Human Toll-like receptor 8 can be cool too: implications for foreign RNA sensing

Identifying RNA Sensors in Antiviral Innate Immunity

The innate immune system plays a pivotal role in pathogen recognition and the initiation of innate immune responses through its Pathogen Recognition Receptors (PRRs), which detect Pathogen-Associated Molecular Patterns (PAMPs). Nucleic acids, including RNA and DNA, are recognized as particularly significant PAMPs, especially in the context of viral pathogens. During RNA virus infections, specific sequences in the viral RNA mark it as non-self, enabling host recognition through interactions with RNA sensors, thereby triggering innate immunity. Given that some of the most lethal viruses are RNA viruses, they pose a severe threat to human and animal health. Therefore, understanding the immunobiology of RNA PRRs is crucial for controlling pathogen infections, particularly RNA virus infections. In this chapter, we will introduce a "pull-down" method for identifying RIG-I-like receptors, related RNA helicases, Toll-like receptors, and other RNA sensors.

Porcine TLR8 signaling and its anti-infection function are disturbed by immune checkpoint receptor TIM-3 via inhibition of P13K-AKT pathway

Toll-like receptor 8 (TLR8), an important innate immune receptor recognizing single stranded RNA and the antiviral imidazoquinoline compounds, can activate intracellular signaling pathway and produce an inflammatory response to kill and eliminate pathogens. However, the molecular regulation mechanisms of TLR8 signaling and its anti-infection activity are not fully elucidated. Our previous transcriptome analysis of porcine TLR8 (pTLR8) signaling suggested the immune checkpoint receptor TIM-3 as the potential regulator for pTLR8. Here we investigated TIM-3 in the regulation of pTLR8 signaling and its anti-infection activity. Our results showed that porcine TIM-3 is upregulated by pTLR8 signaling and TIM-3 inhibits pTLR8 signaling activity in a negative feedback way. Accordingly, TIM-3 disturbs pTLR8 mediated anti-bacterial and anti-viral activity. Mechanistically, TIM-3 suppresses PI3K-AKT pathway by inhibiting the TLR8-PI3K p85 interaction and subsequent AKT phosphorylation which is essential for TLR8 signaling and anti-infection activity. Therefore, our study reveals new insights into innate immune TLR8 signaling and its anti-infection function.

Antibody preparation and age-dependent distribution of TLR8 in Bactrian camel spleens

BACKGROUND

Toll-like receptor 8 (TLR8) can recognize specific pathogen-associated molecular patterns and exert multiple immunological functions through activation of signaling cascades. However, the precise distribution and age-related alterations of TLR8 in the spleens of Bactrian camels have not yet been investigated. This study aimed to prepare a rabbit anti-Bactrian camel TLR8 polyclonal antibody and elucidate the distribution of TLR8 in the spleens of Bactrian camels at different age groups. The methodology involved the construction of the pET-28a-TLR8 recombinant plasmid, followed by the expression of TLR8 recombinant protein via prokaryotic expression. Subsequently, rabbits were immunized with the purified protein to prepare the TLR8 polyclonal antibody. Finally, twelve Alashan Bactrian camels were categorized into four groups: young (1-2 years), pubertal (3-5 years), middle-aged (6-16 years) and old (17-20 years). These camels received intravenous sodium pentobarbital (20 mg/kg) anesthesia and were exsanguinated to collect spleen samples. Immunohistochemical techniques were employed to observe and analyze the distribution patterns and age-related changes of TLR8 in the spleen.

RESULTS

The results showed that the TLR8 recombinant protein was expressed in the form of inclusion body with a molecular weight of 52 kDa, and the optimal induction condition involved 0.3 mmol/L IPTG induction for 8 h. The prepared antibody yielded a titer of 1:32 000, and the antibody demonstrated specific binding to TLR8 recombinant protein. TLR8 positive cells exhibited a consistent distribution pattern in the spleen across different age groups of Bactrian camels, primarily scattered within the periarterial lymphatic sheath of the white pulp, marginal zone, and red pulp. The predominant cell type expressing TLR8 was macrophages, with expression also observed in neutrophils and dendritic cells. Statistical analysis revealed that there were significant differences in the distribution density of TLR8 positive cells among different spleen regions at the same age, with the red pulp, marginal zone, and white pulp showing a descending order (P<0.05). Age-related changes indicated that the distribution density in the marginal zone and red pulp exhibited a similar trend of initially increasing and subsequently decreasing from young to old camels. As camels age, there was a significant decrease in the distribution density across all spleen regions (P<0.05).

CONCLUSIONS

The results confirmed that this study successfully prepared a rabbit anti-Bactrian camel TLR8 polyclonal antibody with good specificity. TLR8 positive cells were predominantly located in the red pulp and marginal zone of the spleen, signifying their pivotal role in the innate immune response of the spleen. Aging was found to significantly reduce the density of TLR8 positive cells, while leaving their scattered distribution characteristics unaffected. These findings provide valuable support for further investigations into the immunomorphology and immunosenescence of the spleen in Bactrian camels.

