BioWorld International Correspondent
LONDON - A live, genetically modified malaria parasite can induce immunity that protects against malaria infection in a laboratory model of the disease. All the mice given the altered parasites were protected against subsequent infection with malaria, which lasted for at least a month.
That approach has the potential to be developed into a live, attenuated whole-organism vaccine to protect against malaria in humans. At the very least, the work will provide information about what sort of immune response is capable of protecting humans against malaria, which might help in the development of a sub-unit vaccine against the disease.
Chris Janse, research scientist at Leiden University Medical Center in the Netherlands, told BioWorld International: "Our study suggests that genetic modification offers real possibilities to create attenuated whole organisms as vaccines. But we need to know if we can get 100 percent protective immunity all the time and how long it can last. Many extra studies will need to be carried out in humans in order to find out if this approach has a future."
Janse, working with Andrew Waters and colleagues in Leiden, and with collaborators in Portugal, Germany and elsewhere in the Netherlands, reported the advance in the Aug. 8, 2005, issue of Proceedings of the National Academy of Sciences. The title of the paper is "Genetically attenuated, P36p-deficient malarial sporozoites induce protective immunity and apoptosis of infected liver cells."
Malaria is caused by the bite of a mosquito infected by parasites of the genus Plasmodium. The parasites, which are called sporozoites at that stage of the infection, migrate to the liver and are able to infect only hepatocytes. Within seven days, each sporozoite that infects a hepatocyte has divided to form 30,000 new parasites, now called merozoites. Those are capable of infecting only red blood cells. Each merozoite that infects a red blood cell again divides, this time taking only 48 hours, into 15 to 20 new merozoites. It is the synchronous release of new generations of parasites from the red blood cells that causes the anemia and intermittent fever typical of malaria.
Malaria kills several million people every year in sub-Saharan Africa, many of them children. Much research has been focussed on identifying protein(s) of the parasite that, when given as a sub-unit vaccine, would stimulate a protective immune response. Ideally, such a vaccine would stimulate an immune response that would target the sporozoites in blood in the short period after infection, or while they develop in the hepatocytes, so completely preventing the signs of infection.
Sub-unit vaccines have the appeal of being easy to manufacture and purify. Yet progress on that front has been slow, so researchers recently have reinvestigated the use of whole attenuated Plasmodium. That vaccine strategy first was tested in 1967, when researchers observed that, in a mouse model of malaria, giving irradiated malarial parasites could bring about protective immunity to the disease.
Despite that promising observation, irradiated sporozoites are unlikely to form the basis for a vaccine for humans - irradiate them too much and they die, failing to stimulate the desired immune response; irradiate them too little, and they can infect the recipient.
Janse and his colleagues set out to generate a genetically modified sporozoite that could invade the hepatocytes but would develop no further. In their previous work, they had identified a gene encoding a protein called P36p, which is expressed during the development of the sporozoites.
Janse said: "We knocked out the gene for P36p, and we observed that the sporozoites produced were viable, but that their development came to a halt in the hepatocytes."
Further investigations using a mouse model of malaria showed that giving these genetically attenuated sporozoites to mice induced protective immunity in all the animals for at least one month.
Encouragingly, P36p also is present in Plasmodium falciparum, which infects humans. The team already has shown that it is possible to knock out the P36p gene in P. falciparum, and that the genetically altered organism can invade human liver cells in culture but, as in the mouse model, develops no further.
"We are now planning to do studies in human volunteers, but we know it is not easy to get permission to infect humans with transgenic parasites," Janse said. "We also want to investigate how this parasite induces a protective immune response, and how it compares to the immune response generated by radiation-attenuated sporozoites."