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Discovery of retroviruses
• Retroviruses possess a unique enzyme known as RT (reverse transcriptase)
• RT uses viral RNA as a template for making DNA copy which integrate in to the chromosome of the host cell and serves either as basis for viral replication or as oncogene.
• Howard Temin and David Baltimore received Nobel Prize for discovery of RT enzyme.
Discovery of HIV
• In 1981 new clinical syndrome characterized by profound immunodeficiency was recorded in male homosexual and termed AIDS.
• Unusual prevalence of Pneumocystis carinii pneumonia in in a group of young previously healthy male homosexual.
Kaposi’s sarcoma (rare cancer) in previously healthy male homosexual??
Discovery of HIV
First isolation of HIV-1 made by Luc Montagnier and Barre-Sinoussi at Pasteur institute in Paris in 1983.
This observation is confirmed by Robert Gallo in the USA.
Discovery of HIV
HIV-2 isolated from mildly immunosuppressed patient n west Africa.
5000 cases of HIV-1 cases per Day?
41 million people have been infected world wide.
HIV-2 account for 4.5% of HIV cases.
HIV-1
Visna
MMTV
SRV-1
HERV-K
RSV
HTLV-I
HTLV-II
BLV
MLV
FLV
WDSV
FFV
SFV
Lentivirus
Spumavirus
Gamma
Delta
Alpha
Beta
Epsilon
Retroviridae
Lentiviruses
25 0 my
MacaqueYellow baboonChacma baboonColobusLangur
MandrillDrillSooty mangabey
HumanChimpanzeeBonoboGorillaOrangutanGibbonSykes'sVervetGrivetTantalusSabeausPatasL'Hoest'sSun-tailedPreussi's
Agile mangabey
Primates infected with lentiviruses
> 30 species of African primates naturally infected with SIV
SIV infections: natural acquired not known
Natural infections:• >50% of adults• nonpathogenic
Chimpanzee the only ape
Primate Lentiviruses
HIV-2
Photograph by Karl Ammann
Photograph by Karl Ammann
Properties of HIV
Classification
The family Retroviridae is named for RT.
(Retro= Backwards)
Seven genera is now recognised (only two of them cause disease in human):
Lentivirus: containing HIV-1 and HIV-2, characterised by:
Cone shaped Nucleocapsid, absence of oncogenicity, and the lengthy and insidious onset of clinical signs.
Properties of HIV
BLV-HTLV retroviruses: contain HTLV-I and II: characterised by ability to cause tumours rather than immunosuppression.
Spumavirus : Causes characteristic foamy appearance in infected primate cell culture. (they are not pathogenic).
Global estimates for adults and children 2010
People living with HIV 34.0 million [31.6 million – 35.2 million]
New HIV infections in 2010 2.7 million [2.4 million – 2.9 million]
Deaths due to AIDS in 2010 1.8 million [1.6 million – 1.9 million]
Total: 34.0 million [31.6 million – 35.2 million]
Western & Central Europe
840 000[770 000 – 930 000]
Middle East & North Africa470 000
[350 000 – 570 000]
Sub-Saharan Africa22.9 million
[21.6 million – 24.1 million]
Eastern Europe & Central Asia1.5 million
[1.3 million – 1.7 million]
South & South-East Asia4.0 million
[3.6 million – 4.5 million]
Oceania54 000
[48 000 – 62 000]
North America1.3 million
[1.0 million – 1.9 million]
Latin America1.5 million
[1.2 million – 1.7 million]
East Asia790 000
[580 000 – 1.1 million]Caribbean200 000
[170 000 – 220 000]
Adults and children estimated to be living with HIV 2010
Estimated number of adults and children newly infected with HIV 2010
Western & Central Europe
30 00030 000[22 000 – 39 000]
Middle East & North Africa59 00059 000
[40 000 – 73 000]
Sub-Saharan Africa1.9 million1.9 million
[1.7 million – 2.1 million]
Eastern Europe & Central Asia
