HIV and AIDS Flashcards
1
Q
When was AIDS first defined?
A
First defined in 1981
2
Q
How many people have been infected since the start of the HIV epidemic?
A
74.9 million
3
Q
How many people have died since the start of the HIV epidemic?
A
32 million
4
Q
Why has the number of deaths decreased?
A
Related to the introduction of generic antiviral drugs driving down the price of antiviral therapy
5
Q
Where are most HIV cases?
A
Africa
6
Q
What are the HIV basic properties?
A
- Lentivirus = slow growing retrovirus
- Spherical enveloped virus
- Approximately 100nm diameter
- Prominent peplomers on the virus surface - spikes
- Contain dense core 70-80 nm - shape variable
- Positive stranded, diploid RNA genome - 2 RNA molecules are held together at the 5’ end by non-covalent interactions
- RNA genome is protected by the nucleocapsid protein
- Nucleocapsid layer is encapsulated by the capsid
- Capsid is surrounded by a coat of matrix protein
- All the proteins involved in the structure are encoded by the GAG gene
- The outside layer is a lipid layer taken from the infected host cell
- 2 glycoproteins are expressed on the surface encoded by the ENV gene - surface glycoprotein = gp120 and transmembrane protein = gp41
7
Q
Describe HIV infection and cell entry
A
- HIV recognises and attaches the CD4 antigen on the T cell lymphocyte
- Requires cellular co-receptor - beta cytokine receptor CCR-5 or alpha cytokine receptor CXCR4
- Both co-receptors are found on many cell types - including CD4/CD8 and macrophage
- HIV in peripheral blood uses mainly CCR-5 - main tropism
- gp120 binds to CCR5 (macrophage) or CXCR4 (T cell) co-receptor - allows tighter binding
- gp41 inserts fusion peptide into the host membrane
- Fusion peptide brings the viral envelope and host cell membrane together - allows fusion which releases the viral capsid into the host cytoplasm
8
Q
What alleles does CCR-5 exist as?
A
- Normal allele
- Defective truncated allele
9
Q
How does being homozygous for delta32ccr5 affect HIV?
A
Highly resistant to HIV
10
Q
What does Gag produce?
A
p55
11
Q
What does the cleavage of p55 produce?
A
- p24 (CA)
- p17 (M)
- p7 (NC)
- p9 (pro)
12
Q
What does Pol produce?
A
p100
13
Q
What does p100 cleavage produce?
A
- p14 (PR)
- p32 (IN)
- p66 (RT)
14
Q
What does Env produce?
A
gp160
15
Q
What does gp160 cleavage form?
A
- gp120 (SU)
- gp41 (TM)
16
Q
How does the Tat protein work?
A
- Tat is an RNA binding protein that binds an RNA regulatory element called TAR
- Tat responsive element (TAR) is within the 5’ untranslated region of the viral mRNA
- In the early stages of infection the transcription of viral mRNA doesn’t occur efficiently as the RNA polymerase is not highly processive so only produces a set of abortive RNA
- Abortive mRNA contains the TAR element that Tat binds to
- Tat complex then recruits Cyclin T1 and Cyclin dependent kinase 9 (CDK9)
- CDK9 has the ability to hyperphosphorylate the RNA polymerase II which increases processive transcription
- This increases transcription of viral RNA
- Increased processive transcription means the RNA polymerase II can now produce full length viral mRNAs
- The complex assembled by TAR and Tat also induces phosphorylation of the negative effectors bound to RNA polymerase which causes them to be released - adds to the boost in efficacy of the polymerase
17
Q
How does the Rev protein work?
A
- Rev is a positive regulator of mRNA export
- Binds the Reve Responsive Element (RRE)
- RRE is a small RNA stem-loop
- Binding of Rev recruits the exporting complex - Ran protein provides energy in the form of GTP - exportin
- Results in the export of viral mRNA into the cytoplasm
- Once in the cytoplasm Rev is released and shuttled back into the nucleus by an importin protein
- CRS inhibits mRNA export
- RRE counteracts their effect to trigger mRNA export by binding Rev
- Single spliced mRNA contain the RRE and so require Rev for export
- Double spliced mRNA don’t contain CRS or RRE so don’t require Rev for export
- In the early phase all the mRNAs are produced but only the Rev independent mRNA can be exported into the cytoplasm where they are used to make Tat and Rev
- In the late phase of transcription, Tat is able to promote the transcription of full mRNAs which are able to be exported by Rev
18
Q
How does vpu work?
A
- While protein synthesis occurs in the ER, CD4 can bind gp160, a precursor of gp120 and gp41
- If this premature binding occurs, the viral particle that is subsequently formed would be tethered to the receptor of the host cell
- May also lead to viral particles without gp120 and gp41 on the surface due to being sequestered by CD4 - would mean these viral particles would be unable to infect cells
- vpu binds to the transmembrane domain of CD4 at the ER surface
- This induce ubiquitination and proteasome targeting of CD4
- Means gp160 is free to assemble in the virus
19
Q
How is it decided if an mRNA will be translated or used in virion assembly?
A
- Regulated by the Gag protein and NC protein processed from Gag
- First the mRNA acts as mRNA to produce proteins - produces Gag protein and NC protein as well as other proteins
- NC protein will bind mRNA - prevents efficient recognition by ribosomes
- NC promotes the dimerisation and encapsidation of the viral RNA
- Thing that drives the switch from mRNA action to encapsidation is the production of the NC protein - the more NC protein around, the more encapsidation will occur
20
Q
How is HIV transmitted?
