Background: As antitumoral immunity requires the generation of local immunity directed against tissue proteins, we attempted to recreate within tumors the same environment found within tissues affected by autoimmune diseases (i.e., prolonged cytokine expression). Vaccinia virus (VV) has not been widely used as a cytokine gene therapy vector because of presumed high immunogenicity that would likely make repeated injections impossible; therefore, we modified it by inserting the cytokine gene into the thymidine kinase region, rendering it replication-restricted. The cytokine chosen was human interleukin-2 (IL-2), a molecule with powerful antitumoral effects.Methods: Six patients with the treatment-resistant tumor malignant mesothelioma received intratumoral (i.t.) VV-IL-2 therapy for 12 weeks by injection of 10(7) plaque-forming units of VV-IL-2 per dose. Serial tumor biopsies, sputum, urine, and blood samples were tested for VV-IL-2 mRNA expression; VV culture and T-cell infiltrates were evaluated by immunohistochemistry. Patients and contacts of patients were monitored for changes in VV immunoglobulin G (Igc) levels and clinical evidence of VV infection.Results: VV-IL-2 was not excreted and was only cultured in one patient from tumor biopsies. A T-cell infiltrate was detected in 50% of tumor biopsies. VV-IL-2 mRNA expression was highest on days 1-3 postinjection and was detected for up to 3 weeks after each injection even though VV IgG levels rose in all patients. No significant toxicities, infection of patient contacts, or tumor regressions were observed.Conclusions: I.t. VV-IL-2 administration is safe, is associated with minimal toxicity, and results in i.t. expression of VV-IL-2 for up to 3 weeks postinjection regardless of the level of anti-VV Ige titers generated. This suggests that VV may be a good vector for repeated cytokine gene therapy of solid human cancer.