Lecture 22 - HIV (1)

Question 1

Which groups are most commonly represented with new HIV infection?

Answer 1

Men who have sex with men (MSM)

Question 2

What are some factors associated with the HIV epidemic?

Answer 2

Behavioural & social:
* little/no condom use
* multiple, overlapping sexual partners
* large sexual networks

Biology:
* high STI rate → predisposes to HIV and other STIs
* low rates of male circumcision

Question 3

In which family is HIV?

What are some features of this family?

Answer 3

Lentiviridae family (cause disease slowly)

retroviruses
icosahedral capsid symmetry
enveloped
ssRNA, + sense
genome replicated in the nucleus
capsid assembled in the cytoplasm

Question 4

What are the two major categories of retroviruses?

To which does HIV belong to?

Answer 4

• Non-primate retroviruses
  o CAEV/Visna Caprine arthritis encephalitis / Visna Virus
  o EIAV Equine Infectious anaemia virus
  o BIV Bovine Immunodeficiency virus
  o FIV Feline Immunodeficiency virus
• Primate viruses
  o African Green Monkey SIVagm
  o Sooty Mangabey SIVsm (HIV-2)
  o Macaque SIVmac
  o Mandrills SIVmnd
  o Sykes monkeys SIVsyk
  o Chimpanze SIVCPZ (HIV-1)

HIV belongs to the primate category

Question 5

Outline the HIV genome

Answer 5

• ssRNA
• • sense
• 3 main genes:
  1. gag (structural proteins): MA-matrix, CA-caspid, NC-nucleocapsid
  2. pol Reverse transcriptase (RT): Integrase (IN), Protease (PR)
  3. env (envelope glycoproteins) – gp120 and gp41; the entrance tetramer that binds CD4, leading to entry, SU-surface: cell attachment, TM-transmembrane, fusion domain

Question 6

What can be said about + sense RNA?

Answer 6

Same as mRNA, and thus can be translated directly

Question 7

Describe the structure of the HIV virion

Answer 7

Env: gp120, gp41
Envelope
Matrix
Capsid
Nucleocapsid
Reverse transcriptase
Integrase
Protease

Question 8

What are HIV clades?

What can we say about their global distribution?

Answer 8

These are the HIV subtypes
Clades allows for HIV to inherently have diversity
US, Europe & Australia share clade

Subsaharan Africa & India share clades

South America

Question 9

Describe the life cycle of HIV

Answer 9

1. CD4 - gp120 binding
2. CoR engagement
3. Fusion
4. Reverse transcription of viral genome into DNA
5. Integration of proviral DNA into host genome
6. Transcription of DNA into viral RNA
7. Translation
8. Assembly
9. Budding
10. Maturation into new HIV virion

The step of integration is the key to retroviruses and the reason as to why we can’t cure HIV

Question 10

What are the key features of HIV replication?

Answer 10

• Rapid
• Error prone reverse transcriptase leads to rapid evolution of multiple quasispecies

Question 11

Describe the engagement of receptors that leads to fusion of the HIV virion with the host cell

Answer 11

1. Viral gp120 protein binds CD4 on the host cell
2. Conformational change, revealing CoR binding site
3. CoR binds gp120-CD4 complex which binds to CCR5 or CXCR4 on host cell
4. Virus fuses with host cell membrane, facilitating viral entry

Question 12

What and where is the CoR?

Answer 12

The Co-receptor
* CCR5
* CXCR4
It is on the CD4+ T cell

Question 13

What are the various chemokine receptors on CD4+ T cells that HIV can bind to?

Answer 13

These are the Co-receptors

CCR5
* Engaged by R5-HIV
CXCR4
* Engaged by X4 HIV
NB D/M HIV expressed gp120 that can bind both CCR5 & CXCR4
This ability to infect both cells occurs over time

Question 14

Describe a case of natural resistance to HIV

Answer 14

• There exists a mutation in CCR5: CCR5 Δ32

Heterozygotes:
* delayed progression to AIDS / death

Homozygous for the mutation
* rare infection with R5

NB The CCR5 Δ32 mutation does not affect immune function –> Those without CCR5 or low levels of its expression still have normal immune functioning as it is not required to survive

Question 15

Which cells does HIV infect?

Answer 15

Cells expressing CD4:
* ‘helper’ T cells
* monocytes & macrophages
* DCs

Question 16

What are the innate anti-viral cellular factors and their roles?

Answer 16

APOBEC3G: Edits RNA
TRIM 5a: Blocks uncoating of retroviruses
Tetherin: Blocks release of virus
LEDGF: Tethers HIV to host chromatin

Question 17

What is the corresponding HIV protein to APOBEC3G?

Answer 17

vif

Question 18

What is the corresponding HIV protein to TRIM 5a?

Answer 18

caspid - inactivates human TRIM 5a

Question 19

What is the corresponding HIV protein to tetherin?

Answer 19

vpu - inhibits tetherin from exiting

Question 20

What is the corresponding HIV protein to LEDGF?

Answer 20

integrase - facilitates HIV integration

Question 21

What are the two major forms of HIV infected cells?

