Sensory neurons: An integrated component of innate immunity

The sensory nervous system possesses the ability to integrate exogenous threats and endogenous signals to mediate downstream effector functions. Sensory neurons have been shown to activate or suppress host defense and immunity against pathogens, depending on the tissue and disease state. Through this lens, pro- and anti-inflammatory neuroimmune effector functions can be interpreted as evolutionary adaptations by host or pathogen. Here, we discuss recent and impactful examples of neuroimmune circuitry that regulate tissue homeostasis, autoinflammation, and host defense. Apparently paradoxical or conflicting reports in the literature also highlight the complexity of neuroimmune interactions that may depend on tissue- and microbe-specific cues. These findings expand our understanding of the nuanced mechanisms and the greater context of sensory neurons in innate immunity.

A mouse DRG genetic toolkit reveals morphological and physiological diversity of somatosensory neuron subtypes

Dorsal root ganglia (DRG) somatosensory neurons detect mechanical, thermal, and chemical stimuli acting on the body. Achieving a holistic view of how different DRG neuron subtypes relay neural signals from the periphery to the CNS has been challenging with existing tools. Here, we develop and curate a mouse genetic toolkit that allows for interrogating the properties and functions of distinct cutaneous targeting DRG neuron subtypes. These tools have enabled a broad morphological analysis, which revealed distinct cutaneous axon arborization areas and branching patterns of the transcriptionally distinct DRG neuron subtypes. Moreover, in vivo physiological analysis revealed that each subtype has a distinct threshold and range of responses to mechanical and/or thermal stimuli. These findings support a model in which morphologically and physiologically distinct cutaneous DRG sensory neuron subtypes tile mechanical and thermal stimulus space to collectively encode a wide range of natural stimuli.

The neuroimmune CGRP-RAMP1 axis tunes cutaneous adaptive immunity to the microbiota

The somatosensory nervous system surveils external stimuli at barrier tissues, regulating innate immune cells under infection and inflammation. The roles of sensory neurons in controlling the adaptive immune system, and more specifically immunity to the microbiota, however, remain elusive. Here, we identified a mechanism for direct neuroimmune communication between commensal-specific T lymphocytes and somatosensory neurons mediated by the neuropeptide calcitonin gene-related peptide (CGRP) in the skin. Intravital imaging revealed that commensal-specific T cells are in close proximity to cutaneous nerve fibers in vivo. Correspondingly, we observed upregulation of the receptor for the neuropeptide CGRP, RAMP1, in CD8+ T lymphocytes induced by skin commensal colonization. The neuroimmune CGRP-RAMP1 signaling axis functions in commensal-specific T cells to constrain Type 17 responses and moderate the activation status of microbiota-reactive lymphocytes at homeostasis. As such, modulation of neuroimmune CGRP-RAMP1 signaling in commensal-specific T cells shapes the overall activation status of the skin epithelium, thereby impacting the outcome of responses to insults such as wounding. The ability of somatosensory neurons to control adaptive immunity to the microbiota via the CGRP-RAMP1 axis underscores the various layers of regulation and multisystem coordination required for optimal microbiota-reactive T cell functions under steady state and pathology.

Gut complement induced by the microbiota combats pathogens and spares commensals

Canonically, the complement system is known for its rapid response to remove microbes in the bloodstream. However, relatively little is known about a functioning complement system on intestinal mucosal surfaces. Herein, we report the local synthesis of complement component 3 (C3) in the gut, primarily by stromal cells. C3 is expressed upon commensal colonization and is regulated by the composition of the microbiota in healthy humans and mice, leading to an individual host's specific luminal C3 levels. The absence of membrane attack complex (MAC) components in the gut ensures that C3 deposition does not result in the lysis of commensals. Pathogen infection triggers the immune system to recruit neutrophils to the infection site for pathogen clearance. Basal C3 levels directly correlate with protection against enteric infection. Our study reveals the gut complement system as an innate immune mechanism acting as a vigilant sentinel that combats pathogens and spares commensals.

Basophils are important for development of allergic skin inflammation

BACKGROUND

Atopic dermatitis skin lesions exhibit increased infiltration by basophils. Basophils produce IL-4, which plays an important role in the pathogenesis of atopic dermatitis.

OBJECTIVE

We sought to determine the role of basophils in a mouse model of antigen-driven allergic skin inflammation.

METHODS

Wild-type mice, mice with selective and inducible depletion of basophils, and mice expressing Il4-driven enhanced green fluorescent protein were subjected to epicutaneous sensitization with ovalbumin or saline. Sensitized skin was examined by histology for epidermal thickening. Cells were analyzed for surface markers and intracellular expression of enhanced green fluorescent protein by flow cytometry. Gene expression was evaluated by real-time reverse transcription-quantitative PCR.

RESULTS

Basophils were important for epidermal hyperplasia, dermal infiltration by CD4+ T cells, mast cells, and eosinophils in ovalbumin-sensitized mouse skin and for the local and systemic TH2 response to epicutaneous sensitization. Moreover, basophils were the major source of IL-4 in epicutaneous-sensitized mouse skin and promote the ability of dendritic cells to drive TH2 polarization of naive T cells.

CONCLUSION

Basophils play an important role in the development of allergic skin inflammation induced by cutaneous exposure to antigen in mice.

Neuronal-immune axis alters pain and sensory afferent damage during dental pulp injury

ABSTRACT

Dental pulp tissue is densely innervated by afferent fibers of the trigeminal ganglion. When bacteria cause dental decay near the pulpal tissue, a strong neuronal and immune response occurs, creating pulpitis, which is associated with severe pain and pulp tissue damage. Neuroimmune interactions have the potential to modulate both the pain and pathological outcome of pulpitis. We first investigated the role of the neuropeptide calcitonin gene-related peptide (CGRP), released from peptidergic sensory afferents, in dental pain and immune responses by using Calca knockout (Calca -/- ) and wild-type (Calca +/+ ) mice, in a model of pulpitis by creating a mechanical exposure of the dental pulp horn. We found that the neuropeptide CGRP, facilitated the recruitment of myeloid cells into the pulp while also increasing spontaneous pain-like behavior 20% to 25% at an early time point. Moreover, when we depleted neutrophils and monocytes, we found that there was 20% to 30% more sensory afferent loss and increased presence of bacteria in deeper parts of the tissue, whereas there was a significant reduction in mechanical pain response scores compared with the control group at a later time point. Overall, we showed that there is a crosstalk between peptidergic neurons and neutrophils in the pulp, modulating the pain and inflammatory outcomes of the disease.

