Western equine encephalitis virus (WEEV) is an arbovirus that historically caused large outbreaks of encephalitis throughout the Americas. WEEV binds protocadherin 10 (PCDH10) as a receptor, and highly virulent ancestral WEEV strains also bind low-density lipoprotein receptor (LDLR)-related proteins. As WEEV declined as a human pathogen in North America over the past century, isolates have lost the ability to bind mammalian receptors while still recognizing avian receptors. To explain shifts in receptor dependencies and assess the risk of WEEV re-emergence, we determined cryoelectron microscopy structures of WEEV bound to human PCDH10, avian PCDH10, and human very-low-density lipoprotein receptor (VLDLR). We show that one to three E2 glycoprotein substitutions are sufficient for a nonpathogenic strain to regain the ability to bind mammalian receptors. A soluble VLDLR fragment protects mice from lethal challenge by a virulent ancestral WEEV strain. Because WEEV recently re-emerged in South America after decades of inactivity, our findings have important implications for outbreak preparedness.

Infections after psychological stress are a major health care problem. Single-cell transcriptomics and lipidomic profiling in a mouse model of stress show that dermal fibroblasts undergoing adipogenesis have defective responses to Staphylococcus aureus skin infection. Adrenalectomy or adrenergic inhibition restores the fibroblast adipogenic response to S. aureus and enables mice to effectively resist infection during stress. Increased susceptibility to S. aureus from stress is attributed to suppression of the antimicrobial peptide cathelicidin (Camp) because adrenaline directly inhibits Camp production by fibroblasts, and mice lacking Camp in fibroblasts do not increase infection after stress. Transforming growth factor β (TGFβ) is induced by stress and adrenergic signaling, and inhibition of TGFβ or deletion of the TGFβ receptor on fibroblasts increases Camp expression and restores protection against infection. Together, these data show that stress initiates a brain-skin axis mediated by TGFβ that impairs the immune defense function of dermal fibroblasts to produce the Camp antimicrobial peptide.

The nervous system, like the immune system, constantly interfaces with the environment, encountering threats, including pathogens. Recent discoveries reveal an emerging role for sensory neurons in host defense and immunity. Sensory neurons detect infections either by directly sensing microbial signals or through immune mediators. Beyond pathogen detection, they modulate immune responses and local inflammation by interacting with immune cells, influencing inflammation and pathogen clearance. Additionally, sensory neurons trigger protective reflexes - such as pain, coughing, sneezing, and itching - that can help expel pathogens but may also facilitate their spread. Sensory neurons may also encode and shape long-term immunity. Understanding the roles of neurons in pathogen defense could offer new insights into infectious diseases and highlight therapeutic opportunities for immune modulation.

Astrocytes promote neuroinflammation and neurodegeneration in multiple sclerosis (MS) through cell-intrinsic activities and their ability to recruit and activate other cell types. In a genome-wide CRISPR-based forward genetic screen investigating regulators of astrocyte proinflammatory responses, we identified the C-type lectin domain-containing 16A gene (CLEC16A), linked to MS susceptibility, as a suppressor of nuclear factor-κB (NF-κB) signaling. Gene and small-molecule perturbation studies in mouse primary and human embryonic stem cell-derived astrocytes in combination with multiomic analyses established that CLEC16A promotes mitophagy, limiting mitochondrial dysfunction and the accumulation of mitochondrial products that activate NF-κB, the NLRP3 inflammasome and gasdermin D. Astrocyte-specific Clec16a inactivation increased NF-κB, NLRP3 and gasdermin D activation in vivo, worsening experimental autoimmune encephalomyelitis, a mouse model of MS. Moreover, we detected disrupted mitophagic capacity and gasdermin D activation in astrocytes in samples from individuals with MS. These findings identify CLEC16A as a suppressor of astrocyte pathological responses and a candidate therapeutic target in MS.

