Macrophage Polarization and Osteoblast Differentiation by Photobiomodulation

Photobiomodulation therapy, represented by red light, has been established to promote tissue healing and treat bone diseases. The osteoimmune microenvironment is increasingly recognized as a critical regulator of bone regeneration, in which the transition of pro-inflammatory M1 macrophages toward a pro-regenerative M2-like phenotype plays an important role. In this study, we aimed to investigate the effects of red-light PBM on macrophage polarization and the following osteoblast differentiation. First, the M1- and M2-like macrophage polarization models were established using lipopolysaccharide (LPS) and interleukin-4 (IL-4), respectively. The pre-treatment of red light (Pre-IR + LPS) decreased the expression of M1-associated markers while increasing the expression of M2-associated markers. Morphological analysis also showed that macrophages in the Pre-IR + LPS group were elongated, resembling the M2-like macrophages. At the functional level, Pre-IR + LPS-treated macrophages showed decreased levels of LPS-induced pro-inflammatory cytokines and enhanced osteogenic gene expression. When osteoblasts were cultured with conditioned medium derived from Pre-IR + LPS-treated macrophages, the migration and osteogenic differentiation were promoted. In contrast, Pre-IR treatment had no significant effects on IL-4-induced M2-like macrophages polarization. Taken together, these findings suggest that red light promotes the transition of M1-to-M2-like macrophage transition rather than further enhancing established M2-like polarization. Furthermore, red-light PBM creates a more osteogenesis-supportive osteoimmune microenvironment by suppressing pro-inflammatory signals and enhancing osteogenic cues in M2-like macrophages.