One approach to developing a malaria vaccine is to elicit antibodies to the sexual stages of the malaria parasite so that when an infectious blood meal is taken by the mosquito, the development of the sexual stages of the parasite is blocked. The genes encoding a series of transmission-blocking target antigens have been isolated; however, few of these vaccines candidates have been expressed in a recombinant form that elicit transmission-blocking antibodies. In addition, there are undoubtedly many more target antigens that have yet to be identified. We have found that rPfs25, a recombinant form of the major surface antigen of the early extracellular sexual stage parasite, and rPfs28, a recombinant form of the protein that replaces Pfs25 on the surface of the late sexual stage parasite, elicit transmission-blocking antibodies in laboratory animals. We have now tested in humans two Pfs25-based malaria transmission-blocking vaccines: 1) a highly attenuated vaccinia virus that encodes Pfs25-sera collected from volunteers vaccinated three times had low levels of anti-Pfs25 antibodies that did not block transmission; 2) yeast-secreted Pfs25 adsorbed to alum sera collected after three vaccinations had moderate levels of anti-Pfs25 antibodies that also did not block transmission. Interestingly though, rabbits or primates previously vaccinated with the vaccinia virus (neither of which elicited transmission-blocking antibodies) and then given a single boost of clinical grade Pfs25/alum developed high titer transmission-blocking antibodies. This prime-boost strategy is now being tested in human volunteers recruited from the original clinical trial of highly attenuated vaccinia virus. In addition, we plan to test in humans a fusion protein of rPfs25 - rPfs28 that appears to be more potent than either recombinant protein alone in eliciting transmission-blocking antibodies. Of the six or so identified transmission-blocking target antigens, the genes encoding all but one have been cloned- chitinase. This parasite-produced enzyme is essentially for the parasite~s eggress from the blood meal. Four approaches have been initiated in an attempt to isolate the chitinase gene: 1) purification of the enzyme for microsequencing; 2) vaccination of experimental laboratory animals with purified parasite-produced enzyme, with fungal chitinase and with peptides derived from the consensus active site to elicit antibodies for immunoscreening; 3) amplification of the gene using DNA sequences derived from conserved amino acid regions of other chitinase genes; and 4) expression cloning by transfecting malaria genomic and cDNA libraries into chitinase deficient yeast.
