Effect of GM-CSF and IL-2 co-expression on the anti-tumor immune response.

We evaluated the effect of potential therapeutic genes, GM-CSF and IL-2 respectively, or in combination of both cytokines, on the activation of systemic antitumor responses. CT26 tumor cells were modified to secrete GM-CSF and/or IL-2. The growth rate of the modified tumor cells versus the parental CT26 cells did not show any difference. When we implanted the CT26 tumor cells which secrete either GM-CSF or IL-2, delayed and suppressed tumorigenicity was observed. However, another CT26 cell line which expresses both GM-CSF and IL-2 (CT26/GMCSF/IL-2) did not form any tumor mass in the immunocompetent syngeneic Balb/c mice, showing the potential immune responses. Immunohistochemical examination of the modified tumor masses implanted with the cells expressing GM-CSF or IL-2 showed increased necrosis and infiltration of NK (CD56+) lineage cells and macrophage/monocytes. In the vaccination model, the growth of rechallenged wild-type CT26 was more suppressed int he mice which were injected with GM-CSF or IL-2, however, the wild-type CT26 tumor formed normal tumor mass in the mice vaccinated with CT26/GM-CSF/IL-2 showing acute non-T-cell mediated immune response. As a treatment, we injected those modified tumor cells into the established tumor. There we could find tumor growth suppression by the injection of cytokine-modified CT26 cells, especially by the CT26/GM-CSF/IL-2. In the present study we could induce the eradication of tumorigenicity by the transfection of both GM-CSF and IL-2 genes and a potent role in the growth suppression of an established tumor.