Macrophage activation and skeletal muscle healing following traumatic injury

Following injury to different tissues, macrophages can contribute to both regenerative and fibrotic healing. These seemingly contradictory roles of macrophages may be related to the markedly different phenotypes that macrophages can assume upon exposure to different stimuli. We hypothesized that fibrotic healing after traumatic muscle injury would be dominated by a pro-fibrotic M2a macrophage phenotype, with M1 activation limited to the very early stages of repair. We found that macrophages accumulated in lacerated mouse muscle for at least 21 days, accompanied by limited myofibre regeneration and persistent collagen deposition. However, muscle macrophages did not exhibit either of the canonical M1 or M2a phenotypes, but instead up-regulated both M1- and M2a-associated genes early after injury, followed by down-regulation of most markers examined. Particularly, IL-10 mRNA and protein were markedly elevated in macrophages from 3-day injured muscle. Additionally, though flow cytometry identified distinct subpopulations of macrophages based on high or low expression of TNFα, these subpopulations did not clearly correspond to M1 or M2a phenotypes. Importantly, cell therapy with exogenous M1 macrophages but not non-activated macrophages reduced fibrosis and enhanced muscle fibre regeneration in lacerated muscles. These data indicate that manipulation of macrophage function has potential to improve healing following traumatic injury.