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Peripheral nerve stimulation and immunity: the
expanding opportunities for providing mechanistic
insight and therapeutic intervention
AidanFalvey1, ChristineN.Metz1,2, KevinJ.Tracey1,2 andValentinA.Pavlov1,2
1The Feinstein Institutes for Medical Research, Northwell Health, Manhasset, NY 11030, USA
2Donald and Barbara Zucker School of Medicine at Hofstra/Northwell, Hempstead, NY 11549, USA
Correspondence to: A.Falvey; E-mail: afalvey@northwell.edu or V.A. Pavlov; E-mail: vpavlov@northwell.edu
Received 13 June 2021, editorial decision 7 September 2021; accepted 7 September 2021
Abstract
Pre-clinical research advances our understanding of the vagus nerve-mediated regulation of
immunity and clinical trials successfully utilize electrical vagus nerve stimulation in the treatment of
patients with inammatory disorders. This symbiotic relationship between pre-clinical and clinical
research exploring the vagus nerve-based ‘inammatory reex’ has substantially contributed to
establishing the eld of bioelectronic medicine. Recent studies identify a crosstalk between the
vagus nerve and other neural circuitries in controlling inammation and delineate new neural
immunoregulatory pathways. Here we outline current mechanistic insights into the role of vagal
and non-vagal neural pathways in neuro-immune communication and inammatory regulation. We
also provide a timely overview of expanding opportunities for bioelectronic neuromodulation in the
treatment of various inammatory disorders.
Keywords: bioelectronic medicine, carotid sinus nerve, inflammation, inflammatory disorders, vagus nerve
Introduction
Recent advances in our understanding of the regulatory
scopes of the nervous system and the immune system have
revealed that these two major control and defense systems,
once considered to be autonomous, interact and commu-
nicate with each other (1, 2). In this dialogue, the nervous
system is an important partner of the immune system in the
regulation of inflammation. Although inflammation is a vital
protective process against pathogen invasion and tissue in-
jury, various forms of excessive and unresolved inflammation
mediate disease pathogenesis. It is known that there is exten-
sive neural regulation of immune responses that are triggered
by cytokines and other inflammatory molecules interacting
with peripheral neurons, which results in subsequent activa-
tion of brain-orchestrated neuroimmunomodulatory mechan-
isms (1, 3). Aprominent example is the ‘inflammatory reflex’,
in which afferent and efferent vagus nerve signaling provide
a physiological, brain-integrated reflex mechanism that sup-
presses excessive cytokine release and inflammation (2, 4).
Substantial mechanistic insights into the inflammatory reflex
have highlighted the efficacy of several anti-inflammatory ap-
proaches in various inflammatory conditions (5–7).
An effective means of stimulating vagus nerve activity and
initiating the inflammatory reflex is by wrapping or attaching
an extra-neural electrode on the cervical part of the nerve and
supplying it with an external current (8). This approach was in-
strumental for the discovery of the anti-inflammatory efficacy
of vagus efferent cholinergic signaling (9). This discovery was
followed by investigations demonstrating the efficacy of elec-
trical vagus nerve stimulation (VNS) in many animal disease
models and led to the initiation and completion of several re-
cent successful bioelectronic VNS-based clinical trials for pa-
tients with chronic inflammatory diseases (3, 10–13). These
advances substantially contributed to establishing the field of
bioelectronic medicine, launching further studies to catalog a
myriad of peripheral neural circuitries involved in the regula-
tion of inflammation (4, 14).
Here, we outline important aspects of neuro-immune com-
munication and the role of the vagus nerve and other neural
circuitries in the regulation of immune function. We provide
a timely discussion on physiological and molecular insights
into this regulation and summarize the anti-inflammatory utility
of stimulating the vagus nerve and other peripheral neural
circuitries.
The immune system and the regulation of inammation
The immune system is a uniquely diffuse system present
throughout the entire body in dedicated tissues/organs,
International Immunology, Vol. 34, No. 2, pp. 107–118
https://doi.org/10.1093/intimm/dxab068
Advance Access publication 8 September 2021
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including lymph nodes, lymphatic vasculature and the
spleen, as well as resident and circulatory components in
every major organ. This distribution is crucial as the immune
system is adequately positioned to detect, respond, resolve
and remember threats that may damage thebody.
Innate immune defense mechanisms are mediated through
pattern-recognition receptors (PRRs), including toll-like re-
ceptors (TLRs) in and on macrophages and other myeloid
cells recognizing pathogen-associated molecular patterns
(PAMPs), for example, lipopolysaccharide (LPS). Activation
of specific intracellular pathways downstream of TLRs that in-
volve nuclear factor-kappa light chain enhancer of activated
B cells (NF-κB), activator protein 1 (AP1) and other transcrip-
tion factors results in the production of cytokines and the gen-
eration of protective pro-inflammatory signaling (15). Tumor
necrosis factor (TNF), interleukin 6 (IL-6) and IL-1β among
others are prototypical pro-inflammatory cytokines, whereas
IL-6 has been linked to both pro- and anti-inflammatory ef-
fects (16). These cytokines also play important roles in re-
cruiting immune cells, including neutrophils and dendritic
cells (DCs), to the sites of injury or infection and promote
their subsequent activation. DCs link the innate and adap-
tive immune systems and recruit cellular components of the
adaptive immune system, including T cells and B cells, to
potentiate the immune response. The pro-inflammatory re-
sponse is finely tuned and balanced by the release of IL-10,
soluble cytokine-receptors and other anti-inflammatory mol-
ecules (3, 17).
In a typical physiological scenario, inflammation is resolved
in a timely manner through an active and complex multi-stage
process involving both cellular and humoral regulators (18).
A series of lipid-based pro-resolving molecules collectively
known as specialized pro-resolving mediators (SPMs) are crit-
ical molecules implicated in the resolution of inflammation (19).
Among other functions, these SPMs have been shown to inter-
fere with neutrophil infiltration into the inflamed tissue, as well
as promoting macrophages to phagocytose apoptotic bodies
(18). Local and timely resolution of inflammation is protective
and advantageous. However, when this beneficial scenario is
not followed various forms of unresolved, chronic and systemic
inflammation drive the pathogenesis of a wide range of inflam-
matory and autoimmune disorders (1, 19). Therefore, control-
ling inflammation is vital and there is a growing appreciation
that in addition to immune mechanisms, neural signaling has a
key role in inflammatory regulation (1, 3).
The nervous system and the neuro-immune
partnership in the regulation of inammation
The nervous system is composed of the central nervous
system (CNS, the brain and the spinal cord) and the periph-
eral nervous system. The peripheral nervous system can be
further divided into the somatic nervous system associated
with voluntary control of muscles and the autonomic nervous
system (ANS), which regulates non-voluntary body functions,
such as heart rate and blood flow, digestion and breathing.
The ANS is subdivided into sympathetic and parasympa-
thetic divisions. In addition to these two major ANS divisions,
the enteric nervous system, mainly associated with gastro-
intestinal regulation, is classified as the third component (20).
Sympathetic neurons residing in the thoracic and
lumbar parts of the spinal cord project to paravertebral or
prevertebral ganglia, where through the release of acetyl-
choline (Ach) they interact with postganglionic neurons (21,
22). These postganglionic neurons provide innervations of
many visceral organs and tissues, including spleen, lungs
and airways, gastrointestinal tract, liver and kidneys, blood
vessels, lymphoid tissue and organs, bone marrow and
joints, and release mainly norepinephrine (NE), but also other
catecholamines and neuropeptide Y (21). Preganglionic
sympathetic neurons also innervate the adrenal medulla
and control the release of epinephrine and small quantities
of dopamine and NE that are released in the circulation
and act as hormones (22, 23). The brain and specifically
locus coeruleus and the rostroventrolateral medulla regulate
the sympathetic neuronal activity by interacting with spinal
cord preganglionic neurons (21, 22, 24). Catecholamine
interaction with metabotropic, G-protein-coupled beta- and
alpha-adrenergic receptors (β-ARs and α-ARs, which are
further subdivided) mediates sympathetic regulation of car-
diovascular, pulmonary and gastrointestinal function, hem-
atopoiesis and metabolism.
The vagus nerve is the main parasympathetic nerve and
the largest cranial nerve. The vagus nerve is a complex nerve
that contains efferent (motor, about 15%) and afferent (sen-
sory, about 85%) neurons (23, 25). Efferent vagus neurons
reside in the brainstem dorsal motor nucleus of the vagus
(DMN) and nucleus ambiguus (NA). They provide long
axonal projections to several visceral organs, including the
heart, lungs, liver, pancreas and gastrointestinal tract where,
through the release of ACh, they interact with short postgan-
glionic cholinergic neurons (26). Afferent vagus neurons res-
iding in the nodose ganglia are pseudounipolar cells with
peripheral axonal endings in visceral organs, including the
heart, lungs, pancreas and gastrointestinal tract, and central
axonal projections to the brainstem nucleus tractus solitarius
(NTS). Vagus nerve afferent and efferent signaling provides
a major communication channel for brain-integrated, neural
reflex regulation of physiological functions, including heart
rate, respiration, hepatic gluconeogenesis, pancreatic se-
cretion and gastrointestinal motility and secretion (27). This
regulation is mediated through the release of ACh acting on
metabotropic G-protein-coupled muscarinic receptors on
smooth muscle cells, glandular cells and cardiac myocytes
in the innervatedorgans.
