Toll-like receptor 3: involvement with exogenous and endogenous RNA

Is There a Role for Immunoregulatory and Antiviral Oligonucleotides Acting in the Extracellular Space? A Review and Hypothesis

Here, we link approved and emerging nucleic acid-based therapies with the expanding universe of small non-coding RNAs (sncRNAs) and the innate immune responses that sense oligonucleotides taken up into endosomes. The Toll-like receptors (TLRs) 3, 7, 8, and 9 are located in endosomes and can detect nucleic acids taken up through endocytic routes. These receptors are key triggers in the defense against viruses and/or bacterial infections, yet they also constitute an Achilles heel towards the discrimination between self- and pathogenic nucleic acids. The compartmentalization of nucleic acids and the activity of nucleases are key components in avoiding autoimmune reactions against nucleic acids, but we still lack knowledge on the plethora of nucleic acids that might be released into the extracellular space upon infections, inflammation, and other stress responses involving increased cell death. We review recent findings that a set of single-stranded oligonucleotides (length of 25-40 nucleotides (nt)) can temporarily block ligands destined for endosomes expressing TLRs in human monocyte-derived dendritic cells. We discuss knowledge gaps and highlight the existence of a pool of RNA with an approximate length of 30-40 nt that may still have unappreciated regulatory functions in physiology and in the defense against viruses as gatekeepers of endosomal uptake through certain routes.

Focusing on the Cell Type Specific Regulatory Actions of NLRX1

Cells utilize a diverse repertoire of cell surface and intracellular receptors to detect exogenous or endogenous danger signals and even the changes of their microenvironment. However, some cytosolic NOD-like receptors (NLR), including NLRX1, serve more functions than just being general pattern recognition receptors. The dynamic translocation between the cytosol and the mitochondria allows NLRX1 to interact with many molecules and thereby to control multiple cellular functions. As a regulatory NLR, NLRX1 fine-tunes inflammatory signaling cascades, regulates mitochondria-associated functions, and controls metabolism, autophagy and cell death. Nevertheless, literature data are inconsistent and often contradictory regarding its effects on individual cellular functions. One plausible explanation might be that the regulatory effects of NLRX1 are highly cell type specific and the features of NLRX1 mediated regulation might be determined by the unique functional activity or metabolic profile of the given cell type. Here we review the cell type specific actions of NLRX1 with a special focus on cells of the immune system. NLRX1 has already emerged as a potential therapeutic target in numerous immune-related diseases, thus we aim to highlight which regulatory properties of NLRX1 are manifested in disease-associated dominant immune cells that presumably offer promising therapeutic solutions to treat these disorders.

TLR3 is involved in paraquat-induced acute renal injury

AIMS

To investigate the role of Toll-like receptor 3 (TLR3) in mouse paraquat-induced acute renal injury.

MATERIALS AND METHODS

Acute renal injury was established in C57BL/6J mice by intraperitoneal injection of paraquat (28 mg/kg). The mice were also injected intraperitoneally with TLR3 agonist poly I:C (20 mg/kg) or TLR3/dsRNA complex inhibitor (1 mg) 1 h before paraquat exposure. At 72 hour post paraquat exposure, the mice were sacrificed and the blood and renal tissues were collected to examine TLR3 expression in renal tissues, pathological injury in renal tissues, renal function, inflammation, and cell apoptosis.

KEY FINDINGS

After paraquat exposure, TLR3 expression in mouse renal tissues was significantly increased, and pathological changes to the renal tissues and remarkable renal impairment were present. Compared to the paraquat group, the poly I:C group showed no significant difference in renal pathology, renal function, inflammation, or cell apoptosis. However, TLR3 inhibitor treatment significantly alleviated injury to the renal tissues, improved renal function, inhibited NF-κB activation, suppressed the infiltration of neutrophils, and lessened the expression of IL-1β, TNF-α, and keratinocyte chemoattractant (KC) in renal tissues. TLR3 inhibitor treatment also suppressed the activation of caspase-8 and caspase-3 and reduced apoptosis in the renal tissues.

