GENETIC ENGINEERING APPROACHES FOR MALARIA VACCINES

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Subunit vaccines for the two predominant species of malaria parasites, Plasmodium falciparum and Plasmodium vivax will be developed. We have demonstrated that hybrid Hepatitis B virus surface antigen (HBVsAg) polypeptides (composed of insertions of part of the circumsporozoite gene of P. falciparum into the pre-S region of the HBVsAg gene) are expressed in genetically engineered yeast vector systems. These hybrid polypeptides assemble into multimeric particles that have antigenicities of both HBV and the malaria parasite. We propose to produce hybrid HBV polypeptides containing parts of the circumsporozoite gene of P. vivax. We also propose to use yeast expression vectors to produce the entire circumsporozoite proteins of both malaria species. In Phase I, the antigenicity of the hybrid HBVsAg particles and circumsporozoite proteins made in yeast will be assessed with antisera to the malaria parasite; hyperimmune human sera and various animal sera raised to either intact parasites or to synthetic peptides representing the repeat units of the circumsporozoite antigen will be tested. Also, the immunogenicity of these preparations will be evaluated in rodents and in sub-human primates. These studies will focus on measuring immune responses that correlate with protection. For Phase II, expression systems will be optimized and purification procedures will be developed to produce large quantities of the most immunogenic form of these malaria antigen(s). Challenge studies will be conducted in sub-human primates. Various immunization procedures as well as the effects of adjuvants will be tested. The results of these challenge experiments will be used to determine whether or not antigens of other stages of the parasites' life cycles should be included to stimulate effective and durable immunity. The procedures developed in Phase I, testing the use of hybrid HBVsAg as a carrier for presentation of portions of the circumsporozoite antigen and testing the entire circumsporozoite protein expressed in yeast, may provide the means to augment the immunogenicity of the surface proteins of merozoite and gametocyte stages of malaria parasites.