The scheme is the brainchild of Professor Andrew McMichael, head of the Human Immunology Unit at Oxford's Institute of Molecular Medicine. The unit is funded by the Medical Research Council, and has now won a prestigious pounds 2m grant from the US-based International Aids Vaccine Initiative. The money is intended to fund a new approach to finding a vaccine to limit HIV, which is now infecting 16,000 people a day, worldwide.
The grant - which will be matched by MRC input to the unit - is seen as a coup for British research in an area where the US predominates. But it was the medical condition of a group of Kenyan prostitutes - and the collaboration of Kenyan scientists - that not only convinced Professor McMichael that his vaccination system might work, but won him the money to find out for sure.
Almost 20 years of attempts to find a vaccine against HIV have so far drawn a blank, chiefly because of its stunning ability to change shape. In the end, it is this ability that not only outflanks the immune system but destroys it. A key reason for this, Professor McMichael believes, is the slow arrival of a section of immune system troops called CTLs, cytotoxic or "killer" T-cells - an immune response that the virus has evolved strategies to avoid.
When HIV invades a cell, it takes about 24 hours before it can persuade the cell to turn rogue and start churning out new virus particles. But it takes only three to four hours for the virus proteins which alert the immune system to the presence of an invader to be displayed on the cell's surface.
"The infected cell is making virus proteins, but the cell machinery fights back by degrading those proteins as fast as it can and the degraded proteins appear as fragments on the cell surface," explains Professor McMichael. "At that point the killer T-cells come in and destroy the cell. But if the response is slow or weak, virus particles are released."
The reason why T-cell response is slow is that the virus is unfamiliar to the immune system and the T-cells take time to recognise it as a threat.
"In people who have never encountered HIV, only one in a million of their lymphocytes may be capable of recognising the virus," explains Professor McMichael. "Our aim is to boost the num- ber of CTLs that recognise HIV up to, say, one in thousand or higher by means of a vaccine. To get to this level naturally would take 10 cell divisions - and that could take five days. We are hoping to prime these killer T-cells so that as soon as the virus infects the cells, they can start eliminating them before they release the next batch of virus."
One virus-infected cell produces hundreds of thousands of virus particles. So if this window of opportunity is missed, the immune system has a huge problem on its hands. And it's not just a numbers game: the virus's phenomenal ability to mutate - it can undergo as much genetic change in a decade as humans experience in millions of years - means every time the killer T-cells home in on one form, several other mutations escape to multiply. Remarkably, says Professor McMichael, our immune systems manage to keep pace with this for a while, which explains why many people infected with HIV remain well for years. In the end, it is the continuous emergence of viral variants that undermines the body's ability to cope; the immune system collapses and the complications of Aids ensue.
The Oxford plan is to artificially weight things so heavily in favour of the immune response that the virus never really gets established, and is cleared up before it does any damage.
Traditionally, vaccines have tried to stimulate antibodies - the immune system's other chief shock troops - to do this, but 10 years of trying this with HIV have proved disappointing. "It seems that the protein coating on the outside of the virus has evolved in such a way that it's almost impossible to neutralise it, or if a vaccine does, the virus can change and escape," says Professor McMichael.
So what's to say that boosting CTLs will have more success? There are two tantalising threads of evidence. The first is that studies on animals have shown that it is possible to multiply killer T-cells in the way required. Indeed, in next month's Journal of Virology, researchers led by Professor Ian Ramshaw from the Australian National University report that vaccinating pigtailed macaque monkeys with a combination of vaccine techniques similar to those Professor McMichael intends to use not only stimulates CTLs, but also protects the animals when they are exposed to HIV.
But even more interesting is information gleaned from the experiences of a group of Kenyan prostitutes. Sarah Rowland-Jones, a scientist in Professor McMichael's team, studied two groups of prostitutes in Kenya and the Gambia where infection rates can be as high as 35 per cent per year. "We were intrigued to discover that about 5 per cent of the women had sexually transmitted diseases just like the other prostitutes, but they were not infected with HIV. They weren't making antibodies, but we found that more than half of them were making strong CTL responses."
Somehow, says Professor McMichael, these women clear HIV before it gets going on its spiral of destruction. "They've either been vaccinated by their exposure to the virus without being infected, or they've been infected but cleared the virus, and this immune response is now protecting them from reinfection," he says.
Other possible explanations (meticulous use of condoms or genetic resistance) are unlikely, as the 5 per cent still get STDs at the same rate as the other women, and because it is easy to infect the women's cells with HIV in the lab which shows they are not genetically resistant to the virus.
"My guess is that it's a series of lucky events. Maybe their first exposure to the virus was at very low levels, but just enough to get the immune system started and primed, and because they're being exposed often but at very low levels they've built up this immune response which always keeps ahead of the infection. Which is exactly what we're trying to do with the vaccine."
What Professor McMichael and crew will do in their pounds 4m trial is to stimulate this protection with a new vaccine combination which appears to produce the CTL response, and to test its protective power, first on healthy British and Kenyan volunteers and then on the prostitutes' less fortunate colleagues, the 95 per cent likely to be infected.
"The reason we were chosen for this grant is that we have ways of stimulating this type of immune response efficiently. There's been a lot of interest in the approach but it's always been a problem to get reliable, high-level induction of the CTL response."
