Human immunodeficiency virus HIV Flashcards
in the scope of understanding viral infections.
what is the clinical hall mark of HIV
reduced CD4 below …
in 1981 what were the two main diseases observed in a cluster in certain groups of people that lead to the discovery of AIDs
– pneumonia caused by a yeast, Pneumocystis carinii,
– and an unusual tumour called Kaposi’s sarcoma caused by human herpes virus 8
AIDS was first identified in what groups of high risk individuals
homosexual men, intravenous drug users and haemophiliacs.
what was the unusual aspect of the cluster of diseases observed in 1981
they were only seen in immunosuppressed people not in those with fully functional immune systems
when was the virus that causes Acquired immunodeficiency syndrome first isolated
• 1983 - 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
when was the second strain of HIV identified
1986
how do the two strains HIV-1 and HIV-2 differ
in their virulence and geographical location.
HIV-2 is less virulent than HIV-1 and is found primarily in western Africa.
what primate species do HIV-1 and HIV-2 come from
HIV-1 came from chimpanzees and HIV-2 from the sooty mangabey.
HIV-1 and HIV-2 are both natural viruses of what and how did they initially infect humans
natural viruses of primates
animals are killed for food and assumed that it was during this process that the virus initially infected humans.
what is the major difference between the HIV-1&2 infectivity in primates vs humans
HIV-1 and HIV-2 do not cause immunosuppression in chimpanzees or sooty mangabeys - only when the virus crossed species to humans does it cause profound immunosuppression
how many people were infected with HIV in 2020
1.5 million
where is the highest rate of increase in HIV infections
sub-Saharan Africa; estimated 20-40% of young adults are infected. (life expectancy now halved to 30-40)
what is the difference in the spread of HIV between genders in africa compared to europe, usa and oceania
in africa it has similar incidence between men and women
in europe and oceania it is much more prevalent in men than women - used to be the same for the usa but the pattern is changing with more women being infected
at the end of 2008, women accounted for ___ of all adults living with HIV worldwide
50%
what family of viruses is HIV a part of
Lentivirus family
what type of virus is HIV
a retrovirus
what is the genome of HIV like
HIV genome contains 2 molecules of single-stranded RNA, each bound by a molecule of reverse transcriptase. (RNA->DNA)
what two major HIV elements are in its genome
p10 protease and a p32 integrase.
what is the HIV genome surround by
a nucleocapsid consisting of inner layer of protein p24 and outer layer of protein p17 (both part of Gag (group specific antigen) polyprotein complex)
what is the outer portion of the virus made from
Consists of lipid envelope derived from host cell membrane
Contains two viral proteins, gp120 and gp41,
(viral envelope proteins)
what does the gag-group specific antigen gene code for in HIV
encodes core viral structural capsid proteins: p24, (a nucleoid shell protein); several internal proteins, p7, p15, p17 and p55
what does env envelope gene of HIV encode
the viral envelope glycoproteins gp120 and gp41
what does the pol polymerase gene encode in HIV
the viral enzymes, protease (p10), reverse transcriptase (p66/55; alpha and beta subunits) and integrase (p32).
what are the three largest genes in the HIV-1 genome
gag (2000bp), pol (2900bp) and env (1800bp)
how does membrane associated Gag trigger budding of virion from surface of cell
attracts two copies of viral RNA along with cellular and viral proteins that trigger budding of virion from surface of cell.
what viral protein of HIV facilitates binding to the CD4 on the host cell
gp120 envelope protein
what cells does HIV infect
glycoprotein-CD4+ cells (T cells, monocytes, dendritic cells)
what cells express CD4
CD4 T cells, monocytes and dendritic
what is CD4
is a co-receptor assisting T cell receptor (TCR) in communicating with an antigen-presenting cell (APC) by binding to MHC in the non-polymorphic region
describe the normal function of CD4
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 the CD4 cytoplasmic tail is able to phosphorylate tyrosine on 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
describe the steps of the HIV lifecycle
1) . Binds to CD4 +ve cells through gp120 to CD4; interactions between virus and chemokine co-receptors.
2) . Nucleocapsid enters the cell, unfolds, releasing viral RNA, which is reverse-transcribed to DS DNA.
3) . The viral DNA integrates in the host genome, where it lies dormant as a provirus.
4) . Following cell activation, viral DNA directs the transcription of viral RNA.
5) . Viral proteins are translated from the RNA.
6) Viral proteins and single-stranded viral RNA assemble to form new viral particles.
