Infectious cycle of influenza virus
They bind to respiratory epithelial using viral HA to sialic acid linked to galactose -> entry via endocytosis
Acidic environment of endosome causes conformational change of haemagglutinin
Genome is transcribed into a positive and negative sense strand
Expression of proteins
- Proteins that need to be glycosylated (NA and HA) are transported to the cell surface via the Golgi
Virus forms at cell surface
Neuraminidase clips HA from sialic acid
Exit from endosome
In order for virus to fuse with the membrane, the haemagglutinin has to be cut after pH drop
Cut occurs at the bottom of the HA stem, revealing a hydrophobic fusion peptide that wants to bury itself into the membrane
Cut performed by Clara cells in the respiratory tract
Tryptase Clara
Immediate immunity from influenza
Epithelial cells are lined with innate proteins
Collectins: carbohydrate binding proteins with globular domains, are soluble pattern recognition receptors
Bind to virus via the carbohydrate side chains on HA and NA, inhibiting the virus engaging its receptor
Trigger lectin complement pathway
Fosters uptake of virus by macrophages and the lysis of infected cells
Delayed: type 1 interferons
Infected epithelial cells and airway immune cells produce and respond to type 1 interferons (IFN alpha and beta)
Infected cells produce type 1 interferons
Interactions between PAMP and PRR triggers an anti-virus response in macrophages and dendritic cells
- Produce type 1 interferons
Become resistant to infection
- induces the influx and activation of natural killer cells that target stressed cells
- upregulate MHC class I for better lysis
Antivirals: ion channel blockers explanation
- HA needs to be cleaved by tryptase clara so when the pH drops, there is a conformational change that allows pore formation
- During endocytosis phase (fusion and pore), viral ribonuclear proteins are tethered to the matrix protein unless the hydrogen ions go into the endosome, and then through the M2 ion channel to enter the virion
No hydrogen ions = VRNPs stay tethered and become stuck
- During endocytosis phase (fusion and pore), viral ribonuclear proteins are tethered to the matrix protein unless the hydrogen ions go into the endosome, and then through the M2 ion channel to enter the virion
examples of ion channel blockers
amantadine and rimantadine, mostly used for children because of drug resitance
NA inhibitors
- NA inhibitor can bind to active site of NA where sialic acid usually binds
- Sialic acid is anchored by carbohydrate and glycerol
Zanamivir and oseltamivir permanently binds in a different way (guanine or hydrophobic group) that stops it from being able to be cleaved
- Sialic acid is anchored by carbohydrate and glycerol
Hemagglutination inhibition test
- Ability of influenza virus to bind to sialic acid on red blood cells and cause them to agglutinate > forms a network
- Chicken RBC are used because they are nucleated and settle quickly, and can’t take the virus in (endocytose) so the virus just sits on the surface
- Stops the cells from settling at the bottom of the cell
Antibodies can block hemagglutination -> hemagglutination inhibition
use this method to identify strains
how is influenza cultured/antisera prepared
influenza virus multiplies rapidly when injected into the allantoic fluid of embryonated chicken eggs
antisera is prepared by injecting ferrets with a human strain of influenza
Antigenic drift
- random advantageous mutations in overlapping epitopes within 5 antigenic sites
- that neutralising antibodies bind to
- mutations in all 5 of these sites will cause the virus to be unrecognisable to the strain
Why is antigenic shift so common
- frequent mutations because of faulty replication by RNA dependent RNA polymerase without proofreading machinery
Cytotoxic T cells role in immune response
- Recognise peptides derived from internal antigens of the virus
- therefore broadly cross-reactive within subtypes of A but not between type A and B
CD8+ cell memory is not long lived, can be boosted by repeated exposure
Antibody role in immune response
speeds clearance by:
- inhibiting attachment of virus
- promotes phagocytosis
- activate complement system to cause lysis
Pre-existing Ab will protect against infection by neutralising virus -> is lifelong!
How are influenza viruses grouped? strain vs subgroup
subgroups are grouped based on differences in internal antigens
strains are differences within subgroups
do people die from influenza often?
No. Influenza targets ciliated epithelial cells in the RT. After infection, most people die from secondary infection such as bacterial pneumonia
How is influenza virus may be recognised by a rapid antigen test?
Based on internal nucleoprotein, which is shared within types but not across
