Irradiated sporozoite vaccine induces HLA-B8-restricted cytotoxic T lymphocyte responses against two overlapping epitopes of the Plasmodium falciparum sporozoite surface protein 2

Vaccines designed to protect against malaria by inducing CD8+ cytotoxic T lymphocytes (CTL) in individuals of diverse HLA backgrounds must contain multiple conserved epitopes from various preerythrocytic-stage antigens. Plasmodium falciparum sporozoite surface protein 2 (PfSSP2) is considered an important antigen for inclusion in such vaccines, because CD8+ CTL against the P. yoelii SSP2 protect mice against malaria by eliminating infected hepatocytes. To develop PfSSP2 as a component of malaria vaccines, we investigated the presence of anti- PfSSP2 CTL in two HLA-B8+ volunteers immunized with irradiated P. falciparum sporozoites and characterized their CTL responses using PfSSP2-derived 15-amino acid peptides bearing the HLA-B8-binding motif. Peripheral blood mononuclear cells from both volunteers stimulated with recombinant vaccinia expressing PfSSP2 displayed antigen-specific, genetically restricted, CD8+ T cell-dependent CTL activity against autologous target cells expressing PfSSP2. Of the five HLA-B8 motif- bearing 15-mers identified in the PfSSP2 sequence, two peptides sharing a 10-amino acid overlap sensitized HLA-B8-matched target cells from both volunteers for lysis by peptide-stimulated effectors. The CTL activity was HLA-B8 restricted and dependent on CD8+ T cells. Analysis of the three shorter peptides representing HLA-B8 motif-bearing sequences within the two positive peptides for their ability to bind to HLA-B8 in vitro, and to sensitize target cells for lysis by effectors stimulated with the 15-mers, identified two overlapping HLA-B8- restricted CTL epitopes. Available data indicate that the sequence of one CTL epitope is conserved and the other is variant among P. falciparum isolates. Circulating activated CTL against the conserved epitope could be directly identified in one of the two volunteers. The identification of two HLA-B8-restricted CTL epitopes on PfSSP2 provides data critical to developing an epitope-based anti-liver stage malaria vaccine.