Lecture 3 - HIV & Vaccines Flashcards
What are the three main types of vaccine manufactured at present?
Outline them
- Whole killed vaccine
• Virion can not replicate / cause infection
• Immune response launched - Live attenuated
• Virion can not cause pathology
• Immune response launched - VLPs
• Specific structural component of virus administered
• Immune response launched
Describe the changes in viral load over a course of 10 years in HIV+ patients
Acute, initial phase: high viral load
Seroconversion: decrease in viral load
Over the years: slow increase in viral load
What is the desired effect of an HIV-1 vaccine?
Viral load is kept very low, but not fully removed
This greatly reduces transmission
Individual never develops AIDS, despite some decrease in CD4+ T cell count
Which molecule in HIV is the target for vaccines?
CD4 binding site on gp120 molecule
Describe HIV entry into cells
- gp120 binds to CD4
- Conformational change in gp120, revealing CoR binding site
- Binding of the co-receptor (CoR); CCR5 to newly exposed binding site on gp120
- Fusion of membranes and insertion of contents into host cell
What is the CoR?
Co-receptor
• CCR5
Considering HIV entry into cells, what could be an approach for a vaccine?
VLP vaccination:
• Vaccination of Env: gp120/gp41
• Immune system launches a response against it
• When exposed to virus, Ab’s block the binding site, so the virion can not enter host cells
NB Adjuvant would also be required
What are the three really large trials for an HIV vaccine?
What were the outcomes?
- Vaxgen
• No protection - STEP
• No protection
• Possible increased susceptibility to infection - RV144
• Moderate efficacy: 20%
Compare the genetic changes in HIV and Influenza
Influenza:
• Variability due to recombinant between different strains
• Once an individual is infected with a given strain, one can launch a sterilising immune response against it
• Linear antigenic drift;
• One predominate variant at a given time
HIV:
• Highly mutable
• Great number of variants circulating at a given time
• Mutating faster than the immune system can keep up with
• Thus, cannot launch a sterilising immune response
Why is HIV highly mutable?
What does this mean for our immune system?
Error prone reverse transcriptase
The immune system may make antibodies against an antigen of the HIV, only for mutation in this antigen to render the antibodies useless.
Outline the escape mechanisms of HIV
- HIV is highly mutable
• Due to error prone reverse transcriptase
• Immune system can not keep up with the escape variants
• Env can withstand much antigenic variation - Unstable association between gp120 and gp41
• gp120 release to expose irrelevant Ag - N-glycan carbohydrate shielding
• Immune system not good at making high affinity Ab against carbohydrates
• Antigenic sites of gp120 hidden - Hidden CoR binging site on gp120
Describe the unstable association between gp120 and gp41 and the implication
- gp120 head can dissociate
2. Exposes irrelevant epitopes which can dominate the immune response
Describe carbohydrate shielding and the effect
The trimeric head of Env can be glycosylated to form a ‘glycan shield’
This shields the antigenic sites on gp120
Describe the function of the hidden CoR binding site on gp120
This binding site is only revealed once CD4 binds to gp120
The site is not available for neutralising antibodies
What features of the HIV Env protein that render it difficult for the immune system to control?
Env can withstand many mutations and still be functional, thus ESCAPING from the immune system
What is some evidence of HIV escape from immune surveillance?
In a given patient, the virus was recovered at different point in time (time of infection, 2 weeks, month, 100 days, 150 days)
Even after a few weeks there was accumulation of mutations in this individual
These ‘escape variants’ are selected for by the immune system
What / who are elite controllers?
These are individuals who are HIV+, but the infection does not progress
They develop antibodies against multiple HIV isolates (broadly neutralising Ab)
What are RSC3 and ΔRSC3?
Why are these useful?
RSC3: A probe that mimics the CD4 binding site on gp120
ΔRSC3: Identical to RSC3, except it is missing the CD4 binding site
Using these two probes we can sort out B cells whose antibodies bind RSC3 and not ΔRSC3, indicating that their Ab binds to the CD4 portion of RSC3 (and thus are broadly neutralising)
What are broadly neutralising Abs?
Antibodies that neutralise HIV not just from themselves, but other people
bnAbs recognise core, invariant, components of the HIV virion
Describe the various methods of enrichment of bnAbs from elite controllers
– FACS using RSC3 –
- FACS using and RSC3 ΔRSC3
- Enrichment of antibodies that only recognise the CD4 binding site on RSC3
- Ignore those that bind both RSC3 and ΔRSC3, because these must be binding elsewhere
- Same goes for those only binding ΔRSC3
– 2CC probe isolation –
- Sorting of cells that bind to 2CC probe
- Recovery of genes from these B cells
- Re-expression of these genes in cells in test tube
– High throughput sequencing –
- Isolation of lymphocytes during HIV immune response in elite controllers
- Sequencing of Ig genes in these B cells
- Identification of common sequences present in elite controllers
– Binding assay –
- Abs isolated from a patient
- Binding tested in culture
Describe the VRC01 story
- VRC01 is a bnAb (binds the CD4 binding site on gp120)
- VRC01 is highly mutated from its original form in the naive B cell
- ‘Germ line’ Ab, i.e. before it underwent somatic hypermutation, did not bind gp120 and thus was not neutralising
Question: how did it ever come to be neutralising?
- ‘Arms race’ between HIV and host response
- During the response, the Abs become no longer neutralising
- B cells re-enter the GC, are exposed to AID, and the variable regions are again mutated
- Some of the new Abs will be neutralising
How did these germline Abs ever become specific for HIV antigen, if they don’t react at all?
Remember, the HIV virus mutates rapidly due to the ‘selective pressure’ of the immune system
These Abs were specific for the strain of the virus at an early stage
Describe the ‘arms race’ between HIV and the host’s immune system
Once the host launches an immune response against HIV, there will be certain virions that have mutations and will not be affected.
These are selected for
Thus, over time, the virus changes strain and a new immune response must be launched
Describe rational design of anti-HIV Ab
Using high-resolution imaging, we can see the CD4-gp120 binding site at a molecular level.
Observation:
• CD4 bind gp120 binding site with a Phe aromatic ring
By observing how CD4 binds to the CD4 binding site, we can create our Ab to mimic this
Do this by changing single amino acid bases
Describe how we can infect mice with HIV in the lab
- Mice with mutation in RAG, and thus, defective immune systems
- Transplant lymphocytes from humans into the mice
- The HIV will now infect these human lymphocytes in the mice
Describe the results of the bnmAb therapy study in mice
- Mice with human lymphocytes able to be infected by HIV
- Monotherapy:
• 5 bNt mAbs individually were not neutralising
• Mice were infected - Tri-therapy:
• Some mice controlled the infection
• Others didn’t - Penta-therapy:
• All mice controlled the therapy
• Low viral load - Removal of the therapy
• Decay of bnmAbs
• Infection comes back
Are there any down sides to penta-therapy?
Once administration of the antibodies ceases, the viral load rebounds
What is the generalised method of vaccine development for HIV?
Identification of the structures in the HIV virions against which we want to direct the immune response
(eg. CD4 binding site on gp120)
What were the conclusions made about penta-therapy with bnmAbs?
- Proof of principle
• bnmAbs can be effective at suppressing the viral load - Adenoviral vectors
• Containing genes for bnmAbs
• These genes become expressed in muscle cells
• Long term production of bnmAbs - Prophylaxis for vertical transmission
