Cross-regulation between the nervous system and type 2 immunity

Abstract

Interactions between the nervous and immune systems are critical to healthy physiology and are altered in many human diseases. Many of the major players in type 2 immune responses, including type 2 lymphocytes and cytokines, mast cells, and immunoglobulin E, have been implicated in neuronal function and behavior. Conversely, neurons in both the central and peripheral nervous systems can affect type 2 immune responses and behaviors relevant to allergy, such as food avoidance. Defining this complex circuitry and its molecular intermediates in physiology may reveal type 2 immunomodulators that can be harnessed for therapeutic benefit in neurologic diseases including Alzheimer’s disease, brain injury, and neurodevelopmental disorders. Conversely, modulation of the nervous system may be an important adjunct to treating immunologic disorders including atopic dermatitis, asthma, and food allergy. This Review covers recent work defining how the nervous system can both regulate and be regulated by type 2 immune responses.

INTRODUCTION

The nervous system encodes data in the synaptic connections between neurons that build functional neuronal circuits. In contrast, the immune system encodes information in cells that can move throughout the entire body. The innate immune system responds to patterns of tissue injury, infection, and remodeling, whereas the adaptive immune system targets an array of self and foreign antigens. Both the nervous and immune systems are shaped by experience and exposures to promote host survival and include a diversity of cellular and functional types. For many years, the complexity of neuroscience and immunology, respectively, hampered intellectual cross-talk between the disciplines, but the past few years has led to a renaissance of work at the interface of these two key organizers of health and disease.

A particular challenge in understanding the neuroimmune interface is that both the nervous and immune systems have tremendous cellular and molecular diversity. For example, immune responses can be broadly categorized into “flavors” optimized for different classes of perturbations. In this Review, we focus on how the nervous system interacts with one of these flavors—type 2 immunity—the branch of the immune system that responds to infectious parasites, allergens, irritants, and toxins. Healthy type 2 immune responses require a coordinated response across the body, recruiting necessary immune cells to the infected or injured tissue concurrent with behavioral changes that modify the risk of future exposure and further organ injury (1). Here, we review recent studies showing that molecules associated with type 2 immune responses also affect the nervous system. We also review work demonstrating that the nervous system modulates the magnitude and impact of type 2 immune responses and allergy-related behaviors (Fig. 1). In each of these settings, we discuss how type 2 immune signaling may be linked to a variety of diseases. These include brain disorders like brain injury and Alzheimer’s disease (AD), as well as allergic diseases such as atopic dermatitis, asthma, and food allergy.

Fig. 1. Interactions between the nervous system and type 2 immunity occur across multiple organs depending on the context of allergic triggers or injury.

The nervous system comprises the CNS and associated sensory nerves (blue) and the autonomic nervous system, which includes sympathetic and parasympathetic nerves (red) and the ENS (purple). The immune system is composed of cells that span the entire body. This includes the meningiocranial immune system, which comprises immune cells important for neural development, brain repair, and brain border protection. Repeated exposure to allergens at mucosal sites (for example, lung, skin, and gut) can trigger local type 2 immunity. Type 2 immune cells signal to neurons across the periphery and brain through cytokines, whereas neurons signal to immune cells via neurotransmitters and neuropeptides. This is not an exhaustive list of cellular players, given that other tissue resident cells (e.g., fibroblasts and glial cells) may also respond to these signals.

TYPE 2 IMMUNITY AND THE NERVOUS SYSTEM

Type 2 immunity is activated to contain large extracellular parasites, such as helminths (worms), protozoa, and mites, as well as allergens and toxins (1, 2). Parasites are tissue-invasive organisms, potentially accounting for the well-known roles of type 2 cells and cytokine signals in tissue remodeling and repair. These tissue-reparative functions are also integrated into mammalian biology that is beyond traditional settings of infection, including early organ development and responses to sterile injury (3, 4). Prominent upstream actors of type 2 responses are the “alarmins.” Interleukin-33 (IL-33) is one such alarmin, an IL-1 family member widely expressed in subsets of fibroblasts, epithelial cells, and endothelial cells, that is sequestered in the nucleus under steady-state conditions. Upon tissue damage or stress, IL-33 is released into the interstitial fluid; signals to cells expressing its cognate receptor IL-33R, a dimer of IL-1RL1 (also known as ST2), and IL-1RAP; and drives proliferation, activation, and cytokine production (5). IL-25 and thymic stromal lymphopoietin (TSLP) are also critical initiators of type 2 immunity, signaling via their receptors IL-17RB and TSLPR, respectively (1). IL-25 is particularly important for type 2 immunity in the gut, where it is expressed by intestinal tuft cells—specialized sensory and secretory epithelial cells—that release IL-25 after sensing of protist- and helminth-derived molecules to promote downstream type 2 immune responses (6–8). TSLP is more broadly expressed across tissues in epithelial and fibroblast cell populations and is released after exposure to allergens, pathogens, or irritants to activate a wide variety of immune cells (9). There is considerable redundancy in these alarmin signaling pathways, which can synergize during inflammation (10).