7) . Virus buds from the cell, picking up some of cell membrane, complete viral particles can infect other cells. Takes the host cell membrane as its envelope
how does HIV circumvent normal dendritic cells to increase t cell interaction for infection spread
An HIV-infected dendritic cell shoots out thin projections called filopodia with virus particles 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
describe the new model of HIV binding
- 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.
- Once bound, 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 is the relationship between gp160, gp120 and gp41
gp160 is sparsely distributed on the virus envelope and is displayed as trimers. Each gp160 is comprised of an external component gp120 and gp41,
gp41 is a transmembrane protein that binds to and anchors gp120.
what are M tropic variants (now called R5 viruses)
variants that use the beta-chemokine receptor CCR5 for entry and are thus able to replicate in macrophages and CD4+ T-cells.
(CCR5 present on CD4 t cells, monocytes and dendritic cells)
what are T trophic variants (now called X4 viruses)
use the alpha-chemokine receptor, CXCR4, for entry. only infect t cells.
how can HIV entry be inhibited (chemokine receptors)
by downregulation of the expression of the coreceptors or by saturating them with their ligands
what is the normal structure and function of chemokine receptors
G-protein-coupled with seven membrane-spanning domains
normally function by binding to chemokines in order to direct leukocytes to migrate to sites of inflammation.
what natural mutations can effectively protect people against HIV
A nonfunctional mutant allele of CCR5 with an internal deletion of 32 bp (CCR5Δ32) is found with high frequency in European and North American populations, and this effectively protects individuals against M-tropic virions
what mutation may affect the co receptor binding and disease progression of HIV
A guanine to adenine point mutation (3′UTR801G/A) in stromal-cell-derived factor 1 (SDF-1), a ligand for CXCR4, has been reported in 40% of healthy Asian Indians.
Insertions in the promoter region of macrophage inflammatory protein 1α (MIP-1α), a ligand for CCR5, have been reported in 1 in 5 Indians
which protein mediates fusion between the viral envelope and host cell membrane for entry of HIV
gp41
why do so many mutations arise in HIV
poor fidelity - when reverse transcriptase copies RNA into dsDNA it has no proofreading ability and creates around 10 mistakes per replication cycle
how is virus production initiated after the latent stage of HIV
by cellular transcription factors like NK-κB which is upregulated when T-cells become activated
at what stage is HIV known as a provirus
when the viral DNA is integrated into the host and it enters the latent stage
what does RNase H do in HIV infection/replication
as the virus integrates itself into the host cell DNA RNase H degrades the copied RNA template
what part of the host cell machinery is key form assembly of the new viral particles
endoplasmic reticulum - important for protein glycosylation (eg gp120)
the clinical progression of HIV can be divided into three stages, what are they and how long do they last
- Infection (4-6 weeks)
- The ‘latent’ stage (0.5 - 17 years)
- Development of AIDS (0.5 - 2 years)
beifly why does AIDs develop
the loss of CD4 T cell function results in failure to control the virus and the development of AIDS.
what level of CD4 cells is considered an indicator of AIDs
below 200 cells/mm^3
what is the normal range for CD4 levels
The CD4 count normal range is 500 to 1500 cell/mm^3.
what happens following infection of HIV
generation of anti-HIV antibodies and T cells clears most of the virus and the disease enters the latent phase
what symptoms are shown by people immediately after infection with HIV
most people show no symptoms immediately after infection
15% demonstrate influenza-like symptoms: fever, malaise, aching muscles, sore throat and swollen lymph nodes
what are the three types of tests available for HIV
nucleic acid tests (NAT), antigen/antibody tests, and antibody tests.
what is seroconversion? in HIV?
a sign that the immune system is reacting to the presence of the virus in the body. (change from seronegative to seropostivie state)
The point at which the body produces antibodies to HIV. - can be detected by antibody tests
33% of people in the latent phase of HIV infection will have this
swollen lymph nodes
what is the average time from infection by HIV to development of AIDS
about 10 years
what symptoms indicate progression to AIDS (without treatment)
weight loss, night sweats, fever and diarrhoea
can also include infections such as oral candidiasis, herpes simplex and shingles caused by minor opportunistic infectious agents.
how is AIDS clinically defined
appearance of major opportunistic infections or by a drop in the CD4 T cell count to below 200 cells/µl of blood.
what causes death in people with AIDS
caused by the combination of opportunistic infections
in europe, usa and oceania what is the most common type of infection associated with AIDS
pneumonia caused by Pneumocystis carinii
what infections are most commonly associated with AIDS in africa and asia
infections such as Cryptosporidium, a protozoan that causes severe diarrhoea and weight loss, and Mycobacterium tuberculosis
what changes occur in the lymph nodes with HIV infection
- swollen lymph nodes during initial infection and potentially continued into the latent stage
- increasing disruption of normal lymph node architecture
- influx of CD8 T cells
- eventual loss of germinal centres
briefly what do dendritic cells usually do
Dendritic cell patrol tissues, on the lookout for microbial invaders.
