Ultrasound Pressure-Dependent Cytokine and Immune Cell Response Lost in Aged Muscle

Abstract

ObjectiveTherapeutic ultrasound remains a highly discussed topic in physical therapy due to uncertainty between treatment regimens and biological benefits. Its impact on aged populations, who are vulnerable to insufficient healing after muscle injury due to sarcopenia, is understudied. Despite the coupling between muscle inflammation and regeneration, research on the immune response after therapeutic ultrasound is limited. The objective of this study was to evaluate structure, inflammatory cytokine signaling and immune cell infiltration after therapeutic ultrasound in young and aging murine muscle. MethodsYoung (6-weeks-old) and Adult (52-weeks-old) male and female mice non-injured gastrocnemii were treated with either low intensity pulsed ultrasound at 2 W/cm2 (∼0.243 MPa) or high intensity pulsed focused ultrasound at 554 W/cm2 (∼5.96 MPa). Cytokine expression was evaluated at 1-, 8-, and 24-hours, cell infiltration was measured via flow cytometry at 1- and 24-hours, and immunofluorescence assessed muscle fiber area, fibrosis and satellite cells at 24-hours after sonication. ResultsLow intensity pulsed ultrasound induced an early, transient inflammatory response where IL-15 and macrophages (M2 > M1) were increased 1-hour post-sonication. High intensity pulsed focused ultrasound caused a late, extended immune response where MCP-1, neutrophils, monocytes, and macrophages (M1 > M2) were increased 24-hours post-sonication. Notably, these changes manifested solely in Young gastrocnemius. The Adult gastrocnemius exhibited decreased cytokine expression (IL-1α, IL-6, IL-15, M-CSF) and no alteration in immune cell recruitment post-sonication. There was no damage to muscle structure. ConclusionsTherapeutic ultrasound induced a pressure dependent inflammatory response that can augment or mitigate intrinsic muscle cytokine signaling and cell recruitment in adolescent or aged muscle, respectively.