HIV vaccine development Flashcards
Vaccine history + active passive immunization + vaccine types examples
Smallpox = Jenner variolation 1798
Cowpox can infect humans but causes mild disease, and gives protection for smallpox.
Active immunization strategies:
inactivated vaccine
cholera, anthrax, plague
Inactivated vaccines are created with, eg. formaline treatment.
Live attenuated:
rabies
Most of the first vaccines are either live attenuated or inactivated, newer ones also use purified proteins etc. It’s learned critical to utilize neutralizing Ab responses. Only in some cases, T cells were also important
Passive immunization
inject Abs
Why vaccines fail
Complex life cycle of virus
Malaria caused by Plasmodium, parasites enter to body once you are bitten by a mosquito, they first enter to skin, and travel to liver, and blood versions of plasmodium and liver versions are clearly different. = complex life cycle. Difficult to target the right stage.
Virus can infect other hosts too - If your pathogen can infect other hosts too, you need to immunize them as well.
Virus is so variable
Malaria, influenza, rhinoviruses have many different variants = so it’s hard to make a vaccine that will cover all of them.
Virus is very persistent
When some of these viruses enter the body, they can turn to some kind of latent stage, they stay in body in a dormant way, eg. Herpes Simplex in nerve cells. They can’t be recognized by Abs, so they form a permanent reservoir.
HIV vaccine problems
HIV: extreme variability/antigen diversity and persistence
Persistence: Latent state in resting memory T cells = reservoirs in immune priviledged sites eg. microglia in brain.
RNA virus: Once it integrates host genome, only way to kill it is to kill the infected cell.
Immune evasion
Targets T helpers/APCs = reduced immune function
Has direct immunesuppression = downregulate MHC Class I
Depletes T cells rapidly from gut = bacteria enters blood, chronic inflammation increases HIV infection even more.
RT is very error prone = high mutation, high variation, easier to escape
For defenses: inducing Inducing T cells for mucosa is a very new concept, and tissue resident memory T cells are studied for defenses.
So the vaccine created must be almost %100 protective for every possible variant.
Other virus vaccines are good if they can only reduce the virus load a bit, then immune system deals with it = but it’s not the case here.
HIV types
HIV has 2 types: HIV-1 and HIV-2. HIV-2 is only circulating in Western Africa, and its derived from SIV. HIV-1 is everywhere, and its derived from chimpanzees. Main group of HIV is group M, and those groups are divided into clades, clade B is in America & Europe, China clade C-E, in Africa, all of them are found. Highly diverse virus.
Animal models - why we need them
Safety evaluation
Immunogenicity evaluation
Efficacy evaluation
Evaluations can’t be only done with in vitro cultures, since there’s a lot of interactions with other cell types – tissues that needs to be evaluated for vaccine development. To check whether vaccine induces desired immune response = immunogenicity evaluation is required. Do they really give protection = efficacy.
Chimpanzee at first = because chimpanzees are the only animals that can be infected w HIV. Chimpanzees are very clever, so it causes a lot of ethical consideration. They can be infected w HIV-1, they are also the original source of virus, but no disease progression observed = not a good model. Expensive too.
Monkeys don’t get ill from HIV-2, but macaques do. And they show similar response like humans.
Problem is that HIV-1 and HIV-2 differ too much.- especially in envelope, you can’t study an envelope targetted vaccine.
So there are models, but they aren’t that good.
Mice have replaced its immune system with human. = Humanized mice
Induction of immune response against HIV is very low inside these models. = Still being improved.
Vaccine types for HIV - Ab
General understanding = If you can create Ab response for Envelope = you can design a vaccine.
Neutralizing Ab response = clinical approach.
1) Live attenuated = not safe, integrates into genome, can reverse mutate and become pathogen, induces disease in fetal macaques
2) Whole inactivated: Modest immunity, but no protection against HIV
Since it has low no of Env spikes per virion, they got denaturated.
Env spikes are damaged It’s still sufficient for virus to enter the host, but its not a good immunogen.
3) Protein subunit vaccines
HIV envelope protein (monomeric)
Protects against HIV-1 challenge in chimpanzees
Protects against SIV in macaques
But only protects for 1 virus type - lab adapted one
Administering only Env only works when the virus entering has the same Spike with the vaccine.
HIV was grown on in vitro cultured cell lines, and those cell lines were different from HIV inside humans are different, they became less virulent in vitro.
Failed in human clinical study too.
Why it’s hard to get neutralizing Ab response?
Trimeric configuration of Envelope
Complex receptor interaction mechanism:
It consists of 2 subunits, gp120 and gp41. First step is the attachment to CD4, this leads to a conformational change, exposing the co-receptor binding site that binds to CCR5/CXCR4 co-receptors. 2nd conformational change occurs, which exposes gp41, and it forms a fusion complex leading to the infection.
