Review
The role of neutrophils in neuro-immune modulation

https://doi.org/10.1016/j.phrs.2019.104580Get rights and content

Highlights

  • Neutrophils are an immunological and diffuse source of neurotransmitters.

  • Neutrophils regulate sensory neuron functions and vice-versa.

  • Neutrophils play a crucial role in inflammatory disorders in the central nervous system.

Abstract

Neutrophils are peripheral immune cells that represent the first recruited innate immune defense against infections and tissue injury. However, these cells can also induce overzealous responses and cause tissue damage. Although the role of neutrophils activating the immune system is well established, only recently their critical implications in neuro-immune interactions are becoming more relevant. Here, we review several aspects of neutrophils in the bidirectional regulation between the nervous and immune systems. First, the role of neutrophils as a diffuse source of acetylcholine and catecholamines is controversial as well as the effects of these neurotransmitters in neutrophil’s functions. Second, neutrophils contribute for the activation and sensitization of sensory neurons, and thereby, in events of nociception and pain. In addition, nociceptor activation promotes an axon reflex triggering a local release of neural mediators and provoking neutrophil activation. Third, the recruitment of neutrophils in inflammatory responses in the nervous system suggests these immune cells as innovative targets in the treatment of central infectious, neurological and neurodegenerative disorders. Multidisciplinary studies involving immunologists and neuroscientists are required to define the role of the neurons-neutrophils communication in the pathophysiology of infectious, inflammatory, and neurological disorders.

Introduction

Neutrophils are short-lived polymorphonuclear leukocytes that are continuously generated from myeloid precursors in the bone marrow. Neutrophils are activated by bacterial and tissue damaged products, such as cytokines, damage-associated molecular patterns (DAMPs), and growth factors. These factors increase the neutrophil lifespan and ensure their migration and infiltration into the inflammatory focus through a concentration gradient of chemotactic stimulus [1]. Neutrophils are a critical component of the innate immune system essential to fight microorganisms and clear cellular debris in both septic and aseptic processes. Neutrophils can kill pathogens through different mechanisms: phagocytosis, degranulation of proteinases, and the release of reactive oxygen/nitrogen species (ROS and RNS), and neutrophil extracellular traps (NETs). ROS are products of the “cellular respiratory burst”, which is initiated by reducing oxygen to superoxide anions through the NADPH oxidase NOX2, an enzyme assembled in the phagosome membrane. From the formation of superoxide, hydrogen peroxide (H2O2) is produced and released into the phagosome space [213]. Neutrophils also release myeloperoxidase (MPO) into the phagosome by degranulated lysosomes. As a consequence, chloride ions are oxidized by H2O2 to generate hypochlorous acid (HOCl), a strong cell membrane oxidant [214]. The nitric oxide (NO) is produced by inducible NO synthase isoform (iNOS). iNOS produces high levels of NO in response to inflammatory mediators and/or to pathogen-associated molecular patterns (PAMPs). iNOS is regulated at transcriptional level and its activity is calcium-independent [2]. In addition to inflammation-induced iNOS expression, this enzyme has a constitutive expression in both murine and human resting neutrophils [3]. The iNOS-derived NO is a microbicidal and host cell-cytotoxic mediator by itself, but it can react with superoxide resulting in peroxynitrite, which is a stronger cytotoxic factor [4].

NETs were first described as a stick web of DNA conjugated with antimicrobial enzymes, such as elastase and MPO, that capture and kill bacteria in the extracellular milieu [5]. This process is not specific for bacteria and many other pathogens including fungi, parasites, and viruses, can also activate neutrophils to produce NETs. Depending of the stimulus, NETosis can occur through different pathways. For example, incubation of neutrophils with phorbol-12-myristate-13-acetate (PMA) dissociates azyrophilic granules containing elastase and MPO via the oxidative burst. These enzymes are then translocated into the nucleus, where they activate the protein-arginine deiminase 4 (PAD4), which is responsible for the deamination of arginine into citrulline. This process results in chromatin decondensation, followed by cell membrane lyse, and NETs release. This pathway is known as a ‘suicidal NETosis’, because it induces cell death [6]. By contrast, ‘vital NETosis’ does not induce cell suicide. Vital NETosis occurs in response to bacteria and fungi, and results in the release of NETs via vesicles, allowing neutrophils to still perform phagocytosis and chemotaxis [7,8]. Although NETs release may help to control infection, it can also cause organ damage. In animal models of autoimmune diseases, such as systemic lupus erythematosus, rheumatoid arthritis, and psoriasis, NETs are spontaneously induced causing tissue damage [9]. As described later in this review, NETs production has also implications in CNS disorders including multiple sclerosis (MS) [10,11], Alzheimer’ disease [12] and stroke [13,14].

