NK1.1+ cells mediate the antitumor effects of a dual Toll-like receptor 7/8 agonist in the disseminated B16-F10 melanoma model

Toll-like receptor 7/8 agonist R848 alters the immune tumor microenvironment and enhances SBRT-induced antitumor efficacy in murine models of pancreatic cancer

BACKGROUND

Stereotactic body radiotherapy (SBRT) has been increasingly used as adjuvant therapy in pancreatic ductal adenocarcinoma (PDAC), and induces immunogenic cell death, which leads to the release of tumor antigen and damage-associated molecular patterns. However, this induction often fails to generate sufficient response to overcome pre-existing tumor microenvironment (TME) immunosuppression. Toll-like receptor (TLR) 7/8 ligands, such as R848, can amplify the effect of tumor vaccines, with recent evidence showing its antitumor effect in pancreatic cancer by modulating the immunosuppressive TME. Therefore, we hypothesized that the combination of R848 and SBRT would improve local and systemic antitumor immune responses by potentiating the antitumor effects of SBRT and reversing the immunosuppressive nature of the PDAC TME.

METHODS

Using murine models of orthotopic PDAC, we assessed the combination of intravenous TLR7/8 agonist R848 and local SBRT on tumor growth and immune response in primary pancreatic tumors. Additionally, we employed a hepatic metastatic model to investigate if the combination of SBRT targeting only the primary pancreatic tumor and systemic R848 is effective in controlling established liver metastases.

RESULTS

We demonstrated that intravenous administration of the TLR7/8 agonist R848, in combination with local SBRT, leads to superior tumor control compared with either treatment alone. The combination of R848 and SBRT results in significant immune activation of the pancreatic TME, including increased tumor antigen-specific CD8+ T cells, decreased regulatory T cells, and enhanced antigen-presenting cells maturation, as well as increased interferon gamma, granzyme B, and CCL5 along with decreased levels of interleukin 4 (IL-4), IL-6, and IL-10. Importantly, the combination of SBRT and systemic R848 also resulted in similar immunostimulatory changes in liver metastases, leading to improved metastatic control. CD8+ T cell depletion studies highlighted the necessity of these effector cells at both the local and hepatic metastatic sites. T cell receptor (TCR) clonotype analysis indicated that systemic R848 not only diversified the TCR repertoire but also conditioned the metastatic foci to facilitate entry of CD8+ T cells generated by SBRT therapy.

CONCLUSIONS

These findings suggest that systemic administration of TLR7/8 agonists in combination with SBRT may be a promising avenue for metastatic PDAC treatment.

Chemical reagents modulate nucleic acid-activated toll-like receptors

Nucleic acid-mediated interferon signaling plays a pivotal role in defense against microorganisms, especially during viral infection. Receptors sensing exogenous nucleic acid molecules are localized in the cytosol and endosomes. Cytosolic sensors, including cGAS, RIG-I, and MDA5, and endosome-anchored receptors are toll-like receptors (TLR3, TLR7, TLR8, and TLR9). These TLRs share the same domain architecture and have similar structures, facing the interior of endosomes so their binding to nucleic acids of invading pathogens via endocytosis is possible. The correct function of these receptors is crucial for cell homeostasis and effective response against pathogen invasion. A variety of endogenous mechanisms modulates their activities. Nevertheless, naturally occurring mutations lead to aberrant TLR-mediated interferon (IFN) signaling. Furthermore, certain pathogens require a more robust defense against control. Thus, manipulating these TLR activities has a profound impact. High-throughput virtual screening followed by experimental validation led to the discovery of numerous chemicals that can change these TLR-mediated IFN signaling activities. Many of them are unique in selectivity, while others regulate more than one TLR due to commonalities in these receptors. We summarized these nucleic acid-sensing TLR-mediated IFN signaling pathways and the corresponding chemicals activating or deactivating their signaling.

