Improving Malaria Vaccines
Malaria vaccines offer limited protection. These vaccines target proteins on a parasite that transmits malaria to humans by mosquito salvia. Kristian Swearingen in the Moritz Lab, with Seattle Children’s Research Institute, is developing a deeper understanding of these parasite proteins to help meet the World Health Organization’s goal of having a vaccine with protective efficacy of at least 75%.
Close up of lab work at ISB. Photo credit: Scott Eklund / Red Box Pictures.
In 2016, ISB researcher Kristian Swearingen, PhD, and his colleagues made an important discovery: Two key proteins in the malaria parasite bear never-before-seen modifications on their surfaces. These modifications are known as glycosylation and involve the addition of a tiny, stable sugar molecule, or glycan, to certain parts of the protein. Since this discovery, other teams have gone on to show that glycosylation is critical for the parasite to complete its life cycle. One of these glycosylated proteins, CSP, forms the basis for the only two WHO-recommended malaria vaccines, which are known as RTS,S and R21. Swearingen and his colleagues at ISB and Seattle Children’s Research Institute (SCRI) are now working to further understand the role of protein glycosylation in Plasmodium falciparum, the parasite that is responsible for the deadliest form of malaria, and whether including these modifications in vaccine production could lead to a more effective malaria vaccine.
- Funded by National Institute of Allergy and Infectious Diseases
- Led by Kristian Swearingen, PhD
- Key collaborators:
- Robert Moritz, PhD, ISB
- Ashley Vaughan, PhD, and Noah Sather, PhD, Seattle Children’s Research Institute
How a tiny sugar molecule could be malaria’s undoing
Plasmodium’s sugary modifications happen on specific pieces of the proteins. These pieces, or domains, are called thrombospondin type-1 repeats or TSRs and they are found in a wide variety of organisms, including us. Because TSRs are commonly glycosylated in other organisms, Swearingen wondered whether they were also glycosylated in the malaria parasite. The team has so far found two proteins that are glycosylated in their TSR regions, CSP and TRAP.
CSP is the basis for two malaria vaccines, both of which use a portion of the CSP protein that includes its TSR. However, the RTS,S and R21 vaccines are produced in yeast cells, which do not glycosylate the protein. It’s possible that including the sugar modification in the manufacturing of the vaccines could boost their efficacy — currently these vaccines are only around 50 percent effective at preventing infection.
Another experimental malaria vaccine, MVA ME-TRAP, similarly contains the TSR region of the TRAP protein. Unlike the CSP vaccines, this vaccine delivers DNA instead of protein (similar to the J&J COVID vaccine) so that the host’s own body produces the vaccine in its own cells. It has not yet been studied whether the TRAP TSR produced by this vaccine is glycosylated, but Swearingen and colleagues have shown that Plasmodium attaches glycans to TSRs in different places than human cells do, so the version of TRAP made by the vaccine probably doesn’t look the same as what the parasite produces.
In their current study, the ISB researchers have made small genetic changes to the Plasmodium parasite that prevent it from glycosylating the CSP and TRAP proteins. When the parasite can no longer modify these proteins, it can’t complete its life cycle in the mosquito, a critical part of the parasite’s infectious journey.
Currently, the joint ISB-SCRI team led by Swearingen is working to understand glycosylation in the other eight TSR-containing proteins in Plasmodium. They’re also planning to study whether glycosylation of CSP, TRAP, or other Plasmodium proteins affects humans’ immune response to these proteins. If so, that would imply that glycosylation could improve the efficacy of vaccines based on these proteins.
As part of their study, they’ve also developed new methods to characterize glycosylation on any protein. The team uses a method of protein analysis called mass spectrometry, which characterizes proteins by their mass with great specificity. However, mass spectrometry analysis methods typically miss glycosylation completely due to the way the technique captures the proteins. Swearingen’s team developed new software tools that enable detection of glycosylated proteins in mass spectrometry data.
Citations
- Swearingen KE, Lindner SE, Shi L, Shears MJ, Harupa A, Hopp CS, Vaughan AM, Springer TA, Moritz RL, Kappe SH, Sinnis P. Interrogating the Plasmodium Sporozoite Surface: Identification of Surface-Exposed Proteins and Demonstration of Glycosylation on CSP and TRAP by Mass Spectrometry-Based Proteomics. PLoS Pathog. 2016. doi: 10.1371/journal.ppat.1005606.
- Swearingen KE, Eng JK, Shteynberg D, Vigdorovich V, Springer TA, Mendoza L, Sather DN, Deutsch EW, Kappe SHI, Moritz RL. A Tandem Mass Spectrometry Sequence Database Search Method for Identification of O-Fucosylated Proteins by Mass Spectrometry. J Proteome Res. 2019 Feb 1. doi: 10.1021/acs.jproteome.8b00638. Epub 2018 Dec 21. PMID: 30523691
Technologies Developed
Software to detect glycosylated proteins in mass spectrometry data
Contact Dr. Kristian Swearingen
Senior Research Scientist
ISB