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Brown researchers identify malaria vaccine candidate

Kurtis lab finds new vaccine target in protein expressed by malaria-causing parasite

A study partially conducted at the University has identified a new vaccine target for malaria with a novel mechanism underlying its ability to confer immunity. Malaria produces a detrimental effect on children in areas most gravely afflicted by the disease — but this vaccine candidate, which has not yet been tested in humans, may provide some hope for the future. 

The paper was published in Nature, a renowned research journal, on April 22, just prior to World Malaria Day on April 25.

A team of scientists — co-lead by Patrick Duffy and Michal Fried of the National Institutes of Health and Jonathan Kurtis ’89 PhD’95 MD’96 — discovered a system of conferring immunity through a novel mechanism of activating cell death. Kurtis is a chair and professor of pathology and laboratory medicine at the University, as well as the director of laboratories for the Lifespan Center for International Health Research. 

The project is the culmination of roughly two decades of work that began shortly after Kurtis completed his MD and PhD at Brown.  

Duffy, who is a laboratory chief at the National Institute of Allergy and Infectious Disease, said his partnership with Kurtis precedes Kurtis’ tenure as a professor at the University. After completing his undergraduate, medical and doctoral education at the University, Kurtis traveled to Kisumu, Kenya, where he did postdoctoral work under Duffy at the U.S. Army’s Walter Reed Army Institute of Research. There, the 20-year project began.

This study, Duffy said, “gives us a brand new strategy for developing vaccines.” The strategy relies on identifying healthy individuals in areas where malaria is common and determining how they acquire immunity.

A novel approach: vaccine candidate creates "a natural collaboration" 

Despite promising initial results on prior malaria vaccine candidates, previous candidates have proven ineffective in their transition from cell plates and animal models to humans, said Dipak Raj, assistant professor of pathology and laboratory medicine (research) and first author of the study. This discrepancy is largely due to mice constituting an inadequate model of the human body. The malaria-producing plasmodium parasite can also evolve and become resistant to these other vaccines, Raj added.

Once the parasite enters the bloodstream, marked by the uncomfortable bite of a mosquito, it travels to the liver, infects red blood cells and spreads until it reaches a threshold at which it can induce severe illness. The parasite has evolved to avoid purposefully killing its host, Raj said, but that doesn’t mean the infection is never fatal. Malaria remains a catastrophic illness, especially when caused by the lethal plasmodium falciparum parasite. 

As the “leading single-agent killer … in children” globally, malaria takes about a quarter of lives in toddlers between ages two and five living in some nations, and even these numbers are likely underestimated, Raj said. 

“We’re all really familiar with the new denominator 10,000” when thinking of COVID-19-related deaths, Kurtis added. This is “the same number of deaths of children under the age of five every single week (from malaria), and it hasn’t just been going on since March.” This already staggering statistic can approach 50,000 deaths weekly, Kurtis added.

The vulnerable population also often does not have access to adequate health care where they live, Raj said.

Although many other potential malaria vaccines have attempted to block infection, these researchers’ new candidate instead aims to kill the infected cell — and thereby its unwanted stowaway — once the parasite is stuck inside. Given the low amount of parasites within the bloodstream early on, this does not harm the body but rather opens a window for a person’s immune system to develop a defensive response to malaria, Raj added.

While it may seem counterintuitive, this approach has succeeded so far because two out of five kinds of malaria parasites have a gene that produces the protein GARP, which protrudes out of the red blood cell’s surface. 

 In 2012, the researchers discovered that GARP proteins can be identified by specific antibodies, molecules that bind to the protein and also instigate cell death in culture, Raj said. The potential vaccine or drug would ideally either consist of these antibodies or produce the same effect as the antibodies do, he added. 

This vaccine candidate “is not preventing the infection, so the parasite is happy, (but) it’s preventing the disease, so the host is happy. If this … vaccine works out, that’s pretty much a natural collaboration,” Raj said.

Experiments for this study that were conducted solely on the cells exposed to the antibodies, excluding other immune system components, succeeded in killing the infected cells, Kurtis said. Subsequent studies involving primates showed that those receiving the vaccine candidate were able to fight malaria on their own, whereas those who were not immunized required additional treatment, Raj added. 

