The
complement cascade is a principal component of innate immunity. Recent studies have underscored the importance of C5a and other components of the
complement system 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 was accompanied by C5a upregulation and was markedly reduced by
C5a receptor (C5aR1) knock-out or treatment with the C5aR1 antagonist PMX53. Direct administration of C5a into the mouse hindpaw produced strong
thermal hyperalgesia, an effect that was absent in TRPV1 knock-out 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 Ca(2+) mobilization in macrophages. Macrophage depletion in transgenic macrophage Fas-induced apoptosis mice abolished C5a-dependent
thermal hyperalgesia. Examination of inflammatory mediators following C5a injection revealed a rapid upregulation of
NGF, a mediator known to sensitize TRPV1. Preinjection 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
complement fragment C5a induces
thermal 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.
SIGNIFICANCE STATEMENT: This study provides mechanistic insight into how the
complement system, a key component of innate immunity, regulates the development of
pain hypersensitivity. We demonstrate a crucial role of the
C5a receptor, C5aR1, in the development of inflammatory thermal and mechanical sensitization. By focusing on the mechanisms of C5a-induced
thermal hyperalgesia, we show that this process requires recruitment of macrophages and initiation of macrophage-to-nociceptor signaling. At the molecular level, we demonstrate that this signaling depends on
NGF and is mediated by the heat-sensitive nociceptive channel TRPV1. This deeper understanding of how immune cells and neurons interact to regulate
pain processing is expected to facilitate mechanism-based approaches in the development of new
analgesics.