HIV is a retrovirus that causes AIDS by infecting CD4+ T cells. It is estimated that 33.3 million people were living with HIV at the end of 2009 according to UNAIDS. HIV belongs to the lentivirus genus and has an RNA genome that is reverse transcribed into DNA. Its replication cycle involves attachment to CD4+ cells, uncoating, reverse transcription, integration, gene expression through transcription and translation, assembly of new virus particles, and release of particles from the cell. Accessory proteins such as Vpu, Nef, Vpr, and Vif assist in various stages of the replication cycle and modulation of host immune response.

1. Presented by
Upasana Ganguly

2. •Acquired Immune Deficiency Syndrome (AIDS) is a
disease of the human immune system caused by the
Human Immunodeficiency Virus (HIV).
•HIV infection in humans is considered pandemic by the
World Health Organization (WHO).
•UNAIDS estimated that 33.3 million people were living
with HIV at the end of 2009.

3. Systemic position of HIV
 HIV is a complex retrovirus.
 A retrovirus is an RNA virus that is duplicated in a host cell
using the reverse transcriptase enzyme to produce DNA
from its RNA genome.
 HIV belongs to the Lentivirus genus of the Retroviridae
family.
 Infections with lentiviruses typically show a chronic course
of the disease, with long periods of clinical latency,
persistent viral replication and involvement of the Central
Nervous System.

4. HIV Variability: Types, Sub types and
Groups
 HIV variability is the
consequence of
i) “error-prone”
mechanism of action of
the viral enzyme reverse
transcriptase
ii) Very rapid viral
replication
iii) Occurrence of
recombination between
two or more HIV viruses
within the same infected
individual.

5. Origin of HIV
 It is now generally accepted that HIV is a descendant of a
Simian Immunodeficiency Virus (SIV) because certain
strains of SIV bear a very close resemblance to HIV-1 and
HIV-2.
 HIV-2 corresponds to SIVsm.
 In February 1999, a group of researchers from the
University of Alabama announced that they had found a
type of SIVcpz that was almost identical to HIV-1.
 Several theories have been hypothesized to explain how
this ‘zoonosis’ may have taken place.

6. Structure of HIV
 Roughly spherical
 Diameter : 100-120nm
 Outer envelope or coat is
composed of a double layer of
lipid envelope that bears
numerous spikes.
 Beneath the envelope is a layer of
matrix protein that surrounds the
core or the cone shaped capsid.
 Capsid contains two copies of
positive sense ss RNA and the
viral enzymes Reverse
Transcriptase, Integrase and
Protease as well as two other
proteins p7 and p6.

7. Genome organization
1.HIV genome consists of a
homodimer of linear, positive-
sense, single-stranded RNA of
approximately 9.2 kb in size.
2.Two RNA strands are capped
and polyadenylated.
3.HIV genome is composed of
nine genes :
A. Structural genes : gag, pol,env
B.Regulatory genes : tat,rev
C.Accessory
genes:vpr,vif,nef,vpu

8. HIV Replication Cycle
The HIV replication cycle can be divided into the following
steps :
1.Virus entry - Attachment and penetration
2.Uncoating
3.Reverse transcription and Nuclear Import
4.Integration
5.Gene expression–Transcription, RNA export and translation
6.Virus particle assembly and release.

9. Step-1: Attachment,fusion and
penetration
 HIV specifically targets the CD4+ subset of T- lymphocytes.
 gp120 interacts with the N-terminal extracellular domain of CD4.
 Binding of gp120 with the CD4 receptor results in
conformational changes in gp120 which exposes the co-receptor
binding sites.
 The co-receptors are mainly the α-chemokine receptor CXCR4
and the β-chemokine receptor CCR5.
 The differential expression of chemokine receptors on target
cells has been shown to be a major determining factor for HIV
tropism.

10. HIV-1 Tropism

11. Diagrammatic representation of Virus
entry

12. Step-2: Uncoating
 During the process of “uncoating” the core is converted to a
complex referred to as the reverse transcription complex
(RTC) and then the pre-integration complex (PIC) in a
series of steps.
 CA appears to be lost while at least some MA, NC, the pol-
encoded enzymes RT and IN, and the accessory protein
Vpr, remain associated.

