ABSTRACT
<p><b>OBJECTIVE</b>To establish an tachyzoite-brachyzoite interconversion system for Toxoplasma gondii RH strain in vitro.</p><p><b>METHODS</b>COS-7 cells were inoculated with purified tachyzoites of T.gondii RH strain and cultured in vitro. The morphology of the cultured cells and parasites was observed and the total cellular RNA extracted on days 1 to 6 following the inoculation for detecting the expression of tachyzoite-specific protein (SAG1) and bradyzoite-specific proteins (BAG1 and SAG2C) using RT-PCR.</p><p><b>RESULTS</b>With the passage of time, the number of parasites in COS-7 cells increased but the proliferation rate was lowered gradually. The intracellular tachyzoites proliferated by means of budding and binary fission, which led to the changes in the alignment of the parasites in the cells from curved pairs, rosette or clustered, and semi-circular patterns to spherical encapsulation-like structures. These changes indicated the gradual transformation of the tachyzoites into bradyzoites. The expressions of the tachyzoite-specific SAG1 gene were detected throughout the 6 days of in vitro culture. The expression of the bradyzoite-specific BAG1 gene had been detected since the second day after the inoculation and SAG2C gene since the fifth day. Alteration of the culture condition resulted in gradual transformation of the bradyzoites into tachyzoites.</p><p><b>CONCLUSION</b>An in vitro tachzoites-bradyzoite interconversion system for T.gondii has been successfully established, which provides the basis for further study of the mechanism of interconversion.</p>