A novel miR1983-TLR7-IFNβ circuit licenses NK cells to kill glioma cells, and is under the control of galectin-1

Although pharmacological stimulation of TLRs has anti-tumor effects, it has not been determined whether endogenous stimulation of TLRs can lead to tumor rejection. Herein, we demonstrate the existence of an innate anti-glioma NK-mediated circuit initiated by glioma-released miR-1983 within exosomes, and which is under the regulation of galectin-1 (Gal-1). We demonstrate that miR-1983 is an endogenous TLR7 ligand that activates TLR7 in pDCs and cDCs through a 5'-UGUUU-3' motif at its 3' end. TLR7 activation and downstream signaling through MyD88-IRF5/IRF7 stimulates secretion of IFN-β. IFN-β then stimulates NK cells resulting in the eradication of gliomas. We propose that successful immunotherapy for glioma could exploit this endogenous innate immune circuit to activate TLR7 signaling and stimulate powerful anti-glioma NK activity, at least 10-14 days before the activation of anti-tumor adaptive immunity.

Toll-Like Receptor Agonists and Radiation Therapy Combinations: An Untapped Opportunity to Induce Anticancer Immunity and Improve Tumor control

The premise that therapies targeting immune checkpoints can enhance radiation therapy (RT)-induced antitumor immunity is being explored rigorously in the preclinical setting, and early clinical trials testing this hypothesis are beginning to report. Although such approaches might prove efficacious in certain settings, it is likely that many tumor types, particularly those that have a deeply immune-suppressed microenvironment with little or no T cell infiltration, will require alternative approaches. Thus, there is now considerable drive to develop novel immune modulatory therapies that target other areas of the cancer immunity cycle. Toll-like receptors (TLRs) are expressed on sentinel immune cells and play a key role in the host defense against invading pathogens. Innate sensing via TLR-mediated detection of pathogen-derived molecular patterns can lead to maturation of antigen-presenting cells and downstream activation of adaptive immunity. After demonstrating promising efficacy in preclinical studies, drugs that stimulate TLR have been approved for use clinically, albeit to a limited extent. There is a growing body of preclinical evidence that novel agonists targeting TLR3, TLR7/8, or TLR9 in combination with RT might lead to enhanced antitumor immunity. Mechanistic studies have revealed that TLR agonists enhance dendritic cell-mediated T cell priming after RT, in some cases leading to the generation of systemic antitumor immunity and immune memory. In this report, we describe results from preclinical studies that advocate the strategy of combining RT with TLR agonists, discuss reported mechanisms of action, and explore the exciting opportunities of how this approach may be successfully translated into clinical practice.

Harnessing the immune system to fight cancer with Toll-like receptor and RIG-I-like receptor agonists

Cancer immunotherapy has come of age with the advent of immune checkpoint inhibitors. In this article we review how agonists for receptors of the innate immune system, the Toll-like receptors and the RIG-I-like receptors, impact anticancer immune responses. Treatment with these agonists enhances the activity of anticancer effector cells, such as cytotoxic T cells and NK cells, and at the same time blocks the activity of immunosuppressive cell types such as regulatory T cells and myeloid-derived suppressor cells. These compounds also impact the recruitment of immune cells to the tumor. The phenomena of pattern-recognition receptor tolerance and reprogramming and their implications for immunotherapy are discussed. Finally, novel delivery systems that target the immune-stimulating drugs to the tumor or the tumor-draining lymph nodes to enhance their efficacy and safety are presented.

