Macrophage and microglial plasticity in the injured spinal cord

• The diversity and plasticity of microglia and macrophage responses in vitro and in spinal cord injury are discussed.
• We discuss the evolving thinking in the field of macrophage polarization.
• Phagocytosis of myelin and red blood cells, and iron uptake modulates macrophage polarization in vitro and in vivo.
• TNF modulates macrophage polarization in vivo in the injured spinal cord.

Macrophages in the injured spinal cord arise from resident microglia and from infiltrating peripheral myeloid cells. Microglia respond within minutes after central nervous system (CNS) injury and along with other CNS cells signal the influx of their peripheral counterpart. Although some of the functions they carry out are similar, they appear to be specialized to perform particular roles after CNS injury. Microglia and macrophages are very plastic cells that can change their phenotype drastically in response to in vitro and in vivo conditions. They can change from pro-inflammatory, cytotoxic cells to anti-inflammatory, pro-repair phenotypes. The microenvironment of the injured CNS importantly influences macrophage plasticity. This review discusses the phagocytosis and cytokine-mediated effects on macrophage plasticity in the context of spinal cord injury.