The neuroimmune CGRP-RAMP1 axis tunes cutaneous adaptive immunity to the microbiota

The somatosensory nervous system surveils external stimuli at barrier tissues, regulating innate immune cells under infection and inflammation. The roles of sensory neurons in controlling the adaptive immune system, and more specifically immunity to the microbiota, however, remain elusive. Here, we identified a novel mechanism for direct neuroimmune communication between commensal-specific T lymphocytes and somatosensory neurons mediated by the neuropeptide Calcitonin Gene-Related Peptide (CGRP) in the skin. Intravital imaging revealed that commensal-specific T cells are in close proximity to cutaneous nerve fibers in vivo . Correspondingly, we observed upregulation of the receptor for the neuropeptide CGRP, RAMP1, in CD8 + T lymphocytes induced by skin commensal colonization. Neuroimmune CGRP-RAMP1 signaling axis functions in commensal-specific T cells to constrain Type 17 responses and moderate the activation status of microbiota-reactive lymphocytes at homeostasis. As such, modulation of neuroimmune CGRP-RAMP1 signaling in commensal-specific T cells shapes the overall activation status of the skin epithelium, thereby impacting the outcome of responses to insults such as wounding. The ability of somatosensory neurons to control adaptive immunity to the microbiota via the CGRP-RAMP1 axis underscores the various layers of regulation and multisystem coordination required for optimal microbiota-reactive T cell functions under steady state and pathology.

Significance statement

Multisystem coordination at barrier surfaces is critical for optimal tissue functions and integrity, in response to microbial and environmental cues. In this study, we identified a novel neuroimmune crosstalk mechanism between the sensory nervous system and the adaptive immune response to the microbiota, mediated by the neuropeptide CGRP and its receptor RAMP1 on skin microbiota-induced T lymphocytes. The neuroimmune CGPR-RAMP1 axis constrains adaptive immunity to the microbiota and overall limits the activation status of the skin epithelium, impacting tissue responses to wounding. Our study opens the door to a new avenue to modulate adaptive immunity to the microbiota utilizing neuromodulators, allowing for a more integrative and tailored approach to harnessing microbiota-induced T cells to promote barrier tissue protection and repair.

S. aureus drives itch and scratch-induced skin damage through a V8 protease-PAR1 axis

Itch is an unpleasant sensation that evokes a desire to scratch. The skin barrier is constantly exposed to microbes and their products. However, the role of microbes in itch generation is unknown. Here, we show that Staphylococcus aureus, a bacterial pathogen associated with itchy skin diseases, directly activates pruriceptor sensory neurons to drive itch. Epicutaneous S. aureus exposure causes robust itch and scratch-induced damage. By testing multiple isogenic bacterial mutants for virulence factors, we identify the S. aureus serine protease V8 as a critical mediator in evoking spontaneous itch and alloknesis. V8 cleaves proteinase-activated receptor 1 (PAR1) on mouse and human sensory neurons. Targeting PAR1 through genetic deficiency, small interfering RNA (siRNA) knockdown, or pharmacological blockade decreases itch and skin damage caused by V8 and S. aureus exposure. Thus, we identify a mechanism of action for a pruritogenic bacterial factor and demonstrate the potential of inhibiting V8-PAR1 signaling to treat itch.

The role of cellular and molecular neuroimmune crosstalk in gut immunity

The gastrointestinal tract is densely innervated by the peripheral nervous system and populated by the immune system. These two systems critically coordinate the sensations of and adaptations to dietary, microbial, and damaging stimuli from the external and internal microenvironment during tissue homeostasis and inflammation. The brain receives and integrates ascending sensory signals from the gut and transduces descending signals back to the gut via autonomic neurons. Neurons regulate intestinal immune responses through the action of local axon reflexes or through neuronal circuits via the gut-brain axis. This neuroimmune crosstalk is critical for gut homeostatic maintenance and disease resolution. In this review, we discuss the roles of distinct types of gut-innervating neurons in the modulation of intestinal mucosal immunity. We will focus on the molecular mechanisms governing how different immune cells respond to neural signals in host defense and inflammation. We also discuss the therapeutic potential of strategies targeting neuroimmune crosstalk for intestinal diseases.

Dynamics of Innate Immune Response in Bacteria-Induced Mouse Model of Pulpitis

INTRODUCTION

During pulpitis, as bacteria penetrate deeper into the dentin and pulp tissue, a pulpal innate immune response is initiated. However, the kinetics of the immune response, how this relates to bacterial infiltration during pulpitis and an understanding of the types of immune cells in the pulp is limited.

METHODS

Dental pulp exposure in the molars of mice was used as an animal model of pulpitis. To investigate the kinetics of immune response, pulp tissue was collected from permanent molars at different time points after injury (baseline, day 1, and day 7). Flow cytometry analysis of CD45+ leukocytes, including macrophages, neutrophils monocytes, and T cells, was performed. 16S in situ hybridization captured bacterial invasion of the pulp, and immunohistochemistry for F4/80 investigated spatial and morphological changes of macrophages during pulpitis. Data were analyzed using two-way ANOVA with Tukey's multiple comparisons.

RESULTS

Bacteria mostly remained close to the injury site, with some expansion towards noninjured pulp horns. We found that F4/80+ macrophages were the primary immune cell population in the healthy pulp. Upon injury, CD11b + Ly6Ghigh neutrophils and CD11b + Ly6GintLy6Cint monocytes constituted 70-90% of all immune populations up to 7 days after injury. Even though there was a slight increase in T cells at day 7, myeloid cells remained the main drivers of the immune response during the seven-day time period.