Identification of an Optimal TLR8 Ligand by Alternating the Position of 2′-O-Ribose Methylation

Recognition of RNA by receptors of the innate immune system is regulated by various posttranslational modifications. Different single 2′-O-ribose (2′-O-) methylations have been shown to convert TLR7/TLR8 ligands into specific TLR8 ligands, so we investigated whether the position of 2′-O-methylation is crucial for its function. To this end, we designed different 2′-O-methylated RNA oligoribonucleotides (ORN), investigating their immune activity in various cell systems and analyzing degradation under RNase T2 treatment. We found that the 18S rRNA-derived TLR7/8 ligand, RNA63, was differentially digested as a result of 2′-O-methylation, leading to variations in TLR8 and TLR7 inhibition. The suitability of certain 2′-O-methylated RNA63 derivatives as TLR8 agonists was further demonstrated by the fact that other RNA sequences were only weak TLR8 agonists. We were thus able to identify specific 2′-O-methylated RNA derivatives as optimal TLR8 ligands.

TLR-8, TNF-α, and ESR-1α Gene Polymorphism Susceptibility in Onset of Arthritis

Arthritis is a genetic disorder characterized by bones and joint degradation assisted by severe pain and inflammation. It is evident by the studies that 0 candidate genes variations play vital role in its development and progression. Therefore, we investigated the genetic variation of TLR-8, TNF, and ESR-1α genes in the Pakistani population. A case-control study comprising 300 RA, 316 OA, and 412 control subjects was conducted. PCR-RFLP and direct sequencing methods were used for determining genetic variations. Analysis was performed by using PLINK and MEGA 6.0 software. Allelic and genetic frequencies of polymorphisms identified on rs3764879 (TLR-8), rs3764880 (TLR-8), rs5744080 (TLR-8), rs1800629 (TNF), rs2228480 (ESR-1α), and rs1451501590 (ESR-1α) were significantly varied among RA, OA, and controls. Novel functional mutations SCV000844945 and SCV000844946 on TLR-8 as well as a non-functional SCV000804801 and functional variation SCV000804802 on ESR-1α were also identified and reported for the first time in the studied population. Multiple site analyses indicated that polymorphisms on TLR-8 and ESR-1α genes were significant risk factors in disease onset to the next generation. In conclusion, TLR-08 and ESR-1α were significant in the onset of arthritis whereas the TNF was not found as a significant risk factor in the onset of RA and OA.

TIRAP/Mal Positively Regulates TLR8-Mediated Signaling via IRF5 in Human Cells

Toll-like receptor 8 (TLR8) recognizes single-stranded RNA of viral and bacterial origin as well as mediates the secretion of pro-inflammatory cytokines and type I interferons by human monocytes and macrophages. TLR8, as other endosomal TLRs, utilizes the MyD88 adaptor protein for initiation of signaling from endosomes. Here, we addressed the potential role of the Toll-interleukin 1 receptor domain-containing adaptor protein (TIRAP) in the regulation of TLR8 signaling in human primary monocyte-derived macrophages (MDMs). To accomplish this, we performed TIRAP gene silencing, followed by the stimulation of cells with synthetic ligands or live bacteria. Cytokine-gene expression and secretion were analyzed by quantitative PCR or Bioplex assays, respectively, while nuclear translocation of transcription factors was addressed by immunofluorescence and imaging, as well as by cell fractionation and immunoblotting. Immunoprecipitation and Akt inhibitors were also used to dissect the signaling mechanisms. Overall, we show that TIRAP is recruited to the TLR8 Myddosome signaling complex, where TIRAP contributes to Akt-kinase activation and the nuclear translocation of interferon regulatory factor 5 (IRF5). Recruitment of TIRAP to the TLR8 signaling complex promotes the expression and secretion of the IRF5-dependent cytokines IFNβ and IL-12p70 as well as, to a lesser degree, TNF. These findings reveal a new and unconventional role of TIRAP in innate immune defense.

Cytomegalovirus Infection Downregulates Vitamin D Receptor in Patients Undergoing Hematopoietic Stem Cell Transplantation

BACKGROUND

Allogeneic hematopoietic stem cell transplantation (HSCT) is a potentially curative option for patients with hematologic diseases but is associated with high mortality and morbidity. Cytomegalovirus (CMV) infection is common in HSCT patients and modulates vitamin D metabolism in vitro. We aimed at validating CMV-associated vitamin D metabolism in vivo in HSCT.

METHODS

Patients treated for significant CMV viremia after HSCT were evaluated for CMV load before, during, and after antiviral treatment. RNA was isolated from whole-blood samples to test for regulation of key components of the vitamin D receptor (VDR) pathway during different phases of CMV viremia.

RESULTS

CMV viremia developed a mean time of 102 (±34) d post-HSCT. Maximum levels of CMV-DNA reached a mean of 5668 (±7257) copies/mL. VDR expression was downregulated to a mean of 64.3% (±42.5%) relative to the VDR expression pre-CMV viremia (P = 0.035) and lagged in recovery following antiviral treatment. Toll-like receptor (TLR) 2 mRNA was upregulated to 225.4% during CMV viremia relative to the expression pre-CMV viremia (P = 0.012) but not TLR6/7/8 and the TLR-adaptor protein MyD88. Levels of 25-OH vitamin D were reduced in all viremic patients (48.0 ± 4.8 versus 25.1 ± 3.7 ng/mL) and were even lower after periods of CMV viremia compared with the control group (48.3 ± 3.5 versus 17.8 ± 1.8 ng/mL; P = 0.008).

