Viral infections - HIV

Question 1

distribution of HIV

Answer 1

[linked to socioeconomic reasons]

high % in southern African countries

Question 2

HIV viral particle

Answer 2

Enveloped, ssRNA virus

Question 3

key components of HIV viral particle

Answer 3

Gp120 (viral protein)

Gp41 (viral protein)

Reverse transcriptase protein

Integrase protein

Viral RNA

Question 4

role of Gp120/Gp41

Answer 4

attach to cell and infect it

Question 5

main role of HIV viral particles

Answer 5

primarily infects CD4+ T cells of the immune system

On surface of CD4+ = receptors

Question 6

key stages of replication cycle

Answer 6

1. Attachment of gp120 to CD4 receptors
2. Fusion of viral and host cell membranes
3. Uncoating of viral particle and release viral material
4. Reverse transcription of viral RNA into DNA
5. Import of viral DNA into nucleus
6. Integration of viral DNA into host DNA
7. Transcription of viral DNA to create new viral RNA
8. Export of new viral RNA
9. Translation of viral RNA to create new viral protein
10. Assembly of new viral RNA and protein at the cell surface
11. Budding of new viral particles at cell surface
12. Release of new viral particles from cell surface
13. Maturation of viral proteins (conversion of single protein and individual proteins)

Question 7

fusion of viral and host cell membranes

Answer 7

HIV gp120 protein mainly binds to CD4 receptors, but other receptors are also involved in infection
= co-receptors

Question 8

main co-receptors

Answer 8

[CCR-5; CXCR4]

A person’s HIV usually uses 1 receptors or the other

Either have CCR5 or CXCR4 receptors used to attach to CD4 cell

But some people can use either to attach to CD4 cell to replicate

Question 9

characterisation of HIV

Answer 9

chronic immune activation and CD4 T cell depletion => leads to dysfunction of immune system

Question 10

what is the main contributor to the pathogenesis of HIV?

Answer 10

immune activation

Manifested in T cell proliferation, increased expression of CD38 on CD4 and CD8 T cells

Despite immunodeficiency, virtually all cellular components of immune system, B cells, NK cells, T cells and macrophages show evidence of immune activation

Elevated T cell proliferation is observed in vitro and in vivo as well as increased expression of T cell receptor

Question 11

why is there elevated CD4 T cell proliferation, yet CD4 T cell depletion is also observed?

Answer 11

T cell proliferation is advantageous to HIV, as T cells act as the host for replication i.e. more T cells = more places for virus to replicate

Also, infected T cells don’t function properly; although T cells are proliferating, the new T cells are not able to produce an effective immune response

Question 12

Opportunistic infections

Answer 12

[HIV-TB interaction and co-infection]

HIV infection increases likelihood that new infection with M. tuberculosis (due to immune suppression) will progress rapidly with TB disease

HIV = most potent factor known to increase risk of progression from M. tuberculosis infection -> disease

Question 13

HIV treatment

Answer 13

AZT = first drug

Most drugs developed against reverse transcriptase and protease

Question 14

problem with HIV medication?

Answer 14

resistance -> virus undergoes mutations

Treatment with single drug = short-term benefit
- Long term only serves to select resistant
mutants

Current treatment involves combinations of drugs acting on different targets

Question 15

types of anti-viral drugs

Answer 15

1. Nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs)
2. Non-nucleoside reverse transcriptase inhibitors (NNRTIs)
3. Protease inhibitors (PI)
4. Fusion inhibitors
5. Integrase inhibitors
6. CCR5 antagonists (entry inhibitors)

Question 16

NRTIs

Answer 16

AZT = pro-drug
- Needs to be converted to AZT-
triphosphate to be active as a chain

NRTIs act as chain terminators at the substrate binding site of RT

Question 17

NNRTIs

Answer 17

Inhibit viral DNA replication by binding at the allosteric non-bonding site of RT, causing a conformational change of the active site (DNA polymerisation blocked)

Structurally diverse

Question 18

Protein inhibitors

Answer 18

During the reproduction cycle of HIV, a specific protease is needed to process GAG and POL polyproteins into mature HIV components

All contain hydoxyethylene bond instead of a normal peptide bond

Competitive inhibitors

Question 19

significance of hydroxyethylene bond

Answer 19

makes protein inhibitors non-scissile substrate analogs for HIV protease (i.e. can’t be broken)

Sp2 -> sp3 (intermediate is now tetrahedral)

Question 20

Fusion inhibitors

Answer 20

Works by blocking entry of HIV into cells (CD4 or T cells)

Fuzeon was rationally designed to mimin components of GP41 and displace them, prevent normal fusion

Peptide-based inhibitors

Must be given intra-venously (not orally)

Question 21

mechanism of HIV fusion

Answer 21

1. HIV approaches a host CD4+ or T cell. It’s viral membrane contains trimeric glycoprotein spikes. Each spike contains a gp41 and gp120 unit
2. Fusion begins with the binding of gp120 to the CD4 and chemokine receptors on the cell membrane
3. Binding induces a conformational change in gp120 moving it aside and exposing gp41
4. Fusion = mediated by gp41, which contains 2 heptad repeating domains, HR1 and HR2
5. As gp41 = exposed, hydrophobic terminus of gp41 embeds itself into cell membrane
   • HR2 domain begins to coil into grooves exposed on the trimeric HR1 domain of gp41 = zipping
   • Destabilises both cell and viral membranes by punching hole (fusion pore) into both membranes
     Allows HIV capsid to pass through cell membrane and infection occurs

Question 22

Mechanism of HIV fusion inhibition

Answer 22

Fuzeon is a peptide mimic of HR2 region of gp41

It binds to the HR1 region

Zipping can’t take place and so infection is blocked

Question 23

Entry inhibitors

Answer 23

Blocks the attachment of HIV gp120 protein to the CCR5 receptor, halting HIV replication

CCR5 inhibitors = only effective in the blocking the CCR5 receptor site

HIV that uses CXCR4 receptors will be unaffected, meaning replication can continue

Question 24

Integrase inhibitor

Answer 24

HIV uses host cells’ genetic material to make its own DNA (= reverse transcription)

virus has to integrate its genetic material into the genetic material of the host cell via integrase enzyme

integrase inhibitors block this enzyme and prevent virus from adding its DNA into the DNA in your CD4 cells

Prevents virus from replicating and making new viruses