Extensive studies in humans and animal models indicate that acquired immunity to malaria develops. Yet, the goal of reducing morbidity and mortality due to malaria through active immunization has not been achieved. Our efforts to identify plasmodial antigens that trigger protective responses are focused on an in vivo system of immunization-induced protection. Using this Plasmodium yoelii model, we identified a novel merozoite surface antigen and a unique family of antigens secreted from infected erythrocytes. We provided evidence of their vaccine potential in the P. yoelii model and of their conservation in Plasmodium falciparum. The objective of this application is to evaluate the efficacy of immunization with conserved domains of these protective antigens. The hypothesis is that the tryptophan-rich domain (WRD) of secreted blood-stage malarial antigens and the epidermal growth factor (EGF)-like domains of merozoite surface protein-6 (MSP-6) are targets of protective immune responses. The efficacy of immunization with conserved domains of pyWRD and pyMSP-6 antigens will be measured as protection against blood-stage P. yoelii malaria. Vaccine formulations and delivery systems will be manipulated to optimize protection induced by immunization with recombinant P. yoelii WRD and MSP-6 antigens administered in combination with merozoite surface protein -1. T cell and B cell responses that correlate with protection induced by immunization with combinations of P. yoelii antigens expressing WRD and EGF-like domains will be defined. As the immune responses to these domains in the P. yoelii model are characterized, the findings will be directly applied to studies of P. falciparum. The ability of rabbit antisera raised against recombinant pfWRD and pfMSP-6 to inhibit the in vitro growth of P. falciparum will be evaluated. Using an integrated approach, novel blood-stage vaccine targets that can be effectively formulated in combination to increase the efficacy of immunization will be characterized. The results will provide a logical basis for planning and evaluating human trials of a multivalent, blood-stage vaccine that includes the WRD and EGF-like domains of these P. falciparum antigens.