Respiratory Influenza A Virus Infection Triggers Local and Systemic Natural Killer Cell Activation via Toll-Like Receptor 7

The innate immune system senses influenza A virus (IAV) through different pathogen-recognition receptors including Toll-like receptor 7 (TLR7). Downstream of viral recognition natural killer (NK) cells are activated as part of the anti-IAV immune response. Despite the known decisive role of TLR7 for NK cell activation by therapeutic immunostimulatory RNAs, the contribution of TLR7 to the NK cell response following IAV infection has not been addressed. We have analyzed lung cytokine responses as well as the activation, interferon (IFN)-γ production, and cytotoxicity of lung and splenic NK cells following sublethal respiratory IAV infection in wild-type and TLR7ko mice. Early airway IFN-γ levels as well as the induction of lung NK cell CD69 expression and IFN-γ production in response to IAV infection were significantly attenuated in TLR7-deficient hosts. Strikingly, respiratory IAV infection also primed splenic NK cells for IFN-γ production, degranulation, and target cell lysis, all of which were fully dependent on TLR7. At the same time, lung type I IFN levels were significantly reduced in TLR7ko mice early following IAV infection, displaying a potential upstream mechanism of the attenuated NK cell activation observed. Taken together, our data clearly demonstrate a specific role for TLR7 signaling in local and systemic NK cell activation following respiratory IAV infection despite the presence of redundant innate IAV-recognition pathways.

Polymer-based nanoparticles loaded with a TLR7 ligand to target the lymph node for immunostimulation

Small-molecule agonists for the Toll-like receptors (TLR) 7 and 8 are effective for the immunotherapy of skin cancer when used as topical agents. Their systemic use has however been largely unsuccessful due to dose-limiting toxicity. We propose a polymer-based nanodelivery system to target resiquimod, a TLR7 ligand, to the lymph node in order to focus the immunostimulatory activity and to prevent a generalized inflammatory response. We demonstrate successful encapsulation of resiquimod in methoxypoly(ethylene glycol)-b-poly(DL-lactic acid) (mPEG-PLA) and mixed poly(DL-lactic-co-glycolic acid) (PLGA)/mPEG-PLA nanoparticles. We show that these particles are taken up mainly by dendritic cells and macrophages, which are the prime initiators of anticancer immune responses. Nanoparticles loaded with resiquimod activate these cells, demonstrating the availability of the immune-stimulating cargo. The unloaded particles are non-inflammatory and do not have cytotoxic activity on immune cells. Following subcutaneous injection in mice, mPEG-PLA and PLGA/mPEG-PLA nanoparticles are detected in dendritic cells and macrophages in the draining lymph nodes, demonstrating the targeting potential of these particles. Thus, polymer-based nanoparticles represent a promising delivery system that allows lymph node targeting for small-molecule TLR7 agonists in the context of systemic cancer immunotherapy.

Anti-tumor Activity of Toll-Like Receptor 7 Agonists

Toll-like receptors (TLRs) are a class of pattern recognition receptors that play a bridging role in innate immunity and adaptive immunity. The activated TLRs not only induce inflammatory responses, but also elicit the development of antigen specific immunity. TLR7, a member of TLR family, is an intracellular receptor expressed on the membrane of endosomes. TLR7 can be triggered not only by ssRNA during viral infections, but also by immune modifiers that share a similar structure to nucleosides. Its powerful immune stimulatory action can be potentially used in the anti-tumor therapy. This article reviewed the anti-tumor activity and mechanism of TLR7 agonists that are frequently applied in preclinical and clinical investigations, and mainly focused on small synthetic molecules, including imiquimod, resiquimod, gardiquimod, and 852A, etc.

Radiotherapy combined with TLR7/8 activation induces strong immune responses against gastrointestinal tumors

In addition to local cytotoxic activity, radiotherapy may also elicit local and systemic antitumor immunity, which may be augmented by immunotherapeutic agents including Toll-like receptor (TLR) 7/8 agonists. Here, we investigated the ability of 3M-011 (854A), a TLR7/8 agonist, to boost the antigen-presenting activity of dendritic cells (DC) as an adjuvant to radiotherapy. The combined treatment induced marked local and systemic responses in subcutaneous and orthotopic mouse models of colorectal and pancreatic cancer. In vitro cytotoxicity assays as well as in vivo depletion experiments with monoclonal antibodies identified NK and CD8 T cells as the cell populations mediating the cytotoxic effects of the treatment, while in vivo depletion of CD11c⁺ dendritic cells (DC) in CD11c-DTR transgenic mice revealed DC as the pivotal immune hub in this setting. The specificity of the immune reaction was confirmed by ELISPOT assays. TLR7/8 agonists therefore seem to be potent adjuvants to radiotherapy, inducing strong local and profound systemic immune responses to tumor antigens released by conventional therapy.

