Structural vaccinology for malaria

National Institute Of Allergy And Infectious Diseases

Investigators

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Abstract

Malaria is caused by eukaryotic parasites that display distinct surface antigens during three independent stages of the life cycle: 1) initial infection caused by the pre-erythrocytic stage, 2) clinical symptoms as a result of the blood stage, and 3) transmission by the mosquito stage. While both T-cell and B-cell responses play a role in naturally acquired immunity to malaria, focusing the B-cell responses on conserved broadly-neutralizing functional epitopes significantly improves protection and may lead to sterile immunity. Four aspects of parasite biology confound malaria vaccine development: 1) antigenic variability across strains and species, 2) the presence of immunodominant but non-neutralizing epitopes in antigens, 3) the diverse, numerous, and often redundant parasite antigens required for each stage of the life cycle, and 4) poor immune response upon vaccination with designed parasite antigens. Rapid major advances in the structural definition of neutralizing epitopes on key malaria antigens, as well as in nanoparticle technology, motivate structure-guided design of immunogens for malaria vaccines targeting all stages of the Plasmodium life cycle. We propose to leverage existing structural information of malaria antigen and neutralizing antibody complexes to design improved immunogens and nanoparticles that will elicit protective immune responses. Immunogens will be improved through protein design to retain neutralizing epitopes, eliminate non-neutralizing epitopes and present optimized immunogens on nanoparticles for efficient delivery and immunogenicity. Proteins will be designed using computational approaches to stabilize protein conformations, reduce large proteins to immunogenic subdomains, and scaffold epitopes for efficient presentation. Diverse established nanoparticle platforms will be evaluated for their ability to effectively present antigens, and novel nanoparticles will be developed for delivery. All designed immunogens and nanoparticles will be structurally characterized through x-ray crystallography and cryo-electron microscopy to ensure the correct conformational 3D structure of the antigen is retained. We have developed a computational design procedure using the NIAID Locus high performance computer cluster, and an in vitro screening platform to validate computationally designed antigens. In FY2021, we expanded the use of our immunogen design pipeline to produce lead candidates for 7 malaria antigens targeting multiple parasite species and all three stages of the parasite life cycle. These lead candidates retain desired neutralizing epitopes and have improved biophysical characteristics compared to the native antigen. The designed immunogens typically have higher production yields, higher thermostability, and lack undesired immunodominant or non-neutralizing epitopes. These immunogens are now being tested in pre-clinical animal models. We have demonstrated that one of these immunogens successfully elicits higher titers of functional antibodies in rodents than the native antigen, and we are continuing to develop this antigen as a nanoparticle to maximize vaccine efficacy. We have developed nine novel nanoparticle-platforms using Plasmodium proteins, which are assembled into heptamers, dodecamers, tetradecamers and even 24-mers. Three established platforms were employed to display designed immunogens up to 48 subunits, increasing immunogenicity and protectivity. These platforms have been designed in a modular fashion to allow plug-and-play screening of designed immunogens in various expression systems, expediting the identification of optimized vaccines. Nanoparticles have been produced displaying several different antigens, and these vaccine candidates are now in pre-clinical testing in rodents. Animal studies have been completed for one antigen-nanoparticle platform, demonstrating a successful increase in antibody titers upon nanoparticle display.