New malaria vaccine candidate seeks to block transmission

Survey team members in Bo, Sierra Leone.
Survey team members in Bo, Sierra Leone. (Image provided by Rhoel Dinglasan’s research team.)

In the global fight against malaria, many strategies seek to protect individuals from becoming infected with disease-causing Plasmodium parasites, without addressing the reservoir of parasites that exist in mosquitoes. But focusing on only people examines only half the problem, because the parasites need both mosquitoes and people to complete their lifecycle.

What if the answer to ending malaria were to focus instead on breaking transmission chains and preventing mosquitoes from spreading the parasites? Even better, what if mosquitoes could be immunized against the parasites that cause malaria?

That’s precisely what University of Florida researcher Rhoel Dinglasan seeks to do by developing a vaccine for people that ultimately immunizes mosquitoes and prevents them from passing Plasmodium parasites on to a new host. Dinglasan, a professor of infectious diseases in UF’s College of Veterinary Medicine, joined the faculty under the state’s preeminence initiative. He is also a faculty member of UF’s Emerging Pathogens Institute.

In a recent feature story, EPI reported on how new funding will help advance Dinglasan’s transmission-blocking vaccine to first-in-human trials in Gabon. Related work newly published in NPJ Vaccines details why his team’s formulation has a high chance of success.

In prior work, Dinglasan’s team identified a mosquito protein, Anopheline alanyl aminopeptidase N, or AnAPN1 for short, that is a potent target for a transmission-blocking vaccine. When laboratory mammals such as mice were exposed to it, they produced antibodies which — when introduced into mosquitoes infected with Plasmodium — prevented the parasites from completing their life cycle.

The concept

To understand how the vaccine works, it helps to understand the Plasmodium life cycle, which starts when a mosquito acquires both male and female Plasmodium gametocytes (mature reproductive cells of the parasite) through a blood meal from infected people. The gametocytes then morph into gametes, which in turn develop into motile zygotes called ookinetes. These ookinetes must attach themselves to the epithelium, or tissue lining, of the mosquito’s midgut where they burrow through to further their development as an oocyst About two weeks later, these structures burst open and release thousands of sporozoites that ride in circulatory fluids to the mosquito’s salivary glands. There, they await transport into a new host when the mosquito pokes its needle-like proboscis into a person for a new meal.

Understanding this life cycle between mosquito and human hosts is key to the concept behind Dinglasan’s new vaccine. It is designed to be administered to people who are at high risk of infection or already infected with Plasmodium; they may have symptoms of malaria, but they may also be symptomless. (Adults and older children can become reservoirs of the parasites. They may not feel ill enough to seek treatment, but the parasites they harbor then pass into biting mosquitoes, which then pass the parasites on to other people in the community.)

People who are infected with the parasites, but who also receive the vaccine, will begin producing antibodies against the AnAPN1 mosquito protein. When a mosquito bites this individual, it then picks up these antibodies too. And when the parasite tries to infect the mosquito midgut tissue, the antibodies disrupt this process.

“The parasite cannot establish an infection in the mosquito,” Dinglasan says. “This breaks the transmission chain of malaria.”

Infographic shows how a transmission-blocking vaccine breaks the malaria transmission cycle.
This infographic, adapted from Dinglasan’s NPJ Vaccines paper, shows how a transmission-blocking vaccine breaks the malaria transmission cycle.

In prior work, Dinglasan’s team identified two epitopes — these are regions on an antigen’s surface where antibodies bind themselves — that induced transmission-blocking antibodies in laboratory experiments. In the current work, his team tested various combinations of antigen design incorporating these two epitopes to assess which formula produced the strongest immune response in mice.

Why it’s promising

In the new paper, Dinglasan’s team screened several immunogens — antigens that stimulate a protective immune response — which were created by redesigning the new constructs, derived from the AnAPN1 protein in laboratory mice. They hypothesized that rearranging some of the amino acids in AnAPN1 would focus the immune response to the two previously identified target epitopes. But first they had to select the best construct and then establish the most cost-effective way to manufacture these immunogens, keeping in mind that this ingredient would be mass produced should the vaccine formula prove successful.

They tested six different immunogen constructs, named UF1 through UF6, in mice. These immunogens were paired with different adjuvants known to be safe for people. The team then evaluated which immunogen-adjuvant combination generated the most potent response.

They found that both UF5 and UF6 generated strong responses against at least one of the two epitopes, but that ultimately UF6 was a better candidate since it generated more effective antibodies that blocked parasite transmission in pilot studies in Cameroon. They also determined that the human-safe, liposome adjuvant developed by the Infectious Disease Research Institute performed better than other adjuvant options.  Finally, they established a basic but scalable production process for UF6b (a purification-tag-free version of UF6).

A recent $6 million funding award from the Global Health Innovative Technology Fund will underwrite testing of the formula in people in phase 1 clinical trials in Gabon. Work leading to the trials began on April 1, 2021.

Will people take it?

To best gauge whether people will accept a vaccine geared toward protecting a community immediately but with a delayed protective benefit for the individual who is vaccinated, Dinglasan’s team did a mixed-method, quantitative and qualitative survey in Bo, Sierra Leone. The findings were recently published in the Malaria Journal. They wanted to know how the unusual design of a transmission-blocking vaccine affects acceptance.

The study was designed as a cross-sectional population survey of 615 adults, in addition to six focus groups of parents, and 20 interviews with key informants. The investigators found that most adults engaged in behaviors such as using bed nets to prevent malaria infection and expressed willingness to accept a transmission-blocking vaccine. Some of those surveyed expressed concern over potential vaccine costs but nearly all of those surveyed felt that their community members would accept it.

Malaria primarily kills children. Dinglasan notes that those individuals who live to adulthood because of some level of immunity to malaria could be unwitting contributors to their own child’s death, or another child in their village, because they themselves can infect mosquitoes when experiencing a non-life-threatening malaria illness.

“Parents will do whatever it takes to protect their children,” Dinglasan says. “They have a strong sense of community and care not only about their own children but other children in the village where they live.”

Written by: DeLene Beeland