Lecture 16 & 17 - Influenza Flashcards
What is influenza?
A specific respiratory syndrome
What are the symptoms of influenza?
Fever Chils Cough Headache Muscle aches Fatigue Loss of appetite
What is the appearance of a chest X ray in patients with influenza?
Normal
How long does influenza infection last?
7 days
However, the cough may last several weeks
Which groups are at risk of influenza infection?
Young
Old
Immuno-compromised
How may influenza be spread?
Droplets from coughing / sneezing
How long is the incubation period for influenza?
1-5 days
Does influenza affect many people each year?
Yes: 10-20 % of pop.
What is the impact of influenza on the population
Burden on:
• doctors
• hospitals
• employers
It’s an economical burden
How many deaths due to influenza each year?
250,000 to 500,000
Describe the pathogenesis of influenza
- Droplets enter respiratory tract
- Virus binds to sialic acid receptors in URT
- Replication in epithelial cells of RT
- Tissue damage → inflammatory response
- Immune response clears the infection in immunocompetent individuals
To which types of cells does influenza bind once it enters the body?
Unciliated epithelial of the respiratory airways
Where does the virus replicate?
Epithelial cells, especially in the LARGE AIRWAYS
Which cytokines are inducted in the inflammatory response to influenza?
What is their effect?
IL-1 → fever
IFN → malaise, head and muscle aches
Is the ciliated epithelium of the airways infected by influenza?
Initially, no.
Later on in the infection, this does occur
What does infected of the ciliated infection sometimes lead to?
Secondary bacterial pneumonia
What are the causes for pneumonia?
Primary viral: rare
Secondary bacterial: more common, esp. in elderly
To which family does Influenza virus belong?
Orthomyxoviridae
Describe the genome of Influenza
-ve sense
ssRNA
segmented
How many types of influenza are there?
3: A B and C
What can we say about the three types of influenza?
They show no immunological cross-reactivity
How do we differentiate the different types of virus?
Antibodies to the internal antigens of the virus
What do types A and B do?
Cause human influenza
Which types of influenza can infect other species?
Type A
How many RNPs does influenza have?
8
Which proteins does influenza have?
HA: haemagglutinin
NA: neuraminidase
M1: matrix protein
M2: matrix protein (channel)
What is the structure of the viral RNP?
-ve sense ssRNA
Protein
3 polymerase subunits
What is the structure of HA?
Trimer
Head with a binding pocket for Sialic acid
What is the structure of NA?
Tetramer
What is different about the different subtypes of Type A influenza?
What is the same?
Same: internal proteins
Different: HA and NA
How many subtypes of HA are there?
16
How many subtypes of NA are there?
9
What is an influenza subtype?
eg. H1N1
What is the ancestral host of influenza A?
Aquatic birds
All subtypes are endemic in birds
Which other animals can be infected by influenza?
Pigs
Horses etc.
Which subtypes of influenza are currently in the human population?
H1N1
H3N2
How are influenza viruses named?
Type Place of isolation Isolate number at that place Year of isolation Subtype
Describe the replication cycle of influenza virus
- HA binds to sialic acid
- RME into endosome
- Endosome becomes more acidic
- HA changes conformation
- Virus leaves endosome, RNPs go to the nucleus
- Replication, transcription, translation
- Budding out of cell, acquiring HA and NA
- NA cleaves Sialic acid receptor on host cell
- Tryptase clara action → active virion disseminates
What allows the viral membrane to fuse with the endosomal membrane, so that the RNPs are released into the cytosol?
Tryptase clara must cleave one amino acid on HA
HA can now change conformation in the low pH conditions of the endosome → fusion
What produces Typtase clara?
Where?
Clara cells
In the small airways (bronchioles)
Why does influenza only infect cells of the respiratory tract?
Needs Typtase clara action to be able to get out of the endosome
Clara cells are only found in the respiratory tract
What is the structure of a collectin?
