Systemic injections of lipopolysaccharide accelerates myelin phagocytosis during Wallerian degeneration in the injured mouse spinal cord

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Abstract
The phagocytic cell response within the injured spinal cord is inefficient, allowing myelin debris to remain for prolonged periods of time within white matter tracts distal to the injury. Several proteins associated with this degenerating myelin are inhibitory to axon growth and therefore prevent severed axons from regenerating. Inflammatory agents such as lipopolysaccharide (LPS) can stimulate both the migration and phagocytic activity of macrophages. Using in situ hybridization, we found that the expression of the LPS membrane receptor, CD14, was enhanced in the mouse dorsal column following a dorsal hemisection. Double labeling studies showed that microglia and macrophages are the two major cell types expressing CD14 mRNA following spinal cord injury (SCI). We therefore tested whether systemic injections of LPS would increase the number and phagocytic activity of macrophages/microglia in the ascending sensory tract (AST) of the mouse dorsal column following a dorsal hemisection. Mice were treated daily via intraperitoneal injections of either LPS or phosphate-buffered saline (PBS). At 7 days post-SCI, greater numbers of activated mononuclear phagocytes were present in the AST undergoing Wallerian degeneration (WD) in LPS-treated animals compared with controls. Animals treated with LPS also exhibited greater Oil Red O staining, which is specific for degenerating myelin and macrophages phagocytosing myelin debris. Myelin clearance was confirmed at 7 days using Luxol Fast Blue staining and on toluidine blue-stained semi-thin sections. These results indicate that it is possible to manipulate the innate immune response to accelerate myelin clearance during WD in the injured mouse spinal cord.