Influenza Virus

Front 1

What are the three types of Influenza virus?

Back 1

There are three types: A, B and C. Influenza A type is likely the most dangerous of the three. It kills 500,000 humans each year.

Front 2

Do all the strains infect all species and humans?

Back 2

Type A influenza infects animals such as birds, pigs seals ferrets and horses.
Type B and C are primarily human viruses.

Front 3

Which type of cell is a preferred target?

Back 3

It is the epithelial cells which line the airways. It infects epithelial cells that line both the upper and the lower respiratory tract. Both of these airways comprise about 1,000 square feet. Within several hours thousands of new viral particles are created.

Front 4

The lower part of the respiratory tract(alveoli) is patrolled by macrophages. True/False

Back 4

True.

Front 5

A sneeze may expel app. how many viral particles?

Back 5

About 10,000.

Front 6

How is binding achieved with the influenza virus?

Back 6

The RNA genome is coated with virus encoded proteins, and enclosed in an envelope, picked up from the previously infected cells. On the outside of the envelope there are about 500 copies of a protein called" viral hemagglutinen". It binds to sialic acid(neuraminic acid) residues of proteins on the surface of the target cells.

Front 7

How did hemagglutinen get its name?

Back 7

When the influenza virus were mixed with RBCs, which have heme, the blood cells clumped. This occurred because there are proteins with sialic acid residues on the surface of the RBCs.
Different influenza virus strains have slightly different hemagglutnin(Hg) molecules.

Front 8

Is the following statement true? "The type of cell or the species infected(host range) is limited by the ability of the viral Hg proteins that bind to the different types of sialic acid.

Back 8

It is true.

Front 9

Before the virus can enter the cell what must first happen regarding the Hg protein?

Back 9

The Hg protein must first be cleaved into two pieces. This splitting is carried out either by enzymes, or by enzymes that are in the environmental vicinity. Only certain cells produce these enzymes. This ability can limit the type of cells that can be infected.

Front 10

Considering that most human cells including airway epithelial cells lack the enzyme(serine protease) required to cleave the Hg proteins of type A influenza, how is the virus able to infect this region so successfully?

Back 10

The important cleavage step must be performed after newly made viruses have been released into the respiratory tract. There are cells in the airway Clara cells, which produce the necessary serine proteases and export them out of the mucus.

Front 11

How is uncoating achieved with the influenza virus?

Back 11

The Influenza virus employs a receptor mediated endocytosis. It first binds to sialic acid residues on the cell surface. The cell then encloses it in a portion of the plasma membrane called an endosome) and transports the cell inside.

Front 12

How does the genome exit the endosome?

Back 12

The plasma membrane of a cell has pumps which normally transports protons out of the cell. However, when the endosome is formed the membrane is inverted so the protons are now pumped into the interior of the endosome. The endosome within now becomes more acidic. When the pH reaches about 5.0, a conformational change takes place in the envelope which permits it and the endosome to fuse, releasing the RNA genome into the cytoplasm.

Front 13

Describe the genome of the influenza virus,

Back 13

The genome is composed of eight short pieces of a single stranded RNA that can encode a total of 11 proteins.

Front 14

What happens once the segments enter into the cellular cytoplasm?

Back 14

These segments quickly enter of the cell, being directed there by nuclear localization signals present on the viral proteins that remain bound to the segments of RNA.

Front 15

Does replication of the Influenza virus take place in the cytoplasm?

Back 15

No. Replication takes place in the nucleus. RNA virus' generally replicate in the cytoplasm.

Front 16

Define positive strand viral RNA.

Back 16

This is defined as RNA which is ready to be translated into proteins, it is synonymous with messenger RNA.
negative strand RNA is the opposite, so a complimentary copy of it must first be made in order to obtain viral messenger RNA.

Front 17

Do human cells possess polymerase enzymes which are capable of replicating RNA molecules?

Back 17

No. In this case, every virus that has an RNA genome must first encode its own polymerase. The RNA strand of negative strand viruses cannot be directly translated into proteins, so they must carry their own polymerase proteins.

Front 18

Regarding the Influenza virus, how does infection proceed once inside the cell?

Back 18

During an infection, one of the polymerase molecules remains associated with each of the eight RNA segments. Once the cells nucleus is reached, its polymerases can activate and can then synthesize complimentary copies of its viral RNAs(vRNA) to yield positive protein, protein coding, messenger RNA(mRNA).