CONCLUSIONS

As bacteria proliferate within the pulp chamber, innate immune cells, including macrophages, neutrophils, and monocytes, predominate as the major immune populations, with some signs of transitioning to an adaptive immune response.

A collaboration between immune cells and the choroid plexus epithelium in brain inflammation

The choroid plexus (ChP) is a vital brain barrier and source of cerebrospinal fluid (CSF). Here, we use chronic two-photon imaging in awake mice and single-cell transcriptomics to demonstrate that in addition to these roles, the ChP is a complex immune organ that regulates brain inflammation. In a mouse meningitis model, neutrophils and monocytes accumulated in ChP stroma and surged across the epithelial barrier into the CSF. Bi-directional recruitment of monocytes from the periphery and, unexpectedly, macrophages from the CSF to the ChP helped eliminate neutrophils and repair the barrier. Transcriptomic analyses detailed the molecular steps accompanying this process, including the discovery of epithelial cells that transiently specialized to nurture immune cells, coordinate their recruitment, survival, and differentiation, and ultimately, control the opening/closing of the ChP brain barrier. Collectively, we provide a new conceptual understanding and comprehensive roadmap of neuroinflammation at the ChP brain barrier.

Gasdermin-E mediates mitochondrial damage in axons and neurodegeneration

Mitochondrial dysfunction and axon loss are hallmarks of neurologic diseases. Gasdermin (GSDM) proteins are executioner pore-forming molecules that mediate cell death, yet their roles in the central nervous system (CNS) are not well understood. Here, we find that one GSDM family member, GSDME, is expressed by both mouse and human neurons. GSDME plays a role in mitochondrial damage and axon loss. Mitochondrial neurotoxins induced caspase-dependent GSDME cleavage and rapid localization to mitochondria in axons, where GSDME promoted mitochondrial depolarization, trafficking defects, and neurite retraction. Frontotemporal dementia (FTD)/amyotrophic lateral sclerosis (ALS)-associated proteins TDP-43 and PR-50 induced GSDME-mediated damage to mitochondria and neurite loss. GSDME knockdown protected against neurite loss in ALS patient iPSC-derived motor neurons. Knockout of GSDME in SOD1G93A ALS mice prolonged survival, ameliorated motor dysfunction, rescued motor neuron loss, and reduced neuroinflammation. We identify GSDME as an executioner of neuronal mitochondrial dysfunction that may contribute to neurodegeneration.

On-person adaptive evolution of Staphylococcus aureus during treatment for atopic dermatitis

The opportunistic pathogen Staphylococcus aureus frequently colonizes the inflamed skin of people with atopic dermatitis (AD) and worsens disease severity by promoting skin damage. Here, we show, by longitudinally tracking 23 children treated for AD, that S. aureus adapts via de novo mutations during colonization. Each patient's S. aureus population is dominated by a single lineage, with infrequent invasion by distant lineages. Mutations emerge within each lineage at rates similar to those of S. aureus in other contexts. Some variants spread across the body within months, with signatures of adaptive evolution. Most strikingly, mutations in capsule synthesis gene capD underwent parallel evolution in one patient and across-body sweeps in two patients. We confirm that capD negativity is more common in AD than in other contexts, via reanalysis of S. aureus genomes from 276 people. Together, these findings highlight the importance of the mutation level when dissecting the role of microbes in complex disease.

An airway-to-brain sensory pathway mediates influenza-induced sickness

Pathogen infection causes a stereotyped state of sickness that involves neuronally orchestrated behavioural and physiological changes1,2. On infection, immune cells release a 'storm' of cytokines and other mediators, many of which are detected by neurons3,4; yet, the responding neural circuits and neuro-immune interaction mechanisms that evoke sickness behaviour during naturalistic infections remain unclear. Over-the-counter medications such as aspirin and ibuprofen are widely used to alleviate sickness and act by blocking prostaglandin E2 (PGE2) synthesis5. A leading model is that PGE2 crosses the blood-brain barrier and directly engages hypothalamic neurons2. Here, using genetic tools that broadly cover a peripheral sensory neuron atlas, we instead identified a small population of PGE2-detecting glossopharyngeal sensory neurons (petrosal GABRA1 neurons) that are essential for influenza-induced sickness behaviour in mice. Ablating petrosal GABRA1 neurons or targeted knockout of PGE2 receptor 3 (EP3) in these neurons eliminates influenza-induced decreases in food intake, water intake and mobility during early-stage infection and improves survival. Genetically guided anatomical mapping revealed that petrosal GABRA1 neurons project to mucosal regions of the nasopharynx with increased expression of cyclooxygenase-2 after infection, and also display a specific axonal targeting pattern in the brainstem. Together, these findings reveal a primary airway-to-brain sensory pathway that detects locally produced prostaglandins and mediates systemic sickness responses to respiratory virus infection.

Bacteria hijack a meningeal neuroimmune axis to facilitate brain invasion

The meninges are densely innervated by nociceptive sensory neurons that mediate pain and headache1,2. Bacterial meningitis causes life-threatening infections of the meninges and central nervous system, affecting more than 2.5 million people a year3-5. How pain and neuroimmune interactions impact meningeal antibacterial host defences are unclear. Here we show that Nav1.8+ nociceptors signal to immune cells in the meninges through the neuropeptide calcitonin gene-related peptide (CGRP) during infection. This neuroimmune axis inhibits host defences and exacerbates bacterial meningitis. Nociceptor neuron ablation reduced meningeal and brain invasion by two bacterial pathogens: Streptococcus pneumoniae and Streptococcus agalactiae. S. pneumoniae activated nociceptors through its pore-forming toxin pneumolysin to release CGRP from nerve terminals. CGRP acted through receptor activity modifying protein 1 (RAMP1) on meningeal macrophages to polarize their transcriptional responses, suppressing macrophage chemokine expression, neutrophil recruitment and dural antimicrobial defences. Macrophage-specific RAMP1 deficiency or pharmacological blockade of RAMP1 enhanced immune responses and bacterial clearance in the meninges and brain. Therefore, bacteria hijack CGRP-RAMP1 signalling in meningeal macrophages to facilitate brain invasion. Targeting this neuroimmune axis in the meninges can enhance host defences and potentially produce treatments for bacterial meningitis.

