TLR4/IFNγ 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 γ (IFNγ) or TLR4. TLR4 and IFNγ 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 IFNγ 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 IFNγ-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/IFNγ/NOS II pathway in immunotherapy was investigated.

Lipid A-induced responses in vivo.

The lipid A analogs used in preclinical studies and clinical trials are not naturally-occurring forms of lipid A; they are synthetic molecules produced to be less toxic than lipid A itself and they do not reproduce the effects of natural lipid A molecules especially in vivo. The responses induced by lipid A analogs are summarized in this chapter: their fate in the blood stream and their toxicity as well as the lipid A tolerance and the tumor immune responses they induce. Lipid A is not found in the mammalian organism under normal circumstances so its use in cancer therapy raises important questions as to its different effects in vivo and its toxicity, particularly in cancer patients. Lipid A has to be injected intravenously (i.v.) to study its effects. Injections of chemically synthesized lipid A in humans and in animals produce sepsis symptoms, such as tachycardia, tachypnea, hyper or hypothermia and leukocytosis or leukopenia. Similar manifestations are observed after injection of purified lipopolysaccharide (LPS), which is why lipid A is usually thought of as the active part of LPS. While lipid A injection is therefore expected to induce reactions similar to septic shock, the lipid A molecules used to treat cancer are not natural forms but analogs, produced by chemical synthesis or genetic engineering, specifically selected for their low toxicity. The in vivo effects of such low-toxicity lipid A analogs are summarized in this chapter.

Nitric oxide-induced resistance or sensitization to death in tumor cells.

This report summarizes the present state of our knowledge pertaining to the NO-induced resistance or sensitization of tumor cell death. The effects of NO and its synergy with members of the TNF family, with cytotoxic drugs, and with ionizing radiations have been investigated. The dual effect of NO-induced resistance or sensitization and the underlying molecular mechanisms are discussed.