Objective     To study the effect of Toxoplasma gondii(T. gondii) RH strain on mouse placental trophoblastic cells in vitro and the mechanism of Toxoplasma gondii RH strain underlying pathological pregnancy.     Methods    Mouse placental trophoblastic cells were inoculated into different cell culture plates and divided into control group and experimental group. Trophoblastic cells in control group were incubated in DMEM and those in experimental group were i

ncubated with tachyzoites at a concentration of 2×10

6

/ml

4×10

6

/ml

and 8×10

6

/ml for 2

4 and 8 h

respectively. Apoptosis of trophoblastic cells was detected by FCM  with Annexin-V-FITC/PI staining. Changes in cell morphology were observed under fluorescence microscope. Expression or activity of apoptosis-related Bax and Bcl-2 proteins in trophoblastic cells was detected by Western blotting.     Results    FCM showed that the apoptosis rate of trophoblastic cells infected with T. gondii RH strain was decreased in a dose-dependent manner and was lower in experimental group than in control group

with the lowest apoptosis rate of (15.81±0.19)% observed in experimental group 8 h after the concentration of tachyzoites was increased to 8×10

6

/ml. Typical early apoptotic cells were found under fluorescence microscope with their membrane green stained and their nuclei not stained. Western blotting showed that the expression levels of Bcl-2 and Bax proteins was higher in experimental group than in control group（P0.05）.     Conclusion    T. gondii RH strain can resist apoptosis of mouse placental trophoblastic cells in vitro by down-regulating the Bax expression and up-regulating the Bcl-2 expression.