SIGNIFICANCE

Paraquat exposure significantly upregulates TLR3 expression in renal tissues, and activation of the TLR3 signaling pathway is an important contributor to paraquat nephrotoxicity. TLR3 activation exacerbates inflammation and cell apoptosis in renal tissues by activating NF-κB and caspase-8, thus promoting paraquat-induced acute renal injury.

Nucleic Acid Sensing in Mammals and Plants: Facts and Caveats

The accumulation of nucleic acids in aberrant compartments is a signal of danger: fragments of cytosolic or extracellular self-DNA indicate cellular dysfunctions or disruption, whereas cytosolic fragments of nonself-DNA or RNA indicate infections. Therefore, nucleic acids trigger immunity in mammals and plants. In mammals, endosomal Toll-like receptors (TLRs) sense single-stranded (ss) or double-stranded (ds) RNA or CpG-rich DNA, whereas various cytosolic receptors sense dsDNA. Although a self/nonself discrimination could favor targeted immune responses, no sequence-specific sensing of nucleic acids has been reported for mammals. Specific immune responses to extracellular self-DNA versus DNA from related species were recently reported for plants, but the underlying mechanism remains unknown. The subcellular localization of mammalian receptors can favor self/nonself discrimination based on the localization of DNA fragments. However, autoantibodies and diverse damage-associated molecular patterns (DAMPs) shuttle DNA through membranes, and most of the mammalian receptors share downstream signaling elements such as stimulator of interferon genes (STING) and the master transcription regulators, nuclear factor (NF)-κB, and interferon regulatory factor 3 (IRF3). The resulting type I interferon (IFN) response stimulates innate immunity against multiple threats-from infection to physical injury or endogenous DNA damage-all of which lead to the accumulation of eDNA or cytoplasmatic dsDNA. Therefore, no or only low selective pressures might have favored a strict self/nonself discrimination in nucleic acid sensing. We conclude that the discrimination between self- and nonself-DNA is likely to be less strict-and less important-than assumed originally.

Single-Stranded Nucleic Acids Regulate TLR3/4/7 Activation through Interference with Clathrin-Mediated Endocytosis

Recognition of nucleic acids by endosomal Toll-like receptors (TLR) is essential to combat pathogens, but requires strict control to limit inflammatory responses. The mechanisms governing this tight regulation are unclear. We found that single-stranded oligonucleotides (ssON) inhibit endocytic pathways used by cargo destined for TLR3/4/7 signaling endosomes. Both ssDNA and ssRNA conferred the endocytic inhibition, it was concentration dependent, and required a certain ssON length. The ssON-mediated inhibition modulated signaling downstream of TLRs that localized within the affected endosomal pathway. We further show that injection of ssON dampens dsRNA-mediated inflammatory responses in the skin of non-human primates. These studies reveal a regulatory role for extracellular ssON in the endocytic uptake of TLR ligands and provide a mechanistic explanation of their immunomodulation. The identified ssON-mediated interference of endocytosis (SOMIE) is a regulatory process that temporarily dampens TLR3/4/7 signaling, thereby averting excessive immune responses.

Stimulation of TLR3 triggers release of lysosomal ATP in astrocytes and epithelial cells that requires TRPML1 channels