13. Step-3 : Reverse Transcription
 Reverse transcription is a critical step in the life cycle of all
retroviruses.
 This complex process is performed exclusively by the retroviral
reverse transcriptase (RT) that converts the viral ssRNA into
integration competent dsDNA.
 RT has three activities : i)an RNA dependent DNA polymerase
ii) a DNA dependent DNA polymerase
iii) a ribonuclease H activity (RNase H)
 RT is a heterodimer containing a 560 residue subunit p66 and a
440 residue subunit p51 both derived from the Pol polyprotein.

14. Process of Reverse Transcription
 Step-1: Reverse Transcription Is initiated
as the tRNA Primer anneals to the Primer
Binding Site of the Viral RNA template.
 Step-2: The First-Strand Transfer is made
possible by Repeat Sequences (R) present
on both ends of the RNA template.
 Step 3: Synthesis of Plus-Strand Strong-
Stop DNA Is initiated from purine-rich
Regions of genomic RNA.
 Step 4: The Second-Strand Transfer is
facilitated by plus- and minus-Strand
DNA Copies of the PBS.
 Step 5: Continuation of DNA synthesis
requires strand displacement of the short
DNA stretch.
 Step 6: Termination of the upstream plus-
strand DNA requires the central
termination sequence.

15. Step -4 : Integration  Integration is an essential step
in the life cycle of the HIV.
 Integration proceeds via a well-
defined series of steps that are
quite common among
retroviruses:
step-1:IN removes two
3´nucleotides from each strand
of the linear viral DNA.
Step-2:The processed 3´ends are
covalently joined to the 5´ends
of the target DNA.
Step-3:Unpaired nucleotides at
the viral 5´ends are removed
and the ends are joined to the
target 3´ends.

16. Step- 5 : Gene expression
Transcription
 In HIV, the 5´ LTR serves as the promoter for the cellular
RNA Polymerase.
 The promoter contains binding sites for both general
Transcription factors as well as tissue-specific
Transcription factors.
 The basal transcriptional activity from the HIV LTR is very
low. Significant expression of HIV genes occur only in the
presence of Tat protein.
 Transcription from the HIV-1 LTR leads to the generation
of a large number of viral RNAs.

17. Fig.
Early and late transcripts
derived from the viral HIV-1
genome. The integrated copy of
the viral genome produces Rev
and Tat proteins from the 2 kb
early transcripts. Both Tat and
Rev are RNA binding proteins
that enter the nucleus and
mediate transcription
transactivation and export of 4
and 9 kb late transcripts,
respectively. The late
transcripts have an RNA
binding site for Rev allowing
their export from the nucleus.

18. RNA export :
 RNA export from the nucleus to the cytoplasm is mediated by
Rev.
 Rev contains a Leucine rich export signal that frequently causes
it to shuttle between the nucleus and cytoplasm.
Translation:
 Viral mRNAs are translated in the cytoplasm and the Gag and
Gag-Pol polyproteins become localized to the cell membrane.
 gag gene overlaps the pol gene by 241 nucleotides. So a ribosomal
frameshifting mutation results in a Gag-Pol fusion protein in 5-
10% cases.
 Env mRNA is translated in the Endoplasmic Reticulum.

19. Step – 6 : Virus particle production
 Gag membrane binding and targeting :
 RNA encapsidation
 Assembly
 Env transport and incorporation
 Budding
 Maturation

20. Role of Accessory proteins in HIV
replication
Protein Function
1. Vpu ( viral protein u ) i) Promotes the degradation of CD4 in
complexes with envelope glycoproteins in
the ER.
ii) Downregulates the cell surface expression
of Class-I MHC molecules.
iii) Promotes viral particle release from the
cell.
2.Nef (Negative factor) i) Downregulating CD4 and MHC-I
molecules from the cell surface
ii) stimulation of virus infectivity
iii) modulation of cellular activation pathways
3.Vpr (viral protein r) i) Arrests Vpr expressing cells in G2 phase of
the cell cycle
ii) might play a role in nuclear import of the
viral PIC
4.Vif (viral infectivity factor) i) Enhances infectiveness of progeny virus
particles

21. CONCLUSION
 Although HIV-1 has been the most studied infectious agent
in the last 30 years, the new available technologies have
allowed the acquisition of new information about virus
structure and replication.
 Extensive study on the different viral subtypes and
recombinant forms has led to the understanding of the
phylogenetic and geographic distribution of HIV isolates.
 A detailed understanding of the HIV biology is required to
design new preventive and therapeutic approaches aimed
at counteracting the molecules of the virus.

22. THANK YOU