Neutrophils deficiency to kill microorganisms can cause immunosuppression and increases the risk of opportunistic infections. For example, individuals with chronic granulomatous disease, a hereditary condition impairing NADPH oxidase, are more susceptible to microbial infection and sepsis [15]. However, neutrophils’ mediators are unspecific as they affect both microbial and host cells, leading to tissue and organ damage as found in auto-immune, infectious, and traumatic disorders [16]. Therefore, neutrophils are key players of the immune response being either a friend or foe for the host according to the inflammatory context.

Emerging evidences show a complex and bidirectional communication between the nervous and the immune systems [[17], [18], [19], [20], [21]]. The nervous system encompasses both central (brain and spinal cord) and the peripheral (autonomic and enteric) systems. The autonomic nervous system controls organ functions through the balance between the sympathetic and parasympathetic systems. In the sympathetic network, preganglionic neurons originated along the thoracolumbar segments of the spinal cord synapse with ganglionic neurons in the pre- or paravertebral ganglia. These ganglionic neurons release norepinephrine on peripheral tissues and activate local adrenergic receptors. In the parasympathetic network, preganglionic neurons originated in the brainstem nuclei and along the sacral spinal cord synapse with ganglionic neurons located near the target organ. These ganglionic neurons release acetylcholine that subsequently activates local cholinergic receptors. The vagus nerve is the principal nerve of the parasympathetic system and plays a pivotal role connecting the brain with the most important organs including the heart, lungs, liver, and the adrenal glands. The adrenal medulla acts as a sympathetic ganglion releasing catecholamines directly into the bloodstream and inducing a systemic effect rather than modulating specific organs. Several studies demonstrated the regulation of the immune system by autonomic nervous networks. Most of these neuro-immune interactions has been described in monocytes/macrophages and lymphocytes [[22], [23], [24]]. However, the role of neutrophils in the neuro-immune panorama in (patho)-physiological conditions is poorly understood.

Previous neuro-immune studies reported neutrophil recruitment as a response to pathological conditions, as determined by blood cytokine levels as inflammatory markers. We have used neutrophil recruitment as a biological signal of local/acute inflammation. We investigated neuromodulation of inflammation in experimental arthritis [[25], [26], [27], [28]], using neutrophil migration as the main hallmark for local inflammation. Despite the key role of neutrophils in tissue damage, few studies investigated their role in the neural circuits, probably because of their short lifespan [29,30]. The half-life of neutrophils is approximately 10−19 h in mice and humans, and treatment with adrenergic or cholinergic drugs cannot be performed for long periods of time after their isolation from the blood. Moreover, mature neutrophils are found almost exclusively in the bloodstream and in inflamed tissue, but not in secondary lymphoid organs such as the lymph nodes or the spleen. The presence of mature neutrophils in the blood represents the first line of defense and, their quick migration into the injured site is essential to fight infections [31]. In contrast to neutrophils, direct interactions between the nervous and the immune systems are mediated through neuro-immune synapses between peripheral nerves and lymphocytes/macrophages. Lymphocytes are distributed in primary (thymus and bone marrow) and secondary (spleen and lymph nodes) lymphoid organs, which are innervated by post-ganglionic sympathetic nerves that interact with resident lymphocytes through synapsis-like structures [24,32,33]. In the thymus and spleen, these sympathetic innervations are responsible for the maturation of T and B lymphocytes [34,35], respectively. On the other hand, macrophages are present in many non-lymphoid organs, where they are regulated through direct sympathetic innervations as described in the liver, and intestine [22,36]. The barrier tissues are the major sites where immune cells traffic and reside; in particular, the intestinal mucosa alone harbors more lymphocytes than all the lymphoid organs combined. Therefore, the interference of such neural inputs in tissue-resident lymphoid populations cannot be excluded. Moreover, lymphoid structures rich in lymphocytes, such as thymus, are innervated by parasympathetic vagal fibers [37]. Moreover, considering the importance of chronic low-grade inflammation as a key factor in the development of cardiovascular diseases and metabolic syndrome [[38], [39], [40], [41]], it is also essential to mention the implications of neuro-immune interface for many pathological states, such as obesity and insulin resistance, and their related diseases including hypertension, atherosclerosis, diabetes, and stroke [[42], [43], [44], [45], [46], [47]].