Cross-reactions between innate immunity, lysosomal function, and purinergic pathways may link signaling systems in cellular pathologies. We found activation of toll-like receptor 3 (TLR3) triggers lysosomal ATP release from both astrocytes and retinal pigmented epithelial (RPE) cells. ATP efflux was accompanied by lysosomal acid phosphatase and beta hexosaminidase release. Poly(I:C) alkalinized lysosomes, and lysosomal alkalization with bafilomycin or chloroquine triggered ATP release. Lysosomal rupture with glycyl-L-phenylalanine-2-naphthylamide (GPN) eliminated both ATP and acid phosphatase release. Secretory lysosome marker LAMP3 colocalized with VNUT, while MANT-ATP colocalized with LysoTracker. Unmodified membrane-impermeant 21-nt and "non-targeting" scrambled 21-nt siRNA triggered ATP and acid phosphatase release, while smaller 16-nt RNA was ineffective. Poly(I:C)-dependent ATP release was reduced by TBK-1 block and in TRPML1-/- cells, while TRPML activation with ML-SA1 was sufficient to release both ATP and acid phosphatase. The ability of poly(I:C) to raise cytoplasmic Ca2+ was abolished by removing extracellular ATP with apyrase, suggesting ATP release by poly(I:C) increased cellular signaling. Starvation but not rapamycin prevented lysosomal ATP release. In summary, stimulation of TLR3 triggers lysosomal alkalization and release of lysosomal ATP through activation of TRPML1; this links innate immunity to purinergic signaling via lysosomal physiology, and suggests even scrambled siRNA can influence these pathways.

Cathelicidin promotes inflammation by enabling binding of self-RNA to cell surface scavenger receptors

Under homeostatic conditions the release of self-RNA from dying cells does not promote inflammation. However, following injury or inflammatory skin diseases such as psoriasis and rosacea, expression of the cathelicidin antimicrobial peptide LL37 breaks tolerance to self-nucleic acids and triggers inflammation. Here we report that LL37 enables keratinocytes and macrophages to recognize self-non-coding U1 RNA by facilitating binding to cell surface scavenger receptors that enable recognition by nucleic acid pattern recognition receptors within the cell. The interaction of LL37 with scavenger receptors was confirmed in human psoriatic skin, and the ability of LL37 to stimulate expression of interleukin-6 and interferon-β1 was dependent on a 3-way binding interaction with scavenger receptors and subsequent clathrin-mediated endocytosis. These results demonstrate that the inflammatory activity of LL37 is mediated by a cell-surface-dependent interaction and provides important new insight into mechanisms that drive auto-inflammatory responses in the skin.

Double-stranded RNA upregulates the expression of inflammatory mediators in human aortic valve cells through the TLR3-TRIF-noncanonical NF-κB pathway

Calcific aortic valve disease is a chronic inflammatory condition, and the inflammatory responses of aortic valve interstitial cells (AVICs) play a critical role in the disease progression. Double-stranded RNA (dsRNA) released from damaged or stressed cells is proinflammatory and may contribute to the mechanism of chronic inflammation observed in diseased aortic valves. The objective of this study is to determine the effect of dsRNA on AVIC inflammatory responses and the underlying mechanism. AVICs from normal human aortic valves were stimulated with polyinosinic-polycytidylic acid [poly(I:C)], a mimic of dsRNA. Poly(I:C) increased the production of IL-6, IL-8, monocyte chemoattractant protein-1, and ICAM-1. Poly(I:C) also induced robust activation of ERK1/2 and NF-κB. Knockdown of Toll-like receptor 3 (TLR3) or Toll-IL-1 receptor domain-containing adapter-inducing IFN-β (TRIF) suppressed ERK1/2 and NF-κB p65 phosphorylation and reduced inflammatory mediator production induced by poly(I:C). Inhibition of NF-κB, not ERK1/2, reduced inflammatory mediator production in AVICs exposed to poly(I:C). Interestingly, inhibition of NF-κB by prevention of p50 migration failed to suppress inflammatory mediator production. NF-κB p65 intranuclear translocation induced by the TLR4 agonist was reduced by inhibition of p50 migration; however, poly(I:C)-induced p65 translocation was not, although the p65/p50 heterodimer is present in AVICs. Poly(I:C) upregulates the production of multiple inflammatory mediators through the TLR3-TRIF-NF-κB pathway in human AVICs. The NF-κB activated by dsRNA appears not to be the canonical p65/p50 heterodimers.

