Switching Rat Resident Macrophages from M1 to M2 Phenotype by Iba1 Silencing Has Analgesic Effects in SNL-Induced Neuropathic Pain

Resident macrophages from dorsal root ganglia are important for the development of traumatic-induced neuropathic pain. In the first 5–7 days after a traumatic sciatic nerve injury (i.e., spinal nerve ligation (SNL), spared nerve injury (SNI), sciatic nerve transection or sciatic nerve ligation and transection), Ionized binding adapter protein 1 (Iba1) (+) resident macrophages cluster around dorsal root ganglia neurons, possibly contributing to nerve injury-induced hypersensitivity. Since infiltrating macrophages gradually recruited to the lesion site peak at about 7 days, the first few days post-lesion offer a window of opportunity when the contribution of Iba1 (+) resident macrophages to neuropathic pain pathogenesis could be investigated. Iba1 is an actin cross-linking cytoskeleton protein, specifically located only in macrophages and microglia. In this study, we explored the contribution of rat Iba1 (+) macrophages in SNL-induced neuropathic pain by using intra-ganglionic injections of naked Iba1-siRNA, delivered at the time the lesion occurred. The results show that 5 days after Iba1 silencing, Iba1 (+) resident macrophages are switched from an M1 (pro-inflammatory) phenotype to an M2 (anti-inflammatory) phenotype, which was confirmed by a significant decrease of M1 markers (CD32 and CD86), a significant increase of M2 markers (CD163 and Arginase-1), a reduced secretion of pro-inflammatory cytokines (IL-6, TNF-α and IL-1β) and an increased release of pro-regenerative factors (BDNF, NGF and NT-3) which initiated the regrowth of adult DRG neurites and reduced SNL-induced neuropathic pain. Our data show for the first time, that it is possible to induce macrophages towards an anti-inflammatory phenotype by interacting with their cytoskeleton.