1. Retroviruses and AIDS 2

2. 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.

3. 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??

4. 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

5. 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.

6. Retroviridae

7. Lentiviruses

8. 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

9. Photograph by Karl Ammann

10. Photograph by Karl Ammann

11. 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):

12. Properties of HIV
Lentivirus: containing HIV-1 and HIV-2, characterised by:
Cone shaped Nucleocapsid
absence of oncogenicity
and the lengthy and insidious onset of clinical signs.

13. 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).

14. 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.

15. 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.

17. HIV genome tat: transactivation. vif: viral infectivity
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

18. HIV genome
The genome is flanked at each end by LTR (Long Terminal Repeats)
3’ LTR has the polyadenylation signal and 5’LTR has the enhancer promotor sequence for viral transcription.
The pol gene code for reverse transcriptase RT, integrase and protease.

19. The HIV-1 genome rev vif nef tat 5’ LTR 3’ LTR vpu vpr gag pol env
p17 matrix antigen
p24 capsid antigen
p6/7 nucleocapsid
reverse transcriptase
protease
integrase
envelope glycoprotein (gp120)
transmembrane
glycoprotein (gp41)

20. Binding
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.

22. HIV lifecycle maturation virus binding fusion
virion assembly and release
ssRNA (+)
reverse transcription
translation of viral proteins
dsDNA
nuclear transport
integration
transcription
nucleus
cytoplasm

24. HIV life cycle
After attachment the virus penetrate the cell by fusion from without (Mediated by gp120 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).

25. 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)
Can exist extra-chromosomally as linear or circular form
the integration of dsDNA to the host chromosome occurs (forming pro-viral DNA) by the enzyme integrase
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 .

26. 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.

27. 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

28. Effects of Variability
Immune escape by changing/masking antigenic determinants
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

29. 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.