Virtual Screening Approaches towards the Discovery of Toll-Like Receptor Modulators

This review aims to summarize the latest efforts performed in the search for novel chemical entities such as Toll-like receptor (TLR) modulators by means of virtual screening techniques. This is an emergent research field with only very recent (and successful) contributions. Identification of drug-like molecules with potential therapeutic applications for the treatment of a variety of TLR-regulated diseases has attracted considerable interest due to the clinical potential. Additionally, the virtual screening databases and computational tools employed have been overviewed in a descriptive way, widening the scope for researchers interested in the field.

Polyinosinic-polycytidylic acid inhibits the differentiation of mouse preadipocytes through pattern recognition receptor-mediated secretion of tumor necrosis factor-α

Viral infections can disturb the functions of adipose tissues and thus result in metabolic diseases. Polyinosinic-polycytidylic acid (poly(I:C)), a synthetic analog of viral double-stranded RNA, induces innate antiviral responses by mimicking viral infection through the activation of pattern recognition receptors (PRRs) such as Toll-like receptor 3 (TLR3), retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated gene 5 (MDA5). Poly(I:C) also inhibits the differentiation of mouse preadipocytes but the mechanism underlying this process remains unclear. In this study, poly(I:C) inhibited preadipocyte differentiation in a dose-dependent manner, but not in a time-dependent manner. Endogenously transfected poly(I:C) severely impaired the adipogenesis of preadipocytes compared with exogenously added poly(I:C). Low concentration of tumor necrosis factor-α (TNF-α) could effectively inhibit the preadipocyte differentiation. The effect of exogenously added poly(I:C) on inhibition of differentiation was significantly diminished in the preadipocytes of TLR3 knockout mice. By contrast, endogenously transfected poly(I:C) still inhibited the differentiation of TLR3-deficient preadipocytes. Hence, MDA5/RIG-I signaling was involved in the poly(I:C)-induced inhibition of preadipocyte differentiation. The effect of poly(I:C) stimulation, either through endogenous transfection or exogenous addition, on inhibition of differentiation was significantly diminished in the preadipocytes of TNF-α knockout mice. These results confirmed the evidence that poly(I:C) inhibited the differentiation of mouse preadipocytes through PRR-mediated secretion of TNF-α.

Evidence for possible role of toll-like receptor 3 mediating virus-induced progression of pituitary adenomas

Tumor-related viruses are known to be involved in initiation and progression of certain tumors. However, the relationship between virus and pituitary adenomas (PAs) remains unknown. Here, we investigated infection status of three types of viruses (HPV16, HHV6B and HSV1) and expression level of toll-like receptor 3 (TLR3) in 60 human PA samples. We also determined the role of TLR3 signaling pathway on a PA cell line (GH3). We firstly found that positive rates of HPV16 and HHV6B infection were significantly higher in invasive PA samples than in noninvasive samples (P < 0.01). Similarly, TLR3 mRNA and protein expression also increased in invasive PA samples (P < 0.01). In vitro analysis indicated that GH3 cell proliferation and survival were enhanced by TLR3 activation, which was accompanied by NF-κB activation. Our data indicate that HPV16 and HHV6B viruses may be involved in promoting the progression of PA by activating the TLR3 signaling pathway.

Attenuated Innate Immunity in Embryonic Stem Cells and Its Implications in Developmental Biology and Regenerative Medicine

Embryonic stem cells (ESCs) represent a promising cell source for regenerative medicine. Intensive research over the past 2 decades has led to the feasibility of using ESC-differentiated cells (ESC-DCs) in regenerative medicine. However, increasing evidence indicates that ESC-DCs generated by current differentiation methods may not have equivalent cellular functions to their in vivo counterparts. Recent studies have revealed that both human and mouse ESCs as well as some types of ESC-DCs lack or have attenuated innate immune responses to a wide range of infectious agents. These findings raise important concerns for their therapeutic applications since ESC-DCs, when implanted to a wound site of a patient, where they would likely be exposed to pathogens and inflammatory cytokines. Understanding whether an attenuated immune response is beneficial or harmful to the interaction between host and grafted cells becomes an important issue for ESC-based therapy. A substantial amount of recent evidence has demonstrated that the lack of innate antiviral responses is a common feature to ESCs and other types of pluripotent cells. This has led to the hypothesis that mammals may have adapted different antiviral mechanisms at different stages of organismal development. The underdeveloped innate immunity represents a unique and uncharacterized property of ESCs that may have important implications in developmental biology, immunology, and in regenerative medicine.

