HIV Flashcards
LO
- To know about the history, incidence and clinical progression of AIDS.
- To understand the biology of the AIDS virus and how it causes immunosuppression.
- To learn about the immune response to the AIDS virus
- Be aware of forms of chemotherapy and issues of vaccine development in AIDS.
Key topics
- History of AIDS (Acquired Immunodeficiency Syndrome)
- The human immunodeficiency virus (HIV)
- Structure
- Replication cycle
- Clinical course of HIV infection
- Immunology of HIV infection
Effects of HIV on the immune system
Immune response to HIV
- Chemotherapy of AIDS
- HIV vaccines and microbiocides
The story of AIDS began in 1981, when a cluster of unusual disease were observed in certain groups of people. What were the two main diseases and what were they caused by?
- pneumonia caused by a yeast, Pneumocystis carinii
- and an unusual tumour called Kaposi’s sarcoma caused by human herpes virus 8
Initially AIDS was noticed in homosexual men before later symptoms appearing in who? What did this suggest about the nature of the virus?
- Initially in homosexual men but later the same symptoms appeared in intravenous drug users and haemophiliacs who were injecting blood-clotting factors.
- This pattern of disease occurrence suggested that a transmittable agent was responsible for the diseases
What was an unusual aspect noticed about AIDS during its discovery?
. Unusual aspect of these diseases was they were only seen in immunosuppressed people and not in people with a fully functioning immune system.
What were some observations made about the individuals who had the disease?
The observation that individuals with these diseases had low numbers of CD4 T cells was consistent with immunosuppression
When was the term for the diseases used by the CDC?
1982 the term acquired immunodeficiency syndrome, or AIDS, was used by the Centres for Disease Control (CDC) in Atlanta to describe the disease
In 1983, shortly after AIDS discovery, what was discovered?
1983 virus causing isolated from lymph node of an infected individual by Luc Montagnier’s group in Paris and was called the human immunodeficiency virus, or HIV for short.
What was identified in 1986?
Second strain of HIV was identified in 1986; called HIV-2 and first strain was renamed HIV-1
Tell me how HIV-1 and HIV-2 differ?
HIV-1 and HIV-2 differ in their virulence (severity and how harmful it is) and geographical location.
HIV-2 is less virulent than HIV-1 and is found primarily in western Africa.
What have genetic studies shown about HIV-1 and HIV-2?
Tell me the origins of both HIV-1 and HIV-2
Genetic studies showed that both HIV-1 and HIV-2 are natural viruses of primates that have jumped species to infect humans.
HIV-1 came from chimpanzees
and HIV-2 from the sooty mangabey.
How did we as humans get HIV from animals?
Both animals are killed for food and assumed that it was during this process that the virus initially infected humans.
Tell me an effect we as humans had from HIV that the animals didnt?
HIV-1 and HIV-2 do not cause immunosuppression in chimpanzees or sooty mangabeys; only when virus crossed into humans did it cause the profound immunosuppression seen in AIDS.
Estimated since 1981, how many worldwide have been infected, and died?
Estimated since 1981 over 75 million people worldwide were infected with HIV and over 32 million died (43% dead); 2 million died in 2007 and 2008.
Where is the highest rate of increase in HIV infection seen?
How has this expected life-expectancy in this area?
The highest rate of increase in HIV infection is seen in sub-Saharan Africa; estimated 20-40% of young adults are infected.
Thus, life expectancy in sub-Saharan Africa almost halved; it is now in the 30s-40s instead of approaching 70, the estimated life expectancy if AIDS pandemic had not occurred.
What is the pattern of spread of HIV in Africa? Compare this to that seen in more developed countries?
- Pattern of spread in Africa appears to be primarily by heterosexual contact and has a similar incidence in men and women.
- By contrast, spread in Europe, the USA and Oceania are still mostly among ‘high-risk’ groups such as homosexuals and intravenous drug users, much more prevalent in men than women.
- However, signs in USA that pattern of spread is changing, with more women being infected.
- End of 2008, women accounted for 50% of all adults living with HIV worldwide
Sub-sahara stats
Women can pass HIV onto their children, how do they do this?
Women can pass on HIV to their children via childbearing and breastfeeding
What are the two 1 strains of HIV?