Protein
What is the function of collectins?
They bind to carbohydrate side chains of HA and NA
This prevents the influenza from binding to the host cell
How do collectins interact with the complement cascade?
Collectin binding triggers the lectin pathway
→ opsonisation
→ MAC in infected cells
What are the early innate defences against Influenza?
Collectin binding
What are the late innate defences against influenza?
Viral infection triggers: IFN-a IFN-B IL-1 IL-6 TNF-a
Inflammation
Viral immunity
What are the type 1 interferons?
IFN-alpha
IFN-beta
What are the roles of type 1 interferon?
- binds to uninfected cells → antiviral function of cell, so it is protected from infection
- induction of NKs
- Upregulation of MHC I
Which cells release the cytokines in the anti-viral inflammatory response?
Why?
When viruses are taken up by macrophages and DCs, they start to produce all these cytokines
Describe the function of NK cells
- Recognise virally infected cells through receptors that recognise ‘stress’ (decreased MHC I expression)
- Release toxic granules → apoptosis
Are NK cells enough to clear a viral infection?
No, it just keeps the infection in check
To clear the virus we need an adaptive IR
In general, how does the adaptive immune response fight influenza infection?
CD8+ cells:
• kill virally infected cells
• recognition of internal antigens
Antibody:
• against HA and NA
Are CD8+ cells cross-reactive between Influenza types A and B?
Why / why not?
No, however, they are cross-reactive between subtypes.
CD8+ cells recognise internal proteins, which are conserved between influenza subtypes
Are the antibody and CD8+ cell response against influenza long lasting?
Antibody: lifelong
CD8+ cell: not long lived (can be boosted by repeat exposure)
If our antibodies against HA and NA are lifelong, why do we get continual infection with influenza?
Antigenic drift
Describe the mechanism of antigenic drift
- Error in replication
- No proof-reading
- Single amino acid changes in viral proteins (inc. HA and NA)
- Antibodies can no longer bind
- Resistant strains are selected for
How many antigenic sites are there on HA?
5
Where does neutralising Ab bind to HA?
The 5 antigenic sites surrounding the receptor binding pocket
In general, when does an epidemic of influenza come about?
When all 5 antigenic sites mutate
The population has no pre-existing antibodies to this strain
Antigenic drift brings about new viral …
Strains within a subtype
Evolution of influenza is a … process
Linear
Why is evolution of influenza described as linear?
Once a new strain is created through mutation, it replaces all older strains
Describe the evolution of the H3 subtype
From 1968 to 1987, there were mutation at the five sites, such that the globular head became completely different
When is influenza present in the population?
Throughout the year
In winter, however, there are peaks
How often do we see epidemics of seasonal influenza?
Every 2-3 years
Why is surveillance of influenza important?
- Detection of new antigenic variants that could cause an epidemic
- Vaccine production
- New strains
- Checking antibody levels to see how effective the vaccines are
Why is rapid diagnosis important?
Rapid anti-viral therapy
What are the three groups of lab diagnosis?
What is the time frame for each?
Rapid: hours
Culture: days
Detection of antibody responses: days-weeks
How is rapid diagnosis of influenza performed?
Reverse transcriptase - PCR
‘X/pect’ test for antigen
What does rapid diagnosis tell us?
Type A vs. Type B
What does culture tell us?
Subtype and strain analysis
How is culture analysis performed?
Embryonated eggs with specific reference antibodies
Haemagglutination-inhibtiion test
Why do we detect for antibody responses?
To evaluate the success of a vaccine
How do we detect for antibody responses?
Haemagglutination-inhibition test with patient’s serum + virus
Describe the process of the Haemagglutination-inhibtion test
- Chicken RBCs with sialic acid
- Add virus
- Virus binds to sialic acid receptor
- Agglutination → sinking, ‘button’
If there are antibodies present, the virus will not bind to the RBCs → no button
Differentiate between a positive H-I test and a negative
Antibodies present: no agglutination, button
No Ab: agglutination, no button
How does the H-I assay let us subtype the virus?
