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Fig. 5. Activity of initiator caspases is inhibited in Toxoplasma-infected cells. Caspase 8 (A,B) and caspase 9 (C,D) activity were measured following treatment with either STS (A,C) or TNF{alpha} (B,D) using the direct cleavage of the fluorescent substrates LEHD-MCA and IETD-MCA, respectively. Activity was determined based on the fluorescence caused by the release of MCA. Based on the fluorometric analysis, infected cells (white bars) exhibit considerably lower levels of activity than uninfected cells (gray bars) for both caspase 8 (A,B) and caspase 9 (C,D) across a broad range of concentrations of STS (A,C) and TNF{alpha} (B,D). In the case of induction of apoptosis by TNF{alpha}, the addition of cycloheximide is noted by a plus (+) sign. STS-treated samples were harvested 4 hours after apoptotic induction. Error bars represent standard deviations using triplicate samples.