[Interferon-γ Induced by Toxoplasma gondii Excreted/Secreted Antigens Promotes Apoptosis of CD4+CD25+ Regulatory T Cells].

Objective: To investigate the effect of excreted/secreted antigens (ESAs) from Toxoplasma gondii RH strain and TgCtwh3 strain on apoptosis of CD4+CD25+ regulatory T cells and interferon-γ (IFN-γ) secretion.
Methods: ESAs of Toxoplasma gondii RH strain and TgCtwh3 strain were prepared. Splenic mononuclear cells were isolated from C57BL/6 mice and randomly divided into RH ESA group（2×106 cells/well with addition of 10 μg/ml RH ESA）, TgCtwh3 ESA group （2×106 cells/well with addition of 10 μg/ml TgCtwh3 ESA） and control group（2×106 cells/well with addition of 10 μg/ml ovalbumin）. Flow cytometry was performed to examine the early apoptosis of CD4+CD25+ regulatory T cells after treatment for 48 h and 72 h. ELISA was conducted to determine the level of IFN-γ in the supernatant after treatment for 72 h. In another experiment, the 3 groups of splenic mononuclear cells were added with 10 μg/ml anti-IFN-γ antibody for 72 h and flow cytometry was performed to examine the early apoptosis of CD4+CD25+ regulatory T cells. Meanwhile, splenic mononuclear cells from IFN-γ knockout and wild-type C57BL/6 mice were also divided into the above-described 3 groups, and flow cytometry was performed to examine the early apoptosis of CD4+CD25+ regulatory T cells after treatment for 72 h.
Results: The concentrations of RH ESA and TgCtwh3 ESA were 0.54 mg/ml and 2.14 mg/ml, respectively. Flow cytometry showed that the early apoptosis rate of CD4+CD25+ regulatory T cells in the RH ESA group and the TgCtwh3 ESA group after 48 h treatment was （12.90±1.26）% and （9.71±1.04）%, respectively （P＜0.05）, both significantly higher than that in control group （4.48±0.48）% （P<0.01） . The early apoptosis rate of CD4+CD25+ regulatory T cells after 72 h in the RH ESA group was（15.21±1.11）%, significantly higher than that in the TgCtwh3 ESA group[（11.02±0.92）%] （P<0.05） and the control group[（10.10±1.49）%]（P<0.01）. ELISA showed that the level of interferon-γ in the RH ESA group and the TgCtwh3 ESA group after 72 h was（4 764.0±118.7） pg/ml and （3 629.0±33.6） pg/ml, respectively （P＜0.01）, both significantly higher than that in the control［（679.4±30.6） pg/ml］（P<0.01）. Flow cytometry revealed lower early apoptosis rate of CD4+CD25+ regulatory T cells in the RH ESA group added with anti-IFN-γ antibody［（10.44±1.44）%］ compared with that without the addition of the antibody［（14.96±0.83）］（P<0.05）. But this difference was not observed for the TgCtwh3 ESA group. Moreover, the RH ESA-induced apoptosis rate of regulatory T cells from IFN-γ knockout mice［（10.64±0.55）%］ was significantly lower than that from the wild-type mice ［（15.21±1.11）%］（P<0.01）. But this difference was not found for the TgCtwh3 ESA treatment.
Conclusion: T. gondii RH ESA induces apoptosis of CD4+CD25+ regulatory T cells and IFN-γ secretion, and these effects are stronger than those of TgCtwh3 ESA. The T. gondii ESA-induced IFN-γ stimulates generation of anti-Toxoplasma immunity during acute Toxoplasma infection through mediation of regulatory T cell apoptosis.