Microbiome induced complement synthesized in the gut protects against enteric infections

Canonically, complement is a serum-based host defense system that protects against systemic microbial invasion. Little is known about the production and function of complement components on mucosal surfaces. Here we show gut complement component 3 (C3), central to complement function, is regulated by the composition of the microbiota in healthy humans and mice, leading to host-specific gut C3 levels. Stromal cells in intestinal lymphoid follicles (LFs) are the predominant source of intestinal C3. During enteric infection with Citrobacter rodentium or enterohemorrhagic Escherichia coli, luminal C3 levels increase significantly and are required for protection. C. rodentium is remarkably more invasive to the gut epithelium of C3-deficient mice than of wild-type mice. In the gut, C3-mediated phagocytosis of C. rodentium functions to clear pathogens. Our study reveals that variations in gut microbiota determine individuals’ intestinal mucosal C3 levels, dominantly produced by LF stromal cells, which directly correlate with protection against enteric infection.

Highlights

Gut complement component 3 (C3) is induced by the microbiome in healthy humans and mice at a microbiota-specific level.Gut stromal cells located in intestinal lymphoid follicles are a major source of luminal C3 During enteric infections with Citrobacter rodentium or enterohemorrhagic Escherichia coli, gut luminal C3 levels increase and are required for protection. C. rodentium is significantly more invasive of the gut epithelium in C3-deficient mice when compared to WT mice. In the gut, C3-mediated opsonophagocytosis of C. rodentium functions to clear pathogens.

Immunity to the microbiota promotes sensory neuron regeneration

Tissue immunity and responses to injury depend on the coordinated action and communication among physiological systems. Here, we show that, upon injury, adaptive responses to the microbiota directly promote sensory neuron regeneration. At homeostasis, tissue-resident commensal-specific T cells colocalize with sensory nerve fibers within the dermis, express a transcriptional program associated with neuronal interaction and repair, and promote axon growth and local nerve regeneration following injury. Mechanistically, our data reveal that the cytokine interleukin-17A (IL-17A) released by commensal-specific Th17 cells upon injury directly signals to sensory neurons via IL-17 receptor A, the transcription of which is specifically upregulated in injured neurons. Collectively, our work reveals that in the context of tissue damage, preemptive immunity to the microbiota can rapidly bridge biological systems by directly promoting neuronal repair, while also identifying IL-17A as a major determinant of this fundamental process.

Nociceptor neurons direct goblet cells via a CGRP-RAMP1 axis to drive mucus production and gut barrier protection

Neuroepithelial crosstalk is critical for gut physiology. However, the mechanisms by which sensory neurons communicate with epithelial cells to mediate gut barrier protection at homeostasis and during inflammation are not well understood. Here, we find that Nav1.8+CGRP+ nociceptor neurons are juxtaposed with and signal to intestinal goblet cells to drive mucus secretion and gut protection. Nociceptor ablation led to decreased mucus thickness and dysbiosis, while chemogenetic nociceptor activation or capsaicin treatment induced mucus growth. Mouse and human goblet cells expressed Ramp1, receptor for the neuropeptide CGRP. Nociceptors signal via the CGRP-Ramp1 pathway to induce rapid goblet cell emptying and mucus secretion. Notably, commensal microbes activated nociceptors to control homeostatic CGRP release. In the absence of nociceptors or epithelial Ramp1, mice showed increased epithelial stress and susceptibility to colitis. Conversely, CGRP administration protected nociceptor-ablated mice against colitis. Our findings demonstrate a neuron-goblet cell axis that orchestrates gut mucosal barrier protection.

Catching a killer: Mechanisms of programmed cell death and immune activation in Amyotrophic Lateral Sclerosis

In the central nervous system (CNS), execution of programmed cell death (PCD) is crucial for proper neurodevelopment. However, aberrant activation of these pathways in adult CNS leads to neurodegenerative diseases including amyotrophic lateral sclerosis (ALS). How a cell dies is critical, as it can drive local immune activation and tissue damage. Classical apoptosis engages several mechanisms to evoke "immunologically silent" responses, whereas other forms of programmed death such as pyroptosis, necroptosis, and ferroptosis release molecules that can potentiate immune responses and inflammation. In ALS, a fatal neuromuscular disorder marked by progressive death of lower and upper motor neurons, several cell types in the CNS express machinery for multiple PCD pathways. The specific cell types engaging PCD, and ultimate mechanisms by which neuronal death occurs in ALS are not well defined. Here, we provide an overview of different PCD pathways implicated in ALS. We also examine immune activation in ALS and differentiate apoptosis from necrotic mechanisms based on downstream immunological consequences. Lastly, we highlight therapeutic strategies that target cell death pathways in the treatment of neurodegeneration and inflammation in ALS.

A neuropeptide regulates immunity across species

Communication between the nervous system and immune system is important for regulating immunity in health and disease. Yu et al. (2022) show that neuropeptide Y and its homolog NPF serve as a "language" to facilitate crosstalk between these two systems across species, enabling neurons to downregulate harmful immune responses.

Neuroimmune Interactions in Peripheral Organs

Interactions between the nervous and immune systems were recognized long ago, but recent studies show that this crosstalk occurs more frequently than was previously appreciated. Moreover, technological advances have enabled the identification of the molecular mediators and receptors that enable the interaction between these two complex systems and provide new insights on the role of neuroimmune crosstalk in organismal physiology. Most neuroimmune interaction occurs at discrete anatomical locations in which neurons and immune cells colocalize. Here, we describe the interactions of the different branches of the peripheral nervous system with immune cells in various organs, including the skin, intestine, lung, and adipose tissue. We highlight how neuroimmune crosstalk orchestrates physiological processes such as host defense, tissue repair, metabolism, and thermogenesis. Unraveling these intricate relationships is invaluable to explore the therapeutic potential of neuroimmune interaction. Expected final online publication date for the Annual Review of Neuroscience, Volume 45 is July 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Enteric glial cells mediate gut immunity and repair

In the gut, coordinated cell interactions regulate tissue repair and immunity. How enteric glial cells (EGCs) mediate these processes remained elusive. In a recent paper, Progatzky et al. demonstrate that EGCs interact with immune and mesothelial cells under homeostasis and helminth infection, revealing an indispensable role of an interferon-γ (IFNγ)-EGC-CXCL10 axis in tissue repair.

