Infiltrating macrophages are broadly activated at the early stage to support acute skeletal muscle injury repair

Acute skeletal muscle injury repair requires an adequate inflammatory response predominated by macrophage infiltration. We studied the activation of infiltrating macrophages by analyzing the expression of M1/M2 signature genes. Most of the intramuscular macrophages were Ly6C(hi) at day 1 after BaCl2 injection, while many were Ly6C(Io) at day 3. Ly6C(hi) macrophages at day 1 expressed a high level of both M1 and M2 genes, and the Ly6C(hi) and Ly6C(Io) macrophages at day 3 expressed a similar level of many M1/M2 genes. Infiltrating macrophages are broadly activated rather than polarized at the early stage to support acute skeletal muscle injury repair.