The best way of getting a good, fast burst of CTL response is to infect someone with a live virus. But although HIV could possibly be weakened to produce an attenuated live vaccine in the same way as tuberculosis and measles have been, Professor McMichael believes it's just too dangerous to try. The traditional way of making a vaccine - immunising with bits of viral proteins - is also no good, as the proteins don't infiltrate cells well enough to stimulate CTL production.
But two other possible methods are the focus of much intense research. The first is a series of vaccines that use a disarmed virus (such as the vaccinia or fowl pox created to counter small pox) to carry an HIV gene into a cell, so that it can be expressed harmlessly on the surface of the cell and therefore prime CTL recognition. The second is vaccines made from pure viral DNA.
But there are still problems: with the first method, the target gene is bundled in with, say, one in a hundred vaccinia proteins, says Professor McMichael. "The immune system doesn't know you want it to respond to that particular gene - all it's trying to do is get rid of vaccinia - and in practice it's proved unreliable for stimulating the immune response - sometimes it works well, but in monkeys and humans it's been disappointing."
A similar thing seems to happen when you immunise just with HIV DNA. Although some researchers have found bits of the HIV genome that do stimulate CTLs well, on its own it isn't enough. But if you use the two together in the right order ...
"We were working with a colleague who's trying to get a CTL-based vaccine for malaria, and we found that if we primed mice with DNA, then boosted them with modified vaccinia Ankara virus (MVA) which has the same bit of HIV DNA inserted, we got killer T-cell responses 10 times higher than either DNA on its own, MVA on its own, or any other combination. It was all rather serendipitous. We were trying different combinations, and we're not sure exactly why it works like this, but my guess is that the DNA primes and focusses the immune response on this particular gene and the MVA comes in and makes more of it."
This discovery, made two years ago, laid the groundwork for the trial Professor McMichael now plans. Their confidence has been spurred by the Australian findings, which show that macaque monkeys immunised with a similar combination will clear all traces of the virus from their bodies within one to two weeks. The Australians plan to start testing their vaccine early next year on people already infected with HIV, to see whether it will work as a treatment. Professor McMichael and co, however, have their sights firmly set on developing the combination as a prevention.
"A prophylactic vaccine is desperately needed in Africa, India and Asia. It's not so much needed here because we have effective treatments, but over there it's a huge crisis and since they can't afford the drugs, their only real hope is to get a vaccine."
Much of the vaccine work going on around the world is based on the B- strain of HIV, which is prevalent in North America and Europe but only accounts for around 10 per cent of cases worldwide. The C-strain, prevalent in India, the fastest growing epidemic area, accounts for around 50 per cent of cases. But it is from the A-strain, which affects about 25 per cent of the world and is the most common strain in Africa, that the Oxford team will make their candidate vaccine.
"If our A-strain vaccine works, it will be relatively simple to make other strain versions. What we don't want to do is go to Kenya, test the vaccine, say thank you very much, then come back here and make a vaccine for people here and ignore their huge need," says McMichael.
This focus and the collaborations they've forged with Kenyan scientists and health authorities are key reasons for the success of their grant application to the International Aids Vaccine Initiative.
"The IAVI is taking a very strong stance. They want vaccines that are not just tested in developing countries - Kenya in this case - but that are available at a price these countries can afford."
The cost of developing a vaccine right through to availability in health centres is huge - anything between pounds 20m and pounds 100m - and IAVI is also trying to establish a purchase fund through the World Bank and G8 countries that will ensure that these vaccine can be bought by the countries that need them. This is primarily to encourage the pharmaceuticals industry to invest in the current research.
As yet, Professor McMichael's team is the only group set to test the combination of a DNA primer vaccine and a modified virus booster to prevent HIV infection on humans. The IAVI grant, plus the MRC's input, will cover the first round of trials, kicking off with Phase One, a trial in Oxford on about 40 low-risk volunteers in late 1999. This will be followed closely by a similar trial in Nairobi. These studies will investigate the extent of the CTL response that this combination provokes in humans, as well as any side-effects, although the latter are unlikely, says Professor McMichael: "We know the combination is safe in monkeys and in mice. We know that MVA has been tested in humans before, although not with an inserted gene, and we know that other DNA vaccines have been used in humans without problems. But there is always the possibility of an unexpected reaction of some sort.
"One reason for doing the trial in Oxford first is that we can assure the Kenyans that we are not just practising on them. We will also be training people from Kenya to work on this kind of trial, which they haven't done before. A lot of the costs in our proposal concerns setting up the laboratory infrastructure in Nairobi."
If things go well and funding continues, the next step will be a Phase Two trial targeting those at high risk, like the Nairobi prostitutes who have been so important in pointing the way.
"The basic model will be to set up a clinic that treats STDs and gives advice and condoms and counselling. With support from social workers and nursing staff, these clinics can be very popular because they help the women a lot, giving rapid free treatment and advice. That alone can reduce the transmission of HIV by half, from 30 per cent to 15 per cent, but that's still very high. In this population, we could get answers on efficacy from a study of just 200 women."
And if Phase Two looks promising, the next and final stage will be a trial of thousands of people at normal risk, possibly in rural Kenya.
But that, says Professor McMichael, is for the future. "I feel as if we're about to set off for the South Pole, and we may be gone some time, so don't hold your breath. This will take five to 10 years."
An excruciatingly long time, but a vital journey if the developing world is to have any chance at all of breaking free of HIV.Reuse content