Developing novel blood-stage malaria vaccines

Thesis

Natural exposure to Plasmodium falciparum’s asexual blood-stage results in protection against severe disease, but no vaccine using the widely-studied blood-stage antigens apical membrane antigen 1 (AMA1) or merozoite surface protein 1 (MSP1) has proven convincingly protective in clinical trials. Challenges include antigenic polymorphism, the apparent requirement for exceptionally high antibody concentrations for protection, and clinical-grade production of conformationally-accurate recombinant protein antigens followed by formulation with a human-compatible adjuvant. This thesis describes the generation of viral-vectored vaccines targeting ten less-studied blood-stage antigens, focusing upon antigens implicated in erythrocyte invasion. These vaccines were immunogenic in mice and rabbits. The rabbit antibodies raised were functionally active in the in vitro assay of parasite growth inhibitory activity (GIA). GIA with antibodies against one antigen, RH5, exceeded that achieved with antibodies against the ‘gold standard’ AMA1 or MSP1 antigens. This antigen’s amino acid sequence is relatively conserved between parasite strains. Importantly, and unlike anti-AMA1 and MSP1 antibodies, the GIA effects transcend genetically diverse strains.

It was hypothesised that blockade of the interaction of RH5 with its receptor basigin was likely to be a mechanism of action of anti-RH5 antibodies. Vaccine-induced polyclonal anti-RH5 serum was found to be capable of blocking this interaction, as well as merozoite attachment to erythrocytes. A panel of RH5-specific monoclonal antibodies were raised: those which block the RH5-receptor interaction were capable of neutralising parasites. Minimal linear epitopes recognised by these antibodies were mapped, and are likely to be within or close to RH5’s receptor binding site.

These data support prompt clinical testing of RH5-based vaccines, and shed light upon the mechanism of action of anti-RH5 antibodies. However substantial challenges remain in establishing whether this antigen, selected on the basis of the in vitro assay of GIA, will be capable of achieving in vivo protection against P. falciparum. Further work presented in this thesis addresses the use of quantitative PCR data to assess blood-stage vaccine efficacy in experimental human challenge with P. falciparum, and the use of surface plasmon resonance to establish more detailed characterisation of vaccine-induced antibody responses. Finally, the results of P. falciparum challenge of RH5-vaccinated Aotus nancymaae non-human primates are presented.

Authors: Douglas, AD

• Publication date: 2015
• DOI: 10.5287/ora-j11xzvxy6
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Malaria; Blood-Stage; Antibody; PfRH5; Vaccine; Viral vector
• Subjects: Immunology; Medical sciences; Malaria; Infectious diseases; Parasitology; Vaccinology; Disease prevention