From a clinical perspective, the study of neuro-immune interactions is allowing the design of new therapeutic strategies for infectious and inflammatory disorders. For example, electrical stimulation of the vagus nerve activates the splenic nerve to release norepinephrine, which in turn activates splenic lymphocytes to produce acetylcholine. Acetylcholine activates the alpha7 subunit of nicotinic acetylcholine receptors (α7nAChR) on macrophages and inhibits the production of inflammatory factors [17,24]. This neural circuits (“inflammatory reflex”) inspired the design of bioelectronic devices for the treatment of autoimmune conditions such as rheumatoid arthritis and Crohn’ disease [[48], [49], [50]]. Furthermore, the vagal signals to the spleen decrease the activation of circulating neutrophils by modulating the expression of CD11b [51]. These results evidence that vagal stimulation can be exploited to modulate neutrophil recruitment in infectious and inflammatory disorders.

In this review, the first section has focused on how neutrophils contribute to the neuronal regulation of the immune system in response to the catecholaminergic/ cholinergic neurotransmitters produced by specific neuronal networks. In return, neutrophils can also produce both neurotransmitters, to feedback the neuronal network, and cytokines to modulate the immune system. These atypical neural mechanisms are behind those classical anti- and pro-inflammatory mediators already described as chemokines and cytokines (Fig. 1A). The second section will discuss the bidirectional crosstalk between neutrophils and sensory neurons and their contribution to pain, and neurogenic inflammation. Pain, one of the cardinal points of inflammation, has relevant clinical importance that, together with fever, shows some neuro-immune peculiarities (Fig. 1B). Finally, the third section of this article discusses the role of neutrophils in neurologic and neurodegenerative disorders affecting the central nervous system (CNS) (Fig. 1C).

Section snippets

Neutrophils as an immunological and diffuse source of neurotransmitters

Recent studies show that immune cells are an important non-neuronal source of neurotransmitters that allow the bidirectional crosstalk between the nervous and the immune system. When activated, neutrophils produce acetylcholine and catecholamines that can feedback the original neuronal network and also to transfer the neuronal signal to other immune cells, including neutrophils themselves. In neurons, tyrosine hydroxylase (TH) initiates the synthesis of catecholamines converting the amino acid l

Bidirectional regulation of sensory neuron-neutrophil functions

The tissue injury caused by physical trauma or infection generates a local synthesis of mediators and DAMPs by neuronal and non-neuronal cells. In an initial stage, the generation of arachidonic acid metabolites enhances the production of prostanoids, a subclass of eicosanoids (e.g., prostaglandin E2), which increase vascular tissue permeability and activate afferent neurons (Fig. 1A–C). These events are associated with the appearance of critical cardinal signs of inflammation, including

Neutrophil in the central nervous system

Early studies suggested that the CNS is an “immune privileged site” due to the impenetrable blood-brain barrier (BBB), and the lack of antigen-presenting cells and lymphatic vasculature [168]. Nonetheless, this concept has been changed over the past few years [169], and recent studies show the presence of lymphatic vessels in the brain, and alterations of the BBB during CNS disorders allows a bidirectional communication between peripheral leukocytes and CNS [170,171]. Neutrophils are immune

Conclusion and perspectives

Although neutrophils are classical innate immune cells, they are a significant non-neural source of neurotransmitters (e.g. catecholamines and acetylcholine) exerting both paracrine and autocrine, self-regulatory modulation in inflammatory conditions. From a clinical perspective, neutrophils can interact with the central and peripheral nervous systems being responsible for the genesis of central inflammatory/neurodegenerative conditions and pain, respectively. Thus, inhibition of neutrophils

Declaration of Competing Interest

None.

Acknowledgments

The research leading to these results has received funding from the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP, grants 11/20343-4, 11/19670-0, 12/04237-2, 13/08216-2, 15/25364-0, and 16/08385-7). AK is supported by National Council for Scientific and Technological Development (CNPq) and Coordination for the Improvement of Higher Education Personnel (CAPES). AB is supported by Instituto Serrapilheira/Serra (Grant 1708-15285).

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