CONCLUSIONS

CMV viremia is associated with significant dysregulation of vitamin D metabolism in HSCT patients.

Genetic characterizations of Toll-like receptors in the brown rat and their associations with pathogen infections

Toll-like receptors (TLRs) are important initiators of innate immune responses that target host-pathogen interactions. However, further research into the molecular characteristics of TLRs in wild populations is required, as well as how TLRs genetically influenced pathogen infections in the brown rat (Rattus norvegicus). Here, we explored the genetic characterization and evolution of 2 sensing nucleic acid TLRs (TLR7 and TLR8) and 2 sensing non-nucleic acid TLRs (TLR2 and TLR4) in the wild brown rat, and assessed their associations with 2 RNA viruses (Seoul hantavirus and rat hepatitis E virus (HEV)) and 2 bacteria (Leptospira and Bartonella). In these 4 TLRs, we discovered a total of 16 variants. Furthermore, TLR8 had high genetic diversity among 7 variants, while TLR2 had low genetic diversity with only 1 variant. According to selective pressure analyses, TLR4, TLR7, and TLR8 genes evolved under purifying selection. Interestingly, significant associations were found between 3 TLR8 variants and HEV infection, as well as 1 TLR2 variant and Bartonella infection. Overall, our findings provided a glimpse into the genetic characterization of TLRs in the brown rat, and further demonstrated that TLR2 and TLR8 genetic variations were related to Bartonella and HEV infection, respectively. Especially, TLR8 may be a good candidate immune gene for future research on molecular ecology and functional adaptation in wild populations.

Endosomal TLR-8 Senses microRNA-1294 Resulting in the Production of NFḱB Dependent Cytokines

The primary function of toll-like receptor 8 (TLR-8) is the detection of viruses and other microbial pathogens. Recent evidence suggests that TLR-8 also senses host microRNAs (miRNAs) and implicate TLR-8 in autoimmune disorders. This study examined the interaction between miR-1294 and TLR-8. We first performed a BLAST search to identify miRNAs with the same sequences as two core motifs of miR-1294. Next, we examined NFḱB activation induced by the binding of miR-1294 mimic to endosomal TLR-8. HEK-Blue™ hTLR-8 cells (Invivogen), a HEK293 cell line co-transfected with human TLR-8 gene, were incubated with miR-1294 mimic. A TLR-8 agonist ssRNA40, was used as a positive control. Using the same experimental set up, we also examined the effects of miR-1294 and its two core motifs (Integrated DNA Technologies) on IL-8, IL-1β, and TNFα. Data were analyzed using t-test or one-way ANOVA and Dunnets post-hoc test. Using miRCarta we identified 29 other mature human miRNAs or their precursors which contain the same core motifs as miR-1294. Our data show that miR-1294 activates NFḱB in cells expressing TLR-8 (p < 0.05). miR-1294, and its core motifs induce expression of IL-8, IL-1β, and TNFα via TLR8 activation (p < 0.05). This constitutes a novel mechanism by which endosomal TLR-8 senses host miRNAs resulting in the release of pro-inflammatory cytokines and thus potentially contributing to autoimmune disorders.

Human Macrophages Clear the Biovar Microtus Strain of Yersinia pestis More Efficiently Than Murine Macrophages

Yersinia pestis is the etiological agent of the notorious plague that has claimed millions of deaths in history. Of the four known Y. pestis biovars (Antiqua, Medievalis, Orientalis, and Microtus), Microtus strains are unique for being highly virulent in mice but avirulent in humans. Here, human peripheral lymphocytes were infected with the fully virulent 141 strain or the Microtus strain 201, and their transcriptomes were determined and compared. The most notable finding was that robust responses in the pathways for cytokine-cytokine receptor interaction, chemokine signaling pathway, Toll-like receptor signaling and Jak-STAT signaling were induced at 2 h post infection (hpi) in the 201- but not the 141-infected lymphocytes, suggesting that human lymphocytes might be able to constrain infections caused by strain 201 but not 141. Consistent with the transcriptome results, much higher IFN-γ and IL-1β were present in the supernatants from the 201-infected lymphocytes, while inflammatory inhibitory IL-10 levels were higher in the 141-infected lymphocytes. The expressions of CSTD and SLC11A1, both of which are functional components of the lysosome, increased in the 201-infected human macrophage-like U937 cells. Further assessment of the survival rate of the 201 bacilli in the U937 cells and murine macrophage RAW 264.7 cells revealed no viable bacteria in the U937 cells at 32 hpi.; however, about 5–10% of the bacteria were still alive in the RAW264.7 cells. Our results indicate that human macrophages can clear the intracellular Y. pestis 201 bacilli more efficiently than murine macrophages, probably by interfering with critical host immune responses, and this could partially account for the host-specific pathogenicity of Y. pestis Microtus strains.

