Skeletal muscles induce recruitment of Ly6C+ macrophage subtypes and release inflammatory cytokines 3 days after downhill exercise

Macrophages are one of the most versatile cells of the immune system that can express distinct subtypes and functions depending on the physiological challenge. After skeletal muscle damage, inflammatory macrophage subtypes aid muscles to regenerate and are implicated in physical training adaption. Based on this information, this study aimed to evaluate two classic mice macrophage subtypes and determine whether some inflammatory cytokines might be involved in the muscle adaption process after exercise. For this purpose, mice were exposed to an intermittent experimental protocol of downhill exercise (18 bouts of running, each bout 5 min with a 2-min rest interval, slope -16 degrees) and were euthanized before [control (CTRL)] and 1, 2 (D2), and 3 (D3) days after exercise. After euthanasia, the triceps brachii was harvested and submitted to protein extraction, immunostaining, and mononuclear digestion procedures. The muscle size, macrophage accumulation, and cytokines were determined. We observed an increase in the Ly6C(+) macrophage subtype (P <= 0.05) in D2 and D3 compared with CTRL, as well as a significant inverse correlation coefficient (-0.52; P <= 0.05) between Ly6C(+) and Ly6C(-) macrophage subtypes. Moreover, we also observed elevation in several cytokines (IL-1 beta, IFN-gamma, TNF-alpha, IL-6, and IL-13) at D3, although not IL-4, which tended to decrease at this time point (P = 0.06). Downhill exercises preferentially recruited Ly6C(+) macrophages with important proinflammatory cytokine elevation at D3. Moreover, despite the elevation of several cytokines involved with myogenesis, an increase in IL-6 and IL-13, which potentially involve muscle adaption training after acute exercise, was also observed.