8. The Pathogenicity of Viruses - HIV

Question 1

What family of viruses does the HIV virus belong to?

Answer 1

Retroviruses

Question 2

What disease is caused by HIV?

Answer 2

AIDS

Question 3

Describe the genome of HIV

Answer 3

Two positive sense single stranded RNA molecules coding for 9 genes.

Question 4

How many HIV subtypes are there?

Answer 4

1. HIV-1

2. HIV-2

Question 5

What are the effects of HIV infection?

Answer 5

Immune suppression and an inability to fight infection

Question 6

What cells can be infected by HIV?

Answer 6

The main cellular target of HIV is CD4+ T lymphocytes however they are also capable of infecting CD4+ Macrophages and some populations of Dendritic cells.

Question 7

How many subtypes of HIV-1 are identified and what are they?

Answer 7

Three:

1. X4
2. R5
3. R5/X4

Question 8

How are the X4 and R5 subtypes of HIV-1 infection distinguished?

Answer 8

HIV-1 subtypes are distinguished form one another based on the co-receptor used during the attachment process.

X4: CXCR4
R5: CCR5

Dual tropic R5/X4 strains are capable of both.

Question 9

Which of these HIV-1 subtypes is of increased cytopathicity?

Answer 9

CXCR4 binding strains (mainly X4)

Question 10

Which of these HIV-1 subtypes is currently predominant in infected individuals?

Answer 10

CCR5 binding strains (mainly R5)

Question 11

On what T lymphocytes are the CXCR4 and CCR5 receptors located?

Answer 11

CXCR4 receptors are located on both naive and memory CD4+ T lymphocytes while CCR5 receptors are exclusive to memory CD4+ T lymphocytes.

Question 12

What host cell surface proteins are used as receptors for the attachment process of HIV?

Answer 12

CD4+ is the primary receptor for the HIV followed by an alpha or beta chemokine co-receptor. In the X4 strain this co-receptor is CXCR4 receptor while in the R5 this co-receptor is CCR5

Question 13

What HIV ligands bind to host cell receptors during attachment?

Answer 13

GP120 of the HIV binds to CD4 and coreceptors strengthen this binding.

Question 14

Describe the process of HIV entry into the host cell once attachment has occurred?

Answer 14

1. The binding of HIV to CD4 and the chemokine coreceptor results in a change in conformation of the HIV ligand GP 120.
2. This causes the GP120-bound GP41 to unfold and insert its hydrophobic terminus into the host cell plasma membrane.
3. This association brings HIV and host cell in an increasingly stable close proximity allowing from plasma membrane fusion and insertion of the HIV genome into host cells.

Question 15

What three proteins are inserted into the host cell along with vRNA of HIV?

Answer 15

1. Integrase
2. Protease
3. Reverse transcriptase

Question 16

Once HIV viral entry is complete, what is the nextt obstacle the HIV virus must overcome?

Answer 16

The cortical Actin Barrier

Question 17

How does HIV overcome the cortical actin barrier?

Answer 17

Actin remodelling by Nef protein of HIV

Question 18

Describe the process of reverse transcription of HIV once it is inserted into the host cell.

Answer 18

While in the cytoplasm Reverse transcription of the HIV virus genome is catalysed by the enzyme REVERSE TRANSCRIPTASE in order to form a double stranded DNA molecule from the positive sense single stranded RNA genome.

1. The polymerase active site of RT catalyses the formation of a RNA/DNA double helix from the + sense single stranded vRNA.
2. The ribonuclease H (RNaseH) active site of RT continues to remove the RNA nucleotides in order to form a single strand of DNA.
3. The polymerase active site of RT then catalyses the formation of a DNA double stranded molecule from the single stranded DNA template.

Question 19

How does the reverse transcription process contribute to the development of resistance to antiviral drugs?

Answer 19

Mutations often occur during the RT process due to the poor read-through ability of RT. These mutations are speculated to contribute to the development of resistance to antiviral drugs

Question 20

What protein is associated with transfer of the HIV DNA into the nucleus?

Answer 20

Integrase

Question 21

Describe the integration process of HIV DNA into the host genome by integrase.

Answer 21

1. The protein integrase cleaves a dinucleotide from each 3’ end of the DNA double helix to create two “sticky ends” in preparation for insertion of the HIV DNA into the host cell genome.
2. Integrase then catalyses the
   cleavage of target DNA
3. Fusion of 3’ sticky ends to 5’ termini of the host genome occurs.
4. 5’ ends of HIV are fused with 3’ ends of host genome by a process of GAP REPAIR catalysed by host cell repair machinery.

Question 22

Once HIV DNA is incorporated into the host genome how does transcription occur?

Answer 22

In the same manner as host genes.

Question 23

Why are activated T cells more affected by the HIV virus?

