Robust immune cell infiltration and macrophage senescence occur within a week of recovery after limb immobilization in older adult skeletal muscle

Immune cells are critical for modulating inflammation and extracellular matrix remodelling for effective muscle regrowth following disuse atrophy. However, disrupted macrophage function and accumulation of cellular senescence may limit muscle recovery in ageing. The present study aimed to compare changes in the cellular dynamics of muscle macrophages, cellular senescence and collagen content during early recovery following 14 days of unilateral limb immobilization in young (n = 18; ∼24 years) and older male and female adults (n = 18; ∼69 years). Participants underwent 14 days of immobilization followed by 7 days of re-ambulation. Muscle biopsies were collected at baseline, post-immobilization, and at 2 and 7 days of recovery. During re-ambulation, older adults exhibited elevated immune cell infiltration (haematoxylin and eosin, CD45⁺), higher CD68⁺ CD206⁺ macrophage content and greater muscle collagen deposition (Sirius Red) compared to their young counterpart. Furthermore, cellular senescence (SA-β-galactosidase⁺) was elevated, including a high number of macrophages co-labelled with p21 in older skeletal muscle during recovery. At 7 days of recovery, the amount of macrophage infiltration was positively associated with cellular senescence, whereas the senescent macrophage cell population was significantly correlated to the Sirius Red percent area. Our findings suggest that an age-altered immune cell response and the accumulation of senescent macrophages may disrupt collagen remodelling during early muscle recovery following disuse. KEY POINTS: Age-related impairments in muscle recovery following disuse remain a significant challenge. Immune cells, particularly macrophages, play critical role in muscle remodelling. Muscle macrophage characteristics during the early phase of recovery following disuse in older adults remain unclear. We compared changes in immune cell content, cellular dynamics, cellular senescence, and collagen content during recovery (2 days and 7 days) following 14 days unilateral limb immobilization in young (18-35 years) and older male and female adults (≥60 years). During muscle recovery, older adults (vs. young), increased muscle collagen content concurrent with an infiltration of macrophages. This response was characterized by a distinct predominance of CD68⁺ CD206⁺ macrophages and a parallel increase in senescent macrophages. Our findings suggest that an altered immune cell response and accumulation of senescent macrophages may disrupt tissue remodelling during muscle recovery in aged muscle.