The effects of myelin on macrophage activation are phenotypic specific via cPLA 2 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 (cPLA 2 ) in myelin-mediated potentiation of pro-inflammatory macrophage activation. We hypothesized that cPLA 2 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 cPLA 2 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 (cPLA 2 inhibitor) blocked myelin’s detrimental effects. Collectively, we provide novel spatiotemporal evidence that myelin and cPLA 2 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.