Amyotrophic Lateral Sclerosis (ALS) is a fatal neurodegenerative disease characterized by progressive motor neuron loss, microgliosis, and neuroinflammation. While pyroptosis, an inflammatory form of programmed cell death, has been implicated in ALS, the specific role of Gasdermin D (GSDMD) - the primary executioner of pyroptosis - remains unexplored. In this study, we examined the function of GSDMD in the well-established SOD1G93A mouse model of ALS. Our results showed robust GSDMD activation in the spinal cords of SOD1G93A animals with elevated expression in Iba1+ microglia. To explore its role in disease progression, we bred C57Bl/6 J.SOD1G93A mice onto a C57Bl/6NJ.GSDMD-deficient background. In comparing SOD1G93A; Gsdmd+/+ and SOD1G93A; Gsdmd-/- mice, we found that Gsdmd loss did not affect disease onset, weight loss, or grip strength decline in either male or female animals. Notably, GSDMD deficiency resulted in a modest but statistically significant increase in mortality in SOD1G93A mice. Moreover, GSDMD absence had minimal impact on astrogliosis, microgliosis and motor neuron loss. These findings show that while GSDMD is activated in the ALS mouse model, its loss does not mitigate key ALS behavioral phenotypes, gliosis or motor neuron loss. This study provides insights into the potential therapeutic relevance of targeting pyroptosis and inflammatory pathways in ALS.

Many species regenerate lost body parts following amputation. Most limb regeneration research has focused on the immediate injury site. Meanwhile, body-wide injury responses remain largely unexplored but may be critical for regeneration. Here, we discovered a role for the sympathetic nervous system in stimulating a body-wide stem cell activation response to amputation that drives enhanced limb regeneration in axolotls. This response is mediated by adrenergic signaling, which coordinates distant cellular activation responses via the α2Α-adrenergic receptor, and local regeneration responses via β-adrenergic receptors. Both α2A- and β-adrenergic signaling act upstream of mTOR signaling. Notably, systemically activated axolotls regenerate limbs faster than naive animals, suggesting a potential selective advantage in environments where injury from cannibalism or predation is common. This work challenges the predominant view that cellular responses underlying regeneration are confined to the injury site and argues instead for body-wide cellular priming as a foundational step that enables localized tissue regrowth.

It is increasingly clear that the nervous system and immune system share a common molecular dialogue for intersystem communication. One of the key mechanisms of this communication is via neurotransmitters and neuropeptides. Diverse neuronal subtypes interact with various immune cell populations via the release of a wide variety of these neuromodulators that bind to receptors on immune cells. In the gut, this communication occurs via gut-intrinsic enteric neurons, extrinsic sensory and autonomic neurons. Here, we highlight a few key neurotransmitters and neuropeptides that have been shown to play a role in gut inflammation and host defense by acting on immune cells. Aberrations in this communication can lead to disorders including autoimmunity and tissue inflammation. We also discuss the need to better understand the molecular code of neuroimmune communication, which could lead to approaches to improve gut function and health.

Efficient mitophagy is essential for neuronal health. A study now shows that loss of VPS13D in neurons impairs mitochondrial clearance, gasdermin E activation, mitochondrial DNA release and microglial STING signaling. This neuroimmune mechanism promotes microglial responses that lead to neuronal dysfunction and loss.

Neuroimmune interactions-signals transmitted between immune and brain cells-regulate many aspects of tissue physiology1, including responses to psychological stress2-5, which can predispose individuals to develop neuropsychiatric diseases6-9. Still, the interactions between haematopoietic and brain-resident cells that influence complex behaviours are poorly understood. Here, we use a combination of genomic and behavioural screens to show that astrocytes in the amygdala limit stress-induced fear behaviour through epidermal growth factor receptor (EGFR). Mechanistically, EGFR expression in amygdala astrocytes inhibits a stress-induced, pro-inflammatory signal-transduction cascade that facilitates neuron-glial crosstalk and stress-induced fear behaviour through the orphan nuclear receptor NR2F2 in amygdala neurons. In turn, decreased EGFR signalling and fear behaviour are associated with the recruitment of meningeal monocytes during chronic stress. This set of neuroimmune interactions is therapeutically targetable through the administration of psychedelic compounds, which reversed the accumulation of monocytes in the brain meninges along with fear behaviour. Together with validation in clinical samples, these data suggest that psychedelics can be used to target neuroimmune interactions relevant to neuropsychiatric disorders and potentially other inflammatory diseases.

