HMGB1 Activates Proinflammatory Signaling via TLR5 Leading to Allodynia

Compartmentalized Antimicrobial Defenses in Response to Flagellin

Motility is often a pathogenicity determinant of bacteria targeting mucosal tissues. Flagella constitute the machinery that propels bacteria into appropriate niches. Besides motility, the structural component, flagellin, which forms the flagella, targets Toll-like receptor 5 (TLR5) to activate innate immunity. The compartmentalization of flagellin-mediated immunity and the contribution of epithelial cells and dendritic cells in detecting flagellin within luminal and basal sides are highlighted here, respectively. While a direct stimulation of the epithelium mainly results in recruitment of immune cells and production of antimicrobial molecules, TLR5 engagement on parenchymal dendritic cells can contribute to the stimulation of innate lymphocytes such as type 3 innate lymphoid cells, as well as T helper cells. This review, therefore, illustrates how the innate and adaptive immunity to flagellin are differentially regulated by the epithelium and the dendritic cells in response to pathogens that either colonize or invade mucosa.

VLR Recognition of TLR5 Expands the Molecular Characterization of Protein Antigen Binding by Non-Ig-based Antibodies

Variable lymphocyte receptors (VLRs) are unconventional adaptive immune receptors relatively recently discovered in the phylogenetically ancient jawless vertebrates, lamprey and hagfish. VLRs bind antigens using a leucine-rich repeat fold and are the only known adaptive immune receptors that do not utilize an immunoglobulin fold for antigen recognition. While immunoglobulin antibodies have been studied extensively, there are comparatively few studies on antigen recognition by VLRs, particularly for protein antigens. Here we report isolation, functional and structural characterization of three VLRs that bind the protein toll-like receptor 5 (TLR5) from zebrafish. Two of the VLRs block binding of TLR5 to its cognate ligand flagellin in functional assays using reporter cells. Co-crystal structures revealed that these VLRs bind to two different epitopes on TLR5, both of which include regions involved in flagellin binding. Our work here demonstrates that the lamprey adaptive immune system can be used to generate high-affinity VLR clones that recognize different epitopes and differentially impact natural ligand binding to a protein antigen.

Small-molecule inhibition of TLR8 through stabilization of its resting state

Endosomal Toll-like receptors (TLR3/7/8/9) are highly analogous sensors for various viral or bacterial RNA/DNA molecular patterns. Nonetheless, few small-molecules can selectively modulate these TLRs. In this manuscript, we identified the first human TLR8-specific small-molecule antagonists via a novel inhibition mechanism. Crystal structures of two distinct TLR8-ligand complexes validated a unique binding site on the protein-protein interface of the TLR8 homodimer. Upon binding to this new site, the small-molecule ligands stabilize the preformed TLR8 dimer in its resting state, preventing activation. As a proof of concept of their therapeutic potential, we have demonstrated that these drug-like inhibitors are able to suppress TLR8-mediated proinflammatory signaling in various cell lines, human primary cells, and patient specimens. These results not only suggest a novel strategy for TLR inhibitor design, but also shed critical mechanistic insight into these clinically important immune receptors.

Elevated microRNA-129-5p level ameliorates neuroinflammation and blood-spinal cord barrier damage after ischemia-reperfusion by inhibiting HMGB1 and the TLR3-cytokine pathway

Background

Ischemia-reperfusion (IR) affects microRNA (miR) expression and causes substantial inflammation. Multiple roles of the tumor suppressor miR-129-5p in cerebral IR have recently been reported, but its functions in the spinal cord are unclear. Here, we investigated the role of miR-129-5p after spinal cord IR, particularly in regulating high-mobility group box-1 (HMGB1) and the Toll-like receptor (TLR)-3 pathway.

Methods

Ischemia was induced via 5-min occlusion of the aortic arch. The relationship between miR-129-5p and HMGB1 was elucidated via RT-PCR, western blotting, and luciferase assays. The cellular distribution of HMGB1 was determined via double immunofluorescence. The effect of miR-129-5p on the expression of HMGB1, TLR3, and downstream cytokines was evaluated using synthetic miRs, rHMGB1, and the TLR3 agonist Poly(I:C). Blood-spinal cord barrier (BSCB) permeability was examined by measuring Evans blue (EB) dye extravasation and the water content.

