Inflammation is a protective and complex biological response to harmful stimuli, such as infection and tissue damage, that involves cell infiltration and production of inflammatory cytokines. Rheumatoid Arthritis (RA) is the most common type of inflammatory autoimmune disease affecting over 1.5 million adult Americans. It is characterized by chronic inflammation of synovial tissues, swelling, stiffness, and loss of joint functions due to excessive leukocyte infiltration, mainly neutrophils, and local cytokine production, such as tumor necrosis factor (TNF) [1, 2]. RA represents a typical example of the harmful effects of excessive inflammatory response in joints and cartilage due uncontrollable or chronic cytokine production.
In addition, RA also increases the risk of major comorbid conditions, including pericardial and pleural effusion, pleuritis, heart diseases, diabetes and pulmonary infections . There is no cure for RA and the actual best clinical treatments are based on the use of the new biological disease-modifying anti-rheumatic drugs (DMARDs) acting by neutralizing TNF and neutrophil activity [2, 4–7]. However these new treatments are expensive and may cause immunosuppression, infections, malignancies and organ dysfunctions when used chronically. Thus, actual clinical and experimental studies focus on local control of inflammatory cytokines production to avoid potential side effects of current RA therapies.
Several studies highlighted the beneficial effects of the autonomic nervous system to control inflammation [8–11]. In fact, studies showed that parasympathetic stimulation, mainly through vagus nerve stimulation (VNS), controlled systemic and local articular inflammation by reducing neutrophil infiltration and TNF production [12–18], allowing novel therapeutic strategies for infectious and inflammatory diseases treatment and highlighting VNS as a potential treatment for joint inflammatory disorders [19, 20]. As an advantage to conventional pharmacological anti-inflammatory strategies in RA, the activation of neuronal networks may regulate the inflammatory response in loco, without unspecific systemic side effects and inducing fast and controllable temporal immunomodulatory effects making possible a real time control of the immune response and a quick reversible effect, if needed.
Anatomical studies have shown that the vagal stimulation inhibits TNF production through several neuroimmune pathways. In the first one, VNS activates splenic sympathetic terminals via a peripheral vagal-splenic neural connection to inhibit TNF production and release by splenic macrophages (e.g: the cholinergic anti-inflammatory pathway) [21, 22]; in the second one, vagal stimulation induces the production of dopamine in the adrenal medulla . However, other studies suggested a third mechanism in which vagal stimulation can inhibit peripheral inflammation through unknown afferent neural pathways, indicating the involvement of central nervous system structures in inflammation control [24–30].
In fact, our group previously showed that stimulation of the aortic depressor nerve (an exclusively afferent nerve) attenuated joint inflammation in experimental arthritis independently on the integrity of classical neuroimmune structures (e.g: spleen, celiac vagus, or adrenal glands), but by the inhibition of the sympathetic drive to the knee, suggesting a central neural arc . In the present thesis, we aim to dissect the neural components of this new central inflammatory reflex.
In the present study, we electrically stimulated the vagus nerve of Wistar rats and assessed joint inflammatory process induced by intra-articular administration of zymosan (100 µg/50µL). We report that a low-intensity, subtle, afferent VNS (5Hz, 0.1 ms, 1V) decreased knee inflammation independently of the integrity of classical peripheral immunomodulatory structures (e.g. spleen, celiac vagus, adrenal glands or acetylcholine-producing lymphocytes) and does not induce cardiovascular alterations, showing the existence of a new neuroimmune pathway.
In fact, VNS effect was inhibited by the systemic administration of propranolol (a non-selective beta-blocker), suggesting that vagal effects could be dependent on the sympathetic system integrity. In fact, vagal stimulation reduced rat tail temperature similarly to the observed during sympathetic chain stimulation and increased norepinephrine (NE) levels only in sympathetically innervated joints. VNS anti-inflammatory effect was abrogated by surgical or local chemical sympathectomy, but intra-articular NE administration replicated VNS results. These data strongly indicate the sympathetic system activation is the main efferent arm for the immunomodulatory vagus nerve effect and the existence of a central neuroimmune axis.
To investigate the existence of potential immunomodulatory central structures, we performed VNS and analyzed the c-Fos expression (a neuronal activity marker) in brain and spinal cord. We found that low-intensity VNS increased c-Fos expression in several sympathomodulatory central nervous system components, as the nucleus of the solitary tract, locus coeruleus (LC), and the hypothalamic paraventricular nucleus (PVN) in the brain, and in neurons of the intermedio-lateral column of the spinal cord. Of note, electrical stimulation of either LC or PVN attenuated joint inflammatory response, suggesting these brain nuclei as potential inflammatory processing brain centers. In addition, VNS anti-inflammatory effect was suppressed only when the LC, but not the PVN, was lesioned, suggesting a central anti-inflammatory neuronal network.
Figure 1. Neuroanatomical description of the central inflammatory reflex. Afferent VNS excites NTS projections to the PVN and LC increasing the activity of these nuclei. The neural information is then funnelled into the LC that, in turn, directly stimulates ipsilateral spinal cord pre-ganglionic neurons to increase the release of norepinephrine into the ipsilateral knee joint and reducing articular inflammation.
Of note, this effect was only observed in the ipsilateral, but not in the contralateral, knee joint to the hemispheric LC lesion, suggesting the existence of a unilateral neuroimmune pathway connecting the LC and spinal cord preganglionic neurons. From a pharmacological point-of-view, intra-articular β1 or β2-adrenergic antagonists inhibited VNS effect and intra-articular administration of β1 or β2-adrenergic agonists decreased joint inflammation. At last, vagal stimulation reduced synovial ICAM-1 expression and this effect was replicated by intra-articular NE, suggesting local mechanisms for inflammation control.
The present study provides, for the first time, the comprehensive dissection of a new central neuroimmune anti-inflammatory pathway describing autonomous inflammatory processing brain centers that are also activated through subtle afferent vagal stimulation (Figure 1). Our data support two current theories related to the mind-body interaction: the existence of the immunological homunculus with a direct role in brain-immune responses  and the polyvagal theory where different vagal subsystems are critical to control behavior and emotions . We do not rule out the possibility that both complex systems could act in synchrony to influence immune response-directed behaviors.
Since the central immunomodulatory information apparently flows through established brain networks, our results reveal a novel neuro-immune brain map with afferent vagal signals controlling side-specific articular inflammation through specific inflammatory-processing brain centers and joint sympathetic innervations.
Author’s Current Affiliation:
Department of Clinical and Experimental Medicine, Translational Research Center for Gastrointestinal Disorders – TARGID, Center of Neuroimmune interaction and Mucosal Immunology, Katholieke Universiteit, KU Leuven, Belgium; Email: firstname.lastname@example.org
Note: PhD thesis completed between 2012 and 2016 at the Department of Immunology, Ribeirão Preto Medical School, University of São Paulo, Ribeirão Preto, SP, Brazil.
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