Understanding inflammation requires neuroscience

Abstract

Inflammation and its resolution are processes subject to neural regulation (1). The best-characterized immune-regulating reflex is the “inflammatory reflex”, in which the efferent branch of the vagus nerve plays a central role in regulating cytokine-release in the periphery. This neural pathway is fundamental to maintaining host homeostasis and preventing potentially damaging inflammation (1–3). Hence, this biology has already been exploited in several clinical trials regarding potentially new therapies for chronic inflammatory diseases (4–6). Of note, cardiovascular disease (CVD) represents the first cause of death worldwide, and its most common manifestation is atherosclerosis which is an inflammatory disease (7,8). Little is known about the neural control of inflammation in this pathology. Atherosclerotic plaques are not innervated (9), and neurotransmitter signaling in atherosclerosis has not been investigated. The 1998 Nobel Prize for “nitric oxide (NO) as a signaling molecule in the cardiovascular system” reveals the importance of the neurotransmitter acetylcholine (ACh) for regulation of vascular relaxation (10). ACh is also a key component of the inflammatory reflex, in which cholinergic signals regulate the course of inflammation. In the inflammatory reflex, ACh is produced by nerves and by acetyl-cholinesterase (ChAT)+ T cell under the control of the nervous system, and interacts with alpha 7 nicotinic acetylcholine receptor subunit (α7nAChR)-expressing macrophages (MΦ) (11).

Most of the current knowledge on the inflammatory reflex was obtained from numerous experiments performed in animals, mice in primis. Much is still unclear about the details of neural regulation of inflammation and its resolution, and understanding these mechanisms in detail will require further experimental studies. At the same time, it is also important to translate these findings to human pathophysiology, and investigate whether it may inform the design of therapeutic strategies for treatment of inflammatory diseases.

This thesis addresses several aspects of this biology. In Project I, we discovered that human ChAT+ T cells participate in cholinergic regulation of vascular function and are found in blood collected from patients in circulatory distress. In Project II, we found components of neurotransmitter signaling in human atherosclerosis, observed an association between low glutamate-receptor expression and adverse clinical events, and found that glutamate signaling regulates smooth muscle cell phenotypic modulation. In Project III, we describe an effective and simple method to electrically stimulate the cervical vagus nerve in mice for the study of experimental inflammation. In Project IV, we provide evidence that electrical activation of the cervical vagus nerve accelerates inflammation resolution in mice through a cholinergic mechanism that involves synthesis of specialized pro-resolving mediators. Technological limitations in vagus nerve stimulation methods for mice has hampered mechanistic studies of peripheral nerve activation in chronic diseases. Hence, the understanding of mechanisms of vagus nerve regulation of inflammation in chronic diseases is yet incomplete. To solve this, in Project V we used a novel approach and developed noninvasive activation of peripheral nerves using temporally-interfering electrical fields. This technology attempts to address methodological shortcoming of “traditional” electrical vagus nerve stimulation (VNS) and enable studies of VNS in genetic mouse models of chronic inflammatory diseases and beyond.

In summary, this thesis studies aspects of neural signaling in inflammation and reveal new details on glutamatergic and cholinergic signals in inflammation and vascular pathophysiology. The work also contributes new methodology which we postulate will be helpful in further understanding of the neural signals that regulate inflammation and for clinical translation of discoveries in this field.