When they encounter a pathogen they chop it up into tiny pieces and carry samples of it to local lymph nodes.
There, they display their finds to the CD4 T cell, which then mounts full-fledged immune response against the invader
how does HIV exploit the dendritic cell surveillance network
HIV is picked up by DCs that patrol mucosal tissues but avoids being killed by them, and instead hitches a ride to lymph nodes before transfer to the CD4 T cell
how do dendritic cells recognise HIV
• Sialylactose head on GM3 ganglioside (glycosphingolipid) exposed on HIV membrane is recognised by DC and virus is taken in
give one consequence of loss of T helper cell activity
reduced ability to mount a delayed-type hypersensitivity reaction
what is delayed-type hypersensitivity
the recruitment of T cells into tissues to be activated by antigen-presenting cells to produce cytokines that mediate local inflammation
what immune functions are lost after a HIV infection
reduction of CD4 cells
loss of CD4 cell function
ability to create antibodies is lost
what antibody abnormalities are seen in HIV
HIV+ individuals can demonstrate increased total levels of serum Ig despite the impaired ability to generate specific antibody responses.
HIV+ individuals can show increased production of autoantibodies (to antigens such as red blood cells, spermatozoa or myelin)
what is the immune response immediately after infection with HIV
strong antibody responses to gp120 envelope protein and the p24 protein of the nucleocapsid (aka gag)
a powerful CD8 cytotoxic T cell against gp120, p24 and some of the proteins that make up reverse transcriptase (p01 antigens).
what cause be the reason for a slower progression to AIDS
low level of viraemia progress (slow viral replication)
what is the difference in levels of infected CD4 t cells in the blood vs lymph nodes
lymph nodes have shown 10-100 times more productively infected CD4 T cells than seen in the blood
why might blood tests of CD4 levels be misleading in HIV
during HIV infection the relative proportion of the total number of CD4 T cells in blood increases, so actual loss of CD4 T cells is greater than that reflected in the blood
what have new RNA measurement techniques shown about HIV replication during the latent stage
that there is a persistant level of active viral replication in the blood during the latent stage
what are the cellular dynamics in the latent stage HIV infection
high viral destruction (30%/day) and replication and extensive CD4 T cell (2x10^9/ day) destruction and replacement
how does HIV survive in the face of a powerful immunological onslaught
very high replication rate
can hide as provirus where it is not detectable by the immune system
very high mutation rate (antigens recognised, mutate not recognised etc etc)
what is thought to be the cause of CD4 T cell loss
a combination of direct killing by the virus and destruction of virally infected cells by the immune system.
what ways can CD4 T cells be killed?
- HIV directly causing lysis of the cell
- infected cell bearing HIV antigens killed antibodies and complement
- infected cell bearing HIV antigens killed by antibody-dependent cell-mediated toxicity
- infected cells presenting HIV peptides on their MHC1 cells killed by CD8 T cells
how are CD4 T cells killed by antibodies and complement
Anti-gpl20 binds to the gp120 bound to CD4, resulting in complement fixation and activation and cell lysis.
how are infected CD4 T cells killed by ADCC
Macrophages and NK cells have Fc receptors on their surface. This enables them to bind to the Fc portion of the anti-gpl20 antibody bound to the CD4 T cell and kill the cell.
what happens when HIV productively infects permissive CD4 T cells in the spleen
death occurs through apoptosis mediated by the enzyme caspase-3.
what happens when HIV abortively infects non permissive CD4 T cells
death occurs by pyroptosis, which depends on activation of caspase-1.
