Influenza

Question 1

What are the symptoms of seasonal influenza?

Answer 1

• subclinical to severe
• fever/chills
• cough
• headache
• muscle aches
• fatigue
• loss of appetite
• lasts ~7 days

Question 2

How is influenza spread?

Answer 2

droplet infection from coughing and sneezing

Question 3

What is the incubation period of influenza?

Answer 3

1-5 days

Question 4

What is the infectious period of influenza?

Answer 4

5-6 days

Question 5

Who is more at risk of getting influenza?

Answer 5

• young
• elderly
• w/underlying chronic heart, lung, renal, or metabolic conditions
• obese
• pregnant women

Question 6

What is the pathogenesis of seasonal influenza?

Answer 6

• viral droplets enter respiratory tract
• virus vinds ti sialic acid on receptors on non-ciliated respiratory epithelium
  
  • SA alpha2-6 linkage to galactose in humans
  • found elsewhere but virus remains localized to RT
• replicates in epithelial cells of U and LRT, esp in large airways
• tissue damage as a result of virus killing epithelial cells and the resulting inflammatory response
• production of cytokines:
  
  • IL-1 from SCs and macros –> fever
  • IFN –> lethargy, aches
• later, infects ciliated epithelium of trachea and bronchi
  
  • leads to secondary commensal bacterial infections
    
    • H. influenzae, S. aureus, S. pneumoniae)
      
      • death from bacterial pneumonia esp in elderly
• rarely infects parenchyma directly (viral pneumonia)

Question 7

What is the general structure of inluenza viruses?

Answer 7

• orthomyxoviridae family
• enveloped virus
• genome is segments of ssRNA -ve sense
  
  • tf carries its own RNA-dep RNA-pol on infection
• 3 genomically different types: A, B, C (mild)
  
  • differetniated by Abs to the internal Ags
• Types A & B cause human influenza
• only type A can infect other species

Question 8

What are the critical features of the influenza envelope?

Answer 8

• hemagglutinin (HA), the dominant glycoprotein in the envelope surface
• nueraminidase, NA

Question 9

What is the structure of the influenza genome?

Answer 9

• 8 segments of -ssRNA
• each twisted in a panhandle structure with a nucleoprotein around it
• each carry a RNA-dep RNA-pol
• each encodes at least one gene
• there are 10+ proteins depeniding on the strain

Question 10

Which part of the influenza virus interacts with sialic acid on receptors?

Answer 10

• hemagglutinin and neuraminidase
• HA binds SA and gets the virus into the cell
• NA cuts SA to free viruses that bud out of the host cell

Question 11

What are the Type A influenza subtypes?

Answer 11

• all Type A share similar internal proteins but differ in HA and NA they encode
  
  • this generates the different subtypes:
  • HA = H1-H16
  • NA = N1-N10
  • named accordingly eg H5N1
• all subtypes of HA and NA are found on viruses endemic in avian species as they are the ancestral host of influenza A
• H1N1, H2N2, and H3N2 have previously become endemic in humans
• currrently H1N1, H3N2, and Type B are the only circulating viruses that are endemic in humans

Question 12

How does influenza enter the cell and start the replication cycle?

Answer 12

• viral hemagglutinin binds sialic acid linked to a galactose on its receptor (unknown) on the surface of a non-ciliated respiratory epithelial cell
• virus is taken up by receptor-mediated endocytosis
• as the endosome pH drops, HA changes conformation
  
  • viral envelope fuses with the endosomal membrane
  • forms a pore
  • releases the 8 viral RNPs (ribonucleoproteins, panhandle genome segments)
• 8 viral RNPs enter nucleus
• viral RNA is amplified to produce mRNA
• mRNA leaves the nucleus to produce proteins

Question 13

How are influenza viruses assembled in the host cell?

Answer 13

• the hemagglutinin and neuraminidase are glycosylated proteins and therefore go through the ER and golgi –> host cell membrane
• proteins are made eg nucleopeptide, RNA-dep RNA-pol
  
  • these are shunted back to the nucleus to form the RNPs
• RNPs exit the nucleus
• come up underneath the host cell membrane
• bud off with envolpe containing HA and NA

Question 14

How is the influenza virus released from the host cell?

Answer 14

• viral RNPs bud out of the host cell membrane with HA and NA in their envelope
• the NA will naturally target sialic acid on receptors on the host cell
• tf NA snips sialic acid off receptors as it exits the host cell to prevent re-binding by the HA

Question 15

What has to happen to newly-formed influenza virus for it to be infectious?

Answer 15

• it must be cut by tryptase Clara, present only in the RT
• occurs at a cleavage site to reveal a hydrophobic fusion peptide
• only viruses that have undergone this cut can undergo the endosomal pH change required to fuse and form a pore for RNP escape

Question 16

What is the adaptive immune response to influenza virus infection?

Answer 16

CD8+ cytotoxic T cells:

• kill virus-infected cells
• important in recovery by keeping viral load down
• recognize peptides derived from the internal antigens
  
  • ​these are conserved within Type A tf CD8s recognize Type A subtypes (but do not see Type B)
• CD8 T cell immunity is not long-lived but can be boosted by repeated exposure

antibody:

• develops predominantly to HA but also to NA
• speeds viral clearance
• Abs bind HA, tf it cannot bind to its recetors
• Ab & complement cascade promote lysis, phagocytosis of virus
• pre-existing Abs will protect against infection in the same season and next by neutralizing the incoming virus
  
  • Ab response to infecting strain is lifelong
  • but rendered ineffective by antigenic drift

Question 17

What is the common site for favourable mutation in antigenic drift of influenza virus?