Answer 21

Productive infection: DNA positive, RNA positive, HIV protein positive, DEATH

Latent infection: DNA positive, RNA negative, HIV protein negative, SURVIVAL

Question 22

When is the initial peak of viraemia in HIV?

Answer 22

21 days after HIV exposure

Question 23

What are the early events in HIV transmission in females?

Answer 23

1. Crossing the barrier: In the female reproductive tract, HIV is capable of infecting both the endocervix and the cervicovaginal epithelium
   * Can infect the cervicovaginal epithelium through a microbreak in the epithelium
   * Can infect the endocervix by binding to the dendritic cell
2. Local expansion (3-4 days): envelope on HIV binds with co-receptor on CD4 T cell –> activated and resting CD4+ T cells are infected
3. Dissemination to lymphatic tissue: local proliferation (1-2 weeks), peak plasma virus levels + CD4 memory cell loss –> partial immune control

Question 24

What is HIV?

Answer 24

HIV (Human Immunodeficiency Virus) infection

In the early stages of HIV infection, individuals may experience flu-like symptoms, such as fever, fatigue, and swollen lymph nodes.

As the disease progresses, the virus attacks and destroys the immune system, leading to the development of opportunistic infections and cancers.

Question 25

How do Antigen presenting cells (APCs) present foreign material, and to which cells?

Answer 25

APCs present foreign antigens to CD4+ T cells in the context of MHC II (major histocompatibility complex) alpha chain and the T cell receptor beta chain (TCR)

Similar process for CD8+ T cells and MHC class I

Both CD4 and CD8 make very robust response to HIV, its the over-exuberant immune response that makes people sick

Question 26

Describe the partial immune control of HIV

Answer 26

Partial immune control of HIV refers to the ability of the immune system to partially suppress HIV replication in the body, leading to lower viral load levels and slower disease progression.

This is typically seen during the chronic phase of HIV infection, which can last for many years. During this phase, the virus replicates slowly and the immune system mounts a response, which results in a decline in viral load levels. However, the virus is not completely eradicated

Question 27

What is the immune response to HIV in steps?

Answer 27

1. Initial immune response - HOURS/DAYS
2. Acute phase: a rapid increase in HIV viral load, which is accompanied by a surge in the number of activated CD4+ T cells. During this phase, the immune system also mounts a specific response against HIV by producing antibodies and cytotoxic T lymphocytes (CTLs) that can recognize and kill HIV-infected cells. - WEEKS/MONTHS
3. Chronic phase: a stable or slowly increasing viral load and a gradual decline in the number of CD4+ T cells. During this phase, the immune response against HIV becomes less effective, with HIV-specific CTLs becoming exhausted and antibodies becoming less able to neutralize the virus - YEARS
4. AIDS: This occurs when the immune system is severely compromised, and opportunistic infections and cancers can develop.

Question 28

How does HIV evade the immune response?

Answer 28

Sequence variation:
* Lack of recognition (both CTL and antibody)
* Antagonism
Altered antigen presentation
* Down regulation of MHC class I molecules by Tat, Vpu and Nef
Loss of effector cells
* Clonal exhaustion
* Loss of CD4 T cell help
* Replicative senescence
Latency
* Particularly in resting T-cells, macrophages and astrocytes
Privileged sites of viral replication
* Brain, testis, gastrointestinal tract

Question 29

What is the major distinction between a newly acquired and newly diagnosed infection?

Answer 29

A newly acquired infection is defined as an infection diagnosed in a patient who has had a negative HIV test in the past 6 months. Newly diagnosed infection is a positive result in a patient whose time of acquiring the virus is not clear as there are no recent negative tests.

Question 30

When HIV binds to CD4 receptor, a conformational change occurs. What part of the HIV virus goes through conformational change?

Answer 30

It’s the gp120 envelope glycoprotein that undergoes conformational changes, however this then also induces conformational changes in gp41 which is critical to fusion of the viral and cell membranes.

Question 31

Does the delta32 CCR5 mutation, when heterozygote, confer resistance to HIV? Or, does one need to be homozygous?
Even when homozygous, how can we call people ‘resistant’ when there could still be infection with x4 viruses?

Answer 31

Those individuals who are heterozygous for the delta 32 CCR5 mutation express a significantly lower level of CCR5 on their CD4+ cells. This makes these cells somewhat “harder” to infect. Thus what you find that these individuals don’t have any greater level of protection from infection, however once they are infected, the disease course tends to be much slower compared to those individuals who are homozygous for wild type CCR5. Individuals who are homozygous for the delta 32 CCR5 mutation express no CCR5 on their cell surface. They are very resistant to infection. This is because the major transmitted strain of HIV is R5.

Question 32

How does lymph node fibrosis lead to decline in CD4 T cells?

Answer 32

Lymph nodes are an important secondary lymphoid tissue where CD4 T cells reside and can recognise antigens presented by APCs and become activated. Fibrosis of the lymph node would result in abnormal scarring and deposition of collagen, and the lymph node would not function effectively. In this way, there would be ineffective activation of T cells, and therefore reduced proliferative signalling.