Novel epitope derived from Plasmodium berghei liver-stage antigen induces a subset of protective CD8 T cells against sporozoite challenge

Abstract CD8 T cells specific for Plasmodium liver stage antigens (LS Ags) are considered key effectors in protective immunity induced by immunization with radiation-attenuated sporozoites (RAS). Sporozoite (spz)-stage epitopes that induce protective CD8 T cells have previously been identified, however, fine specificities of LS Ags-induced protective CD8 T cells remain unknown. Using transcriptome data based on LS forms of Plasmodium falciparum parasites, we identified several orthologue Plasmodium berghei (Pb) LS Ags that, as plasmid DNA vaccines delivered by Gene Gun immunization, significantly reduce liver parasite burden (LPB). Using algorithms that predict binding of peptides to H-2b alleles, we designed ~600 8–9 mer peptides derived from protective Pb LS Ags and screened them utilizing caged MHC class I-tetramer technology. Spleen cells from C57Bl/6 mice immunized thrice with Pb RAS and challenged with infectious spz were tested for binding to the conditional tetramers. We identified an H-2Kb-restricted 9-mer peptide, Kb-17, derived from the novel protective Pb LS Ag, MIF4G-like protein, that formed MHC class I-tetramer specifically binding CD8 T cells; the Kb-17 peptide also recalled IFN-γ responses in splenocytes and liver mononuclear cells at different time points after RAS immunization/challenge. The Kb-17 epitope, delivered as a minigene expressed in adenovirus 5, expanded Kb-17 specific CD8 T cells expressing effector memory phenotype and significantly reduced Pb LPB after infectious spz challenge. Identification of protective MHC class I-restricted epitopes on LS Ags as well as establishing a role for LS Ags-specific CD8 T cells in protection are crucial elements for vaccine development against malaria.