TLR8 and its endogenous ligand miR-21 contribute to neuropathic pain in murine DRG

TLRs are known to be essential for innate and adaptive immunity. Zhang et al. show the involvement of TLR8 and its endogenous ligand miR-21 in neuropathic pain via inducing ERK-dependent proinflammatory mediators’ production and neuronal hyperexcitability in the DRG.

Toll-like receptors (TLRs) are nucleic acid–sensing receptors and have been implicated in mediating pain and itch. Here we report that Tlr8^−/− mice show normal itch behaviors, but have defects in neuropathic pain induced by spinal nerve ligation (SNL) in mice. SNL increased TLR8 expression in small-diameter IB4⁺ DRG neurons. Inhibition of TLR8 in the DRG attenuated SNL-induced pain hypersensitivity. Conversely, intrathecal or intradermal injection of TLR8 agonist, VTX-2337, induced TLR8-dependent pain hypersensitivity. Mechanistically, TLR8, localizing in the endosomes and lysosomes, mediated ERK activation, inflammatory mediators’ production, and neuronal hyperexcitability after SNL. Notably, miR-21 was increased in DRG neurons after SNL. Intrathecal injection of miR-21 showed the similar effects as VTX-2337 and inhibition of miR-21 in the DRG attenuated neuropathic pain. The present study reveals a previously unknown role of TLR8 in the maintenance of neuropathic pain, suggesting that miR-21–TLR8 signaling may be potential new targets for drug development against this type of chronic pain.

Tumor-derived exosomes modulate PD-L1 expression in monocytes

In chronic lymphocytic leukemia (CLL), monocytes and macrophages are skewed toward protumorigenic phenotypes, including the release of tumor-supportive cytokines and the expression of immunosuppressive molecules such as programmed cell death 1 ligand 1 (PD-L1). To understand the mechanism driving protumorigenic skewing in CLL, we evaluated the role of tumor cell-derived exosomes in the cross-talk with monocytes. We carried out RNA sequencing and proteome analyses of CLL-derived exosomes and identified noncoding Y RNA hY4 as a highly abundant RNA species that is enriched in exosomes from plasma of CLL patients compared with healthy donor samples. Transfer of CLL-derived exosomes or hY4 alone to monocytes resulted in key CLL-associated phenotypes, including the release of cytokines, such as C-C motif chemokine ligand 2 (CCL2), CCL4, and interleukin-6, and the expression of PD-L1. These responses were abolished in Toll-like receptor 7 (TLR7)-deficient monocytes, suggesting exosomal hY4 as a driver of TLR7 signaling. Pharmacologic inhibition of endosomal TLR signaling resulted in a substantially reduced activation of monocytes in vitro and attenuated CLL development in vivo. Our results indicate that exosome-mediated transfer of noncoding RNAs to monocytes contributes to cancer-related inflammation and concurrent immune escape via PD-L1 expression.

RNA sensors of the innate immune system and their detection of pathogens

The innate immune system plays a critical role in pathogen recognition and initiation of protective immune response through the recognition of pathogen associated molecular patterns (PAMPs) by its pattern recognition receptors (PRRs). Nucleic acids including RNA and DNA have been recognized as very important PAMPs of pathogens especially for viruses. RNA are the major PAMPs of RNA viruses, to which most severe disease causing viruses belong thus posing a tougher challenge to human and animal health. Therefore, the understanding of the immune biology of RNA PRRs is critical for control of pathogen infections especially for RNA virus infections. RNA PRRs are comprised of TLR3, TLR7, TLR8, RIG-I, MDA5, NLRP3, NOD2, and some other minorities. This review introduces these RNA PRRs by describing the cellular localizations, ligand recognitions, activation mechanisms, cell signaling pathways, and recognition of pathogens; the cross-talks between various RNA PRRs are also reviewed. The deep insights of these RNA PRRs can be utilized to improve anti-viral immune response. © 2017 IUBMB Life, 69(5):297-304, 2017.

Assessing the Inhibitory Activity of Oligonucleotides on TLR7 Sensing

Aberrant sensing of self-nucleic acids by Toll-like receptor (TLR) 7, 8, or 9 is associated with several autoimmune disorders, including systemic lupus erythematosus (SLE), rheumatoid arthritis, psoriasis, or systemic sclerosis. In recent years, several classes of synthetic oligonucleotides have been shown to antagonize sensing of immunostimulatory nucleic acids by TLR7/8/9, indicating that these molecules could have therapeutic applications in such autoimmune diseases. Conversely, synthetic oligonucleotides used in therapeutic technologies such as antisense and microRNA inhibitors also have the potential to inhibit TLR7/8/9 sensing, rendering patients more susceptible to viral/bacterial infections. This chapter describes a protocol to define the inhibitory activity of synthetic oligonucleotides on TLR7.