The ANS has also been implicated in immune regulation.
The neuro-immune communication underlying this regulation
is facilitated by the expression of receptors for neurotransmit-
ters on macrophages, DCs and other immune cells, including
several types of adrenergic receptors and muscarinic and
nicotinic acetylcholine receptors (28, 29). The sympathetic
division of the ANS has a documented role in the regulation of
immune function and inflammation—a topic that has been de-
scribed in detail elsewhere (1, 21, 25). Catecholamines modu-
late immune function in a context-dependent manner and
exert anti- or pro-inflammatory effects (21, 30). For instance,
activation of β2-ARs on macrophages and other immune cells
is associated with increased production of anti-inflammatory
cytokines and decreased production of TNF and other pro-
inflammatory cytokines, whereas activation of α-ARs results
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in increased synthesis of TNF and other pro-inflammatory
cytokines (1, 25, 30).
Recent advances in our understanding of the neural regu-
lation of immunity and inflammation have pointed to the key
role of the vagus nerve (9). Cytokines, including IL-1β and
TNF and other inflammatory molecules, interact with vagal
paraganglia (31–33) or with afferent vagal neurons (34, 35)
and activate neural signaling to the NTS (Fig. 1). These inter-
actions are mediated by the expression of cytokine recep-
tors such as the IL-1 receptor (IL-1R) and TLRs on sensory
vagal neurons (Fig. 1). The signals are further communicated
to the DMN and trigger activation of efferent vagus nerve
cholinergic output that suppresses pro-inflammatory cytokine
release and inflammation (36). This brainstem-integrated af-
ferent and efferent signaling provides the anatomical basis
of the inflammatory reflex that controls inflammation (2). In
addition to several abdominal organs, efferent (motor) vagal
neurons innervate the celiac-superior mesenteric ganglion
complex, where the splenic nerve originates (37–41) (Fig. 1).
This complex is composed of the left and right celiac gan-
glion and the superior mesenteric ganglion.
There is growing evidence for anatomical and functional
interactions between the efferent vagus nerve and the
splenic catecholaminergic nerve in the celiac-superior mes-
enteric ganglion complex in the inflammatory reflex (36, 39,
40, 42). VNS results in the release of NE from splenic neur-
onal terminals in the spleen (43). This NE interacts with β2-
AR which are expressed on a subset of T cells that contain
choline acetyltransferase (T-ChAT cells)—the enzyme that
synthesizes ACh. ACh produced and released by these
T-ChAT-expressing cells (44) in turn binds to the α7 nico-
tinic acetylcholine receptors (α7nAChR) on macrophages
to ultimately suppress pro-inflammatory cytokine production
(23) (Fig. 1). Intracellular mechanisms with a role in cholin-
ergic α7nAChR-mediated anti-inflammatory signaling include
suppression of NF-κB activity (45, 46) and activation of the
JAK2–STAT3 pathway (47). In addition, ACh suppresses the
release of mitochondrial DNA, preventing inflammasome acti-
vation (48). Additional evidence has also implicated adenylyl
cyclase 6, the enzyme that catalyzes the formation of cyclic
adenosine monophosphate (cAMP), and the transcription
factor cAMP response element-binding protein (CREB) in
cholinergic-mediated inhibition of inflammation (49).
Another physiological mechanism that regulates immune
responses and inflammation is the hypothalamic–pituitary–
adrenal (HPA) axis, which is a neuro-hormonal pathway with
brain components involving neural communication between
the hypothalamic paraventricular nucleus (PVN) and the pi-
tuitary gland. This circuitry triggers the release of adrenocor-
ticotropic hormone (ACTH) from the anterior pituitary gland
that stimulates the release of anti-inflammatory glucocortic-
oids (cortisol in humans and corticosterone in mice) from
the adrenal cortex (50). Glucocorticoids are steroid hor-
mones that bind to the glucocorticoid receptor (GR), which
is expressed by immune and other cells in the body (51).
Glucocorticoids activating GRs induce a myriad of effects
that are cell-dependent. Within the context of immune cells,
glucocorticoids induce potent anti-inflammatory effects via
non-genomic, ‘bind & block’, and genomic mechanisms such
as transcription (52). The non-genomic mechanism directly
affects pro-inflammatory transcription factors AP1 (53) and
Fig. 1. Vagus nerve-mediated reflex circuitry in immunity and inflam-
mation. In the inflammatory reflex, the activity of afferent vagus nerve
fibers residing in the nodose ganglion is stimulated by cytokines
and PAMPs. The signal is transmitted to the NTS. Reciprocal con-
nections between the NTS and DMN mediate communication with
and activation of efferent vagus nerve fibers from the DMN (yellow
line). The signal is propagated to the celiac-superior mesenteric gan-
glion complex in the celiac plexus, where the splenic nerve origin-
ates. NE released from the splenic nerve interacts with β2-ARs and
causes the release of ACh from T cells that contain functional ChAT
(T-ChAT cells). ACh interacts with α7nAChRs on macrophages and
suppresses pro-inflammatory cytokine release and inflammation (or-
ange line). The inflammatory reflex can be activated through brain
mAChR-mediated mechanisms by centrally acting M1 mAChR agon-
ists and AChE inhibitors (purple line). Somatosensory activation by
electroacupuncture at the Hegu point also causes activation of brain
mAChR signaling, which then results in activation of efferent vagus
and splenic anti-inflammatory signaling (red line). Electroacupuncture
at a different acupuncture point activates sciatic nerve signals, which
by unknown mechanisms convert to efferent vagus nerve signaling
to the adrenal medulla, resulting in dopamine release (blue line).
Dopamine suppresses inflammation and improves survival in a model
of sepsis. Vagus nerve and splenic nerve signaling mediated through
α7nAChR on splenocytes controls inflammation in acute kidney in-
jury and alleviates the condition. mAChR, muscarinic acetylcholine
receptor. Figure reprinted from Pavlov and Tracey 2017 (25) with per-
mission from the authors in conjunction with Springer/Nature.
Peripheral nerve stimulation and immunity 10 9
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NF-κB (54). The genomic mechanism induces the production
of anti-inflammatory molecules such as IL-10, SOCS proteins,
STAT proteins and IκBα—the natural inhibitor of NF-κB (55).
Glucocorticoids are commonly prescribed for the treatment
of a large range of inflammatory disorders (51). Systemic cor-
ticosteroid therapy has also demonstrated promising results
in attenuating the cytokine storm and alleviating the disease
severity in patients with coronavirus disease 2019 (COVID-
19) infected with severe acute respiratory syndrome corona-
virus 2 (SARS-CoV-2). (56, 57).
Discoveries delineating peripheral and brain regulatory
mechanisms of the inflammatory reflex have provided a ra-
tionale for current exploration of pharmacological agents,
including α7nAChR agonists and acetylcholinesterase
(AChE) inhibitors in controlling inflammation (4, 5, 58, 59).
In parallel, the anti-inflammatory efficacy of VNS has been
increasingly evaluated in pre-clinical research and recently
bioelectronic VNS has been clinically exploited for treating
inflammatory diseases—including rheumatoid arthritis and
inflammatory bowel disease (IBD) (10–13). These develop-
ments are summarized below.
Vagus nerve stimulation
Now, 20 years after publication of the seminal paper
demonstrating the anti-inflammatory effects of VNS (9), we are
witnessing an enormous growth in pre-clinical research and,
more recently, in clinical trials exploring VNS in the treatment
of many inflammatory disorders. COVID-19 is just a very re-
cent example of this interest in targeting the vagus nerve and
the inflammatory reflex in alleviating the associated inflam-
matory state (6). In dozens of laboratories around the world,
left cervical VNS has been shown to activate the efferent arm
of the inflammatory reflex—‘the cholinergic anti-inflammatory
pathway’—in various inflammatory conditions (8, 9, 22, 60,
61). Right cervical VNS also inhibits inflammation (62); how-
ever right cervical VNS could theoretically affect the heart rate
to a greater degree than left cervical VNS does, which can
be a limitation (63). Whereas VNS causes anti-inflammatory
effects through efferent cholinergic signaling, activation of af-
ferent signaling also results in lower circulating TNF levels in
endotoxemic mice (64).
VNS activates cholinergic signaling with the release of
anti-inflammatory ACh in the heart, liver and other directly in-
nervated organs, or through the efferent vagus nerve–splenic
nerve–T-ChAT cell axis in the spleen (41, 44). There is also
experimental evidence that vagus nerve cholinergic signaling
regulates the expression of pro-resolving proteins and SPMs.