Anthrax toxins regulate pain signaling and can deliver molecular cargoes into ANTXR2+ DRG sensory neurons

Bacterial products can act on neurons to alter signaling and function. In the present study, we found that dorsal root ganglion (DRG) sensory neurons are enriched for ANTXR2, the high-affinity receptor for anthrax toxins. Anthrax toxins are composed of protective antigen (PA), which binds to ANTXR2, and the protein cargoes edema factor (EF) and lethal factor (LF). Intrathecal administration of edema toxin (ET (PA + EF)) targeted DRG neurons and induced analgesia in mice. ET inhibited mechanical and thermal sensation, and pain caused by formalin, carrageenan or nerve injury. Analgesia depended on ANTXR2 expressed by Na_v1.8⁺ or Advillin⁺ neurons. ET modulated protein kinase A signaling in mouse sensory and human induced pluripotent stem cell-derived sensory neurons, and attenuated spinal cord neurotransmission. We further engineered anthrax toxins to introduce exogenous protein cargoes, including botulinum toxin, into DRG neurons to silence pain. Our study highlights interactions between a bacterial toxin and nociceptors, which may lead to the development of new pain therapeutics.

VLDLR and ApoER2 are receptors for multiple alphaviruses

Alphaviruses, like many other arthropod-borne viruses, infect vertebrate species and insect vectors separated by hundreds of millions of years of evolutionary history. Entry into evolutionarily divergent host cells can be accomplished by recognition of different cellular receptors in different species, or by binding to receptors that are highly conserved across species. Although multiple alphavirus receptors have been described^1–3, most are not shared among vertebrate and invertebrate hosts. Here we identify the very low-density lipoprotein receptor (VLDLR) as a receptor for the prototypic alphavirus Semliki forest virus. We show that the E2 and E1 glycoproteins (E2–E1) of Semliki forest virus, eastern equine encephalitis virus and Sindbis virus interact with the ligand-binding domains (LBDs) of VLDLR and apolipoprotein E receptor 2 (ApoER2), two closely related receptors. Ectopic expression of either protein facilitates cellular attachment, and internalization of virus-like particles, a VLDLR LBD–Fc fusion protein or a ligand-binding antagonist block Semliki forest virus E2–E1-mediated infection of human and mouse neurons in culture. The administration of a VLDLR LBD–Fc fusion protein has protective activity against rapidly fatal Semliki forest virus infection in mouse neonates. We further show that invertebrate receptor orthologues from mosquitoes and worms can serve as functional alphavirus receptors. We propose that the ability of some alphaviruses to infect a wide range of hosts is a result of their engagement of evolutionarily conserved lipoprotein receptors and contributes to their pathogenesis.

Studies using viral coat glycoproteins show that alphaviruses can enter cells via the very low-density lipoprotein receptor (VLDLR) and apolipoprotein E receptor 2 (ApoER2), members of an evolutionarily conserved family of lipoprotein receptors.

Nociceptive Sensory Neurons Mediate Inflammation Induced by Bacillus Anthracis Edema Toxin

Bacterial products are able to act on nociceptive neurons during pathogenic infection. Neurogenic inflammation is an active part of pain signaling and has recently been shown to impact host-pathogen defense. Bacillus anthracis Edema Toxin (ET) produces striking edema in peripheral tissues, but the cellular mechanisms involved in tissue swelling are not completely understood. Here, we find that nociceptive neurons play a role in ET-induced edema and inflammation in mice. Subcutaneous footpad infection of B. anthracis Sterne caused ET-dependent local mechanical allodynia, paw swelling and body weight gain. Subcutaneous administration of ET induced paw swelling and vascular leakage, the early phases of which were attenuated in the absence of Trp_v1⁺ or Na_v1.8⁺ nociceptive neurons. Nociceptive neurons express the anthrax toxin receptor ANTXR2, but this did not mediate ET-induced edema. ET induced local cytokine expression and neutrophil recruitment, which were dependent in part on Trp_v1⁺ nociceptive neurons. Ablation of Trp_v1⁺ or Na_v1.8⁺ nociceptive neurons also attenuated early increases in paw swelling and body weight gain during live B. anthracis infection. Our findings indicate that nociceptive neurons play an active role in inflammation caused by B. anthracis and Edema Toxin to potentially influence bacterial pathogenesis.

Gut-licensed IFNγ+ NK cells drive LAMP1+TRAIL+ anti-inflammatory astrocytes

Astrocytes are glial cells that are abundant in the central nervous system (CNS) and that have important homeostatic and disease-promoting functions1. However, little is known about the homeostatic anti-inflammatory activities of astrocytes and their regulation. Here, using high-throughput flow cytometry screening, single-cell RNA sequencing and CRISPR-Cas9-based cell-specific in vivo genetic perturbations in mice, we identify a subset of astrocytes that expresses the lysosomal protein LAMP12 and the death receptor ligand TRAIL3. LAMP1+TRAIL+ astrocytes limit inflammation in the CNS by inducing T cell apoptosis through TRAIL-DR5 signalling. In homeostatic conditions, the expression of TRAIL in astrocytes is driven by interferon-γ (IFNγ) produced by meningeal natural killer (NK) cells, in which IFNγ expression is modulated by the gut microbiome. TRAIL expression in astrocytes is repressed by molecules produced by T cells and microglia in the context of inflammation. Altogether, we show that LAMP1+TRAIL+ astrocytes limit CNS inflammation by inducing T cell apoptosis, and that this astrocyte subset is maintained by meningeal IFNγ+ NK cells that are licensed by the microbiome.

Mechanisms of microbial-neuronal interactions in pain and nociception

•

Molecular mechanisms of how microorganisms communicate with sensory afferent neurons.