Type 2 immunity drives classic “weep and sweep” responses at barrier tissues and systemic and local adaptive type 2 memory, including CD4⁺ T helper 2 (T_H2) cell generation and immunoglobulin E (IgE) production and binding to mast cells, which together mediate quicker responses to future insults (1). IL-4, IL-5, IL-9, IL-13, and amphiregulin are key cytokine mediators of type 2 immunity. IL-13, in particular, signals to diverse tissue immune, epithelial, endothelial, and mesenchymal cells to propagate the immune response, promote tissue remodeling and mucus production, and regulate wound repair (4, 10). Type 2 innate lymphoid cells (ILC2s) are early responders to alarmins, enriched at epithelial-rich barrier tissues and structural fibroblast-dense borders across the body, including the lung, skin, intestine, and brain meninges (10). ILC2 alarmin receptor expression is associated with residence within a specific organ and topographic niche (e.g., IL-18R in skin, IL-17RB in gut, and IL-33R in border and adventitial niches across organs) but can be dynamically regulated during inflammation (11, 12). ILC2s are, therefore, poised to rapidly respond to the local release of alarmins and produce the “effector” type 2 cytokines IL-5 and IL-13. T_H2 cells respond to specific antigens via their T cell receptor (TCR) but, once generated, can take up residence in the tissues and cooperate with ILC2s, including responding directly to alarmins (1, 10). Mast cells are tissue-resident granulocytes that express IL-33R and the high-affinity IgE receptor FcεRI. Mast cells degranulate upon cross-linking of FcεRI-bound IgE molecules to cognate allergens, releasing histamine, eicosanoids, proteases, and later cytokines, such as IL-4/13, that are responsible for anaphylaxis and participate in many other allergic responses (13). Allergic diseases such as atopic dermatitis, allergic asthma, and food allergy are associated with a break in tolerance to commensal, environmental, or food antigens that drive maladaptive and chronic type 2 immune responses with associated tissue irritation, hyperreactivity, and, in severe cases, anaphylaxis (2).

The nervous system is broadly divided into the central nervous system (CNS), including the brain and spinal cord, and the peripheral nervous system (PNS). Information in the CNS is partially encoded within the synaptic connections between neurons and the ultimate circuit patterns these create. Neurons are broadly classified as excitatory, inhibitory, or neuromodulatory on the basis of the neurotransmitter that they produce: Excitatory neurons form synapses that release glutamate, whereas most brain inhibitory neurons release γ-aminobutyric acid (GABA) from synaptic and extrasynaptic sites. These synaptically connected circuits can be further tuned via neuromodulatory neurons that release diffusible neuromodulators such as serotonin, acetylcholine, norepinephrine, and dopamine. In addition to their primary neurotransmitter, neurons often express neuropeptides that modulate the activity of neurons and other cells locally, thereby adding additional functions to neurons beyond classical synaptic excitation or inhibition (14).

The PNS consists of all neurons that extend outside the brain and spinal cord. Sensory neurons transmit afferent information from peripheral tissues to the brain about touch, temperature, proprioception, pain, and itch via sensory nerves. Sensory neurons have cell bodies outside of the CNS and two primary processes: one that connects within the CNS and another that ends in a peripheral site. Sensory neuron cell bodies are in the dorsal root ganglia (DRGs) along the spinal cord or the jugular/nodose ganglia of the vagus nerve and some cranial nerves. Autonomic neurons in the PNS exist entirely outside the brain but release the same types of neuromodulators that are made by neuromodulatory neurons within the CNS. These consist of sympathetic and parasympathetic divisions. The sympathetic nervous system promotes the “fight or flight” response via local release of norepinephrine, which increases heart rate and dilates blood vessels and bronchioles. The parasympathetic nervous system, via release of acetylcholine, promotes the “rest and digest” response by reducing heart rate, constricting peripheral blood vessels, and promoting digestion of food. In addition to its afferent sensory roles, the vagus nerve is also the major parasympathetic efferent from the brainstem to the rest of the body (14). The enteric nervous system (ENS) is a self-contained nervous system that innervates the small and large intestines and acts through many of the same neurotransmitters described above. Although it can be modulated by CNS/PNS inputs, it can also function independently to regulate gut motility, secretion, and other functions (15, 16).

TYPE 2 IMMUNE REGULATION OF THE BRAIN

Cytokines are critical messengers in neuroimmune communication and coordination. Neurons express various cytokine receptors, enabling them to directly respond to local immune cues (Fig. 2). Growing evidence demonstrates that type 2 cytokines can affect early life brain physiology, CNS injury, and disease, through both direct signaling to neurons and indirect mechanisms. Type 2 cytokines also signal to peripheral neurons throughout the body and relay this inflammatory information back to the CNS. Type 2 cytokine signaling within the CNS and across the PNS present interesting similarities and differences that reveal key insights into how these neuroimmune circuits mediate homeostasis across the body.

Fig. 2. Molecular mechanisms of type 2 neuroimmune cross-talk.

ILC2s/T_H2 cells, mast cells, and neurons all communicate with each other during a type 2 immune response. IL-33, released by tissue damage, activates both ILC2s/T_H2 cells and mast cells, which both can produce IL-4/13. IL-4/13 can, in turn, signal to neurons. IL-31, which can be produced by T_H2 cells, can trigger neuronal firing and itch responses. Neurons can modulate ILC2/T_H2 responses via neurotransmitters and neuropeptides, such as NMU, norepinephrine, and CGRP. Mast cell degranulation products, such as histamine, can also modulate neuronal responses. OSMRβ, oncostatin M receptor β; RAMP1, receptor activity modifying protein 1; CALCRL, calcitonin receptor-like.

CNS development, synapses, and cognition

Type 2 cytokines can affect brain physiology and behavior by modulating synaptic formation and neural circuit maturation both in development and in adulthood. Both the upstream initiator cytokine IL-33 and effector cytokines IL-4 and IL-13 have been implicated in these contexts, although they appear to have different functions and targets: IL-33 primarily signals to myeloid cells such as microglia to promote tissue remodeling, indirectly affecting synapse numbers, whereas IL-4/13 signal directly to neurons to mediate synaptic changes.

During postnatal development, IL-33 is expressed primarily by astrocytes, and deficiency of IL-33 (Il33^−/− mice) or conditional deletion of its receptor in microglia and other myeloid cells (Cx3cr1^creERT2; Il1rl1^fl/fl) led to excess excitatory synapses in the spinal cord and thalamus, as quantified by immunohistochemistry (17). This resulted in lowered seizure thresholds as assessed via pentylenetetrazol challenge (18). A parallel study found similar synaptic effects in the cortex, as well as increased suscepti bility to kainic acid–induced seizures (19). In both cases, conditional deletion experiments (using pan–myeloid Cx3cr1^creER) suggest that IL-33 signaled directly to ST2 on microglia and myeloid cells (17–19). Mechanistically, IL-33 signaling on microglia increased phagocytosis and promoted elimination of excitatory synapses, visualized using immunohistochemistry, in a manner that partly depended on expression of the pattern recognition receptors macrophage receptor with collagenous structure (MARCO) and Toll-like receptor 2 (TLR2) (18, 19).

