Outcomes of Viral Infection L19

Question 1

Do viruses always kill the cells which they infect?

Answer 1

No.

Question 2

If we infect cells with a virus in vitro a number of outcomes are possible:

• Null infection
• Abortive infection
• Cytopathic effect
• Cell fusion
• Persistent infection
• Latent infection
• Transformation

Define each term.

Answer 2

Null infection – the virus cannot attach to or penetrate the cell

Abortive infection – the virus can enter the cell but can’t produce infectious progeny

Cytopathic effect – cell damage/death (by lysis/apoptosis)

Cell fusion – syncitium formation

Persistent infection – the virus replicates and progeny are continuously released but cell survives (or cell population survives)

Latent infection – virus genome stays in cell, but no progeny are produced

Transformation – viral infection results in immortalisation of cell line

Question 3

Do all virus infections cause disease?

Answer 3

No. Most viruses have little impact on the health of the host
If host dies, virus must infect another host quickly, or virus will die too.

Question 4

Cell damage in host organism may be caused by what?

Answer 4

• Virus effects on cell (cytopathic effect)

- Immune response (immune mediated damage)

Question 5

Name the 2 innate immune responses to viral infection.

2

Answer 5

• Natural killer cells

- Interferons

Question 6

Describe the adaptive immune response to viral infection.

4

Answer 6

• Activation of CD4+ T-helper cells by antigen presenting cells
• Destruction of virus infected cells by CD8+ cytotoxic T cells
• Production of antiviral antibody by B-cells
• Maintenance of memory B-cells and T-cells

Question 7

Which cells produce IFNalpha and IFNbeta?

Answer 7

Virus infected cells.

Question 8

Which cells produce IFNgamma?

Answer 8

T cells and NK cells.

Question 9

What effects to interferons produce?

5

Answer 9

• Induces antiviral state in target cells: viral replication inhibited (IFNa & IFNb only)
• Inhibits protein synthesis and growth of target cells
• Activates macrophage, NK cells and cytotoxic T cells
• Increases antigen presentation by MHCI and MHCII
• Induces fever

Question 10

What does an antibody do to a free virus in viral infection?

3

Answer 10

• Blocks binding to the cell
• Blocks entry to the cell
• Blocks uncoating of virus

Question 11

What does an antibody + compliment do to a free virus in viral infection?
(2)

Answer 11

• Damage to virus envelope

- Opsonisation of virus

Question 12

What does an antibody + compliment do to virus infected cells?
(2)

Answer 12

• Lysis of infected cell

- Opsonisation of infected cell

Question 13

What do antibodies bound to virus infected cells do?

Answer 13

• ADCC by NK cells, macrophage and neutrophils

Question 14

Define a Neutralising antibody.

Answer 14

An antibody which blocks or inhibits the process of viral infection.

Question 15

Name the antibody:

Always the first isotype to be produced. Presence indicates a recent infection.

Answer 15

IgM.

Question 16

Name the antibody:

The major isotype in mucosal secretions. Passed from mother to child in breast milk.

Answer 16

IgA.

Question 17

Name the antibody:

Major isotype in blood and extracellular fluid. Passed from mother to child transplacentally.

Answer 17

IgG.

Question 18

Describe “Passive evasion” as a way in which to avoid the host immune response.
(3)

Give examples.

Answer 18

• Mutagenic escape – antigenic drift/antigenic shift (eg influenza A)
• Compartmentalisation – eg mucosal (Respiratory Syncitial Virus, RSV), inside neurons (Herpes simplex virus, HSV)
• Immune tolerance (Hepatitis B virus, HBV)

Question 19

Describe “Active evasion” as a way in which to avoid the host immune response.
(3)

Give examples.

Answer 19

• Interfering with IFN production/signalling (eg Hepatitis C virus, HCV)
• Blocking MHC antigen presentation (eg MHCI by HSV)
• Immunosuppression (eg HIV)

Question 20

Possible outcomes of viral infection in vivo:

Describe Asymptomatic/subclinical infection.

Answer 20

No adverse symptoms – virus cleared by immune response.

Or establishes persistent infection

Question 21

Possible outcomes of viral infection in vivo:

Describe Acute infection.

Answer 21

Illness lasts a few days/weeks – virus then cleared by immune response.
(Or establishes persistent infection or death).

Question 22

Possible outcomes of viral infection in vivo:

Describe Chronic infection.

Answer 22

Virus not cleared by immune response; continuous production of virus progeny.

Question 23

Possible outcomes of viral infection in vivo:

Describe Latent infection.

Answer 23

Viral genome maintained in host cells for many years until re-activated (eg by stress). No progeny during latent period.

Question 24

Possible outcomes of viral infection in vivo:

Describe Slowly progressive disease.

Answer 24

Virus not cleared by immune response; disease symptoms only apparent after prolonged infection.

Question 25

Possible outcomes of viral infection in vivo:

Describe Tumorigenesis.

Answer 25

Virus induces transformation of infected cell leading to cancer (virus may or may not be cleared by immune response).