Answer 17

• sites on HA or NA that neutralizing antibodies bind to
  
  • if the Ab can no longer recognize the mutated protein sequence
• leads to that strain of virus being selected for
  
  • eg can have multiple strains even within H1 with slight mutative differences that confer a selective advantage for infection of hosts
• there are 5 antigenic sites surrounding the receptor binding pocket
  
  • once all 5 have mutated, majority of population have no antiboides
  • this results in an epidemic
• new strains replace older strains (linear evolution)

Question 18

What are the targets of vaccine-induced immunity?

Answer 18

• Abs produced to the vaccine will target HA to block viral attachment, and NA to block viral release

Question 19

What does the influenza vaccine contain?

Answer 19

• inactivated forms of 3 different influenza viruses ie the most recent circulating strains of:
  
  • Type A: H1N1 and H3N2
  • Type B

Question 20

What type of immune response does the influenza vaccine induce?

Answer 20

• the vaccine is inactivated therefore it induces Ab formation but not cytotoxic T-cell responses

Question 21

What are the targets of influenza antiviral drugs?

Answer 21

• ion channel blockers
  
  • inhibit the M2 ion channel on the influenza virus, preventing the RNPs from escaping the endosome in the host cell
• NA inhibitors
  
  • block NA from snipping sialic acid off receptors on the host cell
  • this impairs the virus from being released from the host cell

Question 22

How do ion channel blocker influenza antivirals work?

Answer 22

• eg amantadine and rimantadine (except for H5N1)
• block the influenza virus M2 ion channel
• normally M2 allows the H+ that is decreasing the pH of the endosome to pass into the viral cell through the pore that is formed when the endosomal and viral membranes fuse
• this H+ is required to induce a conformational change that unlocks the RNPs for release via the fusion pore into the cytoplasm

Question 23

Amantadine & Rimantadine

Answer 23

• influenza antivirals (adamantanes)
• block M2 ion channel to inhibit uncoating of influenza A virus in the endosome
• NOT ACTIVE AGAINST TYPE B
• oral, daily administration
• tx in children, propx in nursing homes to prevent outbreaks
• not used widely in the community because drug-resistant influenza mutants arise
  
  • ​the M2 protein also mutates via drift, can cause epidemics

Question 24

Relenza (Zanamivir) & Tamiflu (Oseltamivir)

Answer 24

• influenza antivirals
• block the action of NA by binding to its active site and blocking its enzymatic activity to release viral particles from host cells
• ACTIVE AGAINST INFLUENZA TYPE A AND B
• reduce duration and severity of the flu
• DO NOT STOP INFECTION
• only effective if administered within 2 days of developing symptoms
• administration is 2x daily:
  
  • Relenza: inhalation by mouth
  • Tamiflu: orally administered prodrug (activated in liver –> lungs)
• some older H1N1 strains show resistance to Tamiflu; susceptible to Relenza
• they are analogs of sialic acid

Question 25

What is antigenic shift?

Answer 25

• sudden appearance of a new HA (sometimes NA) subtype of a Type A influenza virus within the human population
  
  • occurs ~10 years, usually zoonoses from birds, sometimes pigs
• because we have no Abs to these new HA (NA)s, these strains have pandemic effects
• 1-2 AA change in receptor binding pocket of HA switches it from a2-3 (avian) bound sialic acid specificity on receptors to a2-6 sialic acid specificity on human receptors
  
  • influenza jumps from the avian population to humans

Question 26

Why are pandemics rare?

Answer 26

• human influenza virus sees sialic acid bound alpha2-6 to galactose (human RT)
• avian influenza virus sees sialic acid bound alpha2-3 to galactose (bird RT and GIT)
• this normally prevents zoonosis of avian influenza to humans
  
  • ​just 1-2 AA changes can convert HA receptor binding sites from being a2-3 (avian) specific to a2-6(human) specific = zoonosis –> pandemic
• pigs have both of these lined SA receptors in their respiratory tract and can act as a mixing vessel
  
  • can produce a dangerous combination if a human strain and an avian strain coinfect
  • get specificity for human a2-6 in the pig (or first few infected humans) with the HA and NA of the avian strain to which we are niave
    
    • this can be a deadly, pandemic combination

Question 27

Swine-origin H1N1

Answer 27

• 2009
• respiratory symptoms as in seasonal flu
• not highly lethal
  
  • most deaths people had underlying conditions
• far greater ability to replicate in lungs compared to seasonal
  
  • tf younger adults getting viral pneumonia
  • deaths in younger people
• pregnant women, obese, and inigenous pop’ns most susceptible
• susceptible to NA inhibitors
  
  • resistant mutants developing to seasonal H1N1
• replaced seasonal H1N1 tf now included in Aus vaccine

Question 28

avian H5N1

Answer 28

• mutated in birds to become very virulent and endemic in the bird population
• not yet endemic in the human population
• it is a highly pathogenic avian influenza
  
  • it can have systemic effects because it doesn’t depend on tryptase clara and therefore is not restricted to the RT
• can be lethal in humans
• even lethal in eggs in which it is grown
• first cases in 1997 (halted), 2003 (re-emergence)
  
  • indonesia has most cases and highest fatality rate (82%)
• susceptible to NA inhibitors but people are not always treated in time