Relationship of Inflammatory Cytokines From M1-Type Microglia/Macrophages at the Injured Site and Lumbar Enlargement With Neuropathic Pain After Spinal Cord Injury in the CCL21 Knockout (plt) Mouse

Abstract

Spinal cord injury (SCI) causes loss of normal sensation and often leads to debilitating neuropathic pain (NeP). Chronic NeP develops at or below the SCI lesion in as many as 80% of patients with SCI and may be induced by modulators of neuronal excitability released from activated microglia and macrophages. In the inflammatory response after SCI, different microglia/macrophage populations that are classically activated (M1 phenotype) or alternatively activated (M2 phenotype) have become of great interest. Chemokines have also recently attracted attention in neuron-microglia communication. CCL21 is a chemokine that activates microglia in the central nervous system (CNS) and is expressed only in neurons with an insult or mechanical injury. In this study using an SCI model in mutant (plt) mice with deficient CCL21 expression, we assessed post-SCI NeP and expression of microglia/macrophages and inflammatory cytokines at the injured site and lumbar enlargement. SCI-induced hypersensitivities to mechanical and thermal stimulation were relieved in plt mice compared with those in wild-type (C57BL/6) mice, although there was no difference in motor function. Immunohistochemistry and flow cytometry analysis showed that the phenotype of microglia/macrophages was M1 type-dominant in both types of mice at the lesion site and lumbar enlargement. A decrease of M1-type microglia/macrophages was seen in plt mice compared with wild-type, while the number of M2-type microglia/macrophages did not differ between these mice. In immunoblot analysis, expression of M1-induced cytokines [tumor necrosis factor-α (TNF-α), interferon-γ (IFN-γ)] was decreased in plt mice, while that of M2-induced cytokines interleukin-4 (IL-4, IL-10) did not differ in the two types of mice. The results of this study indicate that suppression of expression of inflammatory cytokines by decreasing the number of M1-type microglia/macrophages at the injured site and lumbar enlargement is associated with provision of an environment for reduction of NeP. These findings may be useful for the design of new therapies to alleviate NeP after SCI.

Spinal microglia-neuron interactions in chronic pain

“…In spinal cord injury model of neuropathic pain, CCL21 depletion resulted in less M1‐type microglia/macrophages cell numbers, decreased M1‐type proinflammatory cytokines (e.g., TNF‐α and IFN‐γ) and attenuated pain hypersensitivity from day 14 after injury. In contrast to the peripheral nerve injury model, in which primary sensory neurons‐derived CCL21 are required for early microglia proliferation and pain initiation, CCL21‐knockout mice produced pain‐like responses similar to wide‐type mice at the early stage of injury (e.g., day 4) 93 . These findings indicated that CCL21 was dispensable for initiation of pain hypersensitivity but contributed to chronic pain maintenance in this model.…”

Serum CCL21 as a Potential Biomarker for Cognitive Impairment in Spinal Cord Injury

“…CCL21 is an effective microglia-activating chemokine, and its role in SCI has recently been reported. Research by Honjoh and his colleagues showed that compared with wild-type mice, the number of classically activated (M1 phenotype) microglia in mice with low expression of CCL21 gene mutations in the SCI model was significantly reduced [ 27 ]. This study suggests that CCL21 may be an important target for intervention in neuroinflammation after SCI.…”

Gas6 Inhibits Toll-Like Receptor-Mediated Inflammatory Pathways in Mouse Microglia via Axl and Mer

“…For example, Gas6 has previously been shown to have a suppressive effect on TLR-mediated pro-inflammatory cytokine production in cardiomyocytes (Grommes et al, 2008), microglial cell lines (Li et al, 2019), mouse macrophages (Deng et al, 2011) and primary murine microglia (Binder et al, 2008). There is growing evidence linking alleviation of inflammation to morphological alterations in microglia which are associated with pro-inflammatory signaling responses and cytokine expression (Zhang et al, 2014(Zhang et al, , 2019Kalakh and Mouihate, 2017;Honjoh et al, 2019). This study used WT microglia to observe the potential modulatory effects of Gas6 on microglial morphological characteristics.…”