IL-12 triggers a programmatic change in dysfunctional myeloid-derived cells within mouse tumors

Solid tumors are complex masses with a local microenvironment, or stroma, that supports tumor growth and progression. Among the diverse tumor-supporting stromal cells is a heterogeneous population of myeloid-derived cells. These cells are alternatively activated and contribute to the immunosuppressive environment of the tumor; overcoming their immunosuppressive effects may improve the efficacy of cancer immunotherapies. We recently found that engineering tumor-specific CD8 ⁺ T cells to secrete the inflammatory cytokine IL-12 improved their therapeutic efficacy in the B16 mouse model of established melanoma. Here, we report the mechanism underlying this finding. Surprisingly, direct binding of IL-12 to receptors on lymphocytes or NK cells was not required. Instead, IL-12 sensitized bone marrow-derived tumor stromal cells, including CD11b ⁺F4/80 ^hi macrophages, CD11b ⁺MHCII ^hiCD11c ^hi dendritic cells, and CD11b ⁺Gr-1 ^hi myeloid-derived suppressor cells, causing them to enhance the effects of adoptively transferred CD8 ⁺ T cells. This reprogramming of myeloid-derived cells occurred partly through IFN-&γαμμα;. Surprisingly, direct presentation of antigen to the transferred CD8 ⁺ T cells by tumor was not necessary; however, MHCI expression on host cells was essential for IL-12-mediated antitumor enhancements. These results are consistent with a model in which IL-12 enhances the ability of CD8 ⁺ T cells to collapse large vascularized tumors by triggering programmatic changes in otherwise suppressive antigen-presenting cells within tumors and support the use of IL-12 as part of immunotherapy for the treatment of solid tumors.