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 completed three human clinical trials of 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 completely block transmission; 3) a prime boost strategy in which volunteers previously vaccinated three times with the attenuated vaccinia construct were boosted with a single injection of the yeast- secreted, alum-adsorbed rPfs25?one of nine volunteers developed potent transmission-blocking activity and sera from six others reduced substantially infectivity but did not completely block transmission. Of the six or so identified transmission-blocking target antigens, the genes encoding all but two had been cloned- chitinase and Pfs40. This parasite-produced enzyme is essentially for the parasite?s egress from the blood meal. We have now succeeded in cloning the chitinase gene by microsequencing highly purified enzyme. A similar approach is now being taken to isolate the gene encoding Pfs40. In addition, a signal sequence trap method has been developed to identify the whole repertoire of surface proteins on gametocytes, gametes, zygotes and ookinetes. One of the interesting molecules we hope to identify is one that we recently described that accounts for the phenomenon of erythrocyte-binding during exflagellation.