The danger model approach to the pathogenesis of the rheumatic diseases

The danger model was proposed by Polly Matzinger as complement to the traditional self-non-self- (SNS-) model to explain the immunoreactivity. The danger model proposes a central role of the tissular cells' discomfort as an element to prime the immune response processes in opposition to the traditional SNS-model where foreignness is a prerequisite. However recent insights in the proteomics of diverse tissular cells have revealed that under stressful conditions they have a significant potential to initiate, coordinate, and perpetuate autoimmune processes, in many cases, ruling over the adaptive immune response cells; this ruling potential can also be confirmed by observations in several genetically manipulated animal models. Here, we review the pathogenesis of rheumatic diseases such as systemic lupus erythematous, rheumatoid arthritis, spondyloarthritis including ankylosing spondylitis, psoriasis, and Crohn's disease and provide realistic approaches based on the logic of the danger model. We assume that tissular dysfunction is a prerequisite for chronic autoimmunity and propose two genetically conferred hypothetical roles for the tissular cells causing the disease: (A) the Impaired cell and (B) the paranoid cell. Both roles are not mutually exclusive. Some examples in human disease and in animal models are provided based on current evidence.

Activation of TLR3 induces osteogenic responses in human aortic valve interstitial cells through the NF-κB and ERK1/2 pathways

Calcific aortic valve disease (CAVD) is characterized by chronic inflammation and progressive calcification in valve leaflets. Aortic valve interstitial cells (AVICs) play a critical role in the pathogenesis of CAVD. Previous studies show that stimulation of Toll-like receptor (TLR) 2 or TLR4 in AVICs in vitro up-regulates the expression of osteogenic mediators. Double-stranded RNA (dsRNA) can activate pro-inflammatory signaling through TLR3, the NLRP3 inflammasome and RIG-I-like receptors. The objective of this study is to determine the effect of dsRNA on AVIC osteogenic activities and the mechanism of its action. Methods and results: AVICs isolated from normal human valves were exposed to polyinosinic-polycytidylic acid [poly(I:C)], a mimic of dsRNA. Treatment with poly(I:C) increased the production of bone morphogenetic protein-2 (BMP-2), transforming growth factor beta-1 (TGF-β1) and alkaline phosphatase (ALP), and resulted in calcium deposit formation. Poly(I:C) induced the phosphorylation of NF-κB and ERK1/2. Knockdown of TLR3 essentially abrogated NF-κB and ERK1/2 phosphorylation, and markedly reduced the effect of poly(I:C) on the production of BMP-2, TGF-β1 and ALP. Further, inhibition of either NF-κB or ERK1/2 markedly reduced the levels of BMP-2, TGF-β1 and ALP in cells exposed to poly(I:C). Conclusion: Poly(I:C) up-regulates the production of BMP-2, TGF-β1 and ALP, and promotes calcium deposit formation in human AVICs. The pro-osteogenic effect of poly(I:C) is mediated primarily by TLR3 and the NF-κB and ERK1/2 pathways. These findings suggest that dsRNA, when present in aortic valve tissue, may promote CAVD progression through up-regulation of AVIC osteogenic activities.

