Author: Yejin Kang
Institution: Rice University
An international network of scientists from Singapore, The Netherlands, and France has developed a unique method of immunization that can provide key information on how to prevent Plasmodium falciparum from infecting humans. This parasite is the cause of malaria, a disease that kills an average of one million people per year. The results were recently published in the July 30 issue of The New England Journal of Medicine.
Conventional immunization methods use weakened forms of the infectious organism to help induce protection. However, a successful vaccine for malaria has not been developed because of the challenges posed by the different forms that the parasite takes in its complex life cycle. The parasite invades the host through a mosquito's bite. Once inside the host, the parasites travel through the bloodstream to the liver. Then, they rapidly multiply, causing the liver cells to burst. Now the parasites can enter the bloodstream and infect red blood cells.
In order to study the immune response to malaria, scientists departed from the conventional technique of immunization and thought of using full, whole parasites to induce an immune response. Two groups participating in a clinical trial received separate treatments at Radboud University Nijmegen Medical Centre in the Netherlands. Over a period of three months, one group received monthly exposure to mosquitoes infected by P. falciparum while the other group received exposure to uninfected mosquitoes. All of the participants in the study received chloroquine, a drug that inhibits the malaria parasite from multiplying in the blood. After 8 weeks, which was sufficient for the effects of chloroquine to decline, everyone received exposure to infected mosquitoes.
Surprisingly, no parasites were found in the blood of those in the group that were exposed to the infected mosquitoes over three months. Therefore, the new method of utilizing whole and intact parasites, instead of weakened parasites, helped condition the immune system to an earlier stage of its life cycle. Pluripotent effector memory T cells, which can effectively coordinate the elimination of pathogens from the body, were found in the blood of those who had developed resistance to the parasite.
While using infected mosquitoes as vaccines is not feasible in the real world, the success of this study provides hope for alternative approaches to vaccine development. The study will also help scientists discover more information about anti-malarial immunity. Dr. Laurent Renia, principal investigator of the Laboratory of Malaria Immunobiology at the Singapore Immunology Network, reports that, "this method of immunization could be applied successfully to similar investigations to find biological markers which would indicate the extent of protection against malaria. It would thus advance the currently limited knowledge of what constitutes protective anti-malaria immunity in humans."