Most efforts to find a vaccine against malaria have tried to spur the immune system to make antibodies that seek out the parasite and mark it for destruction. But that's a daunting challenge. Now, a small study finds early promise in an alternative approach: a vaccine that triggers an immunological response mediated by immune cells without the production of antibodies.
The idea behind an antibody vaccine is that injections of proteins from the malaria parasite will urge the immune system to make antibodies. If vaccinated people become infected, these existing antibodies can leap into action and attack the parasite. The problem is the parasite can sport at least 5000 proteins, and many of these come in multiple variations. Worse, the parasite displays different proteins on its surface during the stages of its life, so a vaccine may not work at all times.
A team led by molecular immunologist Michael Good of the Queensland Institute of Medical Research in Brisbane, Australia, decided to try an entirely different approach: stimulating the production of immune cells called T lymphocytes. Once exposed to the parasite, these cells can recognize and kill infected red blood cells. The researchers injected five volunteers with red cells taken from people previously infected with an easily treatable strain of the malaria parasite Plasmodium falciparum. Participants were given antimalarial medicines after each treatment.
Although the volunteers did not develop symptoms of malaria, they were clearly fighting the infection. Levels of parasite DNA in their blood jumped about a week after the first infection, as did the concentration of T cells. But by the fourth inoculation, parasite DNA was no longer detectable, indicating that the parasite's growth had been halted, the team reports in the 24 August issue of The Lancet. Additional experiments confirmed that the parasite was stopped by T cells, not by antibodies.
"These findings open new roads for understanding immunity to Plasmodium falciparum and developing new strategies for malaria vaccines," says malaria expert Dror Baruch of the Laboratory of Parasitic Diseases at the National Institute of Allergy and Infectious Diseases in Bethesda, Maryland.