Researchers from the Johns Hopkins Bloomberg School of Public Health have located a new - and likely more promising, they say - target for a potential vaccine against malaria, a mosquito-borne illness that kills as many as 750,000 people each year.
The findings, published June 15 in the journal Nature Structural & Molecular Biology, detail how the researchers created a 3-D crystal structure of the protein believed central to the transmission of the malaria parasite through mosquitoes. In looking anew at the AnAPN1 protein, an enzyme in the gut of the Anopheles mosquito, they determined that previous incarnations of a proposed vaccine included irrelevant regions of the protein - something they say explains why a vaccine that looked promising in mice didn't provide the optimum protection.
Half the world's population is at risk for malaria, but there is no commercially available vaccine. The vast majority of those who succumb to the parasite - as many as 90 percent - are children under age five, most of them in sub-Saharan Africa.
The researchers had originally looked at fragments of the AnAPN1 protein in order to understand its role in parasite transmission.
"If the protein is like a tree, our original examination is similar to cutting a tree into planks - you don't really know what the tree looked like when all you have are planks," says the study's leader Rhoel R. Dinglasan, PhD, MPH, an assistant professor in the Bloomberg School's Department of Molecular Microbiology and Immunology and member of the Johns Hopkins Malaria Research Institute. "Here we have for the first time a chance to look at the whole tree and have discovered a part we never appreciated before. The missing link had been understanding what was the most important part of the protein to target to prevent transmission of the malaria parasite. Now we believe we have found our new target."
Malaria is spread to humans by the bite of a mosquito infected with the Plasmodium parasite. The goal of the type of vaccine being developed by Dinglasan is to interrupt the transmission of malaria by destroying the parasite's ability to move on to infect the next person. A mosquito becomes infected by the parasite when the parasite is able to invade the mosquito's gut before it can be digested. This process can be blocked by obscuring the newly discovered portion of AnAPN1, "preventing the parasite from essentially grasping the handle to open the door to the gut," Dinglasan says.
Dinglasan and his team, including Natalie A. Borg, of Monash University in Melbourne, Australia, mined the crystal structure of AnAPN1 to generate a map of each part of the protein.
Using this information, they were able to identify the binding site of a particularly potent antibody to malaria as the new region of AnAPN1. They tested these antibodies using blood samples taken from children carrying the malaria parasite in the nation of Cameroon, one of the countries greatly impacted by malaria. They found that very small amounts of the antibody completely prevented transmission of the parasite to the mosquito across all of their samples.
While this brings the team one step closer to a clinical trial, Dinglasan cautions that there is more work to be done. He also thinks a vaccine based on this concept could complement a current vaccine being tested that is somewhat effective in lessening the effects of malaria in people. People who survive malaria or have low-grade malaria are carriers of the disease. When those people are bitten by uninfected mosquitoes, the mosquitoes can then become infected, perpetuating the disease in a community. If scientists can "vaccinate" mosquitoes, it could go a long way toward eradicating the disease, he says.
"We are focused on the mosquito as a way to stamp out malaria," he says. "For more than 100 years, we have studied the parasite as the key to stamping out malaria but we haven't gotten very far. The parasite has to go through the mosquito to continue its life cycle before being able to infect humans. Our approach could stop the disease by halting the infection of the mosquito."
These new findings suggest researchers were probably asking the immune system to target too many regions on AnAPN1, which only diluted the response to the relevant regions of the protein. Dinglasan has now redesigned his vaccine to focus on this newly understood region of AnAPN1.
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