  regions
• Consequently Herpesvirus genomes exist as a
  mixture of 4 isomers

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HerpesvirusesGenomes

• There are generally 3 genomic types
• 1.
• 2.
• 3.

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HerpesvirusesGenomes
HSV about 150 kbp
VZV about 130 kbp
EBV about 170 kbp
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Type Species Herpes Simplex Virus Classification

• Two members HSV1 and 2 share about
  40-50 DNA homology
• Systematic names Human Herpes type 1
• Human Herpes type 2
• Sub family Alpha
• Genome size 152kbp
• Genomic isomers 4
• Genome type 1
• GC HHV1 67 HHV2 69

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Herpes SimplexStructure

• Herpesvirus particles are complex
• Core region large DNA genome wound around a
  proteinaceous core.
• Capsid surrounds the core
• Tegument a protein-filled region which appears
  amorphous in electron micrographs which is
  outside the capsid
• Envelope outside of tegument and contains
  glycoproteins
• here

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Herpes SimplexStructure
http//www-micro.msb.le.ac.uk/335/Herpesvirus.gif
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Herpes SimplexStructure
Lipid coat
Integument
dsDNA
Glycoprotein
Core protein
Capsid
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Herpes Simplex VirusDiagnosis

• Histology
• Cell culture (embryonated egg)
• Gold standard, form syncytia and inclusion bodies
• Grow well in epithelial cell lines eg BHK
• IFT Useful, needs expertise
• Elisa (IgG IgM)
• EM (including Immune EM)
• PCR
• DNA probes

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Herpes Simplex VirusPathogenesis

• Stability highly infectious even if freeze dried
• Inactivation readily inactivated by ether (2hrs)
  or heat ( 560 ) 30 minutes
• EpidemiologyUnder natural conditions infects
  only humans

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Herpes Simplex VirusPathogenesis

• 2 serological types defined
• HSV1
• Mainly oral infections
• Less frequent in genitalia
• HSV 2
• Mainly genital isolation
• Less frequent orally

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Herpes Simplex Virus PathogenesisHSV1

• Primary infection usually occurs in neonate
• Mainly droplet or contact infection
• Most common sources are maternal and hospital
  staff
• In youth mainly by contact
• Infection rate is about 50 up to puberty and
  gt90 in adult hood

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Herpes Simplex Virus PathogenesisHSV2

• Commences with sexual activity
• About 10-20 of Caucasian population in age group
  20 to 30 years are infected
• May also be transmitted at birth
• Is more common in Western cultures

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HSV pathogenesisInfectious replication

• 1o exposure of HSV onto mucosal area or
  integument defects
• Virus shell fuses with cell membrane
• Nucleocapsid reaches cytoplasm and DNA released
• Penetration occurs following fusion with cell
  membrane
• Viral DNA release occurs in region of nucleopores
• DNA enters cell nucleus and takes on a circular
  structure

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HSV pathogenesisInfectious Replication

• Over 30 structural proteins are made
• Replication proceeds in 3 defined stages
• Alpha proteins (early antigens)
• Initiated by an integument protein the viral DNA
  is transcribed into alpha proteins
• Beta proteins
• Alpha proteins regulate synthesis of beta
  proteins
• Beta proteins regulate DNA polymerases
• Gamma proteins are late proteins which are
  structural proteins and together with new DNA
  form new viral particles

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HSV Pathogenesisreplication

• Viral propagation occurs in nucleus.
• Nucleic acid and capsid molecules are formed in
  nucleus (nucleus shows inclusion bodies)
• Virus gains integument by budding from inner
  nuclear membrane
• Virus transfers directly to neighbouring cells by
  fusing cells to form syncytia CPE
• Virus can propagate in the face of an immune
  person

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HSV PathogenesisClinical aspects

• Local propagation in epithelial cells virus is
  followed by migration by intraaxonal route to
  sensory and autonomic ganglia
• Usually in the trigeminate ganglion in facial
  infection sacral ganglia in genital infections
• Lymphohaematogenous dispersion may occur in
  immunosuppressed patients
• Normally immune responses within about 7 days
  usually suppress HSV activity in neurons
• Infection then alters from acute to latent
• Both humoral and cell mediated responses
  important in recovery

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HSV PathogenesisClinical aspects

• Primary infection
• 90 of patients do not display symptoms
• 9 display minor symptoms
• 1 display clinically significant manifestations
• Superinfection with different strains can occur

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HSV PathogenesisClinical aspects

• Re-activation associated with environmental
  triggers
• Stress
• UV radiation
• Menstruation
• Fever
• Pregnancy
• Other infections
• Immunosuppression

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HSV PathogenesisClinical aspects

• Re-activation from endogenous infection
  associated with virus migrating intraaxonally to
  peripheral epithelial cells
• Recurrence is not prevented by immune competency
• HSV is re-propagated in target cells forming
  highly infectious vesicles
• Recurrence may be symptomatic or asymptomatic
• HSV relapses occur in the following frequency
• Genital HSV2 gt oral HSV1 gt genital HSV1 gt oral
  HSV2