Science: Holy Grail of the tropics
Malaria has confounded scientists for decades, but now there may be a vaccine that works.
Friday 14 May 1999
The small, white-walled room is bare save for a sink and a couple of laboratory benches. On one bench is an array of what look like washing- up bowls, each containing a few inches of water. Inside these, swarms of brown larvae dart frantically, or cluster around pellets of dog meal. On the other bench are a pair of cages draped in white mesh. From these comes a familiar, slightly irritating high-pitched whine.
The stuffy, humid room is a mosquito hatchery. It is part of a facility - unique in this country - capable of growing cultures of Plasmodium falciparum - the most important of the four species of the Plasmodium parasite responsible for one of the world's most insidious and devastating diseases, one that kills about three million people each year, mostly children. Professor Hollingdale is on the trail of the Holy Grail of tropical medicine - the development of a malaria vaccine.
The problem of malaria is worsening, and many experts believe that the only realistic solution is the development of a vaccine. But more than two decades of apparent breakthroughs followed by disappointing setbacks have shown that the challenge is far from straightforward.
Part of the difficulty undoubtedly lies in the complex life-cycle of the parasite. P. falciparum is a single-celled organism transmitted by the female mosquito. The form of the organism when it is inside the mosquito is called the sporozoite. When an infected mosquito bites someone, it injects around 100 sporozoites into their blood.
These travel in the bloodstream until they reach the liver. Here, a special protein on the surface of the sporozoite, termed CSP, recognises that it has arrived at the liver and anchors itself to the outside of the cell. The sporozoite then crosses into the liver. Once inside the liver cell, the organism changes shape, from being long and thin to being bloated and spherical. This is termed the schizont stage, and within the schizont thousands of nuclei begin to develop. Eventually maybe 20,000 daughter parasites will form.
These are called merezoites and their sheer volume causes the liver cell to burst open, releasing them into the bloodstream. When a merezoite meets a circulating red blood cell it attaches itself to the surface and enters. Once inside it divides into 15 or 20 daughters in 48 hours. These rupture the blood cell and are released to infect new cells. It is at this stage that the symptoms of malaria appear.
Vaccines work by giving the body's immune system a "preview" of a harmful pathogen - or the relevant bit of it. This allows us to manufacture the appropriate defence hardware rapidly in the event of an invasion by the real thing.
In the early 1970s it was shown that irradiated sporozoites, created by irradiating infected mosquitoes, could confer protection. This, however, is not a feasible way of delivering mass vaccination.
"The experiments with the irradiated sporozoites told us that it is possible to develop a vaccine," says Professor Brian Greenwood, of the London School of Hygiene and Tropical Medicine, who has been involved with vaccine trials in Africa. "The problem is how to devise a simple and practical way of doing it."
In the late Eighties, a putative vaccine was developed by a Colombian scientist, Manual Patarroyo. This consisted of fragments of proteins found on the surface of the parasite artificially stitched together. Early trials suggested that the vaccine produced a high degree of protection, but later studies have cast serious doubt on its effectiveness.
A series of vaccines have been developed based on the CSP. These have given varying degrees of, usually limited, protection. A field trial in The Gambia has just been completed on the latest version of a CSP vaccine, and the results are being analysed. Altogether, says Professor Greenwood, there are about 20 candidate vaccines in development.
The Leeds laboratory is one of the few places in the world able to produce the parasite under conditions which effectively mimic what goes on in sub-Saharan Africa. The parasite is first grown in blood cultures. The infected cultures are then covered by a latex membrane to simulate human skin (condoms were used in early tests). Mosquitoes are allowed to settle on the membrane and take up the parasite. These are then removed from the mosquito's body and transferred to the cultured liver cells.
Under these controlled conditions, scientists can arrest the parasite's life-cycle at any point to scrutinise the biochemical interactions taking place - which genes are active at a given time, or which proteins are essential to a particular series of events. Professor Hollingdale's team is especially interested in a protein called LSA-1. This is a large protein secreted by the parasite as it sits in the liver cells. The protein is an antigen - it elicits an immune response - in other words it stimulates the immune defences to attack it, which makes it a good candidate for a vaccine.
LSA-1 appears to activate one particular aspect of the immune defence, T-cells. These are specialised cells that are produced by the body to seek out and destroy any of the body's cells which are infected by foreign invaders. Professor Hollingdale's team has been working with adult volunteers in Papua New Guinea. By taking samples of blood and adding LSA-1 or specific fragments of the protein, they wanted to see if T-cells were produced by the blood. "Some of the volunteers had previously built up a natural protection to malaria while others had not. The group who were protected had T-cells to LSA-1 in their blood, while the group who were not protected didn't have any. This had never previously been shown and to us strongly implicated that T-cells were important in conferring protection.
"We believe that from these studies and others that there is a strong rationale for producing a vaccine based on LSA-1," says Professor Hollingdale.
The team is looking at a variety of methods to deliver the antigen as a vaccine, and is looking for funding to begin human trials within two years. The European Union has recently earmarked a budget for malaria research and Professor Hollingdale is hoping to help set up a European network to develop a concerted research effort into the development of a vaccine.
"Europe has generated a lot of the candidates for vaccines but has not really had the capital or unity to take these into the complex areas of clinical testing," says Dr David Arnot of Edinburgh University, who has been studying the molecular biology of the malaria parasite for 20 years. "Now we have the opportunity to work in close harness, which I think is an excellent thing. I am reasonably optimistic that within 10 years of good support, and with all the groups pulling hard in a concerted effort, we have a good chance of getting something to work."
Professor Greenwood agrees on the value of a united European approach. "We have been strong on the laboratory side but not had a great presence in testing potential vaccines. Hopefully this EU initiative will go some way to bridging that gap."
As for the prospects for a vaccine, Professor Greenwood remains cautious but optimistic. "For the past 20 years people have been saying `we'll have a vaccine within five years' - and they are still saying it. My own feeling is that we are unlikely to see a dramatic breakthrough, but rather an accumulation of gradual improvements. I think it is right that we follow the diverse approaches that we are pursuing now, and hope that out of this someone will hit upon something that works."
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