A
- Blood
- Blood products - transfusion with contaminated blood or exchange of needles for IV drug users
- Sex
21
Q
Describe HIV transmission during sex
A
- Infected cells in semen/vaginal fluids are more important than free virus
- Infected lymphocytes/macrophages attach to epithelia
- Macrophages introduced from vagina traffic to lymphoid organs
- Localised retention of lymphocytes aids virus transmission from one to another
- Attachment stimulates virus release
- Genital and anal ulcers increase the likelihood of transmission - increased inflammation means increased infected immune cells which means increased transmission
22
Q
What is the probability of HIV transmission during sex?
A
Probability of transmission per coital act ranges from 0.1-2%
23
Q
Describe the acute stage of HIV
A
- First and second weeks
- Burst of viraemia
- Flu-like symptoms, headache, rash, lymphadenopathy
- Majority of people don’t develop symptoms when first infected
- Symptoms disappear within a week so are often mistaken for another viral infection
- People are highly infectious during acute infection
24
Q
What happens in seroconversion?
A
- First to 6th month
- Production of antibody against HIV antigens
- T cell activation to eradicate HIV particles
25
Q
Describe by asymptomatic stage of HIV
A
- Up to 20 years
- Clinical latency - no symptoms
- No virological latency - virus keeps replicating in the lymph nodes which acts as a reservoir
- Constant T cell activation decreases defence against other infections
- With treatment can make this phase last for 3-4 decades
26
Q
Describe clinical immunodeficiency
A
- AIDS develops
- Infections with conditions rare in immunocompetent people - can’t fight opportunistic infections
- Neurological illness
- Ultimately leads to death
27
Q
Describe the destruction of cells by HIV
A
- Destruction of T cells is central to pathogenesis in AIDS
- Infects CD4 positive T cells
- HIV is cytolytic, budding viruses decreases membrane strength leading to cell death
- Exposure of viral gp120 on the surface of infected cells induces immune response
- Release of soluble gp120 can bind CD4 and target immune response to uninfected cells
- Immune response will cause destruction of CD4+ T cells
28
Q
Describe the HIV pathogenesis in acute infection
A
- High viral load
- Initial replication leads to transient decrease in T cells and burst of viraemia
- CD4+ T cells are killed by the viral budding which leads to a decrease in T cell numbers
- Infected cells targeted by immune system, cytotoxic T cell and antibodies - start clearing CD4+ cells
- Amount of available cells for infection will start to decrease - the virus will then go into latency
- Viral load decrease, viruses clears from blood, remains in lymph nodes - establishes clinical latency in the lymph node
29
Q
How does HIV establish clinical latency?
A
- HIV drives the differentiation of more cells that are susceptible to HIV
- Viral promoter within LTR are under the control of cellular activator of transcription (ATF) like NFkappaB
- Virus binding to TCR CD4 activates NFkappaB to induce T cell proliferation
- Also stimulate LTR and viral transcription - the virus kills T cells that are attacking them
- Virus replicates in lymph nodes
- Activated T cells are infected and then a subfraction of these differentiate into latently infected memory T cells
- Memory T cells last for a long time in the body and keep the HIV infection through the integrated viral genome
30
Q
Describe the molecular mechanisms driving HIV latency
A
- Upstream of the TAR element in the LTR there are lots of different promoter binding sites - a lot of TFs are required to stimulate transcription of the HIV genome
- 2 proteins produced by infected cells are able to act as restriction factors for the transcription of HIV - TRIM22 and IFI16
- These proteins are interferon inducible proteins which have the ability to inhibit the Sp1 transcription factor - a key transcription factor in controlling HIV transcription
- This means they are able to block the reactivation of HIV from latency
- Other restriction factors - TRIM25 - ZAP
- These proteins have the ability to suppress HIV gene expression at a later stage - prevent either mRNA packaging or the translation of HIV
- Expression of genes is also controlled by chromatin packaging - epigenetic level
- In the latent stage a number of proteins called histone deacetylase complexes can bind to chromatin and deacetylate histone proteins - inhibits the binding and recognition of promoters by the RNA polymerase II, blocking transcription
- When HIV reactivates the HDACs is reverted by histone acetyltransferase proteins - acetylate histones to activate binding of promoters
- When HATs is active the HIV genes can be transcribed
- When HIV is latent multiple molecular mechanisms work together to suppress expression of HIV genes and production of HIV particles
31
Q
What is the error rate of reverse transcriptase?
A
1-10 errors per RNA synthesised
32
Q
What are the benefits to the virus for coming out of latency?
A
- Transmission - in latency can’t transmit to new cells or infect another person
- Gives a chance for the virus to mutate - can produce new strains
33
Q
How is HIV detected?
A
- ELISA and western blot
- Nucleic acid detection
- CD4+ T cell count
34
Q
Describe using ELISA to detect HIV
A
- Serological diagnosis
- Detects immune response
- Very sensitive
- Detects all persons except the first few weeks of infection
- Very rapid
35
Q
Describe using western blot to detect HIV
A
- Detects HIV proteins
- Can give false positives
36
Q
Describe using nucleic acid detection for detecting HIV
A
- Viral load testing = measurement of HIV levels in the blood
- High levels are 10,000 viruses per ml of blood
37
Q
Describe the onset of AIDS-Kaposi Sarcoma
A
- Common in elderly and usually mild - usually takes a long time to develop
- AIDS patients develop KS young and in an aggressive form
- Affects internal organs
- Tumour of spindle cells of vessel walls - leads to visible lesions
- Vessel leakage gives visible lesions
- Oral Kaposi’s pathogenic for HIV
- Associated with human herpes virus-8 co-infection
38
Q
How is HIV infection controlled?
A
- Behaviour modification - wear condoms - get tested
- Chemotherapy - largely after event and expensive
