Prospects for enhancing malaria vaccine efficacy by combining pre-erythrocytic antigens

Malaria causes almost half a million deaths each year. Existing interventions will almost certainly not be enough to tackle this enormous public health problem on their own. An effective vaccine is urgently needed. The leading malaria vaccine, RTS,S, confers suboptimal protective efficacy, and in addition targets only Plasmodium falciparum and not the other major species of human malaria, P. vivax. This thesis investigates the potential of combining pre-erythrocytic malaria vaccines as a means of enhancing protective efficacy. A novel mathematical model was developed which expresses probability of protection as a function of vaccine-induced humoural and cellular responses. The model predicts that combining partially effective vaccines should result in more than additive improvements in protective efficacy. This was supported by an experiment combining Rv21, a P. vivax circumsporozoite virus-like particle, with viral vectored P. vivax TRAP, the two leading pre-erythrocytic malaria vaccine antigens; this combination raised protective efficacy from 50% and 0%, respectively, to 100% sterile protection. It was also found that antigenic interference, a reduction in anti-CSP titres when Rv21 and PvTRAP are combined, occurred only in the presence of Matrix M adjuvant, and not when using alum, AddaVax or no adjuvant. With a view to creating a single-component multi-antigen vaccine, which would be more cost-effective than a multi-component vaccine, experiments were carried out to establish the virus-like particle Qβ as a platform capable of eliciting protective immunity via the display of short peptides derived from the CSP repeat region of both P. vivax and P. falciparum. For the first time, a tetramer peptide derived from the CSP repeat region of P. vivax VK210, AGDR, was shown capable of eliciting protective immunity alone. Finally, five novel linear B-cell epitopes were discovered, one from P. falciparum CSP, three from P. vivax TRAP and one from TRSP, each capable of conferring partial protection on mice. These epitopes were identified using novel screening methods, using sera from whole-protein vaccinated mice or by exploiting conservation within invasion protein sequences. Two of the protective epitopes, (NANP)6 and (ADGN long) were combined and found to enhance protective efficacy as predicted by the mathematical model. Thus this thesis lays the groundwork for the development of a single-component multi-epitope malaria vaccine with enhanced protective efficacy.