In a mouse model of zymosan-induced peritoneal inflam-
mation, unilateral cervical vagotomy decreased the expres-
sion of Netrin 1—a key pro-resolving protein involved in the
chemoattraction of immune cells, as well as the production of
SPMs (65). Further, it has been confirmed in ex vivo mouse
and human vagus nerves that VNS enhances the production
of SPMs—findings that indicate the involvement of the vagus
nerve in a pro-resolution response (66).
VNS mitigates inflammation and ameliorates disease se-
verity in animals with IBD (67) and collagen-induced arthritis
(68). These pre-clinical findings provided a rationale for as-
sessing the efficacy of VNS in patients with rheumatoid arthritis
(10, 13) and IBD (11, 12). The results of these trials demon-
strated that VNS generated by an implanted bioelectronic
device is well tolerated, alleviates inflammation and im-
proves disease scores in patients. Clinical trials assessing
VNS continue for patients with IBD or rheumatoid arthritis,
endeavoring to refine and optimize these treatments to im-
prove patient outcomes.
In addition to implanted bioelectronic devices that activate
the cholinergic anti-inflammatory pathway, non-invasive trans-
cutaneous auricular VNS is being increasingly used in the
clinic (69, 70). Bioelectronic devices such as ‘gammaCore’
also generate transcutaneous stimulation of the cervical
vagus nerve, with anti-inflammatory efficacy demonstrated in
healthy volunteers (70) and patients with Sjogren’s syndrome
(69). In an open-label study, this approach was also recently
demonstrated to alleviate inflammation and improve disease
scores in patients with rheumatoid arthritis with specific
benefit for a subset of patients with high disease activity (71).
Another very recently published prospective, multicenter,
open-label study reported alleviation of disease scores and
anti-inflammatory effects in patients with rheumatoid arthritis
treated with non-invasive transcutaneous auricular VNS (72,
73). Adetailed summary of these clinical trials has recently
been compiled in a distinct review (74).
Near-organ stimulation
In addition to VNS performed at the cervical level, there is
considerable interest in ‘near-organ’ stimulation that theoret-
ically offers a more selective approach to modulate inflam-
matory responses. Examples of near-organ stimulation with
anti-inflammatory efficacy are stimulation of the hepatic and
abdominal vagus nerve branches (75) as well as the splenic
nerve (76) (Fig. 2).
The anti-inflammatory efficacy of stimulation of the hepatic
branch of the vagus nerve in concanavalin A-induced liver
injury in rats was recently reported (77). This approach re-
sulted in decreased pro-inflammatory cytokine expression in
Kupffer cells (77). The relative expression of pro-inflammatory
cytokines is markedly reduced for several subsequent days
following a single hepatic VNS. In addition, hepatic branch
VNS noticeably diminishes the presence of CD4+ and CD8+
T cells in the liver (77). The authors’ observations also sug-
gest the involvement of α7nAChR and phospho-STAT3 in
mediating cholinergic suppression of hepatic inflammatory
responses (77). Interestingly, this study points to a physio-
logical feedback loop mechanism underlying hepatic branch
VNS anti-inflammatory effects—a brainstem-integrated vagal
hepatic afferent pathway and a DMN efferent cholinergic
pathway (Fig. 2, (4)) (77).
In another study, the effectiveness of right cervical VNS
and, anterior or posterior, abdominal VNS was assessed (Fig.
2, (5)). Cervical and abdominal, anterior and posterior, VNS
were effective means of mitigating LPS-induced inflammation
(75). Further, it was demonstrated in a model of post-operative
ileus in mice that both cervical and abdominal VNS improved
gastrointestinal transit to a comparable degree (75). In add-
ition, using a porcine model, the authors observed that ab-
dominal VNS does not affect heart rate variability, contrary
to right cervical VNS (75). Similarly, in rats, abdominal VNS
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decreases histological scores of inflammation and improves
indices of disease in a 2,4,6-trinitrobenzenesulfonic acid
(TNBS)-induced colitis model (78). Although no clear mech-
anism of the anti-inflammatory efficacy of abdominal VNS was
identified, this approach was not associated with fluctuations
in blood pressure, respiratory rate or heart rate that were ap-
parent following left cervical VNS (78).
The splenic nerve originating at the celiac-superior mes-
enteric ganglion complex is a crucial component of the cho-
linergic anti-inflammatory pathway (38, 41). Neurectomy of
the splenic nerve abolishes the anti-inflammatory impact of
VNS (41). Consequently, it is not surprising that there has
been a growing interest in targeting the splenic nerve with
bioelectronic modalities, and several groups have demon-
strated that stimulation of the splenic nerve attenuates inflam-
mation (Fig. 2, (6)). Initial studies on ex vivo rat splenic nerves
descending to the spleen, when electrically stimulated,
were capable of inhibiting local inflammation (79). Further,
mouse splenic nerve stimulation attenuates LPS-induced
serum TNF levels (43). The splenic nerve that originates at
the celiac-superior mesenteric ganglion complex runs in a
neurovascular bundle that terminates at the parenchyma of
the spleen (36). An additional nerve of currently uncertain
origin was also found at the crown of the spleen in mice—
the apical nerve (76, 80). Acute apical nerve stimulation also
suppresses LPS-induced pro-inflammatory cytokine levels
and chronic stimulation of this nerve alleviates the severity of
collagen-induced arthritis, as well as improved histological
hallmarks of inflammation (76).
The anti-inflammatory potential of modulating other distinct
neurocircuitries using miniaturized bioelectronics is currently
being explored, as discussed in the next section.
Anti-inammatory exploration of the crosstalk between
non-vagal and vagus nerve pathways
Recent studies have revealed a crosstalk between non-vagal
and vagus nerve circuitries in the control of inflammation.
Aseries of recent studies using sacral nerve stimulation have
indicated the relationship of neural circuitry activated by this
stimulation and the efferent vagus nerve in the regulation of
inflammation in rats with colitis (81–83). VNS and sacral nerve
stimulation exert comparable anti-inflammatory and disease-
ameliorating effects in rats with TNBS-induced colitis; inter-
estingly in addition to VNS, sacral nerve stimulation also
increases efferent vagus nerve activity (83).
Sacral nerve stimulation (1h daily for 10days) using chron-
ically implanted electrodes also promotes recovery of rats
with colitis induced by 5% dextran sulfate sodium (DSS)
(81, 84). This is indicated by decreased disease activity
index, histological scores, myeloperoxidase activity and co-
lonic tissue TNF levels, and increased numbers of colonic M2
Fig. 2. Induction points to electrically activate the vagus nerve and the splenic nerve in anti-inflammatory approaches. It has been dem-
onstrated that left VNS [1] inhibits pro-inflammatory cytokine release and inflammation in several inflammatory conditions. In addition, the
anti-inflammatory effects of right VNS [2] have been shown. However, right VNS is not frequently used, because of potential deleterious car-
diac effects. Electrical stimulation of the aortic depressor nerve [3] suppresses zymosan-induced inflammation via afferent vagus nerve fibers,
which potentially instigate the baroreflex. Hepatic vagus branch stimulation [4] induces afferent vagal signaling to the brain, which transforms
into an efferent response—decreasing inflammation in the liver. Subdiaphragmatic VNS [5], anterior and posterior, mitigates pro-inflammatory
cytokine production through undescribed mechanisms. Stimulation of the splenic nerve [6], arising from the celiac-superior mesenteric gan-
glion complex, induces the end-point effect of the cholinergic anti-inflammatory pathway, i.e. amelioration of inflammation. The stimulation loci
indicated are effective in endotoxemia and in a variety of disorders as specified.
Peripheral nerve stimulation and immunity 111
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macrophages (81). This treatment also results in an increase
in the high-frequency component of heart rate variability,
plasma pancreatic peptide and colon tissue ACh—indicative
of increased efferent vagus nerve activity (81). In addition, sa-
cral nerve stimulation increases the levels of the anti-inflam-
matory cytokine IL-10 and decreases myeloperoxidase, as
well as pro-inflammatory cytokines IL-2, IL-17A and TNF in
the colon tissues of rats with TNBS-induced colitis (82). This
treatment is also associated with increased parasympathetic
activity (determined by heart rate variability analysis) and
ACh levels in colon tissue (82). The anti-inflammatory effects
of sacral nerve stimulation are abrogated in animals with sur-
gically ablated sacral nerve afferent and vagus nerve efferent
pathways (82). As stated by the authors, these findings delin-
eate a physiological mechanism by which sacral nerve stimu-
lation is carried out by spinal cord afferents to the brainstem
that coalesces with efferent vagal anti-inflammatory signaling
to the colon tissue (82).
