Increasing infiltration and activation of CD8+ tumor-infiltrating lymphocytes after eliminating immune suppressive granulocyte/macrophage progenitor cells with low doses of interferon ? plus tumor necrosis factor ?

By secreting granulocyte/macrophage colony-stimulating factor (GM-CSF), metastatic Lewis lung carcinoma (LLC-LN7) tumors induce the appearance of myelopoiesis-associated immune-suppressor cells that resemble granulocytic-macrophage (GM) progenitor cells. The presence of these GM-suppressor cells in mice bearing LLC-LN7 tumors was associated with a reduced capacity of splenic T cells to proliferate in response to interleukin-2 (IL-2). Administration of low doses of 100 U interferon gamma (IFN gamma) plus 10 U tumor necrosis factor alpha (TNF alpha) to the tumor bearers, a combination treatment that we previously showed to diminish the presence of GM-suppressor cells synergistically, restored proliferative responsiveness of the splenic T cells to IL-2. These LLC-LN7-bearing mice were also examined for whether cells that phenotypically resemble GM-progenitor cells (ER-MP12+ cells) infiltrate the tumor mass. ER-MP12+ cells composed approximately 10% of the cells isolated from dissociated tumors of mice that had been treated with placebo or with either IFN gamma or TNF alpha alone, but IFN gamma/TNF alpha therapy markedly reduced the number of tumor-infiltrating ER-MP12+ suppressor cells. The IFN gamma/TNF alpha treatment to eliminate GM-suppressor cells and restore T cell responsiveness to IL-2 was next coupled with low dose IL-2 therapy (100 U twice daily). Addition of IL-2 to the treatment regimen did not significantly influence the effectiveness of the IFN gamma/TNF alpha treatment in eliminating GM-suppressor cells from the LLC-LN7 tumor mass. However, inclusion of IL-2 with the IFN gamma/TNF alpha treatment regimen enhanced the CD8+, but not the CD4+, cell content within the tumor, and diminished the number of metastatic lung nodules within the mice. When these tumors were excised, dissociated, and bulk-cultured with a low dose of IL-2, an increased level of cytotoxic T lymphocyte (CTL) activity was generated in the TIL cultures from mice that had received IFN gamma/TNF alpha plus IL-2 treatments. A lesser but detectable level of CTL activity was generated in TIL cultures from mice that were treated with only IFN gamma/TNF alpha, while no CTL activity was generated in tumor cultures from mice receiving only placebo or low-dose IL-2. These results suggest the effectiveness of IFN gamma plus TNF alpha therapy in restoring IL-2 responsiveness in mice bearing GM-suppressor cell-inducing tumors and at enhancing both the intratumoral CD8+ cell content and the generation of CTL activity in bulk cultures of these tumors.