Modes of action of TLR7 agonists in cancer therapy

From the numerous Toll-like receptor agonists, only TLR7 agonists have been approved for cancer treatment, although they are current restricted to topical application. The main target cells of TLR7 agonists are plasmacytoid dendritic cells, producing IFN-α and thus acting on other immune cells. Thereby dendritic cells acquire enhanced costimulatory and antigen-presenting capacity, priming an adaptive immune response. Besides NK cells, antigen-specific T cells are the main terminal effectors of TLR7 agonists in tumor therapy. This qualifies TLR7 agonists as vaccine adjuvants, which is currently being tested in clinical trials. However, the systemic application of TLR7 agonists shows insufficient efficacy, most likely owing to toxicity-limited dosing. The use of TLR7 agonists in combinational therapy holds the promise of synergistic activity and lower required doses.

Additive melanoma suppression with intralesional phospholipid-conjugated TLR7 agonists and systemic IL-2

There remains a compelling need for the development of treatments for unresectable melanoma. Agents that stimulate the innate immune response could provide advantages for cell-based therapies. However, there are conflicting reports concerning whether toll-like receptor (TLR) signaling controls tumor growth. The objective of this study was to evaluate the effect of intralesional administration of a TLR7 agonist in melanoma therapy. B16cOVA melanoma was implanted to TLR7 mice to evaluate the roles of stromal TLR7 on melanoma growth. To capitalize on the potential deleterious effects of TLR7 stimulation on the tumor growth, we injected melanoma tumor nodules with a newly developed and potent TLR7 agonist. B16 melanoma nodules expanded more rapidly in TLR7-deficient and MyD88 mice compared with TLR9 and wild type mice. Repeated injections with low doses of unconjugated TLR7 agonist were more effective at attenuating nodule size than a single high dose injection. To improve the efficacy we conjugated the agonist to phospholipid or phospholipids-polyethylene glycol, which retained TLR7 specificity. The phospholipid conjugate was indeed more effective in reducing lesion size. Furthermore, intralesional administration of the phospholipid TLR7 agonist conjugate enhanced the antimelanoma effects of systemic treatment with interleukin (IL)-2 and prolonged the survival of mice compared with IL-2 alone. Our study showed that: (1) TLR7/MyD88 signaling in the stroma is involved in melanoma growth; and (2) intralesional administration of a TLR7 agonist reduces the growth of melanoma nodules and enhances the antimelanoma effects of IL-2.

Comparing the effect of Toll-like receptor agonist adjuvants on the efficiency of a DNA vaccine

We have investigated whether poly(I:C) Toll-like receptor 3 (TLR3) and resiquimod Toll-like receptor 7 (TLR7) agonists can serve as vaccine adjuvants and promote the efficiency of therapeutic DNA vaccination against tumors expressing the human papilloma virus 16 (HPV-16) E7 protein. For this purpose, C57BL/6 mice were inoculated with 2 × 10(5) TC-1 cells, and they were then immunized with HPV-16 E7 DNA vaccine alone or with 50 μg of resiquimod or poly(I:C) individually. We found that poly(I:C) and resiquimod could induce more antigen-specific lymphocyte proliferation and cytolytic activity compared to vaccination with E7 DNA alone. While E7 DNA had no significant inhibitory effect on tumor growth, co-administration of poly(I:C) and resiquimod with E7 DNA induced significant tumor regression. Peripheral and local cytokine assays demonstrated that co-administration of poly(I:C) and resiquimod with E7 DNA induced circulating antigen-specific IFN-γ and nonspecific intratumoral IL-12. TLR3 and TLR7 agonists can be used to enhance the immune response to DNA vaccine immunogens. Taken together, these data indicate that combined vaccination with DNA encoding HPV-16 E7 plus TLR agonists provides a strategy for improving the efficacy of a vaccine as a possible immunotherapeutic strategy for cervical cancer.

