Epitopic analysis of the Toxoplasma gondii major surface antigen SAG1

T and B cell epitopes of the major Toxoplasma gondii surface antigen SAG1 were studied following CNBr fragmentation. Three fragments, F1, F2 and F3, were obtained, of 19, 16.5 and 14 kDa, respectively. The positions of F1 F2 and F3 within the SAG1 protein were identified by N-terminal sequence determination. The F1 fragment located on residues 125–269 contains the C-terminus, and the fragment F2 (residues 1–124) is located at the N-terminal region. F3 is a C-terminal peptide about 40 amino acids shorter than the F1 fragment (residues 165–269). Polyclonal antibodies obtained from infected animals or humans and a monoclonal anti-SAG1 antibody did not recognize either the reduced protein or the reduced fragments on immunoblotting. The monoclonal antibody 1E5 did not recognize fragment F1. Mouse IgA and IgG antibodies from infected mouse sera and intestinal secretions, as well as human IgG antibodies, only recognized the whole protein and the F1 fragment. These results suggest that the fragment F1 encompasses all B cell epitopes recognized on the SAG1 protein after infection with the parasite and that the sequence 125–165 is essential for the structural integrity of these B cell epitopes. Murine anti-SAG1 T cell proliferation was observed in SAG1 immunized CBA/J mice (H-2 k) and BALB/c mice (H-2 d), but not in C57BL/6 mice (H-2 b). The three fragments F1, F2 and F3 were able to induce specific proliferation of anti-SAG1 T cells from CBA/J mice, while only the F1 and F2 fragments induced specific blastogenesis of anti-SAG1 T cells from BALB/c mice. These results suggest that at least one epitope recognized by the MHC of the two haplotypes is located on the same fragments, F1 and F2. Three peptides P13 (residues 125–140), P14 (residues 138–154) and P15 (residues 152–167) covering this 40-amino acid sequence of the F1 fragment were synthesized. Specific blastogenesis was observed for all three peptides with CBA/J T cells, and for one peptide (P15) with BALB/c T cells. This suggests that P15 expresses one or more T cell epitopes which can be recognized under two different haplotype restrictions. The approach described here has thus allowed the localization of the immunodominant B cell epitopes of SAG1 to sequence 125–276 and highlighted the requirement of sequence 125–165 for the structural conservation of these epitopes. It has allowed the synthesis of a peptide carrying 1 or 2 T cell epitopes restricted by the MHC haplotypes H-2 k and H-2 d.