Intensity modulated radiotherapy induces pro-inflammatory and pro-survival responses in prostate cancer patients

Intensity modulated radiotherapy (IMRT) is one of the modern conformal radiotherapies that is widely used within the context of cancer patient treatment. It uses multiple radiation beams targeted to the tumor, however, large volumes of the body receive low doses of irradiation. Using γ-H2AX and global genome expression analysis, we studied the biological responses induced by low doses of ionizing radiation in prostate cancer patients following IMRT. By means of different bioinformatics analyses, we report that IMRT induced an inflammatory response via the induction of viral, adaptive, and innate immune signaling. In response to growth factors and immune-stimulatory signaling, positive regulation in the progression of cell cycle and DNA replication were induced. This denotes pro-inflammatory and pro-survival responses. Furthermore, double strand DNA breaks were induced in every patient 30 min after the treatment and remaining DNA repair and damage signaling continued after 18-24 h. Nine genes belonging to inflammatory responses (TLR3, SH2D1A and IL18), cell cycle progression (ORC4, SMC2 and CCDC99) and DNA damage and repair (RAD17, SMC6 and MRE11A) were confirmed by quantitative RT-PCR. This study emphasizes that the risk assessment of health effects from the out-of-field low doses during IMRT should be of concern, as these may increase the risk of secondary cancers and/or systemic inflammation.

Danger signals - damaged-self recognition across the tree of life

Multicellular organisms suffer injury and serve as hosts for microorganisms. Therefore, they require mechanisms to detect injury and to distinguish the self from the non-self and the harmless non-self (microbial mutualists and commensals) from the detrimental non-self (pathogens). Danger signals are "damage-associated molecular patterns" (DAMPs) that are released from the disrupted host tissue or exposed on stressed cells. Seemingly ubiquitous DAMPs are extracellular ATP or extracellular DNA, fragmented cell walls or extracellular matrices, and many other types of delocalized molecules and fragments of macromolecules that are released when pre-existing precursors come into contact with enzymes from which they are separated in the intact cell. Any kind of these DAMPs enable damaged-self recognition, inform the host on tissue disruption, initiate processes aimed at restoring homeostasis, such as sealing the wound, and prepare the adjacent tissues for the perception of invaders. In mammals, antigen-processing and -presenting cells such as dendritic cells mature to immunostimulatory cells after the perception of DAMPs, prime naïve T-cells and elicit a specific adaptive T-/B-cell immune response. We discuss molecules that serve as DAMPs in multiple organisms and their perception by pattern recognition receptors (PRRs). Ca(2+)-fluxes, membrane depolarization, the liberation of reactive oxygen species and mitogen-activated protein kinase (MAPK) signaling cascades are the ubiquitous molecular mechanisms that act downstream of the PRRs in organisms across the tree of life. Damaged-self recognition contains both homologous and analogous elements and is likely to have evolved in all eukaryotic kingdoms, because all organisms found the same solutions for the same problem: damage must be recognized without depending on enemy-derived molecules and responses to the non-self must be directed specifically against detrimental invaders.

Mouse models of gene-environment interactions in schizophrenia

Gene-environment interactions (GEIs) likely play significant roles in the pathogenesis of schizophrenia and underlie differences in pathological, behavioral, and clinical presentations of the disease. Findings from epidemiology and psychiatric genetics have assisted in the generation of animal models of GEI relevant to schizophrenia. These models may provide a foundation for elucidating the molecular, cellular, and circuitry mechanisms that mediate GEI in schizophrenia. Here we critically review current mouse models of GEI related to schizophrenia, describe directions for their improvement, and propose endophenotypes to provide a more tangible basis for molecular studies of pathways of GEI and facilitate the identification of novel therapeutic targets.

Mouse embryonic stem cells are deficient in type I interferon expression in response to viral infections and double-stranded RNA