Results

The temporal miR-129-5p and HMGB1 expression profiles and luciferase assay results indicated that miR-129-5p targeted HMGB1. Compared with the Sham group, the IR group had higher HMGB1 immunoreactivity, which was primarily distributed in neurons and microglia. Intrathecal injection of the miR-129-5p mimic significantly decreased the HMGB1, TLR3, interleukin (IL)-1β and tumor necrosis factor (TNF)-α levels and the double-labeled cell count 48 h post-surgery, whereas rHMGB1 and Poly(I:C) reversed these effects. Injection of miR-129-5p mimic preserved motor function and prevented BSCB leakage based on increased Basso Mouse Scale scores and decreased EB extravasation and water content, whereas injection rHMGB1 and Poly(I:C) aggravated these injuries.

Conclusions

Increasing miR-129-5p levels protect against IR by ameliorating inflammation-induced neuronal and BCSB damage by inhibiting HMGB1 and TLR3-associated cytokines.

Canonical NFκB signaling in myeloid cells is required for the glioblastoma growth

Tumor development and therapeutic resistance are linked with tumor-associated macrophage (TAM) and myeloid-derived suppressor cell (MDSC) infiltration in tumors via chemokine axis. Chemokine expression, which determines the pro or anti-inflammatory status of myeloid cells, are partly regulated by the nuclear factor-kappa B (NF-κB) pathway. Here, we identified that conditional deletion of canonical NF-κB signaling (p65) in myeloid cells inhibited syngeneic glioblastoma (GBM) through decreased CD45 infiltration in tumors, as characterized by decreased TAMs (CD206+) and MDSCs (Gr1+ CD11b+), increased dendritic cells (CD86+) and cytotoxic T cells (CD8+) in the p65 knockout (KO) mice. Proinflammatory cytokines (IFNγ, MCP1, MIP1α, and TNFα) and myeloid differentiation factor (Endoglin) were increased in myeloid cells from p65 KO tumor, which demonstrated an influence on CD8+T cell proliferation. In contrast, p65KO athymic chimeric mice with human GBM, failed to inhibit tumor growth, confirming the contribution of T cells in an immune competent model. The analysis of human datasets and GBM tumors revealed higher expression of p65 in GBM-associated CD68+ macrophages compared to neighboring stroma. Thus, canonical NF-κB signaling has an anti-inflammatory role and is required for macrophage polarization, immune suppression, and GBM growth. Combining an NF-κB inhibitor with standard therapy could improve antitumor immunity in GBM.

Toll-like receptors and their role in persistent pain

One of the fundamental mechanisms whereby the innate immune system coordinates inflammatory signal transduction is through Toll-like receptors (TLRs), which function to protect and defend the host organism by initiating inflammatory signaling cascades in response to tissue damage or injury. TLRs are positioned at the neuroimmune interface, and accumulating evidence suggests that the inflammatory consequences of TLR activation on glia (including microglia and astrocytes), sensory neurons, and other cell types can influence nociceptive processing and lead to states of exaggerated and unresolved pain. In this review, we summarize our current understanding of how different TLRs and their accessory or adaptor molecules can contribute to the development and maintenance of persistent pain. The challenges and opportunities of targeting TLRs for new treatment strategies against chronic pain are discussed, including the therapeutic context of TLR-mediated signaling in opioid analgesia and chemotherapy-induced pain. Considering the prevalence of persistent pain and the insufficient efficacy and safety of current treatment options, a deeper understanding of Toll-like receptors holds the promise of novel therapies for managing pathological pain.

