Transmission blocking vaccines (TBVs) against malaria are intended to induce immunity against the stages of the parasite that infect mosquitoes so that malaria transmission is reduced or halted. Malaria transmission occurs locally and 'focally', i.e. spatially confined to an infectious source, thus TBVs used in a community can effectively suppress malaria transmission to others. TBVs will induce antibodies that will target antigens expressed on the surface of sexual and mosquito midgut stages of the malaria parasite. The target antigens include proteins synthesized in the gametocytes (pre-fertilization antigens, in P. falciparum: Pfs230 and Pfs48/45) and in the zygotes-ookinetes (post-fertilization antigens, in P. falciparum: Pfs25 and Pfs28) and the epitopes recognized by transmission blocking antibodies are cysteine-rich reduction-sensitive conformational in nature. The inability to express properly folded proteins is one of the major factors that have severely hampered protein based TBV development. DNA based vaccination was envisaged to overcome the conformational problem in recombinant proteins, and indeed studies in mice and monkeys have firmly established the value of DNA based TBV approach. This vaccine platform can facilitate evaluation of a cocktail of pre- and post-fertilization antigens in pre clinical setting. Studies proposed are aimed at (1) identifying immunologically relevant domains in the pre-fertilization antigens, (2) optimizing the combination of pre- and post-fertilization antigens by vaccine formulation in cationic lipids and vaccine delivery by in vivo electroporation, (3) evaluating a candidate DNA vaccine by in vivo electroporation in nonhuman primates (Macaca mulatta) and testing the concept that immunity against pre-fertilization antigens can be maintained by boosting during natural infection using an Aotus model for P. falciparum infection. Moreover, the development of Pfs25 transgenic P. berghei will provide an approach for in vivo evaluation of human malaria TBV based on Pfs25, as compared to a standard in vitro membrane feeding assay. These are critical and essential issues that need to be systematically addressed in pre clinical studies prior to the development of an ideal and effective TBV for clinical trials in human volunteers.
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