The purpose of this study was to test the hypothesis that macrophage polarization is altered in old compared to young skeletal muscle, possibly contributing to the poor satellite cell response observed in older muscle tissue. Muscle biopsies were collected prior to and at 3, 24, and 72 h following a muscle-damaging exercise in young and old individuals. Immunohistochemistry was used to measure i.m. macrophage content and phenotype, and cell culture experiments tested macrophage behavior and influence on primary myoblasts from older individuals. We found that macrophage infiltration was similar between groups at 24 (young: 3712 ± 2407 vs. old: 5035 ± 2978 cells/mm3) and 72 (young: 4326 ± 2622 vs. old: 5287 ± 2248 cells/mm3) hours postdamage, yet the proportion of macrophages that expressed the proinflammatory marker CD11b were markedly lower in the older subjects (young: 74.5 ± 15 vs. old: 52.6 ± 17%). This finding was coupled with a greater overall proportion of CD206+, anti-inflammatory macrophages in the old (group: P = 0.0005). We further demonstrate in vitro that proliferation, and in some cases differentiation, of old primary human myoblasts increase as much as 30% when exposed to a young macrophage-conditioned environment. Collectively, the data suggest that old macrophages appear less capable of adapting and maintaining inflammatory function, which may contribute to poor satellite cell activation and delayed recovery from muscle damage.-Sorensen, J. R., Kaluhiokalani, J. P., Hafen, P. S., Deyhle, M. R., Parcell, A. C., Hyldahl, R. D. An altered response in macrophage phenotype following damage in aged human skeletal muscle: implications for skeletal muscle repair.
Keywords: immune cells; inflammation; muscle damage; primary myoblasts; satellite cell.