PURPOSE Systemic treatment with an anti-ICAM-2 monoclonal antibody (mAb; EOL4G8) eradicates certain established mouse tumors through a mechanism dependent on the potentiation of a CTL-mediated response. However, well-established tumors derived from the MC38 colon carcinoma cell line were largely refractory to this treatment as well as to intratumor injection of a recombinant adenovirus encoding interleukin-12 (IL-12; AdCMVIL-12). We sought to design combined therapy strategies with AdCMVIL-12 plus anti-ICAM-2 mAbs and to identify their mechanism of action. EXPERIMENTAL DESIGN Analysis of antitumor and toxic effects were performed with C57BL/6 mice bearing established MC38 tumors. Anti-ovalbumin T-cell receptor transgenic mice and tumors transfected with this antigen were used for in vitro and in vivo studies on activation-induced cell death (AICD) of CD8(+) T cells. RESULTS Combined treatment with various systemic doses of EOL4G8 mAb plus intratumor injection of AdCMVIL-12 induced complete regression of MC38 tumors treated 7 days after implantation. Unfortunately, most of such mice succumbed to a systemic inflammatory syndrome that could be prevented if IFN-gamma activity were neutralized once tumors had been rejected. Importantly, dose reduction of EOL4G8 mAb opened a therapeutic window (complete cure of 9 of 18 cases without toxicity). We also show that ICAM-2 ligation by EOL4G8 mAb on activated CTLs prevents AICD, thus extending IFN-gamma production. CONCLUSIONS Combination of intratumor gene transfer of IL-12and systemic anti-ICAM-2 mAb display synergistic therapeutic and toxic effects. CTL life extension resulting from AICD inhibition by anti-ICAM-2 mAbs is the plausible mechanism of action.