Orthomyxoviruses

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Orthomyxoviruses
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Characteristics

• Name originates from the Greek word ortho
  (correct), myxo (mucus). Essentially virions
  infect epithelial cells the right way, as opposed
  to paramyxoviruses
• 3 Types A, B and C
• Hosts Birds, various mammals and humans
• Enveloped virion, 80-120 nm diameter
• Linear -ssRNA genome
• 6-8 different segments
• Total genome length 10-15 Kb
• Each segment encodes for 1-2 proteins
• Envelope proteins Hemagglutinin (HA),
  Neuroaminidase (NA)
• Symptoms High fever, sore throat, cough,
  headache
• Can be fatal, often from secondary bacterial
  infections

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Influenza Virions Infecting Epithelial Cells
(Upper Resp. Tract)
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Influenza History

• The Name Originates From 18th Century Italy,
  Disease Thought To Be Due To Influence of Stars
• In 1918 Spanish Flu Caused 20 Million Deaths
• Many People Confuse Cold With Flu
• Flu is severe! You cannot get out of bed!
• Cold is due to rhinoviruses, coronaviruses,
  adenoviruses
• 20,000-30,000 People Die From Flu and Flu Related
  Complications in USA Every Year Despite Vaccines
  and Anti-viral Drugs
• Pandemics
• Spanish Flu, 1918 (20 million deaths)
• Asian Flu, 1956 (60,000 deaths)
• Hong Kong Flu, 1967

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Influenza Pathogenicity

• Infection and Destruction of Epithelial Cells
  Leads to Loss of Mucociliary Flow
• Loss of Mucociliary Flow Leads to Secondary
  Bacterial Infections.
• Interferon Production Leads to Strong Local and
  Systemic Inflammatory Responses
• These responses cause the severe symptoms
• Flu is severe! You cannot get out of bed!
• Cold is due to rhinoviruses, coronaviruses,
  adenoviruses
• Spanish Flu Pathogenicity Thought to be Due to
  Cytokine Storms
• Affected primarily young people with healthy
  immune systems, rather unusual
• Typically children and older individuals are more
  vulnerable
• Pandemics
• Spanish Flu, 1918 (20 million deaths)
• Asian Flu, 1956 (60,000 deaths)
• Hong Kong Flu, 1967

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Viral Structure
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Viral Entry and HA

• The HA envelop protein is used as an
  attachment/entry receptor
• HA binds to sialic acid residue found on many
  receptors
• Ex. Intestinal cells, lung epithelial cells
• Hemaglutinin name derived from the fact that HA
  can agglutinate RBCs
• Other viruses can cause hemaglutination, not just
  influenza
• HA is 549 a/a long, type I transmembrane
• C-terminus in cytosol, N-terminus outside virion
• HA is initially fusion incompetent
• Cleavage by cellular proteases gives rise to 2
  subunits
• HA1 and HA2

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Fusion Competent HA
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HA and Endosomal Virion Release

• Once virion enters propagates endosomal pathway,
  pH decreases progressively
• This is the typical endosomal maturation process
• Decrease in pH results in conformational change
  of HA
• HA2 extends outwards and inserts into outer
  endosomal membrane
• See next schematic

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M2 Ion Channel Protein

• M2 via formation of a tetramer pore allows H to
  enter virion
• ? in H is needed for nuclecapsids release into
  cytosol
• Release is thought to be due to ? interaction
  between M1 proteins and nucleocapsids
• The antiviral drug amantadine is a specific
  blocker of M2 H channel
• In presence of amantadine nucleocapsid release is
  incomplete

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Nucleocapsids

• Influenza nucleocapsids use nucleocapsid protein
  (NP) as building block
• -ve strand RNA rubs around NP proteins
• Each nucleocapsid contains a trimeric RNA
  polymerase complex
• PA, PB1 and PB2
• Sequences at 5' and 3' ends are highly conserved
  in all genome segments
• 5' and 3' are complimentary but are not thought
  to hybridize

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Nucleocapsids
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Nucleocapsid Entry Into Nucleus

• Orthomyxoviruses replicate inside nucleus
• This is contrary to most RNA viruses
• Nucleocapsids enter nucleus via nuclear pores
• Both NP, PA, PB1 and PB2 contain NLS (nuclear
  localization sequences)
• They all interact with importin-?
• The complex then interacts with importin-? and
  nuclear translocation occurs

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Genome Transcription
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Viral Genome Transcription

• Viral polymerase cannot transcribe independently!
• Cellular mRNAs used as primers
• PB2 binds cellular pre-mRNAs via 5' cap (found on
  all eukaryotic mRNAs)
• PB1 then acts as nuclease cleaving cellular RNAs
• Cleaved mRNA is about 10-13 nt
• PB1 cleaves at an A or G residue
• A residue is hydrogen bonded to U residue on
  viral genome
• PB1 then acts as polymerase moving towards 5' end
  of genome
• Role of PA not understood at this point

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Viral Genome Transcription
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Neuraminidase Inhibitors
NJEM, 2005
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Oseltamivir Clinical Trial
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Vaccines

• Most Common Vaccine, TIV (trivalent influenza
  vaccine)
• Comprised of 2 type A and 1 type B influenza
  viruses
• Less Common Vaccine, LAIV (live attenuated
  influenza virus)
• Vaccine May Be Effective For One Year But Not
  Next Year
• 2008-2009 Flu Vaccine
• A/Brisbane/59/2007(H1N1)-like virus
• A/Brisbane/10/2007 (H3N2)-like virus
• B/Florida/4/2006-like virus
• Many Companies Manufacture Flu Vaccines Worldwide
• CSL Biotherapies (Australia)
• Sanofi-Aventis (US, largest worldwide producer)
• Novartis Vaccines
• Medimmune-Avirion (US)
• Chiron Corporation(Global)
• Wyeth Lederle (US)

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Transmission And Humidity

• Flu Outbreaks Take Place Predominantly From
  December to April
• Why?
• One Recent Study Looked At Relative Humidity As A
  Key Factor
• 2008-2009 Flu Vaccine
• A/Brisbane/59/2007(H1N1)-like virus
• A/Brisbane/10/2007 (H3N2)-like virus
• B/Florida/4/2006-like virus
• Many Companies Manufacture Flu Vaccines Worldwide
• CSL Biotherapies (Australia)
• Sanofi-Aventis (US, largest worldwide producer)
• Novartis Vaccines
• Medimmune-Avirion (US)
• Chiron Corporation(Global)
• Wyeth Lederle (US)