Preclinical Assessment of the Immunogenicity, Durability and Efficacy Induced by Pre-erythrocytic Vaccines Against Plasmodium falciparum Malaria
Authors:   , Klingenberg Hayley , Burns James M. , Vaidya Akhil B. 
Published: November 2023

Malaria is an ongoing global health problem caused by Plasmodium parasites that results in the death of over half a million people each year. The majority of these deaths occur in children under five years old who lack memory immune responses to protect them from infection. A major step towards prevention of malaria is through vaccination with RTS,S, the only WHO approved vaccines to date. RTS,S contains a portion of the central repeat region and the C-terminus of the pre-erythrocytic stage Plasmodium falciparum circumsporozoite protein (PfCSP). CSP is essential for sporozoite entry into hepatocytes making it a strong vaccine target. Naturally acquired and vaccine-induced immune responses towards this protein have been associated with protection against infection. Pre-erythrocytic stage vaccines aim to clear sporozoites and prevent progression to the blood-stage of the infection, which is the cause of disease pathology. RTS,S makes use of a Hepatitis B surface antigen (HbsAg) carrier to help improve immunogenicity and vaccine delivery. Extensive clinical trials have found that immunization with RTS,S results in low vaccine efficacy that wanes over time. This study aimed to improve upon the specificity, efficacy, and durability of RTS,S through the inclusion of the N-terminal region of PfCSP and the use of a Plasmodium-specific carrier protein. This project extended prior work that assessed the magnitude, specificity, and avidity of the antibody response induced by immunization with four novel PfCSP-based vaccine constructs adjuvanted with glucopyranosyl lipid adjuvant (GLA-SE). Three out of four of our constructs are fused to the blood stage protein, merozoite surface protein 8 (PfMSP8), which has been shown to improve the production and immunogenicity of other Plasmodium protein-based vaccines. The focus of the current thesis project was to assess the T cell responses, epitope recognition, antibody persistence and protective efficacy in vivo of our novel recombinant PfCSP-based vaccines. We demonstrate that inclusion of the N-terminal domain of PfCSP broadens the antibody response to include recognition of known linear B cell epitopes. Vaccination with our constructs drives robust antibody responses to conformational C-terminal epitopes as well as high anti-PfCSP antibody titers that remain durable for at least 6 months. Our constructs induce strong T cell responses, measured by the production of IFN-[gamma] by stimulated splenocytes, but this response shifts in specificity away from PfCSP towards PfMSP8 when mice were immunized with our fused chimeric constructs. However, this did not impact overall efficacy, as immunization with our chimeric PfCSP/8 construct resulted in significant protection against blood stage infection in mice when challenged intravenously with transgenic P. yoelii sporozoites that express PfCSP. These data demonstrate improvements over RTS,S and warrant further testing of the chimeric PfCSP/8 construct for use in a multivalent vaccine for P. falciparum malaria.