In the adult hippocampus, a key region for memory encoding, IL-33 was detected via a genetic reporter in neurons. In adulthood, loss of neuronal IL-33 or its receptor on microglia surprisingly led to fewer dendritic spines, a proxy for excitatory synapses (20), whereas viral IL-33 delivery increased dendritic spines, consistent with another study (21). These apparently contradictory results in development versus adulthood may reflect the role of IL-33 in promoting microglial remodeling of the extracellular matrix, which has age-dependent impacts on synapses (20). Regardless, in all of the above studies, the homeostatic role of IL-33 appears to act more like other IL-1 family members in promoting activation of myeloid cells, rather than its role in regulating downstream cytokines like IL-4/13, and thus may not fit into the classical model of type 2 allergic immunity.

In contrast with IL-33, IL-4 and IL-13 are the effector cytokines of type 2 immune responses. IL-4/13 may modulate synaptic development and cognition via a very different mechanism than IL-33: by directly signaling to neurons that express their shared receptor IL-4Rα. Earlier studies reported that T cell–depleted and Il4^−/− mice had deficits in learning and memory, which could be rescued by adoptive transfer of wild-type bone marrow or T cells (22). These deficits were later phenocopied in Il13^−/− and Il4ra^−/− mice (23, 24). However, IL-4Rα is widely expressed across brain regions and both neuronal and nonneuronal cells (25–27), raising the question of which cell types are sensing these cytokines.

Several recent studies suggest that IL-4/13 signal directly to inhibitory interneurons. Interneurons are critical to brain function and act as the “regulatory T cells of the nervous system” by shutting down activating signals, thereby enhancing the most robust signals while suppressing noise (28). Conditional deletion of IL-4Rα in inhibitory neurons (via Gad2^cre) phenocopied memory deficits (contextual fear memory) seen in T cell–deficient mice, implicating direct IL-4Rα signaling to inhibitory neurons (29). Studies of IL-4/13 signaling in the hippocampus also point toward impacts on inhibitory neuron function. Using slice electrophysiology to acutely measure synaptic firing, deletion of Il4ra from all neurons (Syn1^cre) but not excitatory neurons (Camk2a^cre) led to reduced excitatory and inhibitory synapse frequency and alterations in overall excitability patterns. Syn1^cre;Il4ra^fl/fl mice also had fewer presynaptic vesicles (by electron microscopy), demonstrating that IL-4 or IL-13 can directly influence the release of neurotransmitters from neurons that mediate memory encoding. This study also showed that neuronal loss of IL-4Rα led to deficits in anxiety-like behavior and contextual fear learning (26). Application of IL-4 or IL-13 to cultured neurons (typically a mixture of excitatory and inhibitory subtypes) acutely regulated protein phosphorylation, including neurotransmitter receptors (27, 30), and similar studies in human cultured neurons showed impacts on gene expression and synaptic firing (26). Together, these studies strongly argue for a role of IL-4/13 signaling in directly modulating neuronal function.

Importantly, some of these impacts of IL-4/13 signaling on inhibitory neurons may be particularly critical during early development of the brain, when the innate immune system is predominant, and suggest that meningeal lymphocytes are an important source of IL-13. Using sensitive lineage reporters in mice, IL-13 was detected in ILC2s, which populated the brain meninges over the first 15 days of life and produced IL-13 in an early postnatal wave [post-natal day 5 (P5) to P15]. Mice with an ILC2 deficiency (Il5^cre;R26^DTA) or loss of inhibitory neuron-specific IL-4Rα (Slc32a1^cre;Il4ra^fl/fl) had reduced inhibitory synapse numbers in early life (P15 to P30) and altered social behavior that persisted into adulthood despite a normalization of synapse numbers. These effects were not observed in mice with loss of IL-4Rα in microglial and myeloid cells (Cx3cr1^creERT2;Il4ra^fl/fl) (25).

Despite these strongly concordant data, some areas of discrepancy remain. One has to do with the cellular sources of IL-4/13. Several studies suggest that lymphocytes are an important source, including T cells in adulthood (22, 29) and ILC2s during development (25). This raises the question of how these lymphocyte-derived signals are able to cross the blood-brain barrier (BBB) to access CNS neurons. Given emerging evidence that the BBB is a dynamic structure that can be selectively permeable (31) and the meningeal membranes contain pores that allow passage of small molecules across the blood–cerebrospinal fluid (CSF) barrier (32), studies in the coming years may help to resolve this key point. Alternatively, some have reported that IL-13 may be locally produced by neurons within the adult CNS and that postsynaptic neurons express its receptor IL-13Rα1 (30), although sensitive lineage reporters did not indicate any CNS sources of IL-13, at least in development (25). Other studies have reported IL-4Rα expression at presynaptic terminals in both mice and humans, using super-resolution and electron microscopy (26). This would suggest the unexpected hypothesis that IL-13 signaling can act as a classical neurotransmitter, an idea that requires further investigation.

A more modest area of discrepancy relates to behavioral impacts: Some studies observed impacts on contextual learning (26, 29), whereas others did not (25), although behavioral outcomes can vary substantially because of differences in experimental design. Last, IL-4 may signal to microglia in certain contexts, given that it was reported that IL-4 inhibits microglial-dependent remodeling of granule cells in the cerebellum during postnatal development, resulting in hyperactive behaviors in mice that were dependent on direct IL-4 signaling in microglia (Cx3cr1^creERT2;Il4ra^fl/fl) (33). Together, these studies strongly argue that type 2 cytokines affect synapse function and behavior, raising the question of how this signaling may be altered in the setting of type 2 challenges, like allergy, injury, and parasitic infection.

