Our goals are, first, continue development of a 25 kDa sexual stage surface antigen, Pfs25, as a potential vaccine candidate; second, clone the genes for the other known target antigens of transmission blocking immunity; third, identify new target antigens on sexual stage parasites; and finally, define the molecular mechanisms involved in fertilization of malarial parasites. In addition, we are studying the parasite glucose-6-phosphate dehydrogenase (G6PD) enzyme and gene and its role in the protection afforded by G6PD deficiency in humans. Previously we had cloned the gene encoding Pfs25, a prime candidate antigen for a transmission blocking vaccine. Pfs25 has now been expressed in bacteria, yeast, vaccinia ad adenovirus infected mammalian cells, transiently transfected COS cells, and stably transfected CHO cells. Data from mice and Aotus monkeys immunized with vaccinia-produced Pfs25 are very encouraging: sera from mice and monkeys inoculated with live, recombinant vaccinia block transmission of malaria. We have also found that mice and monkeys immunized with purified yeast-produced Pfs25 also acquire transmission blocking immunity. Several adjuvant systems have been examined and alum or DPT has been found adequate. Cloning the other target tissue antigens has been a problem, although we now believe that we have cloned, sequenced, and expressed in E. coli Pfs40, a potential target antigen based on immunogenetic data. A microgamete (male) specific monoclonal antibody has been developed and may provide the entre we need to understand the molecular mechanisms involved in fertilization in the malaria parasite. Finally, we have demonstrated that the malaria parasite expressed G6PD constitutively, and independently of the G6PD status of the host. We have cloned the P. falciparum G6PD gene.