Nucleic acid-sensing TLRs and autoimmunity: novel insights from structural and cell biology

Invasion of pathogenic microorganisms or tissue damage activates innate immune signaling receptors that sample subcellular locations for foreign molecular structures, altered host molecules, or signs of compartment breaches. Upon engagement of innate immune receptors an acute but transient inflammatory response is initiated, aimed at the clearance of pathogens and cellular debris. Among the molecules that are sensed are nucleic acids, which activate several members of the transmembrane Toll-like receptor (TLR) family. Inappropriate recognition of nucleic acids by TLRs can cause inflammatory pathologies and autoimmunity. Here, we review the mechanisms involved in triggering nucleic acid-sensing TLRs and indicate checkpoints that restrict their activation to endolysosomal compartments. These mechanisms are crucial to sample the content of endosomes for nucleic acids in the context of infection or tissue damage, yet prevent accidental activation by host nucleic acids under physiological conditions. Decoding the molecular mechanisms that regulate nucleic acid recognition by TLRs is central to understand pathologies linked to unrestricted nucleic acid sensing and to develop novel therapeutic strategies.

Innate immune receptor Toll-like receptor 4 signalling in neuropsychiatric diseases

The innate immunity is a stereotyped first line of defense against pathogens and unspecified damage signals. One of main actors of innate immunity are the Toll-like receptors (TLRs), and one of the better characterized members of this family is TLR-4, that it is mainly activated by Gram-negative bacteria lipopolysaccharide. In brain, TLR-4 organizes innate immune responses against infections or cellular damage, but also possesses other physiological functions. In the last years, some evidences suggest a role of TLR-4 in stress and stress-related neuropsychiatric diseases. Peripheral and brain TLR-4 activation triggers sickness behavior, and its expression is a risk factor of depression. Some elements of the TLR-4 signaling pathway are up-regulated in peripheral samples and brain post-mortem tissue from depressed and suicidal patients. The "leaky gut" hypothesis of neuropsychiatric diseases is based on the existence of an increase of the intestinal permeability which results in bacterial translocation able to activate TLR-4. Enhanced peripheral TLR-4 expression/activity has been described in subjects diagnosed with schizophrenia, bipolar disorder and in autistic children. A role for TLR-4 in drugs abuse has been also proposed. The therapeutic potential of pharmacological/genetic modulation of TLRs signaling pathways in neuropsychiatry is promising, but a great preclinical/clinical scientific effort is still needed.

Norovirus Infection: Replication, Manipulation of Host, and Interaction with the Host Immune Response

Noroviruses (NoVs) belong to the Caliciviridae family of viruses and are responsible for causing the majority of gastroenteritis outbreaks worldwide. In the past decade, research on NoV biology has intensified because of the discovery of murine NoV and subsequently the first cell culture system and small animal model for NoV replication and pathogenesis. In this review, we discuss the current literature on NoV biology, focusing particularly on NoV replication and the interaction between NoV and the host immune response. Understanding the NoV replication cycle and its interaction with cellular processes and innate immune immunity will help develop molecular targets to control human NoV infection and prevent outbreaks. In addition to the innate immune response, we have documented the current efforts to develop NoV vaccines to control outbreaks.

Expression and regulation of Schlafen (SLFN) family members in primary human monocytes, monocyte-derived dendritic cells and T cells

Schlafen (SLFN/Slfn) family members have been investigated for their involvement in fundamental cellular processes including growth regulation, differentiation and control of viral replication. However, most research has been focused on the characterization of Slfns within the murine system or in human cell lines. Since little is known about SLFNs in primary human immune cells, we set out to analyze the expression and regulation of the six human SLFN genes in monocytes, monocyte-derived dendritic cells (moDCs) and T cells. Comparison of SLFN gene expression across these three cell types showed high mRNA expression of SLFN11 in monocytes and moDCs and high SLFN5 expression in T cells, indicating functional importance within these cell types. Differentiation of monocytes to moDCs leads to the gradual upregulation of SLFN12L and SLFN13 while SLFN12 levels were decreased by differentiation stimuli. Stimulation of moDCs via human rhinovirus, lipopolysaccharide, or IFN-α lead to strong upregulation of SLFN gene expression, while peptidoglycan poorly stimulated regulation of both SLFNs and the classical interferon-stimulated gene MxA. T cell activation was found to downregulate the expression of SLFN5, SLFN12 and SLFN12L, which was reversible upon addition of exogenous IFN-α. In conclusion, we demonstrate, that SLFN gene upregulation is mainly dependent on autocrine type I interferon signaling in primary human immune cells. Rapid decrease of SLFN expression levels following T cell receptor stimulation indicates a role of SLFNs in the regulation of human T cell quiescence.

Negative feedback circuit for toll like receptor-8 activation in human embryonic Kidney 293 using outer membrane vesicle delivered bi-specific siRNA

Background

TLR8 assists in antiviral approach by producing Type 1 INF via MyD88 dependent IRF7 pathway. However, over expression of INFα/β molecule poses threat by developing tolerance in chronic infection cases and enhancing inflammatory response. Here we report a bi-specific siRNA based complex which differentially activates and silences the TLR8 and MYD88 respectively in a negatively regulated fashion.

