A filter paper method for the detection of Plasmodium falciparum gametocytes by reverse transcription polymerase chain reaction.

Plasmodium falciparum gametocytes are obligate parasite sexual stages required for transmission of malaria from human hosts to the mosquito vector. Assessment of gametocyte carriers in the population is critical in understanding malaria transmission dynamics and in epidemiology studies. We applied a reverse transcription-polymerase chain reaction (RT-PCR)-based approach to detect pfs25 transcripts from blood dried on different filter papers in the laboratory. The detection limit was 1-2 gametocytes/microL. We further validated this assay by analyzing RNA in 10 matched blood samples (liquid blood and blood spotted on filter papers) collected from subjects under field conditions in Zambia. These results thus establish feasibility of detection of Plasmodium falciparum gametocytes by RT-PCR method from dried blood on filter paper. This assay will greatly facilitate bulk analysis of gametocyte RNA transcripts on filter paper, especially in areas where collection and preservation of liquid blood is not feasible.

Markedly enhanced immunogenicity of a Pfs25 DNA-based malaria transmission-blocking vaccine by in vivo electroporation.

Pfs25 is a promising target antigen for the development of a malaria transmission-blocking vaccine and prior research has demonstrated induction of high and functionally effective antibodies in mice with IM injection of Pfs25 encoding DNA plasmid. Likewise, Pfs25 DNA vaccine was immunogenic in rhesus macaques but required a protein boost to elicit significant transmission-blocking antibodies. The translation of these encouraging findings to human clinical trials has been impeded largely by the relatively poor immunogenicity of DNA plasmids in larger animals. In vivo electroporation (EP) has revealed significant enhancement of the potency of DNA plasmids. The results reported here compared the immunogenicity and functional transmission-blocking effects of immunization with DNA plasmid (25 microg) by the traditional IM route compared to coupling the IM injection (0.25, 2.5 and 25 microg doses) with in vivo EP. Significantly, a 0.25 microg dose of DNA plasmid, when administered with EP, induced antibody titers (1:160,000) and functional transmission-blocking effects that were equivalent to those achieved by a one hundred fold higher (25 microg) dose of DNA plasmid given without EP. At a 25.0 microg DNA dose with or without EP there was sufficient antigenic stimulation to result in effective antibody titers; however EP method yielded antibody titer of 1:1,280,000 as compared to only 1:160,000 titer without EP. This observed two log reduction in the amount of DNA plasmid required to induce significant transmission-blocking effects makes a compelling argument in favor of further evaluation of DNA vaccines by in vivo EP method in larger animals. Further experiments in non-human primates and eventually in phase I human trials will determine if the use of EP will induce effective and sustained malaria transmission-blocking effects at acceptable doses of plasmid DNA.