•

How pathogenic microorganisms directly communicate with nociceptor neurons to inflict pain on the host.

•

Symbiotic bacterial communication with gut-extrinsic sensory afferent neurons.

•

Plausible roles on how gut symbionts directly mediate pain and nociception.

Nociceptor sensory neurons innervate barrier tissues that are constantly exposed to microbial stimuli. During infection, pathogenic microorganisms can breach barrier surfaces and produce pain by directly activating nociceptors. Microorganisms that live in symbiotic relationships with their hosts, commensals and mutualists, have also been associated with pain, but the molecular mechanisms of how symbionts act on nociceptor neurons to modulate pain remain largely unknown. In this review, we will discuss the known molecular mechanisms of how microbes directly interact with sensory afferent neurons affecting nociception in the gut, skin and lungs. We will touch on how bacterial, viral and fungal pathogens signal to the host to inflict or suppress pain. We will also discuss recent studies examining how gut symbionts affect pain. Specifically, we will discuss how gut symbionts may interact with sensory afferent neurons either directly, through secretion of metabolites or neurotransmitters, or indirectly,through first signaling to epithelial cells or immune cells, to regulate visceral, neuropathic and inflammatory pain. While this area of research is still in its infancy, more mechanistic studies to examine microbial-sensory neuron crosstalk in nociception may allow us to develop new therapies for the treatment of acute and chronic pain.

Transcriptional and proteomic insights into the host response in fatal COVID-19 cases

Coronavirus disease 2019 (COVID-19), the global pandemic caused by SARS-CoV-2, has resulted thus far in greater than 933,000 deaths worldwide; yet disease pathogenesis remains unclear. Clinical and immunological features of patients with COVID-19 have highlighted a potential role for changes in immune activity in regulating disease severity. However, little is known about the responses in human lung tissue, the primary site of infection. Here we show that pathways related to neutrophil activation and pulmonary fibrosis are among the major up-regulated transcriptional signatures in lung tissue obtained from patients who died of COVID-19 in Wuhan, China. Strikingly, the viral burden was low in all samples, which suggests that the patient deaths may be related to the host response rather than an active fulminant infection. Examination of the colonic transcriptome of these patients suggested that SARS-CoV-2 impacted host responses even at a site with no obvious pathogenesis. Further proteomics analysis validated our transcriptome findings and identified several key proteins, such as the SARS-CoV-2 entry-associated protease cathepsins B and L and the inflammatory response modulator S100A8/A9, that are highly expressed in fatal cases, revealing potential drug targets for COVID-19.

Microbiota-neuroimmune cross talk in stress-induced visceral hypersensitivity of the bowel

Visceral hypersensitivity of the lower gastrointestinal tract, defined as an increased response to colorectal distension, frequently prompts episodes of debilitating abdominal pain in irritable bowel syndrome (IBS). Although the pathophysiology of IBS is not yet fully elucidated, it is well known that stress is a major risk factor for development and acts as a trigger of pain sensation. Stress modulates both immune responses as well as the gut microbiota and vice versa. Additionally, either microbes themselves or through involvement of the immune system, activate or sensitize afferent nociceptors. In this paper, we review current knowledge on the influence of stress along the gut-brain-microbiota axis and exemplify relevant neuroimmune cross talk mechanisms in visceral hypersensitivity, working toward understanding how gut microbiota-neuroimmune cross talk contributes to visceral pain sensation in IBS patients.

Neuro-immune Interactions in the Tissues

The ability of the nervous system to sense environmental stimuli and to relay these signals to immune cells via neurotransmitters and neuropeptides is indispensable for effective immunity and tissue homeostasis. Depending on the tissue microenvironment and distinct drivers of a certain immune response, the same neuronal populations and neuro-mediators can exert opposing effects, promoting or inhibiting tissue immunity. Here, we review the current understanding of the mechanisms that underlie the complex interactions between the immune and the nervous systems in different tissues and contexts. We outline current gaps in knowledge and argue for the importance of considering infectious and inflammatory disease within a conceptual framework that integrates neuro-immune circuits both local and systemic, so as to better understand effective immunity to develop improved approaches to treat inflammation and disease.

Neuronal, stromal, and T-regulatory cell crosstalk in murine skeletal muscle

A distinct population of Foxp3+CD4+ regulatory T (Treg) cells promotes repair of acutely or chronically injured skeletal muscle. The accumulation of these cells depends critically on interleukin (IL)-33 produced by local mesenchymal stromal cells (mSCs). An intriguing physical association among muscle nerves, IL-33+ mSCs, and Tregs has been reported, and invites a deeper exploration of this cell triumvirate. Here we evidence a striking proximity between IL-33+ muscle mSCs and both large-fiber nerve bundles and small-fiber sensory neurons; report that muscle mSCs transcribe an array of genes encoding neuropeptides, neuropeptide receptors, and other nerve-related proteins; define muscle mSC subtypes that express both IL-33 and the receptor for the calcitonin-gene-related peptide (CGRP); and demonstrate that up- or down-tuning of CGRP signals augments or diminishes, respectively, IL-33 production by muscle mSCs and later accumulation of muscle Tregs. Indeed, a single injection of CGRP induced much of the genetic program elicited in mSCs early after acute skeletal muscle injury. These findings highlight neural/stromal/immune-cell crosstalk in tissue repair, suggesting future therapeutic approaches.

Hyperactivation of sympathetic nerves drives depletion of melanocyte stem cells

Empirical and anecdotal evidence have associated stress with accelerated hair greying (formation of unpigmented hairs)^1,2, but the scientific evidence linking the two is scant. Here, we report that acute stress leads to hair greying through fast depletion of melanocyte stem cells (MeSCs). Combining adrenalectomy, denervation, chemogenetics^3,4, cell ablation, and MeSC-specific adrenergic receptor knockout, we found that stress-induced MeSC loss is independent of immune attack or adrenal stress hormones. Rather, hair greying results from activation of the sympathetic nerves that innervate the MeSC niche. Upon stress, sympathetic nerve activation leads to burst release of the neurotransmitter norepinephrine, which drives quiescent MeSCs into rapid proliferation, followed by differentiation, migration, and permanent depletion from the niche. Transient suppression of MeSC proliferation prevents stress-induced hair greying. Our studies demonstrate that acute stress-induced neuronal activity can drive rapid and permanent loss of somatic stem cells, and illustrate an example in which somatic stem cell maintenance is directly influenced by the overall physiological state of the organism.

