C5aR1 signaling in sensory neuron-associated macrophages drives neuropathic pain

Technical Summary: C5aR1 Signaling in Sensory Neuron-Associated Macrophages Drives Neuropathic Pain

Problem Statement
Neuropathic pain, a chronic condition resulting from somatosensory system injury, remains difficult to treat due to incomplete understanding of its pathophysiological mechanisms. While neuroimmune interactions, particularly involving the complement system and the C5a/C5aR1 axis, are known to contribute to pain development, the specific cellular loci and molecular pathways through which C5aR1 signaling induces neuropathic pain following peripheral nerve injury have not been fully elucidated. Previous research has implicated C5aR1 in various pain states, but the relative contributions of C5aR1 signaling in different cell types (e.g., infiltrating leukocytes at the injury site, spinal microglia, primary nociceptive neurons, and sensory ganglia macrophages) remain unclear.

Methodology
The study employed a multi-faceted approach combining genetic, pharmacological, and behavioral techniques in murine models, alongside translational analysis of human tissue:

• Animal Models: The study utilized C5ar1 knockout mice (C5ar1−/−) and wild-type (WT) controls. To dissect cell-specific roles, the authors generated and used four complementary conditional knockout strains:
  • Lyz2-Cre × C5ar1^flox/flox (targeting resident macrophages, sparing microglia).
  • Cx3cr1-CreER × C5ar1^flox/flox (targeting both microglia and sensory neuron-associated macrophages, sNAMs).
  • Tmem119-CreER × C5ar1^flox/flox (specifically targeting microglia).
  • Scn10a-Cre (Nav1.8) × C5ar1^flox/flox (targeting nociceptive neurons).
• Pain Model: Peripheral nerve injury was induced via partial sciatic nerve ligation (PSNL).
• Pharmacology: A selective allosteric C5aR1 antagonist (DF2593A) was administered either systemically (oral) or intrathecally (i.t.) to assess therapeutic potential and site-specific effects. Recombinant C5a was injected intrathecally to induce pain hypersensitivity.
• Behavioral Assays: Mechanical (von Frey, electronic von Frey), thermal (Hargreaves, hot plate), and cold (acetone) sensitivity were measured. Motor function was assessed via rotarod.
• Molecular and Cellular Analysis:
  • Gene Expression: qPCR and RT-PCR were used to quantify C5ar1, cytokine (Il1b, Tnf), and marker (Aif1, Gfap) expression in dorsal root ganglia (DRG) and sciatic nerves.
  • Immunofluorescence & Flow Cytometry: Confocal microscopy and flow cytometry were used to identify cell types expressing C5aR1 (co-staining with Iba-1, CX3CR1, Ly6G, Kir4.1) and to quantify leukocyte infiltration.
  • Transcriptomics: Re-analysis of public single-cell RNA sequencing (scRNA-seq) data from mouse DRG and single-nucleus RNA sequencing (snRNA-seq) data from human DRG was performed to map C5AR1 expression.
  • Human Tissue: PCR and immunofluorescence were performed on human DRG samples from organ donors.

Key Results

1. Requirement of C5aR1 for Neuropathic Pain: C5ar1−/− mice (both male and female) exhibited significantly reduced mechanical and cold hypersensitivity following PSNL compared to WT mice. Baseline nociception and motor performance were unaffected in knockout mice. Pharmacological blockade of C5aR1 with DF2593A (both prophylactic and therapeutic) significantly reduced pain hypersensitivity in WT mice.
2. Cellular Localization of C5aR1:
   • Injury Site: While C5aR1 was expressed by infiltrating monocytes/macrophages at the site of nerve injury, its signaling was not required for leukocyte recruitment or the production of pro-nociceptive cytokines (TNF, IL-1β, IL-6) at this site. C5ar1−/− mice showed no difference in immune cell infiltration or cytokine levels at the injury site compared to WT.
   • Sensory Ganglia (DRG): C5aR1 expression was robustly upregulated in the DRG post-injury. scRNA-seq and immunofluorescence confirmed that C5aR1 is exclusively expressed in sensory neuron-associated macrophages (sNAMs) (Iba-1+ and CX3CR1+), and not in satellite glial cells, nociceptive neurons, or other DRG cell types. This expression pattern was conserved in human DRG tissues.
3. Cell-Specific Mechanisms:
   • Conditional deletion of C5ar1 in sNAMs (using Lyz2-Cre and Cx3cr1-CreER lines) recapitulated the pain-resistant phenotype of global knockouts.
   • Conditional deletion of C5ar1 specifically in microglia (Tmem119-CreER) or nociceptive neurons (Scn10a-Cre) did not alter the development of neuropathic pain, indicating these cells are not the primary drivers in this context.
4. Molecular Mediator (IL-1β):
   • In the DRG, PSNL-induced upregulation of Il1b was significantly reduced in C5ar1−/− mice and in mice with sNAM-specific C5ar1 deletion. Expression of Tnf and glial activation markers (Gfap, Aif1) remained unchanged.
   • Intrathecal injection of recombinant C5a induced mechanical and cold hypersensitivity and increased Il1b expression in the DRG. This effect was abolished in C5ar1−/− mice and sNAM-specific knockouts.
   • The pronociceptive effect of intrathecal C5a was significantly reduced in Il1r1 null (Il1r−/−) mice, confirming that C5aR1 signaling in sNAMs drives pain via IL-1β production.

Significance and Claims
The authors claim to have identified a previously undetermined mechanism driving neuropathic pain: the activation of C5aR1 specifically in sensory neuron-associated macrophages (sNAMs) within the sensory ganglia. The study establishes that:

• C5a/C5aR1 signaling in sNAMs, rather than in infiltrating macrophages at the injury site, spinal microglia, or primary neurons, is the critical driver of pain hypersensitivity following peripheral nerve injury.
• This signaling axis functions by stimulating the production of the pro-nociceptive cytokine IL-1β within the DRG.
• The translational relevance is supported by the confirmation of C5AR1 expression in human sNAMs.
• Consequently, the C5a/C5aR1 axis in sNAMs represents a promising and specific therapeutic target for the treatment of neuropathic pain, distinct from targets acting on the peripheral injury site or spinal microglia.

The paper concludes that inhibiting this specific neuroimmune interaction could offer a novel strategy for managing neuropathic pain, a condition currently refractory to many existing treatments.