TLR4/IFN gamma pathways induce tumor regression via NOS II-dependent NO and ROS production in murine breast cancer models

Toll-like receptor (TLR) 4 agonists have emerged as a new group of molecules used for cancer therapy. They have been exploited to enhance the immunogenicity of current chemotherapeutic regimens. However, their effects on cancer cells remain elusive. Here, we showed that a TLR4 agonist, namely a synthetic lipid A analog (ALA), OM-174, exhibits antitumor effects in several mammary tumor mouse models. We also showed that immune components are involved in such effects, as attested to by the failure of ALA to induce tumor regression or an increase of animal survival in mice knocked-out for interferon gamma (IFN gamma) or TLR4. TLR4 and IFN gamma receptor (INFR2) expressed by cancer cells are involved in the antitumor efficacy of ALA since this last did not inhibit tumor growth in mice bearing a tumor but lacking TLR4 or IFN gamma receptor 2 (IFNR2). Mechanistic investigations revealed that nitric oxide (NO), superoxide and peroxynitrite produced by uncoupling of inducible NO synthase (NOS II) in cancer cells are key mediators of ALA and IFN gamma-mediated tumor growth inhibition. We present here a comprehensive picture of tumor cell death induction, in vivo and in vitro, by immunotherapy and for the first time the involvement of the TLR4/IFN gamma/NOS II pathway in immunotherapy was investigated.