The two 1 strains of HIV are called HIV-1 and HIV-2.
Also, many different subtypes of HIV-,1 so vaccines needed to protect against all subtypes.
Why are there different varients of HIV and why is this important?
- Also, HIV has very high mutation rate, giving rise to different forms of the virus known as variants.
- Variants important because, as described later, they differ in which cell types they can infect.
- Although different strains, subtypes and variants of HIV, very similar in structure and replication; therefore, will be described together and referred to collectively as HIV.
What type of virus is HIV? What does this mean?
- HIV retrovirus (lentivirus family)
- contains RNA as genetic material
- Retrovirus: any of a group of RNA viruses which insert a DNA copy of their genome into the host cell in order to replicate, e.g., HIV.
What does the HIV genome contains and what is this bound by?
HIV genome contains…
- 2 molecules of single-stranded RNA;
- each bound by a molecule of reverse transcriptase
- Within genome are also a p10 protease and a p32 integrase
What is the HIV genome surrounded by?
Genome is surrounded by nucleocapsid consisting of inner layer of protein p24 and outer layer of protein p17
(both part of Gag (group specific antigen) polyprotein complex)
What does the outer portion of the virus consist of?
Lipid envelope derived from host cell membrane
What does the outer portion of the HIV contain?
Contains two viral proteins, gp120 and gp41, which collectively are called viral envelope proteins.
Label this HIV structure
inner circle= p24
outer circle= p17
(of my arrows drawn onto diagram)
This is the HIV genome, explain what each section is associated with or encodes?
- LTR-long terminal repeats; repetitive sequence of bases
- gag-group specific antigen gene, encodes viral nucleopcapsid proteins: p24, a nucleoid shell protein, MW=24000; several internal proteins, p7, p15, p17 and p55.
- pol-polymerase gene; encodes the viral enzyme, protease (p10), reverse transcriptase (p66/55; alpha and beta subunits) and integrase (p32).
- env-envelope gene; encodes the viral envelope glyocproteins gp120 (extracellular glycoprotein, MW=120 000) and gp41 (transmembrane glycoprotein, MW=41000).
- tat: encodes transactivator protein
- rev: encodes a regulator of expression of viral protein
- vif: associated with viral infectivity
- vpu: encodes viral protein U
- vpr: encode viral protein R
- nef: encodes a ‘so-called’ negative regulator protein
What does HIV GAG encode?
HIV GAG encodes structural capsid proteins, produced as a GAG precursor polyprotein, which is processed by viral protease.
What does the ‘p’ represent?
P number refer to their molecular size. The larger the number to larger the number of AA in the protein
What does membrane associated GAG attract?
Membrane-associated Gag attracts two copies of viral RNA along with cellular and viral proteins that trigger budding of virion from surface of cell.
Tell me about the HIV particle, its main structures and proteins
- HIV is an enveloped retrovirus and has two single-stranded RNA molecules as its genetic material. The RNA is associated with reverse transcriptase, integrase and polymerase enzymes, which are necessary for viral DNA and RNA synthesis.
- Surrounding this is the nucleocapsid, which consists of an inner layer of p24 protein and an
- outer layer of p17 protein. The outer portion of the virus consists of a lipid layer derived from the host cell into which is inserted the viral gp41 envelope protein. Each gp41 protein molecule is associated with a molecule of the gp120 envelope protein.
- Many Glycan side chains on gp120
Tell me the replication cycle of HIV?
- Like any other virus, HIV must infect a host cell before replicating, with the viral progeny leaving the cell to infect others
- Retrovirus: any of a group of RNA viruses which insert a DNA copy of their genome into the host cell in order to replicate, e.g., HIV.
Tell me how HIV infects the host?
Whats infected and what is initially bound to?
- HIV infects glycoprotein-CD4+ cells (helper T cells recognize antigens on the surface of a virus-infected cell and secrete lymphokines that stimulate B cells and killer T cells; BUT helper T cells are infected and killed by the AIDS virus).
- Initial binding to host cell involves gp120 protein on surface of HIV particle binding to CD4 on host cell surface.
In addition ot CD4 T cells, what else expresses CD4?
In addition to CD4 T cells, monocytes and dendritic cells also express CD4, although at lower levels than T cells
What are the two stages to viral infection?
