The cell wall skeleton of Mycobacterium bovis BCG has been investigated as an immunopotentiating adjuvant for immunotherapy of malignant tumors via Toll-like receptor (TLR) 2 and TLR4. However, due to its high molecular weight, highly complicated lipoglycan structure, and complicated purification and isolation procedure, its exact structure-activity relationship has not been well established. We have newly isolated the cell wall skeleton from M. bovis BCG Tokyo (SMP-105) and examined the binding of SMP-105 with TLR. It was revealed that highly purified SMP-105 activates the nuclear factor-kB promoter in a TLR2-dependent manner, not a TLR4-dependent manner, using a reporter gene assay system. Peritoneal exudated cells of TLR2 and MyD88 knockout mice severely reduced the induction of tumor necrosis factor-a and interleukin-6 in the presence of SMP-105, whereas cells from TLR4 knockout mice produced similar levels of cytokines to wild-type mice. Dendritic cells and macrophages accumulated in the draining lymph nodes of treated mice. When mice were administered both SMP-105 and mitomycin C-inactivated Lewis lung carcinoma cells simultaneously, interferon-g-producing cells reacting to the tumor were increased distinctly in draining lymph nodes. When C57BL/6 mice, into which splenocytes from OT-I transgenic mice had been transferred, were administered with both SMP-105 and E.G7-OVA, OVA-specific cytotoxic T lymphocytes ( A nticancer immunotherapy is an attractive alternative for the treatment of patients with inoperable cancer, without the application of radiotherapy or chemotherapy, and it is expected that the side effects experienced will be lower than with classical antitumor chemotherapeutics. Although several immunotherapeutic agents have been approved for some types of cancer patients, they are not generally accepted as reliable tools that show sufficient efficacy.(1) One reason for the insufficient results of immunotherapy is the poor immunogenecity of most tumor antigens, (2) and the lack of evidence and theoretical basis of immunotherapeutic agents for regulating the antitumor immune response. Recent progress in innate immunity, in particular the function and role of dendritic cells and Toll-like receptors (TLR), offers a great opportunity to overcome previous problems. Appropriate activation of antigen presentation enhances the immunogenecity of tumor antigens to effectively increase cellular immune responses against tumors.Toll-like receptors are pathogen-associated molecular pattern recognition receptors, and play an important role in induction of the immune response via activation of innate immunity. (3,4) TLR agonists have been shown to enhance immune responses against tumors in both animal models and clinical studies. Poly I-C is a ligand for TLR3 (5) and RIG-I. (6) Imiquimod is a ligand for TLR7 and TLR8, (7) and is approved as an immunotherapeutic drug for basal cell carcinoma. CpG DNA is a ligand for TLR9, (8) and its antitumor activity against various types of tumors has been investigated in several...