Tfh responses to novel vaccine candidates and protection from pediatric falciparum malaria

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ABSTRACT Our overall aim is to advance PfSEA-1 as a vaccine candidate for pediatric falciparum malaria by identifying broadly reactive, PfSEA-1 specific T follicular helper (Tfh) epitopes, which are critical for long-lived antibody responses. In previous R01 funded studies, We discovered Schizont Egress Antigen-1 (PfSEA-1), a 244-kDa parasite antigen that is the target of antibodies which arrest parasites at the schizont stage. Active vaccination with rPbSEA-1 results in a 3-fold reduction in parasitemia and a 2 fold longer survival time after challenge with P. berghei ANKA parasites (P = 0.001). Children in our Tanzanian birth cohort (N=785) experienced a dramatically increased incidence of severe malaria during periods with undetectable anti-PfSEA-1 antibody levels (45 cases/23,806 child weeks) compared to periods with detectable antibody levels (0 cases/1,688 child weeks; adjusted OR 4.4; Type III fixed effects P < 0.01). In our cohort of Kenyan males (12-35 yrs old, N= 138), individuals with detectable anti-PfSEA-1 IgG antibodies had 50% decreased parasite density compared to individuals with undetectable anti-PfSEA-1 IgG antibodies (P < 0.04) over an 18-week high transmission season. This work has culminated in a comprehensive, full length Research Article in Science 1. A major obstacle to malaria vaccine development is the generation of high-titer functional antibodies with the induction of long-lived plasma and memory B-cells. In the past 10 yrs, Tfh cells have been recognized as essential for somatic hypermutation, isotype switching, germinal center formation, long lived plasma cell formation as well as memory B cell formation 2. Intriguingly, in the only report of Tfh cells in human malaria infection, inefficient activation of the CXCR3- subset of Tfh cells was observed during acute malaria infections in children, and this defect may be responsible for the poor anti-malarial antibody responses seen in early childhood 3. Based on the known function of Tfh cells and their role in generating protective antibody responses to vaccine antigens 4, 5, identifying Tfh epitopes is essential for malaria vaccine optimization. In the current application, we propose to map Tfh stimulating, MHC Class II T cell epitopes in PfSEA-1 using both recombinant protein as well as overlapping peptide approaches. We will initially identify all possible peptide specific epitopes in a cross-sectional sample of semi-immune adults. We will then relate antibody, cytokine, and T-cell subset responses to these epitopes with resistance to infection in a longitudinal cohort study conducted in 2-7 yr old children living in a holoendemic region of western Kenya. The deliverables from this study will be a list of validated T cell epitopes within the novel egress blocking vaccine candidate PfSEA-1 and a comprehensive, prospective analysis of the relationship between antibody, cytokine and T-cell subset responses to these epitopes and resistance to infection with P. falciparum. These data will allow us to enhance the immunogenicity of second generation rPfSEA-1A based vaccines by including broadly reactive T cell epitopes from the flanking regions, or increasing the copy number of T-cell epitopes within the aa 810-1083 region. In addition, these results will significantly increase our understanding of the role and kinetics of Tfh cell responses during pediatric malaria infections.