Type 2 immunity in CNS injury

Type 2 immune responses modulate tissue repair and remodeling in peripheral tissues (4), and the same is true in the CNS after damage. The alarmin IL-33 is widely expressed in the CNS, particularly in astrocytes and oligodendrocytes (17, 20), and is released after spinal cord injury (SCI) (34). Global deficiency of IL-33 or its receptor ST2 (Il33^−/− or Il1rl1^−/− mice) resulted in larger lesions and functional deficits in mice after SCI, ischemic stroke via the middle cerebral artery occlusion (MCAO) model, or traumatic brain injury (TBI) via the controlled cortical impact (CCI) model (34–36). These works clearly demonstrate a role for IL-33 signaling after CNS injury; however, IL-33 is an IL-1 family member with well-described roles in direct modulation of microglia (and macrophage) function, including in development and aging.

Although IL-33 can signal directly to multiple immune cell types, ILC2s and subsets of “type 2–like” tissue regulatory T cells (T_reg cells) express particularly high levels of IL-33R and the type 2 transcription factor GATA-3, which themselves can coordinate downstream macrophage responses. Indeed, Il33^−/− mice had reduced alternatively activated macrophages, defined by Arg1 and CD206 expression, after SCI and stroke (MCAO) (34, 36), whereas Il33^−/− and Il1rl1^−/− mice had fewer T_reg cells after stroke (MCAO) or TBI (CCI) (35, 37). Adoptively transferred ST2-sufficient T_reg cells, but not ST2-deficient T_reg cells, survived in the brain and reduced astrogliosis after MCAO, suggesting that direct IL-33 signaling in T_reg cells is important for their protective function after injury (37). Meningeal and CNS-associated ILC2s also expand after SCI, stroke (MCAO), and intracerebral hemorrhage in mice (34, 38, 39). Depletion of ILCs using genetic (Rag^−/−;Il2rg^−/− mice) or antibody-mediated (anti-Thy1.2) approaches was associated with larger lesions after injury (38, 39), whereas adoptive transfer of ILC2s reduced lesion sizes (40). Conversely, treatment with exogenous IL-33 increased Arg1⁺ macrophages (41, 42) and ILC2s (38), reduced lesion sizes, and improved functional outcomes after SCI and MCAO (35, 39, 41–43). Together, these results suggest key roles for IL-33 signaling after CNS injury, acting, at least in part, via ILC2s and type 2–like T_reg cells. Roles for CNS injury–associated IL-33 signaling on additional cell types, including mast cells, other lymphocytes (e.g., natural killer cells and CD8⁺ T cells), and microglia and macrophages, are likely but remain poorly defined.

With CNS injury, type 2 lymphocytes may also exert beneficial effects via multiple mechanisms, including IL-4 and IL-13: ILC2s up-regulated IL-4 and IL-13 expression after stroke (MCAO) and SCI, respectively (39, 40). These type 2 effector cytokines have, in turn, been shown to affect CNS damage. Il4^−/− mice had increased lesion sizes and worsened neurological scores after stroke (MCAO) (44). Treatment of mice with exogenous IL-13 decreased lesion size, rescued motor deficits, and increased Arg1⁺ macrophages after stroke (MCAO) or TBI (45–48). Together, these data suggest that the IL-33/ST2 and IL-4/IL-13/IL-4Rα axes are important for limiting the extent of damage after CNS injury, and these effects are mediated by ILC2s, T_reg cells, and/or alternatively activated microglia and macrophages. Future work is required to elucidate the precise cellular targets and mechanisms through which IL-33 and IL-4/13 mediate their functional impacts on CNS damage.

AD and aging

As human life span has increased over the past century, rates of AD have also risen sharply, affecting one in nine individuals over the age of 65. Alzheimer’s is the most common neurodegenerative disease and is characterized by progressive loss of memory and cognitive ability associated with the pathological accumulation of extracellular amyloid-β (Aβ) plaques and subsequently tau fibrils within neurons (49). Of note, most mouse models used to study Alzheimer’s are pure models of amyloid deposition, because mice do not spontaneously develop tau fibrils. Nevertheless, given strong genetic evidence linking amyloid to AD (50–52) and recent clinical studies showing modest benefit of anti-amyloid therapies in humans (53), studies of how immune signals affect amyloid pathology are highly likely to be relevant to human disease.

Type 2 immunity is implicated in both Alzheimer’s and normal aging. ILC2s accumulated in the meninges and choroid plexus (CP) of aged mice (54), and T cells in the CP acquired T_H2 properties during aging, increasing their IL-4 and CCL11 expression while decreasing interferon-γ (IFN-γ) production (55). ILC2s were reduced and functionally deficient in IL-5 production in the amyloid-β precursor protein (APP)/presenilin 1 (PS1) mouse model of amyloid pathology (56). Treatment with type 2 cytokines may partly reverse amyloid pathology in mouse models: Viral overexpression of IL-4 or injection of IL-4/IL-13 into the CNS reduced gliosis and Aβ deposition in the APP/PS1 and APP23 mouse models, respectively, while improving spatial learning and memory (assessed via the Morris water maze) (57, 58). Type 2 immunity may also independently reduce age-related cognitive decline. Adoptive transfer of ILC2s or treatment with exogenous IL-33 or IL-5 rescued the cognitive function of aged mice and Alzheimer’s mouse models, possibly by inhibiting proinflammatory CD8⁺ T cells and their tumor necrosis factor–α (TNF-α) production and by promoting hippocampal neurogenesis (54, 56). These data suggest that cognitive decline associated with Alzheimer’s and aging is, at least in part, driven by a reduction in type 2 cytokine signaling, in particular IL-4 and IL-5. Treatment with these cytokines may be a therapeutic intervention to ameliorate cognition in aged individuals and those suffering from AD.

IL-33 treatment may also improve cognition and memory in amyloid models, but this again appears to act like other IL-1 family cytokines by directly promoting the phagocytic function of myeloid cells. IL-33 improved performance on a spatial memory task in APP/PS1 mice by reducing Aβ deposition and promoting its phagocytic clearance by microglia (59, 60). Levels of soluble ST2 (sST2), a splice variant and decoy receptor of ST2 that inhibits IL-33 signaling, are also correlated with AD severity: Individuals who have the ApoE4 genotype and bear a genetic variant of an IL1RL1 enhancer that reduces sST2 levels have a reduced AD risk, which was corroborated by mouse studies showing that sST2 levels modulate microglial phagocytosis and clearance of Aβ (61). Additional work is required to understand the mechanisms by which type 2 cytokines are reduced in aging and AD, the sources and targets of these signals, and the mechanism by which they promote learning and memory retention.

