Activation of an anticancer innate immune response is highly desirable because of its inherent ability to generate an adaptive antitumor T-cell response. However, insufficient safety of innate immune modulators limits clinical use to topical applications. Tolllike receptor 5 (TLR5) agonists are favorably positioned as potential systemic immunotherapeutic agents because of unusual tissue specificity of expression, uniquely safe profile of induced cytokines, and antitumor efficacy demonstrated in a number of animal models. Here, we decipher the molecular and cellular events underlying the metastasis suppressive activity of entolimod, a clinical stage TLR5 agonist that activates NF-κB-, AP-1-, and STAT3-driven immunomodulatory signaling pathways specifically within the liver. Used as a single agent in murine colon and mammary metastatic cancer models, entolimod rapidly induces CXCL9 and -10 that support homing of blood-borne CXCR3-expressing NK cells to the liver predominantly through an IFN-γ signaling independent mechanism. NK cell-dependent activation of dendritic cells is followed by stimulation of a CD8 + T-cell response, which exert both antimetastatic effect of entolimod and establishment of tumor-specific and durable immune memory. These results define systemically administered TLR5 agonists as organ-specific immunoadjuvants, enabling efficient antitumor vaccination that does not depend on identification of tumor-specific antigens.cancer immunotherapy | liver | colorectal cancer | breast cancer | innate immunity R ecent advancements in the field of anticancer immunotherapy have been primarily focused on development of T-cellbased approaches because of recognition of the inherent ability of adaptive immunity to efficiently eradicate neoplastic disease (1, 2). Innate immune responses play important roles in T-cell activation, but their potential relevance for prevention and treatment of cancer remains underappreciated (3-5). Toll-like receptors (TLRs) are gaining attention as potential therapeutic targets capable of stimulating antitumor immunity by initiating innate responses (6) and subsequent adaptive T-cell-based immunity (7). Although proof-ofprinciple for this concept has been demonstrated with agonists of several TLRs (TLR3, -7, and -9) (8), only one, the TLR7 agonist Imiquimod, has been approved for clinical use [however, this is limited to topical treatment of basal cell carcinoma (9)]. The major clinical limitations of many TLR agonists are the risk of dose-limiting toxicities associated with their systemic delivery (10-12) and metastasis stimulation (13-15). Furthermore, some previously investigated TLR agonists are restricted to injection directly into tumor tissue (3,(16)(17)(18), an approach that will likely have limited therapeutic value in cancer patients with metastatic disease.TLR5 is unique among TLRs as a potential target for systemic anticancer immunotherapy. Studies have shown that the only known natural TLR5 agonist, flagellin, flagellin-expressing Salmonella bacteria, and a pharmacolog...

Toll-like receptor 5 (TLR5) is considered an attractive target for anticancer immunotherapy. TLR5 agonists, bacterial flagellin and engineered flagellin derivatives, have been shown to have potent antitumor and metastasis-suppressive effects in multiple animal models and to be safe in both animals and humans. Anticancer efficacy of TLR5 agonists stems from TLR5-dependent activation of nuclear factor-κB (NF-κB) that mediates innate and adaptive antitumor immune responses. To extend application of TLR5-targeted anticancer immunotherapy to tumors that do not naturally express TLR5, we created an adenovirus-based vector for intratumor delivery, named Mobilan that drives expression of self-activating TLR5 signaling cassette comprising of human TLR5 and a secreted derivative of Salmonella flagellin structurally analogous to a clinical stage TLR5 agonist, entolimod. Co-expression of TLR5 receptor and agonist in Mobilan-infected cells established an autocrine/paracrine TLR5 signaling loop resulting in constitutive activation of NF-κB both in vitro and in vivo. Injection of Mobilan into primary tumors of the prostate cancer-prone transgenic adenocarcinoma of the mouse prostate (TRAMP) mice resulted in a strong induction of multiple genes involved in inflammatory responses and mobilization of innate immune cells into the tumors including neutrophils and NK cells and suppressed tumor progression. Intratumoral injection of Mobilan into subcutaneously growing syngeneic prostate tumors in immunocompetent hosts improved animal survival after surgical resection of the tumors, by suppression of tumor metastasis. In addition, vaccination of mice with irradiated Mobilan-transduced prostate tumor cells protected mice against subsequent tumor challenge. These results provide proof-of-concept for Mobilan as a tool for antitumor vaccination that directs TLR5-mediated immune response toward cancer cells and does not require identification of tumor antigens.

Singlet oxygen, hydrogen peroxide, hydroxyl radical and hydrogen peroxide are the reactive oxygen species (ROS) considered most responsible for producing oxidative stress in cells and organisms. Singlet oxygen interacts preferentially with guanine to produce 8-oxo-7,8-dihydroguanine and spiroiminodihydantoin. DNA damage due to the latter lesion has not been detected directly in the DNA of cells exposed to singlet oxygen. In this study, the singlet oxygen-induced lesion was isolated from a short synthetic oligomer after exposure to UVA radiation in the presence of methylene blue. The lesion could be enzymatically excised from the oligomer in the form of a modified dinucleoside monophosphate. Using liquid chromatography-tandem mass spectrometry (LC-MS/MS), the singlet oxygen lesion was detected in the form of modified dinucleoside monophosphates in double-stranded DNA and in the DNA of HeLa cells exposed to singlet oxygen. Pentamer containing the singlet oxygen-induced lesion and an isotopic label was synthesized as an internal standard for quantifying the lesion and served as well as for correcting for losses of product during sample preparation.

