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Old pro-inflammatory macrophages impair response to acute muscle injury in the elderly, and ablation of disturbed macrophages can enhance regeneration

Miranda Grounds (1), Hannah Radley-Crabb (2), Danielle Dye (2), Bridgette Lee (2), Federica Tomay (2), Kelsi Wells (2), Delia Nelson (2) and Connie Jackaman (2)

  1. School of Human Sciences, the University of Western Australia, Perth, Western Australia, Australia

  2. Curtin Health Innovation Research Institute, Bentley, Western Australia, Australia

While there is little intrinsic myonecrosis (and hence regeneration) in normal old skeletal muscles, elderly people are prone to accidental musculoskeletal injury and experience more complications with delays in healing, as also occurs after surgery. In grafted muscles, we showed (in 2010) that old autografts in mice aged 27-29 months, had a striking delay in inflammation with consequent delayed onset of myogenesis, although subsequent excellent new muscle formation. To investigate how old macrophages impact regeneration (Study 1), muscles were injured (using notexin) in young (3 months) and old (22-24 months) female mice. Muscles, blood, bone marrow, spleen and lymph nodes were sampled at days 1-14. In young mice, inflammation of necrotic muscles was pronounced by 4 days and myotubes pronounced by 7 days, with excellent new muscle at 10-14 days. In contrast, in old mice the inflammation and progression of myogenesis was delayed at 4 and 7 days, although healthy new myofibres were present by 14 days. This delayed regeneration in old muscles involved disturbed muscle-macrophages with heightened, prolonged inflammation, and increased systemic cytokines including TNF, IL-6, IFNγ, MIG (monokine induced by interferon-gamma), IP-10 (interferon-gamma induced protein-10), MIP-1a (macrophage inflammatory protein-1a) and TARC (thymus and activation regulated chemokine), plus a secondary infiltrating monocyte population from the spleen only in elderly mice. To blockade disturbed old macrophages, mice were treated with anti-F4/80 antibody daily. When given from 2 days before injury (Study 2), this accelerated muscle regeneration and reduced systemic inflammation in elderly mice. When given from 4 days after injury (Study 3), the later stages of muscle regeneration were delayed in all mice, emphasising the importance of different macrophage populations throughout regeneration. These data indicate that disturbed macrophages are key drivers of inflammatory dysregulation in the elderly and that interventions can correct this and improve regeneration; with clinical implications.