Defining conserved epitopes on polymorphic malaria antigens

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PROJECT SUMMARY Malaria is still responsible for an estimated 429,000 deaths and 212 million infections per year, the large majority among young children. Individuals living in endemic regions gradually develop an immune response that protects against symptomatic disease. Naturally acquired immunity targets the blood stage of the parasite’s life cycle and is largely dependent on antibody reactivity. Elicitation of a similar immune response by vaccination in young, non-immune children would dramatically reduce malaria morbidity and mortality. Various parasite antigens have been associated with protection against clinical disease. Most of these antigens are expressed in the merozoite stage and are involved in invasion of the red blood cell. However, vaccine trials based on these blood-stage antigens have thus far been unsuccessful. One factor contributing to the failure of blood stage vaccines is the extensive genetic diversity between different parasite strains that has resulted in strain-specific, but not strain- transcending, antibody responses. Studies of naturally acquired immunity suggest that protected individuals harbor antibodies against conserved epitopes. The identification of such conserved epitopes could serve as a blueprint for structure-based design of a malaria blood stage vaccine. We hypothesize that immunity against malaria is dependent on high affinity IgGs with cross-strain reactivity against merozoite antigens and that these antibodies can be used to define conserved epitopes on merozoite vaccine candidates. To test this hypothesis, we will compare the molecular and functional characteristics of antibodies directed against merozoite antigens among immune adults, semi-immune children and non-immune children living in Tororo, Uganda, a region with extremely high malaria transmission intensity, and use the most potent and broadly reactive antibodies for epitope mapping. In Specific Aim 1, we will isolate merozoite antigen-specific memory B cells from these individuals, test the ability of these antibodies to inhibit various malaria parasite strains in vitro, and map the epitopes of cross-strain reactive antibodies to define conserved epitopes on merozoite antigens. In Specific Aim 2, we will deep-sequence the memory B cell receptor repertoire to uncover the impact of life-long malaria exposure on the humoral immune response, analyze the repertoire of anti-merozoite serum antibodies and determine the cross-strain inhibitory activity and epitopes of dominant serum antibody clonotypes. Collectively, the results from these experiments will provide detailed information about the types of antibodies that a blood-stage vaccine should ideally elicit and will enable the design of novel immunogens to elicit strain- transcending antibodies against the malaria blood stage. In addition, it will provide insight into the overlap and differences between the memory B cell and serological compartments, and the selective mechanisms that drive B cell differentiation into plasma cells and memory B cells.