Lecture 19 - HIV Flashcards
Number of people infected with, or dead from HIV in history
75.8 million
Where are most deaths due to HIV?
Sub-Saharan Africa (a third of deaths)
New trend in HIV in Australia
Slowly increasing number of newly diagnosed cases
Major Australian demographic at risk of HIV
Male homosexuals
Prevalence of chronic blood-borne viral infections in Australia
1)
2)
3)
1) Chronic hep C - 210,000
2) Chronic hep B - 160,000
3) HIV - 17,000
Worldwide most common mode of HIV transmission
Heterosexual contact (80-85%)
Size of HIV
80-130nm
HIV capsid symmetry
Icosahedral
Size of HIV genome
9.2kb
Where is the HIV genome replicated?
Nucleus
Where is the HIV virion assembled?
Cytoplasm
Where did HIV originate?
Zoonosis from chimpanzees. Probably from eating bush meat in the Belgian Congo.
Who discovered HIV?
Professor Francois Barre-Sinoussi
What did HIV-1 evolve from?
SIVcz (chimpanzee)
What did HIV-2 evolve from?
SIVsmm (sooty mangabey)
Where does reverse transcription occur?
In the cytoplasm, within a loosened viral protein cone.
Proteins encoded by pol 1) 2) 3) 4)
1) Protease
2) RT
3) RNase H
4) Integrase
How is pol expressed?
As a polyprotein (gag/pol) before autocleavage.
Error rate of reverse transcription
1 in 10,000 bases
Current most important drug target in HIV
Reverse transcriptase
What acts as the HIV gene promoter?
5’ LTR.
What increases the transcription of HIV genome?
Tat protein. Binds to the 5’ LTR.
What occurs to HIV genome transcription after initial replication?
Silencing of 5’ LTR promoter, until activation of host T cell (EG: NF-kB)
Function of tat protein
Tat (transactivator of HIV translation) binds to TAR (trans-activation response) RNA element.
Binding to TAR promotes transcriptional elongation.
How does Tat binding to TAR promote transcriptional elongation?
1)
2)
3)
1) Tat binds to bulge in TAR RNA tertiary structure.
2) Cyclin T is recruited. CDK9 binds cyclin T.
3) RNA polymerase II is phosphorylated, activated.
Rev function.
1)
2)
1) Binds to REV response element (RRE
2) This stabilises, transports unspliced (9kb) and partially-spliced (4kb) HIV RNA into the cytoplasm.
How does HIV regulate early and late gene expression? 1) 2) 3) 4)
1) Early gene expression from tat binding to TAR in a low-REV environment.
2) Unspliced RNA is degraded, non-structural genes are translated.
3) REV is translated, increases in concentration in the nucleus.
4) In a high-REV environment, REV binds to REV response element (RRE), prevents degradation of unspliced RNA. Structural genes are translated (gag, gag/pol).
HIV-1 proteins necessary for replication in vivo and in vitro.
Tat, rev
HIV-1 proteins not necessary for replication in vitro, but necessary for in vitro pathogenesis 1) 2) 3) 4)
1) Vif
2) Vpr
3) Vpu
4) Nef
Vif role
1)
2)
1) Promotes infectivity of cell-free virus
2) Blocks cell defences targeting ssDNA
Vpr role
1)
2)
3)
1) cDNA nuclear import
2) Cell growth arrest
3) Weak transcription transactivation
Vpu role
1)
2)
1) Regulator of particle release, env processing.
2) Promotes degradation of MHC-I, CD4
Nef role
Downregulates MHC-I and CD4
Cell tropism of C5 viruses
Macrophages, T cells
Cell tropism of X4 viruses
T cells
HIV type that forms syncitia
X4
Initial exposure to HIV
Exposed to polyclonal HIV. A C5 HIV clone is selected for, as it can most-effectively bind a DC, which takes HIV to a regional lymph node.