160 000 160 000 [110 000 – 200 000]
South & South-East Asia270 000270 000
[230 000 – 340 000]
Oceania33003300
[2400 – 4200]
North America58 000
[24 000 – 130 000]
Latin America100 000100 000
[73 000 – 140 000]
East Asia88 00088 000
[48 000 – 160 000]
Caribbean12 000
[9400 – 17 000]
Total: 2.7 million [2.4 million – 2.9 million]
Estimated adult and child deaths from AIDS 2010
Western & Central Europe
9900[8900 – 11 000]
Middle East & North Africa35 000
[25 000 – 42 000]
Sub-Saharan Africa1.2 million
[1.1 million – 1.4 million]
Eastern Europe & Central Asia
90 000 [74 000 – 110 000]
South & South-East Asia250 000
[210 000 – 280 000]
Oceania1600
[1200 – 2000]
North America20 000
[16 000 – 27 000]
Latin America67 000
[45 000 – 92 000]
East Asia56 000
[40 000 – 76 000]Caribbean
9000[6900 – 12 000]
Total: 1.8 million [1.6 million – 1.9 million]
Western & Central Europe
1400[<1000 – 1800]
Middle East & North Africa40 000
[27 000 – 52 000]
Sub-Saharan Africa3.1 million
[2.8 million – 3.5 million]
Eastern Europe & Central Asia
17 000 [14 000 – 23 000]
South & South-East Asia160 000
[110 000 – 210 000]
Oceania4600
[3600 – 5800]
North America4500
[4000 – 5800]
Latin America42 000
[30 000 – 54 000]
East Asia16 000
[11 000 – 21 000]Caribbean
16 000[12 000 – 19 000]
Children (<15 years) estimated to be living with HIV 2010
Total: 3.4 million [3.0 million – 3.8 million]
Western & Central Europe
<100[<200]
Middle East & North Africa6800
[4800 – 8800]
Sub-Saharan Africa350 000
[300 000 – 410 000]
Eastern Europe & Central Asia
2200 [1700 – 2900]
South & South-East Asia20 000
[14 000 – 28 000]
Oceania<1000
[<500 – <1000]
North America<100[<200]
Latin America3500
[2100 – 5000]
East Asia2100
[<1000 – 3800]Caribbean
1200[<1000 – 1700]
Estimated number of children (<15 years) newly infected with HIV 2010
Total: 390 000 [340 000 – 450 000]
Western & Central Europe
<100[<200]
Middle East & North Africa3900
[2700 – 5000]
Sub-Saharan Africa230 000
[200 000 – 260 000]
Eastern Europe & Central Asia
1200 [<1000 – 1800]
South & South-East Asia14 000
[8300 – 20 000]
Oceania<500
[<500 – <500]
North America<100[<200]
Latin America2400
[1300 – 3500]
East Asia1100
[<1000 – 1700]Caribbean
1000[<1000 – 1300]
Estimated deaths in children (<15 years) from AIDS 2010
Morphology of HIV
HIV particle is 100-150 nm in diameter.
Outer envelope of lipid penetrated by 72 glycoprotein spike (the lipid envelope protein)
The envelope protein is composed of two subunits: the outer glycoprotein knob (gp120) and transmembrane protein (gp41)
The receptor binding site for CD4 is present on gp120 as well as very important antigen such as V3 loop.
Morphology of HIV
The inner surface of virus lipid envelope is lined by matrix protein (p17)?.
There is also abundant cellular proteins in the lipid envelope (MHC class I and II) antigens.
In HIV-1 the lipid envelope encloses an icosahedral shell of protein (p17), within which is a vase or cone shaped protein core (p24, p7, and p9) containing two molecules of positive sense ssRNA
The RNA genome is associated with several copies of RT, integrase, and protease.
HIV genome
Positive sense ssRNA genome
The genome is approximately 10kb in size
The genome contain control genes which can enhance viral replication:
rev: regulator of virus
tat: transactivation.
vif: viral infectivity
repressor genes:
nef: negative factor
HIV genome
The genome is flanked at each end by LTR
3’ LTR has the polyadenylation signal and 5’LTR has the enhancer promotor sequence for viral transcription.