Type 2 cytokine signaling in the periphery

Some of the biggest strides in peripheral neuroimmunology research have been in the skin, where signaling from type 2 cytokines and upstream alarmins have been shown to regulate itch across various dermatitis models. For example, the pruritogenic cytokine IL-31, an upstream regulator of allergic phenotypes, has long been linked to itch associated with atopic dermatitis (62). T_H2 cells are one immune source of IL-31, with overexpression of Il31 in T cells being sufficient to induce spontaneous itch and dermatitis in mice (63). A subset of DRG neurons controls this itching behavior: IL-31 drives ERK1/2 phosphorylation and firing of IL-31R⁺ sensory neurons that innervate the skin. These sensory neurons express the classic channels transient receptor potential cation channel subfamily V member 1 (TRPV1) and subfamily A member 1 (TRPA1), and consequently, Trpv1^−/− or Trpa1^−/− mice have significantly reduced IL-31–mediated itch (64). Interestingly, despite reduced scratching, Il31^−/− mice actually had higher numbers of IL-4/IL-13–producing T_H2 cells, skin IL-4Rα⁺ macrophages, and serum IgE during house dust mite (HDM)–induced allergic dermatitis. Instead, selective Il31ra ablation in sensory neurons (Avil^cre;Il31ra^fl/fl) increased T_H2 abundance upon HDM exposure (65). These findings collectively suggest that IL-31 acts on skin sensory neurons not only to drive itch but also to limit excessive type 2 inflammation during chronic dermatitis, although it remains unclear whether this is a direct mechanism or an indirect effect through itch and scratch.

Older studies suggest that IL-33 and TSLP contribute to various models of itch and pain, although it is unclear whether these alarmins directly signal to neurons or indirectly via immune cells (66–68). During allergic conjunctivitis, IL-33 recruited ST2⁺ T_H2 cells to the eye. Immunohistochemical staining revealed axonal elongation of neurons in the conjunctiva, a phenotype that has been associated with injury responses and can contribute to chronic hypersensitivity. Adoptive transfers of ovalbumin (OVA)–specific Il1rl1^−/− T_H2 cells resulted in fewer scratching bouts and less axonal elongation upon OVA treatment, suggesting that these T cells exacerbated itch via regulation of peripheral nerves in the tissue (69). The role of IL-33 in immune infiltration also occurs within the PNS itself. In one study exploring the role of TLR2 in pain, treatment of mice with the TLR2 agonist FSL-1 led to IL-33 expression in DRG satellite glial cells, as observed via immunofluorescence, resulting in increased macrophage infiltration. Interestingly, intraplantar FSL-1 injection also triggered IL-33 expression in the DRG. This study suggests that tissue inflammation could also activate immune responses along the cell bodies of neurons that innervate peripheral organs (70).

The effector type 2 cytokines IL-4 and IL-13 may directly increase sensory neuron firing during allergic and parasitic responses in the skin. In murine models of atopic dermatitis, Nav1.8⁺ sensory nociceptors in the DRG expressed type 2 cytokine receptors, such as IL-4Rα and IL-13Rα1, and administration of IL-4 increased Janus kinase 1 (JAK1) phosphorylation and neuronal firing, as measured by ex vivo calcium imaging (71). Similarly, vagal nociceptors treated ex vivo with IL-13 displayed increased signal transducer and activator of transcription 6 (STAT6) phosphorylation and transcript expression of Npy1r, which encodes a G_i-coupled neuropeptide receptor (72). Patients with JAK1 gain-of-function variants develop atopic dermatitis, yet they can also display systemic allergic phenotypes including asthma and eosinophilia (73), demonstrating that IL-4 signaling to sensory neurons could be relevant in other organs.

In the gut, where enteric infections can drive neuronal death, type 2 cytokines aid in neuroprotection via signaling to macrophages. During infection with the intestinal nematode Strongyloides venezuelensis, eosinophil-derived IL-4 and IL-13 activated Arg1⁺ muscularis macrophages. Mice lacking macrophages (via anti-CSF1R–mediated broad myeloid depletion) or mice lacking myeloid Arg1 expression (Lyz2^Cre; Arg1^fl/fl) had fewer enteric neurons after infection. This suggests that IL-4/13–activated Arg1⁺ macrophages may limit postinfection neuronal death long term, given that mice were protected from neuronal loss during subsequent infections (74). It remains unclear whether enteric neurons may also directly respond to IL-4/IL-13 signaling, as in the case of skin sensory neurons.

It is possible that neurons innervating different tissues also differentially respond to type 2 cytokines. For example, unlike in the skin, allergic inflammation in the lung may be dependent on JAK1 signaling downstream of other cytokines besides IL-4, as Scn10a^cre;Il4ra^fl/fl mice displayed no differences compared to their wild-type counterparts during Alternaria alternata–induced allergic inflammation. Neuron-intrinsic JAK1 signaling is nevertheless required to regulate the lung allergic response, given that Scn10a^cre;Jak1^fl/fl mice experienced more severe inflammation during papain-induced allergy (75). This finding suggests that a different signaling pathway is involved in activating neuronal JAK1 during allergic inflammation in the lung. Other JAK1-dependent cytokines, including IL-10 and IFN-γ, are able to trigger sensory neurons in other inflammatory contexts (76). Indeed, there is a large body of literature describing how IFN-γ, IL-17, and IL-10, among other cytokines, can shape peripheral neurons, with recent reviews on this topic (77).