The mannose-sensitive hemagglutination pilus strain of Pseudomonas aeruginosa shift peritoneal milky spot macrophages towards an M1 phenotype to dampen peritoneal dissemination

Peritoneal dissemination (PD) of tumor cells is the most frequent pattern of gastric cancer recurrence and the leading cause of death. Peritoneal milky spots are deemed as the site of origin of gastric cancer PD wherein the main cellular components are macrophages. A vaccine derived from the mannose-sensitive hemagglutination pilus strain of Pseudomonas aeruginosa (PA-MSHA) has exhibit strong immune modulatory properties. In the present study, we tested the hypothesis whether the PA-MSHA vaccine activated peritoneal milky spot macrophages (PMSM) in a manner that would attenuate PD. It was observed that PA-MSHA activated PMSM towards a classical activation phenotype via a toll-like receptor4/9-dependent mechanism, which increased interleukin-12 levels and promoted the expression of co-stimulatory and antigen-presenting molecules like CD80, CD86, and MHC-II (P < 0.05). In addition, PA-MSHA-treated PMSM exhibited strong nonspecific antitumor effects in both contact-dependent and contact-independent modes of action (P < 0.05). In mice treated with PA-MSHA before inoculating gastric cancer cells, we noted alleviated PD toward the untreated mice. In conclusion, our findings demonstrated that PA-MSHA can stimulate PMSM towards an M1 phenotype and that activated PMSM inhibit gastric cancer growth and PD both in vitro and in vivo. The results of the current study provide a mechanistic insight that is relevant to the potential application of PA-MSHA in the treatment of gastric cancer-mediated PD.

Treatment of autoimmune inflammation by a TLR7 ligand regulating the innate immune system

The Toll-like receptors (TLR) have been advocated as attractive therapeutic targets because TLR signaling plays dual roles in initiating adaptive immune responses and perpetuating inflammation. Paradoxically, repeated stimulation of bone marrow mononuclear cells with a synthetic TLR7 ligand 9-benzyl-8-hydroxy-2-(2-methoxyethoxy) adenine (called 1V136) leads to subsequent TLR hyporesponsiveness. Further studies on the mechanism of action of this pharmacologic agent demonstrated that the TLR7 ligand treatment depressed dendritic cell activation, but did not directly affect T cell function. To verify this mechanism, we utilized experimental allergic encephalitis (EAE) as an in vivo T cell dependent autoimmune model. Drug treated SJL/J mice immunized with proteolipid protein (PLP)_139–151 peptide had attenuated disease severity, reduced accumulation of mononuclear cells in the central nervous system (CNS), and limited demyelination, without any apparent systemic toxicity. Splenic T cells from treated mice produced less cytokines upon antigenic rechallenge. In the spinal cords of 1V136-treated EAE mice, the expression of chemoattractants was also reduced, suggesting innate immune cell hyposensitization in the CNS. Indeed, systemic 1V136 did penetrate the CNS. These experiments indicated that repeated doses of a TLR7 ligand may desensitize dendritic cells in lymphoid organs, leading to diminished T cell responses. This treatment strategy might be a new modality to treat T cell mediated autoimmune diseases.

Dendritic cells in the pathogenesis and treatment of human diseases: a Janus Bifrons?