Results

Outer membrane vesicle from Escherichia coli used for siRNA delivery was observed more efficient when attached with invasive protein Ail along with OmpA (P < 0.001) in HEK293-TLR8 cell line. siRNA complexed with p19 protein was efficient in activating TLR8, confirmed by the increment of INFβ molecules (P < 0.001) in HEK293-TLR8 compared to its counterpart. Fusion of lipid bilayer of endosomal compartment was significant at pH 4.5 when fusogenic peptides (diINF-7) were incubated in membrane vesicle, thus facilitating the escape of siRNA complex to the host cytoplasm in order to silence MyD88 transcript (P < 0.001).

Conclusions

We investigated the activation of TLR8 by bi-specific si-RNA for the production of INFβ. In the same setting we showed that bi-specific si-RNA was able to silence MyD88 transcript in a delayed manner. For the cases of auto immune disease and inflammation where over activation of endosomal TLRs poses serious threat, bi specific siRNA could be used as negative feedback controlled system.

Electronic supplementary material

The online version of this article (doi:10.1186/s12865-015-0109-9) contains supplementary material, which is available to authorized users.

Sequence-dependent off-target inhibition of TLR7/8 sensing by synthetic microRNA inhibitors

Anti-microRNA (miRNA) oligonucleotides (AMOs) with 2'-O-Methyl (2'OMe) residues are commonly used to study miRNA function and can achieve high potency, with low cytotoxicity. Not withstanding this, we demonstrate the sequence-dependent capacity of 2'OMe AMOs to inhibit Toll-like receptor (TLR) 7 and 8 sensing of immunostimulatory RNA, independent of their miRNA-targeting function. Through a screen of 29 AMOs targeting common miRNAs, we found a subset of sequences highly inhibitory to TLR7 sensing in mouse macrophages. Interspecies conservation of this inhibitory activity was confirmed on TLR7/8 activity in human peripheral blood mononuclear cells. Significantly, we identified a core motif governing the inhibitory activity of these AMOs, which is present in more than 50 AMOs targeted to human miRNAs in miRBaseV20. DNA/locked nucleic acids (LNA) AMOs synthesized with a phosphorothioate backbone also inhibited TLR7 sensing in a sequence-dependent manner, demonstrating that the off-target effects of AMOs are not restricted to 2'OMe modification. Taken together, our work establishes the potential for off-target effects of AMOs on TLR7/8 function, which should be taken into account in their therapeutic development and in vivo application.

Endosomal localization of TLR8 confers distinctive proteolytic processing on human myeloid cells

Nucleic acid-sensing TLRs are involved in both antimicrobial immune responses and autoimmune inflammation. TLR8 is phylogenetically and structurally related to TLR7 and TLR9, which undergo proteolytic processing in the endolysosomes to generate functional receptors. Recent structural analyses of human TLR8 ectodomain and its liganded form demonstrated that TLR8 is also cleaved, and both the N- and C-terminal halves contribute to ligand binding. However, the structures and ssRNA recognition mode of endogenous TLR8 in human primary cells are largely unknown. In this study, we show that proteolytic processing of TLR8 occurs in human monocytes and macrophages in a different manner compared with TLR7/9 cleavage. The insertion loop between leucine-rich repeats 14 and 15 in TLR8 is indispensable for the cleavage and stepwise processing that occurs in the N-terminal fragment. Both furin-like proprotein convertase and cathepsins contribute to TLR8 cleavage in the early/late endosomes. TLR8 recognizes viral ssRNA and endogenous RNA, such as microRNAs, resulting in the production of proinflammatory cytokines. Hence, localization sites of the receptors are crucial for the nucleic acid-sensing mode and downstream signaling.

Cutting edge: the UNC93B1 tyrosine-based motif regulates trafficking and TLR responses via separate mechanisms

Sensing of nucleic acids by TLRs is crucial in the host defense against viruses and bacteria. Unc-93 homolog B1 (UNC93B1) regulates the trafficking of nucleic acid-sensing TLRs from the endoplasmic reticulum to endolysosomes, where the TLRs encounter their respective ligands and become activated. In this article, we show that a carboxyl-terminal tyrosine-based sorting motif (YxxΦ) in UNC93B1 differentially regulates human nucleic acid-sensing TLRs in a receptor- and ligand-specific manner. Destruction of YxxΦ abolished TLR7, TLR8, and TLR9 activity toward nucleic acids in human B cells and monocytes, whereas TLR8 responses toward small molecules remained intact. YxxΦ in UNC93B1 influenced the subcellular localization of human UNC93B1 via both adapter protein complex (AP)1- and AP2-dependent trafficking pathways. However, loss of AP function was not causal for altered TLR responses, suggesting AP-independent functions of YxxΦ in UNC93B1.