These seminal studies in cytokine-mediated neuroimmune cross-talk focused on cellular interactions within local tissue niches. However, akin to how tissue immune cells bring antigenic signals to lymph nodes, peripheral neurons ultimately relay information to the body’s central control system, the brain. Therefore, a new frontier in neuroimmunology lies in uncovering how immune signals in the periphery are conveyed to the brain.

NEURONAL AND IMMUNE REGULATION OF PHYSIOLOGY AND BEHAVIOR

Interoception is the ability of the brain to sense the body’s autonomic functions, such as heart rate, respiration, and satiety. Research within the past 5 years suggests that the brain can also monitor and control the immune system, a concept termed “immunoception” (78–80).

Recent studies have begun to define the brain circuitry that mediates sensing of peripheral immune status, although none of these have focused on type 2 responses. For example, during dextran sulfate sodium (DSS) colitis, neurons in the insular cortex (also termed “interoceptive” cortex) were activated, as determined by lineage “trapping” neurons that expressed the immediate early gene Fos (via FosCreERT2). Chemogenetic reactivation of these trapped neurons after resolution of inflammation was sufficient to induce IL-6⁺ CD4⁺ T cells, TNF-α⁺ monocytes, and activated γδ T cells, partially recapitulating the inflammatory state in the colon that was observed initially during DSS treatment (81). Another recent example highlights the parabrachial nucleus as a region activated in response to systemic IL-1β to drive corticosteroid release, and reactivation of parabrachial neurons was sufficient to induce corticosteroid-mediated immune suppression (82). Afferent sensory neurons in the DRG/spinal cord and vagus nerve deliver sensory information to the brain, including information about inflammation: Vagal and DRG neurons can respond to TNF-α and IL-1β (83). Various studies using murine models of peripheral viral and bacterial infections demonstrated how proinflammatory cytokines, including IL-1β, TNF-α, and interferons, trigger hypothalamic activation to induce sickness behavior, which in mice is characterized by reduced motor activity, appetite, and social interaction (84–88). In a recent example, chemogenetic activation of TRPA1⁺ vagal neurons before immune challenge, such as lethal lipopolysaccharide (LPS) injection or intestinal Salmonella infection, instead dampened inflammation and increased bacterial dissemination, respectively. Such phenotypes may be potentially mediated by the heterogeneous response of vagal neurons to cytokines (89). A detailed review on this topic can be found elsewhere (77).

Unlike sickness behavior, type 2 immune responses may regulate a uniquely allergic response: avoidance of allergens and toxins. Two recent studies showed that exposure to food allergens activates IL-4 secretion and mast cells decorated with antigen-specific IgE in the gut. In parallel, food allergens induced activation of neurons in the brainstem and the amygdala, a brain region important for emotion processing, as measured by expression of Fos. Mast cell–deficient (Cpa3^cre) mice and chimeric mice reconstituted with hematopoietic cells unable to bind IgE (Fcer1a^−/−) had decreased long-term avoidance behavior against the food allergen, suggesting that this behavioral response is, in part, mediated by mast cell/IgE immune responses (90, 91). Nevertheless, it remains unclear how mast cells and IgE are affecting behavior, given that neuronal activation is a broad response that can be induced both directly and indirectly. Deletion of TRPV1⁺ sensory neurons via resiniferatoxin treatment or vagotomy did not rescue this response, suggesting either direct sensing by CNS neurons or a peripheral neuron subset that was not captured in these experiments (91).

On the other hand, brainstem neurons can receive sensory input about respiratory allergens to control physiological and immune responses to allergy (92). For example, vagotomized, Scn10a^cre;R26^DTA, or Trpv1-DTR mice had dampened brainstem activity, allergic inflammation, and airway hyperreactivity across various murine respiratory allergy models (92–94). Specifically, one study used single-nuclear RNA sequencing to identify a subset of norepinephrine-producing neurons, marked by Dbh expression, in the brainstem that were activated after HDM challenge. Viral tracing studies showed that Dbh⁺ brainstem neurons synapsed onto parasympathetic ganglia that innervate the trachea. Depletion of those brainstem neurons via targeted diphtheria toxin A (DTA)–mediated ablation (using Dbh^cre;R26^DTR mice) dampened airway hyperreactivity after HDM challenge (92).

A different brainstem region, the lateral parabrachial nucleus, also mediates a key feature of allergic anaphylaxis in mice: hypothermia. In a model of OVA-induced IgE-mediated anaphylaxis, a study showed that mast cell–derived chymase (a protease) activated TRPV1⁺ DRG neurons, as measured by calcium imaging. Activation of these sensory neurons correlated with increased Fos expression of neurons in the lateral parabrachial nucleus of the brainstem, a region associated with thermoregulation, and subsequently reduced heat generation in brown adipose tissue resulting in hypothermia (95).

Collectively, these studies show that systemic allergic immune responses modulate neural activity in the brain and allergy-related behavioral and physiological responses, partly mediated by information transmitted from peripheral organs to the brain via the vagus nerve. However, it remains unclear why different allergic contexts induce different behavioral and physiological responses. Therefore, defining which cytokines are relevant in activating vagal and DRG neurons across allergic contexts and how these affect neural circuits in the brain will be important areas for future study.

PERIPHERAL NEURON REGULATION OF TISSUE TYPE 2 IMMUNE RESPONSES

Peripheral neurons can regulate type 2 immunity via secretion of neurotransmitters or neuropeptides. Neuropeptide expression is more complex and longer lasting compared with stereotyped neurotransmitters expressed by neuronal subtypes, raising the possibility that a “neuropeptide code” for immunoregulation could exist (Fig. 3). Multiple recent studies have demonstrated that type 2 immune cells express receptors for both neurotransmitters and neuropeptides and respond to these signals (Fig. 2).

Fig. 3. A neuropeptide code for type 2 immune regulation.

Neuropeptides released from neurons regulate the activity of type 2 immune cells. NMU and SP amplify type 2 immunity, whereas GGRP and NMB inhibit type 2 immune activation and may conversely promote type 1 (or other) immunity. In certain contexts, type 2 immune cells may also produce certain neuropeptides (e.g., CGRP) and regulate themselves in an autocrine manner. The identity of the neurons producing these peptides, whether neuropeptide coexpression occurs, and how the upstream production and release of these peptides are regulated across tissues and anatomical niches remain poorly defined. How this neuropeptide code interfaces with cytokines to tweak the dial on type 2 immunity also remains unclear.

