IRRADIATED SPOROZOITE VACCINE INDUCES HLA-B8-RESTRICTED CYTOTOXIC T-LYMPHOCYTE RESPONSES AGAINST 2 OVERLAPPING EPITOPES OF THE PLASMODIUM-FALCIPARUM SPOROZOITE SURFACE PROTEIN-2

Vaccines designed to protect against malaria by inducing CD8(+) cytoto xic T lymphocytes (CTL) in individuals of diverse HLA backgrounds must contain multiple conserved epitopes from various preerythrocytic-stag e antigens. Plasmodium falciparum sporozoite surface protein 2 (PfSSP2 ) is considered an important antigen for inclusion in such vaccines, b ecause CD8(+) CTL against the P. yoelii SSP2 protect mice against mala ria by eliminating infected hepatocytes. To develop PfSSP2 as a compon ent of malaria vaccines, we investigated the presence of anti-PfSSP2 C TL in two HLA-B8(+) volunteers immunized with irradiated P. falciparum sporozoites and characterized their CTL responses using PfSSP2-derive d 15-amino acid peptides bearing the HLA-B8-binding motif. Peripheral blood mononuclear cells from both volunteers stimulated with recombina nt vaccinia expressing PfSSP2 displayed antigen-specific, genetically restricted, CD8(+) T cell-dependent CTL activity against autologous ta rget cells expressing PfSSP2. Of the five HLA-B8 motif-bearing 15-mers identified in the PfSSP2 sequence, two peptides sharing a 10-amino ac id overlap sensitized HLA-B8-matched target cells from both volunteers for lysis by peptide-stimulated effector;. The CTL activity was HLA-B 8 restricted and dependent on CD8(+) T cells. Analysis of the three sh orter peptides representing HLA-B8 motif-bearing sequences within the two positive peptides for their ability to bind to HLA-B8 in vitro, an d to sensitize target cells for lysis by effecters stimulated with the 15-mers, identified two overlapping HLA-B8-restricted CTL epitopes. A vailable data indicate that the sequence of one CTL epitope is conserv ed and the other is variant among P. falciparum isolates. Circulating activated CTL against the conserved epitope could be directly identifi ed in one of the two volunteers. The identification of two HLA-B8-rest ricted CTL epitopes on PfSSP2 provides data critical to developing an epitope-based anti-liver stage malaria vaccine.