Macrophages modulate skeletal muscle wasting and recovery in acute lung injury in mice.

Skeletal muscle dysfunction in critical illnesses leaves survivors weak and functionally impaired. Macrophages infiltrate muscles; however, their functional role is unclear. We aim to examine muscle leukocyte composition and the effect of macrophages on muscle mass and function in the murine acute lung injury (ALI)-associated skeletal muscle wasting model. We performed flow cytometry of hindlimb muscle to identify myeloid cells pre-injury and time points up to 29 days after intratracheal lipopolysaccharide ALI. We evaluated muscle force and morphometrics after systemic and intramuscular clodronate-induced macrophage depletions between peak lung injury and recovery (day 5-6) versus vehicle control. Our results show muscle leukocytes changed over ALI course with day 3 neutrophil infiltration (130.5 ± 95.6cells/mg control to 236.3 ± 70.6cells/mg day 3) and increased day 10 monocyte abundance (5.0 ± 3.4%CD45⁺CD11b⁺ day 3 to 14.0 ± 2.6%CD45⁺CD11b⁺ day 10, p = 0.005). Although macrophage count did not significantly change, pro-inflammatory (27.0 ± 7.2% day 3 to 7.2 ± 3.8% day 10, p = 0.02) and anti-inflammatory (30.5 ± 11.1% day 3 to 52.7 ± 9.7% day 10, p = 0.09) surface marker expression changed over the course of ALI. Macrophage depletion following peak lung injury increased muscle mass and force generation. These data suggest muscle macrophages beyond peak lung injury limit or delay muscle recovery. Targeting macrophages could augment muscle recovery following lung injury.