T cell clones raised from chronically infected healthy humans by stimulation with Toxoplasma gondii excretory-secretory antigens cross-react with live tachyzoites: Characterization of the fine antigenic specificity of the clones and implications for vaccine development

Excreted-secreted Ags (ESA) of Toxoplasma gondii (Tg) play an important rol e in the stimulation of the host immune system in both acute and chronic in fections. To identify the parasite Ag(s) involved in the maintenance of T c ell-mediated long term immunity, 40 ESA-specific T cell clones were derived from three chronically infected healthy subjects. All the clones were CD4( +) and recognized both ESA and live tachyzoites in a HLA-DR-restricted mann er. Conversely, CD4(+) tachyzoite-specific T cell clones from the same subj ects proliferated in response to ESA, pointing to shared immunodominant Ags between ESA and Tg tachyzoites, By T cell blot analysis using SDS-PAGE-fra ctionated parasite extracts, the following patterns of reactivity were dete cted. Of 25 clones, 6 recognized Tg fractions in the 24- to 28-kDa range an d proliferated to purified GRA2, 5 reacted with Tg fractions in the 30- to 33-kDa range; and 4 of them proved to be specific for rSAg1, Although surfa ce Ag (SAg1) is not a member of ESA, small amounts of this protein were pre sent in ESA preparation by Western blot. Of 25 clones, 8 responded to Tg fr actions in the 50- to 60-kDa range but not to the 55-kDa recombinant rhoptr ies-2 parasite Ag, and 6 did not react with any Tg fraction but proliferate d in response to either ESA or total parasite extracts. In conclusion, CD4( +) T cells specific for either ESA (GRA2) or SAg1 may be involved in the ma intenance of long term immunity to Tg in healthy chronically infected indiv iduals.