The pol gene code for RT, integrase and protease.
rev
tat
vpr vpu
vif
polgag env
nef
5’ LTR
3’ LTR
The HIV-1 genomeThe HIV-1 genome
p17 matrix antigenp24 capsid antigenp6/7 nucleocapsid
reverse transcriptaseproteaseintegrase
envelope glycoprotein (gp120)transmembraneglycoprotein (gp41)
HIV genome
HIV binds to specific receptor on the surface of CD4+ T lymphocytes (T-helper cells)
It also infects:
• B lymphocytes
• Macrophages
• dendritic cells
• brain cells.
Second subsidiary receptor belongs to chemokine receptor family CXCR4 on the T-cells and CCR5 on the surface of macrophages.
Retroviruses
virus binding fusion
ssRNA (+)
reverse transcription
dsDNA
nuclear transportintegration
nucleus cytoplasmtranscription
virion assembly and release
translation of viral proteins
maturation
HIV lifecycleHIV lifecycle
HIV life cycle
After attachment the virus penetrate the cell by fusion from without (Mediated by gp21 and gp41)
Synthesis of viral cDNA starts when the virion enters the cell cytoplasm.
The viral RT enzyme directs the synthesis of cDNA strand (the minus strand) using host positive RNA as a primer and the viral genomic RNA as a template.
Viral RNAse enzymatically remove the viral RNA while the RT synthesize the second DNA strand (plus strand).
HIV life cycle
Viral dsDNA will enter the Nucleolus of the host cell as a pre-integration complex (compose of viral protein M, Vpr, integrase, and dsDNA)
the integration of dsDNA to the host chromosome occurs (forming pro-viral DNA)
After integration viral and cellular factors are needed to activate HIV transcription.
Initial expression of viral RNA is stimulated by vpr and further stimulated by cellular transcription factors .
HIV life cycle
The primary RNA transcript is spliced to give 30 plus strand viral mRNAs.
Viral and cellular factors are required for early and late viral protein expression.
Early viral gene product include (tat, rev, and nef), accessory viral proteins (vif, vpr, and vpu)
Late viral gene products include (gap, pol and. env).
HIV life cycle
Assembly of new virion can begin by proteolytic cascade by viral proteases.
Different viral structural proteins begin to assemble with the p24 as a core and p7 enclosing viral RNA.
Viral genome assemble in the cytoplasm.
Retroviruses including HIV are release from the infected cells by budding from the infected cells.
The pro viral DNA may reside quietly in the chromosome for years.
Genetic Variability
RT has NO proof-reading mechanism therefore mutations (point point mutations and deletions/insertions) occur
Quasispecies = swarm of genetically distinct yet related viruses
Effects of Variability
Immune escape by changing/masking antigenic determinants
• CTLs and Abs
Resistance to anti-retroviral drugs
• Point mutations in enzymatic proteins RT - resistance to nucleoside and non-nucleoside analogues Protease - resistance to protease inhibitors
Altered cytopathogenicity
• Env and particularly V3 mutations alter co-receptor usage Different cell tropism, eg. Macrophages, T-cells, glial cells,
langerhans cells etc. Different tissue tropisms, e.g. brain
Integration
Double stranded cDNA (provirus) migrates to nucleus
Can exist extra-chromosomally as linear or circular form
Can integrate via the enzyme integrase
Activation
Once integrated the provirus responds to cellular nuclear factors e.g. SP1, NF-kB
Mediated through control regions in the 5’ LTR
Once active viral factors take over
Transactivation then control of RNA splicing events
Translation
Translation I.e.viral protein production
Virus release via budding on cell membrane
Morphological characteristics of budding virus is used for classification
• type C, Type D morphology refers to morphology of budding/maturing virus
Summary
HIV member of the Retroviridae family (reverse transcriptase)
Entry mediated by CD4 plus co-receptor
Reverse transcription leads to errors
Virus can become integrated into chromosome (can be latent)
Transcription – short (spliced) then long RNAs
New virus buds at surface
Three main targets for therapy