Another example of crosstalk between non-vagal and
vagal pathways is the interaction of the sciatic nerve with
efferent vagus nerve signaling in inflammatory regulation
(Fig. 1). Electroacupuncture at the ST36 Zusanli acupoint of
the sciatic nerve decreases the LPS-induced expression of
pro-inflammatory cytokines, as well as improves the survival
rate of mice with endotoxemia and mice with sepsis induced
by cecal ligation and puncture (85). The mechanism by
which this occurs is dependent upon the subdiaphragmatic
vagus nerve, the adrenal glands and ultimately the release
of dopamine. As suggested, this form of bioelectronic
neuromodulation could have clinical implications for patients
with sepsis (85). Electroacupuncture at a point located at the
junction of the first and the second metacarpal bones (Hegu
acupoint) also suppresses serum TNF, Il-1β and IL-6 levels
and improves survival in lethal murine endotoxemia (86). The
underlying mechanism involves brain muscarinic ACh recep-
tors, efferent vagus nerve and catecholaminergic signaling to
the spleen (86). These observations importantly link somato-
sensory activation with signaling dependent on brain cholin-
ergic muscarinic receptors, which has a previously identified
role in the regulation of the inflammatory reflex (61, 87, 88)
(Fig. 1).
A specific sensory (afferent) branch of the vagus nerve—
the aortic depressor branch—has an essential role in car-
diovascular reflex regulation (baroreflex) (89). Interestingly,
electrical stimulation of the left aortic depressor nerve that trig-
gers baroreflex activation also results in lower synovial levels
of TNF, IL-1β and IL-6, as well as decreased joint edema and
neutrophil counts in rats with experimental arthritis induced
by administration of zymosan into the femorotibial joint (Fig.
2, (3)). Aseries of surgical interventions reveals that these
effects are abrogated by lumbar sympathectomy, but not
by adrenalectomy, celiac subdiaphragmatic vagotomy or
splenectomy. These observations point to a mediating role of
sympathetic fibers originating in the L2–L5 spinal cord (90).
Although this approach reportedly attenuates joint inflamma-
tion in rats, it does not alter systemic or organ (spleen and
heart) pro-inflammatory cytokine levels during endotoxemia
(91). However, somewhat surprisingly it attenuates brain
hypothalamic TNF, IL-6, IL-1β and IL-10 levels in endotoxemic
rats through an unknown mechanism (91). Stimulation of
other nerves such as the trigeminal (92) and the carotid sinus
nerve (CSN), as briefly discussed below, also presents op-
portunities for exploring novel anti-inflammatory therapies for
treating various inflammatory disorders.
CSN stimulation
The CSN innervates the carotid body and synapses at the
NTS (93, 94). The carotid body, localized at the common
carotid artery bifurcation into the internal and external ca-
rotid arteries, is a complex formation characterized as a
paraganglion, which contains neural chemoreceptor cells,
glial-like cells and vascular cells surrounded by connective
tissue. The carotid body is a polymodal sensor, which de-
tects fluctuations in oxygen, carbon dioxide and metabolic
molecules including angiotensin II, glucose, leptin and in-
sulin through chemoreceptive mechanisms and cognate re-
ceptors (95, 96), and alterations in blood pressure through its
baroreceptors (97). Subsequent activation of CSN signals to
the brain to trigger sympathetic or parasympathetic activity
aimed at balancing these changes.
The carotid body is also a prolific sensor of immune al-
terations; it expresses cytokine receptors and PRRs (98)
and detects cytokines TNF (99), IL-6 (100), IL-1β (101),
as well as zymosan (102) and LPS (103). Subsequent ac-
tivation of the nerve fibers of the CSN has been recorded
following detection of TNF in the carotid body (104) and
IL-1β (101) in rats, and LPS (99) in cats. Further, it has been
demonstrated that, after bilateral CSN transection in rats,
TNF levels are higher and survival is significantly reduced
following endotoxemia (105).
There is growing interest in utilizing CSN stimulation in con-
trolling inflammation (106, 107). In mice, left CSN electrical
stimulation acutely and chronically attenuates LPS-induced
inflammation (108). This mitigation of inflammation is inde-
pendent of efferent signaling to the carotid body, the vagus
nerve, the spleen, catecholaminergic and cholinergic nerves
(108). However, it results in activation of neural signaling in
the PVN of the hypothalamus, and activation of the HPA axis
with the release of corticosterone from the adrenals acting
through a GR-mediated mechanism on myeloid immune
cells (108) (Fig. 3). Furthermore, chronic CSN stimulation
using implanted electrodes in mice improves survival in lethal
endotoxemia (108). This is an important observation that in-
dicates that this unique mechanism triggered by CSN stimu-
lation does not merely suppress pro-inflammatory cytokines
levels, but also improves outcomes. Together these findings
identify the CSN as a prime target in bioelectronic medicine.
Further considerations on the differential therapeutic
utility of neuroimmunomodulation
The core concept of bioelectronic medicine is to utilize the
function of a nerve to target specific molecular mechanisms
for therapeutic benefit (109). Bioelectronic cervical VNS
offers an efficient approach to stimulate the inflammatory re-
flex and its efferent arm—the cholinergic anti-inflammatory
pathway—and this approach is progressing in the clinical
treatment of inflammatory disorders (10, 11, 110). These ad-
vances have been facilitated by the relative ease of access of
112 Peripheral nerve stimulation and immunity
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the cervical vagus nerve and previous experience with VNS
in the treatment of pharmacoresistant epilepsy and depres-
sion (110, 111).
Bioelectronic devices, such as gammaCore, can effect-
ively attenuate inflammation through transcutaneous acti-
vation of the cervical vagus nerve (69, 70). In humans, the
transverse plane of the cervical vagus nerve is in proximity to
the CSN, the cervical sympathetic trunk and the glossopha-
ryngeal and the hypoglossal nerve (112). Therefore, a the-
oretical possibility exists for cross-activation of other nerves
and undesirable side-effects, requiring that the stimulation be
administered by a medical professional or by patients who
are reliably trained in the use of the device. Bioelectronic mo-
dalities utilizing implanted nerve stimulators greatly mitigate
the risk of cross-activation, while also reducing non-compli-
ance to treatment, a characteristic disadvantage of pharma-
cological therapies (10, 11). Technological advances in
bioelectronics for neuromodulation generated by either
non-invasive transcutaneous devices or implanted devices
will undoubtedly benefit from further refining and optimizing
prior to treating acute and chronic inflammatory conditions,
as well as many other disorders (109). In the near future, it
is entirely possible that implanted bioelectronic devices may
become widespread (113).
It should be noted that, in addition to activating efferent
signaling with sufficient anti-inflammatory efficacy, afferent
VNS has also been shown to cause anti-inflammatory
effects—i.e. suppression of serum TNF levels in endotoxemic
mice (64). These observations corroborate other findings
demonstrating the effects of VNS performed 24 hours prior
to renal ischemia–reperfusion injury in mice (114). Although
VNS ameliorates the severity of injury and suppresses circu-
lating TNF levels through α7nAChR-mediated efferent vagus
nerve signaling to the spleen (Fig. 1), stimulation of afferent
vagus fibers is also protective in this model (114). There is
also evidence that vagotomy increases serum corticosterone
levels during murine endotoxemia (9), which can be associ-
ated with interruption of afferent vagus nerve signaling that
reportedly mediates activation of the HPA axis (115).
These findings demonstrate the broader scope of vagus
nerve involvement in the regulation of inflammation in which
afferent signaling can trigger activation of other brain-derived
immunoregulatory pathways. This view is supported by ana-
tomical considerations that, in the brain, the NTS is linked with
the rostral ventrolateral medulla and locus coeruleus, brain
loci that regulate spinal cord-derived catecholaminergic
immunomodulatory signaling (23, 89). In addition, the
NTS connectivity with the hypothalamic PVN provides a
neurocircuitry for activation of the HPA axis (23, 89). As de-
scribed in previous sections, efferent vagus nerve signaling is
also involved in mediating anti-inflammatory effects triggered
by sciatic nerve stimulation resulting in dopamine release from
the adrenals (Fig. 1). Efferent cholinergic signaling also regu-
lates important mediators in the resolution of inflammation
Fig. 3. Electrical stimulation of the CSN attenuates inflammation via activation of the GR in myeloid immune cells. Acute and chronic electrical
stimulation of the left CSN in mice reduce the levels of blood-borne pro-inflammatory cytokines during endotoxemia. Chronic electrical stimula-
tion of the CSN also improves survival in lethal endotoxemia. CSN simulation results in enhanced neuronal activity in the PVN of the hypothal-
amus, which activates the HPA axis. As the CSN terminates in the NTS, a neuronal connection between NTS and PVN mediating this activation
can be suggested. The HPA axis, a major neuro-hormonal anti-inflammatory pathway is triggered by the PVN production and release of CRH
into the median eminence which reaches the anterior pituitary and induces the release of ACTH. ACTH travels through the circulation to the
adrenal glands, generating the production and release of glucocorticoids (corticosterone in mice; cortisol in humans). The anti-inflammatory
effects of CSN stimulation in endotoxemia requires corticosterone signaling through GR in myeloid cells with presumed subsequent activation
of a genomic, or non-genomic, pathway that involves suppression of NF-κB nuclear translocation—ultimately mitigating cytokine production
and enhancing survival. CB, carotid body; CRH, corticotropin-releasing hormone.
