Granulocyte-macrophage colony-stimulating factor (GM-CSF) secreted by cDNA-transfected tumor cells induces a more potent antitumor response than exogenous GM-CSF

Clinical cancer gene therapy trials have generally focused on the transfer of cytokine cDNA to tumor cells ex vivo and with the subsequent Vaccination of the patient with these genetically altered tumor cells. This approach r esults in high local cytokine concentrations that may account for the effic acy of this technique in animal models. We hypothesized that the expression of certain cytokines by tumor cells would be a superior immune stimulant w hen compared with local delivery of exogenous cytokines. Granulocyte-macrop hage colony-stimulating factor (GM-CSF) cDNA in a nonviral expression vecto r was inserted into MDA-MB-231 (human breast cancer), M21 (human melanoma), B16 (murine melanoma), and P815 (mastocytoma) cells by particle-mediated g ene transfer. The ability of transfected tumor cells to generate a tumor-sp ecific immune response was evaluated in an in vitro mixed lymphocyte-tumor cell assay and in an in vivo murine tumor protection model. Peripheral bloo d lymphocytes cocultured with human CM-CSF-transfected tumor cells were 3- to 5-fold more effective at lysis of the parental tumor cells than were per ipheral blood lymphocytes incubated with irradiated tumor cells and exogeno us human CM-CSF. Mice immunized with murine CM-CSF-transfected irradiated B 16 murine melanoma cells or P815 mastocytoma cells were protected from subs equent tumor challenge, whereas mice immunized with the nontransfected tumo rs and cutaneous transfection of murine GM-CSF cDNA at the vaccination site developed tumors more frequently. The results indicate that GM-CSF protein expressed in human and murine tumor cells is a superior antitumor immune s timulant compared with exogenous GM-CSF in the tumor microenvironment.