Complete protection against lethal Toxoplasma gondii infection in mice immunized with a plasmid encoding the SAG1 gene

Infection with the protozoan parasite Toxoplasma gondii is transmitted to h umans from infected animals by tissue cysts and oocysts excreted by cats. I mmunization with inactivated parasites or recombinant proteins has at best shown partial protection. We constructed a plasmid expressing the SAG1 surf ace antigen of T, gondii, p1tPASAG1, and showed that animals immunized with the plasmid produce anti-SAG1 antibodies which recognize the native SAG1. Mice immunized with p1tPASAG1 showed 80 to 100% protection against challeng e with the non-cyst-producing, virulent RH isolate, compared to an 80% mort ality in mice immunized with empty plasmid. which is the greatest efficacy of any vaccine against T. gondii produced so far. The SAG1 molecule was ana lyzed for potential cytotoxic T-lymphocyte (CTL) epitopes, and four peptide s with the best fit were synthesized. The ability of the peptides to stimul ate gamma interferon production by CD8(+) T cells from p1PASAG1-immunized m ice was tested in an ELISPOT assay; and one new CTL epitope was identified. Adoptive transfer of CD8(+) T cells from p1tPASAG1-immunized to naive mice showed partial protection. In conclusion, DNA vaccination with p1tPASAG1 g ave effective protection in mice against T. gondii infection and the protec tion could be adoptively transferred by purified CD8(+) T cells.