Untreated HIV progress
1)
2)
3)
1) Primary infection
2) Asymptomatic infection
3) AIDS
Symptoms of primary HIV infection
Flu-like Fever Myalgia Vomiting, diarrhoea Lethargy Rash Lymphadenopathy Pharyngitis Oral thrush Transient CD4+ cell depletion
Number of mucosal CD4+ cells killed by initial HIV infection
60% killed within days
What leads to initial decline in HIV viral titres?
Depletion of permissive cells (all dead)
How does HIV infection lead to breakdown of tight junctions between enterocytes?
Th cells secrete IL-17, which promotes tight junction formation between enterocytes
Incubation period of primary HIV
2-4 weeks
Duration of primary HIV symptoms
1-4 weeks
Proportion of cases where ELISA can detect HIV antibodies in early primary infection
~50%
Time taken for HIV RNA to be detectable after infection
1-18 days
Will a Western blot show a positive result before or after ELISA will?
Before.
Three main HIV tests
1) vRNA PCR
2) p24 antigen EIA
3) HIV-1/HIV-2 ELISA
Initial immune response to HIV infection
1)
2)
1) 1-5% of all CTLs are specific to a single immunodominant HIV epitope
2) ~5% of total IgG specific to HIV envelope
Problem with anti-HIV antibody response
1)
2)
1) Most antibodies aren’t neutralising
2) Virus-bound gp120 oligomers less immunogenic than free gp120 in plasma
Why can the virus cause acute disease after asymptomatic phase?
During ‘quasi steady state’, virus is mutating.
Immune system is exhausted.
Number of HIV virions produced per day
10^9
Where is HIV mutation rate the greatest?
V regions of env, which promote antibody escape
V3 loop involved in co-receptor binding
Causes of CD4+ T cell destruction 1) 2) 3) 4)
1) Direct destruction of infected cells
2) Indirect destruction
3) Chronic immune activation, as GIT mucosal barrier integrity is lost, leading to chronic immune activation
4) Interference with migration of thymocytes to thymus
Indirect death of infected T cells
1)
2)
1) CTL, NK cytolysis
2) Incorporation of healthy cells into syncitia (with X4 virus)
Chronic immune activation from HIV 1) 2) 3) 4) 5) 6)
1) Integrity of GIT mucosa is lost
2) Microbes enter blood stream, stimulate PRRs
3) Elevation of pro-inflammatory cytokine levels
4) CD4+ enter cell cycle, die from HIV activation
5) CD8+ trapped in lymph nodes
6) B cells make autoantibodies
Effect of HIV infection on neutrophils
Reduced killing of bacteria
Effect of HIV on B cells 1) 2) 3) 4)
1) General increase in antibodies
2) Generation of autoantibodies
3) Poor response to vaccines
4) Reduced killing of capsulated bacteria
Effect of HIV on macrophages
1)
2)
3)
1) Reduced phagocytosis
2) Reduced chemotaxis
3) Reduced microbial killing
Common first sign of AIDS
Pneumocystis jirovecii pneumonia (a fungus)
Hairy leukoplakia
EBV reactivation during AIDS. Forms plaques in the mouth.
What causes Kaposi’s sarcoma?
Human herpesvirus (HHV) 8 activation during AIDS
Viral factors affecting HIV disease progression
1)
2)
3)
1) Attenuated strain (EG: nef deficient)
2) R5 phenotype
3) Co-infection with hepatitis G reduces pathogenicity
Host factors affecting HIV disease progression 1) 2) 3) 4) 5) 6) 7)
1) CCR5 delta32 mutation
2) CCR5 promotor mutations
3) CCR2-64 I mutation
4) SDF-1 3’ untranslated region
5) HLA type
6) Ability to mount an immune response
7) Age
HLA types that can confer increased resistance to HIV
B27, B57
Reservoirs of HIV latency 1) 2) 3) 4) 5)
1) Resting CD4+ memory T cells in lymph nodes, blood
2) Accumulation of virions in LN germinal centres, trapped on follicular dendritic cells
3) Lymph node, tissue macrophages
4) In brain microglia and astrocytes
5) Testes, seminal secretions
Proportion of resting T cells harbouring HIV genome
1/10^5 - 1/10^6
Cell type primarily infected with HIV
Central memory T cells