Neurotransmitters: Amplifying immune responses

The sympathetic nervous system mediates fight or flight responses via release of catecholamines (dopamine, norepinephrine, and epinephrine), and these same neurotransmitters may broadly suppress type 2 immunity. In contrast, rest and digest parasympathetic activation (via acetylcholine) may promote type 2 reparative and allergic immune responses.

Several studies show that sympathetic activation suppresses ILC2 function. For example, in the lung, dopamine suppressed ILC2 responses by dampening cellular oxidative phosphorylation, and deletion of lung-innervating dopaminergic neurons via intranasal administration of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahy dropyridine augmented allergic lung inflammation (96). Similarly, genetic ablation of tyrosine hydroxylase (TH)–expressing sympathetic neurons by deleting their neurotrophin receptor (Th^cre;TrkA^fl/fl) led to IL-33–mediated type 2 innate responses after LPS challenge (97). Norepinephrine-secreting TH⁺ neurons also innervate adipose tissues, and viral tracing demonstrated that these neurons synapse to the hypothalamic paraventricular nucleus. In this setting, adrenergic neurons were shown by immunohistochemistry to be in close proximity to β2-adrenergic receptor (β2AR)–expressing mesenchymal cells. In vitro studies suggested that β2AR agonists drove the secretion of glial-derived neurotrophic factor in these mesenchymal cells, which subsequently triggered ILC2s to release IL-5 and IL-13 (98). ILC2s themselves express β2AR in the gut, where they colocalize with adrenergic neurons. β2AR agonists in vivo inhibit ILC2 proliferation and effector function, and mice lacking adrenergic signaling in all lymphocytes (Il7r^cre;Adrb2^fl/fl) had enhanced type 2 inflammation in both gut and lung (99). Other innate immune cells can express β2AR: For example, in the gut, β2AR⁺ muscularis macrophages protect enteric neurons from bacterial infection–induced death (74, 100). Overall, these studies suggest that catecholamines across various tissues dampen type 2 inflammation.

In contrast, acetylcholine, the major neurotransmitter of the parasympathetic nervous system, amplifies type 2 immune responses. Interestingly, in several settings of type 2 activation, immune cells themselves may contribute to acetylcholine production. Many immune cells express choline acetyltransferase (ChAT), the enzyme catalyzing the rate-limiting step of acetylcholine production. During helminth infection, ILC2s in the lung, gut, and mesenteric lymph nodes produced acetylcholine upon activation by IL-25 and IL-33 (101, 102). ILC2s also expressed the acetylcholine receptor, creating an autocrine signaling loop (102). Accordingly, Rora^cre;Chat^fl/fl mice have impaired ILC2 proliferation and activation, resulting in fewer tuft cells, decreased mucin expression, and impaired helminth expulsion (101). Both CD4⁺ and CD8⁺ T cells can not only express ChAT and secrete acetylcholine (103) but also respond to acetylcholine released by neurons. During acute pain, glutamatergic neurons from the somatosensory cortex synapse onto parasympathetic cholinergic neurons in the vagus nerve that innervate the spleen, which can subsequently increase splenic T_H2 cells via acetylcholine release. Chronic pain has the opposite effect: GABAergic neurons of the amygdala, identified via a combination of retrograde viral tracing and GABA immunolabeling, inhibit this same circuit, decreasing splenic T_H2 cells (104). These findings indicate that acetylcholine plays a critical role in amplifying type 2 immunity in both an autocrine and neuroimmune paracrine manner.

A neuropeptide code for immune cell modulation

Calcitonin gene-related peptide (CGRP; encoded by Calca), a potent vasodilator and modulator of the autonomic nervous system, generally dampens type 2 immunity, although the exact signaling mechanisms may be more complex and context dependent. As with other neurotransmitters in the PNS, CGRP can be made by multiple cell types, including neurons; immune cells; and pulmonary neuroendocrine cells (PNECs), a rare epithelial cell type residing at airway branch points in the lung (69, 105, 106). In response to IL-33 or helminth infection in the lung, ILC2-derived CGRP acted in an autocrine fashion, dampening ILC2 activation, independent of adaptive immunity (105, 107). Neural sources of CGRP can act on ILC2s as well. For example, lung sensory neurons secreted CGRP upon JAK1 activation, resulting in ILC2 suppression and reduced allergic airway inflammation (75). In contrast, one study reported that CGRP stimulates IL-5 expression in ILC2s in vitro and demonstrated, via imaging and genetic depletion models, that PNECs release CGRP to amplify allergic asthma responses (106). The spatiotemporal dynamics of PNEC-ILC2 cross-talk along airways may regulate this amplification loop. In other barrier organs such as skin and gut, CGRP from sensory nociceptors, enteric neurons, ILC2s, and T_H2 cells all polarize macrophages toward repair phenotypes, drive mucus production by goblet cells, dampen the immune response against food allergens, reduce type 2 immune recruitment, and attenuate scratching behavior (69, 108–110).

Whereas CGRP tends to dampen type 2 immunity, neuromedin U (NMU) activates ILC2s, tissue T_H2 cells, and eosinophils, which specifically express the NMU receptor (NMUR1) (111, 112). NMU was shown to activate eosinophils to promote goblet cell differentiation, given that eosinophil-specific deletion of NMUR1 (Epx^cre;Nmur1^fl/fl) had fewer small intestinal goblet cells and eosinophils, as well as less eosinophil degranulation as observed by electron microscopy (111, 112). Neuron-derived NMU increased during intestinal helminth and protist infection, thereby promoting amphiregulin expression in neighboring IL-25–stimulated ILC2s, which further drove eosinophilia and was critical for worm expulsion (111, 113–115). Accordingly, NMUR1 deficiency in eosinophils ultimately resulted in delayed clearance of the intestinal helminth Nippostrongylus brasiliensis (111). In the lung, patients with asthma displayed increased sputum NMUR1⁺ ILC2s, suggesting that a similar mechanism in the gut may be at play (116). On the other hand, neuromedin B (NMB), a neuropeptide of the same neuromedin family, instead limits the type 2 response during helminth infection. Basophils promoted NMB receptor expression in ILC2s, resulting in ILC2 suppression upon NMB exposure in the lung (117). This finding implies that different neuromedin peptides may be working in conjunction to fine-tune type 2 inflammation at various stages of the immune response. Defining which neurons secrete neuromedins may help to elucidate this highly type 2–specific component of the neuropeptide code.