So there’s several conformational changes = complex receptor interaction mechanism. So the epitopes you want to target keep changing all the time!
Virus itself is heavily glycosylated = this glycosylation also produced inside the host – but by host machinery! It also makes human glycoproteins to cover the virus = and they aren’t very immunogenic anymore. Difficult to make immune response against it.
HIV sequences are too variable/most is covered with glycans.
Focus on CTL response!
Role of CTLs in HIV infection
Reduces viremia
At first: Virus replicates like crazy, then it’s repressed = repression occurs with CTLs. CTLs are important.
Elite controllers: People with natural capabilities to suppress the virus well without therapy. They usually have very strong high CTL response
Certain MHC alleles are overrepresented in them.
If virus mutates, it can escape from elite controllers too, and disease progresses.
. How to get a vaccine expressed inside cytosol? - CTL response
Vaccine types for CTL
Problem: If you give a peptide from outside, that’s mostly taken up by APCs, to bring it to endolysosomes. It’s clipped and loaded on MHC Class II. But it’s presented to CD4.
So you need to have this peptide produced inside the cytosol, cleaved and loaded on MHC Class I and bringed to CTLs.
How to get a vaccine expressed inside cytosol?
Nucleic Acid (RNA/DNA) vaccines DNA is stable, but DNA is negatively charged; cell surface is negatively charged = so they aren’t taken up. Trick is to do it with electroporation 2nd problem: Nuclear membrane to take up DNA only opens up in cell division. RNA doesn't depend on cell division but broke down easily . there’s been several techniques developed to shield the RNA and make it useful = works in Corona.
Live recombinant viral vectors
Adenovirus / poxvirus + HIV
Immunogenic
Problem: Also you get immunogenicity against your own virus that you used as a vector. 2nd or 3rd administration = the vaccine doesn’t work anymore.
Prime/boost strategies = different combinations of vaccines. = depending on the timeline of HIV development.
So at first you can vaccinate individuals with DNA, then wait and vaccinate them again with adenovirus approach. Can be pox virus first / adenovirus second. Or use same virus from different serotypes that doesn’t cross-react. Found to be partially effective for SIV.
Results: Transient reduction of virus replication so it escapes.
More sustained reduction in less susceptible MHC alleles in animals
No protection against infection
Might be due to too high viral dose is used for the study, but humans only get infected with 1/100 of it.
But you have a control and an experiment group, and you want all your controls to be infected .
Solution: give low dose, but repeatedly.
Mosaic vaccines
HIV mutates to escape from CTL response
Then include mutated peptides
Studied.
STEP trial + follow up
Only adenovirus vaccines are used, so it’s not that good of an approach.
Huge fail: Vaccine group got more infected! This is seen to be occurring in people that have some kind of responses to Adenoviruses before, and it’s bad.
People with no response to Adenovirus before basically had no difference.
Reason is that some T cells in mucosa that is Adenovirus specific gets activated and apparently activated T cells get more easily infected.
A DNA boost + only with low Adenovirus reactive people are selected later = it just doesn’t work.
HVTN 505 (2013)
Follow on to the STEP trial but included only Ad5 negative and circumcised gay men and
composed of plasmid prime
Again more transmissions appearing in the vaccine group opposed to the placebo)
RV144 - combine Abs and T cells
Boost with canarypox vaccine + give Env protein vaccine
Again monomers of spike proteins are used = they are definitely not optimal.
There was a bit of a protection. Looking at actual numbers shows its actually very veeery small effect. Very low statistical significance. Difference occurred in early infection only, then the line started running parallel = well probably escaped.
Maybe it’s not a good thinking that you can induce good Abs with 1 vaccination, multiple shots:
Envelope A epitopes can somehow trigger the Ab response, but it’s still not so easy since Envelope A is very similar to host proteins as well, so it’s hard to trigger the initial response.
So you need different type of epitopes administered at different times to guide the immune response to the point you want.
Vaccine for first Env A, then B, then C. In trials.
Problem of HIV drugs in protection + What can you use as drugs
Large surface area to be protected
- Needs to cover all cellular interactions
- Needs to penetrate the mucosal layer (and active)
- Needs to function in the context of
bodily secretions (milk/sperm/CVS/mucus)
You can use RT inhibitors, integration inhibitors, or protease inhibitors
Drug development takes 15 years until clinically used.
Impact of male circumcision on HIV
Reduces it to %60
VaxGen trial - 2003
3 shots of recombinant protein aimed at inducing Ab responses but not CD8 cellular
responses (CTL)
gp120 targetting
Worked 0
