L19. Acute Infection: Influenza

Question 1

What are the two types of influenza infection?

Answer 1

Seasonal

Pandemic

Question 2

What are the typical symptoms of influenza?

Answer 2

Fever/chills
Cough
Headache
Muscle aches
Fatigue
Loss of appetite

Question 3

Describe the infection course (timing) of influenza

Answer 3

it is an acute infection
INCUBATION: 1-5 days
INFECTIOUS for 5-6 days
Lasts about 7 days or longer
NO persistence of the virus (weakness and cough may last for several weeks)

Question 4

Who are the at-risk groups for severe infections?

Answer 4

Young
Elderly
Chronically ill (heart, lung, renal, metabolic)

Question 5

Despite being a largely subclinical disease, why is influenza considered an important infection?

Answer 5

Because it has a very large economical burden

Because the worldwide mortality rate of influenza per year is very large (250,000 to 500,000 per year)

Question 6

How is the influenza virus spread?

Answer 6

By droplet inhalation spread by coughing and sneezing and thus enter and infect the respiratory tract.

Question 7

To what does the virus bind on the cell surface in the humans? What is important about this receptor?

Answer 7

The exact receptor is unknown
On non-ciliated respiratory epithelial cells
We know the virus binds to the SIALIC ACID which is S2alpha-6 linked to galactose
It is important because it is only expressed in the RT (localised infection)

Question 8

What are the local symptoms of influenza infection? What causes them?

Answer 8

Caused by both the tissue damage by the virus and the subsequent host inflammatory response.
Fever (IL-1): cytokines and interferon
Malaise, head and muscle aches: IFN

Question 9

What is meant by the synergistic interaction of influenza with bacterial populations?

Answer 9

Bacteria (H influenza, S aureus, S pneumonia) can take opportunity of the damaged cilia and RT environment and cause disease - secondary bacterial infection when they normally wouldn’t have

Question 10

What family does the influenza virus belong to?

Answer 10

Orthomyxoviridae family

Question 11

Describe the influenza virus structure

Answer 11

Enveloped

negative sense ssRNA with a segmented genome (8RNPs)

Question 12

What are the three types of influenza why are they different?

Answer 12

Types A, B and C have no immunological cross reactivity (serologically different)
They cause the production of different antibodies to internal antigens

Question 13

Which types of influenza virus are important to human disease?

Answer 13

Types A and B

Type A in particular is able to cross species barriers

Question 14

Describe the influenza virus ribonucleoprotein (RNP) of Type A influenza

Answer 14

Has 8 gene segments (RNPs) of RNA wound in a helical structure protected by a capsid protein.
It has 3 RNA polymerase subunits

Question 15

What the the most important proteins expressed on the viral surface?

Answer 15

1. HA - Haemoagglutinin
2. NA - Neuraminidase
3. M2 - an ion channel for H+
4. NS1 - Nonstructural protein 1

Question 16

What is the purpose of the surface protein Non-structural protein 1?

Answer 16

It counteracts any interferon activity: protects the virus from the antiviral environment normally set up by interferon.
Thus it is anti-interferon

Question 17

Which of the surface proteins interact with the sialic acid-containing receptors on human cells?

Answer 17

Both HA and NA interact and bind with the receptor (both have binding sites)

Question 18

Describe the differences between the structure and roles of HA vs. NA

Answer 18

HA: is a trimer with 3 binding regions
It ATTACHES to the cell and this allows entry into the cell

NA: is a tetramer with 4 binding regions (mushroom shaped cell)
It also binds to sialic acid but it cuts it off the host cell

Question 19

How do the different subtypes of influenza arise?

Answer 19

Different subtypes of influenza A share the same internal proteins (matrix, polymerase etc) but they have different expression of both the HA and NA proteins and hence have different antigenicity.
16 types of HA and 9 types of NA

Question 20

Who are the original/ancestral hosts of the influenza A virus?

Answer 20

Aquatic birds

Question 21

What are the three main influenza A subtypes that have circulated in humans in the recent decades?

Answer 21

H1N1
H2N2
H3N2
Currently only H1N1 and H3N2 are circulating

Question 22

Describe the viral replication steps of influenza

Answer 22

1. Viral HA binds to sialic acid linked to galactose receptor and it enters the cell by endosome fusion
2. pH change occurs and the endosome membrane fuses with the viral envelope
3. RNPs escape the cell to the nucleus
   4, Viral RNA replication and mRNA protein synthesis
4. Protein modification and synthesis through the golgi and Er for HA and NA
5. Virus buds out of the cell
6. NA binds back to sialic acid and cleaves it off the cell
7. Tryptase enzyme cleaves the virus HA proteins at a single site
8. Virus goes to infect another cell

Question 23

Why does NA cleave the sialic acid off the host cell?