Dendritic cells (DCs) represent the bridging cell compartment between a variety of nonself antigens (i.e., microbial, cancer and vaccine antigens) and adaptive immunity, orchestrating the quality and potency of downstream immune responses. Because of the central role of DCs in the generation and regulation of immunity, the modulation of DC function in order to shape immune responses is gaining momentum. In this respect, recent advances in understanding DC biology, as well as the required molecular signals for induction of T-cell immunity, have spurred many experimental strategies to use DCs for therapeutic immunological approaches for infections and cancer. However, when DCs lose control over such 'protective' responses - by alterations in their number, phenotype and/or function - undesired effects leading to allergy and autoimmune clinical manifestations may occur. Novel therapeutic approaches have been designed and currently evaluated in order to address DCs and silence these immunopathological processes. In this article we present recent concepts of DC biology and some medical implications in view of therapeutic opportunities.

In vivo activity of novel anti-ErbB2 antibody chA21 alone and with Paclitaxel or Trastuzumab in breast and ovarian cancer xenograft models

It was well studied that ErbB2 (HER2/p185(her2/neu)) overexpression in human malignant cancers correlates with poor prognosis and chemo-resistance. Although Trastuzumab (Herceptin) has been widely used in patients with ErbB2-overexpressing metastatic breast cancer, many patients either do not respond to Trastuzumab therapy or progress within 1 year of initiating Trastuzumab treatment. Previously, we reported a novel tumor-inhibitory antibody chA21, which recognized ErbB2 extracellular domain with an epitope distinct from other tumor-inhibitory anti-ErbB2 antibodies. Here, we report that chA21 combined with Paclitaxel or Trastuzumab significantly enhances the tumor-inhibition effects on ErbB2-overexpressing breast and ovarian cancer in xenograft mice. Moreover, the study reveals that the effects by chA21 to cause an enhanced inhibition on cancer cell proliferation and angiogenesis was highly associated with the intrinsic ability of chA21 to down-regulate ErbB2 receptor, inhibit downstream MAPK and PI3K-AKT signal transduction and activate natural killer cells. Our findings show that chA21 may represent a unique anti-ErbB2 antibody with potentials as therapeutic candidate alone or combination with other anti-ErbB2 reagents in cancer therapy.

Toll-like receptor agonists in cancer therapy

Toll-like receptors (TLRs) are pattern-recognition receptors related to the Drosophila Toll protein. TLR activation alerts the immune system to microbial products and initiates innate and adaptive immune responses. The naturally powerful immunostimulatory property of TLR agonists can be exploited for active immunotherapy against cancer. Antitumor activity has been demonstrated in several cancers, and TLR agonists are now undergoing extensive clinical investigation. This review discusses recent advances in the field and highlights potential opportunities for the clinical development of TLR agonists as single agent immunomodulators, vaccine adjuvants and in combination with conventional cancer therapies.

Activation of cytokine-producing and antitumor activities of natural killer cells and macrophages by engagement of Toll-like and NOD-like receptors

Macrophages and natural killer (NK) cells are important antitumor effectors by virtue of their ability to produce cytokines, chemokines and interferons (IFNs) and to mediate tumor cytotoxicity. Little is known about the impact of Toll-like receptor (TLR) and nucleotide binding and oligomerization domain (NOD)-like receptor (NLR) pathways on NK cell functions, and the role of TLRs and NLRs in macrophage activation is incompletely understood. In this study, we examined the capacities of expressed TLRs and NLRs to elicit cytokine production in human NK cells and THP1 macrophages, and to activate NK cytotoxicity against the squamous cell carcinoma of head and neck cell line Tu167 and erythroleukemia K562 cells. We found that NK cells express high levels of NOD2, NLRP3, TLR3, TLR7, and TLR9, while NOD1 was expressed at low levels. All tested NLR and TLR agonists potentiated NK cytotoxicity against Tu167 cells, whereas only poly (I:C) increased NK cytotoxicity against K562 cells. Poly (I:C) and Escherichia coli RNA markedly up-regulated TNF-α and IFN-γ expression in the NK92 cell line and human CD56(+)CD3(-) primary NK cells. High levels of NOD2, TLR7 and TLR9 proteins were observed in human THP1 cells, followed by TLR3, NOD1, and NLRP3. Stimulation of NLRP3 with E. coli RNA led to the highest induction of TNF-α, IL-6, IL-12p40, RANTES and IFN-β, whereas TLR7, TLR3, TLR9, NOD1 and NOD2 agonists had lower effects. Our data reveal involvement of TLRs and NLRs in potentiation of antitumor cytotoxicity and cytokine-producing activities of human NK cells and macrophages.