Functional polymorphisms of TLR8 are associated with hepatitis C virus infection

Chronic hepatitis C virus (HCV) infection is a worldwide threat to public health. Toll-like receptor 8 (TLR8) is critical for eliminating RNA viruses, and variation within the TLR8 gene may alter the function of TLR8 in response to HCV infection. Our previous study demonstrated that the TLR8-129G>C (rs3764879) and TLR8+1G>A (rs3764880) variants were in complete linkage disequilibrium, and that the frequency of TLR8-129C/+1A was significantly higher in male patients with HCV infection compared with the healthy controls. In the present study, we found that the promoter activity of TLR8-129G was higher than that of TLR8-129C in THP-1 cells. Moreover, TLR8-129G mRNA stability and competitive DNA-binding ability were significantly lower than that of TLR8-129C. To investigate the functional effects of TLR8 polymorphisms, we compared the nuclear factor-κB (NF-κB)-driven luciferase activity in HEK293 cells transfected with the TLR8 variants. TLR8+1A plasmids induced less NF-κB signalling than did those transfected with TLR8+1G after 20 μm CL075 (P = 0.011) stimulation. We also analysed the mRNA expression and cytokine production in whole blood and monocytes from people of various genotypes stimulated ex vivo by the interferon-γ and TLR7/8 agonist CL075, R848. TLR8 expression in CD14⁺ cells derived from volunteers with TLR8-129G/+1G was significantly higher than that derived from TLR8-129C/+1A, and interleukin-12p40 production was higher in volunteers with TLR8-129G/+1G after stimulation. The data indicate that variations in TLR8 genes may modulate immune responses during HCV infection.

RNA recognition by human TLR8 can lead to autoimmune inflammation

High expression level of human TLR8 in mice results in spontaneous, multiorgan inflammation attributable in part to increased DC activation.

Studies on the role of the RNA receptor TLR8 in inflammation have been limited by its different function in human versus rodents. We have generated multiple lines of transgenic mice expressing different levels of human TLR8. The high copy number chimeras were unable to pass germline; developed severe inflammation targeting the pancreas, salivary glands, and joints; and the severity of the specific phenotypes closely correlated with the huTLR8 expression levels. Mice with relatively low expression levels survived and bred successfully but had increased susceptibility to collagen-induced arthritis, and the levels of huTLR8 correlated with proinflammatory cytokines in the joints of the animals. At the cellular level, huTLR8 signaling exerted a DC-intrinsic effect leading to up-regulation of co-stimulatory molecules and subsequent T cell activation. A pathogenic role for TLR8 in human diseases was suggested by its increased expression in patients with systemic arthritis and the correlation of TLR8 expression with the elevation of IL-1β levels and disease status. We found that the consequence of self-recognition via TLR8 results in a constellation of diseases, strikingly distinct from those related to TLR7 signaling, and points to specific inflammatory diseases that may benefit from inhibition of TLR8 in humans.

Inosine-mediated modulation of RNA sensing by Toll-like receptor 7 (TLR7) and TLR8

RNA-specific adenosine deaminase (ADAR)-mediated adenosine-to-inosine (A-to-I) editing is a critical arm of the antiviral response. However, mechanistic insights into how A-to-I RNA editing affects viral infection are lacking. We posited that inosine incorporation into RNA facilitates sensing of nonself RNA by innate immune sensors and accordingly investigated the impact of inosine-modified RNA on Toll-like receptor 7 and 8 (TLR7/8) sensing. Inosine incorporation into synthetic single-stranded RNA (ssRNA) potentiated tumor necrosis factor alpha (TNF-α) or alpha interferon (IFN-α) production in human peripheral blood mononuclear cells (PBMCs) in a sequence-dependent manner, indicative of TLR7/8 recruitment. The effect of inosine incorporation on TLR7/8 sensing was restricted to immunostimulatory ssRNAs and was not seen with inosine-containing short double-stranded RNAs or with a deoxy-inosine-modified ssRNA. Inosine-mediated increase of self-secondary structure of an ssRNA resulted in potentiated IFN-α production in human PBMCs through TLR7 recruitment, as established through the use of a TLR7 antagonist and Tlr7-deficient cells. There was a correlation between hyperediting of influenza A viral ssRNA and its ability to stimulate TNF-α, independent of 5'-triphosphate residues, and involving Adar-1. Furthermore, A-to-I editing of viral ssRNA directly enhanced mouse Tlr7 sensing, when present in proportions reproducing biologically relevant levels of RNA editing. Thus, we demonstrate for the first time that inosine incorporation into immunostimulatory ssRNA can potentiate TLR7/8 activation. Our results suggest a novel function of A-to-I RNA editing, which is to facilitate TLR7/8 sensing of phagocytosed viral RNA.

Development of Magnetic Nanoparticles for Cancer Gene Therapy: A Comprehensive Review

Since they were first proposed as nonviral transfection agents for their gene-carrying capacity, magnetic nanoparticles have been studied thoroughly, both in vitro and in vivo. Great effort has been made to manufacture biocompatible magnetic nanoparticles for use in the theragnosis of cancer and other diseases. Here we survey recent advances in the study of magnetic nanoparticles, as well as the polymers and other coating layers currently available for gene therapy, their synthesis, and bioconjugation processes. In addition, we review several gene therapy models based on magnetic nanoparticles.