Embryonic stem cells (ESCs) are considered to be a promising cell source for regenerative medicine because of their unlimited capacity for self-renewal and differentiation. However, little is known about the innate immunity in ESCs and ESC-derived cells. We investigated the responses of mouse (m)ESCs to three types of live viruses as follows: La Crosse virus, West Nile virus, and Sendai virus. Our results demonstrated mESCs were susceptible to viral infection, but they were unable to express type I interferons (IFNα and IFNβ, IFNα/β), which differ from fibroblasts (10T1/2 cells) that robustly express IFNα/β upon viral infections. The failure of mESCs to express IFNα/β was further demonstrated by treatment with polyIC, a synthetic viral dsRNA analog that strongly induced IFNα/β in 10T1/2 cells. Although polyIC transiently inhibited the transcription of pluripotency markers, the stem cell morphology was not significantly affected. However, polyIC can induce dsRNA-activated protein kinase in mESCs, and this activation resulted in a strong inhibition of cell proliferation. We conclude that the cytosolic receptor dsRNA-activated protein kinase is functional, but the mechanisms that mediate type I IFN expression are deficient in mESCs. This conclusion is further supported by the findings that the major viral RNA receptors are either expressed at very low levels (TLR3 and MDA5) or may not be active (retinoic acid-inducible gene I) in mESCs.

Toll-like receptor 3: implications for proinflammatory microenvironment in human breast cancer

Under many circumstances, the host constituents that are found in the tumor microenvironment support a malignancy network and provide the cancer cells with advantages in proliferation, invasiveness and metastasis establishment at remote organs. It is known that Toll like receptors (TLRs) are expressed not only on immune cells but also on cancer cells and it has suggested a deleterious role for TLR3 in inflammatory disease. Hypothesizing that altered IFNγ signaling may be a key mechanism of immune dysfunction common to cancer as well CXCR4 is overexpressed among breast cancer patients, the mRNA expression of TLR3, CXCR4 and IFNγ in breast cancer tumor tissues was investigated. No statistically significant differences in the expression of CXCR4 mRNA, IFNγ and TLR3 between healthy and tumor tissues was observed, however, it was verified a positive correlation between mRNA relative expression of TLR3 and CXCR4 (p < 0.001), and mRNA relative expression of TLR3 was significantly increased in breast cancer tumor tissue when compared to healthy mammary gland tissue among patients expressing high IFNγ (p = 0.001). Since the tumor microenvironment plays important roles in cancer initiation, growth, progression, invasion and metastasis, it is possible to propose that an overexpression of IFNγ mRNA due to the pro-inflammatory microenvironment can lead to an up-regulation of CXCR4 mRNA and consequently to an increased TLR3 mRNA expression even among nodal negative patients. In the future, a comprehensive study of TLR3, CXCR4 and IFNγ axis in primary breast tumors and corresponding healthy tissues will be crucial to further understanding of the cancer network.

Single-stranded DNA oligonucleotides inhibit TLR3-mediated responses in human monocyte-derived dendritic cells and in vivo in cynomolgus macaques

TLR3 is a key receptor for recognition of double-stranded RNA and initiation of immune responses against viral infections. However, hyperactive responses can have adverse effects, such as virus-induced asthma. Strategies to prevent TLR3-mediated pathology are therefore desired. We investigated the effect of single-stranded DNA oligonucleotides (ssDNA-ODNs) on TLR3 activation. Human monocyte-derived dendritic cells up-regulate maturation markers and secrete proinflammatory cytokines on treatment with the synthetic TLR3 ligand polyinosine-polycytidylic acid (poly I:C). These events were inhibited in cultures with ssDNA-ODNs. Poly I:C activation of nonhematopoietic cells was also inhibited by ssDNA-ODNs. The uptake of poly I:C into cells was reduced in the presence of ssDNA-ODNs, preventing TLR3 engagement from occurring. To confirm this inhibition in vivo, we administered ssDNA-ODNs and poly I:C, alone or in combination, via the intranasal route in cynomolgus macaques. Proinflammatory cytokines were detected in nasal secretions in the poly I:C group, while the levels were reduced in the groups receiving ssDNA-ODNs or both substances. Our results demonstrate that TLR3-triggered immune activation can be modulated by ssDNA-ODNs and provide evidence of dampening proinflammatory cytokine release in the airways of cynomolgus macaques. These findings may open novel perspectives for clinical strategies to prevent or treat inflammatory conditions exacerbated by TLR3 signaling.