The TLR7 agonists imiquimod and gardiquimod improve DC-based immunotherapy for melanoma in mice

Toll-like receptors (TLRs) are a family of highly conserved germline-encoded pattern-recognition receptors that are essential for host immune responses. TLR ligands represent a promising class of immunotherapeutics or vaccine adjuvants with the potential to generate an effective antitumor immune response. The TLR7/8 agonists have aroused interest because they not only activate antigen-presenting cells but also promote activation of T and natural killer (NK) cells. However, the exact mechanism by which stimulation of these TLRs promotes immune responses remains unclear, and different TLR7/8 agonists have been found to induce different responses. In this study, we demonstrate that both gardiquimod and imiquimod promote the proliferation of murine splenocytes, stimulate the activation of splenic T, NK and natural killer T (NKT) cells, increase the cytolytic activity of splenocytes against B16 and MCA-38 tumor cell lines, and enhance the expression of costimulatory molecules and IL-12 by macrophages and bone marrow-derived dendritic cells (DCs). In a murine model, both agonists improved the antitumor effects of tumor lysate-loaded DCs, resulting in delayed growth of subcutaneous B16 melanoma tumors and suppression of pulmonary metastasis. Further, we found that gardiquimod demonstrated more potent antitumor activity than imiquimod. These results suggest that TLR7/8 agonists may serve as potent innate and adaptive immune response modifiers in tumor therapy. More importantly, they can be used as vaccine adjuvants to potentiate the efficiency of DC-based tumor immunotherapy.

Cancer immunotherapeutic potential of novel small molecule TLR7 and TLR8 agonists

Toll-like receptor (TLR)-mediated signaling is proposed as an immunotherapeutic target against tumorigenesis. Natural killer (NK) cells play a critical role in host defense against tumors. Specifically, formation of tumor metastasis in various organs can be suppressed by the local activity of NK cells. In this study, we present a novel TLR7 agonist (termed SC-1) that induces pro-inflammatory cytokines in human blood cells, activates NK cell function, and is highly efficient in preventing lung metastases in a pulmonary metastatic Renca model. Furthermore, a second compound (termed SC-2), acting as dual-specific TLR7 and TLR8 agonist, was evaluated with respect to its immunostimulatory and NK cell-activating capacities. The release of pro-inflammatory cytokines was shown to be even more pronounced with this compound. Additional experiments showed a significant up-regulation of activation marker CD69 on NK cells and increased cytolytic activity of peripheral blood cells compared to the effect of a monospecific TLR7 agonist SC-1. Normally, TLR7 and TLR8 are expressed on different immune cell subpopulations. TLR7 expression on antigen-presenting cells is detected in plasmacytoid dendritic cells, CD34+-derived dendritic cells, and B-cells, whereas TLR8 is mainly expressed on cells of the myeloid lineage, such as monocytes, macrophages, and myeloid dendritic cells. Therefore, a compound that activates both TLR7 and TLR8 would result in a highly efficient immune system activation and may give rise to an enhanced anti-tumor activity in vivo compared to that elicited by a monospecific TLR7 agonist.

