Precise timing of expression of a Plasmodium falciparum-derived transgene in Plasmodium berghei is a critical determinant of subsequent subcellular localization.

The development of transfection technology for malaria parasites holds significant promise for a more detailed characterization of molecules targeted by vaccines or drugs. One asexual blood stage vaccine candidate, apical membrane antigen-1 (AMA-1) of merozoite rhoptries has been shown to be the target of inhibitory, protective antibodies in both in vitro and in vivo studies. We have investigated heterologous (trans-species) expression of the human malaria Plasmodium falciparum AMA-1 (PF83/AMA-1) in the rodent parasite Plasmodium berghei. Transfected P. berghei expressed correctly folded and processed PF83/AMA-1 under control of both pb66/ama-1 and dhfr-ts promoters. Timing of expression was highly promoter-dependent and was critical for subsequent subcellular localization. Under control of pb66/ama-1, PF83/AMA-1 expression and localization in P. berghei was limited to the rhoptries of mature schizonts, similar to that observed for PF83/AMA-1 in P. falciparum. In contrast the dhfr-ts promoter permitted PF83/AMA-1 expression throughout schizogony as well as in gametocytes and gametes. Localization was aberrant and included direct expression at the merozoite and gamete surface. Processing from the full-length 83-kDa protein to a 66-kDa protein was observed not only in schizonts but also in gametocytes, indicating that processing could be mediated outside of rhoptries by a common protease. Trans-species expressed PF83/AMA-1 was highly immunogenic in mice, resulting in a response against a functionally critical domain of the molecule.

Transfection systems for animal models of malaria.

Transfection of malaria parasites is a rapidly emerging technology that offers great promise for the investigation of many aspects of infection. Animal models of malaria have always played an important role in the investigation of the disease. In this article, two of the Dutch groups that have been involved in combining transfection with animal models describe the relevant techniques and recent vector developments for the expression of transgenes, giving examples of their application.

Transfection of the primate malaria parasite Plasmodium knowlesi using entirely heterologous constructs.

The recently developed transfection systems for Plasmodium berghei and Plasmodium falciparum offer important new tools enabling further insight into the biology of malaria parasites. These systems rely upon artificial parasite-host combinations which do not allow investigation into the complex interactions between parasites and their natural hosts. Here we report on stable transfection of Plasmodium knowlesi (a primate malaria parasite that clusters phylogenetically with P. vivax) for which both natural and artificial experimental hosts are available. Transfection of this parasite offers the opportunity to further analyze the biology of antigens not only in a natural host but also in hosts that are closely related to humans. To facilitate future development of integration-dependent transfection in P. knowlesi, completely heterologous plasmids that would reduce homologous recombination at unwanted sites in the genome were constructed. These plasmids contained the pyrimethamine-resistant form of dihydrofolate reductase-thymidylate synthase (dhfr-ts) from Toxoplasma gondii or P. berghei, under control of either (a) P. berghei or (b) P. falciparum promoters. Plasmids were electroporated into mature P. knowlesi schizonts and these cells were injected into rhesus monkeys (Macaca mulatta). After pyrimethamine treatment of these monkeys, resistant parasites were obtained that contained the plasmids. Promoter regions of both P. berghei and P. falciparum controlling dhfr-ts expression were effective in conferring pyrimethamine resistance in P. knowlesi, indicating that common signals control gene expression in phylogenetically distant Plasmodium species.