EPITOPES RECOGNIZED BY HUMAN T-LYMPHOCYTES IN THE ROP2 PROTEIN ANTIGEN OF TOXOPLASMA-GONDII

The ROP2 protein of Toxoplasma gondii possesses immunological and biol ogical properties which have led to its proposal as a vaccine candidat e. To identify epitopes recognized by human T cells in the ROP2 antige n, we submitted the sequence of this protein to three reported T-cell epitope prediction algorithms. Three sequences that were predicted by all three methods were selected (sequences 197 to 216, 393 to 410, and 501 to 524), and the corresponding peptides were synthesized. The pep tides were first tested in a proliferation assay with a DPw4-restricte d. ROP2-specific human T-cell clone, and the peptide corresponding to residues 197 to 216 was shown to stimulate the T-cell clone. The three peptides were further tested in proliferation assays with peripheral blood mononuclear cells from a panel of T. gondii-seropositive and -se ronegative individuals. We found that cells from a high proportion of the seropositive donors (64%) recognized at least one of the three pep tides. The most frequently recognized ones were peptides 197 to 216 (4 5%) and 501 to 524 (36%). None of the seronegative donors responded to any peptide. These results show that the ROP2 antigen of T. gondii co ntains T-cell epitopes recognized by a high percentage of the immune p opulation and further strengthen its potential as a vaccine candidate.