Gut-Innervating Nociceptor Neurons Regulate Peyer's Patch Microfold Cells and SFB Levels to Mediate Salmonella Host Defense

Gut-innervating nociceptor sensory neurons respond to noxious stimuli by initiating protective responses including pain and inflammation; however, their role in enteric infections is unclear. Here, we find that nociceptor neurons critically mediate host defense against the bacterial pathogen Salmonella enterica serovar Typhimurium (STm). Dorsal root ganglia nociceptors protect against STm colonization, invasion, and dissemination from the gut. Nociceptors regulate the density of microfold (M) cells in ileum Peyer's patch (PP) follicle-associated epithelia (FAE) to limit entry points for STm invasion. Downstream of M cells, nociceptors maintain levels of segmentous filamentous bacteria (SFB), a gut microbe residing on ileum villi and PP FAE that mediates resistance to STm infection. TRPV1+ nociceptors directly respond to STm by releasing calcitonin gene-related peptide (CGRP), a neuropeptide that modulates M cells and SFB levels to protect against Salmonella infection. These findings reveal a major role for nociceptor neurons in sensing and defending against enteric pathogens.

Neuronal Regulation of Immunity in the Skin and Lungs

The nervous and immune systems are classically studied as two separate entities. However, their interactions are crucial for maintaining barrier functions at tissues constantly exposed to the external environment. We focus here on the role of neuronal signaling in regulating the immune system at two major barriers: the skin and respiratory tract. Barrier tissues are heavily innervated by sensory and autonomic nerves, and are densely populated by resident immune cells, allowing rapid, coordinated responses to noxious stimuli, as well as to bacterial and fungal pathogens. Neural release of neurotransmitters and neuropeptides allows fast communication with immune cells and their recruitment. In addition to maintaining homeostasis and fighting infections, neuroimmune interactions are also implicated in several chronic inflammatory conditions such as atopic dermatitis (AD), chronic obstructive pulmonary disease (COPD), and asthma.

Single-cell RNA-seq identifies the neuropeptide NMU as a novel regulator of ILC2 function

Type 2 innate lymphoid cells (ILC2s) are important for both mucosal homeostasis as well as for initiation and progression of allergy and asthma. Alarmin cytokines produced by damaged or stressed epithelial cells activate ILC2s, however, the role of other pathways regulating the homeostatic versus inflammatory functions of ILC2s remain poorly defined. We generated transcriptional profiles of thousands of lung-resident ILCs isolated from mice challenged with either PBS or the alarmin cytokines IL-25 or IL-33 using massively parallel droplet-based single-cell RNA sequencing (scRNA-seq). We found significant transcriptional heterogeneity within activated ILCs and identified differentially expressed genes as potential novel regulators of ILC function. One of the genes, neuromedin U receptor 1 (NMUR1), is a receptor for the neuropeptide neuromedin U (NMU) and is specifically expressed by ILC2s and not by other immune cells. Nmur1 is particularly expressed by ILC2s at steady-state and after IL-25 treatment. In vivo NMU synergized with IL-25 to induce more severe airway inflammation than was seen with either IL-25 or NMU alone and this effect was dependent on ILCs. Mechanistically, IL-25 + NMU expanded highly proliferative and inflammatory ILCs with distinct transcriptional profiles and increased expression of IL-5 and IL-13. We observed reduced ILC2 frequency and function in NMU- or NMUR1-deficient mice after allergen challenge and found that ILCs from these mice had a lower score for an inflammatory ILC2 gene signature. Our data therefore establish a novel neuro-immune pathway that amplifies ILC2-driven airway inflammation.

Neuro-immune interactions in allergic diseases: novel targets for therapeutics

Recent studies have highlighted an emerging role for neuro-immune interactions in mediating allergic diseases. Allergies are caused by an overactive immune response to a foreign antigen. The peripheral sensory and autonomic nervous system densely innervates mucosal barrier tissues including the skin, respiratory tract and gastrointestinal (GI) tract that are exposed to allergens. It is increasingly clear that neurons actively communicate with and regulate the function of mast cells, dendritic cells, eosinophils, Th2 cells and type 2 innate lymphoid cells in allergic inflammation. Several mechanisms of cross-talk between the two systems have been uncovered, with potential anatomical specificity. Immune cells release inflammatory mediators including histamine, cytokines or neurotrophins that directly activate sensory neurons to mediate itch in the skin, cough/sneezing and bronchoconstriction in the respiratory tract and motility in the GI tract. Upon activation, these peripheral neurons release neurotransmitters and neuropeptides that directly act on immune cells to modulate their function. Somatosensory and visceral afferent neurons release neuropeptides including calcitonin gene-related peptide, substance P and vasoactive intestinal peptide, which can act on type 2 immune cells to drive allergic inflammation. Autonomic neurons release neurotransmitters including acetylcholine and noradrenaline that signal to both innate and adaptive immune cells. Neuro-immune signaling may play a central role in the physiopathology of allergic diseases including atopic dermatitis, asthma and food allergies. Therefore, getting a better understanding of these cellular and molecular neuro-immune interactions could lead to novel therapeutic approaches to treat allergic diseases.

Nociceptor sensory neurons suppress neutrophil and γδ T cell responses in bacterial lung infections and lethal pneumonia

Nociceptor sensory neurons suppress innate immunity during bacterial lung infection.

