Smoldering inflammation often increases the risk of progression for malignant tumors and simultaneously matures myeloid dendritic cells (mDCs) for cell-mediated immunity. PolyI:C, a dsRNA analog, is reported to induce inflammation and potent antitumor immune responses via the Toll-like receptor 3/Toll–IL-1 receptor domain-containing adaptor molecule 1 (TICAM-1) and melanoma differentiation-associated protein 5/IFN-β promoter stimulator 1 (IPS-1) pathways in mDCs to drive activation of natural killer cells and cytotoxic T lymphocytes. Here, we found that i.p. or s.c. injection of polyI:C to Lewis lung carcinoma tumor-implant mice resulted in tumor regression by converting tumor-supporting macrophages (Mfs) to tumor suppressors. F4/80⁺/Gr1⁻ Mfs infiltrating the tumor respond to polyI:C to rapidly produce inflammatory cytokines and thereafter accelerate M1 polarization. TNF-α was increased within 1 h in both tumor and serum upon polyI:C injection into tumor-bearing mice, followed by tumor hemorrhagic necrosis and growth suppression. These tumor responses were abolished in TNF-α^−/− mice. Furthermore, F4/80⁺ Mfs in tumors extracted from polyI:C-injected mice sustained Lewis lung carcinoma cytotoxic activity, and this activity was partly abrogated by anti–TNF-α Ab. Genes for supporting M1 polarization were subsequently up-regulated in the tumor-infiltrating Mfs. These responses were completely abrogated in TICAM-1^−/− mice, and unaffected in myeloid differentiation factor 88^−/− and IPS-1^−/− mice. Thus, the TICAM-1 pathway is not only important to mature mDCs for cross-priming and natural killer cell activation in the induction of tumor immunity, but also critically engaged in tumor suppression by converting tumor-supporting Mfs to those with tumoricidal properties.