Engineering CD8⁺ T cells to deliver interleukin 12 (IL-12) to the tumor site can lead to striking improvements in the ability of adoptively transferred T cells to induce the regression of established murine cancers. We have recently shown that IL-12 triggers an acute inflammatory environment that reverses dysfunctional antigen presentation by myeloid-derived cells within tumors and leads to an increase in the infiltration of adoptively transferred antigen-specific CD8⁺ T cells. Here, we find that local delivery of IL-12 increased the expression of Fas within tumor-infiltrating macrophages, dendritic cells, and myeloid-derived suppressor cells (MDSC), and that these changes were abrogated in mice deficient in IL-12-receptor signaling. Importantly, upregulation of Fas in host mice played a critical role in the proliferation and antitumor activity of adoptively transferred IL-12-modified CD8⁺ T cells. We also observed higher percentages of myeloid-derived cell populations within tumors in Fas-deficient mice, indicating that tumor stromal destruction was dependent on the Fas death receptor. Taken together, these results describe the likely requirement for costimulatory reverse signaling through Fasl on T cells that successfully infiltrate tumors, a mechanism triggered by the induction of Fas expression on myeloid-derived cells by IL-12 and the subsequent collapse of the tumor stroma.