Week 11 Part 2 - HIV

Question 1

HIV/AIDS Virology

Answer 1

HIV-1&2 are retroviruses, having a single stranded RNA genome (9 genes encoding 15 proteins)
Structural proteins: encoded by Gag, Pol, Env
Regulatory proteins encoded by Tat, Rev
Accessory proteins encoded by Vpu, Vpr, Vif and Nef
Three major classes of HIV:
- M = main
- N = new
- O = outlier
M accounts for 90% of HIV

Question 2

HIV/AIDS - Virus Core

Answer 2

Genetic material (RNA), enzymes required for reproduction (protease, reverse transcriptase, RNAse integrase)
Held in capsid of structural protein p24

Question 3

HIV/AIDS - Matrix Protein Membrane

Answer 3

Structural protein, p17

Question 4

HIV - Viral Envelope

Answer 4

Lipids from host cells, spikes of envelope glycoprotein gp160, composed of gp120 and gp41

Question 5

HIV Infection - Overview

Answer 5

Infection begins with binding of virus to host cell
1st step: binding of gp120 and CD4 -> conformational change in gp120 exposing a co-receptor binding site
Co-receptors are chemokine receptrs: CCR5 or CXCR4
The fusion of viral and host cell membranes allows the viral contents to enter the cell

Question 6

HIV Infection Steps - Inside Host T Cell

Answer 6

1. Copying of virus RNA into cDNA
2. Pre-integration complex (cDNA and viral & host proteins) enters nucleus
3. Viral enzyme integrase mediates insertion of viral cDNA into host chromosomal DNA -> provirus
4. Provirus maybe latent and be retained in the DNA of the T cell for the life of the cell
   OR
5. T cell activation can induce transcription of the provirus into RNA
6. Viral RNA transported out of nucleus - RNA may be full-length or spliced
7. New virus particles assemble at plasma membrane incorporating gag, pol, nef, env, vpr and viral genomic RNA
8. HIV protease acts after virus assembly to cleave viral proteins into functional molecules

Question 7

HIV/AIDS Infection Steps

Answer 7

1. Virion binds to CD4 and co-receptor on T cell
2. Viral envelope fuses with cell membrane and viral genome enters cell
3. Reverse transcriptase copies viral RNA genome ino DS-cDNA
4. Viral cDNA enters nucleus and integrates into host DNA -> provirus
5. T cell activation induces transcription of provirus
6. RNA transcripts are spliced to allow synthesis of the early proteins Tat and Rev
7. Tat amplifies transcription of viral RNA and Rev increases transport of RNA to cytoplasm
8. Gag, Pol and Env are made and assembled with viral RNA into virions which bud from the cell

Question 8

HIV - Dendritic Cells Overview

Answer 8

HIV can attach to cell surfaces without infecting the cell (no CD4 involved in binding)
DC surface protein, DC-SIGN can capture HIV and promote infection of permissive cells
At low virus titre, binding to DC-SIGN is required for infection of CD4+/CCR5+ cells
DC-SIGN is expressed on cells that contact virus early in the infection process

Question 9

HIV - Dendritic Cells Steps

Answer 9

Mucosal epithelium are sites where infection is initiated
- contains DCs
If mucosal surface is injured, HIV has easy access to DCs
HIV attaches to DCs by binding DC-SIGN -> triggers internalisation of the virus into a mildly acidic endosomal compartment which protects the virus
DCs containing virus migrate to lymphoid tissue - where HIV is transferred back to the cell surface and onto CD4+ T cells in lymph node

Question 10

HIV Primary Infection

Answer 10

Period from initial infection to development of an antibody response
Replication first occurs in inflammatory cells at the site of infection
Major site of replication then shifts to lymphoid tissues
- lymph nodes and gut act as reservoirs for HIV
Macrophages and Langerhans cells/DCs in epithelia can be HIV infected but not destroyed allowing virus to be carried elsewhere

Question 11

Primary Infection Findings

Answer 11

Antibody response takes weeks or months to develop
Routine HIV tests are negative, but plasma viral loads reach very high levels
Viral replication peaks and then falls concurrently with the appearance of CD8+ T cells
- expansion of HIV specific CD8+ T cells which lyse infected cells

Question 12

Progression of HIV Primary Response - T cells

Answer 12

Initial drop in CD4+ cells
- recovers as disease enters asymptomatic phase
- tests for HIV positive at this stage
Steady state plasma viral load in asymptomatic stage
Progressive loss of CD4+ cells and of immune function
CD8+ expansion persists into advanced stages
Progression to AIDS -> decrease in CD4+ and CD8+ cells with an increased viral load

Question 13

What does the Breadth and Strength of CD8+ T Cells Correlate to?

Answer 13

Correlates positively with degree of viral control and inversely with rapidity of clinical progression

Question 14

Progression of HIV Primary Response - Humoral Response

Answer 14

Vigorous IgG response but antibodies mainly against viral debris with little neutralising activity
- due to neutralising epitopes on gp41-gp120 oligomer being poorly accessible
- infected mucosal DCs migrate to regional lymph nodes -> virus out of reach of neutralising antibody
HIV produces B cell dysfunction and caused by viral protein toxicity
Also B cell depletion

Question 15

Progression of HIV Primary Response - Macrophages

Answer 15

Macrophages serve as reservoir for viral replication but have impaired function
- decreased ability to present antigens to T cells and decreased phagocytosis

Question 16

Which Pathways does HIV Activate Complement

Answer 16

Alternative and classical pathways
Has poor function of C5-9 complex

Question 17

Progression of HIV Primary Response - DCs

Answer 17

Decrease in DCs number and function
Follicular DCs, key APCs in lymphoid tissue - often loaded with virus or viral antigens

Question 18

HIV Immune Escape

Answer 18

HIV mutates and replicates rapidly
CD8+ T cells reduce viral load
- but -> escape mutations render virus infected cells undetectable by host CD8+ T cells
Therapy can suppress viremia but latent virus in resting memory CD4+ cells can escape drug treatment and the immune response

Question 19

HIV Virus Tropism

Answer 19

Virus may be macrophage-tropic (M-tropic) or T cells tropic (T-tropic)
M-tropic strains use CCR5 as their coreceptor (R5 viruses)
- transmitted sexually more easily
T-tropic strains use CXCR4 as their co-receptor (X4 viruses)
- induce formation of syncytia in infected cells
Isolates early in M-tropic and late in T-tropic

Question 20

HIV Opportunistic Infections

Answer 20

CMV
Hep C
Pneumocystis carinii
Mycobacterial infections (tuberculosis)
Fungal infections (Candida)
Kaposi’s sarcoma - most common neoplasm associated with AIDS

Question 21

HIV Antibody Testing

Answer 21

Serum antibodies to HIV measured by ELISA
Positive results confirmed by Western blot
Period shortly after infection may be negative ELISA, but high viral titre

Question 22

HIV CD4+ T Cell Testing

Answer 22

CD4 count or % CD4 T cells diagnostic marker for disease progression
Normal = 1000 cells/uL, AIDS = 200 cells/uL
Initial drop in first few months, slower delcine in asymptomatic seropositive patients, accelerated decline in late disease

Question 23

HIV Viral Load Testing

Answer 23

Viral RNA in blood by either:
- RT-PCR
- nucleic acid sequencing
- branched DNA amplification
Sensitivity to 100-200 copies/mL

Question 24

HIV Antigen Testing

Answer 24

E.g. p24 used to screen donated blood products
ELISA and Western Blot

Question 25

HIV Medication Steps

Answer 25

1. Block reverse transcriptase enzyme
2. Block protease enzyme
3. Inhibit fusion of viral and host membranes