Our understanding of the meningeal immune system has recently burgeoned, particularly regarding how innate and adaptive effector cells are mobilized to meet brain challenges. However, information on how meningeal immunocytes guard brain homeostasis in healthy individuals remains limited. This study highlights the heterogeneous, polyfunctional regulatory T cell (Treg) compartment in the meninges. A Treg subtype specialized in controlling interferon-γ (IFN-γ) responses and another dedicated to regulating follicular B cell responses were substantial components of this compartment. Accordingly, punctual Treg ablation rapidly unleashed IFN-γ production by meningeal lymphocytes, unlocked access to the brain parenchyma, and altered meningeal B cell profiles. Distally, the hippocampus assumed a reactive state, with morphological and transcriptional changes in multiple glial cell types. Within the dentate gyrus, neural stem cells underwent more death and were blocked from further differentiation, which coincided with impairments in short-term spatial-reference memory. Thus, meningeal Tregs are a multifaceted safeguard of brain homeostasis at steady state.

Influenza viruses are a major global cause of morbidity and mortality. Although vagal TRPV1+ nociceptive sensory neurons are known to mediate defenses against harmful agents, including pathogens, their function in lung antiviral defenses remains unclear. Our study demonstrates that both systemic and vagal-specific ablation of TRPV1+ nociceptors reduce survival in mice infected with influenza A virus (IAV). Despite no difference in viral load, mice lacking TRPV1+ neurons exhibited increased viral spread, exacerbated lung pathology, and elevated levels of proinflammatory cytokines. Loss of TRPV1+ neurons altered the lung immune landscape, including an expansion of neutrophils and monocyte-derived macrophages. Transcriptional analysis revealed impaired interferon signaling in myeloid cells and an imbalance in distinct neutrophil subpopulations in the absence of nociceptors. Furthermore, antibody-mediated depletion of myeloid cells during IAV infection substantially improved survival after nociceptor ablation, underscoring the role of TRPV1+ neurons in preventing pathogenic myeloid cell states that contribute to IAV-induced mortality.

Objective:The gastrointestinal (GI) tract is densely innervated, forming a critical network that secretes neuropeptides essential for gut function. Tumour cells are highly adaptive and exploit their microenvironment, including nerves, to support and accelerate growth. However, the mechanisms by which tumour cells interact with neuropeptides in human GI cancers remain poorly understood. We aimed to investigate the expression and function of the sensory neuropeptide calcitonin gene-related peptide (CGRP) and its receptor component, receptor activity-modifying protein 1 (RAMP1), and to elucidate novel mechanisms by which cancer cells exploit neuropeptides. Methods and analysis:We analysed 180 patient samples using multiplex immunohistochemistry to assess CGRP and RAMP1 expression in primary colorectal cancer (CRC), CRC liver metastases and gastric cancers (GC). RAMP1 expression was correlated with patient demographics (age and gender) and tumour characteristics, including pathological features and molecular and genomic subtypes. RAMP1 expression and association with patient survival were evaluated using data from the Cancer Genome Atlas. The function of CGRP on tumour cell lines and patient-derived tumour organoids was assessed via in vitro stimulation assays and RNA sequencing. Results:RAMP1 expression in tumours was significantly associated with reduced survival in both CRC and GC. Over 50% of CRC and 60% of GC cells from patient samples expressed RAMP1. RAMP1 expression was enriched in tumours with microsatellite instability (MSI) and in patients with GC younger than 50 years. CGRP was abundantly expressed in stromal regions indicative of nerve fibres near tumour cells, and unexpectedly, CGRP was also produced by CRC and GC cells. Finally, CGRP stimulation enhanced tumour cell growth in a RAMP1-dependent manner, inducing genes linked to proliferation, metabolism and migration. Conclusion:This study reveals novel mechanisms by which the neuropeptide CGRP promotes tumour growth in GI cancers. We expand upon existing knowledge by demonstrating that tumour cells are a source of CGRP, highlighting potential therapeutic targets within the tumour-nerve axis.