Answer 23

In order for incorporated HIV DNA to be transcribed the activity of the NF-KB protein is required which is upregulated when T cells are activated. Increased expression of viral DNA results in increased cytotoxicity of the virus to the infected cell.

Question 24

What are the first HIV protein products to be produced and why?

Answer 24

Tat and rev are the first protein products to be produced as HIV mRNA is retained in the nucleus until splices and these two proteins are the products of spliced forms of HIV mRNA.

Question 25

What is the role of the Tat protein?

Answer 25

Tat drastically enhances the efficiency of viral transcription.

Question 26

What is the role of the rev protein?

Answer 26

HIV mRNA in both the spliced and unspliced forms are required to produce all the necessary protein products of the HIV virus, however unspliced mRNAs are retained in the nucleus until splicing occurs. Rev, with its NLS is localised to the nucleus of host cells and facilitates the export of unspliced or partially spliced HIV mRNAs.

Question 27

Tat and rev enable the nuclear export of unspliced HIV mRNA. What protein products are made from these unspliced mRNAs?

Answer 27

Gag and Env proteins are made from the full length mRNA.

Question 28

Besides the formation of gag and env proteins, what is the importance of full length HIV mRNA?

Answer 28

This full length HIV mRNA is identical to original positive sense, single stranded RNA molecules of the HIV genome, therefore the genome is essentially copied and exported to the cytoplasm for packaging.

Question 29

What three phases are associated with the HIV infection?

Answer 29

1. Acute phase 2. Asymptomatic phase 3. Symptomatic phase and AIDS

Question 30

How long does it take to progress through the three stages of disease?

Answer 30

The course of the disease varies enormously. It has been known to progress as quickly at 6 months and as long as 25 years. The exact reasons for this are not fully understood.

Question 31

What can be used to determine whether the HIV disease will progress rapidly or not?

Answer 31

Counts of the following during the 6-12 month mark of infection. 1. Plasma viral load 2. CD4 counts

Question 32

What are the clinical characteristics and symptoms of the acute phase of HIV infection?

Answer 32

The initial acute phase of HIV infection is characterised by a large number of HIV viral particles, an a rapid decrease in CD4+ T lymphocytes. At the end of this phase HIV levels begin to fall due to the immune response to relatively stable levels known as the "viral set" and CD4+ T cell levels begin to rise however often never reaching pre-infection levels. Flu-like symptoms may develop including fever, swollen glands, sore throat, rash, muscle and joint aches and pains, fatigue and heatache.

Question 33

What are the clinical characteristics and symptoms of the Asymptomatic phase of HIV infection?

Answer 33

After the acute phase, a latent reservoir of HIV-infected resting memory CD4 T cells is established where the virus is replicated at slow rates due to the transcriptionally inactive resting state of the infected CD4+ T cells. Mild to no symptoms are experiences during this phase.

Question 34

What are the clinical characteristics and symptoms of the symptomatic phase of HIV infection?

Answer 34

At this stage CD4+ T cells have fallen below 200 cells/uL or an opportunistic infection has taken advantage of the host immunocompromised state irrespective of CD4+ T cell levels. At this point AIDS has effectively developed.

Question 35

Once AIDS has developed what is the life expectancy of the infected individual?

Answer 35

3 years without treatment, however if an opportunistic infection has established life expectancy falls to ~1 year.

Question 36

Describe the initial innate response to HIV infection.

Answer 36

1. Toll like receptors (TLR) of Dendritic cells are activated by HIV-1 RNA and other microbial products to produce the cytokines to activate the immune response: - Interferon-alpha (IFN-alpha) - IL12 - Tumor necrosis factor- alpha (TNF-alpha) - IL-6 2. Rapid increase in NK cells thought to be associated with the decrease of viral load of the end of the acute infection.

Question 37

How does the HIV virus alleviate the innate immune response?

Answer 37

Infection of dendritic cells impair their function. Over time a pool of nonfunctional NK cell accumulate .

Question 38

Describe the adaptive immune response during HIV infection.

Answer 38

CD8 T cell expansion allowing for the destruction of virus infected cells. A large number of antibodies are developmed in a matter of weeks from initial infection.

Question 39

How does the HIV virus alleviate the adaptive immune response?

Answer 39

While most of the antibodies produced bind, few actually neutralise the virus. This is largely accounted for by the high mutation rate of the HIV during RT allowing new HIV mutants to espace recognition by old antibodies. Studies also suggest a functional impairment of CD8 Cytotoxic T cells to kill infected CD4 T cells. Potential defects speculated include an inabiliy to express multiple cytokines simultaneously, a failure to kill infected target cells or inhibit HIV replication, proliferation and maturation. CD8+ T cell number also decreases towards the symptomatic stage of infection.