Front 19

What happens after the mRNA is made?

Back 19

The mRNA molecules are then transported into the cytoplasm, and are translated onto cellular ribosomes to make new viral proteins, including more polymerase molecules. These newly made viral polymerases then reenter the nucleus where they can produce more complimentary copies(cRNAs) of the original viral RNA.

Front 20

What happens to these positive strand cRNAs?

Back 20

They are recopied many times by these virasl polymerases to make the negative strands that will form the genomes of newly made viruses.

Front 21

What is meant by cap snatching of viral RNA?

Back 21

In order to be translated by cellular ribosomes the virus' RNA must have a cap structure at its 5' end. The virus can excise the cap off of a cellular mRNA molecule and paste it onto its own mRNA.
In the cells that the virus infects there are already thousands of mRNA molecules. They are all competing for ribosomes.

Front 22

Considering the above explanation, why does the virus need to "snatch" end caps?

Back 22

Regarding all of the cellular mRNAs, without their caps, they cannot be translated. By snatching the caps the virus focuses on the protein making machinery of the cell on making viral proteins. Viral replication at this point takes only 6-7 hours to complete.

Front 23

Considering how fast the virus replicates, how does this directly affect the cell?

Back 23

Infected cells are quickly killed after new viruses are released. It is a cytolytic virus.

Front 24

How do newly made viruses exit the cell?

Back 24

After new vRNAs are coated with virus encoded proteins, they exit the nucleus. They then proceed to the inner surface of the plasma membrane. They must wait there until all eight gene segments have been gathered together. Eventually the whole collection of protein coasted vRNAs buds from the cell surface, taking a piece of cell membrane which creates the viral envelope.

Front 25

Prior to budding, several viral proteins,( including the viral hemagglutinin) are inserted into the cell membrane. True/False

Back 25

True.

Front 26

It is believed that enveloped viruses always have viral proteins inserted into the cell membranes through which they bud. Why is this so?

Back 26

This is true because one or possibly more of these viral proteins will form a type of plug that will engage the socket(recepter) on the surface of the cell the virus wants to infect next.

Front 27

Considering the above explanation, why is it that newly minted viruses are not reinfecting or are recaptured on the cell itself when they try to leave?

Back 27

In fact some new viruses are recaptured. The influenza virus produces a second protein, which like the Hg protein is inserted into the cell membrane during infection. This is the neuraminidase protein. This protein shaves off sialic acid residues from the surface of the infected cell. Once the sialic acid residues are cleaved, new viruses can bud through the shaved membranes and escape unimpeded by the receptors.

Front 28

Does the influenza virus have the ability to defend itself from the Interferon system?

Back 28

It actually utilizes three methods to achieve this.
The first is to block the production of interferon.
The second is to interfere with the interferon induced activation of antiviral genes.
Lastly, some viruses attempt to blunt the effects of those antiviral proteins.

Front 29

What might be considered a drawback of an RNA virus in this case regarding replication?

Back 29

As the genome of the influenza genome is being replicated, long pieces of double stranded RNA are made. When the virus' vRNA is used as a template to make positive strand mRNA, stretches of double stranded RNA exist in the nucleus. In uninfected cells, this type of RNA does not exist.

Front 30

How do infected cells respond to infection?

Back 30

Inside the host cells are pattern recognition receptors like TLR3 which can detect double stranded RNA and activate the interferon system.

Front 31

What is another defense reaction assumed by the infected cell?

Back 31

This involves the 5" triphosphate at the end of the single stranded RNA created by the virus' RNA polymerase. It produces this abnormal 5" end. When this unusual termination is detected, by the cellular pattern recognition receptor, RIG-1, the interferon warning system is activated.

Front 32

What is the RIG-1 pattern receptor?

Back 32

RIG-I-like receptors, abbreviated RLRs, are a type of intracellular pattern recognition receptor involved in the recognition of viruses by the innate immune system.

Front 33

Does the influenza virus possess a defense against the RIG-1 system?

Back 33

Yes. It can encode a protein NS1 that can bind to the double stranded RNA, masking it so that it cannot be recognized by the pattern recognition receptors. NS1 also binds to RIG-1 and inhibits its ability to signal the presence of viral RNA which has a 5' triphosphate.