Recent advances have contributed to a mechanistic understanding of neuroimmune interactions in the intestine and revealed an essential role of this cross talk for gut homeostasis and modulation of inflammatory and infectious intestinal diseases. In this review, we describe the innervation of the intestine by intrinsic and extrinsic neurons and then focus on the bidirectional communication between neurons and immune cells. First, we highlight the contribution of neuronal subtypes to the development of colitis and discuss the different immune and epithelial cell types that are regulated by neurons via the release of neuropeptides and neurotransmitters. Next, we review the role of intestinal inflammation in the development of visceral hypersensitivity and summarize how inflammatory mediators induce peripheral and central sensitization of gut-innervating sensory neurons. Finally, we outline the importance of immune cells and gut microbiota for the survival and function of different neuronal populations at homeostasis and during bacterial and helminth infection.

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.

Visceral pain is a major clinical problem and one of the most common reasons patients with gastrointestinal disorders seek medical help. Peripheral sensory neurons that innervate the gut can detect noxious stimuli and send signals to the central nervous system that are perceived as pain. There is a bidirectional communication network between the gastrointestinal tract and the nervous system that mediates pain through the gut-brain axis. Sensory neurons detect mechanical and chemical stimuli within the intestinal tissues, and receive signals from immune cells, epithelial cells and the gut microbiota, which results in peripheral sensitization and visceral pain. This Review focuses on molecular communication between these non-neuronal cell types and neurons in visceral pain. These bidirectional interactions can be dysregulated during gastrointestinal diseases to exacerbate visceral pain. We outline the anatomical pathways involved in pain processing in the gut and how cell-cell communication is integrated into this gut-brain axis. Understanding how bidirectional communication between the gut and nervous system is altered during disease could provide new therapeutic targets for treating visceral pain.

The choroid plexus (ChP) is a vital brain barrier and source of cerebrospinal fluid (CSF). Here, we use longitudinal two-photon imaging in awake mice and single-cell transcriptomics to elucidate the mechanisms of ChP regulation of brain inflammation. We used intracerebroventricular injections of lipopolysaccharides (LPS) to model meningitis in mice and observed that neutrophils and monocytes accumulated in the 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 and revealed that ChP epithelial cells transiently specialize to nurture immune cells, coordinating their recruitment, survival, and differentiation as well as regulation of the tight junctions that control the permeability of the ChP brain barrier. Collectively, we provide a mechanistic understanding and a comprehensive roadmap of neuroinflammation at the ChP brain barrier.

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.

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.

ABSTRACT Our knowledge about the meningeal immune system has recently burgeoned, particularly our understanding of how innate and adaptive effector cells are mobilized to meet brain challenges. However, information on how meningeal immunocytes guard brain homeostasis in healthy individuals remains sparse. This study highlights the heterogeneous and polyfunctional regulatory-T (Treg) cell compartment in the meninges. A Treg subtype specialized in controlling Th1-cell responses and another known to control responses in B-cell follicles were substantial components of this compartment, foretelling that punctual Treg-cell ablation rapidly unleashed interferon-gamma production by meningeal lymphocytes, unlocked their access to the brain parenchyma, and altered meningeal B-cell profiles. Distally, the hippocampus assumed a reactive state, with morphological and transcriptional changes in multiple glial-cell types; within the dentate gyrus, neural stem cells showed exacerbated death and desisted from further differentiation, associated with inhibition of spatial-reference memory. Thus, meningeal Treg cells are a multifaceted bulwark to brain homeostasis at steady-state. One sentence summary A distinct population of regulatory T cells in the murine meninges safeguards homeostasis by keeping local interferon-γ-producing lymphocytes in check, thereby preventing their invasion of the parenchyma, activation of hippocampal glial cells, death of neural stem cells, and memory decay.