Apoptosis and S Phase of the Cell Cycle in BeWo Trophoblastic and HeLa Cells are Differentially Modulated by Toxoplasma gondii Strain Types

Transplacental transmission of Toxoplasma gondii causes congenital toxoplasmosis, one of the most severe forms of infection. The ability of the parasite to survive intracellularly largely depends on the blocking of different proapoptotic signaling cascades of the host cells. During pregnancy, however, alterations in the incidence of apoptosis are associated with abnormal placental morphology and function. The aim of this study was to evaluate the incidence of apoptosis and cell proliferation in trophoblastic (BeWo cell line) and uterine cervical (HeLa cell line) cells infected with a highly virulent RH strain or a moderately virulent ME49 strain of T. gondii. BeWo and HeLa cells were infected with RH or ME49 tachyzoites (2:1 and 5:1; parasite:cell) or medium alone (control). After 2 h, 6 h and 12 h of incubation, cells were fixed in 10% formalin and analyzed by immunohistochemistry to determine the apoptosis (expression of cytokeratin 18 neo-epitope – clone M30) and cell in S phase (expression of proliferating cell nuclear antigen – PCNA) indices. RH strain-infected BeWo and HeLa cells showed a lower apoptosis index than non-infected controls, whereas a higher apoptosis index was found in ME49 strain-infected cells compared to controls. In addition, RH-infected cells displayed lower apoptosis index than ME49-infected cells, even though active caspase-3 was detected in both cell types infected with either RH or ME49 strains as well in non-infected cells in all analyzed times of infection. Also, the cell S phase indices were higher in ME49 strain-infected BeWo and HeLa cells as compared to non-infected controls and RH strain-infected cells. These results indicate that RH and ME49 strains of T. gondii possess opposing mechanism of interference in apoptosis and cell cycle S phase of both BeWo and HeLa cells and these differences can be associated to evasion strategies of the parasite to survive inside the host cells.