Human CD4⁺ and CD8⁺ T lymphocytes are both cytotoxic to Toxoplasma gondii-infected cells

Studies to determine if Toxoplasma gondii-specific human T cells lyse parasite-infected cells have yielded conflicting results. Furthermore, attempts to obtain human cytotoxic CD8⁺ T lymphocytes have been difficult because of the lack of a reproducible system for their generation. By using paraformaldehyde-fixed, T. gondii-infected peripheral blood mononuclear cells as antigen-presenting cells, we developed a method whereby T. gondii-specific T-cell lines can be reproducibly generated. Six T. gondii-specific T-cell lines were generated from an individual chronically infected with T. gondii. Cytofluorimetric analysis of these lines revealed >99% CD3⁺, 85 to 95% CD3⁺ αβ T-cell-receptor-positive (TCR⁺), 5 to 9% CD3⁺ γδ TCR⁺, 50 to 70% CD4⁺, and 20 to 40% CD8⁺ cells when cells were examined during the first 3 weeks of stimulation and >99% CD3⁺, > 99% CD3⁺ αβ TCR⁺, < 1% CD3⁺ γδ TCR⁺, 20 to 40% CD4⁺, and 60 to 80% CD8⁺ cells when cells were examined between 5 and 11 weeks. Both CD4⁺ and CD8⁺ T cells had remarkable cytotoxic activity against T. gondii-infected target cells (30 to 50% specific Cr release at an effector-to-target ratio of 30:1) but not against uninfected target cells (< 10% at an effector-to-target ratio of 30:1). Cytotoxic activity by the whole T-cell lines was not T. gondii strain specific. Whole T-cell lines were cytotoxic for target cells infected with the C56 and ME49 strains and the RH strain (which was used to infect peripheral blond mononuclear cells). T. gondii-specific T-cell lines displayed the predominant expression of Vβ7 TCR. The CDR3 regions of the Vβ7 TCRs of these T-cell lines showed a striking degree of sequence identity (oligoclonality). T-cell lines obtained by the method reported here can be used to characterize functional activity of T-lymphocyte subsets in humans infected with T. gondii.