Front 34

Is the following statement correct?

Back 34

Once the virus takes a cap from the cellular mRNA and attaches it on, the viruses mRNA no longer has a 5' triphosphate. This process helps evade detection.

Front 35

What is the function of kinase and PKR?

Back 35

When an interferon induced gene encodes a kinase, and PKR, which is activated by a double stranded RNA, it can shut down protein synthesis in an infected cell.

Front 36

Are other genes activated when interferon bind to a cell's receptors?

Back 36

Yes. 2,5, oligo(A) synthetase and RNAse L, is activated.
Double stranded viral RNA activates the synthetase which then activates RNAse L. When RNAse L is in an active form it can destroy both cellular and viral RNA.

Front 37

Regarding viral defense protein, does the NS1 protein have a role to play concerning the above explanation?

Back 37

Yes. The NS1 protein can cover or cloak the viral double stranded viral RNA and reduce the activation of these destructive enzymes, allowing protein synthesis to proceed in infected, interferon warned cells.

Front 38

Can the influenza virus establish a latent or chronic infection?

Back 38

No.

Front 39

Is the influenza virus known for its mutational nucleotides?

Back 39

Yes. The virus' RNA dependent RNA polymerase does not possess the capacity to proofread its progression. Mutations in the genome will arise as incorrect nucleotides are inserted into the growing RNA chains.

Front 40

Can the influenza's mutational aspects serve as a possible advantage?. Is this what is meant by antigenic drift?

Back 40

Yes. Due to the fact that its polymerase makes about 1 mistake in every 10,000 nucleotides, it copies nearly every virus produced in an influenza infected cell will be a mutant. The genetic code "drifts" as mutations are introduced during copying, and some of these mutations can change the shape of the Hg and neuraminidase proteins enough to prevent binding by neutralizing antibodies.

Front 41

How are influenza strains defined?

Back 41

They are defined by the neutralizing antibodies that can bind to either the Hg protein or to the neuraminidase protein.

Front 42

How many Hg and Nm subtypes have been identified?

Back 42

16 Hg (H1-H16)
9 Nm (N1-N9)

Front 43

It is because of antigenic drift that individuals with outdated antibodies who are immune to the original virus can be reinfected by the mutant strain. True/False

Back 43

True. Drifts are responsible for the yearly epidemics.

Front 44

Can antigenic shift be seen in all types of Influenza?

Back 44

No. It is unique to type A. Types B and C are unable to demonstrate shift because these viruses do not have a non human host which can produce a strain of virus that infects humans.

Front 45

Why specifically is shift possible with type A influenza?

Back 45

Type A can infect birds and water fowl. Human strains were originally avian strains that by mutations acquired the ability to infect humans.

Front 46

Type A influenza causes an acute respiratory disease in humans, and in ducks it infects cells in the digestive tract, and is spread by the fecal oral route. True/False

Back 46

True. In ducks, defecation releases many newly made viruses. Young ducks drink infected water.

Front 47

How does the influenza virus survive in the acidic conditions in the digestive tractr of ducts?

Back 47

This is likely due to mutations introduced by the polymerase, duck versions of influenza A virus. They are less prone to acidic insult than are human strains of the flu. Via mutation, route of entry is altered.

Front 48

Why are birds and ducks reservoirs for new human influenza strains?

Back 48

Avaian influenza viruses usually reproduce poorly in human cells. A pig can in facct be a host to both avaian and human influenza A viruses, and sometimes viruses of both origins infect the same pig cell. As a result, new strains are produced that are part human and part bird. This is possible because of a segmented genome.

Front 49

Regarding this large reassortment of viral genes, where is this most likely to occur?

Back 49

It is greatest in places where large numbers of pigs, ducks and humans are in close contact, such as in China.

Front 50

Why are serious forms of pneumonia occasionally seen with the flu virus?

Back 50

The virus destroys large areas of ciliated epithelial cells. Most cases of influenza associated pneumonia are due to superinfecting bacteria which obtain easy access to the damaged respiratory airways.

Front 51

How did a pandemic such as the Spanish flu of 1918 come about?

Back 51

This can occur when the pig virus hybrid includes a bird gene segment that encodes a new Hg protein. In fact, it is so new that none of the neutralizing antibodies produced during earlier flu infections can recognize the bird Hg protein.