Lung-innervating nociceptor sensory neurons detect noxious or harmful stimuli and consequently protect organisms by mediating coughing, pain, and bronchoconstriction. However, the role of sensory neurons in pulmonary host defense is unclear. Here, we found that TRPV1⁺ nociceptors suppressed protective immunity against lethal Staphylococcus aureus pneumonia. Targeted TRPV1⁺-neuron ablation increased survival, cytokine induction, and lung bacterial clearance. Nociceptors suppressed the recruitment and surveillance of neutrophils, and altered lung γδ T cell numbers, which are necessary for immunity. Vagal ganglia TRPV1⁺ afferents mediated immunosuppression through release of the neuropeptide calcitonin gene–related peptide (CGRP). Targeting neuroimmunological signaling may be an effective approach to treat lung infections and bacterial pneumonia.

Neuro-immune interactions in inflammation and host defense: Implications for transplantation

Sensory and autonomic neurons of the peripheral nervous system (PNS) play a critical role in regulating the immune system during tissue inflammation and host defense. Recent studies have identified the molecular mechanisms underlying the bidirectional communication between the nervous system and the immune system. Here, we highlight the studies that demonstrate the importance of the neuro-immune interactions in health and disease. Nociceptor sensory neurons detect immune mediators to produce pain, and release neuropeptides that act on the immune system to regulate inflammation. In parallel, neural reflex circuits including the vagus nerve-based inflammatory reflex are physiological regulators of inflammatory responses and cytokine production. In transplantation, neuro-immune communication could significantly impact the processes of host-pathogen defense, organ rejection, and wound healing. Emerging approaches to target the PNS such as bioelectronics could be useful in improving the outcome of transplantation. Therefore, understanding how the nervous system shapes the immune response could have important therapeutic ramifications for transplantation medicine.

Sensory neuron regulation of gastrointestinal inflammation and bacterial host defence

Sensory neurons in the gastrointestinal tract have multifaceted roles in maintaining homeostasis, detecting danger and initiating protective responses. The gastrointestinal tract is innervated by three types of sensory neurons: dorsal root ganglia, nodose/jugular ganglia and intrinsic primary afferent neurons. Here, we examine how these distinct sensory neurons and their signal transducers participate in regulating gastrointestinal inflammation and host defence. Sensory neurons are equipped with molecular sensors that enable neuronal detection of diverse environmental signals including thermal and mechanical stimuli, inflammatory mediators and tissue damage. Emerging evidence shows that sensory neurons participate in host-microbe interactions. Sensory neurons are able to detect pathogenic and commensal bacteria through specific metabolites, cell-wall components, and toxins. Here, we review recent work on the mechanisms of bacterial detection by distinct subtypes of gut-innervating sensory neurons. Upon activation, sensory neurons communicate to the immune system to modulate tissue inflammation through antidromic signalling and efferent neural circuits. We discuss how this neuro-immune regulation is orchestrated through transient receptor potential ion channels and sensory neuropeptides including substance P, calcitonin gene-related peptide, vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide. Recent studies also highlight a role for sensory neurons in regulating host defence against enteric bacterial pathogens including Salmonella typhimurium, Citrobacter rodentium and enterotoxigenic Escherichia coli. Understanding how sensory neurons respond to gastrointestinal flora and communicate with immune cells to regulate host defence enhances our knowledge of host physiology and may form the basis for new approaches to treat gastrointestinal diseases.

Nociceptor neurons regulate lung infections and bacterial host defense

The sensory nervous system densely innervates the respiratory tract, mediating breathing, cough and bronchoconstriction. However, the role of the nervous system in lung host defenses has not been defined. Nociceptor neurons sense noxious/harmful stimuli and protect organisms from danger. Here, we find that nociceptor neurons play a critical role in regulating innate immunity against bacterial lung infections and lethal pneumonia. Staphylococcus aureus is a major cause of human disease and pneumonia in hospital acquired infections. We find that S. aureus directly induces nociceptor neuron ionic influx and activation. Nociceptor neurons released the neuropeptide CGRP upon bacterial activation, which suppressed neutrophil recruitment and bacterial clearance. Targeting nociceptor neurons by genetic and chemical strategies led to significantly enhanced innate immunity and survival in a mouse model of S. aureus lethal pneumonia. Therefore, the nervous system plays an important role in lung infections, and targeting sensory neurons could lead to better treatments for bacterial pneumonia.

Bacterial Signaling to the Nervous System through Toxins and Metabolites

Mammalian hosts interface intimately with commensal and pathogenic bacteria. It is increasingly clear that molecular interactions between the nervous system and microbes contribute to health and disease. Both commensal and pathogenic bacteria are capable of producing molecules that act on neurons and affect essential aspects of host physiology. Here we highlight several classes of physiologically important molecular interactions that occur between bacteria and the nervous system. First, clostridial neurotoxins block neurotransmission to or from neurons by targeting the SNARE complex, causing the characteristic paralyses of botulism and tetanus during bacterial infection. Second, peripheral sensory neurons-olfactory chemosensory neurons and nociceptor sensory neurons-detect bacterial toxins, formyl peptides, and lipopolysaccharides through distinct molecular mechanisms to elicit smell and pain. Bacteria also damage the central nervous system through toxins that target the brain during infection. Finally, the gut microbiota produces molecules that act on enteric neurons to influence gastrointestinal motility, and metabolites that stimulate the "gut-brain axis" to alter neural circuits, autonomic function, and higher-order brain function and behavior. Furthering the mechanistic and molecular understanding of how bacteria affect the nervous system may uncover potential strategies for modulating neural function and treating neurological diseases.

Infection, Pain, and Itch

Pain and itch are unpleasant sensations that often accompany infections caused by viral, bacterial, parasitic, and fungal pathogens. Recent studies show that sensory neurons are able to directly detect pathogens to mediate pain and itch. Nociceptor and pruriceptor neurons respond to pathogen-associated molecular patterns, including Toll-like receptor ligands, N-formyl peptides, and bacterial toxins. Other pathogens are able to silence neuronal activity to produce analgesia during infection. Pain and itch could lead to neuronal modulation of the immune system or behavioral avoidance of future pathogen exposure. Conversely, pathogens could modulate neuronal signaling to potentiate their pathogenesis and facilitate their spread to other hosts. Defining how pathogens modulate pain and itch has critical implications for sensory neurobiology and our understanding of host-microbe interactions.