A different approach to the measurement of DNA damage has been developed based on the fact that many lesions can be excised from DNA in the form of modified dinucleoside monophosphates. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) is used in conjunction with isotopically labeled internal standards to quantify the lesion. The method has several advantages, including high sensitivity for the detection of dinucleoside monophosphates. The method was applied to the measurement of the 5,6-dihydroxy-5,6-dihydrothymine (thymine glycol) lesion in the DNA of mouse fibroblast cells exposed in culture to various treatments including ionizing radiation, UVC light and buthionine sulfoximine (BSO), an inhibitor of glutathione synthesis. The application of the method to the measurement of other DNA lesions is discussed.

UVC-radiation-induced DNA damage was measured in mouse fibroblast cells using liquid chromatography-tandem mass spectrometry (LC-MS/MS) in conjunction with isotopically labeled internal standards. The thymine glycol and formamide lesions were assayed in the form of modified dinucleoside monophosphates. The 8-oxo-7,8-dihydroguanine lesion was measured as the modified nucleoside. DNA damage in cells treated with tirapazamine was also measured. Tirapazamine is a chemotherapeutic agent that acts via a free radical mechanism. The two agents, UVC radiation and tirapazamine, produce markedly different profiles of DNA damage, reflecting their respective mechanisms of action. Both agents produce significant amounts of thymine glycol and formamide damage, but only the former produced a measurable amount of the 8-oxo-7,8-dihydroguanine lesion. The merits of measuring DNA damage at the dimer level are discussed.

Acute radiation syndrome (ARS) is a major cause of lethality following radiation disasters. A TLR5 agonist, entolimod, is among the most powerful experimental radiation countermeasures and shows efficacy in rodents and non-human primates as a prophylactic (radioprotection) and treatment (radiomitigation) modality. While the prophylactic activity of entolimod has been connected to the suppression of radiation-induced apoptosis, the mechanism by which entolimod functions as a radiomitigator remains poorly understood. Uncovering this mechanism has significant and broad-reaching implications for the clinical development and improvement of TLR5 agonists for use as an effective radiation countermeasure in scenarios of mass casualty resulting from accidental exposure to ionizing radiation. Here, we demonstrate that in contrast to radioprotection, neutrophils are essential for the radiomitigative activity of entolimod in a mouse model of lethal ARS. Neutrophils express functional TLR5 and rapidly exit the bone marrow (BM), accumulate in solid tissues, and release MMP-9 following TLR5 stimulation which is accompanied by an increase in the number of active hematopoietic pluripotent precursors (HPPs) in the BM. Importantly, recombinant MMP-9 by itself has radiomitigative activity and, in the absence of neutrophils, accelerates the recovery of the hematopoietic system. Unveiling this novel TLR5-neutrophil-MMP-9 axis of radiomitigation opens new opportunities for the development of efficacious radiation countermeasures to treat ARS following accidental radiation disasters.

Innate immune modulators can generate an antitumor T cell response. However, significant toxicities associated with systemic administration have significantly limited their clinical use. The natural TLR5 agonist flagellin is unique amonginnate immune modulators because the tissue specificity of TLR5 expression induces a uniquely safe profile of cytokines following systemic TLR5 activation. Entolimod, a pharmacologically optimized flagellin derivative, was initially developed to treat and prevent acute radiation syndrome. Systemic administration of entolimod also showed antitumor effects in multiple preclinical mouse models mimicking clinically occurring liver metastasis. Entolimod suppresses liver metastasis through activation of NF-κB-, AP-1-, and STAT-3-driven immunomodulatory pathways in hepatocytes and a highly coordinated CD4+ T cell independent NK-DC-CD8+ T cell response. Although these studies characterized entolimod as a liver tropic immunotherapy, entolimod also suppresses spontaneous mammary lung metastasis. In contrast to the liver metastatic model, CD4+ T cells are required for the antitumor NK and CD8+ T cell response, indicating that the etiology of the cancer may explain the difference in the therapeutic effects ofTLR5 agonists. These results further strengthen that TLR5 agonists are a safe, effective and broadly applicable immunotherapeutic agent against metastases, which are currently a major cause of cancer-associated mortality. Recent completion of a phase I trial of entolimod in patients with advanced metastatic solid tumors has provided the rationale to test that the efficacy demonstrated in animal cancer models can be translated into immunotherapy of human tumors.

The increased risk of exposure to life-threating nuclear and radiation emergencies warrants the development of FDA-approved medical radiation countermeasures (MRC) as radio mitigators, which can be used after exposure to total body irradiation. Currently, Neupogen and Neulasta are the only FDA-approved radio mitigators but are unfortunately limited by adverse side effects and the need for additional medical supportive care. Bacterial flagellin, the natural agonist of Toll-like receptor (TLR) 5, and apharmacologically optimized derivative entolimod developed by us have powerful activity as a single agent MRC in rodents and non-human primates. Currently entolimod is being developed as a MRC under the FDA’s Animal Efficacy Rule, which guides development of drugs for which efficacy testing in humans would be unethical. The lack of induction of cytokine storm post systemically administered entolimod was demonstrated in rodents, non-human primates, and Phase I clinical trials involving nearly 200 subjects. Entolimod stimulates TLR5 on hepatocytes followed by activation of immunoregulatory signaling pathways, production of pro-inflammatory cytokines, and rapid recruitment of immune cells to the liver. Neutrophils are among the first immune cells that are rapidly recruited to both non-lymphoid and lymphoid tissues post-entolimod. We found that this entolimod-elicited neutrophil response was essential to mitigating radiation damage by promoting the recovery of hematopoietic stem cells in the bone marrow. This work underscores the underappreciated importance of TLR5-elicited neutrophils in promoting there generation of hematopoiesis following total body irradiation.