"This is the first malaria vaccine clinical trial to clearly demonstrate antiparasitic activity by vaccine-induced antibodies," writes Pierre Druilhe of the Pasteur Institute in Paris, who led the study. Malaria--a parasite carried by certain mosquitoes--sickens more than 300 million people worldwide every year and causes at least one million deaths, primarily of young children, according to the World Health Organization. Vaccine development has been hindered by the microscopic parasites adaptability and complexity.
Druilhe and his colleagues focused on a malaria protein--merozoite surface protein 3 (MSP-3)--that prior research had identified as the focus of the immune systems of adults who had proven resistant to the disease. When paired with monocytes (a type of white blood cell), antibodies to MSP-3had cleared a blood sample of the parasite.
In the new work, described in a report published online yesterday by the Public Library of Science,the team injected an MSP-3-based vaccine into 30 European volunteers who had never had malaria, readministering it after one month and again after four months. Blood samples were taken one month after each injection. These blood samples were then compared to French blood samples from individuals with no immunity to malaria and African blood samples from people with immunity.
Nearly every vaccinated sample produced an immune response to malaria when it was introduced in vitro and 77 percent produced anti-MSP-3 antibodies. Plus, these antibodies proved to be as good at killing the parasite as those from immune adults and, in some cases, better, destroying up to twice as much. "This type of immune response, characteristic of immune adults living in malaria-endemic regions, requires under natural conditions 10 to 15 years of daily exposure to billions of infected red blood cells," Druilhe notes.
Due to ethical concerns, the volunteers themselves were not exposed to malaria to test the vaccine's efficacy. But the scientists did conduct in vivo experiments in mice, infecting them with the parasite and then injecting a small amount of the vaccine-produced antibodies. The antibodies significantly cut the number of parasites in the blood of these animals and in some cases wiped it out entirely--outperforming even the immune systems of naturally resistant humans.
The vaccine also showed long-lasting promise, with blood from some of the immunized individuals showing strong resistance a year later. Thus encouraged, Druilhe and his fellow scientists have begun a study of the vaccine's ability to fight the parasite in a larger group of people already suffering from malaria.