Date of Degree
PhD (Doctor of Philosophy)
Usachev, Yuriy M.
First Committee Member
Second Committee Member
Sluka, Kathleen A.
Third Committee Member
Hammond, Donna L.
Fourth Committee Member
Brennan, Tim J.
The complement system is a principal component of innate immunity. Recent studies have underscored the importance of C5a and other complement components in inflammatory and neuropathic pain, although the underlying mechanisms are largely unknown. In particular, it is unclear how the complement system communicates with nociceptors and which ion channels and receptors are involved. Here we demonstrate that inflammatory thermal and mechanical hyperalgesia induced by complete Freund’s adjuvant were accompanied by C5a upregulation and were markedly reduced by C5a receptor (C5aR1) knockout (KO) or treatment with the C5aR1 antagonist PMX53. Direct administration of C5a into the mouse hindpaw produced strong thermal and mechanical hyperalgesia, an effect that was absent in TRPV1 KO mice, and was blocked by the TRPV1 antagonist AMG9810. Immunohistochemistry of mouse plantar skin showed prominent expression of C5aR1 in macrophages. Additionally, C5a evoked strong Ca2+ mobilization in macrophages. Macrophage depletion in transgenic macrophage Fas-induced apoptosis (MAFIA) mice abolished C5a-dependent thermal and mechanical hyperalgesia. Examination of inflammatory mediators following C5a injection revealed a rapid upregulation of nerve growth factor (NGF), a mediator known to sensitize TRPV1. Pre-injection of an NGF-neutralizing antibody or Trk inhibitor GNF-5837 prevented C5a-induced thermal hyperalgesia. Notably, NGF-induced thermal hyperalgesia was unaffected by macrophage depletion. Collectively, these results suggest that C5a induces thermal and mechanical hyperalgesia by triggering macrophage-dependent signaling that involves mobilization of NGF and NGF-dependent sensitization of TRPV1. Our findings highlight the importance of macrophage-to-neuron signaling in pain processing and identify C5a, NGF and TRPV1 as key players in this cross-cellular communication.
Chronic pain conditions are a life-long and debilitating set of diseases which are severely undertreated despite a prevalence of 1 in 3 Americans. Examples include rheumatoid arthritis, neuropathic pain, chronic low back pain, fibromyalgia, cancer pain, and many others. Chronic pain is difficult to treat due to, in part, the diversity of chronic pain conditions and not understanding the physiology that promotes chronic pain. Understanding the basic mechanisms by which chronic pain is produced and maintained will help develop better treatments for chronic pain. Here, I examine how a part of the immune system called the complement system promotes pain. Using a model of inflammatory pain in mice, I found that reducing the activity of the complement system reduced pain related behaviors. I further determined that the complement system produced pain by activating a type of immune cell, called macrophages. When activated by the complement system, macrophages released inflammatory molecules that sensitized the nerves responsible for sensing pain, resulting in enhanced pain behaviors. As there are different causes of chronic pain, I found that blocking the complement system in a model of nerve damage-induced pain also reduced pain behaviors. This suggests that the complement system contributes to multiple types of chronic pain and could be exploited as for pain relief. In conclusion, the activity of the complement system enhances pain sensitivity in both inflammatory and nerve damage-induced models of pain, and its blockade promotes pain relief from these chronic pain conditions.
C5a, Complement, Inflammation, Macrophage, Pain, TRPV1
xvii, 158 pages
Includes bibliographical references (pages 148-158).
Copyright © 2017 Charles A. Warwick
Warwick, Charles A.. "The role of complement component C5a in nociceptive sensitization." PhD (Doctor of Philosophy) thesis, University of Iowa, 2017.