Immunization with the RTS,S/AS malaria vaccine induces IFN-γ(+)CD4 T cells that recognize only discrete regions of the circumsporozoite protein and these specificities are maintained following booster immunizations and challenge.

In a Phase 2a trial of the RTS,S/AS vaccine, we described significant association between protection against infection and vaccine-induced CD4 T cells. To determine whether processing of the circumsporozoite protein as a component of the RTS,S particulate antigen yields the same HLA-DR-restricted epitopes as those recognized by CD4 T cells from donors immunized by exposure to attenuated or infectious sporozoites we mapped the specificities of the RTS,S primed CD4 T cells by measuring IFN-γ cultured Elispot responses to pairs of overlapping 15 a.a. peptides that span the protein's C-terminus. Peptide pairs representing the previously described TH2R, T* and CS.T3 epitopes, were immunoprevalent and immunodominant. There was no response to the peptides corresponding to the human thrombospondin homology region. Responses to the CD4 T cell epitopes were restricted by multiple HLA-DR haplotypes. Of note, HLA-DR4 and HLA-DR11 restricted epitopes in the T* region and in the location on the CS protein defined by peptide pair 4, respectively. We conclude that processing of the CS protein derived from the RTS,S antigen leads to the generation of HLA-DR-restricted epitopes that are similar to those identified previously using CD4 T cells from subjects immunized with and protected by attenuated sporozoites or exposed to infectious sporozoites. This may in part account for the protective efficacy of the RTS,S/AS vaccine.

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