Other neuropeptides have been shown to modulate the immune response between type 2, type 17, and immunoregulatory phenotypes. For example, substance P (SP), typically secreted by nociceptors, drove IgE secretion by B cells, amplified the airway T_H2 response, and triggered mast cell degranulation and dendritic cell migration across lung and skin models of allergy and pain, respectively (118–122). On the other hand, an ultraviolet-mediated model of skin damage triggered nociceptor secretion of TAFA4, which instead promoted IL-10 production in dermis-resident macrophages (123, 124). Vasoactive intestinal peptide (VIP), secreted by enteric neurons during feeding, activated neighboring ILC2s as well as ILC3s. Cosignaling with IL-33 or IL-22 further skewed ILC activation toward IL-5–secreting ILC2s versus IL-22–producing ILC3s, respectively (125). This observation highlights the fact that many of these neuropeptides work synergistically with local cytokine signaling to amplify the appropriate immune responses during an immune challenge.

Despite recent advances in identifying the neural chemical messengers involved in shaping the immune system, some major outstanding questions remain (Fig. 3). Which neuronal subsets are producing the relevant neuropeptides? How is neuropeptide production spatially and temporally regulated across tissue niches? If the same sensory neuron expresses two different neuropeptides, then what upstream signals selectively induce the production and release of each? Many diseases display mixed inflammatory phenotypes that include multiple arms of lymphocytic immunity. How do these mixed immune signals shape the neuropeptide expression pattern of neurons during allergic contexts? Lastly, how does the neuropeptide code interface with cytokine signaling within tissue niches across different allergic contexts to coordinate the immune response over the course of an allergy response? Collectively, unveiling these mechanisms could unlock new therapeutic opportunities for the treatment of autoimmune and neurodegenerative diseases.

CONCLUSIONS

Suppression of type 2 immune responses shows therapeutic benefit for the treatment of allergic diseases, which may act, in part, via suppressing cytokine signaling in neurons. Targeting of the IL-4/JAK1 signaling axis observed in skin-innervating neurons during chronic itch is a prime success story of neuroimmune therapy. JAK inhibition is now used to treat human patients with chronic idiopathic pruritus, atopic dermatitis, psoriasis, and other itching diseases and reduce their itching behavior (71, 126). The biologic dupilumab, a humanized anti–IL-4Rα antibody, targets the IL-4/IL-13/IL-4Rα axis and may also reduce IL-4/13 signaling in neurons and itch responses. Dupilumab has been approved by the US Food and Drug Administration to treat atopic dermatitis and asthma and is in clinical trials to treat chronic rhinosinusitis with nasal polyps, eosinophilic esophagitis, and food allergies (127). In mice, IL-4Rα signaling affects neurodevelopment (25, 33), raising the possibility of whether dupilumab or other IL-4/13 blocking agents could have similar effects on human neurodevelopment, which warrants further research. By targeting neuroimmune axes that go awry during chronic inflammation, we may be able to ameliorate the symptoms and pathology of many type 2–mediated diseases in humans.

Although still in preclinical phases, it is also possible that, in settings where type 2 cytokines are beneficial, treatments that augment type 2 activation could be a promising therapeutic avenue. Treatment with IL-33 promotes the activity of anti-inflammatory ST2⁺ T_reg cells, as well as the phagocytic function of microglia and other myeloid cells, which have both been shown to be beneficial in mouse models of brain injury and AD (35–37, 41, 59, 60). Treatment with IL-4/13 has also been shown to ameliorate outcomes after models of CNS damage and AD in mice, potentially via inhibition of pathologic cytotoxic responses or augmentation of alternatively activated myeloid cells (46–48, 56). IL-5 treatment has also shown promise for improving cognition of aged mice (54). Local overexpression of these cytokines within diseased tissues (possibly via adeno-associated viruses or local injections) remains a promising potential therapeutic route for the treatment of many CNS injuries and disease states. Moreover, the treatment with neuropeptides (such as NMU) alone or in combination with type 2 cytokines may provide additional specificity to therapeutic responses, although impacts on other branches of immunity (e.g., type 1 and type 3/17) should be considered.

Although substantial advances have been made at this type 2 neuroimmune interface, several critical open questions remain. Given that type 2 cytokines can affect synapse maturation and behavior, does exposure to allergens, parasites, and toxins, particularly during critical postnatal development periods, affect brain development and function? If the nervous system is truly necessary for behavioral responses to type 2 challenges, then can we modulate the nervous system to prophylactically promote avoidance to allergies or parasitic infections? In addition, the precise anatomical and biochemical mechanisms underlying cytokine signaling in neurons remain unclear. Defining these mechanisms will require deciphering how lymphocyte-derived cytokines gain access to the highly protected brain parenchyma, especially under homeostasis. Although recent work has identified small pores in the arachnoid membrane (termed arachnoid cuff exits) that could allow trafficking of cytokines or cells from the dural meninges or periphery into the brain (32), it is still unclear how specific neuronal subsets in specific regions are exposed to these cytokines. Finally, neuronal populations across the CNS and PNS respond differently to cytokines. Characterizing the cytokine map of the nervous system and functionally assessing how cytokines alter neuronal firing, as well as the function of supporting cells like astrocytes, oligodendrocytes, microglia, fibroblasts, and endothelial cells, may also be key to better understanding the mechanisms regulating cytokine signaling to neurons.