These findings also suggest that people living in areas where malaria is common may not need constant revaccinations, because their immune system would gradually reinforce its defense after every exposure, Raj said. 

“Each time the person gets (the) infection, (they) will naturally get a booster dose of vaccine.” 

How researchers discovered GARP and its antibody partner: "A scientific game of spot the difference" 

Starting in the mid-1990s, Duffy led a team in Tanzania collecting data on large cohorts of pregnant mothers. After birth, their babies were monitored by the team biweekly for four years. Hundreds of children were tracked to determine whether or not they became infected with malaria or developed immunity to the infection. “Those data sets in the samples we collected in Tanzania became the starting point for Dr. Kurtis to start working in the laboratory to ... see if he could identify whether antibodies targeting specific parasite proteins might be related to protection from malaria,” Duffy said. 

Some newborn babies had antibodies passed down from their mothers, but the effects of the antibodies dissipated within two years, Raj said. Others had innate immunity to malaria, but this could not be introduced into others through a vaccine, Kurtis added. 

Another objective was identifying confounding factors, or the external conditions that could affect someone’s susceptibility to malaria. These include having sickle cell disease, which confers immunity to malaria, or simply a person’s living environment. Most interesting, however, were the children who did not have unique circumstances protecting them from infection but still developed immunity over time. Those children offered the most promising prospects for aiding in the vaccine design. 

The researchers then applied differential screening, a technique used by the late University Professor Paul Knopf in his separate research on rats to determine the parasitic genes “that encoded proteins that were only recognized by the resistant rats, not the susceptible rats,” Kurtis said. 

Through this process resembling a scientific game of spot the difference, the researchers compared the parasite genomes and the blood samples of those who acquired protection to malaria and those who did not, thereby identifying the factor present in children who could control the disease but absent from those who were not resistant to malaria.

GARP was that key factor. 

“We saw in the initial study (that the GARP antibody vaccine candidate is) really, really effective, and it’s totally different from all other vaccine candidates so far (that had been) tried, and it carried a lot of promise,” Raj said. 

According to Kurtis, the researchers now “have two of the three stages of the blood cycle targeted” for malaria. The GARP vaccine kills the parasite inside the blood cell, and another known vaccine prevents it from escaping. “We actually have a new candidate … that actually blocks invasion,” but this project is still in development, Kurtis said. “The idea is to get the three combined.” Kurtis summed up the primary goal: “Kill (the parasite) while it’s inside. Prevent it from getting outside.”

Moving forward: "A brand new concept"

Kurtis and Duffy emphasized that the study was a team effort: with scientists, graduates and undergraduate students across institutions lending their minds and their time to the study. 

“We formed a research partnership but also a friendship that has now gone on since the mid 1990s,” Duffy said, adding,“I don't think any team could have done this alone.”

“A lot of people (were) involved in this project to get all the information and make this study really robust and literally turn every possible (stone) to get the information,” Raj added.

Richard Markham, an investigator at Johns Hopkins University focused on vaccine development, including in the context of malaria, was not involved in this study but provided an external perspective on its findings. “Dr. Kurtis has defined a totally novel mechanism by which an immune response to a malaria antigen can assist in the control of infection, a mechanism which is very different than the way by which classical immune mechanisms control disease,” he wrote in an email to The Herald. 

But according to Markham, there are some key limitations to malaria vaccine design that have not yet been remedied by these findings. 

“The issue that (Kurtis) faces now is the one that has challenged all malaria vaccine development: how to elicit the high and sustained levels of immunity that appear to be required for protection against this disease,” Markham wrote.

Though the issue of funding for these kinds of studies persists, the ultimate aim is to apply the vaccine candidate to a phase one clinical trial in humans, Kurtis said.

“No one previously has described what Professor Kurtis described, which is a protein that the parasite expresses that can be a target of antibodies that cause the parasite to die inside of the red cell. This is a brand new concept,” Duffy said. 

“We can cut severe disease in monkeys without optimization ... in half. (If) you could do that for human beings, just imagine what that world would look like. It would be a totally different world,” Kurtis said.


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