Acute or chronic depletion of macrophages in the dorsal root ganglia induces neuropathic pain after unilateral cervical spinal cord injury in rats

The inflammatory response at the spinal cord injury (SCI) epicenter and heightened macrophage presence in the dorsal root ganglia (DRG) has been well characterized after SCI and correlates with neuropathic pain. CCL2, a chemokine that acts as a macrophage chemoattractant and neuromodulator, is implicated in pain development, however, the role of the CCL2-CCR2 axis in the development of pain after SCI has not been explored. Here, we examined the role of CCL2-CCR2 signaling in macrophage recruitment to the DRG as well as the prolonged presence of macrophages in the DRG on the development and persistence of pain after SCI. Adult female Sprague-Dawley rats received a moderate, unilateral C5 contusion. Sandwich ELISA revealed that CCL2 is upregulated in the ipsilesional C7 and C8 DRGs in the first 24 hours post injury (hpi) and returns to naive levels by 72 hpi. To prevent monocyte-derived macrophage recruitment to the DRG, additional SCI rats received vehicle or INCB3344, a CCR2 antagonist, intravenously at the time of SCI and at 24 and 48 hpi. INCB3344 administration induced transient forepaw allodynia at 7dpi in nearly all rats (88%) compared to only 33% in vehicle controls that resolves partially by 28 dpi, as measured by von Frey and mechanical conflict avoidance paradigms. As expected, qPCR analyses of whole DRG revealed that INCB3344 reduced macrophage markers and inflammatory cytokines in the ipsilesional C7 and C8 DRGs at 7 dpi compared to vehicle treated rats. By 28 dpi, there were no significant differences between INCB3344 or vehicle-treated groups, indicating that SCI-induced macrophage presence in the DRG is delayed by INCB3344 treatment. Moreover, gene expression of markers of macrophage polarity and cytokines suggest a pro-inflammatory environment in the DRG at 28dpi. DRG macrophage ablation via liposomal clodronate at 21dpi did not ameliorate hypersensitive pain behavior, though their ablation did reduce paw withdrawal thresholds in SCI rats that did not previously demonstrate pain behavior. Collectively, these data suggest that driving macrophages to a pro-reparative phenotype may be a viable and effective analgesic strategy that acts by modulating both the immune response and the experience of pain.