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Recombinant vectored vaccines
If recombinant subunit vaccines are the 'next best thing' to a killed virus, then recombinant vectored vaccines have been seen as the next best thing to a live attenuated virus. The idea is simple. Take a successful vaccine which has a good safety record, and incorporate the harmless parts of HIV in it. In theory, an immune response of the kind that vaccine is known to generate, should be induced against HIV.
Pox viruses
The most advanced and popular vectors used for HIV vaccines are pox viruses. One big attraction of these is that they induce strong cytotoxic T-lymphocyte (CTL) immune responses, although some are better at doing this than others.
The very first attempts at creating vectored vaccines used vaccinia, the original smallpox vaccine. Animal studies have found the vaccinia vector virus produced a strong CD8 T-cell response to HIV Gag and Pol proteins. However, after the early death of one immunocompromised trial volunteer from vaccinia infection, most researchers are now working on pox viruses that are unable to reproduce fully in mammalian cells.
These include two viruses native to birds, fowlpox and canarypox, and also bird-adapted strains of vaccinia itself, such as NYVAC and modified vaccinia ankara (MVA). Canarypox is already the basis of a commercial rabies vaccine and MVA was given safely to tens of thousands of people in the 1970s.
While MVA has long been studied in animals, its first clinical use as the basis for an HIV vaccine is as a booster in the International AIDS Vaccine Initiative (IAVI)'s most advanced HIV vaccine project. The ‘HIV-A’ vaccine has been designed by Dr Tomas Hanke in Oxford, based on virological and immunological studies of HIV in Kenya. The idea is to use a DNA vaccine (see DNA vaccines and replicons ) followed by a recombinant MVA vaccine. This combination has been shown in mice, macaques, and human volunteers (in an earlier trial of a malaria vaccine) to be a powerful way of inducing cellular immune responses to target proteins. An Australian team using a parallel system, DNA followed by recombinant fowlpox, has been able to show protection against HIV (grown in monkey cell cultures) in pigtail macaques. Furthermore, strong HIV-specific CD8 and CD4 T-cell responses occurred (Kent 1998).
The first two stages of the Oxford/Nairobi trials recruited low-risk volunteers in Oxford, London, Nairobi and in Uganda to begin testing the DNA and MVA components of the vaccine separately and in combination.
Unfortunately, initial results from the Oxford/Nairobi (Betts 2004; Guimaraes-Walker 2004; Jacko 2004; McMichael 2004) and Australian (Kelleher 2004; Kent 2004) approaches taken into phase I clinical trials in low-risk HIV-negative volunteers have been no better than those seen with the canarypox vaccines described in the next section. IAVI has already said that unless substantially better results emerge from their trials, they will not take DNA/MVA forwards into full-scale testing but will focus their efforts on other vaccine candidates. These still include other MVA-based vaccines for subtype C that are being developed at the Aaron Diamond AIDS Research Center in New York and in a collaboration between the United States company Therion and Indian researchers.
Both NYVAC and MVA are being developed as vectors for subtype B and C gene sequences identified by researchers working with the EuroVac project, and have been entered into clinical trials in London and Switzerland.
ALVAC trials
Aventis Pasteur, previously known as Pasteur-Mérieux Connaught, has been working closely with the United States National Institutes of Health (NIH) to test their ALVAC series of canarypox-based vaccines. One of these vaccines is currently being tested in a controversial full-scale Phase III efficacy trial in Thailand. Another ALVAC product is due to enter a phase II trial as a primer, followed by lipopeptide boosting, as a collaboration between the French ANRS and the HIV Vaccine Trials Network.
The first ALVAC product, vCP125, consisted of gp160 inserted in the canarypox vector. A second product, vCP205, contained more HIV genes (env, gag and protease). Early studies using low doses of vCP205 showed that 25 to 50% of participants had new cell-mediated responses to HIV. Higher doses of the vaccines appear to generate greater cell-mediated immune responses. vCP300 contains additional HIV genes (including Nef), although trials did not show it to be better than vCP205 in inducing immune responses.
The most extensively tested candidate in the western hemisphere, ALVAC-HIV vCP1452, incorporates env, gag and pol genes from subtype B HIV, plus a series of very short sequences from the nef and pol genes which are known to give rise to cellular immune responses in people living with HIV.
Early phase I studies of ALVAC vaccines were conducted in Thailand and Uganda. These studies were to examine whether cell-mediated immune responses generated by subtype B vaccines would be effective against non-subtype B HIV virus and also to explore the feasibility of evaluating this kind of vaccine in those countries.
There is a widespread view that for optimal protection, it is desirable to have both antibodies and cellular immune responses to HIV. Given that subunit vaccines appeared better at inducing antibodies, and vectored vaccines better at inducing cellular immune responses, why not mix the two in the hope of getting both responses?
A United States trial known as AVEG 202/HIVNET 014 was set up to evaluate the immune response in individuals given the ALVAC vCP205 combined with a gp120 envelope protein. Early results showed that 50% of participants who received only vCP205 developed neutralising antibodies, compared to 90% of those who received the combination. One third of both these groups developed a detectable CTL response. Unfortunately, it was reported in October 1999 that 11 of 435 study participants had seroconverted - six on the placebo, three on vCP205 alone and two on the combination. Further research has found that vCP205, vCP1433 and vCP1452 plus a gp160 hybrid booster failed to stimulate neutralising antibodies (Bures 2000).
