Hydrogen Attenuates Inflammation by Inducing Early M2 Macrophage Polarization in Skin Wound Healing

The heterogeneous and highly plastic cell populations of macrophages are important mediators of cellular responses during all stages of wound healing, especially in the inflammatory stage. Molecular hydrogen (H₂), which has potent antioxidant and anti-inflammatory effects, has been shown to promote M2 polarization in injury and disease. However, more in vivo time series studies of the role of M1-to-M2 polarization in wound healing are needed. In the current study, we performed time series experiments on a dorsal full-thickness skin defect mouse model in the inflammatory stage to examine the effects of H₂ inhalation. Our results revealed that H₂ could promote very early M1-to-M2 polarization (on days 2-3 post wounding, 2-3 days earlier than in conventional wound healing), without disturbing the functions of the M1 phenotype. Time series analysis of the transcriptome, blood cell counts, and multiple cytokines further indicated that peripheral blood monocytes were a source of H₂-induced M2 macrophages and that the functions of H₂ in macrophage polarization were not only dependent on its antioxidant effects. Therefore, we believe that H₂ could reduce inflammation in wound care by shifting early macrophage polarization in clinical settings.