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Lessons from the past from vaccines against other viruses
1. Many vaccines stimulate immunologic memory but do not block infection
2. Most successful vaccines induce potent antibody production
3. Most vaccines were developed empirically
4. The best vaccines involve live attenuated or whole killed virus
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1. It must provide sterilizing immunity. It will need to act against free virus and HIV-infected cells.
2. If this cannot be achieved, it must depress initial rates of HIV replication and lower the set point of infection maintaining a low viral load and therefore a state of long term non-progression. By decreasing the viremia it should lower load in blood and semen and therefore reduce transmission.
These are designed to boost the immune system of an already-infected person.
1986: Zagury et al. used fixed B lymphocytes expressing HIV proteins but there was no evidence of improved cellular immunity. They then tried a vaccine consisting of the fixed B cells, inactive interferon a, fixed PHA-activated CD8 T cells, formalin-stabilized RNA-less HIV and a mix of HIV peptides. It appeared safe. All 6 patients had higher T4 cell count. Other therapeutic vaccines have followed but no sustained clinical benefit has been demonstrated in any of them.
The major problem in development of a protective vaccine is that we know too little about the immune responses that correlate with protection. As noted elsewhere, it has been observed that a small number of people remain uninfected despite the fact that they have extensive exposure to the virus. This means that immunological protection is possible.
Initially, after exposure, there is a high level of viremia with infection of T4 helper cells and dissemination to the lymphoid system. Clearly, this initial viremia is effectively controlled and HIV-specific CD8+ cytotoxic T lymphocytes are produced. There are also HIV-binding antibodies early in infection although neutralizing antibodies only appear after viremia has been controlled. The asymptomatic phase then ensues for 8 to 10 years
Recapitulation of the problems involved in the production of an anti-HIV vaccine:
1) HIV is a retrovirus...A vaccine strain that protects against AIDS will still have LTRs and may still be oncogenic
2) HIV is a retrovirus...The use of RT and RNA polymerase II which have no proof reading leads to rapid polymorphism. Therefore a vaccine against a laboratory strain will be unlikely to affect the strains in the population.
3) The major antigen is covered by immuno-silent sugars. The crystal structure of gp120 shows the problem. The conserved epitopes that bind to the CD4 antigen are, for most of the time, hidden deep in the molecule. The variable loops and the sugar chains protect these epitopes
4) Antibodies against HIV elicited by vaccine may increase uptake by macrophages
5) Antibodies against HIV elicited by vaccine may give rise to autoimmunity.
6) We still do not know how HIV leads to CD4+ T4 cell depletion
7) We have no good animal models. Chimpanzees can be infected but do not develop AIDS. So human trials are necessary early on. Alternatively, for development of vaccine, we can use SIV and monkeys but this is not the same virus.
8) Clinical latency means we need other markers than onset of AIDS. Viral load has been shown to be a useful correlate that seems to predict clinical outcome.
9) How do we design trials? If volunteers are counseled, could this skew the results? If a false sense of security is given to vaccinees could this skew the results in the opposite direction?
10) Cell to cell transmission may make standard humoral antibodies elicited by a vaccine unimportant.
Nevertheless, the initial course of the disease shows that a good cytotoxic T-cell mediated immunological response against HIV can be mounted and the virus can be cleared from the circulation, albeit not entirely.
For a vaccine, the following questions are important:
What are correlates of protection?
How should antigens be presented?
What key antigens confer protection?
Should we even think of using a killed or
attenuated whole virus preparation?
What about mucosal immunity? Since most HIV
entry is rectal or vaginal, this is essential.
Since most people are not at risk, should we aim
at a therapeutic vaccine despite past lack of success?
Are animal models relevant?
What should a vaccine elicit? We need to kill
infected cells since virus may be spread from cell to cell. So CTL and cellular response
are very important.
What viral components should be in a vaccine? Whole virus? Should it be attenuated or dead? Originally whole viral vaccines were completely ruled out as problem
with LTRs that could cause cancer after integration. There is also the problem of failure
to inactivate formalin-killed virus completely. High-tech approaches, such as subunit
vaccines, seemed more promising (but they have failed to keep their promise)
THE STATE OF PROTECTIVE ANTI-HIV VACCINES
Nine Years and Counting: Will We Have an HIV Vaccine by 2007?
International AIDS Vaccine Initiative
Subunits (e.g. gp120) presented in a soluble form or expressed from a recombinant vector (vaccinia or vaccine strain of polio have been used)
Original vaccine attempts in chimpanzees used soluble gp120 antigen. Immunized chimps can resist subsequent intra-venous. challenge by virus of the same type from which the antigen was made.
Since 1986, there have been >15 anti-HIV vaccines based on gp160/gp120/gp41. These can protect chimps against HIV under very controlled conditions but fail to protect against clinical isolates. In the SIV/Macaque model, vaccines based on envelope proteins do not seem to protect against subsequent challenge. Note also that the immune response is usually short-lived (often a matter of weeks). So, in the case of glycoprotein-based subunit vaccines, we can say: All are safe! None are likely to work!
This has caused a complete rethink on approaches to vaccines in the last few years.
Whole virus (killed) vaccines in animals
In early studies with the SIV/Macaque model, a whole virus vaccine was produced. This seemed to give protection although later studies showed that the observed protection was due to responses to cellular proteins in the vaccine preparation and in the challenge virus rather than a specific response to the virus itself.
Initial chimpanzee studies also used whole virus preparations and showed some degree of protection. But these were designed for success. The chimps were vaccinated with a killed laboratory strain of the virus and challenged i.v. with small doses of the same strain at the peak of antibody production. No rectal or vaginal immunity was detected and protection only extended to the same strain of HIV that was used as the immunogen. Clearly, a successful vaccine will need to act against the myriad of of sub-strains of HIV that arise during a normal infection or will need to neutralize the initial infecting virions. Using a killed virus preparation, a very complex vaccine that would elicit antibodies against numerous strains of virus would be required.
