ENHANCEMENT OF INTERLEUKIN-4-MEDIATED TUMOR-REGRESSION IN ATHYMIC MICE BY IN-SITU RETROVIRAL GENE-TRANSFER

Intratumoral grafting of genetically engineered cells that produce int erleukin-4 (IL-4) has been shown to produce tumor regression as well a s prolong survival of mice harboring intracerebral gliomas. We sought to determine whether retroviral-mediated gene delivery into tumor cell s in situ resulted in enhanced tumor regression by IL-4. Two mouse fib roblast lines were obtained: they both secreted similar levels of IL-4 but one produced a retrovirus vector bearing the IL-4 gene (CRE-MFG-I L-4 cells), whereas the other did not (NIH3T3-IL-4 cells). In mixed tr ansplantation assays in the subcutaneous flanks of athymic mice, CRE-M FG, IL-4 cells were more effective than NIH3T3-IL-4 cells in inhibitin g the growth of rat C6 glioma cells (p < 0.005, ANOVA). Subcutaneous t umors injected with fibroblasts that produced a control retrovirus vec tor without producing IL-4 (CRE-MFG-LacZ cells) did not inhibit subcut aneous tumor growth. An intracranial assay was used to evaluate surviv al of athymic mice harboring intracranial gliomas. Three days after im planting rat C6 glioma cells into the right frontal lobes of athymic m ice, NIH3T3-IL-4 cells (n = 10) or CRE-MFG-IL-4 cells (n = 10) were st ereotactically inoculated into the tumor bed. The average survival of mice treated with CRE-MFG-IL-4 cells was 38 days (+/-2.4, SE), whereas that of mice treated with NIH3T3-IL-4 cells was 31 days (+/-0.8, SE) (p < 0.005, ANOVA; p < 0.001, log-rank analysis). The mean survival of control mice treated with CRE-MFG-LacZ cells was only 25 days (+/-0.7 , SE) (p < 0.0001, compared to CRE-MFG-IL-4-treated mice). Histologic analysis of animals' tumors revealed extensive eosinophilic infiltrate s in mice treated with the IL-4-producing cells (both NIH3T3-IL-4 and CRE-MFG-IL-4) but not in mice treated with IL-4 nonproducing cells (CR E-MFG-LacZ). This infiltrate started to decrease 3 weeks after fibrobl ast implantation, implying that its maintenance was needed for prolong ed tumor regression. This study demonstrates that in situ retroviral m ediated transfer of the IL-4 gene enhances the anticancer effect of ce lls that produce the cytokine.