Answer 23

To stop inefficient reentry of the viral particles into the dying cell

Question 24

Why does tryptase clara enzyme cleave the viral HA?

Answer 24

Cleaves at a specific amino acid site to expose a hydrophobic fusion peptide that is important for the molecule to undergo membrane fusion and endosomal escape in the next cycle,

Question 25

Describe the CD8 response to influenza infection

Answer 25

Memory CD8 T cells can kill the virus-infected cells as they recognise INTERNAL ANTIGENS of the virus due to the broad cross reactivity between the Type A subtypes.
This is not long lived and can be boosted with repeated exposure

Question 26

Describe the antibody response to influenza infection

Answer 26

Inhibit the attachment or release of the virus (against HA or NA) and can kill by complement or can opsonise for phagocytosis.
Pre-existing antibodies don’t protect against new virus subtype infection

Question 27

Why are antibodies ineffective in preventing recurrent influenza infections?

Answer 27

ANTIGENIC DRIFT which creates new strains within the subtype.
Mutations due to errors in polymerase (no proof reading) leads to accumulation of mutations called DRIFT
If the changes occur in the antibody neutralising regions (HA or NA) then these are selected for and this is called ANTIGENIC SHIFT

Question 28

How many binding epitopes are in HA?

Answer 28

5 different antigenic sites surrounding the binding region and accumulate mutation here.

Question 29

What causes an epidemic?

Answer 29

When there are mutations in all 5 antigenic sites and thus the vast majority of the population have antibodies that are not at all effective at binding to the HA and neutralising it.

Question 30

What are the targets of vaccine-induced immunity for influenza?

Answer 30

HA blocking attachment

NA blocking release

Question 31

Describe the influenza vaccine

Answer 31

An INACTIVATED trivalent vaccine with 3 influenza viruses representing the most recent strains of influenaa A and B
Viruses is grown in eggs and purified from allantoic fluid. The virus is then chemically inactivated and detergent disrupted.

Question 32

Who is the influenza vaccine given to and how?

Answer 32

Intramuscularly administered to at risk people (children, elderly, chronically ill and health workers at risk of passing on the virus to at risk people)

Question 33

What are the targets for antiviral drugs in influenza?

Answer 33

1. NA inhibitors

2. Ion channel blockers

Question 34

Describe the NA inhibitors (examples and mechanism of action)

Answer 34

Relenza (Zanamivir) - inhaled and Tamiflu (Oseltamivir) - oral (prodrug)

Active against both A and B
reduces duration and severity of disease but is only fully effective within 2 days of developing symptoms

2x daily

Question 35

Describe the M2 ion channel blockers (examples and mechanism of action)

Answer 35

ADAMANTANES: Amantadine and Rimantadine

Block the M2 ion channel and inhibit the uncoating of influenza A in the endosome (prevents pH change) and thus prevents infection.

Oral, daily

Question 36

What is meant by designer drugs?

Answer 36

Small molecules (analogs of sialic acid) which bind irreversibly to the active site of NA and blocks its enzymatic activity and interferes with the release of new virus particles from the cell.

Question 37

What is antigenic shift? What does it cause?

Answer 37

When there is a SUDDEN appearance of a new subtype influenza A virus of new HA (sometimes NA) in the human population

Causes pandemic = global spread due to a complete lack of protective immunity. (often with high morbidity and mortality)

Question 38

Why are pandemics/antigenic shift rare?

Answer 38

Because humans have a S2α-6 sialic acid-containing receptor
While acquatic birds (the source of the new subtype) have S2α-3 sialic acid-containing receptors for the virus

Hence the virus doesn’t recognise any receptor on the human if it tried (without the very rare mutation to do so - even if the mutation occurred it would need to meet a human or die in the bird)

Question 39

What is the major means of creating a new human subtype of influenza?

Answer 39

REASSORTMENT in a mixing vessel: the pig
The pig contains both the S2α-6 (human) and S2α-3 (bird) receptors and thus infection with a human and a bird specific virus (CO-INFECTION) can lead to ASSORTMENT of the viral segments when they infect the SAME CELL.