Neurogenic inflammation and the peripheral nervous system in host defense and immunopathology

Abstract

The peripheral nervous and immune systems are traditionally thought of as serving separate functions. The line between them is, however, becoming increasingly blurred by new insights into neurogenic inflammation. Nociceptor neurons possess many of the same molecular recognition pathways for danger as immune cells, and, in response to danger, the peripheral nervous system directly communicates with the immune system, forming an integrated protective mechanism. The dense innervation network of sensory and autonomic fibers in peripheral tissues and high speed of neural transduction allows rapid local and systemic neurogenic modulation of immunity. Peripheral neurons also seem to contribute to immune dysfunction in autoimmune and allergic diseases. Therefore, understanding the coordinated interaction of peripheral neurons with immune cells may advance therapeutic approaches to increase host defense and suppress immunopathology.

Figure 1

Noxious stimuli, microbial and inflammatory recognition pathways trigger activation of the peripheral nervous system. Sensory neurons possess several means of detecting the presence of noxious/harmful stimuli. 1) Danger signal receptors, including TRP channels, P2X channels, and danger associated molecular pattern (DAMP) receptors recognize exogenous signals from the environment (e.g. heat, acidity, chemicals) or endogenous danger signals released during trauma/tissue injury (e.g. ATP, uric acid, hydroxynonenals). 2) Pattern recognition receptors (PRRs) such as Toll-like receptors (TLRs) and Nod-like receptors (NLRs) recognize Pathogen associated molecular patterns (PAMPs) shed by invading bacteria or viruses during infection. 3) Cytokine receptors recognize factors secreted by immune cells (e.g. IL-1beta, TNF-alpha, NGF), which activate map kinases and other signaling mechanisms to increase membrane excitability.

Figure 2

Neuronal factors released from nociceptor sensory neurons directly drive leukocyte chemotaxis, vascular hemodynamics and the immune response. When noxious stimuli activate afferent signals in sensory nerves, antidromic axon reflexes are generated that induce the release of neuropeptides at the peripheral terminals of the neurons. These molecular mediators have several inflammatory actions: 1) Chemotaxis and activation of neutrophils, macrophages and lymphocytes to the site of injury, and degranulation of mast cells. 2) Signaling to vascular endothelial cells to increase blood flow, vascular leakage and edema. This also allows easier recruitment of inflammatory leukocytes. 3) Priming of dendritic cells to drive subsequent T helper cell differentiation into Th2 or Th17 subtypes.

Figure 3

Timeline of advances in understanding of the neurogenic aspects of inflammation from Celsus to the present day.

Figure 4

Sensory and autonomic nervous systems modulate local and systemic immune responses respectively. Nociceptors innervating epithelial surfaces (e.g. skin and lung) induce localized inflammatory responses, activating mast cells and dendritic cells. In allergic airway inflammation, dermatitis and rheumatoid arthritis, nociceptor neurons play a role in driving inflammation. By contrast, autonomic circuits innervating the visceral organs (e.g. spleen and liver) regulate systemic immune responses by blocking macrophage and NKT cell activation. In stroke and septic endotoxemia, these neurons play an immunosuppressive role.