Myelin as an Inflammatory Mediator: Myelin Interactions with Complement, Macrophages, and Microglia in Spinal Cord Injury

Spinal cord injury (SCI) triggers chronic intraspinal inflammation consisting of activated resident and infiltrating immune cells (especially microglia/macrophages). The environmental factors contributing to this protracted inflammation are not well understood; however, myelin lipid debris is a hallmark of SCI. Myelin is also a potent macrophage stimulus and target of complement‐mediated clearance and inflammation. The downstream effects of these neuroimmune interactions have the potential to contribute to ongoing pathology or facilitate repair. This depends in large part on whether myelin drives pathological or reparative macrophage activation states, commonly referred to as M1 (proinflammatory) or M2 (alternatively) macrophages, respectively. Here we review the processes by which myelin debris may be cleared through macrophage surface receptors and the complement system, how this differentially influences macrophage and microglial activation states, and how the cellular functions of these myelin macrophages and complement proteins contribute to chronic inflammation and secondary injury after SCI

Predictive Screening of M1 and M2 Macrophages Reveals the Immunomodulatory Effectiveness of Post Spinal Cord Injury Azithromycin Treatment

Spinal cord injury (SCI) triggers a heterogeneous macrophage response that when experimentally polarized toward alternative forms of activation (M2 macrophages) promotes tissue and functional recovery. There are limited pharmacological therapies that can drive this reparative inflammatory state. In the current study, we used in vitrosystems to comprehensively defined markers of macrophages with known pathological (M1) and reparative (M2) properties in SCI. We then used these markers to objectively define the macrophage activation states after SCI in response to delayed azithromycin treatment. Mice were subjected to moderate-severe thoracic contusion SCI. Azithromycin or vehicle was administered beginning 30 minutes post-SCI and then daily for 3 or 7 days post injury (dpi). We detected a dose-dependent polarization toward purportedly protective M2 macrophages with daily AZM treatment. Specifically, AZM doses of 10, 40, or 160 mg/kg decreased M1 macrophage gene expression at 3 dpi while the lowest (10 mg/kg) and highest (160 mg/kg) doses increased M2 macrophage gene expression at 7 dpi. Azithromycin has documented immunomodulatory properties and is commonly prescribed to treat infections in SCI individuals. This work demonstrates the utility of objective, comprehensive macrophage gene profiling for evaluating immunomodulatory SCI therapies and highlights azithromycin as a promising agent for SCI treatment

The Effects of Myelin on Macrophage Activation Are Phenotypic Specific via cPLA\u3csub\u3e2\u3c/sub\u3e in the Context of Spinal Cord Injury Inflammation

Spinal cord injury (SCI) produces chronic, pro-inflammatory macrophage activation that impairs recovery. The mechanisms driving this chronic inflammation are not well understood. Here, we detail the effects of myelin debris on macrophage physiology and demonstrate a novel, activation state-dependent role for cytosolic phospholipase-A2 (cPLA2) in myelin-mediated potentiation of pro-inflammatory macrophage activation. We hypothesized that cPLA2 and myelin debris are key mediators of persistent pro-inflammatory macrophage responses after SCI. To test this, we examined spinal cord tissue 28-days after thoracic contusion SCI in 3-month-old female mice and observed both cPLA2 activation and intracellular accumulation of lipid-rich myelin debris in macrophages. In vitro, we utilized bone marrow-derived macrophages to determine myelin’s effects across a spectrum of activation states. We observed phenotype-specific responses with myelin potentiating only pro-inflammatory (LPS + INF-γ; M1) macrophage activation, whereas myelin did not induce pro-inflammatory responses in unstimulated or anti-inflammatory (IL-4; M2) macrophages. Specifically, myelin increased levels of pro-inflammatory cytokines, reactive oxygen species, and nitric oxide production in M1 macrophages as well as M1-mediated neurotoxicity. PACOCF3 (cPLA2 inhibitor) blocked myelin’s detrimental effects. Collectively, we provide novel spatiotemporal evidence that myelin and cPLA2 play an important role in the pathophysiology of SCI inflammation and the phenotype-specific response to myelin implicate diverse roles of myelin in neuroinflammatory conditions