Messenger RNA-based vaccines: progress, challenges, applications

Twenty years after the demonstration that messenger RNA (mRNA) was expressed and immunogenic upon direct injection in mice, the first successful proof-of-concept of specific protection against viral infection in small and large animals was reported. These data indicate wider applicability to infectious disease and should encourage continued translation of mRNA-based prophylactic vaccines into human clinical trials. At the conceptual level, mRNA-based vaccines-more than other genetic vectors-combine the simplicity, safety, and focused immunogenicity of subunit vaccines with favorable immunological properties of live viral vaccines: (1) mRNA vaccines are molecularly defined and carry no excess information. In the environment and upon physical contact, RNA is rapidly degraded by ubiquitous RNases and cannot persist. These characteristics also guarantee tight control over their immunogenic profile (including avoidance of vector-specific immune responses that could interfere with repeated administration), pharmacokinetics, and dosing. (2) mRNA vaccines are synthetically produced by an enzymatic process, just requiring information about the nucleic acid sequence of the desired antigen. This greatly reduces general complications associated with biological vaccine production, such as handling of infectious agents, genetic variability, environmental risks, or restrictions to vaccine distribution. (3) RNA can be tailored to provide potent adjuvant stimuli to the innate immune system by direct activation of RNA-specific receptors; this may reduce the need for additional adjuvants. The formation of native antigen in situ affords great versatility, including intracellular localization, membrane association, posttranslational modification, supra-molecular assembly, or targeted structural optimization of delivered antigen. Messenger RNA vaccines induce balanced immune responses including B cells, helper T cells, and cytotoxic T lymphocytes, rendering them an extremely adaptable platform. This article surveys the design, mode of action, and capabilities of state-of-the-art mRNA vaccines, focusing on the paradigm of influenza prophylaxis.

A novel antagonist of Toll-like receptors 7, 8 and 9 suppresses lupus disease-associated parameters in NZBW/F1 mice

Systemic Lupus Erythematosus is an autoimmune disease characterized by production of autoantibodies against nucleic acid-associated antigens. Endogenous DNA and RNA associated with these antigens stimulate inflammatory responses through Toll-like receptors (TLRs) and exacerbate lupus disease pathology. We have evaluated an antagonist of TLR7, 8 and 9 as a therapeutic agent in lupus-prone NZBW/F1 mice. NZBW/F1 mice treated with the antagonist had lower serum levels of autoantibodies targeting DNA, RNP, Smith antigen, SSA and SSB than did untreated mice. Reduction in blood urea nitrogen and proteinuria and improvements in kidney histopathology were observed in antagonist-treated mice. The antagonist treatment also reduced serum IL-12 and IL-1β and increased IL-10 levels. Levels of mRNA for IL-6, iNOS and IL-1β were lower in the kidneys and spleen of antagonist-treated mice than in those of untreated mice. Levels of mRNA for IP-10, TNFRSF9 and FASL were lower and IL-4 mRNA were higher in spleens of antagonist-treated mice than in spleens of untreated mice. mRNA for the inflammasome component NLRP3 was lower and mRNA for the antioxidant enzymes, catalase and glutathione peroxidase 1 was higher in the kidneys of antagonist-treated mice than in those of untreated mice. These results show that the antagonist of TLR7, 8 and 9 effectively inhibits inflammatory pathways involved in the development of lupus in NZBW/F1 mice and constitutes a potential therapeutic approach for the treatment of lupus and other autoimmune diseases.

Critical differences in hematopoiesis and lymphoid development between humans and mice

During the last five decades, elegant mouse models of hematopoiesis have yielded most of the seminal insights into this complex biological system of self-renewal and lineage commitment. More recent advances in assays to measure human stem and progenitor cells as well as high resolution RNA profiling have revealed that although the basic roadmap of blood development is generally conserved across mammals, evolutionary pressures have generated many differences between the species that have important biological and translational implications. To enhance the utility of the mouse as a model organism, it is more important than ever that research data are presented with regard to how they might be influenced by the species of origin as well as the developmental source of the hematopoietic tissue.

A Toll-like receptor 7, 8, and 9 antagonist inhibits Th1 and Th17 responses and inflammasome activation in a model of IL-23-induced psoriasis

Psoriasis is a chronic inflammatory skin disease that involves the induction of T-helper 1 (Th1) and T-helper 17 (Th17) cell responses and the aberrant expression of proinflammatory cytokines, including IL-1β. Copious evidence suggests that abnormal activation of Toll-like receptors (TLRs) contributes to the initiation and maintenance of psoriasis. We have evaluated an antagonist of TLR7, 8, and 9 as a therapeutic agent in an IL-23-induced psoriasis model in mice. Psoriasis-like skin lesions were induced in C57BL/6 mice by intradermal injection of IL-23 in the ear or dorsum. IL-23-induced increase in ear thickness was inhibited in a dose-dependent manner by treatment with antagonist. Histological examination of ear and dorsal skin tissues demonstrated a reduction in epidermal hyperplasia in mice treated with the antagonist. Treatment with antagonist also reduced the induction of Th1 and Th17 cytokines in skin and/or serum, as well as dermal expression of inflammasome components, NLRP3 and AIM2, and antimicrobial peptides. These results indicate that targeting TLR7, 8, and 9 may provide a way to neutralize multiple inflammatory pathways that are involved in the development of psoriasis. The antagonist has the potential for the treatment of psoriasis and other autoimmune diseases.