Two stages to viral infection:
- binding to the host cell,
- and fusion with the cell membrane to allow the virus to enter the cell
What is CD4?
CD4 is a co-receptor assisting T cell receptor (TCR) in communicating with an antigen presenting cell (APC)
What does CD4 bind to?
CD4 binding to MHC (Major Histocompatibility Complex) non-polymorphic region
Tell me about CD4 binding to the MHC moleucle during antigen presentation
- During antigen presentation, TCR complex and CD4 recruited to bind to different regions of the MHCII molecule (α1/β1 and β2, respectively)
- Close proximity between the TCR complex and CD4 means Lck kinase bound to CD4 cytoplasmic tail able to tyrosine-phosphorylate the Immunoreceptor tyrosine activation motifs (ITAM) present on the cytoplasmic domains of CD3
- Phosphorylated ITAM motifs on CD3 recruits and activates SH2 domain-containing protein tyrosine kinases (PTK) e.g. Zap70 to further mediate downstream signal transduction via tyrosine phosphorylation, leads to transcription factor activation including NF-κB and consequent T cell activation.
Tell me the stages to the life cycle of HIV?
- Binds to CD4 +ve cells through gp120 to CD4; interactions between virus and chemokine co-receptors.
- Nucleocapsid enters the cell, unfolds, releasing viral RNA, which is reverse transcribed to DS DNA.
- The viral DNA integrates in the host genome, where it lies dormant as a provirus.
- Following cell activation, viral DNA directs the transcription of viral RNA.
- Viral proteins are translated from the RNA.
- Viral proteins and single-stranded viral RNA assemble to form new viral particles.
- Virus buds from the cell, picking up some of cell membrane, complete viral particles can infect other cells.
HIV life cycle diagram
Image to show budding of HIV particle
Tell me about Hijacked immune cell- HIV circumvents normal dendritic cell- T-cell interaction
An HIV-infected dendritic cell shoots out thin projections called filopodia (red) with virus particles (white) at their ends. Actin filament rearrangement projects filopodia in a waving arc towards T-cells at μm/sec, facilitating 800 dendritic cell - T-cells interactions/hour.
Anupriya Aggarwai and Stuart Turville, PLOS Pathogens, doi:10.1371/journal.ppat.1002762
Binding of g120 to CD4 is not enough for HIV to infect cells.
What was originally thought?
- Originally thought gp41 component of envelope protein binds to a second protein on cell surface. Second protein differs between variants of HIV.
- In some variants originally considered that gp41 binds to the β-chemokine receptor CCR-5, which is on the surface of CD4 T cells, monocytes and dendritic cells.
- These variants can therefore infect all of these cell types and have been called M-tropic.
- Other HIV variants originally thought to bind to another α-chemokine receptor, CXCR-4, present on CD4 T cells but not on monocytes or dendritic cells.
- These variants only infect T cells and are called T-tropic.
In the old binding model, what was an M-tropic variant?
In some variants originally considered that gp41 binds to the β-chemokine receptor CCR-5, which is on the surface of CD4 T cells, monocytes and dendritic cells.
These variants can therefore infect all of these cell types and have been called M-tropic.
In the old binding model, what were T-tropic variants?
Other HIV variants originally thought to bind to another α-chemokine receptor, CXCR-4, present on CD4 T cells but not on monocytes or dendritic cells.
These variants only infect T cells and are called T-tropic.
In the new binding model what is now thought to help binding?
Tell me about this protein type, role and structure
gp160, is a fusion glycoprotein to overcome the energy barrier associated with the fusion of two membranes. gp160 is in trimer formation, meaning three molecules of gp160 are arranged together, often called “spikes”.
What often happens during the early fusion process of biding?
- During early fusion process, gp160 cleaves into gp41 (considered the transmembrane glycoprotein) and gp120 (considered the surface glycoprotein)
- However, gp41 and gp120 remain in trimer formation and noncovalently attached to each other. In this configuration, gp41 is thought to be in high energy state, with its fusion peptide buried inwards.
- Fusion peptide is molecule composed of linked amino acids.
What is the function of gp120?
gp120 binds onto host cell surface to the receptor CD4
Causes a conformational or structural change of gp120, which in turn, allows gp120 to bind again, this time to a chemokine coreceptor, usually CXCR4 or CCR5.
