Toxoplasma gondii Inhibits Apoptosis in Infected Cells by Caspase Inactivation and NF-κB Activation Fig. 5. Increased NF-κB and decreased IκBα of mouse spleen cells by T. gondii infection was observed. (A) NF-κB activity was apparent at 120 min in spleen cells treated with AD and infected with T. gondii on EMSA. (B) De- creased cytosolic IκBα, pNF-κB and increased nuclear NF-κ B proteins were detected in spleen cells treated with AD/T. gondii by Western blot. (+): TNF-α (20 ng/mL); (AD): actinomycin-D; (AD + T. gondii): actinomycin-D + T. gondii; (CP): cytosolic proteins; (NP): nuclear proteins. TEM shows inhibition of apoptosis in T. gondii infected spleen cells Condensation of nuclear chromatin and nuclear fragmentation was observed in spleen cells treated with AD (Fig. 6A & B); however, these two events were not observed in spleen cells treated with AD and infected with T. gondii tachyzoites (Fig. 6C & D). In spleen cells infected with T. gondii only, neither nuclear condensation nor fragmentation was observed (Fig. 6E). DISCUSSION Apoptosis, a mechanism of programmed cell death, is a result of both physical and pathological events. Transduction of pro-apoptotic stimuli via signaling pathways results in the activation of the caspase family of cysteine proteases. Bcl-2 family pro-apoptotic proteins promote cell death by inducing mitochondrial damage, cytochrome c release, and subsequent caspase activation, and Fig. 6. Ultrastructural morphology of mouse spleen cells treated with actinomycin-D and T. gondii after cultivation in vitro for 6h. Condensation of nuclear chromatin and fragmentation of the nucleus were observed in spleen cells treated with AD (A & B); however, these events were not observed in cells treated with AD/T. gondii tachyzoites (C & D) or T. gondii only(E). A, B: Spleen cells treated AD (5 μg/mL); C, D: Spleen cells treated with AD and T. gondii; E: Spleen cells treated with T. gondii; F: Normal spleen cells. A, B, E and F: 6000 ×, C: 7000 ×, D: 4000 ×, Bar = 2 μm. The arrows and arrow heads indicate apoptotic spleen cells (A & B) and T. gondii tachyzoites, respectively (C, D & E). the Bcl-2 family of anti-apoptotic proteins inhibit apoptosis by preventing the release of cytochrome c; however, the mechanism by which parasites utilize this method of apoptotic inhibition has not been elucidated. Down-regulation of host cell apoptosis may also provide an explanation for the persistence of intracellular pathogens. T. gondii has several strategies for inhibiting the initiation of the apoptotic cascade. In the present study, we found that apoptosis was not induced in mouse spleen cells infected with T. gondii when com- pared to uninfected controls (data not shown); however, apoptosis was induced in spleen cells by treatment with AD only. One biochemical analysis of apoptosis activity is the presence of oligonu- Yonsei Med J Vol. 47, No. 6, 2006