Immunology - Viral Infection

Question 1

What basic features do all viruses share?

Answer 1

DNA or RNA core (never both)

Protein coat made up of individual capsomeres

Helical, cubic or more complex arrangement

No cytoplasm

May have envelope derived from host cell

Requires an intracellular organism to replicate

Question 2

What are the 6 stages of viral replication?

Answer 2

1. Attachment
2. Entry
3. Uncoating
4. Replication
5. Assembly
6. Release

Question 3

What does the attachment stage include?

Answer 3

The virus becomes attached to the cell by specific cellular receptors (glycoproteins, phospholipids or glycolipids).

Question 4

What does the entry stage include?

Answer 4

Following attachment the virus can enter the cell, most commonly via receptor mediated endocytosis.

Question 5

What does the uncoating stage include?

Answer 5

Once inside the host cell, the viral capsid must be uncoated to release the viral nucleic acid. Uncoating may be achieved by host or viral enzymes that will degrade the capsid.

Question 6

What does the replication stage include?

Answer 6

Once uncoated, viruses (DNA or RNA) replicate by switching the host machinery from cellular protein synthesis to viral synthesis and viral proteins are produced.

Question 7

What does the assembly stage include?

Answer 7

Newly synthesised viral proteins are post-transcriptionally modified and packaged into virions that can be released from the infected host cell to infect other cells.

Question 8

What does the release stage include?

Answer 8

Virions are released from the cell either by lysis or budding. In lysis, the infected cell dies and the virions are released. In budding, the virion takes some of the host cell’s membrane with it as it leaves – this normally does not kill the infected cell.

Question 9

What are the different ways in which viruses can evade the immune system?

Answer 9

1. Mutations in many different viral sites including changing the proteins at binding sites for antibodies –> antigenic drift
2. Latency: slowly growing and remaining dormant in cells can make it difficult for the immune system to recognise them
3. Production of immunosuppressive molecules: impair immune function.

Question 10

What immune cells does HIV target?

Answer 10

CD4+ Th cells

Question 11

What protein does HIV use to attach itself to CD4+ cells?

Answer 11

Using a protein called gp120 on its viral envelope to attach to the CD4 molecule of the T cell, assisted by the T cell co-receptor CXCR4.

Question 12

What enzyme does HIV use to combine itself into the host cell’s genome?

Answer 12

viral integrase enzyme

Question 13

Once HIV is inside the cell, what does it do?

Answer 13

The virus undergoes reverse transcription and single strands of viral RNA are converted into double-stranded DNA by the viral reverse transcriptase enzyme. The virion is then assembled, buds and leave the host cell, ready to infect other cells.

Question 14

In the first one to two weeks following HIV infection, what are the typical symptoms?

Answer 14

Flu like symptoms

Question 15

What is seroconversion in regards to HIV infection? When does it occur?

Answer 15

Seroconversion is the period when immune responses to HIV start to develop thoughout the body.

This is usually 1-3 weeks after infection. During this time up to 80% of people have symptoms.

Question 16

Describe the CD4+ levels are infection with HIV?

Answer 16

After acute HIV infection, the viral load and CD4+ T cell count remain relatively stable until the virus progresses into clinical AIDS.

Once AIDS has developed the levels of CD4+ T cells decline steadily, leading to opportunistic disease and eventual death.

Question 17

What is the treatment for HIV?

Answer 17

Antiretroviral Therapy (ART), aimed at targeting virally encoded proteins that are essential for viral replication but are absent from normal human cells. The therapy involves a combination of different medications to minimise the risk of the virus evolving to evade the treatment.