TLR signaling-mediated differential histone modification at IL-10 and IL-12 promoter region leads to functional impairments in tumor-associated macrophages

Tumor-associated macrophages (TAM) are severely compromised for the induction of proinflammatory mediators following toll-like receptor (TLR) activation. Here, we reported that the defective TLR response in TAM was due to the malfunctioning of the myeloid differentiation primary response gene 88 (MyD88)-dependent signaling cascade in concert with downregulation of tumor necrosis factor receptor-associated factor (TRAF) 6 and interleukin-1 receptor-associated kinase (IRAK) 1. However, the expression of toll-interleukin1 receptor domain-containing adapter-inducing interferon beta (TRIF) and TRAF 3, which act via the TRIF-dependent pathway of TLR signaling, were found to be unaffected in TAM. Although, TRIF-mediated signal inducers, lipopolysaccharide or poly (I:C), induced high level of extracellular signal-regulated kinase (ERK)-1/2 mitogen-activated protein kinase (MAPK) phosphorylation, but they were failed to induce significant p38MAPK phosphorylation in TAM. Consequently, ERK-1/2-dependent histone phosphorylation at the IL-10 promoter elicited enhanced interleukin (IL)-10 production by TAM. Whereas, the lack of transcription favorable histone phosphorylation at the IL-12 promoter was accompanied with a very low amount of IL-12 expression in TAM. Moreover, ERK-1/2 MAPK activation resulted in enhanced IRAK M induction in TAM, a specific inhibitor of MyD88 pathway. Therefore, for the first time, we decipher an unexplored TLR signaling in TAM where ERK-1/2 activation in a MyD88-independent pathway results in transcription favorable histone modification at the IL-10 promoter region to enhance IL-10-mediated immunosuppression. Additionally, by enhancing IRAK M induction, it also polarizes TAM toward a more immunosuppressive form.

Human endogenous RNAs: Implications for the immunomodulation of Toll-like receptor 3

Toll-like receptors (TLRs), a family of mammalian receptors, are able to recognize nucleic acids. TLR3 recognizes double-stranded (ds)RNA, a product of the replication of certain viruses. Polyinosinic-polycytidylic acid, referred to as poly(I:C), an analog of viral dsRNA, interacts with TLR3 thereby eliciting immunoinflammatory responses characteristic of viral infection or down-regulating the expression of chemokine receptor CXCR4. It is known that dsRNA also directly activates interferon (IFN)-induced enzymes, such as the RNA-dependent protein kinase (PKR). In the present study, the mRNA expression of TLR3, CXCR4, IFNγ and PKR was investigated in a culture of peripheral blood mononuclear cells (PBMCs) stimulated with poly(I:C) and endogenous RNA from human PBMCs. No cytotoxic effect on the cells or on the proliferation of CD3(+), CD4(+) and CD8(+) cells was observed. TLR3 expression in the PBMCs in the presence of poly(I:C) was up-regulated 9.5-fold, and TLR3 expression in the PBMCs treated with endogenous RNA was down-regulated 1.8-fold (p=0.002). The same trend was observed for IFNγ where in the presence of poly(I:C) an 8.7-fold increase was noted and in the presence of endogenous RNA a 3.1-fold decrease was observed. In the culture activated with poly(I:C), mRNA expression of CXCR4 increased 8.0-fold and expression of PKR increased 33.0-fold. Expression of these genes decreased in the culture treated with endogenous RNA when compared to the culture without stimulus. Thus, high expression of mRNA for TLR3, IFNγ, CXCR4 and PKR was observed in the presence of poly(I:C) and low expression was observed in the cells cultured with endogenous RNA. In conclusion, TLR3 may play major physiological roles that are not in the context of viral infection. It is possible that RNA released from cells could contain enough double-stranded structures to regulate cell activation. The involvement of endogenous RNA in endogenous gene expression and its implications in the regulation thereof, are still being studied, and will have significant implications in the future.