what happens in the HIV infection of non permissive CD4 T cells
there is a failure to complete reverse transcription and so incomplete HIV transcripts accumulate triggering cell death (by pyropotosis (caspase-1)
what is pyroptosis and how does it help HIV
a highly inflammatory form of lytic programmed cell death - lures more CD4 T cells to the area thereby creating a vicious cycle that contributes to disease progression and tissue damage
what detects the incomplete HIV transcripts and triggers pyroptosis
DNA sensor IFI16 - results in inflammasome assembly and caspase-1 activation
what cells naturally resist pyroptosis (due to what)
peripheral blood-derived CD4 T cells, partly due to their deeper resting state
how do blood derived CD4 T cells become sensitised to pyroptosis
in co-culture/ in lymphoid tissue microenvironment with lymphoid derive CD4 T cells
why is there a gradual decrease in CD4 T cells level in HIV
Because the production of CD4 T cells never quite matches their destruction
what is antiretroviral therapy and how can it have a longer clinical benefit in the treatment of HIV
combinations of two or preferably three antiretroviral drugs taken every day - longer clinical benefits because there is less chance of the virus mutating to become resistant to all three drugs at once
what was the first anti-HIV drug and why is it no longer used
First anti-HIV drug was zidovudine (AZT), a reverse transcriptase inhibitor introduced in 1987.
No longer used on it own because the virus very quickly mutated and becomes resistant to the drug
what are the three categories of HIV inhibitory drug
- nucleoside analogue reverse transcriptase inhibitors,
- non-nucleoside analogue reverse transcriptase inhibitors
- HIV protease inhibitors
how do nucleoside/nucleotide analogue reverse transcriptase inhibitors (NRTIs and NtRTIs) work?
Nucleoside/Nucleotide analogues lack a 3’-hydroxyl group on the deoxyribose moiety. Following incorporation, the next incoming deoxynucleotide cannot form the next 5’-3‘ phosphodiester bond needed to extend the DNA chain. Thus, viral DNA synthesis is halted, (chain termination). (classified as competitive substrate inhibitors)
how do non-nucleoside analogue reverse transcriptase inhibitors (NNRTIs) work?
Non-nucleoside analogues block reverse transcriptase by binding at a different site on the enzyme, compared to NRTIs and NtRTIs. They are not incorporated into the viral DNA but instead inhibit the movement of protein domains of reverse transcriptase that are needed to carry out the process of DNA synthesis. (Classified as non-competitive inhibitors)
how do protease inhibitors work
Inhibit the cleavage of viral polyproteins to viral proteins - inhibit the active site of the HIV aspartic protease, (site of HIV polyprotein cleavage). Peptide linkage –NH-CO- is replaced by uncleavable hydroxyethylene group (-CH2-CH(OH)-)
what is combination chemotherapy for HIV
(use of one or medications)
usually consists of two reverse transcriptase inhibitors and a protease inhibitor
what are the problems with combination therapy
- considerable toxicity (especially to bone marrow and gut)
- complicated and intrusive regime (difficult to stick to)
- very expensive ($15000 a year)
give an example of a new antiviral HIV strategy
Pa457
- destroys the protein shield that surrounds the RNA strands (so forms a leaky sphere that leaves the core RNA exposed). now the protein capsid is destroyed the virus is suseptible and quickly dies off
what are the two types of vaccine were initially considered for HIV treatment
Prophylactic (to protect those not yet infected)
Therapeutic (originally thought HIV infected people didn’t make a good immune response so boosting it would clear the virus)
what are the immunological and logistical issues for prophylactic vaccines
- many subtypes and variants of the virus
- high mutation rate
- the type of immune response to stimulate
- there are no good animals models as it is primarily in humans
- testing effectiveness of the vaccine would take to long because AIDS is a chronically progressive disease
no clinical trials using DNA, recombinant and antigen/peptide approaches have demonstrated convincing protection so far
give one new strategy for treating HIV using antibodies
naturally occurring, broadly neutralising HIV antibodies - they mimic the CD4 binding site on the envelope site.
cloned from 4 individuals whose immune system naturally developed antibodies with broad-spectrum serum activity (antibodies have similar sequences and originate from two independent genes - valuable in designing future HIV vaccines)
what vaccine has been proved capable of neutralising a wide array of HIV strains in laboratory assays
those containing Broadly Neutralizing Human Monoclonal Antibodies that target (4) HIV-1 epitopes - presenting these epitopes to the immune system in a conformationally precise manner may induce the body to produce such antibodies and provide a high level of protection
what is the monkey version of HIV called
simian immunodeficiency virus (SIV)
what microbicide has been shown to block acute infection in rhesus macaques (and cultured human cells) exposed to the virus?
Glycerol monolaurate (GML) - In cultured human vaginal epithelial cells exposed to HIV, GML blocks the production of molecules that appear during inflammation and that are thought to increase susceptibility to HIV infection. (stops luring by DCs)
