Novel malaria transmission-blocking vaccine development using cell-free protein synthesis system

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Although not providing direct protection for vaccinees, vaccines targeting antigens expressed on the surface of the sexual stages of the parasites are considered one promising strategy for malaria control, and these malaria transmission-blocking vaccines (TBV) have received increased attention over the past decade. This blocks parasite developmental cycles within the mosquito midgut, consequently reducing or preventing parasite transmission to other individuals. TBVs are also expected to prevent the spread of escape mutants emerged during the course of anti-malaria drug treatment or other prophylactic vaccines targeting asexual stages of the parasites. Only four candidate molecules of this type of vaccine are studied for their potential to induce protective immunity, to date. Since the genomic sequence of P falciparum was completed in October 2002, we have now free access to the genome data to search for novel vaccine candidates. However, one of the bottlenecks for the vaccine development is the high AT content in exons of P. falciparum, which will considerably inhibit the recombinant protein expression using conventional methods. Our strategy is to express recombinant proteins for the characterization of each protein based on the genome sequences of P. falciparum without using synthetic gene. In order to identify the novel TBV candidates, we selected 190 genes, which are expected to be expressed only in gametocyte stage of P falciparum. These genes were cloned into plasmids and templates were prepared for transcription through PCR-based procedures, followed by high throughput recombinant protein synthesis by wheat germ cell-free system. Using this approach, we succeeded in obtaining 120 recombinant proteins. After the screening of these recombinant proteins to identify novel TBV candidates with transmission-blocking human sera, we have identified 15 novel gametocyte antigens as TBV candidates.