Cytokines, pain and regulation of inflammation

Abstract

A mechanism termed the “inflammatory reflex” has been proposed as the basis for autonomic regulation of immune function. The efferent arm of this reflex – the neural-to-immune link – is thought to be the “cholinergic anti-inflammatory pathway”. According to this concept, an immune challenge is relayed by afferent nerves and/or by humoral signals to the brain, whereupon the brain drives efferent nerve fibres in the vagus that act ultimately to prevent the damaging consequences of excessive inflammation. The cholinergic anti-inflammatory pathway has been extensively studied in vivo and in vitro in terms of its protective effects against a wide range of inflammation-related diseases and immunomodulating functions. However, whether the in vivo anti-inflammatory effect of the cholinergic system can be mirrored and modelled by in vitro culture systems remains controversial and needs to be readdressed.

The reported inhibitory effect of nicotine and the selective cholinergic agonist, AR-R17779, on the disease severity in murine collagen-induced arthritis could not be confirmed. Moreover, in a systematic in vitro study with LPS-stimulated murine macrophage populations and human monocyte/macrophage populations, it was found that nicotine did not reduce tumour necrosis factor (TNF) release even though in vivo it inhibited such expression in splenic macrophages. This suggests caution should be taken in the use of these particular in vitro systems as surrogate assays to study how the cholinergic pathway suppresses systemic inflammation.

Inflammatory pain is multifaceted. Granulocyte macrophage-colony stimulating factor (GM-CSF) and TNF are potential drug targets for inflammatory and pain-related disorders based on the successful translation of animal data into clinical outcomes. In this thesis, TNF and GM-CSF were found to induce pain in an inflammatory footpad model in mice. It was found that neutrophil infiltration was required for TNF- and GM-CSF-induced pain. Colony stimulating factor-1 receptor signaling in macrophages was also required for inflammatory pain. In addition, TNF- and GM-CSF-induced pain was found to be abrogated in B6-KitW-sh/W-sh mice suggesting mast cell involvement. Ion channels, such as TRPV1, TRPA1, NaV1.7, and the neuropeptides, substance P and calcitonin gene-related peptide, were also found to be required for TNF- and GM-CSF-induced inflammatory pain development. The requirement for similar cell types and ion channels suggests that the pathways leading to TNF- and GM-CSF-driven pain might be similar and that TNF and GM-CSF may, in fact, be linked in this system.

Utilizing GM-CSF-/- mice and an anti-GM-CSF mAb-based strategy, it was found that GM-CSF is required for TNF-induced pain. Moreover, treatment with anti-TNF mAb abolished GM-CSF-induced pain, indicating that TNF and GM-CSF are interdependent likely causing an amplification of the inflammatory response. Mice that lacked CCL17 were protected from inflammatory pain evoked by TNF and GM-CSF; CCL17 itself was able to induce inflammatory pain which remained unaffected in TNF-/- and GM-CSF-/- mice. These data indicate that CCL17 is a novel key downstream mediator of TNF and GM-CSF in these cytokine-driven inflammatory pain models. The pathway(s) linking these cytokines which ultimately leads to pain induction was found to be COX-2 dependent.