Certain anti-neoplastic agents at subtoxic doses may exert immunomodulatory effects, which alter the expression of specific tumor cell surface molecules. We reasoned that potential increases in tumor cell surface markers, such as those important for facilitating effector-target contact, as well as triggering cell death pathways, might then improve antigen (Ag)-specific T-cell-mediated tumor cytolysis. Here, in a human colon carcinoma cell model in vitro, we examined whether the anti-neoplastic agents 5-fluorouracil (5-FU), CPT-11 or cisplatin (CDDP) could upregulate the expression of specific tumor cell surface markers, which may then enhance productive lytic interactions between CD8+ CTL and Ag-bearing tumor cells. Based on our earlier studies, IFN-gamma treatment was included as a control for sensitization to CTL-mediated lysis. Pretreatment of the SW480 primary colon carcinoma cell line with IFN-gamma, 5-FU, CPT-11 or CDDP enhanced ICAM-1 and Fas expression, resulting in Ag-specific CTL-mediated lysis involving Fas-dependent and -independent mechanisms. In contrast, pretreatment of the SW620 metastatic isolate, derived from the same patient, with IFN-gamma, CPT-11 or CDDP, but not 5-FU, enhanced ICAM-1 expression, resulting in Ag-specific CTL-mediated lysis via Fas-independent mechanisms only. Flow cytometric-based assays were then developed to measure the effects of drug treatment on caspase signaling and apoptosis incurred by tumor targets after interaction with CTL. We found that the lytic enhancement caused by drug treatment of SW480 or SW620 targets was accompanied by an increase in caspase-3-like protease activity. A peptide-based caspase inhibitor abrogated CTL-mediated apoptosis, suggesting that "chemomodulation" involved regulation of the caspase pathway. These results revealed for the first time an important role for components of the caspase pathway, such as caspase-3-like proteases, in the sensitization of human colon carcinoma cells by anti-neoplastic agents to Ag-specific CTL. Thus, certain anti-neoplastic agents may display unique immunoregulatory properties that facilitate human colon carcinoma death by engaging the lytic capacity of Ag-specific CTL, which may have implications for chemoimmunotherapy strategies.