Tell me what happens to gp41?
gp41 is released from its high energy state and the previously buried fusion peptide springs out towards the host cell membrane, bridging the divide between the virion and the host cell membrane.
What happens to gp120 after chemokine coreceptor binding?
After chemokine coreceptor binding, gp120 is thought to disassociate away.
At this point, glycoprotein 41 transiently becomes an integral component of two membranes: the viral membrane in which it is anchored, and the cellular membrane that it has gaffed.
How does HIV enter cells?
HIV enters cells by binding of the HIV glycoprotein gp120 to the host CD4 molecule; this process requires chemokine receptors as obligate accessory proteins or ‘co-receptors’.
What are chemokine receptors?
Chemokine receptors are G-protein-coupled and have seven membrane-spanning domains
How do chemokine receptors normally function?
Chemokine receptors normally function by binding to chemokines in order to direct leukocytes to migrate to sites of inflammation.
What members of the chemokine receptor familt are used in macrophages(M)-tropic and T-cell(T)-tropic viruses?
What can this entry be inhibited by?
CCR5 for M-tropic HIV and CXCR4 for T-tropic HIV
This entry can be inhibited by downregulating the expression of the co-receptors or by saturating them with their ligands.
The clinical progression of HIV infection can be divided into three stages. What are these stages?
- Infection
- the “latent” stage
- development of AIDS
Opportunistic infections in HIV
Sialylactose head on GM3 ganglioside
Tell me about the loss of CD4 T cell function in regard to HIV infection?
Why is it lost?
- As infection progresses HIV+ individuals show loss of CD4 T cell function that cannot be explained simply on basis of reduced CD4 T cell numbers and is somehow associated with the effects of the virus.
- Transplantation recipients taking immunosuppressive drugs can show drops in CD4 T cell numbers similar to those seen in HIV+ individuals but the transplant recipients demonstrate much better CD4 T cell function.
- Reasons for loss of CD4 T cell function are not clear. One consequence of loss of Th activity is reduced ability to mount a delayed-type hypersensitivity reaction
- As infection progresses ability to generate antibody responses is also lost.
Because of the expression of viral antigens, CD4 T cells can be killed in a number of ways. What are these ways?
Targeting caspase 1 and pyroptosis of lymphoid CD4 T-cells via orally bioavailable and safe drug prevents lymphoid CD4 T-cell death by HIV-1
a, b, VX-765 efficiently blocks CD4 T-cell death in HIV-infected tonsillar and splenic lymphoid tissues. No toxicity was observed at any of these drug concentrations. Error bars represent s.e.m. from three independent experiments using tonsil or spleen cells from three different donors.
c, Pyroptosis in HIV-infected lymphoid tissues may establish a chronic cycle of CD4 T-cell death and inflammation, which attracts new CD4 T cells and ultimately contributes to disease progression and tissue damage. Inhibitors of caspase 1 such as VX-765 may inhibit pyroptosis in a manner that both preserves CD4 T cells and reduces inflammation.
Pyroptosis CD4 T-Cell location
Anti-HIV drugs
HIV and JAK/STAT pathway…
New antiviral strategy
What are the main logistical problems in testing the prophylactic vaccines?
- Traditionally vaccines were developed against acute diseases that often occurred in waves or epidemics (e.g. measles or mumps) and had a low mortality rate.
- Therefore comparatively straightforward to vaccinate a section of the population and see whether these people were protected from the disease compared with non-vaccinated individuals.
- Possible this way to get an answer to the effectiveness of the vaccine in a relatively short time.
- Because AIDS is a chronically progressive disease the conventional approach would take many years to give an answer - too long for most people to wait.
- A number of vaccines in clinical trials at present using DNA, recombinant and antigen/peptide approaches, although none has demonstrated convincing protection so far.
As there are some issues with HIV vaccines, what was the new strategy developed?
- Found: naturally occuring, broadly neutralising HIV antibodies which mimic the CD4 binding site on the envelope spike that the virus uses to enter and infect its host cells.
- Cloned 576 new HIV antibodies from four individuals whose immune systems naturally developed antibodies with broad serum activity. Sequencing showed antibodies have similar sequence and originate from two independent genes.
- These antibodies will be valuable in designing future HIV vaccines