MiR-150 alleviates neuropathic pain via inhibiting toll-like receptor 5

MicroRNAs (miRNAs) are reported as vital participators in the pathophysiological course of neuropathic pain. However, the underlying mechanisms of the functional roles of miRNAs in neuropathic pain are largely unknown. This study was designed to explore the potential role of miR-150 in regulating the process of neuropathic pain in a rat model established by chronic sciatic nerve injury (CCI). Overexpression of miR-150 greatly alleviated neuropathic pain development and reduced inflammatory cytokine expression, including COX-2, interleukin IL-6, and tumor necrosis factor (TNF)-α in CCI rats. By bioinformatic analysis, 3'-untranslated region (UTR) of Toll-like receptor (TLR5) was predicted to be a target of miR-150. TLR5 commonly serves as an important regulator of inflammation. Overexpression of miR-150 significantly suppressed the expression of TLR5 in vitro and in vivo. Furthermore, upregulation of TLR5 decreased the miR-150 expression and downregulation of TLR5 increased miR-150, respectively. Overexpression of TLR5 significantly reversed the miR-150-induced suppressive effects on neuropathic pain. In conclusion, our current study indicates that miR-150 may inhibit neuropathic pain development of CCI rats through inhibiting TLR5-mediated neuroinflammation. Our findings suggest that miR-150 may provide a novel therapeutic target for neuropathic pain treatment.

Pain in ankylosing spondylitis: a neuro-immune collaboration

Clinicians have commonly differentiated chronic back pain into two broad subsets: namely, non-inflammatory (or mechanical) back pain and inflammatory back pain. As the terminology suggests, the latter category, in which ankylosing spondylitis (AS) is prominent, presupposes a close link between pain and inflammation. Advances in research into the genetics and immunology of AS have improved our understanding of the inflammatory processes involved in this disease, and have led to the development of potent anti-inflammatory biologic therapeutic agents. However, evidence from clinical trials and from biomarker and imaging studies in patients with AS indicate that pain and inflammation are not always correlated. Thus, the assumption that pain in AS is a reliable surrogate marker for inflammation might be an over-simplification. This Review provides an overview of current concepts relating to neuro-immune interactions in AS and summarizes research that reveals an increasingly complex interplay between the activation of the immune system and pain pathways in the nervous system. The different types of pain experienced by patients with AS, insights from brain imaging studies, neurological mechanisms of pain, sex bias in pain and how the immune system can modify pain in patients with AS are also discussed.

Suppression of MyD88-dependent signaling alleviates neuropathic pain induced by peripheral nerve injury in the rat

Background

MyD88 is the adaptor protein of MyD88-dependent signaling pathway of TLRs and IL-1 receptor and regulates innate immune response. However, it was not clear whether and how MyD88 and related signaling pathways in the dorsal root ganglion (DRG) and spinal dorsal horn (SDH) are involved in neuropathic pain.

Methods

Chronic constriction injury (CCI) was used to induce neuropathic pain in the rat. The expression of MyD88, TRIF, IBA1, and GFAP was detected with immunofluorescent staining and Western blot. The expression of interleukin-1 beta (IL-1β), high mobility group box 1 (HMGB1), NF-κB-p65, phosphorylated NF-κB-p65, ERK, phosphorylated ERK, and tumor necrosis factor-alpha (TNF-α) was detected with Western blot. Pain-related behavioral effects of MyD88 homodimerization inhibitory peptide (MIP) were accessed up to 3 weeks after intrathecal administration.

Results

Peripheral nerve injury significantly increased the protein level of MyD88 in the DRG and SDH, but had no effect on TRIF. MyD88 was found partly distributed in the nociceptive neurons in the DRGs and the astrocytes and microglia in the SDH. HMGB1 and IL-1β were also found upregulated in nociceptive pathways of CCI rats. Intrathecal application of MIP significantly alleviated mechanical and thermal hyperalgesia in the CCI rats and also reversed CCI-induced upregulation of MyD88 in both DRG and SDH. Further investigation revealed that suppression of MyD88 protein reduced the release of TNF-α and glial activation in the SDH in the CCI rats.

Conclusions

MyD88-dependent TIR pathway in the DRG and SDH may play a role in CCI-induced neuropathic pain. MyD88 might serve as a potential therapeutic target for neuropathic pain.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-017-0822-9) contains supplementary material, which is available to authorized users.