TNF and increased intracellular iron alter macrophage polarization to a detrimental M1 phenotype in the injured spinal cord

Macrophages and microglia can be polarized along a continuum toward a detrimental (M1) or a beneficial (M2) state in the injured CNS. Although phagocytosis of myelin in vitro promotes M2 polarization, macrophage/microglia in the injured spinal cord retain a predominantly M1 state that is detrimental to recovery. We have identified two factors that underlie this skewing toward M1 polarization in the injured CNS. We show that TNF prevents phagocytosis-mediated conversion from M1 to M2 cells in vitro and in vivo in spinal cord injury (SCI). Additionally, iron that accumulates in macrophages in SCI increases TNF expression and the appearance of a macrophage population with a proinflammatory mixed M1/M2 phenotype. In addition, transplantation experiments show that increased loading of M2 macrophages with iron induces a rapid switch from M2 to M1 phenotype. The combined effect of this favors predominant and prolonged M1 macrophage polarization that is detrimental to recovery after SCI.