Fig. 6. Toxoplasma cannot block activated caspases. Uninfected cells were treated with either 150 nM STS (C) or 25 ng ml^–1 TNF (A,C) to induce apoptosis. Cell extracts of these uninfected apoptotic (UA) cells were mixed with cell extracts of either uninfected non-apoptotic (UNA) cells or infected non-apoptotic (INA) cells in a range of concentrations [shown as UA:UNA (gray bars) or UA:INA (white bars) ratios]. The mixtures of cell extracts were measured for caspase activity using the appropriate fluorogenic substrate for caspase 3 (A), caspase 8 (B) and caspase 9 (C). The line represents an expected decrease of 10% that corresponds to the 10% decrease in apoptotic cell extract (A-C). Error bars represent the standard deviation from triplicate samples. (D) Predictions from this experiment are presented in a hypothetical model. Accordingly, the presence of a specific inhibitor in extracts from infected cells (UA:INA, white bars), would result in a non-linear decrease in caspase activity (dashed line). By contrast, the absence of this specific inhibitor in infected cell extracts, would result in a decrease in activity, reflecting the dilution of the apoptotic cell extract (solid line). The absence of inhibitor in UA:INA (white bars) would result in a profile identical to that found in UA:UNA (gray bars).

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