The early whole virus vaccine studies were done with syncytium-inducing virus (which binds to X4 co-receptor and is found in the patient late in infection) but we now know non-syncytium-inducing, macrophage-tropic, viruses are the infectious form. Clearly, what is required is a vaccine that acts against the initial infection by non-syncytium inducing virus (that bind to the R5 co-receptor). Recently (1999), attempts have been made to do this but primary HIV isolates are a problem because they seem to be far better at concealing their neutralization sites from antibodies by burying vulnerable epitopes within the protein. Nevertheless, these critical conserved epitopes must be exposed at some time in the virus life cycle, for example when the virus binds its receptors at which time a fusion structure is exposed. Cells were fixed with formaldehyde at the point of initiation of virus-cell fusion to capture this fusion structure. To do this, monkey fibroblasts were constructed that express the surface glycoprotein of a primary HIV isolate. Human cell lines were made that express both CD4 antigen and CCR5 co-receptor. After mixing the cells so that the two types bound to each other, they were fixed to capture the fusion epitope and used to make antibodies in mice. The antibodies that the mice produced neutralized 23 of 24 primary isolates, many of which were non-syncytium-inducing.
There have been several demonstrations that attenuated whole virus can protect monkeys against subsequent challenge by SIV. In December, 1992, a live, attenuated vaccine was found to protect Macaque monkeys against SIV. All vaccinated animals were protected, all controls died. This confirms that cell-mediated immunity important. Indeed, in countering HIV infection, cell-mediated immunity is the key. This was clear this from the start as anti-gp120 antibodies rise after virus is cleared. It is the CTL response that rises quickly before the virus goes from the circulation. In the human population, there has always been the phenomenon of the long-term HIV-infected survivors. Why some people survive a long time is unclear but some people who have clearly had many contacts with the virus (e.g. prostitutes or other promiscuous persons) show no antibody response but some of these people show evidence of having mounted a very strong anti-HIV cell-mediated response.
What about an attenuated vaccine for human use? As noted above, there are many problems that have caused researchers to shy away from anti-HIV live attenuated vaccines but there does seem to be a natural attenuated from of HIV which might be a good candidate vaccine strain. Some people, who are actively infected and show no signs of AIDS, harbor HIV NEF deletion mutants. For example, there is a cohort of Australians who have an HIV strain with multiple deletions in the NEF/LTR region of the genome. All of these people got the virus from transfusions and have shown no symptoms for more than 15 years.
NEF is not required for HIV replication in vitro and, indeed, the high mutation rate leads to the closure of the reading frame in vitro. However, NEF is important for a productive infection in vivo since when a virus with a closed NEF reading frame is used to infect a chimp, the reading frame opens. This led to the idea that an attenuated HIV with a NEF deletion would be a good potential vaccine. A deletion rather than a point mutation is needed since it less likely to be reopened and therefore reversion of the attenuated virus to the wild type form should be less of a problem.
These NEF deletion mutants give a bright spot in an, at present, gloomy picture since the above observation has been followed up to produce a candidate attenuated vaccine. It has been found that NEF deletion mutants of SIV can very effectively protect monkeys against simian AIDS (SIV) without causing disease. A NEF deletion mutant against HIV has now been prepared. This is the one that made news (September, 1997) as a result of the announcement that a group of 50 volunteers wanted to participate in trials. However, there are arguments against the NEF deletion mutants as vaccine strains since they may cause lower frequency disease or a disease with a longer latency than normal virulent HIV. Also there is still the problem of reversion, even if it is small, and unfortunately this has occurred. A NEF deletion of SIV reverted to a virulent strain by a process of gene duplication of an adjacent sequence. Moreover, juvenile monkeys developed AIDS when given high dose of the vaccine. So there might be a good chance of an infection among vaccine recipients especially those who are immune compromised (cancer patients or the elderly). It is thought that the vaccine strain may continue to replicate at low levels in some compartment of the immune system.
Declining CD4 T-Cell Counts in a Person Infected with nef-Deleted HIV-1
WHAT IS CURRENTLY GOING ON WITH ANTI-HIV VACCINE DEVELOPMENT?
Current strategies
1. Live attenuated vaccines based on deletion of non-essential NEF gene in monkeys show that a vaccine can be developed against SIV. The vaccine is effective against highly pathogenic strains of SIV. Because of the problems with reversion, changes in the deletion strategy have been used to make potential vaccine strains with multiple NEF deletions. But these also caused disease in juvenile monkeys and adult monkeys with the vaccine strain are also now showing symptoms of disease. To make matters worse although the Australian cohort remained free of disease for more than 17 years (versus 7.5 years for a similar cohort with typical HIV infection), there is now evidence that they too are losing CD4+ T cells.
Declining CD4 T-Cell Counts in a Person Infected with nef-Deleted HIV-1
2. Whole killed virus vaccine is again being studied (see above). The problem remains, however, that failure in inactivation would cause serious disease. Also this is a retrovirus that has the potential to integrate into the host genome and cause disease at a much later time.
3. Recombinant subunits of the envelope and core do induce high levels of neutralizing antibody but poor cross neutralization and there are no killer T cells to attack infected cells.
4. Peptide epitopes directed against the principal neutralizing domain of HIV gp120
5. Pseudovirus may be used to make vaccines. One could insert a gene for the env protein into the genome of a harmless virus. These live vectors that can induce antibody and killer T cells.
6. DNA immunization--a safe, inexpensive, transportable. Gives both humoral and cell-mediated immunity. Naked DNA vaccines have already been effective against SIV. This is another good prospect.
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