Treating tumor-bearing mice with low-dose gamma-interferon plus tumor necrosis factor alpha to diminish immune suppressive granulocyte-macrophage progenitor cells increases responsiveness to interleukin 2 immunotherapy.

Production of granulocyte-macrophage (GM) colony-stimulating factor by murine metastatic Lewis lung carcinoma cells (LLC-LN7) increases the number and distribution of GM progenitor cells that are suppressive to T cell responsiveness to interleukin 2 (IL-2). The presence of these GM suppressor cells can be diminished by treatment of LLC-LN7-bearing mice with low doses of 100 units IFN-gamma plus 10 units tumor necrosis factor alpha (TNF-alpha). The aim of this study was to determine whether treatment of LLC-LN7-bearing mice with IFN-gamma/TNF-alpha to diminish GM suppressor cell presence would increase the responsiveness to IL-2 immune stimulatory therapy (100-1000 IU, twice daily for 5 days). Treatment first with IFN-gamma/TNF-alpha and then also with low dose IL-2 increased both the numbers of CD4+ and CD8+ cells within the tumor and the levels of their expression of the p55 IL-2 receptor. These intratumoral T cells also had an increased cytolytic capacity toward autologous tumor cells and an increased capacity to proliferate and secrete IL-2. Such effects were observed to a lesser extent in mice that were treated with either IFN-gamma/TNF-alpha alone or with low doses of IL-2 only. The combination treatment regimen of IFN-gamma/TNF-alpha and then IL-2 was also significantly more effective at reducing the size of the primary tumor and the formation of metastatic lung nodules than were the individual treatments. These results show that treatment to minimize the presence of GM suppressor cells enhances the effectiveness of IL-2 to stimulate anti-tumor immune responses and to diminish tumor growth and metastasis.