Apoptosis plays crucial roles for the outcome of infection with various infectious agents. The host's apoptotic program may be modulated after infection in order to combat the pathogen or to restrict the immune response. In addition, distinct microorganisms alter the apoptotic program of the host in order to meet the requirements for their further distribution. The activation of caspases (i.e., cysteine proteases with specificity for aspartic acid residues) preludes the disassembly of the cell in response to apoptosis-inducing stimuli. This depends on the proteolytic cleavage of inactive proforms into catalytically active subunits. Analyses of the proteolysis and the enzymatic activity of caspases therefore represent valuable tools to study apoptotic programs during infection. The apicomplexan parasite Toxoplasma gondii interferes with the caspase cascade of its host cell in order to facilitate intracellular survival. The modulation of caspase activation by T. gondii is determined by SDS-PAGE and immunoblotting with caspase-specific antibodies. Furthermore, the impact of the parasite on caspase activity is fluorimetrically determined by measuring the cleavage of caspase-specific substrate analogues.

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