Abstract
Whole-sporozoite vaccination/immunization induces high levels of protective immunity in both rodent models of malaria and in humans. Recently, we generated a transgenic line of the rodent malaria parasite P. berghei (Pb) that expresses the P. falciparum (Pf) circumsporozoite protein (PfCS), and showed that this parasite line (PbVac) was capable of (1) infecting and developing in human hepatocytes but not in human erythrocytes, and (2) inducing neutralizing antibodies against the human Pf parasite. Here, we analyzed PbVac in detail and developed tools necessary for its use in clinical studies. A microbiological contaminant-free Master Cell Bank of PbVac parasites was generated through a process of cyclic propagation and clonal expansion in mice and mosquitoes and was genetically characterized. A highly sensitive qRT-PCR-based method was established that enables PbVac parasite detection and quantification at low parasite densities in vivo. This method was employed in a biodistribution study in a rabbit model, revealing that the parasite is only present at the site of administration and in the liver up to 48 h post infection and is no longer detectable at any site 10 days after administration. An extensive toxicology investigation carried out in rabbits further showed the absence of PbVac-related toxicity. In vivo drug sensitivity assays employing rodent models of infection showed that both the liver and the blood stage forms of PbVac were completely eliminated by Malarone® treatment. Collectively, our pre-clinical safety assessment demonstrates that PbVac possesses all characteristics necessary to advance into clinical evaluation.
Highlights
Despite a recent decrease in malaria-related mortality, the goal of eradication remains distant and is unlikely to be achieved in the absence of an effective vaccine against this disease
The entire genomes of the P. berghei (Pb) and PbVac parasites collected prior to and following cyclical propagation were subsequently sequenced, in order to extensively characterize the genetic differences between the wild-type and the genetically modified Pb parasites, as well as any changes that might have been introduced during the propagation procedure
No differences were detected between the PbVac sequences prior to and after cyclical propagation and a single base change was detected between the wild-type and transgenic lines, which is consistent with the known overall Plasmodium mutation rate.[19]
Summary
Despite a recent decrease in malaria-related mortality, the goal of eradication remains distant and is unlikely to be achieved in the absence of an effective vaccine against this disease. Wholesporozoite (WSp) vaccination approaches have been shown to elicit sterile protection against malaria both in rodent models and in humans. Protection mediated by such vaccine candidates relies on the immune responses elicited by the parasite’s preerythrocytic forms, following administration of live sporozoites to the mammalian host. WSp-based malaria immunization generally employs P. falciparum (Pf) sporozoites rendered safe by radiation or genetic attenuation, or by concomitant prophylactic administration of antimalarial drugs acting on the erythrocytic stage of the parasite[1]. The relative similarity between the genomes of Pb and human-infective Plasmodium spp., Pb’s non-pathogenicity to humans, the ability of Pb sporozoites to infect various types of hepatic cells and cell lines, the availability of rodent models to study Pb infection in vivo, and Pb’s amenability to genetic modification, have made this parasite one of the preferred models for the investigation of Plasmodium infection and the analysis of Plasmodium gene function
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