The cellular immune responses seen with canarypox constructs appear to be weak, transient and only seen in a minority of trial volunteers. For this reason, the NIH concluded that it was not worth taking vCP1452 (boosted by AIDSVAX B/B gp120) forwards into a full-scale trial through the HIV Vaccine Trials Network. In essence, to test the theory that a strong cellular immune response is protective against HIV, you need to be able to get a strong cellular immune response. If all that is seen is a weak response, then the absence of protection is not going to prove anything.
Despite this, Thai researchers have decided to press on with a large scale clinical trial of vCP1521 boosted by AIDSVAX B/E, in Chon Buri and Rayong provinces. They are seeking to recruit up to 16,000 volunteers through healthcare centres. vCP1521 includes env sequences from Thai subtype E virus and gag and pol sequences from subtype B. This trial is sponsored jointly by the Royal Thai Army and the former United States military HIV research programme (transferred during the course of the project to the civilian NIH) and the Thai Ministry of Health.
Adenovirus
Adenovirus is a relatively harmless and common human virus associated with cold-like illnesses, which has been widely studied and is being developed for gene therapy as well as for use in vaccines.
Two major American companies have developed HIV vaccines based on adenovirus. In 1997, researchers from Wyeth-Lederle, part of American Home Products, reported durable protection against HIV in chimpanzees innoculated with a recombinant adenovirus followed by an injected recombinant subunit booster. However, it appears that they have no plans to take this further.
More recently, Merck & Co. has focused its vaccine development programme on a prime-boost strategy in which a DNA vaccine is followed by an adenovirus booster. Relatively large animal studies have shown that such a combination can protect monkeys against a pathogenic HIV-related virus. However, in early stage clinical trials Merck concluded that the DNA vaccine they were using did not add to the response they saw from the adenovirus.
Public sector agencies working with adenovirus systems include the NIH, where the Vaccine Research Center is incorporating HIV gene sequences from viruses of several subtypes into adenovirus vectors and has taken these into early stage clinical trials.
One problem is that the adenovirus strain initially chosen by Merck, known as adenovirus 5 (Ad5), is very widely distributed across the world, so that a substantial proportion of the population are naturally exposed to it. Worse, it seems that levels of natural immunity are higher in countries such as South Africa and Thailand than they are in the United States and Western Europe. This causes problems in using it as an HIV vaccine, if all the vaccine does is re-awaken natural immunity to Ad5 (Isaacs 2004).
Current plans are for the HIV Vaccine Trials Network to set up an intermediate-scale trial, somewhere between phase II and phase III, to recruit volunteers at high risk of HIV in communities where subtype B predominates (i.e., North and South America and the Caribbean). Volunteers will be screened for pre-existing immunity to Ad5 and only those without such immunity will enter the trial. The trial might be under-powered to show moderate efficacy, but could demonstrate efficacy if this is higher than expected. If the vaccine alters the course of HIV infection – for example, by lowering the viral load and so delaying the need for treatment – this trial could also give preliminary evidence to that effect.
Crucell, a Dutch biotechnology company, is now working both with Merck (on subtype-B HIV) and with IAVI (on non-subtype-B) to incorporate HIV gene sequences in newly developed vaccine strains of Ad5s 11 and 35, with a view to getting around the problem of pre-existing immunity to Ad5 (Barouch 2004).
Other vectors
Other viral vectors currently being studied with HIV or simian immunodeficiency virus (SIV) in animals include rabies, measles, poliovirus, herpes simplex, human rhinovirus, influenza and pertussis.
Measles is of particular interest because the live attenuated measles vaccine in common use is extremely effective in generating long-lasting immune responses when given to infants, which can be boosted in adolescents. This might be ideal to protect young people in countries where HIV is widespread (Lorin 2004; Tangy 2004; Zuniga 2004).
However, before going down this path, it is necessary to be sure that the HIV-related component of these vaccines will still be relevant ten or 20 years after the vaccine is first given. Another vital consideration is to avoid undermining the acceptability of the existing vaccine and the protection it provides against measles, a disease which continues to kill many children in developing countries
Poliovirus vaccine strains have been tested experimentally as a basis for HIV vaccines. In monkeys, preliminary experiments suggested they might be particularly effective (Andino 2004).
Here, there is a problem arising from the prospect that poliovirus itself may become extinct in the near future. If this happens, then immunisation with any live strain of poliovirus which might conceivably revert to a 'wild type' virus that causes illness will become unacceptable, even though the risk of this happening may be less than one in a million. Already, some of the last countries to use live polio vaccines for mass immunisation – such as the United Kingdom – are abandoning them in favour of less effective, but safer, inactivated viruses.
The recombinant rabies virus vaccine potentially has a number of advantages:
- Few people are vaccinated against rabies.
- The attenuated rabies virus infects most human cells but does no damage.
- It will potentially produce ongoing exposure to HIV antigens in the body.
HIV proteins can be expressed by a recombinant rabies vaccine and there is in vitro evidence that part of the rabies virus may intensify the immune response to HIV.
American researchers have unveiled their research into an HIV vector vaccine using the live rabies virus vaccine (Schnell 2000). Research in mice found that the vaccine produced HIV-specific neutralising antibodies and cytotoxic T-cells which targeted HIV-infected cells expressing gp160 but not cells expressing HIV gag. A safety study in ten chimpanzees found none developed rabies despite rabies seroconversion.
A comprehensive review of live vectors as potential HIV vaccines, including bacteria (see DNA vaccines and replicons ) and a yeast, was published in IAVI Report early in 1999. A fundamental problem with using complex vectors such as bacteria is that these express very large numbers of different potentially immunogenic proteins, so the HIV component will only ever be a small part of what is exposed to the immune system.