The host defense peptide cathelicidin is required for NK cell-mediated suppression of tumor growth

Tumor surveillance requires the interaction of multiple molecules and cells that participate in innate and the adaptive immunity. Cathelicidin was initially identified as an antimicrobial peptide, although it is now clear that it fulfills a variety of immune functions beyond microbial killing. Recent data have suggested contrasting roles for cathelicidin in tumor development. Because its role in tumor surveillance is not well understood, we investigated the requirement of cathelicidin in controlling transplantable tumors in mice. Cathelicidin was observed to be abundant in tumor-infiltrating NK1.1(+) cells in mice. The importance of this finding was demonstrated by the fact that cathelicidin knockout mice (Camp(-/-)) permitted faster tumor growth than wild type controls in two different xenograft tumor mouse models (B16.F10 and RMA-S). Functional in vitro analyses found that NK cells derived from Camp(-/-) versus wild type mice showed impaired cytotoxic activity toward tumor targets. These findings could not be solely attributed to an observed perforin deficiency in freshly isolated Camp(-/-) NK cells, because this deficiency could be partially restored by IL-2 treatment, whereas cytotoxic activity was still defective in IL-2-activated Camp(-/-) NK cells. Thus, we demonstrate a previously unrecognized role of cathelicidin in NK cell antitumor function.

Immunostimulatory RNA oligonucleotides induce an effective antitumoral NK cell response through the TLR7

RNA oligonucleotides containing immune-activating sequences promote the development of cytotoxic T cell and B cell responses to Ag. In this study, we show for the first time that immunostimulatory RNA oligonucleotides induce a NK cell response that prevents growth of NK-sensitive tumors. Treatment of mice with immunostimulatory RNA oligonucleotides activates NK cells in a sequence-dependent manner, leading to enhanced IFN-gamma production and increased cytotoxicity. Use of gene-deficient mice showed that NK activation is entirely TLR7-dependent. We further demonstrate that NK activation is indirectly induced through IL-12 and type I IFN production by dendritic cells. Reconstitution of TLR7-deficient mice with wild-type dendritic cells restores NK activation upon treatment with immunostimulatory RNA oligonucleotides. Thus, by activating both NK cells and CTLs, RNA oligonucleotides stimulate two major cellular effectors of antitumor immunity. This dual activation may enhance the efficacy of immunotherapeutic strategies against cancer by preventing the development of tumor immune escape variants.

Accessory-cell-mediated activation of porcine NK cells by toll-like receptor 7 (TLR7) and TLR8 agonists

The induction of innate immune responses by toll-like receptor (TLR) agonists is the subject of intense investigation. In large part, this reflects the potential of such compounds to be effective vaccine adjuvants. For that reason, we analyzed the activation of innate cells in swine by TLR7 and TLR8 agonists. These agonists activated porcine NK cells by increasing gamma interferon (IFN-gamma) expression and perforin storage. The activation of porcine NK cells was mediated by accessory cells, since their depletion resulted in reduced cytotoxicity toward target cells. Accessory cells were stimulated to produce interleukin 12 (IL-12), IL-15, IL-18, and IFN-alpha after treatment with TLR7 or TLR8 agonists. Neutralization of these cytokines reduced but did not completely inhibit the induction of NK cell cytotoxicity. Direct stimulation of NK cells with TLR7 or TLR8 agonists resulted in minimal cytotoxicity but levels of IFN-gamma equivalent to those detected in the presence of accessory cells. Porcine NK cells express both TLR7 and TLR8 mRNAs, and treatment with these TLR agonists induced higher mRNA expression levels of TRAIL and IL-15Ralpha, which may contribute to the activity of NK cells. These data indicate that TLR7 and TLR8 agonists indirectly or directly activate porcine NK cells but that optimum levels of activation require cytokine secretion by accessory cells activated by these compounds. Interestingly, NK cells activated by TLR7 or TLR8 agonists were cytotoxic against foot-and-mouth disease virus (FMDV)-infected cells in vitro, indicating that these TLR agonists may be beneficial as adjuvants to stimulate the innate immunity against FMDV.