The antibodies used in the test are for different subtypes and strains
Agglutination or lack of in the presence of certain antibodies will indicate the subtype
What are the antibody targets of the H-I test?
HA and NA
Which types of influenza viruses are contained in the influenza vaccine?
Type A:
• H1N1
• H3N2
Type B
How in the influenza vaccine administered?
Intramuscularly
Which people are strongly advised to get the influenza vaccine?
Elderly Young Chronic heart, lung, kidney disease Cancer / diabetes sufferes Immunosuppressed Health workers
How is the influenza vaccine made?
Viruses are grown in eggs
Purification
Chemical inactivation
Who can’t take the influenza vaccine?
People with egg allergies
What type of vaccine is the influenza vaccine?
Split virus vaccine
Which responses are induced by the influenza vaccine?
Why?
B cell
CD4+ cell
No CD8+ cell response
This is because it is a inactivated virus
Does the influenza vaccine work well?
Young and healthy: 70%
Elderly: less
Why must the vaccine be made again every year?
Because there are new strains every year due to antigenic drift
What are the targets of antiviral drugs?
- M2 ion channels
* Neuraminidase
Describe how M2 ion channels work
How is this targeted by an antiviral drug?
- Ion channels pump H+ into the virus
- Conformational change of HA
- Fusion of virus with endosome
- Release of RNPs
By blocking the ion channel, there is no fusion, and the virus can not release its RNPs
What are some examples of M2 ion channel blockers?
Amantadine
Rimantadine
Which types of influenza are susceptible to M2 ion channel blockers?
Type A
Are M2 ion channel blocker widely used?
Why / why not?
They are not widely used
Because drug resistant variants readily arise
What are some antivirals that block neuraminidase?
Relenza (zamamivir)
Tamiflu (oseltamivir)
Which types of influenza are susceptible to neuraminidase inhibitors?
Types A and B
What is the result of administering neuriminidase inhibitors?
Reduction in severity and duration
How is Relenza administered?
Inhalation by mouth
How is Tamiflu administered?
Oral prodrug
Describe how neuraminidase inhibitors work
- Bind to the active site of NA
- NA can no longer cleave sialic acid from host cell
- Virions are stuck onto the host cell and can not disseminate
For how many days is one infectious?
5-6 days
Describe the bond between Sialic acid and HA
alpha 2-6 galactose on Sialic acid
+
HA
Which bacteria may cause secondary bacterial pneumonia?
Strep. pneumoniae
H. influenzae
Staph. aureus
Describe the structure of the Influenza virion
8 RNPs M1: matrix M2: ion channel Envelope Glycoproteins: HA, NA
Describe the symmetry of Influenzavirus
Helical symmetry
Describe how the virion acquires its glycoproteins on its envelope
- The glycoproteins are made using the host machinery (ribosomes)
- Vesicular transport through ER and Golgi, eventually to host cell membrane
- Viruses bud out, acquiring the glycoproteins
What is one way that you could describe collectins?
Soluble PRR:
• bind PAMPs
Describe the stages of immune response to influenza
- Innate early
• Collectins
2. Innate, delayed • PAMP-PRR → DC activation • IFN release • Inflammatory cytokine release • NK activation
- Adaptive
• B cells produce antibodies
• CD8+ T cells kill infected cells
Describe cross reactivity of CD8+ T cells
Recognise internal antigens
Thus, broadly cross-reactive within a type
However, not crossreactive between types
Describe the role of antibody in the immune response to influenza.
- HA and NA are present on the cell membrane of infected cells
- Ab binds to HA and NA
- Complement activation
• lysis
• phagocytosis
Where are the ‘advantageous’ mutations occurring during antigenic drift?
At the Antigenic Sites:
• (sites on HA and NA where antibodies bind)
