Influenza Flashcards
What kind of virus is influenza?
- negative sense ssRNA
- A, B + C -
A = public health issue
What is the current status of influenza vaccination?
- available and provides good protection
- new variants and strains can appear regularly due to antigenic shift
- vaccines must be regulated constantly
What are the symptoms of the flu?
- headache, fever, cough etc
- routinely life-threatening in vulnerable patients
- usually recover in 7-14 days
Describe the epidemiology of influenza
- peaks a troughs as the weather changes throughout the year especially in temperate climates
- in an edpidemic year 20-30,000 people might die in the UK
- elderly and young often die as a result of secondary bacterial infecitons
What is the structure of influenza A?
- 8 segments encoding 10 proteins and acessories
- HA - Haemagglutinin
- N - neuraminidase
- MI = matrix
- M2 = ion channel
- RNA polymerase
- Nuclear export protein
- NP - nucleoprotein
What is the role of the hemagglutinin (HA) segment of the influenza genome?
- major determinant of host range
- virus attachment factor
- binds sialic acid on the host cell surface which is different in different animals
- major antigenic target for antibodies and vaccines
What is the role of the neuraminidase (NA) of influenza virus?
- appears on the infected cell surface and cleaves sialic acids
- prevents new virus from infected already infected cells to make disease more efficient
- also allows virus release from the cell surface during budding
What are the roles of the M1 and M2 proteins in influenxa?
- Matrix of the viral capsid that contains everything
- acid-dependent ion channel on the viral envelope that allows ions into the virion as a decrease in pH is required for release
What is the role of the nuceloprotein (NP) of influenza virus?
- associated with viral RNA
- one NP monomer per 24 nucleotides
What is the role of the PA, PB1 and PB2 (RNA polymerase)?
What is the role of the nuclear export protein?
production and export of viral ribonuclear proteins
How does influenza virus enter the cell and then the nucleus?
- HA attaches and is taken into an endosome
- endosome is acidified
- HA is folded and inserts itself into the endosome membrane to allow fusion
- M2 ion channel allows H+ ions into the virus particle leading to dissociation of the genome from the matrix - leaves the particle and enters the nucleus
What are the basic steps of influenza replication?
- mRNA made from the genome and translated into viral proteins
- polymerase makes antigenomes that act as a template for viral replication
- progeny genomes make more mRNA or are packaged and exit at the cell membrane
How does influenza virus use host mRNA to transcribe its own mRNA?
- cap snatching
- polymerase binds to host mRNA and cuts near the 5’ cap
- uses the host mRNA as a primer
- can sometimes result in the production of viral-host fusion proteins that contribute to virulence
Describe influenza serology?
- 16 H serotypes
- 9 N serotypes
- many sequence variations within them
- H1N1 e.g.
What is antigenic drift?
- polymerase can make mistakes that result in new genomes
- if changes are in major surface proteins such as H/N it can reduce protective immunity in a population
- accumulation of minor changes can render a vaccine useless in around 4 years
What results from influenza’s ability to infect a variety of animals?
- human trachea epithelium express one type of sialic acid
- birds express another
- bigs both
- pigs can act as mixing pots for different chimeras of the flue
- human flu doesnt replicate well in birds and vice vera
Why can genome assortment in mixing pots occur in influenza?
- genome is segmented with H and N on diffferent segments
- can give rise to flu that can infect humans + birds or that are different to other flus previously encountered
What ist eh structure of the influenza HA protein?
- made as HA0 and cleaved into HA1 and HA2 linked by a disulphide bond
- cleavage is required for infection
- when and how cleavage occurs depends on strain and host
What other ways can influenza genome reassortment leading to antigenic shift occur other than in pig mixing pots?
