Functionally active serum and monoclonal antibody responses targeting the pre-erythrocytic stage of Plasmodium falciparum in Tanzanian adults after vaccination with purified, live-attenuated sporozoites

Abstract
With 212 million cases worldwide and an estimated 429,000 deaths in 2015, malaria continues to be a serious global health threat. An effective vaccine could significantly decrease malaria-associated morbidity and mortality, especially in children under the age of five years. At present, there is no licensed malaria vaccine available. The most advanced malaria vaccine candidate, the RTS, S/AS01 subunit formulation, showed limited and short-term vaccine efficacy against clinical malaria in phase 3 clinical studies in malaria-endemic populations in sub-Saharan Africa.
The only vaccination approach that has ever induced complete sterile protection against controlled human malaria infection (CHMI) in malaria-naïve individuals, is the immunization with live, whole-attenuated sporozoites. The Sanaria® PfSPZ Vaccine (PfSPZ Vaccine) is a pre-erythrocytic vaccine, which is composed of live, radiation-attenuated, aseptic, purified, non-replicating, cryopreserved, whole Plasmodium falciparum (Pf) sporozoites (PfSPZ). In a study at the National Institutes of Health (NIH), the PfSPZ Vaccine was safe and well tolerated and protected 100% (6/6) of malaria-naïve volunteers against homologous CHMI when administered intravenously (IV) at 5 doses of 1.35 x 105 PfSPZ.
The overall aims of this thesis included i) the evaluation of the safety, immunogenicity and protective efficacy against homologous CHMI of the PfSPZ Vaccine by direct venous inoculation (DVI) of malaria pre-exposed volunteers from Tanzania and ii) the study of functionally active serum and monoclonal antibody responses against Pf sporozoites induced by malaria pre-exposed Tanzanian volunteers upon immunization with the PfSPZ Vaccine. Building on these objectives, the here presented thesis is structured around three manuscripts:
Manuscript 1: Safety, immunogenicity and protective efficacy against controlled human malaria infection of PfSPZ Vaccine in Tanzanian adults
This manuscript depicts the first clinical phase I trial on the safety, immunogenicity and protective efficacy against homologous CHMI of PfSPZ Vaccine performed in Africa. The study was conducted in healthy male volunteers aged 20 to 30 years in Bagamoyo, Tanzania. Similar regimen as tested in the NIH study of 5 doses of 1.35 x 105 (low-dose group) or 2.7 x 105 (high-dose group) of PfSPZ Vaccine was administered via DVI to Tanzanian adults. Homologous CHMI was conducted at 3 and 24 weeks after the last vaccination. The PfSPZ Vaccine was safe and well tolerated, but less immunogenic and protective against CHMI as in malaria-naïve US Americans. It is hypothesized that higher individual doses and altering intervals will be required to induce improved protective efficacy and immunogenicity in malaria endemic target populations.
Manuscript 2: Intravenous application of irradiation attenuated Plasmodium falciparum sporozoites elicits long-lived IgG and IgM invasion inhibitory antibodies in malaria pre-exposed volunteers
In this study we aimed i) to gain a better understanding of the vaccine-induced humoral immune responses in malaria pre-exposed individuals after immunization with DVI-administered PfSPZ Vaccine and ii) to investigate if sporozoite binding and invasion inhibitory IgM antibodies are induced following PfSPZ vaccination. To address this, we used serum and plasma samples collected from a subset of volunteers who participated in the PfSPZ Vaccine trial described in manuscript 1. Our findings demonstrate that malaria pre-exposed volunteers develop sporozoite invasion inhibitory antibodies and induce anti-CSP IgG and IgM antibodies after repeated PfSPZ vaccination. A subset of these plasma samples was depleted from IgG and IgA antibodies to obtain fractions containing only IgM antibodies. Depleted IgM antibody fractions of 3 volunteers demonstrated sporozoite invasion inhibitory capacity in in vitro inhibition of sporozoite invasion (ISI) assays. These results indicated for the first time that sporozoite-specific IgM antibodies could contribute to protection against malaria infection.
Manuscript 3: CSP-specific human monoclonal antibodies inhibiting Plasmodium falciparum liver stage infection target a distinct sequence at the N-terminus
To investigate the mechanisms of protection elicited by antibodies following PfSPZ vaccination, we isolated a panel of monoclonal antibodies from the PfSPZ-immunized Tanzanian volunteers who were found protected from homologous CHMI in the study described in manuscript 1. Using an antigen-agnostic approach to identify antibodies that can bind the surface of intact sporozoites, we found that all monoclonal antibodies isolated target the circumsporozoite protein (CSP), confirming its known immunodominance. Functional characterization of these antibodies revealed, that the most effective antibodies reducing Pf liver burden in a humanized mouse model target a distinct epitope located at the junction between the N-terminal end and the NANP repeat, a region which is not included in the RTS, S vaccine. Our results suggest that this identified region could be a component of interest in the future design of effective second generation PfCSP subunit vaccines.