Peripheral nerve stimulation and immunity 11 3
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(65). Thus, VNS anti-inflammatory efficacy may be associated
with activation of multiple physiological mechanisms that can
be targeted in a condition-dependentmanner.
An array of VNS parameters attenuate inflammation (8, 64,
116). Intriguingly, a recent study demonstrated that applying
distinct VNS parameters to mice in the absence of immune
challenge increased serum pro-inflammatory cytokine levels
(117). This suggests that utilizing different VNS parameters
induces distinct immunoregulatory signaling. Although the
underlying mechanisms remain to be further elucidated,
they may be related to fiber-specific activation. The vagus
nerve is composed of A, B and C-fibers, which can be fur-
ther subdivided. Generally, A and B-fibers are myelinated,
whereas C-fibers, the predominant vagus nerve fiber type,
are not. Consequently, activation of Aand B-fibers requires
lower amplitude and pulse widths (key components of stimu-
lation regimens) compared with C-fibers (27, 118, 119). Are-
cent study has concluded that involvement of C-fibers in the
cholinergic anti-inflammatory pathway is unlikely (120) and
previous studies have suggested the involvement of Aand,
or uniquely, B-fibers (64, 117, 121). Conversely, it has been
shown that anterior abdominal VNS can attenuate histo-
logical scores of inflammation via the preferential activation of
C-fibers (78). Interestingly, a recent report has suggested that
vagus nerve anti-inflammatory activity is mediated efferently
through B-fibers and afferently via C-fibers (122). These re-
sults indicate that individual immunoregulatory effects of VNS
can be induced through distinct fiber activation.
The opportunities to regulate inflammation using electrical
stimulation of peripheral nerves are growing. Localized near-
organ stimulation is enticing as it is theoretically associated
with a reduced risk of adverse events during stimulation (Fig.
2). This type of neuromodulation may be more suitable for
treating conditions that predominantly affect a specific organ
or to target an organ that plays a dominant role as a source
of pathogenic systemic inflammation such as the spleen. For
instance, stimulation of the hepatic branch of the vagus nerve
may prove particularly effective for conditions affecting the
liver, i.e. hepatitis (77). However, it should be considered that
even near-organ neuromodulation may result in triggering af-
ferent signaling with the consequent engagement of other
signaling pathways. In the case of hepatic branch VNS in
the setting of experimental hepatitis, activation of afferent
signaling converging into a brainstem-integrated reflex drive
along efferent hepatic fibers (not direct stimulation of efferent
signaling) has been evaluated as an anti-inflammatory mech-
anism (77) (Fig. 2).
The efferent vagus nerve–splenic nerve axis in the inflam-
matory reflex and the resultant catecholaminergic-induced
T-ChAT cell release of ACh are essential for suppressing ex-
cessive pro-inflammatory responses. Splenic nerve stimu-
lation, which is not associated with antidromic or afferent
effects, is generating increased interest as a bioelectronic
modality (43, 76, 79) (Fig. 2). Arecent study examined ex
vivo human splenic tissue and nerves, to determine thera-
peutically relevant stimulation parameters (123). Studying
the anti-inflammatory efficacy of splenic nerve stimulation in
large animal models is ongoing and in humans undergoing
esophagectomy, the safety and potential therapeutic benefit
of splenic neurovascular bundle stimulation is currently being
explored [NCT04171011].
Splenic nerve stimulation requires the development of new
methods to gain access to the nerve. Thus, any overall risks
and benefits of this new approach will have to be assessed
in light of decades of experience with cervical vagus nerve
stimulation, and other non-invasive approaches, e.g. trans-
cutaneous VNS or ultrasound stimulation (124). However,
despite some current limitations, splenic nerve stimulation
may one day find utility in the future lexicon of bioelectronic
medicine.
Stimulation of the CSN, which results in HPA axis activation
and the release of glucocorticoids (Fig. 3) may have also found
a niche in the growing list of mechanism-based bioelectronic
medicines (108). Glucocorticoids are prescribed to patients
with numerous conditions, including those who receive organ
transplants (to induce immune suppression and prevent
organ rejection) (125, 126). The continued administration
of exogenous glucocorticoids is associated with complica-
tions and side effects (127), threatening non-compliance.
Perhaps, a finely tuned CSN stimulation-induced release of
endogenous glucocorticoids could ameliorate inflammation
to an adequate degree to substitute for, or at least reduce
the need for, glucocorticoid therapies—with a decreased risk
of non-compliance. One of the main obstacles in advancing
CSN stimulation to clinical settings is the relatively small size
of this nerve, which makes access to the nerve and CSN ma-
nipulations challenging (112).
It should be noted that, in addition to electrical stimu-
lation of peripheral nerves that is discussed in this review,
other neuromodulation approaches under the umbrella of
bioelectronic medicine, including optogenetics and ultra-
sound and magnetic stimulation, have also been increasingly
utilized (128). Optogenetic stimulation has been instrumental
for gaining specific mechanistic insights into the role of brain
cholinergic regions in controlling pro-inflammatory cytokine
responses through the vagus nerve inflammatory reflex (36,
129). Very recently, selective optogenetic stimulation of ef-
ferent vagus neurons was achieved in a large mammal, a
sheep, which opens new avenues in bioelectronic medicine
(130). Further, two independent studies have demonstrated
that precise ultrasound stimulation of the spleen attenuates
inflammation comparably to cervical VNS (131) and that daily
precise ultrasound stimulation of the spleen reduces clinical
scores of arthritis (124).
Conclusion
Active ongoing research continues to chart the anatomical
and functional landscape of the neuro-immune dialogue and
delineate the role of neural circuitry in immune regulation. New
insights at the interface between immune cells and afferent
and efferent vagus nerve signaling in the brain-integrated
inflammatory reflex indicate novel treatment opportunities.
The anti-inflammatory utility of electrical stimulation of the
vagus nerve and other peripheral nerves has been demon-
strated in a growing number of conditions characterized by
dysregulated cytokine release and aberrant inflammation.
This exploration has also improved our understanding of
neural control of inflammation by linking the neural circuitry
triggered by sacral or sciatic nerve stimulation with signaling
along the efferent vagus nerve. Other modalities, including
the CSN, have been linked to activation of the HPA axis—a
114 Peripheral nerve stimulation and immunity
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major neuro-hormonal immunoregulatory pathway. These
insights inform novel neuromodulation treatments of inflam-
matory and autoimmune diseases in bioelectronic medicine.
Implanted device-generated VNS and non-invasive transcu-
taneous cervical and auricular branch vagus nerve stimula-
tion occupy a key role for clinical translation in this field; recent
studies have validated their efficacy in treating rheumatoid
arthritis, IBD and other chronic diseases. Ongoing and future
pre-clinical research and clinical validation of the efficacy of
bioelectronic stimulation of other peripheral nerves will un-
doubtedly broaden the therapeutic opportunities to intervene
in multiple diseases and help a wide range of patients.
Funding
This work was supported by the National Institutes of Health (NIH),
National Institute of General Medical Sciences Grants: R01GM128008
and R01GM121102 (to V.A.P.) and 1R35GM118182 (to K.J.T.).
Acknowledgements
The authors apologize to colleagues whose work was not cited
because of space limitations. Figures 2 and 3 were created using
Biorender.
Conflicts of interest statement: C.N.M., K.J.T and V.A.P. have
co-authored patents broadly related to the content of this review.
They have assigned their rights to the Feinstein Institutes for Medical
Research. A.F.declares no conflict of interest.
References
1 Olofsson, P. S., Metz, C. N. and Pavlov, V. A. 2017. The
neuroimmune communicatome in inflammation. In Cavaillon,J.
and Singer,M., eds., Inflammation: From Molecular and Cellular
Mechanisms to the Clinic, p.1485. Weinheim, Berlin, Germany.
2 Tracey,K.J. 2002. The inflammatory reflex. Nature 420:853.
3 Pavlov,V.A., Chavan,S.S. and Tracey,K.J. 2018. Molecular and
functional neuroscience in immunity. Annu. Rev. Immunol. 36:783.
4 Chatterjee, P. K., Yeboah, M. M., Solanki, M. H. et al. 2017.
Activation of the cholinergic anti-inflammatory pathway by GTS-
21 attenuates cisplatin-induced acute kidney injury in mice. PLoS
One 12:e0188797.
5 Metz,C.N. and Pavlov,V.A. 2020. Treating disorders across the
lifespan by modulating cholinergic signaling with galantamine. J.
Neurochem., in press. doi:10.1111.jnc.15243.
6 Pavlov, V. A. 2021. The evolving obesity challenge: targeting
the vagus nerve and the inflammatory reflex in the response.
Pharmacol. Ther. 222:107794.
7 Tracey,K.J. 2007. Physiology and immunology of the cholinergic
antiinflammatory pathway. J. Clin. Invest. 117:289.
8 Caravaca,A. S., Gallina,A.L., Tarnawski,L. etal. 2019. An ef-
fective method for acute vagus nerve stimulation in experimental
inflammation. Front. Neurosci. 13:877.
9 Borovikova,L.V., Ivanova,S., Zhang,M. etal. 2000. Vagus nerve
stimulation attenuates the systemic inflammatory response to
endotoxin. Nature 405:458.
10 Koopman,F.A., Chavan,S.S., Miljko,S. etal. 2016. Vagus nerve
stimulation inhibits cytokine production and attenuates disease
severity in rheumatoid arthritis. Proc. Natl Acad. Sci. USA
113:8284.
11 Bonaz, B., Sinniger,V., Hoffmann,D. etal. 2016. Chronic vagus
nerve stimulation in Crohn’s disease: a 6-month follow-up pilot
study. Neurogastroenterol. Motil. 28:948.
12 Sinniger,V., Pellissier,S., Fauvelle,F. etal. 2020. A 12-month pilot
study outcomes of vagus nerve stimulation in Crohn’s disease.
Neurogastroenterol. Motil. 32:e13911.
13 Genovese,M., Gaylis,N., Sikes,D. etal. 2020. Safety and efficacy
of neurostimulation with a miniaturised vagus nerve stimulation
device in patients with multidrug-refractory rheumatoid arthritis: a
two-stage multicentre, randomised pilot study. Lancet Rheumatol.
2:527.
14 Birmingham, K., Gradinaru, V., Anikeeva, P. et al. 2014.
Bioelectronic medicines: a research roadmap. Nat. Rev. Drug
Discov. 13:399.
15 Akira,S., Uematsu,S. and Takeuchi,O. 2006. Pathogen recogni-
tion and innate immunity. Cell 124:783.
16 Turner, M. D., Nedjai, B., Hurst, T. et al. 2014. Cytokines and
chemokines: at the crossroads of cell signalling and inflammatory
disease. Biochim. Biophys. Acta 1843:2563.
17 Koch,U. and Radtke,F. 2011. Mechanisms of T cell development
and transformation. Annu. Rev. Cell Dev. Biol. 27:539.
18 Serhan,C.N. 2014. Pro-resolving lipid mediators are leads for
resolution physiology. Nature 510:92.
19 Serhan,C.N. and Levy,B.D. 2018. Resolvins in inflammation:
emergence of the pro-resolving superfamily of mediators. J. Clin.
Invest. 128:2657.
20 Rao,M. and Gershon, M.D. 2016. The bowel and beyond: the
enteric nervous system in neurological disorders. Nat. Rev.
Gastroenterol. Hepatol. 13:517.
21 Elenkov,I.J., Wilder,R.L., Chrousos,G.P. etal. 2000. The sympa-
thetic nerve–an integrative interface between two supersystems:
the brain and the immune system. Pharmacol. Rev. 52:595.
22 Chavan,S.S., Pavlov,V.A. and Tracey,K.J. 2017. Mechanisms
and therapeutic relevance of neuro-immune communication.
Immunity 46:927.
23 Pavlov, V. A., Wang, H., Czura, C. J. et al. 2003. The cho-
linergic anti-inflammatory pathway: a missing link in
neuroimmunomodulation. Mol. Med. 9:125.
24 Pavlov,V.A. and Tracey,K. J. 2004. Neural regulators of innate
immune responses and inflammation. Cell. Mol. Life Sci. 61:2322.
25 Pavlov,V.A. and Tracey,K.J. 2017. Neural regulation of immunity:
molecular mechanisms and clinical translation. Nat. Neurosci.
20:156.
26 Olofsson, P. S., Rosas-Ballina, M., Levine, Y. A. et al. 2012.
Rethinking inflammation: neural circuits in the regulation of
immunity. Immunol. Rev. 248:188.
27 Yuan, H. and Silberstein, S. D. 2016. Vagus nerve and vagus
nerve stimulation, a comprehensive review: part I. Headache
56:71.
28 Rosas-Ballina,M. and Tracey,K. J. 2009. Cholinergic control of
inflammation. J. Intern. Med. 265:663.
29 Pongratz, G. and Straub,R. H. 2014. The sympathetic nervous
response in inflammation. Arthritis Res. Ther. 16:504.
30 Pongratz, G. and Straub,R. H. 2014. The sympathetic nervous
response in inflammation. Arthritis Res. Ther. 16:504.
31 Niijima,A. 1996. The afferent discharges from sensors for inter-
leukin 1 beta in the hepatoportal system in the anesthetized rat.
J. Auton. Nerv. Syst. 61:287.
32 Goehler, L. E., Relton, J. K., Dripps, D. et al. 1997. Vagal
paraganglia bind biotinylated interleukin-1 receptor antagonist:
a possible mechanism for immune-to-brain communication. Brain
Res. Bull. 43:357.
33 Ek,M., Kurosawa, M., Lundeberg, T. et al. 1998. Activation of
vagal afferents after intravenous injection of interleukin-1beta:
role of endogenous prostaglandins. J. Neurosci. 18:9471.
34 Zanos,T.P., Silverman,H.A., Levy,T. etal. 2018. Identification
of cytokine-specific sensory neural signals by decoding murine
vagus nerve activity. Proc. Natl Acad. Sci. USA 115:E4843.
35 Steinberg, B. E., Silverman, H. A., Robbiati, S. et al. 2016.
Cytokine-specific neurograms in the sensory vagus nerve.
Bioelectron. Med. 3:7.
36 Kressel,A.M., Tsaava,T., Levine,Y.A. etal. 2020. Identification
of a brainstem locus that inhibits tumor necrosis factor. Proc. Natl
Acad. Sci. USA 117:29803.
37 Vida,G., Peña,G., Kanashiro,A. etal. 2011. β2-Adrenoreceptors
of regulatory lymphocytes are essential for vagal neuromodulation
of the innate immune system. FASEB J. 25:4476.
38 Huston, J. M., Ochani, M., Rosas-Ballina, M. et al. 2006.
Splenectomy inactivates the cholinergic antiinflammatory
pathway during lethal endotoxemia and polymicrobial sepsis. J.
Exp. Med. 203:1623.
Peripheral nerve stimulation and immunity 11 5
Downloaded from https://academic.oup.com/intimm/article/34/2/107/6366295 by guest on 27 November 2022
39 Berthoud,H.R. and Powley,T.L. 1993. Characterization of vagal
innervation to the rat celiac, suprarenal and mesenteric ganglia.
J. Auton. Nerv. Syst. 42:153.
40 Murray,K., Barboza,M., Rude, K.M. etal. 2019. Functional cir-
cuitry of neuro-immune communication in the mesenteric lymph
node and spleen. Brain Behav. Immun. 82:214.
41 Rosas-Ballina,M., Ochani,M., Parrish,W.R. etal. 2008. Splenic
nerve is required for cholinergic antiinflammatory pathway control
of TNF in endotoxemia. Proc. Natl Acad. Sci. USA 105:11008.
42 Pavlov,V.A. and Tracey,K.J. 2015. Neural circuitry and immunity.
Immunol. Res. 63:38.
43 Vida, G., Peña,G., Deitch, E. A. etal. 2011. α7-cholinergic re-
ceptor mediates vagal induction of splenic norepinephrine. J.
Immunol. 186:4340.
44 Rosas-Ballina, M., Olofsson, P. S., Ochani, M. et al. 2011.
Acetylcholine-synthesizing T cells relay neural signals in a vagus
nerve circuit. Science 334:98.
45 Guarini,S., Altavilla,D., Cainazzo,M.M. etal. 2003. Efferent vagal
fibre stimulation blunts nuclear factor-kappaB activation and
protects against hypovolemic hemorrhagic shock. Circulation
107:1189.
46 Parrish, W. R., Rosas-Ballina, M., Gallowitsch-Puerta, M. et al.
2008. Modulation of TNF release by choline requires alpha7 sub-
unit nicotinic acetylcholine receptor-mediated signaling. Mol.
Med. 14:567.
47 deJonge,W. J., vanderZanden, E.P., The,F.O. et al. 2005.
Stimulation of the vagus nerve attenuates macrophage activation
by activating the Jak2-STAT3 signaling pathway. Nat. Immunol.
6:844.
48 Lu,B., Kwan,K., Levine, Y.A. etal. 2014. α7 nicotinic acetyl-
choline receptor signaling inhibits inflammasome activation by
preventing mitochondrial DNA release. Mol. Med. 20:350.
49 Tarnawski,L., Reardon,C., Caravaca,A.S. etal. 2018. Adenylyl
cyclase 6 mediates inhibition of TNF in the inflammatory reflex.
Front. Immunol. 9:2648.
50 Turnbull, A. V. and Rivier, C. L. 1999. Regulation of the
hypothalamic-pituitary-adrenal axis by cytokines: actions and
mechanisms of action. Physiol. Rev. 79:1.
51 Kadmiel, M. and Cidlowski, J. A. 2013. Glucocorticoid re-
ceptor signaling in health and disease. Trends Pharmacol. Sci.
34:518.
52 Coutinho,A.E. and Chapman,K.E. 2011. The anti-inflamma-
tory and immunosuppressive effects of glucocorticoids, recent
developments and mechanistic insights. Mol. Cell. Endocrinol.
335:2.
53 Yang-Yen, H. F., Chambard, J. C., Sun, Y. L. et al. 1990.
Transcriptional interference between c-Jun and the gluco-
corticoid receptor: mutual inhibition of DNA binding due to direct
protein-protein interaction. Cell 62:1205.
54 Ray, A. and Prefontaine, K. E. 1994. Physical association and
functional antagonism between the p65 subunit of transcription
factor NF-kappa B and the glucocorticoid receptor. Proc. Natl
Acad. Sci. USA 91:752.
55 Bellavance, M.A. and Rivest, S. 2014. The HPA—immune axis
and the immunomodulatory actions of glucocorticoids in the
brain. Front. Immunol. 5:136.
56 RECOVERY Collaborative Group,Horby,P., Lim,W.S. etal. 2021.
Dexamethasone in hospitalized patients with Covid-19—prelim-
inary report. N. Engl. J.Med. 384:693.
57 WHO Rapid Evidence Appraisal for COVID-19 Therapies (REACT)
Working Group; Sterne,J.A.C., Murthy,S., Diaz,J.V. etal. 2020.
Association between administration of systemic corticosteroids
and mortality among critically ill patients with COVID-19: AMeta-
analysis. JAMA 324:1330.
58 Pavlov,V.A., Ochani,M., Yang,L.H. etal. 2007. Selective alpha7-
nicotinic acetylcholine receptor agonist GTS-21 improves survival in
murine endotoxemia and severe sepsis. Crit. Care Med. 35:1139.
59 Sitapara,R.A., Gauthier,A.G., Valdés-Ferrer,S.I. etal. 2020. The
α7 nicotinic acetylcholine receptor agonist, GTS-21, attenuates
hyperoxia-induced acute inflammatory lung injury by alleviating
the accumulation of HMGB1 in the airways and the circulation.
Mol. Med. 26:63.
60 Chavan,S.S. and Tracey,K.J. 2017. Essential neuroscience in
immunology. J. Immunol. 198:3389.
61 Pavlov, V. A., Ochani, M., Gallowitsch-Puerta, M. et al. 2006.
Central muscarinic cholinergic regulation of the systemic inflam-
matory response during endotoxemia. Proc. Natl Acad. Sci. USA
103:5219.
62 Matteoli,G., Gomez-Pinilla,P.J., Nemethova,A. et al. 2014. A
distinct vagal anti-inflammatory pathway modulates intestinal
muscularis resident macrophages independent of the spleen.
Gut 63:938.
63 Bernik, T. R., Friedman, S. G., Ochani, M. et al. 2002.
Pharmacological stimulation of the cholinergic antiinflammatory
pathway. J. Exp. Med. 195:781.
64 Olofsson,P.S., Levine, Y.A., Caravaca, A. et al. 2015. Single-
Pulse and unidirectional electrical activation of the cervical vagus
nerve reduces tumor necrosis factor in endotoxemia. Bioelectr.
Med. 2:37.
65 Mirakaj,V., Dalli,J., Granja,T. etal. 2014. Vagus nerve controls
resolution and pro-resolving mediators of inflammation. J. Exp.
Med. 211:1037.
66 Serhan,C. N., dela Rosa,X. and Jouvene,C.C. 2018. Cutting
edge: human vagus produces specialized proresolving me-
diators of inflammation with electrical stimulation reducing
proinflammatory eicosanoids. J. Immunol. 201:3161.
67 Meregnani, J., Clarençon, D., Vivier, M. et al. 2011. Anti-
inflammatory effect of vagus nerve stimulation in a rat model of
inflammatory bowel disease. Auton. Neurosci. 160:82.
68 Levine, Y. A., Koopman, F. A., Faltys, M. et al. 2014.
Neurostimulation of the cholinergic anti-inflammatory pathway
ameliorates disease in rat collagen-induced arthritis. PLoS One
9:e104530.
69 Tarn,J., Legg,S., Mitchell,S. etal. 2019. The effects of noninvasive
vagus nerve stimulation on fatigue and immune responses in pa-
tients with Primary Sjögren’s syndrome. Neuromodulation 22:580.
70 Lerman,I., Hauger,R., Sorkin,L. etal. 2016. Noninvasive transcu-
taneous vagus nerve stimulation decreases whole blood culture-
derived cytokines and chemokines: a randomized, blinded,
healthy control pilot trial. Neuromodulation 19:283.
71 Drewes,A.M., Brock,C., Rasmussen,S.E. etal. 2021. Short-term
transcutaneous non-invasive vagus nerve stimulation may reduce
disease activity and pro-inflammatory cytokines in rheumatoid
arthritis: results of a pilot study. Scand. J.Rheumatol. 50:20.
72 Marsal, S., Corominas, H., de Agustín, J. J. et al. 2021. Non-
invasive vagus nerve stimulation for rheumatoid arthritis: a proof-
of-concept study. Lancet Rheumatol. 3:262.
73 Tracey, K. J. 2021. Hacking the inflammatory reflex. Lancet
Rheumatol. 3:237.
74 Courties,A., Berenbaum,F. and Sellam,J. 2021. Vagus nerve
stimulation in musculoskeletal diseases. Joint Bone Spine
88:105149.
75 Stakenborg,N., Wolthuis,A.M., Gomez-Pinilla,P.J. etal. 2017.
Abdominal vagus nerve stimulation as a new therapeutic ap-
proach to prevent postoperative ileus. Neurogastroenterol. Motil.
9:29.
76 Guyot, M., Simon, T., Panzolini, C. et al. 2019. Apical splenic
nerve electrical stimulation discloses an anti-inflammatory
pathway relying on adrenergic and nicotinic receptors in myeloid
cells. Brain Behav. Immun. 80:238.
77 Jo, B.G., Kim, S. H. and Namgung,U. 2020. Vagal afferent fi-
bers contribute to the anti-inflammatory reactions by vagus nerve
stimulation in concanavalin Amodel of hepatitis in rats. Mol. Med.
26:119.
78 Payne, S. C., Furness, J. B., Burns, O. et al. 2019. Anti-
inflammatory effects of abdominal vagus nerve stimulation on ex-
perimental intestinal inflammation. Front. Neurosci. 13:418.
79 Kees, M. G., Pongratz, G., Kees, F. et al. 2003. Via beta-
adrenoceptors, stimulation of extrasplenic sympathetic nerve
fibers inhibits lipopolysaccharide-induced TNF secretion in per-
fused rat spleen. J. Neuroimmunol. 145:77.
80 Buijs,R.M., vanderVliet,J., Garidou,M. L. etal. 2008. Spleen
vagal denervation inhibits the production of antibodies to circu-
lating antigens. PLoS One 3:e3152.
116 Peripheral nerve stimulation and immunity
Downloaded from https://academic.oup.com/intimm/article/34/2/107/6366295 by guest on 27 November 2022
81 Pasricha,T.S., Zhang,H., Zhang,N. etal. 2020. Sacral nerve
stimulation prompts vagally-mediated amelioration of rodent
colitis. Physiol. Rep. 8:e14294.
82 Tu,L., Gharibani, P., Zhang,N. etal. 2020. Anti-inflammatory
effects of sacral nerve stimulation: a novel spinal afferent and
vagal efferent pathway. Am. J. Physiol. Gastrointest. Liver
Physiol. 318:G624.
83 Guo, J., Jin, H., Shi, Z. et al. 2019. Sacral nerve stimula-
tion improves colonic inflammation mediated by autonomic-
inflammatory cytokine mechanism in rats. Neurogastroenterol.
Motil. 31:e13676.
84 Pasricha,T.S., Zhang,H., Zhang,N. etal. 2020. Sacral nerve
stimulation prompts vagally-mediated amelioration of rodent
colitis. Physiol. Rep. 8:e14294.
85 Torres-Rosas, R., Yehia,G., Peña,G. et al. 2014. Dopamine
mediates vagal modulation of the immune system by
electroacupuncture. Nat. Med. 20:291.
86 Song,J.G., Li,H.H., Cao,Y.F. etal. 2012. Electroacupuncture
improves survival in rats with lethal endotoxemia via the auto-
nomic nervous system. Anesthesiology 116:406.
87 Pavlov, V. A., Parrish, W. R., Rosas-Ballina, M. et al. 2009.
Brain acetylcholinesterase activity controls systemic cytokine
levels through the cholinergic anti-inflammatory pathway. Brain
Behav. Immun. 23:41.
88 Ji,H., Rabbi,M. F., Labis,B. et al. 2014. Central cholinergic
activation of a vagus nerve-to-spleen circuit alleviates experi-
mental colitis. Mucosal Immunol. 7:335.
89 Berthoud, H. R. and Neuhuber, W. L. 2000. Functional and
chemical anatomy of the afferent vagal system. Auton. Neurosci.
85:1.
90 Bassi,G.S., Brognara,F., Castania,J.A. etal. 2015. Baroreflex
activation in conscious rats modulates the joint inflammatory
response via sympathetic function. Brain Behav. Immun.
49:140.
91 Brognara,F., Castania,J.A., Dias,D.P.M. etal. 2018. Baroreflex
stimulation attenuates central but not peripheral inflammation in
conscious endotoxemic rats. Brain Res. 1682:54.
92 Chiluwal, A., Narayan, R. K., Chaung, W. et al. 2017.
Neuroprotective effects of trigeminal nerve stimulation in severe
traumatic brain injury. Sci. Rep. 7:6792.
93 Conde, S. V., Sacramento, J. F. and Martins, F. O. 2020.
Immunity and the carotid body: implications for metabolic dis-
eases. Bioelectron. Med. 6:24.
94 Porzionato,A., Macchi,V., Stecco,C. et al. 2019. The carotid
sinus nerve—structure, function, and clinical implications.
Anatom. Record 302:575.
95 Conde, S. V., Sacramento, J. F., Guarino,M. P. et al. 2014.
Carotid body, insulin, and metabolic diseases: unraveling the
links. Front. Physiol. 5:418.
96 Kumar,P. and Prabhakar,N.R. 2012. Peripheral chemorecep-
tors: function and plasticity of the carotid body. Compr. Physiol.
2:141.
97 Lohmeier, T. E. and Iliescu, R. 2015. The baroreflex as a
long-term controller of arterial pressure. Physiology (Bethesda)
30:148.
98 Conde, S. V., Sacramento, J. F. and Martins, F. O. 2020.
Immunity and the carotid body: implications for metabolic dis-
eases. Bioelectron. Med. 6:24.
99 Fernández, R., González, S., Rey, S. et al. 2008. Lipo-
polysaccharide-induced carotid body inflammation in cats:
functional manifestations, histopathology and involvement of
tumour necrosis factor-alpha. Exp. Physiol. 93:892.
100 Fan,J., Zhang,B., Shu,H.F. etal. 2009. Interleukin-6 increases
intracellular Ca2+ concentration and induces catecholamine
secretion in rat carotid body glomus cells. J. Neurosci. Res.
87:2757.
101 Shu,H.F., Wang,B.R., Wang,S.R. etal. 2007. IL-1beta inhibits
IK and increases [Ca2+]i in the carotid body glomus cells and
increases carotid sinus nerve firings in the rat. Eur. J.Neurosci.
25:3638.
102 Ackland,G.L., Kazymov,V., Marina,N. etal. 2013. Peripheral
neural detection of danger-associated and pathogen-
associated molecular patterns. Crit. Care Med. 41:e85.
103 Fernández, R., Nardocci, G., Simon, F. et al. 2011.
Lipopolysaccharide signaling in the carotid chemoreceptor
pathway of rats with sepsis syndrome. Respir. Physiol.
Neurobiol. 175:336.
104 Lam,S.Y., Tipoe,G.L., Liong,E.C. etal. 2008. Chronic hypoxia
upregulates the expression and function of proinflammatory
cytokines in the rat carotid body. Histochem. Cell Biol. 130:549.
105 Nardocci, G., Martin,A., Abarzúa,S. et al. 2015. Sepsis pro-
gression to multiple organ dysfunction in carotid chemo/
baro-denervated rats treated with lipopolysaccharide. J.
Neuroimmunol. 278:44.
106 Ribeiro,A. B., Brognara,F., daSilva,J. F. etal. 2020. Carotid
sinus nerve stimulation attenuates alveolar bone loss and in-
flammation in experimental periodontitis. Sci. Rep. 10:19258.
107 Santos-Almeida, F.M., Domingos-Souza, G., Meschiari, C.A.
et al. 2017. Carotid sinus nerve electrical stimulation in con-
scious rats attenuates systemic inflammation via chemo-
receptor activation. Sci. Rep. 7:6265.
108 Falvey,A., Duprat,F., Simon,T. et al. 2020. Electrostimulation
of the carotid sinus nerve in mice attenuates inflamma-
tion via glucocorticoid receptor on myeloid immune cells. J.
Neuroinflammation 17:368.
109 Pavlov,V. A. and Tracey,K.J. 2019. Bioelectronic medicine:
updates, challenges and paths forward. Bioelectron. Med. 5:1.
110 Levine,Y.A., Faltys,M. and Chernoff,D. 2020. Harnessing the
inflammatory reflex for the treatment of inflammation-mediated
diseases. Cold Spring Harb. Perspect. Med. 1:10.
111 Bonaz, B., Picq, C., Sinniger, V. et al. 2013. Vagus nerve
stimulation: from epilepsy to the cholinergic anti-inflammatory
pathway. Neurogastroenterol. Motility. 25:208.
112 Schulz,S.A., Wöhler,A., Beutner,D. etal. 2016. Microsurgical
anatomy of the human carotid body (glomus caroticum): fea-
tures of its detailed topography, syntopy and morphology. Ann.
Anat. 204:106.
113 Musk, E.; Neuralink. 2019. An integrated brain-machine inter-
face platform with thousands of channels. J. Med. Internet Res.
21:e16194.
114 Inoue,T., Abe,C., Sung,S.S. etal. 2016. Vagus nerve stimu-
lation mediates protection from kidney ischemia-reperfusion in-
jury through α7nAChR+ splenocytes. J. Clin. Invest. 126:1939.
115 Gaykema, R. P., Dijkstra, I. and Tilders, F. J. 1995.
Subdiaphragmatic vagotomy suppresses endotoxin-induced
activation of hypothalamic corticotropin-releasing hormone
neurons and ACTH secretion. Endocrinology 136:4717.
116 Kwan,H., Garzoni,L., Liu,H.L. etal. 2016. Vagus nerve stimu-
lation for treatment of inflammation: systematic review of animal
models and clinical studies. Bioelectron. Med. 3:1.
117 Tsaava, T., Datta-Chaudhuri, T., Addorisio, M. E. et al. 2020.
Specific vagus nerve stimulation parameters alter serum cyto-
kine levels in the absence of inflammation. Bioelectron. Med. 6:8.
118 Krahl,S.E. 2012. Vagus nerve stimulation for epilepsy: a review
of the peripheral mechanisms. Surg. Neurol. Int. 3:S47.
119 Yuan,H. and Silberstein,S.D. 2016. Vagus nerve and vagus nerve
stimulation, a comprehensive review: part III. Headache 56:479.
120 Stakenborg, N., Gomez-Pinilla, P.J., Verlinden,T.J. M. et al.
2020. Comparison between the cervical and abdominal vagus
nerves in mice, pigs, and humans. Neurogastroenterol. Motil.
32:e13889.
121 Bonaz,B. 2020. Parameters matter: modulating cytokines using
nerve stimulation. Bioelectron. Med. 6:12.
122 Chang,Y.C., Cracchiolo,M., Ahmed,U. etal. 2020. Quantitative
estimation of nerve fiber engagement by vagus nerve stimula-
tion using physiological markers. Brain Stimul. 13:1617.
123 Gupta,I., Cassará,A.M., Tarotin,I. etal. 2020. Quantification of
clinically applicable stimulation parameters for precision near-
organ neuromodulation of human splenic nerves. Commun.
Biol. 3:577.
124 Zachs,D.P., Offutt,S.J., Graham,R.S. etal. 2019. Noninvasive
ultrasound stimulation of the spleen to treat inflammatory arth-
ritis. Nat. Commun. 10:951.
125 DeLucena,D.D. and Rangel,É.B. 2018. Glucocorticoids use
in kidney transplant setting. Expert Opin. Drug Metab. Toxicol.
14:1023.
Peripheral nerve stimulation and immunity 11 7
Downloaded from https://academic.oup.com/intimm/article/34/2/107/6366295 by guest on 27 November 2022
126 Czock,D., Keller,F., Rasche,F.M. etal. 2005. Pharmacokinetics
and pharmacodynamics of systemically administered gluco-
corticoids. Clin. Pharmacokinet. 44:61.
127 Paragliola,R.M., Papi,G., Pontecorvi,A. etal. 2017. Treatment
with synthetic glucocorticoids and the hypothalamus-pituitary-
adrenal axis. Int. J.Mol. Sci. 10:18.
128 Cho,Y., Park, J., Lee,C. et al. 2020. Recent progress on per-
ipheral neural interface technology towards bioelectronic medi-
cine. Bioelectron. Med. 6:23.
129 Lehner,K. R., Silverman, H. A., Addorisio, M. E. et al. 2019.
Forebrain cholinergic signaling regulates innate immune re-
sponses and inflammation. Front. Immunol. 10:585.
130 Booth, L. C., Yao, S. T., Korsak, A. et al. 2021. Selective
optogenetic stimulation of efferent fibers in the vagus nerve of a
large mammal. Brain Stimul. 14:88.
131 Cotero, V., Fan, Y., Tsaava, T. et al. 2019. Noninvasive sub-
organ ultrasound stimulation for targeted neuromodulation. Nat.
Commun. 10:952.
118 Peripheral nerve stimulation and immunity
Downloaded from https://academic.oup.com/intimm/article/34/2/107/6366295 by guest on 27 November 2022