- avian infects human and adapts (spanish flu)
- avian and human infect a human at the same time and reassortment occurs within the human
What was the Spsnish flu pandemic:
- worst known flu pandemic
- mostly healthy 18-30 year olds
- flu death rate was 25 times higher then usual
- origin unkown
- first mild wave followed by a second lethal one
Describe the conformational change that occurs in influenza HA in the endosome in more detail
- cleaved protein changes to expose a fusogenic region on HA2
- enabled fusion of the endox=some and viral membranes
What are the 2 possible pathogenic determinants in NA of influenza?
- influenza mutants with no NA aggregate with each other
- NA affects pathogenicity through HA cleavage
- mutant NA binds plasminogen on the cell surface and converts it into plasmin which cleaves H0 with no arginine rich region increasing pathogencity
What is a likely pathogenic determinant of HA proteins in influenza? on the cell surface
- HA cleavage site changes between strains
- HA proteins with additional arginines at the cleavage site can be recongnise by intracellular proteins
- HA proteins can be primed for action as soon as they reach teh cell surface
What is a likely pathogenic determinant of HA proteins in influenza? intracellular/extracellular
- HA proteins that aren’t cleaves intracellularly must be matured extracellularly and are less pathogenic
- in birds, viruses that cleave H0 intracellularly can infect a wide range of organs. those that do it extracellularly can only infect the respiratory tract as the enzymes required for maturation are only available extracellularly in this location
How is the genetics of the 1918 Spanish influenza strain analysed? What was seen
- can create recombinant viruses with the genes seen in the strain using reverse genetics
- HA + NA support lethal infections even without mutations
- acted synergistically but HA the major determinant
- NA allows the strain to grow without extracellular trypsin
Describe the reverse genetics used to make spanish flu recombinantly
- 8 plasmids each encode 1 segment of the genome and a promoter
- each produce a segment of RNA like in virus
- 4 more plasmids contain polymerase and NP with promoters that generate mRNA for these proteins
- NP is wrapped around the genome and polymerase and creates particles as seen in the nucleus of infected cells
What is the role of the 1918 spanish Influenza strain polymerase in its high pathogenecity?
- produces large amounts of mini viral RNAs that bind RIG-1 and trigger expression of IFN-B
- provides a strong innate immune reponse, cytokine storm and high lethality
What was different about the 1997 Hong Kong flu?
- direct transfer from avian to human does not normally occur
- luckily virus did not apart or reassort in humans so was limited
- 33% mortality rate
- has an ariginine rich cleavage site on HA - high pathogenicty
What gene doesn’t allow avian flu to replicate in humans?
- ANP32
- a larger protein in birds than in humans
- means the polymerase can’t interact with it
- a point mutation in avian polymerase along with HA sialic acid binding can allow avian flu to jump to humans
Are there antivirals against influenza?
- some block M2 function
- some block NA function
- side effects
- only reduce severity + duration
- must be given witht=in 2 days
- RNA viruses build resistance very quickly
How do Amantadine and Flumadine block M2 function?
- block the M2 ion channel
- virion is not acidified in the endosome
- prevents M1 and viral RNA/N protein complexes ffrom dissociating and interferes with uncoating
How is a recombinant HSN1 vaccine against influenza produced?
- influenza’s with high growth in chickn embyros have HA and NA genes replaced with H5N1 genes
- work well in the lab but not as well in practice as you have to guess the strain
What vaccine IS routinely used?
- live attenuated vaccine
- cold-adapted viruses with mutations in polymerase and/or NP
- given to young children
- similar to the traditional split vaccine that is made of virus killed by detergents
Why is M2 a future target for influenza vaccine?
- M2e extracellular domain is highly conserved among humans strains and overlaos the M1 gene
- has very little scope for mutational escape
- reduces mortality and morbidiy but not always disease
What are defective interfering particles?
- virus particles with deletions in their largest RNA segments
- have a replicative advantage due to having a shorter genome but slow or block infection
- cannot propagate and need normal influenza to infect the same cell to survive
- can be created by radiation which affects larger segments first
- works on any strain and may induce an immune response
- still has HA and N
What is Tamiflu and Relenza?
- NA blocking antivirals
- cause aggregation
