HIV 1

Question 1

Basics of HIV?

type of virus, family, size, capsid, envelope, genome…

Answer 1

Lentivirus
family: retroviridae
Icosahedral capsid
80-130 nm
diploid linear +ssRNA

Question 2

what does the HIV ‘env’ gene encode?

Answer 2

envelope components:
gp 120: surface protein
gp 41: transmembrane protein
(important in attachment/fusion)

Question 3

what does the HIV ‘gag’ gene encode?

Answer 3

Structural proteins:

matrix, capsid, nucleocapsid

Question 4

what does the HIV gene ‘pol’ encode?

Answer 4

Viral enzymes:
reverse transcriptase
integrase
protease
RNase

Question 5

Outline the HIV replication cycle

Answer 5

binding -> fusion -> entry -> reverse transcription -> nuclear localisation -> integration -> transcription and splicing
-> RNA transport -> translation of protein -> virion assembly -> envelopment -> budding -> maturation

Question 6

Describe the process of HIV entry/fusion

Answer 6

1. Attachment by non-specific cell receptors - CLRs eg DC-sign, Mannose-R
2. CD4+ binding -> structural changes in gp120 -> chemokine coreceptor sites exposed
3. Binding to CCR5, CXCR4 promotes gp41 fusion and peptide insertion
4. Structural rearrangement of gp41 trimers drives membrane fusion

Question 7

Compare CCR5 and CXCR4 co-receptors in regards to their use in the course of HIV, their tropism, syncitium induction and pathogenicity

Answer 7

CCR5: early in disease course, M-tropic, doesn’t induce syncitium, moderate pathogenicity
CXCR4: late in disease course, T-tropic, induces syncitium, high pathogenicity

Question 8

How does HIV end up disseminating thru the body after mucosal exposure?

Answer 8

mucosal exposure -> selective infection by R5 strains -> In lamina propria, HIV binds to dendritic cells by DC-SIGN -> virus transported to regional LNs -> spread of virus to activated CD4+ T cells -> entry of virus-infected cells into bloodstream -> widespread dissemination

Question 9

what is the function of reverse transcriptase?

Answer 9

converts the viral genomic RNA to proviral DNA
high error rate!
Also duplicates the sequences at the ends of the viral RNA forming a DNA structure called the LTR (long terminal repeat)

Question 10

what is the function of HIV integrase?

Answer 10

catalyses the random integration of HIV cDNA into cell DNA

*integration can occur in resting and terminally differentiated cells

Question 11

What is the function of the LTR?

Answer 11

• acts as the HIV gene promoter
  ++ increased expression in response to HIV Tat
• silences HIV expression soon after init. replication
• responds to cellular proteins made during T-cell immune activation (eg NF-KB) to +++ increase HIV production

Question 12

give examples of cis- and trans-acting factors that regulate basal HIV transcription

Answer 12

Cis- (act at integration site): HDACs, histone methyltransferases
Trans- (from T cell activation): NF-KB, NFAT

Question 13

T/F: Tat and Rev are not essential for HIV replication, but are important in it’s pathogenicity in vivo.

Answer 13

False. Tat and Rev are essential for HIV replication.

Question 14

T/F. Vif, Vpr, Vpu and Nef are not essential for HIV replication, but are important for its pathogenicity in vivo.

Answer 14

True. They are not essential for HIV replication, but play important roles in its pathogenicity in vivo

Question 15

What is Tat?

Answer 15

transactivator of HIV transcription (through TAR RNA element)

Question 16

What is TAR

Answer 16

trans-activation response RNA element (activated by Tat)

Question 17

What is Rev?

Answer 17

regulator of structural gene expression through binding of the Rev-responsive RNA element

Question 18

How does the Tat/TAR interaction promote transcriptional elongation?

Answer 18

By forming a complex that increases the activity of RNA Polymerase II

Question 19

What is the vital function of Rev?

Answer 19

stabilises and transports unspliced and partially spliced RRE+ HIV RNA to the cytoplasm

Question 20

What do the non-structural proteins Nef, Vpu and Tat do to MCH class I?

Answer 20

down-regulate MHC class I -> prevent HIV peptides from being presented to CTLs

Question 21

What are the 3 molecules involved in our ‘natural combination therapy’ against HIV?

Answer 21

TRIM5a: destabilises the viral capsid
APOBEC3G: induces lethal hyper-mutations in the viral genome while ssDNA
Tetherin: inhibits viral release

Question 22

How does Vpu contribute to HIV virulence?

Answer 22

Facilitates release of fully infectious virions and counteracts innate immunity factors.

degrades CD4
antagonizes Tetherin
inhibits surface expression of CD1d

Question 23

How does HIV Vif protein contribute to virulence?

Answer 23

Degrades APOBEC3G

preventing it from inducing lethal hypermutations in the viral genome

Question 24

T/F: the protease activity of the Gag-Pol precursor protein causes cleavage of the polyproteins into individual proteins

Answer 24

True

Question 25

Where is the main latent HIV reservoir?

Answer 25

In central and transitional memory T cells

Question 26

T/F: after >10 years on HAART, patients have decreased size of the latent HIV reservoir.

Answer 26

False.

Question 27

How does Tat activate HIV RNA expression?

Answer 27

Recruitment of HATs -> acetylation of histones in chromatin -> exposes the DNA promoter for access by RNA polymerase II