Crosstalk between the nociceptive and immune systems in host defence and disease

An interdisciplinary perspective on peripheral drivers of pain in rheumatoid arthritis

Pain is one of the most debilitating symptoms of rheumatoid arthritis (RA), and yet remains poorly understood, especially when pain occurs in the absence of synovitis. Without active inflammation, experts most often attribute joint pain to central nervous system dysfunction. However, advances in the past 5 years in both immunology and neuroscience research suggest that chronic pain in RA is also driven by a variety of abnormal interactions between peripheral neurons and mediators produced by resident cells in the local joint environment. In this Review, we discuss these novel insights from an interdisciplinary neuro-immune perspective. We outline a potential working model for the peripheral drivers of pain in RA, which includes autoantibodies, resident immune and mesenchymal cells and their interactions with different subtypes of peripheral sensory neurons. We also offer suggestions for how future collaborative research could be designed to accelerate analgesic drug development.

Activation of TGR5 in the injured nerve site according to a prevention protocol mitigates partial sciatic nerve ligation-induced neuropathic pain by alleviating neuroinflammation

ABSTRACT

Neuropathic pain is a pervasive medical challenge currently lacking effective treatment options. Molecular changes at the site of peripheral nerve injury contribute to both peripheral and central sensitization, critical components of neuropathic pain. This study explores the role of the G-protein-coupled bile acid receptor (GPBAR1 or TGR5) in the peripheral mechanisms underlying neuropathic pain induced by partial sciatic nerve ligation in male mice. TGR5 was upregulated in the injured nerve site and predominantly colocalized with macrophages. Perisciatic nerve administration of the TGR5 agonist, INT-777 according to a prevention protocol (50 μg/μL daily from postoperative day [POD] 0 to POD6) provided sustained relief from mechanical allodynia and spontaneous pain, whereas the TGR5 antagonist, SBI-115 worsened neuropathic pain. Transcriptome sequencing linked the pain relief induced by TGR5 activation to reduced neuroinflammation, which was further evidenced by a decrease in myeloid cells and pro-inflammatory mediators (eg, CCL3, CXCL9, interleukin [IL]-6, and tumor necrosis factor [TNF] α) and an increase in CD86-CD206+ anti-inflammatory macrophages at POD7. Besides, myeloid-cell-specific TGR5 knockdown in the injured nerve site exacerbated both neuropathic pain and neuroinflammation, which was substantiated by bulk RNA-sequencing and upregulated expression levels of inflammatory mediators (including CCL3, CCL2, IL-6, TNF α, and IL-1β) and the increased number of monocytes/macrophages at POD7. Furthermore, the activation of microglia in the spinal cord on POD7 and POD14 was altered when TGR5 in the sciatic nerve was manipulated. Collectively, TGR5 activation in the injured nerve site mitigates neuropathic pain by reducing neuroinflammation, while TGR5 knockdown in myeloid cells worsens pain by enhancing neuroinflammation.

B cells drive neuropathic pain-related behaviors in mice through IgG-Fc gamma receptor signaling

Neuroimmune interactions are essential for the development of neuropathic pain, yet the contributions of distinct immune cell populations have not been fully unraveled. Here, we demonstrate the critical role of B cells in promoting mechanical hypersensitivity (allodynia) after peripheral nerve injury in male and female mice. Depletion of B cells with a single injection of anti-CD20 monoclonal antibody at the time of injury prevented the development of allodynia. B cell-deficient (muMT) mice were similarly spared from allodynia. Nerve injury was associated with increased immunoglobulin G (IgG) accumulation in ipsilateral lumbar dorsal root ganglia (DRGs) and dorsal spinal cords. IgG was colocalized with sensory neurons and macrophages in DRGs and microglia in spinal cords. IgG also accumulated in DRG samples from human donors with chronic pain, colocalizing with a marker for macrophages and satellite glia. RNA sequencing revealed a B cell population in naive mouse and human DRGs. A B cell transcriptional signature was enriched in DRGs from human donors with neuropathic pain. Passive transfer of IgG from injured mice induced allodynia in injured muMT recipient mice. The pronociceptive effects of IgG are likely mediated through immune complexes interacting with Fc gamma receptors (FcγRs) expressed by sensory neurons, microglia, and macrophages, given that both mechanical allodynia and hyperexcitability of dissociated DRG neurons were abolished in nerve-injured FcγR-deficient mice. Consistently, the pronociceptive effects of IgG passive transfer were lost in FcγR-deficient mice. These data reveal that a B cell-IgG-FcγR axis is required for the development of neuropathic pain in mice.

RNA sequencing of the thalamus and rostral ventral medulla in rats with chronic orofacial pain

Diagnosing and treating chronic orofacial pain is challenging due to its complex structure and limited understanding of its causes and mechanisms. In this study, we used RNA sequencing to identify differentially expressed genes (DEGs) in the rostral ventral medulla (RVM) and thalamus of rats with persistent orofacial pain, aiming to explore its development. DEGs were functionally analyzed using gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis. Results showed a significant association between immune response and pain in this model. Key DEG mRNA expression trends were further validated using real-time quantitative polymerase chain reaction (RT-PCR), confirming their crucial roles in chronic orofacial pain. After injecting complete Freund's adjuvant (CFA) into the bilateral temporomandibular joint cavity for 14 days, we observed 293 upregulated genes and 14 downregulated genes in the RVM, and 1086 upregulated genes and 37 downregulated genes in the thalamus. Furthermore, we identified 27 common DEGs with altered expression (upregulation) in both the thalamus and RVM, including Cd74, C3, Cxcl13, C1qb, Itgal, Fcgr2b, C5ar1, and Tlr2, which are pain-associated genes. Protein-protein interaction (PPI) analysis using Cytoscape revealed the involvement of Toll-like receptors, complement system, differentiation clusters, and antigen presentation-related proteins in the interaction between the thalamus and RVM. The results of this study show that the immune system seems to have a more significant influence on chronic orofacial pain. There may be direct or indirect influence between the thalamus and RVM, which may participate in the regulation of chronic orofacial pain.

Discovery of a CCR2-targeting pepducin therapy for chronic pain

Targeting the CCL2/CCR2 chemokine axis has been shown to be effective at relieving pain in rodent models of inflammatory and neuropathic pain, therefore representing a promising avenue for the development of non-opioid analgesics. However, clinical trials targeting this receptor for inflammatory conditions and painful neuropathies have failed to meet expectations and have all been discontinued due to lack of efficacy. To overcome the poor selectivity of CCR2 chemokine receptor antagonists, we generated and characterized the function of intracellular cell-penetrating allosteric modulators targeting CCR2, namely pepducins. In vivo, chronic intrathecal administration of the CCR2-selective pepducin PP101 was effective in alleviating neuropathic and bone cancer pain. In the setting of bone metastases, we found that T cells infiltrate dorsal root ganglia (DRG) and induce long-lasting pain hypersensitivity. By acting on CCR2-expressing DRG neurons, PP101 attenuated the altered phenotype of sensory neurons as well as the neuroinflammatory milieu of DRGs, and reduced bone cancer pain by blocking CD4+ and CD8+ T cell infiltration. Notably, PP101 demonstrated its efficacy in targeting the neuropathic component of bone cancer pain, as evidenced by its anti-nociceptive effects in a model of chronic constriction injury of the sciatic nerve. Importantly, PP101-induced reduction of CCR2 signaling in DRGs did not result in deleterious tumor progression or adverse behavioral effects. Thus, targeting neuroimmune crosstalk through allosteric inhibition of CCR2 could represent an effective and safe avenue for the management of chronic pain.

BDNF in Neuropathic Pain; the Culprit that Cannot be Apprehended

In males but not in females, brain derived neurotrophic factor (BDNF) plays an obligatory role in the onset and maintenance of neuropathic pain. Afferent terminals of injured peripheral nerves release colony stimulating factor (CSF-1) and other mediators into the dorsal horn. These transform the phenotype of dorsal horn microglia such that they express P2X4 purinoceptors. Activation of these receptors by neuron-derived ATP promotes BDNF release. This microglial-derived BDNF increases synaptic activation of excitatory dorsal horn neurons and decreases that of inhibitory neurons. It also alters the neuronal chloride gradient such the normal inhibitory effect of GABA is converted to excitation. By as yet undefined processes, this attenuated inhibition increases NMDA receptor function. BDNF also promotes the release of pro-inflammatory cytokines from astrocytes. All of these actions culminate in the increase dorsal horn excitability that underlies many forms of neuropathic pain. Peripheral nerve injury also alters excitability of structures in the thalamus, cortex and mesolimbic system that are responsible for pain perception and for the generation of co-morbidities such as anxiety and depression. The weight of evidence from male rodents suggests that this preferential modulation of excitably of supra-spinal pain processing structures also involves the action of microglial-derived BDNF. Possible mechanisms promoting the preferential release of BDNF in pain signaling structures are discussed. In females, invading T-lymphocytes increase dorsal horn excitability but it remains to be determined whether similar processes operate in supra-spinal structures. Despite its ubiquitous role in pain aetiology neither BDNF nor TrkB receptors represent potential therapeutic targets.

Navigating the blurred path of mixed neuroimmune signaling

Evolution has created complex mechanisms to sense environmental danger and protect tissues, with the nervous and immune systems playing pivotal roles. These systems work together, coordinating local and systemic reflexes to restore homeostasis in response to tissue injury and infection. By sharing receptors and ligands, they influence the pathogenesis of various diseases. Recently, a less-explored aspect of neuroimmune communication has emerged: the release of neuropeptides from immune cells and cytokines/chemokines from sensory neurons. This article reviews evidence of this unique neuroimmune interplay and its impact on the development of allergy, inflammation, itch, and pain. We highlight the effects of this neuroimmune signaling on vital processes such as host defense, tissue repair, and inflammation resolution, providing avenues for exploration of the underlying mechanisms and therapeutic potential of this signaling.

Somatosensory cortex and central amygdala regulate neuropathic pain-mediated peripheral immune response via vagal projections to the spleen

Pain involves neuroimmune crosstalk, but the mechanisms of this remain unclear. Here we showed that the splenic T helper 2 (TH2) immune cell response is differentially regulated in male mice with acute versus chronic neuropathic pain and that acetylcholinergic neurons in the dorsal motor nucleus of the vagus (AChDMV) directly innervate the spleen. Combined in vivo recording and immune cell profiling revealed the following two distinct circuits involved in pain-mediated peripheral TH2 immune response: glutamatergic neurons in the primary somatosensory cortex (GluS1HL)→AChDMV→spleen circuit and GABAergic neurons in the central nucleus of the amygdala (GABACeA)→AChDMV→spleen circuit. The acute pain condition elicits increased excitation from GluS1HL neurons to spleen-projecting AChDMV neurons and increased the proportion of splenic TH2 immune cells. The chronic pain condition increased inhibition from GABACeA neurons to spleen-projecting AChDMV neurons and decreased splenic TH2 immune cells. Our study thus demonstrates how the brain encodes pain-state-specific immune responses in the spleen.

C-Reactive Protein (CRP) is Associated With Chronic Pain Independently of Biopsychosocial Factors

Inflammation is linked with chronic pain but the extent to which this relationship is associated with biopsychosocial factors is not known. We investigated relationships between blood C-reactive protein (CRP) and regional chronic pain conditions adjusting for a large range and number of potential confounders. We performed cross-sectional analyses using the UK Biobank (N = 415,567) comparing CRP in people reporting any of 9 types of regional chronic pain with pain-free controls. Using logistic regression modelling, we explored relationships between CRP and the presence of chronic pain, with demographic, socioeconomic, psychological/lifestyle factors, and medical comorbidities as covariates. CRP was higher in chronic pain at any site compared with controls (Females: median [interquartile range] 1.60 mg/L [2.74] vs 1.17 mg/L [1.87], P < .001; Males: 1.44 mg/L [2.12] vs 1.15 mg/L [1.65], P < .001). In males, associations between CRP and all types of chronic pain were attenuated but remained significant after adjustment for biopsychosocial covariates (OR range 1.08-1.49, P ≤ .001). For females, adjusted associations between CRP and pain remained significant for most chronic pain types (OR range 1.07-1.34, P < .001) except for facial pain (OR 1.04, P = .17) and headache (OR 1.02, P = .07)-although these non-significant findings may reflect reduced sample size. The significant association between CRP and chronic pain after adjustment for key biopsychosocial confounders implicates an independent underlying biological mechanism of inflammation in chronic pain. The presence of yet unknown or unmeasured confounding factors cannot be ruled out. Our findings may inform better-targeted treatments for chronic pain. PERSPECTIVE: Using a large-scale dataset, this article investigates associations between chronic pain conditions and blood C-reactive protein (CRP), to evaluate the confounding effects of a range of biopsychosocial factors. CRP levels were higher in those with chronic pain versus controls after adjusting for confounders-suggesting a possible independent biological mechanism.

Interleukin-10 signaling in somatosensory neurons controls CCL2 release and inflammatory response

Appropriate regulation of the inflammatory response is essential for survival. Interleukin-10 (IL-10), a well-known anti-inflammatory cytokine, plays a major role in controlling inflammation. In addition to immune cells, we previously demonstrated that the IL-10 receptor (IL-10R1) is expressed in dorsal root ganglion sensory neurons. There is emerging evidence that these sensory neurons contribute to immunoregulation, and we hypothesized that IL-10 signaling in dorsal root ganglion (DRG) neurons facilitates the regulation of the inflammatory response. We showed that mice that lack IL-10R1 specifically on advillin-positive neurons have exaggerated blood nitric oxide levels, spinal microglia activation, and cytokine upregulation in the spinal cord, liver, and gut compared to wild-type (WT) counterparts in response to systemic lipopolysaccharide (LPS) injection. Lack of IL-10R1 in DRG and trigeminal ganglion (TG) neurons also increased circulating and DRG levels of proinflammatory C-C motif chemokine ligand 2 (CCL2). Interestingly, analysis of published scRNA-seq data revealed that Ccl2 and Il10ra are expressed by similar types of DRG neurons; nonpeptidergic P2X purinoceptor (P2X3R + ) neurons. In primary cultures of DRG neurons, we demonstrated that IL-10R1 inhibits the production of CCL2, but not that of the neuropeptides substance P and calcitonin-gene related peptide (CGRP). Furthermore, our data indicate that ablation of Transient receptor potential vanilloid (TRPV)1 + neurons does not impact the regulation of CCL2 production by IL-10. In conclusion, we showed that IL-10 binds to its receptor on sensory neurons to downregulate CCL2 and contribute to immunoregulation by reducing the attraction of immune cells by DRG neuron-derived CCL2. This is the first evidence that anti-inflammatory cytokines limit inflammation through direct binding to receptors on sensory neurons. Our data also add to the growing literature that sensory neurons have immunomodulatory functions.

The Known Biology of Neuropathic Pain and Its Relevance to Pain Management

Patients with neuropathic pain are heterogeneous in pathophysiology, etiology, and clinical presentation. Signs and symptoms are determined by the nature of the injury and factors such as genetics, sex, prior injury, age, culture, and environment. Basic science has provided general information about pain etiology by studying the consequences of peripheral injury in rodent models. This is associated with the release of inflammatory cytokines, chemokines, and growth factors that sensitize sensory nerve endings, alter gene expression, promote post-translational modification of proteins, and alter ion channel function. This leads to spontaneous activity in primary afferent neurons that is crucial for the onset and persistence of pain and the release of secondary mediators such as colony-stimulating factor 1 from primary afferent terminals. These promote the release of tertiary mediators such as brain-derived neurotrophic factor and interleukin-1β from microglia and astrocytes. Tertiary mediators facilitate the transmission of nociceptive information at the spinal, thalamic, and cortical levels. For the most part, these findings have failed to identify new therapeutic approaches. More recent basic science has better mirrored the clinical situation by addressing the pathophysiology associated with specific types of injury, refinement of methodology, and attention to various contributory factors such as sex. Improved quantification of sensory profiles in each patient and their distribution into defined clusters may improve translation between basic science and clinical practice. If such quantification can be traced back to cellular and molecular aspects of pathophysiology, this may lead to personalized medicine approaches that dictate a rational therapeutic approach for each individual.

The Impact of the Nervous System on Arteries and the Heart: The Neuroimmune Cardiovascular Circuit Hypothesis

Three systemic biological systems, i.e., the nervous, the immune, and the cardiovascular systems, form a mutually responsive and forward-acting tissue network to regulate acute and chronic cardiovascular function in health and disease. Two sub-circuits within the cardiovascular system have been described, the artery brain circuit (ABC) and the heart brain circuit (HBC), forming a large cardiovascular brain circuit (CBC). Likewise, the nervous system consists of the peripheral nervous system and the central nervous system with their functional distinct sensory and effector arms. Moreover, the immune system with its constituents, i.e., the innate and the adaptive immune systems, interact with the CBC and the nervous system at multiple levels. As understanding the structure and inner workings of the CBC gains momentum, it becomes evident that further research into the CBC may lead to unprecedented classes of therapies to treat cardiovascular diseases as multiple new biologically active molecules are being discovered that likely affect cardiovascular disease progression. Here, we weigh the merits of integrating these recent observations in cardiovascular neurobiology into previous views of cardiovascular disease pathogeneses. These considerations lead us to propose the Neuroimmune Cardiovascular Circuit Hypothesis.

Macrophages and microglia in inflammation and neuroinflammation underlying different pain states

Pain is a main symptom in inflammation, and inflammation induces pain via inflammatory mediators acting on nociceptive neurons. Macrophages and microglia are distinct cell types, representing immune cells and glial cells, respectively, but they share similar roles in pain regulation. Macrophages are key regulators of inflammation and pain. Macrophage polarization plays different roles in inducing and resolving pain. Notably, macrophage polarization and phagocytosis can be induced by specialized pro-resolution mediators (SPMs). SPMs also potently inhibit inflammatory and neuropathic pain via immunomodulation and neuromodulation. In this review, we discuss macrophage signaling involved in pain induction and resolution, as well as in maintaining physiological pain. Microglia are macrophage-like cells in the central nervous system (CNS) and drive neuroinflammation and pathological pain in various inflammatory and neurological disorders. Microglia-produced inflammatory cytokines can potently regulate excitatory and inhibitory synaptic transmission as neuromodulators. We also highlight sex differences in macrophage and microglial signaling in inflammatory and neuropathic pain. Thus, targeting macrophage and microglial signaling in distinct locations via pharmacological approaches, including immunotherapies, and non-pharmacological approaches will help to control chronic inflammation and chronic pain.

A review of the pathophysiology and the role of ion channels on bronchial asthma

Asthma is one of the main non-communicable chronic diseases and affects a huge portion of the population. It is a multifactorial disease, classified into several phenotypes, being the allergic the most frequent. The pathophysiological mechanism of asthma involves a Th2-type immune response, with high concentrations of allergen-specific immunoglobulin E, eosinophilia, hyperreactivity and airway remodeling. These mechanisms are orchestrated by intracellular signaling from effector cells, such as lymphocytes and eosinophils. Ion channels play a fundamental role in maintaining the inflammatory response on asthma. In particular, transient receptor potential (TRP), stock-operated Ca2+ channels (SOCs), Ca2+-activated K+ channels (IKCa and BKCa), calcium-activated chloride channel (TMEM16A), cystic fibrosis transmembrane conductance regulator (CFTR), piezo-type mechanosensitive ion channel component 1 (PIEZO1) and purinergic P2X receptor (P2X). The recognition of the participation of these channels in the pathological process of asthma is important, as they become pharmacological targets for the discovery of new drugs and/or pharmacological tools that effectively help the pharmacotherapeutic follow-up of this disease, as well as the more specific mechanisms involved in worsening asthma.

Neuropathic pain; what we know and what we should do about it

Neuropathic pain can result from injury to, or disease of the nervous system. It is notoriously difficult to treat. Peripheral nerve injury promotes Schwann cell activation and invasion of immunocompetent cells into the site of injury, spinal cord and higher sensory structures such as thalamus and cingulate and sensory cortices. Various cytokines, chemokines, growth factors, monoamines and neuropeptides effect two-way signalling between neurons, glia and immune cells. This promotes sustained hyperexcitability and spontaneous activity in primary afferents that is crucial for onset and persistence of pain as well as misprocessing of sensory information in the spinal cord and supraspinal structures. Much of the current understanding of pain aetiology and identification of drug targets derives from studies of the consequences of peripheral nerve injury in rodent models. Although a vast amount of information has been forthcoming, the translation of this information into the clinical arena has been minimal. Few, if any, major therapeutic approaches have appeared since the mid 1990's. This may reflect failure to recognise differences in pain processing in males vs. females, differences in cellular responses to different types of injury and differences in pain processing in humans vs. animals. Basic science and clinical approaches which seek to bridge this knowledge gap include better assessment of pain in animal models, use of pain models which better emulate human disease, and stratification of human pain phenotypes according to quantitative assessment of signs and symptoms of disease. This can lead to more personalized and effective treatments for individual patients. Significance statement: There is an urgent need to find new treatments for neuropathic pain. Although classical animal models have revealed essential features of pain aetiology such as peripheral and central sensitization and some of the molecular and cellular mechanisms involved, they do not adequately model the multiplicity of disease states or injuries that may bring forth neuropathic pain in the clinic. This review seeks to integrate information from the multiplicity of disciplines that seek to understand neuropathic pain; including immunology, cell biology, electrophysiology and biophysics, anatomy, cell biology, neurology, molecular biology, pharmacology and behavioral science. Beyond this, it underlines ongoing refinements in basic science and clinical practice that will engender improved approaches to pain management.

Differential regulation of cutaneous immunity by sensory neuron subsets

The nervous and immune systems have classically been studied as separate entities, but there is now mounting evidence for bidirectional communication between them in various organs, including the skin. The skin is an epithelial tissue with important sensory and immune functions. The skin is highly innervated with specialized subclasses of primary sensory neurons (PSNs) that can be in contact with skin-resident innate and adaptive immune cells. Neuroimmune crosstalk in the skin, through interactions of PSNs with the immune system, has been shown to regulate host cutaneous defense, inflammation, and tissue repair. Here, we review current knowledge about the cellular and molecular mechanisms involved in this crosstalk, as depicted via mouse model studies. We highlight the ways in which different immune challenges engage specialized subsets of PSNs to produce mediators acting on immune cell subsets and modulating their function.

Identification and validation of biomarkers related to Th1 cell infiltration in neuropathic pain

Neuropathic pain (NP) is a widespread chronic pain with a prevalence of 6.9-10% in the general population, severely affecting patients' physical and mental health. Accumulating evidence indicated that the immune environment is an essential factor causing NP. However, the mechanism is unclear. This study attempted to analyze NP-related immune infiltration patterns. We downloaded the expression profiles from the Gene Expression Omnibus (GEO) database. The novel method of single-sample gene set enrichment analysis (ssGSEA) algorithm and weighted gene co-expression network analysis (WGCNA) was applied to identify immune-related genes and verified in vitro and in vivo experiments. The spared nerve injury (SNI) group was closely related to type1 T helper cells (Th1 cells), and two key genes (Abca1 and Fyb) positively correlated with Th1 cell infiltration. At the single-cell level, Abca1 and Fyb were significantly expressed in macrophages. In addition, we verified that Abca1 could affect the function of macrophages. Finally, we hypothesized that Abca1 is involved in the infiltration of Th1 cells into dorsal root ganglion (DRG) tissues and induces NP via immunoinflammatory response. Hence, the present study aimed to elucidate the correlation between NP and neuroinflammation and identify a new therapeutic target for treating NP.

Cardiovascular Brain Circuits

The cardiovascular system is hardwired to the brain via multilayered afferent and efferent polysynaptic axonal connections. Two major anatomically and functionally distinct though closely interacting subcircuits within the cardiovascular system have recently been defined: The artery-brain circuit and the heart-brain circuit. However, how the nervous system impacts cardiovascular disease progression remains poorly understood. Here, we review recent findings on the anatomy, structures, and inner workings of the lesser-known artery-brain circuit and the better-established heart-brain circuit. We explore the evidence that signals from arteries or the heart form a systemic and finely tuned cardiovascular brain circuit: afferent inputs originating in the arterial tree or the heart are conveyed to distinct sensory neurons in the brain. There, primary integration centers act as hubs that receive and integrate artery-brain circuit-derived and heart-brain circuit-derived signals and process them together with axonal connections and humoral cues from distant brain regions. To conclude the cardiovascular brain circuit, integration centers transmit the constantly modified signals to efferent neurons which transfer them back to the cardiovascular system. Importantly, primary integration centers are wired to and receive information from secondary brain centers that control a wide variety of brain traits encoded in engrams including immune memory, stress-regulating hormone release, pain, reward, emotions, and even motivated types of behavior. Finally, we explore the important possibility that brain effector neurons in the cardiovascular brain circuit network connect efferent signals to other peripheral organs including the immune system, the gut, the liver, and adipose tissue. The enormous recent progress vis-à-vis the cardiovascular brain circuit allows us to propose a novel neurobiology-centered cardiovascular disease hypothesis that we term the neuroimmune cardiovascular circuit hypothesis.

Tenascin C+ papillary fibroblasts facilitate neuro-immune interaction in a mouse model of psoriasis

Dermal fibroblasts and cutaneous nerves are important players in skin diseases, while their reciprocal roles during skin inflammation have not been characterized. Here we identify an inflammation-induced subset of papillary fibroblasts that promotes aberrant neurite outgrowth and psoriasiform skin inflammation by secreting the extracellular matrix protein tenascin-C (TNC). Single-cell analysis of fibroblast lineages reveals a Tnc⁺ papillary fibroblast subset with pro-axonogenesis and neuro-regulation transcriptomic hallmarks. TNC overexpression in fibroblasts boosts neurite outgrowth in co-cultured neurons, while fibroblast-specific TNC ablation suppresses hyperinnervation and alleviates skin inflammation in male mice modeling psoriasis. Dermal γδT cells, the main producers of type 17 pathogenic cytokines, frequently contact nerve fibers in mouse psoriasiform lesions and are likely modulated by postsynaptic signals. Overall, our results highlight the role of an inflammation-responsive fibroblast subset in facilitating neuro-immune synapse formation and suggest potential avenues for future therapeutic research.

Neuroimmune cardiovascular interfaces in atherosclerosis

Two pairs of biological systems acting over long distances have recently been defined as major participants in the regulation of physiological and pathological tissue reactions: i) the nervous and vascular systems form various blood-brain barriers and control axon growth and angiogenesis; and ii) the nervous and immune systems emerge as key players to direct immune responses and maintain blood vessel integrity. The two pairs have been explored by investigators in relatively independent research areas giving rise to the concepts of the rapidly expanding topics of the neurovascular link and neuroimmunology, respectively. Our recent studies on atherosclerosis led us to consider a more inclusive approach by conceptualizing and combining principles of the neurovascular link and neuroimmunology: we propose that the nervous system, the immune system and the cardiovascular system undergo complex crosstalks in tripartite rather than bipartite interactions to form neuroimmune cardiovascular interfaces (NICIs).

Hallmarks of peripheral nerve function in bone regeneration

Skeletal tissue is highly innervated. Although different types of nerves have been recently identified in the bone, the crosstalk between bone and nerves remains unclear. In this review, we outline the role of the peripheral nervous system (PNS) in bone regeneration following injury. We first introduce the conserved role of nerves in tissue regeneration in species ranging from amphibians to mammals. We then present the distribution of the PNS in the skeletal system under physiological conditions, fractures, or regeneration. Furthermore, we summarize the ways in which the PNS communicates with bone-lineage cells, the vasculature, and immune cells in the bone microenvironment. Based on this comprehensive and timely review, we conclude that the PNS regulates bone regeneration through neuropeptides or neurotransmitters and cells in the peripheral nerves. An in-depth understanding of the roles of peripheral nerves in bone regeneration will inform the development of new strategies based on bone-nerve crosstalk in promoting bone repair and regeneration.

Latent myofascial trigger points injection therapy for adult cough variant asthma: A randomized controlled trial

Background

Cough variant asthma (CVA) is a chronic inflammatory airway disease characterized by airway hyper-responsiveness (AHR), of which cough is the only symptom. The cough is a result of the contraction of the vocal cords, diaphragm, sternocleidomastoid muscle, and other respiratory related muscles caused by the AHR. Long-term chronic coughing can lead to repetitive contraction and chronic strain of the muscles involved in the head and neck, ultimately contributing to the formation of latent myofascial trigger points (MTrPs). In turn, latent MTrPs can also irritate or compress the nerves around them, triggering cough. The date indicated that latent MTrPs can induce autonomic phenomena and are effective in allergic rhinitis. But their roles in asthma are unclear. In this article, the efficacy and safety of latent MTrPs injection therapy in CVA were investigated.

Methods

This randomized controlled trial was conducted with 110 patients. Patients were assigned to the intervention or control group in a 1:1.5 ratio. Intervention group (n = 44): single injection therapy with latent MTrPs. Control group (n = 66): budesonide-formoterol plus montelukast for 8 weeks. During the 36-week follow up period, the recurrence rate at week 36, cough visual analog scale (VAS), ACT (asthma control test)-scores, ACQ5 (asthma control questionnaire)-scores, AQLQ (asthma quality of life questionnaire)-scores, proportion of using rescue medication, and adverse events were evaluated.

Results

The recurrence rate at week 36 was lower in the intervention group than in the control group (36 weeks, 5.0 vs. 34.55%, p = 0.001). There were significant differences between groups in change from baseline to 36 weeks in VAS [36 weeks, 1.70 (1.49) vs. 3.18 (2.04), p < 0.001]; ACT-score [36 weeks, 21.38 (2.65) vs. 18.53 (3.00), p < 0.001]; ACQ5-score [36 weeks, 0.85 (0.55) vs. 1.52 (0.62), p < 0.001]; AQLQ-score [36w, 174.40 (18.22) vs. 151.69 (24.04), p < 0.001]; proportion of using rescue medication (36 weeks, 5.0 vs. 29.1%, p = 0.003). Fewer adverse events occurred in the two groups.

Conclusion

Latent myofascial trigger points injection therapy provided long-acting, practical, short treatment duration and safety methods for CVA.

Clinical Trials Registration

http://www.chictr.org.cn/index.aspx, Chinese Clinical Trial Registry Center, ChiCTR2100044079.

C-Fiber Degeneration Enhances Alveolar Macrophage-Mediated IFN-α/β Response to Respiratory Syncytial Virus

Stimulation of unmyelinated C fibers, the nociceptive sensory nerves, by noxious stimuli is able to initiate host responses. Host defensive responses against respiratory syncytial virus (RSV) infection rely on the induction of a robust alpha/beta interferon (IFN-α/β) response, which acts to restrict viral production and promote antiviral immune responses. Alveolar macrophages (AMs) are the major source of IFN-α/β upon RSV infection. Here, we found that C fibers are involved in host defense against RSV infection. Compared to the control mice post-RSV infection, degeneration and inhibition of C fibers by blockade of transient receptor potential vanilloid 1 (TRPV1) lowered viral replication and alleviated lung inflammation. Importantly, AMs were markedly elevated in C-fiber-degenerated (KCF) mice post-RSV infection, which was associated with higher IFN-α/β secretion as measured in bronchoalveolar lavage fluid (BALF) samples. Degeneration of C fibers contributed to the production of vasoactive intestinal peptide (VIP), which modulated AM and IFN-α/β levels to protect against RSV infection. Collectively, these findings revealed the key role of C fibers in regulating AM and IFN-α/β responses against RSV infection via VIP, opening the possibility for new therapeutic strategies against RSV. IMPORTANCE Despite continuous advances in medicine, safe and effective drugs against RSV infection remain elusive. As such, host-RSV interactions and host-directed therapies require further research. Unmyelinated C fibers, the nociceptive sensory nerves, play an important role in regulating the host response to virus. In the present study, from the perspective of neuroimmune interactions, we clarified that C-fiber degeneration enhanced the AM-mediated IFN-α/β response against RSV via VIP, providing potential therapeutic targets for the treatment of RSV infection.

Emerging Roles of Type-I Interferons in Neuroinflammation, Neurological Diseases, and Long-Haul COVID

Interferons (IFNs) are pleiotropic cytokines originally identified for their antiviral activity. IFN-α and IFN-β are both type I IFNs that have been used to treat neurological diseases such as multiple sclerosis. Microglia, astrocytes, as well as neurons in the central and peripheral nervous systems, including spinal cord neurons and dorsal root ganglion neurons, express type I IFN receptors (IFNARs). Type I IFNs play an active role in regulating cognition, aging, depression, and neurodegenerative diseases. Notably, by suppressing neuronal activity and synaptic transmission, IFN-α and IFN-β produced potent analgesia. In this article, we discuss the role of type I IFNs in cognition, neurodegenerative diseases, and pain with a focus on neuroinflammation and neuro-glial interactions and their effects on cognition, neurodegenerative diseases, and pain. The role of type I IFNs in long-haul COVID-associated neurological disorders is also discussed. Insights into type I IFN signaling in neurons and non-neuronal cells will improve our treatments of neurological disorders in various disease conditions.

Sensitisation of colonic nociceptors by TNFα is dependent on TNFR1 expression and p38 MAPK activity

Abstract

Visceral pain is a leading cause of morbidity in gastrointestinal diseases, which is exacerbated by the gut‐related side‐effects of many analgesics. New treatments are needed and further understanding of the mediators and mechanisms underpinning visceral nociception in disease states is required to facilitate this. The pro‐inflammatory cytokine TNFα is linked to pain in both patients with inflammatory bowel disease and irritable bowel syndrome, and has been shown to sensitize colonic sensory neurons. Somatic, TNFα‐triggered thermal and mechanical hypersensitivity is mediated by TRPV1 signalling and p38 MAPK activity respectively, downstream of TNFR1 receptor activation. We therefore hypothesized that TNFR1‐evoked p38 MAPK activity may also be responsible for TNFα sensitization of colonic afferent responses to the TRPV1 agonist capsaicin, and noxious distension of the bowel. Using Ca²⁺ imaging of dorsal root ganglion sensory neurons, we observed TNFα‐mediated increases in intracellular [Ca²⁺] and sensitization of capsaicin responses. The sensitizing effects of TNFα were dependent on TNFR1 expression and attenuated by p38 MAPK inhibition. Consistent with these findings, ex vivo colonic afferent fibre recordings demonstrated an enhanced response to noxious ramp distention of the bowel and bath application of capsaicin following TNFα pre‐treatment. Responses were reversed by p38 MAPK inhibition and absent in tissue from TNFR1 knockout mice. Our findings demonstrate a contribution of TNFR1, p38 MAPK and TRPV1 to TNFα‐induced sensitization of colonic afferents, highlighting the potential utility of these drug targets for the treatment of visceral pain in gastrointestinal disease.

Key points

The pro‐inflammatory cytokine TNFα is elevated in gastrointestinal disease and sensitizes colonic afferents via modulation of TRPA1 and Na_V1.8 activity. We further develop this understanding by demonstrating a role for p38 MAPK and TRPV1 in TNFα‐mediated colonic afferent sensitization. Specifically, we show that:

TNFα sensitizes sensory neurons and colonic afferents to the TRPV1 agonist capsaicin.

TNFα‐mediated sensitization of sensory neurons and colonic nociceptors is dependent on TNFR1 expression.

TNFα sensitization of sensory neurons and colonic afferents to capsaicin and noxious ramp distension is abolished by inhibition of p38 MAPK.

Collectively these data support the utility of targeting TNFα, TNFR1 and their downstream signalling via p38 MAPK for the treatment of visceral pain in gastrointestinal disease.

Neuroimmune Interaction: A Widespread Mutual Regulation and the Weapons for Barrier Organs

Since the embryo, the nervous system and immune system have been interacting to regulate each other’s development and working together to resist harmful stimuli. However, oversensitive neural response and uncontrolled immune attack are major causes of various diseases, especially in barrier organs, while neural-immune interaction makes it worse. As the first defense line, the barrier organs give a guarantee to maintain homeostasis in external environment. And the dense nerve innervation and abundant immune cell population in barrier organs facilitate the neuroimmune interaction, which is the physiological basis of multiple neuroimmune-related diseases. Neuroimmune-related diseases often have complex mechanisms and require a combination of drugs, posing challenges in finding etiology and treatment. Therefore, it is of great significance to illustrate the specific mechanism and exact way of neuro-immune interaction. In this review, we first described the mutual regulation of the two principal systems and then focused on neuro-immune interaction in the barrier organs, including intestinal tract, lungs and skin, to clarify the mechanisms and provide ideas for clinical etiology exploration and treatment.

Regulation of Carcinogenesis by Sensory Neurons and Neuromediators

Interactions between the immune system and the nervous system are crucial in maintaining homeostasis, and disturbances of these neuro-immune interactions may participate in carcinogenesis and metastasis. Nerve endings have been identified within solid tumors in humans and experimental animals. Although the involvement of the efferent sympathetic and parasympathetic innervation in carcinogenesis has been extensively investigated, the role of the afferent sensory neurons and the neuropeptides in tumor development, growth, and progression is recently appreciated. Similarly, current findings point to the significant role of Schwann cells as part of neuro-immune interactions. Hence, in this review, we mainly focus on local and systemic effects of sensory nerve activity as well as Schwann cells in carcinogenesis and metastasis. Specific denervation of vagal sensory nerve fibers, or vagotomy, in animal models, has been reported to markedly increase lung metastases of breast carcinoma as well as pancreatic and gastric tumor growth, with the formation of liver metastases demonstrating the protective role of vagal sensory fibers against cancer. Clinical studies have revealed that patients with gastric ulcers who have undergone a vagotomy have a greater risk of stomach, colorectal, biliary tract, and lung cancers. Protective effects of vagal activity have also been documented by epidemiological studies demonstrating that high vagal activity predicts longer survival rates in patients with colon, non-small cell lung, prostate, and breast cancers. However, several studies have reported that inhibition of sensory neuronal activity reduces the development of solid tumors, including prostate, gastric, pancreatic, head and neck, cervical, ovarian, and skin cancers. These contradictory findings are likely to be due to the post-nerve injury-induced activation of systemic sensory fibers, the level of aggressiveness of the tumor model used, and the local heterogeneity of sensory fibers. As the aggressiveness of the tumor model and the level of the inflammatory response increase, the protective role of sensory nerve fibers is apparent and might be mostly due to systemic alterations in the neuro-immune response. Hence, more insights into inductive and permissive mechanisms, such as systemic, cellular neuro-immunological mechanisms of carcinogenesis and metastasis formation, are needed to understand the role of sensory neurons in tumor growth and spread.

Neuroimmune cardiovascular interfaces control atherosclerosis

Atherosclerotic plaques develop in the inner intimal layer of arteries and can cause heart attacks and strokes¹. As plaques lack innervation, the effects of neuronal control on atherosclerosis remain unclear. However, the immune system responds to plaques by forming leukocyte infiltrates in the outer connective tissue coat of arteries (the adventitia)^2-6. Here, because the peripheral nervous system uses the adventitia as its principal conduit to reach distant targets^7-9, we postulated that the peripheral nervous system may directly interact with diseased arteries. Unexpectedly, widespread neuroimmune cardiovascular interfaces (NICIs) arose in mouse and human atherosclerosis-diseased adventitia segments showed expanded axon networks, including growth cones at axon endings near immune cells and media smooth muscle cells. Mouse NICIs established a structural artery-brain circuit (ABC): abdominal adventitia nociceptive afferents^10-14 entered the central nervous system through spinal cord T₆-T₁₃ dorsal root ganglia and were traced to higher brain regions, including the parabrachial and central amygdala neurons; and sympathetic efferent neurons projected from medullary and hypothalamic neurons to the adventitia through spinal intermediolateral neurons and both coeliac and sympathetic chain ganglia. Moreover, ABC peripheral nervous system components were activated: splenic sympathetic and coeliac vagus nerve activities increased in parallel to disease progression, whereas coeliac ganglionectomy led to the disintegration of adventitial NICIs, reduced disease progression and enhanced plaque stability. Thus, the peripheral nervous system uses NICIs to assemble a structural ABC, and therapeutic intervention in the ABC attenuates atherosclerosis.

Sex-Specific B Cell and Anti-Myelin Autoantibody Response After Peripheral Nerve Injury

Immunotherapy holds promise as a non-addictive treatment of refractory chronic pain states. Increasingly, sex is recognized to impact immune regulation of pain states, including mechanical allodynia (pain from non-painful stimulation) that follows peripheral nerve trauma. This study aims to assess the role of B cells in sex-specific responses to peripheral nerve trauma. Using a rat model of sciatic nerve chronic constriction injury (CCI), we analyzed sex differences in (i) the release of the immunodominant neural epitopes of myelin basic protein (MBP); (ii) the levels of serum immunoglobulin M (IgM)/immunoglobulin G (IgG) autoantibodies against the MBP epitopes; (iii) endoneurial B cell/CD20 levels; and (iv) mechanical sensitivity behavior after B cell/CD20 targeting with intravenous (IV) Rituximab (RTX) and control, IV immunoglobulin (IVIG), therapy. The persistent MBP epitope release in CCI nerves of both sexes was accompanied by the serum anti-MBP IgM autoantibody in female CCI rats alone. IV RTX therapy during CD20-reactive cell infiltration of nerves of both sexes reduced mechanical allodynia in females but not in males. IVIG and vehicle treatments had no effect in either sex. These findings provide strong evidence for sexual dimorphism in B-cell function after peripheral nervous system (PNS) trauma and autoimmune pathogenesis of neuropathic pain, potentially amenable to immunotherapeutic intervention, particularly in females. A myelin-targeted serum autoantibody may serve as a biomarker of such painful states. This insight into the biological basis of sex-specific response to neuraxial injury will help personalize regenerative and analgesic therapies.

Transcriptional neural-like signaling contributes to an immune-suppressive tumor microenvironment

Tumor innervation has recently been documented and characterized in various settings and tumor types. However, the role that nerves innervating tumors play in the pathogenesis of cancer has not been clarified. In this study, we searched for neural signaling from bulk RNA sequencing from The Cancer Genome Atlas (TCGA) dataset and looked for patterns of interactions between different cell types within the tumor environment. Using a presynapse signature (PSS) as a probe, we showed that multiple stromal cell types crosstalk and/or contribute to neural signals. Based on the correlation and linear regression, we hypothesized that neural signals contribute to an immune-suppressive tumor microenvironment (TME). To test this hypothesis, we performed in vitro dorsal root ganglion (DRG)/macrophage coculture experiments. Compared to the M2 macrophage monoculture, the DRG/M2 macrophage coculture prevented anti-inflammatory M2 to pro-inflammatory M1 polarization by LPS stimulation. Finally, a survey of different TCGA tumor types indicated that higher RNA neural signature is predictive of poor patient outcomes in multiple tumor types.

Analysis of Macrophages and Peptidergic Fibers in the Skin of Patients With Painful Diabetic Polyneuropathy

BACKGROUND AND OBJECTIVES

The mechanisms of pain in patients with diabetic polyneuropathy are unknown. Studies have suggested a role of inflammation and increased neuropeptides peripherally in pain generation. This study examined the possible skin markers of painful diabetic polyneuropathy (P-DPN): macrophages, substance P (SP), and calcitonin gene-related peptide (CGRP).

METHODS

The participants were included from a large Danish cross-sectional clinical study of type 2 diabetes. We diagnosed definite diabetic polyneuropathy using the Toronto criteria and used the Neuropathic Pain Special Interest Group classification for defining P-DPN. We included 60 skin biopsies from patients with diabetic polyneuropathy-30 with P-DPN and 30 with nonpainful diabetic polyneuropathy (NP-DPN)-and 30 biopsies from healthy controls of similar age and sex. The biopsies were stained using PGP 9.5, IbA1, and SP and CGRP primary markers.

RESULTS

There was increased macrophage density in patients with P-DPN (8.0%) compared with that in patients with NP-DPN (5.1%, p < 0.001), and there was increased macrophage density in patients with NP-DPN (5.1%) compared with that in healthy controls (3.1%, p < 0.001). When controlling for neuropathy severity, body mass index, age, and sex, there was still a difference in macrophage density between patients with P-DPN and patients with NP-DPN. Patients with P-DPN had higher median nerve fiber length density (274.5 and 155 mm^-2 for SP and CGRP, respectively) compared with patients with NP-DPN (176 and 121 mm^-2 for SP and CGRP, respectively, p = 0.009 and 0.04) and healthy controls (185.5 and 121.5 mm^-2 for SP and CGRP, respectively), whereas there was no difference between patients with NP-DPN and controls without diabetes (p = 0.64 and 0.49, respectively). The difference between P-DPN and NP-DPN for SP and CGRP was significant only in female patients, although a trend was seen in male patients.

DISCUSSION

The findings point to a possible involvement of the innate immune system in the pathogenesis of neuropathic pain in patients with DPN, although markers of activated macrophages were not measured in this study.

Pituitary adenylate cyclase-activating polypeptide promotes cutaneous dendritic cell functions in contact hypersensitivity

BACKGROUND

Sensory nerves regulate cutaneous local inflammation indirectly through induction of pruritus and directly by acting on local immune cells. The underlying mechanisms for how sensory nerves influence cutaneous acquired immune responses remain to be clarified.

OBJECTIVE

This study aimed to explore the effect of peripheral nerves on cutaneous immune cells in cutaneous acquired immune responses.

METHODS

We analyzed contact hypersensitivity (CHS) responses as a murine model of delayed-type hypersensitivity in absence or presence of resiniferatoxin-induced sensory nerve denervation. We conducted ear thickness measurements, flow cytometric analyses, and mRNA expression analyses in CHS.

RESULTS

CHS responses were attenuated in mice that were denervated during the sensitization phase of CHS. By screening neuropeptides, we found that pituitary adenylate cyclase-activating polypeptide (PACAP) mRNA expression was decreased in the dorsal root ganglia after denervation. Administration of PACAP restored attenuated CHS response in resiniferatoxin-treated mice, and pharmacological inhibition of PACAP suppressed CHS. Flow cytometric analysis of skin-draining lymph nodes showed that cutaneous dendritic cell migration and maturation were reduced in both denervated mice and PACAP antagonist-treated mice. The expression of chemokine receptors CCR7 and CXCR4 of dendritic cell s was enhanced by addition of PACAP in vitro.

CONCLUSION

These findings indicate that a neuropeptide PACAP promotes the development of CHS responses by inducing cutaneous dendritic cell functions during the sensitization phase.

HMGB1 released from nociceptors mediates inflammation

Inflammation, the body's primary defensive response system to injury and infection, is triggered by molecular signatures of microbes and tissue injury. These molecules also stimulate specialized sensory neurons, termed nociceptors. Activation of nociceptors mediates inflammation through antidromic release of neuropeptides into infected or injured tissue, producing neurogenic inflammation. Because HMGB1 is an important inflammatory mediator that is synthesized by neurons, we reasoned nociceptor release of HMGB1 might be a component of the neuroinflammatory response. In support of this possibility, we show here that transgenic nociceptors expressing channelrhodopsin-2 (ChR2) directly release HMGB1 in response to light stimulation. Additionally, HMGB1 expression in neurons was silenced by crossing synapsin-Cre (Syn-Cre) mice with floxed HMGB1 mice (HMGB1f/f). When these mice undergo sciatic nerve injury to activate neurogenic inflammation, they are protected from the development of cutaneous inflammation and allodynia as compared to wild-type controls. Syn-Cre/HMGB1fl/fl mice subjected to experimental collagen antibody-induced arthritis, a disease model in which nociceptor-dependent inflammation plays a significant pathological role, are protected from the development of allodynia and joint inflammation. Thus, nociceptor HMGB1 is required to mediate pain and inflammation during sciatic nerve injury and collagen antibody-induced arthritis.

Identifying the most important confounders when assessing the association between low-grade systemic inflammation and musculoskeletal pain: A modified Delphi study

Objective

The association between low-grade systemic inflammation and musculoskeletal pain may be influenced by multiple factors. However, little is known about the relative importance of these factors, and few studies account for them. This Delphi study aimed to reach consensus on the most important confounders which influence the association between low-grade systemic inflammation and musculoskeletal pain.

Methods

The panel consisted of 48 experts. In Round 1, the experts proposed what they believed were important confounders. In Round 2, the experts indicated for each confounder whether they believed it was important (yes/no). At least 50% of experts had to indicate the confounder was important to be considered in the final round. In Round 3, the experts rated the importance of each confounder on a 7-point Likert scale. Consensus was reached if ≥75% of the experts considered the factor either extremely or moderately important.

Results

In Round 1, 120 confounders were proposed, which were synthesized into 38 distinct factors. In Round 2, 33 confounders met the criterion to be considered important. In Round 3, consensus was reached for 14 confounders: acute illness/trauma, immune disease, medication use, endocrine, nutritional, or metabolic disease, other musculoskeletal conditions, age, handling of blood samples, sex, cancer, body composition, pregnancy, cardiovascular disease, physical activity, and pain characteristics.

Conclusions

These findings provide insight in the complexity of the association between low-grade systemic inflammation and musculoskeletal pain. Some factors currently listed as confounders may be re-classified as moderators or mediators as insights progress.

Effects of Dorsal Column Spinal Cord Stimulation on Neuroinflammation: Revisiting Molecular Mechanisms and Clinical Outcomes on Chronic Lumbar/Leg Pain and Failed Back Surgery Syndrome

OBJECTIVE

In this narrative review, we reviewed and discussed current literature describing the molecular mechanisms leading to neuroinflammation and its role in the onset and progression of chronic neuropathic lumbar and leg pain in patients with persistent spinal pain syndrome. In addition, we reviewed the proposed mechanisms and impact of spinal cord stimulation (SCS) on neuroinflammation.

METHODS

A broad search of current literature in PubMed, Embase, Scopus, Cochrane library, Medline/Ovid, and Web of Science was performed using the following terms and their combinations: "biomarkers", "chronic back and leg pain", "cytokines", "neuroinflammation", "spinal cord stimulation (scs)," and "spinal cord modulation". We selected: 1) articles published in the English language between January 2000 and July 2020 2) preclinical and clinical data 3) case reports 4) meta-analysis and systematic reviews and 5) conference abstracts. Manuscripts not disclosing methodology or without full-text availability were excluded.

DISCUSSION

SCS techniques have gradually evolved since inception to include novel methods such as burst-SCS, high frequency SCS, and differential targeted multiplexed SCS. The incidence of chronic pain after spine surgery is highly variable, with at least one third of patients developing persistent spinal pain syndrome. Novel SCS techniques have been associated with improved clinical and functional outcomes thus increasing patient quality of life.

CONCLUSION

Currently, health care providers rely on different options and methods for SCS when treating patients with refractory chronic lumbar pain and persistent spinal pain syndrome. Nevertheless, compelling clinical trials remain necessary to elucidate the long-term benefits and mechanisms of neuromodulation of all different types of SCS.

Passive transfer of fibromyalgia symptoms from patients to mice

Fibromyalgia syndrome (FMS) is characterized by widespread pain and tenderness, and patients typically experience fatigue and emotional distress. The etiology and pathophysiology of fibromyalgia are not fully explained and there are no effective drug treatments. Here we show that IgG from FMS patients produced sensory hypersensitivity by sensitizing nociceptive neurons. Mice treated with IgG from FMS patients displayed increased sensitivity to noxious mechanical and cold stimulation, and nociceptive fibers in skin-nerve preparations from mice treated with FMS IgG displayed an increased responsiveness to cold and mechanical stimulation. These mice also displayed reduced locomotor activity, reduced paw grip strength, and a loss of intraepidermal innervation. In contrast, transfer of IgG-depleted serum from FMS patients or IgG from healthy control subjects had no effect. Patient IgG did not activate naive sensory neurons directly. IgG from FMS patients labeled satellite glial cells and neurons in vivo and in vitro, as well as myelinated fiber tracts and a small number of macrophages and endothelial cells in mouse dorsal root ganglia (DRG), but no cells in the spinal cord. Furthermore, FMS IgG bound to human DRG. Our results demonstrate that IgG from FMS patients produces painful sensory hypersensitivities by sensitizing peripheral nociceptive afferents and suggest that therapies reducing patient IgG titers may be effective for fibromyalgia.

IL-5 mediates monocyte phenotype and pain outcomes in fibromyalgia

ABSTRACT

Fibromyalgia (FM) is characterized by widespread chronic pain, fatigue, and somatic symptoms. The influence of phenotypic changes in monocytes on symptoms associated with FM is not fully understood. The primary aim of this study was to take a comprehensive whole-body to molecular approach in characterizing relationships between monocyte phenotype and FM symptoms in relevant clinical populations. Lipopolysaccharide-evoked and spontaneous secretion of IL-5 and other select cytokines from circulating monocytes was higher in women with FM compared to women without pain. In addition, greater secretion of IL-5 was significantly associated with pain and other clinically relevant psychological and somatic symptoms of FM. Furthermore, higher levels of pain and pain-related symptoms were associated with a lower percentage of intermediate monocytes (CD14++/CD16+) and a greater percentage of nonclassical monocytes (CD14+/CD16++) in women with FM. Based on findings from individuals with FM, we examined the role of IL-5, an atypical cytokine secreted from monocytes, in an animal model of widespread muscle pain. Results from the animal model show that IL-5 produces analgesia and polarizes monocytes toward an anti-inflammatory phenotype (CD206+). Taken together, our data suggest that monocyte phenotype and their cytokine profiles are associated with pain-related symptoms in individuals with FM. Furthermore, our data show that IL-5 has a potential role in analgesia in an animal model of FM. Thus, targeting anti-inflammatory cytokines such as IL-5 secreted by circulating leukocytes could serve as a promising intervention to control pain and other somatic symptoms associated with FM.

Early Life Nociception is Influenced by Peripheral Growth Hormone Signaling

A number of cellular systems work in concert to modulate nociceptive processing in the periphery, but the mechanisms that regulate neonatal nociception may be distinct compared with adults. Our previous work indicated a relationship between neonatal hypersensitivity and growth hormone (GH) signaling. Here, we explored the peripheral mechanisms by which GH modulated neonatal nociception under normal and injury conditions (incision) in male and female mice. We found that GH receptor (GHr) signaling in primary afferents maintains a tonic inhibition of peripheral hypersensitivity. After injury, a macrophage dependent displacement of injury-site GH was found to modulate neuronal transcription at least in part via serum response factor (SRF) regulation. A single GH injection into the injured hindpaw muscle effectively restored available GH signaling to neurons and prevented acute pain-like behaviors, primary afferent sensitization, and neuronal gene expression changes. GH treatment also inhibited long-term somatosensory changes observed after repeated peripheral insult. Results may indicate a novel mechanism of neonatal nociception.SIGNIFICANCE STATEMENT Although it is noted that mechanisms of pain development in early life are unique compared with adults, little research focuses on neonatal-specific peripheral mechanisms of nociception. This gap is evident in the lack of specialized care for infants following an injury including surgeries. This report evaluates how distinct cellular systems in the periphery including the endocrine, immune and nervous systems work together to modulate neonatal-specific nociception. We uncovered a novel mechanism by which muscle injury induces a macrophage-dependent sequestration of peripheral growth hormone (GH) that effectively removes its normal tonic inhibition of neonatal nociceptors to promote acute pain-like behaviors. Results indicate a possible new strategy for treatment of neonatal postsurgical pain.

Neuroimmune Consequences of eIF4E Phosphorylation on Chemotherapy-Induced Peripheral Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a major dose-limiting side effect that occurs in up to 63% of patients and has no known effective treatment. A majority of studies do not effectively assess sex differences in the onset and persistence of CIPN. Here we investigated the onset of CIPN, a point of therapeutic intervention where we may limit, or even prevent the development of CIPN. We hypothesized that cap-dependent translation mechanisms are important in early CIPN development and the bi-directional crosstalk between immune cells and nociceptors plays a complementary role to CIPN establishment and sex differences observed. In this study, we used wild type and eIF4E-mutant mice of both sexes to investigate the role of cap-dependent translation and the contribution of immune cells and nociceptors in the periphery and glia in the spinal cord during paclitaxel-induced peripheral neuropathy. We found that systemically administered paclitaxel induces pain-like behaviors in both sexes, increases helper T-lymphocytes, downregulates cytotoxic T-lymphocytes, and increases mitochondrial dysfunction in dorsal root ganglia neurons; all of which is eIF4E-dependent in both sexes. We identified a robust paclitaxel-induced, eIF4E-dependent increase in spinal astrocyte immunoreactivity in males, but not females. Taken together, our data reveals that cap-dependent translation may be a key pathway that presents relevant therapeutic targets during the early phase of CIPN. By targeting the eIF4E complex, we may reduce or reverse the negative effects associated with chemotherapeutic treatments.

Autoimmune regulation of chronic pain

Chronic pain is an unpleasant and debilitating condition that is often poorly managed by existing therapeutics. Reciprocal interactions between the nervous system and the immune system have been recognized as playing an essential role in the initiation and maintenance of pain. In this review, we discuss how neuroimmune signaling can contribute to peripheral and central sensitization and promote chronic pain through various autoimmune mechanisms. These pathogenic autoimmune mechanisms involve the production and release of autoreactive antibodies from B cells. Autoantibodies-ie, antibodies that recognize self-antigens-have been identified as potential molecules that can modulate the function of nociceptive neurons and thereby induce persistent pain. Autoantibodies can influence neuronal excitability by activating the complement pathway; by directly signaling at sensory neurons expressing Fc gamma receptors, the receptors for the Fc fragment of immunoglobulin G immune complexes; or by binding and disrupting ion channels expressed by nociceptors. Using examples primarily from rheumatoid arthritis, complex regional pain syndrome, and channelopathies from potassium channel complex autoimmunity, we suggest that autoantibody signaling at the central nervous system has therapeutic implications for designing novel disease-modifying treatments for chronic pain.

Lymph nodes are innervated by a unique population of sensory neurons with immunomodulatory potential

Barrier tissue immune responses are regulated in part by nociceptors. Nociceptor ablation alters local immune responses at peripheral sites and within draining lymph nodes (LNs). The mechanisms and significance of nociceptor-dependent modulation of LN function are unknown. Using high-resolution imaging, viral tracing, single-cell transcriptomics, and optogenetics, we identified and functionally tested a sensory neuro-immune circuit that is responsive to lymph-borne inflammatory signals. Transcriptomics profiling revealed that multiple sensory neuron subsets, predominantly peptidergic nociceptors, innervate LNs, distinct from those innervating surrounding skin. To uncover LN-resident cells that may interact with LN-innervating sensory neurons, we generated a LN single-cell transcriptomics atlas and nominated nociceptor target populations and interaction modalities. Optogenetic stimulation of LN-innervating sensory fibers triggered rapid transcriptional changes in the predicted interacting cell types, particularly endothelium, stromal cells, and innate leukocytes. Thus, a unique population of sensory neurons monitors peripheral LNs and may locally regulate gene expression.

A role for neuroimmune signaling in a rat model of Gulf War Illness-related pain

More than a quarter of veterans of the 1990-1991 Persian Gulf War suffer from Gulf War Illness (GWI), a chronic, multi-symptom illness that commonly includes musculoskeletal pain. Exposure to a range of toxic chemicals, including sarin nerve agent, are a suspected root cause of GWI. Moreover, such chemical exposures induce a neuroinflammatory response in rodents, which has been linked to several GWI symptoms in rodents and veterans with GWI. To date, a neuroinflammatory basis for pain associated with GWI has not been investigated. Here, we evaluated development of nociceptive hypersensitivity in a model of GWI. Male Sprague Dawley rats were treated with corticosterone in the drinking water for 7 days, to mimic high physiological stress, followed by a single injection of the sarin nerve agent surrogate, diisopropyl fluorophosphate. These exposures alone were insufficient to induce allodynia. However, an additional sub-threshold challenge (a single intramuscular injection of pH 4 saline) induced long-lasting, bilateral allodynia. Such allodynia was associated with elevation of markers for activated microglia/macrophages (CD11b) and astrocytes/satellite glia (GFAP) in the lumbar dorsal spinal cord and dorsal root ganglia (DRG). Additionally, Toll-like receptor 4 (TLR4) mRNA was elevated in the lumbar dorsal spinal cord, while IL-1β and IL-6 were elevated in the lumbar dorsal spinal cord, DRG, and gastrocnemius muscle. Demonstrating a casual role for such neuroinflammatory signaling, allodynia was reversed by treatment with either minocycline, the TLR4 inhibitor (+)-naltrexone, or IL-10 plasmid DNA. Together, these results point to a role for neuroinflammation in male rats in the model of musculoskeletal pain related to GWI. Therapies that alleviate persistent immune dysregulation may be a strategy to treat pain and other symptoms of GWI.

The Neuroimmunology of Chronic Pain: From Rodents to Humans

Chronic pain, encompassing conditions, such as low back pain, arthritis, persistent post-surgical pain, fibromyalgia, and neuropathic pain disorders, is highly prevalent but remains poorly treated. The vast majority of therapeutics are directed solely at neurons, despite the fact that signaling between immune cells, glia, and neurons is now recognized as indispensable for the initiation and maintenance of chronic pain. This review highlights recent advances in understanding fundamental neuroimmune signaling mechanisms and novel therapeutic targets in rodent models of chronic pain. We further discuss new technological developments to study, diagnose, and quantify neuroimmune contributions to chronic pain in patient populations.

Neural Regulation of Interactions Between Group 2 Innate Lymphoid Cells and Pulmonary Immune Cells

Emerging evidence supports the involvement of nervous system in the regulation of immune responses. Group 2 innate lymphoid cells (ILC2), which function as a crucial bridge between innate and adaptive immunity, are present in large numbers in barrier tissues. Neuropeptides and neurotransmitters have been found to participate in the regulation of ILC2, adding a new dimension to neuroimmunity. However, a comprehensive and detailed overview of the mechanisms of neural regulation of ILC2, associated with previous findings and prospects for future research, is still lacking. In this review, we compile existing information that supports neurons as yet poorly understood regulators of ILC2 in the field of lung innate and adaptive immunity, focusing on neural regulation of the interaction between ILC2 and pulmonary immune cells.

Brain TNF drives post-inflammation depression-like behavior and persistent pain in experimental arthritis

Patients with rheumatoid arthritis experience chronic pain, depression and fatigue, even when inflammation of the joints is well controlled. To study the relationship between arthritis, depression, and sustained pain when articular inflammation is no longer observed, we tested the hypothesis that brain TNF drives post-inflammation depression-like behavior and persistent pain in experimental arthritis. The murine model of antigen-induced arthritis (AIA) was used to evaluate the effects of knee inflammation on sustained pain and depression-like behavior. We measured joint pain using an automated dynamic plantar algesiometer and depression-like behavior with the tail suspension test. Cytokines were measured by Luminex assay and ELISA. TNF in the brain was blocked by intracerebroventricular injection of anti-TNF antibodies. Histological damage and elevated levels of cytokines were observed in the knee 24 h after antigen treatment, but not at 13 days. Reduced pain thresholds were seen 24 h and 13 days after treatment. Depression-like behavior was observed on day 13. Treatment with the antidepressant imipramine reduced both depression-like behavior and persistent pain. However, blocking joint pain with the analgesic dipyrone did not alter depression-like behavior. Elevated levels of TNF, CCL2, and CXCL-1 were observed in the hippocampus 24 h after treatment, with TNF remaining elevated at day 13. Intracerebroventricular infusion of an anti-TNF antibody blocked depression-like behavior and reduced persistent pain. We have demonstrated that depression-like behavior and pain is sustained in AIA mice after the resolution of inflammation. These changes are associated with elevated levels of TNF in the hippocampus and are dependent upon brain TNF. The findings reveal an important mechanistic link between the expression of chronic pain and depression in experimental arthritis